**Facile Synthesis of Bio-Template Tubular MCo2O4 (M** = **Cr, Mn, Ni) Microstructure and Its Electrochemical Performance in Aqueous Electrolyte**

### **Deepa Guragain 1, Camila Zequine 2, Ram K Gupta <sup>2</sup> and Sanjay R Mishra 1,\***


Received: 17 January 2020; Accepted: 10 March 2020; Published: 16 March 2020

**Abstract:** In this project, we present a comparative study of the electrochemical performance for tubular MCo2O4 (M = Cr, Mn, Ni) microstructures prepared using cotton fiber as a bio-template. Crystal structure, surface properties, morphology, and electrochemical properties of MCo2O4 are characterized using X-ray diffraction (XRD), gas adsorption, scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), cyclic voltammetry (CV), and galvanostatic charge-discharge cycling (GCD). The electrochemical performance of the electrode made up of tubular MCo2O4 structures was evaluated in aqueous 3M KOH electrolytes. The as-obtained templated MCo2O4 microstructures inherit the tubular morphology. The large-surface-area of tubular microstructures leads to a noticeable pseudocapacitive property with the excellent electrochemical performance of NiCo2O4 with specific capacitance value exceeding 407.2 F/g at 2 mV/s scan rate. In addition, a Coulombic efficiency ~100%, and excellent cycling stability with 100% capacitance retention for MCo2O4 was noted even after 5000 cycles. These tubular MCo2O4 microstructure display peak power density is exceeding 7000 W/Kg. The superior performance of the tubular MCo2O4 microstructure electrode is attributed to their high surface area, adequate pore volume distribution, and active carbon matrix, which allows effective redox reaction and diffusion of hydrated ions.

**Keywords:** bio-template; MCo2O4 (M = Cr, Mn, Ni); electrochemical; cyclic voltammetry; specific capacitance

### **1. Introduction**

Supercapacitors (SCs) are the energy storage device. SCs are in high demand because of their greater power density compared to batteries and higher energy density compared to that of capacitors [1]. In the capacitor, there is no time lag during the charging process; hence it can give higher power density, and in the battery, there is low self-discharge so that it can provide higher energy density [2]. Because of this, a supercapacitor is easy to charge within a short time and able to show significant performance even after prolonged use. SCs are of three types: (i) electric double-layer capacitors (EDLCs), (ii) pseudo capacitors (PCs), and (iii) hybrid capacitors. EDLCs are based on the principle that physical adsorption takes place on the interface of a solid electrode, usually carbon-based material, and liquid electrolytes [3,4]. In PCs, surface redox reaction takes place at the electrode-electrolyte interface, which is responsible for storing electronic charges [5], where metal oxides and conducting polymer-based materials are used as active electrode materials [6,7]. EDLCs have lower specific capacitance and energy density as compared to PCs, hence, practically PCs are in higher demand. A hybrid capacitor is a combination of both EDLCs and PCs; examples are carbon nanotubes, graphene,

etc. [8,9]. They display hybrid charge–storage mechanisms and have the ability to deliver higher capacity [10].

The electrolyte ion transport in supercapacitor devices occurs through an ion-transport layer separated from the electrode. The charge storage mechanism follows at the electrode surface during the charging-discharging process [11].

Transition metal oxides (TMOs) with novel nano-architectures and rich in redox reactions are increasingly explored for their application in energy storage devices [12]. Among these, cobalt oxides are highly attractive because of their higher theoretical value for specific capacitance, i.e., 3560 F/g [13]. The nanoarchitecture of these metal-oxides is controlled by the synthesis route, which often requires complex technological strategies, including toxic organic reagents, which might make it difficult for their mass industrial application. Thus, it is highly sought to explore cost-effective facile synthesis strategies and environmentally benign techniques for preparing these electrodes. Furthermore, the ideal electrode should have a high specific surface area for better specific capacitance, controlled porosity for better rate capability as well as specific capacitance, and higher electronic conductivity to improve rate capability and power density of supercapacitor. Nowadays, the bio-templating technique has emerged as a convincing technique for the preparation of oxide supercapacitors [14–17]. Nature offers rich and diverse bio-templates [18–20] like bamboo, lotus pollen grains [21], leaf [22], sorghum straw [23], butterfly wing [24], jute fibers [25], and cotton [12]. Such bio-templates offer elaborate interior and exterior surfaces, and geometries, which make these templates attractive materials to produce multiscale hybrid and hierarchical morphologies.

Numbers of research are published on the study of transition metal oxide-based electrodes such as MnCo2O4 [5,26–28] and NiCo2O4 [29–33], Co3O4 [34], and NiFe2O4 [35] for their supercapacitive application. The benefit of transition metal oxides as electrode materials are innumerable, as their multiple oxidation states facilitate multiple redox reactions during electrochemical reaction vis-a-vise offers stable structure. The co-existence of two different cations provides abundant active sites to perform fast reversible faradic redox reaction on the electrode interface; as a result, higher specific capacity, and excellent rate capability are achieved [25,36]. Additionally, the types of bonds between transition metal ions and ligands are dictated by electronegativity and ionization energies [37]; with the former, the structure is dense, while with later the structure is more open. The valence state, ionic radius, electronegativity [38], and the local environment of the cations are affected by the change in Gibbs free energy and electrochemical potential of the electrode. An increase in the electrochemical potential of cathodes is observed with the increase in the number of electrons in *d* orbitals of transition metal elements. This implies a higher consumption of energy during electron transfer [39]. In mixed transition metal oxides, there is a synergetic effect between metal cations; this produces higher electrical conductivity of single metal oxide where there is low activation energy to transfer electron between metal cations and gives excellent structural stability [40,41].

Here we present a comparative study to understand the electrochemical behavior of MCo2O4 (M = Cr, Mn, and Ni) electrodes prepared via a facile bio-template method. The electronegativity differences among M ions viz. Cr (1.66), Mn (1.55), Ni (1.99), and Co (1.88) could have a substantial effect on the electrochemical performance of the said electrodes. The electronegativity difference determines the structure, covalent vs. ionic, and the electric potential of the electrode for the charge transfer, as discussed above. In the present study, MCo2O4 electrode material is prepared via the bio-template method, where the product assumes the morphology of the microstructure of bio-template and ends up with a carbon matrix. The bio-template method adapted to produce active material inherently fixed in the carbon matrix. The carbon matrix is known to enhance electrode electrochemical performance [42]. The template supported mineralization MCo2O4 at room temperature (RT) produces 3D-hierarchical and porous-MCo2O4 superstructures with tubular-like morphologies. The doped Co3O4 (MCo2O4) is explicitly explored in this study as dopant atoms or vacancies are known to affect the crystal field [43], thus modifying the electronic structure and adjusting the electrochemical potential [44].

### **2. Experimental**

### *2.1. Synthesis*

All the chemicals required for the synthsis, such as Cobalt nitrate hexahydrate; Co(NO3)2.6H2O, Chromium nitrate hexahydrate; Cr(NO3)2.6H2O, Manganese nitrate hexahydrate; Mn(NO3)2.6H2O, and Nickel nitrate hexahydrate; Ni(NO3)2.6H2O were purchased from Sigma-Aldrich, St. Louis, Missouri, USA. The spinel MCo2O4 (M = Cr, Mn, Ni) was synthesized by a facile bio-template method. Quantities of 1.16 g of Co(NO3)2.6H2O and 0.8 gm of Cr(NO3)2.6H2O, 0.55 g of Co(NO3)2.6H2O and 0.17 gm of Mn(NO3)2.6H2O, and 0.86 g of Co(NO3)2.6H2O and 0.43 gm of Ni(NO3)2.6H2O were mixed in 15 ml of distilled water separately, and the mixture was ultra-sonicated for 10 minutes to make a homogenous solution. Then, 1.0 g of cotton was soaked in the mixture solutions for 5 minutes. The resulting soaked cotton was filtered and dried at 150 ◦C for 30 minutes. The dried cotton was later calcined at 520 ◦C for 3 hours in the air to obtain bio-templated CrCo2O4, MnCo2O4, and NiCo2O4 tubular microstructure.

### *2.2. Characterization*

The x-ray diffraction patterns were obtained via Bruker D8 Advance X-ray diffractometer (Bruker Corporation, Madison, WI, USA) using Cu *K*α radiation to check phase purity and determine the crystalline parameters of as-prepared samples. A scanning electron microscope (Phenom) at 10 keV analyzed the morphology of samples. The Brunauer–Emmett–Teller (BET) method was used to measure the specific surface area of the samples. The surface area measurement was carried out by adsorption-desorption isotherms at 77 K, (Quantachrome, Boynton Beach, FL 33426, model No. AS1MP) using nitrogen as adsorbing gas. Thermogravimetric analyses (TGA, Instrument Specialist, Inc., Twin Lakes, WI, USA), were performed in 24 to 550 ◦C temperature range. FTIR spectra were collected via Theromo-Fisher Scientific FTIR spectrometer (Nicolet iS10, Thermo Fisher Scientific, Waltham, MA, USA) between 450 and 1000 cm<sup>−</sup>1.

Versastat 4–500 electrochemical workstation (Princeton Applied Research, USA) was used to perform electrochemical measurements in a standard three-electrode configuration. To prepare an electrode, slurry pastes of 80 wt % of the synthesized powder, 10 wt % of acetylene black, and 10 wt % of polyvinylidene difluoride (PVDF) were mixed in the presence of N-methyl pyrrolidinone (NMP). The thoroughly mixed paste was applied onto a nickel foam. Here, the active mass is 80% out of the total pasted mass in the electrode. The prepared electrodes were dried under vacuum at 60 ◦C for 10 hours. The loading mass of all samples was about 2–3 mg, measured by weighing the nickel foam before and after deposition with an analytical balance (MS105DU, Mettler Toledo, 0.01 mg of resolution). MCo2O4 (M = Cr, Mn, Ni) coated nickel foam was used as a working electrode, a saturated calomel electrode (SCE) as a reference electrode, and a platinum wire as a counter electrode. The electrochemical performance of the electrodes was evaluated at RT in 3M KOH electrolyte via cyclic voltammetry and galvanostatic charge-discharge techniques measurements.

### **3. Results and Discussion**

Figure 1a shows the XRD patterns of the bio-templated CrCo2O4, MnCo2O4, and NiCo2O4 microstructure. The XRD patterns match with the face-centered cubic phase of CrCo2O4, MnCo2O4, and NiCo2O4 (International Centre for Diffraction Data (ICDD) #02-0770). The main peaks at 30.9◦, 36.4◦, 44.3◦, 58.6◦, and 64.3◦ for CrCo2O4, 31.1◦, 36.7◦, 44.7◦, 59.2◦, and 65.9◦ for MnCo2O4, and 31.3◦, 36.8◦, 44.8◦, 59.4◦, and 65.2◦ for NiCo2O4 can be assigned to the (220), (311), (400), (511) and (440) reflections of CrCo2O4, MnCo2O4, and NiCo2O4 respectively [45,46]. The pattern of NiCo2O4 shows a peak at 43.2◦, which indicates the formation of NiO cubic phase as also confirmed by TOPAS fitting. The lattice constants obtained using *d*-spacing for the sample are *a* = *b* = *c* = 0.816 nm, 0.808 nm, 0.807 nm, and for CrCo2O4, MnCo2O4, and NiCo2O4, respectively. The crystallite size of CrCo2O4, MnCo2O4, and NiCo2O4 as calculated using Scherrer's formula [47] is around 10.57 nm, 14.65 nm, and 19.97 nm for

CrCo2O4, MnCo2O4, and NiCo2O4, respectively (Table 1). FTIR spectrum, Figure 1b, further identifies the structure of the bio-templated MCo2O4. The FTIR spectrum displays two distinct bands at 515.7 (ν*1*) and 637 (ν*2*) cm<sup>−</sup>1, which arise from the stretching vibrations of the metal-oxygen bonds [48–50]. The ν*<sup>1</sup>* band is characteristic of M-O (M = Cr, Mn, Ni) vibrations in octahedral coordination, and the ν*<sup>2</sup>* band is attributable to M-O (M - Co) bond vibration in tetrahedral coordination. These frequency bands are the signature vibrational bands for the spinel lattice [51]. Hence FTIR spectrum at 519.1, 519.02, and 519.08 cm−<sup>1</sup> indicate stretching vibration of Co3<sup>+</sup>-O2<sup>−</sup> in the octahedral sites, and at 638.6, 639.9, and 641.3 cm−<sup>1</sup> indicate vibration of Cr3<sup>+</sup>-O−, Mn2<sup>+</sup>-O2−, and Ni2+-O<sup>−</sup> at tetrahedral sites for CrCo2O4, MnCo2O4, and NiCo2O4, respectively [52]. The presence of vibration bands confirms the development of pure phase spinal CrCo2O4, MnCo2O4, and NiCo2O4 nanostructures.

**Table 1.** Crystallite size and physical properties of MCo2O4 (M = Cr, Mn, Ni) determined using XRD, the Barrette–Joyner–Halenda (BJH) method, and Brunauer–Emmett–Teller (BET) surface area analyzer.


**Figure 1.** (**a**) x-ray diffraction pattern, (**b**) FTIR, (**c**) adsorption-desorption curve and inset pore volume distribution, and (**d**) thermogravimetric curve of tubular MCo2O4 (M = Cr, Mn, Ni) structures.

Figure 1c shows the BET specific surface area of tubular MCo2O4 microstructures. The specific surface area was determined from N2 adsorption-desorption isotherms obtained at 77 K between relative pressure P/Po~0.029 to 0.99, and the Barrette–Joyner–Halenda (BJH) method was used for measuring corresponding pore sized distributions. The type IV isotherm hysteresis loops [53] suggest the existence of mesopores in the samples. The BET specific surface area of biomorphic CrCo2O4, MnCo2O4, and NiCo2O4 are 34.4 m2/g, 32.2 m2/g, and 18.9 m2/g, respectively. Figure 1c inset shows the pore size distribution. Inset curves indicate having a more favorable condition for the fast ion transport phenomenon within the electrode surface [54–57], which is confirmed by the presence of a significant number of pores distribution at around 0.4 nm to 4.3 nm with the highest pore volume. Additionally, the large BET surface area of tubular MCo2O4 superstructures can provide plenty of superficial electrochemical active sites to participate in the Faradaic redox reactions.

The thermogravimetric analysis was conducted on the infiltrated samples (cotton dipped in a mixture of chemical solution and filtered it) to understand the temperature dependence mechanism of the formation of biomorphic MCo2O4. Figure 1d shows the TGA plots of MCo2O4 measured in the temperature range of 24 to 550 ◦C. The formation of MCo2O4 from the nitrate salts results in three steps. The weight loss at around 110 ◦C for all three MCo2O4 is due to water desorption, the second weight loss up to 187 ◦C for CrCo2O4, 241 ◦C for MnCo2O4, and 160 ◦C for NiCo2O4 is due to burning of cotton and start of decomposition of Co(NO3)2·6H2O and Cr(NO3)2·6H2O, Co(NO3)2·6H2O and Mn(NO3)2·6H2O, Co(NO3)2·6H2O and Ni(NO3)2·6H2O respectively, there was no weight loss at beyond 315, 320, and 200 ◦C which signifies the completion of the formation of CrCo2O4, MnCo2O4, and NiCo2O4. Upon immersing fiber into the precursor solution, the water and Cr(NO3)2·6H2O, Mn(NO3)2·6H2O, Co(NO3)2·6H2O and Ni(NO3)2·6H2O molecules were absorbed onto the hydroxyl-group-rich cotton fiber substrate. With the heat treatment above 520 ◦C, nitrate salts decomposed in the form CrCo2O4, MnCo2O4, and NiCo2O4 as follow [58],

$$2\text{Co(NO}\_3\text{)}\_2\text{-}6\text{H}\_2\text{O} + \text{Cr(NO}\_3\text{)}\_2\text{-}6\text{H}\_2\text{O} \rightarrow \text{CrCr}\_2\text{O}\_4 + 2\text{O}\_2 + 6\text{NO}\_2 + 18\text{H}\_2\text{O} \tag{1}$$

$$2\text{Co(NO}\_3\text{)}\_2\text{-}6\text{H}\_2\text{O} + \text{Mn(NO}\_3\text{)}\_2\text{-}6\text{H}\_2\text{O} \rightarrow \text{MnCo}\_2\text{O}\_4 + 2\text{O}\_2 + 6\text{NO}\_2 + 18\text{H}\_2\text{O} \tag{2}$$

$$2\text{Co(NO}\_3\text{)}\_2\text{-}6\text{H}\_2\text{O} + \text{Ni(NO}\_3\text{)}\_2\text{-}6\text{H}\_2\text{O} \rightarrow \text{NiCo}\_2\text{O}\_4 + 2\text{O}\_2 + 6\text{NO}\_2 + 18\text{H}\_2\text{O} \tag{3}$$

With the increase in calcination temperature, the removal of organic substance was achieved where the remaining few portions of the organic substance change into carbon.

FE-SEM in Figure 2a–d displays tubular morphology of the cotton fibers, samples CrCo2O4, MnCo2O4, and NiCo2O4, respectively, which resembles a biomorphic structure. Figure 3a–c shows SEM images obtained using elemental mapping at chromium, manganese, and nickel energy peaks and shows that the tubular structure is well decorated with the CrCo2O4, MnCo2O4, and NiCo2O4 nanoparticles. Table 2 gives the elemental composition and element distribution, which is obtained via EDX (energy dispersive x-ray spectroscopy).

**Figure 2.** SEM images of (**a**) cotton fiber, (**b**), (**c**), and (**d**) bio-templated tubular MCo2O4 (M = Cr, Mn, Ni) structures.

**Figure 3.** (**a**), (**b**), and (**c**) shows EDX mapping of tubular MCo2O4 (M = Cr, Mn, Ni) structures, respectively.

**Table 2.** Elemental composition in wt % for MCo2O4 (M = Cr, Mn, Ni) obtained using energy dispersive X-Ray analysis (EDX). The elemental composition is approximately determined using EDX. Ideally, EDX can prove which elements are abundant in the particles, but not obtain the exact chemical composition.


The type of electrolytes and their molar concentration play a vital role in determining the electrochemical behavior of oxide electrodes [59–61]. Therefore, many aqueous electrolytes such as sulfates K2SO4, H2SO4, KNO3, Na2SO4, hydroxyl KOH, NaOH, LiOH, and chlorides KCl, NaCl have been explored to be used in supercapacitors [62–66]. The ultimate performance of the electrode is based on the properties of the electrode material and the intercalation efficiency of the cations [51]. Since KOH electrolyte provides lower electrochemical series resistance with better conductivity as compared to other electrolytes [67], KOH is chosen as an electrolyte in this study for the electrochemical measurement.

Cyclic voltammetry and charge-discharge curves were measured to investigate the electrochemical behavior of MCo2O4 nanoparticles. Figure 4 displays the CV curves for tubular MCo2O4 electrodes measured in the 3M KOH electrolyte. Figure 4a,c, and Figure 4e shows the CV curves measured in the voltage window of 0.0 to 0.6 V and measured at different scan rate from 2 to 300 mV/s. A pair of redox peaks associated with the redox reactions involved in the alkaline electrolyte during the charging and discharging process was observed in all CV plots. The CV curve is asymmetric, which indicates a quasi-reversible redox reaction [68], the anodic and cathodic peak separation are 0.121 V, 0.124 V, and 0.123 V at 2 mV/s and 0.310 V, 0.186 V, and 0.333 V at 300 mV/s for CrCo2O4, MnCo2O4 and NiCo2O4 respectively. The presence of anodic and cathodic peaks, indicating the usefulness of

the materials as a pseudocapacitor. Typical pseudo-capacitance behavior of MCo2O4 nanostructures arises from the reversible surface or near-surface Faradic reactions for charge storage. The reversible redox reaction involved in the charge-discharge process for MCo2O4 can be described as follows by Equations (5)–(7) [69–71].

$$\text{MO} + \text{OH}^- \leftrightarrow \text{MOOH} + \text{e}^- \tag{4}$$

$$\rm CrCo\_2O\_4 + OH^- + H\_2O \leftrightarrow CrOOH + 2CoOH + e^- \tag{5}$$

$$\rm MnCo\_2O\_4 + OH^- + H\_2O \leftrightarrow MnOOH + 2CuOOH + e^- \tag{6}$$

$$\text{NiCo}\_2\text{O}\_4 + \text{OH}^- + \text{H}\_2\text{O} \leftrightarrow \text{NiOOH} + 2\text{CoOOH} + \text{e}^- \tag{7}$$

**Figure 4.** (**a**,**c**,**e**) show cyclic voltammetry curves of tubular MCo2O4 (M = Cr, Mn, Ni) electrode obtained in the scan range of 5 mV/s to 300 mV/s measured in 3M KOH electrolyte. (**b**,**d**,**f**) show cyclic stability curves measured up to 1000 cycles in 3M KOH electrolyte at scan rate of 40 mV/s.

Pseudocapacitive characteristics of electrodes are indicated by a non-rectangular form of CV curves. Within the potential range from 0 to 0.6 V, a pair of reversible redox peaks can be observed. With the increase in the scan rate, a small positive shift of the oxidation peak potential and a negative shift of the reduction peak potential was observed, which can be primarily attributed to the influence of the increasing electrochemical polarization as the scan rate scales up. Pairs of reversible redox curve are indicative of pseudocapacitive behavior of the material with redox peaks attributed to M(II)/M(III) redox process [72]. The redox potentials and shape of the CV curves are comparable to those reported

for CrCo2O4, MnCo2O4, and NiCo2O4 electrodes [24,73–76], suggesting that the measured capacitance mainly arises from the redox mechanism.

Figure 5a shows the specific capacitance, *Csp*, as a function of the voltage scan rate of the tubular MCo2O4 electrode. The specific capacitance, *Csp*, was calculated from the CV plots using the following Equation (8) [77].

$$\mathcal{C}\_{sp} = \frac{\int\_{V1}^{V2} i \ast V \ast dV}{m \ast v \ast (V2 - V1)} \tag{8}$$

where *V1* and *V2* stand for the working potential limits, *i* stands for the current, *m* stands for the mass of the electroactive materials, and *v* is the scan rate in mV/s.

**Figure 5.** (**a**) Specific capacitance vs. scan rate, (**b**) and peak current vs. (scan rate)1/2, and (**c**) diffusion and capacitive contribution to the specific capacitance.

It is evident for this figure that the electrode displays higher *Csp* up to 407.2 F/g for NiCo2O4 in 3M KOH electrolyte at 2 mV/s, which is higher than the value that is observed for either CrCo2O4 (*Csp* ~ 403.2 F/g) or MnCo2O4 (*Csp* ~ 378.1 F/g) electrode, value are given in Table 3. The specific capacitance for higher scan rates (>50 mV/s) remains practically constant because of limited ion movement only at the surface of the electrode material. Hence EDLC becomes a dominant mechanism at higher scan rates. At lower scan rates (<5 mV/s), the majority of active surface are utilized by the ions for charge storage, and hence resulting in the higher specific capacitance. The CV cyclic stability of the electrode was tested for 1000 cycles. Figure 4b,d, and Figure 4f show no significant differences in the CV curves after the 100th, 500th, and 1000th cycle of repetition. The CV curves clearly show that the current response is proportionally increased with the scan rate, indicating an excellent capacitive behavior of the electrode materials. This can be ascribed to facile ion diffusion and large specific surface area of the electrode materials. Furthermore, there is almost no relation between the shape of CV curves and scan rates, which can be associated with the electron conduction and improved mass transportation of electrode material [78].


**Table 3.** Data of specific capacitance, energy density, and power density for MCo2O4 (M = Cr, Mn, Ni)


The total stored charge has a contribution from three components; first is the Faradaic contribution coming from the insertion process of electrolyte ions, second is the faradaic contribution from the charge-transfer process with surface atoms, and third is pseudocapacitance and nonfaradic contribution from the double layer effect [79]. Both pseudocapacitance and double-layer charging are substantial, due to their higher surface area of nanoparticles. The capacitive effects are characterized by analyzing the cyclic voltammetry data at various scan rates according to [80,81],

$$
\dot{a} = a v^{\flat} \tag{9}
$$

where *i, v, a* and *b,* are peak current (A), voltage scan rate (mV/s), and fitting parameters, respectively. The charge storage mechanism is defined based on the value of the constant *b*, where *b* = 1 defines capacitive or *b* = 0.5 defines diffusion-limited charge storage mechanism. Fitting the peak current, *i*, vs. square root of the scan rate, SQRT (scan rate), *v-1*/*2,* curves, Figure 5b, with Equation (6), gives *b* values of ~ 0.646, 0.711, and 0.648 for CrCo2O4, MnCo2O4, and NiCo2O4, respectively. This obtained *b* value for our sample MCo2O4 (M = Cr, Mn, Ni) indicates the diffusive nature of the charge storage mechanism is prominent for NiCo2O4 as compared to the other two.

Usually, the contribution to the current response at fixed potential comes from surface capacitive effects and diffusion-controlled insertion processes [82,83]. These contributions to the specific capacitance could be separated using the following Equation (10):

$$\mathbb{C}\_{sp} = k\_1 + k\_2 \,\,\text{v}^{-1/2} \tag{10}$$

For which *k1* and *k2* can be determined from the *Csp vs. v*−1/<sup>2</sup> linear plot with slope *k2* and intercept *k1*. *k1* and *k2* are fractions of diffusion and capacitive contribution to the net specific capacitance at a given voltage rate. The *Csp* was plotted against the slow scan rate up to 20 mV/s, and a regression fit was performed using Equation (10). The obtained *k1* and *k2* values were used to determine the fractional contribution to the net specific capacitance. Figure 5c shows capacitive and diffusive fractional contributions to net specific capacitance for a slow scan rate of up to 20 mV/s. By comparing the lower green area with the total capacitance, we find that capacitive effects contribute by 48%, 54%, and 38% of the total specific capacitance for CrCo2O4, MnCo2O4, and NiCo2O4, respectively.

Figure 6a,c, and Figure 6e show the galvanostatic charge-discharge (GCD) plots measured in the voltage window of 0.0 to 0.6 V at different current densities between 0.75 A/g to 30 A/g in 3M KOH. From the observed non-linearity between the potential and time, it is confirmed that the capacitance of the studied materials is not constant over the studied potential ranges. The specific capacitance of electrodes was calculated using the following Equation (11):

$$\mathbb{C}\_{sp} = \frac{I \ast t}{m \ast \Delta V} \tag{11}$$

where *Csp*, *I*, Δ*V*, *m*, *and t* are the specific capacitance (F/g), charge-discharge current (A), the potential range (V), and the mass of the electroactive materials, and the discharging time (s), respectively. The GCD curves with a plateau, usually displayed by oxide electrodes, show pseudocapacitive behavior of electrode with respect to their discharging time for all electrolytes. This typical GCD behavior could arise from the electrochemical adsorption-desorption of OH- electrolyte and/or a redox reaction at the interface of electrode/electrolyte [84,85]. It is observed that the discharging time

in biomorphic MCo2O4 is longer for CrCo2O4 in the KOH electrolyte. The specific capacitances of biomorphic CrCo2O4, MnCo2O4, and NiCo2O4 at 1 A/g are 231 F/g, 161 F/g, and 190 F/g in 3M KOH electrolytes, respectively are shown in Table 3. Figure 7a shows the dependence of current density on the specific capacitance of the electrode material. Usually, insufficient Faradic redox reaction is achieved at the high discharge current densities. This leads to increased potential drop due to the resistance of tubular MoCo2O4 electrode resulting in an observed decrease in capacitance with the increased discharge current density. This implies ion penetration is feasible at lower current densities where ions have access to the inner structure, and thus all active area of the electrode. However, at higher current densities, the effective use of the material is limited to only the outer surface of the electrode. The specific capacitance of MCo2O4 electrodes in this study is compared with the literature values at current density 1 A/g, 2 A/g, and 5 A/g and are listed in Table 4. It is evident from Table 4 that electrochemical performance of bio-templated MCo2O4 comparable and, in some cases, outperformed electrodes prepared via other techniques.

**Figure 6.** (**a**,**c**,**e**) show charge-discharge (CD) curves of tubular MCo2O4 (M = Cr, Mn, Ni) electrode measured in the current density window of 1 to 30 A/g in 3M KOH electrolyte, where red color CD curve is for 1A/g, blue color CD curve is for 1.5A/g, orange color CD curve is for 2A/g and continuously time is decreasing with increasing current density. (**b**,**d**,**f**) show cyclic stability (black color) and coulombic efficiency (blue color) tested at 10 A/g current density for 5000 cycles in 3M KOH electrolytes of MCo2O4 (M = Cr, Mn, Ni).

**Figure 7.** (**a**) Comparison of specific capacitance as a function of current density and (**b**) Ragone plot of power density vs. energy density.

**Table 4.** Comparison of electrochemical performance of MCo2O4 (M = Cr, Mn, Ni) as available from the literature.


Estimation of the electrochemical utilization of the active materials (CrCo2O4, MnCo2O4, and NiCo2O4 electrode), was evaluated from the fraction of cobalt sites, *z*. The fraction, *z*, can be evaluated using Faraday's law as following [94]:

$$z = \mathbb{C}\_{\text{sp}} \,\,\,\text{MW} \,\,\Delta V/\text{F} \tag{12}$$

where *Csp*, *MW*, Δ*V and F* is the specific capacitance value, the molecular weight, the applied potential window, and the Faraday's constant, respectively. The *z* value of 1 indicates the complete involvement of electroactive material. i.e., all active metal sites participating in the redox process. The molecular weight of Co (84.03 g/mol) and Cr (51.996 g/mol) in CrCo2O4, Co (84.03 g/mol) and Mn (54.93 g/mol) in MnCo2O4, and Co (84.03 g/mol) and Ni (58.69 g/mol) in NiCo2O4 the specific capacitance at a current density of 1 A/g (Csp~ 231 F/g, 161 F/g, and 190 F/g) for CrCo2O4, MnCo2O4, and NiCo2O4, Figure 5b, and a potential window of 0.6 V gives a *z* value of 0.195, 0.139, and 0.169 respectively. In other words, ~20%, 14%, and 17% of the total active material Cr, Mn, and Ni atoms participate in the redox reaction for the charge storage. The observed low value of *z* suggests that the charge storage in tubular MCo2O4 structure via a redox reaction process occurs mainly at the surface with little bulk interaction due to diffusion of OH− ions into the material. It could be concluded that the charge storage due to the redox process in MCo2O4 mostly occurs only at the redox sites predominantly located on the surface of the particles [71].

Cyclic stability tests were performed to evaluate the practical performance of electrodes as a supercapacitor. The stability test of electrode materials was assessed via galvanostatic CD measurement for 5000 cycles for a current density of 10 A g−<sup>1</sup> in 3M KOH and is shown in Figure 6b,d, and Figure 6f. The Coulombic efficiency (η) of the devices was calculated from its charging (*Tc*) and the discharging (*Td*) times from GCD curves following the relation, η = *Td*/*Tc* × *100*, and is plotted in Figure 6b,d,f as a function of cyclic time. The initial η of the device was ~100%, which remained practically the same even after 5000 cycles. For the practical applications, the study of cycling performance for electroactive material is very significant parameter. The percentage retention in specific capacitance was calculated using,

$$\% \text{ reduction in specific capacitance} = (C\_{\#} \langle C\_1 \rangle \times 100 \tag{13}$$

where *C#* and *C1* are specific capacitance at various cycles and the 1st cycle, respectively. The specific capacitance of the electrode is reduced by 7.68% in CrCo2O4, reduced by 9.12% in MnCo2O4, and increased by 0.48% in NiCo2O4.

Figure 5h shows the Ragone plots of as-synthesized MCo2O4 electrodes. The energy densities (E) and power densities (P) of the electrochemical cells are calculated using the following equations [95]:

$$E = (1/2)CV^2\tag{14}$$

$$P = E/t\tag{15}$$

where *C*, *V*, and *t* are the specific capacitance that depends on the mass of the electrodes, the operating voltage of the cell, and discharge time in seconds, respectively. The essential point for high-performance supercapacitors is to obtain a high energy density and meanwhile providing an outstanding power density. It is observed from Figure 7b that the tubular CrCo2O4, MnCo2O4, and NiCo2O4 electrode display superior performance over energy density up to 11.1 Wh/kg, 7.8 Wh/kg, and 9.3 Wh/kg with a peak power density up to 7287.34 W/kg, 7195.33 W/kg, and 7186.12 W/kg, respectively are given in Table 3. As supercapacitor is expected to provide higher power and energy density at the same time, hence NiCo2O4 displays an overall better energy density of 9.3 Wh/kg and a power density of 7186.12 W/kg as compared to MnCo2O4 and CrCo2O4.

Ideally, to develop a higher energy density battery with a given anode, a cathode with high electrochemical interaction potential is desired. This is because the energy density of the device equals the product of the working voltage, which is obtained from the electrochemical potentials different between the cathode and anode and specific capacity of the electrode materials [63]. On the other hand, the theoretical capacity of electrode materials depend on the number of reactive electrons (*n*) and molar weight (*M*) of the materials and is expressed as Equation (16) [96],

$$C\_I = n \ast F / 3.6 \ast M \tag{16}$$

Here, *F* is the Faraday constant, *n* is the number of reactive electrons, and *M* is the molar weight of materials. Theoretically, the equation predicts that an electrode material having a smaller molecular weight can produce a higher capacity. The molecular weight of CrCo2O4, MnCo2O4, and NiCo2O4 is ~ 233.86, 236.80, and 240.55 g/mol, respectively. Thus, in line with the Equation (11), at low current density, among three electrodes studied, the CrCo2O4 displays higher specific capacitance. However, overall superior performance in terms of energy and power density was displayed by the NiCo2O4 electrode. NiCo2O4 is known to possess rich electroactive sites, narrow pore size, and higher electrical conductivity (at least two magnitudes higher) than that of Co3O4 and NiO, which could be the reason for the observed overall better performance of NiCo2O4 [97,98].

### **4. Conclusions**

In conclusion, biomorphic tubular CrCo2O4, MnCo2O4, and NiCo2O4 nanostructures were prepared using cotton by a cost-effective and straightforward bio-template method. The synthesized tubular MCo2O4 display excellent crystallinity, phase purity and display desirable electrochemical properties, which indicate a good chance for the fabrication of high-performance supercapacitor devices. Electrodes constructed using the tubular MCo2O4 demonstrate high specific capacitance, cyclic stability, power, and energy density when evaluated in 3M KOH electrolyte. The study suggests that it is imperative to account for the nature of the electroactive sites and the conductivity of materials when choosing materials from the series of transition metal oxide as the electrode for the supercapacitor application. Furthermore, the superior electrochemical performance of tubular MCo2O4 microstructure owes to the presence of conducting a carbonaceous structure. The highly porous carbonaceous structure can allow electrolyte access throughout the electrode structure. Thus, it produces a large surface area for ion transfer between the electrolyte and the active materials, which leads to achieving ultrafast storage and release of energy.

**Author Contributions:** Conceptualization, D.G.; methodology, D.G., and C.Z.; software, D.G. and C.Z.; validation, S.R.M., R.K.G., and D.G.; formal analysis, D.G.; investigation, S.R.M. and R.K.G.; resources S.R.M.; data curation, D.G., C.Z., and R.K.G.; writing—original draft preparation, D.G.; writing—review and editing, S.R.M.; visualization, S.R.M. and D.G.; supervision, S.R.M. and R.K.G.; project administration, S.R.M.; funding acquisition, S.R.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This is supported by the grants from FIT-DRONES and Biologistics at the University of Memphis, Memphis; TN. Dr. Ram K. Gupta expresses his sincere acknowledgment of the Polymer Chemistry Initiative at Pittsburg State University for providing financial and research support.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Investigation and Improvement of Scalable Oxygen Reducing Cathodes for Microbial Fuel Cells by Spray Coating**

### **Thorben Muddemann 1,\*,**†**, Dennis Haupt 2,**†**, Bolong Jiang 1, Michael Sievers <sup>2</sup> and Ulrich Kunz <sup>1</sup>**


Received: 5 November 2019; Accepted: 17 December 2019; Published: 19 December 2019

**Abstract:** This contribution describes the effect of the quality of the catalyst coating of cathodes for wastewater treatment by microbial fuel cells (MFC). The increase in coating quality led to a strong increase in MFC performance in terms of peak power density and long-term stability. This more uniform coating was realized by an airbrush coating method for applying a self-developed polymeric solution containing different catalysts (MnO2, MoS2, Co3O4). In addition to the possible automation of the presented coating, this method did not require a calcination step. A cathode coated with catalysts, for instance, MnO2/MoS2 (weight ratio 2:1), by airbrush method reached a peak and long-term power density of 320 and 200–240 mW/m2, respectively, in a two-chamber MFC. The long-term performance was approximately three times higher than a cathode with the same catalyst system but coated with the former paintbrush method on a smaller cathode surface area. This extraordinary increase in MFC performance confirmed the high impact of catalyst coating quality, which could be stronger than variations in catalyst concentration and composition, as well as in cathode surface area.

**Keywords:** microbial fuel cell; wastewater treatment; oxygen reduction reaction; municipal wastewater; MnO2; MoS2; Co3O4; spray method

### **1. Introduction**

The permanent rise in standard of living correlates with an increasing energy demand [1]. Particularly in view of global warming and the intended abandonment of fossil and nuclear fuels, processes must be developed that provide environmentally friendly energy [2]. Another challenge is water scarcity, causing 1.2 billion people to suffer from a lack of water and leaving another 1.6 billion people without any access to hygienically safe drinking water [3]. Microbial fuel cells (MFC) are a promising technology that can contribute to a decentralized solution for these future challenges, enabling expansion and assurance of water availability and energy supply [4,5].

Microbial fuel cells (MFCs) combine electrical energy generation with simultaneous wastewater treatment by microorganisms [5]. Anodic exoelectrogenic bacteria oxidize organic compounds from wastewater [6,7]. In combination with a coupled reduction reaction, a positive potential difference, and therefore a net power, is achieved [8]. Oxygen is usually used as final electron acceptor in the coupled oxygen reduction reaction (ORR) at the cathode, as oxygen is available in ambient air [8]. The schematic structure of the investigated MFC is shown in Figure 1.

**Figure 1.** Schematic drawing of the investigated microbial fuel cell (MFC): a planar anode overgrown with bacteria (**dark gray**), an oxygen-reducing cathode (**light gray**), and cation exchange membrane (**white**). The desired reactions and subsequent products are sketched. An electrical load (El. Load) is placed in the outer electrical circuit to use the generated power.

The net power generation of MFCs is rather low, especially due to the performance-limiting cathode [9–12]. To overcome this bottleneck, large electrode surfaces [13], improved cathode manufacturing methods, and especially scalable methods are needed [5]. In the laboratory, cathodes for MFCs are often prepared by brushing [9,14–18], dipping [19], rolling [20], pressing [21], doctor blading [12], spin coating [22], or electro deposition [19,23]. Most of these methods are either not applicable, or less so, for economical technical scale electrodes because they are time-consuming, complex, or not adaptable. Several patents describe manufacturing methods for large-scale cathodes for chemical fuel cells [24–26] or chlor-alkali electrolysis [27,28], but these methods are not yet adapted for MFCs. For that reason, a cathode-coating method is presented in this study, which is also a promising approach for automation, and for the use of large-scale cathodes for MFCs for the first time.

This coating is realized by an airbrush method for applying a self-developed polymeric solution containing catalysts. The coating process was investigated with different catalysts, their different mixing ratios, and different metal meshes.

Next to carbon, MnO2, MoS2 (with varying mixing ratios), and Co3O4 (with TiO2 as support material) were investigated as additional catalysts for ORR. As a benchmark for ORR [8], one electrode was coated with Pt as the catalyst. Unfortunately, Pt is very expensive and susceptible to catalyst poisoning by S2 <sup>−</sup> or NH4 <sup>+</sup> [29], which are commonly present in municipal wastewater, and as a consequence Pt is not suitable for wastewater treatment applications.

Several publications investigated MnO2 as an ORR catalyst for MFCs, including the influence of its different modifications (alpha, beta, gamma), different mixing ratios, and varying manufacturing methods. Modifications of MnO2 were studied by Zhang et al. in an MFC (glucose as feed). The maximum power densities differ from 125 mW/m2 (alpha) to 172 mW/m2 (beta) and 88 mW/m2 (gamma) [30]. This approach was followed by Roche et al. with similar performances up to 161 mW/m<sup>2</sup> (synthetic wastewater) [31]. Newer studies focus on more complex MnO2-based cathodes. Li et al. studied a manganese oxide catalyst with a cryptomelane-type octahedral molecular sieve structure. This catalyst was additionally doped with Co, Cu, and Ce. The highest power density was measured for the Co-doped system with 180 mW/m<sup>2</sup> (domestic wastewater, acetate) [16], followed by an optimized system with a continuous flow MFC. The maximum power density was 201 mW/m2 (wastewater inoculum; acetate as feed) [17]. Recent studies partially increased the performance of MnO2 even further. Zhou et al. investigated the behavior of the combination of polyaniline and MnO2 nanocomposites and reached power densities of 248 mW/m2 (anaerobic digester sludge inoculum) [32]. Furthermore, nano-scaled systems based on low-cost MnO2 nanowires (on carbon support) achieved a maximum of 180 mW/m2 (combination of domestic and artificial wastewater) [33]. Farahani et al. investigated another MnOx-based cathode and found that nitrogen-doped carbon with MnOx revealed a peak power density of 467 mW/m<sup>2</sup> (acetate as feed) [34]. However, the described cathode production process is complex and is difficult to scale. A further study with modified MnO2 and induced carbon nanotubes (CNT) revealed a drastically lower power density of approximately 12 mW/m2 (calculated with the given volumetric power density of 216 mW/m3) [35].

Jiang et al. investigated the performance of MnO2 and its combination with MoS2 (real wastewater). The best combination of MnO2 and MoS2 revealed peak power densities of up to 165 mW/m<sup>2</sup> [9,14]. Another investigated composition within this study was MoS2, which is sparsely described in literature. The combination of MoS2/C is considerably less investigated than MnOx catalysts. Hao et al. evaluated N-doped MoS2/C catalysts and reached high power densities (815 mW/m2) in a small reactor (28 mL) with artificial wastewater (at 30 ◦C) due to the beneficial catalyst and the optimized process conditions [20].

In comparison, Co3O4/C cathodes are more often studied. Ge et al. revealed power densities of up to 1500 mW/m<sup>2</sup> in an MFC (artificial wastewater). This was confirmed by Xia et al. with power densities of up to 1540 mW/m2. The performance of real wastewater with high chemical oxygen demand (COD; 56,500 mg/L) was investigated by Kumar et al. They reached power densities up to 503 mW/m<sup>2</sup> [36]. A following study with flower-like Co3O4 and lower COD loads (digester sludge and acetate) showed lower maximum power densities of 248 mW/m<sup>2</sup> [15]. Our working group already demonstrated the long-term stability of MnO2 [14], MoS2 [9], and Co3O4 [37] electrodes. It was shown that MnO2, MoS2, and Co3O4 have decent ORR properties and fulfill the requirements of MFC cathodes—they are inexpensive, long-term stable, environmentally friendly [38,39], reduce the overvoltage, and are immune to catalyst poising. Unfortunately, these cathodes were produced by a time-consuming manual paintbrush method. Therefore, a novel spray coating method newly adapted to MFC applications by a dedicated suspension is presented, which is also adaptable to automation processes and scalable electrodes.

### **2. Materials and Methods**

### *2.1. Experimantal Setup*

To compare and characterize all coated electrodes, a laboratory MFC (self-made) was installed. The test facility consists of eight identical test cells, and an overview is given in Figure 2. Purchased graphite compound anodes were used, made of 86% graphite and 14% olefinic polymer binder (Eisenhuth GmbH, Osterode am Harz, Germany). This material allowed the microorganisms to grow on the surface and was also scalable. A low-cost membrane type FKS-PET 130 (Fumatech GmbH, Bietigheim-Bissingen, Germany) was used to separate the anaerobic anode chamber and the aerobic cathode chamber. For more information regarding the cell and anode design, refer to [14].

**Figure 2.** Presentation of the used laboratory system, whereby all measurements were done in the white- marked MFC for comparability.

Each MFC was fed with wastewater via a pump (Pumpen und Anlagentechnik, Lutherstadt Wittenberg, Germany, model: 18ZP-VA 0,68-D30-118 FU) and all cathodes were connected to one water basin aerated with ambient air. For all characterizations, the same test cell (Figure 2, circle) was used to increase the measurements' comparability.

During the operation, the temperature, pH values, electrical potentials, cell voltage, and current were monitored through a LabView system (National instruments, Austin, TX, USA). All experiments were carried out at a temperature of approximately 16.5 ◦C (room temperature in the laboratory). The pH values were kept at approximately 8 (anolyte) and approximately 10 (catholyte) by adding a sodium carbonate solution (20 wt %) (Carl Roth GmbH & Co. KG, Karlsruhe, Germany). A constant current source was used to enable a defined microbial growth at continuously controlled maximum power point conditions. For detailed information about the constant current source, refer to [9,14].

The inoculation process took 2 weeks and was performed with real wastewater taken after the primary clarifier of the sewage treatment plant (STP) Goslar (Eurawasser Betriebsführungsgesellschaft mbH, Goslar, Germany). After filling the anode tank with about 10 L of wastewater and turning on the pumps, the cells were kept in open circuit for 5 days. Afterwards, each cell was connected to its own constant current source for individual cell operation and regulation.

To evaluate the electrodes under real conditions, the effluent of the primary clarifier from the same STP Goslar with an initial COD of 150 to 200 mg/L was treated. Despite weekly wastewater exchanges, 10 mL of a solution of 200 g/L glucose (Carl Roth GmbH & Co. KG, Karlsruhe, Germany). and 200 g/L NaAc (Carl Roth GmbH & Co. KG, Karlsruhe, Germany) was added daily to keep a constant COD concentration level of approximately 200 mg/L, and to ensure the comparability of results at different dates.

### *2.2. Catalysts and Carrier Materials for Cathode*

Printex 6L carbon (Orion Engineered Carbons S.A., Houston, TX, USA) was used for the electrode coating. MnO2 by EMD Millipore Corporation (article number: 805958) (Burlington, MA, USA) and MoS2 by Metallpulver24 Corp. (article number: 22020) (Sakt Augustin, Germany) were used as catalysts. Butanone served as solvent (Sigma-Aldrich, St. Louis, MO, USA, 443468). Due to the high prices of Co3O4, carrier supported catalysts (Co3O4 @ TiO2) were evaluated. For these Co3O4 catalysts, Co(NO3)2 was obtained by Sigma-Aldrich (article number: 239267) (St. Louis, MO, USA) and anatase TiO2 nano particles by Cofermin Chemical GmbH (Essen, Germany). Anatase TiO2 in a whiskers morphology was produced according to reference [40]. Fuel cell grade Pt black by De Nora North America ETEK Division (S990670; Painesville, OH, USA) was used for the Pt coating for benchmarking.

Three different types of metal mesh were used as carrier material for the coating. The physical details of the investigated meshes are given in Table 1. Hereinafter, coarse stainless-steel mesh is abbreviated as "cSS", fine stainless steel mesh as "fSS", and nickel net with a very fine mesh width as "vfNi".


**Table 1.** Physical data of the investigated wire meshes for electrode coating.

### *2.3. Electrode Spray Coating Process*

The coating process for the cathodes is shown in Figure 3. At first, a coating solution has to be prepared, consisting of a basic polymer solution, catalyst, and additional butanone. The basic polymer binder solution was prepared by dissolving three table tennis balls (ink-free with no commercial logos) in 150 mL butanone to obtain a celluloid solution. Due to the strict standards, table tennis balls form high-quality educts at low prices and fit perfectly to the low-cost approach for a low-cost flexible binder. Then, the basic polymer solution (20 mL) was merged with additional butanone (100 mL). To obtain a homogeneous dispersion, the solution was stirred, and the catalysts were added stepwise (4 g carbon + 0.4 g additional catalyst: MnO2 and/or MoS2, Co3O4). Then, the mixture was homogenized for 15 min with a dispersing tool (Heidolph, Germany, model SilentCrusher at 13,500 rpm).

**Figure 3.** Flow diagram of the process steps for electrode preparation using the spray method.

A spray table with heating option and the degreased (degreaser: isopropanol) metal mesh were heated and controlled at a very stable surface temperature of 110 ◦C. The dispersion was manually applied with an airbrush spray gun (Harder & Steenbeck, Norderstedt, Germany, model: Evolution Solo) in cross-coat. Inert nitrogen (Linde Gases Division, Pullach, Germany) was used as carrier gas for spraying (3.5 N grade and a pressure of 2 bar). The applicated mass was controlled by weighting the cathode before, during, and after the coating process. Furthermore, the optimum catalyst loading was evaluated and finally fixed to 0.4 mg/cm<sup>2</sup> by preliminary tests. The coating process was completed when the desired catalyst loading was reached. No calcination step was required after coating; therefore, the coating process was timesaving and avoided further changes to the catalyst.

### *2.4. Measuring Procedure and Analytical Methods*

A Luggin capillary is usually used to investigate the electrode potential compared with a reference electrode. In order to not only measure the potential at one geometric point, a membrane extension operated as a salt bridge between the cathode and the reference electrode. Therefore, the potentials of the electrodes were measured in an integrative way over the entire active area [41]. The membrane extension was kept moist by an applied carbon fleece in order to stabilize the ionic conductivity and the measurement (Figure 4). A reversible hydrogen electrode (RHE, Gaskatel GmbH, Kassel, Germany) served as a reference electrode, which was operated in a buffer solution at pH 7 (Libutec GmbH, Langenfeld, Germany; type: buffer solution pH 7).

**Figure 4.** Sketch of the measurement setup and the salt bridge-like membrane extension for potential determination. This modified version is based on [41].

The cathodes were characterized by measuring the electrode potentials and the cell voltages as a function of the current. Furthermore, the power density characteristic curves were calculated and normalized to the geometric area. The power was adjusted between 0 mA to short circuit current in 1 mA steps. After each step, the stationary state was awaited (average duration: 5 min).

To analyze the chemical composition, an X-ray powder diffraction (XRD) analysis was conducted with a D/max-2200PC-X-ray (Rigaku Corp., The Woodlands, TX, USA) diffractometer (40 kV, 20 mA) using CuKα radiation (0.15404 nm) and a scan range between 10◦ to 80◦, with 10◦/min.

Physical properties were analyzed by the Brunauer-Emmet-Teller (BET) method with a NOVA2000e device (Micromeritics GmbH, Unterschleissheim, Germany) with a prearranged outgassing step at 200 ◦C at a vacuum pressure of 6 mmHg of the samples. Selected cathodes were examined with a scanning electron microscope (SEM) at the Institute of Mechanical Process Engineering at Clausthal University of Technology (Carl Zeiss Microscopy GmbH, Jena, Germany, model: DSM 982 Gemini.M).

### **3. Results and Discussion**

### *3.1. Characterization of Catalysts and Supports*

MnO2 and MoS2 have already been evaluated using XRD and BET; reference is made to [9]. It should be noted that gamma MnO2 was intentionally used, which is significantly cheaper than other modifications. Crystalline phases of the investigated TiO2 supports, and its combination with Co3O4, are shown in Figure 5. The whiskers type of TiO2 was abbreviated as TiO2-W, whereas TiO2-A is the abbreviation for the anatase modification. The XRD pattern of the supports showed peaks at 2θ = 25.2◦, 37.8◦, 48.0◦, and 55.0◦, confirming both modifications were anatase phase TiO2. The analysis of the catalyst-loaded TiO2 supports revealed a successful loading, as the peaks of TiO2 were still present, next to the peaks of Co3O4 at 2θ = 36.9◦ and 65.2◦.

**Figure 5.** X-ray powder diffraction (XRD) pattern of TiO2 supports (**a**) and in combination with Co3O4 catalyst (**b**).

The BET surface areas of the TiO2-supported and prepared Co3O4/TiO2 catalysts are presented in Table 2. The comparison between the investigated TiO2 modifications showed that the TiO2-W had a higher specific surface area (85.9 to 70 m2/g), and larger pore volume and size. After catalyst loading, the surface area decreased, but the loaded TiO2-W support still had a greater surface area.

**Table 2.** Catalyst content and physical properties of TiO2 carrier and its combination with Co3O4. TiO2-A: anatase modification of TiO2, TiO2-W: whiskers type of TiO2.


Figure 6 illustrates the Co 2p3/2 spectra of the prepared catalysts. All patterns revealed two peaks at 786.2 eV and 779.4 eV, whereas the small peak can be attributed to Co2<sup>+</sup> species, and the bigger peak at 779.4 eV to Co3<sup>+</sup>. Using this reasoning, Co3O4 was the predominant phase for all samples.

**Figure 6.** Co 2p3/2 spectra for the supported catalysts. The binding energy for Co3O4 (779.4 eV) and Co2O3 (786.2 eV) is shown.

### *3.2. Performance of Coated Electrodes*

3.2.1. Comparison of MnO2, MoS2, and Co3O4-Based Cathodes

The presented spraying method was applied to a variety of catalysts (MnO2, MoS2, Co3O4, and Pt) and different metal meshes. A summary of the fabricated and tested combinations is given in Table 3.

**Table 3.** Overview of the produced electrodes by the novel spray method on different meshes.


In order to achieve a concise comparison of the investigated cathodes, all electrode configurations were characterized as described. Figure 7 shows an example of a power density curve, including the anode and cathode potential curve, as well as the current–voltage curve.

**Figure 7.** Characteristic curves of an microbial fuel cell (MFC) with a spray-coated cathode based on MnO2-MoS2 (1:1) on fSS wire mesh. This example also underlines the cathodic bottleneck of the system, determined by the stronger decreasing cathodic potential in comparison to the anodic potential.

The maximum performance of each electrode is given in Figure 8. For the sake of clarity, electrodes with different meshes, but identical catalysts, are presented in clusters. A cathode based on the reference catalyst Pt (fuel cell grade Pt at cSS substrate) was produced and used for comparison in Figure 8 (left).

**Figure 8.** Comparison of maximum power densities of different cathodes. All electrodes were manufactured with a catalyst load of 0.4 mg/cm2.

The comparison clearly indicates that all catalysts benefit from an increasing surface area of the metal meshes. The performance increases from coarse (cSS) to fine stainless-steel mesh (fSS) and is outperformed by the cathodes on very fine nickel mesh (vfNi). However, the 5.4-times increase of surface area of vfNi mesh cathode compared to fSS mesh cathodes only led to a slight increase in MFC performance for different catalyst systems, including the best one (MnO2-MoS2 2:1). For the MnO2 catalyst, a significant increase of approximately 80 mW/m2 (30%) was found, but still at a lower level than the best catalyst system. These results indicate an existing limit for the impact of electrode surface area on the improvement of MFC performance.

Electrodes with CO3O4 (on TiO2 support) and MoS2 showed the lowest power density (only 60–88 mW/m2). Both investigated Co3O4 on TiO2 systems (in combination with carbon and polymer) showed similar results, but the Co3O4/TiO2-W cathode was slightly better (74.4 mW/m2) in comparison to Co3O4/TiO2-A (63.6 mW/m2), probably caused by the increased surface area.

The increasing surface area caused by the finer mesh width also had a positive effect on MoS2-based cathodes, but the performance was still low (60.2, 65.1, and 88.5 mW/m2). The oxygen reduction capability of MnO2 was considerably higher. The increase from cSS to fSS enhanced the maximum power density from 145.4 to 153.9 mW/m2. A further improvement was given by the usage of the vfNi mesh, showing power densities up to 240.9 mW/m2. Sulphur compounds in wastewater are known as catalyst poisons, and it was recently shown that a catalyst combination with MoS2 increases the long-term stability of MFC cathode catalysts [9]. Therefore, different mixing ratios were investigated.

The (weight %) mixing ratio of 1:1 (MnO2-MoS2) led to a performance deterioration compared to the pure MnO2 system, and the electrodes showed similar performances to the MoS2 cathodes on all substrates. The mixing ratios of 1:2 and 2:1 performed differently, but they showed an improving effect. In particular, the electrodes with a mixing ratio of 2:1 showed power densities of 198.3 mW/m<sup>2</sup> (MnO2-MoS2 2:1, cSS), 312.3 mW/m2 (MnO2-MoS2 2:1, fSS), and 320.6 mW/m2 (MnO2-MoS2 2:1, vfNi). An even better performance was reached with the fSS and vfNi meshes than for the Pt-coated cathode. Potential/current curves of selected cathodes are shown in Figure 9. The diagram clearly indicates the most stable cathode potential curve for MnO2/MoS2 (2:1). Furthermore, the cathode potential stabilized in terms of current stability from the cSS mesh (grey curve) over the fSS mesh (blue curve) to the vfNi mesh (green curve), with simultaneously increasing cathode surface area. All cathodes showed a potential drop, especially at higher currents. This was probably caused by mass transport limitations.

**Figure 9.** Cathode potentials of selected spray-coated cathodes on different substrates.

Due to the high costs for Co3O4, carrier-supported catalysts (Co3O4 at TiO2) were evaluated. However, these showed low ORR activity and the achieved currents were below published levels. Obviously, pure Co3O4 is expected to show significantly better performance, but this is not suitable for wastewater treatment by MFC due to the low cost-efficiency. In conclusion, the MnO2-based cathodes showed significantly better performances. Especially a combination with MoS2 could surpass the performances related to scalable systems described in the literature to date.

### 3.2.2. Comparison to Paintbrush-Manufactured Cathodes

Novel spray coated cathodes were compared to cathodes previously produced by the paintbrush method [9] by SEM. All cathodes had a catalyst ratio of 2:1 (MnO2/MoS2). According to Figure 10, the paintbrush process produced a tight fit between the interstitial spaces, whereby some were closed, whereas other interstitial spaces remained empty. The coating was not uniform and showed a tendency for detaching. The application consisted mainly of elongated particles attached to each other in a porous structure, which were fixed in position by form closure.

**Figure 10.** Scanning electron microscope (SEM) image of a cathode produced by paintbrush method with a catalyst ratio of 2:1 (MnO2/MoS2) in different scales.

In contrast, the cathodes produced by the spray coating method showed a more regular coating on the surface of the wire mesh. The finer the wire mesh, the more uniform the distribution. The SEM image of coated vfNi substrate is shown as an example in Figure 11.

**Figure 11.** SEM image of a spray coated cathode with a catalyst ratio of 2:1 (MnO2/MoS2) in different scales.

Thus, it was shown that the presented spray method enabled uniform and reproducible catalytic coatings. The developed spray coating method seems to be a promising process for the production of high-performance electrodes and also for upscaling. The method also enabled a suitable comparison between different catalysts through homogenous coatings. No additional calcination steps were necessary, which could modify the catalyst structure.

### 3.2.3. Long-Term Performance of Selected Electrodes

The long-term performance of the best performing spray-coated cathode (MnO2-MoS2 2:1 vfNi) in comparison to a paintbrush-coated cathode (according to [9]) is given in Figure 12. It was noted that the paintbrush method was applied on a cSS net, as the paintbrush method would cause clogging on finer mesh width. Although a drop in power density was noticeable at the beginning of the measurement, a relatively constant power density of 150 mW/m2 was reached from the fourth day onwards. From day 6, the power density rose, reaching 200–240 mW/m<sup>2</sup> from day 10 onwards. The long-term performance of paintbrush-coated electrode was about 60 to 100 mW/m2. The comparison of the long-term performance between both electrodes by different coating methods revealed a performance improvement by factor 2 to 3, or in terms of power density, by 100 to 180 mW/m2 for the spray-coated cathode. The comparison between the improvement of long-term power density and the improvement of maximum power density showed that the improvement in long-term performance was higher than that in maximum performance. Therefore, the spray-coating method was identified as enabling the production of high-performing and long-term stable cathodes for MFC application.

**Figure 12.** Long-term performance of the spray-coated MnO2-MoS2 (2:1) vfNi cathode.

### **4. Conclusions**

To scale-up MFCs to the dimensions of technical-scale wastewater treatment plants, it is necessary to manufacture scalable electrodes. Therefore, the composition of a universal sprayable suspension was developed within this study, which was used for a simplified airbrush spray-coating method. The performance was evaluated regarding different catalysts (Co3O4, MnO2, MoS2, and selected

combinations) on different carrier meshes and materials. The spray-coating method facilitated a homogeneous coating and stood out with an increased cathode performance. This could be enhanced further by suitable substrates with a finer mesh width and a higher surface area. The most promising cathode catalyst composition of the tested systems was the combination of MnO2 and MoS2 mixture (2:1) with a maximum and long-term power density of 320 and 200–240 mW/m2, respectively. The coated electrodes also demonstrated long-term stability. This contribution confirmed the effect of the quality of the catalyst coating of cathodes for wastewater treatment by microbial fuel cells (MFC). A promising and more rapid manufacturing method for better catalyst comparisons and large-scale applications was identified. This process could also be the basis for an automated coating. Moreover, this work demonstrated that not only the catalytic components and their composition influenced electrocatalytic properties. The method of preparation and the carrier mesh also exerted a large effect on the electrode performance.

**Author Contributions:** Conceptualization, methodology, validation, data curation, visualization, writing original draft preparation, T.M. and D.H.; investigation, T.M., D.H., and B.J.; writing—review and editing, all authors; supervision, project administration, funding acquisition, M.S. and U.K. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Federal Ministry of Education and Research (Bundesministerium für Bildung und Forschung), BMBF, Germany, grant number WTER0219813.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Preparation and Characterization of Porous Ti**/**SnO2–Sb2O3**/**PbO2 Electrodes for the Removal of Chloride Ions in Water**

### **Kangdong Xu 1, Jianghua Peng 2, Pan Chen 1, Wankai Gu 1, Yunbai Luo <sup>1</sup> and Ping Yu 1,\***


Received: 14 September 2019; Accepted: 14 October 2019; Published: 18 October 2019

**Abstract:** Porous Ti/SnO2–Sb2O3/PbO2 electrodes for electrocatalytic oxidation of chloride ions were studied by exploring the effects of different operating conditions, including pore size, initial concentration, current density, initial pH, electrode plate spacing, and the number of cycles. In addition, a physicochemical characterization and an electrochemical characterization of the porous Ti/SnO2–Sb2O3/PbO2 electrodes were performed. The results showed that Ti/SnO2–Sb2O3/PbO2 electrodes with 150 μm pore size had the best removal effect on chloride ions with removal ratios amounting up to 98.5% when the initial concentration was 10 g L<sup>−</sup>1, the current density 125 mA cm−2, the initial pH = 9, and the electrode plate spacing 0.5 cm. The results, moreover, showed that the oxygen evolution potential of 150 μm porous Ti/SnO2-Sb2O3/PbO2 electrodes was highest, which minimized side reactions involving oxygen formation and which increased the removal effect of chloride ions.

**Keywords:** electrocatalytic oxidation; chloride ions removal ratio; the porous electrode; influencing factors

### **1. Introduction**

In recent years, China has raised the level of environmental protection, which requires industrial wastewaters not to be discharged from various enterprises. Therefore, in order to ensure the normal operation of its own production, each enterprise must realize the recycling of water. At present, the commonly used method is to properly dispose of the drainage and then replenish it into the industrial water system. However, various ions are continuously enriched in the water when the water is reused, which causes various adverse effects on the operation of equipment. The amount of scale cations (such as calcium ions and magnesium ions, etc.) in water can be reduced by changing the pH of the water body and by inducing flocculation sedimentation [1], but there is no effective method of reducing anions (such as chloride ions) which have corrosive effects on equipment in water [2]. Therefore, there is a need to find a way to quickly and easily reduce chloride ions in water.

At present, the methods for removing chloride ions include biological [3,4], reverse osmosis [5,6], distillation, multi-effect evaporation [7], electrodialysis [8–10], and ions exchange methods. The biological method involves high operating costs, while the removal effect does not work well. The reverse osmosis method is burdened by high consumption of acid and alkali as well as energy consumption. The distillation method and the multi-effect evaporation methods involve high energy consumption. The electrodialysis method involves high operation voltages, and membranes are easily contaminated. The ion exchange method is expensive and tends to cause secondary pollution during the elution process. The electrocatalytic oxidation technology is a new method for removing chloride ions, which has the advantages of high efficiency in removing chloride ions, simple process, simple operation, and low operating cost, and there are few related studies.

In this study, porous Ti/SnO2–Sb2O3/PbO2 electrodes were studied and it was found that these have a significant effect on electrocatalytic oxidation of removing chloride ions. Self-made porous Ti/SnO2–Sb2O3/PbO2 electrodes with different pore sizes were used as anodes to construct an electrolyzer. The high potential of the anode and unique catalytic oxidation characteristics of the surface coating were used for removing chloride ions. Three different kinds of porous Ti/SnO2–Sb2O3/PbO2 electrodes were prepared and characterized systematically with regard to morphology, crystal structure, and various electrochemical performances. NaCl solutions were used for simulating chlorine-containing wastewaters for studying the effects of initial concentration, pore size, current density, initial pH, electrode plate spacing, and the number of cycles. The results showed that the porous Ti/SnO2–Sb2O3/PbO2 electrodes were efficiently reducing the number of chloride ions in water, thus providing a practical method of removing chloride ions from water.

### **2. Experimental**

### *2.1. Materials and Reagents*

The substrates used in this study were commercial samples of porous Ti (20 mm × 10 mm × 1 mm), which had average pore sizes of 50 μm, 100 μm, and 150 μm. In regard to removing chloride ions, porous Ti substrates with larger pore sizes do not meet the requirements for removing chloride ions in terms of hardness, our studies were restricted to the three pore sizes mentioned above. In this paper, all chemicals were analytical grade without any other impurities. SnCl4·5H2O, SbCl3, HCl, Pb(NO3)2, Cu(NO3)2·3H2O, NaF, HNO3, NaOH, H2C2O4, NaCl, and other chemicals were obtained from Shanghai Wo Kai Biotechnology Co., Ltd. (Shanghai, China). All the solutions used in these experiments were prepared with deionized water. For the simulation of chloride ion contaminated waste waters, NaCl solutions were used.

### *2.2. Electrode Preparation*

First, at a temperature of 70 ◦C, porous Ti substrates (50 μm, 100 μm, and 150 μm) were heated in sodium hydroxide (20% m%) for 1 h to remove all traces of oil on the surface and were then washed in deionized water. Thereafter, the porous Ti substrates were etched in oxalic acid (15% m%) at a temperature of 85 ◦C for 1 h to obtain a uniformly rough surface. Finally, the samples were washed in deionized water and stored in deionized water [11,12].

Second, the coating solution, consisting of 1.20 g SnCl4·5H2O and 0.20 g SbCl3 was dissolved in 40 mL ethanol, and 1 mL of concentrated hydrochloric acid was added. The treated porous Ti substrates were immersed in the coating solution for 5 min, and then dried at about 120 ◦C for 15 min, and thereafter calcined at 500 ◦C for 20 min in a muffle furnace. All above processes were repeated ten times and the electrodes were annealed at 500 ◦C for 60 min in the last process [13,14]. Finally, the porous Ti/SnO2–Sb2O3 electrode was prepared.

Third, the deposition solution consisted of 40 g Pb(NO3)2, 15 g Cu(NO3)2·3H2O, 0.5 g NaF, and 0.1 M HNO3. The porous Ti/SnO2–Sb2O3 electrodes were used as anodes and a pure titanium plate as a cathode. The current density was 5 mA cm−<sup>2</sup> and the PbO2 was deposited on the porous Ti/SnO2–Sb2O3 electrode at 65 ◦C for 0.5 h under stirring conditions. Finally, the prepared porous Ti/SnO2–Sb2O3/PbO2 electrodes were washed in deionized water [15–17].

### *2.3. Electrode Characterization*

The surface morphologies of porous Ti/SnO2–Sb2O3/PbO2 electrodes were characterized by Field Emission Scanning Electron Microscopy (Zeiss SIGMA, Carl Zeiss Corporation, Jena, Germany). The composition and the chemical state of the porous Ti/SnO2–Sb2O3/PbO2 electrode was determined by an X-ray photoelectron spectrometer (ESCALAB250Xi, Thermo Fisher Scientific, Waltham, MA, USA). The crystal structure of porous Ti/SnO2–Sb2O3/PbO2 electrodes were determined by an X-ray Diffractometer (XRD–6100, Shimadzu Corporation, Kyoto, Japan) with Cu–Kα (λ = 0.154 nm) incident radiation at a scanning rate of 2 min−<sup>1</sup> in 2θ mode from 20 to 90.

The electrochemical characterization including linear sweep voltammetry (LSV) curves and cyclic voltammetry of porous Ti/SnO2–Sb2O3/PbO2 electrodes were carried out using a CHI760E electrochemical workstation (CH Instruments, Chenhua Co., Shanghai, China) with a conventional three-electrode cell. The working electrode was the PbO2 electrode; the reference electrode was a saturated calomel electrode (SCE) and a platinum electrode served as a counter electrode.

### *2.4. Electrocatalytic Oxidation of Chloride Ions*

The porous Ti/SnO2–Sb2O3/PbO2 electrodes were used as anodes and a Ti electrode as the cathode. The produced chlorine was absorbed in an absorption tank. After the reaction, AgNO3 solution was used as a titrant and K2CrO4 solution as a color developer. In order to obtain optimal removal conditions, the effects of the following factors were considered for electrocatalytic oxidation of chloride ions, including initial concentration (from5gL−<sup>1</sup> to 25 g L−1), pore size (from 50 μm to 150 μm), current densities (from 50 mA cm−<sup>2</sup> to 125 mA cm−2), initial pH (from 3 to 11), and electrode plate spacing (from 0.5 cm to 1 cm). The removal ratio of chloride ions was calculated as follows:

$$\text{Removal ratios of chloride ions} = \frac{B\_1 - B\_2}{B\_1} \times 100\%$$

*B*<sup>1</sup> is the concentration of the original chloride ions in the NaCl solution, and *B*<sup>2</sup> the concentration of the remaining chloride ions in the NaCl solution after the reaction.

### **3. Results and Discussion**

### *3.1. Physicochemical Characterization*

### 3.1.1. Scanning Electron Microscopy (SEM) Characterization

In Figure 1, the scanning electron microscopy (SEM) images of the three different pore sizes (50 μm, 100 μm, and 150 μm) are shown. As shown in Figure 1a,d,g, the surfaces of the porous Ti substrates are very rough. Figure 1b,e,h show that the porous Ti/SnO2–Sb2O3 electrodes still have a lot of irregular pores, which indicates that the porous Ti/SnO2–Sb2O3 electrodes have large specific surface areas. By zooming 500 and 4000 times, it is seen that the SnO2–Sb2O3 intermediate layers are uniformly distributed and crack-free, which is beneficial for prolonging the service life of the electrode. Figure 1c,f,i show that the porous Ti/SnO2–Sb2O3/PbO2 electrodes still have many irregular pores. Although some of the pores became during electrodeposition, large specific surface areas were still retained compared to the planar electrodes. By zooming 500 and 4000 times, it is revealed that the PbO2 grains are also very compact and evenly distributed. In short, the porous Ti/SnO2–Sb2O3/PbO2 electrodes have large surface areas, which can provide many active sites for electrochemical oxidation [18].

**Figure 1.** Scanning electron microscopy (SEM) images: (**a**) 50 μm porous Ti substrate; (**b**) 50 μm porous Ti/SnO2–Sb2O3 electrode; (**c**) 50 μm porous Ti/SnO2–Sb2O3/PbO2 electrode; (**d**) 100 μm porous Ti substrate; (**e**) 100 μm porous Ti/SnO2–Sb2O3 electrode; (**f**) 100 μm porous Ti/SnO2–Sb2O3/PbO2 electrode; (**g**) 150 μm porous Ti substrate; (**h**) 150 μm porous Ti/SnO2–Sb2O3 electrode; (**i**) 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode.

### 3.1.2. X-ray Photoelectron Spectrometer (XPS) Characterization

In order to investigate the chemical state of each element in the porous Ti/SnO2–Sb2O3/PbO2 electrodes, the electrodes were analyzed by XPS. Since the XPS spectra of the three different-pore-size electrodes are the same, only the 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode is analyzed here. Figure 2a is the full spectrum of a porous Ti/SnO2–Sb2O3/PbO2 electrode. It can be seen that there are mainly Ti, Sn, Sb, O, Pb, and C peaks in the whole electrode. Figure 2b shows the Sn 3d spectrum with two characteristic peaks at 487.1 eV and 495.5 eV. Figure 2c shows the Sb 3d and O 1s spectra with characteristic peaks at 540.3 eV and 531.6 eV. Figure 2d shows the Pb 4f spectrum with characteristic peaks at 138.8 eV and 143.7 eV. The composition and chemical state of the porous Ti/SnO2–Sb2O3/PbO2 electrode could be determined by the XPS characterization results, and it was inferred that porous Ti/SnO2–Sb2O3/PbO2 electrode was successfully prepared.

**Figure 2.** (**a**) XPS spectrum of the porous Ti/SnO2–Sb2O3/PbO2 electrode; (**b**) XPS spectrum of Sn 3d; (**c**) XPS spectrum of O 1s and Sb 3d; (**d**) XPS spectrum of Pb 4f.

### 3.1.3. X-ray Diffraction (XRD) Characterization

To further verify the results, the XRD patterns of the electrode coatings prepared on the three different-pore-size Ti substrates are shown in Figure 3. Since the XRD images of the three pore sizes electrodes are the same, only the 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrodes are analyzed here. Figure 3 shows the β–PbO2 diffraction peaks, which are (110), (101), (200), (211), (220), (310), (301), (321), (312), and (411). At the same time, Figure 3 shows the weak α–PbO2 diffraction peaks, which is (041). In addition, there are no diffraction peaks of Ti, SnO2, and Sb2O3, which proves that the PbO2 coating had completely covered the porous Ti/SnO2–Sb2O3 electrodes.

**Figure 3.** The X-ray diffraction (XRD) pattern of 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode.

### *3.2. Electrochemical Characterization*

### 3.2.1. Linear Sweep Voltammetry (LSV) Curves

Figure 4 shows the linear sweep voltammetry (LSV) curves of the three different kinds of porous Ti/SnO2–Sb2O3/PbO2 electrodes as obtained in a 0.5 mol L−<sup>1</sup> H2SO4 solution at a scan rate of 5 mV s<sup>−</sup>1. The result shows that the oxygen evolution potential is 2.02 V, 1.99 V, and 1.96 V at the 150 μm, 100 μm, and 50 μm porous electrodes, respectively. High oxygen evolution potentials indicate that side reactions involving the formation of oxidized species are not very likely to occur [19]. 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrodes therefore have the best removal effect on chloride ions.

**Figure 4.** Linear sweep voltammetry (LSV) curves of porous Ti/SnO2–Sb2O3/PbO2 electrodes in 0.5 mol L−<sup>1</sup> H2SO4 solution at a scan rate of 5 mV s<sup>−</sup>1.

### 3.2.2. Cyclic Voltammetry

Figure 5 shows the cyclic voltammetry of the three different-pore-size Ti/SnO2–Sb2O3/PbO2 electrodes in 0.5 mol L−<sup>1</sup> H2SO4 solution at a scan rate of 5 mV s−1. The results show that the redox peak of the 50 μm porous Ti/SnO2–Sb2O3/PbO2 electrode is highest, while it is lowest at the 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode. The low oxygen evolution potential of the 50 μm porous Ti/SnO2–Sb2O3/PbO2 electrode indicates that it has a high areal density of oxygen evolution sites. The high oxygen evolution potential of the 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode, on the other hand, indicates a low areal density of oxygen evolution sites, which is beneficial for the electrocatalytic oxidation of chloride ions [20].

**Figure 5.** Cyclic voltammetry curve of porous Ti/SnO2–Sb2O3/PbO2 electrodes in 0.5 mol L−<sup>1</sup> H2SO4 solution at a scan rate of 5 mV s<sup>−</sup>1.

### 3.2.3. Linear Sweep Voltammetry (LSV) Curves in Different pH Solutions

Figure 6 shows the LSV curves of a 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode in different pH solutions at a scan rate of 5 mV s<sup>−</sup>1. Solutions with pH values between pH = 3 and pH = 5 were prepared from buffer solution of citric acid and sodium citrate, and solutions with pH = 7 from buffer solution of potassium monohydrogen phosphate and potassium dihydrogen phosphate. Solutions with pH values between pH = 9 and pH = 11, finally, were composed from sodium carbonate and sodium bicarbonate buffer solution. It can be seen from the Figure 6 that under the same voltage conditions, the current density is smallest at pH = 9, which indicates that oxygen evolution is least likely to occur under these conditions and that therefore 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrodes will exhibit an optimum chloride ion removal effect at pH = 9.

**Figure 6.** Linear sweep voltammetry (LSV) curves in different pH solutions of the 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode in 0.5 mol L−<sup>1</sup> H2SO4 solution at a scan rate of 5 mV s<sup>−</sup>1.

### *3.3. Electrocatalytic Oxidation of Chloride Ions*

### 3.3.1. Effect of Pore Size

With the electrolysis time fixed at 4 h, the electrode plate spacing at 1.0 cm, the NaCl concentration at 10 g L<sup>−</sup>1, and the current density at 100 mA cm−2, the chloride ion removal ratios of the electrodes were determined to explore the optimal electrode pore size among three different kinds of porous electrodes in NaCl solution. As shown in Figure 7a, the optimal pore size of porous Ti/SnO2-Sb2O3/PbO2 electrode is 150 μm after 4 h, and the removal ratio of chloride ions is 92.8%, which confirms that the 150 μm pore size electrode performs best among the above three different kinds of porous electrodes. In addition, the effect of removing chloride ions from the 150 μm porous Ti/SnO2-Sb2O3 electrode and porous Ti/SnO2–Sb2O3/PbO2 electrode is also compared. It can be seen from Figure 7b that the porous electrode with PbO2 coating has a much better effect on removing chloride ions than the electrode without PbO2 coating. The reason for this improvement is that the stability and the oxygen evolution potential of the porous Ti/SnO2–Sb2O3/PbO2electrode are further improved by the addition of the PbO2 coating, which is beneficial for achieving an increased removal ratio of chloride ions.

**Figure 7.** (**a**) Effect of pore size on chloride ion removal efficiency; (**b**) chloride ion removal efficiency as observed with 150 μm porous Ti/SnO2-Sb2O3 and 150 μm porous Ti/SnO2-Sb2O3/PbO2 electrodes.

### 3.3.2. Effect of Initial NaCl Concentration

Related studies have shown that the initial ion concentration can affect the chloride ion removal ratio and cell voltage. As shown in Figure 8, with the electrolysis time fixed at 4 h, the plate spacing at 1.0 cm, and the current density at 100 mA cm<sup>−</sup>2, the removal ratio of chloride ions and the cell voltage both decrease as the NaCl concentration is increased. The reason for this effect is that the conductivity is increased and the electrical resistance decreased as the NaCl concentration is increased [1]. In addition, it could be seen from the figure that the 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrode has a superior removal effect on chloride ions at low concentrations. Therefore, considering the combined effects of the removal ratio chloride ions and the cell voltage, 10 g L−<sup>1</sup> NaCl solution were selected as the following study.

**Figure 8.** (**a**) Chloride ions removal efficiency under various initial NaCl concentration; (**b**) change in cell voltage under various initial NaCl concentrations.

### 3.3.3. Effect of Current Density

With the electrolysis time at 4 h, the optimal reaction current density was determined for 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrodes. As shown in Figure 9, with a 150 μm electrode, a NaCl concentration 10 g L<sup>−</sup>1, and the electrode plate spacing 1.0 cm, the removal ratio of chloride ions almost reached an upper limit at 125 mA cm−<sup>2</sup> within 4 h. Current densities higher than 125 mA cm<sup>−</sup>2, tended to damage the electrodes, resulting in reduced electrode lifetimes. So, it could be concluded that 125 mA cm−<sup>2</sup> is the optimal current density.

**Figure 9.** Chloride ion removal efficiency under various current densities.

### 3.3.4. Effect of Initial pH

As can be seen in Figure 10, with the 150 μm electrode, the NaCl concentration 10 g L−1, the electrode plate spacing 1.0 cm, and the current density 125 mA cm−2, the removal ratio of chloride ions is shown under the pH from 3 to 11. The solution of pH = 3 and pH = 5 was prepared by adding diluted concentrated sulfuric acid, and the solution of pH = 9 and pH = 11 was prepared by adding a diluted sodium hydroxide solution. It can be seen from the figure when the initial pH is the weak acidity, weak alkali, or neutral conditions, and the removal ratio of chloride ions is very effective, the removal ratio could reach 98.5% at pH = 9. In addition, the porous Ti/SnO2–Sb2O3/PbO2 electrode has a good removal effect over the whole range of pH values.

**Figure 10.** Chloride ion removal efficiency as a function of initial pH value.

### 3.3.5. Effect of Electrode Plate Spacing

Through the previous work, it was found that a change in the spacing between cathode and anode plates would cause a change in the cell voltage, which will impact the energy consumption of the whole process. As can be seen from Figure 11, the cell voltage at a current density of 125 mA cm−<sup>2</sup> is about 11.7 V, when the spacing between both the plates is 1.0 cm and when the 150 μm electrode is operated in a NaCl solution with a concentration of 10 g L−<sup>1</sup> and at an initial pH = 9. Reduction of the plate spacing to 0.75 cm lowers the cell voltage to about 10.4 V; a further reduction to 9.4 V occurs when the plate spacing is reduced to 0.5 cm. Therefore, it can be seen that the voltage decreases as the spacing between both plates is reduced. A reduction in the spacing of the anode and cathode plates therefore can lower the cell voltage and thereby the power consumption.

**Figure 11.** Change in cell voltage with electrode plate spacing.

### 3.3.6. Number of Cycles

With the 150 μm electrode, a NaCl concentration of 10 g L<sup>−</sup>1, an initial pH = 9, a current density 125 mA cm<sup>−</sup>2, and the spacing between plates 0.5 cm, Figure 12a shows the change in removal ratio of chloride ions of the porous electrode after repeating the removal process under optimal conditions for ten times. It can be seen that the removal ratio of chloride ions decreases after 10 times. However, the removal ratio of chloride ions of the porous Ti/SnO2–Sb2O3/PbO2 electrode is still more than 90%, which indicates that the porous Ti/SnO2–Sb2O3/PbO2 electrode has great advantages in stability. In addition, Figure 12b,c show the SEM and XRD patterns of the electrode after recycling, Figure 12c shows that the XRD spectrum of the electrode has not changed with the previous electrode after recycling, and Figure 12b shows that the PbO2 deposition coating of the electrode has suffered partial damage after recycling. By zooming 4000 times, it is found that the PbO2 deposition coating has developed many cracks, and thus it may be suspected that the reduction in removal rate might be related to these cracks.

**Figure 12.** (**a**) Change in chloride removal rate of a porous Ti/SnO2–Sb2O3/PbO2 electrode during ten successive removal cycles; (**b**) Scanning electron microscopy (SEM) pattern of the porous Ti/SnO2–Sb2O3/PbO2 electrode after recycling; (**c**) X-ray diffraction (XRD) pattern of the porous Ti/SnO2–Sb2O3/PbO2 electrode after recycling.

### 3.3.7. Mechanism of Removing Chloride Ions

During the experiment, the sodium chloride solution was poured into the electrolytic cell. The chlorine evolution reaction and other side reactions mainly occurred at the anode. The hydrogen evolution reaction and other side reactions mainly occurred at the cathode. The chloride ions formed chlorine gas at the anode and the produced chlorine gas was absorbed in the absorption tank, which in turn resulted in the removal of chloride ions.

Anode reaction:

$$2\text{Cl}^- - 2\text{e}^- \rightarrow \text{Cl}\_2\uparrow\tag{1}$$

$$4\text{OH}^- - 4\text{e}^- \rightarrow 2\text{H}\_2\text{O} + \text{O}\_2\uparrow\tag{2}$$

Cathode reaction:

$$2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow 2\text{OH}^- + \text{H}\_2\uparrow\tag{3}$$

$$2\text{ClO}^- + 2\text{H}\_2\text{O} + 2\text{e}^- \rightarrow 2\text{OH}^- + \text{Cl}^- \tag{4}$$

Side reaction in solution:

$$\text{Cl}\_2 + 2\text{OH}^- \rightarrow \text{ClO}^- + \text{Cl}^- + \text{H}\_2\text{O} \tag{5}$$

$$\text{Cl}\_2 + \text{H}\_2\text{O} \rightarrow \text{HClO} + \text{H}^+ + \text{Cl}^- \tag{6}$$

$$\rm HClO + H\_2O \rightarrow H^+ + \rm ClO^- \tag{7}$$

It can be seen from the above reaction formulae that the side reaction (2) competes with the desired chloride removal reaction (1) for holes from the anode. Due to the high oxygen evolution potential of porous Ti/SnO2–Sb2O3/PbO2 electrodes, reaction (2), however, is inhibited. With reaction (2) being inhibited, the generation of OH− ions at the cathode via reaction (3) will also be inhibited. Under alkaline conditions, side reactions (5)–(7) will tend to produce ClO− ions, which, in turn, will become converted into Cl− ions via reaction (4) and which will finally become removed by the main reaction (1). As previous studies have shown that the concentrations of hypochlorite and perchlorate ions which are generated in the electrolysis of sodium chloride solutions are actually quite small [21], side reactions (5)–(7) are not expected to make any major contribution to the overall chlorine removal process [22,23]. Overall, our considerations therefore reveal that porous Ti/SnO2–Sb2O3/PbO2 electrodes can efficiently remove chloride ions from aqueous solutions, thus opening very broad application prospects.

#### **4. Conclusions**

In this paper, 150 μm porous Ti/SnO2–Sb2O3/PbO2 electrodes have been demonstrated to exhibit a good removal effect on chloride ions from NaCl solutions via the process of electrocatalytic oxidation. Factors that were found to influence the process of electrocatalytic oxidation were initial concentration, pore size, current density, initial pH, electrode plate spacing, and the number of removal cycles. Removal ratios of chloride ions up to 98.5% were observed under the following conditions: initial concentration 10 g L<sup>−</sup>1, current density 125 mA cm−2, electrode plate pore size 150 μm, initial pH = 9, and electrode plate spacing 0.5 cm. Under these conditions, the porous electrodes exhibited good stability. Physicochemical and electrochemical characterization results showed that porous Ti/SnO2–Sb2O3/PbO2 electrodes had a high oxygen evolution potential. Overall, it appears that porous Ti/SnO2–Sb2O3/PbO2 electrodes can play an important role in the electrocatalytic oxidation of chloride ions in water, thus opening prospects for a wide range of applications.

**Author Contributions:** Conceptualization, K.X.; Data curation, P.C.; Formal analysis, W.G.; Project administration, P.Y.; Resources, Y.L.; Writing—review & editing, J.P.

**Funding:** The authors declare no funding.

**Acknowledgments:** The study was supported by the college of chemistry and molecular sciences at Wuhan University.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **A Novel Porous Ni, Ce-Doped PbO2 Electrode for E**ffi**cient Treatment of Chloride Ion in Wastewater**

### **Sheng Liu, Lin Gui, Ruichao Peng and Ping Yu \***

College of Chemistry & Molecular Science, Wuhan University, Wuhan 430072, China; sgg520@whu.edu.cn (S.L.); lingui@whu.edu.cn (L.G.); prc@whu.edu.cn (R.P.)

**\*** Correspondence: yuping@whu.edu.cn; Tel.: +86-027-6875-2511

Received: 29 March 2020; Accepted: 9 April 2020; Published: 16 April 2020

**Abstract:** The porous Ti/Sb-SnO2/Ni-Ce-PbO2 electrode was prepared by using a porous Ti plate as a substrate, an Sb-doped SnO2 as an intermediate, and a PbO2 doped with Ni and Ce as an active layer. The surface morphology and crystal structure of the electrode were characterized by scanning electron microscope(SEM), energy dispersive spectrometer(EDS), and X-Ray diffraction(XRD). The electrochemical performance of the electrodes was tested by linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and electrode life test. The results show that the novel porous Ni-Ce-PbO2 electrodes with larger active surface area have better electrochemical activity and longer electrode life than porous undoped PbO2 electrodes and flat Ni-Ce-PbO2 electrodes. In this work, the removal of Cl<sup>−</sup> in simulated wastewater on three electrodes was also studied. The results show that the removal effect of the porous Ni-Ce-PbO2 electrode is obviously better than the other two electrodes, and the removal rate is 87.4%, while the removal rates of the other two electrodes were 72.90% and 80.20%, respectively. In addition, the mechanism of electrochemical dechlorinating was also studied. With the progress of electrolysis, we find that the increase of OH- inhibits the degradation of Cl<sup>−</sup>, however, the porous Ni-Ce-PbO2 electrode can effectively improve the removal of Cl−.

**Keywords:** porous Ni-Ce-PbO2; co-doping; active surface area; removal rate

### **1. Introduction**

The widespread use of chlorinated compounds such as HCl, NaCl, and MgCl2 in the industrial field has increased the content of chloride ion in wastewater [1,2]. If it is discharged into the water body beyond control, the water environment will be seriously damaged. The accumulation of chloride ions will make the soil salinized and alkalized, and excessive intake of chlorine by the human body will cause organ damage. Chloride ions are corrosive to pipelines, boilers, etc., and can erode buildings and reduce durability of the concrete structure. For example, a large amount of Cl− in desulfurization wastewater discharged from thermal power plants can corrode pipelines and equipments [3].

At present, the most widely used method for treating Cl− in wastewater is chemical precipitation [4]. However, the concentration of Cl− in the treated wastewater is still high. The treatment of chloride ion in wastewater by chemical precipitation will be limited in the future [5]. How to treat desulfurization wastewater in depth, meet the discharge requirements, and reduce the impact on the environment has always been a difficult problem in the field of wastewater treatment at home and abroad. Therefore, advanced oxidation processes (AOPs) [6,7] such as catalytic ozonation, Fenton oxidation, supercritical water oxidation, electrochemical oxidation, photocatalytic oxidation, etc., [8] have been studied. Electrochemical oxidation is one of the most eye-catching AOPs. It has the advantages of good treatment effect, small floor area, high degradation efficiency, short residence time and no secondary pollution, and has broad application prospects in the advanced treatment of salt compounds and

organic compounds [9]. In the recent years, electrochemical oxidation technology has been increasingly studied for the treatment of chloride ion in wastewater [10].

The degradation efficiency and degradation products of electrochemical oxidation process change with the anode material [11], which means that anode material is one of the main factors in electrochemical oxidation process [12,13]. Because of the critical role of anode materials, scholars have been studying new anodes for many years [14]. The earliest graphite and carbon electrodes have the disadvantages of low current efficiency and poor mechanical strength [15]. Subsequently, metal anodes were developed. At present, metal oxide electrode is widely used and its preparation process is mature, such as dimensionally stable anode (DSA) (e.g., RuO2, IrO2, PbO2, SnO2) [11]. With the deepening of research, 3D porous structure compounds are widely used in the preparation of anode materials because of their large specific surface area, such as carbon nanotubes (CNT), porous graphene (GE), etc., [16,17], which can significantly change the structure of the coating when doped in the metal oxide coating [11]. Scholars have found that doping metal elements in metal oxide coatings can significantly improve the electrode activity and extend service life [18], such as Ce, Bi, Fe, Co, etc., [11]. In addition, it has been found that many rare earth oxides doped into PbO2 can significantly improve the electrochemical performance of the electrode [19]. Kong et al. reported that doping Er2O3, Gd2O3, La2O3, and Ce2O3 on PbO2 electrode could promote the degradation of 4-chlorophenol [20]. Jin et al. [20] found that doping Ce can form smaller crystal size on the surface of the electrode, resulting in the increase of specific surface area and catalytically active sites. Xia et al. [21] reported that proper doping of Ni can make the grains dense, which not only facilitates electron movement, and improves electrochemical performance, but also helps to extend electrode life.

In this work, we intended to use a porous Ti plate as the substrate, because the porous structure has large specific surface area. We can improve the electrochemical activity and stability of the electrode by doping Ce and Ni in the active layer PbO2 electrode [22]. The doping of Ce and Ni reduces the crystal size of the active layer [11,21], but increases the surface area of the active layer and the electrochemical activity of the electrode, in addition, the service life of the electrode is extended to some extent. Finally, the electrochemical activity and stability of porous Ni-Ce-PbO2 electrodes were investigated and compared with the conventional porous undoped PbO2 electrode and the doped flat Ni-Ce-PbO2 electrode. In this experiment, simulated wastewater with high chlorine content was selected as the target pollutant. The electrochemical performance of the electrode was investigated by comparing the effects of three kinds of electrodes on the treatment of chloride ions in simulated wastewater.

### **2. Experimental**

### *2.1. Materials*

Porous titanium plate with a purity of 99.9% was purchased from Baoji Jinkai Technology Co., Ltd., Jinan, China. Ni(NO3)2 6H2O was purchased from Xiqiao Chemical Co. Ltd., Foshan, China. All other chemicals were purchased from Sino pharm Chemical Reagent Co. Ltd., Shangai, China. All chemicals were of analytical grade and used as received. In this work, deionized water was used in all solutions.

### *2.2. Electrode Preparation*

### 2.2.1. Titanium Surface Preparation

The porous titanium plates (20 mm × 10 mm × 2.8 mm) bought by Baoji Jinkai Technology Co., Ltd., Jinan, China, had a purity of 99.9% and an average pore diameter of 50 μm to pretreat the substrate. The porous titanium plate was ultrasonically cleaned in acetone for 15 min, washed in 20% NaOH solution at 90 ◦C for 1 h, and etched in a 15 wt % oxalic acid solution at 90 ◦C for 1 h until a gray matte titanium matrix was formed, and finally saved in ethanol.

The surface treatment of the flat titanium plate (20 mm × 10 mm × 2.8 mm) is similar to porous titanium plate.

### 2.2.2. Coating SnO2–Sb2O3

The electrode intermediate layer was prepared by thermal decomposition method [11]. Total of 1.2 g SnCl4, 0.2 g Sb2O3, and 10 mL concentrated hydrochloric acid were dissolved in 25 mL of isopropanol to obtain a precursor-coating solution. The solution was colorless, transparent, slightly sticky. Then the pretreated porous titanium plate was immersed in the precursor solution. After soaking, it was taken out and dried in an oven at 110 ◦C, and then calcined in a muffle furnace at 500 ◦C for 15 min. After cooling the plate, the drying and calcining process was repeated several times, and the high-temperature baking time in the last time was extended to 1 h to obtain a porous Ti/Sb-SnO2 electrode. The main purpose of this layer is to improve the conductivity of the electrode and prevent the titanium matrix from being oxidized to form TiO2.

The precursor solution can be directly brushed on the surface of the flat titanium plate with a brush. Other preparation processes of the intermediate layer of the flat electrode are similar to that of the porous electrode.

### 2.2.3. Electrochemical Deposition Ni–Ce–PbO2

Pb(NO3)2, Ni(NO3)2, and Ce(NO3)2 were dissolved in 250 mL water at a ratio of 100:1:1 of Pb, Ni, and Ce. Then, 0.04 M NaF and 4 mL/L of PTFE were added, before adding 0.1 mol/L HNO3 to adjust the pH to 1, to form an electrodeposition solution. The control temperature was 65 ◦C, the current density was 20 mA/cm2, the electrodeposition time was 1 h, so as to deposit a surface layer of lead dioxide with Ce, Ni co-doped on the surface of the intermediate layer tin antimony oxide, that is, porous Ti/Sb-SnO2/Ni-Ce-PbO2 electrode.

In addition to the raw materials (Ni (NO3)2 and Ce (NO3)2), the preparation process of porous PbO2 electrode is similar to that of porous Ni-Ce PbO2 electrode

### *2.3. Electrode Characterization*

The surface morphology was observed using a scanning electron microscope (Quanta 200 of FEI, Hillsboro, OR, USA). X-ray diffraction (XRD) patterns of samples were obtained with an X-ray diffractometer (PANalytical, Almelo, The Netherlands). Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and electrochemical impedance spectroscopy (EIS) were performed at room temperature using a computer-controlled electrochemical workstation (CHI 660E, CH Instruments, Shanghai, China) with a conventional three-electrode system. The prepared PbO2-based electrode (20 mm × 20 mm) was used as the working electrode, a saturated Ag/AgCl electrode was employed as the reference electrode, and a stainless steel sheet was applied as the counter electrode. All potentials were referred to the SCE. The stability tests (up to 20 h) were performed by the accelerated life test with a current density of 1 A·cm−<sup>2</sup> and a temperature of 60 ◦C in 2 M H2SO4 solution for porous undoped PbO2 electrodes, porous Ni-Ce–PbO2 electrodes, and Flat Ni-Ce–PbO2 electrodes. These tests were performed in a three-electrode system.

### *2.4. Electrochemical Oxidation*

A simulated chlorine-containing wastewater with a chloride ion concentration of 4 g/L and a volume of 200 mL was selected as the experimental wastewater; the temperature was 298 K in all experiments. The electrochemical oxidation experiment was conducted by a batch method, and the device was mainly composed of a DC power source, a collector types magnetic stirrer, and a glass reactor. The anode (PbO2-based electrodes) and the cathode (flat titanium plate) were placed parallel to each other and perpendicular to the solution level with a distance of 1 cm. The volume of simulated wastewater in all experiments was 200 mL and the Cl− concentration was 4 g/L. The temperature of all experiments was maintained at 298 K. All pH values in the experiment were determined by a pH meter and all Cl− concentration in the experiment were determined by titration with a standard AgNO3 solution.

### **3. Results and Discussion**

### *3.1. Surface Morphological and Crystallographic Analysis*

The SEM images of the plate-modified Ni-Ce-PbO2 electrode, the porous undoped PbO2 electrode, and the porous Ni-Ce-PbO2 electrode are shown in Figure 1. It can be seen that the porous undoped PbO2 electrode and the porous Ni-Ce-PbO2 electrode still have many small holes in the electrode surface while compared with porous Ti plate (Figure 1a), indicating that the PbO2 coating did not block the porous structure. From Figure 1b,f it can be seen that the porous Ni-Ce-PbO2 electrodes apparently have larger pores and specific surface area compared to the flat Ni-Ce-PbO2 electrodes.

**Figure 1.** SEM of different electrodes, (**a**) (porous titanium plate, 80×); (**b**) (flat Ni-Ce-PbO2 electrode, 1200×) and (**c**) (porous undoped PbO2 electrode, 80×); (**d**) (porous undoped PbO2 electrode, 1200×) and (**e**) (porous Ni-Ce-PbO2 electrode, 80×); (**f**) (porous Ni-Ce-PbO2 electrode, 1200×).

The porous PbO2 electrode doped with Ni and Ce (Figure 1e,f) can make the holes smaller or bigger on the surface of the electrodes, and the specific surface area was larger than porous undoped PbO2 electrodes (Figure 1c,d). Besides, the EDS spectrum of different PbO2 electrodes is shown in Figure 2a, which confirms that there are O, Pb, Ce, Ni elements in the porous Ni-Ce-PbO2 electrode, while there are only O and Pb elements in the porous undoped PbO2 electrodes. Thus, it can be concluded that the Ni and Ce was successfully doped into the PbO2 films and doping of Ni and Ce can reduce the grain size of the PbO2 coating.

**Figure 2.** (**a**) EDS of a (porous PbO2 electrode) and b (porous Ni-Ce-PbO2 electrode); (**b**) XRD diffraction patterns of different Electrodes A (Porous Ti/Sb-SnO2/PbO2 electrode), B (Porous Ti/Sb-SnO2/Ni-Ce-PbO2 electrode), and C (Standard XRD pattern of PbO2).

From Figure 2b, it can be seen that the diffraction peaks of undoped porous show that dioxide electrodes appear at 25.4 degrees, 31.9 degrees, 36.1 degrees, 48.9 degrees, 58.8 degrees, 62.4 degrees, and 66.8 degrees, which are consistent with the JCPDS card (41–1492) in pattern, indicating that the main component of undoped PbO2 surface layer is β-PbO2 and the coating of porous Ni-Ce-PbO2 electrode consists of a mixture of crystalline phases of α and β-PbO2. The content of α-PbO2 is higher than that of undoped PbO2 because the doping of Ce changes the preferred crystalline orientation of the electrode surface and forms smaller grains. In addition, compared with the undoped PbO2, the intensity of the diffraction peaks of Ni-Ce-PbO2 decreases or even disappears because of the doping of Ni and Ce. The reason may be that doping of Ni and Ce changes the nucleation and growth of crystals in the coating, making the electrode have smaller crystal size than the undoped PbO2 electrode, which can also be seen from the SEM image. The average crystallite size calculated from the width of [101] diffraction peaks by Scherrer's formula is 17.64 nm (Ce–Ni) and 28.76 nm (undoped). According to literature [20], the diffraction peak width is inversely proportional to the crystallite size. The result indicates that the deposited Ni-Ce-PbO2 has smaller crystallite size than other electrode. Smaller crystal size may help to form larger specific surface area which may lead to better electrochemical performance.

### *3.2. Electrochemical Performance Test*

As shown in Figure 3a, no redox peak signal was observed on any of the electrodes in the blank Na2SO4 solution, indicating that PbO2 is an electrochemically inert material in the blank Na2SO4 solution. After the addition of Cl− (Figure 3b), a distinct oxidation peak was obtained. There is no doubt that the oxidation peak is attributed to the oxidation of Cl− on the surface of the anode. However, no corresponding reduction peak was observed in the reverse scanning from 3 V to 0 V, indicating that the oxidation of Cl− is a completely irreversible electrode reaction process. The oxidation peak potential of the porous Ni-Ce-PbO2 electrode (2.01 V vs. SCE) was lower than that of the other two electrodes

(2.24 and 2.48 V), but the oxidation peaks current (0.031 A) was significantly higher than the other two electrodes (0.028 A and 0.024 A), which showed that the porous Ni-Ce-PbO2 electrode has higher electro-catalytic activity for Cl−, and the improvement of its electro-catalytic activity is not only related to the increase in electrode surface area caused by its porous structure and doping with Ni and Ce, but also due to changes in the PbO2 band structure. The electrode doping Ni and Ce not only increases the donor area of PbO2, but also increases the donor level of PbO2, making it easier for electrons to jump from the donor level to the conduction band [21]. Therefore, the conductivity of PbO2 is improved.

**Figure 3.** (**a**) Cyclic voltammetry (CV) of different PbO2 electrodes measured in 0.1mol·L−<sup>1</sup> Na2SO4 solution, (**b**) 0.1 mol·L−<sup>1</sup> Na2SO4 solution (pH = 6.5) containing 4 g L−<sup>1</sup> Cl−, scan rate: 50 mV s<sup>−</sup>1, T = 298 K, (**c**) LSV curves of different electrodes measured in 0.5 M Na2SO4, scan rate: 10 mV s<sup>−</sup>1, T = 298 K, (**d**) EIS of different electrodes. Conditions: T = 298 K; [H2SO4] = 1 M. (**e**) Electrode stability tests: electrode potential vs. time for the electrolysis using different electrodes. (**f**) The mass losses of electrodes after accelerated life tests for 20 h.

The oxygen evolution overpotential (OEP) of different electrodes could be measured by LSV. According to the polarization curve in Figure 3c, the OEP of the porous Ni-Ce-PbO2 electrode was the highest with 2.09 V (vs. SCE) compared with the flat Ni-Ce-PbO2 electrode of 1.81 V (vs. SCE) and the porous PbO2 electrode of 1.91 V (vs. SCE), respectively. The electrodes with high OEP values can produce more hydroxyl radicals [23]. In addition, it can be seen from Figure 3c that the porous Ni-Ce-PbO2 electrodes had higher oxygen evolution current than the other two electrodes. The higher oxygen releases current also verified that the active surface area of the porous Ni-Ce-PbO2 electrode is larger than that of the other PbO2-based electrodes.

In Figure 3d, there is an obvious semicircle in the electrochemical impedance spectroscopy (EIS) of the three electrodes. The radius of the semicircle usually reflects the magnitude of the transfer resistance Rct of the chlorine evolution or oxygen evolution reaction, which is available from the EIS spectrum. The Rct sizes of Ni-Ce-PbO2 electrodes, porous PbO2 electrodes, and porous Ni-Ce-PbO2 electrodes were 59.4, 21.2, and 12.2, respectively. It is indicated that the porous Ni-Ce-PbO2 electrode has the best conductivity, the most active surface sites, and the highest chlorine evolution activity.

The service life is a critical factor in the practical application of the electrode. Under the condition of anode current of 1A pa−<sup>2</sup> and temperature of 60 ◦C, the accelerated life test of different PbO2 electrodes was carried out in 2M H2SO4 solution for 20 h. According to Figure 3e,f, the Ni-Ce-PbO2 electrode exhibited better electrochemical stability and less mass loss than the pure PbO2 electrode. It is well-known that the main reasons of mass loss of electrode are the separation and dissolution of PbO2 film, and the mass loss is proportional to the service life of the electrode. Therefore, it can be concluded that the stability of Ni-Ce-PbO2 electrode is much better than that of pure PbO2 electrode by the modification of Ni and Ce. The mass loss of the porous Ni-Ce-PbO2 electrode is slightly larger than that of the flat Ni-Ce-PbO2, mainly because of the loose porosity of the structure. In fact, in the experiments, we found that film peeled off on the pure PbO2 electrode, but not on the other two modified PbO2 electrodes.

### *3.3. Electrochemical Oxidation of Cl*− *in Simulated Wastewater*

The electrochemical oxidation of simulated wastewater was compared with three kinds of electrodes. From Figure 4a, after electrolysis for 100 min, the removal rate of Cl− on the porous Ni-Ce-PbO2 electrode was as high as 87.4%. At the same time, the plate Ni-Ce-PbO2 electrode and the porous PbO2 electrode were used as anodes, and the highest removal rates of Cl<sup>−</sup> were 72.90% and 80.20%, respectively. Obviously, the above results show that the electrochemical oxidation ability of the porous Ni-Ce-PbO2 electrode was much higher than the other two electrodes. This may be due to the fact that the porous structure increases the surface area of the electrode and the doping of Ni and Ce further increases the surface area of the electrode, which facilitates the adsorption of Cl− on the surface of the anode and promotes the mass transfer and exchange of the reactants. Thus, the electrochemical oxidation ability of the electrode is improved. As is shown in Figure 4b that in the early stage of electrolysis, the removal rate of Cl− was higher than that in the later stage because of the enrichment of Cl− in wastewater. The results show that higher current density result in higher Cl− removal rate, and the difference in Cl− removal rate was not obvious at relatively higher current densities. With the progress of electrolysis, the decrease of Cl− concentration inhibited the oxidation of Cl− and reached equilibrium at a certain time, in which Cl− cannot be completely removed. However, the porous Ni-Ce-PbO2 electrode with large specific surface area can effectively improve the removal rate of Cl−. During the process of degradation of Cl−, the chlorine evolution reaction and oxygen evolution side reaction in the anode proceed simultaneously, the content of Cl− is higher and the content of OH− is lower in the initial stage of reaction process (Figure 4c). The reaction formulas are as follows:

$$\text{Anode: } 2\text{Cl}^- \rightarrow \text{Cl}\_2 + 2\text{e}^- \tag{1}$$

$$\text{Cl}\_2 + 2\text{H}\_2\text{O} \rightarrow \text{HClO} + \text{H}\_3\text{O}^+ + \text{Cl}^-\tag{2}$$

$$\rm HClO + H\_2O \to H\_3O^+ + ClO^- \tag{3}$$

$$\text{Cathode: }\text{H}\_2\text{O} \to \text{H}^+ + \text{OH}^-\tag{4}$$

$$\text{H} \, 2\text{H}^+ + 2\text{e}^- \to \text{H}\_2 \tag{5}$$

**Figure 4.** (**a**) Variation of Cl− removal percentage with time during electrochemical oxidation on different electrodes, conditions: current density = 50 mA/cm2; T = 298 K; [Cl−] = 4 g/L, (**b**) at different current densities, conditions: porous Ni-Ce-PbO2 electrode; T = 298 K; [Cl−] = 4 g/L and electrochemical reaction process of electrode dechlorinating (**c**) at 30 min and (**d**) at 100 min.

According to the investigation [24], under the experimental conditions, the anode mainly generates Cl2 (Equation (1)). The cathode produces a large amount of OH−, which exists in the solution to raise the pH of the solution quickly, except a small portion is transferred to the anode for consumption. The increase of OH− concentration promotes the disproportionation of chlorine and forms a large number of ClO− (Equations (2) and (3)). In addition, Figure 4a also shows the fact that the Cl− removal rate increases significantly with the increase of the initial concentration. But after the electrolysis time reaches 90 min, a large amount of Cl− is removed and the OH− concentration in the solution is gradually increased. This can also be seen from Figure 4c,d, the reaction formulas are as follows:

$$\begin{aligned} \text{H}\_2\text{O} &\rightarrow \text{H}^+ + \text{OH}^-\\ \text{Anode:} \, 2\text{OH}^- &-4\text{e}^- \rightarrow \text{O}\_2 + 2\text{H}^+\\ \text{Cathode:} \, 4\text{H}^+ &+ 4\text{e}^- \rightarrow 2\text{H}\_2 \end{aligned} \tag{6}$$

A large amount of OH groups accumulates on the surface of the anode, which hinders the contact between a small amount of Cl− in the solution and the anode, so that the oxygen evolution reaction on the surface of the anode gradually becomes the main reaction. The consumption of OH− on the anode results in a small part of residual Cl− in the solution which cannot be removed, and another small part of Cl− is converted into ClO− and HClO [24] (Equations (1)–(3)), which greatly limits the removal of Cl<sup>−</sup>. Also as a DSA electrode, the porous Ni-Ce-PbO2 electrode degraded 87.4% of Cl<sup>−</sup> at 90 min because of its large active surface area, while the degradation rate of the plated Ni-Ce-PbO2 electrode and the porous PbO2 electrode were only 72.90% and 80.20%, respectively. Therefore, the porous Ni-Ce-PbO2 electrode breaks through the limitation of the conventional electrode in removing Cl−. In addition, we also detected the concentration of Pb2<sup>+</sup> and Ni2<sup>+</sup> in the solution after the 5th oxidation, and found that the concentration of Pb2<sup>+</sup> was only about 0.0085 mg/L, and Ni2<sup>+</sup> was even less, far lower than the standard of the World Health Organization. It shows that it is environment friendly and will not cause secondary pollution.

### **4. Conclusions**

In this paper, porous Ni-Ce-doped PbO2 electrodes were successfully prepared on a porous titanium substrate by thermal deposition and electrodeposition. The surface morphology, crystal structure, electrochemical activity, and electrode stability of the flat Ni-Ce-PbO2, porous undoped PbO2, and porous Ni-Ce-PbO2 were tested, and the electro-catalytic properties of these three electrodes in simulated wastewater were compared.

The porous Ni-Ce-PbO2 electrodes possessed porous structure and smaller grain size than the other two electrodes. The doping of Ni and Ce can change the nucleation and growth of crystals on the surface of the electrode, making the electrode have smaller particles, larger electrochemical active surface area, and better electrode life.

At a current density of 50 mA, the porous Ni-Ce-PbO2 electrode was used to treat Cl<sup>−</sup> in the simulated wastewater. The removal rate was as high as 87.4%, while the highest removal rates of the porous pure PbO2 electrode and the flat Ni-Ce-PbO2 electrode were 72.90% and 80.20%, respectively.

In addition, in the later stage of Cl− oxidation, because of the increase of pH of the electrolyte, oxygen evolution reaction mainly occurs in the anode, which results in a part of Cl− that cannot be removed from the solution and the other part that dissolves in the solution in the form of ClO− and HClO; removal rate of Cl<sup>−</sup> is restricted. The novel porous Ni-Ce-PbO2 electrodes can effectively improve the removal of Cl− because of its greater electrochemically active surface area.

**Author Contributions:** S.L. and P.Y. conceived and designed the experiments; S.L., L.G., and R.P. performed the experiments; S.L. analyzed the data and wrote the paper; S.L. and P.Y. revised the manuscript. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.

### **References**


© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).

### *Article* **Experimental Study of Micro Electrochemical Discharge Machining of Ultra-Clear Glass with a Rotating Helical Tool**

### **Yong Liu 1,\*, Chao Zhang 2, Songsong Li 1, Chunsheng Guo 1,3 and Zhiyuan Wei <sup>1</sup>**


Received: 12 March 2019; Accepted: 29 March 2019; Published: 4 April 2019

**Abstract:** Electrochemical discharge machining (ECDM) is one effective way to fabricate non-conductive materials, such as quartz glass and ceramics. In this paper, the mathematical model for the machining process of ECDM was established. Then, sets of experiments were carried out to investigate the machining localization of ECDM with a rotating helical tool on ultra-clear glass. This paper discusses the effects of machining parameters including pulse voltage, duty factor, pulse frequency and feed rate on the side gap under different machining methods including electrochemical discharge drilling, electrochemical discharge milling and wire ECDM with a rotary helical tool. Finally, using the optimized parameters, ECDM with a rotary helical tool was a prospective method for machining micro holes, micro channels, micro slits, three-dimensional structures and complex closed structures with above ten micrometers side gaps on ultra-clear glass.

**Keywords:** electrochemical discharge machining; rotating helical tool; side gap; micro structures; closed structure; ultra-clear glass

### **1. Introduction**

In recent years, ECDM has gained attention. Micro electromechanical systems (MEMS), including micro reactors and micro medical devices, often consist of the micro structures of nonconductive materials, such as glass, ceramics and silicon nitride. Therefore, the traditional machining method is difficult to use to fabricate micro structures composed of brittle and hard nonconductive materials. However, ECDM can machine micro structures on hard nonconductive materials. ECDM is a hybrid machining method including electrochemical machining and electric discharge machining [1]. When discharge takes place between the tool electrode and the surrounding electrolyte, local high temperatures and chemical reactions remove the workpiece material. Micro structures have been widely applied to micro accelerometers, micro pumps, micro containers and biological medical instruments, which could be machined by electrochemical machining (ECM), electro discharge machining (EDM) or ECDM [2–5]. Glass has superior properties, including transparency, high oxidation resistance, wear resistance, biological compatibility, and low electrical conductivity properties. ECDM, with a different machining method, can fabricate complex micro structures on glass, such as micro holes, micro channels, micro slits and complicated three-dimensional features.

ECDM was first put forward by Kurafuji in 1968. Because of machining brittle and hard nonconductive material at the micro level, this machining method was investigated further. Nasim, Mohammad researched the generation of single gas bubbles at the tool electrode surface. Finally, he found that the wettability of the tool electrode and the surface tension between the bubble and electrolyte affected the gas film thickness [6]. Zhang and Huang explored critical voltage under different machining conditions by using ECDM on glass to investigate the time it took to form the gas film, via the mean current of discharge. They concluded that better machining precision and surface quality could be obtained by selecting optimized parameters [7]. Sathisha proposed the empirical model for the process of machining grooves with multiple regression analysis [8]. Jawalkar fabricated micro channels by electrochemical discharge milling. The experiment results showed that voltage plays a leading role in the parameters of both material removal rate and tool wear [9]. Cao found a new method indicating that the grinding process under polycrystalline diamond tools reduced the surface roughness of ECDM structures from a few tens of a micron to 0.05 μm Ra [10]. Elhami utilized special equipment to generate only a single discharge in ultrasonic-assisted electrochemical discharge machining (UAECDM) and studied two important characteristics: material removal and tool wear [11]. Many scholars conducted further studies of UAECDM [12–14]. Han and Min proposed a method of using the side insulation tool and low concentration electrolytes to reduce undesirable over cutting [15]. Furutani concluded that the width, depth and surface roughness of grooves machined by electrochemical discharge milling increased with higher voltage [16]. Kun investigated the precision and stability of quartz fabricated by ECDM and explored optimal machining parameters including the size of the electrode and the machining speed [17].

Wire electrochemical discharge machining (WECDM) was proposed by Tsuchiya [18]. Jain utilized traveling wire as a tool in WECDM, and studied the effects of voltage, the concentration of the electrolyte on material removal rate and tool wear [19]. Panda and Yadava established a 3D finite element transient thermal model and predicted the temperature field and MRR in traveling wire electrochemical spark machining (TW-ECSM) [20]. Kuo found a new wire ECDM approach to machine quartz glass. In their experiments, electrolytes were supplied by titrated flow and the machining quality and efficiency were improved [21]. Wang studied the surface integrity of alumina machined by WECDM [22]. A host of literature proved that electrolyte circulation plays an important role in machining performance. Many approaches to enhancing the electrolyte circulation in ECDM and wire ECDM have been proposed [23–25]. Fang used rotary helical electrodes in wire ECDM, which accelerated the cycle of the electrolyte [26]. Wang and Zhang researched the flow field of ECDM with a rotating helical tool. In their experiments, the gas–liquid phase distribution and the velocity vectors of the electrolyte in the machining gap were investigated [27].

In this paper, a rotating helical electrode was used in different ECDM processes, including electrochemical discharge drilling, electrochemical discharge milling and wire ECDM. The rotary helical electrode produced an axial velocity and axial force, dragging the electrolyte from the bottom of the workpiece. Therefore, the machining accuracy of ECDM is fine. The machining parameters, including voltage, frequency, duty factors, and feed rate, were considered in the experiments and their effect on the side gap was investigated. The optimized parameters were utilized to successfully machine micro holes, micro channels, micro slits and complicated three-dimensional features with ten several-micron side gaps.

### **2. Experimental Set-Up and Model for Machining Process**

### *2.1. Experimental Set-Up*

Most past efforts have been spent on studying the mechanisms of ECDM. The ECDM process can be depicted as in Figure 1. In all of the following experiments, the tool electrode with Φ105 μm was a rotating helical tungsten carbide (WC) electrode while the auxiliary anode is a graphite plate and the electrolyte was 3 mol/L KOH (Shuangshuang chemical industry, Yantai, China). This process can be divided into five steps. In the first step the pulse power was imposed on the tool electrode and the auxiliary anode, which were immersed in the electrolyte. Because of electrolysis, hydrogen and oxygen gas bubbles were generated around the tool electrode and auxiliary anode, respectively. The second step involved the hydrogen gas bubbles accumulating rapidly and embracing the tool electrode. The third step was when the formation rate of the hydrogen gas bubbles was equal to the rate of that escaping from the electrode. The gas film around the tool electrode was formed and completely separated the tool electrode from the surrounding electrolyte. In the fourth step there was a narrow gap between the tool electrode and the electrolyte according to the third step. When the applied voltage rose to a critical value, there was a spark in the gas film. As is known, a large amount of heat generated by discharge will instantaneously melt the surface material of the workpiece when the tool electrode is close to the workpiece. In addition, some material is removed due to evaporation and localized high temperature, leading KOH electrolytes to corrode the workpiece. The fifth step began when a gas film was staved when the tool electrode contacted with the electrolyte again. Then, the process switched back to the first step, beginning the cycle anew.

**Figure 1.** A schematic view of electrochemical discharge machining (ECDM).

The architecture of this experimental system for ECDM is illustrated in Figure 2. The experimental system contains four subsystems: the power supply system, machine tool system, microelectrode system, and processing control and monitoring system. The power supply system was plays a significant part in ECDM, which provides a series of variable ranges including pulse voltage, duty factor, and pulse frequency. The machine tool system is mainly comprised of an optical precision platform, the L shaped marble frame, feed device, high speed motorized spindle, lifting platform, fixture, and other components. The optical precision platform ensured high accuracy for micro ECDM. To guarantee the verticality of the machine tool, the L shaped marble frame possessing vibration isolation performance was used. The feed device, controlled by the MP-C154 motion control card, accurately controlled the feeding of the electric slipway along the three directions and met the requirements for fabricating complex three-dimensional micro structures. The microelectrode system consisted of a rotary helical WC electrode, electrolytic bath, high speed motorized spindle, fixture, and lifting platform. The glass workpiece was fixed on the electrolytic bath and placed on the lifting platform. The processing control and monitoring system had the motion control card and Supereyes. Supereyes monitored the process and captured images. In this research, electrochemical discharge drilling, electrochemical discharge milling, and wire ECDM were utilized to machine micro structures on glass workpieces with rotary helical WC electrodes via an experimental system.

**Figure 2.** Experimental system of ECDM.

### *2.2. Establishing of Machining Process Model*

To investigate the side gap in the ECDM process, three different types of experiments were carried out, including electrochemical discharge drilling, electrochemical discharge milling, and wire ECDM. During a certain specified experiment, only one parameter could be adjusted and the effect on the side gap recorder, all other parameters remained constant.

Step 1 was the model for electrochemical discharge drilling. Establishing the simplified model of electrochemical discharge drilling on the side gap needed the following hypothetical conditions:


**Figure 3.** A schematic view of electrochemical discharge drilling.

The discharge energy *q* in unit time can be obtained by the equation proposed by Jain [28]:

$$q = \mathcal{U}I - RI^2 \tag{1}$$

where *U* is voltage, *I* is the mean current, and *R* is the resistance between the cathode and the anode. According to Assumption (a), the relationship between the discharge energy *q* and *n* is:

$$q = kn\lambda \tag{2}$$

where *n* is the amount of substance of melted glass in unit time and *λ* is the dissolution heat of ultra-clear glass.

*Processes* **2019**, *7*, 195

The volume of melted glass, *V*, is worked out as

$$V = \frac{nM}{\rho},$$
 
$$\text{(3)}$$

where *M* is the molar mass of the glass and *ρ* is the density of the glass. Therefore, the diameter, *D*, of the machined hole could be calculated together with Equation (3) as:

$$D = 2\sqrt{\frac{n\mathcal{M}}{\pi\hbar\rho'}}\tag{4}$$

where *h* is the drilling depth in unit time. The relationship between *h* and the feed rate *v* is:

$$h = \upsilon + \mathfrak{c},\tag{5}$$

where *v* is the feed rate of the rotary helical electrode, *c* is the distance between the end of the rotary helical electrode and the bottom of the hole, according to Assumption (c).

The side gap Δ*S*<sup>1</sup> can be defined as follows, where the diameter of the rotary helical electrode is *d*:

$$
\Delta S\_1 = \frac{D - d}{2}.\tag{6}
$$

The side gap Δ*S*<sup>1</sup> could be solved simultaneously with Equations (1), (2) and (4)–(6).

$$
\Delta S\_1 = \sqrt{\frac{M(III - RI^2)}{\pi \rho k \lambda (v + c)}} - \frac{d}{2} \tag{7}
$$

We concluded that side gap Δ*S*<sup>1</sup> rose with the increasing of the voltage, but decreased with higher feed rates. In addition, the side gap Δ*S*<sup>1</sup> was affected by material properties.

Step 2 was the model for electrochemical discharge milling and WECDM. The side gap was different between electrochemical discharge drilling and milling. The model of the side gap in electrochemical discharge milling ought to be reconstructed. The side gap model in the electrochemical discharge milling process is shown in Figure 4.

**Figure 4.** Schematic view of electrochemical discharge milling.

In unit time, the shape of the machined glass was considered rectangular in electrochemical discharge milling. Therefore, the volume of the machined glass could be obtained in unit time as follows:

$$V = (d + 2\Delta S\_2)vh\_1. \tag{8}$$

In Equation (8), *v* is the feed rate, *h*<sup>1</sup> is mean milling depth, and *d* is the diameter of the rotary helical electrode.

Therefore, the side gap was obtained by Equations (1)–(3) and (8).

$$
\Delta \mathbf{S}\_2 = \frac{M(\mathbf{L}II - RI^2)}{2\rho v h\_1 k \lambda} - \frac{d}{2} \tag{9}
$$

It was not hard to establish that the side gap Δ*S*<sup>2</sup> became larger with any increase of voltage in the electrochemical discharge milling. However, the side gap Δ*S*<sup>2</sup> became narrower with higher feed rate and higher milling depth. The glass properties also influenced the side gap.

The side gap in WECDM could be substituted, approximately, by Equation (9) from Figure 4, with an *h*<sup>1</sup> thickness of the glass.

### **3. Experiments and Discussion**

### *3.1. Experimental Arrangement*

In this paper, electrochemical discharge drilling, electrochemical discharge milling and wire ECDM were employed to investigate the side gap during the processing of ECDM. To ensure the accuracy of the experiments and to avoid accidental influence, each experiment was carried out repeatedly, at least three times. In all of the following experiments, Φ105 μm tungsten was used as the rotary helical tool and a 600 μm thick graphite plate was selected as the auxiliary electrode. Workpieces in the electrochemical discharge drilling and wire ECDM were ultra-glass with a thickness of 300 μm, while the specifications of the glass workpiece were 46 mm × 25 mm × 1 mm in electrochemical discharge milling. In addition, all feed depths were 100 μm in electrochemical discharge milling. The diameter and slit width were measured by a Nikon SMZ1270 microscope (Tokyo, Japan) and NOVA NANOSEM 450 scanning electron microscope (Hillsboro, OR, USA).

In these experiments, the auxiliary anode (Luhan metal, Shanghai, China), rotary helical electrode (Union tool, Tokyo Metropolitan, Janpan) and glass workpiece (Citoglas, Haimen, China) were immersed in electrolytes. When the pulse voltage was applied to the auxiliary anode and the helical electrode was attached to high speed spindle, a rotary helical electrode moved with a certain feed speed to machine the glass. The main discharge areas were the bottom, the side wall, and the side wall of the rotary helical electrode in the electrochemical discharge drilling, the electrochemical discharge milling, and the wire ECDM, respectively. Therefore, the selected experimental parameters were different between the three machining methods. The details of the experimental arrangements are shown in Table 1. In each group of experiments, only one parameter, the pulse voltage, pulse frequency, duty factor, or feed rate could be adjusted to the desirable range to research the effect on the side gap. Other variables were kept constant. The effects of the pulse voltage, frequency, duty factor, and feed rate on the side gap are displayed in the following table.

**Table 1.** The details of experimental arrangements.


### *3.2. Effect of Pulse Voltage on Side Gap*

There have been many experiments conducted to investigate effects of pulse voltage on the side gap. The side gap was calculated and the influence of the pulse voltage on the side gap is shown in Figure 5. From Figure 5 and Equations (7) and (9), we concluded that the side gap increased with the rise of the pulse voltage. At a lower pulse voltage, the bubbles generated by electrolysis were sparse and thin. Therefore, the thickness of the gas film was thin. The thin gas film and low applied voltage led to shorter discharge distances, which greatly shortened the side gap. While at higher pulse voltages, the formation rate of the bubbles increased rapidly. Plenty of bubbles coalesced intensely, resulting in a thicker gas film. Thus, in this case, the discharge distance was longer, meaning more material was removed. It was not difficult to conclude that the side gap increased with the rise of the discharge distance. The diameter of the hole in the electrochemical discharge drilling, the slit width in the electrochemical discharge milling and the wire ECDM increased with the higher pulse voltage.

**Figure 5.** Effect of pulse voltage on side gap.

### *3.3. Effect of Duty Factor on Side Gap*

To research the effect of the duty factor on the side gap, a series of experiments were carried out, including electrochemical discharge drilling, electrochemical discharge milling and wire ECDM. The results are shown in Figure 6. As the picture depicts, the side gap increases as the duty factor rises, from 40% to 90%. The discharge energy *q* in unit time increased due to the higher duty factor, which resulted in more material removal. Therefore, the diameter of the hole in electrochemical discharge drilling, the slit width in electrochemical discharge milling and the wire ECDM increased with the rise of the duty factor. The optimal duty factor should be low, but the lower duty factors reduced material removal rate.

**Figure 6.** Effect of duty factor on side gap.

### *3.4. Effect of Frequency on Side Gap*

The influence of frequency on the side gap is shown in Figure 7. In this set of experiments, the duty factor remained unchanged at 70% and frequency ranged from 200 Hz to 600 Hz. In electrochemical discharge drilling, electrochemical discharge milling, and wire ECDM the side gap decreased when

the frequency increased, gradually. The number of discharge rose with higher frequency per unit time, but the pulse width was correspondingly reduced. Therefore, the discharge energy of a single discharge decreased, resulting in less material removal and smaller side gaps, eventually. The diameter of the hole in electrochemical discharge drilling, the slit width in electrochemical discharge milling and the wire ECDM decreased with the rise of frequency. The optimal frequency should be high but the higher frequency will reduce the material removal rate.

**Figure 7.** Effect of frequency on side gap.

### *3.5. Effect of Feed Rate on Side Gap*

Numerous experiments were conducted to research the effect of feed rate on the side gap. Different feed rates had different influences on the side gap, as displayed in Figure 8. Better machining location with s higher feed rate could be obtained with a lower side gap. The shorter discharge time with the higher feed rate in unit machining distance along the direction of feed, led to less material being removed. Therefore, the side gap was lower in the electrochemical discharge drilling, electrochemical discharge milling, and wire ECDM. However, the optimal feed rate was not higher. The rotary helical electrode collided with the workpiece when the feed rate rose to critical values.

**Figure 8.** Effect of feed rate on side gap.

### **4. Experimental Results**

According to the above experiments and analysis, the effects of the parameters, including voltage, frequency, duty factor and feed rate, on the side gap were worked out in ECDM with rotary helical electrodes. The parameters, after optimization, were selected based on many experiments exploring fabricated micro holes, micro grooves, micro channels and complicated three-dimensional features with lower side gaps. There were some micro structures displayed.

### *4.1. Electrochemical Discharge Drilling of Array Micro Holes*

According to the above discussion about the effect of the parameters on the side gap, the smaller side gaps needed a low voltage, low duty factor, high frequency, and high feed rate. However, considering material removal rate and machining stability, the experiments were carried out to select a set of optimized parameters for electrochemical discharge drilling. The optimized parameters were: pulse voltage—37 V, frequency—3000 Hz, duty factor—70%, feed rate—1 μm/s, spindle speed—3000 rpm, and electrolyte—3 mol/L KOH. High quality array micro holes were successfully fabricated with a lower diameter, as shown in Figure 9. Thickness of the glass was 300 μm. A minimum side gap of 27.2 μm could be obtained with electrochemical discharge drilling.

**Figure 9.** Array micro holes and partial magnification

### *4.2. Electrochemical Discharge Milling of Micro Structures*

Electrochemical discharge milling was capable of fabricating micro grooves, micro channels and micro three-dimensional structures. Some complex micro structures could be machined with a lower side gap by a set of optimized parameters. The optimized parameters were: pulse voltage—34 V, frequency—500 Hz, duty factor—50%, feed rate—2 μm/s, spindle speed—3000 rpm, and electrolyte—3 mol/L KOH. As shown in Figure 10, the micro groove array was milled on the glass. The mean width was 129.4 μm, the length was 750 μm and depth was about 130 μm. The smallest side gap reached 11.5 μm.

**Figure 10.** Micro groove array on glass.

The micro channel machined on the glass by electrochemical discharge milling is displayed in Figure 11. The groove width was about 135.9 μm and the depth was about 150 μm. The abbreviation of the university name milled on the glass is shown in Figure 12. The three-dimensional step structure with vertical sidewalls and high shape accuracy is shown Figure 13. The three-dimensional convex structure of micro electrochemical discharge milling is shown in Figure 14, which is made of two layers of convex structures. The width of the upper convex plate was about 75 μm, the length was 260 μm and the height was about 70 μm.

**Figure 11.** Complex micro channel on glass.

**Figure 12.** The abbreviation of the university name on glass.

**Figure 13.** Three-dimensional step structure.

**Figure 14.** Three-dimensional convex structure.

### *4.3. Wire Electrochemical Discharge of Micro Structures*

Wire electrochemical discharge with rotary helical electrodes fabricated high aspect ratio structures. According to the above discussion and experiments, a set of optimized parameters was selected for machining the micro structures. Long narrow slits were fabricated on 300 μm thick glass, as shown in Figure 15. The smallest side gap reached 14.95 μm. The optimized parameters were: pulse voltage—34 V, frequency—600 Hz, duty factor—50%, and feed rate—1 μm/s. The closed micro structures were machined as displayed in Figure 16. To improve the refreshment of the electrolyte in the closed micro structures, larger processing parameters were used (40 V, 500 Hz, 50%, 1 μm/s).

**Figure 15.** Long narrow slits on 300 μm thick glass workpiece.

**Figure 16.** Closed micro structure on 300 μm thick glass.

The high aspect ratio structure was manufactured on 1060 μm thick glass with wire ECDM, using a rotary helical electrode. The slit width was about 175.4 μm and the side gap was about 35.2 μm, as shown in Figure 17 (40 V, 300 Hz, 60%, 1 μm/s). In addition, the micro cantilever beam was successfully fabricated on a 35 μm thick glass workpiece, as shown in Figure 18. The length of the micro cantilever beam was 1500 μm and the aspect ratio reached 42:1.

**Figure 17.** High aspect ratio structure on 1060 μm thick glass.

**Figure 18.** Micro cantilever beam on 35 μm thick glass workpiece.

### **5. Conclusions**

This research employed ECDM with a rotary helical electrode to fabricate ultra-clear glass. Using a rotary helical tool in electrochemical discharge drilling, electrochemical discharge milling, and wire ECDM, the effects of pulse voltage, frequency, duty factor, and feed rate on the side gap were investigated. The conclusions can be summarized as follows:


**Author Contributions:** Conceptualization, Y.L. and C.Z.; investigation, S.L. and Z.W.; methodology, C.G.; project administration, Y.L.; resources, Z.W.; writing—original draft, S.L.; and writing—review and editing, Y.L. and C.G.

**Acknowledgments:** The authors acknowledge financial support from the Shandong Provincial Natural Science Foundation (Nos. ZR2018MEE018, ZR2017BEE012), the China Postdoctoral Science Foundation (No. 2018M630772), the Young Scholars Program of Shandong University, Weihai (2015WHWLJH03), and the Shenzhen Science and Technology Project (JCYJ20170818103826176).

**Conflicts of Interest:** The authors declare no conflicts of interest.

### **References**


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