*Article* **Sludge Derived Carbon Modified Anode in Microbial Fuel Cell for Performance Improvement and Microbial Community Dynamics**

**Kaili Zhu, Yihu Xu, Xiao Yang, Wencai Fu, Wenhao Dang, Jinxia Yuan and Zhiwei Wang \***

Key Laboratory of Clean Pulp & Papermaking and Pollution Control of Guangxi, College of Light Industry and Food Engineering, Guangxi University, Nanning 530004, China; zhukaili26@163.com (K.Z.); xuyihuzwj@163.com (Y.X.); yxiao0112@163.com (X.Y.); fl392597@163.com (W.F.); wenhaodang14@126.com (W.D.); yuan09@syr.edu (J.Y.)

**\*** Correspondence: author: wangzhiwei@gxu.edu.cn

**Abstract:** The conversion of activated sludge into high value-added materials, such as sludge carbon (SC), has attracted increasing attention because of its potential for various applications. In this study, the effect of SC carbonized at temperatures of 600, 800, 1000, and 1200 ◦C on the anode performance of microbial fuel cells and its mechanism are discussed. A pyrolysis temperature of 1000 ◦C for the loaded electrode (SC1000/CC) generated a maximum areal power density of 2.165 <sup>±</sup> 0.021 W·m−<sup>2</sup> and a current density of 5.985 <sup>±</sup> 0.015 A·m−2, which is 3.017- and 2.992-fold that of the CC anode. The addition of SC improves microbial activity, optimizes microbial community structure, promotes the expression of c-type cytochromes, and is conducive to the formation of electroactive biofilms. This study not only describes a technique for the preparation of high-performance and low-cost anodes, but also sheds some light on the rational utilization of waste resources such as aerobic activated sludge.

**Keywords:** sludge carbon; extracellular polymeric substance; microbial fuel cell; electroactive biofilm; microbial community dynamics

#### **1. Introduction**

Microbial fuel cells (MFCs) can convert chemical energy in organic matter into electric energy by using the oxidation and metabolism mechanism of anaerobic bacteria on anodic electroactive biofilms (EABFs) [1,2]. Compared with other electrochemical cells, such as liquid flow cells and ordinary fuel cells, MFCs do not need external energy input and have high energy conversion efficiency [3,4]. MFCs can use all biodegradable organics and wastewater as fuels [2]. In wastewater treatment, MFCs can generate electrical energy while efficiently treating wastewater, which not only significantly reduces the operation cost of sewage treatment plants, but also makes efficient use of waste resources [3]. Traditional wastewater biological treatment approaches mainly involve anaerobic digestion and aerobic treatment technologies [5]. Anaerobic digestion technology is mainly applicable to high concentration wastewater [6,7]. Although it can produce fuel gases such as methane or hydrogen from organic wastewater, it also produces gases with no practical value such as carbon dioxide, hydrogen sulfide and nitrogen [5,8]. The energy utilization mode is complex and has significant requirements for production conditions. Aerobic treatment technology is mainly applicable to medium and low concentration wastewater. With large sludge production, aeration is required to maintain oxygen concentration, and the cost of sludge disposal and aeration is high [8]. MFCs have a wide application range, low sludge output and directly utilize electric energy. Thus, MFCs have incomparable technical advantages compared with traditional wastewater biological treatment technologies [3,7]. Therefore, the application of MFCs in wastewater treatment has broad prospects. However,

**Citation:** Zhu, K.; Xu, Y.; Yang, X.; Fu, W.; Dang, W.; Yuan, J.; Wang, Z. Sludge Derived Carbon Modified Anode in Microbial Fuel Cell for Performance Improvement and Microbial Community Dynamics. *Membranes* **2022**, *12*, 120. https:// doi.org/10.3390/membranes12020120

Academic Editor: Sophie Tingry

Received: 15 December 2021 Accepted: 17 January 2022 Published: 20 January 2022

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**Copyright:** © 2022 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 (https:// creativecommons.org/licenses/by/ 4.0/).

recently, it was reported that the maximum current and power densities of MFCs are 3.85 mA·cm<sup>2</sup> and 0.66 mW·cm2, respectively, and the power production is still relatively low, i.e., 2–4 orders of magnitude below that of chemical fuel cell [9]. In addition, the cost of MFCs is also relatively high, notably due to the commonly used proton exchange membrane and cathode platinum catalyst, which greatly increase the cost [10]. Thus, practical applications for microbial fuel cells are limited.

The anode, as the core of MFCs, plays a key role in the extracellular electron transfer (EET) between electroactive microorganisms (EAMs) and electrodes [8]. The properties of the anode materials significantly affect the growth of EABFs. An ideal anode material should have high biocompatibility and conductivity, cost efficiency, and easy commercial production [11]. Traditional carbon-based materials, such as carbon cloth (CC), carbon felt, carbon paper and graphite rod, have been widely used in MFCs. However, due to the low electrochemical activity of carbon-based materials, their application in MFC is limited [12]. Therefore, it is necessary to modify carbon-based materials to improve the power generation performance of MFCs. Given the poor durability of carbon paper [4], the high resistance of carbon felt [11] and the smooth surface of graphite rods [13], CC is often used to modify carbon-based materials because of its good durability, low resistance and easy modification [14]. Carbon-based nanomaterials, such as graphene, carbon nanotubes, conductive polymers and carbon nanoparticles have been widely used to modify carbonbased electrodes because of their good electrocatalytic activity and conductivity [11,12]. Liu et al., modified CC with tungsten carbide nanoparticles, which significantly improved the electrocatalytic performance of the resulting electrode. The power density reached 3.26 W·m−2, which was 2.14 times that of the bare CC anode (1.52 W·m−2) [15]. Li et al., modified a CC with polydopamine (PDA) and graphene oxide (rGO), which improved the hydrophilicity and significantly reduced charge transfer resistance. Its power density reached 2.047 W·m−2, which was 6.1 times that of the bare CC electrode [10]. Liu et al., modified CC with graphene to enhance its biocompatibility. The power density and energy conversion efficiency were 2.7 times and 3 times higher than those of the bare CC electrode, respectively [16]. The above studies show that the modification of CC anodes can significantly improve the performance of MFCs. However, the preparation process of these nano materials is complex and yields are low [11]. It is estimated that the preparation cost of these nanomaterials is high, accounting for more than half of the total cost of MFCs [17]. Therefore, reducing the cost of MFC anode materials while maintaining excellent performance is very important for further practical applications of MFCs.

Recently, carbon materials derived from natural biomass materials have attracted extensive interest because of their low cost and sustainable resource utilization. Porous carbon derived from biomass wastes such as almond shells, pomelo peels, towel gourds, kenaf stems, silkworm cocoons and chestnut shells have been used to produce the anodes of MFCs, showing excellent power generation performance; such approaches also open up a new path for the utilization of natural waste [18–20]. Sewage sludge, a byproduct of wastewater treatment, is abundant but remains expensive to dispose of. According to statistical reports, China produces approximately 11.2 million tons of dry sludge every year, while the countries of the European Union produce 10.0 million tons [21]. The cost of sludge treatment and disposal is approximately 60% of the total operating cost of sewage treatment plants [22]. In addition, the sludge contains pathogenic bacteria, organic pollutants, heavy metals and other harmful substances, and its improper treatment can be harmful to human health and the environment. Compared to incineration, landfill, anaerobic digestion, and composting, pyrolysis treatment can effectively reduce the toxicity of dry sludge, yielding sludge carbon (SC) and high value-added fuel [23]. It is considered a safe, stable, and lowcost sludge treatment method. Previously, researchers have applied SCs prepared by onestep pyrolysis to lithium-ion batteries, supercapacitors, and environmental catalysts [23]. Sludge-derived carbon has been shown to be feasible as a low-cost conductive material. However, the effects of the physical and chemical properties of SC (such as specific surface area, porosity, functional groups and metal phase structure) on MFC anodes, especially on

extracellular polymers (EPS) in anode biofilms, and the mechanisms of electron transfer, are still unclear. As an electrode material, SC still poses scientific and technical challenges.

The purpose of this study was to investigate the effects of SC characteristics on the EABFs of MFC anodes. The parameters assessed included power generation, wastewater treatment ability, electrochemical performance, EPS secretion, microbial activity, diversity, and metabolic pathways. It aimed to provide a theoretical basis for the practical application of sludge carbon in MFCs, and to lay out a safe and effective method for the utilization of sludge resources.

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

#### *2.1. Preparation of Electrodes*

The anode and cathode were made of carbon cloth, purchased from WENTE Co., Ltd., Nanjing, China. The CC for the anode substrate (1.0 × 2.0 cm2) was relatively hydrophilic, with a thickness of 0.36 mm and a resistivity of less than 5 mΩ·cm2. In order to increase the hydrophilicity of the anode carbon cloth, the treatment method was as follows: 10% nitric acid solution and 10% sulfuric acid solution were mixed in a 3:1 ratio; then, the carbon cloth was soaked in this solution for 12 h [15]. The CC for the cathode substrate was hydrophobic, with a thickness of 0.41 mm and a resistivity of less than 13 mΩ·cm2.The preparation process of the anode was as follows: Aerobic activated sludge was collected from a sewage treatment plant in Nanning, China. The sludge was dried at 60 ◦C for 24 h and broken by grinding. It was then pyrolyzed at 600, 800, 1000, and 1200 ◦C for 2 h to obtain SC powder, filtered, and dried to obtain SC600, SC800, SC1000 and SC1200 samples. The obtained sludge carbon samples (1.5 mg cm−2) were sprayed onto a carbon cloth to obtain SC600/CC, SC800/CC, SC1000/CC and SC1200/CC electrodes. Bare carbon cloth (CC) (1.0 × 2.0 cm2) was used as the control electrode. The cathode was prepared to use activated carbon as the catalyst, 5 wt% Nafion solution as the binder, and carbon cloth as the collector [24]. The effective working area of the cathode CC was 4.91 cm2.

#### *2.2. MFC Construction*

For air cathode microbial fuel cells, the elimination of the proton exchange membrane can significantly reduce the power output and cost of the cell. The reactors used in this study were 100 mL single-chamber membrane-free air cathode bottle MFCs. The MFC was inoculated on a super clean workbench. Firstly, 10 mL of pre-acclimated anaerobic granular sludge was sucked into the anode chamber with a disinfection syringe. Then, 80 mL of prepared artificial wastewater was sucked. The artificial wastewater was mainly composed of sodium acetate (1.5 g·L−1), phosphoric acid buffer solution (50 mM), trace elements (12.5 mL·L<sup>−</sup>1), and vitamin solution (5 mL·L<sup>−</sup>1) [24]. The formula of the nutrient solution is shown in the Tables S1–S3. Then, an external resistance of 1000 Ω was employed to connect the electrodes [24]. Intermittent water inlet mode was applied to the MFCs. The anode medium was replaced when the cell voltage passed below 50 mV. All experiments were conducted in triplicate, and average values were calculated. The external voltage generated by the MFCs was measured using a data acquisition instrument (Keithley6510, Cleveland, OH, USA). The polarization and power density curves were measured by gradually changing the external resistance (2000 Ω to 80 Ω) [7]. According to the formulas I = U/R and P = UI, the current and power under the corresponding resistances were calculated, respectively [25]. Cyclic voltammetry (CV) tests were performed in a threeelectrode system using an electrochemical workstation (Chenhua, Shanghai). The prepared anode, a saturated calomel electrode (SCE), and a platinum wire were used as the working, reference and counter electrodes. The measurement range was −0.6~0.6 V, the scanning speeds were (1, 5, 10, 15 and 20 mV/s), and the static time was2s[9].

#### *2.3. Materials Characterizations*

The surface morphology and properties of the samples were characterized by scanning electron microscopy (SEM-EDX, SU8020, Hitachi hi tech, Tokyo, Japan), high-resolution electron microscopy (HR-TEM, FEI TECNAI G2 F30, FEI NanoPorts, Hillsboro, OR, USA), X-ray powder diffraction (XRD, RIGAKU D/MAX 2500V, Japan Science Corporation, Tokyo, Japan), X-ray photoelectron spectroscopy (XPS, ESCALAB 250XI+, Thermo Fisher Scientific, Waltham, MA, USA) and Raman spectroscopy (Raman, InVia Reflex, Renishaw, London, UK). The pore size distribution and specific surface area of the samples were measured by Barrett-Joyner-Halenda (BJH) method and Brunauer-Emmett-Teller (BET) theory (NOVA4200E, Cantar Instruments, Boynton Beach, FL, USA). The surface functional groups of the samples were characterized in the range of 4000–300 cm−<sup>1</sup> by Fourier transform infrared spectroscopy (FTIR, TENSOR II, Bruker, Karlsruhe, Germany). A Diamond TG/DTA instrument (DTG-60(H), Hitachi hi tech, Tokyo, Japan) was used for a thermogravimetric (TG) analysis of the samples. Confocal scanning electron microscopy (CLSM, LSM800, zeroK Nanotech, Gaithersburg, MA, USA) was used to characterize the cell activity of the EABFs. Chemical oxygen demand (COD) was detected using spectrophotometry and a multiparameter water quality analyzer (Lianhua Technology Co., Ltd, Shanghai, China). The total suspended solids (TSS) in the anaerobic granular sludge was calculated by drying and weighing at 105 ◦C, while the volatile suspended solids (VSS) were calculated by calcination at 600 ◦C. The zeta potential of the samples was characterized using a zeta potential particle sizer (Nano-ZS90X, Marvin Instrument Equipment Co., Ltd, London, UK).

#### *2.4. EPS Extraction and Analysis*

Extracellular polymers (EPS) on anode biofilms were extracted by the water bath heating method [15]. After power generation, the CC was cut with sterile scissors and placed into centrifuge tubes containing 0.9% sodium chloride solution, rotated for 10 min, and centrifuged at 4000× *g* rpm for 15 min. The supernatant was discarded, replenished with sodium chloride solution, the above operations were repeated, and vibrated in an 80 ◦C water bath shaker for 30 min. Finally, the supernatant was centrifuged at 9000× *g* rpm for 10 min and filtered through a 0.22 μm membrane to obtain tightly bound EPS. The contents of polysaccharides, humic substances, protein, and outer membrane c-type cytochromes (OM c-Cyts) in EPS were detected using UV/visible spectrophotometer (Agilent 8453, Agilent Technology Co., Ltd., Santa Clara, CA, USA). EPS was characterized using a three-dimensional excitation-emission matrix (3D-EEM) fluorescence spectrometer (Hitachi F-7000, Hitachi hi tech, Tokyo, Japan). The main test parameters were as follows: the scanning ranges of the excitation spectrum (Ex) and emission spectrum (EM) were 220–500 nm and 220–550 nm, respectively, while the scanning step was 5 nm.

#### *2.5. Microbial Community Analysis*

The anodes were collected after power generation, and the genomic DNA was extracted from the biofilm samples using the E.Z.N.A.® Mag-Bind Soil m DNA Kit (OMEGA) according to the manufacturer's instructions [25]. The V3-V4 region of the 16S rRNA gene was amplified using polymerase chain reaction (PCR) with primers 338F (5 -ACTCCTACGG GAGGCAGCA-3 ) and 806 R (5 -GGACTACHVGGGTWTCTAAT-3 ) [26]. The same loading buffer volume was mixed with the PCR product, and electrophoresis was performed on a 2% agarose gel. The products were purified using a pre-QUS kit™. The PCR products were detected and quantified using a fluorometer. The NEXTFLEX Rapid DNA-Seq kit was used to build the libraries. After purification and quantification, the samples were tested for 16S rRNA gene sequencing based on the Illumina MiSeq platform. Based on the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, the macrogenomic information of microbial metabolic function was predicted using the PICRUSt pipelines program [9].

#### **3. Results**

#### *3.1. Characterization of Material*

Figure 1 shows that the morphological changes in SC were significantly related to the carbonization temperature. The SC600 and SC800 samples showed disordered sheet structures. For the SC1000 sample, the surfaces were relatively rough, and carbon microspheres and small pores appeared due to the release of volatile substances and the sintering effect, which converted a large number of inorganic parts into mineral-like compounds and induced the carbon phase to warp some metals [27]. For SC1200 samples, high temperature enhanced the shrinkage of carbon, hindered the development of pores, led to the collapse or deformation of coke, and reduced the pore volume. Furthermore, it can be observed from the TEM images (Figure S1) that the morphology of SC changed significantly with the increase of pyrolysis temperature, with carbon particles appearing in SC1000 and SC1200 samples, which is consistent with the SEM results.

**Figure 1.** SEM images of SC samples under different carbonization temperature: (**a**) SC600, (**b**) SC800, (**c**) SC1000, and (**d**) SC1200.

As shown in Table 1, the pH, zeta potential, and ash content of sludge carbon was linearly related to the carbonization temperature. Increasing the pyrolysis temperature can increase the zeta potential of the sludge carbon surface. When the carbonization temperature was low, the measured zeta potential was negative because the original sludge was rich in negative groups such as carboxyl and hydroxyl groups, and the sludge carbon surface was negatively charged [21]. When the carbonization temperature was 1000 ◦C or above, the zeta potential was positive and the mud carbon surface was positively charged. This was because the oxygen-containing functional groups with negative charge were basically decomposed, and a large number of amino and metal groups were exposed on the carbon surface of the sludge [28]. The electrical properties of the material surface changed from negative to positive, which is useful for the electrical absorption of microorganizations with a negative charge. Another important feature of an ideal anode is high conductivity. The results showed that the SC1000 sample had the highest conductivity (57.43 s·m<sup>−</sup>1). In summary, the SC1000 sample may be the most suitable anode material. Its high conductivity and low absolute zeta potential value enable the adhesion of EAMs and the formation of EABFs. As shown in Figure S2, TGA showed a weight-loss temperature range of 200–1300 ◦C, indicating that sludge carbonization began at 200 ◦C and mainly ended at 1300 ◦C, and the mass was reduced by 51%. As shown in Figure 2, The FTIR spectra of all samples showed the following stretching vibrations: -OH of ethanol with a peak at 3450 cm−1, C-H at 2849 cm−1, N-H at 1650 cm−1, unsaturated aldehyde with

C=O at 1406 cm<sup>−</sup>1, C-O of amine at 1100 cm−1, and C-H at 789 cm−<sup>1</sup> and 2928 cm−<sup>1</sup> [21]. The raw sludge was rich in carboxyl and hydroxyl groups [21]. As the carbonization temperature increased, the intensity of these peaks gradually decreased. This was due to the decomposition of oxygen-containing functional groups. Because of their negatively charged surface, oxygen-containing functional groups are not favorable for bacterial adhesion, thus affecting the electron transfer path [25].


**Table 1.** Physical parameters of SC samples.

**Figure 2.** FTIR spectra obtained from raw sludge and SC samples.

As shown in Figure 3 and Table S4, the specific surface area and pore structure of SC were significantly affected by the carbonization temperature, which greatly influenced the catalytic activity of EABFs. The N2 adsorption/desorption curves of the SC samples belonged to type IV isothermal curves, and there were notable H3 hysteresis curves (Figure 3a). This adsorption hysteresis phenomenon is related to the mesoporous and microporous characteristics of the slit [29]. Table S4 summarizes the BET and pore-size distributions of the SC samples. The results showed that the specific surface areas of the SC samples gradually increased as the carbonization temperature increased. The specific surface area of the SC1000 sample was the largest (216.00 ± 0.005 m2·g<sup>−</sup>1). However, when

the carbonization temperature was further increased to 1200 ◦C, the specific surface area of the sample decreased (176.55 ± 0.010 m2·g−1). The pore size distribution of the SC was further studied (Figure 3b and Table S1). There were two types of pores in all SC samples: micropores and mesopores, with an average size of approximately 8 nm. The total pore volume and mesopore number of the SC800 sample were the largest, while the total pore volume and mesopore volume of the SC1000 sample reduced, but the micropore volume increased, which may have been due to the collapse of some mesopores under high-temperature conditions, resulting in pore blockage [30]. It could be predicted that the special mesoporous and microporous structures produced on the surface of SC may provide active sites for the catalytic reaction and enhance the electrocatalytic ability, thus enhancing proton transfer and charge transfer [15,25,31]. The increase in the specific surface area also has a considerable effect on the decrease in the internal resistance [31]. In short, the SC1000 sample had a good specific surface area and rich pore structure for proton and electron transfer, substrate transport, and biofilm formation in MFCs [11,25,29].

**Figure 3.** (**a**) N2 adsorption-desorption isotherms, (**b**) pore size distribution, (**c**) powder XRD patterns and (**d**) Raman spectra of the SC samples.

As shown in Figure 3c, an XRD analysis of the structure and crystallinity of the SC samples at varying carbonization temperatures indicated pyrolysis differences in the samples at different temperatures. The characteristic diffraction peaks at 26.3◦ and 41.0◦ correspond to the (002) and (101) lattice planes of graphite carbon, respectively, confirming the existence

of graphite carbon. The characteristic diffraction peaks at 40.9◦ and 54.1◦ corresponded to the (110) and (116) lattice planes of α-Fe2O3 (JCPDS. Card 33-0664). The characteristic diffraction peak at 35.4◦ corresponded to the (111) lattice plane of Fe3O4 (JCPDS. Card 72-2303). The characteristic diffraction peaks of Fe3C (PDF No. 01-089-3689) are 43.7◦ and 54.7◦ [29,32]. As the carbonization temperature increased, the intensities of α-Fe2O3 and Fe3O4 decreased, while the intensities of graphite carbon and Fe3C increased. The existence of graphite carbon and Fe3C supported the conductivity of the SC [29]. The graphitization degree of SC was studied by testing the carbonized samples at different temperatures using Raman spectroscopy (Figure 3d). The SC samples showed two characteristic strong peaks at 1350 cm−<sup>1</sup> and 1599 cm−1, corresponding to the defect D and G peaks of graphite, respectively [25]. The strength ratios (ID/IG) of the D and G bands decreased as the carbonization temperature increased, indicating that the graphitization degree of SC improved. In addition, compared to the SC600 and SC800 samples, the SC1000 and SC1200 samples had wider peaks at 1350 cm<sup>−</sup>1, indicating that the nanoparticles in the sludge carbon samples were smaller.

EDX was performed to detect the types and distribution of elements in SC1000 (Figure S3). According to the EDX results, SC1000 samples were rich in elements. In addition to the main elements, i.e., C, N, O, and Si, these samples also had low contents of metals such as K, Ca, Mg, Al, Fe, Cu, Zn, Ti, as well as trace amounts of P and S; these heteroatoms were evenly distributed. Among them, C, N, P, and S are conducive to enhancing the graphite properties and hydrophilicity of SC [25]. Al, Ca, and Mg are useful for the formation of biochar metal skeletons [23]. It is speculated that transition metal elements such as Fe, Ti, Cu and Zn can provide rich active sites for redox reactions and improve the electrocatalytic activity of EAMs [28,29]. The specific content of each element in the SC samples was determined using XPS (Table S5). Interestingly, with the increase in carbonization temperature, the contents of the other elements continued to decrease, except for C and S. This was mainly due to the loss of volatile substances during pyrolysis. In addition, with the increase in carbonization temperature, the contents of oxygen- and nitrogen-containing functional groups decreased, which was consistent with the FTIR results. It is well known that the reduction of oxygen content and pyridine groups enhance electron transfer [25,33]. The types and contents of the elements in the sludge carbon were further confirmed by XPS. Figure S4 shows the XPS full spectrum results of the SC samples carbonized at 600, 800, 1000, and 1200 ◦C. According to the results, the elements in the SC samples mainly comprised C, N, O, P, Si, Ca, Mg, Al, Fe, and P; this was consistent with the EDX test results.

Furthermore, changes in the chemical forms of the main elements, i.e., C, N, and Fe, in the SC samples prepared at different carbonization temperatures were analyzed by XPS. As shown in Figure 4, the XPS high-resolution spectrum of C 1s mainly included C-C and C=C graphite carbon with a binding energy of 284.80 eV, C-N with a binding energy of 285.78 eV, C-O with a binding energy of 286.88 eV and C=O with a binding energy of 289.12 eV [23]. It was further indicated that with the increase in carbonization temperature, the oxygen-containing functional groups gradually decreased, and most amorphous carbon was transformed into SP<sup>2</sup> graphite carbon and C-N. Graphite carbon improves the conductivity of electrodes, and the C-N bond could increase its hydrophilicity, reduce charge transfer resistance, and facilitate bacterial adhesion [10,12,31,32]. The XPS high-resolution spectra of N 1s mainly indicated pyridinic-N with a binding energy of 398.80 eV, graphitic-N with a binding energy of 401.50 eV, Fe-N with a binding energy of 399.60 eV, oxidized-N with a binding energy of 402.90 eV and pyrrolic-N with a binding energy of 400.69 eV [29,32]. When the carbonization temperature was lower than 1000 ◦C, the proportion of pyridinic-N and oxidized-N decreased, while the contents of graphitic-N, pyrrolic-N, and Fe-N increased as the temperature progressed. It is well known that graphitic-N can improve the conductivity of materials and accelerate electron transfer, and pyrrolic-N can improve the electrochemical reaction rate. Fe-N improves the electrocatalytic activity of EABFs and accelerates electron transfer [34]. The XPS high-resolution spectrum

of Fe 2p showed that the oxidation state of Fe on the surface of the SC sample was complex, and that the difference was significant with the change of carbonization temperature. Due to the presence of various iron species, sludge carbon exhibits ferromagnetism, which is conducive to microbial adsorption [23]. Particularly, for the SC1000 sample, the presence of a Fe-C binding site with a binding energy of 720.7 eV confirmed the existence of Fe3C. The surface of the CC anode was smooth, which was unfavorable for microbial adsorption (Figure S5). The surfaces of CC loaded with sludge carbon were relatively rough, which increased the microbial contact area. For SC1000/CC, the adhesion between the sludge carbon and CC was closer, which was better for microbial adsorption (Figure S6).

**Figure 4.** C1s spectra for (**a**) SC600, (**b**) SC800, (**c**) SC1000 and (**d**) SC1200; N1s spectra for (**e**) SC600, (**f**) SC800, (**g**) SC1000 and (**h**) SC1200; Fe2p spectra for (**i**) SC600, (**j**) SC800, (**k**) SC1000 and (**l**) SC1200.

#### *3.2. Electrode Electrocatalytic Activity*

CV analyzed the redox medium composition and redox potential of biofilms. All CV curves showed a typical S-type anode catalyst curve with sodium acetate as the substrate (Figure 5). This indicated the formation of electroactive biofilms and showed that all MFC systems may adopt similar electron transfer paths. However, except for the CC anode, the CV curves of other SC anodes showed a pair of redox main peaks, i.e., mainly redox pairs centered on −0.38 V (cathode) and−0.06 V (anode). The midpoint potential was−0.2 V, which is within the electron transfer activity range of outer membrane c-type cytochromes (OM c-Cyts), indicating that the electron transfer path between the biofilm and the electrode is mainly short-range direct electron transfer (DET) mediated by OM c-Cyts [35–37]. The capacitance area of the SC anodes was much larger than that of the CC anode. The above results show that the EABFs on SC electrodes have high electrocatalytic activity. In addition, the peak current density and capacitance area of the anodes revealed a significant linear relationship with the sludge carbonization temperature, and increased with increasing scanning speed.

**Figure 5.** CV plots of (**a**) CC, (**b**) SC600/CC, (**c**) SC800/CC, (**d**) SC1000/CC, (**e**) SC1200/CC with 1, 5, 10 and 15 mV/s scanning rates and (**f**) CV plots of the electrodes with 10 mV/s scanning rates at three-electrode electrochemical systems after 54 d of batch mode operations.

#### *3.3. MFC Performance*

As shown in Figure 6a, in the first six cycles (approximately 25 days), the COD removal rates of all MFCs showed an increasing trend, and after the sixth cycle of power generation, the COD degradation rate stabilized. There was a correlation between COD removal efficiency and carbonization temperature. MFCs equipped with SC1000/CC had the largest COD removal efficiency (97.63 ± 0.039%), i.e., higher than that of CC (90.97 ± 0.035%). In addition, during stable power generation, the VSS/TSS of the granular sludge in MFCs equipped with SC electrodes was significantly higher than that of MFCs equipped with CC (Figure 6b), and the value of VSS/TSS increased with the increase of sludge carbonization temperature. This indicated that sludge carbon can increase the organic components in granular sludge, thus improving the microbial biomass and microbial activity. Interestingly, the trend regarding the COD removal efficiency of the MFC reactor was consistent with that of VSS/TSS. This may have been because the addition of SC promoted the enrichment of microorganisms, stimulated microorganisms to secrete EPS, and accelerated the carbohydrate metabolism of exoelectrogens and methanogens, thereby accelerating the degradation rate of organic matter [5]. Additionally, carbonization temperature also strongly affected the internal resistance, power density and current density of SC anodes (Figure 6c,d). According to the polarization curve, there was a significant difference in anode open circuit voltage. The open circuit voltage of the SC anode was 0.640 V (SC600/CC), 0.686 V (SC800/CC), 0.725 V (SC1000/CC) and 0.698 V (SC1200/CC), i.e., higher than that of the CC anode (0.626 V). Under the same voltage conditions, the current density of SC1000/CC was the largest, indicating that SC1000/CC has the highest electrochemical oxidation activity. The SC1000/CC anode generated a maximum areal power density of 2.165 ± 0.021 mW·m−<sup>2</sup> and current density of 5.985 ± 0.015 A·m−2, which was 3.017- and 2.992-fold that of the CC anode. The SC1000/CC anode had lower internal resistance and produced higher current density and power density, which meant the electroactive biofilm had higher electrocatalytic activity. As shown in Figure 7, the sludge carbonization temperature significantly affected the voltage output of the MFC system. Compared to the MFCs equipped with CC electrodes, the output voltage of the MFCs equipped with SC electrodes significantly improved. Among them, the MFCs equipped with SC1000/CC anodes showed the best power generation performance, with a maximum closed-circuit voltage of 0.501 V and an average power generation cycle of 146 h. The maximum closed-circuit voltage of the MFC equipped with CC anodes was only 0.323 V, and the average power generation cycle was 100 h.

**Figure 6.** (**a**) COD removal efficiency of MFCs, (**b**) VSS/TSS of inoculated anaerobic granular sludge at MFC systems, (**c**) polarization curves, and (**d**) power density of MFCs.

**Figure 7.** Reproducible cycles of output voltages produced in MFCs equipped with different anodes.

#### *3.4. Anode Biofilm Characterizations*

As shown in Figure S7, in the process of power generation, the modification of SC significantly promoted the secretion of EPS, further confirming that SC can improve microbial biomass and microbial activity. It is well known that in the biofilm-producing current, c-Cyts play a key role in interspecific electron transfer and extracellular electron transfer [36]. It is believed that the higher the concentration of c-Cyts, the higher the electron transfer efficiency between EAMs and electrodes [35]. Some studies have shown that the c-Cyts in EPS can closely bind to the active sites of transition metals Fe (III), Cu (II), and Zn (II) to promote electron transfer [23,38]. It can be seen in Figure 8a–f that in all MFCs, the absorbance of c-Cyts at a wavelength of 419 nm in the anode biofilm gradually increased with power generation, and that the addition of sludge carbon at different carbonization temperatures changed the absorbance of c-Cyts, indicating that sludge carbon affected the secretion of c-Cyts and further influenced the power generation. SC1000/CC had the highest absorbance at 419 nm, indicating that its c-Cyts concentration was the highest, so it can continuously absorb electrons from the embedded bacteria and transfer them to the electrode. As shown in Figure 8g–l, EPS extracted from the anode biofilm was analyzed using EEM fluorescence spectroscopy. Among the observed peaks, peak A (Ex/Em = 280–295/320–335 nm) was tryptophan-like acid and soluble microbial by-products, Peak B (Ex/Em = 380/435–450 nm) was humic-like acid, and peak C (Ex/Em = 420–425/450–470 nm) was coenzyme F420, which plays an important role in the hydrogen nutrition pathway and is related to the methanogenic metabolic activity [5,39]. Peak D (Ex/Em = 230/325–330 nm) was protein-like. Compared to the inoculated sludge, the intensity of peaks A and B in the MFC anode biofilm gradually increased, and peak C appeared. Compared to CC, the intensities of peaks A, B, and C in the SC anode biofilms gradually increased, and peak D appeared. This was consistent with the conclusion in Figure S7, which further proved that the SC can change the composition and content of EPS. As shown in Figure S8, SC significantly promoted the increase in microbial biomass in the biofilm and the secretion of EPS. The EPS matrix covered carbon defects on the electrode surface and strengthened cell adhesion. As shown in Figures S8g–l and S9, SCmodified CC can improve microbial activity and promote the growth of EABFs. The ratio of live cells on the SC1000/CC anode biofilm was the largest, indicating that the anode had high biocompatibility.

#### *3.5. Microbial Community Analysis*

The α diversity represents the microbial community diversity on the anode EABFs. As shown in Table S6 and Figure S10, compared to the CC anode biofilm, the biofilm community diversity, richness, and the total number of species in the SC anode biofilms increased. These factors were linearly related to carbonization temperature, indicating that the SC has a significant impact on the community diversity of the anode biofilm. The effect of the SC characteristics on the microbial community structure of EABFs was further explored by high-throughput sequencing (Figure 9). SC significantly affected the community structure of the anode biofilm, thus directly affecting the power generation of MFCs. As shown in Figure 9a, at the phylum level of archaea, Euryarchaeota, and halobacteria were dominant. Among them, Euryarchaeota represent an irreplaceable functional microorganism in the anaerobic digestion process [40], and their numbers increased with an increase in the sludge carbonization temperature. In contrast, the content of halobacteria decreased with an increase in the sludge carbonization temperature, indicating that SC changes the structure of archaea, and that the characteristics of SC will selectively enrich functional microorganisms. At the genus level of archaea (Figure 9b), Methanobacterium and Methanosaeta were dominant. They can participate in EET and interspecific direct electron transfer (DIET) [5,39,40]. Methanosacrina is an obligate anaerobic bacterium that was significantly enriched in the SC1000/CC anode. It can possibly perform an EET. Hence, SC optimizes the archaeal community structure, selectively enriches functional microorganisms, and enhances the hydrogen methane production pathway.

**Figure 8.** Absorbance of c-Cyts for inoculated sludge and different anodic biofilms at MFCs on (**a**) 6D, (**b**) 18D, (**c**) 30D, (**d**) 42D, and (**e**) 54D. Absorbance of c-Cyts for (**f**) SC1000/CC. EEM fluorescence spectra of EPS extracted from (**g**) inoculated sludge, (**h**) CC, (**i**) SC600/CC, (**j**) SC800/CC, (**k**) SC1000/CC, and (**l**) SC1200/CC anode biofilms at MFCs after 54 d of batch mode operations.

As shown in Figure 9c, at the phylum level, the microbial communities on all anodes were similar, but the relative abundance of dominant bacteria was significantly different. Compared to the CC anode, the relative abundance of Proteobacteria, Firmicutes, and Bacteroidetes in the SC anodes decreased, and there was a correlation with the carbonization temperature. In contrast, the relative abundance of Desulfobacterota, Synergistota, and Actinobacteria increased. These results showed that SC could promote a synergistic effect between exoelectrogens and Synergistota. Desulfobacteria and Actinobacteria are related to organic matter degradation and usually control the acetate oxidation community [41], which contains a large number of electrochemically-active species [2,42]. The relative

abundance of Desulfobacterota on the SC1000/CC biofilm was the highest (35%), indicating that the biofilm was rich in electroactive bacteria.

**Figure 9.** Microbial community structures of anodic biofilms of Archaea and bacteria attached on CC and SC anodes at the (**a**,**c**) phylum and (**b**,**d**) genus levels.

As shown in Figure 9d, at the genus level, SC increased the diversity of microbial community. The dominant bacteria in each MFC were Geobacter and Lentimicrobium. Geobacter is a common exoelectrogen that can secrete cytochromes and participate in direct electron transfer [35,37,42]. This may be reflected in the power generation performance of MFC systems. Direct electron transfer may occur between Geobacter and methanogens, which can cooperate in power generation [43]. Lentimicrobium is a strictly anaerobic gram-negative bacterium. Studies have shown that it may form a consortium with other EAMs to convert acetate into electric energy [42]. Interestingly, SC significantly increased the relative abundance of Thermovirga, which was positively correlated with carbonization temperature, and may produce cytochrome to participate in EET [37]. In contrast, the relative abundance of Pseudomonas was significantly reduced due to the presence of SC, which promoted EET by secreting phenazines [42]. The results showed that the electron transfer path of SC anode biofilms was mainly short-range DET of c-type cytochromes, which was consistent with the data presented in the CV curves in Figure 5. The above results show that the microbial community structure of EABFs is directly related to the characteristics of the anode materials, and that there may be electron transfer between Archaea and EAMs. The metabolic pathways of microorganisms determine the flow of

electrons and protons, which affect the performance of electricity production. In metabolism clusters, all reactors used amino acid, carbohydrate, and energy metabolism as the main metabolic pathways (Figure S11). The SC1000/CC anode biofilm had the highest amino acid, carbohydrate, energy, and lipid metabolism. High carbohydrate metabolism indicates that microorganisms decompose organic matter quickly, which is related to increased output voltage [43].

#### **4. Conclusions**

The sludge carbon prepared by one-step pyrolysis had the advantages of high conductivity, good pore structure, high biocompatibility, high carbon content, rich heteroatom composition, and low cost. It significantly improved the activity and diversity of microorganisms on the anode biofilm, optimized the composition of the microbial community, regulated the metabolic pathway of microorganisms, promoted the secretion of EPS and the expression of cytochrome, and strengthened the electron transfer ability of EABFs. Therefore, an anode processed at 1000 ◦C generated a maximum areal power density of 2.165 ± 0.021 W·m−<sup>2</sup> and current density of 5.985 ± 0.015 A·m−2, which was 3.017- and 2.992-fold that of the CC anode. This study provides a theoretical basis for the practical application of sludge carbon in MFCs and provides a new direction for the rational utilization of biomass waste resources.

**Supplementary Materials:** The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/membranes12020120/s1. Figure S1: TEM images of SC samples under different carbonization temperature: a) SC600, b) SC800, c) SC1000, and d) SC1200, Figure S2: TG-DSC plots of raw sludge under N2 atmosphere, Figure S3: EDX element mappings of the SC1000 sample, Figure S4: XPS survey spectrum of the SC samples under different carbonization temperature. Figure S5: SEM image of bare carbon cloth electrode, Figure S6: SEM images of loaded electrodes with different SC samples, Figure S7: The composition and content of extracellular polymer substances for inoculated sludge and different anode biofilms at MFCs, Figure S8: SEM and CLSM images of raw sludge and anodes biofilms from MFCs after 54 d of batch mode operations, Figure S9: Ratio of dead and alive cells for raw inoculated sludge and anode biofilms at MFCs, Figure S10: Venn diagram of the biofilms attached on CC and SC anodes, Figure S11: Predictive abundances of metagenomic functional genes on the anode biofilms based on PICRUSt of KEGG analysis, Table S1: PBS solution composition, Table S2: Trace element solution compositio, Table S3: Vitamin solution, Table S4: Specific area and total surface area of SC samples, Table S5: Atomic contents of SC samples, Table S6: Species diversity and abundance indexes.

**Author Contributions:** Conceptualization, K.Z. and Y.X.; methodology, X.Y.; validation, W.F. and J.Y.; formal analysis, K.Z.; investigation, W.F.; resources, Z.W.; data curation, K.Z.; writing—original draft preparation, K.Z.; writing—review and editing, K.Z. and Z.W.; supervision, W.D. and W.F.; funding acquisition, Z.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research were funded by the National Natural Science Foundation of China, (grant number: 21868004), the Key Research and Development Plan of Guangxi Province (grant number: GuikeAB19259013), and Guangxi Natural Science Foundation (grant number: 2021GXNSFBA196094).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors thank Meiji biological platform for high-throughput sequencing of microorganisms.

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

#### **References**


### *Article* **Influence of Degassing Treatment on the Ink Properties and Performance of Proton Exchange Membrane Fuel Cells**

**Pengcheng Liu, Daijun Yang \*, Bing Li, Cunman Zhang and Pingwen Ming**

Clean Energy Automotive Engineering Center, School of Automotive Studies, Tongji University, Shanghai 201804, China; 2011683@tongji.edu.cn (P.L.); libing210@tongji.edu.cn (B.L.); zhangcunman@tongji.edu.cn (C.Z.); pwming@tongji.edu.cn (P.M.)

**\*** Correspondence: yangdaijun@tongji.edu.cn

**Abstract:** Degradation occurs in catalyst inks because of the catalytic oxidation of the solvent. Identification of the generation process of impurities and their effects on the properties of HSC ink and LSC ink is crucial in mitigating them. In this study, gas chromatography-mass spectrometry (GC-MS) and cyclic voltammetry (CV) showed that oxidation of NPA and EA was the primary cause of impurities such as acetic acid, aldehyde, propionic acid, propanal, 1,1-dipropoxypropane, and propyl propionate. After the degassing treatment, the degradation of the HSC ink was suppressed, and the concentrations of acetic acid, propionic acid, and propyl propionate plummeted from 0.0898 wt.%, 0.00224 wt.%, and 0.00046 wt.% to 0.0025 wt.%, 0.0126 wt.%, and 0.0003 wt.%, respectively. The smaller particle size and higher zeta potential in the degassed HSC ink indicated the higher utilization of Pt, thus leading to optimized mass transfer in the catalyst layer (CL) during working conditions. The electrochemical performance test result shows that the MEA fabricated from the degassed HSC ink had a peak power density of 0.84 W cm<sup>−</sup>2, which was 0.21 W cm−<sup>2</sup> higher than that fabricated from the normal HSC ink. However, the introduction of propionic acid in the LSC ink caused the Marangoni flux to inhibit the coffee ring effect and promote the uniform deposition of the catalyst. The RDE tests indicated that the electrode deposited from the LSC ink with propionic acid possessed a mass activity of 84.4 mA·mgPt−1, which was higher than the 60.5 mA·mgPt−<sup>1</sup> of the electrode deposited from the normal LSC ink.

**Keywords:** catalyst ink; PEMFC; rheology; catalyst layer; impurity

#### **1. Introduction**

Proton exchange membrane fuel cells (PEMFCs) have received significant research attention in recent decades, due to their high efficiencies, low operation temperature, and zero emissions [1–4]. Membrane electrode assemblies (MEAs), which comprise a proton exchange membrane (PEM), cathode and anode catalyst layers (CLs), microporous layers (MPLs), and gas diffusion layers (GDLs), are considered to be the heart of PEMFCs [5]. The complete working principle of an MEA consists of the following process: the oxidation reaction of H2 at the anode catalyst layer (ACL) provides electrons to an external circuit and releases protons to the internal electrolyte, while the reduction reaction of O2 at the cathode catalyst layer (CCL) receives electrons (from the external load) and protons (from the internal electrolyte). Both the CCL and ACL of an MEA are critical components of the system, because they represent energy conversion sites, where charge and mass transfer and the electrochemical reaction occur coinstantaneously [6]. The cost, performance, and durability of PEMFCs are closely dependent on the structure and morphology of CLs, which face several challenges, such as the coupling effects of corrosion in a strong acid environment, humidity stress, thermal shock stress, and mechanical stress during the service period [7]. Therefore, the optimization of the CL microstructure is a considerably critical issue to ensure a high performance of PEMFCs.

**Citation:** Liu, P.; Yang, D.; Li, B.; Zhang, C.; Ming, P. Influence of Degassing Treatment on the Ink Properties and Performance of Proton Exchange Membrane Fuel Cells. *Membranes* **2022**, *12*, 541. https://doi.org/10.3390/ membranes12050541

Academic Editors: Marc Cretin, Sophie Tingry and Zhenghua Tang

Received: 24 March 2022 Accepted: 9 May 2022 Published: 22 May 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 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 (https:// creativecommons.org/licenses/by/ 4.0/).

Understanding and optimizing the preparation process of MEAs are imperative to obtaining CLs with the perfect microstructure for the most effective PEMFCs [8,9]. Generally, the process of preparing CLs involves the following procedures: (i) dispersion of the catalyst (such as Pt-loaded carbon) and the proton-conductive ionomer, which also works as a binder in a dispersion medium (such as water/NPA/isopropanol), in the CLs; (ii) coating of the catalyst ink on the PEM or GDLs; and (iii) drying to evaporate the dispersion medium [6,10]. The effects of the catalyst ink quality and process control exist throughout the above processes, which determine the CL microstructure and therefore the characteristics of the fuel cells [11]. Previous works have focused on the construction and optimization of the CL microstructure based on the coating process and ink formulation [5,12]. Coating and drying parameters influence the distribution of materials and pores in CLs and also have significant impacts on performance. Commonly, the ink formulation, including the alcohol content and type, ionomer content, and Pt dispersion, also affects the ink initial properties such as rheology, stability, and coatability, thereby exerting an influence on the fabrication of CLs [13–16]. However, the degradation of the catalyst ink quality after preparation also affects the catalyst ink viscosity, the size of agglomerates, which are a mixture of the catalyst and ionomer, the quality of the coated catalyst layer, and thus the performance of the fuel cells. Therefore, it is crucially necessary to understand the degradation process of catalyst inks and its impacts on the storage and rheologic properties, as well as well-constructed CLs.

Based on extensive studies on CLs, many researches have demonstrated that the microstructure of CLs was closely realated to the catalyst properties, which was dependent on the size of agglomerates [11]. Catalyst particles are generally found to be agglomerated, forming primary agglomerates with a particle size of 200–300 nm under the effect of van der Walls attractive force. Further agglomeration of primary aggregates happens to generate secondary agglomerates on the microscale. Additionally, the addition of an ionomer can reduce the size of agglomerates due to the electrostatic repulsion and steric hindrance interactions. The larger agglomerates become, the greater negative effect they will have on the construction of CLs, leading to a reduced output performance. Therefore, the state of the catalyst ink should be controlled at a uniform and stable condition to fabricate high-quality MEAs.

The change in composition of the catalyst ink will lead to a change in its properties, especially the generation of impurities. To date, only a handful of studies have described the generation of impurities in catalyst inks and their effects on the processability of catalyst inks [14,17,18]. For instance, some previous works have demonstrated the effects of various impurities, such as acids and aldehydes, on the agglomerate behavior of inks and ultimately on the final structure of CLs [14,17–20]. Uemura [14,18,19] used X-ray computed tomography to detect the presence of air bubbles and the third phase in a catalyst ink and proved the catalyst caused alcohol to decompose [19]. Kameya combined nuclear magnetic resonance (NMR) with magnetic resonance imaging (MRI) to analyze the internal state of an ink during the preparation process and detected the presence of air bubbles in the ink during the main mixing process [21]. In addition, 19F NMR spectra revealed dramatic changes in the dispersion states of Nafion during the mixing period. Other previous studies targeting NPA oxidation on platinum electrodes in acid solutions have demonstrated that NPA is converted to propionic acid, whereas isopropanol is highly selectively converted to acetone, due to the difficulty in breaking the C−C bond [22,23]. Catalyzed oxidation of the dispersion medium and the deuterogenic reaction affect the state of the catalyst ink. These generated impurities induce the generation of larger agglomerates in the catalyst ink, and thus cracking of the CLs because of the capillary stress. Kumano [13] identified the structural parameters that control the dispersion state and stability of Pt/C agglomerates. In inks containing 48–75% of water, the amount of adsorbed ionomers decreased with decreasing water content, resulting in increases in the viscosity, storage modulus, and electrical conductivity. The adsorption rate of the ionomer into the Pt/C decreased, and the average size of agglomerates, viscosity, and storage modulus increased with the increase in

the hydrophobicity of the solvent. The impurity produced in inks undoubtedly changes the hydrophobicity of the solvent and thus affects the properties of the ink. Hence, it is important to obtain an understanding of the generation process of impurities and their effects on processability.

HSC ink is often used in the coating procedure across industrial applications, whereas LSC ink is applied in the spraying procedure and rotating disk electrode (RDE) tests in laboratories [13,24]. High-quality RDE measurements need a thin, uniform film over the entire surface area of the glassy carbon to accurately evaluate the electroactivity of the catalyst [24]. The quality of the working electrode is delicately determined by the drying conditions, alcohol content and type, Pt dispersion, and surface state of the glassy carbon. Therefore, the effects of impurities on the properties of LSC ink require a detailed investigation, owing to their effect on the electrode structure, and the lack of clarity regarding their underlying mechanism of action. At present, there is limited cognition of the formation mechanism of impurities and their effects on the catalyst ink rheology and drying behavior. To fully explore the generation process of impurities, we first investigated the oxidation of the solvent and the effects of the temperature and atmosphere on catalyst ink degradation. In order to understand how impurities affect the rheology and drying process of catalyst inks, we present a comparative study of HSC ink before and after degassing treatment. The range of comparison includes rheological behavior and the property (ink)–structure (catalyst layer)–performance (MEA) relationship.The influence of impurities on the properties of LSC ink cannot be ignored because the LSC ink is widely used in the RDE experiments to evaluate the characteristics of catalysts. The effects of the introduction of propionic acid in the LSC ink on the microscale structure of the RDE were investigated using optimal microscopy, cyclic voltammetry (CV), and line sweep voltammetry (LSV). Based on the understanding of the impurity evolution process and the relationship between the impurity properties and ink quality for the two ink types, we provide insight into optimizing the preparation of catalyst inks to obtain excellent processability and coatability. This also helps in the construction of the desired catalyst layers.

#### **2. Experimental Procedures**

#### *2.1. Preparation of Catalyst Ink*

This section describes the procedures and instrumentation for the preparation of the HSC ink. Briefly, 5.7 g of Pt/C powder (Johnson Matthey, Alfa Aesar, Shanghai, China, Vulcan XC-72 with 60 wt% Pt and 20.5 g of Nafion® solution (Dupont™, New Castle, DE, USA, Nafion® PFSA polymer dispersions D-520) were mixed with 27 g of the NPA and ultrapure water (the ratio of NPA to ultrapure water was 1) dispersion medium. The process of fabricating the HSC catalyst ink followed the steps shown in Figure 1. Firstly, 5.7 g of catalyst powder was added to 13.5 g of water and stirred with a glass rod, and then 13.5 g of NPA and 20.5 g of Nafion solution were added successively (premixed process), followed by ultrasonic dispersion (35 kHz, 5 min, 15 ◦C). Secondly, the catalyst ink was homogenized by high-speed shear (1600 rpm, 30 min) and finally degassed using a magnetic stirrer at 30 rpm for 30 min at −0.1 MPa. The ink and raw material for the above process were contained in glass containers. The catalyst ink that was not degassed was measured after being kept for 24 h and marked as "I-ink", whereas the degassed ink was denoted "D-ink" and left to stand for at least 24 h before subsequent measurement. After preparation of the two catalyst inks, the container was filled with nitrogen as the protective gas.

**Figure 1.** Schematic representation of the process of HSC ink preparation.

The LSC ink for oxygen reduction reaction (ORR) catalyzed activity was evaluated using a rotating disk electrode (RDE). Briefly, 2 mg of catalyst powder was added to a 1 mL mixture of 5 wt% Nafion® solution and NPA (volume ratio of 1:30), and the mixture was fully mixed by ultrasonication for 40 min (35 kHz, 15 ◦C). This LSC ink was denoted "Nink". To characterize the effect of impurities on the LSC ink, we added 10 μL of propionic acid (playing the role of impurities) to the N-ink to produce a comparison ink known as "P-ink". A summary of the compositions of the HSC ink and LSC ink is shown in Table 1.

**Table 1.** Sample composition of catalyst inks.


#### *2.2. Electrochemical Evaluation*

The HSC catalyst ink was directly coated on the proton exchange membrane (Gore, Newark, DE, USA, thickness of 18 μm) using a slot-die coating system. The slot die moved at a horizontal velocity of 10 mm/s above the proton exchange membrane with a coating gap of 100 μm. The baseplate temperature was maintained at 60 ◦C to remove the solvent from the wet film. The MEA was assembled by sandwiching a catalyst coating membrane between two pieces of gas diffusion layer (Freundenberg, Shanghai, China, H24CX483). The Pt loadings were controlled to be 0.4 mg·cm−<sup>2</sup> and 0.2 mg·cm−<sup>2</sup> in the cathode and anode, respectively. To evaluate the fuel cell performance, the polarization curve was measured with a 25 cm2 three-serpentine cell fixture and tested with a fuel cell test system (Dalian New Sunrise Testing Technology Co., Ltd., Dalian, China, NSR-FTCS100B-1802-3). The temperature of cell was controlled at 75 ◦C. The stoichiometry of H2/air was 1.5/2.5. The inlet gauge pressures of anode and cathode were maintained at 100 and 80 kPa. The relative humidity at the anode and cathode sides were both 55%. The electrochemical impedance spectra of single cells were recorded at 1.6 A·cm−<sup>2</sup> using a scanning frequency from 103 to 0.1 Hz. A detailed electrochemical analysis of the oxidation behavior of NPA and EA under an acid environment was performed using a three-electrode cell system (PINE, AFCPRBE). This system comprises a thin film of catalyst ink deposited on a glassy carbon substrate as the working electrode (WE), a reversible hydrogen electrode (RHE) as the reference electrode, and a platinum sheet as the counter electrode. The electrolyte solution was de-aerated before each measurement with N2 and O2 for 30 min, and all electrochemical measurements were performed using an electrochemical workstation (CHI. Instrument company, Shanghai, China, 760E). The working electrode was prepared by transferring 10 μL of ink into the RDE (S = 0.196 cm2), followed by natural drying. CV

and LSV were carried out in 0.1 M HClO4 at 25 ◦C. CV data were recorded at a potential range from 0.05 to 1.10 V, at a scanning rate of 0.05 V·s−1, in a N2-saturated electrolyte solution. ORR polarization curves were obtained in an O2-saturated electrolyte solution at a scanning rate of 0.005 V·s−<sup>1</sup> and an RDE rotation rate of 1600 rpm. The electrochemical surface area (ECSA) of the WE was calculated based on the CV curves, using the following equation [25]:

$$\text{ECSA} = \frac{\text{QH}}{210 \times \text{mp}} \tag{1}$$

where QH (mC) is the charge of hydrogen species' electro-adsorption peak; the value of <sup>210</sup> <sup>μ</sup>C·cm−<sup>2</sup> corresponds to monolayer adsorption of hydrogen atoms on a polycrystalline Pt; and mPt represents the mass load of Pt on the working electrode. ORR's catalytic activity, which is the kinetic current density at 0.9 V (vs. the RHE) from the LSV curve, was calculated based on the Koutecky–Levich (K–L) equation as follows [26]:

$$\frac{1}{\dot{i}} = \frac{1}{\dot{i}\_k} + \frac{1}{\dot{i}\_d} \tag{2}$$

where *i* is the current density value measured at E = 0.9 V; *id* is the diffusion-limited current density at E = 0.4 V (vs. the RHE); and *ik* represents the kinetic current. The specific mass activity (MA) of the catalyst is the kinetic current per unit mass loading of Pt [24,27].

#### *2.3. GC-MS Instrumentation*

Impurities in the catalyst ink were analyzed by GC-MS (Agilent, Shanghai, China, 7890B-5977B), which can detect various volatile components in a solution. Patterns of the mass spectra were analyzed using NIST-2008.

#### *2.4. Rheological Measurements*

The rheological property of the inks was measured using a stress-controlled rheometer (Anton Paar, Shanghai, China, MCR302), with a coaxial cylinder mold. Prior to measurement, the ink was kept quiescent at 25 ◦C for 5 min to remove any previous disequilibrium status and ensure that the constituent material established new equilibrium-status structures. A pre-shear treatment was first used to eliminate the shear history and ensure the repeatability of the test data. During this operation, the shear rate was controlled at 0.01 s−<sup>1</sup> for 100 s. Thereafter, steady status flow measurements were carried out by step-wisely increasing the shear rate from 0.01 to 1000 s<sup>−</sup>1, to test the viscosity function of the formulated inks. Three interval thixotropy tests were used to determine the structural regeneration of the HSC ink, and a typical step test with three intervals depicted as a time-dependent viscosity function was as follows: (1) the shear rate was kept at 0.1 s−<sup>1</sup> for 60 s, at the beginning, to simulate the ink at rest; (2) the shear rate was maintained at 100 s−<sup>1</sup> for 10 s to simulate the structural breakdown of the ink; (3) the rate was kept at 0.1 s−<sup>1</sup> for 60 s to simulate the structural recovery of the ink at rest. Furthermore, the strain dependency of the storage modulus (G ) and loss modulus (G) was applied to change the strain from 0.01 to 100% at 1 Hz after tests of the steady flow viscosity. All rheology experiments were performed at 25 ± 0.1 ◦C.

#### *2.5. Measurement of Ink Cluster Size and Zeta Potential*

The cluster size and zeta potential measurements of HSC were performed with dynamic light scattering (DLS) (Colloid Metrix, Shanghai, China, Nano-fiex) and a particle potential titrator (Colloid Metrix, Shanghai, China, Stabino), respectively. For testing purposes, 0.1 mL of the inks was diluted using 100 mL of a solution with the original solvent composition. The diluted inks were dispersed in an ultrasonic bath for 2 min prior to the DLS and zeta potential measurements.

#### *2.6. Determination of Contact Angle and Deposition of LSC Ink*

The morphology of the LSC ink (5 μL) drying on the RDE was measured using a digital microsystem (KEYENCE, Osaka, Japan, VH-S30B). The contact angle between the catalyst ink and the glass was determined, using the side view of the microscope, and the deposition process of the catalyst ink droplets was observed from the top view.

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

The total ion current (TIC) spectrum of I-ink, based on GC-MS detection, and the spectra of D-ink are shown in Figures 2 and S1; the signals of impurities classified by their corresponding mass spectra are illustrated in Figure 3; and the results of the quantitative analysis are listed in Table 2.

**Figure 2.** Total ion current (TIC) spectrum of I-ink.

The impurities in the ink were indexed as acetaldehyde, propanal, 1,1-dipropoxypropane, propyl propionate, acetic acid, and propanoic acid. The corresponding regions of the TIC spectrum were detected after 1.865, 2.194, 9.136, 6.338, 18.620, and 20.562 s. The presence of acetaldehyde was confirmed by the fragment ions (*m*/*z*) at 29, 43, 44, and 15 (Figure 3), whereas the propanal segments were ionized into *m*/*z* of 26, 27, 28, 29, 57, 58, and 59. Other impurities were identified through additional analysis of the mass spectra, such as propyl propionate, 1,1-dipropoxypropane, acetic acid, and propanoic acid. Interestingly, these impurities were found to simultaneously occur in I-ink and D-ink, albeit with significant differences in concentrations. Therefore, the degassing treatment efficiently suppressed the oxidation process of the solvent. Previous studies have demonstrated that Pt metal can catalyze solvents to produce complex oxidizing compounds, including acetaldehyde, propanal, acetic acid, and propanoic acid [22,28,29]. These catalytic products are then condensed to form esters. We observed significant differences in impurity concentrations between I-ink and D-ink. It is worth noting that the oxidation products exhibited more hydrophobic characteristics than the original solvent composition. Nafion®, a binder and stabilizer in catalyst inks, is essential for optimization of the properties of catalyst inks [30]. In fact, its hydrophobic backbone is attracted to the hydrophobic surface of the carbon support, whereas its hydrophilic sidechains are ionized to generate numerous ionic charges on the surfaces of the carbon support [31]. However, the presence of hydrophobic impurities improved the compatibility between the ionomer and solvent, thereby increasing the amount of free ionomer on the solvent. Consequently, this affected the interaction between

the internal components and rheology of the catalyst ink [16,32]. Next, we investigated the effects of the atmosphere and temperature on the impurity generation process.

**Figure 3.** Mass spectra of impurities derived from electron impact ionization of I-ink: (**a**) acetaldehyde; (**b**) propanal; (**c**) propyl propionate; (**d**) 1,1-dipropoxypropane; (**e**) acetic acid; (**f**) propanoic acid.

**Table 2.** Summary of impurity concentrations of the catalyst inks derived from GC-MS data.


The CV of Pt/C electrodes in 0.05 M H2SO4 + 0.1 M NPA solution at 0.002 V·s−<sup>1</sup> shows the electro-oxidation process of NPA (Figure 4a). The first oxidation peak was detected at 0.90V vs. the RHE in the O2-purged solution, in the positive scanning process. In contrast, this peak was found at 0.95 V vs. the RHE in the N2-purged solution, due to the overpotential required to overcome the concentration polarization caused by the lack of O2. Both oxidation peaks corresponded to the poisonous intermediate formation in the NPA oxidation reaction [28]. However, an increase in the potential generated the second oxidation peak at 1.29 V in the O2-purged solution, whereas a similar peak was observed at 1.35 V in the N2-purged solution. This oxidation peak indicated the formation of reaction intermediates during NPA oxidation. Furthermore, several higher peaks' current densities were recorded in the O2-purged solution relative to the N2-purged solution, because sufficient electro-oxidation of NPA produces a larger reaction current. A similar phenomenon was observed during the oxidation of alcohol (Figure 4b). The oxidation peaks' current densities of NPA and EA are summarized in Table 3. The high impurity concentration of I-ink derived from quantitative analysis of GC-MS also supported this phenomenon (Table 2). As previously mentioned, the CVs of NPA and EA in the O2 atmosphere exhibited a significantly higher oxidation overpotential and smaller oxidation current density, suggesting that the anoxic environment can reduce the intensity of solvent oxidation reactions. The effects of the temperature on the electro-oxidation of NPA and EA are discussed in the Supporting Information, and the CVs of NPA and EA under various temperatures are shown in Figure S2. Summarily, high temperatures promoted the solvent's oxidation behavior, suggesting the need to regulate the N2 atmosphere and control the temperature for an effective reduction in solvent electro-oxidation during the ink preparation process. A summary of the mechanism underlying the formation of impurities in the catalyst inks is shown in Figure 5. Briefly, EA and NPA can be oxidized to their respective aldehydes and acids, in the presence of platinum catalysis; meanwhile, propyl propionate is generated by esterification of propionic acid and propanol. Specifically, the aldolization reaction of NPA and propanal, via Pt catalysis, is the cause of 1,1-dipropoxypropane [14].

**Figure 4.** (**a**) Cyclic voltammograms recorded with a Pt/C electrode in 0.05 M H2SO4 + 0.1 M NPA solution and (**b**) 0.1 M EA solution, with a scan rate of 20 mV·s−<sup>1</sup> and a temperature of 25 ◦C.


**Table 3.** Current densities of oxidation peaks recorded from the CV of NPA and EA.

Rheological characterization of the catalyst ink is an essential index for each step during MEA fabrication. The catalyst ink is taken as the working fluid in a slot die, and its viscosity is perceived as the most crucial rheological property during the coating procedure [33,34]. It directly influences the behavior of the ink formulation during mixing and production of the wet catalyst layer [35]. The relationship between viscosity and shear stress is shown in Figure 6a. Obviously, shear thinning behavior occurred in D-ink and I-ink, which means the viscosity was negatively correlated with the shear rate. Catalyst inks are multi-component, complex solid–liquid mixtures that consist of a catalyst, ionomer, and solvent medium. The dynamic viscosity at low shear rate stages is an index of the settling degree of the solid content, while that at high shear rate stages is an index of the coating processability [36]. Both inks showed a high viscosity at a low shear rate stage, which is preferred owing to the lack of significance in the settlement of the solid content. Notably, at a higher shear rate stage, the strong shear rate force tended to destroy the microstructure of the catalyst inks, which subsequently realigned the internal structure and significantly reduced the viscosity. This behavior means that D-ink and I-ink are non-Newtonian fluids, a property that is quite suitable for the actual production process. At the ink storage stage, the particles in the ink were subjected to external forces, including gravity and shear forces. The shear rate ranged from 10−<sup>6</sup> to 10−<sup>2</sup> s−1. The high viscosity indicates the excellent anti-sedimentation properties of these inks. During coating, the fluid with a high shear rate

requires a low viscosity. After coating, the advection of ink occurs on the proton exchange membrane under surface tension and the action of gravity. The viscosity of I-ink was slightly higher than that of D-ink in most of the shear rate scope, but with the shear rate increasing, the gap between the viscosities of the inks gradually narrowed. This implies that the network structure of the catalyst and binder grew after degassing, and dispersion states in the ink were changed. The rising hydrophobicity of the solvent caused an increase in free ionomers, which subsequently increased the viscosity [37,38]. The difference in the network structure's strength between these two inks was further evidenced by the hysteresis flow curves, as shown in Figure 6b,c. Notably, hysteresis phenomena, where different shear stress values appeared in the positive scan and negative scan, were observed in the inks. Moreover, any destabilization of this steady state would destroy the ionomer structure, owing to entanglements of ionomer chains and fluctuations in the arrangement of the catalyst particles brought about by changes in shear and relaxation processes [39,40]. The degassing treatment improved the adsorption of the ionomer into the catalyst, due to the removal of the microbubbles in the aggregates.

**Figure 5.** A schematic illustration of the proposed NPA and EA conversion process.

Furthermore, the continuous increase in bridging within the catalyst, yield stress, level of shear thinning, and equilibrium G within the inks' linear viscoelastic regime were all strengthened, and both types of inks exhibited a shear stress plateau at shear rates from 1 s−<sup>1</sup> to 10 s−1, indicating the existence of yield stress in both inks (Figure 6b,c). We calculated the numerical value of the yield stress by averaging the initial five points in this stress plateau and found a higher value in I-ink (2.5 Pa) than in D-ink (1.7 Pa). The decrease in the yield stress of the inks contributed to their self-leveling, which suppressed the uneven thickness distribution in the catalyst layer [5,41].

**Figure 6.** (**a**) Dynamic viscosity data of tested catalyst inks; (**b**,**c**) shear stress as a function of the shear rate for I-ink and D-ink; (**d**) three interval thixotropy test of catalyst inks, and amplitude oscillation test of the inks; (**e**,**f**) strain-dependent storage modulus (G ) and loss modulus (G) at 1 Hz and phase angle.

According to the determined yield stress, the level of shear thinning of the inks can be quantified by implementing the Herschel–Bulkley model [42,43]:

$$
\sigma = \sigma\_0 + \mathbb{K}\gamma^n \tag{3}
$$

where σ and σ<sup>0</sup> represent the measured shear stress and yield stress measured at a specific shear rate (Pa), respectively; <sup>K</sup> denotes the consistency index (Pa·sn); <sup>γ</sup> is the shear rate (s<sup>−</sup>1); and n is the dimensionless flow index. Only information obtained from a shear rate above 2 s−<sup>1</sup> was considered in the modeling procedure, because this range of the shear rate matches the actual coating process. The consistency index indicates the degree of viscous contribution during the increase in the shear rate, whereas the function of shear stress and the shear rate of the catalyst inks were presented by the flow index [44]. As shown in Table 4, the results show that I-ink had a significantly higher consistency index than D-ink because I-ink exhibited a significantly higher level of shear thinning and was more viscous than D-ink. All of the inks had *n* < 1, a phenomenon that corresponds to the shear thinning behavior as illustrated in Figure 6a. Notably, a small dimensionless flow index resulted in stronger shear thinning behavior [42], and the gap in viscosity between D-ink and I-ink almost disappeared when the shear rates were increased to about 100 s−1. On the other hand, an increase in the low-shear viscosity (LSV) resulted in a coating layer with a sharper edge, implying less cut-off waste during subsequent processing [45].

**Table 4.** Calculated Herschel–Bulkley parameters for the catalyst inks.


Catalyst ink coating entails a high-shear-rate process, while self-leveling of the ink onto the PEM is a low-shear-rate process. The essential requirements for this structural regeneration process include: (1) applying a slow reconstruction rate for good leveling; and (2) ensuring the rate is not too slow to prevent sagging and to allow a sufficient wet layer thickness and flatness. To investigate this time-dependent behavior, we performed a rotational test with three intervals and present the result as a time-dependent viscosity function (Figure 6d). At the first stage, a very low shear rate (0.1 s<sup>−</sup>1) was used to simulate behavior at rest, and as the hydrophobicity of the dispersion solvent increased, the viscosity of D-ink became lower than that of I-ink. This difference in viscosity resulted from the increase in the free ionomer and generated aggregation, which was related to the change in the rate of adsorption of the ionomer into the catalyst [13,38,46]. An increase in the shear rate to 100 s−<sup>1</sup> (stage 2) caused the strong shear to simulate the structural breakdown of the catalyst inks during the coating process [47]. Moreover, both ink types exhibited very low viscosity due to the shear thinning behavior. At the final stage, the low shear rate simulated structural regeneration for the ink self-leveling process, although the structural strength and viscosity of the inks gradually recovered with time. The thixotropic recovery rates of D-ink and I-ink were 75.8 and 46.6%, respectively. We hypothesized that the high ionomer adsorption on the catalyst's surface strengthened the interaction between the ionomer and the catalyst, and this behavior was also observed in the lithium-ion battery field [47,48].

Next, we used oscillatory shear to investigate the inks' microstructure, and amplitude sweep to characterize the inks' linear viscoelastic regime (LVR). Subsequently, we applied the LVR to accurately measure the breakdown of the network structure and acquire the structural strength of the initial state [42]. The results reveal lower G values for both inks at the low-strain region compared to G , indicating an elastic-dominant response property of the inks (Figure 6e) [42]. At a strain range of 5% to 7%, the G value fell below that of G, indicating that only a slight increase in the shear strain could promote the inks' shift from elastic-dominant to viscous-dominant [49]. The phase angle δ of the ink, which is calculated using Equation (4) below, is shown in Figure 6f.

$$\delta = \tan^{-1} \left( \frac{\mathbf{G}''}{\mathbf{G}'} \right) \tag{4}$$

Briefly, the value of δ for D-ink reached 1 at a faster rate, indicating that the gel–sol transformation occurred more easily. For D-ink, the better self-leveling effect, during the drying procedure, resulted in the gradual development of pores in the wet catalyst layer. Therefore, a homogenized cavity structure is beneficial to the reduction in capillary stress during the drying process. In contrast, a wet film with low self-leveling after drying generates a hierarchic pore structure with larger fluctuations, thereby enhancing capillary stress and increasing the risk of CL cracks [46,50–53].

Figure 7a shows the cell polarization curves of the MEAs fabricated from different HSC inks. According to Figure 7a, the MEA prepared from D-ink exhibited excellent improvements compared with that made from I-ink. At the electrochemical polarization control region, the voltage of the two MEAs showed no distinct differences, which can be attributed to the catalyst having the same catalytic intrinsic activity in both MEAs. With the increase in the current density, the gap in output voltages of the different MEAs expanded. Especially under high current densities, mass transfer loss led to a significant performance reduction for the MEA fabricated with I-ink. The electrochemical impedance spectra were recorded to analyze the H2/air performance and fitting using an equivalent circuit (Figure 7b) [54]. *R*<sup>Ω</sup> denotes the ohmic resistance of the cell. *R*anode and *R*cathode are faradaic resistances, which represent the kinetics of the electrochemical reactions occurring on the anode and cathode, respectively. The finite Warburg circuit element (*W*mt) is used to reflect the mass transport loss on the cathode side. As observed, the MEA performance improved with the decrease in the impedance arc. The fitted values of *R*<sup>Ω</sup> were 0.0043 Ω and 0.0045 Ω for D-ink and I-ink, respectively. The impedance spectra consist of semicircles in the high-, medium-, and low-frequency regions, and each of these semicircles corresponds to the

resistances of anode activation, cathode activation, and mass transport. The increments in the semicircles in the medium- and low-frequency regions reflect the greater resistance of the activation kinetics and mass transport. The *R*anode values for D-ink and I-ink were similar, with 0.0211 Ω and 0.0223 Ω, respectively, due to them having the same anode catalyst layer. The MEA fabricated with I-ink exhibited a higher *R*cathode and *R*mt than that fabricated with D-ink. The *R*cathode values decreased from 0.0604 Ω to 0.0353 Ω, and the *R*mt values varied from 0.0255 Ω to 0.0183 Ω, with the degassing treatment of the ink. Therefore, the cathodic mass transport process and ORR kinetics dominated the H2/air performance. Jian Xie [12] reported a similar phenomenon where the increase in the NPA ratio in the solvent could intensify the resistance of the ORR kinetics and mass transport limitations.

**Figure 7.** (**a**) H2/air polarization curves of membrane electrode assemblies fabricated with HSC inks, and (**b**) corresponding Nyquist plots obtained at 1.6 A·cm−<sup>2</sup> from 0.1 Hz to 1 kHz; (**c**) size distribution and (**d**) zeta potential of I-ink and D-ink.

To better understand how the impurities affect the MEA performance, the catalyst cluster size distributions and zeta potential in different inks, which determine the catalyst/ionomer interface and CL structure, need to be studied. The microscale mass transport in the CLs depends on the aggregate structure and ionomer distribution [55]. As evidently shown in Figure 7c, the intensity signals in I-ink showed a double-peak structure, indicating quite a few of the larger clusters. On the contrary, the intensity signals of D-ink were concentrated in small-size regions. The average diameter of I-ink clusters reached 244.7 nm, while the size of the D-ink clusters was 199.7 nm. The zeta potential results

show the stability differences for D-ink (−32.64 mV) and I-ink (−29.52 mV). A hydrophobic impurity causes the ionomer to adsorb into the solvent and desorb from the catalyst. This reduces the adsorption capacity of the ionomer, leads to the inhibition of steric hindrance between the clusters, and increases the risk of cluster agglomeration.

Apart from electrochemical contamination, impurities also affect the drying behavior of LSC ink. High-quality ORR tests require a thin, uniform film over the entire surface area of the GC electrode [56–58]. However, the catalyst dispersed in a drying ink drop migrates towards the edge of the ink drop to form a "coffee ring" [59,60]. The effects of impurities on catalyst activity were tested in a half-cell using the as-prepared ink coating on a GC electrode. Deegan et al. [59,61,62] postulated that the "coffee ring" effect occurs because the evaporation rate at the edge of droplet is higher than that at the center, resulting in an outward capillary flow within the droplet. This, in turn, transfers the suspended particles to the edge of the droplet and deposits them into a ring at the edge. As shown in Figure 8a, the "coffee ring" phenomenon appeared in the N-ink electrode, leading to an ununiform distribution of the catalyst, but this phenomenon was alleviated in the P-ink electrode. Notably, the introduced propionic acid has a higher boiling point and lower surface tension than the original solvent. As evaporation proceeds, the water evaporation rate at the edge of the droplet exceeds that at the center, whereas the evaporation rate of the propionic acid at the edge of the droplet becomes slower. Therefore, the propionic acid gradually becomes enriched at the edge. The difference in surface tension between the edge and center of the droplet creates the Marangoni effect [63]. On this basis, the enhanced Marangoni flow moves the catalyst particles radially from the edge to the center of the droplet surface, thereby inhibiting the "coffee ring" effect. Therefore, an RDE with a uniform catalyst deposition layer shows a better ORR performance.

**Figure 8.** (**a**) Optical photograph and ORR performance of the electrodes in an RDE: (**b**) CV and (**c**) the corresponding ORR polarization curves.

The CV curves of electrodes from N-ink and P-ink are shown in Figure 8b. The anodic H waves and the cathodic H waves in the CV represent the H from the electrochemical desorption and adsorption process, respectively [24,27,64]. The Hupd charge is estimated after the conventional correction for the pseudocapacity seen in the double-layer region by a straight line. As an electron is transferred during the oxidation of the adsorbed Hupd, the charge of Hupd is therefore given by *Q*<sup>H</sup> = *i*d*E <sup>v</sup>* with the potential *E*, the sweep rate *v*, and the current *i*. The amounts of Hdes in the curves of P-ink and N-ink, with double-layer charges subtracted, are 1.58 mC and 2.83 mC, respectively. In agreement with the report of Garsany et al. [24], the film quality affects the electrochemical surface area measurement, with an ECSA of 62.7 m2·gPt−<sup>1</sup> for the electrode fabricated from N-ink, compared to 112.2 m2·gPt−<sup>1</sup> for the electrode fabricated from P-ink (cf. Table 5—data from tables). Notably, the data shown in Figure 8c indicate that the electrode deposited by P-ink had a higher mass activity (84.4 mA·mgPt−1) than that deposited by N-ink. Furthermore, the normal LSC ink exhibited lower electrochemical properties of electrode deposition than

the LSC ink with propionic acid, under similar conditions of the catalyst ink formation and drying process. The superior catalyst performance of the N-ink electrode was attributed to the morphology of the deposited catalyst, due to the effect of impurities on the ink [65]. The records of the drying process for N-ink and P-ink are presented in the Supporting Information to reveal the drying behavior of the ink droplets on the glass substrate.

**Table 5.** Properties of electrodes deposited by N-ink and P-ink.


#### **4. Conclusions**

In the traditional ink preparation procedure, the catalytic oxidation of alcohols and its effects on the quality of the catalyst ink are generally ignored. In this study, we explored the mechanism by which impurities are generated, and the effects of such impurities on HSC ink and LSC ink. The GC-MS results indicate that the impurities in the inks included propionic acid, acetic acid, propanal, acetaldehyde, propyl propionate, and 1,1-dipropoxypropane. Together with the electrochemical behavior of NPA and EA, the impurity evolution process is as follows: NPA and EA are catalytically oxidized to propionic acid and acetic acid. Then, they are under esterification and aldolization reactions. The strength of the catalytic reaction is strongly correlated with the oxygen atmosphere and temperature. This result demonstrates the importance of temperature control and degassing treatment during the ink preparation process.

The effects of impurities on the HSC ink and LSC ink were identified, and the production of impurities was suppressed by the degassing treatment, causing the concentrations of acetic acid, propionic acid, and propyl propionate to decrease from 0.0898 wt.%, 0.00224 wt.%, and 0.00046 wt.% to 0.0025 wt.%, 0.0126 wt.%, and 0.0003 wt.%, respectively. For the HSC ink, the viscosity and yield stress of D-ink were lower than those of I-ink, and its structural resilience, as a result of the stronger interactions between the ionomer and catalyst, was 29.2 % higher than that of I-ink. For the LSC ink, the addition of propionic acid reduced the surface tension of the original solvent, thereby suppressing the "coffee ring" effect. This creates a thin, uniform catalyst deposition layer. Based on this, the ECSA of electrodes derived from N-ink was 62.7 m2·gPt−1, but the parameter for P-ink reached 112.2 m2·gPt−1. Therefore, the inhibition of impurity generation in HSC ink will provide better coatability, but the introduction of impurities will promote the uniform deposition of the catalyst in LSC ink. In the end, extending this study to the industrial scale will be particularly valuable for the improvement of the electric generation performance of PEMFCs, which is accessible by controlling of the HSC ink quality to establish the desired catalyst layer microstructure.

**Supplementary Materials:** The following supporting information can be downloaded at: https: //www.mdpi.com/article/10.3390/membranes12050541/s1, Figure S1: Total ion current (TIC) spectrum of D-ink; Figure S2: Cyclic voltammograms recorded with Pt/C electrode in 0.05 M H2SO4 + 0.1 M NPA solution (a) and 0.1 M EA solution (b) with a scan rate of 20 mV·s−<sup>1</sup> under N2 atmosphere. Figure S3: A side view of catalyst ink droplets at the initial state and top view of ink droplets with the evolution of time for N-ink (a) and P-ink (b).

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

**Funding:** This work was supported by National Nature Science Foundation of China (No. 52176198).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

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

#### **References**

