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Article

Electrooxidation Performance of a Cotton-Cloth-Derived, Ni-Based, Hollow Microtubular Weave Catalytic Electrode for Methanol and Urea

by
Guangya Hou
*,
Jiaxuan Wei
,
Qiang Chen
,
Jianli Zhang
* and
Yiping Tang
College of Materials Science and Engineering, Zhejiang University of Technology, Hangzhou 310014, China
*
Authors to whom correspondence should be addressed.
Metals 2023, 13(4), 659; https://doi.org/10.3390/met13040659
Submission received: 2 March 2023 / Revised: 22 March 2023 / Accepted: 23 March 2023 / Published: 26 March 2023
(This article belongs to the Special Issue Metallic Functional Materials)
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Abstract

:
Increasing consumption produces a large amount of cotton textile waste, the conversion of which into porous metals used for energy purposes is of practical value. In this paper, a porous, Ni-based, hollow microtubular weave (Ni-HTW) is obtained from cotton weave by high-temperature carbonization and hydrothermal synthesis with high-temperature reduction. The Ni-based, hollow microtubules in this weave have a diameter of 5–10 μm and a wall thickness of about 1 μm, and every 15 microtubules form a loose bundle with a diameter of 150–200 μm. For improved performance, Ni(OH)2 nanosheets are further electrodeposited on the fibers’ surface of the Ni-HTW to form a nano-Ni(OH)2/Ni-HTW composite electrode with a core–shell heterostructure where Ni is the core and Ni(OH)2 the shell. The combination of hollow microtubule weave morphology and nanosheet structure results in a large specific surface area and abundant active sites, and the composite electrode shows excellent electrocatalytic performance and long-term stability for methanol oxidation (MOR) and urea oxidation (UOR). The current densities can reach 303.1 mA/cm2 and 342.5 mA/cm2 at 0.8 V, and 92.29% (MOR) and 84.41% (UOR) of the pre-cycle current densities can be maintained after 2000 consecutive cycles.

1. Introduction

Biomass materials are diverse, easily accessible, renewable, biodegradable, and have potential applications in a broad range of fields. Among these materials, agricultural and forestry waste biomass, such as eggshell membranes [1], ginkgo leaves [2,3], grapefruit peels [4], and cotton fibers [5,6], which are mainly composed of lignin and cellulose, can be carbonized to obtain functional carbon materials. Due to the special morphology and protein-rich property of biomass materials, the resultant carbon materials often have special three-dimensional structures and excellent electrical conductivity [7,8,9]. In particular, cotton has a uniquely flexible and highly porous structure, which can give the material a high specific surface area after carbonization, and its rigid three-dimensional structure can be easily modified by doping and other means to further improve its electrochemical properties. In addition, the use of cotton, a worldwide commodity, generates a large amount of textile waste every year. Exploiting the residual value of cotton through biomass carbonization also helps reduce the economic loss and environmental pollution caused by simple landfill dumping and incineration [10].
The development of renewable and clean energy sources is among the hotspots in the academic community in recent years [11,12]. Direct methanol fuel cells (DMFCs) and direct urea fuel cells (DUFCs) are considered potentially efficient methods for converting chemical energy into electrical energy thanks to the easy oxidation reaction of methanol and urea and the clean and non-polluting reaction products [13,14,15,16,17]. It is important to obtain cost-effective anode catalysts with high performance in order to drive the applications of DMFCs and DUFCs. The transition metal nickel and nickel-based compounds have been extensively studied for their outstanding cost efficiency, insolubility in alkaline solutions, and good catalytic activity for methanol and urea [18,19,20,21,22,23,24,25,26,27]. For example, Ni(OH)2 nanosheet arrays grown in situ on nickel foam have void-rich nanostructures, which facilitate the interaction with the reactants and enable the arrays to show high methanol oxidation reaction (MOR) catalytic activity in alkaline aqueous methanol solutions [28]. In addition, doping of heteroatoms such as vanadium helps Ni(OH)2 nanosheets expose more active sites, which promotes the electrocatalytic oxidation of urea [29,30]. Further, optimization of the edge structure of Ni(OH)2 nanosheets can form a large number of NiOOH species with strong adsorption catalytic properties on urea molecules, thus accelerating the catalytic kinetics of urea oxidation reaction (UOR) and allowing for the fast electrochemical process of urea oxidation [31].
When cotton weave is used as a catalyst template, the inherent three-dimensional network of the weave structure can provide rich active sites for catalytic reactions, enhance electrocatalytic performance, and give the material a degree of flexibility. The carbonized cotton fabrics coupled with a nanostructured Ni(OH)2 coating for aqueous symmetric supercapacitors prepared by Xia et al., with the use of a facile high-temperature carbonization process and electrochemical deposition treatment exhibited extremely excellent long-cycle stability, which benefited from the high flexibility and high conductivity of the substrate material [32]. By assembling hydrophobic metal nanoparticles and small molecular linkers layer by layer onto cotton fibers through chemical deposition, Ko et al. [33] obtained NiFe LDH/Ni-Cotton electrodes with excellent oxygen evolution reaction (OER) performance.
In this paper, nickel-based, hollow microtubular weave structures with large specific surface areas and more exposed active sites were prepared by a combination of hydrothermal and thermal treatment processes for which carbonized cotton cloth was used as a template. Subsequently, Ni(OH)2/Ni hollow tubular weave (Ni(OH)2/Ni-HTW) composite electrodes were prepared by an electrodeposition process. The resulting Ni(OH)2/Ni-HTW has shown improved catalytic performance for methanol and urea. The effects of the above processes on the physical phase and microstructure of the electrode and its electrooxidation performance were systematically investigated. This catalytic electrode prepared using cotton fabric as a template has a unique three-dimensional hollow microtubular weave structure without binders and conductive agents; is thin, light, and flexible; and is expected to be used as an anode for portable or small DUFCs and DMFCs, providing ideas for solving large amounts of waste cotton fabric and designing catalytic electrodes with high specific surface area.

2. Materials and Methods

2.1. Material Preparation

Chemicals: Nickel chloride hexahydrate (NiCl2·6H2O, AR, >98%, Sinopharm Chemical Reagent Co., Ltd., Hangzhou, China), potassium sodium tartrate tetrahydrate (C4H4KNaO6·4H2O, AR, >99%, Xilong Science Co., Ltd., Shantou, China), sodium hydroxide (NaOH, AR, 95%, Maclean’s Reagent Co., Ltd., Shanghai, China), sodium hypophosphite (NaH2PO2, AR, 99%, Shanghai Myriad Biochemical Technology Co., Ltd., Shanghai, China), glucose (C6H12O6·H2O, AR, Guangdong Guanghua Technology Co., Ltd., Zhuhai, China), potassium hydroxide (KOH, 85%, Sinopharm Chemical Reagent Co., Ltd.), methanol (CH3OH, AR, ≥99.5%, Sinopharm Chemical Reagent Co., Ltd., Hangzhou, China), and urea (CH4N2O, AR, 99%, Maclean’s Reagent Co., Ltd., Shanghai, China).
Typically, the cotton weave was washed with ethanol and deionized water and dried at 60 °C. It was subsequently heated to 800 °C in a tube furnace under an N2 atmosphere at a heating rate of 5 °C/min, calcined continuously for 90 min, and then cooled to room temperature. Finally, it was cleaned several times with acetone and ethanol and dried at 60 °C to obtain a carbonized cotton weave (CW).
Briefly, 2.37 g NiCl2·6H2O, 5.64 g C4H4KNaO6·4H2O, 4 g NaOH, 4 g NaH2PO2, and 0.1 g C6H12O6·H2O were dissolved in 40 mL deionized water and sonicated for 20 min to obtain a well-mixed solution. The solution was transferred to a hydrothermal autoclave reactor with an inner tank (100 mL), and then a piece of 3 cm × 2.5 cm CW was added. The reactor containing the solution and the CW was heated to 160 °C in an oven and then held for 3 h. After cooling, the sample was washed three times with deionized water and ethanol and dried in an oven at 60 °C to obtain the Ni/cotton weave (Ni/CW) electrode.
The previously prepared Ni/CW electrode was placed in a tube furnace, heated to 550 °C under air atmosphere at a heating rate of 5 °C/min, and held, followed by successive replacement of N2 for 30 min and H2 for 40 min. The Ni-based, hollow tube weave (Ni-HTW) electrode was obtained after the sample was cooled to room temperature.
With the Pt sheet as the counter electrode, Ni-HTW as the working electrode, and saturated glycerol electrode as the reference electrode, the Ni(OH)2/Ni-HTW electrode was obtained by electrodeposition in a 50 mL solution containing 25 mM Ni(NO3)2·6H2O (0.3635 g) at a constant potential of −1.0 V for 180 s. Samples prepared with different electrodeposition durations were named accordingly, e.g., Ni(OH)2/Ni-HTW-120 s and Ni(OH)2/Ni-HTW-600 s. The general preparation procedures are shown in Figure 1.

2.2. Material Characterization

Physical phase characterization was performed with an X-ray diffractometer (XRD) (Ultima IV) from Rigaku Electric Co., Ltd. (Tokyo, Japan) with a Cu target (wavelength 1.546 Å), a test range of 2θ = 10–80°, and a scan rate of 20°/min.
The morphology of the resulting samples was characterized by an FEI Nova Nano 450 field emission scanning electron microscope (SEM) with an acceleration voltage of 15 kV. The elemental composition of the samples was analyzed with the SEM companion energy spectrometer (EDS).
The microstructure and crystal structure of the samples were characterized with a field emission transmission electron microscope (TEM, Tecnai G2 F30 S-Twin, Philips-FEI, Eindhoven, The Netherlands) at an acceleration voltage of 300 kV.
An X-ray photoelectron spectrometer (XPS) from Thermo Fisher Scientific K-Alpha+ was used to analyze the valences of elements and element contents in the samples.
A Raman spectroscopy measurement (JobinYvon, Labor Raman HR-800, Longjumeau, France) was used to analyze the surface composition, a sweeping range of 200–800 cm−1.

2.3. Electrochemical Test

An electrochemical workstation (CHI 760D) with a maximum range of 0.35 A and a detection sensitivity of 0.1 A/V was used for electrochemical tests at room temperature. The test was performed on a standard three-electrode system for which the sample was the working electrode, a platinum sheet (1.5 cm × 1.5 cm) was the counter electrode, and a saturated calomel electrode (SCE, E = 0.268 V vs. NHE) was the reference electrode. The base electrolyte tested was 1 M KOH. Subsequently, 0.5 M methanol solution and 0.33 M urea solution were, respectively, added for testing the catalytic oxidation performance of methanol and urea. The effect of concentration on the electrocatalytic performance was measured through different concentrations of methanol or urea. Cyclic voltammetry (CV) measurements were taken at a potential window of −0.3–1 V with a scan rate of 50 mV/s. Before the CV tests for MOR and UOR, the electrodes were activated in 1 M KOH for 10 cycles for obtaining stability in the test curves. Electrochemical impedance spectroscopy (EIS) was performed through the application of a 0.3 V overpotential over a frequency range of 10 MHz to 100 kHz. Long-term stability testing was performed using a chrono-current technique at a constant potential of 0.8 V for 3600 s and a 2000-turn CV long-cycle test.

3. Results and Discussion

3.1. XRD and XPS

Figure 2a shows the XRD patterns of the Ni/CW, Ni-HTW, Ni(OH)2/Ni/CW, and Ni(OH)2/Ni-HTW electrodes. All samples had three diffraction peaks at 44.51°, 51.85°, and 76.37°, which are attributed to the (111), (200), and (220) crystallographic planes of nickel metal hollow microtubules (JCPDS No. 04-0850) [34]. Peaks corresponding to Ni(OH)2 were not observed, a possible result of the small thickness of Ni(OH)2 overlay formed during the electrodeposition process and its amorphous nature, which makes the diffraction peaks of Ni(OH)2 less discernible and eventually masked by the diffraction peaks of the hydrothermally formed Ni. Nevertheless, the presence of a broad and low-intensity peak at 475 cm−1 in the Raman spectrum of the Ni(OH)2/Ni-HTW electrode (Figure S1) indicates the successful preparation of amorphous Ni(OH)2 [35,36]. For the hollow microtubules formed after the heat treatment, the diffraction peaks of Ni relative to Ni/CW became significantly stronger and narrower (Tables S1 and S2), which indicates that the crystallinity of Ni in the sample was significantly increased after high temperature treatment. In contrast, the intensity of the diffraction peaks decreased after the electrodeposition process due to the encapsulation of some Ni and amorphous Ni(OH)2 over the walls of the hollow microtubules.
The surface chemical state and composition of the prepared electrodes were examined by XPS analysis. The full XPS spectra for the Ni-HTW and Ni(OH)2/Ni-HTW electrodes are shown in Figure 2b, where the intensities of peaks corresponding to Ni 2p and O 1s increased significantly after electrodeposition, indicating the presence of foreign Ni and O. Figure 2c,d show the fine spectra of Ni 2p and O 1s for Ni-HTW and Ni(OH)2/Ni-HTW electrodes. For the Ni-HTW electrode, the diffraction peaks at 852.41 and 869.51 eV are attributed to Ni0 2p3/2 and Ni0 2p1/2 [28,37], and the two pairs of diffraction peaks at 855.89/873.41 eV and 853.53/870.64 eV may be from the Ni oxides formed on the electrode surface due to the electrode’s exposure to air [28]. In the O 1s fine spectrum, the peaks at 529.18 and 531.47 eV can be attributed to the lattice oxygen of Ni oxides and the peak at 533.1 eV to loosely bound oxygen such as adsorbed O2 and H2O [38,39]. After the electrodeposition process, the peaks corresponding to metallic Ni at 852.41 eV and NiO at 853.53 and 855.89 eV disappeared for the Ni(OH)2/Ni-HTW electrode when compared with the Ni-HTW electrode. Instead, the peaks of Ni2+ 2p3/2 and Ni2+ 2p1/2 at 855.60 and 873.2 eV correspond to Ni(OH)2 [40,41] and the peak at 531.12 eV corresponds to the lattice oxygen of Ni(OH)2. These results indicate that Ni(OH)2 was successfully deposited on the surface of the Ni-HTW electrode.

3.2. Microstructure

Figure 3 shows the SEM images of the CW, Ni/CW, Ni- HTW, and Ni(OH)2/Ni-HTW electrodes obtained at each stage of the preparation. As shown in Figure 3a1–a3 for the carbon weave obtained after carbonization, loose fiber bundles of about 150–200 μm in diameter containing 15 curved or spiral carbon fibers of about 5–10 μm in diameter were observed; these bundles were eventually interwoven into a fabric-like three-dimensional network structure. It is seen in Figure 3b1–b3 that the Ni/CW electrode obtained after hydrothermal treatment is shown that the three-dimensional weave structure of the original activated carbon cloth was well preserved. Obviously, after the hydrothermal reaction process, a metallic nickel shell with a thickness of about 1 μm was formed on the surface of the carbon fiber, and the metallic nickel particles were scattered unevenly on the surface. This hydrothermal treatment can further increase the specific surface area of the material and boost the exposure of chemically active sites in the three-dimensional weave structure.
After the high-temperature calcination treatment under air and H2 atmospheres, it can be observed that carbon fiber templates were removed, leaving hollow nickel-metal microtubes with pores sized approximately 400 nm—a porous structure probably generated by the removal of glucose in the solvent during the heat treatment (Figure 3c1–c3). The hollow microtubular weave structure after heat treatment allows a further increase of the specific surface area of the sample and maintains good macroscopic flexibility. Figure 3d1–d3 and Figure S2 show that the electrodeposition process does not cause large-scale changes in the macroscopic structure of the electrode, but uniformly deposited layers of nanosheet could be observed on the inner and outer surfaces of the hollow microtubes. Combined with the above XRD and XPS analyses, these results indicate that Ni(OH)2 nanosheets were successfully deposited and prepared on the surface of the hollow microtubule electrode. It should be noted that the deposition volume increased significantly with increasing deposition time, and an excessively thick deposited layer will fill the voids between the 3D weave fibers (Figure S2), a result that may hinder the exposure of active sites and, thus, damage the material’s electrochemical properties. When the deposition time is 180 s, uniformly deposited layers attach to both the inner and outer surfaces of the hollow microtubes, which, together with the three-dimensional weave provided by the hollow microtubes, allows for a higher specific surface area of the electrode and more exposed active sites, which is favorable to the electrochemical performance.
TEM and HR-TEM characterizations also revealed the structure and phase composition of the electrodes. The TEM images in Figure 4a,b show the surface microstructure of the Ni(OH)2/Ni-HTW electrode, where a nanosheet structure was successfully deposited on the surface of the Ni-based hollow tube. As shown in the HR-TEM image, the lattice stripes at d = 0.203 nm can be attributed to the (111) crystalline plane of Ni. In the corresponding selected electron diffraction pattern, a broad and diffuse halo ring is observed, an indication of an amorphous state, which implies that the material is not highly crystallized. With the literature and the analysis of the electrodeposition process taken into consideration, it can be concluded that a large amount of amorphous material is Ni(OH)2 nanosheets, from which it is inferred that the surface microstructure after electrodeposition is a crystalline–amorphous, core–shell heterostructure in the form of Ni-Ni(OH)2 [42].

3.3. Electrochemical Performance

The specific MOR and UOR properties of the electrodes were evaluated by a three-electrode system. Figure 5a and Figure S3a show a pair of redox peaks for each electrode, a result of the interconversion between Ni2+ and Ni3+. The redox peak area increased after the heat treatment, which indicates that the hollow microtubular Ni-HTW structure obtained by the heat treatment helps increase the electrochemically active area of the electrode. What is more, the redox peak area was significantly increased after electrodeposition, indicating that the nanosheet structure of the deposited product further increases the electrochemically active area and exposes more electrocatalytically active sites. In addition, it is observed that the redox peak area of the Ni(OH)2/Ni-HTW electrode was larger than that of the Ni(OH)2/Ni/CW electrode, marking that the hollow microtubular weave structure has a larger electrochemical active area and, therefore, contributes to electrochemical performance.
Figure 5b–d and Figure S3b,c show the electrocatalytic oxidation performance of each electrode for methanol and urea. Both heat treatment and electrodeposition showed a positive role in increasing the current density of an electrode. Figure S4 shows the comparison of the electrochemical performance of Ni-HTW electrodes with a thickness of 0.24 mm and weight of ~15 mg prepared in this work and commercial Ni foam electrodes with a thickness of 1.52 mm and weight of ~37.5 mg. Ni-HTW electrodes have similar area activity and higher mass activity than those of Ni foam. It is evident that the Ni-based hollow tube weave obtained with cotton cloth template is light, thin, and material-saving and has good performance. For the Ni(OH)2/Ni-HTW electrode, the hollow microtubular weave structure obtained by high-temperature treatment was compounded with the nanostructure grown by the electrodeposition process, which has a larger specific surface area than the other three electrodes, providing more active sites with the best performance at a current density of 303.1 mA/cm2 at 0.8 V. However, it can be seen in Figure S3b,c that excessive deposition time leads to a decrease in electrochemical performance, resulting from a thick deposited layer that hinders the exposure of active sites. Furthermore, no significant methanol oxidation peaks were observed for any electrode other than the Ni/CW electrode. This is because, during the methanol oxidation process, the electrode surface conversion generates enough Ni3+ active sites to completely oxidize methanol molecules on the surface to CO2 and desorb them [43]. A similar trend was observed in the urea solution (Figure 5c), for which the current density of the Ni(OH)2/Ni-HTW electrode for the oxidation of urea reached 342.5 mA/cm2 at 0.8 V. The case in Ni/CW electrodes was different: two oxidation peaks were observed in the positive and negative sweep stages. This is caused by the desorption of CO2 and N2 products during urea oxidation. The urea is oxidized to intermediates during the positive sweep and further oxidized into CO2 and N2 during the negative sweep, resulting in the extra oxidation peak [44].
It can be seen from Figure 5e, where the CV curves of the Ni(OH)2/Ni-HTW electrode at different sweep rates in 1 M KOH are shown, that the redox peaks appeared at gradually shifted backward potentials and the peak area increased as the sweep rate increased. Figure 5f shows the plots of current densities of the oxidation and reduction peaks versus v1/2 for the three electrodes at different sweep rates in 1 M KOH, and it can be inferred that diffusion is the dominating form of mass transfer in the electrochemical reactions for all three electrodes.
Figure 5g shows the current densities of the Ni(OH)2/Ni-HTW electrodes at 0.8 V for different methanol and urea concentrations. In methanol solutions, the current density increased with increasing methanol concentration and peaked when the methanol concentration increased to 0.5 M; however, it did not change notably when the concentration increased further due to the saturation of catalytic sites for the adsorption of methanol molecules. In 1 M KOH and urea solutions (where the urea concentrations were varied), the current density peaked when the urea concentration increased to 0.33 M and then declined when the urea concentration increased further. The reason for the decline is that too many urea molecules adsorbed on the electrode surface impede the desorption of oxidation products and, therefore, reduce the catalytic oxidation efficiency. Figure 5h,i and Table S3 show the EIS curves and the relevant data of different electrodes in methanol and urea solutions, from which it can be seen that the values of Rct of the electrode decreased after heat treatment and electrodeposition and that the Ni(OH)2/Ni-HTW electrode had the smallest values of Rct. This indicates that the composite structure of the Ni(OH)2/Ni-HTW electrode, i.e., a combination of hollow microtubule weave and nanosheet, facilitates electron transfer and contributes to electrochemical performance. In addition, a comparison of the MOR and UOR performance among electrodes (shown in Table S4 [45,46,47,48,49,50,51]), shows that Ni(OH)2/ Ni-HTW in this work has a comparable or even better electrocatalytic performance.

3.4. Electrochemical Stability

Figure 6a,b show the current density at 0.8 V versus cycle times for Ni(OH)2/Ni/CW and Ni(OH)2/Ni-HTW electrodes in 1 M KOH + 0.5 M methanol solution and 1 M KOH + 0.33 M urea solution for 2000 cycles of cyclic voltammetry testing. In the 1 M KOH + 0.5 M methanol solution, the current density of the Ni(OH)2/Ni-HTW electrode at 0.8 V after 2000 cycles was 55.49% of the initial value, and reached 92.29% of that after the 2001st cycle for which a new electrolyte solution was used. Similarly, in the 1 M KOH + 0.33 M urea solution, the current density values of the sample were 54.47% and 84.41% of the initial value, respectively. The main reason for the remarkable drop in current density after about 2000 cycles is the consumption of methanol or urea in the solution [52,53]. The facts that the current density recovers and remains at a high level after the test solution is updated and that the physical phase and surface nanosheet structure on the electrode is well preserved after 2000 cycles (Figure S5) indicate that the Ni(OH)2/Ni-HTW electrode has an excellent long-cycle life for the electrocatalytic oxidation of methanol and urea. Figure 6c,d show the 3600 s chrono-current curves of Ni(OH)2/Ni/CW and Ni(OH)2/Ni-HTW electrodes at 0.8 V. It can be seen that the curves flattened out for both methanol and urea solutions, an indication of good electrochemical stability of the electrodes. We also performed reproducibility experiments with cyclic voltammetry tests of refabricated samples; the electrochemical performance of those resulting samples was good and consistent, indicating that the fabricating process and the properties of the samples are reproducible (Figure S6).

4. Conclusions

In this paper, Ni/CW, Ni-HTW, and Ni(OH)2/Ni-HTW composite catalytic electrodes with a hollow microtubular weave structure were successfully prepared from cotton weave templates by the combination of heat treatment, hydrothermal method, and constant potential electrodeposition. The transition metal Ni layer has good electrical conductivity, and the unique three-dimensional hollow microtubule weave ensures a stable, thin, and flexible structure and allows for a large specific surface area. For the Ni(OH)2/Ni-HTW electrode, the heterogeneous structure of the electrochemically deposited crystalline Ni core-amorphous Ni(OH)2 shell further increases the specific surface area and, thus, makes an extra contribution to electrochemical performance. The current density of the electrode can reach 303.1 mA/cm2 (MOR) and 342.5 mA/cm2 (UOR) at 0.8 V potential. In addition, 92.29% (MOR) and 84.41% (UOR) of the pre-cycle current density of the electrode can be maintained after 2000 CV cycles, providing evidence of good long-term stability. The electrode is also free of binder and conductive agents, which helps increase the catalytic efficiency and long-lasting stability of anode catalysts in fuel cells. As such, the electrode prepared in this paper is intended for use in portable or small-sized DUFC and DMFC anodes and is also a potential catalytic electrode for the electrooxidation of alcohols and small molecules.

Supplementary Materials

The following supporting information can be downloaded at https://www.mdpi.com/article/10.3390/met13040659/s1: Figure S1: Raman patterns of Ni-HTW and Ni(OH)2/Ni-HTW electrodes. Figure S2: SEM images of Ni-HTW electrodes electrodeposited at different times: (a1–3) Ni(OH)2/Ni-HTW-120 s and (b1–3) Ni(OH)2/Ni-HTW-600 s. Figure S3: Cyclic voltammetric curves of Ni(OH)2/Ni-HTW electrode electrodeposited at different at different times (0, 120s, 180s, and 600s) in different solutions: (a) 1 M KOH, (b) 1 M KOH + 0.5 M methanol, and (c) 1 M KOH + 0.33 M urea. Figure S4: Cyclic voltammetric curves of Ni-HTW electrode and Ni foam electrode in different solutions: (a,d) 1 M KOH, (b,e) 1 M KOH + 0.5 M methanol, and (c,f) 1 M KOH + 0.33 M urea. (a–c) Area activity and (d–f) mass activity. Figure S5: SEM images of Ni(OH)2/Ni-HTW electrode after 2000 cycles in different solutions: (a) 1 M KOH + 0.5 M methanol and (b) 1 M KOH + 0.33 M urea and (c) XRD patterns of those samples. Figure S6: CV curves of three sets of Ni(OH)2/Ni-HTW electrodes in (a) 1 M KOH solution, (b) 1 M KOH + 0.5 M methanol solution, and (c) 1 M KOH + 0.33 M urea solution (scan rate: 50 mV/s). Table S1: FWHW(full width at half maximum) values of XRD diffraction peaks for different electrodes. Table S2. Intensity ratios of XRD diffraction peaks for different electrodes. Table S3: The data of electrode impedance spectrum fitting results. Table S4: Comparison of electrooxidation performance of several anode catalyst electrodes for methanol and urea.

Author Contributions

Conceptualization, validation, investigation, data curation, and writing—original draft, G.H.; validation, investigation, data curation, consult the literature, and writing, J.W.; writing—review and editing and supervision, Q.C.; writing—review and editing and supervision, Y.T.; conceptualization, resources, writing—review and editing, supervision, project administration, and funding acquisition, J.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The authors would like to thank the National Natural Science Foundation of China (grant numbers 5187120, 51101140, and 52002315) and the Natural Science Foundation of Zhejiang Province of China (grant number LY23E010008, LY16E010004) for financially supporting this research.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Illustration of the preparation of the catalyst electrode.
Figure 1. Illustration of the preparation of the catalyst electrode.
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Figure 2. (a) XRD patterns of Ni/CW, Ni-HTW, Ni(OH)2/Ni/CW, and Ni(OH)2/Ni-HTW electrodes. (b) XPS full spectra of Ni-HTW and Ni(OH)2/Ni-HTW electrodes. Fine spectra of (c) Ni 2p and (d) O 1s for Ni-HTW and Ni(OH)2/Ni-HTW electrodes.
Figure 2. (a) XRD patterns of Ni/CW, Ni-HTW, Ni(OH)2/Ni/CW, and Ni(OH)2/Ni-HTW electrodes. (b) XPS full spectra of Ni-HTW and Ni(OH)2/Ni-HTW electrodes. Fine spectra of (c) Ni 2p and (d) O 1s for Ni-HTW and Ni(OH)2/Ni-HTW electrodes.
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Figure 3. SEM images of (a1–a3) CW, (b1–b3) Ni/CW, (c1–c3) Ni-HTW, and (d1–d3) Ni(OH)2/Ni-HTW electrodes.
Figure 3. SEM images of (a1–a3) CW, (b1–b3) Ni/CW, (c1–c3) Ni-HTW, and (d1–d3) Ni(OH)2/Ni-HTW electrodes.
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Figure 4. (a) TEM and (b) HR-TEM images of Ni(OH)2/Ni-HTW electrode.
Figure 4. (a) TEM and (b) HR-TEM images of Ni(OH)2/Ni-HTW electrode.
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Figure 5. CV curves of Ni/CW, Ni-HTW, Ni(OH)2/Ni/CW, and Ni(OH)2/Ni-HTW electrodes in (a) 1 M KOH solution, (b) 1 M KOH + 0.5 M methanol solution, (c) 1 M KOH + 0.33 M urea solution at a scan rate of 50 mV/s. (d) Current density at 0.8 V for different electrodes in different solutions. (e) CV curves of Ni(OH)2/Ni-HTW electrode in 1 M KOH at different scan rates. (f) Redox peak current densities versus v1/2 for Ni/CW, Ni-HTW, and Ni(OH)2/Ni-HTW electrodes at different scan rates in 1 M KOH. (g) Current density at 0.8 V for Ni(OH)2/Ni-HTW electrode in methanol and urea solutions of different concentrations. EIS plots of different electrodes in (h) 1 M KOH + 0.5 M methanol solution and (i) 1 M KOH + 0.33 M urea solution.
Figure 5. CV curves of Ni/CW, Ni-HTW, Ni(OH)2/Ni/CW, and Ni(OH)2/Ni-HTW electrodes in (a) 1 M KOH solution, (b) 1 M KOH + 0.5 M methanol solution, (c) 1 M KOH + 0.33 M urea solution at a scan rate of 50 mV/s. (d) Current density at 0.8 V for different electrodes in different solutions. (e) CV curves of Ni(OH)2/Ni-HTW electrode in 1 M KOH at different scan rates. (f) Redox peak current densities versus v1/2 for Ni/CW, Ni-HTW, and Ni(OH)2/Ni-HTW electrodes at different scan rates in 1 M KOH. (g) Current density at 0.8 V for Ni(OH)2/Ni-HTW electrode in methanol and urea solutions of different concentrations. EIS plots of different electrodes in (h) 1 M KOH + 0.5 M methanol solution and (i) 1 M KOH + 0.33 M urea solution.
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Figure 6. Current densities at 0.8 V for Ni(OH)2/Ni/CW, Ni(OH)2/Ni-HTW electrodes in (a) 1 M KOH + 0.5 M methanol solution and (b) 1 M KOH + 0.33 M urea solution for 2000 cycles. Chrono-current test for Ni(OH)2/Ni/CW and Ni(OH)2/Ni-HTW electrodes in (c) 1 M KOH + 0.5 M methanol solution and (d) 1 M KOH + 0.33 M urea solution at 0.8 V for 3600 s.
Figure 6. Current densities at 0.8 V for Ni(OH)2/Ni/CW, Ni(OH)2/Ni-HTW electrodes in (a) 1 M KOH + 0.5 M methanol solution and (b) 1 M KOH + 0.33 M urea solution for 2000 cycles. Chrono-current test for Ni(OH)2/Ni/CW and Ni(OH)2/Ni-HTW electrodes in (c) 1 M KOH + 0.5 M methanol solution and (d) 1 M KOH + 0.33 M urea solution at 0.8 V for 3600 s.
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Hou, G.; Wei, J.; Chen, Q.; Zhang, J.; Tang, Y. Electrooxidation Performance of a Cotton-Cloth-Derived, Ni-Based, Hollow Microtubular Weave Catalytic Electrode for Methanol and Urea. Metals 2023, 13, 659. https://doi.org/10.3390/met13040659

AMA Style

Hou G, Wei J, Chen Q, Zhang J, Tang Y. Electrooxidation Performance of a Cotton-Cloth-Derived, Ni-Based, Hollow Microtubular Weave Catalytic Electrode for Methanol and Urea. Metals. 2023; 13(4):659. https://doi.org/10.3390/met13040659

Chicago/Turabian Style

Hou, Guangya, Jiaxuan Wei, Qiang Chen, Jianli Zhang, and Yiping Tang. 2023. "Electrooxidation Performance of a Cotton-Cloth-Derived, Ni-Based, Hollow Microtubular Weave Catalytic Electrode for Methanol and Urea" Metals 13, no. 4: 659. https://doi.org/10.3390/met13040659

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