CoNi₂O₄ Coated on Activated Carbon Wheat Husk (ACWH) as a Novel Nano-Electrocatalyst for Methanol and Ethanol Electro-Oxidation

Abstract

In this paper, for the first time, a CoNi₂O₄ nanocatalyst coated on the surface of activated carbon wheat husk (ACWH) was synthesized in the form of CoNi₂O₄/ACWH through a hydrothermal process. The electrocatalytic activity of this catalyst was evaluated using methanol oxidation reaction (MOR) and ethanol oxidation reaction (EOR) processes for use in anodes of alcohol fuel cells. Adding ACWH, as a cheap carbon biomass with an excellent active surface area, improves the performance of the catalyst in the oxidation of alcohols. The current density of CoNi₂O₄/ACWH in the MOR process is 160 mA/cm² at an optimal methanol concentration of 2 M; this oxidation current density in the EOR process and at a concentration of 1.5 M ethanol is 150 mA/cm². The stability of CoNi₂O₄/ACWH in MOR and EOR processes, after 1000 consecutive CV cycles, is 98.6% and 94.6%, respectively.

1. Introduction

Today, energy demand is increasing continuously in the advanced and developing world [1]. Limited sources of fossil fuels not only lead to global energy and fuel shortages, but also endanger human lives due to the enhanced amounts of greenhouse gases and consequences of environmental change [2,3]. A low-cost and environmentally friendly fuel could be used as a suitable alternative to fossil fuels to overcome this problem. There is no doubt that using sustainable and renewable fuel sources, such as sunlight, wind power, geothermal power, etc., is a wise solution for overcoming the energy crisis across the world [4,5].

Reviewing the yearly statistics and data of developed countries on their use of renewable fuel sources indicates that countries allocate enormous budgets for this purpose yearly [6,7]. Although almost all countries have access to renewable and clean energy sources, the production and industrialization of energy transformation to be used in this field need special technical and technological knowledge [8,9]. As an example, a specific photovoltaic cell should be designed and constructed in each location to exploit electric energy from sunlight with the minimum cost and maximum efficiency; the same is true for other cases like wind turbines, devices, and equipment for using sea waves and geothermal energy.

In recent years, with the considerable advancements in nanoscience in energy storage and production, we have witnessed the industrialization of some equipment in this field, such as supercapacitors, various electrochemical batteries, and fuel cells, which are very appealing to customers due to being portable and having small sizes [10,11]. Furthermore, extensive equipment applications in different fields, including pharmaceutical, military, etc., have made developed countries allocate a suitable budget to this field.

Among the mentioned equipment, supercapacitors and electrochemical batteries are used for energy storage [12,13], and fuel cells directly produce energy by transforming chemical energy into electrical power [14]. One of the advantages of fuel cells is the production of water as a side product [15]. Nowadays, fuel cells have found a special place in energy production. In addition to being clean and efficient, they can be used in any application that needs energy since they can produce 1 W to 10 MW power [16]. Fuel cells are divided into high- and low-temperature cells; alcoholic fuel cells comprise low-temperature cells [17]. Among the different alcohols, methanol and ethanol are significant sources of energy production due to their high energy density, availability, and easy storage and transportation [18,19,20].

Alcoholic fuel cells consist of three main parts: the anode, cathode, and membrane [21,22]. Fuel electrochemical oxidation occurs on the anode and oxygen reduction occurs on the cathode [23]. One of the parameters that determines the effectiveness of cell efficiency is the structure of the anode electrode, which is responsible for the alcohol oxidation process. Although platinum-based electrocatalysts [24,25], composites, and composite-based hybrids [26,27,28,29,30] have the maximum catalytic activity in alcohol oxidation, their high cost and limited availability are among the challenges of the practical utilization of fuel cells [31]. Other metals, except platinum, with electrocatalytic properties, are ruthenium, palladium, rubidium, and gold [32,33,34].

Examples of catalyst applications, especially metal oxide-based catalysts, include the use of catalysts based on nickel and cobalt in oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) processes. This broad application of catalysts has attracted much attention recently and has been highly welcomed by researchers [35,36,37]. The electrode material is a crucial factor in methanol fuel cells due to their slow reaction kinetics. The structure of the electrode material plays an essential role in its electrocatalytic activity. This material should be electrically conductive, with a suitable effective surface area and adequate electrocatalytic activity [38,39].

Metal oxides were studied as electrodes and catalyst materials that can be theoretically applied to electrochemical processes. However, these materials have low electric conductivity and effective surface area. Various methods are reported to improve the performance of metal oxides. Doping or compositing these materials with efficient and precious catalysts like platinum and palladium, or hybridizing metal oxides with surface-active and conductive materials such as MOF, conductive polymers, etc., are among the proposed methods [40,41,42,43].

One of the most attractive, cost-effective, and efficient methods of improving the electrocatalytic activity of metal oxides is using carbon in their structures. Various types of carbonaceous material, such as graphene oxide nanotubes and hollow carbon nanospheres (HCNs), facilitate electrochemical processes by increasing the their conductivity and improving their catalytic activity [44]. Among the metal oxides, due to the synergistic effect of two metal oxides in one structure in the form of AB₂O₄, it seems that binary intermediate metal oxides (BTMOs) might be excellent candidates to be used as catalysts in fuel cell structures like NiCo₂O₄/CuO-C [45], MnNi₂O₄-MWCNTs [8], and MnCo₂O₄/NiCo₂O₄/rGO [11].

Among the carbonaceous materials, using the carbon obtained from industrial and agricultural wastes, such as active and conductive carbon, in catalyst structures can be very interesting for scientists. The production processes of these carbons are almost environmentally friendly processes. These carbonaceous materials have attracted much attention due to their availability and low-cost synthesis. Banana fiber [46], rice husk [47], and natural wood [48] are some of these carbons. In this study, wheat husk wastes are used as the source of biomass carbon production, and their combination with CoNi₂O₄, as a low-cost and efficient binary metal oxide nanocatalyst, is used in methanol and ethanol oxidation in alcoholic fuel cell anodes. One of the reasons for using wheat husk for carbon production is its abundance and availability, as well as the ease and low cost of preparing biomass carbon from wheat husk waste compared to other materials. According to studies, this type of catalyst, which includes binary metal oxide and biomass carbon, is not used in fuel cell anodes. EIS, LSV, CV, and chronoamperometry analyses of the catalysts have revealed that CoNi₂O₄/ACWH and CoNi₂O₄ have the ability to oxidize methanol and ethanol. Although both of these catalysts are suitable candidates to be used in alcoholic fuel cell anodes, the presence of ACWH in the CoNi2O4 structure improves its electrocatalytic properties due to the active surface and electric conductivity of this carbon.

2. Materials and Methods

All the materials used were Merck Company products with 99% purity. X-ray diffraction (XRD) analysis was performed using an XRD Xpert Pro Panalytical device (Almelo, The Netherlands). FE-SEM ZEISS equipment (GeminiSEM 300, Carl Zeiss, Oberkochen, Germany) was used for FESEM imaging. Raman spectroscopy was performed using Confocal Microscopy (LabRAM ARAMIS, Horiba Jobin Yvon, Longjumeau, France). All the electrochemical analyses were carried out using Potentiostat/Galvanostat Autolab 302 N (Metrohm, Herisau, Switzerland) with a three-electrode system.

2.1. Synthesis of Carbon Biomass

A total of 50 g of pure wheat husk was poured into 450 mL of 1 M nitric acid (HNO₃) and left for one day under stirring; then, it was dried at 60 °C. The product was poured into 500 mL of 1 M NaOH and stirred for 24 h; then, it was dried at 65 °C. The dried product was put in a 400 °C furnace for 3 h. The obtained black material was mixed with 300 mL of 1 M NaOH and stirred for 30 min. The product was dried at 70 °C. The obtained material was mixed with KOH in a 1:4 ratio (WHC/KOH) and put in a CVD at 800 °C. After an acid wash, the powder reached pH 7, and the activated carbon prepared from the wheat husk (ACWH) was ready.

2.2. Synthesis of CoNi₂O₄ and CoNi₂O₄/ACWH

The hydrothermal method was used to synthesize CoNi₂O₄ and CoNi₂O₄/ACWH. For this purpose, 0.581 g nickel nitrate hexahydrate and 0.291 g cobalt hexahydrate molecules were poured into 15 mL of deionized water and stirred. Then, 3.60 g urea was added to the solution and stirred for 45 min. The resultant homogeneous solutions were placed in a 40 mL autoclave for 12 h at 90 °C. Then, the dilute product was washed with water and ethanol and dried at 80 °C. The obtained powder was annealed at 400 °C.

The CoNi₂O₄/ACWH was prepared using a similar method, with the difference that nickel and cobalt nitrate hexahydrates were added to 10 mL deionized water and mixed with urea. Then, 0.001 g of ACWH that was sonicated in 10 mL water was added to the solution and placed in an autoclave with a similar temperature and conditions to those of the synthesis process; the preparation procedure for the powdered product was then followed. The formation mechanism of the CoNi₂O₄ nanocatalyst was as follows:

$$3\mathrm{C}\mathrm{o}\left(\mathrm{N}{\mathrm{O}}_3\right){}_2+6\mathrm{N}\mathrm{i}\left(\mathrm{N}{\mathrm{O}}_3\right){}_2+14\mathrm{C}{\mathrm{H}}_4{\mathrm{N}}_2\mathrm{O}\stackrel{}{\to }3\mathrm{C}\mathrm{o}\mathrm{N}{\mathrm{i}}_2{\mathrm{O}}_4+23{\mathrm{N}}_2+14\mathrm{C}{\mathrm{O}}_2+28{\mathrm{H}}_2\mathrm{O}$$

2.3. Preparation of Electrode

A setup was prepared to investigate the catalysts’ behavior in methanol and ethanol oxidation reactions to be used in alcoholic fuel cell anodes. For this purpose, some slurries containing 0.4 g each of CoNi₂O₄ and CoNi₂O₄/ACWH catalyst in 0.5 mL Naphion 5% and 0.5 mL isopropyl alcohol were separately prepared with ultrasonication for 30 min to disperse the particles completely. An amount of 0.6 μL of this slurry was poured on the surface of the GCE electrode using a micropipette. The electrochemical tests were performed after drying. The electrode system used in this study consisted of a glass carbon electrode (GCE) that was modified by the catalysts to be used as the working electrode. The auxiliary and reference electrodes were a platinum rod with a 0.5 mm diameter and a Ag/AgCl electrode, respectively.

3. Results

3.1. X-ray Diffraction (XRD)

To study the structure and confirm the successful synthesis of nanocatalysts, their crystalline structure was investigated via X-ray diffraction analysis. Figure 1 shows the X-ray diffraction of nanocatalysts and ACWH. As seen in this figure, two intensive peaks of CoNi₂O₄ and CoNi₂O₄/ACWH appeared at about 38 and 44 degrees. Also, one peak was observed at 29 degrees in the ACWH diffraction pattern. The diffraction patterns of CoNi₂O₄ and CoNi₂O₄/ACWH comply with JCPDS No. 00.040.1191. The characteristic peaks of CoNi₂O₄ and CoNi₂O₄/ACWH were occurred at 18.801, 31.005, 38.269, 44.508, 55.187, 58.930, 64.779, and 76.707 degrees, and are related to the Miller planes of (111), (220), (222), (400), (422), (511), (440), and (533), respectively. A comparison of the two diffraction patterns of CoNi₂O₄ and CoNi₂O₄/ACWH indicates that CoNi₂O₄/ACWH peaks have lower intensity at the (220), (222), and (400) Miller planes, which is due to the presence of ACWH in the crystal structure.

3.2. FE-SEM

The surface morphology of the prepared nanocatalysts was investigated via Field Emission Scanning Electron Microscopy (FESEM) images. Figure 2 shows the FESEM images of CoNi₂O₄, CoNi₂O₄/ACWH, and ACWH nanoparticles. Figure 2a–c show images from the CoNi₂O₄ nanocatalyst which were taken at 100 nm, 200 nm, and 1 μm, respectively. A surface with contiguous spheres with relatively homogeneous sizes is observed in the picture. Figure 2d–f show images of the CoNi₂O₄/ACWH nanocatalyst; as seen, there is a smooth surface consisting of separate spheres that are attached to nanorods, and the surface particles are at a long distance from each other compared to the nanocatalysts. Figure 2g–i present images taken from ACWH that show a porous surface consisting of nanoparticles and nanosheets.

3.3. EDX-Mapping

EDX-mapping elemental analysis was used to identify the composition of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts. As seen in Figure 3 and Figure 4, Co, O, Ni, and C elements are present in the synthesized nanostructures. According to mapping images of CoNi₂O₄ nanocatalyst in Figure 3a, the presence of Co, Ni, and O elements is confirmed. Besides, the mapping images of CoNi₂O₄/ACWH in Figure 3b indicate the presence and uniform distribution of Co, Ni, O, and C elements.

3.4. Raman Spectroscopy

Raman spectroscopy is used for the structural study of the nanocatalysts to confirm the presence of carbon in their structures. Figure 5 presents the Raman spectra of the nanocatalysts. Two peaks with high resolution are observed in the Raman spectra of the nanocatalyst containing ACWH that are related to the D and G bonds of carbon. Bond D in the 1300 to 1400 (cm⁻¹) range belongs to carbonaceous atoms that experienced irregularity. Bond G at about 1600 (cm⁻¹) is related to the E₂g state of the graphite structure. These two peaks confirm the carbonaceous and graphene structure of ACWH, which are evident in the Raman spectra of the CoNi₂O₄/ACWH nanocatalyst and indicate the presence of ACWH in this structure.

3.5. BET Method

The BET method is used to check the specific surface area and porosity of samples based on nitrogen adsorption and desorption isotherms at a nitrogen boiling temperature of 77 degrees Kelvin. Figure 6 shows the adsorption–desorption isotherms of the CoNi₂O₄ and CoNi₂O₄/ACWH samples. According to the AUPAC isotherms, the CoNi₂O₄ and CoNi₂O₄/ACWH isotherms are of type IV, which shows that the samples have a mesoporous structure and the relationship between the surface of the sample and the adsorbent is relatively strong. Investigating the CoNi₂O₄ and CoNi₂O₄/ACWH isotherms indicated that adding ACWH to CoNi₂O₄ increases the specific surface area. Based on the results obtained from the analysis, the specific surface area of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts is 69.686 and 75.089 m²/g, respectively. These results confirm the claim that by increasing the effective level of the catalyst, the performance of the catalyst improves, and these results can be attributed to the addition of ACWH to the CoNi₂O₄ catalyst.

3.6. Electrochemical Studies

3.6.1. Investigation of CoNi₂O₄ and CoNi₂O₄/ACWH Nanocatalysts’ Abilities in Methanol Oxidation Process

As far as we know, metal oxides exhibit maximum efficiency in alcohol oxidation processes in alkaline environments. A 0.5 M KOH solution was prepared, and cyclic voltammetry (CV) analysis was carried out on the modified electrodes with CoNi₂O₄ and CoNi₂O₄/ACWH, at a scan rate of 20 mV/s. The results are presented in Figure 7a. As seen, CoNi₂O₄/ACWH has a higher capacitive current density due to its higher electrical conductivity and more electrochemically active surface compared to CoNi₂O₄. The reason for this superiority is the presence of biomass carbon from the wheat husk in the CoNi₂O₄/ACWH structure. The results of the Electrochemical Impedance Spectroscopy (EIS) analysis of CoNi₂O₄ and CoNi₂O₄/ACWH catalysts, in the frequency range of 1 MHz to 10 KHz, are presented in Figure 7b. The resistance against charge transfer for CoNi₂O₄ and CoNi₂O₄/ACWH is 30.5 and 18.2 Ω, respectively, which shows the positive effect of carbon biomass present in the catalysts’ structure.

To investigate the behavior of CoNi₂O₄ and CoNi₂O₄/ACWH in the methanol oxidation process, some solutions were prepared containing 0.5 M KOH and various methanol concentrations (0.5–3 M). As can be seen in the CV analysis of the two nanocatalysts, in the potential range of 0–0.8 V and at a scan rate of 20 mV/s, enhanced methanol concentration causes an increase in the current density of the electrode that was modified with CoNi₂O₄. This enhancement in the current density continues up to a 1.5 M concentration, a further increase in the concentration causes a reduction in current density, and the oxidation peak has a descending trend. The same trend is observed in CoNi₂O₄/ACWH; the current density of the methanol oxidation peak increases up to a methanol concentration of 2 M, but lower current density is observed at higher concentrations. It seems that the nanocatalyst surface becomes saturated at a concentration higher than the optimum concentration, and electrolyte/methanol cannot penetrate easily to the catalyst core; this causes a reduction in current density. The behavior of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts at different methanol concentrations is presented in Figure 8a,b.

The CV analysis was carried out at different scan rates of 20 to 100 mv/s to study the methanol oxidation mechanism of CoNi₂O₄ and CoNi₂O₄/ACWH. An enhanced current density of the oxidative peak was observed for CoNi₂O₄ and CoNi₂O₄/ACWH after increasing the scan rate. This ascending trend is presented in Figure 8c,d, respectively. The square of the scan rate as a function of peak current density (Jp) is shown in the insets of these figures. The linear relationship between these two parameters, with R² = 0.996 and R² = 0.991, indicates that diffusion is the controlling mechanism of methanol oxidation by CoNi₂O₄ and CoNi₂O₄/ACWH. The proposed six-electron mechanism for these nanocatalysts is in the following form [49]:

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{\mathrm{C}\mathrm{H}}_3\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+4{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{\left(\mathrm{C}\mathrm{O}\right)}_{\mathrm{a}\mathrm{d}\mathrm{s}}+4{\mathrm{H}}_2\mathrm{O}+4{\mathrm{e}}^{-}$$

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{e}}^{-}$$

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{\mathrm{C}\mathrm{O}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+{\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}^{-}$$

The stability of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts in methanol oxidation can be assessed via CV. The CoNi₂O₄ nanocatalyst shows 95.8% stability after 1000 consecutive CV cycles at 0.5 KOH/1.5 methanol solution at a scanning rate of 100 mV/s (Figure 9a), while CoNi₂O₄/ACWH shows 98.6% stability after the same number of cycles and the same scan rate, but in 0.5 KOH/2 methanol solution (Figure 9b).

3.6.2. Investigation of CoNi₂O₄ and CoNi₂O₄/ACWH Nanocatalyst Capabilities in Ethanol Oxidation Process for Use in Ethanol Fuel Cells

To investigate the capability of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts in the oxidation of ethanol in an alkaline environment, various amounts of ethanol were added to KOH 0.5 M, and CV analysis was performed at a 20 mV/s scan rate and in the potential range of 0–0.8 V. Figure 10a shows CoNi₂O₄ nanocatalyst behavior and Figure 10b shows CoNi₂O₄/ACWH nanocatalyst behavior at different ethanol concentrations. The surfaces of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts become saturated at 1 and 1.5 M ethanol concentrations, and the oxidative peak of current density undergoes a descending trend at higher ethanol concentrations. Here, it also seems that the catalyst surface became saturated at concentrations higher than the critical concentration, and ethanol could not penetrate the depth of the catalyst anymore.

The methanol oxidation mechanism of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts was determined via a CV test at the optimum ethanol concentration and a scan rate of 10–100 mV/s. Figure 10c,d show the behavior of CoNi₂O₄ and CoNi₂O₄/ ACWH nanocatalysts at different scan rates. An increase in anodic current density is observed with increasingscan rate.

The squared scan rate as a function of the peak current density (Jp) is presented in the insets of these figures. The linear relationship of these two parameters with R² = 0.994 and R² = 0.996 for CoNi₂O₄ and CoNi₂O₄/ACWH, respectively, implies that diffusion is the controlling mechanism in the EOR process.

The proposed mechanism for ethanol oxidation can be expressed in the following form:

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{e}}^{-}$$

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2\mathrm{O}\mathrm{H}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{(\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2\mathrm{O}\mathrm{H})}_{\mathrm{a}\mathrm{d}\mathrm{s}}$$

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{(\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{H}}_2\mathrm{O}\mathrm{H})}_{\mathrm{a}\mathrm{d}\mathrm{s}}+3{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{(\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O})}_{\mathrm{a}\mathrm{d}\mathrm{s}}{+3\mathrm{H}}_2\mathrm{O}+3{\mathrm{e}}^{-}$$

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{(\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O})}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{\mathrm{O}\mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{(\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H})}_{\mathrm{a}\mathrm{d}\mathrm{s}}+\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}$$

$$\mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}-{{(\mathrm{C}\mathrm{H}}_3\mathrm{C}\mathrm{O}\mathrm{O}\mathrm{H})}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{O}\mathrm{H}}^{-}\to \mathrm{C}\mathrm{a}\mathrm{t}\mathrm{a}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{t}+{{\mathrm{C}\mathrm{H}}_3{\mathrm{C}\mathrm{O}\mathrm{O}}^{-}}_{}{+\mathrm{H}}_2\mathrm{O}$$

The stability of CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts in the EOR process was evaluated through 1000 consecutive CV analyses at the optimum ethanol concentration and a scan rate of 60 mV/s. Figure 11a,b present the cyclic stability of CoNi₂O₄ and CoNi₂O₄/ACWH. After these cycles, a stability of 91.5% for CoNi₂O₄ and 94.6% for CoNi₂O₄/ACWH is obtained.

In

Table 1. Comparison of MOR and EOR performance of CoNi₂O₄/ACWH nanocatalyst with other recent research.

Electrocatalyst	Electrolyte Composition	Peak Potential (V)	Current Density (mA cm−2)	Scan Rate (mV/s)	Reference
CoNi2O4/ACWH	2 M methanol/0.5 M KOH	0.58	160	60	This work
CoNi2O4/ACWH	1.5 M ethanol/0.5 M KOH	0.63	150	60	This work
ZrO2/NiO/rGO	0.7 M methanol/0.5 M KOH	0.52	26.6	20	[10]
ZrO2/NiO/rGO	0.5 M ethanol/0.5 M KOH	0.52	17.3	20	[10]
Mn3O4-CeO2-rGO	0.8 M methanol/1 M KOH	0.51	17.7	90	[49]
Ni3S4–NiS-rGO	0.7 M methanol/1 M KOH	0.54	55	80	[50]
Ni3S4–NiS-rGO	0.5 M ethanol/1 M KOH	0.59	11	60	[50]
NiCo2O4	0.5 M methanol/1 M KOH	0.7	129	10	[51]
NiCo2O4/carbon xerogel	0.5 M methanol/0.5 M KOH	0.29	98	50	[52]
3D Ni/NiO/RG	1 M methanol/1 M KOH	About 0.6	79.21	50	[53]
NiO 2.5CeO2–NiO	1 M methanol/1 M KOH	About 0.55	159.62	50	[54]
Cu-Ni/CuO-NiO/GNs	0.5 M methanol/1 M KOH	0.7	150	50	[55]
NiO NS@NW/NF	0.5 M methanol/1 M KOH	1.62	89	10	[56]
PEDOT:PSS/MnO2/rGO	0.5 M methanol/1 M NaOH	0.32	56.38	50	[57]

, we compare the performance and efficiency of CoNi₂O₄/ACWH nanocatalyst in the MOR and EOR processes with other recent research.

4. Conclusions

In this research study, CoNi₂O₄ and CoNi₂O₄/ACWH nanocatalysts were synthesized and their capabilities in the oxidation of methanol and ethanol, to be used in alcohol fuel cell anodes, were investigated. The CoNi₂O₄/ACWH nanocatalyst was more efficient than the CoNi₂O₄ nanocatalyst, and its higher efficiency was due to the presence of synthesized biomass from the wheat husk in the CoNi₂O₄/ACWH structure. The synthesized carbon with a large active surface and acceptable electric conductivity provides a higher catalytic surface for methanol and ethanol and facilitates the oxidation of these two alcohols by the catalyst. In methanol oxidation at a scan rate of 100 mV/s, CoNi₂O₄/ACWH had an oxidative current density of 160 mA/cm² at the excessive potential of 0.58 V, and its stability was 98.6% after 1000 consecutive CV cycles. CoNi₂O₄/ ACWH was also a cost-effective and stable catalyst for ethanol oxidation; its stability was 94.6% after 1000 consecutive CV cycles. The oxidative current density for this nanocatalyst was 150 mA/cm² at ethanol oxidation with a peak voltage of 0.63 V. This is the first time that the proposed nanocatalysts have been assessed in MOR and EOR processes, and the results are promising regarding their application in alcoholic fuel cells.