# Electrodeposition of Hierarchical Nanosheet of NiCo2O4/CC as Highly Active and Stable Electrode for Water Oxidation

Authors: Zahra Albu; Nawal Al Abass; Bandar AlOtaibi
Journal: International Journal of Energy Research
 (2023)

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## Abstract

Direct growth of NiCo2O4 on 3D textile and wavy carbon cloth (CC) using facile and low-cost electrodeposition results in the creation of distinct morphologies. The homogeneity and conformality of the coating of NiCo2O4 on CC can be readily controlled by adjusting or optimizing the electrodeposition parameters as shown by SEM and TEM images. Various loadings of NiCo2O4 were prepared under different conditions. The electrochemical characterization measurements for the oxygen evolution reaction (OER) show that hierarchical NiCo2O4 on CC has a low onset potential at 270 mV, low overpotential of 290 mV at 10 mA/cm2, and low electron transfer resistance across the electrode and electrolyte. NiCo2O4/CC electrode showed excellent stability when evaluated for ~500 hours. This synthesis method holds high potential for large-scale electrode fabrication on carbon substrates. The method is comprised of controllable optimizing parameters that can readily affect the materials’ catalytic performance.


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## Body

## 1. Introduction

It is envisioned that the artificial conversion of solar energy to clean fuels can achieve net-zero emission targets [1]. Clean hydrogen generation is considered a promising route for a green economy. Green hydrogen produced from renewable resources such as photovoltaic cells, wind turbines, or hydro energy coupled with electrolyzers to split water offers an attractive route for current and forthcoming sustainable energy transitions. However, water electrolysis has been intensively studied with limited success in producing green hydrogen at a very competitive price point [2–4].The major bottleneck for widely adopting such green technology is finding cost-effective electrocatalysts that can efficiently oxidize water and produce hydrogen at a practical cost. That is, the high price of green hydrogen compared to other types of hydrogen (e.g., grey hydrogen produced from fossil fuels) is mainly due to the high-priced materials used in the electrolyzer cells. Specifically, precious metals, such as Ru and Ir oxides, are the best-known catalysts for oxygen evolution reaction (OER) [5]. The rationale for using these metals is to overcome the sluggish kinetics of the water oxidation process compared to water reduction. That is, four electrons are involved in the OER as opposed to the two-electron process for hydrogen reduction. Furthermore, the electrocatalytic materials can be synthesized using different techniques such as physical, mechanical, and solution-based methods [6, 7]. The solution-based process in which the powder is converted to a film or deposited directly on the desired electrode substrate is attractive due to its low cost and scalability [8–10]. However, the synthesized powders usually convert into a film using a polymeric binder and conductive agent. This kind of processing imposes limitations on the final film properties, including conductivity and conformality of the coating [11]. In addition, other methods such as hydrothermal processes, chemical vapour deposition (CVD), physical sputtering, and molecular beam epitaxy (MBE) are widely used to develop nanostructured materials of different shapes [6, 12–24]. Although the capability of these methods to produce high-quality films and diverse compositions has been proven, they suffer from pressure or vacuum requirements, temperature constraints, the need for suitable substrates, and prolonged processing time, usually in the range of 4–12 hours [6, 7, 11, 25–28]. Moreover, some deposition techniques, such as physical methods, cannot uniformly deposit catalysts on the 3D substrate. More flexible and versatile synthesis and deposition methods are required for direct deposition on the desired substrate to overcome the aforementioned limitations. Thus, synthesizing and searching for alternative electrocatalysts based on metal oxide is a sensible approach for low-cost electrolysis.Consequently, material selection for OER and their preparation methods are two cornerstones of the electrolysis process. Numerous research efforts have been dedicated toward finding new earth-abundant electrocatalytic materials to enhance OER. The high commercial suitability of transition metal oxides has attracted research interest for further improving their utility in OER. For example, the use of a binary transition metal oxide was shown to be a promising approach for achieving high performance [3]. Nickel cobaltite (NiCo2O4), as an example, has gained widespread research attention for its use in various energy applications [8–10, 12, 13, 15–17, 26, 27, 29–34]. However, NiCo2O4 is typically prepared using techniques that add complexity and present issues for scaling-up electrode fabrication, thereby increasing the electrode cost and negating the benefits of using less scarce materials. It is worth mentioning that the performance of NiCo2O4 differs from one preparation method to another (Table S3). Therefore, the preparation methods and thus the morphological structure and surface properties will ultimately influence the effectiveness of the catalysts.Herein, we synthesized a highly efficient catalyst of hierarchical NiCo2O4 nanosheet in a 3D flexible carbon cloth (CC) substrate as a binder-free and high-purity adhesive deposit that offers a robust and stable material on a CC substrate for water oxidation. The NiCo2O4 was fabricated by a facile, green, cost-effective, fast, and scalable electrodeposition method that produces high-quality and diverse nanostructure shapes via controlling of the various deposition parameters. Prior to electrodeposition, CC was electro-etched to improve its wettability and increase the surface area via creating a deep, groove-like of hierarchical NiCo2O4 structure on a conductive carbon cloth (CC) which was prepared. The groove-structure created on the CC was obtained via electrochemical treatment in an acid solution followed by an electrodeposition step to conformally decorate the CC with an NiCo2O4 catalyst. The quality of nucleation and growth at various deposition parameters of the deposited nanostructure are thoroughly studied. The developed NiCo2O4 catalysts show excellent catalytic performance and stability, suggesting that such a facile preparation method not only can create novel nanostructures but also the material quality can also benefit the performance. The high performance may be attributed to the 3D hierarchical nanosheet on the CC, which offered high porosity and surface area and also facilitated charge transfer and reduced the resistance of catalysis. Moreover, the relative high amount of Co+2, Co+3, Ni+2, and Ni+3 oxidized species that can easily reduce from one state to another and also an abundant of oxygen vacancies. Through these fast and efficient nanostructured fabrications, real-time applications and the commercialization of electrodes can be realized.


## 2. Experimental Section

### 2.1. Materials and Apparatus

All the reagents were 99.9% pure and have been used as received. Nickel nitrate (Ni(NO3)2·6H2O), cobalt nitrate (Co(NO3)2.6H2O), sodium hydroxide (NaOH), sulfuric acid (H2SO4), ethanol (C2H5OH), acetone (C3H6O), and deionized (DI) water were purchased from Sigma-Aldrich (UK). The CC was purchased from Fuel Cell Store. The CC consists of woven cloth without a microporous layer (MPL) and MPL has a thickness of 330 μm, and the electrical resistivity through its surface plane is less than 5 mΩ.Scanning electron microscopy (SEM, JEOL), high-resolution transmission electron microscope (HR-TEM, FEI Talos F200), corresponding EDS, and the selected area electron diffraction (SAED) pattern were used for probing the morphology of the prepared catalysis and its crystal structure. X-ray diffraction (XRD) measurements were recorded using a Bruker D8 advance diffractometer with Cu Kα1 radiation to study the crystal structure of the catalysis. X-ray photoelectron spectroscopy (XPS, JEOL, Tokyo, Japan) was used to study the elements and their oxidation states. The Brunauer-Emmett-Teller (BET) specific surface area was measured on a nitrogen environment using Micromeritics ASAP-2020.


### 2.2. Preparation of the NiCo2O4 Electrode

#### 2.2.1. Preparation of Carbon Cloth as a Substrate

To prepare the CC substrate for electrodeposition, a large piece of the CC was first cut into small pieces (1.6×2 cm2) and then cleaned using acetone, DI water, and ethanol under sonication for 20 minutes. After the sonication, the CC was left to air dry. Then, each CC was subjected to electrochemical etching for 20 minutes at a potential of 2 V vs. Ag/AgCl in 1 M H2SO4. The electrochemical etching was conducted using a 3-electrode configuration. These electrodes are CC as the working electrode, Pt mesh as the counter electrode, and Ag/AgCl as the reference electrode. After the etching process, each substrate was cleaned with DI water and sonicated for 5-10 minutes in acetone. The sample was then washed with DI water and finally left to air dry.


#### 2.2.2. Electrodeposition and Nucleation of Co-Ni-Based Film

The electrodeposition starts with nucleation and growth at the essential stage and by maintaining the supersaturation ratio of the electrodeposition component, such as the electrolyte concentration at its interface with the electrode [35]. From a redox potential perspective, the electrodeposition is according to the following equation:
(1)Meaqz++ze−⟶Mes,where Meaq is the metal ions in an aqueous state, z represents the oxidation number, and Mes is the metal in solid state. This reaction occurs at the Nernst potential (EMe/Mez+). For Ni and Co, their Nernst potentials are ~-0.26 and -0.28 V vs. RHE, respectively. It is worth mentioning that metal (Me) electrodeposition on foreign substrates (S) also depends strongly on the Me–S interaction. In a situation in which the Me–S interaction is weak, metal deposition starts at supersaturation in the overpotential deposition (ηOPD) in the range of E<EMe/Mez+ with nucleation and growth of the 3D Me solid phase. Whereas in the case of a strong Me–S interaction, the deposition process can start even at undersaturation at underpotential deposition (ηUPD) range E>EMe/Mez+ with the formation of low-dimensional metal phases, which act as precursors for the nucleation and growth of the 3D Me solid phase in the OPD range. The supersaturation ratio can be controlled by electrodeposition parameters such as concentration, overpotential or current density, pH, and temperature [12, 36].Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were conducted to determine the correct range for depositing Ni and Co ions on a carbon electrode. A 3-electrode configuration was used, and the concentration of the precursor was set at 0.1 M. The precursor consists of 40 ml of Co(NO3)2.6H2O and 20 ml of Ni(NO3)2.6H. Either potentiostatic or chronoamperometry was applied to test the effect of potential or current density while changing the depositing time to control the nucleation and growth of NiCo2O4 on the electrochemically etched CC. After electrodeposition, each electrode was washed with ethanol and DI water several times; then, all were placed in the oven to dry at 60°C for several minutes and then annealed for 2 hours at 300°C.CV measurements were recorded under room temperature conditions to define the potential reduction range for depositing Co and Ni metals on the glassy carbon (GC) as the CC exhibits strong capacitive behavior. Shown in Figure1(a) is the cyclic voltammetry scan swept from 1.5 to -2 V vs. Ag/AgCl as the CC electrode is placed on the solution of Ni2+ and Co2+ ions at a scan rate of 100 mV/sec. As the scan goes to the negative direction, the reduction current increases at point A (-0.65 V vs. Ag/AgCl), which is the beginning of the reduction region for Ni2+ ions in the UDP range [37]. At point A, the nucleation takes place, and as the potential further moves into the negative direction, the reduction current increases until point B (~ -1 V vs. Ag/AgCl), where it is suggested to be the reduction region of Co2+ to Co [38] in the ODP range. The work investigates in the potential range between points A and B as beyond B (close to -1.4 V vs. Ag/AgCl) the hydrogen coevolution reaction can strongly dominate other reduction reactions. That is, beyond the potential point B, the current increases sharply, and the hydrogen evolution becomes more prominent due to the hydrogen bubbles observed at the deposited layer. On the reverse potential scan, the current start decreases until it reaches a potential indicated by point B and then increases again until reaching point D and decreases as the potential goes toward the positive direction. Oxidation peaks were not observed in the potential range of -2 to 1.5 V vs. Ag/AgCl, indicating the absence of the dissolution of deposited materials. After determining the reduction range of Ni and Co ions on GC, the following different scenarios are applied to investigate the morphology of deposited metals on the CC.Figure 1
(a) Cyclic voltammetry recorded for a 0.1 M solution of Ni(NO3)2·6H2O and Co(NO3)2.6H2O, for finding the reduction waves for an Ni-Co electrolytic solution deposited on an electro-etched carbon cloth as a substrate, where the scan rate is 100 mV/Sec at room temperature. (b) Current time transient for Ni-Co nucleation and growth over a CC electrode at different over potential (-1 V, -1.1 V, -1.2 V, and -1.4 V) vs. Ag/AgCl.
(a)(b)(1) Chronoamperometry at a Fixed Charge. To determine the required current density to deposit exact quantities of the catalyst, chronoamperometry measurements were conducted at a fixed charge at a different potential range of -1 to -1.4 V vs. Ag/AgCl. Figure 1(b) shows the current transient for depositing NiCo2O4 on CC (or for simplification, NiCo2O4/CC) at different potentials. All recorded curves show the same characteristic behavior except the current transient differs due to applying more anodic potential. All start with an initial current spike that decayed to the point when the nucleation was initiated, followed by a slow increase on the in current transient. Figure 1(c) shows an optical image of CC in its original state, after electrochemical etching and after NiCo2O4 deposition. The change in color after treatment and after deposition is evident, from black to dark black to green (for as-prepared without annealing).(2) Chronoamperometry at Fixed Potential. In this scenario, electrodeposition was also conducted at a fixed potential of -1 V vs. Ag/AgCl, and different deposition times were applied to investigate the nucleation and growth process. Figures 2(a) and 2(b) illustrate the electrodeposition process on the CC and the corresponding current transient curve for depositing for 30, 90, and 180 s. The formation of a continuous film of NiCo2O4 onto CC requires electrical potential difference to drive the electron transfer process. Once a cathodic potential is applied, the metal ions are reduced, transferred toward the electrode surface and tend to cluster together [39]. As the deposition process continues, more aggregates of the metal particles are formed. These aggregate particles are called nuclei and are thermodynamically unstable due to their high surface energy. Only nuclei with sufficient size remain and build up into large entities; meanwhile, small portions will dissolve into the solution. With continuous loading of the metal nuclei, more clusters develop to form a continuous film. Figure 2(d) shows an SEM image of the corresponding stage of electrodeposition at the same potential of -1 V vs. Ag/AgCl with different time periods (i.e., 30, 90, and 180 s). Figure 2(d) shows an initial phase of deposition where the small nuclei form along with the carbon fiber and are located on the groove of the fiber. These grooves’ edges could be active site centers for electrodeposition due to their lower surface energy. Then, with further loading of the catalysis, a semicontinuous film started to grow and finally covered the whole CC fiber when sufficient time was allowed for the deposition.Figure 2
Nucleation and growth of Ni-Co over a CC electrode (top) illustration of nucleation and growth process, (middle) current-time transient, and (down) the corresponding SEM images for nucleation and growth for a deposition potential of -1 V at different deposition times (a) 30 s, (b) 90 s, and (c) 180 s.
(a)(b)(c)(3) Deposition at a Different Current Density. At a fixed concentration of 0.1 M of Ni2+ and Co2+, the supersaturation ratio of NiCo2O4 with galvanostatic deposition on CC can vary from high to low by changing the current density. At a high current density of 15 mA/cm2, the supersaturation ratio becomes more elevated, thus overloading NiCo2O4 particles on the CC obtained. This along with high hydrogen coevolution can lead to the development of microcracks along the cloth fiber (see Figure S1(b) and (c) in Supporting Information and more SEM images are shown in Figure S2 and S3 for -10 mA/cm2 and -5 mA/cm2. Figures S4 and S5 in the Support information show that there were overloading and crack development along the carbon fibers. The parameters were -1 V vs. Ag/AgCl for 3- and 5-minute deposition times. On the other hand, ion reduction in the current density to -10 mA/cm2 clearly shows that the coating is uniform, but cracks developed along the fibers, as shown in Figure S2(a) and (b). The optimum deposited film is achieved by lowering the current density to 5 mA/cm2 (see Figure S3). Similarly, when deposition potential is applied, in this case, the control factor for the supersaturation ratio is the current density that passes through the electrode. Therefore, the development of uniformly crack-free coating is easily achieved by controlling the current density, which adjusts the supersaturation ratio (see Figure S5) and additional SEM images for optimized electrodeposition parameter range (-1: -1.3) V vs. Ag/AgCl last for 6 minutes (see Figure S6). It is worth mentioning that other parameters, such as concentration, pH, and temperature, can also be utilized to control the supersaturation ratio [35].Overall, the nucleation and growth of NiCo2O4 on 3D textile and wavy CC using facile and low-cost electrodeposition has been optimized using various deposition parameters including overpotential, current density, and deposition time. A diagram that summarizes the correlation between the supersaturation ratio and the deposition parameters applied with some SEM images corresponding to some parameters used in this study is given in Figure 3. This highlights the importance of the deposition method which can ultimately influence the overall performance of the electrode.Figure 3
Diagram summarizing the correlation of the supersaturation ratio with deposition parameters such as deposition potential, current density, and time along with some SEM images showing the variation of the film when changing these parameters.


## 2.1. Materials and Apparatus

All the reagents were 99.9% pure and have been used as received. Nickel nitrate (Ni(NO3)2·6H2O), cobalt nitrate (Co(NO3)2.6H2O), sodium hydroxide (NaOH), sulfuric acid (H2SO4), ethanol (C2H5OH), acetone (C3H6O), and deionized (DI) water were purchased from Sigma-Aldrich (UK). The CC was purchased from Fuel Cell Store. The CC consists of woven cloth without a microporous layer (MPL) and MPL has a thickness of 330 μm, and the electrical resistivity through its surface plane is less than 5 mΩ.Scanning electron microscopy (SEM, JEOL), high-resolution transmission electron microscope (HR-TEM, FEI Talos F200), corresponding EDS, and the selected area electron diffraction (SAED) pattern were used for probing the morphology of the prepared catalysis and its crystal structure. X-ray diffraction (XRD) measurements were recorded using a Bruker D8 advance diffractometer with Cu Kα1 radiation to study the crystal structure of the catalysis. X-ray photoelectron spectroscopy (XPS, JEOL, Tokyo, Japan) was used to study the elements and their oxidation states. The Brunauer-Emmett-Teller (BET) specific surface area was measured on a nitrogen environment using Micromeritics ASAP-2020.


## 2.2. Preparation of the NiCo2O4 Electrode

### 2.2.1. Preparation of Carbon Cloth as a Substrate

To prepare the CC substrate for electrodeposition, a large piece of the CC was first cut into small pieces (1.6×2 cm2) and then cleaned using acetone, DI water, and ethanol under sonication for 20 minutes. After the sonication, the CC was left to air dry. Then, each CC was subjected to electrochemical etching for 20 minutes at a potential of 2 V vs. Ag/AgCl in 1 M H2SO4. The electrochemical etching was conducted using a 3-electrode configuration. These electrodes are CC as the working electrode, Pt mesh as the counter electrode, and Ag/AgCl as the reference electrode. After the etching process, each substrate was cleaned with DI water and sonicated for 5-10 minutes in acetone. The sample was then washed with DI water and finally left to air dry.


### 2.2.2. Electrodeposition and Nucleation of Co-Ni-Based Film

The electrodeposition starts with nucleation and growth at the essential stage and by maintaining the supersaturation ratio of the electrodeposition component, such as the electrolyte concentration at its interface with the electrode [35]. From a redox potential perspective, the electrodeposition is according to the following equation:
(1)Meaqz++ze−⟶Mes,where Meaq is the metal ions in an aqueous state, z represents the oxidation number, and Mes is the metal in solid state. This reaction occurs at the Nernst potential (EMe/Mez+). For Ni and Co, their Nernst potentials are ~-0.26 and -0.28 V vs. RHE, respectively. It is worth mentioning that metal (Me) electrodeposition on foreign substrates (S) also depends strongly on the Me–S interaction. In a situation in which the Me–S interaction is weak, metal deposition starts at supersaturation in the overpotential deposition (ηOPD) in the range of E<EMe/Mez+ with nucleation and growth of the 3D Me solid phase. Whereas in the case of a strong Me–S interaction, the deposition process can start even at undersaturation at underpotential deposition (ηUPD) range E>EMe/Mez+ with the formation of low-dimensional metal phases, which act as precursors for the nucleation and growth of the 3D Me solid phase in the OPD range. The supersaturation ratio can be controlled by electrodeposition parameters such as concentration, overpotential or current density, pH, and temperature [12, 36].Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were conducted to determine the correct range for depositing Ni and Co ions on a carbon electrode. A 3-electrode configuration was used, and the concentration of the precursor was set at 0.1 M. The precursor consists of 40 ml of Co(NO3)2.6H2O and 20 ml of Ni(NO3)2.6H. Either potentiostatic or chronoamperometry was applied to test the effect of potential or current density while changing the depositing time to control the nucleation and growth of NiCo2O4 on the electrochemically etched CC. After electrodeposition, each electrode was washed with ethanol and DI water several times; then, all were placed in the oven to dry at 60°C for several minutes and then annealed for 2 hours at 300°C.CV measurements were recorded under room temperature conditions to define the potential reduction range for depositing Co and Ni metals on the glassy carbon (GC) as the CC exhibits strong capacitive behavior. Shown in Figure1(a) is the cyclic voltammetry scan swept from 1.5 to -2 V vs. Ag/AgCl as the CC electrode is placed on the solution of Ni2+ and Co2+ ions at a scan rate of 100 mV/sec. As the scan goes to the negative direction, the reduction current increases at point A (-0.65 V vs. Ag/AgCl), which is the beginning of the reduction region for Ni2+ ions in the UDP range [37]. At point A, the nucleation takes place, and as the potential further moves into the negative direction, the reduction current increases until point B (~ -1 V vs. Ag/AgCl), where it is suggested to be the reduction region of Co2+ to Co [38] in the ODP range. The work investigates in the potential range between points A and B as beyond B (close to -1.4 V vs. Ag/AgCl) the hydrogen coevolution reaction can strongly dominate other reduction reactions. That is, beyond the potential point B, the current increases sharply, and the hydrogen evolution becomes more prominent due to the hydrogen bubbles observed at the deposited layer. On the reverse potential scan, the current start decreases until it reaches a potential indicated by point B and then increases again until reaching point D and decreases as the potential goes toward the positive direction. Oxidation peaks were not observed in the potential range of -2 to 1.5 V vs. Ag/AgCl, indicating the absence of the dissolution of deposited materials. After determining the reduction range of Ni and Co ions on GC, the following different scenarios are applied to investigate the morphology of deposited metals on the CC.Figure 1
(a) Cyclic voltammetry recorded for a 0.1 M solution of Ni(NO3)2·6H2O and Co(NO3)2.6H2O, for finding the reduction waves for an Ni-Co electrolytic solution deposited on an electro-etched carbon cloth as a substrate, where the scan rate is 100 mV/Sec at room temperature. (b) Current time transient for Ni-Co nucleation and growth over a CC electrode at different over potential (-1 V, -1.1 V, -1.2 V, and -1.4 V) vs. Ag/AgCl.
(a)(b)(1) Chronoamperometry at a Fixed Charge. To determine the required current density to deposit exact quantities of the catalyst, chronoamperometry measurements were conducted at a fixed charge at a different potential range of -1 to -1.4 V vs. Ag/AgCl. Figure 1(b) shows the current transient for depositing NiCo2O4 on CC (or for simplification, NiCo2O4/CC) at different potentials. All recorded curves show the same characteristic behavior except the current transient differs due to applying more anodic potential. All start with an initial current spike that decayed to the point when the nucleation was initiated, followed by a slow increase on the in current transient. Figure 1(c) shows an optical image of CC in its original state, after electrochemical etching and after NiCo2O4 deposition. The change in color after treatment and after deposition is evident, from black to dark black to green (for as-prepared without annealing).(2) Chronoamperometry at Fixed Potential. In this scenario, electrodeposition was also conducted at a fixed potential of -1 V vs. Ag/AgCl, and different deposition times were applied to investigate the nucleation and growth process. Figures 2(a) and 2(b) illustrate the electrodeposition process on the CC and the corresponding current transient curve for depositing for 30, 90, and 180 s. The formation of a continuous film of NiCo2O4 onto CC requires electrical potential difference to drive the electron transfer process. Once a cathodic potential is applied, the metal ions are reduced, transferred toward the electrode surface and tend to cluster together [39]. As the deposition process continues, more aggregates of the metal particles are formed. These aggregate particles are called nuclei and are thermodynamically unstable due to their high surface energy. Only nuclei with sufficient size remain and build up into large entities; meanwhile, small portions will dissolve into the solution. With continuous loading of the metal nuclei, more clusters develop to form a continuous film. Figure 2(d) shows an SEM image of the corresponding stage of electrodeposition at the same potential of -1 V vs. Ag/AgCl with different time periods (i.e., 30, 90, and 180 s). Figure 2(d) shows an initial phase of deposition where the small nuclei form along with the carbon fiber and are located on the groove of the fiber. These grooves’ edges could be active site centers for electrodeposition due to their lower surface energy. Then, with further loading of the catalysis, a semicontinuous film started to grow and finally covered the whole CC fiber when sufficient time was allowed for the deposition.Figure 2
Nucleation and growth of Ni-Co over a CC electrode (top) illustration of nucleation and growth process, (middle) current-time transient, and (down) the corresponding SEM images for nucleation and growth for a deposition potential of -1 V at different deposition times (a) 30 s, (b) 90 s, and (c) 180 s.
(a)(b)(c)(3) Deposition at a Different Current Density. At a fixed concentration of 0.1 M of Ni2+ and Co2+, the supersaturation ratio of NiCo2O4 with galvanostatic deposition on CC can vary from high to low by changing the current density. At a high current density of 15 mA/cm2, the supersaturation ratio becomes more elevated, thus overloading NiCo2O4 particles on the CC obtained. This along with high hydrogen coevolution can lead to the development of microcracks along the cloth fiber (see Figure S1(b) and (c) in Supporting Information and more SEM images are shown in Figure S2 and S3 for -10 mA/cm2 and -5 mA/cm2. Figures S4 and S5 in the Support information show that there were overloading and crack development along the carbon fibers. The parameters were -1 V vs. Ag/AgCl for 3- and 5-minute deposition times. On the other hand, ion reduction in the current density to -10 mA/cm2 clearly shows that the coating is uniform, but cracks developed along the fibers, as shown in Figure S2(a) and (b). The optimum deposited film is achieved by lowering the current density to 5 mA/cm2 (see Figure S3). Similarly, when deposition potential is applied, in this case, the control factor for the supersaturation ratio is the current density that passes through the electrode. Therefore, the development of uniformly crack-free coating is easily achieved by controlling the current density, which adjusts the supersaturation ratio (see Figure S5) and additional SEM images for optimized electrodeposition parameter range (-1: -1.3) V vs. Ag/AgCl last for 6 minutes (see Figure S6). It is worth mentioning that other parameters, such as concentration, pH, and temperature, can also be utilized to control the supersaturation ratio [35].Overall, the nucleation and growth of NiCo2O4 on 3D textile and wavy CC using facile and low-cost electrodeposition has been optimized using various deposition parameters including overpotential, current density, and deposition time. A diagram that summarizes the correlation between the supersaturation ratio and the deposition parameters applied with some SEM images corresponding to some parameters used in this study is given in Figure 3. This highlights the importance of the deposition method which can ultimately influence the overall performance of the electrode.Figure 3
Diagram summarizing the correlation of the supersaturation ratio with deposition parameters such as deposition potential, current density, and time along with some SEM images showing the variation of the film when changing these parameters.


## 2.2.1. Preparation of Carbon Cloth as a Substrate

To prepare the CC substrate for electrodeposition, a large piece of the CC was first cut into small pieces (1.6×2 cm2) and then cleaned using acetone, DI water, and ethanol under sonication for 20 minutes. After the sonication, the CC was left to air dry. Then, each CC was subjected to electrochemical etching for 20 minutes at a potential of 2 V vs. Ag/AgCl in 1 M H2SO4. The electrochemical etching was conducted using a 3-electrode configuration. These electrodes are CC as the working electrode, Pt mesh as the counter electrode, and Ag/AgCl as the reference electrode. After the etching process, each substrate was cleaned with DI water and sonicated for 5-10 minutes in acetone. The sample was then washed with DI water and finally left to air dry.


## 2.2.2. Electrodeposition and Nucleation of Co-Ni-Based Film

The electrodeposition starts with nucleation and growth at the essential stage and by maintaining the supersaturation ratio of the electrodeposition component, such as the electrolyte concentration at its interface with the electrode [35]. From a redox potential perspective, the electrodeposition is according to the following equation:
(1)Meaqz++ze−⟶Mes,where Meaq is the metal ions in an aqueous state, z represents the oxidation number, and Mes is the metal in solid state. This reaction occurs at the Nernst potential (EMe/Mez+). For Ni and Co, their Nernst potentials are ~-0.26 and -0.28 V vs. RHE, respectively. It is worth mentioning that metal (Me) electrodeposition on foreign substrates (S) also depends strongly on the Me–S interaction. In a situation in which the Me–S interaction is weak, metal deposition starts at supersaturation in the overpotential deposition (ηOPD) in the range of E<EMe/Mez+ with nucleation and growth of the 3D Me solid phase. Whereas in the case of a strong Me–S interaction, the deposition process can start even at undersaturation at underpotential deposition (ηUPD) range E>EMe/Mez+ with the formation of low-dimensional metal phases, which act as precursors for the nucleation and growth of the 3D Me solid phase in the OPD range. The supersaturation ratio can be controlled by electrodeposition parameters such as concentration, overpotential or current density, pH, and temperature [12, 36].Linear sweep voltammetry (LSV) and cyclic voltammetry (CV) were conducted to determine the correct range for depositing Ni and Co ions on a carbon electrode. A 3-electrode configuration was used, and the concentration of the precursor was set at 0.1 M. The precursor consists of 40 ml of Co(NO3)2.6H2O and 20 ml of Ni(NO3)2.6H. Either potentiostatic or chronoamperometry was applied to test the effect of potential or current density while changing the depositing time to control the nucleation and growth of NiCo2O4 on the electrochemically etched CC. After electrodeposition, each electrode was washed with ethanol and DI water several times; then, all were placed in the oven to dry at 60°C for several minutes and then annealed for 2 hours at 300°C.CV measurements were recorded under room temperature conditions to define the potential reduction range for depositing Co and Ni metals on the glassy carbon (GC) as the CC exhibits strong capacitive behavior. Shown in Figure1(a) is the cyclic voltammetry scan swept from 1.5 to -2 V vs. Ag/AgCl as the CC electrode is placed on the solution of Ni2+ and Co2+ ions at a scan rate of 100 mV/sec. As the scan goes to the negative direction, the reduction current increases at point A (-0.65 V vs. Ag/AgCl), which is the beginning of the reduction region for Ni2+ ions in the UDP range [37]. At point A, the nucleation takes place, and as the potential further moves into the negative direction, the reduction current increases until point B (~ -1 V vs. Ag/AgCl), where it is suggested to be the reduction region of Co2+ to Co [38] in the ODP range. The work investigates in the potential range between points A and B as beyond B (close to -1.4 V vs. Ag/AgCl) the hydrogen coevolution reaction can strongly dominate other reduction reactions. That is, beyond the potential point B, the current increases sharply, and the hydrogen evolution becomes more prominent due to the hydrogen bubbles observed at the deposited layer. On the reverse potential scan, the current start decreases until it reaches a potential indicated by point B and then increases again until reaching point D and decreases as the potential goes toward the positive direction. Oxidation peaks were not observed in the potential range of -2 to 1.5 V vs. Ag/AgCl, indicating the absence of the dissolution of deposited materials. After determining the reduction range of Ni and Co ions on GC, the following different scenarios are applied to investigate the morphology of deposited metals on the CC.Figure 1
(a) Cyclic voltammetry recorded for a 0.1 M solution of Ni(NO3)2·6H2O and Co(NO3)2.6H2O, for finding the reduction waves for an Ni-Co electrolytic solution deposited on an electro-etched carbon cloth as a substrate, where the scan rate is 100 mV/Sec at room temperature. (b) Current time transient for Ni-Co nucleation and growth over a CC electrode at different over potential (-1 V, -1.1 V, -1.2 V, and -1.4 V) vs. Ag/AgCl.
(a)(b)(1) Chronoamperometry at a Fixed Charge. To determine the required current density to deposit exact quantities of the catalyst, chronoamperometry measurements were conducted at a fixed charge at a different potential range of -1 to -1.4 V vs. Ag/AgCl. Figure 1(b) shows the current transient for depositing NiCo2O4 on CC (or for simplification, NiCo2O4/CC) at different potentials. All recorded curves show the same characteristic behavior except the current transient differs due to applying more anodic potential. All start with an initial current spike that decayed to the point when the nucleation was initiated, followed by a slow increase on the in current transient. Figure 1(c) shows an optical image of CC in its original state, after electrochemical etching and after NiCo2O4 deposition. The change in color after treatment and after deposition is evident, from black to dark black to green (for as-prepared without annealing).(2) Chronoamperometry at Fixed Potential. In this scenario, electrodeposition was also conducted at a fixed potential of -1 V vs. Ag/AgCl, and different deposition times were applied to investigate the nucleation and growth process. Figures 2(a) and 2(b) illustrate the electrodeposition process on the CC and the corresponding current transient curve for depositing for 30, 90, and 180 s. The formation of a continuous film of NiCo2O4 onto CC requires electrical potential difference to drive the electron transfer process. Once a cathodic potential is applied, the metal ions are reduced, transferred toward the electrode surface and tend to cluster together [39]. As the deposition process continues, more aggregates of the metal particles are formed. These aggregate particles are called nuclei and are thermodynamically unstable due to their high surface energy. Only nuclei with sufficient size remain and build up into large entities; meanwhile, small portions will dissolve into the solution. With continuous loading of the metal nuclei, more clusters develop to form a continuous film. Figure 2(d) shows an SEM image of the corresponding stage of electrodeposition at the same potential of -1 V vs. Ag/AgCl with different time periods (i.e., 30, 90, and 180 s). Figure 2(d) shows an initial phase of deposition where the small nuclei form along with the carbon fiber and are located on the groove of the fiber. These grooves’ edges could be active site centers for electrodeposition due to their lower surface energy. Then, with further loading of the catalysis, a semicontinuous film started to grow and finally covered the whole CC fiber when sufficient time was allowed for the deposition.Figure 2
Nucleation and growth of Ni-Co over a CC electrode (top) illustration of nucleation and growth process, (middle) current-time transient, and (down) the corresponding SEM images for nucleation and growth for a deposition potential of -1 V at different deposition times (a) 30 s, (b) 90 s, and (c) 180 s.
(a)(b)(c)(3) Deposition at a Different Current Density. At a fixed concentration of 0.1 M of Ni2+ and Co2+, the supersaturation ratio of NiCo2O4 with galvanostatic deposition on CC can vary from high to low by changing the current density. At a high current density of 15 mA/cm2, the supersaturation ratio becomes more elevated, thus overloading NiCo2O4 particles on the CC obtained. This along with high hydrogen coevolution can lead to the development of microcracks along the cloth fiber (see Figure S1(b) and (c) in Supporting Information and more SEM images are shown in Figure S2 and S3 for -10 mA/cm2 and -5 mA/cm2. Figures S4 and S5 in the Support information show that there were overloading and crack development along the carbon fibers. The parameters were -1 V vs. Ag/AgCl for 3- and 5-minute deposition times. On the other hand, ion reduction in the current density to -10 mA/cm2 clearly shows that the coating is uniform, but cracks developed along the fibers, as shown in Figure S2(a) and (b). The optimum deposited film is achieved by lowering the current density to 5 mA/cm2 (see Figure S3). Similarly, when deposition potential is applied, in this case, the control factor for the supersaturation ratio is the current density that passes through the electrode. Therefore, the development of uniformly crack-free coating is easily achieved by controlling the current density, which adjusts the supersaturation ratio (see Figure S5) and additional SEM images for optimized electrodeposition parameter range (-1: -1.3) V vs. Ag/AgCl last for 6 minutes (see Figure S6). It is worth mentioning that other parameters, such as concentration, pH, and temperature, can also be utilized to control the supersaturation ratio [35].Overall, the nucleation and growth of NiCo2O4 on 3D textile and wavy CC using facile and low-cost electrodeposition has been optimized using various deposition parameters including overpotential, current density, and deposition time. A diagram that summarizes the correlation between the supersaturation ratio and the deposition parameters applied with some SEM images corresponding to some parameters used in this study is given in Figure 3. This highlights the importance of the deposition method which can ultimately influence the overall performance of the electrode.Figure 3
Diagram summarizing the correlation of the supersaturation ratio with deposition parameters such as deposition potential, current density, and time along with some SEM images showing the variation of the film when changing these parameters.


## 3. Results and Discussion

### 3.1. Structural and Elemental Characterization

An overview of the morphology structure of a sample, i.e., NiCo2O4/CC-1, deposited at -1.1 V for 6 minutes where the current density has not exceeded 5 mA/cm2 is shown in Figure 4. The low magnification images show the uniformity of the coating, and the high magnification images show the structural morphology that can be formed by applying static potential or galvanostatic electrodeposition. When a fixed potential is used or fixed current density is used for electrodeposition, hierarchical nanosheets were formed regardless of the deposition time with some flower-like features at the top as shown in Figures 4(a)–4(c). NiCo2O4 deposited on the CC substrate and tends to form a continuous coating with the distinct structure of hierarchical nanosheet decorated with a flower-like microstructure on the top of the film. The formation of this morphology is obtained by controlling the kinetic of the electrodeposition process. Both structures show high porosity and surface area, both of which are advantageous for the electrode transfer for OER. Figures 4(d)–4(h) show the elemental mapping for the same electrode NiCo2O4/CC-1.Figure 4
Film surface morphology. SEM images of (a) NiCo2O4/CC-1 potential deposition at -1.1?V for 6 minutes at different magnifications and (b–e) EDS elemental mapping of C, Ni, Co, and O for NiCo2O4/CC-1.
(a)(b)(c)(d)(e)The NiCo2O4/CC electrode was scratched to peel off the deposited catalysts from CC substrate and also another used substrate after taking LSV and impedance measurements several times for transmission electron microscopy (TEM) investigation. Shown in Figure 4(a) and Figure S7 under low magnification is a fragment of NiCo2O4. As can be seen in the figure, the fragment exhibited large aggregates of nano-sized crystallites as can be observed with high magnification as shown in Figures 5(b) and 5(c). Figure 5(c) clearly shows the lattice spacing which suggests a high degree of crystallinity of NiCo2O4 with random orientation and grain boundaries with no symmetrical relationships. The crystallinity of NiCo2O4 is also confirmed with TEM diffraction patterns as shown in Figures 5(d) and 5(e). The diffraction pattern can be correlated with the XRD spectrum. Low magnification STEM imaging and the corresponding STEM-EDX mapping showing the elemental distributions of Co, Ni, and O are provided in Figures 5(f)–5(i). These images confirm the presence of single nanocrystals of NiCo2O4 by an intergrowth texture.Figure 5
Structural properties. TEM images and corresponding selected area electron diffraction (SAED) pattern of a NiCo2O4 structure peeled off from carbon cloth: (a) low-magnification of the peeled NiCo2O4 structure and (b) higher magnification showing that each piece of NiCo2O4 has semicircular particles. (c) High-resolution TEM images of the nanosheet showing the interface boundary between NiCo2O4 particles and (d) and (e) are the corresponding images of the SEAD pattern. (f–i) TEM elemental mapping images of the NiCo2O4 nanosheet showing the elemental distribution of Co, Ni, and O.
(a)(b)(c)(d)(e)(f)(g)(h)(i)Figure6(a) shows the XRD data for pristine CC, electrochemically treated CC, and NiCo2O4/CC. The corresponding XRD data for pristine CC and treated CC show that the material consists mainly of two peaks at 26° and 43°, which are the characteristic diffraction peaks of carbon elements. Meanwhile, the NiCo2O4/CC XRD data shows a few peaks that correspond to those for pure carbon materials’ and NiCo2O4 diffraction. These characteristic peaks of NiCo2O4 occur at 31.2°, 36.9°, 59°, and 65.5°, which can be indexed to (220), (311), (422), and (440) planes, respectively, of NiCo2O4 (JCPDS, No. 73-1702).Figure 6
Crystal properties: (a) X-ray diffraction pattern of CC (line a), electro-etched CC (line b), and NiCo2O4/CC (line c), and the onset shows the unit cell of NiCo2O4. XPS spectra for NiCo2O4 deposited onto CC: (b) Co (2p), (c) Ni (2p), and (d) O (1s), C(1s) in the support.
(a)(b)(c)(d)Further investigation of the elemental compositions and oxidation states of the prepared electrode (NiCo2O4/CC) was performed using X-ray photoelectron spectroscopy (XPS). The full spectroscopy scan shows that NiCo2O4/CC mainly has elements of C, Ni, Co, and O (see Figure S8). The XPS spectra of the corresponding data for each element are shown in Figures 6(b)–6(d), and the chemical state of each element was determined using the deconvolution of XPS curves with Gaussian fitting. XPS spectra of both specimens Ni and Co show that both elements consist of bivalence and trivalence states [40–42]. The peaks for Ni occur at 856 eV and 873.29 eV, which account for the chemical state of Ni2+, and the peaks at 855 eV are assigned for the chemical state of Ni3+. Additional satellite peaks of Ni elements occur at 861.49 and 877.9 eV corresponding to the high binding energy of Ni 2p 3/2 and 1/2.Similarly, the peaks of Co elements occur at 780.92 eV and 796.41 eV are associated with the valence state of Co3+, while the peaks at 785.35 and 800.78 eV are designated to the bivalence of Co2+ [40, 41]. Also, satellite peaks occur at 789.77 eV correspond to the spin-orbital of Co 2p 3/2. The fitting peaks of 1O show that its spectrum consists of two prominent peaks that corresponding to different oxygen functionality. These peaks are at 530.89 eV and 532.48 eV, which can be assigned to OI and OII, respectively. The OI is assigned to hydroxyl absorbance at the NiCo2O4 surface, and the OII corresponds to the absorbance of material with oxygen vacancy. Further XPS images comparing O1s and C1s for pure, electro-etched, and NiCo2O4 are shown in Figure S9.Additionally, the surface area and porosity of NiCo2O4/CC, electro-etched CC, and pure CC electrodes were characterized using Brunauer-Emmett-Teller (BET) on a Micromeritics ASAP 2020 under nitrogen adsorption and desorption at 77 K. The details are fully described in the support information in Figure S10 and Table S1. Moreover, the thermal weight losses of NiCo2O4/CC, electro-etched CC, and pure CC electrodes were evaluated using thermogravimetric analysis (TGA) in the air from room temperature to 1000°C (see Figure S11), and their percentage is shown in Table S2.


### 3.2. Electrochemical Performance

The electrocatalytic activities for OER of the NiCo2O4/CC electrocatalyst were investigated using electrochemical measurements in 1 mol/L NaOH at a pH of ~13 and scan rate of 5 mV/s using a three-electrode configuration in which Pt is as a counter electrode, Ag/AgCl is the reference electrode, and NiCo2O4/CC is the working electrode. Three samples were prepared with different loadings (NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3) deposited at -1.1, -1.3, and -1.4 V for 6 minutes and prepared for electrocatalytic evaluation. First, the electrochemical processes were conducted LSV. As shown in Figure 7(a), all the LSV curves show a distinct oxidation peak before the water oxidation, which could be attributed to the redox potential of Co(III)/Co(IV) and Ni(III)/Co(IV), which are considered active sites and thus contribute to water oxidation [26, 29]. Compared to the pristine CC, the onset potential and the overpotential of NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3 were cathodically shifted toward the water oxidation potential (1.23 V vs. RHE). The highest onset potential shift (~ 70 mV) was recorded for NiCo2O4/CC-3 compared to the one for the CC electrode. The other electrodes were shifted by 60 and 30 mV for NiCo2O4/CC-2 and NiCo2O4/CC-1, respectively. Moreover, the overpotential at which the electrocatalytic activities occur at 10 mA/cm2 is crucial in determining the catalytic activities toward OER. Here, NiCo2O4/CC-3 shows outstanding electrocatalytic activity and stability, which may be attributed to its hierarchically porous nanostructure, channel pathways for charge transport, and abundance of active sites, which has not been affected by gas bubbles detachment [1]. Furthermore, the presence of oxygen vacancy on NiCo2O4/CC-3, as shown in the XPS of NiCo2O4 O 1s, enhances the adsorption energies of the intermediates at the surface and thus fosters the electrochemical performance of the catalysis and also the mixed valence state of Co and Ni. NiCo2O4/CC-3 shows a lower overpotential (of ~290 mV) compared to NiCo2O4/CC-2 and NiCo2O4/CC-1 which have overpotential of 340 and 370 mV, respectively. This result shows higher catalytic activity than the best-reported performance of NiCo2O4 in the literature (Table S3). In addition, the corresponding Tafel slope values are 76, 92, and 146 mV/dec for NiCo2O4/CC-3, NiCo2O4/CC-2, and NiCo2O4/CC-1, respectively, as shown in Figure 7(b). This clearly suggests the higher charge transfer across NiCo2O4/CC-3 surfaces and electrolyte compared to the other electrodes.Figure 7
Electrochemical performance in the oxygen evolution reaction (OER). Electrochemical measurements recorded using a three-electrode configuration in 1 M NaOH. (a) The IR-corrected curve for bare CC, electro-etched CC, NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3. The numbers are referred to as deposition parameter 1 for deposition at -1 V, 2 for -1.3 V, and -1.4 V, all for 6 minutes. (b) Tafel plots calculated from (a). (c) Nyquist plots at the onset potential for each electrode.
(a)(b)(c)To evaluate the electrical conductivity of the prepared electrodes, electrochemical impedance spectroscopy (EIS) was recorded at the onset potential for each electrode as shown in Figure7(e). Nyquist plots fit the equivalent circuit for all the tested electrodes for which the curves compose two semicircles, one at a high frequency and the second at a low frequency. The high frequency of the Nyquist plot semicircle is assigned for charge-transfer resistance Rct, which indicates the electrocatalytic kinetics, the smaller the resistance, the faster the reaction rate. At the same time, the intersection of the semicircle with x-axis in the low frequency range in the Nyquist plot is ascribed to the internal resistance Rin. Figure 7(c) shows that all NiCo2O4 electrodes have lower charge-transfer resistance than the pristine CC. However, NiCo2O4-3 offers the lowest resistance, 3 Ω, compared to 6 Ω, 9 Ω, and 124 Ω for NiCo2O4/CC-2, NiCo2O4/CC-1, and pristine CC, respectively. The enhancement of the electrical conductivity can be ascribed to the porous microstructure of NiCo2O4 directly grown on the 3D CC support electrode and the facile ion diffusion through the 3D porous structure.Stability measurements were conducted for a similar NiCo2O4/CC-3 up to 21 days in 1 M NaOH. Long-term stability was confirmed by measuring its LSV every seven days (~168 hours) (see Figure 8(a)). Figure 8(b) shows that NiCo2O4/CC-3 electrode performance remains almost stable after 21 days (~500 hours). Additional chronoamperometry measurements were also tested for bare CC, NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3 at an overpotential of 593 mV for 9 hours, as shown in the inset of Figure 8(b). The results show that all the electrodes maintained excellent stability. However, NiCo2O4/CC-3 shows the highest current density, about 40 mA/cm2 delivered among the other electrodes bare CC, NiCo2O4/CC-1, and NiCo2O4/CC-2, which is consistent with the previous data presented for LSV in Figure 7(a) and the stability data in Figure 8(b). These results indicate that the hierarchically porous nanostructure is well adhered to the CC substrate and maintained its porosity after long-term stability measurements in alkali media (see SEM Figure S12).Figure 8
Stability measurements were recorded using a three-electrode configuration in 1 M NaOH. (a) The IR-corrected curve NiCo2O4/CC-3 measured after 7 days of chronoamperometry measurements. (b) Stability diagram obtained from LSV at overpotential of 593 mV, and the inset is the chronoamperometry measurement for the catalysis shown in Figure 6, where the measurements were conducted at overpotential of 593 mV for 9 hours. (c) Schematic diagram illustrating the four-step mechanism for OH∗, O∗, and OOH∗ intermediates adsorbed on the surface of the catalysis during water oxidation (OER). (d) Illustration of the electron-hole mechanism at the surface of the catalysis and the process of water decomposition along with the fibers.
(a)(b)(c)(d)The outstanding electrocatalytic activity and stability of NiCo2O4/CC-3 may be attributed to its hierarchically porous nanostructure, channel pathways for charge transport, and abundance of active sites, which have not been affected by gas bubble detachment [43]. Figure 8(c) illustrates the Gibbs free energy diagram for OER. Figure 8(d) illustrates the electron-hole pair transfer mechanism of NiCo2O4 in electro-etched carbon fiber in alkaline solution under applied overpotential. Under the right overpotential, the electrocatalytic elements of NiCo2O4 were converted into different oxidation states in large quantities. As a result, more Ni+2 and Co+2 were oxidized back and forth into Ni+3 and Co+3, which can efficiently react with O-, OH, and OOH- intermediate to form an oxygen molecule.


## 3.1. Structural and Elemental Characterization

An overview of the morphology structure of a sample, i.e., NiCo2O4/CC-1, deposited at -1.1 V for 6 minutes where the current density has not exceeded 5 mA/cm2 is shown in Figure 4. The low magnification images show the uniformity of the coating, and the high magnification images show the structural morphology that can be formed by applying static potential or galvanostatic electrodeposition. When a fixed potential is used or fixed current density is used for electrodeposition, hierarchical nanosheets were formed regardless of the deposition time with some flower-like features at the top as shown in Figures 4(a)–4(c). NiCo2O4 deposited on the CC substrate and tends to form a continuous coating with the distinct structure of hierarchical nanosheet decorated with a flower-like microstructure on the top of the film. The formation of this morphology is obtained by controlling the kinetic of the electrodeposition process. Both structures show high porosity and surface area, both of which are advantageous for the electrode transfer for OER. Figures 4(d)–4(h) show the elemental mapping for the same electrode NiCo2O4/CC-1.Figure 4
Film surface morphology. SEM images of (a) NiCo2O4/CC-1 potential deposition at -1.1?V for 6 minutes at different magnifications and (b–e) EDS elemental mapping of C, Ni, Co, and O for NiCo2O4/CC-1.
(a)(b)(c)(d)(e)The NiCo2O4/CC electrode was scratched to peel off the deposited catalysts from CC substrate and also another used substrate after taking LSV and impedance measurements several times for transmission electron microscopy (TEM) investigation. Shown in Figure 4(a) and Figure S7 under low magnification is a fragment of NiCo2O4. As can be seen in the figure, the fragment exhibited large aggregates of nano-sized crystallites as can be observed with high magnification as shown in Figures 5(b) and 5(c). Figure 5(c) clearly shows the lattice spacing which suggests a high degree of crystallinity of NiCo2O4 with random orientation and grain boundaries with no symmetrical relationships. The crystallinity of NiCo2O4 is also confirmed with TEM diffraction patterns as shown in Figures 5(d) and 5(e). The diffraction pattern can be correlated with the XRD spectrum. Low magnification STEM imaging and the corresponding STEM-EDX mapping showing the elemental distributions of Co, Ni, and O are provided in Figures 5(f)–5(i). These images confirm the presence of single nanocrystals of NiCo2O4 by an intergrowth texture.Figure 5
Structural properties. TEM images and corresponding selected area electron diffraction (SAED) pattern of a NiCo2O4 structure peeled off from carbon cloth: (a) low-magnification of the peeled NiCo2O4 structure and (b) higher magnification showing that each piece of NiCo2O4 has semicircular particles. (c) High-resolution TEM images of the nanosheet showing the interface boundary between NiCo2O4 particles and (d) and (e) are the corresponding images of the SEAD pattern. (f–i) TEM elemental mapping images of the NiCo2O4 nanosheet showing the elemental distribution of Co, Ni, and O.
(a)(b)(c)(d)(e)(f)(g)(h)(i)Figure6(a) shows the XRD data for pristine CC, electrochemically treated CC, and NiCo2O4/CC. The corresponding XRD data for pristine CC and treated CC show that the material consists mainly of two peaks at 26° and 43°, which are the characteristic diffraction peaks of carbon elements. Meanwhile, the NiCo2O4/CC XRD data shows a few peaks that correspond to those for pure carbon materials’ and NiCo2O4 diffraction. These characteristic peaks of NiCo2O4 occur at 31.2°, 36.9°, 59°, and 65.5°, which can be indexed to (220), (311), (422), and (440) planes, respectively, of NiCo2O4 (JCPDS, No. 73-1702).Figure 6
Crystal properties: (a) X-ray diffraction pattern of CC (line a), electro-etched CC (line b), and NiCo2O4/CC (line c), and the onset shows the unit cell of NiCo2O4. XPS spectra for NiCo2O4 deposited onto CC: (b) Co (2p), (c) Ni (2p), and (d) O (1s), C(1s) in the support.
(a)(b)(c)(d)Further investigation of the elemental compositions and oxidation states of the prepared electrode (NiCo2O4/CC) was performed using X-ray photoelectron spectroscopy (XPS). The full spectroscopy scan shows that NiCo2O4/CC mainly has elements of C, Ni, Co, and O (see Figure S8). The XPS spectra of the corresponding data for each element are shown in Figures 6(b)–6(d), and the chemical state of each element was determined using the deconvolution of XPS curves with Gaussian fitting. XPS spectra of both specimens Ni and Co show that both elements consist of bivalence and trivalence states [40–42]. The peaks for Ni occur at 856 eV and 873.29 eV, which account for the chemical state of Ni2+, and the peaks at 855 eV are assigned for the chemical state of Ni3+. Additional satellite peaks of Ni elements occur at 861.49 and 877.9 eV corresponding to the high binding energy of Ni 2p 3/2 and 1/2.Similarly, the peaks of Co elements occur at 780.92 eV and 796.41 eV are associated with the valence state of Co3+, while the peaks at 785.35 and 800.78 eV are designated to the bivalence of Co2+ [40, 41]. Also, satellite peaks occur at 789.77 eV correspond to the spin-orbital of Co 2p 3/2. The fitting peaks of 1O show that its spectrum consists of two prominent peaks that corresponding to different oxygen functionality. These peaks are at 530.89 eV and 532.48 eV, which can be assigned to OI and OII, respectively. The OI is assigned to hydroxyl absorbance at the NiCo2O4 surface, and the OII corresponds to the absorbance of material with oxygen vacancy. Further XPS images comparing O1s and C1s for pure, electro-etched, and NiCo2O4 are shown in Figure S9.Additionally, the surface area and porosity of NiCo2O4/CC, electro-etched CC, and pure CC electrodes were characterized using Brunauer-Emmett-Teller (BET) on a Micromeritics ASAP 2020 under nitrogen adsorption and desorption at 77 K. The details are fully described in the support information in Figure S10 and Table S1. Moreover, the thermal weight losses of NiCo2O4/CC, electro-etched CC, and pure CC electrodes were evaluated using thermogravimetric analysis (TGA) in the air from room temperature to 1000°C (see Figure S11), and their percentage is shown in Table S2.


## 3.2. Electrochemical Performance

The electrocatalytic activities for OER of the NiCo2O4/CC electrocatalyst were investigated using electrochemical measurements in 1 mol/L NaOH at a pH of ~13 and scan rate of 5 mV/s using a three-electrode configuration in which Pt is as a counter electrode, Ag/AgCl is the reference electrode, and NiCo2O4/CC is the working electrode. Three samples were prepared with different loadings (NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3) deposited at -1.1, -1.3, and -1.4 V for 6 minutes and prepared for electrocatalytic evaluation. First, the electrochemical processes were conducted LSV. As shown in Figure 7(a), all the LSV curves show a distinct oxidation peak before the water oxidation, which could be attributed to the redox potential of Co(III)/Co(IV) and Ni(III)/Co(IV), which are considered active sites and thus contribute to water oxidation [26, 29]. Compared to the pristine CC, the onset potential and the overpotential of NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3 were cathodically shifted toward the water oxidation potential (1.23 V vs. RHE). The highest onset potential shift (~ 70 mV) was recorded for NiCo2O4/CC-3 compared to the one for the CC electrode. The other electrodes were shifted by 60 and 30 mV for NiCo2O4/CC-2 and NiCo2O4/CC-1, respectively. Moreover, the overpotential at which the electrocatalytic activities occur at 10 mA/cm2 is crucial in determining the catalytic activities toward OER. Here, NiCo2O4/CC-3 shows outstanding electrocatalytic activity and stability, which may be attributed to its hierarchically porous nanostructure, channel pathways for charge transport, and abundance of active sites, which has not been affected by gas bubbles detachment [1]. Furthermore, the presence of oxygen vacancy on NiCo2O4/CC-3, as shown in the XPS of NiCo2O4 O 1s, enhances the adsorption energies of the intermediates at the surface and thus fosters the electrochemical performance of the catalysis and also the mixed valence state of Co and Ni. NiCo2O4/CC-3 shows a lower overpotential (of ~290 mV) compared to NiCo2O4/CC-2 and NiCo2O4/CC-1 which have overpotential of 340 and 370 mV, respectively. This result shows higher catalytic activity than the best-reported performance of NiCo2O4 in the literature (Table S3). In addition, the corresponding Tafel slope values are 76, 92, and 146 mV/dec for NiCo2O4/CC-3, NiCo2O4/CC-2, and NiCo2O4/CC-1, respectively, as shown in Figure 7(b). This clearly suggests the higher charge transfer across NiCo2O4/CC-3 surfaces and electrolyte compared to the other electrodes.Figure 7
Electrochemical performance in the oxygen evolution reaction (OER). Electrochemical measurements recorded using a three-electrode configuration in 1 M NaOH. (a) The IR-corrected curve for bare CC, electro-etched CC, NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3. The numbers are referred to as deposition parameter 1 for deposition at -1 V, 2 for -1.3 V, and -1.4 V, all for 6 minutes. (b) Tafel plots calculated from (a). (c) Nyquist plots at the onset potential for each electrode.
(a)(b)(c)To evaluate the electrical conductivity of the prepared electrodes, electrochemical impedance spectroscopy (EIS) was recorded at the onset potential for each electrode as shown in Figure7(e). Nyquist plots fit the equivalent circuit for all the tested electrodes for which the curves compose two semicircles, one at a high frequency and the second at a low frequency. The high frequency of the Nyquist plot semicircle is assigned for charge-transfer resistance Rct, which indicates the electrocatalytic kinetics, the smaller the resistance, the faster the reaction rate. At the same time, the intersection of the semicircle with x-axis in the low frequency range in the Nyquist plot is ascribed to the internal resistance Rin. Figure 7(c) shows that all NiCo2O4 electrodes have lower charge-transfer resistance than the pristine CC. However, NiCo2O4-3 offers the lowest resistance, 3 Ω, compared to 6 Ω, 9 Ω, and 124 Ω for NiCo2O4/CC-2, NiCo2O4/CC-1, and pristine CC, respectively. The enhancement of the electrical conductivity can be ascribed to the porous microstructure of NiCo2O4 directly grown on the 3D CC support electrode and the facile ion diffusion through the 3D porous structure.Stability measurements were conducted for a similar NiCo2O4/CC-3 up to 21 days in 1 M NaOH. Long-term stability was confirmed by measuring its LSV every seven days (~168 hours) (see Figure 8(a)). Figure 8(b) shows that NiCo2O4/CC-3 electrode performance remains almost stable after 21 days (~500 hours). Additional chronoamperometry measurements were also tested for bare CC, NiCo2O4/CC-1, NiCo2O4/CC-2, and NiCo2O4/CC-3 at an overpotential of 593 mV for 9 hours, as shown in the inset of Figure 8(b). The results show that all the electrodes maintained excellent stability. However, NiCo2O4/CC-3 shows the highest current density, about 40 mA/cm2 delivered among the other electrodes bare CC, NiCo2O4/CC-1, and NiCo2O4/CC-2, which is consistent with the previous data presented for LSV in Figure 7(a) and the stability data in Figure 8(b). These results indicate that the hierarchically porous nanostructure is well adhered to the CC substrate and maintained its porosity after long-term stability measurements in alkali media (see SEM Figure S12).Figure 8
Stability measurements were recorded using a three-electrode configuration in 1 M NaOH. (a) The IR-corrected curve NiCo2O4/CC-3 measured after 7 days of chronoamperometry measurements. (b) Stability diagram obtained from LSV at overpotential of 593 mV, and the inset is the chronoamperometry measurement for the catalysis shown in Figure 6, where the measurements were conducted at overpotential of 593 mV for 9 hours. (c) Schematic diagram illustrating the four-step mechanism for OH∗, O∗, and OOH∗ intermediates adsorbed on the surface of the catalysis during water oxidation (OER). (d) Illustration of the electron-hole mechanism at the surface of the catalysis and the process of water decomposition along with the fibers.
(a)(b)(c)(d)The outstanding electrocatalytic activity and stability of NiCo2O4/CC-3 may be attributed to its hierarchically porous nanostructure, channel pathways for charge transport, and abundance of active sites, which have not been affected by gas bubble detachment [43]. Figure 8(c) illustrates the Gibbs free energy diagram for OER. Figure 8(d) illustrates the electron-hole pair transfer mechanism of NiCo2O4 in electro-etched carbon fiber in alkaline solution under applied overpotential. Under the right overpotential, the electrocatalytic elements of NiCo2O4 were converted into different oxidation states in large quantities. As a result, more Ni+2 and Co+2 were oxidized back and forth into Ni+3 and Co+3, which can efficiently react with O-, OH, and OOH- intermediate to form an oxygen molecule.


## 4. Conclusion

In summary, a facile, cost-effective, controllable, and scalable fabrication method of NiCo2O4 on a carbon substrate (i.e., CC) was investigated. TEM images show the intergrowth of Ni and Co to form nanostructured binary metal oxide (NiCo2O4). The OER of various loadings of NiCo2O4 was evaluated under different growth conditions. The NiCo2O4 on CC exhibited low overpotential of 290 mV at 10 mA/cm2. Also, the NiCo2O4/CC electrode showed excellent stability when evaluated for ~500 hours. This synthesis method with highly controllable parameters holds high potential to create nanostructured electrodes on nonmetallic substrates for large-scale fabrication. Further investigation in forming different morphological structures of ternary or quadruple metal oxides will be conducted.



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