Composition and Morphology Modulation of Bimetallic Nitride Nanostructures on Nickel Foams for Efficient Oxygen Evolution Electrocatalysis

Abstract

Metal-nitrides-based electrocatalysts for efficient oxygen-evolution have been extensively studied as one of the most promising candidates to fulfil the demand for future energy-conversion and storage. Herein, a series of Ni_xCo_1−xO- and Ni_xCo_1−xN-based nanostructures on nickel foams were reported to show excellent activities for oxygen-evolution reaction. The catalysts were prepared and modulated rationally via a facile-hydrothermal method, followed by high-temperature calcination under air or nitrogen atmosphere. The optimal bimetallic-nitride catalyst Ni_0.3Co_0.7N shows a small overpotential of 268 mV at 20 mA cm⁻², and a Tafel slope of 66 mV dec⁻¹ with good stability. The enhanced OER-performance is ascribed to the synergetic effect of the unique morphology and the intrinsic catalytic property of the nanostructure after nitridation.

1. Introduction

As one of the most promising future energy solutions, hydrogen has gained much attention for its unique advantages such as high energy-density, high conversion efficiency and environmental friendliness in energy conversion and storage technologies [1,2]. However, the most commonly used approach for industrial hydrogen-production is fossil-energy conversion, with much CO₂ emission [3]. Hydrogen production from water splitting and its utilization in fuel cells have been developed as part of a clean-energy cycle with zero-carbon emission, using hydrogen as the energy carrier [4]. For efficient water electrolysis, both the oxygen-evolution reaction (OER) and the hydrogen-evolution reaction (HER) are crucial. The OER process has been studied more extensively in recent years for sluggish surface-reaction kinetics, due to the multiple steps of the proton-coupled electron-transfer process [5,6,7,8].

In order to substitute the utilization of noble metals in electrocatalytic oxygen-evolution, numerous novel cost-efficient catalytic materials have been investigated over the past decades. Among the most promising materials, transition metal-based oxides [9,10,11,12], sulfides [13,14,15], selenides [16,17,18], phosphates, and nitrides [19,20] have drawn much attention, due to their abundance, low cost and excellent performance. Composition control [21,22], morphology design [8,23,24,25], surface engineering [26,27] and hybrid structure [28,29,30] are the most important and common methods of improving the catalytic performance.

In particular, transition metal-based nitrides have been recently developed as highly promising catalysts, demonstrating excellent efficiency and stability for both electrochemical hydrogen-evolution and oxygen evolution [31,32,33,34,35,36,37]. The incorporation of the N atom can introduce superior electronic structure into metal nitrides for high electron-conductivity, to accelerate interface catalysis. Cobalt nitride has been studied as a promising metallic interstitial catalyst for advantageous electronic conductivity [38]. Co₄N porous nanowire-arrays with a 3D electrode-configuration, metallic electronic-property and 1D porous-structure was prepared via a precursor-nitridation method, to show a low overpotential of 257 mV at 10 mA cm⁻² and a low Tafel slope of 33 mV dec⁻¹ [39]. Nickel nitride nanosheets achieved a much enhanced OER performance compared with bulk Ni₃N and NiO nanosheets, due to the enhanced electronic conductivity and atomically disordered structure [40]. Multi-metallic nitrides were also synthesized, to optimize the electronic structure by heteroatom incorporation for improved catalytic-performance. Ordered-mesoporous NiCo₂N was reported using a hard-templating pathway to show a low overpotential of 289 mV at 10 mA cm⁻² geometric current-density, due to its large surface area and enhanced infiltration capacity for electrolyte [21]. Porous nickel–cobalt nitride nanosheets on nickel foams (NF) were prepared as bifunctional electrocatalysts, through facile electrodeposition followed by annealing in NH₃ atmosphere for superior catalytic performance with good stability [22]. Holey cobalt−iron nitride nanosheets were synthesized via a facile-hydrothermal and subsequent thermal-nitridation method, to exhibit high catalytic performance for both HER and OER [23].

However, the research on the composition modulation of bimetallic nitrides and their morphology control is not sufficient [41,42,43]. The transformation and comparison of bimetallic oxides and nitrides has not yet been reported. The detailed and in-depth experiments can be helpful to understand the structure-activity relationship in the electrocatalytic oxygen-evolution reaction. We suppose that the combination of composition control and morphology modulation on bimetallic nitrides will be beneficial for the synergetic effect, and promote the catalytic performance.

Herein, we report a series of bimetallic oxides and nitrides (Ni_xCo_1−xO and Ni_xCo_1−xN) grown on nickel foams (NF) with optimized composition and morphology modulation for efficient electrochemical oxygen-evolution. The bimetallic oxides were synthesized with various composition ratios through a facile-hydrothermal method followed by calcination in air. The morphologies of the catalysts show the obvious tendency that a higher Ni/Co ratio leads to 2D-nanosheet nanostructures and a lower Ni/Co ratio leads to 1D-nanowire nanostructures. Then, the corresponding hydrothermal precursors were calcinated under NH₃ atmosphere to produce bimetallic nitrides with tuned composition and morphologies. The Ni_0.3Co_0.7N sample shows a unique nanostructure composed of nanocorals and nanosheets. SEM, XPS and physical characterizations confirmed the composition, morphology and oxidation states of the catalysts. Based on the comparative research, the bimetallic nitrides showed far superior OER catalytic activities to the oxides, and the composition ratio was optimized. The enhanced OER performance was attributed to both the composition and the morphology optimization, which was also verified by electrochemical-impedance analysis. This research develops a new and efficient nickel-cobalt nitride electrocatalyst with a unique nanostructure, and provides a useful guidance for the controllable synthesis of other efficient metal nitrides.

2. Results and Discussion

2.1. Morphology

The nickel-cobalt oxides and nitrides were synthesized from the high-temperature treatment of the hydrothermal precursors in an air and ammonia atmosphere, respectively (Figure 1). The surface morphology of the Ni_xCo_1−xO and Ni_xCo_1−xN series of nanoarrays grown on nickel foams were characterized using SEM and TEM. From the SEM images, NiO shows clearly a uniform, ultrathin 2D-nanosheet array-structure grown vertically on nickel-foam substrate, and the average size of a single nanosheet is 1–1.5 μm (Figure 2a and Figure S1). With cobalt incorporation, Ni_0.9Co_0.1O also maintains the nanosheet structure while the average size of a single nanosheet increases to 2–2.5 μm (Figure 2b). For Co₃O₄, uniform 1D-nanowire arrays grown vertically on NF can be clearly observed, with an average diameter of around 100 nm (Figure 2f). With the incorporation of nickel, Ni_0.1Co_0.9O and Ni_0.3Co_0.7O mainly keep a similar nanowire morphology to Co₃O₄, while the nanowires tend to gather as a nanoplate (Figure 2d,e). The Ni_0.5Co_0.5O sample shows the structure of nanoplates composed of nanowires, which can be the result of hybridization of nanosheets and nanowires (Figure 2c). The difference in the resulting morphology can be ascribed to the intrinsic properties of the different metal centers for crystal growth. It is likely that CO(NH₂)₂ and NH₄F lead to the 1D-nanostructures with a high aspect-ratio for Co₃O₄ and 2D-nanostructures for NiO [44]. The nanosheets and nanowires structures of the nickel and cobalt oxides can both provide more active catalytic sites for the oxygen-evolution reaction. Hence, the nanostructures of the catalysts can be regulated by adjusting the composition reasonably. Furthermore, after high-temperature nitridation in NH₃ atmosphere, the hydrothermal precursors can be transformed into metal nitrides. Unlike the nanosheets structure of NiO, the obtained Ni₃N shows a coral-like nanoporous morphology, with an average particle diameter of 50–100 nm (Figure 3a and Figure S2). In other words, after nitridation, the nanosheet morphology mostly transformed into nanocorals composed of particles for nickel-cobalt nitrides. Continuing incorporation of nickel leads to more nanowire morphology with more surface roughness for Ni_xCo_1−xN. For Ni_0.3Co_0.7N, the surface of the nanocorals arrays became rougher, due to nitridation, and the surface was composed of ultrathin nanosheets (Figure 3d), demonstrating a unique nanostructure of nanocorals with nanosheets, which could be beneficial for surface-water oxidation reactions. The beneficial nanostructure of Ni_0.3Co_0.7N was well retained after the oxygen-evolution reaction tests, as shown in Figure S3. The main structure of nanocorals with the nanosheets of the catalyst remained unchanged, with only a slight variation on the nanosheets sizes.

Further insights into the morphology of the metal oxides and nitrides were investigated using HRTEM (Figure 4, Figure 5 and Figures S4–S7). Obviously, the nanowire morphology can also be seen in the TEM images for Ni_0.3Co_0.7O, and the nanowires are composed of smaller, interconnected nanoparticles. For Ni_0.3Co_0.7O nanowires, the distinct lattice-fringe of 0.244 nm is in consistent with the (3 1 1) lattice-plane of Co₃O_4, and the distinct lattice-fringes of 0.208 nm are ascribed to the (2 0 0) lattice-plane of NiO. TEM-mapping images further confirm the uniform elemental-distribution of Co, Ni, and O. For Ni_0.3Co_0.7N nanocorals, the distinct lattice-fringe of 0.204 nm is inconsistent with the (1 0 1) lattice-plane of Co₂N_0.67, and the distinct lattice-fringes of 0.231 nm are ascribed to the (1 1 1) lattice-plane of Ni₃N. The HRTEM results clearly indicate the morphology and hybrid structure of the bimetallic nitrides. TEM-mapping images also confirm the uniform elemental-distribution of Co, Ni, and N. The additional HRTEM images for all the samples also show similar results (Figures S4–S7). The above results and analysis demonstrate that the series of metal-oxides and nitrides nanostructures were successfully prepared, and the morphologies were effectively modulated by nitridation and composition-adjustment.

2.2. Crystal Structure and Composition

To investigate the crystal structure and phase information of all the oxide and nitride catalysts, X-ray diffractometry measurements were carried out. The prepared metal oxides and nitrides on nickel foams all show clear and strong signals in the XRD patterns, revealing the successful synthesis of the catalysts. As shown in Figure S8, the characteristic peaks of nickel-oxide patterns are well indexed to those of cubic NiO [JCPDS: 78-0643], and the peaks of cobalt oxide are assigned to the structural phase of Co₃O₄ [JCPDS: 80-1533] [31]. With the molar ratio of Ni and Co changing, the XRD patterns of Ni_xCo_1−xO show the features of the combination of NiO and Co₃O₄, inconsistent with the HRTEM results. From the comparison of XRD patterns in Figure 6, the transformation of the oxides and nitrides can be clearly observed. The strong signals at 44.6°, 51.9° and 76.5° can be ascribed to the nickel-foam substrate (JCPDS No.70-0989). After thermal nitridation under NH₃ atmosphere, the XRD patterns of nickel nitride match well with the structural phase of Ni₃N [JCPDS:10-0280], and the peaks of cobalt nitride are assigned to the Co₂N_0.67 structural phase [JCPDS:06-0691] (Figure S9) [45], which matches well with the HRTEM results. The XRD patterns of Ni_xCo_1−xN show the features of the combination of Ni₃N and Co₂N₀₆₇. The XRD results confirm the successful synthesis of the cobalt- and nickel-based bimetallic oxides and the nitrides composites from the calcination reaction in different atmospheres of hydrothermal precursors. In addition, the XRD patterns of Ni_0.3Co_0.7N after the OER tests remained almost unchanged, as shown in Figure S9, indicating the good stability and robustness of the Ni_xCo_1−xN catalysts.

The catalysts were further characterized by XPS to gain deep insights into the oxidation states of the surface elements and electronic configurations. The calibrated XPS-spectra are fitted and shown in Figure 7, and survey spectra are shown in Figure S10. The XPS survey-spectra confirm the existence of Co, Ni, O, N elements for the corresponding catalysts. The XPS semiquantitative-analysis results to estimate the true x values in the Ni_xCo_1−xO/NF and Ni_xCo_1−xN/NF catalysts are also shown in Table S1. For nickel-cobalt oxides Ni_xCo_1−xO, in Co 2p signals, two featured peaks at binding energies of 780.9 eV and 782.5 eV are ascribed to Co³⁺(2p_3/2) and Co²⁺(2p_3/2) configurations, respectively. The corresponding peaks for Co 2p_1/2 are located at 795.6 eV and 797.2 eV, respectively. With the incorporation of the Ni element, the XPS peaks tend to shift to lower binding-energies, generally. The emerging signals at ~778.7 eV and ~793.4 eV in nickel-cobalt oxides can be attributed to Co⁰ 2p configurations [46], which are caused by the incorporation of Ni heteroatoms during thermal treatment. The deviation of the XPS signals indicate the interaction in the nickel-cobalt oxides, which might strongly influence the electronic structures and intrinsic catalytic properties. The peaks at ~802.9 eV and ~784.2 eV are the Co²⁺ shake-up satellite-peaks. In Ni 2p signals of nickel-cobalt oxides, the featured peaks at binding- energies of 853.5 eV, 855.6 eV, and 857.0 eV are ascribed to the Ni⁰(2p_3/2), Ni²⁺(2p_3/2) and Ni³⁺(2p_3/2) configurations, respectively. The Ni⁰ species might be from the Ni foam substrate. The O 1s signals show three characteristic peaks at 529.1 eV, 531.3 eV, and 533.9 eV, corresponding to the metal-O bond, metal-OH bond and surface-adsorbed oxygen, respectively [8].

On the other hand, after thermal nitridation, the surface valence-states of the bimetallic nitrides Ni_xCo_1−xN were also characterized, for further detailed information. The Co 2p signals and Ni 2p signals show similar results to those of the nickel-cobalt oxides. The peaks at 778.4 eV, 780.2 eV and 781.1 eV correspond to the Co 2p_3/2 configuration, while the peaks at 793.9 eV, 795.5 eV and 796.9 eV correspond to the Co 2p_1/2 configuration. The corresponding shake-up satellite-peaks locate at 802.4 eV and 784.5 eV. For the Ni 2p signals, the featured peaks at binding-energies of ~855.3 eV and ~856.3 eV are ascribed to the Ni²⁺(2p_3/2) and Ni³⁺(2p_3/2) configurations, respectively. The shake-up satellite-peaks located at 861.4 eV and 879.6 eV confirm the existence of Ni²⁺ species [22]. In contrast to the XPS results for metal oxides, no obvious signals for the Ni⁰ species can be found. This can be explained by the fact that the nickel-foam substrate at the phase interface partially reacted with the precursors during thermal nitridation under the NH₃ atmosphere. The comparison of oxides and nitrides in valence-states is important to gain deeper information on the modulation of electronic configuration. Specifically, for N 1s signals, the featured peaks at binding-energies of 396.9 eV and 398.0 eV can be ascribed to Ni-N and Co-N bonds, respectively [43]. The peaks at 399.0 eV, 400.4 and 403.2 eV can be ascribed to pyridinic N, pyrrolic N and oxidized pyridinic-N bonds, respectively. From Co 2p, Ni 2p and N 1s XPS of Ni_0.3Co_0.7N post OER tests (Figure S11), the peaks positions show no obvious shift, except that the relative intensities of the Ni-N and Co-N bonds changed slightly. XPS analysis confirms the existence and valence-states of the surface elements, verifying the electronic-structure changes due to nitridation and composition-adjustment.

2.3. Electrochemical Analysis

The electrochemical measurements of the Ni_xCo_1−xO and Ni_xCo_1−xN series of nanoarrays was conducted to evaluate the OER properties in a three-electrode system tested in 1.0 M KOH aqueous solution. The ohmic-potential-drop (iR) losses caused by electrolyte resistance were iR-compensated, firstly. From the LSV current-density tests of the metal oxides (Figure 8a), it can be seen that the Ni_0.3Co_0.7O sample exhibits the highest catalytic activity. Ni_0.4Co_0.6O and Ni_0.1Co_0.9O also exhibit similar high activities to that of Ni_0.3Co_0.7O (Figure S12), and are much higher than that of NiO and Co₃O₄. To achieve a current density of 20 mA/cm², an overpotential of only 303 mV is required for the Ni_0.3Co_0.7O sample, which is also much lower than that of NiO (401 mV) and Co₃O₄ (346 mV). Moreover, most of the bimetallic oxides show enhanced OER current-densities when compared to NiO and Co₃O₄ (Figure S12a), proving the strategy of heteroatom-incorporation effective. Ni_0.4Co_0.6O and Ni_0.1Co_0.9O show the overpotentials of 304 mV and 312 mV, respectively, which are close to but higher than that of Ni_0.3Co_0.7O. Moreover, the Tafel slope of the Ni_0.3Co_0.7O sample is 130 mV dec⁻¹, also much smaller than that of NiO (222 mV dec⁻¹), Co₃O₄ (163 mV dec⁻¹) and the rest of the oxide catalysts (Figure 8c). The results of LSV tests and Tafel analysis together show evidence that Ni_0.3Co_0.7O is the best oxide catalyst for the oxygen-evolution reaction. These results reveal that the rational composition-tuning can significantly enhance the electrocatalytic activities of the materials, which agree well with the previously reported works [9,47].

On the other hand, thermal nitridation to transform hydrothermal precursors into nitrides has also been proved to be an effective routine for preparing metallic metal nitrides as highly efficient HER and OER catalysts. In this regard, bimetallic nitrides (Ni_xCo_1−xN) on nickel-foam substrates with optimized composition-tuning were investigated to achieve desirable electrocatalytic performance. All the metal nitrides samples show far superior catalytic activities to the corresponding metal oxides (Figure 8b and Figure S12), which validates the fact that nitridation transformation of metal oxides is an effective approach for better catalytic efficiencies. Furthermore, the rational composition-tuning helps to screen out the best oxygen-evolution catalysts. Most of the bimetallic nitrides Ni_xCo_1−xN show much-enhanced activities compared with Ni₃N without metal-heteroatom incorporation. This result demonstrates that rational composition-tuning based on nitridation can contribute to further enhancement of catalytic activities. In addition, it is noteworthy that bimetallic nitrides Ni_xCo_1−xN show the morphologies of the nanocorals and nanowires arrays combined-structure from the SEM and TEM morphology-analysis. This nanostructure, composed of nanowires and nanocorals, might contribute to the more exposed catalytic sites and larger surface-areas. In addition, the introduction of Co into cobalt-based compounds can be helpful for NiOOH transformation at low overpotentials for enhanced electrochemical-performance [44]. As expected, the Ni_0.3Co_0.7N sample exhibits the highest current-density and achieves a much smaller overpotential of 268 mV than that of Co₂N_0.67 (307 mV), Ni₃N (374 mV) and the rest of the nitride catalysts at a current-density of 20 mA cm⁻². The overpotentials in this work are more favorable, in comparison with the literature results [21,22]. The unique nanostructure of ultrathin nanosheets on nanocorals for Ni_0.3Co_0.7N tends to be more favorable for the surface electrocatalysis-reaction. To gain further insights into the bimetallic nitride catalysts, the Tafel slopes for all catalysts were investigated. The Tafel slope of the Ni_0.3Co_0.7N is only 66 mV dec⁻¹, much smaller than that of Co₂N_0.67 (81 mV dec⁻¹), Ni₃N (100 mV dec⁻¹) and the rest of the nitride catalysts (Figure 8d), indicating that Ni_0.3Co_0.7N is a relatively better candidate for an efficient OER catalysis-process. Although Ni_0.1Co_0.9N shows a close LSV curve to that of Ni_0.3Co_0.7N (Figure S12b), the overpotential (276 mV) and Tafel slope (69 mV dec⁻¹) are still slightly inferior to that of Ni_0.3Co_0.7N. Among the nitride catalysts, Co₂N_0.67 shows an unexpected high catalytic-activity, which is superior to some of the bimetallic nitrides. This unusual result can be explained by the fact that that nickel from the substrate at the phase interface partially reacted with the precursors during thermal nitridation, and a slight amount of Ni element was injected into the Co₂N_0.67 catalyst, as shown in Table S1. This result can be inferred from the XPS analysis, and is the same for other catalysts. Based on the above analysis, we can conclude that the intrinsic OER catalytic-properties can be effectively improved by thermal nitridation, composition tuning and morphology modulation, synergistically.

Electrochemical impedance spectra (EIS) of the metal oxides and nitrides on NF was carried out to evaluate the oxygen-evolution catalytic kinetics. The nickel-foam substrate was also tested for comparison. A reasonable equivalent circuit is adopted to simulate the electrode and interface behaviors (Figure S13), in which R_s is the solution resistance in series, R_ct is the charge-transfer-resistance and CPE is the space-charge capacity. In the Nyquist plots, a smaller radius of the semicircle for Ni_0.3Co_0.7N is shown, compared to Co₂N_0.67 and Ni₃N, which reflects the significant charge-transfer-resistance decrease and thus the enhanced-interface reaction kinetics after heteroatom incorporation. From the fitted data, Ni_0.3Co_0.7N nanocorals have an R_ct of about 0.65 Ω, which is lower than that of Co₂N_0.67 (1.22 Ω) and Ni₃N (0.67 Ω). On the other hand, the metal nitrides all show smaller radii and lower resistances compared with the corresponding metal oxides, which is in good accordance with the LSV and Tafel results. The results clearly suggest that the Ni_0.3Co_0.7N oxygen-evolution catalyst has the best intrinsic charge-transfer property for electrochemical water splitting among all the catalysts, which can be ascribed to the incorporation of nitride, the optimal proportion of elements and the advantageous nanostructure. To evaluate the stability of the electrocatalysts for OER in the alkaline electrolyte, continuous OER-electrolysis measurements using chronopotentiometry at two current-densities (10 and 50 mA/cm²) were performed for a continuous period of 20 h. As shown in Figure 5, the time dependence of corresponding overpotentials remains quite stable, suggesting bimetallic nickel-cobalt nitride nanostructures are of good stability and efficiency for electrocatalytic oxygen-evolution.

The excellent OER performance of Ni_0.3Co_0.7N could be ascribed to the synergetic effect of the unique morphology from rational composition-modulation and the intrinsic catalytic property of the nanostructure after nitridation, based on structural, composition and electrochemical characterizations. First of all, nitrogen introduction leads to the superior metallic conductivity of the transition-metal oxides. The decreased charge-transfer resistance can be verified by the EIS results. Secondly, the introduction of Co into cobalt-based compounds can be helpful for NiOOH transformation at low overpotentials for enhanced electrochemical performance. The detailed experiments indicate that Ni_0.3Co_0.7N is the best candidate, with optimal composition. Thirdly, the unique nanostructure of Ni_0.3Co_0.7N composed of nanocorals and nanosheets can be advantageous for efficient interface-reaction and higher activity.

3. Materials and Methods

3.1. Synthesis of Ni_xCo_1−xO/NF and Ni_xCo_1−xN/NF

Nickel foams were used as substrates, and cleaned before synthesizing the precursors. A piece of cut nickel-foam (1 × 2 cm²) was put into hydrochloric acid to remove the oxide layers on its surface by ultrasonication for 10 min, then cleaned with deionized water, acetone and ethanol, separately, and finally dried after ultrasonication, for 10 min. The precursors were firstly prepared using the hydrothermal method. The proportion of cobalt and nickel was changed by mixing different proportions of Co(NO₃)₂·6H₂O and Ni(NO₃)₂·6H₂O, while the total molar weight was controlled at 1 mmol. The proportion of the metal elements in Ni_xCo_1−xO and Ni_xCo_1−xN varied from 0–1, with an interval of 0.1. In a typical process, 0.5 mmol Co(NO₃)₂·6H₂O, 0.5 mmol Ni(NO₃)₂·6H₂O, 5 mmol CO(NH₂)₂, and 2 mmol NH₄F were added to deionized water (20 mL) whilst stirring in a 50mL Teflon-lined stainless-steel autoclave. The dried nickel-foam was immersed in the solution and transferred into a 120 °C drying oven for 4 h. The precursors of the cobalt-nickel catalysts were obtained and then washed with water and ethanol, separately. In the preparation of Ni_xCo_1−xO/NF, the precursor was heated to 400 °C at 3 °C/min in a muffle-furnace system in air, and then Ni_xCo_1−xO/NF was prepared by oxidation for 2h. In the preparation of Ni_xCo_1−xN/NF, under the gas flow of high-purity NH₃, the precursor was heated to 380 °C at 5 °C/min in a tube-furnace system, and then the required Ni_xCo_1-xN/NF was prepared by nitridation for 3 h.

3.2. Structural Characterization

The morphology of the samples was investigated using a scanning electron microscope (SEM, JEOL JSM-7800FE, Tokyo, Japan) and transmission electron microscope (TEM, FEI Tecnai F30, Atlanta, GA, USA) at 300 kV. The X-ray-diffraction (XRD) patterns were characterized by a PANalytical X’pert MPD Pro diffractometer (Malvern, UK) operated at 40 kV and 40 mA using Ni-filtered Cu-Kα irradiation (wavelength 1.5406 Å). The chemical composition and valence-states of the samples were determined by X-ray photoelectron spectroscopy (XPS, Axis UltraDLD, Kratos, Manchester, UK) with monochromatic aluminum Kα radiation.

3.3. Electrochemical Characterization

Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and long-term stability tests were conducted with an electrochemical workstation (CHI760E, Chenhua Shanghai, Shanghai, China) using the nickel foam (1 × 1 cm²) as the working electrode, a platinum wire as the counter-electrode and a saturated Ag/AgCl electrode as the reference-electrode. All the electrochemical measurements were performed in 1.0 M KOH, which served as the electrolyte. The LSV was recorded by cycling between 0 and 0.9 V (vs. Ag/AgCl) at a scan rate of 5 mV/s in an oxygen-saturated electrolyte. The LSV plots were iR-compensated in advance, for comparison. The EIS was carried out in the frequency range between 100 kHz and 0.1 Hz at 0.6 V (vs. Ag/AgCl), and the AC amplitude of 5 mV. A three-electrode system was adopted in the electrochemical measurement conducted on an electrochemical workstation (CHI 760E, Chenhua Shanghai) at room temperature. The current-time (i-t) curve was obtained to assess the stability of the working electrodes. All the potentials gauged in the Ag/AgCl electrode were calibrated to the reversible hydrogen electrode (RHE), according to the following equation:

$$\mathrm{ERHE}=\mathrm{EAg}/\mathrm{AgCl}+0.197+0.059\mathrm{pH}$$

4. Conclusions

In summary, we developed a series of Ni_xCo_1−xO and Ni_xCo_1−xN catalysts on nickel foams via a facile strategy of precursor-oxidation and precursor-nitridation reactions, as efficient OER-electrocatalysts. By adjusting the ratio of the Ni:Co and calcination atmospheres, the composition and morphology of the catalysts can be rationally modulated. From a systematical analysis and comparison, the Ni_xCo_1−xN catalysts show far superior activities to that of the Ni_xCo_1−xO, due to the metallic electronic-structures. The optimal Ni_0.3Co_0.7N catalyst shows the unique nanostructure of nanosheets embedded in nanocorals. Benefiting from the unique morphology and advantageous electronic-conductivity, bimetallic Ni_0.3Co_0.7N shows a small overpotential of 268 mV at 20 mA cm⁻², and a Tafel slope of 66 mV dec⁻¹, with good stability. This work demonstrates the transformation and comparison of bimetallic oxides and nitrides, and provides new insights for composition and morphology modulation of bimetallic catalysts.