Chrysanthemum-like spinel nanomaterials assembled with bundled nanowires as an efficient catalyst for the oxygen evolution reaction

Abstract

The fabrication of low-cost and high-performance electrocatalysts is essential for water splitting to achieve a sustainable oxygen evolution reaction. In this work, spinel oxides (CuCo₂O₄ and Co₃O₄) are mounted on nickel foam (NF) to form chrysanthemum-like hybrid crystal structures assembled from nanowires. The as-synthesized CuCo₂O₄–Co₃O₄/NF shows a high surface area, a large number of mesoporous structures and excellent charge transfer channels, which is conducive to the generation and overflow of O₂. And the synergistic effect, as well as the transfer of electron density between CuCo₂O₄ and Co₃O₄, contributes to the enhancement of OER activity. As a result, CuCo₂O₄–Co₃O₄/NF exhibits excellent OER performance with an overpotential of only 267 mV at 10 mA cm⁻², a low Tafel slope (114.6 mV dec⁻¹) and excellent stability with a scarce loss of the initial catalyst activity over 24 h under alkaline conditions. This work provides an effective method to show higher catalytic activity by combining two spinel oxides.

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Introduction

With the growth of the fossil fuel crisis and environmental problems, the development of renewable energy sources has become a global common view.¹ Hydrogen has been considered as an ideal substitute for traditional fossil fuels, and electrocatalytic water splitting is believed to be a convenient and efficient method to produce hydrogen. Electrochemical water splitting, which includes the hydrogen evolution reaction (HER) on the cathode and the oxygen evolution reaction (OER) on the anode, is an efficient and sustainable way to produce hydrogen (H₂) and oxygen (O₂).² However, four proton-coupled electron transfer and slow kinetics of the OER process result in high overpotential and a bottleneck in the entire process.³ Normally, noble metal oxides like ruthenium dioxide (RuO₂) and iridium dioxide (IrO₂) benchmark electrocatalysts are considered to have the best electrochemical activity for the OER.⁴ However, the high cost, scarcity and poor stability of precious metals have seriously hindered the large-scale application of renewable energy systems.⁵ Therefore, it is urgent to explore efficient OER catalysts with low cost, earth abundance and good stability for widespread applications.

In recent decades, it has been found that transition metals and their derivatives (such as metal sulfides, metal phosphides, and metal oxides or hydroxides) have high activity and low price, which are very suitable as substitutes for noble metal catalysts.^1,3,6 Among transition metal oxides, especially spinel oxides AB₂O₄ (A, B = 3d transition metals, such as MnCo₂O₄, CoFe₂O₄, CoAl₂O₄ and NiCo₂O₄) have attracted considerable attention as OER catalysts.^1,7 In spinel oxides, oxygen is closely packed in a cubic form, divalent metal cations occupy one eighth of tetrahedral sites, and trivalent metal cations are filled in one half of octahedral sites. And the remaining unoccupied gaps endow the spinel with an open structure to accommodate the migration of cations.⁸ At the same time, the mixed valence of metal cations also provides donor–acceptor sites to adsorb oxygen and promote the occurrence of electrolytic water reactions.⁹ In addition, the introduction of other metal atoms into the crystal lattice can greatly affect the electronic structure of the original catalyst and enhance the intrinsic activity as well as the conductivity of the catalyst.

Among them, a Cu-based electrocatalyst has attracted a lot of attention due to its biomimetic chemistry similar to that of O₂.^10,11 The introduction of Co into the Cu-based electrocatalyst to form CuCo₂O₄ spinel oxides can not only change the 3d orbitals of Cu and the surrounding environment of the 3d electrons,¹² but also increase the adsorption energy of water and change the Gibbs free energy of the adsorption intermediate, thereby improving the catalytic activity of the catalyst.^11,13 However, powder catalyst materials easily agglomerate and fall off during the test process, resulting in the reduction of catalyst activity and stability. A self-supporting electrode can directly grow the catalyst on a common substrate surface (such as NF or carbon cloth), which can perfectly solve the problem caused by powder catalysts. Moreover, it has a larger specific surface area and faster carrier release, which can enhance the interaction and synergy between various species.¹⁴ Nevertheless, the spinel oxides on the substrate are mostly two-dimensional layered structures, with a large layer thickness, few active centers and low catalytic activity, which is difficult to meet the industrial requirements of high activity.¹⁵

Another special metal oxide Co₃O₄ is considered to be one of the best OER catalysts due to its large cobalt reserves, low price and high oxygen evolution efficiency in alkaline solution.¹⁶ Zhu et al. prepared a hollow Co₃O₄ microtube array with a hierarchical structure and only required a battery voltage of 1.63 V to reach a current density of 10 mA cm⁻².¹⁷ Kuang et al. synthesized a copper–cobalt mixed oxide, which can reach a current density of 10 mA cm⁻² at only 1.61 V for overall water splitting in alkaline medium.¹⁸ It is generally believed that the 3d empty or semi-empty orbital of Co can interact with the π electrons of oxygen and reduce the adsorption energy of O*, OH* and OO* in the center of the 3d band.^3,7,19 Using one-dimensional Co₃O₄ microtube array materials as templates, 3D catalysts were grown on a conductive substrate to form a three-dimensional (3D) structure, which can also increase the surface area and active sites of the catalyst to a greater extent, promoting the diffusion of reactants and generated gases.²⁰

Inspired by this, we prepared a chrysanthemum-like CuCo₂O₄–Co₃O₄ electrocatalyst on Co₃O₄ microtube arrays by a two-step hydrothermal reaction and annealing in air. A 3D chrysanthemum-like morphology forms outward active sites and a large specific surface area, which accelerates electron transfer and facilitates the desorption of generated oxygen. And the synergistic effect of redox pairs of active catalytic centers makes it show higher catalytic activity. The CuCo₂O₄–Co₃O₄ electrocatalyst exhibits outstanding OER activity with a low overpotential of 267 mV at 10 mA cm⁻², a small Tafel slope of 114.6 mV dec⁻¹, and excellent durability in alkaline medium. It is a promising way to optimize the electronic configuration and greatly enhance the OER activity.

Experimental section

Materials

Cobalt sulfate heptahydrate (CoSO₄·7H₂O, 98%), copper sulfate nonahydrate (CuSO₄·9H₂O, 98%), urea (CH₄N₂O, 99%), ammonium fluoride (NH₄F, 96%), potassium hydroxide (KOH, 85%), Ni foam (NF, purity ≥99%, porosity ≥90%), concentrated hydrochloric acid (HCl, 37%) and ethanol (CH₃CH₂OH, 99.7%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Commercial RuO₂ was purchased from Aladdin Reagent Co. Ltd. Nafion (5 wt%) was purchased from Dupont China Holding Co., Ltd. All the chemical reagents were of analytical grade and used as received without further purification. Water deionized with a Millipore system was used throughout all experiments.

Synthesis of Co(OH)₂/NF

The NF was treated before the experiment. Firstly, the NF was ultrasonically washed in absolute ethanol and HCl solution (3.0 M) sequentially for 20 minutes to remove greasy dirt and the oxide layer, and then cleaned with deionized water. After that, 2.81 g (10 mmol) of CoSO₄·7H₂O, 3.00 g (50 mmol) of CH₄N₂O and 0.74 g (20 mmol) of NH₄F were dissolved into 50 mL of deionized water by an ultrasonic treatment to form a homogeneous solution. Then, both the solution and the cleaned NF were placed into a 50 mL Teflon autoclave and then sealed and kept at 120 °C for 5 h. After cooling down to room temperature, the obtained Co(OH)₂/NF was washed several times with deionized water and dried using a freeze dryer.

Synthesis of CuCo LDH-Co(OH)₂/NF

In a typical synthesis, 337.32 mg (1.20 mmol) of CoSO₄·7H₂O, 149.814 mg (0.60 mmol) of CuSO₄·9H₂O, and 360.36 mg (6 mmol) of CH₄N₂O were dissolved into 30 mL of deionized water by an ultrasonic treatment to form a homogeneous solution. Then, both the solution and the prepared Co(OH)₂/NF were placed into a 50 mL Teflon autoclave and then sealed and kept at 120 °C for 6 h. After cooling down to room temperature, the samples were removed from the autoclave, washed with water several times and freeze-dried to obtain CuCo LDH-Co(OH)₂/NF.

Synthesis of CuCo₂O₄–Co₃O₄/NF

The prepared CuCo LDH-Co(OH)₂/NF was calcined at 350 °C for 2 h with a heating ramp of 5 °C min⁻¹ in air to obtain CuCo₂O₄–Co₃O₄/NF.

Synthesis of other control samples

For comparison, CuCo₂O₄/NF and Co₃O₄/NF were prepared under identical conditions of CuCo₂O₄–Co₃O₄/NF, except replacing CuCo LDH-Co(OH)₂/NF with CuCo LDH/NF and Co(OH)₂/NF, respectively. CuO–Co₃O₄/NF and Co₃O₄–Co₃O₄/NF were also prepared under the same conditions as CuCo₂O₄–Co₃O₄/NF but without CoSO₄·7H₂O and CuSO₄·9H₂O, respectively.

Material characterization

The morphology of the synthesized materials was characterized by field emission scanning electron microscopy (FESEM, GeminiSEM 300) with an energy dispersive spectroscopy (EDX) detector and transmission electron microscope (TEM, JEOL JEM 2100). High-resolution TEM (HRTEM, JEOL JEM 2100) was used to obtain the information on the lattice fringe. X-ray powder diffraction (XRD, X'Pert Pro MRD) was carried out with Cu Kα radiation at a scan rate of 5° min⁻¹. X-ray photoelectron spectroscopy (XPS, ESCALAB XI⁺) was used to analyze the valence state with Al Kα monochromatized radiation.

Electrochemical measurements

All the electrochemical measurements were conducted in a classical three-electrode glass cell using an Electrochemical Workstation (CHI 760E) in 1 M KOH (pH = 14) aqueous solution at room temperature. The three electrode system is composed of a graphite rod as the counter electrode, a saturated calomel electrode (SCE) electrode as the reference electrode, and the sample (CuCo₂O₄–Co₃O₄/NF, CuCo₂O₄/NF, Co₃O₄/NF, CuO–Co₃O₄/NF, Co₃O₄–Co₃O₄/NF, CuCo LDH-Co(OH)₂/NF, RuO₂/NF or blank Ni Foam) as the working electrode. In all tests, the potentials were carefully calibrated to a reversible hydrogen electrode (RHE) potential in 1.0 M KOH electrolyte solution. The SCE electrode was calibrated with respect to the RHE: E(RHE) = E(SCE) + 0.059 × pH + E⁰(SCE). Linear sweep voltammetry (LSV) was conducted from 0 to 0.7 V (vs. SCE) at a scan rate of 2 mV s⁻¹ with 85% iR-compensation. Electrochemical impedance spectroscopy (EIS) analyses were conducted at a fixed potential of 0.50 V and performed in the frequency range of 0.01 Hz to 10 kHz with an amplitude of 5 mV. The electrochemical double layer capacitance (C_dl) of the synthesized samples was measured from 1.043 to 1.143 V (in the non-faradaic region) versus the RHE with different scan rates (10, 20, 30, 40, 50, 60, 70, 80, 90 and 100 mV s⁻¹) to estimate the effective electrode surface area. The C_dl value is half of the linear slope. A long-term durability and stability test was performed using chronopotentiometry with a polarization current density of 10 mA cm⁻¹. The electrochemical stability was actualized using CV sweeps at 50 mV s⁻¹ between −0.1 V and 0.1 V versus the RHE for 5000 cycles at 25 °C.

For comparison, RuO₂ was deposited onto NF as the working electrode with a loading of about 2.7 mg cm⁻². Typically, 4 mg RuO₂ (99.9%) and 40 μL Nafion solution (5 wt%) were ultrasonically scattered in 960 μL solvent (720 μL ultrapure water and 240 μL ethanol), and then the ink solution was loaded onto a cleaned NF, and dried under an infrared drying lamp.

Results and discussion

The preparation of chrysanthemum-like CuCo₂O₄–Co₃O₄ on nickel foam (NF) is mainly divided into three steps, as displayed in Scheme 1. Initially, uniform and vertical Co(OH)₂ nanowire arrays (Co(OH)₂/NF) were grown on the surface of Ni foam by the first solvothermal method. Subsequently, CuCo LDH-Co(OH)₂ was deposited onto the surface of NF by the second hydrothermal reaction in an alkaline environment. Finally, the obtained CuCo LDH-Co(OH)₂/NF was annealed in air to get chrysanthemum-like CuCo₂O₄–Co₃O₄/NF.

Scheme 1 Schematic illustration of the synthesis of CuCo₂O₄–Co₃O₄ on NF.

The crystalline phases of Co(OH)₂/NF, CuCo LDH-Co(OH)₂/NF and CuCo₂O₄–Co₃O₄/NF were characterized by X-ray powder diffraction (XRD). As a comparison, the XRD patterns of CuCo₂O₄/NF, Co₃O₄/NF, CuO–Co₃O₄/NF and Co₃O₄–Co₃O₄/NF were also recorded. As shown in Fig. 1 and S1,† the three high-intensity peaks of all the products at 44.5°, 51.8° and 76.4° can be indexed to the (111), (200) and (220) planes of Ni (JCPDS No.04-0850)²¹ derived from NF. In Fig. 1, the diffraction peaks of Co(OH)₂/NF nanowire arrays at 9.50°, 19.16°, 38.03° and 58.19° can be assigned to the (001), (002), (102) and (110) planes of Co(OH)₂ (JCPDS No.51-1731).²² For CuCo LDH-Co(OH)₂/NF, the peaks of Co(OH)₂ still remain, while the new reflection peaks at 35.8° may be attribute to CuCo LDH.²³ The diffraction peaks of CuCo₂O₄–Co₃O₄/NF at 19°, 31.27°, 36.85°, 59.36° and 66.23° are well ascribed to the (111), (220), (311), (551) and (440) crystal planes of Co₃O₄ (JCPDS No.42-1467),²⁴ respectively, while the diffraction peaks at 19.07°, 31.36°, 36.96° and 38.96° are assigned to the (111), (220), (311) and (222) planes of spinel CuCo₂O₄ (JCPDS No. 01-1155),²⁵ which verifies the formation of CuCo₂O₄–Co₃O₄/NF. In Fig. S1,† the XRD patterns of CuCo₂O₄/NF, Co₃O₄/NF, CuO–Co₃O₄/NF and Co₃O₄–Co₃O₄/NF show the peaks of CuCo₂O₄, Co₃O₄ and CuO respectively, which proved that the comparison samples were also successfully prepared.

Fig. 1 XRD patterns of the samples.

The scanning electron microscope (SEM) images of the as-prepared samples are displayed in Fig. 2, S2 and S3.† For Co(OH)₂/NF, it can be seen from Fig. S2† that the Co(OH)₂ grown on the surface of NF has an obvious sea urchin like structure. Each “sea urchin ball” is surrounded by nanowires, which are connected to each other. After the growth of CuCo LDH, the “sea urchin ball” disappeared, evolved into a flower like structure, and the rudiment of chrysanthemum has appeared. It is speculated that the addition of Cu²⁺ caused the rearrangement of Co(OH)₂. In Fig. 2a, flower like CuCo₂O₄–Co₃O₄ balls are grown on the NF. As shown in Fig. 2b, the flower structure is like a “chrysanthemum”, and each “chrysanthemum” is connected with each other. The “petals” of the “chrysanthemum” are actually composed of nanowires, and the ends of each nanowire are bound together, further increasing the mass transfer channel.^1,2 As shown in Fig. 2d, the EDX mapping of CuCo₂O₄–Co₃O₄/NF verifies the presence of Cu, Co and O, and the ratio of Cu : Co : O atoms is close to 1 : 6 : 8 (Fig. S4†).

Fig. 2 SEM images (a–c) and EDX-mapping images (d) of CuCo₂O₄–Co₃O₄/NF.

The structure and morphology of CuCo₂O₄–Co₃O₄ were further investigated by TEM and HRTEM. It can be seen in Fig. 3a–c that these CuCo₂O₄–Co₃O₄ nanowires are composed of numerous primary nanoparticles, and mesoporous structures can be observed among the tiny particles. These structures are beneficial to the diffusion or immersion of electrolyte ions during the redox reaction, provide channels for efficient diffusion of evolved O₂, and even offer more edges to facilitate the OER.⁹ The HRTEM images of CuCo₂O₄–Co₃O₄ (Fig. 3c and d) confirm that CuCo₂O₄ and Co₃O₄ nanoflakes were well crystallized, and the lattice fringes with an interplanar spacing of 0.286 and 0.244 nm are assigned to the spacing of CuCo₂O₄ (220) and Co₃O₄ (311) planes, respectively. Meanwhile the lattice fringes with an interplanar spacing of 0.465 and 0.467 nm can be indexed to CuCo₂O₄ (111) and Co₃O₄ (111), respectively. The hybrid crystal structure can produce a rich interface structure, which can optimize the valence electron state of the active center and improve the electronic conductivity of the catalyst.^26,27 These results illustrated that CuCo₂O₄ and Co₃O₄ were successfully loaded on NF, which are consistent with the results of XRD patterns.

Fig. 3 TEM images (a and b) and HRTEM images (c) and (d) of CuCo₂O₄–Co₃O₄/NF.

The chemical valence states in CuCo₂O₄/NF, Co₃O₄/NF and CuCo₂O₄–Co₃O₄/NF were analyzed by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 4. The Cu 2p spectrum shows two peaks at binding energies of 933.7 and 953.6 eV, which could be assigned to Cu 2p_3/2 and Cu 2p_1/2, respectively. Two satellite peaks could also be detected at 941.6 and 961.6 eV.¹⁶ In the Co 2p spectra (Fig. 4c), the fitting peaks at 778.97 eV and 793.96 eV are assigned to Co³⁺, while the other two fitting peaks at 780.26 eV and 795.43 eV are attributed to Co²⁺.²⁸ The spectrum of O 1s could be resolved into two peaks at binding energies of 529.4 eV and 530.8 eV, which correspond to metal–oxygen bonding and defect sites. In addition, by comparing the valence distribution of each element of the target sample, CuCo₂O₄/NF (Fig. S5†), and Co₃O₄/NF (Fig. S6†), it can be found that this electron density distribution is conducive to enhancing the electron transfer between interfaces, thus improving the catalytic activity of CuCo₂O₄–Co₃O₄/NF.^16,29

Fig. 4 XPS full-spectrum survey (a) and core-level spectra of Cu 2p (b), Co 2p (c) and O 1s (d) for CuCo₂O₄–Co₃O₄/NF.

The high surface area and mesoporous structure of CuCo₂O₄–Co₃O₄ were further analyzed by Brunauer–Emmett–Teller (BET). The N₂ adsorption–desorption isotherm of CuCo₂O₄–Co₃O₄ is a Type IV isotherm with a H1 hysteresis loop in the P/P₀ range of 0.5–1.0 (Fig. S7†), identifying the mesoporous structure of as-synthesized CuCo₂O₄–Co₃O₄. The BET surface area is 31.5 m² g⁻¹ for CuCo₂O₄–Co₃O₄. The high specific surface area of CuCo₂O₄–Co₃O₄ is beneficial to expose many more electrochemically active sites and promote the mass transport of the electrolyte. According to the pore size distribution curve of CuCo₂O₄–Co₃O₄ calculated by using the BJH model, most of the pore sizes are 4.9 nm and 10 nm, confirming the mesoporous structure of the sample. Hence, it can be concluded from the isotherm and pore size distribution that the as-synthesized CuCo₂O₄–Co₃O₄ possesses a high surface area and mesoporous structure.

The electrocatalytic performance of the obtained catalysts for the OER was tested in a conventional three-electrode configuration in a 1.0 M KOH aqueous electrolyte at room temperature. As shown in the linear scan voltammogram (LSV, Fig. 5a), the CuCo₂O₄–Co₃O₄/NF electrode requires only 267 mV versus the RHE to achieve a current density of 10 mA cm⁻², which is superior to that of CuCo₂O₄/NF (318 mV), Co₃O₄/NF (299 mV), CuO–Co₃O₄/NF (323 mV), Co₃O₄–Co₃O₄/NF (288 mV), CuCo LDH-Co(OH)₂/NF (284 mV), RuO₂/NF (339 mV) and NF (345 mV). In addition, CuCo₂O₄–Co₃O₄/NF only needs an overpotential of 407 mV to reach a high current density of 100 mA cm⁻² (Fig. 5b). The excellent OER performance of CuCo₂O₄–Co₃O₄/NF can be attributed to the unique chrysanthemum-like structure with a three-dimensional mesoporous structure and the synergistic effect between active substances, which is beneficial to the formation and diffusion of oxygen. Some recently studied materials similar to CuCo₂O₄–Co₃O₄/NF are listed in Table S1.† By comparing the performance of the catalysts, such as A_2.7B-MOF-FeCo_1.6 (η₁₀ = 288 mV),³⁰ CoMn LDH@Cu(OH)₂/CF (η₁₀ = 280 mV),³¹ CoBDC–Fc_0.17 (η₁₀ = 291 mV),³² P-doped Co@NC-3/1 (η₁₀ = 340 mV),³³ O–CoSe₂-HNT (η₁₀ = 252 mV),³⁴etc., it was found that CuCo₂O₄–Co₃O₄/NF showed excellent OER activity in an alkaline electrolyte.

Fig. 5 Electrocatalytic properties of the electrodes in 1.0 M KOH for the OER. LSV curves (a), corresponding Tafel plots (b), Nyquist plots (c), and average capacitive current obtained from the center of the potential window at 1.093 V vs. the RHE of (d) CuCo₂O₄–Co₃O₄/NF, CuCo₂O₄/NF, Co₃O₄/NF, CuO–Co₃O₄/NF, Co₃O₄–Co₃O₄/NF, CuCo LDH-Co(OH)₂/NF, RuO₂/NF and blank NF.

To evaluate the intrinsic catalytic activity, Fig. 5b presents the Tafel slopes of the different samples. The data from the LSV can be fitted according to the Tafel equation: η = a + b log(|j|),³⁵ where η is the overpotential, j is the current density, a is the constant and b represents the Tafel slope. CuCo₂O₄–Co₃O₄/NF exhibits the smallest Tafel slope of 114.59 mV dec⁻¹, which is close to that of RuO₂/NF (109.7 mV dec⁻¹) and outperforms that of CuCo₂O₄/NF (155.02 mV), Co₃O₄/NF (135.69 mV dec⁻¹), CuO–Co₃O₄/NF (132.26 mV dec⁻¹), Co₃O₄–Co₃O₄/NF (142.53 mV dec⁻¹), CuCo LDH-Co(OH)₂/NF (140.87 mV dec⁻¹) and NF (140.91 mV dec⁻¹), respectively. The results indicate favorable OER kinetics on the CuCo₂O₄–Co₃O₄/NF surface.

The electrochemical impedance spectroscopy (EIS) spectrum of CuCo₂O₄–Co₃O₄/NF was recorded to illustrate the interfacial properties between the electrode and the electrolyte. As shown in Fig. 5c, all the samples show almost the same solution resistance (R_s).³⁶ The R_s values can be neglected in contrast with the large charge transfer resistance (R_ct). As is known to all, a smaller semicircle in the low frequency region reflects a faster charge transfer process in electrodes, namely a smaller R_ct. The R_ct of CuCo₂O₄–Co₃O₄/NF was estimated to be 1.20 Ω at an overpotential of 300 mV versus the RHE, which is much smaller than those of CuCo₂O₄/NF (5.85 Ω), Co₃O₄/NF (1.75 Ω), CuO–Co₃O₄/NF (31.05 Ω), Co₃O₄–Co₃O₄/NF (1.48 Ω) and CuCo LDH-Co(OH)₂/NF (1.67 Ω), respectively. In CuCo₂O₄–Co₃O₄/NF, the connection between the nanowires forms more electron transport pathways, which is conducive to the rapid transfer of charge. In addition, the interface interaction produced in the formation process makes it have excellent electrical conductivity.³⁷ Generally, electrochemical activity is highly dependent on the nature and number of uncoordinated metal sites. Thus, the electrochemical surface areas (ECSAs) were measured, as shown in Fig. 5d. The ECSA values were estimated from the double electric layer capacitances (C_dl),³⁸ which were derived from cyclic voltammograms (CV) at different scan rates (Fig. S8†) and proportional to the active sites of the catalyst. The C_dl value of CuCo₂O₄–Co₃O₄/NF (26.68 mF cm⁻²) is significantly larger than those of CuCo₂O₄/NF (14.65 mF cm⁻²), Co₃O₄/NF (16.67 mF cm⁻²), CuO–Co₃O₄/NF (17.32 mF cm⁻²), Co₃O₄–Co₃O₄/NF (16.63 mF cm⁻²) and CuCo LDH-Co(OH)₂/NF (12.62 mF cm⁻²), respectively. It is verified that the 3D chrysanthemum bundle grown on NF provides a higher surface area for redox reactions.

In order to investigate the stability and durability of the CuCo₂O₄–Co₃O₄/NF electrocatalyst, continuous CV scans were conducted between −0.1 and 0.1 V (versus the RHE) at an accelerated sweep rate of 50 mV s⁻¹. Long-term electrolysis was carried out at a controlled current density of 10 mA cm⁻² for 20 h in 1 M KOH solution. The performance of the CuCo₂O₄–Co₃O₄/NF electrocatalyst just decreased by 5.2% during the 20 h test (Fig. 6a). In Fig. 6b, the overpotential at 10 mA cm⁻² almost retained its initial value after 5000 cycles. The high stability of the electrode was further confirmed by XRD (Fig. S9†), SEM (Fig. S10†) and XPS (Fig. S11†). After electrolysis, most of the characteristic diffraction peaks, the micromorphology and the main valence state of the original structure were retained. These results demonstrate that the CuCo₂O₄–Co₃O₄/NF electrocatalyst is stable for the OER in strong alkaline media for a long time.

Fig. 6 Chronopotentiometry measurement at a constant current density of 10 mA cm⁻² for 20 h (a) and LSV curves before and after 5000 cyclic voltammetry cycles at a scan rate of 50 mV s⁻¹ (b) of CuCo₂O₄–Co₃O₄/NF in 1.0 M KOH.

Conclusions

In summary, a chrysanthemum-like hybrid electrocatalyst was successfully prepared via a facile hydrothermal reaction and annealing in air. Spinel oxide nanowires (CuCo₂O₄–Co₃O₄) were assembled into a chrysanthemum like hybrid crystal structure and grown on NF. CuCo₂O₄–Co₃O₄/NF exhibited excellent OER performance with an overpotential of only 267 mV to obtain a current of 10 mA cm⁻² in an alkaline electrolyte with a low Tafel slope (114.59 mV dec⁻¹) and excellent stability with a scarce loss of the initial catalyst activity over 20 h under alkaline conditions. The special structure of CuCo₂O₄–Co₃O₄/NF shows a high surface area and excellent charge transfer channels, endowing it with a high surface area and mesoporous structure for the easy evolution of O₂. And the synergistic effect, as well as the transfer of electron density between CuCo₂O₄ and Co₃O₄, contributes to its high OER activity. This study provides a pathway for developing high-performance OER electrocatalysts, resulting in new advances in the field of the oxygen evolution reaction.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (22078251 and 51808414) and the Research Project of Hubei Provincial Department of Education of China (D20191504) as well as by the Graduate Innovation Fund of Wuhan Institute of Technology (CX2021012).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2se01297a

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