Mo modified Co₃O₄ nanosheets array by a rapid quenching strategy for efficient oxygen evolution electrocatalysis

Abstract

The development of transition metal oxides (TMOs) as electrocatalysts for oxygen evolution reaction (OER) has the potential to surpass the performance of noble-metal-based catalysts. In this work, a quenching rapidly strategy was used to synthesize Mo-modified Co₃O₄ nanosheet arrays as advanced catalysts. The resulting Mo-Co₃O₄ electrodes showed superior activity and reaction kinetics, with an overpotential of only 341 mV to drive a current density of 100 mA cm⁻² and a Tafel slope of 69.0 mV dec⁻¹. This improved performance is thought to be due to the formation of high-valence Co sites, which creates a synergistic effect. The ability to regulate the synthesis without causing obvious agglomeration and nucleation growth during annealing makes this method a promising approach for the design of other advanced functional materials.

1 Introduction

As more and more new energy productions come into play now, most people see the use of traditional energy as damaging to the environment (Jeffrey, et al., 2003; Zhang, 2017; Zhi, et al., 2017; Hu, et al., 2021). From the perspective of new energy acquisition, it is a great selection to use the electrochemical method to realize energy conversion technology with high efficiency and stability (Wang et al., 2015). Hydrogen production from the electrolysis of water, for example, consists of two half-cell reactions, the hydrogen evolution reaction (HER) at the cathode and the oxygen precipitation reaction (OER) at the anode, where the OER plays a crucial role (Sayed, 2018; Wu, 2018; He, 2019). The development of high activity/high stability/low-cost OER catalysts for large-scale production is an important issue for academia and the industry today (Suen 2017). Although RuO₂ and IrO₂ noble metal oxides are considered the most efficient OER electrocatalysts, their high costs, apparent scarcity, and low stability have greatly impeded their large-scale application as OER electrocatalysts (Tang et al., 2014; Mao et al., 2015; Yu, 2015; Zhao, 2017; Fang, 2018; Zang et al., 2018; Zhang, 2018; Liu Z. et al, 2019). Owing to the scarcity and high costs of these noble metals, there has been an increasingly growing interest among researchers regarding the use of Earth-abundant elements for the development of efficient electrocatalysts for OER (Liu et al., 2020; Wang L. et al, 2018; Wang B. et al, 2018; Gong et al., 2018; Xu et al., 2018; Yu et al., 2019; Liu G. et al, 2019; Cai et al., 2019; Laguna-Bercero et al., 2019). Transition metal oxides, which contain variable valence metal oxidized redox couples, have recently been considered one promising alternative to noble-metal-based catalysts for efficient OER in alkaline media (Nai et al., 2017; Zheng et al., 2018; Li et al, 2018a; Li et al., 2018b). Numerous studies have confirmed that transition metals and their compounds can be used for electrocatalytic processes through phase transitions, defect generation and migration, and electronic valence changes (Xu H. et al, 2020; Bai et al., 2022). In electrocatalytic processes, transition metal catalysts have the advantage of quickly creating and stabilizing the active center, which can be helpful to reduce the overall energy consumption in the electrocatalytic process and improve the overall reaction stability (Xu Q. et al, 2020).

Nevertheless, the OER electrocatalytic behavior is greatly restricted by intrinsic activity and adsorption ability for active *O and *OOH species (Tan et al., 2019; Chen et al., 2020; Wang Z. et al, 2022). Experiments and theoretical calculations have revealed that adjusting the energy band structure and exposing novel active sites can be applied to regulate the adsorption ability for promoting electrocatalytic performance (Shen et al., 2022; Wang et al., 2019). Thereby, various strategies such as forming composites, doping, interface engineering, etc. are developed to improve these shortcomings (Lei et al., 2014; Wang Z. et al, 2018; Lu et al., 2019). Among these, surface modification can effectively improve the intrinsic activity of the active site by rapidly reconstructing low-potential barrier surfaces (Bai et al., 2022). In some cases, just a slight ions modification of an electrode results in a synergistic catalytic effect enhancing the rate of OER by orders of magnitude (Sadiek et al., 2012), which can introduce oxygen vacancies and mesoporous structures to adjust energy band structure and enhance the charge transfer capacity of the catalyst (Yang et al., 2021; Zhang et al., 2021).

To accomplish modification for improving electrocatalytic performance, some feasible methods were raised such as solvothermal, precipitation, molten salt, and magnetron sputtering (Zhu et al., 2019; Xu H. et al, 2020; Zhang et al., 2020; Fan et al., 2022), which either lead to inevitable agglomeration and nucleation growth or cost too much for mass production (Yang et al., 2021). It is pivotal to develop a succinct and low-cost strategy to maintain the uniformity of microstructure. Herein, we used a one-step quenching strategy that can precisely tailor the surface chemistry by rapid cooling to modify Mo cations on the cobalt oxide self-supporting electrodes (Ye et al., 2021; Liu et al., 2022). The Co₃O₄ modified by Mo cations (Mo-Co₃O₄) has no nucleation growth and remains highly uniform. Moreover, the Mo- Co₃O₄ electrode shows the best OER stability and catalytic activity among all samples, with a low over-potential of 341 mV to reach the current density of 100 mA cm⁻² and a small Tafel slope of 69.0 mV dec⁻¹. It is believed that this process is suitable for industrial mass production with minimal pollution.

2 Experimental section

2.1 Materials

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O, Sigma-Aldrich), Sodium nitrate (NaNO₃, Sigma-Aldrich), Molybdenum acetylacetonate ([CH₃COCH = C(O-)CH₃]₂MoO₂, Macklin), Sodium molybdate (Na₂MoO₄, Macklin), Sodium tungstate (Na₂WO₄, Macklin), Ammonium metavanadate (NH₄VO₃, Macklin), ethanol (C₂H₆O, Aladdin), deionized water and an ultrathin carbon film on holey carbon (400 mesh, Cu, Ted Pella Inc.) were used as received without any further purification.

2.2 Materials synthesis

2.2.1 Co(OH)₂ precursors synthesis

Co(OH)₂ precursors were obtained by loading Co onto the substrate using electrodeposition: the 1*2 cm² nickel foam substrate was all immersed in an electrolyte consisting of 0.1 M cobalt nitrate and 0.01 M sodium nitrate, which was electrostatically anodized at 2 mA cm⁻² to achieve 1,200 s of cobalt hydroxide deposition on the nickel foam substrate evenly.

2.2.2 Molybdenum cation modification

The Co(OH)₂ loaded on nickel foam (10 mm*20 mm*1.1 mm) was taken directly into a muffle furnace (stable at 250°C) and held for 5 min to obtain Co₃O₄. Next, the high-temperature Co₃O₄ was rapidly placed in a molybdenum solution (Molybdenum cation) of acetylacetonate at a concentration of 0.2 mg ml⁻¹ at low temperatures (−40°C, −20°C, 0°C, and 20°C). Then, we kept it in a molybdenum solution of acetylacetonate under low temperatures for 10 min to obtain molybdenum-modified Co₃O₄ (Mo-Co₃O₄) by quenching. Finally, we used deionized water and ethanolic to rinse all samples and then dried them at room temperature.

2.3 Characterization

An X-ray diffractometer (XRD, D/max 2500 V) was used to investigate the crystal structure of the samples. We used a scanning electron microscope (SEM, Zeiss Ultra Plus), and transmission electron microscopy (TEM, JEM-2100F, Japan) to analyze the microstructure of samples. The distribution of elements in Mo-Co₃O₄ was obtained by the energy dispersive spectrometer (EDS). X-ray photoelectron spectroscopy (XPS, Escalab 250 Xi) was used to study the surface chemistry of these samples.

2.4 Electrochemical measurements

All electro-catalytic tests were conducted in a conventional three-electrode electrochemical system containing 1 M KOH solution electrolyte at room temperature, using an Autolab PGSTAT-204 potentiostat equipped with the Nova 2.13 software. The counter electrode and reference electrodes are graphite rod and Hg/HgO (filling with 1M KOH) respectively. The potential of the Hg/HgO reference electrode was regularly calibrated in potassium hydroxide solution before the experiments. All potentials applied herein were calibrated to the RHE using the following equation: E_RHE = E_Hg/_HgO+0.098 + 0.059 × pH. The loading of the commercial catalyst IrO₂ is 48 μg cm⁻². Before LSV measurements, the catalyst had been activated with the scan rate of 50 mV s⁻¹ for at least 50 cycles until stable cyclic voltammetry (CV) curves were obtained in the test range while Tafel slopes were derived from the LSV curves. Stability evaluation was performed by its chronoamperometric response, which was implemented at a current density of about 50 mA cm⁻². The accelerated durability test (ADT) was carried out at the voltage range of 1.10–1.60 V (vs. RHE) for 5,000 cyclic voltammetry cycles with a scan rate of 100 mV s⁻¹. Nyquist plots were obtained from EIS measurements at 1.6 V (vs. RHE) in N2-saturated electrolytes. Chrono analysis (CA) test was further carried out at a constant voltage of 1.5 V vs. RHE in 1 M N₂-saturated KOH for 24h.

3 Results and discussion

3.1 Material structure

The preparation flow of the synthetic Mo ion-modified cobalt hydroxide electrode is shown in Figure 1. The growth of cobalt hydroxide nanoarrays on a nickel foam substrate skeleton is first completed at the anode by conventional electrodeposition. The Co(OH)₂ loaded on nickel foam was taken directly into a muffle furnace to obtain Co₃O₄. Next, the high-temperature Co₃O₄ was rapidly placed in a molybdenum solution (Molybdenum cation) of acetylacetonate at low temperatures. Then, we kept it in the molybdenum solution of acetylacetonate under low temperatures to obtain molybdenum-modified Co₃O₄ (Mo-Co₃O₄) by quenching. For comparison, we also involved anions containing high-valence metals modification by just changing the quenching precursor solution corresponding to Tungsten or Vanadium cation of acetylacetonate and sodium molybdate solution. And we labeled the corresponding samples modified by different ions as W-Co₃O_4, V-Co₃O_4, and A-Mo-Co₃O₄.

FIGURE 1

Two main factors affect the quenching strategy: heating temperature and cooling temperature. Scanning electron microscopy (SEM) was used to further observe the surface morphology of the electrode material. A series of experimental investigations about heating temperature has been done (Supplementary Figure S1), which shows that 250°C heating maintains the original spatial structure of the catalyst stably and regularly. We designed a series of cooling temperature gradients to investigate the effects of low temperatures, therefore. Under 250°C heating temperature, the nanosheets of the sample microstructure grow into disorder and the uniformity of the array decreases (Figure 2) slightly as the temperature of the precursor solution increases. When the precursor solution temperature is at room temperature (−20°C) conditions, the microscopic structure of the material appears significantly irregular. Besides, the EDS elemental mapping pictures (Supplementary Figure S2) of Mo-Co₃O₄ confirm that Co, Mo, and O elements are uniformly distributed throughout the nanosheets.

FIGURE 2

High-angle annular dark field (HAADF) and TEM images (Figures 3A,B) show the micro-morphology of Mo-Co₃O₄ (heated at 250°C, cooled at −40°C). Clear lattice fringes with interplanar spacings of 0.147 nm and 0.245 nm are observed from high-resolution TEM (HR-TEM) images (Figure 3C), respectively. Besides, the selected area electron diffraction (SAED) pattern shows marked diffraction rings corresponding to (220), (311), and (400) planes of the Co₃O₄ phase (Figure 3D). The result confirms that the Mo-Co₃O₄ still well maintains the Co₃O₄ structure, and such microstructure possesses nanoscale grains with a high density of grain boundaries, which could contribute to many active sites for catalytic reactions (Yu et al., 2015). The crystalline phase of the sample (Mo-Co₃O₄) is characterized by X-ray powder diffraction (XRD) to monitor the possible structure and morphology changes induced by ions modification (Figure 4A). To eliminate the strong background of Ni foam substrate, we further amplify the XRD pattern as shown in Figure 4B, which shows the XRD diffraction peaks of samples all can be well indexed to pure Co₃O₄ (JCPDS card No. 43–1,003). There are no additional peaks ascribed to molybdenum phases or compounds after modification, suggesting this method did not convert the structure of the Co₃O₄.

FIGURE 3

FIGURE 4

X-ray photoelectron spectroscopy (XPS) further examines the surface chemical composition of samples. Four typical peaks corresponding to the binding energies of Mo 3 days, O 1 s, Co 2p, and Ni 2p can be observed in Figure 4C. Regarding the Co 2p region in samples, the existence of two fitting peaks belongs to Co 2p3/2 and Co 2p1/2 (Figure 4D) with binding energies at 781.5 and 797.4 eV (Huang et al., 2020). It is noteworthy that the binding energy of Co species increased after Mo modification. And the induced high valence Co is deemed to increase intrinsic activity (Guan et al., 2018; Kou et al., 2020). As shown in Figure 4E, the XPS spectrum of O 1s can be deconvoluted into three pairs of 529.8, 531.3, and 533.3 eV, which can be attributed to the lattice oxygen (O1), coordinative oxygen vacancy or hydroxyl group (O2) and adsorbed H₂O (O3), respectively (Gao et al., 2019; Bao et al., 2015; Zhuang et al., 2017). For Mo-Co₃O₄, the O2’s relative enhancement suggested Mo cation surface modification will produce a large concentration of oxygen vacancies and boost the adsorption/desorption of the OH group (Yang et al., 2017; Ran et al., 2020). This would be beneficial to regulating the ΔG of OH* adsorption/desorption, thus accelerating the kinetic process and finally enhancing the OER activity (Greeley et al., 2006). For Mo 3 days spectrum (Figure 4F), the Mo 3 days peaks of Mo-Co₃O₄ correspond to Mo 3d3/2 (232.3 eV) and Mo 3d5/2 (235.2 eV), demonstrating that Mo was successfully adsorbed into nanosheets. Compared with the binding energies of the Mo precursor (Supplementary Figure S3), the binding energy of Mo species decreased after modification (Wang T. et al, 2022), confirming the electron interaction between Co and Mo species.

3.2 Electrorheological property

The polarization curves after iR-compensation were obtained by linear sweep voltammetry (LSV) at a scan rate of 5 mV s⁻¹ for all samples. The modifying reaction temperature plays an important role in the OER activity of the catalyst. Taking Mo-Co₃O₄ as an example, we first confirmed that the sample obtained at a heating temperature of 250°C had the best electrochemical performance (Supplementary Figure S4). Under the fixed healing temperature of 250°C, as the temperature of the precursor solution increases (−40°C, −20°C, 0°C, 20°C), the samples reach the current density of 100 mA cm⁻² at overpotentials of 341, 356, 373, and 390 mV. In addition, the Tafel slopes of the corresponding electrodes were 69.0 mV dec⁻¹, 91.1 mV dec⁻¹, 114.6 mV dec⁻¹, 146.2 mV dec⁻¹ (Figure 5B), indicating that Mo-Co₃O₄ quenched at -40°C had better electro-catalytic property and OER-kinetics than other as-synthesized materials catalysts. To further explore the role of molybdenum ions in different valence states, our design uses sodium molybdate solution as a precursor to synthesize. As shown in Figures 5D,E, under the same modifying condition (heated under 250°C and quenched under −40°C), Mo-Co₃O₄ exhibited a remarkably electro-catalytic property, which is better than Co₃O₄ modified by molybdenum acid ions (named A-Mo-Co₃O₄) and bare Co₃O₄ sample. Besides, the Mo-Co₃O₄ electrode (heated under 250°C, quenched under −40°C) exhibits the smallest semicircle associated with charge transfer resistance, which indicates optimal electrochemical kinetics (Figures 3C,F, Supplementary Figure S5 and Supplementary Table S1). The electrochemical double layer capacitance (C_dl) is to evaluate the electrochemically active surface areas (ECSA) by measuring the cyclic voltammetry curve of the samples prepared in the range at different scan rates (10, 20, 30, 40, 50 mV s⁻¹). The corresponding CV curves are shown in Supplementary Figure S9, meanwhile, the calculation of C_dl (Supplementary Figures S7, S8) shows the Mo-Co₃O₄ (heated under 250°C, quenched under −40°C) sample reaches the largest value of 44.85 mF cm⁻². The ECSA calculated from C_dl-EIS follows a similar trend as the OER catalytic activity, and Mo-Co₃O₄ (heated under 250°C, quenched under −40°C) owns the largest ECSA value, which means the Mo-Co₃O₄ (heated under 250°C, quenched under −40°C) sample can expose the most active sites to the electrolyte during OER among all samples. Furthermore, we tested the modification function of different high valence metal ions (modified through different quenching precursor solutions we called Mo-Co₃O₄, W-Co₃O₄, V-Co₃O₄, A-Mo-Co₃O₄, and commercial IrO₂) as a contrast (Supplementary Figures S6, S10), and Co₃O₄ catalyst modified by Mo cation had a better performance. Mo-Co₃O₄ (heated under 250°C, quenched under −40°C) has competitive advantages in catalytic activity and Tafel kinetics over related literature (Supplementary Table S2).

FIGURE 5

The improvement observed in the OER electrocatalytic properties of the Mo-Co₃O₄ electrode is attributed to the synergistic effects of the binary metal ions, including Co and Mo (Yang et al., 2018b; Yang L. J. et al, 2018; Bezerra et al., 2020; Guirguis et al., 2020), which acted as active sites. Furthermore, the metal/ions−support interactions (geometric effects, charge transfer) and the interaction between the different oxides resulted in better OER catalysis (Gerber et al., 2019; Bezerra et al., 2020) for the Mo-Co₃O₄ catalyst. It is worth noting that the microstructure of the sample after modification (heated under 250°C and quenched under −40°C) does not appear to noticeable change, which provides stable reaction space and thus provides excellent stability. Figure 5G shows the stability of the electrode investigated by the ADT test, with negligible degradation after 5000 CV cycles. The OER activity of the sample decreased by less than 5% after 24 h of the constant current test (Figure 5H). Obtained under suitable reaction temperatures, the Mo-Co₃O₄ catalysts have a more uniform and regular microstructure which leads to better stability.

4 Conclusion

In the present work, we promoted Co₃O₄ by a surface modification to obtain a cheap and industrially productive electrode for OER. On the one hand, the incorporation of high valence Mo cations brought a synergistic effect with Co and modified electron interaction. On the other hand, the quenching method sustained surface microstructure without heating agglomeration and nucleation growth, which is also advantageous to electrochemical properties. The best sample was obtained at a heating temperature of 250°C and a cooling temperature of −40°C with Mo cation modified, compared to samples modified in other synthetic temperature conditions. Our self-supported Mo-Co₃O₄ electrode avoids the use of expensive polymer binders to fix active material to the substrate. This work provides a valuable strategy to modify the electronic structure and OER catalytic performance of TMO, which obtains better OER activation (Wang et al., 2019; Li et al., 2018b; Yang J. et al., 2018; Zheng et al., 2018; Yu et al., 2019; Liu et al., 2020; Fan et al., 2022).

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