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Article

Surface Modified CoCrFeNiMo High Entropy Alloys for Oxygen Evolution Reaction in Alkaline Seawater

by
Zhibin Chen
1,
Kang Huang
1,
Tianyi Zhang
2,*,
Jiuyang Xia
1,
Junsheng Wu
1,
Zequn Zhang
1 and
Bowei Zhang
1,*
1
Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China
2
School of Energy Power and Mechanical Engineering, North China Electric Power University, Beijing 102206, China
*
Authors to whom correspondence should be addressed.
Processes 2023, 11(1), 245; https://doi.org/10.3390/pr11010245
Submission received: 15 December 2022 / Revised: 5 January 2023 / Accepted: 10 January 2023 / Published: 12 January 2023
(This article belongs to the Special Issue Design and Optimization of Clean Energy Systems)
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Abstract

:
Electrolysis of seawater is a promising technique to desalinate seawater and produce high-purity hydrogen production for freshwater and renewable energy, respectively. For the application of seawater electrolysis technique on a large scale, simplicity of manufacture method, repeatability of catalyst products, and stable product quality is generally required in the industry. In this work, a facile, one-step, and metal salt-free fabrication method was developed for the seawater-oxygen-evolution-active catalysts composed of CoCrFeNiMo layered double hydroxide array self-supported on CoCrFeNiMo high entropy alloy substrate. The obtained catalysts show improved performance for oxygen evolution reaction in alkaline artificial seawater solution. The best-performing sample delivered the current densities of 10, 50, and 100 mA cm−2 at low overpotentials of 260.1, 294.3, and 308.4 mV, respectively. In addition, high stability is also achieved since no degradation was observed over the chronoamperometry test of 24 h at the overpotential corresponding to 100 mA cm−2. Furthermore, a failure mechanism OER activity of multi-element LDHs catalysts was put forward in order to enhance catalytic performance and design catalysts with long-term durability.

1. Introduction

Foreseeable energy crises and global environmental pollution have been considered irreconcilable issues of human society [1,2]. Fortunately, hydrogen fuel, which could be fabricated via electrochemical water splitting, is a new type of energy resource with high heat density and harmless combustion product, and widely assumed as the ideal and achievable alternative to non-renewable energy resources [3,4,5,6,7,8]. Electrochemical-driven water splitting, comprised of two half-reactions including hydrogen evolution reaction (HER) taking place on the cathode and oxygen evolution reaction (OER) occurring on the anode to generate H2 and O2 respectively represents a facile and sustainable approach with zero emission and zero resource consumption [9,10,11]. Due to multiple proton-coupled electron transfer processes during the reaction, OER shows sluggish intrinsic kinetics and requires a large overpotential than HER, which results in additional energy loss and severely impedes the commercial application of electrochemical water splitting [12]. Additionally, in comparison with seawater which accounts for 96.5% of water resources on Earth’s surface and represents a nearly unlimited resource, a freshwater resource could not afford the unreasonable allocation of hydrogen preparation of water splitting, which may lead to unnecessary conflicts, especially in water-deficient regions [13,14,15]. Therefore, a pressing demand for the practical application of direct seawater electrolysis was stimulated despite more challenges that remained to surmount.
The major challenge originates from the existence of chloride anions, which could result in the degradation or collapse of the electrodes, poor long-term stability, and chlorine evolution reaction (CER) as well [16]. CER represents an undesirable reaction occurred on the anode during seawater electrolysis which leads to extra and harmful products, and also possesses a kinetic advantage when competing with OER as a result of its facile two-electron oxidation path [17]. Meanwhile, low concentrations of ions in natural seawater could not afford high-speed electrons transfer on the electrodes, further leading to undesirably low-efficient performance for hydrogen [18]. The insoluble precipitates formed on the surface of electrodes due to the presence of Mg2+/Ca2+ ions, along with the inevitable fluctuations of local pH nearby electrodes, would also attenuate the long-term durability of electrodes [19]. Fortunately, alkalizing the electrolyte could totally avoid the unsatisfied ions content; the insoluble precipitates, as well as CER on account of its overpotentials, is about 480 mV larger than that of OER in alkaline media according to the Pourbaix diagram [20]. Additionally, reinforcing the intrinsic activity of electrodes (catalysts) is another key part of boosting effective seawater electrolysis [21,22,23,24]. In general, Ir/Ru-based materials (such as IrO2, RuO2) have been regarded as state-of-the-art candidates for water electrolysis, while their scarcity and expensiveness of them act as the bottleneck for their widespread use [25,26]. Currently, various non-precious metal-based catalysts, outperforming IrO2/RuO2 in OER, have been explored as the thriving development of catalysts [27,28,29,30,31,32]. However, the complicated composition and cumbersome preparation procedures inevitably limit the large-scale preparation of catalysts regardless of the size of electrodes in industrial production, making seawater electrolysis still an airy-fairy technology for civil use. To this end, it is highly imperative to explore a facile and one-step manufacturing technique for catalysts with long-term durability.
Over the past decades, tremendous efforts have been dedicated to seeking the fabrication of catalysts assembled on electrodes, and the most commonly employed routes include hydro/solvothermal [33,34,35,36], electrodeposition [37,38,39], co-precipitation [40,41,42], cathode plasma electrolytic deposition [43,44,45], and cation exchange [46,47,48], used solely or cooperatively for catalysts [49,50]. Nevertheless, while realistic reproducibility, controllability, and practicability are taken into consideration, a large proportion of reported catalysts could not satisfy the requirements of low cost, low energy consumption, short manufacturing cycle, and high yield rate in the industry. Therefore, exploring and developing self-supported catalysts with outstanding activity for OER in a one-step and industrially compatible approach is still a challenge. Metal corrosion is a spontaneous oxidation–reduction reaction process in the natural environment, and always brings functional deterioration and is greatly inevitable to the industrial metallic products, and causes economic loss to the industry as well. However, taking advantage of the spontaneity and convenience, the harmful corrosion which generally occurs on the surface of metallic material and forms the corrosion layers could be applied to modify the surface and prepare catalysts directly used as electrodes, and this surface modification method by corrosion is named corrosion engineering. In-situ grown and tridimensional construction prepared by corrosion engineering increases the active surface with active sites and improves the catalytic performance of metallic electrodes, which also omits the assemblage work of electrodes with catalysts and minimizes labor and time cost. Wu et al. [51] reported that the highly active catalyst consisted of ultrathin Fe-doped Ni3S2 arrays self-supported onto Ni foam and was obtained via a low-cost and scaled-up chemical etching method for both OER and HER. In 1.0 M KOH, the improved Fe0.9Ni2.1S2@NF catalyst requires overpotentials of only 72 mV and 252 mV to achieve the current density of 10 mA cm−2 and 100 mA cm−2 for HER and OER, respectively. Xia et al. [52] developed a microorganism corrosion strategy with the help of anaerobic sulfate-reducing bacteria to prepare Ni(Fe)OOH–FeSx catalysts on Ni foam, which exhibited remarkable electrocatalytic performance for OER in alkaline electrolyte, requiring a low overpotential of 220 mV to achieve the current density of 10 mA cm−2. Thus, the bridge between corrosion engineering and energy conversion technologies was preliminarily constructed, showing the potential for infinite energy resources. Moreover, benefiting from the synergistic effect, numerous catalysts consisting of multiple elements show improved electrochemical activity and intrinsic charge transfer than unary or binary material [53,54]. However, limited by substrate, few efforts have been devoted to multiple-element catalysts.
Generally, the direct product prepared by corrosion engineering without extra additive is metal (oxy)hydroxides such as layered double hydroxides (LDHs), which have been widely considered the most promising candidates owing to their tremendous specific surface areas, high-density active sites, and flexible valence states, and has also been confirmed to be the actual species with catalytic performance for OER [55,56,57]. Furthermore, hydroxide layers on the electrodes could block the invasion of Cl− to avoid the collapse of electrodes [24,58]. Strasser et al. [16] first reported that 100% Faraday efficiency towards OER was realized on a NiFe-LDH catalyst with brilliant electrocatalytic performance in a weak alkaline stimulated seawater (0.1 M KOH + 0.5 M NaCl). Du and co-workers [59] synthesized NiFe-LDH nanosheet arrays on a carbon fiber cloth, and the as-prepared NiFe-LDH/CC composites with a Ni/Fe ratio of 6:4 showed the best performance for OER, requiring the overpotential of 238 mV to achieve 10 mA cm−2 in alkaline seawater. Nevertheless, considering the simplicity of the corrosion engineering approach without the auxiliary additive of metal salt, each kind of metal element should originate from the metallic substrate. Thus, the option of metallic substrate significantly impacts the electrochemical activities of the product, which would serve as an electrocatalyst/electrode further. The high entropy alloy (HEA) as an emerging metal material exhibits a remarkable diversity of composed elements and satisfactory conductivity, conforming to the requirements for electrodes prepared via the corrosion engineering method. The significant advantage of high entropy alloys is the diversity of composed elements, along with their content leads to no additional metal salts or binders being required in the fabrication process for the catalyst. Thus, HEA is allowed to form new and tailorable active sites in multiple elements adjacent to each other, and the interaction could be tailored by rational selection of element configuration and composition, leading to an achievable operation of the various catalytic properties in one material. Moreover, the bulk high entropy alloys could be reused for a super long service life, which is beneficial to reduce the cost of catalysts [60,61,62,63,64].
Herein, we establish the facile and one-step corrosion engineering approach to in-situ synthesize CoCrFeNiMo quinary LDHs (CoCrFeNiMo-LDHs) array onto CoCrFeNiMo high entropy alloy (CoCrFeNiMo-HEA), named CoCrFeNiMo-LDHs/HEA. During the hydrothermal process, corrosion occurred on the surface region with the stimulation of NaOH electrolyte and heated environment to release and offer metal ions for LDHs material. The samples with corrosion engineering treatment in this work exhibited improved catalytic performance for OER. More specifically, the CoCrFeNiMo-LDHs/HEA catalyst prepared in 3 M NaOH solution at 130 °C with 12 h treatment duration (including heat-up time and holding time) possesses the best performance among the samples, manifesting a low overpotential of 260.1 mV, 294.3 mV, and 308.4 mV were required in a simulated alkaline seawater electrolyte (containing 1.0 M KOH + 0.5 M NaCl) for the current densities of 10 mA cm−2, 50 mA cm−2, and 100 mA cm−2, respectively. This work offers a failure mechanism and a new thought to rational design and large-scale manufacture for durable catalysts for seawater splitting.

2. Materials and Methods

2.1. Preparation of CoCrFeNiMo-LDHs on HEA

The chemical composition of CoCrFeNiMo high entropy alloy was provided in Table S1. A series of CoCrFeNiMo-LDHs/HEA electrodes were prepared by a one-step corrosion engineering approach. CoCrFeNiMo-HEA (10 × 15 × 0.5 mm) underwent the surface pretreatment of grind and ultrasonic wash with deionized water and ethanol to remove original metal oxides and enlarge the region for corrosion. After that, the CoCrFeNiMo-HEA was dried at 60 °C for 12 h under a vacuum. For the best electrocatalytic performance, 0.06 mol NaOH was dissolved in 20 mL of deionized water with vigorous stirring for 30 min to disperse the solution evenly. Subsequently, the NaOH solution was transferred into a Teflon autoclave, followed by that as-prepared CoCrFeNiMo-HEA was submerged into the NaOH solution in the Teflon autoclave. Next, the Teflon autoclave was maintained at 130 °C for 12 h. After naturally cooling the system to room temperature, the sample was taken out and cleaned with deionized water to remove the attached solution. Finally, the golden yellow sample was dried at 60 °C overnight, and a kind of CoCrFeNiMo-LDHs/HEA electrode was acquired. The obtained electrodes are labeled as LDHs-x M-y h (x M and y h corresponding to the concentration of NaOH solution and the kept duration at 130 °C), based on the parameter of preparation.

2.2. Characterization

Scanning electron microscope (SEM, Regulus 8100) and transmission electron microscopy (TEM, TECNAI F20 equipped with energy-dispersive spectrometry) were employed to investigate the morphology and microstructure of the samples. Grazing incident X-ray diffraction patterns of the products were recorded on a Brucker D8 ADVANCE X-ray diffractometer with Cu-Kα radiation and a 2θ angle ranging from 10° to 90° at a scanning rate of 5° min−1. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250XI. All data of the binding energies were calibrated using the value of C 1s of 284.8 eV. Inductively coupled plasma-optical emission spectroscopy (ICP-OES) measurement was conducted by using an Agilent ICPOES730.

2.3. Electrochemical Measurements

Since the dominantly composed ions of Na+ and Cl− are in seawater, 0.5 M NaCl aqueous solution is generally assumed as an artificial substitute for seawater. Electrochemical measurements in 1 M KOH + 0.5 M NaCl aqueous solution for the OER test were performed on an electrochemistry workstation (Autolab PGSTAT 302N) at room temperature in a standard three-electrode system where the as-prepared CoCrFeNiMo-LDHs/HEA was used as the working electrode, a graphite rod was employed as the counter electrode, and Ag/AgCl electrode saturated KCl was employed as the reference electrode, respectively. All the potentials in the linear sweep voltammetry (LSV) test were referenced to the reversible hydrogen electrode according to the formula [65]:
ERHE = EAg/AgCl + 0.197 V + 0.059 × pH
The polarization curves of OER were evaluated by LSV with a scan rate of 10 mV·s−1, and all the LSV curves in this work were corrected with iR compensation because of the resistance from the electrolyte. The overpotential of samples was calculated via the formula [66]:
η (V) = ERHE − 1.23 V
Electrochemical impedance spectroscopy (EIS) measurements for samples were performed with the same condition of a frequency range from 0.01 Hz to 100 KHz. The value of double layered capacitance (Cdl) of the samples was calculated and used for the comparison of the electrochemical surface area (ECSA) among samples. The Cdl value was estimated by CV curves in the non-faradaic potential range at different scan rates of 10, 30, 50, 70, 90, and 110 mV·s−1. The charging current density differences (j = ja − jc) were linear with the scan rates, and the half value of the relevant slope corresponds to the Cdl.
A Chronoamperometry test was employed to obtain stability of the catalyst at the potential corresponding to the current density of 100 mA cm−2.

3. Results and Discussion

The ultra-evenly quinary CoCrFeNiMo LDHs nanosheets anchored onto CoCrFeNiMo-HEA, serving as the high-efficiency catalyst and practical anode without further treatment in electrocatalytic seawater splitting, were fabricated via a one-step and facile hydrothermal protocol as schematically presented in Scheme 1. Therein, each metal ion, continuously released from the surface of CoCrFeNiMo-HEA during the sequential corrosion process under the stimuli of heating and alkaline condition, continuously and simultaneously combined with OH− to synthesize the compound of CoCrFeNiMo-LDHs, and ultimately, followed by the nucleation on the surface of CoCrFeNiMo-HEA, and growth in the morphology of CoCrFeNiMo-LDHs as time progress. For further investigation, NaOH solutions with different concentrations and different processing duration, including heating time together with holding time, were set up at 130 °C in the hydrothermal procedure illustrated in Scheme 1 to produce samples possessing different LDHs morphology and catalytic performance. Therein, the concentration of NaOH solution, including 0.1 M, 1 M, 3 M, 5 M, and 7 M, were carried out for concentration-dependency evolution of CoCrFeNiMo-LDHs. On the other hand, processing durations of 8 h, 12 h, 16 h, and 24 h were selected in time-dependency evolution experiments, respectively.
In the context described above, the surface topography of each sample observed by SEM was used to extrapolate the growth mechanism of CoCrFeNiMo-LDHs. Figure 1b,c, corresponding to the surface morphology of LDHs-0.1 M-12 h and LDHs-1 M-12 h, respectively, reveal the initial stage of the growth state of LDHs nanosheets wherein LDHs were evenly distributed and divided into two sizes including the small size of LDHs in the majority and slightly lager LDHs in the minority in Figure 1b, however, on the contrary in the field of vision of Figure 1c. Additionally, a potential variation tendency of increased concentration of NaOH solution would lead to boosting effect in the morphology of LDHs was preliminary exhibited via Figure 1b,c, and subsequently, this tendency was further confirmed by the comprehensive analysis of Figure 1d–f. As shown in Figure 1d, universally uniform LDHs nanosheets with smooth surfaces were successfully produced under the condition of a concentration of 3 M NaOH. Figure 1e displays the SEM image of LDHs-5 M-12 h, which illustrate LDHs products with larger sheet size, and curly shape broken structure occasioned by deficient room for stretch and collision among LDHs sheets. Obviously, the size of LDHs enlarged as the concentration of NaOH solution increased, and no difference in the morphology of LDHs was found in each image mentioned above. Surprisingly, the growth tendency appears to have no limit due to the extraordinarily enormous size of interlaced and stacked LDHs after the CoCrFeNiMo-HEA suffers from hydrothermal reaction even in 7 M NaOH solution (Figure 1f). With regard to time-dependency evolution experiments, the same growth tendency mentioned in the above description was clearly verified through the exhibition of Figure 1d,g–i, which shows the size of LDHs increased as time progressed. Significantly, the morphology of as-prepared LDHs products maintained sheet shape in this work, demonstrating brilliant constructional stability of LDHs.
To evaluate the practical performance of the corrosion engineering method for high-efficiency multivariate-based catalysts, catalytic activity for OER of as-prepared samples was performed using a standard three-electrode system in alkaline saline water (1 M KOH + 0.5 M NaCl). The freshwater added with 0.5 M NaCl additive is generally assumed to be a simplified artificial substitute for natural seawater solution. Curves of linear sweep voltammetry, obtained from catalysts produced for 12 h in different NaOH solutions, were displayed in Figure 2a, demonstrating advanced OER activity of as-prepared catalysts than the original CoCrFeNiMo-HEA substrate. Furthermore, LDHs-3 M-12 h manifests low overpotential of 260.1 mV, 294.3 mV, and 308.4 mV were required to reach current densities at 10 mA cm−2, 50 mA cm−2, and 100 mA cm−2, respectively, which is evaluated as an optimal catalyst compared with other samples and CoCrFeNiMo-HEA (Table S2). Notably, the current density on LDHs-3 M-12 h rises rapidly with the increase in potential, indicating the admirable performance of promoting OER activity under the condition of high current density. In the range from 0 M to 3 M, increasing active surface area leads to gradually advanced catalytic activity while the performance of samples gradually degrades since the concentration of NaOH surpasses 3 M due to stacked LDHs sheets hindering the electrolyte diffusion and gaseous product release during the catalytic reaction. As LSV curves in Figure 2d show, LDHs-3 M-12 h also possess superior catalytic activity among samples produced in time-dependency evolution experiments, and relevant values of overpotential are depicted in Table S2. Tafel plots in Figure 2b,e exhibit that LDHs-3 M-12 h catalyst possesses the smallest Tafel slope of 39.69 mV dec−1 by contrast with that of CoCrFeNiMo-HEA (98.18 mV dec−1), LDHs-0.1 M-12 h (52.3 mV dec−1), LDHs-1 M-12 h (49.89 mV dec−1), LDHs-3 M-8 h (54.8 mV dec−1), LDHs-3 M-16 h (90.24 mV dec−1), LDHs-3 M-24 h (92.97 mV dec−1), LDHs-5 M-12 h (58.63 mV dec−1), and LDHs-7 M-12 h (77.03 mV dec−1), conforming its relatively rapid OER catalytic kinetics. In addition, to explore the charge-transfer resistance (Rct) of catalysts, EIS measurements were employed, and typical Nyquist plots for OER were shown in Figure 2c,f. Commonly, the diameter of half arc in Nyquist plots could be used to reflect the numerical relationship of the Rct value. Apparently, the smallest diameter of the impedance arc was manifested by LDHs-3 M-12 h, representing its lowest charge-transfer resistance and the fastest electron-transfer rate at the interface among all samples. Moreover, the electrochemical active surface area of catalysts was also investigated by the value of double-layer capacitance (Cdl) from CV curves (Figures S1 and S2). The value of Cdl for LDHs-3 M-12 h was calculated as 136 μF cm−2, which is relatively smaller than LDHs-0.1 M-12 h (91.1 μF cm−2), LDHs-1 M-12 h (97.3 μF cm−2), LDHs-3 M-8 h (121 μF cm−2), LDHs-3 M-16 h (121.7 μF cm−2), LDHs-3 M-24 h (110.6 μF cm−2), LDHs-5 M-12 h (112.2 μF cm−2), LDHs-7 M-12 h (110 μF cm−2), and 3.8 times that of CoCrFeNiMo-HEA (35.5 μF cm−2) demonstrating significantly improved active surface provided by LDHs nanosheet which offers an increased number of active sites for OER (Figure 2g,h). The features, including the fastest OER reaction kinetics, smallest electron transfer resistance, and largest electrochemical active surface, impel LDHs-3 M-12 h to possess outstanding catalytic performance, which is consistent with its smallest value of overpotential. Notably, LDHs-7 M-12 h has a smaller Rct and larger ECSA than that of LDHs-0.1 M-12 h, while the catalytic performance for OER of LDHs-0.1 M-12 h is superior to LDHs-7 M-12 h, and same manifestation takes place between LDHs-3 M-8 h and LDHs-3 M-16 h as well, indicating that reaction kinetic and mass-transport process play a crucial role in enhanced catalytic activity. In general, supported by the satisfactory catalytic performance, the facile and one-step corrosion engineering method was verified as an effective way to synthesize LDH-based material and fabricate highly active catalysts towards seawater oxidation, and the optimal preparation condition for CoCrFeNiMo-HEA was the hydrothermal reaction in 3 M NaOH solution at 120 °C with 12 h heating duration. Subsequently, OER stability of the typical catalyst of LDHs-3 M-12 h was measured by chronoamperometry test under a constant overpotential which achieved the current density of 100 mA cm−2. As the current density-time curve revealed in Figure 2i, the increase in current density at the initial stage originated from the activation of the catalyst, and no obvious fluctuation was found from the curve, suggesting remarkable stability during successive measurements. Meanwhile, there is a noticeable improvement in catalytic performance as revealed in the LSV curves measured before and after the 24 h chronoamperometry test (Figure S3), verifying its extraordinary durability as well, which consists of uphill at the beginning of the current density-time curve. This catalytic activity of LDHs-3 M-12 h for seawater oxidation outperforms several multicomponent-based OER catalysts reported recently (Table 1). According to the electrocatalytic performance for OER of catalysts, the enhancement of electrocatalytic performance originated from the LDHs products on the surface of high entropy alloy, which is composed of five metallic elements. The intrinsic nature of the electronic structure of the metal centers in LDHs nanosheets could be modulated by synergistic effect due to the integration of multiple elements into the one structure and the changes in the electronic environment around metal ions, leading to fast electron transfer of active sites and the improvement of electrocatalytic activities for OER. Moreover, the reinforced specific surface area, electrochemical surface area, and lattice defects obtained by synergistic effect are advantageous to catalytic performance for OER [67,68,69,70,71,72,73].
For further investigation of crystal structures, microstructure, chemical composition of the product, and surface chemical state, a typical sample of LDHs-3 M-12 h was selected to pursue deep details because of its ultra-evenly morphology and superior electrochemical performance than other samples. Unfortunately, extremely thin LDHs layer anchored onto the substrate lead to difficulty identifying peaks corresponding to as-prepared materials. The XRD pattern of LDHs-3 M-12 h exhibits only three peaks located at 43.6°, 50.8°, and 74.7°, which are indexed as (111), (200), and (220) planes of CoCrFeNiMo-HEA substrate as shown in Figure S4a, respectively. The SAED pattern (Figure S4b) recorded from the LDHs nanosheet distinctly proves the certainty of the crystalline material of LDHs, confirming the deduction that the absence of diffraction peaks of LDHs material in the XRD pattern arises from extremely low diffraction intensity compared with the intensity of substrate. TEM image of LDHs nanosheets in Figure 3a further detail its ultra-thin and interlaced morphology, revealing the size and the thickness of the CoCrFeNiMo-LDHs nanosheet is about 400 nm and 10 nm, consisting of SEM image. Figure 3b displays the high-resolution TEM image recorded from the edge of the LDHs nanosheet, showing lattice fringes with the interplanar spacings of 0.242 nm, which is ascribed to the (101) plane of as-synthesized CoCrFeNiMo-LDHs. The EDS mapping analysis (Figure 3c) clearly exhibits that Co, Cr, Fe, Ni, and Mo are homogeneously distributed throughout the entire nanosheet and further verifies the uniform corrosion occurred on the surface of CoCrFeNiMo-HEA substrate during the hydrothermal process and the successful synthesis of the novel material of CoCrFeNiMo-LDHs as well. It is worth noting that the disordered distribution could be observed in the EDS mapping images of Co, Fe, and Mo, originating from the low content of these elements in the LDHs nanosheet, which would further be confirmed by ICP and XPS measurement. Ultrasonic treatment to LDHs-3 M-12 h electrode in deionized water was conducted to obtain pure LDHs products, then followed by the addition of HCl solution to dissolve metal ions from LDHs nanosheet. Subsequently, ICP measurement was conducted to investigate the concrete proportion of metallic elements in LDHs products, and relevant results were displayed in Figure 3d, showing that the ratio of Co:Cr:Fe:Ni:Mo is equal to 1:3.95:1.42:9.49:0.17, which is consistent with the conclusion of EDS mapping.
The surface element state of LDH nanosheets was studied via XPS measurement on LDHs-3 M-12 h, and the high-resolution XPS spectrums of Co, Cr, Fe, Ni, Mo, and O were displayed in Figure 4. Six peaks could be recognized from the high-resolution XPS spectrum of Co 2p (Figure 4a). Therein, two peaks located at a binding energy of 780.9 eV and 795.9 eV indicate the coexistence of Co2+ and Co3+ in LDHs nanosheets [74]. Additionally, the peaks at 790.5 eV and 775.5 eV are assigned to Co0 due to the incomplete surface oxidation of CoCrFeNiMo-HEA, as well as two surplus peaks at 801.6 eV and 784.6 eV corresponding to the relevant satellite peaks [75]. As the Cr 2p narrow scan XPS spectrum shown in Figure 4b, two peaks of Cr 2p1/2 and Cr 2p3/2 appeared at 586 eV with 577.3 eV are ascribed to the species of chromium hydroxide while the peaks at 585.1 eV with 575.8 eV are attributed to the Cr 2p1/2 and Cr 2p3/2 of chromium oxides [76]. The Fe signal displayed in Figure 4c could be divided into three peaks at 725 eV, 710 eV, and 721.9 eV, which are assigned to Fe 2p3/2, Fe 2p1/2 and the satellite peak, respectively, demonstrating the feature of Fe3+ state [77]. The Ni 2p XPS spectrum in Figure 4d indicates that the broad Ni 2p3/2 peak is divided into four peaks at 855.3 eV, 856.7 eV with 864.6 eV, and 861.3 eV corresponding to Ni−OH, Ni−O and satellite signal, as well as characteristic peaks of Ni−OH (872.8 eV), Ni−O (875.1 eV and 882.0 eV) and satellite peak (879.0 eV) originated from Ni 2p1/2, according to the same components as used by Minakshi et al. [78,79]. For the high-resolution XPS of Mo 3d (Figure 4e), the peaks located at 235.4 eV, 231.7 eV (Mo 3d3/2), and 229.6 eV (Mo 3d5/2) indicate two valence states of Mo6+ and Mo3+ [24]. Figure 4f displays the O 1s XPS spectrum with the peaks corresponding to M-OH and M-O information located at 531.3 eV and 529.7 eV, respectively.
To reinforce the catalytic activity for OER, the composition of active sites and failure mechanism was studied via the comprehensive investigation on LDHs-3 M-12 h, which underwent a chronoamperometry test for 100 h and a degradation in performance. The labels of CoCrFeNiMo-BT and CoCrFeNiMo-AT are given to the electrode with original state and with degradation, respectively. As LSV curves in Figure 5a shows, CoCrFeNiMo-AT required overpotentials of 303.1 mV, 374.8 mV, and 431.6 mV to obtain current densities of 10 mA cm−2, 50 mA cm−2, and 100 mA cm−2, which are 43 mV, 80.5 mV and 123.2 mV larger in comparison with CoCrFeNiMo-BT to achieve the same current density. Furthermore, CoCrFeNiMo-AT yielded a higher Tafel slope (79.8 mV dec−1) and a larger diameter of the impedance arc (Figure 5b,c), indicating that the abilities of OER kinetics and electron transfer of the catalyst suffer damage under the condition of electric current act throughout the electrode. Unexpectedly, the value of Cdl (486.8 μF cm−2) increased after the chronoamperometry test due to the exposure of more active sites to the electrolyte, which is 3.57 times and 13.7 times larger than CoCrFeNiMo-BT and CoCrFeNiMo-HEA. Subsequently, SEM, XRD, XPS, and ICP were employed to investigate the causation of attenuated performance. SEM image of CoCrFeNiMo-AT (Figure S5) demonstrated that the structure of LDH nanosheets remained well after the chronoamperometry test, verifying the structural robustness of the as-prepared catalyst. Meanwhile, the rough surface of LDH nanosheets also confirmed our speculation about the variation of the electrochemically active surface. Excellent stability of chemical composition was proved by the XRD pattern due to unchanged peaks in comparison with CoCrFeNiMo-BT (Figure S6), suggesting that no new product was synthesized during the test. Additionally, XPS measurement was conducted to further study the chemical state of CoCrFeNiMo-AT, and comparing with XPS analysis of CoCrFeNiMo-BT, all main peaks identified in XPS spectra including Co 2p, Cr 2p, Fe 2p, Ni 2p, Mo 3d, and O 1s show negligent shift (Figure S7). Due to continuous oxidation on the surface of the catalyst, the peaks, which are assigned to Co0 species and located at 790.5 eV and 775.5 eV corresponding to CoCrFeNiMo-BT, disappeared in Co 2p XPS spectrum. Thus, the collapse of active sites was assumed as the primary reason resulting in the degradation of the catalyst and electrolyte after the test was taken for ICP measurement to validate the assumption. As for the detection result of ICP in Figure 6, there is no big difference among the amount of dissolution of Ni, Cr, Fe, and Mo, and therein, the concentration of Co ions was too low to be taken into consideration. It has been proved that Ni(OH)2 can improve the electrocatalytic performance, which is the result of the Ni element constituting the active site [80,81]. Due to the synergistic effect, the electrocatalytic performance can be further improved by adding elements such as Co, Cr, Fe, and Mo [82,83,84,85]. Thus, the role of the five metal elements is to form the active site. According to the ICP results, on the premise of low content of Fe and Mo elements in LDHs, their amount of dissolution in the electrocatalytic process is relatively large, accompanied by the reduction in catalytic performance. Therefore, we proposed that the activity of active sites containing Mo and/or Fe elements is more effective than that of active sites without these two elements. This discovery offers cogent evidence to inform that active sites containing Mo and Fe elements on LDHs nanosheets possess brilliant kinetics and high efficiency for OER reaction, and the dissolution of Mo and Fe from LDHs material is speculated as the primary cause of degraded OER catalytic performance. Thus, reinforcing the robustness of active sites or increasing the content of specific elements to further enhance active sites should be considered as a potential strategy, as well as a challenge, to produce a highly efficient catalyst with an ultra-long operating lifespan.
Table
Table 1. Activity for OER of catalyst in this work in comparison with other catalysts with multi-elements.
Table 1. Activity for OER of catalyst in this work in comparison with other catalysts with multi-elements.
CatalystElectrolyteOverpotentialDurabilityRef.
CoCrFeNiMo-LDHs @CoCrFeNiMo-HEA1 M KOH + 0.5 M NaClη100 ~ 308.4 mV24 h ~ 100 mA cm−2This work
Fe-Ni(OH)2/Ni3S21 M KOH + 0.5 M NaClη100 ~ 320 mV27 h ~ 100 mA cm−2[86]
NiMo film@NF1 M KOH + 0.5 M NaClη100 = 450 mV15 h ~ 10 mA cm−2[87]
RuNi-Fe2O3/IF1 M KOH + 0.5 M NaClη100 ~ 350 mV20 h ~ 100 mA cm−2[88]
NiFe LDH@Co3O4/NF1 M KOH + 0.5 M NaClη100 = 330 mV—[89]
CoCH@CFP1 M KOH + 0.5 M NaClη100 = 385 mV—[90]
0.5Fe-NiCo2O4@CC1 M KOH + 0.5 M NaClη10 = 273 mV—[91]
NiCoHPi@Ni3N/NF1 M KOH + 0.5 M NaClη100 = 365 mV120 h ~ 200 mA cm−2[92]
NiCoP/NiCo−LDH@NF1 M KOH + 0.5 M NaClη50 = 350 mV50 h ~ 15 mA cm−2[93]
Ni3S2-MoS2-Ni3S2@NF1 M KOH + 0.5 M NaClη100 = 330 mV100 h ~ 100 mA cm−2[94]
CoSe/MoSe21 M KOH + 0.5 M NaClη10 = 320 mV48 h ~ 10 mA cm−2[95]
Mo-CoPX/NF1 M KOH + 0.5 M NaClη100 = 420 mV100 h ~ 10 mA cm−2[96]
B-CoNiOOH/PANI@
C-TiO2/Ti		1 M KOH + 0.5 M NaClη100 = 398 mV72 h ~ 200 mA cm−2[97]
AlNiCoIrMo HEA0.5 M H2SO4η10 = 233 mV48 h ~ 10 mA cm−2[98]
np-UHEAs0.5 M H2SO4η10 = 258 mV10 h ~ 10 mA cm−2[99]
FeCoNiIrRu/CNFs0.5 M H2SO4η10 = 241 mV14 h ~ 10 mA cm−2[100]
H-FeCoNiMnW0.5 M H2SO4η10 = 512 mV—[101]
TiTaFxC2 NP/rGO1 M HClO4η100 = 490 mV40 h ~ 30 mA cm−2[102]
Ru@MoO(S)30.5 M H2SO4η10 = 226 mV—[103]

4. Conclusions

In summary, in-situ fabrication of ultra-evenly CoCrFeNiMo-LDHs was achieved using a facile and one-step corrosion engineering approach. As-formed CoCrFeNiMo-LDHs material exhibit improved catalytic performance for OER in artificial alkaline seawater electrolyte due to its highly porous morphology with large surface area, which ensures efficient mass transfer and offers numerous active metal sites. The best-performing sample named LDHs-3 M-12 h, which is prepared in 3 M NaOH solution and maintained at 12 h at 130 °C, shows a low overpotential of 272.3, 332 mV at 10, 50, and 100 mA cm−2, respectively. LDHs-3 M-12 h also yields a small Tafel slope of 63.4 mV dec−1 and possesses low electron transfer resistance. Moreover, LDHs-3 M-12 h demonstrated good stability, with no obvious fluctuation observed in the current density-time curve. The construction, chemical composition, and surface chemical states of LDHs-3 M-12 h are well maintained after the chronoamperometry test in comparison with that before the test. It is found that the sites composed of Mo and Fe are real active sites for OER, and their dissolution of them under the effect of current is significantly responsible for attenuated catalytic activity and the collapse of CoCrFeNiMo-LDHs/HEA catalyst. This facile fabrication of CoCrFeNiMo-LDHs/HEA catalyst represents a potential strategy to develop more multi-element LDHs-based catalysts for highly efficient electrochemical seawater splitting and clean energy production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/pr11010245/s1, Figure S1: CV curves of as-prepared samples prepared in different NaOH solution and CoCrFeNiMo-HEA. (a) CoCrFeNiMo-HEA (b) LDHs-0.1 M-12 h. (c) LDHs-1 M-12 h. (d) LDHs-3 M-8 h. (f) LDHs-5 M-8 h. (e) LDHs-7 M-8 h. Figure S2: CV curves of as-prepared samples underwent different treatment duration. (a) LDHs-3 M-8 h. (b) LDHs-3 M-16 h. (c) LDHs-3 M-24 h. Figure S3: LSV curves of LDHs-3 M-12 h before and after 24 h stability test. Figure S4: (a) XRD patterns of LDH-3 M-12 h and CoCrFeNiMo-HEA. (b) SAED pattern of CoCrFeNiMo-LDHs nanosheet from LDH-3 M-12 h. Figure S5: SEM image of surface of CoCrFeNiMo-AT. Figure S6: XRD patterns of LDHs-3 M-12 h before and after 100 h chronoamperometry test. Figure S7: High-resolution XPS of (a) Co 2p, (b) Cr 2p, (c) Fe 2p, (d) Ni 2p, (e) Mo 2p, and (f) O 1s of CoCrFeNiMo-AT in comparison with CoCrFeNiMo-LDHs-BT; Table S1: Chemical composition of CoCrFeNiMo high entropy alloy (wt.%). Table S2: Overpotential values (mV) at 10, 50, and 100 mA cm−2 of samples.

Author Contributions

Conceptualization, Z.C. and B.Z.; Investigation, Z.C.; Methodology, Z.C., B.Z. and J.W.; Data Curation, Z.C. and J.X.; Formal Analysis, Z.C., K.H., B.Z. and T.Z.; Writing—Original Draft Preparation, Z.C.; Supervision, Z.Z.; Writing—Review & Editing, Z.C., B.Z. and T.Z.; Project Administration, Z.C.; Funding Acquisition, B.Z. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant No. 51901018), the Natural Science Foundation of Beijing Municipality (Grant No. 2212037), the Young Elite Scientists Sponsorship Program by China Association for Science and Technology (YESS, 2019QNRC001), the National Science and Technology Resources Investigation Program of China (Grant No. 2019FY101400).

Data Availability Statement

Data available upon reasonable request and will be provided by corresponding author.

Conflicts of Interest

The authors declare that they have no conflict of interest or personal relationships that influence the work.

References

  1. Bilgen, S. Structure and environmental impact of global energy consumption. Renew. Sustain. Energy Rev. 2014, 38, 890–902. [Google Scholar] [CrossRef]
  2. Peter, S.C. Reduction of CO2 to chemicals and fuels: A solution to global warming and energy crisis. ACS Energy Lett. 2018, 3, 1557–1561. [Google Scholar] [CrossRef]
  3. Abe, J.O.; Popoola, A.P.I.; Ajenifuja, E.; Popoola, O.M. Hydrogen energy, economy and storage: Review and recommendation. Int. J. Hydrogen Energy 2019, 44, 15072. [Google Scholar] [CrossRef]
  4. Winter, C.J. Hydrogen energy—Abundant, efficient, clean: A debate over the energy-system-of-change. Int. J. Hydrogen Energy 2009, 34 (Suppl. 1), S1–S52. [Google Scholar] [CrossRef]
  5. Dawood, F.; Anda, M.; Shafiullah, G.M. Hydrogen production for energy: An overview. Int. J. Hydrogen Energy 2020, 45, 3847–3869. [Google Scholar] [CrossRef]
  6. Yue, M.; Lambert, H.; Pahon, E.; Roche, R.; Jemei, S.; Hissel, D. Hydrogen energy systems: A critical review of technologies, applications, trends and challenges. Renew. Sustain. Energy Rev. 2021, 146, 111180. [Google Scholar] [CrossRef]
  7. Yan, Y.; Xia, B.Y.; Zhao, B.; Wang, X. A review on noble-metal-free bifunctional heterogeneous catalysts for overall electrochemical water splitting. J. Mater. Chem. A 2016, 4, 17587–17603. [Google Scholar] [CrossRef] [Green Version]
  8. Anantharaj, S.; Ede, S.R.; Sakthikumar, K.; Karthick, K.; Mishra, S.; Kundu, S. Recent trends and perspectives in electrochemical water splitting with an emphasis on sulfide, selenide, and phosphide catalysts of Fe, Co, and Ni: A review. ACS Catal. 2016, 6, 8069–8097. [Google Scholar] [CrossRef]
  9. Li, X.; Hao, X.; Abudula, A.; Guan, G. Nanostructured catalysts for electrochemical water splitting: Current state and prospects. J. Mater. Chem. A 2016, 4, 11973–12000. [Google Scholar] [CrossRef]
  10. Huang, K.; Zhang, B.; Wu, J.; Zhang, T.; Peng, D.; Cao, X.; Zhang, Z.; Li, Z.; Huang, Y.J. Exploring the impact of atomic lattice deformation on oxygen evolution reactions based on a sub-5 nm pure face-centred cubic high-entropy alloy electrocatalyst. J. Mater. Chem. A 2020, 8, 11938–11947. [Google Scholar] [CrossRef]
  11. Roger, I.; Shipman, M.A.; Symes, M.D. Earth-abundant catalysts for electrochemical and photoelectrochemical water splitting. Nat. Rev. Chem. 2017, 1, 0003. [Google Scholar] [CrossRef]
  12. Anantharaj, S.; Noda, S. Amorphous catalysts and electrochemical water splitting: An untold story of harmony. Small 2020, 16, 1905779. [Google Scholar] [CrossRef] [PubMed]
  13. Dresp, S.; Dionigi, F.; Klingenhof, M.; Strasser, P. Direct electrolytic splitting of seawater: Opportunities and challenges. ACS Energy Lett. 2019, 4, 933. [Google Scholar] [CrossRef]
  14. Wang, C.; Shang, H.; Jin, L.; Xu, H.; Du, Y. Advances in hydrogen production from electrocatalytic seawater splitting. Nanoscale 2013, 13, 7897–7912. [Google Scholar] [CrossRef]
  15. Li, J.; Sun, J.; Li, Z.; Meng, X. Recent advances in electrocatalysts for seawater splitting in hydrogen evolution reaction. Int. J. Hydrogen Energy 2022, 47, 29685–29697. [Google Scholar] [CrossRef]
  16. Dionigi, F.; Reier, T.; Pawolek, Z.; Gliech, M.; Strasser, P. Design criteria, operating conditions, and nickel–iron hydroxide catalyst materials for selective seawater electrolysis. ChemSusChem 2016, 9, 962–972. [Google Scholar] [CrossRef]
  17. Khatun, S.; Hirani, H.; Roy, P. Seawater electrocatalysis: Activity and selectivity. J. Mater. Chem. A 2021, 9, 74–86. [Google Scholar] [CrossRef]
  18. Liu, J.; Duan, S.; Shi, H.; Wang, T.; Yang, X.; Huang, Y.; Wu, G.; Li, Q. Rationally Designing efficient electrocatalysts for direct seawater splitting: Challenges, achievements, and promises. Angew. Chem. 2022, 134, e202210753. [Google Scholar]
  19. Wang, X.; Zhai, X.; Yu, Q.; Liu, X.; Meng, X.; Wang, X.; Wang, L. Strategies of designing electrocatalysts for seawater splitting. J. Solid State Chem. 2022, 306, 122799. [Google Scholar] [CrossRef]
  20. Tong, W.; Forster, M.; Dionigi, F.; Dresp, S.; Erami, R.S.; Strasser, P.; Cowan, A.J.; Farràs, P. Electrolysis of low-grade and saline surface water. Nat. Energy 2020, 5, 367. [Google Scholar] [CrossRef]
  21. Wu, L.; Yu, L.; Zhang, F.; McElhenny, B.; Luo, D.; Karim, A.; Chen, S.; Ren, Z. Heterogeneous bimetallic phosphide Ni2P-Fe2P as an efficient bifunctional catalyst for water/seawater splitting. Adv. Funct. Mater. 2021, 31, 2006484. [Google Scholar] [CrossRef]
  22. Wang, Z.; Xu, W.; Yu, K.; Feng, Y.; Zhu, Z. 2D heterogeneous vanadium compound interfacial modulation enhanced synergistic catalytic hydrogen evolution for full pH range seawater splitting. Nanoscale 2020, 12, 6176–6187. [Google Scholar] [CrossRef]
  23. Bennett, J.E. Electrodes for generation of hydrogen and oxygen from seawater. Int. J. Hydrogen Energy 1980, 5, 401. [Google Scholar] [CrossRef]
  24. Yu, L.; Zhu, Q.; Song, S.; McElhenny, B.; Wang, D.; Wu, C.; Qin, Z.; Bao, J.; Yu, Y.; Chen, S.; et al. Non-noble metal-nitride based electrocatalysts for high-performance alkaline seawater electrolysis. Nat. Commun. 2019, 10, 5106. [Google Scholar] [CrossRef] [Green Version]
  25. Li, Y.; Sun, Y.; Qin, Y.; Zhang, W.; Wang, L.; Luo, M.; Yang, H.; Guo, S. Recent advances on water-splitting electrocatalysis mediated by noble-metal-based nanostructured materials. Adv. Energy Mater. 2020, 10, 1903120. [Google Scholar] [CrossRef]
  26. Zhang, K.; Zou, R. Advanced transition metal-based OER electrocatalysts: Current status, opportunities, and challenges. Small 2021, 17, 2100129. [Google Scholar] [CrossRef] [PubMed]
  27. Xu, H.; Ci, S.; Ding, Y.; Wang, G.; Wen, Z. Recent advances in precious metal-free bifunctional catalysts for electrochemical conversion system. J. Mater. Chem. A 2019, 7, 8006–8029. [Google Scholar] [CrossRef]
  28. Guo, Y.; Park, T.; Yi, J.; Henzie, J.; Kim, J.; Wang, Z.; Jiang, B.; Bando, Y.; Sugahara, Y.; Tang, J.; et al. Nanoarchitectonics for transition-metal-sulfide-based electrocatalysts for water splitting. Adv. Mater. 2019, 31, 1807134. [Google Scholar] [CrossRef] [PubMed]
  29. Zhang, H.; Maijenburg, A.; Li, X.; Schweizer, S.L.; Wehrspohn, R. Bifunctional heterostructured transition metal phosphides for efficient electrochemical water splitting. Adv. Funct. Mater. 2020, 30, 2003261. [Google Scholar] [CrossRef]
  30. Peng, X.; Pi, C.; Zhang, X.; Li, S.; Huo, K.; Chu, P.K. Recent progress of transition metal nitrides for efficient electrocatalytic water splitting. Sustain. Energy Fuels 2019, 3, 366–381. [Google Scholar]
  31. Hong, W.T.; Risch, M.; Stoerzinger, K.A.; Grimaud, A.; Suntivich, J.; Shao-Horn, Y. Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis. Energy Environ. Sci. 2015, 8, 1404–1427. [Google Scholar] [CrossRef] [Green Version]
  32. Burke, M.S.; Enman, L.J.; Batchellor, A.S.; Zou, S.; Boettcher, S.W. Oxygen evolution reaction electrocatalysis on transition metal oxides and (oxy) hydroxides: Activity trends and design principles. Chem. Mater. 2015, 27, 7549–7558. [Google Scholar] [CrossRef]
  33. Li, W.; Gao, X.; Xiong, D.; Wei, F.; Song, W.G.; Xu, J.; Liu, L. Hydrothermal synthesis of monolithic Co3Se4 nanowire electrodes for oxygen evolution and overall water splitting with high efficiency and extraordinary catalytic stability. Adv. Energy Mater. 2017, 7, 1602579. [Google Scholar] [CrossRef]
  34. Khan, M.A.; Woo, S.I.; Yang, O.B. Hydrothermally stabilized Fe (III) doped titania active under visible light for water splitting reaction. Int. J. Hydrogen Energy 2008, 33, 5345–5351. [Google Scholar] [CrossRef]
  35. Li, Y.; Zhang, H.; Jiang, M.; Kuang, Y.; Sun, X.; Duan, X. Ternary NiCoP nanosheet arrays: An excellent bifunctional catalyst for alkaline overall water splitting. Nano Res. 2016, 9, 2251–2259. [Google Scholar] [CrossRef]
  36. Fominykh, K.; Feckl, J.M.; Sicklinger, J.; Döblinger, M.; Böcklein, S.; Ziegler, J.; Peter, L.; Rathousky, J.; Scheidt, E.W.; Bein, T.; et al. Ultrasmall dispersible crystalline nickel oxide nanoparticles as high-performance catalysts for electrochemical water splitting. Adv. Funct. Mater. 2014, 24, 3123–3129. [Google Scholar] [CrossRef]
  37. Jiang, N.; You, B.; Sheng, M.; Sun, Y. Electrodeposited cobalt-phosphorous-derived films as competent bifunctional catalysts for overall water splitting. Angew. Chem. 2015, 54, 6251–6254. [Google Scholar] [CrossRef] [PubMed] [Green Version]
  38. Jiang, N.; You, B.; Sheng, M.; Sun, Y. Bifunctionality and mechanism of electrodeposited nickel–phosphorous films for efficient overall water splitting. ChemCatChem 2016, 8, 106–112. [Google Scholar] [CrossRef]
  39. Pu, Z.; Luo, Y.; Asiri, A.; Sun, X. Efficient electrochemical water splitting catalyzed by electrodeposited nickel diselenide nanoparticles based film. ACS Appl. Mater. Interfaces 2016, 8, 4718–4723. [Google Scholar] [CrossRef] [PubMed]
  40. Zhu, W.; Zhang, T.; Zhang, Y.; Yue, Z.; Li, Y.; Wang, R.; Ji, Y.; Sun, X.; Wang, J. A practical-oriented NiFe-based water-oxidation catalyst enabled by ambient redox and hydrolysis co-precipitation strategy. Appl. Catal. B Environ. 2019, 244, 844–852. [Google Scholar] [CrossRef]
  41. Du, X.; Yang, Z.; Li, Y.; Gong, Y.; Zhao, M. Controlled synthesis of Ni(OH)2/Ni3S2 hybrid nanosheet arrays as highly active and stable electrocatalysts for water splitting. J. Mater. Chem. A 2018, 6, 6938–6946. [Google Scholar] [CrossRef]
  42. Li, Y.; Zhang, L.; Xiang, X.; Yan, D.; Li, F. Engineering of ZnCo-layered double hydroxide nanowalls toward high-efficiency electrochemical water oxidation. J. Mater. Chem. A 2014, 2, 13250–13258. [Google Scholar] [CrossRef]
  43. Xiang, J.; Huang, K.; Yao, Z.; Zhang, B.; Li, S.; Chen, Z.; Wu, F.; Wu, J.; Huang, Y. Ternary duplex FeCoNi alloy prepared by cathode plasma electrolytic deposition as a high-efficient electrocatalyst for oxygen evolution reaction. J. Alloys Compd. 2022, 891, 161934. [Google Scholar]
  44. Wu, F.; Yao, Z.; Huang, K.; Zhang, B.; Xia, J.; Chen, Z.; Wu, J. Boosting OER activity of stainless steel by cathodic plasma surface modification. J. Mater. Res. Technol. 2021, 15, 6721–6725. [Google Scholar] [CrossRef]
  45. Huang, K.; Peng, D.; Yao, Z.; Xia, J.; Zhang, B.; Liu, H.; Chen, Z.; Wu, F.; Wu, J.; Huang, Y. Cathodic plasma driven self-assembly of HEAs dendrites by pure single FCC FeCoNiMnCu nanoparticles as high efficient electrocatalysts for OER. Chem. Eng. J. 2021, 425, 131533. [Google Scholar] [CrossRef]
  46. Zhang, H.; Jiang, Q.; Hadden, J.; Xie, F.; Riley, D. Pd ion-exchange and ammonia etching of a prussian blue analogue to produce a high-performance water-splitting catalyst. Adv. Funct. Mater. 2021, 31, 2008989. [Google Scholar] [CrossRef]
  47. YWu, Y.; Liu, Y.; Li, G.; Zou, X.; Lian, X.; Wang, D.; Sun, L.; Asefa, T.; Zou, X. Efficient electrocatalysis of overall water splitting by ultrasmall NixCo3−xS4 coupled Ni3S2 nanosheet arrays. Nano Energy 2017, 35, 161–170. [Google Scholar]
  48. Yu, Y.; Zhang, J.; Wu, X.; Zhao, W.; Zhang, B. Nanoporous single-crystal-like CdxZn1−xS nanosheets fabricated by the cation-exchange reaction of inorganic–organic hybrid ZnS–amine with cadmium ions. Angew. Chem. 2012, 51, 897–900. [Google Scholar] [CrossRef] [PubMed]
  49. Yao, M.; Hu, H.; Sun, B.; Wang, N.; Hu, W.; Komarneni, S. Self-Supportive mesoporous Ni/Co/Fe phosphosulfide nanorods derived from novel hydrothermal electrodeposition as a highly efficient electrocatalyst for overall water splitting. Small 2019, 15, 1905201. [Google Scholar] [CrossRef]
  50. Hsieh, C.; Chuah, X.; Huang, C.; Lin, H.; Chen, Y.; Lu, S. NiFe/(Ni,Fe)3S2 core/shell nanowire arrays as outstanding catalysts for electrolytic water splitting at high current densities. Small Methods 2019, 3, 1900234. [Google Scholar] [CrossRef]
  51. Fei, B.; Chen, Z.; Liu, J.; Xu, H.; Yan, X.; Qing, H.; Chen, M.; Wu, R. Ultrathinning nickel sulfide with modulated electron density for efficient water splitting. Adv. Energy Mater. 2020, 10, 2001963. [Google Scholar] [CrossRef]
  52. Yang, H.; Gong, L.; Wang, H.; Dong, C.; Wang, J.; Qi, K.; Liu, H.; Guo, X.; Xia, B. Preparation of nickel-iron hydroxides by microorganism corrosion for efficient oxygen evolution. Nat. Commun. 2020, 11, 5075. [Google Scholar] [CrossRef] [PubMed]
  53. Chen, M.; Liu, D.; Zi, B.; Chen, Y.; Liu, D.; Du, X.; Li, F.; Zhou, P.; Ke, Y.; Li, J.; et al. Remarkable synergistic effect in cobalt-iron nitride/alloy nanosheets for robust electrochemical water splitting. J. Energy Chem. 2022, 65, 405–414. [Google Scholar] [CrossRef]
  54. Huang, H.; Cho, A.; Kim, S.; Jun, H.; Lee, A.; Han, J. Structural design of amorphous CoMoPx with abundant active sites and synergistic catalysis effect for effective water splitting. Adv. Energy Mater. 2020, 30, 2003889. [Google Scholar]
  55. Fan, G.; Li, F.; Evans, D.; Duan, X. Catalytic applications of layered double hydroxides: Recent advances and perspectives. Chem. Soc. Rev. 2014, 43, 7040–7066. [Google Scholar] [CrossRef]
  56. Xu, M.; Wei, M. Layered double hydroxide-based catalysts: Recent advances in preparation, structure, and applications. Adv. Funct. Mater. 2018, 28, 1802943. [Google Scholar] [CrossRef]
  57. Xu, Z.; Zhang, J.; Adebajo, M.; Zhang, H.; Zhou, C. Catalytic applications of layered double hydroxides and derivatives. Appl. Clay Sci. 2011, 53, 139–150. [Google Scholar] [CrossRef]
  58. Gupta, S.; Forster, M.; Yadav, A.; Cowan, A.; Patel, N.; Patel, M. Highly efficient and selective metal oxy-boride electrocatalysts for oxygen evolution from alkali and saline solutions. ACS Appl. Energy Mater. 2020, 3, 7619–7628. [Google Scholar] [CrossRef]
  59. Dong, G.; Xie, F.; Kou, F.; Chen, T.; Wang, F.; Zhou, Y.; Wu, K.; Du, S.; Fang, M.; Ho, J. NiFe-layered double hydroxide arrays for oxygen evolution reaction in fresh water and seawater. Mater. Today Energy 2021, 22, 100883. [Google Scholar] [CrossRef]
  60. Zhang, Y.; Wang, D.; Wang, S. High-entropy alloys for electrocatalysis: Design, characterization, and applications. Small 2022, 18, 2104339. [Google Scholar] [CrossRef]
  61. Katiyar, N.; Biswas, K.; Yeh, J.; Sharma, S.; Tiwary, C. A perspective on the catalysis using the high entropy alloys. Nano Energy 2021, 88, 106261. [Google Scholar] [CrossRef]
  62. Qiu, H.; Fang, G.; Wen, Y.; Liu, P.; Xie, G.; Liu, X.; Sun, S. Nanoporous high-entropy alloys for highly stable and efficient catalysts. J. Mater. Chem. A 2019, 7, 6499–6506. [Google Scholar] [CrossRef]
  63. Li, K.; Chen, W. Recent progress in high-entropy alloys for catalysts: Synthesis, applications, and prospects. Mater. Today Energy 2021, 20, 100638. [Google Scholar] [CrossRef]
  64. Li, H.; Lai, J.; Li, Z.; Wang, L. Multi-sites electrocatalysis in high-entropy alloys. Adv. Funct. Mater. 2021, 31, 2106715. [Google Scholar] [CrossRef]
  65. Han, L.; Guo, L.; Dong, C.; Zhang, C.; Gao, H.; Niu, J.; Peng, Z.; Zhang, Z. Ternary mesoporous cobalt-iron-nickel oxide efficiently catalyzing oxygen/hydrogen evolution reactions and overall water splitting. Nano Res. 2019, 12, 2281–2287. [Google Scholar] [CrossRef]
  66. Spöri, C.; Kwan, J.; Bonakdarpour, A.; Wilkinson, P.D. The stability challenges of oxygen evolving catalysts: Towards a common fundamental understanding and mitigation of catalyst degradation. Angew. Chem. 2017, 56, 5994–6021. [Google Scholar] [CrossRef] [PubMed]
  67. Jiang, J.; Sun, F.; Zhou, S.; Hu, W.; Zhang, H.; Dong, J.; Jiang, Z.; Zhao, J.; Li, J.; Yan, W.; et al. Atomic-level insight into super-efficient electrocatalytic oxygen evolution on iron and vanadium co-doped nickel (oxy)hydroxide. Nat. Commun. 2018, 9, 2885. [Google Scholar] [CrossRef] [Green Version]
  68. Wen, Y.; Wei, Z.; Liu, J.; Li, R.; Wang, P.; Zhou, B.; Zhang, X.; Li, J.; Li, Z. Synergistic cerium doping and MXene coupling in layered double hydroxides as efficient electrocatalysts for oxygen evolution. J. Energy Chem. 2021, 52, 412–420. [Google Scholar] [CrossRef]
  69. Yu, M.; Zheng, J.; Guo, M. La-doped NiFe-LDH coupled with hierarchical vertically aligned MXene frameworks for efficient overall water splitting. J. Energy Chem. 2022, 70, 472–479. [Google Scholar] [CrossRef]
  70. Xu, H.; Shan, C.; Wu, X.; Sun, M.; Huang, B.; Tang, Y.; Yan, C. Fabrication of layered double hydroxide microcapsules mediated by cerium doping in metal–organic frameworks for boosting water splitting. Energy Environ. Sci. 2020, 13, 2949–2956. [Google Scholar] [CrossRef]
  71. Zhao, G.; Wang, B.; Yan, Q.; Xia, X. Mo-doping-assisted electrochemical transformation to generate CoFe LDH as the highly efficient electrocatalyst for overall water splitting. J. Alloys Compd. 2022, 902, 163738. [Google Scholar] [CrossRef]
  72. Xu, H.; Wang, B.; Shan, C.; Xi, P.; Liu, W.; Tang, Y. Ce-doped NiFe-Layered double hydroxide ultrathin nanosheets/nanocarbon hierarchical nanocomposite as an efficient oxygen evolution catalyst. ACS Appl. Mater. Interface 2018, 10, 6336–6345. [Google Scholar] [CrossRef]
  73. Bera, K.; Karmakar, A.; Kumaravel, S.; Sankar, S.; Madhu, R.; Dhandapani, H.; Nagappan, S.; Kundu, S. Vanadium-doped nickel cobalt layered double hydroxide: A high-performance oxygen evolution reaction electrocatalyst in alkaline medium. Inorg. Chem. 2022, 61, 4502–4512. [Google Scholar] [CrossRef] [PubMed]
  74. Minakshi, M.; Barmi, M.; Jones, R. Rescaling metal molybdate nanostructures with biopolymer for energy storage having high capacitance with robust cycle stability. Dalton Trans. 2017, 46, 3588–3600. [Google Scholar] [CrossRef] [PubMed]
  75. Faraji, M.; Arianpouya, N. NiCoFe-layered double hydroxides/MXene/N-doped carbon nanotube composite as a high-performance bifunctional catalyst for oxygen electrocatalytic reactions in metal-air batteries. J. Electroanal. Chem. 2021, 901, 115797. [Google Scholar] [CrossRef]
  76. Yang, J.; Baker, A.; Liu, H.; Martens, W.; Forster, R. Size-controllable synthesis of chromium oxyhydroxide nanomaterials using a soft chemical hydrothermal route. J. Mater. Sci. 2010, 45, 6574–6585. [Google Scholar] [CrossRef]
  77. Wang, T.; Xu, W.; Wang, H. Ternary NiCoFe layered double hydroxide nanosheets synthesized by cation exchange reaction for oxygen evolution reaction. Electrochim. Acta 2017, 257, 118–127. [Google Scholar] [CrossRef] [Green Version]
  78. Cao, L.; Wang, J.; Zhong, D.; Lu, T. Template-directed synthesis of sulphur doped NiCoFe layered double hydroxide porous nanosheets with enhanced electrocatalytic activity for oxygen evolution reaction. J. Mater. Chem. A 2018, 6, 3224–3230. [Google Scholar] [CrossRef]
  79. Minaksh, M.; Mitchell, D.; Jones, R.; Pramanik, N.; Fulcrand, A.; Garnweitner, G. A hybrid electrochemical energy storage device using sustainable electrode materials. Chem. Sel. 2020, 5, 1597–1606. [Google Scholar] [CrossRef]
  80. Kong, X.; Zhang, C.; Hwang, S.; Chen, Q.; Peng, Z. Free-standing holey Ni(OH)2 nanosheets with enhanced activity for water oxidation. Small 2017, 13, 1700334. [Google Scholar] [CrossRef]
  81. Wu, J.; Subramaniam, J.; Liu, Y.; Geng, D.; Meng, X. Facile assembly of Ni(OH)2 nanosheets on nitrogen-doped carbon nanotubes network as high-performance electrocatalyst for oxygen evolution reaction. J. Alloys Compd. 2018, 731, 766–773. [Google Scholar] [CrossRef]
  82. Kou, T.; Wang, S.; Hauser, J.; Chen, M.; Oliver, S.; Ye, Y.; Guo, J.; Li, Y. Ni foam-supported Fe-doped β-Ni(OH)2 nanosheets show ultralow overpotential for oxygen evolution reaction. ACS Energy Lett. 2019, 4, 622–628. [Google Scholar] [CrossRef]
  83. Wu, Y.; Ji, S.; Wang, H.; Pollet, B.; Wang, X.; Wang, R. A highly efficient water electrolyser cell assembled by asymmetric array electrodes based on Co, Fe-doped Ni(OH)2 nanosheets. Appl. Surf. Sci. 2020, 528, 146972. [Google Scholar] [CrossRef]
  84. He, Y.; Yu, T.; Wen, H.; Guo, R. Boosting the charge transfer of FeOOH/Ni(OH)2 for excellent oxygen evolution reaction via Cr modification. Dalton Trans. 2021, 50, 9746–9753. [Google Scholar] [CrossRef] [PubMed]
  85. Zhang, H.; Xi, B.; Gu, Y.; Chen, W.; Xiong, S. Interface engineering and heterometal doping Mo-NiS/Ni(OH)2 for overall water splitting. Nano Res. 2021, 14, 3466–3473. [Google Scholar] [CrossRef]
  86. Cui, B.; Hu, Z.; Liu, C.; Liu, S.; Chen, F.S.; Hu, S.; Zhang, J.; Zhou, W.; Deng, Y.; Qin, Z.; et al. Heterogeneous lamellar-edged Fe-Ni(OH)2/Ni3S2 nanoarray for efficient and stable seawater oxidation. Nano Res. 2021, 14, 1149–1155. [Google Scholar] [CrossRef]
  87. Yuan, W.; Cui, Z.; Zhu, S.; Li, Z.; Wu, S.; Liang, Y. Structure engineering of electrodeposited NiMo films for highly efficient and durable seawater splitting. Electrochim. Acta 2021, 365, 137366. [Google Scholar] [CrossRef]
  88. Cui, T.; Zhai, X.; Guo, L.; Chi, J.; Zhang, Y.; Zhu, J.; Sun, X.; Wang, L. Controllable synthesis of a self-assembled ultralow Ru, Ni-doped Fe2O3 lily as a bifunctional electrocatalyst for large-current-density alkaline seawater electrolysis. Chin. J. Catal. 2022, 43, 2202–2211. [Google Scholar] [CrossRef]
  89. Lyu, C.; Cheng, J.; Wu, K.; Wu, J.; Wang, N.; Guo, Z.; Hu, P.; Lau, W.; Zheng, J. Interfacial electronic structure modulation of CoP nanowires with FeP nanosheets for enhanced hydrogen evolution under alkaline water/seawater electrolytes. Appl. Catal. B Environ. 2022, 317, 121799. [Google Scholar] [CrossRef]
  90. Li, G.; Li, F.; Zhao, Y.; Li, W.; Zhao, Z.; Li, Y.; Yang, H.; Fan, K.; Zhang, P.; Sun, L. Selective electrochemical alkaline seawater oxidation catalyzed by cobalt carbonate hydroxide nanorod arrays with sequential proton-electron transfer properties. ACS Sustain. Chem. Eng. 2021, 9, 905–913. [Google Scholar] [CrossRef]
  91. Yang, J.; Wang, Y.; Yang, J.; Pang, Y.; Zhu, X.; Lu, Y.; Wu, Y.; Wang, J.; Chen, H.; Kou, Z.; et al. Quench-induced surface engineering boosts alkaline freshwater and seawater oxygen evolution reaction of porous NiCo2O4 nanowires. Small 2022, 18, 2106187. [Google Scholar] [CrossRef] [PubMed]
  92. Sun, H.; Sun, J.; Song, Y.; Zhang, Y.; Qiu, Y.; Sun, M.; Tian, X.; Li, C.; Lv, Z.; Zhang, L. Nickel–cobalt hydrogen phosphate on nickel nitride supported on nickel foam for alkaline seawater electrolysis. ACS Appl. Mater. Interfaces 2022, 14, 22061–22070. [Google Scholar] [CrossRef] [PubMed]
  93. Wu, Y.; Tian, Z.; Yuan, S.; Qi, Z.; Feng, Y.; Wang, Y.; Huang, R.; Zhao, Y.; Sun, J.; Zhao, W.; et al. Solar-driven self-powered alkaline seawater electrolysis via multifunctional earth-abundant heterostructures. Chem. Eng. J. 2021, 411, 128538. [Google Scholar] [CrossRef]
  94. Li, Y.; Wu, X.; Wang, J.; Wei, H.; Zhang, S.; Zhu, S.; Li, Z.; Wu, S.; Jiang, H.; Liang, Y. Sandwich structured Ni3S2-MoS2-Ni3S2@Ni foam electrode as a stable bifunctional electrocatalyst for highly sustained overall seawater splitting. Electrochim. Acta 2021, 390, 138833. [Google Scholar] [CrossRef]
  95. Sun, J.; Li, J.; Li, Z.; Li, C.; Ren, G.; Zhang, Z.; Meng, X. Modulating the electronic structure on cobalt sites by compatible heterojunction fabrication for greatly improved overall water/seawater electrolysis. ACS Sustain. Chem. Eng. 2022, 10, 9980–9990. [Google Scholar] [CrossRef]
  96. Yu, Y.; Li, J.; Luo, J.; Kang, Z.; Jia, C.; Liu, Z.; Huang, W.; Chen, Q.; Deng, P.; Shen, Y.; et al. Mo-decorated cobalt phosphide nanoarrays as bifunctional electrocatalysts for efficient overall water/seawater splitting. Mater. Today Nano 2022, 18, 100216. [Google Scholar] [CrossRef]
  97. Hao, W.; Fu, C.; Wang, Y.; Yin, K.; Yang, H.; Yang, R.; Chen, Z. Coupling boron-modulated bimetallic oxyhydroxide with photosensitive polymer enable highly-active and ultra-stable seawater splitting. J. Energy Chem. 2022, 75, 26–37. [Google Scholar] [CrossRef]
  98. Jin, Z.; Lv, J.; Jia, H.; Liu, W.; Li, H.; Chen, Z.; Lin, X.; Xie, G.; Liu, X.; Sun, S.; et al. Nanoporous Al-Ni-Co-Ir-Mo high-entropy alloy for record-high water splitting activity in acidic environments. Small 2019, 15, 1904180. [Google Scholar] [CrossRef]
  99. Cai, Z.; Goou, H.; Ito, Y.; Tokunaga, T.; Miyauchi, M.; Abe, H.; Fujita, T. Nanoporous ultra-high-entropy alloys containing fourteen elements for water splitting electrocatalysis. Chem. Sci. 2021, 12, 11306–11315. [Google Scholar] [CrossRef]
  100. Zhu, H.; Zhu, Z.; Hao, J.; Sun, S.; Lu, S.; Wang, C.; Ma, P.; Dong, W.; Du, M. High-entropy alloy stabilized active Ir for highly efficient acidic oxygen evolution. Chem. Eng. J. 2022, 431, 133251. [Google Scholar] [CrossRef]
  101. Chang, S.; Cheng, C.; Cheng, P.; Huang, C.; Lu, S. Pulse electrodeposited FeCoNiMnW high entropy alloys as efficient and stable bifunctional electrocatalysts for acidic water splitting. Chem. Eng. J. 2022, 446, 137452. [Google Scholar] [CrossRef]
  102. Feng, M.; Huang, J.; Peng, Y.; Huang, C.; Yue, X.; Huang, S. Tuning electronic structures of transition metal carbides to boost oxygen evolution reactions in acidic medium. ACS Nano 2022, 16, 13834–13844. [Google Scholar] [CrossRef] [PubMed]
  103. Chen, D.; Yu, R.; Wu, D.; Zhao, H.; Wang, P.; Zhu, J.; Ji, P.; Pu, Z.; Chen, L.; Yu, J.; et al. Anion-modulated molybdenum oxide enclosed ruthenium nano-capsules with almost the same water splitting capability in acidic and alkaline media. Nano Energy 2022, 100, 107445. [Google Scholar] [CrossRef]
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Scheme 1. Schematic illustration for the preparation process of CoCrFeNiMo-LDHs/HEA catalysts.
Scheme 1. Schematic illustration for the preparation process of CoCrFeNiMo-LDHs/HEA catalysts.
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Figure 1. SEM images of surface morphology of samples. (a) CoCrFeNiMo-HEA. (b) LDHs-0.1 M-12 h. (c) LDHs-1 M-12 h. (d) LDHs-3 M-12 h. (e) LDHs-5 M-12 h. (f) LDHs-7 M-12 h. (g) LDHs-3 M-8 h. (h) LDHs-3 M-16 h. (i) LDHs-3 M-24 h.
Figure 1. SEM images of surface morphology of samples. (a) CoCrFeNiMo-HEA. (b) LDHs-0.1 M-12 h. (c) LDHs-1 M-12 h. (d) LDHs-3 M-12 h. (e) LDHs-5 M-12 h. (f) LDHs-7 M-12 h. (g) LDHs-3 M-8 h. (h) LDHs-3 M-16 h. (i) LDHs-3 M-24 h.
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Figure 2. Electrocatalytic performance of as-prepared samples in 0.5 M NaCl + 1 M KOH solution. (a,d) OER polarization curves. (b,e) corresponding Tafel slope. (c,f) Nyquist plots. (g,h) Cdl values. (i) current density-time curve.
Figure 2. Electrocatalytic performance of as-prepared samples in 0.5 M NaCl + 1 M KOH solution. (a,d) OER polarization curves. (b,e) corresponding Tafel slope. (c,f) Nyquist plots. (g,h) Cdl values. (i) current density-time curve.
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Figure 3. Details of CoCrFeNiMo-LDHs from LDHs-3 M-12 h (a) TEM image (b) HRTEM image (c) elemental mapping (d) elemental content.
Figure 3. Details of CoCrFeNiMo-LDHs from LDHs-3 M-12 h (a) TEM image (b) HRTEM image (c) elemental mapping (d) elemental content.
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Figure 4. Surface chemical states of LDHs-3 M-12 h and high-resolution XPS of (a) Co 2p. (b) Cr 2p. (c) Fe 2p. (d) Ni 2p. (e) Mo 3d. (f) O 1s.
Figure 4. Surface chemical states of LDHs-3 M-12 h and high-resolution XPS of (a) Co 2p. (b) Cr 2p. (c) Fe 2p. (d) Ni 2p. (e) Mo 3d. (f) O 1s.
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Figure 5. Electrocatalytic performance in 0.5 M NaCl + 1 M KOH of catalyst before and after 100 h chronoamperometry test. (a) LSV curves. (b) Tafel slope. (c) Nyquist plots. (d) Cdl values.
Figure 5. Electrocatalytic performance in 0.5 M NaCl + 1 M KOH of catalyst before and after 100 h chronoamperometry test. (a) LSV curves. (b) Tafel slope. (c) Nyquist plots. (d) Cdl values.
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Figure 6. The concentration of metal ions from LDHs dissolved in the electrolyte.
Figure 6. The concentration of metal ions from LDHs dissolved in the electrolyte.
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Chen, Z.; Huang, K.; Zhang, T.; Xia, J.; Wu, J.; Zhang, Z.; Zhang, B. Surface Modified CoCrFeNiMo High Entropy Alloys for Oxygen Evolution Reaction in Alkaline Seawater. Processes 2023, 11, 245. https://doi.org/10.3390/pr11010245

AMA Style

Chen Z, Huang K, Zhang T, Xia J, Wu J, Zhang Z, Zhang B. Surface Modified CoCrFeNiMo High Entropy Alloys for Oxygen Evolution Reaction in Alkaline Seawater. Processes. 2023; 11(1):245. https://doi.org/10.3390/pr11010245

Chicago/Turabian Style

Chen, Zhibin, Kang Huang, Tianyi Zhang, Jiuyang Xia, Junsheng Wu, Zequn Zhang, and Bowei Zhang. 2023. "Surface Modified CoCrFeNiMo High Entropy Alloys for Oxygen Evolution Reaction in Alkaline Seawater" Processes 11, no. 1: 245. https://doi.org/10.3390/pr11010245

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