High-entropy Cr(NiFeCoV)₂O₄ catalysts via CO₂ laser thermal shock: advancing electrochemical water oxidation with multi-metal synergy

Abstract

Electrocatalytic overall water splitting (OWS) is a promising technology for sustainable hydrogen (H₂) production. However, its practical application is hindered by the sluggish kinetics of the anodic oxygen evolution reaction (OER). To address this challenge, high-entropy oxides have emerged as promising OER electrocatalysts owing to their tunable composition and synergistic effects among constituent elements. In this study, we present the fabrication of a spinel-structured high-entropy Cr(NiFeCoV)₂O₄ (HE-Cr(NiFeCoV)₂O₄) catalyst using a rapid continuous-wave CO₂ laser thermal-shock method. The resulting HE-Cr(NiFeCoV)₂O₄ catalyst demonstrated excellent electrochemical OER performance in 1 M KOH electrolyte, achieving a low overpotential of 284 mV at 10 mA cm⁻² and maintaining long-term stability over 100 h at 50 mA cm⁻². Furthermore, an OWS electrolyzer assembled with HE-Cr(NiFeCoV)₂O₄ as the anode and Pt/C as the cathode operated at a low cell voltage of 1.57 V at 10 mA cm⁻² for efficient H₂ production. In situ Raman spectroscopy confirmed the surface formation of active FeOOH species during OER, while density functional theory calculations revealed how the multi-metal synergy within a single lattice modulated the electronic structure, thereby enhancing the OER activity of Cr(NiFeCoV)₂O₄. This study establishes a cost-effective and energy-efficient pathway for developing advanced multicomponent electrocatalysts for clean energy applications.

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1. Introduction

The increasing global energy demands and the adverse effects of climate change driven by reliance on nonrenewable fossil fuels underscore the urgent need for sustainable energy solutions.^1,2 In this context, renewable hydrogen (H₂) has emerged as a promising clean energy carrier owing to its high energy density, environmental compatibility, and zero carbon emissions.^3,4 Among various H₂ production technologies, water electrolysis is widely recognized as an efficient and environmentally friendly process that facilitates overall water splitting (OWS) powered by renewable energy sources.^5,6 This process comprises the hydrogen evolution reaction at the cathode and the oxygen evolution reaction (OER) at the anode, yielding H₂ and O₂ gases, respectively.^7,8 However, the large-scale implementation of OWS for H₂ production is hindered by the slow kinetics and high overpotential of the anodic OER, necessitating the development of efficient and robust electrocatalysts to minimize energy input and enhance overall efficiency of the OWS. Although noble metal-based catalysts such as IrO₂ and RuO₂ exhibit exceptional electrochemical OER activity, their scarcity, high cost, and limited durability restrict their widespread application.^9,10 This has prompted the search for low-cost, durable, and efficient alternatives for OER electrocatalysts.

Transition metals and their compounds, including oxides, sulfides, phosphides, carbides, and hydroxides, have been extensively studied as potential OER electrocatalysts.^11–13 Among these, transition metal oxides-particularly those with rock-salt, perovskite, and spinel crystalline structures-have demonstrated exceptional electrocatalytic performance owing to their strong OH⁻ adsorption and surface-active *OOH intermediate generation capabilities via metal–oxygen (M–O) bonding, which enhances OER kinetics.^14,15 Spinel-structured metal oxides particularly stand out owing to their straightforward synthesis, structural and compositional versatility, mixed metal valence states (+2 and +3), excellent catalytic redox properties, and stability in strongly alkaline environments.^16,17 Recently, high-entropy oxides (HEOs), comprising five or more equimolar cations alongside oxygen anions, have emerged as a transformative class of electrocatalysts, offering viable alternatives to noble metal catalysts in energy conversion and storage applications.^18–20 The incorporation of multiple metal cations in a single HEO structure facilitates extensive electron transfer between metal cations, resulting in strong electron redistribution and abundant active sites for electrochemical reactions.^21,22 Additionally, the lattice distortion induced by multiple metal species generates efficient surface-oxygen vacancies, enhancing electron redistribution and creating unsaturated coordination environments in the HEOs.^23–25 Remarkably, the high-entropy effect in HEOs, induced by multiple metal components, increases configurational entropy and reduces Gibbs free energy (ΔG↓ = ΔH − TΔS↑), thereby improving catalytic activity, structural stability, and synergistic interactions among constituent metals.^26–28 These unique features endow HEO-based OER electrocatalysts with exceptional electrochemical performance and long-term stability, making them promising candidates for advancing OWS efficiency.

Encouraged by the superior advantages of HEO catalysts in OER, we aimed to develop a spinel-structured high-entropy Cr(NiFeCoV)₂O₄ (denoted as HE-Cr(NiFeCoV)₂O₄) electrocatalyst. The integration of Cr, Ni, Fe, Co, and V cations within a single oxide matrix is anticipated to enhance electrocatalytic activity for OER. Additionally, the synthesis method plays a decisive role in determining the production cost, purity, and catalytic performance of nanomaterials. Conventional furnace-based heating methods are frequently utilized for the synthesis of metal oxides, including HEO nanomaterials.^29,30 However, these conventional approaches often require extended heating durations, substantial energy input, and may generate impurities as byproducts, thereby constraining their practical applicability. To mitigate these challenges, thermal-shock synthesis employing a continuous-wave CO₂ laser has been introduced, facilitating rapid, localized heating that allows for precise control over phase formation, crystallinity, and defect engineering.^31–33 Furthermore, this innovative technique reduces thermal gradients, prevents undesired phase transitions, and drastically reduces synthesis time and energy consumption. These benefits position thermal-shock synthesis as a versatile and efficient method for the fabrication of advanced nanomaterials, offering a viable alternative to traditional synthesis techniques.

Herein, we successfully synthesized a spinel-structured HE-Cr(NiFeCoV)₂O₄ catalyst for the first time via a rapid thermal-shock approach using continuous-wave CO₂ laser technology and evaluated its OER performance. The HE-Cr(NiFeCoV)₂O₄ catalyst demonstrated outstanding OER performance, achieving a low overpotential of 284 mV at 10 mA cm⁻² and long-term stability over 100 h at 50 mA cm⁻² in 1 M KOH electrolyte. Furthermore, when integrated as the anode in an OWS electrolyzer with Pt/C as the cathode, the system required a low cell voltage of only 1.57 V at 10 mA cm⁻² for H₂ production and maintained long-term operational stability over 100 h at 50 mA cm⁻², highlighting its practical applicability. To elucidate the origins of the superior performance of the HE-Cr(NiFeCoV)₂O₄ catalyst, advanced in situ Raman spectroscopy and theoretical density functional theory (DFT) simulations were employed. In situ Raman spectroscopy revealed the formation of surface-active FeOOH species on HE-Cr(NiFeCoV)₂O₄ during OER, providing real-time insights into catalyst surface transformations. Concurrently, DFT simulations showed strong electronic modulations within the HE-Cr(NiFeCoV)₂O₄ structure and lower energy barriers for reaction intermediates, contributing to its exceptional OER performance. Overall, this study integrates advanced synthesis techniques and in-depth structural analyses to pave the way for the development of efficient, cost-effective, and durable multicomponent electrocatalysts for renewable energy applications.

2. Experimental section

2.1. Chemicals and reagents

Nickel chloride tetrahydrate (NiCl₂·2H₂O, 96%) was obtained from Yakuri Chemicals, South Korea. Iron chloride tetrahydrate (FeCl₃·6H₂O, 98%), cobalt chloride tetrahydrate (CoCl₂·6H₂O, 98.5%), chromium chloride tetrahydrate (CrCl₃·6H₂O, 98%), vanadium chloride (VCl₃·4H₂O, 99%), absolute ethanol (C₂H₅OH, HPLC grade, 99.9%), potassium hydroxide (KOH, extra pure flakes, 93%), and commercial iridium oxide (IrO₂) were sourced from Daejung Chemicals, South Korea. All chemicals used were of analytical-grade quality and were employed without further purification.

2.2. Synthesis of the HE-Cr(NiFeCoV)₂O₄ catalyst

The spinel-structured HE-Cr(NiFeCoV)₂O₄ catalyst was synthesized using a novel CO₂ laser-induced rapid thermal-shock technique. The synthesis began with the preparation of metal hydroxide precursors via coprecipitation. Specifically, 0.2 mmol of CrCl₃·6H₂O, 0.1 mmol of NiCl₂·2H₂O, 0.1 mmol of FeCl₃·6H₂O, 0.1 mmol of CoCl₂·6H₂O, and 0.1 mmol of VCl₃ were dissolved in 20 mL of deionized water in a beaker under magnetic stirring for 30 min. Next, 20 mL of 0.1 M NaOH solution was added to the reaction mixture, followed by stirring for additional 3 h to ensure complete precipitation. The resulting metal hydroxide precipitates were collected by centrifugation, thoroughly washed with deionized water and absolute ethanol, and dried at 80 °C for 3 h. Subsequently, 5 mg of the dried metal hydroxide precursors was placed in an alumina crucible and subjected to CO₂ laser irradiation at 14 W power using a wide-lens configuration to ensure uniform exposure across the sample surface. This thermal-shock treatment, lasting 2 min, caused the metal hydroxides to decompose and form the spinel-structured HE-Cr(NiFeCoV)₂O₄. The resultant HE-Cr(NiFeCoV)₂O₄ product was washed with absolute ethanol to remove surface moisture and dried at 80 °C for 3 h before characterization and experimentation. For comparison, a bimetallic spinel-structured CrCo₂O₄ catalyst was synthesized using 0.2 mmol of CrCl₃·6H₂O and 0.4 mmol of CoCl₂·6H₂O during the coprecipitation step, while all other procedures remained identical. This comparison aims to investigate the influence of incorporating multiple metals (Ni, Fe, Co, and V) HE-Cr(NiFeCoV)₂O₄, as opposed to using only Co in CrCo₂O_4, on the OER performance.

Details on material characterization techniques, electrochemical assessments, in situ Raman spectroscopy, and computational methods are provided in the ESI.†

3. Results and discussion

3.1. Material synthesis and physicochemical characterizations

Fig. 1a illustrates the schematic of the two-step synthesis procedure for the HE-Cr(NiFeCoV)₂O₄ catalyst. The synthesis involves preparing metal hydroxide precursors via simple coprecipitation, followed by their transformation into HE-Cr(NiFeCoV)₂O₄ through rapid thermal-shock treatment using continuous-wave CO₂ laser technology. This innovative method generates high-temperature thermal shocks, enabling the rapid phase transformation of metal hydroxides into corresponding metal oxides within a very short time-period. The continuous-wave CO₂ laser technology presents a promising alternative to conventional, time-consuming furnace-heating methods for synthesizing various nanomaterials for diverse applications.

Fig. 1 (a) Schematic of the synthesis process, (b–d) HR-TEM images, (e) SAED pattern, and (f) TEM-EDS mapping images of the HE-Cr(NiFeCoV)₂O₄ catalyst.

The surface morphologies and elemental compositions of the synthesized HE-Cr(NiFeCoV)₂O₄ were analyzed using field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS). FE-SEM (Fig. S1a and b†) images revealed agglomerated nanoparticles in both CrCo₂O₄ and HE-Cr(NiFeCoV)₂O₄ catalysts. SEM-EDS mapping confirmed the uniform distribution of Cr, Ni, Co, Fe, V, and O in the HE-Cr(NiFeCoV)₂O₄ structure. The SEM-EDS spectra (Fig. S2†) showed the elemental composition of CrCo₂O₄: 18.93 wt% Cr, 43.48 wt% Co, and 37.59 wt% O. In contrast, HE-Cr(NiFeCoV)₂O₄ catalyst showed elemental compositions of 19.75 wt% Cr, 12.53 wt% Ni, 11.85 wt% Fe, 12.92 wt% Co, 7.30 wt% V, and 35.65 wt% O, confirming the successful incorporation of multiple metals and its high-entropy nature. HR-TEM images of the HE-Cr(NiFeCoV)₂O₄ catalyst (Fig. 1b–d) further demonstrated agglomerated nanoparticles with well-defined lattice fringes, showing an interplanar spacing of 0.18 nm corresponding to the (311) crystalline plane of spinel CrCo₂O₄ structure. The selected-area electron diffraction (SAED) pattern (Fig. 1e) confirmed the polycrystalline nature of HE-Cr(NiFeCoV)₂O₄, with distinct crystal planes matching the spinel CrCo₂O₄ framework. Furthermore, TEM-EDS spectra (Fig. S3†) and mapping (Fig. 1f) validated the uniform distribution of Cr, Ni, Fe, Co, V, and O with compositions of 21.45 wt%, 12.75 wt%, 11.33 wt%, 13.04 wt%, 7.06 wt%, and 34.37 wt%, respectively, closely aligning with SEM-EDS results. Based on these compositions, the total mixing entropy (ΔS_mix) was calculated using the equation: , where R is the gas constant, C_i represents the mole fraction of the ith element, and n denotes the total number of components. The calculated ΔS_mix value for HE-Cr(NiFeCoV)₂O₄ was 1.65R, further emphasizing its high-entropy characteristics.

To verify the high-entropy formation and phase purity of the HE-Cr(NiFeCoV)₂O₄ catalyst, X-ray diffraction (XRD) analysis was performed. The XRD patterns (Fig. 2a) of both CrCo₂O₄ and HE-Cr(NiFeCoV)₂O₄ catalysts exhibited similar diffraction peaks, consistent with cubic spinel oxide structure (PDF#22-1084).^34,35 Notably, HE-Cr(NiFeCoV)₂O₄ showed peak shifts toward higher 2θ values compared to CrCo₂O₄, indicating lattice distortion caused by the incorporation of additional Ni, Fe, and V cations into the CrCo₂O₄ structure. This lattice distortion likely modulates the electronic properties of the HE-Cr(NiFeCoV)₂O₄ catalyst, enhancing its OER catalytic activity. These findings confirm the successful synthesis of a single-phase HE-Cr(NiFeCoV)₂O₄ catalyst with multiple metal cations and no additional phases, highlighting its excellent phase purity. Further structural characterization was performed using Raman spectroscopy, which revealed characteristic peaks for the CrCo₂O₄ catalyst (Fig. 2b) at 188, 459, 539, 592, and 660 cm⁻¹ corresponding to the typical F_2g, E_g, F_2g, F_2g, and A_1g vibrational modes of cubic spinel oxides, respectively.^17,36 These characteristic peaks shifted to higher wavenumbers in the HE-Cr(NiFeCoV)₂O₄ catalyst, attributable to modified vibrational modes resulting from enhanced M–O interactions and lattice distortions induced by the integration of multiple metals into a single spinel oxide structure. In addition, Fourier transform infrared spectra (Fig. S4†) revealed distinct vibrational peaks at 487 and 632 cm⁻¹ in CrCo₂O₄, corresponding to M–O stretching vibrations at octahedral and tetrahedral sites, respectively. These peaks were noticeably reduced in intensity for the HE-Cr(NiFeCoV)₂O₄ catalyst, likely owing to lattice distortions from Ni, Fe, and V cation incorporation into the CrCo₂O₄ framework. Brunauer–Emmett–Teller (BET) surface area analysis was performed to assess the influence of multiple metal components on the surface area of the HE-Cr(NiFeCoV)₂O₄ catalyst (Fig. 2c), exhibiting considerably higher nitrogen adsorption and desorption capacities, resulting in a higher BET surface area of 116.11 m² g⁻¹ compared to CrCo₂O₄ (85.37 m² g⁻¹). This increased surface area provides abundant surface-active sites and enhances surface adsorption of reaction intermediates, contributing to improved catalyst's OER performance.

Fig. 2 (a) XRD patterns, (b) Raman spectra, and (c) BET N₂ adsorption–desorption isotherms of CrCo₂O₄ and HE-Cr(NiFeCoV)₂O₄ catalysts. (d–i) Core-level XPS plots of (d) Cr 2p, (d) Ni 2p, (f) Fe 2p, (g) Co 2p, (h) V 2p, and (i) O 1s for the HE-Cr(NiFeCoV)₂O₄ catalyst.

Advanced X-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical composition and oxidation states of the CrCo₂O₄ and HE-Cr(NiFeCoV)₂O₄ catalysts. The core-level XPS plots of the HE-Cr(NiFeCoV)₂O₄ catalyst are shown in Fig. S5† and 2d–i, while those of the CrCo₂O₄ catalyst are shown in Fig. S6a–d.† The wide-range XPS plots confirmed the presence of Cr, Ni, Co, Fe, V, and O in HE-Cr(NiFeCoV)₂O₄ (Fig. S5†), whereas only Cr, Co, and O were detected in CrCo₂O₄ (Fig. S6a†), verifying the successful incorporation of multiple metal components into the HE-Cr(NiFeCoV)₂O₄ structure. The Cr 2p XPS plots of both catalysts revealed two doublet peaks at ∼575.8/585.5 and 578.1/587.3 eV (Fig. 2d and S6b†), corresponding to Cr 2p_3/2/2p_1/2 spins associated with Cr³⁺ and Cr⁴⁺, respectively.³⁷ The Ni 2p XPS plots of HE-Cr(NiFeCoV)₂O₄ splits into two doublet peaks at ∼854.8/872.4 and 857.6/875.1 eV (Fig. 2e), along with satellite peaks, corresponding to Ni 2p_3/2/2p_1/2 spins of Ni²⁺ and Ni³⁺, respectively.³⁵ Similarly, the Fe 2p XPS plots of HE-Cr(NiFeCoV)₂O₄ (Fig. 2f) exhibited two doublet peaks at ∼709.9/712.3 and 723.2/725.9 eV along with satellite peaks, ascribed to Fe 2p_3/2/2p_1/2 spins of Fe²⁺ and Fe³⁺, respectively.³⁸ The Co 2p XPS plots of both catalysts (Fig. 2g and S6c†) showed two doublet peaks at ∼780.0/795.6 and 782.6/797.5 eV, with satellite peaks, corresponding to the Co 2p_3/2/2p_1/2 spins of Co³⁺ and Co²⁺, respectively. The V 2p XPS plots of HE-Cr(NiFeCoV)₂O₄ (Fig. 2h) showed two peaks at 516.2 and 523.6 eV, corresponding to the V 2p_3/2 and V 2p_1/2 spins of V³⁺, respectively.^39,40 The O 1s XPS plots of both catalysts (Fig. 2i and S6d†) exhibited a peak at ∼529.4 eV, corresponding to the lattice M–O bonds (where M = Cr/Co in CrCo₂O₄ or M = Cr/Ni/Fe/Co/V in HE-Cr(NiFeCoV)₂O₄) while additional peaks at 531.2 and 532.3 eV are ascribed to surface-adsorbed O₂ and H₂O molecules, respectively.³⁶ These results confirmed the presence of Ni, Fe, and Co in mixed +2 and +3 oxidation states, Cr in mixed +3 and +4 oxidation states, and V in the +3 oxidation state in HE-Cr(NiFeCoV)₂O₄. The incorporation of multiple cations with mixed oxidation states substantially modulates the electronic structure, enhancing the intrinsic electrocatalytic activity of HE-Cr(NiFeCoV)₂O₄. In addition, ultraviolet photoelectron spectra of the CrCo₂O₄ and HE-Cr(NiFeCoV)₂O₄ (Fig. S7a and b†) revealed E_cut-off values of 7.26 and 8.68 eV, respectively, where E_cut-off represents the kinetic energy cutoff of secondary electrons. The work function for HE-Cr(NiFeCoV)₂O₄ was calculated as 12.54 eV, compared to 13.96 eV for CrCo₂O₄, reflecting its modulated electronic structure, enhanced electron transport, and improved electrical conductivity.⁴¹ Reflection electron energy loss spectra (Fig. S8†) revealed a prominent decrease in elastic peak intensity for HE-Cr(NiFeCoV)₂O₄ compared to CrCo₂O₄, highlighting the enhanced surface and electronic properties of the HE-Cr(NiFeCoV)₂O₄ catalyst—potentially accelerating its electrocatalytic activity for OER.⁴²

3.2. Electrocatalytic performance of HE-Cr(NiFeCoV)₂O₄ toward OER

The electrocatalytic OER performance of the synthesized HE-Cr(NiFeCoV)₂O₄ catalyst was evaluated using a three-electrolyte configuration in 1 KOH electrolyte (Fig. 3a). For comparison, the OER performance of CrCo₂O₄ and commercial IrO₂ catalysts was also assessed. As shown in Fig. 3b, the linear sweep voltammetry (LSV) plots recorded at 5 mV s⁻¹ reveal that HE-Cr(NiFeCoV)₂O₄ exhibited considerably enhanced OER performance, as characterized by a greater negative shift in the onset potential and higher current densities compared to CrCo₂O₄ and IrO₂ catalysts. Specifically, Fig. 3c indicates that HE-Cr(NiFeCoV)₂O₄ achieved a lower overpotential of 284 mV at mA cm⁻², compared to the bare nickel foam (NF) substrate (440 V), CrCo₂O₄ (404 V), and IrO₂ (375 V). This reduced overpotential underscores the superior OER activity of HE-Cr(NiFeCoV)₂O₄, attributed to its optimized electronic structure and abundant active sites resulting from the synergistic contributions of multiple metal cations.³⁸ To further validate the OER kinetics of HE-Cr(NiFeCoV)₂O₄, Tafel slope analysis was conducted.⁴³ As depicted in Fig. 3d, HE-Cr(NiFeCoV)₂O₄ exhibited a considerably smaller Tafel slope of 99 mV dec⁻¹, compared to the bare NF substrate (243 mV dec⁻¹), CrCo₂O₄ (186 mV dec⁻¹), and commercial IrO₂ (132 mV dec⁻¹). This lower Tafel slope indicates an optimized reaction pathway with faster charge transfer kinetics, leading to increased intrinsic catalytic activity and lower energy barriers for reaction intermediates in HE-Cr(NiFeCoV)₂O₄ compared to CrCo₂O₄, thereby accelerating the OER process.²³ In addition, the intrinsic catalytic activity of HE-Cr(NiFeCoV)₂O₄ was further assessed through mass activity and turnover frequency (TOF) analyses. Fig. S9† shows that HE-Cr(NiFeCoV)₂O₄ possessed the highest mass activity of 79.78 A g⁻¹ at 1.7 V vs. reversible hydrogen electrode (RHE) among the tested catalysts. Similarly, the TOF values derived from cyclic voltammetry (CV) measurements (Fig. S10a–d†) demonstrated that HE-Cr(NiFeCoV)₂O₄ exhibited the highest TOF of 0.71 s⁻¹ at 1.7 V vs. RHE among the tested catalysts (Fig. 3e). These findings highlight the role of multi-metal environment in enhancing the utilization of active sites and intrinsic catalytic activity of HE-Cr(NiFeCoV)₂O₄, thereby boosting OER performance.¹³ The charge transfer kinetics of HE-Cr(NiFeCoV)₂O₄ during OER were analyzed using electrochemical impedance spectroscopy.³⁸ Nyquist plots recorded at 1.5 V vs. RHE (Fig. 3f) show that HE-Cr(NiFeCoV)₂O₄ exhibited a charge transfer resistance (R_ct) of 0.75 Ω—notably lower compared to CrCo₂O₄ (6.26 Ω) and commercial IrO₂ (3.87 Ω). This reduced R_ct manifests the efficient charge transfer abilities of HE-Cr(NiFeCoV)₂O₄, attributed to its enhanced electrical conductivity and optimized electronic structure, which are facilitated by the synergistic interaction within its multicomponent composition, ultimately resulting in superior OER performance.⁴⁴

Fig. 3 Electrocatalytic OER performance of the HE-Cr(NiFeCoV)₂O₄ catalyst in 1 M KOH electrolyte: (a) schematic of the three-electrode setup, where WE, CE, and RE denote the working, counter, and reference electrodes, respectively. (b) LSV curves, (c) corresponding overpotential values, (d) Tafel plots, (e) TOF values, (f) Nyquist plots, (g) long-term stability assessment, and (h) comparison of overpotential with previously reported catalysts.

Further examination of the electrochemical surface area (ECSA) and double-layer capacitance (C_dl) provided critical insights into the active surface properties of the HE-Cr(NiFeCoV)₂O₄ catalyst.⁴⁵ CV measurements at varying scan rates (Fig. S11a–d†) were used to determine the ECSA and C_dl of the catalysts. As shown in Fig. S12 and S13,† HE-Cr(NiFeCoV)₂O₄ exhibited a higher C_dl of 18.30 mF cm⁻² and ECSA of 1.83 cm² compared to CrCo₂O₄ (16.62 mF cm⁻² and 1.66 cm²) and commercial IrO₂ (14.02 mF cm⁻² and 1.40 cm²). These results indicate that HE-Cr(NiFeCoV)₂O₄ possesses a greater number of accessible active sites, attributed to its high-entropy structure, which enhances both the intrinsic activity and availability of catalytically active regions during OER.^30,46 In addition to catalytic activity, long-term stability of electrocatalysts is a critical parameter for real-world applications. Long-term stability testing results of the HE-Cr(NiFeCoV)₂O₄ catalyst are displayed in Fig. 3g. Notably, during the prolonged stability test conducted at 50 mA cm⁻² for over 100 h, HE-Cr(NiFeCoV)₂O₄ maintained a nearly stable potential throughout continuous OER operation, highlighting its excellent long-term stability. Post-stability analyses, including SEM, EDS, and XPS, further confirmed the robustness of the HE-Cr(NiFeCoV)₂O₄ electrode before and after OER stability testing at 50 mA cm⁻² over 100 h. SEM and EDS images (Fig. S14†) showed that the surface morphology and elemental composition of the HE-Cr(NiFeCoV)₂O₄ electrode were retained after extended OER operation. Additionally, XPS plots (Fig. S15a–f†) indicated slight increases in Cr⁴⁺ intensities and shifts in the Cr 2p, Ni 2p, Fe 2p, Co 2p, V 2p, and O 1s peaks of the HE-Cr(NiFeCoV)₂O₄ electrode following OER stability testing, suggesting electrochemical modifications to its surface during continuous OER. This surface modification likely contributed to the enhanced OER performance of the HE-Cr(NiFeCoV)₂O₄ catalyst. Remarkably, Fig. 3h and Table S1† illustrate that the HE-Cr(NiFeCoV)₂O₄ catalyst exhibited the lowest overpotential among many recently reported OER catalysts. Overall, these comprehensive electrochemical results highlight the exceptional OER performance of the HE-Cr(NiFeCoV)₂O₄ catalyst, which combines low overpotential with excellent stability. These advantages can be primarily attributed to the synergistic effects of its multicomponent composition, optimized electronic structure, large surface area, enhanced electrical conductivity, increased configurational entropy, and improved surface-active sites. Thus, the CO₂ laser-engineered HE-Cr(NiFeCoV)₂O₄ catalyst is a promising candidate for OER in water electrolysis, advancing sustainable H₂ production.

3.3. Electrochemical performance of HE-Cr(NiFeCoV)₂O₄ toward OWS

Based on the exceptional OER performance of the HE-Cr(NiFeCoV)₂O₄ catalyst, an alkaline OWS electrolyzer was assembled using HE-Cr(NiFeCoV)₂O₄ as the anode and standard Pt/C as the cathode in 1 M KOH electrolyte. A schematic illustration of the assembled OWS electrolyzer with the HE-Cr(NiFeCoV)₂O₄‖Pt/C system is provided in Fig. 4a. As shown in Fig. 4b, the LSV profiles recorded at 5 mV s⁻¹ revealed that the HE-Cr(NiFeCoV)₂O₄‖Pt/C system exhibited superior OWS performance compared to the benchmark IrO₂‖Pt/C system. At current densities of 10, 50, and 100 mA cm⁻², the HE-Cr(NiFeCoV)₂O₄‖Pt/C system required substantially lower cell voltages of 1.57, 1.77, and 1.96 V, respectively, compared to the 1.61, 1.87, and 2.15 V for the IrO₂‖Pt/C system under identical conditions (Fig. 4c). This improved OWS efficiency of the HE-Cr(NiFeCoV)₂O₄‖Pt/C system can be primarily attributed to the improved OER performance facilitated by the HE-Cr(NiFeCoV)₂O₄ anode. Furthermore, gas chromatography was employed to quantitatively evaluate H₂ production by the HE-Cr(NiFeCoV)₂O₄‖Pt/C system. As illustrated in Fig. 4d, the HE-Cr(NiFeCoV)₂O₄‖Pt/C system produced 0.592 mmol of H₂ within 60 min, substantially exceeding the 0.516 mmol generated by the IrO₂‖Pt/C system under identical conditions. These results confirm the superior catalytic efficiency of the HE-Cr(NiFeCoV)₂O₄ catalyst in facilitating OWS for efficient H₂ production. Long-term stability testing was then conducted by maintaining the HE-Cr(NiFeCoV)₂O₄‖Pt/C system at 50 mA cm⁻² for 100 h. The results, presented in Fig. 4e, exhibited minimal performance degradation, with only a 100 mV increase in cell voltage over the continuous 100 h OWS operation. This prolonged operational stability demonstrates the robustness of the HE-Cr(NiFeCoV)₂O₄ anode under alkaline conditions, making it suitable for practical water electrolysis applications. Finally, a comparative analysis with previously reported OWS systems (Fig. 4f and Table S2†), highlights the superior performance of the HE-Cr(NiFeCoV)₂O₄‖Pt/C system, particularly in achieving low cell voltages. Therefore, the combination of high catalytic activity, low energy consumption, and long-term stability positions the HE-Cr(NiFeCoV)₂O₄-based system as a promising candidate for sustainable and efficient H₂ production through OWS.

Fig. 4 OWS performance of Cr(NiFeCoV)₂O₄‖Pt/C and IrO₂‖Pt/C electrolyzers: (a) schematic of the OWS system, (b) LSV curves, (c) corresponding cell voltages, (d) quantitative analysis of H₂ production at 2 V, (e) long-term stability evaluation, and (f) comparison of cell voltage requirements with previously reported OER catalysts for OWS systems.

3.4. In situ Raman and theoretical probes on the HE-Cr(NiFeCoV)₂O₄ catalyst

Advanced in situ Raman spectroscopy was employed to monitor the real-time formation of surface-active intermediates on the HE-Cr(NiFeCoV)₂O₄ catalyst during OER. As schematically depicted in Fig. 5a, a 532 nm laser was focused on the surface of the HE-Cr(NiFeCoV)₂O₄ electrode during the OER process to capture the real-time Raman spectra at potentials ranging from 1.2 to 1.7 V vs. RHE. The recorded spectra (Fig. 5b) revealed two distinct Raman peaks at 699 and 802 cm⁻¹, corresponding to FeOOH. The intensities of these peaks increased as the potential rose from 1.2 to 1.7 V vs. RHE, verifying the formation of surface-active FeOOH on the HE-Cr(NiFeCoV)₂O₄ electrode during the OER process.^47,48 This surface transformation is pivotal to enhancing the overall catalytic OER performance of the HE-Cr(NiFeCoV)₂O₄ catalyst.

Fig. 5 (a) Schematic representation of the in situ Raman spectroscopy setup. (b) In situ Raman spectra of the HE-Cr(NiFeCoV)₂O₄ catalyst during OER. (c) DFT-derived top-view crystal structures of the CrCo₂O₄ catalyst. (d) DFT-derived top-view crystal structures of the HE-Cr(NiFeCoV)₂O₄ catalyst. (e) Charge density difference map of the HE-Cr(NiFeCoV)₂O₄ catalyst. (f) PDOS plots for the CrCo₂O₄ catalyst. (g) PDOS plots for the HE-Cr(NiFeCoV)₂O₄ catalyst. (h) TDOS comparison between the CrCo₂O₄ and HE-Cr(NiFeCoV)₂O₄ catalysts.

To gain a deeper understanding of how multiple metal components influence the electronic structure and OER performance of the HE-Cr(NiFeCoV)₂O₄ catalyst, DFT calculations were performed. The optimized top- and side-view crystal structures of CrCo₂O₄ (Fig. 5c and S16†) and HE-Cr(NiFeCoV)₂O₄ (Fig. 5d and S17†) confirmed the successful incorporation of Cr, Ni, Fe, Co, and V into a single spinel oxide structure. The charge density difference plots (Fig. 5e and S18†) revealed considerable redistribution of charge around the metal centers, indicating electronic modulation throughout the HE-Cr(NiFeCoV)₂O₄ structure. This is ascribed to the incorporation of multiple transition metal cations, which further enhances the electronic properties of the catalyst and is consistent with its superior OER performance.⁴⁴ Moreover, the partial and total density of states (PDOS and TDOS) plots (Fig. 5f–h) reveal a substantially higher TDOS near the Fermi level for HE-Cr(NiFeCoV)₂O₄ compared to CrCo₂O₄. This enhancement primarily arises from the combined contributions of Cr(d), Ni(d), Fe(d), Co(d), and V(d) orbitals, suggesting improved electrical conductivity that facilitates rapid charge transport during OER. These theoretical results further corroborate the experimentally observed enhancement in OER performance of HE-Cr(NiFeCoV)₂O₄.⁴⁹

To evaluate the intrinsic OER activity of various metal sites, we calculated the Gibbs free energy changes associated with the adsorption and transformation of key OER intermediates (*OH, O*, and *OOH) on the Cr, Ni, Fe, Co, and V sites of the HE-Cr(NiFeCoV)₂O₄ surface. The 4e⁻ OER pathway at these active sites, along with the corresponding Gibbs free energy diagrams, is illustrated in Fig. 6a–e. The reaction mechanism proceeds via the adsorption and dissociation of H₂O to form the *OH intermediates, followed by the sequential formation of *O and *OOH species. The final step involves the evolution of O₂, thereby regenerating the active site. Gibbs free energy diagrams calculated at U = 0 V and U = 1.23 V indicate that the formation of *OOH constitutes the potential-determining step (PDS) across all active sites. Among these, the Fe site exhibits the lowest overpotential of 0.45 V, suggesting it is the most favorable active site for OER. This theoretical finding is consistent with Raman spectroscopy results, which demonstrate the in situ formation of FeOOH during OER, likely due to progressive oxidation at the Fe sites. To further investigate this phenomenon, we constructed a model featuring a surface FeOOH layer on HE-Cr(NiFeCoV)₂O₄ (Fig. S19†), optimized the adsorption of OER intermediates on the Fe site of this modified structure, and calculated the corresponding Gibbs free energy profile (Fig. 6f). Notably, the presence of FeOOH further decreased the OER overpotential from 0.45 V to 0.41 V, emphasizing the critical role of surface-formed FeOOH in enhancing OER activity. Collectively, these DFT results highlight the synergistic effects of multi-metallic sites in modulating the electronic structure and reducing reaction energy barriers, thereby conferring superior OER performance to the HE-Cr(NiFeCoV)₂O₄ catalyst.^1,50 Consequently, the continuous-wave CO₂ laser-synthesized HE-Cr(NiFeCoV)₂O₄ catalyst emerges as a promising candidate for water electrolysis, facilitating efficient H₂ production.

Fig. 6 (a–e) OER mechanism and corresponding Gibbs free energy diagrams for the (a) Cr site, (b) Ni site, (c) Fe site, (d) Co site, and (e) V site of the HE-Cr(NiFeCoV)₂O₄ catalyst. (f) OER mechanism along with the corresponding Gibbs free energy diagrams at the Fe-site of the surface-formed FeOOH on the HE-Cr(NiFeCoV)₂O₄ catalyst.

4. Conclusions

Herein, we present a novel, simple, and energy-efficient continuous-wave CO₂ laser technology for synthesizing spinel-structured HE-Cr(NiFeCoV)₂O₄ HEOs via a rapid thermal-shock approach. This approach facilitates the efficient transformation of metal hydroxide precursors into HE-Cr(NiFeCoV)₂O₄ HEOs with a homogeneously distributed multicomponent structure within a short processing time. This method offers several advantages over traditional synthesis techniques, including substantial time and energy savings, prevention of impurity formation, and reduced production costs. Notably, the HE-Cr(NiFeCoV)₂O₄ catalyst demonstrated superior OER performance, requiring a lower overpotential of 284 mV at 10 mA cm⁻² in 1 M KOH electrolyte, outperforming the bimetallic CrCo₂O₄ catalyst (404 mV at 10 mA cm⁻²). The catalyst also exhibited excellent long-term stability, maintaining consistent performance over 100 h of continuous OER operation at 50 mA cm⁻². When integrated HE-Cr(NiFeCoV)₂O₄ as the anode in an OWS electrolyzer with Pt/C as the cathode, the system achieved efficient H₂ production with an exceptionally low cell voltage of 1.57 V at 10 mA cm⁻² and sustained remarkable stability over 100 h at 50 mA cm⁻². These findings underscore the potential of HE-Cr(NiFeCoV)₂O₄ catalyst as a highly suitable candidate for OER in large-scale H₂ production. Further insights into its superior OER performance were provided via in situ Raman spectroscopy and theoretical DFT analyses. Raman studies confirmed the formation of active FeOOH surface species, notably contributing to the enhanced OER performance. DFT calculations revealed that the multi-element composition of HE-Cr(NiFeCoV)₂O₄ optimized its electronic structure, lowering energy barriers for reaction intermediates during OER. The synergistic interaction between Cr, Ni, Fe, Co, and V cations within a single oxide lattice endows the catalyst with unique physicochemical properties that boost OER performance. Overall, this study not only demonstrates the potential of HEOs as robust and efficient electrocatalysts for sustainable energy applications but also paves the way for designing tailored HEOs for multifaceted catalytic systems using the facile and innovative continuous-wave CO₂ laser technology.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

No conflict to declare.

Acknowledgements

This research was supported by the Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education (No. 2019R1A6C1010042 and RS-2024-00434932) and the Glocal University 30 Project Fund of Gyeongsang National University in 2024. The authors acknowledge financial support from the National Research Foundation of Korea (NRF) (2022R1A2C2010686 and 2022R1I1A1A01073299). MU acknowledges the funding support from the Researchers Supporting Project (No. RSPD2025R682), King Saud University, Riyadh, Saudi Arabia.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5ta01573a

‡ These authors contributed equally to this work.

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