A facile approach to deposit high performance electrocatalyst high entropy oxide coatings using a novel plasma spray route for efficient water splitting in an alkaline medium

Abstract

Electrocatalytic water splitting is a promising technique for producing sustainable hydrogen, but its effectiveness depends on the development of cost-effective and high-performance electrodes. In this work, phase-pure high entropy oxide (HEO) (Ni, Fe, Co, Cu, Mn)₃O₄ nanostructured coating electrodes were fabricated using a solution precursor plasma spray coating technique under optimized conditions with two different molar concentrations (1 M and 2 M) of solution precursors. This process enables precise deposition of a porous catalyst coating on stainless steel substrates, with an average thickness of 30 micrometers. The as-deposited coating shows a spinel structure, and its degree of crystallinity increases with higher molar concentrations of the solution precursors. The HEO coating electrodes demonstrate excellent activity in alkaline media for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER), with low overpotentials of 129 mV and 220 mV, respectively, at a current density of 10 mA cm⁻². A two-electrode device was fabricated, and the results reveal that the required overall potential to achieve a current density of 10 mA cm⁻² is 1.47 V only. This work highlights the potential of solution precursor plasma spray coating as a versatile and scalable approach for producing phase-pure HEO-based water-splitting electrodes, paving the way for large-scale hydrogen generation in sustainable energy systems.

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1. Introduction

The sustainable production of hydrogen (H₂) via electrochemical water splitting is widely considered as a potential solution to address critical global challenges, including energy shortages and clean environmental degradation.¹ Globally, it's portrayed as a transformative alternative to traditional fuels such as coal, oil, and natural gas. However, the advancement of efficient hydrogen production technologies has been constrained by the limited success in replacing noble-metal-based catalysts, which remain the benchmark electrocatalysts for reducing the overpotentials associated with key electrochemical reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode.² Platinum (Pt)-based materials are widely acknowledged as optimal electrocatalysts for the hydrogen evolution reaction (HER), while iridium (Ir) and ruthenium (Ru)-based materials are established as the benchmark electrocatalysts for the oxygen evolution reaction (OER).³ However, the large-scale commercialization of these catalysts remains a formidable challenge, primarily due to the limited natural abundance of these noble metals and the associated high material costs.

To address this critical research gap, efforts are focused on the development of materials composed of highly active, stable, and Earth-abundant non-noble metals, which could provide both cost-effectiveness and superior catalytic performance for water oxidation.⁴ Extensive investigations have been conducted on a variety of transition metal-based compounds, including metal oxides, carbides, nitrides, phosphides, borides, selenides, and sulphides, as potential substitutes for noble metals. These materials are being explored to reduce catalyst costs while simultaneously improving catalytic efficacy. Numerous studies have demonstrated that first-row transition metals, such as nickel (Ni), manganese (Mn), and iron (Fe), exhibit exceptional intrinsic electrochemical activity for water splitting.⁵ Particularly, spinel structure metal oxides comprising multiple transition metals demonstrate superior catalytic activity compared to single-metal and bimetallic spinel oxides. This enhanced performance is attributed to their distinctive electronic structure, characterized by densely packed O²⁻ anions and transition metal cations occupying both tetrahedral and octahedral coordination sites.⁶

Recent studies emphasize the emerging class of multi-metal oxides, termed high entropy oxides (HEOs), which integrate five or more metal cations into a single-phase crystal structure, positioning them as promising catalysts for replacing noble-metal-based catalysts in water splitting. These materials exhibit exceptional properties, including tuneable electrical, optical, and mechanical characteristics, as well as enhanced structural stability, arising from intrinsic elemental interactions.⁷ In contrast to conventional oxide materials, HEOs are characterized by distinctive features such as high entropy, lattice distortion, sluggish kinetics, and cocktail effects, all of which contribute to their unique catalytic performance.⁸

Duan et al. synthesized HEO (Fe, Co, Ni, Cr, Mn)₃O₄ nanoparticles using a solvothermal approach and utilized them as an electrocatalyst for the oxygen evolution reaction. This catalyst demonstrates excellent electrocatalytic performance with an overpotential of 288 mV at a current density of 10 mA cm⁻² and outstanding stability for 95 h in a 1 M KOH solution.⁹ Amarnath et al. synthesized spinel structured (Ni, Co, Cr, Mn, V)₃O₄ HEO nanoparticles through a thermal plasma route and used them as an electrode material for electrochemical water splitting in an alkaline medium. They demonstrated outstanding OER and HER activity with excellent stability.¹⁰ Numerous other methods have been adopted to produce HEO nanoparticles, including hydrothermal synthesis,¹¹ solid-state synthesis,¹² sol–gel,¹³ co-precipitation,¹⁴ flame spray pyrolysis,¹⁵ and nebulized spray pyrolysis.¹⁶ However, in the fabrication of HEO electrodes for electrochemical performance studies, binders and other additives are commonly employed to form an integrated microstructure that ensures stable operation. The inclusion of these additives, along with the coating interface between the electrode surface and the HEO, can undermine the catalytic activity of the material. Therefore, there is a potential need to develop innovative synthesis methods that enable the deposition of additive-free HEO coatings with a uniform phase and microstructure, while ensuring robust adhesion to the electrode surface.

Solution precursor plasma spraying (SPPS) is a versatile technique for producing nanostructured metal oxide coatings on different substrates.¹⁷ This novel process involves injecting a formulated precursor solution containing the desired metal cations into a hot plasma jet, where the solution undergoes various stages of transformation to form in situ metal oxide particles before melting to form splat deposits on the substrate. It is pertinent to note that the single step consolidated deposits exhibit a fine structured microstructure with desirable porosity and adhesive strength.¹⁸ The SPPS technique has the potential to produce both equilibrium and non-equilibrium phases in multi-component oxide systems by optimizing the plasma spray conditions and liquid injection parameters. To the best of the authors' knowledge, single-phase HEO coatings have not yet been achieved using the solution precursor plasma spray technique. Depositing a phase pure HEO coating using thermal plasma is a difficult operation due to the high temperature exposure and heterogeneous reaction environment of the plasma medium. When utilising multi-component oxide-forming precursors, exposure to a high-temperature plasma jet often leads to different rates of vaporisation of certain substances in the feedstock solutions. This process can produce significant phase segregation within the high-entropy oxide (HEO) structure, promoting secondary phase formation. Therefore, meticulous choice of plasma spraying parameters, such as input power, solution feed rate, carrier gas, injection position, substrate-to-gun distance, substrate temperature, scanning velocity, and number of scans, is crucial for the production of single-phase HEO coatings.

In this study, an attempt is being made for the first time to develop an additive-free HEO nanostructured electrocatalyst coating using the solution precursor plasma spray technique under optimal operating conditions. Following preliminary parametric optimization studies, a spinel-structured HEO (Ni, Fe, Co, Cu, Mn)₃O₄ coating was deposited onto a stainless steel (SS-304) substrate using two different molar concentrations of the feedstock precursor solution (1 M and 2 M). The as-sprayed HEO coatings were used as electrode materials, and their electrochemical water-splitting performance was studied in a 1 M KOH electrolyte solution. Linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry tests were conducted to evaluate the water-splitting performance of the HEO coating electrode.

2. Experimental section

2.1. Chemicals and substrate materials

For this work, analytical grade precursors, ferric nitrate nanohydrate [Fe(NO₃)₃·9H₂O, Loba], nickel(II) nitrate hexahydrate [Ni(NO₃)₂·6H₂O, Sigma Aldrich], manganese(II) nitrate [Mn(NO₃)₂·xH₂O, Sigma Aldrich], copper(II) nitrate trihydrate [Cu(NO₃)₂·3H₂O, Sigma Aldrich], and cobalt(II) nitrate hexahydrate [Co(NO₃)₂·6H₂O, Sigma Aldrich] were used without further purification. A stainless steel (SS-304) plate with the dimensions of 30 × 10 × 2 mm was used as a substrate. Before the coating deposition, the SS substrates were grit blasted with 40 mesh alumna grit to obtain an average roughness (R_a) of around 5–7 μm and ultrasonically cleaned with acetone to remove dust.

2.2. Fabrication of nanostructured HEO coatings

Towards the feedstock preparation, all the constituent (Ni, Fe, Mn, Cu, and Co) nitrate precursors were weighed as per the desired stoichiometry, maintaining 1 M & 2 M concentrations, and were dissolved in double distilled water. The solution was stirred continuously for few hours until a clear and transparent precursor solution was obtained. HEO coatings were deposited using an 80 kW DC plasma spray torch (Model: 9 MB, Oerlikon Metco, Wohlen, Switzerland) equipped with a liquid feeding unit (Model: SPS II, Inframat Corporation, Connecticut, USA). The typical operating parameters for solution precursor plasma spray coatings are listed in Table 1.

Table 1 Typical plasma spray operating parameters

Parameters	Value
Plasma gun	9 MB
Arc current (A)	450
Arc voltage (V)	70
Primary gas, Ar (lpm) @ 5.5 bar	65
Secondary gas, H2 (lpm) @ 3.5 bar	4
Solution feed rate (mL min^−1)	50–55
Solution molar concentration (mole)	1 & 2
Atomizing gas, Air (psi)	20
Stand-off distance (mm)	50
Nozzle exit diameter (mm)	5.5

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The plasma spray torch was mounted on a six-axis robot arm and the substrates were fixed in the sample holder. The movement path of the spray torch and the standard raster scan pattern were controlled by using an ABB robot arm (Model: IRB4400, ABB, Sweden). An atomizer nozzle was positioned to inject the precursor solution along the radial direction perpendicular to the plasma jet. Atomised precursor droplets are subjected to swift transformations while traversing along the plasma plume, starting with evaporation of solvents, gelation, pyrolysis, sintering, and melting, and ending with the deposition of fine structured coatings on the substrate. The schematic representation of the solution precursor plasma spraying process with coating formation steps is illustrated in Fig. 1. The HEO coatings deposited at 1 M and 2 M concentrations were named HEO-A1 and HEO-A2, respectively.

Fig. 1 Schematic representation of the solution precursor plasma spray process detailing the coating formation steps.

2.3. Materials characterization

The structure of the HEO coatings was determined by using an X-ray diffractometer (XRD; Rigaku, SmartLab SE, Japan), at a scan rate of 2° min⁻¹ with Cu-Kα radiation (wavelength: 1.54 Å). The oxidation state of the HEO coatings was ascertained using an X-ray photoelectron spectrometer (XPS; K-Alpha-KAN995413, Thermo Scientific). The produced HEO coatings were placed on a transparent cold mount and dried. Then the metallographic sample was prepared using a standard methodology including sectioning, polishing with the sequence of SiC grit sheets, and final polishing with 1 μm diamond paste. The surface morphology and the microstructural observations were carried out using a field emission scanning electron microscope (FE-SEM; SIMGA HV-Carl Zeiss with Bruker Quantax 200) and HR-TEM (HR-TEM; Tecnai G2 TF20). The surface roughness of the HEO coatings was measured by using a surface profilometer (Alpha-Step D-120).

2.4. Electrochemical measurements

The electrochemical measurements were carried out using a conventional three-electrode system electrochemical workstation (Biologic) at room temperature. Commercial Ag/AgCl and a graphite rod were used as reference and counter electrodes, respectively and the as-deposited HEO coating was maintained as the working electrode. The working electrode was covered with cellophane tape except for the active area (1 cm²). For comparative purposes, the same configuration of as-synthesized HEO nanopowder, produced via the thermal plasma route,⁷ was used to prepare the working electrode. This process mimics the fabrication of SPPS-coated electrodes, utilizing polyvinylidene fluoride (PVDF) as a binder and N-methyl-2-pyrrolidone (NMP) as a slurry preparation solvent. Specifically, a powder catalyst to PVDF ratio of approximately 4 : 1 (relative to the total catalyst loading on the electrode surface) was mixed in a mortar. NMP solvent was then added, and the mixture was continuously stirred using a pestle. Subsequently, a precise volume of catalyst ink was applied to a 1 cm² area of a graphite sheet to form the working electrode. All requisite electrochemical studies such as cyclic voltammetry (CV), linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS), and chronopotentiometry were systematically performed. Prior to the electrochemical studies, Ar gas was purged for 30 min in a 1 M KOH solution. LSV measurements were employed to acquire the polarisation curve in 1 M KOH solution at a sweep rate of 5 mV s⁻¹. The expression for calculating the overpotential, reversible hydrogen electrode (RHE) conversion, Tafel slope and turnover frequency is provided in the ESI.†

3. Results and discussion

Fig. 2 shows the XRD pattern of as-deposited HEO coatings at two different molar concentrations. XRD diffraction peaks obtained at 18.5, 30.4, 35.8, 37.5, 43.5, 54.2, 56.6, and 63.3° correspond to the (111), (220), (331), (222), (400), (422), (511) and (440) planes, respectively. The observed phase contents were matched to iron oxide (Fe₃O₄) (ICDD card no. 01-089-0688), confirming the spinel structure with the Fd3̄m space group. The absence of discernible secondary peaks in the XRD pattern of the HEO coatings indicates that the chosen processing parameters are ideal for achieving phase-pure HEO coatings. It is observed that the phase composition of HEO coatings remains largely unchanged with variations in the molar concentration of the precursor solution. However, the increase in the precursor solution's mole concentration leads to a noticeable increase in peak intensity, suggesting improved crystallinity and density of the coating. As the molar concentration of the precursor solution increases, the amount of material available for deposition during the plasma spray process also increases, thereby promoting nucleation and crystallite growth within the resulting microstructure.¹⁹ The average crystallite sizes of the HEO-A1 and HEO-A2 coatings were determined using the Debye–Scherrer equation, yielding values of 6.5 nm and 8.1 nm, respectively. Additionally, the crystallinity of the HEO-A1 and HEO-A2 coatings was quantified using the following equation, resulting in values of 82% and 90%, respectively.²⁰

Fig. 2 XRD pattern of as-deposited HEO coatings.

The observed differences in crystallinity can be attributed to variations in the metal cation concentration of the precursor solution, which influences the evaporation rates of the precursor droplets within the high-temperature plasma. The increased concentration of metal cations, for a given plasma heat flux, likely promotes enhanced nucleation and subsequent crystal growth. Furthermore, the HEO coating's surface roughness also indicates an interesting trend. The obtained results showed that the solution precursors with high concentrations (HEO-A2) had lower surface roughness (8 ± 2 μm), whereas those with low concentrations (HEO-A1) had higher surface roughness (14 ± 3 μm). It was reported that the high roughness on the surface implies superior surface area and porousness, resulting in increased adsorption and desorption capacity and thereby, improved water splitting performance.²¹

Fig. 3 shows the cross-sectional and surface microstructure of as-deposited HEO coatings with two different molar concentrations. The successive steps of atomization, evaporation, fragmentation, gelation, sintering, and melting, and finally, a fine structured coating were achieved with smaller splat deposits (few micron size splats).²² The cross-sectional features of coatings show the typical characteristic morphology of a solution-derived microstructure, containing fine splats and pores with a few vertical cracks. The presence of un-pyrolyzed precursor materials in the coating composition promotes vertical crack formation due to the thermal stresses generated during spraying.²² The coating shows a highly porous and discontinuous microstructure at a low concentration of the solution precursor, as shown in Fig. 3a and b. The higher mole concentration of the solution precursor can achieve a well-bonded and relatively dense microstructure as observed in Fig. 3d and e.²³ The HEO-A1 and HEO-A2 coatings show a thickness of about ∼30 μm. The surface of the coatings exhibits a cauliflower-like morphology, which is a characteristic feature of the SPPS process-derived microstructure.¹⁸ It contains fine-structured spherical clusters evenly distributed through the surface as shown in Fig. 3c and f. The fine-structured particles in the range of a few tens of nanometres were agglomerated on the spherical clusters resulting in the formation of nano-structured coatings. The porous microstructure of HEO-A1 coating has a high specific surface area, which is expected to enhance overall performance.

Fig. 3 Cross-sectional and surface microstructure of as-deposited coatings: (a–c) HEO-A1 and (d–f) HEO-A2 coatings.

TEM images were acquired to conduct a detailed analysis of the microstructure of the as-deposited coating. Fig. 4a and b present the TEM image and SAED pattern of the as-deposited HEO-A1 coating. The SAED pattern exhibits well-defined diffraction rings, which are characteristic of polycrystalline materials, confirming the polycrystalline nature of the coating. The observed diffraction rings correspond to the (331), (422), and (511) crystallographic planes, respectively. This diffraction pattern provides further evidence of the structural integrity and crystallographic nature of the HEO-A1 coating, demonstrating that the coating formed via the plasma spray process maintains a well-defined polycrystalline microstructure. The high-resolution transmission electron microscopy (HR-TEM) images (Fig. 4c and d) reveal well-defined lattice fringes, which suggest the crystalline nature of the material. These lattice fringes are indicative of well-ordered atomic arrangements within the coating microstructure. The measured interplanar distances between adjacent lattice planes are 0.32, 0.26, 0.42, and 0.21 nm, corresponding to the (220), (311), (111), and (400) crystallographic planes, respectively. These values are consistent with the expected d-spacing for spinel-type structures, providing further evidence of the crystalline nature of the coating. The HR-TEM lattice fringes and SAED pattern are in strong agreement with the XRD result. Collectively, these complementary techniques confirm the phase purity, crystallinity, and structural integrity of the as-deposited coating through the SPPS process, validating the successful formation of a single-phase spinel structure.

Fig. 4 TEM images (a), SAED pattern (b), and HR-TEM images (c and d) of the as-deposited HEO-A1 coating.

XPS analysis was conducted to investigate the surface chemistry and oxidation states of the as-deposited HEO coatings. Fig. 5 confirms the presence of all constituent elements (Ni, Fe, Mn, Cu, and Co) in both HEO coatings. The fine scan of Co 2p XPS spectra (Fig. 5a) was deconvoluted into four peaks with the spin–orbit characteristics of Cr²⁺ and Cr³⁺. The peaks present at 780.8 (Co 2p_3/2) and 795.4 eV (Co 2p_1/2) belong to Co³⁺, whereas, the peaks at 783.1 (Co 2p_3/2) and 797.3 eV (Co 2p_1/2) belong to Co²⁺. The peak at 803.8 eV is the satellite peak of Co 2p_1/2. Fig. 5b illustrates the two main spin–orbital lines in Fe 2p spectra, and the peaks centered at 713.3 and 724.3 eV are ascribed to Fe 2p_3/2 and Fe 2p_1/2, respectively. The XPS peaks at 712.3 and 723.4 eV are attributed to Fe²⁺, while the peaks at 714.3 and 726 eV belong to Fe³⁺. The peak at 733 eV corresponds to the satellite peak of Fe³⁺. The XPS spectra of Mn 2p are shown in Fig. 5c, the peaks centered at 642.5 and 653.4 eV were ascribed to Mn²⁺ and the peaks located at 644.4 and 655.3 eV belong to Mn³⁺. In the Ni 2p XPS spectra (Fig. 5d), the peaks at 854.9 and 872.1 eV are ascribed to Ni²⁺, whereas the peaks at 856.3 and 873.6 eV are associated with Ni³⁺. Furthermore, the peaks at 861.9 and 880.6 eV correspond to the Ni 2p satellite peaks. The Cu 2p XPS spectra (Fig. 5e) were deconvoluted into Cu¹⁺ peaks at 933.5 and 953.2 eV, and Cu²⁺ peaks at 935.2 and 955.1 eV. Satellite peaks at 942.2 and 962.6 eV were attributed to Cu²⁺. The XPS spectra of O 1s (Fig. 5f) were deconvoluted into three peaks O₁, O₂ and O₃, which correspond to the metal–oxygen bond (529.8 eV), oxygen vacancies (531.5 eV) and surface absorbed oxygen (533.2 eV), respectively. HEO-A2 coatings have a higher intensity than HEO-A1 coatings due to the higher metallic concentration of the precursor solution. The obtained result was in agreement with the XRD result. The XPS results reveal variations in the intensities of each element in the HEO-A1 and HEO-A2 coatings without any shifts in binding energy, confirming that compositional differences occurred without changes in oxidation states during the deposition process, corresponding to the mole concentration of the precursor solution.

Fig. 5 XPS spectra of as-deposited HEO coatings: (a) Co 2p, (b) Fe 2p, (c) Mn 2p, (d) Ni 2p, (e) Cu 2p, and (f) O 1s.

3.1. Electrocatalytic performances

The electrocatalytic performances towards the OER and HER were demonstrated in a three-electrode set-up in an Ar-saturated 1 M KOH electrolyte. Fig. 6a shows the iR-compensated LSV curves for the OER over HEO-A1, HEO-A2, commercial RuO₂, and a bare substrate (SS 304). Among them, HEO-A1 exhibited a lower overpotential of 220 mV at 10 mA cm⁻² than HEO-A2 (330 mV), RuO₂ (340 mV), and the bare substrate (420 mV). HEO-Al coatings have higher porosity, surface roughness, and surface area than HEO-A2, which promote increased electrical conductivity, active sites, structural stability, and reduced charge carrier recombination. Such features collectively produce more efficient charge transfer, lower overpotentials, and higher catalytic efficiency for water-splitting reactions.²⁴Fig. 6b illustrates that the overpotential increases on increasing the current density. As shown in Fig. 6c, the HEO-A1 electrocatalyst exhibited a lower Tafel value of 46 mV dec⁻¹, than the HEO-A2 electrocatalyst (96 mV dec⁻¹) which indicates that HEO-A1 has faster reaction kinetics during the electrocatalytic process. The electron transfer kinetics are revealed by the EIS test, and Fig. 6d shows the Nyquist plots of the electrocatalyst along with the corresponding equivalent circuit. As shown in the Nyquist plot, HEO-A1 (0.8 Ω) has the smallest R_ct compared to HEO-A2 (1.5 Ω), suggesting that HEO-A1 exhibits faster charge transfer and outstanding catalytic reaction kinetics for the electrocatalytic process. The turnover frequency (TOF) has been evaluated and is presented as a bar chart in Fig. S4a.† The TOF data indicate that HEO-A1 demonstrates a TOF value of 0.048 × 10⁻³ s⁻¹, which is significantly higher than that of HEO-A2, which has a TOF value of 0.011 × 10⁻³ s⁻¹. Typically, double-layer capacitance (C_dl) of different electrocatalysts is assessed using CV in a non-Faradic window (Fig. S1†). The HEO-A1 electrocatalysts have the largest C_dl value of 24.05 mF compared to HEO-A2 (3.17 mF), which indicates that a greater number of electrochemical active sites can be present in HEO-A1 (Fig. S2†). Additionally, the ECSA values were calculated using the following equation; ECSA = C_dl/C_s, where C_s is the specific capacitance value of the flat electrode (40 μF cm⁻²). The calculated ECSA values of HEO-A1 and HEO-A2 were 601.25 and 79.25 cm², respectively (Fig. 6e). Notably, even after normalizing the LSV results based on ECSA (ECSA-normalized) (Fig. S4b†), the activity trend remains consistent with the observed geometric activity trend. The long-term electrochemical stability of HEO-A1 was then examined using the chronopotentiometry test and the results are displayed in Fig. 6f. The HEO-A1 electrocatalyst exhibited excellent electrochemical durability for 45 h at 10 mA cm⁻² current density. The LSV curve after 45 h of testing was nearly identical, suggesting minimal degradation in OER activity (Fig. 6f (inset)). Table S1† compares different HEO-based electrocatalysts for the OER in 1 M KOH solution.

Fig. 6 (a) OER LSV polarization curve of HEO-A1, HEO-A2, commercial RuO₂ and a bare substrate, (b) overpotential calculated at various current densities (10, 50 and 100 mA cm⁻²), (c) Tafel slope, (d) Nyquist plot of HEO-A1 and HEO-A2; the equivalent circuit is shown in the inset image, (e) ECSA of HEO-A1 and HEO-A2, and (f) chronopotentiometry analysis of HEO-A1 at 10 mA cm⁻² current density over 45 h, and the inset image contains the LSV polarization curve of HEO-A1 before and after stability testing.

Fig. 7a shows the LSV polarization curves of various electrocatalysts towards HER performance, including HEO-A1, HEO-A2, a bare substrate (SS 304), and a commercial Pt/C catalyst. Notably, HEO-A1 achieved a significantly lower overpotential of 129 mV at a current density of −10 mA cm⁻², outperforming both HEO-A2 (185 mV) and the bare substrate (270 mV) as shown in Fig. 7b. This indicates that HEO-A1 demonstrates superior electrocatalytic activity for the hydrogen evolution reaction, highlighting its potential as an efficient electrocatalyst compared to the other materials tested. The Tafel slope is used to assess the electrocatalytic activity of different catalysts and explain the reaction kinetics. As shown in Fig. 7c, HEO-A1 demonstrated a smaller Tafel value of 43 mV dec⁻¹ than HEO-A2 (76 mV dec⁻¹), which indicates that HEO-A1 involves a faster kinetic reaction. The following equations represent the HER mechanism in an alkaline medium:

H₂O + M + e⁻ → M–H* + OH⁻ (Volmer)

H₂O + e⁻ + M–H* → H₂ + M + OH⁻ (Heyrovsky)

2M–H* → H₂ + 2M (Tafel)

Fig. 7 (a) HER LSV polarization curve of HEO-A1, HEO-A2, commercial Pt/C and a bare substrate, (b) overpotential calculated at various current densities (10, 50 and 100 mA cm⁻²), (c) Tafel slope, (d) TOF values of HEO-A1 and HEO-A2, (e) chronopotentiometry analysis of HEO-A1 at 10 mA cm⁻² current density over 45 h, and (f) LSV polarization curve of HEO-A1 before and after stability testing.

Initially, H₂O is electrochemically adsorbed on the catalysts (M–H*), resulting in the production of OH⁻ (Volmer path). In the second stage, hydrogen is formed and released either chemically (Tafel path) or electrochemically (Heyrovsky path). Consequently, HEO-A1 indicates the Volmer–Heyrovsky mechanism where the electrochemical desorption is the rate-determining step. To assess the intrinsic HER electrocatalytic activity, turnover frequency (TOF) was measured and the results are displayed in Fig. 7d. It is shown that the HEO-A1 electrocatalyst exhibited a TOF valus of 9.12 s⁻¹, which is greater than that of HEO-A2 (4.64 s⁻¹). The chronopotentiometry test of HEO-A1 for the HER is shown in Fig. 7e. It can be observed that the voltage remains stable at a current density of −10 mA cm⁻² in strong alkaline electrolyte, even after 45 h. Furthermore, the LSV curve after 45 h (Fig. 7f) demonstrates that the changes in overpotential are minimal, thus indicating the excellent stability of the coating microstructure. Table S2† compares different HEO-based electrocatalysts for the HER in 1 M KOH solution.

3.2. Overall water splitting

The overall water electrolysis system was built by using the optimum electrocatalysts for both the anode and cathode in 1 M KOH, and was inspired by the superior OER and HER performance as well as the longevity of HEO-A1 electrocatalysts. The overall water-splitting activity of HEO-A1‖HEO-A1 is displayed in Fig. 8a. The LSV curve requires the lowest operational potential of 1.47 V to attain a current density of 10 mA cm⁻². The chronopotentiometry test was conducted with a current density of 10 mA cm⁻² as displayed in Fig. 8b. The chronopotentiometry test demonstrated the excellent performance of the overall electrode with no discernible loss even after 24 h. Table 2 compares cell voltages with different HEO-based electrocatalysts for overall water splitting performance in an alkaline medium.

Fig. 8 (a) LSV polarization curve of HEO-A1‖HEO-A1 towards overall water splitting, (b) long-term stability test of HEO-A1‖HEO-A1 for overall water splitting at an applied current density of 10 mA cm⁻² for 24 h in 1 M KOH solution.

Table 2 Comparison of high entropy oxide (HEO) based electrocatalysts for overall water splitting performance in 1 M KOH electrolyte solution

S. No.	Electrocatalyst	Substrate	Cell voltage (V)	Current density (mA cm^−2)	Ref.
1	(Li, Fe, Co, Ni, Cu, Zn)O	Carbon paper	1.89	30	24
2	(Fe, Co, Ni, Cu, Zn)3O4	Ni foam	1.65	10	25
3	(Ni, Co, Cr, Mn, V)3O4	Ni foam	1.596	10	10
4	(Ni, Co, Cr, Mn, Mo)3O4	Ni foam	1.58	10	2
5	(Ni, Fe, Co, Cu, Mn)3O4	SS plate	1.47	10	This work

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Following a comprehensive electrochemical investigation, it was significant to explore the electrocatalyst's structural, morphological, and oxidation state tenacity. The XRD analysis was performed to assess the structural change in HEO-A1 after the stability test, as shown in Fig. S4.† The results obtained indicate that the XRD pattern remains largely unchanged, with only a slight shift in the diffraction peaks. During the stability test, the electrochemical process encounters numerous redox cycles, which results in crystal lattice expansion or contraction. This leads to peak shifts in the XRD pattern. This suggests that the overall crystal structure of the material is stable, with minimal alterations occurring under the tested conditions. After the stability test, the electrocatalyst's surface chemistry and oxidation state were analysed using XPS, as shown in Fig. S5.† The results reveal that all the constituent metal components were still present in the HEO coatings following the stability test. In contrast to additive-based catalysts, additive-free catalysts show strong adherence to the substrate, reducing mechanical failure, preventing chemical degradation, and improving electrical conductivity during long-term operation. Hence, the utilization of the additive-free SPPS coating deposition technique not only improves long-term stability but also provides a significant advantage in terms of electrochemical performance. Similar to the observations made in phase analysis, the XPS data also confirm that there was no significant change in the oxidation states of the constitution metals, although a marginal shift in the binding energies was observed. This suggests that the metal cations were directly or indirectly involved in the water-splitting reaction, with only minimal changes in their chemical state. The HEO-A1 coating was characterized after the stability test using a FE-SEM to observe any changes in surface morphology. The microstructural observation results, as shown in Fig. S6,† confirm that there were no changes in the surface morphology of the coating after the electrochemical test, and the as-sprayed cauliflower-like structure was retained. The aforementioned investigations demonstrate that the electrocatalytic efficacy of the water splitting reaction is enhanced by the synergistic effect of the constituent metal components in high-entropy oxides and the processing approach.

3.3. Comparison of electrocatalytic performance between the as-synthesized HEO nanopowder and the SPPS coating

A comparative study of the electrocatalytic performance of the as-synthesized HEO nanopowder and the SPPS-coated HEO was conducted to assess the impact of the coating microstructure on the water-splitting process. For this purpose, nanopowder with the same HEO composition (Ni, Fe, Co, Cu, Mn)₃O₄ was synthesized via the thermal plasma route. Detailed descriptions of the processing methodology and structural characterization of the synthesized HEOs are provided in prior publications.⁷ The HEO nanopowder was subsequently coated onto a graphite sheet, forming a layer with a thickness of approximately 30–50 μm, using binders as outlined in Section 2.4. The electrocatalytic activity of the resulting coatings was investigated under identical experimental conditions, including the same electrolyte solution, as well as identical working and reference electrodes, to ensure a fair comparison of the electrocatalytic performance. The LSV polarization curves of the HEO nanopowder-coated electrode were recorded at a scan rate of 5 mV s⁻¹ in an alkaline medium. Fig. 9a illustrates the OER LSV polarization curve for the HEO powder-coated electrode, which requires an overpotential of 260 mV to achieve a current density of 10 mA cm⁻². In contrast, the HER LSV polarization curve, shown in Fig. 9b, indicates that a 670 mV overpotential is required to reach the same current density of −10 mA cm⁻².

Fig. 9 LSV result of the (a) OER, and (b) HER of the as-synthesized HEO nanopowder coated electrode.

Compared to the SPPS coated electrode, the HEO powder-coated electrode exhibits significantly lower electrocatalytic performance in water splitting, with approximately 18% lower activity for the OER and 419% lower activity for the HER. The reduced performance can be attributed to the presence of an additive in the dense coating and the formation of a large air gap between the coating and the electrode interface. This air gap increases the resistivity, hindering charge mobility across the interface during electrolysis and consequently impairing the overall electrocatalytic efficiency of the electrode. In contrast, the SPPS-produced additive-free coating exhibits a microstructure with considerable porosity and vertical cracks. This distinctive microstructure facilitates the clear passage of the electrolyte flow through most of the coating microstructure, thus enriching charge transport and promoting more efficient electrolysis. Moreover, the coating formation leads to strong mechanical bonding with the substrate, which significantly reduces the interface resistance in the electrocatalytic-current collector. This improvement in interface conductivity enhances charge mobility and contributes to greater durability during electrochemical cycling. Additionally, the rough surface of the coating with a higher surface area-to-volume ratio and its inherent porosity, play a crucial role in facilitating bubble formation during the evolution of oxygen and hydrogen. The porous nature of the coating provides nucleation sites for gas bubbles, which can improve the overall efficiency of both reactions by promoting effective gas release and minimizing bubble accumulation at the electrode surface.²⁶ These factors collectively contribute to enhancing the water-splitting performance of the coating microstructure.

Thermodynamic calculations indicate that a potential of 1.23 V is required to split water molecules under ideal conditions.²⁷ However, achieving this thermodynamic limit through electrochemical processes presents significant challenges due to the intrinsic and structural properties of electrode materials, as well as the complexities associated with processing conditions. Currently, a potential of approximately 1.4 V has been attained for water splitting through electrochemical methods, utilizing noble metals such as platinum, iridium, and ruthenium, in addition to materials based on selenides, carbides, phosphides, and nitrides.²⁸ These materials have been engineered with various structural and morphological modifications to enhance their catalytic efficiency, reduce overpotentials, and optimize their electrochemical performance under typical operational conditions.

Interestingly, the present study identifies a highly promising water-splitting electrode material composed of transition metals (Ni, Fe, Co, Cu, and Mn) arranged in a spinel structure within a high-entropy oxide configuration. This additive-free and structurally unique microstructure was synthesized via the SPPS process. Notably, the specific microstructural features of the HEO, such as its porosity, rough surface, and mechanical bonding with the substrate, significantly reduce the interface resistance and improve charge mobility. As a result, this material requires only a modest overpotential of 1.47 V to achieve a current density of 10 mA cm⁻² for water splitting, demonstrating its highly efficient catalytic activity, positioning HEO-based materials prepared through the SPPS coating process as a promising alternative for scalable and efficient water-splitting applications.

4. Conclusions

In summary, an additive-free, non-noble metal-based HEO (Ni, Fe, Co, Cu, Mn)₃O₄ nanostructured electrocatalyst coating was developed using a solution precursor plasma spray technique and is being reported for the first time. The HEO-A1 coatings exhibit a superior electrocatalytic activity under OER and HER processes. In light of the OER and HER results, HEO-A1 was constructed as a full-cell device, and its activity and stability were tested. It exhibits remarkable activity to reach a current density of 10 mA cm⁻² at a low potential of 1.47 V and has an outstanding long-term stability of 24 h. The HEO-A1 coating characteristics in terms of higher porosity, surface roughness, and surface area collectively improved electrical conductivity, provided active sites, exhibited structural stability, and performed with a good charge transfer rate, and better catalytic efficiency in water-splitting reactions. These pioneering efforts propose an innovative and additive-free approach for depositing diverse metal oxide coatings including high entropy oxides with a desirable phase and microstructure for energy storage and conversion applications.

Data availability

Data will be made available on request.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgements

The authors thank the Central Instrumentation Facilities (CIF), Pondicherry University for characterization support. The authors would like to acknowledge DST-FIST level-II, at the Department of Physics, Pondicherry University for XRD analysis. The authors PK and MWL acknowledge the Korea Institute of Science and Technology, South Korea; Institutional Program for post-analysis. This work was funded by the Korea Institute of Science and Technology, South Korea, Institutional Program and Innopolis Foundation through Technology Commercialization services funded by the Ministry of Science and ICT (RS-2024-00419982). This study was supported by a National Research Foundation of Korea grant funded by the Korean Government (MSIT) (No. 2022R1A2C1004283), and a Korea Basic Science Institute (National Research Facilities and Equipment Center) grant funded by the Ministry of Education (2019R1A6C1010046).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5se00479a

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