Rapid synthesis of a highly dispersed FeCoNiRuPt high-entropy alloy bifunctional electrocatalyst and exploration of the catalytic mechanism

Abstract

The unique advantages and potential of high-entropy nanoalloys in catalysis are reflected in their ability to transcend the limitations of conventional single-component catalysts through elemental synergies. Herein, an electrocatalyst was synthesized via the microwave rapid heating method, consisting of FeCoNiRuPt high-entropy alloy nanoparticles supported on reduced graphene oxide. The electrocatalytic hydrogen evolution reaction performance of this sample in acidic and alkaline solutions significantly surpasses that of the commercial 20% Pt/C catalysts. Moreover, its oxygen evolution reaction performance in alkaline solution outperforms commercial IrO₂ catalysts. Microstructural characterization indicates that the superior hydrogen evolution reaction activity is attributed to enhanced electron transfer and surface element concentration gradients induced by the dissolution of transition metals during catalysis. For the oxygen evolution reaction, the performance enhancement is ascribed to the formation of a stable high-entropy oxyhydroxide layer on the alloy surface. Stability tests confirm that the catalyst maintains consistent performance for over 120 hours under both acidic and alkaline conditions. These findings highlight the significant potential of high-entropy alloys as bifunctional catalysts for efficient electrochemical water splitting.

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Ningyan Cheng

Ningyan Cheng is currently affiliated with the Institutes of Physical Science and Information Technology, Anhui University, China. She received her PhD degree in 2020 from the Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong (UOW), Australia. She conducted a two-year research stay at the Max-Planck-Institut für Eisenforschung (MPIE) in Germany as a Humboldt Research Fellow. Her research interests focus on the application of advanced transmission electron microscopy techniques to investigate the structural and chemical properties of materials at the nanoscale and atomic level, aiming to establish fundamental structural–property relationships.

1 Introduction

In recent years, high-entropy alloys (HEAs) have attracted significant attention in the field of catalysis owing to their unique elemental tunability.^1–4 The diversity of their constituent elements enables the optimization of both the design and electronic structure of HEAs, thereby enhancing their catalytic performance. This also facilitates the customization and functionalization of high-performance electrocatalysts.^5–7 Furthermore, the application of HEAs can reduce the consumption of noble metal resources and minimize environmental pollution while enhancing material utilization and energy efficiency. This aligns with the principles of sustainable development. Owing to their unique high-entropy characteristics and surface structures, HEAs have demonstrated significant potential in various applications, including water electrolysis,^8–10 methanol oxidation,^11–13 and oxygen reduction reactions.^14–16

Previous studies have demonstrated that HEA catalysts exhibit high stability and activity, capable of operating stably in both acidic and alkaline environments while retaining high catalytic activity and selectivity. Several researchers have introduced transition metals in conjunction with noble metals to form HEAs, thereby modifying the electronic structure of the noble metals. This approach promotes lattice contraction and a downward shift of the d-band center.^17,18 This approach not only facilitates the adsorption and desorption of reaction intermediates but also enhances the stability of active sites. For example, Gao et al. loaded FeCoPdIrPt HEA nanoparticles (NPs) on the surface of graphene oxide (GO) using the fast-moving bed pyrolysis method. The resulting material exhibited an overpotential of 42 mV (vs. RHE) to achieve a current density of 10 mA cm⁻² for the hydrogen evolution reaction (HER) in 1 M KOH solution.¹⁹ Liu et al. employed a pyrolysis method to load Pt₃Co/Co composite nano-catalysts onto porous carbon, achieving a current density of 100 mA cm⁻² for the HER in 0.5 M H₂SO₄ solution at an overpotential of 187 mV (vs. RHE).²⁰ Sharma et al. synthesized CoFeGaNiZn HEA NPs using the low-temperature solid-state synthesis method with an overpotential of 370 mV (vs. RHE) at a current density of 10 mA cm⁻² for the oxygen evolution reaction (OER) in 1 M KOH solution.²¹ Currently reported synthesis methods for HEA catalysts frequently involve complex procedures and rigorous requirements for equipment or substrates.²² During electrochemical reactions, catalysts inevitably undergo structural reconstruction, including surface or subsurface atomic rearrangements, compositional changes, adsorption of reaction intermediates, and oxidation of metals or metal compounds.²³ Different types of catalysts exhibit varying structural reconstruction phenomena in diverse reaction media, which can exert both beneficial and detrimental effects on their catalytic performance.²⁴ This has resulted in the current HEA catalysts being less adaptable to catalytic reactions in diverse media, exhibiting superior single-function catalytic performance only in specific reaction environments.

In this work, we synthesized iron–cobalt–nickel–ruthenium–platinum HEA NPs supported on reduced GO (Fe₂Co₂Ni₂Ru₂Pt_x, where x represents different platinum contents) using the Microwave Rapid Heating (MRH) method. Performance testing indicates that the synthesized Fe₂Co₂Ni₂Ru₂Pt_0.5 (x = 0.5 mmol) exhibits outstanding bifunctional catalytic performance for both the HER and the OER. Post-reaction characterization using spherical-aberration-corrected transmission electron microscopy (AC-TEM) and X-ray diffraction (XRD) reveals that Fe₂Co₂Ni₂Ru₂Pt_0.5 undergoes distinct structural reconstruction phenomena under different reaction conditions. During the HER process, HEA NPs show dissolution of transition metal elements, while during the OER process, they exhibit surface self-reconstruction to form a layer of high-entropy oxyhydroxide. Microstructural analysis and theoretical studies elucidate the electrocatalytic mechanisms of Fe₂Co₂Ni₂Ru₂Pt_0.5 during the reaction process, providing valuable guidance for the development of efficient and stable HEA catalysts.

2 Results and discussion

2.1 Characterization of Fe₂Co₂Ni₂Ru₂Pt_x

We synthesized Fe₂Co₂Ni₂Ru₂Pt_x using the MRH method. The synthesis process is illustrated in Fig. 1a. Initially, the metal salt solution was thoroughly mixed with a GO dispersion, followed by freeze-drying. The freeze-dried samples were then subjected to MRH treatment to obtain Fe₂Co₂Ni₂Ru₂Pt_x. The schematic shows a graphene coating on the HEA surface (Fig. S1†), which is caused by carbon dissolution and segregation during synthesis.^25,26Fig. 1b shows a representative high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Fe₂Co₂Ni₂Ru₂Pt_0.5. It is evident that the particles of Fe₂Co₂Ni₂Ru₂Pt_0.5 are uniformly distributed on the surface of reduced graphene oxide (rGO), with an average particle size of approximately 7.8 nm (Fig. S2†). Additionally, the scanning electron microscope (SEM) images presented in Fig. S3† further corroborate the above results. Fig. 1c shows the atomic-resolution HAADF-STEM image of a Fe₂Co₂Ni₂Ru₂Pt_0.5 particle and the corresponding locally magnified image, indicating that the synthesized Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits a face-centered cubic (FCC) structure. Fig. 1d presents the XRD pattern of the synthesized Fe₂Co₂Ni₂Ru₂Pt_0.5. From the XRD pattern, characteristic diffraction peaks located at 42.08°, 49.06° and 71.60° correspond to the (111), (200) and (220) crystal planes of the FCC structure, respectively. Due to the changes in lattice constant caused by high-entropy alloying, the positions of the characteristic diffraction peaks exhibit a noticeable shift. This shift confirms the successful synthesis of Fe₂Co₂Ni₂Ru₂Pt_0.5. The HAADF-STEM image and corresponding energy-dispersive X-ray spectroscopy (EDS) elemental mappings of a single particle of Fe₂Co₂Ni₂Ru₂Pt_0.5 are shown in Fig. 1e–k (with atomic percentages listed in Table S1†), indicating that the five elements are well mixed and uniformly distributed without compositional segregation.

Fig. 1 (a) Schematic of the Fe₂Co₂Ni₂Ru₂Pt_x preparation process. (b) Representative HAADF-STEM image of Fe₂Co₂Ni₂Ru₂Pt_0.5. (c) Atomic-resolution HAADF-STEM image of Fe₂Co₂Ni₂Ru₂Pt_0.5 (inset shows a partial enlarged image). (d) XRD pattern of Fe₂Co₂Ni₂Ru₂Pt_0.5. (e–j) HAADF-STEM and the corresponding EDS elemental mapping images of Fe₂Co₂Ni₂Ru₂Pt_0.5. (k) Laser confocal Raman spectra and (l) FTIR spectra of GO, rGO and Fe₂Co₂Ni₂Ru₂Pt_0.5. (m) N₂ adsorption/desorption isotherm of Fe₂Co₂Ni₂Ru₂Pt_0.5 (inset shows the pore size distribution).

Due to the preferential reduction of the noble metals Pt and Ru during material synthesis,²⁷ when the Pt content is low, its reduction rate is slower than that of Ru, resulting in a higher relative content of Ru in the synthesized Fe₂Co₂Ni₂Ru₂Pt_0.5 sample compared to other elements. When a larger amount of Pt is added, the characteristic diffraction peaks in the XRD pattern of the synthesized HEA shift closer to those of pure Pt (Fig. S4†). The HAADF-STEM image (Fig. S5a†) demonstrates that increasing Pt content leads to enhanced size heterogeneity of the synthesized HEA NPs, resulting in a decrease in both specific surface area and active sites.²⁸ Additionally, multi-particle EDS elemental mapping (Fig. S4b–h and Table S2†) confirms the increased heterogeneity in elemental distribution, which has a detrimental effect on the catalytic performance. Furthermore, we measured the molar ratios of five elements (Fe/Co/Ni/Ru/Pt) in Fe₂Co₂Ni₂Ru₂Pt_0.5 and Fe₂Co₂Ni₂Ru₂Pt₂ using an inductively coupled plasma optical emission spectrometer (ICP-OES), finding that they ranged from 5% to 35%, consistent with the characteristics of HEAs (Fig. S6†).

To compare the changes in functional groups before and after MRH, we conducted laser confocal Raman spectroscopy and Fourier transform infrared spectroscopy (FTIR) analyses on three samples: GO, rGO and Fe₂Co₂Ni₂Ru₂Pt_0.5. The Raman spectra (Fig. 1k) reveal distinct D and G peaks both before and after MRH. The D peak reflects structural defects in graphene, corresponding to the transition from sp² hybridized carbon to sp³ hybridized carbon, while the G peak represents the typical ordered graphene structure, attributed to the vibrations of sp²-bonded carbon.²⁹ The I_D/I_G ratio of the GO sample is 0.881, while that of the rGO sample is 0.866, and for the Fe₂Co₂Ni₂Ru₂Pt_0.5 sample, it is 1.057. The slight decrease in the I_D/I_G value of the rGO sample compared to GO indicates a reduction in structural defects following MRH, leading to improved conductivity. In contrast, the I_D/I_G ratio of Fe₂Co₂Ni₂Ru₂Pt_0.5 has increased compared to rGO, indicating that the introduction of HEA NPs induced significant distortions in the arrangement of carbon atoms on the rGO surface.³⁰ To further validate the Raman spectroscopy results, we conducted FTIR analyses on the three aforementioned materials, with results shown in Fig. 1l. The characteristic absorption peaks located at approximately 3400 cm⁻¹, 1600 cm⁻¹ and 1200 cm⁻¹ correspond to the hydroxyl (–OH) vibration, carbon–carbon double bond (C=C) stretching vibration and –C–O–C functional group, respectively. These functional groups exhibit a slight reduction in intensity after MRH. The characteristic absorption peaks at approximately 1730 cm⁻¹, 1380 cm⁻¹ and 1050 cm⁻¹ correspond to the stretching vibration of carbon–oxygen double bonds (C=O), the deformation vibration of hydroxyl (–OH), and the vibration absorption peak of C–O–C. These peaks show a significant decrease in intensity after MRH treatment, indicating that most of the oxygen-containing groups were dissociated during the heating process.³¹ This indicates that defects on GO were reduced after MRH, as evidenced by the decreased I_D/I_G value in the Raman spectroscopy results. In contrast, there was no significant change in the FTIR spectrum of Fe₂Co₂Ni₂Ru₂Pt_0.5 compared to rGO. Studies have shown that the electronic state density of carbon atoms at specific defect sites in defective graphene changes, and the introduction of HEA NPs on the rGO surface may promote the emergence of these defect carbon atoms, thereby enhancing catalytic reactions.³²

To determine the specific surface area and pore size distribution of Fe₂Co₂Ni₂Ru₂Pt_0.5, we utilized an automatic physical adsorption analyzer for testing. Fig. 1m shows the measured nitrogen adsorption/desorption isotherms and pore size distribution curves. The isotherm exhibits a distinct hysteresis loop, characteristic of a type IV isotherm, with the hysteresis loop most pronounced in the mid-pressure range (P/P₀ = 0.45–0.9), indicating the presence of a significant amount of mesopores in the structure. In the high-pressure range (P/P₀ = 0.9–1.0), the adsorption amount significantly increases, indicating the presence of larger pores in the material. The inset pore size distribution graph shows an average pore size of 2.4 nm, with most pores falling within the mesopore range. The presence of nanopores can effectively prevent the aggregation of Fe₂Co₂Ni₂Ru₂Pt_0.5 during MRH, thereby maintaining dispersion and enhancing catalytic performance. Additionally, the porous structure reduces the diffusion resistance of intermediates in the HER and OER and accelerates kinetic processes. The results indicate a BET-measured specific surface area of 183.04 m² g⁻¹, providing additional active sites and enhancing the effective contact area between the electrolyte interface and the catalyst. Consequently, this significantly improves mass transfer efficiency and promotes catalytic reactions, leading to enhanced overall performance.

To further confirm the surface chemical state of the synthesized HEA NPs, we conducted X-ray photoelectron spectroscopy (XPS) tests. Fig. S7a–f† display the full XPS spectrum of Fe₂Co₂Ni₂Ru₂Pt_0.5 and high-resolution spectra of Fe 2p, Co 2p, Ni 2p, Ru 3p and Pt 4f. These results indicate that the surface chemical states of Fe, Co, Ni, Ru and Pt in Fe₂Co₂Ni₂Ru₂Pt_0.5 are predominantly metallic, suggesting a high degree of reduction and stability of these elements on the nanoparticle surface. Additionally, given the relatively high chemical activity of transition metals, surface oxidation is inevitable during storage and testing. Consequently, the oxidized states of metals can be derived from the high-resolution spectra of Fe 2p, Co 2p and Ni 2p. We also conducted XPS analysis on the comparison sample Fe₂Co₂Ni₂Ru₂Pt₂, with results shown in Fig. S8.† These results reveal a highly similar distribution of elemental valence states to that of Fe₂Co₂Ni₂Ru₂Pt_0.5, further validating the consistency between the two samples. The XPS results effectively confirm the elemental valence states, demonstrating that we successfully synthesized Fe₂Co₂Ni₂Ru₂Pt_x, with only a small amount of metal elements oxidized during storage or sample preparation.

2.2 Electrocatalytic performance

We compared the electrocatalytic HER and OER performances of the aforementioned catalysts using linear sweep voltammetry (LSV) curves obtained at a scan rate of 10 mV s⁻¹ on a glassy carbon (GC) electrode, with IR compensation applied. The measured HER and OER performances are summarized in Fig. 2. Fig. 2a displays the HER polarization curves of Fe₂Co₂Ni₂Ru₂Pt_0.5, Fe₂Co₂Ni₂Ru₂Pt₂ and 20% Pt/C catalysts in 1 M KOH solution. As can be observed from the figure, to achieve a current density of 10 mA cm⁻², Fe₂Co₂Ni₂Ru₂Pt_0.5 demands an overpotential of merely 49.1 mV, which is notably lower than that of the 20% Pt/C catalyst (61.6 mV). We have summarized the HER performance of similar catalysts reported in recent years in 1 M KOH solution (Table S3†) and found that our HEA demonstrates superior alkaline HER performance. Meanwhile at high overpotential Fe₂Co₂Ni₂Ru₂Pt_0.5 shows more remarkable catalytic performance (Fig. 2b), and a smallest Tafel slope of 47.1 mV dec⁻¹ (Fig. 2c) as well as a remarkable catalytic durability exceeding 120 hours (Fig. 2d). Fig. 2e displays the HER polarization curves of Fe₂Co₂Ni₂Ru₂Pt_0.5, Fe₂Co₂Ni₂Ru₂Pt₂ and 20% Pt/C catalysts in 0.5 M H₂SO₄ solution. From the figure, it can be seen that Fe₂Co₂Ni₂Ru₂Pt_0.5 requires an overpotential value of 40.8 mV when the current density is 10 mA cm⁻². As shown in Table S4,† Fe₂Co₂Ni₂Ru₂Pt_0.5 ranks among the top performers in HER performance. Additionally, Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits superior catalytic performance at higher overpotentials (Fig. 2f). The Tafel slope in the acidic medium is only 18.1 mV dec⁻¹ (Fig. 2g), implying that the catalyst follows the Volmer–Tafel mechanism,³³ and the catalytic durability exceeds 120 hours (Fig. 2h). Fig. 2i displays the OER polarization curves of Fe₂Co₂Ni₂Ru₂Pt_0.5, Fe₂Co₂Ni₂Ru₂Pt₂ and IrO₂ catalysts in 1 M KOH. As can be observed from Fig. 2i and Table S5,† Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits superior OER catalytic activity, particularly at higher overpotentials (Fig. 2j). In order to further elucidate the kinetics of the OER, the Tafel slope was derived from the corresponding polarization curves. As shown in Fig. 2k, Fe₂Co₂Ni₂Ru₂Pt_0.5 delivers a Tafel slope value of 54.9 mV dec⁻¹, smaller than that of Fe₂Co₂Ni₂Ru₂Pt₂ and IrO₂. Similarly, its OER durability test results (Fig. 2l) demonstrate a catalytic durability exceeding 120 h. The above results indicate that the synthesized Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits excellent electrocatalytic HER and OER performance in 1 M KOH (or 0.5 M H₂SO₄), and at higher overpotentials, our synthesized electrocatalyst is more competitive than commercial 20% Pt/C or IrO₂ catalysts. Stability test results show that Fe₂Co₂Ni₂Ru₂Pt_0.5 retains a stable current density with almost no decay after 120 hours in alkaline HER, acidic HER and alkaline OER processes, demonstrating its remarkable electrochemical stability.

Fig. 2 (a–c) HER polarization curves, the overpotentials needed to achieve a current density of 100 or 150 mA cm⁻², and the corresponding Tafel plots of Fe₂Co₂Ni₂Ru₂Pt_0.5, Fe₂Co₂Ni₂Ru₂Pt₂, and 20% Pt/C electrocatalysts; (d) HER stability test of Fe₂Co₂Ni₂Ru₂Pt_0.5 in 1 M KOH solution. (e–g) HER polarization curves, the overpotentials needed to achieve a current density of 100 or 150 mA cm⁻², and the corresponding Tafel plots of Fe₂Co₂Ni₂Ru₂Pt_0.5, Fe₂Co₂Ni₂Ru₂Pt₂, and 20% Pt/C electrocatalysts; (h) HER stability test of Fe₂Co₂Ni₂Ru₂Pt_0.5 in 0.5 M H₂SO₄ solution. (i–k) OER polarization curves, the overpotentials needed to achieve a current density of 300 or 350 mA cm⁻², and the corresponding Tafel plots of Fe₂Co₂Ni₂Ru₂Pt_0.5, Fe₂Co₂Ni₂Ru₂Pt₂, and IrO₂ electrocatalysts; (l) OER stability test of Fe₂Co₂Ni₂Ru₂Pt_0.5 in 1 M KOH solution.

We employed electrochemical impedance spectroscopy (EIS) to measure the impedance of these three catalysts in the two solutions to evaluate the catalytic activity of the prepared electrocatalysts. The results are shown in Fig. S9a, b and Table S6.† In both solutions, Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits the smallest R_ct value, facilitating the acceleration of slow reaction mechanisms in catalytic reactions.³⁴ Moreover, as shown in Fig. S10,† we utilized the double-layer capacitance (C_dl) method to measure the electrochemically active surface area (ECSA) of the catalysts. Fig. S10d and h† show the C_dl values calculated under different scan rates in alkaline and acidic solutions, respectively. It can be observed that in both solutions, the C_dl values of Fe₂Co₂Ni₂Ru₂Pt_0.5 and Fe₂Co₂Ni₂Ru₂Pt₂ are greater than those of commercial 20% Pt/C catalysts. A larger C_dl value indicates a higher number of active sites, which enhances the hydrogen adsorption reaction rate. Due to the varying activity of the active sites, the C_dl value of Fe₂Co₂Ni₂Ru₂Pt₂ in Fig. S10d† is slightly higher than that of Fe₂Co₂Ni₂Ru₂Pt_0.5. These results clearly demonstrate that Fe₂Co₂Ni₂Ru₂Pt_0.5 possesses excellent catalytic performance as a water-splitting catalyst. Additionally, we conducted water electrolysis experiments in the same electrolytic cell (Fig. S11†), where the generation of H₂ and O₂ was clearly observed on the GC electrode, successfully validating the practical dual-functional catalytic effect of Fe₂Co₂Ni₂Ru₂Pt_0.5.

2.3 Electrocatalytic structural evolution of Fe₂Co₂Ni₂Ru₂Pt_0.5

To investigate the structural changes and corresponding electrocatalytic mechanisms of Fe₂Co₂Ni₂Ru₂Pt_0.5 after the three aforementioned reactions, we collected XRD and XPS data for the same batch of Fe₂Co₂Ni₂Ru₂Pt_0.5 before and after the reactions. Fig. 3a–c show the comparative XRD patterns of Fe₂Co₂Ni₂Ru₂Pt_0.5 before and after the HER in 1 M KOH, HER in 0.5 M H₂SO₄, and OER in 1 M KOH, respectively. Notable low-angle shifts are observed in the XRD characteristic peaks of the tested samples after the HER in both 1 M KOH (Fig. 3a) and 0.5 M H₂SO₄ solutions (Fig. 3b). This indicates that the interplanar spacing of the HEA slightly increases after the HER in these two media, which may be attributed to the dissolution of non-precious metal elements during the reaction process. In contrast, the XRD characteristic peaks of the sample undergoing the OER in 1 M KOH (Fig. 3c) show no significant shift. To exclude the influence of acidic or alkaline media on sample corrosion, the samples were subjected to stirring corrosion tests by soaking them in 0.5 M H₂SO₄ and 1 M KOH solutions for 30 hours. The experimental results are shown in Fig. S12,† and the XRD characteristic peaks of the sample are not significantly shifted after the corrosion control test, which proves that the XRD characteristic peak shift is caused by the electrocatalytic process. To verify the dissolution rate of transition metals, we performed a 12 h i–t test in the two solutions, as shown in Fig. S13.† After 12 h of operation, XRD peaks showed only a slight shift in characteristic peaks, confirming slow dissolution of transition metals in Fe₂Co₂Ni₂Ru₂Pt_0.5 and its excellent catalytic stability.

Fig. 3 XRD comparison patterns of original Fe₂Co₂Ni₂Ru₂Pt_0.5 before and after different reactions: (a) HER in 1 M KOH solution, (b) HER in 0.5 M H₂SO₄, and (c) OER in 1 M KOH. The high-resolution XPS spectra of (d) Fe 2p, (e) Co 2p, (f) Ni 2p, (g) Ru 3p, (h) Pt 4f, and (i) O 1s in Fe₂Co₂Ni₂Ru₂Pt_0.5 after different reactions.

We also performed XPS measurements on Fe₂Co₂Ni₂Ru₂Pt_0.5 after different reactions. As shown in Fig. 3d–h, it is evident that the metal peaks of Fe, Co and Ni after the HER in both 1 M KOH and 0.5 M H₂SO₄ shift to higher binding energy levels, while the metal peaks of Ru and Pt shift to lower energy levels (Fig. 3g and h). This can be attributed to the electron transfer from the transition metal elements (Fe, Co and Ni) to the noble metal elements (Ru and Pt) during the dissolution process, which aligns with their electronegativity differences (Fe: 1.83, Co: 1.88, Ni: 1.91, Ru: 2.20, Pt: 2.28). During this process, the transition metal elements do not oxidize to high-valent states due to the loss of some electrons. The presence of electron transfer results in varying degrees of charge redistribution between the noble metal Ru and Pt atoms and their surrounding atoms, leading to a gradient distribution of electrons on the surface noble metal atoms. This process results in the formation of a dual gradient in electron concentration and elemental composition on the HEA surface, leading to spatially varying H* (adsorption of active H atoms) adsorption free energy (strong adsorption sites favor H* adsorption, while weak adsorption sites promote the generation of H₂). The development of this structure is highly beneficial for enhancing the electrocatalytic HER performance.³⁵ Furthermore, the XPS spectra of the five elements in Fe₂Co₂Ni₂Ru₂Pt_0.5 after the OER (Fig. 3d–h) differ significantly from those of the original sample. It is evident that the surface oxidation states of Fe, Co, Ni, Ru and Pt have markedly increased. In the post-reaction spectra of Fe 2p, Co 2p and Ni 2p, the metal peaks show varying degrees of weakening, evolving into M²⁺/M³⁺ (where M represents Fe, Co and Ni). Ru exhibits mixed valence states of Ruⁿ⁺, while Pt oxidizes to Pt⁴⁺. However, due to the instability of high-valent Pt at low potentials, some Pt⁴⁺ may convert to a more stable mixed valence state Ptⁿ⁺.³⁶

Additionally, the O 1s spectrum in Fig. 3i clearly shows the presence of a metal oxyhydroxide peak after the OER, indicating the formation of a high-entropy oxyhydroxide layer on the surface of the HEA. Studies have shown that the oxyhydroxide layer formed during the reaction process is often the true active material of OER catalysts, and its relatively loose atomic arrangement facilitates faster ionic diffusion during the OER.^37,38 Consequently, this structural feature enhances the catalytic efficiency. Moreover, the stable core–shell structure ensures the stability of the catalyst during the OER.³⁹ For the XPS spectrum of Fe₂Co₂Ni₂Ru₂Pt_0.5 after the OER, zero-valent metal peaks remain detectable, indicating that although part of the HEA surface oxidized to metal oxyhydroxide during the OER, the inherent conductivity and stability of the HEA were retained. This retention facilitates the stable progression of catalytic reactions. We also observed that, compared to the pre-reaction state, the O 1s spectrum exhibits significant high-energy shifts in the peaks after all three reactions. This indicates strong electronic interactions between various elements, which can promote the kinetics of the water electrolysis reaction, thereby enhancing the electrocatalytic performance.⁴⁰

2.4 Electrocatalytic microstructural evolution of Fe₂Co₂Ni₂Ru₂Pt_0.5

To more intuitively characterize the microstructural changes in the samples after the reactions, we conducted quasi-in situ TEM experiments to examine the morphology, atomic structure and elemental distribution of Fe₂Co₂Ni₂Ru₂Pt_0.5, following the three different reactions mentioned above. Due to severe corrosion of the coordinate grids after the reaction, we were unable to locate the original positions, and therefore identified alternative locations for microstructural characterization. Fig. 4a–c show the TEM images of Fe₂Co₂Ni₂Ru₂Pt_0.5 after the HER in 1 M KOH solution, the HER in 0.5 M H₂SO₄ solution, and the OER in 1 M KOH solution, respectively. These images reveal that after the three reactions, the HEA particles are well-dispersed with no significant agglomeration, indicating their excellent electrochemical stability. This observation is further confirmed by preliminary magnified TEM images of other locations shown in Fig. S14.†Fig. 4d–f show the HAADF-STEM images after the three different reactions. In Fig. 4d, the interplanar spacing at the particle edge of Fe₂Co₂Ni₂Ru₂Pt_0.5 after alkaline HER is observed to be 0.244 nm, while that at the core is 0.234 nm. In Fig. 4e, after acidic HER, the interplanar spacing at the particle edge of Fe₂Co₂Ni₂Ru₂Pt_0.5 is 0.121 nm, whereas it is 0.119 nm at the core. These observations suggest that the dissolution of transition metal elements from the surface layer may lead to an increase in the interplanar spacing at the edges of sample particles, while the core retains its original structural dimensions due to the limited dissolution of elements. Combining the EDS results after the reactions (Fig. 4g and h) and their corresponding atomic percentage tables (Tables S7 and S8†), it is observed that both HER processes in 1 M KOH solution and 0.5 M H₂SO₄ solution lead to a reduction in the content of transition metal elements (Fe, Co and Ni) in the sample particles. This indicates that the electrocatalytic HER process in the above two media leads to the dissolution of transition metal elements on the particle surface of Fe₂Co₂Ni₂Ru₂Pt_0.5, effectively verifying the test results shown in Fig. 3. Fig. 4f shows the HAADF-STEM image of a single nanoparticle of Fe₂Co₂Ni₂Ru₂Pt_0.5 after the OER in 1 M KOH solution. This clearly demonstrates that a core–shell coating is formed on the particle surface of Fe₂Co₂Ni₂Ru₂Pt_0.5 after prolonged OER. Combining the EDS image (Fig. 4i), the atomic percentage table after the reaction (Table S9†), and the O 1s spectrum (Fig. 3i), it can be inferred that this coating is an oxyhydroxide layer on the surface of HEA NPs.

Fig. 4 TEM images of Fe₂Co₂Ni₂Ru₂Pt_0.5 after different reactions: (a) HER in 1 M KOH, (b) HER in 0.5 M H₂SO₄, and (c) OER in 1 M KOH. HAADF-STEM images at different magnifications after different reactions: (d) HER in 1 M KOH solution, (e) HER in 0.5 M H₂SO₄ solution, and (f) OER in 1 M KOH solution. EDS elemental mappings of individual particles of Fe₂Co₂Ni₂Ru₂Pt_0.5 after different reactions: (g) HER in 1 M KOH solution, (h) HER in 0.5 M H₂SO₄ solution and (i) OER in 1 M KOH solution.

We characterized the morphology, atomic structure, and elemental distribution of Fe₂Co₂Ni₂Ru₂Pt_0.5 after the HER in 1 M KOH solution, the HER in 0.5 M H₂SO₄ solution and the OER in 1 M KOH solution using AC-TEM. The results show that Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits electronic concentration and elemental gradient effects due to element precipitation during the HER process. In contrast, during the OER process, a surface high-entropy oxyhydroxide core–shell structure forms. Analyzing these phenomena, it is evident that they significantly enhance the progression of electrocatalytic reactions. On the one hand, both the rGO support and the HEA NPs exhibit excellent electron transport properties; on the other hand, the high specific surface area and numerous pores of the rGO support provide ample sites for the attachment of HEA NPs, facilitating rapid diffusion of reaction intermediates. Consequently, Fe₂Co₂Ni₂Ru₂Pt_0.5 demonstrates a lower Tafel slope, lower R_ct value and higher C_dl value, indicating faster reaction kinetics, more efficient electron transfer characteristics, and larger ECSA. This explains why the synthesized Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits such outstanding electrocatalytic performance.

3 Conclusions

In summary, we successfully synthesized Fe₂Co₂Ni₂Ru₂Pt_x using a simple MRH method. We analyzed the microscopic structural changes before and after the reaction using AC-TEM and XRD, revealing that different electrocatalytic processes are accompanied by distinct structural reconstructions. Specifically, the spatially distributed H* adsorption energy induced by the HER and the high-entropy oxyhydroxide core–shell structure formed during the OER continuously promote the electrocatalytic reactions. Consequently, Fe₂Co₂Ni₂Ru₂Pt_0.5 exhibits significantly superior bifunctional catalytic performance compared to commercial catalysts. In this study, the Fe₂Co₂Ni₂Ru₂Pt_0.5 electrocatalyst demonstrates both efficient bifunctional water-splitting catalytic performance and stable catalytic performance lasting over 120 hours. This work successfully synthesizes HEA NPs with low noble metal loading, which highlights the enormous potential of Fe₂Co₂Ni₂Ru₂Pt_0.5 as a bifunctional electrocatalyst for water splitting.

Data availability

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

Author contributions

Yong Wei: conceptualization, data curation, formal analysis, investigation, methodology, and writing – original draft; Zhihao Hu: investigation, methodology, and visualization; Ke Wang: methodology and validation. Dongxu Wu: methodology and validation; Feng Cheng: methodology and visualization; Ningyan Cheng: validation and visualization; Chuanqiang Wu: funding acquisition, project administration, resources, supervision, and validation; Binghui Ge: methodology and validation; Li song: project administration, supervision, validation, and writing – review & editing.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (52203289) and Key Projects of Anhui Provincial Department of Education (2022AH050116). We thank the Electron Microscope Center of AHU.

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Footnotes

† Electronic supplementary information (ESI) available: Materials synthesis, characterization, and electrochemical measurement. See DOI: https://doi.org/10.1039/d5ta03352g

‡ Both authors contributed equally to the manuscript.

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