1 Introduction

The development of green energy conversion technologies has been strongly desired as the ever-increasing energy crisis and environmental pollution caused by the excessive consumption of traditional fossil fuels [1]. Electrocatalytic energy conversion technology (EECT), such as water electrolysis, fuel cells, electrochemical nitrogen fixation and electrochemical reduction of CO2, has attracted extensive attention and exhibited great application potential due to the higher conversion efficiency and less carbon emissions [2,3,4,5,6,7]. However, most of the efficient electrocatalysts for EECT are noble metal-based nanoparticles (NPs). The high cost, low reserves and unsatisfactory performance of these noble metal-based electrocatalysts have been the main bottlenecks that limit the large-scale practical applications [8,9,10,11,12]. Therefore, over the past decades, numerous efforts focused on boosting the catalytic property and reducing the cost of electrocatalysts toward EECT. Among them, alloying noble metals with transition metals is a promising technology. A lot of binary and ternary alloyed noble metal-based electrocatalysts with lower noble metals contents have exhibited outstanding catalytic performance [13,14,15,16,17,18,19,20,21,22].

High-entropy alloys (HEAs) were first reported in 2004, which are defined based on the component as alloys with five or more principal elemental components in equimolar ratios (another definition is an atomic concentration between 5% and 35%). The definition of HEAs is still controversial, and some argue that there is no need to limit the concentration of elements [23, 24]. Besides, HEAs were also defined by mixed configuration entropy (S), and S can be calculated by the following formulas:

$$S = - R\sum {x_{i} \cdot \ln } x_{i}$$
(1)
$$S = R\ln n$$
(2)

where R stands for the molar gas constant, xi is the mole fraction of the elemental component and n represents the amount of composition in the alloy. And when S ≥ 1.36 R, the HEA is turned out be a quaternary alloy; when S ≥ 1.5 R, the HEA is composed of more than five elements [25]. After more than a decade of research, HEAs have shown many fascinating properties and attracted numerous attentions. However, most of the researches have paid close attention to the mechanical property including elasticity modulus, shearing modulus, hardness and strength of extension [26,27,28,29,30]. The chemical property of HEAs has been studied rarely. Recently, HEAs as a novel electrocatalyst for EECT have begun to draw attention. The influence of HEAs properties and characteristics on catalytic performance has been investigated from experimental and theoretical calculations [31, 32].

Compared to binary and ternary alloys, HEAs generally have more advantages in catalytic field, such as cocktail effect, high entropy effect, lattice distortion effect, phase structure and sluggish diffusion effect [24, 33]. Cocktail effect means synergistic response among the alloys with various elemental components, which could be tuned by changing the component elements of HEAs. This cocktail effect could effectively modify the electronic structure such as d-band center, which directly manages the adsorption and activation of reactants and intermediates, determining the catalytic performance of HEAs. Surface environment of HEAs such as surface charge distribution could also be governed by modulating the cocktail effect, which affects the adsorption of reactants and thereby improves the catalytic performance [34]. Moreover, HEAs have a higher degree of mixed configuration relative to that of binary and ternary alloys, which named high entropy effect. Benefiting from the high entropy effect, HEAs usually have a stronger corrosion and oxidation resistance and lower atomic diffusion rate, resulting in a more stable structure and catalytic stability of HEAs [35, 36]. Lattice distortion existing widely in HEAs due to the various atomic sizes of each elemental component, usually the surface strain effect. Fine-tuning the surface strain of electrocatalysts could effectively regulate their d-band center according to the previous reports [37,38,39]. In general, the downshift of d-band center will reduce the adsorption capacity of reactants and intermediates, while the upshift of d-band center will result in a strong interaction [40,41,42]. Therefore, taking advantages of lattice mismatch in HEAs may become a powerful strategy to boost the electrocatalytic performance. Phase structure also plays a significant important role in determining the property of HEAs. The simple phases such as face-centered cubic (fcc), body-centered cubic (bcc) and hexagonal close packed (hcp) are highly associated with their catalytic performance [43,44,45,46,47]. Therefore, controlling the phase structure of HEAs can bring unexpected effect to improve the catalytic performance. Especially in recent studies, fabricating HEAs with desirable mixed-phase could dramatically enhance the catalytic property. Although noble metals have excellent catalytic properties in electrocatalytic reactions, they are not stable enough usually. After forming HEAs, the stability will be greatly enhanced. Owing to the large difference in the atomic size of HEAs, the lattice distortion effect will increase the energy barrier of atom diffusion and hinder the atomic diffusion, which will lead to the sluggish diffusion effect of HEAs. The sluggish diffusion effect and high entropy effect are the main reasons for the HEAs to remain stable in acidic or alkaline environments.

In this review, we focus on the recent research on noble metal-based HEAs (NM-HEAs) electrocatalysts for EECT. The synthetic methods of NM-HEAs are systematically summarized firstly. Then we discuss the catalytic performance and mechanism of NM-HEAs in detail according to the various electrocatalytic reactions through experimental and theoretical calculations. Finally, the challenges, opportunities and development tendency in this field are proposed to the authors’ knowledge.

2 Fabrication and detection of NM-HEA NPs

At the early stage, most studies on HEA have focused on macroscopic HEA materials or bulk solids, which could be obtained by mechanical alloying [48, 49], vacuum smelting [50], powder metallurgic [51] or electrochemical deposition method [52]. However, these methods have difficulty in constructing nano-sized NM-HEA materials [53]. The key problem is the tradition method tends to form nanocrystals with separated phases rather than alloys. Especially once the elements exceed three, the rates of nucleation and growth decrease due to the reduced substitutional diffusion and interaction among the inter-diffusing species during partitioning [54]. Therefore, developing rational method to construct a stable single-phase solid solution structure consisting of more than five elements is highly desirable.

2.1 Top-to-down methods

2.1.1 Mechanical alloying at low temperatures

NM-HEA bulk solids are prepared firstly and milled at 123 K via the single ball cryomilling (Fig. 1a) after several hours to obtain NPs, such as CuAuAgPtPd. Kumar et al. [55] used this method to synthesize Cu0.2Ag0.2Au0.2Pt0.2Pd0.2, which has higher product quality and finer powder than other methods. The cryomilling not only has the advantages of high yield and purity, fine powder, keeping the atomic composition of NM-HEAs and reducing air pollution, but also has the great superiority of large-scale preparation of high-entropy alloy NPs, which has attracted wide attention.

Fig. 1
figure 1

a Customized single ball cryomill (WC-tungsten carbide) diagram. Reproduced with permission from Ref. [55]. Copyright 2018, Springer. b General DC arc discharge system composes of above parts. Reproduced with permission from Ref. [56]. Copyright 2018, Springer. c Diagram on left showing relationship between transient current and time generated by collision of a single nanodroplet on carbon fiber, and diagram on right showing a detailed description of collision process of nanodroplet. Reproduced with permission from Ref. [57]. Copyright 2019, Nature Publishing Group. d Schematic diagram of sample preparation and temperature versus time during 55-ms thermal shock. Reproduced with permission from Ref. [53]. Copyright 2018, American Association for the Advancement of Science. e Experimental diagram for synthesis of HEA NPs by fast-moving bed pyrolysis (FMBP). Reproduced with permission from Ref. [60]. Copyright 2020, Nature Publishing Group. f Process diagram of HEAs synthesis by solvent-thermal method. Reproduced with permission from Ref. [25]. Copyright 2020, American Chemical Society

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2.1.2 Spark discharge

NM-HEA bulk solids prepared were used as the electrode in spark discharge system (Fig. 1b). Wu et al. [56] synthesized CoCrFeNiPt and CoFeNiCr0.5Pd0.8 with uniform elemental distribution by the spark discharge way. Under direct current (DC) pulse power of 500 W and pulse width of 50 μs with a duty ratio of 0.5, atoms of different elements on the HEA electrodes are evaporated, and then recondensed into NPs after encountering the cold water to form HEA NPs.

2.2 Down-to-top methods

2.2.1 Electrodeposition based on nanodroplet-mediation

Glasscott et al. [57] synthesized CoFeLaNiPt high-entropy metallic glasses (HEMGs) by electrodeposition of metal salt precursors onto a conductive surface. Metal salt precursors firstly are dissolved water, and then suspended in dichloroethane to form nanodroplet with assisting of tetrabutylammonium perchlorate and ultrasonic. Delivery of the precursor atoms to the substrate results from the collision of nanodroplets with a biased electrode represents an electro-shock event on the order of 100 ms, rapidly reducing up to eight confined metal salt precursors into HEA NPs with precisely tunable stoichiometric ratios (Fig. 1c). Nanodroplet-mediated electrodeposition is an ideal method used to form HEMG-NPs at room temperature, which can maintain precise control over elemental stoichiometry.

2.2.2 Carbothermal shock (CTS)

Yao et al. [53] utilized CTS method to synthesize PtPdCoNiFeCuAuSn in 2018. In CTS, different metal salt mixtures (Pt, Pd, Ni, Fe, Co, Au, Cu, Sn, etc.) are first loaded onto the carbon support, such as carbon fiber. Then, flash heating is employed to trigger the rapid thermal decomposition of the metal salt at high temperature (about 2000 K, with impact duration of 55 ms and rate about 105 K·S−1) (Fig. 1d), forming small droplets of multimetallic solution. Following, these droplets are rapidly cooled to form a uniform and homogeneous HEA NP without aggregation or phase separation. Through controlling the CTS parameters (substrate, temperature, impact duration and heating/cooling rate), new types of NPs with different structures and different practical values could be prepared. The maximum temperature of the carbothermal method is high enough to promote uniform mixing of almost any metal combination.

2.2.3 Ultrasonication-assisted wet chemistry

Liu et al. [58] reported an ultrasonication-assisted wet chemistry method for preparing noble AuRuRhPdPt HEA NPs, utilizing the acoustic cavitation phenomenon in ultrasonication process in which extremely high temperatures in localized microscopic regions at momentary timespans were derived (up to ∼5000 °C of temperature and ∼2.03 × 108 Pa of pressure were generated in localized microscopic regions at a time scale of less than 1 × 10−9 s). The metal salt could be co-reduced by reducing agents and transform to alloy structures under operation. Compared with other wet chemistry methods, this technology uses ultrasonic technology to produce multimetallic alloy NPs with smaller particle sizes.

2.2.4 Fast cooling and dealloying

Qiu et al. [59] reported the construction of nanoporous AlNiFeCoCuMoPdPtAu HEA NPs by precursor fast cooling and dealloying. The bulk HEA solid is first prepared by melting pure melts using an induction-melting furnace under Ar protection, followed by spinning to prepare the alloy ribbons. Later, the ribbons are immersed in 0.5 mol·L−1 NaOH solution for chemical dealloying to prepare the nanoporous high-entropy alloys (np-HEAs). Compared with other high-entropy alloys, np-NM-HEAs not only reduce the content of noble metals, but also greatly improve their electrochemical activity and stability, making them more widely used in sensing and energy storage.

2.2.5 Fast-moving bed pyrolysis method

Recently, a strategy that through pyrolysis of metal salt precursor loaded on carbon supports in fast-moving bed to form ultrasmall and highly dispersed MnCoNiCuRhPdSnIrPtAu HEA NPs, was investigated by Gao et al. [60]. Fast pyrolysis of precursors at high temperatures results in the formation of HEA NPs due to the low free energy of the formation of nuclei (Fig. 1e). Moreover, after formation, the HEA NPs are directly dispersedly immobilized on granular supports for industrial applications.

2.2.6 Solvothermal synthesis

Solvothermal method is one of the most commonly used methods to prepare noble metal-based NPs. In previous reports, this method has also been employed to fabricate NM-HEAs. Bondesgaard et al. [61] reported a low-temperature solvothermal autoclave synthesis method at 200 °C with a reaction duration of 4–24 h in Teflon-lined steel autoclave. Metal salt precursor solutions are first dissolved in a mixture of acetone-ethanol (50:50 vol%), and then the solution is transferred to Teflon-lined steel autoclave which allows the chemical reaction taking place in a nonaqueous solvent at temperature above the boiling point and pressures above 1 × 105 Pa, to form RuRhPdIrPt HEA NPs (Fig. 1f). Wu et al. [62] successfully prepared NM-HEAs with all six Pt group elements through co-reduction process. The mixture of six metal precursors was dissolved in water, then added dropwise to the preheated TEG/PVP solution with 230 °C at a speed of 2 ml·min−1. After the solution was cooled to room temperature, NM-HEAs with the size of 3 nm were obtained.

2.3 Main characteristic methods of detecting element composition

HEAs are composed of many kinds of elements, the kinds, distribution and content of elements should be confirmed when testing its properties. There are many detection methods to determine the element compositions, such as X-ray diffraction (XRD), X-ray fluorescence (XRF), X-ray photoelectron spectroscopy (XPS). XRD can determine whether the sample is amorphous or crystal according to the measured diffraction information. By comparing with the standard pattern, we can also know what composition the sample contains. XRF is an analytical method that measures elements from Na to U in the periodic table. The analytical method can be used for qualitative and quantitative analysis without destroying the sample, but it is easy to be affected by mutual element interference and superposition peak. XPS can be used not only for qualitative analysis of elements, but also for quantitative analysis of elements. In qualitative analysis, almost all elements except H and He can be identified according to the position of the characteristic spectral line in the energy spectrum. Inductively coupled plasma optical emission spectrometer (ICP-OES), also known as inductively coupled plasma atomic emission spectrometer (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) can detect most elements in the periodic table, but the detection ability of the latter is higher than that of the former.

3 NM-HEAs for electrocatalysis

In recent years, several efforts have focused on application of NM-HEAs for advanced electrocatalysts toward various energy conversion reactions including liquid fuel oxidation reaction (FOR), oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), oxygen evolution reaction (OER) and CO2 reduction reaction (CO2RR). Based on experimental results and theoretical calculations, NM-HEAs electrocatalysts have exhibited great promising on the enhancement of catalytic activity and stability.

3.1 Electrocatalytic FOR

Proton exchange membrane fuel cells (PEMFCs) have attracted numerous attentions for several decades due to the high energy conversion efficiency, pollution-free process and portability [63,64,65]. Besides hydrogen energy, some organic small molecules such as methanol, ethanol and formic acid could also be regarded as ideal liquid fuel used in the anode of fuel cells for oxidation reaction. Pt-based nanostructured catalysts are the most efficient electrocatalysts for liquid FOR [66,67,68,69,70]. However, the high cost, low tolerance of CO and unsatisfactory structural stability hinder its broad applications [71]. In recent studies, Pt-based HEAs exhibit great potential to improve the catalytic performance and structured stability. Wu et al. [62] fabricated the platinum-group metals HEAs (PEG-HEA) with six elements through a facile wet chemical method. These PEG-HEA NPs with the average size of 3.1 nm are uniformly dispersed. The energy-dispersive X-ray spectroscopy (EDX) elemental mapping images reveal the homogeneous distribution of Ru, Rh, Pd, Os, Ir and Pt in the PEG-HEA NPs (Fig. 2a). The composition of PEG-HEA NPs was detected by XRF and XPS. Both results reveal that each element has a ratio of 1%–20% (Fig. 2b), indicating the successful synthesis of HEAs. These PEG-HEA NPs were evaluated as anodic electrocatalysts for ethanol oxidation reaction (EOR) and showed the superior specific and mass activity, indicating the complex 12-electron transfer processes (Fig. 2c–f). This excellent performance of EOR could be attributed to the ideal adsorption/desorption sites provided by the multielement surface of PEG-HEA NPs. Besides EOR, NM-HEAs also have been investigated as electrocatalysts for methanol oxidation reaction (MOR). Li and co-workers synthesized PdNiCoCuFe high-entropy alloyed nanotube arrays (NTAs) through template-assisted electrodeposition method (Fig. 2g) [72]. The content of each element in PdNiCoCuFe NTAs is between 9 at% and 33 at%, which is corresponding to the definition of HEAs. Owing to the high entropy effect and unique structure, PdNiCoCuFe NTAs show higher catalytic activity than commercial Pd/C catalyst with lower Pd content, as well as the excellent tolerance of CO (Fig. 2h, i). Moreover, Zhang and co-workers prepared PtRuCuOsIr catalyst with abundant nanoporous by a mild chemical dealloying method [73]. Compared to commercial Pt/C, PtRuCuOsIr catalyst exhibited enhanced catalytic activity and stability for MOR. These studies indicate that preparing NM-HEAs catalysts is an effective strategy to reduce the content of noble metal and improve the catalytic performance toward liquid FOR.

Fig. 2
figure 2

a HAADF-STEM image with corresponding EDX maps and schematic diagram of PEG-HEA NPs; b composition analysis of PGM-HEA according to XRF and XPS; c–f EOR performances of PEG-HEA NPs and commercial catalysts. Reproduced with permission from Ref. [62]. Copyright 2020, American Chemical Society. g Illustration of preparing PdNiCoCuFe NTAs; h catalytic activity and i stability of PdNiCoCuFe NTAs tested in 0.5 mol·L−1 CH3OH +1.0 mol·L−1 NaOH at 50 mV·s−1. Reproduced with permission from Ref. [72]. Copyright 2014, Elsevier

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3.2 Electrocatalytic ORR

The sluggish kinetics and complex multielectron process of ORR have become a key constraint on improving the efficiency of fuel cell. Therefore, designing and fabricating outstanding electrocatalysts toward ORR has been strongly desired and attracted numerous attentions in recent decades. Until now, a lot of state-of-art catalysts especially noble metal-based catalysts have been obtained [8, 12, 22, 74,75,76,77,78,79]. But the problems of high noble metal usage and unsatisfactory catalytic activity and stability remained. Recently, based on theoretical calculations and experiments, NM-HEAs catalysts have shown a great potential to solve these issues. Rossmeisl and co-workers predicted that NM-HEAs could exhibit an excellent catalytic performance of ORR through density functional theory (DFT) calculated *OH and *O adsorption energies on random sites of IrPdPtRhRu HEAs surface [80]. The comparison between the adsorption energy calculated by DFT of *OH and * O and the predicted value of the model which uses the ordinary least squares algorithm is shown in Fig 3a, the accuracy of these predictions is very high, and the root mean square deviation (RMSD) of * OH and * O is only 0.063 eV. The surface of IrPdPtRhRu HEAs offered a near-continuum of adsorption energies and the adsorption energies can be tuned by changing the bulk composition to match the peak of the volcano curve. As shown in Fig. 3b, the adsorption energies and catalytic activity of IrPdPtRhRu HEAs changed with each element content tuning. When the contents of Ir, Pd, Pt, Rh and Ru are 10.2 at%, 32.0 at%, 9.3 at%, 19.3 at% and 28.9 at%, IrPdPtRhRu HEAs possessed a stable structure and showed the superior performance of ORR. Besides theoretical calculations, the advantages of NM-HEAs on ORR performance were also demonstrated by experiments [81, 82]. Yao et al. [83] reported a high-throughput synthesis of ultrafine and homogeneous NM-HEAs through composition design step and rapid thermal shock synthesis step (Fig. 3c). The components of HEAs can be tuned easily by this method and a series of PtPdRuRhIrFeCoNi alloys with various elemental compositions were prepared. After rapid electrochemical screening by using a scanning droplet cell, PtPdFeCoNi HEAs delivered enhanced catalytic properties of ORR (Fig. 3d–h). Qiu and co-workers fabricated a series nanoporous Pt-based HEAs with a low Pt content of 20 at%–30 at% through top-down dealloying method [84]. As a proof-of-concept application, these Pt-based HEAs were evaluated as electrocatalysts for ORR. Among these catalysts, np-Al–Cu–Ni–Pt–Mn HEAs showed the optimal catalytic activity, which is 16 times higher than that of Pt/C catalysts in mass activity. Based on the first-principles simulations, Mn combined with Ni, Cu and Al could effectively modify the electronic structure of Pt, leading to the excellent ORR performance of Al–Cu–Ni–Pt–Mn HEAs.

Fig. 3
figure 3

a *OH adsorption on 871 symmetric 2×2 unit cells (blue dots) and tested on 76 asymmetric 3×4 unit cells (red crosses); b activities of Re-engineered Compositions of alloys. Reproduced with permission from Ref. [80]. Copyright 2019, Cell. c Schematic images of combinatorial and high-throughput fabrication of MMNCs; d ORR performance of various MMNCs, where size of these circles indicates magnitude of specific current at 0.45 V for ORR); e TEM image and f corresponding EDX maps of PtPdFeCoNi alloys; g cycle voltammograms and h linear-sweep voltammograms of these catalysts at 10 mV·s−1. Reproduced with permission from Ref. [83]. Copyright 2020, National Academy of Sciences

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3.3 Electrocatalytic HER

HER as a semireaction of electrochemical splitting of water has attracted wide attention in past decades to produce hydrogen energy, which is one of the most important clean energy in the future. To date, Pt is generally regarded as the most excellent catalysts for electrocatalytic HER in acidic environment, but the costliness and sluggish kinetics in alkaline electrolyte impede its practical utilization [85,86,87,88,89]. Recent reports showed that fabricating NM-HEAs catalysts could boost the HER performance both in acidic and alkaline solution as well as reduce the usage of Pt. Liu et al. [58] successfully prepared PtAuPdRhRu HEAs NPs through a facile ultrasonication-assisted method by means of the instantaneously massive energy generated by acoustic cavitation (Fig. 4a). Thanks to the strong synergistic effect between various elements and high entropy effect, PtAuPdRhRu HEAs NPs delivered a superior activity. The onset potential and Tafel slope of PtAuPdRhRu HEAs NPs reduced remarkably compared to those of other contrastive Pt-based catalysts (Fig. 4b). In addition, Wu et al. [90] fabricated IrPdPtRhRu HEAs NPs with the average size of (5.5 ± 1.2) nm through co-reduction process of these five metal precursors in triethylene glycol solution at 230 °C, and the amount of each element is basically identical characterized by XPS and XRF (Fig. 4c, d). Especially, diverse local electronic structures on IrPdPtRhRu HEAs NPs caused by the random atomic configurations have been directly demonstrated by hard XPS. In the study of electrocatalytic HER performance, IrPdPtRhRu HEAs NPs exhibited outstanding activity both in acidic and alkaline solution (Fig. 4e, f). In order to prove whether the activity of IrPdPtRhRu HEA NPs is related to the adsorption energy of intermediate H species, the turnover frequency (TOF) value at 50 mV is plotted as a function of experimental d-band center which can reflect the metal–H binding of monometallic and most binary catalysts in solutions of 0.05 mol·L−1 H2SO4 and 1.0 mol·L−1 KOH (Fig. 4g, h). According to Fig. 4g, h, the TOF value of monometallic NPs is approximatively related to the d-band center, the deeper the d-band center is, the higher the activity is. However, it should be noted that the enhanced HER performances do not fit perfectly to the d-band theory all the time, suggesting that the relationship between electronic structure and HER performance of NM-HEAs is complex and needs to be further investigated.

Fig. 4
figure 4

a Schematic image of synthetic method and HER application for PtAuPdRhRu/C; b HER polarization curves and Tafel plots of various catalysts in 1.0 mol·L−1 KOH solution. Reproduced with permission from Ref. [58]. Copyright 2019, Wiley-VCH. c HAADF-STEM images and EDX-mapping of RuRhPdIrPt HEAs NPs; d component of RuRhPdIrPt HEAs NPs characterized by XPS and XRF; HER polarization curves of RuRhPdIrPt HEAs NPs and other monometallic catalysts in e 0.05 mol·L−1 H2SO4 and f 1.00 mol·L−1 KOH solution; interrelation between TOF values at 0.05 (vs. RHE) and d-band center in g 0.05 mol·L−1 H2SO4 and h 1.00 mol·L−1 KOH solution (d-band center relative to Fermi level). Reproduced with permission from Ref. [90]. Copyright 2020, Elsevier

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3.4 Electrocatalytic OER

OER is the other semireaction of electrocatalytic water splitting process, which possesses a complex four-electron transfer process, leading to the high kinetic energy overpotential [91,92,93,94,95,96,97]. Although numerous excellent non-noble metal-based catalysts have been obtained and exhibited excellent OER performance, these electrocatalysts are generally not suitable to acidic solution. Ir, Ru and their oxides are still the most excellent electrocatalysts for the OER in acidic environment, as well as in alkaline electrolyte. Therefore, the content of Ir and Ru in electrocatalysts is highly desirable to reduce and the activity and stability are wished to be further enhanced. Jin et al. [98] synthesized np-AlNiCoIrMo HEAs with uniform distribution of elements by a facile dealloy process (Fig. 5a, b). The atomic ratio of Ir in this np-AlNiCoIrMo HEAs is only ~20%, which is much lower than that of binary and ternary Ir-based alloys. Owing to the high-entropy effect and sluggish diffusion effect of HEA, np-AlNiCoIrMo HEAs show a record-high OER electrocatalytic activity in acidic solution as well as superior stability (Fig. 5c, d). As shown in Fig. 5d, the potential changes of np-AlNiCoIrMo, AlNiCoIrCu and AlNiCoIrNb samples are very small, about 11.5, 2.1 and 11.0 mV, respectively, while np-AlNiIr shows a sharp potential increase of about 70.3 mV at 60 mA·cm−2 after 7000 cycles. These results prove that the quinary HEAs have excellent durability. Based on the calculation of Ir–O band, the covalency of Ir–O bond could be increased through alloying Ir with Ni and Mo, leading to the outstanding OER performance (Fig. 5e). Moreover, beside HEAs, Glasscott et al. [57] showed a generalized platform for fabricating the noble metal-based high-entropy metallic glasses, which were also containing disordered five or more equimolar components, but with amorphous microstructure. These novel catalytic materials exhibit excellent OER performance as well as HER due to the strong synergistic effect (Fig. 5f) and provide novel idea for designing advanced water splitting electrocatalysts.

Fig. 5
figure 5

a HAADF-STEM and b EDX-mapping images of np-AlNiCoIrMo HEAs; c OER polarization and d catalytic stability of Ir-based HEAs and IrO2. e Difference of Ir d-band center and O p-band center for various catalysts. Reproduced with permission from Ref. [98]. Copyright 2019, Wiley-VCH. f Electrocatalytic evaluation of CoFeLaNiPt HEMG-NP electrocatalyst for HER and OER. Reproduced with permission from Ref. [57]. Copyright 2019, Nature Publishing Group

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3.5 Electrocatalytic CO2RR

Electrocatalytic CO2RR has been regarded as a promising technology for the proficient conversion of CO2 into chemical fuels [99,100,101,102,103,104,105,106]. However, the high kinetic barriers, complex multielectron process and competition with HER in CO2RR lead to the unsatisfactory activity and selectivity, which hinder its practical application. In previous reports, CO binding energy of metal electrocatalysts played a key role in tuning the activity and selectivity of CO2RR. For instance, Au NPs as the electrocatalysts for CO2RR could exhibit the superior activity due to the weak Au−CO binding energy, but the products are mainly CO. Cu NPs as the electrocatalysts could result in a variety of products including hydrocarbons and oxygenates due to the intermediate CO binding energy [107,108,109,110,111]. NM-HEAs hold a great potential as electrocatalysts for CO2RR since the cocktail effect, high entropy effect, lattice distortion effect, phase structure and sluggish diffusion effect in HEAs could effectively modify the electronic structure and tune the CO binding energy. Rossmeisl and co-workers systematically calculated the CO and H adsorption energies on (111) surface sites of disordered AgAuCuPdPt HEAs with various components through combining DFT with supervised machine learning (Fig. 6a–f) [24]. The results demonstrated that optimizing the HEA compositions could increase the likelihood for sites with weak H adsorption and strong CO adsorption, which can suppress the competition of HER and enhance the reduction of CO. In an experimental attempt, Nellaiappan et al. [112] fabricated AuAgPtPdCu HEAs as a “single-atom catalyst” for the electrochemical reduction of CO2 to reveal the advantages of HEAs catalysts toward CO2RR (Fig. 6g–k). They find that redox-active Cu metal (Cu2+/Cu0) is the only activity site for electrocatalytic CO2RR in AuAgPtPdCu HEAs, other metal elements only provide a synergistic effect. Different from the pure Cu catalysts, the products of CO2 conversion are complete 100% gaseous at a lower potential on AuAgPtPdCu HEAs, and with an excellent stability. According to the free-energy calculations of intermediates, the destabilization of the *OCH3 and stabilization of the *O on HEAs surface lead to the lower limiting potential. In addition, − 0.45 eV of free-energy adsorption of H is usually the thermoneutral value for efficient HER catalysts, resulting in the lower Faraday efficiency (FE) of H2 than that of CH4 and C2H4.

Fig. 6
figure 6

GPR-predicted (ΔEpred) versus DFT-calculated (ΔEDFT) adsorption energies of AgAuCuPdPt for a on-top, b fcc-hollow H and c hcp-follow H, where blue represents data for 2 × 2 atoms slabs and red 3 × 3 atoms slabs; distributions of H and CO adsorption energies for AgAuCuPdPt d equimolar components, e optimal components and f locally optimal components without constraints. Reproduced with permission from Ref. [24]. Copyright 2020, American Chemical Society. g Schematic diagram, h XRD pattern, i TEM image, j HRTEM image and k atom probe microscope mapping image of AuAgPtPdCu NPs; l chronoamperometric test in CO2-saturated solution at −0.7, −0.8 and −0.9 V for 1000 s; m Faradic efficiencies of different products. Reproduced with permission from Ref. [112]. Copyright 2020, American Chemical Society

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4 Summary and outlook

Noble metal-based catalysts have been investigated widely in electrocatalytic field. However, the high cost and unsatisfactory catalytic performance are still not effectively addressed. Recently, NM-HEAs have exhibited great promising to enhance the catalytic performance as well as decrease the usage of noble metal. In this review, the preparation methods of NM-HEAs have been systematically evaluated and the advantages of HEAs electrocatalysts including high entropy effect, cocktail effect, lattice distortion effect, phase structure and sluggish diffusion effect have been discussed. These characteristics could effectively modify the geometric/electronic structure of NM-HEAs. Moreover, recent progresses in NM-HEAs electrocatalysts for fuel cells, water splitting and CO2 reduction have been summarized, with focus on the roles of high entropy effect, cocktail effect and lattice distortion effect in the enhancement of catalytic property. Although several significant efforts have been devoted to the exploitation of advanced NM-HEAs electrocatalysts for energy conversion. The research is still in the infant stage and many challenges and problems still need to be solved.

4.1 Facile preparation of NM-HEAs

Until so far, despite several different kinds of strategies have been developed to successfully synthesize NM-HEAs, the novel preparation method particularly easy-to-operation under ambient condition is highly desirable. Those methods mentioned in this review above either need specialized equipment (mechanical alloying, sputter deposition, spark discharge, etc.), or need to be operated in extreme condition (carbothermal shock synthesis, sputter deposition, etc.), such as high temperature of thousands degree or high vacuum. Additionally, the small preparation scale is also an issue needed to be addressed. Therefore, the simple, easy-to-operate and scale-up methods of fabricating NM-HEAs under facile conditions are imperative to develop.

4.2 Morphology-controlled synthesis of NM-HEAs

As is well known, the morphology of noble metal alloyed electrocatalysts has great influences on the catalytic performance. Various exposed facets possess different binding energies of intermediates, influencing the catalytic activity and stability. In the past decades, shape-controlled fabricating noble-based catalysts have been investigated in-depth and the structure-performance relationships in many electrocatalytic reactions have been gradually revealed [113]. For instance, concave nanocubic and tetrahexahedral noble-based NPs generally exposed high-index facets (HIFs), which usually enhanced the catalytic performance due to the low coordination numbers of surface atoms [114]. In addition, numerous state-of-art noble-based decahedrons and icosahedrons also exhibit superior performance for electrocatalytic HER, ORR and CO2RR due to the abundant twinned defects on the surface and the introduction of strain effect.

Fine controlling the nucleation/growth kinetics of metal alloyed NPs to desired shape is more difficult with the increase in compositional metals. Especially, NM-HEAs require five or more elements with equal content (or 5 at%–35 at%), leading to the greater difficulty. Therefore, morphology-controlled preparation of NM-HEAs remains a great challenge and is reported rarely. However, based on the shape-controlled synthesis of ternary and quaternary alloys reported previously, it is promising to fabricate NM-HEAs with definite morphology through fine-tuning the capping agents, reducing agents, shape-regulation agents and other control conditions.

4.3 Phase structure regulation of NM-HEAs

Phase structure has been regarded as the most significant structural parameter and played a crucial role in determining the catalytic property of HEAs. Especially, the unconventional phases of metal elements generally endow them with intriguing catalytic properties and innovative applications. 4H Au shows enhanced activity and higher ethylene selectivity in electrochemical CO2 reduction compared to the traditional (face center cubic) fcc Au [115]. Fcc Ru NPs could exhibit superior electrocatalytic HER and OER performance than conventional (hexagonal close packed) hcp Ru catalysts [44]. CoNi NPs with fcc/hcp closest packing polymorphism interface significantly boost the HER activity than CoNi NPs with pure fcc or hcp structure. Therefore, fabricating NM-HEAs with unconventional phase structure or multiple phases has great potential to enhance the electrocatalytic performance and is full of challenges at the same time. In general, we believe that NM-HEAs provide a novel strategy to fabricate advanced electrocatalysts and will be applied widely in the further.