Porous high-entropy alloys as efficient electrocatalysts for water-splitting reactions

,


Introduction
Water-splitting reactions (OER/HER) are highly efficient, green, sustainable energy production and storage sources due to their great energy output and earth-abundant oxygen (O 2 ) and hydrogen (H 2 ).[1][2][3] Ir-and Ru-based catalysts are the most active catalysts for OER, and Pt for HER [4][5]; however, the rarity, intolerable cost, and selfpoisoning of these catalysts preclude their practical utilization.[6][7][8] Unlike noble-metal-based catalysts, transition metals-based catalysts were developed for OER while chalcogenides, perovskite, carbides, and phosphides were used for HER but some carbon-based materials for OER/HER, which are low-cost and earth-abundant.[9][10][11][12][13] Distinct from traditional metal-based catalysts, HEAs possess many outstanding properties like low-level stacking fault energy, mechanical strength, thermal stability, and stability against radiation and corrosion.[14][15][16][17][18] HEAs have plentiful disparate active sites, elemental diversity, multiple functionalities, lattice distortion, and inherent surface complexity, which tune the adsorption of reactants besides retarding the adsorption of intermediates, thus accelerating water electrolysis kinetics.[14][15][16][17][18] Porous HEAs have the inimitable merits of porous morphologies like excellent surface area, low density, accessible active sites, quick molecule diffusion, and maximized atomic utilization, which tune the adsorption energies for reactants and intermediates during OER/HER.[19][20][21][22][23][24] The utilization of HEAs in water-splitting has attracted significant attention recently, culminating in 187 articles in total besides 64 articles for only porous HEAs according to the web of science (Fig. 1).Various recent reviews emphasized the rational fabrication of HEAs for OER/HER and other catalytic applications.[25] However, reviews about porous HEAs for complete water-splitting are not yet addressed as far we found.
This review emphasizes the fabrication of porous HEAs for OER/HER with a particular focus on the effects of physiochemical merits (Scheme 1) besides addressing the current challenges on porous HEAs and future research direction to tailor the design of ideal porous HEAs for OER/ HER.

Fundamental and advantages of HEAs for enhanced OER/HER
The history of crystalline HEAs was dated back to 2004, while amorphous alloys with high mixed entropy are back to the 1990 s. HEAs comprise (≥5 metals) in equiatomic or near equiatomic ratios with concentrations between 5 % and 35 % (Fig. 2a); however, there is no limit for the concentration of elements.The HEAs are famous for their increased configurational entropy (S) (>1.5R), as given in Eqn.(1).[26] where n is the number of elements and R is the molar gas constant, but there is no maximum number of elements in HEAs, and their effects on OER/HER are still ambiguous.HEAs usually form crystalline facecentered cubic, body-centered cubic, and close-packed hexagonal structures with uniform element distribution due to the high thermal energy of their solid solutions.[15,25,27] But amorphous phases with lattice distortion are formed when there are significant differences in the atom sizes.There are various methods for preparing HEAs like carbothermal shock, electrosynthesis, mechanical milling, solvothermal pyrolysis, wet chemical, pulsed laser ablation, reactive sputter deposition, and dealloying (Table 1).[15,25,[27][28][29][30][31] However, other methods like a template, reduction, and polyol may be explored, owing to the tendency of reducing multiple metals simultaneously.[32] HEAs possess various unique merits required for enhanced water electrolysis like phase stability, cocktail effects, slow diffusion, and corrosion resistance (Fig. 2b). [31]

The high configurational entropy effect
With their mixed multimetallic composition, HEAs have high mixed configurational entropy that increases with increasing the number of components.The high configuration entropy effect is beneficial to produce a stable single-phase solid solution structure of HEAs with excellent OER/HER stability.

The lattice distortion effect
The dissimilar atomic sizes and electronic configurations allow random occupation in a crystalline and subsequent lattice distortion in HEAs, which endorses the hardness and thermal stability.The tensile lattice strain induced by the lattice distortion upshifts the d-band results in a more robust interaction with reactants (O 2 /H 2 ); meanwhile, the compressive strain downshifts the d-band to weakening of the interaction during OER/HER.

The sluggish diffusion effect
The lattice distortion enhances the energy barrier of atomic diffusion, thus decreasing the diffusion effect, while a strike hindrance of atomic diffusion protects against aggregation of HEAs during OER/HER.

The cocktail effect
The synergism between metals in HEAs promotes the cocktail effect, improving the thermo-electric, mechanical, magnetic properties and altering the d-band center.Upshifting the d-band strengthens the interaction of metals with the O 2 /H 2 molecule and weakens the binding energy of OER/HER intermediates.Multiple charge redistribution on the surface of HEAs resulting from different work functions of metals enriches multifunctionality.Despite the unique properties of HEAs, there are some limitations for their use for OER/HER, like lower active sites on the surface and thermodynamic stability at operating conditions.Thermodynamically, high Δ S mix necessitates enthalpy formation (Δ H mix ) of intermediates to form a single-phase solution of several elements with low Gibbs free energy (Δ G mix ≤ 0, Eqn. ( 2)).

Working principles of electrolyzers
The electrolyzer (Fig. 3a) consists of three main parts: anode, cathode, and electrolyte/membrane.The anode and cathode are coated with highly active catalysts to allow water-splitting reactions.Under applied voltage (1.23 V vs. RHE), H 2 O is cleaved to H 2 (HER) at cathode and O 2 (OER) at anode using linear sweep voltammogram (LSV) test (Fig. 3b).The OER/HER reactions in different electrolytes are shown in Eqns.( 3)-( 7): Acidic condition: Cathode (HER): Alkaline condition: Cathode (HER): Overall reaction: Scheme 1.The overall review outlines.
The mechanism of HER (Fig. 4a) is classified into three steps with corresponding Tafel slope (b c ), [34] in acid: Volmer (H + + e -→ H ads ,b c = 118.2mV/dec) involves adsorption of hydrogen ions (H + ) and electrons (e -) on catalyst's active sites to afford the intermediate adsorption (H ads ).Then, the H ads is desorbed to form H 2 by Tafel (2H ads → H 2 ; b c = 29.6 mV/dec) or Heyrovsky (H ads + H + + e -→ H 2 ; b c = 39.4 mV/dec).Thus, highly active catalysts must have moderate binding energies with low Δ G H . OER produces O 2 via several H + /e -linked methods (Fig. 4b) involving multi-step reactions via a four-electron pathway (4e -) with a high energy barrier that make OER kinetic very slow with a large overpotential (Ƞ).[35] The quest for low-cost and efficient catalysts for OER/HER led to the emergence of HEAs as promising catalysts for both reactions.

High-entropy alloys for HER
Various porous HEAs were used as cathodes for HER, like Ni 20 Fe 20- Mo 10 Co 35 Cr 15, which had higher activity and stability than Pt sheet with Ƞ 10 107 mV in H 2 SO 4 and 172 mV in KOH.[36] CoCrFeNiAl (HF-HEA a2 ) obtained by mechanical alloying and spark plasma sintering consolidation, then etching by hydrogen fluoride and activation by cyclic voltammetry (CV, 4000 cycles) showed superb HER with Ƞ 10 (73 mV), b c (39.7 mV/dec), and high stability in H 2 SO 4 .[37] This was due to the synergistic effects and atomic mixing of its constituent metals.FeCo-NiAlTi intermetallics with unusual periodically (L˥ 2 -type) ordered structure augmented HER with Ƞ 10 (88.2 mV) and b c (40.1 mV/dec) akin to Pt-catalysts because the unique L˥ 2 -type structure enabled specific site-isolation effect that tuned the H + /H* adsorption/desorption.[38].Monolithic hierarchical CuAlNiMoFe electrode enhanced the HER than CuAlNiFe, CuAlNi, CuAl, Cu, Pt/C/Cu in KOH, as proved by the LSV, EIS, and Tafel plots (Fig. 5a-d).[39] This was due to synergistic effects and hierarchical shape that lowered the H + adsorption/desorption,

Table 1
The main preparation approaches of HEAs.[40] Novel CoCrFeMnNiP formed by a eutectic solvent method had a single metal phosphide phase that increased HER activity with lower Ƞ 10 (136 mV) than its counterparts phosphides and Pt/C.[17] Also, CoCr-FeMnNiP gave full water-splitting in KOH at lower voltage (V 100 = 1.78 V) than Pt/C/IrO 2 ((V 100 ) = 1.87 V).Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C synthesized by oil phase method showed HER with low Ƞ 10 (11 mV) and stability in KOH due to its multi-active sites and fast site-to-site e -that ease H + adsorption/desorption as proved by the Density functional theory (DFT).[41] Similarly, PdFeCoNiCu synthesized by oil phase method gave superb alkaline HER with Ƞ 10 (18 mV), b c (39 mV/dec), high mass activity (6.51A mg -1  Pd at − 0.07 V), and durability for 15 days than non-Pd materials.[42] The DFT study showed that Pd and Co were active for initial H 2 O cleavage and H 2 formation, respectively, while Ni, Fe, and Cu aided e -transfer with tuned binding energies of H ads .Nanosponge-like PdPtCuNiP high-entropy metallic glass (HEMG) with ample active sites achieved by surface dealloying had a great HER activity with Ƞ 10 (32 mV) than most presently available catalysts in KOH.[43] The method was scalable and DFT showed that lattice distortion, chemical complexity, and synergism of PdPtCuNiP accelerated H + adsorption/ desorption.The HER activities of the HEAs are summarized in Table 2. Various porous HEAs with morphologies (Fig. 6a-f) like nanodendrites, nanoporous, nanosponges, and nanosheets were reported for watersplitting.

High-entropy alloys for OER
OER is applicable in energy conversion and storage, with IrO 2 /RuO 2 being the best catalysts.[46] Lately, HEAs outperformed IrO 2 /RuO 2 .For example, AlNiCoFeX (X = Mo, Nb, Cr) were designed by controlled integration of metals into an alloy and dealloying their oxidized surface.
[45] Amongst the HEAs studied, AlNiCoFeMo showed the best OER compared to its counterparts and RuO 2 due to the impact of synergy into a single-phase structure, giving a valuable structural and chemical degree of freedom.MO x nanosheet (M = Mn, Fe, Co, and Ni) was grown on MnFeCoNi to form a core-shell structure by CV with an excellent OER.[44] MnFeCoNiCu nanoparticles@N-doped porous carbon on the surface of carbon cloth (HEAN@NPC/CC) nanorods was formed via the insitu growth of quinary metal-organic frameworks (MOFs) on CC sheets via one-pot solvothermal reaction followed by annealing at different temperatures (400-500 • C) (Fig. 7a-c).[47] HEAN@NPC/CC annealed at 450 • C (HEAN@NPC/CC-450) showed the highest OER activity than its counterparts and RuO 2 as well as FeCoNi/CC, FeCoNiCu/CC, and MnFeCoNi/CC.HEAN@NPC/CC-450 achieved low Ƞ 10 of (302 mV), b a (83.7 mV/dec) and long-term durability over 20 h for OER (Fig. 7d-i).
AlCrCuFeNi prepared by combining vacuum induction melting, gas atomization, and acidic etching methods enhanced OER activity with Ƞ 10 (270 mV), b a (77.5 mV/dec), and durability over 35 h compared to RuO 2 .[48] Fe 29 Co 27 Ni 23 Si 9 B 12 ribbon made by melt spinning and electrochemical corrosion etching methods had improved OER after etching for 3 h with a lower Ƞ 10 (230 mV) than its crystalline form.[49] The amorphous Fe 29 Co 27 Ni 23 Si 9 B 12 had a reduced interface between the catalyst and the intermediates with optimized Δ G H .A multilevel structured (CrFeCoNi) 97 O 3 formed by metallurgy method possessed high OER with Ƞ 10 (196 mV), b a (29 mV/dec), and stability for 120 h, due to the island-like Cr 2 O 3 microdomains formation.[50] Porous core-shell FeCoNiCrNb 0.5 made by the dealloying outperformed other alloys and ceramic catalysts due to its large surface area, fast dynamics, and superb durability.[51] A high entropy MOF (HE-MOF) synthesized by a solution-phase at room temperature exhibited high OER activity (Ƞ 10 = 245 mV) because of its high configurational entropy.[52] CoFeNiMnMoPi was first prepared by a high-temperature fly-through, which had a higher OER activity, lower Ƞ 10 (270 mV), and b a (74 mV/dec) than IrO x .[53] That was because the fly-through allowed metals and phosphorous confinement in one aerosol droplet, in-situ oxide-to-phosphate conversion at high temperature, and uniformly mixed multimetallic elements in milliseconds.Ultra-small 3D porous FeCoNiPB/(FeCoNi) 3 O 4-x (ca.15 nm) formed by air after acid-etching of FeCoNiPB, increased OER activity with low Ƞ 10 (229 mV), Ƞ 100 (406 mV), and good durability due to the rich defect structure.[54] FeNiCoCrMn was prepared via a simple solvothermal process that showed an excellent OER with a small Ƞ 10 (229 mV), and Ƞ 100 (278 mV) with good durability than its subsystems.[55] Flower-like phosphates grown in-situ on porous CoCrFeNiMo to afford P-HF-(CoCrFeNiMo) by hydrothermal-phosphorization gave enhanced OER with low Ƞ 10 (220 mV), b a (30.3 mV/dec), and superior stability.[21] That was due to abundant OH -, P-doping, 3D internal connected nanoporous structure, and high conductivity that accelerated charge mobility.The OER activities of HEAs in KOH are summarized in Table 3.

Conclusions and future perspectives
This review emphasizes the fabrications of porous HEAs and the effects of their properties on OER and HER.Dealloying is the most common and promising approach for synthesizing HEAs with different morphologies like nanoporous, nanosponges, and nanosheets without substrate.The solvothermal method was also explored to prepare HEAs containing various elements like Pt, Pd, Ru, Rh, and Ir.Various porous HEAs were prepared for HER, which showed Ƞ 10 ranged from 11 to 183 mV, as Pt 18 Ni 26 Fe 15 Co 14 Cu 27 /C showed the lowest Ƞ 10 of (11 mV).Various porous HEAs were synthesized for OER, which revealed Ƞ 10 ranged from 196 to 302 mV, as (CrFeCoNi) 97 O 3 revealed the lowest Ƞ 10 (196 mV) followed by P-HF-(CoCrFeNiMo) (220 mV).The outstanding OER/HER performances of HEAs aroused from the coupling between the physiochemical merits of HEAs, and the catalytic merits of porous shapes.
In view of future perspectives, the fabrication process of porous HEAs

Fig. 1 .
Fig. 1.Number of articles from 2014 to 8th November 2021 obtained from Web of Science using keywords "high-entropy alloys for HER and OER").

Table 2
Comparison of HER performance of HEAs measured in different electrolytes.