Alloying has proven power to upgrade metallic electrocatalysts, while the traditional alloys encounter limitation for optimizing electronic structures of surface metallic sites in a continuous manner. High-entropy alloys (HEAs) overcome this limitation by manageably tuning the adsorption/desorption energies of reaction intermediates. Recently, the marriage of nanotechnology and HEAs has made considerable progresses for renewable energy technologies, showing two important trends of size diminishment and multidimensionality. This review is dedicated to summarizing recent advances of HEAs that are rationally designed for energy electrocatalysis. We first explain the advantages of HEAs as electrocatalysts from three aspects: high entropy, nanometer, and multidimension. Then, several structural regulation methods are proposed to promote the electrocatalysis of HEAs, involving the thermodynamically nonequilibrium synthesis, regulating the (sub-)nanosize and anisotropic morphologies, as well as engineering the atomic ordering. The general relationship between the electronic structures and electrocatalytic properties of HEAs is further discussed. Finally, we outline remaining challenges of this field, aiming to inspire more sophisticated HEA-based nanocatalysts.

Ruthenium (Ru) is widely recognized as a low-cost alternative to iridium as anode electrocatalyst in proton-exchange membrane water electrolyzers (PEMWE). However, the reported Ru-based catalysts usually only operate within tens of hours in PEMWE because of their intrinsically high reactivity of lattice oxygen that leads to irrepressible Ru leaching and structural collapse. Herein, we report a design concept by employing large-sized and acid-resistant lattice lead (Pb) as a second element to induce a pinning effect for effectively narrowing the moving channels of oxygen atoms, thereby lowering the reactivity of lattice oxygen in Ru oxides. The Pb-RuO₂ catalyst presents a low overpotential of 188 ± 2 mV at 10 mA cm⁻² and can sustain for over 1100 h in an acid medium with a negligible degradation rate of 19 μV h⁻¹. Particularly, the Pb-RuO₂-based PEMWE can operate for more than 250 h at 500 mA cm⁻² with a low degradation rate of only 17 μV h⁻¹. Experimental and theoretical calculation results reveal that Ru-O covalency is reduced due to the unique 6s−2p−4d orbital hybridization, which increases the loss energy of lattice oxygen and suppresses the over-oxidation of Ru for improved long-term stability in PEMWE.

Construction of gamma-phase transition metal oxyhydroxides for electrode materials is an effective strategy for improving electrochemical properties. However, the preparation of gamma-phase transition metal oxyhydroxides with various defects remains an arduous challenge. Herein, gamma-phase Ni–Co oxyhydroxides with cationic vacancy defects and crystalline–amorphous interfaces are synthesized successfully via electrochemical reconstruction to achieve an ultrahigh mass/areal/volumetric energy density, flexible all-solid-state asymmetric supercapacitor (ASC). The Ni–Co oxyhydroxides consist of γ-NiOOH, γ-CoOOH and NiOOH phases; the γ-NiOOH and γ-CoOOH phases can effectively improve electronic conductivity and theoretical capacitance due to the abundant Ni⁴⁺/Co⁴⁺. The cationic vacancy defects and crystalline–amorphous interfaces not only endow the gamma-phase Ni–Co oxyhydroxides with abundant electrochemical active sites, but also enable fast electron transfer between the electrode and electrolyte. Therefore, the as-obtained gamma-phase Ni–Co oxyhydroxides display a high capacitance of 20.9 F cm⁻² at 4 mA cm⁻² and 3483 F g⁻¹ at 0.67 A g⁻¹, as well as excellent rate characteristics (90.5% capacitance retention at a high current density of 240 mA cm⁻² and 40 A g⁻¹). Density functional theory calculations reveal that Ni–Co oxyhydroxides with cationic vacancies can increase the adsorption energy of H₂O, which is beneficial for the capture of H₂O to occur for subsequent charge storage reactions. Furthermore, the assembled Ni–Co oxyhydroxides//active carbon all-solid-state ASC shows ultrahigh mass/areal/volumetric energy density of 92.6 W h kg⁻¹/3.3 mW h cm⁻²/19.5 mW h cm⁻³ (at 1156 W kg⁻¹/34.6 mW cm⁻²/204.1 mW cm⁻³) and possesses excellent electrochemical stability with 91% retention after 7000 cycles.

Proton exchange membrane fuel cells show great potential as power source for automobiles, yet are now facing technological challenges of low efficiency in the cathodic oxygen reduction reaction and severe degradation from Nafion ionomers. Herein, we report the design and construction of a core/shell nanoparticle, composing of Pt₃Fe intermetallic nanoparticle as core and atomically-thin porous N-doped carbon layer as shell, to alleviate Nafion ionomer poisoning and local oxygen transport at the interfaces, thereby improving the performance of membrane electrode assemblies. Combining electrochemical, spectroscopic and calculation results verify that the sandwiching carbon layer can effectively prevent surface Pt active sites from poisoning by ionomers. Moreover, this deliberate design facilitates a more homogeneous distribution of ionomers in catalyst layer, and drives a H₂-air fuel cell peak power density up to 1.0 W cm^-2. Due to the configuration-induced strong Fe-N coordination, our unique catalyst efficiently preserves transition metals and consequently delivers a notable fuel cell durability at a constant potential of 0.5 V for 100 h.

Nanoarray catalysts supported on substrates provide an opportunity for industrially promising overall water splitting at large current densities. However, most of the present electrocatalysts show high overpotentials at a large current density, inducing a low efficiency for industrial water electrolysis. Herein, using the classic NiCoP nanorod arrays as the basic catalyst model, we presented a trace W and Mo co-doped strategy to boost the overall water splitting electrocatalysis at an industrial current density. After a trace amount of W and Mo atoms was doped, the constructed W and Mo co-doped NiCoP nanorod arrays (W,Mo-NiCoP/NF) show a low overpotential of 249 mV towards the hydrogen evolution reaction (HER) at a very large current density of 1000 mA cm⁻². We deduce that the regulation of the electronic structure caused by the trace W and Mo atoms, as well as the intrinsic features of nanoarrays leads to enhanced catalytic activity. In addition, a significant enhancement towards the oxygen evolution reaction (OER) was also achieved by this co-doped strategy. Finally, an overall water splitting device using W,Mo-NiCoP/NF as both the anode and cathode was assembled to exhibit a low cell voltage of 1.85 V at a large current density of 500 mA cm⁻² and an excellent long-term stability within 50 h, better than most of the state-of-the-art bifunctional electrocatalysts yet reported. Our results highlight the significance of trace-doping engineering in industrial water electrolysis.