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High-entropy oxide (HEO) nanoparticles have been regarded as a promising catalytic material system for oxygen evolution reaction in recent years. However, their traditional physical and chemical synthesis remains challenges due to the limitation of fabricating controllable small size HEO nanoparticles. Herein, a general and novel method of low-energy-recoil ion implantation and subsequent annealing is successfully developed to synthesize high-entropy oxide nanoparticles catalysts. By controlling the fluence of irradiation Ar+ ions, the size and the load of HEO nanoparticles can be accurately controlled. The obtained (FeCoNiCrAl)O HEO nanoparticles exhibit an overpotential of 295 mV at 10 mA cm−2, a small Tafel slope of 39 mV dec−1 and good stability in 1 M KOH, which is much better than the properties of binary and medium entropy oxide counterparts prepared by the same method, showing the good application prospect of low-energy-recoil ion implantation in the preparation of complicated multi-element-metal oxide nanoparticles. [ABSTRACT FROM AUTHOR]

Exploring earth‐abundant, highly effective, and stable electrocatalysts for overall water and urea electrolysis is urgent and essential for developing hydrogen energy technology. Herein, a simple self‐derivation method is used to fabricate a Fe‐doped Ni3S2 electrode. The electrode exhibits an impressive trifunctional catalyst, with low overpotentials of 290, 198, and 254 mV at 100 mA cm−2 for the oxygen evolution reaction (OER), urea oxidation reaction (UOR), and hydrogen evolution reaction (HER). The durability is higher than 3500 h (146 days) at 100 mA cm−2 for the OER without obvious change. In situ Raman spectra reveal the incorporation of Fe inhibited S dissolution and facilitates the catalyst reconstruction. The density functional theory calculations indicate that the doping of Fe optimizes the adsorption of the rate‐determining step and the d‐band center is closer to the Fermi level, which accelerates the OER process. The two‐electrode electrolyzer needs the cell voltages of only 1.76 and 1.57 V to achieve a current density of 100 mA cm−2 and remarkable durability for more than 500 h at 100 and 500 mA cm−2 for overall water and urea splitting. This work holds great promise for industrial water and urea splitting applications. [ABSTRACT FROM AUTHOR]

The electronic structure of V-incorporated Ni-Fe oxide catalyst was further tailored by vacuum annealing to enhance OER activity. [Display omitted] • Vacuum annealing can facilitate the electron interactions between Ni/Fe and V sites. • We confirm directly the occurrence of OER self-reconstruction process. • NiFeV 0.65 -A4 catalyst achieves a low overpotential and a durable stability. • V incorporation promotes the formation of *O intermediates under OER. Tailoring the electronic structure of metal active sites of catalysts and understanding the corresponding mechanism of enhanced oxygen evolution reaction (OER) performance are required to engineer advanced OER catalysts. Herein, we first report that the electron interaction between Ni/Fe sites and V dopants in Ni-Fe oxides deposited on nickel foam by reactive magnetron co-sputtering can be stimulated by vacuum annealing. The subsequent annealing at 300 °C promotes the electron transfer from Ni to V, while the annealing at 400 °C significantly facilitates the electron transfer from Fe to V. The best NiFeV 0.65 -A4 (NiFeV 0.65 annealed at 400 °C) electrode with the optimized V-incorporated amount achieves a low overpotential of 248 mV at 10 mA/cm2 and a durable stability for 130 h. Besides, we demonstrate more intuitively the occurrence of OER self-reconstruction process to derive metal oxyhydroxides on the surface of catalysts. The mechanism studies manifest that V incorporation not only affords a larger electrochemical surface area, but also particularly tailors the electronic structure of both Fe3+ and Ni3+ sites. The density functional theory calculations indicate that the synergetic effect between Ni and Fe induced by V incorporation can promote the formation of *O intermediates under OER to enhance the OER intrinsic activity. [ABSTRACT FROM AUTHOR]

• A novel (FeCrCoNiAl 0.1)O x high-entropy oxide film was deposited by magnetron sputtering. • The NiCo-(FeCrCoNiAl 0.1)O x heterojunction films were prepared by high-temperature treatment in vacuum. • Electrocatalytic properties of (FeCrCoNiAl 0.1)O x and NiCo-(FeCrCoNiAl 0.1)O x heterojunction films were studied. Multi-element (FeCrCoNiAl 0.1)O x high-entropy oxide (HEO) films are deposited by direct current (DC) reactive magnetron sputtering under different oxygen flow rates. The structure of the as-prepared (FeCrCoNiAl 0.1)O x films depends on the oxygen flow rate. When the oxygen flow rate is lower than 70%, (FeCrCoNiAl 0.1)O x is in the face-centered cubic (FCC) phase of the solid solution. When the oxygen flow rate reaches 80%, the prepared HEO film has a spinel-type solid solution structure, which is denoted as (Fe 0.73 Cr 0.71 Co 0.78 Ni 0.81 Al 0.1)O 4.01. After thermal annealing at temperatures from 500 to 700 °C under vacuum, a NiCo alloy precipitate appeared on the surface. The as-prepared (Fe 0.73 Cr 0.71 Co 0.78 Ni 0.81 Al 0.1)O 4.01 HEO film has an overpotential of 381 mV and shows an exceptional long-term electrolysis stability of 120 h in alkaline solutions at a current density of 10 mA cm−2. The oxygen evolution reaction (OER) catalysis of the NiCo-(FeCrCoNiAl 0.1)O x heterojunction film formed by annealing (Fe 0.73 Cr 0.71 Co 0.78 Ni 0.81 Al 0.1)O 4.01 at 700 °C is improved after 120 h of stability testing due to the synergetic effect between high-entropy oxide and NiCo. The present work not only provides a promising electrocatalyst but also broadens the application of high-entropy oxides. [ABSTRACT FROM AUTHOR]