Preparation and electrocatalytic properties of (FeCrCoNiAl0.1)Ox high-entropy oxide and NiCo-(FeCrCoNiAl0.1)Ox heterojunction films

Multi-element (FeCrCoNiAl0.1)Ox 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 (FeCrCoNiAl0.1)Ox films depends on the oxygen flow rate. When the oxygen flow rate is lower than 70%, (FeCrCoNiAl0.1)Ox 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 (Fe0.73Cr0.71Co0.78Ni0.81Al0.1)O4.01. After thermal annealing at temperatures from 500 to 700 °C under vacuum, a NiCo alloy precipitate appeared on the surface. The as-prepared (Fe0.73Cr0.71Co0.78Ni0.81Al0.1)O4.01 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-(FeCrCoNiAl0.1)Ox heterojunction film formed by annealing (Fe0.73Cr0.71Co0.78Ni0.81Al0.1)O4.01 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.