High entropy alloy (HEA) electrodeposits: importance, properties and uses

High entropy alloys combine 5 or more major elements at a concentration between 5 and 35 at.-%. HEAs can show the benefits of good wear resistance and corrosion protection. Some HEAs have been introduced as high performance electrocatalytic coatings in water electrolysis and metal-air batteries. Many authors have preferred ionic liquid baths but there are recent examples of success using aqueous baths, sometimes with the aid of ultrasonic agitation or pulsed electrical control. Fe–Co–Ni–Mo–W coatings have been electrodeposited from a pH 4.5 citrate bath under ambient conditions at a current density of 20–80 mA cm−2 while 8–10 μm thick, soft magnetic Co–Cu–Fe–Ni–Zn alloys were deposited from a boric acid buffered sulphate bath at pH 2.0–3.5 at 25–55°C using pulsed current electroplating. Co–Ni–Mn–W HEAs have been prepared as bifunctional electrocatalysts for acidic water splitting. Pulsed electrodeposition from an acid chloride bath was used to coat a carbon gas diffusion electrode (GDE) at a current density of 10 mA cm−2.