Electrodeposition of CoFeNiMoSn high-entropy alloys in choline chloride-ethylene glycol-based electrolyte for high-performance electrocatalyst toward water splitting

Sn- and Mo-containing high-entropy alloys (HEAs) have recently emerged as promising hydrogen evolution reaction (HER) electrocatalysts owing to their unique inherent activity. The conventional synthesis of these alloys, however, relies on energy- and time-intensive melt-dealloying routes, significantly hindering the scale-up application. Herein, we report a facile strategy to electrodeposit the CoFeNiMoSn HEAs with tunable Sn contents and regulated microstructures in a choline chloride—ethylene glycol-based (CE-based) electrolyte, which allow for tailoring the intrinsic electronic structures and maximizing the exposure of active sites. The CoFeNiMoSn-6 HEA with the highest Sn solubility exhibits an outstanding HER activity in 1 M KOH by delivering a current density of 10 mAcm −2 at only 66 mV, a Tafel slope of 44.2 mVdec -1 , an interfacial charge-transfer resistance ( R ct ) of 0.42 Ω cm −2 , and a large electrochemical surface area ( C dl  = 49.83 mF cm −2 ). Despite its modest OER activity, the symmetric CoFeNiMoSn-6 || CoFeNiMoSn-6 couple drives the overall water splitting at 1.58 V to reach 20 mA cm −2 , lower than the benchmark noble-metal couple (20 wt% Pt-C/NF || RuO 2 /NF, 164 V) under the same current density. This work demonstrates the feasibility for fabricating Sn and Mo co-existing HEAs by a facile electrodeposition route in a CE-based electrolyte, and it could be potentially extended to other group elements that are incompatible in aqueous media, thus paving a scalable route toward low-cost and high-performance HEA electrodes for industrial hydrogen production.