Free-Standing High-Entropy Oxides for Efficient Electrocatalytic Nitrate-to-Ammonia

A novel free-standing high-entropy oxides (HEOs) catalyst, i.e., zinc oxide nano-array decorated with iron, cobalt, nickel, copper, and zinc oxides (ZnO-HEOs), is proposed firstly. It exhibits outstanding electrocatalytic nitrate reduction reaction (NO₃ ^-RR) capacity in the neutral electrolyte, achieving a Faradaic efficiency of ∼99.42% with an ammonia yield of ∼34.09 mg cm^-2 h^-1 in a membrane electrode assembly under optimal conditions. Meanwhile, it delivers an open-circuit potential of ∼1.55 V with a peak power density of ∼5.33 mW cm^-2 in a zinc-nitrate battery. The performance of ZnO-HEOs surpasses that of HEOs powder counterparts and most previously reported catalysts or cathode materials, because the difference in electron work function between ZnO and HEOs triggers their interfacial synergistic effect. It facilitates the charge transfer from ZnO-HEOs to key intermediates, thereby changing the rate-determining step of ZnO and HEOs toward the electrocatalytic NO₃ ^-RR, and decreasing their energy barrier simultaneously. More importantly, the ZnO-HEOs can also sustain exceptional stability in both electrochemical systems, due to the fact that the ZnO supports effectively stabilize the active sites of HEOs. This work not only offers a route for the rational design of efficient and stable self-supporting HEOs, but also promotes their practical applications in energy- and environment-related fields.