Identifying Fe as OER Active Sites and Ultralow‐Cost Bifunctional Electrocatalysts for Overall Water Splitting

Abstract Electrocatalysts based on Fe and other transition metals are regarded as most promising candidates for accelerating the oxygen evolution reaction (OER), whereas whether Fe is the catalytic active site for OER is still under debate. Here, unary Fe‐ and binary FeNi‐ based catalysts, FeOOH and FeNi(OH) x , are produced by self‐reconstruction. The former is a dual‐phased FeOOH, possessing abundant oxygen vacancies (V O ) and mixed‐valence states, delivering the highest OER performance among all the unary iron oxides‐ and hydroxides‐ based powder catalysts reported to date, supporting Fe can be catalytically active for OER. As to binary catalyst, FeNi(OH) x is fabricated featuring 1) an equal molar content of Fe and Ni and 2) rich V O , both of which are found essential to enable abundant stabilized reactive centers (FeOOHNi) for high OER performance. Fe is found to be oxidized to 3.5+ during the *OOH process, thus, Fe is identified to be the active site in this new layered double hydroxide (LDH) structure with Fe:Ni = 1:1. Furthermore, the maximized catalytic centers enable FeNi(OH) x @NF (nickel foam) as low‐cost bifunctional electrodes for overall water‐splitting, delivering excellent performance comparable to commercial electrodes based on precious metals, which overcomes a major obstacle to the commercialization of bifunctional electrodes: prohibitive cost.