Oxygen‐Defect‐Rich Cobalt Ferrite Nanoparticles for Practical Water Electrolysis with High Activity and Durability

The scope of any metal oxide as a catalyst for driving electrocatalytic reactions depends on its electronic structure, which is correlated to its oxygen‐defect density. Likewise, to transform a spinel oxide, such as cobalt ferrite (CoFe2O4), into a worthy universal‐pH, bifunctional electrocatalyst for the hydrogen and oxygen evolution reactions (HER and OER, respectively), oxygen defects need to be regulated. Prepared by coprecipitation and inert calcination at 650 °C, CoFe2O4 nanoparticles (NPs) require 253 and 300 mV OER overpotentials to reach current densities of 10 and 100 mA cm−2, respectively, if nickel foam is used as a substrate. With cost‐effective carbon fiber paper, the OER overpotential increases to 372 mV at 10 mA cm−2 at pH 14. The NPs prepared at 550 °C require HER overpotentials of 218, 245, and 314 mV at −10 mA cm−2 in alkaline, acidic, and neutral pH, respectively. The intrinsic activity is reflected from turnover frequencies of >3 O2 s−1 and >5 H2 s−1 at overpotentials of 398 and 259 mV, respectively. If coupled for overall water splitting, the extremely durable two‐electrode electrolyzer requires a cell potential of only 1.63 V to reach 10 mA cm−2 at pH 14. The homologous couple also splits seawater at impressively low cell voltages of 1.72 and 1.47 V at room temperature and 80 °C, respectively. Active vacancies: The spinel oxide CoFe2O4 is transformed into a pH universal, bifunctional electrocatalyst for the hydrogen and oxygen evolution reactions by regulating the oxygen defects in the material. Application of this electrocatalyst for overall water splitting of freshwater, as well as real and artificial seawater is explored.