Oxygen-Deficient Thermal Treatment Boosts Performance of Co-Ni-Fe Oxide Electrocatalyst in Oxygen Evolution Reaction

Abstract

The production of green hydrogen through water electrolysis using electricity generated from renewable sources is one of the most promising strategies toward achieving a carbon-neutral economy.[1-3] Water electrolysis is an effective and well-established water splitting process consisting of two half-cell reactions, the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) at cathode and anode, respectively. Although hydrogen as the value-added product is generated at the cathode, the anodic reaction plays an important role in determining the reaction kinetics of the entire water splitting process due to the multiple-electron transfer.[4] Therefore, developing low-cost and highly active OER electrocatalysts is one of the keys to further improve overall efficiency of water electrolysis. Among various water electrolysis technologies, alkaline water electrolysis (AWE) has been commercialized for many decades and standing for medium-scale hydrogen production and using non-noble metals as electrocatalysts.[5]Co-, Ni-, and Fe- oxides or (oxy)hydroxides are reported as auspicious OER electrocatalysts for AWE, as well as abundance on earth.[6] In addition, oxygen vacancies in metallic oxides are reported recently as an effective approach to enhance catalyst activity for OER.[7] Herein, we present trimetallic oxides (CoNiFe oxides) annealed in different atmospheres which are oxygen-rich (A; air), an oxygen-poor (M; 1 vol% air in argon), and oxygen-free (H; 5 vol% H2 in argon), respectively, The CoNiFe (2:2:1)/M with a molar ratio of Co : Ni : Fe = 2 : 2 : 1 treated under an oxygen-poor atmosphere (M) exhibits the best performance in 1 M KOH, with an overpotential of 291 mV at 10 mA cm-2 geo, a mass activity at 1.56 V versus the reversible hydrogen electrode (RHE) of ~345 A g-1 catalyst, and a small Tafel slope of 32.0 mV dec-1. Moreover, CoNiFe (2:2:1)/M shows excellent stability in the OER process for at least 50 hours at 100 mA cm-2 geo at lab scale and ~75 h at 400 mA cm-2 in an alkaline electrolyzer. This work presents a simple and effective method to design OER catalysts based on trimetallic oxides with oxygen deficiencies for alkaline water electrolysis.