Concisely Synthesized FeNiWO_x Film as a Highly Efficient and Robust Catalyst for Electrochemical Water Oxidation

The critical bottleneck for water splitting, which is important for sustainable production of hydrogen, has remained in sluggish oxygen evolving reaction (OER) requiring insufficiently low overpotentials, η. We report a facile and versatile method for the preparation of loading-controllable metal oxide films adhered rigidly on electrode substrates, enabling effectual material hunting for superior OER anodes. This allows us to discover a ternary FeNiWOx film on a nickel foam (NF), attaining the lowest overpotentials of η10 = 167 (the superscripts represent the attained current densities of 10 mA cm–2) with a Tafel slope of 49 mV dec–1 and at least 100 h stability in OER, which compare advantageously with only a few state-of-the-art OER anodes with excellent η10 < 200 mV. The electrochemical data indicate synergistic coupling among ternary metal centers of Ni, Fe, and W to decrease the η value. The OER current is pH dependent for the FeNiWOx film, showing a non-proton-concerted process in the rate-determining step for OER. This could be explained by coupling of two neighboring lattice O•– radicals to form an O–O bond. The 3d bands of Fe or Ni could be stabilized by the high positive charge on W6+ to become close to or penetrate the 2p band of lattice O2–. This not only decreases the highest oxidation energy level for OER but also allows fast electron transfer from the 2p band of O2– to the 3d band in the FeIV or NiIV state to form the O•– radicals.