Metal Oxide Catalysts for the Evolution of O2 from H2O

Abstract

This paper describes an iron-nickel oxide catalyst that can eliminate oxygen overvoltage in the electrolysis of alkaline water. Oxygen overvoltage is the largest source of energy loss in water electrolysis. A method for cathodic electrodeposition of first-row transition metal oxides for use as catalysts is described, and the effects of the electrodeposition variables on catalytic performance and catalyst composition were explored. The NiFe oxide catalyst achieved a nearly ideal anodic electron-transfer coefficient, α a = 0.0082 (14.8 mV/decade) in a 1 M KOH solution. The metal oxide catalysts reported here could produce hydrogen and oxygen from water at approximately the thermodynamic potential for small currents. Both transmission electron microscopy and X-ray photoelectron spectroscopy experiments indicated that the NiFe oxide was composed of small crystals (∼1 nm) connected with an amorphous phase and contained a highly disordered arrangement of all nonmetallic nickel and iron oxidation states. Changes in electrochemical impedance spectra (EIS) with applied working potential correlated with known nickel and iron aqueous reduction potentials. A simple catalytic mechanism is presented. EIS showed a high-frequency inductive loop that spanned 3 log of frequency (>10 5 to ∼10 2 Hz) and exhibited both negative capacitance and negative resistance. A scan of electrode material (scraped from the electrode) magnetization as a function of applied magnetic field using a SQUID magnetometer showed the NiFe oxide electrode to have magnetically ordered domains within its structure and to have a magnetization at zero-applied field. Magnetic fields associated with these domains are related to the efficient production of the oxygen triplet state with the electrode. These fields are also responsible for the fact that the oxygen-evolving electrode will not produce a cyclic voltammogram but rather gives an oscillating signal indicating charge rearrangement under those conditions.