TEM Characterization of the Structure and Composition of Nickel-Iron Catalysts for Alkaline Oxygen Evolution Reaction

Alkaline anion exchange membrane (AAEM) electrolyzers have high potential to support renewable energy systems by storing energy in the form of hydrogen. However, the sluggish kinetics of the anodic half reaction, the oxygen evolution reaction (OER), significantly hinders the efficiency of these devices. Large overpotentials of 300 mV or more are needed to overcome the slow kinetics of the OER, which diminishes the feasible efficiency of this process by tens of percent. Ultimately, these higher energy barriers translate to financial costs and limit the practicality of AAEM water electrolysis.

Effective electrocatalysts can lower the overpotential for OER, but these catalysts need to be highly active, stable for thousands of hours, and inexpensive. The state-of-the-art catalysts for the OER in the current generation of AAEM electrolyzers are IrO_x and RuO_x; however, these catalysts contain precious transition metals that are prohibitively costly to deploy on a large scale. Oxidized nickel-iron based catalysts (NiFeO_x) are promising non-precious alternatives, which have shown even higher activity than IrO_x in alkaline conditions. However, the long-term stability of these materials has not been adequately demonstrated to date.

We have synthesized carbon-supported NiFe alloy using a facile wet-impregnation method, which is electrochemically pretreated to generate a catalytically active NiFeO_x phase. We have shown that these alloy-derived NiFeOx catalysts achieve a current density of 10 mA/cm² at approximately 280 mV of overpotential at room temperature and at 240 mV of overpotential at 70°C in alkaline aqueous solution. However, the stability of this catalyst in concentrated alkaline electrolytes is ambiguous. Moreover, there is evidence that after spending up to several hundred hours in a colloidal suspension with Nafion ionomer and isopropyl alcohol the catalyst undergoes structural and compositional changes that lead to unpredictable catalyst stability. Ongoing work is focused on utilizing identical location transmission electron microscopy techniques to investigate how the structure and composition of the catalyst changes throughout its lifetime and impacts catalyst activity and stability.