Session: General Catalysis:

Excess energy from renewable energy conversion technologies can be used to produce hydrogen through water electrolysis. This hydrogen functions as an energy storage medium, as it can be used later in a fuel cell to supply energy when renewable energy production cannot meet demand. Alkaline anion exchange membrane (AAEM) water electrolyzers have high potential to offer inexpensive water electrolysis; however, the sluggish kinetics of the anodic half reaction (i.e., the oxygen evolution reaction, OER), significantly hinders the efficiency of these devices. Specifically, large overpotentials of 0.30 V or greater are needed to overcome the slow kinetics of the OER, which diminishes efficiency to no greater than 80% of the theoretical maximum. These higher energy requirements translate to financial costs that limit the practicality of AAEM electrolyzers.

Remedying this issue will require the design of highly active, stable, and inexpensive electrocatalysts; however, state-of-the-art catalysts for the OER in the current generation of AAEM electrolyzers are IrO_x and RuO_x, which are prohibitively costly to deploy. Oxidized nickel-iron based catalysts (NiFeO_x) are promising cost-effective alternatives that have shown even higher OER activity than IrO_x in alkaline conditions. However, a key outstanding issue is that the long-term stability of these materials has not yet been adequately demonstrated.

We have synthesized a carbon-supported NiFe alloy using a facile wet-impregnation method. The alloy catalyst can then be pretreated to generate a catalytically active NiFeO_x phase. We have shown that these alloy-based NiFeO_x catalysts achieve a current density of 10 mA/cm² at approximately 0.28 V overpotential at room temperature and at 0.24 V overpotential at 70°C in alkaline aqueous solution. However, we also found evidence that the catalyst displays unpredictable stability after spending up to several hundred hours in a colloidal suspension (i.e., catalyst inks) comprising carbon-supported NiFeO_x, Nafion ionomer, and isopropyl alcohol. We have further focused on using identical location transition electron microscopy techniques in tandem with controlled electrochemical treatments to investigate the structure and composition of the catalyst throughout its lifetime.

James McKone

Assistant Professor, University of Pittsburgh

Appearances