(Energy Technology Division Graduate Student Award sponsored by BioLogic) Design Principles for High-performance and Durable Anion Exchange Membrane water Electrolyzers

Abstract

Water electrolysis powered by renewable sources such as wind or solar produces clean hydrogen gas, which is used for many industrial processes and will be essential in a future clean energy economy. Alkaline water electrolysis (AWE) and proton exchange membrane (PEM) electrolysis are mature technologies at megawatt to gigawatt scale. AWE uses earth abundant electrode and catalyst materials submerged in caustic liquid electrolyte (usually KOH or NaOH) separated by a porous diaphragm. This configuration can suffer from gas crossover and shunt current limitations. PEM electrolyzers circumvent these barriers by using an ionically-conductive solid electrolyte membrane that reduces gas crossover and allows for pure water operation that eliminates shunt currents. However, these membranes create a locally acidic environment that necessitates the use of expensive platinum-group-metal (PGM) catalyst materials and hardware coatings. Anion exchange membrane (AEM) electrolysis is an emerging technology that has the potential to combine the benefits of liquid alkaline and PEM. AEM electrolyzers use an anion-selective membrane, maintaining the pure water operation of PEM but creating a locally-alkaline environment allowing for the use of non-PGM materials. However, AEM electrolysis is still immature. In particular, the hydroxide-conducting membrane and ionomer have not yet achieved sufficient stability in pure water to compete with PEM systems. The specific dominant degradation mechanism and location has also not been conclusively identified. Further, while non-PGM catalysts such as Ni- and Fe-based oxyhydroxide catalysts out-compete PGM catalysts in three-electrode KOH studies, this performance does not carry to the pure water electrolyzer configuration. In my talk I report AEM electrolyzer performance below 2 V at 1 A·cm-2 in pure water for both PGM and non-PGM anode catalysts. I will also discuss the specific degradation mechanism of the ionomer and membrane during electrolyzer operation with both catalysts. Understanding the differences in performance and durability with precious metal and earth abundant materials is key for AEMs to compete with current electrolyzer technologies. I compare the performance and stability of five Ni-, Co-, and Fe-based catalysts and compare to IrOx. XPS and cross-sectional SEM data show oxidation of the ionomer binder at the anode for all catalysts, which results in significant degradation and performance loss. I also show additional degradation is induced by introducing soluble Fe species, indicating separate mechanisms beyond ionomer degradation are present in the non-PGM systems. In addition to the fundamental insights into the dynamic behavior of non-PGM catalysts and oxidative durability of ionomeric polymer materials, these findings contribute to bringing low-cost AEM electrolyzer technology to scale. Figure 1