UC Berkeley

Commercial membrane electrolyzers rely on acidic fluorocarbon membranes and ionomers, requiring the use of expensive IrOx-based oxygen-evolution catalysts. Anion-exchange-membrane water electrolyzers (AEMWEs) operate in an alkaline environment, enabling the use of non-precious-metal catalysts. Here, we study and engineer CoOx-based catalyst-coated anodes deposited via hydrothermal synthesis directly onto porous transport layers both with and without thermal annealing. Self-supported, nanoneedle-structured Co3O4 anode, formed by annealing the as-synthesized cobalt carbonate hydroxide, Co(CO3)x(OH)y, outperforms the baseline Co3O4 nanoparticle ink-based anode in pure-water-fed AEMWE, due to improved catalyst layer continuity and thus electroactive surface area. The as-synthesized and unannealed Co(CO3)x(OH)y), however, appears to undergo substantial conversion to a more-active CoOx(OH)y phase predominantly at the surface, with nominally Co3+ present and higher electrical conductivity, lowering the cell voltage ~200 mV at 1.0 A∙cm-2 in pure-water-fed AEMWE compared to the conventional Co3O4 nanoparticle anodes. We analyze the differences in electrode electrochemical response between pure-water and KOH feed modes finding distinct activation and degradation modes. The Co(CO3)x(OH)y anode shows significant activation and slower degradation linked to the conversion to oxyhydroxide. We propose catalyst layer designs that promote both hydroxide and electron transport, alongside interfacial engineering strategies to obtain high performance while mitigating anode degradation.