Comparing NiCoFe Hydroxides and Layered Double Hydroxides on the Performance and Stability of Anion Exchange Membrane Water Electrolysis Anode Catalyst Layers

Anion exchange membrane water electrolysis (AEMWE) promises to be a cost-effective technology for producing green hydrogen as a result of reduced reliance on precious metal catalysts to achieve efficient operation. While great strides have been made in the development of Ni and Co (oxy)hydroxide catalysts to replace IrO2 at the anode, engineering of catalysts into effective electrodes has been lacking. Herein, two NiCoFe hydroxide catalysts with different crystal structures and morphologies were synthesized by hydrothermal routes and developed into high performance AEMWE anodes. β-NiCoFe hydroxide synthesized with hydrazine exhibited a large aggregate structure, with crystals >0.5 μm in diameter. α-NiCoFe hydroxide/layered double hydroxide made with ammonium hydroxide instead aggregated into a densely packed powder. Both catalysts demonstrated superior activity toward the oxygen evolution reaction (OER) via thin film measurement (β-NiCoFe hydroxide = 203 mA mg–1 and α-NiCoFe hydroxide/LDH = 341 mA mg–1) compared to IrO2 (51 mA mg–1) at an overpotential of 0.35 V. Although the high activity is translated into a 0.11 V reduction in overpotential at 100 mA cm–2 in half-cell porous transport electrode (PTE) tests, initial cell voltages of 1.77 V for β-NiCoFe hydroxide and 1.73 V for α-NiCoFe hydroxide/LDH at 1 A cm–2 (1 M KOH, 70 °C, Aemion+ membrane) represent modest improvements over IrO2 (1.77 V) when employed in AEMWEs. However, NiCoFe hydroxide catalysts showed far superior AEMWE stability over the IrO2 catalyst layers. Mechanical instability of the β-NiCoFe hydroxide catalyst was evident in half-cell PTE tests necessitating stabilization strategies. Coating of the catalyst layer with Aemion ionomer is shown to mitigate a 0.09 V increase in overpotential in rotating disk electrode tests (10 mA cm–2), but it is shown to be less effective at higher current densities. Further electrode engineering promises effective and durable non-noble metal anode catalyst layers, for which this contribution provides insights.