Unveiling the ampere-level current density induced local oxidation of the Ni-based cathode in anion exchange membrane water electrolyzers

Abstract Anion exchange membrane water electrolysins (AEMWE) demonstrates compatibility with non-precious metal electrodes, offering promising potential for cost-effective and efficient green hydrogen production. However, the high-performance Ni-based cathodic hydrogen evolution reaction (HER) catalyst suffers from stability issues at high current density. Here, we unveil the unexpected cathodic oxidation of Ni at the continuous ampere-level current density HER working condition. The Ni-based HER electrode demonstrates high stability at low current density, but unexpectedly oxidizes at lower potential when the HER current density increased. Mechanism studies reveal that the local pH increases when catalyzing HER, and the higher the current density, the higher the local pH. The increased pH promotes the oxidation of Ni due to the formation of Ni(OH)₄²⁻. It is further found that introducing Co, which forms hydroxylated Co clusters on the surface electrode and increases the negative charge of the electrode, can repel OH⁻ in the double layer and thus lower the local pH and stabilize Ni sites by preventing their oxidation. The synthesized Co-NiMo/MoO₂ electrode demonstrates simultaneously excellent activity and stability. The precious metal-free AEMWE using Co-NiMo/MoO₂ as cathode maintains an ultra-low cell voltage of 1.706 V at 1 A cm⁻ ² after 3000 h of continuous electrolysis.