Developing durable platinum group metal (PGM)-free catalysts is critical for enabling cost-effective hydrogen production through anion exchange membrane water electrolyzers (AEMWEs). Here, we present NiMo catalysts encapsulated within defective carbon shells. Whereas conventional NiMo catalysts degrade rapidly under intermittent conditions with voltage changes, the carbon-encapsulated NiMo catalysts exhibit remarkable resistance to degradation with good hydrogen evolution reaction (HER) activity, effectively addressing the challenges associated with renewable energy integration. The carbon shells prevent oxidation-induced deactivation by inhibiting the structural transformation of metallic Ni into hydroxides accompanying volumetric expansion under open circuit voltage conditions. Using a reference electrode-integrated AEMWE, we decoupled overpotential contributions and demonstrated that the stability of HER catalyst is decisive for the durable AEMWE operation under intermittent scenarios. This study establishes a strategy for durable PGM-free catalyst development for sustainable and scalable hydrogen production.
