Phase‐Stable Palladium Hydride Derived from PdCoO ₂ for Sustainable Hydrogen Evolution Reaction

Abstract Active and reliable electrocatalysts are fundamental to renewable energy technologies. PdCoO 2 is recently recognized as a promising catalyst template for the hydrogen evolution reaction (HER) in acidic media, thanks to the formation of exceptionally active PdH x . In this article, the transformation process of single PdCoO 2 particles during HER is monitored and elucidated, confirming their almost complete transformation to PdH x . Using operando mass spectrometry, Co dissolution from the PdCoO 2 template is observed under reductive potentials, with a partial current of 0.1% of the HER current, while PdH x is formed simultaneously. High HER activity of this phase is retained with long‐term operation, dry storage, or vacuum exposure. Isotope labeling of hydrogen using D 2 O confirms the formation of a stable PdH x phase by secondary ion mass spectrometry and down to the near‐atomic scale by atom probe tomography. A separation between D‐poor α‐ and D‐rich β‐Pd hydrides is observed with an overall composition of β‐PdD 0.64 . These findings highlight the critical role of a templated growth method for obtaining stabilized PdH x , enabling Pt‐like efficient HER without the commonly slow activation processes observed in Pd due to rate‐limiting material hydrogenation. This offer insights into the design of more efficient electrocatalysts for renewable energy technologies.