Do Rh-Hydride Phases Contribute to the Catalytic Activity of Rh Catalysts under Reductive Conditions?

Rh-hydride phases were believed to
be key causes of the
exceptional
catalytic ability of Rh catalysts under H2 reductive conditions.
Here, we utilize the large-scale machine-learning-based global optimization
to explore millions of Rh bulk, surface, and nanoparticle structures
in contact with H2, which rules out the presence of subsurface/interstitial
H in Rh and Rh-hydride phases as thermodynamically stable phases under
ambient conditions. Instead, an exceptional Rh–H affinity is
identified for surface Rh atoms in Rh nanoparticles that can accommodate
a high concentration of adsorbed H, with the surface Rh to H ratio
reaching ∼2.5, featuring stable six-H-coordinated Rh, [RhH6]. Such [RhH6] species forming at edged Rh sites
are found to be the key intermediates in the electrochemical hydrogen
evolution reaction (HER) on Rh. Guided by theory, our synthesized
Rh concave nanocubes with a high density of edged Rh sites achieve
a Tafel slope of 28.4 mV dec–1 and a low overpotential
of 36.1 mV at jECSA = 1 mA cm–2, which outperforms commercial Pt/C and other morphologies of Rh
catalysts. Our results clarify the active phase in Rh–H nanosystems
and guide the catalyst design by precise morphology control of nanocatalysts