Lattice-Hydride Mechanism in Electrocatalytic CO₂ Reduction by Structurally Precise Copper-Hydride Nanoclusters

Copper electrocatalysts can reduce CO₂ to hydrocarbons at high overpotentials. However, a mechanistic understanding of CO₂ reduction on nanostructured Cu catalysts has been lacking. Herein we show that the structurally precise ligand-protected Cu-hydride nanoclusters, such as Cu₃₂H₂₀L₁₂ (L is a dithiophosphate ligand), offer unique selectivity for electrocatalytic CO₂ reduction at low overpotentials. Our density functional theory (DFT) calculations predict that the presence of the negatively charged hydrides in the copper cluster plays a critical role in determining the selectivity of the reduction product, yielding HCOOH over CO with a lower overpotential. The HCOOH formation proceeds via the lattice-hydride mechanism: first, surface hydrides reduce CO₂ to HCOOH product, and then the hydride vacancies are readily regenerated by the electrochemical proton reduction. DFT calculations further predict that hydrogen evolution is less competitive than HCOOH formation at the low overpotential. Confirming the predictions, electrochemical tests of CO₂ reduction on the Cu₃₂H₂₀L₁₂ cluster demonstrate that HCOOH is indeed the main product at low overpotential, while H₂ production dominates at higher overpotential. The unique selectivity afforded by the lattice-hydride mechanism opens the door for further fundamental and applied studies of electrocatalytic CO₂ reduction by copper-hydride nanoclusters and other metal nanoclusters that contain hydrides.