Dynamic changes in hydrogen evolution catalysis impose an upper bound on electrochemical hydrogen storage in Pd

Efficient storage of renewably produced hydrogen remains a serious bottleneck in the global transition to a hydrogen-based energy system. Production of metal “super- hydrides” with a greater than 1:1 metal to hydrogen stoichiometry, produced by an electrochemical driving force in aqueous electrolyte, is experiencing emerging interest as a possible solution to this critical challenge. In this work, using palladium and its hydride (PdHx) as a model system, we combine state-of-the-art grand canonical density functional theory (DFT) and operando X-ray diffraction to investigate the feasibility and limitations of electrochemical hydrogen storage via formation of metal hydrides. We develop a unique computational model of PdHx with high granularity, particularly in the high loading (0.6 < x ≤1.0) regime. After benchmarking calculated lattice strain to operando X-ray diffraction measurements, we reveal that there is a signifi- cant energetic penalty to filling all six octahedral sites surrounding Pd, and that the well-known α−/β−PdHx coexistence is a macroscopic phenomenon. Using our com- putational model of PdHx, we calculate the bias-, coverage-, and loading-dependent activation barriers and reaction energies for the Volmer and Tafel reactions. Our find- ings reveal that dynamic changes in the activation barriers and reaction energies of hydrogen evolution reaction (HER) sub-reactions impose an upper bound on electro- chemical hydrogen storage, well below theoretically predicted loadings via formation of superhydrides. This work establishes a formalism for theoretical investigations of the electrochemical formation of metal hydrides, and provides fundamental insights into the critical importance of the surface in electrochemical loading of metal hydrides.