Computational unravelling of cathodic hydride formation on palladium surfaces

Palladium (Pd) is well-known for its role in catalyzing hydrogen-based reduction reactions, owing to its excellent catalytic activity and hydrogen storage ability. Its surface and subsurface structures under electrochemical conditions are vital in understanding the hydrogen evolution reaction (HER) mechanism at the Pd cathodes where the most active sites are located on 'in situ' formed Pd-hydride layers. In this work, we investigate the process of Pd-hydride formation as well as the step-by-step formation and stability of Pd-hydride/Pd interfaces under electrochemical conditions using first-principles calculations. Our results reveal that among the lowindexed surfaces (111), (110) and (100), the (111) surface is expected to be the most dominant surface in a Pd nanostructure in addition to being the most preferred surface for hydrogen adsorption. Based on calculated Pourbaix diagrams, we can identify the relevant regions close to the equilibrium electrode potential and pH for proton electroreduction and hydrogen evolution, where the Pd surfaces start to be covered by hydrogen adatoms, and when the electrode potential is decreased, there are clear thermodynamic indications for more and more subsurface hydride layers. Overall, the results provide insights into the stability and formation of hydrogencontaining Pd surfaces, forming PdH/Pd type interfaces. Our idealized model systems are a first step towards elucidation of relevant active sites on Pd.