(515f) Unriddling the Cationic and pH Effect in Alkaline Hydrogen Evolution Reaction on Pt

The alkaline water electrolysis is plagued by sluggish kinetics in cathodic hydrogen evolution reaction (HER) even on the platinum catalysts. The electrode/electrolyte interface in alkaline HER is also far more complex and controversial than the acidic case, due to presence of near-surface alkali metal (AM) cations, surface adsorbates, and different interfacial water structures that they lead to.

Herein, we conduct a density functional theory (DFT) study with explicit solvation, including static calculations, grand canonical DFT calculations, ab initio molecular dynamic (AIMD) simulation, and micro-solvation cluster calculations, to gain molecular level insights into the surface adsorption properties, solvation structure, and the dynamics at the Pt/water interface in presence of cations and surface OHad species, or at different pH values. Electrochemical impedance spectroscopy (EIS) and electrical transport spectroscopy (ETS) are also performed by experimental collaborators to probe the surface species and interfacial capacitance. Our integrated studies suggest that the AM cations play an indirect role in modifying HER kinetics: Smaller cations destabilize surface OHad to a lesser extent in the potential window of HER, leading to a higher OHad coverage on Pt surface. The highly polar surface OHad weakens the O-H in nearby water molecules, which can promote alkaline Volmer kinetics and hence boost the HER activity.

In addition, we combine cyclic voltammetry (CV) and electrical transport spectroscopy (ETS) approach to probe Pt surface at different pH and develop molecular level insights on the pH-dependent HER kinetics in alkaline media. The change in HER Tafel slope from ~110 mV/decade in pH 7–10 to ~53 mV/decade in pH 11–13 suggests considerably enhanced kinetics at higher pH. The ETS studies reveal a similar pH-dependent switch in the ETS conductance signal at around pH 10, suggesting a notable change of surface adsorbates. Fixed-potential calculations and chemical bonding analysis suggests this switch is attributed to a change in interfacial water orientation, shifting from primarily O-down below pH 10 to H-down configuration above pH 10. This reorientation weakens the O–H bond in the interfacial water molecules and modifies the reaction pathway, leading to considerably accelerated HER kinetics at higher pH. Our integrated studies provide unprecedented molecular level understanding of the non-trivial pH dependent HER kinetics in alkaline media.

The collection of works demonstrate the necessity and benefits of including explicit electrolyte and/or constant potential treatment in molecular dynamics simulations to properly understand the interfacial chemistry of even the simplest electrochemical reaction.