Effect of Interfacial Electric Fields on Elementary Electrochemical Kinetics on Transition Metal Surfaces

We use density functional theory (DFT) methods to investigate the impact of interfacial electric fields on elementary electrocatalytic reaction steps. Electric fields over 0.1 V/Å alter the energies of molecular orbitals of adsorbates, they can also change the activation barriers for reactions, making electric fields relevant to the field of catalysis¹. In an electrochemical cell, a potential drop of 1.2 V across the interfacial layer of the order of 3-8 Angstrom can generate an electric field of order 0.5 V/Å. An external field is applied to the metal-vacuum interface in DFT models to investigate the relative sensitivity of different elementary bond cleavages (C-H, N-H, and O-H) relevant to a broad range of electrochemical transformations. Differences in susceptibility to electric field effects can allow interfacial fields to be used to promote/hinder specific bond formation or cleavages, altering the selectivity during electrocatalytic reactions. We invoke electric field effects as a correction to the Bulter-Volmer formalism for elementary electrocatalytic reactions. Our preliminary results show that the shuttling agent such as H₂O both facilitates Heyrovsky-like O-H cleavage steps and increases the field sensitivity of the reaction relative to Tafel-like O-H cleavage. This can be explained by the contrast of field-dipole interactions between a significantly charge-separated Heyrovsky transition state (TS) and a relatively non-polar Tafel TS. To predict this contrast of polarity, we are also developing a computational tool based on the Born Effective Charge (BEC) matrix of a reactant state which can allows us to estimate the variation in dipole-field interaction along a reactive mode. Interestingly, the trends of such contrast seems to depend on the nature of metal surface and the polarity of bond cleavage. We extend these studies to guide design principles for electrocatalytic reactions affected by interfacial electric fields.

References:

[1] Che, Fanglin, et al, ACS Catalysis 8.6 (2018): 5153-5174.