(197ab) Modeling Electrode-Electrolyte Interfacial Effects during Specific Alkali Metal Cation Adsorption Using a DFT/FF-MD Approach

Significant efforts have been focused on developing active and selective electrocatalysts to facilitate electrochemical transformations in batteries, electrolysis cells, fuel cells, and carbon capture technologies. It has been known the altering the identity of the ions in the electrolyte has significant effects on the rates and selective on electrocatalytic reactions such as CO₂ reduction, HER, OER, N₂ reduction, and water oxidation. Recently, Waegele et al. was able to demonstrate that specifically adsorbed alkali metal cations on Au electrodes not only alter the rate of CO₂ to CO but is sensitive to the identity of the cation and ability to shed its hydration shell.¹ Modeling specifically adsorbed cations and its interactions within the electrochemical double layer (EDL) is complex, requiring careful modeling choices on how solvation and electrification within this system to be made. Our prior work for NH to NH₂ reduction on Rh (111) showed that predicted barriers for this system can vary by 1 eV based on the model parameters used to describe the EDL. In this work, we used a DFT/FF-MD approach to investigate the thermodynamic equilibrium of alkali metal cation adsorption across late transition metal surfaces while identifying the sensitivity made from the choices within our model. We used DFT methods to study the interactions between the cation and electrode and nearby explicit static H₂O, representing micro-solvation from the first hydration shell of the adsorbed cation. We utilized a “post-DFT” Helmholtz model to correct the DFT simulation to constant potential and incorporate electrification using two discernable parameters, the dielectric constant and width of the EDL. Force-Field Molecular Dynamics (FF-MD) are incorporated to estimate the dynamic effects of solvation upon cation adsorption, which also informs our Helmholtz model suitable dielectric constants and EDL widths to use.

Thermodynamic equilibrium of specific cation adsorption across different late transition metal surfaces are studied by predicting the equilibrium potential (U⁰), the applied potential of the system where cation adsorption is at equilibrium. We observed that predicted U⁰’s for alkali metal cations can vary over 1 V-NHE based on the model used to predicted solvation (explicit static H₂O, continuum solvation, and FF-MD). For incorporating the interactions of electrification, U⁰’s are sensitive to the choice of the width and dielectric constant of the EDL, showing variations in U⁰ of 3 V-NHE span for dielectric constants from 2 to 8. Magnitude of the EDL effects can be predicted from the magnitude of the dipole moment change upon adsorption. Lastly, we demonstrate that our DFT/FF-MD approach can rationally tune the model parameter to agree with potential ranges observed on Au (111) from Waegele et al.

References:

1. Ovalle et al. Nature Catalysis. Vol 5. July 2022. 624-632.