(123c) Influence of an External Potential on the Solvation Thermodynamics of Intermediates in the Pathway for Methanol Oxidation on Pt(111)

Fuel cells based on methanol oxidation are sustainable alternatives to burning fossil fuels; however, electrocatalysts for these reactions utilize expensive noble metals which inhibits their broader adoption. A first step in designing less expensive electrocatalysts is to understand how current materials function. This has proven challenging, however, due to complexities involved with the liquid reaction conditions and how they are influenced by an external field. Understanding the role of liquid water under an external potential is hence critical to electrocatalyst design. Herein, we use a combination of classical molecular dynamics and density functional theory to illustrate how explicit liquid water under an applied potential influences the solvation thermodynamics of intermediates in the pathway for methanol oxidation. Using this multiscale approach, we are able to compute enthalpies and entropies of solvation under an applied field. Entropies of solvation depend on the structure of liquid water at the electrocatalytic interface. In the absence of an external field, the structure of interfacial water is dictated by the catalytic species, with species comprising hydroxyl functional groups forming hydrogen bonds with H₂O molecules. In these hydrogen bonding configurations, the hydrogens on the H₂O molecules point toward the surface. Under an external field, the structure of water is defined by the field itself. Specifically, at negative potentials, hydrogen atoms point toward the surface, while at positive potentials, they point away from the surface. Hence at negative potentials, adsorbates and the field reinforce each other, whereas at positive potentials, they interfere with each other. The result is that entropies of solvation are negative at positive potentials and positive at negative potentials. In contrast, enthalpies of solvation are largely independent of potential. The free energies of solvation hence depend on potential and can be tuned through application of a field. Implications for electrocatalytic mechanisms will be discussed.