Distribution of alkali cations near the Cu (111) surface in aqueous solution

Abstract

It has been widely realized, in aqueous solution, that metal cations near a catalytic surface in the Helmholtz layer can have a significant influence on catalytic reduction processes, from CO₂ reduction to H₂ generation. However, the exact nature of the cation distribution, the free energy profile, and local coordination with water molecules, as well as the electrode potential dependence are still not known. For example, it is known that water molecules can form some layer structures above the surface. Are the cations in some particular positions in the water layer? What is the free energy profile of a cation as a function of its position? What is the cation density at the surface for a given bulk concentration? How do their energy profiles depend on the electrode potential? What is the trend for different cations due to their size and chemistry difference? Answering these questions is essential to understand the role of cations in aqueous based catalytic processes. In this work, using ab initio molecular dynamics (AIMD) simulations with explicit water, we provide a systematic study of the above questions for the series of alkali cations (Li⁺, Na⁺, K⁺, Rb⁺ and Cs⁺) on both neutral and charged Cu (111) electrodes (corresponding to electrode potentials from −2.16 V to 1.56 V vs. SHE). The results indicate that the alkali cations will remain near the electrode–electrolyte interface over a wide potential range (from −2.16 V to 1.37 V). The free energy profile obtained from thermodynamics integration shows that Na⁺ likes to remain near the interface, and it prefers the odd layer of the water structure. A simple model is provided to explain the underlying reason for such behavior. We also find that the surface concentration can be very high for a moderate bulk cation concentration, indicating a possible strong cation role in the catalytic process.