Hydration of ions on the surface of carbon electrodes and the ion specific electrochemical adsorption

Abstract

Carbon materials such as activated carbons are used as the electrodes of electrochemical capacitors, batteries, and devices for capacitive deionization, because of their high specific surface area, high electronic conductivity, and high chemical stability. Although there are abundant studies on the electrode/electrolyte interface, the state of the hydration of ions and its effects on the physical properties at the carbon-electrode/aqueous-electrolyte interface has not been fully understood. We performed molecular dynamics simulations to analyze the structural and thermodynamic behavior of aqueous electrolytes containing various cation species (Li⁺, Na⁺, K⁺, Cs⁺, Mg²⁺, Ca²⁺, Sr²⁺, or Ba²⁺) near a graphite electrode surface. For elucidating the ion specificity of the properties of those cation species at the molecular level, the influence of the ionic radius, the water structure-maker/breaker tendency, and the Hofmeister series are discussed.