Frozen or dynamic? - An atomistic simulation perspective on the timescales of electrochemical reactions

Electrochemical systems span a wide range of timescales, and several recent works have put forth the idea that the reaction environment should remain frozen and out of equilibrium during electrochemical electron or proton transfer reactions. Furthermore, simplified treatments of the electrochemical interface model the solvent and ions as frozen molecules. However, the claims and practices of a frozen environment strongly clash with most theoretical and simulation approaches developed to study electrochemical reaction rates. It has also been suggested that the electrode potential should not be fixed when simulating reaction rates due to conductivity limitations, which indicates constant potential simulations to be incorrect. In this critical review, these claims re-analyzed from the perspective of non-ergodic rate theory, which provides a rigorous framework to determine when or whether the reaction environment should appear frozen: it is shown that in most activated electrochemical reactions in aqueous media, the reaction environment is completely mobile and equilibrated under constant potential conditions. Only for ultrafast reactions or transient methods should the environment be considered (partly) frozen and in a non-equilibrium state. For both metallic and practical semiconductor electrodes the impact of electrode conductivity is minimal and constant potential calculations are found reliable. The implications of these considerations are be discussed from a viewpoint of computational and theoretical electrochemistry.