Cation Co-Adsorption and Island Formation in the Electrochemical Environment

Abstract

The electrode/electrolyte interface is complex, and its composition, structure, and dynamics affect the rate and mechanism of electrocatalytic reactions. The presence of spectator cations near the electrode surface, for example, is well known to affect the rate and selectivity of carbon dioxide electroreduction, where changing from a lithium to a cesium cation can increase the selectivity of ethylene production on copper from 2% to 31% [1]. While many explanations of the mechanism of the effect of these alkali metal cations has been proposed, no one theory has been well accepted. In previous work, we have shown that alkali metal cations can sit close to an electrode surface, in particular at high pH and low potentials; further, once close to (“adsorbed” on) the surface, these spectator cations can then interact with and alter the adsorption energy of reactive intermediates, such as those present during CO2 electroreduction [2], or adsorbed hydroxide (OH*) on platinum (an intermediate in many electrochemical reactions) [3]. In this work, we find a cation which when present in an alkaline electrolyte has a large effect on the binding strength of hydroxide (larger than that of the alkali metal cations) on a Pt(111) electrode and causes a significant hysteresis in hydroxide adsorption/desorption. Using a combination of detailed experiments on a single crystal Pt(111) electrode and density functional theory simulations, we show that this hysteresis results from the co-adsorption of the cation and of hydroxide, with an interaction between the cation and hydroxide which is so strong that it drives island formation and a first order phase transition in the OH* adlayer. This provides direct evidence for cation (co)adsorption. Implications for electrocatalysis and electrolyte design will be discussed.[1] M.R. Singh, Y. Kwon, Y. Lum, J.W. Ager III, A.T. Bell, J. Am. Chem. Soc., 138, 13006-13012 (2016).[2] S.A. Akhade, I.T. McCrum, M.J. Janik, J. Electrochem. Soc., 163, F477 (2016).[3] I.T. McCrum, M.J. Janik, J. Phys. Chem. C, 120, 1, 457-471 (2016).