The Manifestation of the Electrochemical Potential Gradient in the Kinetic Landscapes of Interfacial Electrochemistry

The electrochemical potential gradient constitutes a free energy driving force. Understanding how this gradient manifests itself and drives electronic and ionic charges through the bulk and across interfaces is important across the natural sciences, including in solar cells, batteries, heterogeneous and biocatalysis, and proton pumps and motors. Despite this, literature often misrepresents the electrochemical potential gradient. In the extreme, it is simply replaced by an electric field as an apparent driving force. Here, we use energy band diagrams to review the basics of electrochemical charge separation and to inform on the nonequilibrium boundary conditions in interfacial electrochemistry. We explain that the thermodynamic driving force along the reaction coordinate needs to be distinguished from chemical potential changes in the extended environment that can modulate activation barriers, without necessarily being a driving force themselves. To this end, we focus on the role of overpotential-dependent excess charge and electric fields, before discussing overpotential-dependent coverages and multistep reactions. We close by explaining how these local chemical potential changes give rise to a multidimensional kinetic landscape that cannot be compressed into single-descriptor kinetics.