A heterogeneous electrocatalyst is one that is present in a different phase of matter from the reactants, for example, a solid surface catalyzing a reaction in solution.  Different types of heterogeneous electrocatalyst materials are shown above in green. Since heterogeneous electrocatalytic reactions need an electron transfer between the solid catalyst (typically a metal) and the electrolyte, which can be a liquid solution but also a polymer or a ceramic capable of ionic conduction, the reaction kinetics depend on both the catalyst and the electrolyte as well as on the interface between them. The nature of the electrocatalyst surface determines some properties of the reaction including rate and selectivity.