Abstract: Electrochemical reactions take place at the electrochemical interface, the region in which the properties change from those in the bulk of the electronic conductor (electrode) to those in the bulk of the ionic conductor (electrolyte). Not surprisingly, then, the efficiency of energy conversion and energy storage processes is almost entirely determined by the richness of the interfacial processes that control the rate of electron transfer and various types of interactions between the substrate and reactants, intermediates, and products of the reaction.

The ​“interfacial bridge” between the bulk of the electrode and the electrolyte manifests itself as

1. Differences in atomic arrangements close to or at the electrode surface
2. Differences in electrode composition close to the surface
3. Adsorption of species from electrolyte onto the electrode
4. Ordering of solvent and/or electrolyte molecules observed in the proximity of the surface
5. Changes in electrolyte composition in the proximity of the electrode
6. Formation of solid film between the electrode and electrolyte

All of these processes are rather complex, requiring a special arsenal of tools that are capable of resolving interfacial properties at the atomic and molecular levels. Moreover, it makes the electrochemical interface extremely hard to control.

In this colloquium, a general outline of our current understanding of aqueous and nonaqueous electrochemical interfaces in energy conversion and storage systems will be given. Our recent advances in understanding the interfaces in water electrolyzers and lithium-ion and lead-acid batteries will be shown as specific examples of how entirely different systems on the macroscale follow very similar fundamental principles on the atomic scale.