Although interfaces between metals and water are the local areas where crucial processes of energy technologies such as water splitting occur, comparably little is known about their structure and changes during such processes. For more than 100 years, the scientific description of such interfaces has been based on the model of the so-called electrochemical double layer. It states that charge carriers in an aqueous solution are increasingly arranged in the boundary region to the metal, to compensate for excess electrical charges on the metal side. In the process, the opposing charges are separated by water molecules. Similar to a standard plate capacitor, this nanoscopic charge separation in the interface allows energy to be stored and released later. Processes in which the molecular structure of the electrochemical double layer changes are relevant to many green technologies, such as supercapacitors and fuel cells.