Concepts and selected experiments on the structure of the Helmholtz double layer at the metal- and semiconductor - electrolyte phase boundary are reviewed. The widely used microcapacitor approach of the double layer and its limitations are assessed. Observations on the influence of the electrode potential on the energetic position of surface states at the Ag-electrolyte contact are compared to the predictions of classical charge transfer models that are based on transition state theory where adiabatic tunneling is assumed. Distance tunneling spectroscopy on Au(111) surfaces shows pronounced variations in tunneling barrier heights that are connected to the inner structure of the Helmholtz layer and implications on electrode kinetics are presented. At the semiconductor-electrolyte contact, the influence of the electrode potential on a charge injecting species that results in photocurrent doubling is reviewed for low- and higher doped Si(111) electrodes, showing that the complex that injects electrons into the conduction band is located outside the semiconductor surface. The observations are correlated with the search for low overpotential earth abundant electrocatalysts for solar fuel generation of solar fuels
