where under a positive overpotential (fuel cell mode) gas is oxidised by the fuel electrode and reduced by the air electrode (full derivation in SI). The formation of the electrostatic surface potential can be described as the difference in electrostatic potential of free electrons in the electrode (ϕ e ) and the adsorbate (ϕ ad ): χ = ϕ e -ϕ ad (8) Under bias, an electrostatic potential shift is defined at the surface as: