Our simulations assume an overall Nernst diffusion length of 50 µm. Dirichlet boundary conditions are used for the potential at the OHP (left-hand boundary) as well as for the concentration of species and potential in the bulk (right-hand boundary). Neumann boundary conditions are used for the flux of species at x=0 as per equations ( ), ( ) and ( ). The initial conditions for the concentrations are assumed to be at bulk values with φ = 0 everywhere in the electrolyte. The scaled form of the GMPNP equations used for the numerical simulations can be found in the SI.