In this reaction the conversion of 𝐻 2 𝐶𝑂 3 to 𝐻𝐶𝑂 3 -is skipped since this reaction occurs quasiinstantaneous such that the 𝐻𝐶𝑂 3 -concentration is divided by the equilibrium constant of this conversion 𝐾 2 = 2 ⋅ 10 -4 M to account for this. The forward reaction rate is 6.6 M -1 s - where the concentration of water is assumed to be 55.6 M and the backward rate is 24.8 s -1 where [H + ] 21 is assumed to correspond to a pH of 6 to match the experiment. The system of equations is closed upon imposing blocking boundary conditions on all solid walls, 𝒏 ⋅ 𝒋 𝑖 = 0 with 𝒏 the (inward) normal on the walls of the device, Ψ = ±𝑉 at the positive and negative electrode respectively. We numerically solve the full (GM)PNP equations within the geometry described below using the FE analysis package COMSOL (the mesh used is shown in SI Figure ). The initial 𝑉 = 0 at the electrodes and we set 𝜌 𝑖 to the equilibrium concentrations for all species.