concentration in experiments and in simulations. As shown in Figure , our simulations show that the CO2 concentration in the diffusion layer, which corresponds to the "surface" in experiments, follows the order of Li + > Na + > K + . The concentration in this diffusion region is determined by mass transport effects; the above concentration sequence is a direct consequence of the fact that the current density of the CO2RR follows the sequence Li + < Na + < K + . However, electrostatic interactions and the cation overcrowding effect dominate within the EDL, leading to the inverse order of CO2 concentration.