For the AEM case, a 50 µm diffusion layer including M + , CO 2 , HCO - 3 , CO and to be transported by electroosmosis ; while the TMA + is present at a fixed concentration throughout the domain (see Supporting Information section 1.3 for further explanation). The bulk concentrations of the species in the ionomer depend on the hydration level of the membrane and were calculated taking into account the Donnan potential (which results from having a fixed background charge in the domain) . The relative permittivity of the domain is assumed to be ε r = 42 following available literature , but data in the literature are scarce and further experimental studies are needed.