A total current density ( j tot ) and CO Faradaic efficiency (FE CO , defined as the ratio of electrons consumed for the production of CO vs. H 2 ) is assumed for the simulations and data is generated for a range of j tot and FE CO values. The flux of the solution species at the surface is then given by equations (3), (4) and (5). Our simulations are in 1D and x=0 is considered to be at the OHP (the boundary between the Zone 1 and Zone 2 in Figure ).