The first observation from these curves is that the CO2 concentration and pH in the electrolyte at low current densities is relatively similar for both configurations (Fig. ). This is to be expected as the decreased CO2 diffusion pathway does not affect the maximum availability of CO2 in an electrolyte, and OH -diffusion is moderated by the catholyte. In each case a sudden increase in local pH is observed for the weakly-buffered electrolytes (1 M KCl, 0.1 M KHCO3) as the hydroxide generated as a by-product of water-splitting cannot diffuse away fast enough or be immediately buffered by the solution (Fig. and). Shown here, and in recent experimental work by the Koper group, the electrode pH can in fact shift by as much as 6 units within the first 1 -2 mA/cm 2 . Not only could the reaction itself be altered by this shift, but large pH differences between the reference and working electrodes in this current density range complicate determination of the equilibrium potential, and subsequently, product onset potentials and Tafel slopes. Furthermore, for slightly acidic CO2-saturated electrolytes, the solvent itself can also act as the sole proton source (via H3O + ) at current densities <1 mA/cm 2 until becoming depleted and replaced by water reduction (Fig. ). It is then entirely possible for a CO2 reduction catalyst to be effective within the Tafel region but exhibit slower kinetics at higher potentials/current densities if the water reduction step to liberate protons becomes rate-determining. Within this low current density region, where the most important electrochemical characterizations of a material take place, the reaction environment is then extremely sensitive, complicating analysis of intrinsic catalyst behaviour and the reaction mechanisms at play.