In each of the described configurations CO2 is supplied to a catalyst layer that is fully or partially immersed in a conductive electrolyte. This CO2 can diffuse through the hydrodynamic boundary layer of a saturated bulk electrolyte as in a standard H-cell configuration (Fig. and), or from a nearby gas-phase with a much shorter diffusion pathway (Fig. and). Configurations where CO2 is provided from the gas-phase use a gas-diffusion layer to form a gas-liquid interface adjacent to the catalyst layer. Electrochemical CO2 reactors using gas-diffusion layers have historically been used in diverse configurations, but range primarily from a Kenis-type reactor with a flowing catholyte (Fig. ), to membrane electrode assemblies which directly combine a gas-diffusion layer, catalyst and ion exchange membrane into one unit (Fig. ). These latter configurations are occasionally denoted as 'catholyte-free' or gas-phase electrolysis due to the lack of a flowing electrolyte between the catalyst layer and ion-exchange membrane. Liquid water, however, is reported to be present in the porous catalyst layer in the majority of cases.