We report here the electrocatalytic behaviour of an unconventional gas-phase reactor for the process of CO2 conversion. Conventional systems for the CO2 electrocatalytic reduction refer to electrodes immersed in a liquid electrolyte, presenting many issues mainly related to the low solubility of CO2 in water. In gas-phase (or electrolyte-less conditions) the working electrode is engineered to be in direct contact with an ion-exchange membrane (forming a zero-gap system) and the CO2 flows directly through the catalyst with no electrolyte. The influence of the reactor design (gas- or liquid-phase) is discussed by processing the same kind of electrode based on copper oxide (CuO) deposited on a gas diffusion layer to form a gas-diffusion electrode (GDE). Results, in terms of >C1 productivity and supported by electrochemical characterizations (such as Electrochemical Impedance Spectroscopy -EIS), showed remarkable difference between the two systems and clarified the role of the proton-diffusion process at the catalyst interface. The reasons can be summarized as follows: i) increase of the local CO2 concentration on the electrode surface, overcoming CO2 solubility limitations in water-based solvents; ii) control/limitation of the proton concentration on the catalyst surface due to the absence of aqueous electrolyte. The process selectivity is strongly influenced by charge transport properties on the catalytic surface beyond the properties of the electrocatalyst itself. As a result, engineering of the reactor assumes a role no less important than the role of the electrocatalyst.