The development of sustainable technologies for in-situ resource utilisation (ISRU) on Mars is critical for future human missions. This study investigates the electrochemical reduction of carbon dioxide (CO2) to carbon monoxide (CO) in laboratory conditions, focusing on the performance of silver-based gas diffusion electrodes (Ag GDE) in various catholytes. We evaluate key metrics such as pH and different salt concentrations on the catholytes to evaluate which current density obtains the highest faradaic efficiency (FE%) towards CO; this means the optimum condition at which CO is preferred over the hydrogen evolution reaction. This is the first step towards developing an ISRU process for Mars utilisation since CO2 is the primary component of the Mars atmosphere, and converting it to CO can benefit other chemical processes that generate fuel. The results indicate that selected sulphate catholytes, especially potassium, significantly increase CO yield while minimising hydrogen formation at specific lower current densities. Moreover, a more acidic media results in more hydrogen generation instead of CO, while a lower but still acidic media (such as pH 3) promotes the reduction of CO2 into CO using an Ag GDE. This research encompasses the use of salts as electrolytes and acidic media to enhance the reduction of gaseous CO2 towards CO by using a silver GDE in a microfluidic cell electrolyser.