Abstract: Among the different strategies for CO2 emissions reduction and climate change mitigation that are being considered, valorisation of CO2 appears as a more interesting option than carbon storage, since it allows the recycling of CO2 into added-value products. The purpose of this work is to study the influence of key variables on the performance of an experimental system for continuous electro-reduction of CO2 to formate in aqueous solutions under ambient conditions, using a filter-press electrochemical reactor with a lead plate as cathode. A 22 factorial design of experiments at different levels of current density ("j") and electrolyte flow rate/electrode area ratio ("Q/A ratio") was followed by additional tests at the mean level of each variable. The obtained results confirmed general suggested trends in previous works, but they also offered a detailed analysis of the influence of these variables. Regarding j, it should be emphasised that a significant increase of the rate of formate production was observed at the expense of lowering the Faradaic efficiency when increasing j up to a limit value of 10.5 mA cm-2, where a Faradaic efficiency of 57% was maintained. The influence of Q/A turned out to be more limited than that of j, since the results revealed that increasing the catholyte flow to overcome mass transport limitations only had beneficial effects for the lowest Q/A ratios (i.e. <=0.76 mL min-1 cm-2). This study is a reference for the evaluation of future improvements in the development of these continuous electro-reduction processes for CO2 valorisation, derived from further research to overcome current limitations.

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