Electrochemical CO₂ conversion Fundamentals and practical aspects

Mitigating industrial CO₂ emissions is one of the main challenges we face today. The electrochemical reduction of CO₂ is one possible renewable strategy. But, the economic viability is rather low. Therefore, the reduction can be coupled with the electro-oxidation of an organic polyol in a paired electrolysis system to increase the economics. The most common anodic reaction in electrolysers is water oxidation. For scalability reasons, this work investigated how the use of low-grade, or rather, materials containing first-row transition metal impurities affects the electrocatalytic performance for the oxygen evolution reaction (OER). It was found that the performance can both be positively and negatively influenced by the impurities, depending on what metal was present. Replacing the OER with a selective electro-oxidation of ethylene glycol increases the economic viability of a possible paired system. Here various nickel supported palladium-silver electrocatalysts were used to study the selective oxidation to glycolic acid. An optimal Pd:Ag ratio was found and the catalyst was tested under industrial flow conditions, achieving a constant high selectivity during 140 h of electrolysis. Besides oxidation reactions, the reduction of CO₂ to CO under high pressure was investigated. The selectivity towards CO is strongly dependent on the applied current density. Faradaic efficiencies > 80% can be achieved at 150 mA/cm², but the system favours hydrogen production when this current is doubled. Pairing CO₂ reduction to polyol oxidation is not as straightforward as it seems. Finding optimal and matching operating conditions for both reactions is a challenge yet to be tackled.