Author: Wenchao Ma (EPFL) - Converting CO2 into multi-carbon (C2+) chemicals and fuels using renewable electricity is potentially an important pathway towards a net-zero-emission society. However, the predominating current approach in CO2 electroreduction, namely, the low-temperature CO2 electrolysis, is inefficient. More importantly, the carbonate problem resulting from the reaction of CO2 with hydroxide ion (OH-), either in the reaction medium or generated in-situ by CO2 reduction, has been shown to plump the energy and carbon efficiencies into unpractically low numbers, generally below 10% and 20% at ≥ 1 A/cm2, respectively. Here we develop a two-step tandem process, where CO2 is first converted to CO by a high-temperature solid oxide electrolyzer cell (SOEC), followed by low-temperature CO electroreduction to C2+ products in a membrane electrode assembly (MEA) electrolyzer, for efficient CO2 utilization. Such a process not only avoids the carbonate problem but also enables a higher energy efficiency and stability for CO2 electroreduction. For the first step, we develop an encapsulated Co-Ni alloy catalyst by Sm2O3-doped-CeO2 that exhibits an energy efficiency of 90% and a lifetime of more than 2,000 h at 1 A/cm2 for high-temperature CO2-to-CO conversion in SOEC. The CO single-pass yield reaches over 90% with 100% selectivity. For the second step, we reveal that catalyst, and water and hydroxide transport at the two-phase interface of the cathode limits the performance of CO electroreduction in MEA. By designing a new copper catalyst with 4% lattice tension and developing a system that allows for sufficiently rapid interfacial mass transport, we reach an energy efficiency of 35% at 1 A/cm2 for low-temperature CO-to-C2+ conversion in MEA. The C2+ single-pass yield reaches over 40% with 100% selectivity. Those values are ~2 times higher than those achieved in direct CO2 electroreduction to C2+ processes.
