CO2 Hydrogenation through Electrochemical Hydrogen Activation in a Gas-Vapor Fed Reactor
Authors: Roenigk Samantha , McKone James 
Published: December 2024

An attractive concept in the chemical manufacturing industry is the adoption of a circular carbon economy, which can include continuous capture and conversion of CO2 into industrially relevant chemical feedstocks. This has driven considerable research interest in electrochemical CO2 reduction.1 Despite this popularity, major challenges remain even for the most advanced CO2 reduction reactors, including low conversion rates and energy efficiencies; moreover, in many cases, multiple carbon-containing products are generated, which add to costs for downstream separation.2 By contrast, thermochemical CO2 hydrogenation is well established and typically demonstrates higher energy efficiency and selectivity compared to electrochemical CO2 reduction.3,4 This notable contrast drives our interest in better understanding the physics that governs differences in reactivity between thermocatalytic and electrocatalytic CO2 reduction reactors. To address these questions, we are pursuing a unique approach that involves feeding humidified CO2 and H2, respectively, to the cathode and anode of an electrochemical membrane-electrode assembly (MEA). This presentation will cover initial work we have undertaken to design and validate this reactor for the study CO2 electro-reduction.
The overall design of our reactor resembles a hydrogen fuel cell fed with humidified H2 and CO2. Thus, it was initially benchmarked through operation as an electrochemical hydrogen pump, wherein humidified Ar was fed to the cathode and humidified H2 was fed to the anode with Pt/C as the catalyst on each side of the membrane. Under these conditions, a cation exchange membrane (CEM) readily generated current densities in excess of 1 A/cm2 with monotonic decreases in polarization with increased temperature at fixed current. We then undertook analogous hydrogen pumping experiments using anion exchange membranes (AEM) that had been ion-exchanged with hydroxide, carbonate, and bicarbonate anions. We observed net proton transport between the anode and cathode chambers in each case, albeit with higher ionic resistance for AEMs in the bicarbonate form.
We then undertook further studies to demonstrate the ability to reduce CO2 using H2 fed to the anode. This operating mode eliminates the need for a liquid anode feed, which dramatically reduces the driving force for CO2 crossover via bicarbonate formation. Moreover, the Pt/C anode catalyst is expected to oxidize hydrogen with minimal polarization, allowing the total cell polarization to be interpreted simply as the cathode potential on the RHE scale. CO2 reduction experiments were carried out using commercial Cu/C catalyst with online gas chromatography and NMR for product analysis of volatile and condensable products, respectively. Operation with a CEM yielded only H2(g) at the cathode, but initial experiments with an AEM in the carbonate/bicarbonate form gave high selectivity to ethanol at 70 oC.
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