(195e) Optimizing CO2 Electrolysis in a Two-Step Solar-Driven Electrocatalytic and Photothermocatalytic Process for Ethylene Hydroformylation

Utilizing a two-step process consisting of CO₂ electrolysis followed by downstream thermocatalysis is a promising method of valorizing waste CO₂ into value-added products. For sustainable, solar-driven CO₂ conversion, we connected the product stream of a photovoltaic-driven CO₂ electrolyzer to a photothermal reactor for ethylene hydroformylation to C₆ products. The optimal product stream composition resulting from the electrolyzer is a 1:1:1 mol ratio of H₂:CO:C₂H_4­. To obtain the desired product distribution, we designed a Cu/Ag tandem electrode, where CO₂ is first reduced to CO on Ag, and CO is subsequently reduced further on Cu to C₂H₄. CO₂ electrolysis experiments were first conducted in the dark to test varying Ag and Cu ratios and operating parameters such as membrane thickness, CO₂ inlet flow rate, and cell compression.

In a 5 cm² zero-gap membrane electrode assembly (MEA), the optimal product distribution along with ~7 vol % ethylene is obtained using a catalyst consisting of a thin film of Cu deposited onto a gas diffusion electrode with a 10 nm thin film of Ag covering a 6 mm x 2.23 cm area near the inlet of the MEA flow field. To increase the product yield from the photothermal reactor, the concentration of gas products from outlet of the electrolyzer was increased by scaling up the MEA from 5 cm² to 25 cm². The areas of Cu and Ag were scaled up accordingly while utilizing the same catalyst design. The product distribution of the 25 cm² MEA was found to be dependent on several factors including type of anode and CO₂ inlet flow rate. In the scaled-up MEA, possible electrochemical-mechanical degradation of the membrane and the anode contributed to fluctuating product distribution over time. This study elucidates potential challenges with MEA scaleup within the context of a co-design approach to couple electrocatalytic and thermocatalytic processes.