Single Pass CO2 Conversion Exceeding 85% in the Electrosynthesis of Multicarbon Products via Local CO2 Regeneration

By Colin P. O’Brien, R. Miao, Shijie Liu, Yi Xu, Geonhui Lee, A. Robb, J. Huang, Ke Xie, Koen Bertens, Christine M. Gabardo and 4 more. ACS Energy Letters, 2021

249 citations recorded by the source corpus.

The carbon dioxide reduction reaction (CO2RR) presents the opportunity to consume CO2 and produce desirable products. However, the alkaline conditions required for productive CO2RR result in the bulk of input CO2 being lost to bicarbonate and carbonate. This loss imposes a 25% limit on the conversion of CO2 to multicarbon (C2+) products for systems that use anions as the charge carrierand overcoming this limit is a challenge of singular importance to the field. Here, we find that cation exchange membranes (CEMs) do not provide the required locally alkaline conditions, and bipolar membranes (BPMs) are unstable, delaminating at the membrane−membrane interface. We develop a permeable CO2 regeneration layer (PCRL) that provides an alkaline environment at the CO2RR catalyst surface and enables local CO2 regeneration. With the PCRL strategy, CO2 crossover is limited to 15% of the amount of CO2 converted into products, in all cases. Low crossover and low flow rate combine to enable a single pass CO2 conversion of 85% (at 100 mA/cm ), with a C2+ faradaic efficiency and full cell voltage comparable to the anion-conducting membrane electrode assembly. The electrochemical CO2 reduction reaction (CO2RR) presents an opportunity to utilize renewable electricity to produce chemical fuels and feedstocks from CO2. Valuable multicarbon (C2+) products, such as ethylene (C2H4) and ethanol (C2H5OH), are of particular interest in view of large existing markets. Providing reactant CO2 gas directly to the catalyst sites with gas diffusion electrodes (GDEs) enables CO2RR systems to attain impressive reaction rates (≫ 100 mA/cm2). Membrane electrode assembly cells combine GDEs and membranes in a zero-gap fashion. This configuration mitigates electrolyte degradation and salt precipitation issues characteristic of alkaline flow cells. Alkaline conditions are required at the cathode to suppress the hydrogen evolution reaction (HER) and enable a high faradaic efficiency (FE) toward CO2RR products. 11,12 Locally, alkaline conditions are maintained during CO2RR by hydroxide anions produced at the catalyst layer (eqs 1 and 2). However, these conditions result in the competing reaction of CO2 with hydroxide forming bicarbonate and carbonate (eqs 3 and 4). These ions electromigrate through the anion exchange membrane (AEM) to the anode where they combine with protons generated by the anodic oxygen evolution reaction to form CO2 and water. 13 Here, the CO2 bubbles out of the locally acidic anolyte and combines with produced oxygen, rendering a gas mixture that is costly to separate. This crossover of CO2 in MEA systems results in a low single pass conversion for CO2RR. When carbonate is the dominant charge carrier through the AEM, CO2 conversion efficiency is limited to 50% in the production of CO. + + → + − − CO H O 2e CO 2OH 2 2 (1) + + → + − − 2CO 8H O 12e C H 12OH 2 2 2 4 (2) + → − − CO OH HCO 2 3 (3) + → + − − CO 2OH CO H O 2 3 2 2 (4) Received: June 1, 2021 Accepted: July 23, 2021 Leter http://pubs.acs.org/journal/aelccp © XXXX American Chemical Society 2952 https://doi.org/10.1021/acsenergylett.1c01122 ACS Energy Lett. 2021, 6, 2952−2959 D ow nl oa de d vi a U N IV O F T O R O N T O o n A ug us t 3 , 2 02 1 at 1 2: 51 :0 1 (U T C ). Se e ht tp s: //p ub s. ac s. or g/ sh ar in gg ui de lin es f or o pt io ns o n ho w to le gi tim at el y sh ar e pu bl is he d ar tic le s.

Subfields: Catalysis, Electrical and Electronic Engineering, Renewable Energy, Sustainability and the Environment

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