Carbon dioxide/monoxide (CO2/CO) electrolysis provides a means to convert emissions into multicarbon products. However, impractical energy and carbon efficiencies limit current systems. Here we show that these inefficiencies originate from uncontrolled gas/ion distributions in the local reaction environment. Understanding of the flows of cations and anions motivated us to seek a route to block cation migration to the catalyst surface—a strategy we instantiate using a covalent organic framework (COF) in bulk heterojunction with a catalyst. The π-conjugated hydrophobic COFs constrain cation (potassium) diffusion via cation–π interactions, while promoting anion (hydroxide) and gaseous feedstock adsorption on the catalyst surface. As a result, a COF-mediated catalyst enables electrosynthesis of multicarbon products from CO for 200 h at a single-pass carbon efficiency of 95%, an energy efficiency of 40% and a current density of 240 mA cm−2. ; This work was financially supported by the Ontario Research Fund – Research Excellence programme, the Natural Sciences and Engineering Research Council (NSERC) of Canada and Natural Resources Canada’s Clean Growth Program. This research used synchrotron resources of the Advanced Photon Source (an Office of Science User Facility operated for the US Department of Energy (DOE) Office of Science by Argonne National Laboratory) and was supported by the US DOE under contract number DE-AC02-06CH11357, as well as the Canadian Light Source and its funding partners. Support from the Canada Research Chairs Program is gratefully acknowledged, as is support from an NSERC E.W.R. Steacie Fellowship to D.S. J.L. thanks the National Natural Science Foundation of China (grant number BE3250011), the National Key Research and Development Program of China (grant number 2022YFA1505100), and Shanghai Jiao Tong University (grant number WH220432516) for support. F.P.G.d.A. acknowledges funding from CEX2019-000910-S (MCIN/AEI/10.13039/501100011033), Fundación Cellex, Fundació Mir-Puig, Generalitat de ...