Advancing reaction rates for electrochemical CO 2 reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO 2 , which is capped at 50% (CO as targeted product) due to unwanted homogeneous reactions. Using bipolar membranes in an MEA (BPMEA) has the capability of preventing parasitic CO 2 losses, but their promise is dampened by poor CO 2 activity and selectivity. In this work, we enable a 3-fold increase in the CO 2 reduction selectivity of a BPMEA system by promoting alkali cation (K + ) concentrations on the catalyst’s surface, achieving a CO Faradaic efficiency of 68%. When compared to an anion exchange membrane, the cation-infused bipolar membrane (BPM) system shows a 5-fold reduction in CO 2 loss at similar current densities, while breaking the 50% CO 2 utilization mark. The work provides a combined cation and BPM strategy for overcoming CO 2 utilization issues in CO 2 electrolyzers.

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Salt precipitation is a problem in electrochemical CO 2 reduction electrolyzers that limits their long-term durability and industrial applicability by reducing the active area, causing flooding and hindering gas transport. Salt crystals form when hydroxide generation from electrochemical reactions interacts homogeneously with CO 2 to generate substantial quantities of carbonate. In the presence of sufficient electrolyte cations, the solubility limits of these species are reached, resulting in "salting out" conditions in cathode compartments. Detrimental salt precipitation is regularly observed in zero-gap membrane electrode assemblies, especially when operated at high current densities. This Perspective briefly discusses the mechanisms for salt formation, and recently reported strategies for preventing or reversing salt formation in zero-gap CO 2 reduction membrane electrode assemblies. We link these approaches to the solubility limit of potassium carbonate within the electrolyzer and describe how each strategy separately manipulates water, potassium, and carbonate concentrations to prevent (or mitigate) salt formation.

Typically, anion exchange membranes (AEMs) are used in CO 2 electrolyzers, but those suffer from unwanted CO 2 crossover, implying (indirect) energy consumption for generating an excess of CO 2 feed and purification of the KOH anolyte. As an alternative, bipolar membranes (BPMs) have been suggested, which mitigate the reactant loss by dissociating water albeit requiring a higher cell voltage when operating at a near-neutral pH. Here, we assess the direct and indirect energy consumption required to produce CO in a membrane electrode assembly with BPMs or AEMs. More than 2/3 of the energy consumption for AEM-based cells concerns CO 2 crossover and electrolyte refining. While the BPM-based cell had a high stability and almost no CO 2 loss, the Faradaic efficiency to CO was low, making the energy requirement per mol of CO higher than for the AEM-based cell. Improving the cathode–BPM interface should be the future focus to make BPMs relevant to CO 2 electrolyzers.

The electrochemical reduction of carbon dioxide (CO2) to value-added materials has received considerable attention. Both bulk transition metal catalysts, and molecular catalysts affixed to conductive non-catalytic solid supports, represents a promising approach towards electroreduction of CO2. Here, we report a combined silver (Ag) and pyridine catalyst through a green and irreversible electrografting process, which demonstrates enhanced CO2 conversion versus the individual counterparts. We find by tailoring the pyridine carbon chain length, a 200 mV shift in the onset potential is obtainable compared to the bare silver electrode. A 10-fold activity enhancement at -0.7 V vs RHE is then observed with demonstratable higher partial current densities for CO indicating a co-catalytic effect is attainable through the integration of the two different catalytic structures. We extended performance to a flow cell operating at 150 mA/cm 2 , demonstrating the approach's potential for substantial adaption with various transition metals as supports, and electrografted molecular co-catalysts.