A bipolar membrane maintains a pH gradient for solar driven water splitting with an acidic cathodic environment and neutral or alkaline anodic environment.

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.

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.

Continued advancements in the electrochemical reduction of CO 2 (CO 2 RR) have emphasized that reactivity, selectivity, and stability are not explicit material properties but combined effects of the catalyst, double-layer, reaction environment, and system configuration. These realizations have steadily built upon the foundational work performed for a broad array of transition metals performed at 5 mA cm –2 , which historically guided the research field. To encompass the changing advancements and mindset within the research field, an updated baseline at elevated current densities could then be of value. Here we seek to re-characterize the activity, selectivity, and stability of the five most utilized transition metal catalysts for CO 2 RR (Ag, Au, Pd, Sn, and Cu) at elevated reaction rates through electrochemical operation, physical characterization, and varied operating parameters to provide a renewed resource and point of comparison. As a basis, we have employed a common cell architecture, highly controlled catalyst layer morphologies and thicknesses, and fixed current densities. Through a dataset of 88 separate experiments, we provide comparisons between CO-producing catalysts (Ag, Au, and Pd), highlighting CO-limiting current densities on Au and Pd at 72 and 50 mA cm –2 , respectively. We further show the instability of Sn in highly alkaline environments, and the convergence of product selectivity at elevated current densities for a Cu catalyst in neutral and alkaline media. Lastly, we reflect upon the use and limits of reaction rates as a baseline metric by comparing catalytic selectivity at 10 versus 200 mA cm –2 . We hope the collective work provides a resource for researchers setting up CO 2 RR experiments for the first time.