Grazing incidence operando EXAFS measurements allow observation of oxygen in oxide-derived silver catalysts, its effect on CO2 reduction is explained.

Bipolar membranes (BPMs) are gaining interest in energy conversion technologies. These membranes are composed of cation- and anion-exchange layers, with an interfacial layer in between. This gives the freedom to operate in different conditions (pH, concentration, composition) at both sides. Such membranes are used in two operational modes, forward and reverse bias. BPMs have been implemented in various electrochemical applications, like water and CO 2 electrolyzers, fuel cells, and flow batteries, while BPMs are historically designed for acid/base production. Therefore, current commercial BPMs are not optimized, as the conditions change per application. Although the ideal BPM has highly conductive layers, high water dissociation kinetics, long lifetime, and low ion crossover, each application has its own priorities to be competitive in its field. We describe the challenges and requirements for future BPMs, and identify existing developments that can be leveraged to develop BPMs toward the scale of practical applications.

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.

A bipolar membrane (BPM) can be used to accelerate water dissociation to maintain a pH gradient in electrochemical cells, providing freedom to independently optimize the environments and catalysts used for paired reduction and oxidation reactions. The two physical layers in a BPM, respectively, selective for the exchange of cations and anions, should ideally reject ion crossover and facilitate ionic current via water dissociation in an interfacial layer. However, ions from the electrolyte do cross over in actual BPMs, competing with the water dissociation reaction and negatively affecting the stability of the electrolytes. Here, we explore the mechanisms of ion crossover as a function of pH and current density across a commercial BPM. Our unique series of experiments quantifies the ion crossover for more than 10 electrolyte combinations that cover 10 orders of magnitude in acid dissociation constant (K a ) and current densities spanning over more than 2 orders of magnitude. It was found that the ion crossover is dominated by diffusion for current densities up to a maximum of 10−40 mA cm −2 depending on the electrolyte, while migration is of higher importance at high current densities. The influence of the electrolyte pK a or pH on the ion crossover is not straightforward. However, ions with a higher valence or ion size show significantly lower crossover. Moreover, high current densities are the most favorable for high water dissociation efficiencies for all electrolyte combinations. This operational mode aligns well with practical applications of BPMs in electrolysis at industrial relevant current densities.

A bipolar membrane (BPM), consisting of a cation and anion exchange layer (CEL and AEL), can be used in an electrochemical cell in two orientations: reverse bias and forward bias....