Characterizing CO2 Reduction Catalysts on Gas Diffusion Electrodes: Comparing Activity, Selectivity, and Stability of Transition Metal Catalysts.

Continued advancements in the electrochemical reduction of CO2 (CO2RR) 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 CO2RR (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 CO2RR experiments for the first time.

High Indirect Energy Consumption in AEM-Based CO2 Electrolyzers Demonstrates the Potential of Bipolar Membranes.

Typically, anion exchange membranes (AEMs) are used in CO2 electrolyzers, but those suffer from unwanted CO2 crossover, implying (indirect) energy consumption for generating an excess of CO2 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 CO2 crossover and electrolyte refining. While the BPM-based cell had a high stability and almost no CO2 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 CO2 electrolyzers.

Cation-Driven Increases of CO2 Utilization in a Bipolar Membrane Electrode Assembly for CO2 Electrolysis.

Advancing reaction rates for electrochemical CO2 reduction in membrane electrode assemblies (MEAs) have boosted the promise of the technology while exposing new shortcomings. Among these is the maximum utilization of CO2, 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 CO2 losses, but their promise is dampened by poor CO2 activity and selectivity. In this work, we enable a 3-fold increase in the CO2 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 CO2 loss at similar current densities, while breaking the 50% CO2 utilization mark. The work provides a combined cation and BPM strategy for overcoming CO2 utilization issues in CO2 electrolyzers.

Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems.

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 CO2 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.

Orientation of a bipolar membrane determines the dominant ion and carbonic species transport in membrane electrode assemblies for CO2 reduction.

A bipolar membrane (BPM), consisting of a cation and an anion exchange layer (CEL and AEL), can be used in an electrochemical cell in two orientations: reverse bias and forward bias. A reverse bias is traditionally used to facilitate water dissociation and control the pH at either side. A forward bias has been proposed for several applications, but insight into the ion transport mechanism is lacking. At the same time, when implementing a BPM in a membrane electrode assembly (MEA) for CO2 reduction, the BPM orientation determines the environment of the CO2 reduction catalyst, the anolyte interaction and the direction of the electric field at the interface layer. In order to understand the transport mechanisms of ions and carbonic species within a bipolar membrane electrode assembly (BPMEA), these two orientations were compared by performing CO2 reduction. Here, we present a novel BPMEA using a Ag catalyst layer directly deposited on the membrane layer at the vapour-liquid interface. In the case of reverse bias, the main ion transport mechanism is water dissociation. CO2 can easily crossover through the CEL as neutral carbonic acid due to the low pH in the reverse bias. Once it enters the AEL, it will be transported to the anolyte as (bi)carbonate because of the presence of hydroxide ions. When the BPM is in the forward bias mode, with the AEL facing the cathode, no net water dissociation occurs. This not only leads to a 3 V lower cathodic potential but also reduces the flux of carbonic species through the BPM. As the pH in the AEL is higher, (bi)carbonate is transported towards the CEL, which then blocks the majority of those species. However, this forward bias mode showed a lower selectivity towards CO production and a higher salt concentration was observed at the cathode surface. The high overpotential and CO2 crossover in reverse bias can be mitigated via engineering BPMs, providing higher potential for future application than that of a BPM in forward bias owing to the intrinsic disadvantages of salt recombination and poor faradaic efficiency for CO2 reduction.