Ion Transport Mechanisms in Bipolar Membranes for Electrochemical Applications

Electrochemistry can be a useful technology to enable the transition toward renewable energy sources and to prevent further climate change. Some electrochemical applications are already industrially implemented, like water electrolysers, while others, like CO2 reduction, need further development to be industrially competitive. Here, a bipolar membrane can provide the conditions for a step forward. It will not only separate (gaseous) products from the two electrodes in an electrochemical cell, but it will also allow the use of different pH and electrolyte at either side. This enables optimization of electrode compartments. As it is composed of a cation and anion exchange layer, no ion transport can theoretically occur across the entire membrane. To still provide the required charge transport, the BPM can dissociate water at the interface layer of the BPM, where often a catalyst is deposited to enhance this reaction. An ideal bipolar membrane has highly conductive membrane layers, fast kinetics at the interface layer and therefore high water permeability to the interface, a long lifetime, and a low ion crossover. As the BPM can be implemented in various electrochemical energy applications, likewater and CO2 electrolysis, batteries, fuel cells and resource recovery, different specific requirements exist per application. For batteries and resource recovery a low ion crossover is crucial, while for fuel cells, water and CO2 electrolysis fast kinetics are essential. Hence, for each application a specifically developed BPM is required for future applications. The development should be based on knowledge gained by studying the performance of BPMs in these conditions with techniques like electrochemical impedance spectroscopy.ChemE/Materials for Energy Conversion & Storag

Reduced Ion Crossover in Bipolar Membrane Electrolysis via Increased Current Density, Molecular Size, and Valence

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 (Ka) 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 pKa 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.ChemE/Materials for Energy Conversion & StorageServicedesk Library / SIDChemE/Transport Phenomen

High Indirect Energy Consumption in AEM-Based CO₂Electrolyzers 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.ChemE/Materials for Energy Conversion & StorageChemE/Transport Phenomen

Electrochemical impedance spectroscopy as a performance indicator of water dissociation in bipolar membranes

A bipolar membrane (BPM) can be used to maintain a pH difference in an electrolysis cell, which provides freedom to independently optimize the environments and catalysts used for paired redox reactions. A BPM consists of two physical layers, of which one is selective for the exchange of cations and the other for anions. The water dissociation reaction (WDR) splits water into protons and hydroxide ions under an electric field that concentrates at the interface of the two membrane layers. However, salt ions in commonly used electrolytes influence this WDR when they are present at the interface. Using electrochemical impedance spectroscopy (EIS), we observed the rate of water dissociation decrease in the presence of salt ions while also observing the diffusion and migration of these salt ions, showing a clear link between the peaks observed in EIS and ion crossover. In addition, we show how EIS can be used to in situ monitor the stability and ageing of a BPM, revealing that degradation of the BPM is more prominent in extreme pH electrolyte pairs compared to non-extreme electrolyte pairs. The in situ monitoring of the WDR and stability of a BPM are vital methods for adequate and consistent comparison of novel designs of BPM-based systems, where EIS allows for discriminating BPM characteristics from other components even during operation.Accepted Author ManuscriptChemE/Materials for Energy Conversion & StorageChemE/Transport Phenomen

Intrinsic bipolar membrane characteristics dominate the effects of flow orientation and external pH-profile on the membrane voltage

The practical energy required for water dissociation reaction in bipolar membrane (BPM) is still substantially higher compared to the thermodynamic equivalent. This required energy is determined by the bipolar membrane voltage, consisting of (1) thermodynamic potential and (2) undesired voltage losses. Since the pH gradient over the BPM affects both voltage components, in this work, pH gradient is leveraged to decrease the BPM-voltage. We investigate the effect of four flow orientations: 1) co-flow, 2) counter-flow, 3) co-recirculation, and 4) counter-recirculation, on the pH gradient and BPM-voltage, using an analytical model and chronopotentiometry experiments. The analytical model predicts the experimentally obtained pH accurately and confirms the importance of the flow orientation in determining the longitudinal pH gradient profile over the BPM in the bulk solution. However, in contrast to the simulated results, our observations show the effect of flow orientations on the BPM-voltage to be insignificant under practical operating conditions. When the water dissociation reaction in the BPM is dominant, the internal local pH inside of the membrane determines its final voltage, shadowing the effect of the external pH-gradient in the bulk solution. Therefore, although changing the flow orientation affects the bulk pH, it does not influence the local pH at the BPM junction layer and hence the BPM-voltage. Instead, opportunities for reducing the membrane voltage are in the realm of improved catalysts and ion exchange layers of the BPM.ChemE/Transport PhenomenaChemE/Materials for Energy Conversion & Storag

Cation-Driven Increases of CO₂Utilization in a Bipolar Membrane Electrode Assembly for CO₂Electrolysis

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. ChemE/Materials for Energy Conversion & Storag

Bipolar Membrane and Interface Materials for Electrochemical Energy Systems

Bipolar membranes (BPMs) are recently emerging as a promising material for application in advanced electrochemical energy systems such as (photo)electrochemical CO2 reduction and water splitting. BPMs exhibit a unique property to accelerate water dissociation and ionic separation that allows for maintaining a steady-state pH gradient in electrochemical devices without a significant loss in process efficiency, thereby allowing a broader catalyst material selection for the respective oxidation and reduction reactions. However, the formation of high-performance BPMs with significantly reduced overpotentials for driving water dissociation and ionic separation at conditions and rates that are relevant to energy technologies is a key challenge. Herein, we perform a detailed assessment of the requirements in base materials and optimal design routes for the BPM layer and interface formation. In particular, the interface in the BPM presents a critical component with its structure and morphology influencing the kinetics of water dissociation reaction governed by both electric field and catalyst driven mechanisms. For this purpose, we present, among others, the advantages and drawbacks in the utilization of a bulk heterojunction formed in 3D structures that recently have been reported to demonstrate a possibility of designing stable and high-performance BPMs. Also, the outer layers of a BPM play a crucial role in kinetics and mass transport, particularly related to water and ion transport at electrolyte-membrane and membrane-catalyst interfaces. This work aims at identifying the gaps in the structure-property of the current monopolar materials to provide prospective facile design routes for BPMs with excellent water dissociation and ionic separation efficiency. It extends to a discussion about material selection and design strategies of advanced BPMs for application in emerging electrochemical energy systems. Accepted Author ManuscriptChemE/Materials for Energy Conversion & StorageChemE/Transport Phenomen

Orientation of a bipolar membrane determines the dominant ion and carbonic species transport in membrane electrode assemblies for CO₂reduction

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. This journal is ChemE/Materials for Energy Conversion & StorageChemE/Transport PhenomenaChemE/Delft Ingenious Desig

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. ChemE/Materials for Energy Conversion & StorageChemE/Transport Phenomen

Operando EXAFS study reveals presence of oxygen in oxide-derived silver catalysts for electrochemical CO₂ reduction

Electrocatalysis of carbon dioxide can provide a valuable pathway towards the sustainable production of chemicals and fuels from renewable electricity sources. One of the main challenges to enable this technology is to find suitable electrodes that can act as efficient, stable and selective CO2 reduction catalysts. Modified silver catalysts and in particular, catalysts electrochemically derived from silver-oxides, have shown great promise in this regard. Here, we use operando EXAFS analysis to study the differences in surface composition between a pure silver film and oxide-derived silver catalysts-a nanostructured catalyst with improved CO2 reduction performance. The EXAFS analysis reveals the presence of trace amounts of oxygen in the oxide-derived silver samples, with the measured oxygen content correlating well with experimental studies showing an increase in CO2 reduction reactivity towards carbon monoxide. The selectivity towards CO production also partially scales with the increased surface area, showing that the morphology, local composition and electronic structure all play important roles in the improved activity and selectivity of oxide-derived silver electrocatalysts. Earlier studies based on X-ray photoelectron spectroscopy (XPS) were not able to identify this oxygen, most likely because in ultra-high vacuum conditions, silver can self-reduce to Ag0, removing existing oxygen species. This operando EXAFS study shows the potential for in situ and operando techniques to probe catalyst surfaces during electrolysis and aid in the overall understanding of electrochemical systems.Accepted Author ManuscriptChemE/Materials for Energy Conversion & Storag