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

Aryl ether-free polymer electrolytes for electrochemical and energy devices

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

Gas-Induced Structural Damages in Forward-Bias Bipolar Membrane CO2 Electrolysis Studied by Fast X-ray Tomography

Forward-bias bipolar membrane (BPM) CO2 coelectrolysis (CO2ELY) aims at overcoming the issues of salt precipitation and CO2 crossover in anion exchange membrane CO2ELY. Increasing the stability of BPM-CO2ELY is crucial for widespread application of the technique. In this study, we employ time-resolved X-ray tomographic microscopy to elucidate the structural dynamics that occur within the electrochemical cell during operation under various conditions. Using advanced image processing methods, including custom 4D machine learning segmentation, we can visualize and quantify damages in the membrane and anode catalyst layer (CL). We compare our results to a CO2 transport model and hypothesize gaseous CO2 as the cause of the observed damages. At any operation condition, CO2 is formed at the junction in the center of the BPM by recombination of carbonate ions. CO2 migrates to the anode by diffusion and goes into the gas phase at the interface of the membrane and anode CL. After sufficient CO2 accumulation and pressure buildup after only tens of minutes, small irreversible holes break into the CL distributed over the entire active area. Additionally, at higher current densities, the CO2 accumulation leads to membrane delamination at the BPM junction. Despite the clear degradation processes, we do not observe an obvious direct effect on the electrochemical performance. The poor stability of BPM-CO2ELY remains an open question.

Backbone Engineering of Polymeric Catalysts for High-Performance CO2 Reduction in Bipolar Membrane Zero-Gap Electrolyzer

Bipolar membranes (BPMs) have emerged as a promising solution for mitigating CO2 losses, salt precipitation and high maintenance costs associated with the commonly used anion-exchange membrane electrode assembly for CO2 reduction reaction (CO2RR). However, the industrial implementation of BPM-based zero-gap electrolyzer is hampered by the poor CO2RR performance, largely attributed to the local acidic environment. Here, we report a backbone engineering strategy to improve the CO2RR performance of molecular catalysts in BPM-based zero-gap electrolyzers by covalently grafting cobalt tetraaminophthalocyanine onto a positively charged polyfluorene backbone (PF-CoTAPc). PF-CoTAPc shows a high acid tolerance in BPM electrode assembly (BPMEA), achieving a high FE of 82.6 % for CO at 100 mA/cm2 and a high CO2 utilization efficiency of 87.8 %. Notably, the CO2RR selectivity, carbon utilization efficiency and long-term stability of PF-CoTAPc in BPMEA outperform reported BPM systems. We attribute the enhancement to the stable cationic shield in the double layer and suppression of proton migration, ultimately inhibiting the undesired hydrogen evolution and improving the CO2RR selectivity. Techno-economic analysis shows the least energy consumption (957 kJ/mol) for the PF-CoTAPc catalyst in BPMEA. Our findings provide a viable strategy for designing efficient CO2RR catalysts in acidic environments.

Bipolar Membrane Seawater Splitting for Hydrogen Production: A Review

The growing demand for clean energy has spurred the quest for sustainable alternatives to fossil fuels. Hydrogen has emerged as a promising candidate with its exceptional heating value and zero emissions upon combustion. However, conventional hydrogen production methods contribute to CO2 emissions, necessitating environmentally friendly alternatives. With its vast potential, seawater has garnered attention as a valuable resource for hydrogen production, especially in arid coastal regions with surplus renewable energy. Direct seawater electrolysis presents a viable option, although it faces challenges such as corrosion, competing reactions, and the presence of various impurities. To enhance the seawater electrolysis efficiency and overcome these challenges, researchers have turned to bipolar membranes (BPMs). These membranes create two distinct pH environments and selectively facilitate water dissociation by allowing the passage of protons and hydroxide ions, while acting as a barrier to cations and anions. Moreover, the presence of catalysts at the BPM junction or interface can further accelerate water dissociation. Alongside the thermodynamic potential, the efficiency of the system is significantly influenced by the water dissociation potential of BPMs. By exploiting these unique properties, BPMs offer a promising solution to improve the overall efficiency of seawater electrolysis processes. This paper reviews BPM electrolysis, including the water dissociation mechanism, recent advancements in BPM synthesis, and the challenges encountered in seawater electrolysis. Furthermore, it explores promising strategies to optimize the water dissociation reaction in BPMs, paving the way for sustainable hydrogen production from seawater.

Reactive Carbon Capture Enables CO2 Electrolysis with Liquid Feedstocks

ConspectusThe electrochemical reduction of carbon dioxide (CO2RR) is a promising strategy for mitigating global CO2 emissions while simultaneously yielding valuable chemicals and fuels, such as CO, HCOO-, and C2H4. This approach becomes especially appealing when integrated with surplus renewable electricity, as the ensuing production of fuels could facilitate the closure of the carbon cycle. Despite these advantages, the realization of industrial-scale electrolyzers fed with CO2 will be challenged by the substantial energy inputs required to isolate, pressurize, and purify CO2 prior to electrolysis.To address these challenges, we devised an electrolyzer capable of directly converting reactive carbon solutions (e.g., a bicarbonate-rich eluent that exits a carbon capture unit) into higher value products. This "reactive carbon electrolyzer" operates by reacting (bi)carbonate with acid generated within the electrolyzer to produce CO2 in situ, thereby facilitating CO2RR at the cathode. This approach eliminates the need for expensive CO2 recovery and compression steps, as the electrolyzer can then then coupled directly to the CO2 capture unit.This Account outlines our endeavors in developing this type of electrolyzer, focusing on the design and implementation of materials for electrocatalytic (bi)carbonate conversion. We highlight the necessity for a permeable cathode that allows the efficient transport of (bi)carbonate ions while maintaining a sufficiently high catalytic surface area. We address the importance of the supporting electrolyte, detailing how (bi)carbonate concentration, counter cations, and ionic impurities impact selectivity for products formed in the electrolyzer. We also catalog state-of-the-art performance metrics for reactive carbon electrolyzers (i.e., Faradaic efficiency, full cell voltage, CO2 utilization efficiency) and outline strategies to bridge the gap between these values and those required for commercial operation Collectively, these findings contribute to the ongoing efforts to realize industrial-scale electrochemical reactors for CO2 conversion, bringing us closer to a sustainable and closed-loop carbon cycle.

Protocol for assembling and operating bipolar membrane water electrolyzers

Renewable energy-driven bipolar membrane water electrolyzers (BPMWEs) are a promising technology for sustainable production of hydrogen from seawater and other impure water sources. Here, we present a protocol for assembling BPMWEs and operating them in a range of water feedstocks, including ultra-pure deionized water and seawater. We describe steps for membrane electrode assembly preparation, electrolyzer assembly, and electrochemical evaluation. For complete details on the use and execution of this protocol, please refer to Marin et al. (2023).1.

Modular Solar-to-Fuel Electrolysis at Low Cell Potentials Enabled by Glycerol Electrooxidation and a Bipolar Membrane Separator

Solar fuel generation through water electrolysis or electrochemical CO2 reduction is thermodynamically limited when it is paired with oxygen evolution reaction (OER). Glycerol electrooxidation reaction (GEOR) is an alternative anodic reaction with lower anodic electrochemical potential that utilizes a renewable coproduct produced during biodiesel synthesis. We show that GEOR on an Au-Pt-Bi ternary metal electrocatalyst in a model alkaline crude glycerol solution can provide significant cell potential reductions even when paired to reduction reactions in seawater and acidic catholytes via a bipolar membrane (BPM). We showed that the combination of GEOR and a BPM separator lowers the total cell potential by 1 V at an electrolysis current of 10.0 mA cm-2 versus an anode performing anode's OER when paired with hydrogen evolution and CO2 reduction cathodes. The observed voltage reduction was steady for periods of up to 80 h, with minimal glycerol crossover observed through the membrane. These results motivate new, high-performance cell designs for photoelectrochemical solar fuel integrated systems based on glycerol electrooxidation.

Effects of Iron Species on Low Temperature CO2 Electrolyzers

Electrochemical energy conversion devices are considered key in reducing CO2 emissions and significant efforts are being applied to accelerate device development. Unlike other technologies, low temperature electrolyzers have the ability to directly convert CO2 into a range of value-added chemicals. To make them commercially viable, however, device efficiency and durability must be increased. Although their design is similar to more mature water electrolyzers and fuel cells, new cell concepts and components are needed. Due to the complexity of the system, singular component optimization is common. As a result, the component interplay is often overlooked. The influence of Fe-species clearly shows that the cell must be considered holistically during optimization, to avoid future issues due to component interference or cross-contamination. Fe-impurities are ubiquitous, and their influence on single components is well-researched. The activity of non-noble anodes has been increased through the deliberate addition of iron. At the same time, however, Fe-species accelerate cathode and membrane degradation. Here, we interpret literature on single components to gain an understanding of how Fe-species influence low temperature CO2 electrolyzers holistically. The role of Fe-species serves to highlight the need for considerations regarding component interplay in general.

Continuous ammonia electrosynthesis using physically interlocked bipolar membrane at 1000 mA cm-2

Electrosynthesis of ammonia from nitrate reduction receives extensive attention recently for its relatively mild conditions and clean energy requirements, while most existed electrochemical strategies can only deliver a low yield rate and short duration for the lack of stable ion exchange membranes at high current density. Here, a bipolar membrane nitrate reduction process is proposed to achieve ionic balance, and increasing water dissociation sites is delivered by constructing a three-dimensional physically interlocked interface for the bipolar membrane. This design simultaneously boosts ionic transfer and interfacial stability compared to traditional ones, successfully reducing transmembrane voltage to 1.13 V at up to current density of 1000 mA cm^-2. By combining a Co three-dimensional nanoarray cathode designed for large current and low concentration utilizations, a continuous and high yield bipolar membrane reactor for NH₃ electrosynthesis realized a stable electrolysis at 1000 mA cm^-2 for over 100 h, Faradaic efficiency of 86.2% and maximum yield rate of 68.4 mg h^-1 cm^-2 with merely 2000 ppm NO₃^- alkaline electrolyte. These results show promising potential for artificial nitrogen cycling in the near future.

Electrosorption Integrated with Bipolar Membrane Water Dissociation: A Coupled Approach to Chemical-free Boron Removal

Boron removal from aqueous solutions has long persisted as a technological challenge, accounting for a disproportionately large fraction of the chemical and energy usage in seawater desalination and other industrial processes like lithium recovery. Here, we introduce a novel electrosorption-based boron removal technology with the capability to overcome the limitations of current state-of-the-art methods. Specifically, we incorporate a bipolar membrane (BPM) between a pair of porous carbon electrodes, demonstrating a synergized BPM-electrosorption process for the first time. The ion transport and charge transfer mechanisms of the BPM-electrosorption system are thoroughly investigated, confirming that water dissociation in the BPM is highly coupled with electrosorption of anions at the anode. We then demonstrate effective boron removal by the BPM-electrosorption system and verify that the mechanism for boron removal is electrosorption, as opposed to adsorption on the carbon electrodes or in the BPM. The effect of applied voltage on the boron removal performance is then evaluated, revealing that applied potentials above ∼1.0 V result in a decline in process efficiency due to the increased prevalence of detrimental Faradaic reactions at the anode. The BPM-electrosorption system is then directly compared with flow-through electrosorption, highlighting key advantages of the process with regard to boron sorption capacity and energy consumption. Overall, the BPM-electrosorption shows promising boron removal capability, with a sorption capacity >4.5 μmol g-C^-1 and a corresponding specific energy consumption of <2.5 kWh g-B^-1.

Analysis of Operational Parameters in Acid and Base Production Using an Electrodialysis with Bipolar Membranes Pilot Plant

In agreement with the Water Framework Directive, Circular Economy and European Union (EU) Green Deal packages, the EU-funded WATER-MINING project aims to validate next-generation water resource solutions at the pre-commercial demonstration scale in order to provide water management and recovery of valuable materials from alternative sources. In the framework of the WATER-MINING project, desalination brines from the Lampedusa (Italy) seawater reverse osmosis (SWRO) plant will be used to produce freshwater and recover valuable salts by integrating different technologies. In particular, electrodialysis with bipolar membranes (EDBM) will be used to produce chemicals (NaOH and HCl). A novel EDBM pilot plant (6.4 m²_, FuMa-Tech) has been installed and operated. The performance of EDBM for single pass under different flowrates (2-8 L·min^-1) for acid, base and saline channels, and two current densities (200 and 400 A·m^-2), has been analyzed in terms of specific energy consumption (SEC) and current efficiency (CE). Results showed that by increasing the flowrates, generation of HCl and NaOH slightly increased. For example, ΔOH^- shifted from 0.76 to 0.79 mol·min^-1 when the flowrate increased from 2 to 7.5 L·min^-1 at 200 A·m^-2. Moreover, SEC decreased (1.18-1.05 kWh·kg^-1) while CE increased (87.0-93.4%), achieving minimum (1.02 kWh·kg^-1) and maximum (99.4%) values, respectively, at 6 L·min^-1.

Advances in Selective Electrochemical Oxidation of 5-Hydroxymethylfurfural to Produce High-Value Chemicals

The conversion of biomass is a favorable alternative to the fossil energy route to solve the energy crisis and environmental pollution. As one of the most versatile platform compounds, 5-hydroxymethylfural (HMF) can be transformed to various value-added chemicals via electrolysis combining with renewable energy. Here, the recent advances in electrochemical oxidation of HMF, from reaction mechanism to reactor design are reviewed. First, the reaction mechanism and pathway are summarized systematically. Second, the parameters easy to be ignored are emphasized and discussed. Then, the electrocatalysts are reviewed comprehensively for different products and the reactors are introduced. Finally, future efforts on exploring reaction mechanism, electrocatalysts, and reactor are prospected. This review provides a deeper understanding of mechanism for electrochemical oxidation of HMF, the design of electrocatalyst and reactor, which is expected to promote the economical and efficient electrochemical conversion of biomass for industrial applications.

Neutralization Short-Circuiting with Weak Electrolytes Erodes the Efficiency of Bipolar Membranes

Bipolar membranes (BPMs) are critical components of a variety of electrochemical energy technologies. Many electrochemical applications require the use of buffers to maintain stable, nonextreme pH environments, yet the impact of buffers or weak acids/bases on the electrochemical behavior of BPMs remains poorly understood. Our data for a cell containing weak electrolytes is consistent with internal pH gradients within the anion exchange membrane (AEM) or cation exchange membrane (CEM) component of the BPM that form via ionic short-circuiting processes at open-circuit. Short-circuiting results from the coupling of co-ion crossover and parasitic neutralization and leads to buffering of the bipolar interface. This phenomenon, which we term neutralization short-circuiting, serves to erode BPM efficiency by attenuating the open-circuit membrane voltage and introducing parasitic reverse bias currents associated with weak acid/base dissociation at the interface. These findings establish a mechanistic basis for the operation of BPM cells in the presence of weak acid/base electrolytes.

Geometric Catalyst Utilization in Zero-Gap CO2 Electrolyzers

The electrochemical reduction of CO₂ (CO₂RR) on silver catalysts has been demonstrated under elevated current density, longer reaction times, and intermittent operation. Maintaining performance requires that CO₂ can access the entire geometric catalyst area, thus maximizing catalyst utilization. Here we probe the time-dependent factors impacting geometric catalyst utilization for CO₂RR in a zero-gap membrane electrode assembly. We use three flow fields (serpentine, parallel, and interdigitated) as tools to disambiguate cell behavior. Cathode pressure drop is found to play the most critical role in maintaining catalyst utilization at all time scales by encouraging in-plane CO₂ transport throughout the gas-diffusion layer (GDL) and around salt and water blockages. The serpentine flow channel with the highest pressure drop is then the most failure-resistant, achieving a CO partial current density of 205 mA/cm² at 2.76 V. These findings are confirmed through selectivity measurements over time, double-layer capacitance measurements to estimate GDL flooding, and transport modeling of the spatial CO₂ concentration.

Electrodialysis with Bipolar Membranes for the Generation of NaOH and HCl Solutions from Brines: An Inter-Laboratory Evaluation of Thin and Ultrathin Non-Woven Cloth-Based Ion-Exchange Membranes

The SEArcularMINE project aims to recover critical raw materials (CRMs) from brines from saltworks, thus facing a CRM shortage within Europe. To promote a fully circular scheme, the project valorises concentrated brines using electrodialysis with bipolar membranes (EDBM) to generate the required amounts of reactants (i.e., acids and bases). Regarding the performances of new non-woven cloth ion-exchange membranes (Suez): (i) an ultra-thin non-woven polyester cloth and (ii) a thin polypropylene cloth acting as the support structures were assessed. Additionally, the anion layer includes a catalyst to promote the water dissociation reaction. The effect of current density (100, 200, and 300 A m^-2) on the performance of two combinations of membranes in an inter-laboratory exercise using 2 M NaCl was evaluated. According to statistical analysis ANOVA, there was an agreement on the results obtained in both laboratories. NaOH/HCl solutions up to 0.8 M were generated working at 300 A m^-2 using both combinations of membranes. Regarding the performance parameters, stack set-ups incorporating thin polypropylene membranes showed lower specific energy consumption (SEC) and higher specific productivity (SP) than ultra-thin polypropylene ones. Hence, for ultra-thin polypropylene membranes, SEC was reported to be between 2.18 and 1.69 kWh kg^-1_NaOH and SP between 974 and 314 kg m^-2 y^-1.

Bipolar Membranes Containing Iron-Based Catalysts for Efficient Water-Splitting Electrodialysis

Water-splitting electrodialysis (WSED) process using bipolar membranes (BPMs) is attracting attention as an eco-friendly and efficient electro-membrane process that can produce acids and bases from salt solutions. BPMs are a key component of the WSED process and should satisfy the requirements of high water-splitting capability, physicochemical stability, low membrane cost, etc. The water-splitting performance of BPMs can be determined by the catalytic materials introduced at the bipolar junction. Therefore, in this study, several kinds of iron metal compounds (i.e., Fe(OH)₃, Fe(OH)₃@Fe₃O₄, Fe(OH)₂EDTA, and Fe₃O₄@ZIF-8) were prepared and the catalytic activities for water-splitting reactions in BPMs were systematically analyzed. In addition, the pore-filling method was applied to fabricate low-cost/high-performance BPMs, and the 50 μm-thick BPMs prepared on the basis of PE porous support showed several times superior toughness compared to Fumatech FBM membrane. Through various electrochemical analyses, it was proven that Fe(OH)₂EDTA has the highest catalytic activity for water-splitting reactions and the best physical and electrochemical stabilities among the considered metal compounds. This is the result of stable complex formation between Fe and EDTA ligand, increase in hydrophilicity, and catalytic water-splitting reactions by weak acid and base groups included in EDTA as well as iron hydroxide. It was also confirmed that the hydrophilicity of the catalyst materials introduced to the bipolar junction plays a critical role in the water-splitting reactions of BPM.

Application of bipolar membrane electrodialysis for simultaneous recovery of high-value acid/alkali from saline wastewater: An in-depth review

With the development of comprehensive utilization of high-salinity wastewater, salt resources regeneration has been considered as the fundamental requirement for process sustainability and economic benefits. As one of the potential candidates, bipolar membrane electrodialysis (BMED) was rapidly developed in recent years for the treatment of saline wastewater. Different from other methods directly obtaining salts or condensed wastewater, BMED could utilize and convert the dissolved waste salt into higher-value acid and alkali simultaneously, which has various advantages including outstanding environmental effects and economic benefits. In this review, the recent applications of BMED for waste salt recovery and high-value acid/alkali generation from saline wastewater were systematically outlined. Based on the summary above, the economy analysis of BMED was further reviewed from the roles of desalination and resources recovery. In addition, the BMED-based processes integrated with in-situ utilization of the generated acid/alkali resources were discussed. Furthermore, the influence of operating factors on BMED performance were outlined. Finally, the strategies for improving BMED performance were concluded. Furthermore, the future application and prospects of BMED was presented. This work would provide guidance for the applications of bipolar membrane electrodialysis in saline wastewater treatment and the high-value conversion of salt resources into acids and alkalis.

Challenges and opportunities in continuous flow processes for electrochemically mediated carbon capture

Carbon capture from both stationary emitters and dilute sources is critically needed to mitigate climate change. Carbon dioxide separation methods driven by electrochemical stimuli show promise to sidestep the high-energy penalty and fossil-fuel dependency associated with the conventional pressure and temperature swings. Compared with a batch process, electrochemically mediated carbon capture (EMCC) operating in a continuous flow mode offers greater design flexibility. Therefore, this review introduces key advances in continuous flow EMCC for point source, air, and ocean carbon captures. Notably, the main challenges and future research opportunities for practical implementation of continuous flow EMCC processes are discussed from a multi-scale perspective, from molecules to electrochemical cells and finally to separation systems.

Freestanding Bipolar Membranes with an Electrospun Junction for High Current Density Water Splitting

Freestanding bipolar membranes (BPMs) with an extended-area water splitting junction were fabricated utilizing electrospinning. The junction layer was composed of a mixed fiber mat that was made by concurrently electrospinning sulfonated poly(ether ether ketone) (SPEEK) and quaternized poly(phenylene oxide) (QPPO), with water splitting catalyst nanoparticles intermittently deposited between the fibers. The mat was sandwiched between solution cast SPEEK and QPPO films and hot-pressed to form a dense trilayer BPM with an extended-area junction of finite thickness, composed of QPPO nanofibers embedded in a SPEEK matrix with the catalyst nanoparticles interspaced between the two polymers. The composition, ion-exchange capacity, and catalyst type/loading in the junction were varied, and the water splitting characteristics of the membranes were assessed. The best BPMs fabricated in this work employed a graphene oxide catalyst and exhibited a low trans-membrane voltage drop of about 0.82 V at 1000 mA/cm² in water splitting experiments with 0.5 M Na₂SO₄ and stable water splitting operation for 60 h at 800 mA/cm².

Lithium recovery using electrochemical technologies: Advances and challenges

Driven by the electric-vehicle revolution, a sharp increase in lithium (Li) demand as a result of the need to produce Li-ion batteries is expected in coming years. To enable a sustainable Li supply, there is an urgent need to develop cost-effective and environmentally friendly methods to extract Li from a variety of sources including Li-rich salt-lake brines, seawater, and wastewaters. While the prevalent lime soda evaporation method is suitable for the mass extraction of Li from brine sources with low Mg/Li ratios, it is time-consuming (>1 year) and typically exhibits low Li recovery. Electrochemically-based methods have emerged as promising processes to recover Li given their ease of management, limited requirement for additional chemicals, minimal waste production, and high selectivity towards Li. This state-of-the-art review provides a comprehensive overview of current advances in two key electrochemical Li recovery technologies (electrosorption and electrodialysis) with particular attention given to advances in understanding of mechanism, materials, operational modes, and system configurations. We highlight the most pressing challenges these technologies encounter including (i) limited electrode capacity, poor electrode stability and co-insertion of impurity cations in the electrosorption process, and (ii) limited Li selectivity of available ion exchange membranes, ion leakage and membrane scaling in the electrodialysis process. We then systematically describe potentially effective strategies to overcome these challenges and, further, provide future perspectives, particularly with respect to the translation of innovation at bench-scale to industrial application.

Engineering Aspects for the Design of a Bicarbonate Zero-Gap Flow Electrolyzer for the Conversion of CO2 to Formate

CO₂ electrolyzers require gaseous CO₂ or saturated CO₂ solutions to achieve high energy efficiency (EE) in flow reactors. However, CO₂ capture and delivery to electrolyzers are in most cases responsible for the inefficiency of the technology. Recently, bicarbonate zero-gap flow electrolyzers have proven to convert CO₂ directly from bicarbonate solutions, thus mimicking a CO₂ capture medium, obtaining high Faradaic efficiency (FE) and partial current density (CD) toward carbon products. However, since bicarbonate electrolyzers use a bipolar membrane (BPM) as a separator, the cell voltage (V_Cell) is high, and the system becomes less efficient compared to analogous CO₂ electrolyzers. Due to the role of the bicarbonate both as a carbon donor and proton donor (in contrast to gas-fed CO₂ electrolyzers), optimization by using know-how from conventional gas-fed CO₂ electrolyzers is not valid. In this study, we have investigated how different engineering aspects, widely studied for upscaling gas-fed CO₂ electrolyzers, influence the performance of bicarbonate zero-gap flow electrolyzers when converting CO₂ to formate. The temperature, flow rate, and concentration of the electrolyte are evaluated in terms of FE, productivity, V_Cell, and EE in a broad range of current densities (10-400 mA cm^-2). A CD of 50 mA cm^-2, room temperature, high flow rate (5 mL cm^-2) of the electrolyte, and high carbon load (KHCO₃ 3 M) are proposed as potentially optimal parameters to benchmark a design to achieve the highest EE (27% is obtained this way), one of the most important criteria when upscaling and evaluating carbon capture and conversion technologies. On the other hand, at high CD (>300 mA cm^-2), low flow rate (0.5 mL cm^-2) has the highest interest for downstream processing (>40 g L^-1 formate is obtained this way) at the cost of a low EE (<10%).

Thermo-electrochemical redox flow cycle for continuous conversion of low-grade waste heat to power

Here we assess the route to convert low grade waste heat (< 100 °C) into electricity by leveraging the temperature dependency of redox potentials, similar to the Seebeck effect in semiconductor physics. We use fluid-based redox-active species, which can be easily heated and cooled using heat exchangers. By using a first principles approach, we designed a redox flow battery system with Fe(CN)₆³⁻/Fe(CN)₆⁴⁻ and I⁻/I₃⁻ chemistry. We evaluate the continuous operation with one flow cell at high temperature and one at low temperature. We show that the most sensitive parameter, the temperature coefficient of the redox reaction, can be controlled via the redox chemistry, the reaction quotient and solvent additives, and we present the highest temperature coefficient for this RFB chemistry. A power density of 0.6 W/m² and stable operation for 2 h are achieved experimentally. We predict high (close to Carnot) heat-to-power efficiencies if challenges in the heat recuperation and Ohmic resistance are overcome, and the temperature coefficient is further increased.

Continuum Modeling of Porous Electrodes for Electrochemical Synthesis

Electrochemical synthesis possesses substantial promise to utilize renewable energy sources to power the conversion of abundant feedstocks to value-added commodity chemicals and fuels. Of the potential system architectures for these processes, only systems employing 3-D structured porous electrodes have the capacity to achieve the high rates of conversion necessary for industrial scale. However, the phenomena and environments in these systems are not well understood and are challenging to probe experimentally. Fortunately, continuum modeling is well-suited to rationalize the observed behavior in electrochemical synthesis, as well as to ultimately provide recommendations for guiding the design of next-generation devices and components. In this review, we begin by presenting an historical review of modeling of porous electrode systems, with the aim of showing how past knowledge of macroscale modeling can contribute to the rising challenge of electrochemical synthesis. We then present a detailed overview of the governing physics and assumptions required to simulate porous electrode systems for electrochemical synthesis. Leveraging the developed understanding of porous-electrode theory, we survey and discuss the present literature reports on simulating multiscale phenomena in porous electrodes in order to demonstrate their relevance to understanding and improving the performance of devices for electrochemical synthesis. Lastly, we provide our perspectives regarding future directions in the development of models that can most accurately describe and predict the performance of such devices and discuss the best potential applications of future models.

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.