Bipolar membranes: A review on principles, latest developments, and applications

Exploring electrodialysis with bipolar membranes for water lentil (duckweed) protein purification: A first investigation into process and membrane characterization with products comparison to chemical acidification

Water lentils are free-floating aquatic plants which could be an inexpensive source of protein due to their high leaf protein content and very rapid reproduction. However, the extraction and purification of leaf proteins from their matrix is a necessary step for human consumption, as undesired compounds can reduce their functional or sensorial properties. Therefore, in this study, water lentil proteins were purified for the first time using electrodialysis with bipolar membrane (EDBM), a technology that has been developed as an ecofriendly alternative to chemical acidification. The EDBM of water lentils successfully produced a protein concentrate that had a similar protein content (approximately 47.7 g/100 g) and protein extraction yield (around 39.4 %) compared to chemical precipitation. Moreover, EDBM allowed the demineralization of the protein concentrate by-product compared to chemical precipitation, reducing by 74 % its ash content (58.4 vs 15.2 g/100 g) and doubled its protein content (20.5 vs 41.1 g/100 g). However, during the EDBM process, the system's resistance tripled, and protein deposits were observed inside spacers and on bipolar membrane cation-exchange layer. Hence, while EDBM shows great promise, further optimization is necessary to enhance process efficiency.

Alkylation as a strategy for optimizing water uptake and enhancing selectivity in polyethyleneimine-based anion-exchange membranes for brine mining via electrodialysis

Brine treatment poses a significant challenge for the growing desalination industry, yet it also holds valuable elements and a substantial amount of water. To efficiently extract these elements and increase water recovery, the development of advanced, highly selective separation technologies is urgently needed. This study addresses this challenge by optimizing polyethyleneimine (PEI)-based anion exchange membranes (AEMs) through an alkylation strategy to enhance water uptake control and ion selectivity. The aim is to achieve the high separation efficiency required for effective reverse osmosis (RO) brine mining via electrodialysis. The careful design of functional amine groups with a mixed composition of alkyl substituents enabled the development of membranes with reduced water uptake and high charge density, providing the best conductivity/selectivity ratio, enhanced ion selectivity, and decreased water-splitting activity. The unmodified PEI-membrane already demonstrated a competitive performance compared to common commercial AEMs membranes used in electrodialysis, such as FujiFilm® AEM Type 1 and 2, Ralex® AM-PP, and Neosepta® AMX. However, the alkylation further improved the performance significantly. Among modified membranes, PEI alkylated with propyl followed by methyl (PEI-Pr-Me) achieved the highest current efficiency of 93 %, while PEI alkylated with a mixture of four (C1C4)n-alkyl groups had the highest Cl⁻/SO42⁻-selectivity coefficients of up to 8.7 and the lowest water transfer across the membrane. This tailored functionalization approach presents a promising pathway for improving AEMs' performance in desalination brine treatment, enabling more efficient water and mineral recovery.

Removal of heavy metals from sewage sludge by sequential acidification with HCl-H3PO4 and precipitation with Na2S-Ca(OH)2

Considering the feasibility of using sulfide generated during anaerobic digestion (AD) of sewage sludge as a heavy metal (HM) precipitant, the removal of Cr, Ni, Cu, Zn, As, Cd, Hg, and Pb, from sludge via sequential acidification by nine acids and precipitation as sulfides and hydroxides was investigated. The results showed that HMs were dissociated with the highest efficiency at pH 1.0 using mixed HCl-H3PO4 and a 6 h reaction time. The eight HMs were then efficiently removed from the acidic centrifugate after the addition of the desired dosages of Na2S and Ca(OH)2, both of which acted as precipitants and raised the pH of the centrifugate to 12.47, satisfying the requirements for further AD treatment of the mixture of acidic centrifuged residual sludge (CRS) and HM-removed supernatant. The increase in soluble organic matter after acidification shortened the duration of further AD from 25 to 10 days. Compared with conventional AD, the proposed HM elimination pretreatment effectively reduced the residual content of eight HMs by 24.04 %∼96.53 % and improved the speciation distribution of HMs in the final dewatered sludge. Moreover, the removal efficiencies of soluble chemical oxygen demand (SCOD), total nitrogen (TN), and total phosphorus (TP) increased from 38.5 % to 73.7 %, 27.4 % to 36.2 %, and 21.8 % to 26.1 %, respectively, and the cumulative biogas production during AD increased from 41.12 to 70.38 L/(kg dry sludge). The treatment protocol proposed in this study provides an alternative strategy to address the challenge posed by HMs in the land disposal of massive amounts of sewage sludge.

Methodology for Selecting Anion and Cation Exchange Membranes Based on Salt Transport Properties for Bipolar Membrane Fabrication

Bipolar membranes (BPMs) are a unique construction of ion exchange membranes with anion exchange and cation exchange layers in series. Due to the unique transport processes in BPMs, they are becoming an increasingly attractive option for many electrochemical devices, especially in water electrolysis and carbon dioxide reduction. However, because a large number of anion and cation exchange membranes are available, it can be difficult to select the layers for BPM fabrication, particularly when targeting specific properties for use in a device. In this study, a survey of nine anion and nine cation exchange membranes was conducted to assess their steady-state ion transport properties. The primary application of this work is seawater electrolysis; therefore, measurements of salt flux and area resistance in 0.5 mol/L sodium chloride solutions were performed. These measurements displayed a trade-off behavior, with membranes displaying higher area resistance and having a lower salt flux. Conversely, membranes with lower area resistance had a higher salt flux. From these individual membrane results, a methodology was formulated to select component membranes for BPM fabrication, primarily considering their transport characteristics. Three BPMs were fabricated using this methodology. A model was developed to integrate the parameters and ion transport properties measured from individual membranes to predict salt flux and area resistance values for a BPM. Values produced from the model were then compared with experimental salt flux and area resistance BPM measurements. Both the model and experimental salt flux and area resistance BPMs exhibited an area resistance-flux trade-off, like that of the component membranes.

Redox-Mediated CO2 Electrolysis for Recovering Transmembrane Carbon Loss

CO2 electrolysis in alkaline media presents advantages by suppressing the competitive hydrogen evolution reaction (HER) and enhancing the CO2 reduction selectivity. However, it suffers from the carbonation issue, leading to substantial carbon loss due to CO2 transmembrane transport. To tackle this issue, we here put forward a redox mediator (RM)-coupled electrolysis strategy. By integrating a highly reversible redox couple, this approach spatially separates the cathodic CO2 reduction and the anodic oxygen evolution reactions (OERs) into two electrolyzers, thereby enabling the recovery and reuse of transmembrane CO2. Anthraquinone-2,7-disulfonic acid (AQDS) was chosen as the redox mediator owing to its suitable redox potential, excellent electrochemical reversibility, high solubility, and nontransmembrane shuttling characteristics. It allowed the RM-coupled electrolysis system to operate continuously at 100 mA/cm2, maintaining a high Faradaic efficiency (FE) for CO2-to-CO conversion consistently around 90%, while effectively capturing the transmembrane CO2. This proof-of-concept demonstration validates the feasibility of RM-coupled electrolysis and highlights its significant potential to advance the practical application of CO2 electrolysis.

Molecularly Thin Nanosheet Films as Water Dissociation Reaction Catalysts Enhanced by Strong Electric Fields in Bipolar Membranes

Bipolar membranes (BPMs) are interesting materials for the development of next-generation electrochemical energy conversion and separations processes. One of the key challenges in optimizing BPM performance is enhancing the rate of the water dissociation (WD) reaction. While electric field effects, specifically the second Wien effect, have been demonstrated to enhance the rate of WD reaction, making BPMs with low overpotentials for WD using primary electric field effects has been difficult to achieve. In this study, we constructed an abrupt interfacial structure between the anion exchange membrane (AEM) and cation exchange membrane (CEM) of BPMs to maximize the intensity of local electric field. A film of densely tiled, molecularly thin titanium oxide nanosheets was deposited as the interfacial layer to create an abrupt interface for studying extreme electric field effects. Although BPMs with titanium oxide nanosheet films exhibited higher WD reaction resistance compared to thicker catalyst layers composed of nanoparticles at low current density, they showed superior performance at higher current densities, where strong electric fields were present, and an apparent WD overpotential of 0.25 V at 300 mA cm-2 was extracted from electrochemical impedance measurements. These results highlight the potential of optimizing BPM performance by maximizing the second Wien effect through the utilization of two-dimensionally assembled nanosheet films.

Fast-selective electro-driven membrane reactor in fluoride/silica crystallization for microelectronic wastewaters recycling

Rapid growth of the microelectronic industry leads to a significant increase in the generation of microelectronic wastewaters containing complex pollutants. Resource recovery technologies offer promising solutions for effective wastewater reuse in the microelectronics sector. However, how to simultaneously achieve high-efficiency crystallization and high crystal purity of ionic resources from complex wastewater remains a challenge. Here, for the first time, we propose an electro-driven membrane reactor (EMR) for the ex-situ crystallization of fluoride/silica from microelectronic wastewaters as high-purity fluorosilicates. This EMR with independent chambers combines a bipolar membrane to produce protons for SiF62- generation from the reaction between fluoride and silica. An internal ultrafiltration membrane is used to reject nanoparticles/organics while providing ion channels for protons and SiF62- migration. Selective recovery of Na2SiF6 from the coexisting ions (Cl-, SO42-, NO3- and PO43-)/nanoparticles (SiO2, Al2O3 and CeO2)/organics (tetramethylammonium hydroxide, isopropyl alcohol, bovine serum albumin, sodium alginate and humic acid) is demonstrated. Over 99.5 % Na2SiF6 purity and 64.5 % crystallization rate are verified under the optimal conditions (voltage of 8 V, UH050 membrane, operation mode Ⅰ, and forward permeate flux of 1 mL min-1). This EMR with the advantages of accurate capture capability may be an innovative strategy for enlarging the scale of pollutant elimination, ionic resources and fresh water recovery from micro-electronic wastewaters.

Strategies for Enhancing Stability in Electrochemical CO2 Reduction

The electrochemical CO2 reduction reaction (CO2RR) holds significant promise as a sustainable approach to address global energy challenges and reduce carbon emissions. However, achieving long-term stability in terms of catalytic performance remains a critical hurdle for large-scale commercial deployment. This mini-review provides a comprehensive exploration of the key factors influencing CO2RR stability, encompassing catalyst design, electrode architecture, electrolyzer optimization, and operational conditions. We examine how catalyst degradation occurs through mechanisms such as valence changes, elemental dissolution, structural reconfiguration, and active site poisoning and propose targeted strategies for improvement, including doping, alloying, and substrate engineering. Additionally, advancements in electrode design, such as structural modifications and membrane enhancements, are highlighted for their role in improving stability. Operational parameters such as temperature, pressure, and electrolyte composition also play crucial roles in extending the lifespan of the reaction. By addressing these diverse factors, this review aims to offer a deeper understanding of the determinants of long-term stability in the CO2RR, laying the groundwork for the development of robust, scalable technologies for efficient carbon dioxide conversion.

Enhanced Schemes for Brine Valorization via Electrodialysis with Bipolar Membranes Powered by Renewable Energy

Powering water treatment technologies with renewable energies by using the process buffering capacity as a way to indirectly store energy has been recently proposed as an effective strategy for the smart use of energy. With this respect, the production of chemicals from waste brines via electrodialysis with bipolar membranes (EDBM) can be particularly suitable due to its high energy intensity along with the extreme flexibility of the process. This study demonstrates, through real-environment experiments at the pilot scale, the feasibility of coupling an EDBM pilot plant with renewable energies (solar). The pilot plant was tested in a continuous process configuration (feed and bleed mode) under two different irradiation scenarios, i.e., summer and winter. The use of the controllers implemented allowed us to maintain the target concentration for acid and base fixed at 0.5 mol L-1 in both scenarios. In the summer scenario, current efficiency (CE) values higher than 90% and specific energy consumption (SEC) values lower than 2 kWh kg-1 were obtained, still maintaining a specific productivity (SP) of about 0.2 kg h-1 m-2. In the winter scenario, a current efficiency >80% was obtained, while SEC and SP values up to 1.6 kWh kg-1 and 0.06 kg h-1 m-2 were found, respectively. Results suggest that EDBM technology is perfectly suitable for the valorization of waste brines by using green energy sources, thus paving the way for its development at an industrial scale.

Electrocatalytic CN Coupling: Advances in Urea Synthesis and Opportunities for Alternative Products

Urea is an essential fertilizer produced through the industrial synthesis of ammonia (NH3) via the Haber-Bosch process, which contributes ≈1.2% of global annual CO2 emissions. Electrocatalytic urea synthesis under ambient conditions via CN coupling from CO2 and nitrogen species such as nitrate (NO3 -), nitrite (NO2 -), nitric oxide (NO), and nitrogen gas (N2) has gained interest as a more sustainable route. However, challenges remain due to the unclear reaction pathways for urea formation, competing reactions, and the complexity of the resulting product matrix. This review highlights recent advances in catalyst design, urea quantification, and intermediate identification in the CN coupling reaction for electrocatalytic urea synthesis. Furthermore, this review explores future prospects for industrial CN coupling, considering potential nitrogen and carbon sources and examining alternative CN coupling products, such as amides and amines.

Challenges and Opportunities of Choosing a Membrane for Electrochemical CO2 Reduction

The urgent need to reduce greenhouse gas emissions, particularly carbon dioxide (CO2), has led to intensive research into novel techniques for synthesizing valuable chemicals that address climate change. One technique that is becoming increasingly important is the electrochemical reduction of CO2 to produce carbon monoxide (CO), an important feedstock for various industrial processes. This comprehensive review examines the latest developments in CO2 electroreduction, focusing on mechanisms, catalysts, reaction pathways, and optimization strategies to enhance CO production efficiency. A particular emphasis is placed on the role of ion exchange membranes, including cation exchange membranes (CEMs), anion exchange membranes (AEMs), and bipolar membranes (BPMs). The review explores their advantages, disadvantages, and the current challenges associated with their implementation in CO2 electroreduction systems. Through careful analysis of the current literature, this report aims to provide a comprehensive understanding of state-of-the-art methods and their potential impact on sustainable CO production, with a special focus on membrane technologies.

Promotion of C─C Coupling in the CO2 Electrochemical Reduction to Valuable C2+ Products: From Micro-Foundation to Macro-Application

The electrochemical CO2 reduction reaction (CO2RR) to valuable C2+ products emerges as a promising strategy for converting intermittent renewable energy into high-energy-density fuels and feedstock. Leveraging its substantial commercial potential and compatibility with existing energy infrastructure, the electrochemical conversion of CO2 into multicarbon hydrocarbons and oxygenates (C2+) holds great industrial promise. However, the process is hampered by complex multielectron-proton transfer reactions and difficulties in reactant activation, posing significant thermodynamic and kinetic barriers to the commercialization of C2+ production. Addressing these barriers necessitates a comprehensive approach encompassing multiple facets, including the effective control of C─C coupling in industrial electrolyzers using efficient catalysts in optimized local environments. This review delves into the advancements and outstanding challenges spanning from the microcosmic to macroscopic scales, including the design of nanocatalysts, optimization of the microenvironment, and the development of macroscopic electrolyzers. By elucidating the influence of the local electrolyte environment, and exploring the design of potential industrial flow cells, guidelines are provided for future research aimed at promoting C─C coupling, thereby bridging microscopic insights and macroscopic applications in the field of CO2 electroreduction.

Review of CO2 extraction from seawater through non-electrochemical and electrochemical approaches

As the most significant carbon sink on Earth, the ocean has been inevitably absorbing excess atmospheric CO2, leading to high concentration of CO2 that threatens marine life. Like emissions of greenhouse gases, this issue demands urgent attention. Hence, comprehensive investigation and comparison, including both non-electrochemical and electrochemical direct ocean capture (DOC) methods, are revealed in this review. The non-electrochemical approach utilizes specialized materials such as gas-permeable membranes (GPM), hollow fiber membrane contactors (HFMC), and ion exchange resins to extract CO2 from seawater. In contrast, the electrochemical method employs chemical reactions to generate H+ or OH- ions, which adjust the pH value of seawater to either release CO2 gas or precipitate carbonate, thereby removing dissolved carbon. This article comprehensively overviews each method, including the latest research findings, underlying principles, employed equipment, and performance metrics. Finally, the achievements, current gaps, corresponding perspectives, and potential solutions in CO2 capture from seawater are also proposed.

Electrodialysis with Bipolar Membranes to valorise saline waste streams: Analysing the fate of valuable minor elements

Brine mining can represent a valuable non-conventional resource for the extraction of Mg, Li, B, Sr and other Trace Elements (TEs) such as Rb, Cs, whose recoveries require chemical reagents such as alkaline and acidic solutions. In a circular strategy, these required chemicals can be produced in-situ through Electrodialysis with Bipolar Membranes (EDBM). In this work, a laboratory EDBM unit was operated using real brines from Trapani saltworks to investigate, for the first time, the migration of minor and trace ions, as Li, B, Sr, Cs and Rb through ion-exchange membranes (IEMs). Two different operating configurations were tested by feeding real brines: i) only in the salt channel or ii) in both salt and alkaline compartments. Trace ions migration was assessed by determining their apparent transport number in IEMs to better understanding their "fate" within the EDBM process. The use of real solutions in the base channel resulted in a 50 % reduction in the process water demand, while achieving similar overall Current Efficiencies (75-78 %) and Specific Energy Consumptions (1.50-1.80 kWh/kgNaOH) compared to the reference layout, where real brine was only fed in the salt compartment. Li, Rb, Sr and Cs were mostly transported across the cation-exchange membrane and concentrated in the alkaline channel. Such results lay the ground for the use of complex (multi-ionic) solutions and new designs of the EDBM process that can be operated in integrated chains to valorise saline wastes, reducing water consumption and avoiding the dilution of trace elements before their selective recovery.

Exploring differential pressure-induced hydraulic flows in pilot-scale Electrodialysis with Bipolar Membranes

Electrodialysis with Bipolar Membranes (EDBM) is an electro-membrane process that produces acid and base from saline solutions using electricity. In previous research, this technology has predominantly been explored at the laboratory scale, with very few examples at the pilot scale. This study investigated, for the first time, how differential pressures applied between the EDBM channels affect its performance, utilizing a semi-industrial scale pilot - the largest ever studied in the literature. For this, inlet pressures from 0.5 to 1.5 barg were applied in the EDBM channels. Results were compared in terms of volume variation, product purities and key performance indicators, such as Current Efficiency (CE) and Specific Energy Consumption (SEC). Results indicate that changing the pressure between the channels induces a volumetric flow between compartments, which impacts the EDBM's performance. Specifically, the SEC ranged from 1.20 to 1.58 kWh kgNaOH-1, considering the energy required for both electricity and pumping at base concentration of ∼0.66 mol L-1. Notably, SEC values were 24% lower than the reference case study when operating with the identified best set of pressures. Under similar conditions, the CE varied between 64 % and 86 %, depending on the pressure applied between the channels. Moreover, using this set of pressures, acid and base product purities remained above 90%. This study advances pilot-scale EDBM process intensification, highlighting its potential for reduced energy consumption, increased sustainability, and industrial competitiveness.

Electrochemical water treatment: Review of different approaches

The continued development in agriculture, the rapid growth of industrialization, and last but not least, the increase in the global population adversely affects the environment. The availability of drinking water decreases every year with the rise in water pollution, which is the consequence of the failure of conventional approaches to the water treatment process. This review will provide a comprehensive and detailed analysis of the electrochemical water treatment processes, as these techniques have several benefits over conventional methods, such as being cost-effective, easily applicable, selective, and broad applicability. This review starts by discussing the traditional methods. It explains their limitations and finishes the introductory part by presenting all the benefits of the electrochemical method over the conventional method for water treatment. Then, the discussion will be carried out on the individual electrochemical method with their detailed analysis of the selected approach, selected material, and optimized parameters for analysis. The elaborative study was targeted, and the different coupled systems, their analysis parameters, and derived removal efficiencies were given in tabular form. In the last section of the article, the conclusive statements present the prospects of the electrochemical method for water treatment.

Using Layer-by-layer Assembled Clay Composite Junctions to Enhance the Water Dissociation in Bipolar Membranes

Bipolar membranes (BPMs) with a layer-by-layer (LbL) assembled montmorillonite (K30 MMT) clay-polyelectrolyte (PE) composite junction coated onto a sulfonated poly(ether ether ketone (SPEEK)) electrospun support are prepared, characterized and their water dissociation performance is analyzed. In particular, the focus is on the effect of the presence of the K30 MMT clay as a catalyst for water dissociation, the bilayer number (three, six, and nine), and the PE strength (poly(ethylenimine) (PEI) as a weak PE and poly(diallyl dimethylammonium chloride) (PDADMAC) as a strong PE) on the BPM performance. The BPMs are prepared by electrospinning and hot pressing SPEEK and the Fumion FAA-3 polymer. Adding the composite multilayers in the BPM junction decreases the membrane area resistance in reverse bias from 560 to 21 Ohms cm2 for the best-performing modified BPM. The bilayer number has limited influence on the overall membrane resistance, while the PDADMAC BPMs outperform the PEI BPMs due to the higher and more stable PE and clay adsorptions. Electrochemical impedance spectroscopy shows that the depletion layer thickness decreases exponentially with the number of bilayers as the water dissociation reaction becomes less dependent on the junction electric field. Furthermore, the higher Donnan exclusion at the modified junctions improves the BPM permselectivity 3-fold compared to the BPM containing no catalyst. Altogether, these improvements lead to 6.7 times less energy being used in BPM electrodialysis for the production of acid and base when a BPM with composite LBL junction is used compared to a BPM without catalyst. Thus, adding MMT clay composite LbL catalyst to BPM junctions is a promising method to improve the efficiency and reduce the energy consumption of electrochemical processes that rely on BPMs.

Tailoring high-performance bipolar membrane for durable pure water electrolysis

Bipolar membrane electrolyzers present an attractive scenario for concurrently optimizing the pH environment required for paired electrode reactions. However, the practicalization of bipolar membranes for water electrolysis has been hindered by their sluggish water dissociation kinetics, poor mass transport, and insufficient interface durability. This study starts with numerical simulations and discloses the limiting factors of monopolar membrane layer engineering. On this foundation, we tailor flexible bipolar membranes (10 ∼ 40 µm) comprising anion and cation exchange layers with an identical poly(terphenyl alkylene) polymeric skeleton. Rapid mass transfer properties and high compatibility of the monopolar membrane layers endow the bipolar membrane with appreciable water dissociation efficiency and long-term stability. Incorporating the bipolar membrane into a flow-cell electrolyzer enables an ampere-level pure water electrolysis with a total voltage of 2.68 V at 1000 mA cm-2, increasing the energy efficiency to twice that of the state-of-the-art commercial BPM. Furthermore, the bipolar membrane realizes a durability of 1000 h at high current densities of 300 ∼ 500 mA cm-2 with negligible performance decay.

Leveraging organic acids in bipolar membrane electrodialysis (BPMED) can enhance ammonia recovery from scrubber effluents

While air stripping combined with acid scrubbing remains a competitive technology for the removal and recovery of ammonia from wastewater streams, its use of strong acids is concerning. Organic acids offer promising alternatives to strong acids like sulphuric acid, but their application remains limited due to high cost. This study proposes an integration of air stripping and organic acid scrubbing with bipolar membrane electrodialysis (BPMED) to regenerate the organic acids. We compared the energy consumption and current efficiency of BPMED in recovering dissolved ammonia and regenerating sulphuric, citric, and maleic acids from synthetic scrubber effluents. Current efficiency was lower when regenerating sulphuric acid (22 %) compared to citric (47 %) and maleic acid (37 %), attributable to the competitive proton transport over ammonium across the cation exchange membrane. Organic salts functioned as buffers, reducing the concentration of free protons, resulting in higher ammonium removal efficiencies with citrate (75 %) and malate (68 %), compared to sulphate (29 %). Consequently, the energy consumption of the BPMED decreased by 54 % and 35 % while regenerating citric and maleic acids, respectively, compared to sulfuric acid. Membrane characterisation experiments showed that the electrical conductivity ranking, ammonium citrate > ammonium malate > ammonium sulphate, was mirrored by the energy consumption (kWh/kg-N recovered) ranking, ammonium sulphate (15.6) < ammonium malate (10.2) < ammonium citrate (7.2), while the permselectivity ranking, ammonium sulphate > ammonium citrate > ammonium malate, aligned with calculated charge densities. This work demonstrates the potential of combining organic acid scrubbers with BPMED for ammonium recovery from wastewater effluents with minimum chemical input.

Perspectives and State of the Art of Membrane Separation Technology as a Key Element in the Development of Hydrogen Economy

Due to the objectives established by the European Union and other countries, hydrogen production will be a key technology in the coming decades. There are several starting materials and procedures for its production. All methods have advantages and disadvantages, and the improvements in their performance and decreases in operational costs will be decisive in determining which of them is implemented. For all cases, including for the storage and transport of hydrogen, membranes determine the performance of the process, as well as the operational costs. The present contribution summarizes the most recent membrane technologies for the main methods of hydrogen production, including the challenges to overcome in each case.

Bipolar Membranes Via Divergent Synthesis: On the Interplay between Ion Exchange Capacity and Water Dissociation Catalysis

Bipolar membranes (BPMs) enable the operation of electrochemical reactors with electrode compartments in different chemical environments or pH. The transport properties at the microscopic scale are dictated by the composition and morphology of the interfacial junctions as well as the specific chemistry of the ion-exchange layers that support the current of protons and hydroxide ions. This work elucidates the relation between water-dissociation efficiency and the physicochemical properties of the individual ion-exchange membrane layers in the poly(styrene-b-poly(ethylene-ran-butylene)-b-polystyrene) (SEBS)-based BPM. The optimal water dissociation performance of three previously reported water-dissociation catalysts in the SEBS-based BPM was examined, with junction thickness of graphene oxide > TiO2 > SnO2, resulting in disparate junction morphologies at the BPM's interface. A hybrid junction system, which included both the effective water dissociation catalyst SnO2 and direct contacting of the ion-exchange membrane layer, exhibited high water dissociation efficiency. This was likely due to the immediate ion transport pathway provided by direct membrane contact around the catalyst, which also improved the interfacial adhesion. A higher ion exchange capacity (IEC) in BPMs substantially enhanced the water dissociation performance in BPMs without water-dissociation catalysts. However, the incorporation of the effective SnO2 catalyst into the BPMs with a lower IEC significantly improved performance, an effect attributed to the hybrid junction system. Additionally, the increase in water uptake and ion conductivity of the cation exchange layer with higher IEC suggested that the cation exchange layer and its interface to the water-dissociation catalyst layer may play a key role in water dissociation. This study identifies the key parameters of individual BPM components and their interactions to water dissociation performance, offering new insights to guide in the construction of future BPMs optimized for enhanced water dissociation efficiency at high current densities.

Toward a Circular Lithium Economy with Electrodialysis: Upcycling Spent Battery Leachates with Selective and Bipolar Ion-Exchange Membranes

Recycling spent lithium-ion batteries offers a sustainable solution to reduce ecological degradation from mining and mitigate raw material shortages and price volatility. This study investigates using electrodialysis with selective and bipolar ion-exchange membranes to establish a circular economy for lithium-ion batteries. An experimental data set of over 1700 ion concentration measurements across five current densities, two solution compositions, and three pH levels supports the techno-economic analysis. Selective electrodialysis (SED) isolates lithium ions from battery leachates, yielding a 99% Li-pure retentate with 68.8% lithium retention, achieving relative ionic fluxes up to 2.41 for Li+ over transition metal cations and a selectivity of 5.64 over monovalent cations. Bipolar membrane electrodialysis (BMED) converts LiCl into high-purity LiOH and HCl, essential for battery remanufacturing and reducing acid consumption via acid recycling. High current densities reduce ion leakage, achieving lithium leakage as low as 0.03%, though hydronium and hydroxide leakage in BMED remains high at 11-20%. Our analysis projects LiOH production costs between USD 1.1 and 3.6 per kilogram, significantly lower than current prices. Optimal SED and BMED conditions are identified, emphasizing the need to control proton transport in BMED and improve cobalt-lithium separation in SED to enhance cost efficiency.

Degradation of a Bipolar Membrane in a Hybrid Acid/Alkali Electrolyzer Studied by X-ray Computed Tomography

The use of a bipolar membrane (BPM) in a hybrid acid/alkali electrolyzer is widely considered as a promising energy technology for efficient hydrogen production. The stability of a BPM is often believed to be largely limited by the anion exchange layer (AEL) due to the hydrophilic attack of AEL polymers by hydroxide groups in alkaline. In this study, we employ X-ray computed tomography (CT) to investigate the degradation behaviors of BPM and found that the cation exchange layer (CEL) experiences more pronounced degradation compared with the AEL during water splitting operations. Despite its chemical stability in both acidic and alkaline environments, the CEL is more prone to electrochemical corrosion under the influence of applied voltages. This susceptibility leads to the formation of micropores and a consequent increase in the porosity. The results of this work provide a new perspective on and highlight the complexity of the degradation behaviors of BPMs in hybrid acid/alkali electrolyzers.

Advanced electrochemical membrane technologies for near-complete resource recovery and zero-discharge of urine: Performance optimization and evaluation

The depletion of nutrient sources in fertilizers demands a paradigm shift in the treatment of nutrient-rich wastewater, such as urine, to enable efficient resource recovery and high-value conversion. This study presented an integrated bipolar membrane electrodialysis (BMED) and hollow fiber membrane (HFM) system for near-complete resource recovery and zero-discharge from urine treatment. Computational simulations and experimental validations demonstrated that a higher voltage (20 V) significantly enhanced energy utilization, while an optimal flow rate of 0.4 L/min effectively mitigated the negative effects of concentration polarization and electro-osmosis on system performance. Within 40 min, the process separated 90.13% of the salts in urine, with an energy consumption of only 8.45 kWh/kgbase. Utilizing a multi-chamber structure for selective separation, the system achieved recovery efficiencies of 89% for nitrogen, 96% for phosphorus, and 95% for potassium from fresh urine, converting them into high-value products such as 85 mM acid, 69.5 mM base, and liquid fertilizer. According to techno-economic analysis, the cost of treating urine using this system at the lab-scale was $6.29/kg of products (including acid, base, and (NH4)2SO4), which was significantly lower than the $20.44/kg cost for the precipitation method to produce struvite. Excluding fixed costs, a net profit of $18.24/m3 was achieved through the recovery of valuable products from urine using this system. The pilot-scale assessment showed that the net benefit amounts to $19.90/m3 of urine, demonstrating significant economic feasibility. This study presents an effective approach for the near-complete resource recovery and zero-discharge treatment of urine, offering a practical solution for sustainable nutrient recycling and wastewater management.

Materials descriptors for advanced water dissociation catalysts in bipolar membranes

The voltage penalty driving water dissociation (WD) at high current density is a major obstacle in the commercialization of bipolar membrane (BPM) technology for energy devices. Here we show that three materials descriptors, that is, electrical conductivity, microscopic surface area and (nominal) surface-hydroxyl coverage, effectively control the kinetics of WD in BPMs. Using these descriptors and optimizing mass loading, we design new earth-abundant WD catalysts based on nanoparticle SnO2 synthesized at low temperature with high conductivity and hydroxyl coverage. These catalysts exhibit exceptional performance in a BPM electrolyser with low WD overvoltage (ηwd) of 100 ± 20 mV at 1.0 A cm-2. The new catalyst works equivalently well with hydrocarbon proton-exchange layers as it does with fluorocarbon-based Nafion, thus providing pathways to commercializing advanced BPMs for a broad array of electrolysis, fuel-cell and electrodialysis applications.

Local ionic transport enables selective PGM-free bipolar membrane electrode assembly

Bipolar membranes in electrochemical CO2 conversion cells enable different reaction environments in the CO2-reduction and O2-evolution compartments. Under ideal conditions, water-splitting in the bipolar membrane allows for platinum-group-metal-free anode materials and high CO2 utilizations. In practice, however, even minor unwanted ion crossover limits stability to short time periods. Here we report the vital role of managing ionic species to improve CO2 conversion efficiency while preventing acidification of the anodic compartment. Through transport modelling, we identify that an anion-exchange ionomer in the catalyst layer improves local bicarbonate availability and increasing the proton transference number in the bipolar membranes increases CO2 regeneration and limits K+ concentration in the cathode region. Through experiments, we show that a uniform local distribution of bicarbonate ions increases the accessibility of reverted CO2 to the catalyst surface, improving Faradaic efficiency and limiting current densities by twofold. Using these insights, we demonstrate a fully platinum-group-metal-free bipolar membrane electrode assembly CO2 conversion system exhibiting <1% CO2/cation crossover rates and 80-90% CO2-to-CO utilization efficiency over 150 h operation at 100 mA cm-2 without anolyte replenishment.

Recycling aluminum from polyaluminum chloride sludge through acid dissolution and cation resin separation/purification

To recycle aluminum (Al) from waterworks sludge resulting from polyaluminum chloride (PAC) used as coagulants, this study proposed an innovative strong acidic cation (SAC) exchange resin treatment strategy for Al separation from coexisting fulvic acid (FA) and heavy metals (HMs) in the H2SO4 leachate of PAC sludge. Fluorescence titration confirmed the breakdown of the Al-FA complex at pH 2.0, which facilitated Al separation from FA in the acidic leachate. The species distribution of the dissociated Al (i.e. Ala, Alb, and Alc) significantly influenced the adsorption of Al onto the cation exchange resin. The continuous release of H+ during the cation exchange reaction greatly promoted the transformation of dissociated Alc and Alb into Ala, thereby improving the adsorption of total Al. Moreover, the SAC resin column successfully separated the codissolved HMs from the Al in the leachate even at an influent pH of 2.8, which was attributed to the greater selectivity of the sulfonate groups on the cation exchange resin for free Al3+. The Al eluted from the exhausted resin with 1.1 M H2SO4 was collected as the recycled coagulant after proper pH adjustment. The Al adsorption capacity of the SAC resin decreased by approximately 5 % with each operation cycle and was regained by complete regeneration with 1.8 M H2SO4 after 5 cycles. Overall, the integrated efficiency of Al recovery from PAC sludge by H2SO4 acidification and SAC resin separation/purification reached 70.10 %. The recycled Al from sludge has a water treatment performance comparable to that of fresh PAC coagulant.

Potential of Progressive and Disruptive Innovation-Driven Cost Reductions of Green Hydrogen Production

Green hydrogen from water electrolysis is a key driver for energy and industrial decarbonization. The prediction of the future green hydrogen cost reduction is required for investment and policy-making purposes but is complicated due to a lack of data, incomplete accounting for costs, and difficulty justifying trend predictions. A new AI-assisted data-driven prediction model is developed for an in-depth analysis of the current and future levelized costs of green hydrogen, driven by both progressive and disruptive innovations. The model uses natural language processing to gather data and generate trends for the technological development of key aspects of electrolyzer technology. Through an uncertainty analysis, green hydrogen costs have been shown to likely reach the key target of <$2.5 kg-1 by 2030 via progressive innovations, and beyond this point, disruptive technological developments are required to affect significantly further decease cost. Additionally, the global distribution of green hydrogen costs has been calculated. This work creates a comprehensive analysis of the levelized cost of green hydrogen, including the important balance of plant components, both now and as electrolyzer technology develops, and offers a likely prediction for how the costs will develop over time.

Advancements in Bipolar Membrane Electrodialysis Techniques for Carbon Capture

Bipolar membrane electrodialysis (BMED) is a promising technology for the capture of carbon dioxide (CO2) from seawater, offering a sustainable solution to combat climate change. BMED efficiently extracts CO2 while generating valuable byproducts like hydrogen and minerals, contributing to the carbon cycle. The technology relies on ion-exchange membranes and electric fields for efficient ion separation and concentration. Recent advancements focus on enhancing water dissociation in bipolar membranes (BPMs) to improve efficiency and durability. BMED has applications in desalination, electrodialysis, water splitting, acid/base production, and CO2 capture and utilization. Despite the high efficiency, scalability, and environmental friendliness, challenges such as energy consumption and membrane costs exist. Recent innovations include novel BPM designs, catalyst integration, and exploring direct air/ocean capture. Research and development efforts are crucial to unlocking BMED's full potential in reducing carbon emissions and addressing environmental issues. This review provides a comprehensive overview of recent advancements in BMED, emphasizing its role in carbon capture and sustainable environmental solutions.

Identification of Fouling Occurring during Coupled Electrodialysis and Bipolar Membrane Electrodialysis Treatment for Tofu Whey Protein Recovery

Tofu whey, a by-product of tofu production, is rich in nutrients such as proteins, minerals, fats, sugars and polyphenols. In a previous work, protein recovery from tofu whey was studied by using a coupled environmental process of ED + EDBM to valorize this by-product. This process allowed protein recovery by reducing the ionic strength of tofu whey during the ED process and acidifying the proteins to their isoelectric point during EDBM. However, membrane fouling was not investigated. The current study focuses on the fouling of membranes at each step of this ED and EDBM process. Despite a reduction in the membrane conductivities and some changes in the mineral composition of the membranes, no scaling was evident after three runs of the process with the same membranes. However, it appeared that the main fouling was due to the presence of isoflavones, the main polyphenols in tofu whey. Indeed, a higher concentration was observed on the AEMs, giving them a yellow coloration, while small amounts were found in the CEMs, and there were no traces on the BPMs. The glycosylated forms of isoflavones were present in higher concentrations than the aglycone forms, probably due to their high amounts of hydroxyl groups, which can interact with the membrane matrices. In addition, the higher concentration of isoflavones on the AEMs seems to be due to a combination of electrostatic interactions, hydrogen bonding, and π-π stacking, whereas only π-π stacking and hydrogen bonds were possible with the CEMs. To the best of our knowledge, this is the first study to investigate the potential fouling of BPMs by polyphenols, report the fouling of IEMs by isoflavones and propose potential interactions.

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.

Advanced membrane-based electrode engineering toward efficient and durable water electrolysis and cost-effective seawater electrolysis in membrane electrolyzers

Researchers have been seeking for the most technically-economical water electrolysis technology for entering the next-stage of industrial amplification for large-scale green hydrogen production. Various membrane-based electrolyzers have been developed to improve electric-efficiency, reduce the use of precious metals, enhance stability, and possibly realize direct seawater electrolysis. While electrode engineering is the key to approaching these goals by bridging the gap between catalysts design and electrolyzers development, nevertheless, as an emerging field, has not yet been systematically analyzed. Herein, this review is organized to comprehensively discuss the recent progresses of electrode engineering that have been made toward advanced membrane-based electrolyzers. For the commercialized or near-commercialized membrane electrolyzer technologies, the electrode material design principles are interpreted and the interface engineering that have been put forward to improve catalytic sites utilization and reduce precious metal loading is summarized. Given the pressing issues of electrolyzer cost reduction and efficiency improvement, the electrode structure engineering toward applying precious metal free electrocatalysts is highlighted and sufficient accessible sites within the thick catalyst layers with rational electrode architectures and effective ions/mass transport interfaces are enabled. In addition, this review also discusses the innovative ways as proposed to break the barriers of current membrane electrolyzers, including the adjustments of electrode reaction environment, and the feasible cell-voltage-breakdown strategies for durable direct seawater electrolysis. Hopefully, this review may provide insightful information of membrane-based electrode engineering and inspire the future development of advanced membrane electrolyzer technologies for cost-effective green hydrogen production.

Membrane-based technology in water and resources recovery from the perspective of water social circulation: A review

In this review, the application of membrane-based technology in water social circulation was summarized. Water social circulation encompassed the entire process from the acquirement to discharge of water from natural environment for human living and development. The focus of this review was primarily on the membrane-based technology in recovery of water and other valuable resources such as mineral ions, nitrogen and phosphorus. The main text was divided into four main sections according to water flow in the social circulation: drinking water treatment, agricultural utilization, industrial waste recycling, and urban wastewater reuse. In drinking water treatment, the acquirement of water resources was of the most importance. Pressure-driven membranes, such as ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) were considered suitable in natural surface water treatment. Additionally, electrodialysis (ED) and membrane capacitive deionization (MCDI) were also effective in brackish water desalination. Agriculture required abundant water with relative low quality for irrigation. Therefore, the recovery of water from other stages of the social circulation has become a reasonable solution. Membrane bioreactor (MBR) was a typical technique attributed to low-toxicity effluent. In industrial waste reuse, the osmosis membranes (FO and PRO) were utilized due to the complex physical and chemical properties of industrial wastewater. Especially, membrane distillation (MD) might be promising when the wastewater was preheated. Resources recovery in urban wastewater was mainly divided into recovery of bioenergy (via anaerobic membrane bioreactors, AnMBR), nitrogen (utilizing MD and gas-permeable membrane), and phosphorus (through MBR with chemical precipitation). Furthermore, hybrid/integrated systems with membranes as the core component enhanced their performance and long-term working ability in utilization. Generally, concentrate management and energy consumption control might be the key areas for future advancements of membrane-based technology.

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.

An Overview of Microbial Fuel Cell Technology for Sustainable Electricity Production

The over-exploitation of fossil fuels and their negative environmental impacts have attracted the attention of researchers worldwide, and efforts have been made to propose alternatives for the production of sustainable and clean energy. One proposed alternative is the implementation of bioelectrochemical systems (BESs), such as microbial fuel cells (MFCs), which are sustainable and environmentally friendly. MFCs are devices that use bacterial activity to break down organic matter while generating sustainable electricity. Furthermore, MFCs can produce bioelectricity from various substrates, including domestic wastewater (DWW), municipal wastewater (MWW), and potato and fruit wastes, reducing environmental contamination and decreasing energy consumption and treatment costs. This review focuses on recent advancements regarding the design, configuration, and operation mode of MFCs, as well as their capacity to produce bioelectricity (e.g., 2203 mW/m2) and fuels (i.e., H2: 438.7 mg/L and CH4: 358.7 mg/L). Furthermore, this review highlights practical applications, challenges, and the life-cycle assessment (LCA) of MFCs. Despite the promising biotechnological development of MFCs, great efforts should be made to implement them in a real-time and commercially viable manner.

Logic gating of low-abundance molecules using polyelectrolyte-based diodes

Bioinspired artificial ionic components are extensively utilized to mimic biological systems, as the vast majority of biological signaling is mediated by ions and molecules. Particular attention is given to nanoscale fluidic components where the ion transport can be regulated by the induced ion permselectivity. As a step from fundamentals toward ion-controlled devices, this study presents the use of ionic diodes made of oppositely charged polyelectrolytes, as a gate for low-abundance molecules. The use of ionic diodes that exhibited nonlinear current-voltage responses enabled realization of a basic Boolean operation of an ionic OR logic gate. Aside from the electrical response, the asymmetric ion transport through the diode was shown to affect the transport of low-abundance molecules across the diode, only allowing crossing when the diode was forward-biased. Integration of multiple diodes enabled implementation of an OR logic operation on both the voltage and the molecule transport, while obtaining electrical and optical output readouts that were associated with low and high logic levels. Similarly to electronics, implementation of logic gates opens up new functionalities of on-chip ionic computation via integrated circuits consisting of multiple basic logic gates.

Multiscale CO2 Electrocatalysis to C2+ Products: Reaction Mechanisms, Catalyst Design, and Device Fabrication

Electrosynthesis of value-added chemicals, directly from CO2, could foster achievement of carbon neutral through an alternative electrical approach to the energy-intensive thermochemical industry for carbon utilization. Progress in this area, based on electrogeneration of multicarbon products through CO2 electroreduction, however, lags far behind that for C1 products. Reaction routes are complicated and kinetics are slow with scale up to the high levels required for commercialization, posing significant problems. In this review, we identify and summarize state-of-art progress in multicarbon synthesis with a multiscale perspective and discuss current hurdles to be resolved for multicarbon generation from CO2 reduction including atomistic mechanisms, nanoscale electrocatalysts, microscale electrodes, and macroscale electrolyzers with guidelines for future research. The review ends with a cross-scale perspective that links discrepancies between different approaches with extensions to performance and stability issues that arise from extensions to an industrial environment.

Bipolar Membrane Electrodialysis for Direct Conversion of L-Ornithine Monohydrochloride to L-Ornithine

In this study, bipolar membrane electrodialysis was proposed to directly convert L-ornithine monohydrochloride to L-ornithine. The stack configuration was optimized in the BP-A (BP, bipolar membrane; A, anion exchange membrane) configuration with the Cl- ion migration through the anion exchange membrane rather than the BP-A-C (C, cation exchange membrane) and the BP-C configurations with the L-ornithine+ ion migration through the cation exchange membrane. Both the conversion ratio and current efficiency follow BP-A > BP-A-C > BP-C, and the energy consumption follows BP-A < BP-A-C < BP-C. Additionally, the voltage drop across the membrane stack (two repeating units) and the feed concentration were optimized as 7.5 V and 0.50 mol/L, respectively, due to the low value of the sum of H+ ions leakage (from the acid compartment to the base compartment) and OH- ions migration (from the base compartment to the acid compartment) through the anion exchange membrane. As a result, high conversion ratio (96.1%), high current efficiency (95.5%) and low energy consumption (0.31 kWh/kg L-ornithine) can be achieved. Therefore, bipolar membrane electrodialysis is an efficient, low energy consumption and environmentally friendly method to directly convert L-ornithine monohydrochloride to L-ornithine.

Bipolar Membranes for Direct Borohydride Fuel Cells-A Review

Direct liquid fuel cells (DLFCs) operate directly on liquid fuel instead of hydrogen, as in proton-exchange membrane fuel cells. DLFCs have the advantages of higher energy densities and fewer issues with the transportation and storage of their fuels compared with compressed hydrogen and are adapted to mobile applications. Among DLFCs, the direct borohydride-hydrogen peroxide fuel cell (DBPFC) is one of the most promising liquid fuel cell technologies. DBPFCs are fed sodium borohydride (NaBH4) as the fuel and hydrogen peroxide (H2O2) as the oxidant. Introducing H2O2 as the oxidant brings further advantages to DBPFC regarding higher theoretical cell voltage (3.01 V) than typical direct borohydride fuel cells operating on oxygen (1.64 V). The present review examines different membrane types for use in borohydride fuel cells, particularly emphasizing the importance of using bipolar membranes (BPMs). The combination of a cation-exchange membrane (CEM) and anion-exchange membrane (AEM) in the structure of BPMs makes them ideal for DBPFCs. BPMs maintain the required pH gradient between the alkaline NaBH4 anolyte and the acidic H2O2 catholyte, efficiently preventing the crossover of the involved species. This review highlights the vast potential application of BPMs and the need for ongoing research and development in DBPFCs. This will allow for fully realizing the significance of BPMs and their potential application, as there is still not enough published research in the field.

Janus membranes at the water-energy nexus: A critical review

Membrane technology has emerged as a highly efficient strategy for alleviating water and energy scarcity globally. As the key component, the membrane plays a fatal role in different membrane systems; however, traditional membranes still suffer from shortcomings including low permeability, low selectivity, and high fouling tendency. Janus membranes are promising to overcome those shortcomings and appealing for applications in the realm of water-energy nexus, due to their special transport behaviors and separation properties as a result of their unique asymmetric wetting or surface charge properties. Recently, numerous research studies have been conducted on the design, fabrication, and application of Janus membranes. In this review, we aim to provide a state-of-the-art summary and a critical discussion on the research advances of Janus membranes at the water-energy nexus. The innovative design strategies of different types of Janus membranes are summarized and elucidated in detail. The fundamental working principles of various Janus membranes and their applications in oil/water separation, membrane distillation, solar evaporation, electrodialysis, nanofiltration, and forward osmosis are discussed systematically. The mechanisms of directional transport properties, switchable permeability, and superior separation properties of Janus membranes in those different applications are elucidated. Lastly, future research directions and challenges are highlighted in improving Janus membrane performance for various membrane systems.

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.

Phosphates Transfer in Pristine and Modified CJMA-2 Membrane during Electrodialysis Processing of NaxH(3-x)PO4 Solutions with pH from 4.5 to 9.9

Phosphate recovery from different second streams using electrodialysis (ED) is a promising step to a nutrients circular economy. However, the relatively low ED performance hinders the widespread adoption of this environmentally sound method. The formation of "bonded species" between phosphates and the weakly basic fixed groups (primary and secondary amines) of the anion exchange membrane can be the cause of decrease in current efficiency and increase in energy consumption. ED processing of Na_xH_(3-x)PO₄ alkaline solutions and the use of intense current modes promote the formation of a bipolar junction from negatively charged bound species and positively charged fixed groups. This phenomenon causes a change in the shape of current-voltage curves, increase in resistance, and an enhancement in proton generation during long-term operation of anion-exchange membrane with weakly basic fixed groups. Shielding of primary and secondary amines with a modifier containing quaternary ammonium bases significantly improves ED performance in the recovery of phosphates from Na_xH_(3-x)PO₄ solution with pH 4.5. Indeed, in the limiting and underlimiting current modes, 40% of phosphates are recovered 1.3 times faster, and energy consumption is reduced by 1.9 times in the case of the modified membrane compared to the pristine one. Studies were performed using a new commercial anion exchange membrane CJMA-2.

High-rate and selective conversion of CO2 from aqueous solutions to hydrocarbons

Electrochemical carbon dioxide (CO₂) conversion to hydrocarbon fuels, such as methane (CH₄), offers a promising solution for the long-term and large-scale storage of renewable electricity. To enable this technology, CO₂-to-CH₄ conversion must achieve high selectivity and energy efficiency at high currents. Here, we report an electrochemical conversion system that features proton-bicarbonate-CO₂ mass transport management coupled with an in-situ copper (Cu) activation strategy to achieve high CH₄ selectivity at high currents. We find that open matrix Cu electrodes sustain sufficient local CO₂ concentration by combining both dissolved CO₂ and in-situ generated CO₂ from the bicarbonate. In-situ Cu activation through alternating current operation renders and maintains the catalyst highly selective towards CH₄. The combination of these strategies leads to CH₄ Faradaic efficiencies of over 70% in a wide current density range (100 - 750 mA cm^-2) that is stable for at least 12 h at a current density of 500 mA cm^-2. The system also delivers a CH₄ concentration of 23.5% in the gas product stream.

How Chemical Nature of Fixed Groups of Anion-Exchange Membranes Affects the Performance of Electrodialysis of Phosphate-Containing Solutions?

Innovative ion exchange membranes have become commercially available in recent years. However, information about their structural and transport characteristics is often extremely insufficient. To address this issue, homogeneous anion exchange membranes with the trade names ASE, CJMA-3 and CJMA-6 have been investigated in Na_xH_(3-x)PO₄ solutions with pH 4.4 ± 0.1, 6.6 and 10.0 ± 0.2, as well as NaCl solutions with pH 5.5 ± 0.1. Using IR spectroscopy and processing the concentration dependences of the electrical conductivity of these membranes in NaCl solutions, it was shown that ASE has a highly cross-linked aromatic matrix and mainly contains quaternary ammonium groups. Other membranes have a less cross-linked aliphatic matrix based on polyvinylidene fluoride (CJMA-3) or polyolefin (CJMA-6) and contain quaternary amines (CJMA-3) or a mixture of strongly basic (quaternary) and weakly basic (secondary) amines (CJMA-6). As expected, in dilute solutions of NaCl, the conductivity of membranes increases with an increase in their ion-exchange capacity: CJMA-6 < CJMA-3 << ASE. Weakly basic amines appear to form bound species with proton-containing phosphoric acid anions. This phenomenon causes a decrease in the electrical conductivity of CJMA-6 membranes compared to other studied membranes in phosphate-containing solutions. In addition, the formation of the neutral and negatively charged bound species suppresses the generation of protons by the "acid dissociation" mechanism. Moreover, when the membrane is operated in overlimiting current modes and/or in alkaline solutions, a bipolar junction is formed at the CJMA- 6/depleted solution interface. The CJMA-6 current-voltage curve becomes similar to the well-known curves for bipolar membranes, and water splitting intensifies in underlimiting and overlimiting modes. As a result, energy consumption for electrodialysis recovery of phosphates from aqueous solutions almost doubles when using the CJMA-6 membrane compared to the CJMA-3 membrane.

Nickel recovery from electroplating sludge via bipolar membrane electrodialysis

In this study, nickel (Ni) was recovered from electroplating sludge in the form of Ni(OH)₂ using a bipolar membrane electrodialysis (BMED) system. The results showed that the H⁺ generated by the bipolar membrane could effectively desorb Ni from the sludge to the solution and the solution pH considerably affected Ni desorption. The desorption process can be described using the first-order kinetic model. The current density and solid/liquid ratio (m/v) considerably affected Ni recovery. Moreover, 100% of Ni was removed from the electroplating sludge and 93.5% of Ni was recovered after 28 h under a current density of 20 mA/cm², a solid/liquid ratio of 1.0:15 and an electroplating-sludge particle size of 100 mesh. As the number of electroplating compartments increased from one to two and three, the current efficiency for recovering Ni changed from 12.1% to 11.8% and 11.9%, respectively, and the specific energy consumption decreased from 0.064 to 0.048 and 0.039 kW·h/g, respectively. Fourier-transform infrared spectroscopy and Raman spectroscopy showed that the precipitate obtained in this study is similar to commercial Ni(OH)₂ and the purity of Ni(OH)₂ in the obtained precipitate was 79%. Thus, the results showed that the BMED system is effective for recovering Ni from electroplating sludge.

Understanding the Impact of the Three-Dimensional Junction Thickness of Electrospun Bipolar Membranes on Electrochemical Performance

The use of electrospun bipolar membranes (BPMs) with an interfacial three-dimensional (3D) junction of entangled nano-/microfibers has been recently proposed as a promising fabrication strategy to develop high-performance BPMs. In these BPMs, the morphology and physical properties of the 3D junction are of utmost importance to maximize the membrane performance. However, a full understanding of the impact of the junction thickness on the membrane performance is still lacking. In this study, we have developed bipolar membranes with the same composition, only varying the 3D junction thicknesses, by regulating the electrospinning time used to deposit the nano-/microfibers at the junction. In total, four BPMs with 3D junction thicknesses of ∼4, 8, 17, and 35 μm were produced to examine the influence of the junction thickness on the membrane performance. Current-voltage curves for water dissociation of BPMs exhibited lower voltages for BPMs with thicker 3D junctions, as a result of a three-dimensional increase in the interfacial contact area between cation- and anion-exchange fibers and thus a larger water dissociation reaction area. Indeed, increasing the BPM thickness from 4 to 35 μm lowered the BPM water dissociation overpotential by 32%, with a current efficiency toward HCl/NaOH generation higher than 90%. Finally, comparing BPM performance during the water association operation revealed a substantial reduction in the voltage from levels of its supplied open circuit voltage (OCV), owing to excessive hydroxide ion (OH-) and proton (H+) leakage through the relevant layers. Overall, this work provides insights into the role of the junction thickness on electrospun BPM performance as a crucial step toward the development of membranes with optimal entangled junctions.

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.

Recovery of copper from electroplating sludge using integrated bipolar membrane electrodialysis and electrodeposition

Electroplating sludge, though a hazardous waste, is a valuable resource as it contains a large amount of precious metals. In this study, copper was recovered from the electroplating sludge using a technology that integrates bipolar membrane electrodialysis (BMED) and electrodeposition. The experimental results showed that Cu²⁺ in the electroplating sludge was successfully separated and concentrated in the BMED system without adding any chemical reagents; the concentrated Cu²⁺ was recovered in the form of copper foil in an electrodeposition system. Current density clearly affected the Cu²⁺ separation and concentration in the BMED system; the current density, solution pH and Cu²⁺ concentration drastically affected the Cu²⁺ electrodeposition ratio and the morphology and purity of the obtained copper foil. Under the optimised experimental conditions, 96.4% of Cu²⁺ was removed from the electroplating sludge and 65.4% of Cu²⁺ was recovered in the foil form. On increasing the number of electroplating sludge compartments from one to two and three, the current efficiency for recovering Cu²⁺ increased from 17.4% to 28.5% and 35.2%, respectively, and the specific energy consumption decreased from 11.3 to 6.7 and 5.3 kW h/kg of copper, respectively. The purity of the copper foil was higher than 99.5%. Thus, the integrated technology can be regarded as an effective method for recovering copper from electroplating sludge.