Cation Exchange Layer Research Articles

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.

Ultra-antiwetting Membrane for Hypersaline Water Crystallization in Membrane Distillation.

Membrane distillation (MD) has great potential in the management of hypersaline water for zero liquid discharge (ZLD) due to its high salinity tolerance. However, the membrane wetting issue significantly restricts its practical application. In this study, a composite membrane tailored for extreme concentrations and even crystallization of hypersaline water is synthesized by coating a commercial hydrophobic porous membrane with a composite film containing a dense polyamide layer, a cation exchange layer (CEL), and an anion exchange layer (AEL). When used in direct contact MD for treating a 100 g L-1 NaCl hypersaline solution, the membrane achieves supersaturation of feed solution and a salt crystal yield of 38.0%, with the permeate concentration at <5 mg L-1. The composite membrane also demonstrates ultrahigh antiwetting stability in 360 h of long-term operation. Moreover, ion diffusion analysis reveals that the ultrahigh wetting resistance of the composite membrane is attributed to the bipolar AEL and CEL that eliminate ion crossover. The literature review elucidates that the composite membrane is superior to state-of-the-art membranes. This study demonstrates the great potential of the composite membrane for direct crystallization of hypersaline water, offering a promising approach to filling the gap between reverse osmosis and conventional thermal desalination processes for ZLD application.

Development of High-Performance Bipolar Membranes with Optimal Junction Properties for Efficient Electro-Membrane Processes

A bipolar membrane (BPM) is a special type of ion-exchange membrane in which an anion-exchange layer (AEL) and a cation-exchange layer (CEL) are combined. Water molecules can be easily dissociated at the interface of the BPM by the strong electric field generated when a reverse bias voltage is applied through the membrane. Such dissociation leads to the generation of proton and hydroxide ions, which are then transported through the CEL and AEL, respectively, resulting in the production of acid and base solutions. Under a forward bias, these ions are drawn back to the bipolar interface, where they neutralize each other. The energy derived from this neutralization process, represented as a junction potential (JP) value, is a key factor in the BPM's efficiency. BPMs with ideal JP values are critical for achieving high separation and energy efficiencies in electro-membrane processes. Furthermore, for the prolonged operation of these processes, BPMs must possess a highly adhesive bipolar junction. Our work includes the development of a novel BPM with an ideal JP and a highly adhesive bipolar junction. This BPM has been applied to various electro-membrane processes, including an acid-base flow battery and direct seawater electrolysis. The BPMs are fabricated using poly(phenylene oxide) (PPO)-based ionomers, and incorporates an iron-based catalyst and a reinforcing material. These additions enhance the water-splitting performance and the durability of the interface between the ion-exchange layers. The prepared BPMs not only demonstrated excellent interfacial adhesiveness and mechanical properties but also achieved a water-splitting efficiency of 98% or higher. In comparison to commercial BPM, the electro-membrane processes utilizing the prepared BPM exhibited superior performance. This work was supported by the National Research Foundation of Korea (NRF) grants funded by the Korean government (MEST) (NRF-2022M3C1A3081178 & NRF-2022M3H4A4097521).

Bipolar Membranes for Photoelectrochemical CO2 Conversion

CO2 capture and conversion represent one of the most critical challenges of our society. One sustainable approach to tackle it is the development of electrochemical processes coupling a solar-driven carbon dioxide reduction to CO/water oxidation to O2 with reactions leading tohigh-value products. In such photoelectrochemical reactors, (photo)electrocatalysts and membranes play a crucial role to achieve high CO2 conversion with minimum energy.The aim of this study is developing polymer membranes with high stability, mechanical resistance and ion conductivity, that can work in the broad pH operation range of CO2/H2O co-electrolysis cells. Consistently with the use of KOH at the anode, the used membrane is often an anion exchange membrane (AEM). However, carbonate crossover through AEM occurs during CO2 electrolysis via diffusion and migration. [1] The use of a bipolar membrane (BPM) allows to mitigate this issue, bringing novel challenges of stability and performance.The BPM consists in a cation exchange layer (CEL) and an anion exchange layer (AEL), with a bipolar junction formed at their interface. Such membrane must guarantee a constant pH level generating H+ and OH- by water dissociation at the junction, avoid product crossover, and optimize mass transfer and reactivity of coupled anodic and cathodic reactions. A crucial point is the control of the electrochemical behavior at the CEL/AEL interface. With this aim, a 3D-structure made of a network of fibers was developed [2, 3] providing a great interfacial area and mechanical interlocking of the anion and cation exchange ionomers preventing delamination and facilitating the exchange of ions and matter at the interface. Furthermore, such fiber webs allow the controlled deposition of inorganic or organic catalysts to enhance the water dissociation rate at the bipolar junction.The bipolar membrane was prepared interposing a web of electrospun fibers bearing water dissociation nanocatalysts at the interface between anion and cation exchange polymer layers. The prepared BPMs were characterized for their morphology, composition, ion conductivity, and performance.This work has been performed with the financial support of the European Union’s Horizon 2020 Research and Innovation Action Program under the project SunCoChem (Grant agreement No 862192).

Trilayer Polymer Electrolytes Enable Carbon-Efficient CO2 to Multicarbon Product Conversion in Alkaline Electrolyzers.

The electrochemical CO2 reduction reaction (CO2RR) is an appealing method for carbon utilization. Alkaline CO2 electrolyzers exhibit high CO2RR activity, low full-cell voltages, and cost-effectiveness. However, the issue of CO2 loss caused by (bi)carbonate formation leads to excessive energy consumption, rendering the process economically impractical. In this study, we propose a trilayer polymer electrolyte (TPE) comprising a perforated anion exchange membrane (PAEM) and a bipolar membrane (BPM) to facilitate alkaline CO2RR. This TPE enables the coexistence of high alkalinity near the catalyst surface and the H+ flux at the interface between the PAEM and the cation exchange layer (CEL) of the BPM, conditions favoring both CO2 reduction to multicarbon products and (bi)carbonate removal in KOH-fed membrane electrode assembly (MEA) reactors. As a result, we achieve a Faradaic efficiency (FE) of approximately 46 % for C2H4, corresponding to a C2+ FE of 64 % at 260 mA cm-2, with a CO2-to-C2H4 single-pass conversion (SPC) of approximately 32 % at 140 mA cm-2-nearly 1.3 times the limiting SPC in conventional AEM-MEA electrolyzers. Furthermore, coupling CO2 reduction with formaldehyde oxidation reaction (FOR) in the TPE-MEA electrolyzer reduces the full-cell voltage to 2.3 V at 100 mA cm-2 without compromising the C2H4 FE.

Trilayer Polymer Electrolytes Enable Carbon‐Efficient CO2 to Multicarbon Product Conversion in Alkaline Electrolyzers

AbstractThe electrochemical CO2 reduction reaction (CO2RR) is an appealing method for carbon utilization. Alkaline CO2 electrolyzers exhibit high CO2RR activity, low full‐cell voltages, and cost‐effectiveness. However, the issue of CO2 loss caused by (bi)carbonate formation leads to excessive energy consumption, rendering the process economically impractical. In this study, we propose a trilayer polymer electrolyte (TPE) comprising a perforated anion exchange membrane (PAEM) and a bipolar membrane (BPM) to facilitate alkaline CO2RR. This TPE enables the coexistence of high alkalinity near the catalyst surface and the H+ flux at the interface between the PAEM and the cation exchange layer (CEL) of the BPM, conditions favoring both CO2 reduction to multicarbon products and (bi)carbonate removal in KOH‐fed membrane electrode assembly (MEA) reactors. As a result, we achieve a Faradaic efficiency (FE) of approximately 46 % for C2H4, corresponding to a C2+ FE of 64 % at 260 mA cm−2, with a CO2‐to‐C2H4 single‐pass conversion (SPC) of approximately 32 % at 140 mA cm−2—nearly 1.3 times the limiting SPC in conventional AEM‐MEA electrolyzers. Furthermore, coupling CO2 reduction with formaldehyde oxidation reaction (FOR) in the TPE‐MEA electrolyzer reduces the full‐cell voltage to 2.3 V at 100 mA cm−2 without compromising the C2H4 FE.

Polyelectrolyte multilayer catalysts in electrospun bipolar membranes

Here we utilize the layer-by-layer (LbL) technique to homogenously integrate polyelectrolyte (PE) multilayers with multiple different catalytic functionalities inside an electrospun BPM without forming an electrically and/or ionically non-conductive catalyst layer. BPMs were fabricated by coating an electrospun cation exchange layer alternately with polyethyleneimine (PEI)-poly (acrylic acid) (PAA), followed by hot-pressing between pristine electrospun anion and cation exchange layers. This resulted in a BPM having a thin entangled 3D junction with high surface area and well-integrated catalysts. Optical reflectometry shows that the amount of PE catalysts inside the BPM can be controlled by tuning coating parameters such as the amount of bilayers and the ionic strength of the coating solution, where especially a higher ionic strength resulted in a significantly higher mass adsorption. The BPMs coated at higher ionic strength (500 mM NaCl) showed the lowest water dissociation potential of 0.84 V. Furthermore, it was found that incorporating the PE multilayers diminished the effect of the electric field and that the catalytic activity becomes more dominant in facilitating the water dissociation reaction. The advantage of creating a 3D junction using electrospinning in combination with the versatility of the LbL deposition technique allows for further optimization to create high performance BPMs that can be tailored to specific applications.

The impact of membrane orientation on ion flux in bipolar membranes

Bipolar membranes (BPMs) are asymmetric, layered ion exchange membranes that are increasingly being explored for use in electrochemical devices. This study aims to investigate the effect that the direction of a diffusive driving force across a salt solution concentration gradient has on the flux through a BPM due to its asymmetric nature. BPMs were fabricated using PiperION poly(aryl piperidinium) A40 as the anion exchange layer (AEL) and Nafion 212 as the cation exchange layer (CEL) with no interfacial junction catalyst. The permeability of sodium chloride through the BPM membranes was measured across a 0.5molL-1 concentration differential for both orientations of the membrane, e.g. AEL facing the 0.5molL-1 NaCl solution versus the CEL facing the 0.5molL-1 NaCl solution. A flux differential of (76.3 ± 4.8)% was measured for the BPM depending on the direction of the driving force. A model based on Fick’s law and the Donnan equilibrium was developed and used to show that the flux differential results from changes in the ionic environment at the AEL–CEL junction due to differences in the ion diffusion coefficients and fixed charge concentrations of the two layers.

Smectite-bitumen interactions in non-aqueous bitumen extraction process

The non-aqueous extraction (NAE) is an alternative to the commercial water-based extraction technology used to liberate bitumen from oil sands. Although the impact of kaolinite, illite, chlorite, illite-smectite and montmorillonite on NAE process has been previously studied, the effect of different types of smectites has not been fully understood yet. In this work, mixtures of bitumen with bentonites dominated by saponite, hectorite, nontronite, beidellite and montmorillonite were reacted for several days and then washed with solvent (cyclohexane) to remove bitumen from the bentonites. The main goal was to better understand the impact of different smectites properties (specific surface area, pore structure characteristics, particle size, crystallite thickness, swelling ability, cation exchange capacity, layer charge and crystal-chemistry of smectites) on NAE process. The results showed that the porosity, particle size and swelling ability were the key parameters affecting the solvent-based bitumen recovery under the studied conditions. Samples with larger initial total pore volume and smaller particle size displayed reduced solvent bitumen extractability. The particle size of studied samples was affected primarily by swelling of smectites during the solvent bitumen extraction experiments. Smectites were swelling in water but not in cyclohexane. Swelling of smectite caused splitting of larger clay particles into many smaller particles. As a consequence, the efficiency of solvent bitumen extraction was significantly lower for fine grained samples, consisting of thin delaminated smectite particles, in which the bitumen was distributed among small-sized clay particles.

Designing and development of stable asymmetric bipolar membrane for improved water splitting and product recovery by electrodialysis

The stability and the performance of the bipolar membrane (BPM) are the critical issues to be resolved. The layer thickness plays an important role in improving the performance of bipolar membrane. Here, an efficient transparent asymmetric bipolar membrane has been fabricated by a layer-by-layer solution casting technique comprising cation and anion exchange layers. The thickness of the anion exchange layer was reduced to improve the performance in term of product purity and recovery by minimizing the co-ion leakage. The interface of BPM acts as a catalytic junction for dissociating the water into H+ and OH− ions. A clear membrane interface was confirmed by the scanning electron microscopy (SEM). All three BPMs show the adequate physicochemical and electrochemical properties in term of ionic conductivity and water uptake. Effect of applied potential was studied on the membrane performance through BMED for the dissociation of NaCl. We also performed the bipolar membrane electrodialysis (BMED) for the hydrolysis of different organic and inorganic salts into corresponding acid and alkali. Among the bipolar membranes, BPM with 40 % less AEL thickness (BPM-60 membrane) shows the better performance. BPM-60 shows the highest water splitting rate and alkali recovery of 0.491 g h−1 and 95.5 %, respectively with lowest energy consumption of 1.11 kW h kg−1 and high current efficiency of 90.32 %. Conversion of NaCl in to NaOH and HCl gives the highest product recovery as compared to other salts, due to higher dissociation of NaCl. BPM-60 with higher water splitting rate, lower energy consumption and higher current efficiency for conversion of salt into corresponding acid and alkali made asymmetric bipolar membrane as industrially applicable with high performance.

Forward-Bias 3D-Junction Bipolar Membranes for Electrochemical CO2 Reduction to CO

The ongoing, rapid increase in atmospheric CO2 concentration has led to a growing interest in the electrochemical reduction of CO2 to value-added products like CO. To attain high current densities, the latter reaction is often performed using an anion exchange membrane(AEM) electrolyte that is well known to operate through the transport of (bi)carbonate ions from cathode to anode. This can in turn result in a CO2 pumping effect that decreases the device’s net CO2-consumption, and that can be prevented by using a bipolar membrane in a so-called forward-bias configuration (i.e., with the anion vs. cation exchange layers (AEL, CEL) contacting the cathode vs. anode electrodes, respectively). However, the recombination of protons and (bi)carbonate and concomitant production of H2O and CO2 at the AEL-CEL interface in this operative mode can also lead to the delamination of the membrane and cell failure.In order to prevent such a delamination, this study proposes the introduction of a three-dimensional (3D) AEL-CEL junction, utilizing Nafion® as the cation exchange layer and PiperION as the anion exchange one. Two different methods were applied to fabricate the 3D junction bipolar membrane. The first method involves the fabrication of a 3D interlocking layer using polystyrene (PS) beads that are sprayed (along with Nafion ionomer) onto a Nafion membrane, and removed using toluene. This results in an inverse-opal structure atop the Nafion membrane whose pores can be filled with anion exchange ionomer (AEI). Complementarily, the second method involves the introduction of a layer, consisting of a mixture of AEI and cation exchange ionomer (CEI) between the AEM and CEM. Additionally, a non-conductive oxide material is added to this ionomer layer to increase the contact area of AEI and CEI. Finally, the electrochemical CO2 reduction performance and durability of the resulting, 3D junction bipolar membranes are evaluated and compared to those of an equivalent, 2D junction bipolar membrane, in all cases in a forward bias configuration.

The Effect of Junction Composition of Polarization Behaviour of Bipolar Membranes

Bipolar membranes (BPMs), i.e, laminates cationic and anionic polymer membranes, have a wide range of applications. Starting from an early application in electrodialysis systems, the application scope has more recently broaden to electrochemical energy conversion systems such as water electrolyzer and CO2 reduction reaction, fuel cells and flow batteries.1 In connection to CO2 reduction reaction, the implementation of BPM effectively mitigates CO2 crossover driven by (bi)carbonate migrations, but the rate capability is limited by the large overpotential across current state-of-the-art BPMs.2 The large overpotential primarily originates from the slow water dissociation (WD) kinetics at the interface between the cation exchange layer (CEL) and anion exchange layer (AEL), and largely depends on junction morphology. The detailed structure-properties relationships remain poorly understood, but physical properties of individual polymeric membranes and the composition and morphology of the WD catalyst are important parameters and the optimal combination of materials may be different for different systems. Furthermore, the WD performance of BPM in MEA water electrolyzer not only depends on BPM’s junction contact, but also the interface contact between electrodes and CEL/AEL.In this work, a series of BPM system featuring different compositions of membrane layers and WD catalysts are explored and characterized by polarization behaviour (J-V curve) via H-cell and MEA water electrolyzer. We attempt to link the junction composition created by different BPM systems to its WD performance and provide a systematic choice of CEL/AEL and WD catalyst in the BPM system.(1) Blommaert, M. A.; Aili, D.; Tufa, R. A.; Li, Q.; Smith, W. A.; Vermaas, D. A. Insights and Challenges for Applying Bipolar Membranes in Advanced Electrochemical Energy Systems. ACS Energy Lett. 2021, 2539–2548. https://doi.org/10.1021/acsenergylett.1c00618.(2) Garg, S.; Giron Rodriguez, C. A.; Rufford, T. E.; Varcoe, J. R.; Seger, B. How Membrane Characteristics Influence the Performance of CO 2 and CO Electrolysis. Energy Environ. Sci. 2022. https://doi.org/10.1039/D2EE01818G.

Degradation Processes in Bipolar Membrane CO2-Electrolysis Studied by Time-Resolved X-Ray Tomography

Electrochemical reduction of carbon dioxide via CO2 co-electrolysis (CO2ELY) is an essential process to produce non-fossil chemical feedstock acting as CO2 sink. The required electrical energy can be harvested from temporal surplus of renewable energy sources like solar or wind. Employing a bipolar electrolyte membrane (BPM) in forward-bias is a promising approach to mitigate fundamental problems of the current technological fore-runner design that employs an anion exchange membrane (AEM). We employed synchrotron time-resolved X-ray tomographic microscopy (XTM) to study degradation processes in BPM CO2ELY.In forward-bias BPM co-electrolysis, the cation exchange layer (CEL) faces the anode to provide favorable acidic conditions for the oxygen evolution reaction. Such acidic anode allows using pure water as electrolyte eliminating the salt precipitation at the cathode observed for AEM CO2ELY. As the second major issue in AEM CO2ELY, (bi)carbonate ions formed at the cathode are transported to the anode presenting a major obstacle for high CO2 conversion efficiency. The then required effort to separate CO2 from the electrolyte further reduces the overall energy efficiency. Employing a BPM and introducing the CEL prevents (bi)carbonate ions reaching the anode. BPM CO2ELY retains at the same time the advantages of AEM CO2ELY of performing CO2 reduction under alkaline conditions with the anion exchange layer (AEL) of the BPM facing the cathode.However, BPM CO2ELY suffers from low current density and poor stability in the range of only a few hours. Current limiting factors and degradation processes as well as the affected components are rarely investigated and poorly understood. Processes that have been brought forward are cathode flooding similar to fuel cells or catalyst poisoning. We employ synchrotron XTM to investigate structural dynamics in CO2ELY during operation. The brilliant synchrotron source allows us to acquire one full high-quality scan with a voxel size of 2.75 um in 1 s. We acquired one scan every minute for one hour during electrochemical operation.The image data reveals the formation of gas bubbles at the AEL-CEL junction and ultimately delamination of the BPM. We explain the gas formation as the recombination of (bi)carbonate ions with hydrogen ions at the junction producing gaseous CO2. This CO2 additionally reaches the anode by diffusion causing partial delamination of the anode catalyst layer. CO2 escaping from the AEL surface at the anode forms high-pressure gas bubbles that break the water wet catalyst layer. A diffusive model considering the transport of ions and neutral CO2 is able to capture the mechanism.Our imaging experiments provide insights not attainable by other diagnostic methods and highlight the need for fundamental research on forward-bias BPM CO2ELY. Knowledge based on AEM CO2ELY and fuel cells is not necessarily directly applicable and specific studies on BPM CO2ELY are required to increase performance and stability.We gratefully acknowledge provision of beamtime at TOMCAT, SLS, Paul Scherrer Insititut, Villigen PSI, Switzerland.Figure 1: Tomographic slice of CO2-electrolyser (cathode to the left) a) before operation and running water through anode b) after 23 minutes operation at 3.5V constant potential showing liquid water dynamics at cathode, membrane swelling, gas formation within the membrane and anode catalyst layer disintegration. Figure 1

Design of Bipolar Membranes to Increase CO Formation Rates in Bicarbonate Electrolysers at Low Voltage

The electrolysis of bicarbonate solutions offers a direct route for converting alkaline CO2 capture solutions into value-added products. Alkaline CO2 capture exploits the reaction of CO2 with OH- to form aqueous HCO3-, which can be converted back into CO2 in-situ ( i-CO2) by protons sourced from a bipolar membrane (BPM). Bicarbonate electrolysis is currently too energy intensive to be economically viable, with the largest energy input coming from the voltage drop across the membrane. BPMs are usually thicker than monopolar membranes, causing high Ohmic losses and limited water transport to the water dissociation junction. This causes the membrane to dry out and require excessively high voltages to operate at high current densities. To date, little research has been directed towards designing BPMs for bicarbonate electrolysis. Our work focuses on designing BPMs with low voltage drops while keeping the amount of i-CO2 suppliedto the cathode high. In this study, we present custom-made BPMs that enable bicarbonate electrolysis at a current density of 100 mA cm-2 and a cell voltage < 3 V, with cell voltage remaining < 10 V at current densities in excess of 1 A cm-2. We also demonstrate a correlation between a thicker cation exchange layer and higher i-CO2 generation and elucidate the cause of this phenomenon.

A Mathematical Model for the Membrane Electrode Assembly of a Bicarbonate Electrolyzer

Bicarbonate electrolyzers are devices designed to convert CO2 released in situ from bicarbonate ions into chemicals and fuels without an external source of CO2 gas. A one-dimensional steady-state isothermal model is developed for the membrane electrode assembly of a bicarbonate CO2 electrolyzer with a bipolar membrane design. The model incorporates species transport in both the anode and cathode electrodes due to convection, diffusion, and migration, and accounts for the catalyzed water splitting reaction at the interface of the anion exchange layer and the cation exchange layer of the bipolar membrane. A direct comparison of model simulations with available experimental data shows that the model can accurately simulate measured Faradaic efficiency and CO yield for all operating current densities. The model can also accurately simulate most of the polarization curve, with the only limitation being in the range dominated by mass transport. Compared to the other parameters studied in this paper, numerical results show that the performance of the bicarbonate CO2 electrolyzer is more sensitive to both aqueous electrolyte saturation in the cathode catalyst layer and the catalyzed water splitting efficiency of the bipolar membrane.

Fabrication of efficient bipolar membranes with functionalized MOF interfacial layer for generation of various carboxylic acids via electrodialysis

Stability and compatibility of two layers with each other are the major drawback of the bipolar membrane (BPM). Sulfonated poly (ether ether ketone) and quaternized poly phenylene oxide are taken as the cation exchange layer (CEL) and anion exchange layer (AEL) respectively to fabricate the bipolar membrane by layer-by-layer assembly. Different functionalized MOF (MIL-101 Cr) are prepared for the interfacial layer (IL) of BPM as water dissociation (WD) catalysts. Various analytical techniques (XRD, FT-IR, SEM, BET, XPS, TGA) are used to examine the chemical structure, textural properties and thermal stability of the synthesized catalysts. Two different catalytic sites of MOF as inorganic metal-ion and organic ligands facilitates the water dissociation rate. The effect of different IL (functionalized MOF) on performance of BPM is studied for the dissociation of NaCl into corresponding acid and alkali. BPM with bi-functionalized MOF as IL (BPM-BMOF) shows the better electro-chemical and physicochemical properties in term of ionic conductivity (8.02 mS cm−1) with 38.1 % water uptake, among the synthesized BPMs. The performance of BPM-BMOF is evaluated for the generation of different organic carboxylic acids from sodium carboxylates using bipolar membrane electrodialysis (BPMED). BPM-BMOF shows the 94.12 % acid generation with 0.496 g h−1 water dissociation rate using 0.2 M NaCl feed solution. The results suggest that product recovery (PR) and energy efficiency (ƞ) gradually reduces with higher homologous of sodium carboxylates under similar experimental conditions. The performance of BPM-BMOF with negligible co-ion leakage and stability made it industrially applicable for water splitting.

Bilayer Heterogeneous Cation Exchange Membrane with Polyaniline Modified Homogeneous Layer: Preparation and Electrotransport Properties.

A bilayer membrane based on a heterogenous cation exchange membrane with a homogeneous cation exchange layer and a polyaniline on its surface is prepared. The intercalation of polyaniline into the membrane with a homogeneous cation exchange layer is performed by oxidative polymerization of aniline. The influence of the homogeneous cation exchange layer and the polyaniline on the structure, conductivity, diffusion permeability, selectivity, and current-voltage curve of the heterogeneous cation exchange membrane is established. Membrane properties are studied in the HCl, NaCl, and CaCl2 solutions. The homogeneous cation exchange layer has a negligible effect on the transport properties of the initial heterogeneous membrane. The polyaniline synthesis leads to a decrease in the macropore volume in the membrane structure, conductivity, and diffusion permeability. The counterion transport number in the bilayer membrane is significantly reduced in a solution of calcium chloride and practically does not change in sodium chloride and hydrochloric acid. In addition, the asymmetry of the diffusion permeability and shape of current-voltage curve depending on the orientation of the membrane surface to the flux of electrolyte or counterion are found.

Mechanism of Water Dissociation on Metal Oxide Surfaces in Bipolar Membrane Electrolyzers: A First-Principles Calculation

Bipolar membranes for water electrolysis, consisting of an acidic cation exchange layer and an alkaline anion exchange layer are a promising means for hydrogen production. The electrolyzer operates the oxygen evolution reaction (OER) under locally basic conditions and the hydrogen evolution reaction (HER) under locally acidic conditions. Water dissociation (WD) occurs within the junction, which is spatially separated from the HER and the OER. It has been shown that the addition of a layer of metal oxide nanoparticles at the junction significantly lowers the overpotential for the process of WD [Science, 2020, 369,1099]. However, the atomistic mechanism by which the metal oxide surface catalyzes the WD is still unclear. We will discuss the theoretical understanding of the WD mechanism at the metal-oxide/water interfaces and investigate the descriptors such as surface pKa, dielectric constants, and adsorption energies, etc. We will combine ab intio molecular dynamics simulations with the electronic structure calculations at the hybrid functional level. Further details of the reaction mechanisms during the WD process will also be explored with the nudged-elastic-band calculations.

CO2 electroreduction to multicarbon products from carbonate capture liquid

CO2 electroreduction to multicarbon products from carbonate capture liquid

Improving the Stability, Selectivity, and Cell Voltage of a Bipolar Membrane Zero‐Gap Electrolyzer for Low‐Loss CO2 Reduction

AbstractElectrolyzers for CO2 reduction containing bipolar membranes (BPM) are promising due to low loss of CO2 as carbonates and low product crossover, but improvements in product selectivity, stability, and cell voltage are required. In particular, direct contact with the acidic cation exchange layer leads to high levels of H2 evolution with many common cathode catalysts. Here, Co phthalocyanine (CoPc) is reported as a suitable catalyst for a zero‐gap BPM device, reaching 53% Faradaic efficiency to CO at 100 mA cm−2 using only pure water and CO2 as the input feeds. It is also shown that the cell voltage can be lowered by constructing a customized BPM using TiO2 water dissociation catalyst, however this is at the cost of decreased selectivity. Switching the pure‐water anolyte to KOH improved both the cell voltage and CO selectivity (62% at 200 mA cm−2), but cation crossover could cause complications. The results demonstrate viable strategies for improving a BPM CO2 electrolyzer toward practical‐scale CO2‐to‐chemicals conversion.