Durable Anion Exchange Membrane Research Articles

(Invited) Development of Cost-Effective Anion Exchange Membranes for Water Electrolyzers

Anion exchange membrane water electrolysis (AEMWE) offers the possibility of combining the advantages of alkaline electrolyzers, which allows the use of cheap and plentiful catalytic materials, with those of proton exchange membrane electrolysis, such as high performance and fast response to changing operating conditions. However, the development of performing and durable anion exchange membranes is still the major challenge for the ultimate industrial adoption of AEMWE [1]. The anion-exchange membrane (AEM) is a crucial component that should allow good charge and water transport, i.e. high ionic conductivity, but should also guarantee good chemical, thermal, and mechanical stability.In this paper, commercial and innovative membranes, also based on a nanocomposite approach, are investigated in AEMWE in terms of performance and stability. Fumasep® FAA3-50 (FumaTech) and PiperIon® (Versogen) membranes were used as benchmark materials to compare the results obtained with the novel materials developed among the partners of an Italian project called AMPERE (FISR2019 call). An anion exchange membrane was synthesized from polysulphone (PSU), which was properly functionalized through the introduction of quaternary ammonium functionalities on the polymeric backbone (qPSU) by using an optimized synthetic procedure, to achieve adequate hydrophilicity features, while preserving its mechanical properties [2]. Furthermore, an innovative nanocomposite AEM based on trimethylammonium functionalized silica nanoscale ionic materials (NIM-N+) incorporated in quaternized polysulfone (qPSU) was developed and investigated [3].Other composite membranes including graphene oxide (GO) as the filler were analyzed to increase the ion conductivity and the stability of the membrane based on commercial Fumion® ionomer.Finally, the investigation comprised the very promising class of AEMs based on poly(aryl piperidiniums)s (PAPs), which combine a heteroatom-free backbone with the stable piperidinium cationic group. The simplest and less-expensive chemistry in this family is poly(biphenyl piperidinium), which was synthesized with suitable mechanical properties for use in water electrolyzers [4].An overview of the electrochemical results obtained with the above-mentioned AEMs in single-cell electrolyzers is presented and discussed.

Reinforced Poly (dibenzyl-co-terphenyl piperidinium) Membranes for Highly Durable Anion-Exchange Membrane Water Electrolysis

Low-cost anion exchange membrane water electrolysis (AEMWE) is an attractive technology to solve global energy shortage and environmental problems. However, AEMWE suffers from poor performance and durability, leading to problems associated with the lack of qualified anion exchange membranes (AEMs). In this study, a reinforced membrane using PDTP (Poly(dibenzyl-co-terphenyl Piperidinium)) and PE (Polyethylen) was developed to improve performance and durability. Additionally, the effects of support layer and polymer membrane thickness on water electrolysis were observed. Due to the excellent electrochemical properties of filled PDTP, the prepared reinforced membranes exhibit excellent mechanical properties with tensile strength > 112 MPa, elongation at break > 84%, conductivity (96.3 mS cm−1), and dimensional stability (swelling). ratio < 50% at 80°C). More importantly, the AEMWE based on the reinforced membrane exhibits an outstanding current density of 4.86 A cm−2@2.0 V at 2.0 V and voltage decay rate, while simultaneously exhibiting in-situ durability of less than 1 and 2 A cm−2 for 1000 hours at 60°C. That it can be achieved. 0.02 mV h−1 and 0.1 mV h−1, respectively. This study shows that PDTP reinforced composite membrane can effectively improve AEMWE performance and durability.

Highly durable anion exchange membranes with sustainable mitigation of hydroxide attacks for water electrolysis

Low hydroxide conductivity and insufficient alkaline stability are the main problems currently facing anion exchange membranes (AEMs). Conventional quaternary ammonium groups have limited resistance to attack by hydroxide roots. In this paper, quaternary ammonium-type (Cn-PFCO (n = 2, 5)) perfluoroanion exchange polymers with carbonyl side chains were synthesized. The C5–PFCO exhibited an impressive conductivity of 113.4 mS cm−1 at 80 °C, and remarkably, it retained 86.87% of its initial conductivity even after 15 days of immersion in a 1 M KOH aqueous solution. This excellent alkali resistance is due to the generation of intermediate oxygen anions from OH− and carbonyl groups through a reversible reaction. These anions effectively reduce the likelihood of SN2 attack by diminishing the electrostatic potential of the QA groups from Density Functional Theory (DFT) calculations. The electrostatic potential of C2–PFCO is 6.28 eV, while it decreases to 2.69 eV after the formation of intermediate anions. Furthermore, we also used the prepared AEM for pure water electrolysis to explore its performance. The electrochemical performance of AEMWE exhibited remarkable results when supplied with pure water, demonstrating an impressive current density of 262 mA cm−2 at 1.9 V. This study introduces a novel strategy for concurrently enhancing hydroxide conductivity and alkali resistance in AEMs by analyzing the potential for further improvement in AEMs' alkali resistance.

Engineering Durable Anion Exchange Membrane Water Electrolyzers through Suppressed Electrochemical Corrosion of a NiFe–Graphitic Carbon Shell Anode Catalyst

Engineering Durable Anion Exchange Membrane Water Electrolyzers through Suppressed Electrochemical Corrosion of a NiFe–Graphitic Carbon Shell Anode Catalyst

Tuning polar discrimination between side chains to improve the performance of anion exchange membranes

Anion exchange membranes (AEMs) are the heart of alkaline fuel cells and water electrolysis, and have made a great progress in recent years. However, AEMs are still unable to satisfy the needs of high conductivity and stability, hindering their widespread commercialization. Side chain regulations have been widely used to prepare highly conductive and durable AEMs. Here, we construct a series of polyaromatic AEMs grafted with fluorinated cation side chains and cation-free alkyl chains with different end groups to explore the polar discrimination of side chains on membrane performance. This work demonstrates that AEMs grafting the cation side chains with superhydrophobic fluorine pendent and alkyl side chains with hydrophilic pendent enhance water content and ion conductivity. This is due to the strong immiscibility between the hydrophilic and hydrophobic head groups which promotes the establishments of microphase separation and ion highways. Specifically, poly(binaphthyl-co-terphenyl piperidinium) containing fluorinated piperidinium side chains and alkyl chains with methoxy pendent (QBNTP-QFM) possesses a satisficed OH− conductivity (170.6 mS cm−1 at 80 °C) and can tolerate 5 M hot NaOH for 2100 h with only 3.4 % conductivity loss. Expectedly, the single cell with QBNTP-QFM yields a prominent maximum power density of 1.62 W cm−2 and the water electrolysis cell with QBNTP-QFM achieves a pronounced current density of 3.0 A cm−2 at 1.8 V, both cells also display a prominent durability for 120 h operation. The results prove that this side chain optimization can improve ion conductivity and is a promising method for AEM development.

Beyond Small Molecular Cations: Elucidating the Alkaline Stability of Cationic Moieties at the Membrane Scale.

A major hindrance in the commercialization of alkaline polyelectrolyte-based electrochemical energy conversion devices is the development of durable anion exchange membranes (AEMs). Despite many alkali-stable cations that have been explored, the stability of these cationic moieties at the membrane scale is in the blind. Herein, we present a molecularly designed polyaromatic AEM with cationic moieties in an alternating manner to unbiasedly compare the alkaline stability of piperidinium and ammonium groups at the membrane state. Using nuclear magnetic resonance spectroscopy, we demonstrate that the pentyltrimethyl group is about 2-fold more stable than piperidinium within a polyaromatic scaffold, either in ex-situ alkaline soaking or in-situ cell operation. This finding challenges the judgment extrapolated from the stability trend of cations, that is, the piperidinium-functionalized AEM is more alkali-stable than the counterparts based on quaternary ammoniums. Moreover, the deterioration mechanism of piperidinium moiety after being embedded in polyaromatic backbone is rationalized by density functional theory.

High-performance and durable anion-exchange membrane water electrolysers with high-molecular-weight polycarbazole-based anion-conducting polymer

Development of high-performance and durable anion-exchange membrane water electrolysis enabled by chain-extender derived high-molecular-weight polycarbazole-based anion-conductive polymer.

Development of durable anion-exchange membranes using polyfluorene–based electrolytes and pore-filling method for direct formate fuel cells

Highly durable ether-linkage-free polyfluorene–based electrolytes with different ion exchange capacities (IECs) were synthesized, followed by the preparation and evaluation of cast and pore-filling (PF) membranes as anion exchange membranes (AEMs). The performance of these membranes was tested in direct formate fuel cells (DFFCs). The membrane properties, including formate permeability, were investigated to clarify the relationship between membrane performance and the performance of DFFCs. The formate permeability of the membrane was drastically affected by the existence of OH− anion, as OH− ions induce larger membrane swelling. In addition, the PF method was more effective in reducing the formate permeation than the cast membranes. The PF membrane using the polyelectrolyte with a low IEC value showed 20-fold lower formate permeability compared to that of the original cast membrane, leading to a higher improvement in open-circuit voltage (OCV), from 0.55 to 0.95 V in DFFCs. For the first time, a quantitative correlation between the formate permeability of AEMs and OCV values in DFFCs was established. From this correlation, formate permeation less than 0.03 mol L−1 h−1 is necessary to keep high OCV value. Using the developed membrane, we also demonstrated the high stability of DFFC at 80 °C for 5 days.

Pathways Toward Efficient and Durable Anion Exchange Membrane Water Electrolyzers Enabled By Electro‐Active Porous Transport Layers

AbstractGreen hydrogen, produced via water electrolysis using renewable electricity, will play a crucial role in decarbonizing industrial and heavy‐duty transportation sectors. Anion exchange membrane water electrolyzers (AEMWEs) can overcome many of the performance and cost limitations of incumbent technologies, however, still suffer from durability challenges due to oxidative instability of anion‐exchange ionomers. Herein, the use of an electro‐active porous transport layer as anode (PTL‐electrode) is demonstrated to enable efficient and durable AEMWEs. The stainless‐steel PTL‐electrodes are shown to have superior performance and durability compared to traditional catalyst layers containing ionomer and nanoparticle catalysts. An AEMWE cell operating at 2 A cm−2 for over 600 h exhibited a degradation rate of just 5 µV h−1. During operation, the surface composition of the stainless steel transforms into a mixture of iron and nickel oxyhydroxides, contributing to enhanced oxygen‐evolution reaction activity. The combination of experimental work and modeling elucidates how the bulk structure of the PTL‐electrode offers an additional design dimension to further improve electrolyzer performance. Lastly, a surface modification strategy is applied to a PTL‐electrode to achieve an even higher performing AEMWE (2.3 vs 2.0 A cm−2 at 1.8 V). Overall, this work lays out pathways toward more efficient, durable, and affordable AEMWEs.

Long side-chain N-heterocyclic cation based ionic liquid grafted poly(terphenyl piperidinium) membranes for anion exchange membrane fuel cell applications

Anion exchange membrane fuel cells (AEMFCs) bring out unique advantages including the use of cheap and plentiful catalytic materials, fast oxygen reduction reaction rate at cathode and low cost of membranes. However, the development of performing and durable anion exchange membranes (AEMs) is still the major challenge for the ultimate industrial adoption of AEMFC. Herein, the ether-free poly(p-terphenyl-co-N-methyl-piperidine) (PTP) is synthesized and employed as the matrix for AEM. Four ionic liquids of long side-chain N-heterocyclic cations bromide are synthesized and chosen as the quaternization reagents for PTP. Thus, four flexible and long side-chain cationic groups grafted PTP based AEMs are fabricated. The effect of end-group cations on the properties of long side-chain AEMs has been investigated systematically. The presence of long side-chain cations endows AEMs with high conductivity and low swellings simultaneously. Among all the membranes, N-methylpiperidinium grafted AEM (PTP-C5-MPi) exhibits the highest alkaline stability owing to ether-free polymer chain, long side-chain linkage and piperidinium cation, which retains 87 % of its original ion conductivity after storage in 1.0 M KOH solution for 2118 h. Furthermore, the H2–O2 fuel cell based on PTP-C5-MPi is performed to determine the technical feasibility.

Reinforced poly(dibenzyl-co-terphenyl piperidinium) membranes for highly durable anion-exchange membrane water electrolysis at 2 A cm−2 for 1000 h

Low-cost anion exchange membrane water electrolysis (AEMWE) is an attractive technology to address global energy shortages and environmental issues. However, AEMWE remains far from industrial application because of its poor performance and durability, issues that are associated with a lack of qualified anion exchange membranes (AEMs). Here, we describe the use of reinforced membrane technology in AEMs to prepare robust membranes, and systematically explore the effect of support-layer and polymer-membrane thicknesses on water electrolysis. Due to the excellent electrochemical properties of filled poly(dibenzyl-co-terphenyl piperidinium) polymers, the prepared reinforced membrane displays outstanding mechanical properties, with a tensile strength > 112 MPa, and an elongation at break > 84 %, conductivity (96.3 mS cm−1), and dimensional stability (swelling ratio < 50 % at 80 °C). More importantly, AEMWE based on the reinforced membrane simultaneously can achieve an exceptional current density of 4.86 A cm−2@2.0 V and in-situ durability under 1 and 2 A cm−2 at 60 °C for 1000 h with a voltage decay rate of 0.02 mV h−1 and 0.1 mV h−1, respectively. These findings provide insight into the development of AEMWEs using reinforced membranes.

Composite anion exchange membranes based on polysulfone and silica nanoscale ionic materials for water electrolyzers

Anion exchange membrane water electrolysis (AEMWE) offers the ambition of combining the advantages of alkaline electrolysers, i.e. the use of cheap and plentiful catalytic materials, with those of proton exchange membrane electrolysis of water, i.e. high performance and fast response to changing operating conditions. However, the development of performing and durable anion exchange membranes is still the major challenge for the ultimate industrial adoption of AEMWE. Here, we introduce an innovative nanocomposite AEM based on trimethylammonium functionalized silica nanoscale ionic materials (NIM-N+) incorporated in quaternized polysulfone (qPSU). The presence of NIM-N+ in the hydrophilic clusters of qPSU produces a remarkable enhancement in the dimensional and thermo-mechanical stability of the composite AEM. Furthermore, Nuclear Magnetic Resonance (NMR) and Electrochemical Impedance Spectroscopy (EIS) investigations highlighted that the nanoparticles are directly involved in the transport process of hydroxide ions. This enabled qPSU/NIM-N+ composite membrane to achieve impressive anionic conductivity values, e.g. about 110 mS cm−1 at 80 °C and 95% relative humidity (RH). The AEMWE single cell, equipped with this membrane, operated properly both with 1 or 0.5 M KOH solution, displaying a current density larger than 3.5 A cm−2 at 2.2 V and 80 °C.

Start–Stop Cyclic Durability Analysis of Membrane–Electrode Assemblies Using Polyflourene-Based Electrolytes for an Anion-Exchange Membrane Water Electrolyzer

Highly durable anion-exchange membranes and ionomers are essential for developing cost-effective green hydrogen generation. The anion-exchange membrane and ionomer must withstand chemical attacks by OH– ions as well as nonlinear mechanical stress caused by irregular bubble formation when the electrolyzer is coupled with intermittent power sources. Herein, we examined the durability of a trimethylammonium-modified poly(fluorene-alt-tetrafluorophenylene) (PFT-C10-TMA)-based membrane and ionomer under dynamic operation to evaluate its competency to couple with intermittent power sources. The durability of membrane–electrode assembly (MEA) prepared using these polymer membrane and ionomer was evaluated by performing 1000 start–stop voltage cycles in 1 M KOH solution at 80 °C while varying the applied cell voltage. The electrochemical performances of the MEA and electrodes were simultaneously evaluated using an electrolyzer with reference electrodes, and the chemical structure of the polymer was examined before and after the durability test using NMR techniques. The MEA based on a commercial Sustainion XB-7 ionomer and X37-50 RT anion-exchange membranes showed the catalyst leaching during start–stop cycles, resulting in reduced cell performance. On the other hand, the MEAs with the PFT-C10-TMA membrane and ionomer demonstrated promising high chemical and mechanical durability under dynamic start–stop operation, encouraging the development of electrolyzers that can function with intermittent renewable power sources.

Durable and highly-efficient anion exchange membrane water electrolysis using poly(biphenyl alkylene) membrane

Anion exchange membrane water electrolysis (AEMWE) is a promising technology for large-scale green-hydrogen production, owing to high current density, low gas permeability and low capital cost. High-performance and durable anion exchange membrane (AEM) is the key to bringing it into practical utilization. In this study, poly(biphenyl alkylene) (PBPA) AEM is synthesized and used for the highly-efficient and durable AEMWE. The fabrication conditions of membrane electrode assembly (MEA) and working condition of water electrolysis are investigated in detail. The ionomer contents and electrolyte type significantly affect the performance and durability of the PBPA-based AEMWE cell. The as-fabricated AEMWE cell with the PBPA membrane achieves a current density as high as 4.0 A cm−2 @ 2.0 V in 1 M KOH at 80 °C and a continuous water electrolysis of over 3,500 h at 1.0 A cm−2 with a slow voltage rising rate of 6.5 μV h−1. The high performance and long durability exhibit that the PBPA-based AEM is promising towards the industrial hydrogen production.

Polyaryl piperidine anion exchange membranes with hydrophilic side chain

Recently, the development of high-performance and durable anion exchange membranes has been a top priority for anion exchange membrane fuel cells. Here, a series of polyaryl piperidine anion exchange membranes with hydrophilic side chain (qBPBA-80-OQ-x) are prepared by the superacid-catalyzed Friedel-Crafts reaction. AFM images show that the hydrophilic side chain and hydrophobic main chain form a distinct microphase separation structure. The AEMs of qBPBA-80-OQ-100 and qBPBA-80 have close mechanical strength, but the ionic conductivity of the former (81 mS/cm, 80 °C) is higher than the latter (73 mS/cm, 80 °C). In addition, qBPBA-80-OQ-100 AEM loses by 15.0% after an alkaline treatment of 720 h, while qBPBA-80 AEM loses by 17.8%. The results indicate that the introduction of hydrophilic side chain not only promotes the formation of microphase separation structure, but also improves the ionic conductivity and alkaline resistance of polyaryl piperidine AEMs.

Alkaline Stability of Anion-Exchange Membranes.

Recently, the development of durable anion-exchange membrane fuel cells (AEMFCs) has increased in intensity due to their potential to use low-cost, sustainable components. However, the decomposition of the quaternary ammonium (QA) cationic groups in the anion-exchange membranes (AEMs) during cell operation is still a major challenge. Many different QA types and functionalized polymers have been proposed that achieve high AEM stabilities in strongly alkaline aqueous solutions. We previously developed an ex situ technique to measure AEM alkaline stabilities in an environment that simulates the low-hydration conditions in an operating AEMFC. However, this method required the AEMs to be soluble in DMSO solvent, so decomposition could be monitored using 1H nuclear magnetic resonance (NMR). We now report the extension of this ex situ protocol to spectroscopically measure the alkaline stability of insoluble AEMs. The stability ofradiation-grafted (RG) poly(ethylene-co-tetrafluoroethylene)-(ETFE)-based poly(vinylbenzyltrimethylammonium) (ETFE-TMA) and poly(vinylbenzyltriethylammonium) (ETFE-TEA) AEMs were studied using Raman spectroscopy alongside changes in their true OH- conductivities and ion-exchange capacities (IEC). A crosslinked polymer made from poly(styrene-co-vinylbenzyl chloride) random copolymer and N,N,N',N'-tetraethyl-1,3-propanediamine (TEPDA) was also studied. The results are consistent with our previous studies based on QA-type model small molecules and soluble poly(2,6-dimethylphenylene oxide) (PPO) polymers. Our work presents a reliable ex situ technique to measure the true alkaline stability of AEMs for fuel cells and water electrolyzers.

Deep reconstruction of Ni–Al-based pre-catalysts for a highly efficient and durable anion-exchange membrane (AEM) electrolyzer

The resulting AEM electrolyzer cell demonstrates exceptional performance, achieving an extraordinarily low cell voltage of 1.89 V and remarkable durability of 500 hours at 5 A (1 A cm−2, 1 M KOH, room temperature).

Effects of hydrophobic side chains in poly(fluorenyl-co-aryl piperidinium) ionomers for durable anion exchange membrane fuel cells

Hydrophobic side chain grafted poly(fluorenyl-co-aryl piperidinium) ionomers simultaneously possess outstanding peak power density of 2.6 W cm−2 at 80 °C along with durable in situ stability of 0.4 mV h−1 voltage decay rate under 0.6 A cm−2 at 70 °C.

Highly Efficient and Durable Anion Exchange Membrane Water Electrolyzer Enabled by a Fe–Ni3S2 Anode Catalyst

Anion exchange membrane water electrolyzer (AEM‐WE) is a promising approach to producing green hydrogen using renewable energy. However, most of the reported AEM‐WEs still use platinum‐group metal‐based catalysts and the performance is far beyond unsatisfactory. Particularly, developing highly active, durable, and earth‐abundant metal‐based oxygen evolution reaction (OER) catalysts is essential to improve energy efficiency and reduce the costs of AEM‐WE. Herein, Ni2Fe8/Ni3S2/NF catalyst is fabricated in situ on nickel foam by a simple one‐pot hydrothermal reaction. The as‐prepared anode OER catalyst exhibits current densities of 500 and 1000 mA cm−2 at an overpotential (η) of 279 and 302 mV, superior to the performance of noble metal‐based catalysts (IrO2, RuO2). Coupled with Ni4Mo/MoO2/NF, the resulting single‐cell AEM‐WE displays high performance (1.65 V @ 1 A cm−2) and high durability (100 h @ 1 A cm−2), outperforming most of the reported AEM‐WEs assembled by non‐noble metal‐based catalysts. Additional characterization of the post‐test anode using different spectroscopic techniques further proved that the Ni2Fe8/Ni3S2/NF is a highly efficient and robust anode in the AEM‐WE device.

Development of Pore-Filling Anion Exchange Membranes for Anion Exchange Membrane Water Electrolysis: Enhancement of Alkaline Stability

Water electrolysis is a process that uses electricity to decompose water into oxygen and hydrogen. There are several types of ion exchange membrane can be used, anion exchange membrane, proton exchange membrane and bipolar membrane for water electrolysis. Alkaline based water electrolysis has several advantages to use non-precious electrocatalysts. However, the development of low resistance and durable anion-exchange membranes is of importance. In this study, several anion-exchange membranes were developed to enhance alkaline stability. Pore-filling anion exchange composite membranes with different contents of cross-linkers were prepared by mixing an electrolyte having good anion conducting ability. The mixture of monomers into a porous polyethylene (PE) substrate were polymerized by UV curing. The pore-filling reinforced composite membranes have been investigated in terms of good chemical stability properties, in particular, the variation of conductivity and mechanical strength in 1 M KOH at 60 oC. Characterization in terms of ion exchange capacity, water uptake, swelling ratio, and mechanical strength were also investigated. Acknowledgments This research was supported in part by "Cooperative Research Program for Agriculture Science and Technology Development (Project No. PJ016253)" Rural Development Administration, Republic of Korea, by the New and Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20213030040520) and by 2022 Green Convergence Professional Manpower Training Program of the Korea Environmental Industry and Technology Institute funded by the Ministry of Environment.