Comparative study of anion exchange membranes for low-cost water electrolysis

Various anion-exchange membranes (AEMs) were studied in the electrolysis cell using the non-precious metal-based catalysts showing the good potential of selected AEMs in low-cost water electrolysis application. The 0.1–1 M potassium hydroxide electrolyte is applied for high performance electrolysis process, whereas the usage of pure water leads to the significant AEM resistance increase. The post mortem MEA analysis, using SEM is performed to study the structure and morphology of catalyst layers transferred from the electrodes prepared by catalyst coated substrate approach. The importance of the catalyst layer–membrane interface and the binder used to bond the catalyst layer is discussed. AEM electrolysis safety aspect in terms of the hydrogen crossover through the 28 μm thin A-201 membrane is studied. The linear dependency of the permeated hydrogen flux on current density is shown. Hydrogen content in the anode outlet gas is less enough to ensure high safety of the AEM electrolysis technology in the operating currents range.     

A review of anion exchange membranes prepared via Friedel-Crafts reaction for fuel cell and water electrolysis

Hydrogen is considered a potential, clean, and renewable energy for the future. Anion exchange membranes (AEMs) are significant components in AEM fuel cells and water electrolysis, crucial devices in the hydrogen industry. Friedel-Crafts (F–C) reaction has been widely used in preparing AEMs due to its versatility, high catalytic efficiency, relatively mild reaction conditions, etc. This review article provides a comprehensive literature survey for AEMs prepared via Friedel-Crafts reaction. Firstly, the fundamentals of the F–C reaction were introduced in detail, including the category, mechanism, catalyst and chloromethylating agent. Different types of AEMs, including polysulfones (PSUs), poly(arylene ether)s (PAEs), poly(ether ketones) (PEKs), and poly(2,6- dimethyl-1,4-phenylene oxide) (PPO), etc. were discussed. The cell performance of fuel cells and water electrolysis was investigated and analyzed. Finally, this review addresses the current challenges facing the development of AEM and proposed research implications for future investigations.     

Dense 1,2,4,5-tetramethylimidazolium-functionlized anion exchange membranes based on poly(aryl ether sulfone)s with high alkaline stability for water electrolysis

Anion exchange membranes with enough alkaline stability and ionic conductivity are essential for water electrolysis. In this work, a class of anion exchange membranes (PAES-TMI-x) with dense 1,2,4,5-tetramethylimidazolium side chains based on poly(aryl ether sulfone)s are prepared by aromatic nucleophilic polycondensation, radical substitution and Menshutkin reaction. Their chemical structure and hydrophilic/hydrophobic phase morphology are characterized by hydrogen nuclear magnetic resonance (¹H NMR) and atomic force microscope (AFM), respectively. The water uptake, swelling ratio and ionic conductivity for PAES-TMI-x are in the range of 23.8%–48.3%, 8.3%–14.3% and 18.22–96.31 mS/cm, respectively. These AEMs exhibit high alkaline stability, and the ionic conductivity for PAES-TMI-0.25 remains 86.8% after soaking in 2 M NaOH solution at 80 °C for 480 h. The current density of 1205 mA/cm² is obtained for the water electrolyzer equipped with PAES-TMI-0.25 in 2 M NaOH solution at 2.0 V and 80 °C, and the electrolyzer also has good operation stability at current density of 500 mA/cm². This work is expected to provide a valuable reference for the selection and design of cations in high-performance AEMs for water electrolysis.     

End-group crosslinked hexafluorobenzene contained anion exchange membranes

A kind of anion exchange membranes (AEMs) with CC bond end-group crosslinked structure was synthesized successfully. Unlike the traditional aliphatic AEMs, the AEMs were prepared in this work by a strategy to realize the CC bond thermal end-group crosslinking reaction, exhibiting an obvious microphase separation structure and a suitable dimensional stability. The well-defined ion channels constructed in the AEMs guarantee the fast OH⁻ conduction, as confirmed via physical and chemical characterization. The conductivity was dramatically enhanced due to the effective ion channels and increased ion exchange capacity. Among the as-prepared AEMs, the PHFB-VBC-DQ-80% AEM has a conductivity of 135.80 mS cm⁻¹ at 80 °C. The single cell based on PHFB-VBC-DQ-80% can achieve a power density of 141.7 mW cm⁻² at a current density of 260 mA cm⁻² at 80 °C. The AEMs show good thermal stability verified by a thermogravimetric analyzer (TGA). Furthermore, the ionic conductivity of PHFB-VBC-DQ-80% only decreased by 7.1% after being soaked in a 2 M NaOH solution at 80 °C for 500 h.     

Ternary NiCoFe nanosheets for oxygen evolution in anion exchange membrane water electrolysis

Tuning nickel-based catalyst activity and understanding electrolyte and ionomer interaction for oxygen evolution reaction (OER) is crucial to improve anion exchange membrane (AEM) water electrolyzers. Herein, an investigation of multimetallic Ni_0.6Co_0.2Fe_0.2 OER activity, coupled with in-situ Raman spectroscopy to track dynamic structure changes, was carried out and compared to other Ni catalysts. The effect of KOH concentration, KOH purity, ionomer type, and electrolyte with organic cations was evaluated. The Ni_0.6Co_0.2Fe_0.2 catalyst achieved 10 mA/cm² at 260 mV overpotential with stability over 50 h and 5000 cycles in 1 M KOH. In-situ Raman spectroscopy showed that Ni_0.6Co_0.2Fe_0.2 activity originates from promoting Ni(OH)₂/NiOOH transformation at low potentials compared to bi- and mono-metallic nickel-based catalysts. Fumion anion ionomer in the catalyst inks led to a lower OER activity than catalysts with inks containing Nafion ionomer. The OER activity of Ni_0.6Co_0.2Fe_0.2 is adversely influenced in the presence of fumion anion ionomer and benzyltrimethylammonium hydroxide (BTMAOH) with possible phenyl oxidation under applied high anodic potentials. The alkaline AEM water electrolyzer circulating 1 M KOH electrolyte, with a Pt/C cathode and a Ni_0.6Co_0.2Fe_0.2 anode, achieved 1.5 A/cm² at 2 V.     

Next-generation anion exchange membrane water electrolyzers operating for commercially relevant lifetimes

Alkaline anion exchange membrane (AEM) water electrolysis has gained increasing attention due to its potential to achieve low-cost, high performance hydrogen production. However, most existing membranes are not durable in industrial settings. Here, we demonstrate good performance relative to industrial parameters using Sustainion® anion exchange membranes. Long-duration tests showed stable performance of 1 A/cm² at 1.85 V with a degradation rate of less than 1 μV/h over 10,000 h. The projected lifetime is thus over 20 years. Daily on/off cycling performance over the course of 30 years was simulated experimentally through accelerated voltage shock tests, resulting in a performance loss of only 0.15 μV/cycle over 11,000 cycles. As shown through impact and crossover testing, an improvement in performance is achieved by the addition of zirconia to the polymer matrix and mechanically reinforcing the membrane.     

Three-dimensional copper cobalt hydroxide electrode for anion exchange membrane water electrolyzer

Anion exchange membrane (AEM) water electrolyzers are promising energy devices for producing low-cost and clean hydrogen using platinum group metals (PGMs). However, AEM water electrolyzers still do not show satisfactory performance due to the sluggish kinetics of the electrodes. In this work, copper cobalt hydroxide (CuCo-hydroxide) nanosheet was synthesized on commercial nickel foam (NF) via electrochemical co-precipitation, and used directly as an oxygen evolution reaction (OER) electrode for an AEM electrolyzer. The interaction between Cu and Co induces a change in the electronic structure of Co(OH)₂ and improves the performance of the OER electrode. In addition, the AEM electrolyzers catalyzed by CuCo(OH)₂ showed high energy conversion efficiency of 73.5%. This work demonstrates that non-PGM based electrodes fabricated using a simple electrochemical co-precipitation apply to AEM electrolyzers for low-cost and clean hydrogen production.     

Water transport analysis during cathode dry operation of anion exchange membrane water electrolysis

The effect of water diffusion through an anion exchange membrane (AEM) on the concentration overpotential (η_conc) during cathode dry operation of AEM water electrolysis was experimentally examined using electrolytic cells with different membrane electrode assemblies (MEAs). The specially designed MEAs were used in the cells to obtain reliable and reproducible data to clarify the influence of membrane thickness (t_mem) and porosity of cathode catalyst layer (CL). The relative humidity of generated hydrogen (?_H2) during electrolysis was also measured based on dew point measurements of the hydrogen. The η_conc analysis for cells with single- and double-AEM MEAs revealed that water diffusion through the membrane was the main contributor to η_conc. The quantitative agreement between ?_H2 data and η_conc revealed that the difference in η_conc between the two types of MEAs is explained by the water concentration difference between anode and cathode via the Nernst equation. The effect of the porosity of the cathode CL on cell performance and on water transport was also examined experimentally. The results revealed that a high-porosity cathode CL tended to keep the cathode in a drier state during electrolysis compared with a low-porosity cathode CL. When ?_H2 is lower than a threshold value in the range from 0.5 to 0.6, the ion conductivity of AEM and ionomer would decrease, and the cell performance would deteriorate due to an increase in cell resistance (R_cell) and/or activation overpotential (η_act).     

Development of efficient membrane electrode assembly for low cost hydrogen production by anion exchange membrane electrolysis

Electrochemical production of hydrogen from water using anion exchange membranes (AEMs) can be achieved with non-noble catalysts, other than traditional proton exchange membranes that use platinum group metals. Using non-noble metals in the catalyst layer will reduce the capital costs associated with water electrolysis systems. The objectives of this study were to develop an effective membrane electrode assembly (MEA) for AEM electrolysis and to determine the effects of various operating parameters on AEM electrolysis. Here, the MEA consisted of the commercially available A-201 AEM and non-noble transition metal oxides as catalysts. The best electrolysis performance recorded was 500 mA cm^?2 for 1.95 V at 60 °C with 1% K₂CO₃ electrolyte. For the purpose of comparison, we also considered commercially available AEMs for AEM electrolysis: Fumapem^® FAA-3 and Fumapem^® FAA-3-PP-75. The performances achieved with these AEMs were comparable with the performance recorded for the conventional AEM A-201. Overall, our results indicated that AEM electrolysis clearly manifests the feasibility of commercial viability.     

Electrochemical- and mechanical stability of catalyst layers in anion exchange membrane water electrolysis

Anion exchange membrane (AEM) water electrolysis is considered a promising solution to future cost reduction of electrochemically produced hydrogen. We present an AEM water electrolyzer with CuCoO_x as the anode catalyst and Aemion as membrane and electrode binder. Full cell experiments in pure water and 0.1 M KOH revealed that the optimum binder content depended on the type of electrolyte employed. Online dissolution measurements suggested that Aemion alone was not sufficient to establish an alkaline environment for thermodynamically stabilizing the synthesized CuCoO_x in a neutral electrolyte feed. A feed of base is thus indispensable to ensure the thermodynamic stability of such non-noble catalyst materials. Particle loss and delamination of the catalyst layer during MEA operation could be reduced by employing a heat treatment step after electrode fabrication. This work summarizes possible degradation pathways for low-cost anodes in AEMWE, and mitigation strategies for enhanced system durability and performance.     

Accurate evaluation of hydrogen crossover in water electrolysis systems for wetted membranes

In fuel cell and electrolysis systems, hydrogen crossover is a phenomenon where hydrogen molecules (H₂) permeate through a membrane, lowering the overall process efficiency and generating a potential safety risk. Many works have been reported to mitigate this undesired phenomenon, but it is yet difficult to accurately measure the rate of hydrogen crossover, particularly when the membrane is fully wetted in water. In this work, we investigated the pressure decay method as a simple, convenient, and low-cost method to characterize hydrogen crossover through wetted membranes for water electrolysis systems. Three different ion exchange membranes were analyzed: Nafion 212, Nafion 115, and in-house sulfonated poly(arylene ether sulfone). We rigorously confirmed our method and data by comparing it to the ANSI dataset with the current state-of-the-art equations of state (EOS) to account for the nonideality of high pressure hydrogen systems. The error from the gas non-ideality was less than 0.03%. As expected, the rate of hydrogen crossover showed high dependency on the temperature; more importantly, hydrogen crossover increased significantly when the membrane was fully soaked in water. For dry membranes, the proposed pressure decay method corroborated well with the literature data measured using other known methods. Moreover, for wetted membranes, the obtained data showed high similarity compared to the GC method which is currently the most reliable method in the literature. We attempted to predict the hydrogen permeability of wetted membranes using the solution diffusion model. The model based on the given thermodynamic parameters overestimated the hydrogen permeability, which can be used to estimate the ion channel tortuosity.     

Operational parameters correlated with the long-term stability of anion exchange membrane water electrolyzers

In this study, we investigated the long-term stability of anion exchange membrane water electrolyzers (AEMWEs) under various bias conditions. The cell performance was relatively stable under conditions of voltage cycling in a narrow range, constant voltage and constant current. On the other hand, a relatively dynamic condition, voltage cycling, in a wide range detrimentally affected the cell stability. Abnormally high negative and positive currents were observed when the cell voltage was switched between 2.1 and 0 V. Impedance results and post-material analyses indicated that the performance degradation was mainly due to anode catalyst detachments, which increased non-ohmic resistance in the wide range voltage cycling. An increase in ohmic resistance was also observed, which was due to the membrane dehydration that occurred in the frequent rest times. Thus, it can be said that the voltage cycling range as well as the frequency of rest times are critical operational parameters in determining the long-term stability of AEMWEs.     

Graphene oxide modified non-noble metal electrode for alkaline anion exchange membrane water electrolyzers

This paper reports the performance of a graphene oxide modified non noble metal based electrode in alkaline anion exchange water electrolyzer. The electrolytic cell was fabricated using a polystyrene based anion exchange membrane and a ternary alloy electrode of Ni as cathode and oxidized Ni electrode coated with graphene oxide as anode. The electrochemical activity of the graphene oxide modified electrode was higher than the uncoated electrode. The anion exchange membrane water electrolyzer (AEMWE) with the modified electrode gave 50% higher current density at 30 °C with deionised water compared to that of an uncoated electrode at 2 V. Performance was found to increase with increase in temperature and with the use of alkaline solutions. The results of the solid state water electrolysis cell are promising method of producing low cost hydrogen.     

Effect of water stoichiometry on deuterium isotope separation by anion exchange membrane water electrolysis

Water electrolysis (WE) is a key technology for a decarbonized society and is essential for hydrogen isotope separation for a future fusion reactor. In this study, deuterium (D) separation was performed by anion exchange membrane WE (AEMWE) and compared with our previous results from proton exchange membrane WE (PEMWE). WE allows D to become concentrated in solution and diluted in hydrogen gas compared with the D concentration in feed water. The separation effect increases with decreasing water stoichiometric ratio, λ, of the water feed to electrolysis volume. Owing to little water drainage from the cathode, AEMWE can be performed at λ ≈ 1.05, while λ ≈ 4.0 is the lowest for PEMWE. When the feed rate is reduced (λ < 2), D in the product water becomes more concentrated, which corresponds to a sharp rise in cell voltage owing to the water shortage at the cathode.     

PPOs having piperidinium-based conducting head groups with extra molecular interaction sites as new anion exchange membranes

Poly(2,6-dimethyl-1,4-phenylene oxide)s [PPOs] with 4-hydroxy-piperidinium and tropinium conducting head groups are developed, and corresponding membranes are investigated as new anion exchange membranes (AEMs). Their properties are compared with piperidinium-functionalized PPO membranes. The additional OH group in the piperidinium unit further enhances the chemo-physical stability of the corresponding membrane (OH-Pip-PPO) due to physical crosslinking through hydrogen bonding. The bulky structure of tropinium provides additional free volume in the corresponding membrane (Trop-PPO). Hydrogen bonding between the polymers and water causes well-developed morphology and high dimensional stability. These membranes also contain ion-conducting highways, facilitating efficient ion transport, in agreement with molecular dynamics simulations. The 4-hydroxy-piperidinium-functionalized PPO showed the highest dimensional and alkaline stability, with slightly lower conductivity and cell performance than piperidinium-functionalized-PPO (Pip-PPO) and Trop-PPO. Trop-PPO further increases free volume and water uptake, resulting in the highest normalized conductivity among the three PPO-based membranes and fuel cell performance similar to Pip-PPO.     

Effect of catalyst distribution and structural properties of anode porous transport electrodes on the performance of anion exchange membrane water electrolysis

Anion exchange membrane (AEM) water electrolyzers are expected to be novel devices for hydrogen (H₂) production that achieve high performance at low capital cost. The effect of catalyst distribution in anode porous transport electrodes (PTEs) on the performance of AEM water electrolysis is experimentally examined. Based on the analysis of the correlation between the PTE structure and the electrolysis performance, it was revealed that the surface catalyst coverage is related to the activation overpotential, and that the location and compactness of the catalyst layer (CL) affects the concentration overpotential. This suggests that the water diffusion through the membrane is related to the concentration overpotential, and that denser CLs can promote water diffusion and thus mitigate the concentration overpotential. Based on the electrolysis data with PTEs of different thickness, it was also revealed that decreasing the thickness of the anode PTE enables good performance with low catalyst loading.     

Bis-pyridinium crosslinked poly(ether ether ketone) anion exchange membranes with enhancement of hydroxide conductivity and alkaline stability

A series of tunable bis-pyridinium crosslinked PEEK-BiPy-x anion exchange membranes (AEMs) are prepared successfully to improve the “trade-off” between ionic conductivity and alkaline stability. The crosslinking density of bis-pyridinium is optimized to promote microphase separation and guarantee the free volume. All the PEEK-BiPy-x membranes have a distinct microphase separation pattern observed by atomic force microscopy (AFM) and the PEEK-BiPy-x membranes also display adequate thermal, mechanical and dimensional stability. Impressively, the PEEK-BiPy-0.5 membrane exhibits maximum tensile strength (58.53 MPa) and highest IEC of 1.316 mmol·g^?1. Meanwhile, its hydroxide conductivity reaches up to 70.86 mS·cm^?1 at 80 °C. Besides, great alkaline stability of PEEK-BiPy-0.5 membrane is obtained with conductivity retention of 91.74% after 1440 h in 1 M NaOH solution, owing to the crosslinked structure of the AEMs and steric effect of bis-pyridinium cations. Overall, the PEEK-BiPy-x membranes possess potential applications in AEMs.     

A facile fabrication of functionalized rGO crosslinked chemically stable polysulfone-based anion exchange membranes with enhanced performance

Introducing more ionic conductive groups in polymer-based anion exchange membranes (AEMs) can improve the ion exchange capacity and further overcome the disadvantage of low ion conductivity for AEMs. However, the excessive swelling of AEMs caused by exorbitant IEC value may reduce the dimensional stability of membranes. So it is extremely important to modify the structures of AEMs. Herein, we proposed a facile strategy to construct reduced graphene oxide (rGO) stable crosslinked polysulfone-based AEMs with improved properties. rGO was non-covalently modified with pyrene-containing tertiary amine small molecule and polymer via π-π interactions. The as-prepared functionalized rGO (TrGO and PrGO) as both cross-linkers and fillers to fabricate quaternized polysulfone (QPSU)-based AEMs (CQPSU-X-TrGO and CQPSU-X-PrGO) for the first time. The cross-linked membranes can tighten the internal packing structure, and enhance the alkaline resistance, ion conductivity and oxidative stability of AEMs. Furthermore, the hydrophilicity and flexibility of the CQPSU-X-PrGO membranes were significantly improved as compared with that of CQPSU-X-TrGO membranes. PrGO-crosslinked membranes (CQPSU-2%-PrGO, σ_OH⁻ = 117.7 mS/cm) displayed higher ionic conductivities at 80 °C than TrGO-crosslinked membranes (CQPSU-1%-TrGO, σ_OH⁻ = 87.2 mS/cm). The remarkable nanophase separation can be observed in the CQPSU-X-PrGO membranes by TEM. This feasible strategy can be efficiently used to prepare new type of crosslinked organic-inorganic nanohybrid AEMs with excellent chemical stability and high ionic conductivity.     

Experimental investigation of electrolytic solution for anion exchange membrane water electrolysis

The effect of species and concentration of electrolytic solution on the performance of anion exchange membrane (AEM) water electrolysis is experimentally examined. When potassium hydroxide (KOH) or potassium carbonate (K₂CO₃) solution is applied for the electrolytic solution, electrolysis is successful under mild alkaline conditions, whereas electrolysis with pure water is quite difficult due to the high resistance of the AEM. AEM electrolysis performance with K₂CO₃ solution is superior to that with KOH solution, even at similar pH of around 12. Hydrogen content in the anode gas compartment and relative humidity of produced hydrogen are also measured during the electrolysis.     

The effect of side chain length on the morphology and transport properties of fluorene-based anion exchange membranes

Poly[(fluorene alkylene)- co(biphenyl alkylene)] (PFBA) compounds with quaternary ammonium (QA) groups (PFBA-nC-QAs) that are linked with side chains of various lengths (n = 1～6 carbon atoms) are designed and synthesized by a superacid catalysis reaction, which has the advantages of low cost, easy synthesis and mild reaction conditions. The correlative properties of PFBA-nC-QAs, including water uptake, thermal stability, morphology, ion conductivity and alkaline stability, are discussed in detail. The side chain length is vital to the morphology and transport performance of PFBA-nC-QAs. As the side chain length increases, the alkaline stability and hydroxide ion conductivity of the prepared membranes improve with decreasing water uptake. Experimental results indicate that the hydroxide conductivity of PFBA-6C-QA is 154 mS cm^?1 at 80 °C. Moreover, no degradation of functional groups of PFBA-6C-QA is observed during 30 days of immersion in 2 M NaOH at 80 °C. The peak power density of PFBA-6C-QA is 278 mW cm^?2 at 60 °C with a hydrogen/air single fuel cell. By controlling the length of the polymer side chain, the method is simple and effective for building anion exchange membranes with high performance.