Radiation-Grafted Polymer Electrolyte Membranes for Water Electrolysis Cells: Evaluation of Key Membrane Properties

Recent advances in proton exchange membrane water electrolysis

Proton exchange membrane water electrolyzers (PEMWEs) are an attractive technology for renewable energy conversion and storage. By using green electricity generated from renewable sources like wind or solar, high-purity hydrogen gas can be produced in PEMWE systems, which can be used in fuel cells and other industrial sectors. To date, significant advances have been achieved in improving the efficiency of PEMWEs through the design of stack components; however, challenges remain for their large-scale and long-term application due to high cost and durability issues in acidic conditions. In this review, we examine the latest developments in engineering PEMWE systems and assess the gap that still needs to be filled for their practical applications. We provide a comprehensive summary of the reaction mechanisms, the correlation among structure-composition-performance, manufacturing methods, system design strategies, and operation protocols of advanced PEMWEs. We also highlight the discrepancies between the critical parameters required for practical PEMWEs and those reported in the literature. Finally, we propose the potential solution to bridge the gap and enable the appreciable applications of PEMWEs. This review may provide valuable insights for research communities and industry practitioners working in these fields and facilitate the development of more cost-effective and durable PEMWE systems for a sustainable energy future.

Multi-Block Copolymer Membranes Consisting of Sulfonated Poly(p-Phenylene) and Naphthalene Containing Poly(Arylene Ether Ketone) for Proton Exchange Membrane Water Electrolysis

Glassy hydrocarbon-based membranes are being researched as a replacement for perfluorosulfonic acid (PFSA) membranes in proton exchange membrane water electrolysis (PEMWE). Here, naphthalene containing poly(arylene ether ketone) was introduced into the poly(p-phenylene)-based multi-block copolymers through Ni(0)-catalyzed coupling reaction to enhance π-π interactions of the naphthalene units. It is discovered that there is an optimum input ratio of the hydrophilic monomer and NBP oligomer for the multi-block copolymers with high ion exchange capacity (IEC) and polymerization yield. With the optimum input ratio, the naphthalene containing copolymer exhibits good hydrogen gas barrier property, chemical stability, and mechanical toughness, even with its high IEC value over 2.4 meq g−1. The membrane shows 3.6 times higher proton selectivity to hydrogen gas than Nafion 212. The PEMWE single cells using the membrane performed better (5.5 A cm−2) than Nafion 212 (4.75 A cm−2) at 1.9 V and 80 °C. These findings suggest that naphthalene containing copolymer membranes are a promising replacement for PFSA membranes in PEMWE.

Novel Filler-Filled-Type Polymer Electrolyte Membrane for PEFC Employing Poly(vinylphosphonic acid)-b-polystyrene-Coated Cellulose Nanocrystals as a Filler

Low-acidity polymer electrolyte membranes are essential to polymer electrolyte fuel cells (PEFCs) and water electrolysis systems, both of which are expected to be next-generation energy and hydrogen sources. We developed a new type of high-performance polymer electrolyte membrane (PEM) in which the core particles are precisely electrolyte polymer coated and filled into binder resin. Cellulose nanocrystals (CNCs), which have attracted attention as light, rigid, and sustainable materials, were selected as the core material for the filler. The CNC surface was coated with a new block copolymer containing a proton conductive polymer of poly(vinylphosphonic acid) (PVPA) and a hydrophobic polymer of polystyrene (PS) using RAFT polymerization with particles (PwP) we developed. The pelletized fillers and the filler-filled polycarbonate membranes achieved proton conductivities of over 10^-2 S/cm with lower activation energies and much weaker acidity than the Nafion membrane.

Crosslinked Sulfonated Polyphenylsulfone (CSPPSU) Membranes for Elevated-Temperature PEM Water Electrolysis

In order to reduce the burden on the environment, there is a need to develop non-fluorinated electrolyte membranes as alternatives to fluorinated electrolyte membranes, and water electrolysis using hydrocarbon-based electrolyte membranes has been studied in recent years. In this paper, for the first time, we report elevated-temperature water electrolysis properties of crosslinked sulfonated polyphenylsulfone (CSPPSU) membranes prepared by sulfonation and crosslinking of hydrocarbon-based PPSU engineering plastics. The sulfone groups of the CSPPSU membrane in water were stable at 85 °C (3600 h) and 150 °C (2184 h). In addition, the polymer structure of the CSPPSU membrane was stable during small-angle X-ray scattering (SAXS) measurements from room temperature to 180 °C. A current density of 456 mA/cm² was obtained at 150 °C and 1.8 V in water electrolysis using the CSPPSU membrane and IrO₂/Ti as the catalytic electrode for oxygen evolution. The stability of the CSPPSU membrane at elevated temperatures with time was evaluated. There were some issues in the assembly of the CSPPSU membrane and the catalytic electrode. However, the CSPPSU membrane has the potential to be used as an electrolyte membrane for elevated-temperature water electrolysis.

H+-Conducting Aromatic Multiblock Copolymer and Blend Membranes and Their Application in PEM Electrolysis

As an alternative to common perfluorosulfonic acid-based polyelectrolytes, we present the synthesis and characterization of proton exchange membranes based on two different concepts: (i) Covalently bound multiblock-co-ionomers with a nanophase-separated structure exhibit tunable properties depending on hydrophilic and hydrophobic components’ ratios. Here, the blocks were synthesized individually via step-growth polycondensation from either partially fluorinated or sulfonated aromatic monomers. (ii) Ionically crosslinked blend membranes of partially fluorinated polybenzimidazole and pyridine side-chain-modified polysulfones combine the hydrophilic component’s high proton conductivities with high mechanical stability established by the hydrophobic components. In addition to the polymer synthesis, membrane preparation, and thorough characterization of the obtained materials, hydrogen permeability is determined using linear sweep voltammetry. Furthermore, initial in situ tests in a PEM electrolysis cell show promising cell performance, which can be increased by optimizing electrodes with regard to binders for the respective membrane material.

Improving the Efficiency of PEM Electrolyzers through Membrane-Specific Pressure Optimization

Hydrogen produced in a polymer electrolyte membrane (PEM) electrolyzer must be stored under high pressure. It is discussed whether the gas should be compressed in subsequent gas compressors or by the electrolyzer. While gas compressor stages can be reduced in the case of electrochemical compression, safety problems arise for thin membranes due to the undesired permeation of hydrogen across the membrane to the oxygen side, forming an explosive gas. In this study, a PEM system is modeled to evaluate the membrane-specific total system efficiency. The optimum efficiency is given depending on the external heat requirement, permeation, cell pressure, current density, and membrane thickness. It shows that the heat requirement and hydrogen permeation dominate the maximum efficiency below 1.6 V, while, above, the cell polarization is decisive. In addition, a pressure-optimized cell operation is introduced by which the optimum cathode pressure is set as a function of current density and membrane thickness. This approach indicates that thin membranes do not provide increased safety issues compared to thick membranes. However, operating an N212-based system instead of an N117-based one can generate twice the amount of hydrogen at the same system efficiency while only one compressor stage must be added.

Synthesis and Membrane Properties of Sulfonated Poly(arylene ether sulfone) Statistical Copolymers for Electrolysis of Water: Influence of Meta- and Para-Substituted Comonomers

Two series of high molecular weight disulfonated poly(arylene ether sulfone) random copolymers were synthesized as proton exchange membranes for high-temperature water electrolyzers. These copolymers differ based on the position of the ether bonds on the aromatic rings. One series is comprised of fully para-substituted hydroquinone comonomer, and the other series incorporated 25 mol % of a meta-substituted comonomer resorcinol and 75 mol % hydroquinone. The influence of the substitution position on water uptake and electrochemical properties of the membranes were investigated and compared to that of the state-of-the-art membrane Nafion. The mechanical properties of the membranes were measured for the first time in fully hydrated conditions at ambient and elevated temperatures. Submerged in water, these hydrocarbon-based copolymers had moduli an order of magnitude higher than Nafion. Selected copolymers of each series showed dramatically increased proton conductivities at elevated temperature in fully hydrated conditions, while their H₂ gas permeabilities were well controlled over a wide range of temperatures. These improved properties were attributed to the high glass transition temperatures of the disulfonated poly(arylene ether sulfone)s.

Stability and Degradation Mechanisms of Radiation-Grafted Polymer Electrolyte Membranes for Water Electrolysis

Radiation-grafted membranes are a promising alternative to commercial membranes for water electrolyzers, since they exhibit lower hydrogen crossover and area resistance, better mechanical properties, and are of potentially lower cost than perfluoroalkylsulfonic acid membranes, such as Nafion. Stability is an important factor in view of the expected lifetime of 40 000 h or more of an electrolyzer. In this study, combinations of styrene (St), α-methylstyrene (AMS), acrylonitrile (AN), and 1,3-diisopropenylbenzene (DiPB) are cografted into 50 μm preirradiated poly(ethylene-co-tetrafluoroethylene) (ETFE) base film, followed by sulfonation to produce radiation-grafted membranes. The stability of the membranes with different monomer combinations is compared under an accelerated stress test (AST), and the degradation mechanisms are investigated. To mimic the conditions in an electrolyzer, in which the membrane is always in contact with liquid water at elevated temperature, the membranes are immersed in water for 5 days at 90 °C, so-called thermal stress test (TST). In addition to testing in air atmosphere tests are also carried out under argon to investigate the effect of the absence of oxygen. The water is analyzed with UV-vis spectroscopy and ion chromatography. The ion exchange capacity (IEC), swelling degree, and Fourier transform infrared (FTIR) spectra of the membranes are compared before and after the test. Furthermore, energy-dispersive X-ray (EDX) spectroscopic analysis of the membrane cross-section is performed. Finally, the influence of the TST to the membrane area resistance and hydrogen crossover is measured. The stability increases along the sequence St/AN, St/AN/DiPB, AMS/AN, and AMS/AN/DiPB grafted membrane. The degradation at the weak-link, oxygen-induced degradation, and hydrothermal degradation are proposed in addition to the "swelling-induced detachment" reported in the literature. By mitigating the possible paths of degradation, the AMS/AN/DiPB grafted membrane is shown to be the most stable membrane and, therefore, it is a promising candidate for a membrane to be used in a water electrolyzer.

2-Acrylamido-2-methyl-1-propanesulfonic Acid Grafted Poly(vinylidene fluoride-co-hexafluoropropylene)-Based Acid-/Oxidative-Resistant Cation Exchange for Membrane Electrolysis

For developing acid-/oxidative-resistant aliphatic-polymer-based cation-exchange membrane (CEM), macromolecular modification of poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP) was carried out by controlled chemical grafting of 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS). To introduce the unsaturation suitable for chemical grafting, dehydrofluorination of commercially available PVDF-co-HFP was achieved under alkaline medium. Sulfonated copolymer (SCP) was prepared by the free radical copolymerization of dehydofluorinated PVDF-co-HFP (DHPVDF-co-HFP) and AMPS in the presence of free radical initiator. Prepared SCP-based CEMs were analyzed for their morphological characteristics, ion-exchange capacity (IEC), water uptake, conductivity, and stabilities (mechanical, chemical, and thermal) in comparison with state-of-art Nafion117 membrane. High bound water content avoids the membrane dehydration, and most optimal (SCP-1.33) membrane exhibited about ∼2.5-fold high bound water content in comparison with that of Nafion117 membrane. Bunsen reaction of iodine-sulfur (I-S) was successfully performed by direct-contact-mode membrane electrolysis in a two-compartment electrolytic cell using different SCP membranes. High current efficiency (83-99%) confirmed absence of any side reaction and 328.05 kJ mol-H2(-1) energy was required for to produce 1 mol of H2 by electrolytic cell with SCP-1.33 membrane. In spite of low conductivity for reported SCP membrane in comparison with that of Nafion117 membrane, SCP-1.33 membrane was assessed as suitable candidate for electrolysis because of its low-cost nature and excellent stabilities in highly acidic environment may be due to partial fluorinated segments in the membrane structure.