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.     

Preparation,characterization and properties of proton exchange nanocomposite membranes based on poly(vinyl alcohol) and poly(sulfonic acid)-grafted silica nanoparticles

Nanocomposite membranes based on the poly(vinyl alcohol) (PVA)/poly(sulfonic acid)-grafted silica nanoparticles (PSA-g-SN) were prepared via solvent casting of PVA cross linked by glutardialdehyde in the presence of various amounts (0–20 wt%) of silica nanoparticles (SN), poly(styrene sulfonic acid)- (PSSA-g-SN) and poly (2-acrylamido-2-methyl-1-propane sulfonic acid)-grafted silica nanoparticles (PAMPS-g-SN) as hydrophilic inorganic modifiers. PSA-g-SN nanoparticles were synthesized by surface-initiated redox grafting of SSA and AMPS monomers from the surface of the aminopropylated silica nanoparticles. Membranes were then characterized by FTIR, impedance spectroscopy, thermogravimetric analysis (TGA), water uptake, tensile strength test and SEM. The best proton conductivity was observed for membranes containing 5 wt% of nanoparticles. Among three nanoparticles used, the highest proton conductivity (10.4 mS/cm) was observed for PVA membrane prepared in the presence of 5 wt% PAMPS-g-SN nanoparticles. Results showed that grafting of sulfonated monomer onto the silica nanoparticles enhances various properties, for example proton conductivity, of the polymer electrolyte membranes (PEMs).     

Amorphous-crystalline cobalt-molybdenum bimetallic phosphide heterostructured nanosheets as Janus electrocatalyst for efficient water splitting

Efficient and sustainable Janus catalysts toward hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are highly desirable for future hydrogen production via water electrolysis. Herein we report an active Janus electrocatalyst of amorphous-crystalline cobalt-molybdenum bimetallic phosphide heterostructured nanosheets on nickel foam (CoMoP/CoP/NF) for efficient electrolysis of alkaline water. As-reported CoMoP/CoP/NF consists of amorphous bimetal phosphide nanosheets doped with crystalline CoMoP/CoP heterostructured nanoparticles on NF. It can efficiently catalyze both HER (η = 127 mV@100 mA cm^?2) and OER (η = 308 mV@100 mA cm^?2) in alkaline electrolyte with long-term durability. Serving as anode and cathode of water electrolyzer, CoMoP/CoP/NF generates electrolytic current of 10, 50 and 100 mA cm^?2 at low voltage of 1.50, 1.59, and 1.67 V, respectively.     

Dynamic modelling of an alkaline water electrolysis system and optimization of its operating parameters for hydrogen production

This work aims at developing an approach for modelling and optimizing the operation of a reference alkaline electrolysis unit operating in transient state using orthogonal collocation on finite elements (OCFE). The main goal is to define the set of operating conditions that minimize the processing cost (associated to electricity cost) given a hydrogen yield. Three components of the electrolyzer are considered: the stack of electrolytic cells and two separators that single out the hydrogen and oxygen gas streams. The dynamic behavior is considered for the mass holdup in the separators as well as the energy accumulation for these three components. The associated mathematical model is derived in the paper. Its solving allows characterizing the influence of the transient operating parameters of the system on its working and associated final hydrogen production. Mathematical optimization aims at defining the ideal operating load in order to minimize costs associated to fluctuating price of electricity consumed by the stack given a defined hydrogen yield. The model has been validated according to experimental test runs and operating conditions have been optimized under a proof of concept scenario saving 17% of electricity costs if compared to constant plant capacity.     

SPE water electrolysis with SPEEK/PES blend membrane

Sulfonated poly(ether ether ketone) (SPEEK) was blended with poly(ether sulfone) (PES) to make solid polymer electrolyte (SPE) membranes for hydrogen production via water electrolysis. The blend membranes were characterized in terms of proton conductivity and the swelling degree in water. Membrane electrode assemblies (MEA), with Ir anode and Pt cathode at the two side of the blended membrane, were prepared by a decal method. The effect of hot pressing conditions in fabricating the MEA and the influence of ionomers in the catalyst layers were investigated. The MEA, with an effective area of 4 cm², were tested using a single cell water electrolysis test stand. An electrolytic current of 1655 mA/cm² were obtained at 2 V and 80 °C with the SPEEK based MEA and under suitable fabrication conditions. The experimental results suggest that SPEEK/PES blend membrane could be an alternative to costly perfluorosulfonate membranes in SPE water electrolysis for 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.     

Hydrogen production via electrolysis of aqueous formic acid solutions

Hydrogen was produced via electrolysis of aqueous formic acid solutions, and the effects of the concentrations of formic acid and NaOH on the electrolytic voltage were systematically investigated. The voltage is found to be related to the actual formic acid concentration. When the actual formic acid concentration is higher than 0.8 × 10⁻⁹ M, the initial electrolytic voltage can be as low as 0.30 V, which is much lower than the open circuit voltage in a proton exchange membrane fuel cell. The electrolytic voltage increases with the increase of the current density. Specifically at 1.0 M NaOH and 4.0 M HCOOH, the steady voltage value increases from 0.62 to 0.70 V as the current density increases from 1.0 to 6.0 mA/cm². At 3.0 M HCOOH and 2.5 M NaOH, the hydrogen production rate is 53 μmol/h under 8.0 mA/cm², which is promising for practical industrial-scale hydrogen production.     

Sulfonated poly(ether ether ketone)/poly(ether sulfone) composite membranes as an alternative proton exchange membrane in microbial fuel cells

Custom-made proton exchange membranes (PEM) are synthesized by incorporating sulfonated poly(ether ether ketone) (SPEEK) in poly(ether sulfone) (PES) for electricity generation in microbial fuel cells (MFCs). The composite PES/SPEEK membranes at various composition of SPEEK are prepared by the phase inversion method. The membranes are characterized by measuring roughness, proton conductivity, oxygen diffusion, water crossover and electrochemical impedance. The conductivity of hydrophobic PES membrane increases when a small amount (3–5%) of hydrophilic SPEEK is added. The electrochemical impedance spectra shows that the conductivity and capacitance of PES/SPEEK composite membranes during MFC operation are reduced from 6.15 × 10⁻⁷ to 6.93 × 10⁻⁵ (3197 Ω–162 Ω) and from 3.00 × 10⁻⁷ to 1.56 × 10⁻³ F, respectively when 5% of SPEEK added into PES membrane. The PES/SPEEK 5% membrane has the highest performance compared to other membranes with a maximum power density of 170 mW m⁻² at the maximum current density of 340 mA m⁻². However, the interfacial reaction between the membrane and the cathode with Pt catalyst indicates moderate reaction efficiency compared to other membranes. The COD removal efficiency of MFCs with composite membrane PES/SPEEK 5% is nearly 26-fold and 2-fold higher than that of MFCs with Nafion 112 and Nafion 117 membranes respectively. The results suggest that the PES/SPEEK composite membrane is a promising alternative to the costly perfluorosulfonate membranes presently used as separators in MFC systems.     

Diaphragms obtained by radiochemical grafting of PTFE

Diaphragms for alkaline water electrolysis are prepared by radiochemical grafting of PTFE fabric with styrene, which is later on sulfonated, or with acrylic acid. The diaphragms obtained are mechanically resistant to potash at temperatures up to 200°C, but show some degrafting, which limits the lifetime. The sulfonated styrene group has been found to be more stable in electrolysis than the acrylic acid. In both cases, the incorporation of a cross-linking agent like divinyl benzene improves the lifetime of the diaphragms. Electrolysis during 500 hours at 120°C and 10 kAm² could be performed.     

Comparison of different types of membrane in alkaline direct ethanol fuel cells

It has been understood that the use of cation-exchange membranes (CEM) and alkali-doped polybenizimidazole membranes (APM) in alkaline direct ethanol fuel cells (DEFC) with an added base in the fuel exhibits performance similar to the use of anion-exchange membranes (AEM). The present work is to assess the suitability of the three types of membrane to alkaline DEFCs by measuring and comparing the membrane properties including the ionic conductivity, the species permeability, as well as the thermal and mechanical properties. The comparison shows that: (i) the AEM is still the most promising membrane for the alkaline DEFC, although the thermal stability needs to be further enhanced; (ii) before solving the problem of the poor thermal stability of AEMs, the CEM is another choice for the alkaline DEFC running at high temperatures (<90 °C); and (iii) the APM can also be applied to the alkaline DEFC operating at high temperatures, but its mechanical property needs to be substantially enhanced and the species permeability needs to be dramatically decreased.     

Feasibility of using thin polybenzimidazole electrolytes in high-temperature proton exchange membrane fuel cells

The use of thin polybenzimidazole membranes in high-temperature polymer electrolyte membrane fuel cells is explored. Membranes in thickness of 10–40 μm are prepared, doped and characterized, including fuel cell test. High molecular weight polymers enable fabrication of membranes as thin as 10 μm with sufficient mechanical strength. The thin membranes, upon acid doping, exhibit comparable conductivity and hence decreased ohmic resistance. Membrane electrode assemblies with thin membranes down to 10 μm show slightly lower open-circuit voltages than that for reference 40 μm but all above 0.97 V. This is in good agreement with the hydrogen permeability measurements, which show a value around 10^?12 mol cm^?1 s^?1 bar^?1, corresponding to a crossover current density of <1 mA cm^?2. The acid transferred from the membrane to the catalyst layer seems constant, as the iR-free polarization plots are nearly the same for membranes of varied thicknesses. The acid remaining in the membrane after the break-in period is estimated, showing an acid inventory issue when thin membranes are used. This is verified by using the membranes of higher acid doping levels.     

Synthesizing spindle-shaped anion exchange membranes to improve conductivity and stability

High ionic conductivity and excellent alkaline stability are very important for solid electrolyte. Therefore, spindle-shaped anion exchange membranes (AEMs) based on poly (arylene ether ketone) and 1-Bromo-N,N,N-trimethylhexane-6-aminium bromide (Br-QA) have been prepared. The obtained Br-QA can be grafted with poly (arylene ether ketone) main chains to form micro-phase separation structure enhancing the ionic conductivity. Especially, the grafting quaternary ammonium (QA) cation groups are separated by alkyl bromine endows the AEMs with alkaline stability features. Simultaneously, the OH⁻ conductivity of the QA-PAEK-0.6 obtained membranes is 0.046 S/cm under fully hydrated conditions at 60 °C. After immersing into 1 M NaOH alkaline solution for 15 days at 60 °C, the anionic conductivity still high to 0.03 S/cm. Meanwhile, the poly (arylene ether ketone) backbones provide excellent mechanical properties and the Br-QA cation groups also possess good thermal stability, which satisfy the requirement of wide applications.     

Electrolytic separators from asbestos cardboard: A flexible technique to obtain reinforced diaphragms or ion-selective membranes

A new electrolytic separator made of asbestos cardboard containing polystyrene-divinylbenzene sulphonic acid was obtained by inhibition of the cardboard with a benzene solution of styrene and divinylbenzene, polymerizing the monomers into the asbestos matrix and sulphonating the reaction product with sulphur trioxide at ～0°C. The structure and chemical and physical properties of the separators containing 12–43% w/w of the organic copolymer, both in the sulphonated and parent hydrocarbon form, were investigated by IR spectra, thermocalorimetric analysis, electron microscopy and weight swelling, hydraulic radius, permeability, electrical resistance and permselectivity measurements. The new separator, compared to new asbestos, exhibits a higher mechanical and chemical stability. A thin (0.065 cm) cardboard reinforced with 12% w/w sulphonated copolymer did not show any chemical or physical damage after 1000 h electrolysis of alkaline (KOH 30% w/w) water at 80°C. Compared to the plain asbestos cardboard of equal thickness, the former can withstand much higher hydraulic (> 100 vs less than 7 cm of NaCl-NaOH 300-200 g l.^?1 (brine solution) and tear point (97 vs 57 kg cm^?2) pressure and exhibits only 0.1 ω · cm² higher electrical resistance at temperatures greater than or equal to 50°C. Heavily loaded separators do not work as well in alkaline water electrolysis. Concentrations of sulphonated copolymer greater than 13% w/w imply decreased porosity and higher electrical resistance, but the separator acquires an increase in the efficiency of ion-selectivity.     

Radiation grafted membranes for superior anion exchange polymer membrane fuel cells performance

A study of radiation grafted polymers on the conductivity and performance of alkaline anion exchange membrane fuel cells (AAEMFCs) is reported. The aminated poly (LDPE-g-VBC), poly (HDPE-g-VBC) and poly (ETFE-g-VBC) membranes were produced by the using the radiation grafting technique. Differences in grafting behaviour are observed between the studied materials caused by differences in the base polymer film properties as molar mass, crystallinity, orientation or grafting technique used. In plane conductivities increased with Degree of Grafting DoG. At a DoG of 68% the LDPE-g-VBC membrane achieved an in-plane ionic conductivity between 0.18 and 0.32 S cm⁻¹ in the temperature range 20–80 °C. Measured through plane conductivities were lower than that of the in plane ones for all studied membranes. Membranes with the highest degree of swelling showed the highest through plane conductivity of 0.07–0.11 S cm⁻¹. The membrane specific resistance (per MEA cm²) of most of the produced membranes was in the range of 0.09–0.18 Ω cm². While membrane conductivity and hence IR loss is a crucial factor in fuel cell performance, membrane water permeability is a similarly crucial key for optimised water transport to the cathode. The main source of performance loss of AAEMFCs is believed to be restricted mass transport of water to the cathode reaction sites. The highly humidified anode stream along with large amount of water produced at the anode at high current densities could lead to flooding if water is not removed quickly to the cathode via the membrane (back diffusion) where it is consumed.     

Polybenzimidazole/zwitterion-coated polyamidoamine dendrimer composite membranes for direct methanol fuel cell applications

The zwitterion-coated polyamidoamine (ZC-PAMAM) dendrimer with ammonium and sulfonic acid groups has been synthesized and used as filler for the preparation of PBI-based composite membranes for direct methanol fuel cells. Polybenzimidazole (PBI)/ZC-PAMAM dendrimer composite membranes were prepared by casting a solution of PBI and ZC-PAMAM dendrimer, and then evaporating the solvent. The presence of ZC-PAMAM dendrimer was confirmed by FT-IR and energy-dispersive X-ray spectroscopy (EDS) mapping of sulfur and oxygen elements. The water uptake, swelling degree, proton conductivity, and methanol permeability of the membranes increased with the ZC-PAMAM dendrimer content. For the PBI/ZC-PAMAM-20 membrane with 20 wt% of ZC-PAMAM, it shows a proton conductivity of 1.83 × 10⁻² S/cm at 80 °C and a methanol permeability of 5.23 × 10⁻⁸ cm² s⁻¹. Consequently, the PBI/ZC-PAMAM-20 demonstrates a maximum power density of 26.64 mW cm⁻² in a single cell test, which was about 2-fold higher than Nafion-117 membrane under the same conditions.     

A novel anion exchange membrane based on poly (2,6-dimethyl-1,4-phenylene oxide) with excellent alkaline stability for AEMFC

We report a novel comb-shaped anion exchange membrane based on poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) and 4-(dimethylamino)butyraldehyde diethyl acetal (DABDA). The Menshutkin reaction successfully introduced DABDA into the brominated PPO backbone, which was proved through FT-IR and ¹H NMR. The distinct hydrophilic/hydrophobic microphase separation structure observed by transmission electron microscope (TEM). As the increase of grafting degree, so does the water uptake, swelling ratio, hydroxide conductivity and alkaline stability. The membranes also possess good mechanical property with tensile strength from 14.2 to 38.11 MPa. This is due to the increasing number of hydroxide and unique steric hindrance effect caused the higher water uptake and higher dimensional stability. Simultaneously, especially PPO/DABDA-60 in comb-shaped membranes demonstrate an excellent long-term alkali resistance stability. In a 576-h alkali resistance stability test, the retained ionic conductivity of the PPO/DABDA-60 membrane is 96% of the initial value. The PPO/DABDA-60 is a potential candidate material for AEM.     

Water management in membrane electrolysis and options for advanced plants

The development of polymer electrolyte membrane electrolysis (PEMEL) is driven by increasing performance to decrease the costs of electrolysis systems. One option for increasing power density is decreasing the Ohmic losses within the cell. This can be enabled by using thinner membranes, although the disadvantage of thin membranes is their lower diffusion resistivity for water, hydrogen and oxygen what influences the efficiency and the operating conditions. In this paper the water transport and the Ohmic resistance of catalyst coated membranes with different thickness are analyzed. The disadvantage of high water permeability in thin membranes can be used to change the feed configuration in stacks and systems. It is possible to feed the electrolysis only from the cathode, which simplifies the mass transport (single phase) in the anode's porous transport layer and reducing stack and system dimensions, as well as costs.     

Constructing micro-phase separation structure by multi-arm side chains to improve the property of anion exchange membrane

In order to improve the performance of anion exchange membrane (AEMs) as the core component of alkaline fuel cell, a novel pentamethyl-contained phenolphthalein multi-arm monomer is synthesized. The highly imidazolium-functionalized poly (arylene ether ketone) membrane (Im-PEK-x) are prepared by introducing 1,2-dimethylimidazole as hydrophilic segments. The monomer, polymer and anion exchange membranes are confirmed by ¹H NMR spectra. The well-defined micro-phase separated structure of membranes is conducive to ion transport and the structure is investigated by TEM and SAXS. The imidazolium-functionalized membranes (Im-PEK-0.8) exhibits high ionic conductivity (0.148 S/cm at 80 °C). The tensile strength of Im-PEK-0.8 membrane is 30.06 MPa. Furthermore, after immersing in 60 °C, 2 M NaOH solution for 240 h, the ionic conductivity remains 0.092 S/cm for Im-PEK-0.8. The 1,2-dimethylimidazole enhance alkaline stability by steric effect of the substituent group at the C2 position. All these results indicate that this is a new method to enhance conductivity and stability performance of AEMs.     

Comparison of ionically and ionical-covalently cross-linked polyaromatic membranes for SO2 electrolysis

It has been shown that hydrogen, which can be used for energy storage, can be produced efficiently by the membrane based Hybrid Sulfur (HyS) process. During the HyS electrolysis step, SO₂ and H₂O are converted to H₂ and H₂SO₄, which implies that membranes to be used for this process should have both a high proton conductivity and acid stability. In this study ionic and ionic-covalently cross-linked polybenzimidazole (PBI) blended membranes were investigated and compared with Nafion^®212 in terms of their acid stability. Characterization of the membranes, which included monitoring the change in weight, swelling, SEM/EDX, TEM, TGA-MS, FTIR and IEC before and after H₂SO₄ treatment showed that all tested membranes were stable in 80 wt% H₂SO₄ at 80 °C for 120 h. Subsequent HyS electrolysis showed that the blend membranes performed better than Nafion^®115 at current densities below 0.3 A/cm², while performing similar above 0.3 A/cm².     

High performance porous polybenzimidazole membrane for alkaline fuel cells

In this study, a highly ion-conductive and durable porous polymer electrolyte membrane based on ion solvating polybenzimidazole (PBI) was developed for anion exchange membrane fuel cells (AEMFCs). The introduction of porosity can increase the attraction of electrolytic solutions (e.g., potassium hydroxide (KOH)) and ion solvation, which results in the enhancement of PBI's ionic conductivity. The morphology, thermo-physico-chemical properties, ionic conductivity, alkaline stability, and the AEMFC performance of KOH-doped PBI membranes with different porosities were characterized. The ionic conductivity and AEMFC performance of 70 wt.% porous PBI was about 2 times higher than that of the commercially available Fumapem^® FAA. All KOH-doped porous PBI membranes maintained their ionic conductivity after accelerated alkaline stability testing over a period of 14 days, while the commercial FAA degraded just after 3 h. The excellent performance and good durability of KOH-doped porous PBI membrane makes it a promising candidate for AEMFCs.