Aryl ether-free polymer electrolytes for electrochemical and energy devices.

Anion exchange polymers (AEPs) play a crucial role in green hydrogen production through anion exchange membrane water electrolysis. The chemical stability of AEPs is paramount for stable system operation in electrolysers and other electrochemical devices. Given the instability of aryl ether-containing AEPs under high pH conditions, recent research has focused on quaternized aryl ether-free variants. The primary goal of this review is to provide a greater depth of knowledge on the synthesis of aryl ether-free AEPs targeted for electrochemical devices. Synthetic pathways that yield polyaromatic AEPs include acid-catalysed polyhydroxyalkylation, metal-promoted coupling reactions, ionene synthesis via nucleophilic substitution, alkylation of polybenzimidazole, and Diels-Alder polymerization. Polyolefinic AEPs are prepared through addition polymerization, ring-opening metathesis, radiation grafting reactions, and anionic polymerization. Discussions cover structure-property-performance relationships of AEPs in fuel cells, redox flow batteries, and water and CO2 electrolysers, along with the current status of scale-up synthesis and commercialization.

An organic proton cage that is ultra-resistant to hydroxide-promoted degradation.

Alkaline polymer membrane electrochemical energy conversion devices offer the prospect of using non-platinum group catalysts. However, their cationic functionalities are currently not sufficiently stable for vapor-phase applications, such as fuel cells. Herein, we report 1,6-diazabicyclo[4.4.4]tetradecan-1,6-ium (in-DBD), a cationic proton cage, that is orders of magnitude more resistant to hydroxide-promoted degradation than state-of-the-art organic cations under ultra-dry conditions and elevated temperature, and the first organic cation-hydroxide to persist at critically low hydration levels ( < 10% RH at 80 °C). This high stability against hydroxide-promoted degradation is due to the unique combination of endohedral protection and intra-bridgehead hydrogen bonding that prevents the removal of the inter-cavity proton and lowers the susceptibility to Hofmann elimination. We anticipate this discovery will facilitate a step-change in the advancement of materials and electrochemical devices utilizing anion-exchange membranes based on in-DBD that will enable stable operation under extreme alkaline conditions.

Cation Exchange Membranes and Process Optimizations in Electrodialysis for Selective Metal Separation: A Review.

The selective separation of metal species from various sources is highly desirable in applications such as hydrometallurgy, water treatment, and energy production but also challenging. Monovalent cation exchange membranes (CEMs) show a great potential to selectively separate one metal ion over others of the same or different valences from various effluents in electrodialysis. Selectivity among metal cations is influenced by both the inherent properties of membranes and the design and operating conditions of the electrodialysis process. The research progress and recent advances in membrane development and the implication of the electrodialysis systems on counter-ion selectivity are extensively reviewed in this work, focusing on both structure-property relationships of CEM materials and influences of process conditions and mass transport characteristics of target ions. Key membrane properties, such as charge density, water uptake, and polymer morphology, and strategies for enhancing ion selectivity are discussed. The implications of the boundary layer at the membrane surface are elucidated, where differences in the mass transport of ions at interfaces can be exploited to manipulate the transport ratio of competing counter-ions. Based on the progress, possible future R&D directions are also proposed.

Performance and Stability of Aemion and Aemion+ Membranes in Zero-Gap CO2 Electrolyzers with Mild Anolyte Solutions.

The dependence of performance and stability of a zero-gap CO2 electrolyzer on the properties of the anion exchange membrane (AEM) is examined. This work firstly assesses the influence of the anolyte when using an Aemion membrane and then shows that when using 10 mM KHCO3 , a CO2 electrolyzer using a next-generation Aemion+ membrane can achieve lower cell voltages and longer lifetimes due to increased water permeation. The impact of lower permselectivity of Aemion+ on water transport is also discussed. Using Aemion+, a cell voltage of 3.17 V at 200 mA cm-2 is achieved at room temperature, with a faradaic efficiency of >90 %. Stable CO2 electrolysis at 100 mA cm-2 is demonstrated for 100 h, but with reduced lifetime at 300 mA cm-2 . However, the lifetime of the cell at high current densities is shown to be increased by improving water transport characteristics of the AEM and reducing dimensional swelling, as well as by improving cathode design to reduce localized dehydration of the membrane.

Spectroelectrochemical Detection of Water Dissociation in Bipolar Membranes.

The potentials at which water dissociation occurs in bipolar membranes (BPM) and the relationship between water dissociation and current-voltage curve characteristics are explored using a novel spectroelectrochemical approach in which an anion exchange membrane is doped with a pH indicator. Using this method, we visually detect a pH change in the BPM resulting from OH- formed during the water dissociation reaction. The color change is measured with a UV/vis spectrometer, while electrochemical characterization of the BPM is performed simultaneously. Additional measurements were performed on BPMs with varying anion and cation exchange membrane layer thickness. Our measurements provide direct evidence of water dissociation occurring within a BPM at cross-membrane potentials below 0.5 V, within the first limiting current density region. We also show that the effects of changing bulk anion and cation exchange layer thickness is highly dependent on the permselectivity of these layers.

Does power ultrasound affect hydrocarbon Ionomers?

The effect of low-frequency high-power ultrasound on hydrocarbon-based ionomers, cation exchange sulfonated phenylated polyphenylene (sPPB-H+) and anion exchange hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), was studied. Ionomer solutions were subjected to ultrasonication at fixed ultrasonic frequencies (f = 26 and 42 kHz) and acoustic power (Pacous = 2.1 - 10.6 W) in a laboratory-grade ultrasonication bath, and a probe ultrasonicator; both commonly employed in catalyst ink preparation in research laboratory scale. Power ultrasound reduced the polymer solution viscosity of both hydrocarbon-based ionomers. The molecular weight of sPPB-H+ decreased with irradiation time. Changes in viscosity and molecular weight were exacerbated when ultrasonicated in an ice bath; but reduced when the solutions contained carbon black, as typically used in Pt/C-based catalyst inks. Spectroscopic analyses revealed no measurable changes in polymer structure upon ultrasonication, except for very high doses, where evidence for free-radical induced degradation was observed. Ionomers subjected to ultrasound were used to prepare catalyst layers and membrane electrode assemblies (MEA)s. Despite the changes in the ionomer described above, no significant differences in electrochemical performance were found between MEAs prepared with ionomers pre-subjected to ultrasound and those that were not, suggesting that fuel cell performance is tolerant to ionomers subjected to ultrasound.

Transition metal-containing nitrogen-doped nanocarbon catalysts derived from 5-methylresorcinol for anion exchange membrane fuel cell application.

Highly active electrocatalysts for electrochemical oxygen reduction reaction (ORR) were prepared by high-temperature pyrolysis from 5-methylresorcinol, Co and/or Fe salts and dicyandiamide, which acts simultaneously as a precursor for reactive carbonitride template and a nitrogen source. The electrocatalytic activity of the catalysts for ORR in alkaline solution was studied using the rotating disc electrode (RDE) method. The bimetallic catalyst containing iron and cobalt (FeCoNC-at) showed excellent stability and remarkable ORR performance, comparable to that of commercial Pt/C (20 wt%). The superior activity was attributed to high surface metal and nitrogen contents. The FeCoNC-at catalyst was further tested in anion exchange membrane fuel cell (AEMFC) with poly-(hexamethyl-p-terphenylbenzimidazolium) (HMT-PMBI) membrane, where a high value of peak power density (Pmax = 415 mW cm-2) was achieved.

Voltammetry at Hexamethyl-P-Terphenyl Poly(Benzimidazolium) (HMT-PMBI)-Coated Glassy Carbon Electrodes: Charge Transport Properties and Detection of Uric and Ascorbic Acid.

We describe the voltammetric behavior of an anion-exchange membrane, hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI). The anion-exchange properties of HMT-PMBI chemically modified electrodes were investigated using K4Fe(CN)6 and K2IrCl6 as redox probes. The permselectivity properties of HMT-PMBI chemically modified electrodes were ascertained using tris(2-2')bipyridyl-ruthenium(II) chloride Ru(bpy)32+. Cyclic voltammetry and chronoamperometry were utilized to extract parameters such as the concentration of the redox mediators inside the films and the apparent diffusion coefficients. We found the concentration of K4Fe(CN)6 and K2IrCl6 redox species within HMT-PMBI-coated films to be on the order of 0.04-0.1 mol·dm-3, and values of Dapp ca. 10-10-10-9 cm2·s-1. To evaluate the possibility of using such a polymer coating in electroanalysis, HMT-PMBI-modified electrodes were utilized for the voltammetric detection of uric acid in artificial urine, Surine® and ascorbic acid in Vitamin C samples. The results showed that HMT-PMBI-coated electrodes can detect uric acid in Surine® with a limit of detection (LoD) of 7.7 µM, sensitivity of 0.14 µA·µM-1·cm-2, and linear range between 5 µM and 200 µM, whereas for Vitamin C tablets, the LoD is 41.4 µM, the sensitivity is 0.08 µA·µM-1·cm-2, and the linear range is between 25 µM and 450 µM.

Improving the water management in anion-exchange membrane fuel cells via ultra-thin, directly deposited solid polymer electrolyte.

Thin ionomer membranes are considered key to achieve high performances in anion exchange membrane fuel cells. However, the handling of unsupported anion exchange membranes with thicknesses below 15 µm is challenging. Typical pre-treatments of KOH-soaking, DI-water rinsing and/or wet assembly with sub-15 µm thin films are particularly problematic. In this work, we report configurations of membrane electrode assemblies with solid polymer electrolyte thicknesses equivalent to 3, 5 and 10 µm, made possible by direct coating of the ionomer onto gas diffusion electrodes (direct membrane deposition). The anion-conducting solid polymer electrolyte employed is hexamethyl-p-terphenyl poly(benzimidazolium) (HMT-PMBI), which is known for its high mechanical stability and low rate of gas crossover. By fabricating membrane-electrode-assemblies with PtRu/C anodes and Pt/C cathodes with a low precious metal loading of <0.3 mg cm-2, reproducible performances beyond 1 W cm-2 in H2/O2 atmosphere are achieved. The thin membranes enable excellent performance robustness towards changes in relative humidity, as well as low ionic resistances (<40 mOhm cm2).

Does power ultrasound affect Nafion® dispersions?

The effect of low frequency power ultrasound on Nafion® ionomer used for fabricating proton exchange membrane fuel cell (PEMFC) and water electrolyzer (PEMWE) catalyst inks was investigated. In this study, a series of Nafion® dispersions having three concentrations (10, 5, and 2.5% w/v) were studied under various irradiation durations (tus), at fixed ultrasonic frequency (f = 42 kHz) and ultrasonic power (P > 2 W), under either controlled or unregulated bulk solution temperature conditions using a laboratory ultrasonic cleaning bath. Viscosity (η), thermal degradation, and glass transition temperature (Tg) for all Nafion® dispersion samples was measured and compared to untreated Nafion® samples. In our conditions, it was found that power ultrasound lowered the viscosity of all tested Nafion® dispersion samples; whilst thermogravimetric and differential scanning calorimetry analyses showed that for all ultrasonically irradiated samples, a negligible overall polymer degradation and no obvious change in Tg was observed under controlled and unregulated bulk temperature conditions. It was found that it is possible that acoustic cavitation causes depolymerisation followed by a polymerisation initiation step during ultrasonication. By comparing the ultrasonically treated and high-shear mixed samples, it was also observed that acoustic and hydrodynamic cavitation played an important role in the reduction of dispersion viscosity.

Poly(bis-arylimidazoliums) possessing high hydroxide ion exchange capacity and high alkaline stability.

Solid polymer electrolyte electrochemical energy conversion devices that operate under highly alkaline conditions afford faster reaction kinetics and the deployment of inexpensive electrocatalysts compared with their acidic counterparts. The hydroxide anion exchange polymer is a key component of any solid polymer electrolyte device that operates under alkaline conditions. However, durable hydroxide-conducting polymer electrolytes in highly caustic media have proved elusive, because polymers bearing cations are inherently unstable under highly caustic conditions. Here we report a systematic investigation of novel arylimidazolium and bis-arylimidazolium compounds that lead to the rationale design of robust, sterically protected poly(arylimidazolium) hydroxide anion exchange polymers that possess a combination of high ion-exchange capacity and exceptional stability.

Sulfophenylated Terphenylene Copolymer Membranes and Ionomers.

The copolymerization of a prefunctionalized, tetrasulfonated oligophenylene monomer was investigated. The corresponding physical and electrochemical properties of the polymers were tuned by varying the ratio of hydrophobic to hydrophilic units within the polymers. Membranes prepared from these polymers possessed ion exchange capacities ranging from 1.86 to 3.50 meq g-1 and exhibited proton conductivities of up to 338 mS cm-1 (80 °C, 95 % relative humidity). Small-angle X-ray scattering and small-angle neutron scattering were used to elucidate the effect of the monomer ratios on the polymer morphology. The utility of these materials as low gas crossover, highly conductive membranes was demonstrated in fuel cell devices. Gas crossover currents through the membranes of as low as 4 % (0.16±0.03 mA cm-2 ) for a perfluorosulfonic acid reference membrane were demonstrated. As ionomers in the catalyst layer, the copolymers yielded highly active porous electrodes and overcame kinetic losses typically observed for hydrocarbon-based catalyst layers. Fully hydrocarbon, nonfluorous, solid polymer electrolyte fuel cells are demonstrated with peak power densities of 770 mW cm-2 with oxygen and 456 mW cm-2 with air.

Morphology of Anion-Conducting Ionenes Investigated by X-ray Scattering and Simulation.

We have studied the morphology of a novel series of benzimidazole-based ionenes, methylated poly(hexamethyl-p-terphenylbenzimidazolium) (HMT-PMBI), in halide form. Materials with anion-exchange capacities ranging from 0 to 2.5 mequiv/g were studied. X-ray scattering reveals three length scales in the materials: ion-polymer spacing (4 Å), polymer-polymer interchain spacing (6 Å), and an intrachain repeat distance (20 Å). No long-range structure is apparent above the monomer length, which is rare in ion-conducting polymer membranes. In preliminary molecular dynamics simulations, water molecules were observed forming chains between ions, even at a modest level of hydration, providing an interpenetrating network where conductivity can occur.

Transparent Bipolar Membrane for Water Splitting Applications.

This study describes the use of a benzimidazolium-based anion exchange membrane for creating bipolar membranes and the assessment of their suitability for solar-driven water splitting. Bipolar membranes were prepared by laminating anion exchange membrane with Nafion NR-211 membrane without modification of the interface. Under acidic and basic conditions, proton and hydroxide ion conductivities of 103 and 102 mS cm-1 were obtained for Nafion and benzimidazolium-based membranes, respectively. The fabricated bipolar membranes have an average thickness of 90 µm and show high transmittance, up to 75% of the visible light. The findings suggest that the two membranes create a sharp hydrophilic interface with a space charge region of only a few nanometers, thereby generating a large electric field at the interface that enhances water dissociation.

Highly Stable, Low Gas Crossover, Proton-Conducting Phenylated Polyphenylenes.

Two classes of novel sulfonated phenylated polyphenylene ionomers are investigated as polyaromatic-based proton exchange membranes. Both types of ionomer possess high ion exchange capacities yet are insoluble in water at elevated temperatures. They exhibit high proton conductivity under both fully hydrated conditions and reduced relative humidity, and are markedly resilient to free radical attack. Fuel cells constructed with membrane-electrode assemblies containing each ionomer membrane yield high in situ proton conductivity and peak power densities that are greater than obtained using Nafion reference membranes. In situ chemical stability accelerated stress tests reveal that this class of the polyaromatic membranes allow significantly lower gas crossover and lower rates of degradation than Nafion benchmark systems. These results point to a promising future for molecularly designed sulfonated phenylated polyphenylenes as proton-conducting media in electrochemical technologies.

Cationic Polyelectrolytes, Stable in 10 M KOHaq at 100 °C.

We report on poly(arylene-imidazoliums), which were synthesized by microwave polycondensation of dialdehyde with bisbenzil and quantitatively functionalized by alkylation. This cationic polyelectrolyte is sterically protected around the C2-position and is stable in 10 M KOHaq at 100 °C (t1/2 of >5000 h). Alkaline stability is rationalized through analyses of model compounds, single crystal X-ray diffraction, and density functional theory. The polyelectrolytes form tough, pliable, transparent, ionically conductive films.

Electrochemical Reduction of Dissolved Oxygen in Alkaline, Solid Polymer Electrolyte Films.

Mass transport of oxygen through an ionomer contained within the cathode catalyst layer in an anion exchange membrane fuel cell is critical for a functioning fuel cell, yet is relatively unexplored. Moreover, because water is a reactant in the oxygen reduction reaction (ORR) in alkaline media, an adequate supply of water is required. In this work, ORR mass transport behavior is reported for methylated hexamethyl-p-terphenyl polymethylbenzimidazoles (HMT-PMBI), charge balanced by hydroxide ions (IEC from 2.1 to 2.5 mequiv/g), and commercial Fumatec FAA-3 membranes. Electrochemical mass transport parameters are determined by potential step chronoamperometry using a Pt microdisk solid-state electrochemical cell, in air at 60 °C, with relative humidity controlled between 70% and 98%. The oxygen diffusion coefficient (DbO2), oxygen concentration (cbO2), and oxygen permeability (DbO2·cbO2) were obtained by nonlinear curve fitting of the current transients using the Shoup-Szabo equation. Mass transport parameters are correlated to water content of the ionomer membrane. It is found that the oxygen diffusion coefficients decreased by 2 orders of magnitude upon reducing the water content of the ionomer membrane by lowering the relative humidity. The limitation of the Shoup-Szabo equation for extracting ORR mass transport parameters using thin ionomer films was evaluated by numerical modeling of the current transients, which revealed that a significant discrepancy (up to 29% under present conditions) was evident for highly hydrated membranes for which the oxygen diffusion coefficient was largest, and in which the oxygen depletion region reached the ionomer/gas interface during the chronoamperometric analysis.

Poly(phenylene) and m-Terphenyl as Powerful Protecting Groups for the Preparation of Stable Organic Hydroxides.

Four benzimidazolium hydroxide compounds, in which the C2-position is attached to a phenyl group possessing hydrogen, bromine, methyl groups, or phenyl groups at the ortho positions, are prepared and investigated for stability in a quantitative alkaline stability test. The differences between the stability of the various protecting groups in caustic solutions are rationalized on the basis of their crystal structures and DFT calculations. The highest stability was observed for the m-terphenyl-protected benzimidazolium, showing a half-life in 3 M NaOD/CD3OD/D2O at 80 °C of 3240 h. A high-molecular-weight polymer analogue of this model compound is prepared that exhibits excellent mechanical properties, high ionic conductivity and ion-exchange capacity, as well as remarkable hydroxide stability in alkaline solutions: only 5% degradation after 168 h in 2 M KOH at 80 °C. This is the most stable hydroxide-conducting benzimidazolium polymer to date.

Determination of O2 Mass Transport at the Pt | PFSA Ionomer Interface under Reduced Relative Humidity.

Oxygen mass transport resistance through the ionomer component in the cathode catalyst layer is considered to contribute overpotential losses in polymer electrolyte membrane fuel cells. Whereas it is known that water uptake, water transport, and proton conductivity are reduced upon reducing relative humidity, the effect on oxygen mass transport remains unknown. We report a two-electrode approach to determine mass transport coefficients for the oxygen reduction reaction in air at the Pt/perfluorosulfonic acid ionomer membrane interface between 90 and 30% RH at 70 °C using a Pt microdisk in a solid state electrochemical cell. Potential-step chronoamperometry was performed at specific mass-transport limiting potentials to allow for the elucidation of the oxygen diffusion coefficient (D(bO2)) and oxygen concentration (c(bO2)). In our efforts, novel approaches in data acquisition, as well as analysis, were examined because of the dynamic nature of the membrane under lowered hydration conditions. Linear regression analysis reveals a decrease in oxygen permeability (D(bO2c(bO2)) by a factor of 1.7 and 3.4 from 90 to 30% RH for Nafion 211 membrane and membranes cast from Nafion DE2020 ionomer solutions, respectively. Additionally, nonlinear curve fitting by way of the Shoup-Szabo equation is employed to analyze the entire current transient during potential step controlled ORR. We also report on the presence of an RH dependence of our previously reported time-dependency measurements for O2 mass transport coefficients.

Structurally-defined, sulfo-phenylated, oligophenylenes and polyphenylenes.

We report the synthesis and molecular characterization of structurally defined, sulfo-phenylated, oligo- and polyphenylenes that incorporate a novel tetra-sulfonic acid bistetracyclone monomer. The utility of this monomer in the [4 + 2] Diels-Alder cycloaddition to produce well-defined, sulfonated oligophenylenes and pre-functionalized polyphenylene homopolymers is demonstrated. Characterization of the oligophenylenes indicates formation of the meta-meta and para-para adducts in a ∼ 1:1 ratio. These functionalized monomers and their subsequent coupling provide a route to prepare novel, sterically encumbered, sulfonated polyphenylenes possessing unprecedented structural control.