Patric Jannasch

Poly(phenylene oxide) functionalized with quaternary ammonium groups via flexible alkyl spacers for high-performance anion exchange membranes

We have attached quaternary ammonium (QA) groups to poly(phenylene oxide) via flexible and stable heptyl side chains by using a straightforward synthetic route involving bromoalkylation and quaternization. Membranes based on these polymers showed efficient phase separation, significantly enhanced hydroxide ion conductivity and far superior alkaline stability in relation to corresponding polymers with QA groups placed in benzylic positions directly on the backbone.

Alkali-stable and highly anion conducting poly(phenylene oxide)s carrying quaternary piperidinium cations

New durable and hydroxide ion conducting anion-exchange membranes (AEMs) are currently required in order to develop alkaline fuel cells into efficient and clean energy conversion devices. In the present work we have attached quaternary piperidinium (QPi) cations to poly(2,6-dimethyl-1,4-phenylene oxide)s (PPOs) via flexible alkyl spacer chains with the aim to prepare AEMs. The bromine atoms of bromoalkylated PPOs were displaced in Menshutkin reactions to attach one or two QPi groups, respectively, via heptyl spacers. The cationic polymers have excellent solubility in, e.g., methanol, dimethylsulfoxide and N-methyl-2-pyrrolidone at room temperature, and form tough and transparent membranes. AEMs with bis-QPi side chains efficiently form ionic clusters and reach very high hydroxide ion conductivities, up to 69 and 186 mS cm−1 at 20 and 80 °C, respectively. The AEMs further have excellent alkaline stability, and 1H NMR analysis showed no degradation of the AEMs after storage in 1 M NaOH at 90 °C during 8 days. Thermogravimetry indicated decomposition only above 225 °C. The AEM properties were further tuned by controlled formation of bis-QPi crosslinks through an efficient reaction between bromoalkylated PPO and 4,4′-trimethylenebis(1-methylpiperidine) during a reactive membrane casting process. In conclusion, alkali-stable and highly conductive AEMs can be prepared by placing cycloaliphatic quaternary ammonium cations on flexible side chains and crosslinks.

Anion conducting multiblock poly(arylene ether sulfone)s containing hydrophilic segments densely functionalized with quaternary ammonium groups

We have prepared poly(arylene ether sulfone) multiblock copolymers with cationic blocks containing single dioxyphenylene rings functionalized with two, three or four quaternary ammonium (QA) groups in order to investigate the influence of the ionic concentration and distribution on the anionic conductivity. Precursor blocks were prepared by polycondensation of 4,4′-difluorodiphenyl sulfone and either di-, tri- or tetramethylhydroquinone. Subsequently, these blocks were combined with precursor blocks prepared from 4,4′-dichlorodiphenyl sulfone and bisphenol A to form alternating multiblock copolymers with different block ratios. The benzylic methyl groups of the hydroquinone monomer units were then fully brominated using N-bromosuccinimide. Quaternization with trimethylamine gave multiblock copolymers with extremely high ion exchange capacities (IECs) of the hydrophilic blocks, i.e., 3.2, 4.9, and 5.8 meq. g−1, respectively, in the Br− form. X-ray scattering and atomic force microscopy of the anion exchange membranes (AEMs) showed a distinct nanophase separation of the blocks. At a given IEC, the ionic conductivity was found to decrease with increasing number of QA groups per dioxyphenylene ring, probably because of limited ionic dissociation resulting from the close proximity of the QA groups. Thus, at a similar IEC, the conductivity of a block copolymer with tetra-functionalized rings reached the same level of conductivity as a corresponding polymer with randomly distributed QA groups, whereas a block copolymer with di-functionalized rings exceeded the conductivity of the latter polymer by a factor 4.2, despite a lower water uptake. These findings strongly highlight the importance of controlling and optimizing the local ionic concentration and distribution for highly anion conductive AEMs based on block copolymers.

Anion-exchange membranes with polycationic alkyl side chains attached via spacer units

Anion-exchange membrane (AEM) fuel cells are promising electrochemical systems for efficient and environmentally benign energy conversion. However, the development of high-performance fuel cells requires new AEMs tailored for high conductivity and chemical stability. Herein, we present the synthesis and characterization of AEMs with polycationic side chains attached to poly(phenylene oxide) (PPO) via flexible alkyl spacer units. Three series of PPOs were functionalized with side chains to study the influence of the number (n = 2–6) of –CH2– groups in between the quaternary ammonium (QA) cations, the ion exchange capacity (IECs), and the number (q = 1–4) of QA cations per side chain. The polymers were prepared by successively reacting bromoalkylated PPO with different tertiary diaminoalkanes and 1,6-dibromohexane. Evaluation of the alkaline stability by 1H NMR spectroscopy and thermogravimetry demonstrated that solvent cast AEMs with n = 2 and 3 quickly degraded via Hofmann β-elimination in 1 M NaOH at 60 °C. In sharp contrast, no degradation was detected for AEMs with n = 4 and 6 after storage in 1 M NaOH at 90 °C over at least 8 days. At similar IECs, the OH− conductivity of the AEMs increased with n up to n = 4, whereafter a plateau was reached. This may be explained by a polyelectrolyte effect leading to counter ion condensation and incomplete ion dissociation when the QA cations were closer than the Bjerrum length (approx. 7 A). The conductivity of AEMs with n = 6 and IEC = 1.9 meq. g−1 increased only slightly with the number of QA cations per side chain up to q = 3 but then increased sharply with q = 4 to reach 160 mS cm−1 at 80 °C. The present work demonstrated that a molecular architecture with poly-QA side chains attached via flexible spacer units affords AEMs that combine efficient phase separation, high alkaline stability and OH− conductivity at moderate water uptake, provided that the side chains are properly designed to avoid Hofmann elimination and counter ion condensation.

Hypersulfonated polyelectrolytes: preparation, stability and conductivity

Specially tailored polyelectrolytes are becoming important as energy-related materials. Here we explore a synthetic strategy to prepare fully aromatic polymers containing single phenylene rings in the backbone functionalized with four sulfonic acid groups. Thioether bridges of semifluorinated poly(arylene thioether)s were oxidized to sulfone bridges, followed by substitution of all fluorines by NaSH and quantitative oxidation of the resulting thiol groups. This gave poly(arylene sulfone)s containing octasulfonated biphenyl units, reaching ion exchange capacities up to 8 meq g−1 and unprecedented high local sulfonic acid concentrations. These polyelectrolytes are stable up to 300 °C under air and achieve proton conductivities of up to 90 mS cm−1 at 120 °C and 50% relative humidity. Despite the excellent performance of this unique new class of hypersulfonated polymers, our data suggests that incomplete proton dissociation may ultimately limit the conductivity of highly sulfonated polymers.

Water Vapor Sorption Thermodynamics of the Nafion Ionomer Membrane

The water interactions of polymer electrolyte membranes are of significant interest when these materials are used in, for example, fuel cells. We have therefore studied the sorption thermodynamics of Nafion with a sorption calorimeter that simultaneously measures the sorption isotherm and the mixing (sorption) enthalpy. This unique method is suitable for investigating the sorption thermodynamics of ionic polymers. The measurements were made at 25 °C on a series of samples dried at different temperatures from 25 to 120 °C. The sorption isotherms indicate that the samples dried at 120 °C lost about 0.8 more water molecules per sulfonic group during the drying than did the samples dried at 25 °C, and this result was verified gravimetrically. The mixing enthalpies showed several peaks or plateaus for the samples dried at 60-120 °C. This behavior was seen up to about 2 water molecules per sulfonic group. As these peaks were not directly related to any feature in the sorption isotherm, they probably have their origin in a secondary process, such as a reorganization of the polymer.

A comparative study of anion-exchange membranes tethered with different hetero-cycloaliphatic quaternary ammonium hydroxides

Quaternary ammonium (QA) cations with high alkaline stability are crucial for the long term performance of anion-exchange membrane (AEM) fuel cells. Here, we have tethered poly(phenylene oxide) (PPO) with 8 different hetero-cycloaliphatic QA cations via pentyl spacer chains. The thermal and alkaline stabilities, as well as hydroxide ion conductivity, were systematically evaluated with the primary aim to identify degradation reactions and establish cation design principles. The study included AEMs functionalized with 1-methylazepanium, 1-methylpyrrolidinium, 1-methylmorpholinium, quinuclidinium, as well as 1-methyl-, 1,4-dimethyl-, 1,3,5-trimethyl-, and 1,2,6-trimethylpiperidinium, all within a narrow ion exchange capacity (IEC) range. For reference, PPO was also functionalized with trimethylammonium and dipropylmethylammonium cations on pentyl spacers, and with trimethylammonium and 1-methylpiperidinium QAs in benzylic positions directly on the PPO backbone. The alkaline stability of hetero-cycloaliphatic QA cations was found to depend critically on their position in the polymer structure, ring size, the presence of an additional heteroatom and ring substitution pattern. For example, 1,2,6-trimethylpiperidinium and 1-methylazepanium degraded via Hofmann elimination and 1-methylmorpholinium via ring opening by both Hofmann elimination and substitution reactions, while no degradation was detected by 1H NMR spectroscopy of other cations after 16 days in 1 M NaOH at 90 °C. The hydroxide ion conductivity of the AEMs in the study reached values between 64 and 150 mS cm−1 at 80 °C, depending on the cation and IEC. AEMs tethered with piperidinium and quinuclidinium cations via pentyl spacers were found to show the best overall properties. Hence, the combined results provide insights that may guide the selection of cationic groups and membrane materials to improve the durability and performance of alkaline electrochemical energy conversion and storage devices.

Single lithium-ion conducting poly(tetrafluorostyrene sulfonate) – polyether block copolymer electrolytes

Solid single-ion conducting polymers continue to attract significant interest as electrolyte materials with great potential to improve safety and performance of energy storage devices. Still, their low conductivity is a significant hurdle presently preventing their application. Here, we report on highly conductive BAB triblock copolymers with A blocks of either poly(ethylene oxide) (PEO) or poly(ethylene oxide-co-propylene oxide) (PEOPO), and B blocks of poly(lithium 2,3,5,6-tetrafluorostyrene-4-sulfonate) (PPFSLi). The copolymers were readily synthesised by atom transfer radical polymerisation (ATRP) of 2,3,4,5,6-pentafluorostyrene from polyether macroinitiators, followed by quantitative thiolation using NaSH and subsequent oxidation to form the sulfonate anions. The copolymers possessed high thermal stability and their ionic content was conveniently controlled by the block ratio during the ATRP. Above the polyether melting point, PEO-based block copolymers with [O]u2006:u2006[Li] = [18]u2006:u2006[1] showed the highest conductivity, close to 1.4 × 10−5 S cm−1 at 60 °C, while at lower temperatures, the PEOPO materials reached the highest conductivity, nearly 1.5 × 10−6 S cm−1 at 20 °C. The high conductivity of the former copolymer suggests weak interactions of the lithium ions with the pentafluorosulfonate anions in combination with a high degree of Li+ dissociation facilitated by PEO. The results of the present study demonstrate that well-designed block copolymers containing lithium pentafluorostyrene sulfonate units can approach the levels of conductivity required for high-temperature lithium battery applications.