Hai-Son Dang

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-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.

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.