Effect of Micromorphology on Alkaline Polymer Electrolyte Stability

Effects of Structurally Different Tertiary Amines on the Properties of Quaternized Anionic Exchange Membranes Potentially Applicable for Water Electrolysis

In this work, two structurally different monoamines (trimethylamine [TMA] and N-methylpiperidine [N-MPip]) are used for the amination of a g-VBC-15 graft copolymer, obtained by the functionalization of a mechanically robust, commercially available styrene-butadiene block copolymer (SB) with vinylbenzyl chloride (VBC) via solution free-radical polymerization. Results demonstrate that g-VBC-15-based membranes quaternized with TMA have superior electrochemical performance than N-MPip counterparts; while, the mechanical properties are good and only slightly inferior to those of N-MPip. Therefore, TMA is the selected monoamine to be alternatively mixed with two polyamines (tetramethyl-1,3-propanediamine [TMPDA] and N,N,N',N'',N''-pentamethyldiethylenetriamine [PMDETA]) into different proportions, in order to modulate the average functionality of the amination mixture in terms of number of amine functional groups available for the quaternization reaction of the membranes. g-VBC-15-based membranes derived therefrom are extensively characterized to assess their thermal, mechanical, and ex situ electrochemical properties. Results indicate that membranes quaternized with a TMA/PMDETA mixture (90:10 in mole) display the highest conductivity among all the investigated membranes aminated with polyamine-based mixtures. Moreover, they have comparable mechanical and electrochemical properties to those quaternized with TMA, while exhibiting a reduced water uptake.

Vinylbenzyl Chloride/Styrene-Grafted SBS Copolymers via TEMPO-Mediated Polymerization for the Fabrication of Anion Exchange Membranes for Water Electrolysis

In this work, a commercial SBS was functionalized with the 2,2,6,6-tetramethylpiperidin-N-oxyl stable radical (TEMPO) via free-radical activation initiated with benzoyl peroxide (BPO). The obtained macroinitiator was used to graft both vinylbenzyl chloride (VBC) and styrene/VBC random copolymer chains from SBS to create g-VBC-x and g-VBC-x-co-Sty-z graft copolymers, respectively. The controlled nature of the polymerization as well as the use of a solvent allowed us to reduce the extent of the formation of the unwanted, non-grafted (co)polymer, thereby facilitating the graft copolymer's purification. The obtained graft copolymers were used to prepare films via solution casting using chloroform. The -CH₂Cl functional groups of the VBC grafts were then quantitatively converted to -CH₂(CH₃)₃N⁺ quaternary ammonium groups via reaction with trimethylamine directly on the films, and the films, therefore, were investigated as anion exchange membranes (AEMs) for potential application in a water electrolyzer (WE). The membranes were extensively characterized to assess their thermal, mechanical, and ex situ electrochemical properties. They generally presented ionic conductivity comparable to or higher than that of a commercial benchmark as well as higher water uptake and hydrogen permeability. Interestingly, the styrene/VBC-grafted copolymer was found to be more mechanically resistant than the corresponding graft copolymer not containing the styrene component. For this reason, the copolymer g-VBC-5-co-Sty-16-Q with the best balance of mechanical, water uptake, and electrochemical properties was selected for a single-cell test in an AEM-WE.

Conductivity and Stability Properties of Anion Exchange Membranes: Cation Effect and Backbone Effect

The rise of heterocycle cations, a new class of stable cations, has fueled faster growth of research interest in heterocycle cation-attached anion exchange membranes (AEMs). However, once cations are grafted onto backbones, the effect of backbones on properties of AEMs must also be taken into account. In order to comprehensively study the influence of cations effect and backbones effect on AEMs performance, a series of AEMs were prepared by grafting spacer cations, heterocycles cations, and aromatic cations onto brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO) or poly(vinylbenzyl chloride) (PVB) backbones, respectively. Spacer cation [trimethylamine (TMA), N,N-dimethylethylamine (DMEA)]-attached AEMs showed general ion transportation and stability behaviors, but exhibited high cationic reaction efficiency. Heterocycle cation [1-methylpyrrolidine (MPY), 1-methylpiperidine (MPrD)]-attached AEMs showed excellent chemical stability, but their ion conduction properties were unimpressive. Aromatic cation [1-methylimidazole (MeIm), N,N-dimethylaniline (DMAni)]-attached AEMs exhibited superior ionic conductivity, while their poor cations stabilities hindered the application of the membranes. Besides, it was found that PVB-based AEMs had excellent backbone stability, but BPPO-based AEMs exhibited higher OH^- conductivity and cation stability than those of the same cations grafted PVB-based AEMs due to their higher water uptake (WU). For example, the ionic conductivities (ICs) of BPPO-TMA and PVB-TMA at 80 °C were 53.1 and 38.3 mS cm^-1 , and their WU was 152.3 and 95.1 %, respectively. After the stability test, the IC losses of BPPO-TMA and PVB-TMA were 21.4 and 32.2 %, respectively. The result demonstrated that the conductivity and stability properties of the AEMs could be enhanced by increasing the WU of the membranes. These findings allowed the matching of cations to the appropriate backbones and reasonable modification of the AEM structure. In addition, these results helped to fundamentally understand the influence of cation effect and backbone effect on AEM performance.

Highly Durable Ether-Free Polyfluorene-Based Anion Exchange Membranes for Fuel Cell Applications

The preparation of anion exchange membranes (AEMs) with excellent chemical and dimensional stability and high conductivity faces several challenges. In the present work, a novel ether-free durable polyfluorene (PF) without fluorine-bearing pendant piperidinium groups was synthesized by the Suzuki cross-coupling reaction. Alkyl groups were introduced into the backbone of PF to enhance the solubility and flexibility of PF-based AEMs, and the transparent and flexible polymer membrane showed a high conductivity of 80.44 mS cm^-1 and excellent alkaline stability in 2 M KOH solution at 80 °C. Although the membrane possesses a high ion exchange capacity (IEC) (2.49 mequiv g^-1), it exhibits a low swelling ratio (9.4% at 80 °C), excellent mechanical properties, and dimensional stability. The H₂/O₂ single cell assembled with PFPE-Pi exhibited a maximum power density of 661 mW cm^-2 at a current density of 1280 mA cm^-2 at 80 °C. The present work provides a simple and effective strategy for the preparation of ether-free polyfluorene-based AEMs with high conductivity, excellent mechanical properties, and dimensional stability for application in alkaline fuel cells.

"Intrinsic" Anion Exchange Polymers through the Dissociation of Strong Basic Groups: PPO with Grafted Bicyclic Guanidines

We synthesized anion exchange polymers by a reaction of chloromethylated poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) with strongly basic 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD). TBD contains secondary and tertiary amine groups in the guanidine portion. To favor the functionalization with the secondary amine, TBD was activated with butyl lithium. The yield of amine formation via the reaction of the benzyl chloride moiety with TBD was 85%. Furthermore, we prepared polymers with quaternary ammonium groups by the reaction of PPO-TBD with CH₃I. The synthesis pathways and ionomer structure were investigated by NMR spectroscopy. The thermal decomposition of both ionomers, studied by thermogravimetry, started above 200 °C, corresponding to the loss of the basic group. The ion exchange capacities, water uptake and volumetric swelling are also reported. The “intrinsic” anion conductivity of PPO-TBD due to the dissociation of grafted TBD was in the order of 1 mS/cm (Cl form). The quaternized ionomer (PPO-TBD-Me) showed an even larger ionic conductivity, above 10 mS/cm at 80 °C in fully humidified conditions.