Highly conductive and chemically stable alkaline anion exchange membranes via ROMP of trans -cyclooctene derivatives

Abstract

Alkaline anion exchange membranes (AAEMs) are an important component of alkaline exchange membrane fuel cells (AEMFCs), which facilitate the efficient conversion of fuels to electricity using nonplatinum electrode catalysts. However, low hydroxide conductivity and poor long-term alkaline stability of AAEMs are the major limitations for the widespread application of AEMFCs. In this paper, we report the synthesis of highly conductive and chemically stable AAEMs from the living polymerization of trans-cyclooctenes. A trans-cyclooctene-fused imidazolium monomer was designed and synthesized on gram scale. Using these highly ring-strained monomers, we produced a range of block and random copolymers. Surprisingly, AAEMs made from the random copolymer exhibited much higher conductivities than their block copolymer analogs. Investigation by transmission electron microscopy showed that the block copolymers had a disordered microphase segregation which likely impeded ion conduction. A cross-linked random copolymer demonstrated a high level of hydroxide conductivity (134 mS/cm at 80°C). More importantly, the membranes exhibited excellent chemical stability due to the incorporation of highly alkaline-stable multisubstituted imidazolium cations. No chemical degradation was detected by 1 H NMR spectroscopy when the polymers were treated with 2 M KOH in CD 3 OH at 80°C for 30 d.alkaline anion exchange membrane | trans-cyclooctene | block and random copolymer | cross-linked polymer | transmission electron microscopy A lkaline anion exchange membranes (AAEMs) are a class of polymer electrolytes constructed from immobilized cations with hydroxide counteranions (1-3). AAEMs have been widely used for a variety of electrochemical applications, such as redox flow batteries, electrodialysis, and water electrolysis (4-7). One of the most important applications for AAEMs is their use as polyelectrolytes in alkaline exchange membrane fuel cells (AEMFCs). These devices can efficiently convert chemical energy in fuels (e.g., H 2 , hydrazine, and direct alcohols) into electricity. In comparison with proton exchange membrane fuel cells and aqueous alkaline fuel cells, AEMFCs are advantageous because they allow for rapid reduction of oxygen at the cathode, limit fuel cross-over, prevent the formation of carbonate precipitates, and are compatible with nonnoble electrocatalysts (8-12). Because of their essential role in AEMFCs, AAEMs have been extensively studied to optimize their performance and, in particular, to improve their hydroxide conductivity and alkaline stability.Hydroxide conductivity is one of the most important parameters used to evaluate AAEMs, because it is directly related to the ohmic resistance and power density of an AEMFC (9). There are several strategies to enhance the hydroxide conductivity of AAEMs. The most straightforward method is to increase the ion exchange capacity (IEC), such that more ions are present in AAEMs. However, a higher IEC typically results in higher water uptake. While the presence of water facilitates h...

Cation–dipole interaction that creates ordered ion channels in an anion exchange membrane for fastOH−conduction

“…[17][18][19][20][21][22] The polarity mismatch between the ionic and neutral segments provides the main driving force for aggregation of ionic moieties (termed ion clusters). 23,24 These ion Jianjun Zhang and Yubin He authors contributed equally to this work. clusters further overlap and organize in the casting solution, yielding continuous ion channels in the final solid-state (membrane) after solvent evaporation.…”

“…One of the most efficient synthetic strategies is to selectively attach cationic groups onto a specific segment in block or graft copolymers via multi‐step reactions 17‐22 . The polarity mismatch between the ionic and neutral segments provides the main driving force for aggregation of ionic moieties (termed ion clusters) 23,24 . These ion clusters further overlap and organize in the casting solution, yielding continuous ion channels in the final solid‐state (membrane) after solvent evaporation.…”

Progress of Alkaline Anion Exchange Membranes for Fuel Cells: The Effects of Micro-Phase Separation

“…Among these testing instruments, TEM is an effective method that can directly exhibit the microphase separation of membranes. However, the complex sample preparation process is a big challenge when collecting the TEM images (You et al, 2019). The microphase separation can be proved by the minimal correlations between the AFM phase images and height images (Lin et al, 2017).…”

Poly(Alkyl‐Terphenyl Piperidinium) Ionomers and Membranes with an Outstanding Alkaline‐Membrane Fuel‐Cell Performance of 2.58 W cm−2

“…AEPs consist of hydrophilic cationic groups and hydrophobic polymer backbones,w hich are responsible for conduction of anions (e.g., OH À )and maintaining dimensional stability,respectively. [10][11][12][13][14][15] Although numerous cationic groups (ammonium, imidazolium, phosphonium, sulfonium, and organometallic cations) [16][17][18][19][20][21] and polymer backbones (polyether ether ketone (PEEK), polysulfone (PSF), polyphenyl ether (PPO), polybenzimidazole (PBI), polyphenylene (PP), polyolefins,poly(norbornene) (PNB), and Tr ogers base (TB)) [22][23][24][25][26][27][28][29] have been explored in AEPs,amajority of AEPs displayed insufficient ion conductivity and durability under alkaline conditions.T hat is,A EMFCs in the presence of OH À ions have several inherent drawbacks compared to PEMFCs.F or instance,A EMs possess lower OH À conductivity than the H + conductivity of PEMs due to the lower diffusion coefficient of the larger OH À ions.Meanwhile,most cationic groups and polymer backbones have been documented to be vulnerable and prone to degrade under harsh alkaline conditions. [30,31] Encouragingly,AEMs have seen great progress in the past four years,r esulting in tremendous advances in AEMFCs.…”