A Single-Step Monomeric Photo-Polymerization and Crosslinking via Thiol-Ene Reaction for Hydroxide Exchange Membrane Fabrication

Abstract

A single step monomeric photo-polymerization and crosslinking via thiol-ene reaction is developed for the preparation of hydroxide exchange membranes (HEMs) in a ternary system with a triallyl triazine, a quaternary ammonium diallyl, and a dithiol. This facile method enables reproducible tuning of the ion exchange capacity and crosslink density. These HEMs demonstrate reasonable hydroxide conductivity, limited alkaline stability, and good thermal stability and have lower water uptakes than other photocrosslinked HEMs produced with much longer reaction times. Furthermore, this new fabrication method allows the incorporation of catalyst nanoparticles in the hydroxide exchange materials to form thin catalyst layers that are resistant to dissolution in methanol which suggests these polymers can be used in direct alcohol fuel cells (DAFCs). Hydroxide exchange membrane fuel cells (HEMFCs) have the potential to decrease the overall fuel cell system cost through the utilization of non-precious metal catalysts (e.g., nickel) and inexpensive bipolar plates (e.g., aluminum).1 As one of the key components for a HEMFC, the hydroxide exchange membrane (HEM) has to be both reasonably ion conductive and have low water uptake (WU). Attempts to increase ion conductivity in HEMs typically lead to increased water uptake and excessive swelling that, in the extreme cases, results in the dissolution of the membrane. Covalent crosslinking of the polymeric HEM is an approach that effectively combats excessive swelling and dissolution that would otherwise occur during fuel cell operation. 2,3Typically, polymer crosslinking methods for HEM fabrication require separate polymerization and crosslinking steps necessitating careful manipulation of reaction conditions including the reaction time, reactant concentrations, and temperature. As a result of this complexity, the reproducible fabrication of cross-linked HEMs is still an ongoing challenge in the development of fuel cell membranes to achieve consistent ion exchange capacities (IECs) and degrees of crosslinking (DC).Several groups have already demonstrated that a single-step polymerization and crosslinking fabrication of HEMs is possible using photo-polymerization, 4-8 thermal polymerization, 9 and ring opening metathesis polymerization (ROMP).10,11 A single-step polymerization and crosslinking reaction enables precise control over the degrees of functionalization (and thus IEC) and crosslinking imparted by the use of small molecules rather than large polymer precursors. Moreover, photo-crosslinking, which allows for rapid initiation and propagation, has been demonstrated by numerous groups in the formation of solvent-resistant networks [4][5][6][7][8][12][13][14] and is a particularly attractive means to fabricate membranes because of the low cost, safety, and spatial-temporal control afforded by the use of light initiation.Thus far, all of the studies involving the photo-copolymerization and crosslinking reaction between vinyl-functionalized comonomers have proceeded under traditional ...

Study of Anion Exchange Membrane Properties Incorporating N-spirocyclic Quaternary Ammonium Cations and Aqueous Organic Redox Flow Battery Performance

“…Whereas, at higher IEC , a much sharper increase (more than 50%) in water uptake between M2.1 and M2.8 membranes was recorded. This sudden increase is due to (i) the increase of proportion of the hydrophilic ionic side chains in the membrane, i.e., the higher the IEC , the longer the lengths of poly(DAPCl) and (ii) the decrease of the crosslinking stiffness [ 45 ].…”

Removal of Metal Ions in Phosphoric Acid by Electro-Electrodialysis with Cross-Linked Anion-Exchange Membranes

“…Moreover, other researchers also synthesized the comb-shaped AEMs with a lower membrane area resistance and better swelling resistance than the non-comb-shaped AEMs. ,− Further, the cross-linked network structure in AEMs has been demonstrated as an effective way to enhance the dimensional and mechanical stabilities . Different strategies, such as thiol-ene click chemistry and olefin metathesis were utilized to cross-link AEMs. − Tibbits et al prepared the cross-linked hydroxide exchange membranes by a single step via the thiol-ene reaction, which demonstrated reasonable conductivity, better stability, and lower water uptake than other non-cross-linked membranes . Xu et al synthesized the cross-linked AEMs for electrodialysis by thermal cross-linking of unsaturated moieties and the resultant membranes had high mechanical stability .…”

A facile approach to prepare crosslinked polysulfone-based anion exchange membranes with enhanced alkali resistance and dimensional stability

“…Particularly, cross-linked AEMs with high IEC values can minimize swelling, increase steric hindrance of degradation in alkaline environments, and decrease the attack of OH À on the polymer backbone or ion exchange groups, thus improving alkali stability. 27,28 Cross-linked AEMs can be prepared by various methods, such as a series of end-linked cross-linked AEMs by "click chemistry", 29 reacting halomethylated polymers with diols, dihalides, and diamines, [30][31][32][33][34] ring-opening metathesis polymerization (ROMP), 35,36 and thermal cross-linking. 37,38 Generally, preparation of cross-linked AEMs requires two steps, which are quaternization and cross-linking.…”