Cross-Linker Effect in ETFE-Based Radiation-Grafted Proton-Conducting Membranes

Abstract

In this study, the effect of cross-linker content on the chemical stability of poly͑ethylene-alt-tetrafluoroethylene͒ ͑ETFE͒-based radiation-grafted and sulfonated membranes was investigated. An ex situ degradation test in H 2 O 2 solution showed a strong increase in stability of cross-linked membranes compared to uncross-linked ones. Excessive cross-linking, however, is detrimental to the chemical and mechanical properties. Furthermore, the stability of grafted membranes based on ETFE was superior to those based on poly͑tetrafluoroethylene-co-hexafluoropropylene͒ ͑FEP͒. An in situ long-term test in a H 2 /O 2 single cell over 2180 h, using an ETFE-based grafted membrane optimized with respect to cross-linker content, with a graft level of 25% was carried out. The performance of the membrane electrode assembly exhibited a voltage decay rate of 13 V h −1 over the testing time at a current density of 500 mA cm −2 and a cell temperature of 80°C, while the hydrogen permeation showed a steady increase over time. This indicates that, to some extent, changes in the membrane morphology occur over the operating period. Local postmortem analysis of the tested membrane reveals that high degradation was observed in areas adjacent to the O 2 inlet and in other areas nearby.The development of cost-effective proton exchange membranes to replace the state-of-the-art and expensive perfluorinated membranes ͑e.g., Nafion, Flemion, and Aciplex͒ is one of the main challenges on the road to commercialization of polymer electrolyte fuel cell technology. The radiation-induced grafting technique, based on the utilization of a low-cost base polymer material with either fluorinated or partially fluorinated polymer films, offers several advantages. 1 The radiation-induced grafting in combination with further chemical modification steps ͑sulfonation͒ allows the functionalization of the base material and the introduction of the desired property ͑proton conductivity͒. The attractiveness of this technique is based on its versatility and the possibility to easily control the grafting parameters ͑irradiation dose, concentration of monomers, time, temperature, solvent type, etc.͒. We can tune the desired membrane properties in a broad range due to the wide selection and combination possibilities of base films and grafting monomers.One of the main research areas in our laboratory at the Paul Scherrer Institut is the development of low-cost polymer electrolyte membranes. Radiation-grafted membranes based on grafted styrene/ divinylbenzene ͑DVB:cross-linker͒ onto poly ͑tetrafluoroethylene-co-hexafluoropropylene͒ ͑FEP͒ exhibited performance comparable to that of commercial Nafion 112 membranes and were able to operate over 4000 h without notable degradation under steady conditions at a temperature of 80°C. 2,3The FEP-based membrane was optimized with respect to its performance and chemical stability by adjusting the graft level and cross-linker content. In a study of the effect of DVB on fuel cell performance, the membrane prepared with 10% DVB ͑in th...

Microstructured proton-conducting membranes by synchrotron-radiation-induced grafting

“…relative degree of degradation ( Fig. 8c and d) [25]. For both tested membranes >80% of the polystyrene was lost in their cathodic part (the oxygen inlet), whereas at the anode side much lower chemical degradation was measured (10-20%).…”

Influence of Radiation‐Induced Grafting Process on Mechanical Properties of ETFE‐Based Membranes for Fuel Cells

“…[7][8][9]. To optimise performance and improve the lifetime of the grafted membrane during fuel cell operation, the crosslinker content has to be optimised.…”

“…We reported previously on radiationinduced grafting of styrene onto ETFE in the presence of DVB as the crosslinker, as well as characterisation of fuel cell relevant properties and fuel cell performance of the resulting membranes [5]. The influence of the crosslinker and graft level (GL) on the fuel cell relevant properties was systematically investigated [6][7][8][9].…”

Quantum-beam technology: A versatile tool for developing polymer electrolyte fuel-cell membranes

“…3). Radiation-grafted sulfonated PEMs crosslinked chemically with divinylbenzene (DVB) have been extensively studied using various fluoropolymer substrates at the Paul Scherrer Institut (PSI) since the early 1990s [18][19][20][21][22][23][24][25][26]. The DVB-based crosslinks in the graft chain also had a significant influence on the water uptake, proton conductivity, and thermal or chemical stability of the PEMs [21][22][23].…”