Investigation of Ionic Liquid/Membrane Matrix for the Mid Temperature PEM Fuel Cell

Abstract

Operating polymer electrolyte fuel cells (PEFC) >100 °C allows a significant simplification of the necessary peripheral devices. It allows operation without feed gas humidification—respectively no water recyclation—and a more efficient cooling system. For operation temperatures <80 °C, PEFCs with NAFION® membranes show excellent electrochemical performance. However, at higher temperatures sulfonated fluoropolymer membranes suffers from the drying effects at atmospheric operation which leads to a dramatic decrease of the proton conductivity. When using phosphoric acid (H3PO4) doped polybenzimidazole membranes (PBI), a PEFCs can be operated in the temperature range of 160–180 °C (HT-PEFC). Nevertheless, due to poisoning effects by H3PO4 and its anionic species on the redox catalyst platinum and a low solubility of oxygen in H3PO4, the ORR kinetics is rather sluggish and leads to an insufficient power density. Moreover, at temperatures <150 °C the proton conductivity is not sufficient.Thus, there is need for new proton conducting membrane materials for operation temperatures of 100–120 °C, whose conductivity is not related to a high degree of water swelling. Proton conducting ionic liquids (PILs) exhibit a high proton conductivity, low volatility and high thermal stability, as well as a high electrochemical stability over a wide temperature range [1-3]. Thus, PILs are the promising candidates as non-aqueous proton conductors.This experimental study is focused on high Brønsted-acidic sulfoalkylammonium type PILs as 2-sulfoethylmethylammonium triflate [MSEA][TfO], 2-sulfoethylmethylammonium hydrogen sulfate [MSEA][HSA], 3-sulfopropyldiethylammonium triflate [DESPA][TfO] and 3-sulfopropyldiethylammonium hydrogen sulfate [DESPA][HSA]. The physico-chemical properties of the neat PILs, i.e. ORR kinetic current density, oxygen solubility, oxygen diffusivity, electric conductivity and thermal stability, are measured at temperatures relevant for fuel cell operation. Commercial polyetheretherketone (PEEK) and PBI are chosen as host membrane materials to immobilize the PILs in a polymer matrix. The (total) electric conductivity and the thermal stability of the PIL-polymer systems are also evaluated.The influences of the cations or anions on the PILs and the different immobilization methods of PILs in membrane are investigated. The results show that with different PIL/membrane combinations, significant differences in thermal stability, mechanical stability and electric conductivity are present.Literature:[1] Yasuda, T. and M. Watanabe, Protic ionic liquids: fuel cell applications. MRS Bull., 2013. 38(7): p.560-566.[2] Noda, A., K. Hayamizu, and M. Watanabe, J. Phys. Chem. B, 2001 105(20): p. 4603-4610.[3] Huang, X.-J., et al., J. Phys. Chem. B, 2009. 113(26): p. 8953-8959.