PBI-Type Polymers and Acidic Proton Conducting Ionic Liquids – Conductivity and Molecular Interactions

Abstract

The operation of polymer electrolyte fuel cells (PEFC) at elevated temperatures above 100°C would allow a much simpler system setup: i) no feed gas humidification, ii) a more efficient cooling system (easier water and heat management), iii) the possibility of reco­vering high-grade waste heat, and iv) a higher tolerance against feed gas impurities. Cur­rently, (high temperature) HT-PEFC, based on phosphoric acid doped polybenzimidazole (PBI) membranes, cannot compete with the performance characteristics of NAFION-based low temperature-PEFCs. The presence of H3PO4 causes a slow cathodic oxygen reduction reaction kinetics (ORR). Thus, there is a necessity for new non-aqueous proton conducting electrolytes operational for the temperature range between 100–120 °C. Proton conducting ionic liquids (PILs) with acidic cations are promising candidates for the use as non-aqueous electrolytes at operation temperatures above 100 °C. In this con­tribution, an experimental study on the interaction of PBI based proton exchange mem­brane (PEM) with highly acidic betaine-type zwitterion PILs is presented. 2-Sulfo­ethyl­methylammonum triflate ([2-Sema][TfO]) exhibits a ~3 times higher ORR current densities on Pt compared to H3PO4. There is a (slow) uptake of [2-Sema][TfO] by PBI due to a swelling process, up a weight increase of 135%. The composition of doped membrane was determined by NMR. The doping process was monitored by Raman spectroscopy, proving the protonation of the imidazole groups in the polymer chains. The thermal sta­bility is measured by TGA. NMR analysis and quasielastic neutron scattering (QENS) has been applied to elucidate the proton dynamics and the molecular interactions between PBI, PIL and residual water, which is present during fuel cell operation. Residual water concentrations up to 6 wt% are investigated. Considering the change of viscosity, the total conductivity in [2-Sema][TfO] depends highly on the H2O concentration. A high acidic PIL is able to protonate H2O. Thus, compared to low and medium acidic PILs, the conductivity of high acidic PIL shows a (quasi-)exponential increase. The self-diffusion coefficients are measured by DOSY 1H-NMR, revealing that the diffusion of the active (acidic) proton is faster than the diffusion of the PIL cation. The results indicate a cooperative mechanism involving a fast proton exchange between PIL cations, H2O and H3O+. Beyond the time scale of the NMR mea­surement, two Lorentzian components can be observed by QENS in [2-Sema][TfO]/H2O. Besides the diffusion of PIL cations and the H2O/cation proton exchange, an additional faster dynamic process of the active protons is present. Therefore, an intra-molecular proton dynamic is inferred in cation. Compared to the neat PIL, the proton conduction in the doped PBI membrane is re­stric­ted due to the constraining network of the polymer chains. To optimise the conductivity but also the uptake of the PIL into the polymer, the use of solution casting methods have been studied for these materials.