Investigation of the proton transport and the interaction between ionic liquids and ionogene PBI-type polymers

  • Untersuchung des Protonentransports und der Wechselwirkung zwischen Ionischen Flüssigkeiten und ionogenen Polymeren vom PBI-Typ

Lin, Jingjing; Korte, Carsten (Thesis advisor); Herrmann, Andreas (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2022

Abstract

Fuel cells (FCs) are electrochemical devices with advanced features such as a high energy conversion efficiency and (nearly) no pollutant emission. Polymer electrolyte fuel cells (PEFCs) operating at elevated temperatures of above 80 °C have become a promising topic in recent years, due to a much simpler system setup compared to low-temperature PEFCs. The conductivity of proton exchange membranes (PEMs) used in low-temperature PEFCs up to 80 °C operation temperature is mainly determined by the uptake of water by the polymer. At elevated temperatures, a new membrane material should maintain its conductivity also in anhydrous conditions. Currently, HT-PEFCs are based on phosphoric acid (H3PO4)-doped polybenzimidazole (PBI) membranes. However, these membranes are difficult to operate below 160 °C without suffering loss of H3PO4 because the equilibrium doping degree is temperature dependent and the conductivity is decreasing to an insufficient level. Meanwhile, the presence of H3PO4 causes a slow cathodic oxygen reduction reaction (ORR) kinetics. Thus, there is a necessity for alternative non-aqueous proton-conducting electrolytes operational for the temperature range of 80-160 °C. Protic ionic liquids (PILs) are promising candidates for the use as non-aqueous electrolytes and have received much attention as a potential electrolyte in PEFCs due to their ionic conductivity, wide electrochemical windows and low flammability. PILs based on very strong acids have a less inhibiting effect than H3PO4. In a water-free PIL, protons of highly acidic PILs can only be transported via the protonated cations by means of a vehicle mechanism, which is coupled with a comparably high viscosity. In a solid PEM electrolyte, the vehicular transport of the relatively large PIL cations (and anions) is sterically restricted due to the constrained space and directed interactions. However, PILs in particular, with strong acidic cations, are usually highly hygroscopic. Under fuel cell operation, water will be generated on the cathode side. A steady state concentration of residual water will result from the hygroscopicity of the strongly acidic PIL and the load-dependent water production of the fuel cell. This may offer the possibility to enhance the technically-utilizable conductivity of PIL electrolytes. Water could be regarded as a proton acceptor, a carrier and a donor, and participate in proton transfer process in ionic liquids.In this dissertation, three PILs, 2-Sulfo¬ethyl¬methylammonum triflate [2-Sema][TfO], 1-Ethylimidazolium triflate [EIm][TfO] and Diethyl-methylammonium triflate [Dema][TfO] are used. The acidity of the proton-carrying cation varies over ten orders of magnitude. The influence of cation acidity, residual water, and acid-base stoichiometry on proton transport mechanism in bulk PILs are investigated by using electrochemical and nuclear magnetic resonance (NMR) methods. Further details of the transport mechanism in the time scale of picosecond to nanosecond are detected by using quasi-elastic neutron scattering (QENS) technique. Polybenzimidazole (PBI) is chosen as the matrix polymer of the PIL-based PEM electrolyte. The PBI-PIL blend membranes are prepared by two routes: swelling doping method and solution casting method. The molecular interaction and the proton transport mechanism of the PIL-PBI blend membranes are investigated. The studies are intended to broaden the understanding of the proton conducting process under operating conditions, and to give possible ways for optimizing the PIL electrolyte doped polymer blend membranes for HT-PEFCs.

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