Investigation of the physical and electrochemical properties of ionic liquid suitable for a future use as fuel cell electrolyte for operation > 100 °C

  • Untersuchung der physikalischen und elektrochemischen Eigenschaften ionischer Flüssigkeiten für eine zukünftige Anwendung als Elektrolyt in Brennstoffzellen für den Betrieb > 100 °C

Hou, Hui; Korte, Carsten (Thesis advisor); Herrmann, Andreas (Thesis advisor)

Aachen : RWTH Aachen University (2022)
Habil / Postdoctoral Thesis (Non-german Habil)

Habilitationsschrift, RWTH Aachen University, 2022

Abstract

This thesis is focused on the investigation and the identification of suitable non-aqueous electrolyte applied in the intermediate temperature polymer electrolyte fuel cells (IT-PEFCs). Herein, we show that protic ionic liquids (PILs) are the promising candidates for fuel cells operation in the temperature range of 100 °C to 120 °C. N,N-diethyl-N-methyl-3-sulfopropane-1-ammonium hydrogen sulfate [DEMSPA][HSA] and triflate [DEMSPA][TfO], N,N-diethyl-3-sulfopropane-1-ammonium hydrogen sulfate [DESPA][HSA] and triflate [DESPA][TfO] are investigated in this work. The physico-chemical properties relevant for IT-PEFC operations are systematically evaluated, including specific conductivity, thermal stability, viscosity, oxygen permeability and electrochemical properties. The triflate-based PILs provide the best combination of the fast oxygen reduction reaction (ORR) kinetics and fast oxygen transport. This applies in particular to [DESPA][TfO]. The physical-, electrochemical properties of non-stoichiometric [DESPA][TfO] are investigated. A series PIL blends are prepared, varying from an excess of the proton acceptor (N,N-diethyl-3-aminopropane-1-sulfonic acid) to an excess of the proton donor (triflic acid). The results show that an excess of the (free) acid is beneficial for the conductivity, oxygen reduction reaction (ORR) kinetics and the oxygen transmission coefficient. Blend membranes are prepared from polybenzimidazole (PBI) as a host polymer and stoichiometric and non-stoichiometric [DESPA][TfO] as the electrolyte. The PIL is immobilized in the PBI membrane by solution casting. The maximum PIL loading amount is determined based on the premise that the obtained blend membrane has a sufficient homogeneity, adequate thermal and mechanical stability and ionic conductivity. The blend membranes exhibit promising properties regarding an improved thermal stability and proton conductivity. The highest protonconductivity of 2 mScm-1 is achieved for PBI-PIL blends with stoichiometric [DESPA][TfO] and of 16 mScm-1 at 120 °C and 40% relative humidity for PBI-PIL blends with an excess acid respectively.

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