New promising proton conducting electrolyte for high-temperature fuel cells based on hydrophobic guanidine salt

Guanidine salts are promising proton conductors due to the high content of dissociable protons in guanidinium cation that ensure an efficient proton transfer along hydrogen-bonded network formed by proton donor and proton acceptor sites. However, the high melting point of most guanidine salts is a serious drawback for their application as proton conducting electrolytes. Reducing the symmetry of guanidinium cations by the substitution of hydrogen atoms on alkyl radicals reduces the melting points but also leads to decreased proton conductivity. In this study, monosubstituted guanidine salt, N-butylguanidinium bis(trifluoromethylsulfonyl)imide (BG-TFSI), has been synthesized by a simple two-step method. It is water immiscible room temperature protic ionic liquid. The structure of BG-TFSI was confirmed by nuclear magnetic resonance spectroscopy, as well as infrared spectroscopy. According to thermal gravimetric analysis data, the ionic liquid has the thermal degradation point (5% weight loss) of 348 °C which indicates its excellent thermal stability for use in high-temperature fuel cells. The ionic conductivity of BG-TFSI determined by the electrochemical impedance method was found to be 9·10-4 S/cm at room temperature. This value increased by almost one order of magnitude above 100 °C thus reaching an acceptable level for use in fuel cells. The activation energy Ea calculated from the Arrhenius plot for BG-TFSI is found to be 16.4 kJ/mol which is similar to those reported for other guanidine salts. Based on the obtained results one can assume that the proton transport in BG-TFSI is dominated by Grotthus-type (hopping) mechanism. The results of this study indicated that BG-TFSI is a promising proton conducting electrolyte for fuel cells operating at elevated temperatures in water-free conditions. The hydrophobicity of the ionic liquid is an important advantage since it can prevent its leaching from the polymer electrolyte membrane during the operation of a fuel cell.