Cation exchange membranes (CEMs) are a critical component in various energy storage and conversion technologies such as solid-state batteries and fuel cells. In proton exchange membrane fuel cells (PEMFC), a highly conductive and mechanically robust PEM that can operate at high temperatures is desired. One method to increase the conductivity of polymer membranes is to include highly delocalized anions, such as trifluoromethanesulfonimide (TFSI), that enable high ion conductivity due to the lower activation energy required for ion transport. In this study, TFSI, which is traditionally an ionic liquid, can be synthesized as the side chain of the polymer via a facile two-step reaction.Commercially available sulfonated polymers, a midblock sulfonated pentablock copolymer (Nexar) and sulfonated poly(arylene ether sulfone) homopolymer (sBPSH) are converted to have TFSI groups. To demonstrate the effectiveness of converting sulfonate groups to TFSI groups, the proton conductivity of sulfonated Nexar and Nexar-TFSI were measured in various relative humidities (RH) and at different temperatures. At 80°C and 95% RH, the proton conductivity increased from 270 mS/cm for SO<sub>3</sub>H form to 390 mS/cm for TFSI form, exhibiting over 40% improvement in proton conductivity. This study opens the possibility of a new series of polymer electrolyte membranes easily synthesized from commercially available sulfonated polymers via the TFSI conversion method. The addition of a TFSI functional group is an efficient strategy to significantly improve proton conductivity, and thus provides a potential route for the synthesis of high-performance membranes for PEMFC application, especially when combined with the tailoring of various membrane properties (hydrophobicity, glass transition temperature, chemical stability). The detailed investigation on the impact of TFSI functionalization for PEM will be discussed in this presentation.

Conversion of sulfonated polymers into highly conductive TFSI polymers

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ACS Fall 2022

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