Co-assembly of perfluorinated sulfonic-acid ionomer and tetraphenylporphyrin tetrasulfonic-acid contributes to high-performance proton-exchange membranes

Abstract

To meet improved requirements for proton-exchange membranes (PEMs) applied in medium- and low-temperature fuel cells, novel PEMs with superior performance are prepared by using commercial perfluorinated sulfonic-acid ionomer (PFSA) and a dopant, tetraphenylporphyrin tetrasulfonic-acid (TPPS). Different from physically blending PFSA with another polymer or a nano-material, the membrane-forming mechanism of TPPS modified PFSA PEMs is highly dependent on the efficient interaction between PFSA and TPPS molecules. The unique three-part nested hydrophilic-hydrophobic-hydrophilic molecular structure of TPPS plays a crucial role in the co-assembly of PFSA with TPPS. The more intensive ionic cross-linking and hydrogen-bond networks are formed through hydrophilic groups in the outer and internal parts of TPPS and sulfonic-acid groups of PFSA. Meanwhile, secondary cross-linking of these hydrophilic groups is induced by rigid hydrophobic groups in the middle part of TPPS. Therefore, these molecular interactions induce the formation of more stable and stronger nano-phase separation with more uniform size. This novel nano-phase separation structure endows PEMs with elevated applicable temperatures, much increased proton conductivity and thus superior fuel cell performances, which are the keys to the development of more advanced hydrogen fuel cells.