Cost-effective Prussian blue analogue composite proton exchange membranes for low humidity fuel cell operation

Abstract

Proton exchange membranes (PEMs) exhibit reduced proton conductivity caused by severe dehydration under low relative humidity (RH), which is a pivotal bottleneck toward widespread commercialization of PEM fuel cells (PEMFCs). Previous PEM optimizing strategies have predominantly pursued increasing water uptake or enlarging hydrophilic domains, for which there is less focus on the mode of proton transport (Grotthuss or vehicular mechanisms) as the underlying reason. Here, a counterintuitive route is proposed by incorporating CuII[FeIII(CN)6]2/3 Prussian blue analogue (CuFe-PBA) additive, which possesses a hydrophobic framework. However, due to the ideal lattice length of CuFe-PBA, the lattice water molecules contained within this framework form well-connected hydrogen bond networks, which promote Grotthuss-type proton transport that is less dependent on RH conditions. Consequently, CuFe-PBA doped in sulfonated polysulfone (SPSf) matrix yields SPSf/CuFe-PBA composite PEMs showing better performance over unfilled PEM in both ex situ and in situ evaluations, especially under low RH conditions, which are competitive with Nafion 212. Preliminary trials of scale-up for CuFe-PBA and composite PEMs with cost estimation, and feasibility of incorporating polytetrafluoroethylene reinforcement, suggest economic and practical potential. Thus, CuFe-PBA composite PEMs may be of particular interest to PEMFC and related materials communities.