Abstract
Proton conductive group is essential to provide ion transport site and tune micro-phase separation in proton exchange membranes for vanadium redox flow battery (VRFB), while nearly all the fabrication methods are based on the benchmark sulfonic acid group. Herein, the sulfate ester group (-OSO3H) with superior acidity is proposed as a novel proton conductive group to achieve high proton transport at lower IECs, therefore disentangles the trade-off between vanadium crossover and conductive ion diffusion. The electron withdrawing ester oxygen atom endows sulfate ester group with 27 kJ mol−1 lower proton binding energy than the sulfonic acid group, leading to higher proton dissociation ability and better micro-phase separation. The 1,3-propanediol cyclosulfate grafted polybenzimidazole with IEC of 1.85 (OPBI-OSO3H-1.85) reaches comparable area resistance (0.19 Ω cm−2) to the sulfonic acid grafted polybenzimidazole with much higher IEC of 2.95 (OPBI-SO3H-2.95), but 19.1% decrease in vanadium permeability. The cell assembled with OPBI-OSO3H-1.85 achieves high energy efficiency of about 86.7% and low decay of discharge capacity (0.41% per cycle), superior to that of OPBI-SO3H-2.95 (0.51% per cycle) and Nafion 212 (0.71% per cycle). The sulfate ester group is proved to be chemically stable in H2SO4/vanadium environment.
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