Pyridine-Containing\nPolyhydroxyalkylation-Based Polymers\nfor Use in Vanadium Redox Flow Batteries

In this work, a class of polymers with pendant pyridine\nmoieties\nand variable <i>p</i>-terphenyl (pTP) content was developed\nvia superacid catalyzed polyhydroxyalkylation (PHA). High controllability\nof the copolymer composition was demonstrated, allowing for straightforward\nfine-tuning of the material properties. Low levels of <i>p</i>-terphenyl were used to obtain materials with improved conductivities\nof up to 83 mS cm<sup>–1</sup> in 3 M H<sub>2</sub>SO<sub>4</sub>, allowing the efficient application at high current densities. Increasing\nthe <i>p</i>-terphenyl content significantly improved the\nselectivities up to 1.88 × 10<sup>13</sup> S s m<sup>–3</sup>, resulting in optimized materials for low current density applications.\nA single cell equipped with BP-pTP10-Py exhibited an energy efficiency\n(EE) of 76.7% at a current density of 200 mA cm<sup>–2</sup> (vs 73.7% for the commercial reference FAPQ330). At lower current\ndensity (50 mA cm<sup>–2</sup>), the single cells equipped\nwith BP-pTP20-Py and BP-pTP35-Py showed the highest EE of 92.3% (vs\n90.9% for the FAPQ330). Ex situ stability tests in 1.6 M V(V)/2 M\nH<sub>2</sub>SO<sub>4</sub> proved high chemical stability with no\nchanges in the NMR spectra and similar V(IV) contents in the stability\nsolution as the reference FAPQ330. In addition, long-term in situ\ntests revealed no evidence of performance degradation for the single\ncell equipped with BP-pTP20-Py.