Dual function organic active materials for nonaqueous redox flow batteries

Abstract

Nonaqueous electrolytes require the inclusion of supporting salts to achieve sufficient conductivity for battery applications. In redox flow batteries (RFBs) wherein solutions contain active species at molar values, the presence of supporting salts can reduce the solubility of organic active materials, limiting battery capacity. Here we sought to design organic materials in which permanently charged substituents keep ionic conductivity high while at the same time increasing the maximum concentration of the charge-storing redox moiety to operate all organic supporting-salt-free full flow cell cycling for the first time. Toward this goal, we synthesized redox-active phenothiazine and viologen derivatives bearing permanent charges. We employed these highly soluble materials as RFB electrolytes without adding supporting salts. Using an anion-selective membrane, a flow cell containing 0.25 M active species cycled stably over 100 cycles (433 h), losing an average of only 0.14% capacity per cycle and 0.75% per day, with post-cycling analysis showing no evidence of decomposition. Further, higher concentration cycling (0.75 M – electron) accessing both reductions of viologen, achieved a cell potential of 1.80 V with 18.3 A h L⁻¹, high volumetric capacity, only losing an average of 0.90% capacity per day. These results show a new avenue to improve two performance aspects with one molecular modification.