Enabling High Concentration Redox Electrolytes for Use in Nonaqueous Redox Flow Batteries

Crystal structure of the radical cation salt of a soluble, two-electron donating phenothiazine (top) and its performance in a symmetric cell (bottom).

Scientific Achievement

Building on prior collaborative JCESR research on iteratively advancing phenothiazines as charge storage materials, we developed a new class of phenothiazine derivatives with high equivalent charge concentration by increasing solubility and stability of multiple electron transfer events.

Significance and Impact

Two-electron-donating phenothiazines with oligoglycol substituents exhibit high solubility across all states of charge. The radical cation form is sufficiently stable that it can be isolated in as a crystalline solid. Lessons learned from this activity may, in turn, inform future design strategies for phenothiazines and may be applicable to other molecular families.

Research Details

  • The new phenothiazine derivative is stable across three states of charge, with a limiting solubility of 0.55 M in non-aqueous electrolytes, enabling solutions with capacity of >1 M e-.
  • Symmetric flow cell cycling demonstrates stability over multiple discharge / charge cycles with only 30% capacity fade over 460 h of operation with an electrolytes containing 0.3 M active material.
  • Post-test analysis indicates performance decay is due to self-discharge and crossover, not materials decomposition.

DOI: 10.1021/acs.chemmater.8b04770

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