Person:

Jing, Yan

Aqueous organic redox flow batteries are promising candidates for large-scale energy storage. However, the design of stable and inexpensive electrolytes is challenging. Here, we report a highly stable, low redox potential, and potentially inexpensive negolyte species, sodium 3,3′,3′′,3′′′-((9,10-anthraquinone-2,6-diyl)bis(azanetriyl))tetrakis(propane-1-sulfonate) (2,6-N-TSAQ), which is synthesized in a single step from inexpensive precursors. Pairing 2,6-N-TSAQ with potassium ferrocyanide at pH=14 yielded a battery with the highest open-circuit voltage, 1.14 V, of any anthraquinone-based cell with a capacity fade rate <10 %/yr. When 2,6-N-TSAQ was cycled at neutral pH, it exhibited two orders of magnitude higher capacity fade rate. The great difference in anthraquinone cycling stability at different pH is interpreted in terms of the thermodynamics of the anthrone formation reaction. This work shows the great potential of organic synthetic chemistry for the development of viable flow battery electrolytes and demonstrates the remarkable performance improvements achievable with an understanding of decomposition mechanisms.

Designing electrolytes based on mixture of different organic redox active molecules brings the opportunity of enhancing the volumetric energy density of flow batteries and removes the requirement of high solubility for individual organic species in the mixture. In the present work, we conduct computational and experimental analysis to investigate the electrochemical performance of mixed redox-active organic molecules. A zero-dimensional transient model is employed to investigate the changes in the half-cell potential and the concentrations and partial currents of individual redox reactions in a mixture of organic molecules over time. The model demonstrates the effects of individual properties of species such as kinetic rate constants, mass transfer coefficients, concentration ratios and standard redox potentials and reports the effect of energy-losing homogenous chemical redox reaction on the voltage efficiency and concentration ratios of the mixed species. Pairs of anthraquinone negolyte species were selected for an experimental case study. A mixture of 2,6-N-TSAQ and 2,6-DHAQ showed 40% increase in the volumetric energy density compared to the performance of 2,6-DHAQ alone. Based on the results of the experimental and computational analysis, we propose guidelines for the design of suitable mixed redox-active organic species.