Understanding and Mitigating Capacity Fade in Aqueous Organic Redox Flow Batteries
Authors: Murali Advaith , Nirmalchandar Archith , Krishnamoorthy Sankarganesh , Hoober-Burkhardt Lena , Yang Bo , Prakash G. K. Surya , Narayanan S. R. 
Published: February 2020

The use of water-soluble organic redox couples is a new and attractive pathway to a sustainable electrical energy storage system with the potential to be inexpensive and environmentally-friendly.[1][2][3] Further, several modifications of this type of redox flow battery arrangement using inorganic materials in combination with organic redox couples have also become the subject of research. [4][5] We have demonstrated the repeated cycling of a redox flow cell based on water-soluble organic redox couples (ORBAT) at high voltage efficiency, coulombic efficiency and power density. Recently, we presented for the first time the synthesis, characterization and properties of 3,6-dihydroxy-2,4-dimethylbenzenesulfonic acid (DHDMBS) as a new positive side electrolyte material for aqueous organic redox flow batteries (ORBAT). [6] DHDMBS overcame the major issue of the Michael reaction with water faced with previously reported positive electrolyte materials such as 4,5-dihydroxybenzene-1,3-disulfonic acid (BQDS) and other unsubstituted benzoquinones.
The present study focuses on the crossover of DHDMBS from the positive side of the cell to the negative side. Specifically, we have explored various approaches to mitigate the effects of crossover. These approaches include low-permeability membranes, a new symmetric cell configuration using mixed electrolytes, and operating protocols that involve polarity switching. These approaches were found to reduce fade rates by 70% - 85%. We also uncover mechanistic pathways that lead to slow chemical modification that cause capacity to decrease under strongly acidic conditions during long-term cycling. The new understanding and methods presented here will contribute towards the development of ORBAT as an inexpensive and sustainable solution for large-scale electrical energy storage.
Acknowledgement
The Authors acknowledge the financial support for this research from ARPA-E Open-FOA program (DE-AR0000337), the University of Southern California, and the Loker Hydrocarbon Research
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Figure 1