Durability and Performance of the Br2-H2 Redox Flow Cell
Authors: Tucker Michael C , Weber Adam Z , Lin Guangyu , Chong Pau Ying , Nguyen Trung Van 
Published: February 2020

The bromine - hydrogen flow cell under development at Berkeley Lab and TVN systems provides high power density and stable long-term cycling. During discharge, Br2 in HBr (aq) is fed to the cathode compartment where bromine is reduced to bromide, generating the theoretical open circuit potential of 1.098 V at 25°C. On charge, H2 and Br2 are generated from HBr at the (-) and (+) electrodes, respectively. The challenges include: high vapor pressure of bromine gas; migration of water and bromine to the hydrogen anode during charging; poisoning of the anode Pt catalyst by bromine; and, formation of mixed aqueous and liquid bromine phases at high states of charge (SOC).
Cycling with 3M HBr electrolyte and closed anode tank (hydrogen stored in system) yielded Coulombic efficiency above 98% and energy efficiency around 80% as shown in Figure 1. The capacity decreases slowly, primarily due to loss of bromine from small leaks. Coulombic efficiency is stable, indicating that bromine crossover rate remains low relative to the charge/discharge current density, 400mA/cm2. The voltage efficiency slowly decreases due to changing electrode activity, presumably due to dissolution or bromide-poisoning of the Pt (-) electrode. Degradation mechanisms and evolution of cell materials, including electrode performance and membrane transport properties, will be discussed in detail.
Peak power of 1.4W/cm2 and >90% efficiency at 0.4W/cm2were reported previously [1].
The performance was significantly improved by pre-boiling the membrane. Boiling increases transport properties, and the improved performance shown in Figure 2 can be attributed to increased proton conductivity in the membrane. There is a trade-off, however, between membrane conductivity and bromine crossover. This is illustrated in Figure 3. Bromine crossover and reduction at the anode results in self-discharge, which lowers coulombic efficiency. The as-received membrane displays low bromine crossover (self-discharge current of <2 mA/cm2), allowing high efficiency to be achieved at low current density. In contrast, the high bromine crossover observed for the boiled membrane (self-discharge current of 13 mA/cm2) greatly reduces coulombic efficiency, limiting the energy efficiency. The impact of membrane features including thickness and membrane type (e.g. equivalent weight, presence of reinforcement) on system performance and efficiency further illustrate the trade-off. Thicker, higher equivalent weight, and reinforced membranes display lower crossover, but higher area-specific resistance. Of the membrane features studied, it is found that pretreatment conditions have the highest impact on performance and efficiency.
References
[1] Cho KT, Albertus P, Battaglia V, Kojic A, Srinivasan V, and Weber AZ, Energy Technol. 2013, 1, 596 – 608.
Acknowledgements
This work was funded by Advanced Research Projects Agency-Energy (ARPA-E) of the U. S. Department of Energy under contract number DE-AC02-05CH11231 with cost share provided by Robert Bosch Corp and Award No. DE-AR0000262 to TVN Systems, Inc.

Figure 1