Performance and Durability of the Br2 – H2 Redox Flow Cell

The bromine - hydrogen flow cell under development at Berkeley Lab provides high power density and stable long-term cycling. Peak power of 1.4W/cm² and >90% efficiency at 0.4W/cm²were reported previously [1]. Long-term cycling with 3M HBr electrolyte yielded Coulombic efficiency above 95% and energy efficiency around 75% [2]. Slow capacity fade was observed, presumably due to loss of bromine from the system. Note the electrolyte was replaced every 125 cycles, after which the capacity and performance recovered, suggesting cell components were not degraded. A total of 625 cycles was achieved. The present work describes efforts to optimize the cell materials and understand degradation mechanisms.

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).

Figure 1 shows impedance and open-circuit voltage (OCV) over the whole range of SOC. For SOC above 70%, bromine liquid forms and the impedance increases due to flooding of the reaction sites with Br2 liquid and depletion of HBr in the aqueous phase. Below 5% SOC, the impedance increases due to depletion of bromine. Cycling between these SOC limits ensures acceptable cell impedance.

Cell materials selection has a large impact on cell performance. During charge, bromine and water cross through the membrane. This is a concern for long-term performance, as bromine can cause dissolution of the anode platinum catalyst. In the short term, crossover bromine and water must be returned to the cathode to maintain concentrations in the electrolyte solution. The choice of membrane type, thickness, and treatment procedure is expected to impact crossover rate, water/bromide/proton selectivity, and ohmic cell impedance. We will report results for Nafion, Aquivion, and 3M membranes.

Figure 2 compares strategies for returning the crossover liquid to the cathode tank for a cell with Nafion NR212 membrane:

- Batch collection: anode exhaust liquid is collected during 15 cycles, and returned to the cathode tank at the 15^thcycle.

- Condenser/drain: anode exhaust is passed through a condenser above the cathode tank and the collected liquid flows down into the tank.

- Condenser/pump: anode exhaust is passed through a closed-bottom condenser; the collected liquid is pumped back to the cathode tank.

Because the exhaust liquid is not cooled for batch collection, permanent bromine loss via evaporation is observed. Upon returning the exhaust to the cathode tank, the capacity recovers only to ~90%. With the condenser drain connected to the cathode tank, bromine is continuously returned to the cathode side, however some bromine gas escapes from the head space of the tank through the condenser. Closing the bottom of the condenser and pumping the liquid back into the sealed cathode tank overcomes this bromine loss mechanism, and stable cycling is achieved.

References

[1] Cho KT, Albertus P, Battaglia V, Kojic A, Srinivasan V, and Weber AZ, Energy Technol. 2013, 1, 596 – 608.

[2] Cho KT, Ridgway P, Weber AZ, Haussener S, Battaglia V, Srinivasan V. (2012) J. Electrochem. Soc. 159 (11) A1806-A1815.

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.