A hybrid regenerative flow system engineered for multi-megawatt, long-duration grid balancing. The architecture combines a proton-exchange membrane (such as reinforced perfluorosulfonic acid, Nafion) separating a gaseous hydrogen negative electrode from a circulating aqueous hydrobromic acid / liquid bromine (HBr/Br2) positive catholyte. During discharge, gaseous H2 fuel oxidises at a gas diffusion anode coated with a poison-resistant catalyst (e.g., ruthenium or tantalum oxide-doped platinum), producing protons that traverse the membrane, while aqueous Br2 is reduced to bromide ions Br–. The reaction exhibits some of the fastest electrochemical kinetics among flow chemistries, delivering power densities up to ten times greater than conventional vanadium redox systems at significantly lower electrolyte capital costs.
| Parameter | Value / Range | Notes / Context |
| Nominal Cell Voltage | 1.05 – 1.09 V | Stable plateau across wide range of state of charge |
| Area Power Density | 0.8 – 1.5 W/cm2 | Roughly 5x – 10x higher than aqueous vanadium redox cells |
| Specific Energy (System) | 60 – 100 Wh/kg | Determined strictly by catholyte storage tank concentration |
| Operating Temperature | 25 to 55 °C | Elevated temperatures accelerate kinetics; requires vapor containment |
| Round-Trip Efficiency (DC) | 70 – 78% | High chemical reversibility; limited by hydrogen gas pumping losses |
| Membrane Requirement | Cation Exchange (PFSA) | Must withstand aggressive halogen oxidation and prevent Br2 crossover |
| Catholyte Composition | 1 – 3 M Br2 in 6 – 8 M HBr | Bromine complexing agents (BCAs) used to suppress toxic vapor pressure |
| Primary Failure Modes | Catalyst poisoning & corrosion | Platinum catalytic surface degradation by bromine species crossover |