› Zinc-Bromine (Zn-Br) Flow Battery Simulator Back
Energy Storage

Zinc-Bromine (Zn-Br) Flow Battery Simulator

A hybrid flow battery that plates zinc as a metal on the negative electrode and complexes bromine with a polybromide agent on the positive side. Higher energy density and lower cost than VRFB, but with a more limited cycle life. Adjust stack configuration and electrolyte volume to size a 4-12 h medium-duration storage system for homes and commercial buildings.

Parameters
Cell area
cm²
Active electrode area. Larger area allows lower current density
Cells per stack
cells
Number of stacks
stacks
Electrolyte volume
L
Total ZnBr₂ + complexing agent solution in the tanks
ZnBr₂ concentration
mol/L
3-4 mol/L is typical; near the solubility limit
Current density
mA/cm²
Polybromide complexing agent
mol/L
MEP / TBAB etc. Binds Br₂ as a polybromide complex
Results
—
Total cells
—
Power output (kW)
—
Energy capacity (kWh)
—
Discharge time (h)
—
Energy density (Wh/L)
—
Total cost (USD)
—
Zn-Br flow battery concept animation

Zn²⁺ plates as zinc metal on the negative electrode; Br⁻ is oxidised to Br₂ on the positive side and is held in an orange polybromide phase via the complexing agent. The reactions reverse during discharge to deliver power.

VRFB vs Zn-Br (energy density / cost / cycle life)
Discharge curve (cell voltage vs DOD)
Theory & Key Formulas

$$E = c\,V_e \cdot z\,F \cdot V_{\text{cell}},\qquad \rho_E = \frac{E}{V_e}$$

Stored energy E and volumetric energy density ρ_E. c: ZnBr₂ concentration, V_e: electrolyte volume, z=2 (electrons), F=96485 C/mol, V_cell: average cell voltage.

$$V_{\text{op}} = V_{\text{ocv}} - j\,R_{\Omega},\qquad P = I\cdot N\,V_{\text{op}}$$

Operating cell voltage V_op (j: current density, R_Ω: areal resistance ≈ 0.7 Ω·cm², V_ocv = 1.85 V) and stack power P (I: current, N: total series cells).

$$\eta_{\text{RT}} = \eta_{\text{C}} \cdot \eta_{\text{V}} \cdot \eta_{\text{aux}}$$

Round-trip efficiency is the product of coulombic (≈90%), voltage (V_op / V_ocv) and auxiliary (≈95%) efficiencies.

Capacity and efficiency of the zinc-bromine (Zn-Br) flow battery

🙋
I've heard of the vanadium flow battery, but the zinc-bromine one is new to me. What's the difference?
🎓
In short, Zn-Br is a "hybrid" flow battery. Unlike VRFB, where both sides use dissolved vanadium ions in solution, in Zn-Br the negative side plates zinc as a solid metal on the electrode. So it's a little bit closer to a regular battery on one side. NASA and Exxon studied it in the 1970s, and today Redflow in Australia and Primus Power in the US are commercialising it.
🙋
Why zinc and bromine? There are plenty of other combinations.
🎓
Three reasons. First, energy density. The standard potential of Zn²⁺/Zn is −0.76 V and of Br₂/Br⁻ is +1.09 V, giving an open-circuit voltage of about 1.85 V — 1.5x higher than VRFB's 1.26 V. Combined with the density of zinc metal, the volumetric energy density is 60-90 Wh/L versus 25-40 Wh/L for VRFB. Second, cost: ZnBr₂ is far cheaper than vanadium per kilogram. Third, it is an aqueous, non-flammable chemistry — no thermal runaway like lithium-ion.
🙋
With 1300 L of electrolyte and 3.5 mol/L on the right, I get 104 kWh capacity and a 4-hour discharge. Is that enough for one house?
🎓
Actually that's more like a small commercial building. A typical home uses 10-15 kWh per day and 3-5 kW of power, so a few Redflow ZBM2 units (10 kWh / 3 kW each) are enough. 26 kW / 104 kWh fits a convenience store or a solar-PV peak-shifting installation. The "4-hour discharge" point is also key: lithium-ion is best for 1-2 h (power applications), pumped hydro for 8-24 h (long-duration), and flow batteries occupy the medium-duration niche right in between.
🙋
Capacity grows with ZnBr₂ concentration, but why is 4.5 mol/L the upper limit?
🎓
That's the solubility limit of ZnBr₂ — about 4.5 mol/L at 25°C. Beyond it you get crystallisation and pump clogging. Zinc is also plated as metal, so there's a physical limit (~200 mAh/cm²) on how much can be deposited per unit electrode area. Charge too deep and Zn dendrites grow and pierce the membrane, causing a short. Real systems keep state-of-charge below ~80% and run a "stripping cycle" periodically to dissolve residual zinc. This is the main reason Zn-Br cycle life (~5000 cycles) is shorter than VRFB's (~20000 cycles).
🙋
There's also a complexing-agent slider on the left. What does it do?
🎓
Good catch. During charge the positive side produces a lot of Br₂, which is a highly volatile brown gas. Left alone in water it would evaporate or diffuse across the membrane and self-discharge against the zinc. So we add a quaternary ammonium salt — 4-methylpyrrolidinone (MEP) or tetrabutylammonium bromide (TBAB) — which forms a complex with Br₂ that separates as an oily orange polybromide phase. This phase is pumped to a separate tank for storage, eliminating self-discharge and enabling shelf life of months. The orange colour on the cathode side of the canvas represents that polybromide phase.

Frequently asked questions

Zn-Br is a hybrid flow battery: zinc is electroplated as a metal on the negative electrode while bromine is complexed with a polybromide agent on the positive side. Compared with all-liquid VRFB, Zn-Br has 2-3x higher volumetric energy density (60-90 Wh/L vs 25-40 Wh/L) and a far lower system cost (around $150/kWh vs $400-500/kWh) because ZnBr₂ is much cheaper than vanadium. The trade-offs are a shorter cycle life (~5000 cycles, vs >20000 for VRFB) due to zinc plating and the need for periodic stripping cycles.
Main applications are residential storage (10-20 kWh, 4-8 h), commercial self-consumption optimisation (100-500 kWh), and microgrids or remote off-grid power (500 kWh to several MWh). Products on the market include Redflow ZBM2 (10 kWh / 3 kW), Primus Power EnerBlock (25 kW / 125 kWh) and EnSync. Compared with lithium-ion, response is slower, but tolerance to deep discharge and resistance to thermal runaway make Zn-Br well suited to medium-duration storage of 4-12 hours.
Br₂ generated on the positive side during charge is highly volatile; in an aqueous solution it would evaporate or diffuse across the membrane to the negative side and cause self-discharge. Adding a quaternary ammonium salt such as 4-methylpyrrolidinone (MEP) or tetrabutylammonium bromide (TBAB) forms a polybromide complex that separates as an oily orange phase in the electrolyte. This phase can be stored in a separate tank, suppressing self-discharge and enabling long-term storage.
Round-trip efficiency is the product η_RT = η_coulombic × η_voltage × η_aux, typically 70-80%. Lowering the current density improves voltage efficiency (less IR drop) and raises η_RT, but lowers power density. Coulombic efficiency is set by membrane sealing and how well the complexing agent retains bromine, with about 90% being a realistic ceiling. Auxiliary loads (pumps, thermal management) consume 2-5% of output, so smart standby pump control matters.

Real-world applications

Residential storage systems: The Redflow ZBM2 (10 kWh / 3 kW) is widely deployed in Australia and South Africa alongside rooftop PV. Compared with lead-acid or lithium-ion, it tolerates 100% depth of discharge and does not undergo thermal runaway, making outdoor installation easier. Cooling is simplified, and self-consumption rates in the peak evening window can reach above 80%.

Peak shifting in commercial buildings and data centres: At 100-500 kWh, batteries discharge during the expensive daytime hours and recharge on cheap overnight electricity. Capital cost is lower than lithium-ion and the 4-8 h discharge profile is ideal for demand-charge reduction, where the economics are most favourable. The default 104 kWh / 4 h setting in this tool falls in this class.

Microgrids and island power: Used as long-duration buffers for solar or wind variability on Pacific islands and remote sites. At 1-10 MWh, Primus Power EnerBlock systems are deployed in cascaded configurations. Where lithium-ion shortens its life in tropical climates, the aqueous Zn-Br electrolyte tolerates ambient temperatures around 50°C reliably.

Renewable + medium-duration storage sizing: Solar PV peaks in the afternoon and a 4-12 h battery is needed to shift it into the evening demand. This tool's capacity, discharge time and cost outputs let you size such a scenario. Comparing lithium-ion (short-duration / high power), Zn-Br (medium-duration / mid-power), and pumped hydro (long-duration / geography-constrained) along both axes of capacity and cost helps choose the right mix.

Common misconceptions and pitfalls

The most common mistake is to assume "all flow batteries are the same". Zn-Br and VRFB share the redox-flow concept but differ in design philosophy. VRFB uses all-liquid ions on both sides and aims at fully reversible, ultra-long life, while Zn-Br accepts metal plating on one side to push energy density. VRFB lasts >20000 cycles versus ~5000 for Zn-Br; in return, Zn-Br is roughly one-third the system cost ($150/kWh vs $500/kWh). Choose VRFB when life dominates, Zn-Br when cost and density dominate — do not treat flow batteries as a single category.

Second pitfall: "More electrolyte means more storage, indefinitely." Capacity E does scale with electrolyte volume V_e, but Zn-Br is constrained by zinc plating capacity per electrode area (~200 mAh/cm²). Even if the tank is full of ZnBr₂, the electrode saturates first. The tool uses an 80% usable fraction; in practice this "electrode-limited" regime forces either a larger cell area or a more aggressive stripping strategy. Simply enlarging the tank does not extend discharge time.

Finally, "Bromine is dangerous, so this can't be used at home." Free Br₂ is indeed toxic and volatile, but in a Zn-Br battery it is bound as a polybromide complex with negligible vapour-phase concentration. With double-walled tanks, leak sensors and forced ventilation, residential ZBM2 systems stay below commercial gas-detector thresholds. That said, acidic electrolyte attacks metals and leaks cause odour and staining, so corrosion-resistant coatings and containment trays are required at installation.

How to Use

  1. Enter cell active area in cm² (typical range 100–1000 cm² for industrial stacks); values affect current density and voltage distribution.
  2. Specify number of cells per stack (series connection); standard designs use 20–40 cells to achieve 40–80 V nominal voltage per stack.
  3. Input number of parallel stacks to scale power output; two stacks at 5 kW each yield 10 kW combined capacity.
  4. Set total electrolyte volume in liters; determines energy capacity since Zn-Br systems store 65–80 Wh/L depending on zinc utilization (typically 30–50%) and bromine complexation efficiency.
  5. Simulator calculates total cell count, power (kW), energy (kWh), discharge duration, volumetric energy density (Wh/L), and capex based on cell cost (~USD 150/kWh) and balance-of-plant (~USD 200/kW).

Worked Example

A 10 kWh/4 kW Zn-Br system: cell area 500 cm², 25 cells per stack, 2 parallel stacks, 125 L electrolyte. Each stack: 25 cells × 2.5 V nominal = 62.5 V; power output 2 kW per stack, total 4 kW. Energy capacity: 125 L × 65 Wh/L ≈ 8.1 kWh; discharge time ≈ 2 h. Volumetric density 65 Wh/L. Estimated capex: (10 kWh × USD 150/kWh) + (4 kW × USD 200/kW) = USD 2,300. Zinc plating efficiency, bromide complexation ratio, and thermal losses affect real performance ±15%.

Practical Notes

  1. Zinc dendrite suppression requires carbon-felt electrodes and bromide-complexing agents (quaternary ammonium salts); poor electrolyte circulation causes uneven plating and voltage sag after 50+ discharge cycles.
  2. Bromine toxicity demands hermetic cell sealing and vapor-recovery systems; annual bromine loss ~2–5% without adequate sealing; design electrolyte reservoirs with 20% headspace for gas evolution.
  3. Temperature control critical: operation above 35°C accelerates side reactions and self-discharge (0.5–1% per day); cooling systems add USD 50–100/kWh to capex for grid-scale (>100 kWh) deployments.
  4. Round-trip efficiency typically 75–85%; charge at C/5 (20 h) and discharge at C/10 (10 h) for maximum cycle life (>3,000 cycles); faster rates reduce efficiency by 5–10% per C-rate increase.