Zinc-Bromine Flow Battery: Electrolyte Loop & Bromine Complexation

Cell Mode

Operating Parameters

Current density 40 mA/cm²

Electrolyte flow rate 50%

Complexing agent ratio 70%

Live Readouts

State of charge

30.0%

Cell voltage

1.80 V

Zn deposit thickness

0.041 mm

Br₂ sequestration

78%

How it works

A zinc-bromine flow battery stores charge in two liquid electrolytes pumped from tanks through a stack, rather than in a fixed solid electrode. The two half-reactions are:

Negative: Zn²⁺ + 2e⁻ ⇌ Zn(s)        E° = -0.76 V
Positive: Br₂ + 2e⁻ ⇌ 2Br⁻          E° = +1.09 V
Cell:     Zn²⁺ + 2Br⁻ ⇌ Zn(s) + Br₂  E°cell = 1.85 V

On charge, zinc metal plates onto the negative electrode (mass grows by Faraday's law, m = QM/(nF)) while bromine is generated at the positive side and immediately trapped by a quaternary-ammonium complexing agent, forming a dense second liquid phase that sinks to the tank bottom — this is what keeps free, corrosive Br₂ away from the membrane-free separator.

Open-circuit voltage follows the Nernst equation around the standard potential:

E = E°cell + (RT/nF)·ln( SOC / (1 - SOC) )

Running current adds an overpotential that grows with current density and shrinks with better electrolyte flow (mass-transport limitation) — shown as the gap between charge/discharge voltage and the idle OCV curve.

Complexing agent ratio — too little agent lets Br₂ diffuse back across the separator unreacted: a self-discharge current with no external current flowing, visible as slow SOC decay in Idle mode.
Flow rate — low flow starves the electrode surface of fresh reactant, raising overpotential and slightly weakening complexation contact time.
Real-world relevance — ZnBr flow batteries can be fully discharged to 0% SOC with no capacity fade, unlike Li-ion, because the "electrode" (plated Zn) is regrown fresh every cycle.

Zn²⁺ electrolyte Br⁻/Br₂ electrolyte Br₂-complex phase

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