Chlor-Alkali Membrane Cell

Cell Operation

Current density 4.0 kA/m²

Membrane Na⁺ selectivity 96%

Anolyte NaCl feed 300 g/L

Live Readouts

Cell voltage

3.00 V

Current efficiency

96.0%

NaOH (catholyte)

0.0 wt%

Cl₂ / H₂ rate

0.0 mol/s

Na⁺ crossing membrane

Cl₂ gas (anode)

H₂ gas (cathode)

Cl⁻ leakage (low selectivity)

How it works

An industrial chlor-alkali membrane cell splits saturated brine into chlorine, hydrogen and caustic soda using a cation-exchange membrane (e.g. Nafion) that separates the anode and cathode compartments and passes only cations:

Anode (Ti, anolyte = NaCl brine):
  2Cl⁻ → Cl₂(g) + 2e⁻

Cathode (Ni, catholyte = dilute NaOH):
  2H₂O + 2e⁻ → H₂(g) + 2OH⁻

Membrane transport:
  Na⁺ migrates anolyte → catholyte to balance charge
  Net cell reaction: 2NaCl + 2H₂O → Cl₂ + H₂ + 2NaOH

Charge balance requires every electron pulled at the cathode to be matched by a cation arriving through the membrane. A perfectly selective membrane passes only Na⁺, so current efficiency (Faradaic efficiency for NaOH) approaches the membrane's Na⁺ transport number t₊. A leakier membrane (lower selectivity slider) lets some Cl⁻ or OH⁻ back-migrate, wasting current and slowing caustic buildup.

Gas evolution rate follows Faraday's law: moles of gas per second = I / (n·F), with n = 2 electrons per mole of Cl₂ or H₂ and F = 96,485 C/mol. Cell voltage is approximated as the thermodynamic decomposition voltage (~2.19 V) plus ohmic and overpotential losses that scale roughly linearly with current density: V ≈ V₀ + i·R_cell. Caustic concentration in the catholyte rises over time as OH⁻ produced at the cathode combines with Na⁺ arriving through the membrane, diluted by water transported osmotically with each ion (modeled here as a simple accumulating concentration with a slow outflow).

t = 0.0 s Na⁺ transported: 0

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