How a Membrane Separates Chlorine and Alkali — Working Principle of Ion-Exchange Membrane Electrolysis Cells

Salt (NaCl) is dissolved in water, and direct current is applied. At the anode, yellow-green chlorine gas is released; at the cathode, hydrogen gas is produced, and sodium hydroxide (NaOH) forms in the solution. Overall reaction: 2NaCl + 2H₂O ⟶ 2NaOH + Cl₂↑ + H₂↑

This reaction does not occur spontaneously — it requires at least 2.19 volts to drive it. The higher the temperature, the lower this minimum voltage, so the electrolysis cell operates at 85–90°C. But the temperature cannot be raised indefinitely, because the membrane that plays the critical role cannot withstand it.

The real challenge of electrolysis is not applying current, but separating the products the moment they are born. If chlorine gas and caustic soda meet, they immediately react back into bleach; if chlorine gas and hydrogen gas mix, they may explode. Major accidents in the chlor-alkali industry almost always trace back to this root cause.

Over the past century, three separation approaches have been developed. The earliest, the mercury process, used flowing liquid mercury as the cathode — sodium dissolved into the mercury to form a liquid alloy, which was pumped to an adjacent room to react with water and produce caustic soda; chlorine and alkali never shared the same chamber, at the cost of mercury's toxicity. The diaphragm process sandwiched a porous asbestos pad between the two electrodes, with brine flowing from anode to cathode, using the flow to prevent caustic soda from flowing back — simple and cheap, but the caustic soda was heavily contaminated with salt, requiring subsequent evaporation and purification. The membrane process is fundamentally different: it uses a dense polymer membrane filled with negative charges that naturally repels negatively charged OH⁻ and Cl⁻, allowing only Na⁺ to pass through.

The structure of a membrane electrolysis cell is a sandwich: titanium mesh anode (coated with ruthenium-iridium oxide) → membrane → nickel mesh cathode. Modern “zero-gap” designs press the electrodes elastically against the membrane, leaving no gap — the gas bubbles generated during electrolysis would cover the electrodes and increase resistance; the zero-gap design allows bubbles to escape through grooves on the back of the electrodes, saving considerable electrical power.

Within the multilayer structure of this membrane, the most critical component is the carboxylic acid layer facing the cathode, which is extremely thin. The carboxylic acid groups (–COOH) are weak acids with a pKa of approximately 2–3. On the acidic anode side (pH 2–4), a large proportion of –COOH groups remain as neutral molecules, with ion channels half-open; on the alkaline cathode side (pH > 14), all –COOH groups dissociate into –COO⁻, forming a dense wall of negative charges that blocks OH⁻ firmly. The membrane exploits the natural pH gradient on both sides — “opening the door” for conduction on the anode side and “closing the door” for blocking on the cathode side. A single-layer sulfonic acid membrane achieves only about 80% current efficiency; with the addition of this carboxylic acid layer, the efficiency jumps to 96–97%.

Driven by the electric field, Na⁺ migrates from concentrated brine through the membrane into concentrated caustic soda. Ideally, for every electron that flows, one Na⁺ crosses the membrane — this ratio equals the current efficiency. However, about 3–4% of the current is still carried by “escaping” OH⁻ — the OH⁻ concentration in the catholyte is a trillion times that in the anolyte, and the concentration-gradient-driven diffusion force is extremely strong. As Na⁺ crosses the membrane, it also drags 3–5 water molecules: the catholyte is thereby diluted and needs water replenishment, while the anolyte loses water and the NaCl becomes more concentrated — at the extreme, salt crystals precipitate and scratch the membrane.

The theoretical voltage is 2.2V, while actual operation is approximately 3.0V. The extra 0.8V breaks down into: anode overpotential, cathode overpotential, electrolyte resistance, membrane resistance (the largest source of loss), electrode and contact resistance, and bubble effects. As the membrane is made thinner, the cell voltage also decreases accordingly.

The membrane's requirements for brine purity are nearly stringent: the total calcium and magnesium entering the cell must not exceed 20 ppb. This is equivalent to dissolving no more than 50 grams of calcium chloride in a standard swimming pool of water — exceeding this amount will poison the membrane. The calcium and magnesium content of ordinary seawater is 200,000 times this value. Therefore, the brine requires two-stage purification: chemical precipitation (the order of reagent addition must never be reversed) reduces calcium and magnesium from several hundred ppm down to 5 ppm; chelating resin towers then capture the remaining ions, bringing the total below 20 ppb. Particular vigilance is needed for iodine — trace amounts of iodine in sea salt, after oxidation at the anode, form permanent precipitates within the membrane, which can cause up to 5% loss in current efficiency.

Operating an electrolysis cell is like turning five interlinked knobs simultaneously. Temperature 85–90°C: higher saves electricity but the membrane cannot tolerate it. Current density 3–6 kA/m²: higher means greater capacity but increased resistive losses. More concentrated brine means higher current efficiency but risks crystallization that scratches the membrane. Caustic soda concentration is approximately 32–35%. The hydrogen-side pressure must always be higher than the chlorine-side pressure, ensuring that if the membrane is breached, only hydrogen leaks into the chlorine side — never giving chlorine the opportunity to enter the hydrogen side and form an explosive mixture. If the chlorine-side pressure exceeds the hydrogen-side pressure: chlorine gas will penetrate the ion-exchange membrane or seals and leak into the hydrogen side. Chlorine mixing into hydrogen not only forms an explosive gas mixture but also causes severe corrosion to hydrogen pipelines and compressors. If the hydrogen-side pressure is higher than the chlorine-side pressure, even in the event of a minor leak, hydrogen will permeate toward the chlorine side. Although hydrogen mixing into chlorine also poses an explosion risk, chlorine systems are typically equipped with more comprehensive dehydrogenation and monitoring facilities. More importantly, under the industrial design principle of “Fail-Safe,” maintaining slight positive pressure on the hydrogen side is the last line of physical defense against the most dangerous scenario of “chlorine intruding into the hydrogen system.”

       From salt to chlorine gas, caustic soda, and hydrogen — the ion-exchange membrane, using a polymer membrane thinner than cling film, achieves unhindered passage for cations and impenetrable barriers for anions under the exquisite regulation of a pH gradient. Lowest energy consumption, purest products, and most environmentally friendly — these threefold advantages have made the membrane process the absolute mainstream of the modern chlor-alkali industry.