A clean and membrane-free chlor-alkali process with decoupled Cl2 and H2/NaOH production

Our plain-English summary of A clean and membrane-free chlor-alkali process with decoupled Cl2 and H2/NaOH production, by M. Hou, L. Chen, Z. Guo, et al (Nature Communications, 2018).

The study

• Authors: M. Hou, L. Chen, Z. Guo, X. Dong, Y. Wang, Y. Xia
• Journal: Nature Communications
• Published: 2018;9(1):438
• Cited by: 31 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Existing chlor-alkali processes generally use asbestos, mercury or fluorine-containing ion-exchange membranes to separate the simultaneous chlorine production on the anode and hydrogen production on the cathode, and form sodium hydroxide in the electrolyte. Here, using the Na+ de-intercalation/intercalation of a Na0.44MnO2 electrode as a redox mediator, we decouple the chlor-alkali process into two independent steps: a H2 production step with the NaOH formation in the electrolyte and a Cl2 production step. The first step involves a cathodic H2 evolution reaction (H2O → H2) and an anodic Na+ de-intercalation reaction (Na0.44MnO2 → Na0.44-xMnO2), during which NaOH is produced in the electrolyte solution. The second step depends on a cathodic Na+ intercalation reaction (Na0.44-xMnO2 → Na0.44MnO2) and an anodic Cl2 production (Cl → Cl2). The cycle of the two steps provides a membrane-free process, which is potentially a promising direction for developing clean chlor-alkali technology.

In plain English

This is a laboratory chemistry study, not a study of asbestos exposure or health. It concerns the chlor-alkali process, which makes chlorine, hydrogen and sodium hydroxide from salt water. The authors note that older diaphragm cells separate the products with a porous asbestos mat. They tested a two-step method with no separator. In the first step, a sodium manganese oxide electrode (Na0.44MnO2) released sodium into a sodium hydroxide solution while hydrogen formed. The electrode was then washed and moved to salt water, where it took sodium back up while chlorine formed. At 100 mA, average cell voltages were about 1.3 V for the hydrogen step and about 1.07 V for the chlorine step, and the process ran for over 50 consecutive cycles. In five repeated tests, Faradaic efficiency was about 100% for hydrogen and averaged 90.2% for chlorine. The authors concluded the approach may lead to a cleaner process. They said it is still some way from practical use, citing electrode transfer and washing, slow electrode reactions and other unresolved issues.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory electrochemistry experiment developing and testing a membrane-free, two-step chlor-alkali electrolysis process
• Who was studied: No people or animals were studied. The subject was a laboratory electrolysis set-up using a Na0.44MnO2 electrode measuring 2.5 × 4 cm2, together with commercial coated titanium-mesh electrodes for producing hydrogen and chlorine.
• Exposure: No asbestos exposure was studied. Asbestos is mentioned only as background, as the porous mat used to separate the products in conventional diaphragm cells.
• Main results:
  • Step 1 (hydrogen and sodium hydroxide production) had an average cell voltage of about 1.3 V at an applied current of 100 mA.
  • Step 2 (chlorine production) had an average cell voltage of about 1.07 V.
  • At a higher current of 1000 mA (a current density of 100 mA per square centimetre), the overall cell voltage for Step 1 plus Step 2 was about 3.65 V.
  • At 100 mA, the process produced hydrogen (with sodium hydroxide) and chlorine stably over more than 50 consecutive cycles.
  • In five repeated experiments, the Faradaic efficiency of hydrogen production was about 100%.
  • In five repeated experiments, the average Faradaic efficiency of chlorine production was 90.2%.
  • In comparison experiments using an ion-exchange membrane (Nafion film), the Faradaic efficiency for chlorine was 97.4%, still below 100%. The authors attributed this to a competing oxygen-producing side reaction.
• The authors' conclusion: The authors concluded that sodium intercalation and de-intercalation was successfully used to separate hydrogen (plus sodium hydroxide) production from chlorine production. They stated that the technology should be cleaner than previous chlor-alkali processes because Na0.44MnO2 is environmentally friendly.
• Limitations the authors state:
  • Moving the electrode between the two steps and washing it requires complex automation and advanced industrial design.
  • The electrode reaction is slower than the hydrogen- and chlorine-producing reactions.
  • The current density tested (100 mA per square centimetre) is much lower than in modern membrane cells (150 to 700 mA per square centimetre).
  • Additional washing is needed to remove residues when the electrode is moved between solutions, which the authors call a disadvantage of the process.
  • Product concentrations, product purity, the amount of Na0.44MnO2 required and the competing oxygen side reaction remain unresolved.

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