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

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.


Response to Reviewer #5
Overall Comment: The paper deals with interesting topic new route of chlor-alkali production. The proposed electrochemical process combines MnO 2 based electrodes from sodium batteries and mercury chlor-alkali process. Despite I found this idea interesting the self-approbation of authors without pointing out several drawbacks of their process makes the paper more like advertisement than serious scientific paper. Significantly low intensity of process in contrast to the standard technologies is only mentioned. Reachable concentrations, purity of products, required amount of Na 0.44 MnO 2 for some relevant production etc. should be mentioned as problematical point necessary to be solved prior conclusion about new technology.
Response: Thank you very much for your kindly reviewing our manuscript (NCOMMS-17-20852A) and giving a lot of important and reasonable suggestions. These problematical points (e.g. the reachable concentrations, purity of products, required amount of Na 0.44 MnO 2 for some relevant production, side reaction of OER, etc.) necessary to be solved prior the application of the new technology have been mentioned in the discussion section of the revised manuscript. In discussion section of the revised manuscript, it is also emphasized that this idea is currently far away from the practical application, and the further research is necessary. Please see these sentences highlighted by yellow background in the discussion section (page 11 of the revised manuscript). Furthermore, we believe your specific comments are very reasonable, which are really important to increase the quality of this work. In corresponding to each specific comment pointed out by you, we would like to answer separately and revise our manuscript according to your comments.
(1) In Supplementary Figure 2 the decomposer reactor is shown with power source in the circuit. The decomposer is short-circuited galvanic cell, therefore it doesn't need any external source of energy.
Response: As correctly pointed out by you, it doesn't need any external source of energy.
Supplementary Figure 2 has been revised carefully.
(2) Declared Cl 2 production issues is hardly believable. In membrane and mercury process the brine is acidified by HCl addition. Main reason for it is suppression of parasitic oxygen production. Due to the thermodynamic preference it is impossible to fully omit oxygen evolution reaction (OER) (see reference 8). Therefore declared analysis about only pure Cl 2 formation in saturated NaCl will be real break through in chlor-alkali technology (Page 9

Figs. 4.).
Response: Thank you very much for your kind comment. After carefully reading the nice paper (ref. 8), we have understood that it is impossible to fully omit oxygen evolution reaction (OER) during the chlorine evolution reaction (CER). The achieved data about DEMS (Figure 4b) can only be used to demonstrate the chlorine evolution, rather than the purity. We have revised the corresponding discussion carefully. Please see the revised manuscript, Page 9, these sentences highlighted by yellow background.
(3) To call Cl 2 production with Na intercalation as efficient approach for sea water desalination is pure speculation without any relevance (Page 9 line 235) Response: According to your kind suggestion, the related sentences have been deleted in the revised manuscript.
(4) The discussion about current yields for Cl 2 productions is incorrect (page 10 line 257-272). If the reason for lower Cl 2 yield is back reduction on cathode how it is possible to reach 100% yield for H 2 production. Some charge must be consumed for back reduction. With Nafion membrane incorporation the penetration of oxidized chlorine compounds to the cathode is prevented. Therefore the observed yield for Cl 2 only confirms presence of OER parasitic reaction.
Response: Sincerely thank you for your kind comment. After carefully reading your comment and reference 8, we totally agree with your opinion that the observed yield for Cl 2 confirms the presence of OER parasitic reaction. Therefore, the corresponding discussion has been revised to clarify this point. Please see the revised manuscript, Page 10, these sentences highlighted by yellow background.
(5) Reference 41 is not in right format.