Selective Hydrogen Evolution in the Chlorate Process in the absence of Cr(VI)
Over 3 million tons of chlorate are produced on a yearly basis, mostly to be converted on-site into chlorine dioxide for bleaching paper . It is industrially produced by electrolysis of sodium chloride in water: chloride ions are oxidized at the anode and react with water to form chlorate, while water is reduced at the cathode (Figure 1, green).
However, the reaction pathway to chlorate involves intermediate hypochlorite and hypochlorous acid species which are at risk of being reduced at the cathode (Figure 1, red). This unwanted side reaction greatly reduces the overall energy efficiency of the process, and is typically avoided by adding a small amount of Cr(VI) to the electrolyte. Cr(VI) forms a barrier on the cathode surface that allows hydrogen formation but completely suppresses reduction of intermediates . Unfortunately, Cr(VI) is highly carcinogenic and new REACH legislation aims to phase out its use in the EU by the sunset date, September 21st, 2017 .
Figure 1: Simplified chlorate process, showing desired (green) and undesired (red) reactions.
This project aims to understand the mechanism behind the selectivity towards hydrogen formation as exhibited by Cr(VI), in order to find new cathode materials and additives for application in the chlorate process, as well as the production of hydrogen.
The project is one of four parts of the ELECTROGAS project, a consortium of two academic, two big and two small private partners, aimed at two things:
- developing a better fundamental understanding of the thermodynamic and kinetic aspects of electrochemical gas evolution and bubble formation;
- the formulation of process conditions and novel stable, cost-effective electrocatalysts for specific reactions to achieve selective gas formation or, conversely, inhibition thereof.
 “A review of chromium(VI) use in chlorate electrolysis: Functions, challenges and suggested alternatives”; Electrochimica Acta, ISSN 0013-4686, E-ISSN 1873-3859, Vol. 234, 108-122 p.
 European Chemicals Agency (ECHA), Authorization List (Annex XIV of REACH), 2015., (2015).