A review of chromium(VI) use in chlorate electrolysis: Functions, challenges and suggested alternatives

Sodium chlorate is industrially produced by electrolysis of an aqueous salt solution, in which chromium(VI) constitutes an important excipient component. It is added to a concentration of a few grams Na2Cr2O7/liter to the electrolyte and has several functions in the process, the most important being to increase the Faradaic efficiency for hydrogen evolution in the undivided electrochemical cells. A thin film of Cr(OH)3 × nH2O formed by reductive deposition on the cathodes decreases the rate of unwanted side reactions, while still enabling hydrogen evolution to occur. In addition chromium(VI) buffers the electrolyte at the optimum pH for operation and promotes the desired homogeneous reactions in the electrolyte bulk. Chromium species also affect the rates of hydrogen and oxygen evolution at the electrodes and are said to protect the steel cathodes from corrosion. Although chromium(VI) stays in a closed loop during chlorate production, chromate is a highly toxic compound and new REACH legislation therefore intends to phase out its use in Europe from 2017. A production without chromium(VI), with no other process modifications is not possible, and today there are no commercially available alternatives to its addition. Thus, there is an urgent need for European chlorate producers to find solutions to this problem. It is expected that chromium-free production will be a requirement also in other parts of the world, following the European example. As the chromium(VI) addition affects the chlorate process in many ways its replacement might require a combination of solutions targeting each function separately. The aim of this paper is to explain the role and importance of chromium(VI) in the chlorate manufacturing process. Previous achievements in its replacement are summarized and critically evaluated to expose the current state of the field, and to highlight the most promising avenues to be followed. An attempt is also made to reveal connections with other research fields (e.g. photochemical water splitting, corrosion science) facing similar problems. Allied effort of these different communities is expected to open up research avenues to the mutual benefit of these fields.