Due to the hazards and costs of shipping and storing chlorine gas, there has been increasing interest in using electrochemical cells for on-site generation (OSG) of hypochlorous acid (HOCl) from NaCl solutions.  Although the Faradaic efficiency for chloride oxidation in brine solutions is high, recent studies have documented that significant quantities of unwanted byproducts, such as: chlorite (ClO2-), chlorate (ClO3-), and perchlorate (ClO4-) may be produced in OSG systems.  This research used density functional theory for investigating reactions on boron-doped diamond (BDD) film electrodes that contribute to chlorite and chlorate formation in HOCl solutions.  A 34-carbon atom cluster containing a single boron atom was used to simulate the BDD electrode surface.  In addition to simulations involving hydrogen-terminated surfaces, the simulations also included reactions with surface sites produced under anodic polarization, namely: =C�, =C-O�, =C�HO, and =C�H.  The activation energies for HOCl and HClO2 oxidation via direct electron transfer were calculated as a function of electrode potential.  Activation energies for reactions of hydroxyl radicals with HOCl and HClO2 were also calculated.  Hypochlorite (�OCl) radicals were found to chemically adsorb to both secondary and tertiary carbon atoms on the BDD surface.  These chemisorbed intermediates could react with hydroxyl radicals to regenerate HOCl, and produce =C-O� and =C�HO sites.  The =C-O� and =C�HO sites also reacted with hypochlorite radicals to form chemisorbed intermediates, which could then be converted to HClO2 via reaction with hydroxyl radicals.  Chlorite radicals (�ClO2) also formed chemisorption complexes with the BDD surface.  There were several paths for oxidation of these species to form chlorate.

Understanding chlorite and chlorate production by boron-doped diamond film anodes during electrochemical hypochlorite generation

246th National Meeting (2013)

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