Mechanistic Study of Electrochemical Processes on A Porous Magnéli Phase Electrode

The substoichiometric TiO2 (TinO2n-1) has been considered as a promising electrode material for use in electrochemical advanced oxidation processes (EAOPs). In this study, the mechanisms of electrochemical processes were investigated on a porous TinO2n-1 electrode. First, we presented a non-invasive and non-destructive electrochemical impedance spectroscopy (EIS) method for studying membrane fouling on a TinO2n-1 reactive electrochemical membrane (REM). A mathematical transmission line model was developed to interpret EIS data and was shown to be sensitive to fouling at multiple interfaces of the REM. Based on the analyses, a chemical free electrochemical regeneration (CFER) scheme in backwash mode was developed. The CFER in anodic treatment mode recovered the flux of a humic acid fouled REM from 3.0% to between 76% and 99% of the initial flux over 5 consecutive fouling/regeneration cycles. Full flux recovery of a polystyrene fouled REM (fouling 31% to 38%) was achieved when using either cathodic or anodic CFER. We also investigated the suitability of four hydroxyl radical probes (coumarin, p-chlorobenzoic acid, terephthalic acid, and p-benzoquinone) for use in EAOPs. Results indicated that both coumarin and p-chlorobenzoic acid were oxidized via direct electron transfer reactions, while p-benzoquinone and terephthalic acid were not. Coumarin oxidation to form the hydroxyl radical adduct product 7-hydroxycoumarin was found at anodic potentials lower than that necessary for hydroxyl radical formation. Density functional theory simulations found a thermodynamically favorable and non-hydroxyl radical mediated pathway for 7-hydroxycoumarin formation. Finally, we examined the loss and regeneration of electronic conductivity and electrochemical activity on TinO2n-1 in three different electrolyte solutions (i.e. H2SO4, HClO4, HCl). Results showed that reversible surface passivation only occurred in the H2SO4 electrolyte, which was attributed to the formation of TiOSO4 and not related to a change in the Magnéli phases. The changes in conductivity and electrochemical activity in both HClO4 and HCl electrolytes were only associated with variation of charge carriers, such as hydrogen doping level and Ti3+ sites. It was also determined that the TinO2n-1 phase directly affected to the hydroxyl radical formation rate, with the highest rate observed for Ti4O7.