Effects of electrolyte concentration and anion identity on photoelectrochemical degradation of phenol: Focusing on the change at the photoanode/solution interface

The effects of electrolytes on phenol degradation by typical photoelectrochemical cell (PEC) composed of TiO₂-nanotube array (TNTA) and Pt were studied in terms of interfacial parameters; quantum efficiency, h⁺ generation rate, relative interfacial electron transfer rate, and equivalent resistances. Three different anions, Cl⁻, ClO₄⁻, and SO₄²⁻, and 10⁻⁵ to 10⁻¹ M of electrolyte concentrations have been covered. For all concentrations of interest, pseudo-1st-order kinetic constants for phenol degradation were 4.775 × 10⁻⁴–1.175 × 10⁻³, 3.974 × 10⁻⁴–8.893 × 10⁻³, and 3.902 × 10⁻⁴–8.810 × 10⁻⁴ min⁻¹ for SO₄²⁻, Cl⁻, and ClO₄⁻, respectively. The results of h⁺-generation rate, and relative electron transfer rate confirms that the rate of reactions at electrode/solution interface is in the order of SO₄²⁻, ClO₄⁻, and Cl⁻; 0.038–3.552%, 0.023–2.900%, and 0.021–2.814% were obtained for the quantum efficiency of SO₄²⁻, ClO₄⁻, and Cl⁻, respectively. The inconsistent trend between phenol degradation kinetic and quantum efficiency in terms of anion species shows that the phenol is mainly decomposed by the oxidant produced through the interfacial reactions, not by direct reaction between TiO₂ and phenol. The difference in terms of anions would be attributed to the product of anions since the radicals other than hydroxyl radical would provide the retardation pathways to prolong the lifetime of radicals.