Selective formation of hydrogen peroxide by oxygen reduction on TiO<inf>2</inf> nanotubes in alkaline media

The oxygen electrochemical reduction at nitrogen treated TiO<inf>2</inf> nanotubes electrode shows distinctive and well-separated 2-electrons reduction peaks. The oxygen reduction was investigated in alkaline solution at titanium oxide nanotube (TON) arrays fabricated by anodizing of titanium foil in hydrogen fluoride solution followed by annealing at 450 °C in nitrogen (TON-N<inf>2</inf>) atmosphere. The morphology of the arrays was characterized by SEM and the electrochemical behavior was studied by cyclic voltammetry in potassium ferrocyanide and alkaline solutions. The treated TON arrays showed a significant enhancement in conductivity and in the activity for reduction of O<inf>2</inf> in alkaline media in comparison to the data of plain TiO<inf>2</inf>. TON arrays annealed in N<inf>2</inf> showed a unique electrochemical behavior of two well-resolved 2-electrons O<inf>2</inf> reduction peaks in contrast to TON arrays annealed air, which only showed a single reduction peak at more negative potential. At TON-N<inf>2</inf> arrays, hydrogen peroxide was detected as the main product during the first reduction. The switch in mechanism is proposed to be due to the presence of oxygen vacancies at the surface of titania nanotube arrays. The finding is of a high importance for the selective production of hydrogen peroxide by electrochemical reduction of oxygen in alkaline media using TiO<inf>2</inf> nanotubes electrode.