Fabrication of Oxidant-generating and Capacitive Anode with Electrochromic Effect of TiO2 Nanotube Array

Abstract

Great interest in anode materials has dramatically emerged with increasing demand for electrochemically generated oxidants in industrial electrochemistry. For the last five decades, these needs have been mostly achieved by the introduction of two well-known anode materials, the dimensional stable anode (DSA®) and boron-doped diamond (BDD) electrodes. Nevertheless, the high cost and complicated process in fabricating these electrodes remains as a big obstacle for further development. Thus, the development of a simple and cost-effective method to fabricate an efficient anode material could be a great challenge, and could provide new opportunities and innovations in industrial electrochemistry. This study aimed to development of innovative oxidant-generating anode using the electrochromic effect of TiO2 nanotube array (NTA) and understanding of the electrochemical and surface characteristics of the electrochromic TiO2 NTAs as oxidant-generating anode. First, we report a novel anode material for the production of oxidants, the dark blue colored TiO2 nanotube array (NTA) (denoted as Blue TiO2 NTA) which has never been successfully achieved with titania-based materials. This titania-based electrocatalyst with irreversible electrochromism and high conductivity was successfully fabricated with simple cathodic polarization of anatase TiO2 NTA and exhibits the excellent electrocatalytic activity in generating chlorine (Cl2) and hydroxyl radical (?OH) which is comparable to the commercial DSA® and BDD electrodes, respectively. Second, novel fabricating strategy using simple cathodic polarization was proposed to fabricate new electrochromic TiO2 NTA distinct from Blue TiO2 NTA as oxidants generating electrocatalyst. The simple cathodic polarization of amorphous TiO2 NTA triggered the black coloration with electrochromic effect (denoted as amorphous Black TiO2 NTA). This amorphous Black TiO2 NTA turned to highly electrocatalytic anatase Black TiO2 NTA in hydroxyl radical production with annealing under oxygen deficient condition. Finally, capacitive and electrocatalytic properties of these electrochromic TiO2 NTAs was investigated. In the oxygen evolution reaction (OER), the hydroxyl radical mediated OER was determined by Tafel analysis of OER on Blue and Black TiO2 NTA. Blue TiO2 NTA revealed the higher electrocatalytic activity in chlorine evolution with the large OER overpotential than Black TiO2 NTA, whereas the Black TiO2 NTA exhibited better performance in the capacitive properties estimated by areal capacitance than Blue TiO2 NTA. These unique electrochemical properties of Blue and Black TiO2 NTA can be explained by the different level of charge carrier density between Blue and Black TiO2 NTA as result of electrochemical self-doping. Thus, this Blue and Black TiO2 NTA is suggested as a potential cost effective anodic material in industrial electrochemistry. In addition, even in other metal oxides other than titania, the cathodic polarization (accompanied with irreversible electrochromism) method not only may be applied to explore a new route for low-cost and novel anodic materials, but also can offer new opportunities for applications of photocatalyst, dye-sensitized solar cell and supercapacitor.