Reduction Mechanism and Capacitive Properties of Highly Electrochemically Reduced TiO2 Nanotube Arrays

• Highly reduced and ordered TiO2 nanotube arrays have been fabricated using two-step anodization and three-electrode reduction.
• The reduced TiO2 nanotube arrays show a high specific capacitance of 24.07 mF cm-2 at a scan rate of 10 mV s-1, which is 1094 times higher than the capacitance of pristine nanotube arrays (0.02 mF cm-2).
• They also show an excellent long-term cycling stability with only 1.9% reduction of capacitance after 2000 cycles.
• Under optimized reduction conditions, about 22% of Ti4+ ions in tube surface regions are converted into Ti3+ ions.
• A proton-electron coupled reduction mechanism has been proposed based on the combined paradigms of a conventional energy-band model and chemical evolution of basic building blocks of TiO2.

Highly reduced and ordered TiO2 nanotube arrays have been fabricated using two-step anodization and three-electrode reduction. A proton-electron coupled reduction mechanism has been proposed based on the combined paradigms of a conventional energy-band model and chemical evolution of basic building blocks of TiO2. Under optimized reduction conditions, about 22% of Ti4+ ions in tube surface regions are converted into Ti3+ ions while the morphology of the highly reduced TiO2 nanotube arrays keeps unchanged. The reduced nanotube arrays show superior electrochemical properties such as high areal capacitance, good rate capability, and high cycling stability. The areal capacitance of the reduced electrode is 24.07 mF cm−2 at a scan rate of 10 mV s−1, much higher than that of the pristine TiO2 nanotube arrays (0.02 mF cm−2). This kind of highly reduced one-dimensional oxide nanostructures can find a large array of applications in supercapacitors, photocatalysis, electrochromic display, and Li ion batteries.