Active Area of Anodic TiO₂ Nanotube Arrays in Photo and Electrochemical Energy Storage Devices

The anodic TiO2 nanotube (TNT) has been promising as both electrocatalysts in electro-energy synthesis and storage devices, and photoelectrocatalysts in solar energy conversion and storage devices. The active area of the (photo)electrocatalyst materials must be clarified because the current density can be normalized to an electrochemically active surface area. Herein, it is discussed whether the active area of anodic TNT electrodes in photo-and electrochemicial energy storage devices reported clearness. In literature, the sizes of the titanium (Ti) metal substrate, the anodic TNT, and the active area of the electrode have been misunderstood in different applications. Clarifying the three sizes, Ti metal substrate, TNT array, and active area, with respect to the Ti substrate shapes in different device applications is important in both academic research and commercialization due to the scale-up size effect.