“…A series of hydrophilic modifiers, such as CeO 2 , 27 TiO 2 , 28 NiO, 29 CoO 30 and etc., have been introduced into the electrode surface to improve the hydrophilic properties, and result in an improved electrochemical surface area (ECSA) and electrocatalytic activity of the electrode. In addition, various etching agents have been applied to form etch pits on the fiber surface under higher temperature, such as KOH, 31 CO 2 , 32 NH 3 , 33 urea 34 and etc. Compared to other methods, pore-making on carbon fiber surface is easier, more efficient and low cost, which is very suitable for large-scale production.…”