A TiO2 layer was deposited on the electrode covered with NiOx using dip-coating. For the preparation of the TiO2 precursor solution, 15 mL of titanium tetraisopropoxide (TTIP) was slowly dropped into the solution of 0.4 mL of concentrated HCl mixed with 170 of mL isopropanol. To get a thin layer the withdrawal speed was 8 mm s -1 . Afterward, the electrode was dried for 1 h in air and calcined at 450 °C for 30 min with a heating ramp of 15 °C min -1 .