We report new insights into the photoelectrochemical reduction of CO2 in a photoelectrochemical system, which is assembled from a nickel foam covered with TiO2 modified by covalent ligands as photocathode and BiVO4 as photoanode. These photoelectrocatalytic cells generate mainly methanol as a product in the liquid phase. Our results show that imine-functionalized TiO2/Ni has the highest activity. The formation rate of methanol in this excellent cell is up to 153 mu M/h.cm(2), which is about 15 times higher than that of the TiO2/Ni electrode. If the Faradaic efficiency is considered as 100%, the light quantum efficiency of this cell reaches 1.2%, that is two times better than that of plants. Isotopic labeling experiments with (CO2)-C-13 confirm that the detected products are produced exclusively by the reduction of CO2 and water splitting. (C) 2017 Elsevier Inc. All rights reserved.