The immobilization of the negatively charged [Co4(H2O)2(PW9O34)2] 10─ (CoPOM) water oxidation catalyst onto polymeric carbon nitride is challenging since the surface of carbon nitride is known to be negatively charged due to large amount of unprotonated surface Brønsted base moieties that can be protonated only by highly concentrated strong acids. In order to effectively immobilize the anionic CoPOM cocatalyst onto the internal surface of our porous CNx-TiO2 photoelectrodes, we have therefore utilized the layer-by-layer technique demonstrated by Jeon et al. for immobilization of CoPOM onto various metal oxides. The CNx-TiO2 electrodes carrying negative surface net charge were sequentially immersed into a solution of the cationic polyethyleneimine (PEI) and a solution of the anionic CoPOM for the desired number of times to fabricate the CoPOM-PEI-CNx-TiO2 photoanode.