For comparison, we also prepared a photoanode without the PEI layer using the same procedure (BiVO4/TiO2). First, we compared the surface morphology of BiVO4/TiO2 and BiVO4/PEI/TiO2 by scanning electron microscopy (SEM). According to SEM measurements, the TiO2 layer was conformally deposited on the highly porous BiVO4 photoanodes, regardless of the presence of PEI interfacial layer (Supplementary Fig. ). The morphology of BiVO4/TiO2 and BiVO4/PEI/TiO2 photoanodes was slightly different. BiVO4/PEI/TiO2 exhibited lower porosity in comparison to BiVO4/TiO2. This result could be attributed to the PEI layer filling the pores within the porous BiVO4 structure (Supplementary Fig. ). The thickness of the PEI layer on the BiVO4 surface was estimated as 50.5 nm through profilometer measurement on an FTO electrode (Supplementary Fig. ).