Photoelectrochemical Solar Water Splitting: The Role of the Carbon Nanomaterials in Bismuth Vanadate Composite Photoanodes toward Efficient Charge Separation and Transport

Photoelectrochemical performance of bismuth vanadate (BiVO₄) photoanode is limited by poor charge separation and transport properties. The roles of carbon nanotube, reduced graphene oxide, or graphitic carbon nitride in BiVO₄ composite photoanode were investigated toward enhancing light absorption and reducing overall impedance during photoelectrochemical water oxidation process. X-ray diffraction and Tauc analysis showed that BiVO₄ retains its monoclinic phase, n-type semiconductor nature, and band gap in all carbon nanomaterials-incorporated composite photoanodes. It was observed that the carbon nanomaterials incorporation in BiVO₄ film increases its surface porosity, ultimately leading to enhanced light absorption. The BiVO₄ photoanode with reduced graphene oxide and graphitic carbon nitride showed same bulk charge separation efficiency, whereas the latter showed better charge transfer. It was found that the graphitic carbon nitride formed composite with BiVO₄ to provide enhanced light absorption efficiency, i.e., 89% in 350-505 nm range. The BiVO₄ with graphitic carbon nitride photoanode showed the best performance with a photocurrent of 2.2 mA cm^-2, charge separation efficiency of 67%, and photocurrent of 4.0 mA cm^-2 with cobalt-phosphate surface catalyst at 1.23 V_RHE for water oxidation under 1 sun illumination. The Mott-Schottky and impedance measurements confirmed the shift of conduction band position toward hydrogen reduction potential and reduction in film resistance, respectively, with carbon nanomaterials addition, and the shift was most significant for graphitic carbon nitride. It is concluded that by concomitant formation of junction during photoanode fabrication between carbon nanomaterials, BiVO₄, and fluorine-doped tin oxide glass substrate, better charge separation, transport, and light absorption can be achieved.