Nanostructured Vanadate-based Metal Oxides as Photoanodes for Solar Hydrogen Production

Photovoltage panels are currently used on an industrial scale to produce solar electricity, but the discontinuous nature of the solar irradiance limits the widespread of their applications. Developing technologies to store solar energy in the form of chemical fuels that can be used at any time is thus necessary for the realization of a solar-powered future. Photoelectrochemical cell (PEC) is among the most attractive technologies to store solar energy in the form of chemical fuel. In the PECs, water can be split into hydrogen gas and oxygen. Then hydrogen can be used as a fuel with zero CO2 emission. Water is an abundant carbon free source of hydrogen. By simple calculation, it can be readily found that ~6.381011 L of water is needed to store the energy of 200 Mtoe in the form of hydrogen bond. This amount of energy is equivalent to the energy that the kingdom of Saudi Arabia uses in 1 year and it can be obtained by splitting very small amount of water either from the worlds oceans or the annual rainfall. Unfortunately, the efficiency of solar-to-hydrogen conversion efficiency is still low. Discovery of stable and efficient photoanodes is thus crucial for enhancing the efficiency of photoelectrochemical solar hydrogen production. In this regard, vanadate-based metal oxides have attracted a great interest due to their relative stability and suitable valance band position for water oxidation. Despite of these merits, this class of materials suffers from the high rate of the recombination of the photogenerated charge carriers in the bulk and at the surface. They also exhibit poor kinetics towards water oxidation reaction. To solve these issues, we aim in this project to investigating the alloying of vanadium with other elements to tune the band position and the kinetics of charge carriers transfer. Wet chemistry based methods will be employed for the fabrication of nanostructured BiVO4 materials as referance. Then, one or more metals will be alloyed into the Bi and/or V sites. The synthesized materials will be characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Since the substitutional alloying is expected to alter the material properties, first-principles density functional theory (DFT) calculations will be employed to investigated the electronic structure (Band structures and density of states (DOS)) and the optoelectronic properties of the pristine and modified materials. The photoelectrochemical activity will be evaluated under the standard illumination condition. The kinetic of charge carrier transfer and recombination will be studied using the intensity-modulated photocurrent spectroscopy (IMPS) and photoelectrochemical impedance spectroscopy (PEIS). The DFT and IMPS results will be correlated to define the optimum composition for the best photoanodes. This will lead to the fabrication of photoelectrochemical cell with enhanced efficiency for solar hydrogen production.