Supercharging Solar Fuel Production: Harnessing Sub Bandgap Energy in Mo-Doped BiVO4 Photoanode to Enhance Photoelectrochemical Reaction via Triplet-Triplet Annihilation Upconversion

Developing solar-based empowered technology is an important issue to realize a zero-carbon world. Among various forms of solar energy utilization, photoelectrocatalysis (PEC), which involves a direct solar to chemical energy conversion by light-matter interaction, exhibits a relatively high theoretical efficiency. PEC uses semiconductors to absorb photons to produce effective electron-hole pairs for a redox reaction. Several concepts have been proposed and demonstrated to improve the separation and utilization of the excitons. However, few attempts have been made to engineer the light to enhance the PEC efficiency by broadening the range of the solar spectrum that the semiconductor can utilize. The upconversion (UC) process induced by triplet-triplet annihilation (TTA) is an intermolecular energy transfer process by restoring the energy in triplet-state molecules to produce a high-energy singlet state to enhance the UC process. It is noted that the UC process, which uses the solar spectrum as the light source can convert the sub-bandgap photon into the absorbing range of the PEC semiconductors. Therefore, incorporating PEC with TTA-UC appears to be the most suitable process to enhance the efficiency of PEC by utilizing the abundant visible light from solar light.<br/>In this study, the benchmark TTA-UC chromophores, 9,10-diphenylanthracene (DPA) sensitized by platinum octaethylprophyrin (PtOEP) are used. They are dissolved in oleic acid to upconvert the green light (532 nm) to blue light (400 – 480 nm) in an ambient environment. The phosphorescence of PtOEP is quenched by DPA based on the Stern-Volmer relation, indicating that the triplet energy transfer between the two chromophores is effective. The optimal concentrations of PtOEP and DPA are 20 μM and 8 mM, respectively. For ease of implementation, PtOEP/DPA containing oleic acid is emulsified and acts as soft templates during the sol-gel process of tetraethyl orthosilicate (TEOS), yielding to UC active SiO₂ nanocapsules. The TEM images suggested that a silica shell encapsulates oleic acid with a diameter of 200 – 300 nm with a thickness of 10 nm. The upconverting composite film is prepared using the SiO₂nanocapsules. Semitransparent Mo: BiVO₄photoanode is synthesized by spin coating technique. The incorporation of upconverting composite film with Mo:BiVO₄ photoanode shows an enhanced photocurrent density from 1.09 mAcm^-2 to 1.24 mAcm^-2, equivalent to an increase of 14%. With a 490 nm long pass filter a 45% increase at 1.23 V_RHE is observed corresponding to a current density increase from 0.24 mAcm^-2 to 0.35 mAcm^-2.