A baby step in assembling and integrating the components of an artificial photosynthesis device with forced heterojunctions towards improved efficiency

Abstract

How to achieve unassisted, economical, scalable, and sustainable artificial photosynthesis for liquid fuels/products with improved solar-to-fuel efficiency (STFE) to address a carbon-neutral economy remains a big question. To a large degree, the extent of charge separation at heterojunction interfaces and charge utilization determine the STFE. Towards this, BiVO₃ is assembled from ionic-precursors into TiO₂ pores, and integrated structurally and electronically with TiO₂ on calcination as BiVO₄ quantum dots (BVQDs). BVQDs in TiO₂ (BVT) pores lead to an all-inorganic system with a sub-quadrillion number of heterojunctions in a 1 cm² device (contains ∼25 μg of BiVO₄ (∼2.5 wt%) in the nanopores of ∼975 μg of TiO₂ (∼97.5 wt%)) and facilitate artificial photosynthesis. We demonstrate 31–38% STFE with a photon to chemical conversion turn over frequency (ToF_P2C) of 2.73 s⁻¹ with a 1 cm² wireless BiVO₄–TiO₂ artificial leaf (BVT-AL) device for HCHO and CH₃OH. The sequential nature of CO₂ reduction to HCHO and then to CH₃OH is evident from the reaction results. ¹³CO₂ isotopic labeling experiments confirm that the input CO₂ is the source for product formation. A large increase in the photocurrent density and incident photon-to-current efficiency (IPCE) of BVT, over 100% for the BiVO₄ photoanode in visible light, demonstrates and supports efficient visible light absorption, charge separation and migration to the redox sites. A device has been demonstrated to show sustainable activity in direct sunlight, and addresses scalability from 1 to 9 cm². Assuming no change (50% decrease) in the STFE, a 6.74 m² device is expected to convert 1 (0.5) kg h⁻¹ CO₂ into C1-oxygenates in sunlight. DFT calculations carried out with anatase TiO₂ (101) and BiVO₄ (121) interfaces support many of the experimental findings, including electron flow from the latter to the former, and interaction of the oxygen of TiO₂ with BiVO₄ and vice versa at the interface towards forced heterojunctions.