Research groups led by Prof. LI Can and Prof. LI Rengui from the Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences proposed and demonstrated a practically feasible approach for scalable solar hydrogen production via water splitting, named “Hydrogen Farm Project” (HFP), which enables achieving a world record solar-to-hydrogen efficiency exceeding 1.8% so far. This work was published on Angew. Chem. Int. Ed.
Harvesting and converting solar energy in the form of chemical fuels is one of appealing solutions for solar energy storage and utilization. Among the solutions for solar-to-chemical energy conversion, photocatalytic overall water splitting using particulate photocatalysts is regarded as an economical approach for large-scale hydrogen production owing to the merits of relatively low investment cost and technical simplicity.
However, photocatalytic overall water splitting still suffers from extremely low solar-to-hydrogen conversion efficiency due to the poor charge separation efficiency and the possible reverse reaction between H2 and O2.
Moreover, most of the reported visible-light-responsive particulate photocatalysts are only active for either hydrogen evolution or oxygen production half reaction in the presence of sacrificial reagents.
Natural photosynthesis in green plants offers a textbook for efficient capturing of solar energy in a large scale, in which water oxidation reaction is initially realized in PSII, where molecular oxygen evolves while supplying protons for the following energy storage step via the synthesis of carbohydrates.
Inspired by the natural photosynthesis, the researches in DICP proposed a practically feasible solar energy storage approach via a redox shuttle ion loop comprising with two sub-systems: one is highly efficient photocatalytic water oxidation for solar energy storage and protons production, and the other to utilize protons to produce H2.
As such approach is principally analogue to the agricultural farm process, namely planting crops in a large scale and then concentrating to harvest once crops were ripe, so we named the approach as "Hydrogen Farm" Project (HFP). In the HFP, water oxidation and proton reduction reactions are spatially separated, consequently leaving out the H2/O2 gas separation.
To fulfill the HFP approach, two difficulties must be overcome, one is highly active photocatalysts required for efficient photocatalytic water oxidation in the presence of shuttle ions, and another obstacle is suppressing the oxidation process of the reduced shuttle ions, named as the reverse reaction between shuttle ions.
Based on the previous findings on spatial charge separation between different facets of semiconductors, the researchers in DICP experimentally demonstrated that BiVO4 crystals exposing both {010} and {110} facets exhibit an extremely high efficiency for water oxidation, resulting an AQE up to 71%, and more importantly, the reverse reaction of Fe2+ shuttle ions can be completely blocked. With this ideal photocatalyst, successful realization of the HFP approach was achieved.
The scheme of hydrogen farm strategy for scalable solar hydrogen production via water splitting. (Image by LI Rengui, ZHAO Yue)
An overall solar-to-chemical efficiency over 1.9% and a solar-to-hydrogen efficiency exceeding 1.8% could be achieved, which is the world record efficiency for photocatalytic water splitting systems so far. Meanwhile, a scalable photocatalyst panel for solar energy storage via HFP was well demonstrated under sunlight irradiation outdoors.
This work offers a promising and practical strategy for solar energy harvesting and solar hydrogen production on a large scale by using particulate photocatalysts.
The work was financially supported by National Natural Science Foundation of China, Strategic Priority Research Program of Chinese Academy of Science and Key Research Program of Frontier Sciences of Chinese Academy of Sciences. (Text by LI Rengui Li, ZHAO Yue)