A team at the Max Planck Institute for Solid State Research, Germany, and collaborators at ETH Zurich and the University of Cambridge, have developed a system that enables time-delayed photocatalytic hydrogen generation—essentially, an artificial photosynthesis system that can operate in the dark. A paper on their work is published in the journal Angewandte Chemie International Edition.
The system uses a carbon nitride-based material that can harvest and store sunlight as long-lived trapped electrons for redox chemistry in the dark. More specifically, the system comprises a partially anionic, cyanamide-functionalized heptazine polymer, which, in the presence of an appropriate electron donor, forms a radical species under irradiation that has a lifetime of more than 10 hours. This ultra-long-lived radical can reductively produce hydrogen in the presence of a hydrogen evolution catalyst in the dark on demand.
The as-modified graphitic nitride is a yellow solid, which turns blue upon exposure to light. This “blue radical” state contains trapped electrons. The scientists found out that when the light was switched off and a hydrogen-evolution co-catalyst was added, the polymer turned yellow again while producing hydrogen by releasing the stored electrons.
Thus it is possible to decouple the generation of photoinduced electrons from their use, for example, in fuel production, within one single, inexpensive material. This could be a significant advance for the production of storable solar fuels independent of the intermittency of solar irradiation.
The storage of trapped electrons within a carbon nitride backbone may open the prospect for overcoming limitations of the diurnal availability of sunlight for solar fuel production, provided that a scalable photo-oxidation process can be identified. This finding not only reveals a hitherto undescribed property of heptazine-based materials exploitable for specialized applications,but may also inspire rational design of photo-catalytic materials with long-lived, photo-induced states, a challenge that currently limits their applicability. Finally, the long-lived radical can also be prepared electrochemically by applying a cathodic voltage. The continuous charging of the carbon nitride with electrons reveals a capacitor-like function, which suggests that energy storage by this carbon nitride photocatalyst in the form of a “solar battery” may ultimately become possible.—Lau et al.
Lau, V. W.-h., Klose, D., Kasap, H., Podjaski, F., Pignié, M.-C., Reisner, E., Jeschke, G. and Lotsch, B. V. (2016) “Dark Photocatalysis: Storage of Solar Energy in Carbon Nitride for Time-Delayed Hydrogen Generation” Angew. Chem. Int. Ed.. http://dx.doi.org/10.1002/anie.201608553