The use of hydrogen energy was proposed in the last century as a ‘perfect’ solution to energy shortage. In hydrogen economy, people would utilize the solar energy to split water into hydrogen and oxygen. Hydrogen burns to produce energy, and turns into water, doing completely no harm to the environment. However, the research on photocatalytic water splitting has remained stagnant with difficulties in separating and safely storing hydrogen. 

Recently, a team led by Prof. JIANG Jun and Prof. ZHAO Jin from the National Laboratory of Physical Science at Microscale of University of Science and Technology of China of Chinese Academy of Sciences proposed a multi-layer structure where the produced hydrogen is completely isolated and stored with a high-density level, significantly promoting the possibility of converting solar energy to hydrogen energy. The paper was published in Nature Communication.

The new structure is like a sandwich where a carbon nitride (C3N4) layer lies between two graphene sheets modified by different functional groups. As a metal-free system, GR–C3N4 composite layers can effectively harvest visible solar light. GO–CxNy (GO: GR modified by hydroxyl and epoxy groups) generates excitons, which soon separate to energetic electrons and holes.

Then the electrons and holes are separately delivered to reductive and oxidative reaction sites. Water molecules are absorbed to the surface of graphene, waiting to be split. Noticeably, the functional groups on the graphene sheets could collect much hole carriers after adsorbing water molecule, making it ready for oxidative reaction.

Moreover, these groups effectively reduce the energy barrier for water splitting. After that, protons produced at the oxidative sites would penetrate through the outer layers to meet the inner ones, thus hydrogen is generated. As hydrogen cannot pass through GR-based material, it would be capsuled inside the sandwich.

The ability of integrating photocatalytic hydrogen generation and safe capsule storage has made the structure an exciting candidate for realistic solar and hydrogen energy utilization. Furthermore, other SP2 hybrid carbon materials modified by functional groups and other photo-catalysis could also fit this structure. This multi-layer design solves the problem of separating and storing hydrogen and may have re-found the path to hydrogen economy.  

This work was funded by Youth 973 Project of Ministry of Science and Technology, the National Natural Science Fund, Leading Programs of Chinese Academy of Sciences, and the Ministry of Education.