New COF material boasts 1,000 times longer charge lifetime, faster hydrogen generation

A coumarin-linked covalent organic framework (COF) material developed by researchers at the Ningbo Institute of Materials Technology and Engineering (NIMTE) at the Chinese Academy of Sciences (CAS) has paved the way for faster separation of hydrogen from water while using sunlight as a source. As the world looks for ways to move away from fossil fuels, renewable energy like wind and solar has gained prominence. Large-scale power generation facilities have been set up to supply carbon-free energy to the electrical grid. However, electricity generated through these sources cannot completely replace fossil fuels in areas where high energy density is required. In these circumstances, hydrogen fares better than renewable energy because it can combust like carbon-rich fuels while producing only water as a by-product and releasing large amounts of energy. The drawback, though, is that hydrogen production can be carbon-intensive, prompting researchers to look for better approaches. One such approach is using sunlight to split water molecules into hydrogen and oxygen, technically referred to as photocatalytic water splitting. Using COF photocatalysts For photocatalytic reactions to occur, scientists typically use a photoorganic catalyst, which is a carbon-based compound that facilitates the absorption of visible or ultraviolet light and lifts electrons to their excited state and helps in the production of hydrogen, which can potentially solve the energy crisis the world faces. However, the electrons and holes generated during the process recombine rapidly, impacting the overall efficiency of the reaction. Scientists have therefore turned to covalent organic frameworks (COFs), which are porous polymers made from organic molecules linked by strong covalent bonds and are highly tunable. In conventional COFs, flexible linkages can undergo out-of-plane rotation, which aggravates energy dissipation. This is also a reason for the reduced efficiency of the catalytic process. So, researchers at NIMTE teamed up with colleagues at the Technical Institute of Physics and Chemistry at the CAS and introduced a rigid and planar coumarin linkage into a fully conjugated COF. 1,000 times longer charge When compared to conventional imine and vinyl linkages, the coumarin linkage showed higher conformational rigidity and suppressed structural fluctuations. The linkage also promoted π-electron delocalization, which facilitates a long-range charge-carrier transport and narrows band gaps. The researchers observed that the coumarin-linked COF marked improved charge separation, with the charge-separated state lasting 1,000 times longer than the imine-linked counterpart. When Pt nanoparticles were used as a cocatalyst, photogenerated electrons from the COF were transferred to Pt in approximately 407 picoseconds, helping hydrogen production. Under 440 nm irradiation, hydrogen evolution rate from the coumarin-linked COF was 531 mmol g-1 h-1, with a quantum yield of 37.95 percent at 405 nm. Under visible light, above 420 nm, the achieved hydrogen evolution rate was 166 mmol g-1 h-1, showcasing the potential for solar-driven hydrogen production. “This work provides a facile and effective strategy for tuning charge dynamics in conjugated COFs via linkage engineering,” explained Zhang Tao, a professor of NIMTE in a press release. “It sheds light on the design and application of high-performance organic photocatalysts for solar hydrogen production.” The research findings were published in the journal Nature Synthesis. Get the latest in engineering, tech, space & science - delivered daily to your inbox.Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.