Plants are one of nature’s greatest pieces of engineering. They capture vast amounts of CO2 from the atmosphere, generate the oxygen we breathe and are capable of creating chemical energy out of sunlight. This last capacity even makes them a major source of inspiration for researchers aiming to develop green alternatives to chemical fuels. But many have hit a brick wall as they tried to mimic this process. “Designing a platform that can harvest sunlight and convert it into the chemical energy of a fuel certainly is challenging,” says Bettina Lotsch, director at the Max Planck Institute for Solid State Research and principal investigator of the ERC-funded project COFLeaf (Fuel from sunlight: Covalent organic frameworks as integrated platforms for photocatalytic water splitting and CO2 reduction). “This is largely a materials challenge. We need to orchestrate a complex suite of physico-chemical processes, each with its own timescale and materials requirements, within an earth-abundant and stable materials platform.” With her ERC grant, Lotsch wanted to develop next-generation photocatalysts capable of doing just that. She decided to move away from inorganic photocatalysts, too: these are often toxic, expensive and difficult to fine-tune. Instead, she decided to focus on organic systems, which are tunable ‘from the atom up’. “Our materials are called covalent organic frameworks (COFs). They are a bit similar to the natural photosynthetic machinery of a plant: carbon-based, highly versatile, molecularly well-defined and amenable to the tools available in organic synthesis,” Lotsch explains. COFs can be seen as a bridge between organic molecules and solid-state materials. Simple organic compounds are linked with each other to form COFs, making the resulting materials compositionally tunable using simple chemistry. Unlike most other organic polymeric materials, COFs also benefit from a structure that can easily be studied: As crystals, they are indeed accessible to a range of diffraction and microscopic probes providing unique insights into their solid-state structures. “COFs have many advantages,” Lotsch notes. “They have an exceptionally high degree of both compositional and structural tunability, which sets them apart from classic polymers. In addition, their structural porosity gives them an advantage over other photocatalysts in terms of surface area. The rule of thumb is, the larger the surface area, the better the catalytic activity.”
A new and promising research field
After 5 years of research, Lotsch and her team could successfully demonstrate that COFs show great potential as earth-abundant and highly tunable energy converting systems. A potential so enormous that it opened up a new field of research called ‘soft photocatalysis’. Perhaps the project’s most important outcome is its demonstration that COFs can harvest light efficiently and convert it into chemical energy such as hydrogen. The new photocatalytic systems are earth-abundant and functional in aqueous conditions. Thanks to innovative chemical strategies, they could even be made chemically robust under harsh photocatalytic conditions. “We have also demonstrated what so far has been – and still is – one of the holy grails of photocatalysis. We can precisely tune the activity-determining parameters with atomic-level precision. Finally, we have developed ‘all-single-site’ heterogenous photocatalytic platforms with molecular-level precision. These can not only reduce the usage of expensive and noble metals during photocatalysis, but also serve as platforms for a better understanding of the photocatalytic reaction mechanism.” Eventually, Lotsch’s efforts could contribute to realising efficient and highly tunable artificial photosynthetic platforms using organic polymers. Whilst commercial applications are still far away, the project’s development of solar batteries for a class of polymers called carbon nitrides, along with concepts such as time-delayed ‘dark photocatalysis’, already point towards promising research avenues. Other possible applications include nitrogen fixation and the valorisation of biomass or microplastics.
COFLeaf, photosynthesis, solar batteries, COFs, sunlight, chemical fuel