Artificial photosynthesis for solar fuels

Tanmay Banerjee, Bettina V. Lotsch

Since the beginning of rapid industrialization in the mid-18th century, fossil fuels have been consumed at an extraordinary rate. Although new deposits are continually being discovered, the overexploitation of the finite reserves is a matter of serious concern. Also, CO2, produced by burning fossil fuels, is a greenhouse gas and thus the substantial increase in its levels in our atmosphere in the last 200 years has caused anthropogenic global warming. The last two decades have thus witnessed a paradigm shift in scientific research; the need to reduce dependence on fossil fuels has led to the investigation of renewable energy resources and solar power is arguably the most promising one. An intrinsic approach to storing solar energy is artificial photosynthesis, where thermodynamically uphill chemical reactions are driven by light, most prominently represented by the splitting of water to make solar fuels like H2.

Photocatalytic H2 evolution using "green" photocatalysts

Covalent organic frameworks (COFs) have recently emerged as promising candidates for H2 evolution photocatalysis. These polymeric photocatalysts have a crystalline and porous backbone of covalently interlinked organic molecules that is very robust and is composed entirely of earth abundant elements. Most importantly, the molecular (and hence modular) nature of the structure implies that the properties of the solid state heterogeneous material can actually be controlled at an atomic level. The limiting factor of the photocatalytic system has however been the need to use metallic platinum as the H2 evolving electrocatalyst, which is very rare and, hence, expensive.

Finding an earth abundant and efficient H2 evolving co-catalyst that can be used with COF photoabsorbers is an ardent task requiring careful consideration of the thermodynamic and kinetic requirements of the COF-co-catalyst system.

The first single-site heterogeneous COF-based system for photocatalytic H2 evolution.

Bettina Lotsch’s Nanochemistry Department at the Max Planck Institute for Solid State Research in Stuttgart, in collaboration with the Theoretical Chemistry group led by Christian Ochsenfeld at LMU Munich, has now developed the first COF-based H2 evolving photocatalytic system completely free of noble metals like platinum. Cobaloximes, which are coordination complexes of the earth abundant element cobalt with dimethylglyoxime ligands, have been used to replace metallic platinum as the co-catalyst. “These complexes act as artificial hydrogenases for the reduction of protons” says Tanmay Banerjee, a scientist in the Lotsch Group at the Max Planck Institute for Solid State Research. Hydrogenases are enzymes that catalyze reversible reduction of protons to hydrogen in organisms. When irradiated with simulated sunlight, the combination of N2-COF photoabsorber with cobaloxime co-catalyst produces H2 from mixed water/acetonitrile solvent as efficiently as some of the best functioning photocatalytic systems based on organic polymeric materials known, like carbon nitrides and covalent triazine frameworks. “The biggest achievement of the system is that it is completely molecular in nature and thus highly tunable, in spite of being heterogeneous and thus robust at the same time”, says Bettina Lotsch. Because of its molecular nature, H2 is now produced at readily identifiable ‘single-sites’ on the co-catalyst. Thus, for the first time, the mechanism of H2 generation in a COF based photocatalytic system has been studied experimentally, which is expected to lead to a better perception of rational design principles in such systems.

“Our result is important because it shows the feasibility of using the COF backbone as a supramolecular polymeric platform for integrating different subsystems of the photoinduced water splitting process”, Banerjee says. In future, the researchers expect to further tune the COF-co-catalyst interactions for optimal performance of the photocatalytic system, and extend the scope of solar fuel production to CO2 reduction, akin to natural photosynthesis.

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