A new study, led by academics at St John’s College, University of Cambridge, has used semi-artificial photosynthesis to explore new ways to produce and store solar energy. They used natural sunlight to convert water into hydrogen and oxygen using a mixture of biological components and manmade technologies. Their method also managed to absorb more solar light than natural photosynthesis.

A new paper, published in Nature Energy, outlines how the researchers at the Reisner Laboratory in Cambridge developed their platform to achieve unassisted solar-driven water-splitting.

Natural photosynthesis stores sunlight in chemical energy carriers, but it has not evolved for the efficient synthesis of fuels, such as H₂. Semi-artificial photosynthesis combines the strengths of natural photosynthesis with synthetic chemistry and materials science to develop model systems that overcome nature’s limitations, such as low-yielding metabolic pathways and non-complementary light absorption by photosystems I and II.

Here, we report a bias-free semi-artificial tandem platform that wires photosystem II to hydrogenase for overall water splitting. This photoelectrochemical cell integrated the red and blue light-absorber photosystem II with a green light-absorbing diketopyrrolopyrrole dye-sensitized TiO₂ photoanode, and so enabled complementary panchromatic solar light absorption. Effective electronic communication at the enzyme–material interface was engineered using an osmium-complex-modified redox polymer on a hierarchically structured TiO₂. This system provides a design protocol for bias-free semi-artificial Z schemes in vitro and provides an extended toolbox of biotic and abiotic components to re-engineer photosynthetic pathways.

—Sokół et al.

Natural photosynthesis is not efficient because it has evolved merely to survive so it makes the bare minimum amount of energy needed - around 1-2 percent of what it could potentially convert and store.

—Katarzyna Sokół, first author and PhD student at St John’s College

Artificial photosynthesis has been around for decades but it has not yet been successfully used to create renewable energy because it relies on the use of catalysts, which are often expensive and toxic.

The Cambridge research is part of the emerging field of semi-artificial photosynthesis which aims to overcome the limitations of fully artificial photosynthesis by using enzymes to create the desired reaction.

Sokół and the team of researchers not only improved on the amount of energy produced and stored, they managed to reactivate a process in the algae that has been dormant for millennia.

Hydrogenase is an enzyme present in algae that is capable of reducing protons into hydrogen. During evolution this process has been deactivated because it wasn’t necessary for survival but we successfully managed to bypass the inactivity to achieve the reaction we wanted - splitting water into hydrogen and oxygen.

It’s exciting that we can selectively choose the processes we want, and achieve the reaction we want which is inaccessible in nature. This could be a great platform for developing solar technologies. The approach could be used to couple other reactions together to see what can be done, learn from these reactions and then build synthetic, more robust pieces of solar energy technology.

—Katarzyna Sokół

This model is the first to successfully use hydrogenase and photosystem II to create semi-artificial photosynthesis driven purely by solar power.

This work overcomes many difficult challenges associated with the integration of biological and organic components into inorganic materials for the assembly of semi-artificial devices and opens up a toolbox for developing future systems for solar energy conversion.

—co-author Dr. Erwin Reisner, Head of the Reisner Laboratory, a Fellow of St John’s College, University of Cambridge

Resources

• Katarzyna P. Sokół, William E. Robinson, Julien Warnan, Nikolay Kornienko, Marc M. Nowaczyk, Adrian Ruff, Jenny Z. Zhang & Erwin Reisner (2018) “Bias-free photoelectrochemical water splitting with photosystem II on a dye-sensitized photoanode wired to hydrogenase” Nature Energy doi: 10.1038/s41560-018-0232-y