It powers most of life on earth, so could it also give us cleaner, greener energy?
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Wouldn’t it be great if we could meet our energy needs from ambient sunlight using cheap, bio-degradable, and easily deployable functional materials?
Let’s think about photosynthesis, the mechanism by which plants convert sunlight into chemical energy, a process which ultimately powers almost all life on Earth. A pressing question in current research is whether improving our understanding of photosynthesis could help us engineer solutions for the sustainable generation of energy.
We typically encounter photosynthesis in high school as a chemical reaction, where carbon dioxide and water molecules are converted into sugar and oxygen. This reaction also requires the input of energy, which is supplied by sunlight. However, the photosynthetic process is very different from simply mixing the requisite reactants in a test tube and letting “chemistry do its thing”. Rather, it builds on a complex sequence of events that are supported by ingeniously designed molecular machinery.
Over recent decades, researchers across the world have been able to unveil the precise structure of some of these photosynthetic complexes, which can comprise many tens of thousands of atoms. For instance, we now have precise models of the position of each atom for the ring-like light-harvesting antenna of purple bacteria, as well as for large cylindrically stacked antennae arrangements found in cyanobacteria.
A remaining challenge is to properly understand the dynamical processes enabled by these marvelously complex structures, and this takes us to the realm of physics. It begins with the interaction between light and matter; once a photon has been converted into a matter excitation, its energy must then be funneled towards a so-called reaction centre, where the conversion to useful chemical energy takes place. This must happen quickly (as otherwise excitations convert to heat) and over a long range (allowing many antennae to share the infrastructure of a single reaction centre).
We increasingly appreciate that nature’s structures appear to do better than our own best attempts of light harvesting with organic molecular structures, at least as far as the crucial initial steps of absorbing light and transporting its energy for processing are concerned. These processes occur at the interface between classical and quantum physics. Intriguingly, they seem to derive benefits from tapping into the toolbox of exotic quantum effects as a key ingredient to their highly optimised operation.
Learning from nature and replicating its finely tuned operation could therefore go some way toward realising efficient artificial molecular light harvesting. This could drive a switch to versatile and green organic photovoltaics, mirroring the transition to organic LEDs that has recently revolutionised high-end display technology in smart phones and TV sets.
An exciting prospect is that artificially designed structures are not subject to the same constraints experienced by living systems. This opens the door to engineering bio-inspired quantum-enhanced light harvesting solutions, which could go far beyond what their natural counterparts achieve.
If these ideas can indeed be turned into practical devices, then quantum-enhanced energy solutions could well prove to be the most impactful quantum technology yet.
Professor Erik Gauger is Professor of Theoretical Physics in the Institute of Photonics and Quantum Sciences at Heriot-Watt University, and is a member of the RSE’s Young Academy of Scotland.
This article originally appeared in The Scotsman on 3 January 2023.
The RSE’s blog series offers personal views on a variety of issues. These views are not those of the RSE and are intended to offer different perspectives on a range of current issues.