The world’s energy supply is in danger, but the power of artificial photosynthesis can save it
Photosynthesis sustains life on Earth by using light to drive the conversion of carbon dioxide and water into useful products such as sugars and releasing oxygen as a by-product. Artificial photosynthesis attempts to mimic this process to produce renewable and storable fuels.
Photosystem II (PSII) is a key nano-sized component in photosynthesis which is responsible for using light to convert water into oxygen. This is a challenging and important process. In my work I investigate strategies to ‘plug in’ to PSII. This allows me to study how it works and to harness its functions in proof of concept devices which split water into oxygen and hydrogen using light. These devices serve as an inspiration for advancing the design of man-made systems with lower cost and higher efficiency.
A device which uses PSII requires careful design. To start with, we need to make an electrical connection. PSII pulls electrons from water so we need to adhere it to an electrode to detect and utilise them. Each PSII unit only produces a small current, so we need an electrode which can be loaded with a high amount of it in order to use our device.
We use 3-D electrodes of so-called inverse opal structure which resemble a porous sponge in order to connect to a large amount of PSII. To assemble these electrodes, we use templated self-assembly. In this process polymer microspheres arrange themselves into a pattern while the voids between them are filled with much smaller nanometre-sized spheres which will form the electrode. The polymer sphere template is then removed to reveal the inverse opal electrode. The thickness and porosity of the material can be easily altered to optimise our devices.
However, even when the enzyme loading is maximised, much of it still has a poor connection to the electrode surface which limits the devices’ efficiency. To improve this, we enhance the electrical wiring of PSII to the electrode surface using a polymer which shuttles electrons between it and the electrode. This allows us to create a device called a photoelectrochemical (PEC) cell which splits water to oxygen and hydrogen with 85% efficiency.
This unique semibiological-based electrode provides the basic concept for the design of future devices which harness the reactions and electron transfer properties available from biology. It will also help to develop tools to probe the function of PSII in order to understand how it uses light to turn water into oxygen. As for going from materials to devices, the inverse opal electrodes have demonstrated the potential to be highly versatile and may be used in various applications outside of PEC cells, including sensing, catalysis and solar energy conversion systems.
NanoDTC PhD Associate 2014