Real vs artificial, the differences
Artificial photosynthesis takes bio-inspiration from its natural cousin but there’s a range of differences between the two systems. Efficiency is a major factor here, and the area where nature offers the far poorer example. Plant proteins embedded in a membrane, each with a unique job to do, only operate at about 3% efficiency. For example, the chemicals of natural photosynthesis that performs water oxidation are very quick to fail. It’s a process that happens at pace but also at a very low turnover number. Luckily for nature, protein synthesis is on hand to reinstate the flow rapidly and compensate for this shortfall. But a 3% efficiency model certainly doesn’t fit industry requirements, making it impossible to take natural photosynthesis directly from a plant and transfer it to the lab.
However, nature’s model for solar conversion offers far stronger commercial possibilities. Nature has taken eons to create an incredibly efficient system of inter-connected, concentric chlorophyll rings. Perfectly spaced, they are positioned at the ideal angles for light-harvesting molecules to harness solar photons. To similarly convert solar power into useable energy or fuel through artificial photosynthesis, Prof. Morris focuses on porous materials, specifically metal–organic frameworks (MOFs). These compound ion structures or clusters connected to organic ligands interact with solar photons to create electrons and holes that reduce and oxidize. Side by side, the manufactured material and the protein look nothing alike, but understanding why nature does what it does, and then recreate it synthetically, is the bio-inspiration.
“These organic frameworks,” Prof. Morris explains, “oriented efficiently to three-dimensional space, are what exploit the solar photons. Depending on their polymer structure, we can change how the chloroforms orientate to each other, their spacing and what they look like to exploit light. In that arena, we’re closely mimicking nature.”
And even nature’s weaknesses are proving themselves to be opportunities. Within the last 10 years, major steps in understanding have exposed why the catalysts fall apart when oxidizing water in natural photosynthesis, contributing to its low efficiency. It has been discovered that water oxidation happens through a cluster of manganese and calcium, bridged together by oxygen. These look like metal oxides or little metal oxo clusters and it’s these cobalt oxygen clusters, and not the manganese, that oxidize the water so rapidly. This insight has helped groups like Prof. Morris’s to develop artificial compounds that will perform the same task.