A team of researchers at the University of Cambridge in the U.K., led by Jenny Zhang from the Yusuf Hamied Department of Chemistry, successfully demonstrated the use of bacteria and photosynthesis for solar-based energy harvesting.
When we think of energy-harvesting options, the obvious possibilities that come to mind are likely solar and photovoltaics, vibration and piezoelectric transducers, wind and water turbines, or perhaps thermal via thermocouples.
But why be limited to such conventional thinking? It turns out there are other substances that can harvest energy, as the research team at the University of Cambridge has shown. They combined solar energy (those pre-packaged pure energy bundles) with a specialized bacteria to generate small, but useful, amounts of electricity.
There’s much more to this initiative, however, than just filling a petri dish with the appropriate bacteria and then attaching a few wires. Rather, the Cambridge team used 3D aerosol jet printing for constructing custom electrode structures using indium tin oxide (ITO) nanoparticles, creating “grids of high-rise ‘nano-housing’ where sun-loving bacteria could grow quickly.”
They then extracted the bacteria’s waste electrons leftover from photosynthesis.
“Other research teams have extracted energy from photosynthetic bacteria, but the Cambridge researchers maintain that providing them with the right kind of ‘home’ increases the amount of energy they can extract by over an order of magnitude,” the University of Cambridge said.
Surprisingly, the photosynthetic bacteria used is not a rare “species.” These cyanobacteria (formally designated as cyanobacterium Synechocystis sp. PCC 6803) are free-living, self-repairing bacteria, which the researchers explained are among the most abundant life form on Earth. It’s not explained how they determined that, but I’ll take their word for it.
For years, researchers have attempted to “re-wire” the photosynthesis mechanisms of cyanobacteria to extract their energy.
“There’s been a bottleneck in terms of how much energy you can actually extract from photosynthetic systems, but no one understood where the bottleneck was,” Zhang said. “Most scientists assumed that the bottleneck was on the biological side, in the bacteria, but we’ve found that a substantial bottleneck is actually on the material side.”
Cyanobacteria require large amounts of sunlight to grow, and they must be attached to electrodes to extract the energy they produce through photosynthesis. To provide both structure and electrical connections for the bacteria, the researchers chose ITO electrodes as the material due to its desired combination of inertness, conductivity, light-scattering, and biocompatibility properties.
“The electrodes have excellent light-handling properties, like a high-rise apartment with lots of windows,” Zhang said. “Cyanobacteria need something they can attach to and form a community with their neighbors. Our electrodes allow for a balance between lots of surface area and lots of light — like a glass skyscraper.”
Because these electrodes were custom-printed by the Cambridge team, the researchers were able to experiment with different electrode lengths, diameters, and areal densities. They performed multiple test and analyses to better understand the relationship among the various parameters. As this is an academic project, there are many to explore.
The team found an optimum combination of these parameters to physically support the bacteria, expose their surface to light, and collect the electricity.
With a micropillar height of 600 µm, they reached photocurrent densities of 245 µA/cm2, with external quantum efficiencies of up to 29%, which they say is about an order of magnitude better than existing approaches.
