Leveraging apparently unrelated technologies links bacteria and photosynthesis to additive manufacturing for energy harvesting.
Additive manufacturing—perhaps better known as 3D printing—has radically changed the production reality for everything ranging from unique, one-of-a-kind structures to spare-parts inventory to lower-volume, tooling-free productions. It’s also enabled the creation of structures that would be difficult if not impossible to fabricate with conventional techniques and so opened up new ways to advance research and development in non-intuitive, truly “out of the box” scenarios.
Consider the popular topic of energy-harvesting possibilities, which offers the potential of getting someone for nothing or very close to it. The obvious options that come to mind are probably solar and photovoltaics (PV), vibration and piezoelectric transducers, wind or fluid with small or large-scale turbines, or thermal via thermocouples.
But why be limited to such conventional thinking? It turns out there are other substances that can be used to harvest energy, as a team at the University of Cambridge (UK) has shown. They have combined solar energy with specialized bacteria to generate small but useful amounts of electricity.
There’s much more to do this than just filling a Petri dish with the appropriate bacteria and then attaching a few wires, of course. The researchers used 3D aerosol jet printing to construct custom electrode structures using indium tin oxide (ITO) nanoparticles, creating high-rise ‘nano-housing’ grids where sun-loving bacteria could grow quickly (Figure 1).
They then extracted the bacteria’s waste electrons left over from photosynthesis. Other research teams have extracted energy from photosynthetic bacteria. Still, 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.
Surprisingly, the photosynthetic bacteria is not a rare “species,” (Figure 2). These cyanobacteria (formally designated as cyanobacterium Synechocystis sp. PCC 6803) are free-living, self-repairing bacteria that the researchers said they among the most abundant life form on Earth (it’s not explained how they determine that, but I’ll certainly take them at their word on this).
Researchers have been attempting to ‘re-wire’ the photosynthesis mechanisms of cyanobacteria to extract their energy for several years. “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,” said project leader Dr. Jenny Zhang. “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.”
To grow, cyanobacteria need lots of sunlight, such as on the surface of a lake in the summertime. They need to be attached to electrodes to extract the energy they produce through photosynthesis. To provide a structure for the bacteria and to provide electrical connections, they chose ITO electrodes, as that material has the desired combination of inertness, conductivity, light-scattering, and biocompatibility properties.
The electrodes were printed as highly branched, densely packed pillar structures, like a tiny city or community. Since these electrodes were custom printed by the team, the researchers experimented with different electrode lengths, diameters, and areal densities. They performed multiple tests and analyses to understand better the relationship among the various parameters (as this is an academic project, such multivariable exploration is the norm). The team found an optimum combination of these parameters to physically support the bacteria, expose their surface to light, and collect the electricity (Figure 3).
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 (Figure 4).
The work is detailed in their paper “3D-printed hierarchical pillar array electrodes for high-performance semi-artificial photosynthesis”, published in Nature Materials (Reference 1);. However, behind a paywall, an open version is posted at ChemRxiv (pronounced ‘chem-archive’), an official free-submission, distribution, and archive service for unpublished preprints in chemistry and related areas (Reference 2). Further, there’s a 21-page “Supplementary information” file which is often the case with these supplemental packages; these provide lots of interesting insight (Reference 3). The reason is that, unlike the primary academic paper, the supplemental file is not length-constrained, while it also discusses aspects of special interest to engineers such as the initial process, tradeoffs, and parameter variations (optimal and sub-optimal), and final-test set-up.
It is clearly not meant to predict yet if this mass of exposed bacteria will ever be viable for solar-based energy harvesting, given its messiness and “ick” factor. Perhaps it is mostly an interesting and informative academic exercise showing how new technology—here, 3D printing—can be used to investigate very different approaches to solving persistent problems, even though the outcome has little likelihood of acceptance due to practical and perhaps esthetic reasons.
- Nature Materials, “3D-printed hierarchical pillar array electrodes for high-performance semi-artificial photosynthesis”
- Chemrxiv, “3D-printed hierarchical pillar array electrodes for high-performance semi-artificial photosynthesis”
- Nature Portfolio, “Supplementary information”
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