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Mini Power Plants from Coated Algae

Blue-green algae are among the oldest living creatures on earth and have perfected the use of sunlight over billions of years. Empa/CH scientists have now equipped these humble unicellular organisms with semiconductor coatings to create mini power plants, which supply biofuels and are photocatalytically active in sunlight. And what’s best: the chemical reactions are circular – they form a sophisticated raw materials cycle.

They were already bustling around in the “primordial soup” on our planet well over two billion years ago and have been helping to enrich the earth’s atmosphere with oxygen ever since. Nowadays, blue-green algae or – more scientifically – cyanobacteria are practically omnipresent: in waters, on the bark of trees or on barren mountain tops. They have perfected the efficient use of sunlight for photosynthesis to such an extent that they can survive even in gloomy caves. Empa researchers led by Laetita Philippe from the Mechanics of Materials and Nanostructures Laboratory in Thun have now used bio-templating technology to coat the undemanding blue-green algae of the genus spirulina in such a way that they can be used to produce biofuels and exhibit a photocatalytic effect.

To achieve a highly efficient and at the same time sustainable process, the researchers used the bio-templating method, in which an architecture given by nature serves as a template for applying novel surface coatings. The blue-green algae spirulina, whose shape is reminiscent of the coil in a tiny immersion heater, was particularly suitable for this, as its compact spiral structure contributes to its highly efficient use of sunlight.

The researchers first coated the approximately four micrometer-thin spiral cords of preserved spirulina unicellular organisms with a fine layer of nickel. Like the skin of an onion, delicate layers of zinc oxide and zinc sulfide nanoparticles followed. “While the spiral-shaped blue-green algae act as a structuring scaffold, the nickel coating, being magnetic, provides an excellent way of recovering the tiny power plants,” said Laetitia Philippe. The subsequent zinc-containing coating has impressive photocatalytic activity. This is because the combination of two zinc compounds leads to a prolonged performance of the photocatalytic reaction.

However, once the tiny immersion heaters have been put to work and decomposed pollutants in the wastewater, their task is far from over. While the zinc and nickel compounds can be recovered and reused, the preserved spirulina structures remain as mini power plants. “Biofuels can be produced from the biomass,” said Laetitia Philippe. In addition to yielding bioethanol and biodiesel, the spirulina remains can also be processed and burned as pellets for energy production. And the ashes can eventually be used as fertilizer for the cultivation of new blue-green algae. The sophisticated raw materials cycle has now been demonstrated in lab experiments. But Laetitia Philippe is confident that the versatile single-cell organisms can be applied on a larger scale.

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