Photocatalytic hydrogen production requires light to reach the material carrying out the reaction. A team from the University of Gdańsk and the Institute of Nuclear Physics of the Polish Academy of Sciences investigated whether luminescent bacteria placed inside the reactor could provide a possible source.

Hydrogen can be used as a fuel and energy source. In a fuel cell, it reacts with oxygen to generate electricity, with water as the product of the reaction. However, before it can be used, it must be produced, which requires energy.

One of the most tempting solutions is to use sunlight. It can stimulate a photocatalyst – a material that absorbs light and, using the energy it generates, helps carry out the reaction leading to hydrogen release. Ultimately, this could involve the direct splitting of water into hydrogen and oxygen.

However, the low-emissions nature of the entire process also depends on the construction of the installation, the materials used and the additional energy required for its operation.

A seemingly simple problem also arises: light must reach the photocatalyst.

In a small laboratory vessel, simply pointing a lamp at the sample is sufficient. In a larger installation, however, the radiation is absorbed and scattered before it penetrates the liquid.

Photoreactors therefore require large transparent surfaces, shallow tanks or a network of thin channels. This makes scaling up the installation difficult.

Sunlight also varies. Artificial light sources offer independence from the time of day and weather, but increase energy consumption and process costs. Their environmental impact depends on how the electricity that powers them is generated.

Scientists from the University of Gdańsk and the Institute of Nuclear Physics of the Polish Academy of Sciences decided to test whether light could instead be produced inside a reactor.

They turned to bioluminescence, the light emitted by living organisms.

The researchers used Vibrio harveyi bacteria for their experiments. These microorganisms produce a blue-green glow, the colour of which closely matches the range of radiation absorbed by the tested photocatalyst.

Instead of conducting light through the vessel walls, it could potentially be used to surround a separated section of the reactor where hydrogen is produced.

This, of course, does not mean extracting energy from nothing. Bacteria require nutrients and oxygen, and maintaining their culture also consumes resources.

However, a biological ‘lamp’ could shine directly inside the installation, including areas difficult to reach with external radiation.

The researchers first prepared a photocatalyst based on zinc and indium sulphide, doped with copper and enriched with platinum nanoparticles.

Illuminated by a standard laboratory lamp, it effectively aided in hydrogen release. However, the tests were conducted in model mixtures containing substances that facilitated the reaction and were consumed during it.

Therefore, this was not yet a technology for splitting pure water or a ready-made method for the industrial production of ‘green’ hydrogen.

The researchers then tested several biological sources of light.

The fluorescent GFP protein required excitation by external diodes, so it alone did not solve the problem.

The reaction of luciferin with luciferase – the substances responsible for bioluminescence, a chemical compound that undergoes oxidation and an enzyme that accelerates this reaction – produced a bright flash, but it faded after about 30 seconds.

Bacteria offered the most hope.

The researchers adjusted the temperature, salinity, aeration and medium composition. They also used a modified strain with the GFP protein, which emitted more intense light in the experiment.

They built five versions of the reactor. In the most sophisticated version, glowing microorganisms surrounded a vessel containing a photocatalyst, while a reflective coating redirected escaping radiation back towards its interior.

Despite these efforts, the signal indicating hydrogen formation remained at the limit of detection, and the results could not be replicated.

Was the problem the colour of the light, the reactor's design or simply too few photons?

To determine this, the scientists replaced the bacteria with LEDs emitting light of a very similar colour and gradually increased the power.

The answer was clear.

In the tested system, hydrogen began to be detectable at a radiation power of approximately 20 microwatts.

The modified bacteria reached a short-term maximum of approximately 0.47 microwatts – almost 43 times less.

This threshold is specific to the catalyst, reactor design and experimental conditions, but for the first time it allowed the researchers to quantify the scale of the problem.

The researchers published these results in the Chemical Engineering Journal.

The findings also show how much would need to change before the concept could become viable.

Brighter bacterial strains, cultures continuously fed with nutrients, better-controlled aeration, more sensitive photocatalysts and thin-flow reactors in which no fragment of material is far from the light source could all help.

In the future, bioluminescence could inspire a new way of designing installations – ones in which light is generated precisely where it is needed.

First, however, the biological glow must become a sufficiently strong and stable radiation source.

Krzysztof Petelczyc (PAP)

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