Wednesday 28 June 2017 News Updated at 09:06 PM IST
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Mimicking photosynthesis - Deccan Herald
Mimicking photosynthesis
Linta Joseph,
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Artificial photosynthetic systems could potentially provide us with a relatively inexpensive supply of clean energy. REPRESENTATIVE IMAGE
Energy seems to be the number one global priority of our times. The world is now going through the twin crises of depleting fossil fuel reserves and the environmental catastrophe wrecked by our over-dependence on unclean energy sources. As we scramble for renewable and clean energy sources with moderate success, around us is the cleanest, cheapest and most efficient method for fuel production, photosynthesis.

Now, researchers from the Indian Institute of Engineering Science and Technology (IIEST), Shibpur, West Bengal, in collaboration with the Flemish Institute for Technological Research (VITO), Belgium, are all set to join the global efforts in mimicking this important photobiochemical process by building the first artificial photosynthetic prototype to be entirely driven by visible light!

End of all our energy woes?

Artificial photosynthetic systems could spell the end of all our energy woes. They could convert carbon dioxide from the atmosphere and potentially provide us with a relatively inexpensive supply of all the clean energy we need to power our lives, that too in a storable form. But mimicking such a complicated natural process is no simple feat. Apart from laboratory level models, artificial photosynthetic devices are nowhere near being used on a larger scale. In the natural process, plants rely on chlorophyll to harvest sunlight. They then proceed to break down water into oxygen, hydrogen ions and electrons using a complex array of proteins and enzymes. While oxygen is released out into the atmosphere, the hydrogen ions and electrons further reduce carbon dioxide into carbohydrates, the biological fuel.

When we try to mimic the process, the photovoltaic cells do the job of chlorophyll by capturing sunlight and turning it into useful electrical energy. Our preference for the end products could also be slightly altered - we might produce pure hydrogen that could be used in fuel cells or hydrogen combustion engine, or liquid fuels like methanol that could run our machines. "The biological process that accomplishes the reduction of carbon dioxide to useful fuel has been streamlined by organisms over billions of years of evolution and is hard for us to mimic. Hence, scientists have been on the hunt for the perfect catalyst that could drive and speed up this process,” explains Dr Snehangshu Patra, the study’s lead researcher.

Though metal-based catalysts containing gallium do come in handy, the limited availability of this metal makes it less ideal. A rather interesting solution to the problem is to directly make use of photosynthetic bacteria or biological enzymes in our devices since they have evolved specifically to handle the biochemical reactions, making the devices worthy of the name 'bioelectrochemical systems’. Bioelectrochemical Systems (BES) carry out artificial photosynthesis under ambient temperature and pressure, yielding a solar to fuel efficiency as high as 10% in a recently proposed system, as compared to photosynthetic efficiency of around 2% found in green plants.

However, these systems need a lot of time for bacterial growth, give us a mixed bag of products resulting in high costs to separate and recover them, and the bacteria are ill equipped to cope with oxygen-rich environments. But what if we got rid of the bacteria and directly used their enzymes responsible for the catalysis? Turns out, this is a hugely advantageous solution. "Biological enzymes have a very high efficiency and product specificity in catalysing an energy expensive reaction such as the reduction of carbon dioxide,” explains Snehangshu. Since enzymes as catalytic agents need stability and sustainability, the challenge lies in immobilising them within the electrode and protecting them from degradation.

Path-breaking technique

Funded by the Department of Science and Technology, Government of India, Snehangshu’s group is working on fabricating and improving upon a recently proposed 'integrated photobioelectrochemical system’ (IPBES) that makes use of the enzyme formate-dehydrogenase (FDH) to reduce carbon dioxide to formic acid. Although it has a low solar to fuel efficiency of 0.042%, with a whopping 99% product purity, this technique is pathbreaking as it is the first artificial photosynthetic system to be entirely driven by visible light, without any input of fossil fuel energy!

The newly developed system consists of a semiconducting material Bismuth Vandate (BiVO4), which, when irradiated with visible light, generates the electrical energy required to run the photosynthesis process. Alternatively, a biocompatible catalyst cobalt phosphate (Co-Pi) could be used for splitting water molecules into hydrogen ions and oxygen.

Further, FDH makes use of the hydrogen ions to reduce carbon dioxide to formic acid. The clever technique of embedding the FDH into a nanoscale thick polymer film provides stability and a suitable physiochemical environment for the reaction. With stable catalysis going on for two weeks, this device sports the longest-lived carbon dioxide reducing biocathode.

(The author is with Gubbi Labs, a Bengaluru-based research collective)
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