March for science

Photosynthesis

All of us encountered the term photosynthesis for the first time in our high school science textbooks. In a nutshell, photosynthesis can be described as a complex process through which green plants convert atmospheric carbon dioxide (CO₂) into carbohydrates in the presence of water and sunlight. Through this process, the energy in sunlight is converted into chemical energy stored in the bonds of the carbohydrates that can be used by the plant at a later time. Also, through photosynthesis, oxygen (O₂) is liberated in the atmosphere, which is a byproduct of the process, and is vital for the survival of all animals.

Artificial Photosynthesis

In spite of its seemingly simple nature, the exact mechanisms of the photosynthetic process are still beyond our understanding. However, scientific efforts in the past three centuries in this pursuit, culminated in several significant developments in the field. At the beginning of the current century, an interesting idea was pitched by an Italian chemist, Giacomo Ciamician, viz. artificial photosynthesis. The proposition was simple : what if we can mimic the ingenious idea of plants, of trapping light energy as chemical energy, in our labs? Thus, if we could scale up such a process, we would be able to develop a completely sustainable energy technology. To provide some perspective, in the article titled “The photochemistry of the future”[A], Ciamician says, “...Europe possesses today about 700 billion tons of coal….The desert of Sahara with its six million square kilometers receives daily solar energy equivalent to 6 billion tons of coal.”. Thus it is obvious that if we have a successful technology to harness even 1% of the solar energy incident on the earth, the chronic energy problem of this era will disappear for eternity.

Needless to say, endeavours of realizing photosynthesis in the lab met with numerous challenges. It is a hot topic of research even in the present day. In this article we will discuss its present day pertinence, review a few strategies to realize artificial photosynthesis and explain how these developments can lead to a paradigm shift in 21st century science.

Global Warming

In the last decade, for the first time, we started feeling the catastrophic outcomes of global warming and climate change. There is no doubt that if not checked, it will severely affect all aspects of our personal and socio-economic life in the future. This is a serious concern and it rings an alarm for the future inhabitants of the earth. Excessive deforestation for urbanization, extensive dependence on fossil fuels, uncontrolled industrial emission of greenhouse gases (for example CO₂) are some of the main contributors to global warming. Several international conferences have been organized to evaluate the pitfalls of climate change and its far-reaching effects on the human society. Notably, in the Paris Climate Accord 2015, it was proposed to reduce fossil fuel dependence as much as possible and move towards renewable energy resources, to control global warming.

Alternative Energy Resources

In order to reduce fossil fuel dependence, we need ample alternative (preferably renewable) energy resources to replace it. However, one should understand why fossil fuels are so successful in serving the global energy market. Their current cheap availability, existing distribution networks, and available energy extraction technologies, make fossil fuels the first choice for energy production. Currently, they contribute to around 80-85% of the global energy demand. Thus we can understand that an overnight replacement of the current fossil fuel economy is impossible, until a comparable energy solution emerges.

Today scientists around the world are searching for alternatives to solve this energy problem. In recent times, several different technologies have emerged for energy production from sustainable sources, such as solar power cells, windmills, hydroelectricity, etc. Also, a few countries have adopted these technologies on a moderate to large scale, but clearly they are still a minority. A big challenge here is the fact that these alternative intermittent energy resources are often limited by several uncontrollable factors, such as the absence of sun during the night/cloudy days, favorable geographical topography, etc. Hence we require a solution that is independent of such constraints.

Strategies of Artificial Photosynthesis

In the current landscape, as a clever solution to the energy problem, some scientists are placing their bet on the idea of artificial photosynthesis. In this process, using solar energy as the sole resource, an integrated system would be able to produce energy-rich molecules (such as hydrogen, methanol, or ammonia) from abundantly available energy-poor molecules (such as water, carbon-di-oxide, and nitrogen). Several energy-rich and easily transportable products such as hydrogen, carbon monoxide, methane, methanol, or ammonia, as well as more complex chemical compounds can be synthesized selectively through this process. Thus, artificial photosynthesis seems to have real potential to replace the fossil fuel economy.

Under the artificial photosynthesis paradigm, different approaches can be adopted: 1) Direct Approach: direct absorption of sunlight induced charge separation in a semiconducting material (photovoltaics), and then the separated negative charges (electrons) can be used to convert the energy-poor molecules to energy-rich molecules; 2) Multistep Approach: a combination of conventional solar cell technology (photovoltaics) with an electrolyzer (electrolysis system with a catalyst), in which the electrical energy, generated from renewable sources, can be used as input and stored into the chemical bonds of energy-rich molecules (i.e. electrofuels) with the help of a viable catalyst (see Figure 1). Hence, in the Multistep Approach, harnessing, conversion, and storage issues of solar energy are tackled cooperatively. Needless to say, the Multistep Approach has become one of the most promising approaches towards future renewable and sustainable energy production.

In fact, some notable progress in this domain was achieved in the past few decades. In 1972, Fujishima and Honda successfully reported the photolysis of water to produce hydrogen and oxygen using titanium dioxide semiconductor [B]. Also, in the last decade, Prof. Daniel Nocera and his research group at Harvard University developed a cheap and self-repairing catalyst based on cobalt and phosphate to split water into oxygen and hydrogen. Later, they demonstrated that they could also integrate their catalyst into a compact system known as an artificial leaf [C][D]. Current scientific and technological progress also focuses on the implementation of feasible large-scale solar fuels and commodity chemicals production, however there is a long way to go before its successful implementation.

Perspectives

Presently, for the development of artificial photosynthesis technology, formidable scientific challenges revolve around coming up with an effective way to develop the catalysts required in the process. Also, achieving long-term stability of the catalysts, or its integration in devices pose some serious hurdles to overcome.

Currently, humankind is thinking of expanding into the solar system. But for example, building a future human habitat on the Moon or Mars will require a self-sufficient and effectively recyclable energy technology. Since it was found that water and CO₂ are reasonably available on the Moon or Mars, water and CO₂ reduction using artificial photosynthesis would probably be the best solution to have a sustainable energy production in these extra-terrestrial projects. Thus, the success of the artificial photosynthesis technology would not only solve the planet’s current energy problem, but also lead the way for mankind’s future space explorations. With the current acceleration of research in this field, we are hopeful of seeing the upcoming H₂O/CO₂/N₂ conversion technology based on artificial photosynthesis, playing a major role in the global energy market in the next few decades.

*Tamal Chatterjee received his PhD in Chemistry from IIT Bombay, India, in 2016. He is also the recipient of the ‘MAKE OUR PLANET GREAT AGAIN’ postdoctoral fellowship from the French Government, and worked as a postdoctoral researcher in the University of Paris, France, from 2018 to 2021.

**Debanuj Chatterjee received his PhD in Physics from Université Paris-Saclay, France, in 2021. Currently, he is a postdoctoral researcher at IIT Madras, India.

References

[A] Ciamician, Giacomo. "The photochemistry of the future." Science 36, no. 926 (1912): 385-394.

[B] Fujishima, Akira, and Kenichi Honda. "Electrochemical photolysis of water at a semiconductor electrode." Nature 238, no. 5358 (1972): 37-38.

[C] Kanan, Matthew W., and Daniel G. Nocera. "In situ formation of an oxygen-evolving catalyst in neutral water containing phosphate and Co2+." Science 321, no. 5892 (2008): 1072-1075.

[D] Lutterman, Daniel A., Yogesh Surendranath, and Daniel G. Nocera. "A self-healing oxygen-evolving catalyst." Journal of the American Chemical Society 131, no. 11 (2009): 3838-3839.