The oneiric Ladyland is a distant realm governed by women only and led by a visionary queen with a strong passion for science. In this far kingdom, two major universities develop cutting-edge technologies, including a piping system to channel and distribute the heat from the sun:


‘The kitchen was situated in a beautiful vegetable garden. […] It was clean and bright. […] There was no sign of coal or fire.
-How do you cook?- I asked.
-With solar heat,- she said, at the same time showing me the pipe, through which passed the concentrated sunlight and heat. And she cooked something then and there to show me the process.’


In this excerpt from the 1905 utopian tale Sultana’s Dream, the Indian activist Rokeya Sakhawat Hossain describes a society where humanity has managed to efficiently harvest the energy from the sun to fuel everyday vital activities.


What if we could utilise the energy from the sun to produce chemical fuels? Not only that: what if we could do that using CO2, a greenhouse gas sadly known for its diabolical reputation, but actually an interesting source of carbon? And what if could simultaneously convert industrial by-products into useful chemicals? What sounds like utopia may become a reality known as “artificial photosynthesis” in the next few decades.


Plants are surprising living beings that collect energy from the sun. Such energy is needed for two processes: i) reduction of CO2 into sugars, and ii) oxidation of water to oxygen. In these reactions, electrons are transferred from side to side, alike batteries, but in reversed direction. While batteries deliver energy from chemical reactions, in photosynthesis an energy input synthesises new chemicals.


Photoelectrochemical (PEC) cells work under the same scheme. A photoactive material absorbs light to produce complementary reactions. The Cambridge-born “artificial leaf” is a device made of two compartments: on one side, CO2 is reduced to syngas – a valuable feedstock for the synthesis of hydrocarbons – while on the other water is oxidised to oxygen.[1]

Simultaneous CO2 conversion and PET plastic recycling in a stand-alone photoelectrochemical device. Reproduced from:[2]

In PEC cells, energy from light is usually insufficient to drive the whole reaction, so an external bias is needed. Nevertheless, if catalytic materials are deposited on both compartments, the cell under irradiation generates a significant current (known as “photocurrent”) even with no “nudging” electrical power supply. The result is a stand-alone device that converts CO2 to added-value fuels using abundant sunlight and water.


However strong our desire to utilise solar energy with simultaneous mitigation of global warming, we are not there yet. For instance, CO2 should be converted to more valuable hydrocarbons – like ethylene – which are not selectively produced, yet. Also, we can be smarter than plants. Oxygen, the result of water oxidation, is essentially a by-product and arguably useless chemical, being everywhere around us. Instead, we should replace water with more interesting substrates to be oxidised. What about ethylene glycol from plastic bottles?[2]


In my PhD, I will aim to design a new artificial leaf capable of converting CO2 to multicarbon, energy-dense products with higher selectivity using catalytic copper alloys. On the other side, I will also investigate inexpensive photoactive materials for the conversion of glycerol, a by-product of biodiesel production, to added-value chemicals. In particular, my first step will be to modify a metal-free, safe, and cheap graphitic material known as “carbon nitride” to improve its ability to harvest sunlight.
Recycling CO2 and industrial by-products with sunlight still sounds like a task only accomplished by superheroes. But apparently, we are on the right track to make Sultana’s dream come true.

References:

  1. V. Andrei et al., “Floating perovskite-BiVO4 devices for scalable solar fuel production,” Nature, vol. 608, no. 7923, pp. 518-522, 2022/08/01 2022, doi: 10.1038/s41586-022-04978-6.
  2. S. Bhattacharjee et al., “Photoelectrochemical CO2-to-fuel conversion with simultaneous plastic reforming,” Nature Synthesis, vol. 2, no. 2, pp. 182-192, 2023/02/01 2023, doi: 10.1038/s44160-022-00196-0.
  3. Cover picture: https://www.cam.ac.uk/research/news/artificial-leaf-successfully-produces-clean-gas

Andrea Rogolino

NanoDTC PhD Student, c2022