Artist's representation of hybrid photoelectrodes at work, converting CO2 into useful products.
Researchers at the Center for Hybrid Approaches in Solar Energy to Liquid Fuels (CHASE) in the US have determined that using three-dimensional silicon scaffolds on photoelectrodes improves the yield of products when sunlight is used to turn carbon dioxide into fuel.
CHASE is a consortium set up by the Department of Energy and features researchers from some of the premier universities in the US, including Princeton and Yale.
With humanity looking to reduce its dependence on fossil fuels, renewable energy sources such as wind and solar have stepped up to meet the electricity demands in many parts of the world. However, heavy industries and applications such as long-haul transport can not function with energy from battery packs and need an energy-dense fuel that can replace diesel and petrol.
Just like leaves turn carbon dioxide (CO2) and water into food in sunlight, scientists are keen to develop an approach to generating energy using an abundant light source.
Doing so with a gas like CO2 is doubly advantageous since it reduces the concentration of the greenhouse gas in the atmosphere while also providing a reliable source of energy, such as methanol, made using sunlight. Scientists refer to them as liquid solar fuels.
How are liquid solar fuels made?
To date, scientists have successfully generated liquid solar fuels using silicon photoelectrodes. The process works by incorporating catalysts on the surface of the silicon, which can absorb light and start the necessary chemical reaction.
In the presence of water, CO2 can be converted into methanol or even carbon monoxide (CO), which serves as a starter molecule for synthesizing a wide range of other useful products.
While scientists have known about high-surface-area silicon for decades, it has never been used in photoelectrodes for liquid solar fuel generation. The research team at CHASE attempted this for the first time, with promising results.
What did CHASE scientists do?
The team at CHASE built the photoelectrodes using high-surface-area silicon material. This allowed them to study the catalysts at a molecular level and better understand their role in carrying out the chemical reactions.
In a setup where cobalt was used as a catalyst and deposited on silicon, which was used in a three-dimensional format in the form of micropillars, the researchers observed that methanol was produced with a higher current density, leading to potentially a state-of-the-art photoelectrode that could be used for liquid fuel generation in the future.
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Two examples of silicon photoelectrodes. Left: Micropillar silicon with cobalt catalysts reduces carbon dioxide (CO2) to methanol (CH3OH). Right: Porous silicon with rhenium catalysts reduces carbon dioxide (CO2) to carbon monoxide (CO). Image courtesy of Daniel Kurtz, Bo Shang, and Eleanor Stewart-Jones/ Department of Energy.
In another approach, the catalyst was changed from cobalt to rhenium after being integrated into nanoporous silicon. The hybrid photoelectrode thus generated displayed higher durability and selectivity as it reduced CO to methanol, an important consideration when looking to scale and commercialize the technology.
The experiments demonstrated the advantages of using high-surface silicon in building photoelectrodes. With further technological advancements, the day will not be far when we will be able to create a closed loop of fuel usage and generation using nothing but CO2, water, and sunlight.
That’s a huge step toward a sustainable planet.
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ABOUT THE EDITOR
Ameya Paleja Ameya is a science writer based in Hyderabad, India. A Molecular Biologist at heart, he traded the micropipette to write about science during the pandemic and does not want to go back. He likes to write about genetics, microbes, technology, and public policy.
