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Taking their inspiration from photosynthesis, researchers have developed a graphene-based system that could use sunlight to produce fuel from carbon dioxide and water.
As the quest for renewable and environmentally friendly fuel sources intensifies, researchers from Linköping University, Sweden, have looked to nature for inspiration. The team, led by Jianwu Sun, considered how plants obtain energy from sunlight in the process known as photosynthesis to develop a method that could not only deliver a renewable fuel source but could also help rid the atmosphere of harmful greenhouse gases in an environmentally beneficial double-punch.
Sun and his team suggest that plants capture carbon dioxide from the atmosphere and use energy from sunlight to convert it to methane, ethanol, and methanol. They could take solar energy and turn greenhouse gases into methane, formic acid, and other chemical compounds. These compounds could then be used as fuel in vehicles adapted to use natural gaseous energy sources. Additionally, carbon monoxide, methane, and formic acid by-products could be further processed or used in the industry.
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Methane is one of the preferred fuel sources in natural gas vehicles, which could be an advantage for the environment if it had not been an extremely tough sell to consumers. Part of the reason for this is the fact that they can be quite uneconomical. As an example, Honda’s CNG Civic — produced between 1998 and 2015 — was more expensive than their traditional Civic model and got significantly less milage from its fuel. Thus, reducing the cost of this fuel by producing it more efficiently, could be the key to consumers rethinking natural fuel vehicles.
Another industry in which one of these by-products, formic acid — the simplest carbolic acid — is beneficial for is livestock farming. Currently, formic acid is used primarily as an antibacterial agent in livestock feed, preserving food such as hay, enabling it to retain nutritive value for longer. Cheaper formic acid means cheaper animal feed and could see savings passed on to the consumer. The key to this bright innovation, still in the early research phase and discussed in a paper published in the journal ACS Nano, is a system built upon the wonder material graphene.
The Fine-Line Between Greenhouse Gas and Fuel
Graphene is the thinnest material known to man, a single layer of carbon allotrope atoms painstakingly extracted from graphite. Graphene has become the backbone of so many significant technological advances — particularly in electronics and biomedicine — due to its flexibility, elasticity, as well as its structural stability.
For the Linköping University team and their intended application, its most interesting and useful property is the fact that it is transparent to sunlight.
A significant issue impeding graphene’s use is the fact that it can have substantial current leakage, making it a poor choice for a material in solar-light harvesting.
To improve the material’s capabilities, the team combined graphene with cubic silicon carbide (3C-SiC) — a semiconductor consisting of carbon and silicon. The team is no stranger to this material, having previously developed methods to layer graphene on a substrate of cubic silicon carbide. They achieved this by heating the silicon carbide, which boils away the silicon, leaving behind carbon atoms reshaped into single-atom layers.
The beauty of combined graphene/cubic silicon carbide is that it creates a graphene-based photoelectrode — an electrode that can absorb light and then kick start an electrochemical transformation. The graphene can function as both a protective layer and a transparent conductor, allowing the cubic silicon carbide to get on with its role of collecting solar energy.
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Jianwu Sun and his team worked extensively with their graphene-based technology finding that increasing performance are directly related to several factors, most importantly, the interface between the graphene and the cubic silicon carbide semiconductor.
Their paper suggests that layers of graphene — which the team can currently build to a stack of four — can be adjusted on-demand to create different properties in the graphene-based photoelectrode. This includes improved stability and, crucially, efficient conversion of carbon dioxide.
The formation of other products by the system hinges on bonding it with cathodes of a variety of different metals. Selecting the correct cathodes from a choice of copper, zinc, and bismuth results in the creation of compounds such as methane, carbon monoxide, and formic acid from carbon dioxide and water.
As mentioned above, this technology must be viewed with the understanding that it is in its very early days. The key that Jianwu Sun emphasizes in his paper is that extracting fuel from water and carbon dioxide using sunlight can be done. It is now up to scientists to discover if the process can be scaled up. If so, a significant modality in the preservation of our environment can be delivered.
References and Further Reading
Li. H, Shi. Y, Shang. H, Sun. J,  Atomic-Scale Tuning of Graphene/Cubic SiC Schottky Junction for Stable Low-Bias Photoelectrochemical Solar-to-Fuel Conversion. ACS Nano14 (4), 4905–4915. https://doi.org/10.1021/acsnano.0c00986