Using solar or wind power to produce carbon-based fuels may seem like an oxymoronic approach to making a greener world. But when the starting material is carbon dioxide, which you can pull out of the air, the approach is as green as it gets.
The technology that makes it economically feasible isn’t available yet, but a paper presents a step forward in the effort to not just sequester CO2, but turn it into a useful fuel that is part of a carbon-neutral future.
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Xiao-Dong Zhou, an associate professor of chemical engineering at the University of South Carolina, is part of a team that is working on a sustainable approach to harnessing renewable energy.
Solar panels and wind turbines are most typically used to produce electricity, but on a large scale, electricity from sources like these pose problems. Utilities need to meet demand at all times, so if a power company is relying solely on wind or solar, what happens when the sun goes behind the clouds or the wind takes a breather?
An alternative long talked about is to use that green electricity to kick CO2 up the energy ladder. Carbon dioxide, the combustion by-product that comes out of power plant smokestacks and is getting too plentiful in the Earth’s atmosphere, is at the bottom of the hill when it comes to carbon-based fuels.
If you could convert CO2 into carbon compounds that are fuels, then that could advance the energy industry on two fronts: Adding a new energy source and reducing the carbon footprint. In chemical terms, it’s called reducing CO2 when you convert it to less-oxidized forms of carbon, all of which have actual fuel value. Some single-carbon molecules to aim for would include carbon monoxide (CO), methanol, and methane.
Any of these could end up stored for a cloudy or windless stretch of time, and in most situations more readily than electricity. Methane is the primary component of natural gas, for which there is already plenty of existing infrastructure. Methanol, or wood alcohol, is a close relative of ethanol, or grain alcohol, and routinely ends up used as a liquid fuel. Carbon monoxide might seem unusual in this context, but it has chemical value as a fuel, in and of itself and as a precursor to other fuels.
The trick is to be able to do the CO2 reduction economically. That means not just efficiently converting the electrical energy into chemical energy, but also making the device that does the job in a cost-effective manner.
Zhou and his research team published a paper that shows progress on both fronts. They have developed potentially inexpensive catalysts that efficiently convert CO2 to CO in an electrochemical cell.
As a starting point for making the catalysts, they used as a model carbon nanotubes, made purely of carbon atoms. But in making their catalysts for CO2 reduction, they departed from the carbon-only motif by sprinkling in a few nitrogen atoms to create a different kind of geometric and electronic structure.
The resulting “nitrogen-doped carbon nanotubes” proved to be adept at reducing CO2 to CO, and the team reports the catalysts are more stable than metal-based catalysts reported in the literature for the same reaction.
Researchers went even further by defining how the microstructure on the nitrogen-doped carbon nanotubes can affect the catalysis. When a nitrogen atom substitutes into a position where a carbon atom belongs in a carbon nanotube, it turns out several distinct chemical bonding patterns can result. The team showed that one of them, termed the pyridinic structure, was the most effective as an electrocatalyst, and was competitive even with more expensive precious metal catalysts reported for CO2 reduction.
“We are working in conjunction with other institutions, and they are developing the other side, the water side, using photovoltaics to split water, and eventually we want to couple those two reactions together,” Zhou said. “So one side will be water splitting, generating protons from the anode that travel through the electrolyte to reach the cathode side and then react with carbon dioxide and with incoming electrons to convert carbon dioxide to fuels. Carbon monoxide is one kind of fuel you can produce, and methane and methanol are other fuels that can be produced.
The paper is available for purchase here.