Researchers have long sought to imitate photosynthesis, harnessing the energy of the Sun to generate chemical fuels. Now, a team has come closer to this goal than ever before. The researchers developed a new copper- and iron-based catalyst that uses light to convert carbon dioxide (CO2) to methane, the primary component of natural gas. If the new catalyst can be improved further, it could help reduce our dependence on fossil fuels.
The new work is an “exciting advance,” says Edward Sargent, a chemist and solar fuels expert at the University of Toronto who was not involved with the research. “The advantage of generating methane is that the infrastructure to store, distribute, and utilize it is widely available already.”
Methane recently surpassed coal as the primary source of fuel for generating electricity in the United States. When burned, methane breaks apart into CO2 and water, releasing heat that’s used to generate electricity. Producing methane using sunlight essentially runs this process in reverse, starting with CO2 and water and adding electricity to reforge methane’s chemical bonds.
This conversion isn’t easy, however. Eight electrons and four protons must be added to one molecule of CO2 to form one molecule of methane. Adding each electron and proton requires energy to propel each step of the transformation. Metal catalysts can help these reactions along. They grab on to each reaction’s molecular partners, making the reactions more likely and less energetically costly.
Several years ago, scientists found that copper particles, when paired with light-absorbing materials, showed initial promise in converting CO2 into more energy-rich compounds. But the efficiency and rate at which they did so were still low. So, researchers tried pairing copper with other metals. They grew two-metal particles atop fields of tiny, hairlike nanowires designed to act like mini–solar cells; the cells absorb sunlight and convert it to electricity that feeds the catalysts’ electrons for the reactions.
In 2016, researchers reported that catalysts containing copper and gold deposited atop light-harvesting, strawlike silicon nanowires helped convert CO2 to carbon monoxide, a compound widely used in industry. In March 2019, Zetian Mi, an electrical engineer at the University of Michigan, Ann Arbor, and his colleagues found that a ruthenium- and zirconium-based catalyst grown on top of arrays of light-absorbing gallium nitride (GaN) nanowires efficiently converted CO2 to formate, another industrially useful compound. But neither of these efforts generated a widely used fuel.
Now, Mi and his colleagues have come up with a recipe for doing just that. They started with the same GaN nanowires grown on top of a commercially available silicon wafer. They then used a standard technique called electrodeposition to add tiny 5- to 10-nanometer-wide particles consisting of a mix of copper and iron. Under light and in the presence of CO2 and water, the setup converts 51% of the energy in light into methane, and works at a fast clip.
Other researchers had previously achieved a higher efficiency for generating solar methane. But it worked so slowly, it was impractical. The new catalyst, reported this month in the Proceedings of the National Academy of Sciences, has the highest ever combination of efficiency and output in converting CO2 to methane for a light-driven catalyst, Mi says. Computer modeling revealed that the two metals in the catalyst work together to bind CO2 molecules, forcing them to bend in a way that makes it easier for them to react and absorb incoming electrons. “It lowers the energy barrier for the critical step,” Mi says.
The setup has another major advantage, he says. In contrast to many other fuel-generating light absorbers and catalysts, all the components of the current approach are cheap, abundant, and already used in industry. Sargent notes that the next steps will likely be to improve both the efficiency and rate of methane production, both of which would be needed to make the current system practical. If that happens, the new approach could offer society a way to use sunlight to make a fuel that can be used long after the Sun goes down.