Solar-powered carbon reduction

The sustainable conversion of atmospheric carbon dioxide into carbon monoxide could open up a new route to fuels, albeit carbon-based fuels. An international team led by Liang-shi Li of Indiana University has achieved an efficient carbon reduction that has lower energy requirements than previous attempts. The team has developed a well-defined nanographene-rhenium complex as an electrocatalyst and photocatalyst for selective carbon dioxide reduction [L-s Li et al. J Am Chem Soc (2017); DOI: 10.1021/jacs.6b12530].

"If you can create an efficient enough molecule for this reaction, it will produce energy that is free and storable in the form of fuels," explains Li. "This study is a major leap in that direction." Burning carbon monoxide releases energy and re-generates carbon dioxide, but the carbon dioxide can be fed back into the system provided it is sufficiently efficient and uses a sustainable energy source for the process, essentially making it recyclable and close to carbon neutral. Li's nanographene-rhenium complex linked with the organic compound bipyridine gives a highly efficient reduction with very low levels of side-products.

"Carbon monoxide is an important raw material in a lot of industrial processes," Li adds. "It's also a way to store energy as a carbon-neutral fuel since you're not putting any more carbon back into the atmosphere than you already removed. You're simply re-releasing the solar power you used to make it." It is the nanographene that makes the system so efficient because it is so dark it absorbs energy from sunlight very effectively. Li adds that bipyridine-metal complexes have been studied before but earlier attempts only grasp a tiny sliver of the energy from sunlight, mainly in the ultraviolet range. Li's team exploit the visible-light absorbing power of nanographene to capture energy up to a wavelength of 600 nanometers and thus a large proportion of the spectrum of visible light.

The nanographene acts as an "energy collector", just as green leaves absorb sunlight for plants. The rhenium component is the engine by which the system converts carbon dioxide into a reduced substance. In green plants ultimately carbon dioxide is converted into sugars, but carbon monoxide is the fundamental chemical unit produced by Li's system. The energy collector pushes an electron flow to the rhenium atom repeatedly binding and converting the otherwise stable carbon dioxide molecule into carbon monoxide.

The next step will be to make the system more long-lived and active as a heterogeneous catalyst that will be easier to handle. The team also hopes to replace the rhenium with the more common and so less costly metal, manganese. Indeed, Li told Materials Today that already, "we are working on a heterogeneous catalyst that has manganese replace rhenium."

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the popular science book "Deceived Wisdom".