IU chemists create molecular 'leaf' that collects and stores solar power without solar panels

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The new molecule harvests sunlight to create useable material from carbon dioxide in the atmosphere
March 8, 2017

FOR IMMEDIATE RELEASE

BLOOMINGTON, Ind. -- An international team of scientists led by Liang-shi Li at Indiana University has achieved a new milestone in the quest to recycle carbon dioxide in the Earth’s atmosphere into carbon-neutral fuels and others materials.

The chemists have engineered a molecule that uses light or electricity to convert the greenhouse gas carbon dioxide into carbon monoxide -- a carbon-neutral fuel source -- more efficiently than any other method of "carbon reduction."

The process is reported today in the Journal of the American Chemical Society.

"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," said Li, associate professor in the IU Bloomington College of Arts and Sciences' Department of Chemistry. "This study is a major leap in that direction."

Burning fuel -- such as carbon monoxide -- produces carbon dioxide and releases energy. Turning carbon dioxide back into fuel requires at least the same amount of energy. A major goal among scientists has been decreasing the excess energy needed.

This is exactly what Li’s molecule achieves: requiring the least amount of energy reported thus far to drive the formation of carbon monoxide. The molecule -- a nanographene-rhenium complex connected via an organic compound known as bipyridine -- triggers a highly efficient reaction that converts carbon dioxide to carbon monoxide.

The ability to efficiently and exclusively create carbon monoxide is significant due to the molecule's versatility.

"Carbon monoxide is an important raw material in a lot of industrial processes," Li said. "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."

The secret to the molecule’s efficiency is nanographene -- a nanometer-scale piece of graphite, a common form of carbon (i.e. the black "lead" in pencils) -- because the material’s dark color absorbs a large amount of sunlight.

Li said that bipyridine-metal complexes have long been studied to reduce carbon dioxide to carbon monoxide with sunlight. But these molecules can use only a tiny sliver of the light in sunlight, primarily in the ultraviolet range, which is invisible to the naked eye. In contrast, the molecule developed at IU takes advantage of the light-absorbing power of nanographene to create a reaction that uses sunlight in the wavelength up to 600 nanometers -- a large portion of the visible light spectrum.

Essentially, Li said, the molecule acts as a two-part system: a nanographene "energy collector" that absorbs energy from sunlight and an atomic rhenium "engine" that produces carbon monoxide. The energy collector drives a flow of electrons to the rhenium atom, which repeatedly binds and converts the normally stable carbon dioxide to carbon monoxide.

The idea to link nanographene to the metal arose from Li's earlier efforts to create a more efficient solar cell with the carbon-based material. "We asked ourselves: Could we cut out the middle man -- solar cells -- and use the light-absorbing quality of nanographene alone to drive the reaction?" he said.

Next, Li plans to make the molecule more powerful, including making it last longer and survive in a non-liquid form, since solid catalysts are easier to use in the real world. He is also working to replace the rhenium atom in the molecule -- a rare element -- with manganese, a more common and less expensive metal.

All of the research on the study was conducted at IU. The first authors on the paper are Xiaoxiao Qiao and Qiqi Li, former graduate students at IU. Additional authors are professor Krishnan Raghavachari and graduate students Richard N. Schaugaard, Benjamin W. Noffke and Yijun Liu, all of the Department of Chemistry; Dongping Li, a visiting professor from Nanchang University; and Lu Liu, a visiting undergraduate from the University of Science and Technology of China.

This study was supported by IU Office of the Vice Provost for Research and the National Science Foundation.

Well-Defined Nanographene–Rhenium Complex as an Efficient Electrocatalyst and Photocatalyst for Selective CO2 Reduction

Xiaoxiao Qiao† , Qiqi Li†, Richard N. Schaugaard, Benjamin W. Noffke, Yijun Liu , Dongping Li, Lu Liu, Krishnan Raghavachari , and Liang-shi Li* 

Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States

J. Am. Chem. Soc., Article ASAP

DOI: 10.1021/jacs.6b12530

Publication Date (Web): March 8, 2017

Copyright © 2017 American Chemical Society

Abstract:

Improving energy efficiency of electrocatalytic and photocatalytic CO2 conversion to useful chemicals poses a significant scientific challenge. We report on using a colloidal nanographene to form a molecular complex with a metal ion to tackle this challenge. In this work, a well-defined nanographene–Re complex was synthesized, in which electron delocalization over the nanographene and the metal ion significantly decreases the electrical potential needed to drive the chemical reduction. We show the complex can selectively electrocatalyze CO2 reduction to CO in tetrahydrofuran at −0.48 V vs NHE, the least negative potential reported for a molecular catalyst. In addition, the complex can absorb a significant spectrum of visible light to photocatalyze the chemical transformation without the need for a photosensitizer.

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