Research aims to turn sunlight into fuel

In their “pie in the sky” vision of solar energy conversion, Art van der Est and his colleagues imagine harnessing sunlight to create a compound that could be used as fuel when other options aren’t feasible.

The Brock University Professor Emeritus of Chemistry is among the solar energy researchers investigating how to use a cheap, clean starting material and energy from the sun to produce environmentally sustainable fuels that can be used in place of gasoline.

The total amount of energy the Earth receives from the sun in 90 minutes is enough to supply all of the energy the world needs for a year.

But it’s challenging to capture that energy because it’s spread unevenly over the vast surface of the planet. This energy, which isn’t always available, is most commonly stored in batteries.

As an alternative to batteries, many scientists look to photosynthesis for inspiration.

During photosynthesis, plants and other photosynthetic organisms use a green pigment called chlorophyll to absorb energy from the sun. They then use a complex series of chemical reactions to store the absorbed energy by producing substances such as glucose from carbon dioxide and water.

“The goal of solar fuels research is to mimic this process and produce energy rich compounds that could be used as fuels,” says van der Est.

Chlorophyll is just one of a wide range of brightly coloured compounds that, in their simplest form, are called porphyrins.

A diagram of orange and blue hexagons connected to each other with lines and chemical letters and numbers.

A diagram depicting the porphyrin that has been modified to absorb a wide range of wavelengths of light. Courtesy of Prashanth Poddutoori.

Porphyrins are of great interest in the field of solar fuels because of their ability to absorb light and transfer electrons between molecules to bring about the chemical reaction needed to store the absorbed energy, van der Est says.

He collaborates with his former postdoctoral research associate Prashanth Poddutoori, now an Associate Professor at the University of Minnesota Duluth, on better harnessing the power of porphyrins.

Poddutoori, a synthetic chemist, recently discovered that attaching certain chemical fragments to the periphery of a porphyrin ring and placing elements such as phosphorus or antimony in the centre of the ring has a profound effect on the porphyrin’s optical properties.

In some instances, these modifications can change the colour of the porphyrin from red to black.

“One of the goals is to find dyes, or porphyrins, that absorb a wide range of wavelengths so that you can use all of the light coming from the sun,” van der Est says. “Dr. Poddutoori’s discovery is important because black substances absorb all the wavelengths across the visible light spectrum, which greatly improves the efficiency of the light absorption.”

Once Poddutoori made this discovery, van der Est carried out highly specialized time-resolved electron paramagnetic resonance experiments and advanced quantum chemical computations to follow what was happening to the electrons in the porphyrin during and following light absorption.

van der Est was able to help show that the porphyrins Poddutoori created transferred electrons from the edges into the centre of the porphyrin ring, thereby increasing the amount of light absorption.

The University of Minnesota Duluth-led research team, which includes van der Est and Poddutoori, published their paper, “Intramolecular Charge Transfer and Spin–Orbit Coupled Intersystem Crossing in Hypervalent Phosphorus(V) and Antimony(V) Porphyrin Black Dyes” in the prestigious Journal of the American Chemical Society late last year.

van der Est says knowledge generated by the team can be used as a first step to potentially create a fuel coming directly from the sun.

“We make this cool molecule, we figure out what it’s doing, we write a paper and it’s in the public realm so that a future engineer who is interested in solving a particular problem such as optimizing solar energy conversion can use it,” he says. “That’s the role of basic science.”


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