Power/Performance Bits: Aug. 2

From sun to hydrocarbon fuel

Researchers at the University of Illinois at Chicago have engineered a solar cell that cheaply and efficiently converts atmospheric carbon dioxide directly into usable hydrocarbon fuel, using only sunlight for energy.

Unlike conventional solar cells, which convert sunlight into electricity that must be stored in heavy batteries, the new device converts atmospheric carbon dioxide into fuel. A solar farm of such “artificial leaves” could remove significant amounts of carbon from the atmosphere and produce energy-dense fuel efficiently.

“The new solar cell is not photovoltaic — it’s photosynthetic,” said Amin Salehi-Khojin, assistant professor of mechanical and industrial engineering at UIC. “Instead of producing energy in an unsustainable one-way route from fossil fuels to greenhouse gas, we can now reverse the process and recycle atmospheric carbon into fuel using sunlight,” he said.

While plants produce fuel in the form of sugar, the artificial leaf delivers synthesis gas, a mixture of hydrogen gas and carbon monoxide. Synthesis gas can be burned directly, or converted into diesel or other hydrocarbon fuels.

Simulated sunlight powers a solar cell that converts atmospheric carbon dioxide directly into synthetic gas. (Source: University of Illinois at Chicago/Jenny Fontaine)

Chemical reactions that convert CO2 into burnable forms of carbon are called reduction reactions, the opposite of oxidation or combustion. Engineers have been exploring different catalysts to drive CO2 reduction, but so far such reactions have been inefficient and rely on expensive precious metals such as silver.

The team focused on a family of nano-structured compounds called transition metal dichalcogenides – or TMDCs – as catalysts, pairing them with an unconventional ionic liquid as the electrolyte inside a two-compartment, three-electrode electrochemical cell.

The best of several catalysts they studied turned out to be nanoflake tungsten diselenide, which is 1,000 times faster than noble-metal catalysts, and about 20 times cheaper.

The UIC artificial leaf consists of two silicon triple-junction photovoltaic cells of 18 square centimeters to harvest light; the tungsten diselenide and ionic liquid co-catalyst system on the cathode side; and cobalt oxide in potassium phosphate electrolyte on the anode side.

When light of 100 watts per square meter – about the average intensity reaching the Earth’s surface – energizes the cell, hydrogen and carbon monoxide gas bubble up from the cathode, while free oxygen and hydrogen ions are produced at the anode.

The technology should be adaptable not only to large-scale use, like solar farms, but also to small-scale applications, Salehi-Khojin said. In the future, he said, it may prove useful on Mars, whose atmosphere is mostly carbon dioxide, if the planet is also found to have water.

Energy from printed images

A group at Aalto University in Finland devised a way that any picture or text could be inkjet-printed as a solar cell. Solar cells have already been manufactured from inexpensive materials with different printing techniques, and organic solar cells and dye-sensitized solar cells in particular are suitable for printing.

“We wanted to take the idea of printed solar cells even further, and see if their materials could be inkjet-printed as pictures and text like traditional printing inks,” said Janne Halme, a lecturer at Aalto.

A semi-transparent dye-sensitized solar cell with inkjet-printed photovoltaic portraits of the Aalto researchers (Ghufran Hashmi, Merve Özkan, Janne Halme) and a QR code that links to the original research paper. (Source: Aalto University)

When light is absorbed in an ordinary ink, it generates heat. A photovoltaic ink, however, coverts part of that energy to electricity. The darker the color, the more electricity is produced, because the human eye is most sensitive to that part of the solar radiation spectrum which has highest energy density. The most efficient solar cell is therefore pitch-black.

The idea of a colorful, patterned solar cell is to combine also other properties that take advantage of light on the same surface, such as visual information and graphics. For example, installed on a sufficiently low-power electrical device, this kind of solar cell could be part of its visual design and at the same time produce energy for its needs.

The inkjet-dyed solar cells were as efficient and durable as the corresponding solar cells prepared in a traditional way. According to postdoctoral researcher Ghufran Hashmi, they endured more than one thousand hours of continuous light and heat stress without any signs of performance degradation.

Water boosts perovskite performance

A team at the Okinawa Institute of Science and Technology Graduate University (OIST) investigated why perovskite solar cells perform better after being exposed to ambient air, and found a surprising answer: water.

Digging in to which component of air caused the phenomena, the researchers looked at the top-most layer, the hole transport layer. “It is known that the dopant of the hole transport layer plays a key role in perovskite solar cells’ performance,” said Zafer Hawash, an OIST PhD student. “But it was not clear how.”

The scientists performed controlled exposure of the hole transport layer to environmental gasses, focusing on oxygen, nitrogen, and moisture.

“What we found is that oxygen and nitrogen do not have any role in the redistribution of the dopants,” Hawash explained. “But in the case of moisture, the solar cells’ efficiency increases. This is the discovery: moisture is the air component that causes the redistribution of the dopant across the material, and thus the enhancement of the electric properties of the solar cells.”

Perovskite solar cells. (Source: OIST)

The scientists explain this phenomenon with the structure of the transport layer, which has many pinholes that allow the passage of gasses between the ambient and the underneath material. The dopant in the transport layer is a salt – Lithium TFSI. Being a salt, the dopant has a hygroscopic nature: it absorbs water. When the solar cells are exposed to moisture, the water absorbed by the transport layer causes the dopant to redistribute. However, long time exposure to moisture has a detrimental effect on the solar cells.

During their experiments, the researchers were also able to document the role of oxygen in the solar cells’ performance. “Oxygen enhances the electrical conductivity of the transport layer as well, but this effect does not last long,” Hawash said. “But with the right amount of exposure to moisture, the electric proprieties are irreversibly enhanced.”