A research team at the University of Illinois, Chicago, has developed a new form of solar cell that operates on the same basis as plant photosynthesis; cheaply and efficiently converting atmospheric carbon dioxide into usable hydrocarbon fuel, with the only energy it requires coming from sunlight.
While conventional solar cells require heavy batteries to store the electricity they produce from sunlight, the new solar cell directly converts carbon dioxide into fuel. These “artificial leaves”, if in operation on a large-scale solar farm, would be able to not only provide energy-dense fuel at an efficiency far beyond that of normal cells, but also remove significant amounts of carbon from the atmosphere in the process.
“The new solar cell is not photovoltaic – it’s photosynthetic,” said Amin Salehi-Khojin, assistant professor of mechanical and industrial engineering at UIC and senior author on the study.
“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.”
The cell produces syngas, a mixture of hydrogen gas and carbon monoxide that can either be burned directly or converted into a range of hydrocarbon fuels, including diesel. Such a process is known as a reduction reaction, as it converts CO₂ into a burnable form of carbon.
Until now producing such reactions was inefficient, and relied on expensive precious metals as catalysts.
Deciding that they required a “new family of chemicals with extraordinary properties”, Salehi-Khojin and his team selected a group of nano-structured compounds called transition metal dichalcogenides (TMDCs) to focus on. These were placed in a two-compartment, three-electrode electrochemical cell along with an unconventional ionic liquid which functioned as an electrolyte.
The purpose of this was to determine the most efficient catalyst, and it worked: the team’s ultimate choice was nanoflake tungsten diselenide, a material which was found to be 1,000 times faster and 20 times cheaper than the previously used noble-metal catalysts.
However, there was still more to do to make the process work, as on its own, the catalyst can’t survive the necessary reaction to produce fuel. The solution was to add an ionic fluid with the catchy name ethyl-methyl-imidazolium tetrafluoroborate, mixed 50-50 with water, which allows the catalyst to endure the reaction.
“The combination of water and the ionic liquid makes a co-catalyst that preserves the catalyst’s active sites under the harsh reduction reaction conditions,” Salehi-Khojin said.
The technology, which has had a provisional patent application filed, should have a fairly high rate of adaptability, as it is usable on both large and small scales. Perhaps the most exciting possibility raised, however, is usage on Mars. Given that the planet’s atmosphere is mostly carbon dioxide, if water is found, these cells could go a long way towards contributing to possible settlement on the red planet.
Robert McCabe, National Science Foundation program director, said: “The results nicely meld experimental and computational studies to obtain new insight into the unique electronic properties of transition metal dichalcogenide.
“The research team has combined this mechanistic insight with some clever electrochemical engineering to make significant progress in one of the grand-challenge areas of catalysis as related to energy conversion and the environment.”