Tungsten diselenide nanoflakes can be used to chemically convert carbon dioxide to carbon monoxide in an ionic liquid. This is the new finding from researchers at the University of Illinois at Chicago whose “photosynthetic” device works using only sunlight. The new type of solar cell could be used to remove carbon from the atmosphere and produce fuel at the same time.
Electrochemically reducing CO2 could, in principle, be a good way of recycling this greenhouse gas back into fuels. However, existing catalysts for this reaction are just too inefficient.
A team led by Amin Salehi-Khojin has now tested the efficiency of a class of 2D materials called transition metal dichalcogenides (TMDCs) as catalysts for this reaction. The researchers paired the materials with an ionic liquid as an electrolyte inside a two-compartment, three-electrode electrochemical cell.
Tungsten diselenide makes artificial leaf
They found that tungsten diselenide was the best, and in its nanoflake form, outperformed bulk catalysts (made of silver, for example) by a factor of 60. It was also at least twice as good as the other nanoflake compounds analysed in the study. It is also 20 times cheaper than a silver catalyst.
The team then used its catalyst to build an artificial leaf made up of two silicon triple-junction photovoltaic cells measuring 18 cm2 to harvest light. The tungsten diselenide and ionic liquid catalyst made up the cathode in the cell while cobalt oxide in a potassium phosphate electrolyte made up the anode.
Mimicking the photosynthesis process
“This artificial leaf mimics the photosynthesis process,” explains Salehi-Khojin. “In a real leaf, the CO2 is converted into sugar but in our leaf it is converted into syngas.” Syngas, or synthetic gas, is a mixture of hydrogen gas and carbon monoxide and it can be burned directly in gas turbines and syngas engines or converted into diesel or other high-density hydrocarbon fuels such as naphtha. Salehi-Khojin says that he and his colleagues can also engineer their catalyst to directly produce sugar or other hydrocarbons.
The researchers measured the catalytic activity per each active site on their artificial leaf when it was exposed to light of 100 W/cm2. This is about the average intensity of sunlight reaching the Earth’s surface. Syngas is produced at the cathode and free oxygen and hydrogen ions are produced at the anode. The hydrogen ions diffuse through a membrane to the cathode side to participate in the CO2 reduction reaction.
“This reaction was 1000 times better than that measured for silver nanoparticles and 60 times better than our previous work conducted on bulk molybdenum disulphide,” Salehi-Khojin tellsnanotechweb.org. “What is more, the solar-to-fuel conversion efficiency for the system is 4.6%, which is 2.5% up from previous systems.”
Such artificial leaves could be used to make solar farms next to chemical and power plants to convert CO2 from exhaust gas streams to fuels using just the energy from the Sun, he adds. “In this way we can not only remediate CO2 but also store the energy of the Sun in the form of chemical bonds, which is the most efficient way of storing this energy.”
The Illinois team, reporting its work in Science DOI: 10.1126/science.aaf4767 says that it now hopes to scale up its system in collaboration with industry and has already filed for a provisional patent.