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90 www.resourceworld.com F E B R U A R Y / M A R C H 2 0 1 8 Developments in Green Technology by Jane Bratun GREEN TECHNOLOGIES ELECTROCATALYST CONVERTS CARBON DIOXIDE INTO FUELS Scientists at the Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab) have developed a new elec- trocatalyst that can directly convert carbon dioxide (CO 2 ) into multicarbon fuels and alcohols using low energy input. As cited on the Berkeley Lab web page, the work creates a clean chemical manufacturing system that can put carbon dioxide to good use. In the new study, published in September 2017, the team led by Berkeley Lab scientist, Peidong Yang, discovered that an electrocatalyst made up of copper nanoparticles provides the con- ditions necessary to break down carbon dioxide to form ethylene, ethanol, and propanol. All these products contain two to three carbon atoms, and all are considered high-value products. Ethylene is the basic ingredient used to make plastic films and bottles as well as polyvinyl chloride (PVC) pipes. Ethanol, commonly made from biomass, is a biofuel additive for gasoline. Propanol is also an effective fuel but is too costly to manufacture to be used for that purpose. To gauge the energy efficiency of the catalyst, scientists con- sider the thermodynamic potential of products – the amount of energy gained in an electrochemical reaction and the amount of extra voltage needed above that thermodynamic potential to drive the reaction at sufficient reaction rates. That extra voltage is called the overpotential; the lower the overpotential, the more efficient the catalyst. "It is now quite common in this field to make catalysts that can produce multicarbon products from CO 2 , but those processes typically operate at high overpotentials of 1 volt to attain appre- ciable amounts," said Yang, a senior faculty scientist at Berkeley Lab's Materials Sciences Division. "What we are reporting here is much more challenging. We discovered a catalyst for carbon dioxide reduction operating at high current density with a record low overpotential that is about 300 millivolts less than typical electrocatalysts." The researchers characterized the electrocatalyst at Berkeley Lab's Molecular Foundry using a combination of X-ray photoelec- tron spectroscopy, transmission electron microscopy, and scanning electron microscopy. The catalyst comprised tightly packed cop- per spheres, each about 7 nanometers in diameter, layered on top of carbon paper and densely packed. The researchers found that during the early electrolysis period, clusters of nanoparticles fused and transformed into cube-like nanostructures. The cube- like shapes ranged in size from 10 to 40 nanometres. "It is after this transition that the reactions to form multicarbon products are occurring," said study lead author Dohyung Kim, a graduate student. "We tried to start off with pre-formed nanoscale copper cubes, but that did not yield significant amounts of multi- carbon products. It is this real-time structural change from copper nanospheres to the cube-like structures that is facilitating the for- mation of multicarbon hydrocarbons and oxygenates." Exactly how that is happening is still unclear, said Yang. "What we know is that this unique structure provides a beneficial chem- ical environment for CO 2 conversion to multicarbon products," he said. "The cube-like shapes and associated interface may be pro- viding an ideal meeting place where the carbon dioxide, water, and electrons can come together." Researchers at Berkeley Lab have taken on various aspects of this challenge, for instance, in 2016, a hybrid semiconductor- bacteria system was developed for the production of acetate from CO 2 and sunlight. Earlier this year, another research team used a photocatalyst to convert carbon dioxide almost exclusively to carbon monoxide. More recently, a new catalyst was reported for the effective production of synthesis gas mixtures, or syngas. Researchers have also worked on increasing the energy efficiency of carbon dioxide reduction so that systems can be scaled up for industrial use. "By utilizing values already established for other components, such as commercial solar cells and electrolyzers, we project electric- ity-to-product and solar-to-product energy efficiencies up to 24.1 and 4.3% for two-to-three carbon products, respectively," said Kim. He estimates that if this catalyst were incorporated into an electrolyzer as part of a solar fuel system, a material only 10 square centimeters could produce about 1.3 grams of ethylene, 0.8 grams of ethanol, and 0.2 grams of propanol a day.