Freestanding metal nanowires and macroporous materials from ionic liquids for battery applications
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Introduction When Paul Walden published his pioneering paper in 1914 on the molecular size and electric conductivity of some molten salts,1 he observed that some of these liquid molten salts, which he made by acid/base reactions, had melting points below 20°C. He concluded from the physiochemical data that these organic salts with melting points of 100°C and below behave similarly to molten salts having much higher melting points of between 300°C and 600°C. At that time, electric cars, which were introduced to the market around 1900, were still en vogue and regarded as the future of transportation, as they were clean and safe.2 Of course, there was no relation to ionic liquids. Powered by lead acid batteries with energy densities of around 0.01–0.015 kWh/kg, cruising ranges of about 50 km could be achieved, and even hybrid cars (the Lohner Porsche)3 were invented at that time. As we know now, electric cars did not make it. As soon as it was possible to start internal combustion engines (ICEs) with a battery powered electric starter, the automobile industry concentrated on the development of ICE-driven cars. Hydrocarbons have an energy density of 12 kWh/kg, and even a very inefficient combustion engine could still transform 0.6 kWh/kg of this into mechanical energy, which was 60 times better than the energy density of a battery. Furthermore, fossil fuels were relatively cheap so that attractive cruising ranges
could be achieved. At the end of the 1930s, electric cars were consequently no longer of interest. The late 20th century saw two oil crises,4,5 and the development of electric cars was reinitiated around 1990. Today, this can be termed the “first renaissance of electric cars.” Some reasons why electric cars did not make it again were insufficient reliability of the cars and batteries, as well as high cost. Furthermore, battery energy densities did not exceed 0.1 kWh/kg at that time. In the following 15 years, ICEs were improved, and the “second renaissance of electric cars” occurred around 2008. This was spurred by concerns about “global warming” and the apparent need to reduce CO2 emissions, combined with predictions of “peak oil,” which would make fossil energy extremely expensive within just a few years. Today, doubts regarding “peak oil” are justified, bearing in mind the enormous amounts of shale gas, shale oil, and oil sands found recently.6 This time, Li-ion batteries are regarded as suitable, and indeed cars such as the “Tesla Roadster” were put on the market. Today’s electric cars again suffer from low cruising ranges and/or high costs. Li-ion batteries have energy densities of only 0.1–0.15 kWh/kg and are rather expensive, whereas modern diesel engines have efficiencies of 40%, thus transforming up to 5 kWh of the 12 kWh/kg of hydrocarbons into mechanical energy.
Frank Endres, Clausthal University of Technology, Germany; [email protected] DOI: 10.1557/mrs.2013.136
© 2013 Materials Research Society
MRS BULLETIN • VOLUME 38 • JULY 2013 • www.mrs.org/bulletin
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FREESTANDING METAL NANOWIRE
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