Carbon Colloids Prepared by Hydrothermal Carbonization Power Indirect Carbon Fuel Cells
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Carbon Colloids Prepared by Hydrothermal Carbonization Power Indirect Carbon Fuel Cells Electrochemical conversion of carbon to electrical energy has its origins in the late 19th century. A carbon fuel cell is conceptually much more efficient than combustion for converting the chemical energy in coal into electrical energy. Because the activation energy for the electrochemical oxidation of carbon is high, direct carbon fuel cells (DCFCs) require high temperatures (between 500°C and 1000°C), which limits their application. Alternatively, indirect carbon fuel cells (ICFCs) employ a secondary oxidant to oxidize carbon. The secondary oxidant is renewable, that is, it is reoxidized at the anode. ICFCs can therefore operate at temperatures in the range of about 80–100°C, although the theoretical decrease in cell voltage is about 50%— from about 1 V (for DCFCs) to 0.5 V. Promising results were previously reported for an ICFC with a Fe3+/Fe2+ secondary redox system in the anodic half-cell using a
fuel of subbituminous fossil coal. Recently, J.P. Paraknowitsch, A. Thomas, and M. Antonietti of Max-Planck-Institute of Colloids and Interfaces, Potsdam-Golm, Germany, have shown that the efficiency of ICFCs can be increased even further with a fuel of carbon colloids. As reported online on March 3 in Chemistry of Materials (DOI: 10.1021/ cm801586c), the research team obtained water-dispersible carbon colloids from a process called hydrothermal carbonization (HTC), which dates back to early 20thcentury studies of coal formation from carbohydrates. Specifically, D-glucose, which the researchers regard as a model compound for several biomass sources, was heated in water in a closed reaction vessel for 4–24 h at a temperature of about 200°C to form HTC coal. In addition to the expected zero balance for CO2, the researchers said that another advantage of HTC coal is the hydrophilic surface structure; Fourier transform infrared spectra show bands of hydroxylic and carbonylic groups. Other advantages include the
6H2O
6H + + 1.5CO2
6Fe2+
Cathode
6Fe3+ Anode
3H2O + 1.5C
6VO2+
6VO2+
Nafion-112 Figure 1. Schematic representation of the secondary redox concept for an indirect carbon fuel cell.
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