Use of Solid Carbon Anodes in the Direct Carbon Fuel Cell
The direct carbon fuel cell (DCFC) offers the possibility of replacing inefficient coal fired power stations with a high efficiency and low emission option. Combustion of the carbon source, transforming chemical energy to thermal, is replaced with electrochemical transformation in a fuel cell arrangement, operating between 600-800°C. One of the major challenges of this technology is the delivery of a solid fuel to the reaction zone of the fuel cell. Fuels cells traditionally operate on gaseous fuels and this therefore presents a unique challenge. Although many different cell arrangements have been trialled, the most promising is that of the hybrid DCFC using a molten carbonate oxide transfer medium in the anodic compartment, and a solid electrolyte separating cathode and anode. A slurry of carbon and carbonate can be used along with a current collector, however this arrangement suffers from mass transport limitations as well as being susceptible to chemical corrosion of the solid fuel not in contact with the current collector via the Bouduard reaction (i.e. carbon partial oxidation by carbon dioxide gas to carbon monoxide), resulting in reduced efficiency of operation. The concept under development at the University of Newcastle involves the use of a solid fuel anode which reduces corrosion via carbon monoxide formation, as well as removing mass transport issues. The cell arrangement is visualised in the supplied figure. High conductivity of this electrode is essential as it must act as a current collector as well as a fuel. Further, mechanical stability is of high importance since the electrode will be consumed and binder materials must be stable under operating conditions as well as being less active to avoid preferential consumption and eventual degradation. Careful design of this anode arrangement is therefore required.
Coal type and impact of ash
Kinetics of carbon electrochemical oxidation have been observed to be sluggish and limiting in a fuel cell arrangement. Enhancing these kinetics is essential for realising an active DCFC able to operate competitively with current technologies available. It has been shown in research conducted at the University of Newcastle that kinetics at a solid carbon electrode can be enhanced through the use of common coal ash constituents. Further, kinetics are seen to be heavily affected by the coal type and pyrolysis conditions. It is believed that this is a result of the change in the distribution of ash particles and type of ash present in these coal fuels. Detailed physical characterisation, including advanced mineral liberation analysis, has been carried out on several coal chars in order to determine the reason for apparent activation of certain coal chars electrochemically when thermally treated at a specific higher heating temperature during slow pyrolysis.
Formulation and design of active solid anode
Solid carbon anodes are commonly used in aluminium electrowinning. However, these anodes require extensive treatment at high temperatures for operation in a commercial aluminium plant and are made from premium coking products. This process would use parasitic process heat, reducing efficiency of the DCFC, as well as using premium coke products which limits DCFC applications. At the University of Newcastle, formulation of an active and stable solid anode has been investigated using integration of thermal coal chars pre-treated in various chemical and thermal processes including acid and base demineralisation, pyrolysis and hydrothermal treatment. Coal chars produced in this way have been tested both separately using a graphite binder to assess activity, as well as in combination in order to demonstrate the possibility of using a combined solid carbon anode in the direct carbon fuel cell. The final solid anode uses minimal graphite as a binder and demonstrates high activity as well as good mechanical strength over an extended testing period.