A direct carbon fuel cell (DCFC) can produce electricity with both superior electrical efficiency and fuel utilisation compared to all other types of fuel cells. Although the first DCFC prototype was proposed in 1896, there was, until the 1970s, little sustained effort to investigate further, because of technology development issues. Interest in DCFCs has recently been reinvigorated as a possible method of replacing conventional coal-fired power plants to meet the demands for lower CO emissions, and indeed for efficient utilisation of waste derived chars. In this article, recent developments in direct carbon conversion are reviewed, with the principal emphasis on the materials involved. The development of electrolytes, anodes and cathodes as well as fuel sources is examined. The activity and chemical stability of the anode materials are a critical concern addressed in the development of new materials. Redox media of molten carbonate or molten metal facilitating the transportation of ions offer promising possibilities for carbon oxidation. The suitability of different carbon fuels in various DCFC systems, in terms of crystal structure, surface properties, impurities and particle size, is also discussed. We explore the influence of a variety of parameters on the electrochemical performance of DCFCs, with regard to their open circuit voltage, power output and lifetime. The challenges faced in developing DCFCs are summarised, and potential prospects of the system are outlined.

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Solid oxide fuel cells (SOFCs) afford an opportunity for the direct electrochemical conversion of biogas with high efficiency; however, direct utilisation of biogas in nickel-based SOFCs is a challenge as it is subject to carbon deposition. A biogas composition representative of a real operating system of 36% CH 4 , 36% CO 2 , 20% H 2 O, 4% H 2 and 4% CO used here was derived from an anode recirculation method. A BaZr 0.1 Ce 0.7 Y 0.1 Yb 0.1 O 3Àd (BCZYYb) infiltrated Ni-YSZ anode was investigated for biogas conversion. The infiltration of BCZYYb significantly promoted the electrochemical reactions and the cells exhibited high power output at the operational temperatures of 850, 800 and 750 C. At 800 C, supplied with a 20 ml min À1 biogas, the cell with a BCZYYb-Ni-YSZ anode, generated 1.69 A cm À2 at 0.8 V with an optimal amount of 0.6 wt% BCZYYb, whereas only 0.65 A cm À2 was produced with a non-infiltrated Ni-YSZ in the same conditions. At 750 C, a maximum power density of 1.43 W cm À2 was achieved on a cell with a BCZYYb-Ni-YSZ anode, a 3 mm dense YSZ film electrolyte, a Gd 0.1 Ce 0.9 O 2 (GDC) buffer layer and a La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 -Gd 0.1 Ce 0.9 O 2 (LSCF-GDC) composite cathode. The cell remained stable, while operating at 0.8 V for 50 hours with a current density of 1.25 A cm À2 . A well-designed cell structure and selected components made it possible to obtain excellent performance at good fuel utilisation. The analysis of gases in open-circuit conditions or under various current loads suggested that the prevalent reaction was reforming of methane without coking. This study demonstrates that the BCZYYb-Ni-YSZ is a promising electrode for carbon-containing fuel.Depending on the reaction conditions, the equilibrium for the water-gas shi can be pushed in either direction. In fuel cell systems, the dominant reactions will be dependent on the gas 133 4463808; Tel: +44 (0) 133 4463817 † Electronic supplementary information (ESI) available. See

Three types of coal, anthracite, bituminous and lignite with no pretreatment, were investigated in hybrid direct carbon fuel cells (HDCFC). The HDCFC is composed of 1 mm YSZ electrolyte-supported cell with a NiO-YSZ anode and a LSM-YSZ cathode, and lithium-potassium carbonate mixed with coal in the anode chamber. Anthracite and bituminous coals show promising performance. Bituminous coal shows good initial cell performance as well as short term durability in two hour tests; whilst anthracite coal shows good initial cell performance but a slightly faster performance drop as compared to bituminous coal. It seems that coal with high carbon content and low sulphur content is desirable for direct carbon fuel cell application. Gas analysis shows that the ratio of carbon to oxygen content in coal, is likely to be of importance for the cell performance in terms of carbon oxidation process.

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