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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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The impregnation of electrode precursor solutions is a very powerful technique for creating novel electrode microstructures constrained within preformed scaffolds. Here we report on the microstructural evolution of Mn-containing perovskites impregnated into yttria stabilized zirconia scaffolds on heating and redox cycling. Good performances have previously been reported for SOFC anodes with similar structure, and our objective is to better understand the origins of this good performance. For La 0.75 Sr 0.25 Cr 0.5 Mn 0.5 O 3-δ a remarkable thin coating with microfissures is formed on the scaffold after firing the electrode precursors at 1200 °C, and such behavior can be considered as wetting of one oxide by another. On further treating this microstructure at 800 °C in H 2 the microstructure changes dramatically forming an interconnected array of ∼10 nm scale particles. This seems to offer a very attractive structure with extensive triple phase boundary regions where electrochemical reactions can occur. On reoxidation at this temperature the particles reagglomerate to form a structure approaching the initial smooth coating. Performing similar procedures on the system La 0.33 Sr 0.67 Ti x Mn 1-x O 3(δ , we find that the wetting only occurs if Mn is present in the oxide and that the degree of wetting increases with Mn concentration. This favorable interaction between the Mn containing perovskites and the zirconia scaffold must be associated with a chemical interaction between impregnated oxide and substrate. The strength of this interaction decreases on reduction allowing the perovskite electrode to form nanoscale particles which along with appropriate additional catalysts provide good electrode functionality.

The operation of 25 solid oxide cells stacks in steam electrolysis mode over 9000 hours is reported. The stack has been operated at current densities of 0.57 and 0.72 A/cm2, using a 50% steam conversion, in the 750-780°C temperature range. Performance degradation rates of 2.30%/ kh are reported, with some non-homogeneity in the stack repeat units behavior observed. Stack repeat units located at the bottom of the stack feature degradation rates below 2%/kh. The possibility to use the operating temperature as a buffer to counter the performance degradation is highlighted. The stack performance was not affected by incidents that occurred during the test.

We report here on the investigation of the direct use of ethanol in solid oxide fuel cells. While the operation on Ni-YSZ anodes leads to detrimental catalytic carbon deposition even at high steam to carbon ratio, diluted ethanol can be directly used on LSCM anodes. Steady operation seems achievable only at high temperatures (900°C) due to gas-phase reaction carbon deposits. However, it is demonstrated that the initial performance is fully recovered after removal of these deposits. The addition of CGO to the LSCM anode is shown to considerably improve the anode performance, by limiting the gas-phase carbon deposition to a small layer beneficial to the performance. Recorded performance was higher on diluted ethanol than on humidified hydrogen. No degradation was observed on short term tests, and a polarization resistance of 0.3 Ω.cm 2 was achieved on ethanol diluted (15 mol. %) in steam.

We report here on the development of a novel type of solid oxide fuel cell anodes that are based on Mn containing perovskites and prepared by infiltration of electrode precursor solutions into yttria stabilized zirconia scaffolds. The presence of Mn, even in small amount, on the B-site of the ABO 3 structure leads to electrode microstructures that offer a large surface area that is available for electrochemical reactions. The oxides form a remarkable thin coating on the electrode scaffold after high temperature sintering. The evolution of those films in reducing environments creates arrays of interconnected ~10nm scale particles covering the electrode scaffold. Such electrodes, based on LSCM with small amounts of added catalyst, provide outstanding performance, both for the direct oxidation of methane in solid oxide fuel cells, and the reduction of pure CO 2 in solid oxide electrolyzers.