1.IntroductionThe worldwide growing demand for electrical energy cannot be compensated by renewable energies in the medium term. Thus, new and improved concepts are required in order to use the limited fossil energy carriers in a more efficient way. A promising development in this context could be direct carbon fuel cell systems (DCFC) that use cheap and abundant coal as fuel and additionally benefit from its high thermodynamic conversion efficiency. In a DCFC in contrast to all other gas-fed fuel cell systems the stored chemical energy of a solid fuel is directly converted into electricity. The overall cell reaction (eqn (1)) is based on the complete electrochemical oxidation of carbon to carbon dioxide (CO2) in a four-electron process.1C + O2→ CO2The thermodynamic efficiency slightly exceeds 100%—almost independent of conversion temperature (Fig. 1). This is due to a positive near-zero entropy change of the cell reaction (ΔS° = 2.9 J K−1mol−1)1. Furthermore, the anodic exhaust gas of a DCFC consists of almost pure CO2that can be captured and sequestered with less difficulty than fore.g.conventional thermal power plants. The latter is due to the fact that either the oxy-fuel process needs to be used or additional energy is required in order to separate nitrogen (N2) from the off-gas-stream. Another advantage of a DCFC is that the fuel utilisation can reach up to 100%, since a solid fuel is used. This is due to the fact that the reaction product, CO2, exists in a separate gas phase and thus does not influence activity of the solid carbon.Thermodynamic efficiency as function of temperature for different reactions; triangles: carbon oxidation; squares: hydrogen oxidation; circles: carbon monoxide oxidation.Recently, three different concepts of a DCFC based on different electrolytes have been discussed: molten carbonate, molten hydroxide or solid ceramic material YSZ (yttria-stabilised zirconia). It should be mentioned that performance data of different concepts, presented in the following, cannot be compared directly, because they strongly depend one.g.used carbon fuel material, operating temperature and the concept itself (anode/electrolyte material).The most developed DCFC-systems are those based on molten carbonate electrolyte.1–3Cooper and coworkers1from Lawrence Livermore National Laboratory (LLNL) achieved power densities in the range of 40 to 100 mW cm−2(0.8 V cell voltage, 800 °C) for different carbon materials. They operated a cell for 30 h at a power output of 27 mW cm−2(∼1 V cell voltage, 800 °C). Nevertheless, carbonate-based cells suffer from corrosion problems which limits the choice of materials.1,4–6Zecevicet al.7at Scientific Applications and Research Associates (SARA) developed a DCFC using a molten hydroxide electrolyte. Peak power densities up to 120–180 mW cm−2were observed and an average power output of 40 mW cm−2(0.3 V cell voltage, 630 °C) was achieved over 540 h. The most important problems with hydroxide cells are (i) corrosion of materials and (ii) degradation of the electrolyte due to formation of carbonates during carbon electrooxidation.4,5,7Another concept of a DCFC is based on the combination of solid oxide fuel cell (SOFC) and molten carbonate fuel cell (MCFC) technology. Balachov and coworkers at SRI International demonstrated peak power densities of 10 to 110 mW cm−2(0.7 V cell voltage) in a temperature range of 700 to 950 °C, using different carbon containing materialse.g.plastic.4Irvine and coworkers5,8,9kept a cell running over ten hours at a power output of 10 mW cm−2(0.5 V cell voltage, 700 °C).Duskin and Gür at Direct Carbon Technologies (DCT) operate a DCFC combining SOFC and fluidized-bed technologies.10,11This concept is based on the gasification of carbon to carbon monoxide (CO). The CO isin situformedviaBoudouard reaction (CO2+ C → 2CO) and then electrochemically oxidized to CO2(CO + 0.5O2→ CO2) in a two electron process. For this cell reaction the thermodynamic efficiency is smaller as compared to thermodynamic efficiency of the direct carbon electrooxidation (Fig. 1). Peak power densities up to 140 mW cm−2(0.5 V cell voltage, 900 °C) have been achieved using this concept.10,11It has been reported6,7that the direct conversion of a solid fuel on a solid electrolyte (e.g.YSZ) suffers from low performance because of poor point-like contact between two solid materials. Additionally, carbon fuel needs to be in close contact with the anode material and the active area is limited to the geometric surface area. But as particle density in solid carbon is about 4 orders of magnitude higher than particle density of a gaseous fuel, a smaller active contact area can even be overcompensated, although for gaseous fuels a porous electrode shows typically an at least 3 orders of magnitude larger surface area12compared to the flat electrode.Solid oxide fuel cells (SOFC) show high fuel flexibility. As oxygen ions are transported through the electrolyte a SOFC can theoretically be operated on any combustible fuel, thus also on carbon. In principle, a DCFC with a solid electrolyte may be advantageous for fundamental investigations because it does not suffer from corrosion or degradation of the electrolyte. Compared to other DCFCs, a SOFC based system is rather simple, as no recycle loops for CO2or water management solutions are needed. Furthermore, SOFC systems allow implementation of catalytically active materials within modified anodes in order to enhance carbon electrooxidation.