1.IntroductionFuel cell technology offers many advantages based on its high energy efficiency and scalability. One of the main difficulties for current fuel cell systems, especially those based on hydrogen, is low energy density. The use of solid carbon as a fuel in direct carbon fuel cells (DCFCs) would certainly remove this problem as the energy density of carbon is higher than alternative sources available both on a volumetric1and a mass density2basis. Carbon fuels can be obtained from coal, cracking of hydrocarbons, or pyrolysis of biomass. Biomass is a very attractive energy source in terms of renewable energy.The DCFC itself has a long history.3The first report relating to DCFC was in 1896 by Jacques.4This fuel cell used molten hydroxide as the electrolyte, operating in the temperature range of 400–500 °C. It, however, suffered from poisoning due to the build up of carbonate in the electrolyte. More recently, DCFCs using molten carbonate electrolyte have been reported,2,5–7as the stability, ionic conductivity and thermal properties of carbonates for molten carbonate fuel cell (MCFC) applications have been widely investigated since the 1960s.8The DCFCs with molten carbonate, however, require complex CO2management and cathode materials that are tolerant to molten carbonate at high temperature. The DCFCs based on solid oxide fuel cell (SOFC) technology have also been reported.9–12It is, however, difficult to get sufficient interaction between solid carbon fuel and solid electrode/electrolyte.The hybrid direct carbon fuel cell (HDCFC) with a binary electrolyte merges SOFC and MCFC technologies.13–15A solid oxide electrolyte is employed to separate the cathode and anode compartments while a molten carbonate electrolyte is utilised in the anode compartment. Oxygen is reduced to O2−at the cathode and transported across the solid electrolyte membrane to the carbon/carbonate slurry, where carbon is oxidised. The ideal overall reaction is the oxidation of carbon to carbon dioxide:1C + O2→ CO2This concept has some advantages in comparison to normal MCFC and SOFC. CO2circulation, which is required in normal MCFC, is not necessary. The cathode is not exposed to carbonate; therefore, the cathode materials already developed for SOFC applications are available. An enhancement of carbon oxidation by the carbonate slurry can be expected.We have already demonstrated this concept using tubular HDCFCs,14,15which are suitable for practical demonstrations, with some model fuels including biomass based fuel. It is, however, difficult to determine the exact nature of the anode reaction with those tubular cells because the active area of the electrolyte/electrode and the state of active sites are difficult to be certain of in a closed cell above temperatures where the carbonate has melted. The purpose of this study is to investigate, in detail, the electrochemistry of the oxidation of solid carbon in the carbon/carbonate slurry using a test cell with planar geometry. Investigating the HDCFC reaction over a specific anode with small area should contribute to better understanding of the anode reaction. A test cell was fabricated employing standard materials for SOFC: a yttria-stablilized zirconia (YSZ) electrolyte, Ni/YSZ cermet anode, and (La0.8Sr0.2)0.95MnO3(LSM) cathode. A eutectic carbonate mixture of lithium carbonate and potassium carbonate was utilised for the molten carbonate.