The hybrid direct carbon fuel cell (HDCFC) with solid oxide and molten carbonate binary electrolyte merges solid oxide fuel cell (SOFC) and molten carbonate fuel cell technologies to achieve direct conversion of solid carbon to electric power. The purpose of this study is to investigate in detail the electrochemistry of the oxidation of solid carbon in the carbon/carbonate slurry in the HDCFC. A planar test cell has been fabricated employing conventional SOFC materials and a eutectic carbonate mixture of lithium carbonate and potassium carbonate. The HDCFC with a model fuel, carbon black XC-72R, shows very high open circuit voltages (OCVs), approximately 1.5 V at 550–700 °C, especially after a high temperature operation at 900 °C, where carbonate decomposes to O²⁻ and CO₂. The carbon/carbonate slurry increases the active reaction zone from a two-dimensional Ni/YSZ anode to a three-dimensional slurry and significantly enhances the carbon oxidation. The high OCV is probably due to the low activity of CO₂ in the slurry, which results from the recombination of CO₂ and O²⁻. Gaseous products were analysed using an online gas chromatograph, and CO₂ and CO were detected, with their selectivity found to be dependent on temperature. Solid carbon is electrochemically oxidised to CO₂ and the final distribution of the products is dominated by the equilibrium of the Boudouard reaction (C + CO₂ ⇄ 2CO).