Analysis of the carbon anode in carbon conversion cells

We examine further the electrochemical oxidation of carbon in molten carbonate, based on analysis of published research. Ascending and descending branches of voltage hysteresis found in current sweeps of atomically-ordered graphite and of disordered carbon (coal char) are separated by about 0.20–0.25 V and by 0.10–0.15 V for ordered and disordered forms, respectively, over a wide band of current density, 0.03–0.10 A/cm2. The higher voltage of the descending branch is in rough agreement with prediction of the Y. Li model for the carbon/carbonate electrode in the same current range, for ordered graphite (La = 70–100 nm) and for disordered structures (La = 3–5 nm), respectively. We suggest that the amplitude of the hysteresis represents the difference between the overvoltage requirements for 2- and 4 electron net transfer processes, respectively. The 2 e− reaction (C + CO32− = CO + CO2 + 2e−) dominates the low current segment (LCS) of our previous analysis, and the more hindered 4e− transfer reaction (C + 2CO32− = 3CO2 + 4e−) dominates the high current segment (HCS). The voltage increase separating LCS from HCS is effected by accumulation of CO2 within small, melt-filled pores to form highly supersaturated solutions of CO2, which enhance anode voltage by a concentration overpotential of 0.10–0.25 V. Overpotential increases with reaction extent until (1) overall polarization inhibits the interior reaction and shifts CO2 production to the more accessible exterior surface, or, (2) at a critical concentration (dependent on surface tension and pore diameter) bubbles nucleate and block current flow in the pores. Further support for this picture comes from the often-reported deviation of the gas composition from the CO/CO2 ratio of the Boudouard equilibrium at atmospheric pressure, as open circuit conditions are approached in an electrochemical cell. Our interpretation accounts for the mole fraction of CO2 at open circuit being greater than predicted from the Boudouard equilibrium.