Rechargeable direct carbon fuel cells combined with CO2 electrolysis for the charge using Ni/ Gd0.1Ce0.9O2-δ and Ni/(Y2O3)0.08(ZrO2)0.92 fuel electrodes

Carbon fuel cells have high theoretical efficiency and volumetric energy density, but ensuring continuous carbon supply is difficult. Rechargeable direct carbon fuel cells (RDCFC) using solid oxide fuel cells can supply carbon repetitively by hydrocarbon pyrolysis. If carbon is provided by CO 2 electrolysis, RDCFC can work as a secondary battery. Nickel (Ni)/yttria-stabilized zirconia (YSZ) fuel electrode is usually used for solid oxide fuel cells/electrolysis cells, however, carbon deposition and redox cycle would deteriorate the electrode due to Ni carbonization and reoxidation. Therefore, developing fuel electrodes is a key to enhancing the robustness of both RDCFC power generation and CO 2 electrolysis. Since CeO 2 -based oxides are known to suppress both carbon deposition and Ni oxidation, Ni/Gd-doped CeO 2 (GDC) electrodes are expected to be more robust than Ni/YSZ fuel electrodes. However, there is no report on a comparative evaluation of the characteristics of RDCFC power generation (discharge) combined with CO 2 electrolysis (charge). In this study, the charge/discharge properties at 800 °C and 100 mA cm −2 are investigated using Ni/GDC and Ni/YSZ fuel electrodes. While the Ni/YSZ electrode showed a large overpotential of up to 0.9 V during the initial charge, the large overpotential disappeared after discharging to 0.75 V (rather than to 0 V), indicating that Ni oxidation during discharge caused the large charge overpotential. On the other hand, the large overpotential was not observed for the Ni/GDC electrodes even when discharging to 0 V. From a thermodynamic perspective, we suggest that GDC suppresses Ni oxidation due to the oxygen non-stoichiometry. • Rechargeable direct fuel cells combined with CO 2 electrolysis are demonstrated. • Ni oxidation in discharging increases the overpotential in the initial charging. • Oxidation of Ni in the fuel electrode is suppressed by using GDC rather than YSZ. • The oxygen non-stoichiometry is suggested as the reason for inhibiting Ni oxidation. • Ni/GDC electrode is more robust than Ni/YSZ electrode for the charging-discharging.