Comparative performance of Ca2Fe2O5 and Fe2O3 oxygen carriers in lignin-based chemical looping fuel cells

Recent research has developed systems for the chemical looping (CL) gasification of biomass using iron (Fe)-based metal oxides as oxygen carriers (OCs). The present study constructed a CL fuel cell utilizing lignosulfonate, a type of technical lignin, as the fuel and compared the performance obtained with Ca2Fe2O5 and Fe2O3 as the OC materials. At 800 °C, the redox reactions of Ca2Fe2O5 and Fe2O3 were found to be continuous and stepwise, respectively, during both chemical and electrochemical processes. This difference affected the reoxidation to regenerate the metal oxides upon discharge rather than the reduction to metallic Fe by the lignosulfonate. The Ca2Fe2O5 was almost completely regenerated to its original structure whereas the Fe2O3 was only reoxidized to FeO and Fe3O4. The power densities of these fuel cells were comparable, with values of 0.353 W cm–2 for the Ca2Fe2O5 cell and 0.364 W cm–2 for the Fe2O3 cell. This equivalent performance is attributed to the similar internal resistances of both cells resulting from the strong effect of metallic Fe. However, the difference in reoxidation between the two OCs significantly affected the energy density. The Ca2Fe2O5 cell showed an energy density of 0.755 Wh g–1 whereas a value of just 0.491 Wh g–1 was obtained from the Fe2O3 cell. Similar results were observed in trials with Miscanthus sinensis, a grassy biomass, as the fuel. The use of Ca2Fe2O5 as the OC evidently increases the power generation efficiency of biomass-based fuel cells.