Research On Fabrication And Performance Of Direct Carbon Fuel Cells With Liquid Antimony Anode

Direct carbon fuel cell(DCFC)is a promising technology converting chemical energy in solid carbon fuels,e.g.coal,biomass,municipal waste etc.,directly into power.In this dissertation,liquid Sb metal is used as carbon converting anode of DCFC.The doctoral research studies reaction mechanisms in liquid Sb anode;stability of the ceramic electrolyte membrane;principles and guidelines for DCFC unit design.This thesis reveals the corrosion mechanisms of the solid oxide electrolyte by the liquid Sb anode,investigates the formation and migration processes of electrochemical reactive interface;developed atmospheric plasma spray technology for mass production of solid oxide fuel cells(SOFC).A DCFC unit was built and operated as a demonstration in the research.In this dissertation,I use yttria stabilized zirconia(YSZ)single crystal electrolyte as a tool to study the reaction mechanism in the liquid Sb anode.During operation of the fuel cell,Sb metal in the anode is first oxidized to its oxide,Sb2O3.Sb2O3 is a liquid form ionic conductor under SOFC working condition.The accumulation and migration of Sb2O3 at the interfacial region between the anode and the electrolyte create ionic transport pathways in the liquid Sb anode.Therefore,oxygen ions can travel through these pathways and react with Sb metal in the anode.In fact,electrochemical reaction in the liquid Sb anode not only takes place at the interface between the anode and the electrolyte,but also proceeds at the interface between Sb2O3 and Sb metal inside the anode.By tuning the ratio between Sb2O3 and Sb metal,one can increase the density of electrochemical reacting interface in the liquid Sb anode.In this research,extra amount of Sb2O3 was introduced into the anode reacting system by discharging the fuel cell at a large current,and current density of the cell was promoted by 14.2 %.Experimental results on the electrolyte's corrosion behavior prove that corrosion phenomenon in the ceramic electrolyte is closely related to electrochemical reaction.Corrosion was observed in both zirconia based and ceria based polycrystalline electrolytes.During fuel cell operation,Sb cation in the anode migrates along the grain boundaries in the solid oxide electrolyte.Sb segregation at the grain boundaries isolates ceramic grains from each other,thus weakening the integrity of the electrolyte.Grain boundary is believed to be the weak point and Sb transport pathway in the electrolyte.In this research,atmospheric plasma spray technology is used to fabricate corrosion-proof electrolyte.The plasma sprayed electrolyte worked continuously for 120 h,and proved its resistivity against liquid Sb metal.This dissertation also investigated the design and development of DCFC reacting units.By combining tubular SOFC unit and fluidized liquid Sb anode,anthracite is delivered continuously to the electrochemical reacting interface.Open circuit voltage of the DCFC rose from 0.73 V to 0.83 V,indicating direct oxidation of coal by electrochemical reaction.The plasma sprayed SOFC unit worked for 48 h,achieving a fuel efficiency higher than 99%,which is the highest value in the literature.