Study on Solid Oxide Carbon Fuel Cells with Molten Carbonate on Anode

Author:Li Shuang Bin

Supervisor:wang shao rong li guo rong


Degree Year:2018





The carbon fuel cells are alternative energy devices which convert chemical energy of solid carbon fuels into electrical energy in high conversion efficiency with less pollution emissions.Hybrid carbon fuel cells(HCFCs)are promising carbon fuel cells,in which the contact properties of solid carbon and anode surface,and carbon electrochemical oxidation rates are improved siginificantly,leading to enhanced performance.Further,this complex could solve the cathode corrosions and the complicated configuration.So far,carbon and carbonate reaction characteristics on HCFCs should be invstigated in depth,especially for anode electrochemical reaction mechanisms.There are no clear conclusions about the effects on various patameters on the performance on HCFCs.In addition,the influence of impurity phases need to be further evaluated.Based on these,the work has been carried out as follows:(1)Through physicochemical characterizations including the crystallization,thermal stability,textural properties,surface functional groups,and particle sizes for five carbon fuels:graphite,carbon black,activated carbon,pyrolyzed sawdust and sawdust,it is found that the carbon fuels with lower crytallization,lower thermal stability,and more surface functional groups show better cell performance.And the textural properties and particle sizes have some relationship with cell performance.The order for the electrochemical performance of five carbon fuels is sawdust>pyrolyzed sawdust>activated carbon>carbon black>graphite.The 20 wt%sawdust proportion in sawdust-carbonate mixture produced the highest peak power density of 789 mWcm-2 at 750°C.The contribution of CO electrochemical oxidation is dominated in pyrolyzed sawdust.But for sawdust,apart from CO contribution,C2H4,C2H6 and other light hydrocarbons produced account for the enhanced performance in sawdust.The introducing CO2 purge gas can promote Boudouard reaction rate dramatically and improve mass transfer process,resulting in enhanced performance.From anode porous structures comparision results,the cell performance with carbon black pore former is better than that of garaphite ones,and the phase inversion methode between these two.(2)Comparision for electrochemical results of active carbon,black carbon and graphite with different carbon-carbonate mass ratios,indicates that addition of carbonate increases the open circuit voltage siginificantly and the highest is concertrated in 30 wt%and 40 wt%carbonate contents.By tuning carbonate contents has a great impact on the cell performance.We also find that Li carbonte displays a higher catalytic effect than K and Na carbonate.The carbonate not only shows chemically catalytic effects but also presents electrochemically catalytic effects.The electrochemical oxygen-reduction cycle mechanisms have been proposed to explain catalytic behaviors in carbon gasification process.The electrochemical oxygen-reduction cycle mechanisms are dominated on the anode surface,and a series of complex electrochemical reactions will take place inside anode porous structure.(3)Etching test reveals that with the longer the etching process,the corrosion of YSZ will be more severe.The order of etching ability in three atmospheres:air>nitrogen>hydrogen.Wetting test indicates that the electrochemical driving force will make mixture more condensed.From the stability testing results,with the increasing carbonate,the cell stability will be worse,and the carbon fuels with lower current densities,the lifetime of cell will be longer.Among them,the lifetime of graohite is longest and sawdust show shortest lifetime.The main degradation factors for hybrid carbon fuel cells are LiK(CO3)and carbon mixture blocking,Li2ZrO3 compound and other impurities appearing,and Ni abnoomal coarsening.(4)From 14 selected metal or metal oxides in feedstocks,Fe3O4 and Sn catalysts present best performance,emphasis is also placed on the analysis of Fe-based and Sn-based catalytic mechanisms;Among the anode catalytic materials,CeO2,Fe3O4 and Nb2O5 are considered to be best catalysts.Based on two doping results,the catalytic mechanisms can be divided two parts:the first one is to catalysize the Boudouard reaction,such as Fe3O4;the second is to be elcectochemical reaction catalyst,such as CeO2.