Experimental Investigation On The Performance Degradation Mechanisms For HDCFC Anode

The direct carbon fuel cell is a fuel cell that directly uses solid carbon(coal)as a fuel,and its theoretical power generation efficiency can reach 100%,so it is a very promising fuel cell technology.However,the power density of the direct carbon fuel cell is relatively low.The mixed direct carbon fuel cell with carbonate added to the anode can change the anode reaction from the solid-solid contact reaction to the solidliquid contact reaction,thereby increasing the anode reaction area and the HDCFC output power effectively.Although the hybrid direct fuel cell has many advantages,its output power could decay rapidly with time.At present,the research on the performance degradation of hybrid direct carbon fuel cells shows that the HDCFC performance degradation is mainly caused by the increase of the anode reaction impedance,but the specific attenuation mechanism is still unclear.In this thesis,after summarizing previous studies on attenuation mechanism,the hypothesis of attenuation of battery performance caused by internal mass transfer and blocked charge transfer in hybrid direct carbon fuel cells is proposed and verified by the following experiments.A hybrid direct carbon fuel cell with GDC as electrolyte was prepared,and a hybrid direct carbon fuel cell performance test bed based on GDC and YSZ was built.In addition,HDCFCs with GDC and YSZ electrolyte were characterized.The rate of decay of HDCFC based on YSZ electrolyte and GDC-based electrolyte was found to be different.Under the same constant current discharge load,the YSZ electrolyte-based HDCFC has a lower decay rate,longer life,and can run for up to 16 hours.The HDCFC with GDC as the electrolyte has a shorter running time(about 30 minutes).After the initial life test,the mass transfer and charge transfer process of the anode were indirectly changed by means of vibration,adding heavy materials,changing the carrier gas flow rate,etc.,in order to study the main factors of the performance degradation of the mixed direct carbon fuel cell,and then analyze the performance degradation mechanism,thereby studying the main factors of the performance degradation of mixed direct carbon fuel cells.Through the research of this thesis:1)Studying the mass transfer of activated carbon and its effect on the performance of HDCFC.The study of mass transfer of carbon fuels is carried out in three ways:(1)Using the tap-performance recovery method to recover the HDCFC performance and the HDCFC output performance can be kept stable after continuous tapping.(2)Adding weight on the carbon layer to study the electrochemical performance changes before and after the addition.(3)Scanning electron microscopy of the carbon layer,anode and electrolyte fault structure before and after the experiment,so the distribution of the carbon fuel on the electrode and the carbon layer after the attenuation of HDCFC was obtained.Therefore,the mass transfer effect of carbon fuel in molten carbonate can be improved by knocking and weighting on the carbon layer,resulting in improved performance,but the improvement of HDCFC performance by knocking is limited.This is because the accumulation of carbonate at the bottom of the carbon layer to a certain extent,will prevent the mass transfer of carbon fuel,concentration impedance increase so that the HDCFC performance rapidly decay.In addition,the weight on the carbon layer increases HDCFC output power,but reduces its lifetime because the weight blocks gas transmission.2)By conducting the current in the anode current collector and the carbon layer to the DC resistance meter,measuring the conductivity of the carbon layer of the HDCFC under different operating times to investigate the variation of the ohmic impedance,and obtaining the increase of the operating time of the HDCFC.The carbon layer resistance will increase from 5W at the start of operation at 700°C to 40W at the end of the reaction.It was found in the SEM image that at the end of the reaction,the carbon powder agglomerated and the agglomerated "carbon block" was wrapped by the molten carbonate and not connected to each other,resulting in an increase in the resistance of the carbon layer,which would hinder the oxidation reaction of the carbon.,eventually causing a decline in performance.3)By investigating the factors affecting the increase of the activation resistance by HDCFC transforming the carrier gas and the anode SEM structure after the performance is completely attenuated,it can be observed that the electrode pores are filled with carbonate and the nickel oxide is corroded by carbonate.When the performance of HDCFC is completely attenuated,the carrier gas has a nitrogen gas converted to hydrogen.Compared with the performance of the HDCFC before the attenuation in the hydrogen atmosphere,the output power is decreased,indicating that the catalytic performance of the anode is affected by the corrosion but the catalytic performance is not completely lost.Transforming the carrier gas(nitrogen to hydrogen)and observing the SEM structure of the anode after the performance is completely attenuated to investigate the factors affecting the increase of the activation impedance.4)Observing its effect on HDCFC performance by changing the carrier gas flow rate.It was found that as the carrier gas flow rate increases,the battery decay rate increases.This thesis mainly studied the mass transfer law of carbon and product gas,the charge transfer law and the change of anode performance in the decay process of HDCFC,which lay the foundation for further research on the performance degradation of HDCFC.