Investigation Of Anode Reactions And Electrochemical Performance Of Direct Carbon Fuel Cell Based On Solid Oxide Electrolyte

Direct carbon fuel cell based on solid oxide electrolyte(SO-DCFC)is a kind of fuel cell,which can use solid carbon as fuel and convert the chemical energy of carbon to electricity directly.For its advantages of high theoretical efficiency,low pollutants emission and low cost of CO2 sequestration,SO-DCFC offers a promising way to generate electricity with carbon materials(such as graphite,carbon black,coal,biomass)consumption.As SO-DCFC technology is still in early stages of development,it is far from actual application because of its low power output and poor stability.Literatures have shown that the electrochemical performance of SO-DCFC is not only governed by the anode reactions,but also significantly influenced by the impurities in carbon materials.On the basis,this thesis conducted an exhaustive study of the critical factors affecting the electrochemical performance of SO-DCFC.The key scientific issues involved are:(?)The main anode reactions and the relationship between carbon fuel properties and cell performance in SO-DCFC;(?)Dispersing carbon particles in molten tin(Sn)is a potential way to improve the electrochemical performance of SO-DCFC.The influence of Sn on the anode reactions and electrochemical performance of SO-DCFC;(?)The role of volatile matter in coal fueled SO-DCFC;(?)The effect of ash on the electrochemical performance of SO-DCFC.With regard to these issues,detailed and in-depth research was carried out.(1)The effects of anode carrier gas flow rate(FAr)and contact condition on the electrochemical performance of SO-DCFC were investigated at 1023-1123 K to identify the contribution of CO and C electro-oxidation to the cell performance,respectively.The results showed that cell polarization performance decreased initially and then maintained almost constant when FAr increased from 0 to 200 mL/min,indicating that CO electro-oxidation in the anode had great impact on the cell performance.At high FAr(150 mL/min),separating the anode and carbon bed did not cause the decrease of cell performance remarkably.It was demonstrated that the contribution of C electro-oxidation to the cell performance was insignificant.The main anode reactions were CO electro-oxidation and C-CO2 gasification.The polarization and durability performance of the four carbon fuels in SO-DCFC was limited by the C-CO2 gasification rate rather than CO electrochemical oxidation rate.(2)To enhance the electrochemical conversion of carbon in the anode,four carbon fuels(GC char,YQchar,CFchar and ACchar)mixed with Sn powder were used as fuels in SO-DCFC respectively.The electrochemical performance of SOFCs in three fuel modes,including Sn mode,C mode and Sn-C mode,was measured and compared to clarify the anode reactions in Sn-C mode.The results show that,in Sn mode,the cell anode was completely destroyed by SnO2,which was produced by Sn electrochemical oxidation.In Sn-C mode,the carbothermal reduction of SnO2 was the key factor determining the cell performance.For GC CHAR,YQchar and CFchar,they reduced SnO2 through solid-solid mechanism.The reduction rate of Sn02 was lower than the electro-oxidation rate of Sn,leading to SnO2 accumulation on the anode surface and subsequent cell degradation.The main anode reactions were Sn electro-oxidation and the carbothermal reduction of SnO2.For ACchar,it reduced SnO2 through gas-solid mechanism.The reduction rate was higher and considerable CO was generated.The cell performance of AC char in Sn-C mode was much higher and more stable.The main anode reactions were CO electro-oxidation and C-CO2 gasification.Sn acted as the catalyst for C-CO2 gasification.(3)The effect of coal volatile matter on the performance of SO-DCFC was investigated by comparing the electrochemical performance of three raw coals and their chars with and without anode carrier gas.The cells fueled by raw coals without carrier gas displayed much higher initial cell performance and worse stability than those of raw coals with carrier gas and the corresponding chars without carrier gas.On one hand,the release of small active gases like H2,CO and CH4 from the decomposition of volatile matter participated in anode electrochemical reactions and improved the initial cell performance significantly.On the other hand,coal pyrolysis near the anode surface caused serious S poisoning and carbon deposit in the anode,resulting in rapid cell degradation during durability test.(4)To explore the role of coal ash in SO-DCFC,the electrochemical performance of char,ash-free char,and ash-added char was tested and compared from 1023 to 1123 K.Besides,five different coal ashes and six common mineral species were evaluated in SO-DCFC.The results indicated that the effect of coal ash on the cell performance was dependent on the ash composition.In the short term durability test at 1123 K,the ash with high content of SiO2 and Al2O3 showed no adverse effect on the cell performance.Inorganic species like Ca,K and Fe in coal ash improved the initial cell performance by catalyzing char gasification.Inorganic species like S and Fe in coal ash tended to react with anode Ni particles and formed large aggregations on the anode surface,leading to the deactivation of the anode.SiO2 in coal ash combined with Fe,Ca and K can form deposits in irregular shape on the anode surface and clog up the anode pores.In summary,this study provides basic understanding of the anode reactions and the effects of fuel impurities in SO-DCFC.Further improvement of the electrochemical performance can be expected according to the conclusions.