All-Solid-State Solid Oxide Fuel Cells Directly Operated On Carbon And Biomass Fuels

Solid oxide fuel cell（SOFC）can directly use solid carbon as fuel,which is called direct carbon solid oxide fuel cell（DC-SOFC）.It is a clean coal technology and a potential technology for efficient use of biomass energy.The operating principle of the DC-SOFC technology is as follows:the electrochemical oxidation reaction of CO occurs on the anode,and the resulting CO₂ diffuses to the carbon fuel in a reverse Boudouard reaction,generating more CO at high temperature（800℃）.DC-SOFC directly uses solid carbon as fuel,realizing the true sense of all solid state structure battery with more safety and reliability.There have been many preliminary studies on DC-SOFC,but there is still a lack of understanding of its dynamic mechanism.Moreover,most of the studies are based on button cells and short tube cells with small effective area.In order to further understand the factors affecting the DC-SOFC dynamic process and promote the transformation of DC-SOFC technology to practical application,in this paper,the influence of gas outflow in the DC-SOFC anode reaction on the cell performance was studied;The feasibility of using biomass fuel directly was discussed;A seal-less DC-SOFC stack with output power greater than 10 W was designed,fabricated and tested;Finally,a stack device which can realize the series and assembly of multiple cells is designed.In view of the phenomenon that CO in the DC-SOFC anode chamber may diffuse to the anode for electrochemical oxidation reaction,or may be directly discharged from the cell through the outlet,the influence of outlet size on the performance of DC-SOFC is studied.The quartz tube（anode chamber）with different outlet size are used to seal the cells and the performance are tested and compared.Results show that cells sealed with a small outlet quartz tube shows a lower polarization impedance and a higher output power density.The theoretical calculation of the corresponding process confirms the experimental results that more CO can participate in the electrochemical oxidation reaction of the anode by reducing the area of the exhaust port,thus improving the performance of the DC-SOFC.To expand the application of DC-SOFC,the performance of cells directly using biomass fuel is studied.Bauhinia tree sawdust is directly used as fuel to test SOFC.The maximum output power density of the cell at 850℃is 328 mW·cm^-2,which is significantly higher than that used activated carbon loaded with 5 wt.%Fe.In order to study the in-situ carbonization of the fuel,the solid and gas products at different temperatures are collected by simulating the heating process of the cells,and their compositions and physicochemical properties are characterized by TGA,SEM,EDX,XRD,Raman,FTIR and GC.Results show that the solid products are incompletly carbonize,so the graphitization degree is low and it can provide many fresh formed active sites.The solid products contain some oxygen-containing functional groups,which is beneficial to the activity of the reaction.They also contains a large number of naturally distributed Ca elements and a small amount of K and Mg elements,which can catalyze Boudouard reaction.H₂,CH₄ and other small molecules of fuel gas also participate in the electrochemical reaction,further improve the performance of the cell.To advance the application of DC-SOFC,a compact DC-SOFC stack is developed.It is composed of multiple cells in series printed on a 100 mm×100 mm electrolyte sheet.Without the need for a seal,the cell stack is simply covered on a container filled with solid carbon-rich fuel,forming a small stack.12 cells in series stacks are tested using activated carbon and bauhinia tree sawdust powder as fuel,respectively.The output power of the former reaches 10W at 800℃,and the latter reaches 13 W.This work demonstrates the feasibility of developing the DC-SOFC as a small generator using off-the-shelf carbon-rich fuels.Finally,in order to further expand the scale of DC-SOFC stacks,a stack device is designed.It can achieve multiple cell plates in series.A stack of two cell stack plates in series is tested.It has a open circuit voltage of about 15 V and a maximum power of 15 W at 800℃.After improving the device,a four-plate-in series stack is tested,showing an increasing open circuit voltage.The research and development of this system provides experimental basis and reference for the practical application of distributed power generation and portable power base on DC-SOFC.The above research work provides new ideas and important theoretical and experimental basis for the transformation of DC-SOFC technology into practical applications,promoting the development of this technology in the field of distributed power generation and portable power supply.