Phase-inversion Fabrication And Carbon Deposition Resistance Of Tubular Solid Oxide Fuel Cells

The rapid development of the world has led to the growing demand for energy as well as more attention to environmental protection.Despite the rapid development of clean energy in recent decades,such as hydropower,wind power and solar energy,the world's energy supply is still dominated by fossil fuels.To balance energy demand and environmental protection,it is necessary to improve the utilization efficiency of currently available fossil fuels while accelerate the development of clean and renewable energy.Solid oxide fuel cell?SOFC?is an energy conversion device that can convert the chemical energy in fuels directly into electricity through electrochemical reactions at medium or high temperature without combustion,which is very promising due to its high efficiency,low noise,fuel flexibility.However,the high cost of key materials and fabrication process has limited the practical application of SOFC technology.Thus,it is necessary to develop low-cost SOFC fabrication process on the basis of the existing reliable material system.Besides,the stability of current widely used anode materials in carbon-based fuels is still far from satisfactory despite SOFC can run in those hydrocarbon fuels that PEMFC cannot directly use.In response to the above problems,low-cost and simple phase-inversion method and co-firing technology were adopted to fabricate several kinds of tubular SOFCs on the basis of cheap and presently reliable material system in this study;a novel electrode optimization process was developed;the conversion precess of methane over nickel catalyst was explored,providing important theoretical basis for future anti-carbon deposition researches.In Chapter 3,a novel electrode optimization process,phase inversion-impregnation method,was developed,by which nano-Ni,Fe and BaO were successfully impregnated into the Ni-YSZ tubular supports respectively.Subsequently,the effect of anode modification on cell performance and stability was investigated.Different from the traditional phase-inversion process,the catalyst precursor solution was innovatively used as the coagulation bath in the as-proposed phase inversion-impregnation method.During phase inversion-impregnation process,catalyst precursor in solution can be dispersed to all parts of the NiO-YSZ tubular support under the driving force of interdiffusion between solvent NMP and non-solvent water,forming nano-catalytic coating in one step.The peak power densities of Ni-YSZ anode-supported tubular SOFC impregnated with Fe and Ni nano-catalyst were increased to 0.40 and 0.48 W cm-2 respectively,when using wet hydrogen as fuel at 800 oC.When shifting the fuel to wet methane,the peak power densities of Ni-YSZ anode-supported tubular SOFCs impregnated with Fe and Ni nano-catalyst were increased to 0.37 and 0.45 W cm-2,respectively.Although the peak power densities have declined compared with unmodified SOFCs,the stability of BaO-modified Ni-YSZ anode-supported tubular SOFCs has been improved running under wet methane fuel.It was found that the voltage degradation was only 0.17 V after 100 h operation under wet methane fuel at 800 oC.The phase inversion-impregnation method proposed in this study simplifies the fabrication process compared with the traditional impregnation method,improves the impregnation efficiency,and has a very good application prospects.In Chapter 4,the interaction between methane and nickel catalyst in SOFC anode as well as the methane conversion process were investigated based on the first-principle calculations.By comparing the activation energies of the individual elementary steps,the minimum energy path of the methane conversion process was determined along the transformation path of the carbon-containing intermediate.It was found in the minimum energy conversion process that CH4 firstly converted to CO through CH4?CH3?CH2?CH?CHO?CO path or CH4?CH3?CH2?CH?CHOH?CHO?CO path,then converted to CO₂ through direct oxidation path of CO?CO₂.The minimum energy conversion process does not contain C intermediate,and carbon deposition occurs at these regions with low concentration of oxygen-containing reaction medium O or OH.It can be inferred that promoting the oxygen-containing reaction medium concentration in the double phase boundary?DPB?between methane and Ni catalyst can effectively inhibit the formation of carbon deposition.Based on our experimental results and theoretical analysis,a new anti-carbon deposition mechanism of BaO modified Ni-YSZ anode was proposed.It is believed that BaO modification enhances the capture and dissociation of H2 O on Ni surface,and subsequently promotes the consumption of CH on Ni surface,leading to the suppression of carbon deposition.Based on our theoretical calculations,many experimental results in previous studies are well explained.It is expected that the theoretical findings in the present study could provide some fundamental information to future research on carbon reduction in SOFC anodes.In Chapter 5,porous TZP ceramic-supported tubular SOFC was prepared by phase-inversion and co-firing methods,and its electrochemical performance was characterized.In order to ensure a good permeability of the tubular support,40 wt.% spherical graphite was added as a pore-former.The porosity of the support is up to 40.15% after sintering at 1400 oC,and the pore structure is transformed from asymmetric structure to symmetrical structure.The maximum power densities of TZP ceramic supported tubular single cell are 0.25 and 0.20 W cm-2 at 800 oC using wet hydrogen and methane as fuel respectively.No significant degradation occurred after stably operating in wet hydrogen for 100 h.The as-prepared TZP ceramic-supported tubular SOFCs showed good redox stability.After 8 redox cycles,the open circuit voltage and peak power density can still reach 1.02 V and 0.26 W cm-2,respectively.The single cell can still stably operate in wet hydrogen fuel for 100 h without significant cell performance degradation after the redox cycling test.Segmented-in-series TZP ceramic-supported tubular SOFCs?SIS-SOFC?were successfully fabricated using silver as the interconnector.Using wet hydrogen as fuel at 800 oC,the open circuit voltage of TZP supported tubular two-cell-SIS and four-cell-SIS SOFCs reached 2.10 V and 4.02 V respectively,and the maximum output power density reached 0.24 W and 0.22 W cm-2,respectively.Based on TZP ceramic-supported tubular SOFCs,the elimination of deposited carbon on the anode was investigated,and a novel strategy of carbon elimination through electrochemical reaction was proposed.It was found that the cell can still recover to the initial performance after a total 35 CH4-H2 cycles in four modes.In Chapter 6,a novel metal-supported SOFC preparation process was developed.Ni-Fe alloy supported tubular SOFC was successfully prepared using phase-inversion,co-firing and in-situ reduction methods with NiO and Fe2O3 as raw materials,and its electrochemical performance was characterized.In traditional metal-supported SOFC preparation process,reducing or inert atmosphere is required,making the process complicated and increasing the equipment requirements.In this study,SOFC was prepared by conventional co-firing technique in air atmosphere using metal oxides as precursors,and then in situ reduced before testing.It was found that co-firing the green tubular support with half cells?anode and electrolyte?is conductive to obtain the YSZ crack-free electrolyte film.The XRD analysis indicated that the tubular support was converted into Ni-Fe alloy after in situ reduction,playing both roles of cell support and anode current collector.The maximum power densities of the Ni-Fe alloy supported tubular SOFCs are 0.26 W cm-2 at 800 oC using wet hydrogen as fuel.Compared with traditional preparation process,the in situ reduction method proposed in the present study greatly simplifies the fabrication process and provides a new idea for the fabrication of metal-supported SOFCs.In summary,the low-cost phase-inversion method is very promising for the preparation of tubular SOFC supports.The proposed phase inversion-impregnation method cannot only be used to introduce active catalyst to improve cell performance,but also to introduce modification reagent to enhance the stability.This method is also expected to be applied to the optimization of other electrochemical devices.The adsorption and diffusion of oxygen-containing media on the nickel catalyst surface of SOFC anode have a great influence on the methane conversion process,and the timely consumption of CH intermediate can significantly inhibit the formation of carbon deposition.Apart from removed by burning,the deposited carbon on SOFC anode can also be eliminated by the electrochemical reaction.