| Solid oxide fuel cell(SOFC)can directly convert the chemical energy of fuel into electrical energy at intermediate and high temperatures(600-1000℃).It is an efficient all-solid-state energy conversion device.SOFC has the advantages of flexible fuel,using non-noble metal as catalysts,and high electrical efficiency.Therefore,SOFC has remarkable potential in the fields of portable power,transportation power,dispersed generation,and heat and power generation.However,to achieve leapfrog development from laboratory scale and demonstration operation to actual industrial application,it is very necessary to further improve the electrical performance of the SOFC stack and ensure its long-term stable operation.This study intends to improve the electrical efficiency of the SOFC stack and reduce its degradation rate at the same time by designing the geometric structure of the interconnector,optimizing the anode microstructure parameters,and changing the fuel reforming mode to design the stack.The main work,results,and conclusions are summarized as follows:(1)By coupling the electrode microstructure,the catalytic reforming reaction on the porous anode,the electrochemical reaction on the electrode functional layer,as well as the mass/momentum/species/energy/charge transports,a high-fidelity micro/macro multi-scale and multi-field fully coupled model is developed.First,based on the coordination number and percolation theory,the microstructure model of the porous electrode is established,and the quantitative relationship between the electrode microstructure parameters and the effective macroscopic parameters(e.g.effective electrochemical reaction triple-phase boundary(TPB)area,catalytic reforming reaction specific area,effective conductivity)is defined by custom functions.Then,the chemical reaction and catalytic reforming mechanism of methane steam reforming on porous anode are discussed,and the electrochemical dynamic field on the porous electrode is described by an extended Butler-Volmer equation reflecting the effect of mass transfer.Then,the governing equations of each transfer process are established,and the source terms of the relevant governing equations are defined by effective macroscopic parameters and chemical/electrochemical reaction kinetic equations.Finally,the solution strategy of the numerical model is given,which lays an efficient and accurate numerical foundation for the structural designs of the interconnector and the stack,the optimization of the electrode microstructure,and the reasonable selection of operating conditions.(2)A novel beam and slot interconnector(BSI)is proposed,which is composed of channels,ribs,beams,and grooves.BSI can significantly reduce the additional electrical loss of the SOFC stack caused by the conventional straight channel interconnector(SCI).The experimental results show that under the same operating conditions,the electrical performance of the BSI-SOFC stack is better than that of the SCI-SOFC stack.At 650,700,750,and 800℃,the peak power density of the BSI-SOFC stack is 12.4%,24.3%,33.0%,and 28.5%higher than that of the SCI-SOFC stack,respectively.Moreover,the fuel utilization of the BSI-SOFC stack is higher than that of the SCI-SOFC stack at the same operating conditions.The field distributions of velocity/vorticity/temperature/pressure drop/concentration,current density,power density,and various polarization overpotentials in the SCI-SOFC stack and BSI-SOFC stack are compared.The results show that compared with SCI,BSI promotes the gas disturbance in the gas channel and makes the gas flow in the direction perpendicular to the channel,which significantly increases the gas velocity and vorticity,enhances the gas diffusion in the porous electrode.BSI makes the distribution of H2 more uniform in the porous anode and can diffuse sufficient O2 to the cathode-TPB of the anode supporting SOFC stack(either under the channel or under the ribs),eliminating the oxygen-free zone,and providing sufficient O2 for the cathode electrochemical reaction.BSI increases the number of the charge transfer channel between the electrode and the interconnector and shortens the charge transfer path in the porous electrode,which almost eliminates the adverse effect of the electrode-interconnector contact resistance.Finally,BSI significantly reduces the activation,concentration,and contact overpotential,and improves the electrical performance of the SOFC stack.(3)The microstructural evolution of the Ni-YSZ anode during the long-term operation is studied experimentally under 800℃ and different fuel components.Through segmentation and quantitative analysis of the SEM and EDS images of the porous anode obtained every 100 h,it shows that the average Ni particle diameter gradually increases with the operating time,especially the increase of the steam accelerates the Ni particle coarsening.The YSZ particle size and pore diameter remain almost unchanged during the whole test period.In addition,the volume fraction of Ni,YSZ,and pore fluctuates slightly with the operating time,but the changes are small.Therefore,only the Ni-particle coarsening is considered when studying the microstructural evolution of anode with operating time.In this study,by introducing the operating current into the Ni-particle sintering model obtained by industrial-scale production of hydrogen,an extended Ni particle coarsening model is proposed,and the accuracy of the extended model is verified by experimental data.The model can quantify the effects of different operating conditions(e.g.working temperature,output current density,and steam to carbon)and anode microstructure(e.g.initial particle size,the volume fraction of each phase in the anode)on the particle aggregation.This model is suitable for ternary gases such as H2/H2O/N2 and multi-component gases such as CH4/H2O/H2/CO/CO2/N2.The extended Ni particle coarsening model and the fully coupled numerical model are combined to quantitatively describe the variation of the effective anode-TPB area and the effective conductivity with operating time caused by the Ni particle coarsening.The comprehensive model analyzes the effect of Ni particle coarsening on the long-term operation of the SOFC stack under different operating conditions such as operating temperatures,steam to carbon ratio,or output current density.Finally,by optimizing the microstructure of the porous anode,the electrical performance of the SOFC stack is improved and its degradation rate is reduced at the same time during the long-term operation(4)A methane steam reformer with 20%Ni/20%MgO/Ni foam as a catalyst is designed,and the intrinsic kinetic model of methane steam reforming of the catalyst is developed based on Langmuir theory,and the kinetic parameters are determined by experiments and quantitative analysis of gas composition.An indirect internal reforming(IIR)-SOFC stack is constructed by combining the reformer and the single SOFC stack.Experimental studies on IIR-SOFC stack and direct internal reforming(DIR)-SOFC stack are carried out under different operating conditions,such as operating temperature,inlet fuel-air flow rate,and S/C ratio.The results show that the current density and power density of the two stacks are almost the same.The numerical results show that for the DIR-SOFC stack,the methane reforming reaction mainly occurs on the porous anode near the fuel inlet;while for the IIR-SOFC stack,the strongly endothermic methane steam reforming reaction is completed in the reformer.The reforming reaction rate on SOFC is zero.Therefore,IIR effectively avoids the carbon deposits on the SOFC anode caused by the side reactions in the process of DIR.Ni foam can enhance heat transfer excellently.Through the close contact between the reformer and the SOFC stack,the efficient heat transfer from the SOFC stack to the reformer is realized.Comparing the temperature and temperature gradient distribution of the DIR-SOFC stack and the IIR-SOFC stack,it can be found that IIR reduces the temperature gradient on the cell,makes the temperature distribution more uniform,and significantly improves the ability of the cell to resist cracks and mechanical failure. |
