| Solid oxide fuel cell （SOFC） is electrochemical power device that can convert thechemical energy of gaseous fuel to electricity directly with high efficiency andpollution-free operation. Anode-supported SOFC has received considerable attention toreduce the ohmic resistance and operating temperature. For the anode-supported SOFC,significant losses may well arise from the concentration polarization due to theresistance to the diffusion of the fuel gas and the by-products through the thick anodesubstrate layer. The electrochemical reaction is decreased with lower operatingtemperature and, as a result, the activation overpotential is significant cause of voltagedrop. In this paper, we devoted to improve the microstructure of anode and interface,aiming at decreasing polarization losses at reduced temperatures.Improving pore structure in anode is ability to reduce concentration polarizationlosses for anode-supported SOFC. Flour is usually used to fabricate porous anode thatcan burn off during heat treatment by leaving irregularly shaped and isolated pores inthe anode. These pores are difficult to connect each other to form continuous andconnected gas transport path, which would limit the fuel gas diffusion in anode. A largeamount of flour should be used to form a continuous gas transport path. This, however,could lead to the decrease in the mechanical strength of the anode. Thus the applicationof fiber with a big advantage in length is considered to improving pore structure inanode. PVA fibers prepared by electrospinning technique generate wire-like pores withthe diameter of1m and the length of10m in anodes via high-temperature treatment.Such pores are good for the formation of the continuous pathways to make hydrogenfluently diffuse to inner of anode and vapour diffuse from inner of anode. Themaximum power density （MPD） of the cell with5wt%ES-PVA fibers is0.75W cm-2,and the short-circuit current density （Js） is2.66A cm-2at800oC, which is larger thanthat of the cell with10wt%flour. However, the diameter of ES-PVA fibers is small thatwould limit the gas diffusion to a certain degree, and the concentration polarization isdecreased only modestly. Additionally the efficiency of production for electrospinningis low, thus the other fibers are considered to prepare to porous anode. The preparingmethod of paper-fibers is simple and the efficiency of production is high. The paper-fibers generate cylindrical pores with the diameter of5m and the length of20-100min anodes via high-temperature treatment. Such pores are good for the formation of thecontinuous pathways for rapid the rate of gas transport. And hydrogen could rapidlydiffuse to inner of anode and vapour diffuse from inner of anode, as a result, theconcentration polarization is significantly decreased. Based on unique morphology offibers, a small amount of paper-fibers could form larger porosity of anode, compared with traditional pore-formers. The MPD and Jsof the cell with10wt%paper-fibers are1.06W cm-2and3.48A cm-2, respectively at800oC. The cylindrical pore is desirablegas transport path by theoretical analysis. The Cellulous are formed by plants inphotosynthesis that is inexhaustible renewable resources. The application of cellulouscould decrease the cost of cell preparation. The cellulous generate cylindrical pores withthe length of20m, and the diameter of pores is as same as the pores formed by paper-fibers.Controlling the orientation of cylindrical pores could further decrease concentrationpolarization. Anode support with the thickness in0.5mm is prepared from directlyuniaxial pressing. The paper-fibers in the anode support are perpendicular to thecompaction axis during the uniaxial pressing, and then cylindrical pores are formed viahigh-temperature treatment. The orientation of cylindrical pores is parallel with surfaceof the anode support. Such cylindrical pores are beneficial to form connected gastransport path, and the path is lank. However, the gas transport path formed by sphericalpores generates necks. Thus, the resistance of gas transport is decreased innercylindrical pores, and the rate of gas transport is enhanced to some extent. However thegas diffusion is limited by the micro-pores between the cylindrical pores, resulting inthe delaying effect. The anode powders are pressed into a rectangular anode brick andorientated paper-fibers are formed. The anode brick is cut in direction of vertical topaper-fibers to obtain anode substrate with desirable orientation of cylindrical pores,which is parallel with the direction of inlet gas flow. The effective diffusion coefficientis increased by such pores by theoretical analysis, and the length of gas transport path isshortened, thus concentration polarization is further decreased. The MPD and Jsof thecell are1.54W cm-2and5.67A cm-2, respectively at800oC.Modifying inside of cylindrical pore could reduce activation polarization. Mixturedirectly and impregnation are usually used to introduce catalyst into anode. The mixturedirectly is simple, but catalyst and YSZ particles coarsen under operating temperature（600-1000oC）. Additionally, the catalyst particle outside of three-phase boundary （TPB）never takes part in electrochemical reaction. Most of catalyst could distribute in TPB byimpregnation, while to attain enough catalyst it is necessary to dip many times. And alarge amount of pores in anode would hinder the electrical connection, resulting inconductivity of anode is decreased. Bathing paper-fibers in nitrate solution of catalystcould introduce catalyst into inside of anode. The method of bath is simple. Somecatalyst particles are distributed in the middle of pores, which could continue electricalconnection of anode. Some catalyst particles are distributed in inner wall of cylindricalpores, which could extend the length of TPB. On the other hand, particles （NiO andYSZ） become round by bathing, which could extend electrochemical reaction and reduce activation polarization.Enhancing electrode/electrolyte interfaces could reduce activation polarization.Controlling interfacial geometry is ability to reduce polarization losses becauseelectrochemical reactions primarily occur at TPB regions （10μm in thickness） in theproximity of electrodes/electrolyte interfaces for electrodes made of Ni/YSZ andLSM/YSZ. Anode function layer （AFL） is usually fabricated to improveanode/electrolyte interface and maximize the length of TPB. While a decrease ofcathode resistance is achievable by roughening electrolyte surface. However,roughening electrolyte and preparation of AFL may make the process of cell preparationcomplicated. The dense electrolyte film with the rough surfaces was fabricatedsuccessfully by using the dual-sized YSZ powders via slurry spin coating techniquewithout additional effort to roughen electrolyte. The rough surface could remedy theanodic surface with the holes, which improves the adhesion of electrolyte film to anodesubstrate significantly and extend electrochemical reaction regions. The ragged surfaceof electrolyte film increases the effective electrolyte surface area, resulting in morecontact points are provided for adhesion of LSM particles and thereby the length of TPBis extended evidently. Ultimately, activation polarization is reduced evidently.In summary, enhancing pore structure could accelerate rate of gas transport, which isfurther improved by controlling the orientation of pores, and thus concentrationpolarization is reduced. And electrochemical reaction regions are extended bymodifying inside of pores, resulting in decreased activation polarization. Enhancing theelectrode/electrolyte interfaces also increases electrochemical reaction regions, and inturn activation polarization is decreased. Polarization losses could be reduced by abovemethods, resulting in cell performance is enhanced significantly. |
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