Research Of Nano-sized Ni/YSZ Anode Prepared By Impregnation Technology

Solid oxide fuel cell （SOFC）, as a kind of cleaning, high efficiency fuel cell,is very promising for the next generation power sources because of its efficiencyof up to80%. However, there are still several obstacles need to be overcomebefore its real application such as high temperaute operating, high internalresistance and low power density at reduced temperatures. In order to resolvethese problems, the following two aspects need to be considered carefully: on onehand, developing new electrolytes with higher ionic conductivity and electrodematerials with higher electrochemical activity at reduced temperatures.On theother hand, changing preparation method to improve the microstructure ofelectrolyte and electrodes and thus to improve the performance. Generally, theabove-mentioned approaches are combined to get optimized performance.This paper focus on fabrication of nanometer Ni/YSZ anode withimpregnation method, which could improve anode performance b y changingtraditional anode micro-structure.. At first the sample mixing of YSZ andpore-forming material was sintered at1400℃to get porous-YSZ skeleton, then itwas impregnated with an mixing aqueous solution of Ni（NO3）2·6H2O and urea bycapillary force, finally, it was fired and reduced blow1000℃to form Ni/YSZanode. The Ni was introduced to anode after the high temperature sintering of YSZskeleton via impregnation. By this way, we can get anode with nano-sized Niactive particles through low temperature procedure. The nano-sized Ni particlesmean an enlarged TPB length. However, the smaller Ni grains increase thesurface energy, decrease the anode stability. So it’s necessary to study anodeperformance and stability under different preparation conditions.Firstly, we study anodes performance produced by different NiO content. Theelectrical conductivity of anodes display that percolation limits of Ni is between5-6vol%. When NiO content increases to10vol%, the electrical conductivity valueincreases significantly Further increasing of Ni content to10to15vol%results ina best conductivity. SEM results show that the shape of NiO grains is polygon, andthe reduced Ni grains approach to sphere The Ni particles tends to grow firstlyalong the YS grain boundary. In the anode of8vol%NiO content, NiO grains can’tcover completely YSZ surface, the reduced Ni grains average size is89nm. Whilethe NiO content increases to12.5vol%and15vol%, the YSZ surface is totallycoverd by NiO particles. In the anode of12.5vol%NiO content, the Ni grainsdistribute uniformly and the average size is143nm. However, the other anode’s Nigrains agglomeration seriously and the average size is367nm. The cell output performances show that at800℃the maximum power density is95mW cm^-2,278mW cm^-2and283mW cm^-2in the cells of NiO content of10vol%,12.5vol%,15vol%, respectively.After keeping at800℃for3.5h, the cells were test againand the result reveals that the cell output performance stability decreases as theincrease of NiO content. The results indicate that the anode with12.5vol%NiOshows better cell stability.This part mainly studied NiO grains growth in the condition of400-800℃，1-10h. The result reveals that the temperature is the primary factor whichinfluences the NiO grains growth, while the holding time is less important for thegrain growth at the low sinter temperature. The growth rate of NiO particlesincreases with the increasing of sintering temperatures. Combined with the graingrowth rate equation and the particle size data, we can get the growth activationenergy of NiO.TNext we test the electrical conductivity of the anode produced bydifferent firing process, and find that when the sinter process set as700℃for1h,the anode initial conductivity is307.6S/cm, after10h the value decreaces to101.1S/cm, and the anode conductivity under this sinter process is better thanother process.At the last part, the effect of pore-former on the performance of anode wasstudied. The results show that p-YSZ derived from cassava powder is helpful toget uniform NiO distribution while it is somewhat difficult to impregnate this kindof anode.On the other hand, the p-YSZ produced by flour is opposite–it’s easy toimpregnate but shows less uniform NiO distribution. It is reasonable to combinethe advantage of these two pore-forming materials to produce anode by directlymixing cassava powder and flour as pore-former or layer forming pore. Theresearch result reveals that the cell maximum power density produced by layeranode is400mW cm^-2, which is better than the performances derived from solepore former.