Stability Research Of Nanometer Ni/YSZ Anode Prepared By Impregnation Technology

Solid oxide fuel cells (SOFCs), as a new energy systems, are very promising.However, there are still several obstacles need to be overcomed before its realapplication such as high cost and long-time stability. Reducing the operatingtemperature of SOFCs to resolve these problems come into notice: on one hand, itcan reduce the cost by chose cheap material as low operating temperature canbroaden the range of materials. On the other hand, low operating temperature canalso improve the long-time stability by reduce the element diffusion betweendifferent component and interface reaction in SOFCs. But low operating temperatureresult in the decrease of electrode chemical activity, polarization resistance becomethe key factor of the SOFCs output performance. Therefore, the anode with highchemical activity and stability have great significance in commercialization processof SOFCs.Impregnation method is an effective way to get high efficiency anode at lowtemperature, which can bring nanometer Ni into porous-YSZ skeleton. It greatlyexpanded the TPB in the anode area, to improve the activity of anode by changingmicrostructure. But nanometer anode also faces a huge challenge, nanometer Niparticle can result in a higher surface area,which means an enlarged TPB. On theother hand，however，the Ni particle with reduced size tend to sintering together inSOFC operating temperature, which greatly affected the stability of anode. In orderto achieve better anode performance, the impregnation parameters need to beoptimized, it’s necessary to study anode performance and long-time stability underdifferent preparation conditions, and eventually obtain reliable performance ofnanometer anode.This work found that the impregnation process parameters have significantinfluence on the micro-structure and performance of nano-Ni/YSZ anode. Firstly,we focus on the effect of different calcination temperature,and the result indicatedthat after400℃baking for30min,the nitrate can be completely resolved into NiO.As one side of anode is covered with electrolyte membrane and cathode bilayer，theimpregnation process is conducted on the other side of the anode. It’s important toconsider the influence of impregnating just on one anode surface. The XRD resultsshow that difference on both sides between the peak of NiO and YSZ which issupposed to as skeleton are less than3.4%,indicated that single-sided dipping bycapillary forces can ensure the diffusion of active component into the porousstructure. Different pore structure would significantly affect impregnating, the workfound that when the ratio of YSZ and Pore formers is lower than5:3, porosity significantly improved with increasing pore formers. Pores prepared from flour arethrough holes with higher porosity, with10%higher porosity than cassava. Theconductivity result at800℃with NiO content of19wt%shows that theconductivity of anode prepared from flour is19S/cm,which is9times than oneprepared from cassava. So flour will help improve the uniformity of impregnation.The anode electrical conductivity shows that samples prepared from cassava powderas pore formers is more stable.For example, when NiO content is21wt%, therecession of conductivity in6000s of anode prepared from flour is46%, whilecassava sample is33%. In addition,urea is used to improve the distribution ofparticles,the concentration of urea will also affect the impregnation. Anode electricalconductivity results with different NiO content show that average decline rate is30%and29%, respectively.But initial value of conductivity is relatively high whenthe concentration is4mol/L.It shows that concentrations of4mol/L is more effectiveto improve the conductivity when compared to1mol/L. Anodes performanceproduced by different NiO content is also studied to find an optimal NiO contentrange. The conductivity results show that the NiO content threshold of formingelectrical conductive network is between10wt%to19wt%. When NiO contentincreases to25wt%, conductivity of nano-Ni/YSZ anode decline qucikly. Suitablecontent for impregnation of the electronic conductive phase is between21wt%to27wt%. Considering with the output performance of single cell at800℃,theMaximum power density（MPD）are194mW cm-2and415mW cm-2when the NiOcontent are19wt%and21wt%. When the content increased to25wt%，the MPD is295mW cm-2. The results indicate that the anode with21wt%of NiO shows bettercell performance.NiO grains growth under different annealing process of500℃~900℃，1h~9hare studied,at low temperature, the annealing temperature is the key factor of theNiO grain growth. With annealing temperature increasing, annealing timesignificantly affect NiO grain growth. Combined with average size of NiO grain indifferent annealing process，we can get the activation energy of NiO graingrowth(Q), Q1=48.75kJ/mol in the temperature range between500℃and800℃，growth takes place mainly in grain boundary migration,and Q2=336.64kJ/mol in thetemperature range between800℃and900℃. We also get rate equations of NiOgrain growth at different temperatures. Different reduction temperature significantlyaffects the anode microstructure and its performance. The anode electrical propertiesunder different reduction temperature and annealing process showed that NiO/YSZanodes annealing at500℃for1h, reduce at700℃get better performancecompared with samples prepared with other process. At the last part, as the skeleton of anode prepared by impregnation technology,the particle size of YSZ plays a vital role in the performance of anode. Thecomparison of different commercial YSZ anode supported SOFC output show thatlarger granularity preparation of Tosoh YSZ is better than Jiangxi Fanmeiya-YSZpowder. Study on thermal cycling treatment and constant-current discharge foundthat the effect of polarization resistance is the key factor for thermal stability and theoutput stability of nano-impregnating anode supported SOFC. Also,we found thatthe decrease of particle size of YSZ electrolyte after ball milling will significantlyimprove the open circuit voltage.