Preparation And Performance Of Anode Materials Ni-SDC And Ni-GDC For Solid Oxide Fuel Cells

Fuel cells can be viewed as devices for electrochemically convertingchemical fuels into electricity,essentially batteries with external fuelsupplies.They offer extremely high chemical –to-electrical conversionefficiencies due to the absence of the Carnot limitation,and further energygains can be achieved when produced heat is used in combined heat andpower,or gas turbine applications.Furthermore,the technology does notproduce significant amounts of pollutants such as nitrogen oxides,especiallycompared with internal combustion engines. The fuel-cell telechnology willgain a significant share of the electrical power market.As the fourth generation fuel cell, SOFC (Solid Oxide Fuel Cell) hasmany outstanding advantages, which is better than other fuel cells. Firstly,equipped with all solid components, it eliminates the problems that liquidelectrolyte fuel cell faces, such as corrosion and leakage of liquidelectrolytes. Secondly, operating at high temperatures, its electrode reactionis so fast that it is unnecessary to use noble metals as electrodes. Thus thecost of the cells can be minimized. At the same time, the high quality heat itemits can be fully used. The overall energy conversion efficiency of thethermal-electric system can be added up to 80%. The most outstandingadvantage of SOFC is that it uses a large scale of fuels, from the hydrogen,carbon monoxide to the natural gas or even other combustive gases.Currently the main difficulty that the SOFC faces is the problem caused byhigh temperature and the ceramic components' match.Decreasing the operating temperature of SOFCs down to intermediaterange(500～700℃),which is desirable due to important economical andtechnological advantages,requires optimization of the anode compositionand microstructure in order to achieve sufficiently high performance in theintermediate temperaturature range.Recently there are many reports on the high temperature anodeNi-YSZ,but a little reports on the intermediate temperature anodeNi-Ce0.85Sm0.15O2-δ(SDC) and Ni-Ce0.9Gd0.1O1.95(GDC).In this study,themicrostructre has been tuned by adjusting the grain size to investigate theeffect of microstructure on electrochemical performance of these two kindsof anode.The effect of Ni content on the performance of Ni-SDC has alsobeen studied.(1)Ni-SDC is a kind of anode used in intermediate temperaturerange.A series of anodes have been fabricated by mechanically mixedmethod with NiO and Ce0.85Sm0.15O2-δ(SDC) powders ,each of which hasthe different grain size ,to investigate the effect of microstructure onelectrochemical performance of Ni-SDC anodes.The weight ratio of NiO is65%.The overpotentials of the anode and the conductivities aremeasured.The results show that the anode performance denpend strongly onthe anode microstructure,which in turn is determined by the anodecompositions and fabrication conditions.Two kinds of powders prepared byglycine-nitrate process and sintered at 600℃ are mechanically mixed tofabricate the anode which has excellent eletrochemic performance. Theconductivity is 3534 Scm-1 and the overpotential is 0.18 V while currentdensity is 0.5 Acm-2 under hydrogen atmosphere at 600 ℃.(2)Ni-GDC composite cermets have been widely used as an anodematerial. Four types of anodes have been fabricated by mechanically mixedmethod with two kinds of NiO and Ce0.9Gd0.1O2-δ(GDC) powders toinvestigate the effect of microstructure on electrochemical performance ofNi-SDC anodes.The weight ratio of NiO is 65%.The overpotentials of theanode and the conductivities are measured.The results show that the anodeperformance denpend strongly on the anode microstructure,which in turn isdetermined by the anode compositions and fabrication conditions.Twokinds of powders prepared by glycine-nitrate process and sintered at 600℃are mechanical mixed to fabricate the anode which has excellenteletrochemic performance. The conductivity is 1654Scm-1,and theoverpotential is 0.18 V while current density is 0.17 Acm-2 under hydrogenatmosphere at 600 ℃.(3)According to the results of microstructure study of Ni-SDC,several Ni-SDC composite anode with different Ni contents weresynthesized by mechanically mixed method with NiO and SDCpowders,which are two kinds of powders prepared by glycine-nitrateprocess and sintered at 600 ℃ for 5h. The results of investigation show thatthe composite powder are mixed conductors with oxide ion and electronhole before reducing, and becoming an excellent electron conductors afterreducing;the conductivity of anode materials strongly depend on thecontent of Ni;its relation of the conductivity and Ni presents "S" trend.Based on the theory of percolation, it can be demonstrated that two kinds ofelectric mechanism are coexistent, namely the electronic channel of the Niphase and the ionic channel of the SDC phase. When Ni content is around60wt.%,The highest performance were achieved for the composite anodematerials.The percolation threshold is 30wt% Ni.The conductivity of theanode containing 30wt% Ni is 779 Scm-1 under hydrogen atmosphere at600℃.