Preparation And Evaluation Of Nickel-based Anode For Solid Oxide Fuel Cell

The micro-structure of the anode for solid oxide fuel cell (SOFC) determines theanode’s performance at a large extent. Therefore, the controllable preparation of theanode is a very important issue. As is all known, impregnation is an effective method tooptimize or fabricate the Ni/YSZ composite anode. The Ni/YSZ anode prepared byimpregnation has a unique micro-structure, which can be changed by adjusting theparameters of the impregnation process.In this thesis, the impregnation process is controlled to modify the structure of theNiO/YSZ anode, focusing on the nature of the NiO particles introduced via theimpregnation method. To accurately observe the morphology of NiO, dense YSZ pelletsare fabricated as the substrates to simulate porous YSZ skeletons. A drop of a Ni(NO3)2solutions was placed onto a pellet, following different treatments. Firstly, theimpregnated pellets are sintered at1400℃. The morphology is characterized byscanning electron microscopy (SEM). The results indicate that the impregnation NiOparticles deposit on the triple junctions consist of YSZ grains at first. Then the particlesdistribute along the YSZ grain boundaries. The neighboring tiny NiO particles tend tosinter into blocks just like several islands. With the increasing amount of NiO, the NiOparticles will form films layer upon layer. Secondly, the pellets are heated or baked at100,300,500,700,1000,1250,1300, and1400℃. The SEM images demonstrate thatthe morphology of NiO particle film on YSZ substrate surface change at differenttemperatures, and the grains grow in the wake of temperature. At300℃, the films ofNi(NO3)2start to break and nano NiO grains can be distinguished at500℃. We canconclude that Ni(NO3)2decomposes between300℃and500℃. When the impregnatedpellets are heated at700℃, uniform NiO particles willcover the surface of thesubstrates. Besides, the activities of migration, agglomeration and sintering of NiOparticles take place at a temperature above1000℃. The Anelli model shows themechanism follow which NiO particles grow. The sintering activation energy is about7.6×103kJ/mol, and the calculated sintering temperature is915℃, close to thetheoretical Taman temperature855℃. These details prove that the model is correct.H2is used to reduce the NiO particles sintered at1400℃, and the SEM images show thatthe micro Ni particles are porous after reduction due to loss of oxygen atoms. Ifsupplied of CH4, the Ni particles will be wrapped by carbon particles, while carbonfibers emerge on the surfaces of Ni by baking on C2H5OH flames.The concentration and composition of the Ni(NO3)2solutions can also impact theimpregnation. Ni(NO3)2solutions of2mol/L and0.2mol/L are used for impregnation inthis research. We find that higher concentration of solutions generate larger NiO particles, and the distribution of NiO is much more nonuniform. Another interestingphenomenon existing in the impregnation progress is that Ni(NO3)2would crystallizeprior to the decomposition. The crystallization will also affect the distribution of NiOparticles. This research takes advantage of this phenomenon, which take place at alower temperature to control the micro-structure of the anodes. A nitrate solution quenchmethod is introduced, and forms acicular NiO crystals in porous YSZ backbone. Themethod shortens the impregnation cycle by40%, and the specific surface area is raisedby14%. This is beneficial for the catalytic Ni particles to adsorb the fuel gas and react.Nevertheless, most of the NiO particles would deposit on the surface and block off thepores, resulting in relatively lower electrical conductivity. Results indicate that thesurface tension of Ni(NO3)2can be reduced obviously by adding C2H5OH, along withmodified distribution of NiO. And the addition of CO(NH2)2can also obtain uniformdistribution by pre-precipitation in the YSZ holes. In return, the improved structure canenhance the stability of the anodes. At700℃, the conductivity loss of the anodesreduces from44.4%to32%and29.4%, respectively, after exposure in H2for10hours.And owing to the better adhesion and uniformity, the anodes show better catalyticproperties. The polarization resistances are reduced to2.83and2.50·cm2from3.99·cm2, respectively, for the adding of C2H5OH and CO(NH2)2.The NiO/YSZ anode must be reduced before the operation. However, this processwill result in the sintering of Ni and NiO particles. Reduction and sintering both affectthe performance, and their speeds are decided by the reduction temperature in return.This thesis establishes a model to describe the competitive action in the reductionprocess. The changes of the performances of the NiO/YSZ anode under differentreduction temperatures are tested. Results show that the electrical conductivity, thesurface area and electrochemical performance of the anode canbe improved if thereduction temperature increases within600℃and700℃.