ICMCTF2003 Session A2: Coatings for use in Fuel Cells, Catalysis, and Membranes
Wednesday, April 30, 2003 1:30 PM in Room Sunset
A2-1 Thin Film Nickel Anodes for Protonic Ceramic Fuel Cells
W. G. Coors (Protonetics International Inc.); D. Zhong, J.J. Moore (Colorado School of Mines)
Protonic ceramic fuel cells are being developed that operate directly on hydrocarbon fuels in the 600 to 800°C temperature range. Traditional approaches used to fabricate electrodes in other solid oxide fuel cell types were abandoned in favor of new methodologies more suitable for high-performance and low cost electrode construction for use with the proton-conducting ceramic electrolytes. Since the oxygen partial pressure is typically low at the anode, nickel and nickel alloys are good candidate materials. In this work, nickel anodes were deposited onto protonic ceramic BaCe0.9Y0.1)3-α specimens using magnetron sputtering. These sputtered nickel anodes have been successfully tested with a variety of hydrocarbon fuels. These anodes, besides being excellent mixed electronic/protonic conductors, also exhibit good electrocatalytic activity and durability in the presence of hydrocarbons, water vapor, and carbon oxides. This paper addresses the electrochemical and mechanical requirements for anodes used with a protonic ceramic fuel cell, and presents recent test data on single cells operated with a variety of fuels. The relationship among process, microstructure and performance will be discussed as well.
A2-2 YSZ Thin Film Coatings using Combustion Chemical Vapor Deposition
Z. Xu, J. Sankar (NSF Center For Advanced Materials And Smart Structures)
Yttria stabilized zirconia (YSZ) is still the most effective of the available electrolyte for high-temperature fuel cells. To act as good oxygen-ion conductor, stabilized zirconia in fluorite structure is preferred to other phases. Thin film electrolyte is highly desired to minimize the current path and hence ohmic loss. New effective processing techniques are required to produce the electrolyte thin film at lower cost. In the past years, thin film YSZ used to be processed with electrochemical vapor deposition and low-pressure metalorganic chemical vapor deposition techniques. In both of techniques, however, the size and shape of the parts to be fabricated are limited by the vacuum reactors. Atmospheric combustion chemical vapor deposition (ACCVD) has been used for deposition of thin films of oxides, such as Al2O3, SiO2, YBCO, etc. ACCVD that works in open air has the advantage of high deposition rate and low operational cost. Moreover, it is a promising technique for conformal deposition on large and non-flat parts. In our research center, YSZ thin films have been successfully synthesized with the ACCVD technique with liquid fuel. Key processing parameters, such as the ratio of oxygen to liquid fuel in the flame, the concentration of metal reagents in the solution, the temperature of the substrate and substrate material, have been investigated. The as-grown films are characterized with X-ray diffraction and scanning electron microscopy. Within the range of experimental parameters, the phase of the film is predominantly of cubic structure. The phase and crystallinity of the films are strongly dependent upon the experimental variables. Ac-impedance experiments are performed on some thin film samples. The correlation between the conductivity and the crystallography of the films and hence the processing parameters are investigated.
A2-3 Effect of Densification Process on the Properties of Plasma-Sprayed Electrolytic ZrO2 coatings
C.-J. Li (Welding Research Institute, School of Mechanical Engineering, PR China); X.-J. Ning, C.-X. Li (Xi'an Jiaotong University, PR China)
Many different processes are used to fabricate yttria-stabilized zirconia (YSZ) coatings for solid oxide fuel cell (SOFC). It is much concerned recently to develop a cost-effective processing method. Plasma spraying process is one promising approach because of its flexibility and high productivity, and consequent low cost. However, the features of pores in the coating limit the gas tightness and ionic conductivity. In the present study a densification process to air plasma-sprayed YSZ coating is conducted to aim at achieving a dense coating of necessary gas-tightness and improved high ionic conductivity at a relative low cost. The deposition of YSZ by plasma spray is carried out at ambient atmosphere with a commercial YSZ powder of 8wt% yttria. The densification of the YSZ coating is performed by impregnating yttrium and zirconium nitrate solution into porous coating following by thermal decomposition of nitrates to YSZ through annealing treatment. The densification process was repeated to investigate the effect of processing times on the gas tightness and ionic conductivity of the YSZ coating. The microstructure of subsequent coating was characterized by scanning electron microscopy, X-ray diffraction and Transmission electron microscopy. It has been found that the gas permeability of the coating under optimized conditions can reach a level comparable to that of fully sintered one. The test cell yielded an OCV of about 1.1V at 930°C. The improvement of the ionic conductivity of the resulted YSZ coating is also discussed with regard to the change of microstructure of the coatings after densification treatment.
A2-4 Process Optimization and Electrochemical Properties of LiMn2O4 Cathode for Rechargeable Batteries
S.R Das, N. Santander, S.B. Majumder, R.S. Katiyar (University of Puerto Rico)
We have compared the phase formation behavior, microstructure evolution and electro-chemical properties of LiMn2O4 powders synthesized by solid-state reaction and sol-gel route. The synthesized powders were annealed in the temperature range of 550oC to 850oC for 1 to 12 hours. On the basis of X-ray diffraction and micro-Raman scattering measurements a time temperature transformation (TTT) diagram has been constructed to identify the evolution of spinel structure as a function of calcinations temperature and time. For LiMn2O4 powder synthesized both by solid-state and sol-gel route, it was found that the lattice parameter and crystallite size increase whereas the lattice strain decreases with the increase in calcinations temperature. The electrochemical properties of these powders were characterized by fabricating a three-electrode electrochemical cell and cyclic voltammetry (CV) and galvanostatic cycling test were performed to optimize the synthesis condition and electrode composition for spinel LiMn2O4 cathode.
A2-6 LaCrO3-Based Coatings on Ferritic Stainless Steel for Intermediate-Temperature SOFC Interconnect Applications
J.H. Zhu, Y. Zhang, Z.G. Lu, A. Basu (Tennessee Technological University); M. Paranthaman, D.F. Lee, E.A. Payzant (Oak Ridge National Laboratory)
The application of chromia-forming ferritic steels as intermmediate-temperature solid oxide fuel cell (SOFC) interconnect must address a number of problems such as Cr migration from the interconnect onto the cathode side and insufficient protectiveness of the thermally grown chromia oxides, which lead to rapid degradation of the cell performance. A thin layer of doped lanthanum chromite on ferritic steel may act as a protective coating to mitigate the Cr volatility problems and facilitate the use of metallic interconnect in solid oxide fuel cells. In this paper, the LaCrO3 thin film was successfully synthesized on a ferritic stainless steel substrate by two novel approaches, i.e., reactive formation and sol-gel processing. Sr dopants were also successfully introduced into LaCrO3 in an attempt to reduce the electrical resistance of the coating. X-ray diffraction and scanning electron microscopy were used to analyze the coating microstructure, surface morphology, and the phase(s) in the coatings. The electrical resistance of the sol-gel processed samples before and after thermal and cyclic oxidation at 800°C in both air and reducing environment was determined from 600°C to 900°C using the four-point DC method. It was found that the (La,Sr)CrO3 coating was effective in reducing the degradation in electrical conduction of the interconnect materials. The two coating processes were compared and their advantages and drawbacks were outlined.
A2-7 Characterization of Pd-Ni Thin Film Coated Electrodes for Electrolytic Cell Applications
G.H. Miley, S.-O. Kim, N. Luo, A.G. Lipson, C. Castano (University of Illinois)
Characterization of thin-film coated electrodes designed for electrolytic cell studies are described. This application is very demanding due to differential stresses created in the films by combined heating and hydrogen loading during cell operation. To do this in a reproducible fashion, pre- and post-run analyses of a variety of test samples were carried out using a several select probe techniques. Various multi-layer Pd-Ni thin films on an alumina oxide substrate were fabricated using with a magnetron sputtering method. Several methods for pre-conditioning of the substrate were experimented with. The samples were also treated by different annealing method using inert gas, air, and vacuum annealing processes. Some samples had an added top coat of Pd or Ni deposited electrolytically on the films. Investigations of samples were performed in an UHV system equipped with scanning electron microscopy (SEM), secondary ion mass spectrometry (SIMS) and x-ray diffraction (XRD). The vacuum annealed samples developed fewer pores, better adhesion and a higher bonding energy than air and inert gas annealed samples. Select top coat samples were superior for reducing micro-cracking. These results and their implications for practical cell design will be discussed.
Work supported by Lattice Energy, LLC.
A2-8 Palladium-silver Composite Membranes by r.f. Magnetron Sputtering for Hydrogen Purification.
R. Checchetto, N. Bazzanella, A. Miotello, B. Patton (Universitát di Trento, Italy)
Thin and dense palladium based films have been deposited on mesopurous stainless steel by r.f. magnetron sputtering for application as composite membrane in hydrogen filtering. Before coating deposition a polymeric layer made of polycarbonate (PC) was deposited on the steel substrate to fill the pores and produce a smooth surface. The adhesion between PC coating and porous steel and between PC coating and Pd based layer was studied by Secondary Electron Microscopy (SEM) and Scratch Test. The performances of the filter have been tested by studying the permeation process of different gases (H2, N2, CO2). The filter show good hydrogen selectivity and mechanical resistance over long time operation. Moreover no interdiffusion was revealed between Pd based coating and steel substrate showing that the PC intermidiate layer properly acts also as diffusive barrier.