Diffusion Limitations in the Porous Anodes of SOFCs

Concentration polarization is important because it determines the maximum power output of a solid oxide fuel cell (SOFC) at high fuel utilization. Anodic concentration polarization occurs when the demand for reactants exceeds the capacity of the porous ceramic anode to supply them by gas diffusion mechanisms. Many models simulate this behavior by assuming an anomolous high value for the tortuosity (eg, t=17), a measure of the bulk diffusional resistance for a porous ceramic. However, recent experiments at several laboratories, including results reported herein, have provided strong evidence that typical sintered powder ceramics (30-50% porosity) have much lower tortuosities (t=2.5-3), indicating that the bulk diffusional resistance is too small to be responsible for concentration polarization. We find evidence that concentration polarization originates in the immediate vicinity of the reactive sites near the anode/electrolyte interface, at the triple phase boundaries (TPBs) between the Ni catalyst particles, the gas, and the oxygen conducting YSZ ceramic. A model is proposed to describe how concentration polarization is controlled by two localized phenomena: competitive adsorption of reactants in areas adjacent to the reactive TPB sites, followed by relatively slow surface diffusion to the reactive sites. The model parameters (adsorption activation energy and surface diffusion coefficients) were more » determined by fitting to well-characterized SOFC voltage-current performance data, and are in good agreement with data from the literature. Results suggest that future SOFC design improvements should focus on optimization of the reactive area, adsorption, and surface diffusion at the anode/electrolyte interface, rather than on anode thicknesses or bulk porosities. « less

[1]  Michael A. Henderson,et al.  The Interaction of Water with Solid Surfaces: Fundamental Aspects Revisited , 2002 .

[2]  Mogens Bjerg Mogensen,et al.  Kinetic and geometric aspects of solid oxide fuel cell electrodes , 1996 .

[3]  Motoaki Tajika,et al.  Kinetics of the electrode reaction at the H[sub 2]-H[sub 2]O porous Pt/stabilized zirconia interface , 1994 .

[4]  S. Jiang,et al.  An electrode kinetics study of H2 oxidation on Ni/Y2O3–ZrO2 cermet electrode of the solid oxide fuel cell , 1999 .

[5]  A. Hamza,et al.  Dynamics of the dissociative adsorption of hydrogen on nickel(100) , 1985 .

[6]  N. Bonanos,et al.  Effect of electrode material on the oxidation of H2 at the metal–Sr0.995Ce0.95Y0.05O2.970 interface , 2000 .

[7]  R. Krishna,et al.  The Maxwell-Stefan approach to mass transfer , 1997 .

[8]  R. Krishna.  Problems and pitfalls in the use of the fick formulation for intraparticle diffusion , 1993 .

[9]  B. C. Garrett,et al.  Embedded diatomics-in-molecules: a method to include delocalized electronic interactions in the treatment of covalent chemical reactions at metal surfaces , 1989 .

[10]  Klaus Christmann,et al.  Interaction of hydrogen with solid surfaces , 1988 .

[11]  I. Vinke,et al.  Reaction of hydrogen/water mixtures on nickel-zirconia cermet electrodes. II. AC polarization characteristics , 1999 .

[12]  Mogens Bjerg Mogensen,et al.  Structure/Performance Relations for Ni/Yttria‐Stabilized Zirconia Anodes for Solid Oxide Fuel Cells , 2000 .

[13]  H. Verweij,et al.  The effect of the presence of fine YSZ particles on the performance of porous nickel electrodes , 2000 .

[14]  Werner Lehnert,et al.  Modelling of gas transport phenomena in SOFC anodes , 2000 .

[15]  Mogens Bjerg Mogensen,et al.  Progress in understanding SOFC electrodes , 2002 .

[16]  Rajamani Krishna,et al.  Multicomponent surface diffusion of adsorbed species: a description based on the generalized Maxwell—Stefan equations , 1990 .

[17]  K. Sasaki,et al.  Characterization of adsorbed water layers on Y2O3-doped ZrO2 , 2001 .

[18]  Manabu Ihara,et al.  Competitive adsorption reaction mechanism of Ni/Yttria-stabilized zirconia cermet anodes in H2-H2O solid oxide fuel cells , 2001 .

[19]  A. Adamson.  Physical chemistry of surfaces , 1960 .

[20]  Kerry D. Meinhardt,et al.  Optimized Lanthanum Ferrite-Based Cathodes for Anode-Supported SOFCs , 2002 .

[21]  Tohru Yamamoto,et al.  Configurational and Electrical Behavior of Ni‐YSZ Cermet with Novel Microstructure for Solid Oxide Fuel Cell Anodes , 1997 .

[22]  Alain Vignes,et al.  Diffusion in Binary Solutions. Variation of Diffusion Coefficient with Composition , 1966 .

[23]  Mogens Bjerg Mogensen,et al.  Microstructural and chemical changes at the Ni/YSZ interface , 2001 .

[24]  H. Livbjerg,et al.  Models of pore diffusion in porous catalysts , 1998 .

[25]  L. Gauckler,et al.  The Electrochemistry of Ni Pattern Anodes Used as Solid Oxide Fuel Cell Model Electrodes , 2001 .

[26]  C. Satterfield,et al.  Gaseous Diffusion and Flow in Commercial Catalysts at Pressure Levels above Atmospheric , 1968 .

[27]  L. Gauckler,et al.  State-space modeling of the anodic SOFC system Ni, H2–H2O∣YSZ , 2002 .

[28]  C. G. Pope.  Flow of sulphur dioxide over the surface of Spheron 6(2700) graphitized carbon black , 1967 .

[29]  L. Gauckler,et al.  Reaction mechanism of Ni pattern anodes for solid oxide fuel cells , 2000 .

[30]  S. Bhatia.  Combined surface and pore volume diffusion in porous media , 1988 .

[31]  Masamichi Ippommatsu,et al.  Preparation of Nickel Pattern Electrodes on YSZ and Their Electrochemical Properties in  H 2 ‐  H 2 O  Atmospheres , 1994 .

[32]  S. Singhal,et al.  Polarization Effects in Intermediate Temperature, Anode‐Supported Solid Oxide Fuel Cells , 1999 .

[33]  Paola Costamagna,et al.  Modeling of Solid Oxide Heat Exchanger Integrated Stacks and Simulation at High Fuel Utilization , 1998 .

[34]  E. Seebauer,et al.  Estimating surface diffusion coefficients , 1995 .

[35]  W. Lehnert,et al.  Structural properties of SOFC anodes and reactivity , 1998 .