Anil V. Virkar

The role of electrode microstructure on activation and concentration polarizations in solid oxide fuel cells

Abstract Activation and concentration polarization effects in anode-supported solid oxide fuel cells (SOFC) were examined. The anode and the cathode consisted respectively of porous, composite, contiguous mixtures of Ni+yttria-stabilized zirconia (YSZ) and Sr-doped LaMnO 3 (LSM)+YSZ. The composite electrode provides parallel paths for oxygen ions (through YSZ), electrons (through the electronic conductor; Ni for the anode and LSM for the cathode), and gaseous species (through the pores) and thereby substantially decreases the activation polarization. The composite electrode effectively spreads the charge transfer reaction from the electrolyte/electrode interface into the electrode. At low current densities where the activation polarization can be approximated as being ohmic, an effective charge transfer resistance, R ct eff , is defined in terms of various parameters, including the intrinsic charge transfer resistance, R ct , which is a characteristic of the electrocatalyst/electrolyte pair (e.g. LSM/YSZ), and the electrode thickness. It is shown that the R ct eff attains an asymptotic value at large electrode thicknesses. The limiting value of R ct eff can be either lower or higher than R ct depending upon the magnitudes of the ionic conductivity, σ i , of the composite electrode, the intrinsic charge transfer resistance, R ct , and the grain size of the electrode. For an R ct of 1.2 Ωcm 2 , σ i of 0.02 S/cm and an electrode grain size of 2 μm, the limiting value of R ct eff is 0.14 Ωcm 2 indicating almost an order of magnitude decrease in activation polarization. The experimental measurements on the cell resistance of anode-supported cells as a function of the cathode thickness are in accord with the theoretical model. The concentration polarization is analyzed by taking into account gas transport through porous electrodes. It is shown that the voltage, V , vs. current density, i , traces should be nonlinear and in anode-supported cells, the initial concave up curvature (d 2 V /d i 2 ≥0) has its origin in both activation and concentration polarizations. The experimental results are consistent with the theoretical model.

Fuel Composition and Diluent Effect on Gas Transport and Performance of Anode-Supported SOFCs

Anode-supported solid oxide fuel cells (SOFCs) with Ni+yttria-stabilized zirconia (YSZ) anode, YSZ-samaria-doped ceria (SDC) bilayer electrolyte, and Sr-doped LaCoO 3 (LSC)+SDC cathode were fabricated. Fuel used consistedof H 2 diluted with He. N 2 , H 2 O, or CO 2 , mixtures of H 2 and CO, and mixtures of CO and CO 2 . Cell performance was measured at 800°C with the above-mentioned fuel gas mixtures and air as oxidant. For a given concentration of the diluent, cell performance was higher with He as the diluent than with N 2 as the diluent. Mass transport through porous Ni-YSZ anode for H 2 -H 2 O, CO-CO 2 binary systems. and H 2 -H 2 O-diluent gas ternary systems was analyzed using multicomponent gas diffusion theory. At high concentrations of diluent, the maximum achievable current density was limited by the anodic concentration polarization. From this measured limiting current density, the corresponding effective gas diffusivity was estimated. Highest effective diffusivity was estimated for fuel gas mixtures containing H 2 -H 2 O-He mixtures (∼0.55 cm 2 /s), and the lowest for CO-CO 2 mixtures (∼0.07 cm 2 /s). The lowest performance was observed with CO-CO 2 mixture as a fuel, which in part was attributed to the lowest effective diffusivity of the fuels tested and higher activation polarization.

Stability of BaCeO3‐Based Proton Conductors in Water‐Containing Atmospheres

The stability of Gd‐doped and La‐doped in water‐containing atmospheres was examined. Samples containing up to 20 mol % Gd and 30 mol % La were synthesized by calcining the requisite mixtures of precursors. Three types of tests were performed: (i) thermal treatment in an atmosphere containing water vapor between 150 and 400°C, (ii) boiling in water, and (iii) simultaneous exposure to boiling water and water vapor at essentially the same temperature. Samples were characterized by X‐ray diffraction. All powders boiled in water decomposed within a few hours, whereas none of the powders decomposed when exposed to ~1 atm water vapor pressure at the same temperature. This difference was rationalized on the premise that the mechanisms of decomposition depend upon the form of water, liquid or vapor. In liquid water, the decomposition kinetics appear to be interface‐controlled. By contrast, in water vapor, the kinetics appear to be dictated by bulk diffusion of water into the lattice.

Fabrication and characterization of SiC-AIN alloys

SiC-AIN alloys were prepared by the carbothermal reduction of silica and alumina, derived from an intimate mixture of silica, aluminium chloride and starch. The resulting single-phase SiC-AIN powder was hot-pressed without additives to a high density. The dense bodies had a fine-grained uniform microstructure. The Youngs elastic modulus, microhardness, fracture toughness, thermal expansion and thermal conductivity were measured as functions of composition. The creep behaviour of the SiC-AIN alloy was compared with that of silicon carbide.

Liquid phase sintering of silicon carbide

Abstract It is shown that the decomposition reactions during the sintering of liquid phase silicon carbide (SiC) can be described well by thermodynamics. This allows for an optimization of the sintering parameters. The use of carbon as a sintering additive, together with, for instance, yttria plus alumina, is of advantage. When C is used, SiO 2 will not occur in the liquid phase during sintering or in the amorphous and crystalline phases after sintering. The microstructure of sintered samples is described.Liquid phase sintering is used to densify silicon carbide based ceramics using a compound comprising a rare earth oxide and aluminum oxide to form liquids at temperatures in excess of 1600° C. The resulting sintered ceramic body has a density greater than 95% of its theoretical density and hardness in excess of 23 GPa. Boron and carbon are not needed to promote densification and silicon carbide powder with an average particle size of greater than one micron can be densified via the liquid phase process. The sintered ceramic bodies made by the present invention are fine grained and have secondary phases resulting from the liquid phase.

A High Performance, Anode-Supported Solid Oxide Fuel Cell Operating on Direct Alcohol

Anode-supported solid oxide fuel cells with a thin film of yttria-stabilized zirconia (YSZ) as the electrolyte were fabricated. The cells were operated directly on pure methanol and on an equivolume mixture of ethanol and water over a range of temperatures. Power density achieved with methanol was between 0.6 W/cm 2 at 650°C and 1.3 W/cm 2 at 800°C, and with ethanol + water between 0.3 W/cm 2 at 650°C and 0.8 W/cm 2 at 800°C. Results were compared with tests on humidified hydrogen as a fuel. No carbon deposition on the Ni-YSZ anode was observed with either methanol or an equivolume solution of ethanol and water as fuels. Differences in performance with different fuels were attributed to differences in anode polarization.

Instability of BaCeO3 in H2O-containing atmospheres

Rare earth oxide doped BaCeO 3 is known to exhibit high protonic conductivity in the temperature range 500 to 900°C and is a potential electrolyte for use in hydrogen sensing and fuel cell applications. Prior work, however, has shown that BaCeO 3 may be thermodynamically unstable at low temperatures. In the present work, the stability of BaCe0 3 in an H 2 O vapor containing environment was investigated by exposing powder and sintered samples to ∼430 Torr H 2 O in the temperature range 500 to 900°C. All BaCeO 3 samples decomposed into CeO 2 and Ba(OH) 2 in relatively short periods of time at temperatures less than 900°C. Doped BaCeO 3 decomposed at a faster rate than the undoped BaCeO 3 . Sintered BaCeO 3 decomposed at a rate comparable to the powder samples. These results establish that BaCeO 3 is thermodynamically unstable when a critical H 2 O vapor pressure is exceeded and that the rapid decomposition of both powder and sintered samples is the result of the high solubility of H 2 O in BaCeO 3 which accelerates the kinetics of decomposition.

Thermodynamic Stabilities of SrCeO3 and BaCeO3 Using a Molten Salt Method and Galvanic Cells

SrCeO 3 and BaCeO 3 , which are known to be excellent proton conductors, are potential candidates as electrolytes in hydrogen concentrators and fuel cells. The anticipated application temperature is in the range of ∼500 to ∼1000 o C. Typical synthesis/densification temperature of these cerates using conventional processing methods is ∼1400 to 1600 o C. The oblective of the present work was to determine thermodynamic stabilities of SrCeO 3 and BaCeO 3 with respect to the individual oxides SrO and CeO 2 , and BaO and CeO 2 in the anticipated application temperature regime. Two approaches were selected: the molten salt method and galvanic cells

Theoretical Analysis of Solid Oxide Fuel Cells with Two‐Layer, Composite Electrolytes: Electrolyte Stability

In this paper theoretical analysis of solid oxide fuel cells (SOFCs) using two-layer, composite electrolytes consisting of a solid electrolyte of a significantly higher conductivity compared to zirconia (such as ceria or bismuth oxide) with a thin layer of zirconia or thoria on the fuel side is presented. Electrochemical transport in the two layer, composite electrolytes is examined by taking both ionic and electronic fluxes into account. Similar to most electrochemical transport phenomena, it is assumed that local equilibrium prevails. An equivalent circuit approach is used to estimate the partial pressure of oxygen at the interface. It is shown that thermodynamic stability of the electrolyte (ceria or bismuth oxide) depends upon the transport characteristics of the composite electrolyte, in particular the electronic conductivity of the air-side part of the electrolyte. For example, the greater the electronic conductivity of the air-side part of the electrolyte, the greater is the interface partial pressure of oxygen and the greater is the thermodynamic stability. The analysis shows that it would be advantageous to use composite electrolytes instead of all-zirconia electrolytes, thus making low-temperature ({approximately}600-800{degrees}C) SOFCs feasible. Implications of the analysis from the standpoint of the desired characteristics of SOFC components are discussed.

Suppression of Sr surface segregation in La1−xSrxCo1−yFeyO3−δ: a first principles study

Based on systematic first principles calculations, we investigate Sr surface segregation (SSS) in La(1-x)Sr(x)Co(1-y)Fe(y)O(3-δ) (LSCF) (a typical perovskite ABO(3) compound), a bottleneck causing efficiency degradation of solid oxide fuel cells. We identify two basic thermodynamic driving forces for SSS and suggest two possible ways to suppress SSS: applying compressive strain and reducing surface charge. We show that compressive strain can be applied through doping of larger elements and surface coating; surface charge can be reduced through doping of higher-valence elements in the Sr- and B-site or lower-valence elements in the La-site and introducing surface A-site vacancies. The net effect of oxygen vacancy is to enhance SSS because its effect of increasing surface charge overrides its effect of inducing compressive strain, while Co substitution of Fe always enhances SSS because it induces tensile strain as well as increases surface charge. Our results explain the recent experimental observation of SSS suppression in LSCF by a La(1-x)Sr(x)MnO(3-δ) (LSM) coating.