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Research Papers

Influence of the Temperature on Oxygen Reduction on SOFC Composite Electrodes: Theoretical and Experimental Analysis

[+] Author and Article Information
C. Nicolella

Dipartimento di Ingegneria Chimica,  Università di Pisa, V. Diotisalvi 2, 56126 Pisa, Italy

A. P. Reverberi, P. Carpanese

DICheP,  Università di Genova, P.le Kennedy 1, 16129 Genova, Italy

M. Viviani

 CNR, IENI, V. De Marini 6, 16149 Genova, Italy

A. Barbucci1

DICheP,  Università di Genova, P.le Kennedy 1, 16129 Genova, Italybarbucci@unige.it

1

Corresponding author.

J. Fuel Cell Sci. Technol 5(1), 011011 (Feb 01, 2008) (5 pages) doi:10.1115/1.2786461 History: Received December 01, 2005; Revised January 29, 2007; Published February 01, 2008

Experimental data on half-cells consisting of YSZ electrolyte pellets and slurry-coated cathodes, and a simplified theoretical model were used to give an insight into the kinetics of oxygen reduction in solid electrolyte composite cathodes. Electrochemical impedance spectroscopy and potentiodynamic polarizations were used to evaluate the main electrochemical parameters of the cathodic process in a temperature range between 500°C and 900°C. The experimental results show that the oxygen reaction is not under activation control at low temperatures, and other phenomena, such as the transfer of oxygen ions to and through the solid electrolyte in the composite cathode, occur and retard the overall rate of oxygen reduction. This working hypothesis was assessed using a simplified theoretical model of the cathode that accounts for charge transfer, mass transfer, and conduction. The model simulations compared satisfactorily with the experimental data, and they show that, at low temperatures, the reaction zone in the cathode is confined to the electrolyte interface. When the temperature is increased, the retarding effects of mass transfer and conduction in the electrolyte become negligible, and the reaction zone progressively extends through the electrode.

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Figures

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Figure 1

Three-electrode impedance plot for the cathode in air at 630°C, 720°C, 780°C, and 840°C

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Figure 2

Arrhenius plot, from the slope of the line an apparent activation of 123kJ∕mol is obtained

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Figure 3

Anodic and cathodic polarization IR-free curves of the composite cathode in air at different temperatures

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Figure 4

Ionic conductivity of the YSZ sintered at 1100 for 1h (◼) and 1500°C for 5h (●) as a function of T

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Figure 5

Estimated anodic mass-transfer resistance as a function of temperature

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Figure 6

Overpotential distribution through the electrode determined by the theoretical model considering three resistances (100Ωcm, 1000Ωcm, 10,000Ωcm) of the ionic conductor (ρel=7Ωcm, a=0.5×106m2m−3, i0=100Am−2, itot=3000Am−2)

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Figure 7

Comparison of the polarization resistance Rp obtained from the experimental measurements and the theoretical analysis with and without mass-transfer resistance

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