Impedance Spectroscopy as a Tool for Chemical and Electrochemical Analysis of Mixed Conductors: A Case Study of Ceria

The AC impedance response of mixed ionic and electronic conductors (MIECs) is derived from first principles and quantitatively compared with experimental data. While the approach is not entirely new, the derivation is provided in a unified and comprehensive manner. Using Sm_(0.15)Ce_(0.85)O_(1.925–δ) with Pt electrodes as a model system, a broad spectrum of electrical and thermodynamic properties is extracted solely from the measurement of impedance spectra over wide oxygen partial pressure and temperature ranges. Here, the oxygen partial pressure was varied from air [po_2=0.21 atm] to H_2 [po_2=10^(−31) atm], and the temperature was varied from 500° to 650°C. It was essential for this analysis that the material under investigation exhibit, under some conditions, purely ionic behavior and, under others, mixed conducting behavior. The transition from ionic to mixed conducting behavior is recognizable not only from the oxygen partial pressure dependence of the total conductivity but also directly from the shape of the impedance spectra. Within the electrolytic regime, the impedance spectra (presented in Nyquist form) take the shape of simple, depressed arcs, whereas within the mixed conducting regime (under reducing conditions), the spectra exhibit the features associated with a half tear-drop-shaped element. Parameters derived from quantitative fitting of the impedance spectra include the concentration of free electron carriers, the mobilities and activation energies for both ion and electron transport, the electrolytic domain boundary, and the entropy and enthalpy of reduction. In addition, the electrochemical behavior of O_2 and H_2 at the Pt|ceria interface has been characterized from these measurements. Under oxidizing conditions, the data suggest an oxygen electrochemical reaction that is rate limited by the dissociated adsorption/diffusion of oxygen species on the Pt electrode, similar to Pt|YSZ (yttria-stabilized zirconia). Under reducing conditions, the inverse of the electrode resistivity obeys aJ_i^(mass)dependence, with an activation energy that is similar to that measured for the electronic conductivity. These results suggest that ceria is electrochemically active for hydrogen electro-oxidation and that the reaction is limited by the rate of removal of electrons from the ceria surface.

[1]  E. Warburg,et al.  Ueber das Verhalten sogenannter unpolarisirbarer Elektroden gegen Wechselstrom , 1899 .

[2]  N. Brandon,et al.  AC Impedance Spectra Arising from Mixed Ionic Electronic Solid Electrolytes , 2004 .

[3]  H. Tuller.  6 – Mixed Conduction in Nonstoichiometric Oxides , 1981 .

[4]  Toshio Suzuki,et al.  Impedance Studies of Diffusion Phenomena and Ionic and Electronic Conductivity of Cerium Oxide , 2005 .

[5]  S. Jiang,et al.  Hydrogen Oxidation at the Nickel and Platinum Electrodes on Yttria‐Tetragonal Zirconia Electrolyte , 1997 .

[6]  S. Adler.  Factors governing oxygen reduction in solid oxide fuel cell cathodes. , 2004, Chemical reviews.

[7]  Ulrich Guth,et al.  Electrochemical studies on cells M/YSZ/Pt (M=Pt, Pt–Ga2O3) in NO, O2, N2 gas mixtures , 2004 .

[8]  Hung-Chih Chang,et al.  Polarization in Electrolytic Solutions. Part I. Theory , 1952 .

[9]  J. Jamnik.  Impedance spectroscopy of mixed conductors with semi-blocking boundaries , 2003 .

[10]  W. B. Johnson,et al.  Fundamentals of Impedance Spectroscopy , 2005 .

[11]  J. Ross Macdonald,et al.  Theory of space‐charge polarization and electrode‐discharge effects , 1973 .

[12]  Bernard A. Boukamp,et al.  Interpretation of the Gerischer impedance in solid state ionics , 2003 .

[13]  Juan Bisquert,et al.  Theory of the Impedance of Electron Diffusion and Recombination in a Thin Layer , 2002 .

[14]  N. Sammes,et al.  Physical, chemical and electrochemical properties of pure and doped ceria , 2000 .

[15]  Koji Amano,et al.  Electrode reaction at Pt, O2(g)/stabilized zirconia interfaces. Part II: Electrochemical measurements and analysis , 1987 .

[16]  H. Tagawa,et al.  Oxygen nonstoichiometry of Ce1−ySmyO2−0.5y−x (y=0.1, 0.2) , 1999 .

[17]  Y. Xiong,et al.  Hole and Electron Conductivities of 20 mol % ­ REO 1.5 Doped CeO2 (RE = Yb, Y, Gd, Sm, Nd, La) , 2004 .

[18]  D. Franceschetti,et al.  Interpretation of Finite‐Length‐Warburg‐Type Impedances in Supported and Unsupported Electrochemical Cells with Kinetically Reversible Electrodes , 1991 .

[19]  Richard P. Buck,et al.  Transmission line equivalent circuit models for electrochemical impedances , 1981 .

[20]  A. Tschöpe.  Interface Defect Chemistry and Effective Conductivity in Polycrystalline Cerium Oxide , 2005 .

[21]  H. Gerischer.  Wechselstrompolarisation von Elektroden mit einem potentialbestimmenden Schritt beim Gleichgewichtspotential I. , 1951 .

[22]  M. Dokiya,et al.  Electrical and Ionic Conductivity of Gd‐Doped Ceria , 2000 .

[23]  L. Gauckler,et al.  Engineering of Solid Oxide Fuel Cells with Ceria‐Based Electrolytes , 1998 .

[24]  N. Minh,et al.  Electrical conduction in ceramics , 1995 .

[25]  Brian C. H. Steele,et al.  Appraisal of Ce1−yGdyO2−y/2 electrolytes for IT-SOFC operation at 500°C , 2000 .

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

[27]  Xin Guo,et al.  Blocking Grain Boundaries in Yttria‐Doped and Undoped Ceria Ceramics of High Purity , 2003 .

[28]  A. Mcevoy,et al.  Impedance Characteristics of Platinum Electrodes on Yttria-Stabilized Zirconia , 1993 .

[29]  Joachim Maier,et al.  Generalised equivalent circuits for mass and charge transport: chemical capacitance and its implications , 2001 .

[30]  E. Warburg,et al.  Ueber die Polarisationscapacität des Platins , 1901 .

[31]  J. Jamnik,et al.  Treatment of the Impedance of Mixed Conductors Equivalent Circuit Model and Explicit Approximate Solutions , 1999 .

[32]  H. Tuller,et al.  Doped Ceria as a Solid Oxide Electrolyte , 1975 .

[33]  Zongping Shao,et al.  A high-performance cathode for the next generation of solid-oxide fuel cells , 2004, Nature.

[34]  Koji Amano,et al.  Electrode reaction at Pt, O2(g)/stabilized zirconia interfaces. Part I: Theoretical consideration of reaction model , 1987 .

[35]  D. Pletcher,et al.  8 – A.C. techniques , 2001 .

[36]  Sangtae Kim,et al.  On the conductivity mechanism of nanocrystalline ceria , 2002 .

[37]  A. Nowick,et al.  Cathodic and Anodic Polarization Phenomena at Platinum Electrodes with Doped CeO2 as Electrolyte I . Steady‐State Overpotential , 1979 .

[38]  S. Badwal,et al.  Electrochemical Techniques in Studies of Solid Ionic Conductors , 1996 .

[39]  A. Nowick,et al.  Cathodic and Anodic Polarization Phenomena at Platinum Electrodes with Doped CeO2 as Electrolyte II . Transient Overpotential and A‐C Impedance , 1979 .

[40]  Neil Leslie Robertson,et al.  Oxygen exchange on platinum electrodes in zirconia cells; Location of electrochemical reaction sites , 1990 .

[41]  A. A. Moya,et al.  Steady-state, transient and small-amplitude AC responses of an electrochemical cell with immobile background charge : A network approach , 2000 .

[42]  H. Wiemhöfer,et al.  Electronic conductivity of Gd-doped ceria with additional Pr-doping , 1999 .

[43]  Bernard A. Boukamp,et al.  Electrochemical impedance spectroscopy in solid state ionics: recent advances , 2004 .