Thin Film Microelectrodes in SOFC Electrode Research

Fleig, Jürgen; Baumann, F.; Brichzin, V.; Kim, H.-R.; Jamnik, J.; Cristiani, G.; Habermeier, H.–U.; Maier, Joachim

doi:10.1002/fuce.200500209

Cited by 94 publications

(74 citation statements)

References 46 publications

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“…42 At elevated temperatures, the electrolyte only causes an intercept instead of a semicircle due to its low capacitance. 45 R a is almost identical for both types of microelectrodes, which indicates high in-plane electronic conductivity in air, see below and Ref.…”

Section: Resultsmentioning

confidence: 99%

Comparison of Electrochemical Properties of La_0.6Sr_0.4FeO_3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

Kogler

Nenning

Rupp

et al. 2015

J. Electrochem. Soc.

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Owing to its mixed ionic and electronic conductivity and high thermochemical stability, La 0.6 Sr 0.4 FeO 3-δ (LSF64) is an attractive electrode material in solid oxide fuel/electrolysis cells (SOFCs/SOECs). Well defined thin film microelectrodes are used to compare the electrochemical properties of LSF64 in oxidizing and reducing conditions. The high electronic sheet resistance in hydrogen can be overcome by the use of an additional metallic current collector. With the sheet resistance being compensated, the area specific electrode resistance is similar in humidified hydrogen and oxygen containing atmospheres. Analysis of the chemical capacitance and the electrode resistance for current collectors on top and beneath the LSF64 thin film allow mechanistic conclusions on active zones and bulk defect chemistry. Cyclic gas changes between reducing and oxidizing conditions, performed on macroscopic LSF64 thin film electrodes with top current collector, reveal a strong degradation of the surface kinetics in synthetic air with very fast recovery in reducing atmosphere. Additional in-situ high-temperature powder XRD on LSF64 demonstrates the formation of small amounts of iron oxides in humidified hydrogen at elevated temperatures. Solid oxide fuel cells (SOFCs) are in the process of gaining more and more commercial success as highly efficient power generation systems. They transform the chemically bound energy of a fuel to electrical energy. Solid oxide electrolysis cells (SOECs) are the counter parts of SOFCs as they use electrical energy, e.g. excess energy from the grid, to form fuel by electrolysis. Owing to their very high efficiencies, also SOECs are promising devices in future energy technologies. 1Currently Ni/YSZ cermet electrodes are the standard electrodes in SOF/ECs for reducing conditions, but they are known to suffer from several problems, like sulfur poisoning (in SOFC operation), sintering, redox cycle stability etc.2 To find an alternative to Ni/YSZ could therefore be favorable for both SOFCs and SOECs.Electrodes in SOE/FCs have to meet numerous requirements: thermochemical stability over a wide oxygen partial pressure range, high catalytic activity for oxygen exchange reactions, compatibility with adjacent cell components, sufficiently high electronic and ionic conductivity, etc. Perovskite-type oxides such as LaMnO 3 , LaCoO 3 and LaFeO 3 based materials are used in state-of-the-art SOF/EC electrodes in oxygen atmosphere. Often they are mixed ionic and electronic conductors (MIECs), which is advantageous in SOF/ECs as the whole electrode surface area may become active in the oxygen exchange reaction. Employing such MIECs also in reducing atmosphere could be highly attractive. However, Sr-doped LaMnO 3 and LaCoO 3 are only stable under comparatively high oxygen partial pressures and thus not suited for the use in hydrogen. Fe 4+ states can be interpreted as electron holes (h • ) and those determine the electronic conductivity at high oxygen partial pressure as they are the majority mobile charge carrie...

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Section: Resultsmentioning

confidence: 99%

Comparison of Electrochemical Properties of La_0.6Sr_0.4FeO_3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

Kogler

Nenning

Rupp

et al. 2015

J. Electrochem. Soc.

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“…In most instances a reference electrode is retained as part of the configuration. [4][5][6][7][8][9] In contrast to this trend, Fleig and co-workers [10][11][12][13][14] have adopted a point electrode geometry that is explicitly reference-less. For dense, surface-limited electrodes, the assumption that the point geometry has effectively eliminated the contribution of the counter electrode is likely valid.…”

Section: Introductionmentioning

confidence: 99%

Geometrically asymmetric electrodes for probing electrochemical reaction kinetics: a case study of hydrogen at the Pt–CsH2PO4 interface

Sasaki

Hao

Haile

2009

Phys. Chem. Chem. Phys.

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Electrochemical reactions can exhibit considerable asymmetry, with the polarization behavior of oxidation at a given metal|electrolyte interface differing substantially from that of reduction. The reference-less, microcontact electrode geometry, in which the electrode overpotentials are geometrically constrained to the working electrode (by limiting its area) is experimentally convenient, particularly for fuel cell studies, because the results do not rely on accurate placement of a reference electrode nor must oxidant and reductant gases be sealed off from one another. Here, the conditions under which the critical assumption of this geometry applies-that the overpotential at the large-area counter electrode can be ignored-is numerically assessed. It is found that, for cells of sufficiently large area, the effective radius of the counter electrode (which defines the area through which the majority of the current passes) can be expressed directly as a function of electrolyte thickness and the materials properties, s, the conductivity of the electrolyte, and k, the reaction rate constant for the electrochemical reaction at zero-bias. From this effective radius and the true radius of the working electrode, the fraction of electrode overpotential at the latter, defined as the extent of isolation, can be readily computed. Experimental studies of hydrogen electro-oxidation/proton electro-reduction at the Pt|CsH 2 PO 4 interface using two cells of differing dimensions both validate the computational results and demonstrate that asymmetry in such reactions are readily revealed in the micro-electrode, reference-less geometry. The study furthermore confirms the insensitivity of the results to the precise placement of the working electrode, while indicating the importance of very high isolation values (499%) to ensure that overpotential contributions of the counter electrode do not influence the measurements, particularly as bias is increased.

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“…One of the main advantages of pattern electrodes is that their geometry can be controlled in a well-defined manner and thus geometry dependencies of electrochemical parameters can be obtained much easier compared to cermet electrodes. 31,32 So far, most research effort has been put into explaining the resistive behavior of Ni/YSZ model electrodes, while capacitive effects are still much less understood, despite their importance in impedance measurements. In previous literature high area specific capacitance (ASC) values of Ni/YSZ electrodes were reported, which could not be explained by a classical Helmholtz-type double layer.…”

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confidence: 99%

The Capacitance of Nickel Pattern Electrodes on Zirconia Electrolyte

Doppler

Fleig

Bram

et al. 2016

J. Electrochem. Soc.

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Micro-structured thin-film electrodes were employed to investigate the capacitive behavior of nickel on yttria-stabilized zirconia (YSZ) electrolytes by means of electrochemical impedance spectroscopy. Electrodes with different shapes and electrode areas clearly showed a linear relationship between capacitance and the electrode area. Electrostatic double layer models, however, could not explain the observed area specific capacitance value of ca. 3 F/m 2 . This fact, the characteristic voltage dependence with a hysteresis, and the effect of H 2 S on the electrode capacitance indicate substantial contributions of a chemical capacitance. Solid oxide fuel cells (SOFCs) present a promising technology for direct conversion of chemical energy into electrical energy due to their high efficiency and fuel flexibility while causing little polluting emissions.1 Much research effort was devoted to the optimization of SOFC cathodes and cathodic polarization losses were substantially reduced.2 Consequently, in modern SOFCs the anode, which usually consists of a nickel/yttria stabilized zirconia (YSZ) cermet, may (again) contribute significantly to the overall cell losses. However, despite many investigations of the electrochemical anode behavior, fundamental properties of Ni/YSZ anodes are still far from being well understood. Therefore, considerable research still needs to be done to gain an in-depth understanding of the electrochemistry of Ni/YSZ electrodes, and to further optimize the performance of SOFC anodes.While having advantageous properties for application in operating SOFCs, Ni/YSZ cermet electrodes are often only of limited use for such fundamental research studies, since transport resistances due to ion conduction or gas diffusion and ill-defined geometry make data analysis very challenging. In contrast, model electrodes and in particular micro-structured thin-film electrodes -often referred to as pattern electrodes -are particularly suited for basic research studies.9-30 One of the main advantages of pattern electrodes is that their geometry can be controlled in a well-defined manner and thus geometry dependencies of electrochemical parameters can be obtained much easier compared to cermet electrodes. 31,32 So far, most research effort has been put into explaining the resistive behavior of Ni/YSZ model electrodes, while capacitive effects are still much less understood, despite their importance in impedance measurements. In previous literature high area specific capacitance (ASC) values of Ni/YSZ electrodes were reported, which could not be explained by a classical Helmholtz-type double layer. 12,33,34 A comparison of Ni, Pt and Au electrodes on YSZ also showed significantly higher capacitances of the Ni electrodes, further suggesting additional contributions to capacitive effects.12 Proposed mechanisms for the increased capacitance values were based on chemical processes at the electrode, such as proton ad/absorption 33 and valence changes of impurity ions. 34 Another capacitive model of metal electrodes on YSZ...

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Thin Film Microelectrodes in SOFC Electrode Research

Cited by 94 publications

References 46 publications

Comparison of Electrochemical Properties of La_0.6Sr_0.4FeO_3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

Comparison of Electrochemical Properties of La_0.6Sr_0.4FeO_3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

Geometrically asymmetric electrodes for probing electrochemical reaction kinetics: a case study of hydrogen at the Pt–CsH2PO4 interface

The Capacitance of Nickel Pattern Electrodes on Zirconia Electrolyte

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Thin Film Microelectrodes in SOFC Electrode Research

Cited by 94 publications

References 46 publications

Comparison of Electrochemical Properties of La0.6Sr0.4FeO3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

Comparison of Electrochemical Properties of La0.6Sr0.4FeO3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

Geometrically asymmetric electrodes for probing electrochemical reaction kinetics: a case study of hydrogen at the Pt–CsH2PO4 interface

The Capacitance of Nickel Pattern Electrodes on Zirconia Electrolyte

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Product

Resources

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Comparison of Electrochemical Properties of La_0.6Sr_0.4FeO_3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions

Comparison of Electrochemical Properties of La_0.6Sr_0.4FeO_3-δThin Film Electrodes: Oxidizing vs. Reducing Conditions