Mogens Bjerg Mogensen

Physical, chemical and electrochemical properties of pure and doped ceria

This paper gives an extract of available data on the physical, chemical, electrochemical and mechanical properties of pure and doped ceria, predominantly in the temperature range from 200 to 1000°C. Several areas are pointed out where further research is needed in order to make a better basis for the evaluation of the real potential and limits for the practical application of ceria in solid oxide fuel cells and other solid state electrochemical devices.

Solid Oxide Fuel Cell

SOFC cell comprising a metallic support 1 ending in a substantially pure electron conducting oxide, an active anode layer 2 consisting of doped ceria, ScYSZ, Ni—Fe alloy, an electrolyte layer 3 consisting of co-doped zirconia based on oxygen ionic conductor, an active cathode layer 5 and a layer of a mixture of LSM and a ferrite as a transition layer 6 to a cathode current collector 7 of single phase LSM. The use of a metallic support instead of a Ni—YSZ anode support increases the mechanical strength of the support and secures redox stability of the support. The porous ferrite stainless steel ends in a pure electron conducting oxide so as to prevent reactivity between the metals in the active anode which tends to dissolve into the ferrite stainless steel causing a detrimental phase shift from ferrite to austenite structure.

Impedance of Solid Oxide Fuel Cell LSM/YSZ Composite Cathodes

A number of lanthanum strontium manganate/yttria-stabilized zirconia (LSM/YSZ) composite electrodes are produced with varying composition and processing parameters. The composites are investigated using impedance spectroscopy. General trends related to the oxygen reduction process are extracted from the impedance data. Literature concerning kinetic studies of LSM/YSZ electrodes and related systems is reviewed and compared to new experimental data. From this it is found that at least five processes affect the impedance. Going from high to low frequency, these processes are (i), (ii) two geometry-related contributions interpreted as transport across LSM/YSZ interfaces and through the YSZ of the composite. (iii) a process reflecting competitive reaction steps such as bond breaking and surface diffusion, (iv) gas diffusion in a stagnant gas layer above the electrode structure. and (v) an activation process (inductive) presumably located at the triple phase boundary of electrode, electrolyte, and gas phase.

Electrochemical Characterization of La0.6Sr0.4Co0.2Fe0.8 O 3 Cathodes for Intermediate-Temperature SOFCs

The electrochemical properties of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 (LSCF) have been assessed for its application as a cathode in intermediate-temperature solid oxide fuel cells. van der Pauw dc conductivity, two-electrode impedance, and three-electrode measurements were carried out to investigate the kinetics of the oxygen reduction reaction at various temperatures, oxygen partial pressures, and polarization values. A change in cathode behavior at temperatures around 600°C was observed. This is interpreted in terms of LSCF behaving as a mixed ionic electronic conductor at temperatures above around 600°C, oxygen reduction being stimulated by the formation of oxygen vacancies with increasing cathode overpotential. However, at temperatures below 600°C the contribution of mixed conductivity is low, and cathode behavior can then be interpreted in terms of the classical triple-phase-boundary model.

Physical Properties of Mixed Conductor Solid Oxide Fuel Cell Anodes of Doped CeO2

Samples of CeO2 doped with oxides such as CaO and Gd203 were prepared. Their conductivities and expansions on reduction were measured at 1000~ and the thermal expansion coefficients in the range 50 to 1000~ were determined. The ionic and electronic conductivity were derived from curves of total conductivity vs. oxygen partial pressure. For both types of conductivity a dependence on dopant valency was observed. The electronic conductivity was independent of dopant radius in contrast to the ionic which was highly dependent. These measured physical properties are compared with the ideal requirements for solid oxide fuel cell anodes. Not all requirements are fulfilled. Measures to compensate for this are discussed.

Highly efficient high temperature electrolysis

High temperature electrolysis of water and steam may provide an efficient, cost effective and environmentally friendly production of H2 using electricity produced from sustainable, non-fossil energy sources. To achieve cost competitive electrolysis cells that are both high performing i.e. minimum internal resistance of the cell, and long-term stable, it is critical to develop electrode materials that are optimal for steam electrolysis. In this article electrolysis cells for electrolysis of water or steam at temperatures above 200 °C for production of H2 are reviewed. High temperature electrolysis is favourable from a thermodynamic point of view, because a part of the required energy can be supplied as thermal heat, and the activation barrier is lowered increasing the H2 production rate. Only two types of cells operating at high temperature (above 200 °C) have been described in the literature, namely alkaline electrolysis cells (AEC) and solid oxide electrolysis cells (SOEC). In the present review emphasis is on state-of-the art electrode materials and development of new materials for SOECs. Based on the state-of-the-art performance for SOECs H2 production by high temperature steam electrolysis using SOECs is competitive to H2 production from fossil fuels at electricity prices below 0.02–0.03 € per kWh. Though promising SOEC results on H2 production have been reported a substantial R&D is still required to obtain inexpensive, high performing and long-term stable electrolysis cells.

Manganite-zirconia composite cathodes for SOFC : influence of structure and composition

The performance of La0.85Sr0.15MnyO3 ± δ (LSMy) as cathode material for Solid Oxide Fuel Cells (SOFC) operating at 1000 °C and pO2 = 0.21 atm. can be improved by increasing the Triple Phase Boundary Length (TPBL) where the gas, the electrode and the electrolyte are in contact and by preventing the formation of highly resistive (Zr, La)-rich layers at the electrode/electrolyte interface. In the present work, the influence of three parameters on the performance of LSMy cathodes has been investigated: the adhesion of the cathode to the electrolyte, the addition of Y2O3-stabilized ZrO2 (YSZ) to the cathode and the excess of Mn (LSM1.1). The electrode resistance has been measured by impedance spectroscopy at 1000 °C, pO2 = 0.21 atm and equilibrium potential. The interface structure and the reactions between the electrode and the electrolyte have been examined by Scanning Electron Microscopy and Transmission Electron Microscopy combined with Energy Dispersive X-ray Spectrometry. The results obtained indicate that the reaction resistance of the cathode for the oxygen reduction process decreases upon increasing the TPBL. Furthermore, the formation of highly resistive (Zr, La)-rich layers at the interface electrode/electrolyte can be prevented by increasing the Mn-content in the LSMy, the amount of Mn necessary being dependent of the amount of YSZ in the cathode.

Detailed Characterization of Anode-Supported SOFCs by Impedance Spectroscopy

Anode-supported thin electrolyte cells are studied by electrochemical impedance spectroscopy (EIS). The aim is to describe how the losses of this type of cells are distributed at low current density (around open-circuit voltage) as a function of temperature. An equivalent circuit consisting of an inductance, a serial resistance (R s ), and five arcs to describe the polarization resistance is suggested. This equivalent circuit is based on previous studies of single electrodes in three-electrode and two-electrode symmetric cell setups. The equivalent circuit components have been assigned to the electrode processes, and the assignments were verified by extensive full cell studies in which the partial pressure of reactant gases on both the electrodes as well as temperature was systematically varied with the aim to identify frequency regions which are dominated by an electrode specific process. Furthermore, the model is applied on a good performing cell with area specific resistance (ASR) = 0.15 Ω cm 2 at 850°C and a poor performing cell with ASR = 0.29 Ω cm 2 at the same temperature. Both cells were fabricated using nominally the same procedure. The EIS analysis indicated that the difference in performance originates from microstructural differences on the cathode. This is further supported by the observation of large differences in the cathode microstructure by scanning electron microscope.

Kinetic and geometric aspects of solid oxide fuel cell electrodes

Abstract The paper gives an overview of the main factors controlling the performance of the solid oxide fuel cell (SOFC) electrodes, emphasizing the most widely chosen anodes and cathodes, Ni-YSZ and LSM-YSZ. They are often applied as composites (mixtures) of the electron conducting electrode material and the ion conducting electrolyte. Some reasons for this choice are: 1) to increase the three-phase-boundary (TPB) length (key reactants must pass the TPB) and 2) to assure good adherence of the electrodes to the electrolyte. In the case of Ni-YSZ cermet anode it is also clear that the electrochemical performance is very much dependent on how it was made (structure and composition). Impedance results show that up to three arcs are present which means that at least three processes may contribute to the polarization resistance. Comparisons with anode microstructure micrographs show that the high frequency arc is much more dependent on the structure than the low frequency arcs. In the case of LSM-YSZ composite it has been demonstrated that both the ratio of LSM to YSZ and the conductivity of the YSZ is of major importance. The length and the nature of the three-phase-boundary between LSM, YSZ and air influence the size of the polarisation resistance greatly and may also change the rate limiting step for oxygen reduction as evidenced by the change in dependence on oxygen partial pressure and in the apparent activation energy. O 16 O 18 isotope exchange measurements have shown that oxygen surface exchange takes place with significant rates on both electrodes and electrolyte types of materials. Results from pointed electrodes indicate that the electrochemical reaction occurs on both the solid electrolyte and the electrode materials but only in a narrow zone (few μm) along the three-phase-boundary.

Performance/structure correlation for composite SOFC cathodes

Many solid oxide fuel cell (SOFC) developers use composite electrodes. The cathode is often a mixture of La(Sr)mnO3 and yttria stabilised zirconia (YSZ). Two kinds of cathode structure were studied by means of impedance spectrsocopy and d.c. electrochemical methods. The measurements were performed in air at 700–1000 °C. Then, the electrode microstructures were examined ceramographically. The cathode performance was improved by increasing the thickness of the composite electrode, and the polarisation resistance was decreased by extending the active triple phase boundary line through the application of a coarse layer of YSZ particles on the electrolyte surface before the composite cathode was applied. The performance proved to be sensitive to structural changes at 700 and 850 °C, whereas the effect of the structure was limited at 1000 °C. Values as low as 0.07 Ω cm2 at 1000 °C and 0.5 Ω cm2 at 850 °C were obtained in air at an overvoltage of − 50 mV. Two simple models relating performance, percolation and triple phase boundary length were used for the interpretation of the results and for assessing the potential for further improvement of the performance by optimising the microstructure.