Three-phase Boundary Length Research Articles

Effect of length of gas/liquid/solid three-phase boundary zone on cathodic and corrosion behavior of metals

Liquid dispersion, depending on the length of gas/liquid/solid three-phase boundary (TPB) zone of dispersed liquid on a metal surface, is an important property of liquid state influencing corrosion behavior of metals in gas/liquid/solid multiphase corrosion systems. The effect of TPB length on the cathode and corrosion process was studied using steady state polarization technique, electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR) technique. It was found that both the cathodic limiting current density and corrosion current density linearly increased with increasing the TPB length, indicating that the cathodic and corrosion process in gas/liquid/solid multiphase corrosion systems not only depended on the thickness of liquid layer but also strongly depended on the dispersed liquid state on a metal surface. In addition, a model is developed to establish the relationship between the cathodic limiting current density and the TPB length. The simulation result is in good agreement with the experimental findings. The model is also useful for evaluating the effect of liquid dispersion on atmospheric corrosion rates.

Assessment of the Cathode Contribution to the Degradation of Anode-Supported Solid Oxide Fuel Cells

The degradation of anode-supported cells was studied over as a function of cell polarization either in air or oxygen on the cathode side. Based on impedance analysis, contributions of the anode and cathode to the increase of total resistance were assigned. Accordingly, the degradation rates of the cathode were strongly dependent on the . Microstructural analysis of the cathode/electrolyte interface carried out after removal of the cathode showed craters on the electrolyte surface where the lanthanum strontium manganite (LSM) particles had been located. The changes of shape and size of these craters observed after testing correlated with the cell voltage degradation rates. The results can be interpreted in terms of element redistribution at the cathode/electrolyte interface and formation of foreign phases giving rise to a weakening of local contact points of the LSM cathode and yttria-stabilized zirconia electrolyte and consequently a reduced three-phase boundary length.

High performance of electroless-plated platinum electrode for electrochemical hydrogen pumps using strontium-zirconate-based proton conductors

The performance of an electroless-plated platinum electrode for electrochemical hydrogen pumps was investigated using SrZr 0.9Y 0.1O 3− α (10Y-SZO) and SrZr 0.5Ce 0.4Y 0.1O 3− α (10Y-SZCO) as proton-conducting electrolytes. The plated electrodes using these electrolytes showed quite low overpotentials, resulting in high performance of the hydrogen pumps. The high electrode performance is interpreted from the viewpoint of the three-phase boundary (TPB) length: the plated electrode has a larger TPB length per unit area than that of the pasted electrode. The difference between 10Y-SZO and 10Y-SZCO in ability to match with platinum electrodes is also discussed, and the possibility that the TPB width of 10Y-SZCO with platinum is larger than that of 10Y-SZO is suggested.

The Effect of Manganese Doping on Chromium Deposition at Pt/YSZ Cathode Interfaces

The effect of yttria-stabilized zirconia (YSZ) doping on Cr deposition at a Pt-electrode/YSZ-electrolyte interface was investigated. Comparison between undoped and Mn-doped YSZ electrolyte surfaces after cathodic polarization for 25 h in moist air at 800°C in the presence of chromia indicated that the presence of Mn in the electrolyte significantly enhances Cr deposition at the electrode/electrolyte interface. Impedance spectroscopy results showed that with a Mn-doped YSZ electrolyte the electrode polarization resistance decreased significantly with polarization time, which was attributed to the increase in the three-phase boundary length by the p-type semiconducting chromia deposits.

Three-phase boundary length in solid-oxide fuel cells: A mathematical model

A mathematical model to calculate the volume specific three-phase boundary length in the porous composite electrodes of solid-oxide fuel cell is presented. The model is exclusively based on geometrical considerations accounting for porosity, particle diameter, particle size distribution, and solids phase distribution. Results are presented for uniform particle size distribution as well as for non-uniform particle size distribution.

Effects of anode surface modification on the performance of low temperature SOFCs

An anode functional layer (AFL, ∼5 μm) for improving the cell performance was fabricated by the slurry spin coating method on the porous surface of an anode substrate. The effects of the AFL on the anode/electrolyte interfacial morphology and the Sm 0.2Ce 0.8O 1.9 (SDC) film deposition process were evaluated. And the electrochemical characteristics of the cells with and without the AFL were tested for comparison. With the AFL layer, the cell performance was greatly improved and the maximum power density was increased from 0.733 to 0.884 W cm −2 at 600 °C and from 1.085 to 1.213 W cm −2 at 650 °C. The systematical analysis indicated that the AFL could effectively reduce the anode polarization loss by increasing the three-phase boundary (TPB) length.

Electrophoretically Coated Wire Meshes as Current Collectors for Solid Oxide Fuel Cell

Metal wire meshes were coated with La0.8Sr0.2MnO3 and NiO/YSZ by electrophoretic deposition technique (EPD) and applied them for surface modified current collectors for a solid oxide fuel cell. The current interrupt analyses for each electrode attached with the wire meshes clearly indicated that coated wire meshes remarkably reduced the cathodic and anodic resistances, especially polarization component, compared to using the uncoated meshes. This enhancement of the electrode performance is due to that EPD coating of the metal meshes increases an effective contact area and/or points between the wire mesh and electrode surface, resulting an increase in three-phase boundary length. A single cell with the EPD coated meshes exhibited higher I-V characteristic compared to that with the uncoated meshes as expected from the result of the electrode resistance measurements.

Development of LSM-based cathodes for solid oxide fuel cells based on YSZ films

In an attempt to achieve desirable cell performance, the effects of La 0.7Sr 0.3MnO 3 (LSM)-based cathodes on the anode-supported solid oxide fuel cells (SOFCs) were investigated in the present study. Three types of cathodes were fabricated on the anode-supported yttria-stabilized zirconia (YSZ) thin films to constitute several single cells, i.e., pure LSM cathode, LSM/YSZ composite by solid mixing, LSM/Sm 0.2Ce 0.8O 1.9 (SDC) composite by the ion-impregnation process. Among the three single cells, the highest cell output performance 1.25 W cm −2 at 800 °C, was achieved by the cell using LSM/SDC cathode when the cathode was exposed to the stationary air. Whereas, the most considerable cell performance of 2.32 W cm −2 was derived from the cell with LSM/YSZ cathode, using 100 ml min −1 oxygen flow as the oxidant. At reduced temperatures down to 700 °C, the LSM/SDC cathode was the most suitable cathode for zirconia-based electrolyte SOFC in the present study. The variation in the cell performances was attributed to the mutual effects between the gas diffusing rate and three-phase boundary length of the cathode.

Monte-Carlo simulation and performance optimization for the cathode microstructure in a solid oxide fuel cell

A 3D micro-scale model is developed to simulate the transport and electrochemical reaction in a composite cathode. This model takes into account the details of the specific cathode microstructure such as random pore structure, active TPB (three phase boundary) site distribution, particle size and composition and their interrelationship to the charge transfer and mass transport processes. Especially, the pore structure and mass diffusion were incorporated into this model. Influence of the microsturcture parameters on the performance was investigated by numerical simulations. Simulation shows that the cathode porosity should be in the range of 0.25–0.45 for the optimized performance. A larger thickness is in favor of increasing the effective reaction sites and reducing total specific resistance. However, the thickness needs to be confined below a certain thickness in order to prevent lower utilization and excessive concentration loss. The fine particles contribute to increase the TPB length and thus decrease the activation overpotential. On the other hand, the extremely small particles might lead to an excessive diffusion resistance. The model was validated based on the continuum model and experimental data to determine its accuracy in predicting the cathode performance. The results generally agree with the experimental data from the literature. However, as compared with the continuum model, the predicted total resistance is slightly higher for a thin electrode.

A novel approach to study the structure versus performance relationship of SOFC electrodes

The aim of this work is to develop a new rigorous model to study the structure performance relationship of solid oxide fuel cell (SOFC) electrodes. A new two-dimensional, geometrical model which captures the inhomogeneous nature of the location of electrochemical reactions based on random packing of electronic and ionic conducting particles has been developed. The results show that the concentration of oxygen inside the cathode in the 2D model is not only a function of the electrode depth but also changes along the width of the electrode. Furthermore the effect of composition of the electrode on the length of active three-phase boundary (TBP) and total polarization resistance has been demonstrated. A parametric study of the effect of the conductivity of ionic conductor and diffusion coefficient on the performance of the electrode has been given.

Electrochemical properties of the Sm 0.5Sr 0.5CoO 3–La 0.8Sr 0.2Ga 0.8Mg 0.15Co 0.05O 3 (LSGMC5)/LSGMC5 interface modified by an LSGMC5 interlayer synthesized using the citrate method

A La 0.8Sr 0.2Ga 0.8Mg 0.15Co 0.05O 3 (LSGMC5) interlayer synthesized using the citrate method was added between an Sm 0.5Sr 0.5CoO 3 (SSC)–LSGMC5 electrode and an LSGMC5 electrolyte pellet synthesized using a solid-state reaction, and we found that the electrode activity was improved dramatically. The SEM images of the samples demonstrated that the contact between the electrode and the interlayer was much better than the contact between the electrode and electrolyte without the interlayer. The addition of the interlayer resulted in an increased three-phase boundary length and electrode/electrolyte two-phase interfacial area. An SSC–LSGMC5 electrode sintered at 1123 K deposited onto an interlayer sintered at 1673 K exhibited the highest performance among the samples studied. The electrode resistance was about 0.08 Ω cm 2 at near equilibrium conditions, and the cathodic overpotential at a current density of 1 A cm −2 was only about 70 mV at 973 K in oxygen. The introduction of the interlayer did not change the oxygen reaction mechanism, and the significant increase in electrode performance was due to the increase in the number of active sites for oxygen reduction.

Three-dimensional reconstruction of a solid-oxide fuel-cell anode

The drive towards increased energy efficiency and reduced air pollution has led to accelerated worldwide development of fuel cells. As the performance and cost of fuel cells have improved, the materials comprising them have become increasingly sophisticated, both in composition and microstructure. In particular, state-of-the-art fuel-cell electrodes typically have a complex micro/nano-structure involving interconnected electronically and ionically conducting phases, gas-phase porosity, and catalytically active surfaces. Determining this microstructure is a critical, yet usually missing, link between materials properties/processing and electrode performance. Current methods of microstructural analysis, such as scanning electron microscopy, only provide two-dimensional anecdotes of the microstructure, and thus limited information about how regions are interconnected in three-dimensional space. Here we demonstrate the use of dual-beam focused ion beam-scanning electron microscopy to make a complete three-dimensional reconstruction of a solid-oxide fuel-cell electrode. We use this data to calculate critical microstructural features such as volume fractions and surface areas of specific phases, three-phase boundary length, and the connectivity and tortuosity of specific subphases.

Current and Overpotential Distribution Mathematical Model of the Porous Electrode Based on the Three-Phase Boundary Length

In accordance with the gas and liquid distribution theory of a porous medium, the current and the overpotential distribution of the porous electrode were studied. Based on the analysis of the electric conductance of the solution, the Ohm’s law of the solution considering the gas and liquid distribution in the porous electrode is given. The effective three-phase boundary length of the electrochemical reaction is defined and calculated. According to the definition, a one-dimensional current and an overpotential distribution mathematical model of the porous electrode is established based on the mathematical model of the single-pore electrode and calculated by the numerical iteration method. The influence of gas saturation and effective three-phase boundary length to the current and overpotential distributions are also discussed. © 2005 The Electrochemical Society. All rights reserved.

Oxygen Transport in the SOFC Cathode

The impedance of the La0.75Sr0.2MnO3-cathode/electrolyte interface for cathodes with different porosity is measured. The impedance spectra are fitted using a developed model of the oxygen transport at this interface. After the measurements, the cathode is removed from the electrolyte. The contact area and the three-phase boundary length (TPBL) at the interface are estimated from SEM images of the electrolyte surface. The dependence of the interfacial electrical resistance on the microstructure is discussed. It is shown that the bulk diffusion of oxygen vacancies at the interface at 950°C is high enough to use the whole La0.75Sr0.2MnO3/YSZ contact area F for the oxygen transport into the electrolyte for microstructures with 2F/TPBL ≤ 2 μm. The impact of the surface diffusion of oxygen species on polarization resistance at operation temperatures <900°C is discussed. The polarization resistance and the morphology of composite cathodes made from La0.75Sr0.2MnO3/YSZ and yttria- or scandia-stabilized zirconia powders (3YSZ, 8YSZ, 10ScSZ) are investigated by impedance spectroscopy at 800–950°C. The polarization (interfacial) resistance decreases gradually with addition of electrolyte powder in the uLSM cathode material independent of the electrolyte powder used. The interfacial resistance of the uLSM/3YSZ, uLSM/8YSZ, and uLSM/10ScSZ composite cathodes is almost the same. The interaction between uLSM and doped zirconia particles is discussed on the basis of the interfacial resistance, activation energies, and high-frequency impedance.

Cathode Micromodel of Solid Oxide Fuel Cell

A comprehensive micromodel considering all forms of polarization in the cathode of the solid oxide fuel cell was developed which governs the complex interdependency among the transport phenomena, electrochemical processes (charge-transfer and surface diffusion), and the microstructure of the electrode and their combined effect on the cathode overpotential under different operating conditions. To make the model more generalized, we consider possible oxygen reduction mechanisms, reactions at the cathode/electrolyte interface, grain interior and grain boundary effects on the total resistance, both ordinary diffusion and Knudsen diffusion, active three-phase boundary length as a function of ionic/electronic particle size ratio and volume fraction, the exchange current density as a function of gases concentration, etc. Incorporated with reliable experimental data, the model can be used as a tool to design a high performance cathodes. © 2003 The Electrochemical Society. All rights reserved.

Diffusion controlled oxygen transport and stability at the perovskite/electrolyte interface

The impedance of the La 0.8Sr 0.2MnO 3 (LSM) cathode/electrolyte interface for cathodes with different porosity was measured. The impedance spectra were fitted using the developed model of the oxygen transport at the La 0.8Sr 0.2MnO 3 cathode/electrolyte interface. Following these measurements, the cathode was removed from the electrolyte. The contact area F and the three-phase boundary length (TPBL) at the interface were estimated from SEM micrographs of the electrolyte surface. The dependence of the interfacial electrical resistance on the microstructure is discussed. It is shown that the bulk diffusion of the oxygen vacancies at the interface at 950 °C is high enough to use the whole La 0.8Sr 0.2MnO 3/YSZ contact area F for the oxygen transport into the electrolyte for microstructures with 2 F/TPBL≤2 μm.

Mixed Conducting Porous SOFC Cathodes: Current Distributions and Polarization Resistances

Numerical multidimensional simulations are presented that deal with the effect of material parameters on the current distribution in porous mixed conducting solid oxide fuel cell cathodes where the oxygen incorporation into the electrolyte takes place through the bulk of the electrode. In particular, it is demonstrated that, depending on the ratio kq/Dq (surface incorporation factor/diffusion coefficient of oxide ions), different regimes can be distinguished: For large kq/Dq values, only small regions close to the three-phase boundaries are relevant with respect to the oxygen reduction; a decreasing kq/Dq ratio activates an increasingly larger portion of the cathode. The impact of geometrical parameters (grain size, threephase boundary length, surface area, etc.), and thus suggested optimum geometries, depend on kq/Dq. The simulations showed that either porous cathodes with high surface areas (for low kq/Dq) or composite cathodes (for high kq/Dq) are recommended to achieve low polarization resistances.

Effect of Anode Porosity and Pore Size on Electrochemical Performance

Solid oxide fuel cells consisting of LSCF, YSZ and Ni-YSZ were fabricated with anodes of controlled porosity and pore size. The effects of porosity and pore size on cell performance were investigated theoretically and experimentally. A geometric simulation was used to determine the three-phase-boundary length (TPBL) which is one of main factors determining the anodic reaction and consequently the cell performance. The TPBL calculation indicated there exists an optimum porosity and pore size in the anode, depending on grain size. A porosity of 35-40% was found to be optimum for fuel cell performance in our cells. Based on the anodes tested with pore size from 1 to 10 μm, it was found that cell performance improved with increasing pore size.

Polarisation resistance of the O2, Au/O2− and H2-H2O, Au/O2− electrode systems

Gold electrodes with known contact geometries were studied using impedance spectroscopy. From these data it was possible to determine the specific polarisation conductivity per unit length of three-phase boundary (TPB). The values were found to be (3÷22)×10−4 S·cm−1 dependent on the electrode history in pure oxygen at 977 °C and 2×10−6 S·cm−1 at 977 °C in “pure” hydrogen (PO2=10−20 atm at 1001 °C). The results are compared with previous data obtained for platinum electrodes.

The geometry dependence of the polarization resistance of Sr-doped LaMnO 3 microelectrodes on yttria-stabilized zirconia

Impedance spectroscopic studies and I– V measurements are performed at Sr-doped LaMnO 3 (LSM) microelectrodes in order to elucidate the mechanism of the oxygen-reduction reaction on yttria-stabilized zirconia. The geometry dependence of the polarization resistance was investigated by systematic variations of the microelectrode's size and thickness. The relation between the resistance and the electrode geometry turns out to be bias-dependent: in the cathodic regime and close to equilibrium, the resistance is proportional to the inverse electrode area. Moreover, measurements without bias revealed an almost linear dependence of the resistance on the electrode thickness. This suggests that the relevant oxygen reduction path involves the transport of oxide ions through the bulk of the LSM cathode. In the anodic regime, however, the resistance becomes proportional to the inverse three-phase boundary length and, hence, a mechanism involving the LSM surface is most probable with a step close to the three-phase boundary being rate limiting. Experiments performed on LSM microelectrodes with thin alumina “discs” beneath the LSM to partly block the oxygen ion transport through the bulk of the electrode support this interpretation.