Current Collector Layer Research Articles

Interaction of (La 1 − xSr x) 1 − yMnO 3–Zr 1 − zY zO 2 − d cathodes and LaNi 0.6Fe 0.4O 3 current collecting layers for solid oxide fuel cell application

Conductivity measurements indicate that lanthanum nickel ferrite (LaNi 0.6Fe 0.4O 3) is a potential cathode current collector material in solid oxide fuel cells (SOFCs). However it is known that lanthanum nickel ferrite reacts with zirconia, commonly employed in SOFCs, to form lanthanum zirconate at typical manufacturing temperatures. The reaction product, which has a low electrical conductivity, may therefore compromise the use of lanthanum nickel ferrite as a cathode current collector in SOFCs that employ a lanthanum strontium manganite–zirconia composite cathode, such as the Rolls-Royce Integrated Planar (IP)-SOFC. Thus the interaction of lanthanum nickel ferrite with lanthanum strontium manganite ((La 0.85Sr 0.15) 0.90MnO 3) and 3 mol% yttria-stabilised zirconia has been investigated by powder X-ray diffraction, electrochemical impedance spectroscopy, electron microscopy and energy dispersive spectroscopy. While no interfacial lanthanum zirconate product was detected, it was found that interdiffusion of manganese and nickel occurred between the perovskite phases of the cathode and current collector layers. Results of electrochemical impedance spectroscopy suggest that the interaction has a strong detrimental effect on cathode performance in the long-term.

Effect of Anode Active Layer on Performance of Single-Step Cofired Solid Oxide Fuel Cells

Anode-supported planar solid oxide fuel cells (SOFCs) with and without the anode active layer were fabricated by a single-step cofiring process. The cells were comprised of a porous -stabilized zirconia (YSZ) anode support, a porous fine-grained anode active layer for some experiments, a dense YSZ electrolyte, a porous fine-grained Ca-doped cathode active layer, and a porous LCM cathode current collector layer. The fabrication process involved tape casting of the anode support followed by screen printing of the remaining component layers. The cells were then cofired at for . Sintered cells were electrochemically tested between 700 and with air as oxidant and various compositions of humidified hydrogen as fuel to simulate the effect of fuel utilization on cell performance. Experimentally measured current density-voltage () characteristics of the cells were fitted into a polarization model, and the cell parameters, including the area-specific ohmic resistance, exchange current density, anodic limiting current density, cathodic limiting current density, effective binary diffusivity of hydrogen and water vapor in the anode, and that of oxygen and nitrogen in the cathode, were obtained. Evaluation of the electrochemical performance and the polarization modeling results indicated that the cell performance is dominated by the cathodic activation polarization at low fuel utilization condition. However, the cell performance loss due to the anodic activation polarization increases as the fuel utilization increases. The anode active layer significantly improves the cell performance at high fuel utilization by lowering the anodic activation polarization under -rich fuel because the anode active layer has finer and less porous microstructures and increases the number of the reaction sites near the anode-electrolyte interface. Cell performance at high fuel utilization and the effect of the anode active layer on the cell performance were discussed in detail.

Analysis of Electrochemical Performance of Single Step Co-fired Solid Oxide Fuel Cell (SOFC) Analyzed Using Polarization Model and Impedance Spectroscopy

AbstractAnode-supported planar solid oxide fuel cell (SOFC) was successfully fabricated by a single step co-firing process. The cell comprised of a porous Ni + yittria-stabilized zirconia (YSZ) anode support, a porous and fine-grained Ni + YSZ anode active layer, a dense YSZ electrolyte, a fine-grained porous Ca-doped LaMnO3 (LCM) + YSZ composite cathode active layer, and a porous LCM cathode current collector layer. The cell was tested between 700˜800oC with humidified hydrogen (97% H2+3% H2O) as fuel and air as oxidant. The measured maximum power densities were 1.42W/cm2 at 800°C, 1.20W/cm2 at 750°C, and 0.87W/cm2 at 700°C. The cell was also tested at 800oC with various compositions of oxidant, and cell parameters including the area specific ohmic resistance, exchange current density, and anodic and cathodic limiting current densities were determined using the polarization model and impedance spectroscopy.

Experimental investigations of the microscopic features and polarization limiting factors of planar SOFCs with LSM and LSCF cathodes

The paper deals with the microscopic and polarization evaluation of planar circular-shaped seal-less SOFC cells from InDEC ® with an outline of performance limiting factors at reduced temperature. The cells consist of porous NiO–YSZ anode as mechanical support, NiO–YSZ anode active layer, yttria-stabilized zirconia (YSZ) electrolyte, and only differ for the cathode design. A first design (ASC1) with strontium doped lanthanum manganate LSM–YSZ cathode functional layer (CFL) and LSM cathode current collector layer (CCCL); the second design (ASC2) with yttria doped ceria (YDC) barrier layer and lanthanum strontium cobalt ferrite oxide (LSCF) cathode. The microscopic analysis was performed using SEM methods. The electrical characterization was performed by taking current–voltage measurements over a range of temperatures between 650 and 840 °C with hydrogen as fuel, and air as oxidant. The analysis of performance showed that at 740 °C the voltage of 700 mV is reached at around a double value of current density in the case of ASC2. Further, the dependence of performance on the various polarization contributions was rationalized on the basis of an analytical model. Through a parameter estimation on the experimental data, it was possible to determine some polarization parameters for the two cells such as the effective exchange current densities, ohmic resistance and anodic limiting current density. It is shown that the performance limitation at low temperature is due to activation polarization for ASC1 and ohmic polarization for ASC2. Based on the results of the investigation, it is concluded that LSCF cathodes are really effective for decreasing the cathode activation polarization, allowing the reduction of operating temperature.

Effect of Fuel Composition on Performance of Single-Step Cofired SOFCs

Anode-supported planar solid oxide fuel cells (SOFCs) were successfully fabricated employing a single-step cofiring process. The cells were comprised of a -stabilized zirconia (YSZ) anode, a YSZ electrolyte, a Ca-doped cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single-step cofiring of the green-state cell in the temperature range of for . The maximum power densities were at , at , and at , with humidified hydrogen as fuel and air as oxidant. The experimentally measured voltage-current density (V-i) curves were fitted into a polarization model to obtain the area specific ohmic resistance, exchange current density (anodic and cathodic), anodic limiting current density, cathodic limiting current density, and effective binary diffusivity of hydrogen and water vapor in the anode as well as that of oxygen and nitrogen in the cathode. The cell was also tested at with various compositions of humidified hydrogen to simulate the effect of practical fuel utilization on the performance of single cells. The V-i curves obtained in various fuel compositions were successfully modeled by fitting only the exchange current density. Anodic and cathodic activation polarizations and the exchange current densities at various fuel compositions were determined. An analytical model describing reaction at the anode triple-phase boundaries was postulated based on the relationship between the anodic exchange current density and the hydrogen partial pressure in the fuel. The model predicted that the formation of water molecules from adsorbed hydrogen and hydroxyl radical was the rate-determining step in the anodic reaction.

Dye-sensitized solar cells: Scale up and current–voltage characterization

Large size, dye-sensitized solar cells (DSSC) have been prepared on silver grid embedded, transparent conductive oxide (TCO) glass substrate by screen printing method. Under one sun condition (AM 1.5, P in 100 mW cm −2), achieved active area energy conversion efficiency of 5 cm×5 cm device approaches that of small-size DSSC prepared at similar condition. To improve the accuracy of efficiency measurement, current–voltage characterizations were carried out with shadow mask and different light reflective backgrounds. It was found that transparent and stripe-like non-active area, arising due to current collector and overcoat layer, of large size DSSC does not strongly influence the energy conversion efficiency.

Multilayer High Performance Ceramic Anodes

A new approach to the design of ceramic anodes that uses a thin catalytically active functional layer that has only modest electronic conductivity sandwiched between the electrolyte and a non-catalytic electronically conducting ceramic layer that is used as the current collector is described. The anode design is flexible and allows various materials to be used in the functional and current-collector layers. Results are presented for anodes with thin functional layers (12 μm) consisting of a porous CeO2/YSZ composite impregnated 1 wt % Pd to optimize catalytic activity and a 100 μm thick layer of porous La0.3 Sr0.7TiO3 (LST) as the current collector. Low anode impedances and excellent overall performance were obtained with cells with these anodes while operating on both humidified hydrogen and hydrocarbon fuels.

Polarization measurements on single-step co-fired solid oxide fuel cells (SOFCs)

Anode-supported planar solid oxide fuel cells (SOFC) were successfully fabricated by a single step co-firing process. The cells comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ or scandia-stabilized zirconia (ScSZ) electrolyte, a (La 0.85Ca 0.15) 0.97MnO 3 (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing of the green-state cells in the temperature range of 1300–1330 °C for 2 h. Cells were tested in the temperature range of 700–800 °C with humidified hydrogen as fuel and air as oxidant. Cell test results and polarization modeling showed that the polarization losses were dominated by the ohmic loss associated with the electrodes and the activation polarization of the cathode, and that the ohmic loss due to the ionic resistance of the electrolyte and the activation polarization of the anode were relatively insignificant. Ohmic resistance associated with the electrode was lowered by improving the electrical contact between the electrode and the current collector. Activation polarization of the cathode was reduced by the improvement of the microstructure of the cathode active layer and lowering the cell sintering temperature. The cell performance was further improved by increasing the porosity in the anode. As a result, the maximum power density of 1.5 W cm −2 was achieved at 800 °C with humidified hydrogen and air.

Electrochemical Performance of Solid Oxide Fuel Cells Manufactured by Single Step Co-firing Process

Anode-supported planar solid oxide fuel cells (SOFC) were fabricated by a single step co-firing process. The cells were composed of a Ni yittria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, an industrial Ca-doped (LCM) (or lab-made LCM) YSZ cathode active layer, and an industrial LCM (or lab-made LCM) cathode current collector layer. The fabrication processes involved tape casting of the anode, screen printing of the electrolyte and the cathode, and one step co-firing of the green-state cells at for . The performance of the cells was greatly improved by optimization of these materials and fabrication processes. The electrochemical performance tests of these cells showed that they could provide a stable power density of 0.2– with hydrogen as fuel and air as oxidant while operating in the temperature range 700–. The effects of various polarization losses including ohmic polarization, activation polarization, and concentration polarization were studied by impedance spectroscopy measurements and curve-fitting experimentally measured voltage vs current density traces into an appropriate model. Based on these measurements and curve fitting results, the relationships between cell performance and various polarization losses and their dependence on temperature and microstructure, were rationalized.

Engineering of microstructure and design of a planar porous composite SOFC cathode: A numerical analysis

The electrochemical performance of a SOFC cathode depends not only on the activity of the cathode, but also on the relative facility by which the reaction participants are transported to the reaction site. The microstructure and the geometry of the cathode influence the relevant transport and reaction characteristics, and thereby the distribution of current density, in a complex manner. In this study, the influence of electrode porosity and thickness as well as current collector layer thickness and interconnect coverage was examined using a 2-dimensional across-the-channel model of a planar SOFC LSM-YSZ composite cathode. The results indicate that for a fixed functional layer thickness for the electrode, increasing the porosity of the electrode actually reduces the electrochemical performance but improves the oxygen distribution. Addition of a porous current collector layer improved both oxygen distribution and overall electrochemical performance. The results of varying interconnect coverage were very interesting in that an improved performance was observed upon reduction of coverage from 50% to 25% but no further change was observed upon further reduction to 12.5%. A closer examination of current density distribution for the cases of 25 and 12.5% coverages showed that a reduction of the coverage from 25% to 12.5% results in improving mass transport limitations under the interconnect but reduces electron transport under the channel. However, the electron transport limitation is offset by enhancement in oxygen distribution. The study highlights the need for simultaneous optimization of electrode microstructure and geometry.

A Strategy for Achieving High Performance with SOFC Ceramic Anodes

A proposal that high solid oxide fuel cell (SOFC) anode performance can be achieved by using a very thin, catalytically active functional layer, with a noncatalytic conduction layer, has been tested. An anode impedance of 0.26 Ω cm 2 was obtained at 973 K in humidified H 2 using a Ag-paste conduction layer and a 12 μm thick functional layer made from 1 wt % Pd and 40 wt % ceria in yttria-stabilized zirconia. Replacing the Ag paste with a 100 μm layer of porous La 0.3 Sr 0.7 TiO 3 (LST) had minimal effect on cell performance. The anode concept is flexible and should allow various materials to be used in the functional and the current-collector layers.

A Self-Breathing Proton-Exchange-Membrane Fuel-Cell Pack With Optimal Design and Microfabrication

An entire set of silicon-based microtechnologies is developed for a high-performance H $ _2/$ air self-breathing microproton-exchange-membrane fuel-cell ( $mu$ PEMFC) pack. For improving the performance of the silicon-based $mu$ PEMFC, microflow-fields together with the electrodes at the cathode and the anode are optimally designed. For simplifying the microfabrication, a bulk-micromachining process is developed for fabricating both the cathode and the anode. Besides that the optimally designed flow-fields and electrodes are accurate fabricated, both the cathodes and the anodes can be fabricated in a same wafer with identical process. Optimized packaging conditions, such as the compression ratio and the current-collecting layer for the membrane electrode assembly (MEA), are experimentally obtained for both high fuel-cell performance and reliable silicon micropackaging. Attributed to the optimized design and the precise microfabrication, the peak power-density of the self-breathing $mu$ PEMFC is measured as high as 141.0–147.2 mW/cm $^-2$ . For adapting the output voltage to handheld electronic systems, a thin-pad planar configuration is designed for the $mu$ PEMFC pack that consists of six single cells connected in series. The planar-configured self-breathing $mu$ PEMFC pack is micropackaged on a silicon-micromachined base-chip, with the specific power as high as 271 mW/cm $^3$ measured. Experimental results demonstrate that the fuel cells can reliably work under normal environmental temperature and humidity. 1200-h continuing power supply of the $mu$ PEMFC pack is performed, resulting in stable output of about 3 V.1643

Measuring the Current Distribution in PEFCs with Sub-Millimeter Resolution: I. Methodology

An experimental technique for measuring the current density distribution with a resolution smaller than the channel/rib scale of the flow field in polymer electrolyte fuel cells (PEFCs) is presented. The electron conductors in a plane perpendicular to the channel direction are considered as two-dimensional resistors. Hence, the current density is obtained from the solution of Laplace’s equation with the potentials at current collector and reaction layer as boundary conditions. Using ohmic drop for calculating the local current, detailed knowledge of all resistances involved is of prime importance. In particular, the contact resistance between the gas diffusion layer (GDL) and flow field rib, as well as GDL bulk conductivity, are strongly dependent on clamping pressure. They represent a substantial amount of the total ohmic drop and therefore require careful consideration. The detailed experimental setup as well as the concise procedure for quantitative data evaluation is described. Finally, the method is applied successfully to a cell operated on pure oxygen and air up to high current densities. The results show that electrical and ionic resistances seem to govern the current distribution at low current regimes, whereas mass transport limitations locally hamper the current production at high loads.

Sintering Behavior of (La,Sr)MnO3 Type Cathodes for Planar Anode-Supported SOFCs

One of the main targets in the development of anode-supported solid oxide fuel cell (SOFCs) is to improve the electrochemical performance. This can be achieved by optimizing processing and microstructural parameters of the SOFCs. Variations of the thickness of the cathode functional layer and the cathode current collector layer, the grain size of the powders used for applying these layers, and the sintering temperature, can influence the electrochemical performance as such that lower operation temperatures become possible without detrimentally affecting the power output to a great extent. In this study the effect of variations of the sintering temperature of the cathode on (1) the microstructure, (2) the gas diffusivity and permeability in the cathode, and (3) electrochemical performance of FZJ-type anode-supported single cells, was investigated. The FZ-Jülich cell design is based on anode-supported type cells, which are characterized by a relatively thick anode (thickness: 1.0-1.5mm) consisting of a NiO/8YSZ cermet, a thin 8YSZ electrolyte, and a bi-layered cathode. The cathode distinguished two separated layers: first a cathode functional layer consisting of La0.65Sr0.3MnO3(LSM)∕Y2O3-stabilized ZrO2 (8YSZ) and a cathode current collector layer of pure La0.65Sr0.3MnO3 (LSM). This study can be considered as a follow-up of that (Journal of Power Sources 141 (2005) 216–226) describing the improvement of the cell performance by a systematic variation of the microstructure. The experiments described in this paper and the corresponding results are part of a more extensive study to investigate in more detail the effect of the sintering temperature on the electrochemical performance of LSM-type SOFCs. Since research is still going on, conclusions, drawn in this contribution, are yet not definitive.

Performance Improvement of (La,Sr)MnO3 and (La,Sr)(Co,Fe)O3-Type Anode-Supported SOFCs

Targets in the development of anode-supported or planar solid oxide fuel cells (SOFCs) are low operation temperatures, high durability, high reliability, high power density, and low production costs. During the past ten years steps have already been taken at Forschungszentrum Jülich to lower the operating temperatures while maintaining the power output. This was achieved by optimizing processing and microstructural parameters of the electrodes. This paper presents the latest results concerning performance improvement through variations of the processing route and the microstructure of La0.65Sr0.3MnO3 (LSM) and La0.58Sr0.4Co0.2Fe0.8O3−δ (LSCF)-type SOFCs. In the case of the LSM-type single cells, the following aspects relating to the electrochemical performance were investigated in more detail: (1) production of the anode substrate by tape casting versus warm pressing; (2) deposition of the anode functional layer (AFL) and electrolyte by screen printing versus vacuum slip casting; (3) use of noncalcined and non-ground YSZ for applying the cathode functional layer (CFL); and (4) sintering temperature of the CFL and cathode current collector layer (CCCL). In the case of LSCF-type cells, a systematic approach was initiated for optimizing the Ce0.8Gd0.2O2−δ (CGO) diffusion barrier layer: (1) deposition techniques of the CGO layer and (2) sintering temperature of the screen-printed CGO layer. Results have shown that certain modifications of the processing route led to a slightly lower electrochemical performance, whereas others did not affect the performance at all. Regarding LSCF-type SOFCs, a slight improvement of the performance was achieved by optimizing the sintering temperature of the CGO layer.

Microstructural Optimization of Anode-Supported Solid Oxide Fuel Cells by a Comprehensive Microscale Model

The microstructural optimization of two-layer cathodes in anode-supported solid oxide fuel cells (SOFCs) was numerically performed by a comprehensive microscale model developed by the authors. The single-cell performances of anode-supported SOFCs, recently reported by Zhao and Virkar, were studied to provide the detailed information on the electrochemical processes occurring in the SOFCs. Good agreements between the numerical and experimental results were observed which also ensured the validity of the present calculation. The dependence of the electrochemical reaction and the mass transport on the particle size of each layer in two-layer cathodes was then studied. The optimal microstructure for two-layer cathodes was determined as a mean particle diameter of and a thickness of for cathode functional layers, and a mean particle diameter of and a thickness of for cathode current collector layers. The stack-cell performances of anode-supported SOFCs were also studied by fully considering the effect of interconnect rib geometry, e.g., the contact resistance, in-plane ohmic loss, and nonuniformity of current generation. Based on the simulation results, the geometrical criterion for interconnect rib geometry to obtain better performance was discussed.

High Performance Single Step Co-Fired Solid Oxide Fuel Cells

AbstractAnode-supported planar solid oxide fuel cell (SOFC) was successfully fabricated by a single step co-firing process. The cell comprised of a Ni + yittria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, a Ca-doped LaMnO3 (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single step co-firing of the green-state cells at 1300°C for 2 hours. The maximum power densities were 1.50W/cm2 at 800°C, and 0.87W/cm2 at 700°C with humidified hydrogen (97% H2-3% H2O) as fuel and air as oxidant. The experimentally measured I-V curves were fitted into a polarization model to obtain cell parameters including the area specific ohmic resistance, exchange current density, effective binary diffusivity of hydrogen and water vapor in the anode, and that of oxygen and nitrogen in the cathode. The cell was also tested at 800°C with various compositions of humidified hydrogen to evaluate the cell performance at high fuel utilization, and the maximum power densities were 1.30W/cm2 with 75.7% H2-24.3% H2O, 0.94W/cm2 with 49.4% H2-50.6% H2O, and 0.49W/cm2 with 27.3% H2-70.7% H2O.

The Electrochemical Performance of Anode-Supported SOFCs with LSM-Type Cathodes Produced by Alternative Processing Routes

The electrochemical performance of La0.65Sr0.3MnO3-type (LSM) anode-supported single cells, produced by alternative production processes, has been investigated at intermediate temperatures. In particular, three different variations of the production route were investigated in more detail: (1) the use of nonground LSM powder for the cathode current collector layer, (2) the use of noncalcined and nonground YSZ powder for the cathode functional layer, and (3) the use of tape casting versus warm pressing as the production process for anode substrates. Results from electrochemical measurements performed between 700 and 900°C with H2 (3vol%H2O) as fuel gas and air as the oxidant showed that performance increased with increasing grain size of the outer cathode current collector layer: the highest performance was achieved with nonground LSM powder. Furthermore, performance was not adversely influenced by the use of noncalcined and nonground YSZ for the cathode functional layer. Also the use of anode substrates with a thickness of about 0.7mm and produced by tape casting, instead of those with a thickness of about 1.5mm and applied by warm pressing, did not detrimentally affect the electrochemical performance of this type of SOFC.

Effect of cathode current-collecting layer on unit-cell performance of anode-supported solid oxide fuel cells

According to the characterization of the microstructure and properties of solid oxide fuel cells (SOFC) electrodes, it is essential to verify various processing variables to control microstructural parameters, such as particle size, composition and spatial distribution of the constituent phases of the electrode in order to reduce the ohmic and diffusional polarization losses of the unit-cell performance. From this viewpoint, a current-collecting layer with controlled microstructure very effectively enhances the unit-cell performance by reducing the ohmic and polarization resistance of the cathode. The maximum power density of a 5 cm × 5 cm unit-cell with the controlled current-collecting layer is ∼1.5 W cm −2 at 750 °C, while a unit-cell without the layer is much lower, viz., 0.9 W cm −2.

Dual-chemistry cathode system for high-rate pulse applications

A novel electrode design was developed and used to construct cells for high-rate pulsing applications. In this electrode design, a cathode material with high energy density, such as carbon monofluoride (CF x ), is sandwiched between two layers of current collectors that in turn are sandwiched between two layers of cathode materials with high power capability, such as silver vanadium oxide (SVO). Lithium cells constructed with such cathodes exhibited power capability similar to SVO cells, but with higher volumetric and gravimetric capacity. The advantages of both SVO and CF x materials were efficiently utilized.