Uday B. Pal

Electrochemical Vapor Deposition of Yttria‐Stabilized Zirconia Films

Tubular solid oxide fuel cells employ yttria-stabilized zirconia electrolyte film as an oxygen ion conductor at high temperatures. These yttria-stabilized zirconia electrolyte films are deposited by an electrochemical vapor deposition (EVD) process. The electrochemical transport of oxygen ions during the EVD process is analyzed by measuring the film growth as a function of EVD reaction time; the film growth is found to be parabolic with time. Wagners transport theory for parabolic growth and the defect model for yttria-stabilized zirconia have been used to calculate the average electronic transport number and the partial electronic conductivity of the electrolyte film. The analysis of the data revealed that the electrolyte film growth is controlled by diffusion of electrons. It is also shown that the electrochemical transport that occurs during EVD of the electrolyte is similar to the phenomena of oxygen semipermeability wherein electrons migrate from the low-oxygen partial pressure site to the high-oxygen partial pressure side, and oxygen ions migrate in the reverse direction maintaining charge neutrality

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 LaMnO 3 (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 1300°C for 2 h. 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 0.2-1.0 W/cm 2 with hydrogen as fuel and air as oxidant while operating in the temperature range 700-900°C. 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.

Analytic Solution for Charge Transport and Chemical‐Potential Variation in Single‐Layer and Multilayer Devices of Different Mixed‐Conducting Oxides

Transport phenomena are described involving one mobile ionic species (oxygen ions) and electronic carriers (electrons and holes) in single- and multilayer devices of different mixed-conducting oxides under the influence of an external load and an oxygen-chemical-potential gradient. The analysis utilizes the intrinsic ionic and electronic transport properties of the oxide layers and explicitly computes the ionic and electronic fluxes, the potential drop across the layers, and the chemical-potential variation in the layers. Some limiting cases are solved to illustrate the generality of the analysis. Based on this analysis, the accuracy, limitations, and physical significance of using an equivalent circuit to represent transport in such devices are discussed. The engineering implications of the analysis, in terms of designing efficient, stable devices with layered structures for fuel cells, sensors, separation membranes, and batteries, are also provided.

Solid oxide membrane technology for environmentally sound production of titanium

Abstract Despite its attractive properties, titanium is limited in use by its high price, due to the cost of smelting and processing the ore. The solid oxide membrane (SOM) process aims to consolidate most of the processing steps required for conventional titanium production into a single step, making it an energy efficient and cost effective method for producing CP billet and ingot and possibly also powders used to make titanium alloys. In the SOM process experiment, a steel crucible contains MgF2–CaF2–TiO2 flux; an inert metal/carbon rod serves as the cathode; an oxygen ion conducting yttrium stabilised zirconia (YSZ) membrane in the form of a one end closed tube contains either a liquid metal that acts as the anode or a liquid MgF2–CaF2 ionic flux that connects the YSZ membrane to an anode. During electrolysis, titanium ions are reduced at the cathode while the oxygen ions pass through the YSZ membrane and are oxidised at the anode by a reducing agent, hydrogen gas or carbon, forming steam or CO(g) respectively. To date, a suitable MgF2–CaF2–TiO2 flux has been selected and an optimum operating temperature has been determined. Several electrolysis experiments have been performed. As expected, lower valence deposits of titanium oxides have been witnessed at the cathode before depositing pure titanium. Continuing work will aim to extend the electrolysis time to pass enough charge to produce a significant amount (100 g) of titanium at the cathode.

Analysis of Electrochemical Performance of SOFCs Using Polarization Modeling and Impedance Measurements

Anode-supported planar solid oxide fuel cells (SOFCs) were fabricated by a single-step cofiring process. It comprised of a porous Ni + yttria-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 LaMnO 3 (LCM) + YSZ composite cathode active layer, and a porous LCM cathode current collector layer. The cell was tested between 700 and 800°C with humidified hydrogen (97% H 2 + 3% H 2 O) as the fuel and air as the oxidant. The measured maximum power densities were 1.42 W/cm 2 at 800°C, 1.20 W/cm 2 at 750°C, and 0.87 W/cm 2 at 700°C. The cell was also tested at 800°C with various compositions of fuel and oxidant, and the cell parameters, which included the area specific ohmic resistance, exchange current densities, and anodic and cathodic limiting current densities, were obtained through polarization modeling. The polarization resistances of the cell were calculated using the cell parameters obtained from modeling, and compared with the corresponding values measured using impedance spectroscopy. The effect of cell operating conditions on various polarization losses and performance was analyzed in detail using the polarization modeling results.

Effect of oxygen-containing species on the impedance of the Pt/YSZ interface

The modifications induced by oxygen on the impedance spectra of Pt/YSZ (platinum/yttria-stabilized zirconia) interfaces at 900 °C have been studied. Oxygen atmosphere and oxygen transfer utilizing both positive and negative direct currents influenced the concentration of oxygen-containing species (OCS) at the interface and that is shown to affect the shape of the impedance response. Two main types of OCS have been identified. The effect of chemical stripping of the OCS by utilizing a reducing COCO2 atmosphere was also investigated. A mechanism for oxygen transfer across the Pt/YSZ interface is suggested. It involves two parallel paths: direct, fast interfacial transfer of the O2− and a slow path, wherein the OCS participate.

Thermodynamic stability and interfacial impedance of solid-electrolyte cells with noble-metal electrodes

AbstractIn solid-electrolyte cells, the electrode-electrolyte interfacial stability and impedance are found to be dependent on temperature, atmosphere, current density, microstructure and the process history of the cell. The modifications induced by temperature and oxygen pressure on the impedance spectra of Pt/Yttria-stabilized zirconia (YSZ) and Pd/YSZ interfaces have been studied. The interfacial impedance was controlled by adsorption/desorption of oxygen with a Langmuir-type dependency. When the surface coverage was small, the interfacial impedance decreased with increase in temperature and

Transient and Permanent Effects of Direct Current on Oxygen Transfer across YSZ‐Electrode Interfaces

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