The present invention relates generally to the field of electrochemical devices, and more particularly solid state electrochemical devices composed of one or more electrodes in contact with a solid state electrolyte and/or membrane.
Solid state electrochemical devices are often implemented as cells including two electrodes, the anode and the cathode, and a dense solid electrolyte/membrane which separates the electrodes. In many implementations, such as in fuel cells and oxygen and syn gas generators, the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as oxygen ions, sodium ions, or hydrogen ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane is composed of a mixed ionic electronic conducting material (“MIEC”). In each case, the electrolyte/membrane must be gas-tight to the electrochemical reactants. In all of these devices a lower total internal resistance of the cell results in improved performance.
The preparation of solid state electrochemical cells is well known. For example, a typical solid oxide fuel cell is composed of a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a ceramic, a metal or, most commonly, a ceramic-metal composite (“cermet”), in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of an ionically/electronically-conductive metal oxide on the oxidant side of the cell. Electricity is generated through the electrochemical reaction between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically air). This net electrochemical reaction involves mass transfer and charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase (fuel or oxygen). The contribution of these charge transfer steps, in particular the charge transfer occurring at the oxygen electrode, to the total internal resistance of a solid oxide fuel cell device can be significant.
Electrode structures including a porous layer of electrolyte particles on a dense electrolyte membrane with electrocatalyst material on and within the porous layer of electrolyte are known. As shown in FIG. 1, such electrodes are generally prepared by applying an electrocatalyst precursor-containing electrode material 102 (such as a metal oxide powder having high catalytic activity and high reactivity with the electrolyte) as a slurry to a porous (pre-fired; unsintered; also referred to as “green”) electrolyte structure 104, and then co-firing the electrode and electrolyte materials to densify the electrolyte and form a composite electrolyte/electrode/electrocatalyst 106.
Oxides containing transition metals such as Co, Fe, Mn, are known to be useful as oxygen electrodes in electrochemical devices such as fuel cells, sensors, and oxygen separation devices. However, if such compounds were to be used with typical zirconia-based electrolytes, such as YSZ, a deleterious reaction in the temperature range of 1000-1400° C. typically needed to densify zirconia would be expected. The product of this reaction would be a resistive film 105 at the electrode/electrolyte interface, thereby increasing the cell's internal resistance.
Similar problems may be encountered with sintering highly catalytic electrode materials on densified (fired) zirconia-base electrolytes since the sintering temperatures of about 1200° C. to 1400° C. are sufficient to cause the formation of a deleterious resistive film at the electrode/electrolyte interface.
In order to avoid deleterious chemical reactions, attempts have been made to use barrier layers, such as ceria, or to use chemically compatible electrolytes, such as ceria with such transition metal oxides. Also, it has been proposed to add an electrocatalytic precursor to a fired electrode/electrolyte composite, but only for a specific type of electrode material. Specifically, prior researchers have sought to fabricate electrodes with interpenetrating networks of ionically conductive and electronically conductive materials with subsequent infiltration of a catalytic electrode. However, this can hinder the performance of ionic devices, particularly at high current densities where ohmic drop due to current collection can lead to substantial efficiency losses.
It would be desirable to have improved techniques for fabricating high performance solid state electrochemical device oxygen electrodes composed of highly conductive materials.