CERMET ELECTRODES

Abstract: Cermet electrodes consisting of metal particles (7) of nickel, cobalt, iron, or alloys or mixtures thereof im­ mobilized by zirconia (8) stabilized in cubic form which contains (9) from 0.1 to 5% by weight of praseodymium, dys­ prosium, terbium, or an alloy or mixture thereof. The solid oxide electrode can be made by covering a substrate with particles of nickel, cobalt, iron, or alloys or mixtures thereof, growing a stabilized zirconia solid oxide skeleton around the particles thereby immobilizing them, contacting the skeleton with a compound of praseodymium, dysprosium, terbium, or an alloy or a mixture thereof, and heating the skeleton to a temperature of at least 500°C. The electrode can also be made by preparing a slurry of nickel, cobalt, iron, or alloy or mixture thereof and a compound of praseodymium, dysprosium, terbium, or an alloy or a mixture thereof, depositing the slurry on a substrate, heating the slurry to dryness, and growing a stabilized zirconia skeleton around the metal particles.

Description: This invention relates to cermet electrodes. Solid oxide cells can be operated in either an electrolysis mode or in a fuel cell mode. In an electrol­ ysis mode, electricity and steam are supplied to the cell which then decomposes the steam to form hydrogen and oxygen. In the fuel cell mode, the cell operates in the reverse, burning a fuel such as hydrogen, or methane to produce heat and electricity. When standard solid oxide cells are operated in an electrolysis mode, they charac­ teristically respond to increases in water vapor partial pressure with an increase in the over-potential. As a consequence, higher driving voltages are required at high water vapor partial pressures. This means that the cell is less efficient because more voltage is required at a given current in order to produce a given amount of hydrogen and oxygen. A similar effect is noticed when the cell is operated in a fuel cell mode as the increased over-poten­ tial (i.e., an increased voltage drop within the cell, results in less electricity being produced for a given amount of fuel. Accordingly, the present invention resides in a cermet electrode characterized in that said electrode comprises particles of at least one of nickel, cobalt and iron, or an alloy thereof, immobilized by zirconia stabil­ ized in cubic form containing from 0.1 to 5% by weight of at least one of praseodymium, dysprosium and terbium, or an alloy thereof. The invention also includes a method of making a cermet electrode characterized by covering a substrate with particles of at least one of nickel, cobalt and iron, or an alloy thereof; growing a stabilized zirconia solid oxide skeleton around said particles, thereby immobilizing said particles and binding them to said substrate; contacting said skeleton with a compound of at least one of praseo­ dymium, dysprosium and terbium, or an alloy thereof; and heating said skeleton at a temperature of at least 500°C. Further according to the invention is a method of making a cermet electrode characterized by preparing a slurry of particles of at least one of nickel, cobalt, and iron, or an alloy thereof; and a compound of at least one of praseodymium, dysprosium, and terbium, or an alloy thereof; depositing said slurry on a substrate; heating said slurry to dryness; and growing a stabilized zirconia skeleton around said particles thereby immobilizing said particles and binding them to said substrate. If a very small amount of praseodymium oxide is added to the fuel electrode of a solid oxide cell, the over-potential can be reduced by about 30% in the elec­ trolysis mode at 200 mA, which means that about 30% of the energy needed to make hydrogen is saved. This effect of lowering the over-potential also occurs when the cell is operated in the fuel cell mode. In addition, the addition of praseodymium oxide lowers the resistance of the cell by about half, which greatly increases the efficiency of the cell in both the electrolysis mode and the fuel cell mode. The praseodymium oxide can be easily added either during the preparation of the fuel cell or after the fuel cell has been made, and its presence does not adversely affect the other desirable qualities of the cell. In order that the invention can be more clearly understood, convenient embodiments thereof will now be described, by way of example, with reference to the accom­ panying drawings in which: Figure 1 is an isometric cross-sectional view of a solid oxide cell; and Figure 2 is an enlarged cross-sectional side view showing details of the electrolyte and fuel electrode portions of the cell of Figure 1. Referring to Figures 1 and 2, a ceramic oxide support tube 1 is coated with a ceramic oxide air electrode 2. Over air electrode 2 is a ceramic oxide electrolyte 3 and a cermet fuel electrode 4. An interconnection 5 is covered with fuel electrode material 6 to connect one cell to the adjoining cell (not shown). In Figure 2, electro­ lyte 3 provides a substrate for the fuel electrode 4. The fuel electrode consists of metal particles 7 which are immobilized and bonded to electrolyte 3 by means of ceramic oxide skeleton 8. The praseodymium oxide 9 is disposed throughout the fuel electrode in a manner depicted in Figure 2. In the fuel cell mode, a fuel is typically passed over the outside of the cell and a source of oxygen is passed through the inside of the cell. The oxygen migrates through the cell as ions and reacts at the fuel electrode to generate electricity which is conducted through the fuel and air electrodes. In the electrolysis mode, steam is typically passed over the outside of the tube and electric­ ity from an external source is provided through the fuel and air electrodes. The steam is electrolyzed in the fuel electrode and the oxygen that is produced migrates to the center of the tube. A full and complete description of solid oxide cells and their operation can be found in U.S. Patent Specification Nos. 4,490,444; 4,395,468; and 4,374,163. The fuel electrode of this invention is prepared from finely-divided nickel metal particles dispersed through zirconia stabilized in cubic form. Typical stabi­ lizers include yttria, calcia, magnesia, and scandia, but the preferred stabilizer is yttria because the highest electrical conductivities can be obtained. From 7 to 13 mol % of the stabilizer is added to the zirconia to stab­ ilize it in cubic form. The stabilized zirconia/nickel cermet should contain from 0.1 to 5% by weight of praseo­ dymium, (based on the total weight of the cermet electrode, including the weight of the metal particles therein). If less than 0.1% is used, very little effect will be seen on the over-potential or cell resistance, and more than about 5% may plug the pores in the electrode and prevent the hydrogen from migrating to the surface in the electrolysis mode. In addition to praseodymium, dysprosium, terbium, or alloys or mixtures of the three elements can also be used in the same manner. The praseodymium, dysprosium, or terbium in the electrode is believed to be in the form of an oxide such as Pr₆O₁₁ or Pr₂O₃. The metal powder in­ cluded in the cermet electrode can be nickel, iron, cobalt, or an alloy or mixture thereof. Nickel is preferred because it displays the most resistance to oxidation. The particles preferably have a particle size of from 3 microns to 5 microns and preferably constitute from 35 to 45% by volume of the volume of the fuel electrode. There are two methods by which the electrode can be prepared. In the first method, the nickel particles are spread on a substrate, such as the electrolyte, and a zirconia skeleton is grown around the particles to immobil­ ize them and bind them to the electrolyte. This is conven­ tionally done by an electrochemical vapor deposition process (EVD). A solution of a compound of praseodymium, dysprosium, or terbium is then prepared. The compound may be, for example, a nitrate, chloride, acetate, or oxide; acetates are preferred as they are more soluble and less corrosive. Any solvent can be used in preparing the solution, but water is preferred because it is non-­ flammable and is not polluting. A suitable concentration is from 1 molar to 2 molar, although other concentrations can also be used. A sufficient amount of this solution is placed over the cermet electrode to provide a pick-up of from 0.1% to 5% by weight of the praseodymium, dysprosium, or terbium (based on the weight of the element Pr, Dy, or Tb as a per cent of the fuel electrode weight) into the electrode. The electrode is then placed into operation at a temperature in excess of 500°C, which decomposes the compound and forms the oxide of praseodymium, dysprosium, or terbium. The second method of preparing the cermet elec­ trode according to this invention is to prepare a slurry of the nickel particles and include a soluble praseodymium, dysprosium, or terbium compound in the nickel slurry at a concentration such that the pick-up of the praseodymium dysprosium, or terbium in the electrode will be from 0.1% to 5.0% (based on the weight of the element Pr, Dy, or Tb as a percent of the total electrode weight). The slurry is then placed over the substrate and is heated to evaporate the liquid that forms the slurry. While any liquid can be used in forming the slurry, water is preferred as it is non-flammable and is not polluting. The zirconia skeleton is then deposited around the coated metal particles. Since the electrochemical vapor deposition conventionally used to deposit the skeleton is normally conducted at about 1300°C, the liquid in the slurry will evaporate if electrochemical vapor deposition is used to prepare the zirconia skeleton. In both methods, care must be taken not to heat the electrode to the melting point of the zirconia, the praseodymia, the dysprosia, or the terbia or the nickel that is formed, because melting will result in a closure of the pores in the electrode and prevent the migration of gases to the electrode. Of the two methods, the first is preferred as it entails less chance of vaporizing the additive material in the electrochemical vapor deposition reactor. The solid oxide cell of this invention is partic­ ularly useful in an electrolysis process where more than 75% steam is present, as the cell tends to resist over-­ potential increases under those conditions much more than does a conventional cell. The invention will now be illustrated with reference to the following Example: EXAMPLE The following is a description of the components of the solid oxide cell used in these experiments. The support tube consisted of 15.1 mol % calcia stabilized zirconia closed at one end with a nominal outside diameter of 12 mm, a nominal wall thickness of 1.5 mm, and a porosi­ ty of approximately 30%. The anode (i.e., the air elec­ trode) consisted of sintered strontium doped lanthanum manganite having a thickness of about 0.9 to 1 mm, a porosity of approximately 40% and a sheet resistance (Ω/□) of approximately 0.2 ohms at 1000°C. The electrolyte was 10 to 13 mol % yttria stabilized zirconia having a thick­ ness of 0.025 to 0.040 mm and a ρ of approximately 10 ohm-cm. The cathode (i.e., the fuel electrode) was a nickel zirconia cermet having a thickness of approximately 0.15 mm, a porosity of about 40% and a sheet resistance of approximately 0.1 ohms at 1000°C. This electrode contained 40% by volume nickel and 20% by volume zirconia. The interconnection was magnesium-doped lanthanum chromite having a thickness of about 0.04 mm and an average ρ of 0.7 ohm-cm. The anode and the cathode contact was nickel felt 1 mm x 5 mm x 220 mm and the anode and cathode bus bars were nickel 3 mm x 7 mm x 220 mm. The cell edge location on the tubes was 9 mm from the closed end. An H₂/H₂O gauge external to the system monitored exiting product gas. This information, coupled with flow meter readings, was used to estimate hydrogen production for comparison with the ammeter/time (coulombic) estimates. The active cell area is, in general, 50 sq. cm. to 100 sq. cm., depending on actual length. A cell containing 1% praseodymium in the cermet electrode was compared to an identical cell that did not contain the praseodymium in the cermet electrode. The voltage on the cells was increased until the desired current was obtained. Figure 3 which is a graph of cell current versus cell voltage gives the results of these tests. The area to the right of the zero line in Figure 1 represents the electrolysis mode and the area to the left represents a fuel cell mode. In the fuel cell mode, fuel is passed to the cell and the voltage and current is measured. In Figure 3, in cell A, the praseodymium was added after cell construction by the absorption of aqueous praseodymium acetate directly onto the nickel cermet electrode. The acetate subsequently decomposed and the praseodymium was incorporated into the electrode as an oxide when it was heated to 1000°C in 10% H₂/90% H₂O vapor. In cell B, the praseodymium was added in solution as the acetate as a component of the aqueous nickel slurry em­ ployed in applying and processing the nickel cermet elec­ trode. In this case, the praseodymium component was reacted up to 1300°C in the electrochemical vapor deposi­ tion process used to make the skeleton portion of the electrode. Cells C and D are controls and do not have any praseodymium added. Figure 3 shows that as a result of adding praseodymium to the electrode, the polarization of the cell was lower in both the electrolysis cell mode and the fuel cell mode of operation. Figure 4 which is a graph of cell resistance versus hours on test shows the results of resistance tests performed upon the cells using the electrolysis mode of operation. Cells E and F contained praseodymium deposited by the same method employed for cell B, and cell G is a control cell that did not contain praseodymium. Figure 4 shows that the cell resistance is much lower when praseo­ dymium is added, an added benefit occurring because of the presence of electrically conductive praseodymium oxides.

Claims: 1. A cermet electrode characterized in that said electrode comprises particles of at least one of nickel, cobalt and iron, or an alloy thereof, immobilized by zirconia stabilized in cubic form containing from 0.1 to 5% by weight of at least one of praseodymium, dysprosium and terbium, or an alloy thereof. 2. A cermet electrode according to claim 1 characterized in that the metal particles are of nickel. 3. A cermet electrode according to claim 1 or 2, characterized in that the zirconia is stabilized with yttria. 4. A ceramic oxide cell comprising a ceramic oxide electrolyte having a ceramic oxide air electrode on one side and a cermet electrode on the other side, charac­ terized in that the cermet electrode is as claimed in claim 1, 2 or 3. 5. A ceramic oxide cell in the shape of a tube with an air electrode on the inside of said tube and a cermet electrode on the outside of said tube, characterized in that the cermet electrode is as claimed in claim 1, 2 or 3. 6. A method of making a cermet electrode charac­ terized by covering a substrate with particles of at least one of nickel, cobalt and iron, or an alloy thereof; growing a stabilized zirconia solid oxide skeleton around said particles, thereby immobilizing said particles and binding them to said substrate; contacting said skeleton with a compound of at least one of praseodymium, dysprosium and terbium, or an alloy thereof; and heating said skeleton at a temperature of at least 500°C. 7. A method according to claim 6, characterized in that the zirconia is stabilized with yttria. 8. A method according to claim 6 or 7, charac­ terized in that the skeleton is grown using electrochemical vapor deposition. 9. A method according to claim 6, 7 or 8, characterized in that the compound is an acetate. 10. A method according to claim 6, 7, 8 or 9, characterized in that the compound is in aqueous solution of 1 to 2 molar concentration. 11. A method according to claim 10, character­ ized in that the amount of the solution is sufficient for a pick-up of praseodymium, dysprosium and/or terbium in said electrode of from 0.1 to 5.0% by weight. 12. A method of making a cermet electrode characterized by preparing a slurry of particles of at least one of nickel, cobalt, and iron, or an alloy thereof; and a compound of at least one of praseodymium, dysprosium, and terbium, or an alloy thereof; depositing said slurry on a substrate; heating said slurry to dryness; and growing a stabilized zirconia skeleton around said particles thereby immobilizing said particles and binding them to said substrate. 13. A method according to claim 12, character­ ized in that the slurry is made with water. 14. A method according to claim 12 or 13, characterized in that the skeleton is grown using electro­ chemical vapor deposition.

Inventors: MASKALICK, NICHOLAS JOHN
Applicants: WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1987
Main classification: H01M8
IPC codes: H01M8, C25B11, H01M4