The present invention generally relates to electrochemical cells. In particular, the present invention relates to a third electrode, charging electrode, or counter-electrode (hereinafter simply referred to as a third electrode) for a metal-air battery formed from a mixture of a lanthanum nickel compound and a conductive metal, and to a method for forming the same.
Recently, in response to increasing environmental concerns, research into alternatively powered vehicles has increased. One area of research which has drawn particular interest is battery powered vehicles. Such vehicles are more environmentally friendly than conventionally powered gas or diesel vehicles because the amount of harmful emissions (e.g., CO2, NOx and hydrocarbons) is vastly reduced or eliminated.
One type of battery that has drawn particular interest is metal-air cells. Metal-air cells are well known and desirable as light-weight sources of power. Metal-air cells utilize oxygen from ambient air as a reactant in an electrochemical reaction. Metal-air cells can include an air permeable electrode as a cathode and a metallic anode surrounded by an aqueous electrolyte. Metal-air cells function through the reduction of oxygen from ambient air which reacts with the metal to generate an electric current. For example, in a zinc-air cell, the anode contains zinc, and during operation, oxygen from the ambient air along with water and electrons present in the cell are converted at the cathode to hydroxyl ions. Conversely, at the anode zinc atoms and hydoxyl ions are converted to zinc oxide and water, which releases the electrons used at the cathode portion of the cell. Thus, the cathode and anode acting in concert generate electrical energy.
Specifically, the reactions which take place at the cathode and anode are shown in detail below.
Cathode Reaction: O2+2H2O+4exe2x88x92xe2x86x924OHxe2x88x92
Anode Reaction: 2Zn+4OHxe2x88x92xe2x86x92ZnO+2H2O+4exe2x88x92.
Cells that are useful for only a single discharge cycle are called primary cells, and cells that are rechargeable and useful for multiple discharge cycles are called secondary cells. An electrically rechargeable metal-air cell is recharged by applying voltage between the anode and the cathode of the cell and reversing the electrochemical reaction. During recharging the cell discharges oxygen to the atmosphere through the air permeable cathode.
There are two main types of rechargeable metal-air cells. The first type includes those with three electrodes (i.e., tricells), namely, an anode, a unifunctional cathode, and a counter-electrode (i.e. a third electrode). The unifunctional cathode is used during the discharge cycle of the metal-air and is incapable of recharging the cell. The counter-electrode is required to recharge the metal-air cell. The second type of metal-air cells include two electrodes. The bifunctional electrodes function in both the discharge mode and the recharge mode of the cell, thus eliminating the need for a third electrode. However, bifunctional electrodes suffer from a major drawback. That is, they do not last long because the recharging cycle deteriorates the discharge system (i.e., bifunctional cells suffer from decreasing performance as the number of discharge/recharge cycles increase). Thus, in view of the above, tricells are advantageous when compared to bifunctional cells in that they offer more stable performance over a greater number of discharge/recharge cycles.
One of the important aspects of a metal-air cell is the two types catalysts used for oxygen reduction during discharge and for oxygen evolution during recharge. For example, catalysts which can be used for the oxygen reduction catalyst include silver, platinum, platinum-ruthenium, nickel spinel, nickel perovskites, and iron, nickel, or cobalt macrocyclics. On the other hand, oxygen evolution catalysts can include, for example, tungsten compounds such as CoWO4, WC, WS2, and WC containing fused cobalt. In addition, metal oxides such as LaNiO3, NiCo2O4 and Co3O4 are known to be useful as oxygen evolution catalysts.
Furthermore, most metals and alloys, even platinum and other precious metals and alloys, are not good oxygen evolution catalysts in strong alkaline electrolytes. They either suffer from low activity or their activity fades quickly after just a small number of discharge/recharge cycles. In addition, some active metal/metal oxides such as Ru tend to dissolve in an alkaline system. Also, most if not all metal and alloys suffer from oxidation due to the high electrical potential generated in the metal-air cell.
Metal oxides such LaNiO3, NiCo2O4 and Co3O4 have been bonded with TEFLON(copyright) (a tetrafluoroethylene fluorocarbon polymer) and tested as a third electrode in a tricell. Such third electrodes suffer from the problem that bonded oxides on the surface of the third electrode flake off (i.e., delaminate) during oxygen evolution tests. Due to delamination, such third electrodes lose their activity over time.
One solution to the above problem is disclosed in U.S. Pat. No. 4,497,698 to Bockris et al. U.S. Pat. No. 4,497,698 discloses sintering pure LaNiO3 powder to a ceramic form, then binding the ceramic LaNiO3 to copper wire with a silver epoxy resin. Such an electrode, when tested, has been found to be stable for over 1000 hours of oxygen evolution. However, such a solid electrode is difficult to use as a third electrode in a secondary metal-air cell (i.e. a tricell), for example a zinc-air cell. This is because the solid electrode cannot be put between the metal electrode (e.g., the zinc electrode) and the air electrode (e.g., a platinum electrode) as there is no opening for the electrolyte (e.g., sodium hydroxide) to flow through the third electrode. Such a structure therefore restricts the flow of ions (e.g., hydroxyl ions) between the metal electrode and the air electrode.
Additionally, it can be difficult to bind/adhere a LaNiO3 ceramic to metal to form an electrode. This is because the interface must be carefully protected with extra epoxy or the like, otherwise it will oxidize quickly. Also the silver in the epoxy used to bind the LaNiO3 ceramic to the metal can also oxidize. Finally, an electrode which utilizes silver epoxy to bind the LaNiO3 ceramic to the metal is thick and therefore contributes to an undesirable increase in both battery size and weight. This in turn leads to a decrease in battery performance.
In view of the above, there is a need in the art for improved metal-air batteries.
The present invention provides a third electrode (i.e. a counter-electrode) for use in a metal-air tricell battery formed from a mixture of an lanthanum nickel compound and at least one metal oxide, and support structure such as a mesh screen, a metal planar sheet or a series of rigid rods or wires, and to a method for forming the same. The present invention provides a third electrode that not only produces oxygen during recharge (or initial charge) for a metal-air tricell but also provides stable performance over a large number of discharge/recharge cycles. This in turn leads to improved metal-air batteries based on tricells which incorporate such a third electrode.
In one embodiment, the present invention relates to a third electrode for use in a metal-air tricell comprising a support structure coated with a layer of a lanthanum nickel compound/at least one metal mixture, wherein the mixture is adhered to the support structure without the use of an adhesive.
In another embodiment, the present invention relates to a metal-air tricell comprising: an air electrode; a metal electrode; and a third electrode, wherein the third electrode comprises a support structure coated with a mixture of a lanthanum nickel compound and at least one metal, wherein the mixture is adhered to the support structure without the use of an adhesive.
In another embodiment, the present invention relates to a method of forming a third electrode for use in a metal-air tricell comprising the steps of: (A) applying a mixture of a lanthanum nickel compound and at least one metal oxide to a support structure, thereby yielding a coated support structure; and (B) heating the coated support structure in order to reduce the metal oxide present in the lanthanum nickel compound/metal oxide mixture to its corresponding metal and to adhere the lanthanum nickel compound/metal mixture to the support structure, thereby yielding a third electrode wherein the third electrode is free of an adhesive.
In one embodiment, step (A) of the above mentioned method comprises: (A-1) mixing a lanthanum nickel compound, at least one metal oxide and at least one dispersant to form a lanthanum nickel compound/metal oxide suspension; and (A-2) applying the lanthanum nickel compound/metal oxide suspension to the support structure to yield a coated support structure.
In one embodiment, step (B) of the above mentioned method comprises heating the coated support structure for about 5 to about 20 hours at a temperature in the range of 150xc2x0 C. to about 1350xc2x0 C.
In another embodiment, step (B) of the above mentioned method comprises: (B-1) heating the coated support structure at a first temperature sufficient to reduce the metal oxide present in the lanthanum nickel compound/metal oxide mixture to its corresponding metal; and (B-2) heating the coated support structure at a second temperature sufficient to adhere the lanthanum nickel/metal mixture to the support structure without the use of an adhesive.
In another embodiment, the present invention relates to a method of forming a third electrode for use in a metal-air tricell comprising the steps of: (A) forming a suspension of a lanthanum nickel compound, at least one metal oxide compound and at least one dispersant; (B) applying the lanthanum nickel compound/metal oxide suspension to a support structure, thereby yielding a coated support structure; (C) heating the coated support structure to drive off the dispersant, reduce the metal oxide present in the lanthanum nickel compound/metal oxide suspension to its corresponding metal and to adhere the resulting lanthanum nickel compound/metal mixture to the support structure, thereby yielding a third electrode wherein the third electrode is free of an adhesive.