Patent Publication Number: US-6986838-B2

Title: Nanomachined and micromachined electrodes for electrochemical devices

Description:
REFERENCE TO RELATED APPLICATION 
     This is a continuation-in-part of provisional application Ser. No. 60/494,965, filed Aug. 14, 2003 and entitled NANOMACHINED AND MICROMACHINED ELECTRODES FOR ELECTROCHEMICAL DEVICES. 
    
    
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates generally electrodes for electrochemical devices and to the method of manufacturing nanomachined and micromachined electrodes. 
     BACKGROUND OF THE INVENTION 
     Electrodes for electrochemical devices are critical elements of the devices. Proper device operation demands that the electrodes are highly electrically and thermally conductive, allow unimpeded transport of gases or liquids through the electrode and preferably provide mechanical support to the overall electrochemical device structure. The unimpeded transport requirement is achieved by fabricating a porous electrode. Reduction of solid electrolyte film thickness to 10 μm and below forces a reduction of the pore sizes to micron or even submicron range. 
     Porous electrodes have been produced through an electroplating process wherein the electrode is produced by electroplating upon an organic surfactant. This simple electroplating process however produces electrodes of irregular shape and random pore orientation and sizing, which will not work properly in electrochemical devices. 
     Accordingly, it is seen that a need remains for a manner to produce nanomachined electrodes, i.e., electrodes having generally regularly oriented and shaped pores with a diameter of less than one micron, and micromachined electrodes, i.e., electrodes having pores with a diameter of greater than or equal to one micron, for electrochemical devices. It is to the provision of such therefore that the present invention is primarily directed. 
     SUMMARY OF THE INVENTION 
     In a preferred form of the invention a nanomachined and micromachined electrode is produced in accordance to the method of providing a layer of aluminum positioned upon a conductive substrate, anodizing the layer of aluminum to produce a layer of aluminum oxide having an array of pores, depositing a sacrificial metal within the pores of the aluminum oxide layer, etching the aluminum oxide layer so as to leave an array of sacrificial metal rods, depositing a layer of electrode material between the array of sacrificial metal rods, and etching the sacrificial metal rods so that a layer of copper remains having an array of pores where the sacrificial metal rods had existed. The layer of copper is the electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1–7  are a series of sequential perspective views showing the production of the electrode. 
     
    
    
     DETAILED DESCRIPTION 
     With reference next to the drawings, there is shown nanomachining and micromachining techniques which produce electrochemical device electrodes  10  with desired pore sizes, hereinafter referred to as nano-porous and/or micro-porous electrodes. 
     A preferred method of producing an electrode commences with positioning a layer or sheet of highly electropolished aluminum  11  upon a substrate  12 , see  FIG. 1 . The substrate  12  is be made of a conductive metal, such as gold, platinum, or copper. The aluminum  11  is then anodized by immersing the aluminum sheet  11  and substrate  12  within a bath of phosphoric acid and oxalic acid, a weakly alumina etching solution, with a voltage of approximately 10 milliamps applied across the aluminum. The anodizing process oxidizes the aluminum  11  so that it is changed to a layer of aluminum oxide  13  or alumina Al 2 O 3 , see  FIG. 2 . This anodizing process also causes a self- assembled array of pores  14  to be formed or “etched” into the aluminum oxide layer  13 . These pores  14  are very regular in shape, diameter and orientation. This self-assembled array of pores  14  serves as a patterning template for the further electrode fabrication steps. The self-assembled aluminum oxide pores  14  have pore diameters in the range of 50 nm or less. The pore diameter and spacing is controlled by the anodization voltage and solution composition and therefore both micromachined and nanomachined electrodes may be formed with the current process. 
     The next step in the nanomachining sequence is the positioning of the sacrificial metal  17 , preferably aluminum and therefore referred hereafter as aluminum. A sacrificial metal  17 , is deposited by a non-aqueous electroplating process into the aluminum oxide layer  13 , this electroplating process builds the aluminum layer  17  from the substrate  12 , upwardly in the drawings, to the top surface of the aluminum oxide layer  13 , as shown in  FIG. 3 . In other words, the aluminum fills the pores  14  within the aluminum oxide layer  13  from the bottom up. The aluminum oxide layer  13  thus can be referred to as a mold or mask. It is believed that other sacrificial metal may be used as an alternative to aluminum, although such is not know at this time. 
     The aluminum oxide layer  13  is then etched away in a bath of phosphoric acid and chromic acid leaving tall aluminum columns  18 , as shown in  FIG. 4 . To do this, the aluminum oxide layer  13  is placed in the bath for approximately thirty minutes at sixty degrees Celsius. Subsequently, an electrode metal  19 , such as copper, nickel, platinum or any other metal, hereinafter referred to as copper for ease of explanation, is electroplated from an aqueous solution. The copper  19  is positioned between the aluminum columns  18  under the conditions that the copper  19  does not plate on the aluminum columns  18 , as shown in  FIG. 5 . As such, the copper  19  fills the spaces between the aluminum columns  18 . 
     Finally, the aluminum columns  18  are etched away leaving a copper electrode  10  structure having an arranged array of nano and micro sized pores  20 , as shown in  FIG. 6 . The aluminum may be etched away by immersing it into a bath of tetra methyl ammonium hydroxide, 25% by weight, for thirty minutes at a temperature of twenty degrees Celsius. Once the aluminum is completely etched away the remaining structure is a copper layer with pores  20  that correspond in shape, size and orientation to the pores originally formed in the aluminum layer  11 . The copper layer is then removed from the underlying substrate, thus completing the formation of a porous copper electrode  10 , shown in  FIG. 7 . The pores within the copper are therefore generally uniform in pattern, shape, size and orientation. 
     It should be understood that the term etching, as used herein, may refer also to other methods of removing metallic material known in the art. 
     While this invention has been described in detail with particular reference to the preferred embodiments thereof, it should be understood that many modifications, additions and deletions, in addition to those expressly recited, may be made thereto without departure from the spirit and scope of invention as set forth in the following claims.