Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2001-05-14
2003-07-08
Bell, Bruce F. (Department: 1746)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C204S290130, C204S290140, C204S290010
Reexamination Certificate
active
06589405
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of Invention
The invention disclosed is an electrode comprising Ti-metal fiber wound on to a Ti-metal plate with an electrocatalytic coating that allows operation at a potential large enough to produce hydroxyl free radicals and oxidize substances dissolved in water or an electrolyte solution, and an electrochemical cell including such electrodes. An improved electrode coating sequence and coating procedure are also provided, providing increased service life and good current yield. Made of different materials, electrodes of this geometry may also be used in other process applications, in a fuel cell or as battery plaques.
BACKGROUND OF THE INVENTION
2. Description of Prior Art
In U.S. Pat. No. 5,419,824 Weres and Hoffmann provided an electrode comprising a titanium metal substrate covered with a thin layer of titanium dioxide doped with about 4 mole percent of niobium in the +4 moxidation state. The single d-electron of the Nb
+4
ions enters the conduction band of the mixed metal oxide, making the mixed oxide an heavily n-doped semiconductor. In U.S. Pat. No. 5,364,508 Weres and Hoffmann disclosed use of this electrode as an anode to generate hydroxyl free radical by oxidizing water and to oxidize organic substances dissolved in water. In U.S. Pat. No. 5,439,577 Weres and Hoffmann provided a water purification device utilizing the electrodes provided in U.S. Pat. No. 5,419,824 and an electrolytic cell wherein these electrodes are made by applying the doped titanium dioxide layer to titanium sheet, and assembled in a bipolar array.
A detailed electrode coating procedure was provided in U.S. Pat. No. 5,419,824. A “white slurry” coating composition was prepared, comprising hydrous titanium dioxide (the precursor of anatase pigment which has been precipitated from titanium sulfate solution and washed, but not dried or calcined) dispersed in water. The water soluble compounds diammonium bilactatotitanium (commercially available) and ammonium niobate were added in the correct proportions to cement the slurry and provide the desired level of Nb-doping. An “overcoat” solution was also used, comprising an aqueous solution of the same titanium and niobium compounds. The Ti-metal substrate was dipped into the “white slurry” composition, then baked in air at 400° C. to dry and bake on the slurry. About three coats of the white slurry were applied in this way, followed by three layers of “overcoat,” which cemented the slurry coat. Finally, the electrodes were annealed at 650-800° C. under hydrogen to reduce the niobium in the coating to the +4 oxidation state, conferring the desired semiconductive properties upon the electrode coating. Adding a bit of water vapor to the hydrogen inhibits hydrogen absorption into the Ti-metal substrate, and small electrodes in the form of disks or rods may be produced in this way. However, annealing plate electrodes under hydrogen warps them severely, and fiber electrodes are embrittled and practically destroyed. Therefore, the utility of the electrode coating method revealed in U.S. Pat. No. 5,419,824 is limited to producing small laboratory test electrodes. Also, electrodes coated in this manner fail after a few days of continuous operation due to passivation of the Ti-metal surface beneath the semiconductive oxide coat, making them useless for practical application. Even with periodic reversals of current, an electrode made of Ti-fiber cannot be operated in bipolar mode, because take-up of hydrogen while cathodically polarized embrittles and eventually destroys the fiber.
In U.S. Pat. No. 3,878,083 De Nora et al. provided a titanium electrode coated with a mixture of iridium dioxide and tantalum pentoxide. In U.S. Pat. No. 4,839,007 Kötz et al. provided a method of purifying industrial waste water using an anode comprising a Ti-metal substrate coated with tin dioxide doped (in the preferred embodiment) with antimony. This coating composition allows the electrode to operate at potential high enough to oxidize organic materials dissolved in water. In U.S. Pat. No. 5,364,509 Dietrich described a titanium anode with a two layer coating. The first coat comprises a mixture of IrO
2
and Ta
2
O
5
, and the second coat comprises SnO
2
doped with Sb.
In U.S. Pat. Nos. 4,444,642 and 4,528,084 Hinden and Beer teach using a solution of iridium trichloride and HCl in an alcohol solvent to apply a protective precoat, noting that the solution should attack the Ti-metal substrate, producing a thick oxide layer comprising IrO
2
and TiO
2
, intimately mixed. This coating solution is strongly reducing and depassivates the Ti-metal surface, causing it to corrode. In trying to use this solution, we also noted that it spoils rapidly once used, probably because Ti
+3
produced by corrosion of the Ti-metal reduced the iridium in solution, causing it to precipitate. U.S. Pat. No. 3,878,083 teaches application of a coating comprising IrO
2
and Ta
2
O
5
using a solution of IrCl
3
and TaCl
5
, in hydrochloric acid. This coating solution is very weakly oxidizing. Scanning electron microscopy of Ti-fiber electrodes that we precoated using a solution comprising H
2
IrCl
6
and TaCl
5
in hydrochloric acid (which is more strongly oxidizing and thereby less corrosive against Ti than the solution recommended in U.S. Pat. No. 3,878,083) revealed that some fibers had thick coatings on them, indicating depassivation and corrosion of the Ti-metal substrate, while other fibers had very thin coats. Because the diameter of the fibers is small, corrosion, if it occurs, can dissolve a large fraction of the fiber's mass, and the thick mixed oxide coating produced fills in the grooves typically present in the surface of the fibers, decreasing their effective surface area.
In process electrochemistry, increasing electrode surface area improves the kinetics of the electrochemical process at low reactant concentration. Increased surface area also decreases the true current density at the surface in proportion, allowing the cell to operate at lower voltage and increasing the service life of the electrode. In batteries, increased surface area of the electrode plaques provides improved contact with the active material, improving energy storage efficiency. In practice, large surface area process electrodes and battery plaques are very similar and their design is governed by much the same criteria, allowing technology to be usefully and easily transferred between the two fields.
In U.S. Pat. No. 3,895,960 Brown et al. provided an electrode plaque made by compressing and diffusion bonding iron fibers, attaching a current collector by mechanical means or by welding, and plating the entire assembly with nickel to provide the needed electrocatalytic surface properties. In Brown's Example 1, iron fibers with length:diameter ratio of about 1,900 were used to produce an electrode plaque with 95% porosity, 0.025 inch thickness, and specific area 100 cm
2
/cm
3
. In U.S. Pat. No. 3,835,514 Pollock provided a similar electrode plaque with L:D of 800 to 8000: 1, porosity of 70 to 97% and a diffusion bonded bus connector.
In U.S. Pat. No. 4,331,523 Kawasaki described electrodes suitable for water electrolysis comprising a perforate current collector, preferably titanium expanded mesh or titanium perforated plate coated with platinum group metals, with a “fibrous assembly” pressed against it to provide large surface area. He noted that the fibrous assembly could comprise a diffusion bonded “web” of titanium fibers coated with platinum groups metals. (Here and throughout, we use the term “platinum group metals” to mean the metallic elements Ru, Rh, Pd, Os, Ir and Pt and also their oxides.) Kawasaki did not specify L:D, porosity or specific area of the “fibrous assembly” in his electrodes, but his examples suggest values similar to those taught in U.S. Pat. Nos. 3,895,960 and 5,294,319.
In U.S. Pat. No. 4,708,888 Mitchell et al. described an electrode produced by applying an electrocatalytic coating to a fine titanium expanded mesh, then spot weldi
O'Donnell Henry Edward
Weres Oleh
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