Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Preparing nonmetal element
Reexamination Certificate
1999-12-13
2001-10-16
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Preparing nonmetal element
C420S454000, C420S441000, C205S631000, C205S632000, C204S291000
Reexamination Certificate
active
06303015
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an improved electrode material for use in electrochemical processes and particularly an amorphous metal/metallic glass electrode material intended for constituting the active surface of an electrode for use in the electrolysis of aqueous solutions and more particularly in the electrochemical production of oxygen and hydrogen by said electrolysis.
BACKGROUND OF THE INVENTION
In electrolytic cells for the production of hydrogen and oxygen, such as those of the bipolar and unipolar type, an aqueous caustic solution is electrolyzed to produce oxygen at the anode and hydrogen at the cathode with the overall reaction being the decomposition of water to yield hydrogen and oxygen. The products of the electrolysis are maintained separate by use of a membrane/separator. Use of amorphous metals/metallic glasses and nanocrystalline materials, as electrocatalysts for the hydrogen and oxygen evolution reaction are known. The terms “amorphous metal” or “metallic glass” are well understood in the art and define a material which contains no long range structural order but can contain short range structure and chemical ordering. Henceforth, in this specification and claims both terms will be used as being synonymous and are interchangeable. The term “nanocrystalline” refers to a material that possesses a crystallite grain size of the order of a few nanometers; i.e. the crystalline components have a grain size of less than about 10 nanometers. Further, the term “metallic glass” embraces such nanocrystalline materials in this specification and claims.
In an electrolysis application, not all of the voltage that is passed through the cell during electrolysis is utilized in the production of hydrogen and oxygen. This loss of efficiency of the cell is often referred to as the cell overpotential required to allow the reaction to proceed at the desired rate and is in excess of the reversible thermodynamic decomposition voltage. This cell overpotential can arise from: (i) reactions occurring at either the cathode or the anode, (ii) a potential drop because of the solution ohmic drop between the two electrodes, or (iii) a potential drop due to the presence of a membrane/separator material placed between the anode and cathode. The latter two efficiencies are fixed by the nature of the cell design while (i) is directly a result of the activity of the electrode material employed in the cell including any activation or pre-treatment steps. Performance of an electrode is then directly related to the overpotential observed at both the anode and cathode through measurement of the Tafel slope and the exchange current density (hereinafter explained).
Superior electrode performance for the electrolysis of water may be achieved by the use of addition of metal salts to the electrolyte as “homogeneous” catalysts that function only in the liquid phase. These “homogeneous” catalysts suffer from the difficulty of having to add these additions to an operating cell to be functional, along with the toxicity of the metal salts in powder form and the disposal of electrolyte containing these additions. A desirable alternative would then be a base alloy comprised of Ni, and one or more of these metallic salt constituents which would still provide the same operating characteristics of a low voltage, high current cell behaviour corresponding to the evolution of hydrogen or oxygen while being electrochemically stable in the alkaline solution.
U.S. Pat. No. 5,429,725, issued Jul. 04, 1995 to Thorpe, S. J. and Kirk, D. W. describes the improved electrocatalytic behaviour of alloys made by combinations of the two elements Mo and Co in a Ni-base metallic glass.
However, this is still a need for higher exchange current densities combined with lower Tafel slopes in the (Cr, V)-containing alloys compared with the Mo-containing alloys and, accordingly, a need for enhanced operating efficiency of electrocatalyst material for alkaline water electrolysis
REFERENCE LIST
The present specification refers to the following publications, each of which is expressly incorporated herein by reference.
PUBLICATIONS
1. Lian, K. Kirk, D. W. and Thorpe, S. J., “Electrocatalytic Behaviour of Ni-base Amorphous Alloys”, Electrochim. Acta, 36, p. 537-545, (1991)
2. Kreysa, G. and Hakansson, “Electrocatalysis by Amorphous Metals of Hydrogen and Oxygen Evolution in Alkaline Solution”, J. Electroanal. Chem., 201, p. 61-83, (1986).
3. Podesta, J. J., Piatti, R. C. V., Arvia, A. J., Ekdunge, P., Juttner, K. and Kreysa, G., “The Behaviour of Ni—Co—P base Amorphous Alloys for Water Electrolysis in Strongly Alkaline Solutions Prepared through Electroless Deposition”, Int. J.
Hydrogen Energy, 17, p. 9-22, (1992).
4. Alemu, H. and Juttner, K., “Characterization of the Electrocatalytic Properties of Amorphous Metals for Oxygen and Hydrogen Evolution by Impedance Measurements”, Electrochim. Acta., 33, p. 1101-1109, (1988).
5. Huot, J. -Y., Trudeau, M., Brossard, L. and Schultz, R. “Electrochemical and Electrocatalytic Behaviour of an Iron Base Amorphous Alloy in Alkaline Solution at 70° C.”, J. Electrochem. Soc., 136, p. 2224-2230, (1989).
6. Vracar, Lj., and Conway, B. E., “Temperature Dependence of Electrocatalytic Behaviour of Some Glassy Transition Metal Alloys for Cathodic Hydrogen Evolution in Water Electrolysis”, Int. J. Hydrogen Energy, 15, p. 701-713 (1990).
7. Wilde, B. E., Manohar, M., Chattoraj, I, Diegle, R. B. and Hays, A. K., “The Effect of Amorphous Nickel Phosphorous Alloy Layers on the Absorption of Hydrogen into Steel”, Proc. Symp. Corrosion, Electrochemistry and Catalysis of Metallic Glasses, 88-1, Ed. R. B. Diegle and K. Hashimoto, Electrochemical Society, Pennington, p. 289-307 (1988).
8. Divisek, J., Schmitz, H. and Balej, “Ni and Mo Coatings as Hydrogen Cathodes”, J. Appl. Electrochem., 19, p. 519-530, (1989).
9. Huot, J. -Y. and Brossard, L., “In-situ Activation of Nickel Cathodes by Sodium Molybdate during Alkaline Water Electrolysis at Constant Current”, J.
Appl. Electrochem., 20, p. 281, (1990).
10. Huot, J. -Y. and Brossard, “In-situ Activation of Nickel Cathodes during Alkaline Water Electrolysis by Dissolved Iron and Molybdenum Species”, J.
Appl. Electrochem., 21, p. 508, (1991).
11. Raj, I. A. and Vasu, K. I., “Transition Metal-based Hydrogen Electrodes in Alkaline Solution- Electrocatalysis on Nickel-based Binary Alloy Coatings”, Int. J. Hydrogen Energy, 20, p. 32, (1990).
12. Jaksic, M. M., Johansen, B., and Ristic, M., “Electrocatalytic In-situ Activation of Noble Metals for Hydrogen Evolution” in
Hydrogen Energy Progress VIII
, T. N. Veziroglu and P. K. Takahashi, Eds., Pergamon Press, NY, p. 461, (1990).
SUMMARY OF THE INVENTION
It is an object of this invention to provide an improved electrode having an electrochemically active surface that can be used for the electrolysis of water.
It is a further object of this invention to provide an improved electrode that is chemically stable in an alkaline environment for both static and dynamic cycling operations of the cell.
It is a further object of the present invention to provide an improved electrode material that is sufficiently active so as to reduce either or both the anodic overpotential for oxygen evolution or the cathodic overpotential for hydrogen evolution.
It is a further object to provide an electrode that contains relatively inexpensive elemental constituents compared to the platinum group metals.
It is a further object to provide an electrode whose total processing operations necessary to final electrode fabrication are minimized in comparison to conventional electrode materials.
It is a further object to provide an electrode which can be operated at elevated temperatures in an alkaline environment to provide enhanced performance since the overpotential required to produce either hydrogen or oxygen is reduced as the operational temperature of the cell is increased.
Accordingly, the invention provides in one aspect a metallic glass of use in electrochemical processes, said metallic glass consisting essentially of a material of
Kirk Donald W.
Thorpe Steven J.
Gorgos Kathryn
Manelli Denison & Selter
Nicolas Wesley A.
Stemberger Edward J.
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