Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Patent
2000-01-04
2000-12-26
Ryan, Patrick
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
429 59, 4292181, H01M 458
Patent
active
061656437
DESCRIPTION:
BRIEF SUMMARY
This application is the U.S. national-phase application of PCT International Application No. PCT/GB98/01272.
The present invention relates to a novel hydrogen storage material; a process for preparing the same; and its use in improving hydrogen transfer during charge and discharge conditions such as those required for battery electrode application.
Rechargeable batteries of secondary cells have been available for many years and include lead-acid accumulators, car batteries and, more recently Ni--Cd cells. Efforts to improve the performance of lead-acid batteries have been effective in many respects, but the basic cell still uses lead with its environmental penalty and high density. Similarly, although Ni--Cd cells have a long lifetime, they use cadmium, which is potentially toxic and therefore also raises many environmental concerns. The demand for rechargeable batteries is ever increasing with the proliferation of portable electrical equipment such as mobile telephones, laptop computers, video cameras, power tools, garden equipment etc, also aircraft batteries, and with the prospective development of electric road vehicles.
There are many potential cell systems which have been evaluated for use in rechargeable batteries and one of the more popular ones use hydrogen storage material as an electrode. These are based on the ability of some metals or metal alloys to store hydrogen (in molecular, atomic or ionic form) within the metal lattice. The most commonly-used hydrogen storage material is a metal hydride. Typically, the metal hydride hydrogen storage electrode is the negative electrode in such a system, wherein the metal (M) is charged by the electrochemical absorption of hydrogen and the electrochemical evolution of a hydroxide ion:
Since the hydrogen storage material functions both as an electrode and as a store, it must be capable of efficiently carrying out several different functions: ie reversibly storing large quantities of hydrogen; rapidly transferring hydrogen across the surface interface during both charge and discharge conditions; and maintaining the rate of hydrogen pick-up and supply to maintain the voltage, this being dependent upon the diffusion of hydrogen through the hydrogen storage material and the rate of surface transfer. It must also be capable of charge transfer rates to match current voltage requirements of the battery; being rechargeable over many cycles without loss of performance, which requires chemical stability, even in overcharge conditions; being physically durable; being cost-effective and being non-toxic.
Amongst the main factors affecting the life and performance of a hydrogen storage material electrode, degradation of the hydrogen storage material leading to loss of its ability either to transport hydrogen and/or charge is important. This loss of ability has been linked to chemical instability where the metallic material becomes contaminated or oxidised. Oxidation can occur by the action of OH.sup.- groups, or by the generation of O.sub.2 during charging or overcharging. The oxide formed on the surface of the hydrogen storage material offers a barrier to both hydrogen and charge transfer, thus degrading performance.
Considerable research has been carried out to attempt to overcome the problems of degradation of these hydrogen materials. At present there appears to be two approaches: the first involves mixing the hydrogen storage material, eg a powdered plated with nickel with another metal such as copper, nickel, cobalt, gold or carbon powder plated with platinum (Zuttel et al, Journal of Alloys and Compounds, 206 (1994) 31-38), and compressing the powders to form a pellet. The second approach involves coating the hydrogen storage material particles with one or more complete coatings of metal. For example, Chen et al (J. Mater. Res., 9(7)(1994) 1802-1804) describe coating fine powders of LaNi.sub.3.8 Co.sub.0.5 Mn.sub.0.4 Al.sub.0.3 with nickel, which led to improved performance of the alloy electrode. Geng (Journal of Alloys and Compounds, 215 (1994) 151-153) describ
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Doyle Mark Laurence
Harris Ivor Rex
Pratt Allin Sidney
Willey David Benjamin
Cooke Colleen
Johnson Matthey Public Limited Company
Ryan Patrick
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