Chemistry: electrical current producing apparatus – product – and – Current producing cell – elements – subcombinations and... – Electrode
Reexamination Certificate
2000-02-11
2001-10-30
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Current producing cell, elements, subcombinations and...
Electrode
C420S900000
Reexamination Certificate
active
06309779
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a hydrogen storage alloy electrode which employs, as an active material, a hydrogen storage alloy capable of electrochemically absorbing and desorbing hydrogen in a reversible manner.
Electrodes employing as an active material a hydrogen storage alloy capable of absorbing and desorbing hydrogen in a reversible manner have a larger theoretical energy density compared to cadmium electrodes. Also, electrodes that employ such hydrogen storage alloy are not susceptible to deformation and subsequent formation of dendrites, which is a problem with zinc electrodes. These advantageous properties, as well as the promise of a longer service life and a reduction in the environmental concerns inherent in cadmium-containing electrode/batteries, have encouraged research into developing alloys suited for hydrogen storage alloy electrodes, and particularly negative electrodes for alkaline storage batteries.
Conventional alloys for hydrogen storage alloy electrodes are typically prepared through either an arc melting process, an induction heating melting process, or some similar process. The hydrogen storage alloys currently used for electrodes are La (or Mm)—Ni system multi-element alloys (wherein Mm (misch metal) is a mixture of rare-earth elements). These multi-element alloys are classified as an AB
5
type (wherein A is La, Zr, Ti or an element with a affinity for hydrogen, and B is Ni, Mn, Cr or any other transition metal with a small affinity for hydrogen).
Another alloy having larger hydrogen storing capability than the AB
5
type is a Ti—V system hydrogen storage alloy. For example, hydrogen storage alloy electrodes employing a Ti
x
V
y
Ni
z
alloy are known in conventional. See, e.g., Japanese Laid-Open Patent Publication Nos. 6-228699, 7-268513, and 7-268514.
The La (or Mm)—Ni system multi-element alloy, however, nearly uses the whole theoretical capacity, so there is little probability of drastic capacity increase.
On the other hand, electrodes made from the Ti—V system hydrogen storage alloy have a larger discharge capacity than electrodes made from the La (or Mm)—Ni system alloy; however, there is a problem that the discharge capacity is still lower than the theoretical discharge capacity. Also, the electrodes have to achieve improved cycle characteristic and cost reduction.
BRIEF SUMMARY OF THE INVENTION
The present invention relates to a hydrogen storage alloy electrode made from particulate active material comprising a hydrogen storage alloy of body-centered cubic or body-centered tetragonal crystal structure, said hydrogen storage alloy being represented by the general formula Ti
a
M
1
b
Cr
c
M
2
d
L
e
, wherein M
1
is at least one element selected from the group consisting of Nb and Mo; M
2
is at least one element selected from the group consisting of Mn, Fe, Co, Cu, V, Zn, Zr, Ag, Hf, Ta, W, Al, Si, C, N, P and B; L is at least one element selected from the group consisting of rare-earth elements and Y; 0.2≦a≦0.7; 0.01≦b≦0.4; 0.1≦c≦0.7; 0≦d≦0.3; 0≦e≦0.03; and a+b+c+d+e=1.0, and said particulate active material having a Ti—Ni system alloy phase in the surface portion thereof.
Of the hydrogen storage alloys, the general formula of preferred ones satisfy 0.4≦a≦0.64, 0.05≦b≦0.2, 0.3≦c≦0.4, 0≦d≦0.2, 0≦e≦0.03 and a+b+c+d+e=1.0. More preferred ones are represented by the general formula Ti
a
M
1
b
Cr
c
M
2
d
L
e
Fe
f
Si
g
wherein M
1
is neither Fe nor Si, 0.003≦f<0.2, 0<g≦0.1, d+f+g≦0.2 and a+b+c+d+e+f+g=1.0.
It is preferred that 70% by volume or more of the Ti—Ni system alloy phase has the body-centered cubic crystal structure of TiNi. It is also preferred that nickel concentration of the particulate active material gradually decreases from the surface towards inside thereof. The particulate active material preferably has a particle size of 40 &mgr;m or below.
The present invention also relates to a method for manufacturing a hydrogen storage alloy electrode comprising the steps of: (A) plating nickel or applying nickel powder onto the surface of a hydrogen storage alloy powder, or mixing a hydrogen storage alloy powder with a nickel carbonyl-containing gas and thermally decomposing the gas to apply nickel onto the surface of the alloy powder, and (B) heating the alloy powder at 500 to 1000° C. in an atmosphere of an inert gas or a hydrogen gas, or under vacuum, said hydrogen storage alloy of starting material having body-centered cubic crystal structure or body-centered tetragonal crystal structure, and being represented by the general formula Ti
a
M
1
b
Cr
c
M
2
d
L
e
, wherein M
1
is at least one element selected from the group consisting of Nb and Mo; M
2
is at least one element selected from the group consisting of Mn, Fe, Co, Cu, V, Zn, Zr, Ag, Hf, Ta, W, Al, Si, C, N, P and B; L is at least one element selected from the group consisting of rare-earth elements and Y; 0.2≦a≦0.7; 0.01≦b≦0.4; 0.1≦c≦0.7; 0≦d≦0.3; 0≦e≦0.03; and a+b+c+d+e=1.0.
It is preferred that the nickel carbonyl-containing gas contains 20 to 90% by volume of nickel carbonyl and 10 to 80% by volume of carbon monoxide. Alternatively, the nickel carbonyl-containing gas preferably contains 20 to 85% by volume of nickel carbonyl, 10 to 75% by volume of carbon monoxide, and 5 to 50% by volume of at least one selected from the group consisting of iron carbonyl, chromium carbonyl, molybdenum carbonyl, and tungsten carbonyl.
It is also preferred that, prior to step (A), the step of: (X) mixing the hydrogen storage alloy powder with a gas containing 20 to 90% by volume of at least one selected from the group consisting of iron carbonyl, chromium carbonyl, molybdenum carbonyl and tungsten carbonyl, and 10 to 80% by volume of carbon monoxide, and then thermally decomposing the gas to apply at least one selected from the group consisting of iron, chromium, molybdenum and tungsten onto the surface of the alloy powder.
It is preferable that, prior to step (A) or (X), the hydrogen storage alloy powder is heated at 1200 to 1400° C. in an atmosphere of an inert gas or a hydrogen gas, or under vacuum.
The present invention further relates to a method for manufacturing a hydrogen storage alloy electrode comprising the step of conducting mechanochemical reaction between hydrogen storage alloy powder and nickel, said hydrogen storage alloy having body-centered cubic crystal structure or body-centered tetragonal crystal structure, and being represented by the general formula Ti
a
M
1
b
Cr
c
M
2
d
L
e
, wherein M
1
is at least one element selected from the group consisting of Nb and Mo: M
2
is at least one element selected from the group consisting of Mn, Fe, Co, Cu, V, Zn, Zr, Ag, Hf, Ta, W, Al, Si, C, N, P and B; L is at least one element selected from the group consisting of rare-earth elements and Y; 0.2≦a≦0.7; 0.01≦b≦0.4; 0.1≦c≦0.7; 0≦d≦0.3; 0≦e≦0.03; and a+b+c+d+e=1.0.
It is preferable that, prior to the step of the mechanochemical reaction, the hydrogen storage alloy powder is heated at 1200 to 1400° C. in an atmosphere of an inert gas or a hydrogen gas, or under vacuum.
DETAILED DESCRIPTION OF THE INVENTION
The particulate active material in the present invention is an improved one of conventional hydrogen storage alloy powder having body-centered cubic crystal structure. In other words, the particulate active material is one obtained by forming Ti—Ni system alloy phase in the surface portion of the hydrogen storage alloy particles having either body-centered cubic or body-centered tetragonal crystal structure and not containing nickel. The use of particulate material with such a structure can increase both hydrogen storing capability and discharge capacity, and improve the corrosion resistance of the alloy so as to provide a long-lived electrode.
(1) The following is a de
Matsuda Hiromu
Toyoguchi Yoshinori
Tsuji Yoichiro
Yamamoto Osamu
Akin Gump Strauss Hauer & Feld L.L.P.
Brouillette Gabrielle
Matsushita Electric - Industrial Co., Ltd.
Wills M.
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