Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination...
Reexamination Certificate
1998-02-24
2002-10-22
Weiner, Laura (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Fluid active material or two-fluid electrolyte combination...
C429S129000, C429S247000
Reexamination Certificate
active
06468688
ABSTRACT:
The present invention relates to a method for preparing a high energy density (HED) electrolyte solution for use in an all-vanadium redox cell, a high energy density electrolyte solution, in particular an all-vanadium high energy density electrolyte solution, a redox cell, in particular an all-vanadium redox cell, comprising the high energy density electrolyte solution, a redox battery, in particular an all-vanadium redox battery, comprising the HED electrolyte solution, a process for recharging a discharged or partially discharged redox battery, in particular an all-vanadium redox battery, comprising the HED electrolyte solution, a process for the production of electricity from a charged redox battery, and in particular a charged all-vanadium redox battery, comprising the HED electrolyte, a redox battery/fuel cell and a process for the production of electricity from a redox battery/fuel cell. The present invention also relates to a method for stabilising an electrolyte for use in a redox cell, in particular for stabilising an electrolyte for use in an all-vanadium redox cell, a stabilised electrolyte, in particular an all-vanadium stabilised electrolyte, a redox cell, in particular an all-vanadium redox cell, comprising the stabilised electrolyte, a redox battery, in particular an all-vanadium redox battery, comprising the stabilised electrolyte, a process for recharging a discharged or partially discharged redox battery, in particular an all-vanadium redox battery, comprising the stabilised electrolyte solution, a process for the production of electricity from a charged redox battery, and in particular a charged all-vanadium redox battery, comprising the stabilised electrolyte solution, a redox battery/fuel cell and a process for the production of electricity from a redox battery/fuel cell.
BACKGROUND OF THE INVENTION
Since the energy density available from batteries based on oxidation/reduction reactions of ions in the electrolyte solution is directly proportional to the concentration of redox ions undergoing oxidation or reduction in the electrolyte solution, the energy density available from batteries based on redox electrolyte solutions is limited generally by the maximum solubility of redox salts of the various oxidation states in the electrolyte solution, and in particular the redox component with the lowest solubility.
In the vanadium redox battery employing V(II)/V(III) and V(IV)/V(V) redox couples in the H
2
SO
4
for the negative and positive half-cell electrolyte solutions respectively, the vanadium concentration has been limited to less than 2M (about 1.8M) due to precipitation of V(II) and V(III) at low temperatures and the thermal precipitation of V(V) at high temperatures. The solubility of the V(II), V(III) and V(IV) ions increases with increasing temperatures, however, V(V) undergoes thermal precipitation to V
2
O
5
at temperatures above 30° C.
For example if a 2M V(V) solution is exposed to temperatures of 30° C., a slight precipitate will start to form after 2 days, with heavy precipitation evident after only 4 days. At 40° C., a heavy precipitate will form after 2 days in a 2M V(V) solution. Even a 1.8M V(V) solution will precipitate after 6 days at 40° C.
This problem in use can be avoided by reducing the vanadium ion concentration to less than 1.8M for applications where the temperature is likely to exceed 40° C. and where the systems will be maintained in fully charged state for long periods. However in many applications it is not desirable to reduce the vanadium ion concentration below 2.0M since such a reduction effectively reduces the capacity and energy density of the battery. Thus, there is a need for a vanadium-based redox electrolyte solution which contains a higher concentration of vanadium ions.
In PCT/AU94/00711, a stabilised vanadium electrolyte solution was described which employed stabilising agents to inhibit the precipitation of vanadium from supersaturated solutions. Thus, 3M V(V) solution could be stabilised for several weeks by addition of 1-3 wt % glycerol, while 3M V(II) was stabilised by 1-3 wt % ammonium oxalate. A mixture of glycerol and ammonium oxalate inhibited precipitation of both V(II) and V(V) ions allowing a 3M vanadium electrolyte solution to operate successfully in a vanadium redox cell for close to six months. A large number of other organic and inorganic additives were also shown to inhibit the precipitation of vanadium from supersaturated solutions.
While these additives play a vital role in inhibiting precipitation of vanadium ions from supersaturated solutions of 2 to 4M vanadium surprisingly, the author has found that in the above case of V(V) solutions, at concentrations above 4M, the thermal precipitation reaction is completely inhibited even without the use of stabilising agents. Thus, a 5.5M V(V) solution produced by oxidation of 5.5M VOSO
4
in 2M H
2
SO
4
showed no signs of precipitation even after 6 weeks at 50° C.
One of the objects of this invention is thus an all-vanadium redox battery employing vanadium solutions of greater than 2M and especially above 1.8M, more typically above 2M, even more typically above 3M, 4M or 5M concentration which can operate over a wide range of temperatures and operating conditions. To avoid the precipitation of V(II), V(III) or V(IV) ions at these concentrations the operating temperature of the system is maintained above 25° C. However, it has also been discovered that with the use of suitable stabilizing agents, the operating temperature can be extended below 25° C.
FUNDAMENTAL PRINCIPLE OF INVENTION
In the vanadium cell however, you cannot use normal chelating or complexing methods to increase the concentration of vanadium in a vanadium electrolyte solution since V(V) is strongly oxidizing and will oxidize most of these compounds eventually to CO
2
, producing gas in the system which stops the pumps and can cause the whole stack to burst if not able to escape.
Surprisingly, however, it has been found by inventors that if used in low concentrations, these type of compounds have a stabilising ability and inhibit precipitation in highly supersaturated solutions of vanadium by adsorbing on the nuclei and preventing ions from approaching the nuclei, therefore stopping crystal growth.
At such low concentrations, these additives do not have sufficient reducing power and can thus not be oxidized by the V(V) in the positive half cell electrolyte solution. The solutions are thus stable for long periods and over so much wider temperature range than unstabilised solutions.
For example if a 2M V(V) solution are exposed to temperatures of 30° C., a slight precipitate will start to form after 2 days, with heavy precipitation evident after only 4 days. At 40° C., a heavy precipitate will form after 2 days in a 2M V(V) solution. Even a 1.8M V(V) solution will precipitate after 3 days at 40° C.
This problem in use can be avoided by reducing the vanadium ion concentration to less than 1.8M for applications where the temperature is likely to exceed 40° C. and where the systems will be maintained in fully charged state for long periods. However in many applications it is not desirable to reduce the vanadium ion concentration below 2.0M since such a reduction effectively reduces the capacity and energy density of the battery. Thus, there is a general need for redox electrolyte solutions which contain higher concentrations of redox ions. Thus, there is a need for a vanadium-based redox electrolyte solution which contains a higher concentration of vanadium ions. There is also a need for redox electrolyte solutions in which the precipitation of redox species from the redox electrolyte solution is prevented or reduced. In particular, there is a need for a vanadium-based redox electrolyte solution in which the precipitation of vanadium species from the vanadium-based redox electrolyte solution is prevented or reduced.
OBJECTS OF INVENTION
Accordingly, it is an object of the present invention to provide a method for preparing a high energy density electrolyte solution for use in an all-vanadium r
Kazacos Maria Skyllas
Kazacos Michael
Morgan & Finnegan , LLP
Pinnacle VRB Limited
Weiner Laura
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