Chemistry: electrical current producing apparatus – product – and – Fluid active material or two-fluid electrolyte combination... – Active material in solution
Patent
1995-06-23
1997-12-30
Willis, Jr., Prince
Chemistry: electrical current producing apparatus, product, and
Fluid active material or two-fluid electrolyte combination...
Active material in solution
429 51, 429 62, 429 63, 429198, H01M 1042
Patent
active
057028427
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for charging and discharging zinc/bromine batteries, having a plurality of electrodes, bipolar ones in particular, and a plurality of anode and cathode spaces into which optionally temperature-stabilized catholyte or catolyte fluid is introduced.
Voltaic cells based on the zinc/bromine pairing can be particularly efficient. The high reactivity of bromine, though, requires a particular choice of materials; plastics, for example polyethylene or polypropylene, are preferred materials, because of their high chemical resistance to bromine. The construction of zinc/bromine batteries with plastic is done in such a way that on the one hand, bipolar electrodes are provided, which are constructed with plastic-bonded carbon, for example graphite, activated charcoal, and the like. Between the electrodes, diaphragms are disposed, which are likewise comprised of plastic, in particular polypropylene or polyethylene. The diaphragms or electrodes have thickenings on their edges so that packets of electrodes, between each of which diaphragms are disposed, create electrode spaces, specifically anode and cathode spaces. These spaces are supplied and emptied via separate lines. The electrolytes, specifically the catholyte and catholyte, each have at their disposal separate loops with separate reservoirs and pumps associated with each of these. If need be, the pumps can be driven by a common electromotor.
The storage of the zinc is effected in metallic form on the surface of the electrode. If the electrolyte composition is inadequate, or if there are excessive current densities, dendrite formation can occur, which on the one hand can cause damage to the diaphragms; on the other hand, for example upon discharging, zinc fragments can be formed, which are no longer connected to the electrode via a first-class electrical conductor. These zinc fragments cause malfunctions, which among other things can lead to individual electrode spaces not being supplied with the electrolyte fluid, since the supply lines or also the drain lines are clogged with the zinc fragments. These malfunctions can be solved, according to European Patent Application Serial No. EP-A-0149448, by introducing bromine-containing electrolyte into the electrolyte space which has the zinc, so that the zinc attains dissolution not electrochemically, but purely chemically, and hence there is no need to connect zinc fragments to the electrode. This introduction of bromine-containing electrolyte fluid into the electrode spaces, in which zinc is precipitated, can take place for example at the end of a discharging process; bromine-containing electrolyte is supplied, by means of which the zinc can be chemically dissolved.
Although zinc/bromine batteries have been developed in which the bromine is also deposited on the electrode surface, nevertheless if a larger capacity is to be achieved, then the bromine must be stored outside the electrode. A preferred process for binding of free bromine is comprised in that a complexing agent is dissolved in the aqueous electrolyte, which agent forms a complex with the bromine, which dissolves in water only with difficulty, so that the electrolyte has an aqueous phase and a hydrophobic phase. The hydrophobic phase can be stored together with the aqueous phase in an electrolyte vessel. The hydrophobic phase is also on the surface of the cathode, next to the aqueous phase, during both charging and discharging. Since the cathode surface as a rule has an increased surface area, for example by means of the embedding of carbon fibers, carbon particles, or the like, the hydrophobic phase additionally adheres to the surface of the cathode. Consequently, after a charging or discharging process, there is a bromine concentration in the cathode space, which is required for carrying out the electrochemical process. In order to decrease the bromine concentration, in the internal prior art, after an electrochemical reaction, the electrode space is rinsed out with the aqueous phase; care is taken that there be n
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Chaney Carol
Elin Energieanwendung Gesellschaft M.B.H.
Willis Jr. Prince
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