1. Field of the Invention
The invention relates to a rebalance cell for a Cr/Fe redox storage system for the reduction of Fe.sup.3+ ions by means of hydrogen.
2. Description of Related Art
Redox flow batteries can function for the storage of electrical energy. The energy storage herein takes place by means of solutions of metal ion pairs with different states of oxidation. If two such ion pairs whose oxidation-reduction potentials are sufficiently far apart are allowed to react at two different electrodes separated from each other by an ion-selective membrane, a potential (i.e., electric energy) is obtained (see for example EP 0 143 300).
Redox pairs of the above stated type are for example Cr.sup.3+ /Cr.sup.2+ and Fe.sup.3+ /Fe.sup.2+. In cells or batteries with such oxidation-reduction pairs (i.e., Cr/Fe redox cells or redox batteries), electrical energy is converted during charging into chemical energy through electrochemical reactions. The process is reversed on discharging. Therein the following processes occur: ##STR1##
The oxidation-reduction reaction of the iron takes place at the positive electrode, that of the chromium at the negative electrode; for this purpose acidic electrolyte solutions are used, in particular hydrochloric acid solutions of iron chloride or chromium chloride.
While the charging reaction of the iron proceeds with nearly 100% current yield, in the reduction of Cr.sup.3+ ions the production of hydrogen occurs as a side reaction during the charging process at the graphite electrodes utilized (see hereto for example U.S. Pat. No. 3,996,064 and U.S. Pat. No. 4,382,116 as well as EPO 0 312 875).
Thereby a marked difference with respect to the state of charge of the two electrolyte solutions exists after several charging and discharging processes of the battery. The imbalance of the states of charge consists therein in a higher state of charge of the iron electrolyte i.e. in an excess of Fe.sup.3+ ions.
The difference between the state of charge of the chromium electrolyte and that of the iron electrolyte must be balanced with the aid of a special cell, a so-called rebalance cell through which the balance of the state of charge is again restored. This takes place in general through a reduction of Fe.sup.3+ ions since a further reduction of Cr.sup.3+ ions is connected to a poorer current yield.
The cathodic reduction of Fe.sup.3+ ions can take place in different ways:
(1) electrochemically with anodic Cl.sub.2 generation (as counter reaction): PA0 (2) electrochemically with anodic Br.sub.2 generation; PA0 (3) electrochemically with anodic H.sub.2 oxidation, i.e. consumption; PA0 (4) chemically with hydrogen.
These reactions are accompanied, however, by different disadvantages.
In method (1) a separate anodic circuit is required since a development of chlorine in the redox cells is not desirable for reasons of stability of the different materials. In addition, the generated gases H.sub.2 and Cl.sub.2 must either be removed from the system or must be recombined to form HCl for balancing the substances. Moreover, method (1) as well as method (2) require electric energy for operating the rebalance cell specifically corresponding to the position of the potential of the different reactions.
In method (2) an addition of hydrogen bromide to the electrolyte is required which increases the electrolyte expenditure.
Method (3) would have the advantage that it supplies energy since the oxidation of hydrogen proceeds at approximately 0 V. However, thus far the technical realization of this method has not been successfully achieved.
Method (4) which involves the direct chemical reduction of Fe.sup.3+ ions with the hydrogen generated at the chromium electrode, represents a favorable possibility for rebalancing the state of charge since during chemical balancing, in contrast to the electrochemical methods, an integration into the redox battery is possible whereby the structure is simplified and the energy efficiency of the system is improved. But the reaction rate of the reduction of Fe.sup.3+ ions with hydrogen is low.
It is an object of the invention to implement a rebalance cell for a Cr/Fe redox storage system for the reduction of Fe.sup.3+ ions by means of hydrogen such that this reduction is accelerated and hence a practical application of the chemical balancing system becomes possible.