1. Field of the Invention
This invention relates to electrical energy storage devices and is directed more particularly to an electrically rechargeable, reduction-oxidation (REDOX) type cell.
2. Description of the Prior Art
REDOX cells utilizing anode and cathode fluids separated by an ion permeable membrane and employing REDOX couples as the electrochemical active materials are generally well known. The electrochemical capacity of these fluids is a function of the amount of active material in the solution and the oxidation state of the material. If the anode fluid and the cathode fluid contain the same number of equivalents of active material and both have the same degree of charge the REDOX system is said to be in balance.
For a REDOX system using a chromous/chromic couple as the anode fluid and the ferrous/ferric couple as the cathode fluid, a balanced system has the same number of equivalents of chromous chloride in the anode fluid as there are equivalents of ferric chloride in the cathode fluid. The anode fluid is an aqueous solution of hydrochloric acid and chromium chloride while the cathode fluid is an aqueous solution of hydrochloric acid and iron chloride.
During the operation of a REDOX system, electrochemical imbalance can gradually build up if reactions other than the desired electrochemical REDOX reactions occur. For example, if a system when new had a capacity of 100 ampere hours and after a certain length of time had a capacity of only 90 ampere hours, it would be clear that an imbalance between the anode and cathode fluids had occurred. This imbalance is defined as a different state of charge in the anode and cathode fluids.
An imbalance in a REDOX system can occur for a number of reasons including, but not limited to, the following: