Apparatus and method for determining state of charge in a redox flow battery via limiting currents

Abstract

The present invention relates to methods and apparatuses for determining the ratio of oxidized and reduced forms of a redox couple in solution, each method comprising: (a) contacting a first stationary working electrode and a first counter electrode to the solution; (b) applying a first potential at the first working electrode and measuring a first constant current; (c) applying a second potential at the first working electrode and measuring a second constant current; wherein the sign of the first and second currents are not the same; and wherein the ratio of the absolute values of the first and second currents reflects the ratio of the oxidized and reduced forms of the redox couple in solution. When used in the context of monitoring/controlling electrochemical cells, additional embodiments include those further comprising (d) oxidizing or reducing the solution, so as to alter the balance of the oxidized and reduced forms of the redox couple in solution, to a degree dependent on the ratio of the absolute values of the first and second currents. These embodiments may be used in the context of maintaining an electrochemical cell, stack, or system.