Cell for electrochemically determining active species concentrations in redox flow batteries

A redox flow battery system comprising:

• a redox flow battery including
• a redox flow cell, and
• a supply/storage system external of the redox flow cell, the supply/storage system including first and second electrolytes for circulation through the redox flow cell, at least the first electrolyte being a liquid electrolyte having electrochemically active species with multiple, reversible oxidation states;
• and
• a secondary cell operable to monitor concentration of one or more of the electrochemically active species, the secondary cell including a counter electrode, a flow passage connecting the counter electrode with the redox flow battery to receive the first or second electrolyte, at least one working electrode, and a separator between the counter electrode and the at least one working electrode, the separator being impermeable to the first and second electrolytes and the electrochemically active species, the at least one working electrode being isolated from receiving the electrochemically active species of the first and second electrolytes except for at least one transport passage that comprises an opening through the separator connecting the flow passage and the at least one working electrode, the opening having a first end that opens to the working electrode and a second end that opens to the counter electrode, the opening limiting movement of the electrochemically active species to the at least one working electrode.