Spectroscopic and electrochemical analyses elucidating capacity degradation mechanism of iron-ligand complex and air in all iron aqueous redox flow batteries

Capacity degradation mechanism of all iron aqueous redox flow battery (ARFB) using iron and 2,2-bis(hydroxymethyl)-2,2′,2′'-nitrilotriethanol (Fe(BIS-TRIS)) complex as redox active material for anolyte is analyzed. The capacity degradation occurs by the contact of Fe(BIS-TRIS) and oxygen in air, while this is confirmed by spectroscopic and electrochemical ways including the use of Beer-Lambert's law. According to the analysis, charging/discharging step of catholyte including Fe(CN)₆ does not affect exposure to oxygen, whereas self-discharge occurs at the step of anolyte including Fe(BIS-TRIS) that is exposed to oxygen, promoting its performance decay. This is proved by ARFB single cell tests using the electrolytes. When the electrolytes are open to air, its capacity is considerably degraded within 20 cycles. However, when anolyte is sealed to avoid contact with air, its capacity preserves 96 % of initial value even after 100 cycles. This is because the undesirable self-discharge of Fe(BIS-TRIS) affects the degradation in performance of ARFB single cell. We elucidate effect of oxygen on performance of ARFBs using Fe(BIS-TRIS) with UV–VIS spectroscopic method.