Investigation of Iron(III) Tetraphenylporphyrin as a Redox Flow Battery Anolyte: Unexpected Side Reactivity with the Electrolyte

Redox flow batteries (RFBs) present an opportunity tobridge thegap between the intermittent availability of green energy sourcesand the need for on-demand grid level energy storage. While aqueousvanadium-based redox flow batteries have been commercialized, theyare limited by the constraints of using water as an electrochemicalsolvent. Nonaqueous redox flow battery systems can be used to producehigh voltage batteries due to the larger electrochemical window innonaqueous solvents and the ability to tune the redox properties ofactive materials through functionalization. Iron porphyrins, a classof organometallic macrocycles, have been the subject of many studiesfor their photocatalytic and electrocatalytic properties in nonaqueoussolvents. Often, iron porphyrins can undergo multiple redox eventsmaking them interesting candidates for use as anolytes in asymmetricalredox flow batteries or as both catholyte and anolyte in symmetricalredox flow battery systems. Here the electrochemical properties ofFe(III)TPP species relevant to redox flow battery electrolytes areinvestigated including solubility, electrochemical properties, andcharge/discharge cycling. Commonly used support electrolyte saltscan have reactivities that are often overlooked beyond their conductivityproperties in nonaqueous solvents. Parasitic reactions with the cationsof common support electrolytes are highlighted herein, which underscorethe careful balance required to fully assess the potential of novelRFB electrolytes.