Dyeing to Carry Charge: Commercial Phthalocyanine Dyes as Charge Carriers for Redox Flow Battery Applications

The development of inexpensive charge carriers with long-term stability is critical to increase the economic viability of redox flow batteries (RFBs). Herein, we compare the performance metrics of a series of synthesized or commercially available inexpensive phthalocyanine metal (PcM) charge carriers with varying metal- or ligand-based substitution patterns. All systems were charged–discharged as slurries in conjunction with a conductive heterogeneous carbon source, Ketjenblack (KB). The synthesized, peripherally substituted 1,4,8,11,15,18,22,25-octaethoxyphthalocyanine (^EtOPc) metal complexes, ^OEtPcVO and ^OEtPcNi, were cycled with KB, and their performance metrics were compared to their commercially unsubstituted analogues, PcVO and PcNi. No significant advantages were found in using the synthesized versus commercial variants despite the increased solubility of the former. Expanding these electrochemical analyses to several commercial first-row variants (PcTiCl₂, PcVO, PcMnCl, PcFe, PcCo, PcNi, and PcCu) as well as one heavy-metal analogue (PcPb) revealed that most of these were effective charge carriers for slurry-based PcM/KB RFB applications. Of these, PcCu proved to have the highest efficiencies, energy density (1.23 Wh/L), and stability (>99% capacity retention) of the series while also having the lowest cost by mass.