Developing terpyridine-based metal complexes for non-aqueous redox flow batteries

This paper describes the design and synthesis of a series of terpyridine-based complexes of the first-row transition metals Cr, Mn, Fe, and Co for non-aqueous redox flow batteries (NARFBs). Electrochemical studies reveal that these complexes can undergo multi-electron transfer redox reactions. In particular, the Mn and Fe-based complexes exhibit both low negative redox potentials and high positive redox potential, permitting them to serve as a bipolar electrolyte for symmetric RFBs with a cell voltage of more than 2 V. The solubility of these complexes can be effectively improved by incorporating a polyether substituent on the terpyridine ligand and counter anion optimization. The iron complex [Fe(tpy-O(CH2CH2O)3CH3)2][TFSI]2 shows a high solubility of 0.76 M in MeCN. The fabricated iron-based symmetric NARFB demonstrates a superior battery performance with a high cell voltage of 2.3 V, columbic efficiency of 97%, energy efficiency as high as 88%, and stable charge-discharge capacity retention of 60% after 160 cycles, corresponding to 99.75 % capacity retention per cycle. The post-cycling cyclic voltammetry (CV), UV-Vis, and 1H-NMR characterizations indicate only minor chemical decomposition of the cycled complex, confirming its good charging-discharging stability.