Leveraging coordination chemistry in the design of bipolar energy storage materials for redox flow batteries

Mathias L. Skavenborg, James N. McPherson, Alexandros Pasadakis-Kavounis, Johan Hjelm, T. David Waite, Christine J. McKenzie*

*Corresponding author for this work

Research output: Contribution to journalJournal articleResearchpeer-review


Tuned by the ligand field, some single metal atoms in coordination complexes can offer two redox processes with a large potential difference, a key characteristic of symmetrical redox flow batteries (RFBs). We have designed a new anionic tridentate ligand N-(quinolin-8-yl)pyridine-2-sulfonamide (psq) with the aim of tuning the CoIII/CoII and CoII/CoI couples compared to those for analogous bis-homoleptic cobalt complexes of 2,2′:6′,2′′-terpyridine (terpy) and derivatives which have been tested as bipolar energy storage molecules. With similar structural parameters to these bis-terpy analogues, the single crystal X-ray structures of [M(psq)2] (M = CoII and ZnII) and [CoIII(psq)2](PF6) show a planar ligand geometry. However the cyclic voltammogram of CoII(psq)2 reveals negative shifts of 0.840 V and 0.175 V for the reversible CoII/CoI and CoIII/CoII redox processes compared to the bis-terpy analogue. Therefore, with a peak to peak separation between the CoII/CoI and CoIII/CoII couples that is 0.68 V larger, bipolar RFBs employing CoII(psq)2 as the energy storage material will operate at higher cell potentials than those offered by [CoII(terpy)2]2+. In an H-cell set-up, CoII(psq)2 delivers an initial effective charge to 83%. A charge capacity fade of 3% per cycle observed over 30 cycles is partly ascribed to ligand reduction. This work shows that psq is a promising scaffold for bipolar energy storage molecules and illustrates the opportunities for innovation in RFB design through the development of fit-for-purpose redox-active coordination complexes.
Original languageEnglish
JournalSustainable Energy and Fuels
Pages (from-to)2179-2190
Number of pages12
Publication statusPublished - 2022


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