SE986: Leveraging ligand-coupled electron transfer with nickel complexes for reversible two-electron redox storage in redox-flow batteries

Multi-electron energy storage in redox-flow batteries (RFB) can greatly increase their energy density but requires reversible, multi-electron transfer reactions with the molecular-based redox-active electrolytes. For organic molecules, this is often achieved through proton-coupled electron transfer (PCET), whereby potential inversion between two 1e- redox couples results in a single 2e- redox couple. In contrast, inorganic molecules must make use of ligand-coupled electron transfer (LCET) reactions to achieve potential inversion. Our group has exploited this chemistry to access reversible two-electron transfer reactions with nickel coordination compounds with a variety of ligand frameworks. Here, we show that nickel(II) bis(diphosphine) complexes can be readily tuned between a single 2e- Ni(II/0) redox couple and sequential 1e- redox couples such as Ni(II/I) and Ni(I/0) based on control of the LCET mechanism. Using a square-scheme analysis, we identify and characterize numerous possible LCET reactions (EEC, ECE, CEE) for achieving overall 2e- redox activity and find that only when the mechanism is guided through the ECE pathway is reversible 2e- storage achieved. Chronopotentiometric cycling experiments performed in H-cells showed that a single 2e- redox couple was superior for energy storage in terms of voltage efficiency, stored potential, and capacity fade. Under optimized conditions, the nickel electrolytes were able to be cycled over 200 times for 8 days (1 mA charge/discharge) with nearly 100% capacity retention.