Zinc-Catalyzed Two-Electron Nickel(IV/II) Redox Couple for Multi-Electron Storage in Redox Flow Batteries

Energy storage is a vital aspect for the successful implementation of renewable energy resources on a global scale. Herein, we investigated the redox cycle of nickel(II) bis(diethyldithiocarbamate), Ni^II(dtc)₂, for potential use as a multielectron storage catholyte in nonaqueous redox flow batteries (RFBs). Previous studies have shown that the unique redox cycle of Ni^II(dtc)₂ offers 2e^- chemistry upon oxidation from Ni^II → Ni^IV but 1e^- chemistry upon reduction from Ni^IV → Ni^III → Ni^II. Electrochemical experiments presented here show that the addition of as little as 10 mol % Zn^II(ClO₄)₂ to the electrolyte consolidates the two 1e^- reduction peaks into a single 2e^- reduction where [Ni^IV(dtc)₃]⁺ is reduced directly to Ni^II(dtc)₂. This catalytic enhancement is believed to be due to Zn^II removal of a dtc^- ligand from a Ni^III(dtc)₃ intermediate, resulting in more facile reduction to Ni^II(dtc)₂. The addition of Zn^II also improves the 2e^- oxidation, shifting the anodic peak negative and decreasing the 2e^- peak separation. H-cell cycling experiments showed that 97% Coulombic efficiency and 98% charge storage efficiency was maintained for 50 cycles over 25 h using 0.1 M Zn^II(ClO₄)₂ as the supporting electrolyte. If Zn^II(ClO₄)₂ was replaced with TBAPF₆ in the electrolyte, the Coulombic efficiency fell to 78%. The use of Zn^II to increase the reversibility of 2e^- transfer is a promising result that points to the ability to use nickel dithiocarbonates for multielectron storage in RFBs.