A Zinc Catalyzed Two-Electron Nickel(IV/II) Redox Couple: New Catholyte Design for 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), NiII(dtc)2, for potential use as a multi-electron storage catholyte in non-aqueous redox flow batteries (RFBs). Previous studies have shown the unique redox cycle of NiII(dtc)2 offers 2e- chemistry upon oxidation from NiII → NiIV but 1e- chemistry upon reduction from NiIV → NiIII → NiII. Electrochemical experiments presented here show that the addition of as little as 10 mol% ZnII(ClO4)2 to the electrolyte consolidates the two 1e- reduction peaks into a single 2e- reduction where [NiIV(dtc)3]+ is reduced directly to NiII(dtc)2. This catalytic enhancement is believed to be due to ZnII removal of a dtc- ligand from a NiIII(dtc)3 intermediate, resulting in more facile reduction to NiII(dtc)2. The addition of ZnII also improves the 2e- oxidation, shifting the anodic peak negative and decreasing the 2e- peak splitting. 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 ZnII(ClO4)2 as supporting electrolyte. If ZnII(ClO4)2 was replaced with TBAPF6 in the electrolyte, the coulombic efficiency fell to 78%. The use of ZnII to increase the reversibility of 2e- transfer is a promising result that points to the ability to use nickel dithiocarbonates for multi-electron storage in RFBs.