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 (ClO 4 ) 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 (ClO 4 ) 2 as the supporting electrolyte. If Zn II (ClO 4 ) 2 was replaced with TBAPF 6 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.