Ni(IV/II) Redox Couple 2e- Reversibility Improvement for Redox Flow Battery

Abstract

Energy storage is a vital aspect for the successful implementation of renewable energy resources on a global scale.1,2 Herein, we investigated the redox cycle of nickel (II) bis(diethyldithiocarbamate) ((Ni(dtc)2) for potential use as a catholyte in non-aqueous redox flow batteries. The unique redox cycle of Ni(dtc)2 offers 2e- chemistry upon oxidation from Ni(II) → Ni(IV) but 1e- chemistry upon reduction from Ni(IV) → Ni(III) → Ni(II). The underlying reasons for this cycle lie in the structural changes that occur between four-coordinate Ni(dtc)2 and six-coordinate [Ni(dtc)3]+.3–5 Cyclic voltammetry and spectroscopic experiments show that the addition of Zn(II) to the electrolyte consolidates the two 1e- reduction peaks into a single 2e- reduction where [Ni(dtc)3]+ is reduced directly to Ni(dtc)2. This enhancement is believed to be due to Zn (II) assistance in removing a dtc- ligand from a Ni(dtc)3 intermediate, allowing for more facile reduction to Ni(dtc)2. The addition of Zn (II) also improves the 2e- oxidation, shifting the anodic peak slightly negative and thus decreasing the 2e- peak splitting. Here we found that Zn (II) coordinates with Ni(dtc)2 to form a new complex with an equilibrium constant 147 M-1. Battery experiments showed that 99% coulombic efficiency was obtained for oxidation of Ni(dtc)2 followed by reduction of [Ni(dtc)3]+ using 0.1 M Zn(ClO4)2 as supporting electrolyte after 100 cycles over 58 h. If Zn(ClO4)2 is replaced with TBAPF6 as the electrolyte, the coulombic efficiency falls to 70% . The use of Zn (II) to increase the reversibility of 2e- transfer is a highly promising result which points to the ability to use nickel dithiocarbonates more effectively in RFBs.Reference(1) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111 (5), 3577–3613. https://doi.org/10.1021/cr100290v.(2) Saha, S.; Sahil, S. T.; Mazumder, Md. M. R.; Stephens, A. M.; Cronin, B.; Duin, E. C.; Jurss, J. W.; Farnum, B. H. Synthesis, Characterization, and Electrocatalytic Activity of Bis(Pyridylimino)Isoindoline Cu( ii ) and Ni( ii ) Complexes. Dalton Trans. 2021, 10.1039.D0DT03030A. https://doi.org/10.1039/D0DT03030A.(3) Mazumder, M. M. R.; Burton, A.; Richburg, C. S.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B. H. Controlling One-Electron Vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. ECS Meet. Abstr. 2021, MA2021-01 (1), 52–52. https://doi.org/10.1149/ma2021-01152mtgabs.(4) Islam, R.; Mazumder, M. M. R.; Farnum, B. H. Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage. ECS Meet. Abstr. 2021, MA2021-01 (1), 54–54. https://doi.org/10.1149/ma2021-01154mtgabs.(5) Mazumder, Md. M. R.; Burton, A.; Richburg, C. S.; Saha, S.; Cronin, B.; Duin, E.; Farnum, B. H. Controlling One-Electron vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate. Inorg. Chem. 2021, acs.inorgchem.1c01699. https://doi.org/10.1021/acs.inorgchem.1c01699. Figure 1