The necessity for new grid energy storage techniques for example redox flow batteries (RFBs) will be vital as the consumption of renewable energy sources continues to increase. Herein, we studied the redox cycle of nickel dithiocarbamates for potential use as catholyte in non-aqueous redox flow batteries. Nickel dithiocarbamates (Ni(dtc)2) unique redox cycle offers the ability to better understand these reactions by displaying 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]+. Cyclic voltammetry and spectroscopic experiments show that these 1e- and 2e- pathways can be controlled by the addition of ancillary ligands such as pyridine derivatives and Lewis acids such as Zn(II). Specifically, the addition of different pyridine based ancillary N-donor ligands (L) to the electrolyte solution results in 1e- oxidation producing a mixture of five-coordinate [Ni(dtc)2L]+ and six-coordinate [Ni(dtc)2(L)2]+ intermediate species which decay to [Ni(dtc)3]+ by parallel pathways. The equilibrium constants for L coordination were determined and found to increase with larger pKa values of the pyridine base. On the reduction side, the addition of Zn(II) to the electrolyte is shown to consolidate the two 1e- reduction peaks into a single 2e- reduction where [Ni(dtc)3]+ is reduced directly to Ni(dtc)2. This improvement is believed to be due to chemical assistance in removing a dtc- ligand from a Ni(dtc)3 intermediate, allowing for easier reduction to Ni(dtc)2. Coulombic efficiency of over 80 % is achieved for 50 cycles when Ni(dtc)2 was used as catholyte in the chrono potentiometric (CP) H cell experiment in presence of 24 mM Zn2+ ion.

Mechanistic study of multi-electron redox cycle of Nickel dithiocarbamate complex for redox flow battery application

ACS Spring Meeting 2021

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