Solvent Dependent Spectroscopic and Electrochemical Studies of Nickel (II) Diethyldithiocarbamate for Energy Storage

Abstract

In the field of electrochemical energy storage, multielectron molecular redox couples play significant roles. These redox couples have been used both as anolytes and catholytes in redox flow batteries (RFBs). The energy stored in an RFB is proportional to the number of electrons transferred per molecule. Thus, increasing the number of electrons transferred per molecule can enhance the amount of stored energy. Previous works in this field mostly looked for molecules containing multiple 1e- redox couples. The advantage of a single 2e- redox couple over multiple 1e- redox couples is that both electrons are stored and released at the same potential. In the case of multiple 1e- redox couples, two redox potentials are usually separated by more than 0.3 V. This can cause significant shifts in battery voltage and power during charging and discharging of an RFB. Nickel (II) diethyldithiocarbamate, NiII(Et2dtc)2 undergoes 2e- oxidation at a single potential to form [NiIV(Et2dtc)3]+. In different nonaqueous solvents NiII(Et2dtc)2 can offer divergent redox behavior based on their coordination properties. This divergent redox behavior means significant changes on the electrochemistry of the complex during oxidation and reduction. Pyridine, dimethyl sulfoxide (DMSO), dimethyl formamide (DMF), acetonitrile (MeCN), methanol (MeOH), acetone, and dichloromethane (DCM) have been studied in this work as they have dissimilar donor numbers with different coordination abilities towards the Ni center. Both cyclic voltammetry and spectroscopic data in the above-mentioned solvents show distinct behavior which is expected as solvent coordination ability varies. Due to its highest coordination ability among the solvents used in this study, pyridine hinders the Ni (II) to Ni (IV) oxidation by coordinating strongly to the Ni (III) center to form [NiIII(Et2dtc)2(Py)2]+. On the contrary, noncoordinating DCM allows oxidation to Ni (IV) albeit through two distinguishable 1e- oxidation processes. MeCN and acetone allow Ni (II)→(IV) oxidation at a single potential. DMSO, DMF, and MeOH show intermediate redox behavior between 1e- and 2e- transfer reactions. The presence of Ni (III) in the case of highly coordinating solvents was confirmed by low temperature EPR and UV-vis absorbance spectroscopy with the use of a chemical oxidant. These studies point to the ability of solvent to aid or hinder multielectron redox strategies which use Ligand Coupled Electron Transfer where changes in coordination environment are necessary.