Kinetic Analysis of Electrochemical Redox Reaction of Nitroxyl Radical in Ionic Liquids
Authors: Maruyama Yuki , Nagamine Kuniaki , Nomura Ayako , Tsuchiya Kazuhiko , Iwasa Shigeyuki , Tokito Shizuo 
Published: February 2020

Organic radical batteries (ORBs) have been attracted much attention as a promising second battery because of its high-power density, flexibility, and fast charge/discharge properties. A typical ORB is composed of a poly (2,2-6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA) cathode, a carbon anode and an electrolyte consisting of carbonate solvents and LiPF6 which has a wider potential window than the formal redox potential of PTMA (3.6 V vs. Li/Li+) [1]. The PTMA shows high stability of radicals and high durability during repeated redox reaction, allowing reversible charging and discharging as an electrode active material for a second battery. The redox reaction of PTMA takes place on the cathode with PF6- moving to/from the cathode. Besides, the PTMA is changed into gel state by the absorption of organic solvent-based electrolyte. In the PTMA gel, charge transportation occurs by the electron self-exchange reaction between nitroxyl radicals and oxoammonium cations. However, the organic solvents-based electrolytes are at risk of inflammation when the electrolytes leak out from the batteries. To solve this issue, we are focusing on an ionic liquid as an electrolyte for the ORB because of its non-volatility and non-flammability [2]. However, to the best of our knowledge, electrochemical fundamental studies such as stability and reaction rate of 2,2-6,6-tetramethylpiperidinyloxy (TEMPO) in ionic liquids have not been reported yet.In this study, the electrochemical properties of TEMPO were investigated in the ionic liquids with the electrochemical potential windows which are wide enough to be applied for the ORB.

The ionic liquids used in this study were as follows: 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), 1-butyl-3-methylimidazolium BF4 (BMI-BF4), and N-N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium BF4 (DEME-BF4). Small amount of water originally absorbed in these ionic liquids were firstly removed by heating at 100 oC for 12 h under vacuum. 0.1 M TEMPO was dissolved in the water-removed ionic liquids, followed by bubbling with argon gas for 30 min to remove dissolved oxygen. A three-electrode system was constructed using a silver-silver ion electrode (Ag/Ag+) for a reference electrode, a glassy carbon electrode (Φ 3.0 mm) for a working electrode, and a platinum wire for a counter electrode. The electrode potential was swept from -2 to + 1 V vs. Ag / Ag + using a potentiostat to induce redox reaction of the TEMPO on the glassy carbon electrode.

Figure 1 shows cyclic voltammograms detected in three kinds of ionic liquids at scan rate of 100 mV/s. When the potential was swept between -2 V to 1 V vs. Ag/Ag+, clear peak currents were detected for oxidation and reduction of TEMPO in the ionic liquid. The separations between the electrochemical potentials at which oxidation and reduction peak currents were observed were 218 mV for EMI-BF4, 380 mV for BMI-BF4, and 403 mV for DEME-BF4, respectively. The peak potential separation is one of the parameters to study reversibility of the target redox reaction. These values were larger than that of ideal reversible reaction, 57 mV, suggesting semi-reversible or irreversible reaction of TEMPO in these ionic liquids.

The peak current densities of TEMPO detected in these ionic liquids were calculated from Figure 1 as 4.91 mA/cm2 for EMI-BF4, 2.58 mA/cm2 for BMI-BF4, and 1.10 mA/cm2 for DEME-BF4, respectively. The viscosities of EMI-BF4, BMI-BF4, and DEME- BF4 were reported in the catalog to be 53, 180, and 272 cPs at 25 °C, respectively. The peak current density for redox specie changes depending on its diffusion coefficient which is inversely proportional to the viscosity of solvent. This result suggested that viscosity of the ionic liquid is one of the factors governing the current density. We are now trying to calculate the diffusion coefficient and the reaction rate constant of TEMPO in each ionic liquid from dependency of peak potential separation on the scan rate using Nicholson method.

[1] K. Nakahara et. al., Chem. Phys. Lett., 359, 351 (2002).

[2] Y. Dai et. al., J. Electrochem. Soc., 158, A291 (2011).

Figure 1