Electrochemical kinetics of Ag|Ag+ and TMPD|TMPD+ in the room-temperature ionic liquid [C4mpyrr][NTf2]; toward optimizing reference electrodes for voltammetry in RTILs

The voltammetry and kinetics of the Ag|Ag⁺ system (commonly used as a reference electrode material in both protic/aprotic and RTIL solvents) was studied in the room-temperature ionic liquid N-buty1-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C₄mpyrr] [NTf₂] on a 10 μm diameter Pt electrode. For the three silver salts investigated (AgOTf, AgNTf₂, and AgNO₃, where OTf- = trifluoromethanesulfonate, NTf₂ ^- = bis(trifluoromethylsulfonyl)imide, and NO ₃ ^- = nitrate), the voltammetry gave rise to a redox couple characteristic of a "deposition/stripping" process at the platinum electrode surface. Using potential step chronoamperometry, the diffusion coefficients of AgOTf, AgNTf₂, and AgNO₃ were found to be 1.05, 1.17, and 5.00 × 10^-11 m² s^-1. All three voltammograms were theoretically modeled to reveal surprisingly slow standard electrochemical rate constants, k⁰, of 2.0, 1.5, and 0.19 × 10^-4 cm s^-1 respectively for the Ag ⁺|Ag⁰ couple. As a potentially faster alternative to the Ag|Ag⁺ system, the voltammetry and kinetics of the TMPD|TMPD ⁺ system (where TMPD = N,N,N′,N′-tetramethyl-p- phenylenediamine) was also studied, using neutral TMPD and two TMPD radical cation salts, with BF₄ ^- and NTf₂ ^- counter anions. Diffusion coefficients for TMPD, TMPD⁺BF ₄ ^-, and TMPD⁺NTf₂ ^- were calculated to be 1.84, 1.35, and 1.43 × 10^-11 m² s^-1 respectively, and a k⁰ value of 2.6-2.8 × 10^-3 cm s^-1 was obtained from theoretical fitting of the cyclic voltammetry. This number is an order of magnitude larger than that for the Ag|Ag⁺ system, allowing for the suggestion that the TMPD|TMPD⁺ system may be more suitable than the Ag|Ag⁺ system as a redox couple for use in reference electrodes for ionic liquids.