Electrochemical Kinetics of Ag|Ag+ and TMPD|TMPD+• in the Room-Temperature Ionic Liquid [C4mpyrr][NTf2]; toward Optimizing Reference Electrodes for Voltammetry in RTILs

• Published in: Journal of physical chemistry. C 2007-09, Vol.111 (37), p.13957-13966
• Main Authors: Rogers, Emma I., Silvester, Debbie S., Ward Jones, Sarah E., Aldous, Leigh, Hardacre, Christopher, Russell, Angela J., Davies, Stephen G., Compton, Richard G.
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp0737754

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-butyl-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 NO3 - = 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, k 0, 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+•BF4 -, and TMPD+•NTf2 - were calculated to be 1.84, 1.35, and 1.43 × 10-11 m2 s-1 respectively, and a k 0 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.