Electrochemical kinetics of Ag
Rogers EI., Silvester DS., Jones SEW., Aldous L., Hardacre C., Russell AJ., Davies SG., Compton RG.
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, [C 4 mpyrr] [NTf 2 ] on a 10 μm diameter Pt electrode. For the three silver salts investigated (AgOTf, AgNTf 2 , and AgNO 3 , where OTf- = trifluoromethanesulfonate, NTf 2- = 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, AgNTf 2 , and AgNO 3 were found to be 1.05, 1.17, and 5.00 × 10 -11 m 2 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 + |Ag 0 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 BF 4- and NTf 2- counter anions. Diffusion coefficients for TMPD, TMPD + BF 4- , and TMPD + NTf 2- were calculated to be 1.84, 1.35, and 1.43 × 10 -11 m 2 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. © 2007 American Chemical Society.