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  • Title: Effect of viscosity on steady-state voltammetry and scanning electrochemical microscopy in room temperature ionic liquids.
    Author: Lovelock KR, Cowling FN, Taylor AW, Licence P, Walsh DA.
    Journal: J Phys Chem B; 2010 Apr 08; 114(13):4442-50. PubMed ID: 20225849.
    Abstract:
    The electrochemical properties of a series of room temperature ionic liquids (RTILs) were studied using voltammetric methods and scanning electrochemical microscopy (SECM). The RTILs consisted of 1-alkyl-3-methylimidazolium cations, [C(n)C(1)Im](+), and either bis[(trifluoromethyl)sulfonyl]imide anions, [Tf(2)N](-), or hexafluorophosphate anions, [PF(6)](-). The effect of RTIL viscosity on mass transfer dynamics within each RTIL was studied electrochemically using ferrocene as a redox probe. In the case of the [C(n)C(1)Im][Tf(2)N] RTILs, the viscosity was altered by changing the alkyl chain length. [C(4)C(1)Im][PF(6)] was used for comparison as its viscosity is significantly higher than that of the [C(n)C(1)Im][Tf(2)N] RTILs. The RTIL viscosity affected the ability to record steady-state voltammograms at ultramicroelectrodes (UMEs). For example, it was possible to record steady-state voltammograms at scan rates up to 10 mV s(-1) in [C(2)C(1)Im][Tf(2)N] using 1.5 mum radius disk UMEs, but non-steady-state behavior was observed at 50 mV s(-1). However, at 12.5 microm radius UMEs, steady-state voltammetry was only observed at 1 mV s(-1) in [C(2)C(1)Im][Tf(2)N]. The RTIL viscosity also affected the ability to record SECM feedback approach curves that agreed with conventional SECM theory. In the most viscous [C(n)C(1)Im][Tf(2)N] RTILs, feedback approach curves agreed with conventional theory only when very slow tip approach speeds were used (0.1 microm s(-1)). These observations were interpreted using the Peclet number, which describes the relative contributions of convective and diffusive mass transfer to the tip surface. By recording feedback approach curves in each RTIL at a range of tip approach speeds, we describe the experimental conditions that must be met to perform SECM in imidazolium-based RTILs. The rate of heterogeneous electron transfer across the RTIL/electrode interface was also studied using SECM and the standard heterogeneous electron transfer rate constant, k(0), for ferrocene oxidation recorded in each RTIL was higher than that determined previously using voltammetric methods.
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