ARTICLE pubs.acs.org/JPCB
Comparison of Diffusivity Data Derived from Electrochemical and NMR Investigations of the SeCNh/(SeCN)2/(SeCN)3h System in Ionic Liquids Amber Solangi,†,‡ Alan M. Bond,*,† Iko Burgar,§ Anthony F. Hollenkamp,|| Michael D. Horne,^ Thomas R€uther,|| and Chuan Zhao†,# †
)
School of Chemistry, Monash University, Clayton, Vic 3800, Australia CSIRO Energy Technology, Box 312, Clayton South, Vic 3169, Australia § CSIRO Materials Science and Engineering, Private Bag 33, Clayton South, Vic 3169, Australia ^ CSIRO Process Science and Engineering, Box 312, Clayton South, Vic 3169, Australia
bS Supporting Information ABSTRACT: Electrochemical studies in room temperature ionic liquids are often hampered by their relatively high viscosity. However, in some circumstances, fast exchange between participating electroactive species has provided beneficial enhancement of charge transport. The iodide (Ih)/iodine (I2)/ triiodide (I3h) redox system that introduces exchange via the Ih þ I2 h I3h process is a well documented example because it is used as a redox mediator in dye-sensitized solar cells. To provide enhanced understanding of ion movement in RTIL media, a combined electrochemical and NMR study of diffusion in the {SeCNh(SeCN)2(SeCN)3h} system has been undertaken in a selection of commonly used RTILs. In this system, each of the Se, C and N nuclei is NMR active. The electrochemical behavior of the pure ionic liquid, [C4mim][SeCN], which is synthesized and characterized here for the first time, also has been investigated. Voltammetric studies, which yield readily interpreted diffusion-limited responses under steady-state conditions by means of a Random Assembly of Microdisks (RAM) microelectrode array, have been used to measure electrochemically based diffusion coefficients, while self-diffusion coefficients were measured by pulsed field gradient NMR methods. The diffusivity data, derived from concentration and field gradients respectively, are in good agreement. The NMR data reveal that exchange processes occur between selenocyanate species, but the voltammetric data show the rates of exchange are too slow to enhance charge transfer. Thus, a comparison of the iodide and selenocyanate systems is somewhat paradoxical in that while the latter give RTILs of low viscosity, sluggish exchange kinetics prevent any significant enhancement of charge transfer through direct electron exchange. In contrast, faster exchange between iodide and its oxidation products leads to substantial electron exchange but this effect does not compensate sufficiently for mass transport limitations imposed by the higher viscosity of iodide RTILs.
1. INTRODUCTION Room temperature ionic liquids (RTILs) continue to find new applications in many areas of chemistry. In the discipline of dynamic electrochemistry, the possibility of combining advantageous physical characteristics with wide potential windows means that RTILs are becoming media of choice for seeking enhanced performance in batteries, fuel cells and photovoltaic cells.1,2 The remarkable growth in the use of RTILs in dynamic electrochemistry does not seem to have been limited by the relative difficulty of establishing exactly how mass transport processes such as diffusion occur in these media. Typically, traditional theories established for molecular solvents with added supporting electrolyte are employed to explain transport phenomena in RTILs, although evidence is emerging that there are nuances that may need to be accommodated.35 An interesting example of a r 2011 American Chemical Society
nuance emerged in studies by Kawano and Watanabe on the voltammetric behavior of the iodideiodinetriiodide {IhI2I3h} system in the RTIL N-ethyl-N-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C2mim][NTf2]), in the context of developing improved electrolytes for dye-sensitized solar cells (DSSCs).6,7 Mixtures of iodide and iodine produce the triiodide ion (I3h), according to equilibrium 1, which lies predominantly to the right in nonaqueous solvents.8 I þ I2 h I3
ð1Þ
Triiodide is one of the redox mediators, or “shuttles”, used in Received: December 16, 2010 Revised: February 24, 2011 Published: May 05, 2011 6843
dx.doi.org/10.1021/jp111977q | J. Phys. Chem. B 2011, 115, 6843–6852
The Journal of Physical Chemistry B
ARTICLE
Gr€atzel’s DSSCs,9 where the following redox reactions occur.10,11
ð2Þ
2I3 f 3I2 þ 2e
ð3Þ
6I f 2I3 þ 4e
In RTIL studies under conditions where the concentration ratio of the different species was varied, Kawano and Watanabe found that the apparent iodide diffusion coefficient (obtained from microelectrode voltammetry) increased with the total concentration of iodine-containing species to values well beyond those that could be explained by simple mass transfer.6 These authors suggested that homogeneous exchange between iodide and iodine could enhance the local concentration of triiodide, thereby leading to enhanced oxidation current. More recently, Zistler et al. reached similar conclusions in studies which extended the range of iodide concentration.12 The principal finding from these investigations was that a process leading to enhanced diffusion helps to offset the decrease in electrochemical response (current) that is typically encountered in relatively viscous RTILs. Concentration-dependent “enhancement” of mass transport was discovered in the late 1960s when diffusion measurements were used to quantify the rates of very fast exchange reactions.13,14 Ruff and co-workers provided the theory for what they termed “transfer diffusion” and showed that both the {IhI2I3h} and {BrhBr2Br3h} systems displayed significant increases in the apparent diffusion coefficients of the halide and trihalide species.15,16 Importantly, the technique used by Ruff et al. required a medium of high ionic strength (total added salt concentration was 4 M), presumably to provide solution conditions that allow sufficiently close approach of the negatively charged exchanging species (eq 4). I3 þ I f ½I 3 3 3 I2 3 3 3 I f I þ I3
ð4Þ
This requirement probably explains why attempts to demonstrate the effect in organic solvent media, with total electrolyte concentration
