Scanning electrochemical microscopy in nonusual solvents: inequality of diffusion coefficients problem

Scanning electrochemical microscopy offers interesting possibilities for investigating both transport properties in room-temperature ionic liquids (RTILs) and reactions occurring at the ionic liquid/substrate interface. Besides the expected difficulties related to the lower diffusion coefficients for species dissolved in RTILs arising from the higher viscosity of RTILs, the major problem comes from the inequality of the diffusion coefficients between the oxidized and reduced forms of the redox mediator used to probe the interfaces. This question was treated by an extension of the model originally presented by Martin and Unwin (Martin, R. D.; Unwin, P. R. J. Electroanal. Chem. 1997, 439, 123) and was adapted to the specific aspects of SECM in ionic liquids. The inequalities of diffusion coefficients lead to large anomalies in the current responses, which in extreme cases impede the recording of stationary approach curves. Conditions for recording steady-state approach curves into ionic liquids and consequences of erroneous data treatment were examined. These discrepancies with the simple models (when diffusion coefficients have been taken as equal), could be transformed in a convenient method for characterizing the transport properties of species dissolved in RTIL. The analysis is based on transient SECM experiments and determinations from nonambiguous dimensionless parameters. Experimental examples based on SECM in common RTILs, using the O2/ O2*- couple, were analyzed taking into account the presented model.