Unbiased assessment of electrochemical windows: Minimizing mass transfer effects on the evaluation of anodic and cathodic limits

The maximum voltage that can be used in electroanalytical experiments and electrochemical devices such as supercapacitors is limited by the electrochemical window within which the solvent and electrolyte resist decomposition. In the absence of a better definition, the limits of this window have been typically defined as the potentials at which a current density of an arbitrary value is achieved, which leads to inconsistencies between values from different sources. We propose a more useful definition of the electrochemical limits that depends less on experimental parameters, permitting more meaningful comparison of data from different investigators. The new method introduced here defines the electrochemical limit as the intercept of the approximately linear portions of the current-voltage relationship at potentials below and above the onset of electrochemical electrolyte or solvent decomposition, as determined by linear sweep voltammetry. To demonstrate the usefulness of this definition, we show that its use nearly eliminates the concentration dependence of the electrochemical limit caused by reduction of tetrabutylammonium perchlorate, and that it minimizes the extent to which the electrochemical limit due to tetraalkylammonium ion reduction depends on the length of the alkyl groups.