Unraveling the photoelectrochemical properties of ionic liquids: cognizance of partially reversible redox activity

Abstract

Ionic liquid based electrolytes are gaining great interest in the field of photoenergy conversion. We have found that the ionic liquids namely BMIm Cl, BMIm PF₆ and BMIm Tf₂N inherently offer redox activity. The device performance of the photoelectrochemical (PEC) cells of the configuration PbO_x (0.25 cm²)|blank ionic liquids|platinum (2 cm²) was analyzed in detail to get insights into the working principle of such systems. It was found that partially reversible redox ion pairs diminish the performance of such cells as power generating devices. The partial redox activity of the ionic liquids was confirmed by a number of observations derived from the PEC spectra. The important parameter, V_redox, which determines the performance of any PEC cell was also calculated for all the ionic liquids. The difficulties that arise in high frequency C–V measurements for ionic liquid systems were overcome by choosing the appropriate probing frequency. The evaluated V_redox of BMIm Cl, BMIm PF₆ and BMIm Tf₂N ionic liquids was found to be −0.30, −0.20 and −0.78 V (vs. NHE), respectively. This study will be beneficial to understand the role of ionic liquids as redox active electrolyte media in several applications.