Anionic Effect on Electrical Transport Properties of Solid Co2+/3+ Redox Mediators

Abstract

In a solid-state dye-sensitized solar cell, a fast-ion conducting (σ25°C > 10−4 S cm−1) solid redox mediator (SRM; electrolyte) helps in fast dye regeneration and back-electron transfer inhibition. In this work, we synthesized solid Co2+/3+ redox mediators using a [(1 − x)succinonitrile: x poly(ethylene oxide)] matrix, LiX, Co(tris-2,2′-bipyridine)3(bis(trifluoromethyl) sulfonylimide)2, and Co(tris-2,2′-bipyridine)3(bis(trifluoromethyl) sulfonylimide)3 via the solution-cast method, and the results were compared with those of their acetonitrile-based liquid counterparts. The notation x is a weight fraction (=0, 0.5, and 1), and X represents an anion. The anion was either bis(trifluoromethyl) sulfonylimide [TFSI−; ionic size, 0.79 nm] or trifluoromethanesulfonate [Triflate−; ionic size, 0.44 nm]. The delocalized electrons and a low value of lattice energy for the anions made the lithium salts highly dissociable in the matrix. The electrolytes exhibited σ25°C ≈ 2.1 × 10−3 (1.5 × 10−3), 7.2 × 10−4 (3.1 × 10−4), and 9.7 × 10−7 (6.3 × 10−7) S cm−1 for x = 0, 0.5, and 1, respectively, with X = TFSI− (Triflate−) ions. The log σ–T−1 plot portrayed a linear curve for x = 0 and 1, and a downward curve for x = 0.5. The electrical transport study showed σ(TFSI−) > σ(Triflate−), with lower activation energy for TFSI− ions. The anionic effect increased from x = 0 to 1. This effect was explained using conventional techniques, such as Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), UV–visible spectroscopy (UV-vis), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA).