Anionic Effect on Electrical Transport Properties of Solid Co^2+/3+ Redox Mediators

In a solid-state dye-sensitized solar cell, a fast-ion conducting (σ_25°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 Co^2+/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 σ_25°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).