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).