Temperature-Controlled Charge Transfer Mechanism in a Polymer Film Incorporating a Redox Molecule As Studied by Potential-Step Chronocouloabsorptometry

Abstract

Charge transport (CT) in a Nafion film incorporating [Ru(bpz)3]2+ (bpz = 2,2‘-bipyrazine) was investigated using a potential-step chronocouloabsorptometry technique. The reductive CT by [Ru(bpz)3]2+/+ was found to take place by a combination mechanism of physical displacement of the complex and charge hopping between the molecules of the complex. This is a different mechanism from that of the oxidative CT (occurring by a charge hopping mechanism) in an earlier reported Nafion/[Ru(bpy)3]2+/3+ (bpy = 2,2‘-bipyridine) system. The combination mechanism in CT was also reported in an earlier Nafion/[(NH3)5Ru−O−Ru(NH3)4−O−Ru(NH3)5]6+ (Ru-red) system. In the present [Ru(bpz)3]2+ system, the activation energy (Ea = 78 kJ·mol-1) of physical displacement is higher than that (40 kJ·mol-1) of charge hopping. In the Ru-red system, on the contrary, the Ea (13 kJ·mol-1) of physical displacement is lower than that (55 kJ·mol-1) of charge hopping. As a result, the fraction (Φc) of the contribution of charge hopping to the initial CT rate decreased with increasing temperature for the [Ru(bpz)3]2+ system, whereas it increased with increasing temperature for the Ru-red system. The physical displacement was enthalpy-controlled in the [Ru(bpz)3]2+ system at 25 °C. By contrast, it was entropy-controlled in the Ru-red system. The featured factors influencing physical displacement and charge hopping were discussed.