Temperature effect on charge transport in a polymer membrane with dispersed [Ru(bpy)3]2+ analogues as studied by spectroelectrochemical methods

Charge transport in a Nafion® membrane with dispersed cis-[Ru(bpy)2(H2O)2]2+ or [Ru(terpy)2]2+ was investigated using potential-step chronoamperospectrometry. The apparent diffusion coefficient of charge (Dapp) for cis-[Ru(bpy)2(H2O)2]2+ increased linearly with increasing temperature in low temperature regions, reaching a plateau at higher temperature suggesting the existence of different mechanisms in the low and high temperature regions. The diffusion coefficient for cis-[Ru(bpy)2(H2O)2]2+ increased with an increase in concentration at 298 K, while it decreased slightly at 278 K, which was attributed to the suppression of self-exchange of electrons between molecules at 278 K. A percolation threshold was observed at 0.3 M in the concentration dependence of the diffusion coefficient for [Ru(terpy)2]2+ at 298 K. The charge hopping distance for [Ru(terpy)2]2+ was estimated as 0.9 nm based on the percolation theory. The self-exchange rate constants of electrons at 298 K were estimated in the order, [Ru(bpy)3]2+≈[Ru(terpy)2]2+>cis-[Ru(bpy)2(H2O)2]2+ suggesting that the self-exchange rate increases with increasing molecular size.