Parallel Variation of Mass Transport and Heterogeneous and Homogeneous Electron Transfer Rates in Hybrid Redox Polyether Molten Salts

Metal complexes can be prepared as highly viscous (semisolid), room temperature molten salts by combining them with oligomeric polyether substituents. The fluidity and transport properties of these hybrid redox polyether melts can be systematically manipulated by changing the oligomeric chain lengths and by adding unattached oligomers as plasticizers. This paper describes the voltammetrically measured transport properties of several Co(II) polypyridine (2,2′-bipyridine, phenanthroline) melts. The properties evaluated are the physical self-diffusion coefficient (D_PHYS) of the cationic complex in its melt, the diffusivity of its counterion (D _CION), the heterogeneous electron-transfer rate constant (k _HEI) of the Co(III/II) oxidation at the electrode surface, and the rate constant (k_EX) for homogeneous electron self-exchange between Co(II) and Co(I) in the mixed valent layer next to the electrode. These dynamics parameters change in parallel manners, over a large (> 10 ³) range of values, when the melt fluidity is changed by plasticizers or temperature, While k_HEI and k_EX both change systematically with D_PHYS, they change on a more nearly proportional basis with D_CION. The latter relationship is interpreted as a kind of solvent dynamics control in which both the homogeneous Co(II/I) and heterogeneous Co(III/II) reaction rates are controlled by the ionic atmosphere relaxation time constant, namely, the time constant of redistribution of counterions following an electron-transfer step that has produced a nonequilibrium charge distribution. D_CION provides a measure of the ion atmosphere relaxation rate.