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Electron hopping of tris (2,2-bipyridyl) transition metal complexes M(bpy)32/3 in nafion
Journal article

Electron hopping of tris (2,2-bipyridyl) transition metal complexes M(bpy)32/3 in nafion

Wayne L Gellett, Krysti L Knoche, Nadeesha P.W Rathuwadu and Johna Leddy
Journal of the Electrochemical Society, Vol.163(7), pp.H588-H597
2016
DOI: 10.1149/2.1121607jes

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Abstract

Voltammetric currents of transition metal complexes in films on electrodes increase with electron self exchange (hopping) rate k11. In films, diffusion coefficients Df∝k11c*f, where k11 sets frequency of electron hopping attempts and c*f film concentration sets distance between probes. Cyclic voltammetry of Nafion films on electrodes for six probes, tris(2,2'-bipyridyl) complexes, M(bpy)z +3 where Mz + is Ru2+, Os2+, Fe2+, Cr3+, Co3+, and Co2+, demonstrates Df varies linearly with kH2O11, literature k11 measured in water. From density of Ru(bpy)2 +3 exchanged Nafion 1.95 g/cm3 and nominal equivalent weight 1100, geometry establishes volume limited, z-independent c*f for all M(bpy)z +3. For concentrations of Ru(bpy)2 +3 in HNO3 where either Ru(bpy)2 +3: HNO3 ratio is fixed or Ru(bpy)2 +3 is 1 mM and HNO3 varies, c*f is statistically invariant. Based on aqueous titrations and dielectric constant sufficient to support ions, electron hopping is confined to hydrated domains of Nafion. From c*f, k11 in Nafion kNaf11 is estimated as , consistent with higher ion concentration in Nafion. Measured Df is 2 × 10− 9 cm2/s for Ru(bpy)2 +3 and Os(bpy)2 +3; 1 × 10− 10 cm2/s for Fe(bpy)2 +3 and Cr(bpy)3 +3; and 1.5 × 10− 12 and 5 × 10− 12 cm2/s for Co(bpy)2 +3 and Co(bpy)3 +3. For physical diffusion Dp lesssim 10− 12 cm2/s. A self consistent view of M(bpy)z +3 self exchange in Nafion is outlined. For Os(bpy)2/33, kH2O11 ≈ 4.5 × 108/Ms.

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