Cyclic proton diffusion coefficient

Figure 13.10 Cyclic voltammetric behavior on reduction of the protonated 9H3+ and deprotonated 92+ rotaxane shown in Fig. 13.9 and of its protonated and deprotonated dumbbell-shaped component (argon-purged MeCN/EtrNPFe 0.05 M, 298 K, glassy carbon electrode, scan rate 50mV/s). The current intensity has been corrected to account for the differences in diffusion coefficients.

An attempt was made by Doblhofer et al. [210] to separate surface from bulk charging processes for thermally prepared Ru02 using the potential step technique. These authors [210] concluded that some bulk diffusion was involved, presumably involving protons, and estimated a diffusion coefficient of 10 19 cm2 s1. Weston and Steele [213] deduced a diffusion coefficient value for protons in porous powder electrodes of Ru02 which is approximately similar to the value of Doblhofer et al. [210]. Iwakura and co-workers [214], on the other hand, employed cyclic voltammetry in deduc-... 

It shonld be noted that high utilization factors measnred with cyclic voltammetry by no means warrant the assnmption that nnder dynamic conditions of fnel cell operation the CLs deliver the same cnrrent as they wonld without mass transport and ohmic constraints. To acconnt for the latter, Gloagnen et al. [185] employed the effectiveness factor the ratio of the actnal reaction rate to the rate expected in the absence of mass and ionic transport limitations. The effectiveness factor is a fnnction of the total catalyst area, the exchange cnrrent density, the overpotential, the diffusion coefficient D, the concentration of electroactive species Co, the thickness of the CL, and the proton conductivity of the electrolyte, and drops sharply below 100% with increased exchange current density and decreased the product DCq. 

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