An osmium complex, [OsII(bpy)2(4-aminomethylpyridine)(H2O)]2+, is attached to a mixed self-assembled monolayer on a gold electrode. The complex exhibits 1-electron, 1-proton redox chemistry (OsIII(OH)/OsII(H2O)) at pHs and potentials that are experimentally accessible with gold electrodes in aqueous electrolytes. The thermodynamic behavior and kinetic behavior of the system are investigated as a function of pH in both H2O and D2O. The two formal potentials and two pKa values are relatively constant for two chain lengths in H2O and in D2O. The standard rate constants at all pHs are strongly and uniformly affected by chain length, indicating that electronic coupling is the dominant factor controlling the rate of electron transfer. In both H2O and D2O, the standard rate constant is weakly dependent on the pH, exhibiting a minimum value midway between the pKa values. The kinetic isotope effect is small; standard rate constants decrease by roughly a factor of 2 in D2O over a wide range of pHs, but not at the more acidic pHs. The Tafel plots and plots of the transfer coefficient vs overpotential are asymmetrical at all pHs. These results are interpreted in terms of a larger reorganization energy for the OsII species and a smaller reorganization energy for the OsIII species. The OsIII reorganization energy is constant at all pHs in both H2O and D2O. The pH dependence of the OsII reorganization energy accounts for some or all of the pH dependence of the standard rate constant in H2O and D2O. The data deviate substantially from predictions of the stepwise proton-coupled electron-transfer mechanism. The observation of a kinetic isotope effect supports the concerted mechanism.
Electrochemical impedance analysis was performed on a Ni-YSZ/YSZ/Ni-YSZ (8 mol.% Yttria Stabilized Zirconia) half cell in order to evaluate the effects of a phosphorous impurity on the performance of the Ni-YSZ cermet anode of a solid oxide fuel cell (SOFC). 20 ppm PH 3 was introduced into the simulated coal-derived syngas (30.6% H 2 , 30% CO, 11.8% CO 2 , and 27.6% H 2 O). Both the charge transfer resistance and the diffusion resistance increased with time during exposure to the P-containing syngas. The diffusion resistance increased faster than the charge transfer resistance. Both the charge transfer resistance and the diffusion resistance values under load (DC current flow) were larger the corresponding values at open circuit.
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