Kinetic Analysis of Oxygen Reduction on Pt(111) in Acid Solutions:  Intrinsic Kinetic Parameters and Anion Adsorption Effects

Abstract

Oxygen reduction reaction (ORR) kinetics in acid solutions was studied by analysis of the polarization curves obtained by rotating disk electrode method for Pt(111) in HClO 4 and H 2 SO 4 solutions. The model for ORR kinetic currents assumes that the intrinsic exchange current and Tafel slope are independent of anion adsorption. The site blocking and electronic effects of adsorbed OH (in HClO 4 ) and bisulfate (in H 2 SO 4 ) were evaluated with the adsorption isotherms incorporated in the model. The best fits yielded the intrinsic Tafel slope in the range from -118 to -130 mV/dec, supporting single electron transfer in the rate-determining step with the corresponding transfer coefficients equal to 0.50 and 0.45, respectively. In addition to site blocking, a negative electronic effect on ORR kinetics was found for both OH and bisulfate with the effect of the latter being much stronger. The deviation of the apparent Tafel slope in HClO 4 from its intrinsic value can be fully accounted for by the site blocking and electronic effects of adsorbed OH ions, which vary with coverage over the mixed kinetic-diffusion controlled region. For Pt nanoparticle catalysts, the apparent Tafel slope is similar to that for Pt(111) in HClO 4 and the positive potential shift is mainly due to the increase in apparent exchange current as effective surface area increases.

Analysis of electrocatalytic reaction schemes: distinction between rate-determining and potential-determining steps

“…The experimentally observed Tafel slope for the ORR on a platinum electrode is typically between 60 and 120 mV/dec. 23,24,25 While I do not want to be lured into the discussion of the exact meaning of this Tafel slope, it does suggest that the first electron transfer of the overall four electron transfer reaction is either rate-determining or directly preceding the rate-determining step. On the other hand, the DFT-based thermodynamic analysis of Nørskov et al 1 has identified the removal of oxygenated species, such as OH ads , from the Pt surface, as the thermodynamic bottleneck in the reaction scheme.…”

A new spectroelectrochemical cell for in situ measurement of Pt and Au K-edge X-ray absorption fine structure

“…The structure and reaction mechanism of platinum (Pt) electrocatalysts in the ORR have been studied to reveal ORR catalysis on an atomic level. [1][2][3][4][5] In recent years, core-shell electrocatalysts have drawn wide attention to improve the mass-specific activity of Pt. 6 A typical effective core-shell electrocatalyst is a gold (Au) core with a Pt shell.…”

Oxygen reduction reaction on stepped platinum surfaces in alkaline media

“…Since the procedure using equation (4) is 10 simpler, this equation will be used in the following to determine the kinetic currents. Figure 3 shows some representative curves for the ORR on the different series of electrodes.…”