Temperature dependent surface electrochemistry on Pt single crystals in alkaline electrolyte - Part 3. The oxygen reduction reaction

The kinetics of the oxygen reduction reaction (ORR) was studied in alkaline electrolyte at 293-333 K on Pt(hkl) surfaces by means of the rotating ring-disk electrode technique with solution phase peroxide detected at the ring electrode. The ORR on Pt( hkl) was found to be highly structure sensitive with activities increasing in the sequence (111) > (100) > (110) (1 x 2). Very similar apparent activation energies (37-45+/- 5 kJ mol(-1), eta = 0.35 V) were found on all three surfaces. Furthermore, at elevated temperature, significantly smaller amounts of peroxide are formed in agreement with enhanced peroxide reduction rates by increasing temperature. We found that the Tafel slopes on all three single crystal surfaces decrease with increasing temperature, indicating that the log i-E relationship is not represented by a classical Butler Volmer expression. Based on the kinetic analysis of the polarization curves and from simulations of log i-E curves, we propose that the rate of the ORR on Pt(hkl) in alkaline solution is mainly determined by the potential/temperature dependent surface coverage by OHad. We propose two modes of action of the OHad : (i) OHad blocks the adsorption of O-2 on active platinum sites; and ( ii) OHad alters the adsorption energy of intermediates which are formed during the ORR on Pt sites.