Temperature dependent oxygen electrochemistry on platinum low index single crystal surfaces in acid solutions

Using the rotating ring-disk technique (RRDPt(hkl)E), the oxygen-reduction reaction (ORR) was studied in sulfuric acid solution over the temperature range 298-333 K. At the same temperature, the exchange current density increased in the order, i(111)(0)<i(100)(0)<i(110)(0) which gives the order of absolute kinetic activities of Pt(hkl) in 0.05 mol/L H2SO4:Pt(111)<Pt(100)<Pt(110). We found that at high current densities every crystal face has an ideally temperature-dependent Tafel slope, i.e., -2 x 2.3(RT/F). The activation energy for the ORR is independent of the surface geometry, Delta H-hkl(#) = 42 kJ/mol. The insensitivity of the activation energy to surface structure implies that the reaction pathway for the ORR is the same on all three single-crystal faces. A "series" pathway for the ORR was proposed, with the first electron transfer being the rate-determining step. The structure sensitivity in the kinetics of the ORR on Pt(hkl) is attributed to the structure-sensitive adsorption of bisulfate and hydroxyl anions and a simple site-blocking effect of these adsorbed anions on the rate of the ORR.