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

Abstract

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) lt i(100)(0) lt i(110)(0) which gives the order of absolute kinetic activities of Pt(hkl) in 0.05 mol/L H2SO4:Pt(111) lt Pt(100) lt 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.