Kinetic Isotope Effect of the Oxygen Reduction Reaction on Carbon-Supported Platinum Electrocatalysts

Proton exchange membrane fuel cells (PEMFC) can partially satisfy the energy need with the least anthropogenic impact on nature. However, they suffer from great costs, partly induced by their expensive catalytic material, platinum, necessary to catalyze the oxygen reduction reaction (ORR) occurring at the cathode. Hence, it becomes essential to either synthesize electrocatalysts without platinum1, or to maximize the performance of the fuel cell per unit mass of platinum2 by increasing the exposure of its active sites for the ORR.3 We studied the oxygen reduction reaction (ORR) mechanism on Pt nanoparticles (NPs) supported on several Vulcan-type carbon blacks with various physicochemical characteristics. Those materials were thoroughly investigated by various microscopic, spectroscopic and electrochemical methods, such as kinetic isotope effect (KIE) studies. KIE studies were used to assess an eventual change of the rate determining step (RDS). The rate constants of the RDS were calculated in 0.5M H2SO4 and 0.5M D2SO4 at the onset potential. The KIE values are reported in Figure 1, as a function of Scarbon/SPt,H . Scarbon/SPt,H corresponds to the ratio between the carbon specific surface and the platinum electrochemically active surface and acts as an indicator of the Pt NPs density onto the carbon surface. A low Scarbon/SPt,H shows KIE = 1, which corresponds to what is commonly observed on Pt electrocatalysts for the ORR and implies that the rate determining step is proton independent.4 An identical value was observed on the commercial Pt/C (40 wt. % on Vulcan XC72R carbon, i.e. with the high density of Pt NPs), i.e. KIE = 1.11 ± 0.08. At high Scarbon/SPt,H a KIE = ca. 1.5 is observed, hence, providing evidence for a shift from a proton independent to a proton dependent mechanism with the increase of the Scarbon/SPt,H : the involvement of the proton in the ORR RDS is dependent on the density of Pt NPs on the electrocatalyst surface. (1)Chen, Y.; Asset, T.; Lee, R.; Artyushkova, K.; Atanassov, P. J. Phys. Chem. C 2019, acs.jpcc.9b01480. (2)Robson, M. H.; Serov, A.; Artyushkova, K.; Atanassov, P. Electrochim. Acta 2013, 90, 656–665. (3)Matanović, I.; Garzon, F. H.; Henson, N. J. J. Phys. Chem. C 2011, 115 (21), 10640–10650. (4)Tse, E. C. M.; Varnell, J. A.; Hoang, T. T. H.; Gewirth, A. A. J. Phys. Chem. Lett. 2016, 7 (18), 3542–3547. Figure 1