1 Unusual surface mass changes in the course of the oxygen reduction reaction on platinum and their explanation by using a kinetic model A. Kriston * , B. B. Berkes. * , P. Simon ** , G. Inzelt * , K. Dobos * , A. Nemes * * Department of Physical Chemistry, Institute of Chemistry, Etvs LorÆnd University, 1117 Budapest, PÆzmÆny PØter sØtÆny 1/A, Hungary ** Department of Applied Analysis and Computational Mathematics, Institute of Mathematics, Etvs LorÆnd University, 1117 Budapest, PÆzmÆny PØter sØtÆny 1/C, Hungary e-mail: info@fuelcell.hu Abstract An unusual change of the surface mass with time has been observed during the oxygen reduction reaction on Pt by using chronopotentiometry and simultaneous electrochemical quartz crystal nanobalance (EQCN) measurements. A simplified kinetic model of Damjanovic and Brusic, which involves two electrochemical and a chemical step, was analyzed by using phase plane analysis. The theoretical analysis predicted that bistability might occur in this system at a certain set of parameter values. The mathematical simulation of the different trajectories explained well the strong influence of the starting potential and the current density on the change of the surface mass observed. Evidence was found that the surface coverage can increase at lower potentials which can lead to the formation of hydrogen peroxide even if it is energetically unfavorable. Key words: ORR, platinum, EQCN, simulation, bifurcation 1. Introduction The understanding of oxygen reduction reaction (ORR) on platinum surface is crucial concerning the development of efficient and reliable fuel cells. A deeper knowledge of the mechanism could help to improve the performance of fuel cells and to decrease the amount of the catalyst used. Many efforts have been done to establish the reaction scheme, however no final agreement has been reached, yet [1-30]. Most of these studies have dealt with the determination of the apparent Tafel-slope at different conditions such as different pH, temperature, oxygen concentration [2, 3], and with the analysis of the Koutecký-Levich plot [4] by determining the number of electrons involved in the ORR. The coverage-potential relationships by using chronoamperometric techniques were also reported [5, 6]. They found that the amount of the oxide, i.e., the coverage, was the same in oxygen containing and oxygen-free media in the presence of water, and it decreased linearly with the potential (usual 0DQXVFULSW &OLFN KHUH WR GRZQORDG 0DQXVFULSW .ULVWRQELVWDELOLW\BRIB255B-266(&BUHYBSGI &OLFN KHUH WR YLHZ OLQNHG 5HIHUHQFHV 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65