Kinetic And Mechanistic Studies Of The Oxygen Reduction Reaction On Pt Electrode

Oxygen reduction reaction is widely studied because it is the cathodic reaction in fuel cells, air fuel cell, and several other important processes. From the catalytic point of view, the reaction mechanism on a Pt electrode has attracted most attention because Pt and Pt-alloys are still the most active catalysts for low-temperature fuel cells. In contrast to hydrogen oxidation on Pt, which is one of the very fast reactions, oxygen reduction on Pt exhibits slow and complicated kinetic behavior. The standard reversible potential for O2 reduction is 1.23 V, in fact, the oxygen in a fuel cell has a working potential below 0.8 V so that there is a 400 mV potential loss, which is about 10 times larger than that for H2 oxidation on anode. This large overpotential loss has been generally attributed to the sluggish kinetics of oxygen reduction reaction (ORR) and OH- (from H2O) or other spectator anion adsorption. Thus, one of the important tasks in electrocatalysis is to understand the mechanism and kinetics of oxygen reduction reaction and improve performance of the catalysts of fuel cell.1. Temperature effect on the Oxygen Reduction ReactionThe temperature and the Pt nano-particles effect on the oxidation of canbon support have been studied using a thermostatic cell with gas diffusion electrode as the working electrode or differential electrochemical mass spectrometry (DEMS) in a dual-thin layer flow cell. It is found that increase temperature can improve the oxidation rate of the corresponding carbon, and furthermore Pt can catalyze the carbon oxidation, however, with 0.2 V≤E≤1.0 V , the carbon support is electrochemical stability, almost not effect on the activity of the precious metal-based electrocatalysts.Using the thermostatic cell with gas diffusion electrode as working electrode, we study the temperature effect on the activity of oxygen reduction reaction on the 3nm, 5nm Pt/C electrode respectively, as temperature increases from 5 to 70℃. The oxygen reduction reaction (ORR) On Pt/C electrode demonstrated that at constant temperature, ORR current density monotonously increases with potential decreases from 1.0 to 0.2 V, and at the same potential it increases with temperature from 5 to 70℃, while under otherwise identical condition, the special current density of ORR on the 5 nm Pt / C larger than on the 3 nm Pt / C. As temperature increases from 5 to 70℃, the 3 nm and 5 nm Pt/C electrodes have similar Tafel slopes in the potential region of 0.35 V≤η≤0.55 V are -120±20mV/dec, which indicates that in this potential region, the oxygen reduction is mostly controlled by kinetics . The exchange current density of ORR increases with temperature, ands at same temperature the 5 nm Pt/C has a larger value, which also proved that the activity of ORR increases with temperature. While the apparent activation energy(Ea) for ORR decreases with increases the overpotential, and the two sizes of electrode have similar Ea.In all ,our result show that the mechanism and kinetics of oxygen reduction reaction cannot change with the temperature and potential, and the rate determining step for ORR is the firstelectron to O22. pH effect on the Oxygen Reduction ReactionWith the Hanging Meniscus Rotating Disk Eelctrode (HMRD), the pH effect on oxygen reduction on Pt (111) electrode has been systemically researched. By comparing the activity of ORR in 0.1M HClO4 (pH=1.0) with 0.1 M NaOH (pH=13) solution, it is found that at the same overpotential the activity of ORR in 0.1 M NaOH solution is smaller, attributed to the coverage of OH- on the surface in 0.1M NaOH is larger than in 0.1MHClO4 solution.We also study the oxygen reduction with the different concentration of HClO4 to control the pH of the solution, it is found that when concentration of HClO4≥7 mM, the onset potential and half wave potential of ORR shift to the more negative potentials for approx. 59mV per pH unit. However,when the concentration of HClO4 <2 mM, the flux of H+ toward the electrode surface is smaller than four times that of O2 and two distinct current plateaus for ORR are observed, furthermore, pH at the electrode/electrolyte interface changes abruptly during the oxygen reduction. In order to keep the pH? invariablenes,we choose to investigated the pH effect on the ORR in phosphate buffer solutions.It is found that the pH dependence of the onset of ORR have a slope is-53mV/pH on the SHE scale in phosphate buffer solutions, the result is similar with in HClO4 solution. However, the half-wave potential of ORR E1/2 with pH of solution changes has a slope, -38mV/pH on the SHE scale, the value is similar with the peak potential of adsorbed phosphate species shifts with pHs of solutions. The pH effect in this region is mostly the consequence of adsorption of phosphate species on the Pt (111) electrode surface: the strength of adsorption of phosphate species becomes weaker with increases pHs of solutions result in the activity of ORR improved. In a word , we found that the reaction order with respect to H3O+ is"1"with the oxygen reduction reaction on Pt. The mechanism and kinetics of oxygen reduction reaction cannot change with the pH and The pH effect on the ORR on platinum single crystal electrode is mostly the consequence of adsorption of spectator species.3 The effect of anion adsorption on the surface of Pt(111) on the oxygen reduction reactionAdsorption of anion on the Pt (111) effect on the ORR has been investigated using hanging meniscus rotating disk electrode system (HMRD). In general, the activity of ORR decreases with the strength of spectator anion adsorbed on the electrode surface. For example, the adsorption of sulfate/phosphate anions is much stronger than OH- adsorption on Pt (111), at constant potential, the current of ORR in H2SO4/H3PO4 solution much smaller than in HClO4 solution, however, the mechanism of ORR was not changed with adsorption of sulfate/phosphate anions, which proceed entirely through the 4e- reduction pathway as in HClO4 solution. When the adsorption of anion become much stronger, such as Br-, I- adsorbed on Pt (111), in this situation, the strongly adsorbed anion can simultaneously suppress both the adsorption of the O2 molecule and the formation of pairs of platinum sites needed for the breaking of the O–O bond, thus the ORR proceed not entirely through the 4e- reduction pathway maybe reduced to H2O2. However, when CN- adsorbed (CNad) on Pt (111), which can form a stable and essentially inert structure on Pt (111) that can prevent adsorption of tetrahedral sulfuric acid and phosphoric acid anions but not O2 and CNad does not influence Pt–O2 or Pt-intermediates energetics, that is, the ability of platinum to chemisorb O2 and tobreak the O–O bond is preserved. Thus to improve the activity of ORR, we need to reduced the coverage of spectator anion on surface of electrode and to provide more active sites for O2 adsorption , in addition to breaking of O-O bond and proceed the 4e- reduction pathway, requires at least two unoccupied adjacent Pt sites, and O2 bridged adsorbed on surface of electrode.