| The fuel cell is an energy crisis to solve human inexpensive energy. Mainly influence of the efficiency of the fuel cell is the cathode and anode overpotential. For the cathodic reaction, the oxygen reduction reaction is the major energy lost, and the anodic reaction is oxidation of small organic molecules, such as formic acid oxidation. Understanding the mechanism of ORR can helps us reduce overpotential for oxygen reduction to improve fuel cell performance, and mechanism of anodic oxidation of formic acid will help us design better anode catalyst to improve the activity of the catalyst.In this thesis, single crystal rotating disk technology has been applied to study these two problems, the major findings are:1. Mechanistic Implications on ORR at Au(100) Electrode:pH, temperature and H-D kinetic isotope effects (KIEs) on ORR at Au(100) has been examined systematically using hanging meniscus rotating disk electrode system. We found that for the cases with pH>7, ORR mainly goes through4-electron reduction to OH-at E>pzc (potential of zero charge) without any pH and H-D KIEs. When the pH at the electrode/electrolyte interface (pHs) is below7, O2only reduces to H2O2, its activity increases with pHs, a H-D KIE of above2is observed in0.1M HClO4. According to the experiment results in acid solution, a mechanism with O2+H++eâ†'Ho2,ad as the rate determining step followed by decoupled electron and proton transfer steps is proposed. The high activation barrier for O-O bond breaking and the fast oxidation of H2O2or HO2-to O2render the ORR can only be observed at potentials negative of the equilibrium potential (Eeq) of the redox of H2O2/O2in acidic media or of HO2/O2in alkaline environment. The apparent activation energy (Ea,app) for O2reduction to H2O2is ca.35±3kJ/mol and to OH" is60±6kJ/mol, while the pre-exponential factor (A) for the former is ca.3-6orders of magnitude smaller than that of the latter. The lower activity for O2reduction to H2O2at Au(100) is attributed to the small pre-exponential factor.2. Modeling of the Potential Oscillation during Galvanostatic Electrooxidation of Formic Acid at Platinum Electrode:Oscillation during electrocatalytic oxidation of formic acid at Pt electrode under galvanostatic conditions in acidic electrolyte has been simulated based on a chemical model, which involves three reaction pathways:(i) the indirect pathway, via COad formation (formic acid dehydration) and oxidation,(ii) a formate pathway, involving adsorption/desorption of bridged adsorbed formate and its oxidation;(iii) a direct pathway with successive cutting of O-H and C-H bond. We found that only when the contribution of formate oxidation to the total formic acid oxidation current is negligible, the simulated results reproduce well the experimentally observed oscillatory patterns for electrode potential, formate and COad coverage. It is found that the fast adsorption/desorption of both formate and OHad are responsible for the hidden negative impedance, the slow COad formation and its fast oxidation at higher potentials leads to a positive impedance. The simulated result further supports that bridge-bonded formate is not the reactive intermediate for the major pathway of formic acid oxidation at Pt electrode. |
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