Several Catalytic Processes On Fuel Cell Electrode Surface: A First Principles Study

In recent years, due to the increasing depletion of fossil fuels and growingenvironmental pollution, the fuel cells as a new type of green energy have receivedconsiderable attention. The slow oxygen reduction reaction （ORR） at cathodes is one of thekey factors limiting the performance of fuel cells. To date, Pt is the best catalyst for the ORR.However, the limited resource and high cost of Pt hinder the large-scale commercializationof fuel cells. Therefore, in the past decade, great efforts have been devoted to developingalternative ORR electrocatalysts for the cathode of fuel cells. Carbon-based catalysts areexpected to be the most promising alternatives of Pt catalysts because of the rich resource,durability and low-cost of carbon. Especially, due to their unique physical and chemicalproperties, carbon nanotubes （CNTs） and garphene provide a new opportunity to develop theORR catalyst. N/B/P/S doped CNTs and graphene are shown to be ORR catalysts though afour-electron process. In alkaline media, they even exhibit higher activity and stability thanthe commercial Pt catalyst.H₂O-metal interaction is one of the most familiar and important phenomena in nature. Anumber of scientific and technological processes, such as corrosion and heterogeneouscatalytic reactions, are based on the H₂O-metal interaction. One important characteristic ofthese properties in an electrochemical environment is the variation of the electrode potential,which modifies the surface charge. This can affect the adsorbate-surface interaction and evenalter the relative stability of the surface adsorption at different sites. A number of reactionsthat occur on electrode surfaces require protons and hydroxyl ions as the reactants, which arethe dissociation products of the H₂O molecule. The rate of H₂O dissociation can determinethe electrochemical reaction rate at a certain degree. The dissociation of H₂O molecules onelectrode surfaces depends on the electrode potential, the electrolyte solution, the electronicproperties of the electrode material, and so on.In this thesis, utilizing the first-principle calculations based on density functional theory,the mechanisms of ORR and CO tolerant properties on doped carbon-based catalysts areinvestigated, and the effect of surface charge on the H₂O-metal interaction is explored. Themain results are divided into four parts as following:Firstly, the electrocatalytic mechanisms of the ORR are studied on different structure of N-doped carbon nanotubes （NCNTs）. The results show that the ORR on NCNTs arestructure-selective and potential-dependent. At graphite-like N groups （NG） and pyridine-likeN groups （NP） of NCNTs, the OOH association mechanism is dominant over the O₂andH₂O₂dissociation mechanisms. ORR at NGand NPdefect sites at low electrode potentials aresimilar. However, at high electrode potentials, ORR at the latter is faster than that at theformer. Although the two-electron H₂O₂mechanism and four-electron OOH mechanism playroles at low electrode potentials simultaneously, the latter is more advantageous than theformer at high potentials.Secondly, the mechanisms of ORR on layered SiC nanosheet are studied. Thesimulations demonstrate that the layered SiC nanosheet exhibits excellent ORR catalyticactivity and is free from CO poisoning. Reaction barrier for the ORR on layered SiCnanosheet in alkaline media is smaller than that in acidic media, implying that the ORR onlayered SiC nanosheet in alkaline solution is faster than that in acidic solution. The catalyticactivity of layered SiC nanosheet is similar to Pt（111） surface in acidic media, while theactivity of layered SiC nanosheet is higher than Pt（111） surface in alkaline media. This workclearly indicates that the layered SiC nanosheet can act as metal-free catalysts for ORR infuel cells.Thirdly, the adsorption and oxidation of CO on four different active structures ofFe/N/C catalyst are invesgated. It is found that CO adsorption and oxidation are verysensitive to the active site structures and the coordinate number of Fe atom. CO adsorption isenergetically more favorable than O₂. And CO is hardly oxidized to CO₂on Fe-N4porphyrin-like CNT and graphene. In contrast, on Fe-N3pyridine-like CNT and Fe-N2nanoribbon, O₂prefers adsorbing and CO can be easily oxidized, suggesting the CO tolerantproperty of these two structures.Lastly, H₂O adsorption and dissociation on charged Cu（111） surface are studied. It wasshown that both adsorption energy （Ead） at different surface sites and favored adsorptiongeometries depend on surface charge （Q）. Eadvalues at all high symmetry adsorption sitesare similar when Q <0e. H₂O still prefers adsorbing at atop site on neutral and positivelycharged surface. The two steps of H₂O dissociation are charge-dependent. Moreover, theeffect of Q on dissociation barrier lies on the relative stability of the adsorbate on a chargedmetal surface.Based on first-principle calculations, the reaction mechanism can be investigated atatomic level, which provides the theoretical base for fabricating new catalysts and tuning the relevant catalytic properties.