| Due to rising global energy demands, depletion of fossil fuel reserves, andenvironmental pollution problem, the dream of the hydrogen economy is of great interest.That is, during the daytime, the electricity is produced by solar cells; while at night, it isfrom the combustion of H2through fuel cells. The pollution-free production H2O could berecycled by photocatalytic splitting. To achieve this dream, the commercialization of fuelcells is of paramount importance.A major challenge to make PEM fuel cells commercially viable lies in the insufficientactivity and poor durability of the carbon-supported Pt nanoparticles (Pt/C) that areemployed at present as cathode catalysts for oxygen reduction reaction (ORR), as a result ofstrong adsorption energies of O*, OH*and OOH*intermediates and large electrochemicalsurface area loss in long-term operation caused by the coarsening of Pt nanoparticles. Inaddition, the high cost of Pt metal is another obstacle of the commercialization. That is,according to a United States Department of Energy′s study in2007,56%of cost in a fuel cellcomes from the Pt cathode catalyst. Furthermore, the price of Pt has been steadily increasingwith no break in this trend expected in the near future. This will serve to escalate thischallenge facing fuel cell commercialization.To circumvent these limitations, in the past decade, great attention has been devoted todevelop the novel ORR catalyst. One of attractive strategies is the alloying technique, wherethe presence of the ligand or/and geometric effects modifies the activity of the surface Ptatoms. In addition, it is realized that the core Pt atoms in the Pt nanoparticle are invalid. Thus,how to increase the Pt utilization is of paramount interest. The answer is the nanostructureengineering.In this thesis, utilizing the first-principle calculation, we design the novel Pt3Al alloycatalysts by alloying. In order to further increase the Pt utilization, based on the Pt-Al system,through nanostructure optimization, the core-shell Al13@Pt42cluster has been proposed asORR catalyst. However, it is realized that the stability of Al13@Pt42remains uncertain due tothe harsh work environment. Then, the question is natural to raise that whether some specialnanostructure with high Pt utilization is appeared to be good candidate for ORR. Here, Ptmonatomic wire supported on zigzag edges of graphene nanoribbon is found to be the answer. The mains results are divided into three parts as following:Firstly, based on the suggestion of Greeley et. al., the stability and activity of Pt3Alalloy as a catalyst for ORR has been determined. The strong Pt-Al covalent bonds not onlygive rise to the excellent kinetic stability, but also produce the hybridization between theAl-3p states and the Pt-5d states, substantially altering the d bands of surface Pt atoms, i.e., aremarkable downshift of d-band centre (0.16eV) for Pt3Al/Pt(111) relative to that ofPt(111). This downshift substantially weakens the adsorption ability of Pt3Al/Pt(111). The Oand OH adsorption energies are corresponding to the peak of the volcano curve, which hintthe optimal activity of Pt3Al alloy. Based on the free energy diagrams, the rate-determinedstep of ORR is changed to be OOH*formation on Pt3Al/Pt(111) relative to OH*remove ofPt(111). Furthermore, at U=0.9V, the free energy barrier is reduced from0.15eV on Pt(111)to0.11eV on Pt3Al/Pt(111). The reduction of free energy barrier is further comfirmed theactivity of this novel alloy. In the work, the presence of the Pt3Al intermetallic compoundwith the superior activity and stability breaks the traditional Pt-based ORR catalysts. That is,from the view of the alloying element, it is changed from the transition or noble metalelements to main group metal elements; from the point of the electronic aspect, it is changedfrom d-d interaction to p-d hybridization.Secondly, the stability and activity of core-shell Al13@Pt42cluster as a catalyst for ORRhas been determined. Similay with Pt3Al intermetallic compound, the strong hybridizationbetween the p orbital of Al core and the d orbital of Pt shell is present for core-shellAl13@Pt42cluster, which suggestes the better stability compared with the pure Pt55cluster.Benefit from alloying with Al in this cluster, the covalent Pt-Al bonding effectively activatesthe Pt atoms at the edge sites. That is, the O adsorption energy is located at the optimal rangewith0.0-0.4eV weaker than Pt(111), while OH-poisoning does not be observed. This isenabling its high utility up to70%. Moreover, as the structural difference between clusterand Pt(111), the reaction mechanism is further studied. Therein, both the O2dissociationmechanism and OOH accociative mechanism are considered. From the data of the reactionbarriers, it is expected that ORR comes from O2dissociation mechanism where therate-determined step is located at OH formation from O and H with a barrier of0.59eV,comparable with0.50eV of Pt(111).At last, the ORR activity of Pt monatomic wire that is supported on the zigzag edges ofgraphene nanoribbon (Pt-GNR) has been determined. Due to the presence of the interactionbetween the5d orbital of Pt atom and the2p orbital of C edge atom, the Pt monatomic wire is fixed at the edge of GNR. The stability of Pt-GNR is further supported by thefirst-principle molecular dynamics simulation at a constant temperature of T=400K, wherethe clustering phenomenon is absent. On the other hand, due to the low coordination,Pt-GNR prefers to be covered by the chain of OH*and H2O*, defined as cPt-GNR. Throughthe free energy diagrams on the hydrated Pt monatomic wire, we demonstrate that thehighest potential U for ORR as an exothermic process is0.82V. When U is larger than0.82V, the rate-determined step is located at the reduction of O2*to OOH*where the energybarrier ΔG is less than0.41eV. These results support cPt-GNR as a candidate for ORR.Therein, it is expected that due to the presence of high reaction activity and coadsorptionphenomenon, the ORR activity comes from the metal oxide or hydrolyzed metal oxide. |
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