Electronic and geometric structures of Pt / Au nanoparticles and their electrocatalytic activity

Abstract

Due to being high efficient and environmentally friendly, proton exchange membrane fuel cells are one of the most promising next generation energy conversion devices to power the energy demands of the future. Substantial efforts are devoted to enhance the sluggish oxygen reduction reaction (ORR) at cathode compared to feasible hydrogen oxidation reaction at anode. The development of DFT-based theoretical understanding of the mechanisms of ORR yielded promising electrocatalyst materials. Pt is the most efficient ORR electrocatalyst due to the moderate intensity with adsorbate. To enhance the activity further, the oxygen species adsorption intensity should be lowered about 0.2 eV by DFT analysis. The Pt/Au structure was predicted that the lower activity owing to the lattice strain effects. The Au which atomic size is larger than Pt, induced the tensile strength and made the Pt d-band center up-shift with low degree of orbital overlap. High d-band center correlates to the high chemisorption energy and low ORR activity. Due to these expectations, few researches were conducted. In this study, the Pt/Au nanoparticle systems were applied to the ORR electrocatalyst beyond the previous unfavorable expectation and single crystal studies. The Au@Pt core-shell nanosized electrocatalysts and AuPt alloy nanoparticles were synthesized. Delicate investigations of nanoparticle structure were conducted using high resolution transmission electron microscopy and x-ray diffraction. X-ray absorption fine structures and photo emission spectroscopy also conducted at synchrotron facilities to confirm the inter-atomic distances, information related to the coordination numbers and electronic structures. From the CO stripping experiments, Pt-CO chemisorption energy was larger than that of the pure Pt until certain Pt surface compositions. However, the deposition of Pt increased, the Pt-CO chemisorption energy is lower than pure Pt which was controversy to previous predictions which the Au induced tensile strain to Pt with high chemisorption energy to CO and oxygen related species. To explain the experimental results, elucidations with quantum mechanics were performed. The chemisorption energy between metal and CO and oxygen species was interpreted two terms

center of d-band structures and orbital repulsion terms. The center of d-band positions was related to the band width. By changing the local environment, the d-band structure was changed due to the changing the orbital overlap between near atoms. Orbital repulsion terms were related to the adsorbate– metal orbital coupling matrix. The shorter the bond induced the more orbital overlap, the adsorbate anti-bonding orbital filled with the interaction between metal s,p orbitals. Due to the different electronegativity between Au and Pt, orbital repulsion should also be considered. Au has higher electronegativity compared to the Pt, induced the charge redistribution from Pt to Au. The lower electron in Pt induced the shorter bond distance with adsorbate, made the repulsion. The two parameters were mixed in Au/Pt systems. In the case of low Pt deposition to surface, the strain induced d-band center overwhelmed the effect of orbital repulsion terms. As amount of surface Pt increased, the strain factors decreased. Orbital repulsion factors were no longer the minor parameter at certain points. In the case of AuPt alloy nanoparticles, their flow of electron redistribution was different to core-shell structures. Considering the electronic structures, the core-shell structures were good candidates for ORR electrocatalysts. The ORR activity trends also followed with the CO stripping results. The different results between previous single crystal studies and current nanoparticle case were explained with the characteristics of nanoparticles. In the case of nanoparticles, their surface atoms have low coordinated which induces the surface atom contraction. Due to the surface contraction effect, the tensile strength was little effects on the Au@Pt system in nanoparticles. To confirm the nanosized atomic contraction effects, the surface coordination number was controlled using sonochemical method. The coordination number control was confirmed using extended x-ray absorption fine structures. The fitting parameter was shown as low coordination number, smaller inter-atomic distances and high Debye waller factors. Hydrogen oxidation reaction was also confirmed the surface low coordination number after sononchemical methods. After the coordination number controls, the Pt deposited to the Au nanoparticles. The lattice parameter of Pt was investigated using Rietveld refinement. The Pt deposited with low coordination numbered Au had low lattice parameter. Controlling the surface coordination number of core nanoparticle, strain could be controlled. The Au@Pt_0.5_S had 1.7 times higher ORR activity compared to without sononchemical method and 2.5 times higher activity compared to Pt nanoparticles. From these researches, the possible factors related to the adsorbate chemisorption energy were considered

center of d-band structures and orbital repulsion under the quantum mechanics. The strain effects could be controlled with change of coordination number of nanoparticles. These considerations can be applied to the strategies to design the nanoscale electrocatalysts.