Synthesis And Electrocatalytic Properties Of Pt-based Nanoalloys

Direct methanol fuel cells(DMFCs)are one of the most promising clean energy sources due to their high energy density,environmental friendliness,safety,and convenience of application.However,the large-scale application of DMFCs are greatly hindered because of the high cost,low catalytic efficiency,and poor stability of the widely used commercial Pt/C catalyst.To overcome these issues,the incorporation of one or two non-noble metals with Pt to form Pt-based nanoalloy has become an effective strategy.In many cases,the resultant Pt-based nanoalloy displayed superior electrocatalytic performance in comparison with the monometallic Pt counterpart owing to the synergistic effect and electronic effect;meanwhile,the alloy compositions mean a reduction in the consumption of noble metal Pt.Apart from the composition,the morphology of nanocrystals also has a big influence on the performance of the catalyst.Nanocrystals with unique morphology usually have larger accessible surfaces and more abundant active sites,which can further improve the catalytic performance of the Pt-based nanoalloys.Therefore,it is of great significance to develop effective strategies for the synthesis of Pt-based nanoalloys with unique structures and explore the influence of the composition and morphology on their catalytic performance.In this dissertation,several kinds of Pt-based nanocrystals with unique structures were successfully prepared using different metal precursors and reducing agents.By performing time-evolution experiments and control experiments,the formation mechanisms of these nanocrystals were elucidated.Owing to the large surface areas and abundant active sites,the prepared nanocrystals all exhibited enhanced electrocatalytic performance for methanol oxidation reaction(MOR)in comparison with commercial Pt/C catalyst,which makes them promising catalysts for DMFCs。The main achievements are outlined as follows:1.PtCu rhombic dodecahedral nanoframes(RDFs)were synthesized by a facile one-pot solvent method.Notably,the prepared PtCu RDFs each contain six nanobranches selectively protruding from their six<100>vertices.Benefiting from the large surface-to-volume ratio of the highly open structure,the abundant active sites on the nanobranches,and the electronic effect between Pt and Cu,the resultant PtCu RDFs exhibited outstanding electrocatalytic properties for MOR.2.We prepared vertex-type-selectively reinforced PtCu@PtCuNi double-layered nanoframes(DNFs)by a two-step route.In the first step,PtCuNi heterogeneous nanocrystals(HNCs)were synthesized by a facile one-pot solvent method.The PtCuNi HNC is composed of a Cu-enriched concave rhombic dodecahedral core,a double-layered nanoframe consisting of an inner vertex-selectively-reinforced rhombic dodecahedral PtCu nanoframe and an outer concave rhombic dodecahedral PtCuNi nanoframe,and a Ni-enriched shell.In the second step,PtCu@PtCuNi DNFs were prepared by selectively removing the leachable components via acid treatment.Strikingly,the inner rhombic dodecahedral nanoframe contained six nanopods sticking out from its(100)vertices rather than(111)ones,and the outer concave rhombic dodecahedral nanoframe is formed by interconnecting the adjacent vertices of the inner nanoframe.The electrocatalytic test for MOR showed that the as-prepared PtCu@PtCuNi DNFs exhibited enhanced electrocatalytic activity and improved stability compared to the commercial Pt/C catalyst.3.We have further fabricated PtCuCo double-layered rhombic dodecahedral nanoframes(DNFs)via a one-pot solvothermal synthesis and an acid etching post-treatment.In the first step,PtCuCo heterogeneous nanocrystals(HNCs)were constructed each consisting of a Cu-enriched core,PtCuCo double-layered nanoframes,and a Co-enriched shell,and in the second step,PtCuCo DNFs were obtained by acid-assisted selective removal of the leachable components.Evolution experiments reveal that the galvanic substitution reaction between Pt precursor and Cu-enriched nuclei,the following outer-layer deposition of Co,and the atomic migration of Pt are responsible for the formation of PtCuCo HNCs.Remarkably,PtCuCo DNFs exhibited enhanced catalytic activity for MOR in comparison with commercial Pt/C catalyst.Moreover,the catalytic stability and structural stability of the PtCuCo DNFs are also significantly superior than that of Pt/C catalyst.4.We introduce a facile one-pot solvent method for the synthesis PtCuMn hexapod(HPDs)nanocrystals.The as-synthesized HPDs each contain a rhombic dodecahedral host and six nanopods protruding from its(100)vertices.Evolution experiments demonstrate that four stages are responsible for the formation of the HPDs:the preferential formation of Cu-enriched nuclei,the galvanic substitution reaction between Cu-enriched nuclei and Pt precursor,the selective growth of Pt,and the deposition of Pt,Cu,and Mn.Moreover,the morphology of the HPDs can be tuned by adjusting the dosage of cetyltrimethylammonium bromide(CTAB)due to its ability to affect the size of the preformed Cu-enriched nuclei and the rate of the galvanic substitution reaction during the synthesis.For methanol electrocatalysis,the PtCuMn HPDs exhibit substantially enhanced electrocatalytic activity and durability in comparison with the commercial Pt/C catalyst.The superior electrocatalytic performances can be attributed to the high Pt atom utilization,the abundant step atoms,and the electronic effect of the Pt-based nanocrystals.