Controlled Synthesis Of Porous/Hollow Pt-based Nanostructured Electrocatalysts And Their Properties

In recent years, with the increasing environmental problems and accelerated depletion of fossil fuels, there are urgent demands for the development of a new technology using alternative energy sources othe than fossil fuels. Fuel cells, which can efficiently convert chemical energy into electricity through electrochemical reactions, are considered as ideal power sources for future mobile and stationary applications due to their high energy efficiency, high power density, as well as low/zero emissions. However, the high cost and low stability of Pt catalysts pose a severe challenge to the commercialization of fuel cells. Thus, the design of novel Pt-based catalysts with enhancing catalytic activity, stability and low amount of Pt used is highly desirable but remains a significant challenge. Understanding the structure and property relationship will lead to better design of those new electrocatlysts with improved catalytic performance.This dissertation will focus on the design, controlled synthesis and catalytic properties of porous/hollow Pt based electrocatalysts. In order to improve the Pt utilization to reduce the cost for commercial application, porous/hollow Pt-based electrocatalysts were synthesized via solvent process by which monodisperse nanoparticles can be prepared. Moreover, through the study of the synthesis mechanism, a series of function-oriented porous/hollow catalysts with large active surface and outstanding catalytic performance were designed and fabricated for specific electrode reaction. These porous/hollow structures exhibited unique advantages and wide potential application in catalysis. The main results can be summarized as follows:1. We described a simple method to synthesize uniform and monodisperse porous PtNi nanoflowers electrocatlysts. The prepared porous PtNi nanoflowes are composed of many tiny branches with a diameter of just about2to3nm. In addition, PtNi multi-arms can be obtained as well by modifing reaction conditions. The crystal surfaces of the branchs possess high density of low-coordination defect sites(steps/edges/kinkes), which facilitate the desorption of the intermidates COaa generated in the methanol oxdation process and therefore lead to the higher poisoning resistance. More importantly, the activity and stability of this PtNi nanoflower catalyst are significantly improved because mass transfer in this three-dimensional (3D) porous structure is much easier than other counterparts.2. A facile approach has been developed to synthesize hollow ternary PtPdCu nanostructures by etching with mild acetic acid. Hollow alloyed nanoparticles represent one kind of promising fuel cell electrocatalysts. However, the formation of single-cavity hollow structures by dealloying process is quite challenging owing to the random leaching/dissolution of non-noble metal, surface passivation and the limited diffusion distance of noble metals. In this process, acetic acid not only acts as a chemical etching agent but also plays an important role in the removal of the residual surfactants on the colloidal nanoparticles. Our findings rectify the known knowledge that hollow alloyed nanoparticles s cannot be reached by dealloying strategy and provide an alternative route to understand the dealloying process in ternary system. Endowed by its Pt-enriched surface layer where stress effect is generated by lattice contraction, along with the special hollow structure that utlized Pt more efficiently, this electrocatalyst exhibits excellent catalytic activity and outstanding durability toward cathodic oxygen reduction reaction.3. Porous heteornanostructures (alloy-metal and alloy-sulfide) can be produced by surface etching of the composition-segregated Pt alloyed polyhedrals. The engineering of the interfaces, not only metal-metal interfaces but also metal-nonmetal interfaces, has been demonstrated to play a crucial role in catalysis. We therefore designed and synthesized porous Pt-based heteronanostructures (PtNi-Au and PtNi-Ni3S4) with controlled morphology. The fabrication of composition-segregated PtNi rhombic dodecahedron, where Pt atoms are on the edge and Ni atoms are on the face, is the precondition for synthesizing the regularly porous heteronanostructures. Employing PtNi polyhedrals as the precursor, we successfully synthesized porous PtNi-Ni3S4heteronanostructureed particles through the sulfurization of the precusor by simply adding sulfur power into the synthetic reaction solution. Additionally, we also synthesized surface-etched porous PtNi-Au heteronanostructures by seed-mediated growth caused by the different reduction potentials of the metals in oleylamine solvent. The enhanced performance of this catalyst toward fomic acid can be attributed to its high specific surface and the "ensemble effect" derived from the etching of nickel. This method offers new insight for the design and fabrication of porous heteronanostructured electrocatalysts based on composition-segregated alloyed precusors. 4. A two-step templated process was developed for preparing one-demensional Pt clusters/hollow selenide hybrid electrocatalysts. One-dimensional nanomaterials are well known for their attractive electronic properties due to high aspect ratio. Meanwhile, late transition metal chalcogenides are a kind of promising non-metal electrocatalyst. By partly replacing Te nanowires which can be prepared in our lab in large scale, SeTe alloyed nanowires were obtained. Successively, using these SeTe alloyed nanowires as templates, Pt clusters/hollow selenide hybrid nanostructures, which could further react with cobalt salt and result in1D clusters/cobalt sulfide hybrid nanostructures, can be through galvanic replacement reaction.