Fabrication Of Nanoporous Alloys And Their Applications In Electrocatalysis

The running out and pollution of fossil fuels are driving force for the increasing interests in the development of alternative energy. Among them, one of the most promising technologies is fuel cell, which converts chemical energy directly into electrical energy with high efficiency and low emission of pollutants. The performance of fuel cell device depends intimately on the properties of their electrocatalysts. One of the serious problems for fuel cells at both anode and cathode is the poor activity and durability of the catalysts, leading to a loss of about one-third of the available energy. Typically for the commercial Pt/C electrocatalyst, the long-term service leads to severe structure coarsening and inevitably results in a low operating efficiency. Individually, the kinetic limitation of the oxygen reduction at cathode catalysts is another problem for fuel cells because the rate of breaking of O-O bonds to form water is strongly hindered by oxygenated species. At anode, the pure Pt surface is easily blocked and poisoned by CO, which is one of the main byproducts during the electrooxidation of small molecules. Thereby, propensity of poisoning of Pt by CO species is another problem for the extensively-studied Pt catalysts. Although the nanoporous Au electrode is of special interest as one of electrocatalysts towards the electrooxidation of small molecules in alkaline solution because of its exceptional resistance to the poisoning of CO. Au generally suffers from a low surface adsorption capability and a coarsening phenomenon, which would congenitally reduce its electrocatalytic properties. Thus developing active, robust and low-cost electrocatalysts lies at the heart of the fuel cell commercialization.Traditionally, the low dimensional catalysts are usually nano paticles, which have the virtue of high specific area. However, during processing the catalysts into a thin film electrode before practical applications, this comes with the risk of counteracting the gains in activity sites and specific area, and the addition of supplementary materials would jeopardize the conductivity of electrode. In contrast to low-dimensional catalysts, three dimensional nanoporous metals possess the virtues of open architecture, self-supported and low contact resistance. Due to the unique structure, the confinement effects, EDL overlapping effects and concave effects of nanoporous metals are thought to be beneficial in catalysis. Furthermore, the nanoporous metals bridge the physicochemical and mechanical properties of bulk materials and the merits of nanostructural materials, which have positive potential to address the issues of electrocatalysis in the fuel cell. We focus on the applications of nanoporous alloys in electrocatalysis. The main contents of this dissertation are as follows:1. Nanoporous Pt-based intermetallics in controllable synthesis and their electrocalysis for oxygen reduction reaction.(1). Mesostructured intermetallic compounds of platinum and non-transition metals for enhanced electrocatalysis of oxygen reduction reaction. The choice of Al is motivated by the low cost and its amphoteric properties, large electronegativity difference between Al and Pt enable Al-3p/Pt-5d hybridization and formation of strong Pt-Al covalent bond. We prepare a new class of mesostructured Pt-Al catalysts consisting of atomic-layer-thick Pt skin and Pt3 Al or Pt5 Al intermetallic compound skeletons by varying dealloying conditions. As a result of strong Pt-Al bonds that not only inhibit the evolution of Pt skin and thus enhance the stability against the further dissolution of Al, but also produce ligand and compressive strain effects, the Pt3 Al and Pt5 Al mesoporous catalysts are exceptionally durable and ?6.3- and ?5.0-fold more active than the state-of-the-art Pt/C catalyst at 0.90 V, respectively. Furthermore, the alloying with Al reduces the loading of Pt and enhances the efficiency. The high performance makes them promising candidates as cathode nanocatalysts in next-generation fuel cells.(2). Scalable nanoporous(Pt-Ni)3Al intermetallic compounds as highly active and stable catalysts for oxygen electroreduction. As we know, the element of Ni could boost up the electrocatalytic activity of Pt, but the low alloy formation energy of Pt-Ni roots in the dissolution of Ni. Whereas, extraordinarily negative formation heat of Pt-Al and Ni-Al bonds could improve the durability after alloying with Al. Meanwhile, in order to enhance the low yield by the traditional bottom-up methods, a cost-effective mass production method to fabricate nano catalysts must be proposed. Combining the alloying/dealloying and mechanical crushing technologies, we fabricate nanoporous Pt-Ni-Al nanocatalysts with core-shell nanostructure, where atomic-layer-thick Pt skins cover the surface of(Pt-Ni)3Al intermetallic cores. All the procedures in the preparation method are usual in laboratory conditions, and the methods are a simple and scalable way to implement the mass production of nanoporous particles. Besides the above discussed strong Pt-Al covalent bond, the dissolution of less noble Ni is circumvented by extraordinarily negative formation heat of new-formed Ni-Al covalent bonds. The mass-produced NP(Pt-Ni)3Al/Pt nanocatalysts exhibit specific and mass activities of 3.63 m A cm-2Pt and 2.35 A mg-1Pt at 0.9 V. As a consequence of the compressive strain and ligand effects exerted on the Pt surface atoms by the Ni and Al element, the NP(Pt-Ni)3Al/Pt have the optimized adsorption energy of oxygenated species.2. Self-grown Ni(OH)2 layer on bimodal nanoporous AuNi alloys for enhanced electrocatalytic activity and stability.NP Au exhibits exceptional resistance to the poisoning effect of CO, thus it’s a good candidate for the electrooxidation of small molecules in alkaline solution. However, Au generally suffers from a low surface adsorption capability and high surface diffusion rate, this inevitably gives rise to the occurrence of a coarsening phenomenon that degrades the property of catalyst. To address the above issues, we synthesize Ni(OH)2 layer decorated nanoporous(NP) AuNi alloys, which are facilely fabricated by a combination of chemical dealloying and in-situ surface segregation. Due to the unique architecture, mesoporous AuNi/Ni(OH)2 composites show enhanced electrocatalytic performance in electrooxidation of glucose. As a result of the self-grown Ni(OH)2 on the AuNi alloys with coherent interface, the NP AuNi skeleton offers high conductivity and traps more analyte molecules in its channels while the Ni(OH)2 layer enhances the adsorption capability and blocks the fast surface diffusion of surface Au atoms. The highly reliable glucose biosensing with exceptional reproducibility and selectivity as well as quick response makes it a promising candidate as electrode materials for the application in nonenzymatic glucose biosensor.3. Nanoporous AuNi/Pt alloy enhances the electroxidation of methanol with exceptionally high CO tolerance.The metal of Pt is one of the most important electrocatalysts, however inhibiting effect deactivated the active sites arising out of the low tolerance toward CO. To circumvent CO poison issue for the commercialization of fuel cell, we develop a discrete Pt particles decorated nanoporous(NP) AuNi alloy with three-dimensional and bimodal porous architecture, which are facilely fabricated by a combination of chemical dealloying and galvanic replacement reaction. As a result of the discrete Pt particles on the AuNi alloy(NP AuNi/Pt) with coherent interface, the NP AuNi/Pt not only unexpectedly eliminates CO poison, but also reduces the cost of Pt and boosts the stability markedly. The outstanding CO tolerance performance results from the combination of ensemble effects and the ligand effects of the AuNi substrate on the Pt islands, where the ensemble effects provide OH* for electrooxidation and ligand effects weaken the adsorption energy of CO. The further electrooxidation experiments of methanol demonstrate that the NP Au Ni/Pt electrode shows genuine CO tolerance and high catalytic activity. The intrinsic to CO tolerance is discussed and approaches to overcome it are proposed.