Controlled Synthesis And Electrocatalytic Properties Of Metal Nanostructures With High-Index Facets

Human beings are now facing the increasingly serious energy crisis and environmental problems. Low-temperature fuel cell, which is one of the most important technical means to address these two issues, has a broad application prospect. It has been the key and bottleneck of technology applications how to improve catalytic performance and save application cost by optimizing the composition, size, structure and shape of a nanocatalyst. Noble metal nanocatalysts including platinum and palladium nanocrystals, as the most efficient, stable and durable catalysts for fuel cells, are commonly used. It has been confirmed by experimental measurements and theoretical simulations that the reaction rate and energy on a nanocatalyst quite sensitively depends on the crystal facets exposed on its surface. High-index facets of a nanocrystal with more steps, defects and low coordination atoms, have a higher catalytic activity than low-index facets. This research is focused on the controllable synthesis of noble metal nanocrystals with high index facets (mainly Pt, Pd and related alloys) by selecting different capping agents, controlling reaction kinetics, designing galvanic displacement reaction, modulating activation and epitaxial growth, employing oxidation etching process, and further optimizes the catalytic performance of some important reaction such as formic acid oxidation and oxygen reduction reaction in the proton exchange membrane fuel cells. Related research works are summarized as follows:1. Control over the branched structures of platinum nanocrystals for electrocatalytic applications. Structural control of branched nanocrystals allows tuning two parameters that are critical to their catalytic activity:the surface-to-volume ratio, and the number of atomic steps, ledges, and kinks on surface. In this work, we have developed a simple synthetic system that allows tailoring the numbers of branches in Pt nanocrystals by tuning the concentration of additional HC1. In the synthesis, HC1plays triple functions in tuning branched structures via oxidative etching:(i) the crystallinity of seeds and nanocrystals;(ⅱ) the number of{111} or {100} faces provided for growth sites;(ⅲ) the supply kinetics of freshly formed Pt atoms in solution. As a result, tunable Pt branched structures-tripods, tetrapods, hexapods, and octopods with identical chemical environment-can be rationally synthesized in a single system by simply altering the etching strength. The controllability in branched structures enables to reveal that their electrocatalytic performance can be optimized by constructing complex structures. Among various branched structures, Pt octopods exhibit particularly high activity in formic acid oxidation as compared with their counterparts and commercial Pt/C catalysts. It is anticipated that this work will open a door to design more complex nanostructures and to achieve specific functions for various applications.2. A unique platinum-graphene hybrid structure for high activity and durability in oxygen reduction reaction. It remains a grand challenge to achieve both high activity and durability in Pt electrocatalysts for oxygen reduction reaction (ORR) in fuel cells. Here we develop a class of Pt highly concave cubic (HCC) nanocrystals, which are enriched with high-index facets and exhibit high ORR activity. The durability of HCC nanocrystals can be significantly improved via assembly with graphene. The unique hybrid structure displays further enhanced specific activity, which is7-fold greater than the state-of-the-art Pt/C catalysts. Strikingly, it exhibits impressive performance in terms of half-wave potential (E1/2). The E1/2of0.967V at the Pt loading as low as46μg cm-2, which stands as63mV higher than that of the Pt/C catalysts, is slightly superior to the record observed for the most active porous Pt-Ni catalyst in literature. This work paves the way to designing high-performance electrocatalysts by modulating their surface and interface with loading substrates.3. Anisotropic growth of palladium twinned nanostructures controlled by kinetics and their unusual activities in galvanic replacement. Five-fold twinned structures are a class of important members in the family of metallic nanocrystals with face-centered cubic (fCC) structures, which can anisotropically grow into nanowires when their{100} facets are protected. In this work, a facile synthetic approach has been firstly developed to synthesize a new structure of palladium nanocrystals, which are palladium nanotapers potentially enclosed by high-index facets. We have revealed that the reaction kinetics holds the key to tuning the growth mode of the nanocrystals while the selective capping effect makes a contribution to facet control. The anisotropic growth of twinned structures here provides a new approach for constructing a high-index surface without the need to use long-chain capping agents. These palladium nanotapers exhibit superior chemical activities compared to their low-index counterparts, as proven in the galvanic replacement. It is anticipated that this work opens a door for the development of new synthetic methods for metallic nanocrystals with high-index facets for various applications such as catalysis.4. Synthesis and eletrocatalytic applications of Pd-Pt concave nanocubes with {730} high-index facets by epitaxial growth methods. A method has been developed for controlled epitaxial growth on cubic nanocrystals by selectively activating their surface via etching. Pd concave nanocubes were produced via seeding growth on their corners and edges, formulating high-index facets and highly active sites for catalysis. This method offers a better capability of preventing atomic addition on undesired locations and maintaining particle size in the seeding process, as compared with the previous technique. With the particle size well maintained, the products fully exhibit superior electrocatalytic performance enabled by active sites and high-index facets in formic acid oxidation. Another contribution of this work is to enable the growth of a noble metal with high catalytic activities on another type of cheaper metal, which greatly reduces the usage of expensive materials while retaining high catalytic activity. In this work, we have demonstrated the deposition of a very limited amount of Pt (only3.3wt%.) on Pd nanocrystals towards high electrocatalytic activities in oxygen reduction reaction. Preliminary studies demonstrate that the synthetic strategy can be also applied to the controllable deposition of a different material on the faces of a nanocrystal by simply altering surface conditions.5. Structural effects of palladium nanocrystal electrodes on electrocatalytic oxidation reaction of formic acid. Palladium nanocrystals with various shapes and sizes were controllably synthesized by altering oxidative etching and reaction kinetics in the presence of appropriate metal precursors, reducing agents, stabilizers and capping agents, based on our previous works. Upon cleaning nanoparticles and assembling them onto electrodes, we systematically investigated the relationship between nanocrystal structures and catalytic performance in the oxidation of formic acid. It demonstrates that the maximum current densities of Pd nanocrystals increase in the order of nanoctahedra<nanowires<nanocubes<nanotapers<concave nanocubes. The onset potentials of all the samples are below0.2V. The results prove that the electrocatalytic performance of Pd nanocrystals is not significantly dependent on their sizes after being normalized by active surface area. Instead, the catalytic activity is mainly determined by surface structures:the activities of various Pd facets in the oxidation of formic acid should be in the order of{111}<{100}<high-index facets. Among various Pd nanostructures, concave nanocubes and nanotapers exhibit much better electrocatalytic performance.