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Functional Morphology of Metal Core and Ti1-XMoXOy Nanosupports for Pt Nanocrystals toward Oxygen Reduction Reaction

為了解決Ken block ig的問題，作者NGUYEN TRUNG THANH 這樣論述：

Fuel cell technology is a promising solution for the energy issues over the world. Among the wide variety of fuel cells, Proton Electrolyte Membrane Fuel Cells (PEMFCs) are highly regarded by scientists as a non-polluting power source with good commercial viability. The main objective in fuel cell

technologies is to develop low-cost, high-performance and durable materials. However, current fuel cell systems have high intrinsic costs and fairly poor durability. For PEMFCs, the improvements both of catalytic activity and stability of Pt nanocrystals for oxygen reduction reaction (ORR) are the p

rerequisite issues before PEMFCs can be commercialized for automotive applications. Previous reports showed that the ORR is kinetically limited at the cathode and theinstability of Pt on the cathode is marked by the loss of Pt electrochemical surface area (ECSA)over time, due to Pt dissolution/aggre

gation/Oswald ripening - these being the major contributors to the degradation of fuel cell performance. Additionally, the predominance of weak interactions between the carbon support and the catalytic metal nanoparticles leads to the sintering of the catalytic metal nanoparticles and a consequent d

ecrease in the active surface area with long-term operation. More importantly, the high potentials that accelerate both electrochemical carbon corrosion and the dissolution of the active elements under normal operating conditions, are issues impacting on fuel cell durability that remain unresolved.

Therefore, this dissertation is focused on the "Functional morphology of metal core and Ti1-XMoXOy nanosupports for Pt nanocrystals toward oxygen reduction reaction". This is a collection of new selected approaches for enhancing ORR performance of Pt nano-electrocatalysts.1) The first approach is en

titled: "Functional Pd tetrapod core of heterogeneous Pd-Pt nanodendrites for enhancing oxygen reduction reaction". Herein, the Pd tetrapod core is demonstrated to effectively enhance the catalytic activity and durability of Pd@Pt nanodendites for the ORR. Special attention is given to the positivel

y coupled effect of oxalate ion-formaldehyde during the hydrothermal synthesis of various Pd morphologies at different temperatures. To be able to synthesize tetrapodal morphology in an aqueous solution, the (110) and (111) planes on truncated tetrahedral Pd seeds are passivated at 150 oC by the ads

orption of oxalate ions and CO, a product from the decomposition of formaldehyde on crystalline Pd surfaces. However, Pd nanotetrahedra can be obtained at 200 oC, due to the weakened effect of CO and oxalate ions. Thus,Pd tetrapod synthesis can be achieved by selective deposition of fresh Pd atoms o

nto (100) planes of truncated tetrahedral Pd seeds without directly using hazardous CO gas. Pt (30 wt%) on tetrapodal and truncated octahedral Pd cores and Pt (50 wt%) on tetrapodal Pd cores were compared against commercial Pt/C (E-Tek) for ORR. The Pdtetrapod@30 wt% Pt catalyst exhibited the highes

t ORR catalytic activity. Overall, the Pd tetrapod core has a functional morphologywhich offers high-index facets for the subsequent deposition of Pt(110)nanodendrites and the bimetallic interaction between two materials allows good electron transfer from Pd core into Pt surfaces. Both effects contr

ibute to increased catalytic activity of Pdtetrapod @30 wt% Pt, even under a lower loading of Pt. Pdtetrapod @30 wt% Pt has rather large particle size (~39.5 nm) which exhibits excellent durability and resistance to the agglomeration or sintering of Pt.2) The second approach is entitled: "Synthesis

of Ti0.7Mo0.3O2 supported-Pt nanodendrites and their catalytic activity and stability for oxygen reduction reaction". This part shows that the integrated material of robust non-carbon Ti0.7Mo0.3O2 nanosupports and Pt dendritic layer was demonstrated to effectively enhance the activity and stability

of Pt nano-electrocatalysts for the ORR. Hetero-nanostructural clusters (HNC) of Ptd/Ti0.7Mo0.3O2 were synthesized by a simple aqueous-phase route, in which the deposition and growth of Pt nanoparticles were controlled to obtain high-index facets of Pt. In the synthesis, Pt4+ ions were reduced and f

ormed nuclei in the presence of L-ascorbic acid and Cetyltrimethylammonium bromide (CTAB). With adequate control of dosage intervals and temperature, the following addition of Pt precursor solution was carried out by injecting the precursor into the suspension solution containing Ti0.7Mo0.3O2. The c

lustering of Pt particles could be driven by their high surface energy due to a large surface area to-volume ratio. 20 wt% Ptd/Ti0.7Mo0.3O2-HNC and support-free Pt nanodendrite catalysts were prepared and compared against commercial 20 wt% Pt/C (E-TEK) for ORR. TEM, XRD, X-ray Absorption Near Edge S

tructure (XANES), and electrochemical techniques were applied to characterize these catalysts. Effects of high index facets on dendritic Pt surface contribute to the enhanced catalytic activity and stability of Ptd/Ti0.7Mo0.3O2-HNC towards the ORR. In addition,electron transfer originating from stro

ng metal-support interactions (SMSI) and corrosion-resistant Ti0.7Mo0.3O2 nanosupport also play important roles.3) The final approach is entitled: "Defect-Structural Ti0.9Mo0.1Oy Supported-Pt nanocrystals Used as High-Performance Catalyst for Oxygen Reduction Reaction". The search for catalysts with

high activity and longer-term stability for ORR in PEMFC is ongoing. In this research, we investigated the idea that advanced defect structural Ti0.9Mo0.1Oy nanosupports significantly enhanced the ORR activity and stability of supported-Pt catalysts. Here the approach of synthesisinvolves anchoring

of Pt(111) nanoparticles on the defect structural Ti0.9Mo0.1Oy nanosupports (d-Ti0.9Mo0.1Oy). While the structural defects of Ti0.9Mo0.1Oy are made by doping Molybdenum into anatase-TiO2 structure and Hydrogen-treatment at a high temperature (300 oC). The structural defects of this material were st

udied by Kroger-Vink defect theory, density functional theory (DFT)calculations, and examined by the experimental Raman, electronic conductivity measurements.Rotating disk electrode (RDE) measurements showed that 20 wt% Pt/d-Ti0.9Mo0.1Oy catalyst had 1.5 times and ∼ 9.1 times higher Pt mass activity

for ORR than those of 10 wt% Pt/d-Ti0.9Mo0.1Oy and commercial Pt/C catalysts, respectively. The observed high activity of metal oxide supported-Pt catalyst is attributed to the role of the advanced oxide support with electron donation from support to Pt catalyst surface, oxygen vacancies on nanosup

port surface and high conductivity. Pt nano-electrocatalysts on the advanced robust non-carbon d-Ti0.9Mo0.1Oy nanosupports also exhibits improved stability against Pt sintering under a potential cycling regime (3000 cycles from 0.4 V to 1.0 V vs. RHE) due to SMSI. Moreover, the role of oxygen vacanc

ies on oxide surface was also investigated as a means to enhance ORR activity by DFT theory.This collection of new approaches not only provides feasible paths for enhacing the efficiency of PEMFCs but also extends different catalysis reactions such as CO oxidation, hydrogenation,methanol oxidation r

eaction in DEMFCs etc., or in different applications including biosensors,optical materials (on tetrapod morphology of Pd-based materials) etc.