High Contrast Ratio and Rapid Switching Organic Polymeric Electrochromic Thin Films Based on Triarylamine Derivatives from Layer-by-Layer Assembly

High Contrast Ratio and Rapid Switching Organic Polymeric Electrochromic Thin Films Based on Triarylamine Derivatives from Layer-by-Layer Assembl

In Situ Transformation of Hydrogen-Evolving CoP Nanoparticles: Toward Efficient Oxygen Evolution Catalysts Bearing Dispersed Morphologies with Co-oxo/hydroxo Molecular Units

Reported herein is elucidation of a novel Co-based oxygen evolution catalyst generated in situ from cobalt phosphide (CoP) nanoparticles. The present CoP nanoparticles, efficient alkaline hydrogen-evolving materials at the cathode, are revealed to experience unique metamorphosis upon anodic potential cycling in an alkaline electrolyte, engendering efficient and robust catalytic environments toward the oxygen evolution reaction (OER). Our extensive ex situ characterization shows that the transformed catalyst bears porous and nanoweb-like dispersed morphologies along with unique microscopic environments mainly consisting of discrete cobalt-oxo/hydroxo molecular units within a phosphate-enriched amorphous network. Outstanding OER efficiency is achievable with the activated catalyst, which is favorably comparable to even a precious iridium catalyst. A more remarkable feature is its outstanding long-term stability, superior to iridium and conventional cobalt oxide-based materials. Twelve-hour bulk electrolysis continuously operating at high current density is completely tolerable with the present catalyst

Feasibility of a Spherical Hollow Carbon Framework as a Stable Host Material for Reversible Metallic Li Storage

A spherical hollow carbon framework decorated with functional heteroatoms is designed and synthesized using ultrasonic spray pyrolysis as a potential anode material for lithium metal batteries (LMBs). The pore structure of the hollow carbon framework can be tailored by melamine, which is a functional additive for integrating abundant nanopores and the uniform decoration of heteroatoms in the structure. The large surface area and pore volume of the hollow carbon framework offer enhanced reversibility and capability for metallic Li storage. In addition, the dendritic growth of Li and volume changes induced by repeated Li plating and stripping can be effectively suppressed during cycling. More importantly, atomic-scale decorations of heteroatoms can effectively lower the overpotential for the nucleation and growth of metallic Li inside the hollow carbon framework. It is mainly responsible for improving the cycle performance and rate capability, even at a high current density. Finally, the hollow carbon framework anode shows stable behavior toward Li plating and stripping without significant capacity fading in the LMBs than conventional Li metal anodes

Effect of Surface Segregation on the Methanol Oxidation Reaction in Carbon-Supported Pt−Ru Alloy Nanoparticles

Ru and Pt−Ru (Pt:Ru = 1:1) nanoparticles supported on carbon black were prepared by the borohydride reduction method using oleylamine as a stabilizer in an anhydrous ethanol solvent. We investigated the effect of Pt segregation to the surface of alloy nanoparticles on the methanol oxidation reaction (MOR). As-prepared Pt1Ru1/C showed a narrow size distribution and a relatively uniform particle distribution on a carbon support. However, its electrocatalytic activity toward the MOR was poor due to the high surface concentration of Ru. As duration time of heat treatment at 200 °C was increased up to 2 h, the surface composition of Pt atoms was increased without significant particle growth due to thermally induced segregation of Pt atoms, which were revealed by TEM images, X-ray photoelectron spectroscopy (XPS) analysis, changes in the potentials of zero total charge (pztc), and increase in the oxidation charge of “reduced CO2”. In particular, from the combination of CO adlayer oxidation and “reduced CO2” oxidation charges, the increased surface concentration of Pt of alloy catalysts was relatively quantified when compared to its as-prepared state. Cyclic voltammograms in 0.1 M HClO4 solution with 0.5 M methanol showed that Pt1Ru1/C annealed for 2 h at 200 °C in a flow of mixture gas of Ar and H2 (5 vol %) had a less positive onset potential for the MOR. These results demonstrate a definitive contribution from segregation of Pt atoms to the MOR activity

Origin of the Enhanced Electrocatalysis for Thermally Controlled Nanostructure of Bimetallic Nanoparticles

The thermal annealing process is a common treatment used after the preparation step to enhance the electrocatalytic properties of the oxygen reduction reaction (ORR). The structure of a Pt-based bimetallic nanoparticle, which is significantly affected by the catalytic properties, is reconstructed by thermal energy. We investigated the effect of structural reconstruction induced by thermal annealing on the improvement of the ORR using various physical and electrochemical methods. We found that the structural evolution of PtNi nanoparticles, i.e., the Pt–Ni ordering with the Pt shell and the surface reorientation into the (111) facet, is the source of the enhanced ORR activity as well as electrochemical stability through the thermal annealing. This result confirms the crucial factors for the ORR properties by the thermal annealing process and proposes a way to design advanced electrocatalysts

Design of an Advanced Membrane Electrode Assembly Employing a Double-Layered Cathode for a PEM Fuel Cell

The membrane electrolyte assembly (MEA) designed in this study utilizes a double-layered cathode: an inner catalyst layer prepared by a conventional decal transfer method and an outer catalyst layer directly coated on a gas diffusion layer. The double-layered structure was used to improve the interfacial contact between the catalyst layer and membrane, to increase catalyst utilization and to modify the removal of product water from the cathode. Based on a series of MEAs with double-layered cathodes with an overall Pt loading fixed at 0.4 mg cm^–2 and different ratios of inner-to-outer Pt loading, the MEA with an inner layer of 0.3 mg Pt cm^–2 and an outer layer of 0.1 mg Pt cm^–2 exhibited the best performance. This performance was better than that of the conventional single-layered electrode by 13.5% at a current density of 1.4 A cm^–2

Influence of Oxide on the Oxygen Reduction Reaction of Carbon-Supported Pt−Ni Alloy Nanoparticles

Pt−Ni alloy nanoparticles supported on carbon black (Pt:Ni = 1:1) were prepared by the borohydride reduction method using acetate anions as a stabilizer in anhydrous ethanol solvent. Here, we surveyed the effect of oxide phases in Pt−Ni alloy nanoparticles on the electrocatalytic activity toward oxygen reduction reaction (ORR). As-prepared Pt1Ni1/C, which showed a relatively high degree of alloying, possessed the lower oxygen reduction reaction (ORR) activity as compared to pure Pt. However, following heat treatment in a flow of Ar at 300 °C for 3 h, Pt1Ni1/C showed oxygen reduction activity higher than that of commercial Pt/C (40 wt % Pt/C, Johnson-Matthey). The potential of zero total charge (PZTC) was calculated from cyclic voltammograms and the CO-displacement charge at dosing potentials at which anions are the main adsorbed species. The calculated value then shifted to a more positive potential after heat treatment. This indicates that the surface of the Pt−Ni nanoparticles became less oxophilic mainly due to the clustering of Pt. This anodic shift of the PZTC is consistent with the results of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption near-edge structure spectroscopy (XANES). Consequently, the observed catalytic enhancement by heat treatment is due to the increase of metallic Pt and NiO and the phase separation between metallic Pt and Ni oxides

Ternary Pt−Fe−Co Alloy Electrocatalysts Prepared by Electrodeposition: Elucidating the Roles of Fe and Co in the Oxygen Reduction Reaction

An electrodeposition-based protocol for the synthesis of ternary Pt−Fe−Co electrocatalysts for the oxygen reduction reaction (ORR) has been developed. The eletrodeposition method suits the purpose of fast catalyst screening and mechanism studies. Here, we survey the composition effect of Fe and Co atoms in ternary Pt−Fe−Co alloy electrocatalysts on the electrocatalytic activity toward the ORR in terms of geometric (Pt−Pt distance) and electronic (core-level binding energy, d-band center) aspects. A wide range of Pt−Fe−Co catalysts can easily be obtained using electrodeposition under simple and mild conditions. Among the various compositions, Pt85Fe10Co5 catalyst shows excellent mass activity that is 3.5 times higher than that of pure Pt. Interestingly, the ORR kinetic current density reveals a double-volcano plot as a function of alloy composition. Extended X-ray absorption fine structure (EXAFS) spectroscopy and high-resolution X-ray photoelectron spectroscopy (HRXPS) experiments were conducted to explain the abnormal double-volcano behavior. The results also reveal that the d-band center of Pt85Fe10Co5 is downshifted by about 0.1 eV compared to that of Pt, which explains its superior activity toward the ORR

Surface Structures and Electrochemical Activities of PtRu Overlayers on Ir Nanoparticles

PtRu overlayers were deposited on carbon-supported Ir nanoparticles with various Pt:Ru compositions. Structural and electrochemical characterizations were performed using transmission electron microscopy (TEM), X-ray diffraction, high-resolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), cyclic voltammetry (CV), and CO stripping voltammetry. The PtRu overlayers were selectively deposited on the Ir nanoparticles with good uniformity of distribution. As a result, the PtRu utilization of the present samples was higher than that of PtRu/C. The mass-specific activities for methanol oxidation were also significantly higher. Single-cell performance using the Pt₂Ru₁ overlayer sample as an anode catalyst was slightly higher than that obtained using commercial PtRu/C despite the fact that the PtRu anode loading for Pt₂Ru₁/Ir/C was only 42% of that of PtRu/C

Surface Structures and Electrochemical Activities of Pt Overlayers on Ir Nanoparticles

Pt overlayers were deposited on carbon-supported Ir nanoparticles with various coverages. Structural and electrochemical characterizations were performed using transmission electron microscopy (TEM), X-ray diffraction, high-resolution powder diffraction (HRPD), X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), cyclic voltammetry (CV), CO stripping voltammetry, and N2O reduction. The surface of Ir nanoparticles was covered with Pt overlayers with thickness varying from the submonolayer scale to more than two monolayers. Surface analyses such as CV and CO stripping voltammetry indicated that the Pt overlayers were uniformly deposited on the Ir nanoparticles, and the resultant Pt overlayers exhibited gradual changes in surface characteristics toward the Pt surface as the surface coverage increased. The distinct CO stripping characteristics and the enhanced Pt utilization affected electrocatalytic activities for methanol oxidation. The electrochemical stability of the Pt overlayer was compared with a commercial carbon-supported Pt catalyst by conducting a potential cycling experiment