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

Methanol Electro-Oxidation on the Pt Surface: Revisiting the Cyclic Voltammetry Interpretation

Methanol is a promising fuel for direct methanol fuel cells in portable devices. A deeper understanding of its electro-oxidation is needed for evaluating electrocatalytic performance and catalyst design. Here we provide an in-depth investigation of the cyclic voltammetry (CV) of methanol electro-oxidation. The oxidation peak in backward scan is shown to be unrelated to residual intermediate oxidation. The origin of the second oxidation peak (I_f2) is expected to the methanol oxidation on Pt–O_<i>x</i>. Electrochemical impedance spectroscopy coupled with CV reveals the origin of CV hysteresis to be a shift in the rate-determining step, from methanol dehydration to OH adsorption by water dissociation, induced by a change in Pt surface coverage with oxygenated species. The peak ratio between forward oxidation peak current (I_f) and backward oxidation peak current (I_b), which is I_f/I_b, is not related to the degree of CO tolerance but to the degree of oxophilicity indeed

Tailoring the Electronic Structure of Nanoelectrocatalysts Induced by a Surface-Capping Organic Molecule for the Oxygen Reduction Reaction

Capping organic molecules, including oleylamine, strongly adsorbed onto Pt nanoparticles during preparation steps are considered undesirable species for the oxygen reduction reaction due to decreasing electrochemical active sites. However, we found that a small amount of oleylamine modified platinum nanoparticles showed significant enhancement of the electrochemical activity of the oxygen reduction reaction, even with the loss of the electrochemically active surface area. The enhancement was correlated with the downshift of the frontier d-band structure of platinum and the retardation of competitively adsorbed species. These results suggest that a capping organic molecule modified electrode can be a strategy to design an advanced electrocatalyst by modification of electronic structures

Low-Cost and High-Performance Anion-Exchange Membrane Water Electrolysis Stack Using Non-Noble Metal-Based Materials

With increasing hydrogen demand, the development of a low-cost and high-performance anion-exchange membrane water electrolysis (AEMWE) stack is crucial. Here, two AEMWE models using all non-noble metal-based components were developed. Three components of the membrane electrode assemblya porous transport layer (PTL), an oxygen evolution reaction (OER) catalyst, and a hydrogen evolution reaction (HER) catalystwere examined to be substituted for a non-noble metal. The results revealed that stainless steel felt and carbon paper were the anode and cathode PTLs, respectively, exhibiting the highest and most durable performance. Additionally, nickel–iron (NiFe) was selected as the most applicable OER catalyst. Further, low-loading platinum and nickel–iron oxide (NiFeOx) were optimized as suitable HER catalysts. For a single cell, the resulting AEMWEs showed outstanding performance of 4633 and 1231 mA cm–2 at 2.1 V, with stable performance for 500 h. Further, they exhibited a higher performance relative to their cost than all noble metal AEMWEs. High performances were also observed for 5-layer stacks, in addition to stable durability and energy conversion efficiency. This work supports the commercialization of a low-cost, high-performance, and durable AEMWE stack

Surface Modification of Stretched TiO₂ Nanotubes for Solid-State Dye-Sensitized Solar Cells

Straight-stranded anatase TiO2 nanotubes were produced by anodic oxidation on a pure titanium substrate in an aqueous solution containing a 0.45 wt % NaF electrolyte (pH 4.3 fixed). The average length of the TiO2 nanotubes was approximately 3 μm, which had an effect on the level of dye adsorption in the dye-sensitized solar cells. The anodic TiO2 nanotubes were applied as a working electrode in a solid-state dye-sensitized solar cell. An approximately 1 nm ZnO shell was coated on the TiO2 nanotube to improve the open-circuit voltage (Voc) and conversion efficiency of the solar cell, and to retard any back reaction. Although the Voc and short-circuit current (Jsc) of the cell were improved, there was a low fill factor as a result of the formation of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate. A parameter on the degradation of fill factor (37%) is related to the formation of a thick TiO2 barrier layer in the anodic TiO2/Ti substrate interface. A hydrogen peroxide treatment was performed in an attempt to narrow the TiO2 barrier layer. This treatment was found to influence not only fill factor (37−49%) but also the conversion efficiency (0.704−0.906%) of the cell by eliminating the remnant after anodic reaction and barrier narrowing through an etching effect. This result was confirmed by X-ray photoelectron spectroscopy (XPS) and photocurrent-voltage measurements. The longer electron lifetime on the ZnO coated TiO2 film was measured by the open-circuit voltage decay. The improvement in the electron lifetime from the thin ZnO coating affects the number of electrons collected on the Ti substrate and the retardation of charge recombination. Therefore, the ZnO coating on the TiO2 nanotube film improves the efficiency of dye-sensitized TiO2 solar cells from the extended Voc from ZnO coating confirmed by the Mott−Schottky plots and the increased Jsc through the inhibition of charge recombination confirmed by IPCE measurements

Surface Structure of Pt-Modified Au Nanoparticles and Electrocatalytic Activity in Formic Acid Electro-Oxidation

Platinum-modified Au nanoparticles on a carbon support were prepared using carbon-supported Au nanoparticles as a substrate and a successive reduction process. These nanoparticles were applied as an electrocatalyst for formic acid electro-oxidation. The uniform Pt-modified Au nanoparticles (<5 nm in diameter) were highly dispersed on the carbon particles, and the Au surface was deposited with nanoscaled Pt. The Pt-modified Au nanoparticles showed higher electrocatalytic activities than the pure Pt electrocatalyst in the area- and mass-specific current densities. These results might be due to the enhancement effect of Au atoms and the high Pt utilization in the formic acid electro-oxidation reaction

Simultaneous Phase- and Size-Controlled Synthesis of TiO₂ Nanorods via Non-Hydrolytic Sol−Gel Reaction of Syringe Pump Delivered Precursors

The simultaneous phase- and size-controlled synthesis of TiO2 nanorods was achieved via the non-hydrolytic sol−gel reaction of continuously delivered two titanium precursors using two separate syringe pumps. As the injection rate was decreased, the length of the TiO2 nanorods was increased and their crystalline phase was simultaneously transformed from anatase to rutile. When the reaction was performed by injecting titanium precursors contained in two separate syringes into a hot oleylamine surfactant solution with an injection rate of 30 mL/h, anatase TiO2 nanorods with dimensions of 6 nm (thickness) × 50 nm (length) were produced. When the injection rate was decreased to 2.5 mL/h, star-shaped rutile TiO2 nanorods with dimensions of 25 nm × 200 nm and a small fraction of rod-shaped anatase TiO2 nanorods with dimensions of 9 nm × 100 nm were synthesized. Pure star-shaped rutile TiO2 nanorods with dimensions of 25 nm × 450 nm were synthesized when the injection rate was further decreased to 1.25 mL/h. The simultaneous phase transformation and length elongation of the TiO2 nanorods were achieved. Under optimized reaction conditions, as much as 3.5 g of TiO2 nanorods were produced. The TiO2 nanorods were used to produce dye-sensitized solar cells, and the photoconversion efficiency of the mixture composed of star-shaped rutile TiO2 nanorods and a small fraction of anatase nanorods were comparable to that of Degussa P-25

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

Enhanced Photovoltaic Properties of a Cobalt Bipyridyl Redox Electrolyte in Dye-Sensitized Solar Cells Employing Vertically Aligned TiO₂ Nanotube Electrodes

Photovoltaic performances of TiO2 nanoparticle (NP) electrodes and TiO2 nanotube (NT) electrodes in dye-sensitized solar cells (DSSCs) employing a cobalt bipyridyl redox electrolyte were compared. The TiO2 NP electrodes had pore sizes ranging from 15 to 20 nm while the NT electrodes had a lager pore size of 80 nm. Highly ordered and vertically oriented TiO2 NT electrodes were prepared by a two-step anodization method. In application to DSSCs employing the cobalt redox electrolyte, the 11-μm-thick NP electrode exhibited an efficiency of 1.60% with a Jsc of 3.96 mA/cm2. Meanwhile, despite nearly half of the amount of adsorbed dye molecules, the 11-μm-thick NT electrode exhibited a slightly enhanced efficiency of 1.84% with a Jsc of 5.86 mA/cm2. In addition, the 35-μm-thick NT electrode showed an efficiency of 2.38% with a Jsc of 9.80 mA/cm2. Compared to the 11-μm-thick NP electrode, the 35-μm-thick NT electrode exhibited a 1.5 times higher efficiency with a 2.5 times higher Jsc in spite of having a similar amount of adsorbed dye molecules. Photocurrent transient measurements revealed that the mass transport limitation of the cobalt redox electrolyte within the conventional NP electrodes was greatly alleviated within the NT electrodes. In addition, the electrochemical impedance spectra indicated that the interfacial contact between the cobalt redox electrolyte and TiO2 electrode was prominently enhanced in the NT electrodes. Furthermore, the electron lifetime and electron diffusion length were all greatly longer within the NT electrodes. These superior photovoltaic properties may be attributed to the large pore size and vertically oriented structures of the NT electrodes