Cathode catalyst layer design for proton exchange membrane fuel cells

To improve water management and enhance the catalyst utilization of the cathode catalyst layer of proton exchange membrane (PEM) fuel cells, the effects of polytetrafluoroethylene (PTFE) addition in the catalyst ink and the loading pattern of the catalyst layer were investigated. Two types of catalyst ink were used: a typical one without PTFE (Pt on carbon support + Nafion) and another type added with PTFE (Pt on carbon support + Nafion + PTFE). In exploring the effect of PTFE addition into the conventional full loading pattern of catalyst layer, the presence of 10% PTFE in the catalyst layer improved the cell performance (34% increase of maximum power density) and the optimum Pt loading for the PTFE-added catalyst layer was 0.25 mg/cm². Two catalyst layer loading patterns created in this work were the strip and chess patterns. Each pattern consists of equal areas of several hydrophilic and hydrophobic segments. The hydrophilic segments were formed by using the ink with PTFE while the hydrophilic had no PTFE. For the catalyst loading pattern effect, the cell achieved the highest performance with the chess pattern, followed by the strip and full loading pattern for the case of 0.5 mg/cm² Pt loading having a thick catalyst layer of 50-μm thickness. On the other hand, for the case of 0.25 mg/cm² Pt loading forming a thin catalyst layer of ～30-μm thickness, the catalyst loading pattern had no effect on the cell’s performance.     

Suppression of wear in graphene polymer composites

Polytetrafluoroethylene (PTFE) is one of the most widely used solid lubricants but suffers from a high wear rate which limits its applications. Here we report four orders of magnitude reduction in the steady state wear rate of PTFE due to graphene additives. The wear rate of unfilled PTFE was measured to be ～0.4 × 10^?3 mm³/N m which is reduced to ～10^?7 mm³/N m by the incorporation of 10 wt% of graphene platelets. We also performed a head-to-head comparison of wear rate with graphene and micro-graphite fillers at the same weight fractions. In general, we find that graphene fillers gave 10–30 times lower wear rates than micro-graphite at the same loading fraction. Scanning electron microscopy analysis indicated noticeably smaller wear debris size in the case of graphene/PTFE composites indicating that graphene additives are highly effective in regulating debris formation in PTFE leading to reduced wear.     

Effects of solid content on the phase assemblages,mechanical and dielectric properties of porous α-SiAlON ceramics fabricated by freeze casting

Porous Y-α-SiAlON ceramics were prepared by freezing camphene-based suspensions at 0 °C and subsequently sintering at 1900 °C for 1 h. The effects of solid loading content in the suspensions on porosities and formation of α-SiAlON as well as mechanical and dielectric properties of the porous ceramics were investigated. An XRD analysis performed on sintered samples indicated that the α-SiAlON did not fully form in the sample with initial solid loading content of 10 vol%, due to the high porosity of 90 vol% and interconnected pore of the green body. With the increase of initial solid loading content from 10 vol% to 30 vol%, the porosity decreased from 62.3% to 23.1% and the average pore size decreased from 19 μm to 8 μm. As a result, the flexural strength increased significantly from 72.4 MPa to 190.2 MPa, fracture toughness increased from 1.20 MPa m^1/2 to 3.48 MPa m^1/2, as well as the dielectric constant increased from 3.3 to 6.3. The dielectric loss (tan δ) of obtained material varied between 1.4×10^?2 and 2.8×10^?2, which did not depend on the porosity of samples.     

Electrochemical and peroxidase-catalyzed oxidation of epinephrine

Electrochemical and peroxidase-catalyzed oxidation of epinephrine (EPI) has been studied. In the electrochemical studies a single well-defined, 4e, 4H⁺, pH-dependent oxidation peak was observed in square wave and cyclic sweep voltammetry at edge plane pyrolytic graphite electrode. In the reverse sweep a redox couple was observed. The decay of the UV-absorbing intermediate generated and the first-order rate constants were calculated at different pH and were found to be ～6.3 × 10^?3 s^?1. The detection limit and sensitivity are found to be 17 × 10^?8 M and 2.325 μA μM^?1 respectively. At pH 7.2, the electro-oxidation product was characterized using NMR and DEPT studies as leucoadrenochrome. The peroxidase-catalyzed oxidation was carried out using horseradish peroxidase and initiated by adding H₂O₂. The identical spectral changes, rate constants and product formed during electrochemical and enzymatic oxidation suggest that the same intermediate species is generated during both the oxidations. A tentative pathway for the oxidation of EPI has been suggested. It is concluded that the electrochemical and peroxidase-catalyzed oxidation of EPI proceed by an identical pathway.     

Localized DLC etching by a non-thermal atmospheric-pressure helium plasma jet in ambient air

Using a versatile atmospheric-pressure helium plasma jet, diamond-like carbon (DLC) films were etched in ambient air. We observed that the DLC films are etched at a nominal rate of around 60 nm/min in the treated area (230 μm in diameter) during a 20-min exposure. The etching rate increased after the initial 10-min exposure. During this period, the flat DLC surface was structurally modified to produce carbon nanostructures with a density of ~ 2.4 × 10¹¹ cm^− 2. With this increase in surface area, the etching rate increased. After 20 min, the DLC film had a circular pattern etched into it down to the substrate where silicon nanostructures were observed with sizes varying from 10 nm to 1 μm. The initial carbon nanostructure formation is believed to involve selective removal of the sp²-bonded carbon domains. The carbon etching results from the formation of reactive oxygen species in the plasma.     

Processing of Foturan® glass ceramic substrates for micro-solid oxide fuel cells

The microfabrication of Foturan^® glass ceramic as a potential substrate material for micro-solid oxide fuel cells (micro-SOFC) was investigated. Foturan^® was etched in 10% aqueous hydrofluoric (HF) acid solution at 25 °C with a linear rate of 22 ± 1.7 μm/min to create structures with an aspect ratio of 1:1 in 500 μm-thick Foturan^® substrates for micro-SOFCs. The concentration of the HF etchant was found to influence the etching rate, whereas the UV-exposure time creating nuclei in the glass for subsequent crystallization of the amorphous Foturan^® material had no significant influence on the etching rates. The surface roughness of the crystallized Foturan^® was determined by the crystallite size in the order of 10–15 μm. Free-standing micro-SOFC membranes consisting of a thin film Pt cathode, an yttria-stabilized-zirconia electrolyte and a Pt anode were released by HF etching of the Foturan^® substrate. An open-circuit voltage of 0.57 V and a maximum power density of 209 mW/cm² at 550 °C were achieved.     

Synthesis of highly dispersed titanium dioxide nanoclusters on reduced graphene oxide for increased glucose sensing

Highly dispersed titanium dioxide nanocluster (TDN) was synthesized on reduced graphene oxide (RGO) in a toluene–water system under microwave irradiation. The prepared RGO–TDN hybrids were used to modify glassy carbon electrode for loading glucose oxidase. The fabricated glucose biosensor exhibits excellent performance for glucose sensing including low work potential (−0.7 V), high sensitivity (35.8 μA mM⁻¹ cm⁻²), low detection limit (4.8 μM), wide linear range from 0.032 to 1.67 mM, small Michaelis–Menten constant (K_m) (0.81 mM), and short response time (10 s).     

Isosynthesis via CO hydrogenation over SO4–ZrO2 catalysts

Catalytic performances of sulfated zirconia catalysts with various contents of sulfur (from 0.1 to 0.75%) on isosynthesis were studied. It was firstly found that undoped-zirconia synthesized from zirconyl nitrate provided higher activity towards isosynthesis reaction (106 μmol kg-cat^?1 s^?1) compared to that synthesized from zirconyl chloride (84.9 μmol kg-cat^?1 s^?1). Nevertheless, the selectivity of isobutene in hydrocarbons was relatively lower. It was then observed that the catalytic reactivity and selectivity significantly improved by sulfur loading. The most suitable sulfur loading content seems to be at 0.1%, which gave the highest reaction rate and selectivity of isobutene. By applying several characterization techniques, i.e. BET, XRD, NH₃- and CO₂-TPD and SEM, it was revealed that the high reaction rate and selectivity towards isosynthesis reaction of sulfated zirconia catalysts are related to the acid–base properties, Zr³⁺ quantity and phase composition.     

Nano-graphite platelet loaded with LiFePO4 nanoparticles used as the cathode in a high performance Li-ion battery

LiFePO₄ nanoparticles were grown on nano-graphite platelet (NGP) using a simple chemical route. The material was used as the cathode in Li-ion rechargeable batteries and exhibited excellent cyclability and rate capability because of the easy electron transport in it. The electrochemical stability of the electrode was improved by the two-dimensional conductive network of the NGP. The resulting electrodes delivered a specific capacity of about 150 mA h g^?1 at a current rate of 135 mA g^?1 (～0.8 C) after 100 cycles with no capacity fade. At elevated current rates, the electrodes exhibited capacities of more than 100 mA h g^?1 at a current density of 2000 mA g^?1 (～12 C) without further incorporation of conductivity agents or coatings.     

Selective solid-phase extraction of rare earth elements by the chemically modified Amberlite XAD-4 resin with azacrown ether

A selective solid-phase extraction procedure using chemically modified Amberlite XAD-4 with monoaza dibenzo 18-crown-6 ether was investigated for the preconcentration and separation of La(III), Nd(III) and Sm(III) in synthetic solution. Before loading samples on synthesized adsorbent adjust pH 4.5 by suitable buffer solution. The adsorbed rare earth elements were eluted by 2 M hydrochloric acid. Various parameters like preconcentration, breakthrough capacity, flow rate were investigated. The limits of detection (n = 5) and limits of quantification (n = 5) for La(III), Nd(III) and Sm(III) were founded 3.9, 4.2 and 7.4 μg L^?1 and 13, 15 and 26 μg L^?1, respectively. The eluted metal ions were determined by ICP-AES.     

Influence of loading rate on nanohardness of sapphire

This work reports the loading rate effect on nanohardness of sapphire. The intrinsic nanoscale contact deformation resistance of sapphire increased with the loading rates following empirical power law dependence with a positive exponent. The results showed a significant enhancement (e.g., ~66%) of the nanohardness of sapphire with the increase in loading rates from 10 to 10,000 μN s⁻¹. These results were explained mainly in terms of the maximum shear stress generated underneath the nanoindenter, dislocation density and critical resolved shear stress of the sapphire.     

Impactor designed to increase mass output rate of nanoparticles from a pneumatic nebulizer

A modular impactor was designed to remove large droplets from aerosols generated by a pneumatic nebulizer, the Six-Jet Atomizer from TSI Inc. (Shoreview, MN), with the aim of generating dry nanoparticles. Three interchangeable nozzle heads were designed to provide droplet cutoff diameters of 0.5, 1, and 2 μm at an air flow rate of 8.3×10^?4 m³ s^?1 (50 L min^?1), which corresponds to all six jets of the nebulizer operated at 25 °C and an air pressure of 241 kPa (35 psi). The collection and output characteristics of the 0.5 μm impactor were evaluated from dry particle size distributions produced by nebulizing an aqueous solution with a NaCl mass fraction of 1% both with and without the impactor present. The impactor characteristic cutoff curve was sharp (impactor geometric standard deviation, GSD_imp=1.15–1.19) with a 50% cutoff diameter d₅₀ that ranged from 0.48 μm at 3.0×10^?4 m³ s^?1 to 0.74 μm at 11.7×10^?4 m³ s^?1. The rate of dry NaCl particle generation ranged from 0.5 to 5 g s^?1 (0.04 to 0.4 g day^?1) with mass median diameters MMD_p=80–123 nm and geometric standard deviations GSD_p=1.6–1.8 (depending on flow rate). Anomalous negative impactor efficiencies were observed at flow rates >8.3×10^?4 m³ s^?1 for 100 to 400 nm droplets and at all flow rates for droplets smaller than 100 nm. This phenomenon will be investigated further as a way to increase the generation rate of nanoparticles. A step-by-step procedure is presented for the selection of an appropriate impactor design and operating flow rate for a desired maximum aerosol particle size.     

Photocatalytic degradation of 17-β-oestradiol on immobilised TiO2

Micro-molar concentrations of aqueous 17-β-oestradiol were 98% destroyed in 3.5 h by photocatalysis over the titanium dioxide powder immobilised on Ti-6Al-4V alloy. The concentration of oestradiol was determined by HPLC with fluorescence detection. The degradation kinetics were fitted to a Langmuir–Hinshelwood model with k(S) = 4.4 × 10⁻² μmol dm⁻³ min⁻¹ and K(S) = 0.347 dm³ μmol⁻¹. The pseudo-first-order rate constant (1.57 × 10⁻² min⁻¹) was in line with the 50% degradation time of 40 min. The apparent quantum yield per electron was φ_e′  = 0.41%. The effect of pH on the initial rate of degradation was similar to that reported for phenol.     

Synthesis,crystal structure and hydrolytic activity of a trispyrazolyl borato cadmium hydroxo complex

A dinuclear tris(3,5-diisopropyl-pyrazolyl borate) (Tpy ^iPr2) based cadmium hydroxo complex, [(Tpy^iPr2)₂Cd₂(μ-OH)(μ-Pz^iPr2)](DCM)₅, has been synthesized and characterized by the single crystal X-ray diffraction technique. Structural analysis shows that the two Cd-Tpy^iPr2 moieties are bridged by one hydroxide and one pyrazolate molecule. Hydrolytic activity of the complex was checked using 4-nitro phenylacetate as the substrate. The pseudo-first order rate constant calculated on the basis of kinetic study was 1.535 × 10^− 4 s^− 1 while the second order rate constant was 0.226 M^− 1 s^− 1.     

High resolution optical microprobe investigation of surface grinding stresses in Al2O3 and Al2O3/SiC nanocomposites

It has previously been suggested that Al₂O₃/SiC nanocomposites develop higher surface residual stresses than Al₂O₃ on grinding and polishing. In this work, high spatial resolution measurements of residual stresses in ground surfaces of alumina and nanocomposites were made by Cr³⁺ fluorescence microspectroscopy. The residual stresses from grinding were highly inhomogeneous in alumina and 2 vol.% SiC nanocomposites, with stresses ranging from ～ ?2 GPa within the plastically deformed surface layers to ～ +0.8 GPa in the material beneath them. Out of plane tensile stresses were also present. The stresses were much more uniform in 5 and 10 vol% SiC nanocomposites; no significant tensile stresses were present and the compressive stresses in the surface were ～ ?2.7 GPa. The depth and extent of plastic deformation were similar in all the materials (depth ～ 0.7–0.85 μm); the greater uniformity and compressive stress in the nanocomposites with 5 and 10 vol% SiC was primarily a consequence of the lack of surface fracture and pullout during grinding. The results help to explain the improved strength and resistance to severe wear of the nanocomposites.     

Response surface optimization of photocatalytic process for degradation of Congo Red using H-titanate nanofiber catalyst

In this study, a new generation H-titanate nanofiber catalyst (TNC) with long fibril morphology and surface covered anatase titanium dioxide (TiO₂) crystals of 10–20 nm was used as the photocatalyst for improved photoactivity, mass transfer resistance and downstream separation. The combined Taguchi method and response surface analysis (RSA) were employed to evaluate the effects of key operational factors of TNC loading, pH, aeration rate and initial Congo Red (CR) concentration on the performances of TNC in an annular slurry photoreactor (ASP). The average CR photocatalytic degradation rate (mg dm^?3 min^?1) was estimated and applied as the response outputs of the L₉ (3)⁴ Taguchi orthogonal array. Results from the RSA interpretations revealed that pH, initial CR concentration and aeration rate were the significant factors, while TNC loading appears to be the least significant factor. On contrary, positive interactions of TNC loading were observed when being coupled with pH and aeration rate. Other interactions of the operation factors were also determined using statistical analysis. A natural logarithmic modified regression equation was developed from multiple regression analysis for response surface modelling. This model predicted that the average photocatalytic degradation rate of CR was 0.1576 mg dm³ min^?1 under the optimal conditions. A subsequent verification experiment showed a photocatalytic degradation rate of 0.1563 ± 0.0282 mg dm^?3 min^?1, which is in good agreement with the model predicted value. This proved the applicability of the developed model as a reliable design and modelling tool for scaling up the photocatalytic reactor process.     

Precise micromachining of yttria-tetragonal zirconia polycrystal ceramic using 532 nm nanosecond laser

High quality micro-sized steps and blind hole structures without microcracks, chips or spatter deposition were machined on yttria-tetragonal zirconia polycrystal (Y-TZP, 3 mol% yttria) by nanosecond laser (wavelength=532 nm, pulse width ~6 ns). The diameter of blind hole is 500 μm and each step is 500±10 μm wide and 100±5 μm deep. The 1.35 mm³/min removal rate and the smooth machined surface with Ra=2.824 μm roughness depicting the high precise and efficient processing were achieved. The ablation characteristics of nanosecond laser process of Y-TZP ceramic were also studied. Based on the study, a reasonable design of the processing path for micromachining of a finer embedded step with 24±2 μm width (smaller than the 60 μm focused spot size) around the inner-wall of a 2×2 mm² cavity was developed. These results and discussion offer new possibilities in the manufacturing of bio-ceramic products by nanosecond laser with high processing quality and efficient.     

Particle size effects in flash sintering

The study of 3 mol% yttria stabilized zirconia (3YSZ) with different particle sizes provides new insights into flash sintering. Four powders, all with the same crystallite size but various particle size were investigated: described as nominally 1 μm (D80 = 0.51 μm, meaning 80 vol% has a size less than 0.51 μm), 2 μm (D80 = 0.90 μm), 5 μm (D80 = 2.11 μm) and 10 μm (D80 = 3.09 μm). While the furnace temperature for flash sintering, at a field of 100 V cm^?1, increased from 920 °C to 1040 °C with particle size, the specimen temperature in all instances remained at ～1200 °C. The quantum increase in density decreased with larger particles. The grain size distribution of conventionally and flash sintered specimens remained similar, with some evidence of a preponderance of nanograins in the flash sintered specimens. Joule heating was well below the temperatures that would have been required for sintering in a few seconds. An explanation based upon the nucleation of Frenkel pairs is proposed.     

Parametric study of intrinsic thermal transport in vertically aligned multi-walled carbon nanotubes using a laser flash technique

The thermal diffusivity of vertically aligned carbon nanotube (VACNT, multi-walled) films synthesized by thermal chemical vapor deposition was measured by a laser flash technique, and shown to be ～30 mm² s^?1 along the tube-alignment direction. The calculated thermal conductivities of the VACNT films and the individual CNTs were ～27 and ～540 W m^?1 K^?1, respectively. The technique was verified to be reliable although a proper sampling procedure is critical. A systematic parametric study of the effects of defects, buckling, tip-to-tip contacts, packing density, and tube–tube interaction on the thermal diffusivity was carried out. Defects and buckling decreased the thermal diffusivity dramatically. An increased packing density was beneficial in increasing the collective thermal conductivity of the VACNT film; however, the increased tube–tube interaction in dense VACNT films decreased the effective thermal conductivity of the individual CNTs in the films. The tip-to-tip contact resistance was shown to be ～1 × 10^?7 m² K W^?1. The study will shed light on the potential application of VACNTs as thermal interface materials in microelectronic packaging.     

Crack healing behaviour of Cr2AlC MAX phase studied by X-ray tomography

The autonomous crack-healing capability of Cr₂AlC MAX phase ceramic by surface oxidation at elevated temperatures has a huge potential for high temperature structural and protective coating applications. In this work we use time-lapse X-ray computed tomography (CT) to track the fine details of local crack filling phenomena in 3 dimensions (3D) with time. The maximum crack width that could be fully healed upon exposure to 1200 °C in air is 4.8 μm in 4 h and 10 μm after 12 h. Furthermore, during healing Cr₇C₃ phase is observed beneath the dense Al₂O₃ layer (average thickness of 1 μm on each crack surface) when the crack width exceeds 2 μm. The 3D image sequences indicated that the rate of healing is essentially independent of position along, or across, the crack faces. The crack healing kinetics of Cr₂AlC at 1200 °C broadly follows a parabolic rate law with a rate constant of 4.6 × 10⁻⁴ μm² s⁻¹. The microstructure, composition and thickness of the oxide scale in the healed crack area are characterized via post mortem SEM-EDS measurements and confirm the formation of an initial dense alumina layer on top of which a more porous layer forms. Impurity Cr particles appear to accelerate the oxidation process locally and correlative SEM imaging of the same region suggests this is by providing Cr₂O₃ nucleation sites.