Recast Nafion‐117 thin film from water solution

Perfluorosulfonated ionomer (PFSI) dispersions in various solvents, usually mixtures of organic compounds and water, were used to prepare the membrane‐electrode system in polymer electrolyte membrane fuel cells (PEMFC), the aim being to increase performance by improving the triple contact of graphite (electron conducting material), Pt (hydrogen dissociation catalyst) and ionomeric membrane (proton conducting). When using PFSI dispersions in water‐organic solvent mixture, care must be taken not to poison the Pt catalyst through organic decomposition products, a consequence of the thermal treatment of the electrode‐polymer system bonded with PFSI dispersion. In the present study some procedures for preparing Nafion water dispersion, starting from a Nafion‐117 membrane, are described. The morphological characteristics of the prepared dispersions were compared with Nafion commercial dispersion (NCD). Moreover, membranes with a thickness of 5–20 μm were prepared and characterised, using both the obtained and the NCD dispersions. The obtained data showed that Nafion water dispersion, which can be used to prepare the membrane/electrode system, results in thin membranes that absorb more water than NCD membranes, and have equal and/or higher proton conduction than the NCD.     

Inhomogeneous distribution of platinum and ionomer in the porous cathode to maximize the performance of a PEM fuel cell

A proton exchange membrane (PEM) fuel cell model, accounting for the combined water transport mechanism, ionomer swelling, water phase‐transfer, two‐phase flow and transport processes, is developed. The inhomogeneous distributions of Pt and ionomer inside the catalyst layer (CL) are numerically studied to achieve an optimal cell performance for two types of oxygen reduction reaction catalysts at different loadings. Results indicate that the optimal variation in loading through the thickness of the electrode (slopes) of Pt catalyst and ionomer vary with conditions of operation. An optimal platinum slope increases the agglomerate effectiveness factor and decreases the second Damköhler number near the CL‐membrane interface. An optimal ionomer slope increases the CL porosity near the GDL‐CL interface and decreases the mass transport resistance of reactant through the ionomer film. Their interaction shows that the optimal platinum slope is a tradeoff between the electrochemical active surface area and porosity at high current densities. © 2017 American Institute of Chemical Engineers AIChE J, 2017     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part I. Experimental investigation

The cathode catalyst layer (CL) in direct methanol fuel cells (DMFCs) has been optimized through a balance of ionomer and porosity distributions, both playing important roles in affecting proton conduction and oxygen transport through a thick CL of DMFC. The effects of fabrication procedure, ionomer content, and Pt distribution on the microstructure and performance of a cathode CL under low air flowrate are investigated. Electrochemical methods, including electrochemical impedance, cyclic votammetry and polarization curves, are used in conjunction with surface morphology characterization to correlate electrochemical characteristics with CL microstructure. CLs in the form of catalyst-coated membrane (CCM) have higher cell open circuit voltages (OCVs) and higher limiting current density; while catalyzed-diffusion-media (CDM) CLs display better performance in the moderate current density region. The CL with a composite structure, consisting both CCM and CDM, shows better performance in both kinetic and mass-transport limitation region, due to a suitable ionomer distribution across the CL. This composite cathode is further evaluated in a full DMFC and the cathode performance loss due to methanol crossover is discussed.     

Preparation of mesoporous carbon templated by silica particles for use as a catalyst support in polymer electrolyte membrane fuel cells

Mesoporous carbons were prepared using commercial silica particles and a formaldehyde–resorcinol resin as a template and carbon precursor, respectively. By changing the molar ratio of template to carbon precursor, mesoporous carbons with different mesoporosities (MC-X, X represents the molar ratio of template to carbon precursor) were produced. The resulting MCs had a high-surface area and large pore volume. In particular, the highest mesoporosity was observed for MC-3. Pt catalysts-supported on MC-X were prepared using formaldehyde as a reducing agent for use as a cathode catalyst in a polymer electrolyte fuel cell (PEMFC). The size of Pt crystallite was dependent on the properties of corresponding carbon support. As a whole, a carbon support with a high-surface area and high-mesoporosity served the best in terms of a high-dispersion of Pt nanoparticles. In a unit cell test of the PEMFC, a Pt catalyst with a high-mesoporosity and fine dispersion of metal showed an enhanced performance. The findings indicate that the surface area combined with the mesoporosity had a positive influence on the metal dispersion and the distribution of ionomer, leading to the enhanced cell performance.     

Infrared reflectance absorption spectroscopy (IRRAS). Study of the thermal stability of perfluorinated sulphonic acid ionomers on Pt

Infrared Reflectance Absorption Spectroscopy (IRRAS) has been used to study the thermal stability of Nafion (du Pont, equivalent weight=1100) and a similar perfluorinated sulphonic acid ionomer (PFSI) made by the Dow Chemical Company (equivalent weight=560). Purified aqueous PFSI electrolytes solutions were prepared, PFSI films (0.5–5 m) were cast from these aqueous PFSI solutions onto Pt foil substrates for spectral study after heating at various temperatures. Spectra were recorded at room temperature in a vacuum after a film on Pt had been heated at a particular temperature in the presence of air. The temperature range was 22–300°C. Spectral changes for a PFSI film on Pt were not significant when heated at up to 200°C in an air atmosphere; however, when heated at 300°C there was a significant decrease in intensity for S-O related bands indicating substantial loss of the sulphonic acid groups from these ionomers.     

Aqueous Phase Glycerol Reforming by PtMo Bimetallic Nano-Particle Catalyst: Product Selectivity and Structural Characterization

A carbon supported PtMo aqueous phase reforming catalyst for producing hydrogen from glycerol was characterized by analysis of the reaction products and pathway, TEM, XPS and XAS spectroscopy. Operando X-ray absorption spectroscopy (XAS) indicates the catalyst consists of bimetallic nano-particles with a Pt rich core and a Mo rich surface. XAS of adsorbed CO indicates that approximately 25% of the surface atoms are Pt. X-ray photoelectron spectroscopy indicates that there is unreduced and partially reduced Mo oxide (MoO₃ and MoO₂), and Pt-rich PtMo bimetallic nano-particles. The average size measured by transmission electron microscopy of the fresh PtMo nano-particles is about 2?nm, which increases in size to 5?nm after 30?days of glycerol reforming at 31?bar and 503?K. The catalyst structure differs from the most energetically stable structure predicted by density functional theory (DFT) calculations for metallic Pt and Mo atoms. However, DFT indicates that for nano-particles composed of metallic Pt and Mo oxide, the Mo oxide is at the particle surface. Subsequent reduction would lead to the experimentally observed structure. The aqueous phase reforming reaction products and intermediates are consistent with both C?CC and C?COH bond cleavage to generate H₂/CO₂ or the side product CH₄. While the H₂ selectivity at low conversion is about 75%, cleavage of C?COH bonds leads to liquid products with saturated carbon atoms. At high conversions (to gas), these will produced additional CH₄ reducing the H₂ yield and selectivity.     

纳米粒子填充PTFE摩擦副材料的研究进展

综述国内外纳米粒子填充聚四氟乙烯(PTFE)摩擦副材料的研究进展,对纳米粒子填充后的减摩抗磨机理进行探讨,提出“滚珠轴承”、膜润滑和界面束缚作用三个观点,分析该项研究在纳米粒子分散技术和纳米粒子与PTFE基体界面处理上存在的不足,并结合复合材料界面处理机理、PTFE表面粘结机理和纳米粒子分散机理,提出一些新的纳米粒子填充PTFE复合材料界面改性和分散技术的方法。     

Modeling nanostructured catalyst layer in PEMFC and catalyst utilization

A lattice model of the nanoscaled catalyst layer structure in proton exchange membrane fuel cells (PEMFC) was established by Monte Carlo method. The model takes into account all the four components in a typical PEMFC catalyst layer: platinum (Pt), carbon, ionomer and pore. The elemental voxels in the lattice were set fine enough so that each average sized Pt particulate in Pt/C catalyst can be represented. Catalyst utilization in the modeled catalyst layer was calculated by counting up the number of facets of Pt voxels where “three phase contact” are met. The effects of some factors, including porosity, ionomer content, Pt/C particle size and Pt weight percentage in the Pt/C catalyst, on catalyst utilization were investigated and discussed.     

Effect of Micro- and Nano-Particle Fillers at Low Percolation Threshold on the Dielectric and Mechanical Properties of Polyurethane/Copper Composites

Polymeric composites based on polyurethane (PU) as the matrix and copper (Cu) particles as the filler were prepared by using solution casting. The effects of micro- and nano-particles size and content on the dielectric and mechanical properties depend upon the interface between metal filler and polymeric matrix. The dispersion of the fillers within the polymeric matrix was investigated by scanning electron microscopy (SEM). The SEM results showed a relatively homogeneous dispersion for the micro-particle size and the existence of the aggregation and poor compatibility for the nano-particle size. Differential scanning calorimetric measurements showed that the glass transition temperature (Tg) in case of micro-particles is quite similar to that of the neat PU, but the increase in Tg was observed when nano-particles were used. The dielectric properties of the composites as a function of the filler concentration and filler size was investigated in the frequency range of 100?Hz–10?kHz, showing an increase in dielectric constant with increasing filler content. This increase was more significant when using the nano-particles. The mechanical properties of the composites were obtained by using a tensile tester (ASTM D412). The tensile modulus generally increased with increasing Cu content, but the extent of increase was lower in case of micro-particles. The tensile strength of composite filled with nano-particle slowly decreased when filler content increased, while there was a significant in case of micro-particle as fillers. In addition, the elongation at break decreased with increasing Cu content, but the effect was more significant when micro-particle were employed. AFM image was used to investigate a topology of the tensile fractured surface, showing the mechanism of failure of the composites.     

Optimization of cathode catalyst layer for direct methanol fuel cells: Part II: Computational modeling and design

The cathode catalyst layer in direct methanol fuel cells (DMFCs) features a large thickness and mass transport loss due to higher Pt loading, and therefore must be carefully designed to increase the performance. In this work, the effects of Nafion loading, porosity distribution, and macro-pores on electrochemical characteristics of a DMFC cathode CL have been studied with a macro-homogeneous model, to theoretically interpret the related experimental results. Transport properties in the cathode catalyst layers are correlated to both the composition and microstructure. The optimized ionomer weight fraction (22%) is found to be much smaller than that in H₂ polymer electrolyte fuel cells, as a result of an optimum balance of proton transport and oxygen diffusion. Different porosity distributions in the cathode CLs are investigated and a stepwise distribution is found to give the best performance and oxygen concentration profile. Influence of pore defects in the CLs is discussed and the location of macro-pores is found to play a dual role in affecting both oxygen transport and proton conduction, hence the performance. The reaction zone is extended toward the membrane side and the proton conduction is facilitated when the macro-pores are near the gas diffusion layer.     

Metallic nano-particles for trapping light

We study metallic nano-particles for light trapping by investigating the optical absorption efficiency of the hydrogenated amorphous silicon thin film with and without metallic nano-particles on its top. The size and shape of these nano-particles are investigated as to their roles of light trapping: scattering light to the absorption medium and converting light to surface plasmons. The optical absorption enhancement in the red light region (e.g., 650nm) due to the light trapping of the metallic nano-particles is observed when a layer of metallic nano-particle array has certain structures. The investigation of the light with incident angles shows the importance of the coupling efficiency of light to surface plasmons in the metallic nano-particle light trapping.

PACS

73.20.Mf, 42.25.s, 88.40.hj     

Variations in interfacial properties during cell conditioning and influence of heat-treatment of ionomer on the characteristics of direct methanol fuel cells

Variations in interfacial properties in the anode catalyst layer during cell conditioning were characterized, and influence of the heat-treatment of ionomer on the characteristics of direct methanol fuel cells was investigated in this work. The anode catalyst layer was made by mixing a solvent-substituted Nafion solution with unsupported Pt/Ru black and curing the mixture in an oven with an inert environment. Materials characterization (SEM and optical microscopy) and electrochemical characterization (cell polarization, anode polarization, electrochemical impedance spectroscopy, and CO-stripping cyclic voltammetry) were performed. During cell conditioning, the enhanced kinetics of MeOH electrochemical oxidation and severe limiting current phenomenon are due to the combination of variations in interfacial properties and swelling of ionomer in the anode catalyst layer over time. Ru oxides at the catalyst surface are reduced continuously during cell conditioning. The nearly constant integrated areas under the CO-stripping CV peaks and broadened peak shapes indicate a stable number of Pt/Ru bimetallic alloy surface sites, yet the surface composition distribution is broadened. Heat-treatment influences ionomer crystallinity, altering its swelling behavior and hence affecting the characteristics of the direct methanol fuel cell (DMFC) anode.     

乙二醇稳定的Pt/C催化剂的制备与表征

以乙二醇(EG)兼作溶剂和稳定剂,分别通过NaBH4和EG还原法制备了高度细化与分散的Pt/C催化剂,对其形貌、组成、结构和电化学活性比表面等进行了表征比较,并测试了它们对甲醇与乙醇电催化氧化的活性. 结果表明,2种催化剂中,Pt均为面心立方结构,粒径小且分布窄,在炭黑载体上分散均匀,单位质量Pt对甲醇与乙醇电催化氧化的活性相当;NaBH4还原法所制Pt/C催化剂中Pt0和Pt(220)晶面含量更高,Pt对甲醇与乙醇电催化氧化的峰电流密度分别为0.68与0.67 mA/cm2,分别是EG还原法所制Pt/C催化剂的1.2倍;2种催化剂对甲醇与乙醇电催化氧化的活性均与商品E-TEK催化剂相当.     

片状氢氧化镁晶种的制备研究

研究了在极稀溶液中制备氢氧化镁晶种时,反应物用量、pH值、分散剂等因素对所制备的氢氧化镁晶种的晶体结构、形态及粒径的影响。用X-射线衍射仪（XRD）表征晶体结构,晶种在体系中的分散性及粒径大小由纳米粒度及电位分析仪来表征,通过透射电镜（TEM）观测晶体形貌。最终制得了粒径约为4．5nm左右的片状结构的氢氧化镁晶种。     

纳米颗粒分散模型及其在纳米CaCO3悬浮液制备中的应用

文章以简单立方体堆积模型作为纳米颗粒在分散体系中的分散模型,分析了纳米颗粒的体积百分含量和质量百分含量与纳米颗粒的颗粒间距离和粒径的关系,制备了1 wt%,2 wt%,10 wt%的纳米CaCO3悬浮液。透射电镜显示纳米CaCO3分散均匀,未出现团聚。模型及实验结果表明颗粒间距离为粒径的2-3倍时,纳米颗粒分散均匀并保持稳定。对于平均粒径为40 nm的纳米CaCO3,悬浮液的最佳质量百分含量为10%左右。     

Improving hydrogen storage/release properties of magnesium with nano-sized metal catalysts as measured by tapered element oscillating microbalance

An effective catalyst doping method was developed for directly depositing nano-particles of various metal catalysts (palladium, platinum and ruthenium) on the outer surface of magnesium powders through a wet chemistry process. The catalyst-doped magnesium was characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDS). Catalysts of nano-meter size were uniformly deposited on the outer surface of the magnesium particles. The hydrogen storage and hydrogen release properties of magnesium and catalysts-doped magnesium were measured in situ by the tapered element oscillating microbalance (TEOM), and also by the volumetric method. Both the hydrogen absorption and hydrogen release kinetics of magnesium were significantly improved by doping the nano-particle catalysts. Among the three metals-doped and examined, palladium showed the best catalytic effect. Upon doping 0.5 mol% nano-particle palladium, the hydrogen absorption and hydrogen release rates of magnesium increased 1 and 14 times, respectively, as revealed by the dynamic measurement of storage/release by TEOM, which indicated a strong catalytic effect.     

Effect of Preparation Conditions of Pt/C Catalysts on Oxygen Electrode Performance in Proton Exchange Membrane Fuel Cells

Supported Pt/C catalyst with 3.2 nm platinum crystallites was prepared by the impregnation—reduction method. The various preparation conditions, such as the reaction temperature, the concentration of precursor H₂PtCl₆ solution and the different reducing agents, and the relationship between the preparation conditions and the catalyst performance were studied. The carbon black support after heat treatment in N₂ showed improved platinum dispersion. The particle size and the degree of dispersion of Pt on the carbon black support were observed by transmission electron microscopy (TEM). The crystal phase composition of Pt in the catalyst was determined by X-ray diffraction (XRD). The surface characteristics of the carbon black support and the Pt/C catalyst were studied by X-ray photoelectron spectroscopy (XPS). The electrochemical characteristics of the Pt/C catalysts were evaluated from current—voltage curves of the membrane electrode assembly (MEA) in a proton exchange membrane fuel cell.     

负载途径对炭气凝胶载Pt催化剂性能的影响

选择孔隙率高、孔隙尺寸小、比表面积大、电导率高、孔径大小及分布可控的新型炭材料炭气凝胶(CAs)为载体,通过在成品CAs表面负载和在CAs制备过程中同步负载两种途径制备了CAs负载的Pt催化剂,利用XRD、TEM、ICP及电化学循环伏安测试等手段对比讨论了负载途径对CAs载Pt催化剂物化性能及甲醇氧化催化活性的影响。结果表明,同步负载虽然可以简化制备工艺,但所制得的催化剂样品中催化活性组分Pt颗粒大、团聚严重、分散性差、负载量低,而且有许多Pt颗粒被包裹在CAs内部,因而导致了催化剂的有效利用率和催化性能降低。相反,通过在成品CAs表面负载的方法可以制得Pt颗粒小、分散均匀、实际负载量接近理论设计的高性能催化剂。     

Improved electrocatalytic performance of Pd nanoparticles with size-controlled Nafion aggregates for formic acid oxidation

This work focuses on the effect of Nafion ionomer aggregation within the Pd catalytic electrode on electrocatalytic oxidation of formic acid. By a simple heat-treatment, the particle sizes of both Nafion ionomers in Nafion solution and congeries formed between Pd nanoparticles and Nafion ionomers in the catalyst ink decrease and their size distribution becomes narrow. Heat treatment of the catalyst ink leads to a significantly enhanced catalytic activity for formic acid oxidation on the Pd catalytic electrode. Such an enhancement is ascribed to the improvement in catalyst utilization verified by CO stripping voltammograms and to the decrease in charge-transfer resistance of oxidation reaction confirmed by impedance analysis. Typical XPS analysis shows that there are at least two kinds of Pd and S surface states within the catalytic electrode with the ink pre-heated at 25 °C and only one kind of Pd and S surface state at 80 °C, indicative of a better dispersion between Pd nanoparticles and smaller Nafion ionomers at a higher heat treatment temperature. Furthermore, the decrease in congeries size within the anode catalyst ink leads to a significant decrease in Nafion loading within the catalytic layer and a remarkable improvement in direct formic acid fuel cell's performance.     

Low temperature CO pulse adsorption for the determination of Pt particle size in a Pt/cerium-based oxide catalyst

Unexpectedly large amounts of CO adsorption have resulted from a pulse adsorption experiment at 323 K, giving about 300% Pt dispersion in a Pt/cerium-based oxide catalyst. An in situ diffuse reflectance infrared Fourier transform spectroscopic investigation on a Pt/cerium-based oxide during CO adsorption has revealed that carbonate species on the cerium oxide surface are responsible for the unrealistically large CO adsorption at 323 K, as a result of CO spillover. Lowering the temperature to 195 K considerably diminished the amount of CO adsorption. The size of the Pt particles in the Pt/cerium-based oxide catalyst was determined by CO pulse adsorption at 195 K and showed good agreement with the particle size determined by X-ray diffraction and low energy ion scattering. This indicates that CO pulse adsorption at 195 K is a useful technique to reliably estimate the Pt particle size in a Pt/cerium-based oxide catalyst.