Mass Transport Paths Research Articles

Overcoming the Limitation of Ionomers on Mass Transport and Pt Activity to Achieve High-Performing Membrane Electrode Assembly.

The membrane electrode assembly (MEA) is one of the critical components in proton exchange membrane fuel cells (PEMFCs). However, the conventional MEA cathode with a covered-type catalyst/ionomer interfacial structure severely limits oxygen transport efficiency and Pt activity, hardly achieving the theoretical performance upper bound of PEMFCs. Here, we design a noncovered catalyst/ionomer interfacial structure with low proton transport resistance and high oxygen transport efficiency in the cathode catalyst layer (CL). This noncovered interfacial structure employs the ionomer cross-linked carbon particles as long-range and fast proton transport channels and prevents the ionomer from directly covering the Pt/C catalyst surface in the CL, freeing the oxygen diffusion process from passing through the dense ionomer covering layer to the Pt surface. Moreover, the structure improves oxygen transport within the pores of the CL and achieves more than 20% lower pressure-independent oxygen transport resistance compared to the covered-type structure. Fuel-cell diagnostics demonstrate that the noncovered catalyst/ionomer interfacial structure provides exceptional fuel-cell performance across the kinetic and mass transport-limited regions, with 77% and 67% higher peak power density than the covered-type interfacial structure under 0 kPagauge of oxygen and air conditions, respectively. This alternative interfacial structure provides a new direction for optimizing the electrode structure and improving mass-transport paths of MEA.

Stabilizing graphene oxide with complementary nanosheets for robust membranes with high flux

Laminar assembles of graphene oxide (GO) are receiving tremendous attention for their interesting nanofluidic behaviors, but suffer from structural instability and frustrated mass transfer. Herein, we stabilize GO assembles by “complementary nanosheets”, and thus obtain highly robust and permeable membranes. Self-exfoliated nanosheets of ionic covalent organic frameworks (iCONs) are intercalated into GO nanosheets. GO and iCONs are physicochemically complementary as they carry opposite charges and hydrogen bonds acceptors and donors, respectively. Upon mixing, they form laminar assembles locked by electrostatic attraction and hydrogen bonds, suppressing swelling in water. Moreover, simulation and experimental results reveal that iCONs expands the interlayer distance and provides additional mass-transport paths. Consequently, the GO/iCONs membranes exhibit ultra-high water permeance up to 11-fold larger than that of pure GO membranes. This work provides a controllable route to stabilize GO assembles, and develops new membranes using complementary two-dimensional structures as building blocks.

Sintering behavior and mechanism of Bi‐Zn‐Nb‐O microwave dielectric ceramics

AbstractBi‐Zn‐Nb‐O‐based microwave dielectric ceramics are promising candidates for wireless communication. Investigations have concentrated on their fabrication methods and doping modification. In this work, the densification mechanism and grain growth kinetics of the Bi‐Zn‐Nb‐O‐based microwave dielectric ceramics were explored. A significantly wide sintering temperature window (800–1000°C) of the monoclinic Bi2Zn2/3Nb4/3O7 (BZN) was found. High‐density stacking faults were observed in BZN grains at the initial stage of sintering, which could act as paths for mass transport, thus remarkably accelerating the densification process and decreasing the densification temperature. On the other hand, BZN possesses a high grain growth exponent and activation energy, which indicates a slow grain growth rate and a high threshold for abnormal grain growth, corresponding to the wide sintering temperature window. Moreover, the BZN ceramics display good microwave dielectric properties stability over a wide temperature range (25–450°C).

Functionalized open-ended vertically aligned carbon nanotube composite membranes with high salt rejection and enhanced slip flow for desalination

Vertically aligned carbon nanotubes (VACNTs) have shown unexpectedly high liquid flux. In this research, we investigated the effect of functionalization on the performance of VACNTs/polystyrene membranes, in which densely packed VACNTs with 2–11 nm inner diameters and areal density around 3.6 × 1010 cm−2 act as the only mass transport paths. The measured water permeance in the open-ended VACNTs membrane revealed an enhancement factor of three orders of magnitude higher than the continuum-based no-slip model. Different functional groups including carboxylic, glycine, octadecylamine, and 1,6-diaminohexane were attached to the VACNTs tips through carbodiimide coupling method, which were characterized by ATR-FTIR. Salt exclusion as a function of ion valence ratios, ionic strength, and pH value of the feed solution was studied. The results were strongly supported by Donnan exclusion mechanism. Interestingly, an increase in the concentration of salts from 500 to 5000 ppm improved the rejection, which is in contrast with other reports. The glycine functionalization was repeated two to three times to find the effect of the functional groups length as well as charge density on the rejection. pH-dependent measurements of glycine functionalized membrane demonstrated that both carboxyl and amide groups are responsible for the ion rejection characteristics of the membrane.

An acid-base molecular assembly strategy toward N-doped Mo2C@C nanowires with mesoporous Mo2C cores and ultrathin carbon shells for efficient hydrogen evolution

Molybdenum carbides are promising electrocatalysts for the hydrogen evolution reaction (HER). Rational design of morphology, composition and interfacial structure in Mo2C materials is essential to enhance their HER performance. Herein, an acid-base molecular assembly strategy is demonstrated for the synthesis of novel N-doped Mo2C@C core–shell nanowires (NWs) composed of mesoporous Mo2C cores with interconnected crystalline walls and ultrathin carbon shells. The strong interactions between the two precursors, adenine (Ade) and phosphomolybdic acid (PMA), lead to the formation of inter-molecular hybrid NWs during a hydrothermal process. The subsequent pyrolysis leads to confined growth of crystalline Mo2C NWs with inter-crystal mesopores (5 ~ 10 nm), formation of ultrathin carbon shells (~1.5 nm in thickness), and effective N doping. Such a structure architecture can provide abundant active sites, fast and diverse mass and electron transport paths, as well as stable reaction interfaces. The typical N-doped Mo2C@C NWs exhibit high HER performance with a low overpotential of 136 mV at 10 mA cm−2, a small Tafel slop of 58 mV dec-1, excellent durability and outstanding anti-poisoning performance against CO and H2S gases. Furthermore, the influences of several important factors, including the pyrolysis temperature, hydrothermal temperature and precursor mass ratio, on the morphology, composition and structural configuration of the resulted materials are elucidated and correlated with their HER performance. This work may provide a general strategy for the synthesis of other nanoscale metal carbides for various catalytic applications.

Highly Reversible Aqueous Zn‐MnO2 Battery by Supplementing Mn2+‐Mediated MnO2 Deposition and Dissolution

AbstractRechargeable Zn‐MnO2 batteries are boosted by the reversible intercalation reactions in mild aqueous electrolytes, but they still suffer from cathode degradation. Herein, Zn‐MnO2 batteries with high durability and high energy density are achieved by supplementing MnO2 deposition and dissolution in a mild aqueous electrolyte. The main finding is that adjusting Mn2+ concentration to a critical range enables a reversible MnO2/Mn2+ redox conversion without the involvement of oxygen evolution. This can recycle the by‐products from MnOOH disproportionation (MnOOH → MnO2 + Mn2+), resulting in a battery with extremely high durability (16 000 cycles without obvious capacity fading), high energy density (602 Wh kg−1 based on the active mass of the cathode), and high‐rate capacity (430 mAh g−1 at 19.5 A g−1). The utilization of a 3D carbon nanotube foam skeleton can accommodate the volume change during MnO2 deposition/dissolution and provide paths for efficient charge and mass transport. This work provides a feasible way to push the development of Zn‐MnO2 batteries in mild aqueous electrolytes.

Isotopic composition of total gaseous mercury at a high-altitude tropical forest site influenced by air masses from the East Asia continent and the Pacific Ocean

Gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM), particulate-bound mercury (PBM), and the isotopic composition of total gaseous mercury (TGM = GEM + GOM) were measured at the Lulin Atmospheric Background Station (LABS), a mountain background site downwind of the East Asia continent, from February 2015 to January 2016. Mean (±S.D.) concentrations of GEM, GOM, and PBM were 1.54 ± 0.34 ng m−3, 14.0 ± 27.8 pg m−3, and 2.6 ± 3.7 pg m−3, respectively. Concentration-Weighted Trajectory (CWT) analysis indicated that East and coastal China were the main source regions of elevated GEM observed at LABS. Weekly integrated TGM showed significant variations in δ202HgTGM (−0.42 to 0.41‰) and Δ199HgTGM (−0.31 to −0.13‰). Both δ202HgTGM and Δ199HgTGM showed clear seasonal variations, but the seasonal pattern of δ202HgTGM was different from that of Δ199HgTGM. The lowest mean δ202HgTGM was observed in spring (−0.15‰), while the lowest mean Δ199HgTGM was observed in summer (−0.24‰). A significant positive correlation (R2 = 0.38, p < 0.01) existed between δ202HgTGM and normalized difference vegetation index (NDVI), suggesting vegetation activity was an important factor controlling the seasonal variation of δ202HgTGM. In contrast, air mass transport path, which influences the source region of TGM to LABS, was likely the driver for the seasonal variations of Δ199HgTGM. Furthermore, results from the air mass classification showed distinguishable Δ199HgTGM signature for different air mass source regions, with air masses from South China Sea and western Pacific Ocean being characterized by more negative Δ199HgTGM (−0.25 ± 0.04‰). Results of this research demonstrated a clear difference in isotopic compositions of atmospheric Hg between continental and marine air masses.

A Platelet Graphitic Nanofiber‐Carbon Nanotube Hybrid for Efficient Oxygen Evolution Reaction

AbstractCarbon nanostructures with a high‐density of active sites are needed as catalysts for the oxygen evolution reaction (OER). Here we report a platelet graphitic nanofiber‐carbon nanotube (PGNF‐CNT) hybrid electrocatalyst prepared by chemical vapor deposition. The material is composed of interconnected CNTs and nanofibers consisting of graphitized carbon layers stacked at different angles to the fiber axis with a large number of exposed edges. These unique structural characteristics give the hybrid a high density of active sites, fast electron transfer and a large number of mass transport paths. As a result, the material has an excellent electrocatalytic activity for OER with an overpotential of 0.28 V at a current density of 10 mA cm−2, which is much better than those of Ir/C and previously‐reported carbonaceous catalysts.

Toluene permeation through solid polymer electrolyte during toluene direct electro-hydrogenation for energy carrier synthesis

For the transportation and storage of hydrogen energy by toluene-methylcyclohexane organic hydride systems, the performance of toluene direct electro-hydrogenation electrolyzers using solid polymer electrolytes (SPE) for the membrane and ionomers must be improved. In this study, we investigate the effect of the equivalent weight (EW) and the side-chain length of the SPE on toluene permeability and electrochemical performance. The SPE membrane with a lower EW and a shorter side chain shows a lower toluene permeability. The lower EW, with higher acidity, decreases toluene solubility by salting-out in the hydrophilic mass transport path. The shorter side chain, which lowers the water uptake, suppresses toluene permeation by decreasing the cross-sectional area of the mass transport path. In the electrochemical measurement using the electrolyzer, the combination of SPEs with long side chain and high EW for both the membrane and ionomer shows the best performance. For long-term anode deactivation by membrane-permeated toluene, the membrane should be designed to reduce toluene permeability. Meanwhile, the ionomer has the advantage of higher EW and long side chain.

Temporal changes in atmospheric mercury concentrations at a background mountain site downwind of the East Asia continent in 2006–2016

Atmospheric mercury (Hg) has been monitored at the Lulin Atmospheric Background Station (LABS) in Taiwan since April 2006 and is still continuing. Here we reported the trend in gaseous elemental Hg (GEM) concentrations at LABS between April 2006 and December 2016, before the Minamata Convention on Mercury entered into force in 2017. Previous research indicated nighttime (0–8 am) data collected at LABS are better representative of regional influence. Therefore, only nighttime GEM data were used for trend analysis. A significant decreasing trend in GEM at a rate of −1.5% yr−1 (−0.022 ng m−3 yr−1, p < 0.01) was found, comparable to the decreasing trends observed in Europe, North America, South Africa, and over the North Atlantic Ocean. Five major GEM source regions to the LABS were identified, including northern Indochina Peninsula, China, Northeast Asia, the Pacific Ocean, and South China Sea. Significant decreasing trends in GEM were found for air masses coming from northern Indochina Peninsula (−0.042 ng m−3 yr−1, −2.6% yr−1, p < 0.01), China (−0.041 ng m−3 yr−1, −2.4% yr−1, p < 0.01), Northeast Asia (−0.031 ng m−3 yr−1, −2.0% yr−1, p < 0.05), and the Pacific Ocean (−0.022 ng m−3 yr−1, −1.7% yr−1, p < 0.05). Decreasing GEM trend (−0.020 ng m−3 yr−1, −1.5% yr−1), but insignificant (p > 0.05), was also found for air masses coming from South China Sea. The decreasing trends observed with air from the Pacific Ocean and South China Sea indicated declining background GEM concentrations in Northern Hemisphere. Decrease in GEM concentrations at the LABS was in agreement with the reduction in atmospheric Hg export from the East Asia continent caused by changes in Hg emission quantity and speciation, and temporal and spatial distribution in emission sources that have been suggested by recent research. Additionally, changes in the frequency distribution of air mass origins and transport paths may also contribute to the changes in GEM concentrations at LABS.

Hollow Multishelled Structures for Promising Applications: Understanding the Structure-Performance Correlation.

The unique structural features of hollow multishelled structures (HoMSs) endow them with abundant beneficial physicochemical properties including high surface-to-volume ratio, low density, short mass transport length, and high loading capacity. As a result, HoMSs have been considered as promising candidates for various application areas including energy storage, electromagnetic wave (EW) absorption, catalysis, sensors, drug delivery, etc. However, for a long time, the general and controllable synthesis of HoMSs has remained a great challenge using conventional soft-templating or hierarchical self-assembly methods, which severely limits the development of HoMSs. Fortunately, the sequential templating approach (STA), which was first reported by our group and further developed by others, has been proven to be a versatile method for HoMS fabrication. By using the STA and through accurate physical and chemical manipulation of the synthesis conditions, the diversity of the HoMS family has been enriched in both compositional and geometrical aspects. Benefiting from the flourishing of synthetic methodology, various HoMSs have been fabricated and showed application prospect in diverse areas. However, the structure-performance correlation remained obscure, which hinders the design of optimal HoMSs to achieve the best application performance. This Account aims to explore the correlation between HoMS structural characteristics and their application performance. We first briefly summarize the achievements in the compositional and geometrical manipulation of HoMSs by physically and chemically tuning the synthesis process. Then, we systematically discuss the effect of structural engineering on optimizing performance in various application areas, especially for energy storage, EW absorption, catalysis, sensors, and drug delivery. Specifically, HoMSs with multiple thin shells can provide numerous active sites for energy storage, leading to a higher volumetric energy density than their single-shelled counterparts. The high shell porosity permits electrolyte access to the interior of HoMSs, along with shortened mass transport path through the thin shells, resulting in a high power density. The adequate inner cavity effectively buffers the ion-insertion strain, leading to prolonged cycling stability. For EW absorption, HoMSs with high surface-to-volume ratio can provide many sites for EW-sensitive material loading. The multiple separated shells with small intershell space enable multiple EW reflection and scattering, thus improving EW absorption efficiency. For catalysis and sensors, the increased reaction sites along with the facilitated transport of reactants and products can enhance the activity and sensitivity. The selectivity can be improved by optimizing the pore structure and hydrophobic or hydrophilic properties of the shells. Also the stability is improved with inner shells being protected by exterior ones. For drug delivery, the increased exposed sites and the inner cavity improve the drug loading capacity. The adjustable pore structure along with accurately designed shell composition leads to well-targeted drug release responding to different stimuli at different targeting sites. The multiple separated shells endow HoMSs with sustained drug release step-by-step from inside to outside. These in-depth understandings on the structure-performance correlation can guide the design of ideal HoMSs to satisfy the specific requirements for different application areas, thus further improving the application performance and expanding the HoMSs family.

Honeycomb‐Inspired Heterogeneous Bimetallic Co–Mo Oxide Nanoarchitectures for High‐Rate Electrochemical Lithium Storage

AbstractNanostructure engineering has been proved to be an efficient approach for improving electrochemical properties for energy storage by accommodating volume changes, facilitating rapid mass transport paths, and enlarging ion storage sites and interfaces. The well‐designed fine nanostructures, unfortunately, are usually destroyed during long‐term cycles and ultimately lose their structural advantages. Herein, stimulated by the extraordinary structural stability, robust mechanical properties, and salient ventilation capacity of natural honeycomb species, bioinspired heterogeneous bimetallic Co–Mo oxide (CoMoOx) nanoarchitectures assembled from 2D nanounits are successfully fabricated via a molybdenum‐mediated self‐assembly strategy for improving the rate capability of electrochemical lithium storage devices. Owing to the robust structural stability and the ultrathin 2D wall structure, CoMoOx nanostructures present well‐maintained honeycomb‐like structure, rapid capacitive insertion–desertion behaviors, and thus significantly enhanced lithium ion storage performance at high rates (5.0 A g−1). It is also revealed that the reversible transition of cobalt and molybdenum phases closely associated with the ultrathin 2D wall structures greatly contribute to the outstanding electrochemical lithium storage performances. This attractive integration of structural and functional advantages achieved by learning from nature offers new insights into the design of cost‐effective electrode materials for high‐performance energy devices.

Computational prediction of nanoscale transport characteristics and catalyst utilization in fuel cell catalyst layers by the lattice Boltzmann method

In the present study, a three-dimensional lattice Boltzmann model based on the quasi-random nanostructural model is proposed to evaluate the mass transport properties and catalyst utilization of fuel cell catalyst layers in pursuance of catalyst performance improvement. A series of catalyst layers is randomly generated with statistical significance at the 95% confidence level to reflect the heterogeneity of the catalyst layer nanostructures. The nanoscale gas transport phenomena inside the catalyst layers are simulated by the D3Q19 (i.e., three-dimensional, 19 velocities) lattice Boltzmann method, and the corresponding mass transport characteristics are mathematically modeled in terms of structural properties. Considering the nanoscale reactant transport phenomena, a transport-based effective catalyst utilization factor is defined and statistically analyzed to determine the structure-transport influence on catalyst utilization. The tortuosity estimation results clearly show that the classic Bruggeman equation underestimates the tortuosity of the catalyst layers and should be modified for PEFC applications. Subsequently, the effective mass diffusion coefficient is calculated by applying the tortuosity factors to the Knudsen diffusion coefficient in the catalyst layers, and it shows good agreement with published experimental data. These results indicate that Knudsen diffusion is the dominant mass transfer mechanism for fuel cell catalyst layers and that the pre-estimated tortuosity accurately reflects the mass transfer phenomena in the catalyst layers. Furthermore, catalyst utilization can be affected by excessive Pt/C catalyst loading due to the lack of pore interconnections, and it is significantly limited by the substantive reactant mass transport path inside the fuel cell catalyst layers.

Se-Ni(OH)2-shelled vertically oriented NiSe nanowires as a superior electrocatalyst toward urea oxidation reaction of fuel cells

The direct urea fuel cell holds great promise for energy-sustainable developments and mitigating water contamination but it still faces a great challenge to overcome the sluggish kinetics of the urea oxidation reaction (UOR). In this work, we report Se-Ni(OH)2-shelled vertically oriented NiSe nanowires on a Ni foam as an electrocatalyst toward UOR, showing a low potential of 0.366V vs. SCE to drive 100mAcm−2 in alkaline solution and out-performing all the reported non-noble-metal UOR catalysts up to date. Experimental results and theoretical calculation reveal that the vertically and distantly arranged nanowires with highly porous structure produce high mass transport paths for urea to fully access the reaction sites, the NiSe core offers high conductivity for fast electron transport, and the Se-Ni(OH)2 shell provides large amount of active catalytic sites while lowering the CO2 adsorption/desorption barrier than Ni(OH)2 for fast reaction kinetics. This work opens up an exciting new direction to design electrocatalysts for high performance fuel cells and other energy applications.

Using ground-penetrating radar, topography and classification of vegetation to model the sediment and active layer thickness in a periglacial lake catchment, western Greenland

Abstract. The geometries of a catchment constitute the basis for distributed physically based numerical modeling of different geoscientific disciplines. In this paper results from ground-penetrating radar (GPR) measurements, in terms of a 3-D model of total sediment thickness and active layer thickness in a periglacial catchment in western Greenland, are presented. Using the topography, the thickness and distribution of sediments are calculated. Vegetation classification and GPR measurements are used to scale active layer thickness from local measurements to catchment-scale models. Annual maximum active layer thickness varies from 0.3 m in wetlands to 2.0 m in barren areas and areas of exposed bedrock. Maximum sediment thickness is estimated to be 12.3 m in the major valleys of the catchment. A method to correlate surface vegetation with active layer thickness is also presented. By using relatively simple methods, such as probing and vegetation classification, it is possible to upscale local point measurements to catchment-scale models, in areas where the upper subsurface is relatively homogeneous. The resulting spatial model of active layer thickness can be used in combination with the sediment model as a geometrical input to further studies of subsurface mass transport and hydrological flow paths in the periglacial catchment through numerical modeling. The data set is available for all users via the PANGAEA database, doi:10.1594/PANGAEA.845258.

Hierarchical nanoporosity enhanced reversible capacity of bicontinuous nanoporous metal based Li-O2 battery.

High-energy-density rechargeable Li-O2 batteries are one of few candidates that can meet the demands of electric drive vehicles and other high-energy applications because of the ultra-high theoretical specific energy. However, the practical realization of the high rechargeable capacity is usually limited by the conflicted requirements for porous cathodes in high porosity to store the solid reaction products Li2O2 and large accessible surface area for easy formation and decomposition of Li2O2. Here we designed a hierarchical and bicontinuous nanoporous structure by introducing secondary nanopores into the ligaments of coarsened nanoporous gold by two-step dealloying. The hierarchical and bicontinuous nanoporous gold cathode provides high porosity, large accessible surface area and sufficient mass transport path for high capacity and long cycling lifetime of Li-O2 batteries.

Statistical prediction of fuel cell catalyst effectiveness: Quasi-random nano-structural analysis of carbon sphere-supported platinum catalysts

In this study, a three-dimensional material network model is developed to visualize the nanoscale structures of carbon sphere-supported platinum (Pt/C) catalysts and to examine the effective transport paths to optimize the performance of randomly disordered, ternary-phase fuel cell catalysts. The catalyst layer domain is modeled using a quasi-random stochastic Monte Carlo-based method that utilizes random number generation processes. Successful interconnections of the three catalyst components are identified, and the catalyst effectiveness is defined to statistically estimate the fraction of the fuel cell catalysts that are utilized. Various fuel cell catalyst compositions are simulated to elucidate the effects of the electron, ion, and mass transport paths on the catalyst effectiveness. The statistical data show that at low ionomer contents, the accessible pore ratio is maximized, enhancing mass transport, and the effective ionomer configuration therefore significantly affects the catalyst effectiveness. In contrast, at high ionomer volume fractions, the ionomers form agglomerate chains that effectively transport ions, whereas the average accessible pore ratios are relatively low. More importantly, this study reveals that the maximum effectiveness depends strongly on the accessible pore ratio and the optimal ionomer volume fraction is inversely proportional to the Pt/C volume fraction.

Integrated Statistical and Nano-Morphological Study of Effective Catalyst Utilization in Vertically Aligned Carbon Nanotube Catalyst Layers for Advanced Fuel Cell Applications

In this study, a three-dimensional quasi-random nano-structural model based on the Monte Carlo method is proposed to evaluate effective catalyst utilization in fuel cell catalyst layers. Reflecting the intrinsically inhomogeneous nano-morphology of fuel cell catalyst layers, statistical analyses are performed to statistically compare various types of fuel cell electrodes to gain fundamental insight into the effects of morphological nano-structures on the catalyst utilization. For the detailed morphological analysis, a series of multi-component distributions at a 95% confidence level is randomly generated to deduce the statistical variations of the effective transport paths of ternary catalyst components. The statistical nano-morphology model is validated against published experimental data with good agreement. Subsequently, the morphological configuration of the vertically aligned carbon nanotube (VACNT) catalyst layers is simulated to determine the principal nano-design factors for improved catalyst utilization in the same stochastic manner which is used for carbon-supported catalyst layers. In the VACNT catalyst layers, all of the Pt/CNTs are successfully interconnected and therefore, all solid conducting carbon nanotubes can be utilized as electric current paths. Numerical results reveal that despite the relatively poor interconnections of the ion and mass transport paths, the statistical average catalyst utilization of VACNT is significantly improved when compared to conventional catalyst layers. It is also found that the ionic current paths of the VACNT catalyst layers can be considered as a catalyst utilization determining factor and an adequate amount of ionomers are necessary to promote successful ionic conduction. Finally, the average catalyst utilizations for both the regularly patterned in-line and staggered VACNT catalyst layers are compared with results for randomly distributed VACNTs.

A model for statistical electromigration simulation with dependence on capping layer and Cu microstructure in two dimensions

A model has been developed to simulate electromigration degradation in an interconnect segment in two dimensions using finite differences. The model was deployed on a parallel computer to statistically assess the lifetimes. The simulation takes into account the diffusion paths for electromigration mass transport along the grain boundaries and the capping layer. The microstructure is generated with a Monte Carlo algorithm, using a modified Potts model. Diffusivities along the grain boundaries and the capping layers were applied as multiples of a base diffusivity and were statistically scattered. The simulation results correlate well with electromigration tests.

연료전지 촉매층 내 촉매활성도에 대한 탄소지지 백금 촉매의 기하학적 비등방성 효과에 관한 연구

Transport phenomena of reactant and product are directly linked to intrinsic inhomogeneous random configurations of catalyst layer (CL) that consist of ionomer, carbon-supported catalyst (Pt/C), and pores. Hence, electrochemically active surface area (ECSA) of Pt/C is dominated by geometrical morphology of mass transport path. Undoubtedly these ECSAs are key factor of total fuel cell efficiency. In this study, non-deterministic micro-scale CLs were randomly generated by Monte Carlo method and implemented with the percolation process. To ensure valid inference about Pt/C catalyst utilization, 600 samples were chosen as the number of necessary samples with 95% confidence level. Statistic results of 600 samples generated under particular condition (20vol% Pt/C, 30vol% ionomer, 50vol% pore, and 20nm particle diameter) reveal only 18.2%~81.0% of Pt/C can construct ECSAs with mean value of 53.8%. This study indicates that the catalyst utilization in fuel cell CLs cannot be identical notwithstanding the same design condition.