Experimental measurements of effective diffusion coefficient of oxygen–nitrogen mixture in PEM fuel cell diffusion media

PEM fuel cells are increasingly designed to operate at high current densities. At these densities, mass transport limitations become very significant, but they are not well understood, with many modeling studies but few experimental observations. The use of accurate transport coefficients to simulate the mass transport at high current densities is crucial. In this study, experimental measurements have been carried out to determine the effective diffusion coefficient in the carbon paper gas diffusion layer that is commonly used in PEM fuel cells. It was found that almost all the existing theoretical models significantly overpredict the effective diffusion coefficient by as much as 4–5 times; thus, underestimating the transport limitations considerably. Further, the effects of temperature, Teflon treatment for hydrophobicity and porosity on the effective diffusion coefficient were investigated. It was found that temperature does not affect the overall diffusibility of the gas. The diffusibility is decreased with the increase of Teflon treatment and decrease in porosity. Further work on better understanding the diffusion process in the gas diffusion layer is under way.     

Verification of the Maxwell–Stefan theory for tracer diffusion in zeolites

Using the Maxwell–Stefan theory for diffusion we derive a simple formula to relate the tracer (i.e. self) diffusivity D¹ and Maxwell–Stefan (MS), or jump, diffusivity $\text{D}\text{–}$. The presence of the interchange coefficient $\text{D}\text{–}{}_{\mathrm{ij}}$ in the MS formulation causes the self diffusivity to be lower than the jump diffusivity. Assuming the interchange coefficient to be given by $\text{D}\text{–}$/F we derive:$\text{D}{}^1\text{=}\text{D}\text{–}\text{1+Fθ}$where F is a factor to take account of topology effects within the zeolite matrix.The validity of the MS formulation is established by performing kinetic Monte Carlo simulations for diffusion of methane, perfluoromethane, 2-methylhexane and iso-butane in silicalite. Furthermore, it is shown that the exchange coefficient $\text{D}\text{–}{}_{\mathrm{ij}}$ is a quantification of correlation effects during the hopping of molecules. For iso-butane, the isotherm inflection leads to a sharp inflection in the diffusion behaviour. The influence of molecular repulsive forces on the loading dependence of the jump and self-diffusivities is also discussed with the aid of published Molecular Dynamics simulations for methane.     

Capacitance–voltage profiling of deuterium passivation and diffusion in diamond Schottky diodes

Capacitance–voltage measurements have been performed on deuterated boron-doped natural type IIb diamond. They demonstrate the electrical passivation of the boron acceptors, which was manifested by a persistent decrease in capacitance after deuteration. Bias annealing experiments have been carried out to determine the charge state of the diffusing deuterium. The differences between annealing with or without bias lead to the conclusion that boron–deuterium complexes dissociate at about 750 K and that the released deuterium is positively charged.     

Experimental investigation of laminar LPG–H2 jet diffusion flame with preheated reactants

This paper presents an experimental investigation of the effect of H₂ addition on flame length, soot free length fraction (SFLF), flame radiant fraction, gas temperature and emission level in LPG–H₂ composite fuel jet diffusion flame for two preheated cases namely, (i) preheated air and (ii) preheated air and fuel. Results show that the H₂ addition leads to a reduction in flame length which may be caused due to an increased gas temperature. Besides this, the flame length is also observed to be reduced with increasing reactants temperature. The soot free length fraction (SFLF) increases as H₂ is added to fuel stream. This might have been caused by decrease in the C/H ratio in the flame and is favorable to attenuate PAH formation rate. Interestingly, the SFLF is observed to be reduced with increasing reactants temperature that may be due to reduction in induction period of soot formation caused by enhanced flame temperature. Moreover, the decreased radiant heat fraction with hydrogen addition is pertinent with the reduction in soot concentration level. The reduction in NO_x emission level with H₂ addition to the fuel stream is also observed. On the contrary, NO_x emission level is found to be enhanced significantly with reactant temperature that can be attributed to the increase in thermal NO_x through Zeldovich mechanism.     

Modelling of long-term diffusion–reaction in a bentonite barrier for radioactive waste confinement

Bentonites have been proposed as buffer material for barriers in geological disposal facilities for radioactive waste. This material is expected to fill up by swelling the void between the canisters containing the waste and the surrounding ground. However, the bentonite barriers may be submitted to changes of humidity, temperature variation, fluid interaction, mass transport, etc. This could modify the physico-chemical performance of the barrier, mainly on the interface with the steel container and with the geological barrier. The engineered barrier development necessitates thus the study of the physico-chemical stability of its mineral component as a function of time under the conditions of the repository in the long-term.The purpose of the present study was two-fold. Firstly, it was hoped to simulate the chemical transformations (geochemical and cation exchange reactions) coupled with diffusion of chemical-elements into the engineered barrier under repository conditions by applying a thermokinetic hydrochemical code (KIRMAT: Kinetic Reactions and Mass transport).Secondly, it was hoped to apply a simplified method to estimate the swelling capacity evolution by a volume balance in the fluid-saturated engineered barrier, considering that the decay of swelling capacity is directly proportional on the volume of transformed montmorillonite and, taking into account that it may be partially compensated by the volume of neo-formed swelling clays.The system modelled herein was considered to consist of 1-m thick zone of water-saturated engineered barrier. This non-equilibrated system was placed in contact with a geological fluid on one side, which was then allowed to diffuse into the barrier, while the other side was kept in contact with a source of metallic iron. Reducing initial conditions(P_O2 ≅ 0; Eh = − 200 mV) and a constant reaction temperature (100 °C) were considered.The results showed that the EB in contact with the geological fluid was highly transformed after 10,000 years, whereas the most significant chemical processes were illitization, cation exchange and saponization, extending up to 20 cm into the EB. Chemical transformations of minor importance in the EB were identified as well, such as a neo-formation of silicates (quartz, cristobalite), anhydrite, laumontite, magnetite and chlorite in the system.A simplified method based on volume balance showed that the swelling capacity of the bentonite barrier is slightly affected after 10,000 years of diffusion–reaction (D close to 1) because the volume of neo-formed swelling-clays is almost directly proportional to the volume of transformed Na/Ca-montmorillonite, except for a strong illitization and/or neo-formation of non-swelling clays. In the present study, this simple approach predicted that the decay of swelling capacity of the engineered barrier is drastically affected close to the geological barrier-engineered barrier interface. Out this zone the swelling capacity decay lies between 5% and 11%.     

Ultrafine-grained β-Sialon fabricated by two-stage sintering and study on diffusion toughening of Ti–22Al–25Nb

The ultrafine-grained β-Sialon ceramics were fabricated by spark plasma sintering at different temperatures with inorganic Al₂O₃–Y₂O₃ and Ti–22Al–25Nb intermetallic powder as composite additives. The research showed that β-Sialon ceramics achieve two-stage sintering densification. Al₂O₃–Y₂O₃ inorganic additives promoted the synthesis and densification of β-Sialon ceramics at 1125–1215°C. Ti–22Al–25Nb intermetallic powder diffused Ti and Nb elements at 1240–1425°C, thereby improving the fracture toughness of β-Sialon ceramics. The maximum fracture toughness (∼9.69 MPa m^1/2) under 19.6 N was obtained for β-Sialon ceramics sintered at 1600°C.     

Fibrous metal–carbon composite structures as gas diffusion electrodes for use in alkaline electrolyte

The fabrication of novel fibre composite electrode structures and the performance assessments for oxygen reduction in alkaline electrolyte is reported. An array of 2m diameter activated carbon fibres interlocked within a network of 2m sinter-bonded metal fibres to form the composite structure was used. The resulting electrode structure is stable, highly conductive and can maintain void fraction exceeding 95%. Electrode physical properties including thickness, macroporosity, volume and mass fractions of constituent carbon and metal fibres have been controlled, characterized, and related to the electrode polarization in a KOH half cell. Comparisons have been made with a commercial Teflon-bonded gas diffusion electrode (GDE). It has been demonstrated that this novel method allows reproducible and low-cost fabrication of GDEs with the optimal balance between macropores for gas access, micropores for liquid access, and conductive paths for electron access.     

Crystal-size–dependent external surface diffusion barriers in Pt/ZSM-5 catalyzed n-pentane isomerization

Surface barriers persist in the molecular transport across external crystal surface of zeolites, which brings strong influences on zeolite catalysis. This study probes the role of crystal-size–dependent surface barriers in zeolite catalysis, with n-pentane isomerization catalyzed by Pt/ZSM-5 as a model reaction system. Chemical liquid deposition of SiO₂ is performed to reduce surface barriers on ZSM-5 crystals with different sizes. After SiO₂ deposition, surface barriers on both small (266 nm) and large (1304 nm) crystals are significantly reduced, while the improvement in catalytic activity for the large crystals (76.1%–129.1%) is much higher than that for the small ones (−17.4% to 16.7%). For the large ZSM-5 crystals, diffusion limitation is stronger, and thus the effect of surface barriers on catalytic activity is more important. This study reveals the unique role of crystal-size–dependent surface barriers in zeolite catalysis, and understanding this is important for the optimal design of zeolite catalysts.     

Evidence of hydrogen–boron interactions in diamond from deuterium diffusion and infrared spectroscopy experiments

We report on the first experimental evidence of hydrogen–boron interactions in boron-doped diamond. Deuterium diffusion studies in homoepitaxial B-doped diamond films reveal that hydrogen diffusion is limited by the B concentration and is characterized by a low effective diffusion activation energy. Infrared spectroscopy experiments show that boron acceptor electronic transitions disappear under hydrogenation. These results are consistent with hydrogen ionization and diffusion of fairly mobile H⁺ which form pairs with B⁻.     

Separation of water–isotopes mixture in continuous-flow thermal diffusion columns for recovery of deuterium

The present study deals with the estimation of deuterium recovery from the separation of water–isotopes mixture (H₂O–HDO–D₂O) by continuous-flow thermal diffusion. First, the equations for predicting the degrees of separation for each component in H₂O–HDO–D₂O system were derived by following the same procedure performed in the previous work. The recovery of deuterium (D) was then estimated from the degrees of separation of HDO and D₂O and confirmed with the experimental results.     

Binary gas diffusivity estimates from transient,one-dimensional sublimation–diffusion experiments in a spherical enclosure

Binary gas diffusivities D_AB’s are extremely useful in the analysis/design of mass transfer systems and to develop correlations. This study used an unsteady experimental method to determine D_AB’s in gas pairs starting with a sublimating solid (A) such as naphthalene or camphor and air (B). The cumulative fractional mass transferred from the surface of a solid A sphere placed concentrically within an isothermal spherical enclosure was followed gravimetrically with time. The experimental D_AB,exp for the gas pair was determined by nonlinear regression using the solution to a transient, one-dimensional (radial) diffusion model. The model’s Case 1 option assumed impermeability (no flux of gas A) at the enclosure’s outer surface, while Case 2 assumed zero concentration of gas A at the same location. For naphthalene–air, D_AB,exp overestimated the literature values, the errors ranging from ?110 to ?185% for Case 1 and ?21 to ?65% for Case 2. For camphor–air, the error in D_AB,exp was ?36% for Case 1 and ?16% for Case 2. D_AB,exp for camphor in atmospheric air is herein reported for the first time. Potential improvements to the experiments include automation of the sphere melt-casting process and tighter control of the enclosure’s environmental conditions. Likewise, the theoretical model could be extended to three dimensions with multicomponent diffusion to assess the effect of air humidity on the transport of gas A. This is the first attempt to determine D_AB,exp for naphthalene–air and camphor–air from an unsteady sublimation–diffusion experiment and to model the results using rigorous mass transport theory.     

Asymptotic exponentiality of the first exit time of the Shiryaev–Roberts diffusion with constant positive drift

We consider the first exit time of a Shiryaev–Roberts diffusion with constant positive drift from the interval [0,A] with A>0 fixed. We show that the moment generating function (Laplace transform) of a suitably standardized version of the first exit time converges to that of the unit-mean exponential distribution as A→+∞. The proof is explicit in that the moment generating function of the first exit time is first expressed analytically and in a closed form and then the desired limit as A→+∞ is evaluated directly. The result is of importance in the area of quickest change-point detection, and its discrete-time counterpart has been previously established—although in a different manner—by Pollak and Tartakovsky (2009a Pollak, M. and Tartakovsky, A. G. (2009a). Asymptotic Exponentiality of the Distribution of First Exit Times for a Class of Markov Processes with Applications to Quickest Change Detection, Theory of Probability and Its Applications 53: 430–442.[Crossref], [Web of Science ®] , [Google Scholar]).     

Proton diffusion in γ-manganese dioxide

Deconvolution of the electrolytic manganese dioxide (EMD) discharge curve has indicated the presence of a number of energetically different reduction processes. This has been used to determine the contribution of each reduction process to the total discharge. Using step potential electrochemical spectroscopy (SPECS), the i-t data were modelled as the sum of the discharge of the individual reduction processes. From this, A√ D for each reduction process as a function of degree of discharge was determined. The maximum A√ D values for each process ranged from 2.3×10⁻² to 4.0×10⁻⁴ cm³ s^−1/2 g⁻¹ values are consistent with previously reported values for A√ D, although in this case we have determined values for the entire compositional range.     

Multiple linear equation of pore structure and coal–oxygen diffusion on low temperature oxidation process of lignite

This work aimed at studying the feasibility of calculating the coal–oxygen diffusion properties during the low temperature oxidation process of lignite so as to predict its spontaneous combustion process. Coal samples were oxidized in air ambient under different temperatures. Scanning Electron Microscope was used to indicate the surface morphology changes of oxidization. Then, based on fractal theory and flow characteristics, the fractal dimension of gas diffusion in the pore ways was calculated under different temperature. Considering pore size distribution, connectivity distribution and Fick diffusion mechanisms, the relationship between the gas diffusivity change with pore area fractal dimension and porosity was investigated, and multiple linear equation of the coal–oxygen diffusion coefficients and pore parameters was obtained. Comparison between the experimental data and model prediction verifies the validity of the model. The research provides a theoretical basis for the prediction model of coal–oxygen diffusion law.     

Effective diffusion constant and adsorption constant of synthesized alumina,zirconia, and alumina–zirconia composite material

In this study, Al₂O₃, ZrO₂, and Al₂O₃–ZrO₂ composite materials were prepared with the sol–gel technique. X-ray diffraction analysis, differential scanning calorimetry–thermogravimetry, scanning electron microscopy–energy-dispersive X-ray spectrometry, nitrogen adsorption isotherm measurements, and helium pycnometry were used to characterize the resultant materials. Effective diffusion coefficients of helium and hydrogen and the adsorption equilibrium constant of hydrogen in the resultant materials were determined using single-pellet moment technique. The effective diffusivities of helium and hydrogen in both ZrO₂ and Al₂O₃–ZrO₂ composite pellets were found to be smaller than the value found for Al₂O_3, due to the lower tortuosity factor values of the Al₂O₃ pellet. It was found that hydrogen was weakly adsorbed on all resultant materials.     

Adsorption and diffusion properties of xylene isomers and ethylbenzene in metal–organic framework MIL-53(Al)

Adsorption and diffusion behaviour of C8 aromatic isomers, including para-xylene (PX), ortho-xylene (OX), meta-xylene (MX), and ethylbenzene (EB), in metal–organic framework Mil-53(Al) has been systematically investigated by using intelligent gravimetric analyzer, thermogravimetric analysis and molecular simulation respectively. The results indicate that adsorptions of xylene isomers and ethylbenzene molecules in Mil-53(Al) at 303 K present type-I isotherms. The deviation from normal Langmuir model of the isotherms at higher temperature can be, however, found because of the breathing effect of Mil-53(Al) framework. In order to well understand the selective adsorption process of above adsorbates, the diffusion behaviour has been determined and the diffusion coefficient is in the order: OX > PX > MX > EB. The adsorption thermodynamics have been determined by the isotherms at different temperature. A sharp increase of the heat of sorption Q _st suggests that a strong interaction between sorbates molecules and between sorbates and the framework appears as the increase of the loading. Two desorption peaks in DTG curves suggests that two sorption locations exist in Mil-53(Al) to C8 alkylaromatics. The molecular simulation results have been used to successfully explain the experimental phenomena and to well understand the underlying adsorption and diffusion features of the systems.     

Sorption in zeolites—II. Tracer diffusion

The objective of the paper is to investigate the relationship between tracer diffusivity and intrinsic diffusivity for sorption in zeolites. It is argued that this relationship cannot be derived from irreversible thermodynamics. Dependence of tracer diffusivity on sorbate concentration for Langmuirian sorbed phase is investigated using irreversible thermodynamics. The analysis indicates that the reported relationship between tracer diffusivity and intrinsic diffusivity derived from molecular models are consistent with irreversible thermodynamics.     

Analytic Pade-like approximations of exp(–sqrt(s)) for simulations of diffusion processes in the semi-infinite geometry

Abstract

Detailed transient behaviors of mass and heat transfer processes are required to solve partial differential equations. When those partial differential equations are coupled, they are still difficult to solve in time domain. For linear mass and heat transfer processes, their Laplace-domain solutions are obtainable and, when they are approximated by rational polynomials in the Laplace variable s, the problems can be transformed to a set of ordinary differential equations solved easily in time domain for various initial conditions. In this approximation, the conventional Pade method based on the Tayler series expansion of the Laplace-domain solutions has been well developed and effective. However, for some mass and heat transfer processes in the semi-infinite geometry, the Pade approximation is not applicable because the Laplace-domain solutions involving exp(–sqrt(s)) are not analytic at s?=?0. Here, for such processes, analytical methods to approximate exp(–sqrt(s)) by rational polynomials are proposed. First it is expanded in series in terms of cosh^–1(2^ksqrt(s)) which converges fast. This series, when truncated, is analytic at s?=?0 and its Pade approximations are available. The proposed method enables partial differential equations be replaced to a set of ordinary differential equations, reducing computations considerably for coupled partial differential equations. Performances of the proposed method are illustrated with several realistic mass and heat transfer processes.