Surface Diffusion Processes Research Articles

Enhancing solid oxide electrolysis cell cathode performance through defect and bond engineering: A study on K-doped PrBaMn2O5+δ perovskites

Collaborative design of defects and bonds is a novel strategy to enhance the activity of cathode materials for solid oxide electrolysis cells (SOECs). In this study, we report the synthesis and characterization of layered PrBaMn2O5+δ (PBM) and Pr0.9K0.1BaMn2O5+δ (PKBM) materials, aimed at developing cathodes for high-temperature H2O electrolysis. The effects of K doping on the structural, physicochemical, and electrochemical properties of these materials are thoroughly investigated. The results reveal that K doping induces lattice expansion and reduces the total cation valence state in PKBM. These changes significantly increase oxygen vacancy concentration, enhancing the chemical adsorption capacity of H2O on the electrode surface and improving the H2O reduction reaction activity. Theoretical calculations support these findings, showing that K doping lowers the oxygen vacancy formation energy and strengthens H2O adsorption. Impedance analyses further demonstrate that K doping reduces the polarization resistance, thereby enhancing surface adsorption and diffusion processes. Consequently, the PKBM cathode exhibits superior electrolysis performance, achieving a current density of 739 mA/cm2 at 1.3 V and 800 °C. This strategy provides valuable insights for designing advanced cathodes for high-temperature H2O electrolysis and other electrochemical catalysis applications.

Sintering Mechanism and Leaching Kinetics of Low-Grade Mixed Lithium Ore and Limestone

With the rapid development of new energy fields and the current shortage of lithium supply, an efficient, clean, and stable lithium resource extraction process is urgently necessary. In this paper, various advanced detection methods were utilized to conduct a mineralogical analysis of the raw ore and systematically study the occurrence state of lithium; the limestone sintering process was strengthened and optimized, elucidating the sintering mechanism and analyzing the leaching process kinetics. Under an ingredient ratio of 1:3, a sample particle size of 300 mesh, a sintering temperature of 1100 °C, a sintering time of 3 h, a liquid–solid ratio of 2:1, a leaching temperature of 95 °C, and a leaching time of 1 h, the leaching rate of Li reached 90.04%. The highly active Ca–O combined with Si–O on the surface of β–spodumene to CaSiO4, and Al–O was isolated and combined with Li to LiAlO2, which was beneficial for the leaching process. The leaching process was controlled by both surface chemical reactions and diffusion processes, and Ea was 27.18 kJ/mol. These studies provide theoretical guidance for the subsequent re-optimization of the process.

Prevention of morphological change for (Ba,Sr)(Co,Fe)O3 cathodes by incorporating (Ce,Gd)O2 for intermediate-temperature solid oxide fuel cells

Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) generally exhibits superior cathode activity compared to La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF) for intermediate-temperature solid oxide fuel cells (IT-SOFCs). However, the chemical stability of BSCF is inferior to that of LSCF. When BSCF cathodes are sintered at a high temperature of 1050 °C, performance was compromised due to the formation of (Ba,Sr)ZrO3 between the scandia-stabilized zirconia (ScSZ) electrolyte and gadolinia-doped ceria (GDC) interlayer. The oxide ionic conductivity of (Ba,Sr)ZrO3 was low, decreasing the cathode performance. Furthermore, the BSCF grains enlarged, and cobalt oxide formed due to BSCF decomposition, decreasing the active specific surface area. However, by incorporating GDC into the BSCF cathode, the morphology remained unchanged and cobalt oxide formation was prevented, despite (Ba,Sr)ZrO3 still forming. The performance of the cell with the BSCF-GDC composite cathode surpassed that with the BSCF cathode due to decreased polarization resistance attributed to the oxygen surface exchange and diffusion processes in the cathode.

Large-Scale Analysis on Accelerated Aging of Pentaerythritol Tetranitrate (PETN) Powders in Detonators.

Pentaerythritol tetranitrate (PETN) has been used extensively in commercial detonators and other explosive applications for many decades. Here, we show the results of a comprehensive 1.5 year aging study of PETN in commercial detonators, addressing batch-to-batch variations, surface area changes, and comparisons of aged loose powders side-by-side with identically aged detonators. Function time analysis of the aged detonators has also been provided and discussed in the context of powder aging. This large-scale, statistically relevant study addresses long-standing questions on PETN aging without the complications from making comparisons between multiple batches of material. We have evaluated the aging time required to reach the maximum measured amount of PETN coarsening and estimated an activation barrier of ∼123 kJ mol-1, which is higher than literature values reported by Gee et al. It is possible that this discrepancy is due to the fact that that this study cannot quantify the relative contributions of surface diffusion versus sublimation processes. At the lower temperatures of 50 and 60 °C, we assume that surface diffusion dominates over sublimation processes, even at longer aging times. At the higher temperature of 75 °C, we assume that both surface diffusion and sublimation contribute at the early time points, which are included in the Arrhenius analysis for coarsening.

Structure-electrochemical performance relationship in Sr2(Fe1-xVx)MoO6 anode

Structural property is becoming a powerful tool to manipulate oxygen vacancy concentration and electrochemical performance of electrode material. In this work, the structure-electrochemical performance relationship of Sr2(Fe1-xVx)MoO6 (x = 0, 0.1, 0.2 and 0.25) anode is investigated based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), Energy Dispersive Spectrometer (EDS), BET surface area, DC four-point probe, I–V and electrochemical impedance spectra (EIS). XRD results show that all the compounds are of single phase, the tetragonal to cubic phase transition occurs at x = 0.1, which plays important role in its electrochemical performance. Firstly, the disappeared structural distortion in Sr2(Fe1.8V0.2)MoO6 and Sr2(Fe1.75V0.25)MoO6 anodes benefits electron hopping between Fe–O–Mo–O–Fe, so electron transfer processes (high-frequency semicircle) in their EIS are facilitated. Secondly, both XPS survey and EDS spectrum confirm the change in oxygen content with structural transition, Sr2(Fe1.9V0.1)MoO6 anode own higher concentration of oxygen vacancy, thus the oxygen surface exchange and diffusion process (low-frequency arc) of the corresponding cell is reduced significantly. Consequently, Sr2(Fe1.9V0.1)MoO6-based cell exhibits the highest power output (810 mW cm−2) at 850 °C. For Sr2(Fe1.8V0.2)MoO6 anode, BET surface area test reveals the slightly higher value, while its lower power density indicates that the influence of oxygen vacancy prevails over that of microstructure. Combining with EIS results, it can be deduced that Sr2(Fe1-xVx)MoO6 anodes with tetragonal symmetry own higher concentration of oxygen vacancy, thus the expected electrochemical performance is better than that of other cells. Understanding the critical role of structure is important for achieving highly active electrode material.

Simultaneously enhancing toluene adsorption and regeneration process by hierarchical pore in activated coke: a combined experimental and adsorption kinetic modeling study.

Activated coke is a type of commonly used adsorbent for benzene series VOCs such as toluene, but traditional microporous activated coke usually faces the challenge of poor regeneration performance. Herein, based on self-made activated cokes with typical pore configuration, we found that adsorption and regeneration of toluene can be simultaneously enhanced by constructing hierarchical pore in activated coke. Correlations of pore configuration with toluene adsorption capacity and regeneration efficiency reveal that micropore contributes for strong toluene adsorption; meso-macropore provides mass transfer channel for toluene desorption and regeneration process. Hierarchical porous activated coke prepared from Zhundong subbituminous coal not only achieves the highest toluene adsorption capacity of 340.92mg·g-1, but also can retain more than 90% of initial adsorption capacity after five adsorption-regeneration cycles. By contrast, micropore-dominant activated cokes can only retain 70% of initial adsorption capacity. Adsorption kinetic modelling on adsorption breakthrough curves shows that hierarchical porous activated coke prepared from Zhundong subbituminous coal exhibits high adsorption and diffusion rate constants of 14.39 and 33.45min-1, respectively, much higher than those of micropore-dominant activated cokes. Due to the accelerated surface adsorption and diffusion processes induced by meso-macropore, toluene adsorption and regeneration behavior can be simultaneously improved. Results from this work validated the role of pore hierarchy in toluene adsorption-regeneration process, providing guidance for designing high-performance activated coke with synergistically improved toluene adsorption capacity and regeneration performance.

Measurement of Surface Diffusion at Electrochemical Interfaces Using in Situ Linear Optical Diffraction

Surface diffusion processes play an important role in many reactions at electrode surfaces and are of relevance to electrocatalysis, electrodeposition, and many other areas. Such processes have therefore been extensively studied on solids under vacuum conditions with a variety of different methods. In sharp contrast, only few studies exist to measure surface diffusion in electrochemical environment due to the lack of suitable methodology. We aim on partially filling this gap by introducing the all-optical in situ LOD methodology, which allows to measure surface diffusion rates over a wide range and thus overcomes limitations of established methods that are mostly restricted to slow diffusion rates. The method is based on the generation of a periodic spatial modulation of adsorbates using two interfering laser pulses. The subsequent equilibration of this pattern and thus the diffusion rate of adsorbates is probed by the linear optical diffraction (LOD) signal from a second laser.First proof-of-principle measurements were performed on sulfur diffusion on the Pt(111) electrode. For this purpose, the optical and electrochemical properties of this systems were investigated first, including studies of the optical reflectance change induced by chemisorbed sulfur (Sad) and investigations of the Sad oxidation mechanism. The subsequent studies of Sad surface diffusion revealed potential and coverage dependent diffusion rates which were at least an order of magnitude faster than under vacuum conditions. Supported by numerical simulations, the implications of strong adsorbate-adsorbate interaction combined with different grating modulation depths to the temporal evolution of the LOD signal were demonstrated and data accuracy and pitfalls of the method were discussed.Kattwinkel and Magnussen, ACS Meas. Sci. Au, in press (2023), https://doi.org/10.1021/acsmeasuresciau.2c00066Kattwinkel and Magnussen, Electrochimica Acta 434, 141297 (2022), https://doi.org/10.1016/j.electacta.2022.141297

Influence of growth interruption on the morphology and luminescence properties of AlGaN/GaN ultraviolet multi-quantum wells.

The influence of growth interruption on the surface and luminescence properties of AlGaN/GaN ultraviolet multi-quantum wells (UV MQWs) is investigated. It is found that when the well and barrier layers of MQW samples are continuously grown at the same temperature, they have lower edge dislocation density and flatter surface of MQWs compared to samples with interrupted well and barrier growth. Moreover, continuous growth of well and barrier layers is more conducive to improving the luminescence efficiency of MQWs. This phenomenon is attributed to more impurity incorporation induced by the growth interruption, while a continuous growth of well and barrier can reduce surface diffusion and migration processes of atoms, reducing the defects and surface roughness of MQWs. In addition, the continuous growth of well and barrier can better control the reaction between Al and N atoms, avoiding the formation of excessively high Al content AlGaN at the well/barrier interface, thus improving the luminescence of UV MQWs.

Highly Efficient Production of Desired Solid Forms of Drugs with Improved Mechanical Properties via an Organic Solvent-Free Sublimation Process

The pharmaceutical industry is required to constantly broaden its development areas and accelerate technical transformation because of the continued global rise in medicine consumption. However, the usage of organic solvents in pharmaceutical production results in up to 20 million tons annually of chemical waste, causing a serious environmental problem. Here, we proposed a solvent-free sustainable chemical technology, sublimation crystallization, for highly efficient screening and preparation of pharmaceutical solid forms (i.e., polymorphs), which eliminates the wastes associated with solvent use from the source. By adjusting the parameters of sublimation crystallization, we have obtained multiple polymorphs of several important drugs, including the antituberculosis drug pyrazinamide (PZA) and nonsteroidal anti-inflammatory drugs (NSAIDs), flufenamic acid (FFA) and mefenamic acid, particularly γ form of PZA and polymorphs IV and VII of FFA, which are difficult to produce from conventional solution crystallization methods. Mechanical property measurements from both powder and single crystal tests reveal that the γ form of PZA readily prepared by sublimation crystallization has the most excellent tabletability properties. In situ crystal growth observations indicate a direct vapor-to-crystal growth mechanism for the sublimation crystallization process consistent with conventional vapor phase deposition. Furthermore, the governing factors and growth mechanisms of sublimation and solution crystallization processes were analyzed and discussed, and it was found that the rate-limiting step of crystal growth is the surface-integration process in solution crystallization, whereas it becomes the surface diffusion process in sublimation crystallization. Overall, our developed sublimation crystallization provides a promising green technology for the development and production of (new) drugs, broadening the thinking behind the development of sustainable chemicals.

A probabilistic microkinetic modeling framework for catalytic surface reactions.

We present a probabilistic microkinetic modeling (MKM) framework that incorporates the short-ranged order (SRO) evolution for adsorbed species (adspecies) on a catalyst surface. The resulting model consists of a system of ordinary differential equations. Adsorbate-adsorbate interactions, surface diffusion, adsorption, desorption, and catalytic reaction processes are included. Assuming that the adspecies ordering/arrangement is accurately described by the SRO parameters, we employ the reverse Monte Carlo (RMC) method to extract the relevant local environment probability distributions and pass them to the MKM. The reaction kinetics is faithfully captured as accurately as the kinetic Monte Carlo (KMC) method but with a computational time requirement of few seconds on a standard desktop computer. KMC, on the other hand, can require several days for the examples discussed. The framework presented here is expected to provide the basis for wider application of the RMC-MKM approach to problems in computational catalysis, electrocatalysis, and material science.

Effects of Surface Chemical Modification by Ethoxysilanes on the Evolution of 3D Structure and Composition of Porous Monoliths Consisting of Alumina Hydroxide Nanofibrils in the Temperature Range 25-1700 °C.

Bulk nanomaterials with an open porosity offer exciting prospects for creating new functional materials for various applications in photonics, IR-THz optics, metamaterials, heterogeneous photocatalysis, monitoring and cleaning toxic impurities in the environment. However, their availability is limited by the complexity of controlling the process of synthesis of bulk 3D nanostructures with desired physicochemical and functional properties. In this paper, we performed a detailed analysis of influence of a silica monolayer chemically deposited on the surface of a monolithic ultraporous nanostructure, consisting of a 3D nanofibril network of aluminum oxyhydroxide, on the evolution of structure and morphology, chemical composition and phase transformations after heat treatment in the temperature range of 20−1700 °C. The experimental results are interpreted in the framework of a physical model taking into account surface and volume mass transport and sintering kinetics of nanofibrils, which made it possible to estimate activation energies of the surface diffusion and sintering processes. It is shown that the presence of a surface silica monolayer on the surface affects the kinetics of aluminum oxyhydroxide dehydration and inhibits diffusion mass transfer and structural phase transformations. As a result, the overall evolution of the 3D nanostructure significantly differs from that of nanomaterials without surface chemical modification.

A kinetic model for multicomponent gas transport in shale gas reservoirs and its applications

An accurate gas transport model is of vital importance to the simulation and production optimization of unconventional gas reservoirs. Although great success has been achieved in the development of single-component transport models, limited progress has been made in multicomponent systems. The major challenge of developing non-empirical multicomponent gas transport models lies in the absence of the quantification of the concentration impact on the fluid dynamic properties. To fill such a gap, this work presents a comprehensive transport model for multicomponent gas transport in shale and tight reservoirs. In developing the model, we first conducted molecular dynamic simulations to qualitatively understand the differential release of hydrocarbons from unconventional shale and tight reservoirs. It is found that the gas slippage, differential adsorption, and surface diffusion are the primary transport mechanisms in the working range of Knudsen number during reservoir production. Based on the molecular dynamic study, a quantitative transport model has been developed and validated, which extends existing models from single-component systems to multiple-component systems. The kinetic theory of gases is adopted and modified to model the multicomponent slippage effect. A generalized Maxwell–Stefan formulation with extended Langmuir adsorption isotherm is used to model the multicomponent surface diffusion process. The accuracy of the proposed model is above 90% for low to moderate Knudsen numbers in modeling the differential release phenomenon in unconventional reservoirs.

Quantitative estimate of the contribution of the surface diffusion process to mass transfer during electrochemical deposition of metals

In this study, an attempt was made to quantify the contribution of the surface diffusion process to mass transfer during electrochemical deposition of metals. The maximum density of nucleation centers is calculated, at which only diffuse overgrowth of a continuous layer of a new phase is possible. The time of diffusion overgrowth of a continuous monomolecular layer of a new phase is calculated. The results of independent experiments on the determination of boundery diffusion coefficients in a two-layer platinum-nickel thin-film system are analyzed. The value of the boundery diffusion coefficient of nickel at a temperature of 393K was found. The part of mass transfer due to surface diffusion of physically adsorbed atoms in relation to the total mass transfer during electrochemical deposition of metals has been quantified. It is proved that the role of the surface diffusion process in the formation of the coating during electrochemical deposition of metals is insignificant. It is shown that this is an indirect proof of the formation of an amorphous coating structure during electrochemical deposition of metal.

Electropolymerized 1,10-phenanthroline as the electrode material for aqueous supercapacitors

Research on novel conducting polymer material is essential in designing high-performance electrodes for the electrochemical energy storage systems. 1,10-phenanthroline is, for the first time, electropolymerized in aqueous solutions on carbon paper, and the resulting electrodes (PPhen/CP) are utilized to store energy electrochemically. The influence of the electropolymerization time and intercalated anions to the morphology and charge storage performance is investigated, and the results show that PPhen/CP with 20 min electropolymerization time and intercalated sulfate ions (PPhen/CP-20) exhibits the best performance, with the specific capacities estimated by galvanostatic charge–discharge of 111.5 mAh g−1 at 1 A g−1 in 1 M H2SO4, and of 97.1 mAh g−1 at 1 A g−1 in 1 M KOH. The solid-state supercapacitor assembled using two PPhen/CP-20 and the H2SO4/PVA gel electrolyte can deliver the energy density of 19.44 Wh kg−1 at the power density of 899.5 W kg−1, and can retain 80.76% of the initial capacity after cycling at 5 A g−1 for 4000 cycles. The charge storage mechanism involves dihydroxyl/diketone groups redox process and double layer charging, and the charge storage performance is affected by the electrochemically active surface area and diffusion processes which are related to the morphology of the electropolymerized film. This study shows that the electropolymerized 1,10-phenanthroline film is a promising candidate as the electrode material for aqueous supercapacitors.

Oxygen Diffusion and Surface Exchange Coefficients of Porous La0.6Sr0.4Co0.2Fe0.8O3−δ Decorated with Co3O4 Nanoparticles

This work presents a comparative study of the diffusion and surface exchange coefficients of porous La0.6Sr0.4Co0.2Fe0.8O3−δ (LSCF) and Co3O4 nanoparticles decorated LSCF electrodes. The study was carried out using the 3DT-EIS method, which combines Electrochemical Impedance Spectroscopy experiments with FIB-SEM tomography data through an adapted Transmission Line–Adler Lane Steele electrochemical model.A reduction of the polarization resistance of about 60% was measured for the Co3O4 decorated LSCF respect to the reference LSCF cathode, in air at 700 °C. The Co3O4 decoration was found to modify the ORR surface reaction limiting mechanism from O2 dissociation to O-ion incorporation, whereas the diffusion coefficient was not modified by the decoration, which represents a surface diffusion process for both electrodes. After the EIS measurements, the Co3O4 particles were almost no longer visible by Field-Emission SEM on the surface of the decorated sample, but signs that these particles play an active role in Sr Segregation were observed by STEM-EDS, in particular by concentrating the segregated SrO in the surroundings of the decorated particles.

Mass transport of CO2 over CH4 controlled by the selective surface barrier in ultra-thin CHA membranes

The adsorption and mass transport of CO 2 and CH 4 in CHA zeolite were studied experimentally. First, large and well-defined CHA crystals with varying Si/Al ratios and morphologies ideal for adsorption studies were prepared. Then, adsorption isotherms were measured, and adsorption parameters were estimated from the data. In the next step, permeation experiments for pure components and mixtures were conducted for a defect-free CHA membrane with a Si/Al ratio of 80 and a thickness of 600 nm over a wide temperature range. A maximum selectivity of 243 in combination with a CO 2 permeance of 70 × 10 −7 mol/(m 2 s Pa) was observed for a feed of an equimolar CO 2 /CH 4 mixture at 273 K and 5.5 bar. Finally, a simple model accounting for adsorption and diffusion through the surface barriers and the interior of the pores of the membrane was fitted to the permeation data. The fitted model indicated that the surface barrier was a surface diffusion process at the pore mouth with higher activation energy than the diffusion process within the pores. The model also showed that the highly selective mass transport in the membrane was mostly a result of a selective surface barrier and, to a lesser extent, a result of adsorption selectivity. • CHA crystals with various Si/Al ratios were studied for gas adsorption. • The adsorption were performed in a wide temperature range of 150–350 K. • Single gas and CO 2 /CH 4 permeation were measured over an ultra-thin CHA membrane. • Adsorption parameters were used to model mass transport through the CHA membrane. • High CO 2 /CH 4 selectivity is mostly due to high surface permeability selectivity.

Low temperature growth of semi-polar InN [formula omitted] on non-crystalline substrate by plasma-assisted laser ablation technique

We report low-temperature growth of semipolar (101¯1) InN films on a non-crystalline substrate (quartz) using a novel route i.e. plasma-assisted laser ablation technique. The structural, morphological, optical, electrical and chemical/elemental environment of these films was investigated using respective techniques. Dramatic changes associated with the surface morphology of these films/ nanostructures are observed to depend on the growth temperature which is changed only by the difference of 100 °C. These changes are mainly the transformation from a continuous planar 2D nanostructured film (deposited at RT) to 3D faceted nanostructured film (deposited at 300 °C). Observed changes/transformations are attributed to the surface diffusion processes, which are temperature-dependent. These structural transformations are observed to affect the optical as well as electrical properties of the films.

Investigation of growth mode and surface roughness during homoepitaxial growth of silver metal using kinetic Monte Carlo simulation

In this work, a kinetic Monte Carlo (KMC) method for deposition is investigated to simulate the homoepitaxial growth of silver metal under experimental conditions. On-lattice and coarse-graining approximations are applied. KMC method is an ideal tool for simulating the Physical Vapor Deposition (PVD) process, which involves deposition and diffusion. The method accounts for surface diffusion processes, in addition to nearest-neighbor hopping and including Schwoebel-Ehrlich (SE). The SE mechanism effect on the growth mode and influences of substrate temperature and deposition rate on surface roughness have been studied in detail. The simulation results show when the Schwoebel hops are active, the growth mode is Frank-van der Merwe (2D). Furthermore, when the Schwoebel hops are not active, the growth mode is Volmer-Weber (3D). In addition, when increasing the deposition rate, the surface roughness increases due to the size of islands increase. Besides, when the temperature increases, the surface roughness decreases because the islands formed are transformed into a large cluster.

Morphological properties of the interfaces growth of composite membranes

In this paper, we investigate the growth dynamics and the scaling law of growth interfaces in where the interlayer transport is characterized by surface diffusion process similar to the Ehrlich-Schwoebel effect. The mound morphological properties of obtained interfaces depends strongly to the competition between deposition process and diffusion one. The results show that the growth and roughness exponents are not depending to the ratio of the constant diffusion and the deposition flux D/F. However, the width interface decreases exponentially with the ratio D/F and the obtained interfaces reduce their roughness morphology i.e. become smoother at high values of the D/F ratio. Finally, the investigated interfaces exhibits a Familly-Vicsek law with universal exponents.

The origin of the surface barrier in nanoporous materials

Surface barriers are influencing the mass transfer in nanopores, but their origin is unclear and can be quite different in different materials. For MFI and CHA membranes studied here, we show that the surface barrier may be a surface diffusion process with higher activation energy than the surface diffusion process in the pores, but other possible mechanisms such as pore blocking and pore narrowing has not been ruled out. The higher activation energy is probably a result of less interaction between adsorbed molecules at the pore mouth than inside the pores, i.e. the barrier is simply a geometrical effect in these materials. For pure components at low concentration in MFI zeolite, we found that barrier is proportional to the product of the molecular weight and heat of desorption. For MFI and CHA zeolite, we observed that the barrier is a function of concentration and approach zero at high concentration and that the barriers of the components become more similar due to interaction between the components in mixtures, which explains the high and selective mass transfer displayed by these nanoporous materials at high concentration.