Diffusion Path Length Research Articles

Effects of defect chemistry and kinetic behavior on electrochemical properties for hydrothermal synthesis of LiFePO4/C cathode materials

Abstract Lithium iron phosphate cathode materials for lithium ion batteries were synthesized successfully by hydrothermal synthesis method using LiOH·H2O and FeSO4·7H2O as resources of Li and Fe, respectively. The X-ray diffraction, the scanning electron microscopy, the surface areas, the Fourier transform infrared spectra and electrochemical properties of LiFePO4/C cathode materials prepared at different pH values were investigated. The solubility product principle is reasonable to illustrate the chemical composition of the final products in the hydrothermal process. A full Rietveld refinement of X-ray diffraction patterns and Fourier transform infrared spectra results of pure LiFePO4/C prepared at pH 8, 9, 10 supports that cation mixing is the intrinsic characteristic of LiFePO4/C synthesized by hydrothermal process. The lithium diffusion kinetics is affected by the interaction between the occupation of Fe on Li sites and the diffusion path length. In the applied pH range, there is an optimal pH value of 10 for the formation, morphology, and lithium diffusion kinetics, and hence the best electrochemical performances of LiFePO4/C. The mechanisms of nucleation and crystal growth, the defect chemistry and the kinetic behavior are widely discussed in the study, and are essential for optimizing the properties of LiFePO4.

Suppression of Electrochemically Driven Phase Transitions in Nanostructured MoS2 Pseudocapacitors Probed Using Operando X-ray Diffraction.

Pseudocapacitors with nondiffusion-limited charge storage mechanisms allow for fast kinetics that exceed conventional battery materials. It has been demonstrated that nanostructuring conventional battery materials can induce pseudocapacitive behavior. In our previous study, we found that assemblies of metallic 1T MoS2 nanocrystals show faster charge storage compared to the bulk material. Quantitative electrochemistry demonstrated that the current response is capacitive. In this work, we perform a series of operando X-ray diffraction studies upon electrochemical cycling to show that the high capacitive response of metallic 1T MoS2 nanocrystals is due to suppression of the standard first-order phase transition. In bulk MoS2, a phase transition between 1T and triclinic phases (Li xMoS2) is observed during lithiation and delithiation in both the galvanostatic traces (as distinctive plateaus) and the X-ray diffraction patterns with the appearance of the additional peaks. MoS2 nanocrystal assemblies, on the other hand, show none of these features. We hypothesize that the reduced MoS2 crystallite size suppresses the first-order phase transition and gives rise to solid solution-like behavior, potentially due to the unfavorable formation of nucleation sites in confined spaces. Overall, we find that nanostructuring MoS2 suppresses the 1T-triclinic phase transition and shortens Li-ion diffusion path lengths, allowing MoS2 nanocrystal assemblies to behave as nearly ideal pseudocapacitors.

Carbon gel monoliths with introduced straight microchannels for phenol adsorption

Adsorption of phenol using carbon adsorbents in the form of a packed bed of particles inherently suffers from the incompatibility between low hydraulic resistance and short diffusion path length. In this work, carbon gel monoliths made from resorcinol–formaldehyde gels with introduced straight microchannels in the range of 60–160 µm were successfully synthesized using polyesters fibers as templates. The advantage of this synthesis method is that both channel size and channel density can be tuned independently. Introduced microchannels were effective in minimizing the pressure drop of a fluid flowing through the sample to less than a hundredth of beds packed with particles having the same diffusion path length. Synthesized monoliths also possess sufficient mechanical strength (more than 12 kg per monolith) to be activated to increase their adsorption capacity. Adsorption experiments of phenol in a continuous flow system show that synthesized monoliths can be used as effective adsorbents for phenol removal. The tunable channel size and channel density of the proposed synthesis method has the potential for the synthesis of monoliths which could be used to fulfill various process demands.

Interfacial Kinetics of HOR/MOR at the AEM/Pt Microelectrode Interface: Investigation of the Influence of CO32− on the Reaction Kinetics and the Mass Transport through Membrane

The kinetics of hydrogen oxidation reaction (HOR) and methanol oxidation reaction (MOR) at the Tokuyama anion exchange membrane/ Pt microelectrode interface were investigated using a solid state electrochemical cell. Carbonate ions in the membrane which are strongly adsorbing anions; suppress both HOR and MOR kinetics. Carbonate ions not only have a site blocking effect, but at low pHs sufficient amount of hydroxyl ions are not available for adsorption. Adsorption effects by the quaternary ammonium species in the membrane, on the Pt electrode is found to be minimal. However, an ionomer film (AS4) coated on the membrane, significantly decreased the rate of HOR. Diffusion of hydrogen molecules through the membrane was not influenced by the carbonate ions due to the smaller size of the gaseous molecule. However, hydrogen concentration in the anion exchange membrane is low in the presence of carbonate ions. Methanol diffusion is facilitated in the anion exchange polymer electrolyte due to its high water content. However, change of the diffusion path length in carbonate polymer electrolytes caused methanol permeability to drop significantly.

Database of Computation-Ready 2D Zeolitic Slabs

Zeolites are nanoporous aluminosilicates widely used in catalysis and separations applications. Though generally formed as 3D crystals, new synthesis techniques have given access to 2D zeolite nanosheets with small diffusion path lengths and accelerated molecular diffusion. Since most previous research has focused on bulk zeolite crystals, there is little understanding of the surface adsorption and diffusion mechanisms likely involved at such length scales and their contributions to the permeability and selectivity of different species. To enable the systematic examination of such surface properties, we constructed a database of more than 800,000 computation-ready 2D zeolite nanosheets from the full range of known zeolite structures in the IZA database of zeolite structure types. The nanosheet surfaces cover a wide range of orientations and were created via the principle of minimizing the number of bonds broken during the termination of a unit cell. The database consists of two sets of nanosheets: one set...

Improved charging and strain rates by laser perforating polypyrrole actuator electrodes

Conducting polymer actuators generate significant strains (1%–10%) when driven at low voltages, and are muscle-like in their linear deformation, mechanical compliance, silent operation, and energy density. However, slow electrochemical processes have hindered widespread application. To operate electroactive polymer actuators in practical applications such as soft robotics, the speed of energy delivery is often important. Recent work has achieved very fast actuation by using very thin films and blending of materials of high ionic conductivity with conducting polymers, allowing fast insertion of ions. Here we make microscopic arrays of holes in relatively thick, easy to handle conducting polymer films in order to speed ion insertion and actuation. A single-step, top-down, non-contact and template-free approach is used, employing femto and picosecond lasers to texturize polypyrrole based actuators while preserving the stress and strain output. A wide range of hole separations and diameters (pitch/diameter from 9.75/5.17 μm to 24.7/13.2 μm) are explored to reduce the ion diffusion path length in the bulk polymer. This led to a speed increase of between 2 and 30 times. A finite element model based on diffusion shows similar improvements, suggesting that the reduced distance of mass transport achieved by laser perforation is the key to faster response times in the thick electrodes used. The demonstrated ability to increase rate of response using micromachined electrodes is potentially applicable to batteries and supercapacitors.

The Effect of Ageing on Chemical and Mechanical Properties of Asphalt Mortar

Asphalt mortar is a mixture of bitumen, filler, and sand. Mortar plays an important role in asphalt mixtures as it serves as the adhesive between the coarser aggregates. Due to the effect of bitumen ageing, the chemical and mechanical properties of asphalt mortar evolve with time. The mortar becomes more brittle and prone to cracking, thus leading to inferior pavement performance. In this study, Fourier transform infrared (FTIR) spectrometry was used to quantify changes in the chemical functional groups related to ageing and to calculate the carbonyl and sulfoxide indices. In addition, frequency sweep tests and uniaxial tension tests were performed by means of dynamic shear rheometer (DSR) tests to determine evolution of the stiffness and strength due to ageing. Two different oven ageing protocols were used to evaluate the effect of fine mineral particles on bitumen ageing. The protocols differed with respect to the order of ageing and mixing of the constituents. The results showed that both the chemical and mechanical properties of mortars significantly changed with ageing. Specifically, the carbonyl index, stiffness, and strength of the mortar increased. Under the same ageing conditions, a higher ageing level was observed for mortars produced by first mixing and then ageing compared to the mortars produced by mixing aged bitumen with filler and sand. This could be due to the presence of sand and filler particles, which resulted in an increased length of diffusion paths and consequently a slower ageing process.

Morphology dependent facile synthesis of manganese oxide nanostructures for oxygen reduction reaction

The present work demonstrates the morphology dependent synthesis of manganese oxide using simple hydrothermal method just by adjusting the reaction temperature. The structural and morphological studies confirm the formation of manganese oxide with different phases and anisotropic structures of manganese oxide. Interestingly, electrochemical studies exhibit significant oxygen reduction reaction (ORR) kinetics by manganese oxide nanowires with improved onset potential of +0.83 V versus reversible hydrogen electrode (RHE) and current density of 1.75 mA/cm2 in alkaline condition. Among three different morphologies, nanowires show better catalytic activity due to improved diffusion path length and higher aspect ratio. However, all the three samples follow two (2) - electron transfer ORR kinetics, though four electron transfer kinetics has been of great interest for energy conversion applications. Experimental results represent that the reaction temperature plays a key role in forming different shapes and affect the crystal phase and growth process thereby affecting electrochemical properties.

Unveiling Mesoporous Graphitic Carbon Nitride as a High Performance Electrode Material for Supercapacitors

AbstractSynthesis of mesoporous graphitic carbon nitride (MGCN) by soft template method is still challenging and its application as an electrode material for supercapacitor remains unexplored. Herein, we introduce MGCN as an electrode material for supercapacitors. MGCN is synthesized by facile direct carbonization of methylolated surfactant‐polymer composite and bulk graphitic carbon nitride (BGCN) is synthesized under similar condition but in the absence of surfactant. MGCN displays excellent capacitance properties in 1 M H2SO4 electrolyte with a specific capacitance as high as 279 F g−1 at a current density of 0.25 A g−1. It also exhibits good rate capability and outstanding cycling stability with excellent coulombic efficiency of 100% over 5000 cycles. This superior capacitive storage performance is attributed to the high surface area, easily accessible mesopores and high pyridinic nitrogen content. High surface area and uniform pores of MGCN provide more active sites for charge storage and reduce diffusion path length for ions while high pyridinic nitrogen enhanced the overall specific capacitance by surface Faradaic reaction.

Volume Fraction Dependent Wear Behavior of Titanium-Reinforced Aluminum Matrix Composites Manufactured by Melt Infiltration Casting

This study aims to investigate the effect of volume fraction of commercially pure titanium (CP-Ti) on microstructural, mechanical, and tribological features of A356 aluminum matrix composites. Vacuum-assisted melt infiltration casting was performed to produce composites with 50%, 65%, 75%, and 80% CP-Ti contents. CP-Ti sawdusts were assembled under mechanical pressure in order to attain porous one-piece CP-Ti preforms which were infiltrated by A356 melt at 730 °C under 10−5 Pa vacuum atmosphere. TiAl3 layer was formed at the interface between A356 and CP-Ti phases. Owing to increased diffusion time through decreased diffusion path length, both thickness and hardness of TiAl3 phase were increased with increasing CP-Ti ratio, whereas the best wear resistance was obtained at 65% CP-Ti ratio. The main reason for decrease in wear resistance of 75% and 80% CP-Ti reinforced composites was fragmentation of TiAl3 layer during wear process due to its excessively increased brittleness. Strongly bonded TiAl3 phase at the interface provided better wear resistance, while weakly bonded ones caused to multiply wear rate.

Morphology-Controlled Synthesis of Co3O4 Materials and its Electrochemical Catalytic Properties Towards Oxygen Evolution Reaction

In the present study, we present a facile strategy to synthesis Co3O4 materials with different morphology. Experimental results show that Co3O4 materials with flower-like, fiber, sheet-like and rod morphologies have been successfully prepared by hydrothermal synthesis in different solvent. The effect of the morphology on the electrochemical catalytic properties were also studied. It is found that sheet-like Co3O4 exhibits the best activity towards oxygen evolution reaction (η10 = 390 mV) in 1 M KOH, which can be attribute to its short electrolyte infiltration diffusion path lengths and low charge transfer resistant. LSV curves measured at 5 mV/s in 1 M KOH solution for OER, the inset image is FE-SEM image of prepared Co3O4 materials. a Flower, b fiber, c sheet and d rod.

Micelles directed preparation of ternary cobalt hydroxide carbonate-nickel hydroxide-reduced graphene oxide composite porous nanowire arrays with superior faradic capacitance performance

Electrode material is the key component of a supercapacitor, the highly accessible surface area, efficient electrons/ions migration channels, robust structural stability and redox activity of electrode material are pivotal prerequisites for harvesting optimal capacitive performance. Herein, a ternary cobalt hydroxide carbonate-nickle hydroxide-reduced graphene oxide composite (CN-rGO) with porous nanowire arrays architecture was deposited onto Ni foam substrate through confined hydrothermal reaction directed by surfactant micelles. The as-prepared CN-rGO nanowire arrays exhibit mesoporous texture with high specific surface area, which allows sufficient soaking of electrolyte with short diffusion path length. Additionally, the vertically aligned nanowires with incorporation of reduced graphene oxide offer efficient channels for migration of electrons generated by faradic components. Both features enable the sufficient faradic reactions and charge storage of the CN-rGO electrode. Under optimal Co:Ni feeding molar ratio of 7:3, the battery typed faradic CN-rGO electrode offers superior specific capacitance (2442 F g−1 at 1 A g−1), good rate capability (65% capacitance retaining ratio within 1–20 A g−1) and cycling stability (70% maintaining ratio after 2000 charge-discharge cycles). When used as faradic electrode of hybrid supercapacitor (HSC), balanced energy density (42.9–26.2 Wh kg−1), power density (393–3519 W kg−1) and cycleability (80% initial capacitance maintaining ratio undergoes 5000 charge-discharge cycles) can be delivered simultaneously, highlighting the potential of the micelles directed CN-rGO nanowire arrays electrode in efficient energy storage device.

Deconvoluting the Competing Effects of Zeolite Framework Topology and Diffusion Path Length on Methanol to Hydrocarbons Reaction

Micropore topology and crystal size are two independently adjustable properties that govern the internal mass transport limitations of zeolite catalysts. Deciphering the relative impact of each factor on catalyst performance is often nontrivial owing to the inability to synthesize zeolites with predetermined physicochemical properties. In this study, a series of ZSM-11 (MEL) and ZSM-5 (MFI) catalysts of equivalent acidity, but differing pore architecture, are prepared with well-defined crystal sizes to elucidate the effects of diffusion path length versus topology on catalyst lifetime and selectivity. For these studies, we selected the methanol to hydrocarbons (MTH) reaction to assess the impact of design variables on the hydrocarbon pool (HCP) mechanism. Operando UV–vis microspectroscopy is used to investigate the evolution of active HCP species and heavier aromatic coking species during the transient start up period over both catalysts. Our findings reveal that slight variations in framework topology be...

Two-dimensional porous nanodisks of NiCo2O4 as anode material for high-performance rechargeable lithium-ion battery

Two-dimensional porous nanodisks of NiCo2O4 have been synthesized by low temperature hydrothermal method followed by calcination at 500 °C/3 h/air. X-ray diffraction pattern confirmed the phase purity of the sample. Microstructure analysis clearly exhibits a unique porous nanodisks shape of NiCo2O4 sample. Brunauer–Emmett–Teller technique is also used to investigate the surface area and pore size of the sample. Electrochemical properties are evaluated via cyclic voltammetry and Galvanostatic cycling studies. NiCo2O4 as an anode exhibits excellent lithium storage capacity (673.9 mAh g−1 at 0.5C after 350 cycles; 596.9 mAh g−1 at 3.0 C after 200 cycles), high initial Coulombic efficiency (75%), long cycling stability (∼80% capacity retention at 350 cycles) and better rate capability (552.5 mAh g−1at 4.0 C), which is much higher than the theoretical capacity of graphite (372 mAh g−1). This improvement can be attributed to the unique porous structure of the electrode, which allows maximum Li+ ion to react with electrode material, shortens the diffusion path length and increasing the electrode/electrolyte contact area.

Diffusion in Nanoporous Materials: Novel Insights by Combining MAS and PFG NMR

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) allows recording of molecular diffusion paths (notably, the probability distribution of molecular displacements over typically micrometers, covered during an observation time of typically milliseconds) and has thus proven to serve as a most versatile means for the in-depth study of mass transfer in complex materials. This is particularly true with nanoporous host materials, where PFG NMR enabled the first direct measurement of intracrystalline diffusivities of guest molecules. Spatial resolution, i.e., the minimum diffusion path length experimentally observable, is limited by the time interval over which the pulsed field gradients may be applied. In “conventional” PFG NMR measurements, this time interval is determined by a characteristic quantity of the host-guest system under study, the so-called transverse nuclear magnetic relaxation time. This leads, notably when considering systems with low molecular mobilities, to severe restrictions in the applicability of PFG NMR. These restrictions may partially be released by performing PFG NMR measurements in combination with “magic-angle spinning” (MAS) of the NMR sample tube. The present review introduces the fundamentals of this technique and illustrates, via a number of recent cases, the gain in information thus attainable. Examples include diffusion measurements with nanoporous host-guest systems of low intrinsic mobility and selective diffusion measurement in multicomponent systems.

Highly dispersed PtO nanodots as efficient co-catalyst for photocatalytic hydrogen evolution

PtO nanodots (∼2.5 nm) with high crystallinity are homogeneously deposited on the surface of g-C3N4 by a two-step refluxing-calcination method. XPS analysis indicates that a strong interaction is formed between PtO and g-C3N4, which is beneficial for the transportation of charge carriers from g-C3N4 to the co-catalyst and therefore results in a prolonged carrier life time in the composite. A series of electrochemical tests, including EIS, LSV and OCVD prove that PtO can significantly enhance the conductivity and light response of g-C3N4 compared with Pt. PtO and Serving as hydrogen evolution sites, the highly dispersed PtO nanodots can shorten the diffusion path lengths for photogenerated electrons in the host photocatalyst as well as suppress the undesired hydrogen oxidation reaction. Remarkably, PtO coupled g-C3N4 (0.19 wt% Pt) exhibits excellent hydrogen production activity, giving a turnover frequency of 552.7 h−1, which is 51 times that of Pt nanoparticles confined g-C3N4. The PtO nanodots also display high photostability and would not agglomerate into clusters or be reduced by the photogenerated electrons even after 40 h of irradiation. The present work also provides a new method for synthesis of high performance co-catalysts with low Pt content used in other photocatalytic reactions and energy applications.

Confinement of mesopores within ZSM-5 and functionalization with Ni NPs for deep desulfurization

Adsorptive desulfurization of transportation fuels can be affected by high diffusion limitations in porous sorbents due to the existence of larger kinetic dimensions of sulfur compounds and aromatics. In this contribution, we explored that metal based hierarchically ZSM-5-based micro-/mesostructures (MMZ) can overcome such scarcities without compromising the desulfurization activity. The mesopores of MMZ reduces path length for diffusion to the zeolite internal sites, and the metals (here, Ni) not only establish strong interactions but also enhances the adsorption configurations. The MMZ was synthesized by subjecting commercial H form ZSM-5 (SiO2/Al2O3 = 28) via sequential dissolution-self-assembly and doped with Ni via ultrasound-assisted impregnation. The samples were characterized by N2 sorption, NH3-TPD, XRD, H2-TPR TEM, SEM, FT-IR, elemental distribution mapping, and XPS techniques. The results revealed that high surface area up to 831 m2.g−1 and well-ordered mesostructures obtained when ZSM-5 was subjected to dissolution in 1.0 mol.L−1 NaOH solution and re-assembly of Al- or Si-containing species using cetyltrimethylammonium bromide (CTAB) as a template. The sulfur adsorption capacity was improved from 4.8 to 14.1 mg/g after Ni species and up to 10 wt% are highly dispersed over MMZ. The adsorption capacities of samples decrease in the following order 10Ni/MMZ > 5Ni/MMZ > 15Ni/MMZ > MMZ > 10Ni/ZSM-5 and the increased performance may be ascribed to increased surface acidity, well distributed Ni NPs and texture of MMZ. The isotherm data of thiophene adsorption were fitted well by Langmuir isotherm while kinetic study described that the adsorption data followed pseudo-second order model.

Mathematical model, validation and analysis of the drying treatment on quality attributes of chicory root cubes considering variable properties and shrinkage

This work presents an exhaustive three dimensional mathematical model to describe chicory root cubes drying process (Cichorium intybus L.) using a Fick’s diffusion model considering a variable diffusion path length in three dimensions. Experimental data obtained at laboratory scale is used to validate the proposed model. Experiments are conducted using a forced convection laboratory dryer at temperatures of 60°C and 80°C and air velocities of 0.2 and 0.7m/s to dry chicory root cubes of 1cm of side. Mathematical modeling is firstly used to represent the performance of the drying process with the temporal variations of the average humidity content, to determine diffusion coefficients, and afterwards, to obtain useful predictions on how the volumetric contraction of the cubes behaves under different operating conditions. The good agreements between experimental and predicted values prove that the proposed model can be applied to the precise description of experimental drying curves for chicory roots in three dimensions (3-D). In the ranges covered, the values of the effective moisture diffusivity were obtained between 5.63×10−5 and 8.21×10−5cm2/s. Results from the analysis of rehydration and browning aspects illustrate that drying conditions have effects on the quality attributes of the dried samples.

Supported Embryonic Zeolites and their Use to Process Bulky Molecules

X-ray amorphous zeolite precursors, embryonic zeolites, are prepared using tetrapropylammonium (TPA+) hydroxide as a structure directing agent. Their physicochemical properties are compared to those of a highly crystalline zeolite ZSM-5. Embryonic zeolites contain fewer acid sites, but their micropore volume and SBET area are higher than crystalline MFI-type material synthesized with TPA+. They can be introduced in the mesopores of a shaped silica-doped alumina matrix by two procedures: (i) impregnation of externally bred embryos and (ii) in situ growth of embryos to prepare composite catalysts. Their catalytic performances in the dealkylation of 1,3,5-triisopropylbenzene, a bulky molecule hardly penetrating the micropores of most zeolites, are superior to their highly crystalline ZSM-5 counterpart and the silica-doped alumina support. This is attributed to the highly accessible active sites of embryonic zeolites, located in an open microporosity leading to shorter diffusion path lengths. They offer inter...

Electrochemical Fabrication of Cu2o-Nanowires-Based Photo-Cathodes for Enhanced Solar Water Splitting

Solar-power driven photoelectrochemical (PEC) water splitting using semiconductor photoelectrodes is one of the most promising approaches for energy conversion, in terms of production of renewable hydrogen with a minimum carbon footprint. There are, however, enormous challenges to be overcome to obtain sufficient solar water-splitting efficiency, such as over-potential losses due to fast recombination rates of charge carriers, electrode photo-degradation, limited light-harvesting capacities, to name a few. In this work, we report an extensive study on Cu2O based photocathode for hydrogen evolution reaction (HER) in terms of enhancing its solar-to-hydrogen (STH) conversion efficiency, while using low-cost fabrication methods. We have addressed the inefficiency of Cu2O which mainly arises due to incompatible light absorption and charge carrier diffusion lengths, by fabricating highly reproducible Cu2O-nanowire arrays on Cu substrate. The reported optimal thickness of Cu2O for efficient absorption of sunlight is around 2-4 µm, while the minority charge carrier diffusion length is limited to 200-250 nm only. The nanostructured Cu2O, especially in the form of nanowire arrays, improves its performance through morphology control, by improving the light-harvesting capacity along the full length of wire and providing a shorter diffusion path length corresponding to the radial distance in the wire, towards the electrolyte. We prepared the photocathode by anodizing Cu foil to Cu(OH)2 nanowires at 10 mA/cm2, followed by oxidation to Cu2O nanowires in an inert atmosphere. Experimental measurements showed that the process was highly reproducible and resulted in an increased current density from 0.5 mA/cm2 for planar Cu2O to 82.3 mA/cm2 for Cu2O nanowires, both at 1 V cathodic bias. The photocurrent density was enhanced from 0.15 mA/cm2 for planar Cu2O to 5.54 mA/cm2 for the nanowire morphology, while using visible light source in all cases. All the photoelectrodes were reproducible in the regime of 4 µm thicknesses of Cu2O nanowires, oxidized over Cu-foil substrates. The STH efficiency calculated for all the fabricated photoelectrodes was as high as 3.54±0.21%, in comparison to 0.24% for the planar Cu2O photoelectrodes. Keywords: Cu2O, nanowires, photocathode, solar water splitting, photoelectrochemistry, STH efficiency, PEC.