Reduction In Active Surface Area Research Articles

Effects of two delivery matrix components, alginate and polyethylene glycol, on the intestinal tract and inflammatory response of juvenile rainbow trout, Oncorhynchus mykiss, Walbaum

AbstractA critical step in the administration of oral vaccines in aquaculture is the effective protection of the vaccine antigens through the delivery route of water to stomach to the mucosal immune system in the gut. Bioactive substances can be incorporated into protective matrices to ensure safe ingestion and controlled release into the posterior digestive tract. Polyethylene glycol and alginate are promising and frequently applied substances in the oral administration of pharmaceutical substances; however, their direct influence on the intestinal tract or inflammatory response is not well investigated in aquaculture fish. In the current study, these two substances were evaluated separately and in combination. They were included in a commercial diet for juvenile rainbow trout (18.7 ± 0.1 g) and applied over 22 days in controlled feeding to determine possible gut alterations and inflammatory responses. Three different sections of the intestine, the anterior pyloric caeca, the mid‐intestine, and the posterior intestine, were histologically examined, and expression of inflammatory genes was measured in each. The inclusion of polyethylene glycol in the diet led to a significant reduction of villus height in the pyloric caeca and thus a significant reduction in active surface area. In the mid‐intestine, alginate inclusion resulted in a significant reduction of the mucus‐secreting goblet cells and a significant increase in villus width. In the posterior intestine, polyethylene glycol inclusion caused a significant increase in number of large vacuoles present; the addition of alginate buffered this effect. The results of the histological evaluation indicated symptoms that are usually evident in intestinal inflammation and can reduce functions of the respective intestinal segments. No significant increases in pro‐inflammatory gene expression tumor necrosis factor‐α, interleukin (IL)‐8, or IL‐1β were present, indicating no severe inflammatory response. Based on the observed histological impairments, negative consequences such as reduced efficiency of an oral vaccine or productivity of aquaculture rearing due to hindered nutrient digestion can be considered unlikely.

(Invited) 3D Pore-Scale Transport Model Incorporating Realistic Cathode Morphology and Intermediate Reactions in Lithium-Air Batteries

The lithium-air (Li-air) battery, with its usable energy density close to 1,700Wh/kg1, has captured worldwide attention as a promising battery solution for electric vehicles. However, a major hurdle facing the development of Li-air battery systems is their poor round-trip efficiency owing to the formation of electrically insulating lithium peroxide (Li2O2) at the cathode surface. Computational modeling has proven to be an indispensable tool in the analysis of battery systems, however, all existing simulations thus far treat the electrode/Li2O2 matrix as a homogenous continuum and utilize simply-shaped electrode morphologies, such as spheres and rods, to construct volume-averaged expressions (see Fig. 1a) for porosity and surface area and are insufficient to probe the effect of precise electrode microstructures and Li2O2 growth2,3. Utilizing a pore-scale transport-resolved model of the Li-air battery, the complex electrode and Li2O2morphologies can be directly incorporated and their effects on the system-level performance can be evaluated. In this work, we present a pore-scale transport resolved model (see Fig. 1b) of the Li-air battery that fully accounts for the electrode microstructure and peroxide growth4. This approach requires no empirical correlations regarding the electrode morphology. 3D reconstructed images of real carbon fiber cathode (Fig. 2) are used as geometric inputs to the model. Results obtained from our pore-scale model agree well with experiments and the validated model is then used to predict the galvanostatic discharge behavior of a Li-air cell for a variety of electrode morphologies and design parameters. Fig. 3 shows the cell voltage versus specific capacity curves for nanofiber electrodes of varying mean pore spacing from our 3D pore-scale model. The cell discharge capacity is limited by the spacing between nanostructures, which may lead to pore blocking and hence the reduction of active surface area. To capture the different growth morphologies two potential peroxide forming reactions, based on an intermediate oxygen reduction reaction where O2 reduces to becomes superoxide (O2 -), are incorporated as shown in Fig. 1c. Superoxide may form at the surface of the electrode and then follow one of two possible paths; reaction at the electrode surface to form lithium peroxide or diffusion into the bulk liquid to nucleate lithium peroxide in the electrolyte5. Lithium peroxide created at the electrode/electrolyte surface will produce a thin film growth morphology while peroxide particles generated further away in the electrolyte contributes to the formation of larger particle shaped or toroidal shaped growth. The competition between the different reactions can be explicitly accounted for in the pore-scale modeling framework. A discrete element method is used to simulate the nucleation and movement of lithium peroxide. Through extensive 3D simulations, we systematically examine the effect of drawing current, ORR rate coefficients, oxygen solubility, mean pore size and distribution on Li2O2growth and cell performance as to better understand the underlying physics of capacity fading during cycling. The methodology presented here can be applied to other electrochemical systems that include an insoluble product formation as a result of the reaction process and will be a valuable tool for rational design of electrode microstructures for improved cell performance.

Synthesis of Ca-doped spinel by Ultrasonic Spray Pyrolysis

MgAl2O4 is a stable catalyst support with potential for replacing gamma-alumina in several applications. However, synthesis of magnesium spinel requires elevated temperatures to avoid phase separation (in MgO and Al2O3) at low temperatures, leading to coarsening and reduction of active surface area. In this work, nano CaO-doped and undoped magnesium aluminate were successfully prepared by Ultrasonic Spray Pyrolysis (USP), using a simple adapted experimental set-up operating at 1100°C. During the process, the particles stay at high temperatures for a short period of time, allowing phase stability and limited coarsening. The influence of calcium oxide on the particles morphology and structure was investigated via X-ray diffraction, N2 adsorption, X-ray fluorescence, scanning electron microscopy and transmission electron microscopy. The spinel nanopowders were obtained as spherical porous agglomerates of ~1µm. The resulting powder showed low crystallite sizes in the 5–10nm range and high specific surface area from 110.0 to 76.6m2g−1.

Lanthanum doping for longevity of alumina catalyst bed in hydrogen peroxide thruster

The effects of doping alumina catalyst supports with lanthanum were investigated in terms of catalyst performance in a high concentration hydrogen peroxide monopropellant thruster. The catalyst support was doped with lanthanum by wet impregnation method in different conditions and characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), and bulk crushing strength (BCS) test. XRD showed adequate crystal structure by doping lanthanum on alumina and BET analysis demonstrated that the reduction of active surface area was modest, even though the catalyst was cured at a temperature higher than the phase transition temperature of alumina. The BCS test exhibited a 57.75% increase in the mechanical strength of the lanthanum doped alumina compared to pure γ-alumina. A 50 N monopropellant thruster was built and firing tests were conducted to examine the longevity of manganese oxide lanthanum doped alumina catalyst. The catalyst pellets were removed from the thruster and analyzed. The experimental results have shown significant improvements in the longevity of the catalyst. The firing tests have shown significant improvement in catalyst volume loss, along with characteristic velocity efficiency higher than 98%. The catalyst with lanthanum doped alumina has also shown better pressure stability and a steady pressure drop across the catalyst bed.

3D Pore-Scale Transport Model Incorporating Realistic Cathode Morphology and Peroxide Growth in Lithium-Air Batteries

The lithium-air (Li-air) battery, with its usable energy density close to 1,700Wh/kg1, has captured worldwide attention as a promising battery solution for electric vehicles. However, a major hurdle facing the development of Li-air battery systems is their poor round-trip efficiency owing to the formation of electrically insulating lithium peroxide (Li2O2) at the cathode surface. Computational modeling has proven to be an indispensable tool in the analysis of battery systems, however, all existing simulations thus far treat the electrode/Li2O2 matrix as a homogenous continuum and utilize simply-shaped electrode morphologies, such as spheres and rods, to construct volume-averaged expressions (see Fig. 1a) for porosity and surface area and are insufficient to probe the effect of precise electrode microstructures and Li2O2 growth3,4. Utilizing a pore-scale transport-resolved model of the Li-air battery, the complex electrode and Li2O2morphologies can be directly incorporated and their effects on the system-level performance can be evaluated. In this work, we present a pore-scale transport resolved model (see Fig. 1b) of the Li-air battery that fully accounts for the electrode microstructure and peroxide growth. This approach requires no empirical correlations regarding the electrode morphology. 3D reconstructed images of real carbon fiber cathode (Fig. 2) are used as geometric inputs to the model. The primary drawback of a pore-scale approach is the large amount of computational resources required in order to resolve the appropriate length and times scales for peroxide growth in the pore-scale framework. However, recent advancements in high performance computing have shown Graphics Processing Units (GPUs) to be a powerful alternative to the traditional parallel computation techniques for solving problems in computational fluid dynamics5and in this work multiple GPUs are used together to overcome the large computational challenge. The model incorporates a thickness-dependent conductivity of Li2O2 based on inputs from the density functional theory and rate-dependent growth morphology of Li2O2 6. Results obtained from our pore-scale model agree well with experiments and the validated model is then used to predict the galvanostatic discharge behavior of a Li-air cell for a variety of electrode morphologies and design parameters. Fig. 3 shows the cell voltage versus specific capacity curves for electrodes with different nanostructure designs from our previous 2D pore-scale model7. The cell discharge capacity is limited by the spacing between nanostructures, which may lead to pore blocking and hence the reduction of active surface area. Through extensive 3D simulations, we systematically examine the effect of drawing current, ORR rate coefficient, oxygen solubility, fiber and pore sizes on Li2O2growth and cell performance as to better understand the underlying physics of capacity fading during cycling. The methodology presented here can be applied to other electrochemical systems that include an insoluble product formation as a result of the reaction process and will be a valuable tool for rational design of electrode microstructures for improved cell performance.

Nucleation and Growth of Lithium Peroxide in the Li/O2 Cell

The aprotic Li–O2 battery offers a vast improvement in theoretical energy density up to ten times that of current Li-ion batteries, but still suffers from shortcomings in cell and material design. Some of these shortcomings stem from the large gap in understanding between fundamental studies and cell cycling experiments. In particular, it is known from computational studies that the solid Li2O2 discharge product is an electronic insulator, but SEM images of discharged cathodes show particles of Li2O2 much larger than what electron conduction and tunneling would allow. In addition, the shape of the discharge curves does not match the expected exponential resistance rise of a cell limited by electron tunneling. In order to bridge Li2O2 properties with experimental observations, we developed a model in which Li2O2 forms as discrete particles on the cathode surface by using a combination of Kolmogorov’s statistical phase transformation theory and the atomistic nucleation theory. Nucleation rate was modeled according to a two-step process involving formation of a LiO2 intermediate. Despite its relative simplicity, the proposed model provides a near-quantitative description of empirically observed discharge curves (Figure 1). We find that the sudden potential drop at the end of discharge is caused by the reduction in active surface area as Li2O2 covers the cathode surface. Thus, the extra overpotential gained during discharge is the kinetic overpotential of the ORR rather than an ohmic overpotential through the Li2O2 bulk. This model can be used as a guide for cathode and electrolyte design, by maximizing the amount of Li2O2 that can be collected for a given amount of electrochemically active surface area. One strategy is to promote the growth of larger Li2O2 particles. For cathodes, this means reducing the rate of nucleation by having a higher nucleation energy barrier or a lower concentration of active sites. For electrolytes, this means solvating the LiO2 intermediate to promote the superoxide disproportionation pathway. In principle, the general form of this model is applicable to any electrochemical cell that produces a solid passivation layer. Figure 1: (a) Empirically observed discharge curves (symbols), compared with fit of the nucleation and growth model (lines). (b) Analysis of model parameters shows that they are independent of operating conditions. Figure 1

Performance degradation of direct formic acid fuel cell incorporating a Pd anode catalyst

Electrochemical and physical analysis is employed to verify the performance degradation mechanism in direct formic acid fuel cells (DFAFCs). The power density of a single cell measured at 200mAcm−2 decreases by 40% after 11h of operation. The performance of the single cell is partly recovered however, by a reactivation process. Various analytical methods such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electrochemical impedance spectroscopy (EIS) are used to investigate the mechanism of performance degradation. The analytical results show that the electrolyte membranes in the DFAFC are stable for 11h of operation after the reactivation process. The major factors causing performance degradation in the DFAFC are an increment in the anode charge-transfer resistance and a growth in the particle size of the Pd anode catalyst. The anode charge-transfer resistance, confirmed by EIS, increases with operation time and is due to poisoning of the catalyst surface. Although it is not clear what chemical species poisons the catalyst surface, the catalyst surface is cleaned by the reactivation process. Performance losses caused by surface poisoning are completely recovered by the reactivation process. Increase in catalyst size induces a reduction in active surface area, and the performance loss caused by the growth in catalyst size cannot be recovered by the reactivation process.

Iridium oxide-based nanocrystalline particles as oxygen evolution electrocatalysts

Iridium-based oxides are highly active as oxygen evolving electrocatalysts in PEM water electrolyzers. In this work XRD reveals that Ir-Sn oxides contain a single rutile phase with lattice parameters between those of pure IrO2 and SnO2. Addition of Ru leads to the synthesis of a core-shell type material due to the strong agglomeration of Ru colloids during the preparation procedure. The shell of this material consists of an Ir-Sn-Ru oxide deficient in Ru relative to the bulk. This leads to a decrease in the surface noble metal concentration (as found by XPS), which in turn results in a significant reduction in electrochemically active surface area. Polarization analysis indicates that the addition of Ru can influence the rate-determining step or mechanism by which oxygen is evolved. In a PEM water electrolysis cell, small additions of Sn do not significantly reduce the operating performance, however larger additions cause a performance loss due to a reduction in active surface area and increased ohmic resistance. When a pure IrO2 anode is used, a cell voltage is 1.61 V at 1 A cm−2 and 90°C.

Simulation of residue hydrodesulfurization reaction based on catalyst deactivation model.

A model was proposed to describe deactivation of hydrodesulfurization catalysts for residues. In this model, demetallization interacts with coking reaction on a catalyst, and these two reactions cause both active site poisoning and pore plugging. According to the pore diffusion theory, the reduction of active surface area and effective diffusivity affects the catalyst deactivation. A simulator for residue hydrodesulfurization was developed on the basis of this deactivation model.The parameters of this simulator were obtained from residue hydrodesulfurization experiments, and they were found to be dependent on the properties of the catalyst and the feedstock. Simulation by use of the above specified parameters enabled to predict the actual operations of bench scale fixed-bed and moving-bed reactors.