Mass Transport Kinetics Research Articles

Characterization of a Raman Spectroscopic and Holographic System for Gas‐Liquid Flows in Microchannels

AbstractThe description and prediction of the highly transient processes on the molecular scale in gas‐liquid Taylor flows in microchannels are a great challenge. Due to a lack of in situ measurement techniques with satisfactory spatial and temporal resolution, existing models and numerical simulations of reactive two‐phase flows are poorly developed. The Raman spectroscopic and holographic system presented herein overcomes shortcomings regarding the high temporal resolution and concomitant difficulties with the high energy densities that must be coupled into microstructured devices to give good limits of detection. It gives insight into the concentration profiles in the liquid phase within the microchannel to study the interaction between hydrodynamics, mass transport, and reaction kinetics.

In-Situ neutron Diffraction and Electron Paramagnetic Resonance Spectroscopy to Examine Aging of Lithium Ion Batteries

In this work, we present a detailed investigation of the aging behavior in commercially produced 18650-type cells with a graphite/LFP cell chemistry. Two different kinds of cells are investigated, which only differ in the graphite type used as anode active material, namely mesocarbon microbeads (MCMB) or needle coke (NC). These graphite materials have a similar BET surface area (2.4 m2 g-1 for MCMB, 1.9 m2 g-1 for NC) but differ in the particle shape and morphology (inset Figure 1) and in the ratio of exposed basal and edge planes. Both cells showed good cycling stabilities with relative capacity losses of 8 % (MCMB) and 23 % (NC) after 4750 cycles with a 1C rate. In the first part, in situ neutron diffraction at the instrument SPODI of the Heinz Maier-Leibnitz Zentrum is used to study the cell aging. We demonstrate that, based on a careful comparison of neutron diffraction and electrochemical data, it is possible to differentiate several aging mechanisms like i) active lithium loss, ii) particle isolation or particle deactivation due to loss of ionic and/or electronic contact, and, iii) destruction of bulk electrode material (metal dissolution or irreversible phase transformation). In all investigated cells, active lithium loss is the only observable capacity loss mechanism. The second part focuses on the contribution of lithium plating to capacity fading. The enhanced capacity fading rate during the initial five hundred cycles for the needle coke cell in comparison to the MCMB cell (Figure 1) is assigned to partially irreversible lithium plating during 1C charging. The two carbons differ in particle shape and corresponding electrode properties (porosity and tortuosity), affecting Li-ion transport within the porous electrode.[1] They also differ in the nature of exposed graphite planes (edge vs. basal), affecting the charge transfer kinetics and solid electrolyte interphase (SEI) formation.[2] The question to what extent the differences in mass transport and electrode kinetics are responsible for the different lithium plating behavior of the MCMB and needle coke cells is investigated using several electrochemical techniques. Furthermore, the fundamental lithium plating mechanism and re-intercalation kinetics are studied in more detail using recently developed electrochemical operando electron paramagnetic resonance spectroscopy (EPR).[3] Figure 1 Capacity loss of 18650-type graphite/LFP cells upon cycling between 2.0 and 3.6 V with a rate of 1C for discharge and charge. Anode active materials are mesocarbon microbeads (MCMB) or needle coke (NC). Inset: SEM images of the different carbon materials.

Pulse Plating of Copper from Deep Eutectic Solvents

In recent years there has been considerable interest in the deposition of metals, alloys and semiconductors from ionic liquids (ILs). These melts possess a number of advantages over traditional aqueous solutions, especially for the deposition of reactive metals and where the current efficiency of the process is inherently low. Deep eutectic solvents (DES) are a new class ionic liquids which possess low toxicity, low cost, and relatively good electrochemical characteristics. They have been used to deposit a wide range of materials and, unlike traditional ILs, it is often possible to obtain acceptable deposit characteristics in the presence of significant amounts of water contamination. This makes them particularly suitable for industrial-scale plating. Until now, most studies employing ILs or DES have concentrated on DC plating and there have been relatively few reports of electrodeposition under pulse conditions. Ionic liquids have differing characteristics to aqueous solutions, particularly in terms of their double layer structure, mass transport properties, kinetics and conductivity. These differences indicate that pulse parameters developed for aqueous systems are unlikely to be optimal for deposition from ILs. For example, the very low diffusivity of the depositing species indicates that the DC and pulse limiting current are expected to be substantially lower compared to aqueous solutions. In this paper we will examine some of the constraints placed on the selection of pulse parameters dictated by the unusual properties of ionic liquids. As an example of a ‘model’ system we will examine copper deposition from a deep eutectic solvent (DES) comprising ethylene glycol and choline chloride. After selection of appropriate pulse parameters some preliminary results obtained for copper electrodeposition will be reported. Of particular importance is the presence of a corrosion reaction in the off-time which puts additional constraints on the choice of pulse parameters.

Utilization of Gastrointestinal Simulator, an in Vivo Predictive Dissolution Methodology, Coupled with Computational Approach To Forecast Oral Absorption of Dipyridamole.

Weakly basic drugs exhibit a pH-dependent dissolution profile in the gastrointestinal (GI) tract, which makes it difficult to predict their oral absorption profile. The aim of this study was to investigate the utility of the gastrointestinal simulator (GIS), a novel in vivo predictive dissolution (iPD) methodology, in predicting the in vivo behavior of the weakly basic drug dipyridamole when coupled with in silico analysis. The GIS is a multicompartmental dissolution apparatus, which represents physiological gastric emptying in the fasted state. Kinetic parameters for drug dissolution and precipitation were optimized by fitting a curve to the dissolved drug amount-time profiles in the United States Pharmacopeia apparatus II and GIS. Optimized parameters were incorporated into mathematical equations to describe the mass transport kinetics of dipyridamole in the GI tract. By using this in silico model, intraluminal drug concentration-time profile was simulated. The predicted profile of dipyridamole in the duodenal compartment adequately captured observed data. In addition, the plasma concentration-time profile was also predicted using pharmacokinetic parameters following intravenous administration. On the basis of the comparison with observed data, the in silico approach coupled with the GIS successfully predicted in vivo pharmacokinetic profiles. Although further investigations are still required to generalize, these results indicated that incorporating GIS data into mathematical equations improves the predictability of in vivo behavior of weakly basic drugs like dipyridamole.

A microfluidic chip for rapid single nucleotide polymorphism (SNP) genotyping using primer extension on microbeads

A microfluidic device, which performs a primer extension technique onto microbeads, is demonstrated to genotype Single Nucleotide Polymorphism (SNP) on genomic DNA. The device has on-chip heaters and sensors to create a uniform and yet stable temperature gradient between 56°C and 94°C across an array of serpentine microchannels, where the microbeads carry allele-specific primers passing through for thermal cycling. As the primers on the beads is designed to be only extended when perfectly matched with the SNP site of the template, the mismatched samples will not be amplified. The SNP discrimination is achieved. In addition that the microfluidic environment promote enhanced mass transport and better hybridization kinetics, the microbeads offer an advantage for faster thermal response and higher signal-to-noise ratio. Three most-frequent variants in one SNP location from genomic DNA of Taiwan country chicken are successfully distinguished in only five thermal cycles, compared to thirty (30) thermal cycles in traditional approaches. Moreover, the SNP genotyping procedures, which typically involves multiple steps in amplification and detection, were integrated onto a single chip.

Scaling the Aspect Ratio of Nanoscale Closely Packed Silicon Vias by MacEtch: Kinetics of Carrier Generation and Mass Transport

Metal‐assisted chemical etching (MacEtch) has shown tremendous success as an anisotropic wet etching method to produce ultrahigh aspect ratio semiconductor nanowire arrays, where a metal mesh pattern serves as the catalyst. However, producing vertical via arrays using MacEtch, which requires a pattern of discrete metal disks as the catalyst, has often been challenging because of the detouring of individual catalyst disks off the vertical path while descending, especially at submicron scales. Here, the realization of ordered, vertical, and high aspect ratio silicon via arrays by MacEtch is reported, with diameters scaled from 900 all the way down to sub‐100 nm. Systematic variation of the diameter and pitch of the metal catalyst pattern and the etching solution composition allows the extraction of a physical model that, for the first time, clearly reveals the roles of the two fundamental kinetic mechanisms in MacEtch, carrier generation and mass transport. Ordered submicron diameter silicon via arrays with record aspect ratio are produced, which can directly impact the through‐silicon‐via technology, high density storage, photonic crystal membrane, and other related applications.

Compactness of the Lithium Peroxide Thin Film Formed in Li-O2 Batteries and Its Link to the Charge Transport Mechanism: Insights from Stochastic Simulations.

We simulated the discharge process of Li-O2 batteries and the growth of Li2O2 thin films at the mesoscale with a novel kinetic Monte Carlo model, which combined a stochastic description of mass transport and detailed elementary reaction kinetics. The simulation results show that the ordering of the Li2O2 thin film is determined by the interplay between diffusion and reaction kinetics. Due to the fast reaction kinetics on the catalyst, the Li2O2 formed in the presence of catalyst (cat-CNF) shows a low degree of ordering and is more likely to be amorphous. Moreover, the mobility of the LiO2 ion pair, which depends largely on the nature of the electrolyte, also impacts the homogeneity of the compactness of the Li2O2 thin film. These results are of high importance for understanding the role of the catalyst and reaction kinetics in Li-O2 batteries.

The effect of mass transfer parameterization and ice retention on the scavenging and redistribution of SO2 by a deep convective cloud.

A chemistry module with the aqueous chemistry coupled with the complex 3D nonhydrostatic atmospheric model is used to investigate how the representation of gas-aqueous mass transfer and ice retention affect the SO2 redistribution in the presence of a convective cloud. Gas uptake to the liquid water is calculated using both Henry's law equilibrium (HE) and kinetic mass transport (KMT). The constant retention coefficients for SO2 (k ret=0.46) and for H2O2 (k ret=0.64) are used. It is shown that the amount of SO2 in the air at higher altitudes (10-12km) is greater when partial retention (PR) is included. All values of k ret between 0 and 1 represented the partial retention (PR), while complete retention (CR) means the entire mass of the gas from the solution remained in the ice phase (k ret=1). Total mass of SO2 in the air in the entire domain was greater in the case of PR than in the case when the CR was assumed (at the end of the integration time, 0.11% for HE and 0.61% for KMT) and in KMT than in the HE case (0.9% for CR and 1.4% for PR). The amount of SO2 in the ice phase was lower in the case of PR for both HE and KMT. The highest concentrations of S(IV) in rainwater were in the case of HE-CR, while the smallest values were in the case of KMT-PR. Total precipitation of S(IV) in PR exhibits 90% relative to CR, if HE was assumed. When KMT was used, PR gives 81.7% S(IV) relative to CR. Scavenging was the highest in the HE-CR case and the lowest in the KMT-PR case. If HE is assumed, averaged cumulative mass (ACM) of S(IV) precipitation per unit of domain surface for the CR case was 11.1% greater than in the PR case (if KMT was assumed, this difference was greater, 22.4%). Similarly, ACM for HE is 24.1% greater than KMT for the CR case and 36.8% for the PR case.

Electrical and hydrogen reduction enhances kinetics in doped zirconia and ceria: I. grain growth study

AbstractThe kinetics of mass transport is central to ceramic processing and device stability. In this work, the effect of electrical and hydrogen reduction on the grain growth behavior of doped zirconia and ceria has been investigated. Faster grain growth has been observed under reducing conditions in all cases. The results firmly establish that a depressed local oxygen potential can enhance cation kinetics in fluorite‐structured oxide ceramics. Meanwhile, a large electrical current can generate a sharp, spatially varied oxygen potential profile, creating a graded microstructure with a dramatic grain size transition across the length of the sample.

Modeling of Oxygen Diffusion Resistance in PEFCs in the Intermediate Potential Region

The current-voltage performance of polymer electrolyte fuel cells (PEFCs) in the intermediate voltage region has received attention from the standpoint of increasing the maximum power of the cell. The performance-limiting factors in this kinetic-mass transport mixed control region were first considered as a protonic resistance in the polymer electrolyte, an oxygen diffusion resistance in the pores of the catalyst layer (CL) and gas diffusion layer (GDL), and a diffusion resistance of the dissolved oxygen in the polymer electrolyte that covers the catalyst. These three factors were taken into consideration in mathematical models that predict the performance, which resulted in the predictions much better than experimental results. To bridge the gap, the diffusion resistance in the catalyst agglomerates, which are composed of carbon-supported catalysts and an ionomer, was proposed. However, huge agglomerates that the models assumed were not observed in the scanning electron micrographs of the cross sections of the catalyst layers. In the meanwhile, Kudo and Morimoto measured the oxygen diffusion resistance using Nafion thin films and concluded that the interfacial resistance at the Nafion-Pt or Nafion-gas interface gave rise to the diffusion resistance [1]. The existence of the resistance at the Nafion-Pt interface was supported by molecular dynamics simulations [2]. In this work, the interfacial resistance estimated from the measurement of limiting current densities was incorporated into the performance model and the model predictions were compared with experimental results. The discrepancy between them was assumed to stem from a dependence of diffusion resistance on the electrode potential and then the dependency was analyzed. In the experiments, the cell was operated at 65 °C with a large excess flow rate of hydrogen and 1% oxygen balanced with nitrogen, both humidified to 80% relative humidity, under atmospheric pressure. The condition was selected to reduce the overpotential variation in the through-plane direction caused by the protonic resistance and to avoid the effect of the produced water and generated heat, and thus to emphasize the effect of oxygen diffusion. Following the seminal studies of Mashio et al. [3], we decomposed the diffusion resistance into two resistances in series, namely, the one caused by molecular diffusion (Rmolec) and the remainder (Rother). Rmolec was estimated with the aid of limiting-current density measurements using 1% oxygen balanced with helium instead of 1% O2-N2. Rother was calculated by subtracting Rmolec from the overall diffusion resistance. We well call Rother determined by this procedure as Rother ref. The main features of the model to analyze the experimental results are as follows: (1) One-dimensional through-plane distribution is considered; (2) The anode CL and GDL are neglected; (3) Overpotential distribution and hence the reaction rate distribution is taken into account; (4) The oxygen reduction reaction rate is expressed by Tafel equation and is proportional to the oxygen concentration at the catalyst surface; (5) Oxygen concentration at the catalyst surface is calculated using the diffusion resistance and reaction rate. In Figure 1a, an experimental result (dotted line) and a model prediction (solid line) of the performance of a cell are compared. In the model, constants in Tafel equation are selected so that the prediction fits to the experiment in the activation-controlled region; the membrane and CL resistances were the values measured by impedance spectroscopy; Rmolec and Rother ref were used as the diffusion resistances. The model prediction gives larger current density than the experiment in the intermediate cathode potential region. We assumed that the discrepancy stemmed from the constant Rother (= Rother ref), and therefore hypothesized that Rother is a function of electrode potential. We therefore increased Rother from Rother ref at each potential to find the value that the current density from model prediction matched the experimental value. The resulting Rother is shown in Fig. 1b as a function of cathode potential (Rother ref is shown as a dashed line). Rother increased with potential except for high potential region, where the effect of diffusion resistance on the performance is less significant and hence the accuracy of the fitting is deteriorated. The effect of catalyst loading will be discussed and a mathematical formulation of Rother will be presented. [1] K. Kudo, Y. Morimoto, ECS Trans., 50, 1487 (2012)[2] R. Jinnouchi, K. Kudo, N. Kitano, Y. Morimoto, Electrochim. Acta, 188, 767 (2016)[3] T. Mashio, A. Ohma, S. Yamamoto, K. Shinohara, ECS Trans., 11, 529 (2007) Figure 1

Performance of Pd/Graphene Nanocomposite Catalysts in H2/O2 Anion Exchange Membrane Fuel Cells

With the recent developments of high-performance anion-exchange membranes (AEM), there has been an increased interest towards studying various electrochemical reactions such as ethanol electrooxidation, hydrogen oxidation reactions and oxygen reduction reactions in AEM fuel cells (AEMFC). This increased attention has been attributed to the faster electro oxidation kinetics, increased stability and lower poisoning of the catalysts in alkaline media. And since AEMFCs are not limited to platinum - unlike in proton exchange membrane fuel cells (PEMFCs) with highly acidic environments - intensive efforts towards developing alternatives such as palladium (Pd) and Pd-based alloys are currently underway.1 , 2 Various studies have shown the enhanced stability and higher activities of Pd-based catalysts compared to Pt in alkaline medium using potentiodynamic and potentiostatic techniques. However, there are only a limited number of studies that have investigated the performance of Pd-based electrocatalysts in AEMFCs. Also, majority of these studies have demonstrated relatively low performances in AEMFCs, mainly due the complications associated with forming the triple-phase boundary (TPB) structure in the membrane layer, where reactions are taking place in the electrolyte, gaseous fuel and electrode interface. The density of the TPB, along with the intrinsic activities of the catalysts can play an important role in determining the overall performance of AEMFCs. Moreover, Pd-based are usually synthesized using surfactants, organic stabilizers or reducing agents, that can get adsorbed onto the surface of Pd and inhibit ionomer-catalyst-fuel interactions in the TPB. To mitigate the limitations associated with TPB structure and subpar catalyst activities, macroporous three-dimensional Graphene nanosheet (3D-GNS) supports with controlled morphologies and porosities were fabricated by utilizing silica sacrificial templates.3 The sacrificial templates were then etched to leave a network of porous channels within its matrix, and utilized as supports for Pd nanoparticles. The porous structure of these highly graphitized 3D-GNS supports can facilitate mass-transport kinetics by enabling the ionomer and polymer electrolyte getting the reactants to get catalyzed by Pd nanoparticles. Also, to enhance ionomer-nanoparticle interactions, the Pd nanoparticles of an average size of 3-5 nm were synthesized using a previously established surfactant-free soft alcohol reduction method.4 In this study, anion exchange catalyst-coated membranes (CCMs) were prepared using Pd nanoparticles supported on both commercial carbon blacks (Vulcan) and 3D-Graphene supports as both cathode and anode catalyst in H2/O2 fed AEMFCs. The Pd catalyst inks were prepared by mixing the dispersed Pd-catalyst in an optimized solution of isopropyl alcohol and quaternary ammonium-functionalized AS4 ionomer. The inks were then applied onto an anion exchange membrane (A201, Tokuyama) with an active area of 5 cm2. The CCMs was then hydrated in 0.5 M KOH for 24 hours, followed by rinsing in de-ionized water for 24 hours. It was then sandwiched between gas diffusion layers and annealed with gaskets by pressing under 500 psi for 5 minutes. The cell was then assembled and activated by operating at 0.3V at 60ºC for 10 minutes under humidified O2 and H2, followed by the measuring the polarization curves. Preliminary results show that Pd nanoparticles synthesized using surfactant-free method SARM and deposited on commercial carbon supports have and OCV of 0.95 V and a peak power output of 200 mWcm-2. These results already show the promising potential of using surfactant-free non-alloyed monometallic Pd nanoparticles. Further tests will also be performed to show the effect of porous 3D-Graphene supports towards modifying the TPB interphase and its effects on MEA performance in both Ethanol/O2 and H2/O2 fed AEMFCs. The results from this study will not only contribute towards the development of Pt-free oxidative and reductive electrocatalysts, but also lead to further advancements in AEMFC technology that will enable it to operate with other fuels such as methanol and hydrazine.

Identification of Polarization Losses in High-Temperature PEM Fuel Cells by Distribution of Relaxation Times Analysis

In this study, Distribution of Relaxation Times (DRT) analysis is applied for identification of polarization losses in High Temperature Polymer Electrolyte Membrane Fuel Cells (HT-PEMFCs). Electrochemical impedance spectra recorded for different air stoichiometries and current densities are analyzed. While basic impedance analysis broadly recognizes the presence of three features, DRT reveals at least six distinct processes occurring on different time scales. By varying the stoichiometry and current bias, three of those processes are attributed to mass transport and cathodic kinetics. Furthermore, the DRT analysis suggests that the cathode kinetics may involve two different processes, as opposed to a single-feature process that is often assumed. These results have improved our understanding of the loss mechanisms in HT-PEMFCs and provide valuable guidance for fuel cell optimization.

Nanowire Array Structures for Photocatalytic Energy Conversion and Utilization: A Review of Design Concepts, Assembly and Integration, and Function Enabling

The increasing human demand for clean and renewable energy and a cleaner environment has stimulated various materials related research and development for efficient solar energy conversion and utilization, photocatalytic environmental remediation, and photoassisted selective conversion of organic compounds. Among the various candidate solutions to improving the efficiency of these processes, semiconductor nanowire (NW) and nanorod (NR) array structures have been offering a unique tool box combining desirable characteristics in structure, function, and applicability. Efficient photocatalytic energy conversion requires excellent light absorption, readily charge separation, transport, and collection, as well as fast kinetics of interfacial reactions and mass transport of reactants. The long axis of the NW enables adequate light absorption, while the radical axis provides short electron–hole separation distance. Additionally, the NW arrays have relatively high surface area to facilitate interfacial kinetics while their open pore structure allows good mass transport properties. This study reviews the current status of research on NW and NR array structures of some most popular semiconducting materials for photocatalytic energy conversion and utilization including photocatalytic water splitting and CO2 reduction/fixation, dye/quantum dot sensitized solar cells, and other photoassisted reactive applications such as pollutant degradation, selective conversion of organic compounds, and biological disinfection.

Face-Discriminating Dissolution Kinetics of Furosemide Single Crystals: In Situ Three-Dimensional Multi-Microscopy and Modeling

A versatile in situ multi-microscopy approach to study the dissolution kinetics of single crystals is described, using the loop diuretic drug furosemide as a testbed to demonstrate the utility of the approach. Using optical microscopy and scanning ion-conductance microscopy in combination, the dissolution rate of individual crystallographically independent crystal faces can be measured quantitatively while providing a direct visualization of the evolution of crystal morphology in real time in three dimensions. Finite element method models using experimental data enables quantitative analysis of dissolution fluxes for individual faces and determination of the limiting process—mass transport or interfacial kinetics—that regulates dissolution. A key feature of the approach is that isolated crystals (typically <60 μm largest characteristic dimension) in solution during dissolution experience high and well-defined diffusion rates. The ability to obtain this quantitative information for individual crystal faces...

Computational Fluid Dynamics Modelling of Photoelectrocatalytic Reactors for the Degradation of Formic Acid

In this study, computational fluid dynamics (CFD) simulation was used to predict the performance of photoelectrocatalytic (PEC) reactors with surface reactions. PEC process is a promising and sustainable method that is capable for simultaneous organic degradation and hydrogen production. However, the overall PEC process efficiency is still unsatisfactory and not ready for scale-up application. Preliminary study using CFD model can help to reduce development time, money and effort in experimental work while providing comprehensive analysis and optimum PEC reactor design prior to its real physical fabrication. CFD model integrates irradiance distribution, hydrodynamics, species mass transport and chemical reaction kinetics within the reactor. The performance of PEC reactor for organic degradation depends on reactor configurations and hydrodynamic conditions. Thus, the main aim of this study was to optimize different PEC reactor designs using CFD modelling by varying the reactor configurations and hydrodynamic flow conditions for improved efficiency in degrading the sample organic pollutant of formic acid. The CFD modelling showed higher formic acid degradation efficiency for the simulated convex surface photoreactor than the flat surface photoreactor due to the former possess the ability to concentrate the absorbed light onto the photoanode surface. Besides, the CFD modelling showed that the formic acid degradation rate increased with decreasing inlet fluid flow velocity. This was due to the uniform flow distribution that enables evenly coverage of photoanode surface for subsequent degradation of formic acid in the PEC reactors. Further experimental work is required to validate the CFD simulation to allow better understanding and improvement of the overall efficiency of PEC reactors.

Graphene aerogel based monolith for effective solid-phase extraction of trace environmental pollutants from water samples

Graphene aerogel (GA), a typical kind of three-dimensional (3D) macroscopic assembly, not only provides inherently excellent properties of graphene sheets (GS), but also exhibits interesting characteristics of the 3D macroporous architecture including large and tunable pore volumes, high specific surface areas and fast mass transport kinetics. Thus, it is rational to expect GA to be an efficient adsorbent for solid-phase extraction (SPE). In this paper, a novel GA monolith based solid-phase extraction method was investigated in the application of environment analysis. The GA monolith based SPE cartridge was fabricated directly in the empty cartridge through template-free “sol-cryo” method. Due to the efficient mass transfer, more adsorption sites as well as effective retention for the analytes, the adsorption property of GA for bisphenol A revealed better performance than that of GS. What’s more, GA also outperformed in loading and eluting for target analysis. On the basis of the above advantages, the obtained cartridge was applied for the separation of environmental pollutants from water samples. Taking endocrine disrupting chemicals and polychlorinated biphenyls as the polar and weak polar model analytes, optimizing several parameters influencing the recoveries, limits of detection in the range of 0.01–0.11ngmL−1 and 0.19–1.53ngL−1 for the two series of compounds were provided by the established methods. The satisfied sensitivity was accessed and recoveries ranging from 76.3 to 112.5% were obtained for all the analytes when the proposed methods were applied in real water samples analysis. The results revealed the potential of GA as an effective sorbent in sample preparation processes.

Modeling and Experimental Study of Micro Enzymatic Biofuel Cells

Miniaturized enzymatic biofuel cells (EBFCs) converting biological energy into electrical energy by using enzyme-modified electrodes are considered as a candidate to power implantable medical devices and portable electronics. In this study, modeling of the EBFC system has been conducted using finite element analysis based on COMSOL Multiphysics 4.3a in terms of cell performance, efficiency and optimum cell configurations. Two modules have been applied to obtain the maximum theoretical cell performance of on-chip microelectrodes: 1) diffusion module to incorporate the mass transport and enzymatic kinetics; 2) conductive module to integrate concentration and potential. The design rule has been developed in the steady state. The effect of orientation of the microelectrode arrays in blood artery has been studied. In the experimental part, integration of the high surface area nanomaterials such as carbon nanotubes (CNTs) and graphene could be one of the effective solutions to further improve the performance of current EBFCs. The EBFC systems employing 3D carbon micropillar arrays integrated with different nanomaterials, such as graphene, reduced graphene oxide (rGO) and rGO/CNTs composite have been investigated. The fabrication process of this micro-system combined top-down carbon microelectromechanical system (C-MEMS) technology to fabricate the 3D micropillar arrays platform and bottom-up electrophoretic deposition (EPD) to deposit various carbon based nanomaterials onto the 3D micropillar arrays. The materials characterization, electrochemical characterization and device evaluation have been conducted. Upon comparison among bioelectrodes integrated with different nanomaterials, the amperometric response of the rGO/CNTs based bioelectrodes exhibited the best electrochemical performance and the as-resulted 3D micro EBFC generated a maximum power density of 196.04 µWcm-2 at 0.61 V. In addition, modeling of the rGO/CNTs EBFC system based on finite element analysis has also been conducted to predict the theoretical cell performance. The power density of actual device has been noted to be 71.1% of the modeling result.

Evaluation of methanol oxidation catalysts by rotating disc voltammetry

Rotating disc voltammetry of 0.1M methanol at thick catalyst layers shows significant increases in current with increasing rotation rate. Although a mass transport limited current was not reached, due to inhibition of methanol oxidation by Pt oxide at high potentials, linear Koutecky-Levich plots were obtain in the mixed kinetic-mass transport region. For a carbon supported Pt catalyst, the slopes of the Koutecky-Levich plots corresponded to a methanol oxidation efficiency of ca. 50% (average n-value of 3.1 electrons per molecule), while the efficiency was ca. 100% for a PtRu black catalyst. The methanol diffusion coefficient obtained with the PtRu black catalyst was in good agreement with literature values. Koutecky-Levich plots provide mass transport corrected kinetic currents, in the mixed kinetic-mass transport region, that complement values obtained at lower potentials by cyclic voltammetry or at constant potential.

Study of mass transport kinetics in co-doped Ba0.9Sr0.1Ce0.85Y0.15O3 − δ by electrical conductivity relaxation

In this work, Sr2+ and Y3+ co-doped BaCeO3-based perovskite Ba0.9Sr0.1Ce0.85Y0.15O3−δ (BSCY) was synthesized by solid-state reaction method, and its mass transport behavior was studied using electrical conductivity relaxation (ECR) measurements in 600–800°C range with respect to variations in oxygen partial pressure (pO2) and water vapor pressure (pH2O). During oxidation/reduction process, the electrical conductivity showed a monotonic relaxation behavior and in −2.65≤log(pO2/atm)≤−0.67 range the values of oxygen chemical diffusivity (D˜vO) and surface exchange coefficient (κvO) varied in 10−6–10−4 cm2⋅s−1 and 10−4–10−2 cm⋅s−1 range, respectively. However, during hydration/dehydration process, the electrical conductivity showed a non-monotonic twofold relaxation behavior, and hydrogen, and oxygen chemical diffusivities (D˜iH) and (D˜vH) values were in 10−5–10−3 and 10−6–10−4 cm2⋅s−1 range, respectively, whereas hydrogen and oxygen surface exchange coefficient (κiH) and (κvH) values were in 10−3–10−2 and ~10−4–10−2 cm⋅s−1 range, respectively. The comparison of H and O diffusivities of BSCY with BaCe0.85Y0.15O3−δ (BCY) showed that during hydration/dehydration the diffusivities of BSCY were lower than that of BCY. The long-term electrical conductivity measurement for ~800h in humidified conditions showed a gradual decrease, which might be due to the gradual appearance of rhombohedral phase in otherwise orthorhombic BSCY, as confirmed by the X-ray diffraction (XRD) measurements.

Two-phase model of hydrogen transport to optimize nanoparticle catalyst loading for hydrogen evolution reaction

With electrocatalysts it is important to be able to distinguish between the effects of mass transport and reaction kinetics on the performance of the catalyst. When the hydrogen evolution reaction (HER) is considered, an additional and often neglected detail of mass transport in liquid is the evolution and transport of gaseous H2, since HER leads to the continuous formation of H2 bubbles near the electrode. We present a numerical model that includes the transport of both gaseous and dissolved H2, as well as mass exchange between them, and combine it with a kinetic model of HER at platinum (Pt) nanoparticle electrodes. We study the effect of the diffusion layer thickness and H2 dissolution rate constant on the importance of gaseous transport, and the effect of equilibrium hydrogen coverage and Pt loading on the kinetic and mass transport overpotentials. Gaseous transport becomes significant when the gas volume fraction is sufficiently high to facilitate H2 transfer to bubbles within a distance shorter than the diffusion layer thickness. At current densities below about 40 mA/cm2 the model reduces to an analytical approximation that has characteristics similar to the diffusion of H2. At higher current densities the increase in the gas volume fraction makes the H2 surface concentration nonlinear with respect to the current density. Compared to the typical diffusion layer model, our model is an extension that allows more detailed studies of reaction kinetics and mass transport in the electrolyte and the effects of gas bubbles on them.