OXYGEN LIMITATION IN MICROFLUIDIC BIOFUEL CELLS

Biofuel cells are devices that use biocatalysts (enzymes or microbes) to convert biochemical energy directly into electrical energy. Microfluidic biofuel cells exploit the lack of active mixing at microscale dimensions to eliminate the use of proton exchange membranes that separate anolyte and catholyte streams. Simulation of this system, by solving the governing 3-D conservation equations (flow, species transport), reveals that oxygen availability limits the performance of the cathode. An exponential decay in the availability of oxygen at the cathode is observed along the length of the microchannel, indicating that increasing the number of electrode pairs reduces the overall current density. This conclusion is consistent with experimental observations. Increasing electrolyte flow rates can reduce the mass transport limitations by decreasing the diffusion boundary-layer thickness, but disparity between the flow rates of the anolyte and catholyte can induce wastage of dissolved oxygen.     

Hybrid layered double hydroxides-polypyrrole composites for construction of glucose/O2 biofuel cell

In this work, two layered double hydroxides, Zn₂Cr–ABTS and Zn₂Al–Fe(CN)₆ LDH, have been synthesized and characterized by X ray diffraction and FTIR spectroscopy to confirm the intercalation of redox anions between inorganic layers. These redox active hybrid materials have been used to electrically connect laccase (Lac) and glucose oxidase (GOx) in biofuel cell devices. Co-immobilization of hybrid LDH and enzymes has been performed by entrapment in electropolymerized films of polypyrrole deposited on porous carbon tubular electrodes. Lac/Zn₂Cr–ABTS cathodic electrode allows the electro-enzymatic reduction of O₂, whereas the anodic electrode GOx/Zn₂Al–Fe^III(CN)₆was used for the electro-enzymatic oxidation of glucose. With a two compartment configuration, a maximum power density of 45 μW cm⁻²was obtained at 0.2 V.     

An enzyme-based microfluidic biofuel cell using vitamin K3-mediated glucose oxidation

Viamin K₃-modified poly-l-lysine (PLL-VK₃) was synthesized and used as the electron transfer mediator during catalytic oxidation of NADH by diaphorase (Dp) at the anode of biofuel cell. PLL-VK₃ and Dp were co-immobilized on an electrode and then coated with NAD⁺-dependent glucose dehydrogenase (GDH). The resulting enzymatic bilayer (abbreviated PLL-VK₃/Dp/GDH) catalyzed glucose oxidation. Addition of carbon black (Ketjenblack, KB) into the bilayer enlarged the effective surface area of the electrode and consequentially increased the catalytic activity. An oxidation current of ca. 2 mA cm⁻² was observed when the electrochemical cell contained a stirred 30 mM glucose, 1.0 mM NAD⁺, pH 7.0 phosphate-buffered electrolyte solution. The performance of glucose/O₂ biofuel cells, constructed as fluidic chips with controllable fuel flow and containing a KB/PLL-VK₃/Dp/GDH-coated anode and an Ag/AgCl or a polydimethylsiloxane-coated Pt cathode, were evaluated. The open circuit voltage of the cell with the PDMS-coated Pt cathode was 0.55 V and its maximum power density was 32 μW cm⁻² at 0.29 V when a pH 7.0-buffered fuel containing 5.0 mM glucose and 1.0 mM NAD⁺ was introduced into the cell at a flow rate of 1.0 mL min⁻¹. The cell's output increased as the flow rate increased. During 18 h of continuous operation of the cell with a load of 100 kΩ, the output current density declined by ca. 50%, probably due to swelling of the enzyme bilayer.     

A conducting polymer/ferritin anode for biofuel cell applications

An enzyme anode for use in biofuel cells (BFCs) was constructed using an electrically connected bilayer based on a glassy carbon (GC) electrode immobilized with the conducting polymer polypyrrole (Ppy) as electron transfer enhancer, and with horse spleen ferritin protein (Frt) as electron transfer mediator. The surface-coupled redox system of nicotinamide adenine dinucleotide (NADH) catalyzed with diaphorase (Di) was used for the regeneration of NAD⁺ in the inner layer and the NAD⁺-dependent enzyme catalyst glucose dehydrogenase (GDH) in the outer layer. The outer layer of the GC-Ppy-Frt-Di-NADH-GDH electrode effectively catalyzes the oxidation of glucose biofuel continuously; using the NAD⁺ generated at the inner layer of the Di-catalyzed NADH redox system mediated by Frt and Ppy provides electrical communication with enhancement in electron transport. The electrochemical characteristics of the electrodes were investigated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). This anode provides a current density of 1.2 mA cm⁻² in a 45 mM glucose solution and offers a good possibility for application in biofuel cells.     

A laccase-glucose oxidase biofuel cell prototype operating in a physiological buffer

Here we report on the design and study of a biofuel cell consisting of a glucose oxidase-based anode (Aspergillus niger) and a laccase-based cathode (Trametes versicolor) using osmium-based redox polymers as mediators of the biocatalysts’ electron transfer at graphite electrode surfaces. The graphite electrodes of the device are modified with the deposition and immobilization of the appropriate enzyme and the osmium redox polymer mediator. A redox polymer [Os(4,4′-diamino-2,2′bipyridine)₂(poly{N-vinylimidazole})-(poly{N-vinylimidazole})₉Cl]Cl (E⁰′ = −0.110 V versus Ag/AgCl) of moderately low redox potential is used for the glucose oxidizing anode and a redox polymer [Os(phenanthroline)₂(poly{N-vinylimidazole})₂-(poly{N-vinylimidazole})₈]Cl₂ (E⁰′ = 0.49 V versus Ag/AgCl) of moderately high redox potential is used at the dioxygen reducing cathode. The enzyme and redox polymer are cross-linked with polyoxyethylene bis(glycidyl ether). The working biofuel cell was studied under air at 37 °C in a 0.1 M phosphate buffer solution of pH range 4.4-7.4, containing 0.1 M sodium chloride and 10 mM glucose. Under physiological conditions (pH 7.4) maximum power density, evaluated from the geometric area of the electrode, reached 16 μW/cm² at a cell voltage of 0.25 V. At lower pH values maximum power density was 40 μW/cm² at 0.4 V (pH 5.5) and 10 μW/cm² at 0.3 V (pH 4.4).     

OXYGEN TRANSFER IN HORIZONTAL PIPELINE AERATORS WITH NOZZLES

Horizontal pipeline and tubular loop aerators are of interest for fermentation and waste water treatment and are ideally suited for continuous processing. A major drawback is that these pipeline contactors invariably operate in the “elongated bubble and plug” regime in which the mass transfer rate is low. This article evaluates the performance of a horizontal pipeline aerator fitted with nozzles equispaced along its length to enhance mass transfer rates by promoting turbulence and augmenting effective interfacial area. Such devices can also be advantageously used in long pipe lines as in the case of treating waste while it is being transported. Pressure drop and overall liquid-side mass transfer coefficient data are reported as functions of liquid (water) and gas flow rates and nozzle size and spacing. It is shown that for all the conditions studied, k_La = 0.026(ΔP/L)^1.036 and that the pressure gradient is given by a simple correlation, provided an empirical parameter which characterises a nozzle is known. Preliminary investigations on the effect of surfactant ad the presence of suspended solids (size 75 μm) on mass transfer coefficient are also reported. Very high values of power dissipation can be achieved in such aerators without mechanically moving parts and high values of mass transfer coefficient can be realized.     

OXYGEN TRANSFER IN HORIZONTAL PIPELINE AERATORS WITH NOZZLES

Horizontal pipeline and tubular loop aerators are of interest for fermentation and waste water treatment and are ideally suited for continuous processing. A major drawback is that these pipeline contactors invariably operate in the “elongated bubble and plug” regime in which the mass transfer rate is low. This article evaluates the performance of a horizontal pipeline aerator fitted with nozzles equispaced along its length to enhance mass transfer rates by promoting turbulence and augmenting effective interfacial area. Such devices can also be advantageously used in long pipe lines as in the case of treating waste while it is being transported. Pressure drop and overall liquid-side mass transfer coefficient data are reported as functions of liquid (water) and gas flow rates and nozzle size and spacing. It is shown that for all the conditions studied, k_La = 0.026(ΔP/L)^1.036 and that the pressure gradient is given by a simple correlation, provided an empirical parameter which characterises a nozzle is known. Preliminary investigations on the effect of surfactant ad the presence of suspended solids (size 75 μm) on mass transfer coefficient are also reported. Very high values of power dissipation can be achieved in such aerators without mechanically moving parts and high values of mass transfer coefficient can be realized.     

Direct bio-electrocatalysis by multi-copper oxidases: Gas-diffusion laccase-catalyzed cathodes for biofuel cells

We have studied the bio-electroreduction of oxygen based on direct electron transfer (DET) between laccase and the electrode. Laccase enzymes from two different sources, namely, tree laccase from Rhus vernicifera, and fungal laccase from Trametes hirsuta were used in the study. The gas-diffusion cathode was made using a mixture of teflonized carbon and untreated carbon black, with a nickel mesh that served as a current collector, sandwiched between a hydrophobic gas diffusion layer, and a hydrophilic biocatalytic layer with physically adsorbed laccase enzyme. High current densities: up to 1 mA cm⁻² under oxygen (for bio-electrocatalytic oxygen reduction) and increased stability (up to 30 days) has been achieved using teflonized carbon blacks at gas–electrode interface, high surface area carbon black for loading the enzyme.Gas diffusion laccase-catalyzed cathode demonstrates a number of advantageous properties including good adhesion, biocompatibility and high bio-electrocatalytic properties. An open circuit potential (OCP) of 600 mV at pH 7 for tree laccase (R. vernicifera) and 725 mV at pH 5 for fungal laccase (T. hirsuta) at zero current densities were obtained with respect to SHE reference electrode. Tafel plots obtained confirmed different DET characteristics for the two sources of laccase enzymes, which could suggest different mechanism of charge transfer: 4-electron electroreduction of oxygen using fungal laccase and 2-electron electroreduction using tree laccase. The performance of the cathode was studied in galvanostatic mode and polarization curves at various conditions are reported including those obtained under air and neat oxygen feed from the gas phase.     

OXYGEN TRANSFER IN THE WATER-JET VESSEL

This is an investigation of the gas holdup and the volumetric mass transfer coefficient of a plunging water jet in an air-water system. We sound k_La to be directly proportional to gas holdup in two regions. For the first time, this has been clarified in the plunging liquid jet system. The volumetric mass transfer coefficient and the gas holdup have been well correlated in terms of the Froude number, liquid jet length, nozzle diameter and vessel diameter.     

Finite element analysis of oxygen transport in microfluidic cell culture devices with varying channel architectures, perfusion rates, and materials

Numerical simulations were done both as an exploration into the nature of oxygen transport within microfluidic cell culture devices and an investigation of the relative importance of various design parameters. A rectangular channel comprised of an oxygen permeable polymer layer bonded to a glass substrate seeded with a monolayer of oxygen-consuming cells was modeled. Oxygen transport by both convection and diffusion within the cell culture media and by diffusion in the polymer layer were explored using finite element analysis. Stiff spring analysis was applied at the interface between these two regions to ensure a continuous flux of O₂ across the boundary. The O₂ utilization of the cells was approximated by a constant flux of oxygen from the bottom of the channel. The model was verified against an analytical solution from the literature. Design parameters including flow rate, diffusive layer thickness, and material selection were manipulated within the model to determine their relative importance in ensuring adequate supplies of oxygen for cell growth. The solubility and diffusivity of oxygen within the polymer layer were found to be key parameters in determining the amount of oxygen available to the cells, along with the flow rate of the media perfusing the system. These explorations will enable rational design choices to be undertaken during the implementation of microfluidic devices for cell culture.     

MASS TRANSFER ENHANCEMENT BY SMALL FLOW OBSTACLES IN ELECTROCHEMICAL CELLS

Mass transfer enhancement by small obstacles attached to the cathode in electrolytic flow cells of 5x5 mm cross-section and 500 mm length was investigated. Double beam laser interferometry was used to observe the local mass transfer boundary layer thicknesses preceding and following rod-shaped dielectric obstacles placed normal to the direction of electrolyte flow. Flow patterns have been visualized by use of suspensions of small inert particles and dark field photography. For the evaluation of the effectiveness of mass transport enhancement, pressure drops, and limiting currents for the reduction of ferricyanide have been measured in the range of Reynolds Number 80 to 3200. The degree of enhancement increases with decreasing obstacle spacing until an optimal spacing of approximately 15 times the obstacle size is reached. A three to five-fold increase in the average mass transfer coefficient is achieved by the use of obstacles with a small fraction of the pumping power required to obtain the same limiting current by increasing the flow rate in the unobstructed channel. Small obstacles produce efficient mixing near the electrode surface, and corresponding improvement in uniformity and magnitude of mass transport rates, without increasing the energy dissipation in the bulk fluid.     

MASS TRANSFER ENHANCEMENT BY SMALL FLOW OBSTACLES IN ELECTROCHEMICAL CELLS

Mass transfer enhancement by small obstacles attached to the cathode in electrolytic flow cells of 5x5 mm cross-section and 500 mm length was investigated. Double beam laser interferometry was used to observe the local mass transfer boundary layer thicknesses preceding and following rod-shaped dielectric obstacles placed normal to the direction of electrolyte flow. Flow patterns have been visualized by use of suspensions of small inert particles and dark field photography. For the evaluation of the effectiveness of mass transport enhancement, pressure drops, and limiting currents for the reduction of ferricyanide have been measured in the range of Reynolds Number 80 to 3200. The degree of enhancement increases with decreasing obstacle spacing until an optimal spacing of approximately 15 times the obstacle size is reached. A three to five-fold increase in the average mass transfer coefficient is achieved by the use of obstacles with a small fraction of the pumping power required to obtain the same limiting current by increasing the flow rate in the unobstructed channel. Small obstacles produce efficient mixing near the electrode surface, and corresponding improvement in uniformity and magnitude of mass transport rates, without increasing the energy dissipation in the bulk fluid.     

Process simulation of desalination by electrodialysis of an aqueous solution containing a neutral solute

A model was developed to describe the desalination process of an aqueous solution consisting of a salt and a neutral solute using electrodialysis (ED). Under the assumption of plug flow in compartments, the ED process was analyzed in two-dimensional directions of the electric field and flow to get the differential equations of mass balance in the flow length. Then the transport equations of solutes and water through the membrane were deduced by the irreversible thermodynamics approach. Under the limited condition of uniform current density, the model composed of a first-order differential equation set was developed. While the model parameters such as transport coefficients, dimensions of ED equipment, operation conditions and characteristics of solutes are given, the model was solved by the numerical method. The variations of current density, concentrations of solutes and velocities in dilute and concentrated compartments vs. flow length can be simulated by the model. While there was no neutral solute, the model was used to simulate the desalination process of a salt solution. By comparing the ED experiments to the simulations, it is shown that the model is well suited to describe the actual desalination process. The effects of the initial values of variables in the model on the desalination process were simulated to attempt to construct the actual ED process; and the general simulation of desalination process can be realized by the model. While the effect of concentration polarization on the desalination process is reflected by the variation of membrane conductivity, the model was verified to describe the ED process successfully under low velocity.     

环流反应器中肌苷发酵液的氧传递研究

根据肌苷发酵过程主要是溶氧传质控制的结论,本文研究了操作变量和装置结构变量对环流发酵反应器中氧传递的影响,定量地得到气含率,液体循环速度和结构因素对氧传递的关系。对发酵反应器中溶氧浓度分布进行了摸拟计算,依据计算的结果提出了较佳的 D_E/D 和 L_E/D 的结构。     

Concepts for the Simulation of Wall‐Catalyzed Reactions in Microchannel Reactors with Mesopores in the Wall Region

The realization of reactions in microchannel reactors demands apart from optimal process conditions also a useful geometric layout of the apparatus. Information about the usefulness of the geometry is available through the simulation of the concentrations and the temperature field and their evaluation. Although the mesopores used in microchannel reactors are very regular, the discrepancies between channel and pore dimensions are very large and so, a simultaneous numerical calculation of the entire reactor is neither meaningful nor possible. Therefore, in this paper, an effective kinetics for isothermal reactions considering the diffusional mass flow through the pore mouth is introduced. Still, numerical calculations are very expensive and have the disadvantage that the influence of the individual parameters is seen only by a lot of calculations with changed parameters. That's why a survey of cases with analytical solutions and with other concepts (using mass transfer coefficients, effectiveness factors, etc.) is given, too.     

Two-dimensional numerical simulations of Marangoni-Bénard instabilities during liquid-liquid mass transfer in a vertical gap

Two-dimensional (2D) simulations of isothermal liquid-liquid mass transfer subject to surface tension- and buoyancy-driven hydrodynamic instabilities have been carried out. Simulation is based on the numerical solution of 2D equations of momentum and mass transport, using a combination of finite difference and finite volume methods. Two different cases have been considered: (1) buoyancy stable mass transfer, only surface tension-driven convection occurs; (2) surface tension-driven instability superseeded by buoyant convection. The faster attenuation of mass transfer coefficients in buoyancy stable situations is attributed to the merging of convection cells leading to a reduction in the number of renewal zones along the interface. Concentration profiles next to the interface reveal the diffusional nature of the mass transfer in the immediate interfacial neighbourhood.     

Functionalized-carbon nanotube supported electrocatalysts and buckypaper-based biocathodes for glucose fuel cell applications

The preparation and testing for electrocatalytic activity of functionalized carbon nanotube (f-CNT) supported Pt and Au–Pt nanoparticles (NPs), and bilirubin oxidase (BOD), are reported. These materials were utilized as oxygen reduction reaction (ORR) cathode electrocatalysts in a phosphate buffer solution (0.2 M, pH 7.4) at 25 °C, in the absence and presence of glucose. Carbon monoxide (CO) stripping voltammetry was applied to determine the electrochemically active surface area (ESA). The ORR performance of the Pt/f-CNTs catalyst was high (specific activity of 80.9 μA cm_Pt⁻² at 0.8 V vs. RHE) with an open circuit potential within ca. 10 mV of that delivered by state-of-the-art carbon supported platinum catalyst and exhibited better glucose tolerance. The f-CNT support favors a higher electrocatalytic activity of BOD for the ORR than a commercially available carbon black (Vulcan XC-72R). These results demonstrate that f-CNTs are a promising electrocatalyst supporting substrate for biofuel cell applications.     

Sol‐Gel and Porous Glass‐Based Silica Monoliths with Hierarchical Pore Structure for Solid‐Liquid Catalysis

This work draws attention to both prospects and challenges for silica monoliths in solid‐liquid catalysis at ultrahigh efficiency. The fundamental advantages of silica monoliths as support structure over particulate fixed beds and organic‐polymer monoliths are illustrated as well as recent applications as flow‐through microreactors for the production of fine chemicals. Preparation routes to sol‐gel and porous glass‐based silica monoliths featuring a hierarchical pore structure are described and their potential use as short‐contact‐time reactors is highlighted together with a view on the multiscale characterization and their rational design by establishing quantitative synthesis‐morphology‐transport relationships.     

高富氧载体促进输送膜的研究

基于献，对高富氧载体促进输送膜的载体特征，基膜结构，输氧机理，富氧性能与寿命，富氧工艺设计及经济分析进行了评述。讨论了富氧性能对载体含量，溶剂，温度和压力等的依赖性。指出最高的透氧系数和氧氮分离系数分别达１．１１×１０＾－７ｃｍ＾３（ＳＴＰ）．ｃｍ／ｃｍ＾２．ｓ．ｃｍＨｇ和８０．０。用该膜进行一级空气分离可获得氧含量９０％和氧流量为１．０６×１０＾－３ｃｍ＾３（ＳＴＰ）／ｓ．ｃｍ＾２的富氧空气，