Donald R. Cahela

Modeling double-layer capacitor behavior using ladder circuits

The double-layer capacitor (DLC) is a very complex device that is best represented by a distributed parameter system. Many different lumped-parameter equivalent circuits have been proposed for the DLC. An examination into utilizing a ladder circuit to model a DLC is presented. Parameters for different ladder circuits are determined from AC impedance data. Variations in circuit parameters with DC bias and manufacturing have been investigated. The performance of the ladder circuit has been evaluated in slow discharge and pulse load applications.

Wet layup and sintering of metal-containing microfibrous composites for chemical processing opportunities

Abstract A new class of composite materials is prepared using traditional high speed and low cost paper making equipment and techniques. In this process, μm diameter metal fibers in a variety of compositions and alloys are slurried in an aqueous suspension with cellulose fibers and other selected particulates and/or fibers. The resulting mixture is then cast into a preform sheet using a wetlay process and dried to create a sheet or roll of preform material. Subsequent sintering of the preform at elevated temperatures (ca. 1000°C) removes the cellulosic binder/pore former and entraps the selected particulates/fibers within a sinter-locked network of conductive metal fibers. Unique physical properties are obtained in terms of: void volume, thermal/electrical conductivity, porosity, surface area, permeability, particle size, layer thickness, etc. To a first approximation these composites possess averaged physical properties over heretofore unavailable regions located between those of the entrapped component and those of the high void volume sintered metal carrier. For chemical processing applications the high void volume of the metallic binder/carrier (i.e. 20–99%) facilitates intralayer heat and mass transport while the ability to trap very small particulates (using the novel pore size-void volume relationship of microfibrous carriers) greatly reduces intraparticle heat and mass transport. A description of the unique and fundamental structure-property relationships and behaviors of these materials will be presented and contrasted with those of the more traditional engineering approaches and practices using fused particulates and pasted-carriers. Opportunities for significant steady-state volumetric processing improvements result when one must balance the competing demands of chemical kinetics (e.g. at the entrapped particulates) with those of the required transport processes (i.e. via the interparticle and intralayer voidage). Examples of beneficial processing applications and opportunities will be discussed in: (a) heat transfer materials, (b) catalysts and sorbents, (c) electrochemical processing and (d) filtration.

Permeability of sintered microfibrous composites for heterogeneous catalysis and other chemical processing opportunities

Abstract Microstructured materials have potential for enhanced mass and heat transfer compared to typical catalyst particulates used in industrial processes. The pressure drop through catalyst-containing materials is a very important reactor design consideration. A model equation to predict porous media permeability (PMP) over the entire range of possible bed voidages is extended to predict properties of sintered metal meshes. A correlation of data for sintered meshes of nickel fibers is presented in the form of a Kozeny constant form drag plot. Comparison of predictions by the PMP equation with data taken on a sintered composite fiber/particle mesh is presented. Use of the PMP equation as a design tool for optimization of media for adsorbents, catalysts, and filters is discussed.

Composite fiber structures for catalysts and electrodes

Abstract We have recently envisioned a process wherein fibers of various metals in the 0.5 to 15 μm diameter range are slurried in concert with cellulose fibers and various other materials in the form of particulates and/or fibers. The resulting slurry is cast via a wet-lay process into a sheet and dried to produce a free-standing sheet of ‘composite apper’. When the ‘preform’ sheet is sintered in hydrogen, the bulk of the cellulose is removed with the secondary fibers and/or particulates being entrapped by the sinter-locked network provided by the metal fibers. The resulting material is unique in that it allows the intimate contacting and combination of heretofore mutually exclusive materials and properties. Moreover, due to the ease of paper manufacture and processing, the resulting materials are relatively inexpensive and can be fabricated into a wide range of three-dimensional structures. Also, because cellulose is both a binder and a pore-former, structures combining high levels of active surface area and high void volume (i.e., low pressure drop) can be prepared as free-standing flow through monoliths.

New structures of thin air cathodes for zinc–air batteries

Thin composite cathodes for air reduction were manufactured using microfibre-based papermaking technology. The electrodes have a thin structural design, less than 0.15 mm in thickness. Composite cathode materials for oxygen reduction applications were fabricated by entrapping carbon particles in a sinter-locked network of 2–8 μm diameter metal fibres. The thin structure not only results in electrodes that are 30–75% thinner than those commercially available, but also offers an opportunity for custom-built air cathodes optimized for high-rate pulse applications. Using a thin composite structure for the air cathode in a zinc–air battery that is part of a zinc–air/capacitor hybrid is likely to increase the pulse capability of the hybrid power system. The thin cathode structure provides a better, more efficient three-phase reaction zone. In a half-cell test, the ultrathin air cathode generated more than 1.0 V vs Zn/ZnO for a current of 200 mA cm−2. Half-cell, full-cell and pulse-power tests revealed that thin composite cathodes have a better rate and pulse performance than the air cathodes commonly used.

A comparison of two equivalent circuits for double-layer capacitors

The double-layer capacitor is a very complex device that is best represented by a distributed parameter system. Many different lumped-parameter equivalent circuits have been proposed for the double-layer capacitor. Presented in this paper is a comparison of the classical equivalent circuit and ladder circuits. Parameters for the classical equivalent circuit are determined from in-situ circuit measurements. AC impedance data is acquired for the capacitor and utilized to determine parameters for the ladder circuits. The performance of the two equivalent circuits is evaluated in slow discharge and pulse load applications. In comparison to the ladder circuits, the classical circuit more accurately calculates the capacitor voltage during a slow discharge. In the pulse load application, the classical circuit and a low order ladder circuit predict approximately the same voltage drop measured in the laboratory.

Overview of electrochemical double layer capacitors

Electrochemical liquid double layer capacitors (ELCC) are energy storage devices with properties intermediate between batteries and electrolytic capacitors. The commercial success of carbon based ELCC is due to their low cost, extremely high cycle life, and wide range of operating temperatures. They are used mainly for power backup for electronic circuits where the properties mentioned above give ELCC advantages over rechargeable batteries. ELCC are a promising candidate for load leveling applications in electric vehicles and also pulse power applications. Nonaqueous electrolytes, such as organics, solid polymers, and inorganics offer higher energy densities due to the greater decomposition voltages. Advanced ELCC will likely use metal oxides or conductive polymers to provide higher energy and power densities than carbon based ELCC.

Ammonia - It's Transformation and Effective Utilization

In practical use, ammonia (NH3) can be burned directly in internal combustion (IC), diesel or Stirling engines. However, because NH3 has such a low flame temperature and is hard to ignite, it has generally not been widely used in these applications. Because NH3 can be easily reformed into hydrogen (H2), and as part of the effort to examine the benefits of ammonia for terrestrial applications, we develop processes for catalytic reformation of NH3 and utilize microfibrous materials to encapsulate reforming catalyst. After demonstrating reformation of NH3 studies were conducted on the stability and feasibility of burning NH3 by itself and burning NH3 with synthetic reformate. Favorable conditions for flame stability of combustion of hydrogen reformed from ammonia through this catalyst-impregnated microfibrous porous media were obtained and the results are presented. Initial findings demonstrated stable catalytic combustion and flame temperatures of 940oC were obtained.

Impedance modeling of nickel fiber/carbon fiber composite electrodes for electrochemical capacitors

A process for producing composite materials from metal fibers and carbon fibers was developed in our laboratory. This process allows for independent adjustment of void volume and macro porosities not attainable by other processes or electrode structures. Contact resistance in the sintered metal fiber-carbon fiber composites is low due to the sintered metal structure which entraps carbon fibers in the finished electrode. Extensive mechanistic discrimination and model testing has yielded an equivalent circuit model which successfully predicts impedance performance from 10/sup -2/ to 10/sup 5/ Hz. An equivalent circuit model for a nickel fiber mesh, represented by a constant phase element (CPE) in parallel with a mesh resistance, is combined with a model developed specifically to describe the impedance of activated carbons. The five parameters in the model circuit were successfully correlated with variations in temperature and electrolyte conductivity, also variations in the equivalent circuit parameters with sintering conditions are also presented. The above noted model is applicable for the simulation and design of electrochemical capacitors for specialized used in various pulse power systems.

Micro Scale Heat Transfer Comparison between Packed Beds and Microfibrous Entrapped Catalysts

Abstract Computational Fluid Dynamics (CFD) was used to compare the micro scale heat transfer inside a packed bed and a microfibrous entrapped catalyst (MFEC) structure. Simulations conducted in stagnant gas determined the thermal resistance of the gas in the micro gaps between the particle-to-particle contact points in the resistance network model of a packed bed. Tube to particle diameter ratios for the simulations were 9 based on particle diameter and 27 for MFEC based on surface area average diameter. The maximum temperature difference used in the simulations was 80°C. It was shown that thermal resistance at the contact points accounted for 90% of the thermal resistance of the solid path. In the MFEC, the thermal resistance of the continuous metal fibers was relatively smaller than that of contact points. As a result, 97.2% of the total heat flux was transported by continuous fiber cylinders, which was the fundamental function of fibers on improving the heat transfer of MFEC structures. Enhanced heat transfer characteristics of MFEC were further demonstrated by simulations performed in flowing gas, where both heat conduction and heat convection were significant.