Material Balance Modification in One-Dimensional Modeling of Porous Electrodes

The material balance used previously in one-dimensional mathematical models of porous electrodes is invalid when there is a non-zero volume change associated with the reaction. It is shown here how the material balance should be modified either to account for a loss in volume, or to account for an inflow of electrolyte from the header into the active pores. A one-dimensional mathematical model is used to illustrate the effect of this correction on the prediction of the delivered capacity and the electrolyte concentration in a lithium/thionyl chloride primary battery

Mathematical Modeling of Electrochemical Capacitors

Analytic solutions to the mathematical model of an electrochemical capacitor (EC) are used to study cell performance under two types of operating conditions: (i) constant current and (ii) electrochemical impedance spectroscopy. The analytic solution under constant-current operation is used to investigate the relative importance of ionic resistance in the separator, and ionic and electronic resistances in the porous electrode in the design and operation of an EC. Model results are presented that show the trade-off between energy and power density, as the physical properties of the cell components are varied (e.g., electrode thickness). The analytic solution is also used to study the effect of cell design and operation on the heat generation during constant-current cycling. The impedance model is presented as an alternative to equivalent-circuit models for data analysis. The analytic solution can be used to gain a physical understanding of the various processes that occur in an EC

Hysteresis during Cycling of Nickel Hydroxide Active Material

The nickel hydroxide electrode is known to exhibit a stable hysteresis loop, with the potential on charge being higher than that on discharge at every state-of-charge (SOC). What we show here is that this loop created during a complete charge and discharge (i.e., boundary curves) is not sufficient to define the state of the system. Rather, internal paths within the boundary curves (i.e., scanning curves) can be generated that access potentials between the boundary curves. The potential obtained at any SOC, as well as how the material charges and discharges from that point, depends on the cycling history of the material. The implication of this phenomenon is that the potential of nickel-based batteries cannot be used as an indication of the SOC of the cell. Analysis of the boundary and scanning curves suggest that the electrode consists of a number of individual units or domains, each of which exhibits two or more metastable states. The cycling behavior of the nickel hydroxide electrode is discussed within the context of previously developed theoretical arguments regarding domain theory. Although the specific cause for the metastability in each domain is not understood, considerable insights are provided into the history-dependent behavior of the nickel hydroxide electrode. Finally, an empirical procedure is developed to predict the scanning curves based on the boundary curve

The Revised Uniform Partnership Act: The Reporters\u27 Overview

This Article is a brief overview of what the Reporters believe to be the four basic contributions of the Revised Uniform Partnership Act (RUPA or Act). First, RUPA changes the law of partnership breakups and gives greater stability to partnerships by abandoning the traditional rule that a partnership is dissolved every time a member leaves. Second, RUPA makes clear that partners are not fiduciaries among themselves in the same sense as disinterested trustees. Specifically, RUPA states that partners legitimately may pursue self-interest without automatically running afoul of their fiduciary duties. On the other hand, RUPA provides an irreducible core of fiduciary duties among partners. Third, RUPA rewrites the rules on the nature and transfer of partnership property. It adopts an entity approach for the sake of simplicity and provides for the filing of partnership statements, including statements of partnership authority, dissociation, and dissolution. Fourth, RUPA for the first time expressly authorizes the conversion and merger of partnerships and provides “safe harbor” rules for those transactions

Quantifying Individual Potential Contributions of the Hybrid Sulfur Electrolyzer

The hybrid sulfur cycle has been investigated as a means to produce clean hydrogen efficiently on a large scale by first decomposing H2SO4 to SO2, O2, and H2O and then electrochemically oxidizing SO2 back to H2SO4 with the cogeneration of H2. Thus far, it has been determined that the total cell potential for the hybrid sulfur electrolyzer is controlled mainly by water transport in the cell. Water is required at the anode to participate in the oxidation of SO2 to H2SO4 and to hydrate the membrane. In addition, water transport to the anode influences the concentration of the sulfuric acid produced. The resulting sulfuric acid concentration at the anode influences the equilibrium potential of and the reaction kinetics for SO2 oxidation and the average conductivity of the membrane. A final contribution to the potential loss is the diffusion of SO2 through the sulfuric acid to the catalyst site. Here, we extend our understanding of water transport to predict the individual contributions to the total cell potential

Electrochemical Filtering of CO from Fuel-Cell Reformate

A proton exchange membrane fuel cell was used as a flow reactor for continuous preferential oxidation of CO over H2 from 1.0% CO in H2 under pulse-potential control. By varying the pulse profile ~e.g., on-time, off-time, pulse potential! the CO and H2 oxidation currents were varied independently. The improvement in faradaic selectivity between CO and H2 oxidation results from the promotion of CO adsorption during the off ~i.e., open-circuit! portion of the pulse. Therefore, during the on portion CO oxidation was preferred while the surface was covered with CO

Effect of Water on the Electrochemical Oxidation of Gas-Phase SO2 in a PEM Electrolyzer for H2 Production

Water plays a critical role in producing hydrogen from the electrochemical oxidation of SO2 in a proton exchange membrane (PEM) electrolyzer. Not only is water needed to keep the membrane hydrated, but it is also a reactant. One way to supply water is to dissolve SO2 in sulfuric acid and feed that liquid to the anode, but this process results in significant diffusion resistance for the SO2. Alternatively, we have developed a process where SO2 is fed as a gas to the anode compartment and reacts with water crossing the membrane to produce sulfuric acid. There was concern that the diffusion resistance of water through the membrane is as significant as SO2 diffusion through water, thus limiting the benefit of a gas-phase anode feed. We show here that water diffusion through the membrane is not as limiting as liquid-phase SO2 diffusion. Therefore, we can control the cell voltage, the limiting current, and the sulfuric acid concentration by varying the diffusion resistance of the membrane via thickness or temperature. Catalyst loading, however, has a negligible effect on cell performance

Electrochemical Removal of Carbon Monoxide in Reformate Hydrogen for Fueling Proton Exchange Membrane Fuel Cells

A twin-cell electrochemical filter is demonstrated to reduce the CO concentration in reformate hydrogen. In this design, the potential and gas flow are switched between the two filter cells so that alternative CO adsorption and oxidation occur in each cell while providing a continuous flow of H2 to a fuel cell. The effects of filter switching time and applied potential on the CO concentration of gas exiting the filter are presented here for a CO concentration of 1000 ppm in nitrogen flowing at 100 cm3/min. The parasitic loss of hydrogen from a corresponding reformate stream was estimated to be 1.5%

Proton Diffusion in Nickel Hydroxide Films: Measurement of the Diffusion Coefficient as a Function of State of Charge

Electrochemical impedance spectroscopy (EIS) was used to measure the solid-state diffusion coefficient of protons in nickel hydroxide films at room temperature as a function of state of charge (SOC). A model for the complex faradaic impedance of the nickel hydroxide active material is presented and used to extract the diffusion coefficient of protons from the EIS data. Impedance data over a range of frequencies can be used to extract a constant diffusion coefficient without the knowledge of the initial mobile proton concentration or the form of the charge-transfer kinetic expression. The proton diffusion coefficient is a strong function of SOC and decreases approximately three orders of magnitude from 3.4 × 10–8 to 6.4 × 10–11 cm2 s–1 as the electrode discharges from the completely charged to the completely discharged state. The measurements were performed on well-conditioned nickel hydroxide films and therefore it is likely that the diffusion coefficients measured correspond to the -phase of the active material. The diffusion coefficient of protons was measured for three different film thicknesses, 1.5, 1.2, and 1.0 µm. The diffusion coefficient is independent of the thickness of the film as predicted by theory. The three orders of magnitude decrease in the diffusion coefficient of protons can be explained on the assumption that the protons move predominantly through the oxidized phase [NiOOH] which is interdispersed along with the reduced phase [Ni(OH)2] in the active material