Sathya Motupally

Diffusion of Water in Nafion 115 Membranes

In this paper, experimental and simulated data for the diffusion of water across Nafion membranes as a function of the water activity gradient are presented. The gradient in the activity of water across the membrane was varied by changing the flow rate and pressure of nitrogen gas on one side of the membrane. The other side of the membrane was equilibrated with liquid water. It was found that the model predictions are very sensitive to the value of the diffusion coefficient of water in Nafion. Using the Fickian diffusion coefficient extracted from self-diffusion measurements reported in the literature, the model simulations matched experimental data with less than 5% error over a wide range of operating conditions.

Hydrogen Peroxide Formation Rates in a PEMFC Anode and Cathode Effect of Humidity and Temperature

Hydrogen peroxide (H 2 O 2 ) formation rates in a proton exchange membrane fuel cell (PEMFC) anode and cathode were estimated as a function of humidity and temperature by studying the oxygen reduction reaction (ORR) on a rotating ring disk electrode. Fuel cell conditions were replicated by depositing a film of Pt/Vulcan XC-72 catalyst onto the disk and by varying the temperature, dissolved O 2 concentration, and the acidity levels in hydrochloric acid (HClO 4 ). The HClO 4 acidity was correlated to ionomer water activity and hence fuel cell humidity. The H 2 O 2 formation rates showed a linear dependence on oxygen concentration and square dependence on water activity. The H 2 O 2 selectivity in ORR was independent of oxygen concentration but increased with the decrease in water activity (i.e., decreased humidity). Potential dependent activation energy for the H 2 O 2 formation reaction was estimated from data obtained at different temperatures.

Proton Diffusion in Nickel Hydroxide Prediction of Active Material Utilization

Galvanostatic charge and discharge experiments reveal that the active material in nickel electrodes cannot be fully accessed at high currents or for thick films. It has been proposed that the utilization of the active material is controlled by the diffusion rate of protons through the film. This hypothesis is supported by the good agreement between mathematical simulations of material utilization and experimental data over a range of charge and discharge currents and film thicknesses. Furthermore, the fraction of material utilized is larger on charge than on discharge. The asymmetry on charge and discharge is due to a diffusion coefficient that is a function of the state‐of‐charge of the active material. The mathematical model is used to perform a parametric study of material utilization as a function of charge and discharge currents, and material loading (i.e., film thickness, concentration of nickel sites) in order to improve battery design and operation.

The Effect of Temperature and Ethanol on the Deposition of Nickel Hydroxide Films

ABSTRACT The objective of this work was to determine the effect of the temperature and the ethanol content of the Ni(NQ)2 solution on: (i) the efficiency of electrochemical deposition of nickel hydroxide; and (it) the molecular weight of the deposited film. An electrochemical quartz crystal nanobalance (EQCN) was used to measure the mass of films electrochem- ically deposited from Ni(NO3)2 solutions and constant current discharges were used to determine the electrochemical capacity of the films. The data indicates that increasing the temperature increases both the efficiency of the deposition reaction and the molecular weight of the deposited film. The increased efficiency at higher temperatures is attributed to a decrease in the concentration of a nickel complex at the surface of the electrode. The lower complex concentration decreases the diffusion rate of this species away from the electrode surface and hence increases the rate at which the complex precipitates from the solution. The increase in the molecular weight at higher temperature is attributed to a combination of increased rate of deposition and an increase in the lattice spacing of the active material. The data also indicate that increasing the ethanol content of the solution had no noticeable effect on the efficiency of deposition, when water was present. In pure ethanol, however, the chemistry of deposition seemed to change considerably. However, increas- ing the ethanol content of the solution resulted in an increase of the molecular weight of the film. Increase in the molecular weight with an increase in the ethanol content of the solution is due to an increase in the relative percentage of ethanol incorporated in the active material. The data also indicate that the number of electrons in the discharge reaction is approximately 1.4 electrons per nickel atom.

The Role of Oxygen at the Second Discharge Plateau of Nickel Hydroxide

It was shown that the appearance of a secondary discharge plateau approximately 400 mV below the primary plateau can result from the reduction of oxygen. During the galvanostatic discharge of planar nickel-hydroxide films at room temperature and in 3 weight percent KOH solutions, the second discharge plateau was observed only in the presence of dissolved oxygen in the electrolyte. When the solution was deoxygenated, no residual capacity could be extracted from the films even at low discharge rates or from overcharged films. In addition, the duration of the second plateau is inversely proportional to the square of the discharge current, which is indicative of a diffusion-controlled process. The nickel hydroxide active material, rather than the electrolyte, seems to be the primary reservoir for the oxygen that is reduced on the second plateau.

Water Transport in Polymer Electrolyte Membrane Electrolyzers Used to Recycle Anhydrous HCl: I. Characterization of Diffusion and Electro-osmotic Drag

bE. I. DuPont de Nemours and Company, Central Research and Development, Wilmington, Delaware 19880-0323, USA In this paper, diffusion and electro-osmotic drag of water across Nafion® membranes in the presence of HCl are characterized. For all the measurements, one side of the Nafion membrane was in contact with liquid water and the other side with gaseous anhydrous HCl. To characterize diffusion of water, the open-circuit flux of water across a catalyst-coated Nafion 115 membrane was measured as a function of HCl flow rate and temperature at a constant cell pressure of 1 atm. Due to the nature of varying driving force for diffusion as a function of HCl flow rate, the experimental data was analyzed in conjunction with a mathematical model. The mathematical model accounts for condensation of water and is used to calculate the concentration of liquid hydrochloric acid in contact with the membrane. The mathematical model presented here is general and can be applied to the characterization of water transport across Nafion membranes in the presence of any gas that is soluble in water. In the case of HCl, at low inlet flow rates ~,1500 cm 3 /min, STP!, the diffusion of water across the membrane is primarily governed by the diffusional limitations of HCl in the condensed phase. At high flow rates ~.3000 cm 3 /min, STP!, the flux of water is a constant and depends on the saturation solubility of HCl in the condensed liquid phase. To measure the electro-osmotic drag parameter, the net flux of water across the membrane was measured as a function of the applied current density at high HCl flow rates ~i.e., uniform water flux !.

Phosphoric Acid Fuel Cells for Stationary Applications

Fuel cells generate power by electrochemically combining fuel such as hydrogen and oxidant such as oxygen in air to produce electrical and thermal energy. Fuel cells generally consist of an anode electrode where fuel is oxidized and cathode electrode where oxygen in air is reduced. The electrolyte which is usually placed between the two electrodes acts as a medium to transport charge carriers (e.g., H+, CO−). Fuel cells are particularly interesting as energy generating devices because they consume reactants without combustion, thus providing higher efficiencies and avoiding the issue of pollution. A fuel cell reaction typically produces water as a by-product which is usually removed from the cell by reactant exhaust.