Barbara Jeffries-Nakamura

Recent advances in PEM liquid-feed direct methanol fuel cells

A direct methanol-air fuel cell operating at near atmospheric pressure, low-flow rate air, and at temperatures close to 60/spl deg/C would tremendously enlarge the scope of potential applications. While earlier studies have reported performance with oxygen, the present study focuses on characterizing the performance of a PEM liquid feed direct methanol-air cell consisting of components developed in house. These cells employ Pt-Ru catalyst in the anode, Pt at the cathode and Nafion 117 as the PEM. The effect of pressure, flow rate of air and temperature on cell performance has been studied. With air, the performance level is as high as 0.437 V at 300 mA/cm/sup 2/ (90/spl deg/C, 20 psig, and excess air flow) has been attained. Even more significant is the performance level at 60/spl deg/C, 1 atm and low flow rates of air (3-5 times stoichiometric), which is 0.4 V at 150 mA/cm/sup 2/. Individual electrode potentials for the methanol and air electrode have been separated and analyzed. Fuel crossover rates and the impact of fuel crossover on the performance of the air electrode have also been measured. The study identifies issues specific to the methanol-air fuel cell and provides a basis for improvement strategies.

Performance and impedance studies of thin, porous molybdenum and tungsten electrodes for the alkali metal thermoelectric converter

Columnar, porous, magnetron-sputtered molybdenum and tungsten films show optinum performance as AMTEC electrodes at thicknesses less than 1.0 μm when used with molybdenum or nickel current collector grids. Power densities of 0.40 W cm−2 for 0.5 μm molybdenum films at 1200 K and 0.35 W cm−2 for 0.5 μm tungsten films at 1180 K were obtained at electrode maturity after 40–90 h. Sheet resistances of magnetron sputter deposited films on sodium beta″-alumina solid electrolyte (BASE) substrates were found to increase very steeply as thickness is decreased below about 0.3–0.4 μm. The a.c. impedance data for these electrodes have been interpreted in terms of contributions from the bulk BASE and the porous electrode/BASE interface. Voltage profiles of operating electrodes show that the total electrode area, of electrodes with thickness <2.0 μm, is not utilized efficiently unless a fairly fine (∼1×1mm) current collector grid is employed.

Recent advances in AMTEC recirculating test cell performance

The alkali metal thermal to electric converter (AMTEC) is an electrochemical device for the direct conversion of thermal energy to electrical energy with efficiencies potentially near Carnot. The future usefulness of AMTEC for space power conversion depends on the efficiency of the devices. Systems studies have projected from 15% to 35% thermal to electric conversion efficiencies, and one experiment has demonstrated 19% efficiency for a short period of time. A recent experiment in a recirculating test cell (RCT) has demonstrated sustained conversion efficiencies as high as 13.2%. The cell was operated at lower current and 12% efficiency for over 1700 hours at the time of this writing. The cell required a maturation period of 355 hours at high temperature. During this period, the cell was operated once at 12% efficiency but was generally operated at lower powers. The maturation period ended with the formation of a reflective sodium film on the condenser surface which reduced the parasitic thermal losses in the cell. After maturation, the cell demonstrated the first experimental demonstration of the maximum efficiency occuring at a lower current than the maximum power. The cell also demonstrated an unexpected decrease in parasitic loss with increasing cell current. The decrease in parasitic loss resulted from the development of a more reflective sodium film at higher sodium fluxes.

Performance projections of alternative AMTEC systems and devices

The AMTEC is a device for the direct conversion of heat to electrical power with no moving parts. Most proposed AMTEC systems have focused on minimizing converter mass for space applications. This paper presents two AMTEC devices that focus on high conversion efficiencies (≳30% at 1100 K) and high volumetric power densities. These high current, low voltage modules could find use in small applications such as EM pumps or large systems requiring many modules to reach the desired power levels. A near term system called the tube bundle system produces a peak power of 426 W/1 and a peak efficiency (at lower power) of 34% at 1100 K hot zone temperature. A more advanced system called a flat plate system produces 2.4 W/1 and 30% efficiency at 1100 K. Further improvements are projected for these devices through the development of optimized porous electrodes or a potassium ion conducting sold electrolyte.

Developments in Amtec Devices, Components and Performance

Improvement of the performance of an AMTEC device requires improvement and development of components as well as of device geometry and construction. The research and development effort at JPL includes studies which address both overall device construction and studies of components. This paper discusses recent studies on components and devices which have been carried out at JPL. Components investigated include the electrode materials titanium nitride (TiN) and rhodium‐tungsten (RhW) and the electrolyte materials sodium β“‐alumina and potassium β” ‐alumina. We have studied the mechanical characteristics of sodium and potassium β“‐alumina ceramic and conditions for fabrication of potassium β”‐alumina. Device studies include fabrication and operation of a wick fed cell using a graded, sintered wick, a higher voltage vapor‐vapor multicell which includes three “ subcells” which are internally series connected, and an AMTEC which uses potassium as the working fluid.

Efficiency of an AMTEC Recirculating Test Cell, Experiments and Projections

The alkali metal thermal to electric converter (AMTEC) is an electrochemical device for the direct conversion of heat to electrical energy with efficiencies potentially near Carnot. The future usefulness of AMTEC for space power conversion depends on the efficiency of the devices. Systems studies have projected from 15 to 35 percent thermal to electric conversion efficiencies, and one experiment has demonstrated 19 percent efficiency for a short period of time. Recent experiments in a recirculating test cell (RTC) have demonstrated sustained conversion efficiencies as high as 10.2 percent early in cell life and 9.7 percent after maturity. Extensive thermal and electrochemical analysis of the cell during several experiments demonstrated that the efficiency could be improved in two ways. First, the electrode performance could be improved. The electrode for these tests operated at about one third the power density of state of the art electrodes. The low power density was caused by a combination of high series resistance and high mass flow resistance. Reducing these resistances could improve the efficiency to greater than 10 percent. Second, the cell thermal performance could be improved. Efficiencies greater than 14 percent could be realized through reducing the radiative thermal loss. Further improvements to the efficiency range predicted by systems studies can be accomplished through the development and use of an advanced condenser with improved reflectivity, close to that of a smooth sodium film, and the series connecting of individual cells to further reduce thermal losses.

High temperature conductivity of potassium-β''-alumina

Abstract Potassium β″-alumina single crystals have been reported by several groups to have higher ionic conductivity than sodium β″-alumina crystals at room temperature, and similar conductivities are obtained at temperatures up to 600–700 K. Potassium β″-alumina ceramics have been reported to have significantly poorer consuctivities than those of sodium β″-alumina ceramics, but conductivity measurements at temperatures above 625 K have not been reported. In this study, K + -β″-alumina ceramics were prepared from Na + -β″-alumina ceramic using a modified version of the exchange reaction with KCl vapor reported by Crosbie and Tennenhouse, and the conductivity has been measured in K vapor at temperatures up to 1223 K, using the method of Cole, Weber and Hunt. The results indicate reasonable agreement with earlier data on K + -β″-alumina ceramic measured in a liquid K cell, but show that the K + -β″-alumina ceramics conductivity approaches that of Na + -β″-alumina ceramic at higher temperatures, being within a factor of four at 700 K and 60% of the conductivity of Na + -β″-alumina at T > 1000 K. Both four-probe dc conductivity and four probe ac impedance measurements were used to characterize the conductivity. A rather abrupt change in the grain boundary resistance suggesting a possible phase change in the intergranular material, potassium aluminate, is seen in the ac impedance behavior.

Electrode, current collector, and electrolyte studies for AMTEC cells

Studies of components for AMTEC devices at JPL have focussed on electrode materials, materials and construction of the current collection network, and the [beta][double prime]-alumina solid electrolyte. Electrode materials include thin films of molybdenum metal and metal alloys such as PtW and RhW. Surface self-diffusion coefficients have been determined for Mo electrodes in the temperature range 1050--1200 K, and for RhW at 1125 K. The diffusion coefficients have been used in a grain growth model to predict electrode operating lifetimes at temperatures in this range. Current collection networks sputtered with a thin film of platinum have decreased total electrical resistance in an operating device by 35%. Electrolyte studies have found no mechanical stress or chemical degradation induced by long term operation. Further electrolyte studies have focussed on synthesis of both sodium and potassium [beta][double prime]-alumina ceramic. Potassium [beta][double prime]-alumina solid electrolyte has potential application in a potassium-based AMTEC device which operates at a hot side temperature of [similar to]1000 K.

AMTEC cell testing, optimization of rhodium/tungsten electrodes, and tests of other components

Electrodes, current collectors, ceramic to metal braze seals, and metallic components exposed to the high ‘‘hot side’’ temperatures and sodium liquid and vapor environment have been tested and evaluated in laboratory cells running for hundreds of hours at 1100–1200 K. Rhodium/tungsten electrodes have been selected as the optimum electrodes based on performance parameters and durability. Current collectors have been evaluated under simulated and actual operating conditions. The microscopic effects of metal migration between electrode and current collector alloys as well as their thermal and electrical properties determined the suitability of current collector and lead materials. Braze seals suitable for long term application to AMTEC devices are being developed.