Showing papers on "Direct methanol fuel cell published in 1998"

Combinatorial Electrochemistry: A Highly Parallel, Optical Screening Method for Discovery of Better Electrocatalysts

Abstract: Combinatorial screening of electrochemical catalysts by current-voltage methods can be unwieldy for large sample sizes. By converting the ions generated in an electrochemical half-cell reaction to a fluorescence signal, the most active compositions in a large electrode array have been identified. A fluorescent acid-base indicator was used to image high concentrations of hydrogen ions, which were generated in the electrooxidation of methanol. A 645-member electrode array containing five elements (platinum, ruthenium, osmium, iridium, and rhodium), 80 binary, 280 ternary, and 280 quaternary combinations was screened to identify the most active regions of phase space. Subsequent “zoom” screens pinpointed several very active compositions, some in ternary and quaternary regions that were bounded by rather inactive binaries. The best catalyst, platinum(44)/ruthenium(41)/osmium(10)/iridium(5) (numbers in parentheses are atomic percent), was significantly more active than platinum(50)/ruthenium(50) in a direct methanol fuel cell operating at 60°C, even though the latter catalyst had about twice the surface area of the former.

Proton and Methanol Transport in Poly(perfluorosulfonate) Membranes Containing Cs + and H + Cations

Abstract: Poly(perfluorosulfonate acid) membranes were doped with cesium ions to several degrees. These, along with the H{sup +}-form membrane, were investigated in relation to methanol permeability as well as hydrogen ion conductivity. While retaining considerable conductivity, the cesium-doped membranes are highly impermeable to methanol. The author found that methanol permeability in the membrane reduced by over one order of magnitude, owing to the presence of cesium ions. These findings are discussed on the basis of alterations produced by cesium in the membrane microstructure. Also discussed is the potential implication of these results in the direct methanol fuel cell technology.

Methanol Tolerant Oxygen Reduction Catalysts Based on Transition Metal Sulfides

Abstract: The oxygen reduction activity and methanol tolerance of a range of transition metal sulfide electrocatalysts have been evaluated in half-cell experiments and in a liquid-feed solid polymer electrolyte direct methanol fuel cell. These catalysts were prepared in high surface area form by direct synthesis onto various surface-functionalized carbon blacks. Of the materials tested, mixed-metal catalysts based on ReRuS and MoRuS were observed to give the best oxygen reduction activities. In addition, significant increases in performance were observed when employing sulfur-functionalized carbon black, which were attributed to the preferential deposition of active Ru sites in the catalyst-preparation process. Although the intrinsic activity of the best material tested, namely, Mo{sub 2}Ru{sub 5}S{sub 5} on sulfur-treated XC-72, was lower than Pt (by ca. 1545 mV throughout the entire polarization curve), its activity relative to Pt increased significantly in methanol-contaminated electrolytes. This was due to methanol oxidation side reactions reducing the net activity of the Pt, especially at low overpotentials.

Performance of a direct methanol fuel cell

Abstract: The performance of a direct methanol fuel cell based on a Nafion® solid polymer electrolyte membrane (SPE) is reported. The fuel cell utilizes a vaporized aqueous methanol fuel at a porous Pt–Ru–carbon catalyst anode. The effect of oxygen pressure, methanol/water vapour temperature and methanol concentration on the cell voltage and power output is described. A problem with the operation of the fuel cell with Nafion® proton conducting membranes is that of methanol crossover from the anode to the cathode through the polymer membrane. This causes a mixed potential at the cathode, can result in cathode flooding and represents a loss in fuel efficiency. To evaluate cell performance mathematical models are developed to predict the cell voltage, current density response of the fuel cell.

Material aspects of the liquid feed direct methanol fuel cell

Abstract: A study of a small scale Liquid Feed Direct Methanol Fuel Cell (LFDMFC), based on solid polymer electrolyte membrane, is reported. Two flow cell designs, one with a parallel flow channel arrangement and the other with a spot design of flow bed, are used. The structure of the DMFC comprises a composite of two porous electrocatalytic electrodes; Pt–Ru–carbon catalyst anode and Pt–carbon catalyst cathode, on either side of a solid polymer electrolyte (SPE) membrane. The performance of three Pt–Ru catalysts is compared. The influence of the degree of Teflon loading on the electrode structure is also reported. The effect of the following parameters: cell temperature, oxygen gas or air pressure, methanol liquid flow rate and methanol concentration on the power performance is described.

A methanol sensor for portable direct methanol fuel cells

Abstract: An aqueous methanol sensor for portable direct methanol fuel cell applications is demonstrated. The design is based on current output limited by methanol diffusion through a Nafion 117 perfluorosulfonic acid membrane. Steady-state polarization measurements demonstrate sensitivity to concentrations of 0 to 4 M over a temperature range of 40 to 80C. Furthermore, a correlation that is first order in concentration and temperature is demonstrated for concentrations of 0 to 3 M, with an accuracy of {+-}0.1 M. Measurements of transient response to step concentration change indicate a response time of about 10 to 50 s, depending primarily on temperature.

Direct methanol fuel cells for vehicular applications

Abstract: Dramatic technological advances for the proton exchange membrane fuel cell have focused attention on this technology for motor vehicles. The fuel cell vehicles (FCVs) have the potential to compete with the petroleum-fueled internal combustion engine vehicles (ICEVs) in cost and performance while effectively addressing air quality, energy insecurity, and global warming concerns. Methanol being a liquid can be easily transported and can be supplied from the existing network of oil company distribution sites. Recently, combining improved catalysts with fuel cell engineering, it has been possible to overcome some of the difficulties that have frustrated previous research and development efforts in realizing a commercially viable direct methanol fuel cell. Direct methanol fuel cells (DMFCs) with power densities between 0.2 and 0.4 W/cm2 at operational temperatures in the range 95–130 °C have been developed. These power densities are sufficient to suggest that stack construction is well worth while. This paper reviews recent advances and technical challenges in the field of DMFCs.

Ionomer impregnation of electrode substrates for improved fuel cell performance

Abstract: Liquid feed fuel cell performance can be increased by impregnating electrode substrates with a proton conducting ionomer prior to incorporation of the electrocatalyst, and optionally also after application of the electrocatalyst. Ionomer impregnation is particularly effective for direct methanol fuel cell anodes that comprise carbonaceous substrates.

Direct methanol fuel cell is operated with gaseous fuel for low power losses

Abstract: A direct methanol fuel cell (DMFC) is operated by evaporating a coolant in the fuel cell, evaporating a methanol/water mixture in an evaporator using heat from the fuel cell and supplying the evaporated methanol/water mixture to the anode space of the fuel cell. An Independent claim is also included for apparatus comprising a DMFC containing a coolant line, an evaporator with a fuel feed line and a fuel vapor line leading to the anode space of the fuel cell, and a system for passing excess heat from the fuel cell to the evaporator.

Recent Studies on Methanol Crossover in Liquid-Feed Direct Methanol Fuel Cells

Abstract: In this work, the effects of methanol crossover and airflow rates on the cathode potential of an operating direct methanol fuel cell are explored. Techniques for quantifying methanol crossover in a fuel cell and for separating the electrical performance of each electrode in a fuel cell are discussed. The effect of methanol concentration on cathode potential has been determined to be significant. The cathode is found to be mass transfer limited when operating on low flow rate air and high concentrations of methanol. Improvements in cathode structure and operation at low methanol concentration have been shown to result in improved cell performance.

Factors Affecting the Design of Direct Methanol Fuel Cell Systems

Abstract: Direct methanol fuel cell based power systems are being developed as alternate power sources for portable applications under the sponsorship of DARPA.

New perovskite-type solid electrolyte

Abstract: A novel solid electrolyte has a perovskite structure which contains either (a) alkaline earth metal sites occupied by barium, rare earth metal sites occupied by praseodymium and another rare earth metal (preferably gadolinium or cerium and another rare earth metal, preferably gadolinium), and oxygen; or (b) alkaline earth metal sites occupied by barium and another alkaline earth metal (preferably magnesium or calcium), rare earth metal sites occupied by cerium and another rare earth metal (preferably gadolinium or praseodymium and another rare earth metal, preferably gadolinium), and oxygen. Also claimed are (i) solid ceramic fuel cells, direct methanol fuel cells, hydrogen pumps, oxygen concentration sensors, vapour concentration sensors and fuel cell systems containing the above solid electrolyte; (ii) a direct methanol fuel cell including a proton conductive solid electrolyte formed by sintering a mixture of metal compounds, at least one of which is a metal oxide; and (iii) hydrogen separation and supply equipment which has a hydrogen pump including the above solid electrolyte.

Electrochemical Factors in Design of Direct Methanol Fuel Cell Systems

Abstract: The design of direct methanol fuel cell based portable power source is being pursued at JPL. Performance data obtained on Nafion-based fuel cells have been used to develop a closed loop system model. The model has been exercised to obtain information on the impact of the various possible operating conditions on size, mass and efficiency of portable systems. Flow rate of air, methanol concentration and temperature are found to key controllable variables that significantly impact system size and efficiency. The results of the modeling studies are discussed.

Electrode performance and design for strip-cell direct methanol fuel cells

Abstract: A feasibility analysis of a mixed-reactant, strip-cell direct methanol fuel cell concept is presented. In this type of cell, selective electrodes are mounted in an altemating fashion on the same side of a membrane electrolyte, which minimizes the need for ancillary equipment, and maximizes power density. At low current density, the fuel efficiency of the direct-methanol strip cell is shown to be higher than that of a bipolar cell with typical cathodes. The effect of geometric parameters on the performance of strip cells is discussed, and design recommendations are given for a simple geometry.

Direct Methanol Fuel Cell: Transport Properties of Polymer Electrolyte Membrane and Cell Performance

Abstract: Methanol and water absorption in 1100 and 1200 e.w. Nafion® membranes was determined by weighing P 2 O 5 dried and methanol solution equilibrated membranes. Both methanol and water absorption in the 1200 e.w. membrane is about 70-74 % of that in the 1100 e.w. membrane. The methanol cross-over rate corresponding to that in a direct methanol fuel cell (DMFC) at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions (temperature, humidification and concentration of feed methanol solution). Accounting for the thickness difference between the membrane samples, the methanol cross-over rate through a 1200 e.w. membrane is 52 % of that through a 1100 e.w. membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode (DHE) was used as a reference electrode. Results show that in DMFC operation the cathode could be flooded due to the high water and methanol cross-over rates, especially through the 1100 e.w. membrane at a cell temperature below 80 °C. An increase in methanol cross-over rate as incurred by increasing the concentration of the feed methanol solution, increasing the cell operating temperature or using a membrane more permeable to methanol decreases the cathode potential of the DMFC at open circuit. As the cell current density is increased, the cathode potential of the DMFC can approach the cathode potential of a H 2 /air cell, thanks to the consumption of methanol at the anode and consequent decrease in methanol cross-over rate.

Direct methanol fuel cells: Cathode evaluation and optimization

Abstract: Direct methanol fuel cell (DMFC) performance has been improved as a result of Pt cathode optimization based upon the process of robust design. Cathode performance has been evaluated using cathode polarization curves generated from DMFC data. The effects of variation of temperature and nature of cathode backings on DMFC cathode potential were investigated. Our results show that Pt rich DMFC cathodes, operating on ambient air at 60°C, can exhibit high performance of >0.85 V vs. RHE at 100 mA/cm 2 .