Fuel cells that can operate directly on fuels such as methanol are attractive for low to medium power applications in view of their low weight and volume relative to other power sources.A liquid feed direct methanol fuel cell has been developed based on a proton exchange membrane electrolyte and Pt/Ru and Pt catalyzed fuel and air/O2 electrodes respectively.The cell has been shown to deliver significant power outputs at temperatures of 60 to 90* C. The cell voltage is near 0.5 V at 300 mA/cm 2 current density and an operating temperature of 90* C. A deterrent to performance appears to be methanol crossover through the membrane to the oxygen electrode.Further improvemerits in performance appear possible by minimizing the methanol crossover rate.

No abstract

No abstract

The two-terminal alternating current impedance of lithium-titanium disulfide (Li/TiS2) rechargeable cells has been studied as a function of frequency, state-of-charge, and extended cycling. Analysis based on a plausible equivalent circuit model for the Li/TiS2 cell leads to evaluation of kinetic parameters for the various physicochemical processes occurring at the electrode/electrolyte interfaces. To investigate the causes of cell degradation during extended cycling, the parameters evaluated for cells cycled five times have been compared with the parameters of cells that have been cycled over 600 times. The findings are that the combined ohmic resistance of the electrolyte and electrodes suffers a ten-fold increase after extended cycling, while the charge-transfer resistance and diffusional impedance at the TiS2/electrolyte interface are not significantly affected. The results reflect the morphological change and increase in area of the anode due to cycling. The study also shows that overdischarge of a cathode-limited cell causes a decrease in the diffusion coefficient of the lithium ion in the cathode. The study demonstrates the value of electrochemical impedance spectroscopy in investigating failure mechanisms. The approach and methodology followed here can be extended to other rechargeable lithium battery systems.Of the several high-energy rechargeable lithium battery systems based on lithium-intercalatable cathodes, ~ the one using TiS2 has been advanced to a status of significant technological development. The Li/TiS2 cell consists of an elemental lithium anode (negative electrode), a titanium disulfide cathode (positive electrode), and a lithium salt dissolved in an aprotic nonaqueous solvent as the electrolyte. During discharge of the cell, lithium ions intercalate in the TiS2 cathode forming Li=TiS2 (0 < x < 1), and elemental lithium at the anode is oxidized to lithium ions. These cells can be charged and discharged between 1.6 and 2.7 V. Specific energy values in the range of 75 to 100 Wh/kg and 300 to 500 charge/discharge cycles have been realized. 2-~ During charge/discharge cycling, the Li/TiS2 cell suffers several changes which are irreversible. Some of the processes of cell degradation during cycling include morphological changes at the anode, formation of blocking surface layers, ~' 8 loss of interparticle contact at the cathode, 1 and development of electronic shorts due to dendritic lithium deposits. Destructive physical and chemical analysis during different stages of cycling also confirm degradation of several cell components. 7 The performance and failure properties of the cells are related to the physicochemical processes occurring in the bulk of the electrodes, in the electrolyte, and at the electrode/electrolyte interfaces. Therefore, in the present study, investigation of the interracial processes such as charge-transfer, double-la:~er relaxation, adsorption/desorption, surface layer formation, diffusion of electroactive species in the bulk and pores of the electrode structures, and evalua...

Transition metal dichlorides in sodium tetrachloroaluminate melts are viable alternatives to sulfur cathodes in sodium batteries in view of their potential advantages. Fundamental electrochemical studies on three candidate materials,i.e., FeCl2 , NiCl2 , and CuCl2 , were carried out using various techniques such as cyclic voltammetry, linear polarization, potentiodynamic polarization, and ac impedance. These studies were aimed at identifying various rate processes in the reduction, elucidating the reaction mechanisms, and determining the kinetic parameters for the reduction. The limitations in the performance of these cathode materials in high power density applications were also examined. Finally, recommendations were made from these studies for the selection of a candidate system among these materials for future NASA applications.