Browsing by Issue Date, starting with "2010-04-29"
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- PEM Fuel Cells: materials ageing and degradationPublication . Silva, R. A.; Paiva Luís, Teresa; Rangel, C. M.As fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials ageing mechanisms is essential to attain specified lifetime requirements for applications. In this work, the membrane-electrode assembly (MEA) degradation of an eight cell PEM low power stack was evaluated, during and after fuel cell ageing in extreme testing conditions. The stack degradation analysis comprised observation of catalytic layer, morphology and composition. Cross sections examination of the MEAs revealed thickness variation of catalytic layer and membrane. Other modes of degradation such as cracking,
- A power management strategy for a stand-alone photovoltaic/fuel cell energy system for a 1kW applicationPublication . Pinto, P.J.R.; Rangel, C. M.In this paper a power management strategy is presented for a stand-alone photovoltaic (PV)/fuel cell (FC) energy system. PV is the primary power source of the system and an FC-electrolyzer combination is used as a backup and a long-term storage system. The energy in the hybrid system is balanced by the common dc bus voltage regulation. A simple hysteresis voltage control is used for dc bus voltage regulation. In this way, the fuel cell and the electrolyzer can be protected from unnecessary utilization or irregular operation (reduction of frequent start-ups and shutdowns). Simulation results obtained using Matlab and Simulink are presented to verify the effectiveness of the proposed control algorithm.
- PEM Fuel Cell performance at sub-zero temperaturesPublication . Rangel, C. M.; Paiva Luís, Teresa; Fernandes, VitorIn this work a study of the performance of a low power fuel cell at sub-freezing temperatures has been undertaken. Knowledge in this area is still scarce. After global characterization of the stack on a wide range of temperatures and relative humidity’s the behaviour at negative temperatures (-5ºC -10ºC, -15ºC) has been established. Furthermore, performance was evaluated after the cell was submitted to cycles from -25ºC to + 25ºC. At the end of 10 cycles only marginal loss in performance was registered, when testing at + 2.5ºC and + 25ºC. On the basis of the obtained results a strategy for start-up and shut-down has been designed in order to be implemented for operation at low temperatures. A failure analysis of the membrane and catalyst layers and GDLs is under way in order to evaluate material degradation
- Towards an electroless deposition of gold on metallic substrates using ionic liquids as electrolytesPublication . Sá, A.I. De; Quaresma, S.; Eugénio, S.; Rangel, C. M.; Vilar, RuiRecent research has suggested a number of applications for gold in fuel cells and related hydrogen fuel processing, which include light weight corrosion resistance bipolar plates and the incorporation of gold as catalyst to provide improvements in electrode conductivity, among others. This paper reports on the electroless deposition of gold on copper substrates from a HAuCl4.3H2O solution in 1-butyl-1- methylpyrrolidinium dicyanamide (BMP-DCA), in laboratory atmospheric conditions. The electrochemical behaviour of the plating solution was studied by potential sweep techniques. Results suggest that gold ions are reduced and deposited on copper without the aid of external polarization, probably by displacement deposition involving copper dissolution, since no other oxidation reaction was identified. Preliminary studies using SEM/EDX showed that compact gold thin films, deposited on copper substrates at 333 K, present a granular nanostructured morphology.
- Polarity reversal in PEM Fuel Cells by fuel starvationPublication . Travassos, Maria Antónia; Rangel, C. M.In this work, the degradation caused by polarity reversal by fuel starvation of a 16 MEA – low power fuel cell is reported. Measuring of the potential of individual cells, while on load, was found instrumental in the location of affected cells which revealed very low or even negative potential. Electrochemical impedance spectroscopy insitu gave insight on the increase in resistance and diffusion processes. Ex-situ analysis of MEA after irreversible degradation by fuel starvation gave as a result delamination of catalyst layers with impacts on fuel cell performance such as development of flooded areas by the pockets created increasing the resistance of reactant transport to the catalyst sites. Striking and thickness variation of the anode layer as well as carbon corrosion were found. The proton exchange membrane is also affected by fluoride depletion.
- Generation of hydrogen from chemical hydrides under pressure up to 70 BarPublication . Fernandes, Vitor; Esteves, M. Alexandra; Ferreira, M. J. F.; Pinto, A. M. F. R.; Rangel, C. M.Hydrogen production by sodium borohydride through hydrolysis in alkaline solutions has been extensively studied as a production/storage option. The potential application of this option is dependent on an easily controllable catalysed hydrolysis reaction at significant rates to comply with fuel cell feeding but also on the increase of the gravimetric density being of paramount importance, the design and implementation of compact and efficient reactors and the reaction characterization under pressure. In this work, hydrogen generation by the catalyzed hydrolysis of sodium borohydride was studied under pressure up to 70 bar with excellent results
- Degradation of lithium Iron phosphate-based cathode in lithium-ion batteries: a post-mortem analysisPublication . Plancha, Maria João; Rangel, C. M.; Rodrigues, B.; Azevedo, F.; Silva, A. A.Commercial Li-ion batteries were studied in view to investigate the degradation of the positive electrode in an end-of-life battery condition. Post-mortem analyses were performed by using SEM and DRX techniques; structural and morphological changes after prolonged cycling were evaluated comparatively to a fresh cathode sample. The cycling procedure based on a constant current (CC)/constant voltage (CV) charge and CC discharge was executed, being the condition of end-of-life battery achieved after submitting the Li-ion battery to nearly 2000 charge/discharge cycles. EDS analysis revealed zirconium element as the dopant of a LiFePO4-based cathode of the battery under study. According to X-ray diffraction results for the fresh (charged condition) cathode, the positive electrode includes in its constitution a mixture of crystalline compounds, LiFePO4 and FePO4. SEM images displayed and DRX patterns obtained for the cycled cathode showed modifications compared to the fresh cathode results, evidencing the degradation of the battery at the end-of-life: decrease in the density of microparticles associated to areas where the insertion/de-insertion occurs; decrease of the LiFePO4/FePO4 ratio; both results pointed out to the occurrence of battery loss capacity with the imposed charge/discharge cycles.
- SPP: portable power technologies for the dismounted soldierPublication . Sousa, Carlos; Rangel, C. M.; Rodrigues, J. Campos; Fernandes, Vitor; Paiva Luís, Teresa; Soares, D. A.The project aims at the development of a portable power system as standard equipment for defence use, and is being developed by a Portuguese consortium leaded by SRE, a fuel cell developer, LNEG (Portuguese National Laboratory for Energy and Geology) and EID (Electronics and Telecommunications for Army) through a SRE/ Ministry of Defence contract. The system uses a SRE H2 PEM Fuel Cell and a Li battery buffer (30Wh). The fuel is provided by an on board H2 generator based on chemical hydrides, aiming to provide 72h autonomy to the dismounted soldier. Main project constraints and challenges are the power density (both gravimetric and volumetric) and the operation under severe atmospheric conditions. A field test pre-series shall take place by the end of 2010 with probable commercialization foreseen by early 2012.
- On-demand hydrogen generation by hydrolysis of sodium borohydride in batch reactors: effect of the buffer pressurePublication . Ferreira, M. J. F.; Coelho, F.; Fernandes, Vitor; Rangel, C. M.; Pinto, A. M. F. R.A study was undertaken in order to investigate the potential of hydrogen generation by hydrolysis of sodium borohydride in batch reactors, operating at moderate pressures, in the presence of a reused nickel-ruthenium based catalyst, to feed on-demand a proton exchange membrane fuel cell. The effect of the buffer pressure is explored and hydrogen generation rates are evaluated by changing catalyst amount, operating pressure and successive refueling