Browsing by Issue Date, starting with "2011-11-10"
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- MEA degradation in PEM Fuel Cell : a joint SEM and TEM studyPublication . Silva, R. A.; Hashimoto, T.; Thompson, G. E.; Rangel, C. M.One of the important factors determining the lifetime of polymer electrolyte membrane fuel cells (PEMFCs) is membrane electrode assembly (MEA) degradation and failure. The lack of effective mitigation methods is largely due to the currently very limited understanding of the underlying mechanisms for mechanical and chemical degradations of fuel cell MEAs. This work reports on the effect of 1500 h operation of an eight-cell stack Portuguese prototype low power fuel cell. A performance decrease of 34%, in terms of maximum power, was found at the end of testing period. A post-mortem analysis by SEM and TEM was done for most cells of the fuel cell. Loss of the PTFE ionomer in the anode and cathode catalytic layers; morphological changes in the catalyst surfaces such as loss of porosity and platinum aggregation, deformation on the MEA components (anode, cathode and membrane) were identified. Others, like delamination and cracking were also detected. Catalyst migration and agglomeration on the interface of the electrodes was observed at cells 2, 4, 6 and 7. A platinum band was also detected on the membrane at 2 μm apart from the anode of cell 4. In some cases, dissolution occurred with re-deposition of the platinum particles with facet
- Novos e velhos conceitos da protecção e conservação da madeiraPublication . Santos, José António dos; Duarte, Maria Carlota Oliveira
- Relaxation time distribution analysis of a polymer electrolyte fuel cell stack from its impedance responsePublication . Lopes, Vitor V.; Silva, R. A.; Novais, Augusto Q.; Rangel, C. M.Electrochemical impedance spectroscopy (EIS) is an analysis technique that is commonly used as a base diagnostics technique for the in-situ analysis of the kinetic and transport properties of proton exchange membrane (PEM) fuel cells. This work proposes to use the distribution of relaxation times (DRT) as a complementary analysis technique for the interpretation of EIS data. For this purpose, the DRT is deduced for a modified Randles electric circuit composed of a constant phase element (CPE) connected in parallel with a resistance in series with a finite diffusion Warburg element. The experimental EIS data collected from an eight cell PEMFC with an open-air cathode was modeled through the use of two modified Randles circuit representing the fuel-cell electrodes. The analysis of the DRT allows to identify further characteristics of the individual processes that occur at both electrodes, while also being instrumental in detecting the effect on the fuel cell performance of some operating conditions, namely hydrogen flow-rate and current.
- Innovative reactor prototype for Hydrogen production in a stationary application using sodium borohydridePublication . Barbosa, Rui; Ferreira, V.; Silva, D.; Condes, J.; Ramos, S.; Amaral, V.; Pinto, A. M. F. R.; Figueiredo, A.; Rangel, C. M.Hydrogen storage has proved to be the greatest obstacle preventing hydrogen from replacing fossil fuels. Hence, a safe, efficient and economical method of storing hydrogen must be available to turn viable a hydrogen economy based on renewable resources [1]. Hydrogen can be stored in chemical hydrides such as sodium borohydride (NaBH4), with large theoretical H2 content of 10,9 wt%. With the aid of catalysts, and at room temperatures, the alkaline hydrolysis of NaBH4 can be enhanced [2]. In this work, a 100 L innovative reactor for hydrogen production was designed, based on the optimized layout of a laboratorial scale reactor [3], as part of a project financed by the Portuguese financial support program NSRF. The developed system has the capability to feed a 5 kW PEM fuel cell with a maximum hydrogen consumption of 75 slpm. The NaBH4 solution is stored in a 50 L reservoir from where seven consecutive 7,0 L injections to the reactor are possible. The Ni-Ru based catalyst applied can be re-used several times without losing its performance [1] and because of this capacity its replacement will be done, manually, every seven NaBH4 solution injections (simultaneously with the residual solution removal and the reactor cleaning). The catalyst should then be recovered for further utilization. Additionally to the reactor, a 400 to 500 L reservoir was also designed to be used as the system buffer since the reactor works in batch mode and it is desired that the PEMFC operates continuously. The system was conceived for stationary applications and eventually to be installed in remote areas, reason why the systemś monitoring and control are fully automatized. Its hydropneumatic circuit layout is characterized by four parts: injection system, reactor, valves bloc and buffer. It is assumed that the designed system can operate continuously throughout 15 hours with a medium hydrogen consumption of 10 slpm, which can supply a daily household energy power demand.
- Energetic and environmental evaluation of microalgae biomass fermentation for biohydrogen productionPublication . Ferreira, Ana F.; Ortigueira, Joana; Alves, Luís; Gouveia, Luisa; Moura, Patrícia; Silva, Carla M.This paper presents an energetic and environmental evaluation of the fermentative hydrogen production from the sugars of Scenedesmus obliquus biomass hydrolysate by Clostridium butyricum. The main purpose of this work was to evaluate the potential of H2 production and respective energy consumptions and CO2 emissions in the global fermentation process: hydrolysis of S. obliquus biomass, preparation of the fermentation medium, degasification and incubation. The scale-up to industrial production was not envisaged. Energy consumption and CO2 emissions estimations were based on SimaPro 7.1 software for the preparation of the fermentation medium and the use of degasification gas, nitrogen. The functional unit of energy consumption and CO2 emissions was defined as MJ and grams per 1 MJ of H2 produced, respectively. The electricity consumed in all hydrogen processes was assumed to be generated from the Portuguese electricity production mix. The hydrogen yield obtained in this work was 2.9 ± 0.3 mol H2/mol sugars in S. obliquus hydrolysate. Results show that this process of biological production of hydrogen consumed 281-405 MJ/MJH2 of energy and emitted 24-29 kgCO2/ MJH2. The fermentation stages with the highest values of energy consumption and CO2 emissions were identified for future energetic and environmental process optimisation.
- Synthesis of azole biophospanates presursors for proton-exchange membrane for application in high temperature PEM fuel cellsPublication . Teixeira, Fatima; Rangel, C. M.This work reports the synthesis and characterization of phosphonate-, hydroxybisphosphonate- and aminobisphosphonatebenzimidazole derivatives substituted at N-1 position and new regioisomers phosphonateand aminobisphosphonatebenzotriazole derivatives substituted at N-1 or N-2 positions. The compounds were characterized by NMR, IR spectroscopy and mass spectrometry (low and high resolution) allowing the assignment of their structure, including the identification of regioisomers. These azoles will be precursors of mesoporous silica host to produce novel membranes materials with high proton conductivity for intermediate temperature PEMFCs.
- Materiais e produtos de construção : ciclo de vida, ecodesign, certificação e inovaçãoPublication . Duarte, Ana Paula; Frazão, Rui
- Modeling and simulation of micro direct methanol Fuel CellsPublication . Falcão, D. S.; Oliveira, V. B.; Oliveira, M. S. N.; Rangel, C. M.Fuel cells have unique technological attributes: efficiency, absence of moving parts and low emissions. The Direct Methanol Fuel Cell (DMFC) has attracted much attention due to its potential applications as a power source for transportation and portable electronic devices. With the advance of micromachining technologies, miniaturization of power sources became one of the trends of evolution of research in this area. Based on the advantages of the scaling laws, miniaturization promises higher efficiency and performance of power generating devices, so, MicroDMFC is an emergent technology. Models play an important role in fuel cell development since they facilitate a better understanding of parameters affecting the performance of fuel cells. In this work, a steady state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in a fuel cell, already developed and validated for DMFC in [1-3], is used to predict Micro DMFC performance. The model takes in account all relevant phenomena occurring in a DMFC. Polarization curves predicted by the model are compared with experimental data existing in literature and the model shows good agreement, mainly for lower current densities. The model is used to predict some important parameters to analyze fuel cell performance, such as water transport coefficient and leakage current density. This easily to implement simplified model is suitable for use in real-time MicroDMFC simulations.
- Current density distribution mapping in polymer electrolyte membrane fuel cellPublication . Sousa, T.; Falcão, D. S.; Pinto, A. M. F. R.; Rangel, C. M.A non-uniform utilization of the active area due to inhomogeneous current density distribution is one of the main factors for poor fuel cell performance. Furthermore, it leads to hot points which can be responsible for thermal stress in the membrane electrode assemble (MEA). Therefore, it became extremely important to have a consistent technic to visualize in real time the current density and temperature distribution on the active area. For this purpose a printed circuit board (current scan lin® form S++) was used to measure the current density and temperature distribution. With this equipment it was possible to generate high resolution counters for these two variables. With these results the effect on the current density distribution by different flow fields design, oxygen stoichiometry, and temperature were analysed. Besides, these results can be used to provide crucial data for simulation work, in particular for validation purpose.
- Development of new organic electroactive materials for rechargeable batteriesPublication . Furtado, Olívia; Rangel, C. M.The need for a clean and sustainable energy supply in the transportation sector have prompted electric vehicles as one of the options to reduce fossil fuel dependency and greenhouse gases emissions. Furthermore, electric vehicles are seen as enablers of the storage capacity of electric grids. In this context, the reasobnable energy density and cyclability reached by lithium-ion batteries based on inorganic cathodes have placed them in considerable advantage regarding energy conversation and storage [1, 2]. For higher energy density, lower cost and more environmentally acceptable batteries research efforts are presently focus on organic-based electrodes as a new approach to conventional inorganic cathodes. [3]. In his work, we carried out the development of a derivative of polymide (PI) for use as electrochemically active cathode material in rechargeable lithium batteries. Even though the application of polyimide as electrode material is scarcely found due to its insulating character, its aromatic imide group can be electrochemically oxidized and reduced in a reversible manner [4, 5]. This paper will present the studies on the synthesis and chemical characterization of a polyimide derivate and its evaluation as cathode organic polymer after electrochemical characterization. Test half-cell testing and battery constitution are being planned. The use of polyimide derivates is considered promising because of the potentiial increase in specific discharge capacity, not flammability, excellent thermal stability and high machanical strength. Furthermore since the polyimide is hydrolysable, its use can also be considered environmentally friendly.