ME - Resumos em livros de actas
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- Integrating hydrogen generation and storage in a novel compact electrochemical system based on metal hydridesPublication . Rangel, C. M.; Fernandes, Vitor; Slavkov, Y.; Bozukov, Latchezar N.A novel electrochemical system has been developed which integrates hydrogen production, storage and compression in only one device, at relatively low cost and high efficiency. The development of efficient and reliable energy storage systems based on hydrogen technology represents a challenge to seasonal storage based on renewable hydrogen. State of the art renewable energy generation systems include separate units such as electrolyser, hydrogen storage vessel and a fuel cell system for the conversion of H2 back into electricity, when required. In this work, a compact unit integrating production and storage is proposed. The developed prototype comprises a six electrode cell assembly using an AB5 type metal hydride and Ni plates as counterelectrodes, in a 35 wt% KOH solution. During charging, hydrogen is absorbed in the metal hydride and corresponding oxygen is conveyed out of the system. Conversely, in the case of discharging hydrogen stored in the metal hydride is released to an external H2 storage. In the present prototype, released hydrogen was delivered into the hydrogen storage up to a pressure of 15 Bar. Metal hydride electrodes with chemical composition LaNi4.3Co0.4Al0.3 were prepared by high frequency vacuum melting followed by high temperature annealing at 1000O C during 8 hours. X-Ray phase analysis showed typical hexagonal structure and no traces of other intermetallic compounds belonging to the La-Ni phase diagram. Thermodynamic study has been performed in a Sieverts type of apparatus produced by Labtech. Int. During cycling, charging was run at 40 A at cell voltages of 1.7 V for two hours which corresponds to C/2 charging time. Hydrogen was released by applying a constant current of 40A for two hours until cell voltage rise from 0.5 to 1.7V, at the end of the processes. The process was studied in-situ using a gas chromatograph from Agilent. It is anticipated that the device will be integrated as a combined hydrogen generator and storage unit in a stand alone system associated to a 1 kW fuel cell.
- Water transport through a PEM Fuel Cell: a one-dimensional model with heat transfer effectsPublication . Pinto, A. M. F. R.; Oliveira, V. B.; Falcão, D. S.; Pinho, C.; Rangel, C. M.One of the critical problems and design issues of PEM fuel cells is the water management because the membrane’s hydration determines the performance and durability of the cell. In this work, a steady state, one-dimensional model accounting for coupled heat and mass transfer in a single PEM fuel cell is presented. Two-phase flow effects are neglected. The anode and cathode flow channels are treated using the continuous stirred tank reactor (CSTR) approach. The cell voltage expression incorporates the anodic and cathodic overpotentials as well as the ohmic losses across the membrane. The reactions in the catalyst layers are considered as homogeneous. The kinetics of the cathodic oxygen reduction is modelled using the Tafel equation while a modified Tafel expression is used to describe the anode losses. Pressure gradients across the layers are assumed as negligible. Mass transport in the diffusion layers and membrane is described using effective Fick models. Local equilibrium at interfaces is represented by partition functions. Water transport through the membrane is assumed to be a combined effect of diffusion and electro-osmotic drag. It is assumed that the membrane proton conductivity and water diffusivity are a function of , the number of water molecules per ionic group. The heat transport through the gas diffusion layers is assumed as a conduction-dominated process. The thermal conductivity for all the materials is assumed as constant. Heat generation or consumption is considered in the catalyst layers. The analytical solutions for concentration and temperature across the cell are computed. Particular attention is paid to the water distribution across the membrane. The influence of different parameters (such as the current density and the level of humidification of inlet gases) over the water transport and on the cell performance is studied. The model is validated with recent published data and with experimental results obtained with an in-house designed PEMFC (25cm2 of active area). This easily implemented simplified model is suitable to define the optimal hydration conditions of the membrane
- Heat and mass transfer effects in direct methanol fuel cell: 1D modelPublication . Pinto, A. M. F. R.; Falcão, D. S.; Oliveira, V. B.; Rangel, C. M.Models play an important role in fuel cell development since they facilitate a better understanding of parameters affecting the performance of fuel cells and fuel cells systems. In this work, a steady state, one-dimensional model accounting for coupled heat and mass transfer, along with the electrochemical reactions occurring in the DMFC is presented. The model accounts for the kinetics of the multi-step methanol oxidation at the anode while the kinetics of the cathodic oxygen reduction is modelled using the Tafel equation. Two-phase flow effects are neglected. The anode and cathode flow channels are treated using the continuous stirred tank reactor (CSTR) approach. The cell voltage expression incorporates the anodic and cathodic overpotentials as well as the ohmic losses across the membrane. The mixed potential of the cathode due to methanol crossover is also included. The reactions in the catalyst layers are considered homogeneous. Pressure gradients across the layers are assumed as negligible. Methanol and water transport through the membrane is assumed to be due to the combined effect of the concentration gradient and electro-osmotic force. Mass transport in the diffusion layers and membrane is described using effective Fick models. Local equilibrium at interfaces is represented by partition functions. The methanol flux in the cathode catalyst layer is considered as well as methanol crossover. The transport of heat through the gas diffusion layers is assumed to be a conduction-dominated process. The thermal conductivity for all the materials is assumed to be constant. Heat generation is considered in the catalyst layers. The analytical solutions for concentration and temperature across the cell are compared with recently data existing in literature and with in-house obtained results, for a wide range of operating conditions. The model shows very good agreement. This easily implemented simplified model is suitable for use in real-time DMFC simulations
- Effects of NaBH4 additions on hydrogen absorption by nanostuctured FeTi powdersPublication . Marques, Sofia Figueiredo; Correia, J.B.; Shohoji, Nobumitsu; Rangel, C. M.; Paiva Luís, TeresaHydrogen is nowadays considered as one of the most promising fuels for the future transportation market, since it is highly energetic and its combustion products are non-toxic. There are however some inherent problems related to its handling and storage that makes its implementation difficult in the energy market [1]. One way of storing hydrogen is in form of intermetallic hydrides. Some intermetallics can store large amounts of hydrogen in their interstitial sites and, in some cases, reversible equilibrium absorption/desorption cycles might be realized near ambient temperature and normal pressure. FeTi is an intermetallic compound that is being widely studied for hydrogen storage purposes. This system has one of the highest volumetric storage capacities and can be produced at low cost [2,3]. However, the FeTi alloy prepared through conventional metallurgical process requires activation treatments at elevated temperature. It has been shown previously that the nanostructured FeTi can be activated at room temperature with the mechanical alloying of pure metallic constituents, Fe and Ti, with NaBH4 [4]. In this work nanostructured FeTi based powders were produced by mechanical alloying, and the effects of adding different amounts of NaBH4 on the hydrogen absorption capacity and on the agglomeration of the powders were studied. The effect of handling powders in a glovebox with oxygen free atmosphere or in atmospheric ambient condition was also examined. Several parameters of the as-milled powders were controlled. Among the characterization performed are phase identification and crystallite size determinations by X-ray diffraction, micro hardness measurements, scanning electron microscopy and absorption isotherms determinations.
- PEM fuel cells: materials ageing mechanisms and performance impactPublication . Rangel, C. M.; Silva, R. A.; Paiva Luís, TeresaPolymer exchange membrane (PEM) fuel cells are considered promising power sources, with a vast application domain that includes consumer electronics, automotive and residential applications. As the technology matures, durability, reliability and cost are amongst the most critical issues, so creating the need for a more comprehensive knowledge of material’s ageing mechanism. In this work, the Membrane-Electrode Assembly, MEA, is considered a key component subject to material’s ageing with considerable impact on fuel cell performance. As it contains the polymer electrolyte membrane, the active catalysts and the gas diffusion layers (GDL), the mechanisms of degradation are complex. Furthermore, performance is also link to components such as gas distributor plates, since the used design and flow channels dimensions (channel width, channel depth, rib width) allow minimization of the diffusion pathway for gases. Effective oxidant supply and water management is greatly affected by cell geometry and materials. In-situ and ex-situ evaluation of MEA degradation were conducted after fuel cell ageing in extreme testing conditions. Humidified and dry gas feeds were also examined and the effect on cell performance and membrane conductivity examined. Variations of membrane conductivity with temperature and water content were considered critical: drying during operation as a result of dragging of water by protons or over saturated conditions cause condensation at the electrodes causing flooding with the consequent voltage degradation. Electrochemical Impedance Spectroscopy was found instrumental in the identification of flooding conditions using an equivalent circuit to model the interfaces at critical current densities, according to the location of identified irreversibility’s in the voltage-current domain of the fuel cell. Electrocatalyst surface area loss due to growth of catalyst particle size and particle agglomeration with the number of load cycles is suggested when using cyclic voltammetry of electrodes, this is thought to be due to a mechanism involving catalyst dissolution/precipitation. Cross sections of the membrane catalyst layers and GDLμs were examined under a FEG-SEM indicating that cathode thickness is considerably reduced as a result of ageing. Catalyst particles were found to migrate outwards and located on carbon backings. Fluoride release was considered as an early predictor of membrane degradation, quantified using an ion selective at gases outlet. MEA degradation mechanisms are discussed together with contributions that might aid design and operating strategies in PEM fuel Cells.
- Nanoscale layered double hydroxide materials for corrosion resistancePublication . Rangel, C. M.; Travassos, Maria AntóniaLayered Double Hydroxides (LDHμs), represented by the general formula [MII (1-x)MIIIx(OH)2[An-x/n].zH2O or [MIMIII2(OH)6[An-1/n].zH2O], where MI, MII, MIII are mono-, di- and tri-valent metal cations, are being researched as anion-exchange materials which interesting intercalation chemistry that accommodate a wide range of applications from heterogeneous catalysis to storage and subsequent controlled release of bioactive agents. In this work, layered double hydroxides containing a monovalent (Li+) and trivalent (Al3+) matrix cations, have been synthesized and characterised using X-ray diffraction, FTIR and SEM. LDHμs were prepared by a simple co-precipitation method using metal hydroxides and metal salts in an alkaline solution. Hybrid systems are produced by intercalation which involves a guest molecule introduced into the host structure replacing the existing interlayer ion, without affecting the host structure opening new applications according to desired functionalities namely as thin films in corrosion protection. Li based conversion coatings are easily formed under open circuit conditions on Al surfaces [1-3]. Formation of LDHμs on the metal surface of copper-rich Al alloys were attempted with excellent results. Pitting corrosion was inhibited on Aluminium 2024-T3 with an extensive capability to withstand the presence of high concentrations of chloride ions. Intergranular corrosion was found to be inhibited in Al-Li 8090 alloy by action on copper containing T-phases located at the grain and sub-grain boundaries. The formation of DHLμs is thought to be responsible for inhibition which is demonstrated to be under diffusion control. The action of DLHμs on copper is demonstrated in separated experiments using pure copper samples in similar experimental conditions as for the alloy, in an extensive electrochemical study.
- Materials for solar hydrogen production with simultaneous mineralization of ethanolPublication . Rangel, C. M.; Silva, R. A.; Paiva Luís, Teresa; Charasse, B.The photo-catalytic production of hydrogen by means of irradiation of a suspension of semiconductor oxides presents attractive features over other methods with higher cost such as water electrolysis. In this work, photocatalytic hydrogen production from water is studied, using ethanol as sacrificial agent. New nanostructured multifunctionalised semiconductor materials based on titanium dioxide, with effective photo-catalytic properties under UV illumination were synthesized using sol-gel technology and characterised by X-Ray diffraction and scanning electron microscopy. Aqueous suspensions of the semiconductor powders were used and the effect of solution pH and temperature (20-70ºC) as well as the effect of concentration of ethanol on hydrogen production were studied, for fixed concentrations of the catalyst. Comparison is made with doped Degussa-P25 TiO2 titanium dioxide. The need to decrease the electron-hole recombination rate was accounted for by metal doping [1] with the ethanol molecule acting as a hole trap. An increase in the hydrogen production rate was found as a result of the percentage of metal on doped titania and optimisation of experimental conditions with rate values being superior to recently published literature data [2]. Particle size, reactive surface area, structure and crystallinity of the semiconductor were found to be determinant in the production of highly photoactive titanium dioxide. Research in progress includes development of catalyst that allows effective utilization of visible light and design of an experimental reactor
- Layered double hydroxides for aluminium alloys corrosion resistancePublication . Rangel, C. M.; Travassos, Maria AntóniaLayered Double Hydroxides (LDHμs), represented by the general formula [MII (1-x)MIIIx(OH)2[An-x/n].zH2O or [MIMIII2(OH)6[An-1/n].zH2O], where MI, MII, MIII are mono-, di- and tri-valent metal cations, are being researched as anion-exchange materials with interesting intercalation chemistry that accommodate a wide range of applications including corrosion resistance. In this work, layered double hydroxides containing a monovalent (Li+) and trivalent (Al3+) matrix cations, have been synthesized and characterised using X-ray diffraction, FTIR and SEM. LDHμs were prepared by a simple co-precipitation method using metal hydroxides and metal salts in an alkaline solution. Formation of LDHμs on the metal surface of Al alloys were attempted with excellent results. Pitting corrosion was inhibited on Aluminium 2024-T3 with an extensive capability to withstand the presence of high concentrations of chloride ions. The formation of DHLμs is thought to be responsible for inhibition which is demonstrated to be under diffusion control. The action of DLHμs on copper is demonstrated in separated experiments using pure copper samples in similar experimental conditions as for the alloy, in an extensive
- Electrochemical characterization of poly(ethylene oxide)- zinc chloride system and its application in rechargeable batteriesPublication . Plancha, Maria João; Rangel, C. M.; Sequeira, C.A.C.The system PEOnZnCl2 with n=4-16 was studied in view of its potential application in a solid state rechargeable zinc battery. A.c. conductivity and cationic transference number measurements, in the temperature range 20-150ºC, were performed and the electrochemical stability window was established for the polymer electrolyte with n=4 composition. The ionic conductivity, σ, of this film, follows a VTF behaviour, with an activation energy of 3.3 ± 0.2 kJ mol-1 and σ values were found between 2.50x10-7 S cm-1 at 24ºC and 4.81x10-4 S cm-1 at 145ºC. Acceptable zinc ion transference numbers of 0.36 (medium value) and decomposition voltage values between 3.19V (20ºC) and 1.44V (150ºC) were estimated. Cyclic voltammetric studies using Zn/PEO4ZnCl2/Zn cell indicated reversibility of the Zn/Zn2+ couple at the electrode/electrolyte interface. Several cells Zn(-)/PEO4ZnCl2/Nb2O5(+) were assembled and studied at 55ºC, with several discharge current densities. Results of cell’s discharge profiles, capacity values, charge-discharge cycles behaviour and stability are reported.
- Effect of anode flow field design in direct methanol fuel cells: preliminary studiesPublication . Schock, L.; Silva, R. A.; Malaquias, J.; Pinto, A. M. F. R.; Rangel, C. M.The direct methanol fuel cells are promising candidates for portable power sources due to their high energy density, however studies continue in order to give solutions for a number of drawbacks that affect cell performance and efficiency. Achieving good fuel cell performance requires that the flowing streams of fuel and oxidizer are evenly distributed over the entire surface of the catalyst layer and also an efficient removal of reaction products. This is achieved through the optimal design of the flow field, which primarily depend upon channel pattern as well as channel (and rib) shape and size. In this work the effect of anode flow field design on the performance of an own built DMFC is studied. Preliminary results are herein presented.