Browsing by Author "Silva, R. A."
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- Assessing cell polarity reversal degradation phenomena in PEM fuel cells by electrochemical impedance sectroscopyPublication . Travassos, Maria Antónia; Lopes, Vitor V.; Silva, R. A.; Novais, Augusto Q.; Rangel, C. M.Electrochemical impedance spectroscopy (EIS) is identified as one of the most promising in-situ diagnostics tools available for assessing fuel cell ageing and degradation. In this work, the degradation phenomena caused by cell polarity reversal due to fuel starvation of an open cathode 16 membrane electrode assembly (MEA) – low power (PEM) fuel cell (15 W nominal power) – is reported using EIS as a base technique. Measuring the potential of individual cells, while the fuel cell is on load, was found instrumental in assessing the “state of health” of cells at fixed current. Location of affected cells, those farthest away from hydrogen entry in the stack, was revealed by very low or even negative potential values. EIS spectra were taken at selected break-in periods during fuel cell functioning. The analysis of impedance data was made using an a priori equivalent circuit describing the transfer function of the system in question –equivalent circuit elements were evaluated by a complex non-linear least square (CNLS) fitting algorithm, and by calculating and analyzing the corresponding distribution of relaxation times (DRT). Results and interpretation of cell polarity reversal due to hydrogen starvation were complemented with ex-situ MEA cross section analysis, using scanning electron microscopy. Electrode thickness reduction and delamination of catalyst layers were observed as a result of reactions taking place during hydrogen starvation. Carbon corrosion and membrane degradation by fluoride depletion are discussed.
- Assessing performance and degradation in PEM Fuel Cells by electrochemical impedancePublication . Rangel, C. M.; Silva, R. A.; Paiva Luís, Teresa
- Assessing performance and degradation mechanisms in proton exchange membrane Fuel CellsPublication . Rangel, C. M.; Silva, R. A.; Paiva Luís, Teresa
- Characterization of MEA degradation for an open air cathode PEM fuel cellPublication . Silva, R. A.; Hashimoto, T.; Thompson, G. E.; Rangel, C. M.As fuel cell technology matures and time scale to commercialization decreases, the need for a more comprehensive knowledge of materials’ aging 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 aging in specified testing conditions of load-cycling that may compromise the durability of the catalyst. The stack degradation analysis comprised observation of catalytic layers, morphology and composition. Examination of the MEAs cross sections, in a joint SEM and TEM study, revealed thickness variation of catalytic layer (up to 47% for the cathode layers), and cracking, delamination, and catalyst migration were observed even though catalyst sintering and consequent loss of electrochemical active area seem to be predominant together with F loss from the ionomer used as binder in the catalytic layers.
- 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.
- Effect of the oxide loading on the surface characteristics of LaNio3 oxide coated electrodesPublication . Soares, C. O.; Silva, R. A.; Carvalho, M. D.; Jorge, M. E. Melo; Gomes, A.; Rangel, C. M.; Pereira, M. I. da SilvaThe LaNiO3 perovskite-type oxide is one of the most tested anode for the oxygen evolution reaction in alkaline solutions. It is well established that the oxide preparation conditions and the electrode fabrication are key factors to control the electrochemical behaviour of oxide coatings. In a previous work the authors studied the influence of preparation conditions of the oxide and support type on the electrochemical behaviour of Ni foam coated LaNiO3 electrodes. Ni foam was selected as support due to its unique characteristics namely low contact resistance between the oxide and support, possibility of high metal oxide loadings and dimensional stability [1]. No studies were performed, concerning the influence of the oxide loading. Studies performed by Singh et al. on LaNiO3 coatings on Ni foil supports have shown that the electrode roughness factor increased with increase in oxide loading at the beginning and finally attained a constant value around 0.03 g cm-2 [2]. The present work reports on the study of the dependence of roughness factor (Rf) and morphology factor (φ) on the oxide loading for Ni foam coated LaNiO3 electrodes with loadings varying between 0.02 and 0.14 g cm-2. Cyclic voltammetry and electrochemical impedance spectroscopy were used to evaluate the Rf and φ values, complemented by optical microscopy observations. A non-linear increase of both Rf and φ with the oxide loading is observed, showing a level off when the oxide loading is increased. The level off was interpreted as a progressive exclusion of the crystallites from the contact with the solution as the oxide coating thickness increases.
- Electrochemical energy conversion in direct methanol fuel cellsPublication . Rangel, C. M.; Silva, R. A.; Verget, J.; Pinto, A.M.F.R.Direct methanol fuel cells (DMFCś) are promising candidates as portable power sources due to their lower weight, volume and high-energy density. They can potentially provide an energy content that exceeds current battery technology, with the possibility of instantaneous recharge. However, DMFCś have several serious drawbacks such as slow methanol oxidation kinetics, poisoning by CO of the catalyst surface, the high methanol crossover through the polymer membrane, the high costs of the Nafion membrane and catalyst. The flowing streams of fuel and oxidizer need to be evenly distributed over the entire surface of the catalyst layer in order to achieve a good performance together with an efficient removal of reaction products. This is achieved through the optimal design of the flow field. In this work the effect of flow field design on cell performance is studied, using polarization curves. A testing fuel cell was designed and implemented consisting of two stainless steel end plates, two sets of graphite collector plates, two carbon cloth diffusion layers, two catalyst layers of platinum black (0.70 mg.cm-2 and 0.75 mg.cm-2 Pt on cathode and anode, respectively) and an electrolyte polymer membrane (Nafion 117). Pt-Ru catalyst on the anode with a load of 4 mgcm-2 of catalyst was also studied. The active surface area of the cell is 25 cm2. An own built fuel cell and a Lyntech testing station were used in this work. The effects of temperature, methanol concentration, feeding flow rates of fuel and flow field design on the cell performance were studied. Results showed that the performance of the fuel cell increases with increase in cell temperature due to improvement in methanol oxidation kinetics and cathode kinetics, as indicated by polarization curves. The effect of temperature is complex; the oxygen partial pressure decreases with temperature increase due to the increase in vapor partial pressure, which causes decreases in the open-cell voltage and increases the concentration overpotential; the rate of methanol crossover increases with temperature, so the cell performance decreases; water transfer from anode to cathode through the membrane increases with temperature and the additional water increases the liquid water fraction in both the cathode catalyst and diffusion layers, thus causing an increase in concentration polarization. Increase in methanol concentration affected the performance of the DMFC, as expected due to a higher methanol crossover with higher concentrations. The methanol transferred from the anode to the cathode through the polymer electrolyte membrane is oxidized at the platinum electrocatalyst and causes a mixed potential, which lowers the cathode performance and thus the overall cell voltage output. The effect of the flow field design, single serpentine and parallel, on cell performance was also investigated. The serpentine flow field was better than the parallel design. The obtained results point towards an enhancement in the mass transfer of methanol improving the cell performance. Another important issue is the eventual blocking of the flow channels, by CO2, at high current densities. A reduction of the effective contact area between the fuel (methanol) and the gas diffusion layer is expected, as CO2 gas bubbles accumulate in the channels, bearing implications in the continuous supply of methanol to the catalyst sites through the gas diffusion layer. This is less likely to occur with the serpentine design. Cathode kinetics indicates slightly lower methanol crossover and better voltage efficiency at low current densities. Comparison is made with results obtained for the two different catalyst layers used emphasizing the effect of Ru.
- Electrochemical energy conversion in direct methanol fuel cells: the effects of flow fieldsPublication . Silva, R. A.; Oliveira, V.; Pinto, A. M. F. R.; Rangel, C. M.
- Enhanced bifunctional activity of LaNiO3-based gas diffusion electrodes for regenerative fuel cellsPublication . Silva, R. A.; Soares, C. O.; Carvalho, M. D.; Rangel, C. M.; Pereira, M. I. da SilvaPerovskites are of great interest when searching replacements for precious metals as catalyst for bifunctional oxygen electrodes involving the oxygen evolution(OER) and oxygen reduction reaction (ORR) as is the case of regenerative fuel cells. In this work a full electrochemical study on the electrochemical properties of gas diffusion electrodes (GDEs) using LaNiO3-based catalysts, conducted in alkaline media, led to a study of cyclability and durability. The incorporation of GDEs in a low power electrolyzer/fuel cell prototype was also attempted. The stability of the electrodes was assessed by potential cycling and at constant current density with good results.
- Fuel cell materials degradation in stack configurationPublication . Silva, R. A.; Travassos, Maria Antónia; Paiva Luís, Teresa; Rangel, C. M.