1,721,046 research outputs found
A simple analytical approach to simulate the electrochemical impedance response of flooded agglomerates in polymer fuel cells
Full text linkA novel (ex situ) method to quantify oxygen diffusion coefficient of polymer fuel cells backing and catalyst layers
No full textLimiting current density of oxygen reduction reaction in polymer electrolyte fuel cells is determined by several mass transport resistances that lower the concentration of oxygen on the catalyst active site. Among them, diffusion across porous media plays a significant role. Despite the extensive experimental activity documented in PEMFC literature, only few efforts have been dedicated to the measurement of the effective transport properties in porous layers. In the present work, a methodology for ex situ measurement of the effective diffusion coefficient and Knudsen radius of porous layers for polymer electrolyte fuel cells (gas diffusion layer, micro porous layer and catalyst layer) is described and applied to high temperature polymer fuel cells State of Art materials. Regression of the measured quantities by means of a quasi 2D physical model is performed to quantify the Knudsen effect, which is reported to account, respectively, for 30% and 50% of the mass transport resistance in micro porous layer and catalyst layer. On the other side, the model reveals that pressure gradient consequent to permeation in porous layers of high temperature polymer fuel cells has a negligible effect on oxygen concentration in relevant operating conditions
Modelling analysis of heterogeneity of ageing in high temperature polymer electrolyte fuel cells: insight into the evolution of electrochemical impedance spectra
No full textDegradation of catalyst electrochemical active surface is a major issue that hinders polymer electrolyte fuel cells commercialization. Several degradation mechanisms that have been reported in the literature are known to result in heterogeneity of ageing, whose effects on performance loss have not been previously investigated. In this work experimental and theoretical methodologies are combined to provide an insight between the effects of heterogeneity of ageing and the evolution of electrochemical impedance spectra. Experimental results collected during a 6000 hours degradation test indicate an increase of cathode charge transfer and mass transport resistances, consistently with cyclic voltammetry that reveals about 60% loss of cathode catalyst active surface. A physical based model is developed and validated to demonstrate that uniform cathode ageing does not to affect the evolution of impedance spectra. On the other side, it is shown that non-uniform ageing of cathode active area improves the agreement with experimental data. The evolution of the polarization curves and impedance spectra recorded during 6000 hours degradation test is critically discussed by including heterogeneity of ageing
Physical modelling of cathode impedance in low temperature fuel cells
No full textLow temperature Polymer Electrolyte Membrane (PEM) fuel cells are a promising energy source for stationary and automotive applications, mainly due to high efficiency and low emissions. However the widely use of this technology is still hindered by some technological issues, among which severe cathode flooding and degradation. The most common technique to monitor system internal losses during real operation is the Electrochemical Impedance Spectroscopy (EIS). Despite the potentiality of this in-situ measurement technique, the interpretation of impedance data is still object of discussion in the literature and physical modelling becomes crucial to analyze experiments.
In this work, the development of a physically based model of cathode impedance is described and validated with respect to Direct Methanol Fuel Cell (DMFC) technology. The presented
approach will be also applied to simulate cathode behavior in hydrogen fed PEM
Experimental analysis of mass transport in high temperature polymer electrolyte fuel cells
No full textMASS TRANSPORT ISSUES IN LOW PLATINUM LOADING CATALYST LAYERS FOR POLYMER FUEL CELLS
No full textState of art cathode catalyst layers for polymer electrolyte fuel cells require the use of Platinum based catalysts to promote the sluggish oxygen reduction reaction, with a Platinum loading of approximately 0.2 mg cm-2. The main goal of research in long term deals with the necessity to reduce the Platinum loading that still impacts the cost of the system. Even though this topic has been researched intensively, low Platinum catalyst layers present mass transport limitations that strongly affect performance. In the present work mass transport overpotential in low Platinum polymer fuel cells is investigated with the aid of a quasi 2D single cell continuum based model
Gas crossover leakage in high temperature polymer electrolyte fuel cells: In situ quantification and effect on performance
No full textHere we report an experimental in situ analysis of internal gas crossover leakage in single high- temperature polymer electrolyte membrane fuel cells. The study is based on the analysis of the exhausts from the anode and the cathode of the fuel cell during operation. An abnormal crossover rate across the membrane of the investigated fuel cell was detected, indicating the presence of an internal leakage. The internal flux shows linear dependence on the pressure difference between fuel cell compartments, attesting for permeation as the driving force. When the average cathode pressure is higher than the anode pressure, air crossover is measured. Conversely, hydrogen crossover is measured when the anode pres- sure is higher than the cathode. The effects on fuel cell performance are evaluated under air or hydrogen crossover conditions. Under both conditions, crossover leakage causes a significant increase in hydrogen consumption that reduces fuel recovery from anode exhaust
Degradation in phosphoric acid doped polymer fuel cells: A 6000 h parametric investigation
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