1,721,173 research outputs found
A 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
DMFC performance and methanol cross-over: Experimental analysis and model validation
No full textA combined experimental and modelling approach is proposed to analyze methanol cross-over and its effect on DMFC performance. The experimental analysis is performed in order to allow an accurate investi- gation of methanol cross-over influence on DMFC performance, hence measurements were characterized in terms of uncertainty and reproducibility. The findings suggest that methanol cross-over is mainly deter- mined by diffusion transport and affects cell performance partly via methanol electro-oxidation at the cathode. The modelling analysis is carried out to further investigate methanol cross-over phenomenon. A simple model evaluates the effectiveness of two proposed interpretations regarding methanol cross- over and its effects. The model is validated using the experimental data gathered. Both the experimental analysis and the proposed and validated model allow a substantial step forward in the understanding of the main phenomena associated with methanol cross-over. The findings confirm the possibility to reduce methanol cross-over by optimizing anode feeding
DMFC anode polarization: Experimental analysis and model validation
No full textAnode two-phase flow has an important influence on DMFC performance and methanol crossover. In order to elucidate two-phase flow influence on anode performance, in this work, anode polarization is investigated combining experimental and modelling approach. A systematic experimental analysis of operating conditions influence on anode polarization is presented. Hysteresis due to operating condition is observed; experimental results suggest that it arises from methanol accumulation and has to be considered in evaluating DMFC performances and measurements reproducibility. A model of DMFC anode polarization is presented and utilised as tool to investigate anode two-phase flow. The proposed analysis permits one to produce a confident interpretation of the main involved phenomena. In particular, it confirms that methanol electro-oxidation kinetics is weakly dependent on methanol concentration and that methanol transport in gas phase produces an important contribution in anode feeding. Moreover, it emphasises the possibility to optimise anode flow rate in order to improve DMFC performance and reduce methanol crossover
A simple analytical approach to simulate the electrochemical impedance response of flooded agglomerates in polymer fuel cells
Full text link
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
- …
