Influence of catalyst structure on PEM fuel cell performance – A numerical investigation

The effect of the catalyst microstructure on a 5 cm2 PEM fuel cell performance is numerically investigated. The catalyst layer composition and properties (i.e. ionomer volume fraction, platinum loading, particle radius, electrochemical active area and carbon support type), and the mass transport resistance due to the ionomer and liquid water surrounding the catalyst particles, are incorporated into the model. The effects of the above parameters are discussed in terms of the polarization curves and the local distributions of the key parameters. An optimum range of the ionomer volume fraction was found and a gain of 39% in the performance was achieved. As regards the platinum loading and catalyst particle radius, the results showed that a higher loading and a smaller radius leads to an increase in the PEMFC performance. Further, the influence of the electrochemical active area produces an overall increase of 22% in current density and this was due to the use of a new material developed as support for Pt particles, an iodine doped graphene, which has better electrical contacts and additional pathways for water removal. Using this parameter, the numerical model has been validated and good agreement with experimental data was achieved, thus giving confidence in the model as a design tool for future improvements of the catalyst structure

PEM fuel cell performance improvement through numerical optimization of the parameters of the porous layers

A numerical model for a PEM fuel cell has been developed and used to investigate the effect of some of the key parameters of the porous layers of the fuel cell (GDL and MPL) on its performance. The model is comprehensive as it is three-dimensional, multiphase and non-isothermal and it has been well-validated with the experimental data of a 5 cm2 active area-fuel cell with/without MPLs. As a result of the reduced mass transport resistance of the gaseous and liquid flow, a better performance was achieved when he GDL thickness was decreased. For the same reason, the fuel cell was shown to be significantly improved with increasing the GDL porosity by a factor of 2 and the consumption of oxygen doubled when increasing the porosity from 0.40 to 0.78. Compared to the conventional constant-porosity GDL, the graded-porosity (gradually decreasing from the flow channel to the catalyst layer) GDL was found to enhance the fuel cell performance and this is due to the better liquid water rejection. The incorporation of a realistic value for the contact resistance between the GDL and the bipolar plate slightly decreases the performance of the fuel cell. Also the results show that the addition of the MPL to the GDL is crucially important as it assists in the humidifying of the electrolyte membrane, thus improving the overall performance of the fuel cell. Finally, realistically increasing the MPL contact angle has led to a positive influence on the fuel cell performance

The effects of cathode flow channel size and operating conditions on PEM fuel performance: A CFD modelling study and experimental demonstration

A comprehensive 3D, multiphase, and nonisothermal model for a proton exchange membrane fuel cell has been developed in this study. The model has been used to investigate the effects of the size of the parallel-type cathode flow channel on the fuel cell performance. The flow-field plate, with the numerically predicted best performing cathode flow channel, has been built and experimentally tested using an in-house fuel cell test station. The effects of the operating conditions of relative humidity, pressure, and temperature have also been studied. The results have shown that the fuel cell performs better as the size of the cathode flow channel decreases, and this is due to the increased velocity that assists in removing liquid water that may hinder the transport of oxygen to the cathode catalyst layer. Further, the modelled fuel cell was found to perform better with increasing pressure, increasing temperature, and decreasing relative humidity; the respective results have been presented and discussed. Finally, the agreement between the modelling and the experimentally data of the best performing cathode flow channel was found to be very good

The Influence of Extrinsic Coloration Factors on Composites

Introduction: As we have observed the multiple color changes of composite restorative fillings, we decided to study the extrinsic factors that lead to their coloration. We have studied this visually and by computer, after a previous immersion of the composites in different colored and coloring substances, including cigarette smoke. Purpose: To determine the substances that produce the color changes of composites (extrinsic coloration), in vitro study, also, the composites that remains aesthetic for a long period of time. Method and material: In celluloid tooth shapes, we made 32 teeth, using four different composites shade A2, two nanocomposites and two microhybrid composites. We placed in each celluloid shape two layers of material, composites of the same group, resulting 16 teeth of nanocomposite and 16 teeth of microhybrid composite. After immersing them for 24 hours in purified water at 37°C, the mesial part of every tooth was polished. The teeth were immersed in 15 different substances and purified water was used as standard. After another 24 hours, we made a professional brushing and we evaluated their color again. Pictures were taken after every stage and they had been analyzed by a software. Results: Some composites changed their color from A2 to A3 and A4, others, even to shades of B, C and D. The most intense coloration was produced by coffee and red wine. Conclusions: The coloring drinks may produce significant alteration of the aesthetic of composites, which can be improved by professional brushing. Coloration depends not only on the coloring substance, but also on its pH level, the thickness of the composite, the texture of the surface and the immersion time

SSITKA Investigation of CO and H2 Competitive Adsorption at PEM Fuel Cell Anode Catalysts

Steady-state isotopic transient kinetic analysis (SSITKA) experiments have been performed using the isotopic exchange between 13CO and 12CO to investigate the competitive adsorption of hydrogen and CO on commercial Pt and PtRu catalysts. PtRu alloys are known to be more tolerant fuel cell anode catalysts than platinum, in the instance where the hydrogen fuel contains ppm levels of CO. It has been recently demonstrated that there is a dynamic equilibrium between CO adsorbed on platinum or platinum/ruthenium nano-particles and CO in the gas phase. In this paper, the effect of the competitive adsorption between hydrogen and CO on this equilibrium has been demonstrated. For 1400ppm CO in hydrogen little difference was observed in the measured exchange rates for Pt and PtRu at room temperature, 9.91×10-4 for Pt compared to 1.15×10-3 for PtRu, however there is a significant effect observed at 100ppm CO in hydrogen, where the rates on PtRu are considerably smaller than on Pt (3.61×10-4 desorption rate constant for PtRu and 5.49×10-4 for Pt). The presented methodology using the SSITKA technique has demonstrated a novel way to measure these rate constants, and the implications of these measurements on the mechanistic understanding of the anode reaction are presented.JRC.D.3-Knowledge Transfer and Standards for Securit

Effects of geometrical dimensions of flow channels of a large-active-area PEM fuel cell: A CFD study

Various flow field designs have been numerically investigated to evaluate the effect of pattern and the cross-sectional dimensions of the channel on the performance of a large active area PEM fuel cell. Three types of multiple-serpentine channels (7-channels, 11-channels and 14-channels) have been chosen for the 200 cm2 fuel cell investigated and numerically analysed by varying the width and the land of the channel. The CFD simulations showed that as the channel width decreases, as in the 14-channels serpentine case, the performance improves, especially at high current densities where the concentration losses are dominant. The optimum configuration, i.e. the 14-channels serpentine, has been manufactured and tested experimentally and a very good agreement between the experimental and modelling data was achieved. 4 channel depths have been considered (0.25, 0.4, 0.6 and 0.8 mm) in the CFD study to determine the effects on the pressure drop and water content. Up to 7% increase in the maximum reported current density has been achieved for the smallest depth and this due to the better removal of excess liquid water and better humidification of the membrane. Also, the influence of the air flow rate has been evaluated; the current density at 0.6 V increased by around 25% when air flow rate was increased 4 times; this is attributed to better removal of excess liquid water