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High oxygen transport resistance in the cathode is a main cause of the low performance of high temperature proton exchange membrane fuel cell (HT-PEMFC), and therefore to determine the contribution of different mass transport processes to the oxygen transport resistance is beneficial to the subsequent research work. In this paper, the molecular diffusion resistance (R M) in the cathode was successfully calculated by measuring the limiting currents with using different diluent gases. And the oxygen transport resistance in phosphoric acid (R P A ) was analyzed by changing the humidity and temperature. It was found that the total oxygen transport resistance (R t o t a l ) increased obviously with the increase of the humidify, and the variation of R t o t a l under different conditions was mainly related to the change of R P A . Combined with the phenomenon that the experimental value of R M was less than the calculated value at high humidity, but higher than that at high operation temperature, we speculate that the cause of that R P A increased obviously at high humidity is the occupancy or blockage of gas phase transport channel in catalyst layer (CL) by phosphoric acid due to the water absorption of phosphoric acid in CL and the swelling of membrane. This consequence indicates that it is the change of phosphoric acid distribution that dominate C i s the R t o t a l at high humidity. Image 1 • The separation of molecular diffusion resistance from the total mass transport resistance of HT-PEMFC cathode was realized. • The relationship between the oxygen transport resistance in phosphoric acid and operating conditions was obtained. • The cause of the variation of oxygen transport resistance at different operating conditions was preliminarily analyzing. [ABSTRACT FROM AUTHOR]

The stoichiometric ratio and flow channel geometry play a vital role in the performance of high temperature proton exchange membrane (HT-PEM) fuel cells. Because of the high cost of experiments or simulations, most analyses and optimization of the stoichiometric ratio and flow channel geometry are limited to several points in the entire design domain. In this study, an analysis and optimization method for HT-PEM fuel cells based on the surrogate model was proposed. Surrogate models were constructed using some of the available budgets of samples to analyze and optimize the entire design domain. With this method, it was indicated that the effect of the cathode stoichiometric ratio is more significant to the cell performance than the anode stoichiometric ratio and there are significant nonlinear interactions among the flow channel geometry parameters. At the fixed operating voltage, the flow channel geometry with the maximum current density and that with the maximum real power were obtained. Compared with the base design, the designs obtained by the surrogate model improve the current density and real power by 10.54% and 3.93%, respectively. Thus, this analysis and optimization method is demonstrated to be helpful and deserves attention in future research. Image 1 • It is proved that the effect of cathode stoichiometric ratio was more significant. • The design obtained by the surrogate model improve current density by 10.54%. • The design obtained by the surrogate model improve real power by 3.93%. [ABSTRACT FROM AUTHOR]

Internal reforming methanol fuel cell (IRMFC) has potential applications in portable or stationary power supply system, but currently performance of the IRMFC is limited by the low hydrogen production of its reformer. In order to produce more hydrogen with less volume, in this paper a single channel serpentine packed bed reformer was designed, and its bed size was optimized by experiment and numerical simulation to enhance heat transfer and increase catalyst utilization. It was found that with the bed diameter from 5.8 mm down to 3.8 mm, the reformer temperature distribution was more uniform but the bed pressure drop increased a lot. Considering performance and pressure drop, the reformer of 5 mm was optimal, per milliliters of which could supply 9.8 mL/min hydrogen at 453 K, almost twice as much as that by A. Mendes et al with one‐third of their catalyst loading. The reformer was quite stable, and less than 10% decline in methanol conversion was observed during the 100 hours period at 473 K. When incorporated into an IRMFC single cell, power density of the single cell reached 0.45‐0.55 W/cm2 at 453‐473 K under CH3OH solution and air feed, the highest in existing reports. The main drawback has to do with low stability of the IRMFC single cell at high current density. [ABSTRACT FROM AUTHOR]

Measurement of flow distribution in the fuel cell stack is a challenging task. This study proposes a new method to measure air flow distribution in high-temperature proton exchange membrane fuel cell stacks. The characteristic frequency related to channel impedance is extracted using the distribution of relaxation times analysis. A simple correlation between the air stoichiometry and the characteristic frequency is developed. The influence of various factors on the correlation, including cell operation conditions, fluid physical properties and channel dimensions, are investigated. The experimental results show a good agreement with the correlation. Error estimation indicates that the temperature distribution of the stack, gas humidity and pressure distribution in the inlet manifold have no significant effect on the proposed method under normal conditions. The accuracy and practicability are verified using a three-cell stack with separate mass flow controller (MFC) systems. The results show less than 3.11% of relative error between the reference air stoichiometry given by the MFC and those measured from our proposed method for air stoichiometry below 3.0 at 0.1 A cm−1. The present method provides powerful support for designing high consistency stacks and the real-time monitoring cells state-of-health in stacks. [Display omitted] • Distribution of relaxation times analysis of the channel impedance in fuel cells. • Correlating the characteristic frequency of mass transport and air stoichiometry. • Measuring the air flow distribution in the fuel cell stack through the correlation. • Understanding the impact of the stack parameters' consistency on the measurement. [ABSTRACT FROM AUTHOR]

Abstract: In this paper, the reversible and the irreversible loss in performance of direct methanol fuel cell (DMFC) single cells induced by freeze/thaw cycling are determined and discussed. The reversible loss in performance is attributed to the decreased activity of electrocatalysts due to the strong adsorption of the intermediates from methanol electro-oxidation at subzero temperatures, while the irreversible loss in performance is due to the damage of membrane electrode assembly (MEA) structure. It is found that the reversible degradation of performance is dominant when the cell operated at low current densities with O2; the irreversible degradation is more obvious when the cell discharged at high current densities or operated with air. A method, applying a reverse current on the single cell, is put forward to recover the reversible loss in performance. By this method, the voltage of the single cell operated with O2 at 100 mA cm−2 drops only about 16 mV after experiencing 100 freeze/thaw cycles between−10 and 60 °C. [Copyright &y& Elsevier]

Suitable flow field designs significantly affect the performance of high temperature proton exchange membrane (HT-PEM) fuel cells. Previous designs both in terms of shape and size optimisation are still inadequate in the category of experiential design method, depend on the experience of designers and lack of explainable theories, leading to uncertainties in the design process. To address this issue, a novel flow field design method for innovative flow fields using combined topology and surrogate models is proposed, in which a topology model is built to optimise to obtain the basic shape of flow field, then a numerical model is established to train a surrogate model for the optimum design. Finally, a radial basis function (RBF) model is built to determine the optimum channel parameters. With this method, a 25 cm2 leaf-like flow field with a diagonal inlet outlet is designed and investigated which outperforms the initial serpentine design with a 7.95% increase in current density, a 8.47% increase in actual power, and a 97.8% reduction in pressure drop at 0.50 V. This novel flow field design method combines the advantages of the topology and surrogate models in shape and size optimisation, respectively. It significantly reduces the design threshold and can be widely used in innovative flow field design in energy and other engineering fields. • A nature-inspired leaf-like flow field is designed for HT-PEM fuel cells. • Topology and surrogate models are combined for shape and geometry optimisation. • Novel leaf-like flow field achieves improved performance over traditional serpentine design. • Current density and real power are improved by 7.95% and 8.47%, respectively, at 0.5 V. • Pressure drop is reduced by 97.8% in comparison with serpentine flow field. [ABSTRACT FROM AUTHOR]