Numerical analyses of non-isothermal self-start behaviors of PEM fuel cells from subfreezing startup temperatures

In this paper, a multiphase multidimensional PEM fuel cell model for cold-start simulations has been employed for numerical analyses of the non-isothermal self-start behaviors of a PEM fuel cell from subfreezing startup temperatures, focusing on the coupled phenomena of the ice formation and temperature increase inside the cell. The roles played by many key influential parameters, including the water vapor concentration in the cathode gas channel, the initial water content inside the membrane, the operating current density, and the startup cell temperature, are carefully examined. Numerical results indicate that decreasing the interfacial water vapor concentration at the gas diffusion layer and gas channel surface on the cathode side of the cell would delay ice precipitation and prolong the cell operation time. Decreasing the operation current density and the initial water content inside the membrane, and increasing the startup cell temperature are beneficial for the non-isothermal cold starts of the PEM fuel cell and could lead to successful self-starts.     

Development of small polymer electrolyte fuel cell stacks

The results on the research and development of small polymer electrolyte fuel cell stacks, including the assembly of single cell. 6-cell and 21-cell modules, are described. The important characteristics of the systems are: (i) membrane and electrode assemblies were made with Nafion^® 115 and 117 membranes and particularly low catalyst loading electrodes presenting a geometric area of 20 cm² and a catalyst loading of 0.4 mg Pt/cm²: (ii) bipolar plates were fabricated using a nonporous graphite material in which a series/parallel flow field was machined out: (iii) external distribution of gases to the cells was done using parallel manifolding; (iv) cooling systems were tested employing water/air cooling plates distributed every three cells throughout the stack; (v) the reactant gases were externally humidified using temperature controlled humidification bottles. Testing of the stacks was conducted in a specially designed test station employing nonpressurized H₂/O₂ reactants and measuring the individual and the overall cell voltage vs. current under several conditions for the overall system operation.     

Experimental investigation of coupling phenomena in polymer electrolyte fuel cell stacks

Propagation of performance changes to adjacent cells in polymer electrolyte fuel cell stacks is studied by means of voltage monitoring and local current density measurements in peripheral cells of the stack. A technical fuel cell stack has been modified by implementing two independent reactant and coolant supplies in order to deliberately change the performance of one cell (anomalous cell) and study the coupling phenomena to adjacent cells (coupling cells), while keeping the working conditions of the later cell-group unaltered.Two anomalies are studied: (i) air starvation and (ii) thermal anomaly, in a single anomalous cell in the stack and their coupling to adjacent cells. The results have shown that anomalies inducing considerable changes in the local current density of the anomalous cell (such as air starvation) propagate to adjacent cells affecting their performance. The propagation of local current density changes takes place via the common bipolar plate due to its finite thickness and in-plane conductivity. Consequently, anomalies which do not strongly influence the local current density distribution (such as a thermal anomaly under the studied working conditions) do not propagate to adjacent cells.     

Longevity test results for reformate polymer electrolyte membrane fuel cell stacks

Polymer electrolyte membrane fuel cell (PEMFC) stacks offer a great potential for combined heat and power (CHP) applications because of their good performance and technical maturity of the key components. Nonetheless, some developmental issues have remained open. Among those are the long-term stability with respect to performance degradation and sudden death phenomena like membrane rupture.In a development program for domestic CHP systems, PEMFC stacks intended for long-term operation on reformate were developed. Development targets were high performance, high media utilization, good longevity and low degradation rates. In this paper, results on long-term performance tests of these stacks are reported. Operating times of more than 15,000 h with degradation rates of approx. 10 μV h⁻¹ have been achieved.     

Computational study of forced air-convection in open-cathode polymer electrolyte fuel cell stacks

A mathematical model for a polymer electrolyte fuel cell (PEFC) stack with an open-cathode manifold, where a fan provides the oxidant as well as cooling, is derived and studied. In short, the model considers two-phase flow and conservation of mass, momentum, species and energy in the ambient and PEFC stack, as well as conservation of charge and a phenomenological membrane and agglomerate model for the PEFC stack. The fan is resolved as an interfacial condition with a polynomial expression for the static pressure increase over the fan as a function of the fan velocity. The results suggest that there is strong correlation between fan power rating, the height of cathode flow-field and stack performance. Further, the placement of the fan - either in blowing or suction mode - does not give rise to a discernable difference in stack performance for the flow-field considered (metal mesh). Finally, it is noted that the model can be extended to incorporate other types of flow-fields and, most importantly, be employed for design and optimization of forced air-convection open-cathode PEFC stacks and adjacent fans.     

Water transport during startup and shutdown of polymer electrolyte fuel cell stacks

A dynamic three-phase transport model is developed to analyze water uptake and transport in the membrane and catalyst layers of polymer electrolyte fuel cells during startup from subfreezing temperatures and subsequent shutdown. The initial membrane water content (λ, the number of water molecules per sulfonic acid site) is found to be an important parameter that determines whether a successful unassisted self-start is possible. For a given initial subfreezing temperature at startup, there is a critical λ (λ_h), above which self-start is not possible because the product water completely engulfs the catalyst layers with ice before the stack can warm-up to 0 °C. There is a second value of λ (λ_l), below which the stack can be self-started without forming ice. Between λ_l and λ_h, the stack can be self-started, but with intermediate formation of ice that melts as the stack warms up to 0 °C. Both λ_l and λ_h are functions of the initial stack temperature, cell voltage at startup, membrane thickness, catalyst loading, and stack heat capacity. If the stack is purged during the previous shutdown by flowing air in the cathode passages, then depending on the initial amount of water in the membrane and gas diffusion layers and the initial stack temperature, it may not be possible to dry the membrane to the critical λ for a subsequent successful startup. There is an optimum λ for robust and rapid startup and shutdown. Startup and shutdown time and energy may be unacceptable if the λ is much less than the optimum. Conversely, a robust startup from subfreezing temperatures cannot be assured if the λ is much higher than this optimum.     

Characterization of flooding and two-phase flow in polymer electrolyte membrane fuel cell stacks

A partially flooded gas diffusion layer (GDL) model is proposed and solved simultaneously with a stack flow network model to estimate the operating conditions under which water flooding could be initiated in a polymer electrolyte membrane (PEM) fuel cell stack. The models were applied to the cathode side of a stack, which is more sensitive to the inception of GDL flooding and/or flow channel two-phase flow. The model can predict the stack performance in terms of pressure, species concentrations, GDL flooding and quality distributions in the flow fields as well as the geometrical specifications of the PEM fuel cell stack. The simulation results have revealed that under certain operating conditions, the GDL is fully flooded and the quality is lower than one for parts of the stack flow fields. Effects of current density, operating pressure, and level of inlet humidity on flooding are investigated.     

Numerical evaluation of various thermal management strategies for polymer electrolyte fuel cell stacks

Thermal management is one of the key factors required to ensure good performance polymer electrolyte fuel cell (PEFC) stacks. The choice of the thermal management strategy depends on the specific application, size, weight, design, complexity, and cost. In this work, we investigate various alternative thermal management strategies for PEFC stacks, e.g., forced convection in specially design cooling plate/channel with either (i) liquid or (ii) air as the coolant; (iii) edge-air cooling with fins and; combine oxidant and coolant flow (open-cathode) with (iv) forced and (v) natural convection air cooling. A three-dimensional two-phase model, comprising of the equations of conservation of mass, momentum, species, energy and charge, is employed to quantify the performance of various cooling strategies. The results demonstrate that thermal management is essential to ensure good stack performance. Liquid cooling, as expected, performs the best compared to air cooling, whereas natural convection cooling is just marginally able to maintain a stack with large number of cells from steep drop in performance. Finally, results presented in this paper can provide useful design guidelines for selection of a suitable thermal management strategy for a PEFC stack and its near-to- or optimum cooling condition.     

Modelling of polymer electrolyte membrane fuel cell stacks based on a hydraulic network approach

Polymer electrolyte membrane (PEM) fuel cells convert the chemical energy of hydrogen and oxygen directly into electrical energy. Waste heat and water are the reaction by‐products, making PEM fuel cells a promising zero‐emission power source for transportation and stationary co‐generation applications. In this study, a mathematical model of a PEM fuel cell stack is formulated. The distributions of the pressure and mass flow rate for the fuel and oxidant streams in the stack are determined with a hydraulic network analysis. Using these distributions as operating conditions, the performance of each cell in the stack is determined with a mathematical, single cell model that has been developed previously. The stack model has been applied to PEM fuel cell stacks with two common stack configurations: the U and Z stack design. The former is designed such that the reactant streams enter and exit the stack on the same end, while the latter has reactant streams entering and exiting on opposite ends. The stack analysed consists of 50 individual active cells with fully humidified H₂ or reformate as fuel and humidified O₂ or air as the oxidant. It is found that the average voltage of the cells in the stack is lower than the voltage of the cell operating individually, and this difference in the cell performance is significantly larger for reformate/air reactants when compared to the H₂/O₂ reactants. It is observed that the performance degradation for cells operating within a stack results from the unequal distribution of reactant mass flow among the cells in the stack. It is shown that strategies for performance improvement rely on obtaining a uniform reactant distribution within the stack, and include increasing stack manifold size, decreasing the number of gas flow channels per bipolar plate, and judicially varying the resistance to mass flow in the gas flow channels from cell to cell. Copyright © 2004 John Wiley & Sons, Ltd.     

Spatially resolved degradation effects in membrane-electrode-assemblies of vehicle aged polymer electrolyte membrane fuel cell stacks

State of the art MEAs were aged in a fuel cell vehicle and degradation effects analyzed using electron microscopy and electrochemical methods. All cells of the stack showed a performance decay along with a loss in the electrochemical surface area. This could be correlated to particle growth and carbon corrosion observed by electron microscopy. Spatially resolved investigations showed a significant deterioration of the cathode, which is particularly pronounced at the hydrogen inlet. Differences in the cell performance of the aged cells could not be attributed to a variation in the catalyst degradation, but are linked to an altered ohmic resistance in the cells. The ohmic resistance of the cells is likely to be affected by the formation of precipitates in the membrane and seems to be correlated with their size.     

Modelling and evaluation of heating strategies for high temperature polymer electrolyte membrane fuel cell stacks

Experiments were conducted on two different cathode air cooled high temperature PEM (HTPEM) fuel cell stacks; a 30 cell 400 W prototype stack using two bipolar plates per cell, and a 65 cell 1 kW commercial stack using one bipolar plate per cell. The work seeks to examine the use of different heating strategies and find a strategy suited for fast start-up of the HTPEM fuel cell stacks. Fast start-up of these high temperature systems enables use in a wide range of applications, such as automotive and auxiliary power units, where immediate system response is needed. The development of a dynamic model to simulate the temperature development of a fuel cell stack during heating can be used for assistance in system and control design. The heating strategies analyzed and tested reduced the start-up time of one of the fuel cell stacks from 1 h to about 6 min.     

Nitride films as protective layers for metallic bipolar plates of polymer electrolyte membrane fuel cell stacks

The idea of using nitride films coated by sputtering Nb and Cr, using N₂ as a reaction gas, as protective layers for metallic bipolar plates (BP) of polymer electrolyte membrane fuel cell (PEMFC) stacks, was explored by experiments. Specimens were fabricated from austenite 304 stainless steels, which are frequently used as bipolar plate materials for fuel cells due to their good corrosion resistance and high strength at elevated temperature. The results of XRD analysis and Gaussian function analysis of the coated films suggest that NbN or NbN/NbCrN films were induced depending on the process parameters. The NbN/NbCrN multiphase films were induced at high Cr target powers and low gas ratios among the process parameters selected in this investigation. The result of ICR measurement of the films suggests that the effect of the Cr target power, i.e. the effect of the Cr amount in the film, on the ICR of the films is not significant, while the effect of the gas ratio on the ICR of the films is noticeable. For the films deposited at different gas ratios, the ICR of the film generally decreases as the gas ratio increases. In general, all the 304 SS specimens coated by the NbN single phase or NbN/NbCrN multiphase films at the given process parameters showed significantly improved corrosion resistance in comparison with the bare 304 SS. Among NbN single phase and NbN/NbCrN multiphase films, the performance of NbN/NbCrN multiphase films was more stable. Depending on the process parameters, the polarization curves of the specimens coated with NbN films showed rapid increase of the current density due to the pitting. Therefore, as corrosion resistance coating for metallic BP of PEMFC stacks, NbN/NbCrN multiphase film may be preferred to NbN single phase films.     

Innovative cathode flow-field design for passive air-cooled polymer electrolyte membrane (PEM) fuel cell stacks

A novel cathode flow-field design suitable for a passive air-cooled polymer electrolyte membrane (PEM) fuel cell stack is proposed to enhance the water-retaining capability under excess dry air supply conditions. The innovative cathode flow-field is designed to supply more air to the cooling channels and further enables deceleration of the reactant air in the gas channels and acceleration of the coolant air in the cooling channels simultaneously along the air flow path. Therefore, the design facilitates the waste heat removal through the cooling channels while the water removal by the reactant air is minimized. The conceptual cathode flow-field design is validated using a three-dimensional PEM fuel cell model. The detailed simulation results clearly demonstrate that the new cathode flow-field design exhibits superior water-retaining capability compared with a conventional cathode flow-field design (parallel flow channel configuration) under typical air-cooled fuel cell operating conditions. This study provides a new strategy to design cathode flow-fields to alleviate notorious membrane dehydration and unstable performance issues in a passive air-cooled PEM fuel cell stack.     

Development of polymer electrolyte membrane fuel cell stack

The proton exchange membrane fuel cell (PEMFC) is one of the strongest contenders as a power source for space, electric vehicle and domestic applications. Since 1988 intensive research is being carried out at our centre to develop PEMFCs. The main RandD activities are: (i) to develop a method for the electrode preparation (ii) to enhance platinum utilisation using low platinum loading and (iii) to design multicell stacks. The results of RandD development of the above activities are discussed in this paper.     

Humidification studies on polymer electrolyte membrane fuel cell

Two methods of humidifying the anode gas, namely, external and membrane humidification, for a polymer electrolyte membrane fuel (PEMFC) cell are explained. It is found that the water of solvation of protons decreases with increase in the current density and the electrode area. This is due to insufficient external humidification. In a membrane-based humidification, an optimum set of parameters, such as gas flow rate, area and type of the membrane, must be chosen to achieve effective humidification. The present study examines the dependence of water pick-up by hydrogen on the temperature, area and thickness of the membrane in membrane humidification. Since the performance of the fuel cell is dependent more on hydrogen humidification than on oxygen humidification, the scope of the work is restricted to the humidification of hydrogen using Nafion^® membrane. An examination is made on the dependence of water pick-up by hydrogen in membrane humidification on the temperature, area and thickness of the membrane. The dependence of fuel cell performance on membrane humidification and external humidification in the anode gas is also considered.     

Water transport characteristics of polymer electrolyte membrane fuel cell

This paper describes the performance of a polymer electrolyte membrane fuel cell (PEMFC) system without humidification of the reactants which consumes a lot of parasitic power, increases the weight of the PEMFC system and thus adds complexity. Such PEMFC systems are preferable for portable applications. The results indicate that dry gas operation depends on various factors like reactant flow field design, area of the electrode and equilibration time for the product water. The performance of the fuel cell can be improved by giving some equilibration time for the product water, produced by the electrochemical reactions, to get transported across the membrane to the anode side, thus increasing the conductivity of the membrane. The water transported through the membrane across the cell was investigated by measuring the amount of product water at the anode side which allows humidification for the anode gas and less condensed water in the fluid flow channels of the cathode.     

Thermodynamic properties of direct methanol polymer electrolyte fuel cell

A new semi-empirical model is established to describe the cell voltage of a direct methanol fuel cell (DMFC) as a function of current density. The model equation is validated experimental data over a wide range of a methanol concentration and temperatures. A number of existing models are semi-empirical. They, however, have a serious mathematical defect. When the current density, j, becomes zero, the equation should reduce to the open circuit voltage, E₀. These models, however, do not meet the mathematical boundary condition. The proposed model focuses on very unfavorable conditions for the cell operation, i.e. low methanol solution concentrations and relatively low cell temperatures. A newly developed semi-empirical equation with reasonable boundary conditions includes the methanol crossover effect that plays a major role in determining the cell voltage of DMFC. Also, it contains methanol activity based on thermodynamic functions to represent methanol crossover effect.