One-dimensional thermal model of cold-start in a polymer electrolyte fuel cell stack

A transient, one-dimensional thermal model for a generic polymer electrolyte fuel cell (PEFC) stack is developed to investigate the cold-start ability and the corresponding energy requirement over different operating and ambient conditions. The model is constructed by applying the conservation of energy on each stack component and connecting the component's relevant boundaries to form a continuous thermal model. The phase change of ice and re-circulation of coolant flow are included in the analytical framework and their contribution to the stack thermal mass and temperature distribution of the components is also explored. A parametric study was conducted to determine the governing parameters, relative impact of the thermal mass of each stack component and ice, and anticipated temperature distribution in the stack at start-up for various operating conditions. Results indicate that 20 cells were sufficient to accurately experimentally and computationally simulate the full size stack behavior. It was observed that an optimum range of operating current density exists for a chosen stack design, in which rapid start-up of the stack from sub-zero condition can be achieved. Thermal isolation of the stack at the end plates is recommended to reduce the start-up time. Additionally, an end plate thickness exceeding a threshold value has no added effect on the stack cold-start ability. Effect of various internal and external heating mechanisms on the stack start-up were also investigated, and flow of heated coolant above 0 °C was found to be the most effective way to achieve the rapid start-up.     

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.     

Macroscopic analysis of characteristic water transport phenomena in polymer electrolyte fuel cells

Comprehensive analytical and numerical analyses were performed, focusing on anode water loss, cathode flooding, and water equilibrium for polymer electrolyte fuel cells. General features of water transport as a function of membrane thickness and current density were presented to illustrate the net effect of back-diffusion of water from the cathode to anode over a polymer electrolyte fuel cell domain. First, two-dimensional numerical simulation were performed, showing that the difference in molar concentration of water at the channel outlet is widened as the operating current density increases with a thin membrane (Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111), which was verified by Dong et al. [Distributed performance of polymer electrolyte fuel cells under low-humidity conditions. J Electrochem Soc 2005; 152: A2114–22]. Then, analytical solutions were compared with computational results in predicting those characteristics of water transport phenomena. It was theoretically estimated that the high pressure operation of fuel cells expedites water condensing and results in shorter anode water loss and cathode flooding locations. In this study, it was also found that a thin membrane (Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111) facilitates water transport in the through-membrane direction and therefore water concentration at the anode and cathode channel outlets reaches an equilibrium state particularly at low operating current densities. Moreover, the difference in the anode water concentration between Nafion^®${\mathrm{Nafion}}^{\circledR }$ 111 and Nafion^®${\mathrm{Nafion}}^{\circledR }$ 115 membranes becomes intensified in the in-plane direction under the same water production condition, while the cathode water concentration profiles remains almost same.     

Super-cooled water behavior inside polymer electrolyte fuel cell cross-section below freezing temperature

This study investigated the phenomenon of water freezing below freezing point in polymer electrolyte fuel cells (PEFCs). To understand the details of water freezing phenomena inside a PEFC, a system capable of cross-sectional imaging inside the fuel cell with visible and infrared images was developed. Super-cooled water freezing phenomena were observed under different gas purge conditions. The present test confirmed that super-cooled water was generated on the gas diffusion layer (GDL) surface and that water freezing occurs at the interface between the GDL and MEA (membrane electrode assembly) at the moment cell performance deteriorates under conditions when remaining water was dry enough inside the fuel cell before cold starting. Moreover, using infrared radiation imaging, it was clarified that heat of solidification spreads at the GDL/MEA interface at the moment cell performance drops. Compared with a no-initial purge condition, liquid water generation was not confirmed to cause ice growth at the GDL/MEA interface after cell performance deterioration. Each condition indicated that ice formation at the GDL/MEA interface causes cell performance deterioration. Therefore, it is believed that ice formation between the GDL/MEA interface causes air gas stoppage and that this blockage leads to a drop in cell performance.     

Relative humidity control in polymer electrolyte membrane fuel cells without extra humidification

The performance of polymer electrolyte membrane fuel cells is highly influenced by the water content in the membrane. To prevent the membrane from drying, several researchers have proposed extra humidification on the input reactants. But in some applications, the extra size and weight of the humidifier should be avoided. In this research a control technique, which maintains the relative humidity on saturated conditions, is implemented by adjusting the air stoichiometry; the effects of drying of membrane and flooding of electrodes are considered, as well. For initial analysis, a mathematical model reveals the relationship among variables that can be difficult to monitor in a real machine. Also prediction can be tested optimizing time and resources. For instance, the effects of temperature and humidity can be analyzed separately. For experimental validation, tests in a fault tolerant fuel cell are conducted.     

Numerical modeling of liquid water motion in a polymer electrolyte fuel cell

A three dimensional transient model fully coupling the two phase flow, species transport, heat transport, and electrochemical processes is developed to investigate the liquid water formation and transport in a polymer electrolyte fuel cell (PEFC). This model is based on the multiphase mixture (M2) formulation with a complete treatment of two phase transport throughout the PEFC, including gas channels, enabling modeling the liquid water motion in the entire PEFC. This work particularly focuses on the liquid water accumulation and transport in gas channels. It is revealed that the liquid water accumulation in gas channels mainly relies on three mechanisms and in the anode and cathode may rely on different mechanisms. The transport of liquid water in the anode channel basically follows a condensation–evaporation mechanism, in sharp contrast to the hydrodynamic transport of liquid water in the cathode channel. Liquid water in the cathode channel can finally flow outside from the exit along with the exhaust gas. As the presence of liquid water in gas channels alters the flow regime involved, from the single phase homogeneous flow to two phase flow, the flow resistance is found to significantly increase.     

Innovative water management in micro air-breathing polymer electrolyte membrane fuel cells

The role of cathodic cover opening ratio on water management was investigated for micro air-breathing polymer electrolyte membrane fuel cells (PEMFCs). The results demonstrate the possibility to manage water content in micro-PEMFC using cover opening ratio variation. By measuring the internal resistance of a cell in various cover configurations (0.33 Ω cm² to 4.0 Ω cm²), the influence of cover opening ratio on water management was shown. Indeed, for a cell situated in a 10% relative humidity atmosphere and operated at 0.5 V, the addition of a 5% opening ratio cover allowed to reach similar current densities (270 mA cm⁻²) to those recorded for the same potential at 70% relative humidity without cover. Although the starting current density for a cell operated at 60 °C without gas humidification was extremely low (15 mA cm⁻²), the total closure of the cover allowed to maintain the water produced and accumulated by the cell at the cathode, and current density of 800 mA cm⁻² were reached after height minutes of operation. The influence of the opening ratio on back-diffused water was also evaluated and the maximum of back-diffused water was observed for a cell operated with a 5% cover opening ratio and represented 33% of the total water product at 150 mA cm⁻².A new method of anodic water evacuation, which does not increase the cell volume and which does not require any control tool was carried out and experimentally evaluated.     

Effects of self-humidification on the dynamic behavior of polymer electrolyte fuel cells

The dynamic behavior of polymer electrolyte fuel cells was investigated experimentally at sudden load change conditions. The present study mainly focused on the variation of membrane hydration due to self-humidification. Steady-state results for various temperatures and humidities were used as the basic data for the analysis of dynamic behavior. Electrochemical impedance spectroscopy (EIS) showed that the ohmic resistance was reduced with the increase of humidity and current while the total polarization resistance including the mass transfer effect showed various trends according to cell temperature. The dynamic behavior of the cells was measured with time. The current increment just after an abrupt voltage reduction jumped to a certain level and then increased gradually, showing a logarithmic-shape curve. The stabilization time to steady-state was determined by using the curve-fitted lines representing the variation of the current increment at each operating condition. The stabilization time showed various trends according to cell temperature, humidity, and voltage range.     

Porosity-graded micro-porous layers for polymer electrolyte membrane fuel cells

In this paper, the effect of porosity-graded micro-porous layer (GMPL) on the performance of polymer electrolyte membrane fuel cells (PEMFCs) was studied in detail. The GMPL was prepared by printing micro-porous layers (MPL) with different content of NH₄Cl pore-former and the porosity of the GMPL decreased from the inner layer of the MPLs at the membrane/MPL interface to the outer layer of the MPLs at the gas diffusion electrode/MPL interface. The morphology and porosity of the GMPLs were characterized and the performance of the cell with GMPLs was compared with those having conventional homogeneous MPLs. The result demonstrates that the fuel cells consisting of GMPL have better performance than those consisting of conventional homogeneous MPLs, especially at high current densities. Micro-porous layer with graded porosity is beneficial for the electrode process of fuel cell reaction probably by facilitating the liquid water transportation through large pores and gas diffusion via small pores in the GMPLs.     

Hybrid electrode effects on polymer electrolyte membrane fuel cells

Research on membrane electrode assemblies (MEA) is focused on reducing cost and increasing durability in polymer electrolyte membrane fuel cells (PEMFC). Development of the electrode structure and reduction of platinum (Pt) contents are studied to improve the efficiency of Pt catalysts. We studied the combined effects of improved electrode structure and reduced Pt loading. To enhance the performance of an MEA, a commercial Pt/C catalyst with micro graphite (MG) was used. The 40 wt% Pt/C catalyst content was reduced about 5, 15, 30 and 60 wt% at the cathode. MG was added as a reduced weight percent of Pt/C. Cell performance was significantly dependent on the content of MG. The MEA with 15 wt% of MG was seen to best performance compare with other MEA. These results showed that the catalyst with mixed MG improved both performance and cost savings with reduced Pt content of PEMFC.     

Dynamic water management of polymer electrolyte membrane fuel cells using intermittent RH control

A novel method of water management of polymer electrolyte membrane (PEM) fuel cells using intermittent humidification is presented in this study. The goal is to maintain the membrane close to full humidification, while eliminating channel flooding. The entire cycle is divided into four stages: saturation and de-saturation of the gas diffusion layer followed by de-hydration and hydration of membrane. By controlling the duration of dry and humid flows, it is shown that the cell voltage can be maintained within a narrow band. The technique is applied on experimental test cells using both plain and hydrophobic materials for the gas diffusion layer and an improvement in performance as compared to steady humidification is demonstrated. Duration of dry and humid flows is determined experimentally for several operating conditions.     

Improving the cold-start capability of polymer electrolyte fuel cells (PEFCs) by using a dual-function micro-porous layer (MPL): Numerical simulations

A novel micro-porous layer (MPL) is designed to enhance the cold-start capability of a polymer electrolyte fuel cell (PEFC). The concept of designing an MPL is to expand the ice storage capacity of the electrode into the MPL region. We impose proton conduction capability and the oxygen reduction reaction (ORR) kinetic activity on the MPL via controlling the platinum (Pt) loading, ionomer fraction and weight ratio of Pt to the carbon support (wt%_Pt–C) of the MPL. Therefore, the MPL is dual-functional, and can work as a typical MPL for normal PEFC operations and as a part of the cathode catalyst layer (CL) for cold-start operations. Three-dimensional (3-D) cold-start simulations are carried out by using a 3-D cold-start model developed in a previous study [1]. The detailed simulation results clearly suggest that the cold-start operational time can be extended significantly using a dual-function MPL, and the extended time is directly proportional to the pore volume of the MPL for ice storage. This study provides a new strategy to enhance the cold-start capability of a PEFC by properly designing and optimizing the MPL.     

Effect of operating conditions on carbon corrosion in polymer electrolyte membrane fuel cells

The influence of humidity, cell temperature and gas-phase O₂ on the electrochemical corrosion of carbon in polymer electrolyte membrane fuel cells is investigated by measuring CO₂ emission at a constant potential of 1.4 V for 30 min using on-line mass spectrometry. Carbon corrosion shows a strong positive correlation with humidity and cell temperature. The presence of water is indispensable for electrochemical carbon corrosion. By contrast, the presence of gas-phase O₂ has little effect on electrochemical carbon corrosion. With increased carbon corrosion, changes in fuel cell electrochemical characteristics become more prominent and thereby indicate that such corrosion significantly affects fuel cell durability.     

Modelling of heterogeneities inside polymer electrolyte fuel cells due to oxidants

In present study, the influences of oxidants (concentration and stoichiometry factor) on the heterogeneities inside the polymer electrolyte fuel cells (PEFC) are analysed and modelled. In situ current density mapping technique is adopted to evaluate the heterogeneities. Heterogeneity factor is being introduced in present study to investigate the effects of the oxidants on the local performances. Local and mean performances of the fuel cells are analysed and modelled as a function of the air stoichiometry factor. It is observed that the minimum values of stoichiometry factors required for maintaining heterogeneities inside the fuel cells at the minimum level increase with the mean current density. Based on the experimental results, a model is being developed for determining the optimum values of air stoichiometry factor to maintain the heterogeneity inside the fuel cell at minimum level. The local performances of the fuel cells are investigated as a function of oxygen concentration at the cathode inlet. It is observed that the heterogeneities inside the fuel cell do not always increase with the decrease in the oxygen concentration at the cathode inlets. However, the heterogeneity factor always increases with the decrease in oxygen concentration.     

Effects of metal foam properties on flow and water distribution in polymer electrolyte fuel cells (PEFCs)

Using a two-phase polymer electrolyte fuel cell (PEFC) model, we numerically investigated the influence of metal foam porous properties and wettability on key species and current distributions within a PEFC. Three-dimensional simulations were conducted under practical low humidity inlet hydrogen and air gases, and numerical comparisons were made for different metal foam design variables. These simulations were conducted to elucidate the detailed operating characteristics of PEFCs using metal foams as a flow distributor, and the simulation results showed that two-phase transport and the resulting flooding behavior in a PEFC are influenced by both the metal foam porous properties and the porous properties of an adjacent layer (e.g., the gas diffusion layer). This paper offers basic directions to design metal foams for optimal water management of PEFCs.     

SPEEK/functionalized silica composite membranes for polymer electrolyte fuel cells

Silica and sulfonic acid functionalized silica were synthesized by condensation of appropriate precursors through a sol–gel approach. SPEEK with three different ion exchange capacities (1.35, 1.75 and 2.1 mequiv. g⁻¹) were prepared by sulfonation of PEEK. Composite membranes with 5% and 10% additive loadings were prepared by solvent casting. Characterization by FTIR spectroscopy confirmed the presence of sulfonic acid groups in the functionalized silica additives. The agglomerate size of the additives was estimated by scanning electron microscopy to be between 2 and 5 μm. The room temperature liquid water uptake of the membranes was evaluated. Water uptake increased with SPEEK IEC. Composite membranes exhibited lower water uptakes when compared to pure SPEEK. Proton conductivities of up to 0.05 S cm⁻¹ at 80 °C and 75% relative humidity and 0.02 S cm⁻¹ at 80 °C and 50% relative humidity were recorded for SPEEK composite membranes prepared using sulfonic acid functionalized silica. Hydrogen crossover through the membrane was determined through linear sweep voltammetry on membrane electrode assemblies (MEAs). Hydrogen crossover current densities for all the MEAs were on the order of 1–2 mA cm⁻². MEAs tested showed reasonable performance at 80 °C and 75% and 50% relative humidities.     

Comparison between numerical simulation and visualization experiment on water behavior in single straight flow channel polymer electrolyte fuel cells

A relationship between a flooding and a cell voltage drop for polymer electrolyte fuel cell was investigated experimentally and numerically. A visualization cell, which has single straight gas flow channel (GFC) and observation window, was fabricated to visualize the flooding in GFC. We ran the cell with changing operation condition, and measured the time evolution of cell voltage and took the images of cathode GFC. Considering the operation condition, we executed a developed numerical simulation, which is based on multiphase mixture model with a formulation on water transport through the surface of polymer electrolyte membrane and the interface of gas diffusion layer/GFC. As a result in experiment, we found that the cell voltage decreased with time and this decrease was accelerated by larger current and smaller air flow rate. Our simulation succeeded to demonstrate this trend of cell voltage. In experiment, we also found that the water flushing in GFC caused an immediate voltage change, resulting in voltage recovery or electricity generation stop. Although our simulation could not replicate this immediate voltage change, the supersaturated area obtained by our simulation well corresponded to fogging area appeared on the window surface in the GFC.     

Numerical analysis on performances of polymer electrolyte membrane fuel cells with various cathode flow channel geometries

This paper investigated numerically the effect of cathode channel shapes on the local transport characteristics and cell performance by using a three-dimensional, two-phase, and non-isothermal polymer electrolyte membrane (PEM) fuel cell model. The cells with triangle, trapezoid, and semicircle channels were examined using that with rectangular channel as comparison basis. At high operating voltages, the cells with various channel shapes would have similar performance. However, at low operating voltages, the fuel cell performance would follow: triangle > semicircle > trapezoid > rectangular channel. Analyses of the local transport phenomena in the cell indicate that triangle, trapezoid, and semicircle channel designs increase remarkably flow velocity of reactant, enhancing liquid water removal and oxygen utilization. Thus, these designs increase the limiting current density and improve the cell performance relative to rectangular channel design.