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.     

Performance and liquid water distribution in PEFCs with different anisotropic fiber directions of the GDL

To maintain the efficiency of proton exchange membrane fuel cells (PEFCs) without flooding, it is necessary to control the liquid water transport in the gas diffusion layer (GDL). This experimental study investigates the effects of the GDL fiber direction on the cell performance using an anisotropic GDL. The results of the experiments show that the efficiency of the cell is better when the fiber direction is perpendicular to the channel direction, and that the cells with perpendicular fibers are more tolerant to flooding than cells with fibers parallel to the channel direction. To determine the mechanism of the fiber direction effects, the liquid water behavior in the channels was observed through a glass window on the cathode side. The observations substantiate that the liquid water produced under the ribs is removed more smoothly with the perpendicular fiber direction. Additionally, the water inside the GDL was frozen to observe its distribution using a specially made cell broken into two pieces. The photographic results show that the amount of water under the ribs is larger than that under the channels using the parallel fiber direction GDL while the water distributions in these two places are almost equal level with the perpendicular fiber direction GDL. This freezing method confirmed the better liquid water removal ability and better reactant gas transportation in the GDL with the fiber direction perpendicular to the channel direction.     

Gravity effect on water discharged in PEM fuel cell cathode

Experimental purpose is to test gravity influence on water discharged in PEM fuel cell cathode. Through changing the way of placement of the cathode and anode, it takes adjusting the electronic load to test the output of voltage and current. Corresponding to the position of the cathode-upward and the anode-upward, different humidification condition, draws the polarization curve using the voltage and current density. According to the placing position of the cathode and anode, cell temperature, humidification temperature of the cathode and anode gas, 4 groups of experimental results are obtained. The experimental conclusion is: when a PEM fuel cell is placed anode-upward, gravity is advantageous to discharge the liquid water in PEM fuel cell cathode. On the contrary, gravity is disadvantageous to discharge the liquid water.     

Analysis of the water and thermal management in proton exchange membrane fuel cell systems

Water and thermal management is essential to the performance of proton exchange membrane (PEM) fuel cell system. The key components in water and thermal management system, namely the fuel cell stack, radiator, condenser and membrane humidifier are all modeled analytically in this paper. Combined with a steady-state, one-dimensional, isothermal fuel cell model, a simple channel-groove pressure drop model is included in the stack analysis. Two compact heat exchangers, radiator and condenser are sized and rated to maintain the heat and material balance. The influence of non-condensable gas is also considered in the calculation of the condenser. Based on the proposed methodology, the effects of two important operating parameters, namely the air stoichiometric ratio and the cathode outlet pressure, and three kinds of anode humidification, namely recycling humidification, membrane humidification and recycling combining membrane humidification are analyzed. The methodology in this article is helpful to the design of water and thermal management system in fuel cell systems.     

25kW车用燃料电池发动机系统的设计

文章介绍了质子交换膜燃料电池发动机系统的设计，提出了将其集成为车用发动机系统的设计方案。     

Quantification of water in hydrophobic and hydrophilic flow channels subjected to gas purging via neutron imaging

Water removal from an idle fuel cell is an important issue for start-up/shutdown down under cold temperature conditions. In our study, we performed an in-situ neutron imaging for a PEM fuel cell with bipolar plates, treated with a super-hydrophobic and super-hydrophilic coating on the flow channels. The coatings were applied to the channels but not on the landings in contact with the GDL. The cells were run at a constant voltage prior to shutdown, then sets of neutron images were taken with purge velocities varied from 1 m s⁻¹ to 4 m s⁻¹, in intervals of 1 m s⁻¹. It was found that changing the wettability of the flow channels can improve the dynamics of water removal during purging. The super-hydrophilic and super-hydrophobic coating had better performance in removing water on the landings and in the channels, respectively. Based on our test cells, we used the amount of water remaining as a metric and found no significant improvement by purging the cell at velocities greater than 3 m s⁻¹.     

Quantitative MRI study of water distribution during operation of a PEM fuel cell using Teflon flow fields

Water management remains a leading challenge in the implementation of small polymer electrolyte membrane (PEM) fuel cells for portable electronic applications. At present there are many excellent models for the distribution of water within PEM fuel cells, but little quantitative data on the water distribution that can be compared to models.     

Effects of humidification temperatures on local current characteristics in a PEM fuel cell

It is well known that water plays a very important role in the performance of proton exchange membrane (PEM) fuel cells. Non-uniform water content in the membrane leads to non-uniform ionic resistance, and non-uniform liquid water fraction in the porous electrode causes varied mass transfer resistances. The objective of this work is to study the effects of different anode and cathode humidification temperatures on local current densities of a PEM fuel cell with a co-flow serpentine flow field. The method used is the current distribution measurement gasket technique [H. Sun, G.S. Zhang, L.J. Guo, H. Liu, J. Power Sources 158 (2006) 326–332]. The experimental results show that both air and the hydrogen need to be humidified to ensure optimal cell performance, and too high or too low humidification temperature can cause severe non-uniform distribution of local current density. From the experimental results of local current density distributions, the local membrane hydration, the optimal humidification temperature, and if flooding occurs can be obtained. Such detailed local measurement results could be very valuable in fuel cell design and operation optimizations.     

Effect of freeze-thaw cycles on the properties and performance of membrane-electrode assemblies

The effect of freezing of a membrane-electrode assembly on its physical properties and performance was investigated. It was found that freeze-thaw cycles caused the electrode (i.e., catalyst layer) of a fully hydrated membrane-electrode assembly (MEA), either as a freestanding piece or as assembled in a cell, to crack. Accompanying the cracking was a reduction in the electrochemical active surface areas of the electrodes as measured by cyclic voltammetry, but the short-term performance of the fuel cell did not show much effect. When dry reactants were used to remove some water from a cell that had been previously tested at fully hydrated condition, freeze-thaw cycling did not cause apparent damage to the appearance of the electrodes. Also, for freestanding MEAs that were taken directly from the manufacturing line and only exposed to ambient temperature (e.g., 23 °C) and relative humidity (e.g., <50% RH), freezing did not cause apparent damage to the appearance of the electrodes.     

Maximum power point tracking of a proton exchange membrane fuel cell system using PSO-PID controller

Fuel cells output power depends on the operating conditions, including cell temperature, oxygen partial pressure, hydrogen partial pressure, and membrane water content. In each particular condition, there is only one unique operating point for a fuel cell system with the maximum output. Thus, a maximum power point tracking (MPPT) controller is needed to increase the efficiency of the fuel cell systems. In this paper an efficient method based on the particle swarm optimization (PSO) and PID controller (PSO-PID) is proposed for MPPT of the proton exchange membrane (PEM) fuel cells. The closed loop system includes the PEM fuel cell, boost converter, battery and PSO-PID controller. PSO-PID controller adjusts the operating point of the PEM fuel cell to the maximum power by tuning of the boost converter duty cycle. To demonstrate the performance of the proposed algorithm, simulation results are compared with perturb and observe (P&O) and sliding mode (SM) algorithms under different operating conditions. PSO algorithm with fast convergence, high accuracy and very low power fluctuations tracks the maximum power point of the fuel cell system.     

In situ measurements of water transfer due to different mechanisms in a proton exchange membrane fuel cell

Water management is of critical importance in a proton exchange membrane (PEM) fuel cell, in particular, those based on a sulfonic acid polymer, which requires water to conduct protons. Yet there are limited in situ studies of water transfer through the membrane and no data are available for water transfer due to individual mechanisms through the membrane in an operational fuel cell. Thus it is the objective of this study to measure water transfer through the membrane due to each individual mechanism in an operational PEM fuel cell. The three different mechanisms of water transfer, i.e., electro-osmotic drag, diffusion and hydraulic permeation are isolated by specially imposed boundary conditions. Therefore water transfer through the membrane due to each mechanism is measured separately. In this study, all the data is collected in an actual assembled operational fuel cell. The experimental results show that water transfer due to hydraulic permeation, i.e. the pressure difference between the anode and cathode is at least an order of magnitude lower than those due to the other two mechanisms. The data for water transfer due to diffusion through the membrane are in good agreement with some of the ex situ data in the literature. The data for electro-osmosis show that the number of water molecules dragged per proton increases not only with temperature but also with current density, which is different from existing data in the literature. The methodology used in this study is simple and can be easily adopted for in situ water transfer measurement due to different mechanisms in other PEM fuel cells without any cell modifications.     

Experimental and numerical analysis of the temperature transition of a suspended freezing water droplet

The objective of this study was to develop a simple experimental and numerical method to study the temperature transition of freezing droplets. One experimental approach and several numerical methods were explored. For the experimental method, a droplet was suspended in a cold air stream from the junction of a thermocouple. The droplet’s temperature transition was able to be accurately measured and the freezing of the droplet observed. The numerical models developed were able to predict the temperature transition and the freezing time of the droplet. Of the numerical methods, a simple heat balance model was determined to be an accurate means of predicting the freezing time of the droplet.     

Analyzing the effects of immobile liquid saturation and spatial wettability variation on liquid water transport in diffusion media of polymer electrolyte fuel cells (PEFCs)

Capillary-induced transport of liquid water inside the porous diffusion media (DM) of polymer electrolyte fuel cells (PEFCs) is strongly dependent on DM pore structure and material properties. As such, excessive liquid in the DM can be expelled more efficiently into flow channels by proper design of the DM structure. The present study is devoted to exploring multiphase transport characteristics by considering the effects of DM pore structure and material properties. Two main effects on overall water removability are examined, namely: (i) the effect of immobile liquid saturation, which is a threshold value for initiating macroscopic capillary transport via connected small liquid droplets, and (ii) the effect of hydrophobic spatial variation, which is encountered in typical DM treated with PTFE. Although these two effects are expected to influence significantly the transport characteristics in the fuel cell DM, they have been ignored in most two-phase fuel cell models reported in the literature. In the present work, these features are implemented into a one-dimensional, multiphase mixture (M²) fuel cell model along the through-plane direction, where both anode and cathode sides and the membrane are fully incorporated. The results of the model simulation clearly demonstrate the dramatic influence of the amount of liquid accumulation and capillary transport characteristics inside the DM. The findings are useful for designing and optimizing DM for the purpose of effective water removal.     

The effects of excess phosphoric acid in a Polybenzimidazole-based high temperature proton exchange membrane fuel cell

A series of experiments are conducted in order to investigate the performance of a proton exchange membrane (PEM) fuel cell using a commercially available polybenzimidazole (PBI)-based high temperature membrane. During the study a drastic degradation in performance is observed over time and a significant amount of solid material built-up is found in the flow field plate and the membrane-electrode assembly (MEA). The built-up material is examined by the use of a Scanning Electron Microscope (SEM). Further elemental analysis using Energy Dispersive X-ray Spectroscopy (EDS) finds that the built-up material contains large amount of phosphorus, thus relating it with the excess phosphoric acid found in the MEA. Additional experimental studies show that the built-up material is caused by the excess acid solution in the MEA, and when the excess phosphoric acid is removed from the MEA the fuel cell performance improves significantly and becomes very stable.     

Sol-gel based sulfonic acid-functionalized silica proton conductive membrane

Sulfonic acid-functionalized glass membranes have been synthesized via sol-gel reactions for low-power direct methanol fuel cells (DMFCs). Minimizing the fuel loss due to methanol crossover is the most important issue for creating long-life, low-power DMFC sources. The inorganic glass membrane is of interest due to its low methanol permeability compared to polymer membranes. Three different alkoxy silane reactants were investigated in the sol-gel reaction: 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-mercaptopropyl trimethoxysilane (3MPS), and tetraethoxy orthosilicate (TEOS). The effect of oxidation time of the thiol group on the 3MPS, the mole fraction within the sol, and the water ratio in the reactant mixture were investigated. The ionic conductivity and methanol permeability has been characterized and optimized. The goal in this study was to find a balance between the ionic conductivity and methanol permeability, which determines the fuel conversion efficiency and device lifetime. The optimum glass membrane had a conductivity of 3.71 mS cm⁻¹, and methanol permeability of 2.17 × 10⁻⁹ mol cm cm⁻² day⁻¹ Pa⁻¹, which was significantly better than Nafion or other previously reported membranes for this application.     

In situ investigation of water transport in an operating PEM fuel cell using neutron radiography: Part 1 – Experimental method and serpentine flow field results

Effective management of liquid water produced in the cathodic reaction of a polymer electrolyte membrane (PEM) fuel cell is essential to achieve high cell efficiency. Few experimental methods are available for in situ measurements of water transport within an operating cell. Neutron radiography is a useful tool to visualize water within a cell constructed of many common materials, including metals. The application of neutron radiography to measurements of water content within the flow field channels of an operating 50 cm² PEM fuel cell is described. Details of the experimental apparatus, image processing procedure and quantitative analysis are provided. It is demonstrated that water tends to accumulate in the 180° bends of the serpentine anode and cathode flow fields used in this study. Moreover, the effects of both the current density and cathode stoichiometric ratio on the quantity of accumulated water are discussed.     

Optimization of hydrogen feeding procedure in PEM fuel cell systems for transportation

The hydrogen feeding sub-system is one of balance of plant (BOP) components necessary for the correct operation of a fuel cell system (FCS). In this paper the performance of a 6 kW PEM (Proton Exchange Membrane) FCS, able to work with two fuel feeding procedures (dead-end or flow-through), was experimentally evaluated with the aim to highlight the effect of the anode operation mode on stack efficiency and durability. The FCS operated at low reactant pressure (<50 kPa) and temperature (<330 K), without external humidification. The experiments were performed in both steady state and dynamic conditions. The performance of some cells in dead-end mode worsened during transient phases, while a more stable working was observed with fuel recirculation. This behavior evidenced the positive role of the flow-through procedure in controlling flooding phenomena, with the additional advantage to simplify the management issues related to hydrogen purge and air stoichiometric ratio. The flow-through modality resulted a useful way to optimize the stack efficiency and to reduce the risks of fast degradation due to reactant starvation during transient operative phases.     

Kinetic studies of cathode degradation on PEM fuel cell short stack level undergoing freeze startups with different states of residual water and current draws

One of the biggest challenges for a wide spread introduction of polymer electrolyte membrane fuel cells in automotive applications is the freeze start at subzero temperatures as this poses a severe threat to fuel cell performance and overall lifetime. Therefore, the impact of current draw during stack freeze startup at various rest water contents and current densities was investigated applying state of the art in-situ testing as quasi cyclic voltammetry.The results indicate a clear dependency between number of freeze startups and performance loss, whereas higher initial water content within the stack reduced its destructive impacts.In our earlier work we were able to show the dependency between residual water and degradation for non-freeze-startup-capable systems at the unit cell level [31]. With this work we confirm that the same physical relationship also applies to freeze-startup-capable systems on the short stack level.Furthermore, reversible performance losses that were encountered during this study can be assigned to oxidizable fuel contaminants, which are believed to be CO, and an easy cleansing procedure is being suggested.     

Numerical studies on rib & channel dimension of flow-field on PEMFC performance

Distributions in reactant species concentration in a PEMFC cause distributions in local current density, temperature and water over the area of a PEMFC. These can lead to effects such as flooding or drying of the membrane and cause stresses in different regions of the fuel cell. Changing flow-field configuration, including channel path length, width, or height to distribute the gas more evenly, is one method of minimizing these stresses. This work numerically investigated how serpentine flow-fields with different channel/rib's cross section areas affect performance and species distributions for both automotive and stationary conditions. Further, the influence of flow direction to performance and its distribution was also reported. The prediction revealed that for stationary condition, narrower channel with wider rib spacing gives higher performance but opposite results when automotive condition is applied.