Microscopic Observations of Freezing Phenomena in PEM Fuel Cells at Cold Starts

In polymer electrolyte membrane (PEM) fuel cells, the generated water transfers from the catalyst layer to the gas channel through microchannels of different scales in a two phase flow. It is important to know details of the water transport phenomena to realize better cell performance, as the water causes flooding at high current density conditions and gives rise to startup problems at freezing temperatures. This article presents specifics of the ice formation characteristics in the catalyst layer and in the gas diffusion layer (GDL) with photos taken with an optical microscope and a cryo scanning electron microscope (cryo-SEM). The observation results show that cold starts at –10°C result in ice formation at the interface between the catalyst layer and the microporous layer (MPL) of the GDL, and that at –20°C most of the ice is formed in the catalyst layer. Water transport phenomena through the microporous layer and GDL are also a matter of interest, because the role of the MPL is not well understood from the water management angle. The article discusses the difference in the water distribution at the interface between the catalyst layer and the GDL arising from the presence of such a microporous layer.     

Effective purge method with addition of hydrogen on the cathode side for cold start in PEM fuel cell

The purge process is essential for successful cold start of fuel cell vehicles during winter, and it plays an important role in the removal of the residual water inside the fuel cell in a short time. In this study, a new purge method is introduced by adding a small amount of hydrogen to the cathode gas flow in order to increase the purge performance. The experimental results demonstrate that the hydrogen addition purge method is very effective in removing the residual water near the catalyst layer. The water removal is verified by measuring the resistance of the fuel cell, dew point temperature of the outlet purge gas, and weights of the membrane electrolyte assembly (MEA) and gas diffusion layer (GDL). In addition, the image of the GDL after the purge process is captured to show the advantage of the hydrogen addition purge method. Cold start experiments are also conducted after the optimal purge process. It is also found that the degradation of the catalyst layer is not serious after the hydrogen addition purge process.     

3D neutron tomography of a polymer electrolyte membrane fuel cell under sub-zero conditions

In this article, we implement both 2D and 3D based neutron imaging techniques on a polymer electrolyte membrane (PEFC) fuel cell under sub-zero conditions. A cell was run at steady state power, purged for 60 s, and then brought down to −5 °C inside an environmental chamber situated in front of a neutron beam. A series of 2D radiographs were taken as the cell dropped in temperature capturing the condensation and redistribution of flow field and gas diffusion layer (GDL) water. Immediately after this, 3D tomography was conducted while the cell remained at −5 °C. The image data was reconstructed into a 3D model in order to highlight regions where water/ice formations occur. The tomography results show where ice forms within the flow field and which regions are subject to blockages. Ice is observed predominately under channel areas due to water rejection by the GDL. The cathode side channel exit region displays higher ice content which correlates with elevated saturation levels from reaction water production during operation. Larger ice formations reside in the lower region of the flow field due to gravity. These blockages may pose significant issues to cold start of the cell as well as highlight potential drawbacks to shorter purge durations.     

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.     

Liquid water transport between graphite paper and a solid surface

We studied the interaction of a water droplet with a solid wall on a hydrophobic gas diffusion layer (GDL). Of particular interest is the stability of the droplet as a function of plate wetting properties and the potential for liquid entrapment in the GDL/land contact area. Such transport is of relevance to breakthrough dynamics and convective liquid droplet transport in polymer electrolyte membrane (PEM) fuel cell cathode gas channels. While a variety of complex coupled transport phenomena are present in the PEM fuel cell gas channel, we utilize a very simplified experimental model of the system where a droplet originally placed on a hydrophobic GDL is translated quasistatically across the GDL surface by a solid surface. Transport and entrapment are imaged using fluorescence microscopy. This work provides new insights into droplet behaviour at the GDL/land interface in a PEM fuel cell and suggests that hydrophobic land areas are preferable for mitigating the accumulation of liquid water under the land area of the gas flow channels.     

Effects of hydrophilic/hydrophobic properties on the water behavior in the micro-channels of a proton exchange membrane fuel cell

The mobility of water droplets and water films inside a straight micro-channel of a proton exchange membrane fuel cell was simulated to study the effects of the hydrophilic/hydrophobic properties on water behavior. The volume-of-fluid model in the FLUENT package was used to keep track of the deformation of the liquid–gas interface. The results show that the water moved faster on a hydrophobic surface. But a hydrophobic channel side-wall was a disadvantage for the gas diffusion when the MEA had a hydrophilic surface. A hydrophilic channel side-wall with a hydrophobic MEA surface could avoid water accumulation on the MEA surface. The water and gas distribution under this condition was advantageous for water discharge and gas diffusion.     

Single-walled carbon nanotube interlayer modified gas diffusion layers to boost the cell performance of self-humidifying proton exchange membrane fuel cells

A tradeoff between the low humidity and the high performance remains a key challenge for the proton exchange membrane fuel cell (PEMFC). In this work, a novel self-humidifying gas diffusion layer (GDL) with a single-walled carbon nanotube (SWCNT) nonwoven layer between the gas diffusion substrate and the hydrophobic microporous layer is controllably prepared to elevate the cell performance under dry conditions. The membrane electrode assembly (MEA) with 0.25 mg cm⁻² SWCNT loading exhibits a current density of 0.69 A cm⁻² at 0.6 V, which is 392.8% higher than that of the counterpart without the SWCNT interlayer at the same relative humidity. Moreover, the SWCNT interlayer with rational pore structure and proper wettability dramatically improves the water retention capacity of MEA, thus enhancing the low-humidity performance of MEA. The structure design of GDL provides an effective strategy for self-humidifying PEMFC control optimization.     

A numerical investigation of the effects of GDL compression and intrusion in polymer electrolyte fuel cells (PEFCs)

The purpose of this work is to numerically investigate the effects of non-uniform compression of the gas diffusion layer (GDL) and GDL intrusion into a channel due to the channel/rib structure of the flow-field plate. The focus is placed on accurately predicting two-phase transport between the compressed GDL near the ribs and uncompressed GDL near the channels, and its associated effects on cell performance. In this paper, a GDL compression model is newly developed and incorporated into a comprehensive three-dimensional, two-phase PEFC model developed earlier. To assess solely the effects of GDL compression and intrusion, the new fuel cell model is applied to a simple single-straight channel fuel cell geometry. Numerical simulations with different levels of GDL compression and intrusion are carried out and simulation results reveal that the effects of GDL compression and intrusion considerably increase the non-uniformity, particularly, the in-plane gradient in liquid saturation, oxygen concentration, membrane water content, and current density profiles that in turn results in significant ohmic and concentration polarizations. The present three-dimensional GDL compression model yields realistic species profiles and cell performance that help to identify the optimal MEA, gasket, and flow channel designs in PEFCs.     

Optimization of GDLs for high-performance PEMFC employing stainless steel bipolar plates

The effects of polytetraflouroethylene (PTFE) content in the gas diffusion layer (GDL) on the performance of PEMFCs with stainless-steel bipolar plates are studied under various operation conditions, including relative humidity, cell temperature, and gas pressure. The optimal PTFE content in the GDL strongly depends on the cell temperature and gas pressure. Under unpressurized conditions, the best cell performance was obtained by the GDL without PTFE, at a cell temperature of 65 °C and relative humidity (RH) of 100%. However, under the conditions of high cell temperature (80 °C), low RH (25%) and no applied gas pressure, which is more desirable for fuel cell vehicle (FCV) applications, the GDL with 30 wt.% PTFE shows the best performance. The GDL with 30 wt.% PTFE impedes the removal of produced water and increases the actual humidity within the membrane electrode assembly (MEA). A gas pressure of 1 bar in the cell using the GDL with 30 wt.% PTFE greatly improves the performance, especially at low RH, resulting in performance that exceeds that of the cell under no gas pressure and high RH of 100%.     

Effects of supplied gas humidity change on PEFC transient power generation

Polymer electrolyte fuel cells (PEFCs, PEMFCs) are gaining increasingly more attention as clean and efficient energy‐conversion devices. Vapor and liquid transport has a strong impact on the power generation characteristics and efficiency of PEFCs, and so proper water management is needed for efficiency and durability. However, water transport factors are not well understood, particularly during unsteady operation—often the case in vehicles and distributed stationary power generators. In this study, to understand and generalize the effects of local water transport on PEFC performance, transient mass transport characteristics inside a PEFC were investigated experimentally and numerically. For this purpose, we developed an unsteady two‐dimensional numerical model based on mass‐ and charge‐conservation equations in the channel, gas diffusion layer (GDL), and membrane electrode assembly (MEA). As necessary parameters for model development, we measured the water content of the MEA, the membrane resistivity, the activation overvoltage, overall mass transfer coefficient, and so on. The membrane resistivity greatly increases as the relative humidity decreases. The activation overvoltage is also affected by the relative humidity, and not only by the current density and oxygen activity. Current load and voltage changes are frequently used as PEFC transient inputs, but lead to very complicated and intractable phenomena such as changes in the amount of generated water and electro‐osmosis, state of the electrical double layer, and so on. Hence, stepwise changes in the relative humidity of the supplied gas were adopted in this study. The experimental and numerical transient responses were in good agreement under most operating conditions, and the reliability of our measurement methods for the water transport properties and our numerical model were confirmed. Here we discuss the dominant factor in the transient responses, and conclude that the transport resistance at the PEM–GDL interface is the largest and most dominant factor in a relatively dry state under unsteady operating conditions. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20371     

Cold-start icing characteristics of proton-exchange membrane fuel cells

Understanding the icing characteristics of proton-exchange membrane fuel cells (PEMFCs) is essential for optimizing their cold-start performance. This study examined the effects of start-up temperature, current density, and microporous layer (MPL) hydrophobicity on the cold-start performance and icing characteristics of PEMFCs. Further, the cold-start icing characteristics of PEMFCs were studied by testing the PEMFC output voltage, impedance, and temperature changes at different positions of the cathode gas diffusion layer. Observation of the MPL surface after cold-start failure allowed determination of the distribution of ice formation at the catalytic layer/MPL interface. At fuel cell temperatures below 0 °C, supercooled water in the cell was more likely to undergo concentrated instantaneous freezing at higher temperatures (−4 and −5 °C), whereas the cathode tended to freeze in sequence at lower temperatures (−8 °C). In addition, a more hydrophobic MPL resulted in two successive instantaneous icing phenomena in the fuel cell and improved the cold-start performance.     

Reduced graphene oxide–coated anode gas diffusion layer for performance enhancement of proton‐exchange membrane fuel cell

In this study, we produced reduced graphene oxide (RGO) by reduction of graphene oxide (GO) in Teflon‐lined autoclave, maintained at 100°C for 12 hours, and coated on the anode gas diffusion layer (GDL) of a proton‐exchange membrane fuel cell (PEMFC) to improve the cell performance. Fourier transform infrared spectroscopy and X‐ray photoelectron spectroscopy analysis showed the presence of residual oxygen‐containing functional groups in RGO. Field‐emission scanning electron microscopy images revealed the uniform and adequate coating of the GDLs with RGO. The wettability of RGO‐coated GDL was determined by the sessile drop method and has optimum contact angle 117°. The power densities for the membrane electrode assembly (MEA) with RGO coated on the anode GDL were 30.92%, 41%, and 36.20% higher than those for the MEA without the RGO coating at anode gas humidified temperatures of 25°C, 45°C, and 65°C, respectively.     

A review of low-temperature proton exchange membrane fuel cell degradation caused by repeated freezing start

Proton exchange membrane fuel cell (PEMFC) has advantages of zero emission, fast response and high-power density. There are still obstacles such as manufacturing cost, life span, infrastructure construction and subzero temperature star-up restricting commercialization of PEMFC. The low-temperature start-up is one of them that needs to be solved in the field of fuel cell vehicle. This paper presents research progresses involving PEMFC degradation caused by the low-temperature start-up. Degradation phenomena and mechanism under component-level caused by repeated freezing start, influencing factors and mitigation strategies are summarized and reviewed. Conclusions are made that frequent ice freezing and melting causes the membrane electrode assembly damaged irreversibly, the quality of cold start and low temperature influence the degradation strongly and purge after shutdown, better materials and optimal fuel cell structure design are helpful to reduce the impact of cold start on fuel cell performances. It is suggested that future work should be focused on optimizing strategies of the shutdown purge, promoting the quality of cold start, enhancing properties of the materials, improving internal structure design of stack and developing low-temperature attenuation models.     

A study of the effect of compression on the performance of polymer electrolyte fuel cells using electrochemical impedance spectroscopy and dimensional change analysis

Compression plays an important role in the performance of polymer electrolyte fuel cells (PEFCs). In this study, dynamic compression is applied using a cell compression unit (CCU) to study the effect on performance of a membrane electrode assembly (MEA) with dimension change. The stress/strain characteristics of the MEA are observed to be dominated by the gas diffusion layer (GDL), with the GDL exhibiting a degree of plasticity. Electrochemical impedance spectroscopy (EIS) is used to delineate the effect of compression on contact resistance and mass transfer losses.     

Effects of electrode wettabilities on liquid water behaviours in PEM fuel cell cathode

Liquid water transport is one of the key challenges for water management in a proton exchange membrane (PEM) fuel cell. Investigation of the air–water flow patterns inside fuel cell gas flow channels with gas diffusion layer (GDL) would provide valuable information that could be used in fuel cell design and optimization. This paper presents numerical investigations of air–water flow across an innovative GDL with catalyst layer and serpentine channel on PEM fuel cell cathode by use of a commercial Computational Fluid Dynamics (CFD) software package FLUENT. Different static contact angles (hydrophilic or hydrophobic) were applied to the electrode (GDL and catalyst layer). The results showed that different wettabilities of cathode electrode could affect liquid water flow patterns significantly, thus influencing on the performance of PEM fuel cells. The detailed flow patterns of liquid water were shown, several gas flow problems were observed, and some useful suggestions were given through investigating the flow patterns.     

Basic evaluation of separator type specific phenomena of polymer electrolyte membrane fuel cell by the measurement of water condensation characteristics and current density distribution

This paper investigates phenomena related to water condensation behavior inside a polymer electrolyte membrane fuel cell (PEMFC), and analyzes the effects of liquid water and gas flow on the performance of the fuel cell. A method for simultaneous measurements of the local current density across the reaction area and direct observation of the phenomena in the cell are developed. Experimental results comparing separator types indicate the effect of shortcut flow in the gas diffusion layer (GDL) under the land areas of serpentine separators, and also show the potential of straight channel separators to achieve a relatively uniform current density distribution. To evaluate shortcut flows under the land areas of serpentine separators, a simple circuit model of the gas flow is presented. The analysis shows that slight variations in oxygen concentration caused by the shortcut flows under the land areas affect the local and overall current density distributions. It is also shown that the establishment of gas paths under the water in channels filled with condensed water is effective for stable operation at low flow rates of air in the straight channels.     

Degradation behaviors of polymer electrolyte membrane fuel cell under freeze/thaw cycles

Degradation behaviors of polymer electrolyte membrane fuel cell (PEMFC) in high current density region were investigated under Freeze/Thaw cycles. Different dehumidification scenarios namely hot purge, cold purge and no purge were adopted for comparison. Micrographs from scanning electron microscopy proved little change in catalyst-coated membrane (CCM) integrity, no delamination or segregation occurred after many freeze/thaw cycles. Cyclic Voltammetry (CV) measurement revealed reduction in electrochemical active surface area of CCM. The observed performance decay in the high current density region was mainly attributed to the increased interface contact resistance and degraded electric and gas coupling characteristics at interfaces between CCM and GDL in this paper. Meanwhile, the performance degradation under low current densities (for example 400 mA cm⁻² or even lower) was mainly ascribed to the degraded characteristics of catalyst layers referring to CCM as cyclic voltammetry indicated. Proper dehumidification through gas purging is effective to maintain stable preference under subzero temperature.     

Design of low-humidification PEMFC by using cell simulator and its power generation verification test

As long as the perfluorinated proton exchange membrane (PEM) is used for the electrolyte, both the cell performance and life are highly dependent upon the water content in the electrolyte. On the other hand, pre-humidification of fuel and oxidant gases complicates the PEMFC system and prevents it from possible cost reduction measures. In this study, in order to maintain a membrane electrode assembly (MEA) with a satisfactory water content by only the water produced in catalyst layer through the electrode reaction without prior humidification of both the fuel and oxidant gases, a novel gas diffusion layer (GDL) was fabricated. This was achieved by coating a water management layer (WML) onto a traditional GDL in order to place the WML between the traditional GDL and the catalyst layer of the PEMFC. This study describes the significant balance of water with WML in the fuel cell using both simulation and experimental analysis.     

Effect of water distribution in gas diffusion layer on proton exchange membrane fuel cell performance

Water management of proton exchange membrane fuel cells remains a prominent issue in research concerning fuel cells. In this study, the gas diffusion layer (GDL) of a fuel cell is partially treated with a hydrophobic agent, and the effect of GDL hydrophobicity on the water distribution in the fuel cell is examined. First, the effect of the position of the cathode GDL hydrophobic area relative to the channel on the fuel cell performance is investigated. Then, the water distribution in the fuel cell cathode GDL is observed using X-ray imaging. The experimental results indicate that when the hybrid GDL's hydrophobic area lies on the channel, water tends to accumulate under the rib, and the water content in the channel is low; this improves the fuel cell performance. When the hydrophobic area is under the rib, the water distribution is more uniform, but the performance deteriorates.     

Temperature distribution on the MEA surface of a PEMFC with serpentine channel flow bed

Knowledge of the temperature distribution on the membrane electrode assembly (MEA) surface and heat transfer processes inside a proton exchange membrane fuel cell (PEMFC) is helpful to improvement of cell reliability, durability and performance. The temperature fields on the surface of MEA fixed inside a proton exchange membrane fuel cell with a serpentine channel flow bed were measured by infrared imaging technology under non-humidification conditions. The temperature distributions over the MEA surface under whole channel region were achieved. The experimental results show that the downstream temperatures are higher than the upstream. The hot region on the MEA surface is easy to locate from the infrared temperature image. The mean temperature on the MEA surface and the cell temperature both increase with the current density. Higher current density makes the non-uniformity of temperature distribution on the MEA surface worse. The loading time significantly affects the temperature distribution. Compared with the electrical performance of the cell, the MEA's temperatures need much more time to reach stable. The results indicate that isothermal assumption is not appropriate for a modeling of PEMFCs, and monitoring the temperature of external surface of the flow field plate or end plate cannot supply accurate reference to control the temperatures on MEA surface.