X-ray imaging of water distribution in a polymer electrolyte fuel cell

At present, water management in a polymer electrolyte fuel cell (PEFC) is a major subject of research. In fact, proper water management is vital to achieve maximum performance and durability from a PEFC. Consequently, this study is conducted to visualize quantitatively the water distribution in a PEFC by means of an X-ray imaging technique. The X-ray images of the PEFC components with and without water are clearly distinguished. Reference to the visualized X-ray images, enables quantitative evaluation of the water distribution in the region between the separator and the gas-diffusion layer (GDL). Likewise, the meniscus of water in the channels of the PEFC is clearly observed.     

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.     

Synchrotron X-ray tomography for investigations of water distribution in polymer electrolyte membrane fuel cells

Synchrotron X-ray tomography is used to visualize the water distribution in gas diffusion layers (GDL) and flow field channels of a polymer electrolyte membrane fuel cell (PEMFC) subsequent to operation. An experimental setup with a high spatial resolution of down to 10 μm is applied to investigate fundamental aspects of liquid water formations in the GDL substrate as well as the formation of water agglomerates in the flow field channels. Detailed analyses of water distribution regarding the GDL depth profile and the dependence of current density on the water amount in the GDL substrate are addressed. Visualizations of water droplets and wetting layer formations in the flow field channels are shown. The three-dimensional insight by means of this quasi in situ tomography allows for a better understanding of PEMFC water management at steady state operation conditions. The effect of membrane swelling as function of current density is pointed out. Results can serve as an essential input to create and verify flow field simulation outputs and single-phase models.     

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.     

Quantification of liquid water accumulation and distribution in a polymer electrolyte fuel cell using neutron imaging

Operating parameters, material properties and flow field geometry have a deterministic role on the water storage and distribution within the flow channels and porous media in a fuel cell. However, their effects are not yet precisely understood. In this study, extensive neutron imaging experiments were conducted to visualize and quantify the amount of liquid water in the fuel cell channels and diffusion media as a function of inlet gas flow rate, cell pressure and inlet relative humidity. A seven-channel parallel flow configuration PEFC was used to isolate these parameters from flow field switchback interaction effects.

The neutron imaging experiments were performed at different inlet gas flow rates, operating cell pressures and inlet relative humidities. At each operating condition, the distribution of liquid water in the diffusion media under the lands, and in or under the channels was obtained. Furthermore, at three different cell pressures (0.2 MPa, 0.15 MPa and 0.1 MPa), liquid water distribution and quantification was obtained. The liquid water mass in the cell decreased with increasing pressure for over-humidified anode inlet conditions. Comparison of the fuel cell performance with the total liquid water mass in the cell indicates a non-monotonic relationship between liquid water content and performance. Furthermore, cell performance was highly sensitive to incremental changes in the membrane liquid water content.     

Fuel crossover and internal current in polymer electrolyte membrane fuel cell from water visualization using X-ray radiography

The fuel crossover and internal current in a polymer electrolyte membrane fuel cell undergo a chemical reaction in the cell without power generation. These are the main phenomena for reduced cell voltage at low current density. This fuel crossover also degrades the fuel cell performance, efficiency, and durability. Thus, observation of these phenomena is important for understanding and developing a polymer electrolyte membrane fuel cell. Using X-ray radiography, the water distribution and membrane swelling, which indicate fuel crossover and internal current, in an operating polymer electrolyte membrane fuel cell under open-circuit conditions were examined. The X-ray images effectively demonstrated the transient changes of each phenomenon, which are related to the properties of each component and the operating conditions.     

Improvement of water management in polymer electrolyte membrane fuel cell thanks to cathode cracks

The role of cathodic structure on water management was investigated for planar micro-air-breathing polymer electrolyte membrane fuel cells (PEMFCs). The electrical results demonstrate the possibility to decrease, with the same structure, both cell drying and cell flooding according to the environmental and operation conditions. Thanks to a simultaneous study of internal resistance and scanning electronic microscope (SEM) images, we demonstrate the advantageous influence of the presence of crack in cathodic catalytic layer on water management. On the one hand, the gold layer used as cathodic current collector is in contact with the electrolyte in the cracked zones which allows water maintenance within the electrolyte. It allows to decrease the cell drying and thus strongly increase the electrical performances. For cells operated in a 10% relative humidity atmosphere at 30 °C and at a potential of 0.5 V, the current density increases from 28 mA cm⁻² to 188 mA cm⁻² (+570%) for the cell with a cathodic cracked network. On the other hand, the reduction in oxygen barrier diffusion due to the cathodic cracks allows to improve oxygen diffusion. In flooding state, the current densities were higher for a cell with a cracked network. For cells operating in a 70% relative humidity atmosphere at 30 °C and at a potential of 0.2 V, a current density increase from 394 mA cm⁻² to 456 mA cm⁻² (16%) was noted for the cell with a cathodic cracked network. Microscopic observations allowed us to visualize water droplets growth mechanism in cathodic cracks. It was observed that the water comes out of the crack sides and partially saturates the cracks before emerging on cathodic collector. These results demonstrate that cathode structuration is a key parameter that plays a major role in the water management of PEMFCs.     

Membrane resistance and current distribution measurements under various operating conditions in a polymer electrolyte fuel cell

The ability to make spatially resolved measurements in a fuel cell provides one of the most useful ways in which to monitor and optimise their performance. Localised membrane resistance and current density measurements for a single channel polymer electrolyte fuel cell are presented for a range of operating conditions. The current density distribution results are compared with an analytical model that exhibited generally good agreement across a broad range of operating conditions. However, under conditions of high air flow rate, an increase in current is observed along the channel which is not predicted by the model. Under such circumstances, localised electrochemical impedance measurements show a decrease in membrane resistance along the channel. This phenomenon is attributed to drying of the electrolyte at the start of the channel and is more pronounced with increasing operating temperature.     

Prevention of the water flooding by micronizing the pore structure of gas diffusion layer for polymer electrolyte fuel cell

In polymer electrolyte fuel cells, high humidity must be established to maintain high proton conductivity in the polymer electrolyte. However, the water that is produced electrochemically at the cathode catalyst layer can condense in the cell and cause an obstruction to the diffusion of reaction gas in the gas diffusion layer and the gas channel. This leads to a sudden decrease of the cell voltage. To combat this, strict water management techniques are required, which usually focus on the gas diffusion layer. In this study, the use of specially treated carbon paper as a flood-proof gas diffusion layer under extremely high humidity conditions was investigated experimentally. The results indicated that flooding originates at the interface between the gas diffusion layer and the catalyst layer, and that such flooding could be eliminated by control of the pore size in the gas diffusion layer at this interface.     

The effects of water accumulation at the interface between gas diffusion layer and gas supplying channel on the water distribution in polymer electrolyte membrane fuel cells

The effects of water accumulation at the interface of gas diffusion layer (GDL) and gas supplying channel on the water distribution in polymer electrolyte membrane fuel cells (PEMFCs) is analyzed. The amount of water at the interface and in the GDL are quantified using X-ray. Quantitative analyses show that the value of the criterion of water accumulation that can affect the water distribution is in the water saturation range of 0.22–0.24. The amount of water in the GDL increases with the water accumulated at the beginning of the water generation cycle. However, it remains constant after the water accumulation exceeds a criterion value. The result shows that the water accumulation at the interface should be investigated to understand the water distribution in PEMFC. The water distribution in PEMFC cannot be analyzed based on the steady state concept but can be analyzed based on the concept of cyclic process.     

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.     

Investigation of freeze/thaw durability in polymer electrolyte fuel cells

This study aims to investigate the effect of different gas diffusion layers (GDLs) on freeze/thaw condition durability in polymer electrolyte fuel cells (PEFCs). Three kinds of GDLs–cloth, felt and paper type—with similar basic properties except thickness and bending stiffness were used. The changes in the properties and cell performance were investigated from the −30 to 70 °C range of freeze/thaw cycles. The I–V performance degradation was observed to be negligible for the felt GDL whereas the cloth and paper GDLs showed a marked I–V performance loss. No distinctive correlation between the changes in electrochemical properties, such as active metal surface area, hydrogen crossover rates and decreased I–V performance, was observed except an increase in ohmic resistance revealed by ac-impedance spectroscopy. The physical destruction of electrodes was also shown by scanning electron microscope (SEM) analysis.     

Gas diffusion layer design focusing on the structure of the contact face with catalyst layer against water flooding in polymer electrolyte fuel cell

High humidity must be maintained inside polymer electrolyte fuel cells to achieve high ion conductivity. However, water condensation blocks the diffusion of the reaction gas in the gas diffusion layer under water saturation conditions which are produced by the product water. This effect is known as flooding and causes a sudden drop in the cell voltage. Therefore, advanced water management is required in such fuel cells. Internal water management is generally carried out by making adjustments to the gas diffusion layer. This study reports that the extremely highly flood-resistant gas diffusion layer has been developed, based on simple carbon paper. It was experimentally revealed that flooding is controlled by a gas diffusion layer with a smaller pore-structure facing the catalyst layer and it is one of the governing factors for flooding in the gas diffusion layer.     

Measurement of liquid water content in cathode gas diffusion electrode of polymer electrolyte fuel cell

Water management in cathode gas diffusion electrode (GDE) of polymer electrolyte fuel cell (PEFC) is essential for high performance operation, because liquid water condensed in porous gas diffusion layer (GDL) and catalyst layer (CL) blocks oxygen transport to active reaction sites. In this study, the average liquid water content inside the cathode GDE of a low-temperature PEFC is experimentally and quantitatively estimated by the weight measurement, and the relationship between the water accumulation rate in the cathode GDE and the cell voltage is investigated. The liquid water behavior at the cathode is also visualized using an optical diagnostic, and the effects of operating conditions and GDL structures on the water transport in the cathode GDE are discussed. It is found that the liquid water content in the cathode GDE increases remarkably after starting the fuel cell operation due to the water production at the CL. At a high current density, the cell voltage drops suddenly after starting the operation in spite of a low water content in the cathode GDE. When the GDL thickness is increased, much water accumulates near the cathode CL and the fuel cell shuts down immediately after the operation. In the final section of this paper, the structure of cathode GDL that has several grooves for water removal is proposed to prevent water flooding and improve fuel cell performance. This groove structure is effective to promote the removal of the liquid water accumulated near the active catalyst sites.     

Effect of water on life prediction of liquid silicone rubber seals in polymer electrolyte membrane fuel cell

Liquid silicone rubber (LSR) is a popular gasket or seal material and is also promising for sealing applications in polymer electrolyte membrane fuel cell (PEMFC). The durability of the LSR gasket/seals in PEMFC is one of the major issues in commercialization of PEMFC. As there are water and humidity inside PEMFC and polymers such as LSR generally exhibit stress relaxation property, it is important to understand the effect of water on the compression stress relaxation of LSR. Our test results show that water has no influence on the stress relaxation in the beginning, but it accelerates the relaxation after a certain time. Higher temperature makes this transition occurs earlier. Further studies reveal that water can diffuse into LSR and exists as free water molecules. It may attack the backbones of the polymer and thus accelerate the stress relaxation. High temperature tends to aggravate the attack of water to the polymer chains. The attack coexists with the thermal degradation of the LSR.     

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.     

Study on water transport in hydrophilic gas diffusion layers for improving the flooding performance of polymer electrolyte fuel cells

For hydrogen-based polymer electrolyte fuel cells (PEFCs), water transport control in gas diffusion layers (GDLs) by wettability distribution is useful to suppress the flooding problem. In this study, the water transport of a novel GDL with hydrophilic-hydrophobic patterns was investigated. First, we clarified that the water motion in the hydrophilic GDL with microstructures could be reproduced by the enlarged scale model. The scale model experiment also showed that the same water behavior in hydrophilic GDL can be obtained from Capillary numbers (Ca) in a range of Ca ~ 10^?5 to 10^?3. As the computational load is inversely proportional to Ca, the computational load could be reduced by 1/100th by using Ca ~ 10^?3, which is 100 times higher than PEFC operation (Ca ~ 10^?5). Finally, the simulation with Ca ~ 10^?3 was performed, and we showed that the GDL with straight region of contact angle 50° minimized the water accumulation.     

An integrated modeling approach for temperature driven water transport in a polymer electrolyte fuel cell stack after shutdown

The concept of using controlled temperature gradients to non-parasitically remove excess water from porous media during PEFC stack shutdown has been numerically investigated. An integrated modeling approach focusing both at stack and single cell level is presented. The stack thermal model is developed to obtain detailed temperature distribution across the PEFC stack. The two-phase unit fuel cell model is developed to investigate the detailed water and thermal transport in the PEFC components after shutdown, which for the first time includes thermo-osmotic flow in the membrane. The model accounts for capillary and phase-change induced flow in the porous media, and thermo-osmotic and diffusive flow in the polymer membrane. The single cell model is used to estimate the local water distribution with land or channel boundary condition, and the experimentally validated stack thermal model provided the transient temperature boundary conditions. Two different stack designs are compared to quantify the residual water in the stack. Model results indicate that a favorable temperature gradient can be formed in the stack to enhance the water drainage rate, esp. at anode end cell locations, where freeze/thaw damage has been observed to occur.     

Effect of air flow on liquid water transport through a hydrophobic gas diffusion layer of a polymer electrolyte membrane fuel cell

The transport of liquid water through an idealized 2-D reconstructed gas diffusion layer (GDL) of a polymer electrolyte membrane (PEM) fuel cell is computed subject to hydrophobic boundary condition at the fibre–fluid interface. The effect of air flow, as would occur in parallel/serpentine/interdigitated type of flow fields, on the liquid water transport through the GDL, ejection into the channel in the form of water droplets and subsequent removal of the droplets has been simulated. Results show that typically water flow through the fibrous GDL occurs through a fingering and channelling type of mechanism. The presence of cross-flow of air has an effect both on the path created within the GDL and on the ejection of water into the channel in the form of droplets. A faster rate of liquid water evacuation through the GDL (i.e., more frequent ejection of water droplets) as well as less flooding of the void space results from the presence of cross-flow. These results agree qualitatively with experimental observations reported in the literature.