Current distribution in polymer electrolyte membrane fuel cell with active water management

Air delivery is typically the greatest parasitic power loss in polymer electrolyte membrane fuel cell (PEMFC) systems. We here present a detailed study of an active water management system for PEMFCs, which uses a hydrophilic, porous cathode flow field, and an electroosmotic (EO) pump for water removal. This active pumping of liquid water allows for stable operation with relatively low air flow rates and low air pressure and parallel cathode channel architectures. We characterize in-plane transport issues and power distributions using a three by three segmented PEMFC design. Our transient and steady state data provide insight into the dynamics and spatial distribution of flooding and flood-recovery processes. Segment-specific polarization curves reveal that the combination of a wick and an EO pump can effect a steady state, uniform current distribution for a parallel channel cathode flow field, even at low air stoichiometries (α_air = 1.5). The segmented cell measurements also reveal the mechanisms and dynamics associated with EO pump based recovery from catastrophic flooding. For most operating regimes, the EO pump requires less than 1% of the fuel cell power to recover from near-catastrophic flooding, prevent flooding, and extend the current density operation range.     

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.     

In situ water distribution measurements in a polymer electrolyte fuel cell

The overall water vapor balance and concentration distribution in the flow channels is a critical phenomenon affecting polymer electrolyte fuel cell (PEFC) performance. This paper presents, for the first time, results of a technique to measure in situ water vapor, nitrogen and oxygen distribution within the gas channels of an operating PEFC. The use of a gas chromatograph (GC) to measure high levels of water saturation directly, without dehumidification of the flow stream, is a unique aspect of this work. Following careful calibration and instrumentation, a gas chromatograph (GC) was interfaced directly to the fuel cell at various locations along the serpentine anode and cathode flow paths of a specially designed fuel cell. The 50 cm² active area fuel cell also permits simultaneous current distribution measurements via the segmented collector plate approach. The on-line GC method allows discrete measurements of the water vapor content up to a fully saturated condition about every 2 minutes. Water vapor and other species distribution data are shown for several inlet relative humidities on the anode and cathode for different cell voltages. For the thin electrolyte membranes used (51 μm), there is little functional dependence of the anode gas channel water distribution on current output. For thin membranes, this indicates that there is little gradient in the water activity between anode and cathode, indicating diffusion can offset electro-osmotic drag under these circumstances (i<0.5 A/cm²). This technique can be used for detailed studies on water distribution and transport in the PEFC.     

Humidification studies on polymer electrolyte membrane fuel cell

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

The effects of non-uniform distribution of catalyst loading on polymer electrolyte membrane fuel cell performance

The catalyst layers are the most important part of the polymer electrolyte membrane (PEM) fuel cells, and the cell performance is highly related to its structure. The gas diffusion layers (GDLs) are also the essential components of the PEM fuel cell since the reactants should pass through these layers. Model prediction shows that electrical current in catalyst layer is non-uniform, influenced by the channel-land geometry. In addition, the compression effect of GDLs and water generation due to the electrochemical reaction may cause non-uniformity in porosity and, therefore, increases the non-uniformity in reactant concentration in GDL/catalyst layer interface. Simulation results suggest that non-uniform catalyst loading distribution in the catalyst layer will improve the performance of the whole catalyst layer by diminishing the variation in current density.     

Estimation of crack propagation in polymer electrolyte membrane fuel cell under vibration conditions

In transportation applications, the main reasons of mechanical damage in polymer electrolyte membrane fuel cell (PEMFC) are road-induced vibrations and impact loads. The most vulnerable place of these cells is the interface between membrane and catalyst layer in the membrane electrode assembly (MEA). Hence, studies on mechanical strength of PEMFC should focus on that interface. The objective of present study lies in the fact that employing a prediction method to investigate the damage propagation behavior of vibration applied PEMFC using artificial neural network (ANN). The data available in the literature are used to constitute an ANN model. Three-layer model; input, hidden and output, are used for construction of ANN structure. Initial delamination length (a), amplitude (A), frequency (ω) and time (t) are used as input neurons whereas delamination length is output. Levenberg–Marquardt algorithm is selected as learning algorithm. On the other hand, number of hidden layer neuron is decided with the use of different neuron numbers by trial and error method. It is concluded that prediction capability of ANN model is in allowable limits and model can be suggested as efficient way of delamination length estimation.     

Nafion nanofibers and their effect on polymer electrolyte membrane fuel cell performance

Current fuel cell research is focused on reducing manufacturing costs by reducing platinum catalyst loading without sacrificing performance. Although improvements have been demonstrated by using platinum supported on porous carbon nanoparticles, significant losses in “active” platinum surface area within the catalyst layer (CL) still occur. Optimizing the reactant gas/Nafion^®/platinum triple phase boundary (TPB) in the CL (i.e., CL morphology) will result in increased “active” catalyst area and overall fuel cell performance. In this study, the effect of temperature on the formation of Nafion^® nanofibers in the CL during fuel cell operation and its subsequent improvement on fuel cell performance was clearly characterized. Post mortem scanning electron micrographs clearly show that Nafion^® nanofibers improve the TPB, where Nafion^® nanofibers act as a more efficient proton transport route from the catalyst particles to the polymer electrolyte membrane reducing ohmic and mass transport resistance.     

Effect of alcohol content on the ionomer adsorption of polymer electrolyte membrane fuel cell catalysts

Catalyst layers (CL) composed of catalyst composites and an ionomer are key components in polymer electrolyte membrane fuel cells (PEMFCs). In particular, the preparation conditions of the CL, starting from the dispersion of the catalyst composite dispersion with an ionomer, largely affect the PEMFC performance. In this study, the effects of alcohol content in the dispersion solvent were investigated using two binary mixtures composed of water and ethanol. In addition, Pt-loaded carbon black (CB) and Pt-loaded polymer-wrapped CB were used as the catalyst composites to study the effects of the alcohol contents on the interaction between ionomer and surface of the carbon supports. The CL prepared using the water-rich (80 wt% water) solvent achieved a higher PEMFC performance compared to that using the alcohol-rich (13 wt% water) solvent, which is ascribed to the stronger interaction between the ionomer and CB surface under water-rich conditions. Using the polymer-wrapped CB, the difference of the PEMFC performance between the CLs from the water-rich and alcohol-rich dispersions was minimal because of the comparable interaction between the ionomer and wrapping polymer surface in both solvents. Therefore, the control of the interaction between the ionomer and catalyst composites is crucial to controlling the PEMFC performance.     

Development of polymer electrolyte membrane fuel cell stack

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

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.     

The effect of diffusioosmosis on water transport in polymer electrolyte fuel cells

An analytical study of the effect of diffusioosmosis caused by the concentration gradient of hydrogen ions on the isothermal transport of water in a fully hydrated membrane of a polymer electrolyte fuel cell (PEFC) is presented. A capillary tube or slit with a negatively charged wall is chosen to model the nanopores of the membrane. The electric double layer adjacent to the capillary wall may have an arbitrary thickness relative to the capillary radius and its electrostatic potential distribution is determined as the solution of the Poisson–Boltzmann equation. Solving a modified Navier–Stokes equation, the fluid velocity in the axial direction of the capillary induced by the macroscopic electric field and protonic concentration gradient is obtained as a function of the radial position in closed forms. The results for the local and averaged electrokinetic velocities in the capillary show that the effect of diffusioosmosis on the water transport in the membrane of a PEFC can be significant in comparison with that of electroosmosis under low-potential-difference operations.     

Calculations of transport phenomena and reaction distribution in a polymer electrolyte membrane fuel cell

The performance of Polymer Electrolyte Membrane fuel cells depends on the design of the cell as well as the operating conditions. The design of the cell influences the complex interaction of activation effects, ohmic losses, and transport limitations, which in turn determines the local current density. Detailed models of the electrochemical reactions and transport phenomena in Polymer Electrolyte Membrane fuel cells can be used to determine the current density distribution for a given fuel cell design and operating conditions. In this work, three-dimensional, multicomponent and multiphase transport calculations are performed using a computational fluid dynamics code. The computational results for a full-scale fuel cell design show that ohmic effects due to drying of polymer electrolyte in the anode catalyst layer and membrane, and transport limitations of air and flooding in the cathode cause the current density to be a maximum near the gas channel inlets where ohmic losses and transport limitations are a minimum. Elsewhere in the cell, increased ohmic losses and transport limitations cause a decrease in current density, and the performance of the fuel cell is significantly lower than that which could be attained if the ohmic losses and transport limitations throughout the cell were the same as those near the gas channel inlets. Thus overall fuel cell design is critical in maximizing unit performance.     

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.     

Capillaries for water management in polymer electrolyte membrane fuel cells

Some of the new liquid water management systems in polymer electrolyte membrane (PEM) fuel cells hold great potential in providing flood-free performance and internal humidification. However, current water management systems entail major setbacks, which either inhibit implementation into state-of-the-art architectures, such as stamped metal flow-fields, or restrict their application to certain channel configurations. Here, a novel water management strategy is presented that uses capillary arrays to control liquid water in PEMFCs. These capillaries are laser-drilled into the land of the flow-fields and allow direct removal (wicking) or supply of water (evaporation), depending on the local demand across the electrode. For a 6.25 cm² active area parallel flow-field, a ～92% improvement in maximum power density from capillary integration was demonstrated. The proposed mechanism serves as a simple and effective means of achieving robust and reliable fuel cell operation, without incurring additional parasitic losses due to the high pressure drop associated with conventional serpentine flow-fields.     

Water transport characteristics of polymer electrolyte membrane fuel cell

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

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.     

Modeling of flow field in polymer electrolyte membrane fuel cell

Isothermal two- and three-dimensional polymer electrolyte membrane (PEM) fuel cell cathode flow field models were implemented to study the behavior of reactant and reaction product gas flow in a parallel channel flow field. The focus was on the flow distribution across the channels and the total pressure drop across the flow field. The effect of the density and viscosity variation in the gas resulting from the composition change due to cell reactions was studied and the models were solved with governing equations based on three different approximations. The focus was on showing how a uniform flow profile can be achieved by improving an existing channel design. The modeling results were verified experimentally. A close to uniform flow distribution was achieved in a parallel channel system.     

In situ measurement of temperature distribution within a single polymer electrolyte membrane fuel cell

A novel test scheme for in situ measurement of temperature within a single polymer electrolyte membrane fuel cell (PEMFC) is proposed, which possesses the following attractive features: measuring interference with the internal environment of the fuel cell is likely reduced to minimum; simultaneous measurements for local temperatures of both sides of the fuel cell are conducted with enough numbers of measurement locations; and the cell temperatures are controlled in relatively careful and stringent strategies. Thermal and electrical behaviors of the cell tested are investigated, including the local and averaged temperatures at the back sides of cathode and anode flow field plates (FFPs), the outlet currents, and their variations with the test time. It is found that both temperatures and outlet currents exhibit complex dynamic behaviors; and the rise of temperature and the non-uniformity of temperature distribution of the back sides of the two FFPs are not negligible.