Passive control of liquid water storage and distribution in a PEFC through flow-field design

Liquid water stored in the diffusion media (DM) in a polymer electrolyte fuel cell (PEFC) can dramatically impact steady and transient performance, degradation, and heat transfer. In this study, seven different flow-field designs, with landing-to-channel (L:C) ratio from 1:3 to 2:1, were investigated at dry and fully humidified conditions, using neutron imaging. The results revealed the impact of flow-field geometry on stored liquid overhead is significant. In some cases, the stored water content in the cell can be nearly double that of another design, despite similar performances at low to medium current density. In general, a smaller L:C ratio reduces flooding losses and minimizes the stored water content. Additionally, the channel–DM interface plays a key role. For the same L:C ratio, a reduced number of channel–DM interfaces was shown to reduce flooding and stored liquid water content at steady state. This also suggests that using proper flow-field design can decrease the parasitic power consumption and the stored water content in the cell without any sacrifice from the cell performance. For dryer operating conditions, however, membrane dehydration becomes a dominant effect and a high landing-to-channel ratio flow-field is higher performing.     

Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

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.     

An adiabatic cell as a model for stack in PEFC cold start study

Inadequate cold start capability impedes the diffusion of fuel cell vehicles in cold regions. While the isothermal cell commonly used in literature is valuable in examining failure mechanism, it is not suitable for exploring startup strategy and the associated degradation of polymer electrolyte fuel cells (PEFCs). A model cell capable of simulating the adiabatic thermal boundary conditions of central cells inside the stack is needed. Herein, we design an adiabatic cell as a model for the central cells in PEFC stack. The adiabatic thermal boundary condition is realized by utilizing thermal adjustment boards, thermocouples, and heating plates. The power of heating plates is adjusted by a feedback proportion-integration-differentiation controller in real time. The system is shown to be capable of providing an adiabatic thermal boundary condition with quick response rate and having no risk of overheating in successful starts with two kinds of strategies. This cell can work as a model system for reproducible and well-controlled study of cold start of PEFC stack.     

Monodimensional modeling and experimental study of the dynamic behavior of proton exchange membrane fuel cell stack operating in dead-end mode

The dynamic behavior of a five cells proton exchange membrane fuel cell (PEMFC) stack operating in dead-end mode has been studied at room temperature, both experimentally and by simulation. Its performances in “fresh” and “aged” state have been compared. The cells exhibited two different response times: the first one at about 40 ms, corresponding to the time needed to charge the double-layer capacitance, and the second one at about 15–20 s. The first time response was not affected by the ageing process, despite the decrease of the performances, while the second one was. Our simulations indicated that a high amount of liquid water was present in the stack, even in “fresh” state. This liquid water is at the origin of the performances decrease with ageing, due to its effect on decreasing the actual GDL porosity that in turn cause the starving of the active layer with oxygen. As a consequence, it appears that water management issue in a fuel cell operating in dead-end mode at room temperature mainly consists in avoiding pore flooding instead of providing enough water to maintain membrane conductivity.     

The impact of rib structure on the water transport behavior in gas diffusion layer of polymer electrolyte membrane fuel cells

Using the multiphase lattice Boltzmann method (LBM), the liquid water transport dynamics is simulated in a gas diffusion layer (GDL) of polymer electrolyte membrane fuel cells (PEMFCs). The effect of rib structure on the water invasion process in the micro-porous GDL is explored by comparing the two cases, i.e., with rib and without rib structures. The liquid water distribution and water saturation profile are presented to determine the wetting mechanism in the GDL. The results show that the liquid water transport in the GDL is strongly governed by capillary force and the rib structure plays a significant role on water distribution and water transport behavior in the GDL. Comparison of two cases confirms that the rib structure influences on the location of water breakthrough. The liquid water distribution and water saturation profile indicate that the high resistance force underneath the rib suppresses the growth of water cluster, resulting in the change of flow path. After water breakthrough, the liquid water distribution under the channel has little variation, whereas that under the rib continues to change. The predicted value of effective permeability is in good agreement with Carman-Kozeny correlation and experimental results in the literature. The results suggest that the LBM approach is an effective tool to investigate the water transport behavior in the GDL.     

Experimental study on enhancing the fuel efficiency of an anodic dead-end mode polymer electrolyte membrane fuel cell by oscillating the hydrogen

This paper investigates how to improve the fuel efficiency of an anodic dead-end mode fuel cell for portable power generation. Generally, a periodic purge process in anodic dead-end operation is required to avoid anode flooding caused by back diffusive water from the cathode. However, during the purge process, small amounts of the hydrogen are discharged with the water, lowering the fuel utilization efficiency. Therefore, hydrogen pulsations are introduced and experimental attempt to minimize the purge frequency is conducted in this study. The experimental results indicate that pulsation reduces partial pressure of the water vapor in the anode channel, increasing the interval between purges by approximately three times, thus improving overall efficiency.     

Influence of current densities in SOFC and PEFC stacks on a SOFC–PEFC combined system

A solid oxide fuel cell (SOFC)–polymer electrolyte fuel cell (PEFC) combined system was investigated by numerical simulation. Here, the effect of the current densities in the SOFC and the PEFC stacks on the system's performance is evaluated under a constant fuel utilization condition. It is shown that the SOFC–PEFC system has an optimal combination of current densities, for which the electrical efficiency is highest. The optimal combination exists because the cell voltage in one stack increases and that of the other stack decreases when the current densities are changed. It is clarified that there is an optimal size of the PEFC stack in the parallel-fuel-feeding-type SOFC–PEFC system from the viewpoint of efficiency, although a larger PEFC stack always leads to higher electrical efficiency in the series-fuel-feeding-type SOFC–PEFC system. The 40 kW-class PEFC stack is suitable for the 110 kW-class SOFC stack in the parallel-fuel-feeding type SOFC–PEFC system.     

A study on energy saving in residential PEFC cogeneration systems

We have been developing technologies for energy saving in the residential sector. Recently, we have been concentrating our resources specifically into the development of polymer electrolyte fuel cell (PEFC) cogeneration systems. The system has excellent energy saving characteristics. However, the total amount of energy saved depends on how the system is operated. The characteristics of residential energy consumption are more complicated than in industrial use and depend on individual living patterns. It is therefore not easy to develop a control method for a system that can be generally applied across a wide variety of residential use.In this paper, we propose a system configuration and operation planning method developed for a residential PEFC cogeneration system. Using an operation planning method we developed, we demonstrate that our system provides higher energy savings than the conventional method. The energy saving rates are 15.9% under a large heat demand, 18.4% under a relatively high electrical demand and low heat demand, 1.3% under relative low electrical and heat demands.     

Modeling of water transport through the membrane electrode assembly for direct methanol fuel cells

In this work, a one-dimensional, isothermal two-phase mass transport model is developed to investigate the water transport through the membrane electrode assembly (MEA) for liquid-feed direct methanol fuel cells (DMFCs). The liquid (methanol–water solution) and gas (carbon dioxide gas, methanol vapor and water vapor) two-phase mass transport in the porous anode and cathode is formulated based on classical multiphase flow theory in porous media. In the anode and cathode catalyst layers, the simultaneous three-phase (liquid and vapor in pores as well as dissolved phase in the electrolyte) water transport is considered and the phase exchange of water is modeled with finite-rate interfacial exchanges between different phases. This model enables quantification of the water flux corresponding to each of the three water transport mechanisms through the membrane for DMFCs, such as diffusion, electro-osmotic drag, and convection. Hence, with this model, the effects of MEA design parameters on water crossover and cell performance under various operating conditions can be numerically investigated.     

An experimental investigation of electro-osmotic drag coefficients in a polymer electrolyte membrane fuel cell

Through the use of a water balance experiment, the electro-osmotic drag coefficients of Nafion 115 were obtained under several conditions (as a function of water content and thermodynamics conditions). For the cases when the anode was fully hydrated (corresponding to water content λ ≈ 14 in the adjacent membrane) and the cathode suffered from drying when dry air was supplied (λ ≈ 2), the electro-osmotic drag coefficients varied from 0.82 (±0.06) to 0.50 (±0.03) H₂O/H⁺ when the current density varied from 0.4 to 1.0 A cm⁻² (95% confidence level). When the current density increased, the electro-osmotic drag coefficient decreased. When the water content at the anode increased from λ ≈ 5 to λ ≈ 14, the cathode was supplied with dry air (λ ≈ 2), and the fuel cell discharged constant current density at 0.6 A cm⁻², the electro-osmotic drag coefficient increased from 0.44 (±0.06) to 0.68 (±0.06) H₂O/H⁺ (95% confidence level). Higher relative humidity gas leads to a higher electro-osmotic drag coefficient at constant current density.     

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.     

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.     

Water transport in polymer electrolyte membrane fuel cells

Polymer electrolyte membrane fuel cell (PEMFC) has been recognized as a promising zero-emission power source for portable, mobile and stationary applications. To simultaneously ensure high membrane proton conductivity and sufficient reactant delivery to reaction sites, water management has become one of the most important issues for PEMFC commercialization, and proper water management requires good understanding of water transport in different components of PEMFC. In this paper, previous researches related to water transport in PEMFC are comprehensively reviewed. The state and transport mechanism of water in different components are elaborated in detail. Based on the literature review, it is found that experimental techniques have been developed to predict distributions of water, gas species, temperature and other parameters in PEMFC. However, difficulties still remain for simultaneous measurements of multiple parameters, and the cell and system design modifications required by measurements need to be minimized. Previous modeling work on water transport in PEMFC involves developing rule-based and first-principle-based models, and first-principle-based models involve multi-scale methods from atomistic to full cell levels. Different models have been adopted for different purposes and they all together can provide a comprehensive view of water transport in PEMFC. With the development of computational power, application of lower length scale methods to higher length scales for more accurate and comprehensive results is feasible in the future. Researches related to cold start (startup from subzero temperatures) and high temperature PEMFC (HT-PEMFC) (operating at the temperatures higher than 100 °C) are also reviewed. Ice formation that hinders reactant delivery and damages cell materials is the major issue for PEMFC cold start, and enhancing water absorption by membrane electrolyte and external heating have been identified as the most effective ways to reduce ice formation and accelerate temperature increment. HT-PEMFC that can operate without liquid water formation and membrane hydration greatly simplifies water management strategy, and promising performance of HT-PEMFC has been demonstrated.     

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.     

A generalized multiphase mixture (M2) model for PEFC flow field design

A lot of effort has gone into designing an optimum flow field for PEFC (Polymer Electrolyte Fuel Cell) that can both efficiently distribute reactants to the reactions sites and remove products through the outlet. Presence of liquid water in the products has been one of the main concerns. Unfortunately, single phase flow solutions have been considered for most of the design optimization studies due to the unavailability of a fast and accurate two-phase flow model. Recently a Multiphase-Mixture (M2) based model has been developed for two-phase flow computations in the cathode channels of a PEFC. This model has now been extended to the anode side. A drawback of implementing this mvodel is that it requires an orthogonal hexahedral mesh which in a real PEFC stack geometry is very difficult to achieve. In this study the model has been extended to non-orthogonal hexahedral and tetrahedral meshes, which can be used to mesh any three-dimensional geometry. Also, in order to reduce the meshing effort, an immersed body approach has been tested successfully on this model. The resulting two-phase flow model valid for arbitrary flow field geometries is fast and accurate and a possible direction to reduce the meshing effort is presented.     

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.     

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.     

Performance evaluation of a polymer electrolyte fuel cell with a dead-end anode: A computational fluid dynamic study

The operation of polymer electrolyte fuel cell (PEFC) with a dead-end anode requires careful gas and water management to achieve optimal operating performance. The amount of water accumulated in the anode and nitrogen crossover are particularly important factors. To ascertain (i) the behavior of a PEFC with a dead-end anode, (ii) the accumulation of water and nitrogen in the anode cell with time, and (iii) efficient purging strategies to manage the gas and water, a transient PEFC model with a dead-end anode was developed and analyzed. The model assumes a two-phase flow and solves the governing equations of conservation of mass, momentum, species, energy, charge, coupled with a phenomenological membrane model and agglomerate model for catalyst layer. The model results indicate that water and nitrogen can accumulate in the anode region with time, such that the amount of available hydrogen decreases and hence the cell performance drops. The accumulation rate is found to be closely linked to the current that is drawn from the cell. Further, it is found that to alleviate the problem of build-up of nitrogen and water, the purge frequency and duration of the purge play important roles in affecting cell performance. The transient behavior and impact of the relevant operating conditions obtained from the simulation results can be used for development of efficient purging strategies.     

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.