Predicting current density distribution of proton exchange membrane fuel cells with different flow field designs

In a proton exchange membrane fuel cell (PEMFC), flow field design is an important factor that influences the distributions of current density and water accumulation. The segmented model developed in prior study is used to investigate the effect of flow field patterns on current density distribution. This model predicts the distributed characteristics of water content in the membrane, relative humidity in the flow channels, and water accumulation in the gas diffusion layers (GDLs).Three single cells with different flow field patterns are designed and fabricated. These three flow field designs are simulated using the segmented model and the predicted results are compared and validated by experimental data. This segmented model can be used to predict the effect of flow field patterns on water and current distributions before they are machined.     

Simultaneous measurement of current and temperature distributions in a proton exchange membrane fuel cell

Using a specially designed current distribution measurement gasket in anode and thin thermocouples between the catalyst layer and gas diffusion layer (GDL) in cathode, in-plane current and temperature distributions in a proton exchange membrane fuel cell (PEMFC) have been simultaneously measured. Such simultaneous measurements are realized in a commercially available experimental PEMFC. Experiments have been conducted under different air flow rates, different hydrogen flow rates and different operating voltages, and measurement results show that there is a very good correlation between local temperature rise and local current density. Such correlations can be explained and agree well with basic thermodynamic analysis. Measurement results also show that significant difference exists between the temperatures at cathode catalyst layer/GDL interface and that in the center of cathode endplate, which is often taken as the cell operating temperature. Compared with separate measurement of local current density or temperature, simultaneous measurements of both can reveal additional information on reaction irreversibility and various transport phenomena in fuel cells.     

Study of the cell reversal process of large area proton exchange membrane fuel cells under fuel starvation

In this research, the fuel starvation phenomena in a single proton exchange membrane fuel cell (PEMFC) are investigated experimentally. The response characteristics of a single cell under the different degrees of fuel starvation are explored. The key parameters (cell voltage, current distribution, cathode and anode potentials, and local interfacial potentials between anode and membrane, etc.) are measured in situ with a specially constructed segmented fuel cell. Experimental results show that during the cell reversal process due to the fuel starvation, the current distribution is extremely uneven, the local high interfacial potential is suffered near the anode outlet, hydrogen and water are oxidized simultaneously in the different regions at the anode, and the carbon corrosion is proved to occur at the anode by analyzing the anode exhaust gas. When the fuel starvation becomes severer, the water electrolysis current gets larger, the local interfacial potential turns higher, and the carbon corrosion near the anode outlet gets more significant. The local interfacial potential near the anode outlet increases from ca. 1.8 to 2.6 V when the hydrogen stoichiometry decreases from 0.91 to 0.55. The producing rate of the carbon dioxide also increases from 18 to 20 ml min⁻¹.     

The conception of in-plate adverse-flow flow field for a proton exchange membrane fuel cell

To improve species concentration and current density distribution uniformity of a proton exchange membrane (PEM) fuel cell, an in-plate adverse-flow (IPAF) flow field is developed. Its utility is conceptually examined through three-dimensional numerical simulation comparison between three typical fuel and air flow combinations out of those it can support. Under isothermal condition and constant velocity reactant feeding mode, as the simulation results indicate, there is no significant cell performance improvement by the new flow filed unless in mass transport limited region, while the species concentration and current density distribution uniformities are substantially improved. As data analysis supports, there are two mechanisms in the new flow field that are responsible for the distribution uniformity improvement: the along-channel offset effect and the across-rib transport effect, and their respective pure contributions to the improvement are well discerned.     

Contamination assessment in metal foam flow field-based proton exchange membrane fuel cell

The relative slippage between the open-cell metal foam flow fields and other parts in a fuel cell due to vehicular and flow-induced vibrations causes fretting. The material degradation due to fretting in nickel struts contaminate the stack and is investigated using simulated experiments. The as-formed strut surfaces are rough and increases the material loss during fretting. The total wear volume associated with a single contact in 22,000 cycles is 4.66 E?04 mm³. The contamination in the stack is estimated assuming a dodecahedron unit cell geometry and neglecting the fretting corrosion. About 47 g of debris is expected to be generated when an 8 ppi nickel foam flow fields used in a 50-cell stack for 8700 hours of operation. In addition, the generated flake shaped debris (<10 μm) can obstruct the flow of gases by clogging the gas diffusion layer. The proposed contamination estimation methodology will aid in performance prediction during service.     

Direct measurement of current density under the land and channel in a PEM fuel cell with serpentine flow fields

One of the most common types of flow field designs used in proton exchange membrane (PEM) fuel cells is the serpentine flow field. It is used for its simplicity of design, its effectiveness in distributing reactants and its water removal capabilities. The knowledge about where current density is higher, under the land or the channel, is critical for flow field design and optimization. Yet, no direct measurement data are available for serpentine flow fields. In this study a fuel cell with a single channel serpentine flow field is used to separately measure the current density under the land and channel, which is either catalyzed or insulated on the cathode. In this manner, a systematic study is conducted under a wide variety of conditions and a series of comparisons are made between land and channel current density. The results show that under most operating conditions, current density is higher under the land than that under the channel. However, at low voltage, a rapid drop off in current density occurs under the land due to concentration losses. The mechanisms for the direct measurement results and general guidelines for serpentine flow field design and optimizations are provided.     

Effects of serpentine flow field with outlet channel contraction on cell performance of proton exchange membrane fuel cells

A serpentine flow field with outlet channels having modified heights or lengths was designed to improve reactant utilization and liquid water removal in proton exchange membrane (PEM) fuel cells. A three-dimensional full-cell model was developed to analyze the effects of the contraction ratios of height and length on the cell performance. Liquid water formation, that influences the transport phenomena and cell performance, was included in the model. The predictions show that the reductions of the outlet channel flow areas increase the reactant velocities in these regions, which enhance reactant transport, reactant utilization and liquid water removal; therefore, the cell performance is improved compared with the conventional serpentine flow field. The predictions also show that the cell performance is improved by increments in the length of the reduced flow area, besides greater decrements in the outlet flow area. If the power losses due to pressure drops are not considered, the cell performance with the contracted outlet channel flow areas continues to improve as the outlet flow areas are reduced and the lengths of the reduced flow areas are increased. When the pressure losses are also taken into account, the optimal performance is obtained at a height contraction ratio of 0.4 and a length contraction ratio of 0.4 in the present design.     

Dynamic characteristics of local current densities and temperatures in proton exchange membrane fuel cells during reactant starvations

Reactant starvation during proton exchange membrane fuel cell (PEMFC) operation can cause serious irreversible damages. In order to study the detailed local characteristics of starvations, simultaneous measurements of the dynamic variation of local current densities and temperatures in an experimental PEMFC with single serpentine flow field have been performed during both air and hydrogen starvations. These studies have been performed under both current controlled and cell voltage controlled operations. It is found that under current controlled operations cell voltage can decrease very quickly during reactant starvation. Besides, even though the average current is kept constant, local current densities as well as local temperatures can change dramatically. Furthermore, the variation characteristics of local current density and temperature strongly depend on the locations along the flow channel. Local current densities and temperatures near the channel inlet can become very high, especially during hydrogen starvation, posing serious threats for the membrane and catalyst layers near the inlet. When operating in a constant voltage mode, no obvious damaging phenomena were observed except very low and unstable current densities and unstable temperatures near the channel outlet during hydrogen starvation. It is demonstrated that measuring local temperatures can be effective in exploring local dynamic performance of PEMFC and the thermal failure mechanism of MEA during reactants starvations.     

Behaviors of proton exchange membrane fuel cells under oxidant starvation

Durability is an important issue in proton exchange membrane fuel cells (PEMFCs) currently. Reactant starvation could be one of the reasons for PEMFC degradation. In this research, the oxidant starvation phenomena in a single cell are investigated. The local interfacial potential, current and temperature distribution are detected in situ with a specially constructed segmented cell. Experimental results show that during the cell reversal process due to oxidant starvation, the local interfacial potential in the oxidant inlet keeps positive while that of the middle and outlet regions become negative, which illustrates that oxygen and proton reduction reactions could occur simultaneously in different regions at the cathode. The current distribution would be more uneven with decreasing air stoichiometry before cell reversal. When cell reversal occurs, the current will redistribute and the current distribution tends more uniform. At the critical point of cell reversal, the most significant inhomogeneity in the current distribution can be observed. The temperature distribution in the cell is also monitored on-line. The local hot spot exists in the cell when cell reversal occurs. The study of the critical reversal air stoichiometry under different loads shows that the critical reversal air stoichiometry increases with the rising loads.     

Three-dimensional two-phase flow model of proton exchange membrane fuel cell with parallel gas distributors

A non-isothermal, steady-state, three-dimensional (3D), two-phase, multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with parallel gas distributors. A key feature of this work is that a detailed membrane model is developed for the liquid water transport with a two-mode water transfer condition, accounting for the non-equilibrium humidification of membrane with the replacement of an equilibrium assumption. Another key feature is that water transport processes inside electrodes are coupled and the balance of water flux is insured between anode and cathode during the modeling. The model is validated by the comparison of predicted cell polarization curve with experimental data. The simulation is performed for water vapor concentration field of reactant gases, water content distribution in the membrane, liquid water velocity field and liquid water saturation distribution inside the cathode. The net water flux and net water transport coefficient values are obtained at different current densities in this work, which are seldom discussed in other modeling works. The temperature distribution inside the cell is also simulated by this model.     

Parallel serpentine-baffle flow field design for water management in a proton exchange membrane fuel cell

In a proton exchange membrane fuel cell (PEMFC) water management is one of the critical issues to be addressed. Although the membrane requires humidification for high proton conductivity, water in excess decreases the cell performance by flooding. In this paper an improved strategy for water management in a fuel cell operating with low water content is proposed using a parallel serpentine-baffle flow field plate (PSBFFP) design compared to the parallel serpentine flow field plate (PSFFP). The water management in a fuel cell is closely connected to the temperature control in the fuel cell and gases humidifier. The PSBFFP and the PSFFP were evaluated comparatively under three different humidity conditions and their influence on the PEMFC prototype performance was monitored by determining the current density–voltage and current density–power curves. Under low humidification conditions the PEMFC prototype presented better performance when fitted with the PSBFFP since it retains water in the flow field channels.     

Matching of water and temperature fields in proton exchange membrane fuel cells with non-uniform distributions

In this work, a three-dimensional multiphase non-isothermal model incorporated with a capillary-extended sub-model in gas channels is used to investigate the coupled phenomena of water and thermal transport in proton exchange membrane fuel cells. Distributions of water and temperature along the flow path in the channel are highlighted and the pros and cons of various operating temperatures are elaborated. In addition, this work also sheds light on the impacts of temperature variations of bipolar plates induced by non-uniform cooling conditions, which have been overlooked by most previous works. An important phenomenon of water distribution, dry-out at inlets and flooding at outlets (DIFO), is observed and this non-uniform distribution is revealed to be greatly influenced by the operating temperature, inlet relative humidity and gas flow stoichiometry. Moreover, temperature variations of bipolar plates are shown to exert remarkable impacts on water distribution. Consequently, optimum matching between water and temperature fields is proposed to be of vital importance in fuel cell design, e.g., strong cooling at the inlet and weak cooling at the outlet are demonstrated to be a feasible way of mitigating the problem of DIFO.     

Behavior of a unit proton exchange membrane fuel cell in a stack under fuel starvation

Durability is an important issue in proton exchange membrane fuel cells (PEMFCs) currently. Fuel starvation could be one of the reasons for PEMFC degradation. In this research, the fuel starvation conditions of a unit cell in a stack are simulated experimentally. Cell voltage, current distribution and localized interfacial potentials are detected in situ to explore their behaviors under different hydrogen stoichiometries. Results show that the localized fuel starvation occurs in different sections at anode under different hydrogen stoichiometries when the given hydrogen is inadequate. This could be attributed to the “vacuum effect” that withdraws fuel from the manifold into anode. Behaviors of current distribution show that the current will redistribute and the position of the lowest current shifts close to the anode inlet with decreasing hydrogen stoichiometry, which indicates that the position of the localized fuel starvation would move towards the inlet of the cell. It is useful to understand the real position of the degradation of MEA.     

Microstructured membranes for improving transport resistances in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) have been identified as one of the most promising renewable energy system for use in automotive applications. However, due to the wide range of weather conditions around the world, the PEMFCs must be stable for operating under these variable conditions. One of the inefficiencies of PEMFCs in automotive applications is during vehicle warm-up, where the low hydration level within the PEMFC can lead to a low performance of the fuel cell. In this study, a proton exchange membrane (PEM) was prepared with regular, microstructured features tuned over a range of aspect ratios. These microstructured membranes were incorporated into MEAs and analyzed for their membrane, proton, and oxygen transport resistances. These fuel cells were tested under different conditions to simulate vehicle start-up, normal operating conditions, and hot operating conditions. It was determined that microstructured PEMs improved performance over planar PEMs under both the start-up and hot conditions. Despite the improved performance of the microstructured PEMs, a high hydrogen cross-over and short-circuit current were also observed for these samples. Adjusting the preparation techniques and tuning the dimensions of the microstructures may provide avenues for further optimization of PEMFC performance.     

Channel aspect ratio effect for serpentine proton exchange membrane fuel cell: Role of sub-rib convection

A complete three-dimensional, two-phase, non-isothermal model for proton exchange membrane (PEM) fuel cells was used to investigate the effect of the sub-rib convection on the performances for the single and triple serpentine flow fields at various channel aspect ratios and different thermal constraints. The occurrence of sub-rib convection, which is affected by the serpentine flow field, significantly influences the cell performance if the oxygen supply or membrane moisture content was limited. For single serpentine flow field in which sub-rib convection presents under all ribs, changing channel aspect ratio has minimal effects on cell performance since the oxygen supply is sufficient. For triple serpentine flow field or for serpentine cell with poor external heat loss, owing to limited sub-rib convection or to low membrane moisture content, decrease in channel aspect ratio significantly enhances cell performance. Blocking up the sub-rib convection markedly reduces cell performance. Flow field design for PEM fuel cell should take into consideration the effects of sub-rib convection flow on cell performance.     

Recent advances in designing and tailoring nanofiber composite electrolyte membranes for high-performance proton exchange membrane fuel cells

As the critical component of proton exchange membrane fuel cell (PEMFC), proton exchange membrane (PEM) determine its overall performance. Current PEMs hardly meet the operating requirements of fuel cells, limiting their commercial applications. With the development of nanocomposite technology, nanofibers introduced into PEMs to prepare nanofiber composite proton exchange membranes (NCPEMs) have been widely studied. In an NCPEM, nanofibers can form long-range channels for proton transport, and reinforced skeleton to reach the target performance of PEMFCs. Focusing on NCPEM, this paper reviews on recent progresses in nanofiber preparation and NCPEM preparation techniques. Furthermore, different types of nanofibers incorporated into NCPEMs are reviewed in terms of fiber composition. The challenges and future perspectives regarding NCPEMs are also discussed.     

Cross-linked PEEK-WC proton exchange membrane for fuel cell

The low cost proton exchange membrane was prepared by cross-linking water soluble sulfonated-sulfinated poly(oxa-p-phenylene-3,3-phthalido-p-phenylene-oxa-p-phenylene-oxy-phenylene) (SsPEEK-WC). The prepared cross-linked membrane became insoluble in water, and exhibited high proton conductivity, 2.9 × 10⁻² S/cm at room temperature. The proton conductivity was comparable with that of Nafion^® 117 membrane (6.2 × 10⁻² S/cm). The methanol permeability of the cross-linked membrane was 1.6 × 10⁻⁷ cm²/s, much lower than that of Nafion^® 117 membrane.     

Planar current collector design and fabrication for proton exchange membrane fuel cell

This paper proposes a novel planar type lightweight current collector for proton exchange membrane fuel cells (PEMFC) designed for low power portable applications. The proposed lightweight current collector, which is composed of a substrate, electrical conduction layer and corrosion resistance layer, combines the conventional metal sheet/mesh for current collecting and substrate together to reduce the possible distortion during operation caused by the mismatch due to large different mechanical properties between components. The current collector adopts FR-4 as the substrate material. The electrical conduction layer is made via coating a copper thin film using a thermo-evaporation layer. The corrosion resistance layer is made via coating a graphene thin film using spin coating and a vacuum oven process. Fabricated current collector sheet resistance measurements are conducted. The complete current collectors are assembled into a single cell PEMFC with both forced convection air-breathing cathode and self-air-breathing cathode. The related performance and stability experiments were conducted to investigate the feasibility for further applications.     

Dynamic modeling and simulation of air-breathing proton exchange membrane fuel cell

Small fuel cells have shown excellent potential as alternative energy sources for portable applications. One of the most promising fuel cell technologies for portable applications is air-breathing fuel cells. In this paper, a dynamic model of an air-breathing PEM fuel cell (AB-PEMFC) system is presented. The analytical modeling and simulation of the air-breathing PEM fuel cell system are verified using Matlab, Simulink and SimPowerSystems Blockset. To show the effectiveness of the proposed AB-PEMFC model, two case studies are carried out using the Matlab software package. In the first case study, the dynamic behavior of the proposed AB-PEMFC system is compared with that of a planar air-breathing PEM fuel cell model. In the second case study, the validation of the air-breathing PEM fuel cell-based power source is carried out for the portable application. Test results show that the proposed AB-PEMFC system can be considered as a viable alternative energy sources for portable applications.     

Novel serpentine-baffle flow field design for proton exchange membrane fuel cells

An appropriate flow field in the bipolar plates of a fuel cell can effectively enhance the reactant transport rates and liquid water removal efficiency, improving cell performance. This paper proposes a novel serpentine-baffle flow field (SBFF) design to improve the cell performance compared to that for a conventional serpentine flow field (SFF). A three-dimensional model is used to analyze the reactant and product transport and the electrochemical reactions in the cell. The results show that at high operating voltages, the conventional design and the baffled design have the same performance, because the electrochemical rate is low and only a small amount of oxygen is consumed, so the oxygen transport rates for both designs are sufficient to maintain the reaction rates. However, at low operating voltages, the baffled design shows better performance than the conventional design. Analyses of the local transport phenomena in the cell indicate that the baffled design induces larger pressure differences between adjacent flow channels over the entire electrode surface than does the conventional design, enhancing under-rib convection through the electrode porous layer. The under-rib convection increases the mass transport rates of the reactants and products to and from the catalyst layer and reduces the amount of liquid water trapped in the porous electrode. The baffled design increases the limiting current density and improves the cell performance relative to conventional design.