Effects of self-humidification on the dynamic behavior of polymer electrolyte fuel cells

The dynamic behavior of polymer electrolyte fuel cells was investigated experimentally at sudden load change conditions. The present study mainly focused on the variation of membrane hydration due to self-humidification. Steady-state results for various temperatures and humidities were used as the basic data for the analysis of dynamic behavior. Electrochemical impedance spectroscopy (EIS) showed that the ohmic resistance was reduced with the increase of humidity and current while the total polarization resistance including the mass transfer effect showed various trends according to cell temperature. The dynamic behavior of the cells was measured with time. The current increment just after an abrupt voltage reduction jumped to a certain level and then increased gradually, showing a logarithmic-shape curve. The stabilization time to steady-state was determined by using the curve-fitted lines representing the variation of the current increment at each operating condition. The stabilization time showed various trends according to cell temperature, humidity, and voltage range.     

Impedance study of membrane dehydration and compression in proton exchange membrane fuel cells

Electrochemical impedance spectroscopy (EIS) is used to measure drying and rehydration in proton exchange membrane fuel cells running under load. The hysteresis between forward and backward acquisition of polarization curves is shown to be largely due to changes in the membrane resistance. Drying tests are carried out with hydrogen and simulated reformate (hydrogen and carbon dioxide), and quasi-periodic drying and rehydration conditions are studied. The membrane hydration state is clearly linked to the high-frequency arc in the impedance spectrum, which increases in size for dry conditions indicating an increase in membrane resistance. Changes in impedance spectra as external compression is applied to the cell assembly show that EIS can separate membrane and interfacial effects, and that changes in membrane resistance dominate. Reasons for the presence of a capacitance in parallel with the membrane resistance are discussed.     

Electrochemical durability of gas diffusion layer under simulated proton exchange membrane fuel cell conditions

An effective ex-situ method for characterizing electrochemical durability of a gas diffusion layer (GDL) under simulated polymer electrolyte membrane fuel cell (PEMFC) conditions is reported in this article. Electrochemical oxidation of the GDLs are studied following potentiostatic treatments up to 96 h holding at potentials from 1.0 to 1.4 V (vs.SCE) in 0.5 mol L⁻¹ H₂SO₄. From the analysis of morphology, resistance, gas permeability and contact angle, the characteristics of the fresh GDL and the oxidized GDLs are compared. It is found that the maximum power densities of the fuel cells with the oxidized GDLs hold at 1.2 and 1.4 V (vs.SCE) for 96 h decreased 178 and 486 mW cm⁻², respectively. The electrochemical impedance spectra measured at 1500 mA cm⁻² are also presented and they reveal that the ohmic resistance, charge-transfer and mass-transfer resistances of the fuel cell changed significantly due to corrosion at high potential.     

Nonlinear frequency response analysis of dehydration phenomena in polymer electrolyte membrane fuel cells

Dehydration phenomena in a PEM fuel cell were investigated by nonlinear frequency response analysis (NFRA) in a differential H₂/H₂ cell. The linear H_1,0 spectra, which are equal to classic EIS spectra, showed not only an increase of the membrane resistance but also an increase of the anode reaction resistance, caused by dehydration leading to the decrease of the protonic conductivity of the polymer network in the catalyst layer. With this, active sites with long protonic pathes to the membrane become inactive. In order to further clarify this effect, modelling work was used. Therefore, proton transport was incorporated into an existing model of a differential H₂/H₂ cell. Finally, the key features of NFRA spectra under dehydration and CO poisoning are compared in order to discuss the suitability of NFRA for unambiguous diagnosis of PEMFC. It can be seen that while the linear spectrum is not sufficient to distinguish between both cases, the second order frequency response functions can be used for discrimination.     

In situ diagnostics for water transport in proton exchange membrane fuel cells

Proton exchange membrane fuel cells (PEMFCs) have attracted considerable attention as energy-conversion systems for future applications in vehicles and for on-site power generation. Major technical challenges exist in achieving a high cell performance over a wide range of operating conditions, such as various cell current densities, operating temperatures, and relative humidities of the supplied gases. Correct water management is critical to achieving a high power density, long-term operation, and increased robustness in PEMFCs. Aspects such as the swelling of the membrane by water, the generation and accumulation of liquid water inside the fuel cells, and the discharge of accumulated water need to be clarified to ensure a fundamental understanding of water transport in PEMFCs. In this article, we examine the state of art regarding in situ diagnostics, particularly visualization techniques, for probing the behaviour of water in PEMFCs, with attention to neutron radiography, X-ray imaging, magnetic resonance imaging, and optical visualization techniques. The recent rapid development of in situ imaging techniques with high spatial and temporal resolutions provides a novel platform for the development of PEMFCs.     

Effects of a microporous layer on the performance degradation of proton exchange membrane fuel cells through repetitive freezing

The gas diffusion layer (GDL) covered with a microporous layer (MPL) is being widely used in proton exchange membrane fuel cells (PEMFCs). However, the effect of MPL on water transport is not so clear as yet; hence, many studies are still being carried out. In this study, the effect of MPL on the performance degradation of PEMFCs is investigated in repetitive freezing conditions. Two kinds of GDL differentiated by the existence of MPL are used in this experiment. Damage on the catalyst layer due to freezing takes place earlier when GDL with MPL is used. More water in the membrane and catalyst layer captured by MPL causes permanent damage on the catalyst layer faster. More detailed information about the degradation is obtained by electrochemical impedance spectroscopy (EIS). From the point of view that MPL reduces the ohmic resistance, it is effective until 40 freezing cycles, but has no more effect thereafter. On the other hand, from the point of view that MPL enhances mass transport, it delays the increase in the mass transport resistance.     

Electrochemical impedance investigation of proton exchange membrane fuel cells experienced subzero temperature

Polarization losses of the fuel cells with different residual water amount frozen at subzero temperature were investigated by electrochemical impedance spectroscopy (EIS) taking into account the ohmic resistance, charge transfer process, and oxygen mass transport. The potential-dependent impedance before and after eight freeze/thaw cycles suggested that the ohmic resistance did not change, while the change of the charge transfer resistance greatly depended on the residual water amount. Among the four cells, the mass transport resistance of the cell with the largest water amount increased significantly even at the small current density region. According to the thin film-flooded agglomerate model, the interfacial charge transfer process and oxygen mass transport within the agglomerate and through the ionomer thin film in the catalyst layer both contributed to the high frequency impedance arc. From the analysis of the Tafel slopes, the mechanism of the oxygen reduction reaction (ORR) was the same after the cells experienced subzero temperature. The agglomerate diffusion changed a little in all cells and the thin film diffusion effect was obvious for the cell with the largest residual water amount. These results indicated that the slower oxygen diffusion within the catalyst layer (CL) was the main contributor for the evident performance loss after eight freeze/thaw cycles.     

Challenges and future developments in proton exchange membrane fuel cells

Fuel cell system is an advanced power system for the future that is sustainable, clean and environmental friendly. The importance of fuel cell as the future power system is discussed in the light of future fossil fuel depletion, impending international law on green house gases control and the national renewable energy policy. The modern development of the proton exchange membrane fuel cell (PEMFC) for the last 20 years is then briefly reviewed and the current status of international and national research and development of this type of are established. The review also discuss the remaining research and development issues that still need to be resolved before these fuel cells are available for commercial application. The main thrust in PEMFC research and development is to lower the cost of the fuel cell by reduction in membrane and electrocatalyst costs. Although Europe, USA, Canada and Japan are leading fuel cell research and development as commercialization, it is not too late for Malaysia to master this technology and to apply it to niche markets in the future.     

Hydrocarbon-based electrode ionomer for proton exchange membrane fuel cells

The electrode ionomer is a key factor that significantly affects the catalyst layer morphology and fuel cell performance. Herein, sulfonated poly(arylene ether sulfone)-based electrode ionomers with polymers of various molecular weights and alcohol/water mixtures were prepared, and those comprising the alcohol/water mixture showed a higher performance than the ones prepared using higher boiling solvents, such as dimethylacetamide; this is owing to the formation of the uniformly dispersed ionomer catalyst layer. The relation between ionomer molecular weight for the same polymer structure and the sulfonation degree was investigated. Because the chain length of polymer varies with molecular weight and chain entanglement degree, its molecular weight affects the electrode morphology. As the ionomer covered the catalyst, the agglomerates formed were of different morphologies according to their molecular weight, which could be deduced indirectly through dynamic light scattering and scanning electron microscopy. Additionally, the fuel cell performance was confirmed in the current-voltage curve.     

Electrochemical impedance analysis with transmission line model for accelerated carbon corrosion in polymer electrolyte membrane fuel cells

The effects of varying the applied voltage and relative humidity of feed gases in degradation tests of polymer electrolyte membrane fuel cells (PEMFCs) were analyzed using electrochemical impedance spectroscopy (EIS). A transmission line model that considers the proton-transport resistance in the cathode catalyst layer was used to analyze impedance spectra obtained from degraded PEMFCs. As the applied cell voltage was increased from 1.3 to 1.5 V to induce accelerated degradation, the cell performance decayed significantly due to increased charge- and proton-transfer resistance. The PEMFC degradation was more pronounce at higher relative humidity (RH), i.e. 100% RH, as compared with that observed under 50% RH. Furthermore, changes in the charge transfer resistance of the electrode accompanied changes in the ionic conductivity in the PEMFC catalyst layer. Although the initial ionic and charge-transfer resistances in the catalyst layer were lower under higher RH conditions, the impedance results indicated that the performance degradation was more significant at higher water contents in the electrode due to the consequential carbon corrosion, especially when higher voltages, i.e. 1.5 V, were applied to the PEMFC single cell.     

Direct ethanol fuel cells using an anion exchange membrane

Direct ethanol fuel cells (DEFCs) with a PtRu anode and a Pt cathode were prepared using an anion exchange membrane (AEM) as an electrolyte instead of a cation exchange membrane (CEM), as in conventional polymer electrolyte fuel cells. The maximum power density of DEFCs significantly increased from 6 mW cm⁻² to 58 mW cm⁻² at room temperature and atmospheric pressure when the electrolyte membrane was changed from CEM to AEM. The anode and cathode polarization curves showed a decrease in the anode potential and an increase in the cathode potential for AEM-type DEFCs compared to CEM-type. This suggests that AEM-type DEFCs have superior catalytic activity toward both ethanol oxidation and oxygen reduction in alkaline medium than in acidic medium. The product species from the exhausted liquid from DEFCs operated at a constant current density were identified by enzymatic analysis. The main product was confirmed to be acetic acid in AEM-type, while both acetaldehyde and acetic acid were detected in 1:1 ratio in CEM-type. The anodic reaction of AEM-type DEFCs can be estimated to be the oxidation of ethanol to acetic acid via a four-electron process under these experimental conditions.     

Micro fluidic structure selection of metal mesh combinations in proton exchange membrane fuel cells for air supply enhancement

One of the most significant factors affecting the performance of a proton exchange membrane fuel cell is the flow path for the passage of air and water, which is responsible for oxygen dispersion. A three-dimensional fine mesh, with optimized flow paths, exhibits the best performance in commercialized fuel cell electric vehicles, but the manufacturing cost is significantly high. To achieve high performance at a lower cost, the possibility of using a combination of commercially available screen meshes was investigated. The overlapped screen meshes should provide improved mass transport similar to a 3-D fine mesh. By using an optimized combination of screen meshes (200 and 100 mesh) and gasket thickness (150 μm thinner than the mesh flow field), an improvement in oxygen mass transport was achieved. The suggested combination shows a lower oxygen gain (0.030 V) than a single mesh (0.050 V) and a conventional single serpentine flow field (0.150 V).     

Investigating effect of different gas diffusion layers on water droplet characteristics for proton exchange membrane (PEM) fuel cells

In this work, advanced x-ray radiographic techniques available at the Canadian Light Source (CLS) were utilized to study water droplet dynamics in a serpentine flow channel mimicking a proton exchange membrane fuel cell (PEMFC). High spatial and temporal resolution coupled with high energy photons of an x-ray beam provided high-resolution images of water droplets. This technique solved the problem caused by the opaqueness of fuel cell materials including the gas diffusion layer by providing a unique way to study water droplet dynamics at different operating conditions. From the captured images, droplet emergence and formation on porous gas diffusion layers (GDLs) were analyzed. Three commercially available GDLs (Sigracet AA, Sigracet BA, and Sigracet BC) were used and droplet detachment height was found to decrease in the following order AA < BA < BC under the same flow condition. Increasing the superficial gas velocity was found to decrease the droplet detachment height for all GDLs tested. Average droplet cycle for various operating conditions was obtained. It was found that humidified air did not show a difference in droplet dimensions at detachment compared to dry air used at the inlet gas. However, it did show an impact on droplet cycle time, which might be due to condensation.     

Dynamic modeling of proton exchange membrane fuel cell using non-integer derivatives

The modeling of proton exchange membrane fuel cells (PEMFC) may work as a powerful tool in the development and widespread testing of alternative energy sources in the next decade. In order to obtain a suitable PEMFC model, which can be used in the analysis of fuel cell-based power generation systems, it is necessary to define the values of a specific group of modeling parameters. In this paper, the authors propose a dynamic model of PEMFC, the originality of which lays on the use of non-integer derivatives to model diffusion phenomena. This model has the advantage of having least number of parameters while being valid on a wide frequency range and allows simulating an accurate dynamic response of the PEMFC.

In this model, the fuel cell is represented by an equivalent circuit, whose components are identified with the experimental technique of electrochemical impedance spectroscopy (EIS). This identification process is applied to a commercially available air-breathing PEMFC and its relevance is validated by comparing model simulations and laboratory experiments. Finally, the dynamic response derived from this fractional model is studied and validated experimentally.     

Anode bleeding experiments to improve the performance and durability of proton exchange membrane fuel cells

Cost, durability, efficiency and fuel utilization are important issues that remain to be resolved for commercialization of proton exchange membrane fuel cells (PEMFC). Anode flow mode, which includes recirculation, dead-ended and exit bleeding operation, plays an important role in fuel utilization, durability, performance and the overall cost of the fuel cell system. Depending on the flow mode, water and nitrogen accumulation in the anode leads to voltage transients and local fuel starvation, which causes cell potential reversal and carbon corrosion in the cathode catalyst layers. Controlled anode exit bleeding can avoid the accumulation of nitrogen and water and improve fuel utilization. In this study, we present a method to control the bleed rate with high precision in experiments and demonstrate that hydrogen utilization as high as 0.9988 for a 25 cm² single cell and 0.9974 for an 8.17 cm² single cell can be achieved without significant performance loss. In the experiments, anode pressure is kept at 1 bar higher than the cathode pressure to decrease nitrogen crossover from the cathode, decreasing the crossover from the cathode. Moreover, four load cycle profiles are applied to observe the cumulative loss in the electrochemical surface area (ECSA), which are acquired from cyclic voltammetry (CV) analysis. Experiments confirm that the ECSA loss and severe voltage transients are indicative of fuel starvation induced by prolonged dead-ended or low exit-bleed operation modes whereas bleed rates that are larger than the predicted crossover rate are sufficient to operate the fuel cell without voltage transients and detrimental ECSA loss.     

Sputtered Pt loadings of membrane electrode assemblies in proton exchange membrane fuel cells

The fabrication of electrodes use in proton exchange membrane fuel cells (PEMFCs) by Pt sputter deposition has great potential to increase Pt utilization and reduce Pt loading without loss of cell performance. A radio frequency (RF) magnetron sputter deposition process (RF power = 100 W and argon pressure = 10^?3 Torr) was adopted to prepare Pt catalyst layers of PEMFC electrodes. The effects of cathode Pt and Nafion loadings on membrane electrode assembly (MEA)/cell performance were investigated using cell polarization, cyclic voltammetry, AC impedance, and microstructure analysis. Among the tested MEAs with various cathode Pt loadings (0.02–0.4 mg cm^?2), the one with 0.1 mg‐Pt cm^?2 (grain size = 3.90 nm, mainly Pt(111)) exhibited the best cell performance (320 and 285 mW cm^?2 at 0.44 and 0.60 V, respectively), which was similar to or better than those of some commercial nonsputtered/sputtered electrodes with the same or higher Pt loadings. The electrode Pt utilization efficiency increased as the Pt loading decreased. A Pt loading of greater than or lower than 0.1 mg cm^?2 yielded a lower electrode electrochemical active surface (EAS) area but a higher charge transfer and diffusion resistance. Nafion impregnation (0.1 to 0.3 mg cm^?2) into the sputtered Pt layer (Pt = 0.1 mg cm^?2) noticeably increased the EAS area, consistent with the decrease of the capacitance of the electrode double layer, but did not improve MEA/cell performance, mainly because of the increase in the kinetic and mass transfer resistances associated with oxygen reduction on the cathode. Copyright © 2011 John Wiley & Sons, Ltd.     

Carbon monoxide poisoning of proton exchange membrane fuel cells

Proton exchange membrane fuel cell (PEMFC) performance degrades when carbon monoxide (CO) is present in the fuel gas; this is referred to as CO poisoning. This paper investigates CO poisoning of PEMFCs by reviewing work on the electrochemistry of CO and hydrogen, the experimental performance of PEMFCs exhibiting CO poisoning, methods to mitigate CO poisoning and theoretical models of CO poisoning. It is found that CO poisons the anode reaction through preferentially adsorbing to the platinum surface and blocking active sites, and that the CO poisoning effect is slow and reversible. There exist three methods to mitigate the effect of CO poisoning: (i) the use of a platinum alloy catalyst, (ii) higher cell operating temperature and (iii) introduction of oxygen into the fuel gas flow. Of these three methods, the third is the most practical. There are several models available in the literature for the effect of CO poisoning on a PEMFC and from the modeling efforts, it is clear that small CO oxidation rates can result in much increased performance of the anode. However, none of the existing models have considered the effect of transport phenomena in a cell, nor the effect of oxygen crossover from the cathode, which may be a significant contributor to CO tolerance in a PEMFC. In addition, there is a lack of data for CO oxidation and adsorption at low temperatures, which is needed for detailed modeling of CO poisoning in PEMFCs. Copyright © 2001 John Wiley & Sons, Ltd.     

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.     

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.