Effect of non-active area on the performance of subgasketed MEAs in PEMFC

Subgaskets are usually applied to a catalyst-coated membrane (CCM) for the edge-protection of the electrolyte membrane and easy handling. They cover the peripheral region (non-active area) of CCM and have a defined window (active area) for accommodating the electrode. In this study, three subgasketed CCMs with different configurations were designed with a laboratory-scale 5 cm² fuel cell and the effects of the components underneath the subgaskets on the electrochemical properties of CCMs and cell performance were investigated by several electrochemical techniques. The results reveal that part of the catalyst layer under the subgaskets is activated for reaction area, leading to slightly higher electrochemical surface area (ESA), higher H₂ crossover, and smaller shorting resistance. The non-active region of subgasketed CCM has little impact on proton resistance in the catalyst layer, oxygen reduction reaction (ORR) kinetics, and limiting current, but has adverse effects on cell performance in the low current region under dry conditions due to increased hydrogen crossover. The findings could provide guidelines for subgasket design and application in laboratory-scale fuel cells.     

Effect of catalytic ink on sub-freezing endurance of PEMFCs

Effect of catalytic ink on sub-freezing endurance of proton exchange membrane fuel cells (PEMFCs) was investigated in this paper. By direct spraying method, a catalyst-coated membrane (CCM) was fabricated with isopropyl alcohol as organic solvent (CCM-A), and CCM-B was fabricated with isopropyl alcohol and butyl acetate. The hydrophobicity of the two CCMs was similar proved by contact angle tests, and CCM-B showed larger pore volume demonstrated by mercury intrusion tests. Initial cell performance and relevant electrochemical characteristics of the two CCMs were measured and compared. CCM-B showed better performance and larger electrochemical active surface area (ECA). By analyzing the electrochemical impedance spectra (EIS) at low current densities, the ionic resistances of the catalyst layers were calculated. Results indicated that adding butyl acetate to the catalytic ink benefited the ionic resistance. Then, the fuel cells with the two CCMs were subzero stored at −20 °C with saturated residual water. After 20 freeze–thaw cycles, the CCM prepared with isopropyl alcohol and butyl acetate showed less degradation in terms of polarization curves and EIS. And the ionic resistances of the both CCMs decreased to a certain extent.     

Investigation of catalyst layer defects in catalyst‐coated membrane for PEMFC application: Non‐destructive method

The commercialization of polymer electrolyte membrane fuel cells has been hindered by durability problems caused by defects in the manufacturing process. We demonstrate for the first time a non‐destructive, non‐contact method that uses optical microscopy and image analysis to identify defects that may lead to failure in catalyst‐coated membranes (CCMs) of polymer electrolyte membrane fuel cells. This method is applied to 2 commercial CCMs produced by the decal transfer technique. Defects in the catalyst layer (CL) at the beginning‐of‐life (BOL) are characterized in terms of their initial size and shape, and their propagation is tracked as the CCMs are aged in a non‐reactive environment. The defected area in one of the commercial CCMs increases from approximately 2.4% of the total CL area at BOL to 10.5% by end‐of‐life (EOL). BOL defects in the CL are found to propagate faster in the CCMs stored for 2 years under atmospheric conditions compared with freshly manufactured CCMs with narrow CL defects. Image analysis of another commercial CCM shows the presence of pores with diameters between 5 and 25 μm that comprise 52% of the total pore area in the CL. Other defects such as scratches and missing/empty catalyst areas are identified and characterized, providing a framework for quality control applications. Finally, the effect of defects on fuel cell performance is characterized by measurement of the open‐circuit voltage (OCV). These experiments show that CCMs with a large number of cracks in the CL exhibit a voltage loss of 2.55 mV/hr, whereas CCMs with thin/missing/empty CL defects show a loss of 1.12 mV/hr.     

A two dimensional partial flooding model for PEMFC

Proton exchange membrane fuel cells (PEMFCs) have attracted much attention in these years. In PEMFCs, liquid/gas two-phase flow is a common phenomenon, which has great influence on fuel cell performance. However, the liquid water transport process has not been satisfactorily modeled yet. In this work, a two dimensional partial flooding model was developed, in which the pore size distribution of the gas diffusion layer (GDL) is taken into consideration in the explanation of fuel cell flooding for the first time. Liquid water produced is considered to flood a fraction of the GDL hydrophobic pores with diameter greater than the capillary condensation threshold diameter, and the unflooded pores will serve as passageway for gas transportation and the corresponding catalyst area is available for electrochemical reaction. Use this model, it is simple to explain membrane dehydration and electrode flooding. Different operation conditions have been studied with the model and the model polarization curves show reasonable accordance with the experimental results.     

Analysis of degradation in PEMFC caused by cell reversal during air starvation

The damage caused by cell reversal during proton exchange membrane fuel cells (PEMFCs) operation with air starvation was investigated by a single-cell experiment. Samples from degraded membrane–electrode assemblies (MEAs) were characterized. The loss of electrochemical surface area of the cathode platinum was detected by in situ cyclic voltammetry, and platinum sintering was detected by transmission electron microscopy (TEM) analysis. Degradation at the anode was not detected in the chemical analysis of the anode catalyst layer of MEA samples by energy dispersive X-ray analysis (EDX) and TEM. An obvious decrease in the performance of PEMFC was observed in a sample degraded by cell reversal for 120 min.     

Exploration of the oxygen transport behavior in non-precious metal catalyst-based cathode catalyst layer for proton exchange membrane fuel cells

High cost has undoubtedly become the biggest obstacle to the commercialization of proton exchange membrane fuel cells (PEMFCs), in which Pt-based catalysts employed in the cathodic catalyst layer (CCL) account for the major portion of the cost. Although non-precious metal catalysts (NPMCs) show appreciable activity and stability in the oxygen reduction reaction (ORR), the performance of fuel cells based on NPMCs remains unsatisfactory compared to those using Pt-based CCL. Therefore, most studies on NPMC-based fuel cells focus on developing highly active catalysts rather than facilitating oxygen transport. In this work, the oxygen transport behavior in CCLs based on highly active Fe-N-C catalysts is comprehensively explored through the elaborate design of two types of membrane electrode structures, one containing low-Pt-based CCL and NPMC-based dummy catalyst layer (DCL) and the other containing only the NPMC-based CCL. Using Zn-N-C based DCLs of different thickness, the bulk oxygen transport resistance at the unit thickness in NPMC-based CCL was quantified via the limiting current method combined with linear fitting analysis. Then, the local and bulk resistances in NPMC-based CCLs were quantified via the limiting current method and scanning electron microscopy, respectively. Results show that the ratios of local and bulk oxygen transport resistances in NPMC-based CCL are 80% and 20%, respectively, and that an enhancement of local oxygen transport is critical to greatly improve the performance of NPMC-based PEMFCs. Furthermore, the activity of active sites per unit in NPMC-based CCLs was determined to be lower than that in the Pt-based CCL, thus explaining worse cell performance of NPMC-based membrane electrode assemblys (MEAs). It is believed that the development of NPMC-based PEMFCs should proceed not only through the design of catalysts with higher activity but also through the improvement of oxygen transport in the CCL.     

PEMFC modeling based on characterization of effective diffusivity in simulated cathode catalyst layer

Oxygen diffusion in the cathode catalyst layer (CCL) is crucial to the high performance of polymer electrolyte membrane fuel cells (PEMFCs), especially in high current density or concentration loss regions. Recently, PEMFC performance has been reported to be enhanced by increasing CCL pore size and pore volume due to the reduction of diffusion resistance by capillary water equilibrium [Yim et al., Electrochimica Acta 56 (2011) 9064–9073]. Herein, we simulate these experimental results utilizing a new one-dimensional PEMFC model considering the effects of accumulated water film in CCL on oxygen diffusion. Two CCL microstructures were numerically generated based on agglomerate models to examine the experimental results obtained for two membrane electrode assembly (MEA) samples with different CCL porosity. The effective diffusivity of oxygen in the CCL was estimated by performing auxiliary simulations of oxygen concentration in CCL microstructures covered by a film of liquid water, with exponential correlation obtained between effective diffusivity and the thickness of the above film. Polarization curves predicted by the present model were in good agreement with experimental results. In agreement with the results of Yim et al., the present model predicts that the MEA featuring a CCL with smaller pores (which are more easily filled by liquid water) should exhibit a larger concentration loss at high current densities.     

Catalyst layer formulations for slot-die coating of PEM fuel cell electrodes

The series of electrodes were fabricated by the scalable and manufacturable slot-die coating method for proton exchange membrane fuel cell (PEMFC) application. The inks with different amounts of solids were studied by rheological methods in order to establish a coating window with minimum manufacturing defects. The obtained electrodes were characterized by SEM, AFM, and optical microscopy, which showed that they were uniform and homogeneous with minimum defects. The electrochemical evaluation of the manufactured gas diffusion electrodes (GDE) showed that the main characteristics of the electrodes, like electrochemical surface area, proton resistivity, and double layer capacitance, were found to be close for all samples confirming the reproducibility of the slot-die process. Additionally, we studied the effects of membrane thickness on the performance of the GDE membrane electrode assemblies and determined that a decrease in membrane thickness favored the performance. The obtained results clearly demonstrated the applicability and feasibility of the approach for the Manufacturing of catalyst layers for the fuel cell application with potential for future mass production.     

Corrosion and interfacial contact resistance of nanocrystalline β-Nb2N coating on 430 FSS bipolar plates in the simulated PEMFC anode environment

The corrosion of metallic bipolar plates in the proton exchange membrane fuel cells (PEMFCs) anode environment would degrade the performance and shorten the lifespan of the fuel cell. Hence, it is essential to develop a conductive coating with good corrosion resistance. Herein we demonstrate a dense, defect-free, and well-adhered nanocrystalline β-Nb₂N coating prepared on 430 ferritic stainless steel (430 FSS) via disproportionation of Nb(Ⅳ) species in molten salts. The corrosion mechanism of bare and β-Nb₂N coated 430 FSS in the simulated PEMFC anode environment is also studied by electrochemical techniques including potentiodynamic polarization, potentiostatic polarization, and electrochemical impedance spectroscopy. Results show that β-Nb₂N coating can significantly improve the corrosion resistance of the steel alloy with acceptable contact resistance. In addition, no obvious degradation is observed for the β-Nb₂N coating after potentiostatic polarization measurement for 500 h. This work offers a promising strategy to develop the corrosion protective coating on metallic bipolar plates for PEMFCs.     

Temperature distribution on the MEA surface of a PEMFC with serpentine channel flow bed

Knowledge of the temperature distribution on the membrane electrode assembly (MEA) surface and heat transfer processes inside a proton exchange membrane fuel cell (PEMFC) is helpful to improvement of cell reliability, durability and performance. The temperature fields on the surface of MEA fixed inside a proton exchange membrane fuel cell with a serpentine channel flow bed were measured by infrared imaging technology under non-humidification conditions. The temperature distributions over the MEA surface under whole channel region were achieved. The experimental results show that the downstream temperatures are higher than the upstream. The hot region on the MEA surface is easy to locate from the infrared temperature image. The mean temperature on the MEA surface and the cell temperature both increase with the current density. Higher current density makes the non-uniformity of temperature distribution on the MEA surface worse. The loading time significantly affects the temperature distribution. Compared with the electrical performance of the cell, the MEA's temperatures need much more time to reach stable. The results indicate that isothermal assumption is not appropriate for a modeling of PEMFCs, and monitoring the temperature of external surface of the flow field plate or end plate cannot supply accurate reference to control the temperatures on MEA surface.     

A mechanism leakage model of metal-bipolar-plate PEMFC seal structures with stress relaxation effects

This work addresses the issues of long-term leakage rate prediction, which is crucial for durability study of proton exchange membrane fuel cells (PEMFCs). A theoretical model is presented for the leakage rate of compressive seal structures in PEMFCs, based on the combination of three numerical techniques, namely Lattice-Boltzmann method (LBM) simulations for rough wall interfacial gaps, a numerical 3D rough-surface generation technique, and Finite-Element-Analysis (FEA) for micro-contact mechanics of single asperity. The model clearly reveals the quantitative influence of various factors on the leakage rate without any empirical regression coefficients, and therefore can be easily integrated with structural mechanics and aging mechanism analyses. Long term sealing performance comparisons with three types of rubber material identify liquid silicone rubber to have the optimal durability. When influences of water environment are taken into account, an accelerated degradation of sealing performance can be observed after approximately 3000 h of operation. In addition, the effect of stress loss on leakage rate can be effectively suppressed by reducing surface roughness, reducing gasket thickness and increasing strain level. The proposed theoretical model provides an effective approach for the design of metal-bipolar-plate PEMFCs seal structures.     

Recycling of membrane electrode assembly of PEMFC by acid processing

A simple, high efficient and environmentally friendly approach was investigated to recycle the key materials of membrane electrode assembly (MEA) applied in proton exchange membrane fuel cell (PEMFC). The catalyst coated membranes (CCMs) was dipped into sulfuric acid until the formation of transparent solution composed of Pt and perfluorosulfonic acid resin. Wherein, the membrane was dissolved, and the amorphous carbon nanoparticles as catalyst supports in catalyst layers were oxidized. Subsequently, both metal Pt and perfluorosulfonic acid resin were separated by centrifugal separation. Then the resin was recast into a membrane and the single fuel cell performance was tested. As a result, the solution to recycle the key materials of MEAs is promising for recycling MEA materials used in PEMFC.     

A numerical analysis of PEMFC stack assembly through a 3D finite element model

A finite element model is developed to investigate the influence of the assembly phase of proton exchange membrane fuel cell (PEMFC) stacks on the mechanical state of the active layer (MEAs). Validated by experimental measurements, this model offers the possibility to analyze the influence of different parameters through the use of a complete parametric set, such as the number of cells and their position in the stack. The simulations show that a better uniformity of the MEA compression is obtained with the greatest number of cells, and at the center of the stack. The finite element analysis (FEA) is finally found to be an effective tool to show the influence of the assembly phase on the performance of PEMFCs, and will help the designer to adapt the future generations of stack to ensure the uniformity of the MEA mechanical strain.     

PEMFC cathode catalyst contamination evaluation with a RRDE-methyl methacrylate

This study was undertaken to provide insight into the mechanism of methyl methacrylate (MMA) contamination on the cathodes in proton exchange membrane fuel cells (PEMFCs). The effect of various concentrations of MMA on the oxygen reduction reaction (ORR) was investigated using a rotating ring-disk electrode (RRDE). The adsorption of MMA on the Pt/C electrode leads to a reduction in the electrochemical surface area (ECSA) and a decrease in the mass activity of the ORR. Increasing the MMA concentration causes an increase in its coverage on Pt and a decrease in the ORR mass activity. At high MMA concentration (0.1 M), the O₂ transportation is also affected, resulting in a large decrease in the diffusion limiting current. For all of the MMA concentrations measured, a shift in the ORR reaction pathway from a 4-electron to 2-electron mechanism is observed, as indicated by the increase in the H₂O₂ production on the ring. Using the RRDE measurements and the Levich plots, the total charge transfer number is confirmed to decrease from 4 for the clean electrode to lower values due to the increase in the H₂O₂ side reactions. The shift in the reaction pathway is hypothesized to be caused by the inhibition of dual adsorption of the O₂ molecules, due to steric hindrance induced by MMA adsorption on the Pt/C electrode. Tafel evaluation indicates a continuous increase in the slope with an increase in the MMA concentration, which indicates that the passage of electrons for the ORR at the electrode surface is altered by the adsorption of MMA. A recovery effect was also investigated and a water rinsing process was used to recover the electrode. Only a partial recovery was obtained with a water rinse followed by potential cycling over the same potential range as the initial ECSA measurements. The unrecovered performance is attributed to MMA remnants on the catalyst surface, which can be removed by potential cycling over a wider potential range with a larger anodic potential limit.     

Effect of carbon black concentration in carbon fiber paper on the performance of low-temperature proton exchange membrane fuel cells

This study uses fuel cell gas diffusion layers (GDLs) made from carbon fiber paper containing carbon black in proton exchange membrane fuel cells (PEMFCs) in order to determine the relationship between carbon black content and fuel cell performance. The connection between fuel cell performance and the carbon black content of the carbon fiber paper is discussed, and the effects of carbon black on the carbon fiber paper's thickness, density, and surface resistivity are investigated. When a carbon fiber paper GDL contains 10 wt% phenolic resin and 2% carbon black, and reaction area was 25 cm² and operating temperature 40 °C, tests show that a carbon electrode fuel cell could achieve 1026.4 mA cm⁻² and maximum power of 612.8 mW cm⁻² under a 0.5 V load.     

The effects of Nafion ionomer content in PEMFC MEAs prepared by a catalyst-coated membrane (CCM) spraying method

We analyzed the effects of ionomer content on the proton exchange membrane fuel cell (PEMFC) performance of membrane electrode assemblies (MEAs) fabricated by a catalyst-coated membrane (CCM) spraying method in partially humidified atmospheric air and hydrogen. When high loading Pt/C catalysts (45.5 wt.%) were used, we observed that catalytic activity was not directly proportional to electrochemical active surface area (EAS). This suggests that ionic conductivity through ionomers in catalyst layers is also an important factor affecting MEA performance. In addition, the effects of mass transport were experimentally evaluated by manipulating the air stoichiometry ratio at the cathodes. MEA performance was more sensitive to flow rates under conditions of higher ionomer content. Due to the combined effect of EAS, ionic conductivity, and mass transfer characteristics (all of which varied according to the ionomer content), an MEA with 30 wt.% ionomer content at the cathode (25 wt.% at the anode) was shown to yield the best performance.     

Numerical modeling and experimental study of the influence of GDL properties on performance in a PEMFC

This work is to study the effect of properties of gas diffusion layer (GDL) on performance in a polymer electrolyte membrane fuel cell (PEMFC) by both numerical simulation and experiments. The 1-dimension numerical simulation using the mixture-phase model is developed to calculate polarization curve. We are able to estimate optimum GDL properties for cell performance from numerical simulation results. Various GDLs which have different properties are prepared to verify accuracy of the simulation results. The contact angle and gas permeability of GDLs are controlled by polytetrafluoroethylene (PTFE) content in micro-porous layers (MPLs). MPL slurry is prepared by homogeneous blending of carbon powder, PTFE suspension, isopropyl alcohol and glycerol. Then the slurry is coated on gas diffusion mediums (GDMs) surface with controlled thickness by blade coating method. Non-woven carbon papers which have different thicknesses of 200 μm and 380 μm are used as GDMs. The prepared GDLs are measured by surface morphology, contact angle, gas permeability and through-plane electrical resistance. Moreover, the GDLs are tested in a 25 cm² single cell at 70 °C in humidified H₂/air condition. The contact angle of GDL increases with increasing PTFE content in MPL. However, the gas permeability and through-plane electrical conductivity decrease with increasing PTFE content and thickness of GDM. These changes in properties of GDL greatly influence the cell performance. As a result, the best performance is obtained by GDL consists of 200 μm thick non-woven carbon paper as GDM and MPL contained 20 wt.% PTFE content.     

Performance improvement in direct formic acid fuel cells (DFAFCs) using metal catalyst prepared by dual mode spraying

In the present study, we investigate performance of direct formic acid fuel cells (DFAFCs) consisting of membrane electrode assembly (MEA) prepared by three different catalyst coating methods - direct painting, air spraying and dual mode spraying. For the DFAFC single cell tests, palladium (Pd) and platinum (Pt) are used as anode and cathode catalyst, respectively, and four different formic acid concentrations are provided as a fuel. In the measurements, dual mode spraying shows the best DFAFC performance. To overhaul how difference in coating method influences DFAFC performance, several characterization techniques are utilized. Zeta potential and TEM are used for evaluating anodic Pd particle distribution and its size. Cyclic voltammogram (CV) is measured to calculate electrochemical active surface (EAS) area in anode electrode of the DFAFCs, while charge transfer resistance (R_ct) is estimated by electrochemical impedance spectroscopy (EIS). As a result of the characterizations, Pd prepared by dual mode spraying induces the most uniform particle distribution and the smallest size, the highest EAS area and the lowest R_ct, which are matched with the DFAFC performance result. Conclusively, by adoption of the dual mode spraying, DFAFC can get the maximum power density as high as 240 mW cm⁻² at 5 M formic acid.     

Comparative study of three different catalyst coating methods for direct methanol fuel cells

The performance of direct methanol fuel cells (DMFCs) with membrane–electrode assemblies (MEAs) made separately by three different catalyst coating methods, namely, air-spray, electro-spray and dual-mode spray, is evaluated. Platinum–ruthenium (PtRu) is incorporated as a catalyst for the anode. Several techniques (XRD, FE-SEM, and TEM) are used to examine whether the coating method affects the morphological features of the PtRu catalyst, whereas cyclic voltammetry is used to evaluate the active surface area. The cell polarization curves attained for the three coating methods that use different methanol concentrations are compared to determine the best method. It is found that the PtRu catalyst coated by the dual-mode spray shows the most uniform nanoparticle distribution and the highest active surface area. The DMFC performance is best when the dual-mode spray is employed (165 mW cm⁻² at 2 M methanol).     

High temperature operation of PEMFC: A novel approach using MEA with silica in catalyst layer

Composite membranes with inorganic substances can retain water and allow the operation of polymer electrolyte membrane fuel cells (PEMFCs) at high temperature under low humidity. In this work, the single cell was operated at high temperature using silica–Nafion composite membrane in addition with silica in catalyst layer. The cell was operated at various temperatures under different relative humidity conditions. We observed that the single cell performance decreased steeply as the cell temperature increased. The role of silica in the catalyst layer at high temperature operation was studied by varying the silica content in the catalyst layers. There was a gradual decrease in cell performance when the silica content increased in catalyst layer. The single cell performance of membrane electrode assemblies (MEAs) with composite membrane and electrode was higher than that of MEA with commercial Nafion 112 membrane for high temperature operation.