Durability Of Proton Exchange Membrane Fuel Cells Research Articles

Liquid Water Uptake and Contact Angle Measurement of Proton Exchange Membrane Fuel Cell (PEMFC) Electrodes

Proton exchange membrane fuel cells (PEMFCs) are promising power sources for various stationary and portable applications. Despite the numerous efforts made the recent years, water management remains a major limiting factor to PEMFC performance and durability especially for ultra-thin catalyst layers (CLs) [1]. Therefore, a basic understanding of the wetting and morphological properties of the CLs is crucial to overcome mass transport limitations. This requires measuring and quantifying several properties including contact angle and ionomer water uptake. In this work, imbibition tests were performed to extract (external) contact angle to water and ionomer liquid uptake of free-standing electrodes (about 8 mm thick) fabricated by the 3M Company. They consisted of Vulcan™ XC-72 carbon (without catalyst) dispersed in a 3M brand ionomer matrix with an equivalent weight (EW) of 825 g/mole. Not stiff enough, these films were first hot-pressed on both sides of an Ethylene Tetrafluoroethylene (ETFE) film (120 μm thick) before being immersed 10 mm into liquid (deionized) water for 900 seconds (see Figure 1.a). The increase in sample mass due to the added liquid weight was determined with respect to time during the measurement using a tensiometer (model K100 from Krüss GmbH, Germany) operated at room temperature. After 900 seconds, the samples were pulled out the liquid to estimate the wetting force and thus the external contact angle to water using the Wilhelmy equation. This approach was described with more details in our previous work [2]. The Figure 1.b shows the time evolution of the gain in mass for carbon-ionomer films with ionomer to carbon (mass) ratios of 0.8, 1.0, 1.2 and 1.4 when immersed 10 mm into liquid water. The amount of absorbed water significantly increased with the ionomer presence. The Figure 1.c shows the (external) contact angle to water as function of the I/C ratios estimated by calculating the capillary force when the samples were pulled out the water at t=900s (method described in [2]). The results reflected a highly hydrophobic behavior of the carbon-ionomer films: average contact angle to water about 140°. These observations were confirmed by optical measurement using the sessile drop technic. The electrode hydrophobicity was mainly ensured by the solid phase and the contact angles were little sensitive to the amount of ionomer (for the material and range of I/C considered in this work). Material with different ionomer were also used to determine the influence of the ionomer chemistry on the electrode liquid uptake and contact angle to water. The effect of the temperature on electrode wettability was also investigated. Figure 1. (a) Krüss tensiometer K100 (Krüss GmbH, Germany) used to measure the mass evolution of carbon-ionomer films after being immersed 10 mm into a liquid (at a constant speed of 100 mm/min). The carbon-ionomer films were hot-pressed on both sides of an ETFE film (120 μm thick) with a total surface of 3 x 2 cm2. (b) Time evolution of the gain in mass for carbon-ionomer films (Carbon Vulcan™ XC-72, 3M brand ionomer matrix with EW 825) with I/C ratios of 0.8, 1.0, 1.2 and 1.4. (c) Contact angle to water as function of the sample I/C ratios determined by calculating the capillary force when the samples were pulled out the water at t=900s (see [2]). The contact angles calculated from this approach were in good agreement with the ones measured using the sessile drop technic.

The Influence of the Platinum Loading and the Ionomer to Carbon Ratio on the Durability of the PEMFC

Cost and durability are the major barriers to the success of the Proton Exchange Membrane Fuel Cells (PEMFCs), and reducing the platinum loading of the catalyst layer could reduce the cost of the device. Thus, in this work, long-term durability tests in constant-current mode were performed using single-cells of 25 cm2 with Membrane Electrode Assemblies (MEAs) prepared with 20% Pt/C (BASF) catalyst and electrodes with platinum loadings of 0.4 and 0.1 mgPt.cm-2. Different ionomer to carbon ratios were also studied with the electrodes containing 0.1 mgPt.cm-2 of catalyst loading. To characterize the electrochemical and structural properties of the MEAs, cyclic voltammograms, electrochemical impedance spectroscopy and field-emission scanning electron microscopy were utilized before and after the long-term durability tests. Concerning the platinum loading, the electrodes prepared with 0.1 mg Pt cm-2 have lost proportionally more electrochemical surface area than the ones prepared with 0.4 mg Pt cm-2 but at the same time, the electrodes with the lowest platinum loading presented lower irreversible performance loss rate when using higher ionomer to carbon ratio. The analyses made by several electrochemical techniques have indicated that the raise of the ohmic and mass transport resistances are the factors that most contribute to the irreversible performance loss, meanwhile the charge transport resistance due to the electrodes flooding is the main responsible for the reversible performance loss. The proportion of ionomer in the catalytic layer was studied and it was possible to infer that the highest ionomer proportion studied has contributed to mitigate the MEA degradation, although it facilitates the occurrence of flooding in the electrodes. Finally, it was observed that the quality of the contact formed between the electrodes and the membrane has remarkable influence on the PEMFCs durability.

Comparison Between the Constant Dummy Load and Step Load Shut‐Down Strategy for PEMFCs

AbstractCarbon corrosion of catalyst layer (CL) caused by start‐up/shut‐down (SU/SD) conditions in proton exchange membrane fuel cells (PEMFCs) decreases the durability of the membrane electrode assembly (MEA). It is reported that the dummy load shut‐down strategy helps alleviating the carbon corrosion. In this paper, constant load strategy and step load strategy are applied during 900 SU/SD cycles each, to investigate the effects of different dummy load forms on the durability of PEMFC. Reversal current is observed during the last phase of 5 A constant loading process, while there is no reversal current during the step load strategy. The results obtained through polarization curves, cyclic voltammetry (CV) and scanning electron microscope (SEM) technologies, prove the step load strategy more efficient to alleviate the performance decay. Therefore, the step load strategy can improve the durability of PEMFC.

Three-Dimensional PEM Fuel Cells Modeling using COMSOL Multiphysics

Proton Exchange Membrane Fuel Cell (PEMFC) has become one of the most promising energy technologies at the present time. Several factors are driving the growing interest in this technology. Modeling different phenomena occurring in PEMFC plays an important role in this development and performance. The performance of a Proton Exchange Membrane Fuel Cell (PEMFC) depends on the characteristics of the membrane, gas diffusion layer (GDL), catalyst and operating parameters such as operating pressure, cell operation temperature, relative humidity, and mass flow rate of feed gases, channel geometries and design of the stack. Recent studies on the compilation of factors affecting durability and performance of PEMFC indicate that the performance of fuel cell strongly depends on the performance of its membrane. In this paper, a three-dimensional PEM fuel Cell model has been developed and is used to investigate the effects of geometry membrane on cell performance. The numerical results indicated that a thinner membrane corresponds to the higher current density, the hydrogen and oxygen consumption and, accordingly water production is high. Finally, the numerical results of the proposed CFD model are compared with the available experimental data and that represent good agreement.

Ca2+ Effects on a Pt/C Catalyst for the Oxygen Reduction Reaction

The proton exchange membrane fuel cell (PEMFC) is one of the most promising renewable energy sources due to benign exhaust emissions and high energy efficiency. Pt/C is currently the primary catalyst for the slow oxygen reduction reaction (ORR) kinetics at the cathode of a PEMFC. The ORR, O2 + 4H+ + 4e- → 2H2O, indicates that this process preferentially occurs at a triple phase boundary where reactant, electrolyte, and catalyst are in contact [1, 2]. The depletion of O2, H+ or e- would significantly affect reaction kinetics. The Nafion® perfluorosulfonic acid (PFSA) ionomer, composed of a carbon-fluorine backbone and side chains ending with sulfonic acid groups, is conventionally used to prepare catalyst layers by exploiting its binding, proton-conducting and O2 transport properties. Foreign cations replace ionomer protons and occupy sulfonic acid sites by ion exchange [3-6]. The replacement of H+ by foreign cations in the ionomer decreases not only the proton conductivity but also O2 permeability. Uddin et al. [6] model results showed that the presence of Na+ in the ionomer of the cathode catalyst layer, which decreases the O2 permeability, causes a decrease in dissolved O2 concentration. Durst et al. [7] found that the diffusion coefficient and the concentration of O2 in the H2SO4 electrolyte, and the kinematic viscosity of the electrolyte decrease in the presence of Co2+. Cationic impurities decrease the membrane water content in turn shrinking the ionomer and lowering the active catalyst surface area. Also, foreign cations promote the generation of H2O2, a side product of the ORR, which exerts an undetermined influence on the performance and durability of PEMFCs. Literature and our group studies indicated that foreign cations have severe effects on the performance and durability of the PEMFC. Among all cations, Ca2+ deserves special attention because it is one of the most abundant elements within the earth’s crust. Prior efforts have focused on the effects of Ca2+ on PEMFC operation [8-10] and membrane transport properties [11]. However, the effects of Ca2+ on the ORR kinetics have hardly been studied. Okada et al. [12] studied the effects of Ca2+ on the O2 reduction kinetics at a Nafion® film covered Pt rotating disk electrode. Their results showed that impurity ions, even in small amounts, significantly hinder the rate of the O2 reduction charge transfer step. However, the Nafion® film covered Pt electrode is different from the supported catalyst layer for a PEMFC. The O2 transport properties in a Ca2+ form ionomer are poorly understood. In addition, the H2O2 yield has not been measured. In this presentation, detailed analyses of the changes in the Pt/C electrochemical surface area, ORR diffusion-limited and kinetic currents, Tafel slope, H2O2 yield, O2 permeability and ionomer sulfonate site occupation will be presented and discussed. Acknowledgments Authors are grateful to the United States Department of Energy (award DE-EE0000467), the Office of Naval Research (award N00014-12-1-0496), and the Hawaiian Electric Company.

Characterization of Water Transport in PEMFC Electrode Using the Washburn Method

Proton exchange membrane fuel cells (PEMFCs) are promising power sources for automotive use. Despite the numerous efforts made the recent years, water management still remains a major limiting factor to PEMFC performance and durability. Specifically, the performance of catalyst layers (CLs), where the electrochemical conversion takes place, is heavily affected by mass transport limitations. Understanding liquid-water transport phenomena inside catalyst layers is vital and must be achieved by a basic understanding of the wetting and morphological properties of the pore structure. This requires measuring and quantifying several properties including surface energy and internal contact angle. In this work we present a method for estimation of the internal contact angle to water (and other different liquids) and the surface free energy of CLs with different ionomer/carbon ratios. The method combines the Washburn technique [1] and the Owens-Wendt model [2]. A similar approach was already used by other researchers to determine the wetting properties of fuel cell gas diffusion layers [3,4] but, to the best of our knowledge, such method was never used for catalyst layers. The CLs (carbon-ionomer films) used in this study were fabricated by the 3M Company. They consisted of Vulcan™ XC-72 carbon dispersed in a 3M brand ionomer matrix. Being very thin, these films are not stiff enough to immerse into the liquids. Therefore, each sample was prepared by hot-pressing the carbon-ionomer film on both sides of an Ethylene Tetrafluoroethylene (ETFE) film (120 μm thick) with a surface of 10 mm x 5 mm (see Figure 1.a). The internal contact angle was measured by a dynamic sorption experiment according to the Washburn method [1] using a Krüss tensiometer K100 (Krüss GmbH, Hamburg, Germany) operated at room temperature. A set of six wetting liquids was tested: n-hexane, acetone, methanol, water, Fluorinert™ FC-3283 (3M Company) and toluene. As shown in Figure 1.a, when the sample (10 mm x 5 mm) came into contact with the surface of the desired liquid, the liquid was drawn up as a result of capillary action. The increase in sample mass due to the added liquid weight was determined with respect to time during the measurement. Using the Washburn method [1], internal contact angle can be obtained from the slope of the linear region of the square of the mass against time, Figure 1.b. These contact angles measurements were used to extract both dispersive and polar component of the solid surface energy using the Owens–Wendt model [2-4]. Materials with different ionomer/carbon ratio were used to determine the influence of the ionomer on electrode water uptake and transport. The effect of platinum loading on water transport was also investigated. Acknowledgement The authors thank Andrew Haug of 3M Company for providing materials tested in this study.

(Invited) Durability Evaluation of PEM Fuel Cells for Automotive Application

Proton exchange membrane fuel cells (PEMFC) technology is already in commercialization phase for automotive applications. However, the biggest challenges for its successful mass commercialization are cost and durability. Current PEMFC technology can meet the lifetime targets but the cost to achieve it can be very high due to material’s cost and system level mitigation complexities. Understanding of the membrane electrode assemblies (MEA) degradations using relevant durability accelerated tests at early stages of development is essential. PEMFCs for automotive applications goes through thousands of cycles during normal operation of the vehicle over the expected life span of 10 years. These cycles include a combination of load or voltage cycles, start and stop cycles, humidity cycles and temperature cycles. Study have shown that start-stop cycling accounts for 44% of operation mode while load cycling and idling for 28% each (1). Under these cycles, fuel cells suffer from severe degradations of its main components; cathode, anode, membrane and bipolar plates. The PEMFC cathode can degrade during start-stop operation via carbon corrosion (2). Cathode degradation can be mitigated by using stable materials or by applying system mitigations. System mitigation by a voltage limiting control (VLC) can be one of the effective measures (3). The VLC can, however, lead to voltage spikes (high positive potentials) at the anode during fuel cell start-up and shut-down. Moreover, during fuel (H2) starvation conditions, the potential at the anode can give rise to high positive potentials leading to anode catalyst layer degradation. During idling operation mode, PEMFC is at or near open circuit conditions which is known to accelerate proton exchange membrane (PEM) decomposition especially under low humidification (4). Peroxy-radicals originating from hydrogen peroxide produced at the anode catalyst surface cause the PEM degradation (5). Pt band formation in the membrane is also known to promote peroxide formation leading to membrane decomposition (6-7). Several accelerated protocols are currently being used to evaluate the durability of PEMFC. At Nissan, our durability protocols are focused on understanding the material degradation under relevant automotive cycles and correlating single cell durability to the stack level. Start-stop accelerated protocol to evaluate the cathode support durability consist of potential cycling from 1.0 V to 1.5 V using triangular waveform for 1000 cycles with diagnostics conducted intermittently after certain number of cycles. Cathode catalyst durability is evaluated using load cycling accelerated protocol wherein the potential is cycled between 0.6 V to 0.95 V to simulate peak load and OCV/idle using a square wave profile with 3 second hold at each potential. The upper potential is known to have significant impact on catalyst degradation (8). Load cycling is performed for 10,000 cycles with diagnostics conducted intermittently. Anode degradation under cell reversal condition is evaluated using a protocol designed to simulate high voltage spikes at the anode. Cell reversal cycling is performed by applying a load of 0.2 A/cm2 for 1 sec while flowing N2 at the anode and air at the cathode while measuring the cell voltage under the applied load for a total of 200 cycles. Membrane durability is evaluated using humidity cycling (mechanical durability) and potential hold at open circuit (chemical durability). Membrane’s chemical durability is evaluated at 90 °C and 30% RH under hydrogen and oxygen by operating MEA at open circuit. The target of OCV hold test is 500 hrs while maintaining potential above 0.8 V. Mechanical durability of membrane is evaluated by cycling RH to simulate dry condition (0%) and wet condition (100%) for target of 20,000 cycles. All these accelerated tests are performed at ambient pressure and summarized in the Table 1 below. Acknowledgments: The author would like to gratefully acknowledge Nissan Technical Center North America Fuel Cell Research team members (Dr. Amod Kumar, Dr. Ramesh Yadav and Dr. Cenk Gumeci) and Nissan Motor Co. Ltd Japan Fuel Cell research team members (Dr. Atsushi Ohma, Dr. Yoshihisa Furuya) for technical discussions and data.

One-pot synthesis of Pt/CeO2/C catalyst for improving the ORR activity and durability of PEMFC

Oxygen reduction reaction (ORR) activity and durability of Pt catalysts should be both valued for successful commercialization of proton exchange membrane fuel cells (PEMFCs). We offer a facile one-pot synthesis method to prepare Pt/CeO2/C composite catalysts. CeO2 nanoparticles, with high Ce3+ concentration ranging from 30.9% to 50.6%, offers the very defective surface where Pt nanoparticles preferentially nuclear and growth. The Pt nanoparticles are observed sitting on the CeO2 surface, increasing the PtCeO2 interface. The high concentration of oxygen vacancies on CeO2 surface and large PtCeO2 interface lead to the strong PtCeO2 interaction, effectively improving the ORR activity and durability. The mass activity is increased by up to 50%, from 36.44 mA mg−1 of Pt/C to 52.09 mA mg−1 of Pt/CeO2/C containing 20 wt.% CeO2. Pt/CeO2/C composite catalysts containing 10–30 wt.% CeO2 loss about 80% electrochemical surface area after 10,000 cycles, which is a fivefold enhancement in durability, compared to Pt/C losing 79% electrochemical surface area after 2000 cycles.

Microscopy Supported Multi-Scale Modeling of PEM Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are being developed as alternative energy sources for both residential and automotive applications. For this technology to reach its full commercial potential, however, a significant reduction in cost is still required while the performance and durability of PEMFCs being maintained or improved. The key in this process is continuous improvement of membrane electrode assembly (MEA) through both the development of new materials and the optimization of the 3D structural arrangement of individual MEA components [1,2] . Structural optimization of MEA remains to be challenging due to multiple materials interacting across 6 orders of magnitude in their characteristic length scales, from several nanometers for the catalyst particle size to hundreds of micrometers for carbon fibers in porous transport layer. In recent years, several microscopy techniques have become widely available to characterize the three-dimensional (3D) structure of PEMFC catalyst layers. Due to the heterogeneous nature of the PEMFCs sample, a correlative, multi-scale imaging protocol is developed in this work. The following technology is used where their corresponding resolution limit is pushed. Transmission Electron Microscope tomography, TEMt [3] (0.60nm X 0.60nm X 0.60nm Voxel Size) Focussed Ion Beam - Scanning Electron Microscope tomography, FIB-SEM [4] (2.5nm X 2.5nm X20 nm Voxel Size) Micro and Nano X-ray Computed Tomography, NanoCT [5] (367nm X 367nm X 367nm Voxel Size) Each technique by itself supports valuable insight to be obtained on the investigated structure at the corresponding length scale. By combining 3D imaging data at multiple scales, an unified structural characterization workflow is developed. Furthermore, an up scaling approach is developed based on TEMt, SEM and NanoCT reconstruction of catalyst layer, micro-porous layer and porous gas transport layer of a PMEFC cathode. Structural properties and effective transport properties are computed at each scale via image-based numerical simulation. Effective transport properties are up-scaled. MEA performances are predicted based on the obtained effective quantities at micro-scale.

The experimental analysis of a dead-end H2/O2 PEM fuel cell stack with cascade type design

Proton exchange membrane fuel cells (PEMFCs) with a dead-ended anode and cathode can reach high hydrogen and oxygen utilization by a relatively simple system. Nevertheless, the accumulation of the water in the anode and cathode channels can lead to a local fuel starvation deteriorating the performance and the durability of PEMFCs. In this study, a novel design for a polymer electrolyte membrane (PEM) fuel-cell stack was presented which could achieve higher fuel utilization without using hydrogen and oxygen recirculation devices such as hydrogen pumps or ejectors that consume parasitic power and require additional control schemes. The basic concept of the innovatively proposed design was to divide the cells of a stack into several stages by conducting the outlet gas of each stage to a separator and reentering it into the next stage; thereby, a multistage anode and cathode system was prepared. In this relatively ingenious design, a higher gaseous flow rate was maintained at the cell outlet, even under dead-end conditions resulted in a reduced purge-gas emission by avoiding the accumulation of liquid water in the cells. The results revealed that proposed design had the same polarization curve as the open-end mode, leading to an enhanced PEMFC performance.

Design, manufacturing, assembling and testing of a transparent PEM fuel cell for investigation of water management and contact resistance at dead-end mode

PEM fuel cell can operate at two different modes. At first mode the outlet of gas flow field is open and at the second mode the outlet of flow field is closed. The second mode is known as dead-end PEMFC. Proton exchange membrane fuel cells (PEMFCs) with dead-ended anode and cathode can obtain high hydrogen and oxygen utilization by a comparatively simple system. Nevertheless, the accumulation of the water in anode and cathode channels can lead to a local fuel starvation degrading the performance and durability of PEMFCs. There are different methods for investigation of water management such as: neutron radiography, gas chromatography, capturing by use of X-ray and capturing by use of infrared ray. Due to high cost and many hazards these methods at most cases cannot be used. According to the above mentioned problem we recommend a transparent PEMFC as a simplest, cheapest and the most suitable method for investigation of water management. Designing and manufacturing this type of PEMFC require special techniques. In this paper at first an optimal flow field is numerically designed and according to numerical results a transparent PEMFC is designed, manufactured and tested. Furthermore the performance of PEMFC at dead-end mode and open-end mode is studied. The applied design with a higher efficiency could have a same polarization curve as open-end mode. The results showed that by setting the purge interval time on 5 s and then opening purge valve for 1 s, there isn't any degradation on PEMFC performance but for purge interval of 10 s gradual performance degradation is recorded.

High performance and durability of order-structured cathode catalyst layer based on TiO2@PANI core-shell nanowire arrays

In this paper, an order-structured cathode catalyst layer consisting of Pt-TiO2@PANI core-shell nanowire arrays that in situ grown on commercial gas diffusion layer (GDL) are prepared and applied to membrane electrode assembly (MEA) of proton exchange membrane fuel cell (PEMFC). In order to prepare the TiO2@PANI core-shell nanowire arrays with suitable porosity and prominent conductivity, the morphologies of the TiO2 nanoarray and electrochemical polymerization process of aniline are schematically investigated. The MEA with order-structured cathode catalyst layer is assembled in the single cell to evaluate the electrochemical performance and durability of PEMFC. As a result, the PEMFC with order-structured cathode catalyst layer shows higher peak power density (773.54mWcm−2) than conventional PEMFC (699.30mWcm−2). Electrochemically active surface area (ECSA) and charge transfer impedance (Rct) are measured before and after accelerated degradation test (ADT), and the corresponding experimental results indicate the novel cathode structure exhibits a better stability with respect to conventional cathode. The enhanced electrochemical performance and durability toward PEMFC can be ascribed to the order-structured cathode nanoarray structure with high specific surface area increases the utilization of catalyst and reduces the tortuosity of transport pathways, and the synergistic effect between TiO2@PANI support and Pt nanoparticles promotes the high efficiency of electrochemical reaction and improves the stability of catalyst. This research provides a facile and controllable method to prepare order-structured membrane electrode with lower Pt loading for PEMFC in the future.

Study on Dissolution Behavior of Pt-M Binary Alloys Under Potential Cycling By Channel Flow Multi Electrodes

Introduction Pt-M alloys catalysts have attracted attention over the years and expected to be used in proton exchange membrane fuel cells (PEMFCs) cathode catalysts due to their high oxygen reduction activity than pure Pt and reduction of Pt usage in PEMFCs [1]. However, Pt-M catalysts is easily degraded by both Pt and M dissolution from Pt-M alloys during on-off cycle and load cycle, which leads the performance loss of PEMFCs [2]. Thus, to understand the nature of dissolution mechanism of both Pt and M from Pt–M alloy catalysts is very significant to advance durability of PEMFCs under its operating conditions. In this study, we investigated that the dissolution mechanism of Pt-50 at% Fe alloy (Pt50-Fe50) and Pt50-Cu50 under potential cycling with a channel flow multi electrodes (CFME), and elucidate the effect of additive element to the dissolution behavior of Pt50-M50. Experimental Pt50-Fe50 and Pt50-Cu50 were subjected to potential cycling tests at 298 K using Ar-purged 0.5 M H2SO4 solution with a CFME. A double junction KCl-saturated Ag / Ag-Cl electrode was used as reference electrode, and Au coil was used as the counter electrode. Flow rate of the electrolyte was 10 cm s-1 in order to establish laminar flow condition. Potential cycling tests were employed between 0.05 V and 1.4 V vs. SHE at 20 mV s-1. Before potential cycling, Pt50-M50 was set at 0.45 V vs. SHE in order to remove Pt oxide layer formed on Pt–M alloy surface in ambient air and to measure baseline currents of collector electrodes (CEs). Reactions and potentials for detecting the dissolved Fe2+, Fe3+, and Cu2+ on Au-CEs are as follows, Fe2+ → Fe3+ + e- (E c = 1.0 V vs. SHE) (1) Fe3+ + e-→ Fe2+ (E c = 0 V vs. SHE) (2) Cu2+ + 2e-→ Cu (E c = 0 V vs. SHE) (3) Dissolved Fe2+, Fe3+, and Cu2+ from Pt50-M50 are monitored by the current change of CEs. Results and discussion Figure 1 shows M detection current with a CFME during potential cycling, and red and blue line show the results of Pt50-Fe50 and Pt50-Cu50, respectively. As shown in inset, Fe dissolution from Pt50-Fe50 starts from around 0.3 V in anodic scan. Then Fe dissolution is clearly enhanced between 0.6 and 1.4 V in both anodic and cathodic scan, and finally terminate at 0.3 V in cathodic scan. Wang et al reported that Pt dissolution occur above approximately 0.6 V [3], accordingly major Fe dissolution is enhanced by Pt dissolution. On the contrary, minor Fe dissolution between 0.3 and 0.6 V in both anodic and cathodic scan is contributed by Pt surface diffusion, because Pt atoms are easily diffuse on Pt-M alloy surface in this potential range due to no adsorbed species on its surface [4]. Cu dissolution from Pt50-Cu50 only appears above 0.6 V, whereas Fe dissolution from Pt50-Fe50 starts from 0.3 V. Moreover, the amount of Cu dissolved from Pt50-Cu50 is smaller than that from Pt50-Fe50, which indicate that Cu is more durable elements than Fe. This is explained by the difference of standard electrode potentials between Cu and Fe. Pt-enriched layer formed on Pt50-Cu50 is denser and flatter than that on Pt50-Fe50, because the amount of dissolved Cu is small due to more noble standard electrode potential than Fe. Flat Pt-enriched layer inhibits Pt surface diffusion between 0.3 and 0.6 V, and Pt dissolution above 0.6 V. Reference [1] V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer, C. A.Lucas, G. Wang, P. N. Ross, and N. M. Markovic: Nat. Mater. 6 (2007) 241–247. [2] H. A. Gasteiger, S. S. Kocha, B. Sompalli, and F. T. Wagner: Appl. Catal. B: Environ. 56 (2005) 9–35. [3] Z. Wang, E. Tada, and A. Nishikata: J. Electrochem. Soc. 161 (4) (2014) F380–F385 [4] Q. Xu, E. Kreidler, and T. He: Electrochim. Acta 55 (2010) 7551–7557. Figure 1

Graphene-Supported Platinum Catalyst-Based Membrane Electrode Assembly for PEM Fuel Cell

The aim of this study is the preparation and characterization of a graphene-supported platinum (Pt) catalyst for proton exchange membrane fuel cell (PEMFC) applications. The graphene-supported Pt catalysts were prepared by chemical reduction of graphene and chloroplatinic acid (H2PtCl6) in ethylene glycol. X-ray powder diffraction, thermogravimetric analysis (TGA) and scanning electron microscopy have been used to analyze structure and surface morphology of the graphene-supported catalyst. The TGA results showed that the Pt loading of the graphene-supported catalyst was 31%. The proof of the Pt particles on the support surfaces was also verified by energy-dispersive x-ray spectroscopy analysis. The commercial carbon-supported catalyst and prepared Pt/graphene catalysts were used as both anode and cathode electrodes for PEMFC at ambient pressure and 70°C. The maximum power density was obtained for the Pt/graphene-based membrane electrode assembly (MEA) with H2/O2 reactant gases as 0.925 W cm2. The maximum current density of the Pt/graphene-based MEA can reach 1.267 and 0.43 A/cm2 at 0.6 V with H2/O2 and H2/air, respectively. The MEA prepared by the Pt/graphene catalyst shows good stability in long-term PEMFC durability tests. The PEMFC cell voltage was maintained at 0.6 V without apparent voltage drop when operated at 0.43 A/cm2 constant current density and 70°C for 400 h. As a result, PEMFC performance was found to be superlative for the graphene-supported Pt catalyst compared with the Pt/C commercial catalyst. The results indicate the graphene-supported Pt catalyst could be utilized as the electrocatalyst for PEMFC applications.

Effect of Contaminants on the Performance of a Proton Exchange Membrane Fuel Cell

The overall cost and performance of proton exchange membrane fuel cells (PEMFCs) are an issue of tradeoffs, and the optimal design depends on the application. For example, avoiding degradation via contamination by using expensive materials increases the initial cost to produce the system, but it can extend life and improve the overall performance. This may be desirable where reliability is paramount. In contrast, using less expensive materials reduces the initial cost, but at the expense of the life-time cost of the system and reduced performance. A mathematical model was developed to quantify the performance loss and recovery of the fuel cell by the balance of plant contaminants. This analysis evolved from a systematic and collaborative investigation to understand, characterize, and quantify the degradation mechanisms due to contaminants that arise from components that make up the fuel-cell system. The degradation mechanisms, and the impact to PEMFC performance and durability of each compound was evaluated by infusion of that compound (i.e. 2,6-diaminotoluene , benzyl alcohol, diethylene glycol monoethyl ether and diethylene glycol monoethyl ether acetate) to the cathode side of the fuel cell followed by measuring the fuel cell performance (i.e. current voltage curve, and high frequency impedance resistance).

Pore Network Modeling of Gas Diffusion Layer As a Support to Better Understand Water Management in Proton Exchange Membrane Fuel Cells

PEMFC (Proton Exchange Membrane Fuel Cells) is considered as a promising solution for producing clean energy. To increase its industrial development for automotive application, performance and durability are still to be improved and cost needs to be reduced. Water management is one of the most important remaining bottlenecks to increase performance and durability of PEMFC. Nevertheless, despite numerous studies, many questions remain open and for instance it is not possible yet to propose reliable optimization of the porous layers (Gas Diffusion Layer, Active Layer…) to optimize transfers (and especially water management) in the MEA and thus increase performance and durability. One main reason is that water transfers in the MEA, and also the link between structural properties of these layers and performance, are not correctly described and understood. For instance, classical modeling used in PEMFC do not account for these structural properties despite the fact that they have a huge influence on performance. More descriptive modeling approaches have been proposed in the last years to better understand the link between structural properties of these layers and transfers in the MEA. Amongst those, Pore Network Modeling (PNM) is more and more applied to GDL. This approach, classically used for soil application, allows simulating two-phase flows in porous media taking into account local properties (Pore/Throat Size Distribution, local structure, local wettability…) which is an important progress compared to classical PEMFC modeling based on Darcy approach. This keynote lecture focuses on PNM and its application to GDL to better understand (gas and liquid) transfers in the MEA and to try and link local structural properties of GDL to the performance of PEMFC. Experimental results are presented all along the discussion either to validate the main hypothesis or to validate the PNM simulations. After a description of the PNM approach, two-phase transfers inside GDL are discussed: influence of wettability, thickness, local structure…; study of GDL degradation; role of the Micro Porous Layer (MPL); validity of classical Darçy “laws”… The two classical scenarii of water transfers in the GDL (liquid injection and/or condensation) are also discussed. The coupling between PNM and performance model, which is a step towards upscaling of PNM, is introduced by the analysis of performance loss of PEMFC as a function of hydrophobicity loss of GDL. Advanced results comparing two-phase PNM based on real 3D images of GDL and X-Ray tomography of liquid pattern inside the GDL are also presented as a step towards even more representative PNM modeling of MEA components. Finally it is shown how PNM can be extended to the Active Layers by coupling two-phase transfers to charge transfers by electrochemistry.

Effect of Contaminants on the Performance of a PEMFC

The overall cost and performance of proton exchange membrane fuel cells (PEMFCs) are an issue of tradeoffs, and the optimal design depends on the application. For example, avoiding degradation via contamination by using expensive materials increases the initial cost to produce the system, but it can extend life and improve the overall performance. This may be desirable where reliability is paramount. In contrast, using less expensive materials reduces the initial cost, but at the expense of the life-time cost of the system and reduced performance. During this study, a systematic and collaborative investigation was conducted to understand, characterize, and quantify the degradation mechanisms due to contaminants that arise from components that make up the fuel-cell system. Initially, the key contaminants that leach out of commercial assembly-aid materials were identified and screened for their detrimental effect on fuel-cell component performance (e.g. catalyst activity, membrane conductivity). Then, model compounds were selected and in-depth parametric studies were performed. In total, four model compounds in the assembly aid materials have been selected to study the degradation mechanisms, and the impact to PEMFC performance and durability of each compound was evaluated by in-situ infusion tests and in-situ membrane resistance tests. The results from these parametric studies were then used to develop mathematical models to quantify the performance loss and recovery of the fuel cell.

Effect of Contaminants on the Performance of a Proton Exchange Membrane Fuel Cell

The overall cost and performance of proton exchange membrane fuel cells (PEMFCs) are an issue of tradeoffs, and the optimal design depends on the application. For example, avoiding degradation via contamination by using expensive materials increases the initial cost to produce the system, but it can extend life and improve the overall performance. This may be desirable where reliability is paramount. In contrast, using less expensive materials reduces the initial cost, but at the expense of the life-time cost of the system and reduced performance. During this study, a systematic and collaborative investigation was conducted to understand, characterize, and quantify the degradation mechanisms due to contaminants that arise from components that make up the fuel-cell system. Initially, the key contaminants that leach out of commercial assembly-aid materials were identified and screened for their detrimental effect on fuel-cell component performance (e.g. catalyst activity, membrane conductivity). Then, model compounds were selected and in-depth parametric studies were performed. In total, four model compounds in the assembly aid materials have been selected to study the degradation mechanisms, and the impact to PEMFC performance and durability of each compound was evaluated by in-situ infusion tests and in-situ membrane resistance tests. The results from these parametric studies were then used to develop mathematical models to quantify the performance loss and recovery of the fuel cell.

Effect of Water Transport on Swelling and Stresses in PFSA Membranes

AbstractThe mechanical durability of membranes used in proton exchange membrane fuel cells (PEMFC) is directly linked to the stresses that evolve in the membrane during fuel cell operation. The stresses are primarily induced due to the swelling of the membrane as it absorbs water, within the mechanical constraints in the fuel cell assembly. Thus, in order to predict the membrane stresses, the water content in the perfluorosulphonic acid (PFSA) membranes is determined numerically via three different absorption models based on experimentally determined water uptake data from the literature. Two models are based on a single, humidity‐dependent Fickian transport coefficient for the bulk PFSA membrane. In the third, two transport properties are modeled to account for a possible difference in transport resistance between the bulk membrane and the outer surface. The membrane sorption behaviors characterized from these three models are then independently incorporated in a representative fuel cell finite element model and subjected to a standard relative humidity (RH) protocol designed for measuring mechanical durability of PEMFCs. The results show that the sorption behavior has a significant effect on the membrane stresses and therefore, may impact the lifetime of the membrane.

The Effect of Common Airborne Impurities and Mixtures on PEMFC Performance and Durability

The effect of exposing the cathode of hydrogen (H2) proton exchange membrane fuel cells (PEMFCs) to the airborne contaminants: sulfur dioxide (SO2), benzene (BZ), and nitrogen dioxide (NO2) on their performance and durability was studied for individual contaminants and mixtures at a total concentration of two parts per million (ppm). The goals were to characterize the effects of contaminant exposure on the: (i) cell performance, (ii) ability of the fuel cell to recover performance using only pure air, (iii) irrecoverable performance loss, and (iv) changes to the durability of the fuel cell as quantified by changes to the electrochemically active surface area (ECSA) of the electrodes. In general, contaminant mixtures decreased performance more than single contaminants. Performance loss was recoverable with pure air when no SO2 was in the stream and incomplete when it was. Contaminant exposure accelerated ECSA loss.