Low Humidity Operation Research Articles

Investigation of a Correlation Between Carbon Corrosion and Proton Conductivity of Ionomers in PEFC Under the Low Humidity Operation

One of the strategies to further increase PEFC performance is elevation of the operation temperature. However, durability of materials used in PEFC is assumed to be lowered at the higher temperature condition, which is also accompanied by the lower humidity condition. We have been studying degradation phenomena of cathode catalyst layers at the higher temperature. During our study, we have found carbon corrosion is related to the proton conductivity of ionomers in cathode catalyst layers when PEFC is operated at the higher temperature with lower humidity. Therefore, in this study, a correlation between carbon corrosion and proton conductivity of ionomers has been investigated.MEAs were prepared using a standard catalyst and two types of electrolytes, Pt/Ketjen black catalyst (TEC10E50E, TKK Corp.), and Nafion® and Aquivion®, respectively. Here, The Aquivion® has higher proton conductivity than Nafion®. The operating condition of the higher temperature with lower humidity, 105 °C-RH57%, was applied. Durability of MEAs was studied through the potential cycles between 1.0 and 1.5 V assuming “stop/start cycles” of FCVs.[1], [2] Durability of two different MEAs (Nafion® MEA and Aquivion® MEA) was evaluated in terms of electrochemical and structural changes before and after the cycle test. Here, structural changes were analyzed using a FIB-SEM technique followed by 3D reconstruction of cathode catalyst layers. Although current–voltage curves were not very much different between two types of MEAs even after 2000 cycles, the macroporous structures of catalyst layers were different. Porosity was almost homogeneous through the catalyst layer in Aquivion® MEA, but larger pores intensely formed near the Nafion® membrane side in the case of Nafion® MEA. The possible degradation mechanism is illustrated in Fig.1 and explained in the following. The carbon oxidation reaction probably occurs mainly by reacting with H2O being formed by a fuel cell reaction if it is under the low humidity condition. If proton conductivity is high enough like Aquivion® ionomer, H2O is probably generated more uniformly and carbon oxidation also occurs uniformly throughout the cathode catalyst layers. On the other hand, when proton conductivity is low like Nafion® ionomer, H2O is mainly generated at the interface between the cathode layer and Nafion® membrane. Then, carbon oxidation intensely occurs at the interface between the cathode and the electrolyte membrane. Reference[1] A. Ohma et al., ECS Trans, 41 (2011) 755.[2] H. Miyamoto et al., ECS Trans, 75 (2016) 329. Figure 1

4-4-4 高温低加湿発電に向けた炭化水素系電解質膜を用いた高分子電解質形燃料電池の開発(4-4 燃料電池1,Session 4 新エネルギー,研究発表)

4-4-4 高温低加湿発電に向けた炭化水素系電解質膜を用いた高分子電解質形燃料電池の開発(4-4 燃料電池1,Session 4 新エネルギー,研究発表)

Experimental study on the membrane electrode assembly of a proton exchange membrane fuel cell: effects of microporous layer, membrane thickness and gas diffusion layer hydrophobic treatment

In this study, electrochemical impedance spectroscopy (EIS) is employed to analyze the influence of microporous layer (MPL), membrane thickness and gas diffusion layer (GDL) hydrophobic treatment in the performance of a proton exchange membrane (PEM) fuel cell. Results show that adding a MPL increases cell performance at low to medium current densities. Because lower ohmic losses are observed when applying a MPL, such improvement is attributed to a better hydration state of the membrane. The MPL creates a pressure barrier for water produced at the cathode, forcing it to travel to the anode side, therefore increasing the water content in the membrane. However, at high currents, this same phenomenon seems to have intensified liquid water flooding in the anode gas channels, increasing mass transfer losses and reducing the cell performance. Decreasing membrane thickness results into considerably higher performances, due to a decrease in ohmic resistance. Moreover, at low air humidity operation, a rapid recovery from dehydration is observed when a thinner membrane is employed. The GDL hydrophobic treatment significantly improves the cell performance. Untreated GDLs appear to act as water-traps that not only hamper reactants transport to the reactive sites but also impede the proper humidification of the cell. From the different designs tested, the highest maximum power density is obtained from that containing a MPL, a thinner membrane and treated GDLs.

Analysis of water film formation and low-humidity operation characteristics of a polymer electrolyte fuel cell (PEFC)

Substantial efforts have been made to advance automotive fuel cell technologies with high performance, low cost, and long durability. The dry or low-humidified reactant inlet conditions are typical of automotive fuel cell operations in order to simplify the fuel cell system and reduce parasitic power consumption for external humidification. The low-humidity conditions can cause severe membrane dehydration and thus lead to lower overall cell performance. On the other hand, even under low-humidity operations, oxygen access to the reaction sites can be significantly limited by the liquid film formation in the catalyst layer (CL) due to the hydrophilic nature of electrolyte inside the CL. The purpose of this work is to numerically investigate the characteristics of low-humidity fuel cell operations using a comprehensive two-phase fuel cell model. The main focus is placed on analyzing the water content distribution in the membrane and film formation in the cathode CL under different inlet pressures and operating current densities. The CL is assumed to exhibit cylindrical and spherical pore structures and thus two liquid film growth modes are analyzed and discussed under different pressures and liquid saturation levels. The simulation results clearly elucidate detailed behaviors of membrane dehydration and liquid film formation/growth phenomena and their influences on overall cell performance.

Cerium Migration in Polymer Electrolyte Membranes

Cerium enhances polymer electrolyte membrane (PEM) fuel cell durability by scavenging damaging radical species which are generated during operation. However, during cell fabrication, conditioning, and discharge, cerium migrates between the PEM and catalyst layers (CLs), which can reduce cell performance [1]. In this work, cerium migration within the cell and washout from it were quantified in membrane electrode assemblies (MEAs) which were fabricated using PEMs containing cerium concentrations of ~6.0µg/cm2. MEAs were subjected to accelerated stress tests (ASTs) at open circuit voltage (OCV) with cell temperatures of 90°C in hydrogen/air. ASTs were performed at three different humidity conditions: 100% RH, 30% RH, and wet/dry cycling ASTs, where the cell was cyclically exposed to humidified and dry reactant gasses for 30/45s, respectively [2]. After ASTs, through-thickness and in-plane cerium migration profiles were characterized in MEAs using X-ray fluorescence. In-plane cerium migration from the gasketed, inactive PEM border region into the active area was observed and correlated to the active area water content during the AST. Cerium concentration in the active area increased from ~6.0 to 15.3µg/cm2 after 2,000h of 100% RH operation. After 456h of 30% RH operation, active area water content was less than at 100% RH, and cerium concentration increased to 9.84µg/cm2. After 823h of humidity cycling, cerium concentration remained unchanged. Through-thickness cerium migration from the PEM into the CLs is enhanced by low humidity operation. 30% RH operation resulted in uniform migration of cerium into both the anode and cathode CLs. PEM cerium was reduced from ~6.0 to 3.7µg/cm2, while anode and cathode CL concentrations were uniformly increased from 0.0 to 2.3 and 3.5µg/cm2, respectively. Conversely, after 100% RH operation, more cerium remained in the active area of the PEM. In-plane concentrations were maintained at ~6.0µg/cm2 near the inlet, however, concentration increased linearly to ~15µg/cm2 near the outlet. These concentration gradients were attributed to the condensation and subsequent flow of liquid water from cell inlet to outlet. Subjecting the MEA to humidity cycling resulted in both significant through-thickness cerium migration out of the PEM, as well as concentration gradients from PEM inlet to outlet. Under these conditions, average PEM concentration was reduced to 0.98µg/cm2. PEM cerium was depleted from the inlet and its concentration was only 1.5µg/cm2 near the outlet. Average anode and cathode CL cerium concentrations increased to 1.9 and 3.1µg/cm2, respectively. Cerium depletion which results from such conditions could reduce the amount of available radical scavengers, leaving the PEM more susceptible to chemical attacks. Effluent cell water samples were collected from the anode and cathode during ASTs and analyzed to measure fluoride and cerium concentrations using ion chromatography and inductively coupled plasma mass spectrometry. Fluoride and cerium emissions are correlated, which suggests that ionomer degradation products serve as counter-ions for emission from the cell. The most aggressive test conditions, however, only reduced the total cerium inventory in the MEA by < 0.5%. Fluoride emission rates (FERs) are also correlated to the average PEM cerium content after ASTs, which indicates a relationship between membrane degradation and cerium migration (Figure 1). We propose that during ASTs, cerium migration from the PEM into the CLs is driven by both concentration gradients between the PEM and CL, which arise due to cell humidification, and from degradation, itself, which is proportional to AST aggressiveness (wet/dry cycling > 30% RH >> 100% RH). We also postulate that cerium interacts with carbon catalyst support particles in the CLs which prevents its ionic equilibration with PEM ionomer. Ex situ experiments demonstrate the ability of carbon black to stabilize quantities of cerium identical to those measured in the CLs after ASTs. In addition to water content and degradation, potential gradients and proton flux strongly influence cerium migration from the PEM into the CLs. Window cell experiments will be discussed, which determine the relative influence of these mechanisms and their effects in operating cells. Further understanding of these mechanisms will enable cerium stabilization within the active area of the PEM, in order to mitigate performance losses and further enhance cell durability. Acknowledgements This research is supported by DOE Fuel Cell Technologies Office, through the Fuel Cell Performance and Durability (FC-PAD) Consortium; Fuel Cells program manager: Dimitrios Papageorgopoulos.

Cerium Migration during PEM Fuel Cell Accelerated Stress Testing

Cerium is a radical scavenger which improves polymer electrolyte membrane (PEM) fuel cell durability. During operation, however, cerium rapidly migrates in the PEM and into the catalyst layers (CLs). In this work, membrane electrode assemblies (MEAs) were subjected to accelerated stress tests (ASTs) under different humidity conditions. Cerium migration was characterized in the MEAs after ASTs using X-ray fluorescence. During fully humidified operation, water flux from cell inlet to outlet generated in-plane cerium gradients. Conversely, cerium profiles were flat during low humidity operation, where in-plane water flux was negligible, however, migration from the PEM into the CLs was enhanced. Humidity cycling resulted in both in-plane cerium gradients due to water flux during the hydration component of the cycle, and significant migration into the CLs. Fluoride and cerium emissions into effluent cell waters were measured during ASTs and correlated, which signifies that ionomer degradation products serve as possible counter-ions for cerium emissions. Fluoride emission rates were also correlated to final PEM cerium contents, which indicates that PEM degradation and cerium migration are coupled. It is proposed that cerium migrates from the PEM due to humidification conditions and degradation, and is subsequently stabilized in the CLs by carbon catalyst supports.

Water Uptake in Novel, Water-Stable, Heteropoly Acid Films

High temperature, low humidity fuel cell operation is one method of increasing the potential of commercialization of fuel cell vehicles. In order for performance to be suitable under these conditions, new polymer electrolyte membrane material needs to be synthesized and characterized. One particularly important property is water content. Water is an important aspect of polymer electrolytes, often having strong influence on ion transport and fuel cell performance. A novel material has been made, containing a heteropoly acid covalently bound to a fluoroelastomer. This material was analyzed with DVS to gain insight on water sorption. The data suggested that the water content is similar to Nafion® with respect to humidity, thus some other parameter is likely to cause improved performance under high temperature and low humidity operation.

A Mechanistic Study of Perfluorosulfonic Acid Membrane Water Permeance Degradation in Air

Chemical degradation of perfluorosulfonic acid (PFSA) membranes of proton exchange membrane (PEM) fuel cells is a well-known process which can limit PEM fuel cell lifetimes (1, 2). Chemical degradation can weaken membranes to the point where they no longer provide an effective barrier to reactant gas permeation or electrical shorting. PEM chemical degradation is the result of a complex interplay between mechanical and chemical degradation processes, both of which are strong functions of operating conditions. In particular, hot and dry operating conditions induce high rates of chemical degradation whereas relative humidity cycling between wet and dry states challenges the mechanical integrity of the membranes. Because chemical degradation of PEMs is accelerated under low humidity operating conditions, membrane lifetimes are significantly enhanced by providing humidified inlet gas streams to the fuel cell stack. In addition, fuel cell performance is improved with the use of humidified input streams. An efficient method of humidifying the cathode inlet stream employs a passive water vapor transfer (WVT) device that delivers water vapor from the wet cathode exhaust to the cathode inlet stream (Figure 1). The key component of a WVT device is a highly water permeable, but gas impermeable membrane. Not surprisingly, ionomeric membranes such as PFSAs function as excellent WVT materials. Ideally, WVT membrane materials will maintain adequate durability and performance characteristics to provide necessary stack humidification throughout the target operational life of an automotive fuel cell system (>10years />8000 operational hours). Several general pathways known to reduce water permeance values include cation contamination, mechanical degradation and chemical degradation. We have observed during in-situ and ex-situ testing of WVT materials in air that the water permeance values of PFSA-based WVT membranes can degrade by 75% over time. This degradation of water permeance is associated with a physical and chemical change of the membrane. The extent of ionomer transformation is readily monitored using transmission FTIR spectroscopy by the appearance of new absorption bands centered at 3200 and 1434 cm-1 and also by the decreases in water stretching and bending bands centered near 2800 and 1700 cm-1, respectively (Figure 2). In addition to water permeance, other membrane properties including proton conductivity and ion exchange capacity decrease as a function of this degradation reaction. The fully reversible, deuterium exchanged FTIR spectrum proves that the degradation product is not the cross-linked sulfonic acid anhydride as previously suggested (3). Through an extensive series of kinetic studies and analytical characterization, we have determined that the membrane degradation product results from a reaction between the ionomer and a trace atmospheric gas. We will demonstrate that the definitive structural assignment is fully consistent with all known properties and behaviors of the degraded membrane. The revelation of the PFSA degradation product and its formation mechanism will aid in development of highly durable fuel cell systems. References R. Borup, J. Meyers, B. Pivovar, Y. Kim, R. Mukundan, N. Garland, D. Meyers, M. Wilson, F. Garzon, D. Wood, P. Zelenay, K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. McGrath, M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A. Nishikata, Z. Siroma, Y. Uchimoto, K Yasuda, K. Kimijima, N. Iwashita, Chem. Rev. 107, 3904, (2007)C. S. Gittleman, F. D. Coms, Y-H. Lai, Membrane Durability: Physical and Chemical Degradation, in Polymer Electrolyte Fuel Cell Degradation, pp 15-88, M. M. Mench, E. C. Kumbur, T. N. Veziroglu, eds, Academic Press, 2012F. M. Collette, C. Lorentz, G. Gebel, F. Thominette, J. Mem. Sci. 330, 21-29, (2009) Figure 1

Development of novel proton exchange membrane fuel cells using stamped metallic bipolar plates

This study presents the development of novel proton exchange membrane fuel cells using stamped metallic bipolar plates. To achieve uniformly distributed and low pressure-drop flow fields within fuel cells, a novel bipolar plate with straight channels is designed and verification of a fuel-cell short stack using this bipolar plate is performed. In the experiments, low-temperature and low-humidity operations and high-temperature and high-humidity operations are adopted to evaluate effects of stack temperature and inlet relative humidity on performance at various outlet pressures. Experimental results show that under low-temperature and low-humidity operations, increasing the outlet pressure enhances stack performance and reduces performance differences between various stack temperatures. Under high-temperature and high-humidity operations, stack performance increases with increasing outlet pressures, while the extent of their increase becomes smaller. Compared to low-temperature and low-humidity operations, high-temperature and high-humidity operations have better electrochemical reactions and membrane hydration and, thus, better stack performance. In this study, the operation with a stack temperature of 80 °C and outlet pressure of 4 atm produces the best performance of 1100 mA cm−2 at 0.646 V.

Effects of operating parameters on the transient response of proton exchange membrane fuel cells subject to load changes

Water management is critical to the steady state and transient performance of proton exchange membrane (PEM) fuel cell systems. The dynamic behavior of PEM fuel cells is profoundly related to the dynamics of water transport in the fuel cell system, which include electro-osmotic drag and back-diffusion of water through membrane and rate at which water is generated, supplied and removed from the electrodes. In particular, for low humidity operations, water transport dynamics plays a dominant role in determining the time taken to reach steady state. Toward an understanding of the fuel cell dynamics for dry operations, numerical simulations are carried out for a single channel polymer electrolyte (PEM) fuel cell undergoing step change in cell voltage over wide range of operating conditions. A detailed model-based parametric study is carried out to analyze the effects of operating conditions on membrane water content and the time taken by the single cell system to reach steady state. Based on the studies, design windows are presented that limit the time taken to reach steady state to a desired value. Optimum parameter combinations that minimize the transient time are also identified from the studies.

New Developments in Electrospun Nanofiber Electrodes for Hydrogen/Air Fuel Cells

The hydrogen/air proton-exchange membrane (PEM) fuel cell is a promising candidate for automotive power plants due to its high power output, moderate operating temperature, and quick start-up. The commercialization of such devices is hindered by the high price and inadequate durability of the platinum catalyst electrodes. Recent R&D efforts have been directed at reducing Pt loading and fabricating catalysts with supports that can withstand the harsh automotive operating environment. Work in this area includes the investigation of metal alloys, core-shell nanostructures, and metal oxide supports. Work on non-platinum group metal catalysts for the oxygen reduction cathode in a hydrogen/air fuel cell has also been reported. The creation of particle/polymer nanofiber mat electrodes via electrospinning represents an entirely different strategy for lowering cathode catalyst loading while maintaining high power output in a hydrogen/air fuel cell. A nanofiber electrode morphology with intra and inter-fiber porosity is advantageous because: (i) it increases catalyst contact with reactant gases, (ii) it provides a sufficient number of pathways for effective proton and electron conduction, and (iii) there is sufficient inter-fiber porosity for release of cathode product water. Previously, we have shown that more power was generated from nanofiber electrode MEAs than a conventional MEA with decal electrodes [1,2]. For example, the maximum power density for a H2/air fuel cell with a nanofiber cathode and anode (at loadings of 0.065 mgPt/cm2 and 0.10 mgPt/cm2, respectively) was 437 mW/cm2 vs. 400 mW/cm2 for a decal MEA with a cathode containing 40% more Pt (a loading of 0.104 mgPt/cm2) and an anode loading of 0.10 mg/cm2. In these experiments, the nanofiber catalyst binder was a mixture of Nafion® and poly(acrylic acid), the binder used in decal electrodes was neat Nafion, and the membrane was Nafion 212. The fuel cell operating conditions were: 80oC, 1 atm (ambient) pressure, 125 sccm H2, and 500 sccm air. As another example, an electrospun cathode with a Pt loading of 0.055 mg/cm2 produced a maximum power density of 906 mW/cm2 at 80°C and 3 atm pressure with 2000 sccm fully humidified air and 500 sccm H2. Additionally, the mass activity (0.16 A/mgPt at 0.9 V) and electrochemical surface area (~41 m2/g) of nanofiber cathodes containing Johnson-Matthey HiSpec™ 4000 catalyst were found to be very high and electrospun cathodes exhibited less degradation than a conventional decal cathode in an accelerated voltage cycling durability test.In this presentation, new catalyst/binder electrode formulations will be discussed, in terms of the conditions required to electrospin well-formed fibers and the performance of the fiber mats as cathodes in a hydrogen/air fuel cell operating at 80oC. MEAs are assessed in terms of the initial power density at high and low humidity operation and the changes in power output during voltage cycling Pt dissolution and carbon corrosion durability experiments. All anode and cathodes were prepared with Johnson Matthey HiSpec 4000 catalyst (40% Pt on Vulcan carbon), at a Pt loading of 0.10 mg/cm2. Acknowledgments This work was funded by EMD Millipore and the NSF-funded TN-SCORE program, NSF EPS-1004083, under Thrust 2. References W. Zhang and P. N. Pintauro, ChemSusChem, 4, 1753-1757 (2011).M. Brodt, R. Wycisk, and P. N. Pintauro, Journal of the Electrochemical Society, 160, F744-F749 (2013).

Influence of membrane properties on the transient behavior of polymer electrolyte fuel cells

Understanding the performance of proton exchange membrane (PEM) fuel cells is critical to the water management in the fuel cell system. Low-humidity operating conditions present a complex interaction between dynamic behavior and water transport owing to different time scales of water transport mechanisms in the transient process. Toward understanding the effects of membrane properties on the dynamic behavior, this paper presents numerical simulations for a single channel PEM fuel cell undergoing changes in load, by subjecting the unit cell to step change in current. The complex interaction between cell voltage response and water transport dynamics for various membrane properties is explored in detail, where the performance is critically related to the water content of the membrane. Detailed computational fluid dynamics (CFD) simulations are carried out to show that step increase in current density leads to anode dryout due to electro-osmotic drag, and to investigate the dependence of transient behavior on the variations in membrane properties. The results show that water uptake by the membrane is a crucial factor in determining the occurrence of anode dryout and hence voltage reversal, and can be avoided by a graded membrane design.

Effects of Incorporation of SiO2 Nanoparticles into Sulfonated Polyimide Electrolyte Membranes on Fuel Cell Performance under Low Humidity Conditions

We have developed new composite membranes of sulfonated polyimide containing triazole groups (SPI-8) as a matrix ionomer and SiO2 nanoparticles. The incorporation of SiO2 nanoparticles remarkably improved the fuel cell performances during low humidity operation at 53% RH and 80°C. Among the cells with SPI-8 membranes with uniformly dispersed SiO2 from 0 to 15 wt%, the single cell with 10 wt% SiO2/SPI-8 was found to exhibit the highest I−E performance, with the highest mass activity at 0.85V and the smallest oxygen-transport overpotential (O2-gain) as well as the lowest ohmic resistance. This strongly indicates that SiO2 nanoparticles were able to promote the back-diffusion of water produced in the cathode catalyst layer to the anode catalyst layer, maintaining high water content in the membrane during the operation. It was found that the cell with a bilayer SPI-8 membrane having 10 wt% SiO2 in the anode-side layer and 3 wt% SiO2 in the cathode-side layer exhibited performance superior to that with a uniform dispersion of 10 wt% SiO2, especially in the higher current density region at low RH, which can be ascribed with certainty to the fact that the concentration gradient of SiO2 in the SPI-8 led to enhancement of the back-diffusion of water through the membrane

Separation Analysis of Electro-Osmosis and Diffusion of Water in Electrolyte Membrane of PEMFC Under Low-Humidity Operation

not Available.

Highly phosphonated polypentafluorostyrene: Characterization and blends with polybenzimidazole

In this study we present results of the conductivity and resistance to thermooxidative and condensation reactions of a highly phosphonated poly(pentafluorostyrene) (PWN2010) and of its blends with poly(benzimidazole)s (PBI). This polymer, which combines both: (i) a high degree of phosphonation (above 90%) and (ii) a relatively high acidity (pKa (–PO3H2↔–PO3H−)∼0.5) due to the fluorine neighbors, is designed for low humidity operating fuel cell. This was confirmed by the conductivity measurements for PWN2010 reaching σ=5×10−4Scm−1 at 150°C in dry N2 and σ=1×10−3Scm−1 at 150°C (λ=0.75). Furthermore, this polymer showed only 48% of anhydride formation when annealing it at T=250°C for 5h and only 2% weight loss during a 96h Fenton test. These properties combined with the ability of the PWN2010 to form homogeneous blends with polybenzimidazoles resulting in stable and flexible polymer films, makes PWN2010 a very promising candidate as a polymer electrolyte for intermediate- and high-temperature fuel cell applications.

Separation Analysis of Electro-Osmosis and Diffusion of Water in Electrolyte Membrane of PEMFC under Low-Humidity Operation

To alleviate performance degradation of proton exchange membrane fuel cell (PEMFC) due to membrane dehydration during low-humidity operation, it is essential to understand and control water transports through electrolyte membrane. In this study, the water vapor and current distributions on the anode side of a PEMFC was firstly measured by using the effective visualization tool proposed in our previous work, and the net water transport coefficient was quantitatively estimated under low-humidity conditions. Furthermore, the electro-osmotic drag (EOD) coefficient of Nafion membrane was also measured based on hydrogen pumping method. Therefore, the electro-osmosis and back-diffusion of water through the electrolyte membrane in the operating PEMFC can be separately analyzed. Results revealed that the cathode inlet humidification promotes the back-diffusion of water rather than the electro-osmotic flow, and prevents the membrane dryout.

Experimental Study of the Activity Change Due to Operation History in PEMFC

We have developed a new method of precisely determining the activity of cathode catalyst layer in MEA by maintaining the effect of platinum oxides at a constant level and have subsequently studied activity changes resulting from various operational histories. Cyclic voltammetry was used to quantitatively correlate activity changes in the cathode catalyst layer with various operational conditions and demonstrated that activity decreased after low humidity operation and recovered after high humidity operation in a N2 atmosphere at potentials below 0.2 V. The activity changes were found to be very slow and a span of more than 8 hours was required for the activity to reach the steady state. This phenomenon is presumably caused by the adsorption/desorption of ionomer sulfonate groups on the Pt surface, based on observations that the activity changes are consistent with sulfonate group adsorption and dependent on the ionomer structure.

An Experimental Study of the Effects of Operational History on Activity Changes in a PEMFC

The accurate evaluation of the activation overpotential of a membrane electrode assembly (MEA) is essential for fuel cell design. We have developed a new method of precisely determining the activity of cathode catalyst layer in MEA by maintaining the effect of platinum oxides at a constant level and have subsequently studied activity changes resulting from various operational histories. Cyclic voltammetry was used to quantitatively correlate activity changes in the cathode catalyst layer with various operational conditions and demonstrated that activity decreased after low humidity operation and recovered after high humidity operation in a N2 atmosphere at potentials below 0.2 V. The activity changes were found to be very slow and a span of more than 8 hours was required for the activity to reach the steady state. This phenomenon is presumably caused by the adsorption/desorption of ionomer sulfonate groups on the Pt surface, based on observations that the activity changes are consistent with sulfonate group adsorption and dependent on the ionomer structure. Oxygen transport resistance in the catalyst layer also varied in conjunction with the activity changes. Two possible Pt/ionomer interface models are proposed in an attempt to explain the above observations.

Experimental analyses of low humidity operation properties of SiO2-containing catalyst layers for polymer electrolyte fuel cells

Electrochemical operation properties of SiO2-containing catalyst layers (SiO2-CLs) under low humidity were clarified experimentally in a polymer electrolyte fuel cell (PEFC). The mass activities at 0.85V and current densities at 0.7V increased with increasing SiO2 content below 53% RH at 80°C. The use of SiO2-CLs led to decreased ohmic resistance due to increased water content. It was found that such an increase in water content in the SiO2-CLs increased the electrochemically active surface area (ECA) of the Pt catalyst, due to enhancement of the proton-conducting network in the electrolyte binder, and suppressed the specific adsorption of sulfonate anions on the Pt surface. It was also clarified that the SiO2 addition in the CL promoted the back-diffusion of water generated at the cathode to the anode, resulting in an increased O2 gas diffusion rate to the cathode-CL/PEM interface, accompanied by an increased water content in the membrane. All of these factors greatly contributed to improve the cell performance under low humidity.

Failure Mode Analysis of Membrane Electrode Assembly (MEA) for PEMFC under Low Humidity Operation

AbstractThe degradation mechanisms and phenomena of membrane electrode assembly (MEA) for proton exchange membrane fuel cell (PEMFC under low humidity operation were investigated by means of several on‐line electrochemical methods and transmission electron microscopy (TEM). Electrochemical active surface area (ECSA) of catalyst and hydrogen crossover of MEA were evaluated by cyclic voltagram (CV) and linear sweep voltagram (LSV), respectively. The decrease of catalytic active site and degradation of membrane were evident by ECSA loss and hydrogen crossover rate increase after long‐term discharging test. Membrane degradation played the major role for MEAs failure under low humidity operation in this study. The Pt clusters were also observed in the membrane, formed by Pt ions migration and reduction in the membrane, which can react with the crossover gases to produce free radicals that attack membrane and ionomers in the catalyst layer.Those phenomena will drive membrane to rupture and cause a short circuit in MEA. By AST experiments and fluoride release rate results, the MEA with Pt‐alloy/C catalyst can show better performance and durability due to its high selectivity on ORR to depress the free radicals formation.