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Air Impurities
Abstract
Commercialization of the proton exchange membrane fuel cell, an efficient energy-conversion device, requires additional gains in system lifetime. Contamination represents a key degradation mode. Its status is summarized and analyzed to identify research needs. Contaminant sources include ambient air, system components located upstream of the fuel cell stack, and fuel and coolant loops. The number of reported contaminants was conservatively estimated at 97, but many contaminant compositions are still unclear and many gaps remain to be explored, including airstream system components and coolant and fuel streams. For the latter cases, contaminants may reach the cathode compartment by diffusion through the membrane or as a result of seal or bipolar plate failure, thus representing potential interaction sources. In view of this large potential inventory of contaminants, recommendations were made to accelerate studies, including the addition of identification tests performed by material developers, development of standard tests, and definition of an exposure scale for ranking purposes. Because anions are excluded from the membrane in contact with weak solutions (Donnan exclusion), mechanisms involving anions need to be reevaluated. Contaminant mechanisms were synthesized, resulting in only eight separate cases. This situation favors the development of two key simple mathematical models addressing kinetic and ohmic performance losses that are expected to positively impact the development of test plans, data analysis, model parameter extraction, contaminant classification (use of apparent rate constants), and hypothetical scenario evaluation. Many mitigation strategies were recorded (41) and were downselected by elimination of untimely material-based solutions. The remaining strategies were grouped into three generic approaches requiring further quantitative evaluation and optimization: cathode compartment wash, cathode potential variations, and manufacturing material and processing specifications.
... The development of additional simple models for other key mechanisms is expected to increase the number of fingerprints available and to positively contribute to ongoing screening efforts. 33 Experimental approach.-Several aspects of the model have not been directly ascertained and should be considered for a general contaminant test procedure to facilitate model development. ...
A mathematical model is presented relating the presence of NO2 in the oxidant stream to the fuel cell performance loss. Both contamination and recovery processes are considered. The model is validated using cell voltage/current density distribution and cell voltage change as a function of NO2 concentration transient data. The model is used to discuss NO2 threshold concentrations and contaminant screening. General and NO2 specific gaps in contaminant characterization methodology are also highlighted. (c) 2008 The Electrochemical Society.
... The majority of research in gas separation is geared towards application of ionic liquids to absorb contaminant gases from flue gas with little attention given to the removal of the pollutants from atmospheric environments where contaminant concentration is relatively low [16– 19]. The similarities in components of flue gas and polluted air imply that ionic liquids have potential application in remediation of pollutants, such as SO 2 , from environmental air [20]. High SO 2 absorption capacities have been found with ionic liquids using pure SO 2 or a mixture of SO 2 /N 2 [17,21222324. ...
Proton exchange membrane fuel cells (PEMFCs) with 0.1 and 0.4 mg Pt cm−2 cathode catalyst loadings were separately contaminated with seven organic species: Acetonitrile, acetylene, bromomethane, iso-propanol, methyl methacrylate, naphthalene, and propene. The lower catalyst loading led to larger cell voltage losses at the steady state. Three closely related electrical equivalent circuits were used to fit impedance spectra obtained before, during, and after contamination, which revealed that the cell voltage loss was due to higher kinetic and mass transfer resistances. A significant correlation was not found between the steady-state cell voltage loss and the sum of the kinetic and mass transfer resistance changes. Major increases in research program costs and efforts would be required to find a predictive correlation, which suggests a focus on contamination prevention and recovery measures rather than contamination mechanisms.
The effects of different airborne particulate contaminants (oxalic acid, soot, ammonium sulfate, and sodium chloride) and the gaseous air impurity NOx on the performance of a passive air-breathing proton exchange membrane fuel cell (AB-PEMFC) were investigated by introducing the pollutants into the oxidant air fed to a small AB-PEMFC module. All contamination data were corrected for variations in air temperature and relative humidity. While 10 ppmv NOx degraded fuel cell performance by 22% within 4 h and showed partial recovery of 70% within another 11 h, the effects of the aerosol particles were considerably less severe. Only oxalic acid and ammonium sulfate exhibited significant performance degradation with upper limits of 24% 1000 h−1 and 3% 1000 h−1, respectively. Sodium chloride as well as soot particles did not cause any significant performance decline. Because diffusion is the main transport mechanism for aerosol particles into a passive AB-PEMFC, only a minor fraction of the total particle mass can actually deposit inside these fuel cells resulting in rather low effects even under very high aerosol loadings. Therefore, aerosol related effects seem negligible under ambient conditions.
Models were derived for the scavenging effect of product liquid water on airborne proton exchange membrane fuel cell (PEMFC) contaminants. A time scale analysis of contaminant mass transfer processes, product water accumulation in the gas diffusion electrode, and dissociation reactions indicated that the contaminant saturates the product liquid water simplifying model derivation. The baseline model only accounts for contaminant solubility. A multi-scale extension to this model was derived for the presence of contaminant dissociation reactions within the product liquid water using SO2 as a model contaminant. The extended model demonstrates the large impact of dissociation reactions at low SO2 concentrations. For both models, explicit expressions for the average gas phase contaminant concentration within the fuel cell were also derived and can be used as a surrogate for the effective contaminant concentration to correlate the fuel cell performance loss and facilitate the definition of tolerance limits and filtering equipment. The model was validated using a non-operating PEMFC. The water was transferred from the anode to the cathode by thermo-osmosis. Model contaminants, methanol and SO2, were injected with an inert carrier gas to avoid reactions. The model proved to be acceptable with parameters approximately equal to published values. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4:0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org.
Contaminants were selected from the Environmental Protection Agency lists. Six qualitative down selection criteria were used to reduce the list to the 21 1st tier contaminants. The effect of 1st tier contaminants, mostly organic with the exception of ozone, on PEMFC performance is reported. The behavior of these contaminants was classified into 5 different cases; no effect, recoverable effect, partly recoverable effect, irrecoverable effect and supra-recoverable effect. The steady state contamination and irrecoverable performance losses respectively varied from 0 to 90% and -2 to 80%. Contamination and recovery time scales significantly varied within a similar to 0.01 to similar to 28 h range. Acetaldehyde and propene were characterized by the new supra-recoverable effect. For trichlorofluoromethane, iso-propanol and propene, a decrease of the relative humidity led to a significant change in cell performance (steady states, time scales). This effect was ascribed to several causes including the scavenging effect of liquid water by contaminant dissolution and a non zero water reaction order. Two quantitative selection criteria based on observed fuel cell performance were used to reduce the 1st tier list to 7 2nd tier contaminants for more resource intensive tests. These 2nd tier contaminants are acetonitrile, acetylene, bromomethane, iso-propanol, methyl methacrylate, naphthalene and propene.
The behavior of PEMFCs exposed to airborne contaminants was characterized in 2 stages. In the first stage, impedance spectroscopy was used to identify resistance loss types. In the second stage, other ex situ and in situ characterization methods were employed to obtain more detailed information about observed resistance loss types: rotating ring/disc electrode, membrane conductivity cell, tracer based method for residence time distribution measurements, segmented cell, and fuel cell coupled with a gas chromatograph/mass spectrograph. All 7 VOCs led to kinetic and mass transfer resistance changes. Only acetonitrile led to an ohmic resistance change. A decrease in catalyst active area (7 to >90 %) and increase in H 2O2 side reaction product (18 to 1027 %) accounts for the smaller oxygen reduction current (12 to 100 %). An acetonitrile decomposition product is presumed to be responsible for the observed in situ ohmic loss because acetonitrile did not show an effect in the conductivity cell. Modest current redistributions were noted (<19 % on an absolute and dimensionless basis) but acetylene also initially showed a strong transient effect (≤100 % on an absolute and dimensionless basis) attributed to a gradual contamination along the flow field length. Only the unaffected contaminant species, CO 2 and CO were detected at the cathode outlet. Contaminant conversion increased at higher cell voltages (62 to 332 % V-1) indicative of electrochemical oxidation. An acetylene contamination mechanism is proposed.
A PEMFC stack was contaminated with 50 ppm propene in air. The average performance decreased by 243 mV until a steady state was reached. After the propene injection was interrupted, the average performance increased until a new steady state was attained that exceeded the initial value by ∼6 mV. Propene contamination amplified the uneven cell performance distribution along the stack length. End cells showed a larger performance change than the other cells with respect to the pre-contamination phase of respectively 24 and 15 mV. The effect was ascribed to interactions between propene and the uneven temperature distribution. Propene contamination affects both oxygen reduction kinetics and mass transfer as revealed by impedance spectroscopy measurements.
Proton exchange membrane fuel cells exposed to 300 ppm C2H2 were studied in situ with chronopotentiometry, chronoamperometry, cyclic voltammetry and gas chromatography. Fuel cells significantly lost and fully regained performance at all tested cell potentials. The performance changes were due to the presence of C2H2 or reaction products on the Pt catalyst surface. Two cyclic voltammetry peak potentials were identified and respectively ascribed to reduced acetylene species or CO (0.57 to 0.76 V vs HRE), and adsorbed C 2H2 (1.04-1.1 vs HRE). Gas analysis partially confirmed the existence of these species because only C2H2, CO 2 and CO were identified and revealed an increase in C 2H2 conversion from ∼1 to 100 % in the 0.55 to 0.85 V cell voltage range.
The main challenges for commercialization of fuel cells are cost and durability. Using off-the-shelf materials for system components in Proton Exchange Membrane Fuel Cells (PEMFCs) may lower the cost provided they do not compromise function, fuel cell performance, or life. The purpose of this paper is to increase the understanding of contamination effects of these materials and the contaminants that leach from them. This paper in-situ screening data for six families of materials (i.e., urethane, silicone, epoxy, acrylic, methacrylate, and perfluoroalkylether) broadly classified as assembly aids that may be used as adhesive and lubricants in PEMFC systems.
The effects of temperature on sulfur dioxide (SO2)
contamination in PEMFCs are investigated by operating single cells with
2 ppm SO2 in the cathode at different temperatures. Cell
performance response shows that voltage degradation was delayed and
appears a transition of multiple processes at low temperatures; a
similar performance loss is observed when performances reached steady
state. The restored performance from the reversible and the irreversible
degradations highly depends on temperature. At low temperature, the
performance recovery is only negligible with neat air operation
(self-recovery), while full recovery is observed after cyclic
voltammetry (CV) scanning. As temperature increased, so did the
self-recovery performance. However, the total recovery performance
decreased. Electrochemical impedance spectroscopy analysis indicates
that the potential-dependent poisoning process was delayed at low
temperature, and the removal of the sulfur species from Pt/C was
inhibited during the self-recovery. Water balance analysis implies that
the delay could be attributed to the effect of liquid water scavenging
and the mass transport of SO2 in the membrane electrode
assemblies. The CV analysis confirms that the decomposition/desorption
of the sulfur adsorbates was inhibited and indicates that the
SO2 crossover from the cathode to the anode side was also
mitigated at low temperature.
The fuel cell performance effects of many contaminants originating from multiple sources such as air, fuel, and system materials release need to be determined to reduce risks associated with market introduction. A significant amount of time and resources are required to test the performance effects of all possible contaminants because contaminant impacts develop slowly at concentrations reflecting practical operation. Thus, a two tiered down selection process was proposed to identify specific contaminants for more detailed studies. The methodology used to generate the second tier down selection is presented and discussed in relation to a specific contaminant, sulfur dioxide. Two quantitative criteria were derived based on parameters defined using the cell performance response to a temporary contaminant injection. These quantitative criteria were applied to sulfur dioxide data to investigate the effects of different operating conditions and determine the most relevant selection criterion. Results showed that the method which considered the ratio of the energy lost during contaminant exposure to the energy recovered subsequent to the contaminant exposure is preferable in this case because values are less dependent on operating conditions. Furthermore, the energy lost to energy recovered ratio is also preferable because high values not only identify the contaminants with the most significant performance loss and least performance recovery but also identifies contaminants with a performance recovery that exceeds the performance loss.
This paper presents a study of the effects of an organic contaminant containing an amide bond (-CONH-), ε-caprolactam, on polymer electrolyte membrane fuel cells (PEMFCs). The ε-caprolactam has been detected in leachates from polyphthalamide materials that are being considered for use as balance-of-plant structural materials for PEMFCs. Contamination effects from ε-caprolactam in Nafion membranes are shown to be controlled by temperature. A possible explanation of the temperature effect is the endothermic ring-opening reaction of the amide bond (-NHCO-) of the cyclic ε-caprolactam. UV-vis and ATR-IR spectroscopy studies confirmed the presence of open ring structure of ε-caprolactam in membranes. The ECSA and kinetic current for the ORR of the Pt/C catalyst were also investigated and were observed to decrease upon contamination by the ε-caprolactam. By comparison of the CVs of ammonia and acetic acid, we confirmed the adsorption of carboxylic acid (-COOH) or carboxylate anion (-COO-) onto the surface of the Pt. Finally, a comparison of in situ voltage losses at 80°C and 50°C also revealed temperature effects, especially in the membrane, as a result of the dramatic increase in the HFR.
The effect of trace quantities of ammonia on oxygen reduction reaction (ORR) on carbon-supported platinum catalysts in perchloric acid solutions is assessed using rotating ring disk electrode (RRDE) technique. The study demonstrates that ammonia has detrimental effects on ORR. The most significant effect takes place in the potential region above 0.7 V vs RHE. The effect is explained by the electrochemical oxidation of ammonia, which blocks Pt active sites and increases the formation of H2O2. This leads to losses in the disk currents and increments in the ring currents. The apparent losses in ORR currents may occur in two ways, namely, through the blocking of the active sites for ORR as well as by generating a small anodic current, which is believed to have a lower contribution. In addition, a detrimental effect of sodium cations in the potential range below 0.75 V vs RHE was demonstrated. This effect is most likely due to the co-adsorption of sodium cations and perchlorate anions on the Pt surface.
The performance of a polymer electrolyte membrane fuel cell (PEMFC) operating on a simulated hydrocarbon reformate is described. The anode feed stream consisted of 80% H2, ∼20% N2, and 8ppm hydrogen sulfide (H2S). Cell performance losses are calculated by evaluating cell potential reduction due to H2S contamination through lifetime tests. It is found that potential, or power, loss under this condition is a result of platinum surface contamination with elemental sulfur. Electrochemical mass spectroscopy (EMS) and electrochemical techniques are employed, in order to show that elemental sulfur is adsorbed onto platinum, and that sulfur dioxide is one of the oxidation products. Moreover, it is demonstrated that a possible approach for mitigating H2S poisoning on the PEMFC anode catalyst is to inject low levels of air into the H2S-contaminated anode feeding stream.
The H2S-tolerance of Palladium–Copper nanoparticle catalysts supported on carbon black employed on PEMFC anodes operating on H2S-contaminated anode feeding streams is described. The anode feeding consists of 80% H2, ∼20% N2 (in vol.), and 8 ppm H2S. Catalyst performances were evaluated by estimating the cell potential reduction due to H2S contamination through lifetime tests. It is found that the H2S-tolerance of PdCu catalysts depends on the concentration of copper, as confirmed by a catalyst pre-leaching procedure. This methodology is employed in order to remove copper atoms from the catalyst surface, which is confirmed by energy dispersive X-ray spectroscopy and X-ray diffraction techniques. This method generates catalysts with lower copper content and higher susceptibility to H2S. Moreover, it is proposed a new and effective catalytic activity recovering technique to restore the performance of PdCu catalysts after H2S exposure.
A generic, transient fuel cell kinetic loss mathematical model was developed for the case of contaminants that partially cover the catalyst surface with irreversibly adsorbed products. The model was derived using step changes in contaminant concentration, constant operating conditions and disregarding liquid water scavenging effects. The closed form solutions were validated using H2S, SO2 and COS data from a single source. The model needs to be validated against other data sets and transient operating conditions more representative of automotive applications. A method is proposed to determine kinetic rate constants and relies on tests with a reactant, a contaminant and, a reactant and a contaminant mixture. The method is useful to evaluate the presence of interactions between reactant and contaminant related adsorbates, and, to minimize electrode potential variations during controlled cell voltage measurements. Model parameters were similar for all contaminants suggesting a common adsorbate configuration. The model also expands the number of previously derived cases. All models in this inventory, derived with the assumption that the reactant is absent, lead to different dimensionless current vs. time behaviors similar to a fingerprint. These model characteristics facilitate contaminant mechanism identification. Model predictions include a limit of 0.7ppb contaminant concentration in the reactant stream to minimize cell performance losses during the 5000h automotive application life. This tolerance limit represents a worse case scenario because it does not take into account performance recovery resulting from drive cycle operation or the addition of mitigation strategies. A cell performance loss increase of 40% is also predicted for a catalyst loading decrease from 0.4 to 0.1mgPtcm−2.
The effects of trace concentrations of SO(2) contaminant present in the cathode feed stream on proton exchange membrane fuel cell (PEMFC) performance are studied. Contaminant concentrations of 1, 2, and 10 ppm were exposed to the cell applying a total dosage of 160 mu mol of SO(2) at 80 degrees C and a current density of 0.6 A cm(-2). All experiments show significant cell performance degradation before the steady-state poisoning state is reached. The performance degradation shows an inflection in the cell voltage, which is attributed to at least two different poisoning processes. The overall poisoning process is shown to consist of an irreversible part and a reversible part. While the performance loss of the reversible part is dependent on the SO(2) concentration and is recoverable during a H(2)/air operation, that of the irreversible part is greatly recoverable by potential cycling in the H(2)/N(2) mode. Evidence is also presented that cathode exposure to SO2 results in a performance impact at the anode. Furthermore, sulfur species that remain in the membrane electrode assembly accelerate the cell performance degradation during a neat H(2)/air operation and subsequent SO(2) contaminant exposure.
A mathematical model is derived describing the effect of an electroactive contaminant on the catalyst surface. Two cases are considered and characterized by different rate determining steps: contaminant reaction and contaminant product desorption. Each case leads to a closed form solution for the transient and scaled current density either during the contamination or recovery process. The model was partially validated using proton exchange membrane fuel cell (PEMFC) data obtained by operating with hydrogen contaminated with small amounts of CO. Results suggest the use of well designed experiments to minimize artifacts and additional model validation using a wider range of operating conditions.
Equations are derived from a generic, transient PEMFC contamination model to predict the effect of CO contaminant concentration in H2 on both steady state performance losses and time constants. The resulting predictions allowed determination of the CO tolerance limit for the cases of a Pt and WC catalyst under specific operating conditions. An increase of the International Organization for Standardization CO tolerance limit of 0.2ppm is possible because the CO concentration leading to a steady state performance loss of less than 1% is estimated at 0.2–0.9ppm. An increase in CO tolerance limit is expected to reduce the analytical verification cost (quality control). The steady state performance loss is independent of catalyst loading thus avoiding a future standard change resulting from PEMFC cost reduction activities.
A unique, generic, transient, and more general PEMFC contamination model able to predict the cell performance impact of foreign cations exchanging with ionomer protons was derived by adding a variable equivalent proton fraction in the flow field channel liquid water drops. Ion exchange phenomena are described with appropriate and recognized relationships. Steady state ohmic performance losses and time constants for both contamination and recovery processes were used to define contaminant tolerance limits for monovalent, divalent and trivalent cations and are respectively equal to 5100, 380 and 4.4 ppm. These tolerance limits are expected to be widely applicable for similarly charged cations because model parameters, ionic conductivity and separation factor, do not vary significantly (first order approximation). The model also describes the effect of ammonia, revealing that its fast reaction with ionomer protons creating ammonium ions at the gas phase/membrane phase boundary is analogous to ion exchange. A 2.1 ppb tolerance limit is proposed which does not take into account the scavenging effect of liquid water (parallel contamination path). The model applicability to ammonia enlarges the existing contamination model library specifically developed to facilitate mechanism identification (fingerprinting) and addresses the large number of untested and unidentified contaminants.
Acetylene adsorption on PEMFC electrodes and contamination in single cells are investigated with 300 ppm acetylene at a cathode held at 80 °C. The results of adsorption experiments suggest that acetylene adsorbs readily on electrodes and is reduced to ethylene and ethane under an open circuit potential of H2/N2, as the adsorbates can be electro-oxidized at high potentials. The cell voltage response shows that 300 ppm acetylene results in a cell performance loss of approximately 88%. The voltage degradation curve is divided into two stages by an inflection point, which suggests that potential-dependent processes are involved in acetylene poisoning. These potential-dependent processes may include acetylene oxidation and reduction as well as accumulation of intermediates on the electrode surface. Electrochemical impedance spectroscopy analysis suggests that acetylene affects the oxygen reduction reaction and may also affect mass transport processes. Acetylene also may be reduced in the steady poisoning state of the operating cell. After neat air operation, the cyclic voltammetry results imply that the cathode catalyst surface is almost completely restored, with no contaminant residues remaining in the MEA. Linear scanning voltammetry measurements show no change in hydrogen crossover caused by contamination, and polarization curves confirm complete recovery of cell performance.
The performance degradation and recovery of a polymer electrolyte fuel cell exposed to ammonia is described. The effect of ammonia contamination on each component of the MEA was studied using Neutron Imaging, lifetime tests, cell polarization measurements, potentiostatic measurements, and AC impedance spectroscopy. It is found that among the detrimental effects of the contaminant, mass transport and ORR kinectic are mostly affected. Reductions in electrolyte conductivity are not playing a major role in cell performance losses for the low levels of contamination studied here. Shifts in electrolyte proton activity reduce oxygen reduction reaction mixed equilibrium potential and so cell potential. In addition, cell potential might also be affected by overpotentials on the oxygen reduction reaction caused by oxidation of ammonia at cell potential above 0.7 V. It is demonstrated that cell initial performance can be recovered after contamination and ammonia-contaminated PEFC recovering mechanisms are discussed.
The durability of proton exchange membrane (PEM) fuel cells is a significant function of operating conditions, material properties and product design. This chapter will focus on the effect of operating conditions, which are shown to have a profound impact on durability, as evidenced by a ten times change in product lifetime for different fuel cell applications. The major operating considerations discussed are thermal management, water management, reactant flow management, contamination and duty cycle operation. The impact of operating conditions are described for the major degradation mechanisms, cathode electrode degradation and membrane degradation, as well as for operation-specific degradation mechanisms, including freeze degradation, air starvation and fuel starvation.
A long-term contamination (for 1000 hrs) test is conducted under constant current operation with 5 ppm acetonitrile in the air stream, and the results are compared to those for a controlled blank single-cell test. The long-term effects of the contaminant on cell performance and the structure of the membrane electrodes assembly are identified by a series of analyses, including electrochemical impedance spectroscopy, cyclic voltammetry, ion chromatography, transmission electron microscopy and scanning electron microscopy. During the long-term test, the acetonitrile significantly decreases the cell performance for a 0.1-mg Pt cm-² catalyst coated membrane. The acetonitrile contamination accelerates catalyst particle growth; however, interestingly, it inhibited membrane degradation. This result may provide a possible approach for improving fuel cell durability.
PtCo-alloy cathode electrocatalysts release Co cations under operation, and the presence of these cations in the membrane electrode assembly (MEA) can result in large performance losses. It is unlikely that these cations are static, but change positions depending on operating conditions. A thorough accounting of these Co cation positions and concentrations has been impossible to obtain owing to the inability to monitor these processes in operando. Indeed, the environment (water and ion content, potential, and temperature) within a fuel cell varies widely from inlet to outlet, from anode to cathode, and from active to inactive area. Synchrotron micro-X-ray fluorescence (μ-XRF) was leveraged to directly monitor Co2+ transport in an operating H2/air MEA for the first time. A Nafion membrane was exchanged to a known Co cation capacity, and standard Pt/C electrocatalysts were utilized for both electrodes. Co Kα1 XRF maps revealed through-plane transient Co transport responses driven by cell potential and current density. Because of the cell design and imaging geometry, the distributions were strongly impacted by the MEA edge configuration. These findings will drive future imaging cell designs to allow for quantitative mapping of cation through-plane distributions during operation.
The catalytic reactions of acetylene in contact with air, the electrochemical reactions of acetylene under different potentials, and the contamination reactions of acetylene in an operated proton exchange membrane fuel cell were investigated by using gas chromatography (GC), chronoamperometry, and cyclic voltammetry (CV) techniques. The GC results indicate that carbon dioxide is the catalytic reaction product of acetylene in the cathode and an electro-oxidation product at high potentials, and it can desorb easily. Ethylene, ethane, and methane are the electrochemical reduction products of acetylene at low cathode potentials, and they can desorb easily. The CV analysis suggests the electroreduction intermediates are vinylidene and ethylidyne at low potentials and electro-oxidation intermediates CO (or COH-type species) at high potentials. In an operating fuel cell, acetylene reactions in the cathode include not only catalytic reactions, but also electrochemical redox reactions. According to these catalytic and electrochemical reactions, acetylene contamination mechanisms and possible effective contamination mitigation strategies are proposed.
Elements constituting a fuel cell laboratory are succinctly discussed using the experience developed at the Hawaii Sustainable Energy Research Facility. The information is expected to be useful to organizations with a desire to create or improve a fuel cell laboratory in view of the recent and anticipated fuel cell commercialization activities. Topics discussed cover a wide range with an emphasis on differentiating aspects from other types of laboratories including safety, fuel cell and test equipment, and methods used to characterize fuel cells. The use of hydrogen, oxygen and specifically introduced chemical species, and the presence of high voltages and electrical short risks constitute the most prominent hazards. Reactant purity, cleaning, test station control including data acquisition, and calibration are the most important considerations to ensure fuel cell characterization data quality.
Cleanliness is also an important consideration for the fuel cell assembly and integration into the test station. The fuel cell assembly also needs to be verified for faults. Fuel cells need to be conditioned for optimum performance before a purposefully designed test plan is implemented. Many fuel cell diagnostic methods are available but novel techniques are still needed in many areas including through plane temperature distribution, stack diagnostics and mass transfer properties. The emphasis is given to commonly and sparingly used electrochemical techniques. In situ techniques include polarization, impedance spectroscopy, voltammetry and current distribution over the active area. Ex situ techniques include the rotating ring-disc electrode and the membrane conductivity cell. Other nonelectrochemical techniques are also useful to understand fuel cell behavior and include the analysis of reactant streams and condensed water, and spectroscopic measurements in combination with electrochemical cells (spectroelectrochemical cells).
Ethylene glycol (EG) and caprolactam are two representative contaminant species either present in the fuel cell system or released by fuel cell system materials. The contamination effects of EG and caprolactam on the electro-catalytic performances of a Pt/C catalyst toward the oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) were investigated in acid media using the rotating ring/disk electrode (RRDE) technique. The Pt surface coverage increased with EG and caprolactam concentrations. The H2O2 yield also significantly increased because of the effects of the two contaminants. The EG- and caprolactam-derived adsorbates reduce the ORR and HOR rates and modify the rate-determining step (change in Tafel slope).
It is known that traffic related air contaminants cause power loss, decreasing lifetime or a complete failure of proton exchange membrane fuel cell (PEMFC). Therefore, the present study aims for a better understanding and the development of a data basis for further decisions in dealing with air contaminants for automobile applications. The first section provides an overview of scientific literature about the influence of important air contaminants on proton exchange membrane fuel cells (PEMFC). The second section describes an extensive study of air contaminants at possible automotive operating conditions using a full factorial matrix test. The specific variation of temperature, cell potential and harmful gas concentration resulted in 27 operating points for each used air contaminant. The gases NO, NO2, SO2, NH3, toluene and ethane were used. The results indicate significant degradation but as well the possibility of regeneration. The degradation caused by different harmful gases is both, dependent on temperature and potential. Furthermore, a clear difference of the influence of NO and NO2 at low concentrations could be shown. The experiments give an overview of the cathode harming potential of relevant air contaminants. Hence, the work provides a basis for the development of cathode air filter and regeneration techniques for automotive applications.
The effects of bromomethane (BrCH3), an airborne contaminant, on the performance of a single PEMFC are compared with that of another halocarbon, chlorobenzene. Under a constant current of 1 A cm−2 and at 45 °C, 20 ppm bromomethane causes approximately 30% cell voltage loss in approximately 30 h, as opposed to much more rapid performance degradation observed with chlorobenzene. Electrochemical impedance spectroscopy, cyclic voltammetry, linear scanning voltammetry, and polarization measurements are applied to characterize the temporary electrochemical reaction effect and permanent performance effects. X-ray absorption spectroscopy is used to confirm that Br is adsorbed on the Pt electrocatalyst surface. We conclude that airborne bromomethane poisons a PEMFC in a different way from chlorobenzene because it is largely hydrolyzed to bromide, Br−, which is then excluded from the Pt catalyst by the negatively charged Nafion ionomer. The little Br− and bromomethane that adsorbs on the Pt surface can be partially removed by cycling but causes some irreversible surface area loss.
A database summarizing the effects of 21 contaminants on the performance of proton exchange membrane fuel cells (PEMFCs) was used to examine relationships between cathode kinetic losses and contaminant physicochemical parameters. Impedance spectroscopy data were employed to obtain oxygen reduction kinetic resistances by fitting data in the 10-158 Hz range to a simplified equivalent circuit. The contaminant dipole moment and the adsorption energy of the contaminant on a Pt surface were chosen as parameters. Dipole moments did not correlate with dimensionless cathode kinetic resistances. In contrast, adsorption energies were quantitatively and linearly correlated with minimum dimensionless cathode kinetic resistances. Contaminants influence the oxygen reduction for contaminant adsorption energies smaller than -24.5 kJ mol-1, a value near the high limit of the adsorption energy of O2 on Pt. Dimensionless cathode kinetic resistances linearly increase with decreasing O2 adsorption energies below-24.5 kJmol-1. Measured total cell voltage losses are mostly larger than the cathode kinetic losses calculated from kinetic resistance changes, which indicates the existence of other sources of performance degradation. Modifications to the experimental procedure are proposed to ensure that data are comparable on a similar basis and improve the correlation between contaminant adsorption energy and kinetic cell voltage losses.
Acetonitrile contamination in proton exchange membrane fuel cells (PEMFCs) was studied in situ with chronoamperometry, chronopotentiometry, cyclic voltammetry, electrochemical impedance spectroscopy, gas chromatography/mass spectroscopy, and ex situ with a membrane conductivity cell, ion selective electrode. PEMFCs significantly lost and partially regained performance for all tested electrodes. A significant effect on the membrane proton conductivity was noted in PEMFC but not in conductivity cells. The impact was dependent on the applied potential. Acetonitrile adsorption inhibits the hydrogen oxidation/evolution and the Pt oxidation and PtOx reduction reactions. Several peaks located at 0.17/0.22, 0.40/0.58, 0.65/0.78 V, and 0.9 V vs the hydrogen reference electrode were attributed to the redox of acetonitrile intermediates and products. Clear mechanistic insights were highlighted but a more complete integration of all these results and others obtained with a rotating ring/disc electrode (RRDE) will lead to a more definitive version of the acetonitrile contamination mechanism.
The effects of the airborne contaminants naphthalene, acetonitrile, and propene on the performance of a single PEMFC were investigated at different operating conditions. The results indicated that higher contaminant concentrations resulted in higher performance losses and faster degradations, while the rate of performance recovery was more rapid. Lower temperatures caused increased performance losses, more rapid degradations, slower recoveries, and irrecoverable losses. Higher contaminant concentrations and lower temperatures caused voltage oscillations when the PEMFC was contaminated with naphthalene. The exposure of the PEMFC to propene at higher temperatures resulted in performance recovery to a level that was temporarily greater than its performance prior to propene exposure. The PEMFC performance loss at different current densities was dependent on specific contaminants. At higher current densities, naphthalene and propene caused greater performance losses as well as faster degradation and recovery. Acetonitrile caused similar performance losses at all current densities. The performance degradation caused by naphthalene and acetonitrile may be dependent on the cathode potential. EIS analysis indicated that contaminant exposure affected catalytic and mass transport processes for all the impurities tested and that acetonitrile increased the membrane resistance.
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 generally accelerated ECSA loss.
A 36-cell proton exchange membrane fuel cell (PEMFC) stack was contaminated with 50 ppm propene in air. Propene contamination amplified the uneven cell performance distribution along the stack length. End cells showed a larger performance change due to contamination than contiguous cells owing to a lower temperature and a larger effect of contamination at lower temperatures. The performance change of the inner cells linearly varied from cell 2 to cell 35 and was attributed to several causes including the uneven sub-saturated air flow distribution and the propene oxidation reaction involving a water molecule. The inner cells performance distribution was also credited to the uneven coolant flow distribution and a large effect of temperature on contamination. Higher cathode potentials acted as a cleaning method that minimized the contamination effect by promoting propene oxidation and led to weakly adsorbing CO2. As a consequence, higher cathode potentials also resulted in smoothing the uneven inner cells performance distribution.
In this paper, we investigate contaminationmechanisms and quantify the effect of organicmodel compounds aniline, diethyleneglycol monoethyl ether acetate, diethyleneglycol monoethyl ether, 4-methyl benzensulfonamide, benzyl alcohol, and 2,6-diaminotoluene that have been observed to originate from degradation of balance of plant materials on PEMFCs. In situ voltage loss can be quantified by contamination sources such as Pt, the ionomer, and the membrane using isotherm curves that are prepared by ex situ studies considering contamination mechanisms: adsorption on Pt, ion-exchange/absorption in membranes or electrodes. Severe kinetic loss of Pt activity on oxygen reduction reaction was observed for aromatic compounds due to the greater coverage on Pt/C than aliphatic compounds. An ion-exchange reaction by amine-containing aromatic compounds results in significant conductivity losses of the membrane/ionomer, which is main contributor of the performance loss in this study. That is, controlling the voltage losses caused by the membrane/ionomer contamination is critical to ensure the stability of the system. Infusion of non-amine containing compounds into PEMFCs also increased performance loss by an absorption mechanism but reached at steady state with reversible recovery by switching into normal operations without contaminants.
Contamination of proton exchange membrane fuel cells is coming to the forefront as commercialization activities multiply and fuel cell systems are confronted to field conditions away from the controlled laboratory environment. The recent progress achieved in understanding the disruptive effects of contaminants is reviewed. The analysis is focused on contaminant effects, contaminant sources, mechanisms, mitigation strategies and knowledge gaps in all these areas.
A multitude of contaminants are present in atmospheric air. A large proportion of these contaminants have unknown effects on fuel cell performance. An assessment of the overall performance impact of all suspected contaminants is a costly and time consuming task. Thus, a method is required to select the most relevant contaminants. A two tier down selection approach is presented. Six qualitative criteria were used to create a shorter list of 19 contaminants (first tier). Two quantitative criteria were developed based on the cell performance response to further down select contaminants for detailed studies (second tier). First tier contaminants were injected into fuel cells. Performance data were used to provide a preliminary evaluation of the second tier down selection criteria.
A mathematical and transient model was developed for the case of a neutral species penetrating an ionomer. It is assumed that transport in the gas or liquid water phases is rate determining. A limited number of equations are used to ensure that the model has an analytic solution. These equations include Fick's first law, a distribution of the neutral species between the gas/liquid phases and the ionomer phase described by a separation factor, a neutral species in the ionomer mass balance, an empirical relationship between the ionomer conductivity and the equivalent foreign neutral species mole fraction in the ionomer phase, and a cell voltage balance. Both separation factor and conductivity relations were experimentally validated. The model solution shows that the dimensionless current density reaches a steady state, recovery is eventually complete after removing the contaminant source, and, contamination and recovery time scales are equal.
A rotating disk electrode (RDE) along with cyclic voltammetry (CV) and linear sweep voltammetry (LSV), were used to investigate the impact of two model compounds representing degradation products of Nafion and 3M perfluorinated sulfonic acid membranes on the electrochemical surface area (ECA) and oxygen reduction reaction (ORR) activity of polycrystalline Pt, nano-structured thin film (NSTF) Pt (3M), and Pt/Vulcan carbon (Pt/Vu) (TKK) electrodes. ORR kinetic currents (measured at 0.9 V and transport corrected) were found to decrease linearly with the log of concentration for both model compounds on all Pt surfaces studied. Model compound adsorption effects on ECA were more abstruse due to competitive organic anion adsorption on Pt surfaces superimposing with the hydrogen underpotential deposition (HUPD) region.
Models were derived for the scavenging effect of product liquid water on airborne proton exchange membrane fuel cell (PEMFC) contaminants. A time scale analysis of contaminant mass transfer processes, product water accumulation in the gas diffusion electrode, and dissociation reactions indicated that the contaminant saturates the product liquid water simplifying model derivation. The baseline model only accounts for contaminant solubility and shows a large effect for ethylene glycol. An extension to this model was derived for the presence of contaminant dissociation reactions within the product liquid water using SO2 as a model contaminant. The extended model demonstrates the large impact of dissociation reactions at low SO2 concentrations. For both models, explicit expressions for the average gas phase contaminant concentration within the fuel cell were also derived and can be used as a surrogate for the effective contaminant concentration to correlate the fuel cell performance loss and facilitate the definition of tolerance limits and filtering equipment.
Acetylene is a welding fuel and precursor for organic synthesis, which requires considering it to be a possible air pollutant. In this work, the spatial performance of a proton exchange membrane fuel cell exposed to 300 ppm C2H2 and different operating currents was studied with a segmented cell system. The injection of C2H2 resulted in a cell performance decrease and redistribution of segments' currents depending on the operating conditions. Performance loss was 20–50 mV at 0.1–0.2 A cm−2 and was accompanied by a rapid redistribution of localized currents. Acetylene exposure at 0.4–1.0 A cm−2 led to a sharp voltage decrease to 0.07–0.13 V and significant changes in current distribution during a transition period, when the cell reached a voltage of 0.55–0.6 V. A recovery of the cell voltage was observed after stopping the C2H2 injection. Spatial electrochemical impedance spectroscopy (EIS) data showed different segments' behavior at low and high currents. It was assumed that acetylene oxidation occurs at high cell voltage, while it reduces at low cell potential. A detailed analysis of the current density distribution, its correlation with EIS data and possible C2H2 oxidation/reduction mechanisms are presented and discussed.
Adsorption of SO(2) on a Pt/C catalyst typically used in proton exchange- membrane fuel cells (PEMFCs) has been investigated by temperature programmed desorption (TPD). SO(2) concentrations in N(2) were varied from 5 ppm to 1% (vol) and adsorption isotherms were determined at 25, 50, and 80 °C. Oxygen assisted (O-assisted) desorption experiments (i.e., successive TPD experiments following exposure to room temperature O(2) after the first TPD event) produced an additional SO(2) peak at a temperature higher than the initial SO(2) peak. These two types of SO(2) adsorption were identified as weakly adsorbed SO(2) species desorbed between 140 and 200 °C, depending on concentration, and a strongly adsorbed, dissociated species. For the strongly adsorbed, dissociative species, (18)O(2) isotope introduction during O-assisted desorption yielded ratios of 50%, 36%, and 14% for SO(2) masses of 64, 66, and 68, respectively. The activation energy and kinetic constant of desorption are reported for weakly adsorbed SO(2) at 1% and 20 ppm SO(2) using the Polanyi-Wigner equation.
In order to reduce the price of the bipolar plates, many researchers have been developing metallic bipolar plates to substitute for the non-porous graphite bipolar plates. However, metallic bipolar plates corrode in the PEMFC environments and resultant metal ions can affect the conductivity of the membrane, and the performance of the fuel cell stack. In this study, the corrosion behaviors of three types of metallic materials (6061 aluminum alloy, A36 steel and Grade 2 titanium) in simulated anode and cathode environments of PEMFCs is analyzed using potential-pH diagrams. Potentiostatic electrochemical testing results and SEM metallography were used to verify the predictions of the potential-pH diagram. The results show that potential-pH diagrams can be used to predict the corrosion of these metals in the PEMFC conditions.
As a typical degradation of the proton exchange membranes (PEMs) for fuel cells, formation of hydrogen peroxide (H2O2) on the cathode surface has been presented to be a key issue which leads to the decomposition of PEM. Using perflurosulfonated ionomeric membranes with different equivalent weights (EW = 900, 1000, and 1100) as test samples, degradation of PEM was investigated systematically in practical fuel cell usage conditions (e.g., 80 degrees C) during the progress of H2O2 treatment. Membranes were characterized for proton conductivity by ac impedance technique, pulsed-field-gradient spin-echo NMR, Fourier transform infrared spectroscopy, thermogravimetric analysis (TGA), and extensile experimentation. Durability studies over a period of 1 month operation revealed evident membrane degradation ascribed to the decomposition of sulfonic acid groups in pendant side chains. The products of cross-linked S-O-S (condensation sulfonates) were strongly demonstrated by IR spectroscopy as a result of long H2O2 treatment times, which suggests oxidation provoked by H2O2. Proton conductivity and the water self-diffusion coefficient decreased significantly due to the loss of water inside the membranes. TGA revealed further changes in the membrane morphology, where the onset and decomposition temperatures of the membranes changed upon exposure to H2O2. Membranes with high EW showed a faster decomposition rate than the other ones, whereas the mass loss step showed the reverse case. Although the membranes still retained their bulk physical properties in that they remained flexible and plastic, the tensile analysis showed decreased tensile strength and increased elongation-to-break accompanied by an increased Young's modulus, which suggests a mechanically weaker membrane after exposure to H2O2. (C) 2006 The Electrochemical Society.
Equilibrium concentrations of dissolved platinum species from a Pt/C electrocatalyst sample in 0.5 M H2SO4 at 80°C were found to increase with applied potential from 0.9 to 1.1 V vs reversible hydrogen electrode. In addition, platinum surface area loss for a short-stack of proton exchange membrane fuel cells PEMFCs operated at open-circuit voltage 0.95 V was shown to be higher than another operated under load 0.75 V. Both findings suggest that the formation of soluble platinum species such as Pt 2+ plays an important role in platinum surface loss in PEMFC electrodes. As accelerated platinum surface area loss in the cathode from 63 to 23 m 2 /gPt in 100 h was observed upon potential cycling, a cycled membrane electrode assembly MEA cathode was examined in detail by incidence angle X-ray diffraction and transmission electron microscopy TEM to reveal processes responsible for observed platinum loss. In this study, TEM data and analyses of Pt/C catalyst and cross-sectional MEA cathode samples unambiguously confirmed that coarsening of platinum particles occurred via two different processes: i Ostwald ripening on carbon at the nanometer scale, which is responsible for platinum particle coarsening from 3t o6 nm on carbon, and ii migration of soluble platinum species in the ionomer phase at the micrometer scale, chemical reduction of these species by crossover H2 molecules, and precipitation of platinum particles in the cathode ionomer phase, which reduces the weight of platinum on carbon. It was estimated that each process contributed to 50% of the overall platinum area loss of the potential cycled electrode.
Rotating disk electrode experiments were used to demonstrate the influence of dissolved Ru species on the oxygen reduction activity of a Pt/ C electrocatalyst. Dissolved Ru in micromolar levels was found to deposit instantly onto Pt, thereby blocking the electrode surface for ORR at low overpotentials. Ru contamination can decrease oxygen reduction kinetics by eightfold or increase the overpotential by ca. 160 mV. This facet of fuel cell durability needs special attention from the perspective of appropriate materials choice, i. e., preventing the leaching of Ru from PtRu anodes and its crossover to the cathode across the membrane electrolyte. (c) 2007 The Electrochemical Society.
A mechanism that may cause accelerated performance decay of fuel cells is presented. The mechanism is explained using a one-dimensional model of the potential profile. The analysis indicates that the electrolyte potential drops from 0 to -0.59 V (vs. RHE) when the anode is partially exposed to hydrogen and partially exposed to oxygen. This causes flow of current opposite to normal fuel cell mode at the oxygen-exposed region and raises the cathode interfacial potential difference to 1.44 V, causing carbon corrosion, which decreases performance. The decay mechanism was validated using two different experimental setups which reproduced the carbon-corrosion phenomenon.
Transference coefficients, ionic conductivities and equilibrium properties are reported for Nafion® 117 membranes containing a mixture of two cations and water. The membranes were equilibrated in 11 kinds of aqueous solutions of HCl and NaCl, with mole fractions of HCl between 0 and 1, all having cCl−=0.03 kmol m−3. Some experiments were repeated for Nafion® 112 and 115 membranes. By electron probe microanalysis (EPMA), it was found that Nafion® 117 membranes have a slightly higher affinity for Na+ than for H+. The water content of the membrane was determined gravimetrically. It decreased from 20.6±0.1 to 18.4±0.2 molecules water per cationic site, as the membrane changed from the pure H-form (HM) to the pure Na-form (NaM). The water transference coefficient, tH2O, obtained by streaming potential measurements, increased in Nafion® 117 from 2.5±0.1 to 9.2±0.1 as the mole fraction of Na+ in the membrane, xNaM, increased from 0 to 1. Most of the increase occurred for xNaM>0.5. The transference number of H+ in the membrane, tH+(m), which was determined by an improved emf-method, showed a rapid decrease for xHM
In polymer electrolyte membrane (PEM) fuel cells, the electrolyte membrane is easily contaminated by foreign cations because of a higher affinity of these cations with the sulfonic acid group than that of H⁺. Thereby three modes of degradation in the membrane which affect fuel cell performance are anticipated. The first is a direct effect, and comes from alteration of the membrane bulk properties, e.g., lowering of membrane ionic conductivity, water content, H⁺ transference number. This effect is normally not so serious unless the membrane is severely contaminated. The second comes from the altered water flux inside the membrane, owing to the terms of electroosmotic drag and diffusion coefficient of water affected by the presence of contaminant ions. This results in membrane drying and lowering of membrane conductivity. The degradation starts when the contaminant level exceeds 5%. Simulation results concerning the water concentration profile, membrane ohmic resistance and membrane overpotential are presented based on water flux equations derived using experimentally obtained transport parameters. The last effect comes from the degradation of the cathode catalyst due to the presence of contaminant ions at the interface between the platinum catalyst and the ionomer layer. This effect is the most serious as regards the fuel cell performance, because a contaminant level as low as 1% is enough to come into effect. Membrane contamination by foreign cations, especially local contamination at the cathode catalyst layer, deteriorates the fuel cell performance significantly, and needs special caution in fuel cell design.
Water management in PEFCs is an important parameter to optimize for peak performance. Its importance is further emphasized by data obtained with impurity-contaminated cells and from life tests carried out using several cell humidification levels. The results show the detrimental effects of impurities on several key cell performance loss types (including kinetic, ohmic and mass transport losses). Furthermore, an improper water balance (either too wet or too dry) has a long term effect on cell degradation rates. Efficient and practical water management strategies to reduce the detrimental effects of impurities and exposure to excess water or dryness are also reviewed.
Metallic bipolar plates for Polymer electrolyte membrane (PEM) fuel cells with and without coatings were tested in single cell tests, Current-voltage curves, lifetime curves and the contamination with metal ions were measured. Additionally the surface of the plates was analyzed by several methods. So far the investigations revealed that principally stainless steel covered with a thin coating is suitable as material for bipolar plates in PEM fuel cells. Cell performance is the same as in PEM fuel cells with graphite bipolar plates. Concerning the cost it has to be considered that not only the material itself but also the coating process has to be evaluated.
Ion and water transport characteristics of perfluorosulfonated ionomer membranes are investigated in the mixed cation form of H/Fe, H/Ni, and H/Cu systems. Nafion membranes, which were equilibrated with HCl/FeCl3, HCl/NiCl2, or HCl/CuCl2 mixed aqueous solutions of various mixing ratios, were prepared as test samples, and equilibrium and transport properties were measured systematically. Membrane cationic composition showed that trivalent cations had more affinity than divalent cations. Also larger valence cations caused less water content in the membrane. The membrane ionic conductivity was markedly influenced by counterions, and H+ mobility uH+ was altered according to the nature of coexisting cations. In the presence of Cu2+, uH+ increased from its inherent value, while in the presence of Fe3+, uH+ decreased to a large extent, Ni2+ bringing about nearly no change in uH+. The ionic transference number of H+ was also influenced by coexisting cations in several ways. Despite the unique influence of impurity cations on the mobility of H+, the mobility of impurity cations was not affected by the presence of H+. The interaction between adjacent cationic species in the membrane ion exchange sites, although plausible in general for multivalence cations, appeared to be not specific due probably to the shielding of the cationic charge by water molecules or by sulfonic acid groups. The water transference coefficient tH2O as measured by streaming potential measurements showed unique changes with membrane ionic composition, and tH2O increased from 2.5 to over 13 by the presence of impurity ions. These impurity ions were found to result in more water molecules dragged than in the case of individual ions, when coexisting with the H+ ion. Overall, it was noted that the water molecules within the influence of impurity cations appeared to play a large role in the H+ movement in the membrane.
Carbon supported platinum metal alloy catalysts (Pt–M/C) are widely used in low temperature fuel cells. Pt alloyed with first-row transition elements is used as improved cathode material for low temperature fuel cells. A major challenge for the application of Pt–transition metal alloys in phosphoric acid (PAFC) and polymer electrolyte membrane (PEMFC) fuel cells is to improve the stability of these binary catalysts. Dissolution of the non-precious metal in the acid environment can give rise to a decrease of the activity of the catalysts and to a worsening of cell performance. The purpose of this paper is to provide a better insight into the stability of these Pt–M alloy catalysts in the PAFC and PEMFC environments and the effect of the dissolution of the non-precious metal on the electrocatalytic activity of these materials, in the light of the latest advances on this field. Additionally, the durability of a PtCo/C cathode catalyst was evaluated by a short test in a single PEM fuel cell.
Water management was discussed theoretically for membranes in polymer electrolyte fuel cells in which the polymer electrolyte membranes were contaminated with foreign impurity cations. Water transport in a contaminated two-cation system membrane was considered by assuming an ‘infected zone’ of finite thickness, a hypothetical layer of mixed cations, H+ and the contaminant ions Qn+, stretching from the catalyst layer ∣ membrane interface into the membrane. Solving the flux equation of water with some boundary conditions, the water concentration profile across the membrane was derived in analytical form. Some characteristic variables such as water content, the net water flux and the membrane resistance overvoltage were calculated systematically as functions of several relevant parameters in fuel cell operations. It was discovered theoretically that both the current density and the membrane thickness are vital parameters in the management of water in fuel cell membranes, and this tendency becomes larger when the membrane is contaminated by impurity ions, especially when such ions are localized at the anode ∣ membrane interface. It is noted from the point of view of water management that special caution should be directed in order to protect the membrane from contamination.
Three different electrochemical corrosion tests were applied to eight stainless steels, Inconel, Ti and a TiN coating in an environment simulating the exposures of a bi-polar current collector in a PEMFC. The three tests provided differing opportunities for the development of a passive film, and so the inferred corrosion rates from these tests also differed in kind. The most relevant test exposed samples for 72 hrs at a controlled potential and in a solution and gas phase that simulated either anode or cathode conditions. This test generally yielded the lowest corrosion rates, and it showed that the anode environment is slightly more aggressive than the cathode environment. The conductivity of Nafion™ was shown to decrease upon absorption of corrosion product cations. A simple model was proposed to estimate the loss of power of a PEMFC due to the contamination of a membrane by corrosion product cations.
The effect of ammonia on polymer electrolyte membrane fuel cells (PEMFC) was tested by adding 10 ppm NH3 to the hydrogen feed to PEMFCs based on Gore™™ PRIMEA® membrane electrode assemblies (MEAs). A significant loss in performance was observed. The poisoning process was slow taking 24 h or more to reach a steady state. In some cases no steady state performance was reached during the experiment. The performance loss was reversible in most cases, but only after operation on neat hydrogen for several days. Additions of 1 ppm NH3 for 1 week also resulted in significant performance loss. An MEA based on carbon supported Pt anode and cathode catalyst did not differ from the Gore™™ MEA based on PtRu anode and Pt cathode catalyst. The performance losses were higher than could be explained by the observed increase in ohmic resistance in the cell. There was also a significant decay in performance in a H2|H2H2|H2 cell, especially at high current density where a reaction limiting current was observed showing that the Hydrogen Oxidation Reaction (HOR) was affected. The Oxygen Reduction Reaction (ORR) on the cathode was also significantly affected by ammonia.
Proton exchange membrane fuel cells (PEMFC) have been selected to replace conventional underground power sources such as diesel engines, to improve underground air quality, to reduce green house gas emissions and operating costs and to facilitate equipment automation. The effects of underground mining conditions, gases, dust and shock and vibration on the performance of PEMFC’s were investigated during extensive testing in an operating underground metal mine. Neither the voltage–amperage nor the power–amperage curves showed significant damage effects, and a post-testing stack inspection showed minor pressure drop, at the higher current density and airflow rate. With the use of an air intake filter, little particle accumulation was registered in the stack, and effluent water testing revealed the presence of rock-derived particles, showing that the stack was able to purge itself of low particle concentrations. No physical damage was imposed to the stack, auxiliary system and hydrogen metal hydride storage unit. Fuel cell performance compared well to pre-test and initial construction power plant data generation. Further tests are recommended to study individual mine gas and particle mineralogy type effects.
A simple but comprehensive analysis of the behavior to be expected from molecules whose Langmuirian adsorption on electrodes is controlled by both their rate of transfer to the surface and the rate of their transformation from the dissolved to the adsorbed state (and vice-versa) is presented. A `master equation' relating coverage and time is derived for cases where the mass transport of the adsorbate to the surface of the electrode is by linear diffusion (stationary electrode) or by convection–diffusion (rotating disk electrode). Five sub-cases are examined in which the influence of one (or a combination) of the factors controlling the adsorption is dominant. In each case, an easily computed, dimensionless, integral equation depending on only one parameter is derived. Corresponding calculated values of coverage vs. dimensionless time are presented and their shapes are discussed. A global description of the dynamics of adsorption for Langmuirian systems is thereby achieved.
Water management in membranes for polymer electrolyte fuel cells during their operational conditions is considered theoretically. Using a linear transport equation based on the diffusion of water and the electroosmotic drag, analytical solutions for water concentration profiles in the membrane are obtained from which membrane resistance overvoltage and other characteristic values are calculated. Specific parameters of the membranes such as water transference coefficient tH2O, water permeability Lp, specific membrane conductivity κ etc., at cell operating temperatures (50 to 80°C) have been obtained from the experiment, and used as input parameters to the analytically derived expressions for water balance calculations. Hydration states of the membrane are simulated for various current densities at the fuel cell operation conditions. The effects of several operational factors of fuel cells on the membrane water content are discussed systematically, among which the membrane thickness and humidification conditions are shown to be the most significant. Contamination of the membrane by foreign impurities turned out to cause a serious problem of the water depletion at the anode side of the membrane. For the purpose of testing the validity of the method, the net water flux and the change in electric resistance inside the membrane are calculated extensively and compared with reported experimental results. The present method turned out to be fairly satisfactory for predictive water management, in spite of its simplicity of the simulation procedure.
The stability of platinum in proton exchange membrane fuel cell (PEMFC) electrodes has been investigated by determining the dissolution of platinum from a thin platinum film deposited on a gold substrate in 1M HClO4 at different temperatures ranging between 40 and 80°C and potentials between 0.85 and 1.4V vs reversible hydrogen electrode (RHE). The loss of Pt during the dissolution process is monitored in situ with the highly sensitive quartz crystal microbalance. By combining the microbalance with the electrochemical cell, the dynamic change of the electrode weight can be observed in situ. It is shown that first an oxide layer is formed, which can be dissolved depending on temperature and potential. The dissolution rate is found to be strongly dependent on the potential and temperature. At a potential close to 1.15V vs RHE, the dissolution rate becomes saturated at 80°C due to the protection of a passivating surface platinum oxide layer at high potential. At 80°C , the dissolution rate starts to diminish when the potential is higher than 1.15V .
Dissolution at the cathode and subsequent transport of platinum to the other cell components causes catalyst degradation in proton exchange membrane (PEM) fuel cells. Deposition of platinum in Nafion membrane was observed after potential cycling under hydrogen/air conditions. The deposited Pt formed a band in the ionomer, and a straightforward model was proposed to describe its location. The predicted position of the Pt band agreed with the experimental data. A simple scanning electron microscopy-energy dispersive spectroscopy analysis was used to estimate that similar to 13% of the platinum initially in the cathode was transported into the membrane following 3000 potential cycles. (c) 2007 The Electrochemical Society.
Three different types of metallic bipolar plates (commercial stainless steels, Ni-based alloys, and nitride-coated steels) were investigated in terms of their interface contact resistance (ICR) and corrosion resistance in conditions typical of a proton exchange membrane fuel cell environment. The results showed that stainless steels are unsuitable because of the formation of nonconductive oxide that leads to high ICR. Ni-based alloys showed to be prone to corrosion in acidic medium, although they have an ICR comparable to commercially available graphite. Endurance tests carried out on nitride-coated stainless-steel specimens showed a low ICR and very good corrosion resistance of this material.
Austenite stainless steels (316L, 317L, and 349™) have been coated with 0.6μm thick SnO2:F by low-pressure chemical vapor deposition and investigated in simulated polymer electrolyte membrane fuel cell (PEMFC) environments. The results showed that substrate steel has a significant influence on the behavior of the coating. Coated 316L showed a steadily increasing anodic current in PEMFC environments, indicating that it is not suitable for this alloy/coating combination. Coated 349™ showed a cathodic current in the PEMFC anode environment, demonstrating its stability in the PEMFC cathode environment. Coated 317L exhibited a stable anodic current after a current peak (at ca. 14min) in the PEMFC anode environment, and showed an extremely stable low current in PEMFC cathode environment, suggesting the possibility of using SnO2:F coated 317L for PEMFC bipolar plate applications.ICP results on the corrosion solutions showed that the PEMFC anode environment is much more corrosive than the cathode one. Fresh 316L showed the highest Fe, Cr, and Ni dissolution rates, and coating with SnO2:F significantly reduced the dissolution. Coating the 317L also showed a significant beneficial effect on the corrosion resistance in the PEMFC environments. Coating 349™ steel further improved the already excellent corrosion resistance of this alloy. Trace Sn ions were detected for all coated steels in PEMFC anode environment, but not in the cathode one.The influence of SnO2:F on the interfacial contact resistance (ICR) is mixed. For 316L and 317L steels, a SnO2:F coating reduced the ICR. For 349™ steel, the SnO2:F coating increased the ICR.
To meet automotive targets for fuel cell operation and allow higher temperature operation an understanding of the factors affecting carbon and platinum stability is critical. The stability of both carbons and carbon supported platinum and platinum/cobalt alloy catalysts was studied during 1.2V versus RHE potentiostatic hold tests using carbon and catalyst coated electrodes in a three-chamber wet electrolyte cell at a range of temperatures. At 80°C the wt% of carbon corroded increases with increasing BET area. Surface oxidation was followed electrochemically using the quinone/hydroquinone redox couple. Increasing temperature, time at 1.2V and wt% platinum on the carbon increases surface oxidation. Although increasing temperature was shown to increase the extent of carbon corrosion, catalysing the carbon did not significantly change how much carbon was corroded. Platinum stability was investigated by electrochemical metal area loss (ECA). Platinum catalysts on commercial carbons lost more ECA with increasing temperature. A platinum/cobalt alloy on a low surface area carbon was demonstrated to be more stable to both carbon corrosion and metal area loss at temperatures up to 80°C than platinum catalysts on commercial carbons, making this material an excellent candidate for higher temperature automotive operation.
The endplate is a crucial component in a proton exchange membrane fuel cell (PEMFC) stack. It can provide the necessary rigidity and strength for the stack. An aluminium alloy is one of the ideal materials for PEMFC endplates because of its low density and high rigidity. But it does not meet the requirements of corrosion resistance and electrical insulation in PEMFC environments. In this work, methods of sealing treatments and the conditions of aluminium alloy anodization were investigated. Corrosion resistances of the samples prepared by different technologies were evaluated in simulated PEMFC environments. The results showed that the corrosion resistance of the samples sealed by epoxy resin was greatly improved compared with those sealed in boiling water, and the samples anodized at a constant current density performed better than those anodized at a constant voltage. By insulation measurements, all of the samples showed good electrical insulation. The aluminium alloy endplate anodized at a constant current density and sealed with thermosetting bisphenol-A epoxy resin exhibited promising potential for practical applications by assembling it in a PEMFC stack and applying a life test.
The effect of environmental contamination (NOx, SO2) on the performance of proton exchange membrane fuel cells (PEMFC) was studied. The performance of PEMFCs was tested for 100h with different cathode reactants. According to the Ambient Air Quality Standard of PRC, three kinds of cathode gases were applied to operate the fuel cells, which were 1ppm NO2/air, 1ppm SO2/air and a mixture of contaminant gases. The gas mixture contained 0.8ppm NO2, 0.2ppm NO and 1ppm SO2. Finally, the poisoning behavior and the mechanisms were analyzed by constant-current discharging and cycle voltammetry (CV). During the 100h test, the potentials of the fuel cell degraded by 65%, 77% and 90% with 1ppm SO2/air, a gas mixture and 1ppm NO2/air, respectively.
A model dealing with the anode catalyst contamination induced by fuel impurities has been developed. This model can be used to describe the transient and steady-state performance losses. Several characteristics such as performance loss, contamination transient time constant and recovery process have also been introduced into the model. The obtained equations can be used to simulate and estimate the chemical and electrochemical reaction rate constants, and make some prediction about the severity of the contamination and the performance recoverability.
The suitability of sulfur compounds (e.g., mercaptan and sulfide) and various sulfur-free smelling compounds for hydrogen odorants were evaluated. The influence of each smelling compound on fuel cell performance was evaluated through the measurement of I–V curves and voltage decline under constant current density, and their condensation properties under high-pressure condition were evaluated by measuring their vapor pressures. The results indicated that all the sulfur compounds evaluated in this study were not suitable as hydrogen odorants since their addition to the hydrogen caused serious degradation of fuel cell performance. Among the sulfur-free compounds, however, some oxygen-containing compounds (2,3-butanedione, ethyl isobutyrate and ethyl sugar lactone) and an unsaturated hydrocarbon (5-ethylidene-2-norbornene) proved to be promising candidates since their adverse effects on the fuel cell performance were minimal and their vapor pressures were adequate.
Data are presented to quantify how common air non-condensable impurities such as NO2, SO2, and H2S show a negative effect on the performance of PEMFCs. The severity of the effects of these impurities varies depending on the impurity, concentration, and dosage but in general the impurities containing sulfur showed a stronger effect than NO2. Complete recovery of the cell performance is obtained after applying neat air following exposure to a total NO2 dosage of 61.8μmol. However, to completely recover the cathode exposed to either 118.5μmol SO2 or 2160μmol H2S, cyclic voltammetry is required to oxidize the sulfur adsorbed on the Pt. The sulfur species formed on the Pt cathode after exposure to these impurities appear to be the same and it appears as strongly and weakly adsorbed sulfur on the Pt.The data show that the rate of poisoning of PEMFCs by NO2 does not strongly depend on NO2 bulk concentration but the rate of SO2 poisoning of the cathode appear to be strongly dependent on the concentration of SO2 in the bulk. Relatively high concentrations of impurities were used and the data also show that the cell performance could be totally recovered from NO2 in neat air after repetitive cycles of exposure/no exposure of 5ppm NO2 for 12h for three cycles (185.4μmol total dosage). On the other hand, only partial recovery from 5ppm SO2 was observed in each cycle and the performance continued to decrease in all the five poisoning cycles (592.5μmol total dosage).
This paper covers our investigation into a decline in fuel cell (FC) performance resulting from hydrogen fuel containing impurities. This is a serious problem in case of adopting the methanol or gasoline reforming approach as a means of supplying fuel to FCs. The results are summarized as follows:(1) Components of the gas generated by the reformer adopting the steam reforming and auto thermal reforming approach were predicted and specific components and concentrations were identified;(2) Various experiments and analyses were conducted to determine the adverse effect of CO, CH4, HCHO and HCOOH poisoning on the performance of FC.
Perfluorosulfonated ionomer (PFSI) dispersions in various solvents, usually mixtures of organic compounds and water, were used to prepare the membrane-electrode system in polymer electrolyte membrane fuel cells (PEMFC), the aim being to increase performance by improving the triple contact of graphite (electron conducting material), Pt (hydrogen dissociation catalyst) and ionomeric membrane (proton conducting). When using PFSI dispersions in water-organic solvent mixture, care must be taken not to poison the Pt catalyst through organic decomposition products, a consequence of the thermal treatment of the electrode-polymer system bonded with PFSI dispersion. In the present study some procedures for preparing Nafion water dispersion, starting from a Nafion-117 membrane, are described. The morphological characteristics of the prepared dispersions were compared with Nafion commercial dispersion (NCD). Moreover, membranes with a thickness of 5–20 μm were prepared and characterised, using both the obtained and the NCD dispersions. The obtained data showed that Nafion water dispersion, which can be used to prepare the membrane/electrode system, results in thin membranes that absorb more water than NCD membranes, and have equal and/or higher proton conduction than the NCD.
The correlation between the degradation behavior of a membrane electrode assembly (MEA) and deposited Pt (Pt band) in the membrane during an open-circuit voltage hold test was studied. The degradation behavior was investigated by measuring the fluoride ion emission in the effluent water from the anode and the cathode for various gas compositions at both electrodes. The results showed that the fluoride ion emission rate (FER) varied in accordance with the gas compositions. Cross sections of the tested MEAs were observed using a transmission electron microscope. It was found that the location of the Pt band correlated with the magnitude of the FERs. The Pt band was located close to where the mixed potential in the membrane, which was determined by the gas compositions, changed dramatically. (c) 2007 The Electrochemical Society.
Oxidation of ammonium on polycrystalline Pt has been studied in perchloric and sulfuric acid solutions at room temperature. Ammonium was electrochemically active, and N-2 and NO were detected using differential electrochemical mass spectroscopy. Cyclic voltammetry and electrochemical quartz crystal microbalance data showed that ammonium affects the formation and reduction of Pt oxides, probably due to formation of very stable adsorbed nitrogen or nitrogen-oxygen species stable down to the H-ads region. An oxidation peak occurred at 0.8 V-RHE in positively going scans, decreasing strongly with increasing sweep rate. A corresponding reduction shoulder at about 0.66 V-RHE was seen, probably caused by formation and reduction of NHx,ads or N-ads species. Formation of Pt oxides was shifted to slightly higher potentials in the presence of ammonium, and the reduction charge of the Pt oxide reduction peak at 0.80 V-RHE was independent of sweep rate, indicating that the amount of Pt oxides formed was limited by other adsorbates. In particular, the most strongly adsorbed hydrogen was shifted to lower potentials by adsorbed species formed at high potentials as well as adsorption of bisulfate mutually stabilized by ammonia. The voltammetric response in the hydrogen desorption region was not affected, showing that all adsorbed species were desorbed at low potentials in the negatively going scan. (c) 2006 The Electrochemical Society. All rights reserved.
The consumption rate of Pt in sulfuric acid under potential cycling has been investigated in 1 mol dm(-3) H2SO4 at 40 degrees C as a basic study to understand the consumption rate of the electrocatalyst of polymer electrolyte fuel cells. With a higher potential limit, EH was 1.5 V vs reversible hydrogen electrode ( RHE ), the consumption rate was a few ng cm(-2) cycle(-1) for rectangular and symmetric triangular waves, and the consumption rate increased with the EH up to 1.8 V vs RHE. Among the triangular waves between 0.5 to 1.8 V vs RHE, the consumption rate of symmetric waves and fast cathodic asymmetric triangular waves were a few ng cm(-2) cycle(-1). However, the slow cathodic triangular wave, which were 20 V s(-1) anodic and 0.5 V s(-1) cathodic, showed 24 ng cm(-2) cycle(-1) consumption rate. The ratio in the difference of charge to the consumption showed that the electron transfer number was about 2 for the slow cathodic triangular wave, and that of the electron transfer number was about 4 for the symmetric waves and the fast cathodic triangular waves. Therefore, PtO2 + 4H(+) + 2e(-). Pt2+ + 2H(2)O that follows the Pt oxidation, Pt + 2H(2)O. PtO2 + 4H(+) + 4e(-) would occur under the enhanced condition, and Pt. Pt4+ + 4e(-) would occur under symmetric conditions. (c) 2006 The Electrochemical Society.
An electrochemical technique using thin Pt wires as working electrodes was employed to determine the concentration of peroxide within the proton exchange membrane (PEM) of operating fuel cells. The existence of hydrogen peroxide was clearly observed, and its concentration depended primarily on membrane thickness, with thinner membranes displaying higher peroxide concentrations. H2O2 is most likely formed on the anode side of the cell through reduction of O-2. © 2005 The Electrochemical Society. [DOI: 10.1149/1.1904988] All rights reserved.
This paper presents a mathematical model of platinum dissolution and movement through the layers of a polymer electrolyte membrane (PEM) fuel cell. The model is based on dilute-solution theory. Dilute solution equations are used to describe the movement of soluble platinum through the PEM. (C) 2004 The Electrochemical Society.
Vulcan carbon is the favored support for fuel-cell electrocatalysts, but as-received it contains high levels of sulfur (ca. 5,000 ppm or greater) which could potentially poison the fuel-cell electrochemistry. The chemical state of the sulfur in platinum/carbon electrocatalysts has been examined by measuring its oxidation state via X-ray photoelectron spectroscopy at different points during the preparation of a mock fuel-cell electrode. Also monitored were the presence of sulfate in the aqueous wash from the electrocatalysts and the cyclic voltammetry of the electrocatalysts after each preparation step. The studies indicate that the platinum catalytically oxidizes some of the covalent sulfur in the vulcan carbon to sulfate when water, heat, and strong physical contact between Pt and C are all present. These conditions are attained during the preparation of typical fuel-cell electrodes. Most of the zero-valent sulfur remains in the carbon after treatment, however, and appears not to be initially in contact with the Pt. This remaining unoxidized sulfur may be a source of poisoning to the Pt electrocatalyst with long-term electrochemical use, particularly at the fuel-cell cathode.
The water transport behavior in Nafion 117 cation exchange membrane was studied using the streaming potential method. The water transference coefficient of the membranes in the H{sup +} form was found to be 2.6. The results also show that Nafion 117 membranes have a good cation selectivity performance in HCl solutions within a concentration range between 0.003 and 1N. The water transference coefficients of the membranes equilibrated with alkali metal chlorides and alkaline earth metal chlorides solutions were investigated as well. It was found that the water transport behavior is related to the surface-charge density, hydration enthalpy of the cations exchanged in the membranes, and water content of the membrane. The effect diameter of the micropore in Nafion 117 was estimated as 0.8 to 1.3 nm.
An advanced kinetic model of the coupled diffusion of two counterions in a fixed-site ion-exchange membrane is developed considering the effect of the varying ionic composition on the membrane water content. The transport problem is solved numerically for a set of ratios of the diffusion coefficients of the two counterions and 1:1 ion-exchange stoichiometry. The model is used to evaluate the diffusion coefficients of alkali metal cations in the as-received and expanded H- and M-form Nafion® (M = Li, Na, K, Rb, Cs) from ion-exchange measurements. Owing to a compensating effect of the electro-osmotic pore fluid flow, the initial rates of ion exchange correspond to a fixed water content which, however, is different in H- and M-form membranes. A strong correlation is revealed between the ratio of the membrane to aqueous ion diffusion coefficients and the polymer-phase volume fraction. It is concluded that the polymer phase presents mainly a steric effect without significantly changing the mechanism of transport of alkali metal cations or protons, which resembles that in bulk water. The different behaviors of the as-received and expanded Nafion® forms are probably associated with the prevailing cluster- or pore-network morphology, respectively
We present a durability test of polymer electrolyte membrane fuel cell (PEMFC) in open circuit condition. Such a condition enhances the deterioration of the membrane electrode assembly, ac impedance spectroscopy measurements and scanning electron microscopy observation suggest that the degradation occurred at the cathode. Direct gas mass spectroscopy of the cathode outlet gas indicated the formation of HF, H2O2, CO2, SO, SO 2, H2SO2, and H2SO3. A kinetic model is presented assuming that the H2 gas cross leakage from the anode caused the cathode degradation. The model determines the rate of degradation using the permeability across the electrolyte membrane measured for crossover H2gas.
The involvement of H2 O2 in the membrane degradation mechanism in a polymer electrolyte membrane fuel cell (PEMFC) was investigated. Measurement of fluoride concentration in the effluent water was used as an indicator of the membrane degradation rate. It was found that H2 O2 is formed in the fuel cell in small concentrations but is not the main source of harmful species, which degrade the membrane. H2 O2 decomposition due to impurities or the catalyst leading to the possible formation of radical species would only account for a small fraction of the membrane degradation rate in a fuel cell.
One of the processes responsible for performance degradation of a polymer electrolyte fuel cell (PEFC) is the loss of the electrochemically active surface area of the platinum-based electrocatalysts, due in part to platinum dissolution. The long-term dissolution behavior of polycrystalline platinum and high-surface-area carbon-supported platinum particles was studied under potentiostatic conditions relevant to PEFC cathode conditions. The equilibrium concentration of dissolved Pt was found to increase monotonically from 0.65 to (vs SHE) and decrease at potentials . Dissolution rates measured at were comparable for the two types of electrodes (1.4 and ).
The vulcanized carbon (VC) used to support nanoscale catalysts in fuel cells contains organosulfur that poisons the upon heating in the absence of water. We have previously shown that in the art of making fuel‐cell electrodes (which includes water and heat), surface‐sited organosulfur in the VC is catalytically oxidized to innocuous sulfate when it contacts . When the carbon surface is desulfurized, electroactive electrodes can be fabricated without water and at temperatures that would otherwise poison when mixed with standard VC. The subsurface organosulfur in VC does not migrate over time to the desulfurized carbon surface under ambient conditions to deactivate supported . Electroactive sulfur‐content carbon electrodes can now be made using nonaqueous protocols. Low‐sulfur‐content carbon supports should reduce performance losses in fuel cells. ©2000 The Electrochemical Society
A novel method allows the monitoring of radical formation and membrane degradation in-situ in a working fuel cell which is placed in the microwave resonator of an electron paramagnetic resonance (EPR) spectrometer. By introduction of a spin trap molecule at the cathode the formation of immobilized organic radicals on the membrane surface is observed for F-free membranes, revealing the onset of oxidative degradation. For Nafion® there is much less evidence of degradation, and the hydroxyl radical is detected instead. At the anode, free radical intermediates of the fuel oxidation process are observed. No traces of membrane degradation are detected on this side of the fuel cell.
The Ag-SiO2/SPSU-BP composite membrane has been fabricated aiming to reduce the degradation of membrane by hydrogen peroxide attacking. The composite membrane was characterized by SEM and XRD analysis. Water uptake study and thermo-gravimetric analysis indicated that the hydrophilic Ag/SiO2 particles retain more water in the composite membrane. The hydrogen peroxide decomposition tests showed that the Ag/SiO2 in the composite membranes had a catalyst activity for the decomposition of peroxide. The accelerated fuel cell life tests were performed via the open-circuit voltage (OCV) tests. The durability of PEMFC with the Ag-SiO2/SPSU-BP composite membrane was improved. The result of hydrogen crossover study means that the addition of the Ag/SiO2 catalyst reduced the degradation of membrane by peroxide attack.
The dynamic performance of a PEM fuel cell is one of the most important criteria in the design of fuel cells, especially when the application of the fuel cell in mobile systems is concerned. To attack this issue, we extend the theoretical model developed by Okada (J. Electroanal. Chem. 465 (1999) 1,18) to an unsteady state model and investigate the transient behavior of water transport across the membrane as well as the influences of several physical parameters on the characteristic time to reach the steady state. We also consider the influence due to the presence of foreign impurity ions, which turn out to be a crucial factor affecting the unsteady state features of water transport across the membrane. The results suggest that a higher initial current density to start the operation, a higher water flux from the anode side, a smaller operational current density, and a lower level of contamination in the membrane (especially at the cathode side) can all result in a shorter time for the water transport to reach the steady state, and thus a better dynamic performance of the fuel cell can be obtained.
A quartz crystal microbalance was employed to monitor the changes in mass of thin protonated Nafion® coatings during the replacement of the protons by other cations. In almost every case the mass of the coatings decreased during the cation exchange, despite the replacement of the protons in the coatings with much more massive cations. The expulsion of water from the coatings, induced by the cation exchange, was responsible for the decreases in mass. With simple aqua cations the quantities of water expelled per cation exchanged were relatively small. However, for multiply charged hexaammine and 2,2′-bipyridine complexes as many as 50 water molecules were shown to be ejected from the coatings for each incorporated cation. Data for the cation exchange of 18 cations are presented and some possible reasons for the accompanying dehydration of the coatings are suggested.
The protonic conductivity of the polymer electrolyte membrane (PEM) in the PEM fuel cell is critical to the overall power density of the fuel cell system. The conductivity can be influenced by the presence of impurity cations in the membrane. By the use of electrochemical impedance spectroscopy with microelectrodes, the local conductivity of Nafion membranes, which had been exposed to part per million (ppm) concentrations of impurity cations, was evaluated. Inorganic impurity cations studied included Cu2+, Fe3+, Na+ and Ni2+. Membranes were immersed in sulphate salt solutions of these cations, prepared in distilled water. Conductivity values at 0.1, 1 and 10 ppm cation impurity level were found to vary little from values for the blank solution. However at 100 ppm, a significant decrease in conductivity was observed. At this higher concentration of impurity, the Ni2+ and Cu2+ contaminated membranes displayed lower conductivity than that contaminated by Na+. Meanwhile Fe3+ contaminated membrane had the lowest conductivity. That this decrease in conductivity was greater for cations of higher valence corresponds with the high affinity of the sulphonic acid sites in Nafion to multivalent foreign cations. The results illustrate the detrimental effect of small amounts of contaminants on conductivity in Nafion membrane.
The overall transport characteristics of different cationic species in Nafion® 117 membrane are presented. Equilibrium salt and solvent uptake by the membrane are determined in order to obtain the membrane chloride concentration, the membrane porosity and membrane water content. Conductivity measurements are investigated using impedance spectroscopy for sodium and proton cationic forms of the membrane. Electrical mobilities of sodium, nickel and silver ions are determined using an electrophoresis technique. By combining the results obtained from conductivity measurements, the sodium transport number is found in the Nafion® 117 membrane equilibrated with NaCl solution.
A major challenge to the application of Pt–transition metal alloys in PEMFCs is the stability of the Pt alloy. This work intends to evaluate the durability of PtCo/C cathode catalyst in a dynamic fuel cell environment with continuous water fluxing on the cathode. A potential cycling test between 0.87 and 1.2V versus RHE was applied to the system to illustrate how cobalt or platinum dissolution might affect the cell performance. The results indicate that cobalt dissolution neither detrimentally reduces the cell voltage nor dramatically affects the membrane conductance. Cell performance enhancement by PtCo/C over Pt/C catalyst has been sustained over 2400 cycles and the overall performance loss of the PtCo/C membrane electrode assemblies (MEAs) was less than that of the Pt/C MEA. Potential cycle testing has been shown to accelerate cobalt dissolution as indicated in a substantial loss of catalyst activity in the 1st 400 potential cycles.
AC impedance studies were performed on Na+ and K+ alkali salt forms of the short sidechain perfluorosulfonate ionomer (PFSI) membrane films. With impressive performances of 4 A/cm[sup 2] current density and power densities near 2.5 W/cm[sup 2], the acid forms of these short sidechain PFSI are very promising candidates for use in fuel cells for future electric vehicles. Since, at present, little is known about the exact transport mechanisms for the ionic species within PFSIs, an ac impedance study of the Na+ and K+ forms has been performed. It is hoped that this will provide some insight and understanding of the transport mechanisms in the PFSI and thus will aid in the development and optimization of fuel cells. Results suggest that there are marked differences with respect to host environments within the Dow membrane as compared to Nafion[reg sign] long sidechain PFSI membrane films. Impedance spectra of the Dow salt form membranes displaying two distinct relaxation peaks while the spectra for all forms of Nafion reveal only a single peak. This second low temperature peak in the Dow membrane has been attributed to a much larger [OH[sup [minus]]]/[SO[sup [minus]][sub 3]] ratio, possibly greater than one, existing within the Dow membrane as compared to Nafion, which contains an [OH[sup [minus]]]/[SO[sup [minus]][sub 3]] ratio between 0.23 and 0.29. The temperature behavior of the dc conductivity was extracted from the temperature Cole-Cole plots and was found to be non-Arrhenius. Room temperature conductivities of the order of 10[sup [minus]2] S/cm were achieved for the Na[sup +] and K[sup +] salt forms of the XUS membrane. It was observed that for all T < 25, [sigma][sub dc](Na[sup +]) < [sigma][sub dc](K[sup +]). It has been proposed that this behavior arises from the hydration phenomena of the alkali ions. These results suggest that substantial differences exist between host environments of the Dow and Nafion PFSI.