Experimental study of water transport in a polybenzimidazole-based high temperature PEMFC

Phosphoric acid electrolyte evaporation in a polybenzimidazole based high temperature polymer electrolyte fuel cell is analyzed as a function of reactant gas stoichiometry and temperature. Based on these results a phosphoric acid vapor pressure curve is derived to predict the fuel cell liftetime with respect to electrolyte inventory. The predicted fuel cell life was validated by means of an accelerated stress test. Additionally, the correlation between electrolyte inventory and fuel cell performance was investigated by recording H 2 /air and H 2 /O 2 polarization curves during the course of the stress test to gain insight into the relation between acid inventory and the different degradation modes. © The Author High-temperature polymer electrolyte fuel cells (HT-PEFC) have the potential to become an important technology for small scale heat and power (CHP) applications. However, today, fuel cell based CHP applications are dominated by low-temperature PEFC (LT-PEFC), 1 even though the possibility to sustain high CO levels of up to 3%, 2 thermal integration of the fuel processing unit and no need of additional gas clean-up render HT-PEFCs especially suitable for operation on hydrocarbon-based fuels, i.e. natural gas. The high operating temperature of 160-200• C, reduced system complexity, due to the absence of additional gas humidification, and high system efficiencies are ideal properties of HT-PEFC for stationary CHP applications.Fuel cell durability, efficiency and cost are essential factors for commercialization. Durability is mainly determined by membrane electrode assembly (MEA) degradation. Amongst other degradation modes that HT-PEFCs share with low temperature PEFC, 3 electrolyte loss by evaporation and migration is exclusive to HT-technology and a limiting factor for CHP applications. We have recently demonstrated that PBI based membrane systems exhibit extensive electrolyte migration from cathode to anode under high current operation. 4 This was attributed to the high mobility of free hydrogen phosphate anions which carry part of the ionic current. While this work focuses on phosphoric acid loss by evaporation and its implication on lifetime and fuel cell performance, it cannot be excluded that the high PA mobility has an effect on electrolyte evaporation as it can influence the PA resupply and saturation of the electrodes.With respect to electrolyte evaporation, the phosphoric acid vapor pressure below temperatures of 300• C is extremely low, nevertheless it is expected to be significant considering the targeted lifetime of 50,000 h for CHP systems set out by the US Department of Energy (DOE) for 2015.5 Up to now, no literature data is available for the vapor pressure of phosphoric acid for temperatures below 200• C. 6-9Determining a phosphoric acid vapor pressure curve at the temperatures of interest for fuel cell operation (160-190• C) is a tedious task, due to the low phosphoric acid concentration in the gas phase and the accompanied analytical measurement complexity. Furthermore, phosphoric acid, b...

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“…Hence, the water drag reported for HT-PEFC 22 can be neglected as influencing factor for the studied parameter range.…”

mentioning

confidence: 99%

Correlating Electrolyte Inventory and Lifetime of HT-PEFC by Accelerated Stress Testing

Eberhardt

Lochner

Büchi

et al. 2015

J. Electrochem. Soc.

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“…Furthermore, at 180 ºC, it is expected an increase in cathode tolerance to methanol, despite the possible increase in the methanol crossover [6]. The observed positive affect for all MEAs can be attributed to the humidification of the MEA by the water present in the synthetic reformate that decreased the charge transfer resistance associated to oxygen reduction reaction; when anode feed is humidified, water transport can occur from anode to the cathode [25]. Araya et al also observed a decreasing trend of low frequency resistance (englobing the cathodic activities and the diffusion associated resistance) with increasing methanol slip [20].…”

Section: Eismentioning

confidence: 95%

“…The ohmic resistance of MEAs 1-4, Figure 6, increased slightly when dry gases were replaced by synthetic reformate at the anode and was fairly constant during the first three cycles, increasing slightly afterwards for MEA 2 until the 7 th cycle and for MEA 3 until the 18 th cycle. The presence of water in the methanol mixture promotes the proton conduction in the membrane (and catalyst layer), but can also have a detrimental effect due to phosphoric acid leaching [25]; maintaining a sufficient amount of phosphoric acid in the membrane and catalyst layer is critical to maintain the fuel cell performance and to assure the durability [26]. Performing the cycles with reformate containing high content of methanol solution may lead to thickness reduction and mechanical degradation of membrane -causing formation of cracks and pinholes-as well as degradation of catalyst layer, especially at 180 ºC.…”

Section: Eismentioning

confidence: 99%

The influence of impurities in high temperature polymer electrolyte membrane fuel cells performance

Boaventura

Alves

Ribeirinha

et al. 2016

International Journal of Hydrogen Energy

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This work investigates the influence of carbon dioxide and non-reacted methanol, present in the reformate stream obtained via methanol steam reforming, in the performance of high temperature polymer electrolyte membrane fuel cells (HT-PEMFC), operating between 160 °C and 180 °C.The HT-PEMFC anode was fed with pure hydrogen, hydrogen balanced with carbon dioxide (75 % / 25 % vol.) and synthetic reformate mixture, considering also vaporized methanol solution in the reformate content (up to 10 % vol.). The synthetic reformate was feed during cycles of 420 min. The fuel cell was characterized based on the polarization curve and electrochemical impedance spectroscopy (EIS) analysis. Additionally, acid-base titrations were performed to access the phosphoric acid content in different sections of the MEAs as well as scanning electron microscopy (SEM).A low impact in the fuel cell performance was observed when three cycles of synthetic reformate containing methanol solution were performed. When the number of cycles was increased, the performance of HT-PEMFC decreases and irreversible degradation of performance was observed. The cycles with synthetic reformate increased the ohmic resistance and high frequency resistance associated with anodic processes, but decreased the intermediate frequency resistance associated with cathodic processes. Additionally, by increasing the number of cycles, the phosphoric acid content of Celtec ® MEAs and the thickness of the membrane decreased. Keywords:Polymer electrolyte membrane fuel cell Polybenzimidazole Reformate gas Methanol Impurities effectElectrochemical impedance spectroscopy

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“…The overall resistance of a HT-PEFC is higher under OCV conditions than under load and an increase in stochiometry also leads to a slight increase in overall resistance. 88 If humidified gases are used the resistance of the electrolyte decreases 89 due to a dillution effect. At the same time the cell performance decreases, presumably due to additional acid redistribution or leaching processes.…”

Section: Electrolyte Degradation In Ht-pefc and Pafcmentioning

confidence: 99%

“…At the same time the cell performance decreases, presumably due to additional acid redistribution or leaching processes. 89,90 In HT-PEFC degradation experiments at relatively high current density of 0.7 A cm −2 the loss of phosphoric acid was reported to have a significant impact. 55 For lower current densities (0.6 A cm −2 to 0.2 A cm −2 ) it was shown that internal resistance does not change significantly, 58,91,92 i. e. the loss of phosphoric acid is not significant.…”

Section: Electrolyte Degradation In Ht-pefc and Pafcmentioning

confidence: 99%

Accelerated Degradation of High-Temperature Polymer Electrolyte Fuel Cells: Discussion and Empirical Modeling

Reimer

Schumacher

Lehnert

2014

J. Electrochem. Soc.

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Accelerated stress tests are commonly applied in order to obtain a prediction of fuel cell life time within a short testing period. The stress test in this work considers a fuel cell which is constantly under load with frequent periods of very high current. Four different cells were operated each with a specific load profile. As a result severe performance degradation was observed in the region of high current densities. A short overview over the phenomena which may contribute to this specific fuel cell degradation is compiled from literature. Based on this overview major degradation modes are identified and combined with a simple polarization curve model. The modeling results allow for two different interpretations. Carbon corrosion reactions can explain the observed effects if cell voltage is assumed as the driving force for degradation. A better fit was obtained by using the overall heat flux as degradation criterion. In this case an increase in local temperature could lead to redistribution and loss of phosphoric acid from the MEA. The expected lifetime of a polymer electrolyte fuel cell (PEFC) ranges from 5 000 h for mobile application to 40 000 h for stationary application.1,2 In order to gain information about time dependent degradation in advance accelerated test protocols are used.3,4 Depending on the design of these tests valuable information about the nature of the main degradation mode under the applied operation conditions can be obtained. Despite the difference in operation conditions for PEFC, HT-PEFC and PAFC the structure and material of the electrodes are almost the same. Consequently, observed degradation phenomena show strong similarities.5 A good description of these phenomena can be found in several review papers 6-13 as well as in at least four books 1,2,14,15 where recent experimental facts and interpretations are summarized. A compact description can also be found in a recent review paper on modeling transport phenomena in PEFCs. 16 The purpose of this paper is to present the results of a specific accelerated degradation test for high temperature polymer electrolyte fuel cells (HT-PEFC) with phosphoric acid doped polybenzimidazole (PBI/H 3 PO 4 ) membranes. The fuel cell is constantly under load with frequent periods of very high current. In order to accelerate degradation effects the fuel cell is operated beyond it's point of maximum power. A simple polarization curve model is used as starting point for the analysis of data. Based on this first analysis a degradation model is proposed which allows for a straightforward physical interpretation. The complexity of degradation effects which occur simultaneousy usually leads to models which either resolve a specific mechanism in depth or describe more general effects. This paper aims at general effects which requires a broader understanding of degradation. In the following it is attempted to provide an overview over the phenomena which may contribute to fuel cell degradation. Based on this overview major degradation modes are identified and ...

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Experimental study of water transport in a polybenzimidazole-based high temperature PEMFC

Cited by 54 publications

References 38 publications

Correlating Electrolyte Inventory and Lifetime of HT-PEFC by Accelerated Stress Testing

Correlating Electrolyte Inventory and Lifetime of HT-PEFC by Accelerated Stress Testing

The influence of impurities in high temperature polymer electrolyte membrane fuel cells performance

Accelerated Degradation of High-Temperature Polymer Electrolyte Fuel Cells: Discussion and Empirical Modeling

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