14 Externe wissenschaftliche Einrichtungen
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Item Open Access Polymer electrolyte membrane degradation and mobility in fuel cells : a solid-state NMR investigation(2010) Ghassemzadeh Khoshkroodi, Lida; Müller, Klaus (Prof. Dr.)It is generally believed that fuel cells will play an important role in energy technology already in the near future. Operating polymer electrolyte membrane fuel cells (PEMFCs) at temperatures higher than 100 °C and reduced humidity is anticipated to avoid most of the shortcomings associated with the low-temperature fuel cell operation, such as CO poisoning of the electrode catalysts, slow electrode kinetics of the oxygen reduction reaction and expensive water/thermal management. To date, the operation temperature of PEMFCs is limited to about 90 °C, and this limit is given by the properties of the perfluorosulfonic acid (PFSA) ionomer, Nafion, which is commonly used as a separator material. Apart from the proton conductivity decay at higher temperature and lower humidification, it is also the limited stability of Nafion preventing it from long term operation. Despite the high stability of the PTFE backbone in Nafion, severe deterioration is observed during fuel cell operation. Formation of pinholes and cracks, thinning of the membranes and decrease of ion exchange capacity were reported. The fluorine release indicated that the bond cleavage process takes place under fuel cell operating conditions. Bond cleavage was initially believed to proceed from radical attacks to the carboxyl groups terminating the PTFE backbone of Nafion, and it was claimed to be controlled by the endcapping of the polymer backbone with a CF3 group. However, the release of fluoride was reported even after endcapping of the materials. The observations proved that bond cleavage limits the stability of PFSA membranes, but the elementary reactions and consequences on the membrane microstructure are not fully understood yet. In this work, it has been tried to get new insights into the problems of long term stability of polymer electrolytes for low temperature fuel cells. The aim was to identify the changes in the chemical structure of the membrane after operating in a fuel cell. This understanding is essential for extending the operation limit of PFSA-type membranes by either improving the membrane properties or adjusting the conditions within the running fuel cell. In the present work, therefore the changes taking place in PFSA membranes after applying in-situ and ex-situ aging protocols have been investigated. While the in-situ experiments provide a global picture, the analysis of membranes after ex-situ tests, with various conditions, allows the separation of different types of reactions. In previous studies the degradation changes were mainly monitored by analyzing the released water of the fuel cell or by using the liquid ionomers. In this work with the help of solid-state NMR spectroscopy, the direct study of the chemical structure and dynamics of the polymer membranes before and after the degradation tests became possible. The structural changes in different parts of the PFSA membranes were first inspected after an in-situ aging test. These examined membranes (Nafion and Hyflon Ion) differed by the length of the side chains. The comparison of the solid-state 13C and 19F NMR data of polymers before and after the in-situ degradation test showed that changes can take place not only in the main chain of the polymer, but also within the polymer side chains, as reflected by changes of NMR signals associated with CFSO3, CF3, OCF2 and CF groups. The degree of degradation is found to decrease with increasing membrane thickness while for a given thickness the short side chain polymer, Hyflon Ion, appears to degrade less than Nafion. In order to understand the reason for these observations, a new ex-situ method has been developed to mimic the degradation of polymer electrolyte membranes in PEM fuel cells (caused by the cross-leakage of H2 and O2). In this ex-situ setup, it was possible to expose membranes to flows of different gases with controlled temperature and humidity. H+-form Nafion films with and without electrode layer (Pt) have been treated in the presence of different gases in order to simulate the anode and cathode side of a PEMFC. The changes of the chemical structure occurring during the degradation tests were primarily examined by solid-state 19F NMR spectroscopy. For completion, liquid-state NMR studies and ion exchange capacity measurements were performed. It was found that degradation occurs only when both H2 and O2 are present (condition of gas cross-leakage), and when the membrane is coated with Pt catalyst. The chemical degradation rate is found to be highest for H2-rich mixtures of H2 and O2, which corresponds to the conditions at the anode under OCV. It is further shown that side chain disintegration is very important for chemical degradation, although backbone decomposition also might take place. The fact that in-situ degradation effects were reproduced by the present ex-situ experiments, suggest that membrane degradation in a running fuel cell is mainly the consequence of chemical aging. Detecting the degradation for the membranes coated with Pt in the presence of both gases, H2 and O2, points toward the importance of radicals in the degradation process, which in a running fuel cell (in-situ conditions) may only form in the presence of some gas cross-over, allowing H2 and O2 to react at the Pt catalyst of the anode or cathode structure. Since the gas cross-over increases for the thinner Nafion membrane, these results indirectly explain the higher degradation rate of thin Nafion in the in-situ degradation test. The chemical degradation and stability of PFSA membranes against radical attacks was also investigated in a Fenton ex-situ degradation test. Liquid and solid-state NMR as well as ATR-FTIR spectroscopy were applied to the samples before and after the Fenton reaction. A Comparison of the degradation rate of Nafion and Hyflon Ion in the ex-situ Fenton test again proved that the Hyflon Ion membrane is more stable than Nafion. Comparing the degradation rate of the side chain in these two polymers showed that the stability of Hyflon Ion is mainly due to the shortening of the side chain in this polymer. Hence, the absence of one ether group and the tertiary carbon reduces the degradation rate of the side chain and makes this polymer less sensitive to the radical attacks than Nafion. For the performance of a membrane not only the chemical structure but also the polymer dynamics is important. Therefore the molecular mobility of the ionomer was investigated by variable temperature 19F NMR lineshape, T1 and T1ρ relaxation experiments. The decrease of the temperature dependent linewidth was explained by the reduction of static disorder in the Nafion membrane. From the relaxation data there was evidence for structural annealing, which is independent of the chemical degradation. Chemical degradation is considered to reduce the chain flexibility (i.e. the motional amplitudes), which may be explained by chain cross-linking and condensation reaction for the side chains. To overcome the problem of Nafion's low conductivity at temperatures above 100 °C and low relative humidity, also composite membranes were introduced. These membranes consist of Nafion modified by inorganic oxide additives. It has been reported that under dry conditions, these membranes show enhanced water uptake and water diffusion when compared with filler-free Nafion. In order to understand the reason for the better performance of these polymers, the impact of the oxide particles on the polymer dynamics has been investigated. [Nafion/(SiO2)x] composite membranes in the dry and wet state with x ranging from 0 to 15 w/w% were investigated by variable temperature solid-state 19F NMR spectroscopy. 19F T1 and T1ρ relaxation times and NMR lineshapes were analyzed in order to get details about the polymer mobility. It is concluded that solid oxide SiO2 particles play an important role in stabilizing the chemical structure and morphology of the polymer especially in the dry state. The filler particles lead to higher mobility of polymer chains, if the filler content has an optimized value of about 9 w/w%. The results were further supported by comparing the sideband intensity as well as the linewidth in 19F NMR and recording the 19F{1H} CP/MAS NMR spectra. Furthermore, it has been shown that the structure of composite membranes is more stable after dehydration and possible condensation reactions are less likely in these membranes. The presence of filler particles decrease the chance for morphology changes and close packing of polymer chains in the dry state. Also the decrease of ionic exchange capacity after dehydration is less severe for the composite membrane as compared to filler-free Nafion. In conclusion, the present results provide a complete picture of solid membrane before and after degradation and of possible mechanisms for radical formation and radical attacks to the polymer. In addition, it is shown which changes can occur in the morphology of polymer chains in low humidification and high temperature. Some general suggestions for the better performance of polymer electrolyte membrane are therefore: For improving the performance of polymer electrode membrane, the sources for the radical formation in the fuel cell should be controlled. This can be possible to some extend by avoiding the use of iron end plates in the fuel cells. Also the chance for the gas crossover through the membrane should be decreased. Thicker membranes show less gas cross-over. By taking into account the higher resistivity of thicker membranes, an optimized membrane thickness should be selected. Hydrocarbon sulfonated polyetherketones possess narrower hydrophilic channels which significantly reduce electroosmotic drag, water permeation as well as gas cross-over. Also the short side chain perfluorinated polymer, Hyflon Ion, with lower electroosmotic drag of water should possess a reduced gas cross-over though the membrane. The more efficient way for decreasing degradation is to use membranes which are stable against radical attacks. At this point the perfluorinated polymers are still the best available membranes. Endcapping of the backbone in these polymers and decreasing the concentration of reactive end groups like COOH during the polymer manufacturing process can significantly decrease degradation. To minimize degradation of the side chains in perfluorinated polymers, short side chain polymers are suggested because of less reactive groups for the radical attacks and higher concentration of acidic groups. When higher operation temperatures are required, composite Nafion membranes might be used. The higher stability of these membranes makes them advantageous for operating at evaluated temperatures and low relative humidity. The novel results from the present work lead to a better understanding of membrane degradation, which still represents a serious problem for fuel cells under operation conditions, and provide important indications for future developments of membranes with improved performance for alternative energy conversion devices.Item Open Access Correlation between the microstructure of porous materials and the adsorption properties of H2 and D2(2011) Krkljus, Ivana; Roduner, Emil (Prof. Dr.)One of the most challenging tasks toward the full implementation of the hydrogen based economy is the reversible storage of hydrogen for portable applications. Three main approaches have been investigated to store the hydrogen, storage as a compressed gas or a liquid, or through a direct chemical bond between the hydrogen atom and the material. The alternative approach, the most recently investigated, is the storage of hydrogen at cryogenic conditions. Storage by physisorption within porous adsorbents has particular advantages of complete reversibility, the fast refueling time, the low heat evolution, and above all increased safety. The nature of interaction of hydrogen, deuterium, and gas mixtures with porous adsorbents was exploited by performing thermal desorption spectroscopy (TDS) measurements. This sensitive experimental technique gives qualitative information about the different adsorption sites, which show different desorption temperatures depending on the interaction energy. After an appropriate calibration the amount of gas desorbed may be quantified. To gain a more fundamental insight into the available adsorption sites multiple TDS spectra were recorded, corresponding to different surface coverages (in the pressure range of 1 to 700 mbar), and different heating regimes. Different kind of porous adsorbents, conventional carbon–based materials and novel Metal Organic Framework Materials (MOFs), were used to investigate the hydrogen/deuterium physisorption mechanism. For carbon materials an increase in the hydrogen interaction potential was observed for adsorbents with narrow pore size. The confined geometry, where hydrogen simultaneously interacts with all the surrounding adsorbent walls, strengthens the interaction potential with the adsorbate molecule, thus, maximizing the total van der Waals force on the adsorbate. Crystalline MOFs are a new class of porous materials assembled from discrete metal centers, which act as framework nodes, and organic ligands, employed as linkers. The material properties can be optimized by changing these two main components. Owing to their high porosity, high storage capacity at low temperature, and excellent reversibility kinetics, MOFs have attracted a considerable attention as potential solid–state hydrogen storage materials. This novel class of porous adsorbents has been extensively investigated within this thesis. The greatest challenge for porous adsorbents is to increase the strength of the H2 binding interaction, and bring adsorption closer to RT conditions. Several strategies, aimed at improving hydrogen adsorption potential in MOFs are closely investigated. These strategies comprise the inclusion of open metal sites and the optimization of the pore size and, thus, the adsorption energy by ligand modification. The influence of the coordinatively unsaturated metal centers, liberated by the removal of metal–bound volatile species, has been particularly investigated. As for carbon materials, the H2–MOF interaction potential is especially enhanced in materials with the pore size comparable to the kinetic diameter of the hydrogen molecule. Such effects may result from the overlap of the potential field due to the proximity of the pore wall, which strengthen the interaction potential with the adsorbate molecule. However, smaller pores prevent hydrogen penetration and induce diffusion limitations. Furthermore, the molecular transport in confined pores at low temperatures may be significantly affected by quantum effects.