Long-life PEM-FCH &CHP systems at temperatures higher than100°C
CONSIGLIO NAZIONALE DELLE RICERCHE
Piazzale Aldo Moro 7
€ 310 355,14
Giuseppe Barbieri (Dr.)
Sort by EU Contribution
UNIVERSITA DEGLI STUDI DI ROMA TOR VERGATA
€ 243 100
UNIVERSITE DE PROVENCE
€ 151 429
UNIVERSITAT DES SAARLANDES
€ 151 130
€ 66 490
FUMATECH BWT GMBH
€ 167 533,31
MATGAS 2000 AIE
€ 204 240
€ 65 949,55
Grant agreement ID: 245339
1 January 2010
31 December 2012
€ 2 677 298,40
€ 1 360 227
CONSIGLIO NAZIONALE DELLE RICERCHE
Final Report Summary - LOLIPEM (Long-life PEM-FCH &CHP systems at temperatures higher than100°C)
Membrane treatment following membrane production significantly increases the operating range (for instance, temperature) in fuel cell applications. Right procedures were identified in this LoLiPEM project for PFSA and SAP (Nafion, fumapem F 950, fumapem F 930 ZrP, SPEEK, SPEEK-WC, fumapem SPEEK-EX-330, fumapem SPEEK-EX-320, fumapem® ST310, fumapem® STO305, fumapem® S360) membranes. Thermal annealing or chemical cross-linking are good ways to improve the membrane endurance up to 120/140°C for these membranes, which also show interesting proton conductivities at a higher temperature (e.g. 0.09 S/cm for EX-330 at 120°C). Mechanical resistance and permeation of humidified gases (lower cross-over)were also positively affected by thermal annealing or chemical cross-linking, according to the polymer used. Specific protocols were developed for these mechanical and permeation measurements.
The results achieved can be considered important progress with respect to the state of the art, specifically in the hydrolytic stability in hot water (at about 80°C more than that of the state of the art) and in the mechanical properties and proton conductivity (at 40°C more than that of the state of the art).
Prepared electro-catalysts showed performance as good as the best commercially available, with a much higher degree of catalyst utilization; these were successfully used in developing GDEs containing SPEEK as an ionomeric binder. MEAs consisting of native Nafion®, hydrothermal treated Nafion® and cross-linked SPEEK were prepared investigating also the hot-pressing temperature (160-190°C) as a function of the degree of cross-linking. The good ionic contact between the ionomer used as binder in the GDE and the membrane was given by preparing the micro porous layer from solutions of SPEEK cross-linked also in-situ; thus good performances were achieved also at 95-105°C.
An accelerated aging test carried out at 110°C showed a lifetime of 8850 hours (the most conservative, when the effect of the temperature, 35°C higher than the reference one is not considered) and 44250 hours for Nafion NRE 212 thermally annealed. The use of a plasticizer reducing the SPEEK stiffness overcomes the mechanical rupture of SPEEK-based MEA that showed no performance decay or membrane degradation after many hours.
The work of the EU/FCH-JU LoLiPEM project was carried out by a consortium of eight partners: National Research Council of Italy – Institute on Membrane Technology, Università di Roma «Tor Vergata», Aix-Marseille Université (former Université de Provence), Universität des Saarlandes, EDISON SpA, FuMA-Tech GmbH, MATGAS and Politechnika Krakowska.
Project Context and Objectives:
The LoLiPEM project was aimed to perform advanced R&D of SPG&CHP systems based on Polymeric Electrolyte Membrane Fuel Cell Hydrogen (PEMFCH). PEMFC technology is used widely in different sectors, for instance in transport and stationary applications, and has a power range up to hundreds of kilowatts. Therefore, PEMFC has dominated shipments by megawatt since 2008. PEMFCs are currently the most promising technology for light duty vehicles and materials handling vehicles, and to a lesser extent for stationary and other applications.
The idea of the project was based on the foreseen assumptions that the SPG&CHP systems based on FCs can help reduce pollution, especially in large towns. The expected results were intended to be useful for reducing petrol dependence and for reducing CO2 emissions.
The previously mentioned co-generation systems based on PEMFCHs are of great interest because they can use very thin and flexible ionomer membranes exhibiting high proton conductivity at relatively low temperature without any addition of mineral acids. The peculiar characteristics of the ionomer membranes make the development of small co-generation systems easier and very suitable for small buildings and for mobile (automotive) applications. The absence of free acids in the membranes avoids lowering of conductivity due to its loss and poisoning of electrode catalysts. Furthermore, their low operating temperature reduces insulating and corrosion problems as well as risks for their management. Finally, being noiseless, very small systems could be provided even for single flats when an easier management is developed.
The PEMFC uses a water-based acidic polymer membrane as its electrolyte with platinum-based electrodes. Usually, PEMFCs operate at relatively low temperatures (below 100 degrees Celsius) and can tailor electrical output to meet dynamic power requirements. Owing to the relatively low operating temperatures and the use of precious metal-based electrodes, these cells must operate on pure hydrogen (CO<10 ppm).
A variant of PEMFC which operates at higher temperatures is emerging in the stationary sector where it is finding application in both micro-CHP and UPS. Running temperatures over 100°C are more tolerant to impurities and, as a result, the fuel processing system can be simpler. The additional by-product heat produced by a high temperature PEMFC can also be used as part of an integrated installation, increasing the overall efficiency of these units.
Furthermore, the higher temperature decreases the anode poisoning due to small amounts of carbon monoxide, always present in the hydrogen produced by carbon containing sources, and improves the fuel oxidation kinetics, leading to an enhancement of fuel cell efficiency. Thus, an enhancement of at least 20-30°C of the state-of-the-art operating temperature of PEMFCHs is highly desirable and it could be decisive for the development of systems based on PEMFCHs.
A decay of the proton conductivity of the ionomer used can lead to a decrease of the cell performance at a higher temperature. In this case, co-generators are more interesting since they recover more and more heat.
In this contest, the main objective of the LoLiPEM project is the development of a long-life PEMFCHs operating at temperatures greater than 100°C.
In order to reach this main objective, various sub-objectives of the project have been identified.
(1) Development of new stable Perfluoro Sulfonic Acid (PFSA) membranes operating at least 20°C higher than that of the state of the art and fulfilling the requirements in terms of long term durability for stationary fuel cell operation.
(2) Development of new stable non-perfluorinated ionomers, such as sulfonated aromatic polymers (SAPs), much less expensive than PFSA and exhibiting a lifetime comparable to that of modified PFSA membranes, in order to reduce the cost of the co-generation systems, investigating innovative thermal and chemical treatments for promoting the cross-linking reactions between adjacent ionomer chains.
(3) Development of new catalytic electrodes more stable at the operating temperatures that have been used in MEA.
(4) Preparation and texting of MEAs based on these innovative materials.
(5) Reducing the cost of the high temperature FCH by facilitating the replacement of MEAs (“use and discard” MEAs).
(6) Establishment of accelerated test techniques and lifetime prediction methods for MEAs testing.
(7) Development of a procedure for “post mortem” analysis on MEAs, to identify the main parameters and conditions affecting the MEA performance and durability.
(8) Development of a prototype of a Modular multi-PEMFCHs system for Combined Heat and Power (MoPEM) based on Polymer Electrolyte Membrane Hydrogen FC (PEMFCH) for facilitating the replacement of the single PEMFCH and hence the MEA.
(9) Exploitation of the results in terms of patents for the industrial partners, publications in high impact international journals and top international conferences, reaching out to the public and increasing the awareness of this technology
To develop these aspects, the main areas of innovation foreseen for the LoLiPEM project were:
Novel polymer membranes - Research focused on the development of membranes with good proton conductivity allowing great system simplification and, as a consequence, a significant reduction in cost of manufacturing. One of the tasks was to develop PEM able to operate in the temperature range up to 130°C, at a high relative humidity, and without any degradation. The objectives were aimed using a new thermal annealing and chemical cross-linking procedure for PFSA and SAP membranes, respectively. The new polymeric membranes developed show an improved stability as confirmed by their mechanical properties and an excellent hydrolytic resistance. Furthermore, a new approach for the systematic measurement of the mass transport properties of a PEM has been proposed for evaluating the membrane.
New MEA with improved durability - Project developed novel catalyst materials capable of stable performance at the higher operating temperatures afforded by the new membrane developments. The development of the electrodes using a new technique offered advantages in terms of catalyst utilization and improved kinetics. The membranes based on PFSA and SAP polymer with improved properties and the developed catalyst are assembled together for producing MEAs. The MEA performance are investigated as a function of several parameters such as the degree of cross-linking, the variation of hot-pressing temperature during MEA assembling, the reduction of membrane thickness and the variation of the fuel cell testing conditions.
Lifetime - The durability of the developed membranes under fuel cell operation is predicted by accelerated test. Post-mortem analysis is identifying some parameters affecting the MEA durability. The lifetime of SAP-based membranes is also increased inserting a frame or adding a plasticizer.
Stabilized Nafion, Fumapem F and Fumapem FZP membranes were obtained by tailor-made annealing
Annealing of Nafion 1100 - The possibility of increasing the operating temperature of fuel cells using Nafion as ionomer membranes by a new annealing procedure in the presence of dimethyl sulfoxide and tributyl phosphate as annealing agents was investigated with the aim to increase the crystallinity of the polymer, creating physical crosslinking that can improve mechanical properties and hinder conformational changes.
The extent of annealing was derived by the shift of the nc/T plots (“nc” counter elastic index) of annealed membranes relatively to the plot of as-received membrane. The obtained nc/T plots can be used as a powerful analytical method that can be very useful not only for deriving quantitative information on the extent of annealing, but also for obtaining information on the melting temperature of the semi-crystalline phase grown during the annealing process (Alberti, Di Vona, Narducci, Int. J. Hydrogen Energy, 2012, 37, 6302-6307; Alberti, Narducci, Di Vona, Giancola, Fuel Cells 2013, 13, 42-47). An important amount of semi-crystalline phase with a melting point of about 155°C was formed by using annealing at temperature 140°C in the presence of an annealing agent. The increased thermal stability of Nafion was related to the formation of the semi-crystalline phase that acts as a physical cross-linker, so that the mechanical properties of the annealed ionomer are no longer lost at the glass transition (about 102°C), but at the melting point of the semi-crystalline phase with a consequent thermal stabilization. The use of nc/T plots as analytical tools for detecting and understanding important ionomer properties such as Tg, Tmelting, the hydration number, lambda, mechanical properties, etc., as a function of temperature and relative humidity, was denominated in URoma2 Laboratories as “INCA - Ionomer nc Analysis“. It was found that this method, specific for ionomers, can be used alone or in connection with other known methods for polymers. In this study the annealing temperature was connected with the crystallization temperature and it became clear that the annealing temperature cannot be higher than the melting temperature of the semi-crystalline phase. It also became clear that a certain amount of molecular rearrangement must take place during crystallization, but this rearrangement cannot take place through large-scale molecular diffusion as the process occurs in the solid state. Thus, in order to facilitate the above crystallization process, the addition of small amounts of proton acceptor solvents with plasticizer effect was considered and experiments in the presence of these substances (in the following called annealing agents) were carried out. Our first choice on DMSO as an annealing agent was surely influenced by knowledge on this solvent acquired during our investigations on the thermal formation of chemical cross-links in sulfonated aromatic ionomers. During these investigations, it was realized that the study of the effect of thermal annealing was enormously facilitated by the use of nc/T plots already described in a recent review (Alberti, Narducci, Di Vona, Chapter 8 in “Solid State Proton Conductors. Properties and Application in Fuel Cells”. Eds. P. Knauth and M.L. Di Vona, Wiley). We also remind the reader that the relations between nc and the water molar fraction, molality and molarity of the inner proton solutions at given relative humidity (RH) values, as well as the inner osmotic pressure at various temperatures in the range 20-140°C, have been recently reported (Alberti, Di Vona, Narducci, Int. J. Hydrogen Energy, 2012, 37, 6302-6307) and are discussed below. From these data, the following general trends were observed: (a) the shifts of the plots increase with increasing time of thermal treatments at 140°C; (b) if the time is taken constant, the shift increases from 130° to 140°C; (c) all nc values linearly decrease with temperature. However, an abrupt change of the slope can be observed at about a given nc level that in the case of Nafion 1100 seems to be placed between 8.5-9.5 (d) it is also evident that all the extrapolations of nc plots above the said nc level converge at about 110°C whereas all the extrapolations of nc plots under the said nc level converge at about 155°C. Since the convergence at 110°C seems to correspond to the glass transition of Nafion, the convergence at about 155°C was associated with the melting temperature of the semi-crystalline phase created during the annealing process.
The obtained experimental data indicate that nc values (hence the mechanical properties of the Nafion matrix) are shifted towards higher temperatures with an increasing degree of annealing. In other words, the ionomer matrix increases its thermal stability with increasing degree of annealing. A plausible explanation of the above strong stabilization could be related to the formation of a semi-crystalline phase giving rise to a physical cross-linking of the remaining amorphous ionomer part. The above hypothesis well explains the phenomenon, since mechanical properties are completely lost (i.e. nc tends to zero) at a certain temperature that corresponds to the melting temperature of the formed semi-crystalline phase. Annealed Nafion was analysed by DSC in order to confirm experimentally by another independent method that the convergence point at 155°C was really the melting point of the Nafion semi-crystalline phase created by the annealing in the presence of DMSO at 140°C.
It is important to notice that a value of nc=7 was obtained at 100°C for the curve at 7 days of annealing, corresponding to a value of lambda=20 and a water uptake of around 32 %, the latter is lower than 40%, which is the target for the LoLiPEM project for PFSA membranes.
Some conclusions on the annealing process can already be drawn: (1) in the absence of annealing agents, no appreciable annealing effect can be obtained even for very long thermal treatments also at 140°C; (2) the annealing effect is strongly influenced by the chosen annealing temperature. In our opinion the annealing temperature must be considered as a crystallization temperature and therefore all the present understanding on the crystal growth from molten polymer crystals must be taken into account; (3) it is very plausible that the reinforcement of the mechanical properties at temperatures higher than the ionomer Tg are due to the formation of a semi-crystalline phase; (4) nc/T plots seem to be a very powerful method for studying annealing effects, giving information both on the melting point and on the amount of the formed semi-crystalline phase.
Independent characterizations by the DSC method confirm that an increased amount of crystalline phase (melting point in the range 150-160°C) is formed during the annealing process. Notice that this melting temperature is in very good agreement with the value found for the convergence temperature of nc/T plots. Our hypothesis that the linear nc/T plots converge at the melting point of the semi-crystalline phase remains very plausible, although other research will be necessary for definitive conclusions.
Stabilized Fumapem F-950, Fumapem F-930 ZrP - The same annealing treatments were performed on PFSA membranes, and then lambda and consequently the nc values are measured. The annealing in the presence of DMSO reduces the lambda values that are proportional to the annealing time. The selected temperature of 120°C was kept constant for 15h. A higher temperature or longer time does not present significant benefits. After these results all PFSA membranes were treated with the various protocols before their use. Hydrothermal treatments were also performed to determinate the effect of the swelling in water.
From the “instability maps” (nc/T plots at fixed RH or T/RH at fixed nc values) for Nafion 117 can be noticed that the non-equilibrium value of lambda=22, obtained by the standard procedure, corresponds to the equilibrium lambda value at about 80°C. When this hydrated membrane is placed in a fuel cell operating at 100°C, its hydration increases to an equilibrium lambda value corresponding to this latter temperature (i.e. lambda =27). An asymmetric swelling of the Nafion membrane can therefore take place (parallel to the catalytic electrodes) with consequent decay of proton conductivity. Notice that it is sufficient to increase the nc value of just two units for avoiding the conductivity decay of the membrane. In fact, the increasing of two nc units is equivalent to decreasing the temperature by about 22°C (i.e. a decrease from 100° to 78°C, where we already know that Nafion membranes can operate even for very long times). It is sufficient to operate at RH of 96% to avoid the decay of its proton conductivity in the temperature range 100-120°C.
Analogous treatments were also done on Nafion 212. In the all cases the conductivity of treated Nafion membranes are similar or even higher than commercial Nafion. In order to have a better understanding of the Nafion behaviour in fuel cells operating at temperatures higher than 80°C, the preparation of membranes containing a large amount of layered morphologies prevalently oriented in the direction parallel to the membrane surface (hence of low through-plane conductivity) was attempted. Successful in-plane oriented samples were obtained by forced swelling of membranes between rigid planar constraints. Other than the expected low through-plane conductivity, a first characterization of these modified membranes clearly showed that the dimension changes during processes of dehydration and successive hydration essentially takes place perpendicular to the membrane surface. It was furthermore found that the forced swelling was accompanied by a strong reduction of ionomer density (from an initial value of 2 to about 1.4 g/cm3). Finally, evident changes of the nc/T plots were also found.
The stability of PFSA membranes was discussed in terms of the counter-pressure index (nc). Nafion morphology depends on RH, temperature and time of equilibration. The introduction of the nc index greatly facilitates the study of this dependence, because this index is proportional to the counter-pressure force of the ionomer matrix and can be taken as an index of the actual ionomer morphology. These studies have also led to the elaboration of the INCA Method, which is still under development for the following aspects: membrane annealing, application to low cross-linked ionomers, membrane reconstruction and water loss processes.
Synthesis of cross-linked (XL) SAP membranes (building bridges via SO2 groups by thermal treatment)
The in situ cross-link mechanism mediated by DMSO was clarified, the presence of XL via SO2 moieties was ascertained
A preliminary screening on cross-linked membranes was made measuring the IEC, the degree of cross-linking, and the water-uptake in fully humidified conditions.
The aim was to obtain cross-linked SPEEK membranes with the best compromise of water uptake, IEC and proton conductivity, compatible with good mechanical properties, low swelling and low permeability. A set protocol was followed during the preparation of cross-linked membranes; a different degree of cross-linking (DXL) can be obtained varying the time of the thermal treatment. The water uptake of XL membranes depends on the degree of cross-linking and the required time to reach equilibrium was very long for samples with short thermal treatments (i.e. low DXL). Lambda was measured after swelling in water at 100°C for different times; samples were then immersed in water at room temperature for 24 h and then weighed. The memory effect was determined. An important consideration is that also samples with low DXL (22%) can resist in water at 140°C. Different strategies were also explored coupling cross-linking with thermal annealing. The Fenton test to check the chemical stability demonstrated that the weight loss was 0.7% and 0.9% for cross-linked SPEEK 120/24-180/24 and 120/24-180/10, respectively.
LoLiPEM project targets for SAP membranes in term of conductivity, water uptake, weight loss and permeability were achieved.
Cross-linked SPEEK –WC membranes - Various treatments were used to improve the stability of the SPEEK-WC membranes cast at 45°C and 120°C. The cast membranes maintain residual traces of solvent necessary for the cross-linking reaction; the number of DMSO molecules for -SO3H group was about 0.7. Membranes were post-treated thermally at 160°C and chemically by a cross-linking using 1,5-Diamino-2-methylpentane (DAMP). The membranes treated with DAMP are more stable (lower swelling) but also less conductive than untreated samples. Improved performance (lower swelling, higher proton conductivity and a higher oxidation resistance) were obtained for a lower cross-linking. The membranes withstand permeation measurements at 120°C, lasting for all the time of measurements without presenting any drop in performance.
After an initial screening, the next work was focalized on series FUMAPEM E-350, E-320 and E-330 cast with the FUMATECH production facility. Two strategies were followed: (1) the increasing of water uptake lowering the degree of cross-linking; (2) the exploitation of the memory effect on membranes with high degree of cross-linking. The regeneration of membranes followed a specific developed protocol. Different treatments and solvents were applied and better results were obtained using DMSO. The use of plasticizers was explored to facilitate the process of MEA formation. In addition, the casting procedure was varied shortening the time to facilitate the industrial scale-up. Three membranes (Fumapem® EX-330 (120/12, 180/3), Fumapem® EX-330 (120/12, 180/7) and Fumapem® EX-330 (140/6, 180/14), ) with different thermal treatments (at 120°C for 12 h or 140°C for 6h and at 180°c for 3, 7 and 14 h) were selected for further investigations.
Cross-linked SAP synthesized by FUMATECH - A key concept of FUMATECH aims at the transfer of the DMSO-mediated cross-linking route for SPEEK onto other highly sulfonated hydrocarbon polymers based on polysulfones. These polymers typically have very high conductivities; however, they are water-soluble or show tremendous swelling in contact with water, when they are non-cross-linked. Different High Sulfonated Polysulfones (Fumion® ST-310 - sulfonated poly (phenylene sulfide sulfone), Fumion® STO-305 - sulfonated poly(phenylene ether sulfide sulfone), Fumion® S-360 - sulfonated poly(phenylene sulfone), Fumion® S-340 - sulfonated poly(phenylene ether sulfone)) were synthesized and then chemically treated. Overall, the findings indicate that the cross-linking reaction is based on an electrophilic aromatic substitution mechanism and DMSO is required for the cross-linking reaction. After the promising results obtained, we decided to focalize on cross-linked Fumapem ST-310 treated at 180°C for 20 h which showed a proton conductivity of 0.09 S/cm at 120°C.
Research for a better understanding of the mechanism of cross-link reactions -
The cross-link reaction between macromolecular chains of sulfonated polyetheretherketone (SPEEK) in the presence of dimethylsulfoxide (DMSO) by thermal treatment above 150°C was investigated by various techniques including elemental analysis, acid-base titration, infrared and NMR spectroscopy, water uptake measurements and thermogravimetry (Knauth, Sgreccia, Donnadio, Casciola, Di Vona, J. Electrochemical Society, 2011, 158(2) B159-B165; Hou, Di Vona, Knauth, J. Membrane Science, 2012, 423–424, 113–127)
The main results can be summarized as follows:
(a) Combining elemental analysis and FTIR, it is possible to conclude that sulfone linkages are responsible for covalent cross-linking occurring by transformation of sulfonic acid groups.
(b) We have verified that membranes cast from dimethylacetamide, N-methyl-2-pyrrolidone, acetone and acetone/water did not undergo cross-linking and that DMSO is essential for the cross-linking.
(c) From NMR analysis, it is clear that DMSO promotes the formation of the electrophilic –SO2+ intermediate both through the formation of a direct interaction with the solvent or by the formation of an anhydride moiety.
(d) A two-step thermal treatment is necessary to perform a successful cross-linking process: the first step is an evaporation treatment that leaves the “optimal” quantity of residual DMSO inside the membrane. The second step is the effective treatment that allows running the cross-linking reaction in the presence of a suitable amount of residual DMSO.
(e) The temperature selected for the second treatment is 180°C to achieve relatively short times of treatment. In fact, when the reaction is carried out at 160°C a sufficiently large amount of cross-linking is obtained only for times around 64 h, whereas a temperature higher than 180°C can lead to desulfonation reactions. Therefore, it is important to find the best compromise between time and temperature of treatment in order to facilitate the industrial scale-up.
The cross-linked membranes show a great stability in boiling water, the water uptake determined by the time of the second treatment, that is, by the degree of cross-linking.
The optimization of conductivity properties of cross-linked SPEEK was also studied focalizing on the maximum achievable conductivity at a certain lambda for cross-linked -SAP membranes, using the study made in these years (Knauth and Di Vona, Solid State Ionics, 2012, 225, 255-259; Maranesi, Hou, Polini, Sgreccia, Alberti, Narducci, Knauth, Di Vona, Fuel cells, 2013, 13(2), 107–117). The hydrolytic stability and hydratation can be improved for these membranes when in similar conditions the untreated SPEEK would not support, so that quite good electrical properties can still be obtained, compatible with recent requirements. These membranes exhibited a conductivity of ca. 0.06 S/cm at 100°C.
Ex-situ characterization of stabilized membranes
Tensile stress was utilized in the determination of the mechanical properties of PFSA and SAP membranes applying a new set protocol.
The protocol specifically developed inside the LoLiPEM project includes the name of the sample, the operator, the testing procedure and identification of the apparatus. It also contains the sampling method and the test conditions, most notably temperature and relative humidity, rate of load application and the membrane shape. The test results include all relevant mechanical properties, such as Young modulus, yield stress, tensile stress, elongation etc. Nafion membranes treated at 20 and 50°C show a Young modulus of 268-269 MPa and an elongation of 318 and 336%, respectively. A higher temperature (100°C) lowers both the Young Modulus and elongation at 227 MPa and 227%, respectively.
Measurements were also done on Fumapem F-950 and Fumapem F-930 ZrP annealed with DMSO at 140°C. These membranes show a significantly higher Young modulus (920-1120 MPa) and lower elongation (36-49%). Measurements on Fumapem SPEEK-EX regenerated and un-regenerated membranes show how regeneration in 5M H2SO4 increases the elongation considerably. Higher plasticity owing to lower interaction between chains (no ionic cross-links) is observed.
The comparison between membranes with or without plasticizer underlines that the use of plasticizers is a way of optimizing the mechanical properties beyond crosslinking.
PFSA and SAP membranes showed very good mechanical integrity: as also required by the LoLiPEM project targets.
Dynamic mechanical analysis (DMA) analysis allows deriving the glass transition temperature of the samples treated for different times at 180°C. Membranes were analysed from 30 to 250°C (3 K/min) in air at a fixed frequency of 1 Hz with 0.05 N as initial static force and oscillation amplitude of 10 micron using a DMA 2980 dynamic analyser (TA Instruments). Amplitude of 10 micron was chosen to keep the linear viscoelastic response of samples during experiments. Mechanical measurements clearly show how thermal treatment significantly enhances the mechanical properties of the membranes: Young modulus values of 850, 1160, 1300 and 1450 are for an untreated membrane and membrane treated for 3, 10 and 24 hours, respectively. If mechanical degradation of membranes is assumed to be related to the existence of plastic deformations during fuel cell operation, it is clear that the enhancement of mechanical properties is of major importance for the improvement of membrane durability inside the fuel cell. It should also significantly influence the water uptake behaviour in various degrees of relative humidity and membrane swelling at high RH.
Water uptake measurements in controlled humidity conditions -
The water uptake measurements were made at steady state, i.e. after 240 h of stabilization in a closed vessel over saturated solutions of inorganic salts with defined water activity, including LiCl, MgCl2, NaBr and KBr. Before the experiments, the membranes were dried 240 h over P2O5 and a final measurement was made in presence of pure water (a(H2O)=1).
The water uptake of treated SPEEK membranes is lower than that of untreated SPEEK at high RH, in agreement with the expectation, but, surprisingly, it is higher than that of untreated SPEEK at low RH. Two changes are observed especially in untreated and 3h treated SPEEK between 50 and 60% RH and above 80% RH. This is where residual DMSO is removed from the membrane by « dissolution » in water vapour. The presence of DMSO in untreated samples and in those treated for short time (3 h) explains the reduced water uptake at low RH, where the water channels are «obstructed», then DMSO is removed and water can fill the void.
For Fumapem E-350 and E-360 before and after cross-linking, the standard procedure removes the residual DMSO and the water uptake is, as expected, lower for cross-linking samples.
The water uptake of the developed membrane meets the specification of the LoLiPEM project. It is 40% for PFSA membranes; 40% is also the project target. For SPEEK (180°C–20h), SPEEK-WC and Fumion E-490 the water uptake is 48, 25 and 18%, respectively, lower than 50%, which is the project target.
Thermogravimetric Analysis was shown to be useful for independent determination of degree of Sulfonation and degree of Cross-Linking of SAP, complementary to the more time-consuming acid-base titration. For SPEEK membranes (initial sulfonation degree=0.9) thermally treated at 180°C for different times, the weight loss around 250°C could be attributed, by on-line Mass Spectroscopy, to the decomposition of sulfonic acid groups. The peak of the evolution of the mass signal corresponding to SO2 is slightly asymmetrical, showing that some sulfonic acid groups decompose at higher temperature. The amount of sulfonic acid groups, and thus the degree of Sulfonation, can be assessed by integration of the weight loss. It can be shown to be comparable to values obtained by titration. Furthermore, the degree of cross-linking can also be deduced, because the sulfone bridges formed decompose with the polymer main chain above 400°C.
Conductivity measurements were performed on various membranes to assess the stability of proton conductivity at high temperature (cycling in the range 80–140°C with isothermal steps of 8–15 hours) and RH (50-95%, with an equilibration time of 6 hours, at least, at each RH value).
Measurements were carried out by the impedance technique using ELAT electrodes (Pt free) pressed on the membrane at 60 kg/cm2 before measurements and pre-treatment of the samples in the conductivity cell at 80°C for 12 hours and 140°C for 8 hours (RH=90%).
Differently from un-treated membranes, the conductivity of thermally treated SPEEK membranes remains stable up to 140°C. It depends on the cross-linking degree but values higher than 0.05 S/cm are achieved at 140°C and RH=90%.
In addition, the optimization of the cross-linking and water uptake led the proton conductivity of Fumapem EX-320 and Fumapem EX-330 membranes to be higher than the LoLiPEM project target of 0.07 S/cm.
Permeation measurements - cross over
As already mentioned, the cross-over of the fuel and oxidant not only reduces the PEMFC efficiency, but also favours the failure of the membrane, because radical species produced at the electrodes can attack the polymer. The evaluation of the membrane transport properties, measured in conditions close to the ones used in real applications, provides a fundamental indication on one of the aspects to take into account in the choice of an electrolyte suitable for PEMFC applications.
A new approach for the systematic evaluation of the mass transport properties of a polymer electrolyte membrane was proposed. A protocol (Brunetti, Fontananova, Donnadio, Casciola, Di Vona, Sgreccia, Drioli, Barbieri, J. Power Sources, 2012 (205), 222-230) for permeation measurements was elaborated to compare the transport properties of different membranes as a function of the operating conditions owing to the strict dependence of the membrane performance on the relative humidity and temperature. The mass transport measurements (permeation tests) started feeding H2 at 80°C and RH=50%. Then hydrogen was replaced by nitrogen keeping the same temperature and RH. Afterwards, nitrogen was replaced by oxygen, still at 80°C and RH=50%. An analogous sequence (hydrogen, nitrogen and oxygen) was repeated once for higher RH (75%) and again for RH=100%, at the same temperature. A total of nine steps complete this first part. The same sequences were repeated at higher temperatures such as 100-120°C. The experiments were performed continuously and the membrane was left overnight under a gaseous stream at 2 bar.
The Permeation through the cross-linked SPEEK membranes of hydrogen, nitrogen and oxygen was evaluated as a function of the relative humidity, for all the three temperatures investigated. At 80°C the permeance slightly decreased as the relative humidity increased. The permeation of all the three gases through polymeric dense membranes essentially depends on the diffusive transport mechanism. The reduction of the permeance with RH could be attributed to the water uptake increase that produces a change in the polymer matrix, creating a reduction of diffusive transport of the gas. In the whole RH range considered hydrogen permeance was higher than that of oxygen and nitrogen. Also at 110°C, the permeances of all gases slightly decreased with RH and that of hydrogen was always higher than other permeances. At 120°C, the permeance decrease correspondent to RH increase was more evident than at the other temperatures investigated. The experimental results confirmed that the cross-linked SPEEK membrane was quite resistant, since it was able to withstand drastic changes in operating conditions. Moreover, it must be pointed out that during permeation measurements the membranes are exposed to high pressure, necessary for creating the driving force responsible for the permeation, therefore they undergo an extra stress that does not occur during their use in MEA. The permeation of gaseous species through the membrane is an activated mechanism that follows the Arrhenius law. For this reason, at each value of RH, the permeance of all the gases used in the experiments increased with the temperature. However, at RH= 100% this permeance increase was less evident than at the lower RH% values.
A hydrogen permeability of 1E-13 (mole cm)/(cm² s kP,) lower than the target of the project of 1E-12 (mole cm)/(cm² s kPa) was measured.
Permeation of Nafion 117 membranes - To evaluate the suitability of thermally annealed SPEEK membranes for high temperature PEMFC applications, it is important to compare their transport properties with the native Nafion117, which is currently the most-used electrolyte in PEMFC and, thus, it can be considered as a reference material. The transport properties of commercial Nafion 117 were measured and compared with the results of the SPEEK, following the same procedure described before.
Contrarily to the permeance trend of the SPEEK membrane, at 80°C the Nafion117 showed quite constant values of the permeances as RH increased, for all the gases investigated (hydrogen, nitrogen and oxygen). Basically, the same trend was observed at 100°C, even if a slight decrease of hydrogen and oxygen permeance was recorded. At 120°C, the membrane started to change its behavior, particularly for RH=75%: a dramatic increase in the hydrogen permeance was obtained and the membrane definitely broke after 24 hours in nitrogen stream. The measurements achieved at 120°C were thus affected by the changes in the conformation of the polymeric matrix.
Owing to the difference in the thicknesses of the two membranes, the results were compared in terms of permeability instead of permeance. Globally, SPEEK showed a lower permeability than Nafion117 at all the operating conditions investigated. This is a positive result since the lower the permeability, the lower the crossover affecting the PEMFC performance. At 80°C the hydrogen of the Nafion117 membrane was higher than the SPEEK one and, showed a decrease of permeability in proportion to the higher RH, whereas the Nafion117 permeability was quite constant in the whole RH range considered. Similar trends were obtained at 100°C. A comparison of the performances of Nafion 117 and SPEEK as a function of temperature shows the permeability of both membranes increases with the temperature at each RH. However, the SPEEK exhibited permeabilities always lower than those of Nafion117. Moreover, at 120°C, Nafion117 showed an unstable behavior, whereas the SPEEK was still stable.
Proton conductivity, the other transport property of the membrane to be considered, and gas permeation have to be taken into account to compare the membrane performance. In fact, the ideal/desired membrane must exhibit low gas permeability and high proton conductivity. Moving in this logic, a single parameter, called Transport Performance Index was defined to give the ratio of the permeability and proton conductivity. It takes into account all the mass transport properties, leading to an immediate idea of the whole mass transport performance of the membrane.
The comparison of Transport Performance Index between cross-linked SPEEK and native Nafion 117 highlighted the advantage offered by the SPEEK membrane. The Transport Performance Index of SPEEK got the higher value at 80°C, afterwards it started to decrease as the temperature increased. On the contrary, the Nafion117 membrane, starting at 80°C from the same value of cross-linked SPEEK, exhibited an increasing trend in the whole temperature range. In addition, the Transport Performance Index of the cross-linked SPEEK was always lower than that of Nafion117, for a temperature higher than 80°C. To explain these data, it must be highlighted that even if the proton conductivity of Nafion117 is higher than the cross-linked SPEEK one, nevertheless, it started to reduce after 100°C. Moreover, Nafion117 permeability was also much higher. Indeed, in the comparison of the ratio of these two variables (permeability and proton conductivity), the SPEEK showed better transport properties than Nafion, particularly at a temperature higher than 100°C.
New concept of MEA with improved durability
Catalyst preparation by pulsed potentiostatic electrodeposition - The influence of fuel cell temperature on MEA performance was studied increasing it from 80°C up to 110°C. Gases were held at RH ≥ 95% and a constant dew point of 85°C were applied for fuel cell temperature >90°C. In a separate measurement the performance increasing effect of a dew point reduction at high fuel cell operating temperatures could be revealed. Benchmark MEA (anode: ELE0162, cathode: ELE0162) as well as ED-GDE based MEA (0.45 mg-Pt/cm2 for anode and cathode) were tested using identical conditions utilizing native Nafion® 212 as PEM, which was bonded to the GDEs via a hot-pressing process. For all applied operating temperatures the JM based benchmark MEA showed a slightly better performance. The losses of the ED-GDE consisting MEA at high current densities are most likely related to porous microstructure of the catalyst layer, leading to a higher mass transport resistance. For both MEAs a decrease of fuel cell performance with increasing operating temperature was observed; a greater loss of performance can be observed at temperatures higher than 100°C owing to the vaporization of water and the accompanying loss of membrane conductivity. The fuel cell performances at different temperatures and a reference potential of 650 mV show a slight difference between the catalyst layers developed (0.91 A/cm2) and that of JM (0.98 A/cm2). This difference decreases at a higher temperature. Additional evaluation of the self-prepared GDEs was made in terms of TEM, XRD, performance tests at the PSI Switzerland and in-situ cyclic voltammetry.
MEA using Hydrothermally treated Nafion® membranes - A principle point of this project is the hydro-thermal treatment of PFSA membranes in order to increase the long term stability at elevated temperatures/relative humidity values. For this purpose comparative current-voltage characteristics of MEAs containing untreated and hydro-thermal treated Nafion®212 were carried out. The treatment consists in hydro-thermal procedure at 140°C under dry conditions, in order to erase the hydro-thermal memory of the PFSA material. The results obtained were measured in the “short-term” (meaning 4-5 h) at each fuel cell operating temperature. The slightly decrease in performance could be compensated by a significantly improved stability at higher temperatures.
MEA development using SPEEK membranes -
GDE improvement is an interdisciplinary challenge, essentially dealing with the development of electro-catalyst deposition techniques as well as the development of porous catalyst layer. In contrast to most papers cited in the literature, which are focused on one of these topics, we focused simultaneously on the advancement of Pt electro-deposition and optimization of SPEEK containing micro-porous layer. The aim of this work was to increase the 3-phase zone contact on the one hand and to improve mass transport properties on the other hand in order to increase fuel cell performance. To achieve a “mud-cracked” structure of the micro-porous layer prepared, the solid fraction of the ink, the dispersion agent as well as the drying procedure after coating was optimized for both SPEEK investigated containing micro-porous layers. The “mud-cracked” structure is thought to improve the transport of reactant as well as water across the micro-porous layer and therefore to reduce the mass transport related voltage losses at high current densities.
Micro-porous layers-based GDE with different catalyst load were developed: micro-porous layer-1 has an average Pt loading of 0.52±0.06 mg-Pt/cm2, micro-porous layer-2-based GDE has 0.33±0.07 mg-Pt/cm2 as measured from XRD analysis. The average crystal size is between 9-10 nm; this higher size with respect to that of the literature (2-3 nm), even though less active owing to the decreased catalytically active surface for a given Pt loading, it is thought to have a beneficial effect in terms of size stability of electro-catalyst during fuel cell operation and therefore to improve long-term performance stability at higher operation temperatures. Current-voltage as well as current-wattage characteristics of the MEAs were investigated using the prepared GDEs as anode. For the investigated micro-porous layer -1 and micro-porous layer -2 a loss in performance is observed with increasing temperature, especially for temperatures ≥ 100°C. This is mainly related to the low relative humidity of the reactants leading to dehydration of the ionomer membrane and therefore to the increase of the Ohmic losses. In the case of micro-porous layer -1, a nearly identical fuel cell performance was observed at 60°C and 80°C indicating good mass transport properties at high current densities. The micro-porous layer-1 related GDEs were chosen for their performance in the further reproducibility tests. In this context, a set of independently prepared GDEs (micro-porous layer-1) were tested in terms of fuel cell performance observing a good reproducibility in the micro-porous layer preparation process.
MEA optimization for cross-linked SPEEK - Preliminary tests revealed the importance of the reduction of mechanical stress on the MEA for increasing life-time. With increasing degree of cross-linking the SPEEK membrane tends to get stiffer and more brittle leading to incisions at the PEM/GDE border. The probability of that incision is thought to be increased by unsuitable preparation/operation of the MEA. In order to minimize the mechanical stress and therefore to increase the life-time and the performance of the MEA during preparation/operation influencing parameters were screened and optimized.
Interfacial contact between the SPEEK material and the GDEs is improved using a hot-pressing temperature close to the glass transition point of the membrane used. Corresponding measurements revealed the increase of the glass transition temperature Tg with increasing degree of cross-linking. Therefore the hot-pressing temperature was increased stepwise starting from 160°C up to 250°C. For all prepared MEA an EX-330 SPEEK membrane (30 micron thick), thermally treated for 7 h at 180°C and commercial GDEs from Johnson Matthey with a platinum loading of 0.4 mg-Pt/cm2 were used. Pressure was held constant at 0.5 kN/cm2. Hot-pressing time was fixed at 6 min. For better comparability the same chronopotentiometric pre-conditioning type for all MEAs was chosen. Influence of the hot-pressing temperature on the performance was investigated by the use of polarization curves. A very high hot-pressing temperature had a negative effect on the fuel cell performance: the MEA hot-pressed at 220°C showed a non-steady performance in the low and mid current density region (~ 0.1-0.5 A/cm2); the MEA hot-pressed at 250°C offered a very low fuel cell performance. The best results in terms of fuel cell performance were obtained using a hot-pressing temperature in the range 160-190°C.
In order to reduce the mechanical stress introduced in the MEA by the hot-pressing step different heating and cooling rates were investigated. For this purpose different MEAs based on an SPEEK EX-320 membrane (20 micron thick) were hot-pressed with JM GDEs (0.4 mg-Pt/cm2). In the case of a fast heating and a fast cooling as well as slow cooling rate, high tensions at the GDE/PEM border and especially at the edges are optically recognizable after the hot-pressing process. For the MEA hot-pressed with a slow heating as well as a slow cooling rate, clearly less pronounced tensions were observed. Therefore, slow heating as well as cooling rates during the hot-pressing step are favoured. Besides the visual analysis test bench measurements were carried out in order to reveal the effect of heating and cooling rate on fuel cell performance and MEA lifetime. The polarization curves confirmed the visual based results. In the case of fast heating/fast cooling and fast heating/slow cooling a non-steady state behaviour of the MEAs was observed. This is demonstrated by a performance increase in the first 6 h of operation, which is followed by a failure of the MEA. The loss of performance is limited in the first instance to low current densities but expanding with time and leading to the MEAs end of life. In the case of slow heating/slow cooling the same effect is observed, but beginning after 12 h of operation instead after 6 h. In all cases the presence of incisions at the PEM/GDE-border indicates that the loss of performance is a consequence of the mechanical failure of the MEA.
A series of membranes differing in cross-linking degree were used to prepare MEAs, utilizing, in all cases, reference commercial JM GDEs (ELE0162. loading: 0.4 mg-Pt/cm2). The influence of RH of the inlet gases was studied using the chronopotentiometric technique at a constant current density. In this context the RH at the cathode side was systematically reduced before reducing the RH at the anode side for Fuel cell temperature of 80°C and 90°C. For both operating temperatures no significant decrease in performance was observed whereas decreasing the RH at the cathode side with constant high RH at the anode side. At 80°C the MEA resisted operation using dry oxygen in combination with highly humidified hydrogen (RH=95%). The average voltage loss amounted to ca. 3 mV/h (0–11.5 h). A decrease of the voltage was observed upon reducing the anode RH from 95 % to 75 % using dry oxygen at a time >10.5 h. Nevertheless, MEA performed in a quasi-stationary manner. A further decrease of anode RH to 50% led to significant and steady losses of the voltage indicating inappropriate operating conditions at a time >11.5 h. Reduction of the anode RH (t>10 h) was accompanied by an increase of the Ohmic resistivity indicating the dry-out of the PEM under these operating conditions. Based on these results the use of 95% RH at the anode combined with 50% RH at the cathode showed the smallest voltage degradation rate. Limiting conditions for a quasi-stationary use were found to be 75% RH at the anode and 50% RH at the cathode. A similar behaviour was observed when the operating temperature was increased to 90°C. In this case the use of 95% RH at the anode and 25% RH at the cathode showed the smallest voltage degradation rate. Limiting conditions for a quasi-stationary use were determined with 75 % RH at the anode and 25 % RH at the cathode. The observed incision at the PEM/GDE border is thought to arise from the increased stiffness of the SPEEK membrane with increasing cross-linking degree. The highest current densities were obtained for SPEEK membranes with a thermal treatment time of 3 h and 7 h (lower cross-linking). A treatment time of 3 h is the best also in term of membrane life-time.
Chronopotentiometric & Polarization Results - The optimized parameters obtained from the screening were used for the preparation/operation of MEAs based on cross-linked SPEEK (EX-330 180/7) membranes. Short- and intermediate-term performance tests were carried out. The chronopotentiometry was identified as the most powerful technique for testing at operating temperatures ≥100°C without the use of backpressure.
An intermediate-term performance test was carried out using a reinforced MEA based on a cross-linked SPEEK membrane EX-330 (30 micron thick, thermally treated at 180°C for 7 h) and JM GDEs (0.4 mg-Pt/cm2 of platinum loading). Hot-pressing was performed at 160°C and the MEA was pre-conditioned using the chronopotentiometric method described above. Up to 90°C a constant fuel cell performance was observed up to a maximum current density of 1.5 A/cm2. In order to prevent the MEA from dry-out the operating temperature increase was made under a high current load of the MEA in order to use the thereby-produced water on the cathode side to maintain high humidity and therefore high conductivity level of the PEM material. Nevertheless, a further increase of operating temperature to 95°C leads to a significant voltage loss combined with high voltage fluctuations. For all investigated temperatures (60-90°C) a quasi-stationary behaviour was reached after several hours of operation.
MEAs with improved durability -
From previous results the border-region between GDE and SPEEK membrane was identified as the source of the mechanical failure of the MEA. This is thought to be related to the stiffness of SPEEK membranes, especially when used in a cross-linked modification. Hence the reinforcement of this region was investigated in order to increase the MEA life-time. As additional parameter the type of the MEA pre-conditioning was studied in the same set of experiments. In this context the liquid water pre-conditioning and the chronopotentiometric pre-conditioning were analysed with regard to their beneficial effect on the MEA performance and life-time.
MEAs consisting of SPEEK EX-330 membrane (30 micron thick, thermally treated for 7 h at 180°C) and JM GDEs (0.4 mg-Pt/cm2) were investigated. The use of a reinforcing frame in combination with a chronopotentiometric pre-conditioning has the most beneficial effect on fuel cell performance. The results reveal that the standard pre-conditioning method using liquid water has a negative effect on the performance and is therefore identified as not appropriate. Unexpectedly, the MEA pre-conditioned with liquid water without any reinforcing frame offered good fuel cell performance, however, featuring a very short lifetime. The reinforcing frame introduced led to a large increase of lifetime.
An additional experiment was carried out using a highly cross-linked SPEEK EX-330 containing a little amount of plasticizer to modulate the stiffness of cross-linked (32%) membrane. In the first chronopotentiometric measurement, the influence of the operational temperature (up to 105°C) and of the relative humidity of the inlet gases was studied with focus on fuel cell performance and lifetime. For this purpose the current density was kept constant for a defined time and the corresponding voltage signal was recorded. Fuel cell temperature was increased stepwise starting from 60°C up to 110°C. Steady fuel cell operation was possible up to ca. 100°C. Up to a temperature of 97.5°C the project aim with regard to fuel cell performances (0.642V at 0.7 A/cm2) was accomplished. Operation at higher temperatures was possible and accompanied by a voltage loss: for comparable current densities (0.7 A/cm2) 0.562 and 0.330 V were measured at 100 and 105°C, respectively. This was mainly due to the low relative humidity of the inlet gases caused on the one hand by the dew point limitation of the gas humidifier and on the other hand by the limit of maximum water content in the feed gases for operation at atmospheric pressure. For temperatures above 105°C a stable fuel cell operation was not possible due to the amplification of the previously-mentioned reasons causing dry-out of the membrane and therefore loss of conductivity.
This MEA based on SPEEK EX-330 including a plasticizer was utilized for a total of 50 h under alternating load also at temperature higher than 100°C without any sign of reduced performance and end of life.
Lifetime test and prediction techniques, establishment of accelerated test techniques
A method to predict membrane lifetime consisting of an in situ accelerated test was developed prior to the LoLiPEM project and consisting in two steps: (1) the determination of the value of a coefficient alfa (degradation coefficient) according to the following equation:
alfa = function of (Experimental endurance value/Load cycling value)
and (2) the evaluation of the life time of the membrane through the following relation
Life-time = alfa * Number of cycles.
The alfa coefficient strongly depends on operating conditions (e.g. protocol) and membrane material. Alfa values for Nafion and sPEEK are 16.7 and 17.7 respectively, as reported by Marrony, Beretta, Ginocchio, 2nd CARISMA International Conference on "Progress in MEA Materials for Medium and High Temperature Polymer Electrolyte Fuel Cells", 19-22 September 2010, la Grande Motte, France, using a specific protocol, and 2400 cycles were experimented for Nafion having a lifetime of ca. 40000 hours.
Alfa will be utilized to predict the life time of other membranes once it is evaluated by experimental measurements on some samples of a specific material. The starting points for the evaluation of membranes lifetime are the alfa values calculated following the mentioned method which are reported below.
Considering the different cycling conditions (e.g. longer time of each cycle) with respect to the literature protocol, the alfa value of the LoLiPEM protocol, when the latter is operated at 75°C and RH=85%, could be at least twice higher than the literature one (i.e. 33.4 and 35.4 for Nafion and SPPEK, respectively).
Measurements on Nafion and thermally annealed Nafion membranes - Experiments were performed on commercial Nafion at 75°C using the LoLiPEM test protocol in order to verify the lifetime hypothesis mentioned above. The expected lifetime for this type of Nafion membrane is 40000 hours. The test lasted ca. 830 number of cycles and was then stopped by the operator for service reasons. The MEA suffered a very low level of degradation. Starting from the Nafion lifetime found out in literature (40000 hours) and the number of cycles (ca. 830) a new and refined alfa value was calculated using the above equations. It was estimated as 48. This value is higher than the minimum value 33.4. This means the value of 33.4 can be considered as a lower limit for the alfa coefficient and was used as the most conservative value in the LoLiPEM protocol. The LoLiPEM protocol was applied to an MEA assembled with a Nafion NRE 212 membrane (developed in the project) thermally annealed in DMSO at 140°C for 48h, and then 120°C for 15h. The test was carried out at the temperature of 110°C, significantly higher than the operating temperature that can be used for a commercial Nafion. The pressure and relative humidity of the inlet gases were 1 bar and 95%, respectively. In these conditions, the MEA operated for a number of cycles of approximately 250. Using the mentioned equation, the lifetime of this sample is given by the product of the number of cycles and the evaluated alfa value (e.g. alfa_minimum=33.4).
However, it has to be pointed out that the lifetime of the MEA is also strictly related to the operating conditions used in the protocol. In this sense, the difference between 75°C of the test carried out on commercial Nafion and the 110°C for the thermally annealed Nafion and the higher relative humidity used for the latter membrane, lead to the presence of another coefficient in the equation that also takes into account the temperature and relative humidity differences. The aforementioned equation thus becomes:
Life-time = alfa_minimum * coefficient * Number of cycles.
Alfa = alfa_minimum * coefficient
The new coefficient takes into account the dependence on the operating temperature and relative humidity. The relative humidity affects the membrane stability to a higher degree than the temperature. Alberti et al. stated that any 1% for relative humidity has the same effect as 5°C. Thus, these effects have to be taken into account in the evaluation of the membrane lifetime.
The predicted lifetimes for this MEA based on Nafion thermally annealed membrane is 8850 if no effect of the higher temperature and relative humidity is taken into account (the most conservative situation). However, it has to be noticed the measurement was taken at 110°C (35°C more than usual) and at 95% of relative humidity (10% more). Therefore, if these much stronger conditions (a temperature and relative humidity higher by 35°C and 10%, respectively) are taken into account then a coefficient value higher than 1 has to be considered. Thus, an alfa value of the ones previously reported as hypothesis becomes realistic and therefore a higher membrane lifetime should be expected. Lifetime prediction values for Nafion under these conditions give values between 8850 (the most conservative) and 44250 hours of operation.
Measurements on SPEEK membranes - Analogous tests to those operated on Nafion were done on SPEEK. However, the accelerated tests with SPEEK-based MEAs were unsuccessful due to failure of the membrane, which displayed holes and cracks always at the same location, the feeding hydrogen point. The hydrogen flux orthogonally reaches the membrane surface and also a change in direction is required. This point is, therefore, of specific significance when SPEEK is operating.
After many tests it was considered to protect this point covering it with a Teflon foil. This simple protection allowed an increase from 2/3 test cycles to 10/11 cycles just in one shot. Furthermore, an improved protection of this point consisting in the use of glue between the Teflon foil and the PEM of the MEA, where the feed stream meets the cell, led to a further improvement (up to 19) in operation cycles.
Long-term single-cell testing -
A single-cell PEM setup for the long-term durability tests to be carried out on the membranes was produced, designed and implemented by the project partners. This system is required to test high-temperature PEM fuel cells and to study durability by means of long-term experiments. The fuel cell test bench was implemented to run long-term single-cell testing, which allows a precise control on humidity, gas flow and temperature. This is accomplished by a system which includes a series of sensors (for temperature, pressure and humidity) and actuators at the different lines of gas supply, humidification and fuel cell housing, providing a flexible set-up that can operate at a different range of conditions and which is operated by a central unit that continuously monitors, controls and records the operating parameters. Different tests were performed on a Nafion commercial MEA in order to set-up the equipment and validate the test-bench performance. Also, the polarization curves were obtained for different anode operation: dead-ended anode and flow-through in order to compare performance and thus select the best option for long-term test. Moreover, the temperature was varied to test the performance of the test-bench. The chronopotentiometric activation protocol was established as the optimum conditioning step before operation. The system developed was benchmarked against commercial MEAs obtained from Arbin Instruments, the suppliers of the fuel cell housing. This is a state-of-the-art Nafion-based MEA. The MEA was tested at different temperatures, backpressures and RH in order to find the optimized parameters to obtain the best performance. The effect of pressure on the operation of the fuel cell was investigated by comparing the time ranges between 50-75 hours (system pressure of 1.5 bar) and the range between 80-140 hours (with the system at atmospheric pressure), an increase in current density at higher pressures was observed.
The design of this apparatus was strongly linked to the specifications of the membranes developed in the project by other partners, in order to achieve the best performance and to determine appropriately the degradation rate and lifetime. It is important to note the difficulties found to measure some critical variables inside the fuel cell stack such as the relative humidity. Therefore, a detailed model based on the work presented in ‘A. J del Real, A. Arce and C. Bordons, development and experimental validation of a PEM fuel cell dynamic model, Journal of Power Sources 173, 2007, 310-324’ was developed in order to estimate immeasurable variables. In addition, this model was used in the design step. Specifically, the model is a semi-empirical model, which mixes first principle equations and empirical correlations. The new contributions of this model are the two-phase fluid dynamics, the effects of the flooding on the stack voltage, the start-up dynamics, the thermal dynamics and the novel mathematical description of the polarization curve. The model was tailored for a stack made up of a membrane of 25 cm2 (the membranes used in the project) and it is divided into different blocks: electrochemical equations, fluid-dynamics equations, thermal-dynamics equations and ancillary devices.
Nafion MEAs were tested using air and at atmospheric pressure. Polarization measurements were performed following the LoLiPEM protocol. The system requires a few hours for the stabilization of the humidity in the pipelines. Thus, the operation at the beginning of each section starts with an open circuit current (0A). Moreover, the stack temperature was increased at slow steps in order to avoid large voltage variations in the system. An electric current of 5 A was maintained in the system monitoring the changes in voltage during the operation of the system. However, after 350 hours the MEA was not performing correctly, and in order to continue testing the long-term operability of the system, a lower current was used. The polarization curves performed at different operation times show the decrease of performance of Nafion NR212 based MEA over time. The largest decrease in performance of the MEA is observed at higher current densities. The shape of the curve is kept, however, the voltages obtained at high current densities are already reduced after 60 hours of accumulated operation. Interestingly, we observed that the performance increases when the MEA temperature is increased to 105°C. However, evolution in time was not very stable and a notable decrease in performance can be observed after 48 hours, owing to a higher membrane degradation caused by the higher temperature of the fuel cell.
Different SPEEK-based MEAs were tested to study the effect of thicknesses, cross-linking degree, and the operation under oxygen or air and at different pressures. Also, different membranes were studied with an additional frame to avoid early mechanical failure as identified. The best performance is given by MEAs prepared with Fumapem EX-330 followed by Fumapem EX-350, even when tested using air and at 60°C. The MEA prepared with the thinnest membrane (Fumapem EX-320 gave the lowest performance. This fact can be related to the poor mechanical stability of this MEA and is also translated into high voltage instability.
The influence of cross-linking degree in durability and performance of the developed MEAs was also studied: the higher cross-linking shows the highest durability. Considerable differences are found between SPEEK-EX330 (180/7) and SPEEK-EX330 (180/3) based MEAs not only in performance but also in durability of the accelerated tests. SPEEK-EX330 (180/3) based MEA has a lower degree of cross-linking, which gives a poorer performance and also poorer mechanical stability that is reflected in the shorter duration of the accelerated test. SPEEK membranes show promising characteristics for the development of high temperature PEM fuel cells in terms of performance. It has been observed that thickness and degree of cross-linking are important variables in order to obtain a good system performance. Mechanical failure is the main limitation at the moment to obtain acceptable lifetime values for SPEEK-based MEAs. However, there are different possible ways to overcome this problem and maintain the good properties that SPEEK membranes offer as, already said, the addition of a plasticizer.
MEAs post-mortem analysis - A strategy was developed analysing all important properties of an MEA and of its constitutive elements before and after the performance decay. The so called “Post-Mortem analysis” starts form a collection of results obtained from the comparative analyses achieved using difference techniques considering all the constitutive MEA materials such as, e.g. gas permeation, IEC, SEM, TGA, etc. This analysis carried out on an MEA series showed the formation of small pinholes during the fuel cell operating life. Possible consequences could be high cross-over and short circuit which determine the drop in fuel cell performance. The water induced a mechanical stress by means of its dynamic sorption generating a not-steady swelling status of the membrane. The interface between the PEM and the blocked PEM in the MEA is the most sensitive area for this and a hole or a fracture takes place, irremediably damaging the PEM mechanically and thus the cell performance. The control of the membrane mobility is fundamental to avoid the mechanical rupture of MEAs.
MoPEM - The Modular PEMs (MoPEM) system designed and built consists of a multi-cells system able to host a series of cells independent of each other, sharing the feeding, discard and co-generation system. No bipolar plates are used and their assembling is designed and built in such a way that the disassembling of the whole system is not necessary anymore to remove a single cell, as currently happens for traditional stacks. With its innovative design, the MoPEM reduces some of the main disadvantages present in common fuel cell stacks, such us, in particular, the time needed to use and discard MEAs and thus, with consequent reduction of the related cost. In addition, the challenge of the proposed MoPEM system is to keep/improve the cell performance when heat produced is simultaneously recovered. When a fuel cell constituting the system does not operate anymore, it can be insulated from the rest of the system, simply putting a plug and guaranteeing the stream by-pass eventually required by the system design. The maximum operating temperature depends on resistance of the different components of MEAs and the recovery plate can also be in stainless steel. The heat produced is recovered closer and closer to the MEAs, the hottest section of the system, where the heat is produced improving the quantity of heat recovered. The whole system is controlled interfaced with a computer by specifically developed software. The computer operates also as a data acquisition system, acquiring information in real time and, thus, allowing the operator easy monitoring of the MoPEM operation.
The tuning of the temperature and flow rate of the cooling stream also allows the setting of the operating temperature inside the system.
The increasing demand for electricity and the willingness to preserve hydrocarbon resources clearly address the need to develop further the use of renewable energies for electricity production, in order also to reduce the increasing greenhouse gas emission. Fully aware of this, the European Union is aiming at promoting initiatives to support the implementation of renewable energy technologies such as that of fuel cell, owing to hydrogen being expected to play an increasing role in large-scale energy storage. This technology is able to produce not only electricity but also heat, working as a cogeneration system and, not less important, it can operate without interruption while the system is supplied with fuel (hydrogen) and oxidant (oxygen). Fuel cell and hydrogen are a great opportunity for Europe, from improving the quality of the air, to reducing the emissions causing climate change and playing a significant role in helping to meet the reduction targets recently established of 20% reduction in greenhouse gas emissions compared to 1990 levels, 20% reduction in global primary energy use (through energy efficiency), 20% of renewable energy in the EU's overall mix. In fact, no carbon dioxide is produced during the reaction between hydrogen and oxygen and for this reason FC technology can help the EU to achieve its societal goals, such as better air quality, decarbonisation etc. Furthermore, in the near future, hydrogen could be produced using mainly renewable feedstock, thus bringing the FC to becoming a real “zero CO2” technology.
PEMFC can be used in several applications such as portable and mobile power (cars, buses, industrial transportation, and utility vehicles) and stationary/distributed power. In order to reach this objective more effort should be spent on the hydrogen distribution infrastructure, and lower cost of all parts of the FCH chain and improved performance are still required, for instance, of PEM. These technologies are not mass manufactured in Europe owing to the elevated total cost of the electricity produced.
In order to reduce the costs, both research and scale economies are needed in this field. This is particularly possible due to their application in vehicles, as replacements for the internal-combustion engine. The more competitive automotive cost targets similarly will result in lower costs for stationary applications and portable power applications. The latter are also energy-efficient, clean and fuel-flexible.
A crucial point is the fuel cell service lifetime, which has been specified by commercial targets as five years on-load operation. Technology improvements can be achieved by the development of innovative materials (PEM, catalyst, bi-polar plates, etc.), better system integration and new manufacturing processes.
The LoLiPEM project falls perfectly into this context owing to its results achieved: where new and innovative polymers, membranes with good hydrothermal stability, even at temperatures higher than 100°C, and suitable catalysts were developed. The objectives achieved can lead to a major commercially attractive PEMFC technology and this could encourage efforts to increase production. This, in turn, will bring the production cost down and thus attract new players and create the opportunity for new employment. Fuel cell operating temperatures above 100°C have several advantages, including easier warm water distribution in buildings, reduced anode poisoning due to carbon monoxide impurities in the fuel and improved fuel oxidation kinetics. The results obtained during the project, focusing on the new membrane design (thermally annealed and/or chemically cross-linked), can be decisive for the development on a large scale of stationary power generation and combined heat and power systems. Moreover, the membrane developed also shows a good mass transport and durability at higher temperatures. Furthermore, the development of new SAP membranes and new GDL, with an improved catalyst deposition technique, contribute to the cost reduction in production and running fuel cells allowing a major diffusion of this technology, which nowadays is expensive.
Dissemination of results is one of the most important issues of the research activities carried out. Dissemination includes several (twenty-one) articles already published or accepted to scientific journals and three books chapters; other papers are in preparation. The project participants also attended several (ca. fifty) conferences and congresses as invited speakers or presenting oral contributions as well as posters. The LoLiPEM project partners organized three workshops during the execution of the project.
Exploitation actions - FumaTech GmbH has been active in the PEM market for 15 years and is ready to commercialize any improved ionomer membrane developed in LoLiPEM project. It is also a member of BWT-AG best-water-technology, the leading water technology group in Europe.
The fuel cell strategy of FumaTech is to supply industries with (a) ionomers in bulk powder or solution, (b) membranes from fluorinated and non-fluorinated ionomers and hybrid organic-inorganic composites for fuel cells application.
The fundamental achievements reached in the research laboratories in membrane design and development, such as improved hydrothermal stability also at a temperature higher than 100°C obtained by thermal annealing and chemical cross-linking of PFSA and SAP polymers, have been exploited on a larger scale production. For instance, the DMSO-mediated annealing process for crosslinking the SPEEK membranes in terms of temperature, time and procedure developed in the LoLiPEM activities has produced more stable membranes at a temperature higher than 100°C with stronger hydrothermal stability.
FumaTech has considered the whole production chain starting from polymer production to ready-to-use membrane. This continuous process consists in (a) polymer production, (b) polymeric solution preparation and filtration, (c) membrane casting and solvent evaporation. The membrane production is followed by annealing and/or cross linking and membrane activation.
The production of highly sulfonated PEEK including industrial scalability, cost and environmental considerations were developed and optimized. The process allows for a cost-effective and process-controlled manufacturing and a target-oriented and reproducible achievement of the degree of sulfonation of the PEEK. The different stages of the production were considered including the sulfonation as a batch process versus a continuous one, the isolation by precipitation, the cleaning step by washing with water, the purification, the drying of the product and even the waste treatment. Optimized solution composition and preparation were developed which allow for reproducible and effective casting on the FumaTech continuous membrane production line. In particular, the solvents, solution concentration, solution viscosity, filtration and some important additives were considered and optimised in order to obtain defect-free membranes. The process of the pilot-scale membrane production is fully compatible with the standard large-scale membrane production enabling the rapid transfer and integration of the new technology into the standard operation of manufacturing processes and the subsequent exploitation of the project results. The corresponding cost structure of polymer and membranes preparation was also evaluated, comprising medium-scale polymer and membrane preparation and optionally the use of PTFE-reinforcement.
FumaTech has commenced exploring the exploitation of crosslinking results to alternative highly sulfonated polymers. This concept has already been applied successfully to some sulfonated polymers – e.g. fumion® STO sulfonated poly(ether sulfide sulfone) and fumion® ST sulfonated poly(sulfide sulfone). The use and exploitation of the developed cross-linked sPEEK membranes and other sulfonated hydrocarbon membranes, developed during the project, characterized by improved hydrothermal stability in other applications is also under consideration.
List of Websites:
National Research Council of Italy - Institute on Membrane Technology
Via Pietro BUCCI
c/o The University of Calabria - Cubo 17C
87036 Rende CS, Italy
tel. +39 0984492029
Grant agreement ID: 245339
1 January 2010
31 December 2012
€ 2 677 298,40
€ 1 360 227
CONSIGLIO NAZIONALE DELLE RICERCHE
Deliverables not available
Grant agreement ID: 245339
1 January 2010
31 December 2012
€ 2 677 298,40
€ 1 360 227
CONSIGLIO NAZIONALE DELLE RICERCHE
Grant agreement ID: 245339
1 January 2010
31 December 2012
€ 2 677 298,40
€ 1 360 227
CONSIGLIO NAZIONALE DELLE RICERCHE