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Porous Nafion membranes
Author’s Accepted Manuscript Porous Nafion membranes Dickson Joseph, Julian Büsselmann, Corinna Harms, Dirk Henkensmeier, Mikkel Juul Larsen, Alexander Dyck, Jong Hyun Jang, Hyoung-Juhn Kim, Suk-Woo Nam www.elsevier.com/locate/memsci
PII: DOI: Reference:
S0376-7388(16)30362-3 http://dx.doi.org/10.1016/j.memsci.2016.08.025 MEMSCI14675
To appear in: Journal of Membrane Science Received date: 13 May 2016 Revised date: 5 August 2016 Accepted date: 17 August 2016 Cite this article as: Dickson Joseph, Julian Büsselmann, Corinna Harms, Dirk Henkensmeier, Mikkel Juul Larsen, Alexander Dyck, Jong Hyun Jang, HyoungJuhn Kim and Suk-Woo Nam, Porous Nafion membranes, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.08.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Porous Nafion membranes Dickson Josepha, Julian Büsselmannb,c, Corinna Harmsb, Dirk Henkensmeiera, d*, Mikkel Juul Larsene, Alexander Dyckb, Jong Hyun Janga,f, Hyoung-Juhn Kima, Suk-Woo Nama,f a
Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5,
Seongbuk-gu, Seoul 02792, Republic of Korea b
NEXT ENERGY • EWE Research Centre for Energy Technology at the University of
Oldenburg, Carl-von-Ossietzky-Str. 15, 26129 Oldenburg, Germany c
Hochschule Emden/Leer, Abteilung Naturwissenschaftliche Technik, Applied Life Sciences,
Constantiaplatz 4, 26723 Emden, Germany d
University of Science and Technology, 217 Gajungro, Yuseonggu, Daejeon, Republic of
Korea e
IRD Fuel Cells A/S, Emil Neckelmanns Vej 15 A&B, 5220 Odense SØ, Denmark
f
Green School, Korea University, Seoul 136-713, Republic of Korea
Abstract By varying the amount of porogene (ortho-dichlorobenzene, ODB), and optimization of the dispersion process, two types of solvent cast Nafion membranes with an equivalent weight of 1100 g/mol sulfonic acid can be obtained reproducibly. One type is a dense membrane with a porous layer on one surface. The other membrane type shows a novel structure, consisting of small closed pores throughout the membrane and a single layer of large open pores on one side. In addition, some membranes showed a structural morphology between these two types, a membrane with a dense part and a porous part on top of each other. The latter membrane structure was not fully reproducible yet, but probably could be by carefully adjusting the formulation of the casting solution. Also the effect of the casting temperature on the morphology is shown. Fully porous membranes were characterized for their water permeability, ion conductivity, mechanical properties, their performance in the fuel cell and the hydrogen crossover. While the fully porous membranes are not expected to be part of a real fuel cell, we
1
expect that the new morphologies will inspire applied research, e.g. in which the pores are filled with electrolyte or material or a catalyst is blended into the polymer.
Nafion (EW 1100)
Graphical abstract
Keywords Nafion dispersion, porous Nafion membranes, porosity, morphology, water transport
1. Introduction Nafion membranes are essential components of fuel cells,[1] electrolyzers,[2] and redox flow batteries.[3] Also the use in gas humidifiers was investigated.[4, 5] In practically all applications Nafion membranes are used as dense, non-porous flat sheet membranes with smooth surfaces. However, there is growing evidence that other surface morphologies may improve electrochemical systems. Klingele et al. reported a performance improvement of polymer electrolyte fuel cells (PEFCs) when the membrane is not casted and assembled between the catalyst electrodes, but when the membrane is ink jet printed on the catalyst electrode, probably because assembling of smooth membranes with catalyst coated electrodes leads to voids at the membrane-electrode interface.[6] Shul et al. showed that the platinum utilisation increases with 2
the surface area of the membranes, which can be increased by casting the membranes in microstructured molds.[7] In our previous work we showed that dense Nafion membranes with a porous surface structure can also be prepared by solution casting, when ortho-dichlorobenzene (ODB) is added to Dupont's SE 20092 Nafion dispersion.[8] In the fuel cell, we observed increased platinum utilization and 16% higher performance in comparison to membranes with a smooth surface.[9] However, Dupont's Nafion SE 20092 dispersion contains Nafion with an equivalent weight of EW = 1000, which swells more than standard Nafion of EW = 1100. Since the formation of the surface porous structures depends on the formulation of the casting solution (e.g. Nafion concentration, solvents, concentration of ODB), it was necessary to develop a new formulation, which allows to extend the formation of surface pores in a one-step casting process to EW 1100 Nafion. The results of this work are presented here.
The second goal of this work was to prepare fully porous Nafion membranes. As far as we know, all procedures reported in the literature are based on two-step processes, involving first formation of a Nafion or Nafion composite membrane, and then introduction of pores by leaching the porogenes,[10] expansion of swollen Nafion membranes by formation of gaseous degradation products,[11] or expansion of membranes swollen by liquid SO2 or supercritical CO2 by changing ambient conditions to a point at which the swelling media enters the gaseous state.[12]
In principal, fully porous membranes can have two kinds of pores: closed pores and open pores (as in filtration membranes). It appears that leaching and expansion of membranes often leads to open pore structures.[12, 13] Based on our previous work, we consider it possible that closed pore structures can be introduced into Nafion by using ODB as porogene. However, by using Dupont’s SE20092 Nafion dispersion and ODB as pore forming agent, it was not possible to produce fully porous membranes.[8, 9] Here, by using different solvents and a Nafion with higher equivalent weight, we demonstrate for the first time fully porous Nafion with a closed pore structure, prepared in a single step process.
While the use of porous membranes in fuel cells and electrolyzers may be counterintuitive, because one of the membranes' main functions (in addition to their role as electrolyte) is to separate anode and cathode gases, there could be some interesting applications for porous Nafion 3
membranes. The pores could be filled with water or another electrolyte (phosphoric acid, ionic liquids, etc.). If the pore walls are wetted sufficiently, they may act as a "proton highway", providing a pathway for proton conduction from anode to cathode with low tortuosity.[14] In some work, the inevitable gas crossover of hydrogen and oxygen is used to form water on catalyst particles embedded in a fuel cell membrane. This direct humidification of the membrane allows to operate the system without humidification of the reactant gases.[15] Combining this concept with porous membranes could lead to membranes of increased humidity and therefore excellent performance even when dry fuel and oxidant gases are used. Furthermore, it was reported that water transport through a membrane is not only limited by the transport resistance inside of the membrane itself, but very strongly affected by the interfacial mass transport resistance across the membrane/gas interface.[16, 17] However, water transport in the gas phase is faster than in the liquid phase or in membranes. This could open a pathway to mitigate the water (or gas) transport properties of Nafion membranes, e.g. for use of Nafion membranes in humidifiers. All these potential applications of porous Nafion membranes cannot be addressed in this work. But they are a strong motivation to investigate if and how it is possible to prepare Nafion membranes with different pore morphologies.
2. Experimental part KIST: Nafion dispersion DE2021 was obtained from Dupont, 1,2-dichlorobenzene (ODB) from Kanto Chemical, ethanol from Sigma. Next Energy: Nafion dispersion DE2021 was obtained from Chemours, ODB from Carl Roth, ethanol from Fischer Chemicals (99.8% pure, E/0650DF/17). N211 and N212 were obtained from Chemours.
2.1 Preparation of Nafion dispersion in ethanol (Nafion dispersion 2.4) A 100 mL round bottom (RB) flask was weighted and 20 mL of Nafion dispersion (DE2021, Dupont) were added. The RB flask was placed in a rotary evaporator and solvents were evaporated. To further remove the solvents the RB flask was placed in a vacuum oven at 60 ˚C for 24 h. The RB flask was then removed from the oven and ethanol was added to achieve a 10
4
wt% Nafion dispersion. The mixture was stirred at room temperature for 48 h and then filtered using a syringe filter (0.45 µm), the obtained dispersion was stored at 4 ˚C.
2.2 Preparation of porous Nafion membranes (PN membranes) A required amount of the 10 wt% Nafion dispersion 2.4 was mixed with a required amount of ortho-dichlorobenzene (ODB) to make up a concentration of 150 mg of ODB per mL of Nafion dispersion. The mixture was stirred at room temperature for 24 h and either drop casted into a petri dish or casted onto a glass plate using a doctor blade, e.g. with a blade gap of 400 µm under ambient conditions. The formation of a white colored membrane was observed as the solvents evaporated, and the glass plate was left to stand under ambient conditions for 24 h. The glass plate along with the membrane was then placed in a vacuum oven at 60 °C for 12 h after which the temperature in the oven was increased to 120 °C and maintained for 4 h and allowed to cool to room temperature. The glass plate was then removed from the oven and soaked in water for 24 h and the membrane with an average thickness (for this membrane) of 33 ± 4 µm was peeled off gently.
2.3 Membrane characterization A scanning electron microscope S-3000H with an accelerating voltage ranging from 0.3 to 30 kV or a Neon 40 ESB (Carl Zeiss) was used to obtain scanning electron micrographs of the prepared Nafion membranes. The membranes were coated with platinum or gold before imaging. TGA curves were measured in a Pyris 1 TGA (Perkin Elmer) under helium gas. The sample weight was about 12 mg. The heating rate was 10 K/min. Off-gases were directly transferred into a GC/MS coupled with the TGA.[18] For mercury porosimetry membrane samples were cut into small pieces, and inserted into a Pascal 140 porosimeter (Thermo Scientific). There the samples were degassed, and filled with mercury up to a pressure of 400 kPa. Then the samples were transferred into a Pascal 440 porosimeter, which can apply pressures up to 400 MPa, ensuring that also small cavities are filled with mercury. 5
The maximum tensile strength and the Young modulus of the PN and N211 membranes were determined from force displacement curves measured with a Cometech QC-508E instrument. For each material, several samples from the same membrane were measured, their average values are given. The sample size was 4×1 cm2, the elongation speed 10 mm min
-1
. The ambient
temperature during the measurement was 24°C and 20-34% relative humidity. In-plane proton conductivity was measured on 3 cm long and 1.8 cm broad membrane stripes via impedance spectroscopy. To be sure that the membranes are fully activated, the samples were immersed consecutively in 3 wt% boiling hydrogen peroxide solution, deionized (DI) water, boiling 0.5 M sulfuric acid, and boiling DI water. For the impedance measurements, the samples were assembled into a BekkTech BT-112 cell, and a G100 Greenlight Innovation fuel cell test stand. Impedance was analyzed by a Gamry Reference 3000 potentiostat. The conductivity was calculated according to
In which d is the distance between the potential sensing electrodes (0.425 cm), R M is the membrane resistance extracted from the impedance spectra, b is the membrane width, and t is the membrane thickness. The water permeability of PN membranes was studied using a simple experiment reported in our previous related article.[9] A hole, 8 mm in diameter, was drilled into a plastic screw cap of a vial and a hole was punched though the sealing. A piece of the membrane with a thickness of 30 µm was placed on the inner side of the vial's plastic screw cap and covered with the sealant. Water was filled in the vials, allowing for a head space with an assumed relative humidity of 100%. Then the vials were placed in an environmental chamber, and at regular intervals the vials were weighed in order to calculate the water flux.
2.4 MEA fabrication and testing Because of the uneven surfaces of the prepared membranes, catalyst coated membranes (CCMs) are expected to give the best performing membrane electrode assemblies (MEAs). However, to 6
avoid differences arising from manual coating, commercial gas diffusion electrodes (GDEs; Quintech BC-H225-05S, 0.5 mg platinum/cm² on Sigracet 25 BC GDL) cut to an active area of 4.8 cm2 were used to prepare the MEAs. The MEAs were assembled into a cell with a serpentine flow field, and sealed with silicone gaskets. The cell was tightened with a torque meter adjusted to 10 Nm, and tested for leaks by pressurizing the cell with hydrogen (anode) and nitrogen (cathode), at a flow rate of 300 Nml/min. First 30 kPa were applied on both sides, then 50 kPa on the anode and 30 kPa on the cathode. Finally also the cathode pressure was increased to 50 kPa. Humidified gas streams were heated to 5 °C above the cell temperature to avoid condensation of water. For activation, the cell was operated at 70 °C, 100% relative humidity (RH) and a stoichiometric ratio (λ value) of 3 (anode, hydrogen) and 2 (cathode, synthetic air) at a load of 0.5 A/cm2. After 12 hours, latest after 20 hours, the cell was defined as activated if the potential remained stable within 5 mV for 3 minutes. The flow rates were kept at the stoichiometric ratio of 3 (anode, hydrogen) and 2 (cathode, synthetic air) and an iV curve was measured (90 seconds per step). The same procedure was repeated at 70%, 50% and 30% RH. At the end of all fuel cell tests, the hydrogen crossover was measured by linear sweep voltammetry (LSV). For that the cathode gas was changed to nitrogen, and both flow rates were fixed at 300 Nml/min and 100% RH, and later 50% RH. The potential was varied between 90mV to 500 mV at a rate of 10 mV/s. The resulting curve was corrected for the slope (resulting from internal shorting), to give the limiting current.
3. Results and Discussion 3.1 Dispersion of Nafion Commercial Nafion dispersions available in the market are either water based (D1020 and D1021) or based on water and alcohol mixtures (D520, D521, D2020, D2021, SE 20092). Using SE 20092 dispersion (Nafion with equivalent weight of 1000) it was shown in previous reports that dense membranes with a porous surface can be prepared.[8, 9] Since that method could not be directly transferred to commercial dispersions based on Nafion with EW 1100, the solvent composition needed to be adjusted. This can be done either by dispersion of Nafion pellets under elevated temperature and pressure, or by re-dispersion.
3.1.1 Dispersion 1: Dispersion of Nafion pellets 7
Direct dispersion of commercial Nafion pellets in a given solvent mixture appears to be a straightforward process. As described by Grot, this should be done in a pressurized reactor at elevated temperatures.[19] First 20 wt% of Nafion beads in ethanol were tried to be dispersed at 200 °C and 80 bar, but instead of a dispersion, a large gel lump was obtained (Figure S1). This behavior is not clearly understood but we believe it could be because too much solvent evaporated into the head space of the pressure reactor. Since ethanol is a low boiling solvent also water was tested as solvent. The reaction conditions are shown in Table S1. Though we were able to disperse Nafion in water at low concentrations (dispersion 1), maintaining a constant, exact pressure was not possible. Hence we were not able to reproduce dispersions with a defined concentration. A maximum of 6 wt% Nafion dispersion was obtained using an initial concentration of 10 wt% in water. Therefore, this process did not efficiently disperse small amounts of Nafion in our lab. One possible reason for this is also the process time, which was kept below 8 hours.
3.1.2 Dispersion by re-dissolution of commerical Nafion dispersion In this approach, Nafion dispersion is evaporated to dryness, and the remaining solid (which is not annealed, while Nafion pellets probably are due to the high temperatures required for melt extrusion) is re-dissolved in the required solvent mixture. As starting material, we used the commercially available dispersion DE2021 (EW1100). Using a rotary evaporator, the solvents were evaporated until no liquid was visible. Then the solid Nafion was re-dispersed by adding an excess amount of ethanol. Then the solution was concentrated to 20 wt% by slowly evaporating ethanol by stirring at ambient conditions in an open vial. The thus obtained 20 wt% Nafion dispersion existed in a viscous gel form (dispersion 2.1). To this dispersion orthodichlorobenzene (ODB) was added. The dispersion was mixed, drop casted on glass petri dishes and left undisturbed at ambient conditions for solvents to evaporate and to form membranes. Using 180 mg of ODB per mL of Nafion dispersion, white membranes with a porous structure along the cross-section, and a dense surface with just a few pores (Figure S2) were obtained. However, these membranes were brittle and the reproducibility of the porous structure was poor especially along the cross-sections of the membranes (Figure S3). We speculate that the 8
brittleness and the non-reproducibility of the porous surface were due to the high viscosity of the Nafion dispersion that prevented uniform distribution of ODB in the dispersion before preparing the membranes. Hence, for easier processing and to obtain good membranes, after solvent evaporation in a rotary evaporator, the amount of ethanol needed for 10 wt% solutions was added. The mixtures were stirred at ambient conditions with a cap on the RB until a clear dispersion was obtained (dispersion 2.2). On a small scale membranes were prepared in glass Petri dishes by adding different amounts of ODB/mL Nafion dispersion. We found that 150 mg ODB/mL Nafion dispersion led to white membranes that were stable but somehow brittle (Figure S4). They showed a porous structure along the cross-section, and dense surface with few pores, but again reproducibility was poor (Figure S5). We assume that the incomplete porous structure could be due to the presence of some trace solvents (especially water) from the original dispersion. Hence care was taken to remove as much solvent as possible during the rotary evaporation step by leaving the round bottom flask in the rotary evaporator for at least 30 minutes after the visible solvents were evaporated to completely remove the solvents before redispersion in ethanol. In addition, the final 10wt% dispersions were filtered using a syringe filter (0.45 µm) to remove any particulate if present (dispersion 2.3).[20] To further ensure that all solvents are completely removed, the solid obtained after rotary evaporation was dried in a vacuum oven at 60 ˚C for 24 h, after which a 10 wt% Nafion dispersion in ethanol was prepared as above (dispersion 2.4).
3.2 Preparation of porous Nafion membranes The dispersions 2.3 (obtained by directly dispersing with ethanol after extensive rotary evaporation) and 2.4 (dispersed in ethanol after vacuum drying Nafion in the oven for 24 h after rotary evaporation) were examined for the preparation of porous membranes. Both dispersions led to the formation of white membranes (visual confirmation of porosity) with 150 mg ODB/mL Nafion dispersion, as shown in Figure S6. Figure 1 and 2 show SEM images of the membranes prepared using dispersion 2.3 and 2.4, respectively. The main difference is the porous structure on the membrane top surface (air side). It appears that dispersion 2.4 formed less and more isolated surface pores than dispersion 2.3. For further experiments, dispersion 2.4 was used. In order to study the effect of the ODB amount on the structural morphology, the amount of ODB was varied. We found that with lesser amount of ODB (130 mg/mL) the porosity of the 9
membrane was lower, but with higher amount of ODB (170 mg/mL) the porosity increased (Figure 2). This observation became even more clearly evident especially along the cross-section when the concentrations were further decreased and increased to 100 mg/mL and 200 mg/mL, respectively (Figure S7). While a concentration of 200 mg ODB/ml led to highly irregular brittle membranes, a concentration of 100 mg ODB/mL led to dense membranes with only surface pores (structures similar to those in our previously reported work).[8, 9] In summary, based on dispersion 2.4, both dense membranes with surface pores and fully porous membranes could be obtained. For further studies the porous Nafion membranes prepared using 150 mg ODB/mL Nafion dispersion 2.4 were used (named as PN membranes).
Figure 1. SEM images A) air surface, B) cross-section and C) glass surface of a porous Nafion membrane prepared using 150 mg ODB/mL dispersion 2.3 (dispersed in ethanol after extensive drying in the rotary evaporator).
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Figure 2. SEM images A) air surface, B) cross-section and C) glass surface of the porous Nafion membranes prepared using (top) 130, (middle) 150 and (bottom) 170 mg ODB/mL dispersion 2.4 (dispersed in ethanol after vacuum drying of Nafion).
3.3 Mechanism of pore formation In previous work a mechanism for the formation of dense membranes with a porous surface was suggested.[9] This mechanism involves the formation of ODB droplets during solvent evaporation, which rise up to the surface due to the different density of ODB and Nafion solutions. If this mechanism is correct, increasing viscosity of the casting solutions would slow down the rise of ODB droplets from the bottom to the surface of the casting solution. Since solution viscosity is highly temperature dependent, the temperature during the membrane formation should affect the morphology. As an example, the viscosity of pure ethanol is 1.2 mPa·s at 25 °C and 0.8 mPa·s at 50 °C.[21] To control the temperature, casting solutions where evaporated from petri dishes floating on a water bath, which was maintained at 0, 20, or 50 °C. 11
Evaporation at 20 °C led to the known porous membrane morphology, showing that the increased humidity around the membrane does not have a strong impact on the process (Figure 3). At 0 °C, large cavities were found in the bottom area of the membranes, indicating that the increased viscosity of the solution prevented ODB droplets from rising. At 50 °C, the viscosity of the casting solution is expected to be lower, and ODB droplets should easily rise up to the surface. The SEM images support this as well, and show a dense membrane with pores on and below the surface. In the following, all discussed membranes were prepared at ambient conditions.
Figure 3: PN membrane after evaporation at 0°C (A), 20°C (B), and 50°C (C).
3.4 Mercury porosimetry Two PN membranes (23 and 69 µm thick) and a 74 µm thick membrane prepared with 180 mg ODB/ml were analyzed by mercury porosimetry (Figure 4). In this method mercury is used to fill the pores. Because of mercury's surface tension, higher pressure is needed to fill smaller pores. Since the pressure is increased stepwise in this method, the relative pore volume is given for pressure ranges in Figure 4. Considering the membrane thickness, only the pore size range between 0 and 30 µm was analyzed. Signals indicating larger pores stem from the space between the membrane parts, and were discarded. The largest found pores are those on the surface of the membranes. For all three membranes, the majority of pores (ca. 65%) is in the range of 0.1 – 4 µm. For the thinnest membrane, the maximum value is found around 1 µm. For the other two membranes, the maximum relative pore volume is around 0.3-0.4 µm. This can be explained by the different thicknesses, because the large surface pores contribute more to the overall porosity in thin membranes.
12
Fig 4. Distribution of pore volume for PN membranes (150 mg ODB/ml) and a similar membrane prepared with 180 mg ODB/ml; the thicknesses of the membranes are given.
3.5 Thermal stability and test for traces of ODB To test for traces of ODB in the prepared membranes, a porous membrane and N212 were tested by TGA coupled with GC/MS. After the initial weight loss, which stems from water, the TGA curves did not show any other solvent losses. The off-gas collected between 180-220°C was analyzed by GC/MS, and did not show any of the signals expected for ODB or chlorinated compounds.
3.6 Mechanical and dimensional stability and water swelling The mechanical properties of PN membranes (150 mg ODB, dispersion 2.4) were investigated by measuring stress-strain curves. The maximum tensile strength and Young modulus extracted from the experimental data were 7.1 ± 0.6 MPa and 65 ± 19 MPa, respectively (membrane thickness was 33.5 ± 3.6 µm). The tensile strength of commercially available Nafion N211 was about three times larger, 24.9 ± 1.1 %. This mismatch can be explained by the porosity and the
13
different hygrothermal history of the membranes. The porosity can be assessed through the densities: Porosity [%] = 100 x (
(
))
In this equation Dporous and Ddense are the densities of a dry porous and a dense membrane (1.92 g/mL for dense Nafion), respectively. For a 39 ± 3 µm thick PN membrane we calculated a porosity of 39%. In a first estimation, the effective area in the tensile strength measurement correlates with the portion of the solid portion of a porous material (61% in this case). Since a dense membrane prepared from dispersion 2.4 but without ODB showed a tensile strength of 13.9 ± 0.5 MPa, the expected tensile strength of PN membranes is 8.5 MPa. This matches well with the experimentally obtained 7.1 MPa. The low tensile strength measured for the dense membrane prepared from dispersion 2.4 but without ODB is similar to values in the literature. For example, Yu Seung Kim et al reported a maximum tensile strength of about 12.8 MPa for membranes casted from ethanol and annealed at 140 ºC, and about 42.5 MPa for Nafion 212 (at 50 ºC, 0% RH).[22] The dimensional stability of the membranes in water was investigated by placing the membranes in water at ambient conditions and at 80 °C for 24 h. The volume expansion was calculated to be 5.9 ± 0.3 % and 28.6 ± 2.2 %, respectively, with an average swelling slightly predominant along the length and width of the membrane, whereas there was only a negligible change in the thickness direction. These values are very low. For comparison, the volume swelling of N211 is reported in the literature as 33% at 23 °C and 81% at 100 °C.[23]
3.7 Water permeability Due to the difference in the porous structures on the surfaces of the casted membranes, the water permeability was tested for two sets of membranes, one with the air surface (named PN-A) and the other with the glass surface (named PN-G) exposed to the dry side. Experiments were carried out at two different relative humidities (9 and 50 % RH) at 70 ˚C (Figure 5). Nafion N211 membranes were used as a standard and experiments were carried out as triplicates. As shown in 14
Figure 5, the water flux is proportional to the humidity gradient over the membranes. Both PN-A and PN-G showed higher water flux at both low and high humidity in comparison to N211. PNA showed 29% and 6% higher fluxes, whereas as PN-G showed 31 % and 10 % higher water flux compared to the dense Nafion N211 membranes at 9% RH and 50% RH, respectively. The improved water flux in PN membranes is a direct consequence of the porous structure. Water diffusion not only occurs in the polymer phase (slow), but also in the gas phase of the pores (fast). Furthermore, the surface pores increase the interfacial area (membrane/gas phase) and thereby reduce the observed water transport resistance. The observation that the flux increases when the less porous glass side (smaller interfacial area membrane/gas phase) is on the dry side (PN-G) indicates that water absorption has a higher resistance than water desorption, which is in accordance with the literature.[24]
0.45
o
9 % RH / 70 C o 50 % RH / 70 C
2
water flux [g/cm h]
0.40
0.35
0.30
0.25
0.20 N211
PN-A
PN-G
Figure 5. Water flux through dense Nafion (N211) and ca. 30 µm thick PN membranes; A: air side (side with higher porosity) on the dry side, G: less porous glass side on the dry side.
PN membranes were also tested in a membrane humidifier. In this experiment membranes were clamped between two flow fields at room temperature. When the test started, hot, humid air was 15
blown over one side of the membrane, and cold, dry air over the other side. It was found that the membranes failed quickly. For one membrane it was noticed that the membrane developed a crack at the hot air entrance and was strongly swollen and pressed deeply into the channels (Figure S14). This is surprising, since swelling experiments showed a very low volume swelling for PN membranes. Likely, the rapid change from dry, cold conditions to humid, hot conditions expanded the air trapped in the pores of the membranes, expanding the pores of the now water plasticized membrane like balloons. This would explain both the observed high swelling and the crack formation. Therefore, the membranes are not robust enough for this application, and no further characterization was done.
3.8 In-plane Proton Conductivity In-plane proton conductivity at 70 °C was measured for commercial membranes (N211 and N212), a homemade non-porous membrane, and three porous membranes (Figure 6). At 10% RH the impedance curves were not reproducible, and no resistance could be extracted. In general, all dense membranes show a similar conductivity, including data for N211 from the literature, which, however, was obtained at 80 °C.[23] This shows that the membrane formation process followed in this work (including casting and annealing) leads to membranes similar to commercial Nafion membranes. Therefore, differences between commercial membranes like N211 and porous membranes are mainly a result of the porous structure. In fact all porous membranes showed lower conductivity values than dense membranes. This is expected, because the pores do not contribute to the overall conductivity. If there are no additional effects, like increased conductivity by surface films along the pore walls, or decreased conductivity due to increased tortuosity, it should be possible to estimate the conductivity of porous membranes by correcting the conductivity observed for a dense membrane with the solid matter contents of the porous membrane. With 120 mS/cm observed for dense Nafion and a solid contents of 61% (obtained for a 39 μm thick PN membrane, thicker membranes will show higher solid contents because of the lower overall contribution of the large surface pores), a conductivity of 73 mS/cm can be estimated. This correlates with the experimental data: The thicker porous membranes (69 16
and 73 μm) have a similar conductivity, around 70-80 mS/cm. close to that of dense membranes. There is an indication that an increase in the ODB concentration decreases the conductivity. This could be an effect of increased tortuosity, since the SEM image of the 73 µm thick membrane prepared with 180 mg ODB seems to show more pores. However, this effect is nearly within the error of the method. The thickness seems to have a much larger effect, and the thinnest porous membrane (23 µm) seems to have the lowest conductivity. A SEM analysis of the tested membrane showed that the surface porous layer (which shows the highest pore volume) is about 9 µm thick for the tested membranes. If the conductivity values are corrected for this 9 μm thick layer (as if it is fully porous), the conductivity values of the thin and thick porous membranes are in fact rather similar (ca. 80 mS/cm and 90 mS/cm, respectively).
Figure 6: Proton conductivity of various membranes, obtained at 70°C and various relative humidities. For comparison also values from the literature (measured at 80 °C) are included.[23]
3.9 Operation in a Fuel Cell: iV curve and H2 crossover Obviously porous membranes will have a huge gas crossover, and therefore are not a good choice for use in a fuel cell. The crossed over gases will not only reduce the efficiency of the 17
system, but will also increase the formation of reactive oxygen species, which are known to severely degrade the polymer membrane.[25] However, porous membranes may have a beneficial effect for niche applications, e.g. if the pores are water filled, and add to the overall conductivity by forming a conductive surface film on the pore walls, for example in a direct methanol fuel cell. The membrane morphology presented in this work could also be used for self-humidifying membranes, if a catalyst is blended into the membranes, and allows for the reaction of H2 and O2 inside of the membrane. Due to the porous structure the crossover would be huge, and much more water could be formed inside of the membranes than in the published systems.[15] Excess water may also be stored in the pores and thereby reduce the gas crossover, leading to a self-balancing system. For some applications, where system weight is more important than the life time, this could be attractive. Therefore, fuel cell performance and gas crossover were tested.
Figure 7: iV curves for commercial Nafion and membranes made with 0 mg, 150 mg and 180 mg ODB/ml.
In Figure 7, the performance of N211 (nominally 25 µm thick) and N212 (nominally 50 µm thick) is compared with the performance of a self-made dense membrane (ca. 37 µm) and ca. 70 µm thick porous membranes, prepared from dispersion 2.4 and 150 and 180 mg ODB/ml. It can be seen that the performance of N211 is best. N212 shows the second best performance up to a current density of about 800 mA/cm2, suggesting that the thinner N211 membrane has a better
18
water management than 212: At high current densities, excess product water blocks the pores in the cathode layer, and the limited mass transfer reduces the potential. In addition, the anode may dry out if the electro-osmotic drag is larger than the back-diffusion of water to the anode. The self-made, dense Nafion membrane behaves very similar to N212. For the porous membranes, it should be noted that the membrane-electrode interfacial resistance should be larger, because the used GDE will have a lower contact area with the porous structure than with a dense, flat membrane. Nevertheless, the 70 µm thick porous membrane prepared with 180 mg ODB/ml showed a stable performance up to 1000 mA/cm2 without severe mass transfer limitation, performing better than N212 above 850 mA/cm2. It seems that the porous structure enhances the back diffusion of water from the cathode to the anode, and thereby prevents cathode flooding. The membrane prepared with 150 mg ODB/ml, however showed an unstable performance. The reason for this is not clear. However, looking at the SEM images of the specific membranes used in this experiment (Figure S8), it can be seen that the “180 mg ODB/ml” membrane used here seems to have finer pores, contrary to the generally expected structure.
In previous work we found that membranes with only surface pores can survive mild hot pressing conditions without collapse of the pores. Now we found that fully porous membranes are compressed in the land and non-active areas (Figure S9). The non-compressed channel areas showed still the white appearance indicating porosity after disassembly of the cell. The SEM images of a MEA (after being assembled into a cell) show that the porosity is indeed strongly reduced.
The electrochemical hydrogen crossover current density of N211, N212, and the lab-made dense membrane is in the expected range (Figure 8),[26] and dry membranes are more permeable for hydrogen than wet membranes. The crossover current density of the membrane prepared with 150 mg ODB/ml reached around 40 mA/cm2 at 100% RH and 100 mA/cm2 at 50% RH, because of a severe membrane failure when the humidity of the fuel cell was lowered from 100 to 70 to 50% RH (see also Figure S12 and S13). However, the crossover current density of the membrane made with 180 mg ODB in Fig. 8 is in a range which is below the 10 mA/cm2 often used as indicator for membrane failure in the literature. In conclusion, porous membranes with catalyst
19
particles blended into the polymer could be interesting for the use in self-humidifying membrane systems.
Figure 8: Hydrogen crossover at 70 °C in a fuel cell setup; the membrane prepared with 180 mg ODB/ml is 73 µm thick.
4. Conclusions A one-step solvent evaporation method has been successfully developed to prepare porous Nafion membranes with EW 1100 using ODB as a porogene. Depending on the amount of the porogene, both dense membranes with a porous surface and a novel membrane type, fully porous membranes with larger pores on the surface can be obtained. This in contrast to our previous work (based on Dupont’s SE20092 dispersion), where systematic variation of the ODB concentration only gave access to dense membranes with surface pores.[8,9] Reproducibility of the new method was confirmed by reproduction of the experiments in two laboratories (KIST 20
and NEXT ENERGY). Best results were obtained with an ethanol based dispersion, which was prepared starting from a commercial Nafion dispersion (DE2021) by solvent evaporation and redissolution. The tensile strength of the porous PN membranes was lower than that of commercially available Nafion N211, which is due to their porous structure. The dimensional stability (volume expansion in water) of the porous membranes is excellent at both ambient conditions and 80 °C, probably because the pore walls can expand into the porous structure. The water flux through PN membranes is higher than through Nafion N211 membranes, and a more porous structure on the humid side increased the flux, indicating that interfacial water transport from the wet vapor phase to the membrane limits the water transport. Due to the porous structure, the in-plane proton conductivities of porous membranes are lower than those of dense Nafion membranes of the same thickness. Porous membranes were also tested in the fuel cell, where they performed reasonably well, but showing an increased gas crossover, which however is less than may have been expected, and is still within a reasonable range.
The work presented in this manuscript is basic research, but we believe that the new structural morphology is inspiring and can be the basis for more applied work, e.g. for the development of self-humidifying membranes, or membranes in which the pores are filled with another material.
5. Acknowledgements This work was supported by the Korea-Denmark green technology cooperative research program, by the Ministry of Science, ICT and Future Planning (No. 2015M1A2A2056554), and the KIAT program on Korea-German collaboration (2MR4380).
6. References [1] F.A. de Bruijn, V.A.T. Dam, G.J.M. Janssen, Review: Durability and Degradation Issues of PEM Fuel Cell Components, Fuel Cells 8 (2008) 3-22. [2] M. Carmo, D. L. Fritz, J. Mergel, D. Stolten, A comprehensive review on PEM water electrolysis, Int. J. Hydrogen Energy 38 (2013) 4901-4934. [3] B. Schwenzer, J. Zhang, S. Kim, L. Li, J. Liu, Z. Yang, Membrane Development for Vanadium Redox Flow Batteries, ChemSusChem 4 (2011) 1388-1406.
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[4] S.-K. Park, E.A. Cho, I.-H. Oh, Characteristics of Membrane Humidifiers for Polymer Electrolyte Membrane Fuel Cells, Korean J. Chem. Eng. 22 (2005) 877-881. [5] S.-Y. Lee, I.-G. Min, H.-J. Kim, S.-W. Nam, J. Lee, S. J. Kim, J. H. Jang, E. Cho, K. H. Song, S.-A. Hong, T.-H. Lim, Development of a 600 W Proton Exchange Membrane Fuel Cell Power System for the Hazardous Mission Robot, J. Fuel Cell Sci. Techn. 7 (2010) 031006/1-031006/7. [6] M. Klingele, M. Breitwieser, R. Zengerle, S. Thiele, Direct deposition of proton exchange membranes enabling high performance hydrogen fuel cells, J. Mater. Chem. A 3 (2015) 1123911245. [7] J.K. Koh, Y. Jeon, Y.I. Cho, J.H. Kim, Y.-G. Shul, A facile preparation method of surface patterned polymer electrolyte membranes for fuel cell applications, J. Mater. Chem. A 2 (2014) 8652–8659. [8] D. Henkensmeier, Q.K. Dang, N.N. Krishnan, J.H. Jang, H.-J. Kim, S.-W. Nam, T.-H. Lim, ortho-Dichlorobenzene as a pore modifier for PEMFC catalyst electrodes and dense Nafion membranes with one porous surface, J. Mater. Chem. 22 (2012) 14602-14607. [9] Q.K. Dang, D. Henkensmeier, N.N. Krishnan, J.H. Jang, H.-J. Kim, S.-W. Nam, T.-H. Lim, Nafion membranes with a porous surface, J. Membr. Sci. 460 (2014) 199-205. [10] M.-K. Song, Y.-T. Kim, J.-S. Hwang, H.Y. Ha, H.-W. Rhee, Incorporation of Zirconium Hydrogen Phosphate into Porous Ionomer Membranes, Electrochem. Solid State Lett. 7 (2004) A127–A130. [11] J.A. Hestekin, E.P. Gilbert, M.P. Henry, R. Datta, E.J. St. Martin, S.W. Snyder, Modified porous Nafion: Membrane characterization and two-phase separations, J. Membr. Sci. 281 (2006) 268–273. [12] E.G. Howard, R.B. Lloyd, E.I. Du Pont de Nemours and Company, Porous highly fluorinated acidic polymer catalyst and process for its preparation, EP Patent, 1152830B1, 2001. [13] D.-J. Guo, S.-J Fu, W.Tan, Z.-D.Dai, A highly porous nafion membrane templated from polyoxometalates-based supramolecule composite for ion-exchange polymer-metal composite actuator, J. Mater. Chem. 20 (2010) 10159–10168. [14] N. Nambi Krishnan, D. Henkensmeier, J.H. Jang, H.-J. Kim, Nanocomposite Membranes for Polymer Electrolyte Fuel Cells, Macromol. Mater. Eng. 299 (2014) 1031–1041. [15] M. Watanabe, H. Uchida, Y. Seki, M. Emori, Self-Humidifying Polymer Electrolyte Membranes for Fuel Cells, J. Electrochem. Soc. 143 (1996) 3847-3852. 22
[16] P.W. Majsztrik, M.B. Satterfield, A.B. Bocarsly, J.B. Benziger, J. Membr. Sci. 301 (2007) 93–106. [17] M. Adachi, T. Navessin, Z. Xie, F.H. Li, S. Tanaka, S. Holdcroft, J. Membr. Sci. 364 (2010) 183–193. [18] W. Germer, J. Leppin, C. Nunes Kirchner, H. Cho, H.-J. Kim, D. Henkensmeier, K.-Y. Lee, M. Brela, A. Michalak, A. Dyck, Phase separated methylated Polybenzimidazole (O-PBI) based Anion Exchange Membranes, Macromol. Mater. Eng. 300 (2015) 497-509. [19] W.G. Grot (E. I. Du Pont De Nemours And Company), Process for making liquid composition of perfluorinated ion exchange polymer, and product thereof, US 4,433,082, filed 1981. [20] D. Henkensmeier, D. Aili, “Techniques for PBI Membrane Characterization”, in “High Temperature Polymer Electrolyte Membrane Fuel Cells - Approaches, Status and Perspective”, eds. Q. Li, D Aili, H.A. Hjuler and J. O. Jensen, ISBN 978-3-319-17081-7; DOI 10.1007/978-3319-17082-4, Springer (2015). [21] A. Pimentel-Rodas, L.A. Galicia-Luna, J.J. Castro-Arellano, Capillary Viscometer and Vibrating Tube Densimeter for Simultaneous Measurements up to 70 MPa and 423 K, J. Chem. Eng. Data, 61 (2016) 45-55. [22] Y.S. Kim, C.F. Welch, R.P. Hjelm, N.H. Mack, A. Labouriau, E.B. Orler, Origin of Toughness in Dispersion-Cast Nafion Membranes, Macromolecules 48 (2015) 2161-2172. [23] J. Peron, A. Mani, X. Zhao, D. Edwards, M. Adachi, T. Soboleva, Z. Shi, Z. Xie, T. Navessin, S. Holdcroft, Properties of Nafion® NR-211 membranes for PEMFCs, J. Membr. Sci. 356 (2010) 44-51. [24] S. Ge, X. Li, B. Li, I.M. Hsing, Absorption, desorption and transport of water in polymer electrolyte membranes for fuel cells 105, J. Electrochem. Soc. 152 (2005) A1149-A1157. [25] L. Ghassemzadeh, K.-D. Kreuer, J. Maier, K. Müller, Chemical Degradation of Nafion Membranes under Mimic Fuel Cell Conditions as Investigated by Solid-State NMR Spectroscopy, J. Phys. Chem. C 114 (2010) 14635–14645. [26] D. Henkensmeier, H. Benyoucef, F. Wallasch, L. Gubler, Radiation grafted ETFE-graftpoly(α-methylstyrenesulfonic acid-co-methacrylonitrile) membranes for fuel cell applications, J. Membr. Sci. 447 (2013) 228–235.
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Highlights
Nafion membranes with three different morphologies were prepared a) dense Nafion membranes with a porous surface, based on EW 1100 b) mixed dense/porous membranes c) fully porous membranes with closed pores and larger open pores on the surface membranes were prepared in a single step (solution casting)
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PII: DOI: Reference:
S0376-7388(16)30362-3 http://dx.doi.org/10.1016/j.memsci.2016.08.025 MEMSCI14675
To appear in: Journal of Membrane Science Received date: 13 May 2016 Revised date: 5 August 2016 Accepted date: 17 August 2016 Cite this article as: Dickson Joseph, Julian Büsselmann, Corinna Harms, Dirk Henkensmeier, Mikkel Juul Larsen, Alexander Dyck, Jong Hyun Jang, HyoungJuhn Kim and Suk-Woo Nam, Porous Nafion membranes, Journal of Membrane Science, http://dx.doi.org/10.1016/j.memsci.2016.08.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Porous Nafion membranes Dickson Josepha, Julian Büsselmannb,c, Corinna Harmsb, Dirk Henkensmeiera, d*, Mikkel Juul Larsene, Alexander Dyckb, Jong Hyun Janga,f, Hyoung-Juhn Kima, Suk-Woo Nama,f a
Fuel Cell Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5,
Seongbuk-gu, Seoul 02792, Republic of Korea b
NEXT ENERGY • EWE Research Centre for Energy Technology at the University of
Oldenburg, Carl-von-Ossietzky-Str. 15, 26129 Oldenburg, Germany c
Hochschule Emden/Leer, Abteilung Naturwissenschaftliche Technik, Applied Life Sciences,
Constantiaplatz 4, 26723 Emden, Germany d
University of Science and Technology, 217 Gajungro, Yuseonggu, Daejeon, Republic of
Korea e
IRD Fuel Cells A/S, Emil Neckelmanns Vej 15 A&B, 5220 Odense SØ, Denmark
f
Green School, Korea University, Seoul 136-713, Republic of Korea
Abstract By varying the amount of porogene (ortho-dichlorobenzene, ODB), and optimization of the dispersion process, two types of solvent cast Nafion membranes with an equivalent weight of 1100 g/mol sulfonic acid can be obtained reproducibly. One type is a dense membrane with a porous layer on one surface. The other membrane type shows a novel structure, consisting of small closed pores throughout the membrane and a single layer of large open pores on one side. In addition, some membranes showed a structural morphology between these two types, a membrane with a dense part and a porous part on top of each other. The latter membrane structure was not fully reproducible yet, but probably could be by carefully adjusting the formulation of the casting solution. Also the effect of the casting temperature on the morphology is shown. Fully porous membranes were characterized for their water permeability, ion conductivity, mechanical properties, their performance in the fuel cell and the hydrogen crossover. While the fully porous membranes are not expected to be part of a real fuel cell, we
1
expect that the new morphologies will inspire applied research, e.g. in which the pores are filled with electrolyte or material or a catalyst is blended into the polymer.
Nafion (EW 1100)
Graphical abstract
Keywords Nafion dispersion, porous Nafion membranes, porosity, morphology, water transport
1. Introduction Nafion membranes are essential components of fuel cells,[1] electrolyzers,[2] and redox flow batteries.[3] Also the use in gas humidifiers was investigated.[4, 5] In practically all applications Nafion membranes are used as dense, non-porous flat sheet membranes with smooth surfaces. However, there is growing evidence that other surface morphologies may improve electrochemical systems. Klingele et al. reported a performance improvement of polymer electrolyte fuel cells (PEFCs) when the membrane is not casted and assembled between the catalyst electrodes, but when the membrane is ink jet printed on the catalyst electrode, probably because assembling of smooth membranes with catalyst coated electrodes leads to voids at the membrane-electrode interface.[6] Shul et al. showed that the platinum utilisation increases with 2
the surface area of the membranes, which can be increased by casting the membranes in microstructured molds.[7] In our previous work we showed that dense Nafion membranes with a porous surface structure can also be prepared by solution casting, when ortho-dichlorobenzene (ODB) is added to Dupont's SE 20092 Nafion dispersion.[8] In the fuel cell, we observed increased platinum utilization and 16% higher performance in comparison to membranes with a smooth surface.[9] However, Dupont's Nafion SE 20092 dispersion contains Nafion with an equivalent weight of EW = 1000, which swells more than standard Nafion of EW = 1100. Since the formation of the surface porous structures depends on the formulation of the casting solution (e.g. Nafion concentration, solvents, concentration of ODB), it was necessary to develop a new formulation, which allows to extend the formation of surface pores in a one-step casting process to EW 1100 Nafion. The results of this work are presented here.
The second goal of this work was to prepare fully porous Nafion membranes. As far as we know, all procedures reported in the literature are based on two-step processes, involving first formation of a Nafion or Nafion composite membrane, and then introduction of pores by leaching the porogenes,[10] expansion of swollen Nafion membranes by formation of gaseous degradation products,[11] or expansion of membranes swollen by liquid SO2 or supercritical CO2 by changing ambient conditions to a point at which the swelling media enters the gaseous state.[12]
In principal, fully porous membranes can have two kinds of pores: closed pores and open pores (as in filtration membranes). It appears that leaching and expansion of membranes often leads to open pore structures.[12, 13] Based on our previous work, we consider it possible that closed pore structures can be introduced into Nafion by using ODB as porogene. However, by using Dupont’s SE20092 Nafion dispersion and ODB as pore forming agent, it was not possible to produce fully porous membranes.[8, 9] Here, by using different solvents and a Nafion with higher equivalent weight, we demonstrate for the first time fully porous Nafion with a closed pore structure, prepared in a single step process.
While the use of porous membranes in fuel cells and electrolyzers may be counterintuitive, because one of the membranes' main functions (in addition to their role as electrolyte) is to separate anode and cathode gases, there could be some interesting applications for porous Nafion 3
membranes. The pores could be filled with water or another electrolyte (phosphoric acid, ionic liquids, etc.). If the pore walls are wetted sufficiently, they may act as a "proton highway", providing a pathway for proton conduction from anode to cathode with low tortuosity.[14] In some work, the inevitable gas crossover of hydrogen and oxygen is used to form water on catalyst particles embedded in a fuel cell membrane. This direct humidification of the membrane allows to operate the system without humidification of the reactant gases.[15] Combining this concept with porous membranes could lead to membranes of increased humidity and therefore excellent performance even when dry fuel and oxidant gases are used. Furthermore, it was reported that water transport through a membrane is not only limited by the transport resistance inside of the membrane itself, but very strongly affected by the interfacial mass transport resistance across the membrane/gas interface.[16, 17] However, water transport in the gas phase is faster than in the liquid phase or in membranes. This could open a pathway to mitigate the water (or gas) transport properties of Nafion membranes, e.g. for use of Nafion membranes in humidifiers. All these potential applications of porous Nafion membranes cannot be addressed in this work. But they are a strong motivation to investigate if and how it is possible to prepare Nafion membranes with different pore morphologies.
2. Experimental part KIST: Nafion dispersion DE2021 was obtained from Dupont, 1,2-dichlorobenzene (ODB) from Kanto Chemical, ethanol from Sigma. Next Energy: Nafion dispersion DE2021 was obtained from Chemours, ODB from Carl Roth, ethanol from Fischer Chemicals (99.8% pure, E/0650DF/17). N211 and N212 were obtained from Chemours.
2.1 Preparation of Nafion dispersion in ethanol (Nafion dispersion 2.4) A 100 mL round bottom (RB) flask was weighted and 20 mL of Nafion dispersion (DE2021, Dupont) were added. The RB flask was placed in a rotary evaporator and solvents were evaporated. To further remove the solvents the RB flask was placed in a vacuum oven at 60 ˚C for 24 h. The RB flask was then removed from the oven and ethanol was added to achieve a 10
4
wt% Nafion dispersion. The mixture was stirred at room temperature for 48 h and then filtered using a syringe filter (0.45 µm), the obtained dispersion was stored at 4 ˚C.
2.2 Preparation of porous Nafion membranes (PN membranes) A required amount of the 10 wt% Nafion dispersion 2.4 was mixed with a required amount of ortho-dichlorobenzene (ODB) to make up a concentration of 150 mg of ODB per mL of Nafion dispersion. The mixture was stirred at room temperature for 24 h and either drop casted into a petri dish or casted onto a glass plate using a doctor blade, e.g. with a blade gap of 400 µm under ambient conditions. The formation of a white colored membrane was observed as the solvents evaporated, and the glass plate was left to stand under ambient conditions for 24 h. The glass plate along with the membrane was then placed in a vacuum oven at 60 °C for 12 h after which the temperature in the oven was increased to 120 °C and maintained for 4 h and allowed to cool to room temperature. The glass plate was then removed from the oven and soaked in water for 24 h and the membrane with an average thickness (for this membrane) of 33 ± 4 µm was peeled off gently.
2.3 Membrane characterization A scanning electron microscope S-3000H with an accelerating voltage ranging from 0.3 to 30 kV or a Neon 40 ESB (Carl Zeiss) was used to obtain scanning electron micrographs of the prepared Nafion membranes. The membranes were coated with platinum or gold before imaging. TGA curves were measured in a Pyris 1 TGA (Perkin Elmer) under helium gas. The sample weight was about 12 mg. The heating rate was 10 K/min. Off-gases were directly transferred into a GC/MS coupled with the TGA.[18] For mercury porosimetry membrane samples were cut into small pieces, and inserted into a Pascal 140 porosimeter (Thermo Scientific). There the samples were degassed, and filled with mercury up to a pressure of 400 kPa. Then the samples were transferred into a Pascal 440 porosimeter, which can apply pressures up to 400 MPa, ensuring that also small cavities are filled with mercury. 5
The maximum tensile strength and the Young modulus of the PN and N211 membranes were determined from force displacement curves measured with a Cometech QC-508E instrument. For each material, several samples from the same membrane were measured, their average values are given. The sample size was 4×1 cm2, the elongation speed 10 mm min
-1
. The ambient
temperature during the measurement was 24°C and 20-34% relative humidity. In-plane proton conductivity was measured on 3 cm long and 1.8 cm broad membrane stripes via impedance spectroscopy. To be sure that the membranes are fully activated, the samples were immersed consecutively in 3 wt% boiling hydrogen peroxide solution, deionized (DI) water, boiling 0.5 M sulfuric acid, and boiling DI water. For the impedance measurements, the samples were assembled into a BekkTech BT-112 cell, and a G100 Greenlight Innovation fuel cell test stand. Impedance was analyzed by a Gamry Reference 3000 potentiostat. The conductivity was calculated according to
In which d is the distance between the potential sensing electrodes (0.425 cm), R M is the membrane resistance extracted from the impedance spectra, b is the membrane width, and t is the membrane thickness. The water permeability of PN membranes was studied using a simple experiment reported in our previous related article.[9] A hole, 8 mm in diameter, was drilled into a plastic screw cap of a vial and a hole was punched though the sealing. A piece of the membrane with a thickness of 30 µm was placed on the inner side of the vial's plastic screw cap and covered with the sealant. Water was filled in the vials, allowing for a head space with an assumed relative humidity of 100%. Then the vials were placed in an environmental chamber, and at regular intervals the vials were weighed in order to calculate the water flux.
2.4 MEA fabrication and testing Because of the uneven surfaces of the prepared membranes, catalyst coated membranes (CCMs) are expected to give the best performing membrane electrode assemblies (MEAs). However, to 6
avoid differences arising from manual coating, commercial gas diffusion electrodes (GDEs; Quintech BC-H225-05S, 0.5 mg platinum/cm² on Sigracet 25 BC GDL) cut to an active area of 4.8 cm2 were used to prepare the MEAs. The MEAs were assembled into a cell with a serpentine flow field, and sealed with silicone gaskets. The cell was tightened with a torque meter adjusted to 10 Nm, and tested for leaks by pressurizing the cell with hydrogen (anode) and nitrogen (cathode), at a flow rate of 300 Nml/min. First 30 kPa were applied on both sides, then 50 kPa on the anode and 30 kPa on the cathode. Finally also the cathode pressure was increased to 50 kPa. Humidified gas streams were heated to 5 °C above the cell temperature to avoid condensation of water. For activation, the cell was operated at 70 °C, 100% relative humidity (RH) and a stoichiometric ratio (λ value) of 3 (anode, hydrogen) and 2 (cathode, synthetic air) at a load of 0.5 A/cm2. After 12 hours, latest after 20 hours, the cell was defined as activated if the potential remained stable within 5 mV for 3 minutes. The flow rates were kept at the stoichiometric ratio of 3 (anode, hydrogen) and 2 (cathode, synthetic air) and an iV curve was measured (90 seconds per step). The same procedure was repeated at 70%, 50% and 30% RH. At the end of all fuel cell tests, the hydrogen crossover was measured by linear sweep voltammetry (LSV). For that the cathode gas was changed to nitrogen, and both flow rates were fixed at 300 Nml/min and 100% RH, and later 50% RH. The potential was varied between 90mV to 500 mV at a rate of 10 mV/s. The resulting curve was corrected for the slope (resulting from internal shorting), to give the limiting current.
3. Results and Discussion 3.1 Dispersion of Nafion Commercial Nafion dispersions available in the market are either water based (D1020 and D1021) or based on water and alcohol mixtures (D520, D521, D2020, D2021, SE 20092). Using SE 20092 dispersion (Nafion with equivalent weight of 1000) it was shown in previous reports that dense membranes with a porous surface can be prepared.[8, 9] Since that method could not be directly transferred to commercial dispersions based on Nafion with EW 1100, the solvent composition needed to be adjusted. This can be done either by dispersion of Nafion pellets under elevated temperature and pressure, or by re-dispersion.
3.1.1 Dispersion 1: Dispersion of Nafion pellets 7
Direct dispersion of commercial Nafion pellets in a given solvent mixture appears to be a straightforward process. As described by Grot, this should be done in a pressurized reactor at elevated temperatures.[19] First 20 wt% of Nafion beads in ethanol were tried to be dispersed at 200 °C and 80 bar, but instead of a dispersion, a large gel lump was obtained (Figure S1). This behavior is not clearly understood but we believe it could be because too much solvent evaporated into the head space of the pressure reactor. Since ethanol is a low boiling solvent also water was tested as solvent. The reaction conditions are shown in Table S1. Though we were able to disperse Nafion in water at low concentrations (dispersion 1), maintaining a constant, exact pressure was not possible. Hence we were not able to reproduce dispersions with a defined concentration. A maximum of 6 wt% Nafion dispersion was obtained using an initial concentration of 10 wt% in water. Therefore, this process did not efficiently disperse small amounts of Nafion in our lab. One possible reason for this is also the process time, which was kept below 8 hours.
3.1.2 Dispersion by re-dissolution of commerical Nafion dispersion In this approach, Nafion dispersion is evaporated to dryness, and the remaining solid (which is not annealed, while Nafion pellets probably are due to the high temperatures required for melt extrusion) is re-dissolved in the required solvent mixture. As starting material, we used the commercially available dispersion DE2021 (EW1100). Using a rotary evaporator, the solvents were evaporated until no liquid was visible. Then the solid Nafion was re-dispersed by adding an excess amount of ethanol. Then the solution was concentrated to 20 wt% by slowly evaporating ethanol by stirring at ambient conditions in an open vial. The thus obtained 20 wt% Nafion dispersion existed in a viscous gel form (dispersion 2.1). To this dispersion orthodichlorobenzene (ODB) was added. The dispersion was mixed, drop casted on glass petri dishes and left undisturbed at ambient conditions for solvents to evaporate and to form membranes. Using 180 mg of ODB per mL of Nafion dispersion, white membranes with a porous structure along the cross-section, and a dense surface with just a few pores (Figure S2) were obtained. However, these membranes were brittle and the reproducibility of the porous structure was poor especially along the cross-sections of the membranes (Figure S3). We speculate that the 8
brittleness and the non-reproducibility of the porous surface were due to the high viscosity of the Nafion dispersion that prevented uniform distribution of ODB in the dispersion before preparing the membranes. Hence, for easier processing and to obtain good membranes, after solvent evaporation in a rotary evaporator, the amount of ethanol needed for 10 wt% solutions was added. The mixtures were stirred at ambient conditions with a cap on the RB until a clear dispersion was obtained (dispersion 2.2). On a small scale membranes were prepared in glass Petri dishes by adding different amounts of ODB/mL Nafion dispersion. We found that 150 mg ODB/mL Nafion dispersion led to white membranes that were stable but somehow brittle (Figure S4). They showed a porous structure along the cross-section, and dense surface with few pores, but again reproducibility was poor (Figure S5). We assume that the incomplete porous structure could be due to the presence of some trace solvents (especially water) from the original dispersion. Hence care was taken to remove as much solvent as possible during the rotary evaporation step by leaving the round bottom flask in the rotary evaporator for at least 30 minutes after the visible solvents were evaporated to completely remove the solvents before redispersion in ethanol. In addition, the final 10wt% dispersions were filtered using a syringe filter (0.45 µm) to remove any particulate if present (dispersion 2.3).[20] To further ensure that all solvents are completely removed, the solid obtained after rotary evaporation was dried in a vacuum oven at 60 ˚C for 24 h, after which a 10 wt% Nafion dispersion in ethanol was prepared as above (dispersion 2.4).
3.2 Preparation of porous Nafion membranes The dispersions 2.3 (obtained by directly dispersing with ethanol after extensive rotary evaporation) and 2.4 (dispersed in ethanol after vacuum drying Nafion in the oven for 24 h after rotary evaporation) were examined for the preparation of porous membranes. Both dispersions led to the formation of white membranes (visual confirmation of porosity) with 150 mg ODB/mL Nafion dispersion, as shown in Figure S6. Figure 1 and 2 show SEM images of the membranes prepared using dispersion 2.3 and 2.4, respectively. The main difference is the porous structure on the membrane top surface (air side). It appears that dispersion 2.4 formed less and more isolated surface pores than dispersion 2.3. For further experiments, dispersion 2.4 was used. In order to study the effect of the ODB amount on the structural morphology, the amount of ODB was varied. We found that with lesser amount of ODB (130 mg/mL) the porosity of the 9
membrane was lower, but with higher amount of ODB (170 mg/mL) the porosity increased (Figure 2). This observation became even more clearly evident especially along the cross-section when the concentrations were further decreased and increased to 100 mg/mL and 200 mg/mL, respectively (Figure S7). While a concentration of 200 mg ODB/ml led to highly irregular brittle membranes, a concentration of 100 mg ODB/mL led to dense membranes with only surface pores (structures similar to those in our previously reported work).[8, 9] In summary, based on dispersion 2.4, both dense membranes with surface pores and fully porous membranes could be obtained. For further studies the porous Nafion membranes prepared using 150 mg ODB/mL Nafion dispersion 2.4 were used (named as PN membranes).
Figure 1. SEM images A) air surface, B) cross-section and C) glass surface of a porous Nafion membrane prepared using 150 mg ODB/mL dispersion 2.3 (dispersed in ethanol after extensive drying in the rotary evaporator).
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Figure 2. SEM images A) air surface, B) cross-section and C) glass surface of the porous Nafion membranes prepared using (top) 130, (middle) 150 and (bottom) 170 mg ODB/mL dispersion 2.4 (dispersed in ethanol after vacuum drying of Nafion).
3.3 Mechanism of pore formation In previous work a mechanism for the formation of dense membranes with a porous surface was suggested.[9] This mechanism involves the formation of ODB droplets during solvent evaporation, which rise up to the surface due to the different density of ODB and Nafion solutions. If this mechanism is correct, increasing viscosity of the casting solutions would slow down the rise of ODB droplets from the bottom to the surface of the casting solution. Since solution viscosity is highly temperature dependent, the temperature during the membrane formation should affect the morphology. As an example, the viscosity of pure ethanol is 1.2 mPa·s at 25 °C and 0.8 mPa·s at 50 °C.[21] To control the temperature, casting solutions where evaporated from petri dishes floating on a water bath, which was maintained at 0, 20, or 50 °C. 11
Evaporation at 20 °C led to the known porous membrane morphology, showing that the increased humidity around the membrane does not have a strong impact on the process (Figure 3). At 0 °C, large cavities were found in the bottom area of the membranes, indicating that the increased viscosity of the solution prevented ODB droplets from rising. At 50 °C, the viscosity of the casting solution is expected to be lower, and ODB droplets should easily rise up to the surface. The SEM images support this as well, and show a dense membrane with pores on and below the surface. In the following, all discussed membranes were prepared at ambient conditions.
Figure 3: PN membrane after evaporation at 0°C (A), 20°C (B), and 50°C (C).
3.4 Mercury porosimetry Two PN membranes (23 and 69 µm thick) and a 74 µm thick membrane prepared with 180 mg ODB/ml were analyzed by mercury porosimetry (Figure 4). In this method mercury is used to fill the pores. Because of mercury's surface tension, higher pressure is needed to fill smaller pores. Since the pressure is increased stepwise in this method, the relative pore volume is given for pressure ranges in Figure 4. Considering the membrane thickness, only the pore size range between 0 and 30 µm was analyzed. Signals indicating larger pores stem from the space between the membrane parts, and were discarded. The largest found pores are those on the surface of the membranes. For all three membranes, the majority of pores (ca. 65%) is in the range of 0.1 – 4 µm. For the thinnest membrane, the maximum value is found around 1 µm. For the other two membranes, the maximum relative pore volume is around 0.3-0.4 µm. This can be explained by the different thicknesses, because the large surface pores contribute more to the overall porosity in thin membranes.
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Fig 4. Distribution of pore volume for PN membranes (150 mg ODB/ml) and a similar membrane prepared with 180 mg ODB/ml; the thicknesses of the membranes are given.
3.5 Thermal stability and test for traces of ODB To test for traces of ODB in the prepared membranes, a porous membrane and N212 were tested by TGA coupled with GC/MS. After the initial weight loss, which stems from water, the TGA curves did not show any other solvent losses. The off-gas collected between 180-220°C was analyzed by GC/MS, and did not show any of the signals expected for ODB or chlorinated compounds.
3.6 Mechanical and dimensional stability and water swelling The mechanical properties of PN membranes (150 mg ODB, dispersion 2.4) were investigated by measuring stress-strain curves. The maximum tensile strength and Young modulus extracted from the experimental data were 7.1 ± 0.6 MPa and 65 ± 19 MPa, respectively (membrane thickness was 33.5 ± 3.6 µm). The tensile strength of commercially available Nafion N211 was about three times larger, 24.9 ± 1.1 %. This mismatch can be explained by the porosity and the
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different hygrothermal history of the membranes. The porosity can be assessed through the densities: Porosity [%] = 100 x (
(
))
In this equation Dporous and Ddense are the densities of a dry porous and a dense membrane (1.92 g/mL for dense Nafion), respectively. For a 39 ± 3 µm thick PN membrane we calculated a porosity of 39%. In a first estimation, the effective area in the tensile strength measurement correlates with the portion of the solid portion of a porous material (61% in this case). Since a dense membrane prepared from dispersion 2.4 but without ODB showed a tensile strength of 13.9 ± 0.5 MPa, the expected tensile strength of PN membranes is 8.5 MPa. This matches well with the experimentally obtained 7.1 MPa. The low tensile strength measured for the dense membrane prepared from dispersion 2.4 but without ODB is similar to values in the literature. For example, Yu Seung Kim et al reported a maximum tensile strength of about 12.8 MPa for membranes casted from ethanol and annealed at 140 ºC, and about 42.5 MPa for Nafion 212 (at 50 ºC, 0% RH).[22] The dimensional stability of the membranes in water was investigated by placing the membranes in water at ambient conditions and at 80 °C for 24 h. The volume expansion was calculated to be 5.9 ± 0.3 % and 28.6 ± 2.2 %, respectively, with an average swelling slightly predominant along the length and width of the membrane, whereas there was only a negligible change in the thickness direction. These values are very low. For comparison, the volume swelling of N211 is reported in the literature as 33% at 23 °C and 81% at 100 °C.[23]
3.7 Water permeability Due to the difference in the porous structures on the surfaces of the casted membranes, the water permeability was tested for two sets of membranes, one with the air surface (named PN-A) and the other with the glass surface (named PN-G) exposed to the dry side. Experiments were carried out at two different relative humidities (9 and 50 % RH) at 70 ˚C (Figure 5). Nafion N211 membranes were used as a standard and experiments were carried out as triplicates. As shown in 14
Figure 5, the water flux is proportional to the humidity gradient over the membranes. Both PN-A and PN-G showed higher water flux at both low and high humidity in comparison to N211. PNA showed 29% and 6% higher fluxes, whereas as PN-G showed 31 % and 10 % higher water flux compared to the dense Nafion N211 membranes at 9% RH and 50% RH, respectively. The improved water flux in PN membranes is a direct consequence of the porous structure. Water diffusion not only occurs in the polymer phase (slow), but also in the gas phase of the pores (fast). Furthermore, the surface pores increase the interfacial area (membrane/gas phase) and thereby reduce the observed water transport resistance. The observation that the flux increases when the less porous glass side (smaller interfacial area membrane/gas phase) is on the dry side (PN-G) indicates that water absorption has a higher resistance than water desorption, which is in accordance with the literature.[24]
0.45
o
9 % RH / 70 C o 50 % RH / 70 C
2
water flux [g/cm h]
0.40
0.35
0.30
0.25
0.20 N211
PN-A
PN-G
Figure 5. Water flux through dense Nafion (N211) and ca. 30 µm thick PN membranes; A: air side (side with higher porosity) on the dry side, G: less porous glass side on the dry side.
PN membranes were also tested in a membrane humidifier. In this experiment membranes were clamped between two flow fields at room temperature. When the test started, hot, humid air was 15
blown over one side of the membrane, and cold, dry air over the other side. It was found that the membranes failed quickly. For one membrane it was noticed that the membrane developed a crack at the hot air entrance and was strongly swollen and pressed deeply into the channels (Figure S14). This is surprising, since swelling experiments showed a very low volume swelling for PN membranes. Likely, the rapid change from dry, cold conditions to humid, hot conditions expanded the air trapped in the pores of the membranes, expanding the pores of the now water plasticized membrane like balloons. This would explain both the observed high swelling and the crack formation. Therefore, the membranes are not robust enough for this application, and no further characterization was done.
3.8 In-plane Proton Conductivity In-plane proton conductivity at 70 °C was measured for commercial membranes (N211 and N212), a homemade non-porous membrane, and three porous membranes (Figure 6). At 10% RH the impedance curves were not reproducible, and no resistance could be extracted. In general, all dense membranes show a similar conductivity, including data for N211 from the literature, which, however, was obtained at 80 °C.[23] This shows that the membrane formation process followed in this work (including casting and annealing) leads to membranes similar to commercial Nafion membranes. Therefore, differences between commercial membranes like N211 and porous membranes are mainly a result of the porous structure. In fact all porous membranes showed lower conductivity values than dense membranes. This is expected, because the pores do not contribute to the overall conductivity. If there are no additional effects, like increased conductivity by surface films along the pore walls, or decreased conductivity due to increased tortuosity, it should be possible to estimate the conductivity of porous membranes by correcting the conductivity observed for a dense membrane with the solid matter contents of the porous membrane. With 120 mS/cm observed for dense Nafion and a solid contents of 61% (obtained for a 39 μm thick PN membrane, thicker membranes will show higher solid contents because of the lower overall contribution of the large surface pores), a conductivity of 73 mS/cm can be estimated. This correlates with the experimental data: The thicker porous membranes (69 16
and 73 μm) have a similar conductivity, around 70-80 mS/cm. close to that of dense membranes. There is an indication that an increase in the ODB concentration decreases the conductivity. This could be an effect of increased tortuosity, since the SEM image of the 73 µm thick membrane prepared with 180 mg ODB seems to show more pores. However, this effect is nearly within the error of the method. The thickness seems to have a much larger effect, and the thinnest porous membrane (23 µm) seems to have the lowest conductivity. A SEM analysis of the tested membrane showed that the surface porous layer (which shows the highest pore volume) is about 9 µm thick for the tested membranes. If the conductivity values are corrected for this 9 μm thick layer (as if it is fully porous), the conductivity values of the thin and thick porous membranes are in fact rather similar (ca. 80 mS/cm and 90 mS/cm, respectively).
Figure 6: Proton conductivity of various membranes, obtained at 70°C and various relative humidities. For comparison also values from the literature (measured at 80 °C) are included.[23]
3.9 Operation in a Fuel Cell: iV curve and H2 crossover Obviously porous membranes will have a huge gas crossover, and therefore are not a good choice for use in a fuel cell. The crossed over gases will not only reduce the efficiency of the 17
system, but will also increase the formation of reactive oxygen species, which are known to severely degrade the polymer membrane.[25] However, porous membranes may have a beneficial effect for niche applications, e.g. if the pores are water filled, and add to the overall conductivity by forming a conductive surface film on the pore walls, for example in a direct methanol fuel cell. The membrane morphology presented in this work could also be used for self-humidifying membranes, if a catalyst is blended into the membranes, and allows for the reaction of H2 and O2 inside of the membrane. Due to the porous structure the crossover would be huge, and much more water could be formed inside of the membranes than in the published systems.[15] Excess water may also be stored in the pores and thereby reduce the gas crossover, leading to a self-balancing system. For some applications, where system weight is more important than the life time, this could be attractive. Therefore, fuel cell performance and gas crossover were tested.
Figure 7: iV curves for commercial Nafion and membranes made with 0 mg, 150 mg and 180 mg ODB/ml.
In Figure 7, the performance of N211 (nominally 25 µm thick) and N212 (nominally 50 µm thick) is compared with the performance of a self-made dense membrane (ca. 37 µm) and ca. 70 µm thick porous membranes, prepared from dispersion 2.4 and 150 and 180 mg ODB/ml. It can be seen that the performance of N211 is best. N212 shows the second best performance up to a current density of about 800 mA/cm2, suggesting that the thinner N211 membrane has a better
18
water management than 212: At high current densities, excess product water blocks the pores in the cathode layer, and the limited mass transfer reduces the potential. In addition, the anode may dry out if the electro-osmotic drag is larger than the back-diffusion of water to the anode. The self-made, dense Nafion membrane behaves very similar to N212. For the porous membranes, it should be noted that the membrane-electrode interfacial resistance should be larger, because the used GDE will have a lower contact area with the porous structure than with a dense, flat membrane. Nevertheless, the 70 µm thick porous membrane prepared with 180 mg ODB/ml showed a stable performance up to 1000 mA/cm2 without severe mass transfer limitation, performing better than N212 above 850 mA/cm2. It seems that the porous structure enhances the back diffusion of water from the cathode to the anode, and thereby prevents cathode flooding. The membrane prepared with 150 mg ODB/ml, however showed an unstable performance. The reason for this is not clear. However, looking at the SEM images of the specific membranes used in this experiment (Figure S8), it can be seen that the “180 mg ODB/ml” membrane used here seems to have finer pores, contrary to the generally expected structure.
In previous work we found that membranes with only surface pores can survive mild hot pressing conditions without collapse of the pores. Now we found that fully porous membranes are compressed in the land and non-active areas (Figure S9). The non-compressed channel areas showed still the white appearance indicating porosity after disassembly of the cell. The SEM images of a MEA (after being assembled into a cell) show that the porosity is indeed strongly reduced.
The electrochemical hydrogen crossover current density of N211, N212, and the lab-made dense membrane is in the expected range (Figure 8),[26] and dry membranes are more permeable for hydrogen than wet membranes. The crossover current density of the membrane prepared with 150 mg ODB/ml reached around 40 mA/cm2 at 100% RH and 100 mA/cm2 at 50% RH, because of a severe membrane failure when the humidity of the fuel cell was lowered from 100 to 70 to 50% RH (see also Figure S12 and S13). However, the crossover current density of the membrane made with 180 mg ODB in Fig. 8 is in a range which is below the 10 mA/cm2 often used as indicator for membrane failure in the literature. In conclusion, porous membranes with catalyst
19
particles blended into the polymer could be interesting for the use in self-humidifying membrane systems.
Figure 8: Hydrogen crossover at 70 °C in a fuel cell setup; the membrane prepared with 180 mg ODB/ml is 73 µm thick.
4. Conclusions A one-step solvent evaporation method has been successfully developed to prepare porous Nafion membranes with EW 1100 using ODB as a porogene. Depending on the amount of the porogene, both dense membranes with a porous surface and a novel membrane type, fully porous membranes with larger pores on the surface can be obtained. This in contrast to our previous work (based on Dupont’s SE20092 dispersion), where systematic variation of the ODB concentration only gave access to dense membranes with surface pores.[8,9] Reproducibility of the new method was confirmed by reproduction of the experiments in two laboratories (KIST 20
and NEXT ENERGY). Best results were obtained with an ethanol based dispersion, which was prepared starting from a commercial Nafion dispersion (DE2021) by solvent evaporation and redissolution. The tensile strength of the porous PN membranes was lower than that of commercially available Nafion N211, which is due to their porous structure. The dimensional stability (volume expansion in water) of the porous membranes is excellent at both ambient conditions and 80 °C, probably because the pore walls can expand into the porous structure. The water flux through PN membranes is higher than through Nafion N211 membranes, and a more porous structure on the humid side increased the flux, indicating that interfacial water transport from the wet vapor phase to the membrane limits the water transport. Due to the porous structure, the in-plane proton conductivities of porous membranes are lower than those of dense Nafion membranes of the same thickness. Porous membranes were also tested in the fuel cell, where they performed reasonably well, but showing an increased gas crossover, which however is less than may have been expected, and is still within a reasonable range.
The work presented in this manuscript is basic research, but we believe that the new structural morphology is inspiring and can be the basis for more applied work, e.g. for the development of self-humidifying membranes, or membranes in which the pores are filled with another material.
5. Acknowledgements This work was supported by the Korea-Denmark green technology cooperative research program, by the Ministry of Science, ICT and Future Planning (No. 2015M1A2A2056554), and the KIAT program on Korea-German collaboration (2MR4380).
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Highlights
Nafion membranes with three different morphologies were prepared a) dense Nafion membranes with a porous surface, based on EW 1100 b) mixed dense/porous membranes c) fully porous membranes with closed pores and larger open pores on the surface membranes were prepared in a single step (solution casting)
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