Nafion Membranes Reinforced with Ceria-Coated Multiwall Carbon

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Nafion Membranes Reinforced with Ceria-coated MWCNTs for Improved Mechanical and Chemical Durability in PEMFCc Andrew Baker, Liang Wang, William Johnson, Ajay K Prasad, and Suresh Advani J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp5078399 • Publication Date (Web): 28 Oct 2014 Downloaded from http://pubs.acs.org on November 10, 2014

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The Journal of Physical Chemistry

Nafion Membranes Reinforced with Ceria-Coated MWCNTs for Improved Mechanical and Chemical Durability in PEMFCs Andrew M. Baker, Liang Wang, William B. Johnson, Ajay K. Prasad, and Suresh G. Advani1

Center for Fuel Cell Research Department of Mechanical Engineering University of Delaware, Newark, DE 19716, USA

Abstract A composite membrane consisting of Nafion proton exchange ionomer and ceria-coated MWCNTs (multiwall carbon nanotubes) was prepared by a solution-casting method. Reinforcement due to the presence of MWCNTs provides increased mechanical strength to the membrane, and the addition of ceria improves the membrane’s chemical durability by scavenging free radicals. The ceria coating also insulates the MWCNTs, which helps to preserve the membrane’s low electrical conductivity in the through-thickness direction. The morphology and loading of the CeO2/MWCNT precursor were verified using transmission electron microscopy and thermogravimetric analysis. The mechanical and chemical durability of synthesized composite [CeO2/MWCNT]/Nafion membranes was compared with that of pure Nafion membranes. Composite membranes demonstrated improved tensile strength and dimensional stability during hydration, without significantly affecting electronic or ionic conductivity and maintain equivalent polarization performance in a fuel cell system. They also showed increased durability in an OCV-hold fuel cell test. Such material property enhancements can extend PEMFC membrane lifetime, increasing the economic viability of fuel cell technology.

Keywords: [CeO2/MWCNT]/Nafion composite membrane; Mechanical reinforcement; Chemical durability; OCV-hold test; Tensile strength; Dimensional stability

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Corresponding author: [email protected]

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1. Introduction Polymer electrolyte membrane fuel cells (PEMFCs) are a promising alternative technology to the internal combustion engine for automotive and stationary power applications due to their high power density, low operating temperatures, low emissions, quiet operation, and fast start-up and shutdown. Presently, however, the widespread commercial adoption of PEMFCs has been hindered due to inadequate reliability and durability, and high cost.1 Perfluorosulfonic acid (PFSA) membranes such as Nafion are widely used in PEMFCs because of their high protonic conductivity, low electronic conductivity, and good chemical and thermal stability.2 However, the major issue limiting the durability, and thus the life, of the PEMFC is the deterioration of the PFSA membrane through cyclical mechanical stresses and chemical degradation.3 During fuel cell operation, the membrane experiences mechanical stresses caused by cyclical swelling and shrinking of the membrane due to changes in the fuel cell’s operating temperature or relative humidity (RH). Such hygrothermal stresses can aggravate internal defects formed during processing and/or improper cell assembly, resulting in the formation of perforations, cracks, tears or pinholes which lead to failure of the membrane.4 In order to improve mechanical stability, membranes are typically modified through chemical crosslinking, or physically reinforced with polytetrafluoroethylene (PTFE) membranes, electrospun polymer webs, or inorganic nanoparticles.5,6 Recently, carbon-based nanomaterials such as multi-walled carbon nanotubes (MWCNTs) and graphene have been investigated for potential membrane reinforcement due to their high tensile strength and stiffness.7,8 MWCNT reinforcement increases the tensile strength and dimensional stability of the membrane,9-11 but owing to their high electrical conductivity,12 the introduction of pristine MWCNTs can cause electronic crossover through the membrane, which reduces fuel cell performance.13 Chemical degradation of the membrane is a multi-step process which is initiated by the formation of hydrogen peroxide (H2O2) by the two-electron reduction of oxygen at the cathode. The reaction of hydrogen peroxide with metal impurities, results in the formation of hydroxyl (HO•) and hydroperoxyl (HOO•) radicals.14 These radicals attack carboxyl (-COOH) groups in the membrane and unzip the polymer backbone which result in the thinning of the bulk ionomer, as well as the formation of local defects identical to those formed by mechanical stresses.4, 15 In addition, chemical attacks can exacerbate defects formed by mechanical stresses due to the increased available membrane surface area.5 The


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formation of these deleterious features results in increased gas crossover and reduces cell performance and lifetime.16 Chemical degradation is mitigated through passive routes, such as the synthesis of short side chain or hydrocarbon polymers, or by active routes, such the destruction of hydrogen peroxide and/or free radicals with an antioxidant additive.17 Ceria is a demonstrated antioxidant in biological systems18 and is regenerative in highly acidic media due of its high surface area and ease of redox between 3+ and 4+ ionic states.19,20 Therefore, it is common to add cerium-containing compounds to the fuel cell membrane electrode assembly (MEA) to scavenge free radicals and mitigate chemical attacks.21-29 Wang proposed that ceria nanoparticles possess a positive surface charge when integrated into an acidic PFSA membrane.25 Coms proposed that during free radical scavenging, the surface Ce3+ neutralizes hydroxyl radicals in the MEA by oxidizing to Ce4+ as follows:21 HO• + Ce3+ + H+ → Ce4+ + H2O Then, the Ce4+ is reduced back to Ce3+ by neutralizing hydrogen peroxide and hydroperoxyl radicals through the following reactions: Ce4+ + H2O2 → Ce3+ + HOO• + H+ Ce4+ + HOO• → Ce3+ + O2 + H+ 2Ce4+ + H2 → 2Ce3+ + 2H+ 4Ce4+ + 2H2O → 4Ce3+ + 4H+ +O2 Which It has been shown that ceria loadings as low as 0.5 wt% in the membrane are highly effective in mitigating chemical attacks and extending the lifetime of PFSA membranes in fuel cell durability tests.21,22,24 In this work, we report on a technique of reinforcing Nafion membranes with ceria-coated MWCNTs. The inclusion of MWCNTs improves the tensile properties of the membrane and its dimensional stability during hygrothermal cycling, while the presence of ceria enhances the membrane’s chemical durability and electronically insulates the MWCNTs in order to eliminate electronic crossover through the membrane.



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2. Experimental 2.1 CeO2/MWCNT nanocomposite synthesis Nanocomposite CeO2/MWCNT powder was synthesized using wet chemical techniques.30 Carboxylic acid-functionalized MWCNTs (MWCNT-COOH) were selected to maximize available sites for precipitation of ceria nanoparticles.30 MWCNT-COOH (Cheaptubes.com, 25–50 nm in diameter, 10– 50 µm long, ≥95% purity) were dried as received for 12 hours at 80°C. They were then immersed in 1 M solution of cerium chloride (Sigma Aldrich, 99.9% purity) at a loading of 5 wt% and agitated for 1 hour. A 35 mM solution of potassium hydroxide (Sigma Aldrich, ≥85% purity) was added dropwise to the MWCNT solution until it reached a pH of 8.5-10. The resulting Ce(OH)3/MWCNT solution was then sonicated for 1 hour, and washed through a ceramic filter. Finally, the Ce(OH)3/MWCNTs were calcined in argon for 20 minutes at 450°C to form CeO2/MWCNTs.

2.2 CeO2-MWCNT nanocomposite characterization Transmission electron microscopy (TEM) was performed on the nanocomposite powder samples using a JEOL JEM-2000FX transmission electron microscope with an accelerating voltage of 200 kV. Samples were prepared by depositing suspensions of CeO2/MWCNTs in isopropanol (Fischer Scientific, 99.9% purity) onto lacey carbon on 200 mesh copper TEM grids (Electron Microscopy Sciences). In addition, thermogravimetric analysis (TGA) of the nanocomposite powder was performed on a MettlerToledo TGA/DSC 1 in oxygen. Mass measurements were taken continuously up to 1200°C at a heating rate of 10°C per minute.

2.3 Composite membrane synthesis Membranes were fabricated by solution-casting a mixture of Nafion ionomer and CeO2/MWCNTs nanocomposite powder using a previously developed technique.13 In order to achieve the desired membrane thickness of 35 µm, 2 wt% CeO2/MWCNTs powder was added to the required mass of Nafion D520 PFSA ionomer (DuPont Fluoroproducts). 10 wt% sodium hydroxide (Fisher Scientific, ≥ 98% purity) was also added to the suspension to convert the ionomer to sodium form to reduce aggregations of the MWCNTs in Nafion.10 The suspension was sonicated for 12 hours and dried at 60°C for 12 hours. The dried mixture was dissolved in excess of dimethylacetamide (Sigma Aldrich, ≥99.5% purity) and stirred at 60°C for 4 hours. The suspension was sonicated again for 2 hours and filtered through a PTFE mesh into a stainless steel casting frame heated to 90°C. The membrane was then removed from the casting frame and annealed for 12 hours at 130°C. Finally, it was sulfonated for 4 hours 4 ACS Paragon Plus Environment

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in 0.1 M sulfuric acid (Sigma Aldrich, 95–98% purity) to return the ionomer to acid form. A baseline membrane was synthesized without CeO2/MWCNTs, using identical procedures. 2.4 Composite membrane characterization Electronic and ionic conductivities were measured using 30 mm x 15 mm membrane specimens were soaked in DI water at 90°C for 12 hours. Specimens were removed from the DI water, immediately spread on a glass plate and the in-plane conductivity was measured using two gold probes.. The probes were 10.9 mm wide and spaced 11.9 mm apart. A 400 g mass was placed on top of the probes to ensure proper contact with the membrane sample. Measurements were taken immediately after removal from DI water to minimize changes in membrane temperature and RH. Four-probe DC conductivity measurements were collected using a Keithley 2601A Sourcemeter with a constant voltage of 100 mV. Two-probe electrochemical impedance spectroscopy (EIS) measurements were carried out using a VersaSTAT 3 potentiostat (Princeton Applied Reserarch) with VersaStudio data acquisition software in the frequency range of 1 MHz to 0.1 Hz. Impedance data were fit to a typical Randle's circuit using ZView plotting software (Scribner Associates). The x-intercepts of the resulting impedance plot was taken to be the ionic resistance of the electrolyte. For both measurements, the in-plane conductivity was calculated as: =

where L is the distance between the probes, R is the electronic or ionic resistance, W is the width of the probes and t is the thickness of the membrane. Three replicates of each sample were tested, with the mean and standard deviation of each value calculated accordingly. The tensile properties of the synthesized membranes were tested in accordance with ASTM standards.31 Fiberglass tabs were bonded to the ends of 51 mm × 6.4 mm membrane samples and thickness measurements were taken at three locations along the gage length of the specimen with a micrometer. The specimens were tested at 23°C and 50% RH in an Instron Model 5848 Micro Tester at a strain rate of 10 min−1. The ultimate tensile strength (UTS) was then calculated using the maximum load and average thickness of each specimen. The elastic and plastic moduli were calculated from the initial and final slopes of the resulting stress strain curves.32 Five replicates of each sample were tested, with the mean and standard deviation of each value calculated accordingly.



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To measure dimensional stability and water uptake, 20 mm × 10 mm membrane specimens were dried for 12 hours under vacuum at 120°C. Initial length , thickness , and mass were measured, and then the specimens were soaked in DI water at 60°C for 12 hours. The specimens were removed from the water, immediately spread onto a glass plate and the final hydrated length , and thickness were measured at 23°C and 50% RH . The samples were then gently patted dry using filter paper and the hydrated mass was measured. These steps were performed successively in 10 second intervals to minimize variations in temperature and RH. The dimensional changes and water uptake were calculated as follows:

% Length change = 100

% Thickness change = 100

% Water uptake = 100

Three replicates of each sample were tested, with the mean and standard deviation of each value calculated accordingly. To test fuel cell performance, 50 mm × 50 mm membrane specimens were dried for 12 hours under vacuum at 60°C. The membranes were then hot-pressed between pieces of commercially available catalyst-coated gas diffusion media (FuelCellsEtc, 0.5 mg/cm2 60% platinum on Vulcan, woven carbon cloth) at 130°C for 2 minutes to fabricate the MEA. MEAs were tested in a single cell, with serpentine gas channels and an active area of 10 cm2. An Arbin Instruments FCT fuel cell test stand with MITS Pro software was used to maintain appropriate gas conditions and cell temperature. Accelerated chemical stress testing protocols were adopted from the United States Department of Energy MEA chemical stability protocol.1 The cell was conditioned at 0.5 A/cm2 for 6 hours with reactant gases supplied at 90°C and 100% RH with H2/O2 flowrates of 200/400 sccm. After conditioning, the humidity was reduced to 30% RH and the cell was held at open-circuit voltage (OCV) until the failure criterion of < 0.6V was reached. Every 48 hours, the cell was conditioned for 2 hours, and then polarization and electrochemical gas crossover measurements were taken. Polarization was measured from 0 to 2.5 A/cm2 with steps of 0.1 A/cm2. Gas crossover was measured by linear sweep voltammetry (LSV) which was performed on a VersaSTAT 3 potentiostat with VersaStudio data acquisition software. Voltage was scanned from 0 to 0.7


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V at a scan rate of 2 mV/s in order to isolate the effects of hydrogen crossover.33 The resulting current measurements were averaged from 0.35 to 0.55 V to calculate the gas crossover current.

3. Results and Discussion 3.1 Nanocomposite powder characterization In order to determine the weight percentages of the constituents in the CeO2/MWCNT precursor, thermogravimetric analysis (TGA) was conducted on the precursor in oxygen up to 1200°C. Since neat MWCNTs completely oxidize by 700°C 34, the remaining weight of the precursor is that of ceria. TGA results (Figure 1) showed that the precursor lost 25% of its weight, therefore we can conclude that the precursor consists of ≈ 75% ceria and ≈ 25% MWCNTs by weight. TEM micrographs confirm the high loading of ceria in the precursor. It is observed that the MWCNT walls are decorated by particles of ceria ranging from 0.6 V for 186 hours, which is a 48% improvement over the unreinforced membrane. The results for gas crossover shown in Figure 8 are consistent with the OCV decay results. The recast Nafion membrane experienced a steady increase in gas crossover until 106 hours, while the reinforced membrane was stable until 96 hours. Electrostatic interactions between the positive surface charge of the ceria nanoparticles and the sulfonate groups in the PFSA serve to stabilize cerium in the MEA.25 However, under fuel cell operation, dissolution of the cerium from the membrane and leaching towards the electrodes is observed.38 This can lead to removal of cerium from the system and, thus, ineffective free radical scavenging. Introducing high aspect ratio nanoparticles, such CeO2/MWCNT, could stabilize the cerium in the membrane by increasing the ionic surface area of the particles, and could explain the increased chemical stability presented in this work. The improved OCV, polarization performance, and reduced gas crossover of the [2% CeO2MWCNT]/Nafion membrane in the accelerated chemical stress test demonstrate its ability to scavenge free radicals, which initiate and propagate chemical attacks on the membrane. The improved ability to neutralize these species can lead to extended chemical durability in a PEMFC system.



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4. Conclusions In this study, composite membranes were synthesized for PEMFCs by coating MWCNTs with a layer of ceria and then incorporating them into the Nafion solution. The MWCNTs reinforce the membrane mechanically which increases the tensile strength and in-plane dimensional stability of the composite membrane by 53% and 37% respectively. The ceria electronically insulates the MWCNTs and scavenges free radicals, which maintains equivalent initial fuel cell performance to neat, untreated Nafion, and further improves its lifetime in an accelerated chemical stress test by 48%. This study investigated a single precursor ratio at a single loading in the membrane. Future studies to optimize these variables could further improve the mechanical and chemical properties of the system. Chemical durability can be further improved by addressing the issues of cerium dissolution and leaching from the membrane. Additional studies to better understand this phenomenon are required to enhance the cerium/PFSA interface to further improve cerium stability. This processing route for composite PEMFC membranes demonstrates mechanical and chemical enhancements, which can improve membrane durability in a fuel cell system to extend lifetime and promote the economic viability of PEMFC technology.

5. Acknowledgements This work was conducted under the University of Delaware’s Fuel Cell Bus Program to research, build, and demonstrate fuel cell powered hybrid vehicles for transit applications. This program is funded by the Federal Transit Administration at the Center for Fuel Cell Research at the University of Delaware. We thank Andrew Speese for his assistance with the synthesis of the precursor nanocomposite.

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31. ASTM Standard D882, Standard Test Method for Tensile Properties of Thin Plastic Sheeting, ASTM International, West Conshohocken, PA, 2002. 32. Satterfield, M. B.; Majsztrik, P. W.; Ota, H.; Benziger, J. B.; Bocarsly, A. B. Mechanical Properties of Nafion and Titania/Nafion Composite Membranes for Polymer Electrolyte Membrane Fuel Cells. J. Polym. Sci. Polym. Phys. 2006, 41, 2327-2345. 33. Edmundson, M.; Busby, F. C.; Overcoming Artifacts in Cyclic Voltammetry Through the Use of Multiple Scan Rates and Potential Windows. ECS Trans. 2011, 41, 661-671. 34. Bom, D.; Andrews, R.; Jacques, D.; Anthony, J.; Chen, B.; Meier, M. S.; Selegue, J. P. Thermogravimetric Analysis of the Oxidation of Multiwalled Carbon Nanotubes: Evidence for the Role of Defect Sites in Carbon Nanotube Chemistry. Nano Lett. 2006, 2, 616-619. 35. Spernjak, D.; Mukherjee, P. P.; MukundanMeasurement of Water Content in Polymer Electrolyte Membranes using High Resolution Neutron Imaging. ECS Trans. 2010, 31, 1451-1456. 36. Kusoglu, A.; Hexemer, A; Jiang, R.; Gittleman, C. S.; Weber, A. Z. Effect of Compression on PFSA-Ionomer Morphology and Predicted Conductivity Changes. J. Membr. Sci. 2012, 421-422, 283-291. 37. Wang, L.; Prasad, A. K.; Advani, S. G. Freeze/Thaw Durability Study of MWCNT-Reinforced Nafion Membranes. J. Electrochem. Soc. 2011, 12, B1499-B1503. 38. Stewart, S. M.; Spernjak, D.; Borup, R.; Datye, A.; Garzon, F. Cerium Migration through Hydrogen Fuel Cells during Accelerated Stress Testing. Electrochem. Soc. Electrochem. Lett. 2014, 3, F19-F22.



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Figure Captions: Figure 1 – TGA of MWCNT and CeO2/MWCNTs precursor powder from 23°C to 1200°C in oxygen. Figure 2 – TEM migrographs showing MWCNTs before (a), and after (b) ceria treatment. An individual MWCNT after ceria treatment is shown in (c). Figure 3 – Typical impedance spectra of recast Nafion and [2% CeO2-MWCNT]/Nafion membranes from 1 MHz to 0.1 Hz at 90°C and 100% RH Figure 4 – Typical stress-strain curves for Nafion and [2% CeO2/MWCNT]/Nafion membranes at 23°C and 50% RH. Figure 5 – Dimensional change and water uptake for Nafion and [2% CeO2/MWCNT]/Nafion membranes at 60°C. Error bars represent one standard deviation. Figure 6 – OCV decay of recast Nafion and [2% CeO2/MWCNT]/Nafion at 90°C and 30% RH with H2/O2 flow rates of 100/200 sccm. Gaps in the data are due to electrochemical measurements. Figure 7 – The polarization curves of Nafion before and after reinforcement with 2% CeO2/MWCNTs at various time intervals throughout the OCV hold test at 90°C and 100% RH with H2/O2 flow rates of 200/400 sccm. Figure 8 – Gas crossover of recast Nafion and [2% CeO2-MWCNT]/Nafion membranes during OCV hold.


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Table captions: Table 1 – Conductivity properties of Nafion and [2% CeO2/MWCNT]/Nafion membranes at 90°C and 100% RH Table 2 – Tensile properties of Nafion and [2% CeO2/MWCNT]/Nafion membranes



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Table of contents figure MWCNTs were coated in CeO2 and integrated into Nafion electrolyte membranes to improve dimensional stability and chemical durability


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Figure 1 – TGA of MWCNT and CeO2/MWCNTs precursor powder from 23°C to 1200°C in oxygen 203x153mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2 – TEM migrographs showing MWCNTs before (a), and after (b) ceria treatment. An individual MWCNT after ceria treatment is shown in (c). 86x28mm (300 x 300 DPI)


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Figure 3 – Typical impedance spectra of recast Nafion and [2% CeO2-MWCNT]/Nafion membranes from 1 MHz to 0.1 Hz at 90°C and 100% RH 215x166mm (300 x 300 DPI)



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Figure 4 – Typical stress-strain curves for Nafion and [2% CeO2/MWCNT]/Nafion membranes at 23°C and 50% RH. 203x153mm (300 x 300 DPI)


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Figure 5 – Dimensional change and water uptake for Nafion and [2% CeO2/MWCNT]/Nafion membranes at 60°C. Error bars represent one standard deviation. 215x166mm (300 x 300 DPI)



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Figure 6 – OCV decay of recast Nafion and [2% CeO2/MWCNT]/Nafion at 90°C and 30% RH with H2/O2 flow rates of 100/200 sccm. Gaps in the data are due to electrochemical measurements. 215x166mm (300 x 300 DPI)


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Figure 7 – The polarization curves of Nafion before and after reinforcement with 2% CeO2/MWCNTs at various time intervals throughout the OCV hold test at 90°C and 100% RH with H2/O2 flow rates of 200/400 sccm. 215x166mm (300 x 300 DPI)



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Figure 8 – Gas crossover of recast Nafion and [2% CeO2-MWCNT]/Nafion membranes during OCV hold. 203x153mm (300 x 300 DPI)


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Table 1 – Conductivity properties of Nafion and [2% CeO2/MWCNT]/Nafion membranes at 90°C and 100% RH 61x32mm (300 x 300 DPI)



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Table 2 – Tensile properties of Nafion and [2% CeO2/MWCNT]/Nafion membranes 69x32mm (300 x 300 DPI)


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Nafion Membranes Reinforced with Ceria-Coated Multiwall Carbon

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