Poly(ethylene glycol) modified activated carbon for
high performance proton exchange membrane fuel
cells
Li-Duan Tsai
c, Hung-Chung Chien
c, Cheng-Hong Wang
b,
Chien-Ming Lai
b, Jiunn-Nan Lin
b, Chao-Yuan Zhu
c,**
,
Feng-Chih Chang
a,c,d,*
aDepartment of Materials and Optoelectronic Science, National Sun Yat-Sen University, Kaohsiung, Taiwan bMaterial and Chemical Research Laboratories, Industrial Technology Research Institute, Chutung, Hsinchu 310,
Taiwan
cDepartment of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan d
R&D Center for Membrane Technology, Chung Yuan Christian University, Chungli, Taoyuan, Taiwan
a r t i c l e i n f o
Article history:Received 20 March 2013 Received in revised form 24 May 2013
Accepted 14 June 2013 Available online 20 July 2013 Keywords:
Nafion
Activated carbon Water uptake
Proton exchange membrane fuel cell
a b s t r a c t
A high water retention membrane is developed by co-assembling poly(ethylene glycol) (PEG) grafted activated carbon (AC-PEG) with Nafion. The AC-PEG is prepared via a solegel process. The use of PEG as a transporting medium in AC-PEG shows a largely improved water retention ability, a higher proton conductivity and a reduced swelling ratio, making it well suited for proton exchange membrane fuel cells (PEMFCs). Further, the composite membranes show improved mechanical properties at high temperature, thus ensuring the structural stability of membranes during the fuel cell operation. Compositional optimized AC-PEG/Nafion composite membrane (15 wt% compared to Nafion) demonstrates a better performance than the commercially available counterpart, Nafion 212, in fuel cell mea-surements. To identify the key factor of the improved performance, current interrupt technique is used to quantitatively verify the changes of resistance under different relative humidity environment.
Copyrightª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Proton exchange membrane fuel cell (PEMFC) is considered to be one of the most promising alternative energy conversion devices for electric vehicles and residential power generation, due to its high energy density and environmental friendly properties[1,2]. The proton exchange membrane is centered
between anode and cathode, and acts as proton ion conductor and fuel separator at the same time. Polyperfluorosulfonic acid ionomer developed by Dupont, with the trademark of Nafion, is the most extensively used proton exchange mem-brane for PEMFCs because of its excellent chemical stability, low fuel permeability, and high proton conductivity. Nafion consists of hydrophilic pathways for proton transporting and
* Corresponding author. Department of Applied Chemistry, National Chiao Tung University, Hsinchu 300, Taiwan. Tel./fax:þ886 3 5131512.
** Corresponding author. Tel.:þ886 3 571 2121; fax: þ886 3 5131512.
E-mail addresses:[email protected](C.-Y. Zhu),[email protected],[email protected](F.-C. Chang).
Available online at
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0360-3199/$e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.06.054
semicrystalline hydrophobic backbones act as a mechanical constraint to avoid the excessive swelling in water and ensure mechanical stability at high temperature. However, Nafion has some problems of maintaining high proton conductivity under low relative humidity due to dehydration of the mem-brane, thus deteriorating the fuel cell efficiency[3e5]. Nafion membranes require external water management to humidify the fuel and oxygen in order to maintain the proton conduc-tivity. The reduction of PEM thickness addresses the afore-mentioned dehydration problems through back-diffusing water from the cathode[6]. However, the H2and O2crossover
become severe and accelerate the degradation of PEMFCs as the thickness reduces. Therefore, the water retention ability of PEMs is considered as an important requirement for PEMFCs.
Many published strategies have suggested that the intro-duction of hydrophilic materials into the Nafion membrane evidently enhances the intrinsic low-water retention draw-back. Alternatively, incorporation of metal oxide, carbon, clays, etc. into Nafion has proved to efficiently extend the window of working temperature and to collectively act as the reinforcement in the hybrid membranes[7e12]. In addition to hybrid inorganic fillers, carbon nano-materials, such as car-bon nanotube, fullerene, graphene and carcar-bon nanofibers, have emerged as promising additives to simultaneously enhance the mechanical stability and proton conductivity of the PEMs [13e15]. However, introduction of relatively conductive carbon materials into PEMs adversely causes shortage of the fuel cell with lower performance[16]. Nafion membranes infiltrated with carbon by hydrothermal reaction processes high proton conductivity and water retention, while accompanied by a detrimental increase in dimensions that deteriorate the performance of the fuel cell[17]. As a result, there remains a pressing need on identifying a cost-effective and high water retention material for PEMFC application.
Activated carbon (AC) with highly porous morphology on the surface has been considered as a promising water storage material for PEMFC. In our previous study, AC/Nafion com-posite membranes show outstanding water uptake, enhanced storage modulus, and improved proton conductivity at low humidity level[18]. However, the rugged terrain-like surface roughness on the PEM greatly limits the possibility of intro-duction functional adducts through synergistic co-assembly, deteriously increasing the contact resistance. In this article, we modified the AC with poly (ethylene glycol) (PEG) via a solegel process. The suitability of these composite membranes for PEMFC application was explored through systematic study of the PEM, including water uptake, swelling ratio and proton conductivity. The AC-PEG based membranes are found to exhibit smoother surfaces as well as desired properties for PEMFC application, including high water uptake with desired proton conductivity, and excellent single cell performances.
2.
Experimental
2.1. Material
Activated carbon from Osaka Gas was ground with fused Zirconia powder before use. NafionNR212 membrane (50mm)
and Nafion DE2020 (18 wt% in low aliphatic alcohols and water) were obtained from Ion power inc., Delaware, United States. Platinum supported on activated carbon (Pt/C) was used with further purification as received from Tanaka, Japan. 2-[Methoxy(polyethyleneoxy)-propyl] trimethoxy silane (90%) from Gelest was used to modify the AC surface.
2.2. Preparation of poly(ethylene glycol) modified activated carbon
The acidification of AC was conducted by soaking AC powders into a mixture of HNO3/H2SO4(1/5, v/v) under ultrasonicator
for 1 h. Then the solution was diluted with deionized water (DI water) and the product was collected after centrifugation. The acidified AC was iteratively washed with DI water until the pH value reached 7. The product was dried in vacuum oven before further implementation into active device stacks.
To modify the AC, 1 g acidified AC was dispersed in 80 mL DI water with a pH value adjusted to 4.5. Subsequently, 1.1 g 90% of 2-[Methoxy (polyethyleneoxy)-propyl] trimethoxy silane was dissolved in 20 mL DI water and added dropwisely into the acidified AC dispersion while maintaining the reaction tem-perature at 80C and stirring for another 12 h. The resulting mixture was then collected with 0.2mm filter. To remove the unreacted silane in the AC surface, the products were washed with absolute alcohol several times. In the final step, the poly(-ethylene glycol) modified AC (hereafter referred as AC-PEG) particles were dried in a vacuum oven at the 125C for 12 h. Functional groups of AC-PEG particles were experimentally characterized by XPS (X-ray photoelectron spectroscopy, Thermo VGESCAlab 250).
2.3. Preparation of AC-PEG/Nafion composite membranes
Nafion ionomers were transferred to N, N-dimethylacetamide (DMAc, Aldrich) by distilling a mixing solution containing 500 mL Nafion DE2020 solution and 500 mL DMAc under reduced pressure (70C) to remove water and low boiling point solvent. To assemble the AC-PEG/Nafion composite membranes, 5, 10 and 15 weight percentage of a modified AC (compared to Nafion) were added to the Nafion/DMAc solution, respectively. Ultra-sonication was used to ensure stable dispersion and the mixture was further stirred for another 12 h. The composite membranes were prepared by blade-coating with controlled thickness. The wet film was dried at 50C to remove low boiling point solvent and DMAc, and cured at the 140C for 2 h. To activate the proton substitution reaction, the AC-PEG/Nafion composite membrane were soaked in 1 M H2SO4(aq)at the 80C for 1 h. The membrane
was immersed into the water bath (80C) for another 1 h to fully remove the residual acid.
2.4. Structural characterizations
The morphologies of modified AC and composite membrane were observed with a scanning electron microscope (SEM, JEOL JSM-6500F) and a transmission electron microscope (TEM, JEOL JEM-2100F). For the directly characterization for the distribution of the ionic cluster, the membrane was immersed in 1 N lead acetate (Pb(Ac)2) solution overnight to stain the
ionic clusters. The membranes were rinsed with DI water and dried under vacuum at 80 C for 12 h. TEM samples were prepared by sectioned the membranes with an ultramicro-tome and picked up with copper grids.
2.5. Evaluation of the proton conductivity
The proton conductivity was obtained using electrochemical impedance spectroscopy (Solartron 1260 Impedance/Gain-Phase Analyzer, Solartron Analytical, United Kingdom). The impedance was measured from the range of 7 M Hz to 0.1 Hz with a perturbation voltage amplitude 0.01 V.
s ¼ L=AR (1)
where s, L, A, and R refer to the proton conductivity, sample thickness, cross-sectional area and the resistance of the membranes, respectively[18].
2.6. Determination of water uptake and volume swelling ratio and ion exchange capacity (IEC)
The water uptake (WU) was obtained by immersing mem-branes in DI water at the 30C for 24 h to ensure the mem-branes fully saturated. Later, memmem-branes were dried in vacuum oven at 70C and weighed. The WU was calculated by following equation.
WUð%Þ ¼ Wwet Wdry
Wdry 100 (2)
where WU (%), Wwetand Wdryare the water uptake, the weight
of the wet membrane, and the weight of the dry membrane, respectively.
The dimensions of the membranes were measured with a micrometer (Mitutoyo). Each membrane was cut into 5 5 cm2
and recorded the average thickness. Typically, the thickness of the membranes was measured more than four times. The volume swelling ratio was conducted at the 30C for 24 h and calculated according to the volume changes.
Ion exchange capacity (IEC) titration was measured with the following procedure. Typically, 0.1 g Nafion or AC-PEG/Nafion composite membrane was soaked in 50 mL 1 M NaCl(aq)for
12 h. The membrane was taken out and titrated the NaCl(aq)with
0.01 M NaOH(aq)until the pH value reached 7 (pH meter, Suntex
SP-2200). The IEC value was calculated based on the equation
IEC¼ VC=W (3)
where V is the volume used for titration, C is the concentration of the NaOH(aq)and W is the sample weight.
2.7. Water desorption
Water desorption measurements were conducted using a thermal gravimetric analysis (TA, Q5000) to determine weight changes over time at 50C. The water diffusion coefficient was calculated with the following equation.
Mt=MN¼ 4 Dt=pL2
1=2
(4) where: D, Mt/MN, and L are the water diffusion coefficient,
the water desorption, and the membrane thickness, respec-tively[19].
2.8. Dynamic mechanical analysis (DMA) of the membrane
Each sample was cut into 1.5 2 cm2and the thickness was
measured with a micrometer (Mitutoyo) before DMA mea-surement. The mechanical properties were measured by dy-namic mechanical analysis (DMA, TAQ800) using the film/ fiber tension clamp within the temperature range from 35C to 150C (Scanning rate 3C/min).
2.9. Preparation of membrane electrode assemble (MEA) and single cell
The catalyst ink was prepared by mixing Pt/C, Nafion, IPA and DI water. The ink was blade-coating onto gas diffusion layers (GDLs, Sigracet 10BC, SGL Technologies GmbH., Ger-many) with controlled thickness. The active area of the MEA was around 25 cm2while the Pt loading of the anode and
cathode were 0.4 and 0.5 mg cm2, respectively. Then, the anodic and cathodic catalysts were adhered on the pristine Nafion 212 and AC-PEG/Nafion composite membranes by hot pressed (140C) with 20 kg/cm2. The MEA and two pieces of
the gaskets were sealed by two graphite plates engraved with a double serpentine. Finally, the unit cell was properly sealed between two conductive plates under controlled pressure. Detail characterization of IeV polarization had been described in our previous study[20]. The fuel cell system was operated at the 90 C under different relative humidity environment. IeV plots were obtained by potentiostatically decreasing the cell voltage stepwise at intervals of 0.025 V. The measurement of the IeV curve started from 1.0 V and each successive step was maintained for 60 s. The mea-surements were performed until the cell voltage reached 0.4 V. The resistances of the fuel cells were determined using current interrupt technique built into the cell testing system (850e, Scribner Associates Inc. North Carolina, United States) at the voltage of 0.6.
3.
Results and discussion
3.1. Characterization of the PEG modified activated carbon
As shown inFig. 1(a), three peaks originated in 284.6, 286.1 and 288.6 (eV), can be assigned to CeC, CeO and C]O, respec-tively. After a solegel process, a predominant C 1s peak of CeO appeared at 286.0 eV, indicating the presence of ether groups. The great enhancement of CeO intensity can be explained by the active surface modified with 2-[methoxy (poly-ethyleneoxy)-propyl] trimethoxy silane. Moreover, the pro-nounced Si 2p peak evidently confirmed that the AC was successfully modified with PEG. The AC-PEG dispersed ho-mogeneously in Nafion/DMAc solution without any visibly distinquishable agglomeration. The TEM images and the en-ergy dispersive X-ray (EDX) mapping results of the AC-PEG in line with the XPS study (Supporting information (SI) Fig. S1). The EDX mapping results confirmed the successful modifica-tion of the PEGs.
3.2. Characterization of the composite membrane The optical image and the corresponding scanning electronic microscopy (SEM) of the AC-PEG/Nafion composite membrane is shown inFig. 2. The SEM image, as shown inFig. 2(a) pro-vides a close-up view to reveal a slightly rough morphology with some sub-micron AC-PEG embedded in the Nafion membrane. However, the matte black composite membrane shows smooth morphology in optical image as shown in Fig. 2(a) inset. Compared to the AC (Fig. 2(b)), the AC-PEG/ Nafion composite membrane showed a smoother surface contributed to the higher density of AC-PEG. The density was increased from 0.22 to 0.35 g cm3after the modification of PEG. Additionally, the surface of AC-PEG was reduced from original 1688.3 m2g1e251.1 m2g1due to some of the mi-cropores were filled with PEG as shown in SI Fig. S2. The unique structure of AC-PEG, composed PEG as water trans-porting valve and micropores as a water reservoir, showed lower surface roughness with a potential water storage property. The surface roughness of the proton exchange membranes determines the coating technique, e.g. spraying coating and hot press coating with catalyst layers. Membranes with larges roughness show higher contact resistances during the membrane assembly electrodes due to the inhomoge-neous contact area. The TEM images showed inFig. 2(c) and (d) further explain the spatial distribution of the sulfonic clusters. Populated Pb ions, represented the sulfonic groups, were clustered around the surface of the AC which suggested the hydrophilic channels can effectively transport water mole-cules inside and outside the AC. PEG offered an extra area for
ionic clusters and AC to exchange water molecules and transport protons.
3.3. Water uptake, water desorption and proton conductivity of the membrane
As shown inTable 1, the WU of the pristine Nafion membrane was 28.2% and it slightly increased with AC-PEG content to 44.5, 51.9 and 64.5% for the 5, 10 and 15 wt% loading, respec-tively. The introduction of hydrophilic materials effectively improves the hydration of Nafion membranes. Several related studies show significant enhancement on WU by introducing inorganic fillers, such as laponite, sulfonated mesoporous silica and Zirconia, into Nafion matrix[21e24]. Moreover, the volume swelling ratios of the AC-PEG/Nafion membranes tended to decrease with the increasing carbon content. The volume swelling ratio of pristine Nafion was 40.9% and obvi-ously declined to 23.0% for 15 wt% composite membrane. Dimensional stability is an important requirement for a pro-ton exchange membrane since excessive swelling is detri-mental to the assembly of the membrane and results in poor performance as well as long-term durability of the fuel cell. The unique swelling characteristics of AC-PEG/Nafion mem-branes are built up of a non-expandable nature of the AC particles, grafted PEG and the ionic clusters. These promising signs of AC-PEG/Nafion membranes demonstrated here un-derscore the importance of introducing the AC-PEG into the Nafion matrix for controlling WU and minimizing membrane swelling during the relative humidity changes.
Water absorption and diffusion in PEMs has a significant effect on proton conductivity, especially at low relative hu-midity. As shown inFig. 3, the velocity of water desorption was slightly reduced with an increase of AC-PEG. These re-sults were consistent with the findings of the WU mentioned above.
The proton conductivity of pristine Nafion membranes generally increases with higher relative humidity (RH) due to the hydrophilic domains (clusters of the sulfonic group) absorbing more water molecules in the high RH and facilitate the proton transfer easily. InFig. 4(a), the incorporation of AC-PEG systematically enhanced proton conductivity throughout the entire RH range. In addition, the proton conductivity was obvious improved under low RH both at 30 and 80C. The conductivity of 15 wt% AC-PEG/Nafion composite membrane was 3 times higher than pristine Nafion at 35% RH (80C). Proton conductivity is greatly associated with the ion ex-change capacity of the PEM. The IEC of the composite mem-brane partially decreases upon the addition of the filler, indicating that the proton conductivity enhancement was contributed to the better water retention ability of the AC-PEG. Moreover, the DSC result of AC-PEG (SI Fig. S3) showed that
Fig. 2e SEM image of 15 wt% AC-PEG/Nafion (a) (inlet is the optical microscopy) and 15 wt% untreatment AC/Nafion composite membrane and (c), (d) TEM images of 15 wt% AC-PEG/Nafion.
Table 1e The water uptake, swelling ratio and ion exchange value of the Nafion and the composite membranes.
Recasted Nafion 5% AC-PEG 10% AC-PEG 15% AC-PEG
Water uptake (%) 28.2 44.5 51.9 64.5
Volume swelling ratio (%) 40.9 37.1 24.5 23.0
IEC (mmol g1) 0.932 0.913 0.907 0.897
Fig. 3e The water desorption of pristine Nafion membrane and different ratio of AC-PEG/Nafion composite
membranes. The numbers correspond to the water diffusion coefficients (310L5cmL2sL1).
the PEG was at its melted state as the temperature surpassed 60C. The PEG had better molecular movement at its melted state which facilitates water molecule and the proton transfer and thus enhances the proton conductivity. These observa-tions indicated that the better proton conductivity of the composite membranes at low humidity environments could be traced back to their superior water uptake, a synergistic effect of high porosity structure and PEG. The illustration of the water transportation of the AC-PEG is shown in the Fig. 5(c). The water molecules could be transfered by PEG into AC because the oxygen on PEG backbone had a high affinity with water molecules through hydrogen bonding, providing effective transportation.
3.4. Mechanical properties of the composite membrane Nafion membranes exhibited different mechanical properties and proton conductivity, depending on the recasting solvent and annealing temperature [24,25]. In this study, all the composite membranes were recast from Nafion/DMAc dispersion (see theExperimentalsection) with identical dry-ing and annealdry-ing processes. The incorporation of AC-PEG particles to the Nafion membranes significantly reinforce the mechanical properties as shown in Fig. 5(a). The storage modulus of the composite membranes enhanced as the loading increased. The obvious improvement in storage
modulus was contributed to the physical cross-linking network between the hydrophilic side chain of Nafion and functionalized AC. The storage modulus of 15 wt% AC-PEG/ Nafion was higher than pristine Nafion by a factor of 2.78 and 4.91 at 40 and 90 C, respectively. The mechanical en-hancements greatly extended the window of fuel cell opera-tion temperature. Nafion composites with high mechanical strength have been extensively studied such as hybrid sys-tems based on TiO2, SiO2, clays and carbon materials[26e28].
In practical applications, the thinner and stiffer PEMs are known to reduce internal resistance without the possibility of generating pinhole and crack. Moreover, during the MEA fabrication, the high mechanical strength membrane allows a wide range of assembling pressure without wrinkle forma-tion, excluding the thickness differences and poor adhesion.
The temperature-dependent tand curves of the composite membranes, obtained from DMA (Fig. 5(b)), reflected the limited interface between Nafion molecules and the AC-PEG particles. The a relaxation is related to the mobility of the hydrophilic site of the polymer chains. Nano-filler/Nafion composite membranes typically display an obvious tempera-ture increase of the a relaxation due to large organ-iceinorganic interface[29,30]. Unlike previous studies, only the minor displacement in this study indicated the limited interface between Nafion and AC-PEG because of submicron size of the AC-PEG particles.
Fig. 4e Proton conductivity of the pristine Nafion membrane and different loading percentage of AC-PEG/Nafion composite membranes as function of RH at (a) 30C and (b) 80C. The mechanism of water transfer of the AC-PEG is shown in (c).
3.5. Performance of the PEMFC under different humidity conditions
The performance of fuel cell under different humidity condi-tion is associated with the properties of the PEMs. The output characteristics of the PEMFCs were measured at 90C with controlled relative humidity. The polarization curves for a PEMFC composed of Nafion 212 and 15 w% AC-PEG/Nafion composite membranes are shown inFig. 6. The performance of Nafion 212 dropped drastically with decreasing RH. At a cell voltage of 0.60 V, fuel cells exhibited current densities of 857 mA cm2, 650 mA cm2, and 187 mA cm2at 100, 50 and 20% RH, respectively. Additionally, the maximum power density dramatically decreased from 582 to 119 mW cm2as the RH was reduced from 100% to 20%. The 15 wt% AC-PEG/ Nafion composite membrane revealed similar output
characteristics with Nafion 212 at high RH condition (100% RH). However, at 50% RH, the composite membrane had the current density 735 mA cm2(0.60 V) and maximum power density 525 mW cm2that was obviously better than Nafion 212. Moreover, at the critical condition (20% RH), the com-posite membrane had nearly twice current density and maximum power density compared to Nafion 212. The great improvement of the output characteristics of the AC-PEG composite membranes, especially at low RH, was a result of the introduction of high water retention material.
The resistance of the fuel cell under different RHs was determined using current interrupt technique. The re-sistances for PEMFC of the Nafion 212 and 15 wt% AC-PEG/ Nafion composite membrane under different RHs are shown inFig. 7. Both membranes showed the identical discharging current density and resistance above 90% RH. However, as the RH decreased, the cell resistances gradually increased because of the dehydration of the Nafion and AC-PEG/Nafion composite membrane. Nafion 212 showed a poor current density and high resistance (59 mU cm2) at 10% RH. In
contrast, the AC-PEG/Nafion composite membrane exhibited only 22.2 mU cm2which was much lower than Nafion 212
membrane. In this light, the AC-PEG/Nafion composite Fig. 5e DMA traces of pristine Nafion and AC-PEG/Nafion
composite membranes; (a) Storage modulus and (b) tand. Color denote; pristine Nafion (black), 5 wt% AC-PEG (red), 10 wt% AC-PEG (green) and 15 wt% AC-PEG (blue). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 6e Polarization curves of PEMFC with (a) commercial Nafion 212, (b) 15 wt% AC-PEG/Nafion under different relative humidity test conditions at 90C in H2/Air system.
membrane could absorb water molecules from the cathode and acted as a water reservoir and thus maintain higher proton conductivity.
4.
Conclusions
A high water retention AC-PEG/Nafion composite membrane was developed and characterized. XPS fitting results showed that the AC particles were indeed modified with PEG. The composite membranes exhibited high water uptake, lower water desorption rate and improved proton conductivity. Additionally, the incorporation of the AC-PEG with Nafion greatly enhanced the mechanical properties, affording to varied assembly process and showed great potential for practical application. The performance of PEMFC composed of 15 wt% AC-PEG/Nafion composite membrane showed superior current density and peak power density over the commercial Nafion 212. The resistance results during the fuel cell operation conclusively explained that the enhanced performance of the composite membrane was contributed to the high water retention ability of the composite membrane. The PEG modified activated carbon could be adopted for the mass production of proton exchange membranes for effective energy conversion fuel cell.
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2013.06.054.
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