Surface modification of mordenite in Nafion composite membrane for direct ethanol fuel cell and its characterizations: Effect of types of silane coupling agent

https://doi.org/10.1016/j.jece.2016.05.005Get rights and content

Highlights

  • Silane treated mordenite composite membrane was used for DEFC.

  • Four types of silane coupling agents were treated on MOR surface.

  • Sulfhydryl group in MPTES provided sulfonic group for enhance proton conductivity.

  • MOR-MPTES/Nafion membrane had the highest selectivity at all temperature.

Abstract

Mordenite (MOR) has been used to solve the alcohol crossover in direct ethanol fuel cells. However, the lack of compatibility has been a problem. This paper shows the compatibility improvement of MOR in a Nafion composite membrane for use in a fuel cell using four types of silane coupling agents: gamma-glycidoxypropyltrimethoxysilane (GMPTS), (3-mercaptopropyl)trimethoxysilane (MPTS), (3-mercaptopropyl)triethoxysilane (MPTES) and (3-mercaptopropyl)methyl-dimethoxysilane (MPDMS). The coupling agents were used to treat the MOR surface before mixing with Nafion. Each type of silane was treated carefully and differently, depending on its structure. Their characterizations were also described. The results showed that better compatibility and a noticeable reduction in ethanol permeability were achieved when using all silane-treated MOR in the composite. It was found that the Nafion/MOR-MPTES membrane had the highest proton conductivity at all temperature ranges from 30 to 70 °C. This was due to the fact that the sulfhydryl (single bondSH) functional group in MPTES provided the sulfonic group in its structure after the oxidation in the surface treatment process. These sulfonic groups at the MOR surface facilitated proton transport and improved the selectivity of the membrane.

Introduction

At present, the use of hydrogen and methanol as fuels in the fuel cell is common and has been commercially applied widely. Direct ethanol fuel cells have been under focus due to low emission levels produced and low cost of fuel. Compared with methanol, ethanol has a higher energy density and specific energy [1], [2]. Methanol is mainly obtained from petrochemical processes. Therefore, ethanol is considered a good choice for use in fuel cell development as it can be produced from renewable sources. The performance of a fuel cell depends mainly on the type of catalysts and the membrane used. A significant problem with membranes in a fuel cell is the crossover of alcohol through the membrane from the anode to the cathode. Nafion is a common polymer membrane used as a proton exchange membrane in fuel cells, due to its high proton conductivity and good mechanical properties. The structure of Nafion consists of two parts: a hydrophilic part which is a sulfonic acid group, and a hydrophobic part which is polytetrafluoroethylene (PTFE) [3]. It has been found that the selectivity, proton conductivity and methanol permeability ratio of Nafion are higher compared to other polymers such as chitosan and SPEEK membranes. However, the main problem of Nafion as a membrane in direct alcohol fuel cells is the permeation of alcohol from the anode side to the cathode side, which is known as crossover. The permeated alcohol can react with O2 and generate carbon dioxide at the cathode. As a result, the performance of the fuel cell is decreased [4]. In the report of Wang et al. [5], the selectivity values of chitosan and a Nafion® 117 membrane were 1.49 × 104 and 1.71 × 104 Ss/cm3, respectively. In the report of Maab and Nunes [6], the permeability of ethanol for Nafion 117 was 5 times higher than a plain SPEEK membrane. However, it was found that the power density of a fuel cell using a Nafion® 117 membrane was higher than that of a SPEEK membrane. It has been a good choice for selection as a matrix polymer for polymer exchange membrane fuel cells (PEMFC), especially if the ethanol crossover is improved.

Nafion polymer can be incorporated with filler, forming a composite membrane to improve its membrane properties [7], [8], [9]. Amiinu et al. [10] reported that doping Imidazole functionalized meoporous silica into Nafion showed an increase in proton conductivity to 1.06 × 10−2 Scm−1 at 130 °C. The presence of silica-imidazole within the matrix functioned as a transporting medium to facilitate proton conductivity. Kim et al. [11] enhanced the Nafion membrane by coating it with a delaminated AMH-3 (microporous layered silicate)/Nafion nanocomposite layer, which resulted in a low methanol permeability and maintained a high proton conductivity. The methanol permeability of the nanocomposite layer was 1.6 times lower than Nafion 115 due to the smaller opening pore size of the filler particles in the nanocomposite layer than methanol molecules. Thus, they played a role as a barrier and blocked the methanol molecules by a tortuous pathway effect. These results were consistent with Kumar and Kundu’s work [12] that lower methanol permeability was achieved from Nafion coated and laminated with sulfonated polyvinyllidinefluoride (PVDF). Barbora et al. [13] studied neodymium oxide modified Nafion membrane for direct alcohol fuel cells. The pure Nafion and 5 wt.% composite membrane showed that the permeability of ethanol (1.22 × 10−7 cm2/s and 0.85 × 10−7 cm2/s, respectively) was less than that of methanol (1.38 × 10−7 cm2/s and 0.95 × 10−7 cm2/s, respectively) due to the larger molecule of ethanol. It was also found that the tensile strength of the composite membranes were higher than that of the pure recast Nafion membrane. Yen et al. [14] prepared sulfonated-silica/Nafion composite membranes, which have a higher selectivity than those of pristine Nafion. Adding silica-SH and silica-SO3H to the membrane decreased the methanol permeability by approximately 30% and 15%, respectively. Another way to reduce alcohol crossover is by using a zeolite such as NaA-zeolite incorporated with Nafion which has been reported to successfully reduce alcohol crossover [15], [16]. The study found that the methanol permeability of the Nafion composite membrane, incorporated with zeolite-NaA which was treated by APTS silane was up to 55.96% lower than that of a Nafion® 117 membrane. However, it was found that zeolite-NaA was not stable when used in a direct methanol fuel cell (DMFC) for long periods [17]. Various types of zeolite were incorporated with polymers, such as a zeolite beta-filled chitosan membrane. It was found that using zeolite beta can decrease the methanol permeability when compared with a pure chitosan membrane [5]. Devrim and Albostan [18] prepared the Nafion/zeolite composite membrane with different zeolite loading for a low humidity proton exchange membrane fuel cell (PEMFC). The results showed that water uptake and proton conductivity were enhanced due to the water retention properties of the zeolite and interaction between the Nafion and zeolite particles. PEMFC tests showed the 10 wt.% zeolite loading in the composite membrane was more stable and better than the Nafion membrane.

Cui et al. [19] synthesized Nafion-based membrane containing 5 wt.% and 10 wt.% of nano ammonium-X (NH4-X) and submicron NH4-X zeolite. The results showed that the water uptake, ion exchange capacity, and proton conductivity of the submicron 5 wt.% NH4-X zeolite/Nafion composite membrane were higher than the Nafion membrane and the selectivity was more than twice of that of the Nafion membrane. The power density was 62.2 mW cm−2 at 60 °C, which was 3 times higher than that of the Nafion membrane. Moreover, the report of Kongkachuichay and Pimprom [20] showed that when incorporating analcime and faujasite as fillers with Nafion in PEMFC, the H2 permeation of both types of Nafion/zeolite composite membranes was lower than that of the Nafion® 117 membrane. Nevertheless, mordenite (MOR) was suitable for use in a direct alcohol fuel cell [21]. MOR has 12-membered and 8-membered rings, resulting in 6.7 × 7.0 and 2.9 × 5.7 Å channels, respectively. The molecular size of water and ethanol is 2.6 Å [22] and 5.2 Å [23], respectively. Therefore, it can be seen that the ethanol molecule is smaller than the channels of the MOR. This can lead to better adsorption of water than ethanol. In summary, MOR as a type of zeolite has advantages of stability, high proton transport and higher water absorption than methanol or ethanol [24], [25], [26]. It can also be used in other applications [27]. It was found that there were some pinholes caused by poor compatibility between the Nafion polymer and the zeolite crystals which has been a problem reported in several studies [28], [29], [30], [31], [32]. Kwak et al. [33] synthesized MOR/Nafion composite membrane for high-temperature operation of PEMFC using various weight percentages of MOR. It was found that a higher MOR content led to lower tensile strength of the composite membranes (45 MPa for Nafion and 33 MPa for 5 wt.% MOR/Nafion). The strength was as low as 12 MPa for 20 wt.% MOR/Nafion membrane. This was due to poor compatibility between Nafion and MOR particles. The reduction in alcohol crossover and the higher compatibility can be achieved by improving the interfacial properties between the inorganic and organic parts in the composite membrane. In our previous work, Yoonoo et al. [21], 3-aminopropyl-triethoxysilane (APTS) and gamma-glycidoxypropyltrimethoxysilane (GMPTS) were used for MOR surface modification to increase the compatibility. It was found that GMPTS showed the best performance in DMFC, when 5 wt.% of ground and coarse (non-ground) MOR in Nafion were used. The proton conductivity of Nafion/MOR-GMPTS was slightly lower, but its methanol permeability was much lower than that of the recast Nafion membrane. Nafion/MOR-GMPTS also showed better performance in DMFC. The power density of the Nafion/MOR-GMPTS membrane was 1.11 times that of the recast Nafion membrane. A silane coupling agent was also used to modify zeolite surface for membranes used in gas separation [29], [34]. Li et al. [29] reported that the permeability and selectivity of membranes made from silane modified zeolite were higher than those of membrane made from unmodified zeolite because the degree of partial pore blockage was decreased. Zhao et al. [35] reported the improvement of the adhesion strength of nanoparticles modified by grafting a silane coupling agent onto a TiO2 nanoparticles surface. Wang et al. [36] used γ-mercaptopropyltrimethoxysilane with sulfhydryl (single bondSH) functional groups modified on the zeolite powders to improve the surface/channel of the zeolite substrates.

In this study, the focused was on reducing the permeation of ethanol and increasing the compatibility of the membrane that has been a crucial problem in DEFC. The main objective was the synthesis of Nafion/silanated-mordenite composite membranes to reduce ethanol permeability together with increasing the compatibility of the filler and the matrix for DEFC. MOR was used as inorganic filler and four types of silane were used as coupling agents to increase the compatibility. Further investigation of the effect of coupling agents in this study used gamma-glycidoxypropyltrimethoxysilane (GMPTS), (3-mercaptopropyl)trimethoxy silane (MPTS), (3-mercaptopropyl)triethoxysilane (MPTES) and (3-mercaptopropyl)methy-dimethoxysilane (MPDMS) to modify the surface of MOR before composite membrane fabrication. MPTS, MPTES, and MPDMS, which contain a sulhydryl group that can convert to a sulfonic group, have not yet been used to improve the MOR surface, were used in this study to compare with GMPTS, as had been used in our previous studies [21]. These mercapto silanes should benefit from an increase in the sulfuric group (single bondSO3). Therefore, these silanes were applied in this study. 5 wt.% MOR was incorporated with Nafion composite membrane synthesis for use in the direct ethanol fuel cell (DEFC). The chemical and physical properties of the composite membranes were characterized and their ethanol permeability and proton conductivity were systematically investigated.

Section snippets

Materials

Nafion solution was purchased from Ion Power. MOR-Na with a Si/Al molar ratio of 13 was purchased from Zeolyst International. Gamma-glycidoxypropyltrimethoxysilane (GMPTS), (3-mercaptopropyl)trimethoxysilane (MPTS), (3-mercaptopropyl)triethoxysilane (MPTES) and (3-mercaptopropyl)methy-dimethoxysilane (MPDMS) were purchased from Sigma-Aldrich. Sulfuric acid, hydrogen peroxide, ethanol, methanol, N,N-dimethylformamide, dichloromethane, ammoniumcholride, toluene and aluminumtrichloride were used.

Characterization of MOR before and after modification

MOR powder purchased from Zeolyst was ground resulting in an average particle size of 3.5 μm. To ensure that the crystallinity of MOR was not changed due to grinding, protonating and the silane treatment, the particles were examined using XRD. The XRD fingerprints of each treatment and each type of MOR are shown in Fig. 1. It can be seen that the surface-modified MOR shows identical characteristic peaks to the pristine MOR. This meant the crystalline structures of the modified MOR were unchanged

Conclusions

MOR/Nafion composite membranes were synthesized to reduce the ethanol crossover in DEFC. Four types of silanes, having different functional groups of epoxy (GMPTS) and sulhydryl (MPTS, MPTES, and MPDMS), were selected to treat on the MOR surface to improve the compatibility of MOR in the composite membrane for DEFC. They successfully modified the surface of MOR, as confirmed by the FT-IR and TGA results. These silanes improved the compatibility between the organic and the inorganic parts as

Acknowledgements

The authors would like to acknowledge the Thailand Research Fund (TRF) for funding the project TRG5780256. Our thanks are also to the Kasetsart University Research and Development Institute (KURDI). The authors gratefully acknowledge the financial support from the Faculty of Engineering, Kasetsart University Research Development Institute (KURDI), and the Center for Advanced Studied in Nanotechnology for Chemical, Food and Agricultural Industries, Kasetsart University. Thanks are also due to

References (57)

  • M. Jafari et al.

    Optimization of synthesis conditions for preparation of ceramic (A-type zeolite) membranes in dehydration of ethylene glycol

    Ceram. Int.

    (2013)
  • K. Scott et al.

    Performance of the direct methanol fuel cell with radiation-grafted polymer membranes

    J. Membr. Sci.

    (2000)
  • Y. Devrim et al.

    Enhancement of PEM fuel cell performance at higher temperatures and lower humidities by high performance membrane electrode assembly based on Nafion/zeolite membrane

    Int. J. Hydrogen Energy

    (2015)
  • Y. Cui et al.

    Enhancement of Nafion based membranes for direct methanol fuel cell applications through the inclusion of ammonium-X zeolite fillers

    J. Power Sources

    (2015)
  • P. Kongkachuichay et al.

    Nafion/Analcime and Nafion/Faujasite composite membranes for polymer electrolyte membrane fuel cells

    Chem. Eng. Res. Des.

    (2010)
  • C. Yoonoo et al.

    Nafion®/mordenite composite membranes for improved direct methanol fuel cell performance

    J. Membr. Sci.

    (2011)
  • B. Van der Bruggen et al.

    Influence of molecular size, polarity and charge on the retention of organic molecules by nanofiltration

    J. Membr. Sci.

    (1999)
  • Y. Gao et al.

    Sulfonation of poly(phthalazinones) with fuming sulfuric acid mixtures for proton exchange membrane materials

    J. Membr. Sci.

    (2003)
  • T. Hibino et al.

    Protonic conduction of mordenite-type zeolite

    Solid State Ionics

    (1993)
  • V. Baglio et al.

    Investigation of the electrochemical behaviour in DMFCs of chabazite and clinoptilolite-based composite membranes

    Electrochim. Acta

    (2005)
  • T. Zhou et al.

    Fe-mordenite/cordierite monolith for the catalytic decomposition of nitrous oxide

    Ceram. Int.

    (2009)
  • Y. Li et al.

    Effects of novel silane modification of zeolite surface on polymer chain rigidification and partial pore blockage in polyethersulfone (PES)–zeolite A mixed matrix membranes

    J. Membr. Sci.

    (2006)
  • D. Metın et al.

    The effect of interfacial interactions on the mechanical properties of polypropylene/natural zeolite composites

    Composites A

    (2004)
  • J.P. Boom et al.

    Transport through zeolite filled polymeric membranes

    J. Membr. Sci.

    (1998)
  • S.-H. Kwak et al.

    Nafion/mordenite hybrid membrane for high-temperature operation of polymer electrolyte membrane fuel cell

    Solid State Ionics

    (2003)
  • T. Takahashi et al.

    Surface modification of porous alumina filters for CO2 separation using silane coupling agents

    J. Membr. Sci.

    (2016)
  • J. Zhao et al.

    Surface modification of TiO2 nanoparticles with silane coupling agents

    Colloids Surf., A

    (2012)
  • C.I. Horvat et al.

    Perfluorosulfonic acid ionomer–silica composite membranes prepared using hyperbranched polyethoxysiloxane for polymer electrolyte membrane fuel cells

    Int. J. Hydrogen Energy

    (2012)
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