Enhancement in proton conductivity and methanol cross-over resistance by sulfonated boron nitride composite sulfonated poly (ether ether ketone) proton exchange membrane

https://doi.org/10.1016/j.ijhydene.2019.06.091Get rights and content

Highlights

  • Sulfonated boron nitride (SBN) Composite PEM shows the better performance.

  • Sulfonated Boron nitride may be the unique material for PEM.

  • Composite membranes are found to be mechanically stable in electro-chemical environment.

Abstract

Fuel cells are the promising new non-conventional power source for vehicles as well as portable devices. Direct methanol fuel cell (DMFC) is especially attractive since it uses low cost liquid methanol as a fuel. Proton exchange membrane is one of the most crucial part of DMFC. Herein, we synthesized the sulfonated boron nitride (SBN) based SPEEK composite membranes for the DMFC application. SBN was synthesized by covalent functionalization of hydroxylated BN by using 3-mercaptopropyl trimethoxysilane and sulfonated by subsequent oxidation of mercapto group. Sulfonated poly (ether ether ketone) is used as a polymer matrix for SBN. With well controlled content of SBN into SPEEK matrix exhibit high proton conductivity, IEC and water content along with excellent mechanical strength. Composite membranes show low methanol cross over and high selectivity, which makes them attractive candidate for proton exchange membrane for direct methanol fuel cells.

Introduction

Fuel cell technology have attracted great attention of researchers because it is a promising alternative for future energy needs combined with ultra clean environment [1]. Among fuel cell technologies, direct methanol fuel cells (DMFCs) have been well-thought-out as a striking renewable energy cradle for both portable and automobile applications because of their high energy conversion competence, simple running operation, extraordinary power density, low greenhouse gas emissions and low cost of fuel [2], [3], [4], [5], [6], [7], [8], [9], [10]. Proton exchange membranes are the key component of DMFCs owing to their unique property of providing an ionic pathway for proton transfer [11]. State-of-the-art, Nafion® is mainly used as polymer electrolyte in direct methanol fuel cell due to its extraordinary property like good chemical and thermal resistance and high ionic conductivity. However, commercialization of the direct methanol fuel cell using State-of-the-art as polymer electrolyte is inhibited by some major technical problems. It has been found that 40% of the methanol (fuel) from anode to cathode crosses through Nafion® membrane results in electro-osmotic drag. Along with high methanol cross over, reduction in water retention at high temperature and its high cost restricts its use in DMFCs [12], [13], [14], [15], [16]. These drawbacks of the State-of-the-art gave way to researchers to find out new materials for proton exchange membranes with minimal methanol crossover, while maintaining good proton conductivity.

Thus, it is necessary to develop and test new materials that reduce the fuel cross over without compromising proton conductivity as well as thermal and chemical resistance of polymer electrolyte membrane. In view of this, various proton exchange membranes based on sulfonated aromatic polymers like poly (ether sulfone) [17], [18], [19], [20], poly (ether ether ketone) [21], [22], poly (4-vinylpyridine) [23], polybenzimidazole [24] and aliphatic polymers like polyvinyl alcohol [12], natural polymers alginate and chitosan [25], [26] have been extensively studied as composite membrane electrolytes and attempts have been made to develop a suitable DMFC membrane.

Among various proton exchange reported till date, Sulfonated poly (ether ether ketone) (SPEEK) has been considered as a promising alternate of State-of-the-art in DMFCs [27], [28], [29]. PEEK is a semi-crystalline polymer combined with its low cost. SPEEK possess low methanol cross over compared with Nafion and exhibit excellent thermal, mechanical and chemical resistance [30], [31]. Poor methanol crossover using SPEEK membrane resulted from the poorly interconnected hydrophilic ionizable sulfonic acid groups connecting to a hydrophobic backbone (methanol transport channels). Ionic domains filled with sulfonic acid groups in the hydrated state are short range that is reason behind poor proton exchange capacity of SPEEK [32], [33]. PEEK has been sulfonated using concentrated sulfuric acid and degree of Sulfonation can be varied by varying reaction parameters to get high proton conductivity and proton exchange capacity [34]. The high degree of sulfonation, however, renders the SPEEK membranes high water uptake leads in solubility in aqueous media as well as aqueous methanol solution, and highly sulfonated PEEK result in poor mechanical strength, which strongly hinders its direct uses as the PEMs [35], [36]. Great efforts have been made to improve proton conductivity and mechanical stability of SPEEK membranes by several strategies such as blending with organic and inorganic fillers like zeolites [37], silica [38], polymer blends [39], [40], [41], graphene oxide [42], [43], [44], semi-interpenetrating polymer networks [35], [45], [46], using composite SPEEK composite membranes swelling behavior and proton conduction can be effectively addressed. Boron nitride found to be a very good proton conductor used as proton exchange membranes in different applications [47].

In this work new type of sulfonated boron nitride (SBN) have been introduced into SPEEK matrix for direct methanol fuel cell application, due to following reasons: firstly SBN has graphene like two dimensional layered structure which facilitates proton transfer and increase overall proton conductivity of composite membranes, secondly, SBN itself mechanically, thermally and chemical resistant, provide stability to SPEEK/SBN membranes and last SBN contains numerous sulfonic acid groups on its surface, not only they provide high proton exchange capacity to SPEEK/SBN membranes but also make hydrogen bond interaction with SPEEK. To the best of our knowledge, first time SPEEK/SBN composite membrane were prepared and tested against methanol permeability measurements for direct methanol fuel cell.

Section snippets

Materials

Poly (ether ether ketone) (PEEK) was supplied by Solvay Chemical and vacuum dried before use. (3-Mercaptopropyl) trimethoxysilane and Boron nitride powder (BN) with particle size of ∼1 μm were purchased from Sigma Aldrich. Dimethylacetamide (DMAc), toluene, KMnO4, H2O2, H2SO4, HCl, methanol and NaCl of analytical reagent (AR) grade were obtained from SD Fine Chemicals India and used as received without further purification. Distilled water used throughout the experiments.

Sulfonation of poly (ether ether ketone) (PEEK) and boron nitride (BN)

Sulfonation of PEEK was

Structural and chemical characterization of BN and composite membranes

XRD pattern for BN, fBN and SBN are presented in Fig. 1 and the characteristic 2θ peaks were observed at 26.40, 41.36, 54.76 and 75.92 with their corresponding d-spacing of 3.37, 2.18, 1.67 and 1.25, and miller indices (002), (100), (004) and (112), respectively [50]. It was observed that on functionalization of BN with silane coupling agent results in increased peak intensity due to disoriented stacking of BN layers and leads in change in scattering factors.

Further attachment of silane

Conclusion

To the best of our knowledge for the first time, current study describes the sulfonation of BN using organosilane coupling agent and its subsequent impregnation to sulfonated poly (ether ether ketone) to form composite membranes. The presence of SBN inside polymer matrix the methanol transport channels becomes more compact with improved tortuosity, results in less methanol crossover. Functionalization process of BN increase ion exchange moiety on its surface and further increase IEC and proton

Acknowledgement

Authors are thankful to UGC-DAE CSR, Indore, Department of Science and Technology, India and Russian Foundation for Basic Research, Russia for providing financial support to carry out the work. Analytical and Environmental Science Division and Centralized Instrument Facility, CSIR-CSMCRI, Bhavnagar is greatly acknowledged for instrumental support.

References (60)

  • P.P. Sharma et al.

    Acid resistant PVDF-co-HFP based copolymer proton exchange membrane for electro-chemical application

    J Membr Sci

    (2019)
  • L.Y. Li et al.

    Polybenzimidazole membranes for direct methanol fuel cell: acid-doped or alkali-doped?

    J Power Sources

    (2015)
  • M. Han et al.

    Sulfonated poly (ether ether ketone)/polybenzimidazole oligomer/epoxy resin composite membranes in situ polymerization for direct methanol fuel cell usages

    J Power Sources

    (2011)
  • C. Li et al.

    Sulfonic acid functionalized graphene oxide paper sandwiched in sulfonated poly (ether ether ketone): a proton exchange membrane with high performance for semi-passive direct methanol fuel cells

    Int J Hydrogen Energy

    (2017)
  • A. Shukla et al.

    A facile synthesis of graphene nanoribbon-quantum dot hybrids and their application for composite electrolyte membrane in direct methanol fuel cells

    Electrochim Acta

    (2019)
  • A. Shukla et al.

    Simultaneous unzipping and sulfonation of multi-walled carbon nanotubes to sulfonated graphene nanoribbons for nanocomposite membranes in polymer electrolyte fuel cells

    J Membr Sci

    (2016)
  • T.Y. Inan et al.

    Sulfonated PEEK and fluorinated polymer based blends for fuel cell applications: investigation of the effect of type and molecular weight of the fluorinated polymers on the membrane's properties

    Int J Hydrogen Energy

    (2010)
  • X. Liu et al.

    Semi-interpenetrating polymer networks toward sulfonated poly (ether ether ketone) membranes for high concentration direct methanol fuel cell

    Chin Chem Lett

    (2019)
  • M. Li et al.

    Cross-linked polyelectrolyte for direct methanol fuel cells applications based on a novel sulfonated cross-linker

    J Power Sources

    (2014)
  • S. Sambandam et al.

    SPEEK/functionalized silica composite membranes for polymer electrolyte fuel cells

    J Power Sources

    (2007)
  • T. Yang et al.

    SPEEK/sulfonated cyclodextrin blend membranes for direct methanol fuel cell

    Int J Hydrogen Energy

    (2011)
  • A. Sahin

    The development of Speek/PVA/TEOS blend membrane for proton exchange membrane fuel cells

    Electrochim Acta

    (2018)
  • C. Manea et al.

    Characterization of polymer blends of polyethersulfone/sulfonated polysulfone and polyethersulfone/sulfonated polyetheretherketone for direct methanol fuel cell applications

    J Membr Sci

    (2002)
  • Z. Jiang et al.

    Sulfonated poly (ether ether ketone) membranes with sulfonated graphene oxide fillers for direct methanol fuel cells

    Int J Hydrogen Energy

    (2013)
  • Y. Heo et al.

    The effect of sulfonated graphene oxide on sulfonated poly (ether ether ketone) membrane for direct methanol fuel cells

    J Membr Sci

    (2013)
  • A.K. Mishra et al.

    Enhanced mechanical properties and proton conductivity of Nafion–SPEEK–GO composite membranes for fuel cell applications

    J Membr Sci

    (2014)
  • Z. Zhao et al.

    Electrochemical properties of SPEEK/epoxy semi-interpenetrating network composites as proton exchange membrane

    Int J Electrochem Sci

    (2018)
  • G. Mittal et al.

    Processing and characterization of PMMA/PI composites reinforced with surface functionalized hexagonal boron nitride

    Appl Surf Sci

    (2017)
  • S. Gahlot et al.

    White graphene based composite proton exchange membrane: improved durability and proton conductivity

    Int J Hydrogen Energy

    (2018)
  • G. Rambabu et al.

    Amino acid functionalized graphene oxide based nanocomposite membrane electrolytes for direct methanol fuel cells

    J Membr Sci

    (2018)
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