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