Guanidine functionalized radiation induced grafted anion-exchange membranes for solid alkaline fuel cells
Introduction
Polymer electrolyte membrane fuel cells (PEMFCs), which convert chemical energy to electrical energy, are considered as promising future energy sources for both stationary and mobile applications. The major advantages associated with PEMFCs are quick startup, high energy density, high efficiency, quiet operation, and environmental friendliness [1], [2]. However, they commonly involve the incorporation of expensive materials including proton-exchange membranes as the solid electrolyte and platinum-based electrocatalysts [3]. Although some of the proton exchange membranes (PEM) have developed which exhibit high ionic conductivity in combination with excellent mechanical, chemical and thermal stabilities. However, the main limitations associated with current generation PEMs (such as Nafion® by DuPont) include (1) high costs of the membranes and catalysts (2) slow electrode kinetics, (3) high fuel permeabilities, and (4) CO poisoning of Pt and Pt-based electrocatalysts at low temperatures [4], [5].
The low pH working condition for PEMs is regarded as the major cause of slow electrode kinetics. At low pH, oxygen reduction reaction at cathode is electro-kinetically slow, resulting in high cathode activation over potentials which cause the power loss in PEMFCs. Moreover, the electro-oxidation of methanol (6e- reaction) at anode is also an inherently sluggish reaction at low pH, which reduces the efficiency of DMFCs even further due to which high loadings of complex and expensive catalysts (e.g., Pt/Ru of approximately 2–4 mg cm2 metal loadings) [6] is required, increasing overall cost of the PEMFCs.
To overcome such limitations, Alkaline Anion-Exchange Membranes (AAEM) have been developed as an alternative electrolyte for use in low temperature fuel cells: such Alkaline Polymer Electrolyte Fuel Cells (APEFCs) have been shown to exhibit lower fuel permeability, across the AAEM (compared to proton exchange membranes) when used in direct alcohol mode [7], and the ability to utilize cheaper and more abundant non-precious-metal electrocatalysts (especially at the cathode due to a more facile oxygen reduction reaction at high pH) [8], [9], [10]. Further, AEMFCs can make use wide range of fuels such as methanol, ethanol, ethylene glycol, etc. due to their low overpotentials for oxidation of hydrocarbon fuel and lower fuel crossover [11], [12].
Among the various methods used for AEM synthesis, considerable efforts have been focused on the preparation of AEMs through chloromethylation of commodity polymers such as poly(ether sulfone) [13], [14], poly(2,6-dimethyl-1,4-phenylene oxide) [15], poly(phenylene) [16], poly(ether ketone) [17], followed by quaternization and alkalization steps. However, there are three main limitations of these type of AEMs including (1) the use of chloromethyl ether which is a potent carcinogen, required for chloromethylation reactions [18] (2) the poor stability of commonly used quaternary ammonium (QA) groups due to the attack by the strongly nucleophilic OH anions via direct nucleophilic displacements, minor side-reactions involving ylide-intermediates, and/or Hofmann elimination reactions. [11], [19], [20], [21]; (3) the lower ionic conductivities due to the weak alkalinity of QA hydroxides (cf. the strong acidity of the perfluorosulfonic acid groups in PEMs) and resultant poor self-dissociation capability (e.g. trimethylamine has a pKa= 10.8), (4), the anion mobility is lower compare to mobility of protons, due to greater size compare to protons [22].
An obvious target to solve these problems is to develop AAEMs with superior alkali stabilities and ionic conductivities, whilst avoiding excessive swelling (when hydrated) and the use of chloromethyl methyl ethers. To achieve this aim, a series of AEMs were developed through radiation induced grafting of vinylbenzyl chloride monomer onto ultrahigh molecular weight polyethylene (will be represented by PE in the manuscript) powder followed by film formation by melt pressing and subsequent quaternization with guanidine derivative groups. The synthesis of AEMs through radiation-induced grafting of monomers onto fluorinated, partially fluorinated, and non-fluorinated polymer films have been extensively studied and shown to be an effective way of avoiding the use of chloromethyl ether [23], [24], [25], [26], [27] while pentaalkylguanidine is a kind of organic superbase which can potentially increase the ionic conductivity. The simplest member of guanidines is penta-methylguanidine, possesses a pKa value equal to 13.8 while the value of trimethylamine is only 10.8 [28]. This suggests that guanidinium hydroxides will express enhanced OH anion dissociation. This higher alkalinity promises an augmentation of both the number of dissociated hydroxides and their associated water molecules, thereby facilitating hydroxide ion conduction. In addition, the guanidinium groups can take part in conjugation and are strong electron donors, both of which enhance their alkaline and thermal stabilities [29]. Therefore, the use of guanidinium head-groups addresses two major issues (alkaline stability and hydroxide conductivity).
The purpose of grafting vinylbenzyl chloride groups is that they are readily amenable to quaternization, ultimately providing inexpensive polyelectrolyte membrane materials. The main advantages of using PE are that (1) it is inexpensive in comparison with fluoropolymers; (2) polyolefin copolymers generally have excellent bulk physical/chemical properties; (3) it has a tendency for crosslinking upon exposure to gamma irradiation; (4) it is relatively stable towards alkaline conditions [27]. Furthermore the film generated from the grafted powder leads to enhanced ionic conduction effect due to uniform distribution of grafted units. Therefore, the preparation of target AEMs are expected to present advantages including less involvement of toxicity chemicals and higher ionic conductivity and thermal stability. Moreover, to the best of our knowledge, the potential of radiation grafted membranes containing guanidine groups as a hydroxide-conducting material has not been investigated so far. As a consequence, the objective of this study was to synthesize and characterize novel polyethylene based radiation induced grafted membranes containing quaternary guanidinium hydroxide groups and evaluate their properties as alkaline anion exchange membrane materials.
Section snippets
Materials
UHMWPE (MW = 300000-600000) powder, vinylbenzyl chloride (mixture of 3- and 4-isomers, 97%) were purchased from Sigma–Aldrich (Milwaukee, WI, USA). Other chemicals, including, methanol, toluene, sodium hydroxide and hydrochloric acid were analytical grade and were also purchased from Sigma–Aldrich and used as received. Water obtained through Millipore water purification system was used throughout this study. Modified guanidine (1,1,3,3-tetramethyl-2-n-butylguanidine (TMBG) was synthesized
Preparation of AEMs
The AEMs were prepared in few quite simple steps including pre-irradiation of PE powder in an inert atmosphere followed by grafting of VBC, membrane fabrication of the grafted powder by compression moulding and finally quaternization and alkalization step. According to our previous experience of pre-irradiation grafting technique [32], [33], it was observed that 1:1 monomer to solvent ratio is the best suitable to obtain uniform and maximum degree of grafting. Thus, in the present study, 50% of
Conclusions
Anion-exchange membranes (AEMs) were prepared by radiation grafting of VBC onto PE powder, followed by membrane fabrication, quaternization and alkalization. TMBG is used first time in functionalization of the radiation grafted membranes. Mi-GOH membranes were successfully prepared and characterized by FTIR. Mi-GOH membranes were investigated for physical properties which have a direct influence on fuel cell performance. A maximum hydroxide conductivity of 27.7 mS cm−1 was obtained at 90 °C for
Acknowledgments
Funding for the project by the COMSATS Institute of Information Technology and Higher Education Commission of Pakistan is gratefully acknowledged.
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