Guanidinium based blend anion exchange membranes for direct methanol alkaline fuel cells (DMAFCs)
Graphical abstract
Introduction
Direct methanol fuel cells (DMFCs) present themselves as a promising power source for application in transportation and electronics. Compared to hydrogen fuel cells, for instance, in construction, cost and safety they are more attractive by virtue of liquid methanol granting higher energy density, improved volumetric fuel supply and storage [1], [2]. However, the widely researched and developed proton exchange membranes (PEMs) used for these fuel cell suffer from severe drawbacks, the most important of which is their high methanol permeability and high price (roughly at least $400 per m2 for the popular Nafion® membrane). These barriers have restricted the mass commercialization of DMFCs. However, use of anion exchange membranes (AEMs) in DMFCs may help relieve these limitations through their several inherent technological advantages: (1) improved electrode kinetics and better catalyst stability in alkaline environment thereby allowing use of the much cheaper non-noble metal catalysts, (2) lower methanol crossover due to the fact that hydroxide transports in the direction opposing that of methanol, and (3) better overall water management with the electro-osmotic drag transporting water away from the cathode [3], [4], [5].
As a result, various kinds of AEMs [6], [7], [8], [9], [10], [11], [12] have been developed in the past few decades and most of them employed the use of quaternary ammonium as anion exchange sites [13], [14], [15]. However, the quaternary ammonium groups are prone to degradation in a strong basic environment, especially at temperatures above 60 °C. In addition, slower migration rate of OH− compared to H+ leads to a lower intrinsic ionic conductivity [16], [17]. Recently, attention has been focused on guanidinium groups for fabricating AEMs and promising results have been shown to potentially overcome the above-mentioned technical barriers [18], [19], [20], [21], [22]. The high basicity of guanidinium has led to conductivity values as high as around 67 mS cm−1 [18] and 80 mS cm−1 [23] at 20 °C. Additionally, membrane stability can be potentially improved due to the inherent charge delocalization from the π-electron conjugated system of the resonance structure [18], [24]. However, guanidinium moieties as ion-exchange groups have been mostly attempted as side-chain groups in AEMs [18], [19], [22], [25]; therefore, the membranes are inevitably prone to chemical degradation due to the OH− attack through the so-called SN2 mechanism [26].
In our previous work, we successfully fabricated guanidinium based composite AEMs reinforced by porous PTFE films to enhance both membrane strength and durability [23]. Particularly, the guanidinium moieties were anchored in the polymer backbone through crosslinking. This helped establish a percolated ionic network in the polymer electrolyte and achieve high membrane conductivity (84.7 mS cm−1) [23]. Though reinforcement using a porous PTFE film improved the mechanical properties; however, impregnation of the hydrophilic guanidinium based polymer into a hydrophobic substrate to get an integrated membrane seems challenging and requires tedious procedure of sample preparation and post-treatment.
In this study we developed guanidinium-chitosan blend polymers, in an attempt to create an improved and easier alternative to the PTFE-reinforced composite membrane. The guanidinium polymer was further tuned with stearate groups which could lead to better membrane integrity and lower fuel crossover. The AEMs were characterized and tested in a direct methanol alkaline fuel cell (DMAFC). This was the first instance to the best of our knowledge that a guanidinium based ion exchange membrane was used in methanol fuel cells and the performance was observed at par with the commercial Tokuyama A201 membrane.
Section snippets
Materials
Guanidinium hydrochloride (GHCl, 99%), chitosan (75–85% deacetylated) and sodium stearate (SS) were purchased from Sigma–Aldrich, and 1, 2-bis (2-aminoethoxy) ethane (AEE) Jeffamine EDR-148 from Huntsman were used as received. Water (DI water, 18 MX cm, Millipore Elix 5), Methanol (HPLC grade, EMD Millipore), Acetic Acid (EMD Millipore), and ethyl alcohol (Decon Labs) were used as solvents.
Synthesis of guanidinium prepolymer
Equimolar mixture of GHCl and a guanidine diamine (AEE) were heated and mechanically stirred in a flask
Results & discussion
Three guanidinium (Gu)-chitosan (Chi) blend AEMs were fabricated according to Fig. 1. These samples are denoted as Gu-Chi5.6, Gu-Chi2.2 and Gu(L)-Chi2.5, where the numbers stand for the mass in grams of chitosan used in the blend with 5.6 g of guanidinium polymer and the capital ‘L’ in the last sample stands for the lipophilic polymeric guanidinium (Scheme 2). They each had a thickness of 55, 75 and 275 μm, respectively. Depending on the amount of solvent left in the polymer slurry, thin
Conclusion
Guanidinium-Chitosan blend AEMs were synthesized and decent performance has been successively demonstrated in DMAFCs. They proved to be promising alternative to the existing commercial membranes because of their ease of fabrication and versatility. In these membranes, chitosan has excellent film forming properties and can provide the much needed reinforcement to the blend membranes while the guanidinium groups act as the ion exchange group. Furthermore, modifying the membranes with the stearate
Acknowledgments
Financial support of this work by Materials Science and Engineering Department and University of Texas at Arlington is gratefully acknowledged.
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