Measurements of water uptake and transport properties in anion-exchange membranes
Introduction
Alkaline fuel cells allow the use of non-platinum (Pt) catalysts because of the inherently fast electro-kinetics in the alkaline environment, thus greatly reducing the cost of the fuel cell systems. Although promising, conventional liquid electrolyte-based alkaline fuel cells are hindered by the poisoning problem of carbon dioxide [1], [2]. However, with the emergence of anion-exchange membranes, anion exchange membrane-based alkaline fuel cells have recently received increased attention [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15]. In particular, the direct ethanol fuel cells that employed the A006 anion-exchange membrane (Tokuyama, Japan) have demonstrated high performance [16], [17], [18].
The water uptake and water transport properties through membranes are of paramount importance in their applications to fuel cell systems. In developing acid proton electrolyte membrane fuel cells (PEMFCs), the water uptake and transport properties of Nafion® membranes, a perfluorosulfonic acid polymer with high proton conductivity developed by DuPont, have been extensively investigated [19], [20], [21], [22], [23], [24], [25], [26], [27], [28]. Zawodzinski et al. [19] measured water uptakes of a Nafion® membrane immersed in liquid water and exposed to water vapor of variable water activity, and indicated that the water uptake in equilibrium with liquid water was 22.0, while the water uptake in equilibrium with water vapor increased with the relative humidity. Hinatsu et al. [20] investigated the water uptake with three pretreatment procedures, and showed that the water uptake depended on the immersion temperature, the ion exchange capacity and the pretreatment conditions. Morris et al. [21] indicated that the water diffusivity was a strong function of the water uptake, and the maximum value was presented at the water uptake of 4.0 (mol H2O/mol −SO3−). Weidner et al. [22] showed that the water diffusivity linearly increased with increasing water uptake. Ren et al. [23], [24] reported that the electro-osmotic drag (EOD) coefficient increased significantly with temperature from 2.0 at 15 °C to 5.1 at 130 °C by an in-situ method in direct methanol fuel cell (DMFC). Recently, Hsing et al. [25] investigated the absorption, desorption, and transport of water in a Nafion® membrane, and indicated that the mass-transfer coefficient for absorption of water was much lower than that for desorption of water.
For anion-exchange membranes, efforts so far have mainly been confined to the synthesis and characterization of membranes [29], [30]. Slade et al. [29] prepared a series of FEP-based anion-exchange membranes with the radiation-grafting method and showed that OH- conductivity could be as high as 0.023 S cm−1 at 50 °C, and the water content and water uptake in equilibrium with liquid water were around 50% and 28.0, respectively. Stoica et al. [30] synthesized an anion-exchange membrane that was composed of two cyclic diamines and demonstrated that this membrane was good in terms of ionic conductivity and thermal stability up to 220 °C. The water uptake in liquid water was 20 at 40 °C. Yamanaka et al. [31] studied the electro-osmotic drag coefficient in an anion-exchange membrane immersed in aqueous solutions of HNO3 and KOH. And the electro-osmotic drag coefficients were determined to be 17.0 and 4.0 in HNO3 and KOH solutions, respectively. Recently, Varcoe [42] studied the effect of relative humidity on the water content and hydroxide ion conductivity of quaternary ammonium-based anion-exchange membranes at 30 °C, and exhibited a hydroxide ion conductivity of 0.030 ± 0.005 S cm−1 without additional incorporation of metal hydroxide salts. Stoica et al. [43] evaluated the physicochemical and electrochemical characteristics of the anion-exchange membranes based on the poly(epichlorhydrin-allyl glycidyl ether) copolymer, and found that ionic conductivity was particularly sensitive to relative humidity, and the best conductivity of 0.013 S cm−1 was obtained at 60 °C and a relative humidity of 98%. More recently, Abuin et al. [44] studied the quaternized polysulfone membrane properties, including water and water-methanol uptake, electrical conductivity and Young’s modulus, the experiment indicated that the anionic polysulfone membrane sorbed more water than Nafion® all over the whole range of water activities, but it uptaked much less methanol as compared to Nafion®.
The objective of this work was to investigate the water uptake and transport properties through the A201 membrane. In order to in-situ determine the EOD coefficient of the membrane in an anion-exchange membrane based direct ethanol fuel cell (AEM-DEFC), the mass-transfer coefficient of water at the interface between the membrane and cathode catalyst layer (CL), which is termed as ‘interfacial mass-transfer coefficient’ hereafter, was also investigated. It is well understood that the transport properties of membranes are a strong function of water uptake [21]. Therefore, in this work, we first present a new method for determining the water uptake of the anion-exchange membrane in equilibrium with the water vapor, followed by presenting the theory for determining the water transport properties of the anion-exchange membrane, including the water diffusivity, the EOD coefficient, and the mass-transfer coefficient of water at the cathode CL/membrane interface.
Section snippets
Determination of the water uptake
The water uptake of the membrane, λ, defined as the average number of water molecules per the conducting functional group, is determined by:where IEC represents the ion-exchange capacity (i.e., OH− ion content per gram of polymer, mmol g−1), Mw, 18 g mol−1, is the molecular weight of water, andis the water content of the membrane, with md and mh representing, respectively, the mass of the dry membrane and the same membrane after hydration. To determine md, a membrane
Determination of water transport properties
This section describes the theory for determining the water transport properties of the anion-exchange membrane, including the water diffusivity, the EOD coefficient, and the mass-transfer coefficient of water at the cathode CL/membrane interface. As shown in Fig. 3, an AEM-DEFC consists of sequentially an anode flow field, an anode diffusion layer (DL), an anode catalyst layer, an anion-exchange membrane, a cathode CL, a cathode DL, and a cathode flow field. Under open-circuit conditions, the
Water uptake
The water uptake and the water content of the anion-exchange membrane in equilibrium with liquid water are shown in Table 1: it can be seen that the water uptake increased from 17 to 19 when the temperature increased from 30 to 60 °C (the corresponding water content was 55% and 62%, respectively). And these values are close to that reported by Stoica [43] using a poly(epichlorhydrin-allyl glycidyl ether) copolymer-based anion-exchange membrane, in which the water uptake is 18 at 25 °C. The trend
Conclusions
As compared with their counterparts – cation-exchange membrane, anion-exchange membranes are an emerging technology that offers the promise of reducing the catalyst cost, while achieving high power density of direct alcohol fuel cells. Hence, understanding of water transport properties through this new type of membrane becomes essential. In this work, we have presented a new method to measure the water uptake of anion-exchange membranes. With the obtained water uptakes, the water transport
Acknowledgements
The work described in this paper was supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (Project No. 623008).
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