Correlation of structural differences between Nafion/polyaniline and Nafion/polypyrrole composite membranes and observed transport properties

https://doi.org/10.1016/j.memsci.2011.01.025Get rights and content

Abstract

Polyaniline/Nafion and polypyrrole/Nafion composite membranes, prepared by chemical polymerization, are studied by scanning electron microscopy, infrared and nuclear magnetic resonance spectroscopy. Differences in vanadium ion diffusion through the membranes and in the membranes’ area specific resistance are linked to analytical observations that polyaniline and polypyrrole interact differently with Nafion. Polypyrrole, a weakly basic polymer, binds less strongly to the sulfonic acid groups of the Nafion membrane. Infrared spectroscopy results suggest that the hydrophobic polymer aggregates in the center of the Nafion channel rather than attaching to the hydrophilic walls containing sulfonic acid groups. This results in a drastically elevated membrane resistance and only slightly decreased vanadium ion diffusion compared to a Nafion membrane. Polyaniline, on the other hand, polymerizes along the sides of the Nafion pores and on the membrane surface, binding tightly to the sulfonic acid groups of Nafion, polyaniline's greater basicity possibly causing the difference in polymerization behavior. This leads to a more effective reduction in vanadium ion transport across the polyaniline/Nafion membranes and the increase in membrane resistance is less severe. The performance of selected polypyrrole/Nafion composite membranes is tested in a static vanadium redox cell. Increased coulombic efficiency, compared to a cell employing a pure Nafion membrane, further confirms the reduced vanadium ion transport through the composite membranes.

Research highlights

► In this study we compare transport properties of different Nafion-based composite membranes. ► The basicity of a polymer allows the prediction of composite membrane properties. ► Polyaniline/Nafion composite membranes are predominantly cation cross-transport suppressing. ► Polypyrrole/Nafion composite membranes mainly reduce the proton conductivity.

Introduction

Proton conducting cation-exchange membranes are used in a wide variety of applications, such as fuel cells [1], [2], [3], batteries [4], [5], electrochemical devices [6], [7] and sensors [8], [9]. At present Nafion® (DuPont) is the most commonly used material to fabricate proton conducting membranes. But for membranes in many of these applications good proton conduction alone is not enough to ensure good device performance. For example, membranes used in redox flow batteries need to exhibit selective cation permeability as well as good proton conduction. Cation transport through Nafion, however, is non-selective, allowing all types of cations other than just protons to cross over as well, which leads to self-discharge in this type of batteries.

Vanadium redox flow batteries (VRFBs) are stationary energy storage systems that allow long-term storage of energy in the multi megawatt range [5], [10], [11]. VRFBs are the most promising solution under consideration for balancing power supply and demand as the percentage of energy from renewable sources increases, and first commercial systems are already available today [4], [12]. As the name implies, energy is stored in, and released from, two electrolyte solutions containing the redox couples V4+/V5+ and V2+/V3+, respectively. The two electrolyte solutions, in the positive and negative half-cell, are separated by a proton conducting membrane acting as a separator [5], [13]. The demands on the membrane in these systems are high. Besides exhibiting good proton conduction properties, they need to be chemically stable in the extremely acidic, highly oxidative environment of the electrolyte solutions, they need to be electrochemically inert during VRFB cycling, and they should exhibit low overall crossover of electrochemically active species through the membrane [14]. Currently Nafion is the material of choice in all kinds of redox flow batteries. The high price of Nafion membranes (41% of the total cost of a VRFB cell stack [15]) is one reason that the technology has not yet been implemented more widely. Additionally, the maintenance cost over the lifetime of a battery is closely related to membrane performance. The less cation-selective the cross-transport through the membrane is, the more often the electrolyte solutions need to be refreshed to their original compositions. Nafion exhibits relatively high crossover rates for the different vanadium ion species in the two electrolyte solutions [16]. Furthermore, V4+ ion binding inside the membrane, irreversible under VRFB operating conditions, has been identified as a major source of fouling for Nafion membranes [17]. Consequently, research on VRFBs in many groups is focused on either replacing Nafion membranes with a different, less expensive material [18], [19], [20] or modifying its properties to minimize Vx+ ion diffusion [21], [22], [23].

Polyaniline/Nafion composite membranes [3], [24] and polypyrrole/Nafion composite membranes [25], [26], [27] have separately been investigated as cation-repellant layers for fuel cell applications [3], [24], [25], [26] or use in VRFBs [27]. This study, to our knowledge, for the first time directly compares the diffusion properties and membrane resistance between the two types of composite membranes. Differences in the physical properties led to an investigation of morphology and chemical structure of the composite membranes by scanning electron microscopy (SEM), Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopy, which revealed fundamental differences in the interaction between Nafion and polyaniline or polypyrrole, respectively.

Section snippets

Materials synthesis

All chemicals used in this study are commercially available from Sigma–Aldrich and were used without further purification. Sheets of Nafion® 117 membranes (N117) were purchased from Ion Power Inc., USA, and cut to size prior to use (6.0 cm × 6.5 cm). The N117 membranes were pretreated by heating them in 5% H2O2 (80 °C for 30 min), followed by boiling in deionized H2O for 30 min, then 0.5 M H2SO4 (80 °C for 30 min) and lastly rinsing them repeatedly in boiling deionized H2O to ensure that all membranes

Results and discussion

In order to improve the performance of VRFBs, crossover of electrochemically active species through the membrane, namely Vx+ ion species, needs to be reduced. As an additional benefit, this would also lower the maintenance costs of the system. Because stand-alone hydrocarbon membranes exhibit stability problems under the harsh VRFB operating conditions, we here report efforts to lower the Vx+ ion permeability of Nafion, which exhibits excellent proton-conducting properties and is most widely

Conclusions

We have demonstrated that modifying Nafion membranes with cationic polymers, such as PANI and PPR, leads to reduced cross-transport of cations through the membranes, as we have shown on the example of V4+ ions. While, unfortunately, this modification also results in lower proton conductivity, our study has shown that careful selection of the cationic polymer allows tuning the inherent trade-off between lower membrane permeability and higher resistance. The physical properties not only depend on

Acknowledgements

The work is supported by the Office of Electricity (OE Delivery & Energy Reliability) (OE), U.S. Department of Energy (DOE) under contract #57558. The NMR work was carried out at the Environmental and Molecular Science Laboratory, a national scientific user facility sponsored by the DOE's Office of Biological and Environmental Research (BER). PNNL is a multiprogram laboratory operated by Battelle Memorial Institute for the Department of Energy under Contract DE-AC05-76RL01830.

References (39)

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