List of abbreviations
Chapter 1: Introduction
1.3 Nafion membrane
The polymer electrolyte membrane is a crucial part in DMFC, which governs its performance. One of the major challenges in current DMFC research is the development of suitable PEM for improving the performance of DMFCs [Othman et al., 2010]. The
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polymer membrane should simultaneously maintain high proton conductivity; a low permeability to reactants; and have reasonable chemical, thermal, and mechanical stability [Neburchilov et al., 2007]. A lot of researches have been conducted in attempting to develop a membrane having all of these properties. Some researchers have proposed thermoplastic polymers such as poly(etheretherketone) (PEEK), polysulfone (PSF) and polybenzimidazole (PBI) as good PEM for DMFC [Ahmad et al., 2010]. However, nafion, a perfluorinated ionomer (PFI) membrane developed by DuPont in 1960s for PEMFC has been the preferred electrolyte for DMFC to date [Othman et al., 2010]. Other PFI membranes like Flemion (Asahi Glass), Aciplex (Asahi Chemical), and Dow (Dow Chemical), were developed but their primary applications remained for PEMFC. The starting raw material for developing PFI is polyethylene. The hydrogen molecules of polyethylene are substituted with fluorine and the product obtained is polytetrafluoroethylene or PTFE, also referred to as Teflon[Larminie and Dicks, 2000].
The strong C-F bonds make PTFE mechanically durable and resistant to chemical attacks [Othman et al., 2010]. A perfluorinated side chain with ionically bonded sulphonic acid (SO3H) group is added to the PTFE and the resulting polymer is referred to as PFI.
Nafion has both hydrophobic and hydrophilic domains. The long chain molecules of PTFE contribute to the hydrophobic domain and the sulphonic acid side chains contribute to the hydrophilic domain. Two models have been proposed to describe the morphology of nafion namely, the cluster network model by Gierke, (1981) (fig.1.1) and the three- region model by Yeager and Steck, (1981) (fig.1.2).
As per the cluster network model (fig 1.1) the morphology of hydrated nafion shows the formation of spherical clusters (pores), which are connected by short narrow channels,
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Fig.1.1 The microstructure of nafion according to Gierke’s cluster-network model [Heitner-Wirguin, 1996]
Fig.1.2 Illustration of Yeager and Steck’s three region model of nafion [Yeager and Steck,1981]
A- Polymer backbone B- Transition region C- Proton conducting region
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due to the separation of ion-exchange sites from the fluorocarbon backbone [Gierke, 1981]. When the membrane is dry, an average cluster has a radius of about 1.8 nm and it contains about 26 SO3 groups distributed on the inner pore surface. In the swollen state, the diameter increases to about 4 nm and the number of fixed SO3 groups goes up to about 70. Under these conditions, each pore is filled with about 1000 water molecules [Smitha et al., 2005].
The morphology of nafion as described by the three-region model distinguishes polymer backbone regions, proton conducting (aqueous) regions, and narrow bridge-like transition regions connecting the large aqueous regions and PTFE polymer backbone [Yeager and Steck, 1981]. Within the hydrated regions for both the models, the H+ ions are relatively weakly attached to the SO3 group and move freely to enable the membrane in transferring hydrogen ions through the membrane from anode to the cathode. The migration of hydrogen ion from anode to cathode takes space by diffusion and/or by the hoping mechanism via hydronium or Zundel and Eigen ions [Kreuer, 2001]. The hydrophobic region primarily makes the nafion highly chemically resistive and mechanically strong. The hydrophilic region of the nafion primarily helps to attain acidity, sufficiently absorptive to water, as well as good proton conductivity [Othman et al., 2010].
Despite showing an effective performance in PEMFC, the nafion membrane has many limitations when used in DMFC. The limitations of the nafion hamper the emergence of the DMFC design. One of the main limitations of nafion is methanol crossover (MCO) through the nafion membrane from the anode to the cathode side [Tricoli, 1998]. The
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MCO is a major problem since it has two detrimental effects on the DMFC. Firstly, the crossed methanol is oxidized by the cathodic electro-catalyst, which depolarizes the electrode. Secondly, it severely lowers the efficiency of fuel utilization [Neburchilov et al., 2007]. Since the energy resulting from crossed-over methanol oxidation is not extracted as electricity, it all ends up as waste heat that increase the cooling load on the cell [Othman et al., 2010]. Nafion is also susceptible to water and methanol uptake and thus experience swelling, which is determinative factor in the longevity and performance of a DMFC.
Various polymers are being investigated to replace nafion for reduced methanol crossover while maintaining high proton conductivity, chemical resistance, mechanical strength etc.
comparable to nafion. However, till now no membrane has achieved all the desired properties to surpass nafion membrane. Any new polymeric membrane suitable to DMFC should possess low fuel crossover, high mechanical and thermal stability along with high chemical or oxidation stability, and high proton conductivity. Besides, these properties must be achieved while maintaining low cost. Nafion, as of date, is the only membrane which has many properties up to the desired level. Therefore, nafion is the most widely accepted PEM for DMFC. However, one of the main challenges to nafion is the reduction of MCO in order to make it suitable for DMFC. The investigators are trying various routes to modify the nafion for reduced MCO. A brief description of these routes is provided in the next section.