PEMs for DMFCs

Methanol can be synthesized in the presence of catalysts from water and carbon oxides. It can also be prepared by pyrolysis of wood. Its combustion produces both 

Jiang et al. [155] modified the surface of Nafion membranes by self-assembling on it a monolayer of Pt-PDDA nanoparticles (where PDDA stands for the positively charged poly(diallyldimethylammonium chloride), which act as catalyst for methanol oxidation. Such a surface modification of Nafion membranes enhanced the power output of DMFC by as much as 34% [155]. 

Novel nanoporous membranes have recently been developed by Peled and coauthors [156]. Such membranes consisted of PTFE as the backbone with a nanosized ceramic powder (Aerosil 200 or Aerosil 130) dispersed in it. An aqueous solution of sulfuric acid adsorbed inside the pores of such membranes acted as an ionic conductor. Thus prepared membranes consisted of 50-200 nm spherical particles with nanovoids between them. They have been found to be quite elastic. Preliminary tests conducted using FCs with 250 pm thick nanoporous membrane, with electrodes on which 4 mg Pt/cm was dispersed on both the anode and the cathode, and fueled by 1 M methanol (in 3 M aq. H2SO4) flowing at the rate of 180 ml/min, against 3 atm of dry air, yielded 50 and 130 mW/cm at 80 °C and at 130°C, respectively. The crossover of methanol in these relatively inexpensive membranes was 0.27 and 0.56 A/cm at 80 and 130°C, respectively. Its selectivity to methanol was estimated to be in the same range as PVDF-ceramic powder hybrid [157]. 

In view of the relatively high permeabihty of the currentiy available PEMs to methanol, several sulfonated aromatic and aliphatic polymers were tested as possible alternative PEMs that will significantly reduce the crossover of methanol to the cathode compartment of DMFCs. Sulfonated copolymers of arylene ether sulfones with carboxylic and sulfonic acid groups have been prepared by polycondensation of 4,4 -dichlorodiphenyl sulfone and of sulfonated 4,4 -dichlorodiphenyl sulfone with phenolphthalein (sDCPDS) (Fig. 1.9). For compositions of such copolymers equal to or higher than percolation thresholds (sDCPDS = 20 mol%), their permeability to methanol (calculated as its diffusion coefficient) is much lower than that of Nafion and their selectivity at 30 °C (conductivity to protons versus permeability to methanol) is nearly five times higher than that of Nafion [158]. 

Fluorinated polyfaryl ether) and its phosphonated derivative, containing the 4-bromophenyl pendant group, was prepared with high conversion yield. It had 

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Nafion (Fu et al. 2008). However, the corresponding proton conductivity value is lower. The methanol crossover, however, is only one-third of that found in Nafion 115, compared at the same thickness. Although the PSf membrane has lower proton conductivity than Nafion, the lower production cost and methanol crossover make it a promising alternative for DMFC. Sulfonated poly(phthalazinone ether ketone) (SPPEK) has been discovered as a new kind of PEM for DMFC due to its superior performance in terms of chemical and oxidative resistances, mechanical strength, and thermal stability (Gao et al. 2003). Tian et al. reported that SPPEK prepared from direct polymerization of presulfonated monomer has better performance than that of postsulfonation (Tian et al. 2005). Unfortunately, with direct polymerization it was hard to control the DS and location of sulfonation. 

PEM for DMFC is mainly characterized with ionic conductivity and methanol crossover. Ionic conductivity is tested with the four-point probe method. Through plane and in plane ionic conductivity is characterized with temperature and humidity [22]. Methanol crossover can be measured with diffusion cell method, pervapora-tion of methanol solution, dictating of CO amount from cathode out stream [23]. An oxidant-impermeable property is also one of the important factors in PEM for DMFC. The properties of PEM in DMFC are also characterized with an interfacial resistance, an electro-osmotic drag, methanol crossover through MEA, and so on. In this chapter, development of PEM for DMFC are discussed. Approaches for PEM in DMFC are classified as fluorinated polymer, hydrocarbon polymer, modification of polymer materials, and the technical approaches are described as well. 

In a review of the properties and structure of the polymer electrolyte membranes for direct methanol PCs, Deluca and Elabd mentioned that some of the proposed replacements of Nafion as PEM for DMFC have higher methanol selectivities and comparable proton conductivities to Nafion [174]. Montmorillonite dispersed in Nafion , described by Song et al. [175] in 2003, and blend of PVA with the copolymer of PS-sulfonic and maleic acids, described by Kang et al. [176], seem to be the most promising ones. However, longitudinal proton conductivities of the respective membranes cited in these references may by different fi om horizontal conductivities of these membranes. 

Although ORR catalysts for DMFCs are mostly identical to those for the PEM fuel cell, one additional and serious drawback in the DMFC case is the methanol crossover from the anode to the cathode compartment of the membrane electrode assembly, giving rise to simultaneous methanol oxidation at the cathode. The... 

The blending of two or more polymers is frequently used to try to combine the separate desirable properties of each system rather than trying to develop one system with all the properties. In the case of PEMs, this has led to the blending of proton-conducting polymers with non-ionic polymers, low lEC polymers, or polymer-containing basic moieties, particularly for DMFC applications in order to decrease MeOH crossover. These different types of blends will be briefly discussed next. 

Fuel cells. Topics include both SOFC and PEM type fuel cells. Also R D for DMFC has been investigated. Emphasis was given to solving problems related to fulfilling the market requirements of the Sulzer Hexis SOFC. The main goal is to increase both reliability, lifetime, and the power conversion rate, while reducing costs. Considerable efforts were also invested in the development and demonstration of the PEM. Outcomes of this work include a 60 kW stack for cars demonstrated successfully in 2002 in the VW Bora and recently in a much improved car. The 1 kW-unit "Power Pac" is a standalone unit its PEM-stack has been demonstrated in various applications like boats and small cars (SAM). 

Cell modelling techniques for flow and electrochemistry are basically the same for DMFC as for other PEM fuel cells, i.e., the techniques described in section 3.1.5 and 3.5.1-3.5.3 have been employed for DMFCs by Fuhrman and Gartner (2003). 

Direct methanol fuel cell (DMFC) was developed in 1950s-1960s, based on the liquid alkaline or aqueous acid solution as the electrolyte. It converts the methanol directly into electricity, instead of using indirectly produced hydrogen from methanol through the reforming process. Today, DMFC commonly refers to as the one that employs PEM as the electrolyte. Fuel for DMFC is a dilute solution of methanol, usually 3-5 wt% in water. The size of DMFC can be considerably smaller than PEMFC because of the elimination of fuel processor, and complex humidification and heat management systems. The performance of DMFC is relatively low compared to that of PEMFC. 

Polybenzoimidazole (PBI) polymers doped with phosphoric acid have emerged in 1995 as a promising PEM for use in DMFC [403] due to its relatively low cost, low methanol permeability. Its excellent chemical and thermal stability up to 200 C [403] has also triggered several studies aiming to test the PBI-based membrane performance in high temperature DMFC [404]. 

In the following sections we will discuss the most common durability tests and the different factors that lead to the power decrease of the fuel cell. Some of them are general to hydrogen-fed PEM fuel cells, while others are specific of DAFC. Kim and Zelenay [85] quoted that performance degradation of DMFC has received little attention compared with the degradation of H2/air PEM fuel cells systems, probably because lifetime requirements for DMFC systems are generally perceived as less stringent, and have not been defined as precisely as for fuel cells... 

H+) Produced by the dissociation of Hj at the anode to the cathode, (3) Prevention of the associated electron flow through the membranes forcing them to flow in the external circuit to the cathode to produce DC current, and (4) Support for the catalyst loaded on the electrodes. When Hj is replaced by methanol as a fuel in liquid form in direct methanol fuel cell (DMFC), the dissociation of methanol solution at the anode produces protons that are transported through the hydrated PEM to the cathode, where a reduction of O2 produces water in the presence of the protons. To qualify PEM for commercial application in PEMFC and DMFC, it should have a combination of properties including (Maiyalagan and Pasupathi 2010 Neburchilov et al. 2007 Nagarale et al. 2010) ... 

At present, the most widely used commercial PEM is Naflon produced by DuPont since 1992. Naflon is a plain perfluorosulfonic membrane that is thermally stable and is excellent for PEMFC because of its high proton conductivity. However, Naflon is not suitable for DMFC applications, partly due to its cost. This type of membrane has high permeability toward methanol even at low temperatures, which drastically reduces the DMFC performance (Neburchilov et al. 2007). This is worsened by high water permeability in perfluorinated membranes that can cause cathode flooding and thus lower cathode performance, which also contributes to lower DMFC performance. 

One of the most important characteristics that reflect the performance of a PEM in DMFC is proton conductivity. It has been reported that the proton conductivity depends on the DS, pretreatment of the membrane, hydration state, and ambient relative humidity and temperature. For ionomeric membranes, the proton conductivity depends on the amount of add groups attached to the polymer ring and their dissociation capability in water, which is accompanied by the generation of protons. The high ionic conductivity demonstrated by the membrane at high sulfonation level suggests that the water-swollen ionic domains in the membrane pores were interconnected to form a network structure. Water molecules also dissodate acid functionality and facilitate proton transport. Therefore, it can be deduced that water uptake is an important parameter in proton conductivity tests (Bauer et al. 2000). 

Biddinger EJ, Knapke DS, von Deak D, Ozkan US (2010) Effect of sulfur as a growth promoter for CNx nanostructures as PEM and DMFC ORR catalysts. Appl Catal B Environ... 

A DMFC is quife similar to a proton exchange membrane fuel (PEMFC) in stack structure and components. They both use a PEM for transporting the protons and Pt-based catalysts at the cathode. The anode catalyst for a DMPC is typically a Pt-Ru alloy that has higher CO tolerance than Pt alone, and this is similar to the PEMFC when H2 contains trace amounts of CO. In fhe infer-mediate sfeps during methanol oxidation, some CO-like species will form, which can seriously poison the anode catalyst. The presence of Ru helps fhe removal of fhe CO-like species from fhe Pt surface trough Reaction 7.6. 

If AEMs are shown to be stable in fuel cells over thousands of hours, an in-depth investigation into effective and cheaper non-noble metal catalysts (e.g., Ni, Ag etc.) is indicated. There would also be a greater chance of finding methanol-tolerant catalysts for use in the cathodes than in related PEM-based DMFCs. 

The composite membrane approach can also be used for membranes alternative to Nafion such as sulfonated polyetherketones and polysulfones. Sulfonated polysulfone is one of the most promising polymers for PEMs due to its low cost, commercial availability, and easy processability. Composite polysulfone-based membranes based on silica have been prepared and characterized for DMFC to extend the operating temperature up to 120 °C [16]. As an example, the following describes the preparation and characterization of a... 

Nafion-115, Nafion-112, and Nafion-212 which have thickness of 175 pm, 125 pm, 50 pm, and 50 pm, respectively. These thiimer composite PEMs contain significantly less amounts of the expensive Nafion resin than the thicker neat Nafion membranes. Thus, another advantage of Nafion/PTFE composite PEMs is the fact that they are inexpensive. Besides porous PTFE films, porous films such as polyethylene (PE) [21, 22] and electro-sptm polymer nanofiber films such as those of poly(vinyhdene fluoride) (PVdF) [23, 24], poly(vinyhdene fluoride-co-hexafluoropropylene) (PVdF-co-HFP) [25], and poly(vinyl alcohol) (PVA) [26-31] have also been used as supporting films for impregnating Nafion ionomer solutions to prepare Nafion/fiber composite PEMs for PEMFC and DMFC applications. 

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