Composite Membranes for High Temperature Direct Methanol Fuel Cells

Abstract

A low cost and high temperature membrane, with suitable ionic conductivity and stability up to 150 8C, would be a potential solution to some of the drawbacks presently affecting reformate-fuelled polymer electrolytes (PEMFCs) as well as direct methanol fuel cells (DMFCs) [1][2][3][4][5][6][7]. Fuel cell operation at elevated temperatures can limit the effects of electrode poisoning by adsorbed CO molecules, increase both methanol oxidation and oxygen reduction kinetics and simplify water and thermal management. Furthermore, high temperature operation can reduce the complexity of the reforming reactor employed for PEMFCs [2]; the temperature range 130 to 150 8C is ideal for application of these systems in electric vehicles and for distributed power generation.Various proton-conducting polymer electrolyte materials have been investigated for high temperature operation. Two categories of membranes can be proposed, depending on whether water is required for proton conduction or is not necessary [1][2][3][4][5][6][7]. Polymer electrolytes involving water molecules in the proton mobility mechanism (e.g., perfluorosulfonic membranes) need humidification to maintain suitable conductivity characteristics. The amount of humidification may vary depending on the operating temperature and membrane properties; it influences the size and complexity of the device. Some other electrolytes do not necessarily involve water molecules in the mechanism of proton conduction (e.g., PBI/H 3 PO 4 [5], blends of PBI and polysulfone [8], hybrids of polymers and proton-conducting inorganic compounds such as Zr(HPO 4 ) 2 [4], etc.); these systems do not strictly need humidification. Yet, there are some drawbacks related to the short-term stability of such systems: phosphoric acid leakage from the membrane during operation, poor extension of the three-phase reaction zone inside the electrodes due to the absence of a proper ionomer, and reduced conductivity levels for inorganic proton conductors. These problems have decreased the perspectives of utilization of water-free protonic electrolytes in low temperature fuel cells. Alternatively, composite perfluorosulfonic membranes containing different types of inorganic fillers such as hygroscopic oxides [6,9,10], surface modified oxides [11], zeolites [12], inorganic proton conductors [13] and so on have shown an increased conductivity with respect to the bare Membranes for Energy Conversion. Volume 2. Edited by Klaus-Viktor Peinemann and Suzana Pereira Nunes

Direct Methanol Fuel Cells: History, Status and Perspectives

“…A rational analysis of filler effects on structural, proton transport properties and the electrochemical characteristics of composite perfluorosulfonic membranes for DMFCs was reported [107,125]. It was observed that a proper tailoring of the surface acid-base properties of the inorganic filler for application in composite Nafion membranes allows appropriate DMFC operation at high temperatures and reduced pressure [107].…”

“…It was observed that a proper tailoring of the surface acid-base properties of the inorganic filler for application in composite Nafion membranes allows appropriate DMFC operation at high temperatures and reduced pressure [107]. An increase in both the strength and amount of acidic surface functional groups in the fillers enhances water retention inside the composite membranes through an electrostatic interaction in the presence of humidification constraints, in the same way as for the adsorption of hydroxyl ions in solution [107,109,125].…”

Membranes for Direct Alcohol Fuel Cells

“…Aricó's group in Italy have studied Nafion composite membranes and other polymers aiming to find low cost membranes with suitable proton conductivity and stability up to 150 C for high temperature DMFC [6]. Nafion composites with SiO 2 , ZrO 2 , Al 2 O 3 , and SiO 2 /PWA [315] exhibit maximum in conductivity at about 135 C, that is, a 15 C higher that the corresponding pure Nafion recast membrane, as shown in Fig.…”

“…6.20) [6,95,96]. Important conductivity increases were also obtained using sulfonated mesostructurated silica (MSU-F and MCM-41) [44], and silica nanoparticles with a sulfonated oligomer layer [50], as shown in Fig.…”

“…6.1, reviews published till 2006 [1][2][3][4] cover only one third of the ionomeric membranes currently developed for DAFC. More recent reviews deal with polyimide ionomer membranes [5], composite membranes for high temperature DMFC [6], non-perfluorated sulfonic acid membranes [7], modified Nafion membranes [8], and hybrid membranes [9][10][11].…”