Polymer-electrolyte fuel cells base materials

Polymer Electrolyte Fuel Cells, Oxide-Based Cathode Catalysts, Table 1 Solubility of platinum black powder and oxide-based materials prepared by various methods in 0.1 mol dm H2SO4 at 30 °C under atmospheric condition... 

Rabis A, Fabbri E, Foelske A et al (2013) Durable oxide-based catalysts for application as cathode materials in polymer electrolyte fuel cells (PEMFCs). ECS Trans 50 9-17... 

Varcoe, J.R., Slade, R.C., Lam How Yee, E., Poynton, S.D., Driscoll, D.J., Apperley, D. C. (2007) Poly(ethylene-co-tetrafluoroethylene)-derived radiation-grafted anion-exchange membrane with properties specifically tailored for application in metal-cat-ion-free alkaline polymer electrolyte fuel cells. Chemistry of Materials, 19, 2686-2693. Slade, R.C., Varcoe, J.R. (2005) Investigations of conductivity in FEP-based radiation-grafted alkaline anion-exchange membranes. Solid State Ionics, Y16, 585-597. 

Over the last decade, several new proton exchange membranes have been developed. The new polymers in fuel cell applications are based mostly on hydrocarbon structures for the polymer backbone. Poly(styrene sulfonic acid) is a basic material in this field. In practice, poly(styrene sulfonic acid) and the analogous polymers such as phenol sulfonic acid resin and poly(trifluorostyrene sulfonic acid), were frequently used as polymer electrolytes for PEMFCs in the 1960s. Chemically and thermally stable aromatic polymers such as poly(styrene) [ 3 ], poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbony 1-1,4-phenylene) (PEEK) [4], poly(phenylenesulfide) [5], poly(l,4-phenylene) [6, 7], poly (oxy-1,4-phe-nylene) [8], and other aromatic polymers [9-11], can be employed as the polymer backbone for proton conducting polymers. These chemical structures are illustrated in Fig. 6.2. 

The membrane electrode assembly (MEA) is a delicate component in low-temperature fuel cells based on polymer electrolyte membranes. Its condition is affected by many factors (1) selection and preparation of MEA materials (catalysts, supporting carbon powder, membrane materials, binder for MEA hot pressing, etc.), (2) history of MEA usage, (3) fuel cell operation parameters, and so on. The resulting MEA condition exerts a strong influence on the fuel cell performance, which is also a function of running time. 

Fujitsu has developed a MEA based on an aromatic hydrocarbon solid electrolyte material, coated with a high density of highly active platinum-based nano-particle catalyst, having less than one-tenth of the methanol crossover rate encountered with typical fluorinated polymers. Fig. 10 shows Fujitsu s 15 W micro fuel cell based on a new hydrocarbon solid electrolyte material that enables use of 30% methanol powering a note book PC. The basic specification of a 3.78 W prototype micro fuel cell is given in Table 2. 

Besides silicon, other materials have also been used in micro fuel cells. Cha et al. [79] made micro-FF channels on SU8 sheets—a photosensitive polymer that is flexible, easy to fabricate, thin, and cheaper than silicon wafers. On top of fhe flow channels, for both the anode and cathode, a paste of carbon black and PTFE is deposited in order to form the actual diffusion layers of the fuel cell. Mifrovski, Elliott, and Nuzzo [80] used a gas-permeable elastomer, such as poly(dimethylsiloxane) (PDMS), as a diffusion layer (with platinum electrodes embedded in it) for liquid-electrolyte-based micro-PEM fuel cells. 

Polyphosphazene-based PEMs are potentially attractive materials for both hydrogen/air and direct methanol fuel cells because of their reported chemical and thermal stability and due to the ease of chemically attaching various side chains for ion exchange sites and polymer cross-linking onto the — P=N— polymer backbone. Polyphosphazenes were explored originally for use as elastomers and later as solvent-free solid polymer electrolytes in lithium batteries, and subsequently for proton exchange membranes. 

The development of conventional room-temperature hydrides based on interme-tallic compounds led to a large number of storage materials exhibiting very favorable sorption enthalpies with values of around 25 kj (mol H2), which can be operated in combination with conventional (80 °C operation temperature) polymer electrolyte membrane (PEM) fuel cells. However, their gravimetric storage capacity is limited to less than 3 wt.% H2. 

O. Savadogo and B. Xing. Hydrogen/oxygen polymer electrolyte membrane fuel cell (PEMFC) based on acid-doped polybenzimidazole (PBI). Journal of New Materials for Electrochemical Systems 3, 343-347 2000. 

In this section, recent advances in the field of polymer electrolyte direct methanol fuel cells, i.e., PEFCs based on direct anodic oxidation of methanol are discussed. A schematic of such a ceU is shown in Fig. 48, together with the processes that take place in the cell. The DMFC has many facets, electrocatalysis materials and components which deserve a detailed treatment. The discussion here will be confined, however, to the very significant performance enhancement demostrated recently with polymer electrolyte DMFCs, and, as a result, to possible consideration of DMFCs as a nearer term technology. 

In the development of fuel-cell technology based on this unique polymer electrolyte, special chapters in electrochemical science and engineering have emerged, addressing the fuel-cell ionomeric membrane itself and the optimized fabrication of MEAs. The invention of Nafion, a poly(perfluorosulfonic acid) (poly(PFSA)) at DuPont in the 1960s, was, in fact, a key (if not the key) milestone in the development of PEFC technology. The chemical and mechanical properties of such poly(PFSA) extruded membranes, which are based on a perfluorocar-bon backbone, enabled to achieve stable materials properties and, consequently,... 

The most diffused material for membranes is based on co-polymers of tetrafluoroethylene (TEE) with perfluorosulfonate monomers. The resulting co-polymer is constituted by polytetrafluoroethylene polymeric chain (PTFE, commercially known as Teflon) in which some fluorine atoms are substituted by sulfonated side chains. The monomer perfluoro-sulfonyfluoride ethyl-propyl-vinyl ether is used in membranes commercialized by Dupont with the registered trademark Nafion (Fig. 3.2), which is the most well-known material used as electrolyte in PEM fuel cells. 

In the past two decades, fuel cells and in particular imi-exchange membranes have become a top priority topic in material research. Fuel cells are seen as promising alternative energy conversion systems replacing the combustion-based techniques. Among the various types of fuel cells, the low-temperature fuel cells like the polymer electrolyte membrane fuel cell (PEMFQ, DMFC, or alkaline fuel cell (AFC) are the most flexible ones concerning range of appUcations e.g. portable, automotive, and stationary. 

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