Phosphate fuel cells

Carbonate and phosphate fuel cells have found limited applications because of the lack of effective ways in dealing with liquid electrolytes and in further... 

Phosphoric Acid Fuel Cell. Concentrated phosphoric acid is used for the electrolyte ia PAFC, which operates at 150 to 220°C. At lower temperatures, phosphoric acid is a poor ionic conductor (see Phosphoric acid and the phosphates), and CO poisoning of the Pt electrocatalyst ia the anode becomes more severe when steam-reformed hydrocarbons (qv) are used as the hydrogen-rich fuel. The relative stabiUty of concentrated phosphoric acid is high compared to other common inorganic acids consequentiy, the PAFC is capable of operating at elevated temperatures. In addition, the use of concentrated (- 100%) acid minimizes the water-vapor pressure so water management ia the cell is not difficult. The porous matrix used to retain the acid is usually sihcon carbide SiC, and the electrocatalyst ia both the anode and cathode is mainly Pt. 

Fluorinated phosphonates exhibit interesting properties as enzyme inhibitors, chelating agents or as fuel cell electrolytes [29] however, only few methods of preparation for these compounds are available. Burton et al. [30] developed several methods to prepare fluorinated phosphates which involve phosphonyl, and likely phosphoranyl radicals as chain carriers (Scheme 11). 

Figure 17.17 Schematic representation of a single-compartment glucose/02 enzyme fuel cell built from carbon fiber electrodes modified with Os -containing polymers that incorporate glucose oxidase at the anode and bilirubin oxidase at the cathode. The inset shows power density versus cell potential curves for this fuel cell operating in a quiescent solution in air at pH 7.2, 0.14 M NaCl, 20 mM phosphate, and 15 mM glucose. Parts of this figure are reprinted with permission from Mano et al. [2003]. Copyright (2003) American Chemical Society.

Yang, C., Srinivasan, S., Bocarsly, A. B., Tulyani, S. and Benziger, J. B. 2004. A comparison of physical properties and fuel cell performance of Nafion and zirconium phosphate/Nafion composite membranes. Journal of Membrane Science 237 145-161. 

A fuel cell utilizing UDMH as fuel and N02 as oxidant is reported. Operated intermittently over a 3-month period with degradation, it consistently produced a power density of 40mw/ cm2 (40w/ft2). The cell consists of a sandwich of Zr acid phosphate in a polyvinylidene fluoride (PVF) binder and diffuse-catalyst layers of Pt black, Zr acid phosphate and PVF. Pt screens are used as current collectors] 3) G.R. Eske-lund et al, Chemical-Mechanical Mine , PATR 3724 (1968) [A mine feasibility study is reported in which the hypergolic system UDMH—... 

Traces of unreacted silica (Si02) in the SiC will produce a fluffy white precipitate, about which little is known. Certainly it is known that when hot concentrated phosphoric acid is in contact with glass hardware, the rapid formation of a fluffy gelatinous white precipitate is seen. From an X-ray diffraction analysis of the precipitate from operating fuel-cells, it was determined that the principal component was Si3(P04)4, although metallographic and SEM/EDS analyses support the presence of the other silico-phosphate complexes in varying amounts. 

Bouwman et al. demonstrated that Pt can be used in the ionic form (Pt" and Pt") by dispersing it in a matrix of hydrous iron phosphate (FePO) via a sol-gel process (Pt-FePO)." The hydrous FePO possesses micropores of approximately 2 nm. It has 3 H2O molecules per Fe atom and is thought to also serve as a proton transport medium. The Pt-FePO catalyst exhibited a higher ORR activity than Pt/C catalysts. This catalyst was also found to be less sensitive to CO poisoning because CO did not adsorb onto the catalyst surface. The ORR catalytic activity was attributed to the adsorption and storage of oxygen on the FePO, presumably as Fe-hydroperoxides. However, these catalysts have poor electrical conductivity. There is no published data on the long-term stability of these catalysts in fuel cell environments. 

Three classes of hybrid HPA are known to be stable to hydrolysis 1. Organometallic derivatives of the type RM (M = Si, Ge, Sn, Pb and R = alkyl or aryl). 2. Cyclopentadienyl-titaninm derivatives. 3. Zirconium alkoxide or phosphate derivatives, all of which are illustrated in Figure 2. We have tested phenyl model compounds of all of these for stability by boiling them in 6M HCl or H2O2 solution. This study showed that only PhP-O-HPA moieties are stable under conditions likely to be encountered in a fuel cell. Never the less we continue to study model compounds of the type RSi-O-HPA due to the large diversity of available ethoxy- and chloro- silanes. 

The amount of specifically adsorbed phosphate anions in the PAFC is much larger than of the (bi)sulfate anions in the PEMFC. The PEM membrane is specifically constmeted to keep the Pt electrode free of anions, while in contrast phosphate anions are known to directly adsorb to high coverage. The amount of chemisorbed OH tracks approximately with the fuel cell currents, the PAFC having low adsorption of OH at 0.85 V RHE while the PEMFC... 

Bauer, F., Willert-Porada, M. (2004). Microstructural charaxterization of Zr-phosphate-Nafion membranes for direct methanol fuel cell (DMFC) application. /. Membrane Science 233,141-149. 

Superphosphoric acid Tetraphosphoric acid. Acid used in the manufacture of phosphates, phosphate esters, catalysts, fuel cell electrolytes, metal cleaning and brightening, organic rextions. Viscous liquid d = 2.1000. Albright Wilson Americas Inc. 

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