Phosphoric acid fuel composition

This survey focuses on recent developments in catalysts for phosphoric acid fuel cells (PAFC), proton-exchange membrane fuel cells (PEMFC), and the direct methanol fuel cell (DMFC). In PAFC, operating at 160-220°C, orthophosphoric acid is used as the electrolyte, the anode catalyst is Pt and the cathode can be a bimetallic system like Pt/Cr/Co. For this purpose, a bimetallic colloidal precursor of the composition Pt50Co30Cr20 (size 3.8 nm) was prepared by the co-reduction of the corresponding metal salts [184-186], From XRD analysis, the bimetallic particles were found alloyed in an ordered fct-structure. The elecbocatalytic performance in a standard half-cell was compared with an industrial standard catalyst (bimetallic crystallites of 5.7 nm size) manufactured by co-precipitation and subsequent annealing to 900°C. The advantage of the bimetallic colloid catalysts lies in its improved durability, which is essential for PAFC applicabons. After 22 h it was found that the potential had decayed by less than 10 mV [187],... 

We have hinted above that by alloying, the surface structure and electronic density of a given surface can be modified. Accordingly, the interaction with adsorbates, and hence the catalytic performance, can be engineered. A great deal of effort has been devoted to the preparation, characterization, and study of alloys of the composition PtgX (X = Fe, Co, Ni, Cr, Mn). In a seminal work, Jalan and Taylor identified carbon supported PtCr alloy as the most active alloy for the ORR in phosphoric acid fuel cells. They also proposed PtNi and PtCo as the next best alloys. This line of research was further explored by other groups Mukeijee et... 

Phosphoric Acid, an Electrolyte for Fuel Cells - Temperature and Composition Dependence ofVapor Pressure and Proton Conductivity... 

Figure 29.2 Assumed change of composition of phosphoric acid species during fuel-cell operation.

Korte, C. (2012) Phosphoric acid, an electrolyte for intermediate-temperature fuel cells — temperature and composition dependence of vapor pressure and proton conductivity, in Fuel Cell Science and Engineering, 1st edn. 

In this chapter we will firstly present a literature survey on vapour pressure, viscosity and conductivity properties of phosphoric acid with an emphasis on the temperature and composition range relevant for fuel cell applications. In a second part we want to elucidate the physico-chemical interactions of a protic electrolyte like phosphoric acid as a doping agent with polybenzimidazole-type polymer membranes. Literature data on m-PBI and AB-PBI as well as own measurements on Fumapem AM-55, a commercial PBI derivative, will be cOTisidered. On the basis of the observed doping behaviour a model describing the thermodynamics of the adsorption process is presented. 

The PBI/PBI-PBz-f (thickness 75 pm) composite membrane containing 20 wt% of the PBI-PBz electrospun nano-fiber, of which 10 wt % was the PBz crosslinker, was doped with a phosphoric acid aqueous solution followed by MEA preparation to perform high-temperature fuel cell tests. The phosphoric acid doping level and proton conductivity, membrane mechanical properties, and fuel cell performance of this PBI/PBI-PBz-f composite are listed in Tables 12.3, 12.4, and 12.5, respectively. The data for neat PBI are given for comparison. The PBI/PBI-PBz-f composite membrane showed higher PAdop and a, higher mechanical strength and strain at break, and better fuel cell performance than the neat-PBI membrane. Compared to the hydrophobic porous PTFE film, the... 

Li M, Scott K, Wu X (2009) A poly(RiR2R3-bF )/ H3PO4 composite membrane for phosphoric acid polymer electrolyte membrane fuel cells. J Power Sources 194 811-814... 

Physiochemical properties and experimental data of phosphoric acid can be found in textbooks and various publications [148-152] and in the literature cited therein. The available works cover, e.g., evaporation and condensation considerations, acidity and proteolytic equilibria, composition specifications and condensation equilibria, vapor pressure of water as a function of composition and temperature, proton conductivity as a function of composition and temperatures, dynamic viscosity, and phase equilibria. The available data builds the foundation for modeling and simulation of a H3PO4-H2O system (and PBI/H3PO4-H2O) which is important to further improve the quality and reliability of HT-PEM fuel cell models. The combination of electrolyte modeling... 

An important aspect of the development of fuel cell stacks is to make them more compact, and a key to that is to develop thinner bipolar plates preferably from metal. Especially low temperature automotive PEMFC stacks have reached impressive power densities with metallic bipolar plates. However, because of the free phosphoric acid and the elevated temperature, research and demonstration of HT-PEMFCs has so far been done almost exclusively with plates of graphite and its composite materials. 

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