Phosphoric acid fuel cells catalysts used

Phosphoric acid fuel cells (PAFC) use liquid phosphoric acid as an electrolyte - the acid is contained in a Teflon-bonded silicon carbide matrix - and porous carbon electrodes containing a platinum catalyst. The PAFC is considered the "first generation" of modern fuel cells. It is one of the most mature cell types, the first to be used commercially, and features the most proven track record in terms of commercial applications with over 200 units currently in use. This type of fuel cell is typically used for stationary power generation, but some PAFCs have been used to power large vehicles such as city buses. 

Recent testing in phosphoric acid fuel cells has shown improved performance using promoted Ft on carbon catalysts in the air cathode. The promoters are oxides of the base transition metals, e.g., Ti (O,... 

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],... 

The earliest models of fuel-cell catalyst layers are microscopic, single-pore models, because these models are amenable to analytic solutions. The original models were done for phosphoric-acid fuel cells. In these systems, the catalyst layer contains Teflon-coated pores for gas diffusion, with the rest of the electrode being flooded with the liquid electrolyte. The single-pore models, like all microscopic models, require a somewhat detailed microstructure of the layers. Hence, effective values for such parameters as diffusivity and conductivity are not used, since they involve averaging over the microstructure. 

With a Pt/metal ratio from 1 1 to 5 1, V, Hf, Zr, Nb, and Ta had been tried. All these metals are nonnoble and are expected to be dissolved in phosphoric acid in the fuel cell under operation conditions. The initially used binary alloys are indeed not stable enough, and the catalyst loses its enhanced catalytic activity during several thousand hours of operation. Recently it was detected and claimed that tertiary and quarternary alloys that contain chromium are remarkably much more stable than the binary alloys, so that the aim of 40,000 hr of operation, which is the usually assumed lifetime for phosphoric acid fuel cells, can be achieved. 

Phosphoric acid fuel cell (PAFC)—Phosphoric acid electrolyte with platinum catalyst. It can use hydrocarbon fuel and is suited for stationary applications. It can generate both electricity and steam. As many as 200 units in sizes ranging from 200 kW to 1 mW are in operation. 

Phosphoric acid fuel cells rely on expensive components and, like pems, use platinum catalysts to accelerate the chemical reactions at the electrodes. Finally, they have not achieved the level of sales needed to significantly reduce manufacturing costs. For these reasons, UTC Fuel Cells is phasing out production of phosphoric acid fuel cells in favor of pem fuel cell technology, which is likely to be significantly less expensive. 

Alkaline fuel cells (AFC) — The first practical -+fuel cell (FC) was introduced by -> Bacon [i]. This was an alkaline fuel cell using a nickel anode, a nickel oxide cathode, and an alkaline aqueous electrolyte solution. The alkaline fuel cell (AFC) is classified among the low-temperature FCs. As such, it is advantageous over the protonic fuel cells, namely the -> polymer-electrolyte-membrane fuel cells (PEM) and the - phosphoric acid fuel cells, which require a large amount of platinum, making them too expensive. The fast oxygen reduction kinetics and the non-platinum cathode catalyst make the alkaline cell attractive. 

Pt and Pt-bimetallic nanoparticle catalysts were employed in commercial prototype phosphoric acid fuel cells even in the mid-1970s [7], so in fact the concept of nanoparticle electrocatalysts is not new. Like many industrial catalysts, however, the catalysts are put into use long before their structure and properties are clearly understood, and that was certainly the case, for example, for the Pt-Co-Cr air cathode catalysts used at United Technologies [8]. In this chapter we review studies, primarily from our laboratory, of Pt and Pt-bimetallic nanoparticle electrocatalysts for the oxygen reduction reaction (ORR) and the electrochemical oxidation of H2 (HOR) and H2/CO mixtures. 

This survey focuses on recent catalyst developments in phosphoric acid fuel cells (PAFC), proton exchange membrane fuel cells (PEMFC), and the previously mentioned direct methanol fuel cell (DMFC). A PAFC operating at 160-220 °C uses orthophosphoric acid as the electrolyte the anode catalyst is Pt and the cathode can... 

During phosphoric acid fuel cell research and development, technical solutions were found, which later were adopted successfully in the development of other fuel cell types. This is true, in particular, for the use of platinum catalysts not in a pme form, but as deposits on carbonaceous supports (for instance, carbon black), leading to a considerable drop in the amounts of platinum needed to manufacture... 

In early 1960s, some of the metal carbides such as silicone carbide (SiC) and boron carbide (BC) were tested for the first time in fuel cell environment. For example, BC was used as catalyst support in both phosphoric and alkaline fuel cells by GE in the USA [18]. Since then, it took 20 years for other carbides to be evaluated in fuel cells, in particular phosphoric acid fuel cell (PAFC). United Technologies Corporation... 

It is known that a phosphoric acid fuel cell (PAFC) can generate a current density around 1 A cm before getting to the mass transport-related steep voltage drop region, indicating that the catalyst layer is not completely flooded by the liquid phosphoric acid, although the liquid acid is used as the electrolyte imbedded in either SiC or polybenzimidazole (PBI), and the catalyst layers are filled with the liquid acid for proton transport. 

Different catalysts are used for AFCs. For the hydrogen oxidation reaction, carbon-supported platinum and platinum-palladium catalysts (e.g., noble metal catalysts) are suitable. However, one of the advantages of the AFC compared with acid electrolyte fuel cells, including the phosphoric acid fuel cell (PAFC) and the... 

To date, the catalysts for low-temperature fuel cell electrodes (phosphoric acid and alkaline cells) have been the precious metal blacks and, more recently, precious metals on carbon supports. Development of fuel cell catalysts using precious metals remains very active. Also, some work is being done on systems that may be substituted for the noble metals. For example, tungsten carbide based anode catalysts have been shown to have good durability over long periods, but they are not as active as platinum. 

Classical phosphoric add fuel cells use phosphoric add as the electrolyte, which is immobilized in a Teflon bonded silicon carbide matrix. Phosphoric acid fuel cells usually work at temperatures around 200 °C and are able to tolerate carbon monoxide levels of up to 2 vol.% [1]. Platinum/ruthenium as the anode catalyst may improve the performance in presence of carbon monoxide, similar to PEM fuel cells [33]. 

The phosphoric acid fuel cell (PAFC) was the first fuel cell to be commercialized and shares some technologies with the PEMFC, such as the porous electrodes and the platinum catalysts. The liquid phosphoric acid allows high operating temperatures, around 200 C. Fuels must be free of carbon monoxide, as with the PEMFCs. With rated power over 50 kW, PAFC systems are used for stationary applications. 

---