Phosphoric acid fuel cells performance

Andrew, M.R., McNicol, B.D., Short, R.T. Drury, J.S. Electrolytes for methanol-air fuel-cells. 1. The performance of methanol electro-oxidation catalysts in sulfuric acid and phosphoric acid electrolytes. J.Appl. Electrochem. 1 (1977), pp. 153-160. 

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,... 

One of the critical issues with regard to low temperamre fuel cells is the gradual loss of performance due to the degradation of the cathode catalyst layer under the harsh operating conditions, which mainly consist of two aspects electrochemical surface area (ECA) loss of the carbon-supported Pt nanoparticles and corrosion of the carbon support itself. Extensive studies of cathode catalyst layer degradation in phosphoric acid fuel cells (PAECs) have shown that ECA loss is mainly caused by three mechanisms ... 

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],... 

M. Aoki, Y. Ueki, H. Enomoto, K. Harashima, "Some Approaches to Improve the Life Performance of Phosphoric Acid Fuel Cell," paper provided to the authors by Fuji Electric Corporate Research and Development, 1992, date of preparation unknown. 

A few demonstrations of phosphoric acid fuel cells were carried out to evaluate and validate the performance and durability of small on-site co-generation systems, including 50 kW at Eniricerche in Milan and 200 kW at ACoSeR in Bologna. 

Operators of the Tokyo demonstration plant have concluded that phosphoric acid fuel cell technology is ready for commercialization. The project demonstrated that (I) fuel cells can be sited in urban areas which are regulated by strict environmental constraints (2) performance and operational characteristics were very close to design goals and (3) utility personnel can efficiently operate and maintain fuel cell plant equipment with minimal additional training. As a consequence of the demonstration plant success, a new 11 -MW power plant will be developed and marketed. A comparison of the new 1 C 23 Unit with the 4,5 MW demonstration plant is given in Table 4. 

The 200 kW phosphoric acid fuel cell (PAFC) was introduced into the market in 1991 by International Fuel Cells/ ONSI, now called UTC Fuel Cells. It is the only commercialized fuel cell technology. PAFC units have been installed in various applications—commercial, small industrial, landfill, and military—and some are used for cooling, heating, and power. To date there have been 250 units sold, at roughly 4500/kW. The U.S. Department of Defense (DOD) has cost-shared the purchase of three-quarters of the units sold to date. The units have performed well they have operated at 95 to 98 percent availability and 99.99 to 99.9999 percent reliability and have served 4 million customers and accumulated 4 million hours of operation. The cost of PAFC units has not decreased and in fact has increased from 3500/... 

The technology of phosphoric acid fuel cells is already fairly advanced. Large (4.3 MW) units have been built and tested in several countries for a number of years. Smaller units (40 kW) have been operated continuously for periods of up to 40,000 hours, with little decline in performance. Since the.se units are rather compact (having a footprint of only 5 m ), several of them could be accommodated in the basement of a typical apartment building, providing on-site electricity and heat for the whole building. 

In Section 3, the slow rate of the ORR at the Pt/ionomer interface was described as a central performance limitation in PEFCs. The most effective solution to this limitation is to employ dispersed platinum catalysts and to maximize catalyst utilization by an effective design of the cathode catalyst layer and by the effective mode of incorporation of the catalyst layer between the polymeric membrane electrolyte and the gas distributor/current collector. The combination of catalyst layer and polymeric membrane has been referred to as the membrane/electrode (M E) assembly. However, in several recent modes of preparation of the catalyst layer in PEFCs, the catalyst layer is deposited onto the carbon cloth, or paper, in much the same way as in phosphoric acid fuel cell electrodes, and this catalyzed carbon paper is hot-pressed, in turn, to the polymeric membrane. Thus, two modes of application of the catalyst layer - to the polymeric membrane or to a carbon support - can be distinguished and the specific mode of preparation of the catalyst layer could further vary within these two general application approaches, as summarized in Table 4. 

The phosphoric acid electrolyte of the acid fuel cell is far from optimum, particularly because of the low catalytic activity of platinum and other similar catalysts for 02 reduction in this electrolyte. A promising approach is to replace this electrolyte with new perfluorinated acids, which have high 02 solubility and do not adsorb on the catalyst surface. This should lead to much-improved performance of the air cathode. Work is in progress in several laboratories on the preparation of these new acids (e.g., perfluorinated sulfonic, phosphonic, and phosphinic acids) as a replacement for phosphoric acid. Reasonably high conductivity at high ratios of acid to water is also an important consideration. 

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... 

The electrolyte also plays an important role in the cell performance. For the propane oxidation in a fuel cell the monohydrate of trifluoromethanesulfonic acid as electrolyte had, at a platinum electrode at 135 °C, a limiting current which was more than 1000% greater than that of phosphoric acid . ... 

These cells operate only with hydrogen as the anode fuel and, moreover, the hydrogen must be pure since sulphur compounds and carbon monoxide adversely affect the performance of the Pt catalyst. Each cell consists of two teflon-bonded gas diffusion electrodes on a porous conducting support (see Fig. 10.21). At both anode and cathode the catalyst is platinum particles dispersed on carbon and a recent success has been a reduction in Pt loading from 10 mg cm to 0.75 mg cm ". The electrolyte is concentrated phosphoric acid absorbed onto a solid matrix and the cell operates at 200°C to improve the electrode kinetics. The cells are then mounted in stacks to increase the power output. 

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