Phosphoric acid fuel cells durability

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],... 

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. 

Shao Y, Yin G, Gao Y (2007) Understanding and approaches for the durability issues of Pt-based catalysts for PEM fuel cells. J Power Sources 171 558-566 Watanabe M, Tsurumi K, Mizukami T, Nakamura T, Stonehart P (1994) Activity and stability of ordered and disordered Co-Pt alloys for phosphoric acid fuel cells. J Electrochem Soc 141 2659-2668... 

Much of the knowledge in Pt/C durability derives from the experience with phosphoric acid fuel cells (PAFCs) at operating temperatures of about 200°C. Catalyst degradation is witnessed as an apparent loss of platinum electrochemical surface area over time, " associated with platinum crystal growth. These changes are ascribed to different processes which include... 

The most important features associated with fuel cell technologies are energy conversion efficiency, durability (or life time), and cost. These are common characteristic features among various fuel cells such as phosphoric acid fuel cells (PAFC), molten carbonate fuel cells (MCFC), polymer electrolyte fuel cells (PEFC), and solid oxide fuel cells (SOFC). For comparison among fuel cells, applications in... 

Loss of catalyst electrochemical surface area (ECSA), as discussed above, could be caused by Pt dissolution and migration into the membrane. In addition, increase in Pt particle size during fuel cell operation is another cause of ECSA loss in the catalyst layer. Loss of Pt surface area vs. time during fuel cell operation has been observed in both the phosphoric acid fuel cell [87-89] and PEMFC operations [9, 33, 90]. An increase in Pt particle size from 2-3 mn up to more than 10 mn during durability testing in the catalyst layer has been reported, determined by X-ray diffraction [46] or TEM image analysis [9, 33-38, 90]. 

The presence of phosphoric acid makes the chemistry in the Fenton test far more complex [69]. Firstly, the presence of phosphoric acid was shown to suppress the formation of radicals due to the lowered pH. Secondly, it was suggested that the presence of phosphoric acid further suppressed the degradation rate, possibly due to the formation of iron phosphates or by the ionization of the polymer. It was concluded that the presence of ferrous ion in the membrane-electrode interface had a negative impact on the fuel cell durability. However, this effect was mainly connected to the deterioration of the catalyst activity [69]. 

PAFCs were the first fuel-cell technology to be commercialised and represented almost 40% of the installed fuel cell units in 2004 (Sammes et al., 2004). Most of the demonstration units are in the range of 50-200 kW, but larger plants (1-10 MW) or smaller systems (1-10 kW) have also been built (Ghouse et al., 2000 Yang et al., 2002). Lifetimes of 5 years with > 95% durability have been demonstrated. Phosphoric acid electrode/electrolyte technology has reached maturity. However, fiirther increases in power density and reduced cost are needed to achieve economic competitiveness (US DOE, 2002 Larminie et al., 2003 Haile, 2003). 

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. 

As an outlook to further improvements of catalyst kinetics and durability in low-and high-temperature polymer electrolyte fuel cells, several possibilities are currently under investigation [73] (1) extended large-scale Pt and Pt-alloy surfaces [70] (2) extended nanostructured Pt and Pt-aUoy films [74] (3) de-alloyed Pt-alloy nanoparticles [75] (4) precious metal free catalyst as described by Lefevre et al. [76], e.g., Fe/N/C catalysts and (5) additives to the electrolyte which modify both adsorption properties of anions and spectator species and also the solubility of oxygen [77]. The latter approach is specific to fuel cells using phosphoric acid as electrolyte. 

In [74], completely phosphoric acid-free inter-mediate-T membranes and MEAs from 1/1 (mole imidazole/mole PO3H2) blend membranes of the PBI B5 (Fig. 4.4) and the phosphonated polymer polyvinyl phosphonic acid (PVPA) S12 (Fig. 4.5) were presented and compared to PA-doped membranes and MEAs. In the MEAs, a Pt electrocatalyst was deposited onto multiwaUed carbon nanotubes (MWNT) which have previously been coated with B5. This technique has previously been used for the preparation of electrocatalysts for anion-exchange membrane fuel cells [75]. In the final step, the MWNTs were coated with a layer of S12. It turned out that these membranes and MEAs possessed much higher durabilities than membranes and MEAs which have been doped with PA, therefore opening leeway for long-lasting intermediate-T fuel cell membranes without the PA leaching problems which are always present in PA-doped intermediate-T membranes and MEAs [76]. 

There are some other fuel cells, such as phosphoric acid (PAFC) or molten carbonate (MCFC) [4], that are commercially available but will probably not find significant implementation due to high cost and durability issues. 

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