Degradation and high-temperature fuel cells

Li Q, Rudbeck HC, Chromik A et al (2010) Properties, degradation and high temperature fuel cell test of different types of FBI and FBI blend membranes. J Membr Sci 347 260-270... 

The two types of high temperature fuel cell are quite different from each other (Table 6). The molten carbonate fuel cell, which operates at 650°C, has a metal anode (nickel), a conducting oxide cathode (e.g. lithiated NiO) and a mixed Li2C03/K2C03 fused salt electrolyte. Sulphur attack of the anode, to form liquid nickel sulphide, is a severe problem and it is necessary to remove H2S from the fuel gas to <1 ppm or better. However, CO is not a poison. Other materials science problems include anode sintering and degradation, corrosion of cell components and evaporation of the electrolyte. Work continues on this fuel cell in U.S.A. and there is some optimism that the problem will be solved within 10 years. 

Even though high-temperature fuel cells have good fuel flexibility with various kinds of fuels, some minor constituents (impurities) coming from, for example, low-purity fuels, raw materials, and system components can also react with electrode materials or can be adsorbed on the electrode reaction sites, hindering electrode reactions. Such poisoning phenomena can lead to fuel cell performance degradation with time. 

Aqueous electrolytes are limited to temperatures of >200 °C because of their high water vapor pressure and/or rapid degradation at higher temperatures. The operating temperature also determines the type of fuel that can be used in a fuel cell. The low-temperature fuel cells with aqueous electrolytes are, in most practical applications, restricted to H2 as a fuel. In high-temperature fuel cells, CO and even CH4 can be used because of the inherently rapid electrode kinetics and the lesser need for high catalytic activity at high tanperature. 

Degradation and durability of electrodes of solid oxide fuel cells, in Materials for High-Temperature Fuel Cells, Wiley-VCH Verlag GmbH, Weinheim, pp. 245-307. 

In Section 8.2.1 we pointed out that a power plant with a battery of tubular SOFCs had been operated more or less successfiiUy for about 16,000 hours. Apart from the communication cited, very few data can be found in the literature as to the results of long-term testing of soUd-oxide fuel cells. Since this type of high-temperature fuel cell is intended primarily for large stationary power plants and thus has large investment needs, information as to potential lifetimes and reasons for gradual performance degradation or possible cases of sudden failure are extremely important. 

On the other hand, there is a great demand for alternative fuel cells operating at moderate temperatures. In this context, intermediate temperature (400-800 °C) fuel cells are very attractive since they combine the advantages of both high- and low-temperature fuel cells such as fast electrode kinetics, fuel flexibility, and fewer degradation problems [7]. Furthermore, the tendency of lower temperatures makes conventional ceramic fuel cells (mainly solid oxide fuel cells SOFCs) a leading candidate for applications such as stationary power plants but also the possibility to replace internal combustion engines in vehicles [8]. Ceramic fuel cells based on ceria-carbonate salt composite electrolytes have been intensively studied for the past decade... 

Porous metallic structures have been used for electrocatalysis (Chen and Lasia, 1991 Kallenberg et al., 2007). Porous electrodes are made with conductive materials that can degrade under high temperatures at high anodic potential conditions. This last problem is of less importance for fuel cell anodes, which operate at relatively low potentials, but it can be of importance for electrochemical reactors. Porous column electrodes prepared by packing a conductive material (carbon fiber, metal shot) forming a bar are frequently used. Continuous-flow column electrolytic procedures can provide high efficiencies for electrosynthesis or removal of pollutants in industrial situations. Theoretical analysis for the electrodeposition of metals on porous solids has been provided by Masliy et al. (2008). 

In addition to loss of the platinum, the carlxm support that anchors the platinum crystallites and provides electrical coimectivity to the gas-diffusion media and bipolar plates is also subject to degradation. In phosphoric acid fuel cell, graphitized carbons are the standard because of the need for corrosion resistance in high-temperature acid environments [129], but PEM fuel cells have not employed fully graphitized carbons in the catalyst layers, due in large part to the belief that the extra cost could be avoided. Electrochemical corrosion of carbon materials as catalyst supports will cause electrical isolation of the catalyst particles as they are separated from the support or lead to aggregation of catalyst particles, both of which result in a decrease in the electrochemical active surface area of the catalyst and an increase in the hydrophUicity of the surface, which can, in turn, result in a decrease in gas permeability as the pores become more likely to be filled with liquid water films that can hinder gas transport. 

Aric6 A S, Stassi A, Modica E, Ornelas R, Gatto I, Passalacqua E and Antonucci V (2008), Performance and degradation of high temperature polymer electrolyte fuel cell catalysts. Journal of Power Sources, 178,525-536. 

Schmidt, T.J. (2006) Durability and degradation in high-temperature polymer electrolyte fuel cells. ECS Trans., 1 (8), 19-31. 

Qi Z, Buelte S. Effect of open circuit voltage on performance and degradation of high temperature PBI-H3PO4 fuel cells. J Power Sources 2006 161 1126-32. 

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