Cathodes, in MCFC

The reaction of H2 and O2 produces H2O. When a carbon-containing fuel is involved in the anode reaction, CO2 is also produced. For MCFCs, CO2 is required in the cathode reaction to maintain an invariant carbonate concentration in the electrolyte. Because CO2 is produced at the anode and consumed at the cathode in MCFCs, and because the concentrations in the anode and cathode feed streams are not necessarily equal, the Nemst equation in Table 2-2 includes the CO2 partial pressure for both electrode reactions. 

Kasai H., Suzuki A., Dissolution and Deposition of Nickel-Oxide Cathode in MCFC, Proc. Third International Symposium on Carbonate Fuel Cell Technology, vol. 93-3, p. 240. 

Weaver [40] studied alternate cathode materials at 650 °C, finding several that performed well. Steady-state polarization on Ni, Co and Fe porous electrodes operating as cathodes in a MCFC, with a standard (Li/K)2 C03 tile is shown in Figs. 30-32. Note that the oxidant gas fed to these cathodes is, in normal MCFC operation, the fuel, composed of 32.5% H2, 17.5% COz, 17.5% H20, the balance N2. Polarizations were first taken with this clean gas where the only reaction can be Eq. (35). After steady-state was attained, 0.65% H2S was added and sufficient time allowed for the electrode to convert to the sulfides. After 24 hours, the outlet H2S reached the inlet level and polarizations were measured. Note in Figs. 30-32, that the performance with H2S is significantly improved over the clean gas. (The Ni sample was a commercial (Gould) MCFC electrode the Co and Fe were pressed from powders. Each gas was 8 sq cm in superficial area). The improvement is probably due to a catalytic mechanism involving sulfur interactions with the electrode, as, for Co ... 

Molten Carbonate Fuel Cell (MCFC) The electrolyte in this fuel cell is usually a combination of alkali carbonates, which is retained in a ceramic matrix of LiA102. The fuel cell operates at 600 to 700°C where the alkali carbonates form a highly conductive molten salt, with carbonate ions providing ionic conduction. At the high operating temperatures in MCFCs, Ni (anode) and nickel oxide (cathode) are adequate to promote reaction. Noble metals are not required. 

Typical cathode performance curves obtained at 650°C with an oxidant composition (12.6% 02/18.4% C02/69% N2) that is anticipated for use in MCFCs, and a common baseline composition (33% 02/67% CO2) are presented in Figure 6-3 (20,49). The baseline composition contains O2 and CO2 in the stoichiometric ratio that is needed in the electrochemical reaction at the cathode (Equation (6-2)). With this gas composition, little or no diffusion limitations occur in the cathode because the reactants are provided primarily by bulk flow. The other gas composition, which contains a substantial fraction of N2, yields a cathode performance that is limited by gas phase diffusion from dilution by an inert gas. 

MCFC - <0.5 ppm sulfur as H2S (at the cathode) equates to <10 ppm at the anode because of fuel exhaust being sent to the cathode in an MCFC system (same amount of sulfur, more gas at the cathode), poisoning is reversible. 

Molten Carbonate Fuel Cell The electrolyte in the MCFC is a mixture of lithium/potassium or lithium/sodium carbonates, retained in a ceramic matrix of lithium aluminate. The carbonate salts melt at about 773 K (932°F), allowing the cell to be operated in the 873 to 973 K (1112 to 1292°F) range. Platinum is no longer needed as an electrocatalyst because the reactions are fast at these temperatures. The anode in MCFCs is porous nickel metal with a few percent of chromium or aluminum to improve the mechanical properties. The cathode material is hthium-doped nickel oxide. 

Also for cathodic oxygen reduction in low-temperature fuel cells, platinum is indispensible as a catalyst whereas the cathodic electrocatalysts in MCFCs and SOFCs are lithiated nickel oxide and lanthanum-manganese per-ovskite, respectively. Appleby and Foulkes in the Fuel Cell Handbook (101) reviewed the fundamental work as well as the technologically important publications covering electrocatalysis in fuel cells till 1989. 

Austenitic stainless steels like 31 OS, 316, or 316L are typically used for the construction of cathode and anode current collectors and bipolar separator plates. Corrosion of these steel components is a major lifetime-limiting factor in MCFC. The corrosion behavior of stainless steel components in molten carbonate conditions has been studied extensively during the past decade. Research is being aimed at increasing the corrosion resistance of these components by altering the alloy composition or by surface modification techniques. ... 

Lair V,Ringuedd A,AlbinV and Cassir M (2008), Characterization in MCFC conditions of high-temperature potentiostaticaUy deposited Co-based thin films on NiO cathode . Ionics, 14,555-561. 

Fuel cells (FCs) are electrochemical devices able to convert the chemical energy of reactions directly into electric energy [1]. The electrochemical conversion is possible in molten carbonate fuel cells (MCFCs) thanks to a ceramic matrix consisting of LiAl02 filled with a combination of molten alkali metal carbonates, which acts as an electrolyte and allows the transfer of carbonate ions from the cathode to the anode [2]. The electrochemical reactions occurring in MCFCs are as follows ... 

Many of the models can be used to describe not only the behavior of single cells, but also that of a whole stack. This extension of the model equations has not been discussed here but, as indicated in Table 28.1, this has been reahzed with many models. Other extensions and modifications, such as the application ofequihbrium assumptions with regard to the reforming reactions, the modeling of a catalytic burner between the anode exhaust and the cathode inlet, or the addition of model equations describing an indirect internal reforming reactor, are frequently apphed in MCFC models. For the sake of brevity, they have not been discussed in detail in this chapter. 

Typical cathode performance curves obtained at 650 °C with an oxidant composition (12.6 percent O2/I8.4 percent CO2/69 percent N2) that is anticipated for use in MCFCs, and a common baseline composition (33 percent O2/67 percent CO2) are presented in Figure 6-4 (22,49). The baseline... 

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