Anode catalysts comprising MoS 2 and composite metal sulfides have been investigated for electrochemical oxidation of hydrogen sulfide in solid oxide fuel cells ͑SOFCs͒ at temperatures up to 850°C. All catalysts exhibited good electrical conductivity and catalytic activity at all temperatures. MoS 2 and composite catalysts were found to be more active than Pt, an established catalyst for high-temperature H 2 S-air fuel cells at 650-830°C. However, MoS 2 itself sublimes above 450°C. In contrast, composite catalysts ͑M-Mo-S͒ derived from a mixture of sulfides of Mo and other transition metals ͑Fe, Co, Ni͒ have been shown to be stable and effective for electrochemical conversion of H 2 S in SOFCs up to 850°C. Electrical contact is poor between platinum current collecting layers and metal sulfide anode catalysts. This problem has been overcome by mechanically mixing conductive Ag powder into the anode layer, instead of applying a thin layer of platinum to the anode.

Performance of a solid oxide fuel cell with the configuration of H 2 S, (MoS 2 ϩ NiS ϩ Ag)/yttria-stabilized zirconia/Pt, air is dependent on anode and cathode compartment gas flow rates. The cell open-circuit voltage ͑OCV͒ was independent of air flow rate but increased with increasing H 2 S flow rate. A linear relationship existed between OCV and the logarithm of H 2 S flow rate. Increasing the flow rate increased the exchange rate of reaction products and H 2 S feed at the anode catalyst, thereby decreasing the local concentration of reaction products and increasing the local concentration of H 2 S; hence, OCV increased from the Nernst effect. The magnitude of the change in OCV with temperature was consistent with calculated values based on reaction equilibria. It was found that increasing either or both air flow rate and H 2 S flow rate improved current-voltage and power density performance. The results were consistent with improved gas diffusion in the cathode with increasing air flow rate, and with both improved gas diffusion in anode and increased concentration of anodic electroactive species with increasing H 2 S flow rate.It has been a long-time goal to utilize the large amount of energy associated with the conversion of H 2 S to S or SO 2 as electrical energy. 1 However, it was not until 1987 that Pujare et al. reported the first direct H 2 S/air solid oxide fuel cell ͑SOFC͒. 2 Even at a very low level, H 2 S is poisonous to some metal anode catalysts, such as Pt, Au, or Ni. 3 H 2 S can also be harmful to operation of any currently available fuel cell systems, e.g., molten carbonate fuel cells ͑MCFCs͒, 4,5 phosphoric acid fuel cells ͑PAFCs͒, 6 as well as proton exchange membrane fuel cells ͑PEMFCs͒. Because of the deleterious effects of H 2 S, it is necessary to develop anode catalysts specifically for use with H 2 S/air SOFCs. Known types of H 2 /air SOFC cathodes have been adopted for use as H 2 S/air SOFCs, such as strontium-doped lanthanum manganite ͑LSM͒ 1,7,8 or Pt. [9][10][11][12] Pujare et al. investigated several thiospinels as well as WS 2 as the anode catalyst. The relative activities, based on empirical kinetics, were found to be in the order NiFe 2 S 4 Ͼ WS 2 Ͼ CuCo 2 S 4 Ͼ CuFe 2 S 4 Ϸ NiCo 2 S 4 Ͼ CuNi 2 S 4 . 7 Pt also has been used as anode catalyst for experimental H 2 S SOFCs 9-13 but was found to degrade over time in H 2 S stream due to the formation of PtS. 12 The choice of anode materials currently is limited mostly to metal sulfides. Anode materials based on WS 2 , or Li 2 S/CoS 1.035 have been shown to be stable in H 2 S atmosphere over extended periods of time. 8 Efforts also have been directed to development of H 2 S-tolerant electrolyte materials. While yttria-stabilized zirconia ͑YSZ͒ has frequently been used as the electrolyte, use of other materials also has been explored. In part of their work, Pujare et al. used calciastabilized zirconia as well as YSZ as electrolyte. 7 Kirk and Winnick investigated the performance of cells using yttria-or samaria-doped ceria or ytterbia-d...

A high-performance composite anode has been developed for H 2 S-air solid oxide fuel cells ͑SOFCs͒. The anode design is based on the requirements of three-phase boundary theory. The anode material comprises a mixture of composite metal sulfide ͑Mo-Ni-S͒ catalyst prepared from MoS 2 and NiS ͑1:1 weight ratio͒, admixed with up to 10% each of Ag as electronic conductor and yttria-stabilized zirconia ͑YSZ͒ as ionic conductor. The optimum composition is about 90 wt % Mo-Ni-S, 5 wt % Ag, and 5 wt % YSZ. A fuel cell using a 0.2 mm thick YSZ membrane produced a maximum sustainable current density over 480 mA cm Ϫ2 at 750°C and over 800 mA cm Ϫ2 at 850°C, and maximum power density 50 mW cm Ϫ2 at 750°C and over 200 mW cm Ϫ2 at 850°C.

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