Sulfur-tolerant anodes

It is well established that sulfur compounds even in low parts per million concentrations in fuel gas are detrimental to MCFCs. The principal sulfur compound that has an adverse effect on cell performance is H2S. A nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Chemisorption on Ni surfaces occurs, which can block active electrochemical sites. The tolerance of MCFCs to sulfur compounds is strongly dependent on temperature, pressure, gas composition, cell components, and system operation (i.e., recycle, venting, and gas cleanup). Nickel anode at anodic potentials reacts with H2S to form nickel sulfide. Moreover, oxidation of H2S in a combustion reaction, when recycling system is used, causes subsequent reaction with carbonate ions in the electrolyte [1]. Some researchers have tried to overcome this problem with additional device such as sulfur removal reactor. If the anode itself has a high tolerance to sulfur, the additional device is not required, hence, cutting the capital cost for MCFC plant. To enhance the anode performance on sulfur tolerance, ceria coating on anode is proposed. The main reason is that ceria can react with H2S [2,3] to protect Ni anode. 

There are a number of informative reviews on anodes for SOFCs [1-5], providing details on processing, fabrication, characterization, and electrochemical behavior of anode materials, especially the nickel-yttria stabilized zirconia (Ni-YSZ) cermet anodes. There are also several reviews dedicated to specific topics such as oxide anode materials [6], carbon-tolerant anode materials [7-9], sulfur-tolerant anode materials [10], and the redox cycling behavior of Ni-YSZ cermet anodes [11], In this chapter, we do not attempt to offer a comprehensive survey of the literature on SOFC anode research instead, we focus primarily on some critical issues in the preparation and testing of SOFC anodes, including the processing-property relationships that are well accepted in the SOFC community as well as some apparently contradictory observations reported in the literature. We will also briefly review some recent advancement in the development of alternative anode materials for improved tolerance to sulfur poisoning and carbon deposition. 

In recent years, there have been numerous studies on alternate anode materials. The areas of interest include carbon-tolerant anode materials, sulfur-tolerant anode materials, and redox-stable anode materials. The idea is that by developing alternative anode materials and structure, the reforming and the desulfurization unit could be eliminated, which would reduce the system complexity and cost dramatically. In this section, the studies into these new, alternative anode materials will be briefly touched upon. Because the number of candidate materials studied is quite large, the amount of study on any individual candidate anode material is rather small, and not much work has been done to reproduce the results reported. Therefore, it is not possible to fully evaluate the real potentials of those new materials proposed by different groups of researchers. Therefore, the focus would be on the fundamental issues for these alternative materials, instead of on the processing and properties of a specific candidate material. 

Summary of Previous Studies on Potential Sulfur-Tolerant Anode Materials for Solid Oxide Fuel Cells... 

Gong M, Liu X, Trembly J, and Johnson C. Sulfur-tolerant anode materials for solid oxide fuel cell application. J Power Sources 2007 168 289-298. 

Kurokawa H, Sholkalapper TZ, Jacobson CP, De Johghe LC, and Visco SJ. Ceria nanocoating for sulfur tolerant Ni-based anodes of solid oxide fuel cells. Electrochem... 

Mukundan R, Brosha EL, and Garzon FH. Sulfur tolerant anodes for SOFCs. Electrochem Solid-State Lett 2004 7 A5-A7. 

Zha S, Tsang P, Cheng Z, and Liu M. Electrical Properties and Sulfur Tolerance of La075Sr025Cr1.xMnxO3 under Anodic Conditions. J Solid State Chem 2005 178 1844-185o ... 

Zha S, Cheng Z, and Liu M. A sulfur-tolerant anode materials for SOFCs GdjTij 4Mo06O7. Electrochem Solid-State Lett 2005 8 A406-A408. 

Smith M and McEvoy AJ. Sulfur-tolerant cermet anodes. In Singhal SC, Mizusaki J, editors. Proceedings of the Ninth International Symposium on Solid Oxide Fuel Cells (SOFC IX), Pennington, NJ The Electrochemical Society, 2005 2005(7) 1437-1444. 

Choi S, Wang J, Cheng Z, and Liu M. Surface modification of Ni-YSZ using niobium oxide for sulfur-tolerant anodes in solid oxide fuel cells. J Electrochem Soc 2008 155 B449-B454. 

Similarly to the case of direct-oxidation anode materials, sulfur-tolerant anode materials based on sulfides [6, 7] or double-perovskite oxides have special requirements for their processing into SOFC layers. For example, nickel sulfide-promoted molybdenum sulfide is tolerant to high sulfur levels [7], However, it has a low melting temperature [6] that has resulted in the development of cobalt sulfide as a stabilizer of the molybdenum sulfide catalyst [6], CoS-MoS2 admixed with Ag has an even higher performance in H2S-containing fuels than in pure H2 [6]. However, processing methods such as PS, infiltration, or sol-gel techniques that can process... 

Alloying the nickel of the anode to improve tolerance for fuel contaminants has been explored. Gold and copper alloying decreases the catalytic activity for carbon deposition, while dispersing the anode with a heavy transition metal catalyst like tungsten improves sulfur resistance. Furthermore, ceria cermets seem to have a higher sulfur tolerance than Ni-YSZ cermets [75],... 

The MCFC has some disadvantages, however the electrolyte is very corrosive and mobile, and a source of CO2 is required at the cathode (usually recycled from anode exhaust) to form the carbonate ion. Sulfur tolerance is controlled by the reforming catalyst and is low, which is the same for the reforming catalyst in all cells. Operation requires use of stainless steel as the cell hardware material. The higher temperatures promote material problems, particularly mechanical stability that impacts life. 

The rapid equilibration of the water gas shift reaction in the anode compartment provides an indirect source of H2 by the reaction of CO and H2O. If H2S poisons the active sites for the shift reaction, this equilibrium might not be established in the cell, and a lower H2 content than predicted would be expected. Fortunately, the evidence (77,78) indicates that the shift reaction is not significantly poisoned by H2S. In fact, Cr used in stabilized-Ni anodes appears to act as a sulfur tolerant catalyst for the water gas shift reaction (78). 

Cheng Z et al (2011) From Ni-YSZ to sulfur-tolerant anode materials for SOFCs electrochemical behavior, in situ characterization, modeling, and future perspectives. Energy Environ Sci 4 4380... 

Kurokawa H, Yang L, Jacobson CP, De Jonghe LC, Visco SJ (2007) Y-doped SrTi03 based sulfur tolerant anode for solid oxide fuel cells. J Power Sources 164 510... 

The sulfur tolerance of these anodes is primarily dictated by the thermodynamics of ceria sulfide and oxysulfide formation, as the sulfur concentration required for CuaS formation is significantly higher [45]. Kim et al. demonstrated that stable operation could be achieved if the sulfur level in the fuel is reduced to 100 ppmv S as thiophene in 5 mol% n-CioH22 - this is below the concentration predicted from thermodynamics for Cc202S formation. This study was followed by work of He et al. who demonstrated stable operation up to 450 ppmv H2S in H2 [46], significantly higher than the tolerance levels reported for Ni-based anodes [47]. 

Pd and Rh may be prohibitively expensive for large-scale SOFC applications, but Ni or even Fe would be suitable. Long-term fuel cell tests using (sulfur-rich) CH4 are required to assess the sulfur tolerance and sintering resistance of Ni in the YSZ/LSCM-Ni anode. 

Zha SW, Tsang P, Cheng Z, Liu ML (2005) Electrical properties and sulfur tolerance of Lao.75Sro.25Cri xMnx03 under anodic conditions. J Solid Slate Chem 178 1844—1850... 

For the anode, a nickel-oxide-YSZ cermet is applied, though the limitations of the current state of that type with respect to redox stability, coking, and sulfur tolerance are known. More recently, the integration of oxide anodes into the concept has been analyzed and tested [78]. 

Further system simplification would occur if a sulfur-free fuel was used or if the fuel cell were sulfur tolerant in that case, the fuel could be provided directly from the reformer to the fuel cell. In order to minimize system volume, (and minimize the associated system weight and start-up time) integration of the system components is a key design issue. By recycling the entire anode tailgas to provide steam, a water management system can be avoided, though a hot gas recirculation system is required. 

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