Anode carbon deposition

Sol-gel technique has also been applied to modify the anode/electrolyte interface for SOFC running on hydrocarbon fuel [16]. ANiA SZ cermet anode was modified by coating with SDC sol within the pores of the anode. The surface modification of Ni/YSZ anode resulted in an increase of structural stability and enlargement of the TPB area, which can serve as a catalytic reaction site for oxidation of carbon or carbon monoxide. Consequently, the SDC coating on the pores of anode leads to higher stability of the cell in long-term operation due to the reduction of carbon deposition and nickel sintering. 

This presentation reports some studies on the materials and catalysis for solid oxide fuel cell (SOFC) in the author s laboratory and tries to offer some thoughts on related problems. The basic materials of SOFC are cathode, electrolyte, and anode materials, which are composed to form the membrane-electrode assembly, which then forms the unit cell for test. The cathode material is most important in the sense that most polarization is within the cathode layer. The electrolyte membrane should be as thin as possible and also posses as high an oxygen-ion conductivity as possible. The anode material should be able to deal with the carbon deposition problem especially when methane is used as the fuel. 

Methane decomposition is the most important reaction step, especially for high-temperature operations. Thus, carbon deposition occurs commonly and is a major problem, especially with the Ni-based anode. However, carbon deposition may not deactivate the anode [10, 11]. In some cases, the anode activity increases due to carbon deposition whieh increases the electrical conductivity of the low-Ni-content anode [II]. 

To reduce the formation of carbon deposited on the anode side [2], MgO and Ce02 were selected as a modification agent of Ni-YSZ anodic catalyst for the co-generation of syngas and electricity in the SOFC system. It was considered that Ni provides the catalytic activity for the catalytic reforming and electronic conductivity for electrode, and YSZ provides ionic conductivity and a thermal expansion matched with the YSZ electrolyte. 

The future remains bright for the use of carbon materials in batteries. In the past several years, several new carbon materials have appeared mesophase pitch fibers, expanded graphite and carbon nanotubes. New electrolyte additives for Li-Ion permit the use of low cost PC based electrolytes with natural graphite anodes. Carbon nanotubes are attractive new materials and it appears that they will be available in quantity in the near future. They have a high ratio of the base plane to edge plain found in HOPG. The ultracapacitor application to deposit an electronically conductive polymer on the surface of a carbon nanotube may be the wave of the future. 

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. 

As stated, one of the fundamental problems encountered in the direct oxidation of hydrocarbon fuels in SOFCs is carbon deposition on the anode, which quickly deactivates the anode and degrades cell performance. The possible buildup of carbon can lead to failure of the fuel-cell operation. Applying excess steam or oxidant reagents to regenerate anode materials would incur significant cost to SOFC operation. The development of carbon tolerant anode materials was summarized very well in several previous reviews and are not repeated here [7-9], In this section, the focus will be on theoretical studies directed toward understanding the carbon deposition processes in the gas-surface interfacial reactions, which is critical to the... 

The development of new, alternative anode materials has recently attracted considerable interest. Several new materials show improved tolerance to sulfur poisoning and carbon deposition. However, critical issues associated with each candidate material are yet to be overcome. The traditional Ni-YSZ cermet anode still offers the best performance when clean hydrogen is used as the fuel and will continue to play an important role in SOFCs. 

A similar infiltration method was used to form anodes with Cu-Co alloys as the electronically conducting phase, with Co added to enhance the anode catalytic activity without catalyzing carbon deposition [85], in contrast with Ni-Cu alloys, which were found to catalyze carbon deposition even when small quantities of Ni were present [86], The authors of the studies [84-86] have reported that the percolation of the infiltrated phases is incomplete following the processing of the cells, such that subsequent carbon deposition can actually serve to connect previously isolated islands of the metallic phase, thus increasing the electronic conductivity and decreasing Rs in the short term. Since carbon deposition was observed by the same authors to cause severe... 

Catalytic promotion of carbon deposition from carbon-containing fuels. Carbon deposited in the anode will generally cause irreversible damage (disintegration) of the anode structure. 

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 addition of H2O and CO2 to the fuel gas modifies the equilibrium gas composition so that the formation of CH4 is not favored. Carbon deposition can be reduced by increasing the partial pressure of H2O in the gas stream. The measurements (20) on 10 cm x 10 cm cells at 650°C using simulated gasified coal GF-1 (38% H2/56% CO/6% CO2) at 10 atm showed that only a small amount of CH4 is formed. At open circuit, 1.4 vol% CH4 (dry gas basis) was detected, and at fuel utilizations of 50 to 85%, 1.2 to 0.5% CH4 was measured. The experiments with a high CO fuel gas (GF-1) at 10 atmospheres and humidified at 163°C showed no indication of carbon deposition in a subscale MCFC. These studies indicated that CH4 formation and carbon deposition at the anodes in an MCFC operating on coal-derived fuels can be controlled, and under these conditions, the side reactions would have little influence on power plant efficiency. 

Solid Particulates These contaminants can originate from a variety of sources, and their presence is a major concern because they can block gas passages and/or the anode surface. Carbon deposition and conditions that can be used to control its formation have been discussed earlier in this section. Solid particles such as ZnO, which is used for sulfur removal, can be entrained in the fuel gas leaving the desulfurizer. The results by Pigeaud (72) indicate that the tolerance limit of MCFCs to particulates larger than 3 pm diameter is <0.1 g/1. 

When designing an MCFC power system, several requirements must be met. An MCFC system must properly condition both the fuel and oxidant gas streams. Methane must be reformed into the more reactive hydrogen and carbon monoxide. Carbon deposition, which can plug gas passages in the anode gas chamber, must be prevented. To supply the flow of carbonate ions, the air oxidant must be enriched with carbon dioxide. Both oxidant and fuel feed streams must be heated to their proper inlet temperatures. Each MCFC stack must be operated within an acceptable temperature range. Excess heat generated by the MCFC stacks must be recovered and efficiently utilized. 

Finally, an additional approach to using hydrocarbon fuels with Ni-based anodes involves using methanol and ethanol, molecules that carry sufficient oxygen to avoid carbon formation.Unlike the case with low-temperature fuel cells, methanol crossover is not an issue with ceramic membranes. Since methanol decomposes very readily to CO and H2. SOFC can operate with a very high performance using this fuel. ° ° In addition, recent work has shown promising performance levels with limited carbon deposition using dimethyl ether as fuel. ° ° ... 

Anodes using carbon deposits as the conductor can exhibit reasonably high performance. This was shown earlier, in Figure 19, where carbon formed by decomposition of /7-butane provided the conductivity... 

A variety of nanomaterials have been synthesized by many researchers using anodic aluminum oxide film as either a template or a host material e.g., magnetic recording media (13,14), optical devices (15-18), metal nanohole arrays (19), and nanotubes or nanofibers of polymer, metal and metal oxide (20-24). No one, however, had tried to use anodic aluminum oxide film to produce carbon nanotubes before Kyotani et al. (9,12), Parthasarathy et al. (10) and Che et al. (25) prepared carbon tubes by either the pyrolytic carbon deposition on the film or the carbonization of organic polymer in the pore of the film. The following section describes the details of the template method for carbon nanotube production. 

Formation of Carbon Nanotubes by Pyrolytic Carbon Deposition on Anodic Film... 

Fig. 10.1.5 SEM photographs of the carbon tubes prepared by carbon deposition of propylene (a, b) carbon deposition period of I h on the anodic oxide film with 30-nm channels (c, d) a period of 12 h on the film with 230-nm channels. (From Ref. 12.)...

By the template technique using anodic oxide films and pyrolytic carbon deposition, one can prepare monodisperse carbon tubes. Since the length and the inner diameter of the channels in an anodic oxide film can easily be controlled by changing the anodic oxidation period and the current density during the oxidation, respectively, it is possible to control the length and the diameter of the carbon tubes. Furthermore, by changing the carbon deposition period, the wall thickness of the carbon tubes is controllable. This template method makes it possible to produce only carbon tubes that are not capped at both ends. Various features of the template method are summarized in Table 10.1.1 in comparison with the conventional arc-discharge method. 

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