In-Situ Investigation on the Crystal and Microstructure Change of Nickel Interacted with Methane at High Temperatures

Abstract

The solid oxide fuel cell (SOFC) is a novel electricity generation technology which converts the chemical energy of fuels into electrical power, with high efficiency and low pollution. One of the superiors of SOFCs over other kinds of fuel cell is that carbon-containing fuels can be directly used, leading to much lower operation cost. Nickel cermet is the most commonly used anode material for SOFCs. However, nickel promotes carbon deposition reaction, which results in significant performance degradation, when carbon-containing fuels are used. While there have been quite a few studies on replacing nickel with other materials in the anode of SOFCs operated with carbon-containing fuels, none of the alternatives can surpass nickel for practical application. Meanwhile, there are some evidences showing that carbon can be used as a fuel for SOFCs (the so-called direct carbon solid oxide fuel cells, DC-SOFCs), suggesting that carbon itself is not the cause of deactivation of Ni-based anode exposed to carbon-containing fuels in SOFCs. It is the interaction between nickel and carbon-containing gas that destroys the structure of nickel bulk, resulting in pulverization of the anode. Some previous researches have shown that carbon fiber may grow when transition metals, including nickel, are exposed to carbon-containing gases. The crystal lattice and microstructure changes were assumed but rarely verified by direct experimental evidences. In this report, we present our recent work of in situ investigation on the crystal lattice and microstructure changes of nickel-based bulk interacted with methane at high temperatures. In situ thermal expansion measurements are carried out, using a thermal dilatometer, on Ni-YSZ bulk samples exposed to methane at temperatures of 650 and 800°C, respectively. The samples for the thermal dilatation measurements are of cylindrical shape with relatively thin wall. In situ XRD characterizations are carried out on Ni-YSZ powder, which also exposed to methane at 650 and 800°C, respectively. The thermal dilatation measurement results show that the Ni-YSZ bulk expands when it is exposed to methane at both 650 and 800°C. The expansion leads to the bulk broken or destroyed. At 800°C, the time it takes, from the moment of nickel exposed to methane to that when it is destroyed, is 14 minutes, which is 4 times shorter than that it takes at 650°C, 70 minutes. Room temperature TEM measurements show that carbon deposits on nickel interacted with methane at 800°C is in the form of encapsulation while that of 650°C is carbon fiber. The in situ XRD characterizations show that when nickel is exposed to methane, its crystal lattice parameter increases, both in the situations of 650 and 800°C. At the beginning of nickel exposed to methane (i.e., 10 minutes) at 650°C, its crystal lattice parameter increases. However, the increased parameter reduces almost back to the initial value after a while (i.e., 20 minutes). In the situation of nickel exposed to methane at 800°C, its lattice parameter increases to maximum in 10 minutes and the value is stable in the following time. The above experimental results are analyzed and discussed in detail in the report.