Electron microscopy
Ni-Yttria-Stabilized Zirconia (YSZ)
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Ni-YSZ cermet is most commonly used as the anode of a solid oxide fuel cell (SOFC) because it has excellent electrochemical performance in both hydrogen fuel and clean blended synthetic coal syngas mixtures (e.g. 30% H2, 26% H2O, 23% CO, and 21% CO2). However, it still possesses two main problems when using fuels such as natural gas:
        i) Carbon deposition. When hydrocarbons are used as fuels, carbon deposition is caused, resulting in direct structural damage to the SOFC surface and blocking the area of activation for reaction processes. In this case, Ni catalysts can be deactivated by the carbon deposited, resulting in rapid cell degradation [4].
        ii) Poisoning of trace impurities: phosphide, chloride, or sulphide. [1,2] For instance, phosphine (PH3) in coal-derived syngas can cause degradation of cell performance because Ni5P2 is formed on the cell surface.

The carbon deposition varies depending on the reaction conditions:
        i) Formation of carbidic species (e.g. C bonded to Ni) at temperatures equal to or below 350 °C. This formation is not normally observed in Ni-YSZ SOFCs that operate at higher temperatures.
        ii) Formation of adsorbed or amorphous (graphitic) carbon layer at temperatures above 350 °C, existing in hydrocarbon-fuelled SOFCs.

Various strategies have been suggested to limit carbon deposition in SOFCs. For instance, direct oxidation of natural gas presented that the carbon deposition on a Ni/YSZ anode can be prevented by increasing the operation current density and lowering the operation temperature [5,6]. However, it is not easy to achieve such operation conditions. Therefore, it is necessary to develop alternative anode materials that are capable of displaying mixed conductivity when subjected to complex fuels [3].

Except for Ni/YSZ as the most common anode material in SOFCs, gadolinium doped ceria oxide (GDC), doped SrTiO3 [7,8], and Cu-CeO-YSZ [9] are also used. Nonetheless, some of them have disadvantages of complicated fabrication processes, low conductivity or poor electrochemical catalyst activity.






[1] F. N. Cayan, M. Zhi, S. R. Pakalapati, I. Celik, N. Wu, R. Gemmen , Journal of Power Sources, 2008. 185(2): p. 595-602.
[2] T. S. Li, H. Miao, T. Chen, W. G. Wang, C. Xuz, Journal of The Electrochemical Society, 2009. 156(12): p. B1383-B1388.
[3] C. Sun, U. Stimming, Journal of Power Sources, 2007. 171(2): p. 247-260.
[4] T. Chen, W. G. Wang, H. Miao, T. Li, C. Xu, Journal of Power Sources, 2011. 196(5): p. 2461-2468.
[5] Y. Lin, Z. Zhan, J. Liu, S. A. Barnett, Solid State Ionics, 2005. 176(23–24): p. 1827-1835.
[6] J. Liu, S.A. Barnett, Solid State Ionics, 2003. 158(1–2): p. 11-16.
[7] J. Canales-Vazquez, S.W Tao, J.T.S Irvine, Solid State Ionics, 2003. 159(1–2): p. 159-165.
[8] S. Hui, A. Petric, Journal of The Electrochemical Society, 2002. 149(1): p. J1-J10.
[9] S. Park, J.M. Vohs, R.J. Gorte, Nature, 2000. 404(6775): p. 265-267.


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