I2CNER
Institute Interest Seminar Series
July 25, 2012
Computational Study of Reaction Mechanisms
on Solid Oxide Fuel Cell Anodes
Teppei Ogura
International Research Center for Hydrogen Energy, I2CNER FC Div., JST-CREST, NEXT-FC Center
teppei.ogura.094@m.kyushu-u.ac.jp
Solid oxide fuel cells (SOFCs) have been expected to be most efficient and versatile system
for chemical to electrical energy conversion. To get more efficiency and long-term durability for
SOFCs, it is essential to understand fundamental physics and chemistry occurred in SOFCs.
Computational analysis such as density functional theory (DFT), kinetic modeling, etc., is one of the
powerful tools to give us an atomic-scale insight. Today, introduced are some of our challenges to
computationally simulate and explain the complex phenomena occurred in SOFCs, especially
focusing on the anode.
Our first challenge is to understand the sulfur poisoning and degradation mechanisms of
nickel anode. It is well known that fuel impurities in practical SOFC fuels such as sulfur compounds
could cause the reduction of SOFC power generation performance. We have calculated the sulfur
effect on reaction energies of main path in direct methane reforming process (see Fig.1). The result
shows direct methane reforming is critically suppressed by the surface sulfur atoms while hydrogen
sulfide poisoning occurs. We also found that sulfur poisoning effect for hydrogen fuel is smaller than
methane fuel due to the difference of adsorbant size and its electronic property after dissociation.
Second challenge is to develop the coverage-dependent kinetic simulation algorism. At the
specific situation such as poisoning, adsorbant-adsorbant interaction is not negligible due to the high
surface coverage. Thus, we treated the thermodynamic and kinetic properties of surface
intermediates and reactions as a function of total surface coverage. Coverage-dependent simulations
were performed under sulfur poisoning conditions (see Fig.2). The sulfur coverage gradually goes up
with increase of hydrogen sulfide, which is qualitatively agreed with experimental measurement.
qS = 0.50
硫黄被覆率0.5
2.48
硫黄被覆率
qS = 0.250.25 1.08
CH4+O
0.0
1.92
1.59
1.47
CH3+H+O CH2+2H+O
0.05
0.1
0.01
CH+3H+O
-0.36
q = 0.000.0
硫黄被覆率
1.20
0.77
Activation energies are
estimated
0.6
S*
H*
0.0
-0.47
CO+4H
-1.05
With
coverage
dependence
0.4
0.2
Fig.1 Energetics change in methane reforming
by sulfur poisoning
Total*
0.8
CHO+3H
0.12
S
Ni(111) surface
Surface coverage
E (eV)
2.53
Without
coverage
dependence
1.0
3.55
0.001
0.01
0.1
1
10
100
1000
Concentration of H2S H2/H2S simulation @ 1073 K
Fig.2 Example of coverage dependent
kinetic simulation