PNNL-13501 - Pacific Northwest National Laboratory

1. Eopen is weakly dependent on temperature. Eopen
changes less than 1% when T changes by 50 K.
2. The change of Eopen versus T is approximately linear.
3. The temperature difference in air and fuel does not
affect Eopen significantly. With 20°C difference
across a PEN structure, computations using the
average T may cause about 1% error.
4. Eopen is strongly dependent on fuel composition. Eopen
drops by 20% from 1.11V at the fuel inlet to 0.89V at
the gas outlet for an 80% utilization.
5. Higher fuel utilization does not always mean higher
efficiency, although this is usually true. There exists
an optimal fuel utilization for the best cell efficiency.
The main goal of the electrochemistry code is to generate
the V-i curves, current-power curves, and heat-current
curves with various temperatures because practical SOFC
performance is usually described in terms of these
parameters. The analyses of the results are summarized
with figures below. The thickness of electrolyte, cathode,
and anode are 10, 50, and 750 μm, respectively.
1. Voltage- and power-current curves for different
temperatures are very different from each other. The
maximum output powers differ by a factor of 5 for
600°C and 900°C (Figure 1).
2. The cell operating voltages corresponding to the
maximum output powers are almost identical. They
are 0.47V at 600°C and 0.49V at 900°C (Figure 1).
3. The electrical heat (generated in the PEN structure
due to the current passage) is large and increases
rapidly with increasing current passage (Figure 2).
4. The heat may increase the PEN temperature and
induce temperature gradient field and thermal stress.
5. The heat generation severely limits the cell
performance.
CO Oxidation at Anode
This task studied the effect of CO on the total current. A
one-dimensional steady-state model of an SOFC stack
was developed. The spreadsheet-based stack model
accounts for overpotentials due to ohmic resistance of the
cell components, to contact resistance at electrode-current
voltage (V)
power (W/cm2)
1.2
1
0.8
0.6
0.4
0.2
0
3
2.5
2
1.5
1
0.5
0
0
0.4
0.9
1.3
1.8
2.2
2.7
I-V curve
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
current (A/cm2)
(a)
current-power curve
3.1
3.6
(b)
4.0
4.5
current (A/cm2)
4.9
5.4
5.8
6.3
6.7
7.2
900C
850C
800C
750C
700C
650C
600C
900C
850C
800C
750C
700C
650C
600C
Figure 1. (a) Voltage vs. current density and (b) Power
density vs. current density at seven temperatures
collector interfaces, to charge transfer at the electrodes, to
diffusion of reactants into and products out of the porous
electrodes, and to leakage of gaseous oxygen into the
anode side of the cell. The model is capable of simulating
anode gases derived from hydrogen fuels. It is assumed
that anode gas compositions are determined by attainment
of equilibrium for the water-gas shift reaction.
heat (W/cm2)
4
3.5
3
2.5
2
1.5
1
0.5
0
0
0.2
0.4
0.6
0.8
1
current-heat curve
1.2
1.4
1.6
1.8
2
current (A/cm2)
2.2
2.4
2.6
2.8
3
Computational Science and Engineering 145
3.2
900C
850C
800
750C
700C
650C
600C
Figure 2. Heat generated by current passage. P(H 2)=0.97
atm, P(O s)=0.21 atm, P(H 2O)=0.03 atm.