Solutions to exercises
299
S
5.2 J K mol
r 298 K
1
1
Δ
= −
−
−
°
and
S
39.37 J K mol
r 1073 K
1
1
Δ
=
−
−
°
The standard free enthalpy of the reaction is given by
G°
H° T S°
nFE
r
r
r
Δ
Δ
Δ
=
−
= −
Consequently,
2 at 298 K
G
802.2 10
298 ( 5.2)
r 298 K
3
Δ
= −
−
−
−
#
°
G
800.65 kJ mol
r 298 K
1
Δ
= −
−
°
E
8 96 480
800.65 10
CH /CO ,298 K
3
4
2
Δ
=
−
#
#
°
from which
E
1.037 V
298 K
Δ
=
°
2 at 1 073 K
G
775.2 10 1073 39.37
r 1073 K
3
Δ
=−
−
#
#
°
G
817.44 kJ mol
r 1073 K
1
Δ
=−
−
#
°
from which
E
1.059 V
1073 K
Δ
=
°
Note that temperature only slightly affects the standard emf of the pair
CH 4 / CO 2 .
3. Given the proportion of carrier gas, the steam/carbon (S/C) ratios allow
us to calculate the partial pressures of the various gases. The expressions
established in the preceding questions allow us to calculate the theoretical
emf ΔE th of the cell. The experimental emf ΔE expt are obtained from the
polarization curves for i = 0 (see fig. 99). Table 53 gives the results. Note
the good agreement between the theoretical values and the experimental
results (pressures are in bars).
Table 53 – Partial pressure of gas and theoretical and measured emf
of a SOFC fueled by methane for various operating modes.
Operating mode
S/C P CH 4 P H 2 O P CO 2 P O 2 ΔE th [V] ΔE expt [V]
Direct internal reforming
1
0.05
0.05 4.10
−3
0.21
1.184
1.180
Gradual internal
reforming
½ 0.066 0.033 4.10
−3
0.21
1.209
1.210
¼
0.08
0.02 4.10
−3
0.21
1.237
1.240