Environmental Engineering Reference
In-Depth Information
contact by blocking the electrons and allows the flow of oxide ions from the
cathode to the anode to maintain the overall electrical charge balance and to close
the current circuit. In addition, the electrolyte determines the operating temperature
of the FC.
In a H 2 -
O 2 SOFC, the electrochemical reactions occurring are
H 2 +O 2− !
H 2 O+2e
ð
RX
:
16
:
1
Þ
at the anode and
1
2 O 2 +2e !
O 2−
ð
RX
:
16
:
2
Þ
at the cathode. The resulting overall reaction thus is
H 2 + 1
2 O 2 !
H 2 O
ð
RX
:
16
:
3
Þ
As there are no irreversible combustion processes, energy conversion in FC is
not limited to the efficiency of the Carnot cycle; thus, they possess a high reversible
efficiency Equation (16.1):
η rev = Δ
g r
ð
Eq
:
16
:
1
Þ
Δ
h r
g r is the change of Gibbs energy (J mol −1 ), and
In Equation (16.1),
Δ
Δ
h r is the
enthalpy change (J mol −1 ).
For an FC operating at a constant temperature and pressure, the maximum electri-
cal work that the FC can perform is given by the negative of the Gibbs free energy
difference Equation (16.2):
Δ
g f =
n F E
ð
Eq
:
16
:
2
Þ
where n is the number of electrons transferred for each molecule of fuel, F is the
Faraday constant (= 96,485 C mol −1 ), and E is the reversible voltage (V).
The following Nernst equation gives the reversible voltage for the total cell
reaction occurring in the FC when hydrogen is used as fuel:
!
×ln p H 2 × p 1 = 2
R u T
2F
O 2
p H 2 O
E = E 0 +
ð
Eq
:
16
:
3
Þ
with E 0 being the standard reversible voltage at the temperature of interest:
g r
2F
Δ
E 0 =
ð
Eq
:
16
:
4
Þ
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