Chemistry Reference
In-Depth Information
Fig. 20.1
Schematic diagram of a fuel
cell system. Fuel cell processes: [a]
diffusion of fuel through anode, [b]
electron transfer at anode/electrolyte
interface, [c] diffusion of charge carrier
through electrolyte, [d] electron
transfer at cathode/electrolyte
interface, [e] diffusion of oxygen
through cathode, [f] diffusion of water
through electrodes, [g] ohmic loss in
anode, [h] ohmic loss in cathode and
[i] electronic conductivity of
electrolyte.
Table 20.1
Electrochemical half-reactions within fuel cells
Fuel cell type
Anode reaction
Charge carrier
Cathode reaction
H
2
+
2OH
-
=
2H
2
O
+
2e
-
0.5O
2
+
2H
2
O
+
2e
-
=
2OH
-
Alkaline
OH
-
H
2
=
2H
+
+
2e
-
=
H
2
O
Solid polymer
+
2e
-
H
+
0.5O
2
+
2H
+
H
2
=
2H
+
+
2e
-
=
H
2
O
Phosphoric acid
+
2e
-
H
+
0.5O
2
+
2H
+
H
2
+
CO
3
2
-
=
H
2
O
+
CO
2
+
2e
-
0.5O
2
+
CO
2
+
2e
-
=
CO
3
2
-
Molten carbonate
CO
3
2
-
CO
+
CO
3
2
-
=
2CO
2
+
2e
-
H
2
+
O
2
-
=
H
2
O
+
2e
-
0.5O
2
+
2e
-
=
O
2
-
Solid oxide
O
2
-
CO
+
O
2
-
=
CO
2
+
2e
-
CH
4
+
4O
2
-
=
2H
2
O
+
CO
2
+
8
e
-
CO
+
H
2
O
=
CO
2
+
H
2
Shift reaction
—
—
CH
4
+
H
2
O
=
CO
+
3H
2
Reforming
CH
4
+
CO
2
=
2CO
+
2H
2
reaction
—
—
operating temperatures. Because fuel mixtures typi-
cally are used, reference often is made to equivalent
hydrogen as the basis for calculating the energy
input, and hence the efficiency, of fuel cells.
For a hydrogen/oxygen fuel cell the maximum
electrical work that can be obtained is given by the
change in Gibbs free energy (D
G
) for the reaction of
hydrogen and oxygen. The ideal cell potential (
E
O
)
may be calculated from the ideal Gibbs free energy
(DG
O
) and has a value of 1.23 V when the water
product is in the liquid state, or 1.18 V when the
water is in the gaseous state:
D
G
O
=-
nFE
O
where
n
is the number of electrons per molecule
participating in the electrochemical reaction (
n
= 2
for H
2
/O
2
;
n
= 8 for CH
4
/O
2
),
F
is Faraday's constant
(= 96 487 C mol
-1
) and
E
O
is the cell potential at
ideal conditions of 298 K and 1 atm.
The theoretical efficiency of a fuel cell is given by
the ratio between the Gibbs free energy and the
enthalpy (D
H
) of the fuel:
D
D
G
H
Efficiency of a fuel cell =
