Power Plant Engineering

2.17.1 REVERSIBLE CELLS
Since oxidation and reduction are physically separate, two things must happen to complete the
reaction in a fuel cell:
1. Ions travel through electrolyte:
— acidic f/c: cations to cathode
— alkaline f/c: anions to anode
2. Electrons travel anode to cathode electrons fall through ‘‘potential gradient’’ and thus do
work.

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74
POWER PLANT ENGINEERING
Cathode: electrode to which cations migrate
Anode: electrode to which anions migrate
Mnemonic
— Reduction occurs at the cathode (redcats)
— Oxidation occurs at the anode
Cell Potential: Cell potential E or E
o
is the difference between the cathode potential E
c
and the
anode potential E
a
.
E = E
c
– E
a
or E
o
= E
c
o
– E
a
o
In Fuel cells: Cathode (+); Anode (–) ; E, E
o
> 0
In Electrochemical cells: Cathode (–); Anode (+); E, E
o
< 0
By definition, the hydrogen reaction is defined to be 0.000 V at standard conditions
she = standard hydrogen electrode; hydrogen electrode in equilibrium at standard conditions
(298 K, unit activity of species)
In Reversible Cell
For H
2
/O
2
fuel cell:
Cathode (reduction) :
E
o
she
/v
O
2
+ 4H
+
+ 4e
–
→
2H
2
O
Anode (oxidation):
H
2
→
2H
+
+ 2e
–
Reversible cell potential
E
o
= E
c
o
– E
a
o
2.17.2 IDEAL FUEL CELLS
Over Potentials. In a real process the electrodes cannot operate at their equilibrium potentials.
Nonidealities in real processes lead to efficiency losses or resistances to the process.
Electrodes must shift to potentials more favorable for oxidation or reduction to overcome effi-
ciency losses. These shifted potentials are called overpotentials.
O + 4H + 4e
2
+
–
2H O
2
0
0.5
1.0
E
N
RHE
Real
anode
H
2
2H + 2e
+
–
Real
cathode
ε
a
E
a
E
c
E
o
E
ε
c
E
o
a
E
c
o
More oxidizing
H /O
2
2
Fuel Cell
More reducing
Fig. 2.18

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NON-CONVENTIONAL ENERGY RESOURCES AND UTILISATION
75
In a fuel cell the chemical energy of the fuel drives the overpotential, which in turn drives the
reaction.
 
O + 4H + 4e
2
+
–
H O
2
0
0.5
1.0
E
N
RHE
Real
anode
2H + 2e
+
–
Real
cathode
ε
a
More oxidizing
Water
Electrolysis
More reducing
ε
c
E
c
E
o
c
E
o
a
E
o
E
H O
2
E
a