Power Plant Engineering

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 Eo is the difference between the cathode potential Ec and the
anode potential Ea.
E = Ec – Ea or Eo = Eco – Eao
In Fuel cells: Cathode (+); Anode (–) ; E, Eo > 0
In Electrochemical cells: Cathode (–); Anode (+); E, Eo < 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) : Eoshe/v
O 2 + 4H+ + 4e– → 2H 2 O
Anode (oxidation):
H 2 → 2H+ + 2e–
Reversible cell potential
Eo = Eco – Eao

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

00 .51.0ENRHE

Real

anode

H 2 2H + 2e+–

Real

cathode

εa

Ea Ec

Eo

E εc

Eao Ec

o

More oxidizing

H/O 22

Fuel Cell

More reducing

Fig. 2.18