Power Plant Engineering

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.

HO 2 O + 4H + 4e 2 +–

00 .51EN .0

RHE Real

anode

2H + 2e+–

Real

cathode

εa

More oxidizing

Water

Electrolysis

More reducing

εc

Ec

Eco E

o

a

Eo

E

HO 2

Ea

Fig. 2.19

Ideal Fuel Cell Potentials. Overpotentials always reduce fuel cell potentials, so that less volt-

age is delivered per electron transferred.

Conversely, over potentials always increase

electrochemical cell potentials so that more voltage is

required per electron transferred.

Ideal Cell Potentials

E = Ec – Ea = (Eeq, c – ec) = (Eeq, a + ea)

Ideal fuel cell Energy Conversion

1. Reactant/product transport


2. Ion transport through e-lyte


3. Electron transport


4. Reaction at electrocatalyst.

2.17.3 Other Types of Fuel Cells

Fuel cells are a means of converting a fuel
to electrical energy using an electrochemical
membrane. The most popular to date has been the
proton exchange hydrogen fuel cell. It takes two mol-
ecules of hydrogen and one molecule of oxygen and
produces two molecules of water leaving behind four
spare electrons to generate an electric current. In
terms of the energy value of the hydrogen, the con-
version process is around 75% to 80% efficient.

Some fuel cells use other chemical fuels as
a source of hydrogen such as methanol, which is
processed into hydrogen for the use by the fuel cell.

HO 2

(CO ) 2

Air

Fuel

(–)

(+)

Anode

Electrolyte

Cathode

Fig. 2.20

Product Water

O + 4H + 4e 2 +–2H O 2 2H 2 4H + 4e+–

Oxygen 4e– Hydrogen

(+) (–)

Oxygen

electrode

Hydrogen

electrode

Proton Exchange

Membrane

Fig. 2.21