Fundamentals of energy conversion – FUEL CELLS

Vitvarova Monika, Pilar Lukas

A fuel cell is an electrochemical energy converter. A
fuel cell consists of the fuel electrode (anode) and
the oxygen electrode (cathode) which are
interconnected by an ion-conducting electrolyte.
Maximum power
output from one
cell is 1W (1V)
Fuels
 Gas – hydrogen, natural gas, biogas and so on
 Liquid – methanol, ethanol, oil and so on
 Solid – coal, biomass, „waste“
Oxidizer
 Pure oxygen
 Air oxygen

It converts chemical energy
into electrical energy by two
separated electrochemical
reactions. In a hydrogenfuelled polymer electrolyte
membrane fuel cell (PEMFC),
hydrogen is oxidised to
protons and electrons at the
anode. Protons migrate
through the membrane
electrolyte to the cathode.
As the membrane is an
electric insulator, electrons
are forced to flow in an
external electric circuit. At
the cathode, oxygen reacts
with protons to produce
water, which is the only
waste product from a
hydrogen-operated PEMFC,
see the figure below.
Hydrogen is oxidised at the anode to protons:
H2 2H+ + 2ewhile oxygen is reduced at the cathode:
½O2 + 2H+ + 2e- H2O
Total cell reaction:
H2 + ½O2 H2O

The driving force in a fuel cell is nature's affinity for lower
(chemical) free energy. Hydrogen and oxygen are unstable in
each other's presence and spontaneously form water in a
redox reaction. The product (water) has a lower free energy
than the reactants (hydrogen and oxygen), see the figure
below, and is therefore preferred by the system (nature). By
forcing this reduction and oxidation to take place on different
sides of an electrolyte, the energy difference between the
reactants and the products can be converted to electricity.
 Carnot
 Fuel
cycle
cell
Where the value ΔH represents the corresponding enthalpy
change of the combustion reaction.
In contrast to normal thermal combustion, however, in which
the total reaction enthalpy ΔH is converted into heat, in a fuel
cell only the energetic fraction ΔG (free reaction enthalpy) is
directly converted into electricity, i.e. the maximum theoretical
efficiency is given by the formula
Fuel cell type
Electric power
output [W]
Electric
efficiency [%]
Panasonic
Ene Farm
PEMFC
Toshiba Ene
Farm
Ene Farm
Typ S
BlueGen
PEMFC
SOFC
SOFC
250 – 750
250 – 700
250 – 700
do 2 kW
40
38,5
Dimension
HxWxL [mm]
1883 x 315
x480
1000 x780 x
300
Fuel
Investment cost
[Kč]
Gas
Gas
720 000
680 000
60 % (při
46,5
1,5 kWe)
1010 x
935 x 600 x
600 x 660
335
mm
Gas
Gas
720 000
910 000