Page 87 - MODERN ASPECTS OF ELECTROCHEMISTRY

70
– 2– +6 H O+ 6 e – Claude LamyAet al.
CH OH + 8 OH → CO 3 2 in alkaline electrolytes
3
(2b)
whereas the electrochemical reduction of oxygen occurs at the cathode
(also containing a platinum-based catalyst), which corresponds to the
positive electrode of the cell:
O +4 H +4 e → 2H O in acid electrolytes (3a)
–
+
2
2
-
–
+2 H O+4 e → 4OH in alkaline electrolytes (3b)
O 2 2
The cell potential E is thus equal to the difference between the electrode
potentials of each electrode:
+ – (4)
E= E c –E a
where the electrode potentials E are defined as the difference of internal
i
potential at each electrode/electrolyte interface.
One main advantage of such a power source is the direct transforma-
tion of the chemical energy of methanol combustion into electrical energy.
Hence, the reversible cell potential, E , can be calculated from the Gibbs
r
energy change, ∆G, associated with the overall combustion reaction of
methanol (l), by the equation:
∆G + nF E r = 0 leading to Er > 0 with ∆G <0 (5)

where F is the Faraday constant (F = 96,500 C) and n is the number of
faradays (per mole of methanol) involved in the half-cell reactions. Under
o
standard conditions (25 C), the heat of combustion, i.e., the enthalpy
change for reaction ( l), ∆H , is –726 kJ/mole of methanol, and the Gibbs
o
o
energy change, ∆G , is -702 kJ/mole of methanol. This corresponds to a
standard reversible potential for the cell, as given by the equation:
o
o
o
Er =Ec –Ea = _ ∆G o = 702 x 10 3 =1.21 V (6)
nF 6 x 96,500
o ,E are the standard potentials of each electrode.
o
where E c a
The main attractions of the DMFC are its high specific energy (W s)
and high energy density (W e ), the values of which are calculated as
follows:
_ o 3
( ∆G ) 702 x 10
Ws = = = 6.09 kWhr kg -l (7)
3600 x M 3600 x 0.032