hydrogen production from water using solar cells powered nafion ...

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285,830J/mol + H 2O(l)
→ H 2(g)
+ O2(g)
2
(2.1)
However by applying the electricity water can be split into hydrogen and oxygen ions at
lower temperatures, the dissociation of water requires the amount of electrical energy
corresponding to ∆G of the water splitting reaction. The electrical potential proportional to
reaction Gibbs free energy is required between the electrodes to initiate water
decomposition. This voltage is found from the definition of Gibbs free energy. In fact,
theoretically it is the minimum electrolysis voltage (i.e. the ideal fuel cell voltage). The
summation of the required electrical potential to compensate the Gibbs free energy and
entropy at a temperature is called the thermoneutral (VTN) voltage at the standard
temperature and pressure, it equals to;
V
t, p
− ∆G
=
nF
t, p
T∆∆ − ∆H
+ =
nF nF
t, p
−(
−285830
)
= = 1,481V
2×
96485
(2.2)
VTN is the voltage at which a perfectly insulated electrolyzer would operate. Thus, VTN is
equal to the sum of higher heating value voltage corresponding to the energy required for
the saturation of hydrogen and oxygen with water vapor (Oi and Sakaki 2004). The cell
efficiency is found through using the thermoneutral voltage. In this case, VTN is divided by
the actual voltage applied to the cell to obtain the efficiency of an electrolyzer (Oi and
Sakaki 2004).
η=
V
V
TN
ACT
1.481
=
V
ACT
(2.3)
Another way to find the efficiency of an electrolyzer is that the energy equivalent of
hydrogen output is divided by the given energy as shown in the equation 2.4 below (Ahmad
et al. 2006).
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