Dissertation

|1.7 Mimicking Photosynthesis|
H 2 + O 2
(fuels)
sunlight
(water splitting)
Cat
H 2 O
(exhaust)
energy utilization
(combustion)
Figure 13: Carbon free cycle for solar energy utilization by photocatalytical hydrogen production
from water.
extracting (depicted in figure 13) is a main focus of current research. Ingenious is the fact that the
raw material as well as the combustion product is cheap and abundant water, whereat valuable
hydrogen gas will be the product as depicted for the one-electron process:
2 H 2 O
[photosynthesis]
hν + [solar cell + electrolysis]
[dye + catalyst]
→ 2 H 2(g) + O 2(g) , ∆G = +1.23 eV
The energy that is required for hydrogen and oxygen formation by water splitting is equivalent
to the energy that is released during combustion of hydrogen (∆ f G = -272,9 kJ/mol). This is
equivalent to an electric potential of -1.23 V in solution or to photons with an energy higher than
hν > 1.23 eV, and accordingly to a wavelength shorter than 1008 nm (IR).
However, water is no chromophore and cannot be split by absorption of light. Otherwise an
electrode reaction against a high overpotential is necessary to drive the thermodynamically
not accessible reaction without direct use of visible light energy. Hence, solar water splitting is
accessible by electrolysis, an inefficient (5%) and expensive method when powered by photovoltaic
cells. [28, 29] The process is pH dependent and occurs at standard conditions at the following
potentials (vs. NHE):
2 H + (aq) + 2 e− → H 2(g) , E 1/2
(pH 0) = 0.00 V
2 H 2 O → 4 H + (aq) + O 2(g) + 4 e − , E 1/2 (pH 0) = +1.23 V |18|