Environmental Engineering Reference
In-Depth Information
accumulate reduced compounds such as ethanol, unbalancing the whole microbiota
in a way that can result in human illness [
24
-
26
]. It is thought that their relative
populations of methanogens, acetogens, and sulfate-reducing bacteria may even be
explained by their hydrogenase activity [
27
].
Inter
cellular H
2
cycling can also be important, with many H
2
-producing microbes
generating H
2
-uptake hydrogenases to permit recycling of this potent fuel. The H
2
is
not necessarily generated by a hydrogenase since both nitrogenases (N
2
-fixing
a source of electrons and protons. Cyanobacteria are photosynthetic organisms which
produce hydrogenases and are capable of H
2
production when exposed to light after a
period under anaerobic dark conditions. However, it is often a nitrogenase which
catalyzes a large proportion of cyanobacterial H
2
production, and in these N
2
-fixing
strains a [NiFe] hydrogenase acts as a complementary H
2
-uptake enzyme so that no
net H
2
production is detected [
17
,
28
]. Conversely, in
Escherichia coli
(
E. coli
), three
[NiFe] hydrogenases have been characterized, and one is a H
2
-producing enzyme
while the other two recycle the gas (Figure
2
)[
29
].
2.3 Solar Dihydrogen Economy
Although human cells do not have the appropriate deoxyribonucleic acid to express
hydrogenases, thanks to our grasp of modern molecular biology techniques we have
the ability to utilize these enzymes via microbial production and purification. A
major driver of this work is to understand hydrogenases in the context of how to
develop catalysts for a solar H
2
economy, an energy cycle which involves using the
Sun's energy to split H
2
O into H
2
and O
2
(Figure
3
). Paradoxically humans often
need least energy when the Sun shines the brightest, and producing H
2
is an
effective method for storing solar energy in the form of a 'clean' fuel.
Harnessing solar power is desirable because Earth receives energy from the Sun
at the rate of approximately 120,000 TW (1 TW
10
12
Js
1
),
and when the human population reaches 10 billion it is projected that the rate of our
energy use will be close to 20 TW [
30
,
31
]. It is estimated that coupling current Si
solar electricity technology with modern commercial water electrolyzers would
give an overall solar energy H
2
O splitting efficiency of 10-11 % [
30
]. This tech-
nology is therefore sustainable in terms of energetic and environmental consider-
ations, but unfortunately it is not scalable or affordable because of the reliance of
commercial electrolyzers on rare earth materials [
32
]. Proof-of-concept devices
have been assembled to show that hydrogenases, which are constructed from
common earth elements, can be utilized to replace platinum as the H
2
-producing
catalyst in solar H
2
devices. The two main approaches have been to either fabricate
molecular photo-electrocatalysis devices using purified enzyme [
33
], or to harness
living photosynthetic H
2
-producing microorganisms. Sections
2.3.1
and
2.3.2
focus
on how an understanding of hydrogenase biology and biochemistry can lead to
advances in these methods of solar H
2
production. The challenges of H
2
storage are
10
12
W
¼
1
¼
1
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