are considerably higher, typically 0.6 V or more on an unmodified surface
(both overpotentials can be a serious consideration on lower-cost electrodes
such as nickel, but the overpotential required for effective oxygen evolu-
tion via water splitting remains nearly a half volt higher than for hydrogen
evolution via proton reduction; US Department of Energy 2003).
Nature has evolved catalysts that perform the reactions involved in elec-
trolysis, with a very low overpotential and in relatively low-ionic-strength
water near neutral pH. The oxygen-evolving complex of photosystem II
in photosynthesis (Chapter 20) catalyzes the sequential removal of four
electrons from two water molecules bound to the manganese cluster.
In the photosystem II reaction center, optical excitation results in the
formation of an oxidized chlorophyll complex with a midpoint potential
of roughly 1 V. This is only slightly higher than what is required thermo-
dynamically to split water, forming oxygen and hydrogen, especially con-
sidering the high H+concentration in the chloroplast lumen compared to
the standard state. The ability of photosystem II to perform this reaction
is discussed in detail in Chapter 20.
Hydrogen gas is naturally produced by a wide variety of natural organ-
isms, including methanogenic archaea, sulfate- and nitrate-reducing
bacteria, and some hyperthermophilic bacteria (Vignas et al. 2001;
Armstrong 2004). In these cases, fermentation of an organic substrate,
such as proteins, sugars, or lipids, yields hydrogen gas. Reduction of H+
to hydrogen gas is used by fermentating bacteria to dispose of excess reduc-
ing equivalents and to maintain a suitable oxidation/reduction potential
in the cell. The hydrogen gas is subsequently used as a low-potential reduc-
tant by bacteria living in the same environment as part of a biogeochemical
cycle. A key component of such processes is the involvement of hydro-
genases that catalyze the reversible conversion of molecular hydrogen
(H 2 ) to protons and electrons and thus play a central role in microbial
hydrogen metabolism. On the basis of their metal content, these enzymes
can be grouped into three structural forms, the vast majority of which
contain either iron and nickel ([Ni–Fe]-hydrogenases) or Fe only ([Fe–Fe]-
hydrogenases) in their H 2 -activating sites (Chapter 10). Although hydro-
genases catalyze a very simple reaction, they do so in many different
metabolic contexts and for a diversity of functions. In many microorganisms,
the [Ni–Fe]-containing hydrogenases often catalyze the reaction in which
H 2 , generated by other metabolic sources, is consumed while the [Fe–Fe]-
containing hydrogenases catalyze the reduction of protons (as terminal
electron acceptors) to produce hydrogen. However, so-called bidirectional
hydrogenases, having the capacity to both take up and produce hydrogen,
are also known from a diversity of microbes. Although hydrogen pro-
duction from natural systems is actively being studied, the sensitivity of
these systems to the presence of oxygen coupled with difficulties in large-
scale production makes the approach conceptually feasible but technically
challenging.
singke
(singke)
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