Understanding and Harnessing Hydrogenases, Biological Dihydrogen Catalysts - The Metal-Driven Biogeochemistry of Gaseous Compounds in the Environment - page 104

Figure 3 The fundamental notion of a possible future solar-H 2 economy. For more detailed
images see [ 83 ].

not discussed, but it is acknowledged that this is problematic. Ultimately, linking
H 2 production to CO 2 sequestration, or using H 2 as a carbon-free additive to natural
gas may be necessary.

2.3.1 Hydrogenase Photoelectrolysis Devices

In solar fuel systems there are three key processes: (i) photoexcitation, (ii) reduction
initiated by the photo-induced excited electron (e.g., H 2 production, equation 11 ), and
(iii) oxidation initiated by the photo-induced hole (e.g., O 2 production, equation 12 ),
see Figure 4 .

2H þ þ 2e !

ð Þ

H 2 11

4H þ þ 4e 2H 2 O ! O 2 þ ð 12 Þ

In molecular photoelectrolysis devices fuel-producing catalysts such as hydrogenases
perform a dual role, both enhancing the rate of the reduction reactions and providing
an interface to enhance electron-hole separation, thus preventing recombination.
A key consideration in choosing an effective catalyst is the overpotential, the
difference between the potential which must be applied to the catalyst for it to
function at a significant rate and the equilibrium potential for the redox couple.
Because [NiFe] and [FeFe] hydrogenases catalyze the reduction of protons with a
minimal overpotential, excited electrons only need to be at a reduction potential
slightly more negative than E o (H + /H 2 )todriveH 2 production at a bimetallic hydrog-
enase. A variety of different visible-light photoexcitation centers have been directly
coupled to [NiFe] or [FeFe] hydrogenases to drive H 2 production from water: organic
dyes [ 34 ], biological photosystem I [ 35 , 36 ], and ruthenium-dyes [ 37 ]. Alternatively,
using external electrical circuitry, a carbon felt electrode with [FeFe] hydrogenase
adsorbed onto the surface was wired to a porphyrin-sensitized nanoparticulate