(Fig. 14e) [116]. However, under acidic conditions, the ligand is protonated and the
complex rapidly decomposes. Taking a cue from the coordination of hydrogenases,
a P 2 S 2 -coordinated complex [Ni(bdt)(dppf)] that catalyzes hydrogen evolution at a
rate in excess of 10
3 s
À1 with only 265 mV of overpotential [117] and stability on
the timescale of hours was reported (Fig. 14f).
By analogy to results obtained from cobalt complexes, recent work has focused
on using macrocyclic ligands to create more stable nickel-based electrocatalysts.
For example, Lau, Robert, and coworkers have created a collection of complexes
employing related N 4 , N 3 S, and N 3 P ligands. Surprisingly, in many cases, the
active catalyst is a nickel nanoparticle, and the active catalyst sometimes depends
whether the experiment is photocatalytic or electrocatalytic. For example, the N 3 P
ligand 2,12-dimethyl-7-phenyl-3,11,17-triaza-7-phospha-bicyclo[11,3,1]heptadeca1(17),13,15-triene supported the most active complex [99]. This compound is
active as a homogeneous catalyst in electrocatalytic experiments but converts to
nickel nanoparticles under photocatalytic conditions. The reason for the change in
mechanism may be that solution conditions such as pH differ between the two
types of experiments. This will be crucial to take into account as researchers begin
to translate basic gains in molecular hydrogen production catalyst synthesis to
larger-scale applications.
3.4 Photocatalytic Production of Hydrogen
Photosynthesis directly converts solar energy to chemical energy. The process
essentially consists of photon capture to generate a charge-separated state and
coupling of this state to catalysts. A number of cyanobacteria and algae are
known to produce hydrogen photosynthetically, but the efficiency and yields are
relatively low [118–122]. This may be because competing metabolic pathways vie
for solar-derived reducing equivalents. Metabolic engineering efforts such as eliminating competing sinks have resulted in modest improvements [120], and Golbeck
and coworkers have demonstrated that directly tethering the electron-donating FeS
cluster of Photosystem I to the accepting cluster of an [FeFe]-hydrogenase in vitro
increases the rate of hydrogen evolution [123].
This understanding of natural photosynthesis and attempts to reengineer it have
served as inspiration for artificial photosynthetic systems [124]; see Fig. 15 for a
schematic view of the components and functional requirements for an artificial
photosynthetic system. Although artificial constructs capable of emulating certain
aspects of photosynthesis, such as light-driven generation of a long-lived chargeseparated state, have been described [124–127], developing complete systems for
efficient utilization of light energy to produce fuel remains a significant challenge.
More than a hundred molecular catalysts for electrocatalytic proton reduction have
been described; in contrast, relatively few molecular systems for photocatalytic
hydrogen production have been reported [128]. The challenge is that a functional
photocatalytic system requires more than just a good catalyst. In most cases, i.e.,
when the catalyst is not itself photochemically active, the catalyst must interact
252
L. Gan et al.
complex rapidly decomposes. Taking a cue from the coordination of hydrogenases,
a P 2 S 2 -coordinated complex [Ni(bdt)(dppf)] that catalyzes hydrogen evolution at a
rate in excess of 10
3 s
À1 with only 265 mV of overpotential [117] and stability on
the timescale of hours was reported (Fig. 14f).
By analogy to results obtained from cobalt complexes, recent work has focused
on using macrocyclic ligands to create more stable nickel-based electrocatalysts.
For example, Lau, Robert, and coworkers have created a collection of complexes
employing related N 4 , N 3 S, and N 3 P ligands. Surprisingly, in many cases, the
active catalyst is a nickel nanoparticle, and the active catalyst sometimes depends
whether the experiment is photocatalytic or electrocatalytic. For example, the N 3 P
ligand 2,12-dimethyl-7-phenyl-3,11,17-triaza-7-phospha-bicyclo[11,3,1]heptadeca1(17),13,15-triene supported the most active complex [99]. This compound is
active as a homogeneous catalyst in electrocatalytic experiments but converts to
nickel nanoparticles under photocatalytic conditions. The reason for the change in
mechanism may be that solution conditions such as pH differ between the two
types of experiments. This will be crucial to take into account as researchers begin
to translate basic gains in molecular hydrogen production catalyst synthesis to
larger-scale applications.
3.4 Photocatalytic Production of Hydrogen
Photosynthesis directly converts solar energy to chemical energy. The process
essentially consists of photon capture to generate a charge-separated state and
coupling of this state to catalysts. A number of cyanobacteria and algae are
known to produce hydrogen photosynthetically, but the efficiency and yields are
relatively low [118–122]. This may be because competing metabolic pathways vie
for solar-derived reducing equivalents. Metabolic engineering efforts such as eliminating competing sinks have resulted in modest improvements [120], and Golbeck
and coworkers have demonstrated that directly tethering the electron-donating FeS
cluster of Photosystem I to the accepting cluster of an [FeFe]-hydrogenase in vitro
increases the rate of hydrogen evolution [123].
This understanding of natural photosynthesis and attempts to reengineer it have
served as inspiration for artificial photosynthetic systems [124]; see Fig. 15 for a
schematic view of the components and functional requirements for an artificial
photosynthetic system. Although artificial constructs capable of emulating certain
aspects of photosynthesis, such as light-driven generation of a long-lived chargeseparated state, have been described [124–127], developing complete systems for
efficient utilization of light energy to produce fuel remains a significant challenge.
More than a hundred molecular catalysts for electrocatalytic proton reduction have
been described; in contrast, relatively few molecular systems for photocatalytic
hydrogen production have been reported [128]. The challenge is that a functional
photocatalytic system requires more than just a good catalyst. In most cases, i.e.,
when the catalyst is not itself photochemically active, the catalyst must interact
252
L. Gan et al.