1 3
evolution process, delivering an ultralow onset potential and a turnover number
exceeding 230,000. Also, Santiago and coworkers have developed an innovative
HER-based catalyst composed of graphitic carbon nitride nanostructures functionalized with different silver contents and α-rich proteins. The as-synthesized nanomaterials rendered an impressive overpotential of 79 mV at a current density of 10 mA/
cm
2
towards molecular hydrogen production, which is comparable with the most
efficient HER electrocatalysts reported in the literature [59].
4.2 Redox Proteins
Current energy requirements have demanded that we find new alternatives for the
development of catalytic systems, which be able to carry out proton-coupled redox
reactions. In particular, ORR has been a focus of attention of the scientific community in recent years, since it is the cathodic reaction of fuel cells [60, 61]. Indeed,
ORR is crucial to the function of hydrogen fuel cells. Therefore, one of the biggest
current challenges is the preparation of efficient catalysts for ORRs, particularly for
regenerative or reversible fuel cells, which are fuel cells, and, in a reversible way,
can produce H 2 and O 2 , by water electrolysis [62].
Despite the enormous progress accomplished so far, the development of electrocatalysts with high activity and low costs is still a great challenge. The current bottleneck of fuel cells is ORR reactions, which are the limiting step for the generation
of electricity. So far, the most used systems for ORR reactions are mainly platinumbased materials (or their alloys). However, due to the high cost of Pt, alternative
catalysts based on other, less expensive, metals, such as transition metals, as well as
other non-metal-containing materials are being actively sought [63].
The last few decades have shown considerable improvements in the design of efficient catalysts for ORR. An important factor that should be considered when choosing a good catalytic system is selectivity towards the 4H
+
/4e
−
reduction of O 2 to
H 2 O, as opposed to the 2H
+
/2e
−
reduction of O 2 to H 2 O 2 . Besides, the reduction of
O 2 by one electron, to yield O 2
−
, is also undesired [64].
In this regard, in nature, biological systems possess specific proteins that favor the
selective 4H
+
/4e
−
reduction of O 2 . For instance, cytochrome c oxidase ©cO) catalyzes O 2 reduction as part of the respiratory complex that drives adenosine triphosphate (ATP) biosynthesis. The active sites of CcO enzymes involve a Fe-containing
heme (called heme α3), a distal Cu, and a post-translationally modified tyrosine
amino acid (tyrosine 244) [65]. Another example of O 2 -reducing metalloproteins is
the family of MCOs. In this case, the mechanism of the oxygen reduction reactions
in MCOs favors the direct 4e
−
/4H
+
reduction process of O 2 to H 2 O. It is worth mentioning that some MCOs mediate ORR close to the thermodynamic potential of the
O 2 /H 2 O couple [66–69].
Several materials have been designed employing proteins that can act as active
catalysts for ORR. For example, synthetic models of iron-porphyrins, incorporating a distal CuI center connected via a meso-position with a pendant ligand that
resembles the active site coordination in CcO have been described. Such samples
have been employed in ORR by immobilization on graphite disk electrodes. As well,
248
Reprinted from the journal
Topics in Current Chemistry (2020) 378:43
evolution process, delivering an ultralow onset potential and a turnover number
exceeding 230,000. Also, Santiago and coworkers have developed an innovative
HER-based catalyst composed of graphitic carbon nitride nanostructures functionalized with different silver contents and α-rich proteins. The as-synthesized nanomaterials rendered an impressive overpotential of 79 mV at a current density of 10 mA/
cm
2
towards molecular hydrogen production, which is comparable with the most
efficient HER electrocatalysts reported in the literature [59].
4.2 Redox Proteins
Current energy requirements have demanded that we find new alternatives for the
development of catalytic systems, which be able to carry out proton-coupled redox
reactions. In particular, ORR has been a focus of attention of the scientific community in recent years, since it is the cathodic reaction of fuel cells [60, 61]. Indeed,
ORR is crucial to the function of hydrogen fuel cells. Therefore, one of the biggest
current challenges is the preparation of efficient catalysts for ORRs, particularly for
regenerative or reversible fuel cells, which are fuel cells, and, in a reversible way,
can produce H 2 and O 2 , by water electrolysis [62].
Despite the enormous progress accomplished so far, the development of electrocatalysts with high activity and low costs is still a great challenge. The current bottleneck of fuel cells is ORR reactions, which are the limiting step for the generation
of electricity. So far, the most used systems for ORR reactions are mainly platinumbased materials (or their alloys). However, due to the high cost of Pt, alternative
catalysts based on other, less expensive, metals, such as transition metals, as well as
other non-metal-containing materials are being actively sought [63].
The last few decades have shown considerable improvements in the design of efficient catalysts for ORR. An important factor that should be considered when choosing a good catalytic system is selectivity towards the 4H
+
/4e
−
reduction of O 2 to
H 2 O, as opposed to the 2H
+
/2e
−
reduction of O 2 to H 2 O 2 . Besides, the reduction of
O 2 by one electron, to yield O 2
−
, is also undesired [64].
In this regard, in nature, biological systems possess specific proteins that favor the
selective 4H
+
/4e
−
reduction of O 2 . For instance, cytochrome c oxidase ©cO) catalyzes O 2 reduction as part of the respiratory complex that drives adenosine triphosphate (ATP) biosynthesis. The active sites of CcO enzymes involve a Fe-containing
heme (called heme α3), a distal Cu, and a post-translationally modified tyrosine
amino acid (tyrosine 244) [65]. Another example of O 2 -reducing metalloproteins is
the family of MCOs. In this case, the mechanism of the oxygen reduction reactions
in MCOs favors the direct 4e
−
/4H
+
reduction process of O 2 to H 2 O. It is worth mentioning that some MCOs mediate ORR close to the thermodynamic potential of the
O 2 /H 2 O couple [66–69].
Several materials have been designed employing proteins that can act as active
catalysts for ORR. For example, synthetic models of iron-porphyrins, incorporating a distal CuI center connected via a meso-position with a pendant ligand that
resembles the active site coordination in CcO have been described. Such samples
have been employed in ORR by immobilization on graphite disk electrodes. As well,
248
Reprinted from the journal
Topics in Current Chemistry (2020) 378:43