added-value compounds production yield, e.g., methanol and formate [56–58]. The
mechanism of CAs is pH-dependent but not redox-dependent. The stability of the
Zn
2+ in its active centre makes difficult the direct electrochemical control of the
enzyme but not its indirect control in multi enzymatic systems, free in solution or
immobilized in different matrices such as polymers, hydrogels or others [55–57].
A different scenario is found with CODHs. As stated earlier, in spite the main
reaction in which these enzymes are involved in the CO oxidation to CO 2 , the
reversible reaction is possible and can be electrochemically controlled. The formal
potential of the CODH active catalytic cluster of M. thermoacetica (free in solution)
was found to be -520 mV (at pH 7), the same theoretical value for the CO/CO 2
reduction at the same pH, with the maximum catalytic efficiency at −570 mV at pH
6.3 [59]. The CODH immobilization seems to be more efficient than its use in
solution since results have shown that its immobilization on gold nanoparticles have
improved the affinity for dissolved CO [60]. Immobilized CODHs from
C. hydrogenoformans were found to efficiently catalyse the reaction in the presence
of a 50:50% CO 2 and CO mixture with a catalytic potential around −520 mV closer
to the thermodynamic theoretical value (−558 mV [25]). The catalysis direction
was demonstrated to be controlled by the applied potential, being favorable the CO
oxidation at values more positive than -520 mV and the CO 2 reduction at more
negative applied values. A drawback in using Ni-CODHs is their inhibition not only
by CO but also by small molecules such as cyanide and sulfide [27]; also most of
Ni-CODHs are inactivated in the presence of oxygen, although some may be
reactivated by reduction [29]. CODH has been recently associated with FDHs to
increase the efficiency to promote the high value-added compound formate from
CO and CO 2 either using all cells or the purified enzymes [61].
Similarly to CODS, FDHs can be electrochemically studied and controlled, as
already stated, and the catalysis direction towards formate oxidation or CO 2
reduction can be controlled by the applied potential. The direct electrochemical
behaviour of the metal-dependent FDH from D. desulfuricans was attained with
values for the formal reduction potential of the enzyme Mo active site of −250 mV
and the catalytic activity (towards CO 2 reduction) starting at values of approx.
−200 mV with the maximum catalytic current observed at −250 mV [62]. The
catalytic behavior is similar to other FDH enzymes of the same class, not only
containing Mo, but also W [63], in spite of, in general, the W containing enzymes
seem to present lower potential catalytic values, such as FDH1 from S. fumaroxidans or M. extorquens [26, 63–65], where the catalysis towards CO 2 reduction is
observed at −400 mV. The last example is particularly interesting for future
applications since its activity is not affected by the presence of oxygen. Also,
metal-independent FDH was demonstrated to be able to efficiently catalyse the CO 2
reduction to formate, although generally presenting lower k cat than the
metal-dependent ones [66]. One relevant example is the C. boidinii FDH that has no
metals in the active centre and is NAD
+ dependent (its redox co-factor). For
practical uses, the need for expensive co-factors as NADH are not favourable. The
use of this particular FDH has the advantages of being commercially available, and
also the possible replacement of the co-factor NAD
+
/NADH by other small redox
Carbon Dioxide Utilization—Bioelectrochemical Approaches
89
mechanism of CAs is pH-dependent but not redox-dependent. The stability of the
Zn
2+ in its active centre makes difficult the direct electrochemical control of the
enzyme but not its indirect control in multi enzymatic systems, free in solution or
immobilized in different matrices such as polymers, hydrogels or others [55–57].
A different scenario is found with CODHs. As stated earlier, in spite the main
reaction in which these enzymes are involved in the CO oxidation to CO 2 , the
reversible reaction is possible and can be electrochemically controlled. The formal
potential of the CODH active catalytic cluster of M. thermoacetica (free in solution)
was found to be -520 mV (at pH 7), the same theoretical value for the CO/CO 2
reduction at the same pH, with the maximum catalytic efficiency at −570 mV at pH
6.3 [59]. The CODH immobilization seems to be more efficient than its use in
solution since results have shown that its immobilization on gold nanoparticles have
improved the affinity for dissolved CO [60]. Immobilized CODHs from
C. hydrogenoformans were found to efficiently catalyse the reaction in the presence
of a 50:50% CO 2 and CO mixture with a catalytic potential around −520 mV closer
to the thermodynamic theoretical value (−558 mV [25]). The catalysis direction
was demonstrated to be controlled by the applied potential, being favorable the CO
oxidation at values more positive than -520 mV and the CO 2 reduction at more
negative applied values. A drawback in using Ni-CODHs is their inhibition not only
by CO but also by small molecules such as cyanide and sulfide [27]; also most of
Ni-CODHs are inactivated in the presence of oxygen, although some may be
reactivated by reduction [29]. CODH has been recently associated with FDHs to
increase the efficiency to promote the high value-added compound formate from
CO and CO 2 either using all cells or the purified enzymes [61].
Similarly to CODS, FDHs can be electrochemically studied and controlled, as
already stated, and the catalysis direction towards formate oxidation or CO 2
reduction can be controlled by the applied potential. The direct electrochemical
behaviour of the metal-dependent FDH from D. desulfuricans was attained with
values for the formal reduction potential of the enzyme Mo active site of −250 mV
and the catalytic activity (towards CO 2 reduction) starting at values of approx.
−200 mV with the maximum catalytic current observed at −250 mV [62]. The
catalytic behavior is similar to other FDH enzymes of the same class, not only
containing Mo, but also W [63], in spite of, in general, the W containing enzymes
seem to present lower potential catalytic values, such as FDH1 from S. fumaroxidans or M. extorquens [26, 63–65], where the catalysis towards CO 2 reduction is
observed at −400 mV. The last example is particularly interesting for future
applications since its activity is not affected by the presence of oxygen. Also,
metal-independent FDH was demonstrated to be able to efficiently catalyse the CO 2
reduction to formate, although generally presenting lower k cat than the
metal-dependent ones [66]. One relevant example is the C. boidinii FDH that has no
metals in the active centre and is NAD
+ dependent (its redox co-factor). For
practical uses, the need for expensive co-factors as NADH are not favourable. The
use of this particular FDH has the advantages of being commercially available, and
also the possible replacement of the co-factor NAD
+
/NADH by other small redox
Carbon Dioxide Utilization—Bioelectrochemical Approaches
89