mediators, such as methyl viologen (MV) [67]. Immobilization of different FDHs
classes, in diverse matrices, e.g. hydrogel, conducting polymers, nanoparticles, has
been extensively pursued trying to enhance the stability and efficiency of these
systems [37, 65, 68, 69].
As mentioned earlier, nitrogenases can also catalyse the CO 2 reduction along
with several other small molecules. An extensive review on this multitude of
nitrogenases’ substrates can be found in Seefeldt et al. [41]. The potentiometric
determination of the redox centres reduction potentials was possible (see Table 1)
showing values of approx. −465 and −40 mV for the M-cluster (the catalytic site),
−300 to −90 mV to the P-cluster and −300 to −790 mV for the [4Fe-4S] cluster
[38, 42, 70]. Voltammetric determinations allowed to obtain values around -
300 mV for the M-cluster [38]. Also, in solution, direct voltammetry of the
nitrogenase isolated co-factors from A. vinelandii, was achieved allowing to
observed the value of −270 mV for the M-cluster, close to the attained value of the
centre reduction potential in the all protein [71]. The electrochemical studies
attained, together with the knowledge that the co-factors retain its catalytic properties [72], allows expecting a possible electrochemical control of the nitrogenase
activity. Studies with the immobilization of a His-tagged MoFe nitrogenase on an
electrode surface with functionalized nanotubes modified with a pyrene moiety
incorporated in a polypeptide shown to be efficient for the bio-electrosynthesis of
ammonia from N 3 and a possible route for other catalytic routes [73]. It was also
demonstrated that MoFe and FeFe nitrogenase immobilized on a polymer-modified
electrode, using cobaltocene as mediator were able to reduce CO 2 through imposition of applied potentials close to −1 V. The reaction is not completely specific
since there is a side reaction where protons are reduced, but opens the possibility to
integrate nitrogenases in BES [74].
Table 1 shows the relevant reduction potentials associated to the main enzymes
that catalyse reactions with CO 2 , allowing observing the diversity of centres and
potentials values found in the literature.
2.3 Bioelectrochemical Enzymatic Approaches
for CO 2 Utilization
An example that illustrates the prospective use of enzymes is the case of FDH,
either immobilized or as free in solution. An FDH from M. extorquens was
immobilized using the highly conductive Ketjen Black carbon material mixed with
PTFE on a waterproof carbon cloth, allowing to obtain a gas-diffusion electrode,
where 1,1′-trimethylene-2,2′-bipyridinium dibromide was used as mediator. In this
mediated-electron transfer (MET) system, formate was produced by the reduction
of gaseous CO 2 , overcoming the question of the low solubility of the molecule [75]
and the dependence of the FDH activity with the pH and the different carbonic
species present in solution [76]. An enzymatic (formate-based) fuel cell
(EFC) based on MET using NAD
+
-dependent FDH was also recently reported [67].
In this study the free in solution FDH reduces CO 2 to formate at the cathodic
90
C. M. Cordas et al.
classes, in diverse matrices, e.g. hydrogel, conducting polymers, nanoparticles, has
been extensively pursued trying to enhance the stability and efficiency of these
systems [37, 65, 68, 69].
As mentioned earlier, nitrogenases can also catalyse the CO 2 reduction along
with several other small molecules. An extensive review on this multitude of
nitrogenases’ substrates can be found in Seefeldt et al. [41]. The potentiometric
determination of the redox centres reduction potentials was possible (see Table 1)
showing values of approx. −465 and −40 mV for the M-cluster (the catalytic site),
−300 to −90 mV to the P-cluster and −300 to −790 mV for the [4Fe-4S] cluster
[38, 42, 70]. Voltammetric determinations allowed to obtain values around -
300 mV for the M-cluster [38]. Also, in solution, direct voltammetry of the
nitrogenase isolated co-factors from A. vinelandii, was achieved allowing to
observed the value of −270 mV for the M-cluster, close to the attained value of the
centre reduction potential in the all protein [71]. The electrochemical studies
attained, together with the knowledge that the co-factors retain its catalytic properties [72], allows expecting a possible electrochemical control of the nitrogenase
activity. Studies with the immobilization of a His-tagged MoFe nitrogenase on an
electrode surface with functionalized nanotubes modified with a pyrene moiety
incorporated in a polypeptide shown to be efficient for the bio-electrosynthesis of
ammonia from N 3 and a possible route for other catalytic routes [73]. It was also
demonstrated that MoFe and FeFe nitrogenase immobilized on a polymer-modified
electrode, using cobaltocene as mediator were able to reduce CO 2 through imposition of applied potentials close to −1 V. The reaction is not completely specific
since there is a side reaction where protons are reduced, but opens the possibility to
integrate nitrogenases in BES [74].
Table 1 shows the relevant reduction potentials associated to the main enzymes
that catalyse reactions with CO 2 , allowing observing the diversity of centres and
potentials values found in the literature.
2.3 Bioelectrochemical Enzymatic Approaches
for CO 2 Utilization
An example that illustrates the prospective use of enzymes is the case of FDH,
either immobilized or as free in solution. An FDH from M. extorquens was
immobilized using the highly conductive Ketjen Black carbon material mixed with
PTFE on a waterproof carbon cloth, allowing to obtain a gas-diffusion electrode,
where 1,1′-trimethylene-2,2′-bipyridinium dibromide was used as mediator. In this
mediated-electron transfer (MET) system, formate was produced by the reduction
of gaseous CO 2 , overcoming the question of the low solubility of the molecule [75]
and the dependence of the FDH activity with the pH and the different carbonic
species present in solution [76]. An enzymatic (formate-based) fuel cell
(EFC) based on MET using NAD
+
-dependent FDH was also recently reported [67].
In this study the free in solution FDH reduces CO 2 to formate at the cathodic
90
C. M. Cordas et al.