Electrocatalytic Transformation of Carbon Dioxide into Low Carbon Compounds on Conducting Polymers Derived from Multimetallic Porphyrins

Article abstract of DOI:10.1002/cssc.201500816

The electrochemical reduction of carbon dioxide is studied herein by using conducting polymers based on metallotetraruthenated porphyrins (MTRPs). The polymers on glassy carbon electrodes were obtained by electropolymerization processes of the monomeric MTRP. The linear sweep voltammetry technique resulted in polymeric films that showed electrocatalytic activity toward carbon dioxide reduction with an onset potential of -0.70 V. The reduction products obtained were hydrogen, formic acid, formaldehyde, and methanol, with a tendency for a high production of methanol with a maximum value of turnover frequency equal to 15.07 when using a zinc(II) polymeric surface. Studies of the morphology (AFM) and electrochemical impedance spectroscopy results provide an adequate background to explain that the electrochemical reduction is governed by the roughness of the polymer, for which the possible mechanism involves a series of one-electron reduction reactions.

Full text of DOI:10.1002/cssc.201500816

DOI: 10.1002/cssc.201500816
Full Papers
Electrocatalytic Transformation of Carbon Dioxide into
Low Carbon Compounds on Conducting Polymers Derived
from Multimetallic Porphyrins
Paulina Dreyse,[a] Jessica Honores,[b] Diego Quezada,[b] and Mauricio Isaacs*[b]
The electrochemical reduction of carbon dioxide is studied
herein by using conducting polymers based on metallotetrar-
uthenated porphyrins (MTRPs). The polymers on glassy carbon
electrodes were obtained by electropolymerization processes
of the monomeric MTRP. The linear sweep voltammetry tech-
nique resulted in polymeric films that showed electrocatalytic
activity toward carbon dioxide reduction with an onset poten-
tial of À0.70 V. The reduction products obtained were hydro-
gen, formic acid, formaldehyde, and methanol, with a tendency
for a high production of methanol with a maximum value of
turnover frequency equal to 15.07 when using a zinc(II) poly-
meric surface. Studies of the morphology (AFM) and electro-
chemical impedance spectroscopy results provide an adequate
background to explain that the electrochemical reduction is
governed by the roughness of the polymer, for which the pos-
sible mechanism involves a series of one-electron reduction re-
actions.
Introduction
A considerable portion of the solar radiation that falls on the
earth’s surface is re-emitted as thermal radiation.[1] This energy
is retained by atmospheric gases (CO2, N2O, CH4, among
others); a phenomenon that is called the Greenhouse Effect.[1]
The concentration of these gases has been increasing since
the Industrial Revolution due to human economic activities,
such as the use of fossil fuels (petroleum, natural gas, and
coal) and deforestation processes.[2] The increase of these
gasses, mainly carbon dioxide, has generated dramatic weather
perturbations and changed the global average temperature of
the earth’s atmosphere and oceans.[3,4] In this context, CO2 con-
version into species that are harmless to the environment or
that can be useful as fuels appears to be a feasible alternative
to decrease the concentration of this gas.[5–7] The electrochemi-
cal reduction of CO2 is a promising process for this aim and,
depending on the number of electrons transferred, it is possi-
ble to obtain several reduction products, the most interesting
of which are those that involve the transfer of more than two
electrons, for example, methane, methanol, and ethanol.[8,9] To
obtain these products, electrocatalysts based on transition-
metal complexes containing multiple metal centers have been
utilized to achieve multielectron transfers.[10,11]
There have been diverse studies on the electrocatalytic re-
duction of CO2 by using macrocycles with transition metals as
electrocatalysts.[12–16] One of the first studies showed that films
of cobalt and nickel phthalocyanines deposited onto electrode
surfaces were active catalysts for the reduction of CO2 in
water.[12] Also, cobalt(II) macrocyclic complexes[13,14] and meso-
cobalt(I) tetracarboxyphenylporphyrins have been used as ho-
mogeneous electrocatalysts, with which formic acid was the
main product.[15,16] In the case of Fe0 porphyrins, CO produc-
tion was verified in DMF at À1.80 V[17] and weak Brønsted
acids at À1.50 V.[18] Recently, a lower reduction potential has
been obtained (À0.80 V) by using a conducting polymer of
cobalt 5,10,15,20-tetrakis(4-aminophenyl)porphyrin on an
indium tin oxide (ITO) electrode with an ionic liquid as the re-
action medium.[19,20]
Tetraruthenated porphyrin (TRP) consists of a tetrapyridylpor-
phyrin (TPyP) coordinated to four ruthenium(II) complexes on
the periphery of the macrocycle. These macrocycles are partic-
ularly attractive because they display unusual electrocatalyt-
ic,[21] electroanalytical,[22,23] photophysics,[24] and photoelectro-
chemical[25–27] properties. In particular, these porphyrins have
been used in the electroanalytical detection of SIV oxoan-
[31]
ions[28–30] and electrocatalytic reduction processes of O2
NO2 ,
À [32,33] and CO2.[34,35] In all cases, multielectron transfer is es-
sential to enhance catalytic activity, which is observed by
changes to current density associated with each process.[31–35]
In addition, reduction products that involve the transfer of
more than one electron are obtained;[32,33] therefore, these
TRPs can be considered as catalysts able to orient reaction se-
lectivity toward one particular product.
[a] Dr. P. Dreyse
Departamento de Química
Universidad TØcnica Federico Santa María
Avenida EspaÇa 1680, Valparaiso (Chile)
[b] Dr. J. Honores, Dr. D. Quezada, Dr. M. Isaacs
Facultad de Química, Departamento de Química Inorgµnica
Pontificia Universidad Católica de Chile
On the other hand, with the aim of obtaining stable electro-
chemical sensors and electrocatalysts, transition-metal com-
plexes can be incorporated into electrode surfaces by different
procedures, such as absorbed monolayers,[36–39] polymeric
Avenida VicuÇa Mackenna 4860, Macul, Santiago (Chile)
E-mail: misaacs@uc.cl
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201500816.
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membranes,[40–43] and conducting polymers.[44–46] Some electro-
polymerizable functional groups found in polypyridyl and mac-
rocyclic complexes are 5-NH2-phen (phen: phenanthroline),[47]
5,6-NH2-phen,[48] and 5,5’-NH2-bpy (bpy: bipyridine).[49] Also, the
role of nitro groups as precursors in the electropolymerization
process has been reported in coordination compounds con-
taining transition metals.[50]
Herein, we describe electrocatalytic reduction of carbon di-
oxide by using conducting polymers derived from metallote-
traruthenated porphyrins (MTRPs). In particular, electrocatalysts
of TPyP (containing NiII, ZnII, or metal free) with four [Ru(5-NO2-
phen)2Cl]+ moieties coordinated to the periphery of the mac-
rocycle.
The conducting polymers were obtained by electropolymeri-
zation on the surface of a glassy carbon (GC) or ITO electrode.
The catalytic activity was evaluated by linear sweep voltamme-
try (LSV) analysis, that is, j versus E curves. The reduction prod-
ucts were obtained by means of potential-controlled bulk elec-
trolysis (PCBE) experiments. The correlation between the mor-
phology and product distribution was studied based on AFM
measurements of the polymeric surfaces. Finally, electrochemi-
cal impedance spectroscopy (EIS) was carried out to under-
stand the relationship between electrical behavior, catalytic ac-
tivity, and morphology of the modified electrodes.
Figure 1. Structure of the [MTPyP{Ru(5-NO2-phen)2Cl}4]4+ complex.
These studies allow us to suggest that electrocatalysts are
efficient for methanol generation, and the roughness of the
polymer surface utilized determines the amount of methanol
produced.
Results and Discussion
The general structure of the electrocatalysts studied is shown
in Figure 1, in which TPyP (containing NiII, ZnII, or metal-free
macrocycle) coordinated to four [Ru(5-NO2-phen)2Cl]+ units on
the periphery of macrocycle can be seen.
Electropolymerization process
The modified electrodes were prepared according to a previ-
ously reported electropolymerization process.[50] Briefly, electro-
polymerization takes place by continuous cyclic voltammetry
between À1.2 and 1.45 V versus Ag/AgCl in organic solution
with a fast scan rate (at least 500 mVsÀ1). It has been demon-
strated that at this scan rate radical species form after the re-
duction of nitro groups, which are present in the phenanthro-
line ligands, and can be coupled in the reverse scan.[50] In
every cycle, an electrodeposit of conducting material is accu-
mulated on the electrode surface. This deposit exhibits charac-
teristics of a conducting polymer and maintains spectroscopic
and electrochemical properties of the whole macrocycle. Fig-
ure 2A displays a representative example of this electrochemi-
cal procedure. An ITO surface was used as the working elec-
trode with the ZnTRP macrocycle. An increase in current with
an increasing number of cycles indicates film growth.[51] Fig-
ure 2B displays UV/Vis characterization results for Poly-ZnTRP
and Poly-NiTRP on modified ITO electrodes. Both electrodes
show similar patterns. In the case of the Poly-ZnTRP-ITO elec-
Figure 2. Modification of an ITO electrode with MTRP. A) Electropolymeriza-
tion of 1 mm ZnTRP/DMF/0.1m tetraethylammonium perchlorate (TEAClO4),
500 mVsÀ1 (first cycle is shown by the red solid line). B) UV/Vis spectra of
Poly-NiTRP-ITO (black) and Poly-ZnTRP-ITO (red).
trode, a well-defined set of absorptions bands centered at l=
275, 440, 570, and 618 nm are observed and assigned as fol-
lows: p!p* NO2-phen, Soret Band p!p*, Q band p!p*, and
Q band p!p*, respectively. It has been largely recognized
that when a metal center ion is coordinated in an N-4 porphy-
rin core, the distribution of Q bands changes from four to two
because of changes in the symmetry from D2h (in H2TRP) to D4h
(MTRP).[52] For the Poly-NiTRP-ITO electrode, an analogous case
is observed, for which the Q bands change from four to one as
a consequence of the Ni2+ ion in the porphyrin core. These
facts provide evidence that zinc or nickel atoms are still coordi-
nated to the N-4 porphyrin core after the electropolymeriza-
tion process.
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AFM characterization
Electrocatalytic activity
AFM measurements were carried out on the conducting poly-
mers attached to ITO electrodes. The images obtained are pre-
sented in Figure 3A and histograms that represent the average
size of the conglomerates shown in Figure 3A are depicted in
Figure 3B. The images of the three modified electrodes show
a random distribution of conglomerates in a pseudospherical
shape. These images are completely different to the morpholo-
gy of the bare ITO surface (see Figure S1 in the Supporting In-
formation), which confirms that the electrodes are totally
modified during the electropolymerization process. The images
show that Poly-NiTRP is smaller and thinner than the other
conducting polymers. The latter is also reflected in the histo-
grams, in which the average sizes of conglomerates are 60, 37,
and 90 nm for Poly-H2TRP, Poly-NiTRP, and Poly-ZnTRP, respec-
tively.
Figure 4 shows j versus E curves of the Poly-MTRP-GC electro-
des recorded in 0.1m NaClO4 at 5 mVsÀ1. Figure 4A shows
Also, for Poly-H2TRP and Poly-ZnTRP, the Z-axis magnitudes
(absolute value) are higher than that for Poly-NiTRP. These
characteristics can be related to the presence of a more homo-
genous and compact polymer in the case of Poly-NiTRP.
On the other hand, the average roughness parameter (Ra)
was determined, and the values for Poly-H2TRP, Poly-NiTRP, and
Poly-ZnTRP were 11.7, 7.81, and 17.2 nm respectively.
Figure 4. A) Electrochemical behavior of the modified electrodes measured
by LSV in 0.1m NaClO4 at 5 mVsÀ1 under N2 (dashed lines) and CO2 (solid
lines). Inset: Magnification of the linear sweep voltammogram under a N2 at-
mosphere. B) Cyclic voltammogram of the Poly-H2TRP-GC electrode in 0.1m
NaClO4 at 100 mVsÀ1 before (black solid line) and after (red solid line) elec-
trolysis under a CO2 atmosphere.
In summary, Poly-ZnTRP appears to have a more voluminous
and rougher surface, with a larger area and bigger conglomer-
ates. In contrast, Poly-NiTRP is a more compact film, as corro-
borated by its smaller Z value and the diameter of conglomer-
ates. Poly-H2TRP appears to be an intermediate case, in which
the Z magnitude, conglomerate size, and Ra value are between
those of Poly-NiTRP and Poly-ZnTRP.
a comparative plot for Poly-MTRP-GC modified electrodes
under N2 and CO2 atmospheres. Under a N2 atmosphere
(Figure 4, inset), a low current density at À0.6 V is observed,
which is probably related to a redox process occurring in the
conducting polymer frame or H2
generation with a maximum cur-
rent density for poly-H2TRP of
5 mAcmÀ2. Under the CO2 atmos-
phere, for all modified electrodes
it is possible to observe an irre-
versible electrochemical process
without diffusion control. How-
ever, a considerable change in
the cathodic current density
with an onset potential of
À0.7 V versus Ag/AgCl is evi-
dent. This change in the current
density can be associated with
the CO2 reduction process, as
described in the literature for
similar electrocatalysts.[35] The
Tafel slope was evaluated in the
low polarization regime for the
three modified electrodes and
showed
values
near
to
200 mVdecÀ1, which indicated
that there was a chemical path-
way as a rate-determining step
for multiple electrochemical
steps.[53]
Figure 3. A) AFM images of the modified electrodes studied, and B) relative conglomerate sizes are presented in
histograms.
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CO2 þ 4 Hþ þ 4 eÀ ! HCHO þ H2O
CO2 þ 6 Hþ þ 6 eÀ ! CH3OH þ H2O
ð4Þ
ð5Þ
Table 1. Product distribution for carbon dioxide reduction at various electrodes.
Property
Bare GC Poly-H2TRP-GC Poly-NiTRP-GC Poly-ZnTRP-GC
amount formed
[mmol]
H2
1.81
–
0.46
–
13.3
–
0.029
–
–
–
–
25.0
–
–
–
2.26
–
3.26
71.6
0.476
–
0.680
15.07
1.8
Table 1 collects the concentration distribution of
these products, along with turnover frequency (TOF)
HCOOH 1.17
HCHO 2.52
CH3OH 1.03
values and faradaic efficiencies. In all experiments,
TOF
H2
–
–
–
–
[sÀ1
]
HCOOH
HCHO
CH3OH
carbon monoxide is below the detection limit.
Bare GC produces a wide distribution of products
with poor selectivity and catalytic activity (Table 1).[54]
0.842
–
4.526
–
faradaic efficiency H2
2.5
In the case of the Poly-H2TRP-GC electrode, hydro-
[%]
HCOOH 1.6
HCHO 3.4
CH3OH 1.4
0.42
–
12.2
–
–
55
–
2.6
56
gen and formaldehyde are not produced. For Poly-
NiTRP-GC, the only product obtained is methanol. Fi-
nally, in the case of Poly-ZnTRP-GC, formic acid is not
produced. The predominance of methanol is clear in
all cases; Poly-ZnTRP-GC is the most efficient electro-
Because the experiments are carried out aqueous media,
possible interference from solvent reduction can occur; hence,
PCBE experiments (see Table 1) were carried out at À0.8 V
versus Ag/AgCl. At this potential, all modified electrodes show
a considerable difference between CO2 and N2 atmospheres,
whereas H2 production from proton reduction is sluggish.
To verify stability or activity loss of these modified electro-
des, Figure 4B displays cyclic voltammetry results for the Poly-
H2TRP-GC electrode in aqueous medium before and after the
PCBE experiment (see the Experimental Section and Table 1)
under a CO2 atmosphere. The cyclic voltammogram of the
modified electrode after electrolysis shows a reversible redox
couple at E1/2 =0.71 V versus Ag/AgCl, which is associated with
the RuIII/RuII redox process of ruthenium complexes on the pe-
riphery of the macrocycle. According to previous work, this
process is a simultaneous four-electron process.[50] After the
PCBE experiments, the cyclic voltammogram remains un-
changed, with only a decrease in charge of 4.96% under the
voltammetric peak of the RuIII/RuII redox couple. Results ob-
tained from the above-described experiments show that the
conducting polymer does not suffer from any transformation
that is detectable by cyclic voltammetry; hence, no redox-
active compounds are coordinated or adsorbed on the poly-
meric matrix. The same behavior was observed with the other
modified electrodes (see Figure S2 in the Supporting Informa-
tion).
catalyst. This fact is remarkable, especially when considering
that methanol is a valuable compound for its use as a liquid
fuel. Additionally, two control experiments were carried out,
with the purpose of checking the ruthenium moiety contribu-
tion and effect of degradation products from the electrocata-
lysts. In the first case, the [Ru(5-NO2-phen)2Cl2] complex was
adsorbed (see the Experimental Section) on the GC electrode
and subjected to PCBE for 3 h under the same experimental
conditions as those used for the MTRP modified electrodes.
The results show no evidence of product formation at that po-
tential. For the second case, Poly-ZnTRP-GC (which was most
active for methanol production) was used, but in this case
without CO2 being dissolved in the supporting electrolyte, that
is, a N2-saturated atmosphere. Again, no products were detect-
ed, which indicated that the origin of the reaction products
was an electrocatalytic process from the synergism between
ruthenium(II) peripheral complexes and the macrocycle ring;
thus the reaction products that were quantified were not from
degradation of the electrocatalysts. Indeed, it has been dem-
onstrated that porphyrin complexes under heterogeneous
electrochemical conditions are mainly active in the production
of CO.[55] In a few cases, the production of methanol (5% fara-
daic efficiency) has been reported, for example, a CoII phthalo-
cyanine adsorbed on graphite at À1.36 V with 0.1m KHCO3 as
supporting electrolyte[55] On the other hand, methanol produc-
tion from TRP analogues has been reported for photolysis ex-
periments, in which the TRPs were sensitizers for TiO2 nanopar-
ticle catalysts[55].The synergism of TRP derivatives has been
largely demonstrated from pioneering works by the groups of
Anson and Toma, the most significant example is related to
the oxygen reduction reaction, for which a four-electron reduc-
tion pathway is achieved by using multimetallic macrocycles,
compared with two-electron reduction if CoIITPyP is used.[31,56]
The synergistic effect is a combination of back-donation and
induced electronic density from the Ru(polypyridyl) moiety to
the porphyrin macrocycle center. This effect has also been cor-
roborated by using a layer by layer assembly of [FeTPyP{Cr-
PCBE experiments
The products of electrochemical reduction of CO2 at the Poly-
MTRP-GC electrodes were determined after PCBE, which was
carried out as indicated in the Experimental Section. The quan-
tified products were hydrogen, carbon monoxide, formic acid,
formaldehyde, and methanol, which corresponded to the
transfer of two, four, or six electrons [see Eqs. (1)–(5)].
2 Hþ þ 2 eÀ ! H2
CO2 þ 2 Hþ þ 2 eÀ ! CO þ H2O
CO2 þ 2 Hþ þ 2 eÀ ! HCOOH
ð1Þ
ð2Þ
ð3Þ
4À
(phen)2Cl}4]8+/SiW12O40 on GC.[57]
These results seem to be directly related to the intrinsic
properties of the conducting polymer. In the case of Poly-
NiTRP, the high selectivity toward methanol production can be
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associated with the presence of a very active redox center in
the macrocycle core, and also to the formation of a very com-
pact and thin film. These factors suggest that reduction takes
place thanks to the possible coordination of carbon dioxide to
the reduced metal center in the conducting polymer, which
should occur in the conducting polymer/solvent interphase.
However, to the best of our knowledge, there are no examples
of nickel(II) macrocycles selectively producing methanol. These
kinds of compounds are well known as efficient electrocata-
lysts towards the production of carbon monoxide and formic
acid.[55,58] Therefore, the catalytic activity must not only be gov-
erned by the coordination property. Other factors must also be
involved in assisting this multielectron reduction process.
In the case of Poly-ZnTRP and Poly-H2TRP, the presence of
an inert coordination metal or the absence of a transition
metal in the porphyrin core discount the possibility of coordi-
nating carbon dioxide to a reduced metal center. The reduc-
tion mechanism must be driven by other phenomena.
One possible explanation could be different rugosities, po-
rosities, and electron densities of the surfaces studied. Accord-
ing to the distribution of products and morphological analysis
results, the formation of methanol as the main product is the
result of multiple monoelectronic pathways, as reported previ-
ously.[5,59] Considering that the other products are obtained in
lower amounts, they should be participating as precursors for
methanol formation, since previously reported CO2 reduction
mechanism on semiconductor surfaces revealed that, after fa-
vorable adsorption of CO2, different products are released as
a consequence of different recombination of radical intermedi-
ates, followed by several of these compounds being readsor-
bed to react again and under-
UV/Vis spectroelectrochemical measurements
UV/Vis spectroelectrochemistry was carried out to elucidate
the electronic structure of the most effective electrocatalyst
under the application of an electrochemical potential. Meas-
urements in solution were carried out with a 1 mm solution of
ZnTRP in DMF.
Figure 5 shows that ZnTRP suffers from changes to the in-
tensity of Soret and Q bands as long as À1.0 V is applied
under a N2 atmosphere; the effect is most evident in the case
of the Soret band. This increase can be explained by the for-
mation of a radical anion, which favors the probability of elec-
tronic transitions, particularly the p!p* transition associated
with the Soret band. The same behavior has been observed in
the Soret band of the dimeric cobalt phthalocyanine macrocy-
cle.[60]
When ZnTRP was studied at À1.0 V in the presence of CO2,
a redshift and a decrease in the Soret band could be observed.
This behavior can be explained by electron transfer from the
reduced macrocycle to CO2, which takes place at this potential.
The applied potential, along with the presence of CO2 mole-
cules, promotes the formation of an intermediary species be-
tween a porphyrin anion radical and CO2 with a less energetic
and favorable transition to diminish the band gap relative to
the original species.
As a consequence, this result strengthens the idea that the
electroreduction of CO2, in the case of the macrocycle with an
inert coordination metal, is influenced by the electronic density
of the macrocycle instead of its specific coordination proper-
ties.
take proton-coupled electron-
transfer paths.[59] Therefore, and
owing to similar morphologies,
Poly-H2TRP-GC and Poly-ZnTRP-
GC yield methanol as the main
product. The only noticeable dif-
ference between the electrodes
is a different amount of active
sites in the polymeric framework,
as evidenced by different rough-
nesses; this fact allows us to
deduce that the active site of
the electrocatalyst could be a re-
duced form of a macrocycle ring,
as described previously for ni-
trite reduction with the same
electrocatalysts.[32]
Finally, the insertion of a transi-
tion metal into the porphyrin
cavity apparently has an impor-
tant effect on the morphology
of the film formed, but it does
not have a significant effect on
product distribution.
Figure 5. Spectroelectrochemical behavior of 1 mm ZnTRP in DMF at À1.0 V.
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EIS analysis
molecule in the double-layer structure, which gives rise to elec-
trochemical reactions in the film.
To relate catalytic activity, morphology, and the electronic be-
havior of the modified electrodes, EIS experiments were carried
out at maximum and minimum TOF values for the production
of methanol, that is, by using the Poly-H2TRP-GC and Poly-
ZnTRP-GC electrodes respectively. The behavior of the electro-
des is revealed by Nyquist plots, which are graphic representa-
tions of the capacitive impedance (ZImaginary) versus faradaic im-
pedance (ZReal). Figure 6 shows the Nyquist plots of the Poly-
H2TRP-GC and Poly-ZnTRP-GC electrodes at À0.70 and À0.80 V
under N2 and CO2 atmospheres (see Bode plots in Figures S3
and S4 in the Supporting Information). An imperfect semicircle
is observed in all cases; thus a pseudo-Randles equivalent cir-
cuit was used, in which a constant phase element was necessa-
ry instead of a capacitor.[61] This nonideal behavior has been re-
ported in the literature,[62] and has been attributed to the
roughness of these electrode surfaces.
As can be seen from the results in Table 2, the resistance to
faradaic processes (RF) decreases when the electrode is im-
mersed in a CO2-saturated solution. An extreme case is pre-
sented at À0.70 V, for which the RF value remains almost un-
changed for both electrodes; this is consistent with the elec-
trochemical behavior presented in Figure 4, in which À0.70 V
corresponds to the onset potential of the reduction wave, and
hence, the electrochemical reaction is in a low polarization
state. At À0.8 V, a decrease in RF can be seen in both electro-
des under a N2 atmosphere, which indicates that the electro-
des are more conductive at this potential, since the polymeric
films are in a reduced state. When the solution is saturated
with CO2, the RF values are significantly lower than those in
a N2 atmosphere, which indicates that both surfaces in a re-
duced state are in the electrocatalytic regime; this is in agree-
ment with results obtained for electrolysis experiments
(Table 1).
Relevant data for the electrodes in different media is sum-
marized in Table 2. Both modified electrodes behave similarly
when they are immersed in a CO2-saturated solution, and
show a decrease in the resistance of the solution (Rs). This de-
crease can be roughly explained by the presence of a new
A comparison between RF values and the morphology of the
electrodes can shed light on the factors that affect the activity
and selectivity of the electrodes.
The Poly-H2TRP-GC electrode is a smoother surface with
smaller conglomerates. It behaves more like a solid electrode
than an amorphous conducting polymer. Due to its density,
Poly-H2TRP-GC shows a low RF value under a N2 atmosphere
that changes dramatically when the system is saturated with
CO2 to go from 489 to 64.4 kW; this value confirms fast elec-
tron transfer, which could be used to perform multiple monoe-
lectronic reductions, finally ending in the formation of metha-
nol.
On the other hand, the Poly-ZnTRP-GC electrode produces
the largest amount of methanol, as well as having a high TOF
value that reaches 15.07 sÀ1. It also presents a high RF value,
which is in concordance with the morphological properties of
the electrode. The presence of H2 in the reduction product dis-
tribution gives an important clue about the possible reduction
mechanism of this modified electrode, in which the local
proton availability, and hence, morphology, must have an im-
portant role in the formation of products that involve multiple
coupled electron–proton transfer processes.
Figure 6. Nyquist plots of Poly-H2TRP-GC at A) À0.70 and B) À0.80 V, and
Poly-ZnTRP-GC at C) À0.70 and D) À0.80 V.
Table 2. Representative constants obtained from EIS measurements.
Material Atmosphere
À0.70 V
À0.80 V
RS [W]
P
RF [kW] RS [W]
P
RF [kW]
N2
H2TRP
201.8 0.904
185.1 0.990
865.9 206.3 0.915
513.0 185.2 0.929
489.7
64.4
CO2
N2
ZnTRP
175.1 0.840 3572
162.0 0.892 3500
168.9 0.862 1390
Scheme 1. Probable mechanism for the reduction of CO2 on Poly-MTRP-
modified electrodes.
CO2
160.5 0.918
341
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0.094, 0.090, and 0.1 cm2 for Poly-H2TRP, Poly-NiTRP, and Poly-
ZnTRP, respectively.
Based on the experimental data above discussed and re-
ports in the literature,[59,63] a probable mechanism is given in
Scheme 1.
The value of the Tafel (chemical electrochemical (CE) mecha-
nism) slope, in accordance with spectroelectrochemical data,
account for the first stage in which the conducting polymer is
reduced and simultaneous adsorption of CO2 takes place, fol-
lowed by several steps that involve monoelectronic reductions
with subsequent product release.
Electrocatalytic activity
The reduction of carbon dioxide was studied by LSV with Poly-
MTRP-GC electrodes in 0.1m aqueous solutions of NaClO4 in CO2-
saturated media (33 mm),[64] between 0.0 and À1.1 V at 5 mVsÀ1
.
PCBE analysis
Conclusions
A gas-tight H-type cell and Poly-MTRP-GC plate modified working
electrode (area: 1.1 cm2) were used at À0.80 V for 3 h of electroly-
The electrochemical reduction of carbon dioxide by using GC
electrodes modified with conducting polymers of TRPs was
studied. The electrocatalysts were composed of TPyP (NiII, ZnII,
or metal free) with four [Ru(5-NO2-phen)2Cl]+ moieties coordi-
nated to the periphery of the macrocycle.
sis.
Determination of reaction products
For the determination of H2, CO, gas chromatography measure-
ments were carried out by using a DANI MASTERS gas chromato-
graph: column: Supelco Mol sieve 5 Š (30 m”0.53 mm) coupled
with a microthermal conductivity detector (mTCD) by using argon
as a gas carrier with an isothermal program (408C). Gas samples
were directly taken from the electrochemical cell after 3 h of elec-
trolysis by using a gas-tight Hamilton syringe. Formic acid and
formaldehyde were determined by UV/Vis spectroscopic methods,
as reported in the literature.[65-68] Methanol content was deter-
mined by using a gas chromatograph coupled with a flame ioniza-
tion detector (FID), with a Supelcowax 10 (30 m”0.32 mm”
0.25 mm film thickness) column. Samples were prepared, after 3 h
of electrolysis, with catholyte (3 mL) and propanol as an internal
standard. Sealed vials containing the sample were heated at 808C
for 20 min, and then headspace analysis was carried out. TOF, as
an index calculated from the number of moles of product formed
per mole of catalyst per unit of time, was evaluated for each prod-
uct.
The electrocatalytic activity was studied by LSV, which re-
vealed that, at potentials more negative than À0.70 V, the con-
ducting polymers showed activity toward CO2 reduction. The
product obtained by PCBE in higher amounts was methanol;
the highest TOF value was recorded for Poly-ZnTRP-GC.
After AFM, spectroelectrochemical/UV/Vis spectroscopy, and
EIS measurements, it was possible to confirm that the rough-
ness, and therefore, number of active sites, was the most sig-
nificant factor in the electrocatalytic activity to obtain metha-
nol as the main product. The previously described characteris-
tics defined the activity of the abovementioned modified elec-
trodes and fit very well with the electrocatalytic performance
of Poly-ZnTRP-GC.
Experimental Section
Reagents
AFM measurements
All reagents and solvents were of analytical grade and used with-
out further purification. The [MTPyP{Ru(5-NO2-phen)2Cl}4](PF6)4
(MTRP) coordination compounds were prepared according to pro-
cedures reported in the literature.[32,50]
AFM images were recorded on a Bruker NanoScope Innova AFM,
along with NanoDrive v8.01 software. The images of the surfaces
were investigated by using tapping mode.
Electrochemical measurements
UV/Vis spectroelectrochemical measurements
LSV and EIS were carried out in a CH Instrument 760C potentiostat.
PCBE was carried out in a BASI POWER MODULE PWR-3 potentio-
stat. A GC working electrode (r=1.5 mm, CH Instruments), a satu-
rated Ag/AgCl reference electrode, and a Pt wire counter electrode
were used. All potential values were measured by using an Ag/
AgCl reference electrode. Working electrode modification was car-
ried out by following a description found in the literature.[32,50]
Modified electrodes were treated in strong acid media and the
amount of metal was determine by inductively coupled plasma op-
tical emission spectroscopy (ICP-OES) analysis (Varian Liberty
Series II), which showed the following elemental composition: 4.9
and 1.3 nmolcmÀ2 for Ru and Zn, respectively, in Poly-ZnTRP. In
case of Poly-NiTRP, the composition was 2.9 and 0.78 nmolcmÀ2
for Ru and Ni, respectively. The electroactive area of the modified
electrodes used for cyclic voltammetry purposes (r=1.5 mm) was
estimated by using the Randles–Sevcik equation to give values of
UV/Vis spectroelectrochemical experiments were performed in
a one-compartment quartz cuvette containing auxiliary and work-
ing Pt electrodes and a Ag/AgCl reference electrode. All spectra
were recorded by using 1 mm of MTRP/0.1m TBAPF6 in acetonitrile,
under an applied voltage of À1.0 V; spectra were recorded every
20 s until 20 spectra were recorded.
EIS measurements
The EIS measurements were performed by using Poly-MTRP-GC
electrodes in 0.1m aqueous solutions of NaClO4 in CO2-saturated
media at À0.70 and À0.80 V versus Ag/AgCl, with an amplitude of
5 mV and measured frequencies from 10À2 to 104 Hz. Experimental
data obtained from the EIS measurements were fitted by using CHI
760C software. All data were fitted within an error of 5%.
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Acknowledgements
We acknowledge financial support from the FONDECYT Project
#1141199, #11130221, and RC 130006, granted by the Fondo de
Innovación para la Competitividad del Ministerio de Economía,
Fomento y Turismo, Chile. P.D. thanks USM-Projects, no. 131439.
J.H. and D.Q. thank CONICYT for scholarships 21130709 and
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Keywords: electrochemistry
reduction · ruthenium
· polymers · porphyrinoids ·
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Received: June 17, 2015
Revised: August 4, 2015
Published online on September 18, 2015
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