Polymer coordination promotes selective CO2 reduction by cobalt phthalocyanine

Article abstract of DOI:10.1039/c5sc04015a

Cobalt phthalocyanine (CoPc) is a known electrocatalyst for the carbon dioxide reduction reaction (CO₂RR) that, when adsorbed onto edge-plane graphite (EPG) electrodes, shows modest activity and selectivity for CO production along with co-generation of H₂. In contrast, electrodes modified with CoPc immobilized in a poly-4-vinylpridine (P4VP) film show dramatically enhanced activity and selectivity compared to those modified with CoPc alone. CoPc-P4VP films display a faradaic efficiency of ～90% for CO, with a turnover frequency of 4.8 s^-1 at just -0.75 V vs. RHE. Two properties of P4VP contribute to enhancing the activity of CoPc: (1) the ability of individual pyridine residues to coordinate to CoPc and (2) the high concentration of uncoordinated pyridine residues throughout the film which may enhance the catalytic activity of CoPc through secondary and other outer coordination sphere effects. Electrodes modified with polymer-free, five-coordinate CoPc(py) films (py = pyridine) and with CoPc catalysts immobilized in non-coordinating poly-2-vinylpyridine films were prepared to independently investigate the role that each property plays in enhancing CO₂RR performance of CoPc-P4VP. These studies show that a synergistic relationship between the primary and outer coordination sphere effects is responsible for the enhanced catalytic activity of CoPc when embedded in the P4VP membrane.

Full text of DOI:10.1039/c5sc04015a

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Polymer coordination promotes selective CO2
reduction by cobalt phthalocyanine†
Cite this: DOI: 10.1039/c5sc04015a
W. W. Kramer and C. C. L. McCrory‡*
2
Cobalt phthalocyanine (CoPc) is a known electrocatalyst for the carbon dioxide reduction reaction (CO RR)
that, when adsorbed onto edge-plane graphite (EPG) electrodes, shows modest activity and selectivity for
CO production along with co-generation of H . In contrast, electrodes modified with CoPc immobilized in
2
a poly-4-vinylpridine (P4VP) film show dramatically enhanced activity and selectivity compared to those
modified with CoPc alone. CoPc-P4VP films display a faradaic efficiency of ꢀ90% for CO, with
ꢁ1
a turnover frequency of 4.8 s at just ꢁ0.75 V vs. RHE. Two properties of P4VP contribute to enhancing
the activity of CoPc: (1) the ability of individual pyridine residues to coordinate to CoPc and (2) the high
concentration of uncoordinated pyridine residues throughout the film which may enhance the catalytic
activity of CoPc through secondary and other outer coordination sphere effects. Electrodes modified
with polymer-free, five-coordinate CoPc(py) films (py ¼ pyridine) and with CoPc catalysts immobilized in
non-coordinating poly-2-vinylpyridine films were prepared to independently investigate the role that
Received 22nd October 2015
Accepted 1st February 2016
2
each property plays in enhancing CO RR performance of CoPc-P4VP. These studies show that
DOI: 10.1039/c5sc04015a
a synergistic relationship between the primary and outer coordination sphere effects is responsible for
the enhanced catalytic activity of CoPc when embedded in the P4VP membrane.
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membrane increases the catalyst’s selectivity for the production
Introduction
33,34
CO from CO
2
over the evolution of H
2
from water.
The exact
The selective electrochemical reduction of CO into fuels is an mechanisms for the observed P4VP-enhanced activity and
2
2
important strategy for the development renewable energy selectivity for CO reduction remain unclear. Two properties of
sources. Polycrystalline Cu is currently the state-of-the-art the P4VP membrane are believed to contribute to the
heterogeneous electrocatalyst for carbon dioxide reduction enhancement of CoPc as a CO
RR catalyst. First, individual
2
ꢁ
reactions (CO
2
RR), producing 2–6 e products at potentials as pyridine residues of P4VP can coordinate to the square planar
positive as ꢀꢁ0.9 V vs. RHE in aqueous neutral, and near- cobalt center of CoPc. Axial coordination of pyridine has been
1
–6
neutral solutions.
However, Cu-catalyzed CO
2
RR tends to implicated in enhanced CO
2
RR activity for CoPc and cobalt
Second, the uncoordinated pyri-
2
5,26,28,33,34
yield a large distribution of carbon containing products, and the porphyrin catalysts.
operating potentials are quite negative compared to the ther- dine residues throughout the membrane, which form the
modynamic potentials of the major products.
secondary and outer coordination spheres of CoPc, also may
In contrast, molecular catalysts show propensity to form contribute to the increased activity. In acidic solution some
ꢁ
7–23
33,35,36
a single, typically 2–4 e CO
2
reduction products.
Cobalt proportion of the pyridine residues will be protonated,
and
24–32
phthalocyanine () is a well-known molecular CO
2
RR catalyst.
the presence of these protonated residues may enable
When adsorbed to graphite electrodes, CoPc is reported to secondary coordination sphere effects such as hydrogen
reduce CO
to CO in aqueous solution. However there is bonding interactions that can stabilize activated intermediates,
2
signicant co-generation of H produced from the reduction of and outer sphere effects like the availability of protons around
2
protons in the aqueous electrolyte. Previous studies have shown catalyst active sites. However, the mechanism by which P4VP
that incorporating CoPc into a poly-4-vinylpyridine (P4VP) enhances CO RR by CoPc has not yet been explicitly studied. It
2
is important to determine how P4VP increases the activity and
selectivity of CoPc for CO reduction, as such information could
2
Division of Chemistry and Chemical Engineering, California Institute of Technology,
Pasadena, CA, 91125, USA. E-mail: cmccrory@umich.edu
aid designs of improved immobilized molecular CO RR catalyst
2
systems that may nd wider applications in the emerging eld
of solar-fuels technology.
In the current work, the properties of P4VP are assessed
2
independently in order determine how each affects CO RR by
†
Electronic supplementary information (ESI) available: Representative cyclic
voltammograms of modied electrodes, representative current–time plots from
controlled potential electrolyses, and tabulated results from control
experiments. See DOI: 10.1039/c5sc04015a
‡
Present address: Department of Chemistry, University of Michigan, Ann Arbor,
CoPc. Electrodes modied with CoPc and CoPc-P4VP were
MI, 48109 (USA).
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electrode surface and allowing the solvent to evaporate at room
temperature. Cyclic voltammograms (CVs) of both lms show
a broad Co(II/I) reduction between ꢁ0.4 and ꢁ0.7 V vs. SCE, and
a more well dened phthalocyanine centered reduction at ca.
ꢁ
0.9 V vs. SCE (Fig. S1 and S2†). Because the peaks in the CVs
were quite broad, the electrochemically active coverage of the
catalyst could not be readily determined. Therefore, the total
ꢁ9
ꢁ2
deposited coverage of 1.3 ꢂ 10 mol cm , calculated from the
5
mL of 0.05 mM CoPc solution deposited on each 5 mm
diameter electrode, was used in all subsequent calculations.
This loading of CoPc was held constant for each deposited lm
in this study.
Cobalt phthalocyanine is known to be a competent HER
45–48
catalyst.
2
Under an atmosphere of N , scans negative of the
phthalocyanine reduction show the onset of a catalytic wave
attributed to HER at approximately ꢁ1.04 V vs. SCE for CoPc
modied electrodes and ꢁ1.08 vs. SCE for electrodes modied
Fig. 1 Proposed molecular structures of CoPc, CoPc-P4VP, CoPc(py),
CoPc-P2VP, and CoPc(py)-P2VP.
with CoPc-P4VP. Under a CO atmosphere, the onset of the
2
catalytic wave shis slightly positive to ꢁ1.0 V vs. SCE, for both

lms as shown in Fig. 2. The peak current in the presence of
studied using controlled potential electrolysis (CPE) to better
understand the base activity of CoPc and the increase in activity
that coincides with P4VP encapsulation. As previously reported,
large increases in activity and selectivity were observed for CoPc-
P4VP over the parent CoPc. We present more a detailed evalu-
ation of CPE data to provide a more comprehensive study of
their properties. In particular, we nd that CoPc-P4VP is
remarkably active for the selective generation of CO, operating
ꢁ2
CO
2
of both lms in static CVs is approximately 0.75 mA cm
.
When performed at 1600 rpm (to ensure steady state delivery of
substrate to the electrode surface), rotating disk electrode vol-
tammograms (RDEVs) of CoPc-P4VP display much greater peak
currents than CoPc (Fig. 3A and B, respectively). The more
negative onset potential for H2 evolution with CoPc-P4VP
modied electrodes suggests that the P4VP lm suppresses
HER compared to free CoPc. However, the onset of the catalytic
ꢁ2
in pH 5 solution with an average current density of 2 mA cm
wave under CO
lm indicating that P4VP does not similarly suppress CO
activity of CoPc.
CPE experiments were performed at ꢁ1.25 V vs. SCE to assess
faradaic efficiencies for CO reduction by CoPc and CoPc-P4VP
2
was not affected by the presence of the polymer
ꢁ1
and a turnover frequency of 4.8 s with nearly 90% faradaic
efficiency for CO production at ꢁ1.25 V vs. SCE (ꢁ0.73 V vs.
RHE). This operating potential is only ꢁ0.61 V from the ther-

2
RR
0
modynamic potential of CO production (E ¼ ꢁ0.125 V vs. RHE)
2
making the CoPc-P4VP system among the most active molecular
modied electrodes (Fig. 4). Electrolyses were conducted for 2 h
in a gas-tight, two-compartment electrolysis cell under a CO2
atmosphere in a constantly stirred, CO2 saturated, 0.1 M
2
catalyst reported for the selective reduction of CO to any single
34,37–44
product in aqueous solution.
To investigate the inuence of axial coordination of CoPc on
the CO RR activity in the absence of the polymer lm, elec-
2 4
aqueous NaH PO buffer solution at pH 4.7. The results of these
2
experiments are summarized in Table 1. Nearly four times as
much charge was passed using CoPc-P4VP modied electrodes
compared to CoPc modied electrodes. Over the 2 h experi-
trodes modied with axially coordinated CoPc(py) were
prepared from deposition solutions of CoPc in the presence of
pyridine. Likewise, to separate the intrinsic properties of the
polymer from axial coordination effects, electrodes with four-
coordinate CoPc, and ve-coordinate CoPc(py) encapsulated in
a non-coordinating poly-2-vinylpyridine (P2VP) membrane
were prepared (Fig. 1). Our results suggest that there is
a synergistic relationship between axial coordination and the
chemical environment imposed by the P4VP membrane that
leads to dramatic enhancements in activity observed for CoPc-
P4VP.
ꢁ2
ments, current densities averaging 0.62 mA cm and 2.0 mA
Results
CoPc and CoPc-P4VP
Electrodes modied with CoPc and CoPc-P4VP were prepared to
investigate the CO RR catalytic activity of the free catalyst and
2
the effect of the P4VP support. Films were deposited onto an
Fig. 2 Representative static cyclic voltammograms of (left) CoPc and
2 2
EPG disc electrode by casting a solution in DMF on the (right) CoPc-P4VP under N and CO .
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(TON) for CO of 4500 and 34 000 were determined for CoPc and
CoPc-P4VP, respectively. From these TONs, empirical turnover
frequencies (TOFs) could be determined for the production of
CO (Table 1). It is important to note that, because the total
deposited catalyst coverage was used to determine TON and
TOF values, these values are likely a lower limit of the actual
values. While the previous study of CoPc immobilized in 100%
P4VP did not report faradaic efficiency data for CoPc and CoPc-
P4VP lms, selectivity data is reported in the form of the ratio of
33
CO to H2. For CoPc, the observed CO/H ratio of 0.9 : 1 is in
2
good agreement with the previously reported ratio of ꢀ1.5 : 1.
However, in our hands, H production by CoPc-P4VP is signif-
2
Fig. 3 Representative rotating disk electrode voltammograms at 1600
rpm of (A) the free catalysts CoPc (light blue) and CoPc(py) (purple) and
icantly reduced. The previous study reported a CO/H
2
ratio of
4
: 1, while we observe a CO/H ratio of 19 : 1. This difference in
2
(B) polymer immobilized catalysts CoPc-P4VP (dark blue), CoPc-P2VP
green) and CoPc(py)-P2VP (red) under an atmosphere of CO
(
2
.
selectivity could be due to a difference in P4VP membrane
thickness—5 mL of the 1% w/v P4VP deposition solution to
prepare the modied electrodes in this study whereas only 2 mL
was used in the earlier report.
A series of control experiments were conducted to ensure that
the observed CO production was due to the electrocatalytic
reduction of CO by CoPc. The results of these control experi-
2
ments are provided in Tables S1–S3.† To summarize, potentio-
static electrolysis experiments were conducted with unmodied
and P4VP coated EPG electrodes. For EPG electrodes under 1 atm
2
CO , the major product was hydrogen. For P4VP coated EPG
electrodes under 1 atm CO , hydrogen was also observed, but in
2
much smaller amounts compared with the bare EPG electrodes
due to the very low charge passed during the experiments. In
both cases only very small amounts of CO could be observed (0.17
mmol), less than 2% of the amount of CO generated by CoPc-P4VP
under identical conditions. Additionally, electrolyses were con-
ducted with CoPc and CoPc-P4VP under an atmosphere of N
2
. As
expected the only product observed was H
2
for both systems.
Axial ligand effect
Fig. 4 Representative electrolyses for CoPc, CoPc-P4VP, EPG, and To investigate the inuence of axial coordination on CO
2
RR
EPG-P4VP conducted at ꢁ1.25 V vs. SCE.
activity of CoPc in the absence of the polymer lm, electrodes
modied with the pyridine coordinated catalyst, CoPc(py), were
prepared using a deposition solution of CoPc in neat pyridine. A
ꢁ
2
cm were observed for CoPc and CoPc-P4VP, respectively. The large excess of the axial ligand was employed in the deposition
only products observed for both lms were CO and H . No solutions to ensure the equilibrium would favor the axially
2
49
liquid products were detected for either CoPc or CoPc-P4VP coordinated species. RDEVs of the CoPc(py) lms under an
within our detection limits (ꢀ10 mM). Faradaic efficiencies (3) atmosphere of CO , at 1600 rpm, display a positive shi in the
2
for CO are 36% for CoPc and 89% for CoPc-P4VP. 3
H
is 41% for onset potential of the catalytic wave, compared to CoPc, and
2
CoPc and only 5% for CoPc-P4VP lms. Turnover numbers a signicant increase in the peak catalytic current over the
Table 1 Results obtained from 2 h CPE experiments at ꢁ1.25 V vs. SCE for CoPc modified electrodes. The reported values are averages of
measurements from at least three experiments with independently prepared electrodes. Errors are given as standard deviations except for those
for 3total which were calculated as standard errors
ꢁ1
Charge/C
3
CO
TONCO (2 h)
TOFCO (s
)
3
H2
3
total
3
4
4
3
4
CoPc
CoPc(py)
CoPc-P4VP
CoPc-P2VP
CoPc(py)-P2VP
0.58 ꢃ 0.24
0.83 ꢃ 0.48
1.9 ꢃ 0.20
0.36 ꢃ 0.08
1.76 ꢃ 0.27
36 ꢃ 7%
68 ꢃ 3%
89 ꢃ 3%
73 ꢃ 8%
83 ꢃ 5%
4.5 ꢃ 2.4 ꢂ 10
1.2 ꢃ 0.7 ꢂ 10
3.4 ꢃ 0.4 ꢂ 10
5.6 ꢃ 1.8 ꢂ 10
3.0 ꢃ 0.5 ꢂ 10
0.6 ꢃ 0.3
1.6 ꢃ 1.0
4.8 ꢃ 0.6
0.8 ꢃ 0.2
4.2 ꢃ 0.7
41 ꢃ 8%
19 ꢃ 5%
5 ꢃ 1%
12 ꢃ 3%
6 ꢃ 5%
77 ꢃ 10%
87 ꢃ 6%
94 ꢃ 3%
85 ꢃ 9%
89 ꢃ 7%
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parent complex, as shown in Fig. 3A. During 2 h electrolysis the same potential as CoPc-P4VP, and the peak current was
experiments, CoPc(py) lms passed ꢀ1.5 times more charge much larger than CoPc-P2VP (Fig. 3B). Average current densities
ꢁ
2
ꢁ2
than CoPc, with average current of 0.96 mA cm (Fig. S6†). of 1.9 mA cm were observed for the CoPc(py)-P2VP system,
Faradaic efficiencies for CO with CoPc(py) were more than which are nearly the same as for CoPc encapsulated in P4VP.
double the parent catalyst at 68%. Only 19% of the total charge The total charge passed in bulk electrolyses increased to 1.7 C
2
passed went towards the production of H in the CoPc(py) lm. for CoPc(py)-P2VP from 0.36 C with CoPc-P2VP. Faradaic effi-
No liquid products were detected. The TON for CO with the ve- ciency for CO was increased from 73% without pyridine to 83%,
ꢁ
1
coordinate complex was around 12 000, with a TOFCO of 1.6 s
Enhanced CO RR performance has been observed with some CoPc in P2VP the CO
heterogeneous catalysts aer the addition of free pyridine to the observed for CoPc-P4VP.
.
and 3H2 decreased to 6%. By reintroducing an axial ligand to
2
2
RR performance was restored to the level
50–52
electrolysis solution.
CPE experiments were conducted with
CoPc lms in the presence of added pyridine and 2,6-lutidine to
determine whether the presence of free ligand increases CO RR
2
Discussion
activity. The results of these experiments are summarized in the In agreement with previous reports, the encapsulation of CoPc
ESI (Table S3†). Electrolyses preformed using CoPc modied in P4VP results in a dramatic increase of activity and selectivity
electrodes with 0.05 mM added pyridine or 2,6-lutidine showed for CO RR over free CoPc. The present study examines the
2
no signicant change in the amount of charge passed or in the activities and product distributions of these catalyst lms in
faradaic efficiencies for CO or H compared to CoPc alone. The greater detail in an effort to better understand the underlying
2
concentration of added pyridine and 2,6-lutidine in these cause of this phenomenon. By itself, CoPc is an unremarkable
experiments is orders of magnitude higher than the concen- CO RR catalyst that displays poor selectivity and relatively low
2
tration of pyridine that could be achieved from the complete activity for the reduction of CO over proton reduction. Once
2
dissociation of pyridine from CoPc(py) into solution (6.3 nM). immobilized in P4VP, the selectivity for CO production jumps to
Because no increase in activity or selectivity was observed with nearly 90% with close to an order of magnitude increase in the
the addition of free pyridine, it appears that the axial ligand TONCO. Under the conditions described herein, CoPc-P4VP
remains bound to CoPc in lms of CoPc(py) during catalysis.
displays an even higher selectivity for CO production over HER
33
than has been previously reported. In fact, CoPc-P4VP is
among the most active and selective molecular CO reduction
catalysts yet studied in aqueous solutions (Table 2). Addition-
2
Polymer encapsulation
To separate the secondary and outer coordination sphere effects ally, at ꢁ0.73 V vs. RHE, or a 0.61 V overpotential for the
of the polymer membrane from the axial coordination effects, production of CO, the high activity and selectivity are notable,
modied EPG electrodes were prepared with CoPc encapsulated even in comparison to heterogeneous catalysts like copper,
in a poly-2-vinylpyridine (P2VP) membrane. Steric congestion which at this potential non-selectively produces CO and formate
around the pyridine nitrogens in P2VP should prevent coordi- with lower faradaic efficiencies along with a large amount of H
nation to CoPc. Though P2VP is not capable of coordinating, it under similar conditions, and gold, which is among the most
2
33
1–6
should maintain the secondary and outer coordination sphere active and selective polycrystalline metal catalysts for the
5,53,54
properties of P4VP. CoPc-P2VP lms were deposited under the reduction of CO
2
to CO.
It is clear that the P4VP membrane
same conditions as described for the deposition of CoPc-P4VP. alters the chemical environment of CoPc in a way that promotes
RDEVs of CoPc-P2VP under CO show the onset of catalysis the reduction of CO . We hypothesize that both the ability of
2
2
at potentials similar to CoPc, with peak currents lower than P4VP to coordinate CoPc, and the secondary and outer coordi-
those of the parent catalyst (Fig. 3B). Measurements of CO nation sphere effects that arise from the partial protonation of
2
catalysis showed that CoPc-P2VP lms passed much less charge free pyridine residues throughout the polymer lm are the
during the electrolyses than CoPc-P4VP, and the average current major contributors to the increased activity, as illustrated in
ꢁ2
density was 0.39 mA cm . However, CoPc-P2VP operated with Fig. 5. Though polymer encapsulation is a common way of
3
CO ¼ 73%, much higher than CoPc, and 3H2 for CoPc-P2VP was immobilizing molecular catalysts onto an electrode
19,41,55–64
only 12%. Compared to CoPc, CoPc-P2VP showed practically surface,
the use of a polymer membrane that offers this
25,39,48,56,65–69
identical TONCO and TOFCO during the 2 h electrolyses. These combination of properties is rare.
experiments indicate that while the P2VP support does not Porphyrin and phthalocyanine catalysts oen display
reduction, it does suppress enhanced catalytic activity upon the coordination of an axial
increase the activity of CoPc for CO
2
2
6,28,70–72
HER.
ligand.
The ability of P4VP to act as an axial ligand
RR activity and selectivity of
Electrodes modied with CoPc(py) dispersed in a P2VP contributes to the enhanced CO
2
membrane (CoPc(py)-P2VP) were prepared to reintroduce the CoPc-P4VP modied electrodes. Other than in a P4VP lm, the
axial ligand to CoPc in the P2VP catalysts lms. These lms were only previous study that suggests an axial ligand effect on
deposited from a 19 : 1 DMF/pyridine solution containing 0.05 CO RR selectivity with CoPc comes from an IR spectroelec-
2
mM CoPc and 1% w/v P2VP. Incorporating a more active, ve- trochemical study which suggests that CoPc lms deposited
coordinate CoPc catalyst in the P2VP lm increased CO RR from neat pyridine are more selective for CO RR over HER than
2
2
73
activity by nearly an order of magnitude. The onset potential of lms deposited from THF. The enhanced activity of CoPc(py)
CO RR catalysis in RDEVs of CoPc(py)-P2VP occurred at nearly shows that the coordinating ligand effect is responsible for the
2
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Table 2 A comparison of reported molecular CO
P4VP and CoPc(py)-P2VP
2
RR electrocatalysts which display high activity and/or selectivity in aqueous media to CoPc-
ꢁ2
ꢁ1
Catalyst
Activity/mA cm
V vs. RHE pH
Products (3)
TOF/s
Ref.
CoPc-P4VP
CoPc(py)-P2VP
2.0 ꢃ 0.2
1.9 ꢃ 0.2
NR
ꢁ0.73
ꢁ0.73
ꢁ0.70
4.7
4.7
4.4
CO (89 ꢃ 3%), H
2
2
(5 ꢃ 1%)
(6 ꢃ 5%)
CO: 4.8
This study
This study
34
CO (83 ꢃ 5%), H
CO: 4.2a
b
b
CoPc-(90% P4VP, 10%
polystyrene)/BPG
CoPc-(90% P4VP, 10%
polystyrene)/BPG
COF-367-Co
CO (71.6%), H
CO (77.2%), H
2
2
(21.0%)
CO: 3.1 (EA 41)
a
NR
ꢁ0.66
6.8
(16.6%)
CO: 2.9 (EA 51)
34
a
b
ꢀ3.5
ꢀ0.5
ꢁ0.67
ꢁ0.67
ꢁ0.75
7.3
7.3
7
CO (91%), H
CO (48%), H
CO (46%), H
2
2
2
(20%)
(51%)
(44%)
CO: 0.05 (EA 0.5)
37
37
38
a
b
COF-367-Co(1%)
CO: 0.2 (EA 2.6)
[
Mn(bpy(tBu)
2
)(CO)
3
Br]/ 0.2
CO: 0.0005
Naon/MWCNT
Ni(cyclam)-PALA
c
NA
NR
ꢁ0.17
ꢁ0.52
ꢁ0.65
ꢁ0.83
ꢁ1.0
8
5.8
7
CO (92%)
NA
NR
39
40
d
e
d
Poly(Cr(vinylterpy)
Re[(bpy)(CO
Co(Ch)/MWCNT
Ir-Pincer (2
Ni(cyclam)
Ni(MTC)
2
)
HCHO (87%)
HCO H (48%), CO (16.5%), H
CO (89%)
3
)Br]/Naon 0.002
2
2
(39%) CO: 0.002, HCO
CO: 0.04
2
H: 0.006 41
NR
0.60
0.64–0.97
0.64–0.97
0.64–0.97
0.64–0.97
4.6
42
43
44
44
44
44
MeCN
f
)
6.95 HCOOH (93%), H
2
(7%)
NR
ꢁ0.67
ꢁ0.67
ꢁ0.67
ꢁ0.67
5
5
5
5
CO (84 ꢃ 4%)
CO (88 ꢃ 7%)
CO (92 ꢃ 2%)
CO (88 ꢃ 7%)
NA
NA
NA
NA
Ni(MCC)
Ni(HTC)
a
These TOF values were recalculated from the literature report using the total loading of catalyst cast onto the surface, as opposed to the amount
detected by CV. We believe using the total amount of catalyst deposited provides a more accurate comparison to other reported values in the
b
c
ꢁ1
literature. Reported TOF values based on the electroactive surface coverage of the catalyst. Solution phase catalyst at 2.0 mg mL
concentration. No time information was provided for the electrolysis in the report, so activity and TOF could not be calculated for the
electrolysis. However, the reported Koutecky–Levich analysis in the manuscript yields a TOF of 5.2 s
saturated with CO
d
ꢁ1
e
.
Estimated pH of 0.1 M NaClO
4
f
2
.
No empirical TOF based on electrolysis data was provided. However, the authors did report a TOF value calculated from
:1
CV data of 7.3 s
.
ꢁ
Fig. 6 Relative energies of the cobalt d orbitals in the 1 e reduced
forms of CoPc and CoPc(py). Shown is the energy increase of the
z
cobalt d orbital that results the coordination of the axial pyridine.
2
Fig. 5 An illustration of CoPc immobilized in P4VP. Three properties
of the polymer membrane which are suggested to be important to the
activity and selectivity of CoPc-PVP are illustrated. Pyridine residues
that can coordinate to CoPc, the ability for uncoordinated pyridine
metal becomes a stronger nucleophile, and is better able to bind
. The signicant
residues to act as proton relays, and the hydrogen bonding interac- and activate the Lewis acidic carbon of CO
2
tions that may occur between protonated pyridines and activated CO
2
.
decrease in 3H2 with CoPc(py) compared to CoPc may be
attributable to the CO binding outcompeting proton reduction.
2
This also suggests that CO2 reduction takes place via a CO2
increases in the rate of CO
coordination raises the energy of the cobalt d
2
reduction. As shown in Fig. 6, axial binding event rather than the formation of a cobalt hydride that
26,34
2
orbital. When then goes on to react with CO2,
as the formation of such
z
the metal center is reduced to Co(I), lling the d
z
2
orbital, the a hydride would presumably also be the rst step in HER.
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The primary and outer coordination spheres of enzyme basic enough to suppress HER activity. The selectivity for CO
active sites are specially adapted to efficiently perform difficult with CoPc-P4VP is much higher than with CoPc-P2VP. This
catalytic transformations for the production of a single desired suggests that in addition to the outer sphere weak acid effect,
product. Protein lm voltammetry has been used to demon- the primary sphere effect of axial coordination on the selectivity
strate that enzymes adsorbed on electrode surfaces can perform for CO
electrocatalytic reactions, including CO RR, with very high important factor for this catalytic selectivity.
although low surface coverages limit the overall Beyond limiting HER activity, the polymer membrane effects
activity. These studies have also established the importance of additional CO RR rate enhancements, but only for the more
In molecular active ve-coordinate CoPc catalysts CoPc-P4VP and CoPc(py)-
2
RR over HER, demonstrated by CoPc(py) is also an
2
74,75
TON,
2
76–78
electron and proton relays in these reactions.
systems, the importance of the hydrogen bonding in the P2VP. Protonated pyridine residues in close proximity to the
secondary coordination sphere to stabilize reactive intermedi- CoPc(L) catalyst active site (L ¼ py, P4VP) may be able to
79–85
ꢁ
2
ates has been demonstrated by several groups.
In particular, stabilize the [(L)Co(II)Pc-CO ] intermediate through hydrogen
second sphere proton relays incorporated into a ligand scaffold bonding interactions, and may increase the rate of proton
ꢁ
can lead to high electrocatalytic activity in reactions that require transfer to the activated [(L)Co(II)Pc-CO
the delivery of multiple protons.
2
]
complex (Fig. 7).
86–89
The P4VP membrane Again though, these secondary coordination sphere effects only
imbues many of these secondary coordination sphere effects to result in rate enhancements with the ve-coordinate CoPc(L)
CoPc without the need for synthetically challenging ligand catalysts. This is demonstrated by comparing the observed
modications.
TOFCO of CoPc-P2VP and CoPc(py)-P2VP to the TOFCO of the
While CoPc-P2VP showed virtually no increase in TON for CO catalysts without the polymer lm. No enhancement of CO RR
2
over free CoPc, HER activity was greatly diminished. Combined activity was observed when the four-coordinate CoPc was
with the decrease in HER performance displayed by CoPc-P4VP immobilized in the P2VP membrane compared to the catalyst
under N and in the EPG-P4VP control experiments, it is clear outside of the polymer membrane. Alternatively, a large
2
that the polymer lm helps to suppress proton reduction increase in TOFCO was observed for CoPc(py)-P2VP compared to
pathways. It may be that limited diffusion of water through the polymer free lms of CoPc(py). The chemical environment

lm is the reason for the lower activity for HER. In the case of around the catalyst active sites in CoPc-P2VP and CoPc(py)-
polymer free CoPc modied electrodes, the catalyst is directly P2VP lms is likely unaffected by the presence of the axial
exposed to solution where there is a large local concentration of pyridine, yet only in the case of CoPc(py)-P2VP does being in
protons. In the polymer immobilized systems, the catalyst is not this chemical environment lead to increased catalytic activity. A
exposed to the solvent directly and the permeability of water plausible explanation for this phenomenon is that the presence
through the PVP membrane at this pH is limited, so the main of the axial ligand may change the rate limiting step of CO
2
ꢁ
source of available protons in the polymer lm is likely from reduction from the formation of Co(II)Pc-CO
2
with the four-
33,35,36
protonated pyridine residues.
At pH 4.7 approximately coordinate catalyst, to the subsequent proton transfer steps in
33
20% of the pyridine residues in the lm are protonated. It is the case of the ve-coordinate catalyst. If this is the case, the
reasonable to conclude that increase in selectivity for CO over secondary coordination sphere effects would have no inuence
H with CoPc-P2VP compared to CoPc alone could be due to of the rate of CO reduction by CoPc, but would result in the
2
2
90
a weak acid effect from the protonated pyridine residues.
dramatic increases in CO RR activity observed for the more
2
While these pyridinium residues may be acidic enough to act as active CoPc(L) catalysts.
a proton donor to activated CO intermediates, they may also be
2
2
Fig. 7 Proposed activated CO complexes and secondary coordination sphere interaction for each catalyst film studied. Also shown are the
faradaic efficiencies and turnover frequencies for each catalyst system determined from CPE measurements.
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electrodes were prepared by evaporation of 5 mL of the deposi-
Conclusions
tion solution on an edge-plane graphite disc electrode (5 mm
2
Immobilization of cobalt phthalocyanine in poly-4-vinyl- diameter, effective electrode area: 0.13 cm , Pine Research
pyridine dramatically improves its activity as a catalyst for the Instrumentation). For all modied electrodes used in this study,
ꢁ9
ꢁ2
reduction of CO
competing HER catalytic pathway while also increasing rate of to catalyst deposition, the electrodes were cleaned by manually
CO
RR compared to the polymer free catalyst. It is clear from polishing the surface with 600 grit Carbimet SiC grinding paper
2
to CO. The polymer membrane slows the the coverage of deposited CoPc was 1.3 ꢂ 10 mol cm . Prior
2
the results obtained for CoPc(py) and CoPc-P2VP that neither (Buehler) followed by sonication in ultrapure water for 10 min.
the primary, or secondary and outer coordination sphere effects
alone are responsible for the large increases in CO RR activity. Electrochemical methods
2
These effects must be combined, as is the case in CoPc-P4VP
and CoPc(py)-P2VP, to produce a CoPc CO RR catalyst that is
2
Experiments were conducted using a Bio-Logic SP200 poten-
tiostat/galvanostat with a built-in electrochemical impedance
spectroscopy (EIS) analyzer or a Bio-Logic VMP3 multichannel
potentiostat/galvanostat with a built-in EIS analyzer. The
modied working electrodes were mounted in a Pine Instru-
ment Company E6-series ChangeDisk rotating disk electrode
assembly in an MSR rotator. The auxiliary electrodes were
carbon rods (99.999%, Strem), and the reference electrodes
were commercial saturated calomel electrodes (SCE) (CH-
Instruments) that were externally referenced to a solution of
ferrocene monocarboxylic acid (Sigma-Aldrich) in a 0.2 M
highly active and selective. In this way CoPc-P4VP and CoPc(py)-
P2VP mimic enzymes, where all aspects of the catalyst chemical
environment are critical to the function of the system overall.
We propose that this electrode-modication strategy using
functional polymer membranes can be applied more generally
to the eld of CO reduction catalysis. The ability of PVP lms to
2
suppress HER without inhibiting CO
tance considering that the cogeneration of H
catalysts, both molecular and heterogeneous, in aqueous media
limits CO RR product selectivity. Additionally, the incorpora-
2
RR is in itself of impor-
2
by CO reduction
2
2
91
phosphate buffer at pH 7 (0.284 V vs. SCE). The supporting
tion of groups other than pyridine in the polymer chain could
allow us to engineer the chemical environment around catalyst
active sites in a facile way, without the need for complicated
ligand synthesis. Each component of these catalyst lms could
be selected based on its ability to maximize the efficiency and
selectivity of the system as a whole. This approach could lead to
the development of new highly active catalyst lms which are
not only selective for a single highly reduced product, but also
cost effective and easily prepared.
electrolyte solution was 0.1 M aqueous NaH PO (BioUltra $
2
4
99.0%, Sigma) with the pH adjusted to pH 5 by the addition of 1
M aqueous NaOH. When saturated with CO the pH of the
2
electrolyte solution was measured to be 4.71. The pH of elec-
trolyte solutions were measured with a VWR Symphony multi-
parameter meter with a Thermo Scientic Orion rellable Ag/
AgCl pH electrode lled with Orion Ag/AgCl reference electrode

lling solution. The pH meter was calibrated with a 4 point
calibration curve at pH ¼ 1.68, 4.00, 7.00, and 10.00. Data were
recorded using the Bio-Logic EC-Lab soware package.
Experimental
Materials
Electrochemical measurements
Cyclic voltammetry (CV) and rotating disc electrode (RDE)
experiments were conducted in a glass two-chamber U-cell. One
chamber held the working and reference electrodes in ꢀ150 mL
of solution, and the other chamber held the auxiliary electrode
in ꢀ20 mL of solution. The two chambers were separated by
a ne-porosity glass frit. Both chambers were purged with either
N2 or CO2 for ꢀ20 min prior to each experiment and then
blanketed with N or CO during data collection. CO and N
2
Ultrapure water (18.2 MU cm resistivity) was puried with
a Thermo Scientic Barnstead Nanopure water purication
system. Carbon dioxide (CO
helium (He, Alphagaz–1 grade, 99.999%) were purchased from
Air–Liquide and used as received. Nitrogen (N ) was boil-off gas
2
, Alphagaz–1 grade, 99.99%) and
2
from a liquid nitrogen source and used without further puri-
cation. CoPc (97%) was obtained from Alfa Aesar and used as
received. P4VP (average M ꢀ 160 000) and P2VP (average M ꢀ
2
2
2
w
w
were water-saturated by bubbling though gas-washing bottle
lled with ultrapure water.
159 000) were purchased from Sigma Aldrich and used as

received. N,N-Dimethylformamide (DMF, ACS grade) was
purchased from VWR. Pyridine (ACS grade) was purchased from
Controlled potential electrolyses
Alfa Aesar. Deuterium oxide (D
Cambridge Isotope Labs, Inc.
2
O, 99.9%) was purchased from
Experiments were conducted in a custom, two-chamber U-cell.
The working and reference electrodes were held in a gas tight
chamber with a total volume of either 81 or 91 mL and lled
with solution to provide 41 mL of headspace. The second
Preparation of modied electrodes
All catalyst lms were prepared from deposition solutions chamber held the auxiliary electrode in 15 mL of solution and
containing 0.05 mM CoPc. The parent CoPc was deposited from was open to air. The two chambers were separated by a Naon
solutions in DMF, as were the polymer encapsulated CoPc lms. 117 cation exchange membrane (Aldrich). The chemically-
Deposition solutions for polymer-encapsulated CoPc lms modied working electrode was held in a Pine Instrument
contained 1% w/v of the polymer. For lms of CoPc(py), neat Company RDE internal hardware kit and mounted in a custom
pyridine was used as the solvent in place of DMF. Modied PEEK sleeve. Prior to each electrolysis experiment the cell was
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2
purged with CO for at least 1 h and then sealed under an
Notes and references
atmosphere of CO . All experiments were conducted at least
2
three time with independently prepared electrodes. All values
reported are the averages of these repetitions and errors are
reported as standard deviations.
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