Linkage Effect in the Heterogenization of Cobalt Complexes by Doped Graphene for Electrocatalytic CO2 Reduction

Article abstract of DOI:10.1002/anie.201906475

Immobilization of planar Co^II-2,3-naphthalocyanine (NapCo) complexes onto doped graphene resulted in a heterogeneous molecular Co electrocatalyst that was active and selective to reduce CO₂ into CO in aqueous solution. A systematic study revealed that graphitic sulfoxide and carboxyl dopants of graphene were the efficient binding sites for the immobilization of NapCo through axial coordination and resulted in active Co sites for CO₂ reduction. Compared to carboxyl dopants, the sulfoxide dopants further improved the electron communication between NapCo and graphene, which led to the increase of turnover frequency of the Co sites by about 3 times for CO production with a Faradic efficiency up to 97 %. Pristine NapCo in the absence of a graphene support did not display efficient electron communication with the electrode and thus failed to serve as the electrochemical active site for CO₂ reduction under the identical conditions.

Full text of DOI:10.1002/anie.201906475

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Accepted Article
Title: Linkage effect in the heterogenization of cobalt complexes by
doped graphene for electrocatalytic CO2 reduction
Authors: Xin Wang, Jiong Wang, Xiang Huang, Shibo Xi, Jong-Min
Lee, Cheng Wang, and Yonghua Du
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Linkage effect in the heterogenization of cobalt complexes by
doped graphene for electrocatalytic CO reduction
2
‡
#
Δ#
§
‡
&
§ǁ
Jiong Wang, Xiang Huang, Shibo Xi, Jong-Min Lee, Cheng Wang, Yonghua Du,
Xin Wang‡*
‡School of Chemical and Biomedical Engineering, Nanyang Technological University, 62
Nanyang Drive, Singapore 639798, Singapore
ΔDepartment of Physics, Southern University of Science and Technology, Shenzhen
5
18055, China.
&Institute for New Energy Materials & Low-Carbon Technologies, Tianjin University of
Technology, Tianjin, 300384, P.R. China
§Institute of Chemical and Engineering Sciences, Agency for Science, Technology and
Research (A*STAR), Singapore 627833, Singapore
ǁPresent address: National Synchrotron Light Source II, Brookhaven National Laboratory
Upton NY, 11973 USA
*
E-mail: WangXin@ntu.edu.sg
Abstract
Immobilization of planar Co(II)-2,3-naphthalocyanine (NapCo) complexes onto doped
graphene resulted in a heterogeneous molecular Co electrocatalyst that was active and
selective to reduce CO into CO in aqueous solution. A systematic study revealed that
2
graphitic sulfoxide and carboxyl dopants of graphene were the efficient binding sites for
the immobilization of NapCo through axial coordination and resulted in active Co sites for
CO reduction. Compared to carboxyl dopants, the sulfoxide dopants further improved the
2
electron communication between NapCo and graphene, which led to the increase of
turnover frequency of Co sites by ~ 3 times for COproduction with a Faradic efficiency up
to 97%. Pristine NapCo in the absence of graphene supports did not display efficient
electron communication with electrode and thus failed to serve as the electrochemical
active sites for CO reduction under the identical conditions.
2
Introduction
Conversion of carbon dioxide (CO ) into fuels or chemicals is a potential solution to
2
reduce our dependence on fossil fuels and address the environmental issues arising from
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the use of such fuels. Of particular interest is electrochemical CO reduction, which can
2
sustainably convert CO
inputs from renewable energies. CO
because CO is chemically inert and the conversion needs to compete with hydrogen
evolution if it is conducted in aqueous solution. The process is also complicated that
2
into carbonaceous fuels at mild conditions through the electrical
[
1]
2
reduction reaction (CO RR) is a challenging task
2
2
[
2]
involves multiple protons coupled electron transfer steps leading to various products. As
a main and profitable product of CO RR, carbon monoxide (CO) can be used in syngas for
2
production of hydrocarbons and their oxygenated derivatives via the Fischer-Tropsch
process.[3] Both homogeneous and heterogeneous catalysts have been explored for the
selective production of CO. In principle, homogeneous catalysts, such as metal complexes
and organometallic compounds, contain molecularly uniform and well-defined active sites
that can be tuned systematically to achieve high selectivity and turnover frequencies
[
4]
(
TOFs). Furthermore, they can be an ideal platform to probe the mechanism of CO RR
2
at molecular levels and guide the rational design of catalysts.[5] However, homogeneous
catalysts often need aprotic solvents, such as N, N-dimethylformamide and acetonitrile, to
promote their dissolution in electrolytes and sustain high TOFs.[6] The integrated
electrolytic cells in practice requires aqueous solutions, where few homogeneous catalysts
exhibit sufficient activity or selectivity.[6b, 7] Another issue that homogeneous catalysts face
is the difficulty to be separated from electrolytes for recycle or products extraction purpose,
[
8]
and they are incompatible with flow cells. Heterogeneous catalysts generally exhibit the
opposite characteristics. Such catalysts are robust and operative in aqueous solutions, while
it is not easy to understand catalytic mechanism and tune the activity at molecular levels
due to the surface structure complexity.[9] Immobilizing homogeneous catalysts onto
conductive substrates combines the advantages of molecular catalysis with the convenience
of heterogeneous catalysis in aqueous solutions, and represents a promising approach to
overcome these drawbacks.
Various studies have been carried out to immobilize homogeneous catalysts onto
conductive substrates, including covalent or noncovalent linking,[6b, 10] assembly into metal
or covalent organic frameworks,[11] polymerization[12] and directly casting water-insoluble
molecules[13]. Among these studies, complexes of metal phthalocyanines and porphyrins
represent an important class of CO RR molecular catalysts. Their planar structures are
2
suitable for surface attachment with substrates to form monolayers with the axial sites of
metal centers providing the active sites.[8b, 14] Except for the selection of appropriate active
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sites, in heterogeneous states, the electrocatalysis on complexes also relies on the electron
communication between the metal centers and substrates, which is closely related to the
structures of linkages between the metal centers and solids. The molecular length, rigidities
and π-conjugation degree of linkages determine the distance and pathways for electron
transfer.[15] Poorly chosen linkages might obstruct such electron communications, which
makes the metal sites electro inactive that cannot serve as the active sites.[11a]
Based on the aforementioned considerations, we developed a heterogeneous CO RR
2
electrocatalyst that is operative in aqueous solution by immobilizing planar Co(II)-2,3-
naphthalocyanine complexes (NapCo) on doped graphene based matrices and investigated
the linkage effects. Graphene is a suitable substrate because it has high conductivity,
stability under reductive environments and good reactivity to allow versatile linkages to
connect with molecules.[16] NapCo has a high π-conjugation degree to allow π-π stacking
with graphene. Besides, the axial coordination for NapCo provides another driving force
for the immobilization, and such an interaction has shown the potential to intrinsically
improve the CO
RR activity/selectivity of the immobilized Co sites.[17] We found that
2
graphitic sulfoxide and carboxyl dopants of graphene served as major linking sites for
NapCo through forming axial Co-O coordination. Such configurations resulted in the
active and molecular Co sites for CO RR. Additionally, the sulfoxide dopants greatly
2
promoted the electron communication between the immobilized Co sites and graphene
comparing to the carboxyl based linking site. The sulfoxide-NapCo moiety derived a high
selectivity of CO
3 times comparing to that of carboxyl-NapCo moiety. The pristine NapCo in the absence
of graphene supports did not have redox response and was thus electrochemically inactive
for CO RR.
2
-to-CO conversion (97% at -0.8 V) and increased the TOF of Co sites by
~
2
Results and Discussion
Heterogenization of molecular Co sites. In a typical synthetic process, GO aqueous
-1
solution (~1 mg mL ) was modified with thiourea under hydrothermal conditions, forming
S/N/O heteroatoms doped graphene (SNG). The SNG was mixed with NapCo in DMF at
o
8
0 C to derive a heterogeneous catalyst (denoted as NapCo@SNG). The complexes can
be immobilized onto SNG through π-π stacking. The heteroatoms doped in graphene
contained extra electrons comparing to the neighboring carbon atoms and they were
proposed to serve as another type of linking site that can tune the coordination
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configuration of NapCo on graphene (Figure 1a). Thus, for comparison, in the control
experiments, O and N/O doped graphene (OG and NG) were synthesized from GO using
the similar hydrothermal method. NapCo was immobilized onto the OG and NG to form
NapCo@OG and NapCo@NG following the same procedures as the synthesis of
NapCo@SNG (details were provided in the experiments of supporting information, SI).
The scanning and transmission electron microscopic (SEM/TEM) images (Figure 1b,
S1) revealed the two-dimensional structure of NapCo@SNG with substantial scrolls and
wrinkles, which are the characteristics of graphene. On these graphene layers, we did not
observe any nanoparticles of metallic Co or Co oxides, indicating that NapCo was not
decomposed under the synthesis conditions. In X-ray photoelectron spectroscopy (XPS,
Figure 1c) surveys, the NapCo@SNG showed two main peaks at 779.8 eV and 795.4 eV
with an intensity ratio of 2:1, assigning to the Co 2p3/2 and 2p1/2 core levels, respectively.
Meanwhile, the immobilization of Co sites generated a strong N 1s peak centered at 397.4
eV assigning to the Nap ligands (Figure 1c inset). The NapCo@SNG was probed by
aberration-corrected high-angle annular dark-field scanning TEM (HAADF-STEM, Figure
1
d and S2). Substantial bright spots (sizes less than 0.15 nm) corresponding to the
molecular Co sites were uniformly distributed over the graphene layers. The ultraviolet-
visible (UV-vis, Figure 1d, S3a) spectra showed that two absorbance bands of NapCo at
2
70 nm and 364 nm, corresponding to the metal-to-ligand charge transition (MLCT) and
d-d transition, respectively. The latter transition on NapCo@SNG exhibited a red shift
from 364 nm to 371 nm) comparing to that of pristine NapCo, which might reflect that the
(
charge transfer occurred between the Co sites and SNG. Such a red shift was also observed
upon immobilizing NapCo onto OG and NG (Figure S3b, c).
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Figure 1. (a) Heterogenization of NapCo onto doped graphene through π-π stacking and
coordination with heteroatoms. (b) A typical bright-field TEM image of NapCo@SNG. (c)
XPS surveys of Co 2p core electron levels of SNG and NapCo@SNG, respectively. Inset
shows the N 1s core electron levels of SNG and NapCo@SNG. (d) HAADF-STEM images
of NapCo@SNG. The Co atoms are indicated by the bright spots. (e) UV-vis spectra of
NapCo before and after immobilizing onto SNG.
Binding between molecular Co sites and graphene. The heterogenized Co sites were
investigated by X-ray absorption spectroscopy (XAS). In the normalized X-ray absorption
near edge spectra (XANES, Figure 2a), the absorption edge of NapCo@SNG clearly
shifted to lower energy than that of NapCo (shift by ~ 1 eV for the energy at edge-jump of
0
.5), indicating a strong electronic interaction between NapCo and SNG. In the Fourier-
transformed extended X-ray absorption fine structure spectra (FT-EXAFS, Figure 2b),
centering on the first shells, the peak position of NapCo occurred at R value of 1.4 Å for
the Co-N bonds. For NapCo@SNG, this Co-N peak slightly shifted to 1.35 Å. Notably, a
shoulder peak appeared near ~1.7 Å. Such a variation suggested that the coordination shell
of Co sites was tuned by SNG.
For the XAS data of NapCo@OG/NG (Figure 2c), it was also observed that the
absorption edges of Co sites shifted to lower energies with the immobilization. However,
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in the FT-EXAFS data (Figure 2d), both the control samples exhibited the distinct signals
on the first shells of Co sites from NapCo@SNG, and we did not observe the shoulder
signal as the one occurred in the FT-EXAFS data of NapCo@SNG. Apparently, the
immobilization of NapCo on the doped graphenes were not simply driven by π-π stacking,
but also was determined by possible interaction between Co sites and the different dopants
of graphene. The S dopants should lead to a distinct binding configuration of NapCo on
graphene comparing to the O and N dopants. By CHNS elemental analysis combining with
inductively coupled plasma mass spectrometric (ICP) measurements, we estimated the
atomic ratios of those dopants (i.e., S, N and O atoms) in different graphenes to the
immobilized Co sites (Table S2, S3), showing that those dopants were sufficient as the
major linking sites for NapCo.
Figure 2. (a) Co K-edge XANES data of NapCo@SNG and NapCo. (b) FT-EXAFS data
of NapCo@SNG, NapCo and a CoO reference without phase correction. (c) Co K-edge
XANES data of NapCo@SNG, NapCo@NG and NapCo@OG, (d) their FT-EXAFS data
without phase correction.
In the XPS surveys (Figure S4), SNG showed the distinct S 2p spectrum with two bands
centering at 163 eV and 167.2 eV, which were assigned to the doped structures of carbon
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sulfides and oxidized sulfurs, respectively.[18] The N 1s band of SNG centered at 398.8 eV
for pyridine or/and pyrrole structures as observed in NG.[19] The O 1s bands indicated that
the O dopants were mainly in the forms of carboxyl (530.5 eV) and hydroxyl (532.5 eV)
structures, which were similar to those in OG and NG.[20] The binding states of NapCo on
these dopants of graphene were investigated by density functional theory (DFT)
calculations. It was found that the pyridinic sulfoxide (SO) or graphitic SO of oxidized
sulfurs were very efficient binding sites to immobilize NapCo through Co-O bonding along
the axial position of Co site, which was revealed by the clear charge transfer occurred
between the Co sites and SO dopants (Figure 3a, b and S5). Because of such a clear
chemical bonding nature, the axial Co-O bonds should be more significant to determine
the immobilization of NapCo comparing to pure π-π stacking, which is van der waals
interaction. The optimized axial bond lengths for these two cases were 2.09 Å and 2.08 Å,
respectively, which were consistent with the Co-O distance of 2.08±0.02 Å fitted by the
EXAFS data of NapCo@SNG. The oxidized sulfurs might involve other structures such as
sulfonate (SO
2
) and sulfonic acid (SO H). These dopants can also form the axial Co-O
3
bonds with NapCo. The calculated bond lengths increased to 2.21 Å and 2.25 Å,
respectively, excluding the possible presence of such groups (Figure 3c, S6a-d).
Additionally, our calculation also showed that there was no electronic interaction between
NapCo and the carbon sulfides of graphene (Figure S6e, f). Therefore, for NapCo@SNG,
we suggested that pyridinic or graphitic SO dopants were the main binding sites for the
immobilization of NapCo.
Both pyridinic and pyrrolic N dopants in NG were not the efficient binding sites to
NapCo as they only exhibited van der waals interaction with the Co sites (Figure S7). In
contrast, carboxyl (COO) structure of O dopants in NG and OG was identified to be a
binding site for NapCo in the absence of the oxidized sulfur dopants. The axial Co-O bond
formed between COO and NapCo as indicated by the clear charge transfer between the Co
and COO dopant (Figure 3d, S8). The optimized bond length was 2.00 Å, which was close
to the Co-O distance of 2.08±0.02 Å fitted by the EXAFS data of NapCo@OG. Although
a hydroxyl oxygen of graphene could also form the axial Co-O bond with NapCo, the
charge transfer was less significant than that observed on the COO site and the Co-O bond
length prolonged to be 2.18 Å (Figure S9).
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Figure 3. Charge density difference of NapCo immobilized on pyridinic SO (a) and
graphitic SO (b) dopants of graphene. (c) The Co-O bond lengths for NapCo on SO, SO
and SO H dopants of graphene. (d) Charge density difference of NapCo on a COO dopant
2
3
of graphene. The charge accumulation regions were colored in pink and charge depletion
regions were colored in green. The charge density difference was obtained via subtracting
the total charge densities of adsorbed NapCo and graphene from that of NapCo@graphene.
3
Iso-surface level was set to ±0.001 e/bohr . The brown, cyan, white, yellow, red, blue and
light blue spheres represent C atoms of graphene, C atoms of NapCo, H, S, O, Co and N
atoms, respectively.
Electrochemical measurements. We next investigated the redox of Co sites by cyclic
voltammetry (CV, Figure S10a-c), after depositing NapCo@SNG on a planar glassy carbon
electrode (GCE). The immobilized Co sites were redox active in 0.1 M KHCO as indicated
3
by a pair of redox peaks (E1/2=~0 V) assigning to Co2+/1+ couples[21] (all potentials were
quoted against the reversible hydrogen electrode, RHE, unless stated otherwise). The peak
currents are linearly correlated with the scan rates to indicate the immobilized states of Co
sites on graphene. The rate constant (k ) of the redox was calculated by the Laviron’s
s
-1
equation to be 27.8 s , which was comparable to those of very redox-active monolayers on
electrodes.[15a, 22] We took the advantage of the insolubility of NapCo in water and directly
cast them onto GCE. Even by using differential pulse voltammetry (DPV) measurement,
the redox of Co sites was not observed in the same potential range (Figure S10d) due to
the low conductivity of bulk NapCo. These results suggested that the redox of molecular
Co sites was realized through the support of SNG, and the electron transfer between the
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molecular Co sites and SNG was rapid.
The CO
continuous purging of CO
V (Figure 4a). The products were analyzed by in-line gas chromatography (GC) and high-
performance liquid chromatography (HPLC). H and CO were the only products and other
possible products (hydrocarbons or acids) were not detected. Ranging the potentials from
0.4 V to -0.8 V, the Faradic efficiency for CO production (FEs-CO) gradually increased
from 67.5% to 97%. The FEs-CO are superior to most heterogenous molecular Co catalysts
and metallic metal catalysts reported recently (Table S4). The CO RR activity of
2
RR electrocatalysis on NapCo@SNG was conducted in 0.1 M KHCO
3
with
2
. CO RR currents were observed with an onset potential of -0.4
2
2
-
2
NapCo@NG and NapCo@OG was studied for comparison. Both the control samples
displayed slightly lower CO selectivity at all potentials and lower stability in continuous
running (Figure 4b, c). Notably, pristine NapCo in the absence of graphene afforded low
FEs-CO (20%-25%) and almost negligible currents as the Co sites were not redox active in
the bulk structure and did not serve as the efficient electrochemical sites to catalyze CO
2
RR
(Figure S11). The FEs-CO of pristine SNG, NG and OG were all much lower than the ones
collected in the presence of NapCo and their CO
2
RR currents were trivial (Figure S12,
RR on SNG and NG. This product was
not observed after NapCo was immobilized onto graphene. These results supported that the
Co sites on graphene were the main active sites responsible for CO RR and the contribution
S13). Additionally, formate was detected for CO
2
2
from graphene was insignificant. The TOFs of Co sites, as an indication of specific activity,
were thus calculated according to the following equation (1),
TOF = (Jco  A) / (2  F  m)
(1)
where Jco is the CO
2
RR current density, A is the geometrical surface area of the electrode,
F is the Faraday constant, and m is the amount of Co sites on the doped graphene. The m
was determined by ICP (Table S2). Assuming that all the Co sites on the electrodes were
involved in CO RR, NapCo@SNG afforded higher TOFs than the ones achieved on
2
-1
-1
NapCo@OG (e.g. at -0.735 V, 0.45 s vs 0.16 s , Figure 4d). The TOFs of NapCo@NG
were also relatively low, in agreement with the DFT results that N dopants had the limited
ability to tune the binding configurations of Co sites on the doped graphene (Figure S7).
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Figure 4. (a) CVs of NapCo@SNG in Ar- and CO
2
-saturated 0.1 M KHCO
3
. (b) FEs-CO of
CO
2
RR on NapCo@SNG/NG/OG, CO
2
-saturated 0.1 M KHCO . (c) The corresponding
3
chronoamperometric curves of NapCo@SNG, NapCo@NG and NapCo@OG at -0.8 V. (d)
TOFs of Co sites@SNG/NG/OG for CO production.
Insights into electrocatalysis of Co sites on S-doped graphene. To verify that the S
dopants are indeed the key to promote the specific activity of the immobilized Co sites, we
further synthesized two S/O-doped graphenes by modifying GO with NaSH or CS
denoted as SG-1 and SG-2). SG-1 and SG-2 have an S content of ~1.7% and ~5.7%,
respectively (~9.3% for SNG, Figure S14). NapCo@SG-1/-2 was then prepared and tested
as CO RR electrocatalysts (details were provided in the experiments of SI). For
NapCo@SG-1/-2, UV-vis spectra showed the characteristic d-d transition of NapCo at 372-
74 nm (Figure S15). XPS surveys presented the clear Co and N signals from NapCo
Figure S16). The CO RR measurements showed that NapCo@SG-1/-2 were selective and
durable for CO production (Figure S17, S18). The activities of pristine SG-1/-2 was
insignificant compared to the Co sites-enabled CO RR (Figure S19). By normalizing the
CO RR currents with the Co amounts, NapCo@SG-1/-2 afforded similar TOFs as
2
(
2
3
(
2
2
2
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10.1002/anie.201906475
-1
-1
NapCo@SNG (0.5 s /0.4 s at -0.735 V, Figure 5a).
XPS surveys showed that the S dopants of SG-1/-2 both contained the oxidized sulfur
structures (Figure 5b). The normalized XANES data of NapCo@SG-1/-2 (Figure S20)
were very close to those of NapCo@SNG. In the EXAFS analysis (Figure 5c), the first
shells of Co sites in NapCo@SG-1/-2 also exhibited the shoulder signals for R value near
1
2
.7 Å as observed in the EXAFS data of NapCo@SNG to derive a fitted Co-O distance of
.06±0.02 Å and 2.08±0.02 Å, respectively. These results suggested that the binding
configurations of NapCo on SG-1/-2 were very close to that on SNG (Figure 3d), and thus
the SO dopants of graphene should serve as the main binding sites to NapCo. We found
that the axial coordination of SO dopants tuned the intrinsic CO
The DFT calculation confirmed that the Co sites were the active sites of CO
protonation of CO to *COOH determines the overpotentials (Figure 5d). For NapCo on a
pyridinic SO dopant, the Co site afforded a relatively large theoretical CO RR
overpotential of 0.42 V. In contrast, NapCo on graphitic SO and COO dopants of graphene
are more active. The theoretical CO RR overpotentials decreased to 0.27 V and 0.29 V,
respectively, which are not very far from the experimentally observed value in the CO RR
2
RR activity of NapCo.
2
RR. The
2
2
2
2
measurements of NapCo@SNG and NapCo@OG (their onset potentials were ~-0.4 V,
Figure S13). Indeed, calculation shows that these two Co sites were relatively more active
than the Co sites on the other possibly involved dopants of graphene (Figure 5e). Therefore,
we considered that the Co sites with axial coordination of the graphitic SO and COO
dopants of graphene were the main configurations in NapCo@SNG and NapCo@OG,
respectively, to dominate the electrocatalysis of CO
2
RR. Our DFT calculation combining
with electrochemical analyses are also consistent with the previous results that the axial
coordination can be prominent to determine the CO
in the heterogeneous catalysis.[17]
2
RR activity of the molecular Co sites
Although DFT calculation shows that the NapCo on graphitic SO and COO dopants
afforded similar intrinsic activity, quite different TOFs were observed experimentally on
NapCo@SNG and NapCo@OG. We therefore investigated the redox responses of the
immobilized Co sites by DPVs to verify if the electron communication between NapCo
and the different doped graphenes was efficient. The redox potentials of Co sites on the
different doped graphenes all occurred near 0 V. While in the presence of S dopants, the
DPV currents of the immobilized Co sites on graphene were clearly improved (Figure 5f,
S21). It was suspected that whether such different redox responses were simply determined
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by the conductivity of graphene tuned by the dopants. We used ferrocyanide/ferricyanide
couple (Fe(CN)
4-/3-) to probe the electron transport ability of the doped graphene.
6
Although this probe at certain levels might be sensitive to the surface compositions of
carbon electrodes, it is generally accepted that the coordination shell of Fe(CN) 4
-/3-
6
remained intact during the redox process to have very weak interaction with the different
dopants in graphene. The electron transfer proceeds through the mechanism of outer sphere
electron transfer to reflect the bulk conductivity of graphene.[23] The CVs showed that the
4
-/3-
redox currents of Fe(CN)
6
on the different doped graphenes were close (Figure S22a).
4
-/3-
were analyzed by the Laviron’s method.[22] It showed
The redox kinetics of Fe(CN)
6
-1
that the k
s
were all near 1 s (Figure S22b, c). Therefore, in our doping levels, the different
dopants have very weak impacts on the electron transport ability of graphene skeletons.
For the immobilized NapCo, the redox of Co sites could be closely related to their binding
configurations with the dopants of graphene. Therefore, the different doped graphenes were
electrochemically probed by CoSO
strongly-adsorbed ligands. They tend to interact more easily with the dopants of graphene
comparing to Fe(CN) /KCl probe the electron
4-/3-, such that the redox currents of CoSO
4
/KCl solutions. Under this condition, Co ions lack
6
4
transfer ability of those different dopants of graphene. It showed that the redox responses
of Co ions on graphene with the S dopants (SG-1/-2/SNG) were all higher than those on
OG (Figure S23). These results well supported that through the SO dopants as the efficient
binding sites of Co sites, it resulted in the improved electron communication between
NapCo and graphene, making the immobilized Co sites electro active as the reacting sites
to enable the electron transfer in CO RR.
2
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Figure 5. (a) TOFs of Co sites for CO production at SNG, SG-1 and SG-2. (b) Comparison
of XPS S 2p core levels of NapCo@SNG, NapCo@SG-1 and NapCo@SG-2. (c) The
corresponding FT-EXAFS data at Co-K edge without phase correction. (d) Free energy
diagrams of CO
and graphitic SO doped graphene. Inset shows the formation of *COOH on Co sites as the
rate-limiting step of CO RR. (e) Calculated overpotentials of CO RR on NapCo bonded
with various dopants of graphene as a function of *COOH. (f) DPVs of NapCo@SG-1/SG-
/SNG/NG/OG collected in Ar saturated 0.1 M KHCO . Amplitude: 50 mV; Pulse width:
.05 s; Sampling width: 0.0167 s; Pulse period: 0.5 s.
2
RR on free-standing NapCo and NapCo bonded with COO, pyridinic SO
2
2
Δ
G
2
0
3
Conclusion
In summary, we investigated the immobilization of planar complexes, NapCo, onto
doped graphene matrices at a mild condition toward the heterogeneous electrocatalysis of
CO
potentials that was not electrochemically active to catalyze CO
was enabled with immobilizing NapCo onto the doped graphene, and the k
2
RR. Pristine NapCo on the electrode did not exhibit redox responses with applying
RR. The redox of Co sites
of redox was
2
s
-1
up to 27.8 s to indicate the rapid electron communication between the Co sites and
graphene. By both experimental and theoretical investigations, we suggested that graphitic
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Angewandte Chemie International Edition
10.1002/anie.201906475
SO and COO dopants of graphene were the efficient binding sites to NapCo through the
axial coordination with Co sites. Such heterogenized Co configurations served as the main
active sites to selective reduction of CO
SO dopants could further facilitate the electron communication between NapCo and
graphene resulted in an enhancement of CO RR activity. The TOF of Co sites were
2
into CO with Faradic efficiency up to 97%. The
2
increased by ~3 times on S-doped graphene comparing to the one on O-doped graphene.
Our results demonstrated a novel and easy strategy of immobilizing molecular active sites
to form heterogeneous catalyst for CO RR.
2
Acknowledgements
This project was funded by the National Research Foundation (NRF), Prime Minister’s
Office, Singapore, under its Campus for Research Excellence and Technological Enterprise
(CREATE) program. We acknowledge financial support from the academic research fund
AcRF tier 1 (M4011784 RG6/17 and M4012076 RG118/18), Ministry of Education,
Singapore and Startup grant (M4081887), College of Engineering, Nanyang Technological
University. This research used resources of the Tender Energy Absorption Spectroscopy
Beamline of the National Synchrotron Light Source II, a U.S. Department of Energy (DOE)
Office of Science User Facility operated for the DOE Office of Science by Brookhaven
National Laboratory under Contract No. DE-SC0012704.
Author Contribution
#These authors equally contribute to the work.
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