Photoelectrochemical CO2Reduction by a Molecular Cobalt(II) Catalyst on Planar and Nanostructured Si Surfaces

Article abstract of DOI:10.1002/chem.201603068

In the presence of a molecular Co^IIcatalyst, CO₂reduction occurred at much less negative potentials on Si photoelectrodes than on an Au electrode. The addition of 1 % H₂O significantly improved the performance of the Co^IIcatalyst. Photovoltages of 580 and 320 mV were obtained on Si nanowires and a planar Si photoelectrode, respectively. This difference likely originated from the fact that the multifaceted Si nanowires are better in light harvesting and charge transfer than the planar Si surface.

Full text of DOI:10.1002/chem.201603068

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Accepted Article
Title: Photoelectrochemical CO2 Reduction by a Molecular Cobalt(II) Catalyst on
Planar and Nanostructured Si Surfaces
Authors: Da He; Tong Jin; Wei Li; Sebastian Pantovich; Dunwei Wang; Gonghu Li
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To be cited as: Chem. Eur. J. 10.1002/chem.201603068
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Photoelectrochemical CO2 Reduction by a Molecular Cobalt(II)
Catalyst on Planar and Nanostructured Si Surfaces
Da He,†[a] Tong Jin,†[b] Wei Li,[a] Sebastian Pantovich,[b] Dunwei Wang,*[a] and Gonghu Li*[b]
Abstract: In the presence of a molecular Co(II) catalyst, CO2
reduction occurred at much less negative potentials on Si
photoelectrodes than on an Au electrode. The addition of 1% H2O
significantly improved the performance of the Co(II) catalyst.
Photovoltages of 580 mV and 320 mV were obtained on Si
al.[11] deposited a macrocyclic cobalt catalyst onto different
titanium dioxide surfaces for use in photochemical CO2
reduction. The researchers demonstrated that titanium
dioxide nanoparticles served to absorb UV light while the
surface cobalt catalyst was responsible for reducing CO2 to
CO. Several molecular catalysts have been coupled with
photoelectrodes for solar fuel production via CO2
reduction.[12] This photoelectrochemical approach combines
light-harvesting materials with homogeneous catalysts, and
eliminates the need for amine-based sacrificial electron
donors that are often required in photochemical CO2
reduction.
nanowires and
a planar Si photoelectrode, respectively. This
difference likely originated from the fact that the multi-faceted Si
nanowires are better in light harvesting and charge transfer than the
planar Si surface.
Photochemical reduction is a promising approach towards
sustainable CO2 utilization since this process employs natural
sunlight as a renewable energy input.[1] Both heterogeneous[2]
and homogeneous systems[3] have been extensively
investigated in attempts to achieve efficient CO2-to-fuel
conversion under photochemical conditions. Homogeneous
molecular catalysts could be highly efficient in mediating multi-
electron CO2 reduction. However, the most active homogenous
CO2-reduction catalysts[4] are often based on rare-earth metals,
and great efforts have been made to seek for catalysts with high
activities based on earth-abundant materials.[5]
Herein, we show photoelectrochemical CO2 reduction by a
system based on earth-abundant elements, in which Si
photoelectrodes serve as robust photosensitizers for
[Co(TPA)Cl]Cl. In addition, we investigate the effects of H2O
addition
and
photoelectrode
morphology
on
photoelectrochemical CO2 reduction using the Co(II) catalyst.
The molecular Co(II) complex was synthesized following an
established procedure.[13] Characterization with UV-visible, 1H
NMR, and infrared spectroscopies (Figures S1-S3 in Supporting
Information) and comparison with results in the literature[5a]
Among these homogeneous catalysts are coordination
complexes of cobalt.[5a, 6] For instance, Chan et al.[5a] recently
reported photochemical CO2 reduction by a molecular catalyst,
[Co(TPA)Cl]Cl where TPA is tris(2-pyridylmethyl)amine. An Ir(III)
photosensitizer was employed to harvest light and transfer
electrons to [Co(TPA)Cl]Cl in the presence of triethylamine as a
indicate the synthesized Co(II) complex has
a
trigonal
bipyramidal geometry. Under an atmosphere of Ar, two
irreversible redox waves were observed with reduction peaks at
-1.93 V and -2.15 V (all the potentials mentioned in this report
are versus Fc+/Fc) in the cyclic voltammogram (CV) of
[Co(TPA)Cl]Cl obtained on an Au electrode (Figure 1).
Corresponding oxidation peaks are also clearly observed. These
two waves are assigned to the CoII/CoI and CoI/Co0 redox
couples, respectively.[5a] Significant current enhancement was
observed when the same solution was saturated with CO2,
indicating that the Co(II) complex is a potential catalyst for
electrochemical CO2 reduction. Under CO2, the oxidation peaks
disappeared since the electrochemically generated species,
such as Co(I), were oxidized upon reducing CO2 molecules.
sacrificial electron donor. Yang et al.[7] synthesized
cyclometalated Ir(III) complex as photosensitizer for
a
a
photochemical CO2 reduction. In both studies, high turnover
numbers and reasonable selectivity towards CO formation were
obtained. However, molecular complexes of rare-earth metals
were needed as photosensitizers to harvest light for this and
many other molecular catalysts,[8] which significantly increases
the cost.
Light-absorbing solid-state materials, including inorganic
semiconductors and metal-organic frameworks,[9] have
been utilized as photosensitizers based on abundant
materials for molecular catalysts.[10] For example, Jin et
[a]
[b]
[†]
D. He, W. Li, Prof. D. Wang
Department of Chemistry
Boston College, Chestnut Hill, MA 02467, United States
E-mail: dunwei.wang@bc.edu
T. Jin, S. Pantovich, Prof. G. Li
Department of Chemistry
University of New Hampshire, Durham, NH 03824, United States
E-mail: gonghu.li@unh.edu
Both authors contributed equally to this work
Supporting information for this article is available on the WWW
under http://

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Figure 1. Cyclic voltammograms of [Co(TPA)Cl]Cl under Ar (gray) and CO2
(blue) on an Au electrode. Supporting electrolyte 0.1 M TBAPF6 in acetonitrile,
scan rate 100 mV/s.
various crystal plans might differ in light harvesting and electron-
transfer kinetics. Under an atmosphere of CO2, significant
enhancement in photocurrent is observed on SiNWs in a wide
range of potentials (Figure 3). This enhancement in photocurrent
is likely associated with CO2 reduction by CoI species.
Adding a small amount of H2O to the acetonitrile solution of
[Co(TPA)Cl]Cl altered its redox properties. In our study, proton
reduction by reduced cobalt species was observed in the CVs
collected in the presence of 1% H2O (by volume), although the
reduction potentials did not change significantly (Figure S4). We
note that while CO2 reduction occurs readily at the first reduction
potential of [Co(TPA)Cl]Cl (Figure 1), proton reduction is likely
mediated by Co0 species as indicated by the CV collected under
Ar (Figure S4). This may explain the excellent selectivity towards
CO formation in CO2 reduction by [Co(TPA)Cl]Cl observed in
Chan’s study[5a] and in our work.
Furthermore, the addition of 1% H2O resulted in significantly
greater current enhancement (2.7-fold in Figure S4 vs 2.0-fold in
Figure 1) when the solution of [Co(TPA)Cl]Cl was saturated with
CO2. This comparison suggests that the addition of 1% H2O
promotes CO2 reduction by the Co(II) catalyst. Detailed study of
the reaction mechanisms in the presence of H2O is still under
investigation.
Figure 3. Cyclic voltammograms of [Co(TPA)Cl]Cl in dry acetonitrile on an
SiNWs photoelectrode under Ar (gray) and CO2 (green). Supporting electrolyte
0.1 M TBAPF6, scan rate 100 mV/s.
However, this Co(II) catalyst is only active at highly negative
potentials in electrochemical CO2 reduction (Figure 1). The
potential for two-electron reduction of CO2 to CO is -0.53 V
The advantages of using SiNWs over the Au electrode were
clearly demonstrated in the presence of H2O. In the CV collected
under Ar (Figure S6, trace b), the first reduction corresponding
to the CoII/CoI redox couple occurs at -1.35 V (determined at
maximum photocurrent), representing a photovoltage of 580 mV
as a result of light irradiation compared to that on the Au
electrode. The second reduction occurs around -1.65 V as a
shoulder to a proton reduction peak at -1.94 V. A significant
versus NHE at pH 7 (corresponding to -1.17 V vs Fc+/Fc),[14]
,
suggesting an overpotential of ~800 mV using the Co(II) catalyst.
Photovoltaic effects of some semiconductors have been utilized
to achieve CO2 reduction at modest potentials.[12e-k] For example,
Kumar et al.[12e] investigated CO2 reduction by a molecular Re(I)
catalyst on a p-type Si photoelectrode. Selective reduction of
CO2 to CO was observed at a potential ~600 mV less negative
than that required on a Pt electrode. Recent work by Liu et al.[15]
demonstrated successful applications of Si nanowires (SiNWs)
in high-efficiency solar energy conversion. In this present study,
we couple the Co(II) catalyst with an SiNWs photoelectrode
consisting of vertically aligned arrays of SiNWs (Figure 2) for
use in photoelectrochemical CO2 reduction. In the absence of
the Co(II) catalyst, a much lower photocurrent was observed on
the SiNWs photoelectrode (Figure S5).
photovoltage was also observed using
a
planar Si
photoelectrode which contains the Si(100) crystal plane. On
planar Si, the two reduction waves of [Co(TPA)Cl]Cl overlap
significantly (Figure S6, trace c), but occur in a much narrower
potential range than on SiNWs. This is attributed to the fact that
only the (100) crystal plane is present on the planar Si surface
while the SiNWs are multifaceted. The presence of 1% H2O
resulted in anodic potential shifts in the reduction potentials of
[Co(TPA)Cl]Cl on the planar Si photoelectrode. In addition, the
CoII/CoI reduction wave was observed at around -1.61 V (Figure
S6, trace d) indicating a photovoltage of 320 mV compared to
that on the Au electrode.
In the presence of 1% H2O, CO2 reduction was observed at -
1.57 V on SiNWs (Figure 4c), compared to -1.80 V on planar Si
(Figure 4b, also Figure S7) and -1.96 V on Au (Figure 4a).
Therefore, the use of SiNWs is advantageous since it allows
CO2 reduction at a less negative potential than using the planar
Si photoelectrode and the Au electrode. The comparison shown
in Figure 4 is similar to the previous study of a Ni(II) catalyst on
Si photoelectrodes by Liu et al.[15b] We attribute this effect of
photoelectrode geometry to the unique multifaceted feature and
excellent light harvesting of SiNWs. Other contributing factors
include the greater surface area of SiNWs in comparison with
the planar Si photoelectrode, orthogonal light absorption and
charge transfer directions in SiNWs.
Figure 2. SEM images of an SiNWs photoelectrode: (a) side view and (b) top
view.
A Xe lamp was used to provide simulated solar irradiation for
SiNWs in our photoelectrochemical studies. In dry acetronitrile,
multiple broadened redox peaks are present in the CV collected
on SiNWs under Ar (Figure 3, see also trace a in Figure S6) in
comparison to two distinct redox waves on the Au electrode
(Figure 1). This broadening in the reduction peaks is likely due
to the heterogeneity of the SiNWs, which are multifaceted
nanowires with a variety of crystal planes exposed.[15b] The

Chemistry - A European Journal
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Experimental Section
The UV-visible spectrum of [Co(TPA)Cl]Cl in acetonitrile was
obtained on a Cary 50 Bio spectrophotometer. Infrared spectra were
recorded on a Thermo Nicolet 6700 spectrometer. The morphology of
SiNWs was analyzed with a JOEL JSM 6340F scanning electron
microscope. Cyclic voltammograms were collected on a PAR model
VersaSTAT 4 potentiostat using a single compartment cell. Conditions for
electrochemical
studies
are:
supporting
electrolyte
0.1
M
tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile, scan
rate 100 mV/s, an Au working electrode, a Pt counter electrode, and a
non-aqueous Ag/AgNO3 reference electrode. The electrochemical cell
was purged with Ar or CO2 (99.999%, Airgas) for 20 min before every
scan. In controlled potential experiments, cyclic voltammograms were
collected prior to bulk electrolysis studies using 10 mL of 1 mM
[Co(TPA)Cl]Cl, 0.1 M tetrabutylammonium hexafluorophosphate, and 1%
H2O (by volume) in acetronitrile. Bulk electrolysis was carried out in an H-
shape electrochemical cell, in which the working electrode was separated
from the counter electrode by a sintered glass frit. Prior to bulk
electrolysis using the SiNWs and planar Si photoelectrodes, the solution
was bubbled with CO2 for 20 min. The solution was then irradiated with a
300 W xenon lamp equipped with a water filter. Light intensity was kept
at 113 mW/cm2. The headspace above the reaction solution was
sampled with a gas-tight syringe at different time intervals for product
analysis using an Agilent 7820 GC equipped with a TCD detector and a
60/80 Carboxen-1000 packed column (Supelco).
Figure 4. Cyclic voltammograms of [Co(TPA)Cl]Cl in an acetonitrile solution
containing 1% H2O on (a) Au, (b) planar Si, and (c) SiNWs under CO2.
Supporting electrolyte 0.1 M TBAPF6, scan rate 100 mV/s.
We estimate an optimal H2O content to be ~1-2% for CO2
reduction by [Co(TPA)Cl]Cl on SiNWs. At higher H2O contents
(>5% by volume), the peak associated with CO2-reduction
catalysis became less distinct as proton reduction starts to
dominate (Figure S8). Bulk electrolysis was carried out in an
acetonitrile solution containing 1% H2O and 1 mM [Co(TPA)Cl]Cl
on the photoelectrodes under an atmosphere of CO2. Faradaic
efficiencies for CO formation were measured to be 69% and
57% on SiNWs and planar Si, respectively. Formation of H2
(Faradaic efficiencies 26% and 22%, respectively) was also
observed. In our study, the production of CO from CO2 using
[Co(TPA)Cl]Cl on SiNWs was confirmed by infrared
spectroscopy combined with isotope labeling (Figure S9). On the
SiNW photoelectrode, a turnover number of ~4.2 was obtained
for CO production after bulk electrolysis of a solution containing
10 µmol [Co(TPA)Cl]Cl on SiNWs for 4.6 hours (Figure S10).
During bulk electrolysis in the presence of 1% H2O, the
photocurrent generated on SiNWs showed negligible decay over
a period of 5 h (Figure S11). In addition, heterogeneous
deposition of the Co(II) catalyst was not observed under the
experimental conditions, as shown by microscopic images of
SiNWs collected before and after bulk electrolysis for 3 h (Figure
S12). The stability of [Co(TPA)Cl]Cl was further confirmed by
surface analysis of SiNWs with X-ray photoelectron
spectroscopy (XPS). In particular, a fresh SiNW photoelectrode
was soaked in the electrolysis solution in the dark. The XPS
spectrum of this soaked photoelectrode was then collected and
compared with that of a photoelectrode used in bulk electrolysis.
Spectra of these two photoelectrodes were found to be almost
identical (Figure S13), indicating that the Co(II) catalyst
remained intact during bulk electrolysis. Our results suggest that
a robust system based on abundant materials for CO2-to-fuel
conversion could be developed using the combination of the
Co(II) catalyst and the SiNWs photoelectrode.
Acknowledgements
This material is based upon work supported by the U.S. National
Science Foundation under grants DMR-1055762 (D.W.) and
CHE-1352437 (G.L.).
Keywords: Solar fuels, photoelectrochemistry, molecular
catalysts, silicon nanowires.
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In summary, we have demonstrated that Si photoelectrodes
are viable solid-state photosensitizers for a molecular Co(II)
catalyst in CO2 reduction. The use of SiNWs is advantageous
since it allows CO2 reduction at less negative potentials than
using planar Si due to the unique multifaceted feature of SiNWs.
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reduction catalysis by [Co(TPA)Cl]Cl on SiNWs.
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Chemistry - A European Journal
10.1002/chem.201603068
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Da He, Tong Jin, Wei Li, Sebastian
Pantovich, Dunwei Wang,* and Gonghu
Li *
Page No. – Page No.
Photoelectrochemical CO2 Reduction
by a Molecular Cobalt(II) Catalyst on
Planar and Nanostructured Si
Surfaces
Text for Table of Content Text for Table of Content Text for Table of Content Text
Photoelectrochemical CO2 reduction was achieved by coupling a molecular Co(II)
catalyst with Si photoelectrodes. The presence of a small amount of water
significantly improved the performance of the Co(II) catalyst. A much greater
photovoltage was obtained on Si nanowires than on a planar Si photoelectrode due
to the multi-faceted feature and better light harvesting of Si nanowires.