Stable Aqueous Photoelectrochemical CO2Reduction by a Cu2O Dark Cathode with Improved Selectivity for Carbonaceous Products

Article abstract of DOI:10.1002/anie.201602973

Photocatalytic reduction of CO₂to produce fuels is a promising way to reduce CO₂emission and address the energy crisis. However, the H₂evolution reaction competes with CO₂photoreduction, which would lower the ov

Full text of DOI:10.1002/anie.201602973

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International Edition: DOI: 10.1002/anie.201602973
German Edition: DOI: 10.1002/ange.201602973
Photocatalysis Very Important Paper
Stable Aqueous Photoelectrochemical CO2 Reduction by a Cu2O Dark
Cathode with Improved Selectivity for Carbonaceous Products
Xiaoxia Chang+, Tuo Wang+, Peng Zhang, Yijia Wei, Jiubing Zhao, and Jinlong Gong*
Abstract: Photocatalytic reduction of CO2 to produce fuels is
a promising way to reduce CO2 emission and address the
energy crisis. However, the H2 evolution reaction competes
with CO2 photoreduction, which would lower the overall
selectivity for carbonaceous products. Cu2O has emerged as
a promising material for suppressing the H2 evolution. How-
ever, it suffers from poor stability, which is commonly regarded
as the result of the electron-induced reduction of Cu2O. This
paper describes a simple strategy using Cu2O as a dark cathode
and TiO2 as a photoanode to achieve stable aqueous CO2
reduction with a high Faradaic efficiency of 87.4% and
a selectivity of 92.6% for carbonaceous products. We have
shown that the photogenerated holes, instead of the electrons,
primarily account for the instability of Cu2O. Therefore, Cu2O
was used as a dark cathode to minimize the adverse effects of
holes, by which an improved stability was achieved compared
to the Cu2O photocathode under illumination. Additionally,
direct exposure of the Cu2O surface to the electrolyte was
identified as a critical factor for the high selectivity for
carbonaceous products.
multiple proton-coupled electron transfers, and the associated
kinetic barriers in each step must be overcome for the forward
reaction.[3] The reduction of H2O is kinetically more favorable
than CO2 reduction, rendering H2 as the major product. Thus,
the design and fabrication of efficient CO2 photoreduction
systems with high FE and SCP remain a major challenge.[5]
Recently, copper-based materials (metallic Cu, CuO, and
Cu2O) have gained much attention for photocatalysis owing
to their low toxicity and high abundance.[6] Lee and co-
workers constructed a photoanode-driven PEC system con-
sisting of a WO3 photoanode and Cu cathode for CO2
photoreduction. The FE of 71.6% for all of the carbonic
products was achieved.[7] In particular, among the copper-
based materials, cuprous oxide (Cu2O) with a direct band gap
of ꢀ 2.0 eV has emerged as a promising material for promot-
ing CO2 conversion and suppressing HER in PC and PEC
CO2 reduction.[2b] Rajeshwar and co-workers first reported
the utilization of Cu2O for CO2 photoreduction. They
synthesized hybrid CuO–Cu2O nanorod arrays (NRs) on Cu
substrates and achieved a FE of ꢀ 95% for the photoelec-
trosynthesis of methanol from CO2 in aqueous solutions.[8]
T
he excessive utilization of fossil fuels accompanied with
Wang and co-workers prepared a core–shell-structured
large amounts of CO2 emissions has led to the global energy
and environmental crisis.[1] Photocatalytic (PC) and photo-
electrochemical (PEC) reduction of CO2 into fuels utilizing
solar energy could address both problems, and have received
increasing attention since the seminal investigation in 1978.[2]
However, the photoreduction of CO2 with H2O suffers from
low Faradaic efficiency (FE) and selectivity for carbonaceous
products (denoted as SCP, defined in the Supporting Infor-
mation) owing to many limiting factors. Thermodynamically,
CO2 is an extremely stable molecule and its activation
requires a large energy input.[3] In addition, the thermody-
namic equilibrium potentials for CO2 reduction are very close
to that of H2 evolution reaction (HER), leading to a strong
competition between the two reactions (Supporting Informa-
tion, Table S1).[4] Kinetically, the reduction of CO2 involves
Pt@Cu2O cocatalyst on TiO2 nanoparticles for PC CO2
reduction in aqueous solutions, and achieved a significant
promotion of CH4 and CO generation. The highest SCP
reached up to 85%. They proposed that the Cu2O shell
provided active sites for the preferential conversion of CO2
molecules and suppressed HER.[9] However, one of the major
drawbacks of Cu2O is its poor stability, which potentially
limits its large-scale utilization in photocatalytic reactions.
Because the redox potentials for the reduction and oxidation
of Cu2O lie within its band gap, it can be easily corroded in
aqueous solutions under illumination, leading to the loss of
preferential CO2 reduction capability and a low SCP.[10] Being
commonly used as a cathode, the deactivation of Cu2O is
generally considered to be due to its reduction to Cu caused
by electrons.[11] To address this issue, many studies have
focused on the coating of suitable protective layers with
favorable energy band positions, such as TiO2, on bare Cu2O
for stable and efficient photoreduction of H2O or CO2,
instead of the self-reduction of Cu2O.[10,12] However, the
coating of protective layers would cover the active sites for
the preferential reduction of CO2 on the surface of Cu2O. As
a result, a large amount of H2 will be generated and the SCP
will decrease sharply. Grꢀtzel and colleagues also noticed this
phenomenon and coupled the TiO2-protected Cu2O photo-
electrode with a Re(tBu-bipy)(CO)3Cl molecular catalyst to
selectively reduce CO2 molecules instead of H2O, which
improved the stability of Cu2O and avoided the extensive
generation of H2.[12b,13]
[*] X. Chang,[+] Dr. T. Wang,[+] Dr. P. Zhang, Y. Wei, J. Zhao,
Prof. Dr. J. Gong
Key Laboratory for Green Chemical Technology of
Ministry of Education
School of Chemical Engineering and Technology
Tianjin University; Collaborative Innovation Center of
Chemical Science and Engineering
Tianjin 300072 (China)
E-mail: jlgong@tju.edu.cn
[+] These authors contributed equally to this work.
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under
http://dx.doi.org/10.1002/anie.201602973.
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This paper describes a simple strategy using Cu2O as
a dark cathode and TiO2 as a model photoanode to devise
a stable system for PEC CO2 reduction with high FE and SCP.
First, in order to utilize the active reaction sites on Cu2O for
the preferential conversion of CO2 molecules and the
suppression of HER, the surface of Cu2O was directly
exposed into the electrolytes and resulted in a higher SCP
than typical Pt/Cu metal cathodes and TiO2-protected Cu2O
cathodes. Second, back-side illumination was utilized to
investigate the deactivation mechanism of Cu2O, and it was
found that the photogenerated holes, instead of electrons, are
the major reason for the corrosion of Cu2O. Therefore, the
Cu2O film was used as a dark cathode to minimize the adverse
effects of holes, exhibiting an unexpected and much better
stability during the 3 h reaction than that directly used as
photocathode under illumination. As a result, a high FE of
87.45% and SCP of 92.65% for all of the carbonaceous
products were achieved, which is among the highest SCPs
ever reported for CO2 photoreduction in aqueous solutions
without hole scavengers.[7,9,14]
The Cu2O electrode was synthesized by an electrodepo-
sition method on Cu substrate and TiO2 nanorod (NR)
photoanode was obtained by a facile hydrothermal method
(see the Supporting Information for experimental details).[15]
The Cu2O grains of the film show a cubic morphology and
exhibit a predominant (111) orientation, as evidenced by the
transmission electron microscopy (TEM), high-resolution
TEM (HRTEM), and field-emission scanning electron mi-
croscopy (FESEM) images (Figure 1). The high photoactivity
Figure 2. a) J–V curves using different cathodes measured in N2-
saturated 0.1m KHCO3 (pH 9.3) under dark conditions by two-elec-
trode configuration with TiO2 NRs as WE and different cathodes as
CE. Amounts of b) H2, c) CO, and CH4 as a function of reaction time
using TiO2 NRs as WE, different cathodes as CE and saturated Ag/
AgCl as the RE under 100 mWcmÀ2 AM 1.5G and 0.75 V vs. RHE.
d) The calculated SCP by different cathodes as a function of reaction
time. Experimental details and calculated methods can be found in the
Supporting Information.
Pt, Cu, and Cu2O cathodes as counter electrodes (CE),
respectively (Figure 2a). The highest onset potential of dark
J–V curves by the Cu2O cathode indicated the highest
reaction overpotential for HER on this surface. The PEC
CO2 reduction reactions were conducted in CO2-saturated
0.1m KHCO3 (pH 6.9) using a typical three-electrode config-
uration with TiO2 NRs as WE, saturated Ag/AgCl as
reference electrode (RE), and Pt, Cu, and Cu2O cathodes as
CE, respectively (Figure S2). To compare with previous
works, the PEC CO2 reduction reactions were all conducted
under 100 mWcmÀ2 AM 1.5G and a constant external bias of
0.75 V versus the reversible hydrogen electrode (RHE).[7] The
reduction of CO2 occurred on the surface of different
cathodes and the main products were CO, CH4, and H2,
with trace amounts of CH3OH (Figure 2b and c), as
summarized in Table 1. For the Cu2O cathode, the amount
of H2 was the lowest, whereas the amounts of CO and CH4
were the highest compared with Pt and Cu cathodes, which
resulted in a high SCP of over 90% during the reaction
(Figure 2d). The current density–time (J–t) curves for the
Cu2O dark cathode during the reaction are shown in Fig-
ure 3a. In addition, the J–V curves under AM 1.5G in 0.1m
KHCO3-saturated by different kinds of gases could provide
more insights into the differences between Pt, Cu, and Cu2O
cathodes in PEC CO2 reduction (Figure S3). It has been
widely accepted that CO is the key intermediate during CO2
reduction and can be further reduced to hydrocarbons and
alcohols.[17] Therefore, the adsorption strength of CO on the
electrode is critical to the final products. Ideally, the
intermediates, such as CO, should be bound strongly enough
to facilitate CO2 reduction, but not so strongly that the
intermediates are unreactive.[18] A comparison study between
the Pt, Cu, and Cu2O electrodes revealed that the CO
Figure 1. a) TEM, b) HRTEM, and c,d) FESEM images of Cu2O films.
A lattice spacing of 0.246 nm corresponds to the (111) plane of Cu2O
in (b).
and stability of TiO2 NRs under air mass 1.5 global (AM
1.5G) make it a suitable photoanode to investigate the
properties of different cathodes (Figure S1).[15b,16] The dark
current–potential (J–V) curves for water splitting were
measured in N2-saturated 0.1m KHCO3 using a two-electrode
configuration with TiO2 NRs as working electrode (WE) and
2
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Table 1: PEC CO2 reduction products after the reaction for 3 hours with different electrodes under 100 mWcmÀ2 AM 1.5G.
Electrodes[a]
(Working/Counter)
E[b] [VRHE
]
Average
Faradaic Efficiency[d] [%]
Sel. [%][e]
Current Density[c] [mAcmÀ2
]
CH4
CO
CH3OH
H2
Total
TiO2 NRs/ Pt
0.75
0.75
0.75
À0.1
0.75
1.37
1.36
1.34
0.71
1.24
15.86
27.36
54.63
35.62
22.72
17.05
23.01
30.03
19.01
30.89
0.11
1.11
2.79
0.34
1.44
65.07
46.82
6.94
31.43
41.61
98.09
98.30
94.39
86.40
96.66
33.66
52.37
92.65
63.62
56.95
TiO2 NRs/ Cu
TiO2 NRs/ Cu2O
Cu2O/ Pt
TiO2 NRs/ Cu2O@TiO2
[a] The working electrodes were under illumination and counter electrodes were shadowed during the reaction. [b] E: the external bias vs. RHE. [c] The
average current densities were achieved by the integration over 3 h reaction. [d] The calculated methods for Faradaic efficiencies can be found in the
Supporting Information. [e] Sel.: the selectivity towards carbonaceous products on an electron basis.
situations indicated a similar amount of charges ( ꢀ 2.7 CcÀ2
in the first 0.5 h) passed through the two electrodes. During
the 3 h reaction, the current densities of Cu2O photocathode
remained stable for less than 0.5 h and decreased sharply. In
contrast, only a slight reduction in the current densities was
observed for the Cu2O dark cathode (Figure 3a). X-ray
diffraction (XRD) patterns of the Cu2O films before and after
the reactions showed that the characteristic peaks of Cu2O
completely disappeared after the 3 h reaction when it was
directly used as photocathode, in notable contrast to the Cu2O
dark cathode (Figure 3b). The corrosion of the Cu2O photo-
cathode led to a higher H2 evolution amount and lower SCP
(Figure 3c and d, dash lines; the carbonaceous products over
the Cu2O photocathode are shown in Figure S5). However,
the SCP remained above 90% during the 3 h reaction over
the Cu2O dark cathode (Figure 3d, solid line). In addition,
there were many surface cracks on the Cu2O photocathode
Figure 3. a) J–t curves during the reactions. b) XRD patterns of Cu2O
films, and the surface profile was almost broken after 3 h
electrodes before and after the 3 h reactions. c) Amounts of H2 and
illumination with external bias (Figure 1c,d and Fig-
d) SCP as a function of the reaction time using different electrodes by
ure S6a,b). In contrast, the surface morphological profile of
Cu2O dark cathode films remained unchanged, with no
surface cracks after 3 h reaction (Figure 1c,d and Fig-
ure S6c,d). The excellent stability of the Cu2O dark cathode
in PEC CO2 reduction was further demonstrated by a longer
reaction time of 6 h (Figure S7). Therefore, using Cu2O as
a dark cathode in our system was an efficient way to maintain
its stability and improve the SCP for PEC CO2 reduction.
These results are somewhat unexpected and deserve
further investigations because only Cu2O with an excellent
stability can result in a high SCP for CO2 photoreduction.
Considering the differences between these two situations,
three factors that may lead to different Cu2O stabilities are
proposed and examined: the electrons ready for surface
reactions, the photogenerated transient highly energetic
electrons, and the photogenerated holes. First, electrons
with different electric potentials will exhibit different
powers to drive the reduction reaction and result in different
Cu2O corrosion rates. The average potential of electrons
photogenerated by TiO2 is lower than those by the Cu2O
photocathode owing to the lower conduction band edge of
TiO2. However, when electrons generated by TiO2 were
transferred to the Cu2O dark cathode through an external
circuit, their potential would be elevated by the higher
external bias of 0.75 V versus RHE. Nevertheless, a lower
external bias of À0.1 V versus RHE was applied on the Cu2O
three-electrode configuration in PEC CO2 reduction.
adsorption on Pt surface is so strong that CO will occupy the
active sites for CO2 reduction (Figure S3). Thus, the majority
of electrons will take part in HER and lead to a low yield of
carbonaceous products on Pt.[17,19] The CO adsorption
strength on Cu and Cu2O is moderate and can be removed
under N2 atmosphere so that it can be further reduced into
hydrocarbons. Therefore, the utilization of Cu2O in PEC CO2
reduction is of great importance for the efficient conversion of
CO2.
However, Cu2O severely suffers from poor stability owing
to corrosion. To investigate the performance of the Cu2O
photocathode in PEC CO2 reduction, Cu2O films were
directly used as WE under illumination, with saturated Ag/
AgCl as RE and a Pt foil as CE. Before the reaction, typical J–
V curves of Cu2O photocathode were acquired under AM
1.5G in CO2-saturated 0.1m KHCO3 (Figure S4). To obtain
a similar initial current value ( ꢀ 1.5 mAcmÀ2) for the Cu2O
dark cathode and photocathode, the photo-assisted reaction
was conducted under a potential of À0.1 V versus RHE. The
reduction of CO2 occurred on the surface of Cu2O, and the
comparison results are shown in Figure 3. The deactivation of
the Cu2O photocathode could be clearly observed during the
reaction. The similar initial current values between the two
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photocathode, which could lead to the similar average
potential of electrons between the Cu2O photocathode and
dark cathode. Meanwhile, the same kinds of products were
generated on both Cu2O cathodes, indicating that the
electrons on both surfaces had similar potentials and
enough driving force to reduce CO2 into CO, CH4, and
CH3OH. Additionally, the similar initial current densities
over the Cu2O photocathode and dark cathode indicated
a similar amount of charges passed through the electrodes.
Therefore, one may expect that the Cu2O photocathode and
dark cathode would exhibit similar stability under the
experimental conditions. However, these two electrodes
showed different stabilities, indicating that the reduction of
Cu2O by electrons is unlikely the main reason for its
corrosion.
In contrast to the electrons ready for surface reactions
mentioned above, the photogenerated transient electrons are
highly energetic and hold a much higher reduction capability.
There is a large number of photons with energy much higher
than the band gap energy of Cu2O in the AM 1.5G spectrum.
The transient electrons excited by these photons possess
much higher potential levels than the conduction band edge
of Cu2O and a large driving force for reduction reactions.
However, these photogenerated transient highly energetic
electrons did not exist on the Cu2O dark cathode, whose
electrons came from the external circuit. Upon generation,
the transient electrons will relax into lower states in the
conduction band (usually the bottom) or interband trap states
on a femtosecond time scale before they take part in redox
reactions, which usually occur on a longer time scale.[17b,20]
This can explain why these transient highly energetic
electrons barely provide a higher reduction potential. To
investigate the effects of highly energetic electrons, a Cu2O
film was electrodeposited on a FTO substrate to perform the
stability tests. The J–t curves were measured at 0 V versus
RHE in N2-saturated 0.5m Na2SO4 using the typical three-
electrode configuration. The light source was AM 1.5G with/
without cut-off filters (all the light intensity was adjusted to
100 mWcmÀ2). With the cut-off filters, a large portion of
highly energetic photons could be filtered out from AM 1.5G,
however, the Cu2O films did not exhibit better stability, as
evidenced by the J–t curves (Figure 4a). Therefore, the
photogenerated highly energetic electron is excluded as the
main reason for Cu2O corrosion.
Figure 4. J–t curves of Cu2O photocathode under a) different light
sources and b) front and back illumination. The measurements were
conducted at 0 V vs. RHE in N2-saturated 0.5m Na2SO4 using Cu2O
film as WE, saturated Ag/AgCl as RE and a Pt foil as CE. All of the
light intensity was adjusted to 100 mWcmÀ2. Under back illumination,
the light intensity was measured behind a piece of bare FTO substrate.
c) Diagram of the front and back illumination showing the different
distances that electrons and holes have to travel.
Cu2O stability. For a photocathode, electrons are consumed at
the photocathode/electrolyte interface, whereas the holes
need to travel to the back contact and the counter electrode.
Therefore, the travel distances of electrons and holes are
much different between front and back illumination (Fig-
ure 4c).[22] Back illumination, in which the travel distance of
electrons is much longer than that of holes, exhibited better
Cu2O stability than front illumination, as evidenced by J–
t curves (Figure 4b), indicating that the holes may have more
adverse effects on the stability of Cu2O than electrons.
Therefore, the shadowing of Cu2O in this work to minimize
the adverse effects of holes is a simple but efficient strategy to
improve its stability and the SCP during the reaction.
It has been reported that the coating of protective layers
(for example, TiO2) on the surface of unstable electrodes can
efficiently improve their stability.[23] In this work, TiO2 was
deposited on the Cu2O electrode by atomic layer deposition
(ALD) to suppress its corrosion. The as-prepared
Cu2O@TiO2 dark cathode indeed exhibited better stability,
but lower SCP than bare Cu2O owing to the cover of surface
active sites (Figure S8). Therefore, the use of Cu2O as a dark
cathode in this work could improve its stability and promote
the exposure of active sites simultaneously for high SCP of
CO2 reduction. The calculated FEs and SCPs using different
electrodes are shown in Table 1.
Additionally, the most obvious differences between the
Cu2O photocathode and dark cathode in Figure 3 are the
photogenerated holes. The Cu2O photocathode harvested
photons to generate both electrons and holes. However, the
oxidative holes were hardly present in the Cu2O dark cathode.
It is known that the redox potentials for Cu2O reduction (into
Cu) and oxidation (into CuO) lie within its band gap,[10] which
means that the oxidation of Cu2O into CuO by holes could
also lead to its deactivation. Wu et al. have found that the
morphology changes from a dense structure to a network of
leaf-like crystals with the transformation from Cu2O to CuO
play a critical role in the degradation of its photocurrent.[21] In
this work, the back-side illumination, which generated
electron–hole pairs near the back contact in contrast to
front illumination, was utilized to investigate the difference in
In summary, we have demonstrated a simple approach to
achieve stable PEC CO2 reduction with high FE and SCP by
shadowing the Cu2O electrode from illumination while
exposing it to the electrolytes. Using Cu2O as a dark cathode,
FE of 87.45% and selectivity of 92.65% for all carbonaceous
products were achieved at a low bias of 0.75 V versus RHE.
Meanwhile, it was found that photogenerated holes may have
more adverse effects on the Cu2O stability than electrons,
which led to the unexpectedly large differences of stability
between the Cu2O photocathode and dark cathode. It could
4
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Acknowledgements
We acknowledge the National Science Foundation of China
(U1463205, 21525626, 51302185), Specialized Research Fund
for the Doctoral Program of Higher Education
(20120032110024, 20130032120018), the Scientific Research
Foundation for the Returned Overseas Chinese Scholars
(MoE), and the Program of Introducing Talents of Discipline
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Keywords: carbon dioxide reduction · cuprous oxide ·
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Received: March 25, 2016
Revised: April 27, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Photocatalysis
Copper on the dark side: Cu2O preferen-
tially reduced CO2 to fuels while sup-
pressing hydrogen evolution. It was
found that the photogenerated holes
primarily account for the instability of
Cu2O. The Cu2O was used as a dark
cathode to minimize the adverse effects
of holes, achieving excellent stability and
a selectivity of 92.6% for carbonaceous
products in CO2 photoreduction.
X. Chang, T. Wang, P. Zhang, Y. Wei,
J. Zhao, J. Gong*
&&&& — &&&&
Stable Aqueous Photoelectrochemical
CO2 Reduction by a Cu2O Dark Cathode
with Improved Selectivity for
Carbonaceous Products
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!