Tuning Cu/Cu2O Interfaces for the Reduction of Carbon Dioxide to Methanol in Aqueous Solutions

Article abstract of DOI:10.1002/anie.201805256

Artificial photosynthesis can be used to store solar energy and reduce CO₂ into fuels to potentially alleviate global warming and the energy crisis. Compared to the generation of gaseous products, it remains a great challenge to tune the product distribution of artificial photosynthesis to liquid fuels, such as CH₃OH, which are suitable for storage and transport. Herein, we describe the introduction of metallic Cu nanoparticles (NPs) on Cu₂O films to change the product distribution from gaseous products on bare Cu₂O to predominantly CH₃OH by CO₂ reduction in aqueous solutions. The specifically designed Cu/Cu₂O interfaces balance the binding strengths of H* and CO* intermediates, which play critical roles in CH₃OH production. With a TiO₂ model photoanode to construct a photoelectrochemical cell, a Cu/Cu₂O dark cathode exhibited a Faradaic efficiency of up to 53.6 % for CH₃OH production. This work demonstrates the feasibility and mechanism of interface engineering to enhance the CH₃OH production from CO₂ reduction in aqueous electrolytes.

Full text of DOI:10.1002/anie.201805256

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Accepted Article
Title: Tuning Cu/Cu2O Interfaces for Reduction of Carbon Dioxide to
Methanol in Aqueous Solutions
Authors: Jinlong Gong, Xiaoxia Chang, Tuo Wang, Zhijian Zhao,
Piaoping Yang, Jeffrey Greeley, Rentao Mu, Gong Zhang,
Zhongmiao Gong, Zhibin Luo, Jun Chen, Yi Cui, and Geoffrey
Ozin
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201805256
Angew. Chem. 10.1002/ange.201805256
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10.1002/anie.201805256
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Tuning Cu/Cu2O Interfaces for Reduction of Carbon Dioxide to
Methanol in Aqueous Solutions
Xiaoxia Chang+, Tuo Wang+, Zhi-Jian Zhao, Piaoping Yang, Jeffrey Greeley, Rentao Mu, Gong Zhang,
Zhongmiao Gong, Zhibin Luo, Jun Chen, Yi Cui, Geoffrey A. Ozin, and Jinlong Gong*
Abstract: Artificial photosynthesis can store solar energy and
reduce CO2 into fuels to potentially alleviate global warming and
energy crisis. Compared to gas products, it remains a grand
challenge to tune the product distribution of artificial photosynthesis
to liquid fuels, such as CH3OH, that are suitable for storage and
transport. This paper describes the introduction of metallic Cu
nanoparticles (NPs) on Cu2O films to change the product distribution
from gas products on bare Cu2O to predominant CH3OH via CO2
reduction in aqueous solutions. The specifically designed Cu/Cu2O
interfaces could balance the binding strengths of H* and CO*
intermediates, which plays critical roles in CH3OH production. With
TiO2 model photoanode to construct a photoelectrochemical cell,
Cu/Cu2O dark cathode exhibited a Faradaic efficiency up to 53.6%
for CH3OH production. This work demonstrates the feasibility and
mechanism of interface engineering to enhance the CH3OH
production from CO2 reduction in aqueous electrolytes.
CH3OH production with nearly unity FE via CO2 reduction in
pyridine,[3a] selectively targeting CH3OH at a high yield in
aqueous solutions is still a grand challenge. Rajeshwar and co-
workers have reported CH3OH production at a FE of ~95% over
CuO-Cu2O arrays in aqueous solution, which is speculated from
the current-potential curves.[4] Wei et al. have performed the
CH3OH generation with a FE of 88% over CdSeTe/TiO2
electrode, which however omits the detection of gas products.[5]
Many recent studies on CO2 reduction and related reactions
have primarily focused on designing copper-based materials
owing to their low toxicity, high abundance, unique catalytic
activity and good stability. Systematic investigations have been
performed to reveal the active sites and Cu-oxide interfacial
interaction over Cu-based industrial and electrochemical CO2
hydrogenation catalysts.[6] In our previous work, a CO2 reduction
system containing Cu2O dark cathode was demonstrated to
continuously suppress HER and enhance the generation of
gaseous CO and CH4 products in CO2 reduction for up to 6 h
reaction.[7] However, the generation of CH3OH was limited since
this process requires an appropriate binding strength of CO*
intermediate and sufficient surface adsorbed hydrogen (H*)
rather than protons in the solution for the further reduction and
hydrogenation of CO*.[8] Although Cu(I) species were proposed
to be the active sites for selective CH3OH generation by some
previous studies on Cu2O NPs,[9] there is no catalytic system
developed yet to successfully manipulate the binding strengths
of H* and CO* as the descriptors to optimize CH3OH production
in CO2 reduction. Therefore, Cu2O is taken as a model cathode,
on which the binding strengths of H* and CO* are too weak and
too strong, respectively (see DFT and experimental results
shown below), to optimize these two descriptors for the selective
production of CH3OH. In this study, metallic Cu NPs were
introduced onto Cu2O films to obtain an explicitly defined
structure of Cu/Cu2O interface for enhanced H* binding and
reduced CO* binding, resulting in an increased FE up to 53.6%
for CH3OH generation via aqueous CO2 reduction in conjunction
with a TiO2 photoanode in photoelectrochemical (PEC) cell,
which is among the highest FE ever reported in aqueous
solutions over non-noble catalysts.[9] Through thermodynamic
and kinetic analysis on CH3OH production over Cu/Cu2O films
with different Cu amounts, Cu/Cu2O interface is demonstrated to
be the dominant factor in CH3OH production since the interfacial
sites play critical roles in catalysis.[6c, 10]
Solar power is a clean, renewable and abundant energy
source, which can be utilized to reduce CO2 into fuels and
chemicals.[1] However, the reduction of CO2 with H2O is
confronted with fierce competition from the H2 evolution reaction
(HER), together with a wide product distribution due to many
possible reaction pathways.[2] Liquid products of CO2 reduction,
such as CH3OH, are more suitable for storage and transport as
fuels or chemical feedstocks. The utilization of CH3OH could
become very important in the development of a sustainable
society.[3] Unfortunately, the involved six-proton-coupled six-
electron-transfer processes make it much more difficult to
produce CH3OH than the two-electron-reduced products of
CO/HCOOH. Although Bocarsly and co-workers have achieved
[*]
Dr. X. Chang, Dr. T. Wang, Dr. Z. Zhao, P. Yang, Dr. R. Mu, G.
Zhang, Z. Luo, 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
Prof. Dr. J. Greeley
Davidson School of Chemical Engineering, Purdue University, West
Lafayette, Indiana 47907, United States
Z. Gong, Dr. Y. Cui
Vacuum Interconnected Nanotech Workstation, Suzhou Institute of
Nano-Tech and Nano-Bionics, Chinese Academy of Sciences,
Suzhou 215123, China
Dr. J. Chen
In order to investigate the properties of Cu/Cu2O interface
individually, Cu2O model cathode with a film thickness of ~1 μm
is electrodeposited on Cu substrate and exhibits a smooth
surface with a predominant (111) orientation (Figure S1 and
S2a).[11] X-ray photoelectron spectroscopy (XPS) of Cu2O film
reveals the absence of Cu2+ peaks, indicating that the obtained
Department of Materials Science and Engineering, Stanford
University, Stanford, California 94305, United States
Prof. Dr. G. Ozin
Department of Chemistry, University of Toronto, Toronto, Canada
Dr. X. Chang, and Dr. T. Wang contributed equally to the creation of
this work.
Supporting information for this article is given via a link at the end of
the document.
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Figure 1. (a) Schematic, (b-f) TEM images and (g-k) HRTEM images of Cu NPs/Cu2O film. The particle sizes of Cu NPs were determined via averaging at least
50 particles.
electrodes were not oxidized in air (Figure S2b). Cu NPs were
loaded onto Cu2O film using e-beam evaporation, employing a
quartz crystal microbalance (QCM) to monitor the loading
amount. Five samples denoted as E1-E5 with different Cu
loading amounts were selected to investigate the interfacial
effects. The nucleation of Cu on Cu2O surface followed an island
growth mechanism,[12] resulting in the formation of Cu NPs with
a predominant (111) orientation (Figure 1). The average particle
diameters of Cu are 1.5 ± 0.1, 3.4 ± 0.2, 5.4 ± 0.4, 7.1 ± 0.5 and
8.6 ± 0.5 nm for E1-E5, respectively, as evidenced by
transmission electron microscopy (TEM) and high-resolution
TEM (HRTEM) images in Figure 1 (size distribution in Figure S3).
TiO2 nanorod (NR) model photoanode was obtained by a facile
hydrothermal method to construct a PEC cell with Cu/Cu2O
cathode for CO2 reduction. In addition, bare metallic Cu NPs
were deposited on inert glassy carbon electrode (GCE) with the
same loading amount as E2 for comparison.
which is ascribed to the interface between Cu NPs and Cu2O
film. With increasing Cu NPs loading amount and particle size,
CH3OH evolution shows a volcano-like trend and E2 exhibits the
maximum CH3OH production with a FE of 53.6%, which is
among the highest for CO2 reduction in aqueous solutions
(Figure 2e, Table S1 and S2). It is worthy to note that the size
range of deposited Cu NPs in this work (1.5~8.6 nm) could rule
out the size effect on product selectivity according to previous
studies on CO2 reduction over Cu NPs with different sizes.[14]
The CO2 reduction reactions were conducted in CO2-
saturated 0.1 M KHCO3 (pH 6.9) in a Nafion-separated two-
compartment cell using a typical three-electrode configuration
with TiO2 NRs as the working electrode (WE) under illumination
and external bias, saturated Ag/AgCl as reference electrode
(RE), and Cu NPs, Cu2O film and Cu/Cu2O films as counter
electrodes (CE) in darkness (dark cathodes), respectively
(Figure S4). TiO2 photoanode with a high photovoltage under air
mass 1.5 global (AM 1.5G) plays a critical role in improving the
stability of Cu2O, attributing to the highly energetic electrons and
large potential exerted on cathode for preferential CO2 reduction
as well as the moderate current density that inhibits corrosion,
which could not be achieved through electrocatalysis over Cu2O
(Figure S5 and S6).[7, 13] Thus, TiO2 is a suitable photoanode to
maintain the stability of Cu2O cathodes and to investigate their
surface catalytic properties. To facilitate the comparison with
previous studies, all the CO2 reduction reactions were conducted
by applying 100 mW cm−2 AM 1.5G and a constant external bias
of 0.75 V vs. the reversible hydrogen electrode (RHE) on TiO2
WE. The potential on Cu2O dark cathode and the average
current density in the circuit were measured to be about −0.7 V
vs. RHE and 1.3 mA cm-2, respectively, during the reaction
under these conditions (Figure S6a). In contrast to the Cu/GCE
and Cu2O film dark cathodes with gaseous H2/CO/CH4 as main
products (Figure 2a and b), Cu/Cu2O films exhibit a great
enhancement for CH3OH generation (Figure 2c, d and S7),
Figure 2. (a, b, c and d) The product amounts normalized to the geometric
area of GCE or Cu2O electrodes as a function of reaction time using TiO2 NRs
as WE, saturated Ag/AgCl as RE, bare Cu NPs, Cu2O film, E1 and E2 as CE,
respectively. (e) The dependence of FE on the Cu NP size. (f) The
dependence of CH3OH generation amount on the Cu/Cu2O interfacial length.
To quantify the interface between Cu NPs and Cu2O film, the
Cu NPs were treated as spheres with half inserted into Cu2O
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10.1002/anie.201805256
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surface, which is a reasonable simplification according to TEM
and HRTEM images (Figure 1). Therefore, the Cu NP numbers
and Cu/Cu2O interfacial lengths could be estimated based on
loading mass and particle size (Table S3), which decrease
gradually with increasing Cu particle size (from E1 to E5). With a
minor Cu loading amount on E1, the Cu NPs are featured by
very small sizes with a high density, thus nearly covering all the
Cu2O surface (as estimated in Table S3 and Figure S8), which is
consistent with the growth mechanism of glancing angle
deposition[12] and leads to insufficient exposure of Cu2O and
Cu/Cu2O interface. Thus, CO2 reduction occurred predominantly
on the surface of metallic Cu, resulting in a similar performance
to the Cu/GCE electrode and yielding a large amount of H2. With
increasing Cu loading amount (E2-E5), the adjacent small Cu
NPs would aggregate to form larger ones and the Cu/Cu2O
interface could be exposed.[15] It can be seen that the CH3OH
evolution amounts show an approximate linear relationship with
the lengths of Cu/Cu2O interface (Figure 2f), strongly suggesting
that most CH3OH come from the interfacial sites. In addition, the
TEM and SEM images of E2 after the reaction indicate the good
maintenance of surface structure (Figure S9).
adsorbed H* was formed to reduce Cu+ into Cu0. Compared to
bare Cu2O (Figure 3a), Cu/Cu2O (Figure 3b) exhibits a much
more prominent generation rate of Cu0 peak (918.4 eV)[17] at 1
mbar H2 as the temperature rises from 300 K to 490 K, which is
visualized by the larger slope for the increase of Cu0/Cu+ peak
area ratio (Figure S11a).
In addition, H2 temperature-programmed desorption (TPD)
results also indicate stronger H* binding on Cu/Cu2O (Figure
S10b). Furthermore, with increasing temperature from 313 K to
353 K, the peak of adsorbed CO* on Cu2O (2117.8 cm−1)[18] in
FTIR decreases much more slowly than that on Cu/Cu2O,
indicating the much stronger CO* binding on Cu2O[19] and the
reduced CO* binding strength on Cu/Cu2O (Figure 3c and S11b).
DFT calculations (Figure 3d) provide further evidence of the
extremely weak and strong adsorption of H* and CO*,
respectively, on Cu2O. In contrast, Cu/Cu2O interface exhibits
enhanced H* adsorption and reduced CO* adsorption, both of
which are favorable to CH3OH production. At microscopic level,
a simple qualitative explanation for the stronger H* adsorption
and weaker CO* adsorption is that the Cu/Cu2O interface offers
appropriate adsorption sites. Specifically, H* can not only bind to
Cu2O but also bind to metallic Cu, leading to a more stable
adsorption. On the other hand, CO* binding is much stronger on
Cu2O due to the coordinatively unsaturated Cu atoms. However,
according to the Bader charge analysis (Table S4), the positive
charge of coordinatively unsaturated Cu at the Cu/Cu2O
interface becomes less positive by 0.12 e compared to that on
Cu2O, leading to a weaker CO* adsorption at the interfacial site.
Figure 3. (a, b) The Cu LMM Auger spectra for Cu2O and Cu/Cu2O under
various conditions. (c) FTIR spectra of CO* adsorbed on Cu2O and Cu/Cu2O.
(d) The calculated free energies of H* and CO* after entropy correction on
different sites. Insets are models of Cu2O(111), Cu(111) and the interface.
Furthermore, ambient pressure XPS (APXPS), Fourier
transform infrared spectroscopy (FTIR) in the reflection mode,
and DFT calculations were applied to investigate the H* and CO*
adsorption properties, which play critical roles in CH3OH
production, on Cu, Cu2O and Cu/Cu2O interface (Figure 3). Prior
to the measurements of APXPS, the samples were annealed at
600 K in ultrahigh vacuum (UHV) to remove surface impurities.
With increasing annealing temperature, the peak of hydroxyl,
formed by adsorbing H* on O atoms in Cu2O,[16] at 531.4 eV
decreases gradually. The Cu/Cu2O sample exhibits a much
slower decrease of hydroxyl signal, indicating its stronger H*
binding strength compared to the Cu2O sample (Figure S10a).
After cooling down in UHV, H2 was introduced and surface
Figure 4. (a) The schematic reaction mechanism of CO2* reduction to CH3OH*
on the surface of Cu/Cu2O. (b) Energy profile from DFT calculations for CO2
reduction to CH3OH on different sites at 0 V vs. RHE. (c) Tafel curves of
CH3OH production for samples E2-E5 measured by using them as WE, Pt as
CE and saturated Ag/AgCl as RE. (d) The relationship between θH and
Cu/Cu2O interfacial length. (e) The logarithm of CH3OH partial current as a
function of lnθH.
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The reaction mechanism was investigated through
thermodynamic and kinetic studies based on DFT calculations
and Tafel analysis. The Cu(111) and Cu2O(111) were chosen as
the model surfaces for DFT calculations according to the
measured lattice spacing values in HRTEM images (Figure 1
and S1) and the fact that Cu2O(111) is the most
thermodynamically stable facet of Cu2O with the lowest surface
energy.[20] Furthermore, the lowest free energy pathway for CO2
reduction to CH3OH at 0 V vs. RHE can be concluded based on
our calculated adsorption energies of various intermediates on
the three reaction sites (Figure 4a, b and S12; Table S5 and S6).
The formation of key intermediate CHO* (step 3 in Figure 4a) is
the H* species for CH3OH generation are adsorbed on interfacial
sites. In addition, equation 1 can also be described as:
nFk3 K2 P
EF
RT
CO
lni  2lnH  ln
 ln[H ]
(3)
K1
which indicates the theoretical two-order dependence between
lni and lnθH. On the other hand, the relationship between lni and
lnθH can be established through the experimental results of CO2
reduction in PEC cell (Figure 2 and S7; Table S1) and Tafel
slope measurements (Figure 4c). The linear fitted line obtained
from experimental results shows a slope of 2.2 (Figure 4e),
which is very close to the theoretical value of 2 in equation 3.
Therefore, both the theoretical analysis and experimental results
confirm that the first hydrogenation of CO* is indeed the rate-
limiting step for all the samples and Cu/Cu2O interface plays a
critical role in the binding of H* and the production of CH3OH.
a
potential-limiting step in thermodynamics for CH3OH
generation over all the reaction sites due to its largest uphill
change in free energy (∆G>0).[8d, 21] Cu/Cu2O interface exhibits
the lowest ∆G for step 3, suggesting that the interface is more
beneficial for CHO* formation.
In addition to thermodynamics, kinetic studies were also
conducted based on Tafel analysis (details in Supplemental
Information). The samples E2-E5 possess Tafel slopes of 66, 57,
51 and 48 mV dec−1, respectively, for CH3OH production (Figure
4c), which are very close to 59 mV dec−1 and suggest a
reduction mechanism involving a fast initial single electron
reduction of CO2 and a subsequent slower hydrogenation
process as the rate-determining step.[22] It has been widely
accepted that CO* is the key intermediate in CO2 reduction and
the precursor for hydrocarbon generation on copper based
catalysts, which indicates that the product distribution is
determined by the subsequent hydrogenation process of CO*.
Therefore, in order to conveniently investigate the rate-limiting
step for CH3OH generation, it is proposed that CO* molecules
initially adsorbed on the surface of catalysts, followed by
subsequent reduction steps into CH3OH (marked with red in
Figure 4a). Since the first hydrogenation of CO* (step 3 in Figure
4a) has an uphill ∆G and may be the rate-limiting step (Figure
4b), the CH3OH partial current can be described as follows with
the other reactions in equilibrium.
In summary, a common challenge exists in CO2 reduction in
aqueous solutions, namely the competition between proton
reduction and surface adsorbed H* reduction, where the latter
largely determines the CH3OH generation. Meanwhile, the
binding strength of CO* intermediates needs to be controlled to
an appropriate level to facilitate the subsequent hydrogenation
process. In this work, an electrode with structurally tunable
Cu/Cu2O interface is constructed for the enhanced H* binding
and reduced CO* binding compared to the original Cu2O surface,
which effectively tunes the product distribution and increases the
production of CH3OH to a high FE of 53.6%. This work will
provide new insights into controlling the product distribution of
CO2 reduction and develop a new method for high-efficiency,
low-cost and stable CH3OH generation. If Cu NPs could be
selectively deposited onto the edge sites of Cu2O film, more
reactive Cu/Cu2O interfacial sites might be generated to further
boost the production of CH3OH, which calls for more future
investigations.
nFk3 K2 P H2
Acknowledgements
CO
i  n F k3 COH

(1)
 EF
K1[H ]exp(
)
We acknowledge the National Key R&D Program of China
(2016YFB0600901), the National Natural Science Foundation of
RT
where i is the partial current of CH3OH, θH is the surface
coverage of H* and E is the applied potential. Meanwhile,
combining Nernst equation and the definition of Tafel slope, the
expression of Tafel slope can be described as equation 2.
China
(21525626,
U1463205,
U1662111,
21722608,
51861125104), and the Program of Introducing Talents of
Discipline to Universities (B06006) for financial support.
Keywords: Artificial photosynthesis • CO2 reduction • Liquid
fuels • CH3OH production • Cu/Cu2O interface
2.3
lni
 E
2.3RT
Tafel slope  

(2)
2 F (1 2H )
[1]
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surface area of Cu NPs (Figure S13), indicating that almost all
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10.1002/anie.201805256
Angewandte Chemie International Edition
COMMUNICATION
Table of Contents
COMMUNICATION
This communication describes
how the Cu/Cu2O interface
balances the binding strength of
H* and CO* intermediates for
photoelectrochemical reduction of
CO2, leading to a CH3OH Faradaic
efficiency (FE) of 53.6% in
aqueous conditions.
Xiaoxia Chang+, Tuo Wang+, Zhi-Jian
Zhao,
Piaoping
Yang,
Jeffrey
Greeley, Rentao Mu, Gong Zhang,
Zhongmiao Gong, Zhibin Luo, Jun
Chen, Yi Cui, Geoffrey A. Ozin, and
Jinlong Gong*
Page No. – Page No.
Tuning Cu/Cu2O Interfaces for
Reduction of Carbon Dioxide to
Methanol in Aqueous Solutions
This article is protected by copyright. All rights reserved.