Investigating the Origin of Enhanced C2+Selectivity in Oxide-/Hydroxide-Derived Copper Electrodes during CO2Electroreduction

Article abstract of DOI:10.1021/jacs.9b11790

Oxide-/hydroxide-derived copper electrodes exhibit excellent selectivity toward C2+ products during the electrocatalytic CO2 reduction reaction (CO2RR). However, the origin of such enhanced selectivity remains controversial. Here, we prepared two Cu-based electrodes with mixed oxidation states, namely, HQ-Cu (containing Cu, Cu2O, CuO) and AN-Cu (containing Cu, Cu(OH)2). We extracted an ultrathin specimen from the electrodes using a focused ion beam to investigate the distribution and evolution of various Cu species by electron microscopy and electron energy loss spectroscopy. We found that at the steady stage of the CO2RR, the electrodes have all been reduced to Cu0, regardless of the initial states, suggesting that the high C2+ selectivities are not associated with specific oxidation states of Cu. We verified this conclusion by control experiments in which HQ-Cu and AN-Cu were pretreated to fully reduce oxides/hydroxides to Cu0, and the pretreated electrodes showed even higher C2+ selectivity compared with their unpretreated counterparts. We observed that the oxide/hydroxide crystals in HQ-Cu and AN-Cu were fragmented into nanosized irregular Cu grains under the applied negative potentials. Such a fragmentation process, which is the consequence of an oxidation-reduction cycle and does not occur in electropolished Cu, not only built an intricate network of grain boundaries but also exposed a variety of high-index facets. These two features greatly facilitated the C-C coupling, thus accounting for the enhanced C2+ selectivity. Our work demonstrates that the use of advanced characterization techniques enables investigating the structural and chemical states of electrodes in unprecedented detail to gain new insights into a widely studied system.

Full text of DOI:10.1021/jacs.9b11790

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Article
Investigating the Origin of Enhanced C2+ Selectivity in Oxide-/
Hydroxide-derived Copper Electrodes during CO2 Electroreduction
Qiong Lei, Hui Zhu, Kepeng Song, Nini Wei, Lingmei Liu, Daliang Zhang, Jun Yin, Xinglong
Dong, Kexin Yao, Ning Wang, Xinghua Li, Bambar Davaasuren, Jianjian Wang, and Yu Han
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b11790 • Publication Date (Web): 10 Feb 2020
Downloaded from pubs.acs.org on February 11, 2020
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Investigating the Origin of Enhanced C2+ Selectivity in Oxide-
/Hydroxide-derived Copper Electrodes during CO2
Electroreduction
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Qiong Lei,†,¶ Hui Zhu,†,¶ Kepeng Song,† Nini Wei,§ Lingmei Liu,† Daliang Zhang,# Jun Yin,‡
Xinglong Dong,†,∑ Kexin Yao,# Ning Wang,† Xinghua Li,†, Bambar Davaasuren, Jianjian Wang,
Yu Han*,†,∑
§
#
⊥
† Advanced Membranes and Porous Materials Center, Physical Sciences and Engineering Division, King Abdullah
University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi Arabia.
§ Imaging and Characterization Core Lab, KAUST, Thuwal 23955-6900, Saudi Arabia.
# Multi-scale Porous Materials Center, Institute of Advanced Interdisciplinary Studies, & School of Chemistry and
Chemical Engineering, Chongqing University, Chongqing 400044, P. R. China.
‡ Physical Sciences and Engineering Division, KAUST, Thuwal 23955-6900, Saudi Arabia.
∑ KAUST Catalysis Center, KAUST, Thuwal 23955-6900, Saudi Arabia.
⊥School of Physics, Northwest University, Xi’an 710069, P. R. China.
ABSTRACT: Oxide-/hydroxide-derived copper electrodes exhibit excellent selectivity toward C2+ products during
electrocatalytic CO2 reduction reaction (CO2RR). However, the origin of such enhanced selectivity remains controversial.
Here, we prepared two Cu-based electrodes with mixed oxidation states, namely HQ-Cu (containing Cu, Cu2O, CuO) and
AN-Cu (containing Cu, Cu(OH)2). We extracted ultra-thin specimen from the electrodes using a focused ion beam to
investigate the distribution and evolution of various Cu species by electron microscopy and electron energy loss
spectroscopy. We found that at the steady stage of CO2RR, the electrodes have all been reduced to Cu0, regardless of the
initial states, suggesting that the high C2+ selectivities are not associated with specific oxidation states of Cu. We verified
this conclusion by control experiments, in which HQ-Cu and AN-Cu were pretreated to fully reduce oxides/hydroxides to
Cu0, and the pretreated electrodes showed even higher C2+ selectivity, compared with their un-pretreated counterparts. We
observed that the oxide/hydroxide crystals in HQ-Cu and AN-Cu were fragmented into nano-sized irregular Cu grains
under the applied negative potentials. Such a fragmentation process, which is the consequence of an oxidation-reduction
cycle and does not occur in electropolished Cu, not only built an intricate network of grain boundaries, but also exposed a
variety of high-index facets. These two features greatly facilitated the C-C coupling, thus accounting for the enhanced C2+
selectivity. Our work demonstrates that the use of advanced characterization techniques enables investigating the
structural and chemical states of electrodes in unprecedented detail, to gain new insights into a widely studied system.
INTRODUCTION
as the optimal binding energy for *CO and *COOH
intermediates, making the catalyst surface can stabilize
*COOH without being poisoned by CO.19 Meanwhile, the
drawback is a moderate binding energy of most reaction
intermediates lowers the selectivity toward a certain
product.17
Electrochemical reduction is one of the most appealing
methods to convert CO2 to value-added products,
including CO, formic acid or formate, methane, methanol
and other multi-carbon oxygenates and hydrocarbons
(denote as C2+).1-7 Among all the reported electrocatalysts
for CO2RR,8-16 Cu or Cu-based catalyst has drawn the most
attention because it can produce deep reduction products
(i.e., oxygenates and hydrocarbons) which are more
When electropolished Cu is used as the catalyst, there
are 16 products detected after CO2RR.20 Methane is the
major product, but its faradaic efficiency (FE) decreased
quickly after the first two hours from about 50% to 10%.21
Many strategies have been developed to improve the
activity and tune the selectivity of Cu, including surface
facet control,22-27 morphology manipulation,28-30 single
desirable, considering their higher energy density and
wider applicability.4,
On the mechanism level, such
17-18
unique performance is attributed to its optimal electronic
structure for the rate-limiting CO protonation step, as well
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atom doping,31 alloying with other metals,32-33 and deriving
and EELS. During FIB process, carbon precursors were
from copper oxides or hydroxide,34-36 etc. Among all the
mentioned strategies, special attention has been paid to
the oxide-/hydroxide-derived copper electrocatalysts,
because their selectivity toward C2+ products can generally
reach a FE more than 50%.34-36 However, there is no
common understanding on the origin of such excellent
performance, and the key oxidation state of Cu is still
under debate.37
used to react with the electron beam to deposit the first
protection layer onto the sample surface and a focused
Gallium ion beam was applied to cut and mill the
specimen. Under the FIB conditions, carbon precursors
and Gallium ions do not react with Cu, Cu2O, CuO, or
Cu(OH)2, and therefore do not affect the oxidation states
of Cu. Our results support the argument that metallic Cu
is the actual active species and indicate that the starting
oxidation state of oxidized Cu, mixed or pure Cu0, would
hardly affect the enhanced selectivity toward C2+ products.
HRTEM image and selected area electron diffraction
(SAED) pattern revealed that the initial micron-sized oxide
crystals in HQ-Cu were fragmented into nano-sized Cu
grains under the negative potential during electro-
reduction pretreatment and CO2RR. Using a recently
developed image processing method, we were able to
identify individual Cu grains and their boundaries from the
HRTEM images. The results show that these Cu grains are
5-15 nm in size, irregular in shape, exposing a large number
of various high-index facets. The crystal fragmentation was
only observed in oxide-/hydroxide-derived Cu and not in
electropolished Cu, indicating that an oxidation-reduction
cycle is its prerequisite. On the basis of the correspondence
between “crystal fragmentation” and the high C2+
selectivity, we conclude that the increased grain
boundaries and high-index facets generated during
fragmentation process are the origin of the observed high
C2+ selectivity of oxide-/hydroxide-derived Cu electrodes.
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Essentially, the controversy mainly lies on the presence
of oxidized Cu phases during the electroreduction of CO2.
Jiang et al. reported a FE of 60% for C2+ with an
electrodeposited Cu2O(100) electrode.38 They found there
is no oxygen signals can be observed with electron energy
loss spectroscopy (EELS) nor energy dispersive X-ray
spectroscopy (EDS) on the electrode surface under CO2RR
condition, and the product selectivity owes to the
promoted C-C coupling on Cu(100) facet. Similarly, Kim
and coworkers found that the chemically deposited
Cu(OH)2 nanowires were completely reduced to metallic
Cu after a few minutes of electrolysis, which is confirmed
by X-ray diffraction (XRD) and electrochemical impedance
spectroscopy (EIS), and they gave the credit of 38% FE for
C2+ to systemically modified mesostructure.39 On the other
hand, several groups have evidenced the presence of
oxygen species on the surface of oxide-/hydroxide-derived
copper electrodes under CO2RR conditions, and related it
to the enhanced C2+ selectivity.34-36, 40-43 For example, Mistry
et al. used scanning transmission electron microscopy
(STEM) equipped with EDS, operando X-ray absorption
fine structure (XAFS) and high resolution transmission
electron microscopy (HRTEM) to prove that the surface
Cu1+ species in their plasma treated Cu foil is stable during
CO2RR, and the superb selectivity on ethylene (FE 60%)
relies on the existence of Cu1+.36 Additionally, grazing
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RESULTS AND DISCUSSIONS
Electrode Preparation and Characterization. Both
Cu oxide-/hydroxide-containing electrodes were prepared
from electropolished Cu. SEM image indicates a flat
surface of the electropolished Cu substrate (Figure 1a), and
the XRD pattern clearly shows three sharp intensive peaks
at 43.30, 50.40 and 74.10 that correspond to the (111), (200)
and (220) reflections of Cu (PDF#70-3039) (Figure 1d). To
prepare the oxide-containing electrode, or HQ-Cu,
electropolished Cu was heated in a tube furnace at 11000C
for 10 sec, and immediately quenched in air. This process
results in the generation of sponge-like surface on top of
Cu substrate (Figure 1b). Apart from the three sharp Cu
peaks, five broad peaks at 29.30, 36.20, 42.10, 60.90, and 72.70
in the XRD pattern of HQ-Cu can be well indexed to the
(110), (111), (200), (220) and (311) planes of Cu2O (PDF#05-
0667) (Figure 1d). In addition, there is one weak peak at
38.80, which could possibly be assigned to CuO. To further
confirm the origin of this peak, we consulted Cu 2p of XPS,
and it provided Cu2+ satellites at 962.1 eV and 943.6 eV to
evidence the presence of CuO (Figure S1). For the
incidence
XRD
(GI-XRD),
X-ray
photoelectron
spectroscopy (XPS), and X-ray absorption near edge
structure (XANES) were involved to evidence the stability
of Cu(OH)2 and Cu2O in an anodized Cu electrocatalyst
under CO2RR conditions according to Lee’s work.34 They
claimed the high FE of ethylene (~ 38%) and long catalytic
lifetime (~ 40 h) to be a result of mixed Cu oxidation states
(Cu, Cu2O, and Cu(OH)2). To end this controversy and
facilitate the mechanism study on the origin of enhanced
selectivity toward C2+ products of oxide-/hydroxide-
derived Cu catalysts, it is of great importance to determine
the oxidation states of Cu species and their spatial
distribution on the electrode during CO2RR.
In this work, we have prepared two Cu-based electrodes
with mixed oxidation states: one containing Cu, Cu2O, and
CuO via a heat-quench process (HQ-Cu), and the other
one containing Cu and Cu(OH)2 via anodization (AN-Cu).
They were submitted directly or after an electro-reduction
pretreatment to CO2RR and characterized along the
process. Focused ion beam (FIB) was used to extract ultra-
thin (~ 100 nm) lamella from the electrode, allowing the
analysis on the distribution and evolution of different Cu
oxidation states on the electrode cross-section using TEM
hydroxide-containing
electrode,
or
AN-Cu,
electropolished Cu was anodized in 3.0 M potassium
hydroxide aqueous electrolyte at a constant current of 10
mA·cm–2 for 60 sec. The produced electrode has a nanowire
surface morphology (Figure 1c), and its XRD pattern
perfectly matches with Cu(OH)2 (PDF#80-0656) and Cu
(PDF#70-3039) (Figure 1d), indicating the formation of
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hydroxide on the Cu substrate. In other words, we have selectivity, but also greatly improve the catalytic activity
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produced two electrodes that consist of mixed Cu
oxidation states: the first one is HQ-Cu, containing Cu,
Cu2O and CuO; and the second one is AN-Cu, containing
Cu and Cu(OH)2.
and stability.
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Figure 1. SEM images of (a) electropolished Cu, (b) HQ-Cu,
and (c) AN-Cu, in which the scale bars all represent 2 µm; (d)
indexed XRD patterns of these three electrodes.
CO2RR performance. As-prepared HQ-Cu and AN-Cu
electrodes were both submitted to CO2RR directly at
different potentials to evaluate their catalytic performance.
In comparison with the pure Cu electrode (an
electropolished Cu foil), HQ-Cu and AN-Cu have
effectively suppressed the CH4 production at more
negative potentials (< -1.1 V vs RHE) (Figure 2a), and
greatly improved the selectivity toward C2+ products at all
tested potentials (Figure 2b). The highest FE for C2+ of
CO2RR with HQ-Cu can reach 68.2% at a potential of -1.05
V vs RHE, while that of CO2RR with AN-Cu is 62.3% at -
1.03 V vs RHE. The main C2+ products include C2H4,
C2H5OH, C2H5CHO, CH3CHO, n-C3H7OH, and CH3COO-
(sequenced by decreasing FE), and detailed product
distribution has been summarized in Figure S2. This result
is consistent with other reported works on oxide-
/hydroxide-derived Cu that increased C2+ selectivity has
been observed.34-35, 44-47 Since C–C coupling is the key step
to form C2+ products and it directly consume CO*
intermediates,48 decreased CO production can be expected
at C2+-favored potentials (< -0.9 V vs RHE) (Figure 2c). In
addition, H2 formation has been suppressed as CO2RR is
apparently favored over water splitting on oxide-
/hydroxide-derived Cu electrodes at raised cathodic
potentials (< -0.8 V vs RHE) (Figure 2d). At their optimum
potential for C2+ production, the partial current densities
for C2+ of HQ-Cu and AN-Cu are approximately 50 and 64
times that of electropolished Cu, respectively (Figure 2e).
By prolonging the reaction time, we found the C2+
selectivities of HQ-Cu and AN-Cu can both be extended to
more than seven hours (Figure 2f). In short, comparing
with electropolished Cu, oxide-/hydroxide-derived Cu
electrodes can not only extensively enhance the C2+
Figure 2. Comparison on faradaic efficiency (FE) of (a) CH4,
(b) C2+, (c) CO and (d) H2 for CO2RR with electropolished Cu,
HQ-Cu and AN-Cu at different potentials. (e) Comparison on
partial current density of C2+ for CO2RR with these three
electrodes. Data has been normalized by electrochemically
active surface area (ECSA). (f) Catalytic lifetime test on CO2RR
at -1.2 V vs RHE with these three electrodes. All CO2RR were
conducted in CO2-saturated 0.1 M KHCO3.
Evolution of electrode species during CO2RR.
Although the improved C2+ selectivity of oxide-/hydroxide-
derived Cu electrode has been well noted, there is no
consensus on the origin of such change. In theory, bulk Cu
oxide/hydroxide species should be reduced to metallic Cu
at the negative potentials of CO2RR, but the presence of
oxygen in electrode surface is still under debate.37 To study
the evolution of electrode species under CO2RR
conditions, we used EELS to depict different Cu oxidation
states of HQ-Cu before and after reaction. FIB technique
was utilized to cut a thin slice out of the electrode, which
allowed us to investigate the distribution and evolution of
different Cu oxidation states along its cross-section. Firstly,
we studied HQ-Cu and found that three types of Cu species
(Cu0, Cu1+, Cu2+) coexisted but resided in separate layers
(Figure 3a), which is in line with the previous reports on
Cu oxidation mechanism.49-50 Specifically, Cu2+ is present
on the top surface, forming
a thin, rugged and
discontinuous layer (0 to 200 nm in thickness). A
continuous layer of Cu1+ is sandwiched between the top
surface of Cu2+ and the substrate of Cu0 with thickness of ~
300-1000 nm. EELS spectra extracted from these three
layers perfectly match with the standard spectra for Cu,
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Cu2O, and CuO (Figures S3a,b).51 This result agrees well
the latter, if it exists, should not reside in the interior of the
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with our XRD and XPS analysis described above. Although
the trace amount of CuO on the outermost surface is
difficult to be detected by XRD, it has been clearly
observed by XPS and electron microscopy. Taken together,
these results show that in HQ-Cu electrode, the oxidation
degree increases from the bulk to the surface, with the
overall thickness of the oxidation layers at several
hundreds of nanometers. We note that in the FIB-
fabricated specimen, there is often a crack at the interface
of the oxide layer and the substrate (Figure 3, indicated by
white arrows), which might be a result of different tension
between Cu2O and Cu crystals, and sometimes further
stretched by FIB operation.
electrode (the cracked interface), given that the electro-
reduction of the electrode takes place from the substrate
outwards.
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Figure 3. High-angle annular dark field (HAADF) STEM
image and the corresponding EELS mapping on different Cu
oxidation states and oxygen of the FIB-fabricated specimen
that corresponds to the cross-section of (a) HQ-Cu; and (b)
HQ-Cu after 1 h of CO2RR at 1.05 V vs RHE in CO2-saturated
0.1 M KHCO3.
Figure 4. (a) Indexed XRD patterns of PHQ-Cu and PAN-Cu;
(b) HAADF-STEM image and the corresponding EELS
mapping on different Cu oxidation states and oxygen of the
FIB-fabricated specimen that corresponds to the cross-section
of PHQ-Cu; FE comparison between (c) HQ-Cu and PHQ-Cu,
and (d) AN-Cu and PAN-Cu. Detailed product distributions
have been summarized in Figures S2 and S6. All CO2RR were
conducted in CO2-saturated 0.1 M KHCO3.
To identify the active Cu species for the enhanced C2+
selectivity, we submitted HQ-Cu to CO2RR at -1.05 V vs
RHE for 1 h, allowing the catalytic performance to reach
the steady stage, and then terminated the reaction to ex-
situ investigate the Cu oxidation states with XRD and FIB-
TEM/EELS. The XRD pattern showed that all reflections
associated with Cu2O and CuO disappeared after 1 h of
CO2RR reaction (Figure S4). Accordingly, there was only
metallic Cu (Cu0) detected by EELS throughout the entire
cross-section of the electrode (Figure 3b). The EELS spectra
collected from the upper layer, where Cu2O and CuO were
present in the initial HQ-Cu electrode, and the lower Cu
substrate are identical, and no oxygen peak has been
observed in both areas (Figures S3c,d). Trace amount of
oxygen was detected by EELS at the surfaces contacting
with the Pt/C FIB-protection layer and along the crack
between the initial Cu2O layer and the Cu substrate (Figure
3b), which we believe is due to the inevitable exposure to
the air during the sample-transfer processes in FIB and
TEM imaging. In fact, the oxygen contamination issue has
been aware of in the literature when the reduced Cu
electrodes are ex-situ characterized, and this issue makes
it difficult to completely rule out the presence of residual
unreduced Cu oxide species.52-53 Although this issue
remains in this study, our characterization using analytical
electron microscopy provides sufficiently high spatial
resolution to reveal that the very little amount of oxygen
resides only at the two surfaces of the oxidized/reduced
layer. This observation suggests that the oxygen is from
contamination instead of unreduced Cu oxides, because
Pretreatment
by
electroreduction.
Our
characterization data suggests that the observed high C2+
selectivities are associated with metallic Cu rather than Cu
oxides. This is consistent with the reported in situ
measurements that have demonstrated the reduction of Cu
oxides is prior to the reduction of CO2, including Cu K-
edge X-ray absorption spectroscopy (XAS),54 GI-XRD,37 and
Raman spectroscopy55. To consolidate this conclusion, we
performed a harsh electro-pretreatment on the HQ-Cu
within the same electrolytic cell, in order to ensure the full
reduction of Cu oxides to metallic Cu prior to the CO2RR
reaction. To avoid CO2RR product accumulation during
this process, we applied a highly negative potential for
shortened time, and the pretreated electrode is denoted as
PHQ-Cu. Specifically, -5.0 V vs Ag/AgCl was used for 10 sec.
The cathodic current of prolonged pretreatment was also
recorded in time, indicating that the current reached a
steady state within the first 5 sec and 10 sec at this potential
is enough to reduce Cu oxides to Cu0 (Figure S5). The XRD
pattern collected after the electro-reduction pretreatment
also indicates the reduction of Cu2O and CuO to metallic
Cu (Figure 4a). It was further confirmed by the results of
FIB-EELS analysis performed in the same way as described
above for HQ-Cu, which shows only metallic Cu
throughout the entire cross-section of the PHQ-Cu
electrode (Figure 4b). The EELS spectra collected from the
upper oxidized/reduced layer and the lower Cu substrate
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are identical, and no oxygen peak has been observed in its C2+ selectivity (Figure S7). This result suggests that such
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both areas (Figures S3e,f). In other words, PHQ-Cu is an
electrode containing pure Cu0.
an electro-reduction pretreatment alone would not
obviously change the catalytic selectivity of the Cu-based
electrodes for CO2RR, whereas the enhanced C2+ selectivity
of oxide-/hydroxide-derived Cu electrode is a consequence
of an oxidation-reduction cycle.
CO2RR performance of PHQ-Cu has been evaluated at
different potentials, and its selectivity toward C2+ products
is even slightly higher than that of HQ-Cu at higher
cathodic potentials (Figure 4c). The highest FE for C2+ can
reach 72.7% at -1.09 V vs RHE. This result supports our
conclusion that the observed high selectivity toward C2+
products is only associated with pure metallic Cu, rather
than any Cu oxides. In addition, exactly same results have
been obtained when we performed the electro-reduction
pretreatment (-5.0 V vs Ag/AgCl for 10 min) on AN-Cu.
That is, the AN-Cu has been fully reduced to metallic Cu
(Figures 4a, S6a-c), and the obtained electrode is denoted
as PAN-Cu. Compared with the hydroxide-containing
electrode (AN-Cu), the pretreated one (PAN-Cu) exhibited
further enhanced C2+ selectivity, and the highest FE can
reach 76.73% at -1.07 V vs RHE (Figure 4d). In other words,
the starting oxidation state of oxidized Cu precursors plays
a minor role in determining the selectivity toward C2+
products, because they will all be reduced to Cu0 at the
negative potentials of CO2RR. This is in line with the result
from Eilert’s observations that the ethylene selectivities are
similar between CuCO3·Cu(OH)2 and Cu2O.54
Crystal Fragmentation at Negative Potential. The
above findings inspired us to study the common features
of Cu electrodes with high C2+ selectivity, such as HQ-Cu,
PHQ-Cu, AN-Cu and PAN-Cu, and how they distinguish
from those with low C2+ selectivity, such as electropolished
Cu. For this purpose, we maximized the utilization of FIB-
TEM and visualized the changes on oxide-/hydroxide-
derived Cu crystals under the negative potential of electro-
reduction pretreatment or CO2RR. Firstly, we studied the
as-prepared HQ-Cu electrode. The previous assignment of
the successive layers (Cu/Cu2O/CuO) in HQ-Cu based on
EELS has been confirmed by SAED (Figure 5a). The SAED
analysis also indicated that the CuO layer is very thin and
discontinuous; the Cu2O layer consists of large grains with
dimensions of ~ 1 µm; the Cu substrate layer consists of
even larger grains, whereas the exact grain sizes cannot be
determined because there is only one grain boundary
observed in the FIB-fabricated specimen. In brief, the three
layers follow the order: Cu > Cu2O > CuO, in terms of grain
size. These results demonstrate that the heat-quench
process leads to the generation of oxide layers on the
surface of Cu foil, with the grain sizes decreasing as the
oxidation degree increases.
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In a separate control experiment, we conducted the
same electro-reduction pretreatment on a pure Cu
electrode (an electropolished Cu foil), and found that the
pretreatment did not change its CO2RR activity or enhance
Figure 5. TEM images of FIB-fabricated specimen from different electrodes: (a) HQ-Cu, (b) HQ-Cu after 1h of CO2RR, (c) PHQ-
Cu, and (d) electropolished Cu foil after 1 h of CO2RR. Each dashed circle represents a selected area with a diameter of ~ 160 nm;
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the insets are the corresponding SAED patterns, in which the scale bars all represent 5 1/nm. In (c), the middle panel shows a
typical HRTEM image of the oxide-derived layer in PHQ-Cu, acquired from the selected area in the left panel; the right panel
shows some Cu grains identified from the HRTEM image using a self-developed image processing software, among which one Cu
grain is determined to be oriented along [011] zone axis and its exposed surfaces are all indexed and labelled. We note that there
is a dim diffraction ring in the upper SAED pattern in (c), which can be assigned to the (111) reflection of CuO; the generation of
CuO in the reduced Cu layer is likely due to unavoidable exposure to the air during the sample transfer processes in FIB and TEM
imaging. In the cross section TEM images, the oxide (or oxide-derived) layer and Cu substrate sections are artificially colored in
light blue and light green, respectively, for clarity.
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Secondly, we investigated two electrodes that had
undergone electro-reduction processes, namely HQ-Cu
after 1h of CO2RR and PHQ-Cu, using the same
characterization method. The SAED results showed that,
in both cases, the oxide layers were fully reduced to
metallic Cu (Figures 5b,c), which is consistent with the
EELS analysis of oxidation states (Figures 3b and 4b).
Remarkably, unlike the as-prepared HQ-Cu electrode in
which the oxide layers show single-crystalline diffraction
patterns (Figure 5a), these two electrodes show
polycrystalline diffraction rings at their oxidized/reduced
layers (Figures 5b & 5c). As the selected area for diffraction
is ~ 160 nm in diameter, the observed polycrystalline
diffraction rings must originate from much smaller,
randomly oriented grains. The single-to-polycrystalline
evolution implies that the initial micron-sized Cu2O/CuO
grains are fragmented into nano-sized Cu grains during the
CO2RR or electro-reduction pretreatment. The generation
associated with crystal fragmentation, or a result of the gas
generation (both CO2RR and HER) under highly negative
potential. Such morphological changes slightly increased
the electrode surface roughness according to our ECSA
analysis (Figure S10 and Table S1). Although we did not
characterize AN-Cu as in detail as HQ-Cu, we believe the
same conclusion also holds for hydroxide-derived
electrodes, because when we characterized the PAN-Cu
electrode with the same FIB-TEM technique, similar
results has been obtained. That is, the upper hydroxide-
derived layer was fully reduced to metallic Cu, and
fragmented into nano-sized irregular grains (Figure S11).
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Finally, we applied -1.05 V vs RHE to an electropolished
Cu electrode for 1 h under CO2RR conditions, and then
characterized it with FIB-TEM as described above for other
samples. We found that the electrode retained large grain
sizes without being fragmented, during the CO2RR process,
as evidenced by the single-crystalline diffraction pattern
(Figure 5d). Based on these findings, we conclude that the
fragmentation process under the negative potential of both
electro-reduction pretreatment and CO2RR is the
consequence of an oxidation-reduction cycle, and
therefore does not occur in electropolished Cu.
of nano-sized Cu grains as
a
consequence of
“fragmentation” has been confirmed by HRTEM imaging.
The middle panel in Figure 5c shows a typical HRTEM
image acquired from a FIB-fabricated specimen of PHQ-Cu,
in which randomly oriented, overlapping lattice fringes are
observed. Using a self-developed image processing method
based on modified inverse Fourier transform, we are able
to isolate each individual grains from the raw image, and
thus identify their exact sizes and shapes (Figure 5c; right
panel). The results show that Cu grains have irregular
shapes and small sizes, ranging from 5 to 15 nm. Although
it is impossible to determine the exposed facets of each
grains due to their random orientations (most grains show
only one set of lattice fringes in the image), we can
occasionally find a grain oriented along a zone axis so that
we can determine the indexes of some exposed surfaces
(edges or facets). One example is outlined in the figure,
which is oriented along the [011] zone axis, exposing a
number of high-index surfaces including (91-1), (4-22), (1-
33), (-4-22), (9-11), (-22-2) and (24-4) (Figure 5c; right
panel). We performed HRTEM study for HQ-Cu after 1 h
of CO2RR as well, and obtained similar results (Figure S8).
Taken together, these results demonstrate that under the
negative potential of both electro-reduction pretreatment
and CO2RR, the oxide crystals in HQ-Cu were fragmented
into small irregular Cu grains with many high-index facets
exposed. Note that the Cu substrate layer remains single
crystalline (i.e., consisting of large grains) in both cases
Given that all Cu electrodes with high C2+ selectivity
share the common feature of fragmentation and those with
low C2+ selectivity do not possess this feature, a strong
connection can be built between the crystal
fragmentations with the improved selectivity toward C2+
products. In fact, the fragmentation process can enhance
C2+ production during CO2RR in two ways. On one hand, it
formed an intricate network of grain boundaries surface
terminations, where highly active sites are enriched and
facilitating CO binding. The improved CO binding can
hold CO* longer, not only increasing the chance for two
CO* intermediates to be coupled to generate C2+ products,
but also limiting the access of H2O molecules to the active
sites and therefore suppressing H2 evolution.56-60
Additionally, grain boundaries effect was also reported to
be responsible for the improved CO2 reduction activity at
lower overpotentials.61 On the other hand, such crystal
fragmentation provides enormous opportunities for high-
index facet exposure, such as (91-1) and (-4-22) (Figure 5c;
right panel). The high-index [n(100) × (111)] surfaces have
been reported to significantly promote C2H4 formation
while suppressing CH4 generation, which can explain the
similar selectivity ratio of C2H4/CH4 between PHQ-Cu and
Cu (911) single crystal surface23. Obviously, different
negative potentials and time would vary the fragmentation
results. Especially for the short-term pretreatment at
highly negative potentials, facets with relatively higher
(Figures 5b
& 5c). In addition, SEM and TEM
characterizations indicate that the spongy HQ-Cu surface
(Figure S9a) forms cracks and porous structures under the
negative potentials of CO2RR (Figure S9b) or
electrochemical pretreatment (Figure S9c), which is likely
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energy would have higher chance to be preserved after because it greatly facilitates C-C coupling with numbers of
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exposure. This can probably explain the slightly higher FE
for C2+ of PHQ-Cu than that of HQ-Cu. Hahn et al. have
investigated the CO2RR performance of Cu(111), (100), and
(751) single crystal surface and found the selectivity toward
C2H4 over CH4 is potential-dependent.22 Taking Cu(100) as
an example, C2H4 production dominates over CH4 at lower
potentials, and the latter reverse the situation at more
negative potentials (e.g., -1.10 V vs RHE). While in our case,
the selectivity toward C2H4 over CH4 is consistent at all
tested potentials ranging from -0.65 to -1.2 V vs RHE. In
this light, the enhanced selectivity toward C2+ products of
our HQ-Cu and PHQ-Cu electrodes should be a result of
the combination of increased grain boundaries and high-
index facets exposed during fragmentation under the
negative potential of electro-reduction pretreatment and
CO2RR.
grain boundaries and high-index facets. Our work provides
new insights into the roles of crystal fragmentation in
determining the product selectivity of oxide/hydroxide-
derived Cu electrodes during CO2RR, and should inspire
more highly selective electrocatalysts to be developed.
EXPERIMENTAL SECTION
Electrode Preparation. All Cu foils (Alfa Aesar,
0.127mm thickness, 99.9%) were first cleaned in ethanol
(VWR, ACS reagent grade) and Milli-Q water (18.2 MΩ·cm-
1) under sonication for 10 min each, then electropolished in
85% phosphoric acid (Scharlau, 85% in water) at 3.0 V
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versus another Cu foil for
5 min. The obtained
electropolished Cu was used as-prepared, or after an
oxidation process.
The Cu-based electrode with mixed oxidation states was
prepared via a successive heat and quench method (denote
as HQ-Cu). Specifically, electropolished Cu was heated in
As we were finalizing this manuscript, we noticed a new
publication by Hwang et al., which reported that
fragmentation of Cu2O nanoparticles under CO2RR
conditions enhances C-C coupling and therefore boosts
C2H4 selectivity.35 These two works coincide in main
conclusions. In their report, the FE for C2H4 climbed to the
plateau after six hours reaction, whereas our catalysts
provide the highest FE for C2H4 within the first hour
(Figure 2f). This difference is possibly due to the very
different sizes of initial Cu2O crystals in the two cases (~ 1
µm vs 20 nm): smaller crystals require longer time to get
further fragmented under the same negative potential of
CO2RR.
a
tube furnace (MTI, GSL-1700X) at the desired
temperature of 11000C for 10 sec, and quenched in air right
after the allotted time.
Anodized Cu electrode (denote as AN-Cu) was
fabricated in a modified method.34 The electropolished Cu
were oxidized in 3.0 M potassium hydroxide (Sigma-
Aldrich, 85%) aqueous electrolyte for 60 sec at a constant
current of 10 mA·cm–2 to form Cu(OH)2 nanowires.
Electroreduction of CO2. All electro-pretreatment and
catalytic measurements were performed on CHI 760e
workstation in a commercial H-type gas-tight three-
electrode cell. Nafion 117 membrane (Fuel Cell Store) was
used to separate the cathode and anode compartments.
Platinum foil and Ag/AgCl electrode filled with saturated
potassium chloride solution (Ida, R0303) were employed as
the counter and reference electrode, respectively. All
potentials measured against Ag/AgCl were converted to
the reversible hydrogen electrode (RHE) scale in this work
using E (vs RHE) = E (vs Ag/AgCl) + 0.197 V + 0.0591 × pH.
0.1 M potassium bicarbonate (Honeywell, 99.5-101.0%
(acidimetric)) aqueous solution saturated with CO2 (Air
Liquide, 99.995%) was the electrolyte, and the pH value
was measured to be 6.8 by Orion 5 star benchtop
multiparameter (Thermo Scientific).
CONCLUSION
In summary, we have prepared two Cu-based electrodes
with mixed oxidation states, including HQ-Cu (containing
Cu, Cu2O, CuO), and AN-Cu (containing Cu, Cu(OH)2).
Comparing with electropolished Cu, they were found to
provide similarly enhanced selectivities for C2+ products.
FIB-TEM/EELS was used to characterize the distribution
and evolution of various Cu species. We found that HQ-Cu
electrode was fully reduced to metallic Cu at the steady
stage of CO2RR, suggesting that the observed high C2+
selectivities do not depend on specific oxidation states of
Cu. In addition, an electro-reduction pretreatment was
performed on these two electrodes under harsh conditions
to ensure full reduction to Cu0 before submitted to CO2RR.
The pretreated electrodes exhibited even slightly higher
selectivity toward C2+ products, further confirming the key
catalytic role of Cu0 and the negligible significance of
starting oxidation state of oxidized Cu. Furthermore, we
found that the oxide crystals in HQ-Cu and the hydroxide
crystals in AN-Cu were all reduced and fragmented into
small irregular Cu grains under the negative potential of
electro-reduction pretreatment or CO2RR, while no
fragmentation has been observed in electropolished Cu
after CO2RR. Based on these findings, we conclude that
such fragmentation is the consequence of an oxidation-
reduction cycle, and accounts for the enhanced C2+
selectivity of oxide-/hydroxide-derived Cu electrodes,
Geometric dimensions of all electrodes were controlled
to be 1 × 6 mm2. The electro-pretreatment is to apply a
highly negative bias potential of −5.0 V vs Ag/AgCl for 10
sec on HQ-Cu, or for 10 min on AN-Cu, prior to CO2RR
within the same cell. During CO2RR, 5.0 standard cubic
centimeters per minute (sccm, monitored by Cole-Parmer
Scientific mass flow controller) of CO2 was continuously
purged into the system and the gas products were routed
into an online gas chromatography (GC, Kechuang
GC2002).
Product Analysis. All gas products were analyzed by
the online GC, which is equipped with one thermal
conductivity detector (TCD) to detect H2 and two flame
ionization detectors (FID) to detect hydrocarbons. Before
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the second FID, a methanizer (Kechuang) is installed for reflection show sharp and clear edges that can be used to
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the detection of CO. Quantification of gaseous products
was determined using calibration curves from standard
gases.
determine the crystal facets. The same operations can be
applied to different reflections as needed. More detailed
theoretical basis and operation procedures of this method
will be disclosed in a separate publication shortly.
1
All liquid products were quantified by 1D H NMR
(Bruker, 600 MHz). Standard curves were made using
purchased chemicals over the concentration range of
interest with 1.67 parts per million (ppm) dimethyl
sulfoxide (DMSO, Sigma-Aldrich, 99.9%) as the internal
standard in 0.1 M KHCO3. The water peak was suppressed
by a presaturation sequence. NMR parameters used were
identical between collected spectra to make standard
curves and in the subsequent quantification of CO2RR
products in the electrolyte. FE could be calculated as
follows:
The high-resolution XPS measurements were carried out
using a Kratos Axis ultra DLD spectrometer equipped with
a monochromatic Al Kα X-ray source (hν = 1486.6 eV)
operating at 150 W, a multi-channel plate and delay line
detector under a vacuum of ~ 10-9 mbar. The high-
resolution spectra were collected at fixed analyzer pass
energy of 20 eV.
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ASSOCIATED CONTENT
Supporting Information
[퐴] ∙ 푉 ∙ (푛퐹)
The Supporting Information is available free of charge on the
ACS Publications website.
퐹퐸 =
푄
where [A] represents the concentration of analyte as
determined by quantitative NMR, V the total volume of
solution (typically 600 μL), n the mole ratio of transferred
electrons, F the Faraday constant, and Q the total charge
passed.
XPS of HQ-Cu; CO2RR product distributions; EELS spectra;
indexed XRD pattern of HQ-Cu after 1 h of CO2RR; EELS
mapping and TEM characterization for PAN-Cu; cathodic
current recorded during the electro-reduction pretreatment;
FE and current density comparison between electropolished
Cu with and without pretreatment; HRTEM image and the
corresponding crystal isolation analysis for the oxide-derived
layer in HQ-Cu after 1 h of CO2RR; SEM and TEM
characterization for HQ-Cu before and after crystal
fragmentation; ECSA analysis.
Material Characterization. SEM (FEI, Magellan) was
used to observe the electrode surface morphology. Each
sample was characterized as-prepared, after electro-
pretreatment and after CO2RR by XRD (Bruker, D8
Advance), which had a Cu Kα radiation wavelength of
0.15406 nm.
AUTHOR INFORMATION
FIB was performed using the standard lift-out technique
on a dual beam FIB-SEM (FEI, Helios Nanolab Dualbeam)
equipped with an OmniProbe micromanipulator to
prepare samples for TEM analysis on the cross-section of
each electrode. Before FIB milling, an electron-beam-
assisted (3 kV) protective carbon layer was firstly deposited
on the selected area of the electrode surface, followed by a
second layer of electron-beam (3 kV) Pt and a third layer of
ion-beam-assisted (30 kV) Pt. The total thickness of
coating is ~2 μm. A Gallium ion beam of 30 kV were used
to mill and thinning the sample to ~100 nm thick lamella.
The prepared lamella was mounted to a copper half-grid,
and immediately transferred to glovebox for storage.
Corresponding Author
* yu.han@kaust.edu.sa
Author Contributions
¶Q.L. and H.Z. contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The financial support for this work was provided by Baseline
Funds (BAS/1/1372-01-01) to Y.H. from King Abdullah
University of Science and Technology. This research used
resources of the Core Labs of King Abdullah University of
Science and Technology.
TEM imaging and SAED were performed on Cs-
corrected electron microscope (FEI, Titan cube 80-300),
and EELS mapping on a double Cs-corrected electron
microscope (FEI, Titan Cubed Themis Z). HRTEM images
were processed using a newly developed method to
identify individual Cu grains. Briefly, (i) the image is
processed by Fast Fourier transform (FFT); (ii) a reflection
in the FFT pattern is selected using a line mask (2 by 20
pixels) for inverse Fourier transform to obtain a calculated
image; (iii) the line mask is rotated by 1 degree around the
center of the selected reflection, followed by inverse
Fourier transform to generate the second image, and this
step is sequentially repeated by rotating the line mask
along a consistent direction to produce 180 calculated
images in total; (iv) the 180 images are merged. In the
merged image, Cu grains associated with the selected
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