Speciation of Cu Surfaces during the Electrochemical CO Reduction Reaction

Article abstract of DOI:10.1021/jacs.0c02354

Cu-catalyzed selective electrocatalytic upgrading of carbon dioxide/monoxide to valuable multicarbon oxygenates and hydrocarbons is an attractive strategy for combating climate change. Despite recent research on Cu-based catalysts for the CO₂ and CO reduction reactions, surface speciation of the various types of Cu surfaces under reaction conditions remains a topic of discussion. Herein, in situ surface-enhanced Raman spectroscopy (SERS) is employed to investigate the speciation of four commonly used Cu surfaces, i.e., Cu foil, Cu micro/nanoparticles, electrochemically deposited Cu film, and oxide-derived Cu, at potentials relevant to the CO reduction reaction in an alkaline electrolyte. Multiple oxide and hydroxide species exist on all Cu surfaces at negative potentials, however, the speciation on the Cu foil is distinct from that on micro/nanostructured Cu. The surface speciation is demonstrated to correlate with the initial degree of oxidation of the Cu surface prior to the exposure to negative potentials. Combining reactivity and spectroscopic results on these four types of Cu surfaces, we conclude that the oxygen containing surface species identified by Raman spectroscopy are unlikely to be active in facilitating the formation of C₂₊ oxygenates in the CO reduction reaction.

Full text of DOI:10.1021/jacs.0c02354

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Article
Speciation of Cu Surfaces During the
Electrochemical CO Reduction Reaction
Yaran Zhao, Xiaoxia Chang, Arnav Malkani, Xuan Yang, Levi Thompson, Feng Jiao, and Bingjun Xu
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Apr 2020
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Speciation of Cu Surfaces During the Electrochemical CO Reduction Reaction
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Yaran Zhao#, Xiaoxia Chang#, Arnav S. Malkani, Xuan Yang, Levi Thompson*, Feng Jiao*, and
Bingjun Xu*
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Center for Catalytic Science and Technology, Department of Chemical & Biomolecular Engineering,
University of Delaware, Newark, Delaware 19716, United States
Emails: ltt@udel.edu, jiao@udel.edu, bxu@udel.edu
#Equal contribution
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Abstract
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Cu-catalyzed selective electrocatalytic upgrading of carbon dioxide/monoxide to valuable
multicarbon oxygenates and hydrocarbons is an attractive strategy for combatting climate change.
Despite recent research on Cu-based catalysts for the CO2 and CO reduction reactions, surface
speciation of the various types of Cu surfaces under reaction conditions remains a topic of discussion.
Herein, in-situ surface enhanced Raman spectroscopy (SERS) is employed to investigate the
speciation of four commonly used Cu surfaces, i.e., Cu foil, Cu micro/nanoparticles,
electrochemically deposited Cu film and oxide-derived Cu, at potentials relevant to the CO
reduction reaction in an alkaline electrolyte. Multiple oxide and hydroxide species exist on all Cu
surfaces at negative potentials, however, the speciation on the Cu foil is distinct from that on
micro/nanostructured Cu. The surface speciation is demonstrated to correlate with the initial degree
of oxidation of the Cu surface prior to the exposure to negative potentials. Combining reactivity and
spectroscopic results on these four types of Cu surfaces, we conclude that the oxygen containing
surface species identified by Raman spectroscopy are unlikely to be active in facilitating the
formation of C2+ oxygenates in the CO reduction reaction.
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Introduction
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Electrochemical CO2 and CO reduction reactions (CO2RR and CORR) are widely recognized
and extensively investigated as a potential strategy to use anthropogenic CO2 as an alternative
carbon source to produce fuels and chemicals and thus reduce its emissions to the atmosphere.1-4 Cu
is the only metal capable of directly converting CO2 and CO to valuable oxygenates and
hydrocarbons with high selectivities, and therefore much effort has been devoted into understanding
Cu-based catalysts.5-9 However, key aspects of the fundamental understanding that could enable
rapid future catalyst design and engineering remain unclear, among which two questions are at the
center of recent discussion in the literature:10-17 1) speciation of Cu surfaces, e.g., surface oxides,
hydroxides and subsurface oxygen, at conditions relevant to the CO2RR and the CORR; and 2) roles
of these surface species in promoting these two reactions. The difficulty in addressing the first
question largely lies in the challenge of characterizing the surface speciation at reaction conditions,
i.e., in the presence of electrolytes and at reducing electrode potentials. Since bulk Cu oxide and
hydroxide phases are known to be unstable at potentials relevant to the CO2RR and the CORR,18
oxygen-containing species are likely to be present only in the form of thin layers or patches of
surface species, if at all. Thus, only in situ/operando techniques with high interfacial specificity are
expected to yield reliable information. For example, any ex situ characterization involving ambient
exposure of Cu surfaces, regardless how fleeting, is unlike to produce conclusive results, owing to
the possibility of the surface oxidation by trace amounts of O2. This is because the open circuit
potential (OCP, ~0.65 V vs. RHE at pH = 11.7, all potentials in this work are referred to the
reversible hydrogen electrode, or RHE, scale) is typically sufficiently high to oxidize Cu with trace
amounts of O2.
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Moreover, no single in situ/operando technique is able to provide a complete picture of the
interface, and thus results from multiple such characterizations are needed. For example, our recent
work employed operando surface enhanced infrared spectroscopy (SEIRAS) to investigate the
surface sites available with CO as the probe molecule,19 and showed that oxide-derived Cu (OD-
Cu) preferentially exposed Cu(100) like facets key to the C-C coupling chemistry in the CORR.
Since surface metal-oxygen/carbon (M-O/M-C) vibrations are invisible to SEIRAS, if they exist at
the CORR conditions, their impact on reactivity cannot be assessed using this technique. This
specific point will be systematically investigated with in situ surface enhanced Raman spectroscopy
(SERS) in this work. An often overlooked fact in interpreting results from in situ/operando
experiments is that they often involve special sample preparation procedures and spectral cell
configurations, which could render the results less representative of the state of catalysts in the
reactivity studies. A good example is that most SEIRAS spectroelectrochemical cells are unstirred,
which could cause mass transport limitations and skew the spectral observations, as demonstrated
in our recent work.20 Another example is that the preparation of SEIRAS active Cu surfaces typically
involves chemically depositing a thin Cu layer on a Si surface,21-23 and whether this type of Cu
surface is representative of the surface of Cu foils and particles commonly used in reactivity studies
is implicitly assumed but remains unverified. Similar challenges exist for SERS when
electrochemically roughened surfaces are prepared to generate surface enhancing features.24 The
advent of shell isolated nanoparticles, e.g., silica encapsulated Au nanoparticles (Au@SiO2),25 as
inert surface signal enhancing agents provides an effective strategy to conduct SERS investigations
without the need to modify catalysts’ surface morphologies. Despite the challenges discussed
regarding the identification of surface species, understanding their roles in the reaction could be an
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even more daunting task. This requires comprehensive knowledge of the various species present at
the reaction conditions, and establishes quantitative correlations between densities of various
species and rates/selectivities. Meanwhile, it is less burdensome to show specific surface species to
be spectators in the reaction of interest by demonstrating the lack of consistent reactivity trend with
the absence/presence, absolute density or relative surface coverage of the species.
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In this work, we employ in situ SERS to investigate the speciation of five common Cu surfaces,
i.e., Cu foil, Cu microparticles (Cu MPs), Cu nanoparticles (Cu NPs), chemically deposited Cu film
on Si (Chem-Cu), and OD-Cu, and correlate the spectral observation with their CORR performance.
Multiple Cu oxide and hydroxide species are identified on all of the Cu surfaces investigated at
potential as negative as -0.8 V. The nature and relative abundance of surface species on Cu foil are
distinct from micro/nano-structured Cu surfaces. The difference in the surface speciation of Cu is
attributed to the initial oxidation state of the surface prior to the exposure to the reducing potential.
Combined with the complementary CORR reactivity results, we conclude that the presence of the
CuOx/(OH)y phase is unlikely the origin of OD-Cu’s unique ability to produce C2+ oxygenates.
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Results and Discussion
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Figure 1. (a) In situ Raman spectra and (b) in situ SHINER spectra of a Cu foil at indicated potentials in CO saturated
KOH/K2SO4 electrolyte (0.01 M KOH and 0.045 M K2SO4, pH=11.7). The spectra were collected on a cathodic
sequence of potential steps from OCP to –0.8 V in 0.1 V intervals.
In situ Raman spectroscopy shows that Cu foils do not possess the surface enhancement effect,
and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) is necessary to detect
surface species at different electrode potentials. Raman spectra on a Cu foil (Figures 1a and S1a)
show no feature in the cathodic potential steps from the OCP (~0.65 V) to -0.8 V in the CO
atmosphere, except for a band at 980 cm-1 attributable to the sulfate ion in the bulk electrolyte.26
The lack of spectral feature of surface species on the bare Cu foil could be explained by the
following two possible reasons: 1) no Raman-active surface species is present at the Cu surface in
the entire potential window investigated in this work; and 2) the Cu foil does not possess the
capability of enhancing the Raman signal at the surface, or enabling SERS. The first possibility
contradicts the common knowledge that Cu surfaces possess a native oxide layer upon exposure to
ambient air, which has been confirmed in multiple works.27-29 The relatively flat surface of Cu foil
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(Figure S2a) supports the second explanation because the SERS effect is typically generated on
rough surfaces, especially at hot spots such as tips, edges and gaps between two nanoparticles.30-32
To test this hypothesis, SERS substrates Au@SiO2 nanoparticles are introduced to the Cu foil
surface to enable SHINERS (Figures 1b and S1b).25 Au@SiO2 particles are synthesized according
to a reported method33 (Figure S2b, inset), with an Au core (50 ± 5 nm in diameter) and a SiO2 shell
(2.0 ± 1.0 nm in thickness). Unless otherwise noted, the electrolyte used throughout the
spectroscopic study is composed of 0.01 M KOH and 0.045 M K2SO4 (pH=11.7), which ensures a
reasonable ionic conductivity and minimizes the damage of the SiO2 shell of Au@SiO2 in the
strongly alkaline conditions. No vibrational band attributable to CO adsorbed on Au (~2100 cm-1)
is observed in any experiment reported in this work, suggesting the absence of exposed Au surface
on Au@SiO2.34 Indeed, in situ SHINERS on the Cu foil reveals multiple surface species present at
various potentials (Figure 1b). At the OCP, two broad bands at ~490 and ~600 cm-1 attributable to
the surface Cu oxide phase are present. These spectral features do not match any stoichiometric Cu
oxide phases, i.e., CuO and Cu2O,16 but are similar to those partially reduced Cu2O phase (see
below). These bands persist to as low as -0.1 V, and are assigned to Cu2O1-x. The presence of these
bands shows that the utilization of Au@SiO2 enables the selective detection of surface species.
SHINERS results of Cu foil at reducing potentials relevant to the CORR show that Cu(OH)x
and CuOx species persist at as low as -0.8 V. A peak centered at 612 cm-1 appears at 0.3 V on the Cu
foil, which grows in intensity and blueshifts to 620 cm-1 as the potential decreases to -0.1 V (Figure
1b). Its intensity attenuates and the peak position shifts to 622 cm-1 as the potential further decreases
to -0.8 V. This Raman band is assigned to a Cu–Oad stretching mode rather than a Cu–OH mode,
because of the lack of shift when replacing H2O with D2O (Figure S3). This is also consistent with
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a recent study assigning the Raman bands at 570-630 cm-1 on Cu(111) to the Cu–Oad species.26 The
broad band located between 800 and 1000 cm-1 appears to vary concurrently with the band at 612-
622 cm-1, which likely originates from the same surface species. In addition, a new peak at 525 cm-
1, which has been previously assigned to a Cu–OH mode,26, 35 arises and the intensity reaches its
maximum at -0.4 V. This band also blueshifts as the potential becoming more negative, and largely
disappears at potentials < -0.6 V. This band only redshifts 7 cm-1 in D2O as compared to in H2O,
which differs from a ~30 cm-1 redshift observed for a band at a similar location in a previous report.26
Therefore, we tentatively attribute this band to the mixed Cu oxide and hydroxide species as
CuOx/(OH)y, which will be discussed further later in this work. The band centered at 390 cm-1
appears at the same potential as the band at 525 cm-1 and the intensities of the two bands vary in
sync, indicating a shared origin. A weak band centered at 700 cm-1 that appears between -0.1 V and
-0.4 V has also been assigned to surface hydroxyl species, referred to as Cu(OH)x.26, 36-37 A band
attributable to linearly bonded CO appears in the range of 2053-2070 cm-1 between 0.1 and -0.2 V
(Figure S1b). Another band at 1863-1873 cm-1 corresponding to bridge-bonded CO appears at more
negative potential accompanied by the disappearance of the linearly bonded CO (Figure S1b). These
observations are largely in line with previous Raman studies on Cu.15, 22 It is worth noting that the
adsorbed CO band only appears after the Cu2O1-x phase is reduced, suggesting that CO adsorption
is not feasible on partially reduced oxide species layer of the Cu surface. We note that only peaks
associated with metallic Cu are observed in the XRD pattern of the Cu foil (Figure S4), indicating
the lack of long-range ordered crystalline phases of oxide or hydroxide on the surface.
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While the main spectral features of the Cu foil are generally consistent, there are considerable
variations in the relative intensities of spectral features depending on the locations at which spectra
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are collected. Raman spectra in this work are acquired with a confocal microscope with a focal spot
of ~2 µm in diameter, which is common in similar studies.25, 38 Thus, the spectra collected only
reflect the state of the part of the surface sampled. To probe the spatial variation of the Cu foil at the
same potential, 10 spots are sampled at each potential on the Cu foil (Figure S5). Although the
general features in the spectra are similar, the relative intensities of individual bands do vary, which
is likely caused by the inhomogeneity of the surface rather than the distribution of Au@SiO2
particles. The distribution of Au@SiO2 particles on the Cu foil is generally uniform (Figure S2b),
and the minor aggregation of Au@SiO2 particles should only impact the intensity, rather than the
lineshape of the observed Raman features. The average distance between Au@SiO2 particles is
much smaller than 2 µm, which indicates the SERS signal obtained will be an average enhancing
effect induced by the majority of isolated and minority of aggregated Au@SiO2 particles. A similar
level of spectral variations is observed on micro/nano-structured Cu samples that do not require
Au@SiO2 particles for SERS, confirming that the spectral variation is due the surface
inhomogeneity. These spatial variations in Raman spectra on Cu surfaces could at least in part
explain different spectral features reported in the literature.26 Therefore, it is important to collect
SERS spectra at multiple locations to ensure the spectral features are not caused by anomalies at the
specific area of the surface. Raman spectra are collected at multiple locations (³3) on all samples
and at all potentials investigated in this work. We note that the pre-treatment with HCl39-40 makes
spectra collected on the Cu foil surface more uniform than those on the sample without the treatment
at any given potential (Figure S6). The absence of Cu-Cl vibrational band (270-290 cm-1)37, 41 in
Figure 1 and the control experiment with Cl- containing electrolyte both indicate that the presence
of Cl- in HCl in the pretreatment does not impact the spectral feature.
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The presence of CO impacts the surface speciation on the Cu foil at reducing potentials. Only
a band at 709 cm-1 attributable to Cu(OH)x is observed on the Cu foil (with Au@SiO2) at potentials
between 0.1 V to -0.4 V in Ar saturated electrolyte (Figure S7), which is consistent with a recent
report.26 The clear difference in the spectra at reducing potentials on the Cu foil in CO and Ar is in
sharp contrast with the similar cyclic voltammograms on Cu in both atmospheres (Figure S8). This
indicates that cyclic voltammetry is not sufficiently sensitive to differentiate subtle changes in the
speciation of the Cu foil surface caused by the presence of CO. Adsorbed CO appears to induce the
formation of and/or stabilize the surface Cu(OH)x and CuOx phases on the Cu foil at reducing
potentials.
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Figure 2. Potential-dependent Raman spectra of (a) Cu MPs, (b) Chem-Cu, and (c) OD-Cu in CO saturated
KOH/K2SO4 electrolyte. The spectra were collected during a cathodic sequence of potential steps from the OCP to
–0.8 V vs. RHE.
In addition to Cu foil, in-situ Raman spectroscopy is also employed to probe the surface
speciation of micro/nano-structured Cu catalysts frequently used in the CO2RR and the CORR
research, i.e., Cu microparticles (Cu MPs, 0.1-2 μm), Cu nanoparticles (Cu NPs, 25-100 nm),
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chemically deposited Cu (Chem-Cu) on Si and oxide-derived Cu (OD-Cu). Cu MPs and NPs are
purchased and used without further treatment, as in several recent studies.42-43 These different types
of Cu surfaces are commonly used in the CO2RR and the CORR in both batch and flow cell
configurations due to their ease of integration with gas diffusion layers.43-44 Chem-Cu is chemically
deposited on the Si surface, which is frequently used in SEIRAS investigations of the CO2RR and
the CORR.19-22 OD-Cu is known for its ability to produce multi-carbon products at lower
overpotential than untreated Cu surfaces.44-45 Here, OD-Cu is prepared by electrochemically
reducing a Cu2O layer on a Cu substrate. Detailed preparation procedures and representative
morphologies of samples are provided in the Supporting Information (Figure S9). As expected, Cu
foil, MPs, NPs, Chem-Cu and OD-Cu all exhibit characteristic XRD patterns of polycrystalline Cu
(Figure S4). Peaks corresponding to Cu2O appear on the XRD pattern of Cu NPs, which is likely
due to the propensity of nanoparticles for oxidation when exposed to air. The cyclic voltammograms
(CVs) of these Cu surfaces show similar general redox features (Figures S8 and S10), which are
consistent with literature reports.37, 46
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Although micro/nano-structured Cu, as well as Cu foil, are generally believed to exhibit similar
polycrystalline Cu surfaces, Raman spectra of micro/nano-structured Cu at negative potentials show
distinct features from those of the Cu foil. Cu MPs, NPs, Chem-Cu and OD-Cu all exhibit surface
enhancement effect for Raman spectroscopy, and Au@SiO2 is not necessary to observe Raman
features of surface species (Figures 2, S11 and S12), as suggested in several recent works.15, 17, 47-48
This is expected as the particulate morphology has been shown to be SERS active.49 Control
experiments show that the Raman spectra on these samples with and without Au@SiO2 are identical
within the experimental errors (Figures 2a and S13). Thus, it is reasonable to conclude that Raman
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spectra of Cu MPs, NPs, Chem-Cu and OD-Cu without Au@SiO2 do not selectively enhance the
Raman signal of a subset of surface species because all Raman active surface species should be
probed with Au@SiO2 in SHINERS. At the OCP, multiple peaks attributable to surface Cu2O at 141,
222, 415, 525, and 613 cm-1 are present on the surface Cu MPs (Figure 2a), which agree with the
bands observed on the electrodeposited Cu2O film, the precursor of OD-Cu (Figure 2c), and are also
consistent with the literature.26 Comparing to the Cu2O1-x species present on the Cu foil at the OCP
(Figure 1b), the bands attributable to surface Cu2O are more well defined on Cu MPs, suggesting a
higher degree of surface oxidation (Figure 2a). This is not surprising because the density of
undercoordinated surface atoms, which are more susceptible to oxidation upon air exposure,
increases as the particle size decreases. The appearance of XRD peaks corresponding to Cu2O on
Cu NPs, which are absent in other forms of Cu surfaces investigated (Figure S4), supports this line
of reasoning. The surface Cu2O phase on Cu MPs persists until -0.1 V in the CO atmosphere, and
all corresponding Raman features disappear at -0.2 V or below, indicating its full reduction. This is
consistent with Soriaga’s work which shows reduction of native oxide at -0.9 V vs. SHE in KOH
(~-0.2 V vs. RHE).50 Cu MPs and foil show distinct Raman features at potentials below -0.2 V,
suggesting differences in the distribution of surface species. At -0.2 V, three well defined bands
appear simultaneously at 392, 524 and 613 cm-1 on Cu MPs. Importantly, the relative intensity of
524 and 613 cm-1 bands on the Cu MPs is the opposite of that on the Cu foil, with the former as the
dominant feature. When conducting in D2O instead of H2O, the peaks at 392 and 530 cm-1 under -
0.4 V exhibit redshifts to 385 and 523 cm-1, respectively (Figure S14), indicating that both bands
correspond to normal modes involving at least one proton. The smaller redshift (7 cm-1) than that
reported previously (30 cm-1) for Cu hydroxide species indicate that there might be surface adsorbed
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oxygen, which plays a constructive role in the bending the H atoms, near the hydroxyl group.32, 51
Such co-adsorbed species is likely a mixed Cu oxide and hydroxide species, i.e., CuOx/(OH)y,
similar to that observed on the Cu foil. A similar assignment of these bands at 527/523 cm-1 to a
patina-like mixed phase containing oxides and/or hydroxide and/or carbonate has been suggested in
the literature.27 Meanwhile, the band at 613 cm-1 displays a negligible shift in D2O, which confirms
its assignment of Cu–Oad species as that on Cu foil (Figure 1b).16 All peaks assignments and the
comparison with reported literatures are summarized in Table S1. Only bridge-bonded CO at 1857-
1870 cm-1 is detected at potentials below -0.1 V (Figure S11a). Similar to the case of Cu foil, it can
be inferred that at potentials > -0.2 V, the presence of oxide phases on Cu MPs prevents the
adsorption of CO. This is supported by the observation that adsorbed CO persists up to 0.2 V in the
anodic potential step due to the lack of the native oxide (Figure S15). Based on the relative
intensities of the Raman bands in the same spectra, CuOx/(OH)y is more abundant on Cu MPs than
foil at potentials below -0.1 V, while the opposite is true for Cu–Oad. We note that the relative
intensities of Raman bands cannot be used to infer absolute coverages of surface species due to the
unknown scattering cross sections for different modes, i.e., the coverage of CuOx/(OH)y could be
lower than that of Cu–Oad even though its Raman band is more intense or vice versa. In contrast to
the results on the Cu foil, Raman spectra on Cu MPs at reducing potentials in CO and Ar saturated
electrolytes are similar (Figures 2a and Figure S16), suggesting that the presence of CO does not
impact the surface speciation on Cu MPs. Spectral features of Chem-Cu and Cu NPs (Figures 2b
and S12) in the potential range of OCP to -0.8 V are similar to those of Cu MPs, suggesting the
presence of similar surface species. This suggests that micro/nano-structured Cu share similar
surface species at potentials relevant to the CORR. Upon reduction, the Raman spectra of the
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electrodeposited Cu2O layer, i.e., OD-Cu, also resemble those from other micro/nano-structured Cu
materials (Figure 2c). A common feature of micro/nano-structured Cu is that there is a relatively
well-defined surface Cu2O layer at the OCP, while the oxide layer of the Cu foil at the OCP is
composed of a more reduced oxide phase (Cu2O1-x). Thus, we hypothesize that the initial state of
the Cu catalyst has a substantial impact on the surface speciation at reducing potentials.
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Figure 3. (a) Square wave potential profile applied on Cu foil electrode in CO saturated KOH/K2SO4 electrolyte. (b)
In situ Raman spectra of Cu foil collected after 5 mins holding at each oxidative/reduced potential presented in a. (c)
Corresponding Raman spectra of CO adsorption on Cu foil at each potential shown in a.
To investigate the impact of the surface oxidation state of Cu catalysts prior to the exposure to
reducing potentials on its speciation at negative potentials, in situ Raman spectra are collected with
a square wave potential profile. During this process, a Cu foil is exposed to increasingly positive
potentials for 5 min and followed by a cathodic step to -0.4 V for 5 min (redox currents typically
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reach a stable value within 5 min, Figure 3a). Raman spectra are collected at the end of each positive
potential segment and -0.4 V (Figure 3b and c). No Au@SiO2 is introduced to the Cu foil in this
experiment, and the appearance of Raman features reflects the development of nanostructures that
enable the surface enhancement effect in the redox treatment. Consistent with the results in Figure
1, no discernable Raman signal of surface species is observed during the potential hold at the OCP
and the subsequent hold at -0.4 V presumably due to the lack of SERS activity of the flat surface of
the newly polished and HCl treated Cu foil. Similarly, no Raman signal is present at the end of the
5 min hold under 0.7 V, but weak bands corresponding to CuOx/(OH)y appear at the ensuing
potential hold at -0.4 V. This is likely due to the roughening of the Cu surface in the redox cycle,
which leads to the formation of SERS active microstructures.52 We note that the morphology of Cu
foil treated at up to 1.2 V observed by SEM does not exhibit discernable difference compared to the
untreated one (Figure S17), suggesting the surface enhancing structure is either nanometer in
dimension (beyond the spatial resolution of SEM) or the features generated at negative potentials
are destroyed by upon air exposure prior to the SEM measurements. The most intense Raman feature
of the Cu foil after exposure to oxidizing potentials (>0.7 V) corresponds to CuOx/(OH)y, rather
than CuOx (Figure 1b). This is a clear indication that the exposure to oxidizing potentials, or the
redox history of the catalyst, could impact the nature and distribution of surface species present at
reducing conditions. Weak and broad bands corresponding to Cu2O1-x are present at the end of the
subsequent potential hold at 0.8 V, which is likely caused by the increase in the coverage of Cu2O1-
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after the potential hold and the enhanced SERS effect with the roughened surface. This trend is
x
more obvious after the potential hold at 0.9 V, in which strong bands corresponding to the Cu2O1-x
phase are present, and intense bands at 390, 536 and 613 cm-1 are observed after the subsequent
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potential hold at -0.4 V. These bands are similar to those observed on Cu MPs, NPs, Chem-Cu and
OD-Cu at the same potential, supporting the hypothesis that the key difference between the surface
speciation at reducing potentials on Cu foil and micro/nano-structured Cu is the surface oxidation
state prior to the reduction. Only bridge-bonded CO band (1885-1893 cm-1) is detected after the Cu
foil is exposed to potentials above 0.7 V, which is likely due to the enhancement of SERS effect
upon the exposure to higher potential. Further increasing the oxidizing potential above 0.9 V, the
intensity of the Cu2O1-x phase bands at positive potential and also the CuOx/(OH)y species at -0.4 V
are largely leveled off. Regardless, the spectral features at -0.4 V in all cases resemble those of
micro/nano-structured Cu. Since electrochemical redox cycling leads to higher CO2RR and CORR
reactivity and selectivity to C2+ products,53 the changed surface species at reducing potentials, e.g.,
OH bound to Cu surface, has been proposed to benefit the reduction of CO2 and the production of
C2+ products.48 A similar experiment with the same potential profile on Cu MPs shows little change
in the Raman spectra collected at the end of the potential hold at high and low potentials, suggesting
that the redox cycling does not modify the surface speciation of Cu MPs (Figure S18). This is likely
due to the surface of Cu MPs upon air exposure is sufficiently oxidized to induce the formation of
the CuOx/(OH)y species at reducing potentials. The dominant surface species on each catalyst
observed from in situ Raman are concluded in Table 1.
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Table 1. Major surface species on Cu at indicated potentials identified by Raman spectroscopy
Cu catalysts
Cu foil
OCP
-0.4 V (vs. RHE)
CuOx
Cu2O1-x
Cu2O1-x
Cu foil after oxidized at +0.9 V
CuOx/(OH)y
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Cu micro/nanoparticles
Chem-Cu
OD-Cu
Cu2O
CuOx/(OH)y
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Figure 4. The comparison of product Faradaic efficiency between CORR on Cu foil (brown), Cu MPs (grey), Chem-
Cu (green), Cu NPs (orange) and OD-Cu (blue) in CO sat. 0.1 M KOH under -0.6 V vs. RHE.
In order to correlate the species present on Cu catalysts at reducing potentials with the CORR
performance, the CORR performance of all Cu surfaces investigated is evaluated in a batch cell. Cu
foil, MPs, NPs, Chem-Cu and OD-Cu are employed as working electrodes in CO saturated 0.1 M
KOH at -0.6 V vs. RHE to highlight the impact of catalysts on the yields of C2+ products. Cu foil
shows the worst CORR performance with the highest Faradaic efficiency (FE) for the side product
H2 (84.6%), and the lowest FE for ethylene (0.68%, brown bars in Figure 4). More ethylene is
produced on the surface of Cu MPs (2.03%, grey) and Chem-Cu (3.78%, green) than on the bare
Cu foil, however, without any detectable amount of liquid product. OD-Cu, in contrast, could
produce more ethylene (5.64%) as well as the liquid products of ethanol and n-propanol with the
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FEs of 4.34% and 0.41%, respectively (blue bars in Figure 4), which is consistent with the literature
and suggests the reduction of overpotential needed for multi-carbon products.54-56 Cu NPs’ product
distribution is close to the average of that of Cu MPs and OD-Cu, which is not surprising as Cu NPs
are more deeply oxidized than Cu MPs but less so than Cu2O (precursor to OD-Cu) before exposing
to the reducing condition in the CORR. Cu MPs, NPs and OD-Cu evaluated in the flow-cell
configuration, in which the gaseous reactant is directly fed to the electrode-electrolyte interface to
form a triple-phase boundary, also show distinct product distributions (Figure S19), especially at
low overpotentials (³ -0.6 V vs. RHE). At -0.6 V, the combined FE for C2+ products on these three
catalysts parallels that of batch results: OD-Cu > Cu NPs > Cu MPs. All samples tested in the flow
cell show much higher FEs to the CORR products than in the batch cell, which is expected with the
improved mass transport of CO to the triple-phase boundary. We note that the OD-Cu used in the
flow cell experiments is prepared by a different procedure due to the need to deposit on the Cu
support, but previous reports have shown they show similar reactivities.44, 54
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It is noteworthy that although Cu MPs, NPs, Chem-Cu and OD-Cu have similar Raman
features for CuOx/(OH)y species under -0.6 V (Figures 2 and S12), OD-Cu is unique in its ability to
produce C2+ oxygenates at this potential (Figure 4, blue). This is a strong indication that the surface
CuOx/(OH)y species is not active in facilitating the formation of multi-carbon oxygenates in the
CORR. Meanwhile, all micro/nano-structured Cu samples exhibit a higher FE for ethylene than that
on Cu foil, suggesting a potential promotional role of this phase in the ethylene formation. Following
the same logic, the surface CuOx species on Cu foil (Figure 1b) is also unlikely to be active in the
CORR. Since SERS is not a quantitative technique, by itself it is insufficient to establish a
quantitative correlation between the density or surface coverage of a species with the production
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rate of one or a group of products expected of rigorous structure-activity relations. However, the
criterion for the proof against a species as the active phase for a reaction is demanding, i.e.,
inconsistency between experimental results and any key predictions based on the assumption of this
species being the active phase would suffice. The relative intensities between bands corresponding
to CuOx/(OH)y and CuOx species on all micro/nano-structured Cu catalysts in the same spectra at
potentials relevant to the CORR are similar (Figure 2), suggesting the relative abundances of these
two species across all micro/nano-structured Cu surfaces under reaction conditions are comparable.
While SERS results do not allow for the determination of absolute densities of these surface species,
it is clear that variations in the site density alone cannot account for the different product
distributions observed in both batch and flow reactors (Figures 4 and S19). In addition, despite the
marked difference in the relative abundance of CuOx/(OH)y and CuOx species (Figures 1b and 2a),
both Cu foil and Cu MPs produce ethylene as the only C-C coupling product. The slightly higher
FE for ethylene on Cu MPs than that of Cu foil could be reasonably attributed to its higher surface
area. Thus, the relative abundance of these two observed oxygen-containing surface Cu species does
not appear to impact the production of oxygenated C-C coupling products.
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The combination of the spectroscopic and reactivity results suggest that oxygen-containing
species do exist on Cu catalysts at potentials relevant to the CORR, but do not appear to play any
major role in facilitating C-C coupling pathways. It follows that the composition of these species is
not diagnostic of other structure changes induced by the redox retreatment at varying conditions that
directly impact the CORR performance. It is worth noting that these conclusions drawn from in situ
SERS, as for any other techniques, investigations are limited by the detection limit of this technique.
It is known that trace amounts of additives or contaminants could drastically change the rate and
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product distribution of catalysts,57-58 and the possibility of trace oxygen containing Cu species that
exist below the detection limit of SERS playing a decisive role in the CORR, though remote, cannot
be ruled out completely. In addition, although no appreciable change in the spectral feature at -0.6
– -0.8 V is observed, the typical time scale of the spectroscopic experiments (~1 h) does not allow
for claims regarding the long term stability of oxygen-containing Cu species at the reducing
conditions. Our recent SEIRAS work showed that the CO adsorption sites are markedly different
on Cu MPs and OD-Cu surfaces.19 Combining the Raman results in this work and previous SEIRAS
observations19, the preferential exposure favorable metallic facets, e.g., Cu(100), is likely
responsible for OD-Cu’s unique CORR activity.
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Conclusion
In summary, in situ SERS investigations show that surface oxygen-containing species indeed exist
in the alkaline electrolyte at potentials relevant to the CORR on all types of Cu surfaces investigated
in this work. However, the relative abundance of surface CuOx on Cu foils is higher than that on
micro/nano-structured Cu, while the opposite is true for the surface CuOx/(OH)y species. The
presence of CO is necessary to stabilize surface oxide/hydroxide species on the Cu foil but not for
micro/nano-structured Cu. The difference in the surface speciation on different types of Cu surfaces
is demonstrated to be correlated to the initial degree of oxidation of the Cu surface prior to reduction.
Combined with reactivity results, we conclude that while CuOx and CuOx/(OH)y species do exist
under the CORR conditions, but they are unlikely to be the active sites facilitating the formation of
C2 oxygenates.
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Supporting Information
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Experimental details and supporting data.
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Acknowledgements
Y.Z. and L.T. acknowledge the support of the University of Delaware startup fund. X.C., A.M. and
B.X. acknowledge the support of the National Science Foundation CAREER Program (Award No.
CBET-1651625). F.J. thanks the National Science Foundation for financial support (Award No.
CBET-1904966). The authors also thank Matthew Jouny for help with the reactivity tests in the flow
cell. Authors thank GaossUnion (Tianjin) Photoelectric Technology Company for sharing the design
of the in situ Raman cell.
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