Boosting CO2-To-CO conversion on a robust single-Atom copper decorated carbon catalyst by enhancing intermediate binding strength

Article abstract of DOI:10.1039/d0ta08496d

The ability to manipulate the binding strengths of intermediates on a catalyst is extremely challenging but essential for active and selective CO2 electroreduction (CO2RR). Single-Atom copper anchored on a nitrogenated carbon (Cu-N-C) structure is still rarely unexplored for efficient CO production. Herein, we demonstrate a plausible hydrogen-bonding promoted strategy that significantly enhances the ?COOH adsorption and facilitates the ?CO desorption on a Cu-N-C catalyst. The as-prepared Cu-N-C catalyst with Cu-N3 coordination achieves a high CO faradaic efficiency (FE) of 98% at-0.67 V (vs. reversible hydrogen electrode) as well as superior stability (FE remains above 90% over 20 h). Notably, in a three-phase flow cell configuration, a remarkable CO2 to CO FE of 99% at-0.67 V accompanying a large CO partial current density of 131.1 mA cm-2 at-1.17 V was observed. Density functional theory calculations reveal that the Cu-N3 coordination is potentially stabilized by an extended carbon plane with six nitrogen vacancies, while three unoccupied N sites are spontaneously saturated by protons during the CO2RR. Therefore, the hydrogen bonds formed between the adsorbed ?COOH and adjacent protons significantly reduce the energy barrier of ?COOH formation. After the first proton-coupled electron transfer process, the adsorbed ?CO species are easily released to boost the CO production.

Full text of DOI:10.1039/d0ta08496d

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Number 12
March 2018
Pages 4883-5230
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Zhenhai Wen et al.
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Boosting CO -to-CO Conversion on a Robust Single-Atom
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Copper Decorated Carbon Catalyst by Enhancing Intermediate
Binding Strength
a ǂ
a ǂ
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Shixia Chen, Yuewei Li, Zhuogang Bu, Fangqi Yang, Junhui Luo, Qizheng An,
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Zheling Zeng , Jun Wang, * and Shuguang Deng *
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aSchool of Resources Environmental & Chemical Engineering, Nanchang University,
Nanchang, 330031, China
bSchool of Material Science and Engineering and Institute for Advanced Study,
Nanchang University, Nanchang, 330031 P. R. China
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cSchool for Engineering of Matter, Transport and Energy, Arizona State University,
551 E. Tyler Mall, Tempe, AZ 85287, USA
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These authors contributed equally to this work.
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*Corresponding authors:
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1. E-mail: shuguang.deng@asu.edu (S. Deng)
2. E-mail: jwang7@ncu.edu.cn (J. Wang)
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Abstract: The ability to manipulate the binding strengths of intermediates on a catalyst
is extremely challenging but essential for active and selective CO electroreduction
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(CO RR). Single-atom copper anchored on nitrogenated carbon (Cu-N-C) structure is
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still rarely unexplored for efficient CO production. Herein, we demonstrate a plausible
hydrogen-bonding promoted strategy that significantly enhances the *COOH
adsorption and facilitates the *CO desorption on a Cu-N-C catalyst. The as-prepared
Cu-N-C catalyst with Cu-N coordination achieves a high CO Faradaic efficiency (FE)
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of 98% at -0.67 V (vs reversible hydrogen electrode) as well as superior stability (FE
sustains above 90% over 20 h). Notably, in the three-phase flow cell configuration, a
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remarkable CO to CO FE of 99% at -0.67 V accompanying a large CO partial current
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density of 131.1 mA cm-2 at -1.17 V was observed. Density functional theory
calculations reveal that the Cu-N coordination is potentially stabilized by an extended
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carbon plane with six nitrogen vacancies, while, three unoccupied N sites are
spontaneously saturated by protons during CO RR. Therefore, the hydrogen-bonds
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formed between the adsorbed *COOH and adjacent protons significantly reduce the
energy barrier of *COOH formation. After the first proton-coupled electron transfer
process, the adsorbed *CO species are easily released to boost the CO production.
Keywords: CO electroreduction; single Cu atoms; CO production; binding strength;
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flow cell
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Introduction
The ever-growing carbon dioxide (CO ) level caused by the excessive use of fossil
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fuels has led to serious environmental problems.1,2 On the other hand, CO is a non-
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toxic, inexpensive, and abundant C1 resource for the production of various chemicals.3
The electrochemical reduction of carbon dioxide (CO RR) has been considered as a
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green, facile, and economical strategy to produce high-value two-electron reduction
product carbon monoxide (CO) due to the superior selectivity and yield compared with
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those of other CO reduction pathways. Thus, efficient CO -to-CO conversions
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catalysts with excellent activity and stability are highly desired, which is a promising
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route to reduce CO emissions and achieve a carbon-neutral cycle.
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Manipulating the binding strength of intermediates on catalysts is of great
importance to design highly active and selective CO RR electrocatalysts for CO to CO
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conversion, especially for emerging single metal ions coordinated N-doped carbon (M-
N-C) materials.7-12 Among the reported M-N-C catalysts, Fe-N-C could only maintain
high CO FEs (above 90%) within a narrow operation potential window of 0.2 V (all
potentials are indicated vs. RHE afterward) due to the strong *CO adsorption on Fe-N
sites.13-15 Ni-N-C exhibited high CO FEs at relatively high operation potentials (-0.6 ~
-1.2 V) owing to the weak binding strength of *COOH on the Ni-N sites.16-18 In contrast,
Cu-N-C catalysts have rarely been reported for efficient CO production, instead, they
tend to produce deeply reduced products. As exemplified by the typical Cu-N4
structures, the methanol production with a FE of 44% at -0.9 V and ethanol production
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with a FE of 55% at -1.2 V have been reported.19,20 Note that instead of spontaneous
desorption of the *CO species, a positive free-energy barrier (0.12 V) of *CO
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desorption on CuN coordination was observed. Thus they prefer to be further reduced
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at relatively high overpotentials. Very recently, our group reported the atomically
dispersed Cu-N moieties for effect CO production with a FE of 92% at -0.7 V.21
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Nevertheless, the efficient CO -to-CO conversion on a Cu-N-C electrocatalyst in
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aqueous solutions is still largely unexplored so far.
Herein, we demonstrate a hydrogen-bond promoted strategy to manipulate the
binding strengths of intermediates to achieve an efficient CO -to-CO conversion on Cu-
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N-C. The high active atomic interface enables a high CO FE of 98% at a relatively low
overpotential of -0.67 V with long-term stability over 20 h continuous electroreduction
process. Meanwhile, by applying Cu-N-C catalyst in a three-phase flow cell, a
remarkable CO FE of 99% was realized at -0.67 V with a large CO partial current
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density of 131.3 mA cm at -1.17 V. Density functional theory (DFT) calculations
reveal that the single Cu atoms are stabilized by a unique proton-saturated Cu-N6
coordination with three proton-saturated N sites and one Cu-N coordination. Notably,
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the hydrogen-bond formed between the O atom of *COOH and the H atom of adjacent
H-saturated N site significantly reduces the energy barrier of *COOH formation.
Meanwhile, the *CO species are easily released to boost the CO production after the
first proton-coupled electron transfer process. This work provides new insights into the
mechanism of CO -to-CO conversion on a Cu-N-C structure at the atomic level.
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Results and Discussion
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Fabrication and characterization of Cu-N-C catalyst
The schematic fabrication procedures of the Cu-N-C catalyst are illustrated in Figure
1a and the detailed synthesis methods are provided in the Supporting Information.
Briefly, Cu and Zn co-doped ZIF-8 (Cu/Zn-ZIF-8) with a Cu/Zn molar ratio of 1:8 is
prepared via a hydrothermal method. After calcination at 1000 ℃ in the Ar atmosphere,
the evaporation of Zn species at 907 ℃ creates abundant micropores and Cu atoms are
stabilized by N coordination instead of agglomeration. For comparison, nitrogen-doped
carbon (N-C) without Cu doping was prepared from pristine ZIF-8 in the same way.
The scanning electron microscopy (SEM) images of Cu/Zn-ZIF-8 and ZIF-8 showed
the regular dodecahedron morphology with a uniform size of 2~4 μm (Figure S1). After
carbonization, the dodecahedron morphology could be retained on Cu-N-C and N-C
with a slightly reduced particle size (Figures 1b and S2). Transmission electron
microscopy (TEM) and high-resolution TEM (HR-TEM) showed that no Cu
nanoparticles or clusters were observed on Cu-N-C (Figures 1c and 1d). The
corresponding selected area electron diffraction (SAED) patterns showed a typical
character of disordered carbon substrate (Figure 1c insert). The high-angle annular
dark-field scanning transmission electron microscopy (HAADF-STEM) with a
spherical aberration corrector was applied to identify the single Cu atoms. As shown in
Figures 1e and 1f, the bright dots that are encased in yellow dashed circles are
recognized as the isolated and well-dispersed Cu atoms. The HR-TEM and HAADF-
STEM images from different regions confirm the homogeneous distribution of isolated
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Cu atoms (Figure S3). Besides, energy-dispersive X-ray spectroscopy (EDS) mappings
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(Figure 1g) revealed the homogeneous distributions of Cu and N elements throughout
the carbon matrix. The carbon matrix primarily provides the substance to well disperse
the Cu atoms to create single Cu active sites, meanwhile, the high conductivity and
large surface area will facilitate the electron transport and boost the mass transfer during
the electrocatalytic process. The Cu content on Cu-N-C was determined to be 0.61 wt.%
by inductively coupled plasma mass spectrometer (ICP-MS).
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Figure 1. (a) Schematic illustration of Cu-N-C catalyst synthesis; (b) SEM, (c) TEM,
(d) HR-TEM, and (e, f) HAADF-STEM images of Cu-N-C; (g) The corresponding EDS
mappings of C, Cu, and N elements.
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X-ray diffraction (XRD) patterns of Cu/Zn-ZIF-8 and ZIF-8 (Figure S4) matched
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well with the simulated pattern, indicating that the impregnation of Cu will not change
the topology of the ZIF-8 framework. In contrast, two broad diffraction peaks were
observed in the XRD patterns of Cu-N-C and N-C (Figure S5), which could be assigned
to the (002) and (100) planes of amorphous carbon planes, suggesting the complete
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transformation from ZIF-8 and Cu/Zn-ZIF-8 to N-C and Cu-N-C. Meanwhile, the
absence of Cu-based diffraction peaks coincides well with the HR-TEM and HAADF-
STEM observations. Raman spectra exhibited two peaks correspond to the D (1350 cm-
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) and G bands (1580 cm ), which represents the vibration of sp dangling and in-plane
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sp carbons, respectively (Figure S6). The chemical compositions of the obtained
samples were investigated by X-ray photoelectron spectroscopy (XPS) as depicted in
Figure S7, and the detailed C, N, and O contents are summarized in Table S2. The Cu
content is measured to be 0.12 at.% and no other metal contaminants are observed. The
XPS N1s spectra of Cu-N-C further confirmed the considerable presence of M-N-C
moieties (Figure 2a). During the high-temperature annealing, the N-5 species in ZIF-8
will evolve to various N species due to the graphitization and oxidation processes.7,8
The high-resolution N spectra of Cu-N-C and N-C could be deconvoluted into pyridinic
N (398.5±0.1 eV), pyrrolic N (401.1±0.1 eV), and graphitic N (402.3±0.2 eV).24-26
Notably, the emerging Cu-N peak located at 399.0±0.2 eV on Cu-N-C demonstrates the
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formed Cu-N coordination. In Figure S7b, the high-resolution Cu 2p peak at 933.7
eV indicates the valence state of Cu is between 0 and +2 on Cu-N-C. 28
The specific Brunauer-Emmett-Teller (BET) surface area of Cu-N-C and N-C
catalysts were probed by the N sorption isotherms at 77 K (Figure S8a). The steep N
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adsorption at low-pressure range manifests their microporous nature. The BET specific
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surface area is calculated to be 845 and 858 m g for Cu-N-C and N-C, respectively.
The pore size distributions are also very similar, with dominant pores centered at 0.78
and 0.77 nm on Cu-N-C and N-C, respectively (Figures S8b and S8c). The CO2
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adsorption capacity at 298 K was measured to be 3.48 and 3.37 mmol g on Cu-N-C
and N-C, respectively (Figure S9). The almost identical CO capacities indicate that the
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isolated Cu atoms showed limited effects on the CO adsorption affinity, which could
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further be verified by a similar CO isosteric adsorption heat (Figure S9d). Therefore,
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the almost coincident morphology and CO affinity could be excluded from
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differentiating the CO RR performances of Cu-N-C and N-C.
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To investigate the coordination environment of Cu species at the atomic level, the
Cu K-edge X-ray absorption near-edge structure (XANES) of Cu-N-C with the
reference samples is illustrated in Figure 2b. The absorption edge of Cu-N-C located
between Cu O and CuO, confirms that the oxidation valence state of the Cu atom is
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between Cu1+ and Cu . The coordination environment was demonstrated by the
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extended X-ray absorption fine structure (EXAFS) spectrum (Figure 2c), the Fourier
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transform (FT) k -weighted χ(k) EXAFS spectrum of Cu-N-C showed a main peak at
1.4 Å that is shorter than that of Cu phthalocyanine (CuPc, Cu-N , 1.65 Å). Notably,
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no Cu-Cu coordination peak was observed at 2.2 Å, which unambiguously implied the
complete absence of Cu nanoparticles or clusters. Moreover, the EXAFS spectrum of
Cu-N-C could be fitted and the coordination number of Cu was determined to be 3.2
(Figure 2d and Table S4). In addition, the wavelet transform (WT) plot of Cu-N-C
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showed the maximum WT centered at 5.5 Å that is close to that of CuPc (Figures 2e
and S10), confirming the existing Cu−N bond on Cu-N-C. In contrast, the highest WT
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plot position of Cu-Cu bond is at about 6~8 Å .
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Figure 2. (a) XPS N1s signal of N-C and Cu-N-C; (b) the normalized XANES spectra
at the Cu K-edge; (c) Fourier transformation of the EXAFS spectra at R space; (d) the
EXAFS fitting for Cu-N-C; and (e) the wavelet transform (WT) plot.
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CO electroreduction performance
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The CO electroreduction activities of Cu-N-C, N-C, and CuPc were evaluated in a gas-
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tight H-type cell (Figure S11). As revealed by the linear sweep voltammetry (LSV)
curves (Figure 3a), Cu-N-C exhibited a substantially larger cathodic current density
compared with the N-C and CuPc in CO -saturated 0.5 M KHCO solution (pH =7.2).
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Meanwhile, the recorded current density on Cu-N-C is considerably larger than that in
Ar-saturated 0.5 M KHCO solution (Figure S12a), confirming that the currents are
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mainly devoted to the electrocatalytic CO reduction. The potential-dependent CO RR
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evaluation is performed at the potential range of -0.37 to -0.97 V and the products were
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detected by gas chromatography (Figure S13) and H nuclear magnetic resonance
(NMR). CO and H are the main gaseous products with trace amounts of C H and C H
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that only yield at high potential (Figures 3b and S14), and no liquid products were
detected (Figure S15). The catalysts exhibit stable time-dependent total current
densities at different applied potentials for 2000 s (Figure. S16). The CO current density
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(j ) of Cu-N-C reached 4.6 mA cm at -0.77 V that outperformed that of N-C and
CO
CuPc (Figure S17a). Particularly, Cu-N-C achieved a maximum CO FE of 98% at -0.67
V; while N-C and CuPc could only achieve a CO FE of 28% and 22% at -0.67 V,
respectively (Figure S17b), suggesting the dominating active sites of Cu-N-C.
Meanwhile, it also outperforms many benchmark M-N-C catalysts, such as Ni-N4
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(97.8%),30 Fe-N-C (87%),11 Co-N (82%), and Fe-N (94%). Furthermore, an
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isotope labeling experiment using CO as the feedstock confirms that the CO product
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originates from the reduction of CO (Figure S18).
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Tafel slopes were calculated to gain further insight into the electro-kinetics of
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CO RR. As shown in Figure 3c, the lowest Tafel slope of 156 mV dec was
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observed on Cu-N-C followed by CuPc (239 mV dec ) and N-C (309 mV dec ),
indicating its superior kinetics for the CO production. Moreover, electrochemical
active surface areas (ECSA) were estimated by the double-layer capacitance (Cdl)
measurements, which is obtained from the CV curves at different scan rates (Figure
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S19a-d). Cu-N-C exhibited a greater C of 2.5 mF cm that is 1.4 and 9.6 fold
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larger than N-C and CuPc, respectively (Figure S19e). Since Cu-N-C and N-C
displayed almost identical BET surface area, the larger ECSA can be attributed to
the exposure of electroactive and accessible Cu sites. The electrochemical
impedance spectroscopy (EIS) displays a smaller charge transfer resistance (Rct)
on Cu-N-C compared with N-C and CuPc, confirming the faster electron-transfer
during the CO RR (Figure S19f). Furthermore, the intrinsic activity of Cu-N-C is
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further disclosed by the turnover frequency (TOF) calculation. The Cu-N-C also
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shows an outstanding CO TOF value of 922 hr normalized to the Cu site at −0.77
V (Figure S20). It is worth noting that the high CO FE and TOF are simultaneously
achieved on Cu-N-C (Figure 3d).11,31,33-37 Meanwhile, Cu-N-C also exhibits a
steady current density for CO production with a nearly undeclined FE during the
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consecutive CO RR over 20 h, indicating its long-term stability (Figure 3e). The
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morphology was well maintained and no Cu particles/clusters were observed in the
post-cycling TEM and HR-TEM images. And the absence of Cu agglomeration or
cluster was also confirmed by the XRD patterns. Undetectable Cu species, in the
electrolyte after cycling tests, was verified by the ICP with a detection limit of ~100
ppb. These analyses suggesting the prepared Cu-N-C catalyst is robust and stable
(Figure S22). Flow cell configuration was further assembled to evaluate the
electrochemistry performance of the as-obtained Cu-N-C within 1 M KOH
electrolyte. Notably, the rapid CO diffusion in the flow cell achieves a
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significantly large CO partial current density of 131.3 mA cm at -1.17 V (Figure
3f). Moreover, the product analysis shows that the high CO FEs above 96% could
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be maintained over a wide voltage window on Cu-N-C, and the maximum CO FE
of 99% was obtained at -0.67 V (Figure 3f inset). This is due to the boosted the
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CO transfer and high alkaline condition that suppresses the H production.
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Moreover, Figure S23 showed no detectable Cu reduction/oxidation redox peaks
in both H- and flow-cell reactors.
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Figure 3. (a) LSV curves, (b) CO and H FE of Cu-N-C; (c)Tafel plots of obtained
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samples; (d) FE and TOF comparison with state-of-the-art CO -to-CO reduction
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catalysts; (e) stability tests on Cu-N-C at the potentials at -0.67 V over 20 h; (f) LSV
curves of Cu-N-C in the flow cell (inset is CO FE and jCO for Cu-N-C in the flow cell).
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Density functional theory (DFT) calculations
DFT calculations were carried out to disclose the intrinsic CO RR activity and the high
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product selectivity of Cu-N-C. For Cu-free N-C substrate, the N4-C structure and active
sites, e.g., N -1, N -2, N -3 for C atoms and N -4 for N atom, for adsorption of different
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intermediates (*COOH for CO RR and *H for HER) have been constructed (Figure
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S24). The lower free-energy barriers for *H formation on N4-C structure than that for
*COOH formation reveal the favorable HER on N4-C structure. In contrast, for Cu-N4,
the free energy change of *H formation is uphill with 1.68 eV, implying the
introduction of Cu atoms could effectively suppress the HER. The detailed discussion
is provided in Supporting Information. Based on the EXAFS results, the Cu atoms
directly coordinated with three N atoms (Cu-N ) in the graphene plane is first
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constructed. The *COOH is commonly regarded as the key intermediate for CO
formation.39,40 In the free energy evaluation plot (Figure 4a), the Gibbs free energy
barrier of CO (g)→*COOH on Cu-N was calculated to be 1.0 eV, which is lower than
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that on Cu-N (1.65 eV). However, the CO desorption on the Cu-N structure is
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extremely difficult with an unreasonable energy barrier of 2.84 eV. The following in-
depth investigation revealed that the lower energy barrier on *COOH or *CO
desorption on Cu-N is caused by the severe structure distortion, not originate from the
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intrinsic activity improvement (Figures 4b and 4c). Thus, the directly coordinated Cu-
N structure is excluded for further investigation. Furthermore, another possible Cu-N3
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structure with three carbon vacancies model (Cu-N C ) was constructed (Figure 4d), in
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which the Cu atom could not be stabilized by three N atoms, but the Cu atom will be
energy-favorably coordinated by one N and two C atoms (Cu-NC , Figure 4e). When
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the structure was evaluated for CO RR activity, the Gibbs free energy diagram is very
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similar to that of Cu-N and displays no superior electrocatalytic activity (Figure 4f).
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Figure 4 (a) Calculated Gibbs free energy diagrams for CO RR on Cu-N and Cu-N ;
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Optimized atomic structures for (b) *COOH and (c) *CO intermediates adsorbed on Cu
sites. The simulated structure of (d) Cu-N C and (e) Cu-NC ; (f) Calculated Gibbs free
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3
2
energy evolution of CO RR reduction to CO on Cu-NC and Cu-N at an applied
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2
4
electrode potential (U) of 0 V.
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In order to provide a plausible explanation for the experiment results, the Cu-N3
structure within six N vacancies in a graphene plane (Cu-N N ) was further constructed
3
3
1
1
1
1
1
1
1
and investigated (Figure 5a).41,42 This structure can stably tri-coordinate with the Cu
atom with Cu-N coordination distances of 1.92, 1.93, and 2.06 Å. The Gibbs free energy
diagram disclosed that the energy barrier of CO (g)→*COOH on Cu-N N efficiently
2
3
3
decreases to 1.36 eV, lower than that of Cu-N (1.65 eV, Figure 5b). The formation
4
energy of Cu-N N is predicted to be 1.87 eV that is slightly higher than that of Cu-N
3
3
4
(1.67 eV), suggesting the modeling rationality and synthesis feasibility of Cu-N N
3
3
structure. However, in the following HER calculation, the H could not be adsorbed on
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the Cu center and will simultaneously transfer to the adjacent N site with a Cu-N
distance of 2.44 Å (Figure 5c). This phenomenon strongly suggested that the three
uncoordinated N sites will be easily saturated by protons in the electrolyte. Therefore,
the proton-saturated CuN model (HS-CuN ) was further built and evaluated (Figure
6
6
5d). Impressively, the energy barrier of CO (g)→*COOH significantly reduced to 0.65
2
eV (Figure 5e), which is even lower than the most advantageous free-energy of Ni-N4
(0.84 eV).24 Interestingly, a unique hydrogen-bond promoted CO RR activity was
2
disclosed after we constructed the *COOH adsorbed structure. As shown in Figure 5f,
the adsorbed *COOH on the Cu atom could form an obvious hydrogen-bond interaction
with an O-H distance of 1.74 Å that will significantly strengthen the *COOH adsorption
1
1
1
1
1
1
1
1
1
1
2
2
2
and lower the CO (g)→*COOH energy barrier. After the first proton-coupled electron
2
transfer process, the hydrogen-bond disappears, and as such the *CO could be easily
released to boost the CO production. The corresponding reaction route for CO RR was
2
displayed in Figure S25. Moreover, even within the HS-CuN system, the energy barrier
6
for HER was calculated to be 0.83 eV, which is higher than that of CO RR (0.65 eV),
2
demonstrating the high selectivity toward CO RR (Figure S26). In addition, Figure S27
2
depicted that the calculated adsorption energy of CO is -0.21 eV on HS-CuN , higher
2
6
than that of Cu-N (-0.16 eV) and Cu-N N (-0.18 eV), suggesting the proton-saturated
4
3
3
model facilitate the initial CO adsorption on the catalyst surface. Thus, the key roles
2
of H-bond in Cu-N N can be attributed to: 1). the boosted chemisorption of CO by the
3
3
2
H bonding; 2). the enhanced adsorption of *COOH intermediate; 3). the facilitated
proton transfer from water. These merits eventually explained the high selectivity for
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CO RR over the HER.
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Figure 5 (a) Atomic structure of Cu-N N ; (b) Calculated Gibbs free energy evolution
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3
4
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6
7
8
9
of CO RR on Cu-N and Cu-N N at an applied electrode potential (U) of 0 V; The
2 4 3 3
structure of (c) Cu-N N active sites with the H simultaneously transfer to the adjacent
3
3
N site, and (d) three H-saturated N sites of HS-CuN ; (e) Calculated Gibbs free energy
6
evolution of CO RR on Cu-N , Cu-N N and HS-CuN at an applied electrode potential
2
4
3
3,
6
(U) of 0 V; (f) The optimized atomic structures of Cu-N-C with an *COOH
intermediate adsorbed on Cu sites.
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CONCLUSION
In summary, we have successfully synthesized a well-defined and atomically
dispersed Cu electrocatalyst for efficient and selective CO production. The catalytically
active center is identified by systematical structure investigations and DFT calculations.
The Cu-N-C delivers a high CO FE of 98% at -0.67 V (vs. the RHE). The durability of
Cu-N-C is verified by the steady consecutive CO RR over 20 h. In flow cell
2
configuration, both high CO FE of 99% at -0.67 V and large partial density of 131.3
-2
mA cm at -1.17 V can be reached. The mechanistic study reveals a novel proton-
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saturated Cu-N structure, which can form an extra hydrogen-bond between the
6
saturated H and adsorbed *COOH, is responsible for the enhanced CO RR activity and
2
selectivity. This work provides in-depth insights into the mechanism of CO production
on Cu-N-C structures and can inspire future rational design of CO RR catalysts with
2
high efficiency and selectivity.
ASSOCIATED CONTENTS
Supporting Information: The Supporting Information is available free of charge on
the ACS Publications website at DOI: Supporting Information section describes
experiment details and more supplementary analysis. Figures S1-S27 address additional
characterization of interest and details regarding the catalytic tests, including SEM
1
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2
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2
images, XRD patterns, XPS data, Raman measurement, CO electroreduction activity,
2
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3
and C isotope labeling experiments. Table S1-S4 provide detailed physical properties.
AUTHOR INFORMATION
Corresponding Author
*Email: jwang7@ncu.edu.cn (J.W)
*Email: shuguang.deng@asu.edu (SG.D)
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
This research work was supported by the National Natural Science Foundation of China
(No. 21908090) and the Natural Science Foundation of Jiangxi Province (No.
20192ACB21015). The authors would like to thank the helpful discussions and
suggestions from Prof. Troels Skrydstrup and Dr. Xinming Hu at Aarhus University,
Denmark and Dr. Yuxing Huang and Dr. Xin Zhuo at Nanchang University, P.R China.
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