Cooperative Copper Single Atom Catalyst in Two-dimensional Carbon Nitride for Enhanced CO2 Electrolysis to Methane

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10.1002/adma.202300713.
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Cooperative Copper Single Atom Catalyst in Two-dimensional Carbon Nitride for Enhanced CO2
Electrolysis to Methane
Soumyabrata Roy1#
, Zhengyuan Li2#
, Zhiwen Chen3
, Astrid Campos Mata1
, Pawan Kumar4
, Saurav Ch.
Sarma5
, Ivo F Teixeira6,7
, Ingrid F Silva7
, Guanhui Gao1
, Nadezda V. Tarakina 7
, Md. Golam Kibria4
,
Chandra Veer Singh3
*, Jingjie Wu2
*, and Pulickel M. Ajayan1
*
1
Department of Materials Science and NanoEngineering, Rice University, Houston, Texas 77005, USA
2
Department of Chemical and Environmental Engineering, University of Cincinnati, Cincinnati, OH
45221, USA
3
Department of Material Science and Engineering, University of Toronto, Ontario, Canada
4
Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University
Drive, NW Calgary, Alberta, Canada
5
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, England, UK.
6
Department of Chemistry, Federal University of São Carlos, 13565-905, São Carlos, SP, Brazil.
7
Department of Colloid Chemistry, Max-Planck Institute of Colloids and Interfaces, Am Mühlenberg 1,
D-14476 Potsdam, Germany
E-mail: chandraveer.singh@utoronto.ca, jingjie.wu@uc.edu, ajayan@rice.edu
Abstract
Renewable electricity powered carbon dioxide (CO2) reduction (eCO2R) to high-value fuels like
methane (CH4) holds the potential to close the carbon cycle at meaningful scales. However, this
kinetically staggered 8-electron multistep reduction still suffers from inadequate catalytic efficiency

2
and current density. Atomic Cu-structures can boost eCO2R-to-CH4 selectivity due to enhanced
intermediate binding energies (BEs) resulting from favorably shifted d-band centers. Herein, we
exploit two-dimensional carbon nitride (CN) matrices, viz. Na-polyheptazine (PHI) and Li-polytriazine
imides (PTI), to host Cu-N2 type single atom sites with high density (1.5 at%), via a facile metal ion
exchange process. Optimized Cu loading in nanocrystalline Cu-PTI maximizes eCO2R-to-CH4
performance with Faradaic efficiency (FECH4) of 68% and a high partial current density of 348 mA cm-
2
at a low potential of -0.84 V vs. RHE, surpassing the state-of-the-art catalysts. Multi-Cu substituted
N-appended nanopores in the CN frameworks yield thermodynamically stable quasi-dual/triple sites
with large interatomic distances dictated by the pore dimensions. First-principles calculations
elucidate the relative Cu-CN cooperative effects between the two matrices and how the Cu-Cu
distance and local environment dictate the adsorbate BEs, density of states, and CO2-to-CH4 energy
profile landscape. The 9N pores in Cu-PTI yield cooperative Cu-Cu sites that synergistically enhance
the kinetics of the rate-limiting steps in the eCO2R-to-CH4 pathway.
Keywords: CO2 electroreduction, methane, Cu single atom catalyst, 2D carbon nitride, cooperative
catalysis
1. Introduction
The growing techno-economic competitiveness of renewable electricity and increasing carbon dioxide
(CO2) emissions in the atmosphere prompt us to rethink the ways we cycle energy and products to
achieve a closed carbon economy.[1–3]
To that end, low-temperature electron-driven CO2 reduction
(eCO2R) using renewable energy can produce a large array of industrial fuels and feedstocks via
multiple proton and electron transfer steps.[4,5]
Among them, methane (CH4) has the highest heat of
combustion (55.5 MJ/kg) and provides 24% of global energy in the form of natural gas, making the
eCO2R-to-CH4 process an effective approach toward closing the carbon loop at a global scale.[6–8]
However, the lack of suitable cathode catalysts impedes the commercial viability of eCO2R, which still
suffers from low selectivity (< 60 %), partial current density (< 200 mAcm-2
), and high overpotentials
for CH4 formation.[9–13]
Among the state-of-the-art metal-based catalysts, copper (Cu) stands out because of its unique
capability to produce oxygenates (C2H5OH) and hydrocarbons (HCs-CH4, C2H4, C3) beyond the 2e-
process.[14–17]
Unfortunately, it has a wide product distribution that needs to be tailored through
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efficient electronic and geometric tuning of active Cu sites.[14,18,19]
Many interesting avenues of tuning
the catalytic outcome and product selectivity (including C1 vs. C2+ and oxygenates vs. HCs) of Cu
catalysts have been explored in recent years, including that of the control of facets (100 vs. 111),[20–22]
grain boundaries,[23]
morphology,[24]
dopants,[25]
and oxidation states.[26]
Although innovations in
catalyst, electrolyte, and electrode microenvironment designs have significantly advanced the eCO2R-
to-C2H4 conversion process (Faradaic efficiency- FE 60–70% at 500–1000 mA cm-2
), CH4 production
efficiency falls far behind.[27–29]
Due to the kinetically sluggish eight-electron transfer process involving
at least seven possible intermediates, it is difficult to achieve high selectivity for CH4, despite its
thermodynamic feasibility.[4,5,9,15,30]
Additionally, unidentical active sites in the catalytic motifs, having
different geometries and electronic structures, affect product selectivity severely.[31]
Only recently,
finely tuned Cu catalysts have started pushing the FECH4 boundary beyond 60%, such as CoO-modified
sputtered Cu ( 80%),[32]
low coordination Cu clusters (62%),[8]
sub-nanometric Cu-clusters (82%),[33]
Cu-O sites in metal organic frameworks (MOFs) (73%),[34]
Cu-based MOF@COF (covalent organic
framework) heterostructure (77%),[35]
dispersed Cu nanoparticles (NPs)- n-Cu/C (80%),[36]
CuS
nanosheet arrays (73%),[37]
sputtered Cu NPs (48%),[6]
Ag@Cu2O (74%),[38]
Cu-based alloys,[39]
Cu-
N/O single atom sites (78%),[40]
covalent triazine framework encapsulated Cu and Cu-complexes
(66%);[41,42]
however, many of these suffer from low partial current densities (≈100-200 mA cm-2
) at
comparatively high overpotential (above 1.0 V vs. RHE) requirements. Many of these catalytic systems,
particularly MOFs, oxides, and single atoms, often function as pre-catalysts or precursors to the actual
catalytic motif, entailing dynamic structural and chemical changes that modulate eCO2R performance
and selectivity.[43–45]
Recently, density functional theory (DFT) calculations of size-dependent eCO2R performance on
metallic Cu clusters have revealed that lower atomic clusters (13 atoms) favor CH4 generation, as the
d-band center of these upshifts with a size reduction, enhancing the binding energies of *CO and *H
intermediates.[46–48]
Evidently, single atom catalysts (SACs) involving Cu are appealing in that sense, to
produce uniformly isolated Cu sites with low coordination, discrete energy levels, and high atom
exposure/economy to drive the multistep eCO2R-to-CH4 pathway selectively and efficiently.
Furthermore, synergistic contributions from SA support matrices that can control the exposed atom
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density, Cu coordination environment, metal loading percentage, stability, and electronic conductivity
are crucial to ameliorate the performance of isolated Cu sites.
Transition metal (TM) SACs with M-Nx-C/O moieties embedded in N-doped carbonaceous scaffolds
have emerged as a strong alternative to noble metal catalysts for various thermo- and electrocatalytic
reactions.[49–52]
In the case of eCO2R, SACs have been able to boost the selectivity of 2e-
reduced C1
product close to 100%.[53,54]
However, FE enhancement for higher electron-reduced products or C2+
HCs remains challenging due to the lack of articulate control over single atom structures and metal
density. Additionally, many SACs often undergo dynamic transformations[55]
or structural evolution
like agglomeration/clustering leading to an enhancement of C2+ selectivity.[43,44,56]
Conventional high
temperature routes of M-Nx-C SACs preparation using N-C precursors and TM salts, or post-synthetic
degradation/carbonization of metal complexes, MOFs, or TM@COFs seldom achieve high isolated
atom density (> 1 at%) and suffers from agglomeration at high metal loadings.[40,57]
Moreover, the high
thermal energy (at T > 800 C) inevitably forces the C-N precursor to reorient around the metal ions
to form the most thermodynamically favored MNxC4 moiety.[58,59]
Alternatively, TM-single atom
pinning in defect-incorporated graphitic lattices usually suffers from low metal loading and
agglomeration due to the asymmetric C and N terminated vacancies and low overall N-content.[58,60,61]
The fabrication of densely populated M-Nx SACs along with better microenvironment control is
essential for the colligative performance enhancement where the proximity of metal centers can
further induce ‘cooperative catalysis’, often leading to altered reaction pathways and enhanced
efficiencies.[61–64]
Herein, we use two highly crystalline two-dimensional (2D) carbon nitride (CN)-based anionic
scaffolds, viz. poly (heptazine imide)-PHI and poly (triazine imide)-PTI,[65,66]
as hosts to prepare Cu-N2
single atom sites with high loading density (1.5 at %, Cu-PHI/PTI) via a simple room temperature
metal ion exchange process. Furthermore, the heptazine (C6N7) and triazine (C3N3) templates allow
controlling the cooperative Cu-Cu site distances that can enhance intermediate
stabilization/activation to kinetically enhance the rate-limiting steps in the multistep eCO2R-to-CH4
pathway. PTI support comprising high surface area nanocrystalline domains offers high metal site
exposure and synergistically tunes the SA coordination environment. As a result, the cooperative
catalytic mechanism inside triazine nanopores boosts eCO2R-to-CH4 performance with FECH4 68% and
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a high partial current density of 348 mA cm-2
at a low potential of -0.84 V vs. reversible hydrogen
electrode (RHE). High-angle annular dark field-scanning transmission electron microscopy (HAADF-
STEM) and X-ray absorption spectroscopy (XAS) reveal the Cu-SA atomic site structure, geometry, and
electronic states. DFT calculations were performed to elucidate the relative Cu-CN cooperative effects
between the two carbon nitride matrices and how the Cu-Cu distance and local coordination
environment dictate the adsorbate binding energies, the density of states (DOS), and energy profile
landscape for CH4 formation.
2. Results and Discussions
2.1. Synthesis and structural characterizations
The recently discovered family of crystalline 2D carbon nitrides (2D-CN) synthesized from C/N
precursors (melamine/dicyandiamide) and alkali chlorides (as solvents) have proved to be excellent
matrices for hosting TM single atoms for a variety of reactions.[65–67]
Here, we synthesized two anionic
crystalline 2D-CN scaffolds of poly(heptazine imide) (Na-PHI) and poly(triazine imide) (Li-PTI) by
thermal condensation of melamine with excess NaCl and LiCl salts (1:10 wt/wt), respectively, at 600
C for 4 h under nitrogen flow (5 L min-1
) (see SI for synthesis details).[65]
Subsequently, we conduct a
simple cation exchange reaction to coordinate Cu2+
ions in the N-appended triangular pores of the CN
scaffolds by replacing the Na+
and Li+
cations, which maintains the charge balance of the anionic CN
frameworks. (Figure 1a) (section 2.2 in SI). This step enables the control of metal sites and boosts the
eCO2R activity per mole of metal. As shown in the scheme, due to the availability of multiple TM
exchange sites within the triangular nanopores, we could obtain the incorporation of multiple Cu ion
sites in proximity which is hypothesized to enhance the activity further through cooperative catalysis
and activation/stabilization of key reactants and intermediates.
Pristine Na-PHI forms large flakes (tens of m) with nanocrystalline domains showing a lattice spacing
corresponding to the {100} and {010} planes, as evident from scanning electron microscopic (SEM)
images, high-resolution transmission electron microscopy (HR-TEM) (Figure 1b-e) and FFT on [001].
The PHI layers are arranged directly on top of each other creating aligned channels along the c
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direction (AAA stacking).[68]
Li-PTI, on the other hand, comprises highly crystalline nanoscale 2D flakes
(50-100 nm lateral sizes) that are unidirectionally oriented. They give rise to clear single-crystal-like
hexagonal selected area electron diffraction (SAED) patterns and show d-spacings corresponding to
{200} and {110} planes (Figure 1f-i) in HR-TEM and FFT on [001]. X-ray diffraction analysis reveals the
characteristic crystalline structure of Na-PHI and Li-PTI. Specifically, hexagonal Na-PHI lattice (P31m
space group) exhibits an intense diffraction peak between 2  26-28 for the {001} planes
corresponding to the graphite-like interlayer stacking and a weak in-plane reflection at 2  8.3 for
{1-10} arising from the periodic heptazine units (Figure 2a).[65,66]
Li-PTI exhibits well-resolved and sharp
diffraction peaks consistent with the hexagonal structure (P63cm space group) with a strong reflection
at 2  26.8 for {002}, while the peak due to the {1-10} in-plane periodicity appears at 2  12.[69]
The expected C/N ratio of 0.67-0.70 is maintained for the scaffolds with the additional presence of Cl-
ions in the case of Li-PTI,[65]
as evident from elemental analysis through X-ray photoelectron
spectroscopy (XPS) (Figure S1) and SEM-energy dispersive X-ray spectroscopy (SEM-EDS) (Table S1-2).
From the cumulative elemental analysis from XPS, SEM-EDS, and inductively coupled plasma-optical
emission spectrometry (ICP-OES), the atomic concentration of Na/Li was found to be between 2.5-2.7
mol% ( 4 wt.% of Li in PTI and 11 wt.% of Na in PHI), in agreement with previous reports.[65,69–71]
The alkali cations in the PHI/PTI were exchanged with Cu2+
by a simple ion exchange reaction in
aqueous media at room temperature (see experimental section/SI for synthesis details). A
combination of elemental analysis from XPS, EDS, and ICP-OES reveals a Cu loading of 4-5 wt.%
corresponding to 1-1.5 at%, with some residual Na+
/Li+
ions, still compensating the remnant charges
(Table S1-2). The Cu2+
exchange in the PHI/PTI scaffolds was found to be irreversible upon post-
treatment with an aqueous Na+
/Li+
solution, indicating the formation of strong Cu-N bonds in the CN
frameworks.
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Figure 1. (a) Synthetic scheme of Na-PHI, Li-PTI via molten alkali salt method and Cu-PHI/PTI samples
through metal ion exchange, (b) SEM image, (c-e) TEM/HRTEM images and SAED pattern of Na-PHI,
(f) SEM image, (g-i) TEM/HR-TEM images and SAED pattern of Li-PTI.
During cation exchange, the Cu2+
ions get coordinated by N atoms at the vertices of the nanometric
pores of the 2D CN layers (Figure 1a), leading to the formation of high-density single atom Cu sites
with Cu-N2 coordination. In the case of PHI, there are two possible sites where the Cu ions can
substitute- (a) in the heptazine pores or (b) intercalated between the 2D polymeric heptazine layers.
For PTI, the Cu2+
ions can be coordinated only in the triazine pores and partially balanced by the
interlayer Cl-
ions. XRD analysis shows that the crystal structure of the CN scaffolds is retained after
the Cu2+
exchange, with no additional peaks from impurity phases/metal clusters (Figure 2a). The only
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difference is in the relative intensities of the peaks between 2  5-30, with no discernible shift in the
diffraction peak position along the c-direction. The decrease in intensity of the (100) peaks at 2 
8.3 in Cu-PHI, can be attributed to a loss of in-plane periodicity and disorder in the CN matrix induced
by the stochastic distribution of Cu ions. In the case of Cu-PTI, an increase in relative intensity of the
(100) peak occurs due to ordered Cu arrangement and thus preserving crystalline order of the PTI
framework after substitution of Li. Regardless, in both samples, the local coordination environment of
Cu2+
ions remains the same and uniform. The peak position of h00 and hk0 reflections in Cu-PHI/PTI
remains intact, and thus the 2D structures of CN layers are preserved during metal ion exchange.
Similarly, the unchanged 00l reflections denote no significant changes in interlayer spacing,
eliminating the probability of metal ion intercalation.
2.2. Analysis of Single atom Cu sites and CN scaffolds
2.2.1. Spectroscopic analysis
XPS analysis reveals that the single atom Cu sites in both Cu-PHI and Cu-PTI predominantly exist as
Cu2+
(934.9 eV-2p3/2, and satellite/shake-up peak centered at 943.5 eV) with some minor Cu1+
sites
(933 eV-2p3/2, Cu2+/
Cu1+
6.5:1) (Figure 2b, S2). Lacking any metallic Cu(0) fingerprint signal is
consistent with the absence of nanoparticles, as evident from the following microscopic analysis. The
C1s spectra consist of the primary C-N3 peak at 288.3 eV corresponding to the heterocyclic rings of the
heptazine and triazine units, in addition to some C-OH and C-C (284.8 eV) species arising from
adventitious carbons on the surface. The N 1s spectra of Cu-PHI can be deconvoluted into a ring
nitrogen mixture of N-C2 (C-N=C) and N-C3 (398.9/399.8 eV), terminal/bridging NHx (401 eV) groups,
and N-Cu (404.4 eV) peaks. The observed NC2/NC3 ratio of 6 is characteristic of heptazine rings.[72,73]
Cu-PTI N1s spectra show similar signatures of NC2 (399 eV), with more intense and slightly shifted
NHx peaks (400.6 eV, NC2/NHx ratio  2), due to the absence of the central N-C3 units in the triazine
polymer. The intense Cu2+
/Cu1+
satellite peak centered around 943 eV is characteristic of the strong
covalent interactions between the Cu2+
-N ligand bonds, causing the charge transfer from ligands to
Cu2+
orbits.[74,75]
The obtained C/N molar ratio of 0.71 for Cu-PHI and 0.68 for Cu-PTI reaffirms the CN
structural integrity in the Cu-coordinated samples. XPS elemental analysis shows a 1.51 and 1.43 at%
Cu loading for the Cu-PHI and Cu-PTI, respectively.
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The X-ray absorption near edge spectra (XANES) for the samples and the related references (Cu(0) foil,
Cu(II)O, Cu-CN (a control Cu-N4-C SAC) and CuNO3) are shown in Figure 2c to understand the well-
resolved oxidation states of the Cu ions. From the comparative XANES spectra, the Cu single atoms in
both PHI and PTI samples are very close to +2 oxidation state, with some minor Cu1+
contributions, in
agreement with XPS. Cu-PHI and Cu-PTI exhibit edge energy (E0) of 8984 eV, while the maximum
absorption appears at 8991.5 eV corresponding to the 1s → 4p transition (8994 and 8998 eV for
Cu(NO3)2 and CuO). The characteristic shoulder at 8984.5 eV can be attributed to the 1s → 4p “shake
down” transition generally observed for Cu2+
systems, which is slightly left shifted compared to CuO
(8985 eV).[76]
The effective intermediate oxidation state can be estimated to be close to +1.8 (from
linear combination fitting),[77]
which is also evinced by a slightly left shifted weak pre-edge peak at
8974.2 eV (1s → 3d dipole forbidden transition) with respect to CuO (8978 eV).[73,76,78]
The
intermediate oxidation state can either be due to the presence of mixed +1 and +2 states or a slight
overall reduction of all Cu2+
sites because of the semiconducting nature of the CN scaffolds or LMCT
transition from the coordinating N atoms.[40,79]
The lower edge of Cu-CN XANES indicates that towards
its 4-N coordination, leading to higher LMCT.
Figure 2. Characterizations of Cu-PHI/PTI samples. (a) XRD, (b) N 1s and Cu 2p XPS (the resolved XPS
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spectra is provided in Figure S2), (c) XANES, (d) EXAFS R-space spectra, (e) C K-edge NEXAFS and (f) N
K-edge NEXAFS.
We conducted extended X-ray absorption fine structure (EXAFS) analysis at the Cu K-absorption edge
to resolve the local atomic structure and coordination environment of the Cu site, in combination with
atomistic modeling based on DFT calculations. The k2
-weighted Cu K edge R-space spectra show the
major scattering peak centered at 1.6-1.7 Å corresponding to the Cu-N bonds (Figure 2d) in the first
coordination sphere. This matches well with the reported literature and is very close to the Cu-N bond
distances obtained from relaxed structures in computational studies.[73,80–82]
The absence of long-
range Cu-Cu scattering paths at 2.2 Å or higher radial distances (at k > 10 Å-1
, R > 2.5 Å) indicates the
absence of direct Cu-Cu interactions from metallic Cu or oxidized CuOx clusters.[42,80]
The first peak
corresponding to Cu-N(C) contribution with a fitted bond length of 1.95 Å can be attributed to the first
shell coordination of Cu-N isolated sites dispersed in PHI and PTI matrices (Figure S3, Table S3). The
minor peaks between 2-3 Å can arise from the combined scattering from the second coordination
shells due to interactions between Cu-C-N2, Cu-C-N3, adjacent Cu-NC2 and Cu-NHx.[66,73,80,81]
XANES
signature spectra can further confirm the atomic structures of SACs as the central atoms with different
coordination numbers (CNs) or ligands usually display distinct absorption curve features.[81]
The near-
edge of Cu in PHI and PTI reveals distinctly different features than CuO, Cu, CuNO3, and most
importantly Cu-CN (a reference square planar Cu-N4-C SA system), signifying a difference in the local
environment of Cu-SA moieties. Rather, the signature matches with previously reported Cu-N2 or O-
Cu-N types of coordination geometry, indicating the presence of Cu-N2 sites in the Cu-PHI/PTI SACs
(Figure 2c).[81,82]
The Cu ions are expected to occupy the corner sites in the larger 9N and 15N pores of PTI, and PHI,
respectively, coordinated by the NC2 nitrogens.[66]
Each Cu atom is found to be coordinated by two N
atoms of the heptazine/triazine pores, with bond distances between 1.8-1.85 Å and the bond angles
(∠N-Cu-N) of 105-120 in the DFT calculations. The quantitative coordination configuration of the Cu-
N sites in Cu-PHI/PTI was further investigated by the R-space curve fitting (Figure S3-4, Table S3). The
coordination numbers for Cu in Cu-PHI/PTI were found to be between 2.7-2.9 for the Cu-N bonds with
an average Cu-N bond distance between 1.85-1.95 Å, matching the DFT structures. From the
theoretical modeling, we find that multiple Cu ion incorporations in the CN nanopores are
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thermodynamically feasible due to the higher stabilization energy of the 2 or 3 Cu sites incorporated
PHI/PTI structures (discussed in section 2.4). However, the long interatomic bond distances even
between the closest Cu sites (3-4 Å) in the relaxed structures eliminate the possibility of direct Cu-Cu
bonding interactions inside the pores. Owing to such long distances, no Cu-Cu scattering paths are
expected to arise in the R-space EXAFS spectra. Overall, from the combined XAS and DFT analysis, the
Cu ions were found to occupy the vertical sites of the heptazine/triazine units in the 2D-CN planes,
having a Cu-N2 type of coordination environment.
The excitation-emission matrix spectroscopy (EEMS) map of both samples displayed sharp emission
bands around ~285-287.5 eV and ~400 eV associated with C and N K-edge, respectively, validating the
N rich character of the CNs (Figure S5-6). The C K-edge near-edge X-ray absorption fine structure
(NEXAFS) spectra of the CN scaffolds using soft X-rays (sXAS) exhibited two signature π* resonance
peaks located at 284.6 and 287.5 eV originating from the π*C=C transition in uncondensed
functionalities and adventitious carbons and π*N-C=N transition in N-linked heptazine (C6N7) or triazine
moieties (Figure 2e, see SI for more details).[83] [84]
The increased π*C=C intensity in PHI might arise from
the presence of two nitrogen sites (NC2 and NC3) which can intensify C=C transitions, unlike in PTI. The
N K-edge NEXAFS spectra of Cu-PHI exhibited two π* resonance peaks at 399.1 and 402.4 eV
corresponding to π*C–N=C transition of nitrogen in heptazine unit and π*N–C3 of bridging nitrogen in PHI
(Figure 2f).[85]
Due to the absence of N-C3 moieties, Cu-PTI shows a singular sharp π*C–N=C transition.
The obtained results confirm the structural integrity of the N-linked heptazine and triazine framework
upon Cu ion exchange.
2.2.2. Microscopic analysis
To visualize the Cu single ion dispersion in the CN matrices, we conducted SEM and TEM studies on
the metal ion exchanged Cu-PHI/PTI samples. As evident from the micrometer scale SEM images
(Figure 3a, e), any large metal clustering or deposition was not observed on the CN flakes, which
maintain their 2D layered morphology. Bright field TEM images do not show any NPs, while the clean
exfoliated 2D polymeric layers exhibit the characteristics SAED patterns corresponding to the {00l}
graphitic planes in the CN structures (Figure 3b, f).
HR-TEM images and FFT/SAED patterns (Figure 3c, f-g) show intact d-spacings as XRD patterns,
signifying the retention of primary structures and crystallinity of the 2D polymeric scaffolds after metal
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exchange. PHI matrix shows distributed crystalline domains, while the PTI nanoflakes were highly
crystalline and unidirectionally oriented. Aberration-corrected high-angle annular dark-field scanning
transmission electron microscopy (AC-HAADF-STEM) studies on the samples show the homogeneous
and high-density distribution of the Cu single atoms (sub-nanometer bright spots) in the CN matrices
(Figure 3d, h). Interestingly, a closer analysis of the AC-HAADF STEM images reveals distinguishable
cooperative Cu atom sites (quasi-dual/triple atom sites) within the PHI/PTI matrices (circled areas in
Figure 3d) that are distinct from conventional dual atom catalysts, reported previously.[86–89]
DFT
studies in combination with EXAFS reveal the thermodynamic feasibility of multiple Cu sites (Cu-N2)
within the triangular nanopores having interatomic distances of  4 Å (in 9N triazine pores) and 9 Å
(in 15N heptazine pores) for PTI and PHI, respectively. AC-HAADF STEM imaging on Cu-PTI with low Cu
ion density (Figure S7) show the formation of several quasi-dual and -triple atom sites with large
interatomic distances in the PTI matrix.[90]
A higher density of the quasi-dual atom sites can be
attributed to its high stabilization energy in the DFT calculations (see section 2.4). Intuitively, the
mixture of Cu2+
/Cu1+
sites observed could arise from reducing some Cu sites in the case of dual or triple
Li/Na substituted PHI/PTI pores to maintain charge balance. For mono-substituted pores, two or more
Li/Na+
ions can leave the system to account for the divalent Cu cations. The adsorption of additional
Cl-
/ OH-
ions on CN surfaces can further preserve charge balance.[67,91]
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Figure 3. Microscopic analysis of Cu-PHI/PTI. (a) SEM, (b) TEM (Inset SAED), (c) HR-TEM, and (d) AC-
HAADF STEM of Cu-PHI. (e) SEM, (f) TEM (Inset FFT), (g) HR-TEM, and (h) AC-HAADF STEM of Cu-PTI.
(i-l) TEM-EDX elemental mapping on Cu-PTI.
Thin cross-sections of the embedded materials were mapped by TEM-energy-dispersive X-ray (EDX)
spectroscopy, indicating a relatively uniform distribution of Cu throughout the PHI and PTI matrices
(Figure 3, S8). From the collated elemental mapping images shown in Figure 3i-l, the C, N, and Cu are
homogeneously distributed across the entire mapping area. Line scanning images and EDX spectra
show a crude elemental distribution for C and N in the average range of 60:40 wt% as expected for CN
scaffolds, while for Cu, an average of 5 wt% was obtained in both samples (Figure 3i-l, S8b-c, g-i).
This sums to about 1.3-1.5 at% of Cu in the samples (Table S1). The elemental distribution is coherent
with other analytical techniques like ICP-OES and XPS, as depicted in Table S1-S2.
2.3. CO2 Electroreduction performance
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The electrocatalytic CO2 reduction performance of Cu-PHI-5wt% and Cu-PTI-5wt% samples, denoted
as Cu-PHI and Cu-PTI, respectively, was evaluated in a flow-cell with 1M KOH electrolyte. For both
catalysts, CO and H2 were found to be the dominant products in the low overpotential range of -0.60
V to -0.65 V vs. RHE (Figure S9). The FE of H2 was particularly high for Cu-PHI (40-50%) in this low
overpotential range, while CO was found to be the dominant product on Cu-PTI (FECO 30-40%) (Figure
S10a-b). The higher overall percentage of H2 production on Cu-PHI can be attributed to the higher
percentage of HER favorable N-C sites (NC2-pyridinic and NC3-graphitic combined) in the structure than
Cu-PTI (no graphitic N).[92]
Both selectivity of H2 and CO on Cu-PHI and Cu-PTI decreased dramatically
at more negative potentials, as CH4 emerged as the major high-order product beyond -0.65V (Figure
4a-b, S9). CH4 reached a maximum FE of 54% at -0.88 V for Cu-PHI and 68% at -0.84 V for Cu-PTI. The
corresponding partial current density for CH4 (jCH4) reached 263 and 348 mA cm-2
for Cu-PHI and Cu-
PTI, respectively (Figure 4b). The jCH4 for Cu-PTI maximizes to 395 mA cm-2
at -0.86 V. Cu-PTI was found
to perform beyond the state-of-the-art electrocatalysts for eCO2R-to-CH4 (Figure 4d, Table S4). In
addition to the predominant CH4 formation, Cu-PHI and Cu-PTI also generate C2 products comprising
C2H4, C2H5OH, and CH3COO-
. However, the overall selectivity toward C2 products is low, with a peak
FEC2 of ~ 13% for Cu-PHI at -0.81 V and ~ 10% for Cu-PTI at -0.71 V (Figure S10c-d). The turnover
frequency (TOF) values for eCO2R-to-CH4 of Cu-PTI and Cu-PHI at -0.84 V (vs. RHE) were found to be
1.14 and 0.56 s-1
, respectively, which is on par with some of the best SAC eCO2R catalysts reported in
recent literature (Table S5).
To investigate the role of other cations and pristine N sites, we tested the pristine Na-PHI and Li-PTI
samples under identical conditions. As controls, pristine Na-PHI and Li-PTI showed similar
performance, producing a majority of H2 with a minor amount of CO (FECO < 10%) and CH4 (FECH4 < 5
%) (Figure 4c, S11). The significant difference in eCO2R performance signifies that Cu SAs are the
primary CO2 reduction sites in the ion exchanged catalysts, with no direct contribution from residual
Na/Li or Cl-
ions or the nitrogen sites in CN scaffolds toward CH4 production. Furthermore, we
investigated the electrochemical CO2 reduction performance of Cu-PTI catalysts at different Cu
loadings to analyze the effect of site density on the eCO2R product selectivity. We varied the Cu
concentration from 0.5-8 wt% on the PHI/PTI scaffolds. NEXAFS spectra on the Cu-PHI samples show
the corresponding peaks associated with C K-edge, N K-edge and Cu L-edge (+2 and +1), spectra
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centered at 287.5 (C), 395.8/398.6 (N), 930.5 (Cu) eV,[93,94]
respectively, for various Cu concentrations
(Figure S12). The eCO2R performance at low Cu concentrations revealed a coherent increasing trend
in eCO2R-to-CH4 activity between 0.5 to 5 wt% Cu loading. As shown in Figure 4c, the FECH4 for Cu-PTI
increased, and the FECH4 peaks moved to more positive overpotentials with increasing Cu
concentrations. Consequently, the FEH2 decreased from 75% to 20% with increasing Cu concentrations
(Figure S13). A similar trend in activity was also observed for the Cu-PHI catalysts at lower Cu
concentrations (0-2 wt%, Figure S14). However, Cu-nanostructures were formed at higher
concentrations of 8 wt% and above (Figure S15). The homogenous distribution of Cu SAs was affected
beyond 5 wt% metal loading, leading to a decrease in FECH4 and an increase in FEC2H4 (Figure S16).
Furthermore, we studied the effect of catalyst loading on the performance by analyzing the variation
in the partial current density and TOF of CH4 as a function of catalyst loading (between 0.2 and 1 mg
cm-2
, Figure S17). The TOF is calculated to be ~ 1.14 s-1
when the loading is lower than 0.8 mg cm-2
at
-0.84 V vs. RHE and decreases to ~ 1.03 s-1
as the loading is increased to 1 mg cm-2
. In the loading range
used in this study (0.5-0.6 mg cm-2
), the electrode or the interface does not seem to affect the TOF.
However, at higher catalyst loadings, the selectivity for CH4 is affected, probably due to mass transport
limitations of CO2 in the thicker electrode/catalyst layer.
To prove the origin of the eCO2R products, we also conducted electrocatalytic tests on the Cu-PTI
sample under pure argon conditions. As shown in Figure S18, H2 was obtained as the only reduction
product without any traces of eCO2R products, eliminating the chances of CH4 production from any
other probable carbon precursors in the reaction environment. The isotopic labeling test using 13
CO2
feedstock confirms that CH4 originates from the gaseous 13
CO2 feed (Figure S19). Electrocatalytic
stability of Cu-PTI at -0.84 V (vs. RHE) was studied in a flow cell using 1 M KOH electrolyte (Figure S20)
applying full cell voltage. The CH4 selectivity remains intact till 12 hours at FECH4 68%, beyond which
severe flooding caused CO2 transport limitation and performance decline, typical for flow cell tests
under alkaline conditions.[95]
The calculated half-cell cathodic energy efficiency for Cu-PTI maximized
34% at -0.84 V (vs. RHE) (Figure S21).
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Figure 4. eCO2R performance of Cu-PHI and Cu-PTI catalysts in the flow cell with 1 M KOH. (a) FECH4
comparison of Cu-PHI/PTI, (b) partial current density of CH4 (jCH4) for the catalysts, (c) FECH4 comparison
of Cu-PTI at different Cu loading from 0-5 wt%, (d) comparison of eCO2R-to-CH4 performance (jCH4 vs.
potential) of Cu-PHI and PTI vs. the best-reported catalysts in recent literature tested using flow cells.
E1-18 represent the first 18 catalyst entries in Table S4. The color scheme and size of the point circles
refer to the FECH4 of the catalysts.
The structural and chemical integrity of the catalysts were analyzed by post-electrochemical XAS and
XPS analysis (see Figure S22-23 in SI for more details and section 4 for detailed). The post-
electrochemical HRTEM analysis on the samples were unsuccessful due to impurities from the carbon
particles in GDL during catalyst recovery, presence of organic binders and fast beam damage at higher
resolutions. XANES analysis shows no major changes in the oxidation state of Cu after electrolysis,
while R-space EXAFS spectra signify the same Cu-N coordination environment after eCO2R (Figure
S22a-b). The rigid CN scaffold maintains its structure completely, as is evident from the nearly
unaltered C- and N-k edge NEXAFS spectra in both samples (Figure S22c-d). XPS analysis reveals that
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the percentage of Cu1+
increases on the surface leading to a Cu2+
/Cu1+
ratio  1, along with some
negligible decrease in Cu at% (Figure S23, Table S1). During the CO2 reduction experiments under a
constant applied reduction potential (chronoamperometric condition), the Cu2+
on the surface is
expected to be fast reduced, while Cu1+
might be maintained due to the high redox potential of
Cu1+
/Cu0
for single atom.[96]
Additionally, Cu2+
is reported to be less active for CO2 electrolysis.[33]
Thus,
based on previous studies and post-electrochemical results, we speculate that a mixed redox state of
Cu1+
/Cu0
on the surface is probably responsible for the high CO2 reduction performance and methane
selectivity.[44,62,97,98]
Overall, the catalysts maintain their single atom structure and Cu at% after eCO2R,
showcasing their structural and chemical robustness.
2.4. Structure-activity rationalizations and DFT analysis
BET surface area analysis reveals that PTI (115.8 m2
g-1
) has 10-fold higher surface area than PHI (12.5
m2
g-1
, Figure S24-26), probably arising from the smaller nanocrystalline flake/particle size in PTI. The
surface area and pore size of both matrices reduce marginally (10% change, Figure S24-25) upon
metal ion exchange and the chemical treatments. CO2 adsorption isotherms at room temperature
show that both PHI and PTI have similar CO2 uptake between 0-1 bar, which enhances with Cu
exchange in the samples (Figure S27-28). Interestingly, despite having 10-fold less surface area, Cu-
PHI shows slightly higher CO2 uptake performance than Cu-PTI (11 and 10 cc g-1
at 1 bar, respectively)
in the entire pressure range (Figure S27). Thus, from the CO2 adsorption isotherms, it is evident that
there is negligible effect of surface area on the interactions and uptake of CO2 in Cu-PHI/PTI to have
any preferential catalytic effect. The same was reflected in the congruent activity profiles of the
pristine PHI and PTI samples under eCO2R tests, mainly exhibiting HER activity.
XPS, XAS, and other spectroscopic analyses revealed that Cu SA sites exist in similar electronic states
in both catalysts. Also, from the combined elemental analysis, it is evident that the Cu wt.% and atomic
percentage are very similar (within the error limit) on both matrices. Thus, we envision that the local
environment dictated by the nanometric pore architecture and dimensions might play a significant
role in controlling the reaction pathways and catalytic outcomes. From the TEM analysis, we have seen
the presence of some quasi-dual and triple atoms arrangements, where Cu ions, during ion exchange,
occupy multiple sites within the pores (Figure S7). The pore diameters and the geometries of edge
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sites determine the Cu-Cu distances and the Cu-N bond lengths. Both computational studies and
EXAFS fitted bond lengths indicate that the Cu-N bonds have slightly shorter distances in Cu-PTI than
in Cu-PHI (Table S3, S6). However, as expected, there were significant differences in the inter-Cu
distances within the pores that are expected to modulate the efficacy of cooperative catalysis through
favorable intermediate binding and controlling the energy barriers of potential limiting steps (PLS).
To get detailed insights into the high catalytic activity of PTI-supported Cu catalysts, density functional
theory (DFT) calculations were performed. As the references, PHI-supported Cu catalysts were built
and named as PHI-Cu, PHI-2Cu, and PHI-3Cu (Figure S29a), which represent different Cu loading and
arrangements inside the 15N heptazine pores of PHI. Similarly, PTI-Cu, PTI-2Cu, and PTI-3Cu were also
generated, as shown in Figure S29b. The binding energy of Cu atoms (ΔECu) on PHI and PTI
demonstrates the stability of the designed catalysts (Table S6-7, Figure S30). The calculated ΔECu are
-2.48 eV, -2.56 eV, and -2.39 eV for PHI-Cu, PHI-2Cu, and PHI-3Cu, respectively. Note that PTI-Cu, PTI-
2Cu, and PTI-3Cu have more negative ΔECu values of -3.38 eV, -3.65 eV, and -3.64 eV, respectively,
indicating that PTI-supported Cu catalysts have higher stability than PHI-supported Cu catalysts.
Moreover, the ΔECu values on PTI-2Cu and PTI-3Cu are much stronger than that on PTI-Cu, signifying
that PTI-2Cu and PTI-3Cu are easier to form during the synthesis. The higher stabilization energy of
multiple Cu sites in 9N triazine pores in PTI can be due to the smaller pore size leading to stronger Cu-
N (primary) and Cu-Cu (secondary) interactions (Table S6-7). In both PHI and PTI the 2Cu systems were
found to be the most stable. We considered the catalytic activity of all the systems toward CO2
reduction reaction, as shown in Figure 5a, c. For the PHI-systems nCu (n = 1, 2, 3), the reaction
mechanisms of CO2 reduction reaction remain the same due to the same active site configuration
(CO2-Cu) of the top site, as shown in the insets of Figure 5a. The larger pore dimensions in PHI lead to
longer Cu-Cu distances, limiting the effect of cooperative catalysis upon the adsorption of CO2 in the
case of bi- or tri-Cu substituted systems. The PLS is the formation of CH* with the reaction free energy
of 1.34 eV, 1.07 eV, and 0.96 eV for PHI-Cu, PHI-2Cu, and PHI-3Cu, indicating that the catalytic activity
gradually increases with the increase of the Cu loading within the pores of the PHI-nCu systems (Figure
5a, S30, Table S8-11).
However, the reaction mechanism of CO2 reduction reaction on PTI-nCu varies with Cu site number in
the pore. Same as that for PHI-nCu, the PLS on PTI-Cu is still the formation of CH* with the reaction
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free energy of 1.80 eV. The PLS changes to the formation of CHO* (CO* protonation step) and CH4
(CH3* protonation step) for PTI-2Cu and PTI-3Cu, respectively. Importantly, the reaction free energy
of PLS dramatically decreases to 0.50 eV and 0.76 eV for PTI-2Cu and PTI-3Cu, respectively. The change
in PLS is mainly due to the different active site configurations of PTI-nCu, as shown in the inset of
Figure 5c. The closer Cu-Cu distance allows for cooperative activation of the adsorbed intermediates
lowering the PLS reaction energy barriers.
Figure 5. DFT results. (a) Reaction process of CO2 reduction reaction on PHI-nCu (n = 1, 2, 3), including
all intermediates and the adsorption structures of CH* on catalysts. (b) Reaction process of CO2
reduction reaction on PTI-nCu (n = 1, 2, 3), including all intermediates and the adsorption structures
of CH* on catalysts. (c) Local density of states of Cu-d orbitals in the systems of PTI-nCu (n = 1, 2, 3).
(d) Charge density difference of CH* absorbed on PTI-2Cu. The yellow and azure area indicates
electron accumulation and loss, respectively. The white, gray, silver, and blue balls represent H, C, N,
and Cu atoms, respectively.
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The electronic structures of PTI-nCu systems show that more electrons accumulate near the Fermi
level for the multi-substituted pores, resulting in higher catalytic activity to CH4 formation (Figure 5b).
The same is also reflected in the calculations of the number of electrons transferred from each Cu site
in the PHI/PTI-nCu systems from the Bader charge analysis (Table S12). As shown in Figure 5d, the
charge density difference of CH* adsorbed on PTI-2Cu displays the formation of strong Cu-C bond,
indicating the strong interaction between CH* and PTI-2Cu. Thus, the PLS changes from the formation
of CH* to other steps. Overall, the theoretical calculations reveal that Cu site arrangement controlled
by the triazine pore framework of PTI leads to stronger Cu-PTI binding energies, at higher Cu
substitutions, which subsequently yields higher catalytic activity by altering the reaction energy profile
and PLS energies through cooperative catalysis by multiple adjacent Cu sites within proximity. The CN
support matrices help modulate the adsorption energy of CO2 and CO, as shown in Table S8, and
regulate the distance between Cu atoms, as shown in Table S6-7, which dictates the catalytic activity
of Cu atoms for CO2 reduction. However, as we can see from the CO2 adsorption isotherms, the CO2
uptake capability of both CN matrices at room temperature is the same, which increases slightly with
Cu incorporations. Thus, CNs do not directly participate in the reaction pathway but have multiple
synergistic and secondary effects and modulate the electrical conductivity of the catalyst as a whole.
3. Conclusion
In summary, we used a simple metal ion exchange process on 2D crystalline CN matrices, PHI and PTI,
to homogenously disperse single atom Cu moieties with high site density in the nanometric N-
appended pores. Detailed spectroscopic studies in corroboration with DFT calculations reveal the Cu-
N2 type of coordination environment at the vertices of the triangular CN pores and their respective
electronic states. Optimization of Cu loading in both matrices yields the best Cu-PHI and Cu-PTI
catalysts with 5 wt% (1.5 at%) Cu that show excellent eCO2R-to-CH4 performance, surpassing many
of the state-of-the-art catalysts. The best Cu-PTI catalyst exhibits a FECH4 of 68% at a low potential of -
0.84 V, yielding a high partial current density of 348 mA cm-2
. Extensive eCO2R studies on controls and
detailed structure-activity rationalizations reveal crucial catalytic insights on the relative eCO2R
performances of the two catalysts. Most importantly, AC-HAADF-STEM characterization and first-
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principles calculation show the presence and thermodynamic feasibility of multiple adjacent Cu sites
beyond bonding distances within the nanopores, beneficial for cooperative catalysis. The triangular
9N pore dimensions in PTI lead to stronger Cu-N binding energies for multiple Cu substitutions that
can synergistically yield higher catalytic activity by altering the reaction energy profile and PLS energies
through cooperative catalysis. This study on Cu-CN SACs advances the eCO2R-to-CH4 process from
fundamental and performance-level perspectives.
4. Experimental Section/Methods
Carbon nitride synthesis: The carbon nitride supports were prepared through the thermal treatment
of melamine (see supporting info for more details). In the case of Na-PHI/Li-PTI, this thermal
condensation was conducted in the presence of sodium chloride (NaCl) or LiCl, respectively, producing
highly crystalline materials composed of heptazine and triazine units. This process favors the reduction
of defects in the structure of carbon nitride during the synthesis. The as-prepared materials are
labeled as Na-PHI and Li-PTI, due to the presence of Na+
and Li+
ions between and in the layers of poly
(heptazine imide) and poly (triazine imide) motifs.
Catalyst synthesis: Crystalline Na-PHI/Li-PTI were then used as the scaffolds for stabilizing Cu single
ions. The method employed is a simple cation exchange of the Na+
/Li+
by Cu2+
ions, performed at room
temperature in aqueous media. An aqueous dispersion of Na-PHI/Li-PTI and the copper precursor (a
CuCl2 solution) were mixed and stirred to produce stable Cu2+
in the PHI/PTI matrices. The obtained
catalysts are labeled as Cu-PHI/PTI. Varying the precursor Cu salt concentrations from 1-100 mmol/L
solutions during cation exchange, a series of Cu-CN samples could be obtained with copper
concentration ranging between 0.1-10 wt% in the CN scaffolds (Table S1), with the remaining charges
still compensated by Na/Li-ions (see ESI for more details).
Specifically, to exchange the alkali cations with Cu2+
, an aqueous dispersion of PHI or PTI (0.4 mg in 18
ml H2O) was sonicated for 30 minutes, to which a concentrated aqueous copper chloride (CuCl2.6H2O)
solution (2 mL, 1 M) was added. The resultant mixture was sonicated for 30 min, followed by room
temperature stirring for 2 h to yield Cu-PHI/PTI samples. After the metal exchange, the materials were
separated by centrifugation, washed extensively with deionized water, and dried overnight at 60 ◦C
to yield Cu-PHI/PTI samples with Cu-Nx single atom sites. The room temperature solution phase
process, use of water-soluble metal precursors, and mild drying conditions reduce the chances of
nanoparticle formation or metal salt deposition. Nevertheless, a rapid dilute acid leaching treatment
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was conducted on the Cu-PHI/PTI samples prior to characterizations or performance analysis to
eliminate the presence of any impure phases or clusters.
Gas diffusion electrode preparation: To prepare the gas diffusion electrodes (GDEs) for eCO2R, the
cathode catalyst inks for all PHI/PTI and Cu-PHI/PTI samples were dispersed and sonicated (for 30
mins) in isopropanol along with a 5 wt% Nafion binder solution. Subsequently, the catalyst inks were
sprayed onto a gas diffusion layer (GDL, Sigracet 35BC) using an air-brush method followed by drying
at 130 °C under vacuum for 1 h to make GDEs. The geometric area of GDE was 1 cm2
. We optimized
the catalyst loading in the 0.2-1 mg cm-2
range and finally used 0.5 mg cm-2
for the eCO2R tests. The
catalyst loading was determined by precisely weighing the GDL before and after the air-brushing.
Electrochemical CO2 reduction measurement: The eCO2R experiments were conducted under ambient
conditions in 1 M KOH electrolyte using a customized flow cell setup, which used Ni foam as the anode
and FAA-3-PK-75 anion exchange membrane as the separator. A CO2 gas flow of 50 sccm (standard
cubic centimeters per minute) was maintained at the cathode using a mass flow controller (Alicat
Scientific). The electrolyte was fed into the cell using syringe pumps (New Era Pump Systems Inc.). The
electrolyte flow rates were fixed at 1.0 mL min-1
for the cathode and 2.0 mL min-1
for the anode. The
eCO2R tests were conducted for the PHI/PTI and Cu-PHI/PTI GDEs under the chronoamperometric
mode using a Gamry Interface 1010E potentiostat by applying a constant voltage to the flow cell. The
Ag/AgCl (3 M KCl) reference electrode was used to measure the cathodic potential. The iR
compensation was determined using potentiostatic electrochemical impedance spectroscopy (EIS). All
potentials were converted to the RHE scale using: ERHE = EAg/AgCl + 0.209 V + 0.0591 × pH. The geometric
current density was reported unless specified otherwise.
The gas products of eCO2R experiments were monitored through an on-line gas chromatograph (GC,
SRI Multiple Gas #5) equipped with a flame ionization detector and thermal conductivity detector. To
determine the actual flow rate of CO2 at the outlet of the flow cell, an argon gas stream at a rate of 10
sccm was mixed with the outlet gas stream of the flow cell before being looped to the GC. The Faradaic
efficiency (FE) for gas products was calculated based on FE (%) = zFxV × 100% / jtotal, where z is the
number of electrons involved to form the target product; F is Faraday constant; x is the molar fraction
of product with respect to CO2 (from GC analysis); V is the molar flow rate of outlet CO2; jtotal is the
total current density. The liquid products in the catholyte were quantified via 1
H NMR (Bruker AV 400
MHz spectrometer). The standard deviations were calculated using three independent electrodes on
triplicate data sets. Any sample used for electrocatalytic tests in 0.5-5 wt% range of Cu loading was
pretreated with mild acid to minimize probable nanoparticle content. However, it is possible that even
in those samples (till 5 wt%), a minor percentage of NPs may remain or form operando, which leads
to some C2+ side products obtained in the eCO2R product profile.
The TOF values for the catalysts were calculated based on the following formula:[40]
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TOF=(j/N/F)/(m/MCu), where j is the partial current of CH4 at a given potential; N is the number of
electrons (eight) transferred per one CH4 molecule formation; F is the Faraday constant, 96,485 C/mol;
m is the mass of Cu sites in the electrode (obtained by mcat x ω, where mca mass of loaded catalyst in
the electrode (g), ω Cu content in the catalyst (wt%)); MCu is the atomic mass of Cu, 63.55 g/mol.
DFT calculations: Spin-polarized density functional theory (DFT) was carried out by using the Vienna
ab initio simulation package (VASP).[99]
The projector-augmented wave (PAW) pseudopotential and
the Perdew-Burke-Ernzerhof (PBE) exchange-correlational functional with generalized gradient
approximation (GGA) were considered to describe the interactions between valence electrons and
ionic cores and the exchange-correlation effects, respectively.[100,101]
The kinetic cutoff energy was set
to 550 eV for the wave function calculation. The vacuum gap of 15 Å was chosen to neglect interactions
between the system and its mirror images. The van der Waals interaction was described by the DFT-
D3 method.[102]
The convergence criteria were set to be 1 × 10−5
eV for energy change and 0.05 eV/Å
for force change during the geometrical optimization. Bader charges were considered to analyze the
properties of charge transfer.[103]
To evaluate the electrochemical catalytic performance of the CO2 reduction reaction on the designed
catalysts, the computational hydrogen electrode model was applied to obtain the reaction free
energies. A reversible hydrogen electrode (RHE) was used as reference potential, and the chemical
potential of the proton-electron pair was determined by one-half of the chemical potential of H2.
Reaction free energy (ΔG) was achieved by
ΔG = ΔE + ΔZPE – TΔS, (Equation 1)
Where, ΔE, ΔZPE, T, and ΔS indicated the reaction energy, zero-point energy change, temperature,
and entropy change, respectively. The stability of the designed catalysts was evaluated by the binding
energy of Cu (ΔEb) on PHI or PTI, as shown below,
ΔEb = (EnCu-PHI/PTI – EPHI/PTI – nECu)/n (Equation 2)
where EnCu-PHI/PTI, EPHI/PTI, and ECu were the energy of nCu-PHI/PTI (n = 1, 2, and 3), PHI/PTI, and Cu atom,
respectively.
Statistical analysis: The error bars in Figure 4 and other electrochemical CO2 reduction data represent
standard deviation based on the measurements of three independent electrodes.
Supporting Information
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Supporting Information is available from the Wiley Online Library and provides additional
information on synthesis, characterizations.
Declaration
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgments
S.R. would like to acknowledge Clarkson Aerospace Corp. (G10000425) and UES-AFRL (fund no.:
G10000097) for financial support. I.F.S. thanks the Alexander von Humboldt Foundation for their
postdoctoral fellowship. S. C. S. acknowledges Marie Skłodowska-Curie Fellowship H2020-MSCA-IF-
2019 (896637). I.F.T thanks the Alexander von Humboldt Foundation for his postdoctoral fellowship
(Capes/Humboldt) and the Brazilian funding agencies CNPq (405752/2022-9 and 403064/2021-0) and
FAPESP (2020/14741-6 and 2021/11162-8). N.V.T. would like to acknowledge the financial support of
the Max Planck Society. N.V.T. would like to acknowledge the financial support of the Max Planck
Society. The authors would like to thank Canadian Light Source (CLS), Saskatchewan for beamline
access (Project: 35G12344). Drs. Ning Chen, Jay Dynes, Tom Regier and Zachary Arthur are kindly
acknowledged for helping in hard/soft X-ray analysis.
Received: ((will be filled in by the editorial staff))
Revised: ((will be filled in by the editorial staff))
Published online: ((will be filled in by the editorial staff))
Author Contributions
Conceptualization: SR, ZL, JW, PMA
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Methodology: SR, ZL, ZC, IFT
Investigation: SR, ZL, ZC
Experiments and Analyses: SR, ZL, ZC, ACM, PK, SCS, IFT, IFS, GG, NT,
Visualization & Computational Studies: SR, ZL, ZC
Supervision: CVS, PMA, JW
Writing—original draft: SR
Writing—review & editing: JW, SR, ZL, MGK, PMA
#
Soumyabrata Roy and Zhengyuan Licontributed equally to this manuscript.
Data Availability Statement
The data that support the findings of this study are available from the corresponding author upon
reasonable request.
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ToC figure
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Nanopore templates in two-dimensional carbon nitride matrices control the interatomic distances of
cooperative Cu single atom sites to synergistically enhance eCO2-to-CH4 production via modulation of
energy barriers of the rate limiting steps in the reaction pathway
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