High-density Cobalt Single Atom Catalysts for Enhanced Oxygen
Evolution Reaction
https://doi.org/10.1021/jacs.3c00537
J. Am. Chem. Soc. 2023

Figure 1. Schematic diagram of the synthesis of (a) nanocluster CoCML and CoCMM using thermal condensation (800 ºC) of
melamine and melem, respectively. (b) Co-N4-pyridinic SACs using thermal condensation of cobalt phthalocyanine tetramer (CoPc)
with melem (CoMM) and CoPc with melamine (CoML). (c) HR-TEM images of CoCMM showing Co entrapped carbonaceous
nanotubular structure; Inset showing FFT of the image. (d) HR-TEM image of CoMM showing porous structure. (e) HR-TEM image
of CoML. (f) HR-TEM of CoMM showing lattice fringes and d-spacing. AC-HAADF STEM images of CoMM at (g) 5 nm and (h) 2
nm scale bar; white arrow and circles displaying bright spots as cobalt single atoms. The inset in (h) is showing the line scan
showing Z-contrast. (i) EELS spectrum of CoMM from the complete area in image k. (j) EELS spectrum of small square marked in
the image k. (l-p) EELS mapping for Co, C, N, O and RGB composite of C, N and Co.

Figure 2. (a) Raman and (b) XRD spectra of CoMM, CoML, CoGML, CoCML and CoCMM (Bottom to top).
Synchrotron-based Wide Angle X-ray Scattering (WAXS) 2D images of (c) CoMM and (d) CoCML, calculated Q value
of (e) CoMM and (f) CoCML. (g) C1s (h) N1s XPS spectra of CoMM, CoML, CoGML, CoCML and CoCMM (Bottom
to top). Npyr. stands for pyridinic Ns, while Npyrr. stands for pyrrolic Ns. (i) N K-edge and (j) C K-edge NEXAFS spectra
of CN, CoPc, CoGML, CoML and CoMM (Bottom to top).

Figure 3. (a) Synchrotron-based soft-X-ray spectra for Co L-edge of Co(II) nitrate, CoPc, CoGML, CoML and CoMM (b)
XANES spectra of CoML, cobalt acetate, CoMM, metallic Co, CoO and Co3O4. (c) Enlarged XANES spectra of image b.
(d) FT-EXAFS spectra of Co Ac. CoML and CoMM. Dots-Experimental data, Lines-Fitted data. (e) The DFT simulated
models show the distance between neighboring Co-Co (in Co metal and Co dual atom catalyst) and Co-C atoms to
correlate with EXAFS results. (f) Wavelet transform (WT) map of (i) CoMM, (ii) CoML, (iii) Co foil, and (iv) Co acetate.
Background subtracted CO-DRIFTS time profile spectra of (g) CoMM (h) CoCMM obtained at room temperature.

Figure 4. Electrocatalytic OER performance of Co based electrodes, (CP: Carbon paper) (a)
OER LSV study at 5 mV S-1 (b) Tafel slopes (c) TOF at 1.65 V (d) Mass activity (e) Cdl values
from DJ vs n in a non-Faradaic region (f) Comparison of OER activities (@10 mA cm-2 ) of
CoMM with other catalysts (g) Long-term stability of CoMM for 300 h at 5 mA cm-2.

Figure 5. (a) Schematics of operando XAS analysis of CoMM: XANES spectra at OCV and 1.773 V vs RHE (b) the chemical state change of
CoMM during OER showing the regeneration of the active sites. (c) OER free energy profile of pyridinic-nitrogen-cobalt (representing CoMM
and CoML) and pyrrolic-nitrogen-cobalt single-atom models. The cobalt, carbon, nitrogen, oxygen, and hydrogen atoms are marked as blue,
brown, grey, red, and pink, respectively. The orange highlight represents the rate-determining steps with the values of the relevant energy barriers
labelled. (d) Projected density of states (PDOS) on the Co atom for both pyridinic-nitrogen-cobalt and pyrrolic-nitrogen-cobalt single-atom
models. Electron density difference for *OH and *O intermediates on the pyridinic model. (e) Top view and (f) side view of *OH. (g) Top view
and (h) side view of *O.

Scheme S1. Synthetic protocol of cobalt phthalocyanine tetramer and amido cobalt
phthalocyanine tetramer.

Scheme S2. Plausible condensation mechanism of CoPc and melamine/melem (1) structure of CoPc (2) formation of CoPc anhydride30, 33 (3)
reaction between CoPc anhydride and melamine/melem to produce imide. It should be noted that CoPc and melamine react at 325 ºC while
CoPc and melem react at 425 ºC to form imide linkage. Melamine also self polymerize at <320 ºC to create a branched crosslinked network
while melem forms a sparse crosslinked network above 525 ºC.34-37 (4) melamine cross-linked network after further condensation translates to
graphenic structure. (5) melem’s spacious structure condenses into a porous structure due to the less availability of molecules for self-
condensation.

Figure S1. FE-SEM images of CoCML at (a) 20 μm (b) (c) 4 μm scale bar showing the cobalt entrapped carbon
nanotubes-like structure. EDS elemental mapping (d) composite of C and Co (e) Co map showing the presence of Co NPs
in nanotubes (f) EDX spectra of CoCML. Key points: SEM images of CoCML showing the presence of a nanotube-like
structure with cobalt embedded at the top of the tips. EDX elemental mapping demonstrates the presence of Co NPs
centered at the tip of nanotubes. EDX spectra confirm a significant concentration of Co in the materials.

Figure S2. FE-SEM images of CoCMM at (a) 100 μm scale bar showing bunches of Co-containing nanotubes. (b) FE-SEM
images at 500 nm scale bar showing the individual nanotubes with earthworm type structure and Co at the tip of the
nanotubes (c) FE-SEM image at 4 μm scale bar displaying bundles of Co entrapped carbon nanotubes-like structure. EDS
elemental mapping (d) composite of C and Co (e) Co map showing the presence of Co NPs at the tip of nanotubes (f) EDX
spectra of CoCMM. Key points: SEM images of CoCMM showing the presence of a nanotube-like structure with cobalt
embedded at the tip of tubes. EDX elemental mapping demonstrates the presence of Co nanoparticles centered at the tip of
nanotubes. EDX spectra confirm a significant concentration of Co in the materials.

Figure S3. HR-TEM image of CoCMM at (a-b) 200 nm (c) 100 nm (d) 50 nm scale bar showing Co entrapped
nanotubular structure. (e-g) HR-TEM image at 20 and 10 nm scale bar displaying cobalt core and carbon shell along with
lattice fringes of the carbon structure. (h) magnified region of g displaying lattice fringes and d-spacing of 0.37 nm.
Bottom inset showing d-spacing calculated from live profile. (i) SAED pattern of the image showing diffraction ring due
to the presence of monocrystalline Co. Key points: Low magnification images in (a-d) show the presence of dark Co NPs
in the carbonaceous matrix. High magnification TEM images in (e-g) displayed lattice fringes of crystalline stacked
carbon around Co NPs. SAED pattern exhibits rings corresponding to crystalline carbon and Co.

Figure S4. HR-TEM image of CoCML at (a) 50 nm (b) 20 nm scale bar showing Co entrapped nanotubular structure. (c) HR-TEM image at
10 nm scale bar displaying lattice fringes of carbonaceous structure. Bottom inset: displaying interplanar d-spacing of 0.36 nm. Top inset:
showing d-spacing calculated from live profile. (d) HR-TEM image at 10 nm scale bar showing crystalline Co core at the tip of nanotubes. (e)
magnified part of the image d showing lattice fringes of Co, Top inset: FFT of the selected region displaying the interplanar distance of 0.224
nm, Bottom inset: d-spacing calculated from live profile. (f) SAED pattern of the image shows a faint diffraction ring due to the presence of
monocrystalline Co. Key points: Low magnification images revealing the presence of dark Co NPs embedded in N-doped carbonaceous
nanotubes. High-magnification TEM images displayed lattice fringes and corresponded to stacked carbon. HR-TEM images in (d-e) show
metallic Co embedded in carbon with a lattice spacing of 0.225 nm. FFT exhibits the presence of a sharp dot corresponding to monocrystalline
cobalt. SAED reveals the crystalline nature of Co in the material.

Figure S5. HR-TEM image of CoGML at (a) 200 nm (b) 100 nm scale bar showing nanosheets
type structure (c) 50 nm and (d) 20 nm scale bar displaying an absence of any nanoparticles and
lattice fringes originating from short-range stacking. Inset in d showing amorphous nature of
CoGML. Key points: HR-TEM images and corresponding FFT of CoGML show amorphous
materials and no trace of Co NPs/clusters.

Figure S6. XPS spectra of CoPc (a) survey scan, HR-XPS in (b) C1s and (c) N1s region. (d)
show the type of carbon and nitrogen in deconvoluted HR-XPS (e) O1s and (f) Co2p region.

Figure S7. Bottom to top (a) N2 adsorption-desorption isotherm (b) pore size distribution of
CoMM, CoML, CoGML, CoCML and CoCMM.

Figure S8. FE-SEM images of CoMM at (a) 10 μm scale bar showing the porous structure (b) EDS elemental mapping
showing a composite of C, N and Co (c) only Co. FE-SEM images at (d) 4 μm (e) 1 μm scale bar displaying magnified
porous structure (f) image showing graphitic sheets structures. (g) elemental mapping for Co (h) composite of C, N, O and
Co. (i) EDX spectra of CoMM. Key points: SEM images of CoMM display a porous structure composed of a graphenic
sheets. Mapping demonstrates the uniform distribution of Co in the carbonaceous scaffold.

Figure S9. FE-SEM images of CoML at (a) 20 μm (b) 4 μm scale bar displaying graphinic structure. (c) EDS elemental
mapping showing the distribution of Co. EDS composites map of (d) C, N and Co (e) C, N, O and Co. (f) EDX spectra of
CoML. Key points: The FE-SEM images of CoML show graphene-type sheets and exclude the possibility of bigger
nanoparticles. Mapping images demonstrate that Co was homogeneously distributed over the sheets. EDX spectra
demonstrate a sharp peak for Co suggesting a higher concentration of Co in the sheets.

Figure S10. HR-TEM image of CoMM (a) at 100 nm scale bar showing porous structure (b) at 10 nm scale bar displaying graphenic structure; bottom
right inset: the enlarged image of the marked area showing localized carbon stacking generated lattice fringes; bottom left inset: d-spacing calculated from
live profile. Top right inset: FFT of complete images showing the amorphous nature of the materials. (c) HR-TEM image and corresponding FFT. (d)
SAED pattern showing absence of any diffraction pattern for metallic Co NPs. (e) AC-HAADF STEM images of CoMM at (e-h) 2 nm scale bar; white
arrow displaying bright spots for single atom cobalt. The inset in (h) is showing the line intensity profile for the bright Co center corroborating the presence
of a single atom. (i) annular dark field (ADF) electron image of the area mapped for the EELS spectrum. EELS mapping for (j) N (k) Co (l) C (m) O. (n)
corresponding EELS spectrum showing the peak for Co L-edge. Inset is an RGB composite of C (red), N (green) and Co (blue). Key points: Low
magnification HR-TEM images of CoMM displaying the highly porous nature of the materials. High-magnification HR-TEM images show the absence of
any nanoclusters. FFT and SAED patterns show the amorphous nature of the material. AC-HAADF-STEM shows the presence of single atoms while the
line scan reveals an intense profile for single atoms. EELS maps show uniformly distributed Co SA over the carbonaceous/nitrogenous scaffold. EELS
spectra show the presence of Co-L3 and Co-L2 energy loss peaks.

Figure S11. AC-HAADF STEM images of CoMM at (a,b) 5 nm scale bar (c,d) 2 nm scale
bar showing the dense distribution of single Co atoms (bright spots).

Figure S12. TEM images of CoML at (a) 0.5 μm (b-c) 100 nm scale bar showing nanosheet morphology (d) SAED pattern of the selected area in
image c showing absence of any crystalline features. (e-g) high-resolution TEM images of CoML showing the absence of nanoparticles or
nanoclusters. Inset in image (f) shows the FFT with no diffraction pattern. (h) enlarged HR-TEM image of the area in figure g showing
amorphous graphenic structure and inset FFT further confirm the absence of any nanoparticle/nanocluster. (i) STEM electron image. Elemental
mapping of CoML for (j) C, (k) Co (l) N (m) O and (n) corresponding EDX mapping. Key points: Low magnification HR-TEM images of
CoML displays graphenic structure. The absence of any additional ring in the SAED pattern excludes the possibility of any
nanoparticles/nanoclusters. Images at high magnification (10 nm scale bar) and their FFT show amorphous nature and the absence of any
nanoclusters. STEM mapping images confirm the uniform distribution of N and Co on the graphenic sheets. EDX further confirms the presence
of high concentration of Co.

Figure S13. AC-HAADF STEM images of CoML at (a) 5 nm and (b) 1 nm scale bar; white arrow and circles displaying bright spots for single atom cobalt. (c-e) at 5 nm and
(f) expanded image at 1 nm scale bar; white arrow and circles displaying bright spots for single atom cobalt. The inset in (f) is showing the size of the bright spot is 0.65 Å
corroborating the presence of a Co single atom. (g) at 5 nm showing single atoms of Co (h) annular dark field (ADF) electron image of the area mapped for the EELS
spectrum. EELS mapping for (i) C (j) O (k) Co (l) N and RGB composite of (m) C (red), N (green) and Co (blue). (n) EELS spectrum of the complete area in image h and (o)
EELS spectrum of small spot marked in the image h. Key points: AC-HAADF-STEM images show the presence of Co single atom structures. Line scan for intensity profile
shows intense regions for Co atoms. EELS mapping and composite images show the absence of any nanoclustering and the presence of Co on graphenic sheets. EELS
spectra of a small pixel in the ADF image show the presence of Co with a populated concentration of isolated Co.

Figure S14. AC-HAADF STEM images of CoML at 5 nm scale bar (a) after 1 min of beam exposure showing the absence
of any NPs/clusters (b) after 10 min of beam exposure showing the formation of nanoclusters. AC-HAADF STEM images
of CoML at 5 nm scale bar (c) after 1 min of beam exposure display the absence of any nanoclusters. (d) ADF electron
image of the same area after 10 min of beam exposure mapped for EELS showing the beam damage and formation of
cobalt clusters. EELS mapping of the selected area showing the distribution of (e) N (f) C (g) O and (h) Co (i) EELS
spectrum of image d showing Co L-edge (Inset showing RGB map of C, N, and Co). Key points: EELS map collected in
just 1 min do not show any clustering however after 15 min beam exposure Co gets agglomerated and showed visible
nanoclusters.

Figure S15. AC-HAADF STEM images of CoMM at 5 nm scale bar (a) after 1 min of beam exposure showing the
absence of any NPs/clusters (b) after 10 min of beam exposure showing the formation of nanoclusters. AC-HAADF
STEM images of CoMM at 5 nm scale bar (c) after 1 min of beam exposure show the absence of any nanoclusters.
(d) ADF electron image of the same area after 10 min of beam exposure mapped for EELS showing the beam
damage and formation of cobalt clusters. EELS mapping of the selected area showing the distribution of (e) Co (f) C
(g) O and (h) N (i) EELS spectrum of image d showing Co L-edge (Inset showing RGB map for C, N and Co). Key
points: AC-HAADF-STEM and EELS ADF images show the absence of any nanoclusters after 1 min of data
collection, however, Co clusters were observed after 15 min of beam exposure.

Figure S16. Electron energy loss spectroscopy of (a) Full range data showing C K-edge, N K-edge and
Co L-edge of CoMM (lower panel) and CoML (upper panel). Overlapped EELS spectra of CoMM and
CoML in (b) C K-edge region and (c) N K-edge region showing π* and σ* transition.

Figure S17. Solid-state X-band EPR spectra lower panel: CoML and upper panel: CoMM.

Figure S18. (a) Comparison of Raman spectra of CoML and CoMM with carbon black (CB), reduced graphene oxide (RGO), and nitrogen-doped reduced
graphene oxide (NRGO). (b) FTIR spectra of NRGO, melem, CN, CoML and CoMM. (c) XRD spectra of melem, CN, CoCML and CoMM. Key points:
The peaks corresponding to the (100) plane of CN were absent while the (002) peak was shifted to a lower 2θ value in CoCML and CoMM due to the
graphitization of the carbon nitride framework. CoMM displayed decreased peak intensity suggesting transformation to the carbonaceous structure. CoCML
displayed peaks for the (111), (200) and (220) due to the presence of metallic α-Co with a face-centered cubic (fcc) structure.

Figure S19. Synchrotron-based WAXS 2D map and obtained Q-1 values of (a and d) CoML (b
and e) CoGML and (c and f) CoCMM.

Figure S20. High-resolution XPS spectra of CN in (a) C1s (b) N1s region.

Figure S21. (a) XPS survey scan (b) O1s (c) Co2p and (d) CoLMM Auger spectra of Bottom to
top: CoMM, CoML, CoGML, CoCML and CoCMM respectively.

Figure S22. XPS spectra of Co-Mel-600 (a) survey scan. Core-level HR-XPS in (b) C1s (c)
N1s (d) O1s (e) Co2p regions (f) Raman spectra of Co-Mel-600.

Figure S23. Excitation-Emission Matrix Spectroscopy (EEMS) map of CoCMM showing (a) C K-edge (b) NEXAFS
spectra in C K-edge showing π* and σ* transition. (c) EEMS map showing faint signal of N K-edge and (d) NEXAFS
spectra in N K-edge showing π* and σ* transition of pyridinic, pyrrolic, graphitic, and C-N/C-N=C nitrogen.

Figure S24. Normalized XANES spectra of CoCML (yellow), CoCMM (violet), CoGML
(green), Co nitrate (blue), CoPc (pink), and Co3O4 (purple).

Figure S25. (a) Bottom to top: DRIFT spectra of the catalysts without CO probe for CoMM,
CoML, CoGML, and CoCMM. Background subtracted CO-DRIFTS time profile spectra
of (b) CoML (c) CoGML obtained at room temperature.

Figure S26. OER study in 1 M KOH. (a) Polarization curves (b) Corresponding Tafel plots for
CoGML (green) and CoCML (yellow) respectively.

Figure S27. EIS of CoML, CoMM, IrC and PtC at 393 mV

Figure S28. Histogram demonstrating the change in the current density of catalysts versus
applied overpotentials for OER.

Figure S29. Double-layer capacitance (Cdl) in non-Faradaic region (0-0.1 V vs Ag/AgCl). (a-e)
Rectangular Cdl response with respect to scan rates (v) with increasing ja and jc for CoMM
(red), CoML (blue), CoGML (green), CoCML (yellow) and CoCMM (violet), respectively.

Figure S30. OER specific activities of CoMM (blue) and CoML (red) after ECSA
normalization. (a) ECSA normalized LSV curves, (b) Mass activity versus potential (RHE),
and (c) TOF versus potential (RHE).

Figure 31. Summary of conductive graphene/N-carbon based catalysts with metal loading
higher than 10% and their synthesis route. Details are summarized in Table S7.

Figure S32. EIS studies at various potentials vs Ag/AgCl in 1 M KOH electrolyte. Nyquist plots of (a) CoMM (b) CoML
(c) Ir/C (d) Pt/C catalysts in 1.0 M KOH at various applied potential bias (e) comparison of Nyquist plots at 0.5 and 0.7 V vs
Ag/AgCl for CoML, CoMM, Ir/C and Pt/C electrodes. (Note – inset of each figure depicts the magnified images especially
in high-frequency regions to analyze the difference in semi-circles (Rct) in the prepared electrodes).

Figure S33. Chronopotentiometric stability study of the best active electrodes (a) CoMM and
(b) CoML compared with IrC for OER at 10 mA cm-2 in 1 M KOH for 16 h.

Figure S34. Long run stability study of CoML as anode and cathode for 200 h at 5 mA cm-2.

Figure S35. Post OER EIS studies of (a) CoMM (b) CoML.

Figure S36. XPS spectra (a) survey scan (b) in C1s (c) N1s (d) O1s (e) F1s (f) Co2p region
of lower to the upper panel: CC (carbon cloth), CoMM as coated on CC, CoMM after
OER at 5 mA cm-2 for 16 h, CoMM after OER at 10 mA cm-2 for 16 h, CoMM after 500 CV
cycles at 100 mV sec-1.

Figure S37. Raman spectra in the frequency range of (a) 100-800 cm-1 (b) 100-800 cm-1 lower
to upper panel: CoMM as coated on CC (green), CoMM after OER at 5 mA cm-2 for 16 h
(pink), CoMM after OER at 10 mA cm-2 for 16 h (blue), CoMM after 500 CV cycles at 100
mV sec-1 (red)

Figure S38. GC chromatogram of gaseous reaction product displaying signals of O2 OER
when CoMM was studied in 1M KOH electrolytes.

Figure S39. Digital photographs of electrochemical cell used for the operando XAS
measurement (a) complete picture of the cell (b) image showing lid with mounted reference and
counter electrode while working electrode was attached to main cell (c) back view of a plastic
window with the sample having a connection with the working electrode (d) front view of the
window showing sample exposed to X-ray beam (e) picture of cell secured and aligned on beam
for the measurement and attached to the electrochemical workstation.

Figure S40. The calculated Co-Co atomic distance of CoMM from HAADF-STEM (a) line
scan (b) live profile of selected line scan.

H2O
H+ + e-
H+ + e-
H2O
H+ + e-
O2 + H+ + e-
Figure 41. Schematics of OER mechanism over pyridinic-nitrogen-cobalt single-atom model.

H2O
H+ + e-
H+ + e-
H2O
H+ + e-
O2 + H+ + e-
Figure S42. Schematics of OER mechanism over pyrrolic-nitrogen-cobalt single-atom model.

1.18 -0.62
1.00 -0.94
1.14 -0.51
1.19 -1.17
(a) (b)
(c) (d)
Figure S43. Bader charge analysis for *OH and *O intermediates. (a) *OH on the pyridinic
model (b) *O on the pyridinic model (c) *OH on the pyrrolic model (d) *O on the pyrrolic
model.

(a) (b)
(c) (d)
Figure S44. Electron density difference for *OH and *O intermediates on the pyrrolic model.
(a) Top view and (b) side view of *OH. (c) Top view and (d) side view of *O.