Cobalt single-atoms anchored on porphyrinic triazine-based frameworks as bifunctional electrocatalysts for oxygen reduction and hydrogen evolution reactions

Article abstract of DOI:10.1039/c8ta09490j

Designing and fabrication of highly active single-atom catalysts (SACs) with maximized atomic efficiency is highly desirable but still remains a great challenge. Herein, highly active and stable cobalt single-atoms with a Co-N₄ moiety were uniformly anchored on a porous porphyrinic triazine-based framework (CoSAs/PTF) by a simple ionothermal method. Due to the abundant single-atom Co-N₄ species, the hierarchical porous structure and the good conductivity, the resultant catalyst is highly active for the electrocatalytic oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER). For the ORR, a more positive half-wave potential of 0.808 V (vs. RHE) was achieved, compared with commercial benchmark Pt/C (0.806 V). Furthermore, a small onset potential of 21 mV and a low Tafel slope of 50 mV per decade were obtained for the HER. The porphyrin-like structure was found to stabilize the CoSAs effectively, thus leading to long-term durability and a remarkable methanol-tolerant behavior. This bifunctional single-atom catalyst might be a promising candidate to replace Pt-based electrocatalysts in electrolysers and fuel cells.

Full text of DOI:10.1039/c8ta09490j

Journal of
Materials Chemistry A
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PAPER
Cobalt single-atoms anchored on porphyrinic
triazine-based frameworks as bifunctional
electrocatalysts for oxygen reduction and
hydrogen evolution reactions†
Cite this: J. Mater. Chem. A, 2019, 7,
1252
a
ab
a
Jun-Dong Yi,
Rui Xu,
Guo-Liang Chai,
Teng Zhang,a Ketao Zang,c
Bing Nan,d Hua Lin,
Yuan-Biao Huang
Yu-Lin Liang,d Jiangquan Lv,ab Jun Luo,c Rui Si,
and Rong Cao
a
d
a
ab
*
*
Designing and fabrication of highly active single-atom catalysts (SACs) with maximized atomic efficiency is
highly desirable but still remains a great challenge. Herein, highly active and stable cobalt single-atoms with
a Co–N4 moiety were uniformly anchored on a porous porphyrinic triazine-based framework (CoSAs/PTF)
by a simple ionothermal method. Due to the abundant single-atom Co–N4 species, the hierarchical porous
structure and the good conductivity, the resultant catalyst is highly active for the electrocatalytic oxygen
reduction reaction (ORR) and hydrogen evolution reaction (HER). For the ORR, a more positive half-wave
potential of 0.808 V (vs. RHE) was achieved, compared with commercial benchmark Pt/C (0.806 V).
Furthermore, a small onset potential of 21 mV and a low Tafel slope of 50 mV per decade were obtained
for the HER. The porphyrin-like structure was found to stabilize the CoSAs effectively, thus leading to
long-term durability and a remarkable methanol-tolerant behavior. This bifunctional single-atom catalyst
might be a promising candidate to replace Pt-based electrocatalysts in electrolysers and fuel cells.
Received 1st October 2018
Accepted 30th November 2018
DOI: 10.1039/c8ta09490j
rsc.li/materials-a
SACs, particularly non-precious metals such as cobalt single-
atoms,17–19 are still a great challenge due to their tendency to
Introduction
aggregate. Therefore, it is highly desirable to develop a facile
and general strategy for achieving highly active and stable metal
SACs on an appropriate support.
The size of the metal particles plays a crucial role in nano-
catalysis, and enormous efforts have been made towards
improving the catalytic performance by downsizing metal
nanoparticles.1,2 Single-atom catalysts (SACs), the ultimate
small-size limit for metal particle catalysts, have attracted
extensive attention in recent years because they have low-
coordination active sites that could maximize the atom utili-
zation and thus offer great potential for achieving high activity
and selectivity.3–5 Furthermore, SACs could be employed as ideal
models to bridge the gap between heterogeneous and homo-
geneous catalysis, which is favorable for understanding
heterogeneous catalysis at the molecular level. Although much
effort has been made,6–16 the fabrication and characterization of
Since rst reported by Thomas,20 covalent triazine frame-
works (CTFs) constructed via the trimerization of aromatic
nitriles have attracted considerable attention in recent years
due to their abundant nitrogen sites, large surface areas,
extremely high thermal/chemical stability and mechanical
robustness.21–26 Furthermore, porphyrin motifs could readily
incorporate and stabilize a variety of metal ions to form met-
alloporphyrins, which could serve as light harvesters, oxygen
carriers and catalysts.27 Incorporating metalloporphyrin-like
architectures into porous materials to form M–Nx active sites
has been proved to be an effective way to develop SACs.28–32 More
recently, we have incorporated Fe porphyrin-like units into
porous robust CTFs by using a one-step ionothermal synthesis
approach.31
aState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, 155 Yangqiao Road West, Fuzhou
350002, P. R. China. E-mail: ybhuang@irsm.ac.cn; rcao@irsm.ac.cn
bUniversity of the Chinese Academy of Sciences, Beijing 100049, China
cCenter for Electron Microscopy, Institute for New Energy Materials and Low-Carbon
Technologies, School of Materials, Tianjin University of Technology, Tianjin 300384,
China
In recent decades, a number of researchers have made
efforts to develop clean and renewable energy devices, such as
fuel cells and water electrolysers,33–37 to deal with the energy
crisis and environmental contamination caused by the massive
consumption of fossil energy. The oxygen reduction reaction
(ORR) and hydrogen evolution reaction (HER) are the key issues
for fuel cells and water electrolysers, respectively. Although
dShanghai Synchrotron Radiation Facility, Shanghai Institute of Applied Physics,
Chinese Academy of Sciences, Shanghai 201204, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ta09490j
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ꢁ
Pt-based catalysts are widely regarded as the most efficient synthesis temperature could not exceed 600 C; otherwise the
catalysts for the HER and ORR, the low abundance and high quartz tube will explode.
cost of Pt impede its widespread application. Therefore, it is
urgent to design and fabricate nonprecious-metal materials.
Among them, cobalt-based materials have been found to be
highly active for both the ORR and HER.
Synthesis of PTF-600
0.722 g TPPCN (1 mmol) and 1.36 g ZnCl2 (10 mmol) were
transferred into a quartz tube in an argon-lled glove box. Aer
the tube was evacuated and sealed, it was heated to 600 C for
40 h. Aer cooling to room temperature, the tube was broken
and opened to obtain a black powder. The obtained product was
ground adequately and then stirred in 0.1 M HCl for 24 h to
remove the residual salt. Aer centrifugation, the precipitate
was washed withꢁwater and methanol two times separately, and
then dried at 70 C under vacuum for 6 h.
Herein, non-precious Co single-atom catalysts based on
porous porphyrinic triazine-based frameworks (denoted as
CoSAs/PTFs) were fabricated by incorporating cobalt porphyrin-
like units into CTFs and subsequent in situ thermalization.
Atomically dispersed Co–N4 active sites were achieved while
maintaining the metalloporphyrin-like moieties. Thanks to the
abundant atomic dispersion of coordinatively unsaturated Co–
N4 sites, hierarchically micro- and mesoporous supports, and
signicantly enhanced conductivity, the resultant SACs exhibi-
ted superior electrocatalytic activity towards both the oxygen
ꢁ
Synthesis of (Co)PTF-600
reduction reaction (ORR) and hydrogen evolution reaction 50 mg of the as-prepared CoSAs/PTF-600 was dispersed in 5 mL
(HER).
triuoromethanesulfonic acid to form a uniform suspension
and stirred for 24 h. Aer centrifugation, the obtained precipi-
tate was washed with deionized water three times and dried at
70 ꢁC under vacuum for 24 h. ICP-AES analysis revealed that the
content of Co was 0.06 wt%.
Experimental
Synthesis of 5,10,15,20-tetrakis(4-cyanophenyl)porphyrin
(TPPCN)
Synthesis of CoNPs/PTF-600
TPPCN was synthesized using a modied method from the
literature.38 4-Formyl benzonitrile (1.13 g, 8.46 mmol), pyrrole
(0.6 mL, 8.46 mmol) and propionic acid (20 mL) were added to
a ask and heated to reux overnight. The reaction mixture was
washed several times using methanol, ethyl acetate, and tetra-
hydrofuran. Then, it was dried in a vacuum to get TPPCN
(0.35 g) as a violet solid with 23% yield. 1H NMR (CDCl3): d 8.83
(s, 8H), 8.35 (d, 8H), 8.10 (d, 8H), ꢀ2.86 (2H).
50 mg CoSAs/PTF-600 was dispersed in 20 mL deionized water
to form a uniform suspension. Then 200 mg CoCl2$6H2O was
added and stirred for 4 h. Aer centrifugation, the black powder
was dispersed in 20 mL deionized water and 228 mg NaBH4 was
added to it. Aer stirring for 24 h, the resultant black product
was collected by centrifugation, washed with deionized water
three times and dried at 70 ꢁC under vacuum for 24 h. ICP-AES
analysis revealed that the content of Co was 25.6 wt%.
Synthesis of [5,10,15,20-tetrakis(4-cyanophenyl)
porphyrinato]-Co (Co-TPPCN)
Structural characterization
Transmission electron microscope (TEM) images were recorded
using a FEIT 20 working at 200 kV. Aberration-corrected high-
angle annular dark-eld scanning transmission electron
microscopy (HAADF-STEM) and EDS of samples were per-
formed with a Titan Cubed Themis G2 300 (FEI) high-resolution
transmission electron microscope operated at 200 kV. Powder
X-ray diffraction (PXRD) patterns were recorded on a Miniex
600 diffractometer using Cu Ka radiation (l ¼ 0.154 nm). The
Raman spectra were obtained on a Labram HR800 Evolution
over a range of 500–3500 cmꢀ1. N2 adsorption–desorption
isotherms and Brunauer–Emmett–Teller (BET) surface area
0.722 g TPPCN (1.0 mmol) and 3.1 g CoCl2$6H2O (12.8 mmol)
were dissolved in 100 mL of DMF and then reuxed for 6 h.
150 mL of H2O was added to the mixture aer cooling down to
ambient temperature. The resultant precipitate was ltered and
washed three times with deionized water. Then the as-prepared
product was dissolved in CHCl3 and washed three times with
deionized water again. Red powders were obtained aer
evaporation.
Synthesis of CoSAs/PTFs
0.779 g Co-TPPCN (1 mmol) and 1.36 g ZnCl2 (10 mmol) were measurements were performed using a Micromeritics ASAP
transferred into a quartz tube in an argon-lled glove box. Aer 2020 instrument. Thermogravimetric analyses (TGA) were per-
the tube was evacuated and sealed, it was heated at certain formed under a nitrogen atmosphere with a heating rate of
temperatures (400, 500, and 600 C) for 40 h. Aer cooling to 10 ꢁC minꢀ1 by using an SDT Q600 thermogravimetric analyser.
ꢁ
room temperature, the tube was broken and opened to obtain X-ray photoelectron spectroscopy (XPS) measurements were
a black powder. The obtained product was ground adequately performed on an ESCALAB 250Xi X-ray photoelectron spec-
and then stirred in 0.1 M HCl for 24 h to remove the residual trometer (Thermo Fisher) using an Al Ka source (15 kV, 10 mA).
salt. Aer centrifugation, the precipitate was washed with water XAFS spectra at the Co K-edge (7709 eV) were measured at the
and methanol two times separately, and then dried at 70 ꢁC BL14W1 beamline station of the Shanghai Synchrotron Radia-
under vacuum for 6 h. Notably, due to the lower boiling point tion Facility, China. The Co K-edge XANES data were recorded in
ꢁ
(732 C at atmospheric pressure) of ZnCl2 under vacuum, the a transmission mode. Co foil was used as a reference.
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displacement and RMS displacement were all set by default.
The energy convergence was set to 10ꢀ8 a.u. for all the calcu-
lations. The free energies of intermediates and the electro-
chemical limiting potentials were calculated according to our
previous papers.42,43 The free energy can be obtained by adding
the zero point energy, entropy, and solvation energy corrections
to the total energy as follows:
Electrochemical characterization
All the electrochemical experiments were performed on an
IM6ex (Zahner, Germany). Typically, 2 mg of catalyst and 40 mL
5 wt% Naon were dispersed in 1 mL of the mixed solvent of
water/ethanol with a volume ratio of 4 : 1 and ultrasonicated for
30 min. Then 20 mL of the resulting ink was added dropwise
onto the surface of a rotating disk electrode (RDE) with a glassy
carbon (GC) disk of 5 mm diameter and dried at room
temperature. For the ORR test, the electrochemical measure-
ments were performed in a three-electrode cell using the Ag/
AgCl electrode as the reference electrode and Pt foil as the
counter electrode. Linear sweep voltammetry (LSV) at a scan
rate of 10 mV sꢀ1 was conducted in 0.1 M KOH at room
temperature. Before the electrochemical measurements, the
electrolyte solution was purged with O2 for 30 min to achieve an
O2-saturated solution, and stable polarization curves were
recorded aer 20 cycles. For the HER test, the electrochemical
measurements were performed in a three-electrode cell with the
use of an Ag/AgCl electrode as the reference electrode and
a graphite rod as the counter electrode. Linear sweep voltam-
metry (LSV) at a scan rate of 10 mV sꢀ1 was conducted in 0.5 M
H2SO4 at room temperature. Before the electrochemical
measurements, the electrolyte solution was purged with N2 for
30 min, and stable polarization curves were recorded aer 20
cycles. All LSV curves were measured with IR compensation.
Electrochemical impedance spectroscopy (EIS) measurements
were conducted by applying AC voltage with 5 mV amplitude in
a frequency range from 0.01 Hz to 100 kHz.
DG ¼ DE + DZPE ꢀ TDS + DGs
(1)
where DG, DE, DZPE, DS and DGs are the free energy, total
energy, zero point energy, entropy and solvation energy. Then,
the limiting potential is dened as:
UL ¼ Mini[ꢀDGi] ¼ ne
(2)
where n is the number of electrons transferred for each elec-
trochemical step (here n ¼ 1 for the one-electron transfer step)
and e is the elementary charge. The meaning of Mini[ꢀDGi] in
eqn (2) is to select the smallest [ꢀDGi] among the ORR
elementary steps. Therefore, the limiting potential is the high-
est potential where all of the electrochemical steps are downhill
in free energy, which can be compared with the half-wave
potential measured in experiments.
Results and discussion
CoSAs/PTFs were fabricated by the trimerization reaction
of [5,10,15,20-tetrakis(4-cyanophenyl)porphyrinato]-Co (Co-
TPPCN) in the presence of molten zinc chloride under high
vacuum at 400 ꢁC, 500 ꢁC and 600 ꢁC for 40 h, respectively,
followed by the acid leaching of ZnCl2 and Co-based particles
(Scheme 1). Here, the molten zinc chloride acted as both a Lewis
acid catalyst and a porogenic agent to generate mesopores.
Notably, based on the TGA curves (Fig. S1†), Co-TPPCN was rst
trimerized into a CTF at low temperature (e.g., 400 ꢁC) and then
subsequently in situ thermally transformed into a cobalt por-
phyrinic triazine-based carbonaceous structure at 500 and
600 ꢁC. For convenience, the samples obtained at different
temperatures are denoted as CoSAs/PTF-x (x ¼ polymerization
temperature). The formation of a triazine-based framework was
conrmed by FT-IR spectra.20–26 In the spectra, the carbonitrile
stretching band at 2227 cmꢀ1 disappeared aer polymerization,
Rotating ring-disk electrode (RRDE) measurements were
recorded with catalyst inks and electrodes prepared by the same
method as that for the RDE measurements. The disk electrode
was scanned at a rate of 10 mV sꢀ1 and the ring potential was
constant at 0.5 V vs. Ag/AgCl. The hydrogen peroxide yield
(H2O2%) and the electron transfer number (n) were calculated
with the following equations:
Ir
N
H2O2ð%Þ ¼ 200 ꢂ
Ir
Id þ
N
Id
n ¼ 4 ꢂ
Ir
Id þ
N
where Id is the disk current, Ir is the ring current and N ¼ 0.397
is the current collection efficiency of the Pt ring measured in N2-
saturated 0.1 M K3[Fe(CN)6] solution (details in the ESI†).
First principles calculations
The total energies and geometric structures were determined by
using density functional theory (DFT) that was implemented in
the Gaussian 03 program. The hybrid Becke three-parameter
Lee–Yang–Parr (B3LYP) functional was employed.39–41 The
basis set for C, H, N and O was set to the 6-31+G* level, while
that for Co was set to LANL2DZ. The convergent values of the
maximum force, root mean-square (RMS) force, maximum Scheme 1 Schematic illustration of the fabrication of CoSAs/PTFs.
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and a new absorption band at 1564 cmꢀ1 appeared, which can highly hierarchically porous structures could expose more
be assigned to triazine moieties (Fig. S2†). The powder X-ray active sites and make reactant species more accessible, thus
diffraction (PXRD) patterns of the CoSAs/PTF samples showed improving the catalytic efficiency.
a broad peak in the 2q range of 20–30ꢁ (Fig. S3†), indicating the
The Co loadings in CoSAs/PTF-400, CoSAs/PTF-500, and
formation of an amorphous structure20–26 or a graphitic carbon CoSAs/PTF-600 were determined to be 5.15, 2.74 and 0.85 wt% by
architecture. Moreover, no peaks related to Co or cobalt oxide ICP-OES measurements, respectively (Table S3†). A decrease in
particles can be detected in the PXRD or Raman spectra the Co loading with increasing polymerization temperature was
(Fig. S4†), ruling out the presence of aggregated Co species. In due to the partial pyrolysis of cobalt porphyrin moieties forming
the Raman spectra, the high ratios of IG/ID and the appearance Co particles that were subsequently removed by acid leaching
of a broad 2D peak at ca. 2880 cmꢀ1 further conrmed the steps. The transmission electron microscope (TEM) images
presence of layered or graphitized structures.20–26 It is worth (Fig. 2a, S6 and S7†) and high-angle annular dark-eld scanning
noting that the graphitization of CoSAs/PTFs occurred at transmission electron microscope (HAADF-STEM) images
elevated polymerization temperatures, which will be favorable (Fig. 2b) of the three CoSAs/PTF samples further conrm that no
for improving the conductivity and thus enhancing the elec- visible Co-based particles can be observed. The elemental
trocatalysis. Electrochemical impedance spectroscopy (EIS) mapping images also showed that Co and N were distributed
measurements (Fig. S5, Table S1†) further proved that CoSAs/ uniformly over the entire architecture (Fig. 2b). To further
PTF-600 has the best conductivity among the three materials. identify the formation of the CoSAs, aberration-corrected
Nitrogen sorption measurements indicated that all the CoSAs/ HAADF-STEM (AC-HAADF-STEM) measurements were conduct-
PTFs have large BET surface areas of up to 741 cm3 gꢀ1 ed. As shown in Fig. 2c, d and S8,† many atomically dispersed
(Fig. 1a and Table S2†). Furthermore, the CoSAs/PTFs display bright dots ascribed to the isolated heavier CoSAs are discerned
typical type IV isotherms with hysteresis loops in the P/P0 range in the representative CoSAs/PTF-600 material due to the contrast
of 0.45–1.0, suggesting the presence of mesopores.44 Non-local between Co and the light elements (C and N).45–47
density functional theory analysis further demonstrates a wide
To further validate the atomic dispersion of Co species and
pore size distribution in the range of 0–14 nm (Fig. 1b). Such their coordination environments, X-ray absorption near-edge
structure (XANES) and extended X-ray absorption ne struc-
ture (EXAFS) measurements were performed. As shown in the
Co K-edge XANES spectra (Fig. 3a), all the CoSAs/PTFs show
similar proles to the precursor Co-TPPCN but they are
different from that of Co foil, which indicates the existence of
a metalloporphyrin-like Co–N4 structure.28 The visible pre-edge
peak at 7715 eV, which is attributed to the 1s / 4pz shake down
Fig. 2 (a) TEM and (b) HAADF-STEM images of CoSAs/PTF-600 and
the corresponding elemental maps showing the distribution of Co, C,
Fig. 1 (a) N2 adsorption–desorption isotherms of CoSAs/PTFs. (b) The and N. (c and d) AC-HAADF-STEM image and enlarged image of
pore size distribution (PSD) profiles of CoSAs/PTFs.
CoSAs/PTF-600. Some Co single atoms highlighted by red circles.
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Fig. 3 (a) The normalized XANES spectra at the Co K-edge of the CoSAs/PTFs, Co-TPPCN, and Co foil. (b) Fourier transform EXAFS spectra of
different samples. (c) The corresponding EXAFS fitting curves of CoSAs/PTF-600. The inset is the proposed Co–porphyrin architecture.
transition, further conrmed the square-planar conguration of environments in the samples. The tting of the EXAFS spectra
the Co–N4 center with high D4h symmetry.28 The white line (Table S5†) implied that the coordination numbers of the Co
position of the CoSAs/PTFs is located between those of the Co- centers for CoSAs/PTF-400, 500, 600 are 4.2, 3.9 and 3.9,
TPPCN and Co foil, implying that Co single atoms carried respectively. These results strongly suggested that the dominant
positive charges with oxidation states between Co0 and CoII.45–47 active Co sites in all the CoSAs/PTFs can be postulated as
Furthermore, compared with the reduced state of cobalt, square-planar and coordination-unsaturated Co–N4 models,
a down-shi of Co 2p in all the CoSAs/PTF samples was retaining the Co porphyrin-like coordination environment
observed in their X-ray photoelectron spectra (XPS, Fig. S9†), similar to that of the monomer Co-TPPCN (Fig. 3c, S11–S13†).
indicating that the CoSAs were coordinated with the N atoms in Hence, all the above results indicate that the metalloporphyrin-
porphyrin moieties.28 The high-resolution N 1s peaks also like moieties in the CoSAs/PTFs are still mainly preserved upon
proved the presence of cobalt-bonded N atoms (Fig. S10, Table polymerization and thermalization. The porphyrin-like archi-
S4†). In the Fourier transform of EXAFS for the CoSAs/PTFs tectures are believed to play pivotal roles in stabilizing the
˚
(Fig. 3b), the absence of Co–Co scattering paths at 2.16 A CoSAs with such high loadings.
clearly excluded the presence of Co particles, conrming that
These unique features could render CoSAs/PTFs as prom-
the Co species exist as atomically dispersed sites, which is ising Pt-alternative electrocatalysts for the oxygen reduction
consistent with the conclusion from the AC-HAADF-STEM reaction (ORR), which plays a prominent role in fuel cells and
images and PXRD patterns. The similarity among the EXAFS metal–air batteries.48–54 The ORR activities for CoSAs/PTFs were
spectra of the three CoSAs/PTFs and Co-TPPCN, which evaluated with linear sweep voltammetry (LSV) on a rotating
˚
show obvious signals at approximately 1.44 A attributed to the disk electrode (RDE) in 0.1 M KOH solution saturated with O2.
Co–N scattering paths,45–47 indicates similar Co coordination As shown in Fig. 4a, among the three Co SACs, CoSAs/PTF-600
Fig. 4 ORR test: (a) LSV of glassy carbon, CoSAs/PTFs and Pt/C at 1600 rpm in O2-saturated 0.1 M KOH (scan rate: 10 mV sꢀ1). (b) LSV of CoSAs/
PTF-600, PTF-600, (Co)PTF-600 and CoNPs/PTF-600. (c) The free energy variations for CoSAs/PTF during the ORR process. (d) Electron
transfer number and H2O2 yield of different samples obtained from the RRDE test. (e) Tafel plots obtained from the RDE polarization curves. (f)
Current–time chronoamperometric responses of CoSAs/PTF-600 and Pt/C in an O2-saturated 0.1 M KOH solution.
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exhibited the best activity with the most positive half-wave conrm the presence of Co active sites, rst principles simula-
potential of 0.808 V (vs. RHE), which was slightly more posi- tion was performed. The atomic structure of the unit of covalent
tive than that of the benchmark 20 wt% Pt/C (0.806 V) and most triazine frameworks is shown in Fig. S17,† and the correspond-
of the non-precious metal catalysts (Table S6†). CoSAs/PTF-600 ing ORR free energy variations are shown in Fig. 4c. During the
also exhibits a large diffusion-limited current density of ORR process, *O, *OH and *OOH intermediates are formed, and
6.14 mA cmꢀ2, surpassing that of the Pt/C catalyst (5.08 mA cmꢀ2). the free energy variations between them can be calculated by rst
This result may be attributed to the fact that CoSAs/PTF-600 has principles simulations. The electrochemical limiting potentials
large BET surface areas and highly porous structures.
of the studied structure can be determined from the calculated
For comparison, catalysts without Co species were also free energy variations.42,43 Here the electrochemical limiting
prepared either by direct polymerization from TPPCN at 600 ꢁC potential that can be compared to the measured experimental
(denoted as PTF-600) or by treating CoSAs/PTF-600 with tri- half-wave potential is determined using the smallest free energy
uoromethanesulfonic acid to remove the Co species (denoted gap between reaction intermediates. The Co single atom is the
as (Co)PTF-600). As shown in Fig. 4b, compared with CoSAs/PTF- studied active site for the ORR. It can be seen from Fig. 4c that all
600, both PTF-600 and (Co)PTF-600 showed lower ORR activities, the free energy variations for the elementary steps are downhill
which proves the crucial signicance of the existence of Co single at 0 V. When the potential is 0.90 V, the rst elementary step is
atoms. Nevertheless, PTF-600 and (Co)PTF-600 still exhibited under equilibrium and the last step only shows a small free
non-negligible ORR activities, which were likely attributed to the energy decrease, which means that the formation of an *OOH
abundant nitrogen-doped carbon sites in the catalysts. There- intermediate and the removal of *OH determines the ORR
fore, the synergistic effect between Co–N4 and nitrogen-doped performance. Thus, the calculated electrochemical potential is
carbon in CoSAs/PTF-600 contributed to the enhanced ORR 0.90 V, and it is comparable to the experimental half-wave
activity. Moreover, Co nanoparticles encapsulated on PTF-600 potential of 0.81 V. In order to indicate the signicance of the
(CoNPs/PTF-600) were also prepared (Fig. S14 and S15†), but porphyrinic triazine-based frameworks, the ORR performance of
they exhibited a higher overpotential and lower diffusion-limited the Co–porphyrin molecule was also investigated for comparison
current density than the SACs (Fig. 4b). Thus, these control as shown in Fig. S18.† Similarly, we can see that the calculated
experiments clearly supported that the superior activity of CoSAs/ electrochemical limiting potential is only 0.38 V, which is far
PTF-600 for the ORR was determined by the atomically dispersed from the experimental half-wave potential. So, the Co activity
Co–N4 sites. The Koutecky–Levich (K–L) plots of CoSAs/PTF-600 center uniformly anchored on a porous porphyrinic triazine-
obtained from the LSV curves at different rotation speeds based framework to form a Co–N4 structure is important to
(Fig. S16†) displayed nearly parallel tting lines, indicating rst- realize high ORR performance.
order reaction kinetics with respect to the O2 concentration and
We also determined the H2O2 product yields and the electron
a potential-independent electron transfer rate.48–54 To further transfer numbers with rotating ring disk electrode measurements
Fig. 5 HER test: (a) LSV of glassy carbon, CoSAs/PTFs and Pt/C at 1600 rpm in N2-saturated 0.5 M H2SO4 (scan rate: 10 mV sꢀ1). (b) LSV of CoSAs/
PTF-600, PTF-600, (Co)PTF-600 and CoNPs/PTF-600. (c) Tafel plots of various samples. (d) Mass activities of CoSAs/PTF-600 and Pt/C at
different potentials.
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(Fig. S19†). The electron transfer numbers (n) for the CoSAs/PTFs employed as one of the most promising Pt-alternatives in fuel
calculated according to the RRDE tests are 3.92–3.97 (Fig. 4d), cells, batteries, and many other applications. The work pre-
suggesting a highly efficient four-electron ORR pathway. As sented here will pave a new way toward designing stable
shown in Fig. 4e, CoSAs/PTF-600 possessed a lowest Tafel slope of non-precious metal single-atom catalysts with high efficiency
57 mV decꢀ1 compared with that of Pt/C (78 mV decꢀ1), further for catalysis.
suggesting its superior activity for the ORR. Furthermore, CoSAs/
PTF-600 also exhibited negligible current density degradation
aer a 10 hour durability test (Fig. 4f), while an 18% current loss
Conflicts of interest
was observed for Pt/C, indicating the superior stability of the Co There are no conicts to declare.
SAC in alkaline medium. In addition, CoSAs/PTF-600 also showed
much better methanol tolerance than Pt/C under the same
conditions (Fig. S20†).
Acknowledgements
The unique features of CoSAs/PTFs could boost electro- We acknowledge the nancial support from the National Key
catalysis, which was also veried by the electrocatalytic HER. Research and Development Program of China (2017YFA0206800
Fig. 5a shows the LSV polarization curves of CoSAs/PTFs and and 2018YFA0208600), Strategic Priority Research Program of the
commercial Pt/C that are tested in 0.5 M H2SO4. As expected, Chinese Academy of Sciences (XDB20000000), NSFC (21671188,
owing to its stronger electron transfer capacity, CoSAs/PTF-600 21871263, and 21331006), Key Research Program of Frontier
showed a smaller onset potential of 21 mV than CoSAs/PTF-400 Science, CAS (QYZDJ-SSW-SLH045), and Youth Innovation
(188 mV) and CoSAs/PTF-500 (58 mV). More importantly, a very Promotion Association, CAS (2014265). We acknowledge the
low overpotential (h) of 94 mV for CoSAs/PTF-600 was obtained beamline BL14W1 station for XAFS measurements at the
at jHER ¼ 10 mA cmꢀ2, which is only 40 mV higher than that of Shanghai Synchrotron Radiation Facility, China.
Pt/C (54 mV). Notably, when compared to PTF-600, (Co)PTF-600,
and CoNPs/PTF-600 catalysts, the best HER activity was ach-
ieved by CoSAs/PTF-600 (Fig. 5b), further suggesting that the
References
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