Porphyrinic coordination lattices with fluoropillars

Article abstract of DOI:10.1039/c7ta06658a

The combined use of porphyrins and fluorides to fabricate pillared coordination sheets is presented. Multiple features, such as coordinatively unsaturated metal sites, an optimal pore space, and a fluoride-decorated internal surface, make the new materials ideal adsorbents with high capacity and affinity for carbon dioxide. In addition, the new materials are shown to be suitable single-source precursors to afford fluorine-doped graphene analogues for application in oxygen reduction reaction electrodes.

Full text of DOI:10.1039/c7ta06658a

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Porphyrinic coordination lattices with fluoropillars†
ac
a
a
b
a
*
Xiang Zhao
Qipu Lin,
Chengyu Mao,
Aiguo Kong,
Xianhui Bu,
Cite this: DOI: 10.1039/c7ta06658a
a
*
and Pingyun Feng
The combined use of porphyrins and fluorides to fabricate pillared coordination sheets is presented.
Multiple features, such as coordinatively unsaturated metal sites, an optimal pore space, and a fluoride-
decorated internal surface, make the new materials ideal adsorbents with high capacity and affinity for
carbon dioxide. In addition, the new materials are shown to be suitable single-source precursors to
afford fluorine-doped graphene analogues for application in oxygen reduction reaction electrodes.
Received 28th July 2017
Accepted 19th September 2017
DOI: 10.1039/c7ta06658a
rsc.li/materials-a
metal ions, either alone or in various combinations, which can
greatly enrich the structure and functionality7 beyond those
Introduction
offered by SIFSIX-pillars. Furthermore, the uoro-pillared met-
alloporphyrins reported here could also offer an unprecedented
opportunity for their use as single-source precursors to prepare
metal-free F-doped carbons which can be potentially used as
efficient oxygen reduction reaction (ORR) catalysts in fuel cells.
Recently, tremendous efforts have been directed towards fabri-
cating low-cost electrode materials.8 F-doped carbons offer
a promising direction.9 However, the F-doped electrodes reported
so far have usually been synthesized with NH4F as the F-source.10
In view of its high volatility, new F-doping methods are desirable.
Even though some MOF-derived electrocatalysts have been
investigated,11 metal-free F-doped porphyrinic carbon materials
derived from robust MOFs remain largely unexplored so far.
Here two new SIFSIX-like MOFs (denoted as CPM-131 and
CPM-132) based on tetra(4-pyridyl)metalloporphyrin (TPyP-M,
M ¼ Fe and Zn, respectively) are presented. CPM-131 consists of
Anthropogenic emissions of CO2 have raised serious concerns
about their impact on climate.1 One method to mitigate CO2
impact involves chemisorption via amine scrubbing.2 However,
the energy penalty in regeneration motivates efforts to search
2ꢀ
for alternatives.3 Recently, a family of SiF6 (named SIFSIX)
pillared square grids, termed SIFSIX-n,4 were found to offer
a highly efficient mechanism for carbon capture, even in the
presence of moisture.5 In SIFSIX-n, 2D (4,4)-grids based on
octahedrally coordinated ions (e.g., Zn and Cu) and ditopic
spacers (e.g., pyrazine and 4,40-bipyridine) are pillared with
2ꢀ
SiF6
to form 3D structures with a primitive cubic (pcu)
topology. Compared to other MOFs, an outstanding feature of
2ꢀ
SIFSIXs is their SiF6 group which promotes favorable inter-
action with CO2 and resistance to water.5a However, beyond
these isoreticular series based on linear bipyridine or its vari-
ants, few studies have been reported for other ligand types for
targeting chemically and topologically different SIFSIX-type
MOFs.6 Given the great signicance of SIFSIX-n, it is highly
desirable to pursue new material types in this system.
pillars while CPM-132 contains mixed Fꢀ/SiF6
2ꢀ
2ꢀ
only SiF6
pillars. Both exhibited reversible adsorption with high capacity
and affinity for CO2, but sluggish CH4 and undetectable N2 uptake
at near room temperature. The enhancement in CO2 uptake/
selectivity was attributed to favorable interactions of sorbent hosts
with gas molecules resulting from the optimized pore partition,
open metal sites, and electrostatic uoropillars. In addition to gas
adsorption and separation, these materials have also been
studied as single-source precursors (without additional C, N, and
F sources) to prepare metal-free F-containing carbon derivatives
for ORR electrodes. CPM-132/C, derived from CPM-132, showed
excellent activity and superior long-term durability as a catalyst for
the ORR. The incorporation of uorine into the porphyrinic
carbons contributes to the much improved ORR activity,
compared to F-free CPM-133/C derived from CPM-133.
We hypothesized that metalloporphyrins functionalized with
4 pyridine groups at the meso positions may form 2D layers that
2ꢀ
can be readily pillared by SiF6 to give 3D SIFSIX-like frame-
works. We chose metalloporphyrins also because their core and
periphery coordination sites permit the use of more diverse
aDepartment of Chemistry, University of California, Riverside, CA 92521, USA. E-mail:
pingyun.feng@ucr.edu
bDepartment of Chemistry and Biochemistry, California State University Long Beach,
Long Beach, CA 90840, USA. E-mail: xianhui.bu@csulb.edu
cState Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China
† Electronic supplementary information (ESI) available: Experimental details,
characterization data (NMR, MS, FT-IR, UV-vis, PXRD, SEM, TEM, TGA, XPS,
uorescence spectra, crystal photographs, sorption isotherms, etc.),
crystallography data (and CIFs), additional structural gures and
electrochemical graphs. CCDC 1061788–1061790. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c7ta06658a
Experimental
All reagents were purchased commercially and used without
further purication, unless otherwise noted. The ligands of
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TPyP-H2 (5,10,15,20-tetrakis(4-pyridyl)porphyrin) and its metal-
containing variants, TPyP-M (M ¼ Fe, Zn, or Cu), were synthe-
sized according to Adler's method12 with some modications
(detailed preparations and characterizations in the ESI†).
Synthetic procedure of CPM-13X/C (X ¼ 1, 2 and 3)
The pyrolysis reaction of CPM-13X to CPM-13X/C was carried
out in a quartz tube (with two open ends) in a tube furnace
under an Ar gas ow. The CPM-13X samples were heated from
room temperature to a preset temperature with a heating rate of
4 ꢁC minꢀ1, and then maintained at this temperature for 4 h in
an Ar atmosphere. On the basis of our initial electrocatalyst
Synthetic procedure of CPM-131 ([(TPyP-Fe)Zn(SiF6)]n-fsc)
A solution of 10 mg of TPyP-Fe in 8 mL of N-methyl-2-pyrroli-
done was layered over a solution of 10 mg of ZnSiF6$xH2O in
3 mL of ethylene glycol in a vial. Interdiffusion of the solutions
(at room temperature or heated at 80ꢁ) for a period of 5 days
yielded square-shaped crystals (42% yield based on TPyP-Fe).
Selected FT-IR: n ¼ 3680 (m), 2990 (s), 2360 (w), 1610 (s), 1410
ꢁ
screening, 1000 C was identied as the optimum calcination
temperature for CPM-132/C and CPM-133/C, and also for
comparison, CPM-13X/C-1000s (abbreviated to CPM-13X/C)
were chosen for characterization by a number of experimental
techniques and subsequent investigation of the performance
assessment for the oxygen reduction reaction (ORR).
(s), 1070 (s), and 715 (s) cmꢀ1
.
Electrocatalytic tests
Synthetic procedure of CPM-132 ([(TPyP-Zn)2F2Zn2(SiF6)]n-fsx)
The electrocatalytic activities of the CPM-13X-derived catalysts
for the ORR were evaluated by cyclic voltammetry (CV) and
rotating disk electrode (RDE) techniques on a CHI-650 electro-
chemical analyzer. A 3-electrode cell was employed using
a glassy carbon RDE (5 mm in diameter, Pine) as the working
electrode, an Ag/AgCl, KCl (3 M) electrode as the reference
electrode, and a Pt wire electrode as the counter electrode. All
the working electrodes were prepared as follows: 5 mg of cata-
lysts and 40 mL of Naon solution (5 wt%) were dispersed in
600 mL of ethanol by sonication for 30 min to form a homoge-
neous ink. Then, 5 (base) or 15 (acid) mL of ink was loaded onto
a polished glassy carbon electrode of 0.196 cm2 surface, and the
electrode was dried at room temperature. The ORR experiments
were carried out in 0.1 M KOH or 0.1 M HClO4 solution. The
electrolyte was saturated and maintained with O2 during the
test. All the potentials were calibrated to ones versus the
reversible hydrogen electrode (RHE). The Koutecky–Levich plots
(Jꢀ1 versus uꢀ1/2, where J is the measured current density and u
is the angular velocity of the disk) were analyzed at various
electrode potentials. The slopes of the linear lines were used to
calculate the number of electrons transferred (n).
A solution of 10 mg of ZnSiF6$xH2O in 3 mL of propylene glycol
was layered over a solution of 10 mg of TPyP-Zn in 10 mL of
dimethylsulfoxide (mixed with 2 mL of chloroform) in a vial.
Low temperature (60ꢁ) diffusion of the solutions for a period of
6 days yielded square-shaped crystals (62% yield based on TPyP-
Zn). Selected FT-IR: n ¼ 2990 (s), 2360 (m), 1650 (m), 1610 (s),
1420 (m), 991 (s), and 717 (s) cmꢀ1
.
Synthetic procedure of CPM-133 ([TPyP-Cu]n-nbo)
First, TPyP-Cu (0.10 g) was dissolved in 4 mL of N,N-dieth-
ylacetamide solution (with 1 drop of concentrated nitric acid);
then 1 mL of ethanol was added. The mixture was heated in
ꢁ
a 120 C oven for 3 days. Aer cooling down to room tempera-
ture, hexagonal prism-shaped crystals were harvested by ltra-
tion (43% yield based on TPyP-Cu). Selected FT-IR: n ¼ 3680 (m),
2990 (s), 2360 (m), 1590 (s), 1410 (s), 991 (s), 789 (s), and
667 (s) cmꢀ1
.
Crystallography
The single-crystal X-ray diffraction intensity data were collected
on a Bruker APEX-II diffractometer with a nitrogen-ow
temperature controller using graphite-monochromated MoKa
Results and discussion
˚
radiation (l ¼ 0.71073 A), operating in the u and 4 scan mode.
Crystals of CPM-131 were prepared by layering an N-methyl-2-
pyrrolidone solution of TPyP-Fe onto an ethylene glycol solution
of ZnSiF6. Single-crystal X-ray diffraction revealed the formula
of CPM-131 to be [(TPyP-Fe)Zn(SiF6)]n. Its framework could be
simplied into a 4,6-connected fsc network (Fig. 1b and d). This
is a new SIFSIX type, built of heterometalloporphyrin (4,4)-grid
sheets (Fig. 1f and h). CPM-131 contains 3D interconnecting
channels with a solvent accessible volume of 44.3%, as calcu-
lated using the PLATON soware. Thermogravimetric analysis
under a nitrogen ow (Fig. 2d) indicates the high thermal
stability (up to ca. 410 ꢁC). Its porosity was examined by nitrogen
Data reduction was carried out with the Bruker Saint program.
The SADABS programme was used for absorption correction.
The structures were solved by a direct method followed by the
successive difference Fourier method. The full-matrix least
squares technique was against F2. Non-hydrogen atoms were
rened with anisotropic displacement parameters during the
nal cycles. Hydrogen atoms were placed in calculated posi-
tions with isotropic displacement parameters set to 1.2 ꢂ Ueq of
the attached atom (Table 1). CCDC 1061788–1061790.†‡
˚
‡ Crystal data for CPM-131: M ¼ 912.00, tetragonal, P4/mmm, a ¼ b ¼ 13.9664(3) A,
c ¼ 7.7060(5) A, a ¼ b ¼ g ¼ 90 , V ¼ 1503.1(11) A , Z ¼ 1, GOF ¼ 0.993 based on adsorption–desorption at 77 K. A N2 uptake of 75 cm3 gꢀ1 (STP)
3
ꢁ
˚
˚
F2, nal R1 ¼ 0.0883, wR2 ¼ 0.2524 [for I > 2s(I)]; CPM-132: M ¼ 2152.47,
and a Brunauer–Emmett–Teller (BET) surface area of 226 m2 gꢀ1
were observed. Evaluation using the Horvath–Kawazoe model
ꢁ
˚
˚
tetragonal, I4/m, a ¼ b ¼ 13.985(10) A, c ¼ 24.40(4) A, a ¼ b ¼ g ¼ 90 , V ¼
3
2
˚
4772.0(9) A , Z ¼ 2, GOF ¼ 0.992 based on F , nal R1 ¼ 0.1011, wR2 ¼ 0.2165
˚
provides a pore size distribution centered at 4.3 A (Fig. 2e,
˚
ꢀ
[for I > 2s(I)]; CPM-133: M ¼ 680.22, rhombohedral, R3, a ¼ b ¼ 33.0984(19) A,
3
ꢁ
ꢁ
inset), consistent with the crystallographic data when van der
Waals radii are considered.
˚
˚
c ¼ 9.3673(11) A, a ¼ b ¼ 90 , g ¼ 120 , V ¼ 8887.1(13) A , Z ¼ 9, GOF ¼ 1.002
based on F2, nal R1 ¼ 0.0444, wR2 ¼ 0.0836 [for I > 2s(I)].
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Table 1 A summary of crystallographic data and porosity measurements
a
˚
a [A]
˚
c [A]
Code
Framework composition
S.G.
Topology
R1
S.A.
CO2
Pore size
Qst
˚
CPM-131
CPM-132
CPM-133
[(TPyP-Fe)Zn(SiF6)]n
[(TPyP-Zn)2F2Zn2(SiF6)]n
[TPyP-Cu]n
13.966
13.985
33.098
P
7.706
24.404
9.367
P4/mmm
I4/m
R3
fsc
fsx
nbo
0.0883
0.0875
0.0444
226/286
204/258
390/490
101 (55)
74 (40)
75 (39)
4.3 A
35
34
27
˚
4.3 A
˚
ꢀ
4.8 A
P
S.G. ¼ space group, R1 ¼ kFobs| ꢀ |Fcalck/ |Fobs| with Fobs > 4.0 s(F), S.A. ¼ (BET/Langmuir) surface area (m2 gꢀ1), auptake (cm3 gꢀ1) at 1 (or 0.15)
bar at 273 K, Qst for CO2 (kJ molꢀ1).
Fig. 1 (a) Tetrakis(4-pyridyl)metalloporphyin, TPyP-M (M ¼ Fe or Zn). (b, d) Simplified structure of CPM-131 built of homo-pillared sheets [(TPyP-
Fe)Zn]n from SiF62ꢀ, and its fsc-topology. (c, e) Simplified structure of CPM-132 built of hetero-pillared sheets [(TPyP-Zn)Zn]n from Fꢀ/SiF62ꢀ, and
its fsx-topology. (f, h) Top and side views of the structure of CPM-131. (g, i) Top and side views of the structure of CPM-132. C, gray; O, red; N,
blue; F, aqua; Si, turquoise; metal, violet or lime; H atoms are not depicted for clarity.
Crystals of CPM-132 were prepared by low temperature (60ꢁ) [TPyP-Cu]n was also prepared. It is built of TPyP-Cu scaffolds by
diffusion of
a propylene glycol solution of ZnSiF6 into self-coordinating to each other (see the ESI for crystallographic
a dimethyl sulfoxide solution of TPyP-Zn. Unlike CPM-131, details†). Isostructural metalloporphyrin-based nbo-nets have
CPM-132 is a rare example of porphyrin-zinc layers hetero-pil- been reported13 but not studied for CO2 trapping. Notable
lared by SiF62ꢀ and Fꢀ (which could arise from the degradation features of CPM-133 are the honeycomb channels and pyridine-
2ꢀ
of SiF6 anions during thermal treatment). The framework of functionalized windows (Fig. S34 and S35†). The permanent
CPM-132 could be simplied into a 5,6-connected fsx net porosity of CPM-133 was conrmed by N2 sorption measure-
(Fig. 1c and e), in which the zinc nodes have two different ments at 77 K (Fig. S47†) that revealed a BET surface area of
coordination environments. The framework can also be viewed 390 m2 gꢀ1, higher than those of CPM-131/132.
as
F-intercalated
zinc-tetra(pyridyl)porphyrin
bilayers
2ꢀ
(Fig. S29†), further pillared by SiF6 in the axial direction,
CO2 trapping
giving rise to a 3D framework (Fig. 2i). CPM-132 is chemically
stable. Powder XRD patterns show that no change occurs during The integration of thermal/chemical stability, narrow channels,
immersion in water or other solvents, such as N-methyl-2-pyrro- unsaturated metal sites, and a high concentration of inorganic
lidone, dimethyl sulfoxide, N,N-dimethylformamide, acetone, uorine, which can improve selectivity and affinity for CO2,
chloroform, tetrahydrofuran, and methanol, for 24 h. The inspired us to study CO2 adsorption properties. Single compo-
solvent-accessible volume of CPM-132 is 38.8%. N2 sorption data nent and low-pressure CO2 and N2 sorption data were collected
showed a type I isotherm with a BET surface area of 204 m2 gꢀ1
slightly lower than that of CPM-131.
,
at 273 and 298 K (Fig. 2f). The CO2 uptake of CPM-131 at 1 atm
and 273 K was 101 cm3 gꢀ1 (equivalent to 4.5 mmol gꢀ1 or
For comparison with uoropillared CPM-131/132, a F-free 198.4 mg gꢀ1), whereas N2 uptakes of 1.0 cm3 gꢀ1 were
porphyrin MOF (CPM-133) with an nbo-topology and formula measured. High affinity for CO2 is shown by the sharp increase
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Fig. 2 (a–c) Crystal photographs of CPM-13X (X ¼ 1, 2 and 3, respectively). (d) TGA of CPM-13X. (e) N2 adsorption–desorption isotherms of CPM-
131 with the inset showing the pore size distribution. (f) CO2, CH4 and N2 adsorption isotherms of CPM-131. (g) CO2 adsorption enthalpy (Qst) of
CPM-131.
in CO2 uptake in the low pressure region. In comparison, catalysts for the ORR. Carbonization was performed to convert
CPM-132 exhibited a lower value of 74 cm3 gꢀ1 (equivalent to the CPM-131/132/133 into (metallo)porphyrinic carbons, deno-
3.3 mmol gꢀ1 or 145.4 mg gꢀ1) at 1 atm and 273 K. Its N2 ted as CPM-13X/C (X ¼ 1, 2 or 3). The carbonization temperature
isotherms at 273 K also showed negligible uptake (0.8 cm3 gꢀ1). is the key parameter that determines the trade-off between the
For CPM-133, the volumetric CO2 uptake was 75 cm3 gꢀ1 at degree of heteroatom doping, surface properties, and electrical
1 atm and 273 K. Ideal adsorbed solution theory (IAST)14 conductivity.8c CPM-131 has active Fe-coordination species,
calculations indicated binary gas adsorption selectivity under which could be destroyed during high-temperature treatment
practically relevant conditions (273 K; N2 mole fractions of 0.9) with a concomitant reduction in catalytic activity, but for
to be dramatically higher for CPM-131 (1211) and CPM-132 precursors without Fe-complexes, low-temperature carboniza-
(942) than CPM-133 (523) for CO2/N2. These ndings can be tion cannot lead to a high degree of graphitization to ensure
attributed to the enhanced isosteric heats of adsorption (Qst). a good conductivity. As such, the optimal calcination tempera-
The values of Qst were calculated using the adsorption data ture was determined to be 700 ꢁC for CPM-131/C and 1000 ꢁC for
collected at 273 and 298 K (Fig. S38, S43 and S48†). At zero CPM-132/C and CPM-133/C (details in the ESI†). A comparative
coverage, the enthalpy of CO2 adsorption for CPM-131/132 is study showed that CPM-132/C-1000 (abbreviated to CPM-132/C)
35/34 kJ molꢀ1, greater than that of CPM-133 (27 kJ molꢀ1).
exhibited the highest catalytic efficiency and is thus described
Similar behaviors have also been observed at room temper- below in detail.
ature. CPM-132 exhibited a CO2 uptake of 59 cm3 gꢀ1 (equiva-
The powder XRD pattern (Fig. S54†) of the CPM-132-derived
lent to 2.6 mmol gꢀ1 or 115.9 mg gꢀ1) at 1 atm and 298 K, while N–F–C composite exhibited pronounced peaks at ca. 24 and 43ꢁ
CPM-131 exhibited a substantially higher value of 76 cm3 gꢀ1 corresponding to diffractions from the (002) and (100) graphitic
(equivalent to 3.4 mmol gꢀ1 or 149.3 mg gꢀ1). Apart from its carbon planes, respectively, suggesting the presence of long
relevance to climate, CO2 is an impurity in natural gas, biogas, range ordering in the carbon matrix. As shown in Fig. 3b, the
and many other gas streams.3a,5a Therefore, CO2/CH4 separation square layered morphology of carbonization product CPM-132/
has also become a primary research objective. CPM-131 showed C was seen in the scanning electron microscopy (SEM) image.
preferential adsorption of CO2 over CH4, with a selectivity index To further observe the local microstructure, we also show the
of 108 (in a CO2/CH4:50/50 mixture), similar to that of CPM-132 high-resolution transmission electron microscopy (HRTEM)
(104), as determined from IAST simulation. The results show image of CPM-132/C in Fig. 3d and attest to the presence of
that open metal sites, SiF62ꢀ pillars, and small pore sizes likely lattice fringes. Solid-state diffuse reectance spectroscopy
contribute to the uptake and Qst of CO2.
(Fig. 3f) indicated that the pyrolytic product did retain some
porphyrinic characteristics. Elemental analysis was performed
to probe the chemical composition of the CPM-132/C and the
F, N, and C contents were found to be 2, 9, and 73 wt%. As
shown in Fig. 3e, N2 sorption isotherms at 77 K showed a steep
increase at low pressure and a hysteretic trait of the desorption
isotherm at high pressure, suggesting the hierarchical porosity
ORR activity
In addition to their potential for application in post-combustion
carbon capture and energy gas purication, the uoropillared
materials reported here exhibit desirable features that support
their use as single-source precursors in the synthesis of efficient
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Fig. 3 (a, b) SEM images of CPM-132 and its carbonization product CPM-132/C. (c, d) TEM images of CPM-132/C. (e) N2 adsorption–desorption
isotherms of CPM-132/C with the inset showing the pore size distribution. (f) UV-vis spectra of CPM-132 and CPM-132/C. XPS (g) N 1s and (h) F 1s
spectra of CPM-132/C.
of the nanocomposite CPM-132/C. CPM-132/C exhibited a BET potential of 0.89 V and a half-wave potential of 0.74 V, deter-
surface area of 278 m2 gꢀ1, slightly greater than that of its mined using the polarization curves at 1600 rpm. A more
crystalline precursor. The pore size distribution was centered at substantial improvement in ORR activity, reected by much
1.0 and 3.8 nm, as determined from the adsorption isotherm more positive onset/half-wave ORR potentials (0.94/0.79 V), is
using the Horvath–Kawazoe model and desorption isotherm achieved in CPM-132/C, showing a synergetic effect from N–F
using the Barrett–Joyner–Halenda model, respectively.
codoping. The electrocatalytic capacity of N–F binary-doped
Because the surface chemistry is of particular interest for CPM-132/C even matched that of CPM-131-derived Fe–N–F
catalytic applications, the electronic states of F, N, and C were ternary-doped carbon. The superior performance of the CPM-
examined by X-ray photoelectron spectroscopy (XPS). As 132/C electrode is attributed to its precursor's unique features
revealed in Fig. 3g, the high-resolution N 1s XPS spectrum is of high density of uorine, ionic bonding of Fꢀ to Zn2+ (boost-
deconvoluted into 3 peaks at 398.5, 399.6 and 400.8 eV, corre- ing the framework stability), macrocyclic porphyrins with
sponding to pyridinic, metal-coordinated, and graphitic a layered network, and microporosity.
nitrogen species, respectively, indicating 3 different bonding
To assess the mechanism of the electrocatalytic activity of
congurations.8d,9c As shown in the high-resolution F 1s XPS of CPM-132/C, the Koutecky–Levich (K–L) plots were calculated
CPM-132/C (Fig. 3h), a dominant peak at 689.3 eV, assigned to from RDE polarization curves (Fig. 5b). The electron transfer
semi-ionic F–C bonding,10a was also observed. Overall, the well number (n) was calculated from the slopes of the K–L plots to be
dispersed N and F-codoped active sites, combined with the 4.0 ꢃ 0.1, suggesting a 4-electron transfer process. However, the
easily accessible porous structure, are highly benecial for the ORR process catalysed by CPM-133/C likely involves mixed 2-
electrocatalytic performance described below.
and 4-electron pathways, as the number of electrons transferred
The ORR catalytic activity and kinetics of CPM-13X/C, along per O2 is 3.6 at 0.5 V. The CPM-132/C catalyst exhibited long-
with 20% Pt/C reference catalysts (Alfa Aesar), were evaluated term durability, superior to that of Pt/C (Fig. 5c and d). Through
using cyclic voltammetry (CV) and rotating-disk electrode (RDE) continuous potential cycling, the deterioration of the cathodic
techniques in 0.1 M KOH and 0.1 M HClO4. All potentials were peak current of CVs underwent a non-signicant change with
referenced to the reversible hydrogen electrode (RHE). Prior to
each experiment, the electrolytes were saturated with Ar or O2,
respectively. In an Ar-saturated solution of 0.1 M KOH, the CV
curves of the composites CPM-13X/C within the entire potential
range showed nearly nonsense shapes, while well-dened
cathodic peaks occurred in the O2-saturated solution (Fig. 4a),
illustrating their effective ORR activities in alkaline medium.
Among the 3 samples, N–F–C based CPM-132/C showed an ORR
peak at the most positive potential of 0.83 V, which is close to
that of Pt/C (0.85 V).
Their electrocatalytic activity sequence was further sup-
ported by linear sweep voltammetry on a RDE. As shown in
Fig. 4b, non-uorinated CPM-133/C exhibited an onset CPM-13X/C and 20% Pt/C in O2-saturated 0.1 M KOH.
Fig. 4 (a) CVs and (b) LSV curves at a rotation rate of 1600 rpm on
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Fig. 5 (a) RDE polarization curves at different rotation rates, (b) K–L plots, and (c) cycling CVs on CPM-132/C, and (d) current–time chro-
noamperometric response on CPM-132/C with reference 20% Pt/C, in O2-saturated 0.1 M KOH. (e) LSV curves at a rotation rate of 1600 rpm on
CPM-13X/C and 20% Pt/C, and (f) current–time chronoamperometric response on CPM-132/C with reference 20% Pt/C, in O2-saturated 0.1 M
HClO4.
only 7% drop aer 50 000 cycles. The tolerance of CPM-132/C to internal surface, such unique hybrids exhibited high uptake
methanol was also proved in 0.1 M KOH solution.
capacity and selectivity for CO2 over N2/CH4 at low pressure.
Finally, we also investigated the ORR performance of the Signicantly, such robust F-enriched porphyrinic architectures
CPM-13X-derived catalysts in 0.1 M HClO4 media. Again, the could be self-supporting precursors for F-doped porphyrinic
CPM-132/C-electrode showed a better electrocatalytic activity carbon composites. Their carbonization products exhibited
with an onset potential (0.86 V) higher than that of non-uori- good ORR activity and durability in both alkaline and acidic
nated CPM-133/C (0.74 V). Compared with Fe–N–F triple-doped electrolytes. Moreover, the design concept presented here could
CPM-131/C, the N–F double-doped CPM-132/C exhibited lower be amenable to the expansion of the compositional scope of the
onset/half-wave potentials but much higher current density heteroatom-doped catalysts, through crystal engineering of the
(Fig. 5e). Based on the K–L equation, the number of electrons precursors of (metallo)porphyrinic MOFs.
transferred (n) per O2 molecule for the CPM-132/C in the acidic
medium was calculated to be 3.9 at 0.5 V (Fig. S97†), suggesting
a 4-electron pathway even under acidic conditions. In addition
Conflicts of interest
to the activity, the materials also show long-term durability and
resistance to methanol fuel. The prominent performance of the
CPM-132-derived electrode, on the same level as best metal-free
catalysts,9,10b,15 may come from advantages of its CPM-132
precursor, such as robust porosity, high thermal stability,
uniform distribution and high content of N/F hetero-dopants
with a synergistic effect, porphyrinic coordination sheets, and
hard-templating effects by ionic Zn–F. In addition, it was also
suggested that variations in the charge and spin densities of the
carbon lattice could be signicant factors for the excellent
performance observed with multiple doped nanocarbons.16
There are no conicts to declare.
Acknowledgements
The work is supported by the US Department of Energy, Office of
Basic Energy Sciences, Materials Sciences and Engineering
Division under award No. DE-SC0010596.
Notes and references
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