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Contents lists available at ScienceDirect
Chemical Engineering Journal
journal homepage: www.elsevier.com/locate/cej
Reaction milling for scalable synthesis of N, P-codoped covalent
organicpolymers for metal-free bifunctional electrocatalysts
Xinxin Lin, Peng Peng, Jianing Guo, Zhonghua Xiang⁎
State Key Laboratory of Organic-Inorganic Composites, College of
Chemical Engineering, College of Energy, Beijing University of
Chemical Technology, Beijing 100029, PRChina
H I G H L I G H T S
• Reaction milling method was pro-posed to synthesize
bifunctional elec-trocatalysts.
• Space-time yield for scalable synthesisreaches 288 kgm−3
day−1.
• RM method provides a solvent-freeand scalable alternative for
electro-catalysts.
• The reaction process was significantproceeded by RM
method.
• 3 days under 120 °C (tradition) V.S.2 h under room temperature
(RM).
G R A P H I C A L A B S T R A C T
A R T I C L E I N F O
Keywords:Reaction millingCovalent organic frameworksNitrogen and
phosphorus co-doped carbonmaterialsMetal free electrocatalystOxygen
reductionOxygen evolution
A B S T R A C T
This study exploits an effective mechanochemical process (termed
as reaction milling) to conduct Schiff-basedcoupling reaction with
melamine and terephthalaldehyde for the synthesis of covalent
organic polymer (RM-COP) as the carbon skeleton and the derivative
phosphorus doped material (RM-COP-PA). Comparing with thetradition
solvothermal method with reaction time of 3 days under 120 °C, the
newly developed reaction millingmethod significantly shorten the
reaction time of the synthesis to 3 h under room temperature as
well as by-passing the usage for hazardous solvents. The space-time
yield of the developed reaction milling method forsynthesis of the
bifunctional electrocatalytic precursor reaches 189 kgm−3 day−1.
Significantly, the optimalproducts followed by further
carbonization (RM-COP-PA-900) demonstrated excellent bifunctional
electro-catalytic activities for an efficient ORR performance with
similar commercialized Pt/C half-potential of 841mVvs RHE as well
as an IrO2-like OER activity with a potential of 1.69 V at 10mA
cm−2 in alkaline media, which isbetter than most metal-free
bifunctional catalysts. Moreover, the obtained RM-COP-PA-900
exhibits much betterdurability and resistance to crossover effect
even than the commercial 20 wt% Pt/C catalysts. Therefore, thiswork
will open up a rapid, solvent-free and scalable approach for, but
not limit to, highly efficient electro-catalysts.
1. Introduction
With the development of the high energy conversion and
storagedevices such as fuel cells, Zn-air batteries, solar cells
and
supercapacitors [1–8], development of stable, effective and
low-costbifunctional electrocatalysts towards oxygen reduction
reaction (ORR)and oxygen evolution reaction (OER) becomes one of
the major chal-lenges for their commercial application. Recently,
well-defined 2D
https://doi.org/10.1016/j.cej.2018.09.185Received 28 July 2018;
Received in revised form 18 September 2018; Accepted 23 September
2018
⁎ Corresponding author.E-mail address: [email protected]
(Z. Xiang).
Chemical Engineering Journal 358 (2019) 427–434
Available online 04 October 20181385-8947/ © 2018 Elsevier B.V.
All rights reserved.
T
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covalent organic polymers (COPs) [9,10] have been widely
developedand hold the promising potential to be used as carbon
skeleton due totheir ultrahigh hydrothermal stability, versatile
elements incorporationand controllable structures comparing with
the famous carbon mate-rials such as graphene, carbon nanotubes,
fullerenes and graphite[4,5,11,12], COP based electrocatalysts with
appropriate design haveshown comparable performance to noble-metal
materials [13].
However, rigorous conditions, high-cost catalysts as well as
slowkinetic are inevitable for the synthesis of these COP based
electro-catalysts which hinder the large-scale production of COPs
[9]. In ad-dition, the using solvent with traditional synthesis
methods, such as,solvothermal [14], ionothermal [15], and microwave
[16], inevitablybrought environmental issues, which needs solvent
recovery and re-generation devices in industry application. In
contrast, mechanicalmethods, without using of large amount of
organic solvent, are appliedto not only strip and mix reactants
[17,18], but also enhance the re-action process due to its
generated high energy in a short time bymechanical friction
[19–21]. Ball milling, as one of the most efficientmechanical
methods, has been applied in synthesis porous covalenttriazine
frameworks [22] and functional modification of the carbonmaterials
[23] under mild condition. Besides organic synthesis in
thelaboratory, mechanical milling has already been applied in
large-scaleindustrialization in many fields, such as mechanical
activation of solids,mechanical alloying and the preparing of nano
materials [24–27],which provides the possibility of large-scale
synthesis of functionalorganic materials.
In this study, we have, for the first time, prepared the
bifunctionalelectrocatalyst with N, P co-doped carbon materials
based on covalentorganic polymer through the mechanical chemistry
process under well-optimized conditions without using catalyst and
organic solvent. Sinceof the existence of chemical reaction, here
we termed this newly de-veloped mechanical synthesis method as
reaction milling (RM) to dis-tinguish the traditional physical
mixing/grinding process. As shown inScheme 1, melamine and
terephthalaldehyde are crosslinked during theRM process. After the
construction of COP backbones, phytic acid wasadded to provide the
phosphorous source. The application of reactionmilling can
significantly shorten the reaction time of the synthesis to 3 hand
adopt mild synthesis conditions (Room Temperature) bypassing
theneed for hazardous solvents while the solvothermal method takes
sev-eral days and requires heating at high temperature (usually 120
°C)[28]. Furthermore, the reaction is carried out in the state
where thesolid reactants are continuously mixed to ensure the
homogeneouslyand high contently doping of phosphorous with high
space-time yield.The resulted RM-COP-PA-900 contained abundant
mesopores and de-monstrated excellent catalytic performance towards
ORR and OER inalkaline media. Significantly, the reaction milling
offers the possibilitiesfor efficient synthesis of the bifunctional
electrocatalytic precursor witharound 3.16 g/100mL grinding jar per
synthetic process (Scheme 1)with high space-time yield of 189 kgm−3
day−1. The realization of
mechanical synthesis applied in bifunctional electrocatalysts
makes itpossible for their scalable production, which has a
significant forcommercial needs for energy conversion and storage
applications.
2. Materials and methods
2.1. Materials
Melamine (99%), terephthalaldehyde (98%) and phytic acid (70%in
H2O) were purchased by Shanghai Aladdin Biochemical TechnologyCo.,
Ltd. The reaction milling synthesis was carried out in a
QM-3SPplanetary ball mill from Nanjing Nanda Instrument Co., Ltd.
Thesynthesis was performed in a 100mL grinding jar made of
stainless steelwith 120 g zircon oxide balls (5mm in diameter) as
milling media.Nafion solution was purchased from DuPont Company.
Commercial Pt/C (20 wt%) was obtained from Alfa Aesar Chemical Co.,
Ltd. IrO2 waspurchased from High-purity (99.99%) argon gas, oxygen
gas and ni-trogen gas were obtained from Shi yuan Jing ye (Beijing)
Air PowerTechnology Development Co., Ltd.
2.2. Synthesis of catalysts
The RM-COP-PA was synthesized through reaction milling.
Briefly,melamine (2 g, 1.5 eq.) and terephthalaldehyde (1.5 g, 1
eq.) weremixed by hands for 30 s and then added into the milling
jar with 120 g5mm milling balls. The milling jar was charged with
argon then milledat 500 Hz for 2 h. After milling for two hours,
phytic acid (2 g) wasadded and kept milling for another one hour
under argon atmosphere.The product after reaction milling was
washed by ethanol for threetimes to remove the residues oligomers
then vacuum dried at 80 °C. Theobtained RM-COP-PA was annealed at
350 °C for 2 h, followed byheating up to 900 °C with a ramp rate of
5 °Cmin−1 under argon at-mosphere. The carbonized products were
termed as RM-COP-PA-T (Trepresents the final carbonization
temperature). To make a comparison,we synthesized the RM-COP under
the same condition without theaddition of phytic acid termed as
RM-COP and RM-COP-T (after car-bonization).
2.3. Chemical and physical characterization
The solid-state 13C and 31P measurements were performed with
aBruker AV300 spectrometer operating at 300MHz. The morphology
ofRM-COP-PA-900 and RM-COP-900 materials was observed by
scanningelectron microscopy (SEM, S4700) equipped with an energy
dispersiveX-ray spectrometer (EDS), and transmission electron
microscope (TEM,H7700). X-ray photoelectron spectroscopy (XPS)
analysis was carriedout using a commercial spectrometer (ThermoVG
Fisher Scientific USA)with A1 Kα as X-ray source. Raman spectra was
collected on theLabRAM Aramis Raman Spectrometer (Horiba Jobin
Yvon) using
Scheme 1. Schematic illustration of the preparation process for
the RM-COP-PA-900.
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514 nm laser as the excitation source. The phase analyses of
RM-COP-PA-900 was performed using X-ray diffraction (XRD) with
D/MAX 2000X-ray diffractometer 125 with Cu Kα line (λ=1.54178 Å) as
the in-cident beam and operating at a scan rate of 5° min−1. The
specificsurface areas were calculated using adsorption data in a
relative pres-sure ranging from 0.05 to 0.3 by the
Brunauer–Emmett–Teller (BET)method. Pore size distribution curves
were computed from the deso-rption branches of the isotherms using
the Barrett, Joyner, and Halenda(BJH) method. Fourier transform
infrared spectra (FITR) was recordedon Nicolet 8700 instrument in
the wavenumber range 4000–400 cm−1.
2.4. Electrochemical characterization
The RM-COP-PA-900 catalyst ink was prepared by dispersing
thecatalyst (5 mg) in Nafion (DuPont, 0.5 wt%, 20 μL) dispersion
solutionin ethanol (980 μL) via sonication for 30min to form a
homogeneoussuspension. Then, the catalyst ink (10 μL) was pipetted
onto the glassycarbon (GC) electrode (0.197 cm2) and dried at room
temperature. Thecatalyst loading for the prepared catalyst on the
GC electrode was0.25mg cm−2. By using the same electrode
configuration, RM-COP-PA-800, RM-COP-PA-1000, RM-COP-900, RuO2 and
Pt/C catalysts with thesame amount were also studied for
comparison. Electrochemical mea-surements were conducted with a
CHI660e electrochemical workingstation (CH Instrument) at room
temperature in O2/N2 saturated 0.1 MKOH electrolyte for ORR/OER. A
typical three-electrode system wasemployed, using a glass carbon
rotating disk electrode (RDE) coveredby catalyst as working
electrode, a platinum wire as counter electrode,and an Ag/AgCl
electrode (saturated with KCl) as reference electrode.All
potentials in this study were converted to potential vs.
reversiblehydrogen electrode (RHE) according to the equation (ERHE=
EAg/
AgCl + 0.197+0.0592× pH). As for ORR experiment, O2 or N2
wasbubbled for 30min prior to the test and maintained in the
headspace ofthe electrolyte throughout the testing process. The
catalyst loadedworking electrode was cycled by cyclic voltammetry
(CV) at a scan rateof 50mV s−1, until stabilized current was
obtained.
3. Result and discussion
3.1. Structure and chemical composition of the catalysts
The successful network formation was analyzed by the
solid-state13C, 31P NMR and FTIR spectroscopy. From the
corresponding carbonbond position of Solid-state 13C NMR, we
confirm the generation of C]N bond (Fig. 1a), which is a sign of
the reaction between melamine withterephthalaldehyde. As we know,
the 31P NMR spectrum shift ofphosphate groups on phytic acid is 0
ppm, however, from solid-state 31PNMR spectra of RM-COP-PA, the 31P
NMR spectrum shifts to low-fre-quency region (Fig. 1b), which may
be the reason that the oxygen ofhydroxyl group attached directly to
phosphorus has changed into ni-trogen causing the electron cloud
density around the phosphorus be-comes smaller [11]. Otherwise, the
spectrum has only one peak withoutmiscellaneous peak, further
confirming the uniqueness of combinationbetween phytic acid with
RM-COP. Moreover, Fourier-transform in-frared (FTIR) spectroscopy
(Fig. 1c) reveals the presence of character-istic N-H vibration
(3343 cm−1) and –NH2 vibration (3537 cm−1 and3416 cm−1) of the
carbazole unit, as well as strong C=N vibrations(1656 cm−1 and 1544
cm−1) resulting from the triazine core [29,30].The appearance of
the peaks at 764, 1059 and 1328 cm−1, which couldbe attributed to
the stretching vibrations of P-C, P-O and P-O-H groups,respectively
[31,32]. Contrast with RM-COP (Fig. S1), the main position
Fig. 1. (a) Solid-state 13C NMR spectra of RM-COP with the
corresponding bond position. (b) Solid-state 31P NMR spectra of
RM-COP-PA. (c) FT-IR spectra of RM-COP.
X. Lin et al. Chemical Engineering Journal 358 (2019)
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of the peaks has no obvious shifting and the relative intensity
of pri-mary amine peaks decreases, indicating the addition of
phytic acid withmultiple phosphate groups did not change the COP
framework, in otherwords the doping occurred on the edge functional
group (marginalamino) of RM-COP to form a cross-linked network
structure throughreaction milling, which is further confirmed by
XRD of RM-COP andRM-COP-PA for the position of the peak did not
change, except therelative intensity (Fig. S2). The edge doping
play a critical role forelectrocatalyst performance compared with
the basal doping [33,34].
From the transmission electron microscopy (TEM) image in Fig.
a,we observed that the RM-COP-PA-900 synthesized by reaction
millingpossess good stratified and mesoporous structure, which is
formedduring the reaction milling and pyrolysis process,
respectively (Fig. S3).The layer structure formed through reaction
milling processes goodthermal stability, meanwhile during pyrolysis
process the release ofammonia gas from melamine decomposes under
300 °C produced amacroporous structure [35] (Fig. S4), which is
beneficial for both theORR and OER process [36]. A scanning
electron microscopy (SEM)image and associated elemental mapping
(Fig. 2b–d) identified thehomogeneous distribution of N and P for a
sample pyrolysis at 900 °C(RM-COP-PA-900). In conclusion, the
materials synthesized by reactionmilling have mesoporous arranged
in a two-dimensional structure,which is favor electrocatalysis
applications, because the active sites(i.e., N, P-doped carbon)
could be almost entirely exposed to reactantmolecules [37].
To gain insight into the local chemical environment of
RM-COP-PA-900, X-ray photoelectron spectroscopy (XPS) was applied.
As expected,the C, N and P peaks were found on the XPS spectra. The
XPS surveyspectrum of RM-COP-PA-900 reveals the presence of C, N,
O, and Pelements, which observed N (4.8 at. %) element is from the
melamineprecursor and P (2.3 at%) element is from phytic acid,
along with an O(12.22 at%) peak (Fig. S5), which indicate both N
and P atoms weredoped into the material and has higher level
content of total nitrogen
and phosphorus doping (7.1 at%) than other carbon-based
materials[38–40]. The C1s spectra of RM-COP-PA-900 reveals three
major peaks(Fig. 2f), which can be attributed to sp2 carbon atoms
of the C=C(284.6 eV), carbon atoms in the triazine node (285.3 eV)
and C-O(286.3 eV) [41,42]. The P2p spectra (Fig. 2g) of
RM-COP-PA-900 weredivided into four different bands, the bands at
132.4 eV (PO43−) and133.8 eV (HPO42−) which correspond to the P
atom in phosphatespecies [43]. Except the P in the phytic acid, the
peak of P-C (131.6 eV)and P-O (133.4 eV) [11,44], indicate the
reaction between the phos-phoric acid groups in phytic acid
molecules and the amino groups inmelamine, in other words, the P
heteroatoms effectively doped into thecarbon network through
reaction milling. The N1s spectra for RM-COP-PA-900 samples can be
divided into four different bands at 398.4,399.3, 400.4 and 401.4
eV (Fig. 2h), which correspond to pyridinic N(N1), nitrile N (N2),
pyrrolic N (N3) and graphitic N (N4), respectively[45–48].
Considering the high content of pyridinic N and graphitic N inthe
RM-COP-PA-900 (Fig. 2e), excellent catalytic performance of
thematerial was anticipated toward oxygen redox catalysis [49–53],
forthe former improved the onset potential for ORR, while the
latter de-termined the limiting current density[4,54].
The Raman spectrum of RM-COP-PA after carbonization given inFig.
2j shows the D-band at 1334 cm−1 associated with the E2g mode,while
the G-band located at 1585 cm−1 corresponded to the defectmode
[55]. The high graphitization degree of the RM-COP-PA-900 isevident
from the high ratio of ID/IG (1.1), leading to an
improvedelectrical conductivity. The corresponding X-ray
diffraction (XRD)pattern (Fig. S6) shows two broad graphitic (0 0
2) and (1 0 1) diffrac-tion peaks centered appeared at about 24.3
and 43.8° [56]. We alsoperformed N2 adsorption measurements to
determine the specific sur-face area and pore structure of the
RM-COP-PA-900. It was found thatthe BET surface area was 943m2 g−1
(Fig. 2k), which significantlyhigher than RM-COP-900 (670 cm2 g−1)
(Fig. S7). The type IV isothermcurve with an obvious hysteresis
confirms the presence of mesoporous.
Fig. 2. (a) TEM images of RM-COP-PA-900, (b) SEM images and the
corresponding elemental mapping images for (c) N, (d) P elements in
RM-COP-PA-900. (e) Thecontents of different types of N in XPS
spectra of N 1 s. High-resolution XPS spectra of (f) C 1 s, (g) P
2p, (h) N 1 s for RM-COP-PA-900. (i) Raman spectrum, (j)Nitrogen
adsorption/desorption isotherms, (k) Pore size distribution of
RM-COP-PA-900.
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The rapid N2 uptake (P/P0 > 0.9) is attributable to the
existence ofsecondary, much larger pores. Barrett–Joyner–Halenda
(BJH) pore sizedistribution curves derived from the N2 desorption
confirms the pre-sence of the main mesoporous with diameters
between 3 and 13 nm ofRM-COP-PA-900 (Fig. 2l), and the amount and
uniformity are betterthan RM-COP-900 (Fig. S8). Clearly, therefore,
the RM-COP-PA-900with stereoscopic holes possesses a large surface
area, high pore volumeand wide pore size distribution for
facilitating the electrocatalysis.
3.2. Electrocatalytic performance of the catalysts
To understand the effect of the phosphorus, we compared the
cat-alytic properties of RM-COP-900 and RM-COP-PA-900. As shown
inFig. 3a, b, we can see that the addition of phytic acid can
improve theORR and OER performance obviously, possibly for the
influence of asynergistic effect of N, P co-doping and the edge
doping effect ofphosphorous by changing the charge distribution and
electronic prop-erties, which is beneficial for enhancement of
electrocatalytic activityand electrochemical kinetics. Meanwhile,
the post-addition of phyticacid leads the phosphorus mostly
distributed in the periphery of COPskeleton, which is the active
site for the ORR process more favorablethan the middle of carbon
surface [57,58]. The charge-transfer me-chanism in ORR and OER
process of RM-COP-900, RM-COP-PA-900 andnoble metal based catalysts
were measured by electrochemical
impedance spectrum (EIS) ranging from 106 to 0.05 Hz to
investigatethe electrode kinetics in the alkaline electrolyte (0.1M
KOH) undertheir respective half-wave potential (ORR) and the
potential corre-sponding to 10mA cm−2 (OER). EIS is an effective
method to studyinterfacial properties and processes of electrodes.
Furthermore, theNyquist plots clearly reveals that RM-COP-PA-900
catalyst shows themuch smaller semicircle radius than RM-COP-900
and Pt/C, indicatingthe higher charge-transfer rates of
RM-COP-PA-900, which is beneficialfor ORR in the kinetic process
(Fig. 3c). Also, in the OER process, thecatalyst with phosphorus
has smaller semicircle diameter at low fre-quency area and steeper
slope at high frequency, due to the higherspeed of charge-transfer
and mass-diffusion [59] (Fig. 3d). To furtherexplore the influence
phosphorus doping for the increase of ORR per-formance, we measured
the electrochemically active surface area ofRM-COP-900,
RM-COP-PA-900 and Pt/C (20 wt%) based on double-layer capacitance
method in the potential range of 0.2–0.4 V vs. RHE,where no obvious
Faradic current were observed for each catalyst.Then, the
differences of capacitive currents 1/2|I+‐I‐|@1.1 V wereplotted as
a function of the scanning rates (Eq. S1, Figs. S9, S10, S11),their
slopes are equal to the electrochemical double-layer
capacitance(Cdl) (calculated by Eq. S2). The results demonstrated
that the RM-COP-PA-900 had a large catalytically active surface
area, which was eval-uated by the Cdl for the linearity with the
area. Catalytically activesurface area of RM-COP-PA-900 was
evaluated to be 4.35 cm−2
Fig. 3. The linear scan voltammogram (LSV) curves at an RDE
(1600 rpm) in O2-saturated 0.1M KOH solution of (a) Pt/C,
RM-COP-900 and RM-COP-PA-900 forORR and (b) IrO2, RM-COP-900 and
RM-COP-PA-900 for OER. (c) Electrochemical impedance spectra of
RM-COP-900, RM-COP-PA-900 and Pt/C in 0.1M KOH forORR at E1/2. (d)
Electrochemical impedance spectra of RM-COP-900 and RM-COP-PA-900
in 0.1M KOH for OER at Ej=10. Electrochemical measurements of
RM-COP-900, RM-COP-PA-900 and Pt/C for ORR in 0.1M KOH solution (e)
to estimate the Cdl and relative electrochemically active surface
area and (f) the correspondingTafel plots with a scan rate of 5mV
s−1. (g) The kinetic current density and the mass activities for
RM-COP-900, RM-COP-PA-900 and Pt/C at 0.85 and 0.9 V. (h)Percentage
of peroxide in the total oxygen reduction products and the number
of electron transfer of RM-COP-PA-900 and Pt/C. (i) The stability
tests for ORR of RM-COP-PA-900 and Pt/C in oxygen-saturated 0.1M
KOH under the addition of 3M methanol at 500 s.
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(Fig. 3e) (calculated by Eq. S3), which was higher than that of
RM-COP-900 (EASA=3.15 cm−2) and Pt/C (EASA=2.12 cm−2),
confirmingthe doping of phosphorus can enhance the electrocatalytic
activity byenlarge the active surface area. Tafel plots were used
to investigate thecatalytic kinetics of the prepared hybrids, which
could be derived fromthe polarization curves. As shown in Fig. 3f,
the Tafel slopes of RM-COP-PA-900 (66mV dec−1) is much smaller than
RM-COP-900(114mV dec−1), close to Pt/C (65mV dec−1), which means
that thecurrent increase faster with the increasing potential. The
kinetic currentdensity (Jk) (calculated by Eq. S4) on the nitrogen,
phosphorus co-doped materials (RM-COP-PA-900) electrode is 5.00mA
cm−2, corre-sponding to 19.63mAmgcat.−1 at 0.85 V vs RHE, which is
comparablewith that for Pt/C (19.91 mAmgcat.−1) and over three
folds higher thanthat for RM-COP-900 (6.01mAmgcat.−1), indicating
excellent ORRactivities for the doping of phosphorus. On the basis
of the above result,we can know that the phosphorus plays an
important role in the processof ORR and OER, not only increase the
active sites, as well as changethe atomic charge distribution to
the faster speed of electron trans-mission [60,61]. Besides, the
cyclic voltammetry (CV) curves of RM-COP-PA-900 showed a
substantial reduction process in the presence ofoxygen, whereas no
obvious response was observed under nitrogen,reveals the existence
of oxygen reduction reaction (Fig. S12). Thepercentage of peroxide
species with respect to the total oxygen reduc-tion products and
the electron transfer numbers of RM-COP-PA-900 andPt/C for ORR were
calculated from the RRDE curves (Eqs. S5, S6). It canbe envisioned
that oxygen molecules were reduced to water via a
nearlyfour-electron pathway (n is over 3.87) with a small ratio of
peroxidespecies (around 6%) (Fig. 3h). The electrochemical
stability and thepossible crossover effect are important issues for
cathode materials infuel cells. In comparison with the Pt/C
catalyst, the RM-COP-PA-900electrode exhibited better long-term
stability, higher resistance to themethanol cross-over effect in
oxygen-saturated 0.1 M KOH (the re-sulting methanol concentration
was 3mol L−1), and comparable cata-lytic activity (Fig. 3i).
To explore the effect of carbonization temperatures on the
catalyticperformance of OER and ORR, we evaluated the polarization
curves ofthe catalysts towards ORR and OER activity of the
as-prepared catalystsand commercial Pt/C and IrO2 catalyst in 0.1 M
KOH solution (Fig. 4a,b). Compared the observed oxygen reduction
peak varies with tem-perature, that the RM-COP-PA-900 electrode has
the highest
electrocatalytic activity with a limit current (JL) of 6.12mA
cm−2 and ahalf-wave potential (E1/2) of 841mV towards ORR (Fig.
4e), which iscomparable to most metal-based electrocatalysts
[62,63]. Meanwhilefor OER the onset potential of RM-COP-PA-900 is
1.5 V and the po-tential corresponded to 10mA cm−2 is 1.69 V. The
Tafel slopes showedthat the catalyst under 900 °C was lower than
other conditions, in-dicating that the catalytic reaction with
faster decrease of current to-wards both ORR and OER process (Fig.
4c, d). This is because the highercarbonization temperature leads
to higher degree of graphitization withhigher conductivity for
improving the catalytic activity, which, in turn,excessive
temperatures can cause decomposition of the polymer alongwith the
active sites such as nitrogen and phosphorus [64]. To
furtherconfirm the best performance of RM-COP-PA-900 as ORR and
OERbifunctional catalyst, we measured the RM-COP-PA-T, Pt/C and
IrO2 bysweeping the RDE potential between 0.2 and 1.8 V vs RHE in a
0.1MKOH electrolyte. The overall oxygen activity of the RM-COP-PA-T
as abifunctional catalyst can be evaluated by the potential
difference (ΔE)between the Ej=10 for OER and E1/2 for ORR (i.e.,
ΔE= Ej=10 – E1/2,with the OER potential being taken at a current
density of 10mA cm−2
while the ORR potential being taken at half-wave) with the
smaller ΔEfor the better reversible oxygen electrode [65]. The
RM-COP-PA-900exhibited a ΔE of 0.84 V (Fig. 4f), which is smaller
than other tem-perature, even better than precious metal catalysts
(shown in Fig. 4g)and other recently reported meta-free
bifunctional catalysts (shown inTable S1).
4. Conclusion
In summary, we have developed a mechanical synthesis
method,termed as reaction milling, for scalable synthesis of
covalent organicpolymer as the carbon skeleton (i.e., RM-COP) and
derivative phos-phorus doped material (i.e., RM-COP-PA). The
space-time yield of RM-COP-PA during reaction milling process
reaches 189 kg cm−3 day−1.The pyrosied N, P-doped COP has
macroporous structure, high contentof nitrogen and phosphorus, as
well as possesses a large specific surfacearea (943m2 g−1).
Meanwhile, it shows excellent bifunctional electro-catalytic
activities towards ORR (onset potential= 965mV vs RHE,half
wave=841mV vs RHE, limiting current density= 6.12mA cm−2)and OER
(onset potential= 1.50 V vs RHE, potential at10mA cm−2= 1.69 V).
Moreover, RM-COP-PA-900 displays better
Fig. 4. (a) Linear scan voltammogram (LSV) curves at an RDE
(1600 rpm.) of RM-COP-PA-800, RM-COP-PA-900, RM-COP-1000, Pt/C in
O2-saturated 0.1M KOHsolution for ORR. (b) LSV curves at an RDE
(1,600 rpm.) of RM-COP-PA-800, RM-COP-PA-900, RM-COP-1000 and IrO2
in O2-saturated 0.1M KOH solution for OER.Tafel plots with a scan
rate of 5mV s−1 (c) of RM-COP-PA-800, RM-COP-PA-900, RM-COP-PA-1000
and Pt/C (wt 20%) for ORR (d) of RM-COP-PA-800, RM-COP-PA-900,
RM-COP-PA-1000 and IrO2 for OER in 0.1M KOH solution. (e) The
corresponding limit current density and half-wave potential of
RM-COP-800, RM-COP-PA-900, RM-COP-PA-1000 and Pt/C for ORR. (f)
Comparison of the ΔE of various catalysts (RM-COP-PA-1000,
RM-COP-PA-900, RM-COP-PA-800, Pt/C and IrO2. (g) thespecific
numerical comparisons of ΔE.
X. Lin et al. Chemical Engineering Journal 358 (2019)
427–434
432
-
durability and resistance to methanol crossover effect than
commercialPt/C (20 wt%) and a four-electron transfer pathway,
suggesting thedirect reduction of oxygen to water during the ORR
process.Accordingly, our study provides the possibility for
mechanical fabri-cation of bifunctional catalysts towards ORR and
OER through reactionmilling, which is a solvent-free,
time-efficient, and scalable alternativeto common synthetic
routes.
Acknowledgements
This work was supported by the National Key Research
andDevelopment Program of China (2017YFA0206500); NSF of
China(21676020; 51502012; 21620102007, 21606015); Beijing
NaturalScience Foundation (17L20060, 2162032); Young Elite
ScientistsSponsorship Program by CAST (2017QNRC001);The Start-up
fund fortalent introduction of Beijing University of Chemical
Technology(buctrc201420; buctrc201714); Talent cultivation of State
KeyLaboratory of Organic-Inorganic Composites; Open project of the
StateKey Laboratory of Organic-Inorganic Composites
(OIC-201801007);Distinguished scientist program at BUCT
(buctylkxj02) and the ‘‘111”project of China (B14004).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.cej.2018.09.185.
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Reaction milling for scalable synthesis of N, P-codoped covalent
organic polymers for metal-free bifunctional
electrocatalystsIntroductionMaterials and methodsMaterialsSynthesis
of catalystsChemical and physical characterizationElectrochemical
characterization
Result and discussionStructure and chemical composition of the
catalystsElectrocatalytic performance of the catalysts
ConclusionAcknowledgementsSupplementary dataReferences