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Chemical Engineering Journal

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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: xiangzh@mail.buct.edu.cn (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.

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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.

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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