Boron and nitrogen co-doped carbon dots for boosting electrocatalytic oxygen reduction

LIU Hui, LIU Zi-hui, ZHANG Jin-qiang, ZHI Lin-jie, WU Ming-bo,

（1.State Key Laboratory of Heavy Oil Processing, College of New Energy, College of Chemical Engineering,China University of Petroleum (East China), Qingdao 266580, China;2.School of Engineering, Edith Cowan University, 270 Joondalup Drive, Joondalup, WA 6027, Australia;3.College of New Energy, China University of Petroleum (East China), Qingdao 266580, China）

Abstract：Carbon dots (CDs) have become an emerging carbon nanomaterial for use in energy-conversion systems because of their large surface area and rapid electron transfer.Carbon dots (BN-CDs) doped with both boron and nitrogen were synthesized by a simple one-step electrochemical etching approach using low-cost petroleum coke as precursor.Compared with CDs doped with only B or N,BN-CDs showed an excellent four-electron oxygen reduction reaction (ORR) activity with a positive onset potential of 0.958 V and a large diffusion-limited current density of −4.32 mA cm−2.Furthermore,the long-term stability and methanol tolerance of BN-CDs were better than those of a commercial Pt/C catalyst.It was found by density functional theory (DFT) calculation that the co-doping of N and B promoted the adsorption of O2 molecules in the ORR process.This work provides new insight into the rational design of carbon nanomaterials and their use in energy conversion.

Key words:Carbon dots；Boron doping；Nitrogen doping；Oxygen reduction reaction

1 Introduction

The cathodic electrocatalytic oxygen reduction reaction (ORR) is of importance in the energy conversion devices,e.g.,fuel cells and metal-air batteries[1].Platinum-based ORR electrocatalysts with high performance have been commercialized in practical.However,the further developments of Platinum-based ORR electrocatalysts is inevitably hindered owing to the scarcity,high cost and poor stability[2].Therefore,searching for non-noble metal catalysts with transcendental activities has become a research hotspot in this field.

More recently,carbon nanomaterials,as alternative metal-free catalysts,has made great progress in electrocatalytic ORR[3,4].Specially,introducing the heteroatoms (such as N,B,S and P) into the carbon matrix was used to further modulate the electronic structure and enhance the catalytic performance of carbon-based catalysts.Generally,carbon atoms in the doped carbon materials with graphite structure are replaced to some extent by heteroatoms,thus enabling the graphite structure change.The introduction of heteroatoms into carbon nanomaterials can induce electron modulation of carbon nanomaterials due to the differences in the size and electronegativity of heteroatoms and carbon atoms,which changes their charge distribution and electronic properties[5].In addition,accompanying with the graphite structural defects created by doping,the electrocatalytic activity of carbon nanomaterials can be further improved[6,7].Compared with single doping,binary doping with heteroatoms can form a more unique electronic structure.For instance,N is more electronegative than C,while B is less electronegative than C.Doping with these two elements together would generate more catalytic active sites due to their synergistic effects,which make the binary doped carbon nanomaterials a better catalytic performance than that of the single doped carbon nanomaterials[8].

Carbon dots (CDs),as a kind of zero-dimensional carbon nanomaterials,have been widely applied in many fields,e.g.,bio-imaging,sensors,photovoltaic,photoelectric converting and supercapacitor[9–12].Recently,CDs have shown great application prospects in electrocatalytic ORR,owing to their unique edge effect and good electron transfer ability[13–15].However,CDs synthesized from the small molecular carbon sources is always restricted to the low graphitization degree and low electrical conductivity,which result in a poor electrocatalytic performance.Although the binary doping strategy is effective for improving performance,it lacks convenient preparation methods and sufficient investigations.Therefore,how to prepare controllable binary doped CDs from abundant carbon sources through a convenient method is expected to further improve the ORR activity of CDs electrocatalysis,which is helpful to promote the application of CD in electrocatalysis.

Herein,we synthesized a novel B and N binarydoped CDs (BN-CDs) with petroleum coke by a facile electrochemical etching method.Petroleum coke is a kind of abundant carbon source with abundant C sp2content and aromatic domain.The as-obtained BN-CDs exhibited excellent electrocatalytic performance for ORR,illustrating better stability and methanol tolerance than the commercial Pt/C catalyst.Density functional theory (DFT) calculations indicated that the synergistic effect of B and N binary-doping plays a pivotal role in boosting the ORR activity compared with the single-doped CDs.

2 Materials and methods

2.1 Materials

Petroleum coke was provided by Pingdingshan Coke Plant,Henan.NaOH,Na2B4O7and NH3·H2O were purchased from Sino pharm Chemical Reagent Co.,Ltd.All reagents used in this experiment were of analytical grade without further purification.

2.2 Preparation of BN-CDs

BN-CDs were prepared from petroleum coke by an electrochemical method,as depicted in Fig.1.Briefly,petroleum coke was made into electrode disks by rolling and calcination according our published work[16].Then two prepared petroleum coke electrodes as negative and positive electrode respectively were put into an electrolyte solution mixed with 20 mL of 0.1 mol L−1of NaOH,10 mL of 0.1 mol L−1NH3·H2O and 10 mL of 0.1 mol L−1Na2B4O7.After 4 h electrochemical reaction at a static potential of 20 V,the electrolyte solution turned from colorless to dark yellow.The solution was centrifuged,washed and filtrated.Then the supernatant was dialyzed with a 3500 Da dialysis bag for 3 days.Finally,the BNCDs powders were obtained after freeze-drying.

Fig.1 Schematic illustration of the synthetic process of BN-CDs from petroleum coke by an electrochemical etching method.

In addition,B-doped CDs (B-CDs) and N-doped CDs (N-CDs) as comparison were prepared using the similar method with BN-CDs without the addition of NH3·H2O and Na2B4O7in the electrolyte,respectively.

2.3 Characterizations

The morphology,crystal structure and elements contents of petroleum coke precursor were characterized by scanning electron microscopy (SEM,S-4800,Japan) with the voltage of 20 kV.X-ray diffraction(XRD,X’Pert PRO MPD,Holland) with CuKα radiation (λ= 0.151 8 nm),and elemental analyzer (Elementar Vario EL Cube,Germany),respectively.The morphology and size distribution of resultant CDs were recognized by a transmission electron microscope (TEM,JEM-2100UHR,JEOL) with a 200 kV accelerating voltage.Raman spectra were recorded on a Renishaw RM2000 micro-Raman spectrometer.The infrared spectra were collected by a Thermo Nicolet Corporation Fourier transform infrared spectrometer(FTIR).The elemental composition and electronic energy level structure of samples were identified by a Thermo scientific ESCALAB 250Xi X-ray photoelectron spectrometer.

2.4 Electrochemical measurements

All electrochemical experiments were carried out on a CHI760E electrochemical workstation (CH Instruments,Shanghai,China) with a typical three-electrode system and 0.1 mol L−1of KOH solution as electrolyte solution.Ag/AgCl and Pt wire were employed as the reference electrode and counter electrode,respectively,and a glassy carbon electrode (GCE,diameter:4 mm) loaded with electrocatalysts was served as the working electrode.Typically,2 mg of BN-CDs powders and 5 μL of Nafion (0.5 wt.%) binder were dispersed into 800 μL ethanol solution.After ultrasonic treatment for 1 h,10 μL of catalyst ink was dropped on the GCE and dried in the air.The catalyst loading was 200 μg cm−2.For comparison,the same amount of commercial Pt/C catalyst was loaded on the GCE by the similar method.During the evaluation of electrocatalytic performance,N2or O2was continuously flowed into the KOH solution to ensure a gas saturation environment.

Cyclic voltammograms (CV) were recorded in N2-saturated or O2-saturated 0.1 mol L−1of KOH solution at a san rate of 50 mV s−1.Liner sweep voltammograms (LSV) were conducted in O2-saturated 0.1 mol L−1of KOH solution at a san rate of 10 mV s−1under different rotation rates (400–2000 r/min).The number of transferred electrons per oxygen molecule(n) can be calculated by the Koutecky-Levich (K-L)equations given below:

wherej,jKandjDare the measured current density,kinetic current density and diffusion-limiting current density,respectively.ωis the electrode rotation speed in rad s−1(1 rad s−1= 9.55 r/min).nis the number of transferred electrons per oxygen molecule.Fis the Faraday constant (96 485 C mol−1).DO2is the diffusion coefficient of O2in 0.1 mol L−1of KOH (1.9×10−5cm2s−1).νis the kinetic viscosity (0.01 cm2s−1) andCO2is the bulk concentration of O2in 0.1 mol L−1of KOH(1.2×10−3mol L−1).

The long-term durability was tested by the chronoamperometry technique at a constant voltage of 0.6 V for 10 h in O2-saturated KOH solution.For the methanol tolerance test,the current was collected at 0.6 V with 47.5 μL methanol injected into the electrolyte at 300 s.All potentials were converted to the reversible hydrogen electrode (RHE) by adding a value of (0.059 pH + 0.197 6 V).

2.5 Density functional theory (DFT) calculations

The DFT calculations were carried out using the Dmol3package in Materials Studio (version 2017 R2,Accelrys Software Inc).The exchange-correlation function was described by the Generalized Gradient Approximation implemented with Perdew-Burke-Ernzerhof (PBE-GGA) method.A double numerical plus polarization function basis sets and a smearing of 0.005 Ha (1 Ha = 27.21 eV) to the orbital occupation were applied.The geometry convergence tolerance for energy change,max force and the max displacement were 10−5Ha,0.002 Ha/Å and 0.005 Å,respectively.The Van der Waals force was corrected by Grimme's DFT-D method[17].A C24H12cluster was used to modeled the initial CDs.The adsorption energy of molecular O2(Eads) can be defined as:

whereEtotal,ECDsandEO2are the total energies for the CDs with the adsorbed O2,the individual CDs,and the gas phase O2,respectively.

3 Results and discussion

3.1 Structural characterizations

The morphology of the petroleum coke precursor was examined by scanning electron microscopy(SEM).As shown in Fig.2a,the petroleum coke has an irregular size and large lamellar graphite-like structure (Fig.2a inset),which can be further proved by the XRD pattern (Fig.2b),contributing to the successfully synthesis of CDs with exquisite graphene structure by subsequent electrochemical etching method.In addition,the elemental analysis results (Table 1) depict that the C,H,N,S,O elements contents (wt.%) of petroleum coke is 95.66%,3.40%,0.46%,0.16% and 0.32%,respectively,indicating the potential of petroleum coke as natural carbon source.The morphology of the synthesized CDs was captured by transmission electron microscopy (TEM).As shown in Fig.3a-c,the prepared CDs are dispersed uniformly and the average size of BN-CDs,B-CDs and N-CDs is 5.4,6.4 and 6.2 nm,respectively.Besides,the high resolution TEM (HRTEM) images suggest that the lattice spacing of BN-CDs,B-CDs and N-CDs is 0.345,0.342 and 0.340 nm,respectively,corresponding to the(002) diffraction plane of the graphite plane[18].The results imply that both single and binary doped CDs have high crystallinity.

Fig.2 (a) SEM images and (b) XRD pattern of petroleum coke precursor.

Table 1 The elements contents (wt.%) from elemental analysis of petroleum coke precursor.

Fourier transform infrared (FT-IR) measurement was performed to investigate the doping process of B and N.As depicted in Fig.3d,two peaks at 3 437 cm−1and 1 378 cm−1appear on the BN-CDs,B-CDs and N-CDs samples,corresponding to the stretching vibration of ―OH and ―COOH[19].Moreover,the peak at 1 125 cm−1of BN-CDs and B-CDs is assigned the stretching vibration of B-C[20],and the peak at 1 115 cm−1of BN-CDs and N-CDs is attributed to the N-C stretching vibration[21].The emerged peaks on BNCDs indicate that the B and N atoms have been expectably incorporated into the carbon skeleton of CDs.The structure of BN-CDs was further characterized by Raman spectroscopy.As shown in Fig.3e,the peaks centered at 1 342 cm−1(D-band) and 1 560 cm−1(Gband) originate from the defective and graphitized sp2carbon[22–24].TheID/IGvalues of BN-CDs,B-CDs and N-CDs are 1.15,1.11 and 1.03,respectively,indicating that the binary-doping produces more defects than the single-doping in CDs.

The chemical composition and the bonding configurations of B and N in BN-CDs,B-CDs and N-CDs were further characterized by the X-ray photoelectron spectroscopy (XPS) measurement.Fig.3f shows the full XPS spectra of BN-CDs,B-CDs and N-CDs.Apart from C 1s centered at 284.5 eV and O 1s centered at 532.5 eV,B 1s centered at 191.0 eV and N 1s centered at 398.6 eV were separately observed in B-CDs and N-CDs,whereas both B 1s and N 1s existed in BN-CDs,indicating that the B and N co-doping using Na2B4O7and NH3·H2O as electrolyte was effective.The chemical composition of the prepared samples is summarized in Table 2.The high-resolution C 1s spectrum of BN-CDs can be fitted into six characteristic peaks (Fig.3g),which assigns to C―B(283.8 eV),C＝C/C―C (284.6 eV),C―N (285.0 eV),C―O (286.0 eV),C＝O (288.3 eV) and O―C＝O(289.0 eV),respectively[25,26].However,C―B and C―N bonding cannot be found in N-CDs and B-CDs,respectively (Fig.3h and 2i),which are consistent with the FTIR spectra and XPS survey spectra results,further confirming the validity of the binary-doping method.Fig.4 shows the high-resolution spectra of B 1s and N 1s of BN-CDs.Four deconvoluted peaks at 191.0,189.8,191.2 and 192.3 eV in the B 1s spectra are assigned to B-N,BC3,BC2O and BCO2bonds,respectively (Fig.4a1,a2)[20].The high-resolution N 1s spectrum of BN-CDs was deconvoluted into five peaks corresponded to N-B (398.4 eV),pyridine N(398.8 eV),pyrrole N (400.3 eV),graphite N(401.0 eV) and oxide N (403.2 eV),respectively(Fig.4b1,b2)[21].By contrast,no B-N bonding can be found in B-CDs and N-CDs.The relative contents of different N and B species are summarized in Table 3 and Table 4,respectively.It can be seen that the content of pyridine N and BC3in BN-CDs is significantly increased compared with that of single-doped NCDs and B-CDs,indicating that the codoping plays important roles in improving the performance of ORR[8,7].

Fig.4 High-resolution B 1s spectra of (a1) BN-CDs and (a2) B-CDs.High-resolution N 1s spectra of (b1) BN-CDs and (b2) N-CDs.

Table 2 The atomic percent (at.%) from XPS in BN-CDs,B-CDs and N-CDs.

Table 3 The surface N contents (at.%) of BN-CDs and N-CDs.

Table 4 The surface B contents (at.%) of BN-CDs and B-CDs.

Fig.3 TEM,HRTEM images and corresponding size distribution of (a) BN-CDs,(b) B-CDs and (c) N-CDs.(d) FTIR spectra,(e) Raman spectra and (f) XPS survey spectra of BN-CDs,B-CDs and N-CDs.The high-resolution C 1s XPS spectra of (g) BN-CDs,(h) B-CDs and (i) N-CDs.

3.2 Formation mechanism

According to the above analysis and characterization results,the possible formation mechanism of BN-CDs from petroleum coke was investigated.As shown in Fig.5,there is a large amount of OH−in the system using NaOH as the electrolyte.The OH−species are moved towards the anode under the electric field,impacting the petroleum coke electrode sheet and cutting off the original graphite valence bonds to produce lots of broken bonds and dangling bonds.Meanwhile,oxidation reaction of the electrolyte occurs at the anode,generating ·OH and ·O radicals[28],which would react with the broken bonds and dangling bonds to form oxygen containing bonds,such as C―O,C―OH and C＝O[29],respectively.These new bonds can act as electrochemical "scissors"to gradually cut away large chunks of petroleum coke to form carbon nanomaterials at the nanoscale[30].NH3·H2O and Na2B4O7,served as N and B source,respectively,could be in situ incorporated into the broken bond to form N species (such as pyridine N,pyrrole N,graphite N and oxide N),B species (such as BC3,BC2O and BCO2),as well as B-N bonding,in the B and N binary-doped CDs.

Fig.5 Schematic diagram for the preparation of BN-CDs from petroleum coke.

3.3 Electrochemical performance

The electrocatalytic activity of the samples was first evaluated by the cyclic voltammetry (CV) in O2-saturated and N2-saturated 0.1 mol L−1of KOH electrolyte using a typical three-electrode system.As shown in Fig.6a,all the prepared catalysts show obvious cathodic reduction peaks in the O2-saturated KOH electrolyte.However,no reduction peaks are found in the N2-saturated KOH electrolyte,indicating the electrocatalytic response to ORR.In addition,the oxygen reduction peak of BN-CDs appears at 0.779 V,which is more positive than that of B-CDs (0.749 V)and N-CDs (0.701 V),suggesting that the binary doped BN-CDs exhibit the enhanced electrocatalytic ORR activity than the single doped B-CDs and NCDs.Linear sweep voltammetry (LSV) curves were recorded at 1 600 r/min,as shown in Fig.6b.The onset potential (Eonset) and diffusion-limiting current density (at 0.3 V vs RHE) are summarized in Fig.6c.It can be seen that BN-CDs exhibit better ORR activity with positive onset potential (Eonset) of 0.958 V and high diffusion-limiting current density (jD) of−4.32 mA cm−2,compared with those of B-CDs(0.930 V ofEonset,−3.87 mA cm−2ofjD) and N-CDs(0.889 V ofEonset,−3.70 mA cm−2ofjD),which is comparable to those of Pt/C (0.987 V ofEonset,−4.65 mA cm−2ofjD),confirming the synergistic effect of B and N in binary doping.

Furthermore,the ORR polarization curves of BN-CDs at different rotating rates (400–2 000 r/min)were tested,as shown in Fig.6d.The results show that the current density increases significantly with the raised rotating speed,which is related to the reduced diffusion layer at high rotating rate[31].In addition,there is a good linear relationship in the curve of thej−1andω−1/2(Fig.6d inset).According to the Koutecky-Levich (K-L) equation,the electron transfer number (n) of BN-CDs is calculated to be 3.93,meaning that the ORR process of BN-CDs is a fourelectron pathway with H2O directly generated.

The electrochemical stability of BN-CDs was then tested by chronoamperometry technique.As depicted in Fig.6e,the current density of BN-CDs remains 80% after continuous polarization for 10 h,whereas the current density of Pt/C decreases to 70%.Methanol tolerance is an important indicator of cathode for fuel cells.Chronoamperometry technique was conducted to compare the sensitivity of BN-CDs and Pt/C to CH3OH,as shown in Fig.6f.When CH3OH was injected into the O2-saturated 0.1 mol L−1KOH electrolyte at 300 s,the ORR current of BN-CDs shows an unconspicuous response to CH3OH,whereas the ORR current of Pt/C experiences a significant decrease.The above results indicate that the electrocatalytic stability and methanol tolerance of BNCDs are better than that of the commercial Pt/C catalyst.

Fig.6 (a) CV curves of the BN-CDs,B-CDs and N-CDs in N2 (dotted lines) and O2 saturated (solid lines) environment.(b) RDE LSV curves of BN-CDs,BCDs,N-CDs and Pt/C at the electrode rotation rate of 1 600 r/min.(c) The onset potential and diffusion-limiting current density at 0.3 V of as-prepared samples.(d) RDE LSV curves and the corresponding K-L plots for the BN-CDs.(e) Chronoamperometric responses of BN-CDs and Pt/C catalysts in O2-saturated 0.1 mol L−1 of KOH solution with addition of methanol at 300 s.(f) Chronoamperometric responses of BN-CDs and Pt/C catalysts in O2-saturated 0.1 mol L−1 of KOH solution for 10 h.

3.4 Interpretation of the active sites

Density functional theory calculations (DFT)were carried out to further investigate the electrocatalytic active sites on B and N doped CDs.Based on the C24H12cluster,CDs models with different doping types were fabricated,as shown in Fig.7.According to the XPS analysis and the previous results,five configurations of B and N doping，named as BN-1,BN-2,BN-3,BN-4 and BN-5 respectively were considered in the BN-CDs,while BC3and pyridine N as the representative of B-CDs and N-CDs,respectively[32].The adsorption energies of O2molecule (Eads) on various models,which is the first and the rate-determining step in ORR,are used as criteria for the evaluation of the ORR activity[33].It can be seen that the calculatedEadsvalues on five BN-CDs models are generally higher than that of N-CDs and B-CDs.These results indicate that the binary doped CDs possess a more favorable adsorption capacity of O2than single doped CDs,which are well consistent with the experimental results.

Fig.7 Optimized adsorption sites of O2 molecule on (a) BN-CDs (BN-1,BN-2,BN-3,BN-4 and BN-5 structures),(b) B-CDs (BC3 structure) and (c) N-CDs(pyridine N structure) models and their corresponding Eads values.The gray,blue,green,red and white colors represent carbon,nitrogen,boron,oxygen and hydrogen atom,respectively.

4 Conclusions

In summary,BN-CDs were successfully prepared from petroleum coke by an electrochemical etching method.Owing to the favorable synergistic effect of B and N dopants,the as-synthesized BN-CDs displayed excellent electrocatalytic four-electron ORR performance with positive onset potential of 0.958 V and large diffusion-limiting current density of−4.32 mA cm−2.Furthermore,BN-CDs exhibited better long-term stability and methanol tolerance than the commercial Pt/C catalyst.This work provides a novel strategy to the regulation of nano-carbon materials as metal-free catalysts for electrochemical energy devices.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No.52072409,U1662113,U20A20131) and Taishan Scholar Project (No.ts201712020).