graphene for oxygen reduction reaction

Accepted Manuscript Title: Facile self-assembly N-doped graphene quantum dots/graphene for oxygen reduction reaction Author: Mengmeng Fan Chunlin Zhu Jiazhi Yang Dongping Sun PII: DOI: Reference:

S0013-4686(16)31901-6 http://dx.doi.org/doi:10.1016/j.electacta.2016.09.014 EA 27944

To appear in:

Electrochimica Acta

Received date: Revised date: Accepted date:

3-5-2016 28-8-2016 4-9-2016

Please cite this article as: Mengmeng Fan, Chunlin Zhu, Jiazhi Yang, Dongping Sun, Facile self-assembly N-doped graphene quantum dots/graphene for oxygen reduction reaction, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.09.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Facile self-assembly N-doped graphene quantum dots/graphene for oxygen reduction reaction

Mengmeng Fan a, Chunlin Zhu a, Jiazhi Yang a, Dongping Sun a*

a

Chemicobiology and Functional Materials Institute of Nanjing University of Science

and Technology, Xiao Ling Wei 200, Nanjing, 210094, China. Fax: 86-25-84431939; Tel: 86-25-84315079; E-mail: [email protected]

1 / 29

Graphical abstract

Contents Graphic: The FE-TEM image of N-doped graphene quantum dots/graphene sheets (N-GQDs/G) and its structural schematic diagram and electrocatalyzing oxygen reduction reaction.

Highlights 

We successfully prepare inexpensive hybrid of N-GQDs/G by simple two-step hydrothermal progresses.



The N-GQDs act as the effective crosslinker and conductive agent for constructing 3D graphene with large specific surface area.



The N-GQDs/G shows high electrocatalytic performance for ORR (dominating four-electron pathway), long-term stability and resistance to methanol crossover.



The pyridinic-N plays an important role for high electrocatalytic activity.

2 / 29

Nitrogen doping carbon nanomaterial has become an important metal-free electrocatalyst for oxygen reduction reaction (ORR) in fue cells. N-doped graphene quantum dots (N-GQDs) are one of the most promising nanomaterials due to abundant electrocatalytic edging and N doping active sites, but low yield, high dispersity and no forming efficient percolative conductive network hinder their direct application as the electrocatalyst. Hydrothermal method is an effective route for preparing high-quality N-GQDs and meanwhile, overcomes the drawbacks of complicated preparing progress and low yield. We further hydrothermally prepare the hybrid material of N-GQDs/Graphene (G) to immobilize N-GQDs on graphene and construct 3D conductive network. Therefore, the inexpensive N-GQDs/G integrates the advantages of rich edges, N doping active sites, high conductivity, and large specific surface area. As confirmed by a series of characterizations and tests, the N-GQDs/G performs high electocatalytic performance (dominating four-electron pathway), long-term stability and resistance to methanol crossover. Moreover, we demonstrate that the type of N specie plays an important role in ORR, especially, the pyridinic-N.

Key words: N-doped graphene quantum dots, Graphene, Hydrothermal method, Pyridinic-N, Oxygen reduction reaction

1. Introduction Among heteroatom-doped carbon nanomaterials, N-doped carbon nanomaterial have attracted extensive attention in many different fields,[1-4] especially as an alternative to Pt-based electrocatalyst for oxygen reduction reaction (ORR)[5, 3 / 29

6] due to their excellent electrochemical properties such as high stability, resistance to methanol crossover, catalytic activity,[7] conductivity.[8] 2D N-doped graphene and 3D N-doped carbon nanotubes (CNTs) have been widely studied as the inexpensive, high efficient metal-free electrocatalysts[7, 9-15]. For example, Yoshikazu Ito et al. prepared nanoporous N-doped graphene whose electrocatalytic performance was much higher than that of commercial Pt catalysts.[16] However, zero-dimensional N-doped graphene quantum dots (N-GQDs) are rarely reported as one electrocatalyst in fuel cells. N-GQDs (less than 10 nm of diameter and less than 10 layers of thickness)[17, 18] have abundant edges, large specific surface area (SSA) and good crystal structure. The small size and good crystal structure provide abundant edge active sites and good conductivity, respectively.[19, 20] The doping of abundant N species can drastically alter their electronic characteristics[5] and offer more electrocatalyzing active sites for ORR.[21] Consequently, N-GQDs are a more promising N-doped carbon electrocatalyst for ORR. However, there are many urgent problems to be solved before practical application. On the one hand, low-yield and complicated preparing, modifying progress of N-GQDs limit the wide study of N-GQDs. Although there are many methods to prepare N-GQDs including solution chemistry method,[20] electrochemical method,[19] chemical scissoring method,[22, 23] etc., hydrothermal synthesis method with small molecules is the most extensively studied and promising 4 / 29

route owing to the simple preparing and modifying progress, high yield, and high quality etc..[24-28] However, there is no paper to report this kind of hydrothermal preparing N-GQDs as metal-free electrocatalyst for ORR. Therefore, in this paper, we obtained large-scale and high-quality raw N-GQDs (Fig. S1 and S2) by a simple hydrothermal method,[23] and then applied these N-GQDs into the electrocatalyst of fuel cells. On the other hand, it is difficult to directly apply pristine N-GQDs as an electrocatalyst, in particular, in liquid reaction environment due to the high water-solubility and moreover, there is high percolation threshold value and low electrochemical stability for blank N-GQDs (Fig. S3).[29] In order to overcome above drawbacks, N-GQDs should be anchored on a conductive substrate such as CNTs,[30] gel,[31] graphene.[32, 33] Undoubtedly, graphene is the more promising substrate because of its excellent electrochemical properties such as high conductivity, large SSA.[34] Hydrothermal method is a simple, effective way to hybridize N-GQDs and graphene. GO with abundant oxygen containing functional groups acts as the starting material and can uniformly mixe with N-GQDs in aqueous solution (Fig S4). Furthermore, the abundant functional groups in GO and N-GQDs are also beneficial to increase the stability of hybrid material by forming more chemical bonds between N-GQDs and graphene during hydrothermal reducing reaction. Although hydrothermal method has been reported for hybridizing graphene quantum dots and graphene,[19] the systematic research of N-GQDs/G haven’t been given. 5 / 29

Therefore, we first produce inexpensive, high-yield and high-active hybrid of N-GQDs/G by hydrothermal progress, and meanwhile, systematically study the structure and properties of N-GQDs/G. In this paper, we prepared raw N-GQD by a hydrothermal method, and then the N-GQDs and graphene were hybridized by second hydrothermal progress. We can observe that the N-GQDs are uniformly distributed on the graphene sheet. A series of electrocatalyzing tests demonstrate that the N-GQDs/G has high electrocatalytic performance (dominating four-electron pathway), large SSA (291.1m2g-1), long-term stability and resistance to methanol crossover. Besides, we affirm that the pyridinic-N plays an important role for the high catalytic performance of ORR.

2. Experimental 2.1 Preparation of pristine N-GQDs The N-GQDs were synthesized by a facile hydrothermal route using citric acid as the carbon source and urea as the N source. 0.21g citric acid and 0.18g urea were dissolved into 5ml deionized water[28] and then the solution was transferred into a 100ml Teflon-lined stainless steel autoclave and heated at 160 °C for 4 h. After hydrothermal reaction, the autoclave was cooled down naturally. The obtained aqueous solution was centrifuged at 5000 rpm for 5 min to dislodge the deposit and obtain the upper aqueous solution for further use.

6 / 29

The upper aqueous were dialyzed with dialysis membrane (3500 MWCO) over all night and the dialyzed aqueous were collected as pristine N-GQDs solution. 2.2 Preparation of N-GQDs/G 2.5 ml homogeneous GO aqueous dispersion (2mgml -1) and 1ml pristine N-GQDs (1.5mgml-1) aqueous were added into 6.5 ml of deionized water and above solution was ultrasonic processing for 20 min. Above solution was sealed in a 100ml Teflon-lined stainless steel autoclave and heated at 180 °C for 12h or 24 h. After hydrothermal treating, the autoclave was cooled to room temperature naturally. The N-GQDs/G was obtained by centrifugation and washed few times with deionized water and then freeze-dried. 2.3 Characterization techniques The Field Emission Transmission Electron Microscope (FE-TEM) images were obtained by a field emission transmission electron microscope (FEI Tecnai 20). The Atomic Force Microscope (AFM) image was completed by atomic force microscope (BRUKER Dimension Icon). The Field-Emission Scanning Electron Microscopy (FE-SEM) was performed with a microscope (FEI Quanta 250F) and energy dispersive spectrometer (EDS) (OXFORD INSTRUMENTS). The N2 adsorption–desorption isotherms and the pore size distribution were measured by Brunauer–Emmett–Teller (BET) equation with instrument (micromeritics, ASAP2020). The X-ray Photoelectron Spectroscopy (XPS) spectra was studied by a RBD upgraded PHI-5000C ESCA system (Perkin 7 / 29

Elmer) with Mg K radiation (h =1253.6 eV). The X-Ray Diffraction (XRD) measurements were carried out on an X-ray diffractometer (Bruker D8 DISCOBER, Bruker AXS GmBH, Karlsruhe, Germany). The Raman Spectra recorded from 500 to 2800 cm-1 on a Renishaw Invia Raman Microprobe using a 514 nm argon ion laser. The Fourier Transform-Infrared Spectroscopy (FT-IR) spectra was obtained on a Bruker Model IFS 66v/s spectrophotometer. The Electrochemical Impedance Spectroscopy (EIS) measurements were completed from 0.1 to 1.0×105 Hz in electrochemical workstation (Shanghai Chenhua CHI760C). The Cyclic Voltammetry (CV) measurements were taken from -1.0V to 0.3V at a scan rate of 5 mVs-1 in 0.1 M KOH solution saturated with N2 or O2 using an electrochemical workstation in a three-electrode with Pt electrode as counter electrode and Ag/AgCl electrode as the reference electrode at room temperature. The ORR characterization was carried out by a rotating disk electrode (RDE) technique (Metrohm chi660d) with a scan rate of 5mVs-1 from 400 rpm to 2025 rpm in 0.1M KOH solution from -1.0V to +0.2V and the RDE consists of a 3 mm diameter glassy carbon disk. The sample solution (1mgmL-1) used for RDE test consists of 80% v/v deionized water, 10% v/v absolute ethyl alcohol and 10% v/v nafion D520 (5 wt.%). The 5μL of above sample solution was dropped on the glassy carbon rotating ring disk electrode. The Pt/C 20 wt.% was purchased from Shanghai Hesen Electrical Co. Ltd (HPT020). Unless otherwise specified, all potentials are against Ag/AgCl as reference. 8 / 29

3. Results and discussion 3.1 Morphology characterizations Pristine N-GQDs can uniformly mix with GO after ultrasonic treatment (Fig. S4) and then the above mixture was hydrothermally treated at 180°C for 12h or 24h in a Teflon-lined stainless steel autoclave (Fig. 1a, the illustration of hydrothermal reaction). The obtained hybrid materials are defined as N-GQDs/G-12 and N-GQDs/G-24, respectively. GO sheets connect with N-GQDs by hydrogen bonds at the initial stage. Then the intramolecular and/or intermolecular dehydration occurs between hydroxyl and carboxyl groups under high pressure and temperature leading to the compact packing of N-GQDs/G by π-π stacking and covalent bonds[35] (Fig. 1e, the possible structure of N-GQDs/G). Hence, there is an absolute precipitate for N-GQDs/GO after hydrothermal reaction in Fig. 1d, which indicates that N-GQDs increase the interaction between GO sheets during hydrothermal progress. To the contrary, no precipitate appears for blank GO, blank N-GQDs (Fig. 1b-c) which can be attributed to the low concentration of GO and the small size of N-GQDs, respectively. In order to observe the morphology of N-GQDs/G-12 and estimate the layers of N-GQDs, the FE-TEM and AFM images were obtained. As shown in Fig. 2a, the particle-like N-GQDs are decorated uniformly on graphene sheets as affirmed by the N element mapping (Fig. 3 Inset). The N-GQDs show spherical morphology (Fig. 2b) with uniform size distribution (the diameter of 2-3nm, 9 / 29

Fig. 1a Inset). The N-GQDs also exhibit a good crystalline structure and the in-plane lattice spacing of 0.21 nm is corresponded to the [110] and [110] planes of graphite (Fig. 2b Inset).[36, 37] The AFM image (Fig. 2c) shows a typical topographic height of 1−2.7 nm (Fig. 2c Inset, the height profile) suggesting that most of the N-GQDs on graphene consist of ca. 3−6 layers (Fig. 2c Inset, the layer distribution).[18, 28, 38] To explore the function of N-GQDs on the hybrid material, the BET SSAs of samples

were

measured.

The

N2

adsorption–desorption

isotherms

of

N-GQDs/G-12 and blank Graphene exhibit the typical Type IV property and H3-type hysteresis loop (Fig. 4). The prompt increase of adsorption at relatively high pressure indicates the presence of macropores[39, 40] as revealed by the distribution of pore size (Fig. 4 Inset), which results from the encapsulating water by reducing graphene oxide in the process of hydrothermal reaction.[35] Different from the blank Graphene, there are many micropores in N-GQDs/G-12 (Fig.4 Inset) due to the appearance of new interstices between N-GQDs and/or 3D graphene.[31, 32] Therefore, the BET SSA of N-GQDs/G-12 is 291.1m2g-1 much larger than that of blank Graphene (94.9m2g-1) and the increasing SSA can be ascribed to three potential reasons: (I) the N-GQDs can built cross-link between graphene sheets leading to more wrinkles,[41] (II) the doping of N-GQDs (Fig. S5) can form small intersheet junctions (Fig. S6 the schematic diagram of intersheet junction), (III) the N-GQDs act as an effective crosslinker between graphene sheets avoiding the 10 / 29

overlapping of graphene sheets and forming new interstices between N-GQDs and/or 3D graphene (Fig. S7). [31, 32] 3.2 XPS analysis The XPS measurements were performed to affirm the elemental composition and N species. Compared to the blank Graphene, there is N element dopant at 400eV for N-GQDs/G-12 in survey spectra (Fig. 5a) as affirmed in the EDS spectra and the N element mapping (Fig. 3). Due to the addition of graphene, the N/C ratios decrease from 13.9% (blank N-GQDs) to 6.7% (N-GQDs/G-12) indicating high N content of pristine N-GQDs, and successful hybridization of N-GQDs and graphene. Moreover, the O/C of blank N-GQDs is up to 41.3% and the abundant oxygen containing functional groups of N-GQDs are beneficial to uniformly mix with GO, and form covalent bonds with graphene by dehydration. In the high resolution deconvoluted N1s spectra, there are three peaks at 399.3eV, 400.1eV and 401.3eV assigned to pyridinic-N, pyrrolic-N and graphitic-N, respectively (Fig. 5b),[42] which is consistent with those of blank N-GQDs and N-GQDs/G-24(Fig. S8). In order to further explore the function of N species in electrocatalyzing ORR, we modulated a key parameter of hydrothermal hybridizing progress, namely, extending the hydrothermal time.[1] With the increase of hydrothermal time, the N/C ratio of N-GQDs/G have little reduction from 6.7 % to 5.7 % (Table 1). However, the graphitic-N greatly raises and this conversion trend may result from the high thermal stability of graphitic-N.[43, 44] Graphitic-N 11 / 29

and pyridinic-N are always the major N species for electrocatalyzing ORR despite

being

controversy.[14,

21,

45-47]

Graphitic-N

maintains

sp2

hybridization of nearby C atoms and can improve conductivity by donating delocalized electrons.[48] Pyridinic-N, with a localized lone pair of electrons, may be the most favorable N specie for improving the onset potential of ORR.[49] 3.3 XRD, Raman spectra, FT-IR, and EIS analyses As shown in Fig. 6a, there are three dispersed and broad diffraction peaks due to the disordered carbon atoms. The peaks at 25.2°, 27.2°, 26.3° correspond to the d spacing of ca. 0.35nm, 0.33nm, 0.34nm, respectively. We can get that the hydrothermal preparing N-GQDs have a good crystal structure according to its distance (0.33nm) in good agreement with the basal plane distance of bulk graphite (0.335nm).[50] Moreover, the distance of blank Graphene is larger than that of pristine graphene (0.34nm) but smaller than GO (ca. 0.694 nm), which suggests the existence of π-π stacking between graphene sheets and the residual of oxygen containing functional groups in blank Graphene.[51] However, the interlayer spacing of N-GQDs/G-12 is 0.34nm smaller than that of blank Graphene indicating the more compact structure. The further reducing spacing in N-GQDs/G-12 can be attributed to the layers π-π stacking and the possible covalent bonds between graphene and N-GQDs (Fig. 1e). Raman analysis is an effective way to characterize the intrinsic property of sp2 carbon with disorder.[52] As shown in Fig. 6b, the two typical peaks are D 12 / 29

peak at 1346 cm-1 and G peak at 1583 cm-1 originating from the structural defects and first-order scattering of the E2g mode of sp2 carbon domains, respectively.[52] A lower intensity ratio of ID/IG acts as an important evidence for fewer defects of carbon material. The blank N-GQDs has a lower ID/IG ratio (ca. 0.9) than those of blank Graphene (ca. 1.1) and N-GQDs/G-12 (ca. 1.1) suggesting the high quality of hydrothermal preparing N-GQDs.[19] Although blank Graphene and N-GQDs/G-12 have the same ratio value, N-GQDs/G-12 shows higher peak intensity. The enhanced D peak of N-GQDs/G-12 can be ascribed to the doping of abundant pyridinic-N and pyrrolic-N locating at defects or edges of N-GQDs.[44, 53] The G peak of N-GQDs/G-12 downshifts to 1583cm-1 (no shown) from 1587 cm-1 (blank Graphene) suggesting a n-type doping because a band downshift was considered to be a symbol of n-type doping.[54] In the FT-IR spectra of blank N-GQDs (Fig. 6c), it shows many hydroxyl groups at 3440 cm-1 and the absorption peaks at 1327 and 1413 cm-1 can be assigned to the C-N stretching vibration and the C-N bending vibration, respectively.[18, 27] The blank Graphene exhibits three characteristic absorption peaks including C=O at 1720cm-1, C=C bending vibration at 1575 cm-1, 1211cm-1 C-OH stretching vibration at 1575 cm-1.[55] However, the absorption peaks of N-GQDs/G-12 comprise of not only C=C, C=O peaks but also the N-related peaks of N-GQDs at 750-1250cm-1, which indicates the successful preparation of N-GQD/G. 13 / 29

The EIS was accomplished to compare the electrochemical properties of samples (Fig. 6d). The two Nyquist plots (blank N-GQDs and N-GQDs/G-12) show small semicircles in the high frequency and straight lines in the low frequency. The semicircle portion corresponds to the charge transfer limiting process at high frequencies[56] and the charge transfer resistance Rt can be directly measured as the semicircle diameter. Compared to the blank Graphene, the semicircle of N-GQDs/G-12 dramatically decreases suggesting that the doping of N-GQDs improves the conductivity of pristine graphene. The steeper gradient in the low frequency represents faster ion diffusion,[57] and hence, the N-GQDs/G-12 has a increasing ion conductivity. The enhanced EIS properties of N-GQDs/G-12 result from the doping of N-GQDs which serve as an effective conductive crosslinker (Fig. 1e) leading to more compact contact and smaller intersheet junctions between graphene sheets (Fig. S6). 3.4 ORR analysis To assess the ORR catalytic activity of as-prepared N-GQDs/G, the CV tests were conducted in N2 or O2-saturated KOH solution (0.1M) in a conventional three-electrode system. As shown in Fig. 7a, N-GQDs/G-12 shows an obvious capacitive current background between −1.0 and +0.2 V in N2-saturated solution. To the contrary, there is a well-defined cathodic peak for N-GQDs/G-12 at -0.28V in O2-saturated KOH solution while blank Graphene and blank N-GQDs exhibit negatively shifted peak potential at -0.35V and -0.39V, respectively (Fig. S9). 14 / 29

The Rotating disk electrode (RDE) measurement was completed to investigate ORR kinetics and onset potential is one of the most important criteria to evaluate the electrocatalytic activity.[58] The onset potentials are -0.09V, -0.14V, -0.1V and -0.21V for N-GQDs/G-12, N-GQDs-24, blank Graphene and blank N-GQDs, respectively (Fig. 7b). The different onset potentials suggest that N-GQDs/G-12 has a pronounced electrocatalytic activity for oxygen reduction.[59] Compared to the N-GQDs/G-24, the more positive onset potential of N-GQDs/G-12 can be ascribed to the relative abundant pyridinic-N (table 1) and many papers also have involved that pyridinic-N is beneficial to positively shift onset potential.[47, 60] During the progress of electrocatalyzing ORR, the nearby C atoms of pyridinic-N have favored atomic charges inducing the absorption of intermediate products, the formation of C-O bond, and the disassociation of O-O bond.[61] The linear sweep voltammetry (LSV) of N-GQDs/G-12 (Fig. 7c), similar to that of N-GQDs/G-24 (Fig S10), shows that the current density typically increases with increasing rotation rate due to the enhanced diffusion of electrolyte at higher speed.[15, 62] In order to further insight the catalytic performance, we compute the electron transfer number (n) by Koutecky – Levich equation (in Supporting Information) and the corresponding curves of N-GQDs/G-12 are plotted at different potentials from -0.6 to -0.8V vs Ag/AgCl in Fig. 7c inset, which indicates a first-order reaction with respect to dissolved oxygen due to the parallel and straight fitting lines.[19]. The electron transfer number (n) of N-GQD/G-12 is 15 / 29

derived to be 3.6−4.1 over potential range from −0.6V to −0.8V suggesting a four-electron process superior to that of N-GQDs/G-24 (n = 3.4-3.8) (Fig. 7d). Compared to the N-GQDs/G-24, N-GQDs/G-12 has a high catalytic performance such as more positive onset potential and high electron transfer number. However, N-GQDs/G-24 shows a higher kinetic current density (Fig. 10) possibly resulting from the relative abundant graphitic-N which can donating delocalized electrons on the graphene plane (table 1).[49] Based on the above characterizations and tests, the optimal electrocatalytic performance of N-GQDs/G-12 comes from: (I) the relative abundant pyridinic-N, (II) the abundant edge active sites, excellent crystal structure of N-GQDs, (III) the increasing SSA and conductivity of hybrid material. Additionally, it can be recognized that the type of N specie is a critical factor to the high electrocatalytic performance, especially, the pyridinic-N. Therefore, these results indicate that the two-step hydrothermal preparing N-GQDs/G, similar to N-doped graphene or N-doped CNT is a promising metal-free catalyst for ORR in an alkaline solution.[15, 63] It has been demonstrated that N-doped carbon materials show superior stability and tolerance to cross over effect compared to a commercial Pt/C catalyst, [15, 63] which also is affirmed by the N-GQDs/G in this paper. After 300 CV cycles in 0.1M KOH, the minimal change is observed in the CV curve of N-GQDs/G-12 (Fig. S11b) and in contrast, the cathodic current is decreased significantly for Pt/C 20 wt.% under similar testing conditions (Fig. S11a). 16 / 29

Moreover,

the

presence

of

5%

wt.%

methanol

doesn’t

hinder

the

electrochemical performance of N-GQDs/G-12 (Fig. S11d), whereas a large anodic current is found in the case of Pt/C as a result of methanol oxidation (Fig. S11c).[64]

4 Conclusions We develop two hydrothermal progresses to prepare the low-cost, high-yield N-GQDs and the hybrid material of N-GQDs/G. The N-GQDs serve as electrocatalytic active centers, and meanwhile as the crosslinker and conductive agent for constructing excellent electrocatalyst leading to increasing SSA and conductivity. The N-GQDs/G performs four-electron pathway resulting from the excellent structural properties of N-GQDs such as abundant pyridinic-N, rich-edges, crystal structure. The N-GQDs/G also shows high electrochemical stability and resistance to methanol crossover. Furthermore, the ratio of different types of N specie can be tuned by changing the hydrothermal time on which we demonstrate that the pyridinic-N is the favourable N specie for high catalytic performance. In brief, we improve the availability of N-GQDs and explore the function of different types of N specie in electrocatalyzing ORR.

Acknowledgements This work was supported by “National Natural Science Foundation of China (Nos. 51272106, 51572124, 21103092)”, “Research Fund for the Doctoral Program of Higher Education of China (RFDP) (No.20123219110015)”, 17 / 29

Program for NCET-12-0629 and Qing Lan Project, “The Fundamental Research

Funds

for

the

Central

Universities

(No.

30920130121001,

30920130111003)” and “A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD, China)”.

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Fig. 1 (a) The illustration of formation process of N-GQDs/G, (b-d) the photograph of samples after hydrothermal reaction for 12h (b, blank Graphene; c, blank N-GQDs; d, N-GQDs/G) and (e) the possible structure of N-GQDs/G (not drawn to scale): graphene sheets (black), N-GQDs (blue) and covalent bonds (red).

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Fig. 2 (a, b) The FE-TEM images of N-GQDs/G-12 (a, Inset: size distribution; b, Inset: HRTEM image of N-GQDs and the corresponding line profile analyses of the diffraction fringes); (c) the AFM image of N-GQDs/G-12 (Inset: layers distribution of N-GQDs (up) and height profile along the white line (down)). 24 / 29

Fig. 3 The EDS spectra and element mappings (Inset C, N, O) of N-GQDs/G-12.

Fig. 4 The Nitrogen adsorption–desorption isotherms and the corresponding pore diameter distribution curves (Inset) of N-GQDs/G-12 and blank Graphene.

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Fig. 5 (a) The survey XPS spectrums of blank Graphene, blank N-GQDs, N-GQDs/G-12 and (b) the peak deconvolution of N1s core level of N-GQDs/G-12.

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Fig. 6 (a) The XRD pattern, (b) the Raman spectra, (c) the FT-IR spectra, and (d) the Nyquist plots (Inset: expanded Nyquist plots at high frequency) of blank N-GQDs, blank Graphene, N-GQDs/G-12.

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Fig. 7 (a) The CV curves of N-GQDs/G-12 on a GC electrode in N2 or O2-saturated 0.1 M KOH, (b) the LSV of samples at a sweep rate of 5 mVs-1 at 1600 rpm in O2-saturated 0.1 M KOH, (c) the LSV of N-GQDs/G-12 with different rotation rates from 400 to 2025 rpm (Inset, Koutecky−Levich plots of N-GQDs/G-12 derived from -0.6 to -0.8V), (d) the electron transfer number (n) of N-GQDs/G-12 and N-GQDs/G-24 .

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Table 1. The ratios of N/C and the relative ratios of different N species The ratios of different N species are based on the equation: Nx%=Nx/(N1+N2+N3)×100%, (N1, N2 and N3 represent the peak areas of pyridinic-N, pyrrolic-N and graphitic-N, respectively, which are obtained from the peak deconvolution of N1s spectra). Materials blank N-GQDs N-GQDs/G-12 N-GQDs/G-24

N/C ca.13.9% ca. 6.7% ca. 5.7%

graphitic-N ca. 5.0 % ca. 19.0% ca. 29.2%

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pyrrolic-N ca. 60.5% ca. 38.5% ca. 55.7%

pyridinic-N ca. 34.5% ca. 42.5% ca. 15.1%