Two-electron oxygen reduction on fullerene C60-carbon nanotubes covalent hybrid as a metal-free electrocatalyst

Nanocarbon materials are considered to be active for electrochemical oxygen reduction reaction (ORR) for hydrogen peroxide (H2O2) synthesis. In the present work, a new type of fullerene 60 (C60)-carbon nanotubes (CNTs) hybrid with covalently attached C60 onto outer surface of CNTs was synthesized. The structure of C60-CNT hybrid was confirmed by physical and chemical characterizations and its conformation is proposed featuring the covalent incorporation of CNTs and C60 derivative. C60-CNT hybrid showed high efficiencies on electro-generating H2O2, owing to huge surface area and intermolecular electron-transfer in the hybrid structure. A high H2O2 production rate of 4834.57 mg L−1 h−1 (426.58 mmol L−1) was achieved at − 0.2 V vs saturated calomel electrode (SCE).

promising electrocatalysts for the ORR because of their higher performance compared to that of their individual forms. The hybridizations of CNTs and C 60 fullerene (or a fullerene derivative), which have been shown as the excellent electron acceptor, can be prepared through either non-covalent or covalent procedures [18][19][20][21] . However, covalent functionalization of C 60 and its derivatives to the outer surface of the CNTs is more potent than the van der Waals interaction between them and thus can impose more remarkable changes on their band and electronic structures. Additionally, the covalent nature renders powerful intermolecular interactions between CNTs and C 60 structures. A main challenge in this context is to develop the effective and convenient approaches for the synthesis of C 60 -CNT covalent hybrids. Here, a new type of C 60 -CNT covalent hybrid was prepared by Birch reduction 22 reaction between multi-walled CNTs and 4-chlorobenzoic acid functionalized-fullerene (CB-C 60 ) via Friedel-Crafts acylation.
Herein, the formation of a new C 60 -CNT hybrid based on the direct covalent linkage of C 60 derivative molecules on the sidewalls of CNTs was explored, and the structural properties of the as-prepared hybrid were studied by means of microscopic and spectroscopic approaches. Subsequently, the selectivity and electrochemical activity of the C 60 -CNT hybrid were studied toward the two-electron ORR for H 2 O 2 generation. Physical and chemical characterization tools were associated with obtained results from electrochemical analyses to clarify the distinctive features of as-prepared nanocarbon hybrid that contribute to the H 2 O 2 electro-generation activity.
functionalization of c 60 with 4-chlorobenzoic acid. 4-Chlorobenzoic acid (0.25 mmol), C 60 (0.45 mmol), and polyphosphoric acid (PPA, 83% assay, 20 g) were added in a 250-mL resin flask containing a mechanical stirrer under nitrogen atmosphere. After being stirred at 130 °C for 3 h, 5 g of phosphorous pentoxide (P 2 O 5 ) was added into the reaction media in one portion. The dark mixture turned into lighter brown. The resulting solution was further stirred at 130 °C for 48 h. Afterwards, the mixture was cooled down to reach the room temperature and it was slowly poured into deionized water to dissolve the PPA and P 2 O 5 . The precipitate was gathered by suction filtration and washed with NH 4 OH. Residual PPA and unreacted CB were eliminated by Soxhlet extraction with deionized water and methanol for four days, respectively. Finally, the sample was vacuum-dried at 60 °C for 12 h, as a result, the gray powder was obtained.
preparation of c 60 -cnt hybrid and mixture. The preparation procedures using Birch reduction 22 were as follows: 0.2 g of multi-walled CNTs were added into a dry 150-mL three-neck round-bottom vessel under argon atmosphere. Then, 60 mL of NH 3 was condensed into the reaction medium, which was cooled down to −77 °C in a liquid nitrogen-butyl acetate bath. Subsequently, 1.2 g of lithium metal was gradually added to the reaction vessel while being stirred. 40 min after the lithium addition, 1.4 g of as-prepared CB-C 60 sample was slowly added and the reaction mixture was endlessly stirred at -33 °C during 24 h. In order to quench the reaction, absolute ethanol was added followed by the addition of deionized water. The resulting suspension was acidified by adding 1.0 mol L −1 HCl and washed with absolute ethanol by centrifuging several times and finally dried at 80 °C for 8 h under vacuum (See Fig. 1).
Fabrication of gas diffusion electrode. 0.2 g of carbon nanomaterials (CB-C 60 , CNTs and C 60 -CNT hybrid), 0.42 g of PTFE, 3% V/V of n-butanol were thoroughly blended and dispersed in 30 mL of N-methyl-2-pyrrolidone (NMP) for 1 h to prepare the gas diffusion electrode (GDE) electrodes. The as-prepared mixture underwent heat treatment at 80 °C until a paste-like ink was obtained and pressed by a stainless steel mesh current collector at 10 MPa for 5 min. The prepared electrodes were sintered under N 2 atmosphere at 350 °C for 30 min, followed by cutting to diameter of 15 mm. The obtained electrodes were put at the bottom of a polypropylene cylindrical holder for manufacturing the GDEs. A graphite disk, which is in contact with a copper wire as the electrical connector, is located in the cylindrical holder. physical and chemical characterization methods. TESCAN (Mira3, Czech Republic) and JEOL (JEM-2200FS, Japan) microscopes were used to record the scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HR-TEM) micrographs, respectively. X-ray diffraction (XRD) analysis was done by Siemens D5000 X-ray diffractometer (Germany) using Cu Kα exciting source (λ = 1.54056 Å). Raman spectra were acquired by a WiTech confocal Raman microscope equipped with a 532 nm NiYAG laser. A Tensor 27 IR-spectrometer (Bruker, Germany) on the KBr pellets over the range of 400-4000 cm −1 was utilized for taking the Fourier transform infrared spectroscopy (FT-IR) spectra. Brunauer, Emmett and Teller (BET) method was performed using nitrogen adsorption/desorption at 77 K with a Belsorp mini II device (Bel, Japan). Atomic force microscopy (AFM) was applied to study the surface topography of the coated electrodes, which was performed by a Nanosurf Mobile S microscope (Nanosurf, Switzerland). The elemental composition was studied X-ray photoelectron spectroscopy (XPS) (PHI 5000 Versaprobe, Al Kα source). The water contact angles of the prepared electrodes were determined using a standard goniometer (200, Rame-Hart, USA). For this aim, 5 μL water droplet were deposited onto surface of prepared electrodes at ambient temperature. For each sample, at least five contact angle measurements were averaged on different areas of electrode surface.
www.nature.com/scientificreports www.nature.com/scientificreports/ electrochemical characterization. The electrochemical measurements were done by means of a computer-controlled potentiostat (PARSTAT 2273). For the rotating ring disk electrode (RRDE) measurements, a three-electrode system was assembled with a standard three-electrode configuration. The as-prepared catalyst ink, consisting of nanocatalyst powder, water, isopropanol and Nafion solution (5 wt%), was carefully dropped on the RRDE electrode (the electrode area is 0.2475 cm 2 ) with a catalyst loading of 0.1 mg cm −2 as working electrode. Graphite rod and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. For the accurate and reproducible measurement of H 2 O 2 selectivity, it is very important to clean the RRDE thoroughly prior to each experiment. Cyclic voltammetry (CV) was performed between −1.2 and 0.3 V (vs. SCE) in N 2 -saturated 0.5 mol L −1 H 2 SO 4 at a scan rate of 5-10 mV s −1 , in which a steady CV response was obtained. O 2 gas was purged into the electrolyte for 5 min (caution: if the time interval between the Pt ring cleaning and ORR measurement is long, the H 2 O 2 selectivity can be underestimated due to the surface passivation of the Pt ring). The electrochemical impedance spectroscopy (EIS) was conducted at −0.2 V (vs. SCE) from 100,000 to 1 Hz to determine the uncompensated resistance (R u ) in a high-frequency range for iR-correction. The H 2 O 2 production activity was assessed by linear sweep voltammetry (LSV) in O 2 -saturated 0.1 mol L −1 H 2 SO 4 at a scan rate of 5 mV s −1 and a rotation speed of 1600 rpm. The ring electrode was set at a constant potential of 0.5 V vs. SCE to detect the generated H 2 O 2 . The electron transfer numbers (n) and H 2 O 2 selectivity were calculated using the following relations 23 : where I r and I d denote the ring current and disk current, respectively. The N was the collection efficiency of Pt ring, which was determined to be 0.3 with [Fe(CN) 6 ] 4−/3− redox probe 24 . The Koutecky-Levich (K-L) plots (J −1 versus ω −1/2 ) represents the relation between the measured current, electron transfer number, and rotation speed as follows:

Friedel-Crafts acylation
Birch reduction The stability of as-prepared C 60 -CNT hybrid was evaluated using the chronoamperometric method performed at a constant potential of −0.2 V vs. SCE. The H 2 O 2 faradaic efficiency was determined from the H 2 O 2 yield against the quantity of charge passed: where C is the H 2 O 2 concentration (mol L −1 ), V is the volume of electrolyte (L), F is the Faraday constant (96485.3 C mol −1 ), and Q is the passed charge amount (C).
To appraise the number of electrochemically active centers on the surface of as-prepared hybrid electrodes, CV analysis in a solution containing potassium hexacyanoferrate-III (1 mmol L −1 ) and potassium chloride (1 mol L −1 ) was fulfilled. The Randles-Sevcik equation 25 (Eq. 6) was applied to calculate electrochemically active surface area (ECSA) of the mentioned electrodes.
= . × υ I 265 10 n ACD (6) P 5 3/2 1/2 1/2 where I p is the peak current (A), n (=1) is the number of electrons transferred, A is the effective area of the electrode(cm 2 ), D is the diffusion coefficient of potassium hexacyanoferrate-III (taken to be 7.60 × 10 −6 cm 2 s −1 at 25 °C) 26 , C is the concentration (mol cm −3 ), υ is the scan rate (V s −1 ). Nafion 117 membrane was applied as a separator in H 2 O 2 electro-generation experiments which were carried out in a cell with two portions. The cathodes were chosen in the form of pure carbon paper, improved cathode with CB-C 60 , CNTs and C 60 -CNT hybrid electrodes (with similar area of 4.9 cm 2 ) and anode was selected in the form of Pt sheet (10 cm 2 ). For electro-generation of H 2 O 2 , the diffusion cathode was steadily provided with pure O 2 gas. 100 mL of Na 2 SO 4 solution with specific concentrations was magnetically stirred (at 300 rpm) and produced as a supporting electrolyte for all tests. The 3100ST pH meter was used to identify the solution pH (Ohaus, Switzerland). The pH was set by H 2 SO 4 and NaOH solutions (0.1 mol L −1 ). The concentration of electro-generated H 2 O 2 was acquired by spectrophotometer based on iodide approach 27 . In this method, the sample (4 mL), potassium hydrogen phthalate (3 mL, 0.5 mol L −1 ) and iodide reagent (3 mL) which contains 0.4 mol L −1 KI, 10 −4 mol L −1 (NH 4 ) 2 MoO 4 and 0.05 mol L −1 NaOH were mixed. Afterwards, the solution absorbance was read at 351 nm by the UV-Vis spectrophotometer (DR3900, Hach, USA). H 2 O 2 quantity was obtained by flow-injection chemiluminescence method following the luminol reaction by H 2 O 2 .

Results and Discussion
functionalization and characterization of cB-c 60 . C 60 was functionalized with CB through direct Friedel-Crafts acylation method in a PPA/P 2 O 5 medium (Fig. 1). 1 H, 13 C NMR, and FT-IR spectroscopic techniques were applied to monitor the functionalization progress. To remove possible impurities, samples were worked-up by adding water and methanol in the Soxhlet extraction for four days to eliminate any residual precursor before characterization. The 1 H NMR spectrum of CB-C 60 in CS 2 /CDCl 3 ( For further investigation of functionalization of C 60 with CB, FT-IR spectra of C 60 , CB, and CB-C 60 were taken (Fig. S1d). The bare C 60 spectrum exhibited four main dominant peaks at 525, 574, 1180, and 1428 cm −1 , which could be related to the F 1u infrared active vibrations 28,29 . CB spectrum showed a strong peak at 1690, a weaker peak at 1230 cm −1 and a broad peak at about 3200-3600 cm −1 , which were attributed to carboxylic acid group (C=O, C-OH, O-H stretching modes, respectively). In the spectrum of the CB-C 60 , the appearance of a peak at 1718 cm −1 was ascribed to the stretching vibrations of carbonyl group (C=O), which demonstrated that the attachment between CB and C 60 molecules had indeed been attained by acylation. In addition, two peaks were detected in the spectrum of the CB-C 60 at 545 and 582 cm −1 , which are assumed to be resulting from the peaks of C 60 at 525 and 572 cm −1 , respectively. Furthermore, the peaks observed around 755 cm −1 and in the range of 1400-1600 cm −1 was assigned to the vibration of C-Cl and C=C aromatic, respectively. These obtained results verify that C 60 has been covalently functionalized by CB as a hydroxyfullerenyl ketone.
Raman spectroscopy analysis is commonly applied to study the lattice and electronic structures of nanocarbon materials. Typical Raman spectra of raw C 60 and as-synthesized CB-C 60 are indicated in Fig. 2a. The spectrum of the raw C 60 exhibited the A g (2) pentagonal pinch vibration frequency at 1469 cm −1 , and the additional H g (7) and H g (8) modes can be distinguished at 1425 and 1575 cm −1 , respectively 30 . In the spectrum of CB-C 60 , because of the covalent bonding of the CB to the C 60 cage the positions of relevant modes were red-shifted by 6 cm −1 with respect to that in the bare C 60 . Moreover, H g (7) and H g (8) modes of CB-C 60 appeared notably broadened, owing to the decreased symmetry of CB-C 60 compared to the pristine C 60 . It can be seen that the A g (2) mode of CB-C 60 spectrum had a slight broadening, which can be ascribed to inhomogeneous broadening phenomenon as it is a non-degenerate mode and hence cannot split upon modification of the C 60 cage 31 . characterization of c 60 -cnt hybrid. A series of spectroscopic and morphological surveys demonstrated the as-prepared C 60 -CNT hybrid structure. Raman spectroscopy was used to verify the covalent incorporation of C 60 to sidewall of CNTs. As can be realized from Fig. 2a, the Raman spectrum of pristine CNTs showed two peaks at 1347 and 1578 cm −1 as D-band and G-band, respectively. The G-band was associated to the sp 2 hybridization of carbon in the graphitic frame. The D-band mainly originates from the vibrations of sp 3 bonds of carbon atoms which show the disorders and defects in the CNTs 32 . In the spectrum of the C 60 -CNT hybrid, besides the D and G bands at 1350 and 1582 cm −1 , respectively, a peak located at 1458 cm −1 was clearly detected, which can be ascribed to the A g (2) mode of the C 60 framework 33 . However, the up-shift of the D and G peak positions and down-shift of the A g (2) mode in the C 60 -CNT hybrid spectrum compared to the individual materials showed that the charge-transfer from CNTs to C 60 may happen as a result of strong electron-withdrawing ability of C 60 34 , which was in agreement with the reported results 19,20 . On the other hand, the intensity ratio of these bands (I D / I G ) was proportional to the disorder degree on the carbon matrix and applied as a probe to identify the covalent attachment of the CNTs surface. The I D /I G value of the C 60 -CNT hybrid (0.96) was greater than that of the CNTs (0.43), indicating that the CB-C 60 covalently bonded onto the CNTs surface 33 . These results readily concur with those obtained from FT-IR analysis (see ESI Fig. S2).
XRD diffractograms of C 60 , CB-C 60 , CNTs and C 60 -CNT hybrid are revealed in Fig. 2b. For bare C 60 , the localized peaks at 2θ = 10.8°, 17.7°, 20.8°, 21.7°, 27.5°, 28.2°, 30.8° and 32.7° that referred to plane reflections of (111), (220), (311), (222), (331), (420), (422) and (511), respectively, associated with face centered cubic (fcc) crystalline phase of C 60 with lattice constant a = 14.17 Angstrom (JCPDS 44-0558) 35,36 . As for CB-C 60 , the detected peaks at 2θ = 10.2°, 17.1°, 20.2°, and 21.5° became slightly broader with decreased intensity compared to those of the unfunctionalized C 60 , indicating that functionalization of C 60 did not completely change the lattice structure of C 60 . As can be observed in Fig. 2b, the pure CNTs 37 displayed two broad peaks at 2θ = 25.74° and 42.87° which can be ascribed to the hexagonal graphite crystal planes (002) and (001), respectively, with an interlayer distance (d) of 0.34 nm. XRD diffractogram of C 60 -CNT hybrid demonstrates a superposition of the peaks of the CB-C 60 and CNTs, evidencing the hybrid structure of these two nanocarbons. The intensity of the (002) diffraction peak at 26.12° remarkably decreased in comparison to the pure CNTs with negligible shift. However, the weaker peak at 42.87° (001) of CNTs pattern almost disappeared in the C 60 -CNT hybrid illustrating that the lattice structure of CNTs was relatively changed upon the linkage of the CB-C 60 moiety. On the other hand, in comparison to the XRD analysis of CB-C 60 , the diffraction peaks of C 60 -CNT hybrid centered at 10.5°, 18.1° and 20.9° appeared which asserted the formation of C 60 -CNT hybrid. For further investigation, the XRD pattern for the mixture of CNTs and bare C 60 (C 60 -CNT mixture) was taken, which indicated a poorly crystalline or amorphous feature to that of CNTs and peaks of C 60 that were hardly detected (see ESI Fig. S3). It was confirmed that physical mixing of C 60 with CNTs slightly influenced the properties of CNTs compared to covalent hybrid structure. In fact, in the mixture, C 60 molecules were attached to the CNTs surface by van der Waals forces 38,39 .  www.nature.com/scientificreports www.nature.com/scientificreports/ The SEM micrographs of synthesized C 60 -CNT hybrid (Fig. 3a) demonstrate the growth in wall width of C 60 -CNT hybrid in comparison with unmodified CNTs (SEM and TEM images are shown in Fig. S4) which can be attributed to the addition CB-C 60 on the CNTs surface. In addition, as observed by the HR-TEM image of C 60 -CNT hybrid in Fig. 3b,c, the spherical CB-C 60 molecules are covalently conjugated on the CNTs sidewall. The diameter of the individual CB-C 60 sphere was determined to be around 1 nm (Fig. 3c, inset), while TEM micrographs of spherical particles of C 60 (Fig. S5a) and CB-C 60 (Fig. S5b) display the facile agglomeration of their molecules resulted from the potent van der Waals attractions and profound π-π forces between them 40 . It should be noted that for removing the unreacted and physically adsorbed CB-C 60 particles, the synthesized samples were washed via ultrasonication in CS 2 solvent several times.
XPS analysis was fulfilled to clarify the bonding configurations and the chemical composition in synthesized C 60 -CNT hybrid. The C 1s scan of the pristine CNTs (Fig. 4a) indicated the C-C bonds (sp 2 carbon) at 284.4 eV, together with different peaks located at 284.9, 286.1, and 288.0 eV could be connected to the C-H, C-O, and C=O bonds, respectively, because of native surface groups and structural defects present in the raw CNTs sample. A minor peak at 290.9 eV can be ascribed to the π-π* changes in sp 2 carbon structures 18 . The XPS spectrum of C1s  www.nature.com/scientificreports www.nature.com/scientificreports/ of C 60 shows the located peaks at 284.6 eV (C-C), 286.1 eV (C-O), and two π-π* shake-up peaks around 290 eV (Fig. 4b). As it can be observed in Cls region of CB-C 60 (Fig. 4c), three new peaks appear at 287.1, 285.3, and 288.0 assigned to aromatic C-Cl, C-OH, and C=O bonds, respectively. XPS spectrum of Cl2p (Fig. 4d) indicates two major peaks centered at 200.5 eV (Cl2p 1/2 ) and 202.1 eV (Cl2p 3/2 ). The results can confirm the successful functionalization of C 60 within the CB. In case of C 60 -CNT hybrid (Fig. 3e), the peaks at 288.0 and 285.3 eV assigned to C=O and C-OH groups, respectively 18 , indicating a higher relative intensity than those of the pristine CB-C 60 and CNTs. It is suggesting that more C=O and C-OH bonds have been introduced to the hybrid structure. These results readily concur with those obtained from the characterizations discussed above. physical characterization of fabricated GDe cathodes. SEM micrographs of prepared electrodes by CB-C 60 (Fig. 5a), CNTs (Fig. 5b), and C 60 -CNT hybrid (Fig. 5c) indicate the mesoporous morphology of CNTs and the slightly smooth plate surface of CB-C 60 , while C 60 -CNT hybrid electrode possesses a rough surface and heterogeneous porous structure. These results were further confirmed by means of BET analysis (Fig. 5d), wherein C 60 -CNT hybrid presented a higher surface area (422.52 m 2 g −1 ) compared to that of the CNTs (185.28 m 2 g −1 ), C 60 -CNT mixture (258.5 m 2 g −1 ), and CB-C 60 (98.2 m 2 g −1 ), respectively.
The hydrophilic feature of the fabricated nanocarbon based electrodes was investigated by water contact angle (Fig. 5a-c, inset). The average contact angles of CB-C 60 , CNTs, and C 60 -CNT hybrid were found to be 76.5°, 64.8°, and 37.2°, respectively. As can be seen, the decrease in contact angle value means the increase hydrophilicity of the samples, which could be caused by the presence of carboxyl and hydroxyl groups in the fabricated C 60 -CNT hybrid electrode, as verified in the Raman and XPS data. www.nature.com/scientificreports www.nature.com/scientificreports/ The surface morphology of the CB-C 60 (Fig. 6a), CNTs (Fig. 6b), and C 60 -CNT hybrid (Fig. 6c) electrodes were examined using AFM analysis, and the 3D images in the scale of 8 × 8 µm 2 are displayed. In these graphs, dark areas show the pores or valleys, while bright areas show the highest point of the fabricated electrodes surface. The www.nature.com/scientificreports www.nature.com/scientificreports/ parameters of roughness were measured by the AFM images. The observed increase in the average roughness of the fabricated electrode with C 60 -CNT hybrid could be attributed to the increased surface porosity as shown in the SEM images and BET analysis. electrochemical activity of oxygen reduction. In the first step, the electrochemical activities of the fabricated hybrid as the GDE materials towards ORR were evaluated by CV method in N 2 -or O 2 -saturated acidic media. As can be seen from Fig. 6d, there were no obvious reduction peaks in the N 2 -saturated solution using C 60 -CNT hybrid electrode, while there were well-known oxygen reduction peaks for all nanocarbon based materials in the O 2 -saturated solution, suggesting their distinct electrocatalytic activity toward ORR. Furthermore, the synthesized C 60 -CNT hybrid demonstrated the highest reduction current density of 2.2 mA cm −2 , followed by CB-C 60 (0.56 mA cm −2 ), CNTs (0.85 mA cm −2 ), and C 60 -CNT mixture (1.2 mA cm −2 ).
The onset potential for CB-C 60 , CNTs, C 60 -CNT mixture, and C 60 -CNT hybrid was −0.32, −0.28, −0.21 and −0.11 (V vs. SCE), respectively. These results suggest that the fabricated C 60 -CNT hybrid cathode had the best ORR activity amongst all the as-prepared nanocarbon catalysts. Besides, the EIS of all the samples was carried out in O 2 -saturated 0.5 mol L −1 Na 2 SO 4 electrolyte solution (Fig. 7a). Evidently, the C 60 -CNT hybrid indicated a lower resistance foe mass-and charge-transfer than those of CB-C 60 , CNTs, C 60 -CNT mixture samples, showing a more promising reactant diffusion and electron transfer for ORR. These observations confirmed that the covalent integration of CB-C 60 molecules into CNTs structure could reduce the resistance of charge transfer because of charge-transfer from CNTs to CB-C 60 , which was effective for improving catalytic activity toward ORR. www.nature.com/scientificreports www.nature.com/scientificreports/ To appraise the number of electrochemically active centers on the surface of the fabricated nanocarbon based electrodes, CV analysis in a solution containing potassium ferrocyanide was performed 25 . The obtained voltammograms (Fig. 7b) demonstrate the high electrochemical performance of the C 60 -CNT hybrid electrode compared to others, where the electrochemical active surface area of C 60 -CNT hybrid was 27.78 ± 0.45 cm 2 (Fig. S6).
Selectivity of hydrogen peroxide generation. The selectivity of the fabricated nanocarbon electrocatalysts toward ORR to H 2 O 2 was evaluated by the RRDE method, using LSV at 1600 rpm rotating speed. As shown in Fig. 8a, among the material, C 60 -CNT hybrid presented the highest overall electocatalytic activity for ORR with disk current density (j disk ) about −5.3 mA cm −2 , and the main hydroperoxyl productivity under ring current density (j ring ) around 0.91 mA cm −2 at −0.4 V (vs. SCE). Furthermore, C 60 -CNT hybrid exhibited the most positive onset potential around −0.12 V (vs. SCE). Figure 8b,c indicate the number of electrons transferred and the H 2 O 2 selectivity trends resulting from the RRDE voltammograms, respectively. In the potential range of −0.3 to −0.8 V (vs. SCE), the mean number of transferred electrons was determined to be close to two (Fig. 8b), suggesting that ORR predominated by two-electron reduction pathway. The H 2 O 2 selectivity within the studied applied potential range followed the order of C 60 -CNT hybrid > C 60 -CNT mixture > CNTs > CB-C 60 .
RDE analyses were further used to prove the ORR kinetics of C 60 -CNT hybrid. As shown in Fig. 8d, higher current intensities were obtained by increasing the rotation rate for the reduction of oxygen due to the decrease in the diffusion layer 23 . The K-L plot (Fig. 8e) with a good linearity at different potentials proved first-order reaction kinetics of the ORR process. In case of the fabricated C 60 -CNT hybrid, the mean number of transferred electrons per O 2 molecule was measured to be ~2.2 (−0.3 to −0.7 V) based on the obtained slope of K-L plots.
For further evaluation of electrocatalytic performance, H 2 O 2 generation rate of the fabricated nanocarbon based electrodes was investigated in O 2 -saturated acidic solution under the obtained optimal conditions (pH = 3 at −0.2 V vs SCE). As showed in Fig. 8f, the H 2 O 2 concentration increased versus reaction times for all as-prepared nanocarbons. Notably, the C 60 -CNT hybrid electrocatalysts produced 426.58 mmol L −1 of H 2 O 2 within 3 h and displayed a remarkably high H 2 O 2 electrogeneration rate of 4834.57 mg L −1 h −1 . This amount was greater than those of C 60 -CNT mixture (307.79 mmol L −1 , 3488.28 mg L −1 h −1 ), CNTs (244.60 mmol L −1 , 2772.13 mg L −1 h −1 ) and CB-C 60 (97.72 mmol L −1 , 1107.49 mg L −1 h −1 ).
It is noticeable that the faradaic efficiency for H 2 O 2 production on C 60 -CNT hybrid electrode could reach 82.6% in the investigated applied potential, which is greater than those of previous studies of electrocatalysts under the same experimental conditions (Table S1). The observations show that the C 60 -CNT hybrid was a promising cathode material for H 2 O 2 electrogeneration.
The higher ORR activity of the C 60 -CNT hybrid can be ascribed to the following reasons: (a) The carbon nanostructured morphology could provide higher active centers on the surface of electrode; (b) CB-C 60 structures, as the electron acceptor, facilitate the electron transportation inside the hybrid and (c) the covalent attachment of CB-C 60 into CNTs framework facilitates the oxygen adsorption and OOH desorption pathways 18 . The schematic of H 2 O 2 production by C 60 -CNT hybrid electrode is represented in Fig. 9.
The stability of as-prepared electrodes is one of the most noteworthy parameters to evaluate their performance toward two-electron ORR. The stability of C 60 -CNT hybrid electrode was studied by sequential scanning of potential for 100 cycles (Fig. 10a), the chronoamperometric method for 10 h (Fig. 10b), and H 2 O 2 generation rate over 15 repeated runs (Fig. 10c). As can be observed, C 60 -CNT hybrid even shows well two-electron ORR efficiency in the 100 th cycle compared to the 1 st cycle. Moreover, the peak current in the chronoamperometric curve, after a prolonged operation, about 98% of the initial current is retained and the rate of H 2 O 2 generation after 15 runs (382.85 mmol L −1 ) was close to 426.58 mmol L −1 . These obtained results proved high stability of the C 60 -CNT hybrid electrode for two-electron ORR.

conclusion
In summary, we demonstrated a method to synthesize the covalent C 60 -CNT hybrid as a novel electrocatalyst for H 2 O 2 production. The C 60 -CNT hybrid exhibited high content of large surface area, intermolecular electron transitions, fast mass transport, and defect sp 3 -C bonds. It was demonstrated here the high performance of C 60 -CNT hybrid, as a cathode electrode, for electrogeneration of H 2 O 2 (112.6-792.6 mmol h −1 g −1 ). In addition, the C 60 -CNT hybrid showed high stability and reusability in ORR. This study may provide a new insight into the design of metal-free and efficient nanocarbon-based electrocatalysts for production of H 2 O 2 .