Nitrogen-Codoped Carbon Nano-Onion Electrocatalysts for the

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Boron/Nitrogen Co-Doped Carbon Nano-Onion Electrocatalysts for the Oxygen Reduction Reaction Adalberto Camisasca, Adriano Sacco, Rosaria Brescia, and Silvia Giordani ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b01430 • Publication Date (Web): 03 Oct 2018 Downloaded from http://pubs.acs.org on October 5, 2018

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ACS Applied Nano Materials

Boron/Nitrogen Co-Doped Carbon Nano-Onion Electrocatalysts for the Oxygen Reduction Reaction Adalberto Camisasca,1,2 Adriano Sacco,*1 Rosaria Brescia,3 and Silvia Giordani*1,4 1

Nano Carbon Materials and Center for Sustainable Future Technologies, Istituto Italiano di Tecnologia (IIT), via Livorno 60, 10144 Torino, Italy

2

Department of Chemistry and Industrial Chemistry, University of Genova, via Dodecaneso 31, 16145 Genova, Italy 3

Electron Microscopy Facility, Istituto Italiano di Tecnologia (IIT), Via Morego 30, 16163 Genova, Italy 4

Department of Chemistry, University of Torino, via Giuria 7, 10125 Torino, Italy

KEYWORDS: carbon nano-onion, oxygen reduction reaction, boron and nitrogen doping, electrocatalysis, fuel cell.

ABSTRACT: Herein, we present the production of dual-atom (B and N) co-doped carbon nanoonions (BN-CNOs) by standard annealing method. The proposed co-doping approach is efficient to introduce both heteroatoms in the graphitic skeleton, as revealed by several characterization

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techniques, and suitable for a low-cost mass production of carbon-based catalysts. The activity of the CNO-based electrocatalysts towards oxygen reduction reaction (ORR) has been investigated in alkaline media, showing comparable catalytic performance, higher long-term stability (retaining 98.7% of the initial current over three hours of testing) and excellent immunity towards methanol crossover compared to the standard Pt/C catalysts. Our findings confirm that boron/nitrogen co-doped CNOs are promising candidates to efficiently catalyze the oxygen reduction in energy devices.

INTRODUCTION The increasing energy demand in our society, together with current environmental concerns, requires new, clean and renewable energy technologies with high efficiency and costeffectiveness.1 Nowadays, fuel cells2 and metal-air batteries3 are the most promising strategies proposed to solve the energy-related problems. The main technological issue, which limits their development, is related to the kinetically slow ORR at the cathode side,4 making the use of a catalyst essential. Carbon-supported Platinum (Pt), through a four-electron reaction pathway, is the most efficient ORR catalyst due to its remarkable current density and low over-potential. However, it suffers from several drawbacks such as low long-term durability, high cost and scarcity of natural reserves, thus limiting the performance and precluding a possible commercialization of fuel cells.5 Therefore, it is extremely important to find an effective alternative to Pt. Many efforts were made to develop novel ORR catalysts, including nonprecious metals and metal oxides6 and metal-free materials.7 Among the latter, carbon nanomaterials (CNMs) have been proposed as efficient cathode materials, due to large specific surface area (thus providing additional catalytic sites), cost effectiveness and outstanding electrical and mechanical properties.8 One efficient strategy to increase the catalytic properties of


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CNMs is to introduce heteroatoms such as nitrogen into the graphitic lattice. Compared to pristine CNMs, nitrogen (N)-doped carbon nanostructures, in particular carbon nanotubes9 and graphene,10 showed higher performance as ORR electrocatalyst to reduce oxygen to water (in acidic media) or to hydroxide (in alkaline media) via a four-electron pathway.11 The enhanced catalytic activity has been related by Kong et al.9 to the positive charging of carbon atoms adjacent to N; the charge delocalization, driven by the presence in the lattice of N atoms, would create new catalytic sites, allowing for the weakening of the O-O bonding. The doping with nonmetal heteroatoms (i.e. nitrogen, boron, sulphur and phosphorus) can efficiently improve the performance of the CNM-based catalysts due to the formation of active sites in the graphitic skeleton together with the synergistic coupling effects between heteroatoms,12-14 making dopedCNMs an interesting and competitive alternative to Pt-based materials as ORR electrocatalysts. Among the several members of the carbon family, carbon nano-onions (CNOs) showed very attractive features in different research fields, in particular in biomedical15 and electrochemical applications.16 CNOs are nearly spherical carbon nanoparticles composed of multiple nested fullerene-like shells17 with size and physico-chemical properties related to the method used for the fabrication. Up to date, the only method which ensures a gram-scale synthesis of small CNOs (typically below 10 nm in size) is the thermal annealing of detonation nanodiamonds (DNDs), performed in vacuum18 or inert gases.19-20High yield-product and low cost are the key factors of this method in the perspective of the industrial scalability. Recent studies reporting the in-situ or post-process doping of CNOs with heteroatoms showed promising performances as ORR catalysts for fuel cells,21-23 while no reports on heteroatom co-doping of CNOs for ORR have been published so far.


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In this work, we present the co-doping of CNOs with boron and nitrogen by one-step thermal annealing process, which ensures large scale production and low cost. Since DNDs precursor contains in its structure residual nitrogen impurities as a consequence of the synthesis process,24 we directly exploit this feature as N source and boric acid (H3BO3) as B source. Our results confirm the formation of doped-CNOs, as consequence of the high affinity between N and B; for comparison, no nitrogen was detected in the undoped CNOs, suggesting that boron plays a key role in the co-doping mechanism. The ORR activity of the as-formed co-doped carbon nanoonions (BN-CNOs) have been tested in alkaline medium and compared with the standard Pt/C catalyst. Our results show remarkable ORR electrocatalytic activity, high selectivity for the fourelectron reduction pathway, excellent durability and immunity towards methanol crossover, thus making these novel electrocatalysts an effective alternative to the commercial Pt-based catalysts. RESULTS AND DISCUSSION Thermal annealing of DNDs was employed to synthetize pristine CNOs (p-CNOs), while the additional presence of boric acid was exploited for boron and nitrogen co-doped CNOs (BNCNOs), as schematically shown in Figure 1A.


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Figure 1. Schematic representation of thermal annealing process (A) and functional groups incorporated on the CNO surface (B); N1,2,3,4 and 5 correspond to different N-types (i.e. pyridinic, pyrrolic, graphitic and oxide, respectively) and B-bonded N atoms; B1,2 and 3 correspond to B-bonded C and N atoms and B oxide, respectively; C1,2 and 3 correspond to hydroxyl, carbonyl and carboxylic functional groups. The successful formation of the two nanomaterials was corroborated by different characterization techniques. We investigated the morphology and the chemical composition of the co-doped CNOs by high-resolution transmission electron microscopy (HRTEM), electron energy loss spectroscopy (EELS) and energy-filtered transmission electron microscopy (EFTEM) elemental mapping. Representative HRTEM images of BN-CNOs reveal the presence of polyhedral-shaped CNOs (Figure 2A and Figure S1A), whose formation is typically achieved by annealing nanodiamonds at temperature above 1900°C.25 This is attributed to the presence of


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B, which produces an enhancement of the graphitization during the annealing,26 allowing for the formation of polyhedral CNOs at lower temperature, namely 1650°C, where quasi-spherical onions are the typical products.20 In addition, a small fraction of the sample consists of CNOs with nearly spherical shape (Figure S1B), which may be tentatively attributed to the formation of temperature gradients during the thermal annealing process. EEL spectrum of BN-CNOs in the energy-loss region between 150 eV and 480 eV shows the three characteristic K-ionization edges of B (onset at 188 eV), C (onset at 284 eV) and N (onset at 401 eV), respectively (Figure 2B). The fine structure of the C K edge reveals features in perfect agreement with what previously reported for CNOs;27 evidence of the efficient incorporation of the dopants in the carbon structure is provided by the inspection of the B and N K edges structure, which also reveals their sp2-hybridization state.28 In addition, the EFTEM elemental mapping in Figure 2C-F shows that boron (in green) and nitrogen (in cyan) are uniformly distributed in the graphitic carbon network, confirming the homogeneous nature of BN-CNOs. These results are consistent with what reported for other BN co-doped carbon nanomaterials,14, 29 confirming the successful heteroatom co-doping approach.


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Figure 2. A) HRTEM image of BN-CNOs, showing polyhedral shape (highlighted with black dotted lines); B) EELS spectrum of BN-CNOs, showing distinct B, C and N K edges; C-G) Zeroloss BF-TEM image and corresponding EFTEM elemental mapping of BN-CNOs, showing C, B and N maps in red, green and cyan, respectively. Nitrogen gas sorption measurements were conducted to evaluate the textural features of the synthetized CNOs, which may have a function in the electrochemical performance of a catalyst. The N2 adsorption/desorption isotherms, the BET area plots and the pore size distribution are depicted in Figure S2. Both samples display a typical IV-type isotherm with a distinctive hysteresis loop (Figure S2A) characteristic of mesoporous materials. In addition, the residual volume adsorbed at low relative pressures may suggest the presence of micropores in all


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samples. BET specific surface areas (SSA), pore volumes and sizes of all investigated materials are shown in Table S1. Compared to the undoped sample, a notable reduction of SSA is observed for the co-doped catalysts (Figure S2B and Table S1). The porous properties of both samples were evaluated by using the Barrett, Joyner and Halenda (BJH) method (insets in Figure S2A). Both CNMs display a mesoporous structure with a wide pore-size distribution between 2 and 30 nm (with larger pore sizes for BN-CNOs) and a small volume of micropores (below 2 nm). In line with the SSA results, the doped CNMs exhibit lower pore volumes, suggesting that the dual-atom doping affects to some extent the mesoporous structure of CNOs. The surface compositions of CNO-based materials were examined via X-ray photoelectron spectroscopy (XPS). Figure S3B shows the XPS survey spectra of the two samples. Compared to p-CNOs, which display two peaks characteristic of carbon (C) and oxygen (O), BN-CNOs reveal, additionally, the existence of boron (B) and nitrogen (N) with atomic percentages of 8% and 7.4%, respectively (Table S2). The appearance of the nitrogen peak in the co-doped catalyst, which is not present in p-CNOs, originates from the precursor DNDs (as shown in the XPS survey spectrum in Figure S3A), where is typically found as impurity as a consequence of the explosive compositions in the detonation process;24 its higher content in BN-CNOs, compared to that of DNDs, suggests that N is included in the carbon lattice in association with boron, as reported for co-doped carbon structures.30-31 Boron has crucial importance in the co-doping process, stabilizing nitrogen in the hexagonal carbon lattice due to the great affinity between B and N. In comparison, in the absence of a direct source of boron, nitrogen has not been introduced in the carbon network during the formation of pristine CNOs.


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In order to evaluate the chemical state of nitrogen and boron as result of the doping process, high-resolution spectra of the C, N and B1s core-levels were acquired, and the different contributes were assigned after peak deconvolution, as depicted in Table S3, S4 and S5. The C1s peak of p-CNOs, shown in Figure 3A, can be deconvoluted into six individual components; the most intense peak is assigned to graphitic carbon (C=C), while the peak at around 285.2 eV is due to carbon atoms with sp3 hybridization. The higher binding energy peaks are attributed to residual hydroxyl (C-O), carbonyl (C=O) and carboxyl (COOH) species onto the surface of CNOs, and the π-π* transition peak.32 After the co-doping process, the C1s peak of BN-CNOs (Figure 3B) shows the presence of additional sp2 and sp3 hybridized C-N bonding states,33 while the new peak appearing at 283.5 eV is assigned to boron atoms in the carbon lattice (B-C bonds), which is further confirmed by the slight broadening of the C1s observed for BN-CNOs, suggesting that the co-doping process increases the structural disorder as a consequence of the different bond lengths formed in the structure.22 The high-resolution N1s spectrum of BN-CNOs, presented in Figure 3C, displays five different deconvoluted contributes of nitrogen species on the surface of BN-CNOs. The peaks at 398.4, 400.1, 401.7 and 403 eV can be ascribed to pyridinic, pyrrolic, graphitic and oxidized type of N atoms in the graphitic structure.33-34 Pyridinic and pyrrolic N species correspond to nitrogen in the hexagonal and pentagonal ring, respectively, while graphitic N to nitrogen replacing carbon atoms in hexagonal ring.34 In addition, the peak at 397.9 eV can be assigned to boronbonded nitrogen (N-B) in the form of B-N-C bonding state.29,

31, 35

From the total amount of

nitrogen found in BN-CNOs (7.4 at.%), pyridinic N-sites, which were reported to display the highest ORR activity among various types of N species,36-37 constitute the most prominent portion in the carbon lattice (i.e. around 55%). On the other side, their beneficial effect may be


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affected somehow by the notable presence of N-B species, which typically show poor ORR activity.

38

Finally, regarding the other N species such as pyrrolic and graphitic, it is unlikely

their direct involvement in the catalytic behavior of the sample because of their negligible content. The B1s core level XPS spectrum of BN-CNOs (Figure 3D) occurs at higher binding energy (around 190 eV) as in pure boron (188 eV), suggesting that boron atoms are included in the carbon network.39 The B1s peak shows three deconvoluted sub-peaks located at around 189.8 eV, 190.5 eV and 191.8 eV, respectively. The first peak can be assigned to boron atoms at trigonal sites included by substitution in the graphitic structure (B-C bonding as BC3), while the third peak can be attributed to B-O bonding (BC2O, BCO2 and B2O3).22,

28

Boron atoms

substituted at the trigonal sites in the carbon lattice, acting as electron acceptor (having three valence electrons), have been reported to be the most ORR active boron group in doped carbon materials.14,

40

The second, intermediate peak can be related to B–N bonding state,29,

35, 41

in

accordance with the XPS spectrum of N1s and represent the preferential bonds type. From XPS analyses, it can be inferred that both hetero-atoms were efficiently incorporated in the hexagonal graphitic lattice of CNOs, as schematically shown in Figure 1B. The presence of isolated N and B active sites in the form of pyridinic nitrogen and substitutional B species are expected to confer catalytic activity to the BN-CNOs, while other species such as pyrrolic or graphitic N, present in a very small amount, are supposed to have a negligible effect on the global performance. Regarding the dominant presence of B-N species, they have been reported to produce a negative effect for the chemisorption of O2 molecules;38 therefore, their presence may have a role in limiting the activity of the co-doped catalyst, affecting to some extent its ORR performance.


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Figure 3. High resolution XPS spectra of p- and BN-CNOs, including peaks deconvolution: A) C1s spectrum of p-CNOs; B) C1s spectrum of BN-CNOs; C) N1s spectrum of BN-CNOs and D) B1s spectrum of BN-CNOs. In the spectra, experimental and fitting curves are showed in black and red, respectively. In order to study the effects of the co-doping on the crystalline structure, p- and BN-CNOs were analyzed by X-ray diffraction (XRD) and Raman spectroscopy. XRD pattern of precursor DNDs (Figure S4) shows two typical diamond diffraction peaks, corresponding to the (111) and (220) diamond faces,42 and the inter-planar distance 2 43.7°, 2.07Å is consistent with the reported value of bulk diamond (i.e., 2.05 Å). XRD patterns of p- and BN-CNOs show significant differences from that of DNDs (Figure 4A), confirming the conversion of


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nanodiamonds, composed of sp3-hybridized carbon atoms, into a sp2 graphitic network. Both samples exhibit typical graphitic peaks, corresponding to the (002), (100+101), (004) and (110) planes;43 compared to the pristine one, the sharpening of the (101) peak is observed for the doped sample, as reported for boron doping of carbon nanostructures.30 Furthermore, no peaks related to the presence of boric acid or B2O3 species are observed in XRD spectrum of BN-CNOs, suggesting that boron precursors have been entirely consumed during the formation of the doped structure. The very low intensity peaks in the XRD spectra of CNOs (Figure 4A) can be ascribed to residual metal impurities formed in the detonation process (Figure S4).24 Metal traces (in a very low concentration, as suggested by the negligible residue after TGA discussed below) are supposed to be embedded into the graphitic layers and thus not exposed at the surface,44 as suggested by the absence of the corresponding signals from XPS analyses. In addition, a slight contraction of the inter-planar distance ( from 3.486 to 3.446 Å) is observed due to the introduction of the heteroatoms in the lattice, which prevents the diffusion of carbon atoms during the graphitization, leading to a more compact graphitic structure.45 Figure 4B displays the Raman spectra of the two catalysts, showing two prominent peaks: the D-band at 1328 cm-1, associated with structural defects in the graphitic domains, and the G-band at 1590 cm-1, related to the E2g vibration mode of sp2 carbon atoms.46-47 A very intense D-band is observed for the pristine material, confirming the defective nature of CNOs synthetized by thermal annealing, as reported in literature.20 The co-doping process causes a slight blue-shift of the G- and D-band by 8 cm-1 (Table S6), while the 2D-band becomes very weak, compared with the sharp 2D peak of the pristine CNOs, suggesting that the incorporation of hetero-atoms into the CNO backbone added disorder in the graphitic domains.22 Moreover, the increase in the ID/IG ratio, which is typically used to evaluate the disorder in graphitic materials,48 provides further


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evidence of the intercalation of B and N in the structure; as shown in Table S6, the intensity ratio increases from p-CNOs (ID/IG = 1.53) to BN-CNOs (ID/IG = 1.72), as a consequence of the different length of the C-C/C-N/C-B bonds.

Figure 4. XRD (A) and Raman spectra (B) of p- and BN-CNOs Thermogravimetric analysis (TGA) has been employed to investigate the thermal stability of pristine and doped CNOs; the weight loss curves and their derivatives, reported in Figure S5A, show a similar trend. Both samples are thermally stable without significant degradation up to 600 °C, while the high weight loss in the temperature range between 600-800 °C is assigned to the


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decomposition of the carbon skeleton. In particular, no weight loss is observed between 100200°C and between 400-500°C in the TGA plot of BN-CNOs, indicating that no unreacted boric acid or B2O3 species are present (the melting points are 170° and 450°C, respectively), in accordance with the XRD results discussed above. The derivative curve of BN-CNOs (blue dash dotted line in Figure S5A) shows a decomposition temperature of 726 °C, which is about 30 °C less than that of p-CNOs (Table S6), due to the introduction of defects as a consequence of the co-doping process. In addition, at 900 °C, BN-CNOs exhibit a mass residue of approx. 17%, which can be ascribed to the formation of boron oxide at high temperature,49 while only 0.05% is observed for the undoped samples, tentatively ascribed to metal traces, as previously discussed. Fourier-transform infrared (FT-IR) spectroscopy was carried out to study the effect of the doping on the chemical structure. FTIR spectra of p- and BN-CNOs are shown in Figure S5B; while pristine CNOs do not show distinctive IR features,27 new vibrational modes can be observed after the doping treatment. The peaks in the range between 800-1600 cm-1 can be ascribed to the in-plane stretching and out-of-plane bending modes of the atoms composing the BCN rings.50 The broad band at around 1550 cm-1 can be assigned to the stretching vibrations of C=C/C=N bonds, while the peaks at around 1367 and 849 cm-1 correspond to the B-N stretching and bending vibrations, respectively; in addition, the small shoulder at around 1400 cm-1 can be attributed to the asymmetric B–O stretching modes. Finally, the very wide band between 10001300 cm-1 can be attributed to the superposition of the B-C and C-N stretching modes.51 All characterization confirmed the successful synthesis of co-doped BN-CNOs, thus allowing us to proceed and examine the electrocatalytic performance of both pristine and doped CNOs. In particular, their activity toward ORR was evaluated through different techniques.


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The cyclic voltammetry (CV) curves related to the two samples acquired in O2- and N2saturated aqueous solutions are shown in Figure 5. Both materials exhibit a pair of oxidationreduction peaks in the range 0.78 – 0.98 V when in oxygen-containing electrolyte: this is a characteristic feature of the ORR catalytic activity,52 as confirmed by the disappearing of the peaks when nitrogen is fluxed in the solution (a small reduction peak can be still distinguished in the voltammogram of BN-CNOs, due to a residual presence of dissolved oxygen53). In addition, from the analysis of Figure 5, it can be observed a quite larger reduction current density for the co-doped CNOs, with respect to pristine sample. This peculiarity has to be related to the higher catalytic performance of BN-CNOs. In fact, similarly to other carbon-based catalysts,31, 54 the codoping of CNOs is effective in increasing the electrochemical activity toward the ORR as result of the synergic effects of dopants and the high amount of active sites (i.e. pyridinic N and substitutional B) in the carbon structure.

Figure 5. CV curves of p- and BN-CNOs in O2- and N2-saturated aqueous solutions Once obtained the evidence of the catalytic activity of the proposed materials, ORR measurements have been carried out. In Figure 6A, the Rotating Disk Electrode (RDE) curves


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acquired for co-doped sample at different rotation speeds are shown. Similar curves were obtained for pristine CNOs and reference Pt/C catalysts (data not shown). As expected, an increase of the cathodic current can be observed for potential lower than the onset (about +0.78 V), followed by a quasi-plateau region, when diffusion-limiting occurs.55 In addition, the diffusion limiting current density J increases (in absolute value) while raising the rotation speed ω, since the diffusion distance reduces for larger speed values.55 From these curves, onset and half-wave potentials were extracted, and these values are displayed in the inset of Figure 6A. In agreement with the results of CV analysis, co-doped BN-CNOs exhibit slightly lower values, implying enhanced electrochemical activity with respect to pristine CNOs. However, it has to be highlighted that BN-CNOs sample is characterized by quite larger values with respect to reference Pt/C catalyst. This feature may be explained considering the large presence of bonded B-N species in the carbon matrix of CNOs. As anticipated above, it was reported that they could reduce to some extent the catalytic performance.38 ORR polarization curves can be further exploited to build the Koutecky-Levich plots, which give information on the electron transfer number. These plots were obtained for all the analyzed catalysts, and are reported in Figure 6B. Both CNOs samples exhibit a linear dependence of the inverse of diffusion-limiting current on the square root of 1/ω, implying a first-order reaction kinetics with respect to the dissolved oxygen concentration.56 The experimental points were fitted through equation (S1) in order to calculate the number of electrons n associated to each catalyst. As reported in Figure 6B, pCNOs exhibit n value equal to 3.51, implying a predominant four-electrons catalytic pathway.57 When subjected to the co-doping process, CNOs increase their activity, as witnessed by the larger electron transfer number, namely 3.94. Such high value is among the largest ever reported for doped carbon-based materials9, 14, 22, 31, 33, 41, 54 and also considering other cost-effective ORR


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catalysts,58-59 being quite close to that of reference Pt/C and theoretical one (i.e. 4), as reported in Figure 6B. These results once again prove that the co-doping of CNOs is an effective route to improve their electrochemical activity. To further confirm the catalytic pathways of the tested materials and their dependence on the applied potential, 4-electrodes Rotating Ring Disk Electrode (RRDE) analyses were carried out. In this technique, the disk potential is scanned at fixed rotation rate, while maintaining the ring potential at fixed large value. The current of both electrodes is acquired: the disk current ID is associated to the four-electron current of the analyzed catalyst, while the ring current IR to the two-electron (intermediate) peroxide species.60 From the analysis of Figure 6C and Figure 6D, it can be noted that both CNOs samples display ring current densities one or two order of magnitude less than the corresponding disk current densities, thus confirming that the reduction reaction mainly proceeds through a four-electrons pathway, in accordance with the results of the RDE measurements discussed above. In addition, co-doped sample exhibits IR values lower than 10 µA/cm2, implying a very low production of peroxide species.52 By exploiting equations (S2) and (S3), the electron transfer number (n) and the peroxide percentages ( %) were acquired, and their dependence on the potential is shown in Figure 6E. Pristine CNOs are characterized by n values in the range 3.46 – 3.70 and % values lower than 27%, with a small variation with the applied potential; this result is in good agreement to what found by Koutecky-Levich analysis reported in Figure 6B. Moreover, the effect of the co-doping on the ORR activity of CNOs is clearly visible in Figure 6E: BN-CNOs, in fact, exhibit a quasiconstant (3.95 ± 0.02) value, similarly to that of reference Pt/C catalyst (3.94 ± 0.01) in the whole potential range. This impressive result is also confirmed by the low amount (< 4%) of peroxide ions, which guarantees a quasi-ideal ORR pathway. If compared to bare N-doping,33 the


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co-doping of CNOs with boron and nitrogen allows obtaining higher performance, thus once again proving that this strategy is effective to improve the catalytic activity of carbon-based materials.

Figure 6. A) RDE polarization curves of BN-CNOs at different rotation speeds, whilethe inset reports the calculated onset and half-wave potentials of p-CNOs, BN-CNOs and reference Pt/C catalysts at 1600 RPM; B) Koutecky−Levich plots of p-CNOs, BN-CNOs and reference Pt/C catalysts at +0.38 V (the numbers reported close to each curve represent the electron transfer numbers). Ring current density (C) and Disk current density (D) obtained from RRDE analyses of CNOs samples and reference Pt/C catalyst at 2500 rpm and a potential of the ring electrode of


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1.18 V. (E) Comparison of the electron transfer number (left axis) and the peroxide percentage (right axis) estimated from RRDE analyses at 2500 rpm and different potentials for all the catalysts under investigation. In order to cast light on the observed difference of activity of the two CNOs samples, Electrochemical Impedance Spectroscopy (EIS) analysis was carried out. The corresponding Nyquist plots are reported in Figure 7.A. A high-frequency arc, that can be related to the charge transport properties inside the material, characterizes both curves along with a larger low frequency arc associated to the charge transfer at the catalyst/electrolyte interface.52 In addition, it can be easily observed that the codoped sample shows lower impedance values with respect to the pristine one, in accordance with the results reported and discussed above. The experimental EIS data were fitted through an equivalent circuit, comprised of a series resistance (accounting for the electrolyte conductivity) and two resistance/constant phase element (CPE,61) parallels, accounting for the high frequency and low frequency processes introduced above; the calculated curves are shown in Figure 7.A superimposed to experimental points. The obtained charge transport resistances were found to be 97.6 Ω and 51.4 Ω for p- and BN-CNOs: this outcome implies that the introduction of the heteroatoms in the carbon lattice can increase the conductivity of the CNOs, as expected as a consequence of a doping process.38 Moreover, also charge transfer resistance values, which are directly related to the ORR activity, resulted to be lower for the co-doped sample: 4888 Ω vs. 9663 Ω. Overall, these results explain the improved performance of CNOs after co-doping: enhanced capability of reducing molecular oxygen accompanied to faster transport inside the catalyst.


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Finally, the durability of the novel B-N-doped CNOs was assessed through chronoamperometry (CA) measurements. From Figure 7.B, it can be appreciated that, after more than three hours of continuous test, BN-CNOs are capable to retain 98.7% of the initial current. Such remarkable outcome can be attributed to the intrinsic low production of peroxide species during the reaction, according to what reported by Jaouen et al..62 By considering the results obtained with single N-21, 33 and B-doping22 of CNOs, it can be inferred that the co-doping with both heteroatoms is responsible for the longer durability of our catalyst. This peculiarity, coupled with the high activity reported above, makes BN-CNOs a good candidate for replace the expensive Pt/C as catalyst material for ORR. In fact, one of the known issue of the latter is its limited durability, caused by the detachment of Pt nanoparticles from the carbon matrix, or on their aggregation.63 These phenomena lead to a reduction of ORR activity for this catalyst, as evidenced by the results reported in Figure 7.B: a current decrease equal to 29% was indeed obtained for this sample. In addition, the resistance of BN-CNOs to methanol crossover was assessed by CA. As depicted from the inset of Figure 7.B, upon addition of 3 M CH3OH, the current response of the novel catalyst remained unchanged, thus confirming its tolerance to methanol crossover.12


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Figure 7. A) Nyquist plots of CNO mateials measured at +0.68 V and 2500 rpm. Points correspond tothe experimental data, while the continuous lines to the calculated curves. B) Chronoamperometric curves of BN-CNOs and Pt/C catalysts measured at +0.68 V and 2500 rpm in 0.1 M KOH solution; both curves have been normalized to the initial current value. The inset shows the CA curves of BN-CNOs acquired with and without the addition of 3 M CH3OH (the arrow indicates the moment of methanol addition). Normally, a high surface area should provide more and better dispersed reaction sites, thus enhancing the catalytic performance; in our case, BN-CNOs exhibit the highest ORR activity, suggesting that the electrocatalytic behavior is not influenced by the surface area. The as-shown


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improved electrocatalytic activity of BN-CNOs compared to the pristine sample can be assigned to the introduction of ORR active sites in the structure, which favor the chemisorption of oxygen molecules. Dopant atoms have a crucial role in the reduction process. Substitutional boron atom in the three-coordinate form (BC3) and pyridinic nitrogen atoms, as a consequence of the difference of electronegativity, induce positive charge in the hexagonal carbon lattice, thus strengthening the interaction with O2 molecules. In the case of boron atoms, boron itself acts as a positively charged active site; it accumulates π electrons and transfer them to the O2 molecules, acting as a bridge.12 With respect to nitrogen, the electrochemical reduction of O2 is driven by the N-adjacent carbon atoms, which are positively charged due to the N strong electron affinity.9 In both cases, the O-O bonds are weakened, thus facilitating the ORR process in the BN-CNOs. Nevertheless, further studies are required to elucidate the exact mechanism behind the improved ORR catalytic behavior of the co-doped materials. On the other hand, as depicted from XPS results, the bonded B and N species are dominant in the samples, thus influencing heavily the performance of the catalyst. Therefore, a systematic investigation on how to regulate and optimize the dopant’s configuration in the CNO catalysts (i.e. to reduce the connected B-N species) is essential. Our results show additionally that undoped CNOs, compared to the typical inactivity of pure carbon nanostructures, exhibited relatively good ORR catalytic performance. A possible explanation for this behavior may be the presence of metal impurities and/or the high defectiveness of the CNO surface. Regarding the possible presence of metal impurities, XRD patterns of CNO materials display few low intensity peaks which can be due to residual metallic traces, while TGA residual mass has been shown to be less than 0.05%, suggesting an extremely low content. Furthermore, XPS analyses do not show any metal-related peaks. Therefore, we


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suppose that metal impurities may be embedded in the carbon layers, thus not acting as active sites. They may indirectly influence the catalytic behavior of the material, as reported by Choi et al., transferring electrons to the carbon atoms, thus enhancing the catalytic activity.64 Conversely, we suggest that the high degree of defects in the pristine catalyst (as supported by Raman analyses) may represent a factor for the quite good activity of p-CNOs. The ORR catalytic performance of doped CNMs is related to their ability to disrupt the integrity of π conjugation in the graphitic network; therefore, the presence of defects in sp2 carbon structures may act likewise, as reported in literature.65-66 In support of this assertion, Jiang et al. proved both experimentally and theoretically that the high defectiveness of undoped carbon nanocages significantly contributed to the ORR activity of their catalyst, which showed similar electron transfer number to our pristine sample (2.9 vs 3 for p-CNOs).65 We suggest that the large surface area and high defectiveness of p-CNOs may offer high numbers of active sites, although with much lower activity, enhancing its activity toward ORR. These results demonstrate that carbon defects have not be overlooked when considering the ORR activity of CNMs and further investigations are needed to fully evaluate the influence of structural defects towards ORR. CONCLUSIONS In summary, we reported a low-cost approach to produce boron and nitrogen co-doped CNOs through one-step thermal annealing of a mixture of detonation nanodiamonds and boric acid directly exploiting the nitrogen impurities included in the precursor. The as-formed co-doped materials show very promising catalytic performance compared to the Pt/C catalyst. The remarkable four-electron ORR activity and durability of BN-CNOs is a result of the synergetic effect of the co-dopants among the carbon framework promoted by the presence of pyridinic


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nitrogen and substitutional boron atoms as active sites. The proposed doping approach is a suitable strategy to break the electro neutrality of sp2 carbon network due to the different electronegativity of the heteroatoms; this allows the creation of charged active sites, favoring the adsorption and reduction of oxygen molecules during the ORR process. Although the co-doped CNOs have a lower surface area compared to the undoped CNOs, much improved ORR activity was observed for BN-CNOs due to the higher conductivity and a quasi-ideal four-electron reduction process with a very low production of peroxide species. Nevertheless, the optimization of the dopant bonding states in the carbon skeleton is of outmost importance to avoid the preferential formation of the connected B-N species, which limit the catalytic behavior of the catalyst. Studies in this specific direction are essential to fully exploit the real potential of material. In addition, the good ORR catalytic performances showed by undoped CNOs, compared to other pristine carbon nano-materials, may be tentatively ascribed to the high defectiveness of the CNO surface, which act as catalytic sites. In conclusion, our findings indicate that co-doped CNO-based catalyst is a promising material to rival or even replace metal-based catalysts in fuel cells and metal-air batteries. MATERIALS AND METHODS Materials: All the chemicals (from Merck) and DNDs (from Carbodeon) were used as received. Synthetic procedures: The synthesis of CNO materials was accomplished by traditional thermal annealing process by using DNDs as precursor.18, 20 BN-CNOs were produced by high temperature annealing of a mixture of DNDs and H3BO3 (30 wt. %).22 The sample was thermally treated at 1650°C under helium atmosphere in a tube furnace at a heating rate of 3.5 K min-1 and


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further annealed at 450°C in air to remove the presence of amorphous carbon. p-CNOs were synthetized in the same way, without the addition of the boron source. Material characterizations: For TEM investigations, the BN-CNOs sample was suspended in ethanol and mildly sonicated; about 100 µL of the suspension were drop-cast onto Cu grids covered with a holey C film. HRTEM imaging, EFTEM mapping and EEL spectroscopy were performed on an image-corrected JEOL JEM-2200FS TEM (Schottky emitter, operated at 200 kV), equipped an in-column energy filter (Ω-type). EFTEM elemental maps were computed applying the three-windows method at the K edges of B (10 eV energy slit width), C (16 eV energy slit width) and N (20 eV energy slit width). The EEL spectra were collected on areas (250 nm diameter) including BN-CNOs suspended on holes, in order to avoid contributions from the amorphous carbon support film. The spectra were acquired in diffraction mode, with a collection semi-angle of 5.5 mrad, an energy resolution of 1.1 eV and a dispersion of 0.3 eV/pixel. N2 adsorption/desorption measurements were performed at 77K using Quadrasorb evo analyzer from Quantachrome Instruments after degassing each sample at 100°C under vacuum overnight to remove any adsorbed species. Specific surface area (SSA) was calculated by BET method between 0.1-0.3 relative pressure values. The pore volumes and the pore-size distributions have been determined by BJH method to the desorption branch of the isotherms. Data were analyzed by using QuadraWin dedicated software from Quantachrome. X-ray Photoelectron Spectroscopy (XPS) analyses were performed in a PHI 5000 XPS instrument equipped with monochromatic Al Kα source. Multipak 9.6 dedicated software was used for data analysis with energy calibration performed with respect to C1s peak (284.5 eV).


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X-Ray Diffraction (XRD) measurements were performed with a PANalytical Empyrean instrument (Cu Kα radiation source) using a zero-diffraction silicon substrate. HighScore 4.5 dedicated software was used for data analysis. Raman analyses were performed using an 800 UV LabRam instrument (Horiba), exciting the sample with a 632 nm laser. For comparison of the Raman intensities, each spectrum was normalized to the G-band. Thermogravimetric measurements (TGA) were conducted with a TG 209 F1 Libra analyzer (Netzsch), using an Al2O3 pan. After equilibrating the samples at 100 °C for 20 minutes, the measurements were carried out under air flow (20 mL/min) at 10°C/min as heating rate until 900°C. Fourier-transform infrared (FTIR) analyses were conducted under vacuum in an Equinox 70 instrument (Bruker) in the 4000-600 cm-1 range. Electrochemical characterizations: The electrochemical characterizations were conducted employing an electrochemical workstation (760D, CH Instrument) and a rotating ring disk electrode apparatus (RRDE-3A, ALS). Catalyst samples were deposited on the working electrode (glassy carbon disk/Pt ring, BioLogic, with active area 0.13 cm2) following the procedure reported elsewhere.50 Commercial Pt/C (purchased from Sigma-Aldrich) was employed as a reference catalyst. For both CNO- and Pt-based catalysts, the amount of active material was fixed at 0.5 mg/cm2.67 The experiments were carried out at room temperature, employing 0.1 M KOH electrolyte saturated with oxygen (unless otherwise specified), and Pt and Ag/AgCl as counter and reference electrodes, respectively. The potentials reported in the manuscript are referred to Reversible Hydrogen electrode (RHE). Cyclic voltammetry (CV) curves were acquired at 10 mV scan rate, in the potential range 0.18 – 1.18 V in oxygen- and


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nitrogen-saturated electrolytes. For the rotating disk electrode (RDE) characterization, the potential range was 0.18 – 1.18 V, the scan rate was 5 mV/s and the rotation speed ω was changed between 400 and 2500 rpm. Electron transfer number n was calculated exploiting Koutecky-Levich plots and the equation: 55

1 1 0.62 /

/! " /

+

1 $

(S1)

In (S1), J and JK represent the measured and the kinetic current densities, respectively; F is the Faraday constant, and are the oxygen bulk concentration and diffusion coefficient, respectively and ν represent the electrolyte kinematic viscosity. The rotating ring disk electrode (RRDE) characterization was carried out employing the same parameters of RDE one, with exception of rotation speed (fixed at 2500 rpm) and ring electrode potential (fixed at 1.18 V). Ring (IR) and disk (ID) currents were acquired and (through the following equations)68

% 200 ×

4 ×

() /* (+ + () /*

(+ (+ + () /*

(S2)

(S3)

used to calculate the percentage of and the electron transfer number (N in (S2) and (S3) represent the Pt ring current collection efficiency). Electrochemical impedance spectroscopy (EIS) characterization was conducted at 2500 rpm, with a small voltage of 10 mV superimposed to a fixed 0.68 V potential, between 10−2 Hz and 104 Hz. Finally, Chrono-amperometry (CA) measurements were carried out at 0.68 V and 2500 rpm. The methanol crossover effect was analyzed by performing a CA test at same potential and rotation speed in 0.1 M KOH electrolyte upon the addition of 3 M CH3OH. 9


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AUTHOR INFORMATION Corresponding Authors * E-mail: [email protected]. * E-mail: [email protected]. ORCID Adalberto Camisasca: 0000-0002-0185-9656 Adriano Sacco: 0000-0002-9229-2113 Rosaria Brescia: 0000-0003-0607-0627 Silvia Giordani: 0000-0002-9212-5067 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS Istituto Italiano di Tecnologia is greatly acknowledged for funding. We wish to thank Dr. Micaela Castellino for XPS measurements, Dr. Stefania Lettieri for the help with the electrode preparation for the methanol resistance experiment and the IIT Nanophysics department for instrumental support.


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Nitrogen-Codoped Carbon Nano-Onion Electrocatalysts for the

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