A Comparative Study of Plasma-Treated Oxygen-Doped Single

Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11396−11406

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A Comparative Study of Plasma-Treated Oxygen-Doped SingleWalled and Multiwalled Carbon Nanotubes as Electrocatalyst for Efficient Oxygen Reduction Reaction Roopathy Mohan, Arindam Modak, and Alex Schechter* Department of Chemical Sciences, Ariel University, Ariel Research Park 40700, Israel

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ABSTRACT: The oxygen reduction reaction (ORR) is pivotal in renewable energy technologies, such as in fuel cells and metal−air batteries. Precious-metal-free electrochemical ORR is a critical component in designing cost-effective electrochemical energy conversion devices. We show here the surface modification of carbon nanotubes (CNT) through one-step oxygen plasma irradiation, which induces doping and charge redistribution around the doped heteroatom oxygen to promote ORR activity. The generation of defect sites owing to oxygen dopant in CNTs was confirmed by Raman spectra, X-ray photoelectron spectroscopy surface composition, and CHNO elemental analysis. The O-doped CNTs were thoroughly characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, and high-resolution transmission electron microscopy measurements. Our research shows that plasma-treated singlewalled carbon nanotubes (SWCNT) are a more effective ORR catalyst compared to multiwalled carbon nanotubes (MWCNT) due to the inherent structure of SWCNT, that can access more defects and surface functional groups than MWCNT. Importantly, for the first time a comparison of doping-induced catalytic activity in ORR is shown here between SWCNT and MWCNT. KEYWORDS: Oxygen plasma, Surface treatment, Defects, Carbon nanotubes, Oxygen reduction

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INTRODUCTION Fuel cells (FCs) are extensively researched as a sustainable power source for transportation and stationary power grid applications because of their high efficiency and low product gas emissions. The oxygen reduction reaction (ORR) is considered a critical fundamental process in membrane fuel cells technology commercialization.1 However, the sluggish nature of the ORR at the cathode of the electrochemical proton exchange membrane fuel cell (PEMFC) urges the discovery of an efficient catalyst with fast ORR kinetics and low cost. Pt-based noble-metal electrocatalysts are so far the most reactive and state-of-the-art catalyst for the ORR, but these catalysts are too expensive and suffer from a lack of long-term durability.2 These drawbacks are limiting their potential applications in viable fuel cells. To address these challenges, current ORR research efforts are mostly directed toward finding an alternative catalyst of low-cost, noble-metal-free materials with high durability for commercial application in fuel cell.3−6 Among the nonprecious-metal-based catalysts, high surface area carbonaceous materials (doped with N, P, B, S) show great promise for the ORR in alkaline and acid electrolytes.7−9 Among the most active materials, heteroatomdoped carbon nanotubes (CNTs) show good stability and reactivity in the ORR.10 The dopant-atom-induced defect sites on CNTs hold unique physicochemical properties suitable for © 2019 American Chemical Society

high ORR electrochemical activity. Although N-doped CNTs were previously explored as a promising catalyst for electrochemical oxygen reduction,11 they are mainly prepared by pretreatment steps using catalytic promoters12 that incorporate metal impurities into the doped CNTs.13 In this work, we demonstrate plasma treatment as a better approach for surface modification than chemical modification, owing to its shorter reaction time, non-polluting processing, and wide range of different functional groups depending on the plasma parameters.14 Several studies reported that the plasmamodified CNTs possess high defect concentration along the wall, which promotes its surface energy and hydrophilicity.15,16 Although there is some uncertainty in the identification of the active sites in the metal-free catalyst, on the basis of experimental and theoretical calculations, non-heteroatomdoped defects may also contribute significantly in ORR research.17 Apart from defects, plasma treatment can also induce several oxygenated functional sites, depending on plasma parameters, which could be responsible for its activity.18 In this regard, Matsubara and Waki demonstrated that the onset potential of oxygen reduction on oxidized Received: February 26, 2019 Revised: May 3, 2019 Published: May 29, 2019 11396

DOI: 10.1021/acssuschemeng.9b01125 ACS Sustainable Chem. Eng. 2019, 7, 11396−11406

Research Article

ACS Sustainable Chemistry & Engineering

Electrochemical Characterization. Electrochemical experiments and the ORR performances of the as prepared catalysts were conducted at room temperature on a CHI 700C bipotentiostat (CH Instrument) using a standard three-electrode system. A rotating ringdisk electrode (RRDE) with a 4 mm diameter glassy carbon (GC) disk (ALS-Japan) serves as the working electrode. Platinum wire and Ag/AgCl electrode were used as counter and reference electrode, respectively. The catalyst-modified GC working electrode was fabricated by applying 25 μL of catalyst ink followed by drying at room temperature. Typically, 2 mg of as prepared catalysts was dispersed in 70% 2-propanol and sonicated for 15 min. To the suspension, 30% of water and 50 μL of Nafion ionomer were added, and the mixture was sonicated for 30 min. The prepared ink was then coated on the glassy carbon electrode disk surface (0.1256 cm2) and allowed to dry. After completely drying, the loading amount of the catalyst was 50 μg cm−2. RRDE measurements were performed with the potential range from 0.20 to −0.90 V vs Ag/AgCl. Cyclic voltammetry and linear sweep voltammetry studies were performed in 0.10 M KOH. The oxidation current of H2O2 emitted from the disk electrode was measured by applying 0.10 V vs Ag/AgCl to the Pt-ring electrode of the ring-disk electrode. Throughout the experiments, highly pure O2 (99.99%) and N2 (99.999%) gases were purged through the electrochemical cell. The yield percentage of hydrogen peroxide and the electron transfer number (n) (kinetic parameter)21 involved in the ORR were calculated from the following equations (eqs 1 and 2) using the RRDE results, where ID refers to the disk current, IR refers to the ring current, and N refers to the collection coefficient.

multiwalled carbon nanotubes (MWCNTs) shifts by 60 mV to a more positive potential compared with the untreated MWCNTs.19 In this research, we modify the graphitic structure of CNTs with oxygen and/or defects by cold oxygen plasma treatment and describe their impact on ORR activity in alkaline electrolytes. The effect of plasma exposure time is explored to control the formation of oxygen-containing functional groups on the surface of single-walled carbon nanotubes (SWCNTs) and MWCNTs. The defect sites along the sidewalls of SWCNTs and MWCNTs were intentionally introduced using oxygen plasma and acted as anchoring sites for adsorbing more oxygen molecules.20 Therefore, oxygenplasma-treated nanotubes as an ORR catalyst opened another possibility to diagnose the surface confinement behavior of oxygen functional groups on the surface of single and multiwalled CNTs along with the ORR activity.

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EXPERIMENTAL SECTION

Materials and Methods. Single-walled carbon nanotubes were provided by OCSiAl Europe S.à.r.l. and multiwalled carbon nanotubes (>98%) were purchased from Sigma-Aldrich USA. Both the nanotubes were used without any further purification. Potassium hydroxide (99.9%) from Sigma-Aldrich was of analytical grade and used without further purification. High-purity 18.2 M Ω deionized water was used. The gas used for plasma treatment was oxygen with a purity of 99.999%. Functionalization of Carbon Nanotubes. Plasma treatment of SWCNTs and MWCNTs was performed in a low-pressure plasma cleaner (Zepto, Diener Electronic GmbH, Ebhausen, Germany) equipped with 40 kHz generator operating between 0 and 100 W. Different plasma parameters (plasma power of 100 W, gas pressure of 1.2 mbar, and predefined exposure time) were carefully selected to achieve desired properties, as shown in the results. Samples of 10 mg (SWCNT or MWCNT) were loaded on a transfer plate and inserted into the plasma chamber; initially, the chamber was completely evacuated to 0.010 mbar and then filled with oxygen (O2 −99.999%) up to a pressure of 1.2 mbar. The plasma was generated at 100 W and maintained for a desired period (0−30 min). Thus, 0−30 min plasma irradiation conditions were shown to be optimal for the ORR activity of the catalysts. For comparison, untreated SWCNTs and MWCNTs were used for control sample preparation. The plasma-modified catalysts are denoted as O-SWCNT and O-MWCNT. Materials Characterization. The scanning electron microscopy (SEM) images of CNTs were obtained using a JEOL JSM 35CF equipped with the EDS-Link system and a reflected light microscope (Axiolab A). Transmission electron microscopy (TEM) and highresolution transmission electron microscopy (HRTEM) images were obtained from a JEOL JEM-2010F equipped with UHR pole piece, operated using an accelerating voltage of 200 kV. Samples for SEM, TEM, and HRTEM measurements were prepared by ultrasonic treatment of CNTs in 2-propanol for 30 min and by drop-casting 10 μL of this aliquot on a copper grid. Nitrogen sorption was measured at Gold AAP Instruments to obtain the Brunauer−Emmett−Teller (BET) surface area. Raman spectra were recorded with a XploRA ONE micro-Raman system (Horiba Scientific) using as 532 nm laser of power 200 m2 g−1 (theoretically) depending on the tube dimensions. In this study, the BET surface area of SWCNT and MWCNT were found to be 305 and 157 m2 g−1, respectively. However, the measured BET surface areas of O-SWCNT and O-MWCNT after 20 min of plasma exposure are 350 and 177 m2 g−1, respectively. This result implies that most of the oxygen plasma effect on the surface should be attributed mainly to the chemical modification of CNTs and to a lesser extent to surface area evolution. The chemical environment of oxygen functional sites in CNTs was explored by the XPS analysis, as shown in Figure 4. The survey spectra of both pristine and 20 min plasma treated O-SWCNT and O-MWCNT and their relative surface concentration is summarized in Table 1. A significant 1 order of magnitude increase of O 1s signal is depicted after plasma treatment on both samples. Table 1. Atomic Ratio of Plasma Treated and Untreated CNT (SWCNT and MWCNT) samples

C/O ratio

C (%)

pristine SWCNT 20 min O2 plasma treated SWCNT pristine MWCNT 20 min O2 plasma treated MWCNT

71.84 6.05 177.57 7.02

98.43 85.61 99.44 87.35

N (%) 0.21 0.26 0.20

O (%) 1.37 14.14 0.56 12.44

The plasma-treated samples exhibit a decrease in C/O ratio from 71.84 to 6.05 for SWCNT and from 177.57 to 7.02 for MWCNT, which indicates a significant increase in oxygen content after the treatment. Hence, the reduction of carbon percentage is ascribed to the introduction of oxygen groups onto the surface by cleaving the sp2 carbon structure and increasing its oxygen defects in both SWCNT and MWCNT.15 Table 1 suggests that after oxygen plasma treatment, the total atomic percentage of oxygen increased by about 1 order of magnitude in SWCNT and MWCNT from 1.37% to 14.14% and from 0.56% to 12.44%, respectively. A slightly higher 11400


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that most of the surface chemical reactions occur between the plasma -generated oxygen radicals and the active species in the plasma atmosphere (e.g., O2+, O+, O2•, O•). Generally, different types of nitrogen functionalities are presented in the graphitic layer of carbon; pyridinic N, pyrrolic N, quaternaric N, and graphitic N are reported as active sites for ORR.11,39,40 Herein, we studied nitrogen-free active O-SWCNT and OMWCNT as ORR catalysts by simple surface oxygen plasma treatment. Electrochemical Properties. The plasma-modified CNTs containing defects and oxygenated functional groups could be used as an active site for ORR.14 The electrochemical behavior of pristine SWCNT and the modified O-SWCNT were studied by cyclic voltammetry (CV). Since pristine CNTs are inherently hydrophobic in nature, the double-layer capacitance of the pristine SWCNT and MWCNT electrodes indicates the wettability of CNT electrodes. Capacitance values of 210 and 95 μF for pristine SWCNT and MWCNT were calculated from the non-Faradaic potential region from 0.10 to 1.10 V vs RHE in alkaline solution (Figure S6, SI). After exposure to 20 min plasma radiation, SWCNT and MWCNT yielded a double layer capacitance of 1422 and 1190 μF, corresponding to 1.32 and 1.60 times higher than pristine SWCNT and MWCNT, as shown in Figure S6a,b (SI). This enhancement in the capacitances is higher than that of the BET surface area measurements (1.1 time increase), which is ascribed to the improved wettability induced by the oxygen hydrophilic groups, namely, carboxylic acid, hydroxides, and lactone functional groups, as seen in the XPS results. In Figure 6a, the CV of SWCNT and 20 min plasma treated O-SWCNT displays mostly capacitive currents with no noticeable redox features under N2 in alkaline electrolytes. However, a pronounced O2 reduction current wave is seen from plasma-treated O-SWCNT, which exhibits a sharp cathodic oxygen reduction peak at 0.72 V vs RHE at 0.1 V lower overpotential than untreated SWCNT (0.62 V), as seen from Figure 6a. Similarly, in Figure 6b, the voltammograms of O-MWCNT showed a positive ORR potential peak, shifting to 0.67 V vs RHE from 0.61 V vs RHE for MWCNT. The calculated result indicates that the electrical double-layer capacitance (Cdl) in N2-saturated KOH is higher for all plasmatreated CNTs, accompanied by an increase in ORR current. It has been measured that O-SWCNT shows 1.3 times higher Cdl and 2 orders of magnitude increase in ORR current than SWCNT. For O-MWCNT, although the Cdl is 1.6 times higher than that for MWCNT, the ORR current increases only 1.2 order. Hence, our investigation suggests a dominance of oxygen reduction current in O-SWCNT over O-MWCNT, which could be related to the increase in electrochemical surface area and the utilization efficiency of oxygen concentration after plasma exposure. Therefore, it suggests that higher ORR current is related only to the incorporated oxygenated species, rather than surface area. It is worth mentioning that the intriguing structure of SWCNT allows more oxygenated species at its surfaces than MWCNT, exhibiting the higher efficiency in oxygen reduction reactions. This comparison of pristine to different plasma exposed SWCNT and MWCNT illustrates the impact of utilization efficiency of nanotubes with a high surface area. A more analytical inspection of the oxygen reduction reaction on OSWCNT and O-MWCNT was done by RRDE measurements of the prepared samples.

Figure 5. High-resolution deconvoluted (C 1s and O 1s) spectra of (a, c) pristine SWCNT, (b, d) plasma-treated SWCNT, (e, g) pristine MWCNT, and (f, h) plasma-treated MWCNT, respectively (note: 0− 20 min of oxygen plasma treatment).

CNTs. These results are quantified and tabulated in Tables S3 and S4 (SI). Hence, it is seen that 20 min oxygen plasma treatment can induce inner chemical reactions on the surfaces of SWCNTs and MWCNTs, e.g., a −CO(HO) moiety at a terminal carbon edge and a neighboring CO bond can form lactone groups upon plasma treatment.34 During the oxygen plasma functionalization, the attack of the oxygen radicals on the CNTs would likely lead to the formation of C−O bonds and then activated sites are generated on the surface of the CNTs. Hydroxyl bonds are formed upon stabilization by hydrogen atom transfer from the adjacent reactive carbon or on exposure to the atmosphere after plasma treatment via reaction with water from the air, while carbonyl bonds are the consequence of an intramolecular reorganization of C−C bonds. Conversely, the carboxylic groups are formed by active sites of previously generated carbonyl bonds, followed by stabilization through proton transfer.38 Furthermore, oxidation of C−OH of a terminal carbon can also produce carboxylic acid sites in the oxygenated environment.39 The results depict 11401


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Figure 6. CV of untreated and plasma-treated (a) SWCNT and (b) MWCNT in N2- and O2-saturated 0.10 M KOH. Catalyst loading: 50 μg cm−2, at a scan rate of 50 mV s−1. Linear sweep voltammetry (LSV) comparison curve of (c) SWCNT, O-SWCNT and (e) MWCNT, O-MWCNT in 0.10 M KOH, collected at a scan rate of 5 mV s−1, (d, f) Ring current density derived from various catalysts.

impact of oxygen plasma exposure time on ORR activities is also reflected in the onset potential and half-wave potential of the RRDE polarization curves, as tabulated in Tables S5 and S6 (SI). The result implies that with increasing plasma time (from 5 to 20 min), defects and vacancies are more likely contributing to the catalysis, as these sites are helpful for oxygen adsorption and desorption processes. Next, we compared the ORR activity of pristine and plasmatreated MWCNTs, which are shown in Figure 6e. At 20 min plasma exposure, MWCNT shows higher ORR activity than different irradiation times (5, 10, 15, 25, or 30 min) and this trend is similar to that of O-SWCNT. An increase in onset potential of 0.78 V vs RHE, together with a 90 mV positive shift in E1/2, is observed with 20 min O-MWCNT, in contrast to 0.74 V vs RHE for pristine MWCNT, as shown in Figure 6e. Further increasing plasma exposure time beyond 20 min creates a reduction in ORR activity, as tabulated in Table S6 (SI). However, beyond 20 min of exposure, the CNT wall collapses due to collective damage, which affects the electronic properties (e.g., sensitivity) of the broken tubes. Hence, the catalytic advantages of tubular CNTs are no longer applicable

Consequently, LSV measurements were done to compare the electrochemical performances and to understand the correct onset potential for oxygen reduction between pristine and treated CNTs. Generally, a positive shift of the onset potential of ORR was observed in the slow scan LSV of plasma-treated samples compared to untreated SWCNT (Figure 6c). Among various plasma exposure times, 20 min oxygen plasma treated SWCNT and MWCNT exhibits ORR activity. The onset potential of 20 min O-SWCNT is 0.78 V compared to 0.74 V for pristine SWCNT, as shown in Figure 6c. However, prolonged plasma exposure had negative influence on ORR activity, which is related to the damage of oxygen-modified graphitic sites of the CNT, as discussed above. Contrary to highly oriented pyrolitic graphite (HOPG),41 a slight decrease in onset potential was observed for prolonged exposure (25, 30 min) of CNTs. Interestingly, all SWCNT samples exhibit a two-step ORR process characterized by the presence of wavy features corresponding to a 2 + 2 electron pathway.42 Specifically, 20 min oxygen plasma treatment shows a two-step ORR process with two threshold potentials, which was observed in Figure 6c. The 11402


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Figure 7. ORR polarization curve of 20 min O2 plasma treated (a) SWCNT and (d) MWCNT rotating electrodes at different rpm values in 0.10 M KOH at a scan rate of 5 mV s−1. Number of electrons transferred calculated from the K−L plot of 20 min O2 plasma treated (b) SWCNT and (e) MWCNT. Tafel plot of 20 min O2 plasma treated (c) SWCNT and (f) MWCNT.

Figure 8. Accelerated stability test of pristine and 20 min O2 plasma functionalized (a) SWCNT and (b) MWCNT electrodes in O2 saturated 0.1 M KOH, cycled from 0.40 to 1.0 V vs RHE at a scan rate of 20 mV/s. (The insets of parts a and b show the polarization curve before and after 1000 potential cycles.)

exposure times, as seen in Figure 6d,f and in Tables S5 and S6 (SI). The peroxide yields of O-SWCNT and O-MWCNT are 8.8% and 15.3% at 0.5 V vs RHE, respectively, as shown in Figure S7 (SI). Figure 7a,d shows the polarization of 20 min treated OSWCNT and O-MWCNT electrodes at different rotation speeds. The number of electrons involved in ORR for 20 min O-SWCNT is calculated using eq 3 and was n = 2.3 at 0.30 V potential and 3.7 at 0.70 V vs RHE, which is closer to a 4e− pathway (Figure 7b). Similarly, the number of electrons transferred for 20 min O-MWCNT calculated from the K−L plot can be seen in Figure 7e, and the electron transfer varies from 3.1e− to 3.6e− at 0.30 to 0.60 V (closer to a 4e− pathway). Further insight into the ORR kinetics can be obtained by the Tafel slope calculation. The measured slope of

after 20 min of plasma treatment. It is noteworthy from these results that the ORR activity of O-SWCNT and O-MWCNT is higher than that reported for N-doped CNTs.43 In their research, Fujigaya et al. showed that the ORR performance of N-doped SWCNT is more facile compared to N-doped MWCNT, having 0.76 V vs RHE onset potential. In our study we showed that nitrogen-free O-SWCNT and O-MWCNT have higher ORR activity (onset potential 0.78 V vs RHE) at 20 min plasma exposure. The corresponding ring currents of various plasma-exposed CNTs were collected to quantify the oxidation of hydrogen peroxide ions (HO2−) on the Pt ring electrode, at the potential of 0.10 V vs Ag/AgCl, and are displayed in Figure 6d,f. The maximal peroxide current of 3.18 and 6.17 μA cm−2 was achieved for 20 min O-SWCNT and OMWCNT, respectively, in comparison to other plasma 11403


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standing, for the first time oxygen-plasma-treated SWCNT and MWCNT show the correlation between surface chemistry and ORR activity under well-controlled conditions toward a nonprecious metal-free catalyst.

the ORR as catalyzed by O-SWCNT and O-MWCNT catalysts are shown in Figure 7c,f. The kinetic current densities in the Tafel plot were obtained by a mass-transfer correction from the K−L plot in the potential range from 0.70 to 0.80 V vs RHE. The kinetic current of O-SWCNT derived from the mass transport correction of the disk current has a Tafel slope of ≈53 mV per decade at low current. In the case of 20 min OMWCNT, the measured Tafel slope is ≈49 mV per decade; these values are similar to ORR catalyzed by platinum, as shown in Figure S8 (SI). Transfer of the first electron catalyzed by 20 min O-SWCNT and O-MWCNT is the ratedetermining step (rds), as the Tafel slope is close to −2.303 RT/n′αF ≈ 60 mV per decade (R, T, n′, α, and F are the molar gas constant, absolute temperature, number of electrons until the rds, transfer coefficient, and Faraday constant, respectively) at room temperature.44 The basic reaction features of the oxygenated nanotubes endorse two important roles of oxygen sites (with edge defects) in general: the accessibility O2 molecules is more in 20 min oxygen plasma treated SWCNT and MWCNT, which (1) reduces O2 into HO2− efficiently and (2) facilitates further reduction of HO2− to OH− in alkaline electrolyte. Thus, the oxygen functional groups introduced by the oxygen plasma treatment provide higher concentrations of C−O−C and C O/COO defects at the surface, which might be the active center for both single and multiwalled CNTs in ORR.45 Previously Nakashima and colleagues also showed that plasma exposure of SWCNT is effective in creating defects such as grooves, which allows more oxygen adsorption and lowers the ORR overpotential.43 Recently, our group also explored that plasma treatment with graphitic electrode provides higher local activity at the nanometric defect sites under SECM-AFM.41 Hence, in this study, we envisage that SWCNT contains more free edges than MWCNT and possesses lower accessibility of self-edging with fewer layers. Figure 8a,b shows the cycling durability performance of 20 min O-SWCNT and O-MWCNT within the potential range 0.40−1.0 V. Figure 8 displays the ORR onset potential and current density at 0.10 V observed before and after a stability run of 1000 potential cycles.4 In this work, 20 min O-SWCNT showed only a 1−3 mV loss of onset potential with 30 mV decline in potential at 2.0 mA cm−2, yet the maximum current density at 0.10 V vs RHE remains the same throughout 1000 potential cycles, as shown in Figure 8a. The durability test of 20 min O-MWCNT from Figure 8b shows a sharp decline in onset (1−2 mV) potential and limiting current density (from 3.95 to 3.60 mA cm−2) that describes the stability only after the first 100 cycles with no further ORR current changes until the 1000th cycle. These results suggest that both materials are quite stable with an improved durability of O-SWCNT and OMWCNT with respect to the pristine electrodes. In comparison with O-MWCNT, the ORR activity of OSWCNT showed better stability. An explanation of the faster instability of O-MWCNT is still elusive. Moreover, to study the practical utility of plasma-treated CNTs, the methanol tolerance of O-SWCNT was compared with that of OMWCNT in alkaline medium. Upon adding 3 M methanol in 0.1 M KOH, no considerable change in onset potential was observed. However, a 5 mV negative shift in half-wave potential is seen, as shown in Figure S9 (SI). In Table S7 (SI), we have given a comparison between O-SWCNT and OMWCNT with other metal-free, non-nitrogenous catalysts, which depicts the potential of our catalyst. To our under-

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CONCLUSION A remarkable increase in the electrochemical activity (Eonset = 0.78 V vs RHE) toward the oxygen reduction reaction of oxygen-plasma-treated CNT shows a new approach for the development of metal- and nitrogen-free heterogeneous electrocatalysts in alkaline electrolyte. Plasma treatment creates sp3 defects at the CNT surfaces, along with the higher concentration of oxygenated species (lactones, carboxylic acid, −OH), which favors surface and electrochemical reactions. Our study demonstrates that oxygen functionality at the CNTs is imperative to improve the electrical double-layer capacitance as well as oxygen reduction current. The intriguing wall structure of SWCNT permits a higher concentration of oxygenated sites, even after prolonged exposure to plasma, which was higher than that in MWCNT. As a result, OSWCNT possesses a higher ORR onset potential, as well as a lower peroxide formation rate and improved stability, than OMWCNT. Compared to the critical synthesis of nitrogendoped CNTs, we show a new model utilizing defects and oxygenated sites developed within CNTs that might be convenient in making a cost-efficient electrocatalyst for ORR.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b01125.

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Additional characterization and electrochemical data (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +97239371470. Fax: +97239076586. ORCID

Alex Schechter: 0000-0002-3464-1936 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.S. would like to acknowledge the Israel Science Foundation (ISF) for funding the research work through the Israel National Research Center for Electrochemical Propulsion (INREP) and Ariel University for financial support. R.M and A.M acknowledge Ariel University for providing their scholarship. The authors are grateful to OCSiAl Europe S.à.r.l. for providing SWCNT.

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REFERENCES

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