- Email: [email protected]
Waste cotton-derived N-doped carbon as a sustainable metal-free electrocatalyst for oxygen reduction
Materials Letters 188 (2017) 33–36
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Waste cotton-derived N-doped carbon as a sustainable metal-free electrocatalyst for oxygen reduction Guomin Jiang, Bing Peng
MARK
⁎
School of Metallurgy and Environment, Central South University, Changsha 410083, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Metal-free carbon material Electrocatalyst Metal/air batteries
A superior electrocatalytic metal-free carbon material for oxygen reduction reaction was fabricated using waste cotton as feedstock through a simple sulfuric acidification and subsequent melamine activation at 1000 °C. The carbon possesses hierarchical micro-meso porous feature and a relatively high content of pyridinic & pyrrolic N. The carbon material exhibits a nice electrocatalytic activity with an onset potential of −0.17 V (vs. Ag/AgCl) in alkaline solutions which indicates its great potential in the application of metal/air batteries.
1. Introduction Alkaline metal/air batteries have become the prospect candidates of next generation energy production device due to their high power density and non-toxic feature [1,2]. As one of the most substantial components in metal/air batteries, Pt-based electrocatalyst is responsible for the effective oxygen reduction reaction [3,4]. However, the cost of Pt-based electrode is very expensive due to the use of noble metals [5,6]. It is highly desirable to develop a cheap and highperformance material to replace Pt. In this context, metal-free Ndoped materials have become a hot topic [7,8]. But the typical fabrication process usually involves complicated and time-consuming procedures, such as the hydrothermal carbonization, high-temperature treatment. Here we reported an effective and low-cost method to fabricate the N-doped carbon using waste cotton as raw materials. The carbon exhibits a superior catalytic performance in oxygen reduction reaction compared with many reported carbon materials (Table S1).
water until the solution pH is close to 7. Then the black cotton was freeze-dried. The cotton was uniformly mixed with melamine with weight ratio of 1:10 (cotton: melamine). Control experiments were conducted to optimize the selection of nitrogen sources and weight ratio of cotton to nitrogen sources (Fig. S1, Fig. S2, Table S2). It is suggested that melamine is the ideal choice and the best ratio is 1:10 (cotton: melamine). The mixture was heated to 1000 °C with ramp rate of 5 °C min−1 in flowing Ar atmosphere. The isothermal time is 2 h and after that the furnace was cooled to room temperature naturally. The product can be named as N-HC-PC. The undoped sample (HC-PC) was synthesized without the addition of melamine. The carbon derived from waste cotton (PC) was obtained by a direct carbonization at 1000 °C for 2 h [9,10]. 2.3. Characterizations The detailed information on the characterizations of sample by SEM, TEM, XPS, N2 adsorption-desorption and Raman was given in the Supplementary Informaiton (SI).
2. Experimental 2.4. Electrochemical characterizations 2.1. Materials The waste cotton was obtained from hospital. Melamine and sulfuric acid (98%) were of analytical grade and used as received. 2.2. Fabrication of waste cotton-derived N-doped carbon 10.0 g of cotton was immersed into 100 mL of sulfuric acid for 0.5 h at room temperature. The pretreated cotton was washed with deionized ⁎
The electrocatalytic measurements of the as-prepared samples were all operated in a three-electrode cell with a rotating disk electrode (PINE, USA). The catalyst was loaded on the glassy carbon electrode with a mass loading of 100 μg cm−2. The catalyst loaded electrode performs as working electrode. Ag/AgCl electrode and Pt wire were employed as reference and counter electrode, respectively. The cyclic voltammogram (CV) and linear sweep voltammetry (LSV) curves were measured using electrochemical work station (Solartron 1470E) to
Corresponding author. E-mail address: [email protected] (B. Peng).
http://dx.doi.org/10.1016/j.matlet.2016.10.080 Received 24 August 2016; Received in revised form 14 October 2016; Accepted 19 October 2016 Available online 20 October 2016 0167-577X/ © 2016 Published by Elsevier B.V.
Materials Letters 188 (2017) 33–36
G. Jiang, B. Peng
Fig. 1. XPS spectra of N-HC-PC: a) full scale spectra, b) the high-resolution of C1s spectrum and c) N1s spectrum. d) Raman spectrum of N-HC-PC..
product. It is believed that the treatment in sulfuric acid destroyed the intact microstructures of cotton since similar morphology was also observed for the product prepared by acid pretreatment without melamine activation (Fig. 2e and f). In fact, the cotton-derived carbon possesses a highly porous architecture, which is in consistent with the analysis of porosity above. To evaluate the electrocatalytic activities of the carbon materials, the CV and LSV measurements in O2-saturated 0.1 M KOH solution for various samples were operated in a three-electrode electrolytic cell with the same catalyst mass loading. As shown in Fig. 3a, the O2 reduction peak of N-HC-PC centers at −0.225 V which is 125 mV higher than that of HC-PC. Namely, the N-HC-PC exhibits the enhancing ORR electrocatalytic activity compared to HC-PC. The LSV curves of different samples (HC-PC, N-HC-PC and Pt/C) are illustrated in Fig. 3b. The PC is also taken into comparison in Fig. S4. The properties of PC were also characterized to support the performance enhancement of N-HC-PC over PC (Fig. S5, Fig. S6, Fig. S7). The N-HC-PC shows more positive onset potential and higher limiting current density than HC-PC and PC, indicating the priority of H2SO4-carbonization method. Moreover, it presents nice electrocatalytic activity. These results indicate that the N doping can significantly improve the ORR electrocatalytic activity due to the formation of more active sites in the process of N doping. The LSV curves of N-HC-PC at different rotation rates are displayed in Fig. 3c. The diffusion current density increases with the enhancement of rotation rate. To further demonstrate the ORR process of N-HC-PC, the Koutecky-Levich (K-L) plots at different potentials is shown in
demonstrate the electrocatalytic activity of the sample in the potential range from 0.2 to −1.0 V in O2-saturated 0.1 M KOH solution. 3. Results and discussion Fig. 1 provides the XPS full-scale, C1s and N1s spectra of the final product. As shown in Fig. 1a, the carbon product (N-HC-PC) activated by melamine has a relatively high N content, which can be further confirmed by the C1s and N1s spectra (Fig. 1b and c). Moreover, the N1s spectrum verifies the N species containing pyridinic and pyrrolic N, which are the useful species to promote the electrocatalytic performance [11,12]. And the nitrogen content of this N-HC-PC is 4.13 at%. The Raman spectrum (Fig. 1d) indicates a high graphitization of N-HC-PC, which improves the electron conductivity of materials. This is no doubt beneficial for the electrochemical application. Fig. 2a and b show the N2 adsorption-desorption isotherm and pore-distribution profile. The isotherm curves of N-HC-PC from Fig. 2a demonstrate a typical Type-IV feature. The specific surface area reaches as high as ~444 m2 g−1. Noticeably, the carbon materials possess hierarchical porosity (micropores/mesopores) based on the pore size distribution in Fig. 2b. This feature for HC-PC obviously weakens when melamine was not used in the final carbonization step (Fig. S3). That is to say, melamine not only provides the N sources for heteroatom doping, but also services as micropores-creating agent. According to the SEM analysis (Fig. 2c and d), the original cotton morphology (e.g., fibrous shape) cannot be observed in the final 34
Materials Letters 188 (2017) 33–36
G. Jiang, B. Peng
Fig. 2. a) N2 adsorption-desorption isotherm of N-HC-PC; b) pore-size distribution of N-HC-PC; SEM image of the carbon product N-HC-PC: c)-d) and HC-PC: e)-f)..
Fig. 3d is derived from Fig. 3c. The K-L equation was shown as below:
represent for the electrode rotation rate and the Faraday constant (96485 C mol−1). v is the kinetic viscosity of the electrolyte (1.0×10−2 cm2 s−1) [13,14]. The calculation result of the K-L plot demonstrates that the average electron transfer number is about 3.58, indicating that the main electron transfer process is 4-electron reduction route (O2+2H2O +4e−=4OH−) [15,16]. As shown in Fig. S9, N-HC-PC exhibits a much higher catalytic selectivity than Pt/C to resist the methanol electrooxidation. Moreover, the durability of the as-prepared carbon was evaluated by analyzing the CV curves at the 1st cycle and 2000th cycle,
1 1 1 1 1 = + = + J JK JL Bω12 JK 2
1
B = 0. 2nF (D0 ) 3 v− 6 C0 In the equation above, J is the practically measured current density and JL refers to the limited diffusion current density, JK is on behalf of the kinetic current density. C0 and D0 are the bulk concentration and the diffusion coefficient of O2, respectively. ω(rpm) and F respectively 35
Materials Letters 188 (2017) 33–36
G. Jiang, B. Peng
Fig. 3. a) CV curves of different samples (N-HC-PC, HC-PC) in O2-saturated 0.1 M KOH solution; b) LSV curves of different samples (N-HC-PC, HC-PC and Pt/C) at rotation rate of 1600 rpm with a scan rate of 5 mV s−1; c) LSV curves of N-HC-PC with various rotation rates; d) K-L plots at different potentials derived from c).. atomic layer deposition for the oxygen reduction reaction, Adv. Mater. 27 (2) (2015) 277–281. [5] C. Hu, L. Dai, Carbon-based metal-free catalysts for electrocatalysis beyond the ORR, Angew. Chem. (2016). [6] S. Lee, M. Choun, Y. Ye, J. Lee, Y. Mun, E. Kang, J. Hwang, Y.H. Lee, C.H. Shin, S.H. Moon, S.K. Kim, E. Lee, J. Lee, Designing a highly active metal-free oxygen reduction catalyst in membrane electrode assemblies for alkaline fuel cells: effects of pore size and doping-site position, Angew. Chem. 54 (32) (2015) 9230–9234. [7] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogendoped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (6271) (2016) 361–365. [8] R. Liu, D. Wu, X. Feng, K. Mullen, Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction, Angew. Chem. 49 (14) (2010) 2565–2569. [9] Y. Zhang, L. Zuo, L. Zhang, Y. Huang, H. Lu, W. Fan, T. Liu, Cotton wool derived carbon fiber aerogel supported few-layered MoSe2 nanosheets as efficient electrocatalysts for hydrogen evolution, ACS Appl. Mater. Interfaces 8 (11) (2016) 7077–7085. [10] H. Bi, Z. Yin, X. Cao, X. Xie, C. Tan, X. Huang, B. Chen, F. Chen, Q. Yang, X. Bu, X. Lu, L. Sun, H. Zhang, Carbon fiber aerogel made from raw cotton: a novel, efficient and recyclable sorbent for oils and organic solvents, Adv. Mater. 25 (41) (2013) 5916–5921. [11] L. Chai, J. Wang, H. Wang, L. Zhang, W. Yu, L. Mai, Porous carbonized grapheneembedded fungus film as an interlayer for superior Li–S batteries, Nano Energy 17 (2015) 224–232. [12] M. Wang, Z. Fang, K. Zhang, J. Fang, F. Qin, Z. Zhang, J. Li, Y. Liu, Y. Lai, Synergistically enhanced activity of graphene quantum dots/graphene hydrogel composites: a novel allcarbon hybrid electrocatalyst for metal/air batteries, Nanoscale 8 (22) (2016) 11398–11402. [13] S. Wang, D. Yu, L. Dai, D.W. Chang, J.B. Baek, Polyelectrolyte-functionalized graphene as metal-free electrocatalysts for oxygen reduction, ACS nano 5 (8) (2011) 6202–6209. [14] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co(3)O(4) nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (10) (2011) 780–786. [15] X. Ge, A. Sumboja, D. Wuu, T. An, B. Li, F.W.T. Goh, T.S.A. Hor, Y. Zong, Z. Liu, Oxygen reduction in alkaline media: from mechanisms to recent advances of catalysts, ACS Catal. 5 (8) (2015) 4643–4667. [16] R. Zhou, Y. Zheng, M. Jaroniec, S.-Z. Qiao, Determination of the electron transfer number for the oxygen reduction reaction: from theory to experiment, ACS Catal. 6 (7) (2016) 4720–4728.
demonstrating slight peak shift, which reveals the good stability of the N-HC-PC. 4. Conclusion In summary, a porous and N-doped carbon was successfully synthesized using a facile H2SO4-based carbonization method using waste cotton as carbon source. This strategy produces superior carbon materials with enhanced efficiency in ORR compared with the normal carbonization method. Specifically, the resultant product N-HC-PC possessed high onset potential, good durability and excellent methanol tolerance, which makes it a potential candidate for the application of alkaline metal/air batteries. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.matlet.2016.10.080. References [1] Z.-L. Wang, D. Xu, J.-J. Xu, X.-B. Zhang, Oxygen electrocatalysts in metal–air batteries: from aqueous to nonaqueous electrolytes, Chem. Soc. Rev. 43 (22) (2014) 7746–7786. [2] Z.L. Wang, D. Xu, J.J. Xu, X.B. Zhang, Oxygen electrocatalysts in metal-air batteries: from aqueous to nonaqueous electrolytes, Chem. Soc. Rev. 43 (22) (2014) 7746–7786. [3] D. Li, C. Wang, D.S. Strmcnik, D.V. Tripkovic, X. Sun, Y. Kang, M. Chi, J.D. Snyder, D. van der Vliet, Y. Tsai, V.R. Stamenkovic, S. Sun, N.M. Markovic, Functional links between Pt single crystal morphology and nanoparticles with different size and shape: the oxygen reduction reaction case, Energy Environ. Sci. 7 (12) (2014) 4061–4069. [4] N. Cheng, M.N. Banis, J. Liu, A. Riese, X. Li, R. Li, S. Ye, S. Knights, X. Sun, Extremely stable platinum nanoparticles encapsulated in a zirconia nanocage by area-selective
36
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Waste cotton-derived N-doped carbon as a sustainable metal-free electrocatalyst for oxygen reduction Guomin Jiang, Bing Peng
MARK
⁎
School of Metallurgy and Environment, Central South University, Changsha 410083, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Metal-free carbon material Electrocatalyst Metal/air batteries
A superior electrocatalytic metal-free carbon material for oxygen reduction reaction was fabricated using waste cotton as feedstock through a simple sulfuric acidification and subsequent melamine activation at 1000 °C. The carbon possesses hierarchical micro-meso porous feature and a relatively high content of pyridinic & pyrrolic N. The carbon material exhibits a nice electrocatalytic activity with an onset potential of −0.17 V (vs. Ag/AgCl) in alkaline solutions which indicates its great potential in the application of metal/air batteries.
1. Introduction Alkaline metal/air batteries have become the prospect candidates of next generation energy production device due to their high power density and non-toxic feature [1,2]. As one of the most substantial components in metal/air batteries, Pt-based electrocatalyst is responsible for the effective oxygen reduction reaction [3,4]. However, the cost of Pt-based electrode is very expensive due to the use of noble metals [5,6]. It is highly desirable to develop a cheap and highperformance material to replace Pt. In this context, metal-free Ndoped materials have become a hot topic [7,8]. But the typical fabrication process usually involves complicated and time-consuming procedures, such as the hydrothermal carbonization, high-temperature treatment. Here we reported an effective and low-cost method to fabricate the N-doped carbon using waste cotton as raw materials. The carbon exhibits a superior catalytic performance in oxygen reduction reaction compared with many reported carbon materials (Table S1).
water until the solution pH is close to 7. Then the black cotton was freeze-dried. The cotton was uniformly mixed with melamine with weight ratio of 1:10 (cotton: melamine). Control experiments were conducted to optimize the selection of nitrogen sources and weight ratio of cotton to nitrogen sources (Fig. S1, Fig. S2, Table S2). It is suggested that melamine is the ideal choice and the best ratio is 1:10 (cotton: melamine). The mixture was heated to 1000 °C with ramp rate of 5 °C min−1 in flowing Ar atmosphere. The isothermal time is 2 h and after that the furnace was cooled to room temperature naturally. The product can be named as N-HC-PC. The undoped sample (HC-PC) was synthesized without the addition of melamine. The carbon derived from waste cotton (PC) was obtained by a direct carbonization at 1000 °C for 2 h [9,10]. 2.3. Characterizations The detailed information on the characterizations of sample by SEM, TEM, XPS, N2 adsorption-desorption and Raman was given in the Supplementary Informaiton (SI).
2. Experimental 2.4. Electrochemical characterizations 2.1. Materials The waste cotton was obtained from hospital. Melamine and sulfuric acid (98%) were of analytical grade and used as received. 2.2. Fabrication of waste cotton-derived N-doped carbon 10.0 g of cotton was immersed into 100 mL of sulfuric acid for 0.5 h at room temperature. The pretreated cotton was washed with deionized ⁎
The electrocatalytic measurements of the as-prepared samples were all operated in a three-electrode cell with a rotating disk electrode (PINE, USA). The catalyst was loaded on the glassy carbon electrode with a mass loading of 100 μg cm−2. The catalyst loaded electrode performs as working electrode. Ag/AgCl electrode and Pt wire were employed as reference and counter electrode, respectively. The cyclic voltammogram (CV) and linear sweep voltammetry (LSV) curves were measured using electrochemical work station (Solartron 1470E) to
Corresponding author. E-mail address: [email protected] (B. Peng).
http://dx.doi.org/10.1016/j.matlet.2016.10.080 Received 24 August 2016; Received in revised form 14 October 2016; Accepted 19 October 2016 Available online 20 October 2016 0167-577X/ © 2016 Published by Elsevier B.V.
Materials Letters 188 (2017) 33–36
G. Jiang, B. Peng
Fig. 1. XPS spectra of N-HC-PC: a) full scale spectra, b) the high-resolution of C1s spectrum and c) N1s spectrum. d) Raman spectrum of N-HC-PC..
product. It is believed that the treatment in sulfuric acid destroyed the intact microstructures of cotton since similar morphology was also observed for the product prepared by acid pretreatment without melamine activation (Fig. 2e and f). In fact, the cotton-derived carbon possesses a highly porous architecture, which is in consistent with the analysis of porosity above. To evaluate the electrocatalytic activities of the carbon materials, the CV and LSV measurements in O2-saturated 0.1 M KOH solution for various samples were operated in a three-electrode electrolytic cell with the same catalyst mass loading. As shown in Fig. 3a, the O2 reduction peak of N-HC-PC centers at −0.225 V which is 125 mV higher than that of HC-PC. Namely, the N-HC-PC exhibits the enhancing ORR electrocatalytic activity compared to HC-PC. The LSV curves of different samples (HC-PC, N-HC-PC and Pt/C) are illustrated in Fig. 3b. The PC is also taken into comparison in Fig. S4. The properties of PC were also characterized to support the performance enhancement of N-HC-PC over PC (Fig. S5, Fig. S6, Fig. S7). The N-HC-PC shows more positive onset potential and higher limiting current density than HC-PC and PC, indicating the priority of H2SO4-carbonization method. Moreover, it presents nice electrocatalytic activity. These results indicate that the N doping can significantly improve the ORR electrocatalytic activity due to the formation of more active sites in the process of N doping. The LSV curves of N-HC-PC at different rotation rates are displayed in Fig. 3c. The diffusion current density increases with the enhancement of rotation rate. To further demonstrate the ORR process of N-HC-PC, the Koutecky-Levich (K-L) plots at different potentials is shown in
demonstrate the electrocatalytic activity of the sample in the potential range from 0.2 to −1.0 V in O2-saturated 0.1 M KOH solution. 3. Results and discussion Fig. 1 provides the XPS full-scale, C1s and N1s spectra of the final product. As shown in Fig. 1a, the carbon product (N-HC-PC) activated by melamine has a relatively high N content, which can be further confirmed by the C1s and N1s spectra (Fig. 1b and c). Moreover, the N1s spectrum verifies the N species containing pyridinic and pyrrolic N, which are the useful species to promote the electrocatalytic performance [11,12]. And the nitrogen content of this N-HC-PC is 4.13 at%. The Raman spectrum (Fig. 1d) indicates a high graphitization of N-HC-PC, which improves the electron conductivity of materials. This is no doubt beneficial for the electrochemical application. Fig. 2a and b show the N2 adsorption-desorption isotherm and pore-distribution profile. The isotherm curves of N-HC-PC from Fig. 2a demonstrate a typical Type-IV feature. The specific surface area reaches as high as ~444 m2 g−1. Noticeably, the carbon materials possess hierarchical porosity (micropores/mesopores) based on the pore size distribution in Fig. 2b. This feature for HC-PC obviously weakens when melamine was not used in the final carbonization step (Fig. S3). That is to say, melamine not only provides the N sources for heteroatom doping, but also services as micropores-creating agent. According to the SEM analysis (Fig. 2c and d), the original cotton morphology (e.g., fibrous shape) cannot be observed in the final 34
Materials Letters 188 (2017) 33–36
G. Jiang, B. Peng
Fig. 2. a) N2 adsorption-desorption isotherm of N-HC-PC; b) pore-size distribution of N-HC-PC; SEM image of the carbon product N-HC-PC: c)-d) and HC-PC: e)-f)..
Fig. 3d is derived from Fig. 3c. The K-L equation was shown as below:
represent for the electrode rotation rate and the Faraday constant (96485 C mol−1). v is the kinetic viscosity of the electrolyte (1.0×10−2 cm2 s−1) [13,14]. The calculation result of the K-L plot demonstrates that the average electron transfer number is about 3.58, indicating that the main electron transfer process is 4-electron reduction route (O2+2H2O +4e−=4OH−) [15,16]. As shown in Fig. S9, N-HC-PC exhibits a much higher catalytic selectivity than Pt/C to resist the methanol electrooxidation. Moreover, the durability of the as-prepared carbon was evaluated by analyzing the CV curves at the 1st cycle and 2000th cycle,
1 1 1 1 1 = + = + J JK JL Bω12 JK 2
1
B = 0. 2nF (D0 ) 3 v− 6 C0 In the equation above, J is the practically measured current density and JL refers to the limited diffusion current density, JK is on behalf of the kinetic current density. C0 and D0 are the bulk concentration and the diffusion coefficient of O2, respectively. ω(rpm) and F respectively 35
Materials Letters 188 (2017) 33–36
G. Jiang, B. Peng
Fig. 3. a) CV curves of different samples (N-HC-PC, HC-PC) in O2-saturated 0.1 M KOH solution; b) LSV curves of different samples (N-HC-PC, HC-PC and Pt/C) at rotation rate of 1600 rpm with a scan rate of 5 mV s−1; c) LSV curves of N-HC-PC with various rotation rates; d) K-L plots at different potentials derived from c).. atomic layer deposition for the oxygen reduction reaction, Adv. Mater. 27 (2) (2015) 277–281. [5] C. Hu, L. Dai, Carbon-based metal-free catalysts for electrocatalysis beyond the ORR, Angew. Chem. (2016). [6] S. Lee, M. Choun, Y. Ye, J. Lee, Y. Mun, E. Kang, J. Hwang, Y.H. Lee, C.H. Shin, S.H. Moon, S.K. Kim, E. Lee, J. Lee, Designing a highly active metal-free oxygen reduction catalyst in membrane electrode assemblies for alkaline fuel cells: effects of pore size and doping-site position, Angew. Chem. 54 (32) (2015) 9230–9234. [7] D. Guo, R. Shibuya, C. Akiba, S. Saji, T. Kondo, J. Nakamura, Active sites of nitrogendoped carbon materials for oxygen reduction reaction clarified using model catalysts, Science 351 (6271) (2016) 361–365. [8] R. Liu, D. Wu, X. Feng, K. Mullen, Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction, Angew. Chem. 49 (14) (2010) 2565–2569. [9] Y. Zhang, L. Zuo, L. Zhang, Y. Huang, H. Lu, W. Fan, T. Liu, Cotton wool derived carbon fiber aerogel supported few-layered MoSe2 nanosheets as efficient electrocatalysts for hydrogen evolution, ACS Appl. Mater. Interfaces 8 (11) (2016) 7077–7085. [10] H. Bi, Z. Yin, X. Cao, X. Xie, C. Tan, X. Huang, B. Chen, F. Chen, Q. Yang, X. Bu, X. Lu, L. Sun, H. Zhang, Carbon fiber aerogel made from raw cotton: a novel, efficient and recyclable sorbent for oils and organic solvents, Adv. Mater. 25 (41) (2013) 5916–5921. [11] L. Chai, J. Wang, H. Wang, L. Zhang, W. Yu, L. Mai, Porous carbonized grapheneembedded fungus film as an interlayer for superior Li–S batteries, Nano Energy 17 (2015) 224–232. [12] M. Wang, Z. Fang, K. Zhang, J. Fang, F. Qin, Z. Zhang, J. Li, Y. Liu, Y. Lai, Synergistically enhanced activity of graphene quantum dots/graphene hydrogel composites: a novel allcarbon hybrid electrocatalyst for metal/air batteries, Nanoscale 8 (22) (2016) 11398–11402. [13] S. Wang, D. Yu, L. Dai, D.W. Chang, J.B. Baek, Polyelectrolyte-functionalized graphene as metal-free electrocatalysts for oxygen reduction, ACS nano 5 (8) (2011) 6202–6209. [14] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co(3)O(4) nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (10) (2011) 780–786. [15] X. Ge, A. Sumboja, D. Wuu, T. An, B. Li, F.W.T. Goh, T.S.A. Hor, Y. Zong, Z. Liu, Oxygen reduction in alkaline media: from mechanisms to recent advances of catalysts, ACS Catal. 5 (8) (2015) 4643–4667. [16] R. Zhou, Y. Zheng, M. Jaroniec, S.-Z. Qiao, Determination of the electron transfer number for the oxygen reduction reaction: from theory to experiment, ACS Catal. 6 (7) (2016) 4720–4728.
demonstrating slight peak shift, which reveals the good stability of the N-HC-PC. 4. Conclusion In summary, a porous and N-doped carbon was successfully synthesized using a facile H2SO4-based carbonization method using waste cotton as carbon source. This strategy produces superior carbon materials with enhanced efficiency in ORR compared with the normal carbonization method. Specifically, the resultant product N-HC-PC possessed high onset potential, good durability and excellent methanol tolerance, which makes it a potential candidate for the application of alkaline metal/air batteries. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.matlet.2016.10.080. References [1] Z.-L. Wang, D. Xu, J.-J. Xu, X.-B. Zhang, Oxygen electrocatalysts in metal–air batteries: from aqueous to nonaqueous electrolytes, Chem. Soc. Rev. 43 (22) (2014) 7746–7786. [2] Z.L. Wang, D. Xu, J.J. Xu, X.B. Zhang, Oxygen electrocatalysts in metal-air batteries: from aqueous to nonaqueous electrolytes, Chem. Soc. Rev. 43 (22) (2014) 7746–7786. [3] D. Li, C. Wang, D.S. Strmcnik, D.V. Tripkovic, X. Sun, Y. Kang, M. Chi, J.D. Snyder, D. van der Vliet, Y. Tsai, V.R. Stamenkovic, S. Sun, N.M. Markovic, Functional links between Pt single crystal morphology and nanoparticles with different size and shape: the oxygen reduction reaction case, Energy Environ. Sci. 7 (12) (2014) 4061–4069. [4] N. Cheng, M.N. Banis, J. Liu, A. Riese, X. Li, R. Li, S. Ye, S. Knights, X. Sun, Extremely stable platinum nanoparticles encapsulated in a zirconia nanocage by area-selective
36