Nitrogen-doped microporous carbon: An efficient oxygen reduction catalyst for Zn-air batteries

Journal of Power Sources 359 (2017) 71e79

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Nitrogen-doped microporous carbon: An efficient oxygen reduction catalyst for Zn-air batteries Li-Yuan Zhang a, 1, Meng-Ran Wang b, 1, Yan-Qing Lai b, Xiao-Yan Li a, * a b

Environmental Engineering Research Centre, Department of Civil Engineering, The University of Hong Kong, Pokfulam, Hong Kong School of Metallurgy and Environment, Central South University, Changsha, China

h i g h l i g h t s
Nanoporous carbon was synthesized from waste biomass using a universal method.
The sample possesses excellent ORR activity and high stability in alkaline media.
The sample further exhibits good discharge performance for zinc/air batteries.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 February 2017 Received in revised form 24 April 2017 Accepted 17 May 2017

N-doped microporous carbon as an exceptional metal-free catalyst from waste biomass (banana peel as representative) was obtained via fast catalysis carbonization, followed by N-doping modification. The method achieves a relatively high C conversion efficiency of ~41.9%. The final carbon materials are doped by N (~3 at.%) and possess high surface area (~1097 m2 g1) and abundant micropores. Compared to commercial Pt/C materials, the as-prepared carbon catalyst exhibits a comparable electrocatalytic activity and much better stability. Furthermore, the metal-free catalyst loaded Zn-air battery possesses higher discharge voltage and power density as compared with that of commercial Pt/C. This novel technique can also be readily applied to produce metal-free carbon catalysts from other typical waste biomass (e.g., orange peel, leaves) as the carbon sources. The method can be developed as a potentially general and effective industrial route to transform waste biomass into high value-added microporous carbon with superior functionalities. © 2017 Elsevier B.V. All rights reserved.

Keywords: Microporous carbon Nitrogen doping Oxygen reduction reaction Zn-air battery

1. Introduction Research into the development of alternative and sustainable energy sources has become an urgent task, due to the increasing consumption of non-renewable fossil fuels and the related environmental problems (e.g., air pollution, greenhouse gas emission, and subsequent global warming) [1e3]. In this context, zinc-air batteries with high energy density and sustainable features are of particular interest [4,5]. Their conversion efficiency is, however, commonly limited by slow oxygen reduction reaction (ORR) at the cathode, due to the high activation energy of the reaction [6]. The activation energy can be reduced with the aid of catalysts, and currently noble metals-based catalysts, typically Pt-based

* Corresponding author. E-mail address: [email protected] (X.-Y. Li). 1 These authors contributed equally to this work. http://dx.doi.org/10.1016/j.jpowsour.2017.05.056 0378-7753/© 2017 Elsevier B.V. All rights reserved.

materials, are commonly used, but unfortunately these suffer from poor stability in aqueous zinc-air batteries conditions [7]. Not to mention that are noble metals also scarce and expensive. Thus, developing low-cost catalysts with high ORR efficiency and potential of large-scale production remains a great challenge for aqueous zinc-air batteries technology. Carbonaceous materials are low-cost and possess high biocompatibility and electrocatalytic activity, making them a suitable metal-free catalyst that can replace Pt-based materials [8e13]. The classical examples include carbon nanotubes [14,15], graphene [16e18], and their hybrids [19e21]. Apart from these choices, waste biomass-derived carbon has attracted increasing attention, due to its almost zero-cost feedstock and the resulting reduction of environment burdens. Currently, ORR catalysts have been reported using amaranthus, typha orientalis, enoki mushroom, and pomelo peel, etc, as raw materials [22e31]. It should be noted that the daily production of waste biomass is very large while its composition is

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very complicated [32e34]. Developing an effective, high C conversion and general approach to transform the waste biomass into superior ORR catalysts becomes an urgent task. In this study, we developed a fast catalysis carbonization at a relatively low temperature, to obtain a microporous carbon as a metal-free ORR catalyst precursor using waste biomass as the carbon source. Banana peel, which has been commonly used as a model waste biomass[ [35]], was selected as the carbon source for the present synthesis and experiments. This new process involves 10 min carbonization and 15 min cooling. It is simple, low-cost, efficient, and can be readily applied on a large scale with high C conversion efficiency. The porous carbon was N-doped by melamine modification at a high temperature to create an excellent metal-free ORR catalyst. The new technology can be readily used as well to produce high-performance catalysts from other typical biomass wastes, such as orange peel, leaves, etc., suggesting its promising potential of extensive applications.

2. Experimental section 2.1. Materials Zinc chloride anhydrous (98%), ethanol absolute (99.5%), and

hydrochloric acid (36%) were purchased from Alfa. Melamine (99%) was obtained from Aladdin. Bananas and oranges were purchased from a local Hong Kong supermarket as the source of peel. Tree leaves were collected from the campus of the University of Hong Kong. Unless otherwise specified, the water used in the laboratory experiments was deionized water.

2.2. Production of microporous carbon from banana peel The banana peel was washed in water to remove any dirt. The peel was then cut into pieces and dried at 105  C in an oven to provide a constant weight. The dried peel was mixed with ZnCl2 powder at a weight ratio of 1:5, the ZnCl2 was utilized as a catalyst for rapid carbonization. The mixture was then processed by ball milling, with the milling time and rate controlled at 30 min and 500 rpm, respectively. The uniform powders were placed into a quartz bottle filled with N2. As illustrated in Fig. 1, the bottle was rapidly inserted into a vertical furnace that had been heated to a pre-determined temperature. After 10 min, the bottle was removed and immediately immersed into water for rapid cooling. The cooling time was 15 min. The carbon product was rinsed in HCl solution and water to remove and recover the ZnCl2, and was then dried at 70  C in an oven. The carbon obtained from the fast

Fig. 1. Illustration of the synthesis process of porous carbon and its N-doped product: (a) Uniform mixture of banana peel and ZnCl2, prepared by ball milling followed by rapid catalysis carbonization, (b) Rapid cooling of the product, and (c) N doping by melamine modification. TEM images of the porous carbon (d) and its N-doped product (e).

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catalysis carbonization was labeled as CZnrapid-X, where X represents the temperature of the furnace. Carbon was also synthesized by the same carbonization process without using ZnCl2, and the product was labeled as Crapid-X. 2.3. N doping of the microporous carbon The dried carbon was thoroughly mixed with melamine at a weight ratio of 1:10. The mixture in a porcelain boat was placed into a horizontal tube furnace for heating at a specific temperature with a ramp rate of 5  C min1 and an isothermal time of 2 h. The N doping process with melamine for carbon modification was protected by N2 gas. The temperature then dropped naturally under ambient conditions. The carbon after N doping was labeled as CZnrapid-X-Melamine-Y, where Y represents the modification temperature. For comparison, the carbon product without melamine treatment for N doping was denoted as CZnrapid-X-Y. The C conversion efficiency of the aforementioned process was calculated using the following equation.

Ec ¼

Mfinal  Cfinal  100% Mraw  Craw

(1)

where Ec is the C conversion efficiency (%), Mfinal and Mraw are the weights of the final carbon product and the raw material (banana peel) (g), respectively, Cfinal and Craw are the C mass percentage of the final carbon product and the raw material (wt.%), respectively. The C mass ratio of the sample was obtained by the elemental analysis described below. The C mass percentage of banana peel (Craw) was determined to be 43.67%.

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mg of the sample and 40 mL of 5 wt% Nafion solution were dispersed in 1 mL of ethanol followed by 40 min sonication to obtain a uniform ink. The working electrode was prepared by drop-casting 5 mL of the catalyst ink onto the glassy carbon (loading ~0.1 mg cm2). An Ag/AgCl with saturated KCl and a spiral Pt wire electrode were used as a reference electrode and counter electrode, respectively. The linear sweep voltammetry (LSV) measurements were operated in the potential range of þ0.2 to 1.0 V with a scan rate of 5 mV s1, at a rotation speed varying from 400 to 2500 rpm, in the O2-saturated 0.1 M KOH aqueous solution. The cyclic voltammogram (CV) curves were acquired in the same potential range as the LSV, using a scan rate of 50 mV s1 in the O2 or N2 saturated 0.1 M KOH. To further analyze the ORR potential of the catalyst, rotating ring disk measurement was performed. The record of the ring current and disk current was used to calculate the electron transfer number and the percentage of HO 2 using the following equations.

%HO 2 ¼ 200 

n¼4

Ir =N Id þ Ir =N

Id Id þ Ir =N

(2)

(3)

where Id is the disk current, Ir is the ring current, and N is the current collection efficiency of the Pt ring. The N value was determined to be 0.40 from the reduction of K3Fe [CN]6 [6,36,37]. 2.7. Zinc/air batteries A zinc/air battery was established in an electrochemical cell

2.4. Comparative carbonization experiment In this study, other typical carbonization methods were also applied to prepare the carbon using banana peel [23,28]. The synthesis process was the same as reported except for the raw material. The C conversion efficiency was also calculated. 2.5. Characterizations The morphology of the samples was analyzed by a fieldemission scanning electron microscope (SEM/Hitachi S4800 FEG) and a transmission electron microscope (TEM/FEI Tecnai G2). The accelerating voltage of SEM and TEM was set to be 20 and 200 kV, respectively. The specific surface area and pore size distribution of the samples were measured by the Micromeritics ASAP 2020 accelerated surface area and porosimetry system. For the measurement, the sample was degassed at 100  C for 24 h and the test was carried out at 196  C. X-ray photoelectron spectroscopy (XPS/ ESCALAB 250Xi) was used to investigate the chemical composition of the samples. The vacuum degree was maintained at 5  1010 mbar. In data processing, all binding energies were referenced to the C1s neutral carbon at 284.6 eV to compensate for the surface charging effect. The quantitative content (mass percentage) of C of the samples was measured by the Vario Macro cube elemental analyzer. Raman scattering spectra of the samples were obtained by using the inVia confocal Raman microscope in the range of 500e4000 cm1. The acquisition time was 10 s and the HeLe laser excitation wavelength was 532 nm. 2.6. Oxygen reduction reaction The electrocatalytic activity of the carbon products for ORR was determined by an electrochemical workstation (Solartron 1470E) using a rotating disk electrode (PINE) at room temperature. Four

Fig. 2. N2 adsorption-desorption isotherm (a) and pore size distribution (b) of CZnrapid400-Melamine-1000.

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and polytetrafluoroethylene emulsion (PTFE, 60 wt%, as a binder), mixed uniformly at a weight ratio of 3:3:1:3. The air diffusion layer was a membrane which was used to prevent electrolyte from leaking and allow air to diffuse through at the same time. The air diffusion layer was fabricated by rolling and pressing the acetylene black and PTFE hybrid slurry (weight ratio: 1:5), which were predispersed in ethanol. The air cathode was fixed on the side with a

under air atmosphere. The carbon sample was used as an air electrode and a Zn (99.99%) plate was used as an anode. Electrolyte is 6.0 M KOH. The commercial Pt/C catalyst (20 wt%, J&M) was also measured as an electrode material for comparison. The air electrode contained three layers: the catalytic layer, air diffusion layer and nickel mesh. The catalytic layer was fabricated with the catalyst sample, active carbon, acetylene black (as a conductive additive)

Table 1 Summary of the chemical composition analysis (high-resolution XPS spectra), surface area, pore volume (N2 adsorption-desorption isotherm) and graphitization degree (Raman spectra) of the different carbon samples. The representations of N1-N5 and C1-C2 are given in the caption of Fig. 3. Sample

CZnrapid-400 CZnrapid-400-Melamine-900 CZnrapid-400-Melamine-1000 CZnrapid-400-Melamine-1100 a b c

Nx:N (%)a

Cx:C (%)b

N1

N2

N3

N4

N5

C1

C2

35.48 34.89 31.09 11.31

9.51 15.18 8.01 9.00

34.47 36.84 44.86 60.06

14.20 10.07 16.04 19.63

6.33 3.02 ec ec

32.90 38.43 48.44 59.86

52.79 43.43 35.94 25.90

Specific surface area (m2 g1)

Pore volume (cm3 g1)

Graphitization degree (ID/IG)

977.6 982.4 1009.6 968.4

0.816 0.774 0.818 0.772

e 2.376 2.024 1.856

Nx:N is the relative molar ratio of the specific N species to the total N. Cx:C is the relative molar ratio of the specific C species to the total C. N5 cannot be detected in CZnrapid-400-Melamine-1000/1100.

Fig. 3. High-resolution C1s (a, c, e, g) and N1s (b, d, f, h) XPS spectra of the carbon products: CZnrapid-400, CZnrapid-400-Melamine-900, CZnrapid-400-Melamine-1000, and CZnrapid400-Melamine-1100. The color of the lines in the Figure represents specific C or N species. C1: sp2 hybridized graphitized carbon, C2: sp3 C-C carbon, C3: C-O or C-N, C4: C¼O; N1: pyridinic-N, N2: amine or imine, N3: pyrrolic N, N4: quaternary N, N5: pyridinic-N-oxide. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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circular hole, while the anode plate was fixed on another side. The constant-current discharge measurement was conducted for estimating the electrochemical performance of the air cathode in the application of zinc/air batteries. The measurement was carried out using a LAND CT2001A battery-testing instrument. All battery testing measurements were operated in atmospheric air. 3. Results and discussion The main steps are illustrated in Fig. 1, and consist of ball milling of the raw materials, fast carbonization followed by rapid cooling, and N doping. Specifically, the mixture of banana peel and ZnCl2 catalyst in the vessel was directly inserted into the vertical furnace pre-heated to 400  C (Fig. 1a) and then pulled out (Fig. 1b). The whole time costs less than 30 min and totally speaking, the new carbonization process can be operated hemi-continuously. This will allow a simple scaling up of the system for large-scale production. The porous carbon product was finally activated by N-doping with melamine at 1000  C for 2 h (Fig. 1c). The microstructures of the waste biomass-derived carbon can be found in Fig. 1d and e. CZnrapid-400 exhibits a highly porous microarchitecture, containing mesopores (<50 nm) and macropores (>50 nm) according to the TEM images (Fig. 1d). The mesopores range from 15 to 30 nm from TEM, and this porosity is typically formed by the accumulation of carbon nanoparticles, which can be further confirmed by the SEM analysis (Fig. S-1). Without ZnCl2, the carbon products (e.g., C-400) are hardly porous (Fig. S-2). In our carbonization conditions, the ZnCl2 rapidly melted into liquid, which would easily penetrate into the inner structure of the waste biomass. In this circumstance, the decomposition and dehydration of carbohydrate polymers were initiated effectively, which then triggered the carbonization and aromatization with the formation of pores [38,39]. As shown in

75

Fig. 1e, however, the numbers of mesopores (15e30 nm) of the carbon after melamine modification (CZnrapid-400-Melamine1000) declined compared to CZnrapid-400. The interconnected graphitic walls (bright contrast) and pores/channels (dark contrast cores) can be seen from the HR-TEM image, which indicates the uniform micropores and amorphous nature. These phenomenons may be due to the fusion of the pristine fine carbon nanoparticles induced by a high temperature. It should be noted that melamine does not, as expected, carbonize throughout the process, but functions as a modification agent only. In fact, no carbon product can be obtained when heating melamine to 700  C in N2. Moreover, the C conversion efficiency of CZnrapid-400-Melamine-1000 (20.4%) is somewhat lower than that of CZnrapid-400-1000 (23.8%, Tables Se1), which further suggests that melamine does not involve in the carbonization but in the modification of the product. In the reaction, -NH2 groups in melamine would react with the oxygen groups on CZnrapid-400, which was also an important prerequisite to achieve effective N-doping. The carbon obtained at a different temperature appears to have a similar morphology as that shown in Fig. 1 (Fig. S-3). The rapid catalysis carbonization process can also achieve a high C conversion efficiency compared to other previously reported methods. As shown in Tables Se1, the C conversion efficiency of CZnrapid-400-Melamine-1000 reached as high as 41.9%. However, without using ZnCl2 as the catalyst for carbonization, the C conversion efficiency of Crapid-400-Melamine-1000 drops to 17.2%. By the typical conventional carbonization process [22,23,29], the C conversion efficiency of the obtained product was less than 31% for the banana peel. The results clearly verify the outstanding resource recovery capability of the new technology developed in the present study. Fig. 2a and b and Table 1 show the N2 adsorption-desorption

Fig. 4. LSV curves for (a) CZnrapid-400-Melamine-1000 and commercial Pt/C in O2-saturated KOH solution at a rotation rate of 1600 rpm, (b) various samples in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm, and (c) CZnrapid-400-Melamine-1000 at different rotation rates in O2-saturated 0.1 M KOH solution at a scan rate of 5 mV s1; and (d) KeL plot of CZnrapid-400-Melamine-1000 derived from (c).

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isotherm, pore size distribution and corresponding information of CZnrapid-400-Melamine-1000. It displays type-II isotherms with a type H3 hysteresis loop in the IUPAC classification. This type of isotherm indicates the formation of slit-shaped pores in the samples, which is in line with the nanoparticles-accumulated & fusedstructures observed with the SEM and TEM. The porosity, based on the Barrett-Joyner-Halenda (BJH) method, verifies a microporous structure (Fig. 2b), which is in line with the results of TEM analysis above. The high micropore content typically may be the main reason for the higher surface area of CZnrapid-400-Melamine-1000 (Fig. S-4). The N2 adsorption-desorption isotherm and pore size distribution of CZnrapid-400-Melamine-900/1100 are given in Fig. S4, which both exhibit similar physiochemical properties to CZnrapid400-Melamine-1000. Generally speaking, the porosity analysis further proves the effectiveness of the new carbonization method in producing highly microporous carbon.

XPS detection results of CZnrapid-400 and CZnrapid-400Melamine-900/1000/1100 are provided in Fig. 3 and Table 1. The C1s high-resolution spectra of these four products show two dominant peaks at ~284.3 and ~284.8 eV, which correspond to the sp2 hybridized graphitized carbon (C1) and sp3 C-C carbon (C2), respectively. The two other weak peaks at ~285.7 and ~286.8 eV are attributable to C-O/C-N (C3) and C¼O (C4), respectively [40e43]. The content of C1 clearly increases with the increase in N-doing temperature, suggesting its improvement in conductivity. In the N1s high-resolution spectra, pyridinic (N1), amine/imine (N2), pyrrolic (N3), quaternary (N4), and pyridinic-N-oxide (N5) can be detected, which confirms that the N doping of the carbon materials, while N1 and N3 account for more than 65 at.% of the total N content in the products [44e48]. Though the amount of N in carbon decreased with the temperature increase, it still reached ~3 at.% even after the melamine modification at 1100  C. The samples were

Fig. 5. CV curves for the ORR at 50 mV s1 for CZnrapid-400-Melamine-1000 (a) and commercial Pt/C (20 wt%) catalyst (b) under different conditions. (c) Rotating ring-disk electrode voltammograms for the ORR of the CZnrapid-400-Melamine-1000, Crapid-400-Melamine-1000 and Pt/C in O2-saturated 0.1 M KOH solution at 1600 rpm, the disk current (Id) is shown on the lower half of the graph and the ring current (Ir) on the upper half; and (d) The percentage of peroxide (solid line) and the electron transfer number (n) (dashed line) of CZnrapid-400-Melamine-1000 (blue line), Crapid-400-Melamine-1000 (black line) and Pt/C (red line) at various potentials, based on the corresponding RRDE data in (c). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. The 1st and the 2000th CV curves of (a) CZnrapid-400-Melamine-1000 and (b) commercial Pt/C with a scan rate of 100 mV s1 in O2-saturated 0.1 M KOH solution.

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also characterized by the Raman spectra (Fig. S-5). For CZnrapid400-Melamine-900/1000/1100, there is a D band at ~1350 cm1 and a G band at ~1600 cm1, and a broad 2D peak at ~2700 cm1, suggesting good graphitization results for these carbon products [24]. Typically, the graphitization degree increases with the raise of temperature (Table 1). To evaluate the electrocatalytic activity of the N-doped porous carbon, linear sweep voltammetry (LSV) was conducted and commercial Pt/C was used as a benchmark material (Fig. 4). Fig. 4a shows that CZnrapid-400-Melamine-1000 exhibits a similar onset potential and reduction current for ORR as Pt/C, revealing a comparable electrocatalytic activity. This can contribute to the synergetic effect of the good conductivity, abundant micropores, and high N doping level of the carbon. For the carbon produced under various experimental conditions, including the use of ZnCl2 and different temperatures for carbonization and modification, CZnrapid-400-Melamine-1000 shows the optimum performance (Fig. 4b and Fig. S-6). In addition, the important effect of N doping on the catalytic activity is verified by comparing the CZnrapid-400Melamine-1000 with CZnrapid-400-1000 (Fig. 4b). The onset potential of CZnrapid-400-Melamine-1000 is 82 mV higher than that of CZnrapid-400-1000. The onset potential is defined as the potential at the current density of 0.5 mA cm2. The LSV curves in Fig. 4c reveal an obvious increase of current density at an increasing rotation rate, which can be ascribed to the shorter diffusion route. Fig. 4d is derived from Fig. 4c based on the Koutecky-Levich (K-L) equation, as below:

1 1 1 1 1 ¼ þ ¼ þ J JK JL Bu12 JK

(4)

B ¼ 0:2nFðD0 Þ3 v6 C0

(5)

2

1

where JK stands for the kinetic current density, JL is the limited diffusion current density, u represents the RDE rotation rate (rpm), F is the Faraday constant (96485 C mol1), D0 is the diffusion coefficient and the bulk O2 concentration is denoted as C0, y is the kinetic viscosity of the electrolyte, and B can be acquired from the slope of the fitting line. Based on the K-L equation, n (electron transfer number) can be calculated from the B value. The oxygen reduction process is mainly directed by a four-electron pathway (O2þ2H2Oþ4e ¼ 4OH), which is consistent with the analysis of K-L plots [49e51]. The CV curves tested in the N2- and O2-saturated KOH solution (Fig. 5a and b) clearly show the O2 reduction peak in the potential range. To further understand the ORR electron transfer route by CZnrapid-400-Melamine-1000, a rotating ring-disk electrode (RRDE) measurement was carried out and subsequently the production of HO 2 in oxygen reduction process was analyzed. The ring and disk currents were measured to obtain the HO 2 yield in the reaction (Fig. 5c). The recorded HO 2 percentages of CZnrapid-400Melamine-1000, Crapid-400-Melamine-1000 and Pt/C in 0.1 M KOH are below 20%, 50% and 30%, respectively, in the potential range of 0.55 to 0.2 V, corresponding to the respective electron transfer numbers of ~3.7, ~3 and ~3.5 (Fig. 5d). The values calculated from the K-L plot of CZnrapid-400-Melamine-1000 coincide well with the RRDE result, which confirms the four-electron transfer route by CZnrapid-400-Melamine-1000 for the ORR process. Fig. 6a shows that the CV curve of CZnrapid-400-Melamine-1000 in the first cycle overlaps perfectly with the 2000th cycle, demonstrating its excellent durability in alkaline solution. The stability of Pt/C in 0.1 M KOH was also evaluated to confirm the superiority of CZnrapid-400-Melamine-1000 for the potential application in fuel cells or metal-air batteries for long-term operation. In Fig. 6b, the

77

CV curves of Pt/C for the first cycle and the 2000th cycle are less consistent with each other, indicating that the durability of the commercial product is not better than that of CZnrapid-400Melamine-1000. The outstanding ORR performance of the N-doped porous carbon should be attributed to its intrinsic properties, for which proper nitrogen doping and high porosity are two important factors (Scheme 1). The N doping, particularly pyridinic and pyrrolic N, promotes the reactivity of the carbon material, while the microporous structures are beneficial for mass transport. More importantly, the high content of micropores provides a large specific surface area to accommodate the N active sites reacting with O2. In the present research, CZnrapid-400-Melamine-1000 possesses a high content of active N species and the highest content of micropores, and is therefore a better ORR catalyst than the other four samples prepared at different temperatures (Fig. 4b and Fig. S-6). Apart from these two property factors, the high graphitization degree of the carbon, as deduced from the XPS and Raman characterizations, is essential to the improvement in conductivity, which potentially promotes the performance, too. The generality of our carbonization technique was further verified with different biomass as the carbon sources. Orange peel and leaves were selected for the additional experiments. As illustrated in Fig. S-7, their corresponding carbon materials also exhibited excellent catalytic activities, which are close to those of commercial Pt/C. It should be noted that the production conditions of these carbon materials are identical to those for CZnrapid-400Melamine-1000. The test results demonstrate the high level of effectiveness of our new method, which can be widely applied to synthesize superior carbon catalysts using a variety of raw waste biomass. The application of CZnrapid-400-Melamine-1000 in the zinc/air battery was further evaluated in an electrochemical cell under air atmosphere, in which the sample CZnrapid-400-Melamine-1000 was used as an air electrode and Zn (99.99%) plate as anode in 6.0 M KOH. These measurements were all executed in a home-made model as described in literature [52]. In Fig. 7a, the CZnrapid-400Melamine-1000 exhibits a much better rate performance and higher power density compared with Pt/C due to its superior rate capability. The discharge curves in Fig. 7b reveal that the CZnrapid-

Scheme 1. Proposed ORR catalytic process by the N-doped microporous carbon.

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Fig. 7. (a) The average discharge voltages and power densities of zinc/air batteries with different catalysts (CZnrapid-400-Melamine-1000,Pt/C) at different current densities; (b)the discharge curves of zinc/air batteries with different catalysts (CZnrapid-400-Melamine-1000,Pt/C) at a current density of 50 mA cm2.

400-Melamine-1000 loaded air electrode demonstrate a higher discharge voltage and more stable discharge platform over Pt/C at a current density of 50 mA cm2. Therefore, the typical CZnrapid-400Melamine-1000 can be an alternative to commercial Pt/C in the practical zinc/air battery application. 4. Conclusions N-doped microporous carbon derived from waste biomass is successfully synthesized by fast catalysis carbonization at 400  C and melamine modification at 1000  C, which possesses a high surface area (>1000 m2 g1), abundant micropores, and relatively high N content. The whole process of fast catalysis carbonization can be shortened to less than 30 min and the operation becomes semi-continuous, which would allow for large-scale production. The C conversion efficiency can reach as high as 41.9%, achieving effective resources recovery. The N-doped porous carbon exhibits an outstanding ORR activity in an alkaline medium compared to commercial Pt/C. The high surface area, well-developed micropores, and high content of pyridinic and pyrrolic N together contribute to the excellent ORR activity of the carbon product. Moreover, the N-doped porous carbon can perform as a highly efficient catalyst for Zn-air batteries and shows an excellent discharge performance over Pt/C. Promising metal-free ORR catalysts can also be produced from other typical waste biomass (e.g., orange peel, leaves). The research presents an effective, low-cost, and generally scalable approach for fabricating waste biomassderived carbon, with a microporous structure, heteroatom doping, and high activities. Such a development will be essential to the applications of porous carbon in the areas of energy conversion and storage. Acknowledgement We acknowledge the funding support from The Research Grants Council of Hong Kong: Collaborative Research Fund (C7044-14G) and Theme-based Research Scheme (T21-711/16R). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2017.05.056. References [1] D. Larcher, J.M. Tarascon, Nat. Chem. 7 (2015) 19e29. [2] X. Luo, J. Wang, M. Dooner, J. Clarke, Appl. Energy 137 (2015) 511e536. [3] G. Ren, G. Ma, N. Cong, Renew. Sustain. Energy Rev. 41 (2015) 225e236.

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