Hemp derived N-doped highly porous carbon containing Co nanoparticles as electrocatalyst for oxygen reduction reaction

Journal of Colloid and Interface Science 559 (2020) 21–28

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Hemp derived N-doped highly porous carbon containing Co nanoparticles as electrocatalyst for oxygen reduction reaction Chao Zhang, Jinhe Shu, Shuxian Shi, Jun Nie, Guiping Ma ⇑ State Key Laboratory of Chemical Resource Engineering, Beijing Laboratory of Biomedical Materials, Beijing University of Chemical Technology, Beijing 100029, PR China

g r a p h i c a l a b s t r a c t Scheme 1. Schematic illustration of the synthesis route of Co/NHPC-X.

a r t i c l e

i n f o

Article history: Received 25 July 2019 Revised 12 September 2019 Accepted 18 September 2019 Available online 24 September 2019 Keywords: Biomass Porous carbon Oxygen reduction reaction

a b s t r a c t Biomass derived porous carbon was wildly used in non-precious metal carbon based electrocatalysts for ORR due to its low cost and sustainability. Here, we develop a facile route to prepare Co/N doped hemp derived highly porous carbon (Co/NHPC) as ORR electrocatalyst. The prepared Co/NHPC-90 possess 3D hierarchically porous nanostructure with high specific surface areas (1251 m2 g
1) and large pore volumes (0.99 cm3 g
1) due to the chemical activation of NaHCO3, which is benefit for the mass/electron transfer and exposure of active sites. In addition, melamine and cobalt nitrate were selected as nitrogen and metal source respectively to enrich the density of active sites. Thus, Co/NHPC-90 exhibits excellent ORR electrocatalytic performance with high half-wave potential (0.826 V), superior catalytic stability and tolerance to methanol. Ó 2019 Elsevier Inc. All rights reserved.

1. Introduction

⇑ Corresponding author. E-mail address: [email protected] (G. Ma). https://doi.org/10.1016/j.jcis.2019.09.064 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.

In order to solve the energy and environmental crisis facing the current society, new generation energy such as fuel cells and metalair batteries have received widespread attention [1]. For fuel cells,

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the sluggish kinetics of the oxygen reduction reaction (ORR) of the cathode has been hindering its development, to explore low cost ORR catalysts with excellent electrocatalytic performance is highly desired [2,3]. Carbon nanomaterials have been extensively studied for their excellent electrical conductivity, stability, and large specific surface area [4]. Although emerging carbon nanotubes [5,6] and graphenes [7,8] have also been used in the field of electrocatalysis, it is difficult to obtain scale applications due to their high cost. Considering the environmental issues and cost, the synthesis of these materials should be relatively simple and energy-efficient. Biomass materials, which are natural, abundant and renewable, are receiving increasing attention as an alternative [9]. In recent years, the conversion of biomass to porous carbon nanomaterials has become an economic strategy for the preparation of carbon nanomaterials [10]. Biomass derived carbon are also considered to have the potential to replace traditional precious metal catalysts. For the preparation methods of biomass derived carbon materials, there are mainly hydrothermal methods [11], chemical/physical activation [12,13] and soft/hard template methods [14], et al. To produce carbon nanomaterials with high specific surface area and high electrocatalytic activity, it is necessary to select a suitable treatment method for different materials. Besides, precursor containing nitrogen or heteroatom is usually selected as precursor, sometimes introduction of transition metal is essential improve catalytic performance. Chemical activation generally involves combining biomass materials with activators such as potassium hydroxide (KOH), zinc chloride (ZnCl2), sodium bicarbonate (NaHCO3), and subsequently pyrolysis at temperature above 300 °C [15–17]. The biomass derived porous carbon was prepared by the leaching effect of gas released by activators or the reaction between biomass and activators [18]. Ammonia treatment has also been reported to introduce nitrogen and form porous structure by etching carbon [19]. During the ammoxidation process, a large number of nitrogen-containing free radicals generate and attack carbon fragments, replace oxygen-containing substances and form nitrogen-containing functional groups, which can act as active sites to catalyze ORR [20]. In this article, hemp was selected as the precursor, sodium bicarbonate as the activator and melamine as the external nitrogen source, one-step carbonization was used to obtain nitrogen-doped porous activated carbon firstly, and then the Co/N doped porous hemp derived carbon can be obtained by adsorbing the transition metal and secondary carbonization. The prepared porous carbon possess large specific surface area and rich active sites, exhibits excellent catalytic activity for oxygen reduction reaction, with good stability and methanol resistance. 2. Experimental 2.1. Materials Hemp power were collected from Shandong province of China. Sodium bicarbonate (99%, NaHCO3), cobalt nitrate hexahydrate (99%, Co(NO3)26H2O), melamine (99%, AR), potassium hydroxide (98%, KOH), methanol (99.5%) and ethanol (99.7%) were purchased from Aladdin Chemical Reagent Co. Nafion solution (5 wt %) was bought from DuPont Co. Common commercial 20% Pt/C catalyst was obtained from Johnson Matthey Co. All chemicals were used without any further purification. The deionized water (18.25 MX) was obtained from a Ulupure System.

boat and pyrolysis at 800 °C under nitrogen atmosphere for 1 h. And then the obtained black power was washed with deionized water until the filtrate is neutral. After drying in a vacuum at 80 °C for 12 h, N-doped hemp derived porous carbon (NHPC) was prepared. For comparison, the product prepared by the mixture of hemp power and NaHCO3 was noted as hemp derived porous carbon (HPC). 2.3. Synthesis of Co/N doped hemp derived porous carbon (Co/NHPC) 90 mg Co(NO3)26H2O was dissolved in 10 mL ethanol firstly and 100 mg NHPC was added into the solution under ultrasonic followed by violently stirring for 1 h. The obtained homogenous black slurry was dried at 60 °C for 24 h to form solid power. The solid power was then carbonized at 800 °C for 1 h under nitrogen atmosphere. The final product was named Co/NHPC-90. For comparison, 30 mg, 60 mg and 120 mg Co(NO3)26H2O were used in precursors to prepare Co/NHPC-30, Co/NHPC-60 and Co/NHPC-120. 2.4. Materials characterizations SEM (S-4700, Hitachi) and TEM (Tecnai G2 T20, FEI) were used to investigate the morphology of the prepared porous carbon. The STEM element mapping was collected using a scanning tunneling electron microscope (Tecnai G2 F20, FEI). XRD patterns was obtained using a powder X-ray diffractometer (D8 Advance, Bruker) with Cu Ka radiation. Raman spectra was investigated by Raman spectrometer (Invia Reflex, Renishaw) with a laser device at a wavelength of 514 nm to measure the graphitic degree of porous carbon. Nitrogen adsorption-desorption method (3H2000PM2, Beishide Instrument) was carried out to analyze the Brunauer–Emmett–Teller (BET) specific surface area and pore size distribution. The surface chemical composition/state of the samples was analyzed using a X-ray photoelectron spectroscopy (ESCALAB 250, Thermo VG Scientific). 2.5. Electrochemical measurements The electrochemical performance was tested with a threeelectrode system using CHI 760E electrochemical workstation. A typically three-electrode system is composed of a glassy-carbon (GC) electrode loaded catalyst ink as the work electrode, a platinum wire as the counter electrode and the Ag/AgCl electrode as the reference electrode. To obtain the catalyst ink, 5 mg of the catalyst was added into 1000 lL ethanol containing 100 lL 5 wt% Nafion solution, sonicated for at least 30 min to make the homogenous solution. The electrode was polished carefully with alumina slurry and washed with the deionized water before used. Then 5 lL of the prepared catalyst ink was dropped onto the polished GC electrode and dried under ambient conditions. Cyclic voltammetry curves for the ORR were carried out in O2saturated 0.1 M KOH aqueous solution at a scan rate of 50 mV s
1 in the potential range of 0.1 V to
0.8 V. Liner sweep voltammograms were measured at a scan rate of 10 mV s
1 with rotation rate of 1600 rpm. Rotating disk electrode (RDE) measurements were carried out from 0.1 V to
0.8 V at a scan rate of 10 mV s
1 with different rotation rates of 625 rpm to 2500 rpm. Koutecky–Levich (K-L) plots were plotted by linear fitting of the reciprocal rotating speed versus reciprocal current density. The electron transfer number (n) and the kinetic current density (J k ) were calculated based on the following Koutecky-Levich equation:

2.2. Synthesis of N-doped hemp derived porous carbon (NHPC)

1 1 1 1 1 ¼ þ ¼ þ J J k J L J k Bx0:5

Hemp power was mixed with NaHCO3 and melamine with a mass ratio of 1:3:1. The mixed powder was transferred into sample

B ¼ 0:2nFD02=3 v
1=6 C 0

C. Zhang et al. / Journal of Colloid and Interface Science 559 (2020) 21–28

where J is the measured current density, J L is the diffusionlimiting current density, x is the rotation rate. B can be determined from the slope of the K-L plot, F is the Faraday constant (96,485 C mol
1),D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9  10
5 cm2 s
1), m is the kinetic viscosity of the electrolyte (0.011 cm2 s
1), andC 0 is the bulk concentration of O2 (1.2  10
6 mol cm
3). The stability test and methanol tolerance test were performed by a i–t chronoamperometric measurement in O2saturated 0.1 M KOH solution at a fixed potential of
0.40 V (vs. Ag/AgCl) and the response current recorded against time at a rotation rate of 1600 rpm. In this study, E (vs. RHE) = E (vs. Ag/AgCl) + 0.197 V + 0.0591  pH. The onset potential was calculated according to the potential of
0.1 mA cm
2. 3. Results and discussion The typical synthesis route of the Co/N doped hemp derived porous carbon is depicted in Scheme 1. Firstly, hemp powder was mixed with NaHCO3 and melamine with subsequently pyrolysis at 800 °C under N2 atmosphere, after washed with deionized water, N-doped hemp derived porous carbon (NHPC) was obtained. And then, NHPC and Co(NO3)26H2O was dispersed in ethanol under ultrasonication to form homogenous black slurry, after drying at 60 °C to form solid power, the obtained power was then carbonized at 800 °C under nitrogen atmosphere to prepare the final product. The obtained product was named Co/N doped hemp derived porous carbon (Co/NHPC). As exhibited in Fig. 1(a) and (b), hemp power possess a smooth layered structure and no pores on the surface. As shown in Fig. 1(c), HPC showed 3D frameworks with hierarchical pores due to the leaching effect of H2O and CO2 produced by the decomposition of NaHCO3 to carbon above 300 °C. After the process of ‘‘adsorptionpyrolysis”, Co nanoparticles was loaded in the hemp-derived carbon and encapsulated in pores. No metal particles can be clearly seen in the SEM images of Co/NHPC (Fig. 1(d) and Fig. S1) and the porous structure was well retained. The corresponding TEM images (Fig. 2(a)–(d)) revealed the Co nanoparticles were uniformly distributed in the carbon matrix. With the mass of metal increasing, metal nanoparticles tend to aggregation and lead to the improve of the diameter of metal nanoparticles. As can be seen in Fig. 2(f), the lattice spacing of 0.203 nm can be distributed to Co (1 1 1), indicating the metal

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nanoparticles can be metallic Co. In addition, the metal particles were wrapped by onion-like graphitic carbon layers, which can be protect metal nanoparticles from corrosion and aggregation in the process of electrocatalysis. STEM image and the corresponding elemental mapping images were shown in Fig. 2(g), revealing the evenly distribution of C, O, N and Co. The crystalline structure of Co/NHPC was detected by XRD. As can be seen in Fig. 3(a), the peak around 26° belonging to the (0 0 2) planes of graphitic carbon, the three peaks at 44.2°,51.5°and 75.9° were associated with (1 1 1), (2 0 0) and (2 2 0) planes of Co-a (JCPDS no. 89-7093) reapectively. With the increasing of the metal mass, the peaks tend to be sharper and stronger, revealing the increase of crystalline [21], which is consistent with the TEM images. Raman spectra was used to evaluate the graphitic degree of Co/NHPC, the peaks at 1350 cm
1 and 1580 cm
1 were attributed to the disorder carbon structure and the in-plane vibration of the sp2 carbon atoms in the graphited crystallites [22], which were noted as D band and G band. The intensity ratio of D band (ID) and G band (IG) was the indication of graphitic degree of samples. As shown in Fig. 3(b), the value of ID/IG was decrease with the increasing of the mass of metal, which means the increase of graphitic degree [23]. The specific surface area and pore structure of samples were quantified by nitrogen adsorption/desorption curve Co/NHPC-90 presents a combination curve of type Ⅰ and type Ⅳ isotherm with a type-H4 hysteresis loop (Fig. 3(c)). Compared with NHPC-1, the BET specific surface area of Co/NHPC-90 decreased from 1571 m2 g
1 to 1251 m2 g
1, which is mainly due to the blockage of the holes by the Co nanoparticles. According to the results of T-plot calculation, the micropore surface area (Smicro) and mesopore surface area (Smeso) of NHPC-1 are 1369 m2 g
1 and 202 m2 g
1, respectively. As shown in Table S1, the Smicro of Co/NHPC-90 (1079 m2 g
1) dramatically decrease, on the contrary, the Smeso of Co/NHPC-90 (172 m2 g
1) was almost maintained. For electrocatalytic reactions, mesopores play a key role in mass transfer. BJH equation and H-K equation was applied to measure the pore diameter distribution of mesopore and micropore, respectively. Fig. 3(d) shows the pore diameter distribution of the Co/NHPC-90, the micropore peaked at 0.64 nm and 0.78 nm, mesopore diameter was mianly below 5 nm. Besides, the total pore volume of Co/NHPC-90 is 0.99 cm3 g
1. With high specific surface area and pore volume, Co/NHPC-90 may exhibit an excellent ORR electrocatalytic performance.

Scheme 1. Schematic illustration of the synthesis route of Co/NHPC-X.

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Fig. 1. (a) and (b) SEM images of hemp power at different magnifications; (c) SEM image of HPC; (d) SEM image of Co/NHPC-90.

Fig. 2. TEM images: (a) Co/NHPC-30, (b) Co/NHPC-60, (c) Co/NHPC-90 and (d) Co/NHPC-120; HRTEM images of Co/NHPC-90: (e) and (f); Elemental mapping: (g) Co/NHPC-90.

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Fig. 3. (a) XRD patterns of Co/NHPC-30, Co/NHPC-60, Co/NHPC-90 and Co/NHPC-120; (b) Raman spectra of Co/NHPC-30, Co/NHPC-60, Co/NHPC-90 and Co/NHPC-120; (c) Nitrogen adsorption/desorption curve of Co/NHPC-90; (d) pore diameter distribution curves of Co/NHPC-90.

XPS analysis was used to detect elemental contents and chemical states of the prepared carbon materials. As shown in Fig. 4(a), C, N, O and Co were co-exist in Co/NHPC samples, which is consistent with the results of element mapping (Fig. 2(g)). Table S2 shows the element contents of samples, NHPC-1 and Co/NHPC exhibit relatively high nitrogen content compared with HPC, indicating the vital role of melamine in doping nitrogen. Besides, with the addition of Co(NO3)26H2O, Co was detected in Co/NHPC. As shown in Fig. 4(b), the high resolution C1s spectrum of Co/NHPC90 can be divided into three peaks. The peaks at 284.9 eV, 285.7 eV and 289.3 eV were assigned to CAC, CAO/CAN and C@O, revealing the compound structure of hemp-derived porous carbon [24]. The fitted high resolution N1s spectrum of Co/NHPC were deconvoluted into four peaks (Figs. 4(c) and S2), the peaks at ~398.4 eV can be attribution to pyridinic-N, the peaks at ~400.2 eV belongs to pyrrolic-N, the peaks at ~401.1 eV corresponds to graphitic-N, the peaks at ~402.2 eV were assigned to oxidized-N [25,26]. Among these four types of nitrogen elements, pyridinic-N and graphitic-N are considered to have catalytic activity for oxygen reduction [27], and these two nitrogen play an important role in promoting oxygen adsorption and four-electron reduction [28,29]. Due to the higher electronegativity of the nitrogen atom, the charge distribution of the nitrogen-doped carbon atom changes, and thus the carbon atom bonded to the nitrogen atom is positively charged, resulting in enhanced adsorption with oxygen molecules [30]. The Co2p spectrum displayed two main peaks around 780 eV and 795 eV (Fig. 4(d)). The peaks at 784.9 eV and 794 eV can be ascribed to metallic Co, which is agreement of the XRD patterns (Fig. 3(a)). The peaks near 782 eV and 797 eV correspond to the 2p3/2 and 2p1/2 peaks of multivalent

cobalt. The formation of multivalent cobalt is mainly due to the metallic cobalt is sensitive to air oxidation [31]. The ORR electrocatalytic performance test was carried out in O2-saturated 0.1 M KOH solution. Fig. 5(a) shows the CV curves of Co/NHPC samples. The four curves show obvious reduction peaks around 0.8 V, and the peak potential of Co/NHPC-90 is the most positive (0.814 V). To better evaluate the performance of the electrocatalyst, we tested the LSV curves at 1600 rpm in an alkaline electrolyte using a rotating disk electrode loaded with catalyst. As shown in Fig. 5(b), Co/NHPC-90 showed the best catalytic activity, and its half-wave potential (0.826 V) was just slightly lower than 20% Pt/C catalyst (0.829 V). And its diffusion-limiting current density (JL) is higher than 20% Pt/C catalyst. In order to understand the kinetic characteristics of the catalyst, we tested the LSV curve of the catalyst at different rotation speeds and processed the curve platform area to obtain the K-L lines. As shown in Fig. 5(c), as the electrode speed increases, the diffusion current density of the catalyst increases uniformly, and the K-L lines are substantially coincident, indicating that the diffusion current is related to the dissolved oxygen concentration and conforms to the first-order reaction. We selected the kinetic control zone data of the LSV curves of Co/NHPC-90 to obtain the Tafel curves. The lower the slope, the higher the efficiency. As shown in Fig. 5(d), the Tafel slope of Co/NHPC-90 (63 mV dec
1) is close to the theoretical value of 60 mV dec
1 of 20% Pt/C catalyst [32], indicating the similar kinetic characteristics. Rapid material/charge transfer in the ORR electrocatalytic process is also related to the porous structure of the catalyst. To further evaluate the performance of the Co/NHPC-90 catalyst, we tested the long-term stability and methanol tolerance of

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Fig. 4. (a) XPS spectra of Co/NHPC-30, Co/NHPC-60, Co/NHPC-90 and Co/NHPC-120; High resolution spectra of (b) C1s, (c) N1s and (d) Co2p.

Fig. 5. (a) CV curves and (b) LSV curves of Co/NHPC-30, Co/NHPC-60, Co/NHPC-90 and Co/NHPC-120; (c) LSV curves at different rotation speed and the corresponding K-L curves of Co/NHPC-90; (d) Tafel curves of Co/NHPC-30, Co/NHPC-60, Co/NHPC-90 and Co/NHPC-120.

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Fig. 6. (a) Cyclic stability test and (b) Methanol tolerance test of Co/NHPC-90.

the catalyst using a chronoamperometry (i-t) method. As shown in Fig. 6(a), after 18,000 s, the current density retention of Co/NHPC90 is reduced to 80.77%, which is better than that of 20% Pt/C (70.88%), indicating excellent long-term stability of electrocatalysts. Which can be attributed to the protection of carbon layers to metal nanoparticles. In the methanol tolerance test, after the addition of methanol at 200 s, the current density of the 20% Pt/C catalyst was significantly reduced, indicating the weak tolerance to methanol. However, the current density of Co/NHPC-90 quickly returned to normal after a slight fluctuation, indicating its excellent methanol tolerance. 4. Conclusions In summary, hemp derived N-doped highly porous carbon containing Co nanoparticles was synthesized by a simple and efficient method. The prepared Co/NHPC-90 possess 3D hierarchically porous nanostructure with high specific surface areas (1251 m2 g
1) and large pore volumes (0.99 cm3 g
1), besides, nitrogen species and Co nanoparticles were uniformly distributed in the porous carbon. Which is benefit for the mass/electron transfer and exposure of active sites. Thus, Co/NHPC-90 exhibits excellent ORR electrocatalytic performance with high half-wave potential (0.826 V), superior catalytic stability and tolerance to methanol. Acknowledgment The authors would like to thank the Six talent peaks project in Jiangsu Province (Grant No. XCL-167) for their financial support. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.09.064. References [1] M. Khalid, A.M.B. Honorato, H. Varela, L. Dai, Multifunctional electrocatalysts derived from conducting polymer and metal organic framework complexes, Nano Energy 45 (2018) 127–135. [2] Y. Wang, L. Tao, Z. Xiao, R. Chen, Z. Jiang, S. Wang, 3D carbon electrocatalysts in situ constructed by defect-rich nanosheets and polyhedrons from NaClsealed zeolitic imidazolate frameworks, Adv. Funct. Mater. 28 (11) (2018) 1705356, https://doi.org/10.1002/adfm.v28.1110.1002/adfm.201705356. [3] Q. Liu, Y. Duan, Q. Zhao, F. Pan, B. Zhang, J. Zhang, Direct synthesis of nitrogendoped carbon nanosheets with high surface area and excellent oxygen reduction performance, Langmuir 30 (27) (2014) 8238–8245. [4] J. Deng, M. Li, Y. Wang, Biomass-derived carbon: synthesis and applications in energy storage and conversion, Green Chem. 18 (18) (2016) 4824–4854.

[5] W. Fan, Z. Li, C. You, X. Zong, X. Tian, S. Miao, T. Shu, C. Li, S. Liao, Binary Fe, Cudoped bamboo-like carbon nanotubes as efficient catalyst for the oxygen reduction reaction, Nano Energy 37 (2017) 187–194. [6] S. Liu, I.S. Amiinu, X. Liu, J. Zhang, M. Bao, T. Meng, S. Mu, Carbon nanotubes intercalated Co/N-doped porous carbon nanosheets as efficient electrocatalyst for oxygen reduction reaction and zinc–air batteries, Chem. Eng. J. 342 (2018) 163–170. [7] Y. Tong, P. Chen, T. Zhou, K. Xu, W. Chu, C. Wu, Y. Xie, A Bifunctional hybrid electrocatalyst for oxygen reduction and evolution: cobalt oxide nanoparticles strongly coupled to B, N-decorated graphene, Angew. Chem. Int. Ed. Engl. 56 (25) (2017) 7121–7125. [8] X.X. Liu, J.B. Zang, L. Chen, L.B. Chen, X. Chen, P. Wu, S.Y. Zhou, Y.H. Wang, A microwave-assisted synthesis of CoO@Co core–shell structures coupled with N-doped reduced graphene oxide used as a superior multi-functional electrocatalyst for hydrogen evolution, oxygen reduction and oxygen evolution reactions, J. Mater. Chem. A 5 (12) (2017) 5865–5872. [9] B. Li, D. Geng, X.S. Lee, X. Ge, J. Chai, Z. Wang, J. Zhang, Z. Liu, T.S.A. Hor, Y. Zong, Eggplant-derived microporous carbon sheets: towards mass production of efficient bifunctional oxygen electrocatalysts at low cost for rechargeable Zn– air batteries, Chem. Commun. 51 (42) (2015) 8841–8844. [10] M. Borghei, J. Lehtonen, L. Liu, O.J. Rojas, Advanced biomass-derived electrocatalysts for the oxygen reduction reaction, Adv. Mater. 30 (24) (2018) e1703691. [11] M.-M. Titirici, R.J. White, C. Falco, M. Sevilla, Black perspectives for a green future: hydrothermal carbons for environment protection and energy storage, Energy Environ. Sci. 5 (5) (2012) 6796. [12] J. Mi, X.-R. Wang, R.-J. Fan, W.-H. Qu, W.-C. Li, Coconut-shell-based porous carbons with a tunable micro/mesopore ratio for high-performance supercapacitors, Energy Fuels 26 (8) (2012) 5321–5329. [13] R.G. Pereira, C.M. Veloso, N.M. da Silva, L.F. de Sousa, R.C.F. Bonomo, A.O. de Souza, M.O.D.G. Souza, R.D.C.I. Fontan, Preparation of activated carbons from cocoa shells and siriguela seeds using H3PO4 and ZnCL2 as activating agents for BSA and a-lactalbumin adsorption, Fuel Proces. Technol. 126 (2014) 476– 486. [14] X. Liu, I.S. Amiinu, S. Liu, K. Cheng, S. Mu, Transition metal/nitrogen dualdoped mesoporous graphene-like carbon nanosheets for the oxygen reduction and evolution reactions, Nanoscale 8 (27) (2016) 13311–13320. [15] J. Deng, T. Xiong, F. Xu, M. Li, C. Han, Y. Gong, H. Wang, Y. Wang, Inspired by bread leavening: one-pot synthesis of hierarchically porous carbon for supercapacitors, Green Chem. 17 (7) (2015) 4053–4060. [16] S. Gao, H. Fan, S. Zhang, Nitrogen-enriched carbon from bamboo fungus with superior oxygen reduction reaction activity, J. Mater. Chem. A 2 (43) (2014) 18263–18270. [17] R. Wang, K. Wang, Z. Wang, H. Song, H. Wang, S. Ji, Pig bones derived N-doped carbon with multi-level pores as electrocatalyst for oxygen reduction, J. Power Sources 297 (2015) 295–301. [18] R. Ma, G. Lin, Y. Zhou, Q. Liu, T. Zhang, G. Shan, M. Yang, J. Wang, A review of oxygen reduction mechanisms for metal-free carbon-based electrocatalysts, npj Computat. Mater. 5 (1) (2019). [19] W. Shen, W. Fan, Nitrogen-containing porous carbons: synthesis and application, J. Mater. Chem. A 1 (4) (2013) 999–1013. [20] C. Zhu, M. Takata, Y. Aoki, H. Habazaki, Nitrogen-doped porous carbon asmediated by a facile solution combustion synthesis for supercapacitor and oxygen reduction electrocatalyst, Chem. Eng. J. 350 (2018) 278–289. [21] H. Jiang, Y. Liu, W. Li, J. Li, Co Nanoparticles confined in 3D nitrogen-doped porous carbon foams as bifunctional electrocatalysts for long-life rechargeable Zn-Air batteries, Small 14 (13) (2018) e1703739. [22] M. Zhang, X. Jin, L. Wang, M. Sun, Y. Tang, Y. Chen, Y. Sun, X. Yang, P. Wan, Improving biomass-derived carbon by activation with nitrogen and cobalt for supercapacitors and oxygen reduction reaction, Appl. Surf. Sci. 411 (2017) 251–260.

28

C. Zhang et al. / Journal of Colloid and Interface Science 559 (2020) 21–28

[23] G. Lin, R. Ma, Y. Zhou, C. Hu, M. Yang, Q. Liu, S. Kaskel, J. Wang, Threedimensional interconnected nitrogen-doped mesoporous carbons as active electrode materials for application in electrocatalytic oxygen reduction and supercapacitors, J. Colloid Interf. Sci. 527 (2018) 230–240. [24] W. Sun, S.M. Lipka, C. Swartz, D. Williams, F. Yang, Hemp-derived activated carbons for supercapacitors, Carbon 103 (2016) 181–192. [25] T. Zhou, R. Ma, Y. Zhou, R. Xing, Q. Liu, Y. Zhu, J. Wang, Efficient N-doping of hollow core-mesoporous shelled carbon spheres via hydrothermal treatment in ammonia solution for the electrocatalytic oxygen reduction reaction, Micropor. Mesopor. Mater. 261 (2018) 88–97. [26] R. Xing, T. Zhou, Y. Zhou, R. Ma, Q. Liu, J. Luo, J. Wang, Creation of triple hierarchical micro-meso-macroporous N-doped carbon shells with hollow cores toward the electrocatalytic oxygen reduction reaction, Nano-Micro Letters 10 (1) (2017). [27] R. Ma, R. Xing, G. Lin, Y. Zhou, Q. Liu, M. Yang, C. Hu, K. Yan, J. Wang, Graphenewrapped nitrogen-doped hollow carbon spheres for high-activity oxygen electroreduction, Mater. Chem. Front 2 (2018) 1489–1497. [28] M. Wu, K. Wang, M. Yi, Y. Tong, Y. Wang, S. Song, A facile activation strategy for an MOF-derived metal-free oxygen reduction reaction catalyst: direct access

[29]

[30]

[31]

[32]

to optimized pore structure and nitrogen species, ACS Catal. 7 (9) (2017) 6082–6088. J. Masa, W. Xia, M. Muhler, W. Schuhmann, On the role of metals in nitrogendoped carbon electrocatalysts for oxygen reduction, Angew. Chem. Int. Ed. Engl. 54 (35) (2015) 10102–10120. Q. Niu, J. Guo, Y. Tang, X. Guo, J. Nie, G. Ma, Sandwich-type bimetal-organic frameworks/graphene oxide derived porous nanosheets doped Fe/Co-N active sites for oxygen reduction reaction, Electrochim. Acta 255 (2017) 72–82. W. Zhao, P. Yuan, X. She, Y. Xia, S. Komarneni, K. Xi, Y. Che, X. Yao, D. Yang, Sustainable seaweed-based one-dimensional (1D) nanofibers as highperformance electrocatalysts for fuel cells, J. Mater. Chem. A 3 (27) (2015) 14188–14194. D. Ji, S. Peng, J. Lu, L. Li, S. Yang, G. Yang, X. Qin, M. Srinivasan, S. Ramakrishna, Design and synthesis of porous channel-rich carbon nanofibers for selfstanding oxygen reduction reaction and hydrogen evolution reaction bifunctional catalysts in alkaline medium, J. Mater. Chem. A 5 (16) (2017) 7507–7515.