From Chlorella to Nestlike Framework Constructed ... - ACS Publications

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From Chlorella to Nestlike Framework Constructed with Doped Carbon Nanotubes: A Biomass-Derived, High-Performance, Bifunctional Oxygen Reduction/Evolution Catalyst Guanghua Wang, Yijie Deng, Jinnan Yu, Long Zheng, Li Du, Huiyu Song,* and Shijun Liao* The Key Laboratory of Fuel Cell Technology of Guangdong Province, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China S Supporting Information *

ABSTRACT: The development of effective bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is significant for energy conversion systems, such as Li−air batteries, fuel cells, and water splitting technologies. Herein, a Chlorella-derived catalyst with a nestlike framework, composed of bamboolike nanotubes that encapsulate cobalt nanoparticles, has been prepared through a facile pyrolysis process. It achieves perfect bifunctional catalysis both in ORR and OER on a single catalyst. For our optimal catalyst Co/M-Chlorella-900, its ORR half-wave potential is positively shifted by 40 mV compared to that of a commercial Pt/C catalyst, and the overpotential at 10 mA cm−2 for the OER is 23 mV lower than that of a commercial IrO2/C catalyst in an alkaline medium. This superior bifunctional catalytic performance is benefited from the simultaneous increase of pyridinic N sites for ORR and graphitic N sites for OER. In addition, N-doped carbon-encapsulated Co nanoparticles improve both ORR and OER performance by forming new active centers. The unique nestlike carbon nanotube framework not only afforded highly dense ORR and OER active sites but also promoted the electron and mass transfer. Our catalyst also displays notable durability during the ORR and OER, making it promising for use in ORR/OER-related energy conversion systems. KEYWORDS: biomass, activity sites, nitrogen-doped carbon nanotubes, oxygen reduction reaction, oxygen evolution reaction

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INTRODUCTION The oxygen electrode reaction in various electrochemical energy conversion devices, for instance, fuel cells and metal− air batteries, and in electrolysis of water is limited by its poor oxygen reduction and/or oxygen evolution due to the complex four-electron process and sluggish kinetics of the ORR and OER.1−3 Currently, noble metals (e.g., Pt)4−6 and metal oxides (e.g., RuO2 and IrO2)7−10 are effective electrocatalysts for the ORR and OER, respectively. However, noble-metal-based catalysts have the drawbacks of prohibitive cost, scarcity, and poor stability, all of which have hampered the development and large-scale commercialization of these devices. Thus, substantial research efforts have been dedicated to exploring novel and effective bifunctional ORR/OER catalysts based on earthabundant materials as substitutes for noble metals.11−16 Carbon materials doped with heteroatoms (e.g., N, S, P, and B) have attracted major interest because of their efficient ORR/ OER activity, stability, and low cost.17−22 Both theoretical calculations and experiments have demonstrated that nitrogen dopants can effectively modulate electrical properties and catalytic activities.14,23,24 So far, the nature of ORR or OER active sites for N-doped carbon catalysts is still controversial. One opinion proposed that pyridinic N sites are responsible for © 2017 American Chemical Society

ORR and graphitic N sites act as active sites for OER, whereas others proposed that both pyridinic and graphitic N sites were not only the active centers for ORR but also responsible for OER.25−27 An ideal bifunctional catalyst should possess sufficient effective ORR and OER active sites simultaneously. Nevertheless, most of N-doped carbon materials show only single catalytic activity for ORR or OER because of their singletype active sites. Previous studies have proved that the introduction of transition metals (e.g., Fe, Co, and Ni) into doped carbon materials can further enhance their electrocatalytic activity because transition metals may either act as part of active sites or assist in the formation of active sites.22,28−33 Therefore, two common strategies have been proposed in the preparation of doped carbon catalysts for ORR or OER. One solution is to increase the amount of active sites by expanding the proportion of pyridinic or graphitic N.34−36 The other is to construct multiple types of active sites by the introduction of transition metals.37−40 Unfortunately, it is difficult to simultaneously improve the ORR and OER performance Received: July 21, 2017 Accepted: August 28, 2017 Published: August 28, 2017 32168

DOI: 10.1021/acsami.7b10668 ACS Appl. Mater. Interfaces 2017, 9, 32168−32178

Research Article

ACS Applied Materials & Interfaces Scheme 1. Schematic Illustration of the Preparation Process of Co/M-Chlorella-900

laboratory water purification system (Hi-tech, Master-S15) in all experiments. All chemicals were directly used as received without further treatment. Material Preparation. The preparation process of the doped carbon catalyst is described in Scheme 1. The procedure involved the impregnation of Chlorella with melamine and cobaltous acetate solution and pyrolysis in inert atmosphere under high temperature. In a typical synthesis, 25 mg (0.2 mmol) of C3H6N6 and 12.5 mg (0.05 mmol) of C4H6O4·Co·4H2O were dissolved in 80 mL of deionized water and continually stirred for 8 h at 70 °C, yielding a solution of melamine and cobalt. Then, 30 mg of Chlorella was added into the solution, and the mixture was continually stirred for 2 h until it was homogeneously mixed. Next, the resultant mixture was filtered and dried at 80 °C for 24 h. Then, Chlorella impregnated with melamine and cobalt salt was pyrolyzed at 900 °C for 1 h in an Ar atmosphere. The obtained black powders were then leached in 1 M H2SO4 at 80 °C for 24 h. The resulting products were separated via centrifugation, washed with deionized water, and dried at 80 °C overnight. Finally, the acid-leached powders were heat-treated a second time at 900 °C to achieve further graphitization. We denote the obtained sample as Co/ M-Chlorella-900, with Chlorella, M, and Co representing the Chlorella, melamine, and cobalt included in the precursor, respectively, and 900 representing the pyrolysis temperature. For comparison, Chlorella-900, M-Chlorella-900, Co-Chlorella-900, and Co/M-900, representing the precursors containing only Chlorella, Chlorella and melamine, Chlorella and cobalt, and melamine and cobalt, respectively, were prepared using the same procedures as for Co/M-Chlorella-900. To study the influence of Co amount on ORR and/or OER, we also prepared other three samples with the addition of 0.01, 0.025, and 0.10 mmol C4H6O4·Co·4H2O in the precursors. Material Characterization. Merlin field emission scanning electron microscopy (SEM, Carl Zeiss), transmission electron microscopy (TEM, JEM-2100, operated at 200 kV), X-ray diffraction (XRD, TD-3500, Tongda, China), Raman spectra (LabRAM ARAMIS Raman spectrometer, HJY, France), X-ray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, Britain), and Brunauer−Emmett− Teller (BET) method on a TriStar II 3020 gas adsorption analyzer were used to characterize the morphology, structure, surface chemical state, specific surface areas, and pore size distributions of the samples. Electrochemical Testing. Electrochemical measurements were conducted on an Autolab electrochemical workstation (PGSTAT302N, Switzerland) with a standard three-electrode system in 0.1 M KOH solution at room temperature. A glassy carbon electrode (GCE, 5 mm diameter), a Hg/HgO electrode in 1.0 M KOH, and a Pt wire were used as the working electrode, the reference electrode, and the counter electrode, respectively. The potential of the Hg/HgO reference electrode in this paper was calibrated versus a reversible hydrogen electrode (RHE). Before every test, the GCE was polished with α-Al2O3 slurry (50 nm), then cleaned by ultrasonication in ethanol, and dried under an infrared lamp. Then, 5.0 mg of the catalyst was suspended in 1 mL of Nafion/ethanol solution (0.25 wt % Nafion) to form a homogeneous ink under ultrasonication. Then, 8 μL of the catalyst ink was dropped onto the GCE surface and then dried by an infrared lamp. The catalyst loading amount was approximately 0.2 mg cm−2.

through the increasing the percentage of pyridinic and graphitic N separately. Besides, added excess transition metals are easy to form transition-metal oxides which suffer from limited catalytic activities because of their chemical instability and low conductivity. In addition to intrinsic activity, building up plentiful cavity construction and larger surface area is also important to enable efficient mass and electron transfer during the ORR and OER process.41−43 Recently, it has been found that transition-metal nanoparticles encapsulated in N-doped carbon nanotubes (CNTs) can partly improve both ORR and OER catalytic activities.24,44,45 However, it is still confusing to distinguish the contribution of Nx−C, M−Nx (M = Fe, Co, and Ni), and transition-metal nanoparticles to ORR or OER performance in N-doped carbon materials. Moreover, the synthesis procedures are time-consuming and cumbersome, carbon precursors are usually environmentally unfriendly, and the specific surface area and N content are usually low. Thus, rational design and bottom-up synthesis of satisfactory ORR and OER bifunctional catalysts through creating multiple effective ORR and OER active sites simultaneously are still very challenging. Chlorella has been deemed as an ideal N and C source because it contains rich proteins, carbohydrates, lipids, and glucosamine. Furthermore, Chlorella owns hollow microsphere architectures with an interconnected porous structure in multiple sizes. We could take full advantage of the chemical composition and specialized structure of Chlorella to fabricate high-performance doped carbon catalysts. Herein, we wellexploited a facile strategy to fabricate nestlike materials composed of N-doped CNTs and cobalt nanoparticles encapsulated in the nanotubes by pyrolyzing a mixture of Chlorella biomass, melamine, and cobaltous acetate. The materials exhibited much better ORR and OER catalytic activity and stability simultaneously not only than most of the reported N-doped carbon-based electrocatalysts but also than the state-of-the-art noble-metal electrocatalysts (Pt/C and IrO2/C). The excellent ORR and OER performance can be attributed to the synchronous increase of the density of ORR (pyridinic N) and OER (graphitic N) active sites. More importantly, the formation of new active sites through Co nanoparticles encapsulated in CNTs further improved both ORR and OER performance, and the unique 3D framework structure accelerated the mass and electron transport.

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EXPERIMENTAL SECTION

Materials and Reagents. Chlorella was purchased from a local market. Melamine (C3H6N6, analytical grade) and cobaltous acetate (C4H6O4·Co·4H2O, analytical grade) were obtained from Sinopharm Chemical Reagent Co., Ltd. Commercial IrO2/C and Pt/C (20 wt %) catalysts were acquired from Sigma-Aldrich and Johnson Matthey, respectively. Deionized water (18.2 MΩ cm) was supplied from a 32169


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Figure 1. (a) SEM image of Chlorella, (b,c) SEM images of Co/M-Chlorella-900, (d) TEM image of Co/M-Chlorella-900; the inset is a HRTEM image of a single Co nanoparticle encapsulated in N-doped CNTs, (e) HRTEM image of the joint structure of the bamboolike CNTs, and (f) HRTEM image of the CNT wall; the inset shows a corresponding whole CNT. The ORR activity of the samples was evaluated via cyclic voltammetry (CV) at a scan rate of 10 mV s−1 and linear sweep voltammetry (LSV) at a scan rate of 5 mV s−1 in 0.1 M KOH electrolyte using a rotating disk electrode (RDE) system (Pine Research Instrumentation, USA). The LSV curves were obtained at various rotating speeds from 400 to 2500 rpm in an O2-saturated electrolyte. The stability measurements were recorded by current− time (i−t) chronoamperometric response at 0.85 V for the ORR. In addition, an accelerated durability test was performed on Co/MChlorella-900 and Pt/C before and after 2000 cycles. The electrontransfer number (n) during the ORR process was calculated from the Koutecky−Levich (K−L) equation J −1 = JL−1 + JK −1 = B−1ω−1/2 + JK −1

(1)

B = 0.62nFD0 2/3C0ν−1/6

(2)

Information). However, Co/M-Chlorella-900 shows a hollow nestlike morphology/structure consisting of large amounts of entangled bamboolike N-doped CNTs (Figure 1b,c), with the length of the CNTs ranging from hundreds of nanometers to several micrometers. This nestlike structure should be beneficial for the transport of electrons and reactants. We found that the doped cobalt plays a crucial role in the transformation of Chlorella into bamboolike CNTs and a nestlike framework; without the addition of cobalt, Chlorella-900 and M-Chlorella900 retained the initial morphology of Chlorella and exhibited plicated, hollow carbon spheres (Figure S1b,c). Additionally, in Co/M-900 (Figure S1d), which contained cobalt but not Chlorella, a few CNTs could be observed in the pyrolyzed products, further demonstrating the crucial role of cobalt in the formation of CNTs. Co-Chlorella-900 shows a hollow nestlike structure (Figure S1e) which is similar to Co/M-Chlorella-900. The magnified SEM image of Co-Chlorella-900 (Figure S1f) revealed that the N-doped CNTs displayed closed nanotube tips with obvious bamboolike joints. Clearly, the formation of CNTs from biomass carbonaceous precursors is attributable to the catalysis of Co during pyrolysis.46,47 No other type of carbon material could be observed using SEM or TEM. Hence, this method can provide an effective way for preparing bamboolike CNTs from biomass. The TEM images shown in Figure 1d,e clearly reveal the bamboolike structure of the CNTs; the thickness of the wall is about 2 nm, corresponding to 12 carbon atom layers (Figure 1f). Some cobalt nanoparticles could be found in the CNTs (Figure 1d and S2a) and encapsulated in the inner space of the nanotubes. As shown in the inset of Figure 1d, the interplanar spacing of the cobalt nanoparticles was 0.2 nm, which corresponds to the (111) plane of Co.48 A high-resolution TEM (HRTEM) image (Figure S2b) clearly shows that a Co nanoparticle is surrounded by a complete graphitic shell consisting of a wellordered arrangement of carbon layers. The interplanar spacing of the graphitic shell was 0.34 nm, which corresponds to the (002) plane of graphite.49 It is believed that the encapsulated

where J, JL, and JK represent the measured current density, the diffusion-limiting current density, and the kinetic density, respectively, B represents the Levich slope, n represents the number of electrons transferred, ω denotes the rotation speed of the electrode, F denotes the Faraday constant (F = 96485 C mol−1), and D0 denotes the diffusion coefficient of O2 in 0.1 M KOH electrolyte (1.9 × 10−5 cm2 s−1). The O2 bulk concentration C0 is 1.2 × 10−3 mol L−1, and the kinetic viscosity of the electrode ν is 0.01 cm2 s−1. The OER performance of the samples was measured by LSV at a scan rate of 0.5 mV s−1 in 0.1 M KOH electrolyte. The i−t chronoamperometric response study at 1.60 V for the OER (vs RHE) and accelerated durability tests were performed on Co/M-Chlorella900 and IrO2/C before and after 2000 cycles in O2-saturated 0.1 M KOH solution. Tafel slopes were used to evaluate the OER kinetics based on the Tafel equation η = (b log J )/JL

(3)

where η denotes the overpotential, b denotes the Tafel slope, J is the current density, and JL is the exchange current density.

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RESULTS AND DISCUSSION The morphologies and structures of the samples were characterized by SEM and TEM. Native Chlorella presents a wrinkled/plicate microsphere (Figures 1a and S1a, Supporting 32170


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Figure 2. (a) CV curves of five catalysts in N2-saturated and O2-saturated 0.1 M KOH. (b) LSV curves at 1600 rpm for five catalysts and 20 wt % commercial Pt/C in 0.1 M KOH solution. (c) LSV curves at 1600 rpm for five catalysts and commercial IrO2/C in 0.1 M KOH solution. (d) Tafel plots of five catalysts and commercial IrO2/C, for the OER. (e) Comparative current density of various samples for O2 reduction at 0.8 V. (f) Comparative overpotential of various samples at a current density of 10 mA cm−2.

than those of Chlorella-900 (0.68 V), M-Chlorella-900 (0.70 V), Co-Chlorella-900 (0.74 V), and Co/M-900 (0.76 V), respectively, indicating its significantly enhanced ORR performance. In addition, Co/M-Chlorella-900 had the largest peak current density of the five catalysts, implying that it had the highest electrochemical activity and the largest active surface area.52 Further investigation of the ORR activity of the catalysts was performed by LSV using a RDE. Figure 2b shows the LSV curves of Co/M-Chlorella-900, Co/M-900, Co-Chlorella-900, M-Chlorella-900, Chlorella-900, and 20 wt % commercial Pt/C at a rotation rate of 1600 rpm in O2-saturated 0.1 M KOH solution. The addition of cobalt induced a large positive shift in the half-wave potentials of Co/M-Chlorella-900, Co/M-900, and Co-Chlorella-900 compared to those of M-Chlorella-900 and Chlorella-900. The half-wave potentials of Co/M-900 and Co-Chlorella-900 were, respectively, 20 and 60 mV lower than that of commercial 20 wt % Pt/C. However, the half-wave potential (0.87 V) of Co/M-Chlorella-900 surpassed that of commercial 20 wt % Pt/C (0.83 V) by 40 mV. To our

Co nanoparticles acted as a catalyst for the formation of bamboolike CNTs (or the transformation of Chlorella into CNTs) and that most of the unstable cobalt oxides were removed during the acid leaching stage, which is why only a few stable Co nanoparticles encapsulated in the carbon matrix could be observed in the Co/M-Chlorella-900 sample. We suggest that the remaining Co nanoparticles contributed to the ORR/OER performance of the catalyst through forming new catalytic active centers containing Co.50,51 In other words, the added cobalt not only played a crucial role in the formation of bamboolike CNTs or the transformation of Chlorella into bamboolike CNTs but also directly enhanced the catalytic activity by forming a new type of active sites. The ORR activity of Co/M-Chlorella-900 was first evaluated by CV in N2-saturated (dash line) and O2-saturated (solid line) 0.1 M KOH solution. The CV curves of all catalysts in O2 saturation displayed a well-defined oxygen reduction peak (Figure 2a). The redox peak potential of Co/M-Chlorella-900 was up to 0.84 V, which was 160, 140, 100, and 80 mV higher 32171


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Figure 3. (a) LSV curves at 1600 rpm for Co/M-Chlorella-900 and 20 wt % commercial Pt/C in O2-saturated 0.1 M KOH before and after 2000 cycles. (b) i−t chronoamperometric responses of Co/M-Chlorella-900 and 20 wt % commercial Pt/C at 0.85 V in O2-saturated 0.1 M KOH. (c) LSV curves at 1600 rpm for Co/M-Chlorella-900 and commercial IrO2/C in O2-saturated 0.1 M KOH before and after 2000 cycles. (d) i−t chronoamperometric responses of Co/M-Chlorella-900 and commercial IrO2/C at 1.60 V in O2-saturated 0.1 M KOH.

commercial IrO2/C. However, Co/M-Chlorella-900 exhibited higher OER performance than IrO2/C; its OER overpotential (352 mV) was 23 mV lower than that of the latter. To further assess the OER kinetics and mechanism, we show Tafel plots for the five samples and IrO2/C in Figure 2d. The Tafel slope of Co/M-Chlorella-900 is 60 mV dec−1, which is much smaller than the slopes of Chlorella-900 (196 mV dec−1), M-Chlorella900 (177 mV dec−1), Co-Chlorella-900 (98 mV dec−1), Co/M900 (105 mV dec−1), and IrO2/C (75 mV dec−1). These results demonstrate that our Co/M-Chlorella-900 catalyst possessed superior OER kinetics and efficiency. In brief, the ORR performance of the catalysts follows the order Co/M-Chlorella900 > Pt/C > Co/M-900 > Co-Chlorella-900 > M-Chlorella900 > Chlorella-900. Their OER performance follows the order Co/M-Chlorella-900 > IrO2/C > Co-Chlorella-900 > Co/M900 > M-Chlorella-900 > Chlorella-900. Clearly, Co/MChlorella-900 exhibited not only a wonderful ORR activity but also a prominent OER activity, and the cobalt doping played a crucial role in its improved performance.53 Noticeably, Co/M-Chlorella-900 shows its significant advantages as compared with most of other similar ORR/OER bifunctional catalysts reported in recent papers (Table S1). Except for remarkable ORR and OER activity, Co/MChlorella-900 also exhibited outstanding long-term stability. As shown in Figure 3a, after 2000 ORR cycles, its onset potential decreased only slightly, by 8 mV, from its initial onset potential. However, this decrease is up to 33 mV for a commercial Pt/C catalyst. Furthermore, for our optimal Co/M-Chlorella-900 catalyst, after 50 000 s of long-term, continuous ORR at 0.85 V, the current density remained at about 96% of its initial value, whereas the attenuation for commercial Pt/C was up to 14%, further confirming the excellent stability of Co/M-Chlorella-900

knowledge, this Co/M-Chlorella-900 catalyst is to date one of the best doped carbon catalysts for the ORR in an alkaline medium. We attribute the high performance to its unique nestlike structure constructed by the N-doped CNTs, which provides a large surface area and is rich in mesopores and macropores, resulting in greater exposure of surface active sites. In addition, the Co nanoparticles encapsulated by the CNTs may act as new active sites. To gain further insight into the kinetics and mechanism of the ORR on Co/M-Chlorella-900, a series of LSV curves at different rotation speeds were recorded and are shown in Figure S3a. Their corresponding K−L plots (J−1 vs ω−1/2) at various electrode potentials are presented in Figure S3b. The average n value (the inset of Figure S3b) of Co/M-Chlorella-900, calculated with the K−L equation, was close to 4. This indicated that Co/M-Chlorella-900 mainly favors a four-electron-transfer pathway during the ORR process. In addition, the current density IK of Co/MChlorella-900 at 0.8 V (RHE) was up to 5.06 mA cm−2, the highest value among all samples (Figure 2e), which was 1.28 times as high as that of Pt/C catalyst (3.95 mA cm−2). Figure 2c shows the LSV curves of the catalysts for the OER. The onset potentials of Co/M-Chlorella-900 (1.33 V), Co/M-900 (1.52 V), and Co-Chlorella-900 (1.49 V) were much lower than those of M-Chlorella-900 (1.58 V) and Chlorella-900 (1.59 V), indicating that the addition of Co significantly promoted the OER activity of the catalysts. At a current density of 10 mA cm−2, the overpotentials (Figure 2f) of Chlorella-900 (565 mV), M-Chlorella-900 (547 mV), Co-Chlorella-900 (418 mV), and Co/M-900 (494 mV) were, respectively, 190, 172, 43, and 119 mV higher than that of commercial IrO2/C (375 mV). Clearly, the OER activity of Chlorella-900, M-Chlorella-900, CoChlorella-900, and Co/M-900 was lower than that of 32172


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Figure 4. (a) XRD for Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900. (b) Raman spectra for Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900.

Figure 5. (a) N2 adsorption−desorption isotherms for Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900. (b) Pore size distributions for Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900.

arose from the N-doped CNTs and cobalt nanoparticles facilitates charge transfer and promotes both ORR and OER performance. The Raman spectra of Chlorella-900, M-Chlorella900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900 are shown in Figure 4b. Each Raman spectrum displays two obvious peaks. The D band at approximately 1350 cm−1 reflects local defects and disorders of carbon materials,56 whereas the G band at approximately 1580 cm−1 indicates the graphitic inplane vibration of the carbon materials.57 The relative peak intensity of D and G (ID/IG) can indicate the degree of the graphitization of carbon materials.58 The ID/IG ratios of Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900 were 1.33, 1.22, 1.15, 1.03, and 1.08, respectively, suggesting that Co-Chlorella-900 and Co/MChlorella-900 had a higher degree of graphitization than Chlorella-900 and M-Chlorella-900 because the bamboolike N-doped CNTs (Co-Chlorella-900 and Co/M-Chlorella-900) had a higher degree of graphitization than the amorphous carbon (Chlorella-900 and M-Chlorella-900) without the addition of Co. The specific surface areas of the five catalysts were detected by nitrogen adsorption−desorption porosimetry, as shown in Figure 5a. Co/M-Chlorella-900 had the highest BET specific surface area, 728 m2 g−1, compared with those of Co/M-900 (236 m2 g−1), Co-Chlorella-900 (604 m2 g−1), M-Chlorella-900 (489 m2 g−1), and Chlorella-900 (157 m2 g−1). Clearly, the large surface area of Co/M-Chlorella-900 should be ascribed to the nestlike framework formed by the N-doped CNTs. The pore size distributions of the five samples were calculated through the Barrett−Joyner−Halenda method. Figure 5b indicates the

(Figure 3b). The OER stability of Co/M-Chlorella-900 was also evaluated through an accelerated durability test. Figure 3c shows that after 2000 cycles at a current density of 10 mA cm−2, the potential of Co/M-Chlorella-900 decreased only by 12 mV, whereas for IrO2/C, it decreased by about 37 mV. Further durability measurements with a chronoamperometer for 5000 s are presented in Figure 3d. After 5000 s of continuous OER at 1.60 V, Co/M-Chlorella-900 showed 1% attenuation relative to its initial current density, whereas for IrO2/C, the attenuation was almost 17%. Figure 4a shows the XRD patterns of Chlorella-900, MChlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900. Chlorella-900 and M-Chlorella-900 present two broad diffraction peaks at 23.8° and 44°, corresponding to C(002) and C(100), respectively. However, Co-Chlorella-900, Co/M900, and Co/M-Chlorella-900 exhibit a sharp peak at 26°, corresponding to the (002) plane of graphitic carbon. This indicates that Co-Chlorella-900, Co/M-900, and Co/MChlorella-900 had higher degrees of graphitization54,55 than Chlorella-900 and M-Chlorella-900. Co-Chlorella-900, Co/M900, and Co/M-Chlorella-900 show three other distinct peaks at approximately 44.3°, 51.6°, and 75.8°, which compare well with the standard JCPDS file 15-0806, indicating the existence of metallic cobalt in Co-Chlorella-900, Co/M-900, and Co/MChlorella-900.46 Except for zerovalent-state Co, no metal-oxide diffraction peaks are observed. This implies that cobalt mainly exists in the form of Co nanoparticles encapsulated in N-doped carbon after acid leaching, and the unstable metal oxides have been removed completely. This is consistent with the results from TEM. These results indicate that the synergistic effect that 32173


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Figure 6. (a) XPS survey spectra of Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900. (b) Elemental contents of Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900, determined by XPS. (c−g) High-resolution N 1s XPS spectra of Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900. (h) N species contents of Chlorella-900, MChlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900.

been generated from the N-doped CNTs, whereas the macropores might have originated from the spaces within the entangled network of CNTs. The large specific surface area and the mesopores and macropores in the Co/M-Chlorella-900

coexistence of rich mesopores and macropores in Co-Chlorella900, Chlorella-900, M-Chlorella-900, and Co/M-Chlorella-900, ranging from 6 to 250 nm. By contrast, Co/M-900 had only mesopores. In Co/M-Chlorella-900, the mesopores might have 32174


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Chlorella-900 first increased and then decreased as the amount of Co increased, with the precursor containing 0.05 mmol Co, achieving the highest ORR and OER activity. Too little Co might not create enough active sites, but too much Co might limit the formation of active sites.62,72 We also optimized the heat treatment temperature from 800 to 1000 °C. As shown in Figure S5c,d, the optimal temperature was 900 °C, as the sample prepared at this temperature showed the best ORR and OER performance.

made possible the sufficient exposure of accessible active sites and facilitated mass and electron transfer. The surface chemical compositions and bonding configurations of Chlorella-900, M-Chlorella-900, Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900 were detected by XPS. Figure 6a shows the survey spectra of the five samples, which indicate the presence of C, O, and N in all samples, and Co appears in the spectra for Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900. Figure 6b shows that the N contents of M-Chlorella-900, Co/M-900, and Co/M-Chlorella-900 significantly increased in comparison with those of Chlorella-900 and Co-Chlorella-900, indicating that the N-doped content had been helpfully supplemented by the introduction of melamine.59,60 It has been widely recognized that transition-metal content and N content can significantly influence the ORR and OER performance of a doped carbon catalyst.61−64 Too little transition metal and N cannot generate enough catalytic active sites, whereas too much transition metal and N content may limit the formation of active sites.65,66 Co/M-Chlorella-900 had suitable Co and N content, compared with solely Co-doped Co-Chlorella-900 or solely N-doped M-Chlorella-900, so codoping may be the main reason for its greatly improved ORR/OER performance. The high-resolution N 1s spectra in the five samples (Figure 6c−g) can be fitted into four N peaks, centered at approximately 398.26, 400.19, 401.16, and 402.14 eV, which correspond to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively.14,67 As shown in Figure 6h, CoChlorella-900 had the lower pyridinic N than that of Co/M-900 and Co/M-Chlorella-900 but higher graphitic N than that of Co/M-900. Co/M-900 had the highest proportion of pyridinic N and the much lower proportion of graphitic N than that of Co-Chlorella-900 and Co/M-Chlorella-900. However, Co/MChlorella-900 had the highest proportion of graphitic N and the second highest proportion of pyridinic N. The XPS data associated with the electrochemical results indicate that the pyridinic N sites are responsible for the ORR and the graphitic N sites are responsible for the OER on the N-doped CNTs. These results are in accord with previous research.35,68,69 The XPS spectra of Co 2p for Co-Chlorella-900, Co/M-900, and Co/M-Chlorella-900 (Figure S6) show two peaks at around 778.8 and 793.9 eV, which we assign to the zerovalent of Co.70,71 These results are consistent with the observations from XRD and TEM, further confirming that the doped Co may have existed in a zerovalent metallic state in the catalysts. On the basis of the above results and analyses, combined with the recent research in transition metal−nitrogen−carbon catalysts for ORR and/or OER, the most reasonable hypothesis can be proposed to explain the excellent bifunctional catalytic activity as follows: (1) the N dopants in Co/M-Chlorella-900 play a leading role in controlling the ORR and OER active sites. N-Doped carbon caused charge redistribution from adjacent C to N dopants and reduced the energy barriers for ORR and/or OER, resulting in the formation of catalytic active sites (pyridinic N and graphitic N). (2) The pyridinic N sites are mainly favorable for the ORR, and the graphitic N sites are mainly favorable for the OER. (3) The Co nanoparticles encapsulated in the N-doped CNTs act as new active sites to further improve both ORR and OER activity. We also investigated how the amount of added Co affected the morphology and ORR/OER activity of Co/M-Chlorella900. The yield of CNTs, consisting of the nestlike framework, increased with the amount of Co (Figure S4). As shown in Figure S5a,b, the ORR and OER performance of Co/M-

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CONCLUSIONS We successfully prepared nitrogen-doped bamboolike CNTs encapsulating cobalt nanoparticles with a nestlike structure using Chlorella as the precursor. The catalyst possessed not only a large surface area and a high degree of graphitization but also high contents of both pyridinic N as active sites for ORR and graphitic N as active sites for OER simultaneously. The catalyst Co/M-Chlorella-900 exhibited an excellent electrochemical activity toward both the ORR and the OER in an alkaline medium, far superior to those of Chlorella-900, MChlorella-900, Co-Chlorella-900, and Co/M-900 and even better than those of commercial Pt/C and IrO2/C. What is more, the catalyst exhibited significantly enhanced durability and efficient selectivity. We ascribe the excellent performance of the catalyst to its high proportion of both pyridinic N and graphitic N, synergistic effects between the enclosed Co and the N-doped carbon, and the 3D framework structure. In particular, the Co doping not only was crucial for the formation of the special primary (CNTs) and secondary (nestlike framework) structures but also contributed to the high performance of the catalyst by creating a new type of active centers. The present work may provide a new pathway to design and develop novel types of ORR/OER bifunctional doped carbon catalysts using abundant, low-cost, and eco-friendly natural biomass as the precursor.

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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/acsami.7b10668. SEM images, TEM images, LSV curves, and XPS spectra of the samples (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.S.). *E-mail: [email protected] (S.L.). ORCID

Li Du: 0000-0003-2394-0727 Shijun Liao: 0000-0003-2481-0377 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the State’s Key Project for Research and Development Plan of China (project no. 2016YFB0101201), the National Natural Science Foundation of China (NSFC project nos. 21476088, 51302091, U1301245), the Natural Science Foundation of Guangdong Province (project nos. 2014A010105041, 2015A030312007), the Guangdong Provincial Department of Science and 32175


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Technology (project no. 2015B010106012), the Educational Commission of Guangdong Province (project no. 2013CXZDA003), and the Guangzhou Science Technology Innovation Committee (project no. 2016201604030012).

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