CoO Embedded Nitrogen-Doped Carbon

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Core@shelled Co/CoO Embedded Nitrogen-doped Carbon Nanosheets Coupled Graphene as Efficient Cathode Catalysts for Enhanced Oxygen Reduction Reaction in Microbial Fuel Cells Liang Tan, Qiu-Ren Pan, Xiao-Tong Wu, Nan Li, Jian-Hua Song, and Zhao-Qing Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00026 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 20, 2019

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ACS Sustainable Chemistry & Engineering

Core@shelled Co/CoO Embedded Nitrogen-doped Carbon Nanosheets Coupled Graphene as Efficient Cathode Catalysts for Enhanced Oxygen Reduction Reaction in Microbial Fuel Cells† Liang Tan, Qiu-Ren Pan, Xiao-Tong Wu, Nan Li*, Jian-Hua Song, Zhao-Qing Liu* School of Chemistry and Chemical Engineering/Institute of Clean Energy and Materials/Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Guangzhou University; Guangzhou Higher Education Mega Center, Outer Ring Road No. 230, China 510006 Fax: 86-20-39366908; Tel: 86-20-39366908; E-mail: [email protected] (N. Li); [email protected] (Z.-Q. Liu).

ABSTRACT High-efficient oxygen reduction reaction (ORR) catalysts are crucial for facilitating the large-scale exploitation of electrochemical energy storage and conversion technologies. Herein, we demonstrate a carbon-based metal hybrid, which offers a higher electrocatalytic activity than that of the individual composite by optimizing the electronic modulation effect from suitable microstructure. The resulting cobalt@cobalt oxide nanoparticles embedded in N-doped carbon shell coupling with hierarchical porous graphene (GCN-Co@CoO), exhibiting a significantly enhanced ORR activity in alkaline solution and highlighting a synergistic effect between N-doped carbon shell and metallic Co species. More precisely, the GCN-Co@CoO hybrid pyrolyzed at 800 oC achieves a more positive half-wave potential of -0.194 V (vs. SCE) and superior limiting current of 4.91 mA cm-2. Moreover, ACS Paragon Plus Environment

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the GCN-Co@CoO composite also shows an outstanding tolerance to methanol cross-over effects and long-term stability. Furthermore, based on the GCN-Co@CoO cathode catalyst, the self-assembled microbial fuel cells (MFCs) performs a maximum power density of 611 ± 9 mW m-2 at a high current density of 1869 ± 24 mA m-2.

Key words: Porous graphene, CoO, Oxygen reduction reaction, Microbial fuel cells.

INTRODUCTION Oxygen reduction reaction (ORR) is a most important half-reaction for electrochemical energy storage and conversion technologies, such as metal-air batteries and microbial fuel cell.1-2 The precious metal-based materials are generally used as ORR electrocatalysts.3 However, the poor stability, high-cost and scarcity of the precious metal-based ORR catalysts inevitably limit the large-scale implementation of the energy storage and conversion technologies.4 Thus, it is of great significance to design and exploit some high-efficient and durable alternatives for electrocatalysis. Up to now, a great deal of replaceable materials, involving carbon species, transition metal oxides, carbides and their composites, have been considered to be the promising electrocatalysts toward the ORR process. Among them, carbon-based materials, including carbon nanotubes, graphene and carbon black, have been proven with high catalytic and stable capacities, which could be further enhanced by the introduction of heteroatoms (such as N, P etc.) to modify the geometric and electronic structure.5-15 However, few reports of these carbon-based materials have so far shown excellent activity comparable to that of Pt/C catalysts. Recently, N-doped carbon coupling with transition-metal hybrid materials (metal-N-C) had emerged as the preferred to substitute Pt and other precious metal catalysts in ORR process. Especially, metallic phrase nanoparticles encapsulated in nitrogen-doped graphite carbon (M@CN) demonstrated an outstanding catalytic activities, stability and methanol cross-over effects.16-17 The development of this ACS Paragon Plus Environment

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kind of M@CN materials can be traced back to 1964, Jasinski et al. have reported that the annealed metal phthalocyanine complexes can exhibit a be used as a great ORR catalytic capability comparable to the expensive Pt-based catalysts.18 Subsequently, Varnell discovered that nitrogen-doped carbon protected iron species structure could be favourable to greatly enhance the activity and stability of catalysts, and this viewpoint is also consistent with these have reported results, such as carbon encapsulated metallic cobalt nanostructure and NiFe embedded carbon nanosheets structure.19-22 This is because, in M@CN species catalysts, the interaction between the graphite/carbon shell and the metal core phase can alter the local electronic modulation of the graphite/carbon shell and provide surprisingly high electrochemical activity, indicating a great promising for substituting the Pt-and precious metal-based in ORR.23-24 Normally, M@CN species catalysts are synthesized by co-pyrolysis of the transition-metal salts and rich-nitrogen organic precursors or metal-organic frameworks (MOFs).25-26 Unfortunately, such method-derived composites are usually tend to agglomeration and show a very poor graphitic degree, which severely limit the electron and ion transport, and further weaken the ORR catalytic activity and long-time stability.27-28 On the basis of these previous efforts, we realized the important role of the conductivity and subtle nanostructure for the nitrogen-doped carbon encapsulated metal in determining their high-efficient ORR catalytic capacities and stability. Thus, it is urgent to develop low-cost, highly active and highly conductive catalyst by effectively combining M@CN with graphene. Herein, we demonstrate a porous GCN-Co@CoO hybrid electrocatalyst via a simple pyrolysis of graphene oxide (GO) and cube-like zeolitic-imidazolate framework (ZIF-67-cube). Notably, the ZIF-67-cubes not only act as the precursor of the nitrogen-doped carbon and metallic cobalt, but also serve as the template of the resultant hierarchically porous graphene; and graphene nanosheets, in the unique carbon-based material, function as a binder and electrical conductor to interconnect individual metallic cobalt species NPs and carbon materials. As-expected, the obtained catalyst exhibits an excellent activity comparable to commercial Pt/C catalyst and superior stability in ORR. Moreover, the ACS Paragon Plus Environment


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practical application of GCN-Co@CoO composite in ORR is assessed by MFCs systems; and the MFCs systems equipped with GCN-Co@CoO composite cathode has achieved a high open circuit voltage of 0.6 ± 0.01 V and a large power density of 611 ± 9 mW m-2. EXPERIMENTAL SECTION Synthesis of ZIF-67 nanocubes In a typical synthesis, 16 mL of aqueous solution involving 464 mg of Co(NO3)2.6H2O and 8 mg of cetyltrimethylammonium bromide (CTAB) was rapidly injected into 112 mL of aqueous solution with 7.264 g of 2-methylimidazole and persistently stirred at 30 oC for 2 h. Afterward, the precipitate was collected by centrifugating with deionized water for at least 8 times before vacuum drying at 50 oC overnight. Synthesis of GCN-Co@CoO composite In a typical synthesis, 80 mL of GO suspension (1 mg mL-1) was rapidly injected into 100 mL of ethanol solution with 500 mg ZIF-67 nanocubes and 500 mg CTAB and vigorously stirred at 30 oC for 6 h. Then, the resulting precipitate was gathered by washing with deionized water and following dried at 50 oC overnight. To synthesize the GCN-Co@CoO composite material, the obtained precursor was transferred to a ceramic boat and annealed at 800 oC for 4 h with a ramping rate of 2 oC min-1 in a N2 atmosphere. Synthesis of CN-Co@CoO and GCN composite The synthesis of CN-Co@CoO was similar to that of the GCN-Co@CoO without the involvement of graphene and CTAB. To prepare the GCN composite material, the as-obtained GCN-Co@CoO was soaked in acid solution (3 M HCl) for 20 h and washed with deionized water to neutral. Construction and evaluation of the MFCs A conventional MFCs system with 28 mL volume was assembled as the previous work.29 Typically, a bare carbon cloth, which heat treated within 450 oC for 30 min, was served as anode; and the ACS Paragon Plus Environment

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air-cathode was composed of carbon cloth (an effective area of 7 cm2) equipped with waterproof layer and catalyst materials (1 mg cm-2) or commercial Pt/C (2.5 mg cm-2, wt 20%). Besides, the activated sludge with anaerobic bacteria, which acquired from the living area of the Pearl River downstream, was used to inoculate all of the MFCs; and the data acquisition instrument (MPS 010602, Beijing) was applied to record the output voltage of MFCs equipped with 1000 Ω external resistance for every minute. In order to prevent deviation, all of relevant evaluation and analysis were tested three parallel samples. Furthermore, polarization curves of MFCs with different cathode catalysts were obtained according to adjust the external resistances from 0.04 KΩ to 5KΩ. The current density I was gained using the formula of I = V /(R × A) (V is cell voltage, R is external resistance and A is efficient area), and power density P was acquired from the formula of P = V × I.30-31 Characterization and measurement The crystal phase and morphology of the GCN-Co@CoO composite and control samples were obtained by powder X-ray diffraction (XRD, PW3040/60), field emission scanning electron microscopy

(FE-SEM,

JEOL

JSM-7001F)

and

transmission

electron

microscopy

(TEM,

JEM2010-HR). Surface electronic states and element compositions of the GCN-Co@CoO composite and control samples were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALab250). Besides, Brunauer-Emmett-Teller (BET) tester was used to investigate the specific surface area and pore size distribution of the sample. Electrochemical measurements In this work, CHI 760D electrochemical workstation (Shanghai, Chenhua) was employed to investigate the electrochemical catalytic capacity of as-prepared catalyst materials and commercial Pt/C in a typical three-electrode system, which reference electrode was the saturated calomel electrode (SCE), counter electrode was the Pt plate and rotating disk electrode coated with catalyst film (about 0.1 mg cm-2 for both as-prepared materials and Pt/C (wt% 20%) catalyst) was acted as the work

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electrode. Besides, Rotating disk electrode (RDE) system was applied to evaluate the ORR catalytic capacity of as-prepared materials under an oxygen saturated 0.1 M KOH electrolyte. Kouteky-Levich (K-L) curve analysis was applied to calculate the number of electrode transferred (n) according to the K-L equation as follows: 32 1 𝑖

1

= 𝑖𝑘 +

(

1 0.62 𝑛F𝐴𝐷

23

𝑉

―1 6

)𝜔

𝐶

―1 2

(1)

Where F symbol is the Faraday constant, A is area of the disk electrode (0.126 cm-2), i symbol is the measured current (A), D symbol is the diffusion coefficient of oxygen, V symbol represent the kinematic viscosity, C symbol is the oxygen content of electrolyte and ω symbol represent the rotation rate for electrode. Furthermore, the electrochemically double-layer capacitance (Cdl) was recorded to evaluate the electrochemically active surface area of the resultant GCN-Co@CoO and control samples according to the reported literatures.33-34 The value of Cdl was obtained from the following equation: 𝐶𝑑𝑙 = △ 𝐼𝑐 △ 𝑣

(2)

Where 𝐼𝑐 is the current density (mA cm-2) and 𝑣 is the scan rates (V s-1). Results and Discussion The fabrication process of the GCN-Co@CoO catalyst is schematically shown in Figure 1. Nanocube structural MOF, zeolitic imidazolate framework (ZIF-67-cube), was prepared and acted as the precursors to obtain GO@ZIF-67 during the self-assembly of GO. After pyrolysis at specific temperature, the cobalt ions in the frameworks were gradually reduced to metallic cobalt and cobalt oxide nanoparticles. Meanwhile, the sustaining decomposition of ZIF-67 nanocubes was also accompanied by melting the volume of MOF, resulting in the formation of porous structure and graphitization. Moreover, the released N-containing gases are beneficial to restore the graphene oxide and nitrogen doping in carbon shells, finally evolving into a multi-dimensional GCN-Co@CoO hybrids. ACS Paragon Plus Environment

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Figure 1. Synthesis schematic illustration of the GCN-Co@CoO composite.

As displayed in X-ray diffraction (XRD) pattern, the as-prepared ZIF-67-cubes exhibited a high crystallinity without any examinable by-products (Figure S1). After thermal pyrolysis at the special temperature, the characteristic peaks of ZIF-67 cubes were disappeared and six significant diffraction peaks at 36.5, 42.4, 61.5, 44.2, 51.5 and 75.9o were clearly observed, which were of great agreement with the crystalline data of the cobalt oxide and metallic cobalt. These obvious peaks in Figure 2a are indexed as (111), (200) and (220) planes of the metallic cobalt (JCPDS: 15-0806, α = b = c = 3.54 Å) and (111), (200) and (220) planes of the cobalt oxide (JCPDS: 48-1719, α = b = c = 4.26 Å), respectively. Moreover, a small peak at 2θ = 26

o

indicated the existence of graphene and graphite

carbon layer. Figure 2b-c shows the SEM images of the GCN-Co@CoO hybrid pyrolyzed at 800 oC. The representative SEM images reveal that the GCN-Co@CoO hybrids were composed of the hierarchical graphene phase and metal species nanoparticles. And many obvious porous structure can be observed in the surface of GCN after etching HCl solution for 20 h (Figure 2d-e), indicating that the metallic cobalt species nanoparticles with uniform particle size (~100 nm) were homogenously embedded on the surface of graphene sheets. More detailed morphology and structure of the GCN-Co@CoO were characterized with transmission electron microscopy (TEM). As shown in Figure 2f-h, an obvious core-shell structure composed of carbon phase and cobalt species phase can be observed, indicating a masterly microstructure exist in the obtained GCN-Co@CoO. The ACS Paragon Plus Environment


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high-resolution TEM (HRTEM) in Figure 2i displays the nature of single cobalt species. The interplanar spacing of the adjacent fringes were measured to be 0.20 nm and 0.25 nm, which can be coincide well with the (111) facet of metallic cobalt and cobalt oxide, respectively. Moreover, the energy dispersive spectrometer (EDS) spectrum analysis result of the sample demonstrates that no

Figure 2. (a) XRD patterns of GCN-Co@CoO and GCN, respectively, (b-c) SEM images of GCN-Co@CoO, (d-e) SEM images of GCN, (f-h) TEM images of GCN-Co@CoO, (i-l) HRTEM image and corresponding FFT pattern of GCN-Co@CoO. 5 other

irrelevant elements can be observed in the resulting sample (Figure S2). Detailed information

about the surface area and pore size of the as-prepared GCN-Co@CoO were determined by N2 adsorption/desorption isotherm analysis. A typical IV type with distinct hysteresis loop indicates the present of numerous mesopores in the GCN-Co@CoO (Figure S3). The BET surface area and pore volume of the GCN-Co@CoO are confirmed as 180.27 m2 g-1 and 0.32 cm3 g-1, respectively, providing 10 a large

surface area for the ORR process.


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Figure 3. (a) High resolution XPS spectra of N 1s, (b) C 1s, (c) Co 2p and (d) O 1s, respectively.

The surface composition and valence state of the GCN-Co@CoO hybrid were evaluated by X-ray photoelectron spectroscopy (XPS) technology. The representative XPS survey spectra of the 5 as-obtained

hybrid mainly composed by C, N, O and Co was consistent with the EDS spectrum results

(Figure S4). In Figure 3a, the high-resolution of the N 1s spectrum could be deconvoluted into four sub-peaks, which may be ascribed to the various nitrogen conditions. The nitrogen species were assigned to pyridinic-N (398.1 eV), Co-Nx (399.2 eV), graphitic-N (400.7 eV) and oxidized-N (404.9 eV) according to the reported related works; 10 GCN-Co@CoO

35-36

Moreover, the percentage of nitrogen dopant in

hybrid is 5.38 % (wt %), which consistent with the result of the EDS measurement. As

the previously work, Pyridine-N in the carbon matrix can induce the delocalization of charge and further enhance the ability to adsorb oxygen, improving its catalytic capacities in ORR process. 37 The C 1s spectrum was exhibited in Figure 3b, revealing a typical characteristic for heteroatom-doped carbon.38 The peak 15 situated

at around 284.5 eV represents a typical sp2-hybridized graphite carbon, and sub-peaks located ACS Paragon Plus Environment


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at higher binding energy regions of 285.5, 287.1 and 290.9 eV

Figure 4. (a) CV curves of different catalysts in Oxygen and Nitrogen-saturated 0.1 M KOH solution, (b) LSV curves, (c) K-L curves, and (d) the electron transfer number of different catalysts, (e) Capacitive current density 5 of the different catalysts as a function of scan rates. (f) EIS curves of different catalysts.

were summarized as the combination of carbon-nitrogen and carbon-oxygen, respectively.39-42 The spectrum of O 1s was divided into three contributions for the hybrid in Figure 3c. The peak centred at 529.6 eV was represent a typical of C-O and M-O bands.43-44 And the pesks at 531.1 and 532.7 eV may be ascribed to O-H bands.45-47 The deconvolution analysis of the Co 2p spectrum in Figure 3d 10 indicates

the presence of cobalt oxides, which may be derived from the surface oxidation of the ACS Paragon Plus Environment

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metallic Co in air.48-52 Although the metallic Co cannot be examined in the spectrum, it’s substantially existed in the core of cobalt species derived from the results of XRD, because the XPS technique is restricted to the material surface.53-55 The electrochemical capacities of the GCN-Co@CoO catalyst was first evaluated with cyclic 5 voltammetry

(CV) measurement in N2 and O2-saturated alkaline condition (0.1 M KOH). As depicted

in Figure 4a, there is not any characteristic peak in the CV curve of the resultant catalysts when the saturating KOH solution with N2, demonstrating the great stability of catalyst in the whole electrochemical window. In contrast, the well-defined cathodic peak for the as-prepared catalyst can be observed in the CV curve when the saturating alkaline solution with O2, indicating an apparent ORR 10 catalytic

capacities of this sample. The ORR activity of the catalyst was further evaluated by linear

sweep voltammetry (LSV) on a rotating disk electrode system in the O2-saturated KOH solution. In Figure 4b, the GCN-Co@CoO catalyst shows a half-wave potential of -0.194 V (vs. SCE), which is obviously superior to those of the CN-Co@CoO and GCN electrode. Moreover, the obtained GCN-Co@CoO acquired an outstanding diffusion limit current of 4.91 mA cm-2, which is very close 15 to

that of the Pt/C electrode (4.92 mA cm-2). The presented half-wave potential and diffusion limit

current are also significantly more positive than the recently reported non-noble metal carbon materials.56-58 To better determine the kinetic parameters of the GCN-Co@CoO catalyst for ORR, the LSV tests were conducted at various rotating speeds (Figure S5). The current density of LSV and the rotating speed occurred with a regular pattern so that the electron transfer number (n) could be 20 caculated

by the K-L equation (Figure 4c). Based on the linear Koutecky-Levich equation, the value of

n for the

GCN-Co@CoO was calculated to be 3.92, which is proximity to that of the commercial

Pt/C (n = 4.0), indicating a predominant four-electron process with improved the ORR kinetics (Figure 4d).59-60 To further comprehend the outstanding ORR performance of the resultant samples,



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Figure 5. (a) I-T curves of the GCN-Co@CoO and commercial Pt/C electrodes, (b) Comparison of chronoamperometric responses of 3D GCN-Co@CoO and Pt/C with or without methanol. (c-d) Comparison of CV curves of the 3D GCN-Co@CoO (c) and Pt/C (d) with or without methanol

electrochemically active surface area (ECSA) and electrochemical impedance spectrum (EIS) measuring techniques were also conducted. According to the reported works, electrochemical double-layer capacitances (Cdl) is generally proportional to the value of ECSA, which was obtained from the linear relationship of current density against to the various scan rates (Figure S6). The Cdl value of the GCN-Co@CoO was confirmed to be 6.38 mF cm-2, which is much higher than that of the CN-Co@CoO (4.61 mF cm-2) and GCN (3.25 mF cm-2) (Figure 4e). This results demonstrated that the introduction of nitrogen-doped graphene was significantly increased the additional electroactive sites in catalytic surface. Secondly, the corresponding Nyquist plots clearly inspect the electrode kinetics of the catalysts under the operating conditions. As displayed in Figure 4f, compared with the CN-Co@CoO, graphene was further used to restrict the cobalt species and enhance the contact among cobalt phrase nanoparticles, further improve the electron transfer capacities of the GCN-Co@CoO, ACS Paragon Plus Environment

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resulting in the more available reaction sites within GCN-Co@CoO. The stability of the as-fabricated GCN-Co@CoO and commercial Pt/C catalysts was studied and compared with chronoamperometric measurements at a constant potential of -0.3 V (vs. SCE) in O2-saturated 0.1 M KOH solution (Figure 5a). According to the figures, Pt/C catalyst exhibited an obviously active attenuation about 24.73 % after operating 7 h. the activity loss of GCN-Co@CoO catalyst about only 17.57 %, indicating a superior durability of GCN-Co@CoO. As shown in Figure 5b, the methanol cross tolerance of resultant catalysts and commercial Pt/C was also evaluated, the current response of GCN-Co@CoO just presented a slight wave when a certain amount of methanol was injected. Unexpectedly, current curve for commercial Pt/C shows a rapid wave after the addition of methanol, which the cathodic current was varied to a reversed anodic current within a very short time. Furthermore, a strong evidence, which confirm the conversion of the dominated oxygen reduction to the methanol oxidation reaction in the Pt/C surface when the methanol was injected, was also provide from the CV curves observed in Figure 5c-d. These results clearly show that the GCN-Co @CoO catalyst has outstanding stability and great tolerance compared to Pt / C. The superior performance of GCN-Co @ CoO can be attributed to its unique microstructure, in which metal species are tightly modified on the surface of nitrogen-doped graphene. Further insights into the obtained-observation, the enhanced ORR capacities of the as-prepared GCN-Co@CoO can be ascribed to its several preponderances. Firstly, linear sweep voltammetry exhibits an outstanding limit current density and a positive half-wave potential for GCN-Co@CoO catalyst, which is clearly superior to those of the GCN and CN-Co@CoO. Moreover, the comparison between GCN and CN in the electrochemical capacities, GCN presented a lower resistance and higher ORR catalytic activity to that of the CN (Figure S7). This suggests that graphene served as carbon substrate and played a cross-linked role. Such intimate contact in catalyst guarantees a highly efficient synergetic catalysis between graphene nanosheets and Co@CoO nanoparticles for ORR. Secondly, it’s worth noting that the embedded metal species nanoparticles effectively conquer etching upon exposure ACS Paragon Plus Environment


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to harsh chemical environment owing to the protection from the layered carbon structure, resulting an excellent stability, which conforms to the results of chronoamperometric measurements. Thirdly, the

Figure 6. (a) Operational voltage-time curves, (b) Cathode and Anode (vs. SCE) potential polarization curves, (c) Power density curves and entire potential polarization curves, (d) Hypothetical catalytic mechanism.

nitrogen dopant as extra active sites further improved the intrinsic ORR activity for the as-prepared samples, in which the charge densities of the carbon matrix were modified by the adjacent nitrogen atoms derive from its stronger electronegativities. The performance of GCN-Co@CoO and commercial Pt/C catalysts were further evaluated in MFCs. All of the MFCs with different cathodes were loaded with an external resistance of 1000 Ω. As displayed in Figure 6a, the stability capacities of catalysts were also assessed by a voltage-time curves operated more than 800 h in MFCs. Obviously, the GCN-Co@CoO revealed a stable platform voltage after one week, and there are just a weak wave presented in the platform voltage even after 800 h, indicating that GCN-Co@CoO possessed a long-term stability in catalytic process. In Figure 6b, the


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output voltage in both GCN-Co@CoO and commercial Pt/C showed a distinct decrease with the increase of current density. The diminishing tendency in MFCs-GCN-Co@CoO was lower than that of the MFCs-Pt/C, demonstrating that GCN-Co@CoO possessed a stable catalytic capacity in MFCs device. The maximum output voltage and maximum power density of GCN-Co@CoO were measured to be 0.6 ± 0.01 V and 611 ± 9 mW cm-2 (Figure 6c), respectively, which was very close to that of the commercial Pt/C. Based on the superior electrochemical catalytic capacities in GCN-Co@CoO catalyst, we proposed a possible catalytic process in the interface of GCN-Co@CoO. In Figure 6d, graphene with excellent conductivity and large surface area can quickly accept and transfer electrons (e-) into the active sites. Then, the reduction reaction of O2 coupled with H+ and e- was rapidly induced on the active sites area, in which, the absorption of O2 was a crucial procedure for the whole catalytic process. More interestingly, porous structure as well as rich pyridinic-N in the GCN-Co@CoO was beneficial to strengthen the absorption for O2, further enhanced ORR catalytic capacities of GCN-Co@CoO. CONCLUSIONS In summary, we have demonstrated a multi-dimensional architecture that composed of cobalt/cobalt oxides nanoparticles encapsulated in nitrogen-doped carbon shell coupling with porous graphene hybrid material. The GCN-Co@CoO displays an excellent ORR activity. The outstanding electrocatalystic capacities in ORR were due to the subtle regulating electronic structure of the outer carbon layers and cobalt metallic core. The active sites and electron densities in graphene and carbon shells can be improved by optimizing the pyrolysis temperature, thus further enhance the ORR catalytic activity. Moreover, a stable MFCs device with high power density was fabricated based on the as-prepared GCN-Co@CoO as the cathode catalyst. The self-assembled MFCs generated a maximum power density of 611 ± 9 mW cm-2 at a high current density of 1869 ± 24 mA m-2. This work not only provides a new approach to fabricate high-performance electrocatalytic materials for



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MFCs, but also opens up new opportunities to Li-batteries and supercapacitors.

AUTHOR INFORMATION Corresponding Author Fax: 86-20-39366908; Tel: 86-20-39366908; E-mail: [email protected] (N. Li); [email protected] (Z.-Q. Liu). Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was supported by Natural Science Foundations of China (Grant No. 21875048, 21576056, and 21576057), Guangdong Natural Science Foundation (Grant No. 2017A030311016), Science and Technology Research Project of Guangdong Province (Grant No. 2016A010103043), Major Scientific Project of Guangdong University (Grant No. 2017KZDXM059), Featured Innovation Project of Guangdong University (Grant No. 2017KTSCX142), Science and Technology Research Project of Guangzhou (Grant No. 201607010232 and 201607010263), Guangzhou University’s 2017 Training Program for Young Top-Notch Personnel (BJ201704). Supporting Information Synthesis and characterization, additional structure figures, XRD, SEM, EDX, N2 adsorption isotherms, XPS, LSV, CV data, Table S1 and Table S2. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Sun, M.; Zhai, L. F.; Li, W. W.; Yu, H. Q. Harvest and Utilization of Chemical Energy in Wastes by Microbial Fuel Cells. Chem. Soc. Rev. 2016, 45, 2847-2870, DOI: 10.1039/c5cs00903k. (2) Yuan, H.; Hou, Y.; Abu-Reesh, I. M.; Chen, J.; He, Z. Oxygen Reduction Reaction Catalysts Used in Microbial Fuel Cells for Energy-Efficient Wastewater Treatment: A Review. Mater. Horiz. ACS Paragon Plus Environment

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Table of Contents Graphic

The optimized cobalt@cobalt oxide nanoparticles embedded in N-doped carbon shell coupling with hierarchical porous graphene (GCN-Co@CoO), exhibiting a significantly enhanced ORR activity in alkaline solution and highlighting a synergistic effect between N-doped carbon shell and metallic Co species.


CoO Embedded Nitrogen-Doped Carbon

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