Homogeneously Dispersed Co9S8 Anchored on Nitrogen and Sulfur

Subscriber access provided by University of Sydney Library

Energy, Environmental, and Catalysis Applications

Homogeneously Dispersed Co9S8 Anchored on Nitrogen and Sulfur Co-Doped Carbon Derived from Soybean as Bifunctional Oxygen Electrocatalysts and Supercapacitor Zhen Xiao, Guozheng Xiao, Minhao Shi, and Ying Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 03 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Homogeneously Dispersed Co9S8 Anchored on Nitrogen and Sulfur Co-Doped Carbon Derived from

Soybean

as

Bifunctional

Oxygen

Electrocatalysts and Supercapacitor †

Zhen Xiao1 , Guozheng Xiao1, Minhao Shi1, Ying Zhu1,2* 1

Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of

Education, School of Chemistry, Beihang University, Beijing, 100191, P.R. China 2

Beijing Advanced Innovation Center for Biomedical Engineering, Beihang University, Beijing,

100191, P.R. China.

KEYWORDS: multifunctional electrocatalyst, soybean precursor, heteroatoms doped carbon, cobalt sulfide nanoparticles, porous structure.

ABSTRACT: Developing low-cost and highly active multifunctional electrocatalysts to replace noble metal catalysts is crucial for the commercialization of future clean energy technology. Herein, homogeneous Co9S8 nanoparticles anchored on nitrogen and sulfur co-doped porous carbon nanomaterials (CoS@NSCs) are fabricated by pyrolysis of natural soybean treated with cobalt nitrate. The unique porous structures of the soybean are utilized to provide space for

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 37

oxidation and complexation reactions for cobalt compounds, thus leading to in-situ generation of homogenously dispersed cobalt sulfide nanoparticles that anchored on the N, S co-doped carbon framework. Due to coupling effect of cobalt sulfide and doping heteroatoms, CoS@NSC-800 not only displays excellent electrocatalytic performances with low overpotential and high current density toward both ORR and OER comparable to commercial Pt/C catalyst and IrO2 catalyst, but also might be a promising candidate for high performance supercapacitors. The method for the preparation of the multifunctional hybrids is simple but effective for formation of uniformly distributed metal sulfide nanoparticles anchored on carbon materials, therefore providing a new perspective for the design and synthesis of multifunctional electrocatalysts for electrochemical energy conversion and storage at a large scale.

1. Introduction Given the pressing worry about the global energy and environmental crisis, considerable attention have been paid to developing renewable clean energy conversion and storage devices of next-generation technologies.[1] Fuel cells and supercapacitors have attracted increasing interest because they can provide benefits of high efficiency, low or zero emission, scalability and ease of maintenance.[2-4] However, several previous works have demonstrated that the sluggish kinetics of the electrode reactions, including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and low charge transfer kinetics of supercapacitors, result in severe energy losses during conversion and storage process, thus hindering their commercial applications.[5-12] Therefore, much effort has been paid to developing new materials that could improve the efficiency of energy conversion and storage system. Although currently, the commercially used ORR catalysts are platinum (Pt) and other Pt-based catalysts, and meanwhile the OER catalysts


2

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


are iridium oxide (IrO2) catalysts,[13] these noble-metal catalysts are subject to high cost and scarce resources, moreover, they have limited capabilities for simultaneously catalyzing ORR and OER at both acidic and alkali medium.[9] Therefore, it is attractive to exploit the possibility of multifunctional materials for energy harvesting and electricity storage on an industrial scale. The

heteroatoms-doped

carbon

nanomaterials

are

under

extensive

investigation

as

electrocatalysts for ORR and OER as well as electrode materials for supercapacitor, due to their extraordinary catalytic activity and micro and nano-structures, as demonstrated in previous works.[14-19] Pyrolysis converting biomass to the heteroatoms-doped carbon nanomaterials is an effective strategy for the synthesis of electrocatalysts and electrode materials of high activity.[2022]

Generally, biomass materials consist of organic compounds containing carbon, nitrogen,

sulfur and phosphor, which can introduce heteroatoms into the carbon framework during pyrolysis process, thus improving the electrochemical activity.[23-25] In addition, natural biomass materials have abundant porous structures, which could offer a high surface area in favor of the mass transport.[26-29] For instance, Yu et al.[30] reported N and S-doped carbon materials with large surface area by carbonizing human hair displayed high ORR activity in alkali medium, comparable to that of commercial Pt/C, which can be attributed to the porous structure with high surface area, abundant catalytic active sites resulting from heteroatoms dopants and the improved conductivity. Huang et al.[21] prepared a series of phosphorus-rich carbon materials derived waste coffee, which was constructed to supercapacitors that showed good stability, high capacitance and high energy density. The narrow micropores for revisable electrochemical hydrogen storage, and the unique electronic rearrangement by those phosphorus-rich functional groups can provide a wide electrochemical window of supercapacitor above theoretical potential for water decomposition. Although doped-carbon materials for electrocatalysts and electrodes have made


3


Page 4 of 37

progress, the improved performance is still needed that meets the requirement of practical applications. Recent researches have proved that transitional metal sulfide nanoparticles with excellent catalytic behavior are mostly likely to be employed as multifunctional electrocatalyst.[32-34] But these nanoparticle catalysts generally tend to aggregate and detach from supports in the catalytic reaction process, which could result in serious malfunction during operation.[35] An effective solution is to anchor these active nanoparticles on a chemically stable carbon support. Surprisingly,

the

hybrid

materials

usually

show

unexpectedly

high

electrocatalytic

performance,[36] which can be determined by the morphological features of nanoparticles and the conductivity of carbon support.[37] For example, Ganesan et al.[38] presented simple solid-state thermolysis of cobalt thiourea and graphene oxide (GO) for the in-situ formation of CoS2 nanoparticles grown on the surface of N and S co-doped GO, and realized the controlled particle dimensions by tuning thermo treatment condition. The strong coupling effect of CoS2 and N, SGO led to the significant decrease of the oxygen overpotential, therefore showing an outstanding oxygen electrode activity comparable to that of commercial Pt/C, Ru/C and Ir/C catalysts. Huang et al.[39] reported N-, O-, and S-tridoped carbon-encapsulated Co9S8 nanomaterial, synthesized via pyrolysis of S- and Co(II)-containing polypyrrole solid precursors, were proven to act as noble metal-free bifunctional electrocatalysts for water splitting with small overpotentials, high anodic current densities, low Tafel slopes as well as nearly 100% Faradic efficiencies in alkaline medium. Its excellent electrocatalytic activity was attributed to the synergistic effects between Co9S8 nanoparticles cores and the heteroatom-doped carbon shells. Moreover, Alshareef et al.[41] developed an electrodeposition method for the preparation of nickel cobalt sulfide nanosheet arrays on carbon substrates as electrode materials for supercapacitors. Due to the high


4

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


conductivity, abundant mesoporous and open framework of the three-dimensional structure, the hybrid electrode materials displayed high specific capacitance and rate capability. The asymmetric supercapacitor composed of the ternary nanosheet arrays and porous graphene film demonstrated outstanding electrochemical performance with high energy density and long-term cycling stability. For the electrochemical applications, a cost-effective method for combination both metal sulfide and doped-carbon is highly desirable. Here, we report Co9S8 nanoparticles anchored on N and S co-doped carbon nanomaterials (CoS@NSC-X), fabricated by pyrolysis of natural soybeans treated with cobalt nitrate [Co(NO3)2], potassium hydroxide (KOH) and ammonium persulfate [(NH4)2S2O8]. Soybean is a species of legume native to East Asia, and now is widely grown for its edible beans, which are not only cheap but also available throughout the whole year. Meanwhile, soybeans are rich in plant proteins, thus offering abundant sources of heteroatoms to form active catalytic sites, in particular high N contents. Furthermore, their unique porous structures were utilized to be container for the chemical reactions of additive reactants, Co(NO3)2, KOH and (NH4)2S2O8 to form Co9S8 nanoparticles during pyrolysis. The reactions within the porous structures of soybean have several advantages: firstly, they encapsulate cobalt compounds in the soybean and lead to the desired homogeneous distribution of Co9S8 nanoparticles, as a result of redox reaction between Co(III), OH- and SO42- in high temperature. Secondly, the presence of NH4+, and SO42as well as other heteroatoms-containing substances from the soybean, further contributed to the formation of N, S co-doped carbon as a support of the nanoparticles. The coupling effect between Co9S8 nanoparticles and doping elements can generate great numbers of electrocatalytic active sites on the carbon material. Meanwhile, the considerable numbers of nanopores from the bean it-self and those formed during KOH activation are in favor of electrolyte and oxygen


5


Page 6 of 37

transportation. As expected, the Co9S8 nanoparticles anchored heteroatoms-doped carbon nanomaterial demonstrated excellent performances as electrocatalysts toward ORR and OER. For ORR, the as-prepared CoS@NSC-800 catalyst had a positive half-wave potential, high limit current density, long-term stability and tolerance to CO poisoning and methanol crossover effect in both alkaline and acidic medium, which was superior to commercial Pt/C catalyst. And for OER, CoS@NSC-800 displayed high current density and low overpotential (10 mA cm−2 at 1.61 V vs. RHE), comparable to IrO2 catalyst as well as other similar catalysts from previous reports. Moreover, the as-prepared CoS@NSC-800 showed versatile electrochemical activities as supercapacitor electrodes that provided a high capacitance of 289 F g-1 at a current density of 1 A g-1, comparable to other previous reported transitional metal compound materials. The method for the preparation of cobalt sulfide nanoparticles anchored on heteroatom-doped carbon nanomaterials with multiple electrochemical performances, whose precursor is agriculturally productive soybeans, is cost-effective at a large scale. It is our hope that such technique could help to contribute to green chemistry and environmental friendly society. 2. Experimental 2.1. Preparation of the catalysts Soybeans were purchased from a local vegetable market in Beijing. After being washed by distilled water to remove dirt, 12 grains of beans, around 1.11 g, were immersed into 30 mL Co(NO3)2·6H2O aqueous solution with a mass fraction of 33.3% for 2 days under room temperature. The beans were then placed in a freeze drier and freeze-dried at -50 ºC for 24 h to completely remove water in the beans. The cobalt-containing soybeans were then obtained, whose mass was increased by 28% of its original, indicating that roughly 1 mmol of


6

Page 7 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Co(NO3)2·6H2O was absorbed into the beans. Next, the beans were treated with 10 mL 0.5 M KOH solution at room temperature for 12 h, and subsequently 1.14 g of (NH4)2S2O8 (5 mmol) was ultrasonically dispersed into the system and treated for another 12 h. Again, the beans were freeze-dried for 24 hours to give a mass of 1.25 g, as a result of loss of inorganic salt during the metathesis reaction in the aqueous solution. The as-treated beans were placed in a tube furnace and heated to 700, 800 or 900 °C at a heating rate of 5 °C min-1 and kept for 3 h under nitrogen atmosphere, respectively. The product was treated with 1 M hydrochloric acid for 15 min to dissolve metals and other dissolvable impurities, and then rinsed with ultrapure water for several times until neutrality (pH value=7.0) was reached, and finally dried in oven at 80 °C for 120 min. The product catalysts were denoted as CoS@NSC-X (cobalt sulfide anchored on nitrogen and sulfur co-doped carbon), with X marking the treatment temperature. For comparison, we prepared NSC-X catalysts, which had no chemical treatment. Another 12 grains of beans were immersed into water for 2 days under room temperature. After being freezedried, the soy beans were directly heated to 700, 800 or 900 °C, then treated with hydrochloric acid and ultrapure water and dried in oven. The doping amount of cobalt was also investigated where the Co(NO3)2·6H2O aqueous solution with a mass fraction of 11.1% and 22.2% was used and finally the as-treated beans are heated to 800 °C followed by the same post treatment with the other samples. 2.2. Characterizations The cobalt-containing freeze-dried soybeans and the series of as-prepared catalysts were characterized by Field Emission Scanning Electron Microscopy (FESEM), Transmission


7


Page 8 of 37

Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and

Raman

spectroscopy.

Thermo-gravimetric

analysis

(TGA)

and

nitrogen

absorption/desorption isotherm was also performed on the CoS@NSC-X samples. Details of the instrumental information can be found in the supporting information (see ESI). 2.3. Electrochemical Measurements The electrochemical characterization method was carried out on the electrochemical workstation (CHI 760D, Shanghai) following our previous report.[17] For ORR and OER, the three electrode system was composed of a catalyst-loaded glass carbon disk (0.3 mg cm-1) as the working electrode, saturated calomel electrode as reference electrode and platinum wire as counter electrode. N2 or O2 saturated 0.1 mol L-1 KOH or 0.1 M HClO4 was the electrolyte. Cyclic voltammetry was performed under a scan rate of 50 mV S-1 between -0.8 V to 0.1 V. Linear sweep voltammetry (LSV) was performed under a scan rate of 10 mV S-1 at different rotation speed. I-t curves were obtained by chronoamperometric technique at a bias potential of -0.3 V under a rotation speed of 900 rpm. For supercapacitor property measurement, the catalysts powder, carbon black, and Poly(vinylidene fluoride) (PVDF) with a mass of 8 mg, 1mg and 1 mg, respectively, were mixed and dissolved in N, N-Dimethylformamide (DMF) to form a homogeneous slurry and then transfered onto a carbon paper (1 cm×1 cm). The loading mass of the catalyst was 8 mg for each electrode. The platinum foil and Hg/Hg2Cl2 electrodes were used as the counter and reference electrode, respectively. All tests were conducted in a standard three electrode configuration in a 6 M KOH electrolyte. 3. Results and Discussion


8

Page 9 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 1. Schematic illustration for the preparation of the CoS@NSC-X. The schematic illustration for the preparation of CoS@NSC-X catalysts was shown in Figure 1. The fresh, untreated soybeans were ellipsoidal, about 5 mm in length and 3 mm in width, both the surface and the inside displaying natural yellow. After immersion in the cobalt nitrate solution and freeze-drying, the beans looked plumper and more spherical, with around 4 mm in diameter, and had red color resemble to that of Co(NO3)2 crystal, thus indicating cobalt salt had been absorbed into the beans. After treated with KOH and (NH4)2S2O8, the beans turned bright orange on the both inside and outside. Due to the oxidation of Co2+ by (NH4)2S2O8, the result Co3+ reacted with NH3 to form [Co(NH3)6]3+ complex, thus turning an orange color covering the whole beans. Therefore, the introduction of (NH4)2S2O8 not only served as an oxidant and complexation agent of Co(III) ions, but also provided abundant resources of N and S to form Co9S8 nanoparticles and doped carbon framework. The reactions took place as follows: Co2+ +2OH- → Co(OH)2 ↓ 2Co(OH)2 + S2O82- +2OH- → 2 Co(OH)3 + 2SO42- + H2O Co(OH)3 + 6NH4+ +3OH- → [Co(NH3)6]3+ + 6H2O


9


Page 10 of 37

[Co(NH3)6]3+ + SO42- + (CH2O)n → Co9S8/N,S-Carbon (CH2O)n represents the carbohydrates in the soybean precursor, which were converted to the carbon framework during pyrolysis. What’s more, the additional KOH served as the activation agent for the resultant carbon, which has been demonstrated by previous research:[66] 6KOH + 2C → 2K + 2K2CO3 + 3H2 Thermogravimetric analysis (TGA) curve of the soybean treated with Co(NO3)2 and KOH(NH4)2S2O8, was presented in Figure S1 (see ESI). It can be seen that the loss of 63 wt% in weight at the first step occurs in the temperature ranging from 100 to 450 °C, which may be related to the release of H2O and decomposition of inorganic composites such as ammonium sulfate and nitrate introduced by chemical treatment, as well as other small organic molecules such as plant proteins from the soybean precursor. The next step of weight loss occurred in the range from 450 to 1000 °C, the remaining mass fraction was 26 wt%, 22 wt% and 19 wt% at 700 °C, 800 °C and 900 °C, respectively, which should be associated to the transformation of soybean into doped carbon framework and the formation of Co9S8 nanoparticles between the redox reaction between cobalt and sulfur.[67-69] The decomposition of organic components and activation of KOH may result in the formation of large numbers of micro and nano-sized pores on the carbon framework.


10

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a, b) SEM images of the freeze-dried soybean after pure water treatment, (c, d) SEM images of the as-prepared CoS@NSC-800; (e, f) TEM images of CoS@NSC-800. The morphology of the soybean free from chemical treatment and the as-prepared CoS@NSC800 was characterized by FE-SEM, as shown in Figure 2. From Figure 2 a, it can be seen from cross-section image that soybean consists of large numbers of protrusion with a diameter of 4 µm. The enlarged SEM images (Figure 2 b) showed that there were numbers of pores on the surface of the protrusion, ranging from 100 to 180 nm in diameter. Its surface was relatively smooth. These pores in the soybean may be responsible for the transportation of oxygen and nutrients during the germination process. As a result of the presence of abundant protrusion structures and nano- or micropores, the resulting carbon materials were expected to have large specific surface areas. After chemical treatment and high temperature pyrolysis, the original


11


Page 12 of 37

porous structures of soybean were partially maintained with an enhanced hierarchical morphology, as shown in Figure 2 c. Meanwhile, more microporous on the surface of soybean can be observed due to the thermal decomposition of the organic compounds and KOH activation under high temperature pyrolysis,[44] which greatly increased the surface area of asprepared carbon material. Note that a great number of uniformed nanoparticles, with an average dimension of 33 nm, were found on the catalyst’s surface, as displayed in Figure 2 d. TEM was employed to further investigate the nature of these nanoparticles mentioned in SEM, as shown in Figure 2 e, f and Figure S2. From Figure 2e, one can see that these nanoparticles were homogeneously distributed in CoS@NSC-800, with a diameter range of 8.1~9.6 nm. Those particles in CoS@NSC-700 were in the range of 3.5~7.1 nm, and those in CoS@NSC-900 were 10.4~11.3 nm as seen in Figure S2 b and d, indicating that temperature played a key role in determining the particle size on the surface of carbon materials, as higher temperature favoured the formation of larger nanoparticles.[38] The high-resolution TEM (HRTEM) image of CoS@NSC-800 (Figure 2 f) indicated the cubic crystalline Co9S8 structure with lattice fringe of 2.97, 2.92 and 3.37 Å, corresponding to the (31-1), (11-3) and (20-2) plane of Co9S8, respectively[62]. We also found that the angle between three planes to be about 50.7 and 65.8°, which was in accordance with the theoretical value in cubic crystalline Co9S8.[39] The nanostructured particles, homogeneously distributed and anchored on the carbon framework, might not only make more contact with the electrolyte and oxygen, but also overcome the problem in aggregation and detachment of particles after long-term operation that is common for metal-based nanoparticle electrocatalysts.[35] As a result, the as-prepared catalysts could possess an improved electrocatalytic performances as well as long-term stability. Element mapping (Figure S2 e-j) further indicated the presence of carbon, oxygen, cobalt, sulphur and nitrogen,


12

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


where a uniform distribution pattern of carbon, oxygen and nitrogen can be seen, while that of cobalt and sulfur matched the distribution of the nanoparticles, which was consistent with previous analysis. Nitrogen sorption/desorption tests were employed to obtain the Brunauer-Emmett-Teller (BET) specific surface area and porosity of the CoS@NSC-X based on the Barrett-Joyner-Halenda (BJH) model, as shown in Figure S3 a and b. The BET surface area, according to Figure S3 a, was calculated to be 209, 350 and 357 m2·g−1 for CoS@NSC-700, CoS@NSC-800 and CoS@NSC-900, respectively, demonstrating that a high surface area can be obtained at high temperature resulted from the combination of thermal decomposition and KOH activation. Moreover, as seen from Figure S3 b, the CoS@NSC-X had good mesoporous structures with a narrow peak at 3.7 nm and a broad peak between 15~45 nm was found. It should also be noticed, porous size of CoS@NSC-X increased with increasing carbonization temperature from 700 to 900 °C, which was in consistent with BET surface area analysis. The chemical compositions were investigated by X-ray Diffraction (XRD) and Raman spectra. The XRD was firstly employed in determine the molecular structure after KOH-(NH4)2S2O8 treatment, to verify the proposition regarding the color change of the soybean. As seen from Figure S4, the intermediate had several narrow and obvious peaks corresponding to [Co(NH3)6]Cl3 (Joint Committee on Powder Diffraction Standards, PDF#36-0783), the peak at 15.1°can be ascribed to (131) lattice plane, the one at 16.8° to (202) plane, and those at 24.3°, 24.8°, 25.1° and 36.2° to (131), (331), (060) and (262) plane, respectively, indicating the presence of [Co(NH3)6]3+. Other peaks present corresponded to the inorganic salts generated during the high temperature decomposition and salt metathesis, for example, the strongest peak at 21.58° can be ascribed to the (021) and (111) plane of K2SO4 (PDF#05-0613) and another


13


Page 14 of 37

strong peak at 29.9° can be ascribed to the (031) plane of (NH4)2SO4 (PDF#41-0621). Therefore, it can be confirmed that the color change within the soybean during the chemical treatment is due to the formation of [Co(NH3)6]3+. The formation of cobalt complex can not only facilitate the even distribution of cobalt compounds, but also preserve NH3 in the soybean precursor as to further increase N content in the product carbon catalysts.

Figure 3. (a) Raman spectra, (b) XRD patterns of the CoS@NSC-X. The Raman spectra and XRD patterns of the as-prepared CoS@NSC-X samples were shown in Figure 3. As is seen in Figure 3 a, all three samples had two distinct peaks: The D band centred around 1329 cm−1 can be ascribed to defects in the carbon framework arisen from sp3 atoms, and the G band centred around 1590 cm−1 caused by stretching of sp2 carbon atoms.[45] According to previous studies, the intensity ratio (ID/IG) of the two bands is used to determine the extent of defect for carbon materials, where a higher ratio indicates more defects in the carbon framework and thus able to form more active sites that in turn enhances the ORR catalytic performance.[46] The ID/IG ratios of CoS@NSC-700, CoS@NSC-800 and CoS@NSC-900 are to be calculated 1.32, 1.24 and 1.18, respectively, indicating that the ID/IG ratio would decrease at higher carbonization temperature, because the graphitization degree and ordered structure of the carbon materials would increase. Furthermore, a relatively weaker peak was observed in the lower


14

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


wavenumber region (centered around 670 cm-1), which can be attributed to Co9S8,[39] indicating the presence of cobalt sulfide in the carbon framework. The XRD patterns of as-prepared CoS@NSC-X nanomaterials were shown in Figure 3 b. After comparison with XRD patterns, it could be analysed the diffraction peaks could mainly be attributed to carbon (PDF#50-0926) and cobalt sulfide (Co9S8: PDF#02-1459). The three samples all displayed two strong and broad peaks at 2θ around 15.5° and 22.3°, corresponding to the (101) and (002) lattice plane of carbon, respectively, indicating carbon as the main component of the as-prepared CoS@NSC-X.[47] Other peaks, weaker but still distinct, at 2θ of 29.9°, 47.3°, 51.9° and 73.3°, could be assigned to the lattice planes of (311), (511), (440) and (731) of Co9S8, respectively, from where it can be deducted that the nanoparticles, found in both SEM and TEM images, should be the crystal of Co9S8, as a result of reduction reaction of sulfate under high temperature and combination with cobalt.[39] It should also be noticed that two peaks at 61.3° and 62.2° can be assigned to (533) and (622) lattice plane of Co9S8, but gradually disappeared with the increase of pyrolysis temperature, as can be seen that the two peaks for CoS@NSC-900 and CoS@NSC-800 were weaker than those of CoS@NSC-700, indicating progressive change in chemical composition of cobalt sulfide compound at different reaction temperature.[51] Meanwhile, no apparent peaks related to cobalt oxide species were found, and their absence may be a result of hydrochloric acid treatment, where cobalt oxide was dissolved and detached.


15


Page 16 of 37

Figure 4. (a) XPS survey, (b-f) high-resolution spectra of C1s, O1s, N1s, S2p, and Co2p of CoS@NSC-800. In order to further investigate the composition of the as-prepared catalysts, X-ray photoelectron spectroscopy (XPS) was carried out for CoS@NSC-X and NSC-X samples as well as the


16

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


soybean precursor, as shown in Figure 4 a. The obvious peaks were observed at 161 eV, 284 eV, 398 eV, 531 eV, and 790 eV, corresponding to sulfur, carbon, nitrogen, oxygen and cobalt, respectively. According to quantitative analysis of the XPS survey (Table S1), both the soybean precursor and the NSC-X samples had no cobalt content, proving that cobalt sulfide could only be introduced by the aforementioned chemical treatment. It can also be observed that the heteroatom contents decreased at higher carbonization temperature. For CoS@NSC-700, CoS@NSC-800 and CoS@NSC-900, the relatively high contents of N were 9.20 at%, 5.31 at% and 2.20 at%, and those of S and Co were 1.96 at%, 1.82 at% and 1.15 at%, and 1.05 at%, 0.78 at% and 0.55 at%, respectively. The phenomenon was mainly due to the decomposition of organic compounds (such as proteins and nucleic acids) from the soybean and inorganic compounds (such as cobalt salt, ammonium salt and sulfate) from chemical reactions under high temperature and formed smaller gaseous molecules that eventually escaped to the atmosphere. On the other side, the relative content of C increased at higher carbonization temperature, leading to less heteroatom defects formation in the carbon framework and higher graphite degree, which was consistent with the Raman spectra. It should be noted due to the etching of hydrochloric acid, sodium, potassium and calcium, common elements in biomass materials, were absent from the CoS@NSC-X series. The removal of these salts could lead to the formation of a large number of mesopores that can further increase surface areas of the carbon materials.[17][48] The high resolution XPS spectra for C1s, O1s, N1s, S2p and Co2p of CoS@NSC-800 were shown in Figure 4 b~f and those of CoS@NSC-700 and CoS@NSC-900 were shown in Figure S5. As presented in Figure 4 b, there are a major peak at 284.4 eV corresponding to sp2-hybridized graphitic carbon species, two other peaks at 285.6 eV and 286.7 eV corresponding to C-N/C-S and C-O carbon species, and one broad peak at 289.3 eV corresponding to C=O/C=N carbon


17


Page 18 of 37

species, demonstrating that nitrogen and sulfur were successfully doped into the carbon framework. In Figure 4 c, the high-resolution spectrum for O1s XPS of CoS@NSC-800 had a major peak at 532.4 eV corresponding to -N-O- oxygen species, a minor peak at 533.9 eV to C=O/O-C=O oxygen species and another at 529.8 eV may result from environmental oxygen, absorbed by the porous carbon material.[49] In Figure 4 d, the high-resolution N1s XPS spectrum was deconvoluted into four peaks, corresponding to four types of nitrogen species: pyridinic N at 398.2 eV, pyrrolic N at 400.2 eV, graphitic N at 401.1 eV and high oxidation state N at 403.3 eV.[30] According to previous reports,[20]-[25] pyridinic and graphitic N species are in favor of the ORR catalytic activity, while the other two species have little effect, as pyridinic N species could donate one p electron to the aromatic π system of carbon framework and improve carbon material’s conductivity, and graphitic N could induce charge delocalization and therefore change the chemisorption mode of O2.[19] Therefore, the species of N atoms is also crucial for the electrochemical performance of N-doped carbon materials.[52][53] As calculated from the integrated area in Figure 4 d, Figure S5 e and f, contents of pyridinic and graphitic N species for CoS@NSC-800 were 23.5% and 43.4% (see Table S2). Compared to the N high-resolution XPS spectra of CoS@NSC-700 (21.4% for pyridinic N and 24.3% for graphitic N) and CoS@NSC-900 (26.8% and 17.5%), CoS@NSC-800 had a higher content of overall effective N species, which would result in a better electrocatalytic performance of CoS@NSC-800. In Figure 4 e of S2p XPS spectrum of CoS@NSC-800, two peaks at 168.5 eV and 169.5 eV can be assigned to high oxidation state sulfur (SOxn−), as a result of incomplete reduction of the sulfate generated during the chemical treatment.[54] The peaks at 163.5 eV and 164.8 eV can be related to sulfur atoms doped in the carbon framework (C-S-C),[57] which can be attributed to the mismatch of the outermost orbitals of sulphur and carbon when sulphur is doped


18

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to carbon, so that the spin density would arise and thus participating ORR electrons transfer process as synergistic factor with nitrogen.[55] Moreover, the peak at 161.6 eV can be assigned to S2−, corresponding to cobalt sulfide.[38] In Figure 4 f of the Co2p XPS spectrum, a sharp peak at 779.6 eV was observed, corresponding to metallic cobalt (Co-Co).[31] At 782.2 eV and 795.6 eV, two main peaks were observed, which can be assigned to 2p3 and 2p1 electron, respectively. And meanwhile, two satellite peaks at 786.5 eV and 802.6 eV were found, as a result of different oxidation number among the cobalt atoms, in accordance with the presence of Co9S8.[36] Taken together, these results confirmed that CoS@NSC-X were composed of N and S co-doped carbon and Co9S8 nanoparticles anchored on the carbon framework, which may improve electrocatalytic ORR performance of the carbon materials.


19


Page 20 of 37

Figure 5. (a) Cyclic voltagrammetry (CV) curves of CoS@NSC-700, CoS@NSC-800, CoS@NSC-900 and commercial Pt/C in N2 (dash lines) or O2 (solid lines) saturated 0.1 M KOH solution with a scan rate of 50 mV s-1. (b) Linear sweep voltammetry (LSV) curves of CoS@NSC-700, CoS@NSC-800, CoS@NSC-900 and Pt/C catalysts in O2 saturated 0.1 M KOH solution with a scan rate of 10 mV s-1 and rotation speed of 2000 rpm. (c) The LSV curves of CoS@NSC-800 at rotation speed from 400 to 1600 rpm. (d) The K-L plots of CoS@NSC-800 catalyst, inset: the electron transfer number (n) under different potentials. The ORR electrocatalytic activity evaluation of the as-prepared catalysts in alkaline media were performed on the electrochemical workstation. Figure 5 a was the CV curves of CoS@NSC-X and commercial Pt/C in N2 or O2-saturated 0.1 M KOH solution. As seen in the CV curves, no apparent peak was found in N2-saturated solution, while in contrast, from -0.20 V to -0.10 V, a single peak was observed in all the curves in O2-saturated solution, corresponding to electrocatalytic reduction of oxygen on the electrode. Among the curves of the three samples, it can be seen that CoS@NSC-800 had the most positive peak potential of -0.121 V, which was comparable to that of commercial Pt/C catalyst (-0.112 V). However, at peak potential, the current density of CoS@NSC-800 (-1.50 mA cm-2) was a lot higher than Pt/C (-0.88 mA cm-2), indicating CoS@NSC-800’s excellent electrocatalytic activity toward ORR. The ORR performances of CoS@NSC-700 and CoS@NSC-900 were clearly inferior to CoS@NSC-800, whose peak potential was -0.180 V and -0.145 V, with a current density of -0.91 mA cm-2 and 1.59 mA cm-2, respectively. Figure 5 b was the LSV curves of the as-prepared samples and Pt/C at a constant rotation rate of 2000. It was easily noted that among the LSV curves, the limit current density of CoS@NSC-800 (-6.24 mA cm-2) was the highest, not only higher than that of CoS@NSC-700 (-4.93mA cm-2) and CoS@NSC-900 (-5.15 mA cm-2), but also exceeded that of


20

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt/C (-5.70 mA cm-2). Meanwhile, the onset potential (0.155 V) and half-wave potential (-0.134 V) of CoS@NSC-800 were both more positive than those of Pt/C, indicating a lower O2 overpotential on the electrode, thus suggesting CoS@NSC-800 have superior electrocatalytic activity toward ORR. Although from the quantitative analysis of XPS, CoS@NSC-800’s content of N and S as well as cobalt was lower than that of CoS@NSC-700, and the specific surface area was smaller than that of CoS@NSC-900, we believe it was the synergistic effect both of heteroatoms dopant and specific surface area that contributed to CoS@NSC-800’s superior ORR catalytic activity. As previously demonstrated, the heteroatoms could induce charge delocalization, which could improve the conductivity of the carbon support, and form strong coupling with the Co9S8 nanoparticles to generate numbers of active catalytic sites, while a large surface area could ensure full contact of electrolytes and oxygen to the catalyst material, thus the synergistic effect of the resultant should present a superior electrocatalytic activity.[39][43] We compared the ORR activity between CoS@NSC-800 and other catalysts with similar structures reported in recent years and the result was shown in Table S3, including Co@Co3O4@PPD[34], CoS2(400)/N,SGO[38], NiCo2O4–G[46], Fe–N–C[58], Co9S8/graphene[62], and Co0.5Fe0.5S@N-MC[63], and found an analogous pattern of ORR reaction where they all possessed comparable or even higher limit current density, compared to commercial Pt/C, suggesting the effectiveness of doping metal composites into carbon framework. What’s more, among them, CoS@NSC-800 not only displayed higher limit current density per loading amount, but it also demonstrated a more positive half-wave potential, suggesting an excellent electrocatalytic activity. It was noteworthy that our method required no sophisticated chemical precursor or preparation procedure, it should be more cost-effective and easily scaled up.


21


Page 22 of 37

As ORR performance in acidic environment is also important in fuel cell systems, the same evaluation was conducted in 0.1 M HClO4 solution and the result was shown in Figure S6. From the CV curves (Figure S6 a), it can be seen that in acidic medium ORR the reduction peak potential for CoS@NSC-700 and CoS@NSC-900 shifted to 0.38 V and 0.40 V, a lot inferior to that of Pt/C at 0.54 V, but CoS@NSC-800 demonstrated an excellent performance with a peak potential of 0.46V and peak current of -1.1 mA cm-2, close to those of Pt/C (-1.4 mA cm-2). From the LSV curves (Figure S6 b), a similar result was found as both CoS@NSC-700 and CoS@NSC-900 had diminished limit current density of -3.6 and -4.3 mA cm-2, while CoS@NSC-800 maintained a limit current density of -5.2 mA cm-2, close to that of Pt/C (-5.8 mA cm-2). Therefore, we found that CoS@NSC-800 demonstrated relatively good ORR catalytic performances in both alkali and acidic media, which was very close to that of commercial Pt/C. In order to further clarify the catalytic kinetic process, RDE tests at rotation speed from 400 to 1600 rpm was carried out for CoS@NSC-X. The result was shown in Figure 5 c and Figure S7 a and c, between the potential of -0.3 to -0.9 V, the catalytic current density of all three samples increased only with increasing rotation speed, regardless of potential change, suggesting a diffusion-controlled oxygen reduction mechanism. The transferred electron number (n) during ORR was calculated by the Koutechy-Levich (K-L) equations:

=

+

.

(1)

= 0.2 ( )/ ν/ (2) according to previous reports.[45] The result of the calculation was shown in Figure 5 d and Figure S7 b and d. The average transferred electron number between -0.2 and -0.5 V for CoS@NSC-700, CoS@NSC-800 and CoS@NSC-900 was 3.59, 3.91 and 4.00, respectively, thus demonstrating a four-electron reaction mechanism, which was similar to the commercial Pt/C


22

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalyst. Moreover, the rotating ring-disk electrode (RRDE) tests were further conducted on the CoS@NSC-X samples as well as Pt/C to confirm the ORR reaction pathway by detecting the formation of HO2- and thus deriving the electron transfer number.[17] The result was shown in Figure S8 a. The formation of HO2- and the number of transferred electrons (n) can be calculated by the following equations:

%( ) = 200 =4

/ ! " /

(3)

! ! " /

(4)

Where, jD and jR represents the disk and the ring current, respectively, and N is current collection efficiency of the ring, which in this case is about 0.24. The result derived from RRDE was presented in Figure S8 b, where n was calculated to be 3.9 for CoS@NSC-800, 3.7 for CoS@NSC-700 and 3.4 for CoS@NSC-900, consistent with the results obtained from RDE measurements. The ORR performance evaluation for NSC-X was also carried out, as shown in Figure S9. For the CV curves, the peak potential was -0.27, -0.24 and -0.19V for NSC-700, NSC-800 and NSC900, respectively, which was more negative than the CoS@NSC-X. Their current densities, in both CV and LSV curves, were also a lot smaller than those of CoS@NSC-X, and the average transferred electron number of NSC-700, NSC-800 and NSC-900 was 2.42, 2.61, 2.63, respectively, indicating a two-electron reduction process, which were a lot inferior to those of CoS@NSC-X catalysts, thus indicating that Co9S8 nanoparticles play an important role for enhancing ORR catalytic performance. It’s also interesting to investigate the effect of the cobalt nitrate treatment on the ORR activity of the final catalysts, therefore, the CV and LSV curves of the catalysts treated by a series of Co(NO3)2 aqueous solution with a mass fraction of 0% (NSC800), 11.1%, 22.2% and 33.3% (CoS@NSC-800) were shown in Figure S10. The foreseeable


23


Page 24 of 37

pattern can be seen from both the CV and LSV curves where a higher mass fraction of Co(NO3)2 led to higher ORR activity, indicated by the positively shifting peaks and half-wave potential as well as increasing current density from 0% to 33.3%. It should also be noted that the improvement from 22.2% to 33.3% was much less that from 0% to 11.1% or to 22.2%, suggesting further increasing the concentration of Co(NO3)2 might not lead to more electrocatalytically active catalysts. To understand CoS@NSC-800 practical applicability, its stability and tolerance to methanol and CO were also studied. At a constant potential of -0.3 V and a rotation speed of 900 rpm, the current density-time curve of both CoS@NSC-800 and commercial Pt/C was shown in Figure 6.

Figure 6. (a) Stability evaluation for CoS@NSC-800 compared by commercial Pt/C catalyst at a constant potential of -0.3 V and a rotation speed of 900 rpm in O2-saturated 0.1 M KOH solution,


24

Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


I-t curves for 12 hours of uninterrupted operation, (b) tolerance against CO at a constant rate of 60 mL min–1, (c) methanol of 5 mL. As seen from Figure 6 a, after 12 hours of uninterrupted operation, there was only 7.6% decrease in the current density of CoS@NSC-800, while commercial Pt/C catalyst displayed 16.3% decrease under the same condition, demonstrating a superior stability of CoS@NSC-800 catalyst. The excellent stability of CoS@NSC-800 can be ascribed to the strong covalent force between carbon and heteroatoms (nitrogen and sulfur), thus making the carbon material more chemically stable. Meanwhile, the anchored cobalt sulfide nanoparticles on the surface of carbon framework may also enhance long-term durability.[50] The resistance to methanol and CO of both CoS@NSC-800 and Pt/C through a current-time chronoamperometry measurement were shown in Figure 6 b and c. When CO was bubbled into the solution at 300 s, both CoS@NSC-800 and Pt/C showed a direct decline of current density. After 2400 s, Pt/C suffered a significant 46% loss, while CoS@NSC-800 maintained more than 78% of its original current density, thus proving an excellent tolerance against CO of CoS@NSC-800. For commercial Pt/C catalyst, carbon monoxide can be a strong ligand with platinum to form coordination complexes thus decreasing the number of active catalytic sites of the commercial Pt/C catalyst, leading to serious malfunction. As for CoS@NSC-800, Co9S8 nanoparticles were supported on carbon framework and coupled by N, S co-doped carbon, which would be reluctant to react with CO, thus showing good tolerance to CO poisoning.[33] Moreover, when 5 mL of methanol (1 M) was introduced to the system at 300 s, the current density of Pt/C dropped more than 60% immediately, and then restored slowly to 75% of its original value at 2400 s. It was reported that methanol is more easily oxidized by Pt/C, which produced a positive current of oxidation,[60] thus leading to the drastic decrease in the current density. Meanwhile the current density on CoS@NSC-800 showed


25


Page 26 of 37

a slight decrease less than 5% at 2400 s, indicating that CoS@NSC-800 had superior selectivity on electrocatalysis toward ORR. As a result, CoS@NSC-800 demonstrated excellent stability and tolerance to methanol and CO, therefore may become a promising candidate for precious metal catalysts toward cathodic ORR of fuel cells. Subsequently, the electrocatalytic performances toward oxygen evolution reaction (OER) of CoS@NSC-X and IrO2 catalyst were measured in O2-saturated 0.1 M KOH solution, as displayed in shown in Figure 7.

Figure 7. (a) OER activity evaluation for CoS@NSC-700, CoS@NSC-800, CoS@NSC-900 and IrO2 catalyst in O2-saturated 0.1 M KOH solution with a scan rate of 10 mV s-1 and rotation speed of 1600 rpm. (b) OER performance comparison of different transitional metal catalysts (NCo9S8/G,[62] Co0.5Fe0.5S@N-MC[63] and CoS2(500)/N,S-GO[38] from literatures and CoS@NSC800 from this work). Form Figure 7 a, it can be seen that between 1.40~1.65 V, CoS@NSC-800 had a similar pattern to that of IrO2, and at above 1.7 V, CoS@NSC-800 displayed larger current density than IrO2 as well as other CoS@NSC-700 and CoS@NSC-900. As OER potential at I = 10 mA cm−2 vs RHE (V) is often used to describe the activity of the OER catalysts,[58] the potential for CoS@NSC-


26

Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


700, CoS@NSC-800 and CoS@NSC-900 was found to be 1.64 , 1.61 and 1.62 V, respectively, indicating CoS@NSC-800’s relatively high OER activity, which was comparable to IrO2 catalyst (1.60 V). Moreover, we compared the OER performances of CoS@NSC-800 with some other transitional metal catalysts from literatures,[38], [62][63] shown in Figure 7b. At the potential of 1.55, 1.6 and 1.7 V, a relatively high current density of CoS@NSC-800 was reached, thus demonstrating a high OER activity, comparable or even better than some other state-of-art transitional metal catalysts reported elsewhere.

Figure 8. (a) CV curve of CoS@NSC-800 at different scan rate. (b) Galvanostatic charge/discharge curves of CoS@NSC-800 with different current density at a potential window of -0.4 V to 0.6 V. (c) Specific capacitance versus current density of CoS@NSC-700,


27


Page 28 of 37

CoS@NSC-800 and CoS@NSC-900 calculated from discharge curves. (d) Performance comparison of different transitional metal nanocomposites (Fe3O4-C nanosheets,[40] Ni-Co HC[64] and Fe3O4-DCN[65] from literatures and CoS@NSC-800 from this work) for Supercapacitors. The capacitance properties of the CoS@NSC-X were tested in 6 M KOH solution, and the results were shown in Figure 8 and Figure S11. As seen from Figure 8 a, at different scan rate, the CV curves of CoS@NSC-800 showed good rectangular characteristics and no apparent peaks were found, indicating a rapid ion transfer and high rate performance.[61] At the scan rate of 10 mV S-1, the specific capacitance of CoS@NSC-800, calculated from the integrated area, reached as high as 315 F/g, indicating its application for electrode material as supercapacitors. The excellent supercapacitor performance can be attributed to its porous structure and improved conductivity, which were in favor of electrolyte and electron transfer. However, compared to CoS@NSC-800, the CV curves of CoS@NSC-700 and CoS@NSC-900 were narrow and had low current density, showing relatively small capacitance. To further study the specific capacitance, galvanostatic charge/discharge experiment was carried out on the CoS@NSC-X, as shown in Figure 8 b and Figure S11 b and d. All the charge/discharge curves at different current density displayed a triangular shape, suggesting a well-balanced charge storage.[61] Also, the smaller current density was, the longer time it took to finish a charge/discharge cycle, typical of supercapacitor. It should be also noted that at smaller current density, the discharging time became longer than charging time than those at higher current density. Such phenomenon might be ascribed to the synergistic effect of the Faradaic pseudocapacitance and the static double-layer capacitance.[41] At higher current density, the reaction time could only accommodate the doped carbon framework in the double-layer capacitor reaction, therefore a symmetric charge/discharge curve was observed. While at smaller current density with extended reaction time, cobalt sulfide


28

Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


participated in the redox reaction and due to the synergistic effect of the pseudocapacitor and the double-layer capacitor, an asymmetric charge/discharge curve was observed where the discharging time was longer than that of charging part.[70] Based on the discharge part of the curves, we calculated the specific capacitance which was presented in Figure 8 c. At the current density of 1 A g-1, a maximum capacitance of 289 F g-1 was reached by CoS@NSC-800, a lot higher than that of CoS@NSC-700 (230 F g-1) and CoS@NSC-900 (207 F g-1). The capacitance declined when current density increased, and gradually became stable between 3 to 10 A g-1, with a capacitance around 120 F g-1 for CoS@NSC-800, 88 F g-1 for CoS@NSC-700 and 64 F g-1 for CoS@NSC-700, suggesting that CoS@NSC-800 had a good performance for supercapacitors with a high and relative stable capacitance. To know whether the carbon material can be of practical use, the cyclic stability of CoS@NSC-800 was studied and the result was shown in Figure S12. It could be clearly seen that after 10,000 cycles of operation, the sample still had a capacitance retention of 80%, demonstrating a excellent cyclic stability, which was consistent with the ORR duration test. At the same current density of 1 A g-1, the supercapacitor performance of CoS@NSC-800 was compared with those of other transitional metal nanocomposites,[40], [64][65] as shown in Figure 8d. The specific capacitance of Fe3O4-C nanosheets, Ni-Co HC and Fe3O4-DCN was 163, 127 and 285 F g-1, respectively, indicating CoS@NSC-800, with a specific capacitance of 289 F g-1, had excellent performance for supercapacitors. 4. Conclusions In summary, Co9S8 nanoparticles anchored on N and S co-doped porous carbon (CoS@NSC-X) multifunctional catalysts were fabricated from natural soybeans through simple chemical


29


Page 30 of 37

treatment and subsquent pyrolysis. We found that the unique hierarchically porous structures of the soybean precursor provided room for chemical reactions of cobalt nitrate, potassium hydroxide and ammonium persulfate, thus leading to homogeneously distributed Co9S8 nanoparticles anchored on the N, S co-doped carbon framework after pyrolysis. The hybrid materials displayed excellent performances on various electrocatalysis due to the strong coupling between Co9S8 and the N, S co-doped carbon support, which greatly improved the electrocatalytic activity and stability. Among them, CoS@NSC-800 exhibited excellent electrocatalytic performance towards ORR and OER in both alkaline and acidic media, with a positive half wave potential of -0.134 V (vs Hg/Hg2Cl2) as well as a high limit current density of -6.24 mA cm-2 for ORR in alkaline media, superior to those of commercial Pt/C catalyst (-0.17 V and 5.70 mA cm-2), and a potential of 1.61 V (vs RHE) at I = 10 mA cm-2 for OER, comparable to IrO2 catalyst and other catalysts in previous reports. Moreover, and as supercapacitor electrodes, CoS@NSC-800 had a specific capacitance of 289 F g-1 at current density of 1 A g-1, superior to other transitional metal carbon composite catalysts. Our preparation method may provide a new perspective for the design and synthesis for multi-functional electrocatalysts of metal sulfide nanoparticles anchored on heteroatoms doped carbon materials.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Supplementary material of characterization method, SEM characterization, BET data, Raman


30

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


spectroscopy measurements, further details of XPS analysis and other electrochemical measurements (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Present Addresses † Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The work is supported by the National Natural Science Foundation of China (51473008, 51672019), the National Key Research and Development Program of China (2017YFA0206900). The author would also like to thank Dr. Vicki Colvin for her suggestion on the analysis of XPS and XRD. REFERENCES [1]. [2]. [3].

Yang, C., An impending platinum crisis and its implications for the future of the automobile, Energy Policy, 2009, 37,1805–1808. Zhang, S., Shao, Y., Yin, G., Lin, Y., Recent progress in nanostructured electrocatalysts for PEM fuel cells, J. Mater. Chem. A, 2013, 15, 4631–4641. Steele, B. C., Heinzel, A., Materials for fuel-cell technologies, Nature, 2001, 414, 345– 352.


31


[4]. [5].

[6]. [7].

[8].

[9].

[10].

[11].

[12].

[13].

[14].

[15].

[16].

[17].

[18]. [19].

Page 32 of 37

Lu, X., Yu, M., Wang, G., Tong, Y., Li, Y., Flexible solid-state supercapacitors: design, fabrication and applications, Energy Environ. Sci., 2014, 7, 2160–2181. Zhou, X., Qiao, J., Yang, L., & Zhang, J., A Review of Graphene‐Based Nanostructural Materials for Both Catalyst Supports and Metal‐Free Catalysts in PEM Fuel Cell Oxygen Reduction Reactions, Advanced Energy Materials, 2014, 4, 1289-1295. Debe, M., Electrocatalyst approaches and challenges for automotive fuel cells, Nature, 2012, 486, 43–51. Morozan, A., Jousselme, B., Palacin, S., Low-platinum and platinum-free catalysts for the oxygen reduction reaction at fuel cell cathodes, Energy Environ. Sci., 2011, 4, 1238– 1254. Candelaria, S. L., Shao, Y., Zhou, W., Li, X., Xiao, J., Zhang, J. G., Cao, G., Nanostructured Carbon for Energy Storage and Conversion, Nano Energy, 2012, 1, 195– 220. Kong, F. D., Zhang, S., Yin, G. P., Zhang, N., Wang, Z. B., & Du, C. Y., Pt/porous-IrO2 nanocomposite as promising electrocatalyst for unitized regenerative fuel cell. Electrochem. Commun., 2012 14, 63-66. Zhou, W., Wu, X. J., Cao, X., Huang, X., Tan, C., Tian, J., Zhang, H., Ni3S2 nanorods/Ni foam composite electrode with low overpotential for electrocatalytic oxygen evolution, Energy Environ. Sci. 2013, 6, 2921−2924. Shao, Y., El-Kady, M. F., Wang, L. J., Zhang, Q., Li, Y., Wang, H., Kaner, R. B., Graphene-based materials for flexible supercapacitors, Chem. Soc. Rev., 2015, 44, 36393665. Shang, Y., Wang, C., He, X., Li, J., Peng, Q., Shi, E., ... & Li, Y., Self-stretchable, helical carbon nanotube yarn supercapacitors with stable performance under extreme deformation conditions, Nano Energy, 2015, 12, 401-409. Antolini, E., Iridium as catalyst and cocatalyst for oxygen evolution/reduction in acidic polymer electrolyte membrane electrolyzers and fuel cells, ACS Catal., 2014, 4, 1426– 1440. Fellinger, T. P., Hasché, F., Strasser, P., Antonietti, M., Mesoporous nitrogen-doped carbon for the electrocatalytic synthesis of hydrogen peroxide, J. Am. Chem. Soc., 2012, 134, 4072–4075. Feng, L., Yan, Y., Chen, Y., Wang, L., Nitrogen-doped carbon nanotubes as efficient and durable metal-free cathodic catalysts for oxygen reduction in microbial fuel cells, Energy Environ. Sci., 2011, 4, 1892–1899. Yang, W., Fellinger, T., Antonietti, M., Efficient metal-free oxygen reduction in alkaline medium on high-surface-area mesoporous nitrogen-doped carbons made from ionic liquids and nucleobases, J. Am. Chem. Soc., 2011, 133, 206–209. Xiao, Z., Gao, X., Shi, M., Ren, G., Xiao, G., Zhu, Y., & Jiang, L., China rose-derived tri-heteroatom co-doped porous carbon as an efficient electrocatalysts for oxygen reduction reaction, RSC Advances, 2016, 6, 86401-86409. Qu, L., Liu, Y., Baek, J., Dai, L., Nitrogen-doped graphene as efficient metal-free electrocatalyst for oxygen reduction in fuel cells, ACS Nano, 2010, 4, 1321–1326. K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Nitrogen-doped carbon nanotube arrays with high electrocatalytic activity for oxygen reduction, Science, 2009, 323, 760–764.


32

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[20].

[21].

[22].

[23].

[24].

[25]. [26].

[27].

[28].

[29].

[30]. [31].

[32].

[33].

[34].

Choi, C., Park, S., Woo, S., Heteroatom doped carbons prepared by the pyrolysis of bioderived amino acids as highly active catalysts for oxygen electro-reduction reactions, Green Chem., 2011, 13, 406–412. Huang, C., Sun, T., Hulicova‐Jurcakova, D., Wide electrochemical window of supercapacitors from coffee bean-derived phosphorus-rich carbons, ChemSusChem, 2013, 6, 2330–2339. Song, M. Y., Park, H. Y., Yang, D. S., Bhattacharjya, D., Yu, J. S., Seaweed-derived heteroatom-doped highly porous carbon as an electrocatalyst for the oxygen reduction reaction, ChemSusChem, 2014, 7, 1755–1763. Liu, S., Tian, J. Q., Wang, L., Zhang, Y. W., Qin, X. Y., Luo, Y. L., Asiri, A. M., AlYoubi, A. O., Sun, X. P., Hydrothermal treatment of grass: A low-cost, green route to nitrogen-doped, carbon-rich, photoluminescent polymer nanodots as an effective fluorescent sensing platform for label-free detection of Cu(II) ions, Adv. Mater, 2012, 24, 2037–2041. Qian, W. J., Sun, F. X., Xu, Y. H., Qiu, L. H., Liu, C. H., Wang, S. D., Yan, F., Human hair-derived carbon flakes for electrochemical supercapacitors. Energy Environ. Sci., 2014, 7, 379–386. Chaudhari, N. K., Song, M. Y., Yu, J.-S., Heteroatom-doped highly porous carbon from human urine. Sci. Rep., 2014, 4, 5221. Zhu, H., Yin, J., Wang, X., Wang, H., Yang, X., Microorganism Derived Heteroatom Doped Carbon Materials for Oxygen Reduction and Supercapacitors, Adv. Mater., 2013, 23, 1305-1312. Chen, F., Gong, A. S., Zhu, M., Chen, G., Lacey, S. D., Jiang, F., Song, J., Mesoporous, Three-Dimensional Wood Membrane Decorated with Nanoparticles for Highly Efficient Water Treatment, ACS Nano, 2017, 11, 4275-4282. Li, Y., Zhao, Y., Cheng, H. H., Hu, Y., Shi, G. Q., Dai, L. M., Qu, L. T., Nitrogen-doped graphene quantum dots with oxygen-rich functional groups, J. Am. Chem. Soc., 2012, 134, 15–18. Cao, R. G., Thapa, R., Kim, H., Xu, X. D., Gyu Kim, M., Li, Q., Park, N., Liu, M. L., Cho, J., Promotion of oxygen reduction by a bio-inspired tethered iron phthalocyanine carbon nanotube-based catalyst, Nat. Commun., 2013, 4, 2076. Chaudhari, K. N., Song, M. Y., Yu, J. S., Transforming hair into heteroatom-doped carbon with high surface area, Small, 2014, 10, 2625–2636. Ren, G., Li, Y., Guo, Z., Xiao, G., Zhu, Y., Dai, L., Jiang, L., A bio-inspired Co3O4polypyrrole-graphene complex as an efficient oxygen reduction catalyst in one-step ball milling, Nano Research, 2015, 8, 3461-3471. Oh, H., Kim, H., The role of transition metals in non-precious nitrogen-modified carbonbased electrocatalysts for oxygen reduction reaction, Power Sources, 2012, 212, 220– 225. Xiang, Z., Xue, Y., Cao, D., Huang, L., Chen, J. F., Dai, L., Highly Efficient Electrocatalysts for Oxygen Reduction Based on 2D Covalent Organic Polymers Complexed with Non‐precious Metals, Angew. Chem. Int. Ed., 2014, 53, 2433-2437. Wang, Z., Li, B., Ge, X., Goh, F. W., Zhang, X., Du, G., Zong, Y., Co@Co3O4@PPD Core@bishell Nanoparticle-Based Composite as an Efficient Electrocatalyst for Oxygen Reduction Reaction, Small, 2016, 12, 2580-2587.


33


[35].

[36].

[37].

[38].

[39].

[40].

[41].

[42]. [43].

[44]. [45].

[46].

[47].

[48].

[49]. [50].

Page 34 of 37

Deng, D., Yu, L., Chen, X., Wang, G., Jin, L., Pan, X., Bao, X., Iron encapsulated within pod‐like carbon nanotubes for oxygen reduction reaction. Angew. Chem. Int. Ed., 2013, 52, 371-375. Liang, Y., Li, Y., Wang, H., Zhou, J., Wang, J., Regier, T., Dai, H., Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nature Materials, 2011, 10, 780-786. Wu, G., More, K. L., Johnston, C. M., Zelenay, P., High-performance electrocatalysts for oxygen reduction derived from polyaniline, iron, and cobalt, Science, 2015, 332, 443447. Ganesan, P., Prabu, M., Sanetuntikul, J., Shanmugam, S., Cobalt sulfide nanoparticles grown on nitrogen and sulfur codoped graphene oxide: an efficient electrocatalyst for oxygen reduction and evolution reactions. ACS Catal., 2015, 5, 3625-3637. Huang, S., Meng, Y., He, S., Goswami, A., Wu, Q., Li, J., Wu, M., N‐, O‐, and S‐ Tridoped Carbon‐Encapsulated Co9S8 Nanomaterials: Efficient Bifunctional Electrocatalysts for Overall Water Splitting, Adv. Funct. Mater., 2017, 27, 1606585 Liu, D.; Wang, X.; Wang, X.; Tian, W.; Liu, J.; Zhi, C.; He, D.; Bando, Y.; Golberg, D., Ultrathin nanoporous Fe3O4-carbon nanosheets with enhanced supercapacitor performance, J. Mater. Chem. A, 2013, 1, 1952−1955. Chen, W., Xia, C., Alshareef, H. N., One-step electrodeposited nickel cobalt sulfide nanosheet arrays for high-performance asymmetric supercapacitors, ACS Nano, 2014, 8, 9531-9541. Zhang, J., Xia, Z., Dai, L., Carbon-based electrocatalysts for advanced energy conversion and storage, Science Advances, 2015, 1, e1500564. Wang, J., Zhou, J., Hu, Y., Regier, T., Chemical interaction and imaging of single Co3O4/graphene sheets studied by scanning transmission X-ray microscopy and X-ray absorption spectroscopy, Energy Environ. Sci., 2013, 6, 926-934. Zhu, Y., Murali, S., Stoller, M., Carbon-based Supercapacitors Produced by Activation of Graphene, Science, 2011, 332,1537–1541. Guo, Z., Xiao, Z., Ren, G., Xiao, G., Zhu, Y., Dai, L., Jiang, L., Natural tea-leaf-derived, ternary-doped 3D porous carbon as a high-performance electrocatalyst for the oxygen reduction reaction, Nano Research, 2016, 9, 1244-1255. Lee, D. U., Kim, B. J., Chen, Z., One-pot synthesis of a mesoporous NiCo2O4 nanoplatelet and graphene hybrid and its oxygen reduction and evolution activities as an efficient bi-functional electrocatalyst, J. Mater. Chem. A, 2013, 1, 4754-4762. Wen, P., Gong, P., Sun, J., Wang, J., Yang, S., Design and synthesis of Ni-MOF/CNT composites and rGO/carbon nitride composites for an asymmetric supercapacitor with high energy and power density, J. Mater. Chem. A, 2015, 3, 13874-13883. Wu, G., Johnston, C. M., Mack, N. H., Artyushkova, K., Ferrandon, M., Nelson, M., Zelenay, P., Synthesis–structure–performance correlation for polyaniline–Me–C nonprecious metal cathode catalysts for oxygen reduction in fuel cells, J. Mater. Chem. 2011, 21, 11392–11405. Collins, P. G., Bradley, K., Ishigami, M., Zettl, A., Extreme oxygen sensitivity of electronic properties of carbon nanotubes, Science, 2000, 287, 1801-1804. Li, Y., Zhou, W., Wang, H., Xie, L., Liang, Y., Wei, F., Dai, H., An oxygen reduction electrocatalyst based on carbon nanotube-graphene complexes. Nat. Nanotechnol., 2012, 7, 394-400.


34

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[51].

[52].

[53].

[54].

[55].

[56]. [57].

[58].

[59].

[60].

[61].

[62]. [63].

[64].

[65].

Ziegelbauer, J. M., Olson, T. S., Pylypenko, S., Alamgir, F., Jaye, C., Atanassov, P., Mukerjee, S., Direct spectroscopic observation of the structural origin of peroxide generation from Co-based pyrolyzed porphyrins for ORR applications, J. Phys. Chem. C, 2008, 112, 8839-8849. Li, L., Liu, E., Yang, Y., Shen, H., Huang, Z., Xiang, X., Nitrogen-containing carbons prepared from polyaniline as anode materials for lithium secondary batteries, Mater. Lett., 2010, 64, 2115–2117. Rincón, R. A., Masa, J., Mehrpour, S., Tietz, F., & Schuhmann, W., Activation of oxygen evolving perovskites for oxygen reduction by functionalization with Fe-Nx/C groups, Chem. Commun., 2014, 50, 14760−14762. Böttger-Hiller, F., Mehner, A., Anders, S., Kroll, L., Cox, G., Simon, F., Spange, S., Sulphur-Doped Porous Carbon from a Thiophene-Based Twin Monomer, Chem. Commun, 2012, 48, 10568–10570. Sun, Y., Liu, C., Grauer, D. C., Yano, J., Long, J. R., Yang, P., Chang, C. J., Electrodeposited cobalt-sulfide catalyst for electrochemical and photoelectrochemical hydrogen generation from water, J. Am. Chem. Soc., 2013, 135, 17699−17702. Gorlin, Y., Jaramillo, T. F., A bifunctional nonprecious metal catalyst for oxygen reduction and water oxidation, J. Am. Chem. Soc., 2010, 132, 13612− 13614. Yang, Z., Yao, Z., Li, G., Fang, G., Nie, H., Liu, Z., Huang, S., Sulfur-doped graphene as an efficient metal-free cathode catalyst for oxygen reduction, ACS Nano, 2011, 6, 205211. Dresp, S., Luo, F., Schmack, R., Kühl, S., Gliech, M., Strasser, P., An efficient bifunctional two-component catalyst for oxygen reduction and oxygen evolution in reversible fuel cells, electrolyzers and rechargeable air electrodes, Energy Environ. Sci., 2016, 9, 2020-2024. Kim, M., Nam, D. H., Park, H. Y., Kwon, C., Eom, K., Yoo, S., Kwon, H., Cobalt-carbon nanofibers as an efficient support-free catalyst for oxygen reduction reaction with a systematic study of active site formation, J. Mater. Chem. A, 2015, 3, 14284-14290. Narayanamoorthy, B., Datta, K. K. R., Eswaramoorthy, M., Balaji, S. Highly active and stable Pt3Rh nanoclusters as supportless electrocatalyst for methanol oxidation in direct methanol fuel cells, ACS Cat., 2014, 4, 3621-3629. Li, Y., Ren, G., Zhang, Z., Teng, C., Wu, Y., Lu, X., Jiang, L., A strong and highly flexible aramid nanofibers/PEDOT:PSS film for all-solid-state supercapacitors with superior cycling stability, J. Mater. Chem. A, 2016, 4, 17324-17332. Dou, S., Tao, L., Huo, J., Wang, S., Dai, L., Etched and doped Co9S8/graphene hybrid for oxygen electrocatalysis, Energy Environ. Sci., 2016, 9, 1320-1326. Shen, M., Ruan, C., Chen, Y., Jiang, C., Ai, K., Lu, L., Covalent entrapment of cobalt– iron sulfides in N-doped mesoporous carbon: Extraordinary bifunctional electrocatalysts for oxygen reduction and evolution reactions, ACS Appl. Mater. Interfaces, 2015, 7, 1207-1218. Zhao, Y., Ma, H., Huang, S., Zhang, X., Xia, M., Tang, Y., Ma, Z. F., Monolayer nickel cobalt hydroxyl carbonate for high performance all-solid-state asymmetric supercapacitors, ACS Appl. Mater. Interfaces, 2016, 8, 22997−23005. Wang, L., Yu, J., Dong, X., Li, X., Xie, Y., Chen, S., Song, Y., Three-dimensional macroporous carbon/Fe3O4-doped porous carbon nanorods for high-performance supercapacitor, ACS Sustainable Chem. Eng., 2016, 4, 1531−1537.


35


[66].

[67].

[68].

[69].

[70].

[71].

[72].

Page 36 of 37

Lv, Y., Zhang, F., Dou, Y., Zhai, Y., Wang, J., Liu, H., Xia, Y., Tu, B. and Zhao, D. A comprehensive study on KOH activation of ordered mesoporous carbons and their supercapacitor application. J. Mater. Chem., 2012, 22, 93–99 Feng, L.L., Li, G.D., Liu, Y., Wu, Y., Chen, H., Wang, Y., Zou, Y.C., Wang, D. and Zou, X. Carbon-armored Co9S8 nanoparticles as all-pH efficient and durable H2-evolving electrocatalysts. ACS Appl. Mater. Interfaces, 2015, 7, 980-988. Feng, L.L., Fan, M., Wu, Y., Liu, Y., Li, G.D., Chen, H., Chen, W., Wang, D. and Zou, X. Metallic Co9S8 nanosheets grown on carbon cloth as efficient binder-free electrocatalysts for the hydrogen evolution reaction in neutral media. J. Mater. Chem. A, 2016, 4, 6860-6867. Liu, Y., Li, Q., Si, R., Li, G.D., Li, W., Liu, D.P., Wang, D., Sun, L., Zhang, Y. and Zou, X. Coupling Sub‐Nanometric Copper Clusters with Quasi‐Amorphous Cobalt Sulfide Yields Efficient and Robust Electrocatalysts for Water Splitting Reaction. Adv. Mater., 2017, 29, 1606200. Owusu, K.A., Qu, L., Li, J., Wang, Z., Zhao, K., Yang, C., Hercule, K.M., Lin, C., Shi, C., Wei, Q. and Zhou, L. Low-crystalline iron oxide hydroxide nanoparticle anode for high-performance supercapacitors. Nat. Commun., 2017, 8, 14264. Wang, M., Lin, M., Li, J., Huang, L., Zhuang, Z., Lin, C., Zhou, L. and Mai, L. Metal– organic framework derived carbon-confined Ni2P nanocrystals supported on graphene for an efficient oxygen evolution reaction. Chem. Commun., 2017, 53, 8372-8375. Huang, S., Meng, Y., He, S., Goswami, A., Wu, Q., Li, J., Tong, S., Asefa, T. and Wu, M. N-, O-, and S-Tridoped Carbon-Encapsulated Co9S8 Nanomaterials: Efficient Bifunctional Electrocatalysts for Overall Water Splitting. Adv. Funct. Mater., 2017, 27, 1606585.


36

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The Table of Contents Homogeneously Dispersed Co9S8 Anchored on Nitrogen and Sulfur Co-Doped Carbon Derived from Biomass as Bifunctional Oxygen Electrocatalysts and Supercapacitor Zhen Xiao, Guozheng, Xiao, Minhao Shi, Ying Zhu* Co9S8 nanoparticles anchored on N and S co-doped porous carbon nanomaterials are fabricated by pyrolysis of natural soybean through simple chemical treatment. Due to coupling effect of Co9S8 and doping heteroatoms of the carbon framework, CoS@NSC-800 not only displays excellent electrocatalytic performances with low overpotential and high current density toward both ORR and OER, but also might be a promising candidate for electrode material of high performance supercapacitors. Keywords: multifunctional electrocatalyst, soybean precursor, heteroatoms doped carbon, cobalt sulfide nanoparticles, porous structure.

TOC Figure


37

Homogeneously Dispersed Co9S8 Anchored on Nitrogen and Sulfur

Recommend Documents