Co9S8-Modified N, S ... - ACS Publications

Sep 27, 2017 - Company, Shanghai, China) in a standard three-electrode system, where a platinum sheet and a saturated calomel electrode (SCE) were use...
0 downloads 0 Views 3MB Size
Download PDF
Research Article Cite This: ACS Sustainable Chem. Eng. 2017, 5, 9848-9857

pubs.acs.org/journal/ascecg

Co9S8‑Modified N, S, and P Ternary-Doped 3D Graphene Aerogels as a High-Performance Electrocatalyst for Both the Oxygen Reduction Reaction and Oxygen Evolution Reaction Xiu-Xiu Ma, Xu-Hong Dai, and Xing-Quan He* School of Materials Science and Engineering, Changchun University of Science and Technology, Weixing Road, No. 7989, Changchun, 130022 Jilin, P. R. China S Supporting Information *

ABSTRACT: Efficient and robust electrocatalysts for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play key roles in energy conversion and storage devices. In this work, we construct a bifunctional catalyst by homogeneously dispersing Co9S8 nanoparticles on a nitrogen (N), sulfur (S), and phosphate (P) ternary-doped 3D graphene aerogel (NSPG) matrix via a facile pyrolysis method. Because of the N, S, and P ternary-doping effects, abundant defects and channels are produced in the edge of graphene, resulting in a high specific BET surface area (SBET) of 657 m2 g−1. With the incorporation of Co9S8 nanocrystals into NSPG, the produced catalyst (Co9S8/NSPG-900) possesses an SBET of 478 m2 g−1 with porous characteristics. The synergy of structure and composition features enables fast electrochemical kinetics of the catalyst, leading to efficient ORR activity with a half-wave potential of 0.800 V versus the reversible hydrogen electrode (RHE), a limiting current density of 7.26 mA cm−2, high stability and CH3OH tolerance, a high OER performance with an overpotential of 343 mV at the current density of 10 mA cm−2, and excellent long-term stability. As a bifunctional catalyst for both the ORR and OER, it delivers a potential gap of 820 mV, comparable to state-of-the-art bifunctional catalysts. KEYWORDS: N, S, and P ternary-doping, oxygen reduction reaction, oxygen evolution reaction

■

INTRODUCTION The oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play key roles in energy conversion and storage devices,1−3 and the two processes have stimulated the development of catalysts. Currently, most of the electrocatalysts rely on noble metals, such as Pt, RuO2, or IrO2; however, the sluggish kinetics of the ORR and OER, the easy polarization, and the high cost make them unpractical for large-scale applications.4−6 Consequently, it is urgent to develop highly efficient catalysts using cost-effective nonprecious-metal-containing materials. Graphene has drawn intensive attention in catalysis, and it is regarded as a semiconductor with zero bandgap. For the tailoring of its electronic properties, heteroatom-doping on the chemical surface is an effective method,4,7 which works in the conductivity and active sites for ORR/OER.4,8 N-doping can shift the bandgap and leads to an n-type semiconductor; specific graphitic N was of high conductivity to promote ORR,9,10 and pyridinic N was the active center for OER.10 S-doping can result in a stable structure with high charge and spin densities,4,11 and P-doping forms a small bandgap. N-, S-, and P-doping in carbon materials can promote more formation of active sites for electrocatalysis due to the synergistic couplings,4 and resulted in improved electrocatalytic activity.12 However, the ORR/OER performance is still unsatisfied. Cobalt composites have been exploited as © 2017 American Chemical Society

alternatives due to their high electrical properties; nevertheless, the intrinsic electrical conductivity and low surface area block its further improvement of catalytic performance. For optimization of this, hybrids combining cobalt composites and carbon materials have been constructed.13,14 For example, Li et al. successfully prepared a hybrid composed of Co2P on multielement codoped graphene, which exhibited excellent bifunctional performance for both the ORR and OER.15 With the introduction of ultra-thin-walled Co9S8 nanotubes on graphene, a large surface area with exposed active sites resulted, and the catalyst showed excellent catalytic activity.16 In addition, porous features of Co9S8/C could advance the ORR catalytic performance; for instance, with the assembly of Co9S8 nanoparticles (NPs) with carbon having a porous structure, electron and reactant diffusion was accelerated, leading to high ORR activity.1,14 Studies have confirmed that the activity of cobalt sulfides depends on the chemical environment of cobalt active sites, and it can be improved by modification of doping ions, other functional groups, and substrates.5,17 Dou et al. successfully realized N-doping in graphene and N-etching Co9S8 simultaneously,18 Received: June 6, 2017 Revised: September 7, 2017 Published: September 27, 2017 9848

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering

ultrasonication to generate a uniform catalyst ink. Then, 27.5 μL of the catalyst slurry was drop-cast on the surface of a glassy carbon electrode (rotating disk electrode, diameter ∼5.00 mm; rotating ring disk electrode, ∼5.61 mm). The catalyst loading was kept at 0.283 mg cm−2 during all of the electrochemical measurements. The electrolyte was 0.1 M KOH for the ORR and 1.0 M KOH for the OER. Before the electrochemical testing, N2/O2 was purged into the electrolyte for ∼0.5 h, and the gas flow was maintained during tests. Cyclic voltammetry (CV) curves were acquired on a rotating disk electrode (RDE) in the potential window from 0.2 to −0.8 V versus SCE at a scan rate of 100 mV s−1 for several tens of cycles until steady CV curves were obtained. Linear sweep voltammetry (LSV) polarization curves for ORR were obtained on a RDE at rotating speeds ranging from 200 to 2500 rpm with a scan rate of 10 mV s−1. LSV plots for the OER were collected on a RDE at 1600 rpm and 10 mV s−1. Rotating ring disk electrode (RRDE) measurements were taken in O2-saturated 0.1 M KOH at 1600 rpm and 10 mV s−1. The electron transfer number (n) and yield of hydrogen peroxide during the ORR were calculated on the basis of the following eqs 1 and 2 according to the RRDE curves:24,25

and the obtained hybrid exhibited outstanding ORR and OER performance because N-doping resulted in a modulating electronic structure and abundant active sites. Additionally, with the decoration of Fe3O4 on Co3S4, hierarchical porous characteristics can be generated, and the synergistic coupling between compositions boosted the OER activity.5 To achieve a cobalt sulfide modified porous carbon structure, three-dimensional (3D) graphene aerogel was an intriguing matrix due to its high surface area with interconnected features in facilitating mass transfer and sufficient room for interior volume expansion and easy insertion of NPs.7,19 Wang’s group explored the defects and oxygendoping effects of 3D carbon cloth on the ORR and OER performance via a plasma treatment,20 and layered double hydroxides were also optimized to exhibit excellent ORR/OER activity.21,22 Therefore, building Co9S8 on N, S, and P ternary-doped graphene aerogels as an ORR and OER catalyst is appealing. In this work, we successfully designed N, S, and P ternarydoped 3D graphene aerogels (NSPGs) grown with Co9S8 NPs. The 3D structure afforded high surface area and abundant accessible channels for mass transport; the introducing of N, S, and P on the edge of graphene created a large number of defects, and enabled the anchored Co9S8 NPs to expose more active actives and strengthen the interaction to improve the stability. Attributing to the above benefits, the proposed Co9S8/ NSPG-900 hybrid showed superior ORR and OER performance.

■

n=

4id id + ir /N

%HO2− = 200 ×

(1)

ir / N id + ir /N

(2)

where id and ir are the disk and ring currents, respectively. N (37%) is the collection efficiency of the Pt ring calculated by a K3Fe(CN)6 redox reaction in a N2-saturated 0.1 M KOH solution. All the recorded potentials in the experiments were reported versus the reversible hydrogen electrode (RHE) on the basis of the following equation:

EXPERIMENTAL SECTION

Chemical Materials. Graphite powder was purchased from Sinopharm Chemical Reagent Co., Ltd. Pt/C and RuO2 were purchased from Alfa Aesar. Chemical regents including thiourea, glucose, Co(OAc)2·4H2O, KOH, KMnO4, H2SO4, and H2O2 were of analytic grade and used directly as purchased. Synthesis of Nitrogen and Sulfur Codoped Graphene Aerogels (NSGs). Graphite oxide (GO) was prepared via a modified Hummers method.23 A 40 mg portion of GO was dispersed in 20 mL of deionized water under ultrasonication for 2 h to form a suspension. Then, 80 mg of glucose and 80 mg of thiourea were slowly added into the GO suspension, and the mixture was ultrasonicated for 15 min. Next, it was transferred into a 50 mL Teflon-lined autoclave and stored at 180 °C for 12 h. After cooling down, a block cake appeared. Then, it was subjected to freeze-drying at vacuum conditions, and the obtained product was N- and S-codoped graphene aerogels (NSGs). Synthesis of Co9S8/NSPG Hybrid. For the synthesis of Co9S8/ NSPG, a pyrolysis procedure was used. A 50 mg portion of Co(OAc)2· 4H2O and 200 mg of NSG were mixed in 10 mL of deionized water, and vibrated for 48 h at ambient temperature for adsorption of Co2+ on the surface of NSG due to the electrostatic interactions. Then, the solution was filtrated to remove unadsorbed Co2+. Subsequently, the wet residue was dispersed in 10 mL of phosphate buffer solution (PBS; pH = 7.4), and vibrated for 12 h. Then, the above mixture was displaced with water several times, and it was subjected to a freeze-drying process to obtain a bulk precursor. Next, the precursor was held to 900 °C for 2 h at a ramp rate of 5 °C min−1 in the center of a furnace under an argon flow. The pyrolyzed product was named as Co9S8/NSPG-900. For comparison, Co9S8/NSPG-800 and Co9S8/NSPG-1000 were also prepared with a similar method to Co9S8/NSPG-900 only tuning the pyrolysis temperature to 800 and 1000 °C, respectively. In addition, Co9S8/NSG-900 and NSPG-900 were also produced like Co9S8/NSPG900 only with the absence of PBS and Co(OAc)2·4H2O, respectively. Electrochemical Measurements. Electrochemical tests for the prepared materials were taken on a CHI workstation (Chenhua Company, Shanghai, China) in a standard three-electrode system, where a platinum sheet and a saturated calomel electrode (SCE) were used as the counter electrode, and reference electrode, respectively. A catalystmodified glassy carbon electrode acted as the working electrode. For preparation of the working electrode, 2 mg of Co9S8/NSPG-900 catalyst was dispersed in 1 mL of ethanol/Nafion (99.8:0.2, v/v) under

E(RHE) = E(SCE) + E ⊖(SCE) + 0.059pH Characterizations. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images were collected from a JSM-6701F instrument at an operating voltage of 10 kV and a Tecnai G220 S-TWIN instrument at 200 kV, respectively. SEM associated energydispersive X-ray (EDX) mapping was performed on a focused ion beam scanning electron microscope. Raman spectra were collected from a Tri vista 555CRS Raman spectrometer under a 532 nm laser excitation wavelength. Nitrogen sorption isotherms were performed on a gas analyzer at 77 K to obtain the BET surface area and BJH pore-size distributions. XPS measurements were conducted using Al Kα (1486 eV) radiation.

■

RESULTS AND DISCUSSION Composition and Structure Characterizations. Raw materials have critical roles in the microstructures of produced materials.5 In the process of fabricating the Co9S8/NSPG-900 catalyst, Co(OAc)2·4H2O, thiourea, glucose, PBS, and graphite oxide were chosen to be the starting materials. Under high temperature, thiourea, a pore-creating agent in the 3D structure, decomposed to produce NH3 and H2S, which simultaneously acted as the N-and S-doping resources and the S source to the formation of Co9S8 due to the affinity between Co2+ and S2−.6,26 The pyrolysis process assisted Co2+ to convert Co9S8.26 Glucose acted as the carbon resource on one hand, and functioned as the ligand to form aerogels on the other hand. During the pyrolysis procedure, thiourea reduced graphene oxide partially, and heat treatment at 900 °C completely reduced it. The released gas replaced carbon atoms,26,27 and successfully allowed N-, S-, and P-doping in graphene. SEM characterizations indicated the formation of 3D interconnected frameworks for nitrogen, sulfur, and phosphate ternary-doped 3D graphene aerogels (NSPG-900), as seen in Figure S1a (Supporting Information). The 3D structure with porous characteristics avoided the stack of graphene sheets, provided a high surface area, afforded abundant channels for 9849

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering

the surface of the catalyst was rough and crumbled because of the shrinkage of graphene sheets under calcination. In Figure 1c−h, the SEM associated with elemental mapping confirmed the uniform distribution of coexisting C, O, N, S, P, and Co elements. For further examination of the microstructure of Co9S8/ NSPG-900, TEM and high-resolution TEM (HRTEM) characterizations were executed. The TEM characterization of Co9S8/NSPG-900 demonstrated a similar structure with the SEM results, as shown in Figure 1i. Obviously, Co9S8 NPs were supported on the NSPG matrix, and the clear lattice fringes with a d-spacing of 0.28 nm were assigned to the (222) plane of Co9S8 in the corresponding HRTEM image in Figure 1j,28 which revealed the high crystallinity of Co9S8 NPs. Benefited from the 3D structure, NSPG-900 retained a high specific Brunauer−Emmett−Teller (BET) surface area of 657 m2 g−1, and 478 m2 g−1 for Co9S8/NSPG-900 measured by the N2 sorption isotherms, as shown in Figure S2a and Figure 2a, respectively. The insertion of Co9S8 NPs reduced the relative BET surface area. The distinct hysteresis loop at the relative pressure of 0.4−0.9 indicated a classical IV curve of Co9S8/ NSPG-900 in Figure 2a, and the rapid adsorption at a high pressure suggested meso- and macroporous characteristics of Co9S8/NSPG-900,18,24 in accordance with SEM results. The Barrett−Joyner−Halenda (BJH) pore-size distribution curve in Figure 2b and Figure S2b displayed a wide pore-size distribution ranging from 1 to 60 nm and 1 to 40 nm, suggesting porous characteristic for Co9S8/NSPG-900 and NSPG-900, respectively. Additionally, the BET surface areas, pore sizes, and pore volumes of NSPG-900, Co9S8/NSPG-800, Co9S8/NSPG-900, and Co9S8/NSPG-1000 are listed in Table S1. It can be seen that Co9S8/NSPG-900 possessed the highest specific BET surface area and average pore size compared with Co9S8/NSPG-800 and

mass transport, and enabled the evolution and release of gas bubbles on the surface of the catalyst, resulting in close contact between catalyst and electrolyte.5 Co9S8 NPs were built on NSPG with average sizes of ∼50 nm in diameter, which can be observed from the low- and high-magnification SEM images of Co9S8/NSPG-900 (Figure 1a,b). The morphology of

Figure 1. (a) Low- and (b) high-magnification SEM images of Co9S8/ NSPG-900. (c−h) EDX elemental mapping of Co9S8/NSPG-900. (i) TEM and (j) HRTEM images of Co9S8/NSPG-900.

Co9S8/NSPG-900 duplicated the nanoporous architectures of NSPG-900, avoiding the aggregation of Co9S8 NPs; additionally,

Figure 2. (a) N2 adsorption−desorption isotherms, (b) BJH pore-size distribution, and (c) XRD profile of Co9S8/NSPG-900. (d) Raman spectra of the investigated samples. 9850

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. (a) XPS survey, high-resolution XPS spectra of (c) S 2p, (e) N 1s, and (g) P 2p of Co9S8/NSPG-800, Co9S8/NSPG-900, and Co9S8/NSPG1000. (b) High-resolution Co 2p XPS of Co9S8/NSPG-900. Content percentages of deconvoluted (d) S 2p, (f) N 1s, and (h) P 2p species in Co9S8/ NSPG-800, Co9S8/NSPG-900, and Co9S8/NSPG-1000, respectively.

Co9S8/NSPG-1000, which enabled facilitation of more exposure of active sites and faster mass transfer kinetics.24 The XRD pattern in Figure 2c shared the feature of Co9S8 phase with characteristic diffraction peaks centered at around 29.8° (311), 31.6° (222), 36.6° (400), 39.4° (331), 41.4° (420),

45.1° (422), 47.8° (511), 51.8° (440), 58.1° (531), 60.8° (533), 61.7° (622), 68.1° (640), and 73.8° (731),6,16 and Co9S8 NPs showed high crystallinity, in good agreement with the HRTEM results. In addition, the absence of diffraction peaks of other NPs indicated the pure phase of Co9S8. For evaluation of the weight 9851

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. (a) CV polarization curves of Co9S8/NSPG-900 in N2-/O2-saturated 0.1 M KOH. (b, d) LSV curves of Co9S8/NSPG-900 and 20 wt % Pt/C, respectively, on an RDE. (c, e) Corresponding K−L plots of Co9S8/NSPG-900 and 20 wt % Pt/C at different potentials, respectively. (f) LSV profiles on an RDE at 1600 rpm and 10 mV s−1 of the investigated samples.

with the EDX mapping. Obviously, the high-resolution of Co 2p XPS showed two characteristic peaks at 779.8 and 781.8 eV for Co 2p3/2 and Co 2p1/2, and a satellite peak at ∼788.0 eV in Figure 3b, suggesting the presence of Co9S8.11,29 Figure 3c displayed the high-resolution S 2p XPS spectrum, as can be seen; split S species at around 163.9 and 165.0 eV were assigned to S 2p3/2 and 2p1/2 in Co9S8, respectively,6,11 and binding energy at 162.0 and 168.3 eV corresponded to thiophene S and SOx, respectively.4,29 The N 1s XPS spectrum in Co9S8/NSPG-900 can be split into four peaks at binding energies centered at around 398.1, 399.2, 401.1, and 402.4 eV, corresponding to pyridinic N, pyrrolic N, graphitic N, and oxidized N, respectively (Figure 3e).29 In addition, the information on P 2p was also analyzed in Figure 3g. P−C, P−N, and C−O−PO3 bonds were at the binding energies of 132.7, 133.5, and 134.5 eV, respectively. In addition, both Co9S8/NSPG-800 and Co9S8/NSPG-1000 shared similar deconvoluted S 2p, N 1s, and P 2p features with Co9S8/NSPG-900, as presented in Figure 3c,e,g. The relative contents of these deconvoluted S 2p, N 1s, and P 2p species in Co9S8/NSPG-800, Co9S8/NSPG-900, and Co9S8/NSPG-1000

content of Co9S8 in the hybrids, thermogravimetric analysis (TGA) was conducted under air atmosphere at a ramp rate of 10 °C min−1. As shown in Figure S3, the final weight at 800 °C was about 10%, 15%, and 12% in Co9S8/NSPG-800, Co9S8/ NSPG-900, and Co9S8/NSPG-1000, respectively, corresponding to Co9S8. Raman spectra for the prepared samples in Figure 2d presented the general features of carbon materials with D and G bands at around 1350 and 1590 cm−1, which were indexed to disorder and graphitic structures, respectively.6 The intensity ratio of the D band to G band (ID/IG) was used to evaluate the extent of defects,25,29 and it followed the following order: 1.26 (Co9S8/ NSPG-800) < 1.28 (NSPG-900) < 1.31 (Co9S8/NSG-900) < 1.34 (Co9S8/NSPG-900) < 1.36 (Co9S8/NSPG-1000). The higher ID/IG intensity of Co9S8/NSPG-900 as compared to that of Co9S8/NSG-900 indicated the role of P in creating more defects.15 Co9S8/NSPG-900 showed an ID/IG value of 1.34, ascribing to the balance between the disorder structure caused by N, S, and P ternary-doping at the edge of graphene and the graphitic structure. XPS visualized the elemental compositions of C, O, N, S, P, and Co in Co9S8/NSPG-900, as presented in Figure 3a, in line 9852

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering

Figure 5. (a) RRDE measurement of Co9S8/NSPG-900 and 20 wt % Pt/C. (b) Electron transfer number and (c) yield of hydrogen peroxide of Co9S8/ NSPG-900 and 20 wt % Pt/C obtained from the corresponding RRDE profiles. (d) Tafel plots of the samples. (e) Current−time chronoamperometric response and (f) methanol tolerance for Co9S8/NSPG-900 and 20 wt % Pt/C.

P, and Co in NSG-900, Co9S8/NSPG-800, Co9S8/NSPG-900, and Co9S8/NSPG-1000 are also listed in Table S2. The specific contents of deconvoluted N, S, and P species are also presented in Table S3. It can be seen that Co9S8/NSPG-900 possessed the highest thiophene S content, the highest total content of pyridinic N and graphitic N, and the highest C−O−PO3 content, which promoted the excellent ORR/OER performance of Co9S8/NSPG-900. Evaluation of ORR Performance. Cobalt at mixed valence in cobalt sulfides is a promoter in electrocatalysis because the variable valences afford donor−acceptor sites for the adsorption of oxygen and enable electron hopping with the surface redox centers, leading to high electrocatalytic activity.5,29,32 Consequently, the designed Co9S8/NSPG-900 electrode catalyst could perform with excellent electrocatalytic properties. The electrochemistry of Co9S8/NSPG-900 toward the ORR was first assessed by cyclic voltammetry (CV) measurements on an RDE in the potential widow 0.2−1.2 V versus RHE at a scan rate of 100 mV s−1 in N2-/O2-purged 0.1 M KOH, as seen in Figure 4a. A distinct cathode reduction peak appeared in an O2-saturated

were shown in Figure 3d,f,h. Clearly, Co9S8/NSPG-900 possessed the highest content of thiophene S (Figure 3d), which was effective for the enhancement of conductivity because of its strong electron-donor and electron-reduction ability.4 The content of the S element in NSG-900 was 1.02%, and the S 2p XPS spectrum was analyzed, as shown in Figure S4. The relative contents for deconvoluted SOx, S 2p3/2, and S 2p1/2 were 37.2%, 38.65%, and 22.56%, respectively. Compared with those corresponding contents in Co9S8/NSPG-900 (Table S2), it can be deduced that the oxidized S at the zigzag edge was converted to Co9S8 and thiophene S.13 The pyrrolic N content decreased with the increase of pyrolysis temperature (Figure 3f), and those for the pyridinic N and graphitic N increased because of the conversion of pyrrolic N to pyridinic N and graphitic N at higher annealing temperatures (see Table S3).29,30 In addition, Co9S8/ NSPG-900 also possessed the highest contents of pyridinic N and graphitic N, and previous studies have confirmed that pyridinic N and graphitic N were active sites in catalysis.4,30,31 A higher temperature promoted a more effective P-doping (C−O−PO3).4 Additionally, the element contents of C, O, N, S, 9853

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. (a) LSV curves of the samples for the OER on an RDE at 1600 rpm and 10 mV s−1. (b) Tafel plots of Co9S8/NSPG-900 and RuO2 for the OER. (c) Durability of Co9S8/NSPG-900 for the OER by LSV plots before and after 1000 cycles. (d) Electrocatalytic evaluation for both the ORR and OER.

ORR activity at 900 °C in regards to onset potential (Eo), E1/2, and JL, as shown in Figure 4f. Simultaneously, Co9S8/NSPG-900 also exhibited the largest peak current density compared to Co9S8/NSPG-800 and Co9S8/NSPG-1000 in the corresponding CV polarization curves, as displayed in Figure S5. Consequently, 900 °C was the optimum temperature, and other counterparts were heat-treated at the same temperature. In Figure S6, NSPG900, Co9S8/NSG-900, Co9S8/NSPG-800, and Co9S8/NSPG1000 showed similar ORR characteristics in the aspects of LSV curves and corresponding K−L plots, and in Figure 4f, the Co9S8/NSPG-900 hybrid showed better ORR activity than Co9S8/NSG-900 and NSPG-900 in terms of E0, E1/2, and JL, confirming that the P coupling and the main active sites of Co9S8 promoted the improvement of ORR activity. Moreover, Co9S8/ NSPG-900 showed the smallest Nyquist circle in the electrochemical impedance spectroscopy (EIS) results compared with Co9S8/NSPG-800 and Co9S8/NSPG-1000 (Figure S7), demonstrating its quickest charge transfer ability.24 The rotating ring disk electrode (RRDE) technique was then used to further shed light on the mechanism of Co9S8/NSPG900 for the ORR, as presented in Figure 5a. It was found that Co9S8/NSPG-900 possessed catalytic parameters with E1/2 of 0.800 V versus RHE, and JL of 7.26 mA cm−2, like those of 20 wt % Pt/C (E1/2, 0.830 V versus RHE; JL, 5.47 mA cm−2), agreeing well with the RDE results. These ORR parameters were comparable to state-of-the-art ORR catalysts, as given in Table S4. The electron transfer number (n) and yield of hydrogen peroxide species (%HO2−) during the ORR were calculated based on eqs 1 and 2, respectively, and the results are displayed in Figure 5b,c. It can be observed that the electron transfer number was 3.90−3.92 in the potential range 0.25−0.55 V versus RHE (Figure 5b); accordingly, the calculated yield of HO2− was less than 5.0% (Figure 5c). These results demonstrated the Co9S8/NSPG-900-catalyzed

electrolyte, whereas no peak was observed in the N2-saturated one, identifying the response to the ORR for the Co9S8/NSPG900 catalyst. Linear sweep voltammetry (LSV) curves of Co9S8/ NSPG-900 in Figure 4b showed a higher current density with the increase of rotating speeds like that of 20 wt % Pt/C (Figure 4d) because of the reduced diffusion routes, and the corresponding Koutecky−Levich (K−L) plots from the mixed kinetics− diffusion control region to the diffusion one exhibited linearity and parallelism (Figure 4c), similar to that of 20 wt % Pt/C (Figure 4e), certifying the first-order kinetics of dissolved O2 and similar kinetic processes.32 The ORR activity is always judged by the half-wave potential (E1/2) and the limiting current density (JL). As seen in Figure 4f, the Co9S8/NSPG-900 catalyst gave rise to an E1/2 of 0.800 V versus RHE, only 30 mV more negative than 20 wt % Pt/C (0.830 V versus RHE), and a JL of 7.26 mA cm−2, higher than the 5.47 mA cm−2 of 20 wt % Pt/C. The close E1/2 of our obtained electrode catalyst to that of 20 wt % Pt/C demonstrated its rapid kinetics process for the ORR. The 1.3 times higher JL of Co9S8/NSPG-900 than that of 20 wt % Pt/C confirmed the better efficiency of the catalyst in catalyzing the ORR in 0.1 M KOH. In addition, Co9S8/NSPG-1000 showed higher ORR activity than Co9S8/NSPG-800 in terms of limiting current density, and according to the XPS analysis (Table S3), Co9S8/NSPG-1000 was of higher content of C−O−PO3 species than Co9S8/NSPG-800; therefore, it can be concluded that C−O−PO3 served as main active sites for the ORR. The higher ORR activity of Co9S8/NSPG-900 than those of Co9S8/NSPG800 and Co9S8/NSPG-1000 was mainly due to the synergistic effects among the active N, S, and P species and Co9S8 active forms. Pyrolysis temperature was a critical parameter in producing active sites, and different pyrolysis temperatures led to different activities. By optimizing the pyrolysis temperature (800, 900, and 1000 °C), we found that the precursor performed with the best 9854

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering ORR via a direct 4e pathway by reducing O2 to OH−, as efficient as that of 20 wt % Pt/C. The kinetics were illustrated by the Tafel slopes in Figure 5d. At a low overpotential region, Co9S8/NSPG-900 gave a Tafel slope of 84 mV dec−1, as low as 79 mV dec−1 of 20 wt % Pt/C, and smaller than the counterparts of Co9S8/NSPG-800 (87 mV dec−1), Co9S8/NSPG-1000 (85 mV dec−1), and Co9S8/NSG900 (92 mV dec−1), suggesting that the rate-determining step was limited by the protonation of adsorbed O2 on the active sites,14 which followed a Temkin isotherm with high coverage of O2.33,34 This Tafel slope was close to those of state-of-the-art ORR catalysts, which are compared in Table S4. For evaluation of the long-term stability of the Co9S8/NSPG900 catalyst, current−time (i−t) chronoamperometric response over a period of 10 000 s on a RDE at 1600 rpm and 10 mV s−1 in O2-saturated 0.1 M KOH was performed. As shown in Figure 5e, after 10 000 s of consecutive operation, ∼89% of the initial current was retained for our catalyst, whereas only ∼78% for 20 wt % Pt/C was kept, demonstrating the excellent durability of the Co9S8/NSPG-900 hybrid. Remarkably, the Co9S8/NSPG900 electrode catalyst exhibited almost no current density fluctuation with the addition of 3 M CH3OH; however, a sharp decrease of current density was observed for 20 wt % Pt/C under the similar condition (Figure 5f), confirming the high CH3OH tolerance of our proposed electrode catalyst. OER Evaluation. The OER activity of the electrode catalyst was assessed by LSV plots collected on a RDE at 1600 rpm and 10 mV s−1 in O2-saturated 1.0 M KOH using a traditional threeelectrode system, and the counterparts including the benchmark RuO2 were also compared under identical conditions. Figure 6a exhibits the OER polarization curves of Co9S8/NSG-900, Co9S8/ NSPG-900, NSPG-900, and RuO2; as observed, the Co9S8/ NSPG-900 electrode catalyst showed the earliest onset potential (Eo) at 1.51 V versus RHE compared with Co9S8/NSG-900 and NSPG-900, implying that P-doping and Co9S8 may be the main active sites for the improvement of the OER.4,11,18 Moreover, Co9S8/NSPG-900 also showed better OER activity than Co9S8/ NSPG-800 and Co9S8/NSPG-1000, as exhibited in Figure S8. Co9S8/NSPG-800 exhibited enhanced OER performance compared with Co9S8/NSPG-1000, demonstrating that thiophene S was the main active center for OER due to the higher content of thiophene S in Co9S8/NSPG-800 than in Co9S8/ NSPG-1000, as summarized in Table S3. The Eo of Co9S8/ NSPG-900 was very close to 1.51 V versus RHE of RuO2. Additionally, Co9S8/NSPG-900 also delivered an overpotential (η10) of 343 mV at the current density of 10 mA cm−2, which was an important indicator to evaluate an OER catalyst.5,6,29,35 The η10 for Co9S8/NSPG-900 was similar to that of RuO2, and this value was also comparable with those of state-ofthe-art OER catalysts including heteroatom-doped carbon and metal-compound-modified carbon materials, as summarized in Table S5. During the OER measurement in 1.0 M KOH, S diffused to the electrolyte,6 and the cobalt was converted to CoOOH and Co(OH)2 in the 1.0 M KOH electrolyte, which were effective species for the OER.6,24,36,37 The OER kinetics of the Co9S8/NSPG-900 electrode catalyst was reflected by its corresponding Tafel slope obtained from its relative LSV curve for the OER, which is shown in Figure 6b. The Tafel slope was 82 mV dec−1 for Co9S8/NSPG-900, and 79 mV dec−1 for RuO2, close to those of other OER catalysts, as summarized in Table S5. The relatively low Tafel slope of Co9S8/ NSPG-900 reflected the intrinsic quick kinetics for the OER,

indicating the higher increase of current density than the increase of overpotential.5 The long-term durability of an OER electrode catalyst is a critical factor that must be taken into account, and the stability of Co9S8/NSPG-900 was evaluated by continuous cycling for 1000 cycles on a RDE at 1600 rpm and 10 mV s−1 in O2-saturated 1.0 M KOH. Figure 6c displays the LSV polarization curves; it can be observed that, after 1000 cycles, Co9S8/ NSPG-900 presented a curve that is similar to the initial one, and there was only a positive shift of 5 mV for the potential at the current density of 10 mA cm−2 in relation to the initial one, confirming the excellent stability of Co9S8/NSPG-900 for the OER. For use as a bifunctional catalyst for both the OER and ORR, it was worthy to study the potential gap of the OER and ORR, e.g., ΔE, and the potentials for the OER and ORR were usually taken at the current density of 10 and −3 mA cm−2, respectively; ΔE = EOER@10 − EORR@−3.29 Figure 6d displays the LSV polarization curves for both the OER and ORR in the entire potential region, and the potential difference between the OER and ORR was achieved to be 820 mV, suggesting Co9S8/NSPG-900 as an ideal bifunctional catalyst. On the basis of the above analysis, Co9S8/NSPG-900 showed excellent ORR and OER activity under corrosion-resistant conditions. Three key factors boosted the performance. First, the catalyst possessed a high specific BET surface area of 478 m2 g−1 with hierarchically porous structure, which was able to expose more active sites and accelerate faster mass transfer. It also showed a balance between active defects and electronic conductivity. Second, it was of the highest specific active forms of thiophene S, pyridinic N, graphitic N, and C−O−PO3, which guaranteed the excellent ORR and OER performance. Sulfurdoping generated a stable structure of graphene (S−C and thiophene S), and thiophene S was able to enhance the conductivity of graphene because of its strong ability to donate electrons and led to an effective ORR process.4,38 The Co9S8/ NSPG-900 was of the highest thiophene S content (6.61%) compared with Co9S8/NSPG-800 and Co9S8/NSPG-1000. Nitrogen broke the electroneutrality of carbon, and the adjacent carbon was of positive charges, which was favorable to adsorb oxygen; the specific pyridinic N was of more favorable energy to promote the reaction kinetics,39 and acted as active sites for the OER.4,10,27 Graphitic N was of high conductivity, which functioned as active centers for the ORR.4,10,27 For Co9S8/NSPG-900, it possessed the highest total content (63.31%) of pyridinic N and graphitic N, which guaranteed that the catalyst would display excellent ORR and OER activity. Phosphorus atoms were of a large covalent radius and electronegativity; they modified the electronic structure of carbon atoms effectively, and acted as bridges to transfer electrons to the carbon atoms. As a result, the induced carbon could function as active sites for the ORR and OER,27 as confirmed by the improved ORR and OER activity of Co9S8/NSPG-900 compared with that of Co9S8/NSG-900. In addition, Co9S8/NSPG-900 was of the highest content (15%) of Co9S8 compared with Co9S8/NSPG-800 and Co9S8/ NSPG-1000, and Co9S8/NSPG-900 exhibited better ORR and OER activity than Co9S8/NSPG-800, Co9S8/NSPG-1000, and NSPG-900, confirming that Co9S8 nanoparticles were active forms for the ORR and OER. Finally, the synergistic coupling among N, S, and P, and the synergistic effects between Co9S8 and NSPG, contributed a lot to the improved activity and stability for both the ORR and OER. 9855

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering

■

(8) Wu, G.; Santandreu, A.; Kellogg, W.; Gupta, S.; Ogoke, O.; Zhang, H.; Wang, H.; Dai, L. Carbon Nanocomposite Catalysts for Oxygen Reduction and Evolution Reactions: From Nitrogen Doping to Transition-Metal Addition. Nano Energy 2016, 29, 83−110. (9) Liu, Y.; Li, J.; Li, W.; Li, Y.; Zhan, F.; Tang, H.; Chen, Q. Exploring the Nitrogen Species of Nitrogen Doped Graphene as Electrocatalysts for Oxygen Reduction Reaction in Al-Air Batteries. Int. J. Hydrogen Energy 2016, 41, 10354−10365. (10) Yang, H.; Miao, J.; Hung, S.; Chen, J.; Tao, H.; Wang, X.; Zhang, L.; Chen, R.; Gao, J.; Chen, H.; Dai, L.; Liu, B. Identification of Catalytic Sites for Oxygen Reduction and Oxygen Evolution in N-Doped Graphene Materials: Development of highly Efficient Metal-Free Bifunctional Electrocatalyst. Sci. Adv. 2016, 2, e1501122. (11) Han, L.; Qin, W.; Zhou, J.; Jian, J.; Lu, S.; Wu, X.; Fan, G.; Gao, P.; Liu, B. Chemical Grafting of Co9S8 onto C60 for Hydrogen Spillover and Storage. Nanoscale 2017, 9, 5141−5147. (12) Cai, J.; Wu, C.; Zhu, Y.; Zhang, K.; Shen, P. Sulfur Impregnated N, P Co-Doped Hierarchical Porous Carbon as Cathode for High Performance Li-S Batteries. J. Power Sources 2017, 341, 165−174. (13) Chen, B.; Li, R.; Ma, G.; Gou, X.; Zhu, Y.; Xia, Y. Cobalt Sulfide/ N,S Codoped Porous Carbon Core−Shell Nanocomposites as Superior Bifunctional Electrocatalysts for Oxygen Reduction and Evolution Reactions. Nanoscale 2015, 7, 20674−20684. (14) Liu, Y.; Shen, H.; Jiang, H.; Li, W.; Li, J.; Li, Y.; Guo, Y. ZIFDerived Graphene Coated/Co9S8 Nanoparticles Embedded in Nitrogen Doped Porous Carbon Polyhedrons as Advanced Catalysts for Oxygen Reduction Reaction. Int. J. Hydrogen Energy 2017, 42, 12978−12988. (15) Jiang, H.; Li, C.; Shen, H.; Liu, Y.; Li, W.; Li, J. Supramolecular Gel-Assisted Synthesis Co2P Particles Anchored in Multielement CoDoped Graphene as Efficient Bifunctional Electrocatalysts for Oxygen Reduction and Evolution. Electrochim. Acta 2017, 231, 344−353. (16) Yuan, H.; Jiao, Q.; Liu, J.; Liu, X.; Yang, H.; Zhao, Y.; Wu, Q.; Shi, D.; Li, H. Ultrathin-Walled Co9S8 Nanotube/Reduced Graphene Oxide Composite as an Efficient Electrocatalyst for the Reduction of Triiodide. J. Power Sources 2016, 336, 132−142. (17) Yang, J.; Zhu, G.; Liu, Y.; Xia, J.; Ji, Z.; Shen, X.; Wu, S. Fe3O4Decorated Co9S8 Nanoparticles In Situ Grown on Reduced Graphene Oxide: A New and Efficient Electrocatalyst for Oxygen Evolution Reaction. Adv. Funct. Mater. 2016, 26, 4712−4721. (18) 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. (19) Liu, X.; Wang, Y.; Wang, Z.; Zhou, T.; Yu, M.; Xiu, L.; Qiu, J. Achieving Ultralong Life Sodium Storage in Amorphous Cobalt-tin Binary Sulfide Nanoboxes Sheathed in N-doped Carbon. J. Mater. Chem. A 2017, 5, 10398−10405. (20) Liu, Z.; Zhao, Z.; Wang, Y.; Dou, S.; Yan, D.; Liu, D.; Xia, Z.; Wang, S. In Situ Exfoliated, Edge-Rich, Oxygen-Functionalized Graphene from Carbon Fibers for Oxygen Electrocatalysis. Adv. Mater. 2017, 29, 1606207. (21) Wang, Y.; Zhang, Y.; Liu, Z.; Xie, C.; Feng, S.; Liu, D.; Shao, M.; Wang, S. Layered Double Hydroxide Nanosheets with Multiple Vacancies Obtained by Dry Exfoliation as Highly Efficient Oxygen Evolution Electrocatalysts. Angew. Chem., Int. Ed. 2017, 56, 5867−5871. (22) Wang, Y.; Xie, C.; Liu, D.; Huang, X.; Huo, J.; Wang, S. Nanoparticles-Stacked Porous Nickel-Iron Nitride Nanosheet: A Highly Efficient Bifunctional Electrocatalyst for Overall Water Splitting. ACS Appl. Mater. Interfaces 2016, 8, 18652−18657. (23) Marcano, D.; Kosynkin, D.; Berlin, J.; Sinitskii, A.; Sun, Z.; Slesarev, A.; Alemany, L.; Lu, W.; Tour, J. Improved Synthesis of Graphene. ACS Nano 2010, 4, 4806−4814. (24) Ma, X.; He, X. Co4S3/NixS6(7 ≥ x ≥ 6)/NiOOH In-Situ Encapsulated Carbon-based Hybrid as a High-Efficient Oxygen Electrode Catalyst in Alkaline Media. Electrochim. Acta 2016, 213, 163−173. (25) Ma, X.; Su, Y.; He, X. Fe9S10-Decorated N, S Co-Doped Graphene as a New and Efficient Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Catal. Sci. Technol. 2017, 7, 1181−1192.

CONCLUSIONS In summary, we have confirmed a Co9S8-modified N, S, and P ternary-doped 3D graphene electrode catalyst as a promising bifunctional catalyst for both the ORR and OER under corrosive conditions. The satisfied ORR and OER performance is mainly due to the following factors: First, a 3D structure was formed, which was of a connected channel and high surface area, and enabled more exposed active sites and rapid mass diffusion and transport. Second, the ternary doping of N, S, and P created a great quantity of defects, and tailored the electronic properties, producing effective species for catalysis. Third, the strong synergistic coupling between Co9S8 species and NSPG gave rise to the satisfied catalytic performance. This work suggested that synergistic cobalt sulfide/heteroatom-doped graphene showed great potential as bifunctional catalysts.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01820. SEM images, adsorption−desorption isotherms, BJH pore-size distribution curves, catalyst properties and compositions, TGA curves, XPS spectra, cyclic voltammetry curves, LSV curves, and Nyquist plots (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 8558 3430. ORCID

Xing-Quan He: 0000-0001-9400-1693 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors acknowledge the support of the Natural Science of Jinlin Province, China (20160101298 JC).

■

REFERENCES

(1) Qu, G.; Geng, H.; Ge, D.; Tang, M.; Zheng, J.; Gu, H. Porous Carbon-Wrapped Mesoporous Co9S8 Fibers as Stable Anode for Li-Ion Batteries. Electrochim. Acta 2016, 211, 305−312. (2) Yang, Z.; Lin, L.; Xu, A. 2D Nanoporous Fe−N/C Nanosheets as Highly Efficient Non-Platinum Electrocatalysts for Oxygen Reduction Reaction in Zn-Air Battery. Small 2016, 12, 5710−5719. (3) Lee, D.; Park, M.; Park, H.; Seo, M.; Wang, X.; Chen, Z. Highly Active and Durable Nanocrystal-Decorated Bifunctional Electrocatalyst for Rechargeable Zinc−Air Batteries. ChemSusChem 2015, 8, 3129− 3138. (4) Duan, J.; Chen, S.; Jaroniec, M.; Qiao, S. Heteroatom-Doped Graphene-Based Materials for Energy-Relevant Electrocatalytic Processes. ACS Catal. 2015, 5, 5207−5234. (5) Du, J.; Zhang, T.; Xing, J.; Xu, C. Hierarchical Porous Fe3O4/Co3S4 Nanosheets as Efficient Electrocatalysts for Oxygen Evolution Reaction. J. Mater. Chem. A 2017, 5, 9210−9216. (6) Long, J.; Gong, Y.; Lin, J. Metal−Organic−Framework-Derived Co9S8@CoS@CoO@C Nanoparticles as Efficient Electro- and PhotoCatalysts for Oxygen Evolution Reaction. J. Mater. Chem. A 2017, 5, 10495−10509. (7) Wei, J.; Hu, Y.; Wu, Z.; Liang, Y.; Leong, S.; Kong, B.; Zhang, X.; Zhao, D.; Simone, G.; Wang, H. A Graphene-Directed Assembly Route to Hierarchically Porous Co-Nx/C Catalysts for High-performance Oxygen Reduction. J. Mater. Chem. A 2015, 3, 16867−16873. 9856

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857

Research Article

ACS Sustainable Chemistry & Engineering (26) Li, M.; Zhou, H.; Yang, W.; Chen, L.; Huang, Z.; Zhang, N.; Fu, C.; Kuang, Y. Co9S8 Nanoparticles Embedded in N, S Co-Doped Graphene-Unzipped Carbon Nanotubes Composite as a High Performance Electrocatalyst for Hydrogen Evolution Reaction. J. Mater. Chem. A 2017, 5, 1014−1021. (27) Zhao, H.; Yuan, Z. Transition Metal−Phosphorus-Based Materials for Electrocatalytic Energy Conversion Reactions. Catal. Sci. Technol. 2017, 7, 330−347. (28) Zhu, H.; Zhang, J.; Yanzhang, R.; Du, M.; Wang, Q.; Gao, G.; Wu, J.; Wu, G.; Zhang, M.; Liu, B.; Yao, J.; Zhang, X. When Cubic Cobalt Sulfide Meets Layered Molybdenum Disulfide: A Core−Shell System Toward Synergetic Electrocatalytic Water Splitting. Adv. Mater. 2015, 27, 4752−4759. (29) Ma, X.; Su, Y.; He, X. Use of Cobalt Polyphthalocyanine and Graphene as Precursors to Construct an Efficient Co9S8/N,S-G Electrocatalyst for the Oxygen Electrode Reaction in Harsh Media. ChemCatChem 2017, 9, 308−315. (30) Ma, X.; He, X. Electronically Tailoring 3D Flower-Like Graphene via Alumina Doping and Incorporating Co as an Efficient Oxygen Electrode Catalyst in both Alkaline and Acid Media. J. Power Sources 2017, 353, 28−39. (31) Zhang, C.; Wang, B.; Shen, X.; Liu, J.; Kong, X.; Chuang, S.; Yang, D.; Dong, A.; Peng, Z. A Nitrogen-Doped Ordered Mesoporous Carbon/Graphene Framework as Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions. Nano Energy 2016, 30, 503−510. (32) Pendashteh, A.; Palma, J.; Anderson, M.; Marcilla, R. NiCoMnO4 Nanoparticles on N-doped Graphene: Highly Efficient bifunctional Electrocatalyst for Oxygen Reduction/Evolution Reactions. Appl. Catal., B 2017, 201, 241−252. (33) Giacomini, M.; Ticianelli, E.; McBreen, J.; Balasubramanian, M. Oxygen Reduction on Supported Platinum/Polythiophene Electrocatalysts. J. Electrochem. Soc. 2001, 148, A323−A329. (34) Markovic, N.; Schmidt, T.; Stamenkovic, V.; Ross, P. Oxygen Reduction Reaction on Pt and Pt Bimetallic Surfaces: A Selective Review. Fuel Cells 2001, 1, 105−116. (35) Zhang, C.; Zhao, J.; Zhou, L.; Li, Z.; Shao, M.; Wei, M. Layer-ByLayer Assembly of Exfoliated Layered Double Hydroxide Nanosheets for Enhanced Electrochemical Oxidation of Water. J. Mater. Chem. A 2016, 4, 11516−11523. (36) Zhan, Y.; Du, G.; Yang, S.; Xu, C.; Lu, M.; Liu, Z.; Lee, J. Development of Cobalt Hydroxide as a Bifunctional Catalyst for Oxygen Electrocatalysis in Alkaline Solution. ACS Appl. Mater. Interfaces 2015, 7, 12930−12936. (37) Yoon, T.; Kim, K. One-Step Synthesis of CoS-Doped βCo(OH)2@Amorphous MoS2+x Hybrid Catalyst Grown on Nickel Foam for High-Performance Electrochemical Overall Water Splitting. Adv. Funct. Mater. 2016, 26, 7386−7393. (38) Wang, Z.; Li, P.; Chen, Y.; He, J.; Zhang, W.; Schmidt, O.; Li, Y. Pure Thiophene−Sulfur Doped Reduced Graphene Oxide: Synthesis, Structure, and Electrical Properties. Nanoscale 2014, 6, 7281−7287. (39) Zhang, C.; Wang, B.; Shen, X.; Liu, J.; Kong, X.; Chuang, S.; Yang, D.; Dong, A.; Peng, Z. A Nitrogen-Doped Ordered Mesoporous Carbon/Graphene Framework as Bifunctional Electrocatalyst for Oxygen Reduction and Evolution Reactions. Nano Energy 2016, 30, 503−510.

9857

DOI: 10.1021/acssuschemeng.7b01820 ACS Sustainable Chem. Eng. 2017, 5, 9848−9857