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Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction
international journal of hydrogen energy xxx (xxxx) xxx
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Cost-effective Co3O4 nanospheres on nitrogendoped graphene used as highly efficient catalyst for oxygen reduction reaction Jiahao Guo*, Qianfu Li, Haitao Hou, Junming Chen, Chengdong Wang, Songlin Zhang, Xuchun Wang College of Chemistry and Materials Engineering, Anhui Science and Technology University, Fengyang, Anhui, 233100, PR China
highlights A Co3O4 NS/N-rGO hybrid has been prepared through a simple two-step method. An interaction between Co3O4 and N-rGO is beneficial to improve ORR activity. The hybrid catalyst has excellent ORR activity with a 4-electron mechanism.
article info
abstract
Article history:
Developing low-cost, efficient and stable catalyst for the oxygen reduction reaction is
Received 28 June 2019
meaningful and necessary for the industrialization of fuel cells. In this work, we report the
Received in revised form
controllable synthesis of Co3O4 nanospheres uniformly anchored on N-doped reduced
13 September 2019
graphene oxide (N-rGO) sheets with significant catalytic activity for the oxygen reduction
Accepted 23 September 2019
reaction via a simple two-step approach. Results show that there is an interaction between
Available online xxx
Co3O4 nanospheres and N-rGO after riveting on N-rGO, which is beneficial to the electron transfer between them. The Co3O4 NS/N-rGO hybrid has excellent catalytic activity, com-
Keywords:
parable to commercial Pt/C catalyst. The transferred electron number of the oxygen
Co3O4 nanospheres
reduction reaction on Co3O4 NS/N-rGO is around 3.95. The hybrid has more excellent
Nitrogen-doped reduced graphene
durability and methanol resistance than commercial Pt/C. The Co3O4 NS/N-rGO catalyst in
oxide
our work provides a cost-effective the oxygen reduction reaction catalyst alternative to the
Oxygen reduction reaction
precious metal Pt in fuel cell.
Catalytic performance
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasingly serious energy crisis and environmental pollution have impelled a great attention to the development and utilization of new renewable clean energy resources, such as fuel cells and metal-air batteries [1e3]. The development of high efficiency catalysts for the oxygen reduction reaction
(ORR) has become the heart and main challenge of these new renewable clean energy technologies because of the sluggish kinetics of ORR [4e6]. Nowadays, Pt and its alloys have long been regarded as the most effective catalyst for ORR; however, the high cost as well as limited supply and poor stability have seriously restricted them from becoming the most effective solution for ORR [7,8]. In this respect, the substitution catalysts based on non-precious metals or metal oxides with high
* Corresponding author. E-mail address: [email protected] (J. Guo). https://doi.org/10.1016/j.ijhydene.2019.09.165 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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3 h to obtain Co3O4 NS/N-rGO. The theoretical mass ratio of Co3O4 to N-rGO in Co3O4 NS/N-rGO catalyst is approximate 1:1. The final product is collected by centrifugation and washed with water and ethanol and dry. The obtained catalyst is recorded as Co3O4 NS/N-rGO-1 when the concentration of CoCl2 is 15 mM. The synthesis procedure of Co3O4 NS is the same as that of Co3O4 NS/N-rGO, except GO was not added. The morphologies of the synthesized samples were analyzed by field emission scanning electron microscopy (FESEM, FEI HITACHI S-4800) and transmission electron microscopy (TEM, JEOJ-2010). XRD tests were carried out on a Rigaku D/max-rA with Cu Ka radiation (l ¼ 1.54178 A). Xray photoelectron spectra (XPS) were carried out on an Thermo ESCALAB 250Xi using a monochromic Al Ka (hv ¼ 1486.6 eV) X-ray source to analyze the surface elemental composition of the samples. Brunauer-EmmettTeller (BET) tests were carried out on a Micromeritics ASAP 2460 system to determine the surface area by nitrogen adsorption. Thermal gravimetric analysis (TGA) were carried out on a Setaram Labsys Thermogravimetric Analyzer in air atmosphere at a heating rate of 10 C$min1 from room temperature to 800 C. 4 mg catalysts and 17 mL 5% Nafion solution were ultrasonically dispersed in 1 mL of 3:1 v/v water/isopropanol mixed solvent to form homogeneous ink. The pre-polished glassy carbon electrode of 5 mm in diameter was dropped with 5 mL catalyst ink, yielding a loading of catalyst 0.1 mg cm2, and dried in air used as a working electrode. All electrochemical tests were carried out in 0.1 M KOH saturated with O2 or N2 by a CHI 700 E electrochemical workstation with the standard three-eletrode cell under room temperature. The counter electrode was Pt wire and the referece electrode was Ag/AgCl (PINE, 4 M KCl). Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were scanned from 1.0 to 0.2 V at 10 mV s1. The disk rotation rates of RDE were ranged from 400 to 2500 rpm. Chronoamperometry tests were carried out at 0.3 V with a rotation speed of 1600 rpm. For RDE tests, the transferred electron number (n) and kinetic current density (Jk) in the ORR process were obtained from the slope and intercept of K-L straight line based on the Koutecky-Levich equation. 1 1 1 ¼ þ J JK B612
(1) 2
1
=
abundance, low cost, environmental friendliness and potential ORR activity have attracted the researchers' interest [9e13]. Recently, non-precious metal oxides with spinel structure, such as Co3O4 [10,14], MnO2 [15e17], Fe3O4 [18], Mn3O4 [19,20] etc. have attracted considerable attention. Nevertheless, pristine metals and metal oxides usually exhibit poor catalytic activity due to agglomeration, sintering and dissolution of catalyst particles during the fuel cells operation [21]. To overcome this obstacle, the loading of catalyst particles on the materials with large electroactive surface area can improve their catalytic activity and durability [22]. According to this idea, considering that both non-precious metal oxides and doped carbon materials have considerable ORR catalytic activities, the hybrid composite loaded non-precious metal oxide nanoparticles on doped carbon materials should have excellent ORR catalytic activity. Wu and co-workers synthesized 3D N-doped graphene aerogelsupported Fe3O4 nanoparticles (Fe3O4/N-GAs) as efficient ORR catalysts [23]. The 3D macroporous structure and high specific surface area of the supporter are responsible for the enhancement of ORR activity. Three kinds of Mn3O4 nanoparticles were loaded on N-doped graphene sheets and the effect of Mn3O4 morphology on their catalytic performance have been researched by Duan and co-workers [21]. It was found that the ORR activity was correlated to the shape of Mn3O4 nanocrystals and the exposed crystalline facets. The ellipsoidal Mn3O4 particles on N-doped graphene exhibited the highest ORR activity. At present, the composite catalysts for ORR based on Co3O4 have attracted considerable attention, such as Co3O4/N-doped graphene hybrid material and porous nitrogen doped carbon supported Co3O4 [24]. However, Co3O4 nanosphere anchored on nitrogen-doped reduced graphite oxide as ORR catalysts have been seldom reported. Herein, we demonstrate Co3O4 NS/N-rGO catalyst, a nanostructual hybrid Co3O4 nanospheres uniformly anchored on N-doped reduced graphite oxide (N-rGO) sheets, prepared through a simple two-step method. After binding with N-rGO, the uniformly dispersed Co3O4 nanospheres exhibit remarkable ORR activity with a representative four-electron pathway, which is superior to Co3O4 nanospheres and comparable to commercial Pt/C catalyst (20 wt% Pt on Vulcan XC-72). The Co3O4 NS/N-rGO hybrid also showed excellent durability and methanol resistance.
=
2
=
B ¼ 0:2nFðDO Þ 3 n 6 cO
Experimental Graphite oxide (GO) was obtained by a modified Hummer’s method according to literature [25]. In a typical synthesis procedure of Co3O4 NS/N-rGO, 3 mM CoCl2 solution was obtained by adding 20 mg CoCl2 into 50 mL ethylene glycol. After added 10 mg GO, the solution was sonicated for 2 h to adsorbed Co(II) ions on GO by electrostatic action. After stirring at 50 C and adding 0.1 M NH4OH drop by drop to adjusted pH to about 9, the stable suspension was transferred to a Telfon-lined autolave and solvothermally treated at 130 C for 24 h. Then, the precursor suspension was slowly added with 50 mL of 3% H2O2 at 60 C to oxidized for
(2)
where J, JK are the measured current and the kinetic current, u is the rotating rate, F is the Faraday constant (96 485 C mol1). DO is the diffusion coefficient of O2 in 0.1 M KOH (1.9 105 cm2 s1). n is the kinetic viscosity (0.01 cm2 s1). CO is the bulk concentration of O2 (1.2 106 mol cm3). The constant 0.2 is adopted when the rotation speed is expressed in rpm. In the RRDE tests, the conditions are the same as the RDE test, except for the rotating speed of the disk electrode was 1600 rpm and the potential of ring electrode set to 0.25 V to oxidize HO 2 intermediate from the disk electrode. The HO 2 % and the transferred electron number (n) were determined by the followed equations [26,27].
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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HO 2%¼
n¼
200 Ir NId þ Ir
(3)
4 Id Id þ Ir=N
(4)
where Id is disk current, Ir is ring current and N is current collection efficiency of the Pt ring and is 0.39 in our experiment.
Results and discussion The morphology of the samples was characterized by SEM and TEM. It can be seen from Fig. 1Sa and 1Sb that Co3O4 NS mainly consist of loose nanospheres with a small amount of nanorods, and the diameter of the nanospheres is about 140 nm. After the addition of GO to the reaction system, the Co2þ ion was firstly adsorbed on the active site graphene as the growth point and gradually grew into compact nanospheres (Fig. 1a and b) [28]. SEM images elaborated smaller Co3O4 nanospheres in the Co3O4 NS/N-rGO hybrid with a diameter of about 110 nm than the Co3O4 NS and uniformly distributed on the graphene sheets. Such geometric configuration of metal oxide nanospheres on graphene sheets contribute to ameliorate their interface contact and effectively prevent the dissolution and aggregation of the nanospheres, thus promoting the electrochemical activity and stability of the hybrid catalyst. TEM images further authenticated the uniform distribution of the Co3O4 nanospheres on the graphene sheets (Fig. 1c). HRTEM images show that the lattice spacing of the composite catalyst and Co3O4 NS is 0.465 nm, corresponding to the (111) plane of
3
the spinel structure of Co3O4 nanocrystals, while the other lattice spacing of the hybrid catalyst is 0.351 nm, corresponding to the (002) planes of the graphite (Fig. 1d and Fig. S1d). In the preparation process of Co3O4 NS/N-rGO, when the concentration of CoCl2 used is 15 mM, the resulting catalyst is a nanosphere aggregate with smaller particle size, attached to the graphene (Fig. S2). The aggregation of Co3O4 nanospheres will lead to the decrease of active sites, which is not conducive to the improvement of catalytic performance of catalysts. XRD patterns are used to analyze the crystal structure of the hybrid catalyst, as shown in Fig. 2a. The diffraction peaks in the XRD patterns of Co3O4 and hybrid catalyst can be indexed to a spinel phase (space group Fd3m) with a lattice constant a ¼ 8.084 A, matched well with JCPDS card No. 43-1003 [29,30]. However, compared with Co3O4, the intensity of the corresponding diffraction peak of hybrid catalyst was significantly reduced due to the influence of the graphene. The hybrid catalyst has a sharp diffraction peak at 25 , which can be attributed to graphene oxide. The Brunauer-Emmett-Teller (BET) surface area of the hybrid catalyst determined by nitrogen adsorption isotherm was 85 m2 g1, which is slightly higher than that of 61 m2 g1 with Co3O4 NS (Fig. 2b). The pore size of the hybrid catalyst was centered at z16 nm, larger than that of Co3O4 NS (z7 nm) (the inset of Fig. 2b). Larger surface area and pore size are beneficial to the adsorption of reactant particles and the transport of materials, which is beneficial to the improvement of catalytic activity of the hybrid catalyst [31e35]. The chemical composition of the surface of the hybrid catalyst was determined by X-ray photoelectron spectroscopy (XPS) and the results are shown in Fig. 3a. XPS spectrum of the
Fig. 1 e (a, b) SEM, (c) TEM and (d) HRTEM images of Co3O4 NS/N-rGO. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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Fig. 2 e (a) XRD spectra and (b) Nitrogen adsorptionedesorption isotherms of Co3O4 NS and Co3O4 NS/N-rGO. The inset in (b) shows the corresponding pore size distribution curves.
Co3O4 NS/N-rGO hybrid revealed the presence of the elements C, N, O and Co, located at 285 eV, 400 eV, 533 eV and 781 eV, respectively. Their contents are 80.93 at.% C, 3.92 at.% N, 14.46 at.% O and 0.69 at.% Co, respectively. However, XPS spectrum of Co3O4 NS only displays O1s, Co2p and petty C1s peaks, which could be caused by adsorbed. In contrast, the peak of Co2p of the hybrid catalyst is obviously weaker than that of Co3O4 NS, due to the presence of graphene oxide. The
deconvolution of the C1s spectrum indicates the presence of three states of C, that is CeC or C]C at 284.7eV, CeN at 285.9 eV, and OeC]O at 288.2 eV, and the corresponding contents are 67.1%, 21.6% and 11.3%, respectively, indicating the effective reduction of GO and successful incorporation of nitrogen (Fig. 4b) [36,37]. The high-resolution N1s could be deconvoluted into three peaks: pyridinic N at 398.8 eV, pyrrolic N at 399.8 eV and graphitic N at 400.5 eV (Fig. 4c) [38e40]. The
Fig. 3 e (a) XPS spectrum of Co3O4 NS/N-rGO and Co3O4 NS; (b) C 1s and (c) N 1s core-level and corresponding deconvoluted spectra of Co3O4 NS/N-rGO; (d) Co 2p core-level and corresponding deconvoluted spectra of Co3O4 NS/N-rGO and Co3O4 NS. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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growth and dispersion of Co3O4 nanospheres on GO sheet were promoted by doping nitrogen serve as the anchor sites of Co3O4 nanocrystalline. The incorporation of N also can effectively improve the catalytic activity of the hybrid catalyst. The deconvolution of O1s spectrum shown in Fig. S3 could be fitted into four peaks at 533.5, 532.5, and 531.3 eV, corresponding to the chemisorbed oxygen (OC), oxygen vacancies (OV), and surface lattice oxygen (OL) in Co3O4, respectively [41,42]. The existence of oxygen vacancy on the surface of Co3O4 NS/NrGO can promote the adsorption and dissociation of O2, thus improving its catalytic performance. In Fig. 4d, the Co2p XPS spectrum can be deconvoluted into a pair of spin-orbit double peaks at 781.4 eV and 797.1 eV, attributed to the 2p1/2 and 2p3/2 peaks of Co3O4, respectively. The Co 2p3/2 peak could be decomposed into two peaks at 781.4 eV and 785.7 eV,
5
corresponding to Co3þ and Co2þ, respectively. Compared with the Co 2p peak of pure Co3O4, the hybrid catalyst positive shifted from 780.5 eV to 781.4 eV for 2p3/2 peak and positive shifted from 796.1 eV to 797.1 eV for 2p1/2 peak, which could probably be attributed to the interaction between Co3O4 and rGO [42]. The positive shift of Co 2p binding energy leads to the decrease of electron density on Co atom, which is beneficial to the adsorption and dissociation of O2. By means of thermogravimetric analysis, the mass percentage of Co3O4 in the hybrid catalyst is determined to be about 76%, which basically coincides with the theoretical yield of hybrid catalyst (Fig. S4). Cyclic voltammetry (CV) measurement was first used to evaluated the ORR catalytic performance of Co3O4/N-rGO in 0.1 M KOH solution (Fig. 4a). In N2-saturated electrolyte, CV curves of all catalysts showed no obvious reduction peak
Fig. 4 e (a)CV of Co3O4/N-rGO, Co3O4 NS, and N-rGO in N2- and O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s¡1. (b) Linear sweep voltammetry (LSV) of four samples and Pt/C in O2- saturated 0.1 M KOH at a scan rate of mV·s¡1 with an RDE rotation rate of 1600 rpm. (c and d) LSV and Calculated K-L plots of Co3O4 NS/N-rGO in O2-saturated 0.1 M KOH at a scan rate of 10 mV s¡1 and at different RDE rotation rates. (e and f) KeL plots at ¡0.6 V and transferred electron number n of the ORR for four samples and Pt/C. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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Table 1 e Comparison of the electrocatalytic performance parameters in different Literature. Catalysts Co/NC-800 HGSeMn3O4 MONPMs/NC 3DFeeWO3 NF/NG Zn/Co@CeNCNFs CoFe2O4/NG Co3O4 NS/N-rGO
Catalyst loading 0.24 mg/cm2 0.32 mg/cm2 0.1 mg/cm2 0.35 mg/cm2 0.1 mg/cm2 283 mg/cm2 0.1 mg/cm2
Eonset/V（vs. Ag/AgCl）
E1/2/V (vs. Ag/AgCl） 0.19
0.14 0.98 (vs.RHE) 0.98 (vs.RHE) 0.099 0.030 0.02
during the process of potential decrease, indicating that ORR did not occur. When the electrolyte was saturated with O2, the reduction current of Co3O4/N-rGO reveals a well-defined ORR reduction peak at 0.17 V with the potential decrease, which is close to 0.12 V of Pt/C (Fig. S5) and significantly higher than 0.45 V of Co3O4 NS and 0.36 V of N-rGO. The peak current density of 0.51 mA cm2 is also significantly larger than those of Co3O4 NS and N-rGO. The results are indicated a considerable ORR activity of Co3O4/N-rGO, which further illuminated that the structure of Co3O4 riveting on N-doped graphitic can obviously improve the ORR activity of catalyst. Linear sweep voltammetry (LSV) measurement for Co3O4 NS/N-rGO was executed on a rotating disk electrode (RDE) in O2-saturated 0.1 M KOH electrolyte at a rotation rate of 1600 rpm to determining the kinetic parameters of ORR, in comparison with Pt/C (20 wt% Pt) with the same loading capacity. As shown in Fig. 4b, Co3O4 NS/N-rGO hybrid occupies
0.81 (vs.RHE) 0.85 (vs.RHE) 0.20 0.144 0.13
n
Reference
3.99 3.69 3.94 3.97 3.69 3.61e3.88 3.95e3.98
[14] [20] [41] [42] [43] [44] This work
an evident positive onset potential of 0.02 V and half-wave potential of 0.13 V, much higher than those of Co3O4 NS (0.31 V, 0.39 V), N-rGO (0.060 V, 0.20 V) and very close to those of Pt/C (0.01 V, 0.18 V). Similarly, the current density of Co3O4 NS/N-rGO catalyst is remarkable higher than that of Co3O4 NS and N-rGO. These results indicate that Co3O4 NS/NrGO occupies an excellent ORR activity, because of the well combination of Co3O4 nanospheres with N-doped graphene. The onset potential and half-wave potential of Co3O4 NS/NrGO-1 are almost equal to those of Co3O4 NS/N-rGO, but the current density is obviously lower, which may be caused by the decrease of activity sites caused by the aggregation of catalyst particles. Typical LSV curves of Co3O4 NS/N-rGO, Co3O4 NS/N-rGO-1, Co3O4 NS, N-rGO and Pt/C at different rotating speeds from 400 to 2500 rpm were measured in Fig.4c and Fig. S6. An obvious increase of current density with the increase in rotating speeds at the same potential was
Fig. 5 e (a) RRDE measurement of four samples and Pt/C for ORR. (b) The number of electrons transferred per O2 and calculated HO¡ 2 production yields of four samples and Pt/C during the ORR. (c) Chronoamperometric response of Co3O4 NS/NrGO and Pt/C. (d) iet of Co3O4 NS/N-rGO and Pt/C before and after the addition of 3 M methanol. Tests were conducted in O2saturated 0.1 M KOH solution at ¡0.3 V. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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observed due to the decrease of the O2 diffusion distance with the increase of rotation speed. The corresponding KouteckyLevich plots from 0.3 V to 1.0 V were calculated from LSVs at different potentials and assume good linear relationship and near parallelism, suggesting first-order reaction kinetics for ORR (Fig.4d and Fig. S6). Fig. 4e shows the comparison of K-L curves of several catalysts at 0.6 V. It can be seen from the diagram that the K-L curve of Co3O4 NS/NrGO is almost coincide with that of Pt/C, which means that the catalytic activity of Co3O4 NS/N-rGO is close to that of Pt/C. The transferred electron number of Co3O4 NS/N-rGO calculated from the slopes of the K-L plots is varied from 3.95 to 3.98 in the range of 0.3-1.0 V, indicating that the ORR process proceeds via a 4-electron mechanism. On the contrary, the transferred electron number of N-rGO and Co3O4 NS are about 3 and 2.1, respectively, which are significantly lower than that of Co3O4 NS/N-rGO. The transferred electron number of Co3O4 NS/N-rGO-1 catalyst is about 3.66, which is also lower than that of Co3O4 NS/N-rGO. The recently reported transition metal oxide catalysts based on nitrogen-doped carbon material was compared and found that Co3O4 NS/N-rGO possesses excellent ORR performance (Table 1). The effect of CoCl2 concentration on the catalytic activity of the prepared Co3O4 NS/N-rGO sample was researched. As can be seen from Fig. S7, the catalytic activity of the Co3O4 NS/N-rGO sample prepared with 3 mM CoCl2 was best. Further characterization of ORR pathway by an additional rotating ring disk electrode (RRDE) voltammograms to monitoring the formation of HO 2 were executed in O2-saturated 0.1 M KOH electrolyte. As displayed in Fig. 5a, compared to Co3O4 NS, N-rGO, the onset potential and diffusion limiting current density of Co3O4 NS/N-rGO have significantly improved, which can be compared with the commercial Pt/C catalyst. Co3O4 NS/N-rGO-1 has the same onset potential as Co3O4 NS/N-rGO, but the limit diffusion current is obviously lower. Fig. 5b shows the hydrogen peroxide yields generated on four catalysts at different potentials. In the potential range from 0.3 to 1.0 V, the H2O2% of Co3O4 NS/N-rGO is lower than 5%, which is obviously lower than those of Co3O4 NS, NrGO, and Co3O4 NS/N-rGO-1. The transferred electron number calculated from RRDE on Co3O4 NS/N-rGO is around 3.98 from 0.3 to 1.0 V. At the same time, the transferred electron number of Co3O4 NS, N-rGO, and Co3O4 NS/N-rGO-1 is about 2.06, 3.15 and 3.65 in the same range of potentials. The catalytic performance of Co3O4 NS/N-rGO-1 is closer to Co3O4 NS/ N-rGO, but it is obviously affected by the aggregation of Co3O4 NS. All these results indicate that the oxygen reduction process on Co3O4 NS/N-rGO mainly takes a 4-electron path, directly reducing O2 to OH ions. The long-term durability and outstanding tolerance to methanol crossover are important concerns of ORR catalysts, which is related to the industrialization development of direct methanol fuel cells (DMFC). Chronoamperometric durability testing for the ORR of Co3O4 NS/N-rGO and Pt/C were performed at 0.3 V in O2-saturated 0.1 M KOH and shown in Fig. 5c. Pt/C shows a 26.2% decrease in activity after 10 h at 0.3 V. However, Co3O4 NS/N-rGO remains a high relative current of 93.5% after 10 h, suggesting a superior durability of Co3O4 NS/N-rGO than commercial Pt/C. The effect of methanol crossover was estimated by chronoamperometric tests at
7
0.3 V with the injection of 3.0 M methanol (Fig. 5d). An abrupt change of Pt/C in the current from negative to positive has been found, implying the occurrence of methanol oxidation. In contrast, the current of Co3O4 NS/N-rGO remained stable after methanol was added, confirming its selectivity to methanol oxidation. These results further demonstrate the potential application value of Co3O4 NS/N-rGO in fuel cells as a low cost, efficient and stable ORR catalyst.
Conclusions This work reported on a highly efficient Co3O4 NS/N-rGO catalyst for ORR, prepared by a simple two-step approach. The uniform riveting of Co3O4 NS on N-rGO has brought about the excellent ORR catalytic activity in alkaline electrolyte, as well as excellent durability and methanol resistance, owed to the enhancement of electron transfer ability caused by the interaction between Co3O4 NS and N-rGO. The high catalytic activity and durability of this catalyst makes it a substitute for commercial Pt/C and be used as an efficient, cheap and stable ORR catalyst in fuel cells.
Acknowledgements This study was supported by the Natural Science Foundation of Anhui Province (Grant 1808085MB31); Students Platform for Innovation and Entrepreneurship Training Program of China (2018S10879018); Stable Talent Foundation of Anhui Science and Technology University (HCWD201601); Science and Technology Major Project of Anhui Province (Grant 18030901087).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.165.
references
[1] Chen C, Kang Y, Huo Z, Zhu Z, Huang W, Xin HL, Snyder JD, Li D, Herron JA, Mavrikakis M, Chi M, More KL, Li Y, Markovic NM, Somorjai GA, Yang P, Stamenkovic VR. Highly crystalline multimetallic nanoframes with threedimensional electrocatalytic surfaces. Science 2014;343:1339e43. [2] Guo D, Shibuya R, Akiba C, Saji S, Kondo T, Nakamura J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016;351:361e5. [3] Wang YJ, Long W, Wang L, Yuan R, Ignaszak A, Fang B, Wilkinson DP. Unlocking the door to highly active ORR catalysts for PEMFC applications: polyhedron-engineered Ptbased nanocrystals. Energy Environ Sci 2018;11:258e75. [4] Erickson EM, Thorum MS, Vasic R, Marinkovic NS, Frenkel AI, Gewirth AA, Nuzzo RG. In situ electrochemical X-ray absorption spectroscopy of oxygen reduction electrocatalysis with high oxygen flux. J Am Chem Soc 2012;134:197e200.
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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[5] Cheon JY, Kim JH, Kim JH, Goddeti KC, Park JY, Joo SH. Intrinsic relationship between enhanced oxygen reduction reaction activity and nanoscale work function of doped carbons. J Am Chem Soc 2014;136:8875e8. [6] Zhang P, Sun F, Xiang ZH, Shen ZG, Yun J, Cao DP. ZIFderived in situ nitrogen-doped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction. Energy Environ Sci 2014;7:442e50. [7] Wei W, Liang H, Parvez K, Zhuang X, Feng X, Mullen K. Nitrogen-doped carbon nanosheets with size-defined mesopores as highly efficient metal-free catalyst for the oxygen reduction reaction. Angew Chem 2014;126:1596e600. [8] Jing SY, Luo L, Yin SB, Huang F, Jia Y, Wei Y, Sun ZH, Zhao Y. Tungsten nitride decorated carbon nanotubes hybrid as efficient catalyst supports for oxygen reduction reaction. Appl Catal B Environ 2014;147:897e903. [9] Yuan K, Zhuang X, Fu H, Brunklaus G, Forster M, Chen Y, Feng X, Scherf U. Two-dimensional core-shelled porous hybrids as highly efficient catalysts for the oxygen reduction reaction. Angew Chem 2016;128:6972e7. [10] Sa YJ, Kwon K, Cheon JY, Kleitz F, Joo SH. Ordered mesoporous Co3O4 spinels as stable, bifunctional, noble metal-free oxygen electrocatalysts. J Mater Chem A 2013;1:9992e10001. [11] Xia W, Zou R, An L, Xia D, Guo S. A metaleorganic framework route to in situ encapsulation of Co@Co3O4@C core@bishell nanoparticles into a highly ordered porous carbon matrix for oxygen reduction. Energy Environ Sci 2015;8:568e76. [12] Lu FL, Sui J, Su JM, Jin C, Shen M, Yang RZ. Hollow spherical La0.8Sr0.2MnO3 perovskite oxide with enhanced catalytic activities for the oxygen reduction reaction. J Power Sources 2014;271:55e9. [13] Zhang GQ, Xia BY, Wang X, Lou XW. Strongly coupled NiCo2O4-rGO hybrid nanosheets as a methanol-tolerant electrocatalyst for the oxygen reduction reaction. Adv Mater 2014;26:2408e12. [14] Wang Q, Qiu X, Hu W, Huang Y. Facile synthesis of threedimensional porous nitrogen doped carbon supported Co3O4 for oxygen reduction reaction and oxygen evolution reaction. Mater Lett 2017;190:169e72. [15] Meng Y, Song W, Huang H, Ren Z, Chen SY, Suib SL. Structure-property relationship of bifunctional MnO2 nanostructures: highly efficient, ultra-stable electrochemical water oxidation and oxygen reduction reaction catalysts identified in alkaline media. J Am Chem Soc 2014;136:11452e64. [16] Lambert TN, Vigil JA, White SE, Delker CJ, Davis DJ, Kelly M, Brumbach MT, Rodriguez MA, Swartzentruber BS. Understanding the effects of cationic dopants on a-MnO2 oxygen reduction reaction electrocatalysis. J Phys Chem C 2017;121:2789e97. [17] Ma Y, Wang R, Wang H, Key J, Ji S. Control of MnO2 nanocrystal shape from tremella to nanobelt for ehancement of the oxygen reduction reaction activity. J Power Sources 2015;280:526e32. [18] Su Y, Jiang H, Zhu Y, Yang X, Shen J, Zou W, Chen J, Li C. Enriched graphitic N-doped carbon-supported Fe3O4 nanoparticles as efficient electrocatalysts for oxygen reduction reaction. J Mater Chem A 2014;2:7281e7. [19] Xiao J, Wan L, Wang X, Kuang Q, Dong S, Xiao F, Wang S. Mesoporous Mn3O4-CoO coreeshell spheres wrapped by carbon nanotubes: a high performance catalyst for the oxygen reduction reaction and CO oxidation. J Mater Chem A 2014;2:3794e800. [20] Lv X, Lv W, Wei W, Zheng X, Zhang C, Zhi L, Yang Q. A hybrid of holey graphene and Mn3O4 and its oxygen reduction reaction performance. Chem Commun 2015;51:3911e4.
[21] Duan J, Chen S, Dai S, Qiao SZ. Shape Control of Mn3O4 nanoparticles on nitrogen-doped graphene for enhanced oxygen reduction activity. Adv Funct Mater 2014;24:2072e8. [22] Wang Y, Wilkinson DP, Zhang J. Noncarbon support materials for polymer electrolyte membrane fuel cell electrocatalysts. Chem Rev 2011;111:7625e51. [23] Wu Z, Yang S, Sun Y, Parvez K, Feng X, Mullen K. 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J Am Chem Soc 2012;134:9082e5. [24] 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. Nat Mater 2011;10:780e6. [25] Wang D, Yan W, Vijapur SH, Botte GG. Electrochemically reduced graphene oxide-nickel nanocomposites for urea electrolysis. Electrochim Acta 2013;89:732e6. [26] Zhang B, Lin Z, Huang J, Cao L, Wu X, Yu X, Zhan Y, Xie F, Zhang W, Chen J, Mai W, Xie W, Meng H. Highly active and stable non noble metal catalyst for oxygen reduction reaction. Int J Hydrogen Energy 2017;42:10423e4. [27] Zhan Y, Yu X, Cao L, Zhang B, Wu X, Xie F, Zhang W, Chen J, Xie W, Mai W, Meng H. Th] influence of nitrogen source and doping sequence on the electrocatalytic activity for oxygen reduction reaction of nitrogen doped carbon materials. Int J Hydrogen Energy 2016;41:13493e503. [28] Wu ZS, Ren W, Wen L, Gao L, Zhao J, Chen Z, Zhou G, Li F, Cheng HM. Graphene anchored with Co3O4 nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance. ACS Nano 2010;4:3187e94. [29] Meher SK, Rao GR. Ultralayered Co3O4 for high-performance supercapacitor applications. J Phys Chem C 2011;115:15646e54. [30] Ma TY, Dai S, Jaroniec M, Qiao SZ. Metalorganic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversible oxygen evolution electrodes. J Am Chem Soc 2014;136:13925e31. [31] Yi H, Yan M, Huang D, Zeng G, Lai C, Li M, Huo X, Qin L, Liu S, Liu X, Li B, Wang H, Shen M, Fu Y, Guo X. Synergistic effect of artificial enzyme and 2D nano-structured Bi2WO6 for ecofriendly and efficient biomimetic photocatalysis. Appl Catal B Environ 2019;250:52e62. [32] Yi H, Jiang M, Huang D, Zeng G, Lai C, Qin L, Zhou C, Li B, Liu X, Cheng M, Xue W, Xu P, Zhang C. Advanced photocatalytic fenton-like process over biomimetic heminBi2WO6 with enhanced pH. J Taiwan Institute of Chemical Engineers 2018;93:184e92. [33] Yang Y, Zhang C, Huang D, Zeng G, Huang J, Lai C, Zhou C, Wang W, Guo H, Xue W, Deng R, Cheng M, Xiong W. Boron nitride quantum dots decorated ultrathin porous g-C3N4: intensified exciton dissociation and charge transfer for promoting visible-light-driven molecular oxygen activation. Appl Catal B Environ 2019;245:87e99. [34] Qin L, Zeng G, Lai C, Huang D, Zhang C, Xu P, Hu T, Liu X, Cheng M, Liu Y, Hu L, Zhou Y. A visual application of gold nanoparticles: simple, reliable and sensitive detection of kanamycin based on hydrogen-bonding recognition. Sens Actuators B Chem 2017;243:946e54. [35] Yi H, Huang D, Zeng G, Lai C, Qin L, Cheng M, Ye S, Song B, Ren X, Guo X. Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production. Appl Catal B Environ 2018;239:408e24. [36] Tang HX, Ehlert GJ, Lin YR, Sodano HA. Highly efficient synthesis of graphene nanocomposites. Nano Lett 2012;12:84e90. [37] Jiang ZX, Wang JJ, Meng LH, Huang YD, Liu L. A highly efficient chemical sensor material for ethanol: Al2O3/
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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[38]
[39]
[40]
[41]
Graphene nanocomposites fabricated from graphene oxide. Chem Commun 2011;47:6350e2. Ghanbarlou H, Rowshanzamir S, Parnian MJ, Mehri F. Comparison of nitrogen-doped graphene and carbon nanotubes as supporting material for iron and cobalt nanoparticle electrocatalysts toward oxygen reduction reaction in alkaline media for fuel cell applications. Int J Hydrogen Energy 2016;41:14665e75. Ratso S, Kruusenberg I, Joost U, Saar R, Tammeveski K. Enhanced oxygen reduction reaction activity of nitrogendoped graphene/multi-walled carbon nanotube catalysts in alkaline media. Int J Hydrogen Energy 2016;41:22510e9. Zeng D, Yu X, Zhan Y, Cao L, Wu X, Zhang B, Huang J, Lin Z, Xie F, Zhang W, Chen J, Xie W, Mai W, Meng H. Insight into the nitrogen-doped carbon as oxygen reduction reaction catalyst: the choice of carbon/nitrogen source and active sites. Int J Hydrogen Energy 2016;41:8563e75. Zhang Y, Wang C, Fu J, Zhao H, Tian F, Zhang R. Fabrication and high ORR performance of MnOx nanopyramid layers
9
with enriched oxygen vacancies. Chem Commun 2018;54:9639e42. [42] Maiti K, Balamurugan J, Gautam J, Kim NH, Lee JH. Hierarchical flowerlike highly synergistic three-dimensional iron tungsten oxide nanostructure-anchored nitrogen-doped graphene as an efficient and durable electrocatalyst for oxygen reduction reaction. ACS Appl Mater Interfaces 2018;10:32220e32. [43] Kong A, Zhang Y, Chen Z, Chen A, Li C, Wang H, Shan Y. Onepot synthesized covalent porphyrin polymer-derived coreshell Fe3C@Carbon for efficient oxygen electroreduction. Carbon 2017;116:606e14. [44] Lu L, Hao Q, Lei W, Xia X, Liu P, Sun D, Wang X, Yang X. Wellcombined magnetically separable hybrid cobalt ferrite/ nitrogen-doped graphene as efficient catalyst with superior performance for oxygen reduction reaction. Small 2015;11:5833e43.
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Cost-effective Co3O4 nanospheres on nitrogendoped graphene used as highly efficient catalyst for oxygen reduction reaction Jiahao Guo*, Qianfu Li, Haitao Hou, Junming Chen, Chengdong Wang, Songlin Zhang, Xuchun Wang College of Chemistry and Materials Engineering, Anhui Science and Technology University, Fengyang, Anhui, 233100, PR China
highlights A Co3O4 NS/N-rGO hybrid has been prepared through a simple two-step method. An interaction between Co3O4 and N-rGO is beneficial to improve ORR activity. The hybrid catalyst has excellent ORR activity with a 4-electron mechanism.
article info
abstract
Article history:
Developing low-cost, efficient and stable catalyst for the oxygen reduction reaction is
Received 28 June 2019
meaningful and necessary for the industrialization of fuel cells. In this work, we report the
Received in revised form
controllable synthesis of Co3O4 nanospheres uniformly anchored on N-doped reduced
13 September 2019
graphene oxide (N-rGO) sheets with significant catalytic activity for the oxygen reduction
Accepted 23 September 2019
reaction via a simple two-step approach. Results show that there is an interaction between
Available online xxx
Co3O4 nanospheres and N-rGO after riveting on N-rGO, which is beneficial to the electron transfer between them. The Co3O4 NS/N-rGO hybrid has excellent catalytic activity, com-
Keywords:
parable to commercial Pt/C catalyst. The transferred electron number of the oxygen
Co3O4 nanospheres
reduction reaction on Co3O4 NS/N-rGO is around 3.95. The hybrid has more excellent
Nitrogen-doped reduced graphene
durability and methanol resistance than commercial Pt/C. The Co3O4 NS/N-rGO catalyst in
oxide
our work provides a cost-effective the oxygen reduction reaction catalyst alternative to the
Oxygen reduction reaction
precious metal Pt in fuel cell.
Catalytic performance
© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The increasingly serious energy crisis and environmental pollution have impelled a great attention to the development and utilization of new renewable clean energy resources, such as fuel cells and metal-air batteries [1e3]. The development of high efficiency catalysts for the oxygen reduction reaction
(ORR) has become the heart and main challenge of these new renewable clean energy technologies because of the sluggish kinetics of ORR [4e6]. Nowadays, Pt and its alloys have long been regarded as the most effective catalyst for ORR; however, the high cost as well as limited supply and poor stability have seriously restricted them from becoming the most effective solution for ORR [7,8]. In this respect, the substitution catalysts based on non-precious metals or metal oxides with high
* Corresponding author. E-mail address: [email protected] (J. Guo). https://doi.org/10.1016/j.ijhydene.2019.09.165 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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3 h to obtain Co3O4 NS/N-rGO. The theoretical mass ratio of Co3O4 to N-rGO in Co3O4 NS/N-rGO catalyst is approximate 1:1. The final product is collected by centrifugation and washed with water and ethanol and dry. The obtained catalyst is recorded as Co3O4 NS/N-rGO-1 when the concentration of CoCl2 is 15 mM. The synthesis procedure of Co3O4 NS is the same as that of Co3O4 NS/N-rGO, except GO was not added. The morphologies of the synthesized samples were analyzed by field emission scanning electron microscopy (FESEM, FEI HITACHI S-4800) and transmission electron microscopy (TEM, JEOJ-2010). XRD tests were carried out on a Rigaku D/max-rA with Cu Ka radiation (l ¼ 1.54178 A). Xray photoelectron spectra (XPS) were carried out on an Thermo ESCALAB 250Xi using a monochromic Al Ka (hv ¼ 1486.6 eV) X-ray source to analyze the surface elemental composition of the samples. Brunauer-EmmettTeller (BET) tests were carried out on a Micromeritics ASAP 2460 system to determine the surface area by nitrogen adsorption. Thermal gravimetric analysis (TGA) were carried out on a Setaram Labsys Thermogravimetric Analyzer in air atmosphere at a heating rate of 10 C$min1 from room temperature to 800 C. 4 mg catalysts and 17 mL 5% Nafion solution were ultrasonically dispersed in 1 mL of 3:1 v/v water/isopropanol mixed solvent to form homogeneous ink. The pre-polished glassy carbon electrode of 5 mm in diameter was dropped with 5 mL catalyst ink, yielding a loading of catalyst 0.1 mg cm2, and dried in air used as a working electrode. All electrochemical tests were carried out in 0.1 M KOH saturated with O2 or N2 by a CHI 700 E electrochemical workstation with the standard three-eletrode cell under room temperature. The counter electrode was Pt wire and the referece electrode was Ag/AgCl (PINE, 4 M KCl). Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were scanned from 1.0 to 0.2 V at 10 mV s1. The disk rotation rates of RDE were ranged from 400 to 2500 rpm. Chronoamperometry tests were carried out at 0.3 V with a rotation speed of 1600 rpm. For RDE tests, the transferred electron number (n) and kinetic current density (Jk) in the ORR process were obtained from the slope and intercept of K-L straight line based on the Koutecky-Levich equation. 1 1 1 ¼ þ J JK B612
(1) 2
1
=
abundance, low cost, environmental friendliness and potential ORR activity have attracted the researchers' interest [9e13]. Recently, non-precious metal oxides with spinel structure, such as Co3O4 [10,14], MnO2 [15e17], Fe3O4 [18], Mn3O4 [19,20] etc. have attracted considerable attention. Nevertheless, pristine metals and metal oxides usually exhibit poor catalytic activity due to agglomeration, sintering and dissolution of catalyst particles during the fuel cells operation [21]. To overcome this obstacle, the loading of catalyst particles on the materials with large electroactive surface area can improve their catalytic activity and durability [22]. According to this idea, considering that both non-precious metal oxides and doped carbon materials have considerable ORR catalytic activities, the hybrid composite loaded non-precious metal oxide nanoparticles on doped carbon materials should have excellent ORR catalytic activity. Wu and co-workers synthesized 3D N-doped graphene aerogelsupported Fe3O4 nanoparticles (Fe3O4/N-GAs) as efficient ORR catalysts [23]. The 3D macroporous structure and high specific surface area of the supporter are responsible for the enhancement of ORR activity. Three kinds of Mn3O4 nanoparticles were loaded on N-doped graphene sheets and the effect of Mn3O4 morphology on their catalytic performance have been researched by Duan and co-workers [21]. It was found that the ORR activity was correlated to the shape of Mn3O4 nanocrystals and the exposed crystalline facets. The ellipsoidal Mn3O4 particles on N-doped graphene exhibited the highest ORR activity. At present, the composite catalysts for ORR based on Co3O4 have attracted considerable attention, such as Co3O4/N-doped graphene hybrid material and porous nitrogen doped carbon supported Co3O4 [24]. However, Co3O4 nanosphere anchored on nitrogen-doped reduced graphite oxide as ORR catalysts have been seldom reported. Herein, we demonstrate Co3O4 NS/N-rGO catalyst, a nanostructual hybrid Co3O4 nanospheres uniformly anchored on N-doped reduced graphite oxide (N-rGO) sheets, prepared through a simple two-step method. After binding with N-rGO, the uniformly dispersed Co3O4 nanospheres exhibit remarkable ORR activity with a representative four-electron pathway, which is superior to Co3O4 nanospheres and comparable to commercial Pt/C catalyst (20 wt% Pt on Vulcan XC-72). The Co3O4 NS/N-rGO hybrid also showed excellent durability and methanol resistance.
=
2
=
B ¼ 0:2nFðDO Þ 3 n 6 cO
Experimental Graphite oxide (GO) was obtained by a modified Hummer’s method according to literature [25]. In a typical synthesis procedure of Co3O4 NS/N-rGO, 3 mM CoCl2 solution was obtained by adding 20 mg CoCl2 into 50 mL ethylene glycol. After added 10 mg GO, the solution was sonicated for 2 h to adsorbed Co(II) ions on GO by electrostatic action. After stirring at 50 C and adding 0.1 M NH4OH drop by drop to adjusted pH to about 9, the stable suspension was transferred to a Telfon-lined autolave and solvothermally treated at 130 C for 24 h. Then, the precursor suspension was slowly added with 50 mL of 3% H2O2 at 60 C to oxidized for
(2)
where J, JK are the measured current and the kinetic current, u is the rotating rate, F is the Faraday constant (96 485 C mol1). DO is the diffusion coefficient of O2 in 0.1 M KOH (1.9 105 cm2 s1). n is the kinetic viscosity (0.01 cm2 s1). CO is the bulk concentration of O2 (1.2 106 mol cm3). The constant 0.2 is adopted when the rotation speed is expressed in rpm. In the RRDE tests, the conditions are the same as the RDE test, except for the rotating speed of the disk electrode was 1600 rpm and the potential of ring electrode set to 0.25 V to oxidize HO 2 intermediate from the disk electrode. The HO 2 % and the transferred electron number (n) were determined by the followed equations [26,27].
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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HO 2%¼
n¼
200 Ir NId þ Ir
(3)
4 Id Id þ Ir=N
(4)
where Id is disk current, Ir is ring current and N is current collection efficiency of the Pt ring and is 0.39 in our experiment.
Results and discussion The morphology of the samples was characterized by SEM and TEM. It can be seen from Fig. 1Sa and 1Sb that Co3O4 NS mainly consist of loose nanospheres with a small amount of nanorods, and the diameter of the nanospheres is about 140 nm. After the addition of GO to the reaction system, the Co2þ ion was firstly adsorbed on the active site graphene as the growth point and gradually grew into compact nanospheres (Fig. 1a and b) [28]. SEM images elaborated smaller Co3O4 nanospheres in the Co3O4 NS/N-rGO hybrid with a diameter of about 110 nm than the Co3O4 NS and uniformly distributed on the graphene sheets. Such geometric configuration of metal oxide nanospheres on graphene sheets contribute to ameliorate their interface contact and effectively prevent the dissolution and aggregation of the nanospheres, thus promoting the electrochemical activity and stability of the hybrid catalyst. TEM images further authenticated the uniform distribution of the Co3O4 nanospheres on the graphene sheets (Fig. 1c). HRTEM images show that the lattice spacing of the composite catalyst and Co3O4 NS is 0.465 nm, corresponding to the (111) plane of
3
the spinel structure of Co3O4 nanocrystals, while the other lattice spacing of the hybrid catalyst is 0.351 nm, corresponding to the (002) planes of the graphite (Fig. 1d and Fig. S1d). In the preparation process of Co3O4 NS/N-rGO, when the concentration of CoCl2 used is 15 mM, the resulting catalyst is a nanosphere aggregate with smaller particle size, attached to the graphene (Fig. S2). The aggregation of Co3O4 nanospheres will lead to the decrease of active sites, which is not conducive to the improvement of catalytic performance of catalysts. XRD patterns are used to analyze the crystal structure of the hybrid catalyst, as shown in Fig. 2a. The diffraction peaks in the XRD patterns of Co3O4 and hybrid catalyst can be indexed to a spinel phase (space group Fd3m) with a lattice constant a ¼ 8.084 A, matched well with JCPDS card No. 43-1003 [29,30]. However, compared with Co3O4, the intensity of the corresponding diffraction peak of hybrid catalyst was significantly reduced due to the influence of the graphene. The hybrid catalyst has a sharp diffraction peak at 25 , which can be attributed to graphene oxide. The Brunauer-Emmett-Teller (BET) surface area of the hybrid catalyst determined by nitrogen adsorption isotherm was 85 m2 g1, which is slightly higher than that of 61 m2 g1 with Co3O4 NS (Fig. 2b). The pore size of the hybrid catalyst was centered at z16 nm, larger than that of Co3O4 NS (z7 nm) (the inset of Fig. 2b). Larger surface area and pore size are beneficial to the adsorption of reactant particles and the transport of materials, which is beneficial to the improvement of catalytic activity of the hybrid catalyst [31e35]. The chemical composition of the surface of the hybrid catalyst was determined by X-ray photoelectron spectroscopy (XPS) and the results are shown in Fig. 3a. XPS spectrum of the
Fig. 1 e (a, b) SEM, (c) TEM and (d) HRTEM images of Co3O4 NS/N-rGO. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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Fig. 2 e (a) XRD spectra and (b) Nitrogen adsorptionedesorption isotherms of Co3O4 NS and Co3O4 NS/N-rGO. The inset in (b) shows the corresponding pore size distribution curves.
Co3O4 NS/N-rGO hybrid revealed the presence of the elements C, N, O and Co, located at 285 eV, 400 eV, 533 eV and 781 eV, respectively. Their contents are 80.93 at.% C, 3.92 at.% N, 14.46 at.% O and 0.69 at.% Co, respectively. However, XPS spectrum of Co3O4 NS only displays O1s, Co2p and petty C1s peaks, which could be caused by adsorbed. In contrast, the peak of Co2p of the hybrid catalyst is obviously weaker than that of Co3O4 NS, due to the presence of graphene oxide. The
deconvolution of the C1s spectrum indicates the presence of three states of C, that is CeC or C]C at 284.7eV, CeN at 285.9 eV, and OeC]O at 288.2 eV, and the corresponding contents are 67.1%, 21.6% and 11.3%, respectively, indicating the effective reduction of GO and successful incorporation of nitrogen (Fig. 4b) [36,37]. The high-resolution N1s could be deconvoluted into three peaks: pyridinic N at 398.8 eV, pyrrolic N at 399.8 eV and graphitic N at 400.5 eV (Fig. 4c) [38e40]. The
Fig. 3 e (a) XPS spectrum of Co3O4 NS/N-rGO and Co3O4 NS; (b) C 1s and (c) N 1s core-level and corresponding deconvoluted spectra of Co3O4 NS/N-rGO; (d) Co 2p core-level and corresponding deconvoluted spectra of Co3O4 NS/N-rGO and Co3O4 NS. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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growth and dispersion of Co3O4 nanospheres on GO sheet were promoted by doping nitrogen serve as the anchor sites of Co3O4 nanocrystalline. The incorporation of N also can effectively improve the catalytic activity of the hybrid catalyst. The deconvolution of O1s spectrum shown in Fig. S3 could be fitted into four peaks at 533.5, 532.5, and 531.3 eV, corresponding to the chemisorbed oxygen (OC), oxygen vacancies (OV), and surface lattice oxygen (OL) in Co3O4, respectively [41,42]. The existence of oxygen vacancy on the surface of Co3O4 NS/NrGO can promote the adsorption and dissociation of O2, thus improving its catalytic performance. In Fig. 4d, the Co2p XPS spectrum can be deconvoluted into a pair of spin-orbit double peaks at 781.4 eV and 797.1 eV, attributed to the 2p1/2 and 2p3/2 peaks of Co3O4, respectively. The Co 2p3/2 peak could be decomposed into two peaks at 781.4 eV and 785.7 eV,
5
corresponding to Co3þ and Co2þ, respectively. Compared with the Co 2p peak of pure Co3O4, the hybrid catalyst positive shifted from 780.5 eV to 781.4 eV for 2p3/2 peak and positive shifted from 796.1 eV to 797.1 eV for 2p1/2 peak, which could probably be attributed to the interaction between Co3O4 and rGO [42]. The positive shift of Co 2p binding energy leads to the decrease of electron density on Co atom, which is beneficial to the adsorption and dissociation of O2. By means of thermogravimetric analysis, the mass percentage of Co3O4 in the hybrid catalyst is determined to be about 76%, which basically coincides with the theoretical yield of hybrid catalyst (Fig. S4). Cyclic voltammetry (CV) measurement was first used to evaluated the ORR catalytic performance of Co3O4/N-rGO in 0.1 M KOH solution (Fig. 4a). In N2-saturated electrolyte, CV curves of all catalysts showed no obvious reduction peak
Fig. 4 e (a)CV of Co3O4/N-rGO, Co3O4 NS, and N-rGO in N2- and O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s¡1. (b) Linear sweep voltammetry (LSV) of four samples and Pt/C in O2- saturated 0.1 M KOH at a scan rate of mV·s¡1 with an RDE rotation rate of 1600 rpm. (c and d) LSV and Calculated K-L plots of Co3O4 NS/N-rGO in O2-saturated 0.1 M KOH at a scan rate of 10 mV s¡1 and at different RDE rotation rates. (e and f) KeL plots at ¡0.6 V and transferred electron number n of the ORR for four samples and Pt/C. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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Table 1 e Comparison of the electrocatalytic performance parameters in different Literature. Catalysts Co/NC-800 HGSeMn3O4 MONPMs/NC 3DFeeWO3 NF/NG Zn/Co@CeNCNFs CoFe2O4/NG Co3O4 NS/N-rGO
Catalyst loading 0.24 mg/cm2 0.32 mg/cm2 0.1 mg/cm2 0.35 mg/cm2 0.1 mg/cm2 283 mg/cm2 0.1 mg/cm2
Eonset/V（vs. Ag/AgCl）
E1/2/V (vs. Ag/AgCl） 0.19
0.14 0.98 (vs.RHE) 0.98 (vs.RHE) 0.099 0.030 0.02
during the process of potential decrease, indicating that ORR did not occur. When the electrolyte was saturated with O2, the reduction current of Co3O4/N-rGO reveals a well-defined ORR reduction peak at 0.17 V with the potential decrease, which is close to 0.12 V of Pt/C (Fig. S5) and significantly higher than 0.45 V of Co3O4 NS and 0.36 V of N-rGO. The peak current density of 0.51 mA cm2 is also significantly larger than those of Co3O4 NS and N-rGO. The results are indicated a considerable ORR activity of Co3O4/N-rGO, which further illuminated that the structure of Co3O4 riveting on N-doped graphitic can obviously improve the ORR activity of catalyst. Linear sweep voltammetry (LSV) measurement for Co3O4 NS/N-rGO was executed on a rotating disk electrode (RDE) in O2-saturated 0.1 M KOH electrolyte at a rotation rate of 1600 rpm to determining the kinetic parameters of ORR, in comparison with Pt/C (20 wt% Pt) with the same loading capacity. As shown in Fig. 4b, Co3O4 NS/N-rGO hybrid occupies
0.81 (vs.RHE) 0.85 (vs.RHE) 0.20 0.144 0.13
n
Reference
3.99 3.69 3.94 3.97 3.69 3.61e3.88 3.95e3.98
[14] [20] [41] [42] [43] [44] This work
an evident positive onset potential of 0.02 V and half-wave potential of 0.13 V, much higher than those of Co3O4 NS (0.31 V, 0.39 V), N-rGO (0.060 V, 0.20 V) and very close to those of Pt/C (0.01 V, 0.18 V). Similarly, the current density of Co3O4 NS/N-rGO catalyst is remarkable higher than that of Co3O4 NS and N-rGO. These results indicate that Co3O4 NS/NrGO occupies an excellent ORR activity, because of the well combination of Co3O4 nanospheres with N-doped graphene. The onset potential and half-wave potential of Co3O4 NS/NrGO-1 are almost equal to those of Co3O4 NS/N-rGO, but the current density is obviously lower, which may be caused by the decrease of activity sites caused by the aggregation of catalyst particles. Typical LSV curves of Co3O4 NS/N-rGO, Co3O4 NS/N-rGO-1, Co3O4 NS, N-rGO and Pt/C at different rotating speeds from 400 to 2500 rpm were measured in Fig.4c and Fig. S6. An obvious increase of current density with the increase in rotating speeds at the same potential was
Fig. 5 e (a) RRDE measurement of four samples and Pt/C for ORR. (b) The number of electrons transferred per O2 and calculated HO¡ 2 production yields of four samples and Pt/C during the ORR. (c) Chronoamperometric response of Co3O4 NS/NrGO and Pt/C. (d) iet of Co3O4 NS/N-rGO and Pt/C before and after the addition of 3 M methanol. Tests were conducted in O2saturated 0.1 M KOH solution at ¡0.3 V. Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
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observed due to the decrease of the O2 diffusion distance with the increase of rotation speed. The corresponding KouteckyLevich plots from 0.3 V to 1.0 V were calculated from LSVs at different potentials and assume good linear relationship and near parallelism, suggesting first-order reaction kinetics for ORR (Fig.4d and Fig. S6). Fig. 4e shows the comparison of K-L curves of several catalysts at 0.6 V. It can be seen from the diagram that the K-L curve of Co3O4 NS/NrGO is almost coincide with that of Pt/C, which means that the catalytic activity of Co3O4 NS/N-rGO is close to that of Pt/C. The transferred electron number of Co3O4 NS/N-rGO calculated from the slopes of the K-L plots is varied from 3.95 to 3.98 in the range of 0.3-1.0 V, indicating that the ORR process proceeds via a 4-electron mechanism. On the contrary, the transferred electron number of N-rGO and Co3O4 NS are about 3 and 2.1, respectively, which are significantly lower than that of Co3O4 NS/N-rGO. The transferred electron number of Co3O4 NS/N-rGO-1 catalyst is about 3.66, which is also lower than that of Co3O4 NS/N-rGO. The recently reported transition metal oxide catalysts based on nitrogen-doped carbon material was compared and found that Co3O4 NS/N-rGO possesses excellent ORR performance (Table 1). The effect of CoCl2 concentration on the catalytic activity of the prepared Co3O4 NS/N-rGO sample was researched. As can be seen from Fig. S7, the catalytic activity of the Co3O4 NS/N-rGO sample prepared with 3 mM CoCl2 was best. Further characterization of ORR pathway by an additional rotating ring disk electrode (RRDE) voltammograms to monitoring the formation of HO 2 were executed in O2-saturated 0.1 M KOH electrolyte. As displayed in Fig. 5a, compared to Co3O4 NS, N-rGO, the onset potential and diffusion limiting current density of Co3O4 NS/N-rGO have significantly improved, which can be compared with the commercial Pt/C catalyst. Co3O4 NS/N-rGO-1 has the same onset potential as Co3O4 NS/N-rGO, but the limit diffusion current is obviously lower. Fig. 5b shows the hydrogen peroxide yields generated on four catalysts at different potentials. In the potential range from 0.3 to 1.0 V, the H2O2% of Co3O4 NS/N-rGO is lower than 5%, which is obviously lower than those of Co3O4 NS, NrGO, and Co3O4 NS/N-rGO-1. The transferred electron number calculated from RRDE on Co3O4 NS/N-rGO is around 3.98 from 0.3 to 1.0 V. At the same time, the transferred electron number of Co3O4 NS, N-rGO, and Co3O4 NS/N-rGO-1 is about 2.06, 3.15 and 3.65 in the same range of potentials. The catalytic performance of Co3O4 NS/N-rGO-1 is closer to Co3O4 NS/ N-rGO, but it is obviously affected by the aggregation of Co3O4 NS. All these results indicate that the oxygen reduction process on Co3O4 NS/N-rGO mainly takes a 4-electron path, directly reducing O2 to OH ions. The long-term durability and outstanding tolerance to methanol crossover are important concerns of ORR catalysts, which is related to the industrialization development of direct methanol fuel cells (DMFC). Chronoamperometric durability testing for the ORR of Co3O4 NS/N-rGO and Pt/C were performed at 0.3 V in O2-saturated 0.1 M KOH and shown in Fig. 5c. Pt/C shows a 26.2% decrease in activity after 10 h at 0.3 V. However, Co3O4 NS/N-rGO remains a high relative current of 93.5% after 10 h, suggesting a superior durability of Co3O4 NS/N-rGO than commercial Pt/C. The effect of methanol crossover was estimated by chronoamperometric tests at
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0.3 V with the injection of 3.0 M methanol (Fig. 5d). An abrupt change of Pt/C in the current from negative to positive has been found, implying the occurrence of methanol oxidation. In contrast, the current of Co3O4 NS/N-rGO remained stable after methanol was added, confirming its selectivity to methanol oxidation. These results further demonstrate the potential application value of Co3O4 NS/N-rGO in fuel cells as a low cost, efficient and stable ORR catalyst.
Conclusions This work reported on a highly efficient Co3O4 NS/N-rGO catalyst for ORR, prepared by a simple two-step approach. The uniform riveting of Co3O4 NS on N-rGO has brought about the excellent ORR catalytic activity in alkaline electrolyte, as well as excellent durability and methanol resistance, owed to the enhancement of electron transfer ability caused by the interaction between Co3O4 NS and N-rGO. The high catalytic activity and durability of this catalyst makes it a substitute for commercial Pt/C and be used as an efficient, cheap and stable ORR catalyst in fuel cells.
Acknowledgements This study was supported by the Natural Science Foundation of Anhui Province (Grant 1808085MB31); Students Platform for Innovation and Entrepreneurship Training Program of China (2018S10879018); Stable Talent Foundation of Anhui Science and Technology University (HCWD201601); Science and Technology Major Project of Anhui Province (Grant 18030901087).
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.09.165.
references
[1] Chen C, Kang Y, Huo Z, Zhu Z, Huang W, Xin HL, Snyder JD, Li D, Herron JA, Mavrikakis M, Chi M, More KL, Li Y, Markovic NM, Somorjai GA, Yang P, Stamenkovic VR. Highly crystalline multimetallic nanoframes with threedimensional electrocatalytic surfaces. Science 2014;343:1339e43. [2] Guo D, Shibuya R, Akiba C, Saji S, Kondo T, Nakamura J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016;351:361e5. [3] Wang YJ, Long W, Wang L, Yuan R, Ignaszak A, Fang B, Wilkinson DP. Unlocking the door to highly active ORR catalysts for PEMFC applications: polyhedron-engineered Ptbased nanocrystals. Energy Environ Sci 2018;11:258e75. [4] Erickson EM, Thorum MS, Vasic R, Marinkovic NS, Frenkel AI, Gewirth AA, Nuzzo RG. In situ electrochemical X-ray absorption spectroscopy of oxygen reduction electrocatalysis with high oxygen flux. J Am Chem Soc 2012;134:197e200.
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
8
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[5] Cheon JY, Kim JH, Kim JH, Goddeti KC, Park JY, Joo SH. Intrinsic relationship between enhanced oxygen reduction reaction activity and nanoscale work function of doped carbons. J Am Chem Soc 2014;136:8875e8. [6] Zhang P, Sun F, Xiang ZH, Shen ZG, Yun J, Cao DP. ZIFderived in situ nitrogen-doped porous carbons as efficient metal-free electrocatalysts for oxygen reduction reaction. Energy Environ Sci 2014;7:442e50. [7] Wei W, Liang H, Parvez K, Zhuang X, Feng X, Mullen K. Nitrogen-doped carbon nanosheets with size-defined mesopores as highly efficient metal-free catalyst for the oxygen reduction reaction. Angew Chem 2014;126:1596e600. [8] Jing SY, Luo L, Yin SB, Huang F, Jia Y, Wei Y, Sun ZH, Zhao Y. Tungsten nitride decorated carbon nanotubes hybrid as efficient catalyst supports for oxygen reduction reaction. Appl Catal B Environ 2014;147:897e903. [9] Yuan K, Zhuang X, Fu H, Brunklaus G, Forster M, Chen Y, Feng X, Scherf U. Two-dimensional core-shelled porous hybrids as highly efficient catalysts for the oxygen reduction reaction. Angew Chem 2016;128:6972e7. [10] Sa YJ, Kwon K, Cheon JY, Kleitz F, Joo SH. Ordered mesoporous Co3O4 spinels as stable, bifunctional, noble metal-free oxygen electrocatalysts. J Mater Chem A 2013;1:9992e10001. [11] Xia W, Zou R, An L, Xia D, Guo S. A metaleorganic framework route to in situ encapsulation of Co@Co3O4@C core@bishell nanoparticles into a highly ordered porous carbon matrix for oxygen reduction. Energy Environ Sci 2015;8:568e76. [12] Lu FL, Sui J, Su JM, Jin C, Shen M, Yang RZ. Hollow spherical La0.8Sr0.2MnO3 perovskite oxide with enhanced catalytic activities for the oxygen reduction reaction. J Power Sources 2014;271:55e9. [13] Zhang GQ, Xia BY, Wang X, Lou XW. Strongly coupled NiCo2O4-rGO hybrid nanosheets as a methanol-tolerant electrocatalyst for the oxygen reduction reaction. Adv Mater 2014;26:2408e12. [14] Wang Q, Qiu X, Hu W, Huang Y. Facile synthesis of threedimensional porous nitrogen doped carbon supported Co3O4 for oxygen reduction reaction and oxygen evolution reaction. Mater Lett 2017;190:169e72. [15] Meng Y, Song W, Huang H, Ren Z, Chen SY, Suib SL. Structure-property relationship of bifunctional MnO2 nanostructures: highly efficient, ultra-stable electrochemical water oxidation and oxygen reduction reaction catalysts identified in alkaline media. J Am Chem Soc 2014;136:11452e64. [16] Lambert TN, Vigil JA, White SE, Delker CJ, Davis DJ, Kelly M, Brumbach MT, Rodriguez MA, Swartzentruber BS. Understanding the effects of cationic dopants on a-MnO2 oxygen reduction reaction electrocatalysis. J Phys Chem C 2017;121:2789e97. [17] Ma Y, Wang R, Wang H, Key J, Ji S. Control of MnO2 nanocrystal shape from tremella to nanobelt for ehancement of the oxygen reduction reaction activity. J Power Sources 2015;280:526e32. [18] Su Y, Jiang H, Zhu Y, Yang X, Shen J, Zou W, Chen J, Li C. Enriched graphitic N-doped carbon-supported Fe3O4 nanoparticles as efficient electrocatalysts for oxygen reduction reaction. J Mater Chem A 2014;2:7281e7. [19] Xiao J, Wan L, Wang X, Kuang Q, Dong S, Xiao F, Wang S. Mesoporous Mn3O4-CoO coreeshell spheres wrapped by carbon nanotubes: a high performance catalyst for the oxygen reduction reaction and CO oxidation. J Mater Chem A 2014;2:3794e800. [20] Lv X, Lv W, Wei W, Zheng X, Zhang C, Zhi L, Yang Q. A hybrid of holey graphene and Mn3O4 and its oxygen reduction reaction performance. Chem Commun 2015;51:3911e4.
[21] Duan J, Chen S, Dai S, Qiao SZ. Shape Control of Mn3O4 nanoparticles on nitrogen-doped graphene for enhanced oxygen reduction activity. Adv Funct Mater 2014;24:2072e8. [22] Wang Y, Wilkinson DP, Zhang J. Noncarbon support materials for polymer electrolyte membrane fuel cell electrocatalysts. Chem Rev 2011;111:7625e51. [23] Wu Z, Yang S, Sun Y, Parvez K, Feng X, Mullen K. 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction. J Am Chem Soc 2012;134:9082e5. [24] 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. Nat Mater 2011;10:780e6. [25] Wang D, Yan W, Vijapur SH, Botte GG. Electrochemically reduced graphene oxide-nickel nanocomposites for urea electrolysis. Electrochim Acta 2013;89:732e6. [26] Zhang B, Lin Z, Huang J, Cao L, Wu X, Yu X, Zhan Y, Xie F, Zhang W, Chen J, Mai W, Xie W, Meng H. Highly active and stable non noble metal catalyst for oxygen reduction reaction. Int J Hydrogen Energy 2017;42:10423e4. [27] Zhan Y, Yu X, Cao L, Zhang B, Wu X, Xie F, Zhang W, Chen J, Xie W, Mai W, Meng H. Th] influence of nitrogen source and doping sequence on the electrocatalytic activity for oxygen reduction reaction of nitrogen doped carbon materials. Int J Hydrogen Energy 2016;41:13493e503. [28] Wu ZS, Ren W, Wen L, Gao L, Zhao J, Chen Z, Zhou G, Li F, Cheng HM. Graphene anchored with Co3O4 nanoparticles as anode of lithium ion batteries with enhanced reversible capacity and cyclic performance. ACS Nano 2010;4:3187e94. [29] Meher SK, Rao GR. Ultralayered Co3O4 for high-performance supercapacitor applications. J Phys Chem C 2011;115:15646e54. [30] Ma TY, Dai S, Jaroniec M, Qiao SZ. Metalorganic framework derived hybrid Co3O4-carbon porous nanowire arrays as reversible oxygen evolution electrodes. J Am Chem Soc 2014;136:13925e31. [31] Yi H, Yan M, Huang D, Zeng G, Lai C, Li M, Huo X, Qin L, Liu S, Liu X, Li B, Wang H, Shen M, Fu Y, Guo X. Synergistic effect of artificial enzyme and 2D nano-structured Bi2WO6 for ecofriendly and efficient biomimetic photocatalysis. Appl Catal B Environ 2019;250:52e62. [32] Yi H, Jiang M, Huang D, Zeng G, Lai C, Qin L, Zhou C, Li B, Liu X, Cheng M, Xue W, Xu P, Zhang C. Advanced photocatalytic fenton-like process over biomimetic heminBi2WO6 with enhanced pH. J Taiwan Institute of Chemical Engineers 2018;93:184e92. [33] Yang Y, Zhang C, Huang D, Zeng G, Huang J, Lai C, Zhou C, Wang W, Guo H, Xue W, Deng R, Cheng M, Xiong W. Boron nitride quantum dots decorated ultrathin porous g-C3N4: intensified exciton dissociation and charge transfer for promoting visible-light-driven molecular oxygen activation. Appl Catal B Environ 2019;245:87e99. [34] Qin L, Zeng G, Lai C, Huang D, Zhang C, Xu P, Hu T, Liu X, Cheng M, Liu Y, Hu L, Zhou Y. A visual application of gold nanoparticles: simple, reliable and sensitive detection of kanamycin based on hydrogen-bonding recognition. Sens Actuators B Chem 2017;243:946e54. [35] Yi H, Huang D, Zeng G, Lai C, Qin L, Cheng M, Ye S, Song B, Ren X, Guo X. Selective prepared carbon nanomaterials for advanced photocatalytic application in environmental pollutant treatment and hydrogen production. Appl Catal B Environ 2018;239:408e24. [36] Tang HX, Ehlert GJ, Lin YR, Sodano HA. Highly efficient synthesis of graphene nanocomposites. Nano Lett 2012;12:84e90. [37] Jiang ZX, Wang JJ, Meng LH, Huang YD, Liu L. A highly efficient chemical sensor material for ethanol: Al2O3/
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165
international journal of hydrogen energy xxx (xxxx) xxx
[38]
[39]
[40]
[41]
Graphene nanocomposites fabricated from graphene oxide. Chem Commun 2011;47:6350e2. Ghanbarlou H, Rowshanzamir S, Parnian MJ, Mehri F. Comparison of nitrogen-doped graphene and carbon nanotubes as supporting material for iron and cobalt nanoparticle electrocatalysts toward oxygen reduction reaction in alkaline media for fuel cell applications. Int J Hydrogen Energy 2016;41:14665e75. Ratso S, Kruusenberg I, Joost U, Saar R, Tammeveski K. Enhanced oxygen reduction reaction activity of nitrogendoped graphene/multi-walled carbon nanotube catalysts in alkaline media. Int J Hydrogen Energy 2016;41:22510e9. Zeng D, Yu X, Zhan Y, Cao L, Wu X, Zhang B, Huang J, Lin Z, Xie F, Zhang W, Chen J, Xie W, Mai W, Meng H. Insight into the nitrogen-doped carbon as oxygen reduction reaction catalyst: the choice of carbon/nitrogen source and active sites. Int J Hydrogen Energy 2016;41:8563e75. Zhang Y, Wang C, Fu J, Zhao H, Tian F, Zhang R. Fabrication and high ORR performance of MnOx nanopyramid layers
9
with enriched oxygen vacancies. Chem Commun 2018;54:9639e42. [42] Maiti K, Balamurugan J, Gautam J, Kim NH, Lee JH. Hierarchical flowerlike highly synergistic three-dimensional iron tungsten oxide nanostructure-anchored nitrogen-doped graphene as an efficient and durable electrocatalyst for oxygen reduction reaction. ACS Appl Mater Interfaces 2018;10:32220e32. [43] Kong A, Zhang Y, Chen Z, Chen A, Li C, Wang H, Shan Y. Onepot synthesized covalent porphyrin polymer-derived coreshell Fe3C@Carbon for efficient oxygen electroreduction. Carbon 2017;116:606e14. [44] Lu L, Hao Q, Lei W, Xia X, Liu P, Sun D, Wang X, Yang X. Wellcombined magnetically separable hybrid cobalt ferrite/ nitrogen-doped graphene as efficient catalyst with superior performance for oxygen reduction reaction. Small 2015;11:5833e43.
Please cite this article as: Guo J et al., Cost-effective Co3O4 nanospheres on nitrogen-doped graphene used as highly efficient catalyst for oxygen reduction reaction, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.09.165