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Co nanoparticles supported on three-dimensionally N-doped holey graphene aerogels for electrocatalytic oxygen reduction
Journal of Colloid and Interface Science 559 (2020) 143–151
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Co nanoparticles supported on three-dimensionally N-doped holey graphene aerogels for electrocatalytic oxygen reduction Kai-Wen Cao a, Hao Huang b, Fu-Min Li b, Hong-Chang Yao c, Juan Bai a,⇑, Pei Chen a, Pu-Jun Jin a, Zi-Wei Deng a, Jing-Hui Zeng a, Yu Chen a,⇑ a Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, People’s Republic of China b School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, People’s Republic of China c College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, People’s Republic of China
g r a p h i c a l a b s t r a c t Co nanoparticles supported on three-dimensionally (3D) N-doped holey graphene aerogels (Co/N-GA-h) hybrids were successfully synthesized by simple high-temperature pyrolysis method. As a non-noble metal electrocatalyst, the Co/N-GA-h hybrids displayed the excellent activity and durability for the ORR in alkaline media.
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Article history: Received 29 August 2019 Revised 8 October 2019 Accepted 8 October 2019 Available online 9 October 2019 Keywords: Co nanoparticles N-doped Holey graphene aerogels
a b s t r a c t The reactive and stable catalysts for the oxygen reduction reaction are highly desirable for low temperature fuel cells. The commercial oxygen reduction reaction electrocatalysts generally reply on noble metal based nanomaterials, which suffer from inherent cost and selectivity issues. At present, it still remains challenge for designing efficient non-noble metal-based oxygen reduction reaction electrocatalysts. Herein, we successfully synthesize Co nanoparticles supported on three-dimensionally N-doped holey graphene aerogels hybrids by the high-temperature calcination of the graphene aerogelspolyallylamine-CoII hybrids. The component optimized hybrids show the excellent electrocatalytic activity for oxygen reduction reaction in alkaline media, which is comparable to commercial Pt/C electrocatalyst. Meanwhile, the hybrids also show eminent tolerance for CO and methanol, attributing to their
⇑ Corresponding authors. E-mail addresses: [email protected] (J. Bai), [email protected] (Y. Chen). https://doi.org/10.1016/j.jcis.2019.10.025 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
144 Electrocatalysts Oxygen reduction reaction
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excellent oxygen reduction reaction selectivity. The three-dimensionally interconnected structure of graphene aerogels, N-doping, uniform dispersion and high crystallinity of Co nanoparticles, and holey structure of graphene contribute to the striking oxygen reduction reaction activity of hybrids. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
2. Experimental
The environmental pollution and rising global energy demand have seriously affected the human health and energy security. One promising solution is the fuel cell technology, which can convert chemical energy into electrical energy and provide sustainable power [1–6]. However, the oxygen reduction reaction (ORR) at cathode in fuel cell primarily limits the energy conversion efficiency. Traditionally, Pt-based nanomaterials are considered as the best electrocatalyst for ORR [7–12]. Unfortunately, scarce resource, low stability, CO deactivation, poor methanol tolerance and high cost are also the primary barrier to their industrial applications [13,14]. Thus, the further efforts are necessary to develop alternative electrocatalysts that are low-cost, stable and selective for ORR [15–17]. The study for efficient alternatives to displace Pt-based ORR electrocatalysts is growing intensively [18–20]. Tremendous efforts have been made to develop low cost ORR electrocatalysts, including transition metal-based materials (Fe, Co, CoO, Co3O4, and Fe3O4, etc.) [21–26], heteroatom doped carbon materials (graphene, fullerene and carbon nanotubes, etc.) [27–30] and heteroatom-coordinated transition metals on carbon materials [31,32]. Among these electrocatalysts, nitrogen coordinated Fe or Co supported on carbon materials have been regarded as one of the most promising non-noble metal based ORR electrocatalysts [33,34]. But they also suffer from sintering, dissolution, and agglomeration during ORR process, resulting in the deactivation of electrocatalyst [35]. To overcome these difficulties, carbon supports like porous carbon, graphene and active carbon have been developed to increase the electroactive surface area of electrocatalysts and improve their electrocatalytic activity and stability [36– 38]. Graphene, as a star material in carbon family, has become a new-generation support due to its unique two-dimensional single-layer sheet structure, high surface area, good chemical stability and excellent conductivity [39–42]. Although tremendous efforts have been made in graphene-based ORR electrocatalysts, there is no literature of Co nanoparticles supported on threedimensionally (3D) porous interconnected graphene based nanostructures for ORR. Such materials are attractive systems, as they allow the maximum utilization of the unique structure of twodimensional (2D) sheets and accelerate the mass transfer during the electrocatalytic reactions due to 3D porous interconnected structure. Furthermore, holes on graphene also accelerate the mass transfer. Thus, a controlled interplay of multiple types of active sites together result in the improvement of the catalytic activity for the sluggish ORR [43–45]. In this work, we synthesized Co nanoparticles supported on 3D N-doped graphene aerogels with holes (Co/N-GA-h) hybrids by simple high-temperature pyrolysis method. The graphene-based hybrids show an interconnected micropores structure and Co nanoparticles uniformly deposit on the mesopores framework of holey graphene sheets. When used as ORR electrocatalyst in alkaline medium, Co/N-GA-h hybrids display a more positive onset potential and half-wave potential than N-doped graphene aerogels (N-GA). Meanwhile, Co/N-GA-h hybrids exhibit low H2O2 yield, excellent anti-methanol performance, weak CO-poisoning effect, good durability and high electron transfer number for ORR.
2.1. Materials Polyallylamine hydrochloride (PAA, molecular weight: 15 0000) was obtained from Japan Nitto Boseki Co., Ltd. Graphene oxide (GO) was purchased from JCNANO Technology Co., Ltd (Nanjing). Cobalt (II) chloride (CoCl2) and ascorbic acid (AA) were acquired from Aladdin Industrial Co. Commercial Pt/C electrocatalyst was purchased from JM Corporation. 2.2. Synthesis of graphene aerogels (GA) Typically, GO (10 mg) and AA (40 mg) was added into 5 mL of H2O by ultrasonic treatment. Then the mixture was given the temperature of 95 °C, GA could be obtained after 2 h. The excess AA was removed by multiple dialysis at 60 °C. 2.3. Synthesis of graphene aerogels-polyallylamine hybrids (GA-PAA) Typically, GA-PAA hybrids were obtained by immersing GA in PAA aqueous solution (5 mL, 0.5 M). Then, a bottle containing above mixture heated 2 h at 95 °C. Finally, GA-PAA hybrids were obtained after the multiple dialysis at 60 °C. 2.4. Synthesis of graphene aerogels-polyallylamine-CoII hybrids (GAPAA-CoII) Typically, GA-PAA hybrids were immersed in 5 mL of 1 mg mL 1 CoCl2 aqueous solution. Then, a bottle containing above mixture heated 2 h at 95 °C to speed up adsorption of Co2+. After the multiple dialysis at 60 °C, the GA-PAA-CoII-1 hybrids were acquired by freeze-drying. For comparison, after immersing GAPAA hybrids into 5 mL of 0.4 mg mL 1 CoCl2 aqueous solution and 5 mL of 16 mg mL 1 CoCl2 aqueous solution, GA-PAA-CoII-0.4 and GA-PAA-CoII-16 hybrids were also fabricated under the same conditions, respectively. 2.5. Synthesis of Co nanoparticles supported on 3D N-doped holey graphene aerogels hybrids (Co/N-GA-h) Co/N-GA-h-1 hybrids were obtained by high-temperature calcination of GA-PAA-CoII-1 hybrids at 900 °C for 2 h under a flow of Ar with a heating rate of 10 °C min 1. Co/N-GA-h-0.4 and Co/N-GA-h16 hybrids obtained by using GA-PAA-CoII-0.4 and GA-PAA-CoII-16 hybrids as reaction precursor under same experimental conditions. For comparison, N-GA hybrids without Co nanoparticles were obtained by high-temperature calcination of GA-PAA hybrids at the same condition. 2.6. Electrochemical measurements Electrochemical experiments were measured on CHI 760E workstations with glassy carbon rotating disk electrode (RDE) and glassy carbon rotating ring disk electrode (RRDE) at 30 ± 1 °C. A standard three-electrode system was used in this work. A carbon rod acted as auxiliary electrode, a saturated calomel electrode was used as reference electrode and RDE or RRDE modi-
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fied with the electrocatalyst served as working electrode. The electrocatalyst suspension was obtained by mixing 4 mg of production and 2 mL Nafion/ethanol/water. After strong sonication for 20 min, 10 lL suspension was carefully dropped on the RDE and dried at room temperature. The linear sweep voltammetry (LSV) curves were evaluated in O2-saturated 0.1 M KOH electrolyte. All potentials in this work were referenced to the reversible hydrogen electrode (RHE), using the equation of ERHE = ESCE + 0.241 V + 0.0591pH. The number of electron transfer (n) was evaluated using the Koutecky–Levich equation: 1/j = 1/jk + 1/Bx0.5, B = 0.2nF(DO2)C2/3m 1/6CO2 (where j was apparent current density, jk was the kineticlimiting current density, x was angular velocity, F was the Faraday constant, DO2 was the O2 diffusion coefficient in 0.1 M KOH solution, m was the kinetic viscosity of 0.1 M KOH solution, and CO2 was the concentration of dissolved O2 in 0.1 M KOH solution). The values of n and hydrogen peroxide yield (yH2O2) with RRDE measurements were calculated by equations: n = 4NId/(NId + Ir), yH2O2 = 200Ir/NId + Ir (where N-the collection efficiency, Id-the disk current and Ir-the ring current). 2.7. Instruments The morphology and microstructure were analysed by scanning electron microscopy (SEM, SU-8020), standard/high resolution transmission electron microscopy (TEM/HRTEM, TECNAI G2 F20) and energy-dispersive X-ray (EDS, TECNAI G2 F20) maps. The crystallographic texture was performed by X-Ray diffraction (XRD, DX2700) and selected area electron diffraction (SAED, TECNAI G2 F20). The electronic property was investigated by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA). The composition of sample was analyzed by EDS (Quanta 200). Thermogravimetric analysis (TGA) was completed by thermoanalysis analyzer (Q600 systems) under air atmosphere. The samples surface charge was taken on by Nano ZS90. The interaction between PAA and CoCl2 was measured by ultraviolet and visible spectroscopy (UV vis, Shimadzu UV3600U). 3. Results and discussion 3.1. Characterization of Co/N-GA-h-1 hybrids GA and GA-PAA were synthesized according to our previous work with a slight modification [36]. Typically, GO was reduced
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by AA and the reduced graphene oxide (rGO) self-assembled into GA with 3D porous structure (Fig. S1). After dialysis and freezedrying, GA interact with PAA and Co2+ in step-by-step process to generate GA-PAA and GA-PAA-CoII-1 hybrids, respectively. Furthermore, Co/N-GA-h-1 hybrids are obtained by pyrolysis of GAPAA-CoII hybrids at 900 °C (Scheme 1). The internal structure of Co/N-GA-h-1 hybrids was characterised by SEM. The SEM image reveals that Co/N-GA-h-1 hybrids have the distinct interconnected structure with 3D porous (Fig. 1A), which will be beneficial to electronic conduction and can accelerate the mass transfer during the electrocatalytic reactions. In order to prove that GO has been reduced to rGO, XRD and XPS were performed. XRD spectrum show the characteristic diffraction peak of Co/N-GA-h-1 hybrids is located at 24° of 2h degree, which is different from that (2h = 12°) of GO (Fig. 1B). And XPS of C1s also show the CAO/C@O bonds decreased sharply in the C1s region, revealing the GO has been transformed into rGO successfully (Fig. 1C) [36]. GA-PAA hybrids were easily achieved by immersing GA in PAA aqueous solution. The zeta-potential of GA is measured to be 24.8 ± 2 mV, and the zeta-potential of GA-PAA hybrids changes to +35.7 ± 2 mV in aqueous solution after soaking, indicating the GA can attract the PAA through the strong electrostatic interaction. Our previous works have proved that PAA can interact with metal ion (Mx) to generate the PAA-Mx complex due to the specific coordination ability of the ANH2 groups. UV–vis spectrum shows the absorption spectrum of PAA + CoCl2 solution is different from the absorption spectrum of CoCl2 and the absorption spectrum of PAA (Fig. 1D), revealing PAA can interact with CoCl2. Another controlled experiment has been also proved the effect of PAA. The optical digital images show the color of PAA + CoCl2 solution is light brown and the color of CoCl2 solution is light pink. After adding the NaOH solution, hydrolysis process occurs quickly, resulting the precipitate in the CoCl2 solution. However, there is no precipitate in the PAA + CoCl2 solution, also confirming the PAA can interact with CoCl2 again (Fig. S2). After heating GA-PAA-CoII-1 hybrids at 900 °C for 2 h, Co/N-GAh-1 hybrids were obtained. TEM image clearly shows the nanoparticles are well distributed on the graphene surface (Fig. 2A). But there is no characteristic diffraction peaks of metallic Co or cobalt oxide in the XRD spectrum (Fig. 1B), likely originating from the low content of Co. So, we synthesize the calcined product in the absence of graphene at the same condition. The XRD of calcined
Scheme 1. The overall synthetic route of Co/N-GA-h-1 hybrids.
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Fig. 1. (A) SEM image of Co/N-GA-h-1 hybrids, (B) XRD patterns of GO and Co/N-GA-h-1 hybrids, (C) C1s XPS of Co/N-GA-h-1 hybrids, (D) UV–vis absorption spectra of CoCl2, PAA and PAA + CoCl2 solution.
product reveal the nanoparticles are metallic Co (Fig. S3). The morphology of Co/N-GA-h-1 hybrids is different from GA-PAA and GAPAA-CoII-1 hybrids (Fig. S4), indicating the addition Co2+ have the influence on the morphology of Co/N-GA-h-1 hybrids. Surprisingly, a significant number of holes appear on the graphene surface (Fig. 2A). Most holes relate with at least one Co nanoparticle. Clearly, the Co nanoparticles catalyzed the oxidation the graphitic carbon by oxidation etching method, resulting in many pores on graphene. And the shape of some holes liked tracks, which are apparently related to the directional motion of the deposited Co nanoparticles, owing to etching-induced motions. The above phenomenon indicates Co nanoparticles catalyzed the oxidation of carbon atoms that associate with them, while the other carbons remain original shape. This holes on graphene can facilitate the diffusion of reactant ions to the active sites [44,45]. HRTEM image clearly shows the lattice distance of nanoparticles on graphene surface is 0.205 nm (Fig. 2B), indicating that these nanoparticles are elemental Co0 nanoparticles rather than cobalt oxide. SAED pattern of Co nanoparticles reveal its polycrystalline structure (Insert in Fig. 2B). EDS elemental maps prove the existence of C, N and Co elements. Co element evenly distribute in the nanoparticles and the patterns of C, N are completely superimposed on each other, this phenomenon further suggest the N element is successfully embedded in the C skeleton (Fig. 2C). Further XPS characterization method were measured to investigate the detailed chemical composition of Co/N-GA-h-1 hybrids. The N and Co content can be detected in XPS measurement, suggesting the successful reduction of Co2+ and N doping (Fig. 2D). The main peaks at 780.1 and 796.0 eV corresponds to 2p3/2 and 2p1/2 of Co, indicating the metallic Co is the predominant species in Co/N-GA-h-1 hybrids. The XPS spectrum of Co 2p (Fig. 2D)
shows two fitted peaks for Co 2p3/2 that can be assigned to Ncoordinated (778.7 eV) and O-coordinated Co (781.6 eV), revealing the inevitable oxidation of partial surface atoms on Co nanoparticles. The previous work have demonstrated that Co centres stabilized by N coordination can act as the active sites in electrocatalysts during the ORR process [31]. Meanwhile, the N1s XPS spectrum match with four chemical states, pyridinic N (398.0 eV), pyrrolic N (399.7 eV), graphitic N (401.0 eV), respectively. We can observe an additional peak at 399.0 eV (Fig. 2E), which come from the Co-N. According to previous work, pyridinic N, graphitic N and Co-N are primary N species, which will offer the active site for ORR [31]. The structural features and surface area of Co/N-GA-h-1 hybrids were further characterized by N2 adsorption–desorption isotherm. BET specific surface area of Co/N-GA-h-1 hybrids is measured to be 361.4 m2 g 1 (Fig. S5), which originates from the particular 2D structure of the nanosheets. The N2 gas adsorption isotherm and the pore size distribution show there are many micropores and mesopores, which is consistent with the 3D porous and holey structure of Co/N-GA-h-1 hybrids in TEM image. The high specific surface area and porous structure will make for the fast mass transfer during the ORR. 3.2. ORR performance of Co/N-GA-h-1 hybrids As a significant half reaction in fuel cells and Zn-air batteries, ORR performance affects the working life and efficiency of these devices. The ORR performance of Co/N-GA-h-1 hybrids was studied by LSV tests (Fig. 3A). The ORR performance of the N-GA hybrids and commercial Pt/C electrocatalyst (20 wt%) electrocatalyst were also measured under the same conditions. As observed, Co/N-GA-
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Fig. 2. (A) TEM image of Co/N-GA-h-1 hybrids, (B) HRTEM image and SAED pattern of Co nanoparticles, (C) EDX maps of Co/N-GA-h-1 hybrids, (D) Co 2p XPS of Co/N-GA-h-1 hybrids (E) N 1s XPS of Co/N-GA-h-1 hybrids.
h-1 hybrids exhibit the best ORR performance. Specifically, the onset and half-wave potential (Eonset = 0.92 V, E1/2 = 0.81 V) of Co/N-GA-h-1 hybrids for ORR are much higher than that (Eonset = 0.86 V, E1/2 = 0.77 V) of N-GA hybrids. This result indicate that the introduction of Co can effectively heighten their electrocatalytic activity for ORR. Although the ORR performance of Co/N-GA-h-1 hybrids is slightly poorer than that of commercial Pt/C electrocatalyst, it is comparable to other carbon based ORR electrocatalysts, such as N-rich graphene (E1/2 = 0.70 V) [46], N/P-codoped carbon (E1/2 = 0.78 V) [47], N-doped porous carbon nanosheets (E1/2 = 0.76 V) [48], plasma-treated graphene (E1/2 = 0.74 V) [49], N-doped porous carbon (E1/2 = 0.75 V) [50]. Even more than some metal-based electrocatalysts, like Au@Pd/ reduced graphene oxide (E1/2 = 0.759 V) [51], Co/N-C-800 (E1/2 = 0.74 V) [52], Fe/N-gCB (E1/2 = 0.80 V) [53] and Co-NC-900 (E1/2 = 0.80 V) [54]. The reaction kinetics of ORR at Co/N-GA-h-1 hybrids was measured by LSV at the different rotation rates using the Koutecky– Levich equation. In general, the limiting diffusion current density increases with the rotation rate (Fig. 3B). The n is 3.96 at the
potential 0.4 V, revealing there is a four-electron process for ORR. The RRDE measurement show a negligible ring current and the percentage of H2O2 is calculated to be <20% in the range of 0– 0.8 V. Meanwhile, the n value is ca. 3.6, also indicating a fourelectron reaction pathway (Fig. 3C and D). To investigate the effect of Co content on ORR activity, Co/NGA-h hybrids with different Co content (i.e., Co/N-GA-h-0.4 and Co/N-GA-h-16) were also synthesized (see experimental section for detail). SEM images show both Co-N-GA-h-0.4 and Co/N-GAh-16 hybrids have the 3D porous interconnected structures (Fig. S6). TGA curves show that the Co content in Co-N-GA-h-0.4 is 5.1 wt%, which is smaller than that (11.2 wt%) of Co/N-GA-h-1 hybrids (Fig. S7). And the Co content in Co/N-GA-h-16 hybrids (12.5 wt%) is almost the same as Co/N-GA-h-1 hybrids, indicating the maximum adsorption of CoCl2 is reached in Co/N-GA-h-1 hybrids. Then, the ORR activity of Co-N-GA-h-0.4 and Co/N-GA-h16 hybrids were evaluated by the LSV. Compared with Co/N-GAh-0.4 hybrids, Co/N-GA-h-1 hybrids exhibit the better ORR Eonset, E1/2 and limiting diffusion current. And LSV of Co/N-GA-h-16 hybrids and Co/N-GA-h-1 hybrids are almost overlapping, indicat-
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Fig. 3. (A) ORR polarization curves of commercial Pt/C electrocatalyst, Co/N-GA-h-1 hybrids and N-GA hybrids. (B) ORR polarization curves of Co/N-GA-h-1 hybrids at different rotation rates. (C) RRDE measurements for Co/N-GA-h-1 hybrids at a rotation rate of 1600 rpm. (D) Electron transfer number and H2O2 yield derived from the RRDE tests.
ing similar ORR activity (Fig. 4A). The above experimental result show that ORR activity of Co/N-GA-h-1 hybrids enhances with Co content increases and then remain constant. The direct methanol fuel cells in alkaline media draw more attention because of its unique advantages, including less CO poisoning, enhanced methanol oxidation reaction (MOR) and ORR kinetics [55,56]. But methanol can permeate into the cathode side across the Nafion membrane, leading to an undesirable competitive reaction between ORR and MOR at the cathode [57–59]. The ORR polarization curves of commercial Pt/C electrocatalyst show extreme descent and rapid poisoning when adding CO (Fig. 4B). In contrast, Co/N-GA-h-1 hybrids exhibit the excellent antimethanol performance and weak CO-poisoning effect. ORR polarization curves of Co/N-GA-h-1 hybrids in the existence of methanol is very similar to that the solution without methanol, only shifting 10 mV in E1/2 (Fig. 4C). In comparison, there is an obvious MOR peak at 0.78 V in the presence of methanol for ORR polarization curves of commercial Pt/C electrocatalyst (Fig. 4D). Thus, ORR measurements reveal Co/N-GA-h-1 hybrids have the unordinary ORR selectivity, which may originate from the low Intrinsic activity of transition metal and carbon materials for MOR. Durability is another key parameter to evaluate the ORR performance of the electrocatalysts. The accelerated durability method [50] was measured to evaluate the electrocatalytic durability of Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst (Fig. 5). After 2000 cycles of CV, Co/N-GA-h-1 hybrids reveal 10 mV degradation in E1/2 value for ORR (Fig. 5A) whereas commercial Pt/C electrocatalyst show 60 mV shifts in E1/2 value for ORR (Fig. 5B). This fact indicates Co/N-GA-h-1 hybrids have better stability than
commercial Pt/C electrocatalyst for ORR. Most cogently, the durability of the non-noble metal electrocatalyst is highly related to the amount of H2O2 releasing during the reaction [60]. The very low H2O2 yield of Co/N-GA-h-1 hybrids will be beneficial to its ORR stability. And the durability of remarkable enhancement on Co/NGA-h-1 hybrids can be attributed to its 3D interconnected graphene framework structure. To verify the structure stability, TEM of Co/N-GA-h-1 hybrids was measured after the ORR reaction. The TEM image shows Co nanoparticles and holes in graphene are maintained well, which results in their good durability (Fig. S8). This unique structure can suppress the structural collapse, enhance the interfacial contact and facilitate the transport of electron and reactant [31]. Thus, Co/N-GA-h-1 hybrids are more stable than commercial Pt/C electrocatalyst. 4. Conclusions In summary, we have successfully prepared Co nanoparticles supported on 3D N-doped graphene aerogels hybrids with holes by high-temperature pyrolysis method. It is remarkable that this work provides a facile and low-cost approach to design high performance carbon materials supported transition metal nonprecious metal hybrids. The prepared Co/N-GA-h-1 hybrids exhibit excellent electrocatalytic performance for ORR in alkaline media due to its porous interconnected structure, high surface area, nitrogen doping, holey graphene and numerous Co-N active sites. In the presence of trace methanol and CO, the anti-methanol and antiCO-poisoning performance of Co/N-GA-h-1 hybrids are much better than that of commercial Pt/C electrocatalyst. Facile preparation
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Fig. 4. (A) ORR polarization curves of the of Co/N-GA-h-0.4 hybrids, Co/N-GA-h-1 hybrids and Co/N-GA-h-16 hybrids, (B) CO tolerance performance of Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst for ORR analysed by chronoamperometry. (C) ORR polarization curves of Co/N-GA-h-1 hybrids and (D) commercial Pt/C electrocatalyst before and after adding methanol.
Fig. 5. (A) ORR polarization curves of Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst before and after 2000 cycles accelerated durability test. (B) The change values of the Eonset and E1/2 for Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst for ORR after the accelerated durability tests.
method, low cost raw materials and outstanding ORR performance make Co/N-GA-h-1 hybrids a promising non-noble metal ORR electrocatalyst in direct methanol fuel cells.
Acknowledgements This research was sponsored by the National Natural Science Foundation of China (51873100), National Key R & D Program of China (2017YFC0107401), Fundamental Research Funds for the Central Universities (GK201901002) and the 111 Project (B14041).
Declaration of Competing Interest Appendix A. Supplementary material The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.025.
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References [1] K. Eid, Y.H. Ahmad, H. Yu, Y. Li, X. Li, S.Y. AlQaradawi, H. Wang, L. Wang, Rational one-step synthesis of porous PtPdRu nanodendrites for ethanol oxidation reaction with a superior tolerance for CO-poisoning, Nanoscale 9 (2017) 18881–18889. [2] L. Zhang, L.X. Ding, H. Chen, D. Li, S. Wang, H. Wang, Self-supported PtAuP alloy nanotube arrays with enhanced activity and stability for methanol electrooxidation, Small 13 (2017) 1604000. [3] Q. Li, T. Wang, D. Havas, H. Zhang, P. Xu, J. Han, J. Cho, G. Wu, Highperformance direct methanol fuel cells with precious-metal-free cathode, Adv. Sci. 3 (2016) 1600140. [4] L. Tan, N. Li, S. Chen, Z.Q. Liu, Self-assembly synthesis of CuSe@graphene– carbon nanotubes as efficient and robust oxygen reduction electrocatalysts for microbial fuel cells, J. Mater. Chem. A 4 (2016) 12273–12280. [5] X.T. Wu, J.C. Li, Q.R. Pan, N. Li, Z.Q. Liu, Gallic acid-assisted synthesis of Pd uniformly anchored on porous N-rGO as efficient electrocatalyst for microbial fuel cells, Dalton Trans. 47 (2018) 1442–1450. [6] X. Yan, S. Yu, Y. Tang, D. Sun, L. Xu, C. Xue, Triangular AgAu@Pt core–shell nanoframes with a dendritic Pt shell and enhanced electrocatalytic performance toward the methanol oxidation reaction, Nanoscale 10 (2018) 2231–2235. [7] H. Wang, S. Yin, Y. Xu, X. Li, A.A. Alshehri, Y. Yamauchi, H. Xue, Y.V. Kaneti, L. Wang, Direct fabrication of tri-metallic PtPdCu tripods with branched exteriors for the oxygen reduction reaction, J. Mater. Chem. A 6 (2018) 8662–8668. [8] L. Zhang, M. Wei, S. Wang, Z. Li, L.X. Ding, H. Wang, Highly stable PtP alloy nanotube arrays as a catalyst for the oxygen reduction reaction in acidic medium, Chem. Sci. 6 (2015) 3211–3216. [9] S. Xue, W. Deng, F. Yang, J. Yang, I.S. Amiinu, D. He, H. Tang, S. Mu, Hexapod PtRuCu nanocrystalline alloy for highly efficient and stable methanol oxidation, ACS Catal. 8 (2018) 7578–7584. [10] M. Bao, I.S. Amiinu, T. Peng, W. Li, S. Liu, Z. Wang, Z. Pu, D. He, Y. Xiong, S. Mu, Surface evolution of PtCu alloy Shell over Pd nanocrystals Leads to Superior hydrogen evolution and oxygen reduction reactions, ACS Energy Lett. 3 (2018) 940–945. [11] X.L. Tian, Y.Y. Xu, W. Zhang, T. Wu, B.Y. Xia, X. Wang, Unsupported platinumbased electrocatalysts for oxygen reduction reaction, ACS Energy Lett. 2 (2017) 2035–2043. [12] Y. Chen, G. Fu, Y. Li, Q. Gu, L. Xu, D. Sun, Y. Tang, L Glutamic acid derived PtPd@Pt core/satellite nanoassemblies as an effectively cathodic electrocatalyst, J. Mater. Chem. A 5 (2017) 3774–3779. [13] Z. Kou, B. Guo, D. He, J. Zhang, S. Mu, Transforming Two-dimensional boron carbide into boron and chlorine dual-doped carbon nanotubes by chlorination for efficient oxygen reduction, ACS Energy Lett. 3 (2018) 184–190. [14] Y. Guo, P. Yuan, J. Zhang, Y. Hu, I.S. Amiinu, X. Wang, J. Zhou, H. Xia, Z. Song, Q. Xu, S. Mu, carbon nanosheets containing discrete Co-Nx-By-C active sites for efficient oxygen electrocatalysis and rechargeable Zn-Air batteries, ACS Nano 12 (2018) 1894–1901. [15] Z.Q. Liu, H. Cheng, N. Li, T.Y. Ma, Y.Z. Su, ZnCo2O4 quantum dots anchored on nitrogen-doped carbon nanotubes as reversible oxygen oeduction/evolution electrocatalysts, Adv. Mater. 28 (2016) 3777–3784. [16] T.Y. Ma, J. Ran, S. Dai, M. Jaroniec, S.Z. Qiao, Phosphorus-doped graphitic carbon nitrides grown in situ on carbon-fiber paper: flexible and reversible oxygen electrodes, Angew. Chem. Int. Edit. 54 (2015) 4646–4650. [17] J. Zhu, W. Li, S. Li, J. Zhang, H. Zhou, C. Zhang, J. Zhang, S. Mu, defective N/Scodoped 3D cheese-like porous carbon nanomaterial toward efficient oxygen reduction and Zn-Air batteries, Small 14 (2018) 1800563. [18] J. Zhang, S. Wu, X. Chen, M. Pan, S. Mu, Egg derived nitrogen-self-doped carbon/carbon nanotube hybrids as noble-metal-free catalysts for oxygen reduction, J. Power Sources 271 (2014) 522–529. [19] B. Tang, S. Wang, R. Li, X. Gou, J. Long, J. Long, Urea treated metal organic frameworks-graphene oxide composites derived N-doped Co-based materials as efficient catalyst for enhanced oxygen reduction, J. Power Sources 425 (2019) 76–86. [20] A. Sumboja, J. Chen, Y. Ma, Y. Xu, Y. Zong, P.S. Lee, Z. Liu, Sulfur-rich colloidal nickel sulfides as bifunctional catalyst for all-solid-state, flexible and rechargeable Zn-Air batteries, ChemCatChem 11 (2019) 1205–1213. [21] Z. Wang, H. Jin, T. Meng, K. Liao, W. Meng, J. Yang, D. He, Y. Xiong, S. Mu, Cucoordinated ZIF-derived carbon framework for efficient oxygen reduction reaction and zinc-air batteries, Adv. Funct. Mater. 28 (2018) 1802596. [22] Y. Qiao, P. Yuan, Y. Hu, J. Zhang, S. Mu, J. Zhou, H. Li, H. Xia, J. He, Q. Xu, Sulfuration of an Fe-N-C catalyst containing Fex C/Fe species to enhance the catalysis of oxygen reduction in acidic media and for use in flexible Zn-Air batteries, Adv. Mater. (2018) e1804504. [23] S. Liu, I.S. Amiinu, X. Liu, J. Zhang, M. Bao, T. Meng, S. Mu, Carbon nanotubes intercalated Co/N-doped porous carbon nanosheets as efficient electrocatalyst for oxygen reduction reaction and zinc-air batteries, Chem. Eng. J. 342 (2018) 163–170. [24] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (2011) 780. [25] Z.S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Müllen, 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction, J. Am. Chem. Soc. 134 (2012) 9082–9085.
[26] S. Guo, S. Zhang, L. Wu, S. Sun, Co/CoO nanoparticles assembled on graphene for electrochemical reduction of oxygen, Angew. Chem. Int. Edit. 51 (2012) 11770–11773. [27] R. Ma, Y. Zhou, C. Hu, M. Yang, F. Wang, K. Yan, Q. Liu, J. Wang, Post irondoping of activated nitrogen-doped carbon spheres as a high-activity oxygen reduction electrocatalyst, Energy Storage Mater. 13 (2018) 142–150. [28] D. Gu, Y. Zhou, R. Ma, F. Wang, Q. Liu, J. Wang, Facile synthesis of N-doped graphene-like carbon nanoflakes as efficient and stable electrocatalysts for the oxygen reduction reaction, Nano-Micro Lett. 10 (2018) 29. [29] R. Ma, X. Ren, B.Y. Xia, Y. Zhou, C. Sun, Q. Liu, J. Liu, J. Wang, Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction, Nano Res. 9 (2016) 808–819. [30] J. Zhang, Y. Sun, J. Zhu, Z. Kou, P. Hu, L. Liu, S. Li, S. Mu, Y. Huang, defect and pyridinic nitrogen engineering of carbon-based metal-free nanomaterial toward oxygen reduction, Nano Energy 52 (2018) 307–314. [31] Y. Jiang, Y. Lu, X. Wang, Y. Bao, W. Chen, L. Niu, A cobalt–nitrogen complex on N-doped three-dimensional graphene framework as a highly efficient electrocatalyst for oxygen reduction reaction, Nanoscale 6 (2014) 15066– 15072. [32] Y. Hu, J.O. Jensen, W. Zhang, L.N. Cleemann, W. Xing, N.J. Bjerrum, Q. Li, Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts, Angew. Chem. Int. Edit. 53 (2014) 3675–3679. [33] C.Y. Su, H. Cheng, W. Li, Z.Q. Liu, N. Li, Z. Hou, F.Q. Bai, H.X. Zhang, T.Y. Ma, Atomic modulation of FeCo-nitrogen-carbon bifunctional oxygen electrodes for rechargeable and flexible all-solid-state zinc-air battery, Adv. Energy Mater. 7 (2017) 1602420. [34] Y. Han, Y. Wang, R. Xu, W. Chen, L. Zheng, A. Han, Y. Zhu, J. Zhang, H. Zhang, J. Luo, C. Chen, Q. Peng, D. Wang, Y. Li, Electronic structure engineering to boost oxygen reduction activity by controlling the coordination of the central metal, Energy Environ. Sci. 11 (2018) 2348–2352. [35] C. Huang, Y. Li, N. Wang, Y. Xue, Z. Zuo, H. Liu, Y. Li, Progress in research into 2D graphdiyne-based materials, Chem. Rev. 118 (2018) 7744–7803. [36] Q. Xue, Y. Ding, Y. Xue, F. Li, P. Chen, Y. Chen, 3D nitrogen-doped graphene aerogels as efficient electrocatalyst for the oxygen reduction reaction, Carbon 139 (2018) 137–144. [37] J. Ding, S. Ji, H. Wang, V. Linkov, R. Wang, Mesoporous cobalt selenide/ nitrogen-doped carbon hybrid as bifunctional electrocatalyst for hydrogen evolution and oxygen reduction reactions, J. Power Sources 423 (2019) 1–8. [38] Y. Yu, J. Ma, C. Chen, Y. Fu, Y. Wang, K. Li, Y. Liao, L. Zheng, X. Zuo, General method for synthesis transition-metal phosphide/nitrogen and phosphide doped carbon materials with yolk-shell structure for oxygen reduction reaction, ChemCatChem 11 (2019) 1722–1731. [39] R. Lv, E. Cruz-Silva, M. Terrones, Building complex hybrid carbon architectures by covalent interconnections: graphene-nanotube hybrids and more, ACS Nano 8 (2014) 4061–4069. [40] R. Lv, T. Cui, M.S. Jun, Q. Zhang, A. Cao, D.S. Su, Z. Zhang, S.H. Yoon, J. Miyawaki, I. Mochida, F. Kang, Open-ended N-doped carbon nanotube-graphene hybrid nanostructures as high-performance catalyst support, Adv. Funct. Mater. 21 (2011) 999–1006. [41] X. Liu, I.S. Amiinu, S. Liu, Z. Pu, W. Li, B. Ye, D. Tan, S. Mu, H2O2-assisted synthesis of porous N-doped graphene/molybdenum nitride composites with boosted oxygen reduction reaction, Adv. Mater. Interfaces 4 (2017) 1601227. [42] X. Cai, B.Y. Xia, J. Franklin, B. Li, X. Wang, Z. Wang, L. Chen, J. Lin, L. Lai, Z. Shen, Free-standing vertically-aligned nitrogen-doped carbon nanotube arrays/graphene as air-breathing electrodes for rechargeable zinc-air batteries, J. Mater. Chem. A 5 (2017) 2488–2495. [43] Z. Fan, Q. Zhao, T. Li, J. Yan, Y. Ren, J. Feng, T. Wei, Easy synthesis of porous graphene nanosheets and their use in supercapacitors, Carbon 50 (2012) 1699–1703. [44] H. Cao, X. Zhou, C. Zheng, Z. Liu, Metal etching method for preparing porous graphene as high performance anode material for lithium-ion batteries, Carbon 89 (2015) 41–46. [45] T. Palaniselvam, H.B. Aiyappa, S. Kurungot, An efficient oxygen reduction electrocatalyst from graphene by simultaneously generating pores and nitrogen doped active sites, J. Mater. Chem. 22 (2012) 23799–23805. [46] S.N. Faisal, E. Haque, N. Noorbehesht, W. Zhang, A.T. Harris, T.L. Church, A.I. Minett, Pyridinic and graphitic nitrogen-rich graphene for high-performance supercapacitors and metal-free bifunctional electrocatalysts for ORR and OER, RSC Adv. 7 (2017) 17950–17958. [47] H. Jiang, Y. Wang, J. Hao, Y. Liu, W. Li, J. Li, N and P co-functionalized threedimensional porous carbon networks as efficient metal-free electrocatalysts for oxygen reduction reaction, Carbon 122 (2017) 64–73. [48] H. Yu, L. Shang, T. Bian, R. Shi, G.I. Waterhouse, Y. Zhao, C. Zhou, L.Z. Wu, Nitrogen-doped porous carbon nanosheets templated from g-C3N4 as metalfree electrocatalysts for efficient oxygen reduction reaction, C.H. Tung, T. Zhang Adv. Mater. 28 (2016) 5080–5086. [49] L. Tao, Q. Wang, S. Dou, Z. Ma, J. Huo, S. Wang, L. Dai, Edge-rich and dopantfree graphene as a highly efficient metal-free electrocatalyst for the oxygen reduction reaction, Chem. Commun. 52 (2016) 2764–2767. [50] B. Men, Y. Sun, M. Li, C. Hu, M. Zhang, L. Wang, Y. Tang, Y. Chen, P. Wan, J. Pan, Hierarchical metal-free nitrogen-doped porous graphene/carbon composites as an efficient oxygen reduction reaction catalyst, ACS Appl. Mater. Interfaces 8 (2016) 1415–1423. [51] S.S. Li, A.J. Wang, Y.Y. Hu, K.M. Fang, J.R. Chen, J.J. Feng, Seedless wet-chemical synthesis of gold@ palladium nanoflowers supported on reduced graphene
K.-W. Cao et al. / Journal of Colloid and Interface Science 559 (2020) 143–151
[52]
[53] [54]
[55]
[56]
oxide with enhanced electrocatalytic properties, J. Mater. Chem. A 2 (2014) 18177–18183. Y. Su, Y. Zhu, H. Jiang, J. Shen, X. Yang, W. Zou, J. Chen, C. Li, Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions, Nanoscale 6 (2014) 15080–15089. R. Zhou, S.Z. Qiao, An Fe/N co-doped graphitic carbon bulb for high-performance oxygen reduction reaction, Chem. Commun. 51 (2015) 7516–7519. C. Guo, Y. Wu, Z. Li, W. Liao, L. Sun, C. Wang, B. Wen, Y. Li, C. Chen, The oxygen reduction electrocatalytic activity of cobalt and nitrogen co-doped carbon nanocatalyst synthesized by a flat template, Nanoscale Res. Lett. 12 (2017) 144. Q. Xue, J. Bai, C. Han, P. Chen, J.X. Jiang, Y. Chen, Au nanowires@Pdpolyethylenimine nanohybrids as highly active and methanol-tolerant electrocatalysts toward oxygen reduction reaction in alkaline media, ACS Catal. 8 (2018) 11287–11295. Y.Q. Kang, Q. Xue, Y. Zhao, X.F. Li, P.J. Jin, Y. Chen, Selective etching induced synthesis of hollow Rh nanospheres electrocatalyst for alcohol oxidation reactions, Small 14 (2018) 1801239.
151
[57] G.R. Xu, B. Wang, J.Y. Zhu, F.Y. Liu, Y. Chen, J.H. Zeng, J.X. Jiang, Z.H. Liu, Y.W. Tang, J.M. Lee, Morphological and interfacial control of platinum nanostructures for electrocatalytic oxygen reduction, ACS Cat. 6 (2016) 5260–5267. [58] G.R. Xu, F.Y. Liu, Z.H. Liu, Y. Chen, Ethanol-tolerant polyethyleneimine functionalized palladium nanowires in alkaline media: the ‘‘molecular window gauze” induced the selectivity for the oxygen reduction reaction, J. Mater. Chem. A 3 (2015) 21083–21089. [59] H. Liu, X. Liu, Y. Li, Y. Jia, Y. Tang, Y. Chen, Hollow PtNi alloy nanospheres with enhanced activity and methanol tolerance for the oxygen reduction reaction, Nano Res. 9 (2016) 3494–3503. [60] F. Jaouen, E. Proietti, M. Lefèvre, R. Chenitz, J.P. Dodelet, G. Wu, H.T. Chung, C. M. Johnston, P. Zelenay, M. Lefèvre, R. Chenitz, J.P. Dodelet, G. Wu, H.T. Chung, C.M. Johnston, P. Zelenay, Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells, Energy Environ. Sci. 4 (2011) 114–130.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Co nanoparticles supported on three-dimensionally N-doped holey graphene aerogels for electrocatalytic oxygen reduction Kai-Wen Cao a, Hao Huang b, Fu-Min Li b, Hong-Chang Yao c, Juan Bai a,⇑, Pei Chen a, Pu-Jun Jin a, Zi-Wei Deng a, Jing-Hui Zeng a, Yu Chen a,⇑ a Key Laboratory of Macromolecular Science of Shaanxi Province, Key Laboratory of Applied Surface and Colloid Chemistry (Ministry of Education), Shaanxi Key Laboratory for Advanced Energy Devices, School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062, People’s Republic of China b School of Chemistry and Chemical Engineering, Shaanxi Normal University, Xi’an 710062, People’s Republic of China c College of Chemistry and Molecular Engineering, Zhengzhou University, Zhengzhou 450001, People’s Republic of China
g r a p h i c a l a b s t r a c t Co nanoparticles supported on three-dimensionally (3D) N-doped holey graphene aerogels (Co/N-GA-h) hybrids were successfully synthesized by simple high-temperature pyrolysis method. As a non-noble metal electrocatalyst, the Co/N-GA-h hybrids displayed the excellent activity and durability for the ORR in alkaline media.
a r t i c l e
i n f o
Article history: Received 29 August 2019 Revised 8 October 2019 Accepted 8 October 2019 Available online 9 October 2019 Keywords: Co nanoparticles N-doped Holey graphene aerogels
a b s t r a c t The reactive and stable catalysts for the oxygen reduction reaction are highly desirable for low temperature fuel cells. The commercial oxygen reduction reaction electrocatalysts generally reply on noble metal based nanomaterials, which suffer from inherent cost and selectivity issues. At present, it still remains challenge for designing efficient non-noble metal-based oxygen reduction reaction electrocatalysts. Herein, we successfully synthesize Co nanoparticles supported on three-dimensionally N-doped holey graphene aerogels hybrids by the high-temperature calcination of the graphene aerogelspolyallylamine-CoII hybrids. The component optimized hybrids show the excellent electrocatalytic activity for oxygen reduction reaction in alkaline media, which is comparable to commercial Pt/C electrocatalyst. Meanwhile, the hybrids also show eminent tolerance for CO and methanol, attributing to their
⇑ Corresponding authors. E-mail addresses: [email protected] (J. Bai), [email protected] (Y. Chen). https://doi.org/10.1016/j.jcis.2019.10.025 0021-9797/Ó 2019 Elsevier Inc. All rights reserved.
144 Electrocatalysts Oxygen reduction reaction
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excellent oxygen reduction reaction selectivity. The three-dimensionally interconnected structure of graphene aerogels, N-doping, uniform dispersion and high crystallinity of Co nanoparticles, and holey structure of graphene contribute to the striking oxygen reduction reaction activity of hybrids. Ó 2019 Elsevier Inc. All rights reserved.
1. Introduction
2. Experimental
The environmental pollution and rising global energy demand have seriously affected the human health and energy security. One promising solution is the fuel cell technology, which can convert chemical energy into electrical energy and provide sustainable power [1–6]. However, the oxygen reduction reaction (ORR) at cathode in fuel cell primarily limits the energy conversion efficiency. Traditionally, Pt-based nanomaterials are considered as the best electrocatalyst for ORR [7–12]. Unfortunately, scarce resource, low stability, CO deactivation, poor methanol tolerance and high cost are also the primary barrier to their industrial applications [13,14]. Thus, the further efforts are necessary to develop alternative electrocatalysts that are low-cost, stable and selective for ORR [15–17]. The study for efficient alternatives to displace Pt-based ORR electrocatalysts is growing intensively [18–20]. Tremendous efforts have been made to develop low cost ORR electrocatalysts, including transition metal-based materials (Fe, Co, CoO, Co3O4, and Fe3O4, etc.) [21–26], heteroatom doped carbon materials (graphene, fullerene and carbon nanotubes, etc.) [27–30] and heteroatom-coordinated transition metals on carbon materials [31,32]. Among these electrocatalysts, nitrogen coordinated Fe or Co supported on carbon materials have been regarded as one of the most promising non-noble metal based ORR electrocatalysts [33,34]. But they also suffer from sintering, dissolution, and agglomeration during ORR process, resulting in the deactivation of electrocatalyst [35]. To overcome these difficulties, carbon supports like porous carbon, graphene and active carbon have been developed to increase the electroactive surface area of electrocatalysts and improve their electrocatalytic activity and stability [36– 38]. Graphene, as a star material in carbon family, has become a new-generation support due to its unique two-dimensional single-layer sheet structure, high surface area, good chemical stability and excellent conductivity [39–42]. Although tremendous efforts have been made in graphene-based ORR electrocatalysts, there is no literature of Co nanoparticles supported on threedimensionally (3D) porous interconnected graphene based nanostructures for ORR. Such materials are attractive systems, as they allow the maximum utilization of the unique structure of twodimensional (2D) sheets and accelerate the mass transfer during the electrocatalytic reactions due to 3D porous interconnected structure. Furthermore, holes on graphene also accelerate the mass transfer. Thus, a controlled interplay of multiple types of active sites together result in the improvement of the catalytic activity for the sluggish ORR [43–45]. In this work, we synthesized Co nanoparticles supported on 3D N-doped graphene aerogels with holes (Co/N-GA-h) hybrids by simple high-temperature pyrolysis method. The graphene-based hybrids show an interconnected micropores structure and Co nanoparticles uniformly deposit on the mesopores framework of holey graphene sheets. When used as ORR electrocatalyst in alkaline medium, Co/N-GA-h hybrids display a more positive onset potential and half-wave potential than N-doped graphene aerogels (N-GA). Meanwhile, Co/N-GA-h hybrids exhibit low H2O2 yield, excellent anti-methanol performance, weak CO-poisoning effect, good durability and high electron transfer number for ORR.
2.1. Materials Polyallylamine hydrochloride (PAA, molecular weight: 15 0000) was obtained from Japan Nitto Boseki Co., Ltd. Graphene oxide (GO) was purchased from JCNANO Technology Co., Ltd (Nanjing). Cobalt (II) chloride (CoCl2) and ascorbic acid (AA) were acquired from Aladdin Industrial Co. Commercial Pt/C electrocatalyst was purchased from JM Corporation. 2.2. Synthesis of graphene aerogels (GA) Typically, GO (10 mg) and AA (40 mg) was added into 5 mL of H2O by ultrasonic treatment. Then the mixture was given the temperature of 95 °C, GA could be obtained after 2 h. The excess AA was removed by multiple dialysis at 60 °C. 2.3. Synthesis of graphene aerogels-polyallylamine hybrids (GA-PAA) Typically, GA-PAA hybrids were obtained by immersing GA in PAA aqueous solution (5 mL, 0.5 M). Then, a bottle containing above mixture heated 2 h at 95 °C. Finally, GA-PAA hybrids were obtained after the multiple dialysis at 60 °C. 2.4. Synthesis of graphene aerogels-polyallylamine-CoII hybrids (GAPAA-CoII) Typically, GA-PAA hybrids were immersed in 5 mL of 1 mg mL 1 CoCl2 aqueous solution. Then, a bottle containing above mixture heated 2 h at 95 °C to speed up adsorption of Co2+. After the multiple dialysis at 60 °C, the GA-PAA-CoII-1 hybrids were acquired by freeze-drying. For comparison, after immersing GAPAA hybrids into 5 mL of 0.4 mg mL 1 CoCl2 aqueous solution and 5 mL of 16 mg mL 1 CoCl2 aqueous solution, GA-PAA-CoII-0.4 and GA-PAA-CoII-16 hybrids were also fabricated under the same conditions, respectively. 2.5. Synthesis of Co nanoparticles supported on 3D N-doped holey graphene aerogels hybrids (Co/N-GA-h) Co/N-GA-h-1 hybrids were obtained by high-temperature calcination of GA-PAA-CoII-1 hybrids at 900 °C for 2 h under a flow of Ar with a heating rate of 10 °C min 1. Co/N-GA-h-0.4 and Co/N-GA-h16 hybrids obtained by using GA-PAA-CoII-0.4 and GA-PAA-CoII-16 hybrids as reaction precursor under same experimental conditions. For comparison, N-GA hybrids without Co nanoparticles were obtained by high-temperature calcination of GA-PAA hybrids at the same condition. 2.6. Electrochemical measurements Electrochemical experiments were measured on CHI 760E workstations with glassy carbon rotating disk electrode (RDE) and glassy carbon rotating ring disk electrode (RRDE) at 30 ± 1 °C. A standard three-electrode system was used in this work. A carbon rod acted as auxiliary electrode, a saturated calomel electrode was used as reference electrode and RDE or RRDE modi-
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fied with the electrocatalyst served as working electrode. The electrocatalyst suspension was obtained by mixing 4 mg of production and 2 mL Nafion/ethanol/water. After strong sonication for 20 min, 10 lL suspension was carefully dropped on the RDE and dried at room temperature. The linear sweep voltammetry (LSV) curves were evaluated in O2-saturated 0.1 M KOH electrolyte. All potentials in this work were referenced to the reversible hydrogen electrode (RHE), using the equation of ERHE = ESCE + 0.241 V + 0.0591pH. The number of electron transfer (n) was evaluated using the Koutecky–Levich equation: 1/j = 1/jk + 1/Bx0.5, B = 0.2nF(DO2)C2/3m 1/6CO2 (where j was apparent current density, jk was the kineticlimiting current density, x was angular velocity, F was the Faraday constant, DO2 was the O2 diffusion coefficient in 0.1 M KOH solution, m was the kinetic viscosity of 0.1 M KOH solution, and CO2 was the concentration of dissolved O2 in 0.1 M KOH solution). The values of n and hydrogen peroxide yield (yH2O2) with RRDE measurements were calculated by equations: n = 4NId/(NId + Ir), yH2O2 = 200Ir/NId + Ir (where N-the collection efficiency, Id-the disk current and Ir-the ring current). 2.7. Instruments The morphology and microstructure were analysed by scanning electron microscopy (SEM, SU-8020), standard/high resolution transmission electron microscopy (TEM/HRTEM, TECNAI G2 F20) and energy-dispersive X-ray (EDS, TECNAI G2 F20) maps. The crystallographic texture was performed by X-Ray diffraction (XRD, DX2700) and selected area electron diffraction (SAED, TECNAI G2 F20). The electronic property was investigated by X-ray photoelectron spectroscopy (XPS, AXIS ULTRA). The composition of sample was analyzed by EDS (Quanta 200). Thermogravimetric analysis (TGA) was completed by thermoanalysis analyzer (Q600 systems) under air atmosphere. The samples surface charge was taken on by Nano ZS90. The interaction between PAA and CoCl2 was measured by ultraviolet and visible spectroscopy (UV vis, Shimadzu UV3600U). 3. Results and discussion 3.1. Characterization of Co/N-GA-h-1 hybrids GA and GA-PAA were synthesized according to our previous work with a slight modification [36]. Typically, GO was reduced
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by AA and the reduced graphene oxide (rGO) self-assembled into GA with 3D porous structure (Fig. S1). After dialysis and freezedrying, GA interact with PAA and Co2+ in step-by-step process to generate GA-PAA and GA-PAA-CoII-1 hybrids, respectively. Furthermore, Co/N-GA-h-1 hybrids are obtained by pyrolysis of GAPAA-CoII hybrids at 900 °C (Scheme 1). The internal structure of Co/N-GA-h-1 hybrids was characterised by SEM. The SEM image reveals that Co/N-GA-h-1 hybrids have the distinct interconnected structure with 3D porous (Fig. 1A), which will be beneficial to electronic conduction and can accelerate the mass transfer during the electrocatalytic reactions. In order to prove that GO has been reduced to rGO, XRD and XPS were performed. XRD spectrum show the characteristic diffraction peak of Co/N-GA-h-1 hybrids is located at 24° of 2h degree, which is different from that (2h = 12°) of GO (Fig. 1B). And XPS of C1s also show the CAO/C@O bonds decreased sharply in the C1s region, revealing the GO has been transformed into rGO successfully (Fig. 1C) [36]. GA-PAA hybrids were easily achieved by immersing GA in PAA aqueous solution. The zeta-potential of GA is measured to be 24.8 ± 2 mV, and the zeta-potential of GA-PAA hybrids changes to +35.7 ± 2 mV in aqueous solution after soaking, indicating the GA can attract the PAA through the strong electrostatic interaction. Our previous works have proved that PAA can interact with metal ion (Mx) to generate the PAA-Mx complex due to the specific coordination ability of the ANH2 groups. UV–vis spectrum shows the absorption spectrum of PAA + CoCl2 solution is different from the absorption spectrum of CoCl2 and the absorption spectrum of PAA (Fig. 1D), revealing PAA can interact with CoCl2. Another controlled experiment has been also proved the effect of PAA. The optical digital images show the color of PAA + CoCl2 solution is light brown and the color of CoCl2 solution is light pink. After adding the NaOH solution, hydrolysis process occurs quickly, resulting the precipitate in the CoCl2 solution. However, there is no precipitate in the PAA + CoCl2 solution, also confirming the PAA can interact with CoCl2 again (Fig. S2). After heating GA-PAA-CoII-1 hybrids at 900 °C for 2 h, Co/N-GAh-1 hybrids were obtained. TEM image clearly shows the nanoparticles are well distributed on the graphene surface (Fig. 2A). But there is no characteristic diffraction peaks of metallic Co or cobalt oxide in the XRD spectrum (Fig. 1B), likely originating from the low content of Co. So, we synthesize the calcined product in the absence of graphene at the same condition. The XRD of calcined
Scheme 1. The overall synthetic route of Co/N-GA-h-1 hybrids.
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Fig. 1. (A) SEM image of Co/N-GA-h-1 hybrids, (B) XRD patterns of GO and Co/N-GA-h-1 hybrids, (C) C1s XPS of Co/N-GA-h-1 hybrids, (D) UV–vis absorption spectra of CoCl2, PAA and PAA + CoCl2 solution.
product reveal the nanoparticles are metallic Co (Fig. S3). The morphology of Co/N-GA-h-1 hybrids is different from GA-PAA and GAPAA-CoII-1 hybrids (Fig. S4), indicating the addition Co2+ have the influence on the morphology of Co/N-GA-h-1 hybrids. Surprisingly, a significant number of holes appear on the graphene surface (Fig. 2A). Most holes relate with at least one Co nanoparticle. Clearly, the Co nanoparticles catalyzed the oxidation the graphitic carbon by oxidation etching method, resulting in many pores on graphene. And the shape of some holes liked tracks, which are apparently related to the directional motion of the deposited Co nanoparticles, owing to etching-induced motions. The above phenomenon indicates Co nanoparticles catalyzed the oxidation of carbon atoms that associate with them, while the other carbons remain original shape. This holes on graphene can facilitate the diffusion of reactant ions to the active sites [44,45]. HRTEM image clearly shows the lattice distance of nanoparticles on graphene surface is 0.205 nm (Fig. 2B), indicating that these nanoparticles are elemental Co0 nanoparticles rather than cobalt oxide. SAED pattern of Co nanoparticles reveal its polycrystalline structure (Insert in Fig. 2B). EDS elemental maps prove the existence of C, N and Co elements. Co element evenly distribute in the nanoparticles and the patterns of C, N are completely superimposed on each other, this phenomenon further suggest the N element is successfully embedded in the C skeleton (Fig. 2C). Further XPS characterization method were measured to investigate the detailed chemical composition of Co/N-GA-h-1 hybrids. The N and Co content can be detected in XPS measurement, suggesting the successful reduction of Co2+ and N doping (Fig. 2D). The main peaks at 780.1 and 796.0 eV corresponds to 2p3/2 and 2p1/2 of Co, indicating the metallic Co is the predominant species in Co/N-GA-h-1 hybrids. The XPS spectrum of Co 2p (Fig. 2D)
shows two fitted peaks for Co 2p3/2 that can be assigned to Ncoordinated (778.7 eV) and O-coordinated Co (781.6 eV), revealing the inevitable oxidation of partial surface atoms on Co nanoparticles. The previous work have demonstrated that Co centres stabilized by N coordination can act as the active sites in electrocatalysts during the ORR process [31]. Meanwhile, the N1s XPS spectrum match with four chemical states, pyridinic N (398.0 eV), pyrrolic N (399.7 eV), graphitic N (401.0 eV), respectively. We can observe an additional peak at 399.0 eV (Fig. 2E), which come from the Co-N. According to previous work, pyridinic N, graphitic N and Co-N are primary N species, which will offer the active site for ORR [31]. The structural features and surface area of Co/N-GA-h-1 hybrids were further characterized by N2 adsorption–desorption isotherm. BET specific surface area of Co/N-GA-h-1 hybrids is measured to be 361.4 m2 g 1 (Fig. S5), which originates from the particular 2D structure of the nanosheets. The N2 gas adsorption isotherm and the pore size distribution show there are many micropores and mesopores, which is consistent with the 3D porous and holey structure of Co/N-GA-h-1 hybrids in TEM image. The high specific surface area and porous structure will make for the fast mass transfer during the ORR. 3.2. ORR performance of Co/N-GA-h-1 hybrids As a significant half reaction in fuel cells and Zn-air batteries, ORR performance affects the working life and efficiency of these devices. The ORR performance of Co/N-GA-h-1 hybrids was studied by LSV tests (Fig. 3A). The ORR performance of the N-GA hybrids and commercial Pt/C electrocatalyst (20 wt%) electrocatalyst were also measured under the same conditions. As observed, Co/N-GA-
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Fig. 2. (A) TEM image of Co/N-GA-h-1 hybrids, (B) HRTEM image and SAED pattern of Co nanoparticles, (C) EDX maps of Co/N-GA-h-1 hybrids, (D) Co 2p XPS of Co/N-GA-h-1 hybrids (E) N 1s XPS of Co/N-GA-h-1 hybrids.
h-1 hybrids exhibit the best ORR performance. Specifically, the onset and half-wave potential (Eonset = 0.92 V, E1/2 = 0.81 V) of Co/N-GA-h-1 hybrids for ORR are much higher than that (Eonset = 0.86 V, E1/2 = 0.77 V) of N-GA hybrids. This result indicate that the introduction of Co can effectively heighten their electrocatalytic activity for ORR. Although the ORR performance of Co/N-GA-h-1 hybrids is slightly poorer than that of commercial Pt/C electrocatalyst, it is comparable to other carbon based ORR electrocatalysts, such as N-rich graphene (E1/2 = 0.70 V) [46], N/P-codoped carbon (E1/2 = 0.78 V) [47], N-doped porous carbon nanosheets (E1/2 = 0.76 V) [48], plasma-treated graphene (E1/2 = 0.74 V) [49], N-doped porous carbon (E1/2 = 0.75 V) [50]. Even more than some metal-based electrocatalysts, like Au@Pd/ reduced graphene oxide (E1/2 = 0.759 V) [51], Co/N-C-800 (E1/2 = 0.74 V) [52], Fe/N-gCB (E1/2 = 0.80 V) [53] and Co-NC-900 (E1/2 = 0.80 V) [54]. The reaction kinetics of ORR at Co/N-GA-h-1 hybrids was measured by LSV at the different rotation rates using the Koutecky– Levich equation. In general, the limiting diffusion current density increases with the rotation rate (Fig. 3B). The n is 3.96 at the
potential 0.4 V, revealing there is a four-electron process for ORR. The RRDE measurement show a negligible ring current and the percentage of H2O2 is calculated to be <20% in the range of 0– 0.8 V. Meanwhile, the n value is ca. 3.6, also indicating a fourelectron reaction pathway (Fig. 3C and D). To investigate the effect of Co content on ORR activity, Co/NGA-h hybrids with different Co content (i.e., Co/N-GA-h-0.4 and Co/N-GA-h-16) were also synthesized (see experimental section for detail). SEM images show both Co-N-GA-h-0.4 and Co/N-GAh-16 hybrids have the 3D porous interconnected structures (Fig. S6). TGA curves show that the Co content in Co-N-GA-h-0.4 is 5.1 wt%, which is smaller than that (11.2 wt%) of Co/N-GA-h-1 hybrids (Fig. S7). And the Co content in Co/N-GA-h-16 hybrids (12.5 wt%) is almost the same as Co/N-GA-h-1 hybrids, indicating the maximum adsorption of CoCl2 is reached in Co/N-GA-h-1 hybrids. Then, the ORR activity of Co-N-GA-h-0.4 and Co/N-GA-h16 hybrids were evaluated by the LSV. Compared with Co/N-GAh-0.4 hybrids, Co/N-GA-h-1 hybrids exhibit the better ORR Eonset, E1/2 and limiting diffusion current. And LSV of Co/N-GA-h-16 hybrids and Co/N-GA-h-1 hybrids are almost overlapping, indicat-
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Fig. 3. (A) ORR polarization curves of commercial Pt/C electrocatalyst, Co/N-GA-h-1 hybrids and N-GA hybrids. (B) ORR polarization curves of Co/N-GA-h-1 hybrids at different rotation rates. (C) RRDE measurements for Co/N-GA-h-1 hybrids at a rotation rate of 1600 rpm. (D) Electron transfer number and H2O2 yield derived from the RRDE tests.
ing similar ORR activity (Fig. 4A). The above experimental result show that ORR activity of Co/N-GA-h-1 hybrids enhances with Co content increases and then remain constant. The direct methanol fuel cells in alkaline media draw more attention because of its unique advantages, including less CO poisoning, enhanced methanol oxidation reaction (MOR) and ORR kinetics [55,56]. But methanol can permeate into the cathode side across the Nafion membrane, leading to an undesirable competitive reaction between ORR and MOR at the cathode [57–59]. The ORR polarization curves of commercial Pt/C electrocatalyst show extreme descent and rapid poisoning when adding CO (Fig. 4B). In contrast, Co/N-GA-h-1 hybrids exhibit the excellent antimethanol performance and weak CO-poisoning effect. ORR polarization curves of Co/N-GA-h-1 hybrids in the existence of methanol is very similar to that the solution without methanol, only shifting 10 mV in E1/2 (Fig. 4C). In comparison, there is an obvious MOR peak at 0.78 V in the presence of methanol for ORR polarization curves of commercial Pt/C electrocatalyst (Fig. 4D). Thus, ORR measurements reveal Co/N-GA-h-1 hybrids have the unordinary ORR selectivity, which may originate from the low Intrinsic activity of transition metal and carbon materials for MOR. Durability is another key parameter to evaluate the ORR performance of the electrocatalysts. The accelerated durability method [50] was measured to evaluate the electrocatalytic durability of Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst (Fig. 5). After 2000 cycles of CV, Co/N-GA-h-1 hybrids reveal 10 mV degradation in E1/2 value for ORR (Fig. 5A) whereas commercial Pt/C electrocatalyst show 60 mV shifts in E1/2 value for ORR (Fig. 5B). This fact indicates Co/N-GA-h-1 hybrids have better stability than
commercial Pt/C electrocatalyst for ORR. Most cogently, the durability of the non-noble metal electrocatalyst is highly related to the amount of H2O2 releasing during the reaction [60]. The very low H2O2 yield of Co/N-GA-h-1 hybrids will be beneficial to its ORR stability. And the durability of remarkable enhancement on Co/NGA-h-1 hybrids can be attributed to its 3D interconnected graphene framework structure. To verify the structure stability, TEM of Co/N-GA-h-1 hybrids was measured after the ORR reaction. The TEM image shows Co nanoparticles and holes in graphene are maintained well, which results in their good durability (Fig. S8). This unique structure can suppress the structural collapse, enhance the interfacial contact and facilitate the transport of electron and reactant [31]. Thus, Co/N-GA-h-1 hybrids are more stable than commercial Pt/C electrocatalyst. 4. Conclusions In summary, we have successfully prepared Co nanoparticles supported on 3D N-doped graphene aerogels hybrids with holes by high-temperature pyrolysis method. It is remarkable that this work provides a facile and low-cost approach to design high performance carbon materials supported transition metal nonprecious metal hybrids. The prepared Co/N-GA-h-1 hybrids exhibit excellent electrocatalytic performance for ORR in alkaline media due to its porous interconnected structure, high surface area, nitrogen doping, holey graphene and numerous Co-N active sites. In the presence of trace methanol and CO, the anti-methanol and antiCO-poisoning performance of Co/N-GA-h-1 hybrids are much better than that of commercial Pt/C electrocatalyst. Facile preparation
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Fig. 4. (A) ORR polarization curves of the of Co/N-GA-h-0.4 hybrids, Co/N-GA-h-1 hybrids and Co/N-GA-h-16 hybrids, (B) CO tolerance performance of Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst for ORR analysed by chronoamperometry. (C) ORR polarization curves of Co/N-GA-h-1 hybrids and (D) commercial Pt/C electrocatalyst before and after adding methanol.
Fig. 5. (A) ORR polarization curves of Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst before and after 2000 cycles accelerated durability test. (B) The change values of the Eonset and E1/2 for Co/N-GA-h-1 hybrids and commercial Pt/C electrocatalyst for ORR after the accelerated durability tests.
method, low cost raw materials and outstanding ORR performance make Co/N-GA-h-1 hybrids a promising non-noble metal ORR electrocatalyst in direct methanol fuel cells.
Acknowledgements This research was sponsored by the National Natural Science Foundation of China (51873100), National Key R & D Program of China (2017YFC0107401), Fundamental Research Funds for the Central Universities (GK201901002) and the 111 Project (B14041).
Declaration of Competing Interest Appendix A. Supplementary material The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.10.025.
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References [1] K. Eid, Y.H. Ahmad, H. Yu, Y. Li, X. Li, S.Y. AlQaradawi, H. Wang, L. Wang, Rational one-step synthesis of porous PtPdRu nanodendrites for ethanol oxidation reaction with a superior tolerance for CO-poisoning, Nanoscale 9 (2017) 18881–18889. [2] L. Zhang, L.X. Ding, H. Chen, D. Li, S. Wang, H. Wang, Self-supported PtAuP alloy nanotube arrays with enhanced activity and stability for methanol electrooxidation, Small 13 (2017) 1604000. [3] Q. Li, T. Wang, D. Havas, H. Zhang, P. Xu, J. Han, J. Cho, G. Wu, Highperformance direct methanol fuel cells with precious-metal-free cathode, Adv. Sci. 3 (2016) 1600140. [4] L. Tan, N. Li, S. Chen, Z.Q. Liu, Self-assembly synthesis of CuSe@graphene– carbon nanotubes as efficient and robust oxygen reduction electrocatalysts for microbial fuel cells, J. Mater. Chem. A 4 (2016) 12273–12280. [5] X.T. Wu, J.C. Li, Q.R. Pan, N. Li, Z.Q. Liu, Gallic acid-assisted synthesis of Pd uniformly anchored on porous N-rGO as efficient electrocatalyst for microbial fuel cells, Dalton Trans. 47 (2018) 1442–1450. [6] X. Yan, S. Yu, Y. Tang, D. Sun, L. Xu, C. Xue, Triangular AgAu@Pt core–shell nanoframes with a dendritic Pt shell and enhanced electrocatalytic performance toward the methanol oxidation reaction, Nanoscale 10 (2018) 2231–2235. [7] H. Wang, S. Yin, Y. Xu, X. Li, A.A. Alshehri, Y. Yamauchi, H. Xue, Y.V. Kaneti, L. Wang, Direct fabrication of tri-metallic PtPdCu tripods with branched exteriors for the oxygen reduction reaction, J. Mater. Chem. A 6 (2018) 8662–8668. [8] L. Zhang, M. Wei, S. Wang, Z. Li, L.X. Ding, H. Wang, Highly stable PtP alloy nanotube arrays as a catalyst for the oxygen reduction reaction in acidic medium, Chem. Sci. 6 (2015) 3211–3216. [9] S. Xue, W. Deng, F. Yang, J. Yang, I.S. Amiinu, D. He, H. Tang, S. Mu, Hexapod PtRuCu nanocrystalline alloy for highly efficient and stable methanol oxidation, ACS Catal. 8 (2018) 7578–7584. [10] M. Bao, I.S. Amiinu, T. Peng, W. Li, S. Liu, Z. Wang, Z. Pu, D. He, Y. Xiong, S. Mu, Surface evolution of PtCu alloy Shell over Pd nanocrystals Leads to Superior hydrogen evolution and oxygen reduction reactions, ACS Energy Lett. 3 (2018) 940–945. [11] X.L. Tian, Y.Y. Xu, W. Zhang, T. Wu, B.Y. Xia, X. Wang, Unsupported platinumbased electrocatalysts for oxygen reduction reaction, ACS Energy Lett. 2 (2017) 2035–2043. [12] Y. Chen, G. Fu, Y. Li, Q. Gu, L. Xu, D. Sun, Y. Tang, L Glutamic acid derived PtPd@Pt core/satellite nanoassemblies as an effectively cathodic electrocatalyst, J. Mater. Chem. A 5 (2017) 3774–3779. [13] Z. Kou, B. Guo, D. He, J. Zhang, S. Mu, Transforming Two-dimensional boron carbide into boron and chlorine dual-doped carbon nanotubes by chlorination for efficient oxygen reduction, ACS Energy Lett. 3 (2018) 184–190. [14] Y. Guo, P. Yuan, J. Zhang, Y. Hu, I.S. Amiinu, X. Wang, J. Zhou, H. Xia, Z. Song, Q. Xu, S. Mu, carbon nanosheets containing discrete Co-Nx-By-C active sites for efficient oxygen electrocatalysis and rechargeable Zn-Air batteries, ACS Nano 12 (2018) 1894–1901. [15] Z.Q. Liu, H. Cheng, N. Li, T.Y. Ma, Y.Z. Su, ZnCo2O4 quantum dots anchored on nitrogen-doped carbon nanotubes as reversible oxygen oeduction/evolution electrocatalysts, Adv. Mater. 28 (2016) 3777–3784. [16] T.Y. Ma, J. Ran, S. Dai, M. Jaroniec, S.Z. Qiao, Phosphorus-doped graphitic carbon nitrides grown in situ on carbon-fiber paper: flexible and reversible oxygen electrodes, Angew. Chem. Int. Edit. 54 (2015) 4646–4650. [17] J. Zhu, W. Li, S. Li, J. Zhang, H. Zhou, C. Zhang, J. Zhang, S. Mu, defective N/Scodoped 3D cheese-like porous carbon nanomaterial toward efficient oxygen reduction and Zn-Air batteries, Small 14 (2018) 1800563. [18] J. Zhang, S. Wu, X. Chen, M. Pan, S. Mu, Egg derived nitrogen-self-doped carbon/carbon nanotube hybrids as noble-metal-free catalysts for oxygen reduction, J. Power Sources 271 (2014) 522–529. [19] B. Tang, S. Wang, R. Li, X. Gou, J. Long, J. Long, Urea treated metal organic frameworks-graphene oxide composites derived N-doped Co-based materials as efficient catalyst for enhanced oxygen reduction, J. Power Sources 425 (2019) 76–86. [20] A. Sumboja, J. Chen, Y. Ma, Y. Xu, Y. Zong, P.S. Lee, Z. Liu, Sulfur-rich colloidal nickel sulfides as bifunctional catalyst for all-solid-state, flexible and rechargeable Zn-Air batteries, ChemCatChem 11 (2019) 1205–1213. [21] Z. Wang, H. Jin, T. Meng, K. Liao, W. Meng, J. Yang, D. He, Y. Xiong, S. Mu, Cucoordinated ZIF-derived carbon framework for efficient oxygen reduction reaction and zinc-air batteries, Adv. Funct. Mater. 28 (2018) 1802596. [22] Y. Qiao, P. Yuan, Y. Hu, J. Zhang, S. Mu, J. Zhou, H. Li, H. Xia, J. He, Q. Xu, Sulfuration of an Fe-N-C catalyst containing Fex C/Fe species to enhance the catalysis of oxygen reduction in acidic media and for use in flexible Zn-Air batteries, Adv. Mater. (2018) e1804504. [23] S. Liu, I.S. Amiinu, X. Liu, J. Zhang, M. Bao, T. Meng, S. Mu, Carbon nanotubes intercalated Co/N-doped porous carbon nanosheets as efficient electrocatalyst for oxygen reduction reaction and zinc-air batteries, Chem. Eng. J. 342 (2018) 163–170. [24] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Co3O4 nanocrystals on graphene as a synergistic catalyst for oxygen reduction reaction, Nat. Mater. 10 (2011) 780. [25] Z.S. Wu, S. Yang, Y. Sun, K. Parvez, X. Feng, K. Müllen, 3D nitrogen-doped graphene aerogel-supported Fe3O4 nanoparticles as efficient electrocatalysts for the oxygen reduction reaction, J. Am. Chem. Soc. 134 (2012) 9082–9085.
[26] S. Guo, S. Zhang, L. Wu, S. Sun, Co/CoO nanoparticles assembled on graphene for electrochemical reduction of oxygen, Angew. Chem. Int. Edit. 51 (2012) 11770–11773. [27] R. Ma, Y. Zhou, C. Hu, M. Yang, F. Wang, K. Yan, Q. Liu, J. Wang, Post irondoping of activated nitrogen-doped carbon spheres as a high-activity oxygen reduction electrocatalyst, Energy Storage Mater. 13 (2018) 142–150. [28] D. Gu, Y. Zhou, R. Ma, F. Wang, Q. Liu, J. Wang, Facile synthesis of N-doped graphene-like carbon nanoflakes as efficient and stable electrocatalysts for the oxygen reduction reaction, Nano-Micro Lett. 10 (2018) 29. [29] R. Ma, X. Ren, B.Y. Xia, Y. Zhou, C. Sun, Q. Liu, J. Liu, J. Wang, Novel synthesis of N-doped graphene as an efficient electrocatalyst towards oxygen reduction, Nano Res. 9 (2016) 808–819. [30] J. Zhang, Y. Sun, J. Zhu, Z. Kou, P. Hu, L. Liu, S. Li, S. Mu, Y. Huang, defect and pyridinic nitrogen engineering of carbon-based metal-free nanomaterial toward oxygen reduction, Nano Energy 52 (2018) 307–314. [31] Y. Jiang, Y. Lu, X. Wang, Y. Bao, W. Chen, L. Niu, A cobalt–nitrogen complex on N-doped three-dimensional graphene framework as a highly efficient electrocatalyst for oxygen reduction reaction, Nanoscale 6 (2014) 15066– 15072. [32] Y. Hu, J.O. Jensen, W. Zhang, L.N. Cleemann, W. Xing, N.J. Bjerrum, Q. Li, Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts, Angew. Chem. Int. Edit. 53 (2014) 3675–3679. [33] C.Y. Su, H. Cheng, W. Li, Z.Q. Liu, N. Li, Z. Hou, F.Q. Bai, H.X. Zhang, T.Y. Ma, Atomic modulation of FeCo-nitrogen-carbon bifunctional oxygen electrodes for rechargeable and flexible all-solid-state zinc-air battery, Adv. Energy Mater. 7 (2017) 1602420. [34] Y. Han, Y. Wang, R. Xu, W. Chen, L. Zheng, A. Han, Y. Zhu, J. Zhang, H. Zhang, J. Luo, C. Chen, Q. Peng, D. Wang, Y. Li, Electronic structure engineering to boost oxygen reduction activity by controlling the coordination of the central metal, Energy Environ. Sci. 11 (2018) 2348–2352. [35] C. Huang, Y. Li, N. Wang, Y. Xue, Z. Zuo, H. Liu, Y. Li, Progress in research into 2D graphdiyne-based materials, Chem. Rev. 118 (2018) 7744–7803. [36] Q. Xue, Y. Ding, Y. Xue, F. Li, P. Chen, Y. Chen, 3D nitrogen-doped graphene aerogels as efficient electrocatalyst for the oxygen reduction reaction, Carbon 139 (2018) 137–144. [37] J. Ding, S. Ji, H. Wang, V. Linkov, R. Wang, Mesoporous cobalt selenide/ nitrogen-doped carbon hybrid as bifunctional electrocatalyst for hydrogen evolution and oxygen reduction reactions, J. Power Sources 423 (2019) 1–8. [38] Y. Yu, J. Ma, C. Chen, Y. Fu, Y. Wang, K. Li, Y. Liao, L. Zheng, X. Zuo, General method for synthesis transition-metal phosphide/nitrogen and phosphide doped carbon materials with yolk-shell structure for oxygen reduction reaction, ChemCatChem 11 (2019) 1722–1731. [39] R. Lv, E. Cruz-Silva, M. Terrones, Building complex hybrid carbon architectures by covalent interconnections: graphene-nanotube hybrids and more, ACS Nano 8 (2014) 4061–4069. [40] R. Lv, T. Cui, M.S. Jun, Q. Zhang, A. Cao, D.S. Su, Z. Zhang, S.H. Yoon, J. Miyawaki, I. Mochida, F. Kang, Open-ended N-doped carbon nanotube-graphene hybrid nanostructures as high-performance catalyst support, Adv. Funct. Mater. 21 (2011) 999–1006. [41] X. Liu, I.S. Amiinu, S. Liu, Z. Pu, W. Li, B. Ye, D. Tan, S. Mu, H2O2-assisted synthesis of porous N-doped graphene/molybdenum nitride composites with boosted oxygen reduction reaction, Adv. Mater. Interfaces 4 (2017) 1601227. [42] X. Cai, B.Y. Xia, J. Franklin, B. Li, X. Wang, Z. Wang, L. Chen, J. Lin, L. Lai, Z. Shen, Free-standing vertically-aligned nitrogen-doped carbon nanotube arrays/graphene as air-breathing electrodes for rechargeable zinc-air batteries, J. Mater. Chem. A 5 (2017) 2488–2495. [43] Z. Fan, Q. Zhao, T. Li, J. Yan, Y. Ren, J. Feng, T. Wei, Easy synthesis of porous graphene nanosheets and their use in supercapacitors, Carbon 50 (2012) 1699–1703. [44] H. Cao, X. Zhou, C. Zheng, Z. Liu, Metal etching method for preparing porous graphene as high performance anode material for lithium-ion batteries, Carbon 89 (2015) 41–46. [45] T. Palaniselvam, H.B. Aiyappa, S. Kurungot, An efficient oxygen reduction electrocatalyst from graphene by simultaneously generating pores and nitrogen doped active sites, J. Mater. Chem. 22 (2012) 23799–23805. [46] S.N. Faisal, E. Haque, N. Noorbehesht, W. Zhang, A.T. Harris, T.L. Church, A.I. Minett, Pyridinic and graphitic nitrogen-rich graphene for high-performance supercapacitors and metal-free bifunctional electrocatalysts for ORR and OER, RSC Adv. 7 (2017) 17950–17958. [47] H. Jiang, Y. Wang, J. Hao, Y. Liu, W. Li, J. Li, N and P co-functionalized threedimensional porous carbon networks as efficient metal-free electrocatalysts for oxygen reduction reaction, Carbon 122 (2017) 64–73. [48] H. Yu, L. Shang, T. Bian, R. Shi, G.I. Waterhouse, Y. Zhao, C. Zhou, L.Z. Wu, Nitrogen-doped porous carbon nanosheets templated from g-C3N4 as metalfree electrocatalysts for efficient oxygen reduction reaction, C.H. Tung, T. Zhang Adv. Mater. 28 (2016) 5080–5086. [49] L. Tao, Q. Wang, S. Dou, Z. Ma, J. Huo, S. Wang, L. Dai, Edge-rich and dopantfree graphene as a highly efficient metal-free electrocatalyst for the oxygen reduction reaction, Chem. Commun. 52 (2016) 2764–2767. [50] B. Men, Y. Sun, M. Li, C. Hu, M. Zhang, L. Wang, Y. Tang, Y. Chen, P. Wan, J. Pan, Hierarchical metal-free nitrogen-doped porous graphene/carbon composites as an efficient oxygen reduction reaction catalyst, ACS Appl. Mater. Interfaces 8 (2016) 1415–1423. [51] S.S. Li, A.J. Wang, Y.Y. Hu, K.M. Fang, J.R. Chen, J.J. Feng, Seedless wet-chemical synthesis of gold@ palladium nanoflowers supported on reduced graphene
K.-W. Cao et al. / Journal of Colloid and Interface Science 559 (2020) 143–151
[52]
[53] [54]
[55]
[56]
oxide with enhanced electrocatalytic properties, J. Mater. Chem. A 2 (2014) 18177–18183. Y. Su, Y. Zhu, H. Jiang, J. Shen, X. Yang, W. Zou, J. Chen, C. Li, Cobalt nanoparticles embedded in N-doped carbon as an efficient bifunctional electrocatalyst for oxygen reduction and evolution reactions, Nanoscale 6 (2014) 15080–15089. R. Zhou, S.Z. Qiao, An Fe/N co-doped graphitic carbon bulb for high-performance oxygen reduction reaction, Chem. Commun. 51 (2015) 7516–7519. C. Guo, Y. Wu, Z. Li, W. Liao, L. Sun, C. Wang, B. Wen, Y. Li, C. Chen, The oxygen reduction electrocatalytic activity of cobalt and nitrogen co-doped carbon nanocatalyst synthesized by a flat template, Nanoscale Res. Lett. 12 (2017) 144. Q. Xue, J. Bai, C. Han, P. Chen, J.X. Jiang, Y. Chen, Au nanowires@Pdpolyethylenimine nanohybrids as highly active and methanol-tolerant electrocatalysts toward oxygen reduction reaction in alkaline media, ACS Catal. 8 (2018) 11287–11295. Y.Q. Kang, Q. Xue, Y. Zhao, X.F. Li, P.J. Jin, Y. Chen, Selective etching induced synthesis of hollow Rh nanospheres electrocatalyst for alcohol oxidation reactions, Small 14 (2018) 1801239.
151
[57] G.R. Xu, B. Wang, J.Y. Zhu, F.Y. Liu, Y. Chen, J.H. Zeng, J.X. Jiang, Z.H. Liu, Y.W. Tang, J.M. Lee, Morphological and interfacial control of platinum nanostructures for electrocatalytic oxygen reduction, ACS Cat. 6 (2016) 5260–5267. [58] G.R. Xu, F.Y. Liu, Z.H. Liu, Y. Chen, Ethanol-tolerant polyethyleneimine functionalized palladium nanowires in alkaline media: the ‘‘molecular window gauze” induced the selectivity for the oxygen reduction reaction, J. Mater. Chem. A 3 (2015) 21083–21089. [59] H. Liu, X. Liu, Y. Li, Y. Jia, Y. Tang, Y. Chen, Hollow PtNi alloy nanospheres with enhanced activity and methanol tolerance for the oxygen reduction reaction, Nano Res. 9 (2016) 3494–3503. [60] F. Jaouen, E. Proietti, M. Lefèvre, R. Chenitz, J.P. Dodelet, G. Wu, H.T. Chung, C. M. Johnston, P. Zelenay, M. Lefèvre, R. Chenitz, J.P. Dodelet, G. Wu, H.T. Chung, C.M. Johnston, P. Zelenay, Recent advances in non-precious metal catalysis for oxygen-reduction reaction in polymer electrolyte fuel cells, Energy Environ. Sci. 4 (2011) 114–130.