- Email: [email protected]
N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis
international journal of hydrogen energy xxx (xxxx) xxx
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis Zhaoxia Cao a,b,*, Jingyi Jia a,b, Haohan Li a,b, Yuhe Wang a,b, Xinxin Mao a,b, Zhennan Zhang a,b, Mingguo Yang a,b, Shuting Yang a,b,** a
National & Local Engineering Laboratory for Motive Power and Key Materials, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China b Collaborative Innovation Center of Henan Province for Green Motive Power and Key Materials, Henan Normal University, Xinxiang, Henan, 453007, PR China
highlights Hierarchically porous graphene-like carbon nanosheets were successfully synthesized. The highly graphitized carbon nanosheets are abundant in Fe related active sites (Fe-Nx and Fe3C). The optimal sample can act as an effective oxygen electrocatalyst for ORR.
article info
abstract
Article history:
Hierarchically porous carbon sheets decorated with transition metal carbides nano-
Received 10 September 2019
particles and metal-nitrogen coordinative sites have been proposed as the promising non-
Received in revised form
precious metal oxygen electrocatalysts. In this work, we demonstrate a facile and low-cost
13 November 2019
strategy to in situ form Fe/N codoped hierarchically porous graphene-like carbon nano-
Accepted 5 December 2019
sheets abundant in Fe-Nx sites and Fe3C nanoparticles (FeeN/C) from pyrolyzing chestnut
Available online xxx
shell precursor. The as-prepared FeeN/C samples with abundant Fe-Nx sites and Fe3C nanoparticles show superior electrocatalytic activity to oxygen reduction reaction (ORR) in
Keywords:
the alkaline medium as well as high stability and methanol tolerance due to the integration
Fe/N codoping
of multi-factors: the high content of Fe-Nx active sites, the coexistence of Fe3C, the unique
Fe3C nanoparticle
hierarchically porous structure and high conductivity of carbon matrix. The optimal FeeN/
Graphene-like carbon nanosheets
C-2-900 sample exhibits a more positive half-wave potential (0.122 V vs. Ag/AgCl (3 M)
Hierarchically porous
reference electrode) than commercial 20 wt% Pt/C catalyst. This study provides a facile
Oxygen electrocatalyst
approach to synthesize Fe3C nanoparticles decorated Fe/N co-doped hierarchically porous carbon materials for effective oxygen electrocatalyst. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. National & Local Engineering Laboratory for Motive Power and Key Materials, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China. ** Corresponding author. National & Local Engineering Laboratory for Motive Power and Key Materials, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China. E-mail addresses: [email protected] (Z. Cao), [email protected] (S. Yang). https://doi.org/10.1016/j.ijhydene.2019.12.030 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction The oxygen reduction reaction (ORR) is a pivotal process due to its sluggish kinetics in fuel cells and metal-air batteries. The normally used Platinum-based electrocatalysts for ORR suffer from inevitable problems such as low abundance, high cost, poisoning by methanol crossover, and poor stability [1]. Thus, it is crucial to explore low-cost and high-performance (nonprecious metal catalysts) for ORR [2]. In the past decades, extensive efforts have been made to designing and developing low-cost and high-performance NPMCs [3]. Nitrogen and transition metal co-modified carbon materials (M-N/C with M ¼ Fe, Co, Ni) [4e6], and metal carbides (Fe3C and MoC) [7e11] have attracted wide concerns in the recent few years. Among these potential substitutes, the FeeN/C catalyst, which may possess multiple active components (e. g. NeC, Fe-Nx, and Fe/Fe3C), is considered one of the most promising alternatives for both oxygen reduction reaction and oxygen evolution reaction (OER) [12e15]. However, the performance of FeeN/C catalysts is unsatisfactory for widespread commercial applications. Especially, the Fe3C-containing FeeN/C catalysts still suffer from the limited transfer pathway and unsatisfactory stability of Fe3C as well as the amorphous/low graphitized carbon which undergoes oxidization at inherently oxidizing potentials. To date, lots of attempts have been devoted to a detailed understanding of the catalytic mechanism of Fe3C containing FeeN/C catalysts since the active sites remain debate [16e21]. Generally, it is widely accepted that FeeN active sites play a critical role in ORR process due to the improvement of oxygen adsorption while NeC active sites can improve surface wettability as well as the conductivity of the carbon matrix. However, the exact nature of the Fe3C active sites in the FeeN/ C catalysts is still controversial until today. Early studies suggested Fe3C itself are highly active for splitting the OeO bond of molecular oxygen on its surface, i.e., the wellaccepted limiting step of ORR process [21,22]. Although some argued that NeC species and Fe3C species are inactive sites and they don’t interact with FeeN sites during the catalysis of the ORR [23,24], more researchers held that Fe3C nanocrystals could boost the ORR activity of adjacent Fe-Nx or activate the surrounding graphitic carbon [17,21,25]. More recently, the Fe/ Fe3C component of FeeFe3C@C could help the surrounding Ndoped carbon layers stabilize peroxide intermediate and promote facile 2 e 2e ORR based on experiment results [20,21]. Whatever, the prevailing belief is there exist a synergistic effect between Fe3C and Fe-Nx as well as Fe3C and carbon which is of benefit to the catalytic activity [26e32]. The synthesis of Fe3C-containing FeeN/C carbon catalysts can be achieved via pyrolyzing Fe, N, and C containing precursors [30,33]. Composition tuning, structure optimization and modulation of the environment surrounding the catalytically active sites are the main strategies for improving the catalytic performance to ORR. Noting that most of the welldeveloped Fe/NeC catalysts may contain varieties of Fe species such as Fe-Nx, Fe, Fe3C, etc., which are determined mainly by the precursor materials and the synthesis protocols [4,5,17,28,34e48]. The density of accessible active sites is closely associated with the specific surface area, pore structure as well as the dopant species distribution in the carbon
framework. Mesopores can facilitate mass transfer while micropores are favorable for the numerous accessible active sites. Thus, hierarchically porous carbons which own a balance of micro/meso porosity may favor the catalytic activity [29]. Generally, graphitization by high-temperature annealing procedure can effectively improve the electrical conductivity. However, such a procedure usually leads to low specific surface area and less active sites due to low-level N doping and the fast crystal growth and agglomeration, thus adverse to the catalytic activity. Therefore, the type, number, and accessibility of active sites and the conductivity of the carbon matrix should be considered as a whole to maximize the electrocatalytic activity [29]. Two-dimensional (2D) hierarchical graphitic carbon sheets are attractive porous materials due to the high surface area, large pore volume, and accredited conductivity [28,47,49e57]. Consequently, despite the great progress in the synthesis of Fe/N modified graphitic carbon sheets, a facile and low-cost method is still highly demanded for the fabrication of 2D carbon sheets in one step, including the in situ formation of ORR-active species. Herein, we report a facile and low-cost strategy to in situ form Fe and N doping hierarchically porous graphene-like carbon nanosheets (FeeN/C) which possess abundant Fe-Nx sites and Fe3C nanoparticles via pyrolyzing lignocellulose-rich chestnut shell precursor in the presence of iron ion and zinc ion. The synthesis strategies can modulate the physicochemical properties and the final catalytic performance. Fig. 1 illustrates the synthesis of FeeN/C-900 catalysts and its catalysis to ORR. Notably, the final carbon matrix presents different morphologies from large block to thin sheets with increasing the dosage of FeCl3. 2D porous graphene-like structure enables the materials to form a continuous electron conduction path and expose the catalytic active sites greatly. Thus, the optimal FeeN/C-2-900 catalyst exhibits superior ORR electrocatalytic activity, high stability and methanol tolerance in alkaline medium, comparable or even better than that of commercial Pt/C catalyst. The reason may be attributed to the high content of Fe-Nx active sites, the coexistence of Fe3C of together with the unique hierarchically porous 2D sheet-like structure and high conductivity of carbon matrix.
Experimental section Synthesis of FeeN/C catalysts Chestnut shell, ZnCl2, and FeCl3 were used as the carbon source, activating agent and graphitic catalyst precursor, respectively. FeCl3 also acts as a precursor of Fe3C. Chestnut shell used in this work were purchased from the market. All the chemicals were of analytical grade and used without any further purification. The FeeN/C catalysts were prepared via a facile two-step process including impregnation and step-pyrolysis similar to a previous report [57] since heat treatment under low and high temperature would result in carbon with a high degree of graphitization. In a typical process, 3 g of the dry Chestnut shell was mixed with 9 g ZnCl2 in 50 mL 3 M ferric trichloride (FeCl3) solution. Then, the mixture underwent an evaporation
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e Schematic illustration for the controllable synthesis of FeeN/C-900 electrocatalysts. step at 80 C for 2 h under stirring and then dried at 100 C. Subsequently, the activation and graphitization process of carbon precursor was carried out in a tubular furnace with a heating rate of 5 C min1 under N2 while maintained at 500 C and 900 C for 1 h, respectively. The pyrolyzed product was leached in 2 M hydrochloric acid to remove inactive iron species thoroughly, then washed to neutral and dried in a vacuum oven at 60 C overnight. The resultant sample was denoted as FeeN/C-2-900. Other control samples with different FeCl3 concentrations (FeeN/C-1-900 and FeeN/C-3900 when the concentration of FeCl3 is 2 and 4 mol/L, respectively) or pyrolyzed at low temperature (FeeN/C-2-700 and FeeN/C-2-800) were also prepared.
Physical characterizations The as-prepared samples were characterized by the scanning electron microscope (FESEM; SU8010), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images (JEOL-2100F) to obtain the morphologies and microstructure. Scanning transmission electron microscopy (STEM) and EDX elemental mapping analysis were conducted (JEOL JEMARM200CF). The thermogravimetric analysis (TGA) was tested on PerkinElmer synchronous thermal analysis instruments. The powder X-Ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 diffractometer. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) was recorded to obtained the surface chemistry of the samples with a monochromatic Al Ka radiation (hn ¼ 1486.6 eV) and the C1s peak at 284.8 eV as an internal standard. The Raman spectra were achieved on Lab-RAM HR800 with excitation by an argon ion laser (514.5 nm). The nitrogen adsorption-desorption measurements were conducted at 77 K on a TriStar 3020 surface area and porosity analyzer.
Electrocatalytic measurements The activity of the catalyst was tested on a rotating disk electrode (RDE, glassy carbon disk with 5 mm of diameter) and a rotating ring-disk electrode (RRDE with 0.196 cm2 of surface area) of Pine Company. The standard three-electrode system was adopted in 0.1 M solution with the catalyst coated on disc
electrode as working electrode, an Ag/AgCl (3 M KCl) as a reference electrode, and a platinum wire as counter electrode. The preparation method of the working electrode is as follows: the catalyst inks were obtained by mixing 5 mg of catalyst sample powders with 95 mL of 5% Nafion solution (Dupont) and 350 mL of ethanol, then the mixture treated by ultrasonication for 45 min and 7 mL of the ink was placed on an RRDE electrode. Catalyst loading of the samples or Pt/C was controlled to be 0.4 mg cm2 on RDE and 0.3 mg cm2 on RRDE. Cyclic voltammetry (CV) test was first measured in an N2saturated or O2-saturated electrolyte solution with a scan rate of 50 mV s1. The linear sweep voltammetry (LSV) were measured in the O2-saturated electrolyte with a scan rate of 5 mV s1 and with different rotating rates of 400, 625, 900, 1225, 1600, and 2025 rpm. The electron transfer number (n) can be calculated by the K-L plot using equations (1) and (2): 1 1 1 1 1 ¼ þ ¼ þ j jk jd jk Bu1=2
(1)
B ¼ 0:2nFDO2 y1=6 CO2
(2)
2=3
where j, jk, and jd are the measured disk current, the kinetic limiting current and the diffusion limiting current, respectively. u is the electrode rotation rate in rpms. The values of F DO2 , CO2 , y are 96485 C mol1, 1.86 105 cm2 s1, 1.21 106 mol cm3 and 0.01 cm2 s1, respectively [41]. A durability test was performed by chronoamperometric technique at 0.3 V for 10000 s with a rotation speed of 1600 rpm. The durability test was also evaluated using the accelerated durability test protocol (the US Department of Energy) by cycling the catalysts between 0.8 and 0.1 V under O2 atmosphere. Methanol durability test was measured by CV in O2saturated electrolyte with or without 1 M CH3OH. The Electrochemical Impedance Spectroscopy (EIS) measurements were conducted at 0 V vs Ag/AgCl reference electrode.
Results and discussion Pyrolyzing cheap and abundant biomass waste was a common strategy to prepare porous carbon [22,57e59]. Previous
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
4
international journal of hydrogen energy xxx (xxxx) xxx
studies displayed that the heat-treatment strategy has a vital effect in the crystal phase as well as the morphology of pyrolyzed Fe-containing carbon-based catalyst, because Fe may exist in different crystal phases such as Fe3C, Fe3N, Fe and iron oxide. Therefore, we first examined the crystal structure of FeeN/C samples pyrolyzed in the range of 700e900 C. The XRD data in Fig. S1 only show the characteristic of Fe3O4 which is believed an inactive species to ORR, and a weak peak at 26 indicating a low graphitization degree when pyrolyzing below 900 C for FeeN/C-2-700 and FeeN/C-2-800. Thus, a low ORR activity was achieved featuring of the negative onset and halfwave potentials and low diffusion-limiting current revealed by LSV tests (Fig. S2). While FeeN/C-2-900 shows the presence of Fe3C phase which is believed could boost the ORR activity. FeeN/C catalysts prepared at 900 C was further optimized with different dosage of FeCl3 to study the relation between electrical conductivity, porosity, and active site density to ensure the material has both activity and conductivity. SEM and TEM analysis revealed that FeeN/C catalysts synthesized at 900 C shared different morphologies but the same porous structure (Fig. S3-5 and Figs. 2 and 3). Notably, the typical FeeN/C-2-900 catalyst, whose precursor contained a middle FeCl3 dosage, mainly features graphene-like carbon sheets (Fig. S3 and Fig. 2a). The highly-graphitized domains can be easily identified with a continuous interlayer distance of 0.337 nm which is well consistent with the d-spacing of (002) crystallographic plane of graphite as shown in Fig. 2b. Fig. 2b also reveals the existence of dispersed Fe3C nanocrystals marked by red circles with an interlayer distance of 0.201 nm assigned to the (031) plane. It should be pointed out that some large agminated Fe3C particles with the size of about 50 nm can also be found (Fig. 2c) in less graphitic domains. HRTEM images further reveal the presence of small Fe3C nanocrystals (Fig. 2d) with the 0.201 nm interlayer distance and pores less than 5 nm marked by red circles (Fig. 2f). The corresponding SAED pattern confirmed the above analysis with the diffraction ring of (031) plane of Fe3C and (002) plane of graphite (Fig. 2e). A high-angle annular dark field (HAADF) and the scanning transmission electron microscopy (STEM) analysis was further conducted to reveal the status of iron and nitrogen in the catalyst (Fig. 2g and h). Fig. 2g and h verified the uniform distribution of carbon, nitrogen, and
dispersed iron for the highly-graphitized domains and less graphitic domains, respectively. The homogeneous distribution of Fe and N in the porous carbon frameworks may suggest the presence of Fe related species (Fe3C and Fe-Nx), which are believed as the catalytically active sites in FeeN/C catalysts. A decrease of FeCl3 amount leads to the generation of a thick block structure (FeeN/C-1-900), as shown in Fig. 3a and Fig. S3. Only a few Fe3C particles in amorphous domains can be found (Fig. 3b). While increasing the amount of FeCl3 (FeeN/C-3-900), only highly graphitic “graphene-like” nanosheets emerged, which is thinner than FeeN/C-2-900 as revealed by Fig. 3c and Fig. S5 and S6. It should be pointed out that no clear large nanoparticles can be found in TEM observation. As seen in Fig. 3d, a lattice distance of 0.337 nm is indexed to the (002) plane of graphite. The local highly distributed Fe3C nanocrystals marked in white circles with a lattice distance of 0.168 nm assigned to the (001) plane of Fe3C, are visible in the HRTEM image (Fig. 3e, enlarged part B in Fig. 3c). The inset of Fig. 3e shows the FFT image indexing the planar distance of (004) plane of Fe3C. The corresponding SAED pattern also shows the diffraction ring of (004) plane of Fe3C and (002) plane of graphite (Fig. 3f). The crystal phase of the FeeN/C-900-2 catalyst was further explored by XRD survey (Fig. 4a). Crystalline Fe3C (JCPDS file No. 35-0071) and amorphous carbon, are discerned according to the diffraction patterns, consistent with the results of HRTEM and SAED [17]. As for FeeN/C-1-900 whose FeCl3 dosage is the smallest, there are only two broad peaks assigned to amorphous carbon and no obvious peaks for Fe3C, suggesting a low crystallinity. While a sharp peak at 26 assigned to the (002) facets of graphite carbon for FeeN/C-3900 appears, indicating enhanced graphitization because of the largest FeCl3 dosage. Thus, the FeCl3 dosage not only affects the degree of graphitization indexed by the (002) facet of graphite carbon but also affects the crystal phase and intensity of Fe3C. Precisely tuning the FeCl3 content to manipulate the surface area, pore architecture, and graphitization degree of the catalysts is of pivotal importance for the ORR activity. The assynthesized three samples all features a hierarchical porous structure, possessing large specific surface area and high pore volume measured by N2 adsorption-desorption analysis
Fig. 2 e TEM (a, c) and HRTEM (b, d, f), magnified HAADF-STEM (g, h) images and element mapping of different area for FeeN/ C-2-900. (a, b, g) for highly-graphitized domains. (c, d, f, h) for less graphitic domains. (e) Corresponding SAED pattern of (c). Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 3 e TEM images of FeeN/C-1-900 (a) and FeeN/C-3-900 (c). HRTEM images of FeeN/C-1-900 (b) and FeeN/C-3-900 (d, e). The inset of (e) shows the FFT image. (f) Corresponding SAED pattern of (d).
Fig. 4 e XRD patterns (a), nitrogen sorption isotherms (b), the corresponding PSD curves and Raman spectra (d) for FeeN/C900 catalysts. The high-resolution spectra of Fe 2p (e) and N 1s (f) of FeeN/C-900 catalysts. (g) Absolute content of different nitrogen species of FeeN/C-900 catalysts.
(Fig. 4b and c). Detailed information is shown in Table S1. Especially, FeeN/C-2-900 owns the largest mesoporosity with a mesopore volume of 1.10 m3 g1 and the second largest specific surface area of 1536 m2 g1 among them, thus was
expected to display excellent ORR performance [60,61]. With the presence of Fe components, the graphitization of carbon source can be catalyzed, and meanwhile, a carburized phase of the Fe formed during the heating process, and the
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
6
international journal of hydrogen energy xxx (xxxx) xxx
formation-decomposition process of the carburized phase can result in the formation of graphene-like nanosheets [17,57]. Moreover, only the Fe content is enough, graphene-like carbon sheets can be created. The larger dosage of the Fe components, the smaller thickness and the higher graphitization of the carbon sheets features. Namely, the Fe content not only affects the degree of graphitization as displayed by XRD patterns but also makes a critical difference in the morphology of carbon matrix discerned by SEM and TEM images. Fig. 4d shows the Raman spectra of the as-prepared FeeN/ C catalysts. Two intense peaks can be observed at 1357 and 1582 cm1 corresponding to the D (disorders or defects in carbon graphitized structure) band and G (well-graphitized carbon) band, respectively. The values of ID/IG for FeeN/C-1900, FeeN/C-2-900, and FeeN/C-3-900 are 1.60, 1.45, and 1.64, respectively, indicate that the high pyrolysis temperature develops rich defect on its surface which can be attributed to the atomic doping of nitrogen and iron into the carbon matrix. Furthermore, the positive shift of G peak position can also be ascribed to the defect sites and structural distortion when increasing Fe dosage [62]. The narrowing of the FWHM of G peak indicates a higher degree of graphitization with increasing Fe dosage, as also demonstrated by XRD. X-ray photoelectron spectroscopy (XPS) was conducted to determine the surface chemistry of the FeeN/C-900 catalysts, as shown in Fig. 4e, f, Fig. S7, and Table S1. The survey XPS spectra reveal the presence of C, O, N, and Fe, suggesting that Fe and N were successfully incorporated into the carbon surface. The content of Fe elements detected in the three catalysts is low (1.7 wt% for FeeN/C-2-900, 2.6 wt% for FeeN/C-3900, and even too low to be detected as for FeeN/C-1-900) (Table S2). The amount of Fe on the sample surface is less than the value test by TG (1.37, 2.03 and 2.61 wt% for FeeN/C-1-900, FeeN/C-2-900, and FeeN/C-3-900 respectively) (Fig. S8). The high-resolution spectra of Fe 2p in Fig. 4e shows two major peaks at 712.8 and 725.3 eV corresponding to Fe 2p3 and Fe 2p1 respectively. There is also a weak peak due to a shakeup satellite peaks of Fe 2p3. The other peak at z710.5 eV can be assigned to FeeN. The undiscernible FeeC peak implied that a large amount of FeeN structure exists on the surface of Fe3C nanoparticles [35,63]. The nitrogen content in the catalysts is 1.91 atom % for FeeN/C-1-900, 2.66 atom % for FeeN/C-2-900, and 2.12 atom % for FeeN/C-3-900, respectively (Table S3). The high-resolution N 1s spectra for the typical FeeN/C-2-900 were further deconvoluted into five peaks corresponding to pyridinic N, FeNx, pyrrolic N, graphitic N and oxidized N [14,64], while FeeN/ C-1-900 lack of Fe-Nx and FeeN/C-3-900 lack of both pyrrolic N and pyridinic N. (Fig. 4f). The relative composition of the N species calculated on the basis of total nitrogen content and its fraction in the catalyst was compared and displayed in Fig. 4g. The larger concentration of FeCl3, the higher content of graphic N and Fe-Nx as revealed by the high-resolution N 1s and Fe 2p spectra (Fig. 4 and Table S2, 3), which indicates that the chemical states of derived materials were strongly affected by the concentration of FeCl3. In particular, FeeN/C2-900 has the highest content of Fe-Nx and pyridinic N, and the second-high content of pyrrolic N and graphic N, which are expected to show a promising ORR activity since Fe-Nx sites
are believed to be mainly responsible for the high catalytic activity. The catalytic activity for ORR of the FeeN/C-900 catalysts was first assessed on a rotating disk electrode (RDE) in O2saturated 0.1 M KOH solution. FeeN/C-2-900 shows the most positive reduction peak among the three FeeN/C-900 catalysts revealed by Cyclic voltammetry (CV) curves in O2- or N2saturated 0.1 M KOH (Fig. S9). The peak potential of FeeN/C-2900 is nearly the same as the commercial Pt/C (20%, Johnson Matthey), suggesting high ORR activity. Fig. 5a shows the linear-sweep voltammogram (LSV) curves of the three catalysts in O2-saturated 0.1 M KOH solution at 1600 rpm. FeeN/C2-900 has the best catalytic activity with the most positive E1/2 of 0.122 V (vs. Ag/AgCl (3 M)) in the LSV curves. In contrast, FeeN/C-1-900 and FeeN/C-3-900 exhibit the degraded ORR activity with an E1/2 of 0.148 V and 0.157 V, respectively. The three samples all delivered a diffusion-limiting current about 5 mA cm2 at 0.6 V and the electron transfer approaching the ideal value of 4.0 obtained from the slopes of K-L plots, as shown in Fig. S10, 11 and Table S4. Fig. 5b also reveals that the catalytic activity of FeeN/C-2-900 is comparable to Pt/C. The RRDE measurements further corroborate that FeeN/C-2-900 shows superior activity for ORR in terms of the low yield of H2O2 and high electron transfer number (Fig. 5c). To gain more information on the catalytic performance, impedance spectra were recorded at 0 V vs Ag/AgCl reference electrode (Fig. S12). The three FeeN/C-900 catalysts show similar shape of the impedance spectra featuring the depressed semi-circles at the high-frequency regions and a straight line in the low frequency at low-frequency regions. There is no obvious difference between the diameters of the three semi-circles. This imply that three samples have similar ohmic resistance (including solution resistance and film resistance) and charge transfer resistance (indicative of the semicircle diameter). While the diffusion resistance (indicative of the slope of the line) of FeeN/C-2-900 is the smallest among them due to its the largest mesoporosity and the second largest specific surface area, which is favorable to the mass transfer. The EIS results further illustrate that the accessibility of active sites is vital to maximize the electrocatalytic activity. Also, it is well-known that for an electrochemical reaction, the reaction should exhibit a low Tafel slope in order to achieve a high current at low overpotential. As shown in Fig. 5d, the Tafel slope of FeeN/C-2-900 (~99.76 mV/dec) is very close to that of Pt/C (~ 96.17 mV/dec). This means that the rate-determining step is probably the transfer of the first electron in ORR catalyzed by FeeN/C-2-900, similar to platinum [21]. The ORR stability and methanol tolerance electrocatalysts with chronoamperometric (it) measurements in O2-saturated 0.1 M KOH. A higher performance attenuation than Pt/C electrocatalyst was observed for FeeN/C-2-900 (Fig. 5e). An accelerated durability test was also done to assess the durability of FeeN/C-2-900 by cycling the catalysts between 0.8 and 0.1 V under O2 atmosphere. After 4500 continuous cycles, a negative shift of the E1/2 on FeeN/C-2-900 by ~ 6 mV at 5 mV s1 (inset of Fig. 5f) and ~8 mV at 50 mV s1 (Fig. S13) appeared. As can be seen in Fig. 5f, there is a slight performance degradation for FeeN/C-2-900 after the injection of 3 M
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 5 e (a) Linear sweep voltammogram (LSV) curves of FeeN/C-900 catalysts. (b) LSV curves of FeeN/C-2-900 and Pt/C. (c) The electron transfer number and the percentage of peroxide of FeeN/C-2-900 and Pt/C at various potentials based on the RRDE results. (d) Tafel slope of FeeN/C-2-900 and Pt/C. (e) Stability test for FeeN/C-2-900 and Pt/C in O2-saturated 0.1 M KOH solution at 0.7 V with a rotation rate of 1600 rpm. Inset: endurance test of FeeN/C-2-900 for 4500 cycles in O2-saturated 0.1 M KOH with a scan rate of 5 mV s¡1. (f) Chronoamperometric response of methanol at FeeN/C-2-900 and Pt/C electrocatalyst at 1600 rpm upon the injection of methanol after 200 s into the O2-saturated 0.1 M KOH solution. methanol into the electrolyte at 200 s, exhibiting a much better fuel selectivity than the Pt/C catalyst. The excellent ORR performance of FeeN/C-2-900 can be ascribed as follows: Firstly, FeeN/C-2-900 has the highest amount of Fe-Nx and pyridinic active sites coexistence with Fe3C nanocrystals (Fig. 6). Secondly, FeeN/C-2-900 displays a high mesoporosity with a large specific surface area (1536 m2 g1), which can offer better mass diffusion and provide more accessible catalytically active sites compared to other FeeN/C samples. Besides, FeeN/C-2-900 owns decent electrical conductivity. Hence the optimum ORR activity of FeeN/C-2-900 was attributed to the synergistic effect of the high content of Fe-Nx, pyridinic-N active sites and the existence of Fe3C nanoparticles in the highly conductive and hierarchically porous carbon sheets. Namely, FeeN/C-2-900 owns the best equilibrium degree of graphitization and the density of the active sites. The ORR performance of FeeN/C-2-
Fig. 6 e Illustration of the role of Fe-Nx and Fe3C Sites for the ORR.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
8
international journal of hydrogen energy xxx (xxxx) xxx
900 is comparable with previously reported Fe3C-containing Fe, N doped nonprecious catalysts, as shown in Table S5. [7]
Conclusions Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically porous carbon nanosheets have been facilely prepared via a cost-effective pyrolysis method using the chestnut shell, ZnCl2, and FeCl3 as precursors. The optimal FeeN/C-2-900 catalyst possesses superior ORR catalytic activity, high stability, and methanol tolerance in an alkaline medium than 20 wt% Pt/C. The half-wave potential is 39 mV more positive than that of Pt/C catalyst. The synergistic effect of the high content of Fe-Nx active sites and the coexistence of Fe3C in the unique hierarchically porous carbon matrix with high conductivity is responsible for the excellent catalytic performance. This report offers a facile approach to synthesize Fe3C nanoparticles decorated Fe/N doped graphene-like hierarchically carbon nanosheets for effective nonprecious metal oxygen electrocatalyst.
Acknowledgments This work was financially supported by the Scientific and Technologies Research Program of Henan Province (Grant No. 182102310827), Natural Science Foundation of Henan Province (Grant No. 162300410170), and Fundamental Research Funds for the Henan Normal University (Grant No. 2017PL07).
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.030.
[15]
references [16] [1] Nie Y, Li L, Wei Z. Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction. Chem Soc Rev 2015;44:2168e201. https://doi.org/10.1039/C4CS00484A. [2] Xia W, Mahmood A, Liang Z, Zou R, Guo S. Earth-abundant nanomaterials for oxygen reduction. Angew Chem Int Ed 2016;55:2650e76. https://doi.org/10.1002/anie.201504830. [3] Gewirth AA, Varnell JA, DiAscro AM. Nonprecious metal catalysts for oxygen reduction in heterogeneous aqueous systems. Chem Rev 2018;118:2313e39. https://doi.org/ 10.1021/acs.chemrev.7b00335. [4] Fan X, Peng Z, Ye R, Zhou H, Guo X. M3C (M: Fe, Co, Ni) nanocrystals encased in graphene nanoribbons: an active and stable bifunctional electrocatalyst for oxygen reduction and hydrogen evolution reactions. ACS Nano 2015;9:7407e18. https://doi.org/10.1021/acsnano.5b02420. [5] Wang M-Q, Ye C, Wang M, Li T-H, Yu Y-N, Bao S-J. Synthesis of M (Fe3C, Co, Ni)-porous carbon frameworks as highefficient ORR catalysts. Energy Storage Mater 2018;11:112e7. https://doi.org/10.1016/j.ensm.2017.10.003. [6] Sun T, Tian B, Lu J, Su C. Recent advances in Fe (or Co)/N/C electrocatalysts for the oxygen reduction reaction in polymer
[17]
[18]
[19]
[20]
electrolyte membrane fuel cells. J Mater Chem 2017;5:18933e50. https://doi.org/10.1039/C7TA04915C. Kong A, Zhang Y, Chen Z, Chen A, Li C, Wang H, et al. Onepot synthesized covalent porphyrin polymer-derived coreshell Fe3C@carbon for efficient oxygen electroreduction. Carbon 2017;116:606e14. https://doi.org/10.1016/ j.carbon.2017.02.046. Li J, Mao S, Hou Y, Lei L, Yuan C. 3D edge-enriched Fe3C@C nanocrystals with a coreeshell structure grown on reduced graphene oxide networks for efficient oxygen reduction reaction. ChemSusChem 2018;11:3292e8. https://doi:10.1002/ cssc.201801084. Ma J, Xiao D, Chen CL, Luo Q, Yu Y, Zhou J, et al. Uric acidderived Fe3C-containing mesoporous Fe/N/C composite with high activity for oxygen reduction reaction in alkaline medium. J Power Sources 2018;378:491e8. https://doi.org/ 10.1016/j.jpowsour.2017.11.091. Yang H, Liu J, Wang J, Poh CK, Zhou W, Lin J, et al. Electrocatalytically active graphene supported MMo carbides (MNi, Co) for oxygen reduction reaction. Electrochim Acta 2016;216:246e52. https://doi.org/10.1016/ j.electacta.2016.09.023. Yan Z, He G, Shen PK, Luo Z, Xie J, Chen M. MoCegraphite composite as a Pt electrocatalyst support for highly active methanol oxidation and oxygen reduction reaction. J Mater Chem 2014;2:4014e22. https://doi.org/10.1039/C3TA14251E. Jiang H, Yao Y, Zhu Y, Liu Y, Su Y, Yang X, et al. Iron carbide nanoparticles encapsulated in mesoporous FeeN-doped graphene-like carbon hybrids as efficient bifunctional oxygen electrocatalysts. ACS Appl Mater Interfaces 2015;7:21511e20. https://doi.org/10.1021/acsami.5b06708. Han S, Hu X, Wang J, Fang X, Zhu Y. Novel route to Fe-based cathode as an efficient bifunctional catalysts for rechargeable Zneair battery. Adv Energy Mater 2018;8:1800955. https://doi.org/10.1002/aenm.201800955. Ahn SH, Yu X, Manthiram A. “Wiring” Fe-Nx-Embedded porous carbon framework onto 1D nanotubes for efficient oxygen reduction reaction in alkaline and acidic media. Adv Mater 2017;29:1606534. https://doi.org/10.1002/ adma.201606534. Li J-S, Li S-L, Tang Y-J, Han M, Dai Z-H, Bao J-C, et al. Nitrogen-doped Fe/Fe3C@graphitic layer/carbon nanotube hybrids derived from MOFs: efficient bifunctional electrocatalysts for ORR and OER. Chem Commun 2015;51:2710e3. https://doi.org/10.1039/C4CC09062D. Sun T, Jiang Y, Wu Q, Du L, Zhang Z, Yang L, et al. Is iron nitride or carbide highly active for oxygen reduction reaction in acidic medium? Catal Sci Technol 2017;7:51e5. https:// doi.org/10.1039/C6CY01921H. Jiang W-J, Gu L, Li L, Zhang Y, Zhang X, Zhang L-J, et al. Understanding the high activity of FeeNeC electrocatalysts in oxygen reduction: Fe/Fe3C nanoparticles boost the activity of Feenx. J Am Chem Soc 2016;138:3570e8. https://doi.org/ 10.1021/jacs.6b00757. Fajrial AK, Saputro AG, Agusta MK, Rusydi F, Nugraha, Dipojono HK. First principles study of oxygen molecule interaction with the graphitic active sites of a boron-doped pyrolyzed FeeNeC catalyst. Phys Chem Chem Phys 2017;19:23497e504. https://doi.org/10.1039/C7CP02390A. Kattel S, Atanassov P, Kiefer B. A density functional theory study of oxygen reduction reaction on non-PGM Fe-Nx-C electrocatalysts. Phys Chem Chem Phys 2014;16:13800e6. https://doi.org/10.1039/C4CP01634C. Kim JH, Sa YJ, Jeong HY, Joo SH. Roles of FeNx and FeFe3C@C species in FeN/C electrocatalysts for oxygen reduction reaction. ACS Appl Mater Interfaces 2017;9:9567e75. https://doi.org/10.1021/acsami.6b13417.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
[21] Hu Y, Jensen JO, Zhang W, Cleemann LN, Xing W, Bjerrum NJ, et al. Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts. Angew Chem Int Ed 2014;53:3675e9. https://doi.org/10.1002/ anie.201400358. [22] Ma M, You S, Wang W, Liu G, Qi D, Chen X, et al. Biomassderived porous Fe3C/tungsten carbide/graphitic carbon nanocomposite for efficient electrocatalysis of oxygen reduction. ACS Appl Mater Interfaces 2016;8:32307e16. https://doi.org/10.1021/acsami.6b10804. [23] Zhao P, Xu W, Hua X, Luo W, Chen S, Cheng G. Facile synthesis of a N-doped Fe3C@CNT/porous carbon hybrid for an advanced oxygen reduction and water oxidation electrocatalyst. J Phys Chem C 2016;120:11006e13. https:// doi.org/10.1021/acs.jpcc.6b03070. [24] Qian Y, Du P, Wu P, Cai C, Gervasio DF. Chemical nature of catalytic active sites for the oxygen reduction reaction on nitrogen-doped carbon-supported non-noble metal catalysts. J Phys Chem C 2016;120:9884e96. https://doi.org/ 10.1021/acs.jpcc.6b02670. [25] Varnell JA, Tse ECM, Schulz CE, Fister TT, Haasch RT, Timoshenko J, et al. Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts. Nat Commun 2016;7:12582. https://doi.org/10.1038/ncomms12582. [26] Kong F, Fan X, Kong A, Zhou Z, Zhang X, Shan Y. Covalent phenanthroline framework derived FeS@Fe3C composite nanoparticles embedding in N-S-codoped carbons as highly efficient trifunctional electrocatalysts. Adv Funct Mater 2018;28:1803973. https://doi.org/10.1002/adfm.201803973. [27] Deng Y, Wang G, Sun K, Chi B, Shi X, Dong Y, et al. Highly effective and stable doped carbon catalyst with threedimensional porous structure and well-covered Fe3C nanoparticles prepared with C3N4 and tannic acid as template/precursors. J Power Sources 2019;417:117e24. https://doi.org/10.1016/j.jpowsour.2019.02.022. [28] Liu Y, Huang B, Zhang X, Huang X, Xie Z. In-situ fabrication of nitrogen-doped carbon nanosheets containing highly dispersed single iron atoms for oxygen reduction reaction. J Power Sources 2019;412:125e33. https://doi.org/10.1016/ j.jpowsour.2018.11.024. [29] Tan H, Tang J, Kim J, Kaneti YV, Kang Y-M, Sugahara Y, et al. Rational design and construction of nanoporous iron- and nitrogen-doped carbon electrocatalysts for oxygen reduction reaction. J Mater Chem 2019;7:1380e93. https://doi.org/ 10.1039/C8TA08870E. [30] Liu K, Peng Z, Wang H, Ren Y, Liu D, Li J, et al. Fe3C@Fe/N doped graphene-like carbon sheets as a highly efficient catalyst in Al-air batteries. J Electrochem Soc 2017;164:475e83. https://doi.org/10.1149/2.0171706jes. [31] Guo D, Han S, Wang J, Zhu Y. MIL-100-Fe derived N-doped Fe/ Fe3C@C electrocatalysts for efficient oxygen reduction reaction. Appl Surf Sci 2018;434:1266e73. https://doi.org/ 10.1016/j.apsusc.2017.11.230. [32] Wei L, Sun H, Yang T, Deng S, Wu M, Li Z. Iron carbide encapsulated by porous carbon nitride as bifunctional electrocatalysts for oxygen reduction and evolution reactions. Appl Surf Sci 2018;439:439e46. https://doi.org/ 10.1016/j.apsusc.2018.01.056. [33] Wei J, Liang Y, Hu Y, Kong B, Simon GP, Zhang J, et al. A versatile ironetannin-framework ink coating strategy to fabricate biomass-derived iron carbide/Fe-N-carbon catalysts for efficient oxygen reduction. Angew Chem Int Ed 2016;55:1355e9. https://doi.org/10.1002/anie.201509024. [34] Zitolo A, Goellner V, Armel V, Sougrati M-T, Mineva T, Stievano L, et al. Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nat Mater 2015;14:937. https://doi.org/10.1038/nmat4367.
9
[35] Li G, Zhang J, Li W, Fan K, Xu C. 3D interconnected hierarchical porous N-doped carbon constructed by flakelike nanostructure with Fe/Fe3C for efficient oxygen reduction reaction and supercapacitor. Nanoscale 2018;10:9252e60. https://doi.org/10.1039/C8NR02337A. [36] Yang W, Liu X, Yue X, Jia J, Guo S. Bamboo-like carbon nanotube/Fe3C nanoparticle hybrids and their highly efficient catalysis for oxygen reduction. J Am Chem Soc 2015;137:1436e9. https://doi.org/10.1021/ja5129132. [37] Sa YJ, Seo D-J, Woo J, Lim JT, Cheon JY, Yang SY, et al. A general approach to preferential formation of active Feenx sites in FeeN/C electrocatalysts for efficient oxygen reduction reaction. J Am Chem Soc 2016;138:15046e56. https://doi.org/10.1021/jacs.6b09470. [38] Liang H-W, Wei W, Wu Z-S, Feng X, Mu¨llen K. Mesoporous metalenitrogen-doped carbon electrocatalysts for highly efficient oxygen reduction reaction. J Am Chem Soc 2013;135:16002e5. https://doi.org/10.1021/ja407552k. [39] Zhu Y, Zhang B, Liu X, Wang D-W, Su DS. Unravelling the structure of electrocatalytically active FeeN complexes in carbon for the oxygen reduction reaction. Angew Chem Int Ed 2014;126:10849e53. https://doi.org/10.1002/ ange.201405314. [40] Wu Z-Y, Xu X-X, Hu B-C, Liang H-W, Lin Y, Chen L-F, et al. Iron carbide nanoparticles encapsulated in mesoporous FeN-doped carbon nanofibers for efficient electrocatalysis. Angew Chem Int Ed 2015;127:8297e301. https://doi.org/ 10.1002/ange.201502173. [41] Xiao M, Zhu J, Feng L, Liu C, Xing W. Meso/macroporous nitrogen-doped carbon architectures with iron carbide encapsulated in graphitic layers as an efficient and robust catalyst for the oxygen reduction reaction in both acidic and alkaline solutions. Adv Mater 2015;27:2521e7. https://doi.org/ 10.1002/adma.201500262. [42] Yasuda S, Furuya A, Uchibori Y, Kim J, Murakoshi K. Ironenitrogen-doped vertically aligned carbon nanotube electrocatalyst for the oxygen reduction reaction. Adv Funct Mater 2016;26:738e44. https://doi.org/10.1002/ adfm.201503613. [43] Cui X, Yang S, Yan X, Leng J, Shuang S, Ajayan PM, et al. Pyridinic-nitrogen-dominated graphene aerogels with FeeNeC coordination for highly efficient oxygen reduction reaction. Adv Funct Mater 2016;26:5708e17. https://doi.org/ 10.1002/adfm.201601492. [44] Hu E, Yu X-Y, Chen F, Wu Y, Hu Y, Lou XW. Graphene layerswrapped Fe/Fe5C2 nanoparticles supported on N-doped graphene nanosheets for highly efficient oxygen reduction. Adv Energy Mater 2018;8:1702476. https://doi.org/10.1002/ aenm.201702476. [45] Ren H, Wang Y, Yang Y, Tang X, Peng Y, Peng H, et al. Fe/N/C nanotubes with atomic Fe sites: a highly active cathode catalyst for alkaline polymer electrolyte fuel cells. ACS Catal 2017;7:6485e92. https://doi.org/10.1021/acscatal.7b02340. [46] Zhang Y, Jiang W-J, Guo L, Zhang X, Hu J-S, Wei Z, et al. Confining iron carbide nanocrystals inside CNx@CNT toward an efficient electrocatalyst for oxygen reduction reaction. ACS Appl Mater Interfaces 2015;7:11508e15. https://doi.org/ 10.1021/acsami.5b02467. [47] Song A, Cao L, Yang W, Li Y, Qin X, Shao G. Uniform multilayer graphene-coated iron and iron-carbide as oxygen reduction catalyst. ACS Sustainable Chem Eng 2018;6:4890e8. https://doi.org/10.1021/ acssuschemeng.7b04319. [48] Zhou T, Ma R, Zhang T, Li Z, Yang M, Liu Q, et al. Increased activity of nitrogen-doped graphene-like carbon sheets modified by iron doping for oxygen reduction. J Colloid Interface Sci 2019;536:42e52. https://doi.org/10.1016/ j.jcis.2018.10.021.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
10
international journal of hydrogen energy xxx (xxxx) xxx
[49] Gu D, Zhou Y, Ma R, Wang F, Liu Q, Wang J. Facile synthesis of N-doped graphene-like carbon nanoflakes as efficient and stable electrocatalysts for the oxygen reduction reaction. Nano-Micro Lett 2017;10:29. https://doi.org/10.1007/s40820017-0181-1. [50] Yuan HR, Zhang X, Yan F, Zhang S, Zhu CL, Li CY, et al. Nitrogen-doped carbon nanosheets containing Fe3C nanoparticles encapsulated in nitrogen-doped graphene shells for high-performance electromagnetic wave absorbing materials. Carbon 2018;140:368e76. https://doi.org/10.1016/ j.carbon.2018.08.073. [51] Chen L, Xiao J, Liu B, Yi T. A bonded double-doped graphene nanoribbon framework for advanced electrocatalysis. ACS Appl Mater Interfaces 2016;8:16649e55. https://doi.org/ 10.1021/acsami.6b02522. [52] Tian H, Wang N, Xu FG, Zhang PF, Hou D, Mai YY, et al. Nitrogen-doped carbon nanosheets and nanoflowers with holey mesopores for efficient oxygen reduction catalysis. J Mater Chem 2018;6:10354e60. https://doi.org/10.1039/ c8ta02319k. [53] Li L, Song L, Guo H, Xia W, Jiang C, Gao B, et al. N-Doped porous carbon nanosheets decorated with graphitized carbon layer encapsulated Co9S8 nanoparticles: an efficient bifunctional electrocatalyst for the OER and ORR. Nanoscale 2019;11:901e7. https://doi.org/10.1039/c8nr07179a. [54] Wang Y, Pan Y, Zhu L, Yu H, Duan B, Wang R, et al. Solventfree assembly of Co/Fe-containing MOFs derived N-doped mesoporous carbon nanosheets for ORR and HER. Carbon 2019;146:671e9. https://doi.org/10.1016/j.carbon.2019.02.002. [55] Wu Q, Wang Z, Li Y, Wang Z, Liang Y, Hu Y, et al. Nitrogenrich, well-dispersed nanoporous carbon materials for superefficient oxygen reduction reaction. Chemelectrochem 2019;6:1894e900. https://doi.org/10.1002/celc.201900198. [56] Zhou N, Li L, Chen S, Peng X, Niu W, Qu Y. Mesoporous, nitrogen-doped, graphitized carbon nanosheets embedded with cobalt nanoparticles for efficient oxygen electroreduction. J Mater Sci 2019;54:4168e79. https://doi.org/ 10.1007/s10853-018-3121-7.
[57] Sun L, Tian C, Li M, Meng X, Wang L, Wang R, et al. From coconut shell to porous graphene-like nanosheets for highpower supercapacitors. J Mater Chem 2013;1:6462e70. https://doi.org/10.1039/C3TA10897J. [58] Balahmar N, Mitchell AC, Mokaya R. Generalized mechanochemical synthesis of biomass-derived sustainable carbons for high performance CO2 storage. Adv Energy Mater 2015;5:1500867. https://doi.org/10.1002/aenm.201500867. [59] Borghei M, Lehtonen J, Liu L, Rojas OJ. Advanced biomassderived electrocatalysts for the oxygen reduction reaction. Adv Mater 2018;30:1703691. https://doi.org/10.1002/ adma.201703691. [60] Mao X, Cao Z, Yin Y, Wang Z, Dong H, Yang S. Direct synthesis of nitrogen and phosphorus co-doped hierarchical porous carbon networks with biological materials as efficient electrocatalysts for oxygen reduction reaction. Int J Hydrogen Energy 2018;43:10341e50. https://doi.org/10.1016/ j.ijhydene.2018.04.100. [61] Mao X, Cao Z, Chen S, Jia J, Li X, Yin Y, et al. Facile synthesis of N, P-doped hierarchical porous carbon framework catalysts based on gelatin/phytic acid supermolecules for electrocatalytic oxygen reduction. Int J Hydrogen Energy 2019;44:5890e8. https://doi.org/10.1016/ j.ijhydene.2019.01.044. [62] Ferrari AC. Raman spectroscopy of graphene and graphite: disorder, electronephonon coupling, doping and nonadiabatic effects. Solid State Commun 2007;143:47e57. https://doi.org/10.1016/j.ssc.2007.03.052. [63] Huang H, Feng X, Du C, Wu S, Song W. One-step pyrolytic synthesis of small iron carbide nanoparticles/3D porous nitrogen-rich graphene for efficient electrocatalysis. J Mater Chem 2015;3:4976e82. https://doi.org/10.1039/C4TA06742H. [64] Wang M-Q, Yang W-H, Wang H-H, Chen C, Zhou Z-Y, Sun SG. Pyrolyzed FeeNeC composite as an efficient non-precious metal catalyst for oxygen reduction reaction in acidic medium. ACS Catal 2014;4:3928e36. https://doi.org/10.1021/ cs500673k.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis Zhaoxia Cao a,b,*, Jingyi Jia a,b, Haohan Li a,b, Yuhe Wang a,b, Xinxin Mao a,b, Zhennan Zhang a,b, Mingguo Yang a,b, Shuting Yang a,b,** a
National & Local Engineering Laboratory for Motive Power and Key Materials, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China b Collaborative Innovation Center of Henan Province for Green Motive Power and Key Materials, Henan Normal University, Xinxiang, Henan, 453007, PR China
highlights Hierarchically porous graphene-like carbon nanosheets were successfully synthesized. The highly graphitized carbon nanosheets are abundant in Fe related active sites (Fe-Nx and Fe3C). The optimal sample can act as an effective oxygen electrocatalyst for ORR.
article info
abstract
Article history:
Hierarchically porous carbon sheets decorated with transition metal carbides nano-
Received 10 September 2019
particles and metal-nitrogen coordinative sites have been proposed as the promising non-
Received in revised form
precious metal oxygen electrocatalysts. In this work, we demonstrate a facile and low-cost
13 November 2019
strategy to in situ form Fe/N codoped hierarchically porous graphene-like carbon nano-
Accepted 5 December 2019
sheets abundant in Fe-Nx sites and Fe3C nanoparticles (FeeN/C) from pyrolyzing chestnut
Available online xxx
shell precursor. The as-prepared FeeN/C samples with abundant Fe-Nx sites and Fe3C nanoparticles show superior electrocatalytic activity to oxygen reduction reaction (ORR) in
Keywords:
the alkaline medium as well as high stability and methanol tolerance due to the integration
Fe/N codoping
of multi-factors: the high content of Fe-Nx active sites, the coexistence of Fe3C, the unique
Fe3C nanoparticle
hierarchically porous structure and high conductivity of carbon matrix. The optimal FeeN/
Graphene-like carbon nanosheets
C-2-900 sample exhibits a more positive half-wave potential (0.122 V vs. Ag/AgCl (3 M)
Hierarchically porous
reference electrode) than commercial 20 wt% Pt/C catalyst. This study provides a facile
Oxygen electrocatalyst
approach to synthesize Fe3C nanoparticles decorated Fe/N co-doped hierarchically porous carbon materials for effective oxygen electrocatalyst. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. National & Local Engineering Laboratory for Motive Power and Key Materials, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China. ** Corresponding author. National & Local Engineering Laboratory for Motive Power and Key Materials, School of Chemistry and Chemical Engineering, Henan Normal University, Xinxiang, Henan, 453007, PR China. E-mail addresses: [email protected] (Z. Cao), [email protected] (S. Yang). https://doi.org/10.1016/j.ijhydene.2019.12.030 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
2
international journal of hydrogen energy xxx (xxxx) xxx
Introduction The oxygen reduction reaction (ORR) is a pivotal process due to its sluggish kinetics in fuel cells and metal-air batteries. The normally used Platinum-based electrocatalysts for ORR suffer from inevitable problems such as low abundance, high cost, poisoning by methanol crossover, and poor stability [1]. Thus, it is crucial to explore low-cost and high-performance (nonprecious metal catalysts) for ORR [2]. In the past decades, extensive efforts have been made to designing and developing low-cost and high-performance NPMCs [3]. Nitrogen and transition metal co-modified carbon materials (M-N/C with M ¼ Fe, Co, Ni) [4e6], and metal carbides (Fe3C and MoC) [7e11] have attracted wide concerns in the recent few years. Among these potential substitutes, the FeeN/C catalyst, which may possess multiple active components (e. g. NeC, Fe-Nx, and Fe/Fe3C), is considered one of the most promising alternatives for both oxygen reduction reaction and oxygen evolution reaction (OER) [12e15]. However, the performance of FeeN/C catalysts is unsatisfactory for widespread commercial applications. Especially, the Fe3C-containing FeeN/C catalysts still suffer from the limited transfer pathway and unsatisfactory stability of Fe3C as well as the amorphous/low graphitized carbon which undergoes oxidization at inherently oxidizing potentials. To date, lots of attempts have been devoted to a detailed understanding of the catalytic mechanism of Fe3C containing FeeN/C catalysts since the active sites remain debate [16e21]. Generally, it is widely accepted that FeeN active sites play a critical role in ORR process due to the improvement of oxygen adsorption while NeC active sites can improve surface wettability as well as the conductivity of the carbon matrix. However, the exact nature of the Fe3C active sites in the FeeN/ C catalysts is still controversial until today. Early studies suggested Fe3C itself are highly active for splitting the OeO bond of molecular oxygen on its surface, i.e., the wellaccepted limiting step of ORR process [21,22]. Although some argued that NeC species and Fe3C species are inactive sites and they don’t interact with FeeN sites during the catalysis of the ORR [23,24], more researchers held that Fe3C nanocrystals could boost the ORR activity of adjacent Fe-Nx or activate the surrounding graphitic carbon [17,21,25]. More recently, the Fe/ Fe3C component of FeeFe3C@C could help the surrounding Ndoped carbon layers stabilize peroxide intermediate and promote facile 2 e 2e ORR based on experiment results [20,21]. Whatever, the prevailing belief is there exist a synergistic effect between Fe3C and Fe-Nx as well as Fe3C and carbon which is of benefit to the catalytic activity [26e32]. The synthesis of Fe3C-containing FeeN/C carbon catalysts can be achieved via pyrolyzing Fe, N, and C containing precursors [30,33]. Composition tuning, structure optimization and modulation of the environment surrounding the catalytically active sites are the main strategies for improving the catalytic performance to ORR. Noting that most of the welldeveloped Fe/NeC catalysts may contain varieties of Fe species such as Fe-Nx, Fe, Fe3C, etc., which are determined mainly by the precursor materials and the synthesis protocols [4,5,17,28,34e48]. The density of accessible active sites is closely associated with the specific surface area, pore structure as well as the dopant species distribution in the carbon
framework. Mesopores can facilitate mass transfer while micropores are favorable for the numerous accessible active sites. Thus, hierarchically porous carbons which own a balance of micro/meso porosity may favor the catalytic activity [29]. Generally, graphitization by high-temperature annealing procedure can effectively improve the electrical conductivity. However, such a procedure usually leads to low specific surface area and less active sites due to low-level N doping and the fast crystal growth and agglomeration, thus adverse to the catalytic activity. Therefore, the type, number, and accessibility of active sites and the conductivity of the carbon matrix should be considered as a whole to maximize the electrocatalytic activity [29]. Two-dimensional (2D) hierarchical graphitic carbon sheets are attractive porous materials due to the high surface area, large pore volume, and accredited conductivity [28,47,49e57]. Consequently, despite the great progress in the synthesis of Fe/N modified graphitic carbon sheets, a facile and low-cost method is still highly demanded for the fabrication of 2D carbon sheets in one step, including the in situ formation of ORR-active species. Herein, we report a facile and low-cost strategy to in situ form Fe and N doping hierarchically porous graphene-like carbon nanosheets (FeeN/C) which possess abundant Fe-Nx sites and Fe3C nanoparticles via pyrolyzing lignocellulose-rich chestnut shell precursor in the presence of iron ion and zinc ion. The synthesis strategies can modulate the physicochemical properties and the final catalytic performance. Fig. 1 illustrates the synthesis of FeeN/C-900 catalysts and its catalysis to ORR. Notably, the final carbon matrix presents different morphologies from large block to thin sheets with increasing the dosage of FeCl3. 2D porous graphene-like structure enables the materials to form a continuous electron conduction path and expose the catalytic active sites greatly. Thus, the optimal FeeN/C-2-900 catalyst exhibits superior ORR electrocatalytic activity, high stability and methanol tolerance in alkaline medium, comparable or even better than that of commercial Pt/C catalyst. The reason may be attributed to the high content of Fe-Nx active sites, the coexistence of Fe3C of together with the unique hierarchically porous 2D sheet-like structure and high conductivity of carbon matrix.
Experimental section Synthesis of FeeN/C catalysts Chestnut shell, ZnCl2, and FeCl3 were used as the carbon source, activating agent and graphitic catalyst precursor, respectively. FeCl3 also acts as a precursor of Fe3C. Chestnut shell used in this work were purchased from the market. All the chemicals were of analytical grade and used without any further purification. The FeeN/C catalysts were prepared via a facile two-step process including impregnation and step-pyrolysis similar to a previous report [57] since heat treatment under low and high temperature would result in carbon with a high degree of graphitization. In a typical process, 3 g of the dry Chestnut shell was mixed with 9 g ZnCl2 in 50 mL 3 M ferric trichloride (FeCl3) solution. Then, the mixture underwent an evaporation
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
3
Fig. 1 e Schematic illustration for the controllable synthesis of FeeN/C-900 electrocatalysts. step at 80 C for 2 h under stirring and then dried at 100 C. Subsequently, the activation and graphitization process of carbon precursor was carried out in a tubular furnace with a heating rate of 5 C min1 under N2 while maintained at 500 C and 900 C for 1 h, respectively. The pyrolyzed product was leached in 2 M hydrochloric acid to remove inactive iron species thoroughly, then washed to neutral and dried in a vacuum oven at 60 C overnight. The resultant sample was denoted as FeeN/C-2-900. Other control samples with different FeCl3 concentrations (FeeN/C-1-900 and FeeN/C-3900 when the concentration of FeCl3 is 2 and 4 mol/L, respectively) or pyrolyzed at low temperature (FeeN/C-2-700 and FeeN/C-2-800) were also prepared.
Physical characterizations The as-prepared samples were characterized by the scanning electron microscope (FESEM; SU8010), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images (JEOL-2100F) to obtain the morphologies and microstructure. Scanning transmission electron microscopy (STEM) and EDX elemental mapping analysis were conducted (JEOL JEMARM200CF). The thermogravimetric analysis (TGA) was tested on PerkinElmer synchronous thermal analysis instruments. The powder X-Ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 diffractometer. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) was recorded to obtained the surface chemistry of the samples with a monochromatic Al Ka radiation (hn ¼ 1486.6 eV) and the C1s peak at 284.8 eV as an internal standard. The Raman spectra were achieved on Lab-RAM HR800 with excitation by an argon ion laser (514.5 nm). The nitrogen adsorption-desorption measurements were conducted at 77 K on a TriStar 3020 surface area and porosity analyzer.
Electrocatalytic measurements The activity of the catalyst was tested on a rotating disk electrode (RDE, glassy carbon disk with 5 mm of diameter) and a rotating ring-disk electrode (RRDE with 0.196 cm2 of surface area) of Pine Company. The standard three-electrode system was adopted in 0.1 M solution with the catalyst coated on disc
electrode as working electrode, an Ag/AgCl (3 M KCl) as a reference electrode, and a platinum wire as counter electrode. The preparation method of the working electrode is as follows: the catalyst inks were obtained by mixing 5 mg of catalyst sample powders with 95 mL of 5% Nafion solution (Dupont) and 350 mL of ethanol, then the mixture treated by ultrasonication for 45 min and 7 mL of the ink was placed on an RRDE electrode. Catalyst loading of the samples or Pt/C was controlled to be 0.4 mg cm2 on RDE and 0.3 mg cm2 on RRDE. Cyclic voltammetry (CV) test was first measured in an N2saturated or O2-saturated electrolyte solution with a scan rate of 50 mV s1. The linear sweep voltammetry (LSV) were measured in the O2-saturated electrolyte with a scan rate of 5 mV s1 and with different rotating rates of 400, 625, 900, 1225, 1600, and 2025 rpm. The electron transfer number (n) can be calculated by the K-L plot using equations (1) and (2): 1 1 1 1 1 ¼ þ ¼ þ j jk jd jk Bu1=2
(1)
B ¼ 0:2nFDO2 y1=6 CO2
(2)
2=3
where j, jk, and jd are the measured disk current, the kinetic limiting current and the diffusion limiting current, respectively. u is the electrode rotation rate in rpms. The values of F DO2 , CO2 , y are 96485 C mol1, 1.86 105 cm2 s1, 1.21 106 mol cm3 and 0.01 cm2 s1, respectively [41]. A durability test was performed by chronoamperometric technique at 0.3 V for 10000 s with a rotation speed of 1600 rpm. The durability test was also evaluated using the accelerated durability test protocol (the US Department of Energy) by cycling the catalysts between 0.8 and 0.1 V under O2 atmosphere. Methanol durability test was measured by CV in O2saturated electrolyte with or without 1 M CH3OH. The Electrochemical Impedance Spectroscopy (EIS) measurements were conducted at 0 V vs Ag/AgCl reference electrode.
Results and discussion Pyrolyzing cheap and abundant biomass waste was a common strategy to prepare porous carbon [22,57e59]. Previous
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
4
international journal of hydrogen energy xxx (xxxx) xxx
studies displayed that the heat-treatment strategy has a vital effect in the crystal phase as well as the morphology of pyrolyzed Fe-containing carbon-based catalyst, because Fe may exist in different crystal phases such as Fe3C, Fe3N, Fe and iron oxide. Therefore, we first examined the crystal structure of FeeN/C samples pyrolyzed in the range of 700e900 C. The XRD data in Fig. S1 only show the characteristic of Fe3O4 which is believed an inactive species to ORR, and a weak peak at 26 indicating a low graphitization degree when pyrolyzing below 900 C for FeeN/C-2-700 and FeeN/C-2-800. Thus, a low ORR activity was achieved featuring of the negative onset and halfwave potentials and low diffusion-limiting current revealed by LSV tests (Fig. S2). While FeeN/C-2-900 shows the presence of Fe3C phase which is believed could boost the ORR activity. FeeN/C catalysts prepared at 900 C was further optimized with different dosage of FeCl3 to study the relation between electrical conductivity, porosity, and active site density to ensure the material has both activity and conductivity. SEM and TEM analysis revealed that FeeN/C catalysts synthesized at 900 C shared different morphologies but the same porous structure (Fig. S3-5 and Figs. 2 and 3). Notably, the typical FeeN/C-2-900 catalyst, whose precursor contained a middle FeCl3 dosage, mainly features graphene-like carbon sheets (Fig. S3 and Fig. 2a). The highly-graphitized domains can be easily identified with a continuous interlayer distance of 0.337 nm which is well consistent with the d-spacing of (002) crystallographic plane of graphite as shown in Fig. 2b. Fig. 2b also reveals the existence of dispersed Fe3C nanocrystals marked by red circles with an interlayer distance of 0.201 nm assigned to the (031) plane. It should be pointed out that some large agminated Fe3C particles with the size of about 50 nm can also be found (Fig. 2c) in less graphitic domains. HRTEM images further reveal the presence of small Fe3C nanocrystals (Fig. 2d) with the 0.201 nm interlayer distance and pores less than 5 nm marked by red circles (Fig. 2f). The corresponding SAED pattern confirmed the above analysis with the diffraction ring of (031) plane of Fe3C and (002) plane of graphite (Fig. 2e). A high-angle annular dark field (HAADF) and the scanning transmission electron microscopy (STEM) analysis was further conducted to reveal the status of iron and nitrogen in the catalyst (Fig. 2g and h). Fig. 2g and h verified the uniform distribution of carbon, nitrogen, and
dispersed iron for the highly-graphitized domains and less graphitic domains, respectively. The homogeneous distribution of Fe and N in the porous carbon frameworks may suggest the presence of Fe related species (Fe3C and Fe-Nx), which are believed as the catalytically active sites in FeeN/C catalysts. A decrease of FeCl3 amount leads to the generation of a thick block structure (FeeN/C-1-900), as shown in Fig. 3a and Fig. S3. Only a few Fe3C particles in amorphous domains can be found (Fig. 3b). While increasing the amount of FeCl3 (FeeN/C-3-900), only highly graphitic “graphene-like” nanosheets emerged, which is thinner than FeeN/C-2-900 as revealed by Fig. 3c and Fig. S5 and S6. It should be pointed out that no clear large nanoparticles can be found in TEM observation. As seen in Fig. 3d, a lattice distance of 0.337 nm is indexed to the (002) plane of graphite. The local highly distributed Fe3C nanocrystals marked in white circles with a lattice distance of 0.168 nm assigned to the (001) plane of Fe3C, are visible in the HRTEM image (Fig. 3e, enlarged part B in Fig. 3c). The inset of Fig. 3e shows the FFT image indexing the planar distance of (004) plane of Fe3C. The corresponding SAED pattern also shows the diffraction ring of (004) plane of Fe3C and (002) plane of graphite (Fig. 3f). The crystal phase of the FeeN/C-900-2 catalyst was further explored by XRD survey (Fig. 4a). Crystalline Fe3C (JCPDS file No. 35-0071) and amorphous carbon, are discerned according to the diffraction patterns, consistent with the results of HRTEM and SAED [17]. As for FeeN/C-1-900 whose FeCl3 dosage is the smallest, there are only two broad peaks assigned to amorphous carbon and no obvious peaks for Fe3C, suggesting a low crystallinity. While a sharp peak at 26 assigned to the (002) facets of graphite carbon for FeeN/C-3900 appears, indicating enhanced graphitization because of the largest FeCl3 dosage. Thus, the FeCl3 dosage not only affects the degree of graphitization indexed by the (002) facet of graphite carbon but also affects the crystal phase and intensity of Fe3C. Precisely tuning the FeCl3 content to manipulate the surface area, pore architecture, and graphitization degree of the catalysts is of pivotal importance for the ORR activity. The assynthesized three samples all features a hierarchical porous structure, possessing large specific surface area and high pore volume measured by N2 adsorption-desorption analysis
Fig. 2 e TEM (a, c) and HRTEM (b, d, f), magnified HAADF-STEM (g, h) images and element mapping of different area for FeeN/ C-2-900. (a, b, g) for highly-graphitized domains. (c, d, f, h) for less graphitic domains. (e) Corresponding SAED pattern of (c). Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
5
Fig. 3 e TEM images of FeeN/C-1-900 (a) and FeeN/C-3-900 (c). HRTEM images of FeeN/C-1-900 (b) and FeeN/C-3-900 (d, e). The inset of (e) shows the FFT image. (f) Corresponding SAED pattern of (d).
Fig. 4 e XRD patterns (a), nitrogen sorption isotherms (b), the corresponding PSD curves and Raman spectra (d) for FeeN/C900 catalysts. The high-resolution spectra of Fe 2p (e) and N 1s (f) of FeeN/C-900 catalysts. (g) Absolute content of different nitrogen species of FeeN/C-900 catalysts.
(Fig. 4b and c). Detailed information is shown in Table S1. Especially, FeeN/C-2-900 owns the largest mesoporosity with a mesopore volume of 1.10 m3 g1 and the second largest specific surface area of 1536 m2 g1 among them, thus was
expected to display excellent ORR performance [60,61]. With the presence of Fe components, the graphitization of carbon source can be catalyzed, and meanwhile, a carburized phase of the Fe formed during the heating process, and the
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
6
international journal of hydrogen energy xxx (xxxx) xxx
formation-decomposition process of the carburized phase can result in the formation of graphene-like nanosheets [17,57]. Moreover, only the Fe content is enough, graphene-like carbon sheets can be created. The larger dosage of the Fe components, the smaller thickness and the higher graphitization of the carbon sheets features. Namely, the Fe content not only affects the degree of graphitization as displayed by XRD patterns but also makes a critical difference in the morphology of carbon matrix discerned by SEM and TEM images. Fig. 4d shows the Raman spectra of the as-prepared FeeN/ C catalysts. Two intense peaks can be observed at 1357 and 1582 cm1 corresponding to the D (disorders or defects in carbon graphitized structure) band and G (well-graphitized carbon) band, respectively. The values of ID/IG for FeeN/C-1900, FeeN/C-2-900, and FeeN/C-3-900 are 1.60, 1.45, and 1.64, respectively, indicate that the high pyrolysis temperature develops rich defect on its surface which can be attributed to the atomic doping of nitrogen and iron into the carbon matrix. Furthermore, the positive shift of G peak position can also be ascribed to the defect sites and structural distortion when increasing Fe dosage [62]. The narrowing of the FWHM of G peak indicates a higher degree of graphitization with increasing Fe dosage, as also demonstrated by XRD. X-ray photoelectron spectroscopy (XPS) was conducted to determine the surface chemistry of the FeeN/C-900 catalysts, as shown in Fig. 4e, f, Fig. S7, and Table S1. The survey XPS spectra reveal the presence of C, O, N, and Fe, suggesting that Fe and N were successfully incorporated into the carbon surface. The content of Fe elements detected in the three catalysts is low (1.7 wt% for FeeN/C-2-900, 2.6 wt% for FeeN/C-3900, and even too low to be detected as for FeeN/C-1-900) (Table S2). The amount of Fe on the sample surface is less than the value test by TG (1.37, 2.03 and 2.61 wt% for FeeN/C-1-900, FeeN/C-2-900, and FeeN/C-3-900 respectively) (Fig. S8). The high-resolution spectra of Fe 2p in Fig. 4e shows two major peaks at 712.8 and 725.3 eV corresponding to Fe 2p3 and Fe 2p1 respectively. There is also a weak peak due to a shakeup satellite peaks of Fe 2p3. The other peak at z710.5 eV can be assigned to FeeN. The undiscernible FeeC peak implied that a large amount of FeeN structure exists on the surface of Fe3C nanoparticles [35,63]. The nitrogen content in the catalysts is 1.91 atom % for FeeN/C-1-900, 2.66 atom % for FeeN/C-2-900, and 2.12 atom % for FeeN/C-3-900, respectively (Table S3). The high-resolution N 1s spectra for the typical FeeN/C-2-900 were further deconvoluted into five peaks corresponding to pyridinic N, FeNx, pyrrolic N, graphitic N and oxidized N [14,64], while FeeN/ C-1-900 lack of Fe-Nx and FeeN/C-3-900 lack of both pyrrolic N and pyridinic N. (Fig. 4f). The relative composition of the N species calculated on the basis of total nitrogen content and its fraction in the catalyst was compared and displayed in Fig. 4g. The larger concentration of FeCl3, the higher content of graphic N and Fe-Nx as revealed by the high-resolution N 1s and Fe 2p spectra (Fig. 4 and Table S2, 3), which indicates that the chemical states of derived materials were strongly affected by the concentration of FeCl3. In particular, FeeN/C2-900 has the highest content of Fe-Nx and pyridinic N, and the second-high content of pyrrolic N and graphic N, which are expected to show a promising ORR activity since Fe-Nx sites
are believed to be mainly responsible for the high catalytic activity. The catalytic activity for ORR of the FeeN/C-900 catalysts was first assessed on a rotating disk electrode (RDE) in O2saturated 0.1 M KOH solution. FeeN/C-2-900 shows the most positive reduction peak among the three FeeN/C-900 catalysts revealed by Cyclic voltammetry (CV) curves in O2- or N2saturated 0.1 M KOH (Fig. S9). The peak potential of FeeN/C-2900 is nearly the same as the commercial Pt/C (20%, Johnson Matthey), suggesting high ORR activity. Fig. 5a shows the linear-sweep voltammogram (LSV) curves of the three catalysts in O2-saturated 0.1 M KOH solution at 1600 rpm. FeeN/C2-900 has the best catalytic activity with the most positive E1/2 of 0.122 V (vs. Ag/AgCl (3 M)) in the LSV curves. In contrast, FeeN/C-1-900 and FeeN/C-3-900 exhibit the degraded ORR activity with an E1/2 of 0.148 V and 0.157 V, respectively. The three samples all delivered a diffusion-limiting current about 5 mA cm2 at 0.6 V and the electron transfer approaching the ideal value of 4.0 obtained from the slopes of K-L plots, as shown in Fig. S10, 11 and Table S4. Fig. 5b also reveals that the catalytic activity of FeeN/C-2-900 is comparable to Pt/C. The RRDE measurements further corroborate that FeeN/C-2-900 shows superior activity for ORR in terms of the low yield of H2O2 and high electron transfer number (Fig. 5c). To gain more information on the catalytic performance, impedance spectra were recorded at 0 V vs Ag/AgCl reference electrode (Fig. S12). The three FeeN/C-900 catalysts show similar shape of the impedance spectra featuring the depressed semi-circles at the high-frequency regions and a straight line in the low frequency at low-frequency regions. There is no obvious difference between the diameters of the three semi-circles. This imply that three samples have similar ohmic resistance (including solution resistance and film resistance) and charge transfer resistance (indicative of the semicircle diameter). While the diffusion resistance (indicative of the slope of the line) of FeeN/C-2-900 is the smallest among them due to its the largest mesoporosity and the second largest specific surface area, which is favorable to the mass transfer. The EIS results further illustrate that the accessibility of active sites is vital to maximize the electrocatalytic activity. Also, it is well-known that for an electrochemical reaction, the reaction should exhibit a low Tafel slope in order to achieve a high current at low overpotential. As shown in Fig. 5d, the Tafel slope of FeeN/C-2-900 (~99.76 mV/dec) is very close to that of Pt/C (~ 96.17 mV/dec). This means that the rate-determining step is probably the transfer of the first electron in ORR catalyzed by FeeN/C-2-900, similar to platinum [21]. The ORR stability and methanol tolerance electrocatalysts with chronoamperometric (it) measurements in O2-saturated 0.1 M KOH. A higher performance attenuation than Pt/C electrocatalyst was observed for FeeN/C-2-900 (Fig. 5e). An accelerated durability test was also done to assess the durability of FeeN/C-2-900 by cycling the catalysts between 0.8 and 0.1 V under O2 atmosphere. After 4500 continuous cycles, a negative shift of the E1/2 on FeeN/C-2-900 by ~ 6 mV at 5 mV s1 (inset of Fig. 5f) and ~8 mV at 50 mV s1 (Fig. S13) appeared. As can be seen in Fig. 5f, there is a slight performance degradation for FeeN/C-2-900 after the injection of 3 M
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
7
Fig. 5 e (a) Linear sweep voltammogram (LSV) curves of FeeN/C-900 catalysts. (b) LSV curves of FeeN/C-2-900 and Pt/C. (c) The electron transfer number and the percentage of peroxide of FeeN/C-2-900 and Pt/C at various potentials based on the RRDE results. (d) Tafel slope of FeeN/C-2-900 and Pt/C. (e) Stability test for FeeN/C-2-900 and Pt/C in O2-saturated 0.1 M KOH solution at 0.7 V with a rotation rate of 1600 rpm. Inset: endurance test of FeeN/C-2-900 for 4500 cycles in O2-saturated 0.1 M KOH with a scan rate of 5 mV s¡1. (f) Chronoamperometric response of methanol at FeeN/C-2-900 and Pt/C electrocatalyst at 1600 rpm upon the injection of methanol after 200 s into the O2-saturated 0.1 M KOH solution. methanol into the electrolyte at 200 s, exhibiting a much better fuel selectivity than the Pt/C catalyst. The excellent ORR performance of FeeN/C-2-900 can be ascribed as follows: Firstly, FeeN/C-2-900 has the highest amount of Fe-Nx and pyridinic active sites coexistence with Fe3C nanocrystals (Fig. 6). Secondly, FeeN/C-2-900 displays a high mesoporosity with a large specific surface area (1536 m2 g1), which can offer better mass diffusion and provide more accessible catalytically active sites compared to other FeeN/C samples. Besides, FeeN/C-2-900 owns decent electrical conductivity. Hence the optimum ORR activity of FeeN/C-2-900 was attributed to the synergistic effect of the high content of Fe-Nx, pyridinic-N active sites and the existence of Fe3C nanoparticles in the highly conductive and hierarchically porous carbon sheets. Namely, FeeN/C-2-900 owns the best equilibrium degree of graphitization and the density of the active sites. The ORR performance of FeeN/C-2-
Fig. 6 e Illustration of the role of Fe-Nx and Fe3C Sites for the ORR.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
8
international journal of hydrogen energy xxx (xxxx) xxx
900 is comparable with previously reported Fe3C-containing Fe, N doped nonprecious catalysts, as shown in Table S5. [7]
Conclusions Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically porous carbon nanosheets have been facilely prepared via a cost-effective pyrolysis method using the chestnut shell, ZnCl2, and FeCl3 as precursors. The optimal FeeN/C-2-900 catalyst possesses superior ORR catalytic activity, high stability, and methanol tolerance in an alkaline medium than 20 wt% Pt/C. The half-wave potential is 39 mV more positive than that of Pt/C catalyst. The synergistic effect of the high content of Fe-Nx active sites and the coexistence of Fe3C in the unique hierarchically porous carbon matrix with high conductivity is responsible for the excellent catalytic performance. This report offers a facile approach to synthesize Fe3C nanoparticles decorated Fe/N doped graphene-like hierarchically carbon nanosheets for effective nonprecious metal oxygen electrocatalyst.
Acknowledgments This work was financially supported by the Scientific and Technologies Research Program of Henan Province (Grant No. 182102310827), Natural Science Foundation of Henan Province (Grant No. 162300410170), and Fundamental Research Funds for the Henan Normal University (Grant No. 2017PL07).
[8]
[9]
[10]
[11]
[12]
[13]
[14]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.ijhydene.2019.12.030.
[15]
references [16] [1] Nie Y, Li L, Wei Z. Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction. Chem Soc Rev 2015;44:2168e201. https://doi.org/10.1039/C4CS00484A. [2] Xia W, Mahmood A, Liang Z, Zou R, Guo S. Earth-abundant nanomaterials for oxygen reduction. Angew Chem Int Ed 2016;55:2650e76. https://doi.org/10.1002/anie.201504830. [3] Gewirth AA, Varnell JA, DiAscro AM. Nonprecious metal catalysts for oxygen reduction in heterogeneous aqueous systems. Chem Rev 2018;118:2313e39. https://doi.org/ 10.1021/acs.chemrev.7b00335. [4] Fan X, Peng Z, Ye R, Zhou H, Guo X. M3C (M: Fe, Co, Ni) nanocrystals encased in graphene nanoribbons: an active and stable bifunctional electrocatalyst for oxygen reduction and hydrogen evolution reactions. ACS Nano 2015;9:7407e18. https://doi.org/10.1021/acsnano.5b02420. [5] Wang M-Q, Ye C, Wang M, Li T-H, Yu Y-N, Bao S-J. Synthesis of M (Fe3C, Co, Ni)-porous carbon frameworks as highefficient ORR catalysts. Energy Storage Mater 2018;11:112e7. https://doi.org/10.1016/j.ensm.2017.10.003. [6] Sun T, Tian B, Lu J, Su C. Recent advances in Fe (or Co)/N/C electrocatalysts for the oxygen reduction reaction in polymer
[17]
[18]
[19]
[20]
electrolyte membrane fuel cells. J Mater Chem 2017;5:18933e50. https://doi.org/10.1039/C7TA04915C. Kong A, Zhang Y, Chen Z, Chen A, Li C, Wang H, et al. Onepot synthesized covalent porphyrin polymer-derived coreshell Fe3C@carbon for efficient oxygen electroreduction. Carbon 2017;116:606e14. https://doi.org/10.1016/ j.carbon.2017.02.046. Li J, Mao S, Hou Y, Lei L, Yuan C. 3D edge-enriched Fe3C@C nanocrystals with a coreeshell structure grown on reduced graphene oxide networks for efficient oxygen reduction reaction. ChemSusChem 2018;11:3292e8. https://doi:10.1002/ cssc.201801084. Ma J, Xiao D, Chen CL, Luo Q, Yu Y, Zhou J, et al. Uric acidderived Fe3C-containing mesoporous Fe/N/C composite with high activity for oxygen reduction reaction in alkaline medium. J Power Sources 2018;378:491e8. https://doi.org/ 10.1016/j.jpowsour.2017.11.091. Yang H, Liu J, Wang J, Poh CK, Zhou W, Lin J, et al. Electrocatalytically active graphene supported MMo carbides (MNi, Co) for oxygen reduction reaction. Electrochim Acta 2016;216:246e52. https://doi.org/10.1016/ j.electacta.2016.09.023. Yan Z, He G, Shen PK, Luo Z, Xie J, Chen M. MoCegraphite composite as a Pt electrocatalyst support for highly active methanol oxidation and oxygen reduction reaction. J Mater Chem 2014;2:4014e22. https://doi.org/10.1039/C3TA14251E. Jiang H, Yao Y, Zhu Y, Liu Y, Su Y, Yang X, et al. Iron carbide nanoparticles encapsulated in mesoporous FeeN-doped graphene-like carbon hybrids as efficient bifunctional oxygen electrocatalysts. ACS Appl Mater Interfaces 2015;7:21511e20. https://doi.org/10.1021/acsami.5b06708. Han S, Hu X, Wang J, Fang X, Zhu Y. Novel route to Fe-based cathode as an efficient bifunctional catalysts for rechargeable Zneair battery. Adv Energy Mater 2018;8:1800955. https://doi.org/10.1002/aenm.201800955. Ahn SH, Yu X, Manthiram A. “Wiring” Fe-Nx-Embedded porous carbon framework onto 1D nanotubes for efficient oxygen reduction reaction in alkaline and acidic media. Adv Mater 2017;29:1606534. https://doi.org/10.1002/ adma.201606534. Li J-S, Li S-L, Tang Y-J, Han M, Dai Z-H, Bao J-C, et al. Nitrogen-doped Fe/Fe3C@graphitic layer/carbon nanotube hybrids derived from MOFs: efficient bifunctional electrocatalysts for ORR and OER. Chem Commun 2015;51:2710e3. https://doi.org/10.1039/C4CC09062D. Sun T, Jiang Y, Wu Q, Du L, Zhang Z, Yang L, et al. Is iron nitride or carbide highly active for oxygen reduction reaction in acidic medium? Catal Sci Technol 2017;7:51e5. https:// doi.org/10.1039/C6CY01921H. Jiang W-J, Gu L, Li L, Zhang Y, Zhang X, Zhang L-J, et al. Understanding the high activity of FeeNeC electrocatalysts in oxygen reduction: Fe/Fe3C nanoparticles boost the activity of Feenx. J Am Chem Soc 2016;138:3570e8. https://doi.org/ 10.1021/jacs.6b00757. Fajrial AK, Saputro AG, Agusta MK, Rusydi F, Nugraha, Dipojono HK. First principles study of oxygen molecule interaction with the graphitic active sites of a boron-doped pyrolyzed FeeNeC catalyst. Phys Chem Chem Phys 2017;19:23497e504. https://doi.org/10.1039/C7CP02390A. Kattel S, Atanassov P, Kiefer B. A density functional theory study of oxygen reduction reaction on non-PGM Fe-Nx-C electrocatalysts. Phys Chem Chem Phys 2014;16:13800e6. https://doi.org/10.1039/C4CP01634C. Kim JH, Sa YJ, Jeong HY, Joo SH. Roles of FeNx and FeFe3C@C species in FeN/C electrocatalysts for oxygen reduction reaction. ACS Appl Mater Interfaces 2017;9:9567e75. https://doi.org/10.1021/acsami.6b13417.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
international journal of hydrogen energy xxx (xxxx) xxx
[21] Hu Y, Jensen JO, Zhang W, Cleemann LN, Xing W, Bjerrum NJ, et al. Hollow spheres of iron carbide nanoparticles encased in graphitic layers as oxygen reduction catalysts. Angew Chem Int Ed 2014;53:3675e9. https://doi.org/10.1002/ anie.201400358. [22] Ma M, You S, Wang W, Liu G, Qi D, Chen X, et al. Biomassderived porous Fe3C/tungsten carbide/graphitic carbon nanocomposite for efficient electrocatalysis of oxygen reduction. ACS Appl Mater Interfaces 2016;8:32307e16. https://doi.org/10.1021/acsami.6b10804. [23] Zhao P, Xu W, Hua X, Luo W, Chen S, Cheng G. Facile synthesis of a N-doped Fe3C@CNT/porous carbon hybrid for an advanced oxygen reduction and water oxidation electrocatalyst. J Phys Chem C 2016;120:11006e13. https:// doi.org/10.1021/acs.jpcc.6b03070. [24] Qian Y, Du P, Wu P, Cai C, Gervasio DF. Chemical nature of catalytic active sites for the oxygen reduction reaction on nitrogen-doped carbon-supported non-noble metal catalysts. J Phys Chem C 2016;120:9884e96. https://doi.org/ 10.1021/acs.jpcc.6b02670. [25] Varnell JA, Tse ECM, Schulz CE, Fister TT, Haasch RT, Timoshenko J, et al. Identification of carbon-encapsulated iron nanoparticles as active species in non-precious metal oxygen reduction catalysts. Nat Commun 2016;7:12582. https://doi.org/10.1038/ncomms12582. [26] Kong F, Fan X, Kong A, Zhou Z, Zhang X, Shan Y. Covalent phenanthroline framework derived FeS@Fe3C composite nanoparticles embedding in N-S-codoped carbons as highly efficient trifunctional electrocatalysts. Adv Funct Mater 2018;28:1803973. https://doi.org/10.1002/adfm.201803973. [27] Deng Y, Wang G, Sun K, Chi B, Shi X, Dong Y, et al. Highly effective and stable doped carbon catalyst with threedimensional porous structure and well-covered Fe3C nanoparticles prepared with C3N4 and tannic acid as template/precursors. J Power Sources 2019;417:117e24. https://doi.org/10.1016/j.jpowsour.2019.02.022. [28] Liu Y, Huang B, Zhang X, Huang X, Xie Z. In-situ fabrication of nitrogen-doped carbon nanosheets containing highly dispersed single iron atoms for oxygen reduction reaction. J Power Sources 2019;412:125e33. https://doi.org/10.1016/ j.jpowsour.2018.11.024. [29] Tan H, Tang J, Kim J, Kaneti YV, Kang Y-M, Sugahara Y, et al. Rational design and construction of nanoporous iron- and nitrogen-doped carbon electrocatalysts for oxygen reduction reaction. J Mater Chem 2019;7:1380e93. https://doi.org/ 10.1039/C8TA08870E. [30] Liu K, Peng Z, Wang H, Ren Y, Liu D, Li J, et al. Fe3C@Fe/N doped graphene-like carbon sheets as a highly efficient catalyst in Al-air batteries. J Electrochem Soc 2017;164:475e83. https://doi.org/10.1149/2.0171706jes. [31] Guo D, Han S, Wang J, Zhu Y. MIL-100-Fe derived N-doped Fe/ Fe3C@C electrocatalysts for efficient oxygen reduction reaction. Appl Surf Sci 2018;434:1266e73. https://doi.org/ 10.1016/j.apsusc.2017.11.230. [32] Wei L, Sun H, Yang T, Deng S, Wu M, Li Z. Iron carbide encapsulated by porous carbon nitride as bifunctional electrocatalysts for oxygen reduction and evolution reactions. Appl Surf Sci 2018;439:439e46. https://doi.org/ 10.1016/j.apsusc.2018.01.056. [33] Wei J, Liang Y, Hu Y, Kong B, Simon GP, Zhang J, et al. A versatile ironetannin-framework ink coating strategy to fabricate biomass-derived iron carbide/Fe-N-carbon catalysts for efficient oxygen reduction. Angew Chem Int Ed 2016;55:1355e9. https://doi.org/10.1002/anie.201509024. [34] Zitolo A, Goellner V, Armel V, Sougrati M-T, Mineva T, Stievano L, et al. Identification of catalytic sites for oxygen reduction in iron- and nitrogen-doped graphene materials. Nat Mater 2015;14:937. https://doi.org/10.1038/nmat4367.
9
[35] Li G, Zhang J, Li W, Fan K, Xu C. 3D interconnected hierarchical porous N-doped carbon constructed by flakelike nanostructure with Fe/Fe3C for efficient oxygen reduction reaction and supercapacitor. Nanoscale 2018;10:9252e60. https://doi.org/10.1039/C8NR02337A. [36] Yang W, Liu X, Yue X, Jia J, Guo S. Bamboo-like carbon nanotube/Fe3C nanoparticle hybrids and their highly efficient catalysis for oxygen reduction. J Am Chem Soc 2015;137:1436e9. https://doi.org/10.1021/ja5129132. [37] Sa YJ, Seo D-J, Woo J, Lim JT, Cheon JY, Yang SY, et al. A general approach to preferential formation of active Feenx sites in FeeN/C electrocatalysts for efficient oxygen reduction reaction. J Am Chem Soc 2016;138:15046e56. https://doi.org/10.1021/jacs.6b09470. [38] Liang H-W, Wei W, Wu Z-S, Feng X, Mu¨llen K. Mesoporous metalenitrogen-doped carbon electrocatalysts for highly efficient oxygen reduction reaction. J Am Chem Soc 2013;135:16002e5. https://doi.org/10.1021/ja407552k. [39] Zhu Y, Zhang B, Liu X, Wang D-W, Su DS. Unravelling the structure of electrocatalytically active FeeN complexes in carbon for the oxygen reduction reaction. Angew Chem Int Ed 2014;126:10849e53. https://doi.org/10.1002/ ange.201405314. [40] Wu Z-Y, Xu X-X, Hu B-C, Liang H-W, Lin Y, Chen L-F, et al. Iron carbide nanoparticles encapsulated in mesoporous FeN-doped carbon nanofibers for efficient electrocatalysis. Angew Chem Int Ed 2015;127:8297e301. https://doi.org/ 10.1002/ange.201502173. [41] Xiao M, Zhu J, Feng L, Liu C, Xing W. Meso/macroporous nitrogen-doped carbon architectures with iron carbide encapsulated in graphitic layers as an efficient and robust catalyst for the oxygen reduction reaction in both acidic and alkaline solutions. Adv Mater 2015;27:2521e7. https://doi.org/ 10.1002/adma.201500262. [42] Yasuda S, Furuya A, Uchibori Y, Kim J, Murakoshi K. Ironenitrogen-doped vertically aligned carbon nanotube electrocatalyst for the oxygen reduction reaction. Adv Funct Mater 2016;26:738e44. https://doi.org/10.1002/ adfm.201503613. [43] Cui X, Yang S, Yan X, Leng J, Shuang S, Ajayan PM, et al. Pyridinic-nitrogen-dominated graphene aerogels with FeeNeC coordination for highly efficient oxygen reduction reaction. Adv Funct Mater 2016;26:5708e17. https://doi.org/ 10.1002/adfm.201601492. [44] Hu E, Yu X-Y, Chen F, Wu Y, Hu Y, Lou XW. Graphene layerswrapped Fe/Fe5C2 nanoparticles supported on N-doped graphene nanosheets for highly efficient oxygen reduction. Adv Energy Mater 2018;8:1702476. https://doi.org/10.1002/ aenm.201702476. [45] Ren H, Wang Y, Yang Y, Tang X, Peng Y, Peng H, et al. Fe/N/C nanotubes with atomic Fe sites: a highly active cathode catalyst for alkaline polymer electrolyte fuel cells. ACS Catal 2017;7:6485e92. https://doi.org/10.1021/acscatal.7b02340. [46] Zhang Y, Jiang W-J, Guo L, Zhang X, Hu J-S, Wei Z, et al. Confining iron carbide nanocrystals inside CNx@CNT toward an efficient electrocatalyst for oxygen reduction reaction. ACS Appl Mater Interfaces 2015;7:11508e15. https://doi.org/ 10.1021/acsami.5b02467. [47] Song A, Cao L, Yang W, Li Y, Qin X, Shao G. Uniform multilayer graphene-coated iron and iron-carbide as oxygen reduction catalyst. ACS Sustainable Chem Eng 2018;6:4890e8. https://doi.org/10.1021/ acssuschemeng.7b04319. [48] Zhou T, Ma R, Zhang T, Li Z, Yang M, Liu Q, et al. Increased activity of nitrogen-doped graphene-like carbon sheets modified by iron doping for oxygen reduction. J Colloid Interface Sci 2019;536:42e52. https://doi.org/10.1016/ j.jcis.2018.10.021.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030
10
international journal of hydrogen energy xxx (xxxx) xxx
[49] Gu D, Zhou Y, Ma R, Wang F, Liu Q, Wang J. Facile synthesis of N-doped graphene-like carbon nanoflakes as efficient and stable electrocatalysts for the oxygen reduction reaction. Nano-Micro Lett 2017;10:29. https://doi.org/10.1007/s40820017-0181-1. [50] Yuan HR, Zhang X, Yan F, Zhang S, Zhu CL, Li CY, et al. Nitrogen-doped carbon nanosheets containing Fe3C nanoparticles encapsulated in nitrogen-doped graphene shells for high-performance electromagnetic wave absorbing materials. Carbon 2018;140:368e76. https://doi.org/10.1016/ j.carbon.2018.08.073. [51] Chen L, Xiao J, Liu B, Yi T. A bonded double-doped graphene nanoribbon framework for advanced electrocatalysis. ACS Appl Mater Interfaces 2016;8:16649e55. https://doi.org/ 10.1021/acsami.6b02522. [52] Tian H, Wang N, Xu FG, Zhang PF, Hou D, Mai YY, et al. Nitrogen-doped carbon nanosheets and nanoflowers with holey mesopores for efficient oxygen reduction catalysis. J Mater Chem 2018;6:10354e60. https://doi.org/10.1039/ c8ta02319k. [53] Li L, Song L, Guo H, Xia W, Jiang C, Gao B, et al. N-Doped porous carbon nanosheets decorated with graphitized carbon layer encapsulated Co9S8 nanoparticles: an efficient bifunctional electrocatalyst for the OER and ORR. Nanoscale 2019;11:901e7. https://doi.org/10.1039/c8nr07179a. [54] Wang Y, Pan Y, Zhu L, Yu H, Duan B, Wang R, et al. Solventfree assembly of Co/Fe-containing MOFs derived N-doped mesoporous carbon nanosheets for ORR and HER. Carbon 2019;146:671e9. https://doi.org/10.1016/j.carbon.2019.02.002. [55] Wu Q, Wang Z, Li Y, Wang Z, Liang Y, Hu Y, et al. Nitrogenrich, well-dispersed nanoporous carbon materials for superefficient oxygen reduction reaction. Chemelectrochem 2019;6:1894e900. https://doi.org/10.1002/celc.201900198. [56] Zhou N, Li L, Chen S, Peng X, Niu W, Qu Y. Mesoporous, nitrogen-doped, graphitized carbon nanosheets embedded with cobalt nanoparticles for efficient oxygen electroreduction. J Mater Sci 2019;54:4168e79. https://doi.org/ 10.1007/s10853-018-3121-7.
[57] Sun L, Tian C, Li M, Meng X, Wang L, Wang R, et al. From coconut shell to porous graphene-like nanosheets for highpower supercapacitors. J Mater Chem 2013;1:6462e70. https://doi.org/10.1039/C3TA10897J. [58] Balahmar N, Mitchell AC, Mokaya R. Generalized mechanochemical synthesis of biomass-derived sustainable carbons for high performance CO2 storage. Adv Energy Mater 2015;5:1500867. https://doi.org/10.1002/aenm.201500867. [59] Borghei M, Lehtonen J, Liu L, Rojas OJ. Advanced biomassderived electrocatalysts for the oxygen reduction reaction. Adv Mater 2018;30:1703691. https://doi.org/10.1002/ adma.201703691. [60] Mao X, Cao Z, Yin Y, Wang Z, Dong H, Yang S. Direct synthesis of nitrogen and phosphorus co-doped hierarchical porous carbon networks with biological materials as efficient electrocatalysts for oxygen reduction reaction. Int J Hydrogen Energy 2018;43:10341e50. https://doi.org/10.1016/ j.ijhydene.2018.04.100. [61] Mao X, Cao Z, Chen S, Jia J, Li X, Yin Y, et al. Facile synthesis of N, P-doped hierarchical porous carbon framework catalysts based on gelatin/phytic acid supermolecules for electrocatalytic oxygen reduction. Int J Hydrogen Energy 2019;44:5890e8. https://doi.org/10.1016/ j.ijhydene.2019.01.044. [62] Ferrari AC. Raman spectroscopy of graphene and graphite: disorder, electronephonon coupling, doping and nonadiabatic effects. Solid State Commun 2007;143:47e57. https://doi.org/10.1016/j.ssc.2007.03.052. [63] Huang H, Feng X, Du C, Wu S, Song W. One-step pyrolytic synthesis of small iron carbide nanoparticles/3D porous nitrogen-rich graphene for efficient electrocatalysis. J Mater Chem 2015;3:4976e82. https://doi.org/10.1039/C4TA06742H. [64] Wang M-Q, Yang W-H, Wang H-H, Chen C, Zhou Z-Y, Sun SG. Pyrolyzed FeeNeC composite as an efficient non-precious metal catalyst for oxygen reduction reaction in acidic medium. ACS Catal 2014;4:3928e36. https://doi.org/10.1021/ cs500673k.
Please cite this article as: Cao Z et al., Fe3C nanoparticles decorated Fe/N codoped graphene-like hierarchically carbon nanosheets for effective oxygen electrocatalysis, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.12.030