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Binary Fe, Cu-doped bamboo-like carbon nanotubes as efficient catalyst for the oxygen reduction reaction
Author’s Accepted Manuscript Binary Fe, Cu-doped Bamboo-like Carbon Nanotubes as Efficient Catalyst for the Oxygen Reduction Reaction Wenjun Fan, Zelong Li, Chenghang You, Xu Zong, Xinlong Tian, Shu Miao, Ting Shu, Can Li, Shijun Liao www.elsevier.com/locate/nanoenergy
PII: DOI: Reference:
S2211-2855(17)30273-2 http://dx.doi.org/10.1016/j.nanoen.2017.05.001 NANOEN1942
To appear in: Nano Energy Received date: 17 October 2016 Revised date: 21 April 2017 Accepted date: 1 May 2017 Cite this article as: Wenjun Fan, Zelong Li, Chenghang You, Xu Zong, Xinlong Tian, Shu Miao, Ting Shu, Can Li and Shijun Liao, Binary Fe, Cu-doped Bamboo-like Carbon Nanotubes as Efficient Catalyst for the Oxygen Reduction Reaction, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.05.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Binary Fe, Cu-doped Bamboo-like Carbon Nanotubes as Efficient Catalyst for the Oxygen Reduction Reaction Wenjun Fana,b, Zelong Lib, Chenghang Youa, Xu Zongb, Xinlong Tiana, Shu Miaob, Ting Shua, Can Lib* and Shijun Liaoa* a
The Key Laboratory of Fuel Cell Technology of Guangdong Province & the Key Laboratory of New Energy Technology of Guangdong University, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong, 510641(China) b State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning, 116023(China) [email protected] [email protected] * Corresponding author. Prof. Can Li, Fax: (+86)-411-84694447 * Corresponding author. Prof. Shijun Liao, Fax: (+86)-020-87113586
Abstract In this work, CuCl2 as a promoter was added into the mixture of polythiophene (PTh), FeCl 3, and melamine for preparing Fe-Cu-N/C catalyst. The catalyst features one-dimensional bamboo-like carbon nanotubes with few metal oxide nanoparticles encapsulated into tubes. The catalyst exhibits excellent activity toward the oxygen reduction reaction (ORR) with half-wave potential 50 mV more positive than the commercial Pt/C in 0.1 M KOH. It also shows comparable ORR activity in 0.1 M HClO 4 solution. Moreover, it exhibits superior long-term stability and excellent methanol tolerance in both alkaline and acidic solutions. The outstanding catalytic performance of Fe-Cu-N/C catalyst can be ascribed to the doping of Cu in the Fe-N-C architecture, which promotes the formation of bamboo-like nanotube structure and the generation of interaction among Cu and Fe-N-C. This synthetic strategy may open new avenues for constructing highly efficient electrocatalysts that adding of an inactive metal can obviously promote the catalytic performance of catalysts.
Graphical abstract
Oxidant
Thiophene
FeCl3, CuCl2
1.Carbonized
Melamine
2.Acid leaching, pyrolyzing
Polythiophene
Fe-Cu-N/C
-2 Current density (mA cm )
0 -1 -2 -3 -4 -5
C-PTh Cu-N/C Fe-Cu-N/C
-6 -7
0.2
0.4
0.6
0.8
Fe/C Fe-N/C 20% Pt/C 1.0
Potential(V vs. RHE)
Keywords: Binary doping; Copper; Carbon nanotube; Electrocatalyst; Oxygen reduction reaction
1. Introduction Fuel cells are promising energy conversion devices that generate almost no operating emissions. However, the kinetically slow ORR at the cathode plays a detrimental role for the energy conversion efficiency. Although platinum (Pt) is the most effective catalyst for ORR, its scarcity, high cost and low stability greatly hinder the practical applications. To overcome those shortcomings, numerous attempts have been made to improve the performance, such as alloying Pt with secondary transition metals [1-4], or depositing atomic layers of Pt onto other fine metal particles to create core-shell structures [5, 6]. Nevertheless, these well-desinged Pt-based catalysts still suffer from the intrinsic problems. Recently, nonprecious metal-based [7-9] or metal-free [10, 11] catalysts have emerged as potential candidates for Pt-based catalysts. In particular, transition metal (especially for Fe or Co)/N-doped carbon materials (M-N-C) [12-14] have been demonstrated as promising non-platinum ORR catalysts, where the interaction between transition metals and nitrogen-doped carbon endows their high activity and stability [8, 15]. It has been indicated that the introduction of a second metal, especially a third metal, into Pt-based catalysts can finely tune the chemical and electronic properties of the surface layer and hence modulate their catalytic performance [16-18]. Based on the modification strategies of Pt-based catalysts, it becomes interesting on how to improve the catalytic performance of M-N-C catalysts by tuning the microstructures of active sites. Previous report has been demonstrated that the doping of nitrogen into carbon could realize the chemical coupling among metal oxide and N-doped carbon, leading to higher ORR activity in alkaline solution [19]. Simultaneously, more nitrogen doped into carbon could further
facilitate the formation of active sites as well as enhance the interaction among metal oxide and the N-doped carbon [20], indicating that more nitrogen species doped into carbon can not only promote the formation of the coupling among metal and nitrogen-doped carbon but also modulate the electronic properties of metal through the interaction. Nitrogen species play a crucial role for the outstanding catalytic performance of M-N-C catalysts, in which nitrogen species can act as potential promoter to construct the active sites and tune the electronic properties of catalysts. Furthermore, the incorporation of another metal species can effectively tune the microstructures of the single metal Fe- or Co-based N-doped carbon electrocatalysts, thus delivering better ORR performance [8]. Therefore, it is highly desired to introduce the synthetic strategy of Pt-metal catalyst into M-N-C catalysts, in which another metal can be added as a promoter to further improve the catalytic performance of M-N-C catalysts [21]. Copper has a rich redox chemistry which presents multiple valence states (Cu (0), Cu (I), Cu (II) or even Cu (IV)), depending on the types of reaction [22]. Cu compounds exhibit biomimetic chemistry with O2, such as the reductive activation of O2 in enzymes and the protein laccase that catalyze the four-electron reduction of oxygen to water at overpotentials lower than those of noble metals [23, 24]. In this work, an efficient strategy was developed for fabricating Fe-Cu-N/C catalyst using PTh, FeCl3, CuCl2 and melamine as precursors, in which Cu was added as a promoter into the Fe-N/C architecture. The introduction of Cu helps to form bamboo-like nanotube structure, with enhanced the mass transfer and increased number of active sites. The resulting catalyst exhibits excellent ORR performance in both alkaline and acidic solutions, and it shows superior long-term stability and excellent methanol tolerance in both solutions. More importantly the activity toward ORR was greatly enhanced after the relatively inactive Cu was doped into Fe-N-C architecture [25]. The unusual catalytic activity also arises from chemical coupling effects among Cu and Fe-N-C that modifies the electronic structures of catalyst. To the best of our knowledge, the present Fe-Cu-N/C catalyst is one of the most efficient non-noble-metal catalysts ever reported for ORR [26].
2. Materials and methods 2.1 Reagents All of the chemicals were analytical grade and used without further purification unless otherwise stated. Thiophene monomer was freshly distilled prior to use. Hydrogen peroxide (H 2O2), anhydrous ferrous chloride (FeCl2), anhydrous ferric chloride (FeCl3) and copper chloride (CuCl2) were obtained
from Aladdin. Nafion (5 wt%) was purchased from Dupont Corp. (USA). Commercial 20 wt% Pt/C electrocatalyst was purchased from Johnson Matthey (UK).
2.2 Preparation of materials Thiophene monomer (1.8 mL) was dispersed in deionized water (200 mL), and a trace amount of FeCl2 (8 mg) was added as a catalyst to form a well-dispersed suspension, which was then stirred for 30 min. Next, oxidant H2O2 (1.3 mL) was slowly added in. The suspension was kept for 24 h under continuous stirring in an ice bath to complete the polymerization, then melamine (7.2 g), FeCl 3 (1.25 g) and CuCl2 (1.0 g) were added. The mixture was stirred and evaporated to remove the solvent, yielding the hybrid precursor. The mechanism for the polymerization of thiophene via chemical oxidation has been described elsewhere [27]. Afterwards, the catalyst was prepared through a two-stage pyrolysis process. First, the hybrid precursor was pyrolyzed at 900 oC in an argon atmosphere for 1 h, then the sample was leached in a solution of 0.5 M H2SO4 and 0.5 M FeCl3 at 80 oC for 10 h, followed by thoroughly rinsing with deionized water. Finally, the catalyst was obtained by graphitizing the leached sample in an argon atmosphere at 900 oC for 1 h. The acid and FeCl3 leaching removes the poor conductive impurity phases [15] and metallic Cu, respectively, thereby creating more active sites and more pores; the second heat treatment repairs the partially oxidized catalyst surface generated during the acid leaching step [28]. The prepared catalysts are labeled as Fex-Cuy-N/C (x, y represent the relative metal doping content). For comparison, a series of doped catalysts labeled as Cu-N/C, Fe-N/C, Fe/C and C-PTh were also prepared using the same procedures (which are prepared without FeCl3, without CuCl2, without melamine and CuCl2, and with only polythiophene as the precursor, respectively).
2.3 Electrochemical measurements Electrochemical measurements were conducted on an electrochemical workstation (Ivium, Netherlands) coupled with a rotating disk electrode (RDE) system (Pine Research Instrumentation, USA). A standard three-electrode electrochemical system equipped with a gas system was used. A glassy carbon electrode (GCE) with a diameter of 5 mm and an electrode area of 0.196 cm2 was used as the working electrode. The as-prepared catalysts or Pt/C catalyst (20 wt%) were dispersed in a mixture of ethanol and Nafion (0.25 wt%) and then casted onto the pretreated GCE surface with a loading of 0.25 mg cm-2 and 0.05 mg cm-2, respectively. An Ag/AgCl electrode (filled with 3 M KCl solution) was used as the reference electrode, and a Pt wire was used as the counter electrode. All potentials in this
study initially measured refer to that of Ag/AgCl electrode were converted to a reversible hydrogen electrode (RHE). The ORR performance of the catalysts was studied using linear sweep voltammetry (LSV) in 0.1 M KOH solution at a scan rate of 10 mV s–1 and at a disk rotation rate of 1600 rpm. Before every measurement, the KOH solution was saturated with pure N2 (99.999%) or pure O2 (99.999%) for at least 30 min. All the LSV measurements were performed at room temperature. All current densities were normalized to the geometric area of the RDE (0.1964 cm2). The chronoamperometric response was obtained at 0.7 V (vs. RHE) in O2-saturated 0.1 M KOH solution. The electron transfer number per oxygen molecule involved was calculated based on the Koutecky-Levich (K-L) equation as follows
J 1 J L1 J K1 B 1 1/2 J K1
(1)
where J is the measured current, Jk is the kinetic-limiting current, and ω is the electrode rotation rate. By changing the electrode rotation rate, we obtained a series of current values (J). A linear plot could then be obtained by plotting J versus ω. The electron transfer number (n) per oxygen molecule involved could be calculated from the slope (B) of the linear plot and the following relationship:
B 0.62nFC O DO2/3v 1/6 2
2
(2)
where n is the overall number of transferred electrons in the ORR process, F is the Faradaic constant (96485 C mol–1),
CO2 is the oxygen concentration (solubility) in 0.1 M KOH (1.2 × 10–6 mol cm–3),
DO2 is the oxygen diffusion coefficient in 0.1 M KOH (1.90 × 10–5 cm2 s-1 ) and v is the kinematic viscosity of 0.1 M KOH (0.01 cm2 s–1). For the rotating ring-disk electrode (RRDE) measurement, the catalyst ink and electrode were prepared by the same method as the RDE. The disk electrode was scanned cathodically at a rate of 10 mv s-1 and the ring potential was constant at 1.5 V (vs RHE). The peroxide species and the electron transfer number (n) were determined using the following equations: HO2− % = 200 × n=4 ×
Ir ⁄N Id + Ir ⁄N
Id Id + Ir ⁄N
(3) (4)
2.4 Characterizations Scanning electron microscopy (SEM) was conducted on a Zeiss Merlin field emission scanning electron microscope (Germany). Transmission electron microscopy (TEM) images were recorded on a
JEM-2100 HR microscope (JEOL, Japan) operated at 200 kV. Sub angstrom resolution HAADF STEM images were obtained on a JEOLJEM-ARM200F STEM/TEM, equipped with a CEOS probe corrector, with an attainable resolution of 0.08 nm. The sample was suspended in ethanol with an ultrasonic dispersion for 5 -10 min and then a drop of the resulting solution was dropped on a holey carbon film supported by a Au TEM grid. X-ray diffraction (XRD) patterns were recorded on Rigaku D/Max-2500/PC powder diffractometer operating at 40 kV and 200 mA with Cu Kα radiation (λ= 0.154 nm) at a scanning rate of 5o/min. The X-ray photoelectron spectrometer (XPS, Kratos-England) employed a monochromated Al-Kα X-ray source (hυ=1486.6 eV). Specific surface areas and pore-size distributions were measured by Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption on a Tristar II 3020 (Micromeritics, USA) gas adsorption analyzer.
3. Results and Discussions
Oxidant
Thiophene
FeCl3, CuCl2
1.Carbonized
Melamine
2.Acid leaching, pyrolyzing
Polythiophene
Fe-Cu-N/C
Scheme 1 Schematic diagram of the process for preparing Fe-Cu-N/C.
The formation of Fe-Cu-N/C is proposed in Scheme 1. First, polythiophene microspheres was synthesized through a facile polymerization and coated with FeCl3, CuCl2 and melamine. Then the sample underwent carbonization at 900oC in an argon environment, followed by acid and FeCl3 leaching. Finally, a second heat treatment process was performed for further carbonization, thus the bamboo-like Fe-Cu-N/C was generated. For comparison, a series of catalysts were prepared and labeled as Fex-Cuy-Nz/C, in which x, y and z represent the relative elemental doping content, respectively.
Figure 1. Morphology characterization of Fe-Cu-N/C catalyst. (a) SEM image; (b) TEM image, and (c,d) HRTEM images.
The morphology of the catalysts from various precursors was investigated using SEM and TEM. C-PTh exhibits a homogeneous spherical morphology with a diameter of ca. 80 nm (Figure S1a), while the resulting Fe/C catalyst retains a well-defined spherical morphology after the addition of Fe into the precursor (Figure S1b and S1d). In contrast, the obvious aggregation is observed in Fe-N/C (Figure S1c and S1e) and Cu-N/C (Figure S2) by introducing nitrogen species into the precursor. Particularly, when Cu and Fe were added simutaneously, the morphology of the catalysts changed dramatically. When the atomic ratio of Fe to Cu was 2:1, both nanospheres and bamboo-like nanotubes were obtained (Figure S3a and S3c), and when the ratio reduced to 1:2, the irregular nanoparticles or porous slice sheets appear (Figure S3b and S3d). It is interesting that when the molar ratio of Fe to Cu was adjusted to 1:1, the Fe-Cu-N/C exhibits a well-defined and uniform nanotube morphology composing of individual cylindrical compartments linked to form long nanotubes with over 1 μm in length and ca. 80 nm in diameter (Figure 1a and b). The TEM images of a single nanotube also reveal numerous stacked cylindrical compartments with porous structure (Figure 1 c and d), which are corresponding to the results of the N2 adsorption-desorption (Figure S5). C-PTh, Fe/C, Fe-N/C and Fe-Cu-N/C exhibit type IV curves with hysteresis loops in the medium- and high- pressure regions, indicating that the samples contain both micropores and mesopores (Figure S5a). The Fe-Cu-N/C catalyst shows the highest BET specific surface area among these catalysts and reaches 588 m2 g-1 (Figure S5c). The pore-size distribution of Fe-Cu-N/C shows a higher pore density in the micropore and mesopore regions, as well as macropores between 100 and 150 nm, confirming its hierarchical porous structure (Figure S5b). Furthermore, the calcination temperature also plays an important role for the formation of nanotubes; at lower temperature (800 oC), only metal-based nanoparticles coated with carbon layers were formed
(Figure S4a); as the temperature reached 1000 oC, graphene-like structures appeared (Figure S4c). These results reveal that the coexistence of Fe and Cu under appropriate molar ratio and calcination temperature is crucial for the formation of bamboo-like nanotubes. (a)
(b)
Figure 2. HAADF/STEM images and the corresponding EDS elemental mapping (C, N, O, Fe, and Cu) of Fe-Cu-N/C (a) a single nanotube, Scale Bar 250 nm; (b) a single nanoparticle, Scale Bar 50 nm.
Element mapping using EDS coupled with HAADF/STEM was used to investigate the compositional distributions of the nanotubes. The images show that C, N, O, Fe and Cu are distributed uniformly along the nanotube (Figure 2a). Furthermore, the homogeneous distribution of O, Fe and Cu components is also presented on a single particle inside the nanotube (Figure 2b), indicating that the interaction has been formed among Fe, Cu and O. The XRD patterns also reveal the effect of the interaction, and the incorporation of the Cu can obviously affect the phase formation of CFe15.1, Fe2O3, and Fe3O4 which has been identified in Fe-N/C (Figure 3a). The Fe-Cu-N/C after pyrolysis is detected to be CuFe2O4, Fe2O3, CuO and Cu (Figure 3a), whereas only CuFe 2O4 and Fe3O4 are detected in Fe-Cu-N/C after the acid/FeCl3 wash and second pyrolysis, indicating that some of the inactive species and exposed particles have been removed (Figure 3b). The peak at about 24 o is assigned to the (002) plane of carbon in all samples, but this peak becomes sharper and shifts positively after metal doping, indicating the increase of the graphitization degrees (Figure 3a). Especially for Fe-Cu-N/C, a typical strong peak is observed at about 26o, in agreement with the morphology feature of bamboo-like carbon nanotubes [29]. After acid/FeCl3 leaching and second pyrolysis, the patterns of the respective carbon peaks show no significant change (Figure 3b). It has been reported that Fe compound as a graphitization catalyst can lead to graphitization by a complex process involving the dissolution of carbon atoms into the catalyst, followed by the precipitation of graphitized carbon [30, 31] and
generating highly porous structure [30], which is significant for creating more active sites and enhancing ORR activity.
Fe-Cu-N/C
CFe15.1 Fe O 3 4 Fe O 2
CuFe O 2 4 Fe O 2
3
Fe-N/C
(c) Oxidized
C0.12Fe1.88
Fe-Cu-N/C
Fe O 2 3 Fe-N/C
Fe-N/C N 1s
Graphitic
3
Intensity (a.u.)
Intensity (a.u.)
(b)
CuFe2O4 Fe2O3 CuO Cu
Intensity (a.u.)
(a)
Pyrrolic
Pyridinic
C-PTh 30
40
50
60
70
80
90
10
100
20
30
40
722.3
711.8
725
720
715
710
705
Intensity (a.u.)
Intensity (a.u.)
730
406
404
Pyrrolic
402
400
398
402
722.3
Pyridinic
Binding Energy (eV)
Pyrrolic
402
400
400
725.8
396
735
730
725
396
394
Cu-N/C Cu 2p
(f)
933.3
398
396
394
954.7
935.5
952.8
Satellite peak
960
955
950
945
940
935
930
925
Binding Energy (eV)
713.5
715
Binding Energy (eV)
710.4
710
705
Fe-Cu-N/C Cu 2p
(i)
Fe-Cu-N/C Fe 2p
720
398
Binding Energy (eV)
Pyridinic
(h)
Fe-Cu-N/C N 1s
Oxidized
404
100 404
90
Binding Energy (eV)
Intensity (a.u.)
Intensity (a.u.)
Graphitic
80
Oxidized
Binding Energy (eV)
(g)
70
Cu-N/C N 1s Graphitic
709.7
735
60
(e)
Fe-N/C Fe 2p
(d) 724.9
50
2 angel (degrees)
2 angel (degrees)
Intensity (a.u.)
20
932.6
952.4 Intensity (a.u.)
10
935.0 Satellite peak
954.1
960
955
950
945
940
935
930
Binding Energy (eV)
Figure 3. XRD patterns of C-PTh, Fe-N/C and Fe-Cu-N/C: (a) after first pyrolysis; (b) after acid/FeCl3 and second pyrolysis. XPS survey spectra of Fe-N/C, Cu-N/C and Fe-Cu-N/C catalysts: XPS patterns of N 1s from (c) Fe-N/C, (e) Cu-N/C and (g) Fe-Cu-N/C; the Fe 2p from (d) Fe-N/C and (h) Fe-Cu-N/C; the Cu 2p from (f) Fe-N/C and (i) Fe-Cu-N/C.
XPS was conducted to further investigate the role of Cu for the prepared catalysts (Figure 3). The Fe 2p XPS spectra for Fe-N/C and Fe-Cu-N/C can be deconvoluted into Fe (II) and Fe (III) species, which is consistent with the XRD results. Notably, the Fe and N contents in Fe-Cu-N/C (Fe: 0.93 at%, N: 6.08%; Table S1) are higher than those of the Fe-N/C catalyst (Fe: 0.57at%, N: 4.22%; Table S1), suggesting that the incorporation of Cu is beneficial for the formation of Fe and N species which may derive from the stronger interaction among metals and nitrogen on the catalysts. Furthermore, the peaks of Fe 2p for Fe-Cu-N/C shift to higher binding energy compared with Fe-N/C, which is attributed to the electron delocalization between Fe and the carbon nanotubes (CNTs), as a result of electron from Fe to CNTs [32]. The electron delocalization also alters the electronic structure of the metallic species, enabling uniform distribution of Fe and Cu species. The N 1s spectra in all the catalysts are
deconvoluted to four peaks locating at 398 ± 0.4 , 399.4 ± 0.4 , 400.8 ± 0.4 , and 402.0 ± 0.4 eV, which are assigned to pyridinic N [33], pyrrolic N [34], graphitic N [35] , and oxidized N species [36], respectively. It is shown that the relative content of inactive oxidized N in Cu-N/C is the lowest, and the relative content of oxidized N in Fe-Cu-N/C is much lower than that in Fe-N/C (Table S2), revealing the role of Cu for suppressing the formation of inactive oxidized N species. Briefly, the incorporation of Cu enhances the interaction among Fe and N species, as a result, the electronic structure is modified and the contents of Fe and N species on the surface of catalyst increase, all of those contribute to the enhanced ORR performance. 0
C-PTh Cu-N/C Fe-Cu-N/C 0.2
0.4
-2 Current density (mA cm )
-2 Current density (mA cm )
(a)
Fe/C Fe-N/C
(b)
-1 -2
50 mV
-3 -4 -5
C-PTh Cu-N/C Fe-Cu-N/C
-6 -7
0.6
0.8
1.0
0.2
1.2
0.4
2
0.18
(c)
-2 -4 -6 -8
1600 rpm 2500 rpm 3600 rpm
-10 -12
0.2
0.4
0.6
2000 rpm 3000 rpm
0.8
1.0
Potential(V vs. RHE)
0.7 V 0.6 V 0.5 V 0.4 V
0.12 0.09 0.05
0.06
0.07
-1/2
Normalized Current (%)
120
0.65 V 0.55 V 0.45 V 0.08
Fe-Cu-N/C Pt/C
12 8 4 4.0 3.9 3.8 3.7 3.6 3.5
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Potential (V vs. RHE) 120
99.2
80
80.4
60 40 20
Fe-Cu-N/C Pt/C 5000
10000
1.0
(d)
16
1/2
(e)
0
0.8
20
(s /rad)
100
0
n
0.15
Normalized Current (%)
-1
0.21
-1
J (mA cm )
0.24
Peroxide yield/%
0.27
0.6
Potential(V vs. RHE)
0
-2
0.30
Current density (mA cm )
Potential(V vs. RHE)
Fe/C Fe-N/C 20% Pt/C
15000
Time (s)
20000
MeOH
(f)
100 80 60 40 20 0
Fe-Cu-N/C Pt/C 0
100
200
300
400
500
600
Time (s)
Figure 4. Electrochemical measurements for catalysts in O2-saturated 0.1 M KOH solution. (a) CVs; (b) LSVs at 1600 rpm; (c) K-L plots of Fe-Cu-N/C, calculated from the LSVs of Fe-Cu-N/C at different rotating rates in the inset; (d) The electron transfer number (n) (bottom) and peroxide yield (top) of the Fe-Cu-N/C and Pt/C. Current-time (i-t) chronoamperometric response of Fe-Cu-N/C and commercial 20% Pt/C electrodes in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm: (e) at 0.7 V for 20,000 s; (f) upon addition of 3 M methanol after 200 s.
The electrocatalytic activity of the as-prepared Fe-Cu-N/C and other samples was first investigated by cyclic voltammogram (CV). As shown in Figure 4a, the obviously enhanced cathodic peak (0.9 V vs RHE) for Fe-Cu-N/C in O2-saturated solution compared with that in N2-saturated electrolyte implies a
significant ORR activity of this sample. By contrast, only weaker peaks for cathodic reduction are observed for C-PTh, Fe/C, Cu-N/C and Fe-N/C (0.64, 0.71, 0.79 and 0.81 V vs RHE, respectively). The linear sweep voltammetry (LSV) curve of C-PTh demonstrates a slow current increase with no current plateau (Figure 4b), indicating predominantly a two-electron reduction of O2 to OOH– [37]. By contrast, Fe/C and Fe-N/C exhibit markedly enhanced ORR activity with high half-wave potential and limiting current density. In particular, the catalytic performance is greatly promoted after doping of Cu into Fe-N/C catalyst, implying that the introduction of Cu improves the ORR performance and this matches with the results of XPS. Moreover, Fe-Cu-N/C catalyst exhibits superior ORR activity in comparison with commercial Pt/C (20%) catalyst, with the half-wave potential (0.89 V) 50 mV more positive than that of Pt/C (0.84 V). Notably, the incorporation of only Cu into nitrogen doped carbon to derive Cu-N/C only exhibits poor ORR activity, which matches well with the work demonstrated by our group [25]. Therefore, the doping of Cu plays a crucial role for the formation of synergic effect among Cu and Fe-N-C architecture and enhancing ORR activity of Fe-N/C, which facilitates the generation of bamboo-like structure and the interaction among Fe and N species by tuning the active sites in microstructure (Figure S6) [8, 38]. The RDE measurements at different rotation rates were performed to determine the electron transfer number of ORR on the Fe-Cu-N/C catalyst (Figure 4c inset). The Koutecky-Levich (K-L) plots of Fe-Cu-N/C show good linearity and parallelism, indicating first-order reaction kinetics with respect to the concentration of dissolved O 2 (Figure 4c). The electron-transfer number (n) is calculated to be approximately 3.98, revealing the Fe-Cu-N/C catalyst follows a four-electron transfer pathway [39]. The ORR pathways were also determined by RRDE measurement. The percentage of peroxide species with respect to the total oxygen reduction products and the electron transfer number (n) are calculated from the RRDE curves according to eqn (3) and (4), and the results are shown in Fig. 4d. The yield of peroxide species for the Fe-Cu-N/C catalyst was less than 10% over the measured potential range, although it is slightly higher than that of the commercial Pt/C catalyst. The electron transfer number calculated from the RRDE measurements was over 3.85 for Fe-Cu-N/C, which indicated that the ORR catalyzed by Fe-Cu-N/C is mainly through the four electron (4e) pathway. To further evaluate the suitability as ORR catalysts, the tolerance to methanol crossover of Fe-Cu-N/C was assessed at 0.7 V (vs RHE). Evidently, the original catalytic current of Fe-Cu-N/C
experienced only a slight attenuation (0.8%) after continuous reaction for 20,000 s, whereas the Pt/C catalyst retained only 80.4% of its initial performance (Figure 4e). After methanol was introduced, the ORR performance of Pt/C catalyst displays a dramatic drop in relative current density, whereas almost no apparent change is observed for the Fe-Cu-N/C electrode (Figure 4f), suggesting its great potential
0
(a)
0.27
-1
0.24
-2
2
J (mA cm )
0.21
-1
-3 -4
-1
-2
Current Density (mA cm )
0
2
0.30
Current Density (mA/cm )
for the cathode in direct methanol fuel cells [40]. (b)
-2 -4 1600 rpm 2000 rpm 2500 rpm 3000 rpm 3600 rpm
-6 -8
-10 0.0
0.2 0.4 0.6 0.8 1.0 Potential vs RHE (V)
0.18 0.5 V 0.4 V 0.3 V 0.2 V
0.15
-5
Pt/C Fe-Cu-N/C
-6 0.0
0.2
0.4
0.6
0.8
0.12 0.05
1.0
0.06
100
0.08
1/2
(s /rad)
(d)
2
Current Density (mA/cm )
Normalized current (%)
(c) 100
90
80
70
60
0.07 -1/2
Potential vs RHE (V)
0.45 V 0.35 V 0.25 V
Pt/C Fe-Cu-N/C 0
5000
10000
15000
20000
Time (s)
50
0
-50
Pt/C Fe-Cu-N/C
-100 0
100
200
300
400
500
Potential vs RHE (V)
Figure 5. Electrochemical measurements of Fe-Cu-N/C in O2-saturated 0.1 M HClO4 solution. (a) comparison of the LSVs of Fe-Cu-N/C with 20% Pt/C catalyst; (b) K-L plots at different electrode potentials for Fe-Cu-N/C. Inset: RDE voltammograms for the ORR at various rotation speeds. Current-time (i-t) chronoamperometric response of Fe-Cu-N/C and commercial 20% Pt/C electrodes (c) at 0.7 V (vs RHE) in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm; (d) when 3 M methanol was added after 100 s.
Notably, the Fe-Cu-N/C catalyst is also active and durable for ORR in acidic solution. In O2-saturated 0.1 M HClO4, the ORR polarization curve of Fe-Cu-N/C exhibits a high half-wave potential (0.71 V vs RHE), which is comparable to that of commercial 20% Pt/C (0.78 V vs RHE, Figure 5a). The electron transfer number (n) calculated from the K-L equation (Figure 5b) reveals nearly a four electron transfer process. Furthermore, the Fe-Cu-N/C catalyst shows much better stability for ORR than the Pt/C catalyst under potentiostatic condition (Figure 5c), and exhibits excellent tolerance to crossover effect (Figure 5d).
4. Conclusion In summary, we have developed a facile and effective synthesis approach for preparing carbon-based material with finely controlled morphology and hierarchical porous structure via simply pyrolyzing
nanospherical polythiophene with a mixture of FeCl3, CuCl2 and melamine. By adjusting the calcination temperature (900oC) and tuning the Cu/Fe ratio (1:1), the Fe-Cu-N/C with one-dimensional bamboo-like carbon nanotube morphology, hierarchical nanostructure, and bimetallic Fe, Cu-doping has been successfully obtained. The resulting Fe-Cu-N/C exhibits high ORR activity, outstanding long-term stability and excellent methanol tolerance in an alkaline and acidic medium, which is superior to the Pt/C catalyst. The outstanding catalytic performance of Fe-Cu-N/C can be attributed to the one-dimensional bamboo-like structure and high surface area, providing enhanced mass transfer and more active sites. Furthermore, the synergic effect among Cu and Fe-N-C could also greatly enhance the activity, modulating the microstructure of the catalyst. This study presented here opens good strategies for the preparation of electrocatalysts with excellent activity and durability materials toward energy storage and conversion.
Acknowledgements This work was supported by 973 National Basic Research Program of the Ministry of Science and Technology (grant 2014CB239400), National Natural Science Foundation of China (no. 21090340), National Science Foundation of China (NSFC Project Nos. 21276098, 21476088, 51302091, U1301245), Natural Science Foundation of Guangdong Province (Project No.2015A030312007), and Educational Commission of Guangdong Province (Project No. 2013CXZDA003)
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Wenjun Fan received his B.S. degree and Master degree from Hebei University of Science and Technology and Kunming University of Science and Technology in 2010 and 2014, respectively. Currently, he is a Ph.D candidate of South China University of Technology under the supervision of Prof. Shijun Liao and Prof. Can Li. His research focuses on catalyst design for fuel cells and energy conversion.
Zelong Li obtained his B.S. degree and Ph. D degree from Dalian University of Technology and Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Science (CAS) in 2008 and 2013, and then worked as Research Assistant in LICP until 2015. Currently, he worked as a post-doctoral of Dalian Institute of Chemical Physics (DICP), CAS under the supervision of Prof. Can Li. His research interests focus on heterogeneous catalysis for CO2 hydrogenation and electrocatalysis for fuel cells.
Chenghang You obtained his master degree in 2012 from Hainan University and Ph.D degree in 2015 from South China University of Technology. Since 2015, he has served as an associate professor in Hainan Normal University. Now, his interest focus on the electrocatalysts and electrode materials for novel electrochemical energy conversion and storage devices, such as PEMFC, DMFC, metal-air batteries, super capacitors, and etc.
Xu Zong received his Ph.D degree in Physical Chemistry from Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS) in 2010. He then worked as a postdoctoral research fellow in the University of Queensland from 2010-2014. In 2014, he returned to DICP as the recipient of Hundred Talent Program of DICP and was promoted to professor in 2015 as the recipient of One Thousand Youth Talents Program of China. His research interests focus on the conversion of solar energy to chemical energy via photocatalytic and photoelectrochemical approaches. He has published more than 50 peer-reviewed papers with over 2800 citations.
Xinlong Tian obtained his bachelor degree in 2010 from Tianjin University of Science & Technology and master degree in 2013 Guangdong University of Technology, and then focused on developing effective catalyst for fuel cells as a Ph.D student at South China University of Technology. He is currently working as a post-doctoral research associate at Huazhong University of Science and Technology under the supervision of Prof. Bao Yu Xia, now his research interests include the synthesis of low platinum core–shell structured catalyst, transition metal nitride nanostructures and metal organic frameworks for fuel cells and energy conversion.
Shu Miao is a full professor at Dalian Institute of Chemcial Physcis, CAS. He received his B.A. and M.Eng.from Tsinghua University in 1999 and 2002, respectively. After receiving his PhD from California Institute of Tecnhology in 2007, he worked as a Postdoc in CEMES-CNRS (2007-2008) and University of Sheffield (2008-2010). He joined Dalian Institute of Chemcial Physics in 2011. His research focuses on development of electron microsocpy techniques and their applications in catalysis and energy materials.
Ting Shu received her Ph.D degree from South China University of Technology (Guangzhou, China) in 2013. Her study focuses on the research of high performance MEA prepared by direct deposition of platinum on the gas diffusion layer using atomic layer deposition technique or electrochemical deposition technique.
Can Li received his Ph.D degree in Physical Chemistry from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in 1989, and he joined the same institute and was promoted to full professor in 1993. He is the director of Dalian National Laboratory for Clean Energy and the director of State Key Laboratory of Catalysis. He has 600 publications, 13000 citations, an H-index 60, 60 granted patents and over 70 plenary and keynote lectures at international
conferences.
His
research
interests
include
Photo(electro)catalysis,
Environmental Catalysis, Chiral Catalysis, Biocatalysis, In-situ Characterization of Catalytic Reaction, Time-resolved Spectroscopy.
Shijun Liao is a chair professor and director of The Laboratory of Fuel Cell Technology of Guangdong Province, South China University of Technology, he received his Ph.D in 1999 in South China University of Technology, and conducted his research works in Prof. Viola Birss group of Calgary University, Canada, and in Dr. Radoslav Adzic group of Brookhaven National Laboratory of USA respectively. He is the board members of six international journals, including Scientific Reports etc, he is also the winner of three national awards two location awards for excellent research, his research interests mainly focus on fundamental investigation and engineering development of PEM fuel cell.
Highlights
Binary Fe- and Cu-codoped carbon nanotubes was proposed as efficient catalyst for ORR.
The Fe-Cu-N/C catalyst exhibits uniform bamboo-like morphology, high surface area and hierarchical porous structure.
The doping of Cu into Fe-N-C architecture was found to effectively tune the electronic structure.
The Fe-Cu-N/C catalyst exhibits superior activity and stability to commercial Pt/C in 0.1M KOH and HClO4 solution.
PII: DOI: Reference:
S2211-2855(17)30273-2 http://dx.doi.org/10.1016/j.nanoen.2017.05.001 NANOEN1942
To appear in: Nano Energy Received date: 17 October 2016 Revised date: 21 April 2017 Accepted date: 1 May 2017 Cite this article as: Wenjun Fan, Zelong Li, Chenghang You, Xu Zong, Xinlong Tian, Shu Miao, Ting Shu, Can Li and Shijun Liao, Binary Fe, Cu-doped Bamboo-like Carbon Nanotubes as Efficient Catalyst for the Oxygen Reduction Reaction, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2017.05.001 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Binary Fe, Cu-doped Bamboo-like Carbon Nanotubes as Efficient Catalyst for the Oxygen Reduction Reaction Wenjun Fana,b, Zelong Lib, Chenghang Youa, Xu Zongb, Xinlong Tiana, Shu Miaob, Ting Shua, Can Lib* and Shijun Liaoa* a
The Key Laboratory of Fuel Cell Technology of Guangdong Province & the Key Laboratory of New Energy Technology of Guangdong University, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, Guangdong, 510641(China) b State Key Laboratory of Catalysis, Dalian National Laboratory for Clean Energy, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning, 116023(China) [email protected] [email protected] * Corresponding author. Prof. Can Li, Fax: (+86)-411-84694447 * Corresponding author. Prof. Shijun Liao, Fax: (+86)-020-87113586
Abstract In this work, CuCl2 as a promoter was added into the mixture of polythiophene (PTh), FeCl 3, and melamine for preparing Fe-Cu-N/C catalyst. The catalyst features one-dimensional bamboo-like carbon nanotubes with few metal oxide nanoparticles encapsulated into tubes. The catalyst exhibits excellent activity toward the oxygen reduction reaction (ORR) with half-wave potential 50 mV more positive than the commercial Pt/C in 0.1 M KOH. It also shows comparable ORR activity in 0.1 M HClO 4 solution. Moreover, it exhibits superior long-term stability and excellent methanol tolerance in both alkaline and acidic solutions. The outstanding catalytic performance of Fe-Cu-N/C catalyst can be ascribed to the doping of Cu in the Fe-N-C architecture, which promotes the formation of bamboo-like nanotube structure and the generation of interaction among Cu and Fe-N-C. This synthetic strategy may open new avenues for constructing highly efficient electrocatalysts that adding of an inactive metal can obviously promote the catalytic performance of catalysts.
Graphical abstract
Oxidant
Thiophene
FeCl3, CuCl2
1.Carbonized
Melamine
2.Acid leaching, pyrolyzing
Polythiophene
Fe-Cu-N/C
-2 Current density (mA cm )
0 -1 -2 -3 -4 -5
C-PTh Cu-N/C Fe-Cu-N/C
-6 -7
0.2
0.4
0.6
0.8
Fe/C Fe-N/C 20% Pt/C 1.0
Potential(V vs. RHE)
Keywords: Binary doping; Copper; Carbon nanotube; Electrocatalyst; Oxygen reduction reaction
1. Introduction Fuel cells are promising energy conversion devices that generate almost no operating emissions. However, the kinetically slow ORR at the cathode plays a detrimental role for the energy conversion efficiency. Although platinum (Pt) is the most effective catalyst for ORR, its scarcity, high cost and low stability greatly hinder the practical applications. To overcome those shortcomings, numerous attempts have been made to improve the performance, such as alloying Pt with secondary transition metals [1-4], or depositing atomic layers of Pt onto other fine metal particles to create core-shell structures [5, 6]. Nevertheless, these well-desinged Pt-based catalysts still suffer from the intrinsic problems. Recently, nonprecious metal-based [7-9] or metal-free [10, 11] catalysts have emerged as potential candidates for Pt-based catalysts. In particular, transition metal (especially for Fe or Co)/N-doped carbon materials (M-N-C) [12-14] have been demonstrated as promising non-platinum ORR catalysts, where the interaction between transition metals and nitrogen-doped carbon endows their high activity and stability [8, 15]. It has been indicated that the introduction of a second metal, especially a third metal, into Pt-based catalysts can finely tune the chemical and electronic properties of the surface layer and hence modulate their catalytic performance [16-18]. Based on the modification strategies of Pt-based catalysts, it becomes interesting on how to improve the catalytic performance of M-N-C catalysts by tuning the microstructures of active sites. Previous report has been demonstrated that the doping of nitrogen into carbon could realize the chemical coupling among metal oxide and N-doped carbon, leading to higher ORR activity in alkaline solution [19]. Simultaneously, more nitrogen doped into carbon could further
facilitate the formation of active sites as well as enhance the interaction among metal oxide and the N-doped carbon [20], indicating that more nitrogen species doped into carbon can not only promote the formation of the coupling among metal and nitrogen-doped carbon but also modulate the electronic properties of metal through the interaction. Nitrogen species play a crucial role for the outstanding catalytic performance of M-N-C catalysts, in which nitrogen species can act as potential promoter to construct the active sites and tune the electronic properties of catalysts. Furthermore, the incorporation of another metal species can effectively tune the microstructures of the single metal Fe- or Co-based N-doped carbon electrocatalysts, thus delivering better ORR performance [8]. Therefore, it is highly desired to introduce the synthetic strategy of Pt-metal catalyst into M-N-C catalysts, in which another metal can be added as a promoter to further improve the catalytic performance of M-N-C catalysts [21]. Copper has a rich redox chemistry which presents multiple valence states (Cu (0), Cu (I), Cu (II) or even Cu (IV)), depending on the types of reaction [22]. Cu compounds exhibit biomimetic chemistry with O2, such as the reductive activation of O2 in enzymes and the protein laccase that catalyze the four-electron reduction of oxygen to water at overpotentials lower than those of noble metals [23, 24]. In this work, an efficient strategy was developed for fabricating Fe-Cu-N/C catalyst using PTh, FeCl3, CuCl2 and melamine as precursors, in which Cu was added as a promoter into the Fe-N/C architecture. The introduction of Cu helps to form bamboo-like nanotube structure, with enhanced the mass transfer and increased number of active sites. The resulting catalyst exhibits excellent ORR performance in both alkaline and acidic solutions, and it shows superior long-term stability and excellent methanol tolerance in both solutions. More importantly the activity toward ORR was greatly enhanced after the relatively inactive Cu was doped into Fe-N-C architecture [25]. The unusual catalytic activity also arises from chemical coupling effects among Cu and Fe-N-C that modifies the electronic structures of catalyst. To the best of our knowledge, the present Fe-Cu-N/C catalyst is one of the most efficient non-noble-metal catalysts ever reported for ORR [26].
2. Materials and methods 2.1 Reagents All of the chemicals were analytical grade and used without further purification unless otherwise stated. Thiophene monomer was freshly distilled prior to use. Hydrogen peroxide (H 2O2), anhydrous ferrous chloride (FeCl2), anhydrous ferric chloride (FeCl3) and copper chloride (CuCl2) were obtained
from Aladdin. Nafion (5 wt%) was purchased from Dupont Corp. (USA). Commercial 20 wt% Pt/C electrocatalyst was purchased from Johnson Matthey (UK).
2.2 Preparation of materials Thiophene monomer (1.8 mL) was dispersed in deionized water (200 mL), and a trace amount of FeCl2 (8 mg) was added as a catalyst to form a well-dispersed suspension, which was then stirred for 30 min. Next, oxidant H2O2 (1.3 mL) was slowly added in. The suspension was kept for 24 h under continuous stirring in an ice bath to complete the polymerization, then melamine (7.2 g), FeCl 3 (1.25 g) and CuCl2 (1.0 g) were added. The mixture was stirred and evaporated to remove the solvent, yielding the hybrid precursor. The mechanism for the polymerization of thiophene via chemical oxidation has been described elsewhere [27]. Afterwards, the catalyst was prepared through a two-stage pyrolysis process. First, the hybrid precursor was pyrolyzed at 900 oC in an argon atmosphere for 1 h, then the sample was leached in a solution of 0.5 M H2SO4 and 0.5 M FeCl3 at 80 oC for 10 h, followed by thoroughly rinsing with deionized water. Finally, the catalyst was obtained by graphitizing the leached sample in an argon atmosphere at 900 oC for 1 h. The acid and FeCl3 leaching removes the poor conductive impurity phases [15] and metallic Cu, respectively, thereby creating more active sites and more pores; the second heat treatment repairs the partially oxidized catalyst surface generated during the acid leaching step [28]. The prepared catalysts are labeled as Fex-Cuy-N/C (x, y represent the relative metal doping content). For comparison, a series of doped catalysts labeled as Cu-N/C, Fe-N/C, Fe/C and C-PTh were also prepared using the same procedures (which are prepared without FeCl3, without CuCl2, without melamine and CuCl2, and with only polythiophene as the precursor, respectively).
2.3 Electrochemical measurements Electrochemical measurements were conducted on an electrochemical workstation (Ivium, Netherlands) coupled with a rotating disk electrode (RDE) system (Pine Research Instrumentation, USA). A standard three-electrode electrochemical system equipped with a gas system was used. A glassy carbon electrode (GCE) with a diameter of 5 mm and an electrode area of 0.196 cm2 was used as the working electrode. The as-prepared catalysts or Pt/C catalyst (20 wt%) were dispersed in a mixture of ethanol and Nafion (0.25 wt%) and then casted onto the pretreated GCE surface with a loading of 0.25 mg cm-2 and 0.05 mg cm-2, respectively. An Ag/AgCl electrode (filled with 3 M KCl solution) was used as the reference electrode, and a Pt wire was used as the counter electrode. All potentials in this
study initially measured refer to that of Ag/AgCl electrode were converted to a reversible hydrogen electrode (RHE). The ORR performance of the catalysts was studied using linear sweep voltammetry (LSV) in 0.1 M KOH solution at a scan rate of 10 mV s–1 and at a disk rotation rate of 1600 rpm. Before every measurement, the KOH solution was saturated with pure N2 (99.999%) or pure O2 (99.999%) for at least 30 min. All the LSV measurements were performed at room temperature. All current densities were normalized to the geometric area of the RDE (0.1964 cm2). The chronoamperometric response was obtained at 0.7 V (vs. RHE) in O2-saturated 0.1 M KOH solution. The electron transfer number per oxygen molecule involved was calculated based on the Koutecky-Levich (K-L) equation as follows
J 1 J L1 J K1 B 1 1/2 J K1
(1)
where J is the measured current, Jk is the kinetic-limiting current, and ω is the electrode rotation rate. By changing the electrode rotation rate, we obtained a series of current values (J). A linear plot could then be obtained by plotting J versus ω. The electron transfer number (n) per oxygen molecule involved could be calculated from the slope (B) of the linear plot and the following relationship:
B 0.62nFC O DO2/3v 1/6 2
2
(2)
where n is the overall number of transferred electrons in the ORR process, F is the Faradaic constant (96485 C mol–1),
CO2 is the oxygen concentration (solubility) in 0.1 M KOH (1.2 × 10–6 mol cm–3),
DO2 is the oxygen diffusion coefficient in 0.1 M KOH (1.90 × 10–5 cm2 s-1 ) and v is the kinematic viscosity of 0.1 M KOH (0.01 cm2 s–1). For the rotating ring-disk electrode (RRDE) measurement, the catalyst ink and electrode were prepared by the same method as the RDE. The disk electrode was scanned cathodically at a rate of 10 mv s-1 and the ring potential was constant at 1.5 V (vs RHE). The peroxide species and the electron transfer number (n) were determined using the following equations: HO2− % = 200 × n=4 ×
Ir ⁄N Id + Ir ⁄N
Id Id + Ir ⁄N
(3) (4)
2.4 Characterizations Scanning electron microscopy (SEM) was conducted on a Zeiss Merlin field emission scanning electron microscope (Germany). Transmission electron microscopy (TEM) images were recorded on a
JEM-2100 HR microscope (JEOL, Japan) operated at 200 kV. Sub angstrom resolution HAADF STEM images were obtained on a JEOLJEM-ARM200F STEM/TEM, equipped with a CEOS probe corrector, with an attainable resolution of 0.08 nm. The sample was suspended in ethanol with an ultrasonic dispersion for 5 -10 min and then a drop of the resulting solution was dropped on a holey carbon film supported by a Au TEM grid. X-ray diffraction (XRD) patterns were recorded on Rigaku D/Max-2500/PC powder diffractometer operating at 40 kV and 200 mA with Cu Kα radiation (λ= 0.154 nm) at a scanning rate of 5o/min. The X-ray photoelectron spectrometer (XPS, Kratos-England) employed a monochromated Al-Kα X-ray source (hυ=1486.6 eV). Specific surface areas and pore-size distributions were measured by Brunauer-Emmett-Teller (BET) nitrogen adsorption-desorption on a Tristar II 3020 (Micromeritics, USA) gas adsorption analyzer.
3. Results and Discussions
Oxidant
Thiophene
FeCl3, CuCl2
1.Carbonized
Melamine
2.Acid leaching, pyrolyzing
Polythiophene
Fe-Cu-N/C
Scheme 1 Schematic diagram of the process for preparing Fe-Cu-N/C.
The formation of Fe-Cu-N/C is proposed in Scheme 1. First, polythiophene microspheres was synthesized through a facile polymerization and coated with FeCl3, CuCl2 and melamine. Then the sample underwent carbonization at 900oC in an argon environment, followed by acid and FeCl3 leaching. Finally, a second heat treatment process was performed for further carbonization, thus the bamboo-like Fe-Cu-N/C was generated. For comparison, a series of catalysts were prepared and labeled as Fex-Cuy-Nz/C, in which x, y and z represent the relative elemental doping content, respectively.
Figure 1. Morphology characterization of Fe-Cu-N/C catalyst. (a) SEM image; (b) TEM image, and (c,d) HRTEM images.
The morphology of the catalysts from various precursors was investigated using SEM and TEM. C-PTh exhibits a homogeneous spherical morphology with a diameter of ca. 80 nm (Figure S1a), while the resulting Fe/C catalyst retains a well-defined spherical morphology after the addition of Fe into the precursor (Figure S1b and S1d). In contrast, the obvious aggregation is observed in Fe-N/C (Figure S1c and S1e) and Cu-N/C (Figure S2) by introducing nitrogen species into the precursor. Particularly, when Cu and Fe were added simutaneously, the morphology of the catalysts changed dramatically. When the atomic ratio of Fe to Cu was 2:1, both nanospheres and bamboo-like nanotubes were obtained (Figure S3a and S3c), and when the ratio reduced to 1:2, the irregular nanoparticles or porous slice sheets appear (Figure S3b and S3d). It is interesting that when the molar ratio of Fe to Cu was adjusted to 1:1, the Fe-Cu-N/C exhibits a well-defined and uniform nanotube morphology composing of individual cylindrical compartments linked to form long nanotubes with over 1 μm in length and ca. 80 nm in diameter (Figure 1a and b). The TEM images of a single nanotube also reveal numerous stacked cylindrical compartments with porous structure (Figure 1 c and d), which are corresponding to the results of the N2 adsorption-desorption (Figure S5). C-PTh, Fe/C, Fe-N/C and Fe-Cu-N/C exhibit type IV curves with hysteresis loops in the medium- and high- pressure regions, indicating that the samples contain both micropores and mesopores (Figure S5a). The Fe-Cu-N/C catalyst shows the highest BET specific surface area among these catalysts and reaches 588 m2 g-1 (Figure S5c). The pore-size distribution of Fe-Cu-N/C shows a higher pore density in the micropore and mesopore regions, as well as macropores between 100 and 150 nm, confirming its hierarchical porous structure (Figure S5b). Furthermore, the calcination temperature also plays an important role for the formation of nanotubes; at lower temperature (800 oC), only metal-based nanoparticles coated with carbon layers were formed
(Figure S4a); as the temperature reached 1000 oC, graphene-like structures appeared (Figure S4c). These results reveal that the coexistence of Fe and Cu under appropriate molar ratio and calcination temperature is crucial for the formation of bamboo-like nanotubes. (a)
(b)
Figure 2. HAADF/STEM images and the corresponding EDS elemental mapping (C, N, O, Fe, and Cu) of Fe-Cu-N/C (a) a single nanotube, Scale Bar 250 nm; (b) a single nanoparticle, Scale Bar 50 nm.
Element mapping using EDS coupled with HAADF/STEM was used to investigate the compositional distributions of the nanotubes. The images show that C, N, O, Fe and Cu are distributed uniformly along the nanotube (Figure 2a). Furthermore, the homogeneous distribution of O, Fe and Cu components is also presented on a single particle inside the nanotube (Figure 2b), indicating that the interaction has been formed among Fe, Cu and O. The XRD patterns also reveal the effect of the interaction, and the incorporation of the Cu can obviously affect the phase formation of CFe15.1, Fe2O3, and Fe3O4 which has been identified in Fe-N/C (Figure 3a). The Fe-Cu-N/C after pyrolysis is detected to be CuFe2O4, Fe2O3, CuO and Cu (Figure 3a), whereas only CuFe 2O4 and Fe3O4 are detected in Fe-Cu-N/C after the acid/FeCl3 wash and second pyrolysis, indicating that some of the inactive species and exposed particles have been removed (Figure 3b). The peak at about 24 o is assigned to the (002) plane of carbon in all samples, but this peak becomes sharper and shifts positively after metal doping, indicating the increase of the graphitization degrees (Figure 3a). Especially for Fe-Cu-N/C, a typical strong peak is observed at about 26o, in agreement with the morphology feature of bamboo-like carbon nanotubes [29]. After acid/FeCl3 leaching and second pyrolysis, the patterns of the respective carbon peaks show no significant change (Figure 3b). It has been reported that Fe compound as a graphitization catalyst can lead to graphitization by a complex process involving the dissolution of carbon atoms into the catalyst, followed by the precipitation of graphitized carbon [30, 31] and
generating highly porous structure [30], which is significant for creating more active sites and enhancing ORR activity.
Fe-Cu-N/C
CFe15.1 Fe O 3 4 Fe O 2
CuFe O 2 4 Fe O 2
3
Fe-N/C
(c) Oxidized
C0.12Fe1.88
Fe-Cu-N/C
Fe O 2 3 Fe-N/C
Fe-N/C N 1s
Graphitic
3
Intensity (a.u.)
Intensity (a.u.)
(b)
CuFe2O4 Fe2O3 CuO Cu
Intensity (a.u.)
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Pyrrolic
Pyridinic
C-PTh 30
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Intensity (a.u.)
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Pyrrolic
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398
402
722.3
Pyridinic
Binding Energy (eV)
Pyrrolic
402
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400
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396
735
730
725
396
394
Cu-N/C Cu 2p
(f)
933.3
398
396
394
954.7
935.5
952.8
Satellite peak
960
955
950
945
940
935
930
925
Binding Energy (eV)
713.5
715
Binding Energy (eV)
710.4
710
705
Fe-Cu-N/C Cu 2p
(i)
Fe-Cu-N/C Fe 2p
720
398
Binding Energy (eV)
Pyridinic
(h)
Fe-Cu-N/C N 1s
Oxidized
404
100 404
90
Binding Energy (eV)
Intensity (a.u.)
Intensity (a.u.)
Graphitic
80
Oxidized
Binding Energy (eV)
(g)
70
Cu-N/C N 1s Graphitic
709.7
735
60
(e)
Fe-N/C Fe 2p
(d) 724.9
50
2 angel (degrees)
2 angel (degrees)
Intensity (a.u.)
20
932.6
952.4 Intensity (a.u.)
10
935.0 Satellite peak
954.1
960
955
950
945
940
935
930
Binding Energy (eV)
Figure 3. XRD patterns of C-PTh, Fe-N/C and Fe-Cu-N/C: (a) after first pyrolysis; (b) after acid/FeCl3 and second pyrolysis. XPS survey spectra of Fe-N/C, Cu-N/C and Fe-Cu-N/C catalysts: XPS patterns of N 1s from (c) Fe-N/C, (e) Cu-N/C and (g) Fe-Cu-N/C; the Fe 2p from (d) Fe-N/C and (h) Fe-Cu-N/C; the Cu 2p from (f) Fe-N/C and (i) Fe-Cu-N/C.
XPS was conducted to further investigate the role of Cu for the prepared catalysts (Figure 3). The Fe 2p XPS spectra for Fe-N/C and Fe-Cu-N/C can be deconvoluted into Fe (II) and Fe (III) species, which is consistent with the XRD results. Notably, the Fe and N contents in Fe-Cu-N/C (Fe: 0.93 at%, N: 6.08%; Table S1) are higher than those of the Fe-N/C catalyst (Fe: 0.57at%, N: 4.22%; Table S1), suggesting that the incorporation of Cu is beneficial for the formation of Fe and N species which may derive from the stronger interaction among metals and nitrogen on the catalysts. Furthermore, the peaks of Fe 2p for Fe-Cu-N/C shift to higher binding energy compared with Fe-N/C, which is attributed to the electron delocalization between Fe and the carbon nanotubes (CNTs), as a result of electron from Fe to CNTs [32]. The electron delocalization also alters the electronic structure of the metallic species, enabling uniform distribution of Fe and Cu species. The N 1s spectra in all the catalysts are
deconvoluted to four peaks locating at 398 ± 0.4 , 399.4 ± 0.4 , 400.8 ± 0.4 , and 402.0 ± 0.4 eV, which are assigned to pyridinic N [33], pyrrolic N [34], graphitic N [35] , and oxidized N species [36], respectively. It is shown that the relative content of inactive oxidized N in Cu-N/C is the lowest, and the relative content of oxidized N in Fe-Cu-N/C is much lower than that in Fe-N/C (Table S2), revealing the role of Cu for suppressing the formation of inactive oxidized N species. Briefly, the incorporation of Cu enhances the interaction among Fe and N species, as a result, the electronic structure is modified and the contents of Fe and N species on the surface of catalyst increase, all of those contribute to the enhanced ORR performance. 0
C-PTh Cu-N/C Fe-Cu-N/C 0.2
0.4
-2 Current density (mA cm )
-2 Current density (mA cm )
(a)
Fe/C Fe-N/C
(b)
-1 -2
50 mV
-3 -4 -5
C-PTh Cu-N/C Fe-Cu-N/C
-6 -7
0.6
0.8
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Potential(V vs. RHE)
0.7 V 0.6 V 0.5 V 0.4 V
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0.06
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-1/2
Normalized Current (%)
120
0.65 V 0.55 V 0.45 V 0.08
Fe-Cu-N/C Pt/C
12 8 4 4.0 3.9 3.8 3.7 3.6 3.5
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Potential (V vs. RHE) 120
99.2
80
80.4
60 40 20
Fe-Cu-N/C Pt/C 5000
10000
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(d)
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n
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-1
0.21
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J (mA cm )
0.24
Peroxide yield/%
0.27
0.6
Potential(V vs. RHE)
0
-2
0.30
Current density (mA cm )
Potential(V vs. RHE)
Fe/C Fe-N/C 20% Pt/C
15000
Time (s)
20000
MeOH
(f)
100 80 60 40 20 0
Fe-Cu-N/C Pt/C 0
100
200
300
400
500
600
Time (s)
Figure 4. Electrochemical measurements for catalysts in O2-saturated 0.1 M KOH solution. (a) CVs; (b) LSVs at 1600 rpm; (c) K-L plots of Fe-Cu-N/C, calculated from the LSVs of Fe-Cu-N/C at different rotating rates in the inset; (d) The electron transfer number (n) (bottom) and peroxide yield (top) of the Fe-Cu-N/C and Pt/C. Current-time (i-t) chronoamperometric response of Fe-Cu-N/C and commercial 20% Pt/C electrodes in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm: (e) at 0.7 V for 20,000 s; (f) upon addition of 3 M methanol after 200 s.
The electrocatalytic activity of the as-prepared Fe-Cu-N/C and other samples was first investigated by cyclic voltammogram (CV). As shown in Figure 4a, the obviously enhanced cathodic peak (0.9 V vs RHE) for Fe-Cu-N/C in O2-saturated solution compared with that in N2-saturated electrolyte implies a
significant ORR activity of this sample. By contrast, only weaker peaks for cathodic reduction are observed for C-PTh, Fe/C, Cu-N/C and Fe-N/C (0.64, 0.71, 0.79 and 0.81 V vs RHE, respectively). The linear sweep voltammetry (LSV) curve of C-PTh demonstrates a slow current increase with no current plateau (Figure 4b), indicating predominantly a two-electron reduction of O2 to OOH– [37]. By contrast, Fe/C and Fe-N/C exhibit markedly enhanced ORR activity with high half-wave potential and limiting current density. In particular, the catalytic performance is greatly promoted after doping of Cu into Fe-N/C catalyst, implying that the introduction of Cu improves the ORR performance and this matches with the results of XPS. Moreover, Fe-Cu-N/C catalyst exhibits superior ORR activity in comparison with commercial Pt/C (20%) catalyst, with the half-wave potential (0.89 V) 50 mV more positive than that of Pt/C (0.84 V). Notably, the incorporation of only Cu into nitrogen doped carbon to derive Cu-N/C only exhibits poor ORR activity, which matches well with the work demonstrated by our group [25]. Therefore, the doping of Cu plays a crucial role for the formation of synergic effect among Cu and Fe-N-C architecture and enhancing ORR activity of Fe-N/C, which facilitates the generation of bamboo-like structure and the interaction among Fe and N species by tuning the active sites in microstructure (Figure S6) [8, 38]. The RDE measurements at different rotation rates were performed to determine the electron transfer number of ORR on the Fe-Cu-N/C catalyst (Figure 4c inset). The Koutecky-Levich (K-L) plots of Fe-Cu-N/C show good linearity and parallelism, indicating first-order reaction kinetics with respect to the concentration of dissolved O 2 (Figure 4c). The electron-transfer number (n) is calculated to be approximately 3.98, revealing the Fe-Cu-N/C catalyst follows a four-electron transfer pathway [39]. The ORR pathways were also determined by RRDE measurement. The percentage of peroxide species with respect to the total oxygen reduction products and the electron transfer number (n) are calculated from the RRDE curves according to eqn (3) and (4), and the results are shown in Fig. 4d. The yield of peroxide species for the Fe-Cu-N/C catalyst was less than 10% over the measured potential range, although it is slightly higher than that of the commercial Pt/C catalyst. The electron transfer number calculated from the RRDE measurements was over 3.85 for Fe-Cu-N/C, which indicated that the ORR catalyzed by Fe-Cu-N/C is mainly through the four electron (4e) pathway. To further evaluate the suitability as ORR catalysts, the tolerance to methanol crossover of Fe-Cu-N/C was assessed at 0.7 V (vs RHE). Evidently, the original catalytic current of Fe-Cu-N/C
experienced only a slight attenuation (0.8%) after continuous reaction for 20,000 s, whereas the Pt/C catalyst retained only 80.4% of its initial performance (Figure 4e). After methanol was introduced, the ORR performance of Pt/C catalyst displays a dramatic drop in relative current density, whereas almost no apparent change is observed for the Fe-Cu-N/C electrode (Figure 4f), suggesting its great potential
0
(a)
0.27
-1
0.24
-2
2
J (mA cm )
0.21
-1
-3 -4
-1
-2
Current Density (mA cm )
0
2
0.30
Current Density (mA/cm )
for the cathode in direct methanol fuel cells [40]. (b)
-2 -4 1600 rpm 2000 rpm 2500 rpm 3000 rpm 3600 rpm
-6 -8
-10 0.0
0.2 0.4 0.6 0.8 1.0 Potential vs RHE (V)
0.18 0.5 V 0.4 V 0.3 V 0.2 V
0.15
-5
Pt/C Fe-Cu-N/C
-6 0.0
0.2
0.4
0.6
0.8
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(s /rad)
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Current Density (mA/cm )
Normalized current (%)
(c) 100
90
80
70
60
0.07 -1/2
Potential vs RHE (V)
0.45 V 0.35 V 0.25 V
Pt/C Fe-Cu-N/C 0
5000
10000
15000
20000
Time (s)
50
0
-50
Pt/C Fe-Cu-N/C
-100 0
100
200
300
400
500
Potential vs RHE (V)
Figure 5. Electrochemical measurements of Fe-Cu-N/C in O2-saturated 0.1 M HClO4 solution. (a) comparison of the LSVs of Fe-Cu-N/C with 20% Pt/C catalyst; (b) K-L plots at different electrode potentials for Fe-Cu-N/C. Inset: RDE voltammograms for the ORR at various rotation speeds. Current-time (i-t) chronoamperometric response of Fe-Cu-N/C and commercial 20% Pt/C electrodes (c) at 0.7 V (vs RHE) in O2-saturated 0.1 M KOH at a rotation rate of 1600 rpm; (d) when 3 M methanol was added after 100 s.
Notably, the Fe-Cu-N/C catalyst is also active and durable for ORR in acidic solution. In O2-saturated 0.1 M HClO4, the ORR polarization curve of Fe-Cu-N/C exhibits a high half-wave potential (0.71 V vs RHE), which is comparable to that of commercial 20% Pt/C (0.78 V vs RHE, Figure 5a). The electron transfer number (n) calculated from the K-L equation (Figure 5b) reveals nearly a four electron transfer process. Furthermore, the Fe-Cu-N/C catalyst shows much better stability for ORR than the Pt/C catalyst under potentiostatic condition (Figure 5c), and exhibits excellent tolerance to crossover effect (Figure 5d).
4. Conclusion In summary, we have developed a facile and effective synthesis approach for preparing carbon-based material with finely controlled morphology and hierarchical porous structure via simply pyrolyzing
nanospherical polythiophene with a mixture of FeCl3, CuCl2 and melamine. By adjusting the calcination temperature (900oC) and tuning the Cu/Fe ratio (1:1), the Fe-Cu-N/C with one-dimensional bamboo-like carbon nanotube morphology, hierarchical nanostructure, and bimetallic Fe, Cu-doping has been successfully obtained. The resulting Fe-Cu-N/C exhibits high ORR activity, outstanding long-term stability and excellent methanol tolerance in an alkaline and acidic medium, which is superior to the Pt/C catalyst. The outstanding catalytic performance of Fe-Cu-N/C can be attributed to the one-dimensional bamboo-like structure and high surface area, providing enhanced mass transfer and more active sites. Furthermore, the synergic effect among Cu and Fe-N-C could also greatly enhance the activity, modulating the microstructure of the catalyst. This study presented here opens good strategies for the preparation of electrocatalysts with excellent activity and durability materials toward energy storage and conversion.
Acknowledgements This work was supported by 973 National Basic Research Program of the Ministry of Science and Technology (grant 2014CB239400), National Natural Science Foundation of China (no. 21090340), National Science Foundation of China (NSFC Project Nos. 21276098, 21476088, 51302091, U1301245), Natural Science Foundation of Guangdong Province (Project No.2015A030312007), and Educational Commission of Guangdong Province (Project No. 2013CXZDA003)
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Wenjun Fan received his B.S. degree and Master degree from Hebei University of Science and Technology and Kunming University of Science and Technology in 2010 and 2014, respectively. Currently, he is a Ph.D candidate of South China University of Technology under the supervision of Prof. Shijun Liao and Prof. Can Li. His research focuses on catalyst design for fuel cells and energy conversion.
Zelong Li obtained his B.S. degree and Ph. D degree from Dalian University of Technology and Lanzhou Institute of Chemical Physics (LICP), Chinese Academy of Science (CAS) in 2008 and 2013, and then worked as Research Assistant in LICP until 2015. Currently, he worked as a post-doctoral of Dalian Institute of Chemical Physics (DICP), CAS under the supervision of Prof. Can Li. His research interests focus on heterogeneous catalysis for CO2 hydrogenation and electrocatalysis for fuel cells.
Chenghang You obtained his master degree in 2012 from Hainan University and Ph.D degree in 2015 from South China University of Technology. Since 2015, he has served as an associate professor in Hainan Normal University. Now, his interest focus on the electrocatalysts and electrode materials for novel electrochemical energy conversion and storage devices, such as PEMFC, DMFC, metal-air batteries, super capacitors, and etc.
Xu Zong received his Ph.D degree in Physical Chemistry from Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences (CAS) in 2010. He then worked as a postdoctoral research fellow in the University of Queensland from 2010-2014. In 2014, he returned to DICP as the recipient of Hundred Talent Program of DICP and was promoted to professor in 2015 as the recipient of One Thousand Youth Talents Program of China. His research interests focus on the conversion of solar energy to chemical energy via photocatalytic and photoelectrochemical approaches. He has published more than 50 peer-reviewed papers with over 2800 citations.
Xinlong Tian obtained his bachelor degree in 2010 from Tianjin University of Science & Technology and master degree in 2013 Guangdong University of Technology, and then focused on developing effective catalyst for fuel cells as a Ph.D student at South China University of Technology. He is currently working as a post-doctoral research associate at Huazhong University of Science and Technology under the supervision of Prof. Bao Yu Xia, now his research interests include the synthesis of low platinum core–shell structured catalyst, transition metal nitride nanostructures and metal organic frameworks for fuel cells and energy conversion.
Shu Miao is a full professor at Dalian Institute of Chemcial Physcis, CAS. He received his B.A. and M.Eng.from Tsinghua University in 1999 and 2002, respectively. After receiving his PhD from California Institute of Tecnhology in 2007, he worked as a Postdoc in CEMES-CNRS (2007-2008) and University of Sheffield (2008-2010). He joined Dalian Institute of Chemcial Physics in 2011. His research focuses on development of electron microsocpy techniques and their applications in catalysis and energy materials.
Ting Shu received her Ph.D degree from South China University of Technology (Guangzhou, China) in 2013. Her study focuses on the research of high performance MEA prepared by direct deposition of platinum on the gas diffusion layer using atomic layer deposition technique or electrochemical deposition technique.
Can Li received his Ph.D degree in Physical Chemistry from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in 1989, and he joined the same institute and was promoted to full professor in 1993. He is the director of Dalian National Laboratory for Clean Energy and the director of State Key Laboratory of Catalysis. He has 600 publications, 13000 citations, an H-index 60, 60 granted patents and over 70 plenary and keynote lectures at international
conferences.
His
research
interests
include
Photo(electro)catalysis,
Environmental Catalysis, Chiral Catalysis, Biocatalysis, In-situ Characterization of Catalytic Reaction, Time-resolved Spectroscopy.
Shijun Liao is a chair professor and director of The Laboratory of Fuel Cell Technology of Guangdong Province, South China University of Technology, he received his Ph.D in 1999 in South China University of Technology, and conducted his research works in Prof. Viola Birss group of Calgary University, Canada, and in Dr. Radoslav Adzic group of Brookhaven National Laboratory of USA respectively. He is the board members of six international journals, including Scientific Reports etc, he is also the winner of three national awards two location awards for excellent research, his research interests mainly focus on fundamental investigation and engineering development of PEM fuel cell.
Highlights
Binary Fe- and Cu-codoped carbon nanotubes was proposed as efficient catalyst for ORR.
The Fe-Cu-N/C catalyst exhibits uniform bamboo-like morphology, high surface area and hierarchical porous structure.
The doping of Cu into Fe-N-C architecture was found to effectively tune the electronic structure.
The Fe-Cu-N/C catalyst exhibits superior activity and stability to commercial Pt/C in 0.1M KOH and HClO4 solution.