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C electrocatalysts for highly efficient oxygen reduction reaction
Journal of Alloys and Compounds 686 (2016) 874e882
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Template-assisted conversion of aniline nanopolymers into nonprecious metal FeN/C electrocatalysts for highly efficient oxygen reduction reaction Chaozhong Guo a, b, c, *, Bixia Wen a, b, Wenli Liao b, **, Zhongbin Li b, Lingtao Sun a, Chao Wang b, Youcheng Wu b, Jing Chen b, Yunqing Nie b, Jianglan Liao b, Changguo Chen c, *** a
Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China School of Materials and Chemical Engineering, Chongqing University of Arts and Sciences, Chongqing 402160, China c College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2016 Received in revised form 22 June 2016 Accepted 23 June 2016 Available online 26 June 2016
Developing high-performance noble-metal-free electrocatalysts for oxygen reduction reaction (ORR) is the key to the commercialization of fuel cells. Here we use a Fe (III)-modified montmorillonite (Fe-MMT) as a solid-state flat-template to synthesize iron and nitrogen-doped carbon electrocatalysts (FeN/C-PANI) for the ORR derived from thermal conversion of aniline nanopolymer at controlled temperatures. The utilization of flat template can hinder thermal agglomeration of aniline nanopolymer and optimize the nitrogen-containing active site density on the surface. We find that this catalyst exhibits a similar ORR electrocatalytic activity with an onset potential of 0.99 V (versus RHE), a major four-electron reaction pathway and a higher stability compared to the state-of-the-art 20 wt% Pt/C catalyst in alkaline medium. Besides, the ORR half-wave potential measured on our catalyst is only 10 mV lower than that on a Pt/C catalyst. Electrochemical poisoning experiments also confirm that the Fe-Nx group inside the FeN/C-PANI can improve the ORR activity and may be one of key components of active sites. The pyrrolic-N species may be mainly responsible for the ORR catalytic activity and may be another essential component of active sites. This study opens a new way to rationally design inexpensive and highly-efficient ORR catalysts by using simple flat compounds as a direct template. © 2016 Elsevier B.V. All rights reserved.
Keywords: Fuel cells Oxygen reduction Electrocatalyst Pyrrolic-N Metal-Nx
1. Introduction The development of inexpensive, pollution-free, and highefficiency electrocatalysts for oxygen reduction reaction (ORR) is very important for various renewable power source applications, e.g., H2eO2 fuel cells and metal-air batteries [1,2]. With respect to the mainstream ORR catalysts, today’s Pt-based materials suffer from limited supply and high cost, which greatly impedes largescale application of these energy technologies [3]. The search for
* Corresponding author. Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (C. Guo), [email protected] (W. Liao), [email protected] (C. Chen). http://dx.doi.org/10.1016/j.jallcom.2016.06.243 0925-8388/© 2016 Elsevier B.V. All rights reserved.
cheap, renewable, and highly active catalysts for ORR to instead of Pt-based catalysts has attracted the attention of many researchers in the past few decades [4e6]. In particular, great efforts have been devoted to developing Pt-free ORR catalysts, including transition metal oxides [7], metal chalcogenides [8], nitrogen-doped carbon (NDC) [9,10], and transition-metal/nitrogen-doped carbon (TMNDC) [11,12]. Among them, NDC and TMNDC have been highlighted as promising candidates owing to their low cost, good stability and high electrocatalytic activity, as well as abundant resource on earth. Unfortunately, the nature of the catalytically active sites for ORR in NDC and TMNDC still remains a controversial issue until now, but it is believed that the doped nitrogen atoms (e.g., pyridine-like, pyrrole-like, and graphite-like N atoms) play an important role for ORR electrocatalysis [13e15]. The nitrogencontaining active sites of the catalysts are significantly expected to be exposed in the surface as much as possible in order to achieve
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Fig. 1. Schematic illustration for synthesis of the FeN/C-PANI catalyst.
high ORR catalytic efficiency. However, for most of current NDC and TMNDC synthesized by direct pyrolysis of nitrogen-containing hydrocarbons, polymers or biomass, abundance of active sites induced by the nitrogen-dopant atoms are embedded in the inner part of NDC or TMNDC, which are not easily accessible to the
reactants resulting in a low ORR efficiency [16]. Thus, the search for new and simple synthesized methods to effectively expose the nitrogen-doped active sites on the surface and increase the surface density of active sites is a key to improve the electrocatalytic activity towards the ORR.
Fig. 2. (a,b) FE-SEM and (c,d) HR-TEM images of the FeN/C-PANI catalyst; (e) Raman spectra and (f) XPS full-scan spectra of N/C-PANI and FeN/C-PANI.
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Fig. 3. C 1s XPS spectra of N/C-PANI (a) and FeN/C-PANI (b); N 1s XPS spectra of N/C-PANI (C) and FeN/C-PANI (d); (e and f) Fe 2p3/2 and O 1s XPS spectrum of FeN/C-PANI, respectively.
We previously developed another approach for green synthesis of NDC or TMNDC material from the precursors of protein pyropolymer derived from animal bloods and carbon nanospheres/ carbon nanotubes [17,18]. The addition of carbon nano-supports can effectively hamper the agglomeration behavior during pyrolysis process at high temperatures and largely expose the ORR active sites on the catalyst surface. More recently, we have prepared a novel CoN/C catalyst by pyrolysis of a mixture of polyanline, melamine and carbon nanotubes, which can also display good ORR activity and stability in alkaline medium [19]. Here we have further utilized the Fe-modified MMT as a cheap solid-state flat-template for synthesizing the FeN/C catalyst by using aniline nanopolymer as nitrogen and carbon sources. Similarly, this method can hinder thermal agglomeration of aniline nanopolymer and optimize the Ncontaining active site density on the prepared catalyst surface. The obtained FeN/C catalyst can demonstrate more excellent ORR performance in alkaline medium compared to the acidic medium. We also confirm that Fe-MMT template-assisted conversion of aniline nanopolymer might be a practicable route to design highperformance FeN/C catalysts for the oxygen reduction.
2. Experimental details The iron modified montmorillonite (Fe-MMT) was obtained by adsorption of Fe (III) ions onto the surface of montmorillonite (MMT). Briefly, 2.0 g sodium modified montmorillonite (Na-MMT) and 3.3 g FeCl3$6H2O were added into 30 mL deionized water. Then the mixture was intensely agitated for 24 h at room temperature to promote the adsorption of Fe ions. After high-speed centrifugation, the product was dehydrated under a vacuum at 80 C for 24 h. The FeN/C-PANI was synthesized by in situ polymerization of aniline (ANI) onto the Fe-MMT surface. Typically, 0.4 g Fe-MMT and 0.4 g ANI were added into 20 mL deionized water and controlled the condition to PH ¼ 2 by 0.5 M sulfuric acid solution. 2.0 g ammonium persulfate was subsequently added dropwise with vigorous stirring for initiating polymerization. The polymerization was carried out at room temperature for 24 h. After centrifugation and vacuum dehydration at 80 C overnight, the Fe-MMT-PANI was obtained and further heat-treated at 900 C for 2 h under nitrogen protection. The produced sample was finally etched off in a 40 wt% HF solution to obtain the FeN/C-PANI catalyst. The schematic
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illustration for synthesis of FeN/C-PANI using a Fe-MMT nanotemplate was shown in Fig. 1. As a control, the N/C-PANI was fabricated through a procedure similar to that of FeN/C-PANI. Differently, the unmodified Na-MMT was used as a nanotemplate to substitute the Fe-MMT for the direct synthesis of the N/C-PANI catalyst. The PANI-900 was prepared by direct pyrolysis of the ANI polymer at 900 C for 2 h under nitrogen protection. The Fe-PANI was synthesized by direct polymerization of ANI monomers in the presence of FeCl3, and was heat-treated at 900 C for 2 h under nitrogen protection to produce the Fe-PANI-900 sample. Field-emission scanning electron microscopy (FESEM) spectroscopy images were obtained by Hitachi UHR SU8020 (Japan). High-resolution transmission electron microscopy (HRTEM) was carried out on a Zeiss LIBRA 200 FETEM instrument operating at 200 kV. Raman spectroscopy data were recorded with a Renishaw inVia unit using the Ar ion laser with an excitation wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos XSAM800 spectrometer equipped with an Al X-ray source (Al Ka, 1.4866 keV). The KouteckyeLevich plots were acquired by linear fitting of the reciprocal rotating speed versus reciprocal current density collected at various potentials. The electron transfer number was calculated from the following equation [13]:
1=jd ¼ 1=jk þ 1=Bu1=2
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polymerization of aniline and followed by a pyrolysis process. This result suggests that the Fe-MMT can function as a solid-state flattemplate for the two dimensional morphology formation. Besides, it is notable that the HR-TEM images display the defected structures on the exposed edges of FeN/C-PANI, which can be ascribed to the doping of N atoms into the carbon layers. Previous research results significantly proposed that more defected structures may provide more nitrogen-containing catalytic sites for ORR and further improve the ORR electrocatalytic activity [20]. The defect sites and disordered structures of two catalysts were investigated by Raman spectroscopy analysis, as shown in Fig. 2e. Each Raman spectrum is deconvoluted into two components, which exhibits the characteristic ‘‘D’’ and ‘‘G’’ peaks, respectively. No obvious shifts in the position of the ‘‘D’’ and ‘‘G’’ bonds are observed, but the Raman intensity of N/C-PANI is relatively higher than that of FeN/C-PANI. It is well-known that the intensity ratio (ID/IG) of the D peak to the G peak can provide the indication of the amount of structural defects and a quantitative measure of edge plane exposure [18]. The ID/IG ratio is 0.90 and 0.92 for N/C-PANI and FeN/C-PANI, respectively, demonstrating the FeN/C-PANI exhibits a lower graphitization degree and more defected sites on the exposed edges owing to the doping of more nitrogen atoms. The element compositions of N/CPANI and FeN/C-PANI were also analyzed by XPS, as indicated in Fig. 2f. The XPS survey spectra reveal the successful incorporation
B ¼ 0:62nFCO DO n1=6 u1=2 2=3
where n is electron transfer numbers per oxygen molecule involved in the ORR, CO is the O2 saturation concentration in the electrolyte, DO is the O2 diffusion coefficient in the electrolyte, n is the kinetic viscosity of the electrolyte, and u is the electrode rotation rate, and 0.62 is a constant when the rotation rate is expressed in rpm. All electrochemical data were collected on a Zahner Zennium-E electrochemical workstation (Germany) with a convential onecompartment three-electrode cell at room temperature. A glasscarbon rotation disk electrode (GC-RDE, 4 mm in diameter, Model 636, Princeton Applied Research), a saturated calomel electrode (SCE), and a Pt foil with geometric area of 1 cm2 were used as working electrode, reference electrode, and auxiliary electrode, respectively. The preparation of the working electrode was performed by a coating method. Typically, the obtained carbon catalyst was well-dispersed in the 0.5 wt% Nafion/isopropanol solution. 5.0 ml of 10 mg ml1 dispersion was transferred onto the GC-RDE surface and then dried at room temperature. The mass loading was estimated to be around 0.40 mg cm2. A commercial Pt/C catalyst (20 wt% Pt, E-ETK) on the GC-RDE surface was prepared in the same way, but its mass loading was kept at 0.32 mg cm2. In this study, all potentials are quoted versus a reversible hydrogen electrode (RHE). All cyclic voltammetry (CV) and linear sweep voltammetry (LSV) experiments were performed over the potential range of 1.2 to 0.2 V at a scan rate of 5 mV s1 in 0.1 M KOH solution, or the potential range of 1.1 to 0.1 V at a scan rate of 5 mV s1 in 0.1 M HClO4 solution. 3. Results and discussion 3.1. Characterization of morphology and structure The morphology of FeN/C-PANI was characterized by FE-SEM (Fig. 2a and b) and HR-TEM (Fig. 2c and d). It can be clearly observed that the FeN/C-PANI is mainly composed of two dimensional ultrathin carbon nanosheets (CNSs), although some irregular granular carbon nanoparticles are also formed by the
Fig. 4. (a) CV of N/C-PANI and FeN/C-PANI in N2 and O2 saturated 0.1 M HClO4; (b) LSV of PANI-900, Fe-PANI-900, N/C-PANI, FeN/C-PANI, and Pt/C in O2-saturated 0.1 M HClO4 at a rotation speed of 1600 rpm.
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Fig. 5. LSV of N/C-PANI (a) and FeN/C-PANI (c) in O2-saturated HClO4 solution at different rotation rates; Koutecky-Levich plots of N/C-PANI (b) and FeN/C-PANI (d) derived from RDE data.
of nitrogen atoms into the graphite structure of two electrocatalysts, which were 2.71 at.% for N/C-PANI and 3.95 at.% for FeN/ C-PANI, respectively. Furthermore, it is also confirmed that the iron atoms with a percentage of 1.10 at.% are incorporated into the FeN/ C-PANI. What’s more, a higher percentage of 28.0 at.% for C-N/C]N bonding structures can be found at the C 1s XPS spectrum of FeN/CPANI compared to the N/C-PANI with a percentage of 22.5 at.% (Fig. 3a and b), further suggesting that more nitrogen atoms were doped into the carbon structure of the FeN/C-PANI compared to the N/C-PANI, which is in accordance with the results of Raman spectra. The N1s XPS spectrum of N/C-PANI can be deconvoluted into three peaks with binding energies of 398.1, 400.8, and 402.9 eV (Fig. 3c), which can be assignable to pyridinic-N, pyrrolic-N, and pyridinic-N-oxides, respectively [13,14]. The N 1s XPS spectrum of FeN/C-PANI can be similarly deconvoluted into four peaks with binding energies of 398.2, 399.5, 400.8, and 402.8 eV (Fig. 3d), which can correspond to pyridinic-N, Metal-N, pyrrolic-N, and pyridinic-N-oxide [13,21], respectively. These results suggest that only Fe-N sites can be formed in FeN/C-PANI, but the pyrrolic-N is still to dominate in all types of nitrogen functionalities. The formation of the Fe-N bond may be derived from iron bound to pyrrolic N [22], based on a reasonable phenomenon that the percentage (17.5 at.%) of Fe-N sites almost approaches the reduced percentage (17.2 at.%) of the pyrrolic-N species. About 10.0 at.% of pyridinic-N is accordingly oxidized to convert into inactive pyridinic-N-oxide after using the Fe-MMT as a solid-state template. For confirming the presence of FeN types of bond in N 1s spectra, we have done the analysis of Fe 2p3/2 spectrum for the same sample (Fig. 3e). In addition to three peaks due to various forms of iron oxides (710e713 eV), there is a distinct peak due to the FeNx bond at 708.4 eV [22,23]. A significant amount of oxygen species are also included in the catalysts. The O 1s XPS spectrum of FeN/C-PANI (Fig. 3f) is deconvoluted into four peaks with binding energies of 530.8, 532.2, 533.7, and 535.8 eV, which can be assigned to the C] O, C(aliphatic)-OH/C(aliphatic)-O-C(aliphatic), metal-bounded
oxygen, and chemisorbed water molecules, respectively. The comprehensive analysis of Fe 2p, N 1s and O 1s spectra for the FeN/ C-PANI catalyst suggests that the Fe atoms are in two forms (FeeNx and FeeO).
3.2. Evaluation of electrocatalytic ORR activity The electrocatalytic activities towards the ORR of the synthesized catalysts are studied using a typical three-electrode device by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements. All electrodes are pre-treated by repeated cycling of the potential from 0 to 1.2 V versus a reversible hydrogen electrode (RHE) with a sweep rate of 50 mV s1 to remove surface contaminations. The CV and LSV curves of catalyzed electrodes made with N/C-PANI, FeN/C-PANI, and Pt/C (20 wt% Pt) are performed in 0.1 M HClO4 solution saturated by O2 or N2. As displayed in Fig. 4a, a significant enhancement of the cathodic peak for ORR with an onset potential (EORR) of 0.82 V and a peak potential (Ep) of 0.62 V (versus RHE) on N/C-PANI is well-defined in an O2-saturated 0.1 M HClO4 solution, but it is featureless and no cathodic peak can be observed in the N2-saturated HClO4 solution, indicates the electrocatalytic activity of the N/C-PANI catalyst. However, a higher current density of the ORR together with more positive EORR (0.84 V) and Ep (0.66 V) can be obtained on the FeN/C-PANI-catalyzed electrode under the same condition, which suggests better ORR electrocatalytic activity the in HClO4 solution. Furthermore, a very weak cathodic peak at about 0.70 V can be found at the CV curve of the FeN/C-PANI catalyst, which can be caused by the electroreduction of noexhausted oxygen molecule in N2-saturated HClO4 solution. Compared with two CV curves in N2-saturated solution, the electrode capacitance increases on FeN/C-PANI, indicating an increase of the solid/electrolyte interface of the electrode [16]. It may be attributed to the increase of effective surface area after using the Fe-modified MMT as a nano-template. RDE tests of PANI-900, FePANI-900, N/C-PANI, FeN/C-PANI, and Pt/C catalysts for ORR were
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carried out in an O2-saturated 0.1 M HClO4 solution at a rotation rate of 1600 rpm, as indicated in Fig. 4b. Similar ORR onset potentials can be observed at PANI-900, Fe-PANI-900 and N/C-PANI catalysts, but a relatively higher ORR limited current density at the same potential is found at N/C-PANI, indicating that using NaMMT as a nano-template can also help to improve the ORR activity of the catalyst. The half-wave potential (E1/2) for the ORR of FeN/ C-PANI is around 0.62 V, higher than that of N/C-PANI (0.50 V), suggesting a relatively higher ORR activity. The use of Fe-MMT for synthesis of the carbon-based catalyst should be more effective to promote its ORR activity in acidic solution compared to the use of Na-MMT nano-template. The difference of E1/2 between FeN/CPANI and Pt/C is about 180 mV, implying that the ORR activity of FeN/C-PANI is still lower than that of the Pt/C catalyst in the acidic solution, however, the limited diffusion current density (jd) of FeN/ C-PANI at 0.2 V is almost identical to that of Pt/C. We have further done a series of RDE measurements at different rotating speeds from 400 rpm to 3600 rpm to further reveal the kinetics of the ORR. As demonstrated in Fig. 5a and c, the jd increases with the increase in the rotation rate of RDE. It can be estimated from the KouteckyLevich plots that the average electron number of the ORR on the N/ C-PANI and FeN/C-PANI-catalyzed electrodes is 3.2 and 3.1, respectively, at the potential range of 0.1e0.3 V versus RHE (Fig. 5b
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and d). This result shows a major four-electron ORR pathway (O2 þ 4Hþ þ 4e / 2H2O) on these catalyzed electrodes. We further measured the ORR activity of PANI-900, Fe-PANI900, N/C-PANI, FeN/C-PANI, and Pt/C catalysts in 0.1 M KOH solution using CV and RDE techniques, as shown in Fig. 6. Fig. 6a displays the ORR onset potentials of 0.95 and 0.99 V for N/C-PANI and FeN/C-PANI in an acidic electrolyte, respectively, which are larger than those in 0.1 M HClO4 solution. The peak potentials of ORR are about 0.80 V and 0.85 V for N/C-PANI and FeN/C-PANI, respectively; but they are almost featureless in the N2-saturated KOH solution. Moreover, compared to CV profiles in N2-saturated electrolyte, a strong reduction peak can be clearly seen with the electrolyte saturated with O2, suggesting the high electrocatalytic activities of N/C-PANI and FeN/C-PANI towards the ORR. These results further show that FeN/C-PANI exhibits the best ORR activity in KOH solution, based on a fact that the onset potential of FeN/C-PANI approaches to that of Pt/C catalyst (see in Fig. 6b). The half-wave potentials for ORR measured on a N/C-PANI-catalyzed electrode and a FeN/C-PANI-catalyzed electrode are only 85 mV and 10 mV lower than that on a state-of-the-art Pt/C catalyst at a mass loading of 320 mg cm2, respectively. Besides, the limited diffusion current density on the FeN/C-PANI-catalyzed electrode is notably larger compared to the Pt/C-catalyzed electrode. Interestingly, the FePANI-900 also exhibits a better ORR electrocatalytic activity with a high limited current density compared to the PANI-900 and N/CPANI catalysts, but its ORR activity is also lower than that of FeN/CPANI in terms of onset potential, half-wave potential and limited diffusion current density. These results not only show that iron atoms may be a key to the formation of the catalyst with high ORR activity, but also confirm that the introduction of Fe-MMT as a nano-template can play a very important role in the enhancement of the ORR activity of the catalyst. The ORR catalytic mechanism of N/C-PANI and FeN/C-PANI was investigated further using the RDE at different rotation rates, as shown in Fig. 7a and c. As can be seen, an increase in the ORR current density with the rotation rate was observed at catalyzed electrodes. The good linearity of the Koutecky-Levich plots (Fig. 7b and d) suggests the first-order dependence of the ORR kinetics at different potentials. The ORR electron transfer number was calculated to be 3.7 and 4.0 at the potential range of 0.2e0.4 V for N/C-PANI and FeN/C-PANI, respectively, based on the slopes of Koutecky-Levich plots. Our results indicate that the ORR on two types of carbon catalysts proceeded mainly with four-electron reduction pathway (O2 þ 2H2O þ 4e / 4 OH), very similar to ORR catalyzed by a state-of-the-art Pt/C catalyst measured in 0.1 M KOH solution [21]. More significantly, the ORR activity of FeN/C-PANI is comparable to that of the best metal/nitrogen-doped carbon electrocatalysts and metal-free carbon-based electrocatalysts reported to date, in particular the catalysts derived from various nitrogen-containing organic molecules [14e21]. Hence we can reasonably conclude that the FeN/C-PANI catalyst is a very promising candidate for commercial Pt-based catalysts. 3.3. Analysis of electrocatalytic stability
Fig. 6. (a) CV of N/C-PANI and FeN/C-PANI in N2 and O2 saturated 0.1 M KOH; (b) LSV of PANI-900, Fe-PANI-900, N/C-PANI, FeN/C-PANI, and Pt/C in O2-saturated 0.1 M KOH at a rotation speed of 1600 rpm.
The electrochemical ORR stability of FeN/C-PANI in alkaline and acidic media is one of the most important factors to assess whether it can be applied to practical fuel cells. We performed the accelerated aging tests (AAT) in O2-saturated 0.1 M HClO4 or 0.1 M KOH solutions by continuous CV measurements for 2000 cycles at a scan rate of 50 mV s1. The ORR electrocatalytic activity of FeN/C-PANI was further evaluated by CV and LSV under the same conditions as above experiments, as displayed in Fig. 8. The CV curves of FeN/ C-PANI before and after the AAT have not obviously changed except for a slightly decrease in ORR current density, whether it was tested
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Fig. 7. LSV of N/C-PANI (a) and FeN/C-PANI (c) in O2-saturated KOH solution at different rotation rates; Koutecky-Levich plots of N/C-PANI (b) and FeN/C-PANI (d) derived from RDE data.
in HClO4 solution or in KOH solution (see Fig. 8a and c). The LSV curves (Fig. 8b and d) show that no noticeable changes in the onset potential of the ORR were observed on the FeN/C-PANI-catalyzed electrode, but the half-wave potential of the ORR was negatively shifted by 18 mV in 0.1 M HClO4 solution and 11 mV in 0.1 KOH solution, respectively, suggesting a relatively better stability of FeN/
C-PANI in alkaline electrolyte compared with the acidic electrolyte. The poor stability of FeN/C-PANI in an acidic solution can be attributed to an accepted reason that the dissolution of metal-Fe contained in Fe-Nx structures can easily occur in acidic solutions, resulting in the partially damage of Fe-Nx structures and the decrease of ORR activity in acidic environments. In addition, the
Fig. 8. (a) CV and (b) LSV of FeN/C-PANI before and after AAT in O2-saturated 0.1 M HClO4 solution; (c) CV and (d) LSV of FeN/C-PANI before and after AAT in O2-saturated 0.1 M KOH solution.
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are bonded to two carbon atoms and contribute to the p system with two p-electrons, which will result in chemically active, localized areas of higher electron density and promotes the catalysis of ORR [18]. Besides, an irreversible fact should be paid much attention, which is that the transformation of pyridinic-N into pyridinic-N-oxide does not result in the decrease of the electrocatalytic activity for the prepared Fe-N/C-PANI catalyst, showing the pyridinic-N may be not the effectively catalytic active sites for ORR in this study. Our previous reports have effectively confirmed that the pyrrolic-N configuration may be the electrocatalytically active site for ORR of nitrogen-containing carbon-based electrocatalyst [17]. Thus, on the basis of the XPS results and electrochemical data, there is sufficient reason to judge that the pyrrolic-N active site may be mainly responsible for the ORR activity of FeN/CPANI and the Fe-Nx active site can be play a key role in the improvement of the ORR activity of FeN/C-PANI. 4. Conclusions
Fig. 9. CV of FeN/C-PANI in (a) O2-saturated HClO4, (b) O2-saturated HClO4 containing 2 mM SCN, and (c) N2-saturated HClO4 containing 2 mM SCN.
above results also demonstrate that FeN/C-PANI has good durability under the alkaline conditions, and may be suitable for the electrocatalysis of the ORR in alkaline electrolytes. 3.4. Discussion of catalytically active sites for the ORR Up to now, the function of the real ‘‘electrocatalytically active sites’’ remains controversial because their contribution to the ORR activity is not well-defined. To clarify the role of metal-Fe atoms in ORR electrocatalysis is very significant for study of ORR catalytic sites. We first designed an additional poisoning experiment by using 2 mM thiocyanate (NaSCN) as a poisoning electrochemical probe based on the strong binding force of SCN and Fe3þ. An acidic medium (0.1 M HClO4) was solely chosen in this study to make the conditions more relevant for ORR in the PEM fuel cell environment. We separately tested the CV curves of FeN/C-PANI in O2-saturated 0.1 M HClO4 (a), O2-saturated 0.1 M HClO4 containing 2 mM SCN (b) and N2-saturated 0.1 M HClO4 containing 2 mM SCN (c), as shown in Fig. 9. We found that both onset potential and peak potential for ORR were negatively shifted by 60 mV and 40 mV, respectively, while the FeN/C-PANI-catalyzed electrode was tested in 0.1 M HClO4 solution containing 2 mM SCN saturated by oxygen. However, the cathodic current density for the ORR was not largely decreased. Interestingly, a well-defined peak at 0.85 V can be clearly observed at the CV anodic scanning process. It is ascribed to the oxidation peak of SCN on the surface of the FeN/C-PANIcatalyzed electrode. When the FeN/C-PANI-catalyzed electrode was tested in 0.1 M HClO4 solution containing 2 mM SCN saturated by nitrogen, a more obvious oxidation peak of SCN can be observed and its peak current density was beyond twice larger than that in O2-saturated 0.1 M HClO4 solution containing 2 mM SCN. It suggests that the metal-Fe can more easily interact with the SCN in the ORR process and then result in a relatively lower current density of SCN oxidation. These results confirm that the FeN/CPANI catalyst formed by a nano-template method may have metal-centered active sites (Fe-Nx) for the ORR, which was supported by many reported researches [24,25]. However, the pyrrolicN species is still to dominate in all types of nitrogen functionalities of the prepared catalysts. Pyrrolic N refers to nitrogen atoms that
Herein, we developed a facile approach based on Fe (III)-modified montmorillonite as a direct solid-state template to the design of a non-noble-metal electrocatalyst (FeN/C-PANI) from the conversion of aniline nanopolymer that acted as a single precursor for both carbon and nitrogen atoms, therefore avoiding the usage of complicated chemicals in the synthesis processes. The utilization of Fe-MMT flat-template can effectively prevent aniline nanopolymer from agglomeration during pyrolysis at high temperatures and cause more nitrogen-containing active sites to be exposed on the catalyst surface, further enhancing the electrocatalytic activity towards the ORR in acidic and alkaline media. The prepared FeN/CPANI catalyst exhibited more excellent ORR activity and better stability in alkaline electrolyte than those in acidic electrolyte. More notably, in alkaline electrolyte, the onset potential for four-electron ORR measured on FeN/C-PANI was about 0.99 V versus RHE, and the half-wave potential for ORR measured on FeN/C-PANI is only 10 mV lower than that on a state-of-the-art 20 wt% Pt/C catalyst. XPS and electrochemical results comprehensively show that the Fe atoms in FeN/C-PANI are in two forms (Fe-Nx and FeeO) and the FeNx sites can be play a key role in the improvement of the ORR activity. The pyrrolic nitrogen may be the electrocatalytically active site for ORR in our prepared catalysts. Therefore, this study demonstrates that Fe-MMT template-assisted conversion of aniline nanopolymer at controlled temperatures might be a practicable route to design high-performance FeN/C electrocatalysts for a range of electrochemical reactions. Acknowledgments This study was financially supported by the Basic and Frontier Research Program of Chongqing Municipality (cstc2015jcyjA50032 and cstc2014jcyjA50038), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501118), the Talent Introduction Project (R2014CJ02) and NSFC (Project No. 21273292). The authors are very grateful to Prof. Zhongli Luo, Haibo Ruan and Yujun Si for helpful discussions. References [1] F. Cheng, J. Chen, Metal-air batteries: from oxygen reduction electrochemistry to cathode catalysts, Chem. Soc. Rev. 41 (6) (2012) 2172e2192. [2] L. Dai, Y. Xue, L. Qu, H.-J. Choi, J.-B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev. 115 (2015) 4823e4892. [3] G. Wu, P. Zelenay, Nanostructured nonprecious metal catalysts for oxygen reduction reaction, Accounts Chem. Res. 46 (2013) 1878e1889. [4] Z. Yang, H. Nie, X. Chen, X. Chen, S. Huang, Recent progress in doped carbon nanomaterials as effective cathode catalysts for fuel cell oxygen reduction reaction, J. Power. Sources 236 (2013) 238e249.
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[5] C.-Z. Guo, W.-L. Liao, L. Sun, C.-G. Chen, Synthesis of non-noble nitrogencontaining catalysts for cathodic oxygen reduction reaction: a critical review, Int. J. Electrochem. Sci. 10 (2015) 2467e2477. [6] K.N. Wood, R.O. Hayre, S. Pylypenko, Recent progress on nitrogen/carbon structures designed for use in energy and sustainability applications, Energy Environ. Sci. 7 (2014) 1212e1249. [7] W.T. Hong, M. Risch, K.A. Stoerzinger, A. Grimaud, J. Suntivich, Y. Shao-Horn, Toward the rational design of non-precious transition metal oxides for oxygen electrocatalysis, Energy Environ. Sci. 8 (2015) 1404e1427. [8] N. Alonso Vante, W. Jaegermann, H. Tributsch, W. Hoenle, K. Yvon, Electrocatalysis of oxygen reduction by chalcogenides containing mixed transition metal clusters, J. Am. Chem. Soc. 109 (1987) 3251e3257. [9] J. Shui, M. Wang, F. Du, L. Dai, N-doped carbon nanomaterials are durable catalysts for oxygen reduction reaction in acidic fuel cells, Sci. Adv. 1e7 (2015) e1400129. [10] S. Chen, J. Bi, Y. Zhao, L. Yang, C. Zhang, Y. Ma, Q. Wu, X. Wang, Z. Hu, Nitrogen-doped carbon nanocages as efficient metal-free electrocatalysts for oxygen reduction reaction, Adv. Mater 24 (2012) 5593e5597. [11] D. Geng, H. Liu, Y. Chen, R. Li, X. Sun, S. Ye, Knights, Non-noble metal oxygen reduction electrocatalysts based on carbon nanotubes with controlled nitrogen contents, J. Power Sources 196 (2011) 1795e1801. [12] C.-Z. Guo, W.L. Liao, C.-G. Chen, Design of a non-precious metal electrocatalyst for alkaline electrolyte oxygen reduction by using soybean biomass as the nitrogen source of electrocatalytically active center structures, J. Power Sources 269 (2014) 841e847. [13] W. Ding, Z. Wei, S. Chen, X. Qi, T. Yang, J. Hu, D. Wang, L.-J. Wan, S.F. Alvi, L. Li, Space confinement-induced synthesis of pyridinic- and pyrrolic nitrogendoped graphene for the catalysis of oxygen reduction, Angew. Chem. Int. Ed. 52 (2013) 11755e11759. [14] C. Guo, W. Liao, Z. Li, L. Sun, C. Chen, Easy conversion of protein-rich enoki mushroom biomass to a nitrogen-doped carbon nanomaterial as a promising metal-free catalyst for oxygen reduction reaction, Nanoscale 7 (2015) 15990e15998. [15] H. Yang, H. Li, H. Wang, S. Ji, J. Key, R. Wang, Fe(III)-induced N enrichment in the surface of carbon materials derived from silk fibroins and its effect on electrocatalytic oxygen reduction, J. Electrochem. Soc. 161 (2014) F795eF802. [16] Y. Nie, X. Xie, S. Chen, W. Ding, X. Qi, Y. Wang, J. Wang, W. Li, Z. Wei, M. Shao,
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Template-assisted conversion of aniline nanopolymers into nonprecious metal FeN/C electrocatalysts for highly efficient oxygen reduction reaction Chaozhong Guo a, b, c, *, Bixia Wen a, b, Wenli Liao b, **, Zhongbin Li b, Lingtao Sun a, Chao Wang b, Youcheng Wu b, Jing Chen b, Yunqing Nie b, Jianglan Liao b, Changguo Chen c, *** a
Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China School of Materials and Chemical Engineering, Chongqing University of Arts and Sciences, Chongqing 402160, China c College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 April 2016 Received in revised form 22 June 2016 Accepted 23 June 2016 Available online 26 June 2016
Developing high-performance noble-metal-free electrocatalysts for oxygen reduction reaction (ORR) is the key to the commercialization of fuel cells. Here we use a Fe (III)-modified montmorillonite (Fe-MMT) as a solid-state flat-template to synthesize iron and nitrogen-doped carbon electrocatalysts (FeN/C-PANI) for the ORR derived from thermal conversion of aniline nanopolymer at controlled temperatures. The utilization of flat template can hinder thermal agglomeration of aniline nanopolymer and optimize the nitrogen-containing active site density on the surface. We find that this catalyst exhibits a similar ORR electrocatalytic activity with an onset potential of 0.99 V (versus RHE), a major four-electron reaction pathway and a higher stability compared to the state-of-the-art 20 wt% Pt/C catalyst in alkaline medium. Besides, the ORR half-wave potential measured on our catalyst is only 10 mV lower than that on a Pt/C catalyst. Electrochemical poisoning experiments also confirm that the Fe-Nx group inside the FeN/C-PANI can improve the ORR activity and may be one of key components of active sites. The pyrrolic-N species may be mainly responsible for the ORR catalytic activity and may be another essential component of active sites. This study opens a new way to rationally design inexpensive and highly-efficient ORR catalysts by using simple flat compounds as a direct template. © 2016 Elsevier B.V. All rights reserved.
Keywords: Fuel cells Oxygen reduction Electrocatalyst Pyrrolic-N Metal-Nx
1. Introduction The development of inexpensive, pollution-free, and highefficiency electrocatalysts for oxygen reduction reaction (ORR) is very important for various renewable power source applications, e.g., H2eO2 fuel cells and metal-air batteries [1,2]. With respect to the mainstream ORR catalysts, today’s Pt-based materials suffer from limited supply and high cost, which greatly impedes largescale application of these energy technologies [3]. The search for
* Corresponding author. Research Institute for New Materials Technology, Chongqing University of Arts and Sciences, Chongqing 402160, China. ** Corresponding author. *** Corresponding author. E-mail addresses: [email protected] (C. Guo), [email protected] (W. Liao), [email protected] (C. Chen). http://dx.doi.org/10.1016/j.jallcom.2016.06.243 0925-8388/© 2016 Elsevier B.V. All rights reserved.
cheap, renewable, and highly active catalysts for ORR to instead of Pt-based catalysts has attracted the attention of many researchers in the past few decades [4e6]. In particular, great efforts have been devoted to developing Pt-free ORR catalysts, including transition metal oxides [7], metal chalcogenides [8], nitrogen-doped carbon (NDC) [9,10], and transition-metal/nitrogen-doped carbon (TMNDC) [11,12]. Among them, NDC and TMNDC have been highlighted as promising candidates owing to their low cost, good stability and high electrocatalytic activity, as well as abundant resource on earth. Unfortunately, the nature of the catalytically active sites for ORR in NDC and TMNDC still remains a controversial issue until now, but it is believed that the doped nitrogen atoms (e.g., pyridine-like, pyrrole-like, and graphite-like N atoms) play an important role for ORR electrocatalysis [13e15]. The nitrogencontaining active sites of the catalysts are significantly expected to be exposed in the surface as much as possible in order to achieve
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Fig. 1. Schematic illustration for synthesis of the FeN/C-PANI catalyst.
high ORR catalytic efficiency. However, for most of current NDC and TMNDC synthesized by direct pyrolysis of nitrogen-containing hydrocarbons, polymers or biomass, abundance of active sites induced by the nitrogen-dopant atoms are embedded in the inner part of NDC or TMNDC, which are not easily accessible to the
reactants resulting in a low ORR efficiency [16]. Thus, the search for new and simple synthesized methods to effectively expose the nitrogen-doped active sites on the surface and increase the surface density of active sites is a key to improve the electrocatalytic activity towards the ORR.
Fig. 2. (a,b) FE-SEM and (c,d) HR-TEM images of the FeN/C-PANI catalyst; (e) Raman spectra and (f) XPS full-scan spectra of N/C-PANI and FeN/C-PANI.
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Fig. 3. C 1s XPS spectra of N/C-PANI (a) and FeN/C-PANI (b); N 1s XPS spectra of N/C-PANI (C) and FeN/C-PANI (d); (e and f) Fe 2p3/2 and O 1s XPS spectrum of FeN/C-PANI, respectively.
We previously developed another approach for green synthesis of NDC or TMNDC material from the precursors of protein pyropolymer derived from animal bloods and carbon nanospheres/ carbon nanotubes [17,18]. The addition of carbon nano-supports can effectively hamper the agglomeration behavior during pyrolysis process at high temperatures and largely expose the ORR active sites on the catalyst surface. More recently, we have prepared a novel CoN/C catalyst by pyrolysis of a mixture of polyanline, melamine and carbon nanotubes, which can also display good ORR activity and stability in alkaline medium [19]. Here we have further utilized the Fe-modified MMT as a cheap solid-state flat-template for synthesizing the FeN/C catalyst by using aniline nanopolymer as nitrogen and carbon sources. Similarly, this method can hinder thermal agglomeration of aniline nanopolymer and optimize the Ncontaining active site density on the prepared catalyst surface. The obtained FeN/C catalyst can demonstrate more excellent ORR performance in alkaline medium compared to the acidic medium. We also confirm that Fe-MMT template-assisted conversion of aniline nanopolymer might be a practicable route to design highperformance FeN/C catalysts for the oxygen reduction.
2. Experimental details The iron modified montmorillonite (Fe-MMT) was obtained by adsorption of Fe (III) ions onto the surface of montmorillonite (MMT). Briefly, 2.0 g sodium modified montmorillonite (Na-MMT) and 3.3 g FeCl3$6H2O were added into 30 mL deionized water. Then the mixture was intensely agitated for 24 h at room temperature to promote the adsorption of Fe ions. After high-speed centrifugation, the product was dehydrated under a vacuum at 80 C for 24 h. The FeN/C-PANI was synthesized by in situ polymerization of aniline (ANI) onto the Fe-MMT surface. Typically, 0.4 g Fe-MMT and 0.4 g ANI were added into 20 mL deionized water and controlled the condition to PH ¼ 2 by 0.5 M sulfuric acid solution. 2.0 g ammonium persulfate was subsequently added dropwise with vigorous stirring for initiating polymerization. The polymerization was carried out at room temperature for 24 h. After centrifugation and vacuum dehydration at 80 C overnight, the Fe-MMT-PANI was obtained and further heat-treated at 900 C for 2 h under nitrogen protection. The produced sample was finally etched off in a 40 wt% HF solution to obtain the FeN/C-PANI catalyst. The schematic
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illustration for synthesis of FeN/C-PANI using a Fe-MMT nanotemplate was shown in Fig. 1. As a control, the N/C-PANI was fabricated through a procedure similar to that of FeN/C-PANI. Differently, the unmodified Na-MMT was used as a nanotemplate to substitute the Fe-MMT for the direct synthesis of the N/C-PANI catalyst. The PANI-900 was prepared by direct pyrolysis of the ANI polymer at 900 C for 2 h under nitrogen protection. The Fe-PANI was synthesized by direct polymerization of ANI monomers in the presence of FeCl3, and was heat-treated at 900 C for 2 h under nitrogen protection to produce the Fe-PANI-900 sample. Field-emission scanning electron microscopy (FESEM) spectroscopy images were obtained by Hitachi UHR SU8020 (Japan). High-resolution transmission electron microscopy (HRTEM) was carried out on a Zeiss LIBRA 200 FETEM instrument operating at 200 kV. Raman spectroscopy data were recorded with a Renishaw inVia unit using the Ar ion laser with an excitation wavelength of 514.5 nm. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Kratos XSAM800 spectrometer equipped with an Al X-ray source (Al Ka, 1.4866 keV). The KouteckyeLevich plots were acquired by linear fitting of the reciprocal rotating speed versus reciprocal current density collected at various potentials. The electron transfer number was calculated from the following equation [13]:
1=jd ¼ 1=jk þ 1=Bu1=2
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polymerization of aniline and followed by a pyrolysis process. This result suggests that the Fe-MMT can function as a solid-state flattemplate for the two dimensional morphology formation. Besides, it is notable that the HR-TEM images display the defected structures on the exposed edges of FeN/C-PANI, which can be ascribed to the doping of N atoms into the carbon layers. Previous research results significantly proposed that more defected structures may provide more nitrogen-containing catalytic sites for ORR and further improve the ORR electrocatalytic activity [20]. The defect sites and disordered structures of two catalysts were investigated by Raman spectroscopy analysis, as shown in Fig. 2e. Each Raman spectrum is deconvoluted into two components, which exhibits the characteristic ‘‘D’’ and ‘‘G’’ peaks, respectively. No obvious shifts in the position of the ‘‘D’’ and ‘‘G’’ bonds are observed, but the Raman intensity of N/C-PANI is relatively higher than that of FeN/C-PANI. It is well-known that the intensity ratio (ID/IG) of the D peak to the G peak can provide the indication of the amount of structural defects and a quantitative measure of edge plane exposure [18]. The ID/IG ratio is 0.90 and 0.92 for N/C-PANI and FeN/C-PANI, respectively, demonstrating the FeN/C-PANI exhibits a lower graphitization degree and more defected sites on the exposed edges owing to the doping of more nitrogen atoms. The element compositions of N/CPANI and FeN/C-PANI were also analyzed by XPS, as indicated in Fig. 2f. The XPS survey spectra reveal the successful incorporation
B ¼ 0:62nFCO DO n1=6 u1=2 2=3
where n is electron transfer numbers per oxygen molecule involved in the ORR, CO is the O2 saturation concentration in the electrolyte, DO is the O2 diffusion coefficient in the electrolyte, n is the kinetic viscosity of the electrolyte, and u is the electrode rotation rate, and 0.62 is a constant when the rotation rate is expressed in rpm. All electrochemical data were collected on a Zahner Zennium-E electrochemical workstation (Germany) with a convential onecompartment three-electrode cell at room temperature. A glasscarbon rotation disk electrode (GC-RDE, 4 mm in diameter, Model 636, Princeton Applied Research), a saturated calomel electrode (SCE), and a Pt foil with geometric area of 1 cm2 were used as working electrode, reference electrode, and auxiliary electrode, respectively. The preparation of the working electrode was performed by a coating method. Typically, the obtained carbon catalyst was well-dispersed in the 0.5 wt% Nafion/isopropanol solution. 5.0 ml of 10 mg ml1 dispersion was transferred onto the GC-RDE surface and then dried at room temperature. The mass loading was estimated to be around 0.40 mg cm2. A commercial Pt/C catalyst (20 wt% Pt, E-ETK) on the GC-RDE surface was prepared in the same way, but its mass loading was kept at 0.32 mg cm2. In this study, all potentials are quoted versus a reversible hydrogen electrode (RHE). All cyclic voltammetry (CV) and linear sweep voltammetry (LSV) experiments were performed over the potential range of 1.2 to 0.2 V at a scan rate of 5 mV s1 in 0.1 M KOH solution, or the potential range of 1.1 to 0.1 V at a scan rate of 5 mV s1 in 0.1 M HClO4 solution. 3. Results and discussion 3.1. Characterization of morphology and structure The morphology of FeN/C-PANI was characterized by FE-SEM (Fig. 2a and b) and HR-TEM (Fig. 2c and d). It can be clearly observed that the FeN/C-PANI is mainly composed of two dimensional ultrathin carbon nanosheets (CNSs), although some irregular granular carbon nanoparticles are also formed by the
Fig. 4. (a) CV of N/C-PANI and FeN/C-PANI in N2 and O2 saturated 0.1 M HClO4; (b) LSV of PANI-900, Fe-PANI-900, N/C-PANI, FeN/C-PANI, and Pt/C in O2-saturated 0.1 M HClO4 at a rotation speed of 1600 rpm.
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Fig. 5. LSV of N/C-PANI (a) and FeN/C-PANI (c) in O2-saturated HClO4 solution at different rotation rates; Koutecky-Levich plots of N/C-PANI (b) and FeN/C-PANI (d) derived from RDE data.
of nitrogen atoms into the graphite structure of two electrocatalysts, which were 2.71 at.% for N/C-PANI and 3.95 at.% for FeN/ C-PANI, respectively. Furthermore, it is also confirmed that the iron atoms with a percentage of 1.10 at.% are incorporated into the FeN/ C-PANI. What’s more, a higher percentage of 28.0 at.% for C-N/C]N bonding structures can be found at the C 1s XPS spectrum of FeN/CPANI compared to the N/C-PANI with a percentage of 22.5 at.% (Fig. 3a and b), further suggesting that more nitrogen atoms were doped into the carbon structure of the FeN/C-PANI compared to the N/C-PANI, which is in accordance with the results of Raman spectra. The N1s XPS spectrum of N/C-PANI can be deconvoluted into three peaks with binding energies of 398.1, 400.8, and 402.9 eV (Fig. 3c), which can be assignable to pyridinic-N, pyrrolic-N, and pyridinic-N-oxides, respectively [13,14]. The N 1s XPS spectrum of FeN/C-PANI can be similarly deconvoluted into four peaks with binding energies of 398.2, 399.5, 400.8, and 402.8 eV (Fig. 3d), which can correspond to pyridinic-N, Metal-N, pyrrolic-N, and pyridinic-N-oxide [13,21], respectively. These results suggest that only Fe-N sites can be formed in FeN/C-PANI, but the pyrrolic-N is still to dominate in all types of nitrogen functionalities. The formation of the Fe-N bond may be derived from iron bound to pyrrolic N [22], based on a reasonable phenomenon that the percentage (17.5 at.%) of Fe-N sites almost approaches the reduced percentage (17.2 at.%) of the pyrrolic-N species. About 10.0 at.% of pyridinic-N is accordingly oxidized to convert into inactive pyridinic-N-oxide after using the Fe-MMT as a solid-state template. For confirming the presence of FeN types of bond in N 1s spectra, we have done the analysis of Fe 2p3/2 spectrum for the same sample (Fig. 3e). In addition to three peaks due to various forms of iron oxides (710e713 eV), there is a distinct peak due to the FeNx bond at 708.4 eV [22,23]. A significant amount of oxygen species are also included in the catalysts. The O 1s XPS spectrum of FeN/C-PANI (Fig. 3f) is deconvoluted into four peaks with binding energies of 530.8, 532.2, 533.7, and 535.8 eV, which can be assigned to the C] O, C(aliphatic)-OH/C(aliphatic)-O-C(aliphatic), metal-bounded
oxygen, and chemisorbed water molecules, respectively. The comprehensive analysis of Fe 2p, N 1s and O 1s spectra for the FeN/ C-PANI catalyst suggests that the Fe atoms are in two forms (FeeNx and FeeO).
3.2. Evaluation of electrocatalytic ORR activity The electrocatalytic activities towards the ORR of the synthesized catalysts are studied using a typical three-electrode device by cyclic voltammetry (CV) and rotating disk electrode (RDE) measurements. All electrodes are pre-treated by repeated cycling of the potential from 0 to 1.2 V versus a reversible hydrogen electrode (RHE) with a sweep rate of 50 mV s1 to remove surface contaminations. The CV and LSV curves of catalyzed electrodes made with N/C-PANI, FeN/C-PANI, and Pt/C (20 wt% Pt) are performed in 0.1 M HClO4 solution saturated by O2 or N2. As displayed in Fig. 4a, a significant enhancement of the cathodic peak for ORR with an onset potential (EORR) of 0.82 V and a peak potential (Ep) of 0.62 V (versus RHE) on N/C-PANI is well-defined in an O2-saturated 0.1 M HClO4 solution, but it is featureless and no cathodic peak can be observed in the N2-saturated HClO4 solution, indicates the electrocatalytic activity of the N/C-PANI catalyst. However, a higher current density of the ORR together with more positive EORR (0.84 V) and Ep (0.66 V) can be obtained on the FeN/C-PANI-catalyzed electrode under the same condition, which suggests better ORR electrocatalytic activity the in HClO4 solution. Furthermore, a very weak cathodic peak at about 0.70 V can be found at the CV curve of the FeN/C-PANI catalyst, which can be caused by the electroreduction of noexhausted oxygen molecule in N2-saturated HClO4 solution. Compared with two CV curves in N2-saturated solution, the electrode capacitance increases on FeN/C-PANI, indicating an increase of the solid/electrolyte interface of the electrode [16]. It may be attributed to the increase of effective surface area after using the Fe-modified MMT as a nano-template. RDE tests of PANI-900, FePANI-900, N/C-PANI, FeN/C-PANI, and Pt/C catalysts for ORR were
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carried out in an O2-saturated 0.1 M HClO4 solution at a rotation rate of 1600 rpm, as indicated in Fig. 4b. Similar ORR onset potentials can be observed at PANI-900, Fe-PANI-900 and N/C-PANI catalysts, but a relatively higher ORR limited current density at the same potential is found at N/C-PANI, indicating that using NaMMT as a nano-template can also help to improve the ORR activity of the catalyst. The half-wave potential (E1/2) for the ORR of FeN/ C-PANI is around 0.62 V, higher than that of N/C-PANI (0.50 V), suggesting a relatively higher ORR activity. The use of Fe-MMT for synthesis of the carbon-based catalyst should be more effective to promote its ORR activity in acidic solution compared to the use of Na-MMT nano-template. The difference of E1/2 between FeN/CPANI and Pt/C is about 180 mV, implying that the ORR activity of FeN/C-PANI is still lower than that of the Pt/C catalyst in the acidic solution, however, the limited diffusion current density (jd) of FeN/ C-PANI at 0.2 V is almost identical to that of Pt/C. We have further done a series of RDE measurements at different rotating speeds from 400 rpm to 3600 rpm to further reveal the kinetics of the ORR. As demonstrated in Fig. 5a and c, the jd increases with the increase in the rotation rate of RDE. It can be estimated from the KouteckyLevich plots that the average electron number of the ORR on the N/ C-PANI and FeN/C-PANI-catalyzed electrodes is 3.2 and 3.1, respectively, at the potential range of 0.1e0.3 V versus RHE (Fig. 5b
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and d). This result shows a major four-electron ORR pathway (O2 þ 4Hþ þ 4e / 2H2O) on these catalyzed electrodes. We further measured the ORR activity of PANI-900, Fe-PANI900, N/C-PANI, FeN/C-PANI, and Pt/C catalysts in 0.1 M KOH solution using CV and RDE techniques, as shown in Fig. 6. Fig. 6a displays the ORR onset potentials of 0.95 and 0.99 V for N/C-PANI and FeN/C-PANI in an acidic electrolyte, respectively, which are larger than those in 0.1 M HClO4 solution. The peak potentials of ORR are about 0.80 V and 0.85 V for N/C-PANI and FeN/C-PANI, respectively; but they are almost featureless in the N2-saturated KOH solution. Moreover, compared to CV profiles in N2-saturated electrolyte, a strong reduction peak can be clearly seen with the electrolyte saturated with O2, suggesting the high electrocatalytic activities of N/C-PANI and FeN/C-PANI towards the ORR. These results further show that FeN/C-PANI exhibits the best ORR activity in KOH solution, based on a fact that the onset potential of FeN/C-PANI approaches to that of Pt/C catalyst (see in Fig. 6b). The half-wave potentials for ORR measured on a N/C-PANI-catalyzed electrode and a FeN/C-PANI-catalyzed electrode are only 85 mV and 10 mV lower than that on a state-of-the-art Pt/C catalyst at a mass loading of 320 mg cm2, respectively. Besides, the limited diffusion current density on the FeN/C-PANI-catalyzed electrode is notably larger compared to the Pt/C-catalyzed electrode. Interestingly, the FePANI-900 also exhibits a better ORR electrocatalytic activity with a high limited current density compared to the PANI-900 and N/CPANI catalysts, but its ORR activity is also lower than that of FeN/CPANI in terms of onset potential, half-wave potential and limited diffusion current density. These results not only show that iron atoms may be a key to the formation of the catalyst with high ORR activity, but also confirm that the introduction of Fe-MMT as a nano-template can play a very important role in the enhancement of the ORR activity of the catalyst. The ORR catalytic mechanism of N/C-PANI and FeN/C-PANI was investigated further using the RDE at different rotation rates, as shown in Fig. 7a and c. As can be seen, an increase in the ORR current density with the rotation rate was observed at catalyzed electrodes. The good linearity of the Koutecky-Levich plots (Fig. 7b and d) suggests the first-order dependence of the ORR kinetics at different potentials. The ORR electron transfer number was calculated to be 3.7 and 4.0 at the potential range of 0.2e0.4 V for N/C-PANI and FeN/C-PANI, respectively, based on the slopes of Koutecky-Levich plots. Our results indicate that the ORR on two types of carbon catalysts proceeded mainly with four-electron reduction pathway (O2 þ 2H2O þ 4e / 4 OH), very similar to ORR catalyzed by a state-of-the-art Pt/C catalyst measured in 0.1 M KOH solution [21]. More significantly, the ORR activity of FeN/C-PANI is comparable to that of the best metal/nitrogen-doped carbon electrocatalysts and metal-free carbon-based electrocatalysts reported to date, in particular the catalysts derived from various nitrogen-containing organic molecules [14e21]. Hence we can reasonably conclude that the FeN/C-PANI catalyst is a very promising candidate for commercial Pt-based catalysts. 3.3. Analysis of electrocatalytic stability
Fig. 6. (a) CV of N/C-PANI and FeN/C-PANI in N2 and O2 saturated 0.1 M KOH; (b) LSV of PANI-900, Fe-PANI-900, N/C-PANI, FeN/C-PANI, and Pt/C in O2-saturated 0.1 M KOH at a rotation speed of 1600 rpm.
The electrochemical ORR stability of FeN/C-PANI in alkaline and acidic media is one of the most important factors to assess whether it can be applied to practical fuel cells. We performed the accelerated aging tests (AAT) in O2-saturated 0.1 M HClO4 or 0.1 M KOH solutions by continuous CV measurements for 2000 cycles at a scan rate of 50 mV s1. The ORR electrocatalytic activity of FeN/C-PANI was further evaluated by CV and LSV under the same conditions as above experiments, as displayed in Fig. 8. The CV curves of FeN/ C-PANI before and after the AAT have not obviously changed except for a slightly decrease in ORR current density, whether it was tested
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Fig. 7. LSV of N/C-PANI (a) and FeN/C-PANI (c) in O2-saturated KOH solution at different rotation rates; Koutecky-Levich plots of N/C-PANI (b) and FeN/C-PANI (d) derived from RDE data.
in HClO4 solution or in KOH solution (see Fig. 8a and c). The LSV curves (Fig. 8b and d) show that no noticeable changes in the onset potential of the ORR were observed on the FeN/C-PANI-catalyzed electrode, but the half-wave potential of the ORR was negatively shifted by 18 mV in 0.1 M HClO4 solution and 11 mV in 0.1 KOH solution, respectively, suggesting a relatively better stability of FeN/
C-PANI in alkaline electrolyte compared with the acidic electrolyte. The poor stability of FeN/C-PANI in an acidic solution can be attributed to an accepted reason that the dissolution of metal-Fe contained in Fe-Nx structures can easily occur in acidic solutions, resulting in the partially damage of Fe-Nx structures and the decrease of ORR activity in acidic environments. In addition, the
Fig. 8. (a) CV and (b) LSV of FeN/C-PANI before and after AAT in O2-saturated 0.1 M HClO4 solution; (c) CV and (d) LSV of FeN/C-PANI before and after AAT in O2-saturated 0.1 M KOH solution.
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are bonded to two carbon atoms and contribute to the p system with two p-electrons, which will result in chemically active, localized areas of higher electron density and promotes the catalysis of ORR [18]. Besides, an irreversible fact should be paid much attention, which is that the transformation of pyridinic-N into pyridinic-N-oxide does not result in the decrease of the electrocatalytic activity for the prepared Fe-N/C-PANI catalyst, showing the pyridinic-N may be not the effectively catalytic active sites for ORR in this study. Our previous reports have effectively confirmed that the pyrrolic-N configuration may be the electrocatalytically active site for ORR of nitrogen-containing carbon-based electrocatalyst [17]. Thus, on the basis of the XPS results and electrochemical data, there is sufficient reason to judge that the pyrrolic-N active site may be mainly responsible for the ORR activity of FeN/CPANI and the Fe-Nx active site can be play a key role in the improvement of the ORR activity of FeN/C-PANI. 4. Conclusions
Fig. 9. CV of FeN/C-PANI in (a) O2-saturated HClO4, (b) O2-saturated HClO4 containing 2 mM SCN, and (c) N2-saturated HClO4 containing 2 mM SCN.
above results also demonstrate that FeN/C-PANI has good durability under the alkaline conditions, and may be suitable for the electrocatalysis of the ORR in alkaline electrolytes. 3.4. Discussion of catalytically active sites for the ORR Up to now, the function of the real ‘‘electrocatalytically active sites’’ remains controversial because their contribution to the ORR activity is not well-defined. To clarify the role of metal-Fe atoms in ORR electrocatalysis is very significant for study of ORR catalytic sites. We first designed an additional poisoning experiment by using 2 mM thiocyanate (NaSCN) as a poisoning electrochemical probe based on the strong binding force of SCN and Fe3þ. An acidic medium (0.1 M HClO4) was solely chosen in this study to make the conditions more relevant for ORR in the PEM fuel cell environment. We separately tested the CV curves of FeN/C-PANI in O2-saturated 0.1 M HClO4 (a), O2-saturated 0.1 M HClO4 containing 2 mM SCN (b) and N2-saturated 0.1 M HClO4 containing 2 mM SCN (c), as shown in Fig. 9. We found that both onset potential and peak potential for ORR were negatively shifted by 60 mV and 40 mV, respectively, while the FeN/C-PANI-catalyzed electrode was tested in 0.1 M HClO4 solution containing 2 mM SCN saturated by oxygen. However, the cathodic current density for the ORR was not largely decreased. Interestingly, a well-defined peak at 0.85 V can be clearly observed at the CV anodic scanning process. It is ascribed to the oxidation peak of SCN on the surface of the FeN/C-PANIcatalyzed electrode. When the FeN/C-PANI-catalyzed electrode was tested in 0.1 M HClO4 solution containing 2 mM SCN saturated by nitrogen, a more obvious oxidation peak of SCN can be observed and its peak current density was beyond twice larger than that in O2-saturated 0.1 M HClO4 solution containing 2 mM SCN. It suggests that the metal-Fe can more easily interact with the SCN in the ORR process and then result in a relatively lower current density of SCN oxidation. These results confirm that the FeN/CPANI catalyst formed by a nano-template method may have metal-centered active sites (Fe-Nx) for the ORR, which was supported by many reported researches [24,25]. However, the pyrrolicN species is still to dominate in all types of nitrogen functionalities of the prepared catalysts. Pyrrolic N refers to nitrogen atoms that
Herein, we developed a facile approach based on Fe (III)-modified montmorillonite as a direct solid-state template to the design of a non-noble-metal electrocatalyst (FeN/C-PANI) from the conversion of aniline nanopolymer that acted as a single precursor for both carbon and nitrogen atoms, therefore avoiding the usage of complicated chemicals in the synthesis processes. The utilization of Fe-MMT flat-template can effectively prevent aniline nanopolymer from agglomeration during pyrolysis at high temperatures and cause more nitrogen-containing active sites to be exposed on the catalyst surface, further enhancing the electrocatalytic activity towards the ORR in acidic and alkaline media. The prepared FeN/CPANI catalyst exhibited more excellent ORR activity and better stability in alkaline electrolyte than those in acidic electrolyte. More notably, in alkaline electrolyte, the onset potential for four-electron ORR measured on FeN/C-PANI was about 0.99 V versus RHE, and the half-wave potential for ORR measured on FeN/C-PANI is only 10 mV lower than that on a state-of-the-art 20 wt% Pt/C catalyst. XPS and electrochemical results comprehensively show that the Fe atoms in FeN/C-PANI are in two forms (Fe-Nx and FeeO) and the FeNx sites can be play a key role in the improvement of the ORR activity. The pyrrolic nitrogen may be the electrocatalytically active site for ORR in our prepared catalysts. Therefore, this study demonstrates that Fe-MMT template-assisted conversion of aniline nanopolymer at controlled temperatures might be a practicable route to design high-performance FeN/C electrocatalysts for a range of electrochemical reactions. Acknowledgments This study was financially supported by the Basic and Frontier Research Program of Chongqing Municipality (cstc2015jcyjA50032 and cstc2014jcyjA50038), the Scientific and Technological Research Program of Chongqing Municipal Education Commission (KJ1501118), the Talent Introduction Project (R2014CJ02) and NSFC (Project No. 21273292). The authors are very grateful to Prof. Zhongli Luo, Haibo Ruan and Yujun Si for helpful discussions. References [1] F. Cheng, J. Chen, Metal-air batteries: from oxygen reduction electrochemistry to cathode catalysts, Chem. Soc. Rev. 41 (6) (2012) 2172e2192. [2] L. Dai, Y. Xue, L. Qu, H.-J. Choi, J.-B. Baek, Metal-free catalysts for oxygen reduction reaction, Chem. Rev. 115 (2015) 4823e4892. [3] G. Wu, P. Zelenay, Nanostructured nonprecious metal catalysts for oxygen reduction reaction, Accounts Chem. Res. 46 (2013) 1878e1889. [4] Z. Yang, H. Nie, X. Chen, X. Chen, S. Huang, Recent progress in doped carbon nanomaterials as effective cathode catalysts for fuel cell oxygen reduction reaction, J. Power. Sources 236 (2013) 238e249.
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