C Electrocatalysts for Oxygen Reduction Reaction in PEM Fuel

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Fe/N/C Electrocatalysts for Oxygen Reduction Reaction in PEM Fuel Cells Using Nitrogen-Rich Ligand as Precursor Lingling Yang, Yumiao Su, Wenmu Li, and Xianwen Kan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511576q • Publication Date (Web): 21 Apr 2015 Downloaded from http://pubs.acs.org on May 7, 2015

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Fe/N/C Electrocatalysts for Oxygen Reduction Reaction in PEM Fuel Cells Using Nitrogen-Rich Ligand as Precursor Lingling Yang†,§, Yumiao Su§,‖, Wenmu Li*,§,‖, Xianwen Kan*,†

†College of Chemistry and Materials Science, Anhui Key Laboratory of Chemo-Biosensing, Anhui Normal University, Wuhu, 241000, P. R. China

§National Key Laboratory, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou, 350002, P. R. China

‖University of Chinese Academy of Sciences, Beijing, 100049, P. R. China

KEYWORDS: Fuel cell, oxygen reduction reaction, Fe-based catalysts, nitrogen-rich precursor Abstract High temperature pyrolysis can significantly improve the activity and stability of Fe-based catalysts. However, unwanted iron nanoparticles, which are proven inactive to oxygen reduction reaction (ORR), will form under this procedure. Herein, a nitrogen-rich and hindrance multifunctional 6,7-di(pyridin-2-yl)pteridine-2,4-diamine (DPPD) monomer was deliberately designed and synthesized. High content of thermally-stable nitrogen in DPPD can increase the degree of coordination with iron and provide a high content of active nitrogen after pyrolysis. Distorted nitrogen-rich ferrous complex polymers were successfully prepared to keep iron ions well separated and


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prevent them from aggregating during the heat treatment. Carbon supported Fe-based catalysts with different initial iron loadings from 0.2 to 4.0 wt % were obtained. Transmission electron microscopy (TEM) revealed that there were no obvious nanocrystals observed, even the initial iron loading was up to 2.0 wt %. The electrochemical performance of the Fe-based catalysts was evaluated via cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The result shows that Fe-based catalyst with 2.0 wt % initial iron loading is the best ORR catalyst in acid media among all the iron loadings. Typically, in basic media, the catalyst with 2.0 wt % initial iron loading exhibits comparable electrocatalytic activity to commercial Pt/C material via an efficient four-electron-dominant ORR pathway coupled with better methanol tolerance as well as durability. XPS measurements confirmed that the outstanding activity of the catalyst with 2.0 wt % initial iron loading was likely attributed to higher content of pyridinic nitrogen, providing the highest density of active site structures. Introduction Proton exchange membrane fuel cells (PEMFCs), the most promising electrochemical devices that is highly potential to replace internal engines for transportation application,1-3 have captured a significant amount of research attention in the past few decades. However, the sluggish oxygen reduction reaction (ORR) occurring at the cathode presents several technical challenges that comprise the largest bottleneck hindering the commercialization of PEMFCs. Until now, Pt and its alloys remain the best electrocatalysts available for the ORR and anodic hydrogen oxidation reaction (HOR),3-4 providing reduced overpotentials and higher kinetic rates in comparison to alternative catalyst technologies. As the ORR is inherently several orders of magnitude slower than the HOR, higher Pt content is required at the cathode as this is the component that limits the overall performance of PEMFCs. Besides high cost together with limited global supply,5 Pt still suffers from multiple deficiencies, including inadequate durability originating from Pt catalyst migration and dissolution during PEMFCs operation, decrease of electrochemically active surface areas, corrosion of the



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carbon support as well as the poisoning6 and crossover effects. To overcome these pertinent challenges, extensive research must be done to explore novel catalysts as alternatives to Pt that possess high performance and durability, along with low cost. Significant progress has been achieved in recent years on this front,7-8 such as non-precious metal chalcogenides,9 carbon nitrides,10-11 conductive polymer-derived materials,12-13 and most notably, metal-N 4 macrocycle based materials.14-15 Jasinski was the first to observe that cobalt phthalocyanine (CoPc) showed catalytic activity for the ORR.16 This finding stimulated intensive research on assessing transition metal macrocycles, mainly Fe-N 4 or Co-N 4 macrocycles, as possible electrocatalysts for the ORR. The onset potential, current density and excellent selectivity towards the fourelectron ORR pathway for FePc are almost on the same level of the commercial Pt/C material in alkaline media,17 whereby these catalysts are commonly prepared by mixing Fe-N 4 or Co-N 4 macrocycles with carbon supports using an impregnation method. Later studies demonstrated that pyrolyzing these transition metal macrocycles with carbon supports could remarkably enhance both the catalytic activity and stability of the catalysts. An unprecedented achievement was reported by Yeager and his coworkers when they observed that these costly transition metal macrocycles could be substituted by a mixture of separate nitrogen, carbon and transition metal precursors that were subjected to high temperature pyrolysis. Following this, a variety of common nitrogen-containing

chemicals,

including

polyacrylonitrile,18

pyrrole,19

ethanediamine,20-21 triethylenetetramine,22 etc. have been subsequently used as nitrogen precursors, and Fe salts have been found to provide the highest ORR activity in comparison to numerous other transition metal based precursors.8, 23 To a large extent, the activity and durability of these catalysts rely greatly on the precursor selection and the synthesis conditions utilized. For instance, the heat treatment temperature, time and molar precursor ratios have all been shown to have a significant impact on the resulting ORR performance.23-26 Though high temperature pyrolysis of precursors can remarkably enhance both the


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activity and stability of the resulting catalysts, it will cause several issues which seriously offset the benefits it brings. One side, it is inevitable that large metal nanoparticles are formed by sintering during the pyrolysis process.27-28 For another, the nitrogen content,29-31 a necessary consideration for the ORR active sites formation, will be significantly reduced as a result of the heat treatment process. Both of the aforementioned issues can seriously impede the formation of active sites. One possible method to resolve the challenges is to synthesize a novel precursor with a high content of thermally-stable nitrogen, and with a structure that can readily coordinate with iron, generating well-distributed Fe-N sites prior to pyrolysis. These well-defined and distributed Fe-N sites can provide segregation between iron ions, thus preventing their aggregation and the formation of iron nanoparticles during the pyrolysis. Additionally, the high amount of thermally-stable nitrogen can increase the degree of coordination with iron and provide a high content of active nitrogen after pyrolysis, promoting the formation of catalysts with high active site densities. In the present work, we design and synthesize a novel, multifunctional nitrogen-rich ligand 6,7-di(pyridin-2-yl)pteridine-2,4-diamine (DPPD) by a one-step synthesis procedure as shown in Scheme 1. DPPD contains approximately 27 wt % of highly thermally-stable nitrogen such as pyridinic nitrogen, among the highest content reported in the literature up to now.4, 32 DPPD is then coordinated with Fe(II) salt to form a Fe(II)-N complex, whereby the distorted structure of the prepared complex is necessary to assure that iron (II) ions are well-isolated from each other. This is in order to prevent the aggregation of Fe (II) ions and provide well-distributed Fe-N sites in the prepared catalysts. Finally, DPPD is intentionally designed to contain –NH 2 groups, which will turn into ammonia during the pyrolysis process, promoting the formation of pores.33 The newly generated porosity will offset the structural collapse and loss of surface area that commonly occur during the pyrolysis, allowing for the efficient transport of O 2 , H 2 O and other ORR-related species through the catalyst structure,34 resulting in the formation of highly catalytic activity catalysts.33,


35

This rationally


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designed precursor also possesses an aromatic structure that can readily interact with graphitic carbon supports36 and graphitize during the pyrolysis process, providing valuable ORR activity and stability benefits, respectively. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption isotherms and X-ray photoelectron spectroscopy (XPS) were used to investigate the surface morphology, specific surface area, pore characteristic and structure information of the as-prepared catalysts. The catalysts developed using this innovatively designed ligand DPPD were subjected to cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements using rotating disk electrode (RDE) techniques. Experiment Section Synthesis of DPPD 2.12 g (10 mmol) of 2-di(pyridin-2-yl)ethane-1,2-dione and 1.40 g (10 mmol) of pyrimidine-2,4,5,6-tetraamine were dissolved in DMF in a three-neck round bottom flask. The mixture was heated to 150 oC and then refluxed for a total 4 hours under pure purity grade nitrogen atmosphere. Then the mixture was allowed to cool down to room temperature and filtered. The filtrate was dried overnight, resulting in a yellow powder. The 1H NMR, EIMS and FT-IR results of the yellow powder were listed as below. 1H NMR δ: 8.27-8.25 (d, 1H), 8.18-8.16 (d, 1H), 8.09-8.07 (d, 1H), 7.94-7.81 (m, 5H); 7.35-7.32 (t, 1H), 7.27-7.23 (t, 1H), 6.8 (s, 2H,); EIMS: (m+1)/z=317.5; calcd for C 16 H 12 N 8 , 316.3. FT-IR (film/KBr): 3445, 3303 cm-1(ν, N–H stretching), 3055 cm-1(ν, aromatic C–H stretching), 1639 cm-1(ν, C=N stretching), 1563, 1417 cm-1(ν, aromatic C=C stretching), 1356 cm-1(ν, C–N stretching), 1088 cm-1(δ, C–H in-plane deformation), 797,744 cm-1(δ, C–H out-of-plane deformation). Synthesis of Fe(II)-DPPD The as-prepared DPPD (50 mg) and DMF (3 mL) were added to a 25 mL three-neck round bottom flask equipped with a magnetic stirrer, and the reactor was purged with pure nitrogen to eliminate oxygen. 1mL [(NH 4 ) 2 Fe](SO 4 ) 2 ·6H 2 O water solution (0.0207g iron salt) was added dropwise into the reactor. The mixture was


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continuously stirred for 1 hour at room temperature under nitrogen atmosphere. The solvent was then evaporated and the complex formed was dried at 120 °C overnight. A resultant brown solid was obtained. Catalyst Preparation For example, a catalyst with an initial iron loading of 2.0 wt % the following procedure was used. DPPD (0.15 g) was dissolved in DMF (8 mL) under nitrogen atmosphere. [(NH 4 ) 2 Fe](SO 4 ) 2 ·6H 2 O water solution (1 mL, 0.049 g of iron salt) was added dropwise into the mixture at 80 °C. The reaction was allowed to proceed for 1 h. Thereafter, 0.15 g of commercial BP 2000 (BET: 1529 m2/g) was added. The mixture was stirred for another 2 hours to permit the resulting compound to uniformly disperse on the carbon black particles. The solvent was then evaporated, and the mixture was dried overnight until a solid product was obtained. This dried sample was pyrolyzed in a horizontal tube furnace under continuous flow of purified nitrogen and using a heating rate of 10 °C/min up to a temperature of 900 °C. This temperature was maintained for 1 hour, after which the furnace was allowed to cool down to room temperature under nitrogen flow and the final catalyst structure was obtained. Catalysts, with initial iron loadings of 0.2, 0.5, 1.0, 4.0 wt %, were prepared in the same way. As reference, BP 2000 was pyrolyzed in the same procedure and denoted as BP-900. Physical Characterization Fourier-transform infrared spectroscopy (FT-IR) obtained on a PerkinElmer Spectrometer was used to confirm the functional groups of DPPD and Fe(II)-DPPD. The samples were prepared with KBr pellets at wavenumbers ranging from 400-4000 cm-1. The UV-vis spectroscopic data of DPPD and Fe(II)-DPPD were collected using Lambda 950 from PerkinElmer. Electron ionization (EI) mass spectrometry and 1H NMR spectrum were used to identify the formation of DPPD. Thermal gravimetric analysis (TGA) of DPPD and Fe(II)-DPPD was performed with Netzsch STA449C instrument from room temperature to 1000 oC using a heating rate of 10 °C/min with pure nitrogen as a carrier gas. The resulting catalysts were characterized using scanning



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electron microscopy (SEM) on a JEOL JSM 6700F microscope, transmission electron microscopy (TEM) on a FEI Tecnai F20 electron microscope and X-ray powder diffraction (XRD) on Rigaku DMAX2500 X-ray diffractometer using a copper target (λ=0.154 nm). The specific surface areas together with the pore characteristics of the obtained catalysts were estimated from low-temperature nitrogen adsorption isotherms measured at 77 K on a Micromeritics ASAP2020. XPS measurements were performed on a Thermo Scientific ESCALAB 250 to probe the electronic states and relative contents of the elements on the surface of the catalysts. Electrocatalytic Properties Measurements The electrochemical properties of the catalysts were investigated by cyclic voltammetry (CV) and linear sweep voltammetry (LSV) based on rotating disk electrode (RDE) techniques using a CHI Electrochemical Station (Model 700E) and a conventional three-electrode electrochemical Teflon cell. Experiments were carried out using an Ag/AgCl electrode in saturated KCl solution (0.198V vs. NHE) as a reference electrode and Pt foil as the counter electrode. Before the preparation of the working electrode, the glassy carbon electrode (5 mm diameter, 0.196 cm2 geometrical surface area) was polished with 0.5 and 0.05 μm alumina powder, sequentially, and then was sonicated several times alternately in ethanol and ultrapure water (2 min in each) to obtain a mirror finish, followed by drying under purified nitrogen flow. The ink35 for electrocatalytic properties measurements was prepared in a 2.5 mL glass vial by sonicating for 30 minutes a mixture of 4 mg catalyst, 38 μL 5 wt % Nafion in alcohol (Aldrich) and 140 μL ethanol. A 7 μL aliquot of the ink was dropcoated onto the glassy carbon disk. The working electrode was dried at room temperature in air until a uniform thin film with a constant loading of 806 μg/cm2 catalyst was obtained in all cases. Prior to the electrochemical analysis, the working electrode surface was activated by repeatedly cycling the potential between 0.05 and 0.95 V νs. NHE for 40 cycles. Then, the CV measurements were conducted in N 2 - or O 2 -saturated 0.1 M HClO 4 by cycling the potential between 0.05 and 0.95 V νs. NHE for 4 cycles. The LSV measurements


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were conducted in the same way as the CV measurements with rotation speeds ranging from 100 to 2500 rpm. The polarization curves were corrected by subtracting background currents measured in N 2 -saturated electrolyte. Please note that the axes corresponding to currents presented in all electrochemical measurements and graphs were all normalized to the geometrical area of the electrodes. All the electrochemical measurements were performed at room temperature. All potentials are referred as NHE scale in this article, unless otherwise stated. Results and Discussion A one-step method was developed to synthesize the nitrogen-rich ligand DPPD and the procedure is shown in Scheme 1. To confirm the formation of the synthesized compound, electron ionization (EI) mass spectrometry (Figure 1), FT-IR spectrum (Figure 2(a)) and 1H NMR spectroscopy (Figure S1) were conducted. As shown in Figure 1, a distinct and intensive peak is displayed at (m+1)/z=317.5, revealing the expected molecular weight of 316.3 g/mol for DPPD. Further characterization of the structure and functional groups of DPPD is confirmed by FT-IR spectroscopy. There are two peaks concentrated around ca. 3445 cm-1 and 3303 cm-1, which could be assigned as the vibration of N–H of –NH 2 groups. The dominating peaks at 1639 cm-1 and 1356 cm-1 are assigned as stretching vibrations of C=N and C–N on aryl ring,37 respectively. Meanwhile,

the

characteristic

double

carbonyl

group

peaks

of

1,2-di(pyridin-2-yl)ethane-1,2-dione at 1717 cm-1 and 1695 cm-1 disappear in the FT-IR spectrum of DPPD. All of these results confirm successful formation of DPPD. The coordination ability of a ligand to Fe plays an important role in the catalytic activity of a catalyst.38 Thus, the FT-IR spectrum of Fe(II)-DPPD was conducted to investigate the coordination ability of DPPD with Fe (Figure 2(a)). It is interesting that the peak at 3445 cm-1 (assigned as N–H vibration of –NH 2 ) disappears when DPPD reacted with ferrous ion. This indicates that the nitrogen of –NH 2 may coordinate with ferrous ions. New peak around 1295 cm-1 is observed in Fe(II)-DPPD, while the peak around 1356 cm-1 disappears in DPPD. An almost 61 cm-1 blue shift of ν C-N stretching



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vibration between DPPD and Fe(II)-DPPD, indicates that there is effective coordination between Fe(II) and the nitrogen.39 Remarkably, compared with the FT-IR spectrum of DPPD, a peak centered at 575 cm-1 attributed to the vibration of Fe-N bond,40 appears for Fe(II)-DPPD. To further confirm the formation of the iron (II) complex, UV-vis absorption spectra of DPPD and Fe(II)-DPPD were carried out as well. As shown in Figure 2(b), there are two characteristic absorption peaks centered at 288 nm and 391 nm for DPPD solution. In case of Fe(II)-DPPD, there is not any difference at 288 nm. However, there is an obvious 18 nm blue shift observed for Fe(II)-DPPD. The sharp blue shift clearly indicates the formation of the Fe (II) complex. High temperature pyrolysis can enhance the activity and stability of a catalyst and a proper pyrolysis temperature will result in higher activity. In order to select an appropriate pyrolysis temperature, the detailed thermal decomposition process of DPPD was conducted via TGA measurements. As shown in Figure 3, the largest mass loss is observed between 207 oC and 365 oC, mainly due to the carbonization of DPPD. When the temperature rises from 365 oC up to 1000 oC, the weight loss rate is slower. At temperature of 900 oC, the total mass loss is 58 %, indicating that more than half of DPPD decomposes during the heat treatment and forms new compounds. A TGA test for Fe(II)-DPPD was also carried out to investigate the impact of ferrous iron on the behavior of nitrogen-rich ligand during pyrolysis. The TGA curve of Fe(II)-DPPD resembles that of DPPD, however, there is a sudden mass loss occurring at about ca. 477 oC, which can be attributed to the accelerated formation of nitrogen gas catalyzed by Fe.41-42 In order to get highly active catalysts, this accelerated nitrogen losing phenomenon should be prevented. Therefore, the optimal catalysts pyrolysis temperature is chosen at 900 oC under inert atmosphere.36, 43 Increasing the surface active site density is a common used method to improve the catalytic activity of Fe/N/C type catalysts.23, 36, 44-46 Generally, high iron loading means that iron is capable of coordinating with more nitrogen, providing a higher surface density of active site. However, iron nanoparticles which are inactive for the ORR, are


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the inevitable byproducts with high initial iron loading during the heat treatment process. In our work, the ligand DPPD is deliberately designed as a novel and multifunctional nitrogen precursor to prevent ferrous ions from aggregating, even in much higher iron loading. XRD test was conducted to determine the formation of nanoparticles. As shown in Figure 4, two broad diffraction peaks around 24.8o and 43.7o are observed for Fe-based catalysts and BP-900, which could be assigned as the inter-plane (002) and the inner-plane (110) reflections of nanographitic in carbon support.47 Fe-based catalyst with 4.0 % initial iron loading displayed much higher intensity around 43.7o than that of other Fe-based catalysts with lower iron loading. The only difference in these Fe-based catalysts was their initial iron loading. Therefore, higher intensity around 43.7o in Fe-based catalyst with 4.0 % initial iron loading maybe due to the formation of iron nanoparticles. Transmission electron microscopy (TEM) was used to further affirm the existence of iron nanoparticles. The results are shown in Figure 5. For catalyst with 4.0 % initial iron loading, many nanoparticles are observed in high resolution TEM image (Figure 5(a) and 5(b)). These nanoparticles are clearly proven to have high iron content from the energy dispersive spectrum (EDS) in Figure 5(c). For catalyst with 2.0 % initial iron loading in Figure 5(d), though there are few large big particles observed, the HRTEM (Figure 5(e)) proves that these big particles are not nanocrystal. The EDS shows that the black particle contains much less iron than that of catalyst with 4.0 % initial iron loading. In addition, there are not any obvious black particles observed in Fe-based catalysts with lower initial iron loading (0.5 %~1.0 %) (Figure S3). These results indicate that DPPD is capable of preventing iron from aggregation, even the initial iron loading is as high as 2.0 %. High specific surface area and micro/mesopores feature of a catalyst are required to obtain highly catalytic activity, efficiently transport ORR-related species such as O 2 , protons and H 2 O.48 In order to evaluate the specific surface area (SSA) and porosity



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characteristic of the as-prepared catalysts, nitrogen adsorption/desorption isotherms were performed. The results are displayed in Figure S4 and Table S1. The Brunauer-Emmett-Teller (BET) surface areas of the catalysts decrease from 966 m2/g to 741 m2/g (Table S1) with increasing iron loading. The pore size distribution curves are obtained (Figure S5) utilizing the density functional theory (DFT). It is clear that all the samples exhibit similar pore size distribution and disclose a multi-level pore feature, mainly mesopores as suggested from the nitrogen isotherms with pore size primarily concentrated at 30, 36 and 43 nm. The presence of meso/micropores feature can favor the efficient mass transportation of ORR-related species, which is the rate-determined step in the ORR. The catalysts also display high pore volumes from 1.30 to 1.69 cm3/g. The above results demonstrate that the as-prepared catalysts possess many favorable structure features including high surface area, pore volume and multi-level meso/micropores. All these unique structure features are keys for the ORR because of effective O 2 transportation and high ratios of accessible catalytic active sites. The performance of Fe-based materials to the ORR strongly relies on the nitrogen bonding configuration of iron active sites.43 To evaluate the surface properties of Fe-based catalysts，XPS measurements were performed. The N 1s spectrums of these Fe-based catalysts are shown in Figure 6(a). There are mainly three types of nitrogen in Fe-based catalysts, pyridinic nitrogen (398.6 ± 0.3eV), pyrrolic nitrogen (401 ± 0.3eV), and oxidized nitrogen (N-O: 402.7-404.7eV), respectively.49-50 It has been reported that the onset potential of a nitrogen-doped catalyst has strong relation with pyridinic form nitrogen, but little effect by pyrrolic nitrogen and oxidized type nitrogen.51 This indicates that Fe-based catalysts with higher pyridinic nitrogen content might display higher catalytic activity. The contents of different type nitrogen in Fe-based catalysts are shown in Table S2. Catalyst with 2.0 % initial iron loading displays the highest pyridinic nitrogen content. The Fe 2p spectrums of all the catalysts are shown in Figure 6(b). The binding energy at 725.0-725.5eV can be assigned to 2p 1/2 signal of Fe(II) and Fe(III).52-53 The


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peaks at 722.5-723.3eV are consistent with the 2p 1/2 binding energy of Fe(II)52 and 710.5-712.2eV are 2p 3/2 of Fe(II) and Fe(III) spectrum.27, 54 These results indicate the co-existence of Fe(II) and Fe(III) in the catalysts and that a large proportion of surface iron is in the presence of ionic state. To roughly assess the catalytic activity, all the materials were first loaded onto glassy carbon electrodes for cyclic voltammetry with testing potentials from 0.05 to 0.95 V νs. NHE in N 2 - and O 2 -statured 0.1 M HClO 4 solution at room temperature. In Figure S6, quasi-rectangular voltammograms with no clear redox peaks are observed for all the samples in N 2 -statured CV curves. On the contrary, in O 2 -saturated solution, the CVs exhibit a steep current associated with the ORR, reaching a sharp and well defined cathodic reduction peak, indicating a distinct catalytic activity of all the samples towards the ORR. In addition, with the initial iron loading increasing from 0.2 % to 4.0 %, the peak potentials shift positively from 0.62 (0.2 %) to 0.68 V νs. NHE (2.0 %), then turn negatively shift to 0.56 V νs. NHE (4.0 %). This suggests that the Fe-based catalyst with 2.0 % initial iron loading is the best for the ORR among all the samples and that higher iron loading does not usually result in higher ORR activity. To shed some light on the catalytic activity of all the five catalysts during the ORR process, linear sweep voltammetry (LSV) measurements of each catalyst modified electrode were performed on RDE instrument with a rotating speed of 900 rpm in 0.1 M HClO 4 saturated with O 2 (Figure 7). Again, Fe-based catalyst with 2.0 % initial iron loading displays the highest activity with a 0.57 V νs. NHE half wave potential, 0.80 V νs. NHE onset potential, and 4.20 mA/cm2 limiting current density (Table S3 in Supporting Information). The high activity of Fe-based catalyst with 2.0 % initial iron loading is due to its higher amount of pyridinic nitrogen and well dispersion of iron ions which form possible higher density of ORR active sites compared with the other iron loading catalysts. The oxygen reduction reaction has two pathways in acidic medium as indicated in equation (1) (the 4 electrons pathway) and (2)、(3) (the 2 electrons pathway):55



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O 2 +4H++4e-

2H 2 O

O 2 +2H++2e-

H2 O2

H 2 O 2 +2H++2e-

2H 2 O

(1) (2) (3)

For PEMFCs, the 4 electrons pathway is preferred. In order to verify the ORR catalytic pathway and reaction kinetics of the resulted catalysts, RDE measurements were carried out with rotation speeds from 100 to 2500 rpm and a series of polarization curves were obtained (Figure 8 and Figure S7). These polarization curves show typical increasing limiting current density with higher rotation rates due to the reduced diffusion distance. The corresponding K-L plots derived from RDE measurements can be obtained at different testing potentials (detailed calculations for K-L plots see equations in Supporting Information). The electron number transferred per oxygen molecule and kinetic current density can be calculated from the slope and intercept of the K-L plots, respectively. The K-L plots show good linearity, regardless of the testing potential values. The slopes remain approximately the same over the testing potentials range from 0.05 to 0.95 V νs. NHE, suggesting a similar electron transfer number for the ORR at different potentials. Linearity and parallelism of the plots are considered as typical features of first-order reaction kinetics with respect to the concentration of dissolved O 2 . In the case of the catalyst with 2.0 % initial iron loading, the number of electrons transferred per oxygen molecule is 3.9, 3.7, 3.6, 3.7, 3.9 for the potential at 0.50, 0.45, 0.40, 0.35, 0.30 V νs. NHE, respectively. The average of n is 3.8, approaching the high-desired 4 electrons process, independent of the potentials tested. Although other Fe-based catalysts with 0.2, 0.5, 1.0 and 4.0 % initial iron loadings show lower activity, their electrons transferred number are still close to 4 at the potential of 0.40 V νs. NHE (Figure S7). This reveals that Fe-based catalysts have high ORR efficiency and are capable of directly reducing O 2 to H 2 O. The performance of Fe-based catalysts was also evaluated in basic media. Fe-based catalyst with 2.0 % initial iron loading shows the highest activity with a 0.04 V νs. NHE half wave potential (Figure 9), which is close to that of the commercial JM 20 % Pt/C.


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Furthermore, the Fe-based catalysts with initial 2.0 % initial iron loading has 18.00 mA/cm2 j kin which is comparable to that of the commercial Pt/C catalyst (19.90 mA/cm2) based on K-L plots (Figure S8). Durability is another critical criterion on the evaluation of high performance Fe-based materials for practical fuel cell applications. For this purpose, linear sweep voltammetry of Fe-based catalyst with 2.0 % initial iron loading has been run for 1000 cycles in 0.1 M KOH aqueous solution and the potential is hold at -0.20 V νs. NHE. The results are shown in Figure 10. Even after 1000 test cycles, our typical Fe-based catalyst only exhibits a 10 mV negative shift on half-wave potential, while the commercial Pt/C catalyst shows a 35 mV potential shift. Besides high ORR stability, the Fe-based catalyst also has high methanol tolerance. As shown in Figure 10(c), the current of Fe-based catalyst still remains 70 % after 3 M methanol is added. In case of Pt/C electrocatalyst, a sharp current decrease is observed upon the addition of 3 M methanol. Conclusion In this work, we have designed a nitrogen-rich ligand DPPD with a deliberately controlled structure with the purpose of preventing the aggregation of iron during the synthesis of the catalysts. The rationally designed, distorted structure assures iron ions are isolated from each other, preventing the formation of large iron nanoparticles during the high temperature pyrolysis. What is more, the high amount of thermally-stable nitrogen is capable of coordinating with more iron and improving the surface active nitrogen content for the resultant catalysts. The as-prepared catalyst with 2.0 % initial iron loading, with an onset potential of 0.80 V vs. NHE and limiting current density of 4.20 mA/cm2, possesses the best activity of all in acidic media. XPS measurements confirmed that the outstanding activity of the catalyst with 2.0 % initial iron loading was likely attributed to higher content of pyridinic nitrogen, providing the highest density of active site structures. AUTHOR INFORMATION Corresponding Author



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*E-mail: [email protected], [email protected]. Tel: 86-591-63173557, Fax: 86-591-63173557

Notes The authors declare no competing financial interest.

Acknowledgment The authors gratefully acknowledge the financial support of the NSFC (Project Nos. 21303206), use of facilities supported by the Nature Science Foundation of Fujian Province, and the kind helps of Dr. Junnan Gu from Suzhou Institute of Nano-Tech and Nano-Bionics (SINANO), CAS.

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Scheme and Figures Scheme and Figure Captions Scheme 1. Figure 1.

Synthesis procedure of DPPD. Electron ionization (EI) mass spectrum of DPPD.

Figure 2. FT-IR (a) and UV-vis (b) absorption spectra of DPPD and Fe(II)-DPPD. Figure 3. TGA curves of DPPD and Fe(II)-DPPD. Figure 4. BP-900.

XRD diffraction grams of the catalysts with various iron loadings and

Figure 5. TEM images of the catalysts with 4.0 % (a) and 2.0 % (d) initial iron loadings. The scale is 20 nm. HRTEM images of the catalysts with 4.0 % (b) and 2.0 % (e) initial iron loadings and the scale is 2 nm. Images (c) and (f) are the energy dispersive spectrum (EDS) of (b) and (e), respectively. Figure 6. (a) N 1s and (b) Fe 2p XPS spectrum of the catalysts with various iron loadings. Figure 7. ORR polarization curves of the catalysts with various iron loadings in O 2 -saturated 0.1 M HClO 4 solution with a rotation rate of 900 rpm after background subtraction. The scan rate is 10 mV/s. Figure 8. (a) Rotating disk electrode (RDE) linear sweep voltammograms of the catalyst with 2.0 % iron in O 2 -saturated 0.1 M HClO 4 solution after background subtraction at various rotation rates. The scan rate is 10 mV/s. (b) The corresponding Koutecky-Levich plots of the catalyst with 2.0 % iron at different testing potentials. Figure 9. Polarization curves of the catalysts with various iron loadings and the commercial JM 20 % Pt/C in O 2 -saturated 0.1 M KOH solution at a rotation rate of 1600 rpm after background subtraction. The scan rate is 10 mV/s. Figure 10. Accelerated durability tests (ADTs) for (a) the catalyst with 2.0 % iron (b) JM 20 % Pt/C in O 2 -saturated 0.1 M KOH solution with a rotation rate of 1600 rpm. The sweep rate is 100 mV/s. (c) Current-time chronoamperometric response of the catalyst with 2.0 % iron and the commercial JM 20 % Pt/C in O 2 -saturated 0.1 M KOH with a rapid addition of 3 M methanol at around 300 s at a rotation speed of 1600 rpm. The scan rate is 10 mV/s.



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Page 22 of 32

N H 2N

O O N

N N

N

NH2

DMF

o NH2 150 C

NH2

Scheme 1.

N

H 2N N

N NH2

Synthesis procedure of DPPD.


N

N

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


\

317.5

l :

100

%

318.5 319.5 0

310

312

Figure 1.

314

316

318 320 m/z

322

324

Electron ionization (EI) mass spectrum of DPPD.


326


(a)

(b)

288

DPPD 0.4

3445 3303

Abs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1356

1639

DPPD Fe(II)-DPPD

3319

575 Fe(II)-DPPD

4000

3500

Figure 2.

373 391 0.2

3000

1639

0.0

1295

2500 2000 1500 -1 Wavenumber / cm

1000

500

100

200

300

400 500 600 Wavenumber / nm

700

800

FT-IR (a) and UV-vis (b) absorption spectra of DPPD and Fe(II)-DPPD.


Page 25 of 32

100

DPPD Weight loss / %

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fe(II)-DPPD

80

60 o

477 C

40

20

0

200

400

600

800

1000

T / oC

Figure 3. TGA curves of DPPD and Fe(II)-DPPD.



BP-900 0.2 %

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

0.5 % 1.0 % 2.0 % 4.0 %

20

Figure 4. BP-900.

40 2Θ / deg.

60

XRD diffraction grams of the catalysts with various iron loadings and


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a

(b

(c

nanocrystal

(d

(e

(f)

Figure 5. TEM images of the catalysts with 4.0 % (a) and 2.0 % (d) initial iron loadings. The scale is 20 nm. HRTEM images of the catalysts with 4.0 % (b) and 2.0 % (e) initial iron loadings and the scale is 2 nm. Images (c) and (f) are the energy dispersive spectrum (EDS) of (b) and (e), respectively.



(a) 0.2 %

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

403.0 402.7

0.5 %

2.0 %

403.5

4.0 %

403.8

Figure 6. loadings.

406

404

400.8

398.3

398.6 398.4

1.0 % 404.7

408

400.7

398.8 398.3

402 400 398 Binding energy / eV

396

394

(a) N 1s and (b) Fe 2p XPS spectrum of the catalysts with various iron


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-2

0 Current density / mA • cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1 -2

4.0 % 2.0 % 1.0 % 0.5 % 0.2 %

-3 -4 0.0

0.2

0.4 0.6 Potential / V vs NHE

0.8

Figure 7. ORR polarization curves of the catalysts with various iron loadings in O 2 -saturated 0.1 M HClO 4 solution with a rotation rate of 900 rpm after background subtraction. The scan rate is 10 mV/s.



(b)

0

0.8

-2

(a)

-1

j-1 / mA-1• cm2

current density / mA • cm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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-2 -3

100 rpm 400 rpm 900 rpm

-4 -5

1600 rpm 2500 rpm

-6 -0.25

0.00

0.25 0.50 Potential / V vs NHE

0.6 0.50 V 0.45 V 0.40 V 0.35 V 0.30 V

0.4

0.2 0.75

0.02

0.04

0.06 -1/2

ω

0.08

0.10

-1/2

/ rpm

Figure 8. (a) Rotating disk electrode (RDE) linear sweep voltammograms of the catalyst with 2.0 % initial iron loading in O 2 -saturated 0.1 M HClO 4 solution after background subtraction at various rotation rates. The scan rate is 10 mV/s. (b) The corresponding Koutecky-Levich plots of the catalyst with 2.0 % initial iron loading at different testing potentials.


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0

Current density / mA•cm-2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-1

4.0 % 2.0 % 1.0 % 0.5 % 0.2 % Pt/C

-2 -3 -4 -5 -0.6

-0.4

-0.2 0.0 Potential / V vs NHE

0.2

0.4

Figure 9. Polarization curves of the catalysts with various iron loadings and the commercial JM 20 % Pt/C in O 2 -saturated 0.1 M KOH solution at a rotation rate of 1600 rpm after background subtraction. The scan rate is 10 mV/s.



(b) 0 -2

0 -1 -2 ∆E1/2=10

mV

-3 -4 -5 -0.6

initial 1000th cycles -0.4


0.2

Current density / mA•cm

Current density / mA•cm

-2

(a)

-1 -2 -3 ∆E=35

-4

mV

-5

initial 1000th cycles

-6 -7 -0.6

-0.4


0.2

(c) 1.2 1.0

Relative current

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 32

Pt/C 2.0 %

0.8 0.6 0.4 0.2 300

600 T/s

900

Figure 10. Accelerated durability tests (ADTs) for (a) the catalyst with 2.0 % initial iron loading and (b) JM 20 % Pt/C in O 2 -saturated 0.1 M KOH solution with a rotation rate of 1600 rpm. The sweep rate is 100 mV/s. (c) Current-time chronoamperometric response of the catalyst with 2.0 % initial iron loading and the commercial JM 20 % Pt/C in O 2 -saturated 0.1 M KOH with a rapid addition of 3 M methanol at around 300 s at a rotation speed of 1600 rpm. The scan rate is 10 mV/s.


C Electrocatalysts for Oxygen Reduction Reaction in PEM Fuel

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