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Noble metal free oxygen reduction reaction catalysts derived from Prussian blue nanocrystals dispersed in polyaniline Xiaojuan Wang, Leran Zou, He Fu, Yifu Xiong, Zixu Tao, Jie Zheng, and Xingguo Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12102 • Publication Date (Web): 03 Mar 2016 Downloaded from http://pubs.acs.org on March 4, 2016

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ACS Applied Materials & Interfaces

Noble Metal Free Oxygen Reduction Reaction Catalysts

Derived

from

Prussian

Blue

Nanocrystals Dispersed in Polyaniline Xiaojuan Wang,a Leran Zou, a He Fu, a Yifu Xiong,b Zixu Tao, a Jie Zheng,*a and Xingguo Li*a a

Beijing National Laboratory for Molecular Sciences (BNLMS), (The State Key Laboratory of

Rare Earth Materials Chemistry and Applications), College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. b

Sichuan Institute of Materials and Technology, China Academy of Engineering Physics,

Mianyang 621900, China Keywords: Polyaniline, Prussian blue, oxygen reduction, electrocatalysts, porous carbon

Abstract：

A highly efficient noble metal free catalyst for the oxygen reduction reaction (ORR) is derived from a composite of polyaniline (PANI) and Prussian blue analogue (PBA, Co3[Fe(CN)6]2) by pyrolysis. The composite consists of 2-5 nm PBA nanocrystals homogeneously dispersed in PANI. During the pyrolysis, the PBA nanocrystals serve as both the template for the pore formation and the precursor for the ORR active sites, which results in a nanoporous structure strongly coupled with the ORR active sites. The catalyst exhibits superior ORR performance in

ACS Paragon Plus Environment

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both alkaline and acidic electrolyte, comparable to that of the commercial Pt/C with 20 wt% Pt loading. 1. Introduction Reducing the cost of polymer electrolyte membrane fuel cells (PEMFC) and metal air batteries is of critical importance for their large scale applications. The Pt based catalysts on the electrodes, particularly on the cathode to catalyze the oxygen reduction reaction (ORR), are major cost of the fuel cell stacks1-3 and metal air batteries4, 5. The ORR is a four electron process with sluggish kinetics and thus requires substantially higher loading of Pt to match the anode reaction.2 Therefore, developing cheap yet efficient non-precious metal (NPM) catalysts is ultimately important but extremely challenging, which has inspired considerably research interest in recent years.6-8 Among various NPM ORR catalysts, the M-N-C (M=Fe, Co) system is of particularly interest, because it may exhibit comparable performance to that of Pt based catalysts by properly designed structure and well controlled processing. The speculated active sites in M-N-C ORR catalysts are the N-coordinated transition metal structures MNx, which has been verified by structural analysis such as X-ray absorption9-11 and Mössbauer spectroscopy1, 12, 13 as well as theoretical approaches14. The M-N-C catalysts are typically prepared by pyrolysis of precursors containing metal and carbon (and possibly also nitrogen) in inert or NH3 atmosphere. The structure of precursors is very critical for the ORR performance of the pyrolysis product. Composite precursors with various structure are extensively investigated during the last decade, which can be roughly classified into three categories: 1) a carbon support doped with metal salts (pyrolyzed in NH3 atmosphere)15,

16

, 2) metal salts and N-containing organic or polymeric

molecules (with or without a carbon support)17,

18

and 3) metal complexes or metal-organic


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frameworks with N-chelated polydentate ligands (with or without a carbon support)

9, 10, 19-21

.

Precursors containing the M-N bonds are more favorable, due to the higher probability to yield the ORR active MNx structure after pyrolysis. Currently, rationally designing the precursor continues to be the research focus for the M-N-C ORR catalysts.22-24 Prussian blue (PB) and its analogues (PBAs) designate a category of compounds with ordered three dimensional framework structure composed of coordinative bonded transition metal cations and cyanide groups.25 The PBA structure inherently contains high density of nitrogen coordinated metal sites MNx, which is ideal to derive ORR catalysts. Somehow surprisingly, the application of PB and PBAs in ORR, either directly or as a pyrolysis precursor has been very rare. Fu et al.26 directly used PB in the cathode in microbial fuel cells. Sanetuntikul et al.27 and Xu et al.28 obtained ORR catalysts by combining Prussian blue with carbon materials, while with only little improvement in performance. Deng et al. obtained a novel metal encapsulated, Ndoped carbon nanotube structure by direct pyrolysis of Co3[Fe(CN)6]2, though the term Prussian blue was not explicitly mentioned in their work.29 The ORR performance of Prussian blue derived catalyst in alkaline electrolyte was further enhanced by Zou et al.30 and Xi et al.31 via optimizing the heat treatment process. However, as to the ORR performance, there still exist large gaps between Prussian blue derived catalysts and the commercial catalysts. Therefore, proper modification is required in order to derive high performance ORR catalysts based on PBA. Hou et al.32 reported a novel nitrogen-doped core shell-structured porous Fe/Fe3C@C nanoboxes supported on RGO sheets by pyrolyzing graphene oxide (GO) and PB nanocubes composite, which exhibited comparable ORR performance to the commercial Pt/C in alkaline electrolyte. However, the selectivity was poor (electron transferred number is only 3.08) and the sample was unstable in acid. There is still large margin to improve the performance,


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especially in acidic electrolyte. In this paper, we demonstrate that a composite composed of PBA and PANI leads to a highly catalytic M-N-C ORR catalyst after pyrolysis and acid leaching. PANI is extensively studied as a precursor to derive ORR catalysts.18, 33, 34 It gives nitrogen doped carbon material with good electron conductivity after pyrolysis. Another advantage of PANI is that the polymerization process can be carried out in aqueous solution. Since PBA tends to form quite stable colloidal suspension, this will greatly facilitate the processibility with PBA, allowing more homogeneous mixing of the two components. The obtained composite consists of 2-5 nm PBA nanocrystals homogeneously dispersed in PANI. During the pyrolysis, the PBA nanocrystals serve as both the template for the pore formation and the precursor for the MNx structure. The obtained catalyst exhibits superior ORR activity in both alkaline and acidic electrolyte. 2. Experimental section 2.1 Starting materials Cobalt nitrate (Co(NO3)2•6H2O, >99%), hydrochloric acid (HCl, 36.0-38.0 %), sulphuric acid (H2SO4, 95.0-98.0) and ammonium peroxydisulfate ((NH4)2S2O8, >99%) were purchased from XILONG Chemical. Potassium ferricyanate (K3Fe(CN)6, >99.5%) was purchased from Sinopharm Chemical Reagent Co., Ltd. Pt/C (nominally 20 % on carbon black) was purchased from Johnson Matthey. Nafion (5 wt% solution in ethanol and water) was purchased from Alfa Aesar. All the reagents are used without any treatment except the Nafion. The Nafion is diluted to 0.5 wt% before the electrode preparation. 2.2 Structural characterization techniques


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High-resolution transmission electron microscopy (HRTEM, JEM 2100 and JEM 2100F, 200 kV) and X-ray diffraction (XRD, Rigaku D/max 2000 diffractometer, Cu Kα) were used to characterize the morphology and structure of samples. Thermogravimetric analysis (TGA) was performed using a Q600 SDT thermoanalyzer (Thermal Analysis Corporation, USA) with a heating rate of 10 °C /min in N2 atmosphere. Raman spectra were collected using a HORIBA Jobin Yvon LabRam ARAMIS Raman spectrometer with excitation laser wavelength of 532nm. An Autosorb IQ gas sorption analyzer (Quantachrome, USA) was used to analyze porosity of samples. The samples were degassed at 200 °C for at least 2 h to remove the solvent molecules before testing. Then nitrogen adsorption-desorption isotherms were obtained on the gas sorption analyzer at 77 K. The specific surface area and the pore size distribution were determined by Brunauer–Emmett–Teller (BET) and the Quenched Solid density functional theory (QSDFT) methods, respectively. The X-ray photoelectron spectroscopy (XPS) measurement was carried out on an AXIS-Ultra spectrometer (Kratos Analytical) using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV). The content of C, H and N was collected by an elemental analyzers of Vario EL. The metal content was obtained by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES), which was performed on a CCD-ICP-AES spectrometer PROFILE SPEC. The samples were first calcinated in air at 900 °C for 12 h. The obtained residue was dissolved in 2 mol L-1 HNO3 to give the solution for ICP-AES analysis. 2.3 Electrode Preparation To prepare the working electrode, the samples are ultrasonically dispersed in a mixed solution of isopropanol and Nafion (0.5 wt% in ethanol and water) to make the suspension with the concentration of 10 mg mL-1. Generally, the ultrasonic time is at least 1 h. Then we drop 7 µL of suspension onto a 5 mm glassy carbon rotating disk electrode (RDE, Pine Research


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Instrumentation) or a 5.61 mm (disk outer diameter) rotating ring-disk electrode (RRDE, Pine Research Instrumentation) surface and dried for 12 h at room temperature. The Pt/C catalyst loading on RDE and RRDE is 0.71 μg Pt cm-2 and 0.57 μg Pt cm-2 . The synthesized catalysts loading on RDE and RRDE is 0.36 mg cm-2 and 0.28 mg cm-2, respectively. 2.4 Electrochemistry measurements Cyclic voltammograms (CVs) and linear scan polarization curves (LSVs) measurements of the samples are carried out at room temperature in 0.1 M KOH or 0.5 M H2SO4. The performance of Pt/C in acidic electrolyte is measured in 0.1 M HClO4 to avoid possible poisoning of Pt by the sulfur species. Prior to each measurement, the electrolyte is bubbled by argon or oxygen for at least 30 min. The data is collected on a CHI 760D bipotentiostat. The reference electrode is a saturated Ag/AgCl electrode in 0.1 M KOH solution and a saturated calomel electrode (SCE) in 0.5 M H2SO4 or 0.1 M HClO4 solution. All potentials reported in this work have been converted to the RHE scale. The counter electrode is a platinum foil. The LSVs results are subtracted by background current recorded in Ar-saturated electrolyte. Based on RDE tests at different rotating rates, the electron transfer number (n) included in a typical ORR process is calculated from the slope of Koutecky-Levich plots by the following equation35, 36 : 1 1 1 = + j j k Bω 1 / 2

Equation 1

Where jk is the kinetic current, ω is the angular velocity of the disk (ω = 2πN, N is the electrode rotating speed), and B is the Levich slope, could be determined from the slope of Koutecky-Levich plots (K-L plots) based on Levich equation as follows: Equation 2

B = 0.62nFADO2 2/ 3ν −1 / 6cO 2


6

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Where F is the Faraday constant (F = 96485 C mol-1). n is electron transfer number per oxygen molecule. DO2 is the diffusion coefficient of O2 in electrolyte (1.9×10-5 cm2 s-1 in 0.1 M KOH, 1.93×10-5 cm2 s-1 in 0.5 M H2SO4). CO2 is the bulk concentration of O2 (1.2×10-3 mol cm-3 in 0.1 M KOH, 1.26×10-3 mol cm-3 in 0.5 M H2SO4). υ is the kinetic viscosity (0.01 cm2 s-1 in 0.1 M KOH, 0.01009 cm2 s-1 in 0.5 M H2SO4). The constant 0.62 is adopted when the angular velocity of the disk is expressed in rad/s. The number of electron transferred and peroxide percentage during the ORR is also calculated based on measurements on a RRDE. The ring potential is set at 1.46 V vs. RHE in 0.1 M KOH and 1.20 V vs. RHE in 0.5 M H2SO4. The number of electron transferred n and peroxide yield H2O2% in calculated using the following equations9.

n=

4Id

Equation 3

I Id + r N

H 2O2 % =

200 I r NI d + I r

Equation 4

Where N=0.37 is the collection efficiency, Id and Ir is the disk current and the ring current, respectively. Collection efficiency of the ring electrode was calibrated by the K3Fe(CN)6 redox reaction. 2.5 Preparation of precursors and catalysts The PBA used in our study is Co3[Fe(CN)6]2. The PBA can be easily prepared by mixing the aqueous solution of Co(NO3)2•6H2O and K3Fe(CN)6 under vigorous stirring. To prepare the PANI/PBA composite, a mixture K3Fe(CN)6, aniline and HCl is cooled in an ice-water bath.


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Aqueous solution containing Co(NO3)2•6H2O and (NH4)2S2O8 is added dropwisely to the mixture under vigorous stirring. The suspension is kept below 10 °C for about 5 h and at room temperature for 24 h to complete the polymerization, resulting in a viscous dark green suspension (Figure S1a). The product is collected by centrifugation and washed by deionized water for at least three times. To prepare the ORR catalyst, the PANI/PBA composite is pyrolyzed at 1000 °C with a heating rate of 10 °C min-1 for 1 h in Ar atmosphere. The product is hydrothermally etched in concentrated hydrochloric acid to remove the unstable species and heated again to obtain a more robust structure. For clarity, the precursors and the derived catalysts are designated as xPANI/PBA and C-xPANI/PBA, respectively, where x indicates the molar ratio of

aniline/(aniline+PBA) in the starting material. Here PBA is represented by the molecular formula Co3[Fe(CN)6]2 in calculating x. That is to say, x=0, 0.50, 0.67, 0.80, 1 represent C-PBA, CPANI/PBA, C-2PANI/PBA, C-4PANI/PBA and C-PANI, respectively. More details of sample preparation, characterization and electrochemical measurements are provided in Supporting Information.

3. Results and discussion 3.1 Structural and morphology characterization of the precursors Figure 1 presents the transmission electron microscopy (TEM) images of the obtained PANI, PBA and the 2PANI/PBA sample. PANI obtained in this way is a crystalline dark green solids (Figure S1a) showing irregular shape (Figure 1a). The obtained PBA can form a dark red colloidal suspension (Figure S1a), which is composed of well crystallized nanocrystals around 30-50 nm, as shown by the X-ray diffraction (XRD) pattern (Figure S1b) and the TEM image


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(Figure 1b). For the composite (illustrated using the 2PANI/PBA sample), the diffraction peaks of both PANI and PBA can be resolved in the XRD pattern (Figure S1b), indicating that the strong acidic and oxidative environment in the polymerization process does not destroy the PBA structure.

Figure 1 TEM images of PANI (a), PBA (b) and the 2PANI/PBA composite (c-d). The PBA nanocrystals are highlighted by the circles in Figure 1(d). The inset in Figure 1(d) is a lattice image of a single PBA nanocrystal. TEM image suggests that the 2PANI/PBA composite is composed of nanoparticles about 2030 nm in size (Figure 1c). These particles are highly aggregated. A closer inspection shows that each nanoparticle contains even smaller nanocrystals about 2-5 nm in size (Figure 1d, S2). The lattice fringes exhibit cubic symmetry with interplanar space of 0.294 nm, which is corresponding to the {200} planes of the Fe-Co PBA (Figure 1d, inset). The composite structure is further characterized by high angle annular dark field-scanning TEM (HAADF-STEM, Figure S3). A nanoparticle about 50 nm in size contains dispersed bright spots about 2-5 nm. The bright


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spots are corresponding to the species containing the heavier elements, i.e. Fe and Co in this case. The above results suggest that the 2PANI/PBA composite is composed of 2-5 nm PB nanocrystals quite homogeneously distributed in PANI. The coating of PANI significantly reduces the size of PBA. This can be attributed to the strong interaction between the aniline monomer which is positively charged in the preparation condition and the Fe(CN)63- anion. The growth of PBA nanocrystals, therefore, will be confined with the presence of aniline or PANI. 3.2 Structural and morphology characterization of the catalysts The catalyst derived by pyrolysis followed by acid leaching (C-2PANI/PBA) is composed of FeCo alloy, cobalt nitride and graphite phases, as indicated by XRD (Figure S4). The acid leaching step eliminates the sulphide phase. The peaks corresponding to the alloy and Co nitride phases remain, while the intensity is significantly reduced, as inferred by their relative intensity compared to that of the graphite peak at 25o. A comprehensive elemental analysis suggests that the C-2PANI/PBA sample is composed of C (79.1 wt%), N (2.7 wt%), Fe (4.68 wt%) and Co (6.92 wt%). Despite of the appreciable residual metal, the metal content is significantly reduced compared to that before acid leaching (Fe 23.6 wt%, Co 33.4 wt%). Assuming that carbon and nitrogen are not affected, the mass of the catalyst is reduced by 51.4% by acid leaching. TEM image suggests that the C-PANI/PBA contains two distinct structure domains (Figure 2a). The dark particles around 100-200 nm are corresponding to the residual metal containing phases, while the region with light contrast is corresponding to the carbon matrix. The dark particles are enclosed by several graphene layers. This explains the stability of the metal species against acid leaching. Acid leaching removes some of these particles, probably those with less


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compact carbon shells, leaving the voids enclosed by the graphene layers. The residual persistent metal species are commonly observed in M-N-C catalysts derived from pyrolysis, most of which are in the form of particles well wrapped by graphene layers.17, 29 Herein we think these large particles are ORR inactive. As shown in Figure S5, acid leaching almost does not affect the ORR activity. The slight difference may come from the decrease of the large dark particles and part of active sites which is vulnerable to acid.

Figure 2 TEM images of the pyrolyzed product of 2PANI/PBA before (a) and after acid leaching (b-d). The structure of the carbon matrix is very similar for the samples with and without acid leaching, which exhibits a highly porous structure with nanoscale pores (Figure 2a and 2b). Graphitic structures can also be discerned from the walls of the pores, while the graphitic layers are much smaller and the stacking is also less ordered compared to those embracing the particles of the metal containing phases. The nanoporous nature of the samples is further confirmed by the nitrogen adsorption isotherms (Figure 3a). Both samples with and without acid leaching exhibit


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features corresponding a hierarchically porous structure. There are two major peaks of the pore width in the sub 2 nm and 3-40 nm region in the pore size distribution curves (Figure 3b). The C2PANI/PBA sample has high specific surface area of 702 m2g-1, compared to that of 296 m2g-1 before acid leaching.

Figure 3 Nitrogen adsorption-desorption isotherms (a) and pore size distributions (b) of the pyrolyzed product of 2PANI/PBA before and after acid leaching. It should be noted that the micropores are readily formed before acid leaching. The only notable difference after acid leaching is the reduced density of the dark solid particles and the voids enclosed by the graphene layers (Figure 2). This is in agreement with the pore size distribution analysis (Figure 3b). The volume of the micropores is 0.102 and 0.249 cm3g-1 for the sample before and after acid leaching. The increase (2.44 times) is in good agreement with the increase of the specific surface area (2.37 times). Considering that acid leaching causes mass loss of 51.4%, the additional pores created by acid leaching is not very significant. Therefore, we conclude that most of micropores are automatically formed during the pyrolysis step.


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Figure 4 Thermogravimetric analysis of PANI, PBA and 2PANI/PBA. To understand the formation mechanism of the micropores during pyrolysis, it is illuminating to note the comparable size of the pores and the PBA nanocrystals. Thus we assume that the PBA nanocrystals may assist the simultaneous pore formation during pyrolysis, which is confirmed by the thermogravimetric analysis (TGA) (Figure 4). PBA shows very high thermostability with a threshold decomposition temperature of 500 °C. On the other hand, PANI shows multistep weight loss above 200 °C. When PBA starts to decompose, the decomposition of PANI is largely complete. This distinct thermal stability allows PBA to serve as a template for the pyrolysis of PANI prior to its decomposition. During the pyrolysis process, PANI will first decompose to give a preliminarily carbonized shell coated on the PBA nanocrystals. The PBA nanocrystals then decompose at higher temperature (>500 °C) and generate the nanoscale pores. Meanwhile, the metal containing phases from the decomposition of PBA undergo aggregation to yield the larger particles. This simultaneous phase segregation process leaves the nanoscale voids in the original location of the PBA nanocrystals. Therefore, the micropores are formed from the template effect of the PBA nanocrystals and the simultaneous phase segregation. The TEM images of 2PANI/PBA and C-2PANI/PBA supports the mechanism very well. Before pyrolysis, the 2PANI/PBA exhibits a structure of 2-5 nm PBA nanocrystals quite homogeneously


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distributed in PANI. After pyrolysis, 2PANI/PBA is converted into a nanoporous carbon with hierarchical pores and metal-containing phases. Obviously, the decomposition of PBA and the aggregation of metal-containing phases resulted in the formation of micropores in the catalyst. The structure of the C-2PANI/PBA catalyst is further studied by X-ray photoelectron spectroscopy (XPS, Figure S6 and S7). The N 1s spectrum can be deconvoluted into three peaks correpsonding to pyrrolic (400.5 eV), pyridinic (398.8 eV) and graphitic N (402.0 eV), respectively. Such N bonding structure is in agreement with PANI-derived N-doped carbon materials reported previously33. In addition, the nitrile or MNx phase also contributes to the 398.8 eV peak34, 37, 38. Moreover, the acid leaching step shows little impact on the N 1s spectra. This inconsistency can be explained by the fact that XPS is only surface sensitive. The nitride particles embedded in multiple graphene layers only contribute very weak signal to the XPS spectra. For the same reason, XPS tends to significantly underestimate the metal content. As shown in Figure S6, even for the sample before acid leaching with high metal content up to 33.4 wt%, only weak Fe and Co peaks are detected. The acid leached sample almost shows no metal peaks, despite of the ~10 wt% metal remaining, indicating that most residue metal species are embedded in the carbon matrix. The Raman spectrum of the C-2PANI/PBA sample can be deconvoluted into four bands: the G band at 1590 cm-1, the D band at ~1340 cm-1, the D” band at 1470 cm-1 and the I band at 1180 cm-1 (Figure S8). The D band is related to the lattice defects within the graphene layers while the D” band is originated from the stacking disorder of the graphene layers39. These defects related bands are in agreement with the disordered graphitic layers of the nanoporous carbon matrix observed in TEM images (Figure 2d).


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3.3 Electrochemical performance of the catalysts Figure 5 suggests that the C-2PANI/PBA sample exhibits very good ORR activity in both alkaline and acidic electrolyte. Here we define the E1/2 as the potential at which the current density is half of that at the lowest potential in the linear scanning voltammetry (i.e. 0.4 V in alkaline electrolyte and acidic electrolyte) to indicate the ORR performance. In alkaline electrolyte, the catalyst shows a half wave potential of 0.85 V, which is comparable to that of the commercial Pt/C catalyst. In acidic electrolyte, the catalyst shows a slightly more negative half wave potential, compared to Pt/C. As far as we know, the C-2PANI/PBA is the most efficient ORR catalyst derived from PB so far. The catalytic performance is much higher than the catalysts based on PB and PBAs without pyrolysis27,

28

, and is also significantly improved

compared to that derived from directly pyrolysis of Co3[Fe(CN)6]2, as reported by Deng et al. 29 and Zou et al.40 Therefore, introducing PANI is of critical importance to improve the performance of the PB and PBAs derived ORR catalysts. By comparing the current on the disk and the ring, the number of electron transferred (n) of the ORR process can be deduced (Equation 3). As shown in Figure 5c-d, the corresponding n value is 3.75-3.98 and 3.70-3.84 in alkaline and acidic electrolyte, respectively, indicating that the ORR process on the M-N-C catalyst is a four electron dominant process, which is consistent with the results from the K-L plots (Figure S9).


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Figure 5 (a and b) The RRDE voltammetric response of C-2PANI/PBA in O2-saturated 0.1 M KOH (a) and 0.5 M H2SO4 (b), respectively, measured at the rotating rate of 1600 rpm with a scan rate of 10 mVs-1. A commercial Pt/C catalyst with 20 wt% Pt loading is used as the reference material. The acidic electrolyte is 0.1 M HClO4 for Pt/C to avoid possible poisoning of Pt by the sulfur species. (c and d) Summary of H2O2 yield and electron transfer number (n) in alkaline (c) and acidic (d) electrolytes. For all RRDE measurements, the catalysts loading is 0.28 mg cm-2. The C-2PANI/PBA catalyst also exhibits very good stability. As shown in Figure S10, after 5000 scanning cycles, the linear scanning voltammetry profile remains unchanged in both alkaline and acidic electrolytes. To investigate the tolerance to methanol, cyclic voltammetry is


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measured in electrolyte containing 3 M methanol. Figure S11 reveals that the oxygen reduction peak of Pt/C disappears after introducing methanol. However, the ORR peak of the C2PANI/PBA sample is unaffected, indicating the catalyst exhibits better tolerance to methanol compared to that of the Pt/C catalyst. Interestingly, the catalysts derived from either PBA or PANI through the same procedure are both rather inefficient for ORR (Figure 6a, 6b). On these two catalysts, ORR occurs at much more negative potential with much lower saturate current density. Clearly, the high ORR performance of the xPANI/PBA derived catalyst cannot be explained by the contribution from either component. To further illustrate this synergic effect of the two components, catalysts derived from xPANI/PBA composites with different aniline/PBA ratio in the starting materials are studied. The linear scanning polarization curves in alkaline and acidic electrolytes are shown in Figure 6a and Figure 6b, respectively. The highest ORR performance in both alkaline and acidic electrolytes is obtained for the C-2PANI/PBA sample, as indicated by the most positive E1/2 value.

Figure 6 (a and b) Linear scanning polarization curves of the C-xPANI/PBA samples with different corresponding molar ratio of PANI in the starting material (i.e. x/(1+x), where x is


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natural number, which is given by the numbers in the figure). Sample designation: x=0, 0.50, 0.67, 0.80, 1 represent C-PBA, C-PANI/PBA, C-2PANI/PBA, C-4PANI/PBA and C-PANI, respectively. As shown previously, the C-2PANI/PBA sample shows a hierarchically nanoporous structure (Figure 3). Indeed, the porosity is one of the major structural parameters accounting for the difference in the ORR performance of this work. As summarized in Figure 7, in both alkaline and acidic electrolytes, the E1/2 values exhibits strong correlation with the specific surface area (SBET, Figure 7a). A more detailed porosity analysis (Figure S12) reveals that all the catalysts derived from the xPANI/PBA composites exhibit hierarchical pore structure, while the C2PANI/PBA sample with the best ORR performance preserves the highest amount of micropores (Table S1). Such micropores are less abundant in the catalysts derived from non-optimal xPANI/PBA ratio. The ORR performance also exhibits a positive correlation with the pore

volume of the micropores (Figure 7b). This result is in agreement with previous studies which suggest that microporous structure provides favorable locations for ORR2, 41.

Figure 7 The correlation of the half-wave potential E1/2 with the specific surface area (SBET) (a) and the micropore volume (b) of the C-xPANI/PBA samples. The number in the figures is the


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corresponding PANI molar ratio in the starting materials (i.e. x/(1+x)), where x is natural number, which is given by the numbers in the figure). Sample designation: x=0, 0.50, 0.67, 0.80, 1 represent C-PBA, C-PANI/PBA, C-2PANI/PBA, C-4PANI/PBA and C-PANI, respectively. Formation of the micropores during pyrolysis requires both effective segregation of the metalcontaining species and the robustness of the carbon shells. Insufficient PANI will leads to thinner carbon shell after pyrolysis which is less robust, while excessive PANI will limit the transport of the metal containing species. This effect can be evidenced by the TEM images, as shown in Figure 8. The C-PBA sample shows a porous carbon structure with a few dark particles corresponding to the residue metal containing phases. Some bamboo-like tubular structure (Figure S13) can also be found, which is similar to those obtained by Deng et al.29 The CPANI/PBA sample shows similar features to that of C-PBA. It is also evident that the introduced PANI effectively enhances the BET surface area and the micropores volume. In these two cases, there is insufficient carbon to sustain a robust nanoporous structure. At high temperature, migration of the metal containing species results in collapse of the nanoporous structure. On the other hand, C-PANI shows a non-porous structure. In the PANI rich sample (C-4PANI/PBA), the particles of the metal containing phases are more homogeneously distributed, indicating hindered phase segregation due to the surplus carbon. Generation of micropores is also a little ineffective in this case. In the optimal 2PANI/PBA ratio, there is sufficient carbon to sustain a robust nanoporous structure while still leaves space allowing effective phase segregation. This is shown for the C-2PANI/PBA sample (Figure 2), which exhibits well-defined nanoporous structure together with the large particles corresponding to the metal containing phases. In the high magnification TEM image (Figure 2d), there is no visible particulate metal containing


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species inside nanoscale pores, while the metal species with strong interaction with the N-doped carbon may still exist.

Figure 8 TEM images of the C-xPANI/PBA samples (scale bar: 100 nm): (a) C-PBA; (b) CPANI/PBA; (c) C-4PANI/PBA; (d) C-PANI. The numbers in the figure are the corresponding molar ratio of PANI in the starting material, i.e. x/(1+x), where x is natural number, which is given by the numbers in the figure. Sample designation: x=0, 0.50, 0.67, 0.80, 1 represent CPBA, C-PANI/PBA, C-2PANI/PBA, C-4PANI/PBA and C-PANI, respectively. In M-N-C ORR catalysts, it has been recognized that the ORR activity is mainly originated from the nitrogen coordinated metal structure (MNx)41-43. The inherent M-N bonding in the PBA structure is highly favorable for the MNx formation after pyrolysis. Meanwhile, the PBA nanocrystals also serve as the templates for the formation of the micropores. This is one unique feature of the xPANI/PBA composite, which allows formation of the MNx sites simultaneously with the nanoscale pores, through the decomposition of the same precursor. As a result, there


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will be strong coupling between the MNx sites and the nanoscale pores. As both structural factors are recognized to be favorable for ORR, this will greatly facilitate the catalytic performance. In recent years, there is an increasing interest in using PANI as a precursor to derive noble metal free ORR catalysts.18,

33, 44, 45

As shown in this paper, direct pyrolysis of PANI gives

nonporous products with very poor ORR activity. Therefore, PANI is often used together with a porous carbon support17, 45 or a template, such as layered oxides33 or porous silica18, to give sufficient porosity after pyrolysis. As most template materials used do not contain transition metals, additional metal doping is required to obtain the M-N-C catalysts. PBA nanocrystals combine the template effect with the high density of M-N bonds as the precursor for the MNx structure. This unique bifunctionality results in a nanoporous structure strongly coupled with the MNx sites, which cannot be achieved by other current template materials.

4. Conclusions In conclusion, a high performance M-N-C ORR catalyst is derived by pyrolysis of a 2PANI/PBA composite with 2-5 nm PBA nanocrystals homogeneously dispersed in PANI. The embedded PBA nanocrystals serve as the template to assist the formation of a nanoporous structure. The inherent M-N bonding in PBA structure also facilitates the formation of the ORR active MNx structure. The unique role of the PBA nanocrystals results in a nanoporous structure strongly coupled with the MNx sites. The resultant catalyst exhibits superior ORR performance in both alkaline and acidic electrolytes. These results imply that PBA is a promising precursor to derive noble metal free ORR catalysts. ASSOCIATED CONTENT


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† Electronic Supplementary Information available: Additional TEM images of precursors, XPS spectra,

XRD

patterns,

Raman

spectra,

nitrogen

adsorption-desorption

isotherms,

electrochemical activity of the samples, additional TEM images of the catalysts. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author * Dr. Xingguo Li, [email protected]; Dr. Jie Zheng, [email protected]

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge MOST of China (No. 2012CBA01207) and NSFC (No. U1201241, 11375020, 51431001 and 21321001). The authors also acknowledge Guangxi Collaborative Innovation Center of Structure and Property of New Energy and Materials & Guangxi Key Laboratory of Information Materials, Guilin University of Electronic Technology. REFERENCES 1.

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