Supporting Information for

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1 Supporting Information for 3D Nitrogen-Doped Graphene Aerogel-Supported Fe 3 O 4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction Zhong-Shuai Wu, Shubin Yang, Yi Sun, Khaled Parvez, Xinliang Feng,* and Klaus Müllen* Max-Planck-Institut für Polymerforschung, Ackermannweg 10, Mainz, Germany School of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, , Shanghai, P. R. China feng@mpip-mainz.mpg.de; muellen@mpip-mainz.mpg.de S1

2 Experimental Section Synthesis of Fe 3 O 4 /N-GAs, Fe 3 O 4 /N-GSs and Fe 3 O 4 /N-CB Graphene oxide (GO) was prepared from natural graphite flakes using a modified Hummers method. [1-3] Fe 3 O 4 /N-GAs were prepared by hydrothermal assembly of GO, iron acetate and polypyrrole (PPy), subsequently combining with freeze-drying and thermal treatment. In a typical experiment, a 6 ml GO (1.5 mg L -1 ) aqueous dispersion with the iron acetate (1~40 mg) was firstly sonicated for 5 mins to achieve the electrostatic adsorption of Fe(II) ions on GO, and then 100 mg PPy was slowly added to form a stable complex solution by sonication for 10 min. Subsequently, the stable suspension was sealed in a Telfon-lined autoclave and hydrothermally treated at 180 C for 12 h. After that, the as-prepared sample was freeze-dried overnight, followed by thermal treatment at 600 C for 3 h in N 2 gas with 400 sccm. For comparison, we also prepared Fe 3 O 4 nanoparticles supported on nitrogen doped graphene sheets (Fe 3 O 4 /N-GSs) and carbon black (Fe 3 O 4 /N-CB) via the same procedure. In the case of Fe 3 O 4 /N-GSs, a 6 ml GO (1.5 mg L -1 ) solution was first diluted to form a 50 ml GO aqueous dispersion, and then followed the same synthetic procedure. In the case of Fe 3 O 4 /N-CB, a 6 ml suspension of carbon black (1.5 mg L -1 ) was used and followed in the same procedure above. Synthesis of Fe/N-GAs, Fe/N-GSs and Fe/N-CB Fe/N-GAs were prepared by hydrothermal assembly of GO, FeCl 3 and cyanamide, and subsequently followed by freeze-drying and thermal treatment. In a typical experiment, a 13.3 ml GO (1.5 mg L -1 ) aqueous dispersion with 2 mg FeCl 3 was firstly mixed and sonicated for 1 h, and then 0.8 ml cyanamide was slowly added into S2

3 the above solution by sonication for 15 min. Subsequently, the stable suspension was sealed in a Telfon-lined autoclave and hydrothermally treated at 100 C for 16 h. After that, the sample was freeze-dried overnight, and heated at 550 C for 4 h, and then 900 C for 1 h, with a heating rate of 2.5 C/min, in N 2 gas with 400 sccm. For comparison, we also prepared Fe nanoparticles supported on nitrogen doped graphene sheets (Fe/N-GSs) and carbon black (Fe/N-CB) via the same procedure. In the case of Fe/N-GSs, a 13.3 ml ml GO (1.5 mg L -1 ) solution was first diluted to form a 80 ml GO aqueous dispersion, and then followed in the same synthetic procedure. In the case of Fe/N-CB, a 13.3 ml suspension of carbon black (1.5 mg L -1 ) was used and followed in the same synthetic procedure. The morphology and structure of the samples were investigated by XRD, SEM (Gemini 1530 LEO), TEM, HRTEM and STEM (Philips Tecnai F20), AFM (Veeco Dimension 3100), XPS (VG ESCA 2000) and thermogravimetric analysis measurements. Nitrogen adsorption and desorption isotherms were measured at 77 K with a Micromeritcs Tristar 3000 analyzer (USA). For electrode preparation, 1 mg Fe 3 O 4 /N-GAs (Fe 3 O 4 /N-GSs, Fe 3 O 4 /N-CB, Fe/N-GAs, Fe/N-GSs and Fe/N-CB) was dispersed in 1 ml solvent mixture of Nafion (5%) and water (V: V ratio = 1:9) for 0.5~1 h under sonication. And then 10 µl portion of Fe 3 O 4 /N-GAs (Fe 3 O 4 /N-GSs, Fe 3 O 4 /N-CB, Fe/N-GAs, Fe/N-GSs and Fe/N-CB) suspension was slightly dropped on the disk surface of the pre-polished glassy carbon electrode. For comparison, a commercially available catalyst of 30 wt% Pt/C powder was used and 1 mg/ml Pt/C suspension was prepared as the same procedure above. The electrodes were then dried overnight at room temperature before measurement. Electrochemical measurements of cyclic voltammetry, rotating disk electrode (RDE) and rotating ring-disk electrode (RRDE) were performed by a computer-controlled S3

4 potentiostat (CHI 760C, CH Instrument, USA) with a three-electrode cell system. A glass carbon RDE (Autolab) after loading the electrocatalyst was used as the working electrode, an Ag/AgCl (KCl, 3 M) electrode as the reference electrode, and a Pt wire as the counter electrode. The electrochemical experiments were conducted in O 2 saturated 0.1 M KOH (0.5 M H 2 SO 4 ) electrolyte for the oxygen reduction reaction. The potential range is cyclically scanned between -1.2 and +0.2 V (between -0.2 and 0.8 V in 0.5 M H 2 SO 4 ) at a scan rate of 100 mv s -1 at the room temperature after purging O 2 or Ar gas for 15 min. RDE measurements were conducted at different rotating speeds from 225 to 2500 rpm, using an Autolab Model, and RRDE measurements were carried out at 1600 rpm with an Autolab Model. The peroxide percentage (% HO - 2 ) was calculated based on the following equation: Ir / N % HO 2 = 200 I + I / N The electron transfer number (n) was determined from RRDE measurements on the basis of the disk current (I d ) and ring current (I r ) via the following equation: Id n = 4 I + I / N where N is current collection efficiency of the Pt ring. d r d r S4

5 Figure S1. (a) Typical AFM image and (b) the height profile of GO, with a thickness of ~1 nm, coated on silicon wafer. Figure S2. (a) Nitrogen adsorption and desorption isotherm and (b) BJH pore distribution of the as-prepared Fe 3 O 4 /N-GAs. S5

6 Figue S3. Thermogravimetric analysis curve of Fe 3 O 4 /N-GAs measured from 25 to 800 ºC in air atmosphere with a heating rate of 10 ºC/min. It was pointed out that, at between 200 and 380 ºC, the Fe 3 O 4 phase was oxidized by air to transform into Fe 2 O 3 phase. Based on the mass of Fe 2 O 3 (47.7wt%) left after 800 ºC, it can be calculated that the Fe 3 O 4 content in hybrid is 46.2 wt%. Figure S4. XPS spectrum of Fe 3 O 4 /N-GAs recorded in the range of ev. S6

7 Figure S5. SEM images of the as-prepared (a,b) Fe 3 O 4 /N-GSs and (c,d) Fe 3 O 4 /N-CB. Both of them reveal the serious agglomeration of Fe 3 O 4 particles on the graphene sheet or carbon black. S7

8 Figure S6. (a,b) Cyclic voltammnetry curves of (a) Fe 3 O 4 /N-GSs and (b) Fe 3 O 4 /N-CB in nitrogen- and oxygen-staturated 0.1 M KOH aqueous electrolyte solution. The scan rate is 100 mv s -1. (c,d) Linear sweep voltammetry curves of (c) Fe 3 O 4 /N-GSs and (d) Fe 3 O 4 /N-CB in an oxygen-saturated 0.1 M KOH at a scan rate of 10 mv s -1 and different rotation rates, recorded by RDE voltammograms. S8

9 Figure S7. (a) Nitrogen adsorption and desorption isotherm and (b) BJH pore distribution of the as-prepared Fe 3 O 4 /N-GSs. The Brunauer-Emmett-Teller (BET) analysis reveals that a lower surface area of ~9.5 m 2 g -1 was obtained for Fe 3 O 4 /N-GSs, and the size of pore distribution is mainly mesoporous, ranging from 3.5 to 8.0 nm; additionally, no macroporous features were observed. S9

10 Figure S8. (a) Nitrogen adsorption and desorption isotherm and (b) BJH pore distribution of the as-prepared Fe 3 O 4 /N-CB. The Brunauer-Emmett-Teller (BET) analysis reveals that a surface area of ~44.5 m 2 g -1 was obtained for Fe 3 O 4 /N-CB, and the size of pore distribution is mainly mesoporous, varying from 3.5 to 30 nm. No macroporous features were observed. S10

11 Figure S9. Current-time chronoamperometric response of Fe 3 O 4 /N-GAs and Pt/C catalysts at -0.4 V in O 2 saturated aqueous solution of 0.1 M KOH at a rotation rate of 1600 rpm. After s, the commercial Pt/C catalyst suffered from a 39% decrease in current density while the Fe 3 O 4 /N-GAs hybrid catalyst showed the 22% loss of current density, suggesting a better durability of the Fe 3 O 4 /N-GAs catalyst. S11

12 Structure, Morphology and ORR Performance of Fe/N-GAs, Fe/N-GSs and Fe/N-CB in Acidic Solution The structure and morphology of Fe/N GAs catalysts were investigated by XRD, SEM, TEM, EDX techniques (Figure S10-S12). XRD patterns reveal the presence of the characteristic diffraction peak of Fe(110) in Fe/N-GAs as well as Fe/N-GSs and Fe/N-CB (Figure S10). SEM images of the Fe/N-GAs display the 3D macroporous structure and uniform distribution of Fe nanoparticles in the graphene aerogel, and TEM and HRTEM images of Fe/N GAs reveal the crystalline Fe nanoparticles with a size of 5-10 nm anchored on the graphene layers (Figure S11). The presence of C, N, and Fe elements in the Fe/N-GAs was confirmed by the EDX spectrum (Figure S11f). Differently, SEM images of the as-prepared Fe/N-GSs and Fe/N-CB demonstrate the serious agglomeration of Fe particles loading on graphene sheet or carbon black, without the continuous macroporous framework of Fe/N-GAs (Figure S12). We further performed the rotating ring disk electrode (RRDE) technique to investigate the ORR performance of the as-prepared Fe/N GAs, Fe/N-GSs and Fe/N-CB in oxygen saturated 0.5 M H 2 SO 4 electrolyte (Figure S13-S14, Table S1). We found that Fe/N-GAs exhibited much better electrochemical activity with larger disk current ( ma at -0.2 V), more positive onset potential (0.46 V vs. Ag/AgCl) and higher electron transfer number (3.92 at -0.2 V) than the Fe/N-GSs and Fe/N-CB due to the 3D macroporous structure of Fe/N-GAs (Figure S13, Table S1). Furthermore, we estimated the durability of Fe/N GAs with respect to commercial Pt/C (30%) in O 2 saturated aqueous solution of 0.5 M H 2 SO 4 at 100 mv s -1 for 5000 cycles (Figure S14). As shown in Figure S14, the deterioration of the commercial Pt/C catalyst results in a 30.2% drop in current density at 0.4 V while the Fe/N-GAs hybrid catalyst showed only a slight loss of current density, with durability S12

13 of as high as ~98.7%, suggesting an excellent stability of the Fe/N-GAs catalyst over Pt/C in acidic medium. Figure S10. X-ray diffraction (XRD) patterns of Fe/N-GAs, Fe/N-GSs and Fe-N-CB, revealing the presence of the characteristic diffraction peak of Fe(110) at about 43. The broad diffraction peak between 20 and 30 in three samples is (002) peak of graphitic carbon derived from the disorderedly-stacked graphenes or carbon black. The XRD pattern of Al (200) is derived from Al substrate that is used to support the measured samples. S13

14 Figure S11. (a-c) Typical SEM images of Fe/N-GAs displaying the 3D macroporous structure with some Fe nanoparticles on the graphene surface (d) Representative TEM and (e) HRTEM images of Fe/N GAs reveal a crystalline structure of Fe nanoparticles with a size of 5-10 nm anchored on graphene sheets. (f) The EDX spectrum of Fe/N-GAs suggests the presence of C, N, and Fe elements. The residual oxygen is from the reduced graphene oxide. The peak of Cu is from the copper TEM grid. S14

15 Figure S12. SEM images of the as-prepared (a, b) Fe/N-GSs and (c, d) Fe/N-CB. Both of them reveal the serious agglomeration of Fe particles on the graphene sheet or carbon black. S15

16 Figure S13. (a) RRDE curves of ORR on Fe/N GAs, Fe/N GSs, Fe/N CB in oxygen saturated 0.5 M H 2 SO 4 electrolyte at a rotation rate of 1600 rpm. (b) The electron transfer number (n) of Fe/N GAs, Fe/N GSs, and Fe/N CB as a function of the electrode potential, calculated from their RRDE curves. These results reveal the much better electrochemical activity of Fe/N-GAs than that of Fe/N GSs and Fe/N CB, showing a larger disk current ( ma at -0.2 V), more positive onset potential (0.456 V), and higher electron transfer number for Fe/N-GAs (also see Table S1). Table S1. The comparison of the electrochemical catalytic properties of Fe nanoparticles loaded on three different nitrogen-doped carbon supports Samples Electron transfer number at -0.2 V a Disk current (ma) at -0.2 V Onset potential (V) b Fe/N-GAs Fe/N-GSs Fe/N-CB < <0.3 a: Electron transfer number calculated at the electrode potential of -0.2 V; b: E /V vs. Ag/AgCl S16

17 Figure S14. The comparison of the durability of (a) Fe/N-GAs and (b) Pt/C (30%) catalysts in O 2 saturated aqueous solution of 0.5 M H 2 SO 4 at 100 mv s -1 for 5000 cycles. As indicated in Figure S14b, the deterioration of the commercial Pt/C catalyst results in a 30.2% decrease in current density at 0.4 V while the Fe/N-GAs hybrid catalyst (Figure S14a) shows only a slight loss of current density, suggesting a superior durability of the Fe/N-GAs catalyst in acidic medium. References [1] Y. Liang, D. Wu, X. Feng, K. Müllen, Adv. Mater. 2009, 21, [2] S. B. Yang, X. L. Feng, K. Müllen, Adv. Mater. 2011, 23, [3] S. B. Yang, X. L. Feng, L. Wang, K. Tang, J. Maier, K. Müllen, Angew. Chem. Int. Ed. 2010, 49, S17