SUPPLEMENTARY INFORMATION

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1 SUPPLEMENTARY INFORMATION An Oxygen Reduction Electrocatalyst Based on Carbon Nanotube- Nanographene Complexes Yanguang Li, Wu Zhou, Hailiang Wang, Liming Xie, Yongye Liang, Fei Wei, Juan-Carlos Idrobo, Stephen J. Pennycook and Hongjie Dai* Calibration of SCE and conversion to RHE The calibration of SCE reference electrode is performed in a standard three-electrode system with polished Pt wires as the working and counter electrodes, and the SCE as the reference electrode. Electrolytes are pre-purged and saturated with high purity H 2. Linear scanning voltammetry (LSV) is then run at a scan rate of 0.1 mv s 1, and the potential at which the current crossed zero is taken to be the thermodynamic potential (vs. SCE) for the hydrogen electrode reactions. For example, in 0.1 M HClO 4, the zero current point is at V, so E (RHE) = E (SCE) V; in 0.1 M KOH, the zero current point is at V, so E (RHE) = E (SCE) V. NATURE NANOTECHNOLOGY 1

2 SUPPLEMENTARY INFORMATION Figure S1. Structural characterization of oxidized NT-NG. (a-b) Aberration corrected TEM images of oxidized NT-NG. (c) Raman spectra of raw CNT (blue) and oxidized NT-NG (black). (d) AFM image of oxidized NT-NG showing length distribution of oxidized nanotubes. The sample is prepared by drop-casting oxidized NT-NG aqueous solution onto (3-aminopropyl)triethoxysilane (APTES) functionalized Si wafer. Unbound nanotubes are rinsed off with DI water. Then the Si wafer is calcined in air at 300oC for 30 min, followed by a vacuum annealing at 900oC for 30 min. 2 NATURE NANOTECHNOLOGY

3 SUPPLEMENTARY INFORMATION Table 1: ORR electrocatalytic performance of previous CNT-based ORR catalysts in acid. Sources Technique Loading Activity (vs RHE) (mg/cm 2 ) this study CV RDE E p : V E 1/2 : V Chem. Commun. 2008, 329. CV RDE unknown ~0.5 E p : ~0.47 V E 1/2 : ~0.31 V J. Power Sources 2011, 196, RDE 0.16 E 1/2 : ~0.23 V J. Phys. Chem. C 2009, 113, RDE unknown E 1/2 : ~0.53 V J. Am. Chem. Soc. 2010, 132, CV unknown E p : ~0.5 V Appl. Catal. B 2011, 103, 362. RDE unknown E 1/2 : ~0.6 V Nanoscale, 2010, 2, RDE unknown E 1/2 : ~0.23 V J. Am. Chem. Soc. 2010, 132, CV RDE unknown unknown E p : ~0.44 V E 1/2 : ~0.44 V Figure S2. Polarization curves of Pt/C (loading 16 µg Pt /cm 2 ) before and after the potential cycling in O 2 saturated 0.1 M HClO 4. The half-wave potential of Pt/C shifts >20 mv to the negative direction, more than the NT-NG catalyst evaluated under the same condition. NATURE NANOTECHNOLOGY 3

4 SUPPLEMENTARY INFORMATION Figure S3. The effect of oxidation conditions. (a-d) Aberration corrected TEM images of 1x CNT (oxidized with a KMnO 4 -to-cnt mass ratio of 1), 3x CNT (oxidized with a KMnO 4 -to- CNT mass ratio of 3), 8x CNT (oxidized with a KMnO 4 -to-cnt mass ratio of 8) and reflux CNT (oxidized by refluxing in 3:1 w/w H 2 SO 4 :HNO 3 at 120 o C for 30 min) respectively. (e) Corresponding Raman spectra and (f) polarization curves (catalyst loading mg/cm 2 ) in O 2 saturated 0.1 M HClO 4. The NT-NG catalyst (5x) possesses the optimal structure containing abundant nanographene sheets attached to intact inner-tubes. 4 NATURE NANOTECHNOLOGY

5 SUPPLEMENTARY INFORMATION Figure S4. An ORR catalyst based on multi-walled CNTs (denoted as MW-NG). (a-b) Aberration corrected TEM images of MW-NG. The material contains many exfoliated nanographene pieces attached to CNTs with intact inner tubes. (c) Raman spectrum of MW-NG. (d) The polarization curve of MW-NG compared to NT-NG (from mainly double-walled CNT) in O 2 saturated 0.1 M HClO 4. The MW-NG is prepared by mixing oxidized MWNTs (oxidized with a KMnO 4 -to-cnt mass ratio of 5) with 20 wt% of Fe (from FeAc) and then annealed in NH 3. Excess Fe species is dissolved by 0.5 M H 2 SO 4 at 80 o C before subjected to a second NH 3 annealing at 900 o C for 30 min to afford the final MW-NG catalyst. NATURE NANOTECHNOLOGY 5

6 SUPPLEMENTARY INFORMATION Figure S5. Structural characterization of the purified NT-NG material with most of the Fe impurities removed. (a) N 1s XPS spectra and (b) Raman spectra of NT-NG without intentional impurity removal (red curve) and purified NT-NG (black curve). (c) Aberration corrected TEM image of purified NT-NG. Purified NT-NG shows similar structure and composition as NT-NG except for much lower Fe content. Figure S6. Polarization curves of NT-NG (red) and purified NT-NG (black) in O 2 -saturated 0.1 M KOH with (solid lines) or without (dotted lines) 10 mm CN -. Both have a catalyst loading of mg/cm 2. 6 NATURE NANOTECHNOLOGY

7 SUPPLEMENTARY INFORMATION Figure S7. RRDE polarization curves and peroxide yields of N-doped graphene (blue) in O 2 - saturated (a) 0.1 M HClO 4 and (b) 0.1 M KOH compared to NT-NG (red). Metal-free N-doped graphene is much less ORR active than NT-NG containing ~1 wt% Fe Figure S8. The 2 nd dataset showing the distribution of Fe and N atoms in the NT-NG ORR catalyst. (a) Bright-field STEM and (b) corresponding ADF images of NT-NG. The area marked by the white square in (b) is further characterized by (c) ADF intensity mapping, (d) N EELS mapping, (e) Fe EELS mapping and (f) overlaid Fe and N EELS maps. The ADF and EELS maps are recorded simultaneously. NATURE NANOTECHNOLOGY 7

8 SUPPLEMENTARY INFORMATION Figure S9. The 3 rd dataset showing the distribution of Fe and N atoms in the NT-NG ORR catalyst. (a) ADF image of NT-NG. The area marked by the white square in (a) is further characterized by (b) ADF intensity mapping, (c) N EELS mapping, (d) Fe EELS mapping and (e) overlaid Fe and N EELS maps. The ADF and EELS maps are recorded simultaneously. 8 NATURE NANOTECHNOLOGY