10 Chemicals All chemicals are purchased from Sigma-Aldrich and used without further treatment. Pt/C used is 20% Platinum on Vulcan XC-72, purchased from Fuel Cell Store. Syntehsis of GO 15 Graphite flake (3 g) was put into concentrated H 2 SO 4 (12 ml), K 2 S 2 O 8 (2.5 g), and P 2 O 5 (2.5 g). The mixture was kept at 80 °C for 4.5 h, then cooled to room temperature and diluted with 0.5 L of DI water. The mixture was filtered and washed with DI water. The product was dried in vacuum at 50 °C

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... ernight. The pretreated graphite was put into cold (0 °C) concentrated H 2 SO 4 (120 mL). Then, KMnO 4 (15 20 g) was added gradually under stirring and the temperature of the mixture was kept to be below 20 °C by ice cooling. Successively, the mixture was stirred at 35 °C for 2 h, and then diluted with DI water (250 mL) using ice bath cooling. The mixture was then stirred for 2 h, and then additional 0.7 L of DI water was added. 20 mL of 30% H 2 O 2 was added to the mixture. The mixture was washed with 1:10 HCl aqueous solution (1 L) and DI water. The resulting paste-like product was diluted to make a 25 graphite oxide dispersion. Exfoliation was carried out by sonicating graphite oxide dispersion under ambient condition for 30 min, followed with centrifuging at 3000 rpm for 30 min to eliminate unexfoliated graphite. For measurement of N 2 adsorption-desorption isotherms, the GO is thermally reduced under 200 °C in vacuum to form rGO. 30 Synthesis of CuO 0.25 ml of 0.1 M CuCl 2 was added to 16 ml ethanol and stirred for 5 min at 25°C. Then, 0.75 ml of 0.1 M NaOH was added to the suspension and stirred for another 1 hour. The suspension was then centrifuged, washed and dispersed in ethanol. Synthesis of CuO/GO. 35 0.25 ml of 0.1 M CuCl 2 was added to 8 ml of GO solution (1 mg ml -1 in water) and was stirred for 5 min. Then 0.75 ml of 0.1 M NaOH was added to the suspension and stirred for another 1 hour at 60°C. The suspension was then centrifuged, washed with water and dispersed in ethanol. For measurement of N 2 adsorption-desorption isotherms, the sample is degased under 200 °C in vacuum where GO is Electronic Supplementary Material (ESI) for Journal of Materials Chemistry A This journal is