Supporting information

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1 Supporting information Cu 2 O-Cu Hybrid Foams as High-Performance Electrocatalysts for Oxygen Evolution Reaction in Alkaline Media Han Xu, Jin-Xian Feng, Ye-Xiang Tong, and Gao-Ren Li* MOE Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chemistry & Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou , China ligaoren@mail.sysu.edu.cn Experimental Section Synthesis of Cu 2 O-Cu foams, Cu foams, Cu 2 O foams and CuO foams: Prior to electrodeposition, Cu wire (the diameter of 0.5 mm) were polished by SiC abrasive paper with 800 grits, then rinsed with ethanol and distilled water thoroughly to remove the oxide on the surface. The electrodeposition was performed in a simple two-electrode cell by galvanostatic method, and the platinum electrode was used as the counter electrode, the Cu wire was directly used as the working electrode with a length of 1.5 cm exposed to the electrolyte. The porous Cu 2 O-Cu foams were fabricated in solution of 0.75 M H 2 SO M CuSO 4 at current density of 3.6 A cm -2 at 25 o C for 120 s. The fabricated Cu 2 O-Cu foams are calcinated in air atmosphere at 300 o C for 3 h to prepare CuO foams. SEM image and XRD pattern of the prepared CuO foams are shown in Figure S7a and S7b, respectively. In addition, single Cu foams and Cu 2 O foams were also fabricated for comparative study. The single Cu foams were fabricated via the reduction of porous Cu 2 O-Cu foams by immersing in solution of 1.5 M hydrazine hydrate at 70 o C for 10 h with stirring. The single Cu 2 O foams were obtained in solution of 0.5 M H 2 SO M CuSO 4 5H 2 O

2 at current density of 0.53 A cm -2 at 25 o C for 5 min. The SEM images and XRD patterns of synthesized single Cu foams and Cu 2 O foams are shown in Figure S7c-f, respectively. Structural Characterizations: The surface morphologies of the synthesized Cu 2 O-Cu foams, Cu foams and Cu 2 O foams were examined by Thermal Field Emission Environmental Scanning Electron Microscopy (SEM, FEI, Quanta 400F) and Transmission Electron Microscope (TEM, FEI, Tecnai G2 F30). X-ray diffractometer (D-MAX 2200 VPC) was employed to characterize the structures of samples. X-ray photoelectron spectroscopy (XPS, ESCALAB 250) was used to analyze the chemical state of Cu in the Cu 2 O-Cu foams (the XPS spectra were corrected using the C 1s line at ev). Relative intensity ratio (RIR) method was used to estimate the mass fraction of Cu and Cu 2 O in the synthesized electrocatalysts that were analyzed by X-ray powder diffraction (XRD). [1] Based on the RIR values which can be obtained from the PDF card database, the mass fraction of Cu and Cu 2 O in Cu 2 O-Cu foams can be calculated using following equation: [2] W Cu ICu (1) ICu (ICu 2O/(RIR Cu 2O/RIR Cu)) Where W Cu and W Cu2O are the mass fractions of Cu and Cu 2 O, respectively, and W 1 W Cu 2 O Cu ; I Cu and I Cu2O are the integrated intensities of the strongest peaks of Cu and Cu 2 O, respectively. According to XRD spectrum, the values of I Cu and I Cu2O can be determined from Cu (111) and Cu 2 O (111) planes, respectively. In addition, the values of RIR Cu (8.85, JCPDS No ) and RIR Cu2O (8.63, JCPDS No ) can be obtained from the matching PDF card database. Electrochemical Characterizations: All electrochemical measurements were carried out by a CHI 660D Electrochemical Workstation (CH instruments, Inc.) in a standard three-electrode cell at room temperature (25 o C), where graphite electrode (spectral grade, 1.8 cm 2 ) and saturated calomel electrode (SCE) were utilized as the counter electrode and the reference electrode, respectively. The synthesized

3 self-supported electrocatalysts were directly used as the working electrode (a length of 1.5 cm immersed in the electrolyte). The total mass loading of Cu and Cu 2 O in the porous Cu 2 O-Cu foams is about 5.5 mg, which was obtained by the Precision Electronic Analytical Balance. The ir-corrected was applied in the liner sweep voltammetry and chronopotentiometry experiments. All the potentials were recorded with respect to the reversible hydrogen electrode (RHE) by converting the potentials measured vs. SCE according to the following equation: E (RHE) = E (SCE) ph (2) The double-layer capacitance was measured to determine the electrochemically-active surface area (ECSA) of Cu 2 O-Cu foams by cyclic voltammetry (CV) in 1.0 M KOH. The cyclic voltammograms were obtained in a non-faradaic region (1.315~1.415 V vs RHE) at the following scan rates: 2, 5, 10, 20, 40, 60 and 80 mv s -1. The double-layer charging current is equal to the average of the absolute value of the cathodic and anodic charging currents measured at V vs RHE. The double-layer capacitance of electrocatalyst is taken as the value of slope of the linear fits, [3] and it is proportional to the ECSA of electrocatalyst. References (1) Dean, C. C.; Dugwell, D.; Fennell, P. S. Energ Environ. Sci. 2011, 4, (2) (a) Al-Jaroudi, S. S.; Ul-Hamid, A.; Mohammed, A.-R. I.; Saner, S. Powder Technol. 2007, 175, (b) Cao, J.; Luo, B. D.; Lin, H. L.; Xu, B. Y.; Chen, S. F. Appl. Catal. B-Environ. 2012, 111, (3) McCrory, C. C.; Jung, S.; Peters, J. C.; Jaramillo, T. F.; J. Am. Chem. Soc. 2013, 135,

4 Figure S1. Optical photographs of the vigorous gas evolution on the electrode surface (please see the place marked with red circle). Figure S2. SEM images of Cu 2 O-Cu foams with electrodeposition time increasing (a) 5 s; (b) 15 s; (c) 30 s.

5 100 µm Figure S3. The thickness of Cu 2 O-Cu foams with deposition time of 120 s. Cu 2p 3/2 Cu 2p 1/ ev Figure S4. XPS spectra of (a) Cu 2p and (b) Cu LMM of Cu 2 O-Cu electrode.

6 Figure S5. SEM images and XRD patterns of Cu 2 O-Cu foams with different deposition time: (a, b) 60 s; (c, d) 90 s and (e, f) 150 s.

7 Figure S6. The magnified XRD patterns of Cu 2 O foams and Cu 2 O-Cu foams at 2 theta of 30º-40º.

8 Figure S7. SEM images and XRD patterns of (a, b) CuO foams; (b, d) Cu foams; (e, f) Cu 2 O foams.

9 Figure S8. (a) LSV curves and (b) Tafel plots of Cu 2 O-Cu foams with different electrodeposition time in the deaerated 1.0 KOH solution at scan rate of 5 mv s -1.

10 Figure S9. (a) SEM image and (b) XRD pattern of Cu 2 O-Cu foams after OER 50 h; (c) LSV curves and (d) Tafel plots of Cu 2 O-Cu foams and CuO foams in deaerated 1.0 M KOH solution at 5 mv s -1.

11 Current (A) (a) (b) Figure S10. XPS spectra of (a) Cu 2p and (b) Cu LMM of Cu 2 O-Cu electrode after water oxidation (the inset in (b) corresponds to the magnified portion of the area marked in black box) nd to 20th cycles 1st cycle Potential (V vs SCE) Figure S11. CVs of the Cu 2 O-Cu foams from first to 20 cycles in solution of 1.0 M KOH at 50 mv s -1.

12 Figure S12. (a, c) CVs of single Cu 2 O foams and Cu foams at the different scan rates from 2~80 mv s -1 in the potential rang of V vs RHE, respectively; (b, d) Capacitive current at V vs RHE as a function of scan rate for single Cu 2 O foams and Cu foams, respectively.

13 Z"/ohm 16 Cu 2 O-Cu foams Cu 2 O foams Cu foams Z'/ohm Figure S13. EIS Nyquist plots of Cu 2 O-Cu foams, Cu 2 O foams and Cu foams at high-frequency. Figure S14. Equivalent circuit used to model the AC impedance of the working electrode/electrolyte. R s represents the uncompensated series resistance; C dl represents the accompanying capacitance of R c ; CPE represents a constant-phase element. The other elements are defined in the text.

14 Table S1. Composition quantitative analysis of Cu 2 O-Cu foams with different electrodeposition time by the RIR method. Electrodeposition Time (s) Cu 2 O (wt %) Cu (wt %)

15 Table S2. Comparisons of electrocatalytic activity of some reported nonprecious OER electrocatalysts in alkaline media. Tafel slop η Catalysts Electrolyte 10 (mv (mv dec -1 Reference ) vs RHE) Cu 2 O-Cu foams 1.0 M KOH This work Cu nanoparticles 0.5 M KOH 483 N.A. J. Mater. Chem. A, 2013, 1, 4728 NiCuO x 1.0 M NaOH >400 N.A. J. Am. Chem. Soc. 2013, 135, Cu-N-C/graphene 0.1 M KOH >770 N.A. Nat. Commun. 2014, 5, 5285 Cu 0.3 Ir 0.7 O δ 0.1 M KOH Chem. Sci. 2015, 6, 4993 CuFe 2 O 4 nanofibers 0.1 M KOH > Nanoscale 2015, 7,8920 N-doped CG-CoO 1.0 M KOH Energy Environ. Sci. 2014, 7, 609 Mn oxide 0.1 M KOH 540 N.A. J. Am. Chem. Soc. 2010, 132, NiCo layered double hydroxide 1.0 M KOH Nano Lett. 2015, 15, 1421 overlapped g-c 3 N 4 and Ti 3 C 2 nanosheets 0.1 M KOH Angew. Chem. Int. Ed. 2016, 55, D Ni@[Ni (2+/3+) Co 2 (OH) 6 7 ] x 0.1 M KOH Adv. Funct. Mater. 2014, 24, 4698 nanotube Co@Co 3 O 4 /NC 0.1 M KOH Angew. Chem. Int. Ed. 2016, 55, 1 N and S codoped graphite foam 1.0 M KOH Adv. Energy Mater. 2016, 6, NanoCOT 0.1 M KOH >540 ~129 J. Am. Chem. Soc. 2015, 137, 11996

16 Table S3. The summary of the values of η 0, η 10 and Tafel plot of Cu 2 O-Cu hybrid foams with different electrodeposition time. Time (s) η 0 (mv vs RHE) η 10 (mv vs RHE) Tafel slop (mv dec -1 )