Three-Dimensional Honeycomb-Like Cu 0.81 Co 2.19 O 4. Nanosheet Arrays Supported by Nickel Foam and. Their High Efficiency as Oxygen Evolution

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1 Supporting Information Three-Dimensional Honeycomb-Like Cu 0.81 Co 2.19 O 4 Nanosheet Arrays Supported by Nickel Foam and Their High Efficiency as Oxygen Evolution Electrode Woo-Sung Choi*,, Myeong Je Jang,, Yoo Sei Park,, Kyu Hwan Lee,, Joo Yul Lee,, Min-Ho Seo, and Sung Mook Choi*, Surface Technology Division, Korea Institute of Materials Science, Changwon, 51508, Republic of Korea Advanced Materials Engineering, Korea University of Science and Technology (UST), Daejeon, 34113, Republic of Korea Department of Materials Science and Engineering, Pusan National University, Busan, 46241, Republic of Korea Hydrogen and Fuel Cell Center, New and Renewable Energy Institute, Korea Institute of Energy Research (KIER), Buan-gun, 56332, Republic of Korea W.-S. C. and M. J. Jang contributed equally to this work. *To whom correspondence should be addressed. S-1

2 * * S-2

3 Experimental Section Preparation: The catalyst layer was electrodeposited onto a Ni foam substrate (Pore size: 580 μm, Alantum, Korea) in an aqueous solution of 10 mm Co(NO 3 ) 2 6 H 2 O, 2 mm Cu(NO 3 ) 2 5 H 2 O. The deposition was performed in a three-electrode cell consisting of a Ni foam substrate as the working electrode (cathode), a platinum wire as the counter electrode (anode), and a saturated calomel electrode (SCE) as the reference electrode. A constant potential of -1 V vs. SCE was applied for 5 min at 25 C without agitation and deaeration. The as-deposited sample was then heat-treated at 250 C in an air atmosphere for 3 h. Characterization: The morphologies and compositions of the samples were analyzed using a field-emission scanning electron microscope (FE-SEM, JSM-7001F, JEOL, Japan) and a field-emission gun transmission electron microscope (FEG-TEM, JEM 2100F, JEOL, Japan). The vibrational modes of the samples were analyzed using a micro-raman spectrophotometer (NRS-3300, JASCO, Japan). The surface S-3

4 compositions of the samples were analyzed using X-ray photoelectron spectroscopy (XPS, K-Alpha, VG Multilab 2000-Thermo Scientific, USA). Alkaline water electrolysis test: Electrochemical measurements were carried out using a potentiostat/galvanostat (ZIVE LAB, BP2C, Korea) in a standard three-electrode electrochemical cell at 298 K in a 1 M KOH solution. Pt mesh and Ag/AgCl (1 M KCl) electrode were used as the counter and the reference electrodes, respectively. Steadystate polarization curves were recorded at a scan rate of 5 mv s 1. Stability tests were performed at a constant current density of 25 ma cm 2 or 100 ma cm 2. The potentials of the working electrodes were calibrated as V RHE = V Ag/AgCl ph V, where V RHE is a potential vs. a reversible hydrogen potential, V Ag/AgCl is a potential vs. Ag/AgCl electrode, and ph is the ph value of the electrolyte. The potential values from all electrochemical measurements were taken with ir compensation using the solution resistance value. Anion exchange membrane water electrolysis cell test: AEMWE cell tests were carried out in a designed cell with active area of 4.9 cm -2. The anode electrode was used pressed CCO S-4

5 electrode with diameter of 31 mm and Pt/C (40 wt.%, Johnson Matthey Corp, 1 mg Pt cm -2 ) was used as cathode electrode with diameter of 25 mm. The cell consists of the end plate, diffusion layer (Ni foam), anode, cathode, and anion exchang membrane (AEM, Fumasep FAA-3-PE-30, FuMA-Tech). Steady-state polarization curves of AEMWE cell were recorded by sweeping the voltage from 1.2 V to 1.9 V at a scan rate of 10 mv s 1 with 0.1 M KOH as an electrolyte. Stability test was performed at a constant current density of 100 ma cm 2 for 100 h at a cell temperature of 30 C. S-5

6 Figure S1. SEM images of samples deposited in the baths of a) 10 mm Co(NO 3 ) 2 6 H 2 O and b) 2 mm Cu(NO 3 ) 2 5 H 2 O. S-6

7 Figure S2. Surface images of CCO. Inset shows the atomic composition ratio of copper to cobalt at each respective points. S-7

8 Figure S3. XRD patterns of NF, CCHO, and CCO, CCO after stability test in 1 M KOH. S-8

9 Figure S4. The first and the second cycle polarization curves of CCHO. S-9

10 Figure S5. Photograph of a) CCHO, b) CCO, and c) electrochemically oxidized (after first cycle LSV) CCO electrocatalysts on the Ni foam. S-10

11 Figure S6. a) Surface SEM images and b) HR-TEM image of CCO after 24 hr stability tests with current density of 25 ma cm -2 in 1 M KOH. The corresponding elemental mapping of c) electron image, d) Co, and e) Cu. The inset in (b) shows a corresponding SAED pattern. S-11

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13 Figure S7. The XPS survey spectra of a) CCHO and b) CCO. c) CCO after stability test. XPS spectra of d) Co 2p and e) Cu 2p of CCO after stability test with current density of 25 ma cm -2 in 1 M KOH. Figure S8. Photograph of AEMWE cell configuration. S-13

14 Current density / ma cm ml min ml min ml min -1 without circulation Cell voltage / V Figure S9. Catalytic activity changes of AEMWE cell with circulation speed of electrolyte solution. S-14

15 Figure S10. a) Chronopotentiometry curve of CCO with constant current densities of 25 ma cm -2. b) LSVs of CCO before/after 24hr stability tests. c) Surface SEM images of CCO after 24 hr stability tests with constant current density of 25 ma cm -2 in 1 M KOH. S-15

16 Table S1. OER activities of noble metal oxide and copper and/or cobalt oxide catalysts in alkaline solutions with a current density of 10 ma cm -2. Active Material Detailed Structure Electrolyte η / mv (at 10 ma cm -2 ) Tafel Slope / mv dec -1 References 3-D CCO Nanoplates 1M KOH This work IrO 2 Commercial powder 1M KOH IrO 2 Nanopowder 1M NaOH RuO 2 Nanopowder 1M NaOH RuO 2 RuO 2 nanowires on CN x 0.5M KOH Co 3 O 4 Ni foam-supported N- CNT@Co 3 O 4 1M KOH Co 3 O 4 Nanoflakes 1M KOH Co 3 O 4 Graphene-Co 3 O 4 nanocomposite 1M KOH CoO Graphene-CoO nanocomposite 1M KOH Cu 0.7 Co 0.3 O 4 Cu x Co 3-x O 4 nanoparticles 1M KOH up to CuO CuO nanoparticles/cuo/cu 2 O 1M NaOH S-16

17 CuO Cu oxide nanowire arrays 0.1 M NaOH CuO Co-Doped CuO Nanoarray: 1 M KOH 330* * η / mv (at 100 ma cm -2 ) S-17

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