Supporting Information

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1 Supporting Information Self-Supported Cedarlike Semimetallic Cu 3 P Nanoarrays as a 3D High- Performance Janus Electrode for Both Oxygen and Hydrogen Evolution under Basic Conditions Chun-Chao Hou, Qian-Qian Chen, Chuan-Jun Wang, Fei Liang, Zheshuai Lin, Wen-Fu Fu, and Yong Chen, * Key Laboratory of Photochemical Conversion and Optoelectronic Materials & CAS-HKU Joint Laboratory on New Materials, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing , P.R. China Key Laboratory of functional catalysis and laser Technology, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing , P.R. China College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming , P.R. China chenyong@mail.ipc.ac.cn S-1

2 Synthesis of RuO 2 nanoparticles. The RuO 2 nanoparticles were synthesized using a hydrothermal method reported before. 1 An aqueous solution of RuCl 3 (20 mm, 80 ml) was kept in a Teflon-lined autoclave with a stainless steel shell. Specifically, the autoclave was heated from room temperature to 180 o C at a rate of 10 o C min 1 in an oven, and the temperature was then kept at 180 o C for 3 h. The precipitates were centrifuged and washed several times with pure water and ethanol. The precipitates were dried in a reduced-pressure oven overnight at room temperature. After that, the sample was transferred to a quartz boat in the tube furnace and maintained at 500 o C for 2 h in air and then allowed to cooling down to room temperature. Synthesis of Nano-CuO nanoparticles. The nano-cuo nanoparticles were synthesized via a simple thermal reaction using Cu(OH) 2 as precursor. In a typical process, 200 mg of Cu(NO 3 ) 2 3H 2 O was added to a 100 ml aqueous solution containing 50 mg sodium citrate and stirred for 120 min. Then 20 ml NaOH solution (0.5 M) was added to the above mixture dropwise. The formed Cu(OH) 2 suspension was separated by centrifugation, washed by water and dried in vacuum oven. Afterwards, this above solid were ground in a mortar to form a uniform distribution and put in a quartz boat of the tube furnace. Subsequently, the samples were maintained at 150 o C for 6 h with a heating rate of 5 o C min 1 in air. After cooling to room temperature, the obtained solid was washed subsequently by water and ethanol three times and dried in vacuum oven. The formation of nano-cuo sample was confirmed by XRD and the main size was about 6 7 nm based on the Scherrer formula. Fabrication of RuO 2 or Nano-CuO NPs electrode. The preparation method of the working electrodes containing investigated catalysts can be described as follows. In short, a certain amount of catalyst was dispersed in 1 ml of water/ethanol (v/v = 46:50) solvent containing 40 L of Nafion solution (5 wt %, Sigma-Aldrich), and then the mixture was ultrasonicated for at least 30 min to generate a homogeneous ink. Next, various amount of the dispersion was transferred onto the CF, leading to the catalyst loading film.. Figure S1. (a) SEM and (b-d) magnified-sem images of cedarlike Cu 3 P film. S-2

3 Counts Cu 3 P film Cu Cu O P Energy (kev) Figure S2. EDX spectrum of Cu 3 P film. Figure S3. XPS survey spectrum (a) and the high-resolution XPS spectra of (b) Cu 2p and (c) P(2p) regions for Cu 3 P film. S-3

4 Figure S4. (a) XRD, (b) SEM and (c-d) magnified-sem images of CuO x film. Figure S5. Characterization of RuO 2 nanoparticles. (a) Powder XRD pattern. (b, c) TEM and magnified- TEM images (inset: size distribution of RuO 2 nanoparticles). (d) HRTEM image of RuO 2 nanoparticles. S-4

5 Current density (ma cm -2 ) Cu 3 P ir-corrected Cu 3 P CF ir-corrected CF CuO x ir-corrected CuO x RuO 2 ir-corrected RuO Potential (V vs. RHE) Figure S6. The polarization curves of the as-prepared Cu 3 P, RuO 2 NPs, and CuO x without or with ir compensation in 0.1 M KOH at a scan rate of 5 mv s 1. Figure S7. (a, c) CVs for CuO x and RuO 2 measured in 0.1 M KOH solution and (b, d) corresponding double layer capacitance (C dl ). S-5

6 j s (ma cm -2 ) Figure S8. (a) CVs for the Cu-P@Cu 3 P measured in 0.1 M KOH solution and (b) corresponding double layer capacitance (C dl ). The electrical double layer capacitor (C dl ) of the as-prepared materials were investigated on the basis of double-layer charging curves using cyclic voltammograms (CVs) recorded at different scan rates in a small non-faradaic potential range from to V vs. RHE to avoid the redox potential of the Cu-P@Cu 3 P sample ir-corrected Cu 3 P ir-corrected CuO x ir-corrected RuO Potential (V vs. RHE) Figure S9. IR-corrected polarization curves for all of the catalysts after ECSA normalization. In order to accurately evaluate the intrinsic electrochemical behavior of all catalysts, the measured currents were normalized by ESCA to obtain the normalized current density (the corresponding ECSA data are shown in Figure S8). As shown in Figure S9, the Cu 3 P exhibits the best OER catalytic performance, superior to the CuO x and comparable to the RuO 2, indicating the intrinsic OER catalytic performance of all of the as-prepared catalysts. The better OER catalytic activity of semimetallic Cu 3 P compared with copper oxides catalyst can be mainly ascribed to its excellent electrical conductivity to expedite the charge transfer from the catalyst surface to the current collector, favoring the high OER performance. S-6

7 Current density (ma cm -2 ) Cu 3 P RuO 2 Nano-CuO Potential (V vs. RHE) Figure S10. OER polarization curves without ir compensation of as-prepared Cu 3 P, RuO 2 and Nano- CuO with the same weight loading in 0.1 M KOH at a scan rate of 5 mv s 1. For ease of comparison of electrocatalytic performance of different catalysts with the same weight loading, the RuO 2 and Nano-CuO NPs were synthesized and the corresponding RuO 2 NPs and Nano-CuO NPs electrodes were also fabricated using the common Nafion method with the same weight loading as our prepared Cu 3 P film. It should be noted that the cedarlike Cu 3 P nanoarrays film was a self-supported 3D electrode and used in topotatic conversion method. This method often leads to a very high catalyst loading, good catalytic performance and electrode stability. 3 The mass of Cu 3 P catalyst on CF was calculated as follows: Cu 3 P loading = x mg ( M Cu3P /M P ) = x mg (221.6/30.97) = 7.15x mg, where x (mg) is the increase of mass of CF after phosphidation treatment (the increase of mass is the P element), M is the molecular weight of Cu 3 P or P. This method has been used in the literature and proved to be reliable. 4-6 By carefully weighing several films, we obtained an average increasing mass about 4.6 mg (0.5 cm 2 ). Then, the weight loading of Cu 3 P catalyst on CF was about 65.8 mg cm 2. S-7

8 40 j / ma cm Cu 3 P RuO 2 10 CF Time / s Figure S11. Bulk electrolysis of Cu 3 P, RuO 2, and bare CF catalysts at applied 1.76 vs. RHE without ir correction in a 0.1 M KOH solution. FigureS12. (a) SEM and (b) magnified-sem images of post-oer Cu 3 P film. S-8

9 Current density (ma cm -2 ) O 2 / mol 90 Theoretical Experiment Time / s Figure S13. O 2 production measured by fluorescent oxygen probe (red line) and the theoretical amount of O 2 (black) in 0.1 M KOH solution at applied 1.76 V vs. RHE bare CF 0.2 mg cm -2 RuO mg cm -2 RuO mg cm -2 RuO mg cm -2 RuO mg cm -2 RuO Potential / Vvs. RHE Figure S14. OER polarization curves of as-prepared RuO 2 without ir compensation in 0.1 M KOH at different loading amount RuO 2 NPs at a scan rate of 5 mv s 1. S-9

10 Current density Figure S15. (a) TEM and (b) HRTEM images of post-oer Cu 3 P samples Potential / V vs. RHE Figure S16. CV cycling test for as-prepared Cu 3 P film at a scan rate of 10 mv s 1 in 0.1 M KOH. S-10

11 Figure S17. Nyquist plots of electrochemical impendence spectra of as-prepared Cu 3 P and CF films in 0.1 M KOH at an applied 0.44 V vs. RHE. Figure S18. (a) Linear sweep voltammetric curves of overall water splitting using the as-prepared Cu 3 P and copper foam (CF) as both anode and cathode electrodes at a scan rate of 5mV s 1 in 0.1 M KOH. Inset is the schematic Cu 3 P two-electrode water electrolysis device. (b) Chronoamperometric curve at an applied bias potential of 2.20 V in such two-electrode device. Inset is the linear sweep voltammetric curve of intial and post-oer 20 h Cu 3 P electrodes. S-11

12 Table S1. Comparison of the OER activities of some representative solid-state Cu-based, partial noble and several recently reported active non-noble metal catalysts supported on different substrates. Catalyst Cu 3 P CuO x Current density j (ma cm 2 ) Overpotential a at the corresponding j (mv) Cu-B i ~ H 2 -CuCat ~ CuO film ~ CuO ~ Electrolytes 0.1 M NaOH 0.2 M borate buffer solution (ph 9) 0.1 M KBi solution (ph 9.2) 0.1 M acetate electrolyte (ph 12.4) 0.1 M borate buffer solution (ph 9.2) Tafel slope (mv dec 1 ) substate Cu foam reference This work 89 FTO 7 85 FTO ITO 9 56 FTO 10 Mono NiTi M MO 10 ~320 1 M KOH 52 GC 11 Ni P@C M KOH 64 RDE 12 Co 3 O 4 -carbon porous M KOH 70 Cu foil 5 nanowire arrays Nitrogen-doped crumpled M KOH 71 GC 13 graphene CoO Ni x Co 3-x O M NaOH Ti foil 14 NiO/FeNC sheets M KOH 76 RDE 15 -Ni(OH) M KOH 42 GC 16 NiSe M KOH 64 Ni foam 4 CoO M KOH Ni foam 17 Co 3 O 4 /NiCo 2 O M KOH 88 Ni foam 18 LiCo 0.8 Fe 0.2 O M KOH 50 RDE 19 Rutile RuO 2 ~ M HClO 4 GC 20 IrO 2 /C ~ KOH (ph=13) GC 21 a The overpotentials reported here for our self-supported Cu-based materials and Co, Fe, Ni-based materials at 10 ma cm 2 are ir corrected, and the overpotentials without ir compensation for other Cu-based films. To get more details, please see recent Co-based water splitting catalysts review. 22 S-12

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