Supporting Information

Transcription

1 Supporting Information Hierarchical design of amorphous Ni-P bilayer on a 3D mesh substrate for high efficiency oxygen evolution reaction Xi Xu, Chaojiang Li,Jiahao Gwendolyn Lim, Yanqing Wang, Aaron Ong, Xinwei Li, Erwin Peng*, and Jun Ding* Department of Materials Science and Engineering, Faculty of Engineering, National University of Singapore, 9 Engineering Drive 1, Singapore , Singapore * Corresponding author mseer@nus.edu.sg, msedingj@nus.edu.sg (Professor Ding) These authors contributed equally to this work. S-1

2 S1. Additional Experimental Procedures Electrode Fabrications: The Zirconia paste was prepared by mixing high purity yttria-stabilized zirconia powders (ZirPro; CY3Z-P; Saint Gobain) with organic additives including polyvinyl alcohol (PVA; Mw 31000; Sigma Aldrich) as binder, poly(ethylene glycol) (PEG-400; Mw 400; Sigma Aldrich) as plasticizer and L-ascorbic acid (AA; Sigma Aldrich) as dispersant. The paste was prepared by manually mixing all the components. 1 The starting powder contains 3 mol% yttria addition and nanosized zirconia particles (average particle size less than 400 nm with a specific area of 9.0 m 2 g -1 ). Every 12 g CY3Z-P powder was added in to a mixture of 0.5 ml dispersant solution (200 mg ml -1 ), 0.25 ml binder (12.5 wt.%) and 0.05 ml plasticizer with 3.0 ml DI water. The paste was further homogeneously mixed mins to obtain a desired viscosity ranging from pa s -1 and exhibits a shear thinning behaviour to enable good flow during printing. Sensitizing is accomplished by immersing the printed parts in acid stannous chloride solution (hydrochloric acid 5wt%, stannous chloride 5%; Transene) followed by activating step by rinsing the part in a solution of palladium chloride (hydrochloric acid 5wt%, palladium chloride 5%; Transene). Each step takes 5 minutes. The reaction progresses catalytically with nickel deposition taking place at the operating temperature of C controlled by silicon oil operating bath. Post heat treatment process enabled by high temperature tolerable ceramic support material was also performed to study the catalytic behavior differences of amorphous and crystalline coated phase. S-2

3 Electrolyte and Glassware Purification: The 1M electrolyte was purified to remove possible iron impurity. Approximately, 2 g Ni(NO 3 ) 2 6H 2 O was made into solution with 4mL DI water in a 50 ml centrifuge tube and then mixed with 20 ml 1M KOH to get the precipitated high purity Ni(OH) 2. The mixture was mechanically agitated and centrifuged, and the liquor supernatant was decanted. The Ni(OH) 2 then was washed three times by adding 20 ml of DI water and 2 ml of 1 M KOH, dispersing the solid, centrifuging, and decanting the supernatant. Finally, 50 ml of 1M KOH was added to the tube for final purification. The solid was again dispersed and shaken for 10 min, followed by resting at least 3 h. The mixture was centrifuged, and the purified KOH supernatant was decanted into a H 2 SO 4 -cleaned centrifuge tube for use. New glassware was adopted with no pre-determination of iron species and the glass cell was cleaned three times with deionized water, detergent, 0.1% hydrochloric acid and boiled water before next use. Electrode Characterization: The surface morphology was observed using Scanning electron microscopy (SEM) with acceleration voltage of 5 kv (Zeiss; FESEM Supra 40). X-ray powder diffraction (XRD) patterns were obtained by Bruker D8 Advanced diffractometer system with Cu Kα radiation source. An Axis Ultra DLD X-ray photoelectron spectrophotometer (XPS) equipped with an Al Kα excitation source ( ev) was used to record compositional information of all the samples. The energy step size of the XPS was 1 ev for the survey scans and 0.1 ev for the fine scans. Raman spectra were conducted on a Horiba Micro Raman HR Evolution System. High-resolution transmission electron microscopy (HR-TEM) of the samples S-3

4 were characterized using a field-emission transmission electron microscope (FE-TEM, JEM-2010F, JEOL, Japan), which was operated at an accelerating voltage of 200 kv. The polishing test was conducted by attaching commercial silicon carbide paper to a flat glass plate and applying the polishing along the width direction ten times followed by a 90 degree rotation of the sample. After polishing, air duster was used to blow off the dust and debris left on the surface before further characterization. Electrochemical measurement: Catalytic behavior of as-prepared integrated electrode was recorded by VMP3 electrochemical workstation (Bio-logic Inc.). All the measurements were conducted based on a three-electrode system concluding self-fabricated electrode as working electrode, a platinum plate as the counter electrode and a saturated calomel electrode (1M KOH) as reference electrode. Before actual data recording, the working electrode was scanned 20 cycles for LSV to obtain steady grams in 1M KOH with scan speed of 5 mv s -1 at room temperature. In order to calculate ECSA, the electrochemical double-layer capacitance (EDLC) of working electrodes was measured by CV scans at different scan rates. EDLC was therefore calculated by plotting graphs of scan rated versus current density at certain potential with respect to reference electrode. Tafel slopes of the experiments were all derived from LSV curves. For stability study, chronopotentiometry was also measured for commercially available Ni foam and self-fabricated electrode at a fixed current density of 15 ma cm -2 over 10 hours. The conductivity was tested using 2638A Hydra Series III data acquisition unit. The four probes measurement method was carried out in the 2638A Hydra Series III data acquisition unit, with the precision of Ohm. S-4

5 Sheet resistance is a measure of resistance of thin films that are nominally uniform in thickness. It is used to characterize materials made by semiconductor doping, metal deposition, resistive paste printing and glass coating. Based on the proposed sample structure, the sheet resistance was used to estimate the conductivity. R S = ρ/t where ρ is bulk resistivity, R S is the sheet resistance, and t is the film thickness. S-5

6 (a) (b) Figure S1. Four views of models prepared by Autodesk Fusion360 for printing. (a) designed model for slicing; (b) extruded model, i.e. printed schematic. Total volume is calculated to mm 3 with a geometric surface area of mm 2 and the printed mesh has an open porosity of 70.3%. A handle was added to the design for better electrochemical test setting. S-6

7 Figure S2. SEM images of 1hr coated (a) mesh and (b) plate templates after water splitting cycles and adhesion test.(insert: Optical and cross-sectional SEM images of corresponding sample) Figure S3. Cross-sectional SEM and EDX images of (a) NiP60 and (b) NiP60-P after electrolysis. (scale bar: 1 µm; yellow: O, blue: Ni, green: P.) S-7

8 Figure S4. Cross-sectional SEM and EDX mapping of all samples (a) NiP3, (b) NiP15, (c) NiP60, (d) NiP120, (e) HT-NiP60, (f) NiP60-P after LSV scans and stability test to show the coating thickness and surface/non-surface oxidized layer. (Scale bar: 500nm for (a); 1 µm for (b)-(f); yellow: O, blue: Ni, green: P.) S-8

9 Figure S5. XPS spectra of (a) NiP3 and (b) NiP60 after electrolysis. S-9

10 Table S1. Phosphorus Content NiP3 NiP15 NiP60 NiP120 HT-NiP60 Atomic Percent Weight Percent S-10

11 Figure S6. Polarization curves of NiP3 at different cycles. Figure S7. Cross-sectional SEM and EDX mapping of Sample NiP120 (a) after, (b) before LSV scans and (c) stability test; (d) Line scan signal of NiP120 after electrolysis. (Scale bar: 1 µm) S-11

12 Figure S8. Images of sets of coated and uncoated samples.(a) Digital Light Projecting(DLP) 3d-printed polymer meshes; (b) DLP 3d-printed polymer tube meshes; (c) Robocasted hexegonal meshes; (d) DLP 3d-printed polymer lucky cats; (e) SEM image of coated DLP 3d-printed polymer mesh; (f) Robocasted meshes. S-12

13 Figure S9. Raman spectra of representative samples. (Range: 350cm -1 LHT-NiP60: 60 minutes coating time; 280 C in vacuum heat treatment) to 750cm -1 ) (Sample S-13

14 Figure S10. High resolution XPS spectra of NiP120 after durability test. (a). Ni 2p; (b). P 2p; (c) O 1s. S-14

15 Figure S11. X-Ray diffraction patterns of NiP120 before and after durability test. (2theta from 20 o to 80 o ) S-15

16 Table S2. Catalytic parameters of all samples after 20 cycles Sample geometric 10mA.cm -2 Tafel slope (mv.dec -1 ) NiP NiP NiP HT-NiP NiP60-P NiP Ni foam S-16

17 Table S3. Comparison of electrolysis behaviour of Nickel based and phosphide catalysts in alkaline solution after activation(1m KOH) Material Substrate 10 ma cm -2 Ref. NiP120 3D-printed ZrO 2 mesh 286 This work Ni(OH) 2 Drop casting onto glassy carbon 331 [2] β-ni(oh) 2 Drop casting onto glassy carbon 444 [3] Ni 2 P nanowires Drop casting onto FTO glass plate 360 [4] Ni 2 P PVC coated Cu wire together with metal sheet 339 [5] Co 2 P PVC coated Cu wire together with metal sheet 367 [5] Fe 2 P PVC coated Cu wire together with metal sheet 390 [5] Ni 2 P Carbon cloth ma cm -2 [6] FeP/Ni 2 P Ni foam 154 [7] Ni x P y -325(Ni x P y with riches Ni 5 P 4 phase synthesis in 325 o C) Drop casting onto carbon fiber paper Overall bias at 1.57 V [8] Bulk NiFeP Free-standing 319 [9] Etched Bulk NiFeP Free-standing Segement 219 [9] S-17

18 References (1) Peng, E.; Wei, X.; Garbe, U.; Yu, D.; Edouard, B.; Liu, A.; Ding, J. Robocasting of Dense Yttria-stabilized Zirconia Structures. J. Mater. Sci. 2017, 53, (2) Stern, L.-A.; Feng, L.; Song, F.; Hu, X. Ni 2 P as a Janus Catalyst for Water Splitting: the Oxygen Evolution Activity of Ni 2 P Nanoparticles. Energy Environ. Sci. 2015, 8, (3) Gao, M.; Sheng, W.; Zhuang, Z.; Fang, Q.; Gu, S.; Jiang, J.; Yan, Y. Efficient Water Oxidation Using Nanostructured Alpha-nickel-hydroxide As an Electrocatalyst. J. Am. Chem. Soc. 2014, 136, (4) Han, A.; Chen, H.; Sun, Z.; Xu, J.; Du, P. High catalytic activity for water oxidation based on nanostructured nickel phosphide precursors. Chemical communications 2015, 51, (5) Read, C. G.; Callejas, J. F.; Holder, C. F.; Schaak, R. E. General Strategy for the Synthesis of Transition Metal Phosphide Films for Electrocatalytic Hydrogen and Oxygen Evolution. ACS Appl. Mater. Interfaces. 2016, 8, (6) Pu, Z.; Xue, Y.; Li, W.; Amiinu, I. S.; Mu, S. Efficient Water Splitting Catalyzed by Flexible NiP2 Nanosheet Array Electrodes under both Neutral and Alkaline Solutions. New. J. Chem. 2017, 41, (7) Yu, F.; Zhou, H.; Huang, Y.; Sun, J.; Qin, F.; Bao, J.; Goddard, W. A., 3rd; Chen, S.; Ren, Z. High-performance Bifunctional Porous Non-noble Metal Phosphide Catalyst for Overall Water Splitting. Nat Commun 2018, 9, (8) Li, J.; Li, J.; Zhou, X.; Xia, Z.; Gao, W.; Ma, Y.; Qu, Y. Highly Efficient and Robust Nickel Phosphides as Bifunctional Electrocatalysts for Overall Water-Splitting. ACS Appl. Mater. Interfaces. 2016, 8, (9) Hu, F.; Zhu, S.; Chen, S.; Li, Y.; Ma, L.; Wu, T.; Zhang, Y.; Wang, C.; Liu, C.; Yang, X.; Song, L.; Yang, X.; Xiong, Y. Amorphous Metallic NiFeP: A Conductive Bulk Material Achieving High Activity for Oxygen Evolution Reaction in Both Alkaline and Acidic Media. Adv. Mater. 2017, 29, S-18