Cobalt Selenide Nanostructures: An Efficient. Bifunctional Catalyst with High Current Density at

Transcription

1 Supporting Information Cobalt Selenide Nanostructures: An Efficient Bifunctional Catalyst with High Current Density at Low Coverage Jahangir Masud, Abdurazag T. Swesi, Wipula, P. R. Liyanage, and Manashi Nath* Department of Chemistry, Missouri S&T, Rolla, MO * nathm@mst.edu S-1

2 Characterizations Powder X-ray Diffraction. Powder X-ray diffraction (PXRD) of the as-synthesized films were obtained using a Philips X-Pert diffractometer with CuKα (1.5418Ǻ) radiation. The PXRD was collected at grazing angle incidence in thin film geometry (GI mode with Göbel mirrors). Scanning Electron Microscopy (SEM). A FEI Helios NanoLab 600 FIB/FESEM operating at 10 kv accelerating voltage with a working distance of 4.5 mm was used to obtain scanning electron microscopy (SEM) images, energy dispersive X-ray analysis (EDX) spectroscopy and the elemental mapping. X-ray Photoelectron Spectroscopy (XPS). X-ray Photoelectron Spectroscopy (XPS) was performed by using a KRATOS AXIS 165 X-ray Photoelectron Spectrometer using monochromatic Al X-ray source. The spectra were collected as is and after sputtering with Ar for 2 min which removes approximately 2 nm from the surface. Calibration of binding energy was carried out by setting the binding energy of C 1s peak to ev. Raman Spectra. Raman spectra from the as-deposited films were obtained from Horiba Jobin Yvon Lab Raman Aramis model using a He-Ne laser. The laser power was maintained at 1.7 mw/cm 2 over a range of cm -1. A typical spectra was iterated over at least 25 scans. Electrochemical Measurements. All the electrochemical measurements were carried out with an electrochemical workstation ( IvumStat potentiostat) in a standard three-electrode cell with Ag AgCl and Pt mesh as the reference electrode and counter electrode, respectively. A glassy carbon (GC) rotating disk electrode (RDE) acted as the working electrode. OER and HER measurements were conducted in 1 M KOH as the electrolyte, which was saturated with nitrogen S-2

3 during the experiments. The LSVs were performed at a scanning rate of 5 mv s 1 while electrode was rotating at 1600 rpm. In this study, we used Ag AgCl KCl(sat.) as the reference electrode in all measurements. This reference electrode was calibrated by measuring open circuit potential (OCP, V) at Pt wire in pure H2 saturated 1.0 M H2SO4 solution. Potentials measured vs Ag AgCl electrode were converted to a reversible hydrogen electrode (RHE) on the basis of Nernst equation as follows o E RHE = E Ag AgCl pH + E Ag AgCl where ERHE is the converted potential vs. RHE, EAg AgCl is the experimentally measured potential against Ag AgCl reference electrode, and E o Ag AgCl is the standard potential of Ag AgCl at 25 o C (0.199 V). Cleaning of GC Electrode: Prior to the electrodeposition, a bare GC electrode (5mm in diameter, cm2 of geometric surface area) was mechanically polished to produce a clean GC surface. Electrode was polished with alumina slurry (0.05 µm) with the help of a polishing microcloth until to get the mirror like surface. To remove the slurries, the electrode was washed ultrasonically in DI water for 15 min. Electrodeposition of RuO2 on GC S1 : Electrodeposition of RuO2 on GC substrate was carried out from a mixture of RuCl3 (0.452 g) and KCl (2.952 g) in 40 ml of 0.01M HCl by using cyclic voltammetry from to V (vs. Ag AgCl) for 100 cycles at a scan rate of 50 mv s -1. Finally heated at 200 o C for 3 h in presence of Air. Qualitative Analysis of the Evolved Gas: RRDE experiment was carried out with Pt as the ring electrode and GC-Co7Se8 as the disk electrode to confirm the composition of the evolved gas where O2 produced at disk electrode S-3

4 (anode) was reduced at Pt ring electrode. The Pt ring electrode was maintained at a constant potential of 0.2 V (vs. RHE) which is sufficient to reduce O2, and the ring current was monitored as a function of applied disk potential. The electrolyte was purged with N2 for 30 min to get rid of any dissolved O2 and blanketed with N2 atmosphere during measurements. Faradaic Efficiency: RRDE technique was employed to calculate the Faradaic efficiency of deposited Co7Se8 catalyst where GC and Pt were used as a disk and ring electrode, respectively following standard procedures. The collection efficiency, N (0.24), of the rotating ring-disk electrode was determined by the ratio of ring and disk current in 0.001M K3Fe(CN)6 containing 1M of KOH. Prior to RRDE measurement, N2 gas was purged in the cell for 30 min and then blanketed with N2 during the experiment. Co7Se8 was deposited at interchangeable GC disk electrode for 2.5 min at -0.8 V (vs. Ag AgCl). Pt ring was polished with Alumina slurry (0.05µm), washed and sonicated in DI water for 5 min. Before the measurements, Pt ring was electrochemically cleaned in N2 saturated 0.5 M H2SO4 until to get the reproducible characteristic CVs of Pt. The disk electrode was held at the open circuit potential of OER where no Faradaic process occurs, while the Pt ring electrode was maintained at 0.2 V vs RHE to get the background current of the ring electrode. The background ring current was lower than 20 μa. Then, the voltage of disk electrode was held at several definite potentials in the OER kinetic region for 30 sec, while, the ring current was recorded at 0.2 V vs. RHE. Turnover Frequency (TOF). The turnover frequency (TOF) was calculated from the following equation (eq. S1): TOF = I 4 F M (S1) S-4

5 Where I is the current in Amperes, F is the Faraday constant and M is number of moles of the active catalyst. Tafel plots. The Tafel slope was calculated from the following equation: η = a RT αnf log(j) (S2) where η is the overpotential, j is the current density and the other symbols have their usual meanings. The Tafel equation as shown Eq. (S2) is a fundamental equation which is acquired from the kinetically control region of OER / HER, and relates the overpotential η with the current density j where the Tafel slope is given by 2.3RT/αnF. To calculate Tafel slopes, LSV plots were obtained with slow scan speed (2 mv s -1 ) in non-stirred solution. Nanostructuring of Electrode by Lithography: E-Beam Lithography (EBL): For the development of tubular pattern on the glassy carbon (GC) substrate by EBL, the e-beam resist was prepared by spin coating two layers of polymethylmethacrylate (PMMA, 495k mol. wt. followed by 950k) polymer on GC. First PMMA layer (mol. wt. 495K) was spin coated, baked for 3 minutes on a hotplate at 180 o C, and allowed to cool to room temperature before coating the second PMMA layer (mol. wt. 950K). Substrate was again baked for 3 minutes on a hotplate at 180 o C and allowed to cool to room temperature. As prepared resist layer had a thickness of approximately 300 nm. These PMMA layers were selectively exposed to the electron beam through image-guided pattern writing in the EBL process. After that the exposed area of the polymer could be removed by dipping the substrate in methyl isobutyl ketone (MIBK) - isopropyl alcohol (IPA) (1:3 v/v) mixture for 55 seconds, while unexposed polymer remained intact. During S-5

6 this pattern development process, the underlying GC was exposed through the shell-like nanoholes defined by EBL thus forming confined nanoelectrodes on the substrate. Nanosphere Photolithography (NPL): Arrays of rod-like structures was developed by Nanosphere Photolithography (NPL) onto GC substrate. Spin coating was used to deposit 450 nm of S-1805 (positive tone photoresist) onto the substrates (5000 rpm for 1 min) and baked at 115 o C for 90s. A solution of polystyrene nanospheres (10% by weight of 1.7 μm polystyrene microspheres which was mixed with a 1:400 Triton X-100 to methanol surfactant solution in 7:1 volume ratio) was then spin-coated onto the surface. During spin coating, the nanospheres self-assemble onto the surface of the photoresist to form a hexagonal close-packed lattice. The photoresist was exposed by illuminating the nanospheres with UV light from an i-line mask aligner for 0.7s at10 mw/cm 2. Following exposure the photoresist was developed in MF-319 developer solution for 30s and rinsed in DI water, before hard baking at145 o C for 30 min. During the development process the microspheres were washed away along with the exposed photoresist leaving the hole array pattern in the photoresist, with open channels to the GC layer. S-6

7 Se Co Se Co Figure S1. Crystal structure of Co7Se8 showing polyhedral view (top) and ball-and-stick model (bottom). S-7

8 Intensity / a.u. Se 3d 5 min 2.5 min 1min Co 3p Binding Energy / ev Figure S2. XPS spectra of Se 3d and Co 3p of Co7Se8/GC S-8

9 Co Se 2 µm (a) Co Se 4 µm (b) Figure S3. EDS elemental mapping of nanorods (a) and line scan of nanotube (b) S-9

10 j / ma cm min Not IR Corrected IR Corrected E / V vs. RHE j / ma cm min Not IR Corrected IR Corrected E / V vs. RHE (a) j / ma cm min Not IR Corrected IR Corrected E / V vs. RHE j / ma cm E / V vs. RHE 1 min Not IR Corrected IR Corrected j / ma cm min Not IR Corrected IR Corrected E / V vs. RHE j / ma cm min -8 Not IR Corrected -12 IR Corrected E / V vs. RHE (b) Figure S4. Comparison of LSVs of (a) OER and (b) HER for IR correction in N2 saturated 1 M KOH solution. S-10

11 j / ma cm (a) j / ma cm (b) As Prepared After 15 h of activity Time / h E / V vs. RHE Figure S5. Extended OER stability of catalyst (a) and (b) comparison of LSVs of the catalyst before and after OER. S-11

12 5 µm 5 µm After 6 h HER After 6 h OER 10 µm 10 µm After 6 h HER After 6 h OER Figure S6. SEM images and EDS line scans of catalyst after 6 h of HER and OER. S-12

13 Intensity / a.u. Intensity / a.u. Intensity / a.u. * * * * * Au As prepared After OER After HER Reference θ / Deg (a) Co 2p As Prepared After 6h OER After 6h HER Se 3d As prepared After OER After HER Co 3p Binding Energy / ev (b) Binding Energy / ev (c) Figure S7. Comparison of (a) XRD; (b) Co 2p XPS and (c) Se 3d XPS spectra of the catalyst before and after 6 h of HER and OER. S-13

14 Overpotential, η / V vs. RHE 0.32 Nanotubes: 45.3 mv / decade 0.30 Nanorods: 53.5 mv / decade Thin films: 44.1 mv / decade log (j / ma cm -2 ) Figure S8. Tafel plots of tubes, rods and thin films of Co7Se8 for OER. S-14

15 Potential / V vs. RHE Pt: 30.2 mv dec Log (j / ma cm -1 ) Figure S9. Tafel plot of Pt for HER. S-15

16 Calculation the mass of catalyst deposited on GC: According to the Faraday s second law of electrolysis, the amount of a material deposited on an electrode is proportional to the amount of electricity used. Thus from the electrodeposition curve, the mass of catalyst can be calculated as- Mass = (Molecular weight of cobalt selenide X Charge)/ (No. of electron involved in process X Faraday Constant) Here, we can get the total charge from the deposition curve at constant potential (-0.8 V vs. Ag AgCl). As selenium cation (Se 4+ ) is electrochemically reduced to Se 2-, thus the number of electron involved in the process is 6. ma Scan Current time s Figure S10: Electrodeposition curve for cobalt selenide. S-16

17 Movie S1: Movie for the full water splitting at 1.6 V S-17

18 Table S1. Comparison of HER Activity Data for Different Catalysts Catalyst Electrolyte Onset, mv Η to achieve Tafel slopes, References 10 ma cm -2 / mv mv dec -1 NiMo(a) H2 sat M S1 NaOH CoNiFe H2 sat M NaOH MoS H2 sat M NaOH CoSe2 nanowire on Carbon fiber paper P-CoSe C Amorphous CoSe CoSe2 NP-CP NiSe2 -CP CoP-Carbon cloth NiS2-GS Mn1Ni1 Mn0.1Ni1 Co7Se8-GC H2 sat M H2SO4 H2 sat M H2SO4 H2 sat M H2SO4 H2 sat M H2SO4 H2 sat M H2SO4 H2 sat M KOH H2 sat M NaOH H2 sat M KOH H2 sat M KOH N2 sat M KOH mv 34 S mv 32 S S S S S S This Work S-18

19 References: S1. McCrory, C.C.L.; Jung, S.; Ferrer, I.M.; Chatman, S.M.; Peters, J.C.; Jaramillo, T.F. Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices. J. Am. Chem. Soc. 2015, 137, S2. Wang, K.; Xi, D.; Zhou, C.; Shi, Z.; Xia, H.; Liu, G.; Qiao, G. CoSe2 necklace-like nanowires supported by carbon fiber paper: a 3D integrated electrode for the hydrogen evolution reaction, J. Mater. Chem. A 2015, 3, S3. Zhang, H.; Yang, B.; Wu, X.; Li, Z.; Lei, L.; Zhang, X. Polymorphic CoSe2 with Mixed Orthorhombic and Cubic Phases for Highly Efficient Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2015, 7, S4. Carim, A. I.;Saadi, F. H.; Soriaga, M. P.; Lewis, N. S.Electrocatalysis of the hydrogenevolution reaction by electrodeposited amorphous cobalt selenide films, J. Mater. Chem. A 2014, 2, S5. Kong, d.; Wang, H.; Lu, Z.; Cui, Y. CoSe2 Nanoparticles Grown on Carbon Fiber Paper: An Efficient and Stable Electrocatalyst for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2014, 136, S6. Shi, Y.; Zhang, B. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem. Soc. Rev. 2016, 45, S7. Wu, X.; Yang, B.; Li, Z.; Lei, L.; Zhang, X. Synthesis of supported vertical NiS2 nanosheets for hydrogen evolution reaction in acidic and alkaline solution. RSC Adv. 2015, 5, S8. Ledendecker, M.; Clavel, G.; Antonietti, M.; Shalom, M. Highly Porous Materials as Tunable Electrocatalysts for the Hydrogen and Oxygen Evolution Reaction. Adv. Funct. Mater. 2015, 25, S-19