Supporting Information. Electropolymerization of aniline on nickel-based electrocatalysts substantially

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1 Supporting Information Electropolymerization of aniline on nickel-based electrocatalysts substantially enhances their performance for hydrogen evolution Fuzhan Song, Wei Li, Guanqun Han, and Yujie Sun* Department of Chemistry and Biochemistry, Utah State University, Logan, UT 84322, USA. *Corresponding Author 1

2 Experimental Procedures Chemicals All chemicals were used as received without any further purification. Ammonium chloride (NH 4 Cl), nickel chloride (NiCl 2 6H 2 O), potassium phosphate (KH 2 PO 4 ) were purchased from Fisher. Sulfur (S) was purchased from Sigma. Nickel foam (Ni foam) was purchased from MTI. Potassium hydroxide (KOH) and potassium hydrogen phosphate (K 2 HPO 4 ) were purchased from Alfa Aesar. Sulfuric acid (H 2 SO 4 ) and hydrochloric acid (HCl) were purchased from Pharmco. Water deionized (18 MΩ cm) with a Barnstead E-Pure system was used in all experiments. Syntheses of electrocatalysts Synthesis of Ni/Ni foam (Ni/NF): Ni microspheres supported on commercial 3D porous Ni foams were prepared by a facile cathodic electrodeposition method. The electrodeposition process was performed in a standard two-electrode electrochemical cell in an electrolyte consisting of 2.0 M NH 4 Cl and 0.1 M NiCl 2 at room temperature. A piece of clean Ni foam (0.5 cm 0.5 cm) was used as the working electrode and a platinum wire as the counter electrode. The electrodeposition was implemented at a constant current density of 1.0 A cm 2 for 500 s to obtain Ni/NF samples. Synthesis of Ni 2 P/Ni foam (Ni 2 P/NF): A Ni/NF was placed at the center of a tube furnace. NaH 2 PO 2 H 2 O (1.0 g) was placed at the upstream side of the furnace at a carefully adjusted location. After calcination at 400 C under Ar flow for 1 h, the metallic nickel on Ni/NF was converted to nickel phosphides, resulting in Ni 2 P/NF. Synthesis of NiS x /Ni foam (NiS x /NF): A Ni/NF was placed at the center of a tube furnace. Sulfur (0.5 g) was placed at the upstream side of the furnace at a carefully adjusted location. After calcination at 300 C under Ar flow for 0.5 h, the metallic nickel was converted to nickel sulfide, resulting in NiS x /NF. Synthesis of polyaniline/ni/ni foam (PANI/Ni/NF): The electropolymerization process was conducted in a standard three-electrode configuration at room temperature. The electrolyte was 0.05 M Na 2 SO 4 and 0.01 M aniline. A platinum wire was used as the counter electrode and a Ag/AgCl electrode (saturated KCl) as the reference electrode. The electropolymerization of aniline was conducted using Ni/NF as the working electrode at a constant current density of 0.5 ma cm 2 for 1000 s to obtain the PANI-coated electrodes (PANI/Ni/NF). To study the effect of the electrodeposition time on the catalytic performance of the resultant catalysts, the catalysts were prepared by electrodeposition for different times (200, 400, 600, 1400 and 1800 s). Following the same procedure, polyaniline was also electropolymerized on fluorine-doped tin oxide (FTO), Ni 2 P/NF, and NiS x /NF to obtain control samples of PANI/FTO, PANI/Ni 2 P/NF, and PANI/NiS x /NF, respectively. 2

3 Characterization Scanning electron microscopy (SEM) measurements were collected on a FEI QUANTA FEG 650. X-ray diffraction (XRD) patterns were obtained on a Rigaku MinifexII Desktop X-ray diffractometer. The X-ray photoelectron spectroscopy (XPS) analyses were performed using a Kratos Axis Ultra instrument (Chestnut Ridge, NY) at the Surface Analysis Laboratory of Nanofab at the University of Utah. Electrochemical Measurements Electrochemical measurements in 1.0 M KOH, 1.0 M phosphate buffer, and 0.5 M H 2 SO 4 were performed by a computer-controlled Gamry Interface 1000 electrochemical workstation with a three-electrode cell system. The resultant electrocatalysts were used as the working electrode, a Ag/AgCl (saturated KCl) electrode as the reference electrode, and a carbon rod as the counter electrode. The electrolytes were saturated with H 2 for HER evaluation. All potentials reported herein were quoted with respect to the reversible hydrogen electrode (RHE) through RHE calibration. ir (current times internal resistance) compensation was applied in all the electrochemical experiments to account for the voltage drop between the reference and working electrodes using Gamary Framework Data Acquisition Software Since it is very challenging to directly measure the absolute electrochemically active surface area (ECSA), a widely adopted method is to derive the relative ECSA based on the measurement of double-layer capacitance (Cdl) in the non-faradaic potential region. Specifically, cyclic voltammetry curves of the electrodes were collected in a non-faradaic region ( V vs. RHE) at different scan rates of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 mv s -1. The electrochemical Cdl was then estimated by plotting the difference between the anodic and cathodic current densities (Δ j = ja jc) at 0.11 V vs. RHE against the scan rate. The resulting linear slope is twice of the Cdl. 3

4 Results and Discussion Figure S1. Digital photograph showing a bare NF (left, silver), a Ni/NF (middle, black), and a PANI/Ni/NF (right, black) 4

5 Figure S2. SEM images of PANI/Ni/NF at different magnifications. 5

6 Figure S3. SEM images of Ni/NF at different magnifications. 6

7 Figure S4. TEM image of PANI/Ni/NF. 7

8 Figure S5. High-resolution Ni 2p XPS spectrum of PANI/Ni/NF. 8

9 Figure S6. HER polarization curves of PANI/Ni/NF with different electropolymerization times measured in 1.0 M KOH (scan rate = 5 mv s 1 ). 9

10 Figure S7. HER polarization curves of PANI/Ni/NF with different electropolymerization times measured in 1.0 M phosphate buffer (scan rate = 5 mv s 1 ). 10

11 Figure S8. HER polarization curves of PANI/Ni/NF with different electropolymerization times measured in 0.5 M H 2 SO 4 (scan rate = 5 mv s 1 ). 11

12 Figure S9. Randle equivalent circuit model. 12

13 Figure S10. Cyclic voltammograms of (a) PANI/Ni/NF and (b) Ni/NF at scan rates from 10 to 100 mv s 1. (c) Scan rate dependence of the current densities of PANI/Ni/NF and Ni/NF at V versus RHE. All experiments were carried out in 1.0 M KOH. 13

14 Figure S11. SEM images of post-her PANI/Ni/NF in 1.0 M KOH. 14

15 Figure S12. Elemental mapping images of post-her PANI/Ni/NF in 1.0 M KOH. 15

16 Figure S13. Nyquist plots of Ni/NF and PANI/Ni/NF catalysts measured at V vs RHE in 1.0 M phosphate buffer of ph 7. 16

17 Figure S14. Cyclic voltammograms of (a) PANI/Ni/NF and (b) Ni/NF at scan rates from 10 to 100 mv s 1. (c) Scan rate dependence of the current densities of PANI/Ni/NF and Ni/NF at 0.8 V versus RHE. All experiments were carried out in 1.0 M phosphate buffer of ph 7. 17

18 Figure S15. HER polarization curves of PANI/Ni/NF before and after 1000 cycles of CV at a scan rate of 100 mv s 1. The inset shows the chronopotentiometry curve of PANI/Ni/NF at J = 10 ma cm 2. All experiments were carried out in 1.0 M phosphate buffer of ph 7. 18

19 Figure S16. HER polarization curves of PANI/Ni/NF before and after 1000 cycles of CV at a scan rate of 100 mv s 1. The inset shows the chronopotentiometry curve of PANI/Ni/NF at J = 10 ma cm 2. All experiments were carried out in 0.5 M H 2 SO 4. 19

20 Figure S17. Nyquist plots of Ni/NF and PANI/Ni/NF measured at V vs RHE in 0.5 M H 2 SO 4 of ph 0. 20

21 Figure S18. XRD of Ni 2 P/Ni/NF and PANI/Ni 2 P/Ni/NF together with the standard XRD patterns of Ni and Ni 2 P. 21

22 Figure S19. SEM images of (a, b) Ni 2 P/Ni/NF and (c, d) PANI/Ni 2 P/Ni/NF. 22

23 Figure S20. XRD of NiS x /Ni/NF and PANI/NiS x /Ni/NF together with the standard XRD patterns of Ni, NiS, NiS 2, and Ni x S 6. 23

24 Figure S21. SEM images of (a, b) NiS x /Ni/NF and (c, d) PANI/NiS x /Ni/NF. 24