Multidimensional Thin Film Hybrid Electrodes. Hydrogen Evolution Reaction

Transcription

1 Multidimensional Thin Film Hybrid Electrodes with MoS2 Multilayer for Electrocatalytic Hydrogen Evolution Reaction Eungjin Ahn 1 and Byeong-Su Kim 1,2 * 1 Department of Energy Engineering and 2 Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Korea bskim19@unist.ac.kr Table of Contents 1. Experimental 2. Characterizations 3. Supplementary Table and Figures 4. References S1

2 1. Experimental Preparation of MoS2 nanosheet suspension. Lithium intercalation of MoS2 was carried out according to the method described by Morrison et al. S1 Briefly, 400 mg of bulk MoS2 powder (Sigma-Aldrich) was immersed in 4.0 ml of n-butyllithium solution 1.6 M in hexane for 2 days under argon. The lithium-intercalated MoS2 (LixMoS2) was washed with hexane to remove excess lithium, and the residue was vacuum dried. The dried LixMoS2 was exfoliated by ultrasonication in deionized (DI) water for 1 h. The mixture was then purified through several cycles of centrifugation. Preparation of amine-functionalized MWNT suspension. Multiwalled carbon nanotubes (MWNT) were purchased from Hanwha Nanotech Corp. (CM-100) synthesized by chemical vapor deposition (CVD) method. The diameter of MWNT is in range of nm, average length of approximately 200 μm, and purity over 95%. The amine-functionalized MWNT (MWNT-NH2) suspension was produced in a two-step process. In the first step, carboxylic acid functional groups were introduced to the MWNTs according to the method from Lee et al. S2 The 1.0 g of MWNTs were mixed with 50 ml of concentrated H2SO4/HNO3 (3/1 v/v) till complete dissolution. The mixture was kept at 70 C for 4 h using a hot plate. After the heating, the mixture was diluted with 1 L of DI water. The mixture was then filtered and washed to remove all traces of acid. In the second step, the carboxyl functional groups on the MWNTs were transformed into amine functional groups by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide (EDC) coupling reaction. 100 ml of carboxylated MWNT solution (0.50 mg/ml) was vigorously mixed with 1.25 g of EDC and 10 ml of ethylenediamine, and the solution was stirred overnight at room temperature. The product mixture was then dialyzed for three days in DI water to remove the remaining reagents and byproducts. S2

3 Preparation of polyaniline (PANi) solution and deposition of PANi priming layer. To guarantee sufficient mass loading of MoS2, an initial PANi layer was deposited on the O2-plasma treated substrates (silicon, quartz, or FTO-coated glass) by the spin-assisted dipping method, before assembling the (MoS2/MWNT)n multilayer. The PANi solution (2.0 mg/ml) was prepared by dissolving PANi (Mw = 20,000) in dimethylacetamide (DMAc) diluted with ph 3.0-adjusted water (DMAc:H2O = 1:9 v/v) based on the method from Stockton et al. S3 The substrates were spin-dipped in the 50 ml of PANi solution for 10 min, then rinsed with ph 3.0-adjusted DI water 3 times (1 min each) to remove loosely adhered PANi layer. Layer-by-Layer assembly of (MoS2/MWNT)n multilayer film. We applied a spin-assisted dipping (spin-dipping) method using automation equipment (nanostrata Inc). The spin-dipping LbL assembly with rinsing steps enables the LbL components to assemble onto the substrate with a uniform layered structure. S4 All suspensions and DI water for rinsing were adjusted to ph = 5.0 before the LbL assembly. The PANi-adsorbed substrates were spin-dipped in the MoS2 suspensions for 10 min, which changed the surface charge from positive to negative. The dipped substrate was rinsed with DI water 3 times (1 min each), in order to remove loosely bound MoS2 nanosheets. Then the substrate was spin-dipped in the MWNT-NH2 suspension for another 10 min, followed by the same rinsing steps. This process constructs 1 bilayer (BL) within the MoS2/MWNT multilayer, and it was repeated till the desired number of BLs (n) was reached. The fully assembled multilayer film was denoted as (MoS2/MWNT)n. For additional thermal treatment, the as-assembled (MoS2/MWNT)n film was placed into a tube furnace for thermal reduction. The temperature rising ratio was 10 o C per min and maintained for 1 h at the target temperature (100, 200, or 300 o C) under Ar atmosphere. Preparation of simple mixture sample for control. The concentration of and MoS2 and MWNT S3

4 suspensions was set to 0.10 mg/ml and 0.40 mg/ml, respectively. Following the mass ratio of MoS2 to MWNT (1.5:1) determined by QCM of LbL assembled (MoS2/MWNT)n multilayers, 6.0 ml of MoS2 suspension was mixed with 1.0 ml of MWNT suspension to form the simple mixture sample. We prepared several drop-casted samples with different amount of loadings on polyaniline (PANi) deposited FTO-glass substrate, and dried in a vacuum at room temperature. We chose the sample that has the most similar thickness with (MoS2/MWNT)14 (confirmed by cross-section SEM image) for comparison. Electrochemical measurements. The electrochemical measurements were carried out in a threeelectrode electrochemical compression cell using a potentiostat (VSP, Bio-Logic Science Instruments). For electrochemical measurement, a 0.5 M H2SO4 solution, saturated calomel electrode (SCE), and Pt foil were utilized as the electrolyte, reference electrode, and counter electrode, respectively. Linear sweep voltammetry (LSV) was recorded at a scan rate of 10 mv/s to obtain the polarization curves. The long-term stability tests were performed by cyclic voltammogram (CV) with a scan rate of 50 mv/s. All the data presented were corrected after ir-correction to account for the Ohmic drop. Electrochemical impedance spectroscopy (EIS) measurement was performed at overpotential of 200 mv with frequency from 100 khz to 100 mhz and an amplitude of 10 mv. The electrochemical double-layer capacitance (EDLC) was determined from the CV curves measured in a potential range of V (vs. RHE) without Faradaic processes at a various scan rates from 10 to 160 mv/s. EDLC was calculated according to the following equation: ic = vcdl, where ic, v and Cdl are the charging current (ma/cm 2 ), scan rate (mv/s), and double-layer capacitance (F/cm 2 ) of the electroactive materials, respectively. S4

5 2. Characterizations The morphology of the MoS2 nanosheets was observed by AFM (Dimension AFM, Veeco). The amine-functionalized MWNTs were characterized with a Fourier-transform infrared (FT- IR) spectrometer (670-IR, Varian). Raman measurements (alpha300r, WITec) were conducted with a 532-nm laser (0.50 mw). The absorbance of the films was characterized using a UV-vis spectrophotometer (Cary 5000, VARIAN). Electron microscopy images of the multilayers on FTO-coated glass substrate were collected by SEM (Cold FE-SEM, Hitachi). The sheet resistances were measured by using a four-point probe (CMT-SR1000N, AIT). S5

6 3. Supplementary Table and Figures Table S1. Summary of all electrochemical catalytic parameters toward HER S6

7 Figure S1. AFM image and the corresponding line scan profiles of chemically exfoliated MoS2 nanosheets. Figure S2. FT-IR spectra of raw MWNTs (black) and amine-functionalized MWNT-NH2, denoted as MWNT(+) (green). S7

8 Figure S3. Zeta-potential distribution of MoS2 nanosheets and amine-functionalized MWNTs in their respective suspensions. The ph of the suspensions was adjusted to 5.0, which is the assembly condition for the (MoS2/MWNT)n multilayers. The measurement was performed three times for each suspension. Figure S4. Photograph of the control (simple mixture of MoS2 and MWNT) and assembled (MoS2/MWNT)n multilayers on FTO-coated glass substrate. Number of bilayers (n = 2 30 as labeled in the photograph. S8

9 Figure S5. (a) Raman spectra of (MoS2/MWNT)14 deposited on FTO-glass substrate (black), and amine-functionalized MWNTs (green) and MoS2 nanosheets (gray) deposited on Si/SiO2 substrate. The Raman characteristic peaks of (MoS2/MWNT)14 are compared to those of (b) MoS2 nanosheets and (c) MWNT-NH2. Figure S6. Polarization curves of (MoS2/MWNT)14 thin film electrode toward HER. (black) before and (red) after 200 cycles. Inset shows the current density changes measured at -0.3 V for every 5 cycle during the cycle stability test. S9

10 Figure S7. Nyquist plots of three different (MoS2/MWNT)n multilayered film electrodes (n = 4, 14, and 24 BL) measured in a frequency range of 100 khz to 100 mhz at overpotential of 200 mv. The orange box region is magnified and shown as the inset. Figure S8. Sheet resistance of (MoS2/MWNT)14 multilayers determined by four-point probe measurement, before and after annealing. Sheet resistance was measured in 7 different spots for each sample. S10

11 Figure S9. (a) Onset potential variation of (MoS2/MWNT)14 samples for HER, depending on the annealing conditions: no thermal treatment (black), 100 C (red), 200 C (blue), and 300 C (green). The onset potential was calculated by the general method, as the crossing point between the baseline current (orange dotted line) and the lines tangent to the polarization curve (blue, red, and black dotted lines). (b) Double layer capacitance (Cdl) measurements for determining the ECSA. CVs were measured in a potential range from -0.1 to 0.1 V (vs. RHE) at various scan rates from 10 to 160 mv/s. S11

12 Figure S10. SEM images of LbL-assembled (MoS2/MWNT)14 film electrode under different annealing conditions. (a) no treatment, (b) 100 C, (c) 200 C, and (d) 300 C for 1 h under Ar. 4. References S1. Joensen, P.; Frindt, R. F.; Morrison, S. R. Single-Layer MoS2. Mater. Res. Bull. 1986, 21, S2. Lee, S. W.; Kim, B.-S.; Chen, S.; Shao-Horn, Y.; Hammond, P. T. Layer-by-Layer Assembly of All Carbon Nanotube Ultrathin Films for Electrochemical Applications. J. Am. Chem. Soc. 2009, 131, S3. Stockton, W. B.; Rubner, M. F. Molecular-Level Processing of Conjugated Polymers. 4. Layer-by-Layer Manipulation of Polyaniline via Hydrogen-Bonding Interactions. Macromolecules 1997, 30, S4. Ahn, E.; Lee, T.; Gu, M.; Park, M.; Min, S. H.; Kim, B.-S. Layer-by-Layer Assembly for Graphene-Based Multilayer Nanocomposites: The Field Manual. Chem. Mater. 2017, 29, S12