Polymer Brushes Ionic Liquid as a Catalyst for Oxygen Reduction and Oxygen Evolution Reactions

Transcription

1 Supporting Information Polymer rushes Ionic Liquid as a Catalyst for Oxygen Reduction and Oxygen Evolution Reactions Thuan Nguyen Pham Truong, Hyacinthe Randriamahazaka*, Jalal Ghilane* Université Paris Diderot, Sorbonne Paris Cité, ITODYS, UMR 7086 CNRS, SIELE group, 15 rue Jean ntoine de aïf, Paris, France. * jalal.ghilane@univ-paris-diderot.fr, hyacinthe.randria@univ-paris-diderot.fr Methods Synthesis. N-methyl-N -vinylimidazolium bis(trifluoromethane)sulfonimide was synthetized as followed. Vinylimidazole (0.94 g, 10 mmol) was dissolved in 20 ml of acetonitrile. Then methyl iodide (1.87 ml, 30 mmol) was added into the solution under vigorous stirring. The mixture was then refluxed for 2 hrs. Finally, the solvent and un-reacted methyl iodide was eliminated under reduced pressure and the product was dried under vacuum for 24 hrs giving 90% yield. For getting the final monomer, anion exchange step was performed by addition of 1.2 equiv. of LiTFSI in 20 ml distilled water into the intermediate product. The mixture was heated at 70 C during 24 hrs under vigorous stirring. The organic layer was then recovered and washed 3 times with 20 ml of distilled water, the water traces were eliminated with MgSO 4 followed by drying under vacuum overnight. Chemicals. Dodecanethiol functionalized gold nanoparticles (2 % (w/v) in toluene, 3-5 nm particle size (TEM)), Platinum supported on graphitized carbon (Pt/C) extent of labeling: 20 wt. % Pt loading, is(trifluoromethane)sulfonimide lithium salt (> 99%), 1-ethyl-3-methylimidazolium ethylsulfate (> 95%), sodium borohydride (98 %), potassium hydroxide (> 99%) and Nafion 117 solution were purchased from Sigma ldrich. ll the chemical products were used as received without further purifications. Electrode preparation. For Poly(VImM) and Poly(VImMM) onto glassy carbon, carbon black dispersed in 5% Nafion 117/EtOH was added to ensure the conductivity (surface coverage density = 0.23 mg.cm -2 ). Pt/C based ink: 2.8 mg of Pt supported on graphitized carbon (Pt/C 20 wt. % loading) (< 5 nm), ECS = 60 m²/g, was dispersed in 0.5 ml of 5% Nafion 117/EtOH under sonication for 3 times 10 minutes.

2 Then 10 µl of as-prepared catalytic ink was drop-casted onto glassy carbon electrode or poly(vimm) modified GC electrode. For the catalyst Pt/C 20 wt%, the Pt loading in the prepared film was measured to be 40 µg.cm -2. The film was then dried in an oven at 50 C for 1 hour before use. u NPs ink: 100 µl of suspension of u NPs in toluene was mixed with 250 µl of absolute ethanol followed by addition of 15 µl of Nafion 117 solution. The as-prepared mixture was sonicated for 3 times ten-minutes. 10 µl of the solution was drop casted onto glassy carbon electrode or poly(vimm) modified GC electrode. The film was dried in an oven at 50 C for 1 hour before use. Electrochemical measurements For catalytic performance evaluations, a E7R9 ThinGap Fixed-Disk RRDE tips with 320 µm gap ( GC disk = cm 2, Pt ring = cm 2 ) was used as working electrode, a stainless-steel mesh and saturated calomel electrode were used as counter electrode and reference electrode, respectively. In all experiments, the potential was converted to Reversible Hydrogen Electrode (RHE) via the following equation: E (vs RHE) = E (vs SCE) ph was used for the electrochemical measurements which are controlled with a CHI920C bipotentiostat (CH Instruments, ustin, Texas). For oxygen reduction and evolution reaction measurements, 0.1 M KOH solution was used as electrolyte and bubbled with purified oxygen gas for 30 minutes. During the measurements, the solution was kept under O 2 atmosphere. Instrumentation. For electrochemical measurements, a rotating ring disk setup was used. saturated calomel electrode and a stainless steel mesh were used as reference electrode and counter electrode, respectively. rotating ring disk electrode (E7R9 series) from Pine Research Instruments was used for the electrochemical measurements. The electrochemical tests including cyclic voltammetry, linear sweep voltammetry (LSV), electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) were performed using CHI920C bipotentiostat (CH Instruments, ustin, Texas). XPS measurements of the modified electrodes were performed using a Thermo VG Scientific ESCL 250 system fitted with a microfocused, monochromatic l Kα (hν = ev) 200 W X-ray source. The FM investigations were performed in tapping mode at room temperature with a Molecular Imaging PicoPlus. Prepartion of polymer ionic liquid.

3 The polymer brushes were obtained using Surface-Initiated tom Transfer Radical Polymerization (SI-TRP). The initiator was attached electrochemically into electrode surface as shown in the Fig.S1. y sweeping the potential to the anodic side, an irreversible peak was observed at 1.3 V vs. SCE attributed to the oxidation of amine giving radical cation which can attach to the electrode surface. The current intensity decreases dramatically after few cycles corresponding to the attachment of a thin layer of initiator at the electrode surface. 3 1st cycle E 2 1 l e c t r o d e 2nd cycle Potential (V) vs SCE Figure S1. Cyclic voltammetry recorded on GC in the presence of 1 mm bromo-ethylamine in 0.1 M u 4 NF 4 acetonitrile solution. Scan rate 0.1 V/s. The polymerization process (Fig.S2) was performed by dipping the initiated electrode into acetonitrile solution containing 3 mm of monomer, 4 µm of CuCl as catalyst, 2.8 µm of CuCl 2 and 5 µl of N,N,N,N,N -Pentamethyldiethylenetriamine (PMDET) as complexing agent under r flux. The reaction was maintained at 70 C for 2 hrs. Then the modified electrode was removed from the reaction medium, washed thoroughly with acetonitrile, acetone, and was sonicated in distilled water for 5 minutes, respectively. Figure S2 Scheme illustrating the principle of SI-TRP polymerization using vinylimidazolium monomer.

4 Surface characterizations. Figure S3. a, SEM image of poly(vimm) onto u substrate. b and c, FM topography and 3D FM image of poly(vimm), image size 4 4 µm 2. d, Cross section after FM scratch experiment. dditional electrochemical investigations. The Koutecky-Levich plot give information not only for the electron transfer number at each potential but also the kinetic current. The Koutechky-Levich equations are givens below: = + = + = 0.2nFν D C = n 1/6 3/2 1/2 O2 j jk jlev ω. where j, j k and j lev are the measured current density, the kinetic current density and diffusion controlled current density. The diffusion limited current is calculated by the Faraday constant (F), the kinetic viscosity of the medium (υ), the oxygen diffusion coefficient (D), the oxygen concentration (C O2 ) and the rotation speed of the electrode (ω) and the constant 0.2 is adopted when rotation speed is expressed in rounds per minute (rpm). The kinetic current is

5 determined by extrapolating the Koutecky-Levich plot (j -1/2 vs ω -1/2 ) at various rotation speeds and could be determined from the slope of K-L plot. The electron transfer number and the percentage of the hydrogen peroxide can be calculated using the equations: 4ID ne = I + I / N D R % HO * IR / N = I + I / N D R where I D is the faradic disk current, I R is the corresponding faradic ring current and N = 0.37 is the collecting efficiency are GC % H 2 O are GC b = 78 mv/dec b = 97 mv/dec Figure S4., Variation of the hydrogene peroxide percentage at bare GC and poly(vimm) as function of the potential obtained from RRDE plots in O 2 -saturated 0.1 M KOH solution., Tafel plots for bare GC and poly(vimm).

6 % H 2 O GC/Poly(VImMM)/C are GC Electron transfer number are GC GC/Poly(VImMM)/C Figure S5., Variation of the hydrogene peroxide percentage at bare GC, poly(vimm) and poly(vimmm) in O 2 -saturated 0.1 M KOH solution as function of the potential., Evolution of electron transfer number as function of the potential for different catalysts Pt/C - Poly(VImM) Pt/C b = 54 mv/dec b = 59 mv/dec Figure S6. Tafel plots for Pt/C and Pt/C/poly(VImM) catalysts for ORR in O 2 -saturated 0.1 M KOH solution.

7 u NPs/GC u NPs/Poly(VImM)/GC Electron transfer number u NPs/GC u NPs/Poly(VImM)/GC Potential (V) vs RHE Potential (V) vs RHE Figure S7.. ORR polarization curves using RRDE at rotation 1600 r.p.m. with 10 mv/s scan rate for unps/gc (black line) and unps/poly(vimm)/gc (red line) in O 2 -saturated 0.1 M KOH solution.. Evolution of electron transfer number as function of the potential for unps/gc and unps/poly(vimm)/gc catalysts. % H 2 O Pt/C Pt/C - Poly(VImM) Electron transfer number Pt/C Pt/C - Poly(VImM) Figure S8., Variation of the hydrogene peroxide percentage at Pt/C and Pt/C/poly(VImM) catalysts in O 2 -saturated 0.1 M HClO 4 solution as function of the potential., Evolution of electron transfer number as function of the potential.