Zhengping Zhang, Junting Sun, Meiling Dou, Jing Ji, Feng Wang*

Transcription

1 Supporting Information Nitrogen and Phosphorus Codoped Mesoporous Carbon Derived from Polypyrrole as Superior Metal-Free Electrocatalyst towards the Oxygen Reduction Reaction Zhengping Zhang, Junting Sun, Meiling Dou, Jing Ji, Feng Wang* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Electrochemical Process and Technology for Materials, Beijing University of Chemical Technology, Beijing , China. Table of Contents 1. General methods (materials, preparation and electrochemical measurements) 2. Characterization of polymer precursors and carbon samples 3. Electrochemical measurements of the as-prepared samples

2 1. General methods Materials: Pyrrole, phytic acid, styrene, FeCl 3, ammonium persulphate, acetone, methanol, ethanol, sodium dodecyl sulfate (SDS), potassium persulfate (KPS), NaOH and KOH were purchased from Sinopharm. 5% Nafion was bought from DuPont. High-purity argon gas was obtained from Beijing AP BAIF Gases Industry Co., Ltd. All chemicals were of analytical grade and applied without further purification. Distilled ultrapure water was used for all solution preparation and cleaning products. Preparation of polystyrene (PS) microsphere: 12 ml styrene and 0.5 g SDS were dissolved in 250 ml distilled water, and then stirred at room temperature for 2 h. Then the aqueous solution was heated to 70 C and slowly added 30 ml KPS solution (0.1 g KPS / 30 ml H 2 O) with vigorous stirring. The polymerization was allowed to proceed for 8 h. All steps were performed under N 2 atmosphere. Then the product was washed with water and ethanol and dried in a vacuum at 40 C for 12 h. Characterizations The morphology was observed by field emission scanning electron microscopy (SEM, JSM- 6701F) and transmission electron microscopy (TEM, JEM-2010) obtained by JEOL Co., Ltd. The specific surface areas and pore size distributions were obtained and calculated on Quadrasorb SI (Quantachrome Instruments). X-ray diffraction (XRD) patterns and Raman spectra were performed on Rigaku D/Max 2500 VB2+/PC (Rigaku Co., Ltd., with Cu Kα radiation, λ = Å), and LabRam HR800 (HORIBA Co., Ltd., with 532 nm laser). The surface chemical structures were carried out on X-ray photoelectron spectroscopy (XPS) by ESCALAB 250 (Thermo Fisher Scientific, with Al Kα = 150W). The electrical conductivity was measured with a Nanometrics HL5550 cryostat based on the standard four-probe technique developed by Van der Pauw at a load pressure of 10 MPa. Electrochemical measurement Electrochemical measurements were obtained with Rotating Ring Disk Electrode Apparatus (RRDE-3A) directly controlled by Model 2323, ALS workstation (BAS INC) and CHI760E electrochemical workstation (CH Instruments). All electrochemical measurements were carried S-1

3 out at room temperature with a normal three-electrode system: a potassium chloride saturated calomel reference electrode (SCE) as the reference electrode, a Pt wire counter electrode as the counter electrode and the modified glassy carbon (GC, d = 40 mm) as the working electrode. 10 mg of the electrocatalyst was dispersed with a mixture of 2 ml ethanol and 5 µl 5 % Nafion solutions before sonicated. The catalyst ink was transferred onto the polished GC electrode and dried for 20 min at room temperature. For comparison, commercial 20 wt% Pt/C (Johnson Matthey, JM) catalyst obtained from Alfa Aesar was measured. The loading mass of all samples on the working electrode is 0.2 mg cm -2. Rotating disk electrode (RDE) measurements: The glassy carbon rotating disk electrode is 4 mm in diameter. The Koutecky Levich (K-L) plots reflecting the relation of I 1 vs.ω 1/2 were constructed according to: I = i +(0.2nFC D γ ) ω 1 1 2/3 1/6 1 1/2 k O2 O2 where I is the measured current, i k is the kinetic-limiting current, n is the number of electrons transferred per oxygen molecule, F is the Faraday constant (96,500 C mol -1 ), concentration of oxygen in 0.1 M KOH, C O 2 is the D O 2 is the diffusion coefficient of oxygen in 0.1 M KOH, γ is the kinematic viscosity of the 0.1 M KOH and ω is the electrode rotation rate. For the Tafel plot, the kinetic current was calculated from the mass-transport correction of RDE by: J k = J JL J J L Rotating ring-disk electrode (RRDE) measurements: The disk electrode was scanned catholically at a rate of 5 mv s 1 and the ring potential was constant at 1.5 V vs. RHE. The electron transfer number (n) was determined by the followed equation: Id n= 4 I + I / N d r S-2

4 where I d is disk current, I r is ring current and N = 0.4 is the current collection efficiency of the Pt ring. The peroxide percentage (% HO - 2 ) was calculated based on the equation: I r / N % HO2 = 200 I + I / N Enhancement factor: The enhancement factor to the N-C sample contains two parts. First, the content of active sites (C sites, including pyridinic N and P-C bond) is divided by the content of pyridinic N in N-C. Second, the BET surface area (S BET ) of other samples is divided by that of N-C. Herein, we hypothesize the enhancements of two parts have the same contribution to the final results. Therefore, the sum of two parts is directly divided by 2. The formula is shown as below: d r Enhancement Fact Csites SBET = ( + ) / 2 C ( N-C) S ( N-C) sites BET S-3

5 2. Characterization of polymer precursors and carbon samples Figure S1 UV spectra of PA, PPy, PA-PPy, PPy/PS and PA-PPy/PS. Figure S2 SEM image of the PS spheres. S-4

6 Figure S3 SEM images of the a) N-C, b) N,P-C and c) N-MC. d) TEM image of N-MC. Figure S4 a) XPS survey spectra of N-C, N,P-C, N-MC and N,P-MC. b) High-resolution XPS scans of Fe 2p for N,P-MC. S-5

7 Figure S5 a) High-resolution XPS scans of N 1s for N-C, N,P-C, N-MC and N,P-MC. b) Highresolution XPS scans of P 2p for N,P-C and N,P-MC. Table S1 Elements contents derived from XPS survey spectra. C (at.%) O (at.%) N (at.%) P (at.%) N-C N,P-C N-MC N,P-MC Table S2 The relative contents of different N-sites derived from high-resolution XPS spectra of N 1s. Pyridinic N (%) Pyrrolic N (%) Graphitic N (%) N-C N,P-C N-MC N,P-MC Table S3 The relative contents of two P-sites derived from high-resolution XPS spectra of P 2p. P-C (%) P-O (%) N,P-C N,P-MC S-6

8 3. Electrochemical measurements of the as-prepared samples Figure S6 Cyclic voltammograms covering the capacitive potential interval as a function of potential sweep rate for a) N-C, b) N,P-C, c) N-MC and d) N,P-MC. Figure S7 Linear fitting of current density ( J, at a potential of 1.1 V vs. RHE) versus scan rate used to estimate the double-layer capacitance (C dl ) for N-C and N,P-C. S-7

9 Figure S8 a) LSV curves of N-C and N,P-C, derived from different additional amounts of 5 wt% PA aqueous solution in O 2 -saturated 0.1 M KOH solution at a sweep rate of 5 mv s -1 with 1600 rpm. The N,P-C sample with the addition of 1.0 ml PA solution was designed as N,P-C in the text and used for other analysis. b) LSV curves of N,P-C by different temperature pyrolysis treatment. The optimized pyrolysis temperature is about 900 C, and hence only the N,P-MC was subsequently prepared in the condition of pyrolysis treatment. Figure S9 a) LSV curves of N-C, N,P-C, N-MC, N,P-MC and 20%Pt/C for ORR in O 2 -saturated 0.1 M KOH solution at a sweep rate of 5 mv s -1 with 1600 rpm. b) LSV curves for the electrocatalytic systems of 20% Pt/C for ORR in O 2 -saturated 0.1 M KOH solution scanned at 5 mv s -1 at rpm. The inset showed the Koutecky-Levich plots. S-8

10 Figure S10 LSV curves of N-C, N,P-C, N-MC, N,P-MC and 20% Pt/C for ORR in O 2 -saturated 0.1 M HClO 4 solution at a sweep rate of 5 mv s -1 with 1600 rpm. Table S4 The electrical conductivities of N-C, N,P-C, N-MC and N,P-MC. Catalyst N-C N,P-C N-MC N,P-MC Electrical conductivity (S m -1 ) ca. 325 ca. 690 ca. 320 ca. 690 Table S5 Comparison of ORR performance for N,P-MC with the other metal-free catalysts. Catalyst Loading mass (µg cm -2 ) Electron transfer number (n) E 1/2 (V vs. RHE) Current density (ma cm -2 ) at 0.85 V Ref. N,P-MC This work NPMC ca ca. 2.0 [1] Meso/micro-PoPD ca. 2.6 [2] NCNT [3] S-9

11 N-CNS [4] C-COP [5] N-Graphene [6] References: [1] Zhang, J.; Zhao, Z.; Xia, Z.; Dai, L. A Metal-Free Bifunctional Electrocatalyst for Oxygen Reduction and Oxygen Evolution Reactions. Nat. Nanotechnol. 2015, 10, [2] Liang, H. W.; Zhuang, X.; Bruller, S.; Feng, X.; Mullen, K. Hierarchically Porous Carbons with Optimized Nitrogen Doping as Highly Active Electrocatalysts for Oxygen Reduction. Nat. Commun. 2014, 5, [3] Deng, H.; Li, Q.; Liu, J.; Wang, F. Active Sites for Oxygen Reduction Reaction On Nitrogen- Doped Carbon Nanotubes Derived from Polyaniline. Carbon 2017, 112, [4] Yu, H.; Shang, L.; Bian, T.; Shi, R.; Waterhouse, G. I.; Zhao, Y.; Zhou, C.; Wu, L. Z.; Tung, C. H.; Zhang, T. Nitrogen-Doped Porous Carbon Nanosheets Templated from g-c3 N4 as Metal- Free Electrocatalysts for Efficient Oxygen Reduction Reaction. Adv. Mater. 2016, 28, [5] Xiang, Z.; Cao, D.; Huang, L.; Shui, J.; Wang, M.; Dai, L. Nitrogen-Doped Holey Graphitic Carbon from 2D Covalent Organic Polymers for Oxygen Reduction. Adv. Mater. 2014, 26, [6] Li, J.; Zhang, Y.; Zhang, X.; et al. Direct Transformation from Graphitic C3N4 to Nitrogen- Doped Graphene: An Efficient Metal-Free Electrocatalyst for Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces, 2015, 7, S-10