www.sciencemag.org/content/351/6271/361/suppl/dc1 Supplementary Materials for Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts Donghui Guo, Riku Shibuya, Chisato Akiba, Shunsuke Saji, Takahiro Kondo,* Junji Nakamura* *Corresponding author. E-mail: takahiro@ims.tsukuba.ac.jp (T.K.); nakamura@ims.tsukuba.ac.jp (J.N.) This PDF file includes: Materials and Methods Supplementary Text Figs. S1 to S10 Table S1 Published 22 January 2016, Science 351, 361 (2016) DOI: 10.1126/science.aad0832
Materials and Methods Preparation of grap-hopg. The HOPG samples (ca. 6 mm 6 mm) were cleaned by annealing at 1073 K under ultra-high vacuum (UHV) for 15 min. Nitrogen doping was performed by mild bombardment with a nitrogen ion beam (50 ev) for 5 sec at ~600 K (total ion emission current: 0.5 ma), followed by annealing at 973 K for 10 min to clean and well-graphitize the surface. Preparation of edge-hopg. edge-hopg samples were prepared by bombarding the sample with an Ar + ion beam through a thin metallic mask. The preparation was illustrated in Fig. S1 and the principle procedure is as follows: i) etching the HOPG surface with an Ar + beam through a slit-patterned Ni mask to form a pattern on the surface with a large density of edges; ii) acid treatment with HNO 3 solution (1 M) to remove Ni impurities; and iii) annealing in dry air at 818 K to oxidize out carbon clusters formed during Ar + bombardments. The slit-patterned Ni mask contained rectangular slits (25 µm 250 µm) with spacings of 50 µm and 100 µm (optical image of edge-hopg in Fig. 1a directly reveals the structure of the mask). During etching, the slit-patterned Ni mask was fixed by a stainless mask with a hole (about 4 mm in diameter) to obtain good contact between the slit-patterned Ni mask and the HOPG sample (the detailed procedures are shown in Fig. S1). Preparation of pyri-hopg. pyri-hopg samples were prepared by exposing edge-hopg samples to NH 3 (flow rate: 60 cm 3 /min) in a silica tube heater at 973 K (see Fig. S1). Preparation of N-GNS catalysts. The graphene nano sheets (GNS) were prepared by reducing graphene oxide (GO) via the conventional Hummer s method followed by ultrasonic exfoliation. Nitrogen atoms were doped into GNS by a treatment with NH 3 at 973 K, in which nitrogen atoms were preferentially doped at the sites containing oxygen functional groups by so-called ammoxidation. Thus, the nitrogen concentration can be controlled by the amount of oxygen functional groups remaining in GNS. We used NaBH 4 as a weak reductant and N 2 H 4 as a strong reductant for the reduction of GO to GNS. The weak reduction leads to a large amount of oxygen functional groups, which is favorable for N doping, resulting in a higher nitrogen concentration. XPS analysis. 2
The binding energy of the XPS spectrum was calibrated based on the peak energy positions of standard samples (Au 4f 7/2 (83.95 ev), Ag 3d 5/2 (368.26 ev), and Cu 2p 3/2 (932.65 ev)) measured mostly using Al-K X-rays. For C 1s and N 1s narrow scans, the pass energy was set to 10 ev. The N 1s spectrum was fitted by combination of the peak components represented by the Gaussian-Lorentzian function after Shirley-type background subtraction. The full-width-at-half-maximum for each type of nitrogen component used for fitting was fixed to 2.10 ev because the best fitting results were obtained by selecting this width. For post-reaction XPS, the sample was left under UHV for 12 hr for degassing and then measured. ORR measurements. Due to the bulk HOPG with a rectangular/square shape was utilized here, the sample could not be deposited or bonded on the electrode for ORR measurement as used in the conventional rotating disk method. The HOPG model catalysts were suspended in an electrochemical cell by a metal wire without using a disc electrode. The setup for ORR measurements is illustrated in the Fig. S3. The ORR activities of the N-GNS powder catalysts were evaluated via the conventional rotating disc method (rotating rate: 300 rpm) in three electrodes cell. The reference electrode is Pt with saturated H 2 in 0.1 M H 2 SO 4. The working electrode for the HOPG model catalyst is the HOPG sample itself. The working electrode for the N-GNS powder catalysts is glassy carbon (GC) with a diameter of 0.6 cm. The loading amount of N-GNS is 0.02 mg. The counter electrode is a Pt wire facing the working electrode. Electrochemical measurements were conducted from 1.1 V to 0.05 V with a scanning rate of 0.005 V/s and a rotating rate of 300 rpm. Electrochemical measurements for the HOPG model and N-GNS powder catalysts were first conducted under N 2 -saturated 0.1 M H 2 SO 4 solutions and then under O 2 -saturated solutions. ORR curve is acquired under oxygen-saturated conditions with subtraction of the data under nitrogen-saturated conditions as a background, in which the currents are divided by the electrode surface area. Exposed surface areas of N-HOPG and their ORR current densities. Each of the HOPG model catalysts is fixed to a rod and covered by an epoxy paste; hence, the N-HOPG surface is partially covered with the epoxy paste. That is, except for the catalyst surface, the epoxy paste covers the entire electrode. We then individually measured the exposed surface area of the N-HOPG by a ruler and software (ImageJ) as shown in fig. S3 (G H). By measuring the number of the pixel points within the exposed surface of HOPG using software (ImageJ) by calibration using a ruler in the same picture, we estimated the exposed catalyst surface area (loading): 3
S electrode (cm 2 ) = (number of pixels in the exposed catalyst surface)/(number of pixels in 1 cm 2 ) Then the ORR current density for the HOPG model is obtained from j ORR = (current in O 2 current in N 2 )/S electrode Evaluation of BET surface areas of N-GNS on electrodes. The catalyst surface area (exposed surface area) of HOPG electrode was only 0.1 cm 2, as same as the geometric surface area of the electrode. In contrast, the BET surface area of the N-GNS sample is about 80 cm 2 (400 m 2 /g 10 4 cm 2 /m 2 0.02 mg 10-3 g/mg, estimated from BET) on an GC electrode with the geometric surface area of 0.283 cm 2, where the loading amount of N-GNS is always 0.02 mg. Post-ORR XPS measurements. For post-orr XPS measurements, the HOPG experienced three cycles of electrochemical ORR measurements (1.1 V 0.05 V 1.1 V as one cycle) in a 0.1 M H 2 SO 4 solution. After the ORR measurement at 1.1 V, the HOPG was removed from the electrochemical cell and was washed using distilled water and dried. Then, the sample was rapidly transferred to an ultrahigh vacuum XPS chamber for degassing for 24 h, followed by XPS measurements. Supplementary Text Absence of diffusion-limited state of ORR for HOPG models. In ORR experiments using the HOPG model catalysts, the diffusion-limited currents were never observed, even at 0 V vs. RHE. This is attributed to the very small amount of catalytic active species (pyridinic nitrogen) on the electrode, which originates from the very low catalyst surface area of HOPG electrode (only about 0.1 cm 2, Fig. S3). This is in stark contrast with the N-GNS BET surface area of about 80 cm 2 (400 m 2 /g 10 4 cm 2 /m 2 0.02 mg 10-3 g/mg) on the electrode (0.283 cm 2 ). Calculation of specific activity per pyridinic N. The ORR activities of the nitrogen doped HOPG model catalysts in Fig. 2 were compared with those of N-GNS powder samples in Fig. 4. The ORR activities of the model catalysts were apparently lower than those of the N-GNS powder samples, even though the nitrogen concentrations and species configuration were similar. This difference can be attributed to differences in the surface area of the catalysts. Thus the specific activities per pyridinic N were compared. Herein, the activities at 0.5 V vs. RHE were compared because under this potential the oxygen-limited phenomenon is not significant for the N-GNS powder catalysts and the oxygen reduction reaction on the HOPG model catalysts can be detected. 4
The specific activity per pyridinic N was calculated from the total number of electrons in the ORR per second divided by the total atomic number of pyridinic N per cm 2 of the electrode surface. For the N-HOPG model and N-GNS powder catalysts, the number of electrons per second was calculated from the ORR current (current densities multiplied by electrode surface area). The number of pyridinic N in the N-HOPG model catalysts was calculated from the atomic ratio of pyridinic N (from XPS) multiplied by the carbon atomic density of the graphene/graphite surface (3.82 10 15 atoms/cm 2 ) and the electrode surface area. The number of pyridinic N in the N-GNS powder catalysts was also calculated from the atomic ratio of pyridinic N, the carbon atomic density of graphene, and the GNS surface area by further taking into account the following conditions: i) the loading of N-GNS powder (0.02 mg) on the disc electrode (surface area: 0.283 cm 2 ), ii) the effective surface area of 400 m 2 g -1 (estimated by BET method). The detailed calculations are as follows: 5
Calculations of specific activity per pyridinic N (pyrin) Activity per pyrin = number of electrons per sec per cm2 number of pyrin per cm 2 (1) For HOPG model catalysts: Activity per pyrin = j ORR( ma cm 2) 0.001( A ma ) N eper coulomb per second ( e A s ) D atom in graphene ( atoms cm 2 ) ρ pyrin(at%) (2) For N-GNS powder catalysts: = j ORR( ma cm 2) 0.001( A ma ) 6.24 1018 ( e A s ) 3.82 10 15 ( atoms cm 2 ) ρ pyrin(at%) Activity per pyrin = j ORR( ma cm 2) 0.001( A ma ) N eper coulomb per second ( e A s ) = W GNS ( g cm 2) S BET( m2 g ) D atom( atoms cm 2 ) ρ pyrin(at%) j ORR ( ma A cm2) 0.001( ma ) 6.24 1018 ( e A s ) 0.0707 10 3 ( g cm 2) S BET( m2 g ) D atom( atoms cm 2 ) ρ pyrin(at%) Here, j ORR is the current density of ORR, N e is a constant indicating electron numbers in 1 Coulomb, W GNS is the loading amount of the N-GNS catalyst per 1 cm 2 of geometric surface of electrode, S BET is the BET surface area of N-GNS, D atom is a constant of carbon atomic density in each graphite layer (3.82 10 15 atoms/cm 2 ), and ρ pyrin is the atomic percentage of pyridinic N in the sample. For example, for N-GNS-1, Activity per pyrin = j ORR( ma cm 2) 0.001( A ma ) N eper coulomb per second ( e A s ) = = W GNS ( g cm 2) S BET( m2 g ) D atom( atoms cm 2 ) ρ pyrin(at%) 0.085 ( ma cm 2) 0.001( A ma ) 6.24 1018 ( e A s ) 0.0707355 10 3 ( g cm 2) 400 (m2 g ) 104 ( cm2 m 2 ) 3.82 1015 0.729 0.01 0.085 6.24 10 15 2.702 10 15 400 0.729 0.01 e- s -1 pyri-n -1 (3) (4) (5) = 5.304 1014 7.879 10 15 = 0.673 e- s -1 pyri-n -1 6
Supplementary Figures Fig. S1. Schematic illustration for the preparation of edge-hopg and pyri-hopg as model catalysts. The edge-patterned HOPG model catalyst (edge-hopg) was prepared by bombarding the sample with an Ar + ion beam through a thin metallic mask. The details of the procedure are as follows: (A) the Ni mask was placed on top of the HOPG sample; (B) a stainless hole mask (hole size: ca. 4 mm in diameter) was placed over the Ni mask to hold the latter; (C) Ar + bombardment was performed at 500 ev under ultra-high vacuum (UHV) for about 2 h and hole patterns were etched in the surface; (D) the sample was washed with HNO 3 solution to remove Ni impurities; (E) the sample was annealed in dry air at 818 K for 2 h to oxidize out the carbon clusters produced during Ar + bombardment; (F the sample was annealed in NH 3 at about 973 K for 3 h for nitrogen doping; (G) the sample was allowed to cool spontaneously in NH 3 and the atmosphere was changed to N 2 at room temperature. 7
Intensity (a. u.) Intensity (a.u.) survey pyri-hopg C 1s grap-hopg edge-hopg clean-hopg 800 600 400 200 0 288 286 284 282 Fig. S2. XPS spectra of survey and C 1s for HOPG model catalysts corresponding to Fig. 1E. The O 1s peak appeared around 531 ev and a ghost peak appeared in spectrum of edge-hopg or grap-hopg at 517.6 ev, which may be originated from X-ray source. ( hν Al kα (1486.6 ev) hν Mg kα (1253.6 ev) + BE of main peak (284.6 ev) ). 8
From (H), electrode surface area (S ) can be obtained electrode S = 48570/625 2 = 0.124 (cm 2 ) electrode Fig. S3. Preparation and setup of HOPG model catalysts electrode for ORR measurements. The sample was first spot-pasted with a thin layer of conductive paste (dotite) at the edge or corner (A). The sample was then bonded with metallic wire using the dotite (B and C). After leaving the sample for about 24 h for thorough drying, all non-catalytic surfaces were protected by epoxy bond, including the sample edge, back-side, as well as the metallic bonded part (D), and dried for 24 h (E). The prepared sample faced downwards to the counter electrode (Pt wire) as a working electrode to replace the conventional rotating disk (F). (G and H) electrode picture and the calculation of the exposed surface of HOPG model catalyst. 9
A pyri-n: 0.0 at% grap-n: 0.0 at% E pyri-n: 2.21 at% grap-n: 2.05 at% B pyri-n: 0.04 at% grap-n: 0.26 at% F pyri-n: 3.11 at% grap-n: 1.83 at% C pyri-n: 0.04 at% grap-n: 0.60 at% G pyri-n: 3.86 at% grap-n: 8.24 at% D pyri-n: 0.57 at% grap-n: 0.03 at% H pyri-n: 6.51 at% grap-n: 3.39 at% Fig. S4. XPS spectra of N 1s for the model catalysts used for the ORR measurements in Fig. 2. Nitrogen concentration was controlled from 0.0 at% to approximately 13 at%, as shown in the figure (A-H) for the HOPG samples in Fig. 2A (1-8). 10
Intensity (a. u.) Intensity (a. u.) A Corresponding to Fig. 2 8 B Corresponding to Fig. 2 7 6 5 8 7 6 4 3 2 1 5 4 3 2 1 800 600 400 200 0 292 288 284 280 Fig. S5. XPS spectra of survey spectra and C 1s for model catalyst used for to ORR measurement in Fig. 2. Nitrogen concentration was controlled from 0.0 t% to about 13 at% as seen in the figure for samples (1-8). 11
0.00 Fig. S6. Relationship between ORR performances (in Fig. 2A) with respect to concentration of graphitic N in HOPG model catalysts. Pearson s r values were -0.58972, -0.58972, and -0.59606 Intensity (arb. unit) for the current density (j) at 0.2 V, 0.3 V, and 0.4 V vs. concentration of graphitic N plots, respectively. Current density/macm -2-0.02-0.04-0.06-0.08-0.10 0 j at 0.2V j at 0.3V j at 0.4V 2 4 6 Graphitic N (at%) 8 C 1s Before ORR After ORR 292 290 288 286 284 282 280 278 Fig. S7. Evolution of C 1s XPS profile of pyri-hopg model catalyst induced by ORR measurement, corresponding to Fig. 3A. After the ORR, a shoulder appeared at the high binding energy side in the C 1s spectrum compared with the spectrum acquired before the ORR. The shoulder corresponds to an oxidized state of carbon C 1s. 12
A N 1s Before H 2 SO 4 B After H 2 SO 4 Fig. S8. Effect of immersion in H 2 SO 4 solution on N 1s. To confirm the ORR-induced change in the N 1s spectra, the effect of immersion in H 2 SO 4 solution on N 1s was evaluated for the N-HOPG model catalyst. The N 1s spectra show no significant change. The intensity of the peak at about 401.1 ev did not increase but slightly decreased with respect to that of pyridinic N, contradictive to the effect of ORR, which indicates the change in the N1s spectra observed in Fig. 3A (increase in the 401.1 ev peak intensity) is indeed induced by the ORR. 13
Current density (macm -2 ) Intensity (arb. units) N 1s D C pyri-hopg (2.4 at%) B grap-hopg (0.8 at%) A edge-hopg clean-hopg 405 400 395 390 Fig. S9. XPS N1s profile for HOPG model catalyst used for CO 2 -TPD measurement. The N concentrations for (A) to (D) are: 0.0 at%, 0.0 at%, 0.8 at% (grap:pyri = 3:1), and 2.4 at% (pyri:grap = 1.7:1), respectively. 0.0-0.4-0.8-1.2-1.6 j at 0.7 V j at 0.6 V j at 0.5 V 0.0 0.2 0.4 0.6 Graphitic N (at %) 0.8 1.0 Fig. S10. ORR activity with respect to concentration of graphitic N in N-GNS catalysts. 14
Table S1. Comparison of specific activities at different potential. specific activity at 0.5 V (e - s -1 pyri-n -1 ) specific activity at 0.6 V (e - s -1 pyri-n -1 ) N-HOPG4 0.12 N-HOPG5 0.066 0.034 N-HOPG6 0.082 0.042 N-HOPG7 0.11 0.037 N-HOPG8 0.14 0.067 N-GNS-1 0.07 0.021 N-GNS-2 0.11 0.043 N-GNS-3 0.13 0.088 15