Electronic Supplementary Material A Co-N/C hollow-sphere electrocatalyst derived from a metanilic CoAl layered double hydroxide for the oxygen reduction reaction, and its active sites in various ph media Jun Wang, Liqun Li, Xu Chen, Yanluo Lu, Wensheng Yang ( ), and Xue Duan State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China Supporting information to DOI 10.1007/s12274-017-1455-0 The XRD pattern (Fig. S1(a)) shows the characteristic series of basal and higher order (00l) reflections at low angles with a layer distance (d 003 ) of 1.47 nm. The size of the gallery height of 0.99 nm, obtained by subtracting the crystallographic thickness of the LDH layer of 0.48 nm from the interlayer distance of 1.47 nm, was much longer than a single perpendicular anion height of 0.64 nm but shorter than its double value. This may indicate that the guest anions are accommodated with an interpenetrating arrangement, leaving interstitial gaps between the monomers and hydroxide sheets. Water molecules and nitrate anions occupy these interstitial spaces. The FT-IR spectrum (Fig. S1(b)) of CoAl-metanilic-LDHs shows four strong bands at 1,601, 1,509, 1,475 and 1,451 cm 1 and a weak band at 1,315 cm 1, which are characteristic absorptions of metanilic anions. Moreover, the broad band at 1,174 cm 1 was attributed to the presence of para-substituted aromatic C H ( in plane) and the aromatic C N ( ). The bands at 1,110 and 1,038 cm 1 were characteristic absorptions of the symmetric vibration of S C and S=O, respectively. The absorption band at 1,382 cm 1 is due to the stretching vibration of NO 3, which further confirm that the CoAl-metanilic-LDHs co-intercalated with metanilic anions and NO 3 have been successfully synthesized. Figure S1 (a) XRD pattern and (b) FT-IR spectrum of CoAl-metanilic-LDHs. Address correspondence to yangws@mail.buct.edu.cn
Nano Res. Figure S2 (a) and (b) SEM images of CoAl-metanilic-LDHs. As shown in Fig. S3, the XRD pattern indicated the Co-ions in the LDH layers have been converted to Co metal and Co9S8 after the calcination at 900 C under a N2 atmosphere. Figure S3 XRD pattern of c-ldh. Figure S4 (a) and (b) SEM images of c-ldh. (c) TEM and (d) HRTEM images of c-ldh. www.editorialmanager.com/nare/default.asp
Figure S5 Scanning transmission electron microscopy image and C-, S-, N- and Co-elemental mappings for Co-N/C composite. Figure S6 XRD pattern of Co-N/C composite. Figure S7 Raman spectrum of of Co-N/C composite. The cycling was carried out with CV within a potential range of 0.6 to 1.0 V in O 2 -saturated 0.1 M KOH, 0.1 M PBS and 0.1 M HClO 4 solutions, respectively. After 3,000 continuous cycles, there is no significant change of CV (Figs. S9(a) S9(c)) and LSV curves in the half-wave potential (Figs. S9(e) S9(g)), suggesting the high durability of the Co-N/C catalyst. Moreover, we recorded the CV (Figs. S9(a) S9(c)) and LSV (Figs. S9(g) S9(i)) www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
of Co-N/C in O 2 -saturated electrolytes before and after the addition of 3 M methanol. It is clearly observed that no obvious performance decay, indicating Co-N/C exhibits a strong tolerance against crossover effect. Figure S8 (a) (c) LSV curves of Co-N/C at different rotation speeds from 400 to 2,500 rpm in O 2 -saturated 0.1 M KOH, 0.1 M PBS (ph = 7) and 0.1 M HClO 4 solutions. Figure S9 (a) (c) RRDE curves of Co-N/C in O 2 -saturated 0.1 M KOH, 0.1 M PBS (ph = 7) and 0.1 M HClO 4 solutions at a rotation speed of 1,600 rpm. www.editorialmanager.com/nare/default.asp
Figure S10 (a) (c) CV curves of Co-N/C in 0.1 M KOH, 0.1 M PBS (ph = 7) and 0.1 M HClO 4 solutions. (d) (f) LSV curves of Co-N/C in O 2 -satureted 0.1 M KOH, 0.1 M PBS (ph = 7) and 0.1 M HClO 4 solutions before and after 3,000 cycles. (g) and (h) LSV curves of Co-N/C in O 2 -satureted 0.1 M KOH, 0.1 M PBS (ph = 7) and 0.1 M HClO 4 solutions before and after addition of 3 M methanol. The Co-N/C catalyst obtained at 900 C exhibits the highest ORR catalytic activity compared with the Co-N/C catalysts obtained at other calcination temperatures. The SEM images show that the Co-N/C catalysts prepared at different calcination temperatures have almost identical hollow spherical structures composed of numerous interconnected nanosheets. www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
Nano Res. Figure S11 (a) (c) LSV curves of Co-N/C obtained at 700, 800 and 1,000 C at various rotation rates in O2-saturated 0.1 M KOH solution. (d) LSV curves of Co-N/C obtained at different calcination temperature in O2-saturated 0.1 M KOH solution at a rotation speed of 1,600 rpm. Figure S12 (a) and (b) SEM images of Co-N/C catalyst obtained at 700 C. (c) and (d) SEM images of Co-N/C catalyst obtained at 800 C. (e) and (f) SEM images of Co-N/C catalyst obtained at 1,000 C. www.editorialmanager.com/nare/default.asp
Figure S13 (a) (c) Nitrogen sorption isotherms of Co-N/C composites obtained at 700, 800 and 1,000 C. (d) (f) Pore size distribution curves of Co-N/C composites obtained at 700, 800 and 1,000 C. The specific surface area is estimated to be 479.4, 447.1, and 286.8 m 2 g 1 for Co-N/C catalysts obtained at 700, 800, and 1,000 C, respectively, which is lower than that of Co-N/C catalyst obtained at 900 C (817.7 m 2 g 1 ). The pore size distribution of Co-N/C catalysts obtained at 700, 800, and 1,000 C suggests their hierarchical micro-/mesoporous structure with pore diameters ranging from 1 50 nm, which is consistent with that of Co-N/C catalyst obtained at 900 C. www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
Figure S14 (a) N 1s XPS spectra and (b) S 2p XPS spectra of Co-N/C composites obtained at 700, 800 and 1,000 C. The Co-N/C prepared at 700 C has the highest N content (7.41 at.%) compared with those of Co-N/C prepared at 800 C (2.96 at.%), 900 C (2.81 at.%), and 1,000 C (2.71 at.%). Moreover, increasing the temperature from 700 to 800 C decreases the percentage of pyrrolic N while increases the percentage of quaternary N due to the lower thermal stability of pyrrolic N, which is consistent with the previous reports [S1, S2]. It is worth noting that extremely high temperature (1,000 C) results in a disappearance of pyridinic N, which is considered as the efficient active sites for ORR [S3, S4]. The S 2p spectra indicate the Co-N/C catalyst prepared at 800 C has the highest S content (1.96 at.%) compared with those of Co-N/C prepared at 700 C (1.35 at.%), 900 C (1.80 at.%), and 1,000 C (1.15 at.%) and the S species changed significantly, especially the oxidized sulfur species SO x. Figure S15 The normalized C 1s XPS spectra of Co-N/C composite obtained at 700, 800, 900 and 1,000 C. The normalized C 1s XPS peaks of Co-N/C catalysts obtained at different calcination temperatures are centered at about 284.8 ev and are slightly asymmetric, which is a common effect for doped carbon materials. A narrower half-peak width suggests an enhanced graphitic character [S5], indicating that the graphitization degree of Co-N/C materials increases with increasing the calcination temperature. www.editorialmanager.com/nare/default.asp
Nano Res. Figure S16 (a) and (b) SEM images of Co-N/C-1 composite. (c) and (d) TEM images of Co-N/C-1 composite. (e) Nitrogen sorption isotherms and (f) pore size distribution curve of Co-N/C-1 composite. The SEM and TEM images show that the Co-N/C-1 still retains the hollow spherical structure, which is similar to the structure of Co-N/C. The specific surface area and pore volume are estimated to be 834.6 m2 g 1 and 1.78 cm3 g 1, both of which are close to those of Co-N/C composite (817.7 m2 g 1 and 1.88 cm3 g 1). The pore size distribution of Co-N/C-1 composite suggests the formation of micropores and mesopores with pore diameters ranging from 1 50 nm, which is almost identical with that of Co-N/C composite. Therefore, the second thermal treatment has an ignorable effect on the morphology, specific surface area, pore volume and pore size distribution of Co-N/C-1 composite. www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
Figure S17 (a) S 2p XPS spectrum of Co-N/C composite. (b) S 2p XPS spectrum of Co-N/C-1 composite. Figure 18 RRDE curves of (a) (c) Co-N/C-1, (d) (f) Co-N/C-2, and (g) (i) Pt/C catalysts in O 2 -saturated (a), (d), and (g) 0.1 M KOH, (b), (e), and (h) 0.1 M PBS (ph = 7), and (c), (f), and (i) 0.1 M HClO 4 solutions with a sweep rate of 5 mv s 1 at a rotation rate of 1,600 rpm. www.editorialmanager.com/nare/default.asp
Figure S19 The H 2 O 2 yield and electron transfer number (n) of (a) (c) Co-N/C-1, (d) (f) Co-N/C-2, and (g) (i) Pt/C catalysts in O 2 -saturated (a), (d), and (g) 0.1 M KOH, (b), (e), and (h) 0.1 M PBS (ph = 7), and (c), (f), and (i) 0.1 M HClO 4 solutions with a sweep rate of 5 mv s 1 at a rotation rate of 1,600 rpm. Table S1 The average n and H 2 O 2 yield derived from RRDE curves of Co-N/C, Co-N/C-1, Co-N/C-2 and Pt/C Co-N/C Co-N/C-1 Co-N/C-2 Pt/C Medium n H 2 O 2 yield n H 2 O 2 yield n H 2 O 2 yield n H 2 O 2 yield 0.1 M KOH a 3.91 4.36% 3.92 3.93% 3.92 4.07% 3.96 2.04% 0.1 M PBS b 3.75 12.23% 3.61 19.49% 3.58 20.97% 3.91 4.29% 0.1 M HClO 4 c 3.71 14.56% 3.54 22.93% 3.50 25.24% 3.88 5.99% a Potential range from 0.15 to 0.85 V; b potential range from 0 to 0.75 V; c potential range from 0.1 to 0.75 V. References [S1] Wu, G.; Mack, N. H.; Gao, W.; Ma, S. G.; Zhong, R. Q.; Han, J. T.; Baldwin, J. K.; Zelenay, P. Nitrogen-doped graphene-rich catalysts derived from heteroatom polymers for oxygen reduction in nonaqueous lithium-o 2 battery cathodes. ACS Nano 2012, 6, 9764 9776. [S2] Wang, H. B.; Xie, M. S.; Thia, L.; Fisher, A.; Wang, X. Strategies on the design of nitrogen-doped graphene. J. Phys. Chem. Lett. 2014, 5, 119 125. [S3] Niu, W. H.; Li, L. G.; Liu, X. J.; Wang, N.; Liu, J.; Zhou, W. J.; Tang, Z. H.; Chen, S. W. Mesoporous N-doped carbons prepared with thermally removable nanoparticle templates: An efficient electrocatalyst for oxygen reduction reaction. J. Am. Chem. Soc. 2015, 137, 5555 5562. www.thenanoresearch.com www.springer.com/journal/12274 Nano Research
[S4] Guo, D. H.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active sites of nitrogen-doped carbon materials for oxygen reduction reaction clarified using model catalysts. Science 2016, 351, 361 365. [S5] Liu, R. L.; Wu, D. Q.; Feng, X. L.; Müllen, K. Nitrogen-doped ordered mesoporous graphitic arrays with high electrocatalytic activity for oxygen reduction. Angew. Chem., Int. Ed. 2010, 49, 2565 2569. www.editorialmanager.com/nare/default.asp