1. G ENERAL I NTRODUCTION
1.5. R EFERENCES
(1) Japan Ministry of Internal Affairs and Communications, Historical Statistics of Japan, http://www.stat.go.jp/english/data/chouki/index.html (accessed Apr 3, 2007).
(2) U.S. Department of Energy, Types of Fuel Cells,
http://www1.eere.energy.gov/hydrogenandfuelcells/fuelcells/fc_types.html (accessed Aug 4, 2011).
(3) Kakati, B. K.; Deka, D. Electrochimica Acta 2007, 52, 7330–7336.
(4) Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Appl. Catal. B 2005, 56, 9−35.
(5) Debe, M. K. Nature 2012, 486, 43−51.
(6) Shao, M.; Chang, Q.; Dodelet, J.-P.; Chenitz, R. Chem. Rev. 2016, 116, 3594−3657.
(7) Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. Angew. Chem., Int. Ed. 2016, 55, 2650−2676.
(8) Kulkarni, A.; Siahrostami, S.; Patel, A.; Nørskov, J. K. Chem. Rev. 2018, 118, 2302−2312.
(9) Gewirth, A. A.; Varnell, J. A.; DiAscro, A. M. Chem. Rev. 2018, 118, 2313−2339.
(10) Marcinkoski, J.; Spendelow, J.; Wilson, A.; Papageorgopoulos, D. DOE Hydrogen and Fuel Cells Program Record # 15015: Fuel Cell System Cost.
(11) Jaouen, F.; Proietti, E.; Lefèvre, M.; Chenitz, R.; Dodelet, J.-P.; Wu, G.; Chung, H. T.; Johnston, C. M.; Zelenay, P. Energy Environ. Sci. 2011, 4, 114−130.
(12) Wu, G.; Zelenay, P. Acc. Chem. Res. 2013, 46, 1878−1889.
(13) Shi, H.; Shen, Y.; He, F.; Li, Y.; Liu, A.; Liu, S.; Zhang, Y. J. Mater. Chem. A. 2014, 2,
21 15704−15716.
(14) Jia, Q.; Ramaswamy, N.; Tylus, U.; Strickland, K.; Li, J.; Serov, A.; Artyushkova, K.; Atanassov, P.; Anibal, J.; Gumeci, C.; Barton, S. C.; Sougrati, M.-T.; Jaouen, F.; Halevi, B.; Mukerjee, S.
Nano Energy 2016, 29, 65−82.
(15) Sa, Y. J.; Kim, J. H.; Joo, S. H. J. Electrochem. Sci. Technol. 2017, 8, 169−182.
(16) Zhu, Y. P.; Guo, C.; Zheng, Y.; Qiao, S.-Z. Acc. Chem. Res. 2017, 50, 915−923.
(17) Sa, Y. J.; Woo, J.; Joo, S. H. Top. Catal. 2018, 61, 1077−1100.
(18) Martinez, U.; Babu, S. K.; Holby, E. F.; Chung, H. T.; Yin, X.; Zelenay, P. Adv. Mater. 2019, 31, 1970224.
(19) Lefèvre, M.; Proietti, E.; Jaouen, F.; Dodelet, J.-P. Science 2009, 324, 71−74.
(20) Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Science, 2009, 323, 760–764.
(21) Qu, L.; Liu, Y.; Baek, J.-B.; Dai, L. ACS Nano 2010, 4, 1321–1326.
(22) Wu, G.; More, K. L.; Johnston, C. M.; Zelenay, P. Science 2011, 332, 443−447.
(23) Proietti, E.; Jaouen, F.; Lefèvre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J.-P. Nat.
Commun. 2011, 2, 416.
(24) Ma, S.; Goenaga, G. A.; Call, A. V.; Liu, D.-J. Chem. Eur. J. 2011, 17, 2063−2067.
(25) Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Nat. Mater. 2011, 10, 780−786.
(26) Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K. J. Am. Chem. Soc. 2012, 134, 9082−9085.
(27) Seo, B.; Sa, Y. J.; Woo, J.; Kwon, K.; Park, J.; Shin, T. J.; Jeong, H. Y.; Joo, S. H. ACS Catal.
2016, 6, 4347−4355.
(28) Gao, M.-R.; Jiang, J.; Yu, S.-H. Small 2012, 8, 13−27.
(29) Fan, X.; Liu, Y.; Peng, Z.; Zhang, Z.; Zhou, H.; Zhang, X.; Yakobson, B. I.; Goddard, W. A., III;
Guo, X.; Hauge, R. H.; Tour, J. M. ACS Nano 2017, 11, 384−394.
(30) Liang, J.; Jiao, Y.; Jaroniec, M.; Qiao, S. Z. Angew. Chem., Int. Ed. 2012, 51, 11496−11500.
(31) Cheon, J. Y.; Kim, T.; Choi, Y.; Jeong, H. Y.; Kim, M. G.; Sa, Y. J.; Kim, J.; Lee, Z.; Yang, T.- H.; Kwon, K.; Terasaki, O.; Park, G.-G.; Adzic, R. R.; Joo, S. H. Sci. Rep. 2013, 3, 2715.
(32) Wang, Q.; Zhou, Z.-Y.; Lai, Y.-J.; You, Y.; Liu, J.-G.; Wu, X.-L.; Terefe, E.; Chen, C.; Song, L.;
Rauf, M.; Tian, N.; Sun, S.-G. J. Am. Chem. Soc. 2014, 136, 10882−10885.
(33) Cheon, J. Y.; Kim, J. H.; Kim, J. H.; Goddeti, K. C.; Park, J. Y.; Joo, S. H. J. Am. Chem. Soc.
2014, 136, 8875−8878.
(34) Sahraie, N. R.; Kramm, U. I.; Steinberg, J.; Zhang, Y.; Thomas, A.; Reier, T.; Paraknowitsch, J.- P.; Strasser, P. Nat. Commun. 2015, 6, 8618.
(35) Shui, J.; Chen, C.; Grabstanowicz, L.; Zhao, D.; Liu, D.-J. Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 10629−10634.
22
(36) Han, J.; Sa, Y. J.; Shim, Y.; Choi, M.; Park, N.; Joo, S. H.; Park, S. Angew. Chem., Int. Ed. 2015, 54, 12622−12626.
(37) Cheon, J. Y.; Kim, K.; Sa, Y. J.; Sahgong, S. H.; Hong, Y.; Woo, J.; Yim, S.-D.; Jeong, H. Y.;
Kim, Y.; Joo, S. H. Adv. Energy Mater. 2016, 6, 1501794.
(38) Sa, Y. J.; Seo, D.-J.; Woo, J.; Lim, J. T.; Cheon, J. Y.; Yang, S. Y.; Lee, J. M.; Kang, D.; Shin, T.
J.; Shin, H. S.; Jeong, H. Y.; Kim, C. S.; Kim, M. G.; Kim, T.-Y.; Joo, S. H. J. Am. Chem. Soc.
2016, 138, 15046−15056.
(39) Wu, H.; Li, H.; Zhao, X.; Liu, Q.; Wang, J.; Xiao, J.; Xie, S.; Si, R.; Yang, F.; Miao, S.; Guo, X.; Wang, G.; Bao, X. Energy Environ. Sci. 2016, 9, 3736−3745.
(40) Yin, P.; Yao, T.; Wu, Y.; Zheng, L.; Lin, Y.; Liu, W.; Ju, H.; Zhu, J.; Hong, X.; Deng, Z.; Zhou, G.; Wei, S.; Li, Y. Angew. Chem., Int. Ed. 2016, 55, 10800−10805.
(41) Kim, J. H.; Sa, Y. J.; Jeong, H. Y.; Joo, S. H. ACS Appl. Mater. Interfaces 2017, 9, 9567−9575.
(42) Jiao, L.; Wan, G.; Zhang, R.; Zhou, H.; Yu, S.-H.; Jiang, H.-L. Angew. Chem., Int. Ed. 2018, 57, 8525−8529.
(43) Woo, J.; Yang, S. Y.; Sa, Y. J.; Choi, W.-Y.; Lee, M.-H.; Lee, H.-W.; Shin, T. J.; Kim, T.-Y.; Joo, S. H. Chem. Mater. 2018, 30, 6684–6701.
(44) Wan, X.: Liu, X.; Li, Y.; Yu, R.; Zheng, L.; Yan, W.; Wang, H.; Xu, M.; Shui, J. Nat. Catal.
2019, 2, 259–268.
(45) Jiao, L.; Zhang, R.; Wan, G.; Yang, W.; Wan, X.; Zhou, H.; Shui, J.; Yu, S.-H.; Jiang, H.-L. Nat.
Commun. 2020, 11, 2831.
(46) Nørskov, J. K.; Rossmeisl, J.; Logadottir, A.; Lindqvist, L. R. K. J.; Kitchin, J. R.; Bligaard, T.;
Jonsson, H. J. Phys. Chem. B 2004, 108, 17886-17892.
(47) Greeley, J.; Stephens, I. E. L.; Bondarenko, A. S.; Johansson, T. P.; Hansen, H. A.; Jaramillo, T.
F.; Rossmeisl, J.; Chorkendorff, I.; Nørskov, J. K. Nat. Chem. 2009, 1, 552–556.
(48) Zhang, J.; Lima, F. H. B.; Shao, M. H.; Sasaki, K.; Wang, J. X.; Hanson, J.; Adzic, R. R. J. Phys.
Chem. B 2005, 109, 22701–22704.
(49) Chen, Y.; Yang, F.; Dai, Y.; Wang, W.; Chen, S. J. Phys. Chem. C 2008, 112, 1645–1649.
(50) Xie, S.; Choi, S.-I.; Lu, N.; Roling, L. T.; Herron, J. A.; Zhang, L.; Park, J.; Wang, J.; Kim, M.
J.; Xie, Z.; Mavrikakis, M.; Xia, Y. Nano Lett. 2014, 14, 3570–3576.
(51) Chen, C.; Kang, Y.; Huo, Z.; Zhu, Z.; Huang, W.; Xin, H. L.; Snyder, J. D.; Li, D.; Herron, J.
A.; Mavrikakis, M.; Chi, M.; More, K. L.; Li, Y.; Markovic, N. M.; Somorjai, G. A.; Yang, P.;
Stamenkovic, V. R. Science 2014, 343, 1339–1343.
(52) Li, M.; Zhao, Z.; Cheng, T.; Fortunelli, A.; Chen, C.-Y.; Yu, R.; Zhang, Q.; Gu, L.; Merinov, B.
V.; Lin, Z.; Zhu, E.; Yu, T.; Jia, Q.; Guo, J.; Zhang, L.; Goddard, W. A.; Huang, Y.; Duan, X.
Science 2016, 354, 1410–1414.
(53) Kim, H. Y.; Kwon, T.; Ha, Y.; Jun, M.; Baik, H.; Jeong, H. Y.; Kim, H.; Lee, K.; Joo, S. H.
23 Nano Lett. 2020, 20, 7413–7421.
(54) Ikeda, T.; Boero, M.; Huang, S. F.; Terakura, K.; Oshima, M.; Ozaki, J. J. Phys. Chem. C 2008, 112, 14706–14709.
(55) Yang, W.; Fellinger, T. P.; Antonietti, M. J. Am. Chem. Soc. 2011, 133, 206–209.
(56) Yang, S. B.; Feng, X. L.; Wang, X. C.; Mllen, K. Angew. Chem., Int. Ed. 2011, 50, 5339–5343.
(57) Gao, S.; Wei, X.; Fan, H.; Li, L.; Geng, K.; Wang, J. Nano Energy 2015, 13, 518–526.
(58) Shi, Z.; Zhang, J.; Liu, Z.; Wang, H.; Wilkinson, D. P. Electrochim. Acta 2006, 51, 1905−1916.
(59) Jiao, Y.; Zheng, Y.; Jaroniec, M.; Qiao, S. Z. J. Am. Chem. Soc. 2014, 136, 4394–4403.
(60) Li, Y. G.; Zhou, W.; Wang, H. L.; Xie, L. M.; Liang, Y. Y.; Wei, F.; Idrobo, J. C.; Pennycook, S.
J.; Dai, H. J. Nat. Nanotechnol. 2012, 7, 394–400.
(61) Xiong, W.; Du, F.; Liu, Y.; Perez, A.; Supp, Jr., M.; Ramakrishnan, T. S.; Dai. L. M.; Jiang, L. J.
Am. Chem. Soc. 2010, 132, 15839–15841.
(62) Deng, D.; Yu, L.; Chen, X.; Wang, G.; Jin, L.; Pan, X.; Deng, J.; Sun, G.; Bao, X. Angew.
Chem., Int. Ed. 2013, 52, 371-375.
(63) Hu, Y.; Jensen, J.O.; Zhang, W.; Cleemann, L.N.; Xing, W.; Bjerrum, N.J.; Li, Q. Angew. Chem., Int. Ed. 2014, 53, 3675-3679.
(64) Wang, Y. J.; Wilkinson, D. P.; Zhang, J. Chem. Rev. 2011, 111, 7625-7651.
(65) Dong, G.; Fang, M.; Wang, H.; Yip, S.; Cheung, H.-Y.; Wang, F.; Wong, C.-Y.; Chu, S. T.; Ho, J.
C. J. Mater. Chem. A 2015, 3, 13080–13086.
(66) Jasinski, R. Nature 1964, 201, 1212-1213.
(67) Jahnke, H.; Schönborn, M.; Zimmermann, G. Top. Curr. Chem. 1976, 61, 133−181.
(68) Gupta, S.; Tryk, D.; Bae, I.; Aldred, W.; Yeager, E. J. Appl. Electrochem. 1989, 19, 19−27.
(69) Jaouen, F.; Dodelet, J.-P. J. Phys. Chem. C 2009, 113, 15422−15432.
(70) Zhang, J.; Vukmirovic, M. B.; Xu, Y.; Mavrikakis, M.; Adzic, R. R. Angew. Chem., Int. Ed.
2005, 44, 2132–2135.
(71) Jia, Q.; Ramaswamy, N.; Hafiz, H.; Tylus, U.; Strickland, K.; Wu, G.; Barbiellini, B.; Bansil, A.; Holby, E. F.; Zelenay, P.; Mukerjee, S. ACS Nano 2015, 9, 12496−12505.
(72) Natoli, C. R. In EXAFS and near edge structure III; Hodgson, K.; Hedman, B.; Penner-Hahn, J., Eds.; Springer: Berlin, Heidelberg, 1984; Vol. 2, pp 38–42.
(73) Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M.-T.; Mineva, T.; Stievano, L.; Fonda, E.; Jaouen, F. Nat. Mater. 2015, 14, 937−942.
(74) Chung, H. T.; Cullen, D. A.; Higgins, D.; Sneed, B. T.; Holby, E. F.; More, K. L.; Zelenay, P.
Science 2017, 357, 479−484.
(75) Yang, C.; Zhao, H.; Hou, Y.; Ma, D. J. Am. Chem. Soc. 2012, 134, 15814−15821.
(76) Xu, H. X.; Cheng, D. J.; Cao, D. P.; Zeng, X. C. Nat. Catal. 2018, 1, 339−348.
(77) Li, Q.; Chen, W.; Xiao, H.; Gong, Y.; Li, Z.; Zheng, L.; Zheng, X.; Yan, W.; Cheong, W. C.;
24
Shen, R.; Fu, N.; Gu, L.; Zhuang, Z.; Chen, C.; Wang, D.; Peng, Q.; Li, J.; Li, Y. Adv. Mater.
2018, 30, 1800588.
(78) Qiao, Y.; Yuan, P.; Hu, Y.; Zhang, J.; Mu, S.; Zhou, J.; Li, H.; Xia, H.; He, J.; Xu, Q. Adv. Mater.
2018, 30, 1804504.
(79) Chen, Y.; Ji, S.; Zhao, S.; Chen, W.; Dong, J.; Cheong, W.-C.; Shen, R.; Wen, X.; Zheng, L.;
Rykov, A. I.; Cai, S.; Tang, H.; Zhuang, Z.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Nat. Commun.
2018, 9, 5422.
(80) Shen, H.; Gracia-Espino, E.; Ma, J.; Zang, K.; Luo, J.; Wang, L.; Gao, S.; Mamat, X.; Hu, G.;
Wagberg, T.; Guo, S. Angew. Chem., Int. Ed. 2017, 56, 13800−13804.
(81) Chen, P. Z.; Zhou, T. P.; Xing, L. L.; Xu, K.; Tong, Y.; Xie, H.; Zhang, L. D.; Yan, W. S.; Chu, W. S.; Wu, C. Z.; Xie, Y. Angew. Chem., Int. Ed. 2017, 56, 610−614.
(82) Mun, Y.; Lee, S.; Kim, K.; Kim, S.; Lee, S.; Han, J. W.; Lee, J. J. Am. Chem. Soc. 2019, 141, 6254−6262.
(83) Zhang, J.; Zhao, Y.; Chen, C.; Huang, Y.-C.; Dong, C.-L.; Chen, C.-J.; Liu, R.-S.; Wang, C.;
Yan, K.; Li, Y.; Wang, G. J. Am. Chem. Soc. 2019, 141, 20118−20126.
(84) Yuan, K.; Lützenkirchen-Hecht, D.; Li, L.; Shuai, L.; Li, Y.; Cao, R.; Qiu, M.; Zhuang, X.;
Leung, M. K. H.; Chen, Y.; Scherf, U. J. Am. Chem. Soc. 2020, 142, 2404−2412.
(85) Shang, H.; Zhou, X.; Dong, J.; Li, A.; Zhao, X.; Liu, Q.; Lin, Y.; Pei, J.; Li, Z.; Jiang, Z.; Zhou, D.; Zheng, L.; Wang, Y.; Zhou, J.; Yang, Z.; Cao, R.; Sarangi, R.; Sun, T.; Yang, X.; Zheng, X.;
Yan, W.; Zhuang, Z.; Li, J.; Chen, W.; Wang, D.; Zhang, J.; Li, Y. Nat. Commun. 2020, 11, 3049.
(86) Jaouen, F.; Dodelet, J.-P. J. Phys. Chem. C 2007, 111, 5963–5970
(87) Chung, H. T.; Johnston, C. M.; Zelenay, P. ECS Trans. 2009, 25, 485–492
(88) Kramm, U. I.; Lefevre, M.; Larouche, N.; Schmeisser, D.; Dodelet, J. P. J. Am. Chem. Soc.
2014, 136, 978–985.
(89) Serov, A.; Artyushkova, K.; Atanassov, P. Adv. Energy Mater. 2014, 4, 1301735 (90) Li, J. K.; Jaouen, F. Curr. Opin. Electrochem. 2018, 9, 198–206
(91) Zitolo, A.; Ranjbar-Sahraie, N.; Mineva, T.; Li, J. K.; Jia, Q. Y.; Stamatin, S.; Harrington, G. F.;
Lyth, S. M.; Krtil, P.; Mukerjee, S.; Fonda, E.; Jaouen, F. Nat. Commun. 2017, 8, 957 (92) Malko, D.; Kucernak, A.; Lopes, T. Nat. Commun. 2016, 7, 13285
(93) Luo, F.; Choi, C. H.; Primbs, M. J. M.; Ju, W.; Li, S.; Leonard, N. D.; Thomas, A.; Jaouen, F.;
Strasser, P. ACS Catal. 2019, 9, 4841–4852.
(94) Primbs, M.; Sun, Y.; Roy, A.; Malko, D.; Mehmood, A.; Sougrati, M.-T.; Blanchard, P.-Y.;
Granozzi, G.; Kosmala, T.; Daniel, G.; Atanassov, P.; Sharman, J.; Durante, C.; Kucernak, A.;
Jones, D.; Jaouen, F.; Strasser, P. Energy Environ. Sci. 2020, 13, 2480–2500.
(95) Bae, G.; Kim, H.; Choi, H.; Jeong, P.; Kim, D. H.; Kwon, H. C.; Lee, K.-S.; Choi, M.; Oh, H.- S.; Jaouen, F.; Choi, C. H. J. Am. Chem. Soc. Au 2021, https://doi.org/10.1021/jacsau.1c00074
25
(96) Wan, H.; Jensen, A. W.; Escudero-Escribano, M.; Rossmeisl, J.; ACS Catal. 2020, 10, 5979−5989.
(97) Yang, S.; Yu, Y.; Dou, M.; Zhang, Z.; Wang, F. J. Am. Chem. Soc. 2020, 142, 17524−17530.
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2
I MPACT OF T EXTURAL P ROPERTIES OF
M ESOPOROUS P ORPHYRINIC C ARBON
E LECTROCATALYSTS ON O XYGEN R EDUCTION
R EACTION A CTIVITY
This chapter includes the published contents:
Woo, J.; Sa, Y. J.; Kim, J. H.; Lee, H.-W.; Pak, C.; Joo, S. H. ChemElectroChem 2018, 5, 1928–
1936. DOI: 10.1002/celc.201800183. Reproduced with permission. Copyright © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
2.1.INTRODUCTION
The oxygen reduction reaction (ORR) is an important reaction for environmentally benign energy conversion devices such as polymer electrolyte fuel cells and metal–air batteries.1,2 Although Pt-based catalysts show the highest ORR activities, high cost and scarcity of Pt and its susceptibility to catalyst poisons (i.e., CO and CH3OH) impede its widespread implementation in energy devices.3,4 As alternatives to Pt-based catalysts, non-precious metal-based catalysts (NPMCs) have emerged for the ORR. Representative classes of NPMCs include transition metal- and N-codoped carbons (M–
N/C), heteroatom-doped carbons, metal oxides/carbon composites, metal carbides, and metal chalcogenides.4–7 M–N/C catalysts have received particular attention because they show high ORR activities.4–25 Recent research based on spectroscopic and microscopic investigations suggests that the plausible active sites in M–N/C catalysts comprise metal ligated to N (M–Nx) moieties, reminiscent of the heme structure in the naturally occurring nature’s ORR catalyst, cytochrome c oxidase (CcO).26–32 In M–N/C catalysts, the overall catalytic activity is predominantly determined by the intrinsic activity of each active site and the number of utilizable active sites. Hence, the design of high-performance M–N/C catalysts requires the successful incorporation of the desired active sites into a conductive carbon framework as well as a well-developed pore structure that can facilitate mass transport.
Many synthetic strategies have been developed to boost ORR catalytic performance. Prominent approaches include the exploitation of metal–organic frameworks (MOFs) as hosts or precursors,20,22 the use of sacrificial templates,23,33–55 preferential generation of active sites,19,25,56,57 and elaborate mimicking of enzymatic catalysts.58–60 Among these methods, the preparation of M–N/C catalysts using sacrificial templates, such as silica spheres,33–35 mesoporous silicas,23,36–45 metal oxide
27
nanoparticles,46–48 Te nanowires,49–51 and salts,52–55 features a multiple advantages for enhancing the ORR activity. In this method, the removal of nanoparticulate or nanoporous templates can yield tunable mesopores with sizes in the range of a few or some tens of nanometers, which, in combination with the microporous framework, permits the facile transport of fuels and byproducts. Highly developed porous structure can yield specific surface areas exceeding 1000 m2 g–1 that can host high densities of active sites. The synthesis is amenable to simple and mild experimental conditions; while the preparation of highly active M–N/C often requires multiple pyrolysis steps or the use of toxic ammonia gas, the templating approach avoids such laborious and toxic synthetic steps. Furthermore, the textural properties of the template materials directly dictate the pore structure of the resulting catalysts, enabling straightforward design.
In this line of research, we have developed highly active catalysts based on ordered mesoporous carbons embedded with porphyrinic or phthalocyanic moieties, which were prepared via a nanocasting route using ordered mesoporous silicas as the templates and metallo-macrocylces as the precursors.23,42 Notably, Fe and Co-doped ordered mesoporous porphyrinic carbon showed very high ORR activity in acidic media.23 Müllen et al. showed that the ORR activities of Co–N/C catalysts prepared from different sacrificial templates were linearly correlated to their surface areas.36 Other research groups also reported active M–N/C catalysts based on sacrificial template-based methods for the ORR.37–45 We note that the ORR activity of templated M–N/C catalysts is affected by multiple textural factors such as external morphology, surface area, and pore sizes. These textural properties are usually interdependent, and it is hard to isolate the effect of each parameter. Therefore, systematic studies identifying the impact of specific textural parameters of M–N/C catalysts are important yet rarely reported.
In this study, we investigated the impacts of the textural properties of mesoporous silica- templated Fe–N/C catalysts, including the pore size, pore connectivity, and particle size, on the ORR activities of the catalysts. The Fe–N/C catalysts were prepared via the nanocasting route by using mesoporous silicas (SBA-15 and KIT-6) as the templates and iron(III) porphyrin as the precursor.
Textural properties such as pore sizes, pore connectivities, and particle sizes could be systematically controlled by the use of different silica templates to reveal the role of each parameter. The resulting four catalysts, denoted as iron doped mesoporous porphyrinic carbons (Fe-MPCs), showed excellent ORR activity in both acidic and alkaline media with half-wave potentials at 0.82 and 0.90 V (vs.
reversible hydrogen electrode, RHE) in acidic and alkaline media, respectively. A comparison of the textual properties of Fe-MPC catalysts and their ORR activities suggested that smaller particle size exerted better ORR activity, while larger pore size and 3D connectivity further promoted ORR activity under diffusion-limited conditions.
28 2.2.EXPERIMENTAL METHODS
2.2.1. Synthesis of Mesoporous Silica Templates
Mesoporous silica SBA-15 with hexagonal symmetry was synthesized as described in previous reports,61,63 with some modifications. 18.5 g of Pluronic® P123, 110.0 g of HCl, and 580.0 g of deionized (DI) water were added to a 1 L polypropylene (PP) bottle, and the mixture was stirred at 35 °C until P123 was completely dissolved. 39.0 g of TEOS was added to P123 solution, and the mixture was stirred for 5 min and aged at 35 °C without stirring for 24 h. The aged reaction mixture was divided into three portions. Each portion of the mixture was then subjected to a hydrothermal treatment at a desired temperature (35, 100, or 150 °C) for 24 h. For samples treated at 100 or 150 °C, the reaction mixture was transferred to a Teflon®-lined autoclave. After the hydrothermal treatments the resulting white-colored precipitates were filtered, washed twice with DI water, and dried in an oven at 60 °C. To remove P123 structure-directing agent, the dried powder was added to a solution containing 3.0 g of HCl and 118.4 g of EtOH. The mixture was stirred for 30 min, filtered, washed with EtOH, and dried in an oven at 60 °C. Finally, the dried sample was calcined in air at 550 °C for 5 h. According to the hydrothermal treatment temperature, the resulting samples were denoted as SBA- 15-X (X = hydrothermal treatment temperature). Mesoporous silica KIT-6 with cubic Ia3d symmetry was synthesized following the previously reported method.62 17.5 g of Pluronic® P123, 33.8 g of HCl, and 625.0 g of DI water were added to a 1 L PP bottle, and the mixture was stirred at 35 °C until P123 was completely dissolved. To this solution, 22.4 g of butanol was added and stirred for 1 h. Next, 55.5 g of TEOS was added and the mixture was stirred at 35 °C for 24 h, and subsequently aged at 100 °C for 24 h under static conditions. Subsequent washing and calcination steps were carried out in the same manner as described above.
2.2.2. Synthesis of Fe-MPC Catalysts
One gram of a mesoporous silica template and a desired amount of FeIIITMPPCl were mixed in an agate mortar for 10 min. The amount of FeIIITMPPCl was adjusted depending on the total pore volume of a mesoporous silica template; 1.0 g of FeIIITMPPCl was used per 1.0 cm3 g−1 of pore volume of a silica template. The mixture was heated from RT to 800 °C for 6 h, and maintained at that temperature for 3 h under N2 gas flow (1.0 L min−1). To remove the silica template, the resulting carbon-silica composite was mixed with 1:1 (v/v) = EtOH:10% aqueous HF solution, and the slurry was stirred for 30 min, filtered, and washed with EtOH several times.13 The HF etching process was repeated in the same manner, and the resulting sample was dried at 60 °C to afford Fe-MPC samples.
2.2.3. Characterization Methods
SEM images were taken on a scanning electron microscope (S-4800, Hitachi) operating at 10 kV. TEM images were obtained using a transmission electron microscope (JEM-2100F, JEOL) with a
29
probe-forming Cs corrector at an acceleration voltage of 200 kV. XRD patterns were measured with a high-power X-ray diffractometer (D/MAX2500V/PC, Rigaku) equipped with Cu Kα radiation, and operated at 40 kV and 200 mA. Small angle (2θ range of 0.6° to 5°) and high angle (15° to 65°) XRD patterns were obtained with scan rates of 0.5° min−1 and 2° min−1, respectively. The textural properties of the samples were analyzed using a nitrogen physisorption analyzer (BELSORP-Max, BEL JAPAN, Inc.) operated at −196 °C. Specific surface areas were calculated using the Brunauer–Emmett–Teller (BET) equation in the relative pressure range of 0.05–0.2, while pore size distributions of the samples were obtained using the BJH method based on adsorption branches of their isotherm. All samples were pre-evacuated at 423 K and 10−2 Pa for 12 h before measurements. The Fe content in Fe-MPC catalysts was analyzed using an ICP-OES analyzer (700-ES, Varian). Carbon, hydrogen, nitrogen, and oxygen contents in Fe-MPC catalysts were determined by an elemental analyzer (Flash 2000, Thermo Scientific), which is based on the dynamic flash combustion of the sample followed by reduction, trapping, complete separation by gas chromatography, and detection of the products by thermal conductivity detector. XPS measurements were performed on an ESCLAB 250XI (Thermo Scientific), equipped with a monochromatic Al Kα X-ray source (1486.6 eV). XPS 1s spectra of Fe-MPC catalysts were deconvoluted using XPS peak41 software. Deconvoluted chemical species of N 1s binding energy region were fitted to the spectra using the mixed Lorentzian-Gaussian-function (Lorentzian:
30% and Gaussian: 70%) after a Shirley-type background subtraction.
Fe K-edge XAS data were collected at the 8C beamline of the Pohang Accelerator Laboratory (PAL), Republic of Korea. The storage ring was operated at 3.0 GeV with the beam current of 300 mA. The beamline is equipped with a focusing Si (1 1 1) double crystal monochromator that was used to filter the incident photon, which was detuned by 20% to remove high-order harmonics and calibrated using a standard Fe foil. The catalyst powder was pelletized for the measurements. The background removal, normalization of the collected XAS data, and Fourier transform to the radial distribution functions were conducted using the Athena software.70
2.2.4. Electrochemical Characterizations
Electrochemical experiments were performed with a bipotentiostat (CHI760E, CH Instruments) and a rotator (AFMSRCE, Pine Research Instrumentation) at 25 °C using a three-electrode electrochemical cell. Ag/AgCl (RE-1B, ALS; saturated KCl filling solution) and Hg/HgO (CHI152, CH Instruments; 1 M KOH filling solution) reference electrodes were used for measurements in acidic and alkaline electrolytes, respectively, and a graphite rod was used as the counter electrode.
Potential was converted with respect to the RHE. To obtain the RHE conversion value, two electrode cell was built with Pt coil and a reference electrode soaked in a H2-saturated electrolyte. In this case, the Pt coil acts as the RHE. With continuous H2 bubbling, a stable open circuit voltage, which corresponds to the RHE conversion factor, could be acquired within 20 min. A catalyst ink coated disk
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in rotating ring-disk electrode (RRDE, AFE7R9GCPT, Pine Research Instrumentation) was used as the working electrode. Prior to every measurement, the RRDE was polished with a 1.0 µm alumina suspension and then 0.3 µm suspension to afford a mirror finish. A catalyst ink was prepared by mixing 30 mg of a catalyst powder with 100 µL of H2O, 1010 µL of EtOH, and 75 µL of Nafion (5 wt%). The mixture was ultrasonicated until catalyst was well dispersed. For Pt/C catalyst (20 wt% Pt, HiSPEC-3000, Johnson-Matthey), a catalyst ink was prepared by mixing 3.5 mg of Pt/C catalyst with 100 µL of H2O, 1070 µL of EtOH and 30 µL of Nafion (5 wt%). 6 µL of a catalyst ink was pipetted with a micro-pipette onto the glassy carbon disk (0.247 cm2) of the RRDE, resulting in a catalyst loading of 0.6 mg cm−2 (70 µg cm−2 for Pt/C). 0.1 M HClO4 and 0.1 M KOH electrolytes were prepared using 70% HClO4 (double distilled, Veritas, GFS chemicals) and 99.99% KOH pellet (Aldrich) with 18.2 MΩ∙cm Millipore water. Before the linear sweep voltammetry (LSV), the catalyst was cleaned by cycling the potential between 0.05 and 1.2 V (vs RHE) for 20 cycles at a scan rate of 100 mV s−1 (500 mV s−1 for Pt/C) in an N2-saturated electrolyte. Subsequently, cyclic voltammetry was performed in the potential rage of 0.05 to 1.2 V at a scan rate of 20 mV s−1 (50 mV s−1 for Pt/C).
ORR polarization curves were obtained by LSV from 1.2 to 0.2 V (from −0.01 to 1.1 V for Pt/C) at a scan rate of 5 mV s−1 (20 mV s−1 for Pt/C) in an O2-saturated electrolyte at a rotation speed of 1600 rpm. To correct the non-Faradaic current (capacitive current) from the LSV curve, the same measurement was conducted in N2-saturated electrolyte. To investigate the kinetics, Tafel plot was generated according to Tafel equation
where E, b, ik, and C stand for the applied potential, the Tafel slope, the kinetic current, and constant, respectively. The kinetic current (ik) was extracted from the correction of diffusion-limited current.
where i and il signify the measured current and the diffusion-limited current (current at the plateau in the polarization curve), respectively. To measure peroxide production yield and electron transfer number (n), the applied potential of the Pt ring was held at 1.3 V (vs. RHE) during the potential sweep.
The peroxide yield and n were then calculated using the following relations
where N, id, and ir represent the collection efficiency (0.37, provided by manufacturer), the disk current, and the ring current, respectively. To measure solution resistance for iR-compensation, electrochemical impedance spectra were obtained at 0.68 V with AC potential amplitude of 10 mV
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from 100,000 to 1 Hz. In the carbon paper test, 40 µL of catalyst ink was deposited on the 1 × 1 cm2 carbon paper (EC-TP-060T, Toray), resulting in a catalyst loading of 1 mg cm−2. The measurement protocol with a carbon paper electrode was the same as that by the RRDE test. The measurements were independently repeated at least three times and the average data are presented.
2.3.RESULTS AND DISCUSSION
2.3.1. Synthesis Optimization of Fe-MPC Catalysts
Fe-MPC catalysts with tailored textural properties were prepared by a nanocasting method using mesoporous silicas with controlled pore sizes, pore connectivity, and particle sizes as the templates. We used three types of SBA-15 mesoporous silicas with hexagonal p6m symmetry and a KIT-6 mesoporous silica with cubic Ia3d symmetry as the templates. The detailed synthesis procedures for SBA-15 and KIT-6 silicas are described in the Experimental Section. For the SBA-15 silicas, three different hydrothermal treatment temperatures (35 °C, 100 °C, or 150 °C) were used, and the resulting SBA-15 silicas were denoted as SBA-15-X (X = 35, 100, and 150). Scanning electron microscopy (SEM) images of the three SBA-15 silicas show that they have very similar particle sizes of approximately 950 nm. In contrast, the KIT-6 silica comprises large particles of 1–20 μm with irregular distribution. Small-angle X-ray diffraction (XRD) patterns of the SBA-15 and KIT-6 silicas exhibited diffraction peaks commensurate with hexagonal p6m and cubic Ia3d symmetry, respectively, as reported earlier.61,62 The N2 adsorption isotherms of the SBA-15 silicas and the corresponding pore size distribution curves calculated by the Barrett–Joyner–Halenda (BJH) method suggested that each template has distinctively different pore sizes according to the hydrothermal treatment temperatures.
As the hydrothermal treatment temperature was decreased, the pore size decreased from 12.2 nm for SBA-15-150, to 8.2 nm for SBA-15-100, and to 4.8 nm for SBA-15-35. The KIT-6 silica had a pore size of 8.2 nm. The SBA-15 silicas also had significantly different pore connectivities. As suggested previously,63–65 SBA-15-150 and SBA-15-100 silicas have small complementary pores in the frameworks that connect adjacent mesopores, thus providing three-dimensionally (3D) interconnected pore structures. In contrast, the complementary pores were absent in SBA-15-35, creating 2D hexagonal arrays of mesopores.