Recent advances in active sites identification and regulation of <sc>M−N/C</sc> electro-catalysts towards ORR

Jie Liu; Zhao Jin; Xian Wang; Junjie Ge; Changpeng Liu; Wei Xing

doi:10.1007/s11426-018-9425-5

SCIENCE CHINA Chemistry

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SCIENCE CHINA Chemistry, Volume 62 , Issue 6 : 669-683(2019) https://doi.org/10.1007/s11426-018-9425-5

Recent advances in active sites identification and regulation of M−N/C electro-catalysts towards ORR

Jie Liu ^1,2, Zhao Jin ^3,4, Xian Wang ^2,3,4, Junjie Ge ^3,4,*, Changpeng Liu ^3,4,*, Wei Xing ^1,3,4,*

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Affiliations：

1. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

2. University of Science and Technology of China, Hefei 230026, China

3. Laboratory of Advanced Power Sources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China

4. Jilin Province Key Laboratory of Low Carbon Chemical Power Sources, Changchun 130022, China

* Corresponding authors (email: gejj@ciac.ac.cn; liuchp@ciac.ac.cn;xingwei@ciac.ac.cn)

ReceivedNov 28, 2018
AcceptedJan 22, 2019
PublishedMar 29, 2019

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Abstract

Funded by

National Science and Technology Major Project(2017YFB0102900)

the National Natural Science Foundation of China(21633008,21433003,U1601211,21733004)

Jilin Province Science and Technology Development Program(20150101066JC,20160622037JC,20170203003SF,20170520150JH)

Hundred Talents Program of Chinese Academy of Sciences and the Recruitment Program of Foreign Experts(WQ20122200077)

Acknowledgment

This work was supported by National Science and Technology Major Project (2017YFB0102900), the National Natural Science Foundation of China (21633008, 21433003, U1601211, 21733004), Jilin Province Science and Technology Development Program (20150101066JC, 20160622037JC, 20170203003SF, 20170520150JH), Hundred Talents Program of Chinese Academy of Sciences and the Recruitment Program of Foreign Experts (WQ20122200077).

Interest statement

The authors declare that they have no conflict of interest.

References

[1] Gewirth AA, Varnell JA, DiAscro AM. Chem Rev, 2018, 118: 2313-2339 CrossRef PubMed Google Scholar

[2] Sa YJ, Kim JH, Joo SH. J Electrochem Sci Te, 2017, 8: 169–182. Google Scholar

[3] Wang Y, Li J, Wei Z. ChemElectroChem, 2018, 5: 1764-1774 CrossRef Google Scholar

[4] Huo P, Zhao P, Wang Y, Liu B, Yin G, Dong M. Energies, 2018, 11: 167 CrossRef Google Scholar

[5] Kang SY, Kim HJ, Chung YH. Nano Converg, 2018, 5: 13 CrossRef PubMed ADS Google Scholar

[6] Kiani M, Zhang J, Luo Y, Jiang C, Fan J, Wang G, Chen J, Wang R. J Energy Chem, 2018, 27: 1124-1139 CrossRef Google Scholar

[7] Li JC, Hou PX, Liu C. Small, 2017, 13: 1702002 CrossRef PubMed Google Scholar

[8] Li R, Zhang D, Zhou Y, Wang X, Guo G. Sci China Chem, 2016, 59: 746-751 CrossRef Google Scholar

[9] Lu S, Jin Y, Gu H, Zhang W. Sci China Chem, 2017, 60: 999-1006 CrossRef Google Scholar

[10] Ma J, Xiang Z, Zhang J. Sci China Chem, 2018, 61: 592-597 CrossRef Google Scholar

[11] Tong X, Wei Q, Zhan X, Zhang G, Sun S. Catalysts, 2016, 7: 1 CrossRef Google Scholar

[12] Chong L, Wen J, Kubal J, Sen FG, Zou J, Greeley J, Chan M, Barkholtz H, Ding W, Liu DJ. Science, 2018, 362: 1276-1281 CrossRef PubMed Google Scholar

[13] Jasinski R. Nature, 1964, 201: 1212-1213 CrossRef Google Scholar

[14] Jahnke HMS, Zimmermann G. Top Curr Chem, 1976, 61: 131–181. Google Scholar

[15] Gupta S, Tryk D, Bae I, Aldred W, Yeager E. J Appl Electrochem, 1989, 19: 19-27 CrossRef Google Scholar

[16] Shao M, Chang Q, Dodelet JP, Chenitz R. Chem Rev, 2016, 116: 3594-3657 CrossRef PubMed Google Scholar

[17] Goellner V, Armel V, Zitolo A, Fonda E, Jaouen F. J Electrochem Soc, 2015, 162: H403-H414 CrossRef Google Scholar

[18] Zagal JH, Koper MTM. Angew Chem Int Ed, 2016, 55: 14510-14521 CrossRef PubMed Google Scholar

[19] Masa J, Xia W, Muhler M, Schuhmann W. Angew Chem Int Ed, 2015, 54: 10102-10120 CrossRef PubMed Google Scholar

[20] Ren Q, Wang H, Lu XF, Tong YX, Li GR. Adv Sci, 2018, 5: 1700515 CrossRef PubMed Google Scholar

[21] Song Z, Cheng N, Lushington A, Sun X. Catalysts, 2016, 6: 116 CrossRef Google Scholar

[22] Huang X, Wang Y, Li W, Hou Y. Sci China Chem, 2017, 60: 1494-1507 CrossRef Google Scholar

[23] Yeager E. 1984, 29: 1527-1537 Google Scholar

[24] Xiong D, Li X, Fan L, Bai Z. Catalysts, 2018, 8: 301 CrossRef Google Scholar

[25] Borghei M, Lehtonen J, Liu L, Rojas OJ. Adv Mater, 2018, 30: 1703691 CrossRef PubMed Google Scholar

[26] Fukuzumi S, Lee YM, Nam W. ChemCatChem, 2018, 10: 9-28 CrossRef Google Scholar

[27] Ji Y, Dong H, Liu C, Li Y. J Mater Chem A, 2018, 6: 13489-13508 CrossRef Google Scholar

[28] Choi CH, Lim HK, Chung MW, Chon G, Ranjbar Sahraie N, Altin A, Sougrati MT, Stievano L, Oh HS, Park ES, Luo F, Strasser P, Dražić G, Mayrhofer KJJ, Kim H, Jaouen F. Energy Environ Sci, 2018, 11: 3176-3182 CrossRef Google Scholar

[29] Barbusinski K. Ecol Chem Eng S, 2009, 16: 347–358. Google Scholar

[30] Nandan R, Gautam A, Nanda KK. J Mater Chem A, 2017, 5: 20252-20262 CrossRef Google Scholar

[31] Singh D, Mamtani K, Bruening CR, Miller JT, Ozkan US. ACS Catal, 2014, 4: 3454-3462 CrossRef Google Scholar

[32] Hu Y, Jensen JO, Zhang W, Martin S, Chenitz R, Pan C, Xing W, Bjerrum NJ, Li Q. J Mater Chem A, 2015, 3: 1752-1760 CrossRef Google Scholar

[33] Hu Y, Jensen JO, Zhang W, Cleemann LN, Xing W, Bjerrum NJ, Li Q. Angew Chem Int Ed, 2014, 53: 3675-3679 CrossRef PubMed Google Scholar

[34] Guo D, Shibuya R, Akiba C, Saji S, Kondo T, Nakamura J. Science, 2016, 351: 361-365 CrossRef PubMed ADS Google Scholar

[35] Cui X, Yang S, Yan X, Leng J, Shuang S, Ajayan PM, Zhang Z. Adv Funct Mater, 2016, 26: 5708-5717 CrossRef Google Scholar

[36] Choi CH, Baldizzone C, Polymeros G, Pizzutilo E, Kasian O, Schuppert AK, Ranjbar Sahraie N, Sougrati MT, Mayrhofer KJJ, Jaouen F. ACS Catal, 2016, 6: 3136-3146 CrossRef Google Scholar

[37] Jia Q, Ramaswamy N, Tylus U, Strickland K, Li J, Serov A, Artyushkova K, Atanassov P, Anibal J, Gumeci C, Barton SC, Sougrati MT, Jaouen F, Halevi B, Mukerjee S. Nano Energy, 2016, 29: 65-82 CrossRef Google Scholar

[38] Wang N, Lu B, Li L, Niu W, Tang Z, Kang X, Chen S. ACS Catal, 2018, 8: 6827-6836 CrossRef Google Scholar

[39] Kramm UI, Herranz J, Larouche N, Arruda TM, Lefèvre M, Jaouen F, Bogdanoff P, Fiechter S, Abs-Wurmbach I, Mukerjee S, Dodelet JP. Phys Chem Chem Phys, 2012, 14: 11673-11688 CrossRef PubMed ADS Google Scholar

[40] Xiao M, Zhu J, Ma L, Jin Z, Ge J, Deng X, Hou Y, He Q, Li J, Jia Q, Mukerjee S, Yang R, Jiang Z, Su D, Liu C, Xing W. ACS Catal, 2018, 8: 2824-2832 CrossRef Google Scholar

[41] Zhong L, Frandsen C, Mørup S, Hu Y, Pan C, Cleemann LN, Jensen JO, Li Q. Appl Catal B-Environ, 2018, 221: 406-412 CrossRef Google Scholar

[42] Lefèvre M, Dodelet JP, Bertrand P. J Phys Chem B, 2002, 106: 8705-8713 CrossRef Google Scholar

[43] Palaniselvam T, Kashyap V, Bhange SN, Baek JB, Kurungot S. Adv Funct Mater, 2016, 26: 2150-2162 CrossRef Google Scholar

[44] Kim D, Li OL, Saito N. Phys Chem Chem Phys, 2014, 16: 14905-14911 CrossRef PubMed ADS Google Scholar

[45] Artyushkova K, Serov A, Rojas-Carbonell S, Atanassov P. J Phys Chem C, 2015, 119: 25917-25928 CrossRef Google Scholar

[46] Jia Q, Liu E, Jiao L, Pann S, Mukerjee S. Adv Mater, 2018: e1805157. Google Scholar

[47] Tylus U, Jia Q, Strickland K, Ramaswamy N, Serov A, Atanassov P, Mukerjee S. J Phys Chem C, 2014, 118: 8999-9008 CrossRef PubMed Google Scholar

[48] Ren H, Wang Y, Yang Y, Tang X, Peng Y, Peng H, Xiao L, Lu J, Abruña HD, Zhuang L. ACS Catal, 2017, 7: 6485-6492 CrossRef Google Scholar

[49] Chung HT, Cullen DA, Higgins D, Sneed BT, Holby EF, More KL, Zelenay P. Science, 2017, 357: 479-484 CrossRef PubMed ADS Google Scholar

[50] Pfisterer JHK, Liang Y, Schneider O, Bandarenka AS. Nature, 2017, 549: 74-77 CrossRef PubMed ADS Google Scholar

[51] Fazio G, Ferrighi L, Perilli D, Di Valentin C. Int J Quantum Chem, 2016, 116: 1623-1640 CrossRef Google Scholar

[52] Holby EF, Zelenay P. Nano Energy, 2016, 29: 54-64 CrossRef Google Scholar

[53] Zheng Y, Yang DS, Kweun JM, Li C, Tan K, Kong F, Liang C, Chabal YJ, Kim YY, Cho M, Yu JS, Cho K. Nano Energy, 2016, 30: 443-449 CrossRef Google Scholar

[54] Chen X, Yu L, Wang S, Deng D, Bao X. Nano Energy, 2017, 32: 353-358 CrossRef Google Scholar

[55] Sebastián D, Serov A, Matanovic I, Artyushkova K, Atanassov P, Aricò AS, Baglio V. Nano Energy, 2017, 34: 195-204 CrossRef Google Scholar

[56] Yang L, Cheng D, Xu H, Zeng X, Wan X, Shui J, Xiang Z, Cao D. Proc Natl Acad Sci USA, 2018, 115: 6626-6631 CrossRef PubMed ADS Google Scholar

[57] Gu W, Hu L, Li J, Wang E. Electroanalysis, 2018, 30: 1217-1228 CrossRef Google Scholar

[58] Wang Y, Luo E, Xiao M, Ge J, Liu C, Xing W. Sci Sin Chem, 2017, 47: 554-564 CrossRef Google Scholar

[59] Lim KH, Kim H. Appl Catal B-Environ, 2014, 158-159: 355-360 CrossRef Google Scholar

[60] Deng J, Ren P, Deng D, Yu L, Yang F, Bao X. Energy Environ Sci, 2014, 7: 1919-1923 CrossRef Google Scholar

[61] Sahraie NR, Kramm UI, Steinberg J, Zhang Y, Thomas A, Reier T, Paraknowitsch JP, Strasser P. Nat Commun, 2015, 6: 8618 CrossRef PubMed ADS Google Scholar

[62] Malko D, Kucernak A, Lopes T. Nat Commun, 2016, 7: 13285 CrossRef PubMed ADS Google Scholar

[63] Ohtsuka M, Kitamura F. Electrochemistry, 2015, 83: 376-380 CrossRef Google Scholar

[64] Kattel S, Wang G. J Mater Chem A, 2013, 1: 10790 CrossRef Google Scholar

[65] Ramaswamy N, Tylus U, Jia Q, Mukerjee S. J Am Chem Soc, 2013, 135: 15443-15449 CrossRef PubMed Google Scholar

[66] Kramm UI, Herrmann-Geppert I, Behrends J, Lips K, Fiechter S, Bogdanoff P. J Am Chem Soc, 2016, 138: 635-640 CrossRef PubMed Google Scholar

[67] Liang W, Chen J, Liu Y, Chen S. ACS Catal, 2014, 4: 4170-4177 CrossRef Google Scholar

[68] Meng H, Jaouen F, Proietti E, Lefèvre M, Dodelet JP. Electrochem Commun, 2009, 11: 1986-1989 CrossRef Google Scholar

[69] Han Y, Wang Y, Xu R, Chen W, Zheng L, Han A, Zhu Y, Zhang J, Zhang H, Luo J, Chen C, Peng Q, Wang D, Li Y. Energy Environ Sci, 2018, 11: 2348–2352. Google Scholar

[70] Wu YJ, Wang YC, Wang RX, Zhang PF, Yang XD, Yang HJ, Li JT, Zhou Y, Zhou ZY, Sun SG. ACS Appl Mater Interfaces, 2018, 10: 14602-14613 CrossRef Google Scholar

[71] Xiao M, Zhang H, Chen Y, Zhu J, Gao L, Jin Z, Ge J, Jiang Z, Chen S, Liu C, Xing W. Nano Energy, 2018, 46: 396-403 CrossRef Google Scholar

[72] Xiao J, Xu Y, Xia Y, Xi J, Wang S. Nano Energy, 2016, 24: 121-129 CrossRef Google Scholar

[73] Hu Z, Guo Z, Zhang Z, Dou M, Wang F. ACS Appl Mater Interfaces, 2018, 10: 12651-12658 CrossRef Google Scholar

[74] Osmieri L, Escudero-Cid R, Armandi M, Monteverde Videla AHA, García Fierro JL, Ocón P, Specchia S. Appl Catal B-Environ, 2017, 205: 637-653 CrossRef Google Scholar

[75] Ratso S, Kruusenberg I, Käärik M, Kook M, Saar R, Kanninen P, Kallio T, Leis J, Tammeveski K. Appl Catal B-Environ, 2017, 219: 276-286 CrossRef Google Scholar

[76] Domínguez C, Pérez-Alonso FJ, Salam MA, Al-Thabaiti SA, Peña MA, García-García FJ, Barrio L, Rojas S. Appl Catal B-Environ, 2016, 183: 185-196 CrossRef Google Scholar

[77] Lee S, Kwak DH, Han SB, Lee YW, Lee JY, Choi IA, Park HS, Park JY, Park KW. ACS Catal, 2016, 6: 5095-5102 CrossRef Google Scholar

[78] Pascone PA, de Campos J, Meunier JL, Berk D. Appl Catal B-Environ, 2016, 193: 9-15 CrossRef Google Scholar

[79] Yasuda S, Furuya A, Uchibori Y, Kim J, Murakoshi K. Adv Funct Mater, 2016, 26: 738-744 CrossRef Google Scholar

[80] Wang B, Wang X, Zou J, Yan Y, Xie S, Hu G, Li Y, Dong A. Nano Lett, 2017, 17: 2003-2009 CrossRef PubMed ADS Google Scholar

[81] Lai Q, Zheng L, Liang Y, He J, Zhao J, Chen J. ACS Catal, 2017, 7: 1655-1663 CrossRef Google Scholar

[82] Wei W, Shi X, Gao P, Wang S, Hu W, Zhao X, Ni Y, Xu X, Xu Y, Yan W, Ji H, Cao M. Nano Energy, 2018, 52: 29-37 CrossRef Google Scholar

[83] Shen H, Gracia-Espino E, Ma J, Tang H, Mamat X, Wagberg T, Hu G, Guo S. Nano Energy, 2017, 35: 9-16 CrossRef Google Scholar

[84] Ahn SH, Yu X, Manthiram A. Adv Mater, 2017, 29: 1606534 CrossRef PubMed Google Scholar

[85] Tan H, Li Y, Kim J, Takei T, Wang Z, Xu X, Wang J, Bando Y, Kang YM, Tang J, Yamauchi Y. Adv Sci, 2018, 5: 1800120 CrossRef PubMed Google Scholar

[86] Hu BC, Wu ZY, Chu SQ, Zhu HW, Liang HW, Zhang J, Yu SH. Energy Environ Sci, 2018, 11: 2208–2215. Google Scholar

[87] Wang W, Luo J, Chen W, Li J, Xing W, Chen S. J Mater Chem A, 2016, 4: 12768-12773 CrossRef Google Scholar

[88] Li J, Chen S, Li W, Wu R, Ibraheem S, Li J, Ding W, Li L, Wei Z. J Mater Chem A, 2018, 6: 15504-15509 CrossRef Google Scholar

[89] Zhong W, Chen J, Zhang P, Deng L, Yao L, Ren X, Li Y, Mi H, Sun L. J Mater Chem A, 2017, 5: 16605-16610 CrossRef Google Scholar

[90] Liu Z, Sun F, Gu L, Chen G, Shang T, Liu J, Le Z, Li X, Wu HB, Lu Y. Adv Energy Mater, 2017, 7: 1701154 CrossRef Google Scholar

[91] Fu X, Hassan FM, Zamani P, Jiang G, Higgins DC, Choi JY, Wang X, Xu P, Liu Y, Chen Z. Nano Energy, 2017, 42: 249-256 CrossRef Google Scholar

[92] Sun M, Davenport D, Liu H, Qu J, Elimelech M, Li J. J Mater Chem A, 2018, 6: 2527-2539 CrossRef Google Scholar

[93] Wang Y, Chen W, Chen Y, Wei B, Chen L, Peng L, Xiang R, Li J, Wang Z, Wei Z. J Mater Chem A, 2018, 6: 8405-8412 CrossRef Google Scholar

[94] Pardo Pérez LC, Sahraie NR, Melke J, Elsässer P, Teschner D, Huang X, Kraehnert R, White RJ, Enthaler S, Strasser P, Fischer A. Adv Funct Mater, 2018, 28: 1707551 CrossRef Google Scholar

[95] Yang ZK, Yuan CZ, Xu AW. Nanoscale, 2018, 10: 16145-16152 CrossRef PubMed Google Scholar

[96] Qiao Z, Zhang H, Karakalos S, Hwang S, Xue J, Chen M, Su D, Wu G. Appl Catal B-Environ, 2017, 219: 629-639 CrossRef Google Scholar

[97] Yan X, Liu K, Wang T, You Y, Liu J, Wang P, Pan X, Wang G, Luo J, Zhu J. J Mater Chem A, 2017, 5: 3336-3345 CrossRef Google Scholar

[98] Liu L, Ci S, Bi L, Jia J, Wen Z. J Mater Chem A, 2017, 5: 14763-14774 CrossRef Google Scholar

[99] Wang S, He Q, Wang C, Jiang H, Wu C, Chen S, Zhang G, Song L. Small, 2018, 14: 1800128 CrossRef PubMed Google Scholar

[100] Cheng Q, Yang L, Zou L, Zou Z, Chen C, Hu Z, Yang H. ACS Catal, 2017, 7: 6864-6871 CrossRef Google Scholar

[101] Li Y, Gao J, Zhang F, Qian Q, Liu Y, Zhang G. J Mater Chem A, 2018, 6: 15523-15529 CrossRef Google Scholar

[102] Zhang Y, Lu L, Zhang S, Lv Z, Yang D, Liu J, Chen Y, Tian X, Jin H, Song W. J Mater Chem A, 2018, 6: 5740-5745 CrossRef Google Scholar

[103] Peera SG, Balamurugan J, Kim NH, Lee JH. Small, 2018, 14: 1800441 CrossRef PubMed Google Scholar

[104] Sun T, Xu L, Li S, Chai W, Huang Y, Yan Y, Chen J. Appl Catal B: Environ, 2016, 193: 1–8. Google Scholar

[105] Wang L, Wurster P, Gazdzicki P, Roussel M, Sanchez DG, Guétaz L, Jacques PA, Gago AS, Andreas Friedrich K. J Electroanal Chem, 2018, 819: 312-321 CrossRef Google Scholar

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Figure 1
Direct pathway and indirect pathway of ORR mechanism [22] (color online).
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Figure 2
Oxygen adsorption form on the surface of catalysts [23].
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Figure 3
Structural and elemental characterization and ORR performance of the 4 types of N-HOPG model catalysts [34] (color online).
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Figure 4
An example of high resolution XPS spectra employed for identification of the active sites: (a) N 1s and (b) Fe 2p [45] (color online).
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Figure 5
Revealing catalytic sites under reaction conditions [50] (color online).
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Figure 6
Illustrations of the synthesis of (a) FeCl1N4/CNS [69] and (b) S-Fe/Z8/2-AT catalyst [70] (color online).
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Figure 7
Illustration of the process to prepare the (a) Co₂N_xC_y catalyst [71] and (b) Fe₂N/MNGCS catalyst [72] (color online).
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Figure 8
Illustration of the preparation of active Fe/Co-NpGr [43] (color online).
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Figure 9
Electrochemical characterizations of the selected catalysts under alkaline environment [73] (color online).
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Figure 10
Electrochemical characterizations of the selected catalysts under acid environment [73] (color online).
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Figure 11
Illustration of the process to yield the Fe–N–C/VA-CNT catalyst [79] (color online).
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Figure 12
Illustration of the synthesis of Fe−N-SCCFs [80] (color online).
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Figure 13
(a) Illustration of the host-guest chemistry strategy to form MOF-derived Fe-N/C catalysts [81]; (b) schematic procedure for the synthesis of Fe–N/C catalysts [40]; (c) the schematic illustration for the non-solution fabrication process of Fe–N/C [82] (color online).
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Figure 14
(a) Synthetic procedure of FeN₂/NOMC; (b) TEM image;(c, d) HAADF STEM images; (e–i) EELS mapping [83] (color online).
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Figure 15
(a) Illustration of the steps for preparing PS-b-P2VP-b-PEO micelle@M-FR spheres and pyrolyzed hollow Fe₃C-Fe,N/C-x spheres [85]; (b) schematic illustration of the formation processes of the up-Fe-NCNF (top) and p-Fe-N-CNF (bottom) catalysts [86] (color online).
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Figure 16
(a) Schematic illustration of the synthesis of the PVP-NaCl-Fe/N/C catalyst [87]; (b) schematic fabrication of the Fe/N/C-ZnCl₂/KCl catalyst based on the porous ZnCl₂+KCl eutectic salt [88] (color online).
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Figure 17
SEM images of the as-prepared catalysts [93] (color online).
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Figure 18
Synthesis illustrations for the route to prepare carbon support-free Fe–N/C catalysts [96] (color online).
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Figure 19
Schematic illustrations for the preparation of activity mentioned as well as corresponding structure models [99] (color online).
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Figure 20
Schematic illustration of the fabrication process of Co@NCNT HMS [101] (color online).
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Figure 21
Preparation of MOF-derived Co@NC catalyst [103] (color online).
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Figure 22
Schematic of the process to prepare the Co-N-OMMCs [104] (color online).
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Figure 23
Structure of synergistic ORR catalysts with an ultralow concentration of Pt-alloy supported upon PGM-free materials, denoted as LP@PF [12] (color online).