Interwoven Molecular Chains Obtained by Ionic Self-Assembly of Two

Subscriber access provided by Macquarie University

Functional Nanostructured Materials (including low-D carbon)

Interwoven Molecular Chains Obtained by Ionic Self-Assembly of Two Iron (III) Porphyrins with Opposite and Mismatched Charges Yan Xie, Qinglu Zhong, Yang Lv, Jia Li, Zhiqiang Hao, Chizhou Tang, Xuming Wei, Yang Su, Jiahui Huang, Anjie Wang, Xinwen Guo, Junhu Wang, Guohui Li, and Yujiang Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b07460 • Publication Date (Web): 28 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Interwoven Molecular Chains Obtained by Ionic Self-Assembly of Two Iron (III) Porphyrins with Opposite and Mismatched Charges Yan Xie,†,⊥,‡ Qinglu Zhong,∥,｜,‡ Yang Lv,† Jia Li,† Zhiqiang Hao,† Chizhou Tang,⊥,｜ Xuming Wei,⊥ Yang Su,⊥ Jiahui Huang,⊥ Anjie Wang,† Xinwen Guo,† Junhu Wang,⊥ Guohui Li,*,∥ and Yujiang Song*,†

†State

Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian

University of Technology, Dalian 116024, P.R. China.

⊥ Dalian

Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan

Road, Dalian 116023, P. R. China.

ACS Paragon Plus Environment

1

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

∥

Page 2 of 28

Laboratory of Molecular Modeling and Design, State Key Laboratory of Molecular

Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, P. R. China.

｜

University of Chinese Academy of Sciences, No.19(A) Yuquan Road, Shijingshan

District, Beijing 100049, P. R. China.

KEYWORDS: Porphyrin; Electrocatalysts; Ionic Self-Assembly; Oxygen Reduction Reaction; Molecular Dynamics Simulation

ABSTRACT: We report ionic self-assembly of positively charged FeIII meso-tetra(Nmethyl-4-pyridyl) porphyrin (FeIIINMePyP) with negatively charged FeIII meso-tetra(4sulfonatophenyl) porphyrin (FeIIITPPS4), leading to the formation of flower-like nanostructures composed of unprecedented three-dimensional (3D) entangled chains of porphyrin dimers. Molecular dynamics (MD) simulations show that the 3D entanglement


2

Page 3 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of porphyrin chains closely correlates to mismatched charges present in porphyrin dimers like [FeIII(H2O)2NMePyP]5+/[FeIII(H2O)2TPPS4]3+ that requires extra interactions or entanglement with neighboring ones to achieve electric neutrality. Interestingly, the interwoven chains bring in excellent thermal stability as evidenced by well maintenance of the flower-like morphology after pyrolysis at 775 °C in argon, which is in good agreement of high-temperature MD simulations. Meanwhile, heat-treatment of the flowerlike porphyrin nanostructure leads to the formation of a non-noble metal electrocatalyst (NNME) with largely inherited morphology. This exemplifies a new approach by combining ionic self-assembly with subsequent pyrolysis for the synthesis of NNMEs with desired control over the morphology of template-free NNMEs that has rarely been achieved prior to this study. Furthermore, our electrocatalyst exhibits excellent activity and durability toward oxygen reduction reaction (ORR) as well as much better methanol tolerance compared with commercial Pt/C in alkaline solutions.


3


Page 4 of 28

1. INTRODUCTION

Pt-based electrocatalysts are currently confronting the issue of prohibitive cost and limited supply, and thus have become a major obstacle preventing polymer electrolyte membrane fuel cells (PEMFCs) from widespread commercialization.1 Non-expensive and abundant non-noble metal electrocatalysts (NNMEs) with a high performance toward oxygen reduction reaction (ORR) are required to replace Pt-based electrocatalysts. In general, NNMEs can be categorized into carbonized transition metallomacrocycles,2-6 carbon supported transition metal oxides,7,8 doped carbon materials,9,10 transition metal carbides,11 sulfides12 and others13. In particular, carbonized transition metallomacrocycles have been considered as highly promising NNMEs for the replacement of Pt-based electrocatalysts. Since cobalt phthalocyanine (CoPc) was identified to possess ORR activity in an alkaline electrolyte,14 continuous efforts have been devoted to the exploration of new synthetic approaches by using porphyrins or phthalocyanines as precursors. Loh and co-workers functionalized both sides of reduced graphene oxides (rGO) with pyridine ligands to link metalloporphyrin nodes, thus building up a novel hybrid metal organic framework (MOF).15 Tang and co-workers synthesized an interesting NNME by incorporating cobalt (II) porphyrin in-between rGO sheets in a layer-bylayer (LBL) assembly process.16 Müllen and co-workers introduced mesoporous NNMEs by using vitamin B12 and ordered mesoporous silica SBA-15 as the metal precursor and the hard template to fabricate well-defined porous NNMEs with high surface area and narrow mesopore size distribution.17 Later, this group fabricated unique NNMEs via template-free pyrolysis on the conjugated mesoporous polymer frameworks based on Co (II) porphyrin.18 Campidelli and coworkers prepared carbon nanotube/porphyrin hybrid NNME by polymerizing cobalt (II) mesotetraethynylporphyrin around multi-walled carbon nano-tubes through Hay-coupling reaction.19


4

Page 5 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Dai and co-researchers designed and synthesized a class of 2D covalent organic polymers (COPs) by controlling locations of both N hetero-atoms and holes with N-containing ligands like porphyrin in a nickel-catalyzed Yamanoto reaction.20 Recently, Dai, Xiang and co-workers employed MOF180 with ultrahigh nano-confined space as the template for in-situ growth of well-defined porphyrin COPs as ORR electrocatalysts.21 In addition, Shelnutt and co-researchers firstly introduced the ionic self-assembly approach to obtain nanostructures over a decade ago,22 nano- to micro-sized structures have been assembled by oppositely charged porphyrins, which can be further ultilized in photo- and electrocatalysis.23,24 Herein, we introduce a new approach for the synthesis of NNMEs by using ionic self-assembly of two oppositely charged iron (III) porphyrins in combination with subsequent pyrolysis. Surprisingly, the flower-like morphology of ionic self-assembled porphyrins was well maintained merely with some overall shrinkage after pyrolysis at 775 oC in argon. This enables the desired manipulation of the morphology of self-supported NNMEs that has rarely been realized prior to this study. MD simulations turn out that alternately linking up of oppositely charged iron (III) porphyrins in a manner of edge-by-edge (J-aggregates) leads to the formation of molecular chains. Besides π-π interaction, van der Waals, steric hindrance and so on, mismatched charges present in each porphyrin dimer of a chain, including [FeIII(H2O)2NMePyP]5+/[FeIII(H2O)2TPPS4]3-, [FeIII(H2O)(OH-)NMePyP]4+/[FeIII(H2O)2TPPS4]3-,

and

[FeIII(OH-)NMePyP]4+/[FeIII(H2O)2TPPS4]3-), require interactions with neighboring chains to reach electric neutrality. The mismatched charge is a primary driving force for the entanglement of porphyrin chains. Furthermore, high-temperature molecular dynamics (MD) simulations show that interwoven molecular chains in the self-assembled flower-like nanostructures would account for the morphological preservation during pyrolysis. In addition, the resultant NNMEs exhibit


5


Page 6 of 28

pronounced ORR activities and much better durability toward ORR and excellent methanol tolerance compared with Pt/C electrocatalyst in alkaline media. This study introduces a new and general synthetic approach by simply combing ionic self-assembly of porphyrins with pyrolysis for the synthesis of high-performance NNMEs with controllable morphology. 2.

EXPERIMENTAL SECTION

2.1. Synthesis In a typical synthesis, 10 mL of 5 mM FeIIINMePyP aq. was mixed with 10 mL of 5 mM FeIIITPPS4 aq. under vigorous stirring for 10 mins, followed by incubation without stirring at 25 oC

for 24 h. Brown precipitates were vacuum-filtered and dried overnight at 25 oC. Subsequently,

the obtained samples were further heat-treated at different temperatures from 600 to 825 oC under Ar at 80 mL min-1 and kept at 600 to 825 oC for 2 h. After cooled naturally down to the room temperature, the heated samples were then acid leached for 0.5 h with 0.5 M H2SO4 aq. at 80 oC and then washed with de-ionized water until the pH value reached neutral. Lastly, samples were dried again in an oven at 65 oC overnight. 2.2. Electrochemical measurements The rotation disk electrode (RDE, PINE Instrument) with a disk area of 0.19625 cm2 and the rotating ring-disk electrode (RRDE, PINE Instrument) with a disk area of 0.2475 cm2 and a ring area of 0.1886 cm2 were polished with aluminum oxide powder and rinsed thoroughly with nanopure water. About 5 mg of electrocatalysts was dispersed into a mixture of Nafion, water and ethanol (VNafion:Vwater:Vethanol=0.06:1:9) to obtain a suspension of 2 mg mL-1. After 5 min of ultrasonication, 60 µL and 120 µL of the suspension were dropped onto the surface of the glassy carbon (GC) electrodes to obtain the electrocatalyst loading on GC as 0.6 and 1.2 mg cm-2, respectively. For comparison, 1 mg mL-1 of suspension of 20 wt% Pt/C electrocatalyst (Johnson Matthey, JM)


6

Page 7 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


was also prepared using the same method. The total loading of Pt is 20 µgPt cm-2, corresponding to 20 µL of the addition of the suspension on GC electrode. Electrochemical measurements were conducted on a three-electrode electrochemical cell. The catalyst modified GC electrode was used as the working electrode. A graphite rod or a platinum mesh (1 cm x 1 cm) was used as the counter electrodes for the ORR performance and durability test. Hg/HgO (1 M NaOH) was used as the reference electrode in 0.1 M KOH aqueous. All reported potentials were calibrated to be vs. RHE. The conversion constants from Hg/HgO to RHE was shown in Figure S1. Thus, the corresponding conversion equation was E(RHE)=E(Hg/HgO)+0.897 V. Cyclic voltammetry (CV) of electrocatalysts was recorded in N2-saturated electrolyte from 0.1 to 1.1 V (vs. RHE) in 0.1 M KOH aq. at a positive scanning rate of 100 mV s-1. The polarization curves of ORR were collected by using RDE and RRDE techniques at 5 mV s-1 and 1600 rpm in O2-saturated 0.1 M KOH aqueous electrolyte. The percentage of HO2- (η) and n was determined based on equations as follows: 𝜼 = 𝟐𝟎𝟎 ×

𝑰𝑹 ⁄𝑵 𝑰𝑫 + 𝑰𝑹 ⁄𝑵

(1)

𝒏=𝟒×

𝑰𝑫 𝑰𝑫 + 𝑰𝑹 ⁄𝑵

(2)

Here, ID and IR denotes the disk current and ring current, respectively. N is the H2O2 collection coefficient of 37% at the ring electrode. The number of electrons transfer can also be evaluated by Koutecky-Levich (K-L) equation: 𝟏 𝟏 𝟏 = + 𝑰𝐃 𝑰𝐊 𝐁𝛚𝟏/𝟐

(3)

ID and IK represent the measured current density at the disk and the kinetic current in amperes at a constant potential, respectively. ω is known to be the electrode rotation speed in rpm. B as the


7


Page 8 of 28

reciprocal of the slope can be further obtained from K-L plot by Levich equation: 𝑩 = 𝟎. 𝟔𝟐𝒏𝑭𝑨𝑪𝟎 𝑫𝟎

𝟐� −𝟏� 𝟑𝝂 𝟔

(4)

The n is the number of electrons transferred per oxygen molecule. The Faraday (F) constant is known to be 96485 C mol-1. The diffusion coefficient and the concentration of O2 are described as D0 (1.93×10-5 cm2 s-1) and C0 (1.26×10-6 mol cm-3) in 0.1 M KOH aq. ν as the kinetic viscosity of the solution is 0.01009 cm2 s-1. Chronoamperometric responses were conducted as a durability test at a constant potential of 0.8 V (vs. RHE) in O2-saturated 0.1 M KOH aq. at 1600 rpm for both as-obtained NNMEs and Pt/C. In this case, graphite rod as the counter electrode was used to avoid the interference from the counter electrode.25 Chronoamperometric responses were also measured for NNMEs and 20 wt% Pt/C at the potential of ~0.9 V (vs. RHE) in O2-saturated 0.1 M KOH aqueous. Meanwhile, 10 vol% methanol was injected into the electrolyte during the chronoamperometric measurement. 3. RESULTS AND DISCUSSION In a typical synthesis, 10 mL of 5 mM FeIIINMePyP (Figure 1a) aqueous solution was mixed with 10 mL of 5 mM FeIIITPPS4 (Figure 1a) aqueous solution at room temperature (RT) under vigorous stirring. The mixture instantaneously turned cloudy and produced brown precipitates denoted as FeIIINMePyP/FeIIITPPS4-RT. For comparison, we also carried out ionic self-assembly of the oppositely charged porphyrins at 5 oC (FeIIINMePyP/FeIIITPPS4-LT) or at RT in the presence of sodium dodecyl sulphate (SDS; FeIIINMePyP/FeIIITPPS4-SDS). These three samples were then heat-treated in Ar and leached at 80 oC with 0.5 M H2SO4 aq. and washed with nanopure water to remove by-products and unstable species.


8

Page 9 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) revealed that the obtained FeIIINMePyP/FeIIITPPS4-RT are globular flower-like nanostructures that are well separated from one another with an average diameter of 338 ± 77 nm in Figure 1b, 1c, S2a and S2b. Interestingly, after heat-treatment and acid-leaching, the overall flower-like morphology was well retained with the average diameter shrinking to 301 ± 62 nm (Figure 1d, 1e, S2c and S2d). FeIIINMePyP/FeIIITPPS4-LT also takes the flower-like morphology with an average diameter of 460 ± 87 nm (Figure S2e and S2f), which is larger than that of FeIIINMePyP/FeIIITPPS4-RT likely due to a smaller number of nucleation centers formed at 5 oC. In contrast, FeIIINMePyP/FeIIITPPS4SDS possesses oval-shaped nanostructures with voids (Figure S3a and S3b). In this case, the sulphate group of SDS might interference the ionic self-assembly process, causing the formation of the oval shape. After pyrolysis and acid leaching, the flower-like morphology of FeIIINMePyP/FeIIITPPS4-LT is also maintained with a shrunk average diameter of 399 ± 59 nm (Figure S2g and S2h). For the case of FeIIINMePyP/FeIIITPPS4-SDS, the original oval morphology was also basically remained with some structural collapse (Figure S3c and S3d) likely due to the small average size. UV-visible spectra of FeIIINMePyP/FeIIITPPS4-RT, FeIIINMePyP and FeIIITPPS4 (Figure 1f) show that the Soret band of FeIIINMePyP/FeTPPS4-RT (454 nm) apparently red-shifts relative to that of FeIIINMePyP and FeIIITPPS4, suggestive of appreciable interactions among porphyrin molecules and the formation of J-aggregates.26 The Q band residing at 551 nm of FeIIINMePyP/FeIIITPPS4-RT further proves the formation of J-aggregates.27 As shown in Figure S3e, the thermogravimetry (TG) behavior of FeIIINMePyP/FeIIITPPS4-RT is quite different from that of the two constituent porphyrins owning to the interactions between oppositely charged porphyrins. The loss of peripheral groups shifts to a much higher temperature relative to that of


9


Page 10 of 28

FeIIINMePyP and FeIIITPPS4. FeIIINMePyP/FeIIITPPS4-RT continuously loses weight with increased temperature after 350 oC and has no specific carbonization temperature that can be identified. Additionally, Fourier transformation infrared (FT-IR) spectra in Figure 1g show characteristic peaks C=N stretching vibration of pyridine group at 1640 cm-1, stretching vibration of C=N at 1512 cm-1 and C=C of porphyrin rings at 1462 cm-1, respectively,28 as well as symmetrical and asymmetrical stretching vibration of sulfonic group29 at 1132 cm-1 of FeIIINMePyP/FeIIITPPS4-RT after pyrolysis at 775 oC and acid leaching. Taken together, it is certain that the nanostructures produced by the ionic self-assembly process can well survive heattreatment and acid leaching, allowing us to manipulate the morphology of self-supported NNMEs, which has rarely been realized previously.

Figure 1. Characterizations of flower-like nanostructures: Molecular structure of FeIIINMePyP and FeIIITPPS4 (a); TEM images of FeIIINMePyP/FeIIITPPS4-RT before (b, c) and after (d, e) pyrolysis and acid leaching (Insets: corresponding size distribution plots based on measurements of over 200 individual nanostructures); typical UV-visible spectra of FeIIINMePyP, FeIIITPPS4 and


10

Page 11 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


ionic self-assembled FeIIINMePyP/FeIIITPPS4-RT (f) in water at room temperature and FT-IR spectra of FeIIINMePyP, FeIIITPPS4, FeIIINMePyP/FeIIITPPS4-RT and FeIIINMePyP/FeIIITPPS4RT-775 (g). For simplicity, axial ligation species were left out. It is necessary to elucidate the origin of the interesting morphological maintenance after pyrolysis. We tried to employ MD simulations to study this phenomenon at molecular level to correlate the structure and the appeared morphological stability. Since FeIIITPPS4 carries two H2O as axial ligands, and FeIIINMePyP carries two H2O, or H2O and OH-, or OH- as axial ligation species,30

there

should

exist

three

types

of

porphyrin

dimers,

including

[FeIII(H2O)2NMePyP]5+/[FeIII(H2O)2TPPS4]3-, [FeIII(H2O)(OH-)NMePyP]4+/[FeIII(H2O)2TPPS4]3-, and [FeIII(OH-)NMePyP]4+/[FeIII(H2O)2TPPS4]3-) as shown in Figure 2. The ionic self-assembly process of porphyrin dimers can be captured by MD simulations (Figure S4 and Figure 3). It is clear that the adjacent oppositely charged porphyrins link up in an edge-to-edge manner, forming porphyrin J-aggregates that agrees well with UV-vis spectra (Figure 1f). It is worth noting that the J-aggregates further evolve into a flexible molecular chain with angulations as shown in Figure 3a-c. We define that each oppositely charged porphyrin dimer with the closest distance between any two non-hydrogen atoms less than 0.32 nm, a rational ionic bond length, starts to form a chain. Besides π-π interaction, steric hindrance and van der Waals, the angulation might be correlated to mismatched charge in each porphyrin dimer, requiring extra electrostatic interactions with neighboring chains to achieve electric neutrality. For the case of an ideal [FeII(H2O)2TPPS4]4/[FeII(H2O)2NMePyP]4+ dimer with matched charge, we do observe the development of straight segment mixed with irregular stacking of the dimer (Figure S5a). This verifies that the mismatched charge is a crucial driving force for the formation of twisted chains. Meanwhile, steric hindrance, π-π interaction and van der Waals should not be ignored. Many twisted molecular


11


Page 12 of 28

chains interweave with one another into a sphere (Figure 3a). Although the sphere is not exactly the same as the nanoflowers, yet does resemble the constituent sphere of the flower-like morphology (Figure S2b) and the MD simulations are deemed to be in good agreement with our experimental results. We tentatively use “net-knots” concept to explain the extraordinary morphological maintenance at nanoscale. As the porphyrin in a porphyrin dimer attracts other oppositely charged porphyrin(s) in the vicinity for electric neutrality, a porphyrin net starts to be woven. When a porphyrin attracts more than two oppositely charged porphyrins, the porphyrin site can be defined as a net knot. As shown in Figure 3d, darkness represents how many neighboring porphyrin molecule(s) a porphyrin can attract. Approximately 50% of the individual porphyrin molecules serve as net-knots, thus accounting for the excellent thermal stability of the flower-like structure (more discussions in the supporting information). Additionally, it turns out three types of porphyrin dimers with mismatched charge all form entangled chains and spheres regardless of the difference in ligation species (Figure 3e, S5b and S5c). MD simulations further show that a spherical structure can be well retained under theoretical high temperature conditions (Figure 3f), corroborating our TEM observations in Figure 1.


12

Page 13 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Three types of porphyrin dimers formed in the ionic self-assembly process due to

the

presence

of

different

[FeIII(H2O)2TMePyP]5+/[FeIII(H2O)2TPPS4]3-,

axial (b)

ligands,

including

(a)

[FeIII(H2O)(OH-)TMePyP]4+/

[FeIII(H2O)2TPPS4]3- and (c) [FeIII(OH-) TMePyP]4+/[FeIII(H2O)2TPPS4]3-.


13


Page 14 of 28

Figure 3. MD simulations: (a) a typical sphere formed by ionic self-assembly of 1008 individual porphyrin molecules in total with a ratio of 378:630 between [FeIII(H2O)2NMePyP]5+ and [FeIII(H2O)2TPPS4]3-

after

enhanced

sampling

and

relaxation

(For

simplicity,

[FeIII(H2O)2TMePyP]5+ and [FeIII(H2O)2TPPS4]3- are represented by red and blue X, respectively.); (b) a twisted porphyrin chain formed by alternated [FeIII(H2O)2TMPyP]5+ and [FeIII(H2O)2TPPS]3-; (c) representative edge-to-edge arrangement (J-aggregate) of [FeIII(H2O)2TMePyP]5+ and [FeIII(H2O)2TPPS4]3- in the selected region of (b); (d) a sphere used for the analysis of the number of “net-knot”, N=0 as white, N=1, 2 as gray and N>2 as black; (e) ionic self-assembly of 1008 porphyrin molecules in total into nanosphere(s) after enhanced sampling and relaxation for [FeIII(OH-)NMePyP]4+/[FeIII(H2O)2TPPS4]3-; (f) heat-treated sphere in (e) at 1048 K for 100 ns. The ratio between postively and negatively charged porphyrin is kept at 432:576 for


14

Page 15 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[FeIII(OH-)NMePyP]4+/[FeIII(H2O)2TPPS4]3-.

(For

simplicity,

[FeIII(OH-)NMePyP]4+

and

[FeIII(H2O)2TPPS4]3- are represented by red and blue X, respectively.) Heat-treated porphyrin represents a class of highly promising NNMEs toward ORR. In general, pyrolysis temperature is a key synthetic parameter and routinely needs to be optimized in order to achieve desired ORR performance.31 Consequently, we chose a series of heating temperatures ranging from 600 to 825 oC to determine the optimum pyrolysis temperature. The ORR polarization curves of FeIIINMePyP/FeIIITPPS4-RT heat-treated at different temperatures were collected on a RDE in 0.1 M KOH aq. in Figure 4a. It appears that 775 oC is the optimum carbonization temperature according to the potentials at 3 mA cm-2 (Figure 4b). Therefore, the identified 775 oC was employed to heat-treat all the samples. As-prepared NNMEs were marked as FeIIINMePyP/FeIIITPPS4-X-775 (X=RT, LT, SDS). For comparison, the mechanical mixture of two pristine porphyrins heat-treated and acid leached under the same conditions was labelled as PFeIIINMePyP/FeIIITPPS4-RT-775. The ORR performances of electrocatalysts were conducted on RDE in 0.1 M KOH aqueous solution. As shown in Figure 4c, all of the three FeIIINMePyP/FeIIITPPS4-X-775 (X=RT, LT, SDS) electrocatalysts outperform P-FeIIINMePyP/FeIIITPPS4-RT-775, which verifies the importance of ionic self-assembly process for gaining a high ORR performance. In addition, FeIIINMePyP/FeIIITPPS4-RT-775 shows an ORR activity a little better than that of FeIIINMePyP/FeIIITPPS4-SDS-775 and FeIIINMePyP/FeIIITPPS4-LT-775, and comparable to that of 20 wt% Pt/C and other reported NNMEs in 0.1 M KOH (Table S1). To understand the electron transfer kinetics of the electrocatalysts,32,33 ORR polarization curves at different rotation rates from 625 to 2500 rpm were recorded in O2-saturated 0.1 M KOH aqueous solution. It appears that the diffusion-limited current density increases with enlarged rotation speed as shown in Figure S6a-


15


Page 16 of 28

c. The corresponding Koutecky-Levich (K-L) plots based on various potentials show good linearity over 0.55 to 0.75 V (vs. RHE) (Figure S6d-f). According to Figure S6, the average electron transfer number of n was determined to be 3.92, 3.72, and 3.99 (Figure 4d) for FeIIINMePyP/FeIIITPPS4-RT-775,

FeIIINMePyP/FeIIITPPS4-LT-775,

and

FeIIINMePyP/FeIIITPPS4-SDS-775, respectively. This indicates that oxygen has been primarily reduced to OH- through a four-electron route without producing much intermediate HO2-.34 We also studied the reaction kinetics by using RRDE. Based on RRDE measurements, the n number of FeIIINMePyP/FeIIITPPS4-RT-775 between 0.1 to 0.8 V (vs. RHE) was 3.94 (Figure S7a), which is consistent with the value of 3.92 based on K-L plots. This n value is close to that of 3.98 of Pt/C electrocatalyst

determined

by

RRDE.

In

addition,

the

calculated

HO2-%

of

FeIIINMePyP/FeIIITPPS4-RT-775 is lower than 4.5%, again close to HO2-% of Pt/C electrocatalyst. Since durability is another key parameter for the evaluation of electrocatalysts, chronoamperometric responses were conducted at 0.8 V (vs. RHE) at 1600 rpm in 0.1 M KOH. FeIIINMePyP/FeIIITPPS4-RT-775 maintains 87.6 % of their initial ORR activity, whereas only 51.9 % of initial activity is retained for Pt/C (Figure 4e). Pt/C experienced a dissolution/re-deposition Ostwald ripening process during durability test similar to realistic working conditions of fuel cell stacks, leading to a serious degradation.35 While for the case of FeIIINMePyP/FeIIITPPS4-RT-775, it is possible there might be no such degradation mechanism involved in the durability test and thus demonstrates a superb ORR durability. For direct methanol fuel cells, resistance to methanol crossover is a crucial property needed for ORR electrocatalysts. As shown in Figure 4f, current-time (i-t) chronoamperometric response suggests that FeIIINMePyP/FeIIITPPS4-RT-775 is slightly influenced by methanol crossover as evidenced by small current density change after the introduction of methanol. However, the


16

Page 17 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chronoamperometric response of commercial Pt/C changes dramatically after methanol is injected. It is clear that our electrocatalyst is superior to commercial Pt/C according to methanol resistance.

Figure 4. Electrocatalytic properties of electrocatalysts: (a) ORR polarization curves of FeIIINMePyP/FeIIITPPS4-RT-T, T=600, 700, 775, 800 and 825 oC (Test conditions: 1600 rpm, 0.1 to 1.1 V vs. RHE, 5 mV s-1, and an eletrocatalyst loading of 0.6 mg cm-2 on GC in O2-saturated 0.1 M KOH), (b) corresponding potentials of FeIIINMePyP/FeIIITPPS4-RT-T at 3 mA cm-2 in (a); (c) ORR polarization curves of P-FeIIINMePyP/FeIIITPPS4-775, FeIIINMePyP/FeIIITPPS4-X-775 (X=RT, LT and SDS) and commercial Pt/C recorded at 1600 rpm with a positive scanning rate of 5 mV s-1, and an electrocatalyst loading of 1.2 mg cm-2 on GC in O2-saturated 0.1 M KOH; (d) the electron transfer number at certain potentials from 0.55 V to 0.75 V (vs. RHE) for FeIIINMePyP/FeIIITPPS4-X-775 (X=RT, LT and SDS); (e) normalized current degradation of FeIIINMePyP/FeIIITPPS4-RT-775 and 20 wt% Pt/C at a potential of 0.8 V (vs. RHE) with the counter electrode of graphite rod; (f) i-t curves of FeIIINMePyP/FeIIITPPS4RT-775 and commercial Pt/C after the addition (at ~400 s) of 10% (v/v) methanol in O2-saturated 0.1 M KOH while holding the potential at ~0.9 V (vs. RHE).


17


Page 18 of 28

To correlate the outstanding ORR performance of our electrocatalysts with the structural and compositional features, we further characterized the electrocatalysts. XRD pattern of self-assembled FeIIINMePyP/FeIIITPPS4-RT is quite different from that of pristine FeIIINMePyP and FeIIITPPS4 (Figure 5a), suggestive of ordered arrangement of oppositely charged porphyrins during the ionic self-assembly process as depicted by the MD simulations. According to selected area electron diffraction (SAED) pattern in Figure 5b, it reveals certain crystallinity and specific growth tropism of FeIIINMePyP/FeIIITPPS4RT, which is again consistent with the MD simulations. FeIIINMePyP/FeIIITPPS4-RT-775 shows characteristic diffraction peaks residing at 26.00o from graphitic (0 0 2), and at 30.29, 35.68, 43.38, 53.77, 57.38 and 62.97o that can be attributed to γ-Fe2O3 (JCPDS 25-1402).36 This suggests that FeIIINMePyP/FeIIITPPS4-RT has been carbonized and some iron species have been converted into Fe2O3 during the pyrolysis process. High-resolution TEM (HRTEM) image in Figure 5c shows well-defined lattice fringes of a typical nanoparticle with d spacing of 2.517 Å, corresponding to (3 1 1) plane of γ-Fe2O3. This is consistent with the XRD pattern. High-angle annular dark-field scanning TEM (HAADF-STEM) image (Figure 5d) of FeIIINMePyP/FeIIITPPS4-RT-775 demonstrates that the average size of Fe2O3 nanoparticles is 36±14 nm encapsulated by carbon layers. The presence of metal oxides wrapped with carbon layers would be an important contributor to the highperformance in alkaline.37 In addition, energy-dispersive X-ray spectrometry (EDX) and elemental mapping of FeIIINMePyP/FeIIITPPS4-RT-775 in Figure S7b, S7c and Figure 5ei show the existence of nitrogen, oxygen, carbon, iron and sulphur. To examine the local environment of iron,

57Fe

Mössbauer spectra of FeIIINMePyP/FeIIITPPS4-RT-775 were

measured as shown in Figure 5j. The obtained curve can be fitted with two doublets (D1


18

Page 19 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and D2) and two sextets (Sext1 and Sext2). As shown in Table S2, the doublets D1 and D2 can be assigned to low-spin and intermediate-spin FeIIIN4. The high content (65.61%) of D1 and D2 indicates the high density of ORR active sites are beneficial for the enhancement of the activity. The other two sextets can be assighed to iron oxides, which is good agreement of XRD pattern (Figure 5a).

Figure 5. Structural and compositional characterizations: (a) XRD patterns of FeIIITPPS4, FeIIINMePyP, FeIIINMePyP/FeIIITPPS4-RT and FeIIINMePyP/FeIIITPPS4-RT-775; (b) SAED pattern of FeIIINMePyP/FeIIITPPS4-RT; (c) HRTEM image of FeIIINMePyP/FeIIITPPS4-RT-775; (d) HAADF-STEM image of FeIIINMePyP/FeIIITPPS4-RT-775 and elemental mapping of N (e), O (f), C (g), Fe (h) and S (i); (j) 57Fe Mössbauer spectra of FeIIINMePyP/FeIIITPPS4-RT-775. FeIIINMePyP/FeIIITPPS4-RT-775 was further evaluated by X-ray photoelectron


19


Page 20 of 28

spectroscopy (XPS). The total N content is calculated to be about 5.8 at% relative to C, including pyridinic, pyrrolic, graphitic, and oxidized nitrogen (Figure 6a-6f and Table S3).38 The Fe content was calculated to be 0.4 wt% measured by the inductively coupled plasma optical emission spectroscopy (ICP-OES). The high-resolution XPS spectrum of C1s reveals 66% C=C, 23% C=N or C-O, and 11% C-N or C=O, supporting the doping of N in carbon matrix (Figure S7d). Figure 6f shows the high-resolution XPS spectrum of S1s, including oxidized-S centred at 168.2 eV and thiophene-S residing at 164.8 and 163.6 eV. The content of thiophene-S is about 0.37 at% relative to C. The relatively high content of N and a small amount of S should also contribute to the high ORR performance.39 We propose that the alkaline ORR activity would be correlated to nitrogen-doped carbon, sulfur-doped carbon, oxides and Fe-N-C sites.

Figure 6. XPS spectra of electrocatalysts: (a-c) high-resolution N1s spectra of FeIIINMePyP/FeIIITPPS4-X-775 (X=RT, LT and SDS); (d) high-resolution N1s spectrum of P-FeIIINMePyP/FeIIITPPS4-775; (e) relative atomic percentage (%) of deconvoluted N 1s peaks in (a-d); (f) S2p spectrum of FeIIINMePyP/FeIIITPPS4-RT-775.


20

Page 21 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


4. CONCLUSIONS We describe a new synthetic approach for the fabrication of NNMEs by combining the ionic self-assembly of oppositely charged porphyrins with pyrolysis. The flower-like morphology formed in the self-assembly process is well retained after pyrolysis. MD simulations shows that oppositely charged porphyrins alternatively link up as Jaggregates to form twisted molecular chains, which further interweave with others into nanospheres. A primary driving force of the angulation of the flexible chain is the mismatched charge in each porphyrin dimer, besides π-π interaction, steric hindrance and van der Waals. MD simulations also exhibit that the interwoven chains with a large number of “net-knots” should explain the morphology maintenance. Our approach enables the desired manipulation over the morphology of NNMEs, which has rarely been realized previously. In addition, our NNMEs demonstrate excellent ORR activity, durability as well as tolerance to methanol relative to 20 wt% Pt/C in alkaline medium. This study gives a new and general avenue for the preparation of high-performance NNMEs with controllable morphology. AUTHOR INFORMATION


21


Page 22 of 28

Corresponding Author * E-mail: [email protected].

* E-mail: [email protected].

Author Contributions ‡ These two authors contributed equally to this work.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was partially supported by National Key Research & Development Program of China (Grant No. 2016YFB0101307), National Natural Science Fund of China (Grant Nos. 91430110 and 21606219), Liaoning BaiQianWan Talents Program (Grant No. 201519), Program for Liaoning Excellent Talents in University (Grant No. LR2015014), Dalian Excellent Young Scientific and Technological Talents (Grant No. 2015R006) and the Fundamental Research Funds for the Central Universities (Grant No. DUT19ZD208, DUT15RC(3)001 and DUT15ZD225). ASSOCIATED CONTENT Supporting Information


22

Page 23 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Materials, characterizations and molecular dynamics simulations in details are listed in the supporting information. REFERENCES (1)

Debe, M. K. Electrocatalyst Approaches and Challenges for Automotive Fuel Cells.

Nature 2012, 486, 43-51. (2)

Cao, R.; Thapa, R.; Kim, H.; Xu, X.; Kim, M. G.; Li, Q.; Park, N.; Liu, M.; Cho, J.

Promotion of Oxygen Reduction by a Bio-Inspired Tethered Iron Phthalocyanine Carbon Nanotube-Based Catalyst. Nat. Commun. 2013, 4, 2076. (3)

Li, J.; Song, Y.; Zhang, G.; Liu, H.; Wang, Y.; Sun, S.; Guo, X. Pyrolysis of Self-

Assembled Iron Porphyrin on Carbon Black as Core/Shell Structured Electrocatalysts for Highly Efficient Oxygen Reduction in both Alkaline and Acidic Medium. Adv. Funct. Mater. 2017, 27, 1604356. (4)

Sun, C.; Li, Z.; Zhong, X.; Wang, S.; Yin, X.; Wang, L. Three-Dimensional

Graphene-Supported Cobalt Phthalocyanine as Advanced Electrocatalysts for Oxygen Reduction Reaction. J. Electrochem. Soc. 2018, 165, F24-F31. (5)

Xu, S.; Li, Z.; Ji, Y.; Wang, S.; Yin, X.; Wang, Y. A Novel Cathode Catalyst for

Aluminum-Air Fuel Cells: Activity and Durability of Polytetraphenylporphyrin Iron (II) Absorbed on Carbon Black. Int. J. Hydrogen Energy 2014, 39, 20171-20182. (6)

Ji, Y.; Li, Z.; Wang, S.; Xu, G.; Li, D.; Yu, X. Durability of Polytetraphenylporphyrin

Cobalt Adsorbed on Carbon Black as an Electrocatalyst for Oxygen Reduction. J.

Electrochem. Soc. 2011, 158, B139-B142. (7)

Liang, Y.; Li, Y.; Wang, H.; Zhou, J.; Wang, J.; Regier, T.; Dai, H. Co3O4

Nanocrystals on Graphene as a Synergistic Catalyst for Oxygen Reduction Reaction. Nat.

Mater. 2011, 10, 780-786. (8)

Sun, M.; Liu, H.; Liu, Y.; Qu, J.; Li, J. Graphene-Based Transition Metal Oxide

Nanocomposites for the Oxygen Reduction Reaction. Nanoscale 2015, 7, 1250-1269.


23


(9)

Page 24 of 28

Gong, K.; Du, F.; Xia, Z.; Durstock, M.; Dai, L. Nitrogen-Doped Carbon Nanotube

Arrays with High Electrocatalytic Activity for Oxygen Reduction. Science 2009, 323, 760764. (10)

Sun, S.; Liang, S. Recent Advances in Functional Mesoporous Graphitic Carbon

Nitride (mpg-C3N4) Polymers. Nanoscale 2017, 9, 10544-10578. (11)

Yang, W.; Liu, X.; Yue, X.; Jia, J.; Guo, S. Bamboo-like Carbon Nanotube/Fe3C

Nanoparticle Hybrids and Their Highly Efficient Catalysis for Oxygen Reduction. J. Am.

Chem. Soc. 2015, 137, 1436-1439. (12)

Gao, M.; Jiang, J.; Yu, S. Solution-Based Synthesis and Design of Late Transition

Metal Chalcogenide Materials for Oxygen Reduction Reaction (ORR). Small 2012, 8, 1327. (13)

Xie, Y.; Li, H.; Tang, C.; Li, S.; Li, J.; Lv, Y.; Wei, X.; Song, Y. A High-Performance

Electrocatalyst for Oxygen Reduction Based on Reduced Graphene Oxide Modified with Oxide Nanoparticles, Nitrogen Dopants, and Possible Metal-NC Sites. J. Mater. Chem. A 2014, 2, 1631-1635. (14)

Jasinski, R. A New Fuel Cell Cathode Catalyst. Nature 1964, 201, 1212-1213.

(15)

Jahan, M.; Bao, Q.; Loh, K. P. Electrocatalytically Active Graphene-Porphyrin MOF

Composite for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 6707-6713. (16)

Tang, H.; Yin, H.; Wang, J.; Yang, N.; Wang, D.; Tang, Z. Molecular Architecture

of Cobalt Porphyrin Multilayers on Reduced Graphene Oxide Sheets for HighPerformance Oxygen Reduction Reaction. Angew. Chem. Int. Ed. 2013, 52, 5585-5589. (17)

Liang, H.; Wei, W.; Wu, Z.; Feng, X.; Müllen, K. Mesoporous Metal-Nitrogen-

Doped Carbon Electrocatalysts for Highly Efficient Oxygen Reduction Reaction. J. Am.

Chem. Soc. 2013, 135, 16002-16005. (18)

Wu, Z.; Chen, L.; Liu, J.; Parvez, K.; Liang, H.; Shu, J.; Sachdev, H.; Graf, R.;

Feng, X.; Müllen, K. High-Performance Electrocatalysts for Oxygen Reduction Drived from Cobalt Porphyrin-Based Conjugated Mesoporous Polymers. Adv. Mater. 2014, 26, 1450-1455. (19)

Hijazi, I.; Bourgeteau, T.; Cornut, R.; Morozan, A.; Filoramo, A.; Leroy, J.; Derycke,

V.; Jousselme, B.; Campidelli, S. p. Carbon Nanotube-Templated Synthesis of Covalent


24

Page 25 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Porphyrin Network for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2014, 136, 63486354. (20)

Xiang, Z.; Xue, Y.; Cao, D.; Huang, L.; Chen, J.; Dai, L. Highly Efficient

Electrocatalysts for Oxygen Reduction Based on 2D Covalent Organic Polymers Complexed with Non-Precious Metals. Angew. Chem. Int. Ed. 2014, 53, 2433-2437. (21)

Guo, J.; Li, Y.; Cheng, Y.; Dai, L.; Xiang, Z. Highly Efficient Oxygen Reduction

Reaction Electrocatalysts Synthesized under Nanospace Confinement of Metal-Organic Framework. ACS Nano 2017, 11, 8379-8386. (22)

Wang, Z. C.; Medforth, C. J.; Shelnutt, J. A. Porphyrin Nanotubes by Ionic Self-

Assembly. J. Am. Chem. Soc. 2004, 126, 15954-15955. (23)

Mazur, U.; Hipps, K. W. A Systematic Approach toward Designing Functional Ionic

Porphyrin Crystalline Materials. J. Phys. Chem. C 2018, 122, 22803-22820. (24)

Mazur, U.; Hipps, K. W.; Eskelsen, J. R.; Adinehnia, M. Functional Porphyrin

Nanostructures for Molecular Electronics: Structural, Mechanical, and Electronic Properties of Self-Assembled Ionic Metal-Free Porphyrins. Handb. Porphyrin Sci. 2016,

40, 69-103. (25)

Li, J.; Liu, H.; Lü, Y.; Guo, X.; Song, Y. Influence of Counter Electrode Material

during Accelerated Durability Test of Non-Precious Metal Electrocatalysts in Acidic Medium. Chinese J. Catal. 2016, 37, 1109-1118. (26)

Wang, Z.; Ho, K. J.; Medforth, C. J.; Shelnutt, J. A. Porphyrin Nanofiber Bundles

from Phase-Transfer Ionic Self-Assembly and Their Photocatalytic Self-Metallization.

Adv. Mater. 2006, 18, 2557-2560. (27)

Martin, K. E.; Wang, Z.; Busani, T.; Garcia, R. M.; Chen, Z.; Jiang, Y.; Song, Y.;

Jacobsen, J. L.; Vu, T. T.; Schore, N. E. Donor-Acceptor Biomorphs from the Ionic SelfAssembly of Porphyrins. J. Am. Chem. Soc. 2010, 132, 8194-8201. (28)

Ma, J.; Wu, J.; Zheng, J.; Liu, L.; Zhang, D.; Xu, X.; Yang, X.; Tong, Z. Synthesis,

Characterization and Electrochemical Behavior of Cationic Iron Porphyrin Intercalated into Layered Niobate. Micropor. Mesopor. Mat. 2012, 151, 325-329. (29)

Zhu, Q.; Maeno, S.; Sasaki, M.; Miyamoto, T.; Fukushima, M. Monopersulfate

Oxidation of 2,4,6-Tribromophenol using an Iron (III)-Tetrakis (p-sulfonatephenyl)


25


Page 26 of 28

Porphyrin Catalyst Supported on an Ionic Liquid Functionalized Fe3O4 Coated with Silica.

Appl. Catal. B: Environ. 2015, 163, 459-466. (30)

Chi, Y.; Chen, J.; Aoki, K. Electrochemical Generation of Fee Nitric Oxide from

Nitrite Catalyzed by Iron Meso-tetrakis (4-N-methylpyridiniumyl) Porphyrin. Inorg. Chem. 2004, 43, 8437-8446. (31)

Li, S.; Zhang, L.; Liu, H.; Pan, M.; Zan, L.; Zhang, J. Heat-Treated Cobalt–Tripyridyl

Triazine (Co-TPTZ) Electrocatalysts for Oxygen Reduction Reaction in Acidic Medium.

Electrochim. Acta 2010, 55, 4403-4411. (32)

Ai, K.; Liu, Y.; Ruan, C.; Lu, L.; Lu, G. Sp2 C-Dominant N-Doped Carbon Sub-

micrometer Spheres with a Tunable Size: A Versatile Platform for Highly Efficient OxygenReduction Catalysts. Adv. Mater. 2013, 25, 998-1003. (33)

Wu, M.; Tang, Q.; Dong, F.; Wang, Y.; Li, D.; Guo, Q.; Liu, Y.; Qiao, J. The Design

of Fe, N-Doped Hierarchically Porous Carbons as Highly Active and Durable Electrocatalysts for a Zn-Air Battery. Phys. Chem. Chem. Phys. 2016, 18, 18665-18669. (34)

You, C.; Zen, X.; Qiao, X.; Liu, F.; Shu, T.; Du, L.; Zeng, J.; Liao, S. Fog-like Fluffy

Structured N-doped Carbon with a Superior Oxygen Reduction Reaction Performance to a Commercial Pt/C Catalyst. Nanoscale 2015, 7, 3780-3785. (35)

Chen, S.; Wei, Z.; Qi, X.; Dong, L.; Guo, Y.; Wan, L.; Shao, Z.; Li, L.

Nanostructured Polyaniline-Decorated Pt/C@PANI Core-Shell Catalyst with Enhanced Durability and Activity. J. Am. Chem. Soc. 2012, 134, 13252-13255. (36)

Guo, S.; Li, Y.; Jiang, J.; Xie, H. Nanofluids Containing γ-Fe2O3 Nanoparticles and

Their Heat Transfer Enhancements. Nanoscale Res. lett. 2010, 5, 1222-1227. (37)

Schneider, D.; Mehlhorn, D.; Zeigermann, P.; Kärger, J.; Valiullin, R. Transport

Properties of Hierarchical Micro-Mesoporous Materials. Chem. Soc. Rev. 2016, 45, 34393467. (38)

Hu, E.; Yu, X.; Chen, F.; Wu, Y.; Hu, Y.; Lou, X. Graphene Layers-Wrapped

Fe/Fe5C2 Nanoparticles Supported on N-Doped Graphene Nanosheets for Highly Efficient Oxygen Reduction. Adv. Energy Mater. 2018, 8, 1702476. (39)

Dong, Q.; Zhuang, X.; Li, Z.; Li, B.; Fang, B.; Yang, C.; Xie, H.; Zhang, F.; Feng,

X. Efficient Approach to Iron/Nitrogen Co-Doped Graphene Materials as Efficient


26

Page 27 of 28 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Electrochemical Catalysts for the Oxygen Reduction Reaction. J. Mater. Chem. A 2015,

3, 7767-7772.


27



Page 28 of 28

Interwoven Molecular Chains Obtained by Ionic Self-Assembly of Two

Recommend Documents