C with Hierarchical

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Energy, Environmental, and Catalysis Applications

3D Networks of S-Doped Fe/N/C with Hierarchical Porosity for Efficient Oxygen Reduction in Polymer Electrolyte Membrane Fuel Cells Yijin Wu, Yucheng Wang, Ruixiang Wang, Pengfang Zhang, Xiaodong Yang, Huijuan Yang, Jun-Tao Li, Yao Zhou, Zhi-You Zhou, and Shi-Gang Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19332 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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ACS Applied Materials & Interfaces

3D Networks of S-Doped Fe/N/C with Hierarchical Porosity for Efficient Oxygen Reduction in Polymer Electrolyte Membrane Fuel Cells † ‡ ‡ † ‡ Yi-Jin Wu , Yu-Cheng Wang , Rui-Xiang Wang , Peng-Fang Zhang , Xiao-Dong Yang , Hui‡ †* † ‡ †‡ Juan Yang , Jun-Tao Li , Yao Zhou , Zhi-You Zhou , and Shi-Gang Sun

†

‡

College of Energy, Xiamen University, Xiamen 361005, China

State Key Lab of Physical Chemistry of Solid Surface, College of Chemistry and Chemical

Engineering, Xiamen University, Xiamen 361005, China *

Corresponding Author: E-mail Addresses: [email protected] (J.T.Li)

KEYWORDS S-doping Fe/N/C, 3D networks, Hierarchical Porosity, Oxygen Reduction Reaction, PEMCFs.

ACS Paragon Plus Environment

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ABSTRACT

Reasonable design and synthesis of Fe/N/C-based catalysts, is one of most promising way for developing precious metal-free ORR catalysts in acidic medium. Herein, we developed a highly active MOFs-derived S-doped Fe/N/C catalyst (S-Fe/Z8/2-AT) prepared by thermal treatment. The S-Fe/Z8/2-AT catalyst with uniform S doping possesses the 3D macro-meso-micro hierarchically porous structure. Moreover, the chemical composition and structural features have been well optimized and characterized for such S-Fe/Z8/2-AT; and their formation mechanism was also revealed. Significantly, applying the optimal S-Fe/Z8/2-AT catalysts into electrocatalytic test exhibits remarkable oxygen reduction reaction catalytic activity with a halfwave potential of 0.82 V (versus RHE) and a mass activity of 18.3 A g-1 at 0.8 V in 0.1 M H2SO4 solution; and the PEMFCs test also confirmed its excellent catalytic activity, which gives a maximal power density as high as 800 mW cm-2 at 1 bar. A series of designed experiments disclosed that the favorable structural merits and desirable chemical compositions of S-Fe/Z8/2AT catalysts are critical factors for efficient electrocatalytic performance. The work provides a new approach to open an avenue for accurate control the composition and structure of Fe/N/C catalyst with highly activity for oxygen reduction reaction.


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1. Introduction Highly effective oxygen reduction reaction (ORR) is a primary condition for polymer electrolyte membrane fuel cells (PEMFCs) with the high output of power.1 Pt-based noble metallic catalysts, though observed with high catalytic activity for ORR, are restricted from a large-scale application in PEMFCs because of their scarcity and high cost.2 To replace the Ptbased catalysts, a variety of non-precious-metal catalysts (NPMCs) have been constantly developed for ORR in alkaline or acidic medium over the past few decades.3-5 In recent years, NPMCs with the catalytic performance towards ORR superior to Pt-based catalysts in alkaline medium have been reported.6-11 However, when ORR runs over a prolonged timeframe in an alkaline solution, the pH value of the medium would decrease inevitably due to the permeated dissolution of CO2 gas from the air; as a consequence, the performance of the relevant ORR would fade gradually. Therefore, the development of NPMCs with high-efficiency for ORR in acidic medium is preferred for practical applications in PEMFCs. Among various NPMCs(M/N/C, M = Fe, Co, Ni, etc.),12 especially for the Fe/N/C catalysts, were reported as the most promising alternative to Pt-based catalysts in both acidic and alkaline medium and have attracted a lot of attention.13-14 Relevant researches mainly fall into two categories for Fe/N/C catalysts. One aims to directly improve the catalytic activity of such Fe/N/C-based catalysts for ORR via optimization of the synthetic process (e.g., exploration of efficient thermal treatment), and/or their material compositions (e.g., usages of highlyconductive carbon supports such as graphene oxide)15-17 or carbon nanotube18-20 to replace conventional carbon black21. The other one focuses on the reaction mechanism, especially the O2 reduction pathway and the active sites of the electrocatalyst.22-30 Regardless of the above efforts,


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the Fe/N/C catalysts remain to be improved in terms of their chemical composition and structure features. For example, the formation of those undesirable inactive and/or unstable compounds (e.g., excessive presence of metallic Fe) or inhomogeneous distribution of Fe species in the carbon framework of Fe/N/C catalysts is almost unavoidable in the preparation of Fe/N/C catalysts.31 Moreover, the structure engineering of Fe/N/C catalysts at meso- and micro-scale, for example optimization of the pore structure, is still necessary to facilitate the mass transfer and thus enhance the interface reaction kinetics during the ORR. Therefore, it is expected that the rational design of Fe/N/C-based catalysts with optimal chemical composition, tunable porosity and favorable spatial structure would further improve their catalytic performance. In line with the above issue, S or N doping of the carbon-based materials was found to modify the electron spin density as well as the electronegativity, which eventually changes the internal charge distribution and facilitates the chemisorptions of oxygen species,32-34 leading to improved catalytic activity for ORR. S-doped Fe/N/C catalysts with enhanced electrocatalytic activity have been reported recently, but most of them were employed in alkaline mediums.35 Moreover, for current synthetic strategies, the S-doped Fe/N/C catalysts are mostly prepared through pyrolyzing a mechanical mixture of the N- or S-rich organic molecules and iron salts,36, 37 which has an disadvantage for homogeneous dispersion of the catalytic sites and often leads to nonadjustable pore structures. Recently, metal-organic frameworks (MOFs),38, 39 a new class of highly porous crystalline framework materials, are considered as the promising precursors and/or templates for facile preparation of Fe/N/C catalysts via simple thermal treatment, and the prepared Fe/N/C catalysts usually show superior ORR catalytic performance.40-42 Such a synthetic strategy has exhibited evident advantages.43 The existence of the spatially ordered multi-coordination sites within the MOFs precursors supports the formation of Fe/N/C structures


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with homogeneous distribution of hetero-atoms within the carbon skeleton.44 And MOFs with tunable ordered pores, after pyrolysis, enable excellent control over the pore structures of resulting Fe/N/C catalysts, which hence allows efficient mass transportation during the ORR process.45 In addition, due to the large specific surface area of those MOFs precursors,46 their resulting pyrolysis products often have relatively large specific surface area compared with those prepared using the traditional amorphous carbon supports.47 With the above merits, such MOFsderived Fe/N/C catalysts have been reported repetitively with improved catalytic performances for PEMFCs in recent years.48, 49 On this basis, it could be expected that MOFs-derived S-doped Fe/N/C with favorable chemical compositions and desirable structure, which so far has not been reported yet, might be a promising ORR catalyst in PEMFCs. Herein, we developed a simple method to prepare three-dimensional (3D) S-doped Fe/N/C (SFe/Z8/2-AT hereafter) networks with hierarchical micro-meso-macro porosity, by pyrolysis of a mixture of ZIF-8, 2-aminothiazole (2-AT), and FeCl2. The selected ZIF-8 is not only has the high specific surface area, but also possesses uniform and high nitrogen content that benefits to form high N doped carbon framework after heating treatment process. The 2-AT, on the one hand, is introduced as nitrogen and sulfur source to form uniform N and S doped into porous carbon. On the other hand, 2-AT just-like a “linker” to connect neighboring ZIF-8 particles in narrow space due to hydrophilic and hydrophobic interaction, which is critical to the formation of such unique 3D connected-network structure during carbonization process. The optimal S-Fe/Z8/2-AT catalyst exhibited superior catalytic activity for ORR with a half-wave potential of 0.82 V (versus RHE) and a mass activity of 18.3 A g-1 at 0.8 V in 0.1 M H2SO4 solution; we also demonstrated the feasibility of the electrode materials for above catalyst in PEMFCs test which displayed its excellent catalytic activity, displaying a maximal power density as high as 800 mW


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cm-2 at 1 bar. 2. EXPERIMENTAL SECTION 2.1 Materials and chemicals Zinc nitrate hexahydrate (99 %), iron (II) chloride tetrahydrate (97%), 2-methylimidazole (97 %), 2-aminothiazole (97 %), sulfuric acid (99.9999 %), hydrochloric acid (98 %), Nafion solution (D520, 5%, Dupont) and Pt/C (20 and 40 wt % Pt) were purchased from Alfa Aesar. Methanol (AR), ethanol (AR) and n-hexane (AR) were bought from Xilong Chemical Reagent Co., Ltd. Ultrapure water (18 MΩ) was obtained by a Millipore System (Millipore Q, USA). 2.2 Synthesis of ZIF-8 particles The ZIF-8 colloidal particles were synthesized using the previously reported method.50 In brief, 12 mmol of zinc nitrate hexahydrate was dissolved in 300 mL of methanol as the solution A. 0.17 mol of 2-methylimidazole was dissolved in 180 mL of methanol as the solution B. Then, the solution B was quickly poured into the solution A followed by sonication for 2 min and incubation at room temperature for 24 h. Afterward, the product was separated via filtration and washed with methanol. The as-obtained milk white solid was dried at 80 °C in vacuum for 12 h. 2.3 Synthesis of S-Fe/Z8/2-AT(x) The as-prepared ZIF-8 particles (0.5 g) and a specific dosage of 2-AT were added into n-hexane (100 mL), which then was stirred for 1 h to make it homogenous. Towards the hydrophobic mixture, an aqueous solution which was prepared by dissolving 10.7 mg of iron (II) dichloride tetrahydrate in 0.7 ml of water was added dropwise within 15 min with vigorous stirring; the resulting mixture, after being stirred for at least 3 h, was decantated to obtain a brownish yellow solid which was dried under 80 °C in vacuum for 12 h. Subsequently, the dried powder was


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subjected to the first round of carbonization, which was pyrolyzed at 950 °C for 1 h with a ramping rate of 15 °C min-1 in Ar2. After cooling down to the room temperature, the mixture was again heated at 950 °C for 15 min with a ramping rate of 15 °C min-1 in a mixture of NH3 and Ar2 (NH3/Ar2 = 1/9). Then the as-obtained product was subjected to acid leaching in 1.0 M HCl solution at 80 °C with continuous stirring, which then was centrifuged and washed with deionized water, and finally dried overnight at 80 °C. Thus generated catalysts were denoted as S-Fe/Z8/2-AT(x), where x= m2-AT/(m2-AT + mFeCl2+ mZIF-8)*100 which depends on the mass fraction of 2-AT added in the mixture of the precursors. 2.4 ORR catalytic activity test All ORR performances were carried out in O2/N2-saturated 0.1 M H2SO4 solution at 30 °C on a rotating ring-disk electrode (RRDE) setup from Pine Instrument Company with a CHI 760 electrochemical workstation, with a constant rotating speed of 900 rpm. Glassy carbon (GC, 5.61 mm diameter) disk-Pt ring RRDE, GC plate and saturated calomel electrode (SCE) were used as the working electrode, the counter electrode and the reference electrode, respectively. Electrode potential reported in this work was consistently referenced to the reversible hydrogen electrode (RHE) potential according to the formula: ERHE / V = E(SCE) + φ(SCE) + 0.059*pH. To prepare a catalyst ink, 6 mg of the catalyst was added in a mixture of 0.5 mL of water, 0.45 mL of ethanol and 0.05 mL of 5 % Nafion solution, which was then ultrasonically dispersed for 1.0 h. Next, 25 μL of the catalyst ink was dropped onto the freshly polished GC disk of RRDE, resulting in a catalyst loading of 0.6 mg cm-2. The Pt/C (20 %) film was also prepared similarly with a catalyst loading of 0.1 mg cm-2. Before the electrochemical measurements, the GC dick electrode was activated by cycling from 0.2 to 1.0 V (versus RHE, thereafter) at 50 mV s-1 in O2-saturated 0.1 M H2SO4 solution until stable voltammogram curves observed. Polarization curves were


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acquired by potential cycling within 0.2 to 1.0 V at 10 mV s-1 with rotating rate of 900 rpm and negative scanning direction. The iR drop was compensated. The capacitive background current was corrected by subtracting the curve recorded in N2-saturated solution from the relevant polarization curve. To determine H2O2 yield, the ring electrode was set to 1.20 V. The hydrogen peroxide yield (H2O2, %) in the ORR is calculated according to Equation 1 where jD, jR are the disk current and the ring current, respectively, and N is 0.37 which is the collection efficiency. The number of electrons transferred (n) is calculated based on Equation 2.

(1)

(2)

The kinetic current (jk) in this work was calculated from experimental data using the Koutecky-Levich equation (Equation 3) where j and jL are the measured current density and diffusion-limiting current densities (j at 0.2 V in this work), respectively.

(3)

The mass activity jm was obtained from Equation 4 where mcat is the catalyst loading.

(4)

2.5 Polymer electrolyte membrane fuel cells test The catalyst ink was obtained by ultrasonic dispersion of a mixture of S-Fe/Z8/2-AT(16.4)


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catalyst (26 mg), 5 wt % Nafion solution (600 μL) and the ultrapure water (1.0 mL) for 1.0 h. The as-prepared ink was directly coated on PTFE-pretreated Toray 060 carbon paper as the cathode, with a catalyst loading of 4.0 mg cm-2. Note that in our case no microporous layer formed by carbon black and nafion was involved. The Nafion content in the cathode catalyst layer was approximately 50 wt%. The anode catalyst is 40 wt% Pt/C with a loading of 0.4 mg Pt cm-2. The membrane electrode assembly (MEA) was prepared by hot-pressing of the cathode, the anode, NRE 211 Nafion membrane, and the gasket at around 3 MPa for 135 s. Fuel cell performance was tested at 80 °C with H2 and O2 flow rates of 0.2 slpm at 100% of relative humidity. The back pressures were set to 1 bar gauge for both anode and cathode sides.

2.6 Material characterizations PXRD was performed to obtain the crystalline information of the catalysts by Panalytical X'Pertpro MPD X-ray power diffractometer (a Cu Kα irradiation source, λ = 1.54056 Å). The morphologies were characterized by FEI Tecnai G2 20 S-TWIN (F20) electron microscope at 200 kV and Hitachi S4800 field emission scanning electron microscopy at 6 kV. HAADF-STEM imaging and EDX elemental mapping were carried out by F20 at 200 kV. Brunauer-EmmettTeller (BET) surface area and pore size were measured using a sorptometer (ASAP-2420, Micromeritics, USA) at 77 K. XPS was recorded on the Thermo Scientific ESCALab 250Xi using 200 W monochromated Al Kα radiation. All binding energies were referenced to the C 1s peak with binding energy at 284.5 eV arising from C-C bonds. The 500 μm X-ray spot was used for XPS analysis. SERS was tested by a Renishaw micro-Raman system accompanied with research grade LeicaDMLM microscope. FTIR spectra of samples were scanned in the region of 700-4000 cm-1 on quartz plate using Nicolet™ iN™ 10 FTIR spectrophotometer.


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3. Results and discussion 3.1 Formation and characterization of 3D S-Fe/Z8/2-AT networks The preparation of 3D S-Fe/Z8/2-AT networks is briefly depicted in Scheme 1. The polyhedral ZIF-8 particles in which 2-methylimidazole tetrahedrally coordinated with zinc ions were employed as the carbon and nitrogen precursor and the template. The as-prepared ZIF-8 particles have high crystallinity and are mono-disperse in size and uniform in shape, as confirmed by the scanning electron microscopy (SEM) shown in Figure S1a and b. Moreover, the mixture of ZIF-8, 2-AT and FeCl2 is observed by SEM in Figure S1d, demonstrating that 3D skeleton of MOFs is well maintained after hydrophilic and hydrophobic interaction. It is found that the addition of 2-AT with different dosages leads to end products with different morphological and compositional features as well as different electrocatalytic performances. Therefore, a series of S-Fe/Z8/2-AT(x) were prepared and evaluated (which will be discussed in latter). Among them, the S-Fe/Z8/2-AT(16.4) was characterized and discussed in detail since our catalytic activity test revealed that the S-Fe/Z8/2-AT(16.4) catalysis demonstrated the best electrochemical performance. Scheme 1. Synthetic procedures of the S-Fe/Z8/2-AT electrocatalyst.


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Representative SEM images of the S-Fe/Z8/2-AT(16.4) with different magnifications, shown in Figure 1a and b, exhibit clearly the formation of porous networks with high structural integrity. One could see that the as-formed network consists of numerous short and curved branches which are well-connected with each other in all the three-dimensions, forming a sponge-like 3D network (Figure 1b, Figure S2). It is expected that carbon matrix with such continuous 3D network structures would support easier electron transport, which, compared with those discrete carbon materials, is an evident advantage when used as electrocatalysts for the ORR. Moreover, as indicated by the red arrows in Figure 1c, TEM observation reveals that numerous irregularly-shaped nanobubble-like or short nanotube-like particulates with sizes in general ranging from a few to around ten nanometers were formed; those particulates have evidently higher TEM image contrast in their edge areas than that in the central parts, which well verifies their hollow nature. Note that the hollow nature of such nanobubbles or short nanotubes is recognizable only for those located at the boundary of the sample under TEM observation as they stack upon each other in large numbers. Hence, the 3D networks of the S-Fe/Z8/2-AT(16.4) are actually porous carbon sponges formed by numerous well-connected hollow nanobubbles or short nanotubes. At the high-resolution TEM in Figure 1d, it can be seen clearly that the shells of those hollow


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particulates are highly graphitized, which demonstrate multiple layers, just likes the walls of the multi-walled carbon nanotubes.51 The graphitization degree was investigated by the Raman spectroscopy, as shown in Figure 2a. The D-band peak at 1350 cm-1 and G-band peak at 1590 cm-1 are assigned to the amorphous carbon and graphitized carbon, respectively.52, 53 And the intensity ratio of these two peaks, i.e., ID/IG, reaches 0.92, demonstrating that more than half of the carbon exists as the graphitic phase in whole S-Fe/Z8/2-AT(16.4).


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Figure 1. (a, b) SEM images; (c,d) TEM images (with different magnifications); and (e-i) HAADF-STEM image and the corresponding EDX element mapping images of S-Fe/Z8/2AT(16.4).


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More interestingly, such 3D networks of S-Fe/Z8/2-AT demonstrate hierarchical porosity, which possesses micro-meso-macro pores simultaneously. Shown in Figure 1a and b, abundant macropores with sizes in the range of 50- 500 nm could be found easily, which are formed likely due to the disordered spatial stacking of the curved and short branches. Meanwhile, as the curved branches of 3D networks are formed from disordered packing of the irregularly-shaped hollow carbon particulates (Figure 1c and d, Figure S3a-d), meso-scale interstitial voids, i.e., mesopores, are expected to occur among the neighboring hollow particulates. Accordingly, based on the N2 adsorption/desorption isotherm in Figure 2c, our 3D networks of S-Fe/Z8/2-AT(16.4) possess a BET specific surface area of 625.3 m2 g-1. The relevant pore size analysis reveals considerable uniform mesopores with an average size of 13.3 nm according to the Barret-JoynerHalenda (BJH) characterization in Figure 2d. In addition to those mesopores, analysis of the N2 desorption isotherm by the Horvath-Kawazoe (HK) method reveals uniformly distributed micropores with sizes centered at 0.88 nm in Figure 2d. Such micropores are expected to exist most likely in the graphitized shells of those hollow particulates, which are formed probably due to the violation of the Zn element and/or the decomposed organic fragments during the pyrolysis at high temperature. With such micropores, the internal surface of the hollow nanobubbles hence is accessible to external reactants with relatively small sizes. Note that the macropores could not be analyzed based on the N2 adsorption/desorption isotherms. The formation of such hierarchically porous structure is expected to support fast mass transfer during the electrocatalytic reaction at the solid-liquid interface.


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Figure 2. Characterizations of S-Fe/Z8/2-AT(16.4): (a) Raman pattern;(b) PXRD patterns before and after acid leaching;(c) N2 adsorption/desorption isotherms; (d) the corresponding micropore (HK method) and mesopore (BJH method) size distributions. The chemical compositions of thus-prepared 3D S-Fe/Z8/2-AT(16.4) networks were further characterized. The relevant energy-dispersive X-ray spectroscopy (EDX) spectrum demonstrated the rich existence of Fe, N and S elements within the sample (Figure S4); and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging and EDX elemental mapping in Figure 1e-i further confirm homogeneous distributions of Fe, N, S and C within the sample, supporting the formation of S-Fe/Z8/2-AT(16.4). High-resolution TEM (HRTEM) observation revealed no existence of the inactive crystalline metal-containing phases (e.g., metallic Fe such as Fe@C, Fe3/C, or species like FexSy, ZnS) in their carbon framework (Figure S3a-d). Consistently, such an observation is also supported by the powder X-ray


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diffraction (PXRD) analysis (Figure 2b, the red curve). Two broad peaks which are centered at 25.9 and 43.6 degree are representative for the graphitized carbon;45 besides these two bands there was no other peak in the relevant PXRD spectrum. Therefore, no Fe-based impurities (e.g., FeS) was formed during the carbonization of the mixture in Ar and NH3 atmosphere, and all the Fe atoms exist in the form of Fe-Nx species within the porous carbon matrix after acid leaching, which is highly active for the ORR process.31 This is understandable. When mixed with ZIF-8 particles and 2-AT molecules, Fe(II) ions would coordinate with the -NH2 or -NH groups in 2-AT or compete with Zn(II) ions from ZIF-8 to coordinate with N atoms of 2-methylimidazole, which, during the following carbonization process, undergoes in-situ pyrolysis to form the Fe-Nx species and hence successfully inhibits the interaction between Fe(II) and S-containing species. Moreover, as the solubility product constants for FeS and ZnS are 6.3*10-18 and 1.6*10-24, respectively, theoretically the formation of ZnS would dominate over that of FeS during the carbonization process. In addition, no signal of ZnS could be observed in the XRD spectrum for the mechanical mixture of ZIF-8 and 2-AT (Figure S5), whereas the PXRD spectrum of the SFe/Z8/2-AT(16.4) before acid leaching (Figure 2b, the black curve) reveals the presence of crystalline ZnS (JCPDS No. 79-2204), suggesting that the ZnS crystals were derived completely from the carbonization process (Scheme 1). It also proved that the acid leaching is necessary, as the ZnS impurities could be removed completely.


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Figure 3. (a) XPS survey; (b) high-resolution S 2p spectra; (c) C 1s spectra; and (d) N 1s spectra of S-Fe/Z8/2-AT(0) and S-Fe/Z8/2-AT(16.4). The black dots are the experimental data and the colored circles are the calculated ones. Furthermore, the element composition, content and chemical state for the catalysts with (i.e., S-Fe/Z8/2-AT(16.4)) and without S-doping (i.e., S-Fe/Z8/2-AT(0)) were investigated by X-ray photoelectron spectroscopy (XPS). The presence of Fe, N, C, O and S elements can be easily identified within the XPS survey scan of the S-Fe/Z8/2-AT(16.4) (the red curve in Figure 3a, and also in Table S1), with the average mass fraction of Fe, N, C, O and S obtained as 0.13%, 2.70%, 90.67%, 4.90%, and 1.64%, respectively. For the case without S-doping (the black curve in Figure 3a), no significant signal of S was found within its XPS survey, with average mass fraction of Fe, N, C and O obtained as 0.29%, 3.86%, 91.67%and 4.18%, respectively. The comparison of the XPS of S 2p would more intuitively reveal the differences between the above


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two samples. The sample S-Fe/Z8/2-AT(0) exhibited no characteristic band of S 2p (Figure 3b, the grey curve). In contrary, the deconvolution of the XPS S 2p of S-Fe/Z8/2-AT(16.4) revealed four distinct characteristic peaks (Figure 3b). The two with binding energy (BE) centered at 164.1 eV and 165.4 eV belong to S atoms connected with carbon (i.e., S-C),35, 54 and the two peaks at 167.6 eV and 169.3 eV are attributed to oxidized S species (i.e., S-Ox).55, 56 Such a result indicates that the S element was partly doped into carbon lattice of Fe/N/C after carbonization. Such successful S doping could improve the catalytic activity for ORR in acidic medium, as discussed later. Analysis of high-resolution C 1s spectrum revealed three types of C species in both samples, namely the C-C at around 284.0 eV, C-O at around 285.0 eV, C-N at around 287.3 eV (Figure 3c).57, 58 Besides, an additional peak at around 284.1 eV was also observed for the case of S-Fe/Z8/2-AT(16.4), which could be attributed to C atoms connected with S (C-S) and proved again the successful S doping in the carbon framework.54 The deconvolution of N 1s XPS for both samples disclosed four peaks at around 398.5 eV, 399.6 eV, 401.0 eV, and 402.2 eV which could be assigned to 42.9 % pyridinic N (N1), 18.4 % pyrrolic N (N2) and 25.8 % quaternary N (N3, also named as graphitic N) and 12.9 % from oxidized N (N4),58 respectively (Figure 3d). It was reported that doping of different N species in the carbon matrix, especially pyridinic N and quaternary N, could improve both the conductivity as well as the ORR catalytic activity.31 Comparing with S-Fe/Z8/2-AT(0) sample, the content of pyridinic N and quaternary N decreased by 2.1 % and 3.2 %, respectively. Although the decline was modest, the catalytic activity of S-Fe/Z8/2-AT(16.4) for ORR is superior to S-Fe/Z8/2-AT(0), further confirming the significance of S doped and 3D network structure for ORR, which more detail will be discussed in the part of electrocatalytic performance. Moreover, the two samples demonstrate evident signals of Fe 2p with similar binding energies in their relevant XPS spectra, which well supports


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the presence of Fe element in both cases (Figure S6a and b). 3. 2 Formation mechanism of 3D S-Fe/Z8/2-AT networks The structural evolution mechanism of such hierarchically porous 3D networks of S-Fe/Z8/2AT was furtherly explored. Our control experiment revealed that, when the polyhedral ZIF-8 colloidal particles were pyrolyzed alone, discrete aggregates of N-doping carbon (N/C) particulates were generated, in which the N/C particles maintain the original rhombododecahedral shape of the ZIF-8 precursors in Figure 4a. Such discrete aggregates of N/C are clearly differentiated from the 3D well-connected networks of our S-Fe/Z8/2-AT(16.4) in which can hardly recognize the original shape of the ZIF-8 colloidal particle (Figure 1b, Figure S2c-d). Such comparative experimental results implied that the presence of the N- and S-rich 2-AT and FeCl2 played important roles in inducing the formation of the hierarchical 3D networks during the carbonization process.


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Figure 4. SEM images of (a) N/C; (b) S-Fe/2-AT; S-Fe/Z8/2-AT(x), x = (c) 0; (d) 16.4; (e) FTIR spectra of 2-AT, ZIF-8 and ZIF-8/2-AT. As a comparison, when the mixture composed of ZIF-8 and FeCl2 was used as the precursor, large continuous aggregates of S-Fe/Z8/2-AT(0) were produced instead of discrete particulates, together with a few carbon nanotubes which were formed due to the catalytic effect of the Fe species during the pyrolysis (Figure 4c).59 Note that within such aggregates of S-Fe/Z8/2-AT(0) can hardly recognize the original shape of the ZIF-8 particulate, showing a thorough fusion of the precursor ZIF-8 particles under fast ramping rate during the thermal treatment. Such a result implies that the presence of the FeCl2 had the strong impact on the internal chemical and structural stability of the ZIF-8 particles. Fe2+ could have chemical interactions with the ZIF-8 by


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competing against the Zn2+ to coordinate with the -NH groups in the 2-methylimidazole. As a result, when subjected to carbonization, with the rich existence of Fe2+, the ZIF-8 particles tend to lost their stability of the crystals as well as the original rhombodo-decahedral shape under high temperature, and eventually undergo fusion with their neighboring particles. The presence of the 2-AT is also necessary to induce the formation of the hyperbranched structure of the porous 3D networks. As can be seen, compared with the porous 3D networks in Figure 1a, for the case prepared with only ZIF-8 and FeCl2, the as-formed aggregate of S-Fe/Z8/2-AT(0) is much less perforated and is hardly branched (Figure 4c). Such bulky aggregate is not desirable as an electrocatalyst as it does not support efficient mass transfer towards its internal active sites. As 2-AT serves as the sole S, its mass fraction employed is expected to directly affect the chemical composition, e.g., the mass fraction of S doped within the carbon matrix and the structure features of the resulting end-products. To understand the role of 2-AT, a series of SFe/Z8/2-AT(x) (x = 0, 8.9, 16.4, 28.1) were prepared using different dosages of 2-AT, as exemplified in Figure 4 where 3D networks with slightly different structural features at microscale were observed. When the mass fraction of 2-AT is x = 8.9, porous carbon network is observed, but with less structural integrity (Figure S8a). Raman spectra of this sample reveals that the intensity ratio of the two bands, i.e., ID/IG, for S-Fe/Z8/2-AT(8.9) is 0.95 (Figure S7, blue line), slightly larger than the case of S-Fe/Z8/2-AT(0) which is 0.91 (Figure S7, black line). One can expect that the 2-AT monomer would chelate with Zn2+ from the surface of ZIF-8 via their N and/or S-containing groups and then adsorbed on the external of the ZIF-8 particles, similar to the adsorption of the anti-cancer drug doxorubicin on the surface of ZIF-8.60 In order to prove further our conjecture, the structures information of 2-AT, ZIF-8 and the mixture of both them (defined as ZIF-8/2-AT) prepared by hydrophilic and hydrophobic interaction were confirmed


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using ex-situ Fourier transform infrared (FTIR) spectrometer. For ZIF-8, the FTIR spectrum displays that the absorption peaks at 3126 and 2930 cm-1 are ascribed to the aromatic and the aliphatic C-H stretch of the imidazole (Figure 4, black line), respectively;61 the peak at 1587 cm1

is assigned to the stretching vibration of C=N; and the intense and convoluted bands at 1350 -

1500 cm-1 are attributed to the entire ring stretching; It also can be confirmed that the bands in the characteristic region of 900 - 1350 cm-1 and those below 800 cm-1 can be associated with inplane bending and out-of-plane bending of the ring, respectively. These typical absorption peak well-demonstrated ZIF-8 successfully prepared. When 2-AT was tested, the IR spectrum shows that two characteristically splitting peaks at 3410 and 3288 cm-1 are assigned to aliphatic N-H asymmetric and symmetric stretching of -NH2 group of 2-AT (Figure 4, red line);62 the peaks at 1628 cm-1 represents the stretching vibration of C=N and the peaks at 1490 cm-1 can be attributed to -N-H in plane bending vibration. It is significant that the characteristic peaks in the spectrum of ZIF-8/2-AT at 3410 and 3288 cm-1 were almost disappeared comparing with that of 2-AT, indicating that the nitrogen in the -NH2 group were involved in Zn2+ chelation (Figure 4, blue line). Such adsorbed 2-AT monomer would decompose and generate abundant gas species such as NH3, H2S and CS2 et al., forming the porous carbon matrix with uniform S-doping during the carbonization process. Therefore, on one hand, the 2-AT monomer serves as a “linker” to connect neighboring ZIF-8 precursor particles via chelation, which is beneficial to form the unique 3D networks structure after the mixture suffering pyrolysis; on the other hand, they impose a steric hindrance which hence avoids close-packing and extensive fusion of the ZIF-8 particles into relatively bulky aggregates during heating treatment. Moreover, the specific surface area of the sample S-Fe/Z8/2-AT(0) is found as 781.6 m2 g-1, with average sizes of mesopores and micropores obtained at 12.6 nm and 1.06 nm (Figure S9a-c), respectively.


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Addition of 2-AT could lead to slight decreases in the specific surface area of the end-products. The S-Fe/Z8/2-AT (8.9) demonstrates a specific surface area of 572.4 m2 g-1, average mesopores of 8.5 nm and micropores of 0.81 nm (Figure S9d-f). This is because that the presence of 2-AT monomer could partially block the original porous-structure of ZIF-8 particles. Further increasing the dosage of 2-AT, x ≥16.4, typical hyperbranched 3D networks with hierarchical porous structure was found (Figure 4d and Figure S8b). However, as depicted in (Figure 2a, c, d, and Figure S7, S9), the fraction of the graphitic carbon, the specific surface area, and the average mesopore and micropore sizes increase with x increasing from 8.9 to 16.4 and then decrease with x further increasing to 28.1, indicating that the addition of 2-AT with appropriate dose can obtain optimal S-Fe/Z8/2-AT catalysts with advanced chemical component, microstructure including morphology and porosity, high electricalconductivity and excellent electrochemical performance which be discussed in detail in the following section. On this basis, the S-Fe/Z8/2-AT(16.4) is expected to be the optimal electrocatalyst considering its relatively large specific surface area and pore sizes. The average mass fraction of S element increases mono-directionally along with increasing dosages of 2-AT (Table S1). In addition to the dosage of 2-AT, it shall also be mentioned that the content of the metal and pyrolytic temperature are also important technical parameters for their catalytic activity, which were also optimized in this work, with the mass fraction of Fe as 1.75 % and a pyrolytic temperature of 950 oC obtained as the optimal conditions (see more details in Figure S16 and Figure S17). 3.3 Electrocatalytic performance of 3D S-Fe/Z8/2-AT networks


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Thus-prepared 3D S-Fe/Z8/2-AT networks with hierarchical macro-meso-micro porosity are expected to have multiple advantages when used as electrocatalysts. The catalytic performance of our S-Fe/Z8/2-AT(16.4) towards ORR was characterized and compared against a variety of control samples. Figure 5a displays the relevant ORR polarization curves of those different catalysts from 0.2 - 1.0 V (vs RHE, similarly thereafter) evaluated in O2-saturated 0.1 M H2SO4 under the same technical conditions. The N/C catalyst displays half-wave potential (E1/2) at 0.462 V and the kinetic-limiting current density (jk) at 1.82 mA cm-2, indicating the poor catalytic activity of the N/C catalyst (i.e., with neither Fe- nor S-doping) in acidic medium (Figure 5a, line 1 in black, and Table 1). However, the catalytic activity of N/C catalyst is better than that of ZIF-8 and intermediates (defined as ZIF-8/2-AT/FeCl2) of end products, respectively, indicating the carbonation process is significant for enhancing ORR performance, showed in Figure S13. In comparison, the E1/2 of the S-Fe/Z8/2-AT(0) catalyst, the one without S-doping catalysts, is at 0.805 V, with jk at 4.0 mA cm-1(Figure 5a, line 3 in pink), significantly greater catalytic activity than that of the N/C catalyst, which shows that the formation of Fe-Nx species could improve catalytic activity for ORR.


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Figure 5. Comparison of electrocatalytic performances of different catalysts: (a) ORR polarization curves in O2-saturated 0.1 M H2SO4 solution; (b) H2O2 yield and the electron transfer number n; (c) the relationship between H2O2 yield %,n and E1/2; and (d) Tafel plots with the inset to compare the mass activity at 0.80 V. The colored curves with different numbers correspond to catalysts, including: (1) N/C, (2) S-Fe/2-AT, (3) S-Fe/Z8/2-AT(0), (4) S-Fe/Z8/2AT(8.9), (5) S-Fe/Z8/2-AT(16.4), and (6) S-Fe/Z8/2-AT(28.1). It is believed that moderate S-doping into the carbon lattice could facilitate oxygen chemisorption, which is beneficial for the ORR process.32 Consistently, the ORR polarization curves of the S-doped Fe/N/C catalysts reveal that the S-doping into the carbon matrix further improves the catalytic activity of such carbonaceous material-based NPMCs. As compared in Table 1, the values of E1/2 for the S-Fe/Z8/2-AT(x) (x = 8.9 or 16.4) are more positive than those presented by either the N/C or the S-Fe/Z8/2-AT(0). For the case of S-Fe/Z8/2-AT(8.9) (Figure


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5a, line 4 in green), E1/2 was obtained at 0.812 V and the jk was 4.63 mA cm-2, overtaking the catalytic performance of S-Fe/Z8/2-AT(0). The above results indicate that S-doping into Fe/N/C skeleton and such unique 3D network structure are very critical for improving ORR activity. Specifically, the S-Fe/Z8/2-AT(16.4) gives even better catalytic performance, with E1/2 obtained at 0.820 V (Figure 5a, line 5 in red), which is comparable to the best MOFs-derived NPMCs which is only a 30 mV gap with S-Fe/Z8/2-AT(16.4),63 and the jk reached at 4.57 mA cm-1 which is slightly inferior than that of S-Fe/Z8/2-AT(8.9) (the biggest one for jk). Nevertheless, the catalyst prepared with further increase of 2-AT, S-Fe/Z8/2-AT(28.1), exhibits E1/2 at 0.803 V (Figure 5a, line 6 in blue). For further comparison, the S-Fe/N/C catalyst derived directly from the mixture of 2-AT and Fe2+ under the same preparing condition (referred to as S-Fe/2-AT thereafter) was also evaluated, which gives a poor catalytic activity with E1/2 obtained at 0.656 V (Figure 5a, line 2), demonstrating the advantages of ZIF-8 colloidal particles as the precursor. It should be mentioned that the doping of different N species in the carbon matrix, especially pyridinic N and quaternary N, could improve both the conductivity as well as the ORR catalytic activity.31 More interestingly, as discussed in the previous XPS section, the contents of pyridinic N and quaternary N of S-Fe/Z8/2-AT(16.4), respectively, are lower than that of S-Fe/Z8/2-AT(0), which demonstrates the unique advantages of S doped and 3D network structure for improving ORR activity. With hierarchical porosity and large surface area within the 3D networks, our SFe/Z8/2-AT catalyst could support efficient electron transport and mass transfer; and with homogeneously doped S, N and Fe species within the porous carbon frameworks, the as-formed S-Fe/Z8/2-AT has large electrochemically active surface area, with catalytic active sites highly exposed. As such, fast reaction kinetics and high catalytic activity are resulted in when they are used for the ORR. Additionally, as discussed in the previous section, among the three S-doped


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electrocatalysts, S-Fe/Z8/2-AT(x) (x = 8.9, 16,4 and 28.1), the S-Fe/Z8/2-AT(16.4) possesses the largest specific surface area and the largest average pore sizes (Figure 5c) and thus enables more efficient mass transfer during the interfacial reaction. Therefore, the comparative catalytic performances presented by these three electrocatalysts confirmed the importance of the structural merits to the catalytic activity. Table 1. Comparison of the half-wave potential (E1/2), the kinetic-limiting current density (jk), the maximum H2O2 yield (H2O2max), the minimum electron transfer number (nmix), and the mass activity (jm) at 0.8 V for different electrocatalysts prepared in this work.

Electrocatalysts

E1/2a)

jkb)

H2O2maxc)

nmixd)

jme)

N/C

0.462

1.82

59.1

2.83

1.2

S-Fe/2-AT

0.656

2.55

26.9

3.45

3.2

S-Fe/Z8/2AT(0)

0.805

4.0

7.1

3.83

8.1

S-Fe/Z8/2AT(8.9)

0.812

4.63

6.6

3.88

10.7

S-Fe/Z8/2AT(16.4)

0.820

4.57

3.8

3.94

18.3

S-Fe/Z8/2AT(28.1)

0.803

4.21

8.0

3.85

7.8

a)

(V) vs. RHE, obtained from RRDE tests; b)(mA cm-2) obtained from RRDE tests; c)(%) read

from Figure 5b; d)read from Figure 5b; e)(A g-1) read from the inset of Figure 5d. During the ORR process, when the diffusion of protons and the transfer of the electrons occur with different kinetics at the solid-liquid interface, side-reactions will occur, generating side products such as H2O2.64 The ORR processes catalyzed by different carbonaceous materials were


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further investigated by the H2O2 yield and the electron transfer number (n) via the rotating ring disk electrode (RRDE) tests, as compared in Figure 5b and Table 1. Among the six catalysts, the case of N/C demonstrated the highest H2O2 yield which is 59.1 %，followed by the cases of the S-Fe/2-AT and the S-Fe/Z8/2-AT(28.1) which were 26.9 % and 8.0 % respectively, and the case of Fe/N/C exhibited a H2O2 yield of 7.1 %. The S-Fe/Z8/2-AT(16.4), the most reactive catalyst in this study, gives the lowest H2O2 yield of 3.8 %, implying excellent match between the proton diffusion rate and the electron transfer kinetics within the catalyst during the ORR reaction. Such a result further proves the compositional and structural merits of our hierarchically porous 3D SFe/Z8/2-AT(16.4) networks. The corresponding n values, namely the number of the electrons transferred in ORR, were also acquired for each catalyst based on the relevant RRDE test, which was found to follow the opposite tendency for that of the H2O2 yield, as shown in Figure 5b and Table 1. The n values for the N/C case and the S-Fe/2-AT change from ca. 4.0 to ca. 2.8 versus decreasing voltage, which indicates that the electron-transfer path involves a mixture of the 2+2 peroxide ORR pathway and a four electron pathway.29, 30 For the other four cases, their n values approach 4.0 at voltage range from 0.8 to 0.2 V, indicating a direct four-electron pathway.65 Furthermore, the linear regression was conducted upon the dataset of the maximum H2O2 yield and E1/2, as well as the minimum n and E1/2. One can see that the value of E1/2 increases linearly with decreasing maximal H2O2 yield, with a correlation coefficient of 0.998 (Figure 5c, the red line), indicating the negative correlation between the H2O2 yield and the catalytic activity. And with a correlation coefficient of 0.997, the linear plot between the minimum n and the value of the E1/2 shows the opposite trend (Figure 5c, the blue line), demonstrating that a higher n value, i.e., the direct fourelectron pathway and low H2O2 yield, favors relatively high catalytic activity for ORR.


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To further assess the intrinsic activity of the catalysts, the kinetic current was calculated by the Koutecky-Levich equation,3 which was then normalized with the catalyst loading to obtain the mass activity (jm). It was found that at 0.8 V the S-Fe/Z8/2-AT(16.4) generates a jm as large as 18.3 A gcat.-1 (Figure 5d, column 5 in the inset), which is significantly larger than those of the other controls in this work(Figure 5d, column 3 in the inset). And based on the normalized polarization curves, the Tafel slope was also calculated. As depicted in Figure 5d and Table 1, the N/C demonstrates the largest Tafel slope at 96 mV decade-1, followed by the cases of S-Fe/2AT and S-Fe/Z8/2-AT(28.1) which are 82 and 71 mV decade-1, respectively. The three cases, namely S-Fe/Z8/2-AT(0), S-Fe/Z8/2-AT(8.9) and S-Fe/Z8/2-AT(16.4), demonstrate similar Tafel slopes at 69, 68 and 65 mV decade-1, respectively.

Figure 6. (a) Polarization (filled squares) and power density (open squares) plots for H2-O2 PEMFCs with S-Fe/Z8/2-AT(16.4) as the cathode catalysts at 80 °C and 1 bar; (b) Durability test of S-Fe/Z8/2-AT(x) (x = 0, 8.9, 16.4 and 28.1). To further evaluate the catalytic performance of our optimal 3D S-Fe/Z8/2-AT networks, the commercial Pt/C catalyst, which was known as the most efficient ORR catalyst, was also tested with a loading of 20 μgPt cm-2 under the same conditions for comparison. As shown in Figure


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S14a, E1/2 of our S-Fe/Z8/2-AT(16.4) has only a 45 mV gap than that of the commercial Pt/C catalyst, , and the jk for the Pt/C catalyst is 4.0 mA cm-2, which is smaller than that of our SFe/Z8/2-AT(16.4), with a gap of 0.57 mA cm-2. The S-Fe/Z8/2-AT(16.4) and the Pt/C demonstrated similar H2O2 yield, with the maximum difference less than 4.0 %; and both cases displayed similar n values which are larger than 3.9 within the range of 0.2 to 0.8 V (Figure S14b), implying that both catalysts assume a direct four electron ORR process. In addition, the two cases exhibit similar Tafel slopes which are 65 and 63 mV decade-1 for S-Fe/Z8/2-AT(16.4) and Pt/C, respectively (Figure S14c). The above results clearly proved that our 3D S-Fe/Z8/2-AT networks are comparable to the commercial Pt/C catalyst and have the enormous potential to be a highly active electrocatalyst for the ORR process in acidic medium. It is noted that the second pyrolysis in NH3 atmosphere was very important for affecting the ORR performance of the obtained products. We prepared different S-Fe/Z8/2-AT samples by controlling the different pyrolysis time under NH3 atmosphere from 0 to 10 min. Catalytic intermediates pyrolyzed at 950 °C with holding time in 0, 5 and 10 min were defined as SFe/Z8/2-AT(16.4)-0 NH3, S-Fe/Z8/2-AT(16.4)-5 NH3 and S-Fe/Z8/2-AT(16.4)-10 NH3, respectively. Figure S16 showed the ORR polarization curves on RDE at O2-saturated 0.1 M H2SO4 solution when the products pyrolyzed in different holding time were employed as the ORR catalysts. Compared with LSVs curves of S-Fe/Z8/2-AT(16.4)-0 NH3, S-Fe/Z8/2-AT(16.4)5 NH3, and S-Fe/Z8/2-AT(16.4)-10 NH3, the S-Fe/Z8/2-AT(16.4) catalyst obtained by holding time 5 min NH3 possessed more positive E1/2 and higher limiting current densities, which suggested that the NH3-treated 5 min was the optimal holding time for obtaining highly active ORR catalysts in the acidic electrolyte. There is demonstrated that the NH3-treated Fe/N/C catalysts have the same Fe-centered moieties comparing with the counterpart without NH3


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treatment, and which the higher activity of NH3-treated Fe/N/C catalysts is rather dependent on its physicochemical properties of the carbon support.28 Hence, there is a significant possibility that the advanced ORR catalytic activity via the NH3 treatment can be ascribed to the enhanced number of accessible active site, and/or the higher basicity of the carbon matrix.28, 66 The performance of S-Fe/Z8/2-AT(16.4) was further investigated as the cathode catalyst for ORR in the acidic solution in the PEMFCs using the Nafion proton exchange membrane. As shown in Figure 6a the maximum power density was 800 mW cm-2 at the cell potential of 0.37 V with 1 bar, a value comparable to the best performance exhibited by MOFs-derived NPMCs in Table S2, and the current density was obtained as 910 mA cm-2 at the cell potential of 0.6 V in the PEMFCs operating conditions, respectively. Note that the peak power density of PEMFCs also depends strongly on the conductivity of the electrocatalysts; it is expected that the performance of S-Fe/Z8/2-AT(16.4) could be even better if tested with highly conductive carbon supports. The durability of these catalysts was investigated by the current-time chronoamperometric response at 0.6 V in O2-saturated 0.1 M H2SO4 solution at rotation rates of 900 rpm in Figure 6b and Figure S18. The relative current of S-Fe/Z8/2-AT(16.4) electrode exhibited a slow decreased up to 12.5 % loss after 40 000 s, in stark contrast, 22.4 %, 15.6 %, 12.4 % and 25.6 % loss of the current density was found at S-Fe/Z8/2-AT(0), S-Fe/Z8/2-AT(8.9), S-Fe/Z8/2-AT(28.1) and Pt/C electrode after just running 20 000 s under the same condition, revealing that the S-Fe/Z8/2AT(16.4) catalyst possessed more superior durability than other catalysts. Further, the S-Fe/Z8/2AT(16.4) was also evaluated at a constant potential of 0.50 V in PEMFCs under long-term running, which demonstrated similar result recorded by RDE. As Shown in Figure S19, the catalytic performance of the S-Fe/Z8/2-AT(16.4) degrades slowly, which retains a current


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density of 0.24 mA cm-2, around 25 % of the initial PEMFCs current density after 67 h. Note that durability in a fuel cell test could be influenced by a wide variety of factors beyond the catalyst itself, such as degradation and cracking of the Nafion membrane during the aging test, the degradation-induced failure of the sealing gasket etc. Further efforts are still needed to improve the long-term durability of such PEMFCs. 4. Conclusions In summary, the rational preparation of S-doped Fe/N/C catalysts has been realized through stepwise pyrolysis of the mixture of ZIF-8 colloidal particles, 2-AT and FeCl2 in the Ar2 and NH3 atmosphere. The obtained catalysts have been well characterized and optimized, which demonstrates uniform element doping of S, N, Fe in the carbonaceous framework and also possesses a 3D connective network with the macro-meso-micro hierarchical porosity. The ORR electrocatalytic performance of such S-Fe/Z8/2-AT catalysts has been characterized systematically. Significantly, such formation mechanism of 3D connective network was also legitimately disclosed by a series of experiments. Due to its favorable chemical composition and structural merits, the optimal catalyst exhibits impressive catalytic activity for ORR in acidic medium, with a half peak potential as high as 0.82 V obtained for ORR in 0.1 M H2SO4. Such hierarchically porous 3D S-Fe/Z8/2-AT networks were further applied to practical PEMFCs application, which displayed a maximal power density of 0.80 W cm-2 at 1 bar, further demonstrating its promising potential for applications as NPMCs in acidicPEMFCs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:


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XRD images of ZIF-8 particles, SEM images ZIF-8 particles, N/C particulates, S-Fe/Z8/2AT(8.9), S-Fe/Z8/2-AT(16.4), and S-Fe/Z8/2-AT(28.1); TEM and EDX images of S-Fe/Z8/2AT(16.4); elemental composition (%) table of S-Fe/Z8/2-AT(x) (Table S1); XPS of highresolution Fe 2p spectra of S-Fe/Z8/2-AT(x) (x = 0 and 16.4) and Survey XPS spectra of SFe/Z8/2-AT(x) (x = 0, 8.9, 16.4 and 28.1); raman spectra of S-Fe/Z8/2-AT(x) (x = 0, 8.9, 16.4 and 28.1); N2 adsorption-desorption isotherm, micropore and mesopore size distributions of SFe/Z8/2-AT(x) (x = 0, 8.9 and 28.1); ORR polarization curves of S-Fe/Z8/2-AT(16.4) with different metal contents and different thermally treated temperature; durability test of PEMFC. AUTHOR INFORMATION Corresponding Author: *E-mail: [email protected] (J.T.Li). Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by National Natural Science Foundation of China (21703182, 21621091), National Key Research and Development of China (2016YFB0100202). The authors wish to thank Dr. Y.-C.Wang, R.-X. Wang and X.-D. Yang for their assistance in electrochemical test. REFERENCES (1)

Gasteiger, H. A.; Kocha, S. S.; Sompalli, B.; Wagner, F. T. Activity Benchmarks and

Requirements for Pt, Pt-alloy, and Non-Pt Oxygen Reduction Catalysts for PEMFCs. Appl. Catal. B 2005, 56, 9−35.


33


(2)

Page 34 of 44

Shao, M.; Chang, Q.; Dodelet, J. -P. Chenitz, R. Recent Advances in Electrocatalysts for

Oxygen Reduction Reaction. Chem. Rev. 2016, 116, 3594−3657. (3)

Xia, W.; Mahmood, A.; Liang, Z.; Zou, R.; Guo, S. Earth-Abundant Nanomaterials for

Oxygen Reduction. Angew. Chem. Int. Ed. 2016, 55, 2650−2676. (4)

Du, L.; Shao, Y.; Sun, J.; Yin, G. P.; Liu, J.; Wang, Y. Advanced Catalyst Supports for

PEM Fuel Cell Cathodes. Nano Energy 2016, 29, 314−322. (5)

Wu, G.; Zelenay, P. Nanostructured Nonprecious Metal Catalysts for Oxygen Reduction

Reaction. Acc. Chem. Res. 2013, 46, 1878−1889. (6)

Seh, Z. W.; Kibsgaard, J.; Dickens, C. F.; Chorkendorff, I.; Norskov, J. K.; Jaramillo, T.

F. Combining Theory and Experiment in Electrocatalysis: Insights into Materials Design. Science 2017, 355, eaad4998. (7)

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. Single Cobalt Atoms with Precise N-Coordination as Superior Oxygen Reduction Reaction Catalysts. Angew. Chem. Int. Ed. 2016, 55, 10800−10805. (8)

Xia, B. Y.; Yan, Y.; Li, N.; Wu, H. B.; Lou, X. W.; Wang, X. A Metal-Organic

Framework-Derived Bifunctional Oxygen Electrocatalyst. Nat. Energy 2016, 1, 15006-15013. (9)

Wu, Y.; Zhao, S.; Zhao, K.; Tu, T.; Zheng, J.; Chen, J.; Zhou, H.; Chen, D.; Li, S. Porous

Fe-Nx/C Hybrid Derived from Bi-Metal Organic Frameworks as High Efficient Electrocatalyst for Oxygen Reduction Reaction. J. Power Sources 2016, 311, 137−143.


34

Page 35 of 44 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


(10)

Tang, H.; Zeng, Y.; Liu, D.; Qu, D.; Luo, J.; Binnemans, K.; De Vos, D. E.; Fransaer, J.;

Qu, D.; Sun, S. -G. Dual-Doped Mesoporous Carbon Synthesized by a Novel Nanocasting Method with Superior Catalytic Activity for Oxygen Reduction. Nano Energy 2016, 26, 131−138. (11)

Gadipelli, S.; Zhao, T.; Shevlin, S. A.; Guo, Z. Switching Effective Oxygen Reduction

and Evolution Performance by Controlled Graphitization of a Cobalt-Nitrogen-Carbon Framework System. Energy Environ. Sci. 2016, 9, 1661−1667. (12)

Xia, Z.; An, L.; Chen, P.; Xia, D. Non-Pt Nanostructured Catalysts for Oxygen Reduction

Reaction: Synthesis, Catalytic Activity and its Key Factors. Adv. Energy Mater. 2016, 6, 1600458. (13)

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. Phenylenediamine-Based FeNx/C Catalyst with High Activity for Oxygen Reduction in Acid Medium and its Active-Site Probing. J. Am. Chem. Soc. 2014, 136, 10882−10885. (14)

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

(15)

Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Mullen, K. 3D Nitrogen-Doped

Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 9082−9085. (16)

Zhang, L.; Xia, Z. Mechanisms of Oxygen Reduction Reaction on Nitrogen-Doped

Graphene for Fuel Cells. J. Phys. Chem. C 2011, 115, 11170−11176.


35


(17)

Page 36 of 44

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. (18)

Chung, H. T.; Won, J. H.; Zelenay, P. Active and Stable Carbon Nanotube/Nanoparticle

Composite Electrocatalyst for Oxygen Reduction. Nat. Commun. 2013, 4, 1922. (19)

Li, Y.; Zhou, W.; Wang, H.; Xie, L.; Liang, Y.; Wei, F.; Idrobo, J. -C.; Pennycook, S. J.;

Dai, H. An Oxygen Reduction Electrocatalyst Based on Carbon Nanotube-Graphene Complexes. Nat. Nanotechnol. 2012, 7, 394−400. (20)

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, 760−764. (21)

Wang, M.-Q.; Yang, W.-H.; Wang, H.-H.; Chen, C.; Zhou, Z.-Y.; Sun, S.-G. Pyrolyzed

Fe-N-C Composite as an Efficient Non-precious Metal Catalyst for Oxygen Reduction Reaction in Acidic Medium. ACS Catal. 2014, 4, 3928−3936. (22)

Malko, D.; Kucernak, A.; Lopes, T. In Situ Electrochemical Quantification of Active

Sites in Fe-N/C Non-Precious Metal Catalysts. Nat. Commun. 2016, 7, 13285. (23)

Jiang, W.-J.; Gu, L.; Li, L.; Zhang, Y.; Zhang, X.; Zhang, L.-J.; Wang, J.-Q.; Hu, J.-S.;

Wei, Z.; Wan, L.-J. Understanding the High Activity of Fe-N-C Electrocatalysts in Oxygen Reduction: Fe/Fe3C Nanoparticles Boost the Activity of Fe-Nx. J. Am. Chem. Soc. 2016, 138, 3570−3578.


36

Page 37 of 44 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


(24)

Ramaswamy, N.; Tylus, U.; Jia, Q.; Mukerjee, S. Activity Descriptor Identification for

Oxygen Reduction on Nonprecious Electrocatalysts: Linking Surface Science to Coordination Chemistry. J. Am. Chem. Soc. 2013, 135, 15443−15449. (25)

Kramm, U. I.; Herranz, J.; Larouche, N.; Arruda, T. M.; Lefevre, M.; Jaouen, F.;

Bogdanoff, P.; Fiechter, S.; Abs-Wurmbach, I.; Mukerjee, S.; Dodelet, J. -P. Structure of the Catalytic Sites in Fe/N/C-Catalysts for O2-Reduction in PEM Fuel Cells. Phys. Chem. Chem. Phys. 2012, 14, 11673−11688. (26)

Velázquez-Palenzuela, A.; Zhang, L.; Wang, L.; Cabot, P. L.; Brillas, E.; Tsay, K.;

Zhang, J. Carbon-Supported Fe-NxCatalysts Synthesized by Pyrolysis of the Fe(II)-2,3,5,6Tetra(2-pyridyl)pyrazine Complex: Structure, Electrochemical Properties, and Oxygen Reduction Reaction Activity. J. Phys. Chem. C 2011, 115, 12929−12940. (27)

Byon, H. R.; Suntivich, J.; Shao-Horn, Y. Graphene-Based Non-Noble-Metal Catalysts

for Oxygen Reduction Reaction in Acid. Chem. Mater. 2011, 23, 3421−3428. (28)

Zitolo, A.; Goellner, V.; Armel, V.; Sougrati, M. T.; Mineva, T.; Stievano, L.; Fonda, E.;

Jaouen, F. Identification of Catalytic Sites for Oxygen Reduction in Iron- and Nitrogen-Doped Graphene Materials. Nat. Mater. 2015, 14, 937−942. (29)

Tylus, U.; Jia, Q.; Strickland, K.; Ramaswamy, N.; Serov, A.; Atanassov, P.; Mukerjee,

S. Elucidating Oxygen Reduction Active Sites in Pyrolyzed Metal-Nitrogen Coordinated NonPrecious-Metal Electrocatalyst Systems. J. Phys. Chem. C 2014, 118, 8999−9008.


37


(30)

Page 38 of 44

Olson, T. S.; Pylypenko, S.; Fulghum, J. E.; Atanassov, P. Bifunctional Oxygen

Reduction Reaction Mechanism on Non-Platinum Catalysts Derived from Pyrolyzed Porphyrins. J. Electrochem. Soc. 2010, 157, B54−B63. (31)

Masa, J.; Xia, W.; Muhler, M.; Schuhmann, W. On the Role of Metals in Nitrogen-Doped

Carbon Electrocatalysts for Oxygen Reduction. Angew. Chem. Int. Ed. 2015, 54, 10102−10120. (32)

Wang, D.; Su, D. Heterogeneous Nanocarbon Materials for Oxygen Reduction Reaction.

Energy Environ. Sci. 2014, 7, 576−591. (33)

Pei, Z.; Li, H.; Huang, Y.; Xue, Q.; Huang, Y.; Zhu, M.; Wang, Z.; Zhi, C. Texturing in

situ: N, S-Enriched Hierarchically Porous Carbon as a Highly Active Reversible Oxygen Electrocatalyst. Energy Environ. Sci. 2017, 10, 742−749. (34)

Tian, W.; Zhang, H.; Sun, H.; Suvorova, A.; Saunders, M.; Tade, M.; Wang, S.

Heteroatom (N or N-S)-Doping Induced Layered and Honeycomb Microstructures of Porous Carbons for CO2 Capture and Energy Applications. Adv. Funct. Mat. 2016, 26, 8651−8661. (35)

Liang, J.; Jiao, Y.; Jaroniec, M.; Qiao, S. Sulfur and Nitrogen Dual-Doped Mesoporous

Graphene Electrocatalyst for Oxygen Reduction with Synergistically Enhanced Performance. Angew. Chem. Int. Ed. 2012, 51, 11496−11500. (36)

Wang, Y. -C.; Lai, Y.-J.; Song, L.; Zhou, Z.-Y.; Liu, J.-G.; Wang, Q.; Yang, X.-D.;

Chen, C.; Shi, W.; Zheng, Y. -P.; Rauf, M.; Sun, S. -G. S-Doping of an Fe/N/C ORR Catalyst for Polymer Electrolyte Membrane Fuel Cells with High Power Density. Angew. Chem. Int. Ed. 2015, 54, 9907−9910.


38

Page 39 of 44 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


(37)

Chen, C.; Yang, X. -D.; Zhou, Z. -Y.; Lai, Y. -J.; Rauf, M.; Wang, Y.; Pan, J.; Zhuang,

L.; Wang, Q.; Wang, Y. -C.; Tian, N.; Zhang, X. -S.; Sun, S.-G. Aminothiazole-Derived N,S,FeDoped Graphene Nanosheets as High Performance Electrocatalysts for Oxygen Reduction. Chem. Commun. 2015, 51, 17092−17095. (38)

Zhu, Q.; Xu, Q. Metal-Organic Framework Composites. Chem. Soc. Rev. 2014, 43,

5468−5512. (39)

Furukawa, H.; Cordova, K.; O'Keeffe, M.; Yaghi, O. The Chemistry and Applications of

Metal-Organic Frameworks. Science 2013, 341, 1230444. (40)

Xia, W.; Mahmood, A.; Zou, R.; Xu, Q., Metal-Organic Frameworks and Their Derived

Nanostructures for Electrochemical Energy Storage and Conversion. Energy Environ. Sci. 2015, 8, 1837−1866. (41)

Zhong, H.; Wang, J.; Zhang, Y.; Xu, W.; Xing, W.; Xu, D.; Zhang, Y.; Zhang, X. ZIF-8

Derived Graphene-Based Nitrogen-Doped Porous Carbon Sheets as Highly Efficient and Durable Oxygen Reduction Electrocatalysts. Angew. Chem. Int. Ed. 2014, 53, 14235−14239. (42)

Zhao, S.; Yin, H.; Du, L.; He, L.; Zhao, K.; Chang, L.; Yin, G.; Zhao, H.; Liu, S.; Tang,

Z. Carbonized Nanoscale Metal-Organic Frameworks as High Performance Electrocatalyst for Oxygen Reduction Reaction. Acs Nano 2014, 8, 12660−12668. (43)

Li, B.; Wen, H. M.; Cui, Y.; Zhou, W.; Qian, G.; Chen, B. Emerging Multifunctional

Metal-Organic Framework Materials. Adv. Mater. 2016, 28, 8819−8860.


39


(44)

Page 40 of 44

Mahmood, A.; Guo, W. H.; Tabassum, H.; Zou, R. Metal-Organic Framework-Based

Nanomaterials for Electrocatalysis. Adv. Energy Mater. 2016, 6, 1600423. (45)

Proietti, E.; Jaouen, F.; Lefevre, M.; Larouche, N.; Tian, J.; Herranz, J.; Dodelet, J, -P.

Iron-Based Cathode Catalyst with Enhanced Power Density in Polymer Electrolyte Membrane Fuel Cells. Nat. Commun. 2011, 2, 416. (46)

Ma, T.; Dai, S.; Jaroniec, M.; Qiao, S. Metal-Organic Framework Derived Hybrid Co3O4-

Carbon Porous Nanowire Arrays as Reversible Oxygen Evolution Electrodes. J. Am. Chem. Soc. 2014, 136, 13925−13931. (47)

Shui, J.; Chen, C.; Grabstanowicz, L.; Zhao, D.; Liu, D. Highly Efficient Nonprecious

Metal Catalyst Prepared with Metal-Organic Framework in a Continuous Carbon Nanofibrous Network. Proc. Natl. Acad. Sci. U S A 2015, 112, 10629−34. (48)

Zhang, C.; Wang, Y. C.; An, B.; Huang, R.; Wang, C.; Zhou, Z.; Lin, W. Networking

Pyrolyzed Zeolitic Imidazolate Frameworks by Carbon Nanotubes Improves Conductivity and Enhances Oxygen-Reduction Performance in Polymer-Electrolyte-Membrane Fuel Cells. Adv. Mater. 2017, 29, 1604556. (49)

Strickland, K.; Miner, E.; Jia, Q.; Tylus, U.; Ramaswamy, N.; Liang, W.; Sougrati, M. T.;

Jaouen, F.; Mukerjee, S. Highly Active Oxygen Reduction Non-Platinum Group Metal Electrocatalyst without Direct Metal-Nitrogen Coordination. Nat. Commun. 2015, 6, 7343. (50)

You, B.; Jiang, N.; Sheng, M.; Drisdell, W. S.; Yano, J.; Sun, Y. Bimetal-Organic

Framework Self-Adjusted Synthesis of Support-Free Nonprecious Electrocatalysts for Efficient Oxygen Reduction. ACS Catal. 2015, 5, 7068−7076


40

Page 41 of 44 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


(51)

Wang, X.; Zhang, H.; Lin, H.; Gupta, S.; Wang, C.; Tao, Z.; Fu, H.; Wang, T.; Zheng, J.;

Wu, G.; Li, X. Directly Converting Fe-Doped Metal-Organic Frameworks into Highly Active and Stable Fe-N-C Catalysts for Oxygen Reduction in Acid. Nano Energy, 2016, 25, 110−119. (52)

Liu, Z.; Lin, X.; Lee, J.-Y.; Zhang, W.-S.; Han, M.; Gan, L.-M. Preparation and

Characterization of Platinum-Based Electrocatalysts on Multiwalled Carbon Nanotubes for Proton Exchange Membrane Fuel Cells. Langmuir 2002, 18, 4054−4060. (53)

Dresselhaus, M. S.; Dresselhaus, G.; Saito, R.; Jorio, A. Raman Spectroscopy of Carbon

Nanotubes. Phys. Rep. 2005, 409, 47−99. (54)

Chang, Y.; Hong, F.; He, C.; Zhang, Q.; Liu, J. Nitrogen and Sulfur Dual-Doped Non-

Noble Catalyst Using Fluidic Acrylonitrile Telomer as Precursor for Efficient Oxygen Reduction. Adv. Mat. 2013, 25, 4794−4799. (55)

Jeon, I. Y.; Choi, H. J.; Jung, S. M.; Seo, J. M.; Kim, M. J.; Dai, L.; Baek, J. B. Large-

Scale Production of Edge-Selectively Functionalized Graphene Nanoplatelets via Ball Milling and Their Use as Metal-Free Electrocatalysts for Oxygen Reduction Reaction. J. Am. Chem. Soc. 2013, 135, 1386−1393. (56)

Yang, S.; Zhi, L.; Tang, K.; Feng, X.; Maier, J.; Müllen, K. Efficient Synthesis of

Heteroatom (N or S)-Doped Graphene Based on Ultrathin Graphene Oxide-Porous Silica Sheets for Oxygen Reduction Reactions. Adv. Funct. Mater. 2012, 22, 3634−3640. (57)

Shui, J.; Wang, M.; Du, F.; Dai, L. N-Doped Carbon Nanomaterials are Durable

Catalysts for Oxygen Reduction Reaction in Acidic Fuel Cells. Sci. Adv. 2015, 1, e1400129−e1400129.


41


(58)

Page 42 of 44

Artyushkova, K.; Kiefer, B.; Halevi, B.; Knop-Gericke, A.; Schlogl, R.; Atanassov, P.

Density Functional Theory Calculations of XPS Binding Energy Shift for Nitrogen-Containing Graphene-Like Structures. Chem. Commun. 2013, 49, 2539−2541. (59)

Zhang, B.; Xu, Z. L.; He, Y.-B.; Abouali, S.; Akbari Garakani, M.; Kamali Heidari, E.;

Kang, F.; Kim, J. K. Exceptional Rate Performance of Functionalized Carbon Nanofiber Anodes Containing Nanopores Created by (Fe) Sacrificial Catalyst. Nano Energy 2014, 4, 88−96. (60)

Vasconcelos, I. B.; da Silva, T. G.; Militão, G. C. G.; Soares, T. A.; Rodrigues, N. M.;

Rodrigues, M. O.; da Costa, N. B.; Freire, R. O.; Junior, S. A. Cytotoxicity and Slow Release of the Anti-Cancer Drug Doxorubicin from ZIF-8. Rsc Adv. 2012, 2, 9437−9442. (61)

Hu, Y.; Kazemian, H.; Rohani, S.; Huang, Y.; Song, Y. In Situ High Pressure Study of

ZIF-8 by FTIR Spectroscopy. Chem. Commun. 2011, 47, 12694−12696. (62)

Xiong, C. H.; Jia, X.; Chen, X. Y.; Wang, G. T.; Yao, C. P. Optimization of

Polyacrylonitrile-2-aminothiazole Resin Synthesis, Characterization, and its Adsorption Performance and Mechanism for Removal of Hg(II) from Aqueous Solutions. Ind. Eng. Chem. Res. 2013, 52, 4978−4986. (63)

Zhang, H.; Hwang, S.; Wang, M.; Feng, Z.; Karakalos, S.; Luo, L.; Qiao, Z.; Xie, X.;

Wang, C.; Su, D.; Shao, Y.; Wu, G. Single Atomic Iron Catalysts for Oxygen Reduction in Acidic Media: Particle Size Control and Thermal Activation. J. Am. Chem. Soc. 2017, 139, 14143−14149.


42

Page 43 of 44 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


(64)

Tse, E. C.; Barile, C. J.; Kirchschlager, N. A.; Li, Y.; Gewargis, J. P.; Zimmerman, S. C.;

Hosseini, A.; Gewirth, A. A. Proton Transfer Dynamics Control the Mechanism of O2 Reduction by a Non-Precious Metal Electrocatalyst. Nat. Mater. 2016, 15, 754−759. (65) Collman, J. P.; Denisevich, P.; Konai, Y.; Marrocco, M.; Koval, C.; Anson, F. C. Electrode Catalysis of the Four-Electron Reduction of Oxygen to Water by Dicobalt Face-to-Face Porphyrins. J. Am. Chem. Soc. 1980, 102, 6027−6036. (66) Li, J.; Ghoshal, S.; Liang, W.; Sougrati, M.-T.; Jaouen, F.; Halevi, B.; McKinney, S.; McCool, G.; Ma, C.; Yuan, X.; Ma, Z.-F.; Mukerjee, S.; Jia, Q. Structural and Mechanistic Basis for the High Activity of Fe-N-C Catalysts toward Oxygen Reduction. Energy Environ. Sci. 2016, 9, 2418−2432.


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