NaCl Crystallites as Dual-Functional and Water-Removable

Tang , J.; Liu , J.; Li , C.; Li , Y.; Tade , M. O.; Dai , S.; Yamauchi , Y. Angew. Chem., Int. Ed. 2015, 54, 588– 593 DOI: 10.1002/anie.201407629. ...
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NaCl Crystallites as Dual-Functional and Water-Removable Template to Synthesize Three-Dimensional Graphene-Like Macroporous Fe-N-C Catalyst Wang Wang, Wenhui Chen, Peiyu Miao, Jin Luo, Zidong Wei, and Shengli Chen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01695 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 4, 2017

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ACS Catalysis

NaCl Crystallites as Dual-Functional and Water-Removable Template to Synthesize Three-Dimensional Graphene-Like Macroporous Fe-N-C Catalyst Wang Wang†, Wenhui Chen†, Peiyu Miao‡, Jin Luo†, Zidong Wei*‡ and Shengli Chen*† †

Hubei Electrochemical Power Sources Key Laboratory, Department of Chemistry, Wuhan University, Wuhan 430072, China. ‡

Chongqing Key Laboratory of Chemical Process for Clean Energy and Resource Utilization, School of Chemistry and Chemical Engineering, Chongqing University, Shazhengjie 174, Chongqing 400044, China

ABSTRACT: Three-dimensional macroporous carbon materials with hierarchical pore structures (3D MPC) have wide applications; but the scale-up synthesis is limited by the cumbersome procedures of template formation and removal. Herein, we show that NaCl crystallites, which form in-situ in a lyophilizing process of NaCl solution containing carbon precursor and are removable simply through water washing, can act as templates to grow 3D MPC materials with graphene-like ultrathin and mesoporous walls through pyrolitic carbonization. What’s further, by using a nitrogen (N) rich polymer (polyvinylpyrrolidone, PVP) as carbon precursor and introducing Fe salt in the precursor, a MPC catalyst with high Fe/N-doping content is achieved due to the NaCl crystallites simultaneously serving as confining agents to prevent the large weight loss and N evaporation, a severe problem in usual pyrolytic synthesis of Fe-N-C catalysts. Benefiting from the mass transport convenience of the macropores as indicated by the impedance spectroscopy results, the Fe/N-doped 3D MPC exhibits high catalytic performance towards oxygen reduction reaction. The dualfunctionality, facile formation and removal, and reusability of NaCl make the present method a promising way to gain costeffective porous Fe-N-C catalysts. KEYWORDS: Macroporous carbon, water-removable templates, Dual-functionality, mass transfer, oxygen reduction reaction. INTRODUCTION Proton exchange membrane fuel cells (PEMFCs) are among the most promising energy conversion technology for a green and sustainable society.1-7 For the large scale application of PEMFCs, a cost-effective electrocatalyst alternative to Pt for the oxygen reduction reaction (ORR) is highly desired, and Fe/N-doped carbon materials (Fe-N-C) have shown great prospect.8-11 The past decade has seen significant progress in pyrolytic synthesis of Fe-N-C ORR catalysts by elaborately selecting precursor substance and optimizing carbonization process.12-16 Despite, the ORR performance of current Fe-N-C catalyst remain significantly deficient as compared with Ptbased materials in acidic electrolytes, mainly owing to the low surface density of catalytic sites.17-19 The activity deficiency in principle may be offset by applying electrodes with higher loading of Fe-N-C catalysts, because of the apparently much lower cost.20 However, the resulting thick catalyst layer could bring about severe problems of mass transport and ionic conductance, making the desired performance practically unachievable in PEMFCs. Three-dimensional macroporous carbon materials (3D MPC) may provide a solution. The simultaneous high macro- and mesoporosity of 3D MPC can greatly increase the exposure of catalytic sites, thus offsetting the low active site density. As

well, the large inter-connected open channels in 3D MPC allow efficient accessibility of catalytic sites to the electrolyte and reactants, thus overcoming the ionic conductivity and mass transport problems in thick catalyst layer. In fact, 3D MPC with hierarchical pore structures represents a class of materials that have wide applications in catalysis and energy conversion.21, 22 3D MPCs are usually synthesized by using colloidal crystals such as silica,23, 24 polystyrene sphere,25 and other templates.26 As well as the relatively complicated template fabrication, a major drawback of the traditional template methods is the cumbersome and maybe also harsh template removal procedures, for example, lengthy immersions in organic solvents or reacting with toxic chemicals such as HF.23 This severely limits the scale-up material production. Therefore, templates with facile and green fabrication and removal processes for 3D MPCs are of great significance and highly desired. Herein, we develop a straightforward approach to fabricate 3D MPCs using NaCl crystallites as templates, which can form in-situ during precursor drying and removed simply through water washing. As depicted in Scheme 1a, sudden freeze of NaCl solution containing water-soluble polymer precursor (polyvinylpyrrolidone, PVP) and Fe ions would produce a homogeneous mixture; while the gradual sublima-

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tion of water in the lyophilization process causes crystallization of NaCl and segregation of PVP, forming packed PVPenclosed NaCl crystallites, with Fe ions uniformly dispersed in PVP. Upon carbonizing the lyophilized precursor mixture at high-temperatures and finally removing the NaCl in water, a graphitized carbon material with macro- and mesoscale pores can be obtained.

Scheme 1. (a) Schematic illustration of the synthesis of 3D MPC. (b) Illustration of pore structure tuning by varying the NaCl/PVP ratios in precursors. There were several reasons to choose NaCl as the template agent. It is among the most soluble salts in water. This offers great ease to control the content of the salt templates and to remove the template through water-wash after carbonization. NaCl has an appropriately high melting point of ~800 °C, before which the main carbonization processes of PVP takes place. This ensures that the templates do not collapse before the precursor carbonization, thus forming macropores during carbonization process. At temperatures higher than 800 °C where the graphitization of carbon takes place, the NaCl crystallites melt to help the formation of mesopores through capillarity. Finally, NaCl is the most abundant salt on the earth. There is hardly another salt that is competitive as compared with NaCl in the view of the above advantages. The reasons of choosing PVP as the C/N-containing precursor compound are two-folds. First, PVP is highly soluble in water, which makes it to be homogeneously dispersed in the NaCl-saturated solution; while PANI tends to precipitate in aqueous solution. Second, as a widely used surfactant, PVP can adsorb on the surface of NaCl crystallites to form enclosed shells, thus preventing NaCl crystallites from aggregating and growing large. In addition, the porous structures can be tuned by changing the NaCl/PVP ratios in the precursor mixtures. As depicted in Scheme 1b, the NaCl/PVP ratios modulated the thickness of the PVP layers enclosing the NaCl crystallites, which in turn determines the porous structures of carbon materials obtained after pyrolysis. At an appropriately high NaCl/PVP ratio, the PVP shells between NaCl crystals would be thin, 3D MPC could be obtained through pyrolysis. At low and intermediate NaCl/PVP ratios, hollow carbons with closed or partially open shells were obtained. EXPERIMENTAL SECTION Synthesis: typically, 3D MPC was prepared by dissolving 1 g Polyvinylpyrrolidone (MW=29000, Sinopharm Chemical Reagent Co., Ltd.) and 10 mg FeCl3·6H2O (Sinopharm Chemical Reagent Co., Ltd.) in saturated NaCl (Sinopharm Chemical Reagent Co., Ltd.) solution (4 g NaCl dissolved in 11 ml water) with vigorous stirring at about 25℃. Then about 30 mL of liquid nitrogen was poured into the solution to freeze the mixture instantly and lyophilization was performed to dry the mixture for about 15 h. For 3D MPC-1, 3D MPC-2, 3D MPC-

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3, the same process was conducted except that 0 g, 1 g, and 2 g NaCl were added respectively. The light yellow sample (denoted as PVP-NaCl-N2(l)) was transferred to ceramic crucible, and heat-treated under argon atmosphere at 900 ℃. After cooling to room temperature, the sample was washed with 0.5 M H2SO4 (Sinopharm Chemical Reagent Co., Ltd.) for 6 h and annealed at 900 ℃ again for 2h. Characterization: The structure and morphology of the prepared materials were characterized by Scanning Electron Microscope (HITACHI S-4800), Transmission Electron Microscope (TEM, JEM-2100F), Raman spectroscopy (Renishaw inVia, Renishaw, 532 nm excitation wavelength), and X-ray photoelectron spectroscopy (Kratos Ltd. XSAM-800 equipped with an Al Kα X-ray source). For XPS, the binding energies were calibrated to the C 1s peak at 284.3 eV and the data is fitted through the public software XPSPEAK. The N2 adsorption isotherms were investigated by an ASAP2020 Surface Area and Porosity Analyzer (Micromeritics, USA). XRD measurements were conducted on Bruker D8 Advance with an interval 0f 0.02˚ between 5˚ to 80˚ at a step of 6˚/min. Thermogravimetric analysis (TGA) was conducted between room temperature to 900 ˚C under N2 atmosphere at a heating rate of 10 ˚C/min by a TGA Q500 (TA instrument, USA). Electrochemical Measurements: All electrochemical measurements were performed using a CHI 400 electrochemical workstation in a standard three-electrode cell with a Pt plate as the counter electrode, saturated calomel electrode (SCE) as the reference electrode when the 0.1 M HClO4 was used as the electrolyte, Hg/HgO electrode as the reference electrode for measurements in 0.1 M KOH, and a glassy carbon (GC) rotating-disk-electrode (RDE) electrode (5 mm in diameter) loaded with catalyst sample as the working electrode. To prepare the working electrode, 5 mg sample catalyst was dispersed in 1 mL Nafion/isopropyl alcohol solution (0.1%) to form the catalyst ink by sonicating for 30 min; 20 µL of the ink was then pipetted onto the GC electrode. In comparison, working electrode loaded with 150 µg/cm2 Pt/C (20 wt. %) catalyst from Johnson-Mathey (JM) was also made. The ORR polarization curves were measured in O2-saturated electrolyte solutions by linearly scanning the potential of working electrode at 5 mV s-1 and with an electrode rotation speed of 900 rpm. The potentials were reported with respect to the reversible hydrogen electrode (RHE). To calibrate the equilibrium potentials of the SCE and Hg/HgO electrodes to the RHE scale, the steady-state polarization curves of the hydrogen electrode reactions on Pt/C-loaded GC electrode in 0.1 M HClO4 (or 0.1 M KOH) saturated with H2 were measured; the corresponding RHE zero potentials were estimated at that the current crossed zero (Figure S1). The rotating ring-disk electrode was employed to detect the H2O2 yield, where the ring potential was set to 1.2 V (vs. RHE). The H2O2 yield was calculated through the following equations: 4, 6

H 2 O 2 (%) = n=

I disk

200( I ring / N 0 ) I disk + I ring / N 0

4 I disk + I ring / N 0

(1)

(2)

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ACS Catalysis

tively. N0 is the ring collecting efficiency, which was calibrated using 10 mM K3[Fe(CN)6] in 0.1 M KNO3. The measured N0 value was 0.241, which is close to the producer’s value of 0.25. For stability test, the catalysts were cycled in the potential range of 0.1-1 V in O2-saturated 0.1 M HClO4 at 50 mV s-1 using a graphite rod as a counter electrode. And the polarization curve of the catalyst was tested before and after the cycling. Membrane electrode for H2-O2 fuel cell was prepared by sandwiching the Nafion 211 membrane with the cathode and anode and hot-pressed at 135 °C for 3 minutes under 135 Mpa. The catalyst “ink” was prepared by dispersing the catalysts (Fe-N-C for cathode and 40 wt. % Pt/C from Johnson-Matthey for anode) in alcohol with 5 wt. % Nafion solution (DuPont). The “ink” was deposited onto the commercial carbon paper with an area of 1 cm2 to prepare the cathode and anode. The Nafion content in the catalyst layer was about 40 wt. %. The loading of the catalyst was 3 mg/cm2 for the cathode (Fe-N-C) and 0.3 mg/cm2 for the anode (40 wt. % Pt/C), respectively. The flow rate of H2 and O2 was 150 mL/min. RESULTS AND DISCUSSION As indicated by the scanning electron microscopy (SEM) images, the as-prepared 3D MPC possess irregular macropores with shapes between cube and sphere (Figure 1a and 1b), and there are relatively smaller macropore channels on the bottom of each large pores, forming an inter-connected pore structure (Figure 1b). The transmission electron microscopy (TEM) images indicated that the 3D MPC sample possesses graphenelike wrinkled ultrathin pore walls (Figure 1c and 1d).

Figure 1. (a, b) SEM and (c, d) TEM images of 3D MPC. It was found that the NaCl/PVP ratios in the precursor mixture crucially affect the porous structures of the carbon materials obtained after pyrolysis and template removal. The details are shown in Figure S2. The carbon materials obtained without using NaCl templates exhibited a large bulk morphology with no macroscale pore. Upon introducing NaCl to a relatively low NaCl/PVP ratio (e.g. below 1:1), hollowstructured carbons with closed pores of approximately cubic shapes were obtained. At intermediate NaCl/PVP ratios, some open macropores appeared. When increasing the NaCl/PVP mass ratio to an appropriately high value, most of the pores became opened, forming 3D MPC. The formation of closed pores at low NaCl/PVP ratios should be due to the formation

of relatively thick PVP layers between NaCl crystallites.

Figure 2. (a) N2 adsorption/desorption isotherm of 3D MPC pyrolyzed with NaCl/PVP ratios of 0 (3D MPC-1), 1:1 (3D MPC-2) and 2:1 (MPC-3) and 4:1 (MPC). (b) The corresponding pore distribution curves (BJH method). The porous structures of the carbon materials obtained under different NaCl/PVP ratios were also characterized with nitrogen adsorption-desorption isotherm (Figure 2a). The typical type-IV isotherm with distinct hysteretic loops indicates the presence of mesopores in these materials, which have diameters around 4 nm (Figure 2b). The formation of mesopores of about 4 nm sizes should be due to the shrinkage of the thin PVP layers, the formation and volatilization of gaseous intermediates during the decomposing carbonization, and the capillarity-driven flowing of the molten NaCl through the carbon layer at high temperature. As indicated by Figure 2, the mesopores also formed without using the NaCl templates, which may indicate the role of gaseous intermediates in the formation of mesopores. However, the mesoporous volume was boomed upon introduction of NaCl, with the mesopore volume increasing with NaCl/PVP ratio, indicating the crucial role of NaCl crystallites in forming the mesoscale pores. The 3D MPC obtained at the optimal NaCl/PVP ratio exhibited a Brunauer-Emmett-Teller (BET) surface area of ca. 424. 3 m2 g-1; while the similar pyrolytic process in the absence of NaCl produced a sample having a fairly low BET surface area of 72.9 m2 g-1. The pore volume of 3D MPC (0.36 cm2 g-1) were 10 times of that of 3D MPC-1 (0.034 cm2 g-1) obtained without using NaCl templates.

Figure 3. SEM images of (a) the lyophilized precursor mixture, and the samples obtained after pyrolyzed at (b) 650 ˚C and (c) 900 ˚C respectively. We have investigated the structure evolution of the precursor in the fabrication process. The XRD (Figure S3) and SEM observations (Figure 3a) suggested that small NaCl crystallites formed after the lyophilization process. The crystallites had irregular shapes from spheres to cubes and are enclosed by PVP. When the temperature was increased to 650 ˚C before the melting point of NaCl (800 ˚C), the originally light yellow precursor turned to gray (Figure S4a and S4b), suggesting the occurrence of PVP carbonization. In the meantime, the NaCl crystallites became more closely squeezed due to the carbonization of the PVP which resulted in shrinking of the whole matrix (Figure 3b). Upon washing the sample pyrolyzed at 650 ˚C with water, the inter-connected porous structure can be

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clearly seen (Figure S5a), suggesting that the NaCl crystallites acted as the rigid templates to form 3D MPC prior to the NaCl melting. Further increasing the temperature to above the melting point of NaCl (900 ˚C), the mixture turned to dark black due to deep carbonization of the precursor (Figure S4c); and the NaCl crystallites became deformed and shrunk (Figure 3c, Figure S5b). NaCl beads were found on the surface of the carbonized sample (Figure S4c,), implying that the molten NaCl flowed out through the pore walls due to the capillarity during the carbonization process, which produced the mesoscale pores. The samples formed at different stages of fabrication process exhibited only the diffraction patterns of NaCl crystallites; and no apparent difference in XRD peak position or peak width for NaCl was observed for these samples (Figure S3), which suggested that the structure of NaCl crystallites did not change obviously. As an N rich polymer, the pyrolysis of PVP in the presence of Fe is expected to produce Fe-N-C materials that can catalyze the oxygen reduction reaction. The X-ray photoelectron spectroscopy (XPS) characterization of the prepared carbon materials showed the presence of Fe, N, C and O (Figure S6 and Table S1). The distributions of different N species in the MPC obtained under optimal NaCl/PVP ratio were estimated from the high-resolution XPS spectra, which were 29.7 % for pyridinic N (398.3 eV), 61.6 % for graphitic N (400.9 eV) and 8.7% for oxidized N (403.0 eV), respectively. We have also conducted XPS measurements to detect Na and Cl in the final product and no obvious Na and Cl signals were seen (Figure S7). The XPS results revealed that the N content of 3D MPC (3.31 at %) was significantly increased comparing to that of 3D MPC-1 (2.93 at %), and that the 3D MPC mainly contained pyridinic and graphitic N species, which have been considered to play indispensable parts in forming the active sites of Fe-N-C catalysts.2,7 The high content of graphite and pyridinic N should be a result of the confining effect of NaCl crystallites during the pyrolysis. Recent reports have shown that introduction of NaCl crystals can serve as confining agents to prevent the weight loss of the precursor, 27, 28 which has been a severe problem in ordinary pyrolytic synthesis of Fe-N-C catalysts. As shown in Figure S8, upon increasing the NaCl content in the precursor, the weight loss of precursor after pyrolysis increasingly alleviated, indicating the confining of PVP and the N-containing pyrolytic intermediates in the stacked NaCl crystallites. The confining effect of NaCl crystallites inhibits the large loss of N-containing pyrolytic intermediates to allow high-density formation of catalytic sites. The Raman spectra indicated that the presence of NaCl crystallites improved the graphitization of the resulted carbon materials (Figure S9). The G band to D band intensity ratio (IG/ID) increased with increasing NaCl/PVP ratio. The IG/ID values were 1.21, 1.132, 1.138, 1.141 respectively for 3D MPC, 3D MPC-1, 3D MPC-2, 3D MPC-3. The D band (≈1360 cm-1) is associated with the defects and disorder in graphitic lattice, while the G (≈1600 cm-1) band can be assigned to the in-plane vibration of the sp2 carbon network.29 In previous studies, the precursor mixture of NaCl and Ncontaining substances were mostly obtained through evaporation recrystallization, 27, 28 which yielded large crystals of millimeter scales (Figure S10). In this study, by employing the

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lyophilizing crystallization strategy, relatively small NaCl crystallites that simultaneously play templating and confining roles can be obtained to prepare 3D macroporous Fe-N-C catalyst with hierarchical pore structures. The facile formation and removal, the reusability, as well as the dual-functionality of NaCl make the present method a promising way to gain costeffective Fe-N-C catalyst. Thanks to the interconnected porous network with large surface area, high graphitization degree and homogeneous dispersion of Fe/N active sites (as inferred from the element mapping in Figure S11), the obtained 3D macroporous Fe-N-C showed remarkable electrocatalytic activity towards ORR in both acidic and alkaline electrolytes. Figure 4a shows the polarization curves obtained with a rotating disk electrode (RDE) in 0.1 M O2-saturated HClO4. The ORR half-wave potential (E1/2) was only 48 mV negative than that of a commercial Pt/C catalyst (20 wt. % Pt, Johnson Matthey), which was among the best non-precious metal catalysts reported for ORR in acidic electrolyte.30-32 In comparison, the samples pyrolyzed from the precursor mixtures with lower NaCl contents showed much deficient activity (Figure S12), demonstrating the importance of hierarchical open porous structure. A pair of redox peaks in the range of 0.7-0.9 V (vs. RHE, potential referred to RHE unless stated) were observed on the blank cyclic voltammogram (CV) conducted in Ar-saturated acidic electrolyte (Figure S13a), which correspond to redox transition of the iron centers.33 An oxygen reduction peak along this redox transition was apparently seen in O2-saturated electrolyte, suggesting its catalytic capability towards ORR. The H2O2 yields on the 3D MPC-loaded electrode were calculated to be about 5.1% at the potential of 0.8 V and decreased gradually to 2.7% as the potential was scanned down to 0.1 V (Figure 4b). Oppositely, the H2O2 yield was about 4.4% on Pt/C-modified GC electrode at the potential of 0.8 V and increased gradually to 4.8% as the potential scanned down to 0.1 V. The corresponding electron transfer number for 3D MPC-loaded electrode was determined to be 3.89-3.94 at the potential range of 0.80.1 V, indicating a dominant 4-electron catalyzing process.

Figure 4. ORR polarization curves of 3D MPC and 20 wt. % Pt/C (Johnson-Matthey) in O2-saturated 0.1 M HClO4 (a) and 0.1 M KOH (c). Scan rate: 5 mV s-1; rotation speed: 900 rpm. Plots of H2O2 yield and number of electron transfer of 3D MPC and 20 wt. % Pt/C in O2-saturated 0.1 M HClO4 (b) and

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(a)

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12 3D MPC mesoporous Fe-N-C

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0.1 M KOH (d). Different from that in 0.1 M HClO4, the CV of 3D MPC in Ar-saturated 0.1 M KOH is featureless except a large doublelayer capacitance (Figure. S13b). However, an obvious oxygen reduction peak at about ~0.8 V was exhibited in O2-saturated 0.1 M KOH. As shown in Figure 4c, the E1/2 of 3D MPC (0.88 V) was 25 mV ahead that of commercial 20% Pt/C. A quasi 4e- catalyzing process for ORR was also inferred in 0.1 M KOH with the electron transfer number ranging from 3.813.96 at the potential range of 0-0.9 V (Figure 4d). These results are comparable or even better than those of the best nonprecious metal catalysts reported so far (See table S2). As iron plays important roles in the formation and ORR active sites of Fe-N-C catalysts, we have optimized the Fe content in the precursors. As shown in Figure S14, the ORR half-wave-potential and onset-potential became more positive as the Fe content was increased from 0 to 0.12 at%, but further increase of Fe content didn’t improve or even lower the ORR activity. This might because that only a limited number of active sites can be hosted in the carbon matrix.34 Therefore, the optimized Fe content of 0.12 at% was used to prepare the 3D MPC catalyst.

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Figure 5. (a) Nyquist plots of the 3D MPC catalyst prepared through lyophilizing recrystallization of NaCl and the mesoporous Fe-N-C catalyst from evaporation recrystallization of NaCl on gas-diffusion electrode in O2-saturated HClO4 at 0.5 V. (b) Polarization (hollow symbol) and power density (filled symbol) curves for H2-O2 PEMFC with 3D MPC and mesoporous Fe-N-C as cathodes respectively and 40 wt.% Pt/C as anode at 80 ˚C under a back pressure of 1.0 bar. (c) CV curves of 3D MPC in 0.5 M H2SO4 at various scanning rates. (d) Mass capacitance of 3D MPC as a function of the scan rates. To demonstrate the superior mass transport property of the 3D MPC catalyst made through lyophilizing crystallization of NaCl, the electrochemical impedance spectroscopy and fuel cell performance were compared with the Fe-N-C catalyst prepared through evaporation recrystallization (denoted as mesoporous Fe-N-C), which possesses only mesopores and no interconnected macropores as have shown in previous study.28 The measurements were conducted using gas diffusion electrodes prepared by depositing the corresponding catalysts on carbon paper with microporous layer. As shown in Figure 5a, a much smaller low-frequency arc, which corresponds to the

diffusion resistance in a gas diffusion electrode, 36 was observed for 3D MPC comparing to mesoporous Fe-N-C, suggesting a much-enhanced mass transfer. The electrochemical impedance spectroscopy of the 3D MPC-1 obtained without using NaCl templates was also measured. As shown in Figure S16, it exhibited much larger charge transfer and mass transport resistance as comparing to those obtained using NaCl templates. The performance of a H2-O2 PEMFCs with 3D MPC and mesoporous Fe-N-C as cathodes were compared in Figure 5b. Similar voltammetric behaviors were observed for the two catalysts at low current density region, indicating comparable intrinsic ORR activity. While a much slower voltage decrease at higher current density was displayed for the 3D MPC catalyst comparing to that from mesoporous Fe-N-C, which could be ascribed to enhanced mass transport in the 3D porous structure of 3D MPC. 35, 38 As a high-surface-area porous carbon material, the electrochemical capacitance of the 3D MPC catalyst was also tested in a three-electrode cell by CV at the potential window of -0.30.7 V (vs. SCE) with various potential scan rates. As shown in Figure 5c, the CV curves collected for the catalyst exhibited characteristic quasi-rectangular shapes for double-layer capacitor at the scan rates from 5 to 100 mV s-1. A maximum specific capacitance of 118 F/g was obtained at a scan rate of 5 mV s-1 (Figure 5d), which is even comparable to that of graphene.39-41 In addition, 3D MPC possessed prominent rate capability with more than 84.1% retention of its initial capacitance as the scan rate increased from 5 to 100 mV s-1, which may be attributed to the excellent mass transport because of the hierarchical carbon network. These results proved the dual-functional template applied in this work to be a promising strategy in the development of carbon-based energy storage materials. The stability of the 3D MPC catalyst was assessed by using the commonly used procedures in literatures for Fe-N-C catalysts, namely, long-term CV in O2-saturated 0.1 M HClO4 and chronoamperometric measurements in O2-saturated 0.1 M KOH. The half-wave potential shifted ca. 17 mV after 1500 cycles of potential cycling in acid solution (Figure S15a), and about 94.5% of the initial current density was retained after 100000s’ potentiostatic polarization at 0.625 V in alkaline solution (Figure S15b), which are comparable to those reported for other excellent Fe-N-C catalysts.8, 27, 35, 36 The superior durability of the 3D MPC should be ascribed to the interconnected graphene-like carbon network with high graphitization degree and evenly dispersed active sites. Comparing to Pt/C, the 3D MPC exhibited higher stability in alkaline solution. In acid solution, however, the 3D MPC was much less durable, although its stability is comparable to that of the best non-precious catalysts reported. For Pt/C, a similar half-wave potential shift occurred after about 5000 cycles of potential cycling in acid. The poor stability of Fe-N-C catalysts in acid media has been widely recognized and currently is a major problem for applying the Fe-N-C catalysts in acidic fuel cells. CONCLUSION We have demonstrated the implementation of packed NaCl crystallites as templates to synthesize three-dimensional macroporous graphene-like Fe-N-C catalyst. The novelties of the present method are two-folds. The simple lyophilizing

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formation and water-removability of NaCl templates make the present synthesis a facile, green and easy scale-up method. Besides, the packed NaCl crystallites can simultaneously confine the precursor to prevent the pyrolyzed intermediates and debris from volatilizing, thus to avoid large C/N loss, a severe problem in pyrolytic preparation of Fe-N-C catalyst. The selection of the water-soluble polymer, PVP, as C/N precursor is also important, ensuring the high density and homogeneity of active sites in the prepared catalyst. The as-formed 3D MPC exhibits both excellent intrinsic ORR activity and superior mass transport property, as well as good capacitive charge storage performance. This method provides new opportunities for broad applications of 3D MPC materials in heterogeneous catalysis, ultracapacitors, and batteries.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENT

Supporting Information.

The authors acknowledge the financial support from the National

Additional experimental details, SEM, XRD, XPS, Raman spectra, electrochemical data.

Natural Science Foundation of China (Grant No. 21633008, 91534205

and

21436003).

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