Article pubs.acs.org/cm
Low-Temperature and Gram-Scale Synthesis of Two-Dimensional Fe−N−C Carbon Sheets for Robust Electrochemical Oxygen Reduction Reaction Dong Young Chung,†,‡ Min Jeong Kim,†,‡ Narae Kang,†,‡ Ji Mun Yoo,†,‡ Heejong Shin,†,‡ Ok-Hee Kim,§ and Yung-Eun Sung*,†,‡ †
Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, South Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 08826, South Korea § Department of Science, Republic of Korea Naval Academy, Jinhae-gu, Changwon 51704, Republic of Korea ‡
S Supporting Information *
ABSTRACT: The Fe−N−C-based carbon materials, which are generally formed by high-temperature annealing, have been highlighted as a promising alternative to expensive Pt electrocatalysts for oxygen reduction reaction. However, the delicate formation of active sites remains an issue because of decomposition and transformation of the macrocycle during heat treatment. Accordingly, we developed a low-temperature and gram-scale approach to synthesizing iron phthalocyanine (Pc)-embedded twodimensional carbon sheets by annealing at 450 °C. The lowtemperature annealing process, which is motivated by the synthesis of carbon nanoribbons, is suitable for maintaining the Fe−N−C structure while enhancing coupling with carbon. Our twodimensional carbon sheets show higher ORR activity than commercial Pt catalyst in alkaline media. Furthermore, the feasibility of real application to alkaline membrane electrolyte fuel cell is verified by superior volumetric current density. In durability point of view, the initial activity is retained up to 3000 potential cycles without appreciable activity loss; this excellent performance is attributed to the structural stabilization and electron donation from the carbon sheet, which occurs via strong electronic coupling. We believe that this low-temperature and large-scale synthesis of a carbon structure will provide new possibilities for the development of electrochemical energy applications.
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INTRODUCTION The oxygen reduction reaction (ORR) is an important reaction in electrochemistry field because it is the reaction in fuel cells1−4 and Li−O2 battery,5−7 which are considered to be promising next-generation energy devices. The slow kinetics of ORR is a key limiting factor to the commercialization of fuel cells. Currently, Pt is the best electrocatalyst in terms of activity; however, its high cost and low abundancy provide substantial incentive to find alternatives that are less expensive and more abundant. Recently, metal−nitrogen−carbon (M− N−C) materials have shown promise as Pt alternatives because of their high activity and low cost.8−12 Phthalocyanine (Pc) is a typical macrocycle molecule that can be used to generate the M−N−C component and has been suggested as a potential alternative for ORR electrocatalysts.13−15 Among the various metals, including Fe, Co, Cu, and Zn, FePc shows the highest ORR activity, which is comparable to that of the Pt electrocatalyst. Its high activity is attributed to the Fe−N−C structure, which promotes an optimal oxygen binding energy;16−18 however, the low stability of FePc is a critical limiting factor to its direct use as an electrocatalyst.19,20 The © 2017 American Chemical Society
detailed degradation mechanism remains unclear yet; however, demetallization, i.e., replacement of the Fe atom with a proton, is likely the main route.20 To improve the long-term stability, several approaches have been suggested: Chen’s group reported a Pc-based catalyst containing an electron-donating group that remains stable up to 100 cycles.20 Other approaches that have been investigated include the construction of a hybrid structure with a carbon support such as reduced graphene oxide,21,22 carbon nanotube,23 and graphene.24−26 To generate the hybrid structure with carbon, simple mixing of Pc with the carbon materials and a subsequent heat treatment has been suggested. Simple mixing with the carbon support is a facile process; however, the resulting material shows low Pc/carbon support interactions, resulting in low durability. A hybrid structure obtained through mixing followed by pyrolysis at high temperatures results in an electrocatalyst with high durability; however, the structural deformation of the macrocycle that Received: December 2, 2016 Revised: February 28, 2017 Published: March 11, 2017 2890
DOI: 10.1021/acs.chemmater.6b05113 Chem. Mater. 2017, 29, 2890−2898
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times. The average usage of DI water is around 2 L, and the time to filtering is about 30 min. After thorough washing, we conducted XPS analysis to confirm; however, we could not find residual NaCl. The SEM and energy dispersive EDS analysis (field emission SEM, MERLIN Compact, ZEISS) were used to characterize the morphology of the samples. The TEM images were obtained by Technai F20 at operation voltage of 200 kV. The XAFS measurements were performed at Pohang Accelerator Laboratory (PAL) 8C Nanoprobe XAFS beamline. UV−vis absorption was measured using a Jasco V-670 spectrometer. X-ray photoelectron spectroscopy (XPS) was measured using a Thermo Sigma Probe with Al Kα radiation. All spectra were calibrated with the C−C peak of C 1s orbitals as 284.6 eV. Raman spectroscopy was analyzed using HORIBA, LabRAM HV Evolution. Nitrogen sorption experiment was measured using a Micromeritics TriStar ll 3020 instrument. Electrochemical Analysis. The electrocatalysts including commercial Pt/C (20 wt %, Johnson Matthey Co.) catalyst was dispersed with Nafion (15 wt %, Sigma-Aldrich) and 2-propanol. A 7 μL aliquot of solution was deposited onto the rotating ring disk electrode (PINE; the disk area is 0.2475 cm2). The total catalyst loading of the non-Pt sample is 0.274 mg cm−2 and that of Pt/C is 8 μgPt cm−2, respectively. Electrochemical analysis was measured using an Autolab potentiostat (PGSTAT302N). Pt wire as counter electrode and Ag/AgCl as reference electrode were used. All potentials in this work are referred to as reversible hydrogen electrode (RHE) by calibrating H 2 oxidation/evolution reaction using Pt wire. The ORR measurement was conducted under O2-saturated 0.1 M KOH electrolyte at 298 K. The Koutecky−Levich analysis was conducted by the following equation.
occurs during high-temperature annealing reduces the electrocatalytic activity.27 Recently, interesting approaches to preserve the active sites structure during high-temperature annealing have been suggested; however, further enhancement of constructing a Fe−N−C site is important.28−30 To address these issues, effective approaches to constructing a hybrid structure with both high activity and high stability are demanded.31,32 Considering that the phthalocyanine structure sublimes above 500 °C resulting in a loss of structure, a lowtemperature annealing process is required to maintain the Fe− N−C structure and increase the interactions with carbon. Graphene nanoribbons are highly promising materials used recently as electronic materials and in various applications.33−36 Graphene ribbons are formed by simple heat treatment at around 400 °C,33 followed by consecutive dehalogenation, C− C coupling, and dehydrogenation steps that occur. It can be utilized as a low-temperature carbonization process for various applications. 37−39 Furthermore, carbon ribbons can be constructed into a two-dimensional structure, which is advantageous for mass transport with high density. Considering mass transport issues are significantly important in electrochemical energy devices including fuel cells40−43 and lithium rechargeable batteries,44 it is promising for active materials in electrochemical devices. Despite its advantage, the carbon ribbon structure has rarely been utilized in electrochemical energy devices. From these analogies, we conjectured that the low-temperature graphene ribbon formation process can be effectively utilized to construct a hybrid carbon and FePc structure while retaining the Fe−N−C structure. Herein, we propose a rationally designed hybrid FePc− carbon hybrid material as an efficient ORR electrocatalyst by low-temperature process. We construct the hybrid structure by embedding the FePc in a bottom-up constructed carbon sheet prepared by a single-step heat treatment process. Using NaCl as the template for the carbon sheet and applying the carbon nanoribbon formation process, we successfully synthesized a large-scale, two-dimensional FePc-embedded carbon sheet structure using gram scale. The Fe−N−C structure was revealed by ultraviolet−visible spectroscopy (UV−vis) and the X-ray absorption near edge structure (XANES). This hybrid structure shows a higher ORR activity than the current state-ofthe-art Pt electrocatalyst in alkaline media. We demonstrated the feasibility of the electrode materials in anion exchange membrane fuel cells (AEMFCs) by testing its performance in a membrane electrode assembly (MEA). The maximum volume power density is 1.36 times higher than commercial Pt/C due to feasible mass transport by two-dimensional structure and high volume density. Finally, there is no appreciable activity loss after accelerated durability test (ADT) up to 3000 potential cycles, verifying its superiority for energy conversion device active materials with high activity and durability
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1 1 1 1 1 = + = + j jk jL jk Bω0.5 where jk is the kinetic current and jL is the limiting current. ω is rotating rate, and B is determined by the slope of the following equation. B = 0.2nF(DO2)2/3 ν−1/6CO2 where n is the electron transfer number per oxygen molecule, F is the Faraday constant (96500 C mol−1), and DO2 is the diffusion coefficient of oxygen in 0.1 M KOH solution (1.9 × 10−5 cm2 S1−). ν is the kinematic viscosity (0.01 cm2 S1−), and CO2 is the oxygen bulk concentration (1.2 × 10−6 mol cm−3). The value 0.2 is referred to as the conversion factor for rotating speed expressed in revolutions per minute. Long-term durability test was conducted by potential cycling from 0.6 to 1.0 V with 50 mV s−1 in O2-saturated condition. The ORR in acid media was measured under 0.5 M H2SO4 at 298 K. All conditions are identical with the condition that electrochemical analysis was conducted in alkaline media except reference electrode (a saturated calomel electrode was used in acid media). Preparation of MEA and Single Cell Test. The catalyst-coated membrane (CCM) was obtained by spraying an ink on the membrane (A901, Tokuyama) directly. The anode catalyst ink used for all membrane electrode assemblies (MEAs) was 40 wt % Pt/C (Johnson Matthey Co.) with a catalyst loading of 0.5 mg cm−2. The synthesized CS-FePc_450 and 40 wt % Pt/C (Johnson Matthey) were deposited on the cathode side of the membrane, respectively, with a catalyst loading of 1 mg cm−2 for both electrodes. All of the catalyst ink contained catalyst powder, isopropyl alcohol, deionized water, and ionomer (AS-4, Tokuyama). The fabricated CCM was dried for 1 day before testing. Carbon paper was used as a gas diffusion layer for each side of electrode. The MEAs were inserted between two graphite plates which contained a serpentine gas flow channel. The active electrode area (geometric) was 5 cm2. The performance of the assembled single cells was carried out using the Fuel Cell Test System (CNL Energy Co., Korea). Humidified hydrogen and oxygen gases were fed into the anode and cathode sides of a single cell, respectively, with constant flows to activate the cell and during cell operation. Polarization curves were measured using the current-sweep method
EXPERIMENTAL SECTION
Materials and Characterization. Commercially available solvents and reagents were used without further purification. Iron phthalocyanine, NaCl, and n-hexane were purchased from Sigma-Aldrich. 10,10′Dibromo-9,9′-bianthryl monomer was prepared by Richest. A 0.3 g amount of iron phtalocyanine and 1.7 g of 10,10′-dibromo-9,9′bianthryl monomer were dispersed in 200 mL of hexane solution. After full dispersal, the solvent was mixed with 300 g of NaCl followed by drying at 70 °C overnight with stirring. After full evaporation, the mixture was annealed at 200 °C for 2 h and 450 or 750 °C for 2 h (ramp rate of temperature is 4 °C min−1). We conducted template removal using vacuum filtering with only distilled (DI) water several 2891
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Figure 1. (a) Schematic for gram-scale bottom-up synthesis of a two-dimensional FePc-embedded carbon sheet by low-temperature annealing process. SEM images of FePc carbon hybrid structure (b) before NaCl template removal, (c) enlarged image, and (d) after NaCl template removal.
Figure 2. HR-TEM images of (a, b) CS-FePc_450, (c) CS-FePc_750, (d) FFT pattern of image c, (e) inverse Fourier transform result of image d, (f) enlarged image of white box in image e, and (g) line scan analysis of image f. with a current-sweep rate of 10 mA cm−2 s−1. The total outlet pressure was 180 kPa. The cell temperature was maintained at 50 °C during the tests.
NaCl to prepare a carbon sheet was added to the solution, which was vaporized overnight to fully remove solvent (see Experimental Section for detailed information). During this process, the NaCl surface became coated with the mixture of FePc and 10,10′-dibromo-9,9′-bianthryl monomers. The solid mixture was annealed under Ar at 200 °C to induce the polymerization of 10,10′-dibromo-9,9′bianthryl monomers by debromination induced single covalent bond formation between each monomer.36 Subsequently, the composite was further annealed at either 450 or 750 °C (consult the Experimental Section for details). During
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RESULTS AND DISCUSSION The overall synthetic procedures are shown in Figure 1a. First, FePc was well dispersed in hexane and mixed with 10,10′dibromo-9,9′-bianthryl monomers. To synthesize a two-dimensional carbon sheet structure, we utilized NaCl as the template because it has a flat surface and can be easily removed by water.45−47 After sufficient dispersion, an adequate amount of 2892
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Figure 3. (a) SEM-EDS results of CS-FePC_450. (b) Ultraviolet visible spectra (inset, CS-FePc_750). (c) Fe K-edge XANES spectra. (d) XANES pre-edge region.
dimensional carbon sheet structure (Figure 2b). In the case of the samples annealed at 750 °C (i.e., CS-FePc_750), a typical graphene sheet comprising a few layers was observed (Figure 2c). The fast Fourier transform pattern shows two sets of signals at 0.12 and 0.21 nm, which is typical for a few layers of graphene (Figure 2d). The inverse Fourier transform image shows that the individual carbon atoms are arranged in a hexagonal array, as shown in Figure 2e. The line-scan analysis revealed splitting of the two peaks, which were separated by 0.15 nm; this further corroborated the formation of a few layers of graphene in Figure 2f,g.49 The good dispersity of the Fe−N−C structure on the carbon sheet was revealed by SEM energy dispersive spectroscopy analysis. As shown in Figure 3a, iron and nitrogen are evenly dispersed over all of the carbon sheet structure. To confirm the presence of the typical Fe−N−C structure of phthalocyanine, UV−vis analysis was conducted (Figure 3b). FePc shows a typical absorption at 660 nm (G band), which is attributed to the π−π* transition of the FePc rings.24,50 The signal is clearly observed in the spectrum of CS-FePc_450, which suggests that the typical Fe−N−C structure is retained after annealing. (Note: this signal is absent in the case of CS_450, which does not contain FePc.). Furthermore, the electronic coupling of the carbon sheet and FePc was confirmed by the bathochromic shift of the G band from 660 (FePc) to 662 nm (CS-FePc_450).24 Strong π−π interactions between the carbon sheet and FePc induce the donation of electrons to FePc. In contrast, the spectrum of CSFePc_750 does not show any UV−vis absorptions, which suggests that the FePc structure deformed during annealing at 750 °C (Figure 3b, inset). The existence of Fe−N−C structures in CS-FePc_450 was further supported by the XANES results. From Figure 3c,d, it is evident that the XANES spectra of FePc
annealing, the fully aromatic structure was obtained, which induced intramolecular cyclodehydration of polymer chain and hybridized with FePc in the carbon sheet. The scanning electron microscopy (SEM) image shows that the samples that were annealed at 450 °C (denoted as CS-FePc_450; CS means carbon sheet and the final number indicates final annealing temperature) feature large hexahedral composites corresponding to NaCl, as shown in Figure 1b. The enlarged SEM image confirms the formation of a large, two-dimensional carbon sheet (Figure 1c). Finally, the mixture was washed several times with distilled water to remove the NaCl template using vacuum filtering (Figure 1d). The total mass of single-step synthesis is about 0.96 g which is enough for large-scale application (Figure 1a).48 Moreover, the scale-up can be easily achieved considering its facile experimental procedure. We conducted Raman analysis to confirm the structure (Supporting Information Figure S1). At 400 °C annealed sample (CSFePc_400) shows four peaks. The 1341 and 1600 cm−1 denote D and G peaks, respectively. And two residual peaks around 1220 and 1260 cm−1 originate from the finite width and low symmetry, which is well supported by a previous work.36 With an increase in the annealing temperature, the D peaks are enhanced and broadened and two peaks (1220 and 1260 cm−1) are attenuated. From the Raman analysis, we can conjecture that the carbon ribbon formation process is successfully progressed during our synthesis process. We also measured X-ray photoelectron spectroscopy (XPS) analysis to confirm the composition of catalyst and the extent of carbonization. The Fe and N signals were clearly observed, and there is no residual signal in the survey peak (Figure S2). Figure 2a shows the transmission electron microscopy (TEM) image of the two-dimensional carbon sheet at low magnification. TEM analysis clearly revealed a large, thin, two2893
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Figure 4. ORR polarization curves for (a) CV results of FePc/C, CS-FePc_450, and CS-FePc_750. (b) Linear sweep voltammograms of FePc/C, CS-450, CS-FePc_450, CS-FePc_750, and CS-FePc_450 without NaCl template and commercial Pt/C. (The ORR was measured in O2-saturated 0.1 M KOH at 298 K, with a rotation speed of 1600 rpm and a sweep rate of 10 mV s−1.) (c) Summary of electron transfer number calculated by Koutecky−Levich analysis. (d) Methanol tolerance test for Pt/C and CS-FePc_450 (methanol was introduced at 110 s during chronoamperometry at 0.75 V).
is the activity parameter which is a potential where current is equal to half of the limiting current). To further investigate the ORR performance of CS-FePc_450, the ORR activity of CS_450 (without FePc) was assessed for comparison (Figure 4b). The significant low activity of CS_450 indicates that the Fe−N−C sites on FePc are the main active sites of CSFePc_450. Furthermore, NaCl template effect can be clearly corroborated by comparison of sample which was annealed without NaCl (w/o NaCl). From these comparison experiments, we confirmed that not only creating and preserving the active sites but also increasing the surface area are important strategies to design electrocatalyst. To confirm surface area, we conducted N2 adsorption/desorption analysis. The Brunauer− Emmett−Teller (BET) surface area of CS-FePc_450 is 212.5 m2 g−1 (Figure S4). However, the BET surface area of w/o NaCl cannot be measured due to their small surface area. Therefore, we compare the surface area of two samples using cyclic voltammograms in Ar, which is a non-faradaic condition and indicates the electrical double layer capacitance. The CV area of CS-FePc_450 is 3.05 times larger than that of w/o NaCl, indicating the design of high surface area is related to the high activity of even the same materials (Figure S5). The electron transfer number per molecule of oxygen gas was calculated using the Koutecky−Levich equation (see the Experimental Section for details and Figure S6). The electron transfer number of CS-FePc_450 is 3.91, indicating dominant four-electron oxygen−reduction pathway, which indicates superior H2O selectivity (Figure 4c). Compared with the low electron transfer number of CS_450 (2.60), FePc-based Fe− N−C samples show superior reaction selectivity for the fourelectron pathway. The chemical selectivity of methanol was also assessed using current−time chronoamperometric analysis as shown in Figure 4d. An abrupt decrease of the reduction current after methanol introduction was observed for the Pt/C electrocatalyst. The decrease of oxygen reduction current is attributed to the methanol oxidation on the surface, simultaneously. However, CS-FePc_450 retained its initial activity without appreciable change, indicating a superior tolerance to methanol, which is an important factor for practical applications.
and CS-FePc_450 show typical characteristic peaks at around 7118 eV, which arise from the transition of the square-planar Fe ion surrounded by four coordinated nitrogen ligands.51,52 In contrast, there is no pre-edge signal in the XANES spectrum of CS-FePc_750, which supports the UV−vis results. The ORR activities of the FePc-based samples in 0.1 M KOH solution were assessed using cyclic voltammograms (Figure 4a). The dotted line indicates the CV under Ar-saturated electrolyte. However, the solid line indicates the CV results under O2saturated condition. The peak potentials of a mixture of FePc and carbon support (Vulcan XC-72R, Cabot, denoted as FePc/ C) and CS-FePc_450 are similar. However, CS-FePc_750 indicates a low peak potential compared to both samples. We also conducted a rotating disk electrode (RDE) experiment with various samples including commercial Pt/C. As shown in Figure 4b, the ORR activity of CS-FePc_450 is comparable to that of FePc/C. However, the activity of CS-FePc_750 is lower than that of both CS-FePc_450 and FePc/C, which is the same trend of CV results. The low activity of the high-temperature annealed sample (CS-FePc_750) is attributed to deformation of the Fe−N−C structure during high-temperature annealing, which is supported by UV−vis and XANES results. The temperature effect was further investigated between temperatures (Figure S3). The CS-FePc-600 shows comparable or even high activity in the high-potential region; however, the overall activity is low compared to CS-FePc_450. The discrepancy between previous reports that high temperature annealed samples show high activity and our results is attributed to the characteristics of our carbon ribbon samples. Generally low temperature annealed samples in previous reports show low electrical conductivity due to the inadequate carbonization. However, our carbon ribbon derived samples have their native carbon π−π structure, inducing sufficient electrical conductivity, leading to high electrical conductivity with the remaining active sites. Even though the onset potential, which is measured by the kinetic current density at 0.01 mA cm−2, of CS-FePc_450 is still low (0.97 V) compared to that of Pt/C (1.01 V), however, the half-wave potential of CS-FePC_450 (0.88 V) is higher than that of Pt/C (0.86 V), which suggests superior ORR activity (note: half-wave potential 2894
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Figure 5. Full cell results of MEA (a) SEM image of commercial Pt/C as cathode. (b) SEM image of CS-FePc_450 as cathode. (c) Polarization curves normalized by electrode area (the electrocatalyst loading on the cathode side is 1 mg cm−2). (d) Summary of current density at 0.6 and 0.4 V. (e) Polarization curves normalized by electrode volume. (f) Summary of volume density at 0.6 and 0.4 V, respectively.
Figure 6. Long-term durability test (a) by potential cycling of CS-FePc_450 (up) and FePc/C (down). (b) Summary of peak potential change during potential cycles. (c) ORR polarization curves in 0.5 M H2SO4 solution. (d) ORR polarization curves of before and after ADT, with a rotation rate of 1600 rpm and a sweep rate of 10 mV s−1.
but also the MEA result are very important.48,53 Catalyst loading on the cathode side is the same for both CS-FePc_450 and commercial Pt/C (40 wt %) as 1 mg cm−2. The average
The feasibility of these materials for practical applications was evaluated using an AEMFC full cell analysis using MEA. From a practical point of view, not only catalytic activity in the half-cell 2895
DOI: 10.1021/acs.chemmater.6b05113 Chem. Mater. 2017, 29, 2890−2898
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Chemistry of Materials electrode thickness of the commercial Pt/C cathode is 38.7 ± 0.5 μm, which is well correlated to our previous report54 in that the average electrode thickness of the catalyst layer is 4.3 μm/ (0.1 mg cm−2) using the same commercial J.M. 40 wt % catalyst (Figure 5a). The slight difference may be originated from the different ink content and different ionomer. The thickness of CS-FePc_450 is 28.5 ± 0.6 μm, which is 26.4% thinner than that of commercial Pt/C (Figure 5b). The polarization curve of CS-FePc_450 showed a low areal current density compared to commercial Pt/C in the analyzed region (Figure 5c,d). However, CS-FePc_450 shows a similar volumetric current density at 0.6 V (Figure 5e). Further, the significant high volumetric current density was achieved at 0.4 V, which is a 30% higher value compared to the commercial Pt/C (Figure 5f). The enhanced high volumetric current density is attributed to a feasible mass transport by the thin electrode thickness. The maximum volume power density of CS-FePc_450 is 72 W cm−3, which is 1.36 times higher than that of commercial Pt/C. Considering that cathode catalyst loading of AEMFC is 5−20 times higher than that of PEMFC,55 the thickness related mass transport issues in AEMFC may cause big problems.56 Furthermore, recent non-noble metal based carbon materials show extensively small density due to the high surface area, potentially inducing a severe mass transport problem. For these reasons, DOE focuses on the target of non-noble metal catalyst development, not the area normalized current density but volumetric current density.42,57 The importance of volume density in an energy device is not only limited by fuel cell applications but also supercapacitance58,59 and lithium rechargeable battery60,61 applications. We further corroborate the mass transfer effect of our CS-FePc_450 structure using a similar thickness of Pt/C cathode electrode (Figure S7, 29.6 ± 0.5 μm). The volumetric density of Pt/C (29.6 ± 0.5 μm) is slightly higher than that of FePc_ 450 (28.5 ± 0.6 μm) in the high-potential region; however, the maximum power density is lower than that of CS-FePc_450, indicating superior mass transfer properties of our carbon sheet in the MEA structure. Our CS-FePc_450, which is a two-dimensional carbon sheet structure with high cell density, has superior advantages on electrode materials with facile mass transport. This result suggests that our FePc-embedded carbon sheet structure is a promising candidate for AEMFC real applications. The long-term durability of electrocatalysts is a very pivotal criterion in catalytic performance. Accordingly, we conducted a long-term durability test of CS-FePc_450 that involved potential cycling between 0.6 and 1.0 V under O2-saturated conditions. We also conducted a long-term durability test of FePc/C for comparison. There was no appreciable potential shift during 3000 potential cycles for CS-FePc_450 (Figure 6a). However, a dramatic activity loss was observed for FePc/C; the summary of the peak potential as increasing the potential cycles clearly shows the durability of CS-FePc_450 (Figure 6b). The peak potential of FePc/C dramatically decreases from 0.86 to 0.62 V; however, a slight activity enhancement from 0.87 to 0.88 V suggests superior durability of CS-FePc-450. The superior long-term durability of CS-FePc_450 is attributed to the strong interactions between the carbon sheet and FePc that form during heat treatment, as discussed in the UV−vis results. These strong interactions stabilize the Fe−N−C structure of Cs-FePc_450 during potential cycles resulting in superior electrocatalyst stability. We also conducted durability tests under acidic electrolyte (i.e., 0.5 M H2SO4) because acidic media are much harsher conditions for promoting demetalliza-
tion by proton. The results revealed that CS-FePc_450 is much more durable than FePc/C for 100 cycles (Figure 6c). After 3000 ADT tests, CS-FePc_450 retain its initial activity without appreciable activity loss, verifying its superior robustness (Figure 6d). It is likely that the electron-rich characteristics of FePc, which are due to the π−π interactions between the carbon sheet and FePc, contribute to the high long-term stability based on a previous report suggesting that electronicdonating groups stabilize the Fe center thereby enhancing the durability of the catalyst.20,62 The strong structural interactions between the carbon sheet and FePc stabilize FePc, while the electronic interactions stabilizing the Fe in the Fe−N−C structure are the main causes for high durability. We further compared the long-term stability using commercial Pt/C with the same protocol (3000 potential cyclings from 0.6 to 1.0 V under O2-saturated condition, 50 mVs−1). After the 3000th potential, the activity decrease was observed in the case of Pt/C (Figure S8); however, there is no appreciable activity in the case of CS-FePc_450, verifying the strong long-term durability of CS-FePc_450.
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CONCLUSION In summary, we developed a facile and effective approach to prepare a robust electrocatalyst comprising an iron phthalocyanine-embedded carbon sheet via a single-step heat treatment process. Using the low-temperature annealing process, as motivated by the carbon nanoribbon formation process, coupled with the NaCl template method, we synthesized a large, two-dimensional carbon sheet, with a remaining macrocycle structure. The existence of the FePc structure was verified by UV−vis and XAFS analyses. The FePc-embedded carbon sheet has a higher ORR activity than the commercial Pt electrocatalyst in 0.1 M KOH. Furthermore, the initial activity is retained for 3000 potential cycles in O2-saturated electrolyte. The superior long-term durability of the catalyst is attributed to structural stabilization of FePc by the carbon sheet and electron donation from the carbon sheet to FePc by strong electronic coupling. Finally, we demonstrated its superior feasibility on AEMFC MEA electrode materials with high volumetric current density. We expect that low-temperature bottom-up synthesis of a two-dimensional carbon hybrid structure could be expanded to other routes for the development of electrochemical energy devices with robustness and a high volume density.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b05113. Physical and electrochemical data (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Yung-Eun Sung: 0000-0002-1563-8328 Notes
The authors declare no competing financial interest. 2896
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ACKNOWLEDGMENTS This work was supported by Institute for Basic Science (IBSR006-G1).
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