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Dual-functional electrocatalyst derived from ironporphyrin-encapsulated metal–organic frameworks Jungwon Park, Hyunjoon Lee, Young Eun Bae, Kyoung Chul Park, Hoon Ji, Nak Cheon Jeong, Min Hyung Lee, Oh Joong Kwon, and Chang Yeon Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08786 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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ACS Applied Materials & Interfaces

Dual-Functional Electrocatalyst Derived from IronPorphyrin-Encapsulated Metal–Organic Frameworks Jungwon Park,†a Hyunjoon Lee,†a Young Eun Bae,†c Kyoung Chul Park,a Hoon Ji,b Nak Cheon Jeong,b Min Hyung Lee,*c Oh Joong Kwon,*ad Chang Yeon Lee*ad a

Department of Energy and Chemical Engineering, Incheon National University, Incheon

22012, Republic of Korea. Email: [email protected], [email protected] b

Department of Emerging Materials Science, DGIST, Daegu 42988, Republic of Korea

c

Department of Applied Chemistry, Kyung Hee University, Yongin, Gyeonggi 17104, Republic

of Korea. Email: [email protected] d

Innovation Center for Chemical Engineering, Incheon National University, Incheon 22012,

Republic of Korea † These authors contributed equally

KEYWORDS: iron carbide, metal-organic frameworks, N-doped carbon, Fe-Nx, oxygen reduction reaction, hydrogen evolution reaction, high durability

ABSTRACT: Active, stable electrocatalysts based on non-precious metals for the oxygen reduction reaction (ORR) and hydrogen evolution reaction (HER) are critical for the

ACS Paragon Plus Environment

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development of cost-effective, efficient renewable energy technologies. Here, Fe/Fe3Cembedded nitrogen-doped carbon was fabricated via pyrolysis of iron-porphyrin-encapsulated mesoporous metal–organic frameworks [PCN-333 (Fe), where 'PCN' stands for 'porous coordination network'] at 700 °C. The various characterization techniques confirmed that Fe- and Fe3C-containing Fe–N–C material (FeP-P333-700) was successfully prepared by pyrolysis of porphyrin-encapsulated PCN-333 (Fe). FeP-P333-700 exhibited superior electrocatalytic performance for the ORR and HER owing to the synergistic effect of Fe/Fe3C and Fe–N–C active sites.

1. INTRODUCTION The depletion of fossil fuel resources and related environmental problems have facilitated a demand for renewable and green energy sources. The combination of electrochemical water splitting and fuel cell technology can be a perfect way to meet this demand. Producing H2 by water electrolysis and then using it as a power source for fuel cells would be an ideal process to create clean, sustainable energy. The main requirement for this strategy is the development of efficient electrocatalysts for the hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR).1,2 Pt and its alloys have been widely adopted to boost the ORR and HER processes to date,3 but the high cost, scarcity, and instability of Pt have hampered its realistic application.4,5 Thus, non-precious-metal electrocatalysts have been intensively explored for the ORR and HER to replace Pt.6-11 Among them, transition metal (Fe, Co) nitrogen- and carbonbased electrocatalysts (M–N–C catalysts) have received much attention as one of the most


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promising alternatives to Pt-based catalysts owing to their low cost and high catalytic activity resulting from nitrogen incorporation and nitrogen-coordinated metal centres.12-20 Metal–organic frameworks (MOFs) constructed from coordination bond between multitopic organic building blocks and metal or metal-cluster secondary building units, have rapidly emerged as new porous materials in the past few years. The high porosity and surface area, tailorable functionality, and structural diversity of MOFs have enabled various applications including

gas

storage,21

separation,22

catalysis,23

sensing,24

drug

delivery,25

and

electrocatalysis.26-29 MOFs are considered as suitable precursors or templates for the synthesis of M-N-C electrocatalysts.30-32 Because MOFs have a highly ordered porous structure with abundant organic ligands and nitrogen-containing ligands, their carbonization efficiently yields nitrogen-doped porous carbon. Moreover, homogeneous catalytically active metal sites can be easily obtained from the regular placement of metal nodes within the pristine MOFs. Various M– N–C electrocatalysts have been derived from several iconic MOF precursors such as ZIF-8, ZIF67, Cu3(BTC)2, and MIL-101(Fe, Cr), and these MOF-derived electrocatalysts have been tested as catalysts for several electrochemical reactions including the ORR, the HER, the oxygen evolution reaction (OER), and CO2 reduction.33-38 Among these MOF-derived M–N–C catalysts, Fe/Fe3C-based nitrogen-doped carbon materials39-42 exhibited strong advantages because of their remarkable electrocatalytic performance. Chen and co-workers obtained a N-doped core–shellstructured porous Fe/Fe3C@C nanobox/reduced graphene oxide hybrid by pyrolysis of grapheneoxide-supported iron-based MOFs.43 The synthesized material exhibited excellent ORR performance. Lan and co-workers recently reported a MIL-101 (Fe)-derived N-doped Fe/Fe3C@graphitic layer/carbon nanotube hybrid.44 The hybrid showed superior bifunctional electrocatalytic activity for the ORR and OER. Lou and co-workers reported a Fe3C-


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nanoparticle-embedded N-doped carbon nanotube composite that was derived from MIL-88B and ZIF-8.45 The catalytic performance for ORR of this composite was comparable with that of a commercial Pt/C catalyst. As reported in the above examples, several representative iron-containing MOFs, such as MIL-101, Fe-MIL-88C, and Fe-MIL-88B, have been applied in the preparation of Fe3C-based nitrogen-doped carbon materials. To investigate the structure–electrocatalytic activity relationship, another iron-bearing MOF needs to be used as a precursor for synthesis of an electrocatalyst. We report here the synthesis of Fe/Fe3C-embedded nitrogen-doped carbon materials derived from PCN-333 (Fe)46 (where 'PCN' stands for 'porous coordination network'), a new ironcontaining mesoporous MOF, and its catalytic activity in the ORR and HER. PCN-333 (Fe) was constructed by reaction between the 4,4’,4’’-s-triazine-2,4,6-triyl-tribenzoate (TATB) ligand and FeCl3. This MOF has the identical topology (MTN) with MIL-100,47 but the expansion of the organic linker in PCN-333 (Fe) results in a larger cage and higher void volume, which can accommodate large enzymes. It is widely accepted that ideal M–N–C electrocatalysts must have a large number of transition metal–nitrogen coordination sites (M–Nx, M = Fe, Co, etc.) in the porous carbon network.48,49 Metalloporphyrins, which are composed of M–N4 centres, could be an ideal precursor for producing M–Nx active sites. Indeed, several groups reported electrocatalytic active sites derived from metalloporphyrin.50-53 Because PCN-333 (Fe) exhibits an extraordinary porosity for carrying large molecules, metalloporphyrin can be easily encapsulated in the void space in PCN-333 (Fe). Through a pyrolysis step, metalloporphyrin-containing PCN-333 (Fe) would be successfully transformed to


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Fe/Fe3C-embedded nitrogen-doped carbon materials having a high density of M–N4 active sites (Scheme 1). Here, this idea is tested to realize a new electrocatalyst.

Scheme 1. Preparation of electrocatalyst.

2. EXPERIMENTAL SECTION Materials N,N-Dimethylformamide (DMF, 99.8%, anhydrous) was purchased from TCI and used as received. TATB and [5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato]-Fe(III) chloride (FeTCPP) were synthesized according to a published procedure.54,55 Synthesis Synthesis of PCN-333 (Fe): PCN-333 (Fe) was prepared according to a method reported in the literature.46 Encapsulation of Fe-TCPP in PCN-333 (Fe) (Fe-TCPP@PCN-333): PCN-333 (Fe) was dried at 85 °C for 0.5 h in a vacuum oven and then activated at 150 °C for 5 h in a vacuum oven. The Fe-TCPP (25.0 mg, 0.0248 mmol) was dissolved in 10 mL of DMF. Then, 160 mg of activated PCN-333 (Fe) was added to the above solution. The resulting mixture was agitated for 5 h at room temperature and the precipitate was collected by centrifugal separation (2600 rpm).


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The greenish brown afforded solid was washed with DMF three times, and dried at 100 °C for 12 h in a vacuum oven. Pyrolysis of Fe-TCPP@PCN-333: Fe-TCPP@PCN-333 inserted into quartz tube furnace and pyrolyzed at 700 °C for 4 h at a heating rate of 3 °C/min under Ar flow. The resulting black solid is denoted as FeP-P333-700. Acid leaching of FeP-P333-700: FeP-P333-700 was placed in a 0.1 M H2SO4 aqueous solution and held at 85 °C for 8 h. The resulting material is denoted as FeP-P333-700 AL. Instrumentation used for characterizations

Rigaku SmartLab diffractometer with Cu Kα radiation (α = 1.5412 Å) was employed to gain powder X-ray diffraction (PXRD) data. Fourier-transform Infrared (FT-IR) spectra for the samples were carried out with a Vertex 80v spectrometer (Bruker). N2 sorption isotherms were obtained volumetrically at 77 K with an Autosorb-iQ from Quantachrome Instruments. Autosorb-iQ Win software package equipped in Autosorb-iQ was utilized to handle sorption data. Brunauer-Emmett-Teller (BET) model was applied to calculate the specific surface areas of porous materials. Pore size distribution was obtained from N2 sorption data with nonlocal density functional theory (NLDFT) method. The morphologies of the samples were confirmed by scanning electron microscopy (SEM, JEOL JSM-7800F). High-resolution and scanning transmission electron microscopy (HRTEM and STEM) images were obtained from a fieldemission transmission electron microscope (FE-TEM, Hitachi HF-3300) operated at an acceleration voltage of 300 kV. Elemental mapping images were obtained using an energydispersive X-ray spectroscopy (EDX, Bruker Xflash® 5030) instrument on the FE-TEM. Raman spectra were collected on a Nicolet Almega XR dispersive Raman spectrometer from Thermo Scientific. The sample was excited by focusing a 1.23 mW, 532‐nm‐wavelength laser beam on a


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crystal with a 10× magnifying objective lens. The instruments used for the TEM, EDX, and Raman analyses are located at the Center for Core Research Facilities at Daegu Gyeongbuk Institute of Science and Technology (DGIST). PHI 5000 VersaProbe ll X-ray photoelectron spectrometer was employed to obtain X-ray photoelectron spectroscopy (XPS) data. . Electrochemical measurements ORR: Catalyst slurry was obtained by blending 10 mg of a electrocatalyst with 66 µL of 5 wt% NafionTM ionomer and 1 mL of isopropyl alcohol (Junsei). The resulting slurry was uniformly dispersed by ultrasonicator for 30 min. Next, 10 µL of the perfectly dispersed electrocatalyst slurry was loaded dropwise onto the glassy carbon (GC) working electrode. A Pt/C catalyst (E-TEK, 20 wt%) slurry was gained by the same method in order to compare the electrochemical activity of the catalysts. Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) measurements were performed on a rotating disk electrode (RDE). The electrochemical analyses were performed using a three-electrode setup. A Pt coil was adopted as the counter electrode, and Ag/AgCl electrode was employed as the reference electrode. A potentiostat (Princeton Applied Research, PARSTAT® 2273) controlled the potential while CV and LSV were performed. CV curves were recorded in a nitrogen-saturated 0.1 M KOH electrolyte at a scan rate of 20 mV/s. The potential was scanned between 0.05 and 1.0 V vs. RHE. LSV was performed in an O2-saturated 0.1 M KOH electrolyte at a scan rate of 5 mV/s The RDE was rotated at 1600 rpm while the LSV curves were monitored. The electrolyte was kept at 25 °C via a thermostatic bath. Accelerated durability tests (ADTs) were conducted for 10,000 cycles between 0.6 and 1.0 V vs. RHE at a scan rate of 50 mV/s in nitrogen-saturated 0.1 M KOH at 25 °C.


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HER: Electrocatalyst solutions were prepared by blending 1 mg catalysts with 25 µL Nafion (5 wt%, Sigma-Aldrich, USA) solution as a binder and then adding 50 µL EtOH and 200 µL deionised H2O; the mixture was then ultrasonicated for 20 min. Then, 20 µL of each cocktail solution was deposited on a GC working electrode (d = 3 mm). All the electrodes were dried at 25 ℃. Polarisation curves were obtained in 0.5 M H2SO4, 1 M KOH, and 1 M phosphate buffer solution (PBS) at a 10 mV/s scan speed using a 3-electrode configuration: a Pt mesh as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and catalysts on GC as the working electrodes.. All of the reported electrochemical data are given in terms of the potential versus RHE by conversion using the equation of E(RHE) = −0.241(SCE) + 0.05916/n*log(pH). Electrochemical impedance spectroscopy (EIS) was carried out using the same experimental setup under a bias of −0.6 V vs. SCE at frequency range of 100,000 to 0.1 Hz with 0.01 V amplitude.

3. RESULTS AND DISCUSSION PCN-333 (Fe) was prepared and activated using a reported method.46 The PXRD patterns of the synthesized PCN-333 (Fe) agreed well with simulated and reported patterns (Fig. S2), indicating successful preparation of PCN-333 (Fe). Iron-porphyrin (Fe-TCPP) was encapsulated into PCN-333 (Fe) by a typical impregnation method. Briefly, activated PCN-333 (Fe) powder was poured into a ca. 2.5 mM iron-porphyrin solution in DMF, and the resultant mixture was agitated for 5 h. After that mixture was centrifuged, washed three times with fresh DMF, and dried at 100 °C for 12 h in a vacuum oven. After the loading process, the color of the solid changed from yellow ochre to greenish brown (Fig. S1). The PXRD pattern of Fe-TCPP-loaded PCN-333 (Fe) (denoted as Fe-TCPP@PCN-333) matched that of pristine PCN-333 (Fe) well,


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indicating that the crystallinity of PCN-333 (Fe) remained unchanged after porphyrin loading (Fig. S2). FT-IR verified the encapsulation of the Fe-TCPP in PCN-333 (Fe) after impregnation through the presence of a vibrational peak at 999 cm−1 corresponding to the ν(Fe-N) moiety in the metalloporphyrin (Fig. S3). Loading of Fe-TCPP on PCN-333 reduced the surface area, but its porosity was retained (Fig. S4).


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Figure 1. (a) PXRD patterns of FeP-P333-700, (b) and (c) TEM images of FeP-P333-700, (d–g) HAADF-STEM and elemental mapping images of FeP-P333-700.

To obtain electrocatalysts, Fe-TCPP@PCN-333 was pyrolyzed at 700 °C (the resulting material is denoted as FeP-P333-700). Pristine PCN-333 and Fe-TCPP were also pyrolyzed at 700 °C for comparison experiments (they are denoted as P333-700 and FeP-700, respectively). The crystallinity and composition of the pyrolyzed samples were confirmed by PXRD (Figs. 1a, S6, and S7). FeP-P333-700 showed broad peaks at 26° and 44.6°, which were corresponded to the (002) and (101) planes of graphitic carbon, and two diffraction peaks corresponding to the (110) and (200) planes of α-Fe were detected at 44.6 and 65.0°, respectively. The other diffractions peaks agree with those of Fe3C (JCPDS No. 89-2867). Similar PXRD patterns were obtained after pyrolysis of PCN-333 and Fe-TCPP. The overall morphology and structure of FeP-P333-700 was confirmed by SEM and TEM. As shown in Fig. S8, the octahedral shape of PCN-333 contracted slightly but was generally retained during pyrolysis. A TEM image of FeP-P333-700 revealed Fe and Fe3C nanocrystals of various sizes dispersed in a 5-nm-thick graphitic carbon matrix (Fig. 1b). An HRTEM image showed that the dark core exhibits lattice fringes with a spacing of 0.24 nm, which corresponds to the (210) plane of Fe3C, and the outer layer displays lattice fringes with a spacing of 0.34 nm, which matches the (002) plane of graphite (Fig. 1c). High-angle annular dark-field STEM (HAADF-STEM) and elemental mapping verified a uniform distribution of carbon, nitrogen, and dispersed iron (Fig. 1d–g), suggesting the presence of Fe–N–C catalytic active sites. Raman spectroscopy revealed the local structure information for carbon and it displayed in Figure 2a. FeP-P333-700 showed separate D and G bands centered at 1344 and 1587 cm−1, due


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to the defective carbon structures and E2g mode, respectively.56The value of IG/ID ratio was 1.157 for FeP-P333-700, which suggested that the graphene sheet was well developed, and both graphene and disordered carbon coexist in the local carbon structure.57 Additionally, two broad peaks were found at 2686 and 2930 cm−1 and corresponded to the 2D and D+D’ bands of graphite,58 implying that the graphite component in FeP-P333-700 is composed of a few layers, which is consistent with the HRTEM results. These well-established graphitic layers can enhance the electrical conductivity within the electrocatalyst networks.

Figure 2. (a) Raman spectrum of FeP-P333-700, (b) N 1s XPS spectra of P333-700 and FePP333-700, (c) N2 adsorption–desorption isotherm of FeP-P333-700, (d) pore size distribution of FeP-P333-700.


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The XPS spectra of the pyrolyzed samples are shown in Fig. 2b. The high-resolution N 1s spectrum of FeP-P333-700 can be resolved into four peaks corresponding to pyridinic N (398.5 eV), metal-coordinated pyrrolic N (399.6 eV), pyrrolic N (400.4 eV), and graphitic N (401.3 eV). On the other hand, the N 1s spectrum of the control system, P333-700, can be fitted by only three peaks without metal-coordinated pyrrolic N. These results indicate that encapsulated porphyrin in PCN-333 (Fe) acts as an efficient precursor for the formation of an Fe–Nx moiety on the catalyst. Type IV isotherm with large hysteresis between adsorption-desorption curves was observed in the N2 isotherms of FeP-P333-700 (Fig. 2c), implying its mesoporous characteristic. In addition, from nitrogen isotherms, a BET surface area of FeP-P333-700 was calculated to be 105 m2/g.The pore size distribution calculated using the NLDFT was broad, with pore widths centred at 1.9 and 3.9 nm (Fig. 2d). This pore size range is suitable for transporting substances through the pore channel. The N2 adsorption–desorption isotherms of P333-700 also displayed a similar shape with FeP-P333-700 and BET surface area of 187 m2/g was calculated from isotherm (Fig. S9). The above results of various characterization techniques indicated that Fe- and Fe3Ccontaining porous nitrogen-doped carbon material was successfully prepared by pyrolysis of the porphyrin-embedded PCN-333 (Fe) MOF. Electrochemical analysis was performed to evaluate the ORR activity of the synthesized catalysts. The CV results for FeP-P333-700 and P333-700 are shown in Figs. 3a and S12, respectively. In a N2-saturated aqueous KOH electrolyte solution, no cathodic current peak was observed in the CV curve. On the other hand, in an O2-saturated aqueous KOH electrolyte


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solution, the cathodic current peaks for the ORR were observed in both samples, where FePP333-700 showed a more obvious cathodic peak than P333-700 did. These results indicate that both FeP-P333-700 and P333-700 have ORR activity in an alkaline medium. To examine the ORR activity of each material [P333-700, FeP-700, FeP-P333-700, and FeP-P333-700 AL (see below)] in more detail, the linear sweep voltammetry (LSV) of the ORR was performed using a rotating disk electrode (RDE) (Fig. 3b). Among the samples, FeP-P333-700 showed the most outstanding ORR catalytic activity. Specifically, its ORR activity was superior to that of P333700, suggesting that Fe–Nx active sites play an essential role in the ORR activity of FeP-P333700. Moreover, FeP-700, which is derived from the encapsulated material, Fe-TCPP, showed unstable and poor ORR activity, indicating that a MOF template is required to synthesize an efficient ORR catalyst. As shown in Fig. 3b, FeP-P333-700 displayed good ORR activity comparable to that of commercial Pt/C. The onset potential and half-wave potential of FeP-P333700 were 0.916 and 0.843 V (vs. RHE), respectively. In particular, the half-wave potential of FeP-P333-700 was better than that of Pt/C (0.810 V). These results demonstrate that the active sites in FeP-P333-700 exhibit activity comparable to those in Pt/C toward the ORR reaction in an alkaline electrolyte. The catalytic performances of Fe3C based catalyst for ORR are summarized in Table S3. The half-wave potential value in this work is relatively superior to previous studies. To further investigate the active sites, FeP-P333-700 was leached by 0.5 M H2SO4 at 80 °C for 8 h to remove or diminish the Fe content; the resulting material is denoted as FeP-P333-700 AL. PXRD confirms that most of the Fe3C was removed and the α-Fe content was diminished after acid leaching (Fig. S10). Fe-based non-noble-metal catalysts generally perform better after acid leaching; the reason is thought to be that more active sites are exposed to the electrolyte.59 However, in this study, both the onset potential and half-wave potential weremoved to negative


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direction and the limiting current became smaller after acid leaching. This observation suggests that Fe/Fe3C can be considered as another active site. The catalytic activity is reduced because the effect of acid leaching is predominantly dissolution of active material rather than exposure of active sites. Our observation and suggestion are strongly supported by a previous study49 in which the removal of Fe/Fe3C retarded the ORR performance. Consequently, the co-existence of Fe–Nx and Fe/Fe3C active sites in FeP-P333-700 is responsible for its superior ORR performance. For comparison purpose, another ORR electrocatalyst have been prepared via pyrolysis of physical mixture of metalloporphyrin (ca. 10 wt %) and PCN-333. Interestingly, catalyst derived from a physical mixture exhibited inferior ORR catalytic performance compared with that of FeP-P333-700 (Fig. S16). This result clearly presented the superiority of metalloporphyrinencapsulated MOFs as electrocatalyst precursors. The Koutecky–Levich (K–L) plots were measured to investigate the reaction pathway in FeP-P333-700. We obtained the K–L plots from LSV polarization experiments conducted at various rpm values (Fig. 3c). The polarization curves exhibited an almost straight line in the mass transfer limit potential region.


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Figure 3. (a) CV curves of FeP-P333-700 in N2- and O2-saturated 0.1 M aqueous KOH electrolyte solutions at a scan rate of 20 mV/s, (b) LSV curves of various electrocatalysts and Pt/C at a rotation rate of 1600 rpm in O2-saturated 0.1 M KOH solution, (c) RDE LSV curves of ORR on FeP-P333-700 at a scan rate of 5 mV/s; inset shows corresponding Koutecky–Levich plot (J−1 vs. rpm−1/2), (d) RDE LSV curves of FeP-P333-700 and Pt/C obtained before and after 10000 ADT cycles. The electron transfer number during the ORR was calculated from the slope of the K–L equation, 0.62 ⁄ 1⁄6 , where is the slope of the K–L plot, is the Faraday constant (96,485 C/mol), is the concentration of O2 (1.2 x 10−3 mol/L), is the diffusion coefficient of O2 in 0.1 M KOH (1.9 x 10−5 cm/s), and

is

the kinetic viscosity of the


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electrolyte.60 The dependence of on the potential is revealed in the inset of Fig. 3c. The average value of n is 3.6, which is smaller than the theoretically calculated value assuming that the ORR follows the four-electron pathway. This means that the unfavourable two-electron pathway occupies some portion of the ORR occurring over Fe and Fe3C nanocrystals. The RDE LSV curves (Fig. 3d) demonstrate the durability of FeP-P333-700 as an ORR catalyst. During the 10000 consecutive ADT cycles, which were conducted by cycling the potential between 0.6 and 1.0 V (vs. RHE) at a scan rate of 50 mV/s, the half-wave potential of FeP-P333-700 decreased by only 19 mV, whereas that of Pt/C decreased by 42 mV. Even after the ADT, the half-wave potential of FeP-P333-700 is still more positive than that of as-prepared Pt/C. FeP-P333-700 degrades more slowly than Pt/C; the reason is thought to be that the degradation mechanism of non-precious-metal catalysts differs from that of Pt/C. Fe, Fe3C, and N-doped carbon are active sites for the reaction; thus, metal particle aggregation, dissolution, and detachment, which are the main causes of Pt degradation, could not be common in the nonprecious-metal catalyst.61,62 To investigate the performance of the synthesized materials as a HER catalyst, various electrochemical characterizations of FeP-P333-700, P333-700, and FeP-P333-700 AL were performed (Fig. 4). Working electrodes were prepared by depositing each sample on a GC electrode and were tested using a typical three-electrode setup in electrolytes with different pH (see Methods for detailed information). To benchmark the HER performance of each sample, they were first evaluated in an acidic condition (0.5 M H2SO4, pH 0.1) for favourable band positioning. The polarization curve of FeP-P333-700 on the GC electrode shows that its onset potential (defined as the potential at Jsc = −10.0 mA/cm2) of −0.207 V vs. RHE for the HER was similar to that of P333-700 (Fig. 4a); however, the Jsc value at −0.237 V was −20 mA/cm2, which


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is a much higher cathodic current than that of P333-700 (−10 mA/cm2). The sample without Fe/Fe3C (FeP-P333-700 AL) exhibits a much more negative onset potential (−0.527 V at −10 mA/cm2) than the samples with Fe/Fe3C (FeP-P333-700 and P333-700). Tafel plots were extracted from the LSV curve data after the potential compensation for the iR drop induced by the resistance between the electrolyte and electrodes. To extract the Tafel slopes, the linear range of the Tafel plot (Fig. 4b) was fitted to the Tafel equation (η = blog j + a, where j is the current density, and b is the Tafel slope). As a standard control sample, Pt/C was also examined, and it shows a value of 28 mV/dec., which is comparable to the other reported values.63 FeP-P333-700 shows the lowest Tafel slopes (37 mV/dec.) among the samples, and its Tafel slope is comparable to those reported for MoS2-based catalysts,64,65 the most widely studied alternative to Pt. P333-700 (TCPP-free) has a higher Tafel slope (67 mV/dec.) than FeP-P333-700, but a much lower Tafel slope than FeP-P333-700 AL (132 mV/dec.). Fe/Fe3C sites might play a significant role in decreasing the overpotential for catalysing the HER. EIS was performed to compare the charge transport and electrode kinetics in the faradaic HER reaction. Nyquist plots were obtained for each sample on the basis of the Randles equivalent circuit model (Fig. S19). The series resistance (Rs) is about 10 Ω for all the samples, indicating good conductivity at the electrode/electrolyte interface. However, the FeP-P333-700 AL sample exhibits a dramatic increase in the charge-transfer resistance (RCT) at the catalyst/GC electrode, which is almost two orders larger than that of the FeP-P333-700 samples. A larger RCT value indicates greater disruption of electron transfer from the sample to the electrolyte. The RCT trend of each sample tends to coincide with that of the Tafel slope (Table S1).


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Figure 4. Electrochemical characterization of electrocatalysts. (a) Polarization curves of each sample measured in an acidic condition (0.5 M H2SO4, pH 0.1) and (b) corresponding Tafel plots extracted from the polarization curves in (a) after iR correction. (c) Comparison of the HER catalytic performance of FeP-P333-700 in various pH conditions. (d) Durability test using 1000 consecutive cycles of CV in acidic condition with potential swept from −0.2 to −1.0 V vs. RHE. These results support the suggestion that the Fe/Fe3C sites in FeP-P333-700 and P333-700 have better catalytic performance for the HER owing to lower resistance to carrier transport, but the Fe/Fe3C-free sample (FeP-P333-700 AL) showed very high resistance and low HER performance. FeP-P333-700 showed slightly better performance than P333-700, indicating that the synergistic effect of Fe/Fe3C and Fe–N–C might facilitate catalytic HER reactions. To date, Fe/Fe3C42 or M–N–C19 active sites have been independently fabricated and demonstrated as


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electrochemical catalysts for the HER. To the best of our knowledge, this is the first study demonstrating improved catalytic activity for the HER due to the co-existence of Fe/Fe3C and M–N–C active catalytic sites. To expand the application potential of the catalysts, the HER activity of FeP-P333-700 was also investigated in universal pH conditions (Figs. S17 and S18). In the basic condition (1 M KOH, pH 14), FeP-P333-700 shows good HER catalytic performance (−10 mA at −0.265 V vs. RHE) except for a detrimental shift in the onset potential compared to those in acidic electrolytes. FeP-P333-700 shows a low Tafel slope (46 mV/dec.) similar to that in the acidic condition. Development of a HER catalyst with good catalytic performance in neutral conditions is desired owing to the corrosion of materials in acidic and basic conditions. However, the stability of the catalysts is usually in a trade-off relationship with the inherently slow kinetics in neutral conditions. The HER catalytic performance of FeP-P333-700 in neutral conditions (1 M PBS, pH 7) was evaluated (Fig. S18). Although the catalyst has a higher onset potential than that under acidic and basic conditions (−0.207 and −0.301 V at pH 0.1 and 14, respectively), FePP333-700 shows superb performance (about −0.420 V at 10 mA/cm2) compared to a previously reported Co-based MOF (ZIF-67) catalyst (about −0.67 V at 10 mA/cm2).66 The performance of the catalysts in different pH conditions are compared in Table S2. To measure the long-term stability of the electrocatalysts, 1000 consecutive CV cycles were monitored in the acidic and basic conditions. For the acidic condition, the cathodic current was not degraded even after 1000 cycles with potential sweeps between −0.2 and −1.0 V vs. RHE, indicating good stability of the catalysts in the acidic condition (Fig. 4d). In the basic condition, the HER performance was slightly degraded after 1000 cycles with an applied bias from −0.2 to 1.0 V vs. RHE, but the performance and stability were still good (Fig. S17c). The good


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robustness of the carbon materials in acidic and basic electrolytes is attributed to the good stability of FeP-P333-700.

4. CONCLUSIONS An iron-porphyrin-encapsulated iron-based MOF (Fe-TCPP@PCN-333) was pyrolyzed to provide an Fe/Fe3C-embedded nitrogen-doped carbon material (FeP-P333-700) for use as an ORR and HER catalyst. Both Fe–N–C (Fe–Nx) and Fe/Fe3C electrochemical active sites were well fabricated in FeP-P333-700 by pyrolysis owing to the pre-existing iron-porphyrin precursor in the carbon-abundant iron-based MOF template. FeP-P333-700 exhibited high efficiency and good stability in electrocatalysis for the ORR and HER. Electrochemical measurements confirmed that a metalloporphyrin precursor incorporated into the MOF template has a key role in the enhanced electrocatalytic performance. These results will open the possibility of synthesis of electrochemical catalysts from various metalloporphyrin-encapsulated MOFs. ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: ##########. Detailed experimental procedures and characterizations (PDF). Photograph image, PXRD patterns, FT-IR spectra, N2 sorption isotherms, pore size distributions, SEM images, XPS spectra, CV, RDE LSVs curves, HER performance data in basic and neutral condition, Nyquist plots, impedance parameters table, HER performance table, ORR electrocatalytic performance comparison table.

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


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research was supported by two mid-career researcher programs of the National Research Foundation of Korea funded by the Ministry of Science, ICT & Future Planning (NRF2016R1A2B4010376 and NRF-2017R1A2B4007641). REFERENCES

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Efficient Fe- and Fe3C-containing Fe–N–C electrocatalyst derived from iron-porphyrinencapsulated mesoporous metal–organic frameworks


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