NiFe2O4@Carbonitride Layers

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Strongly Coupled FeNi Alloys/NiFe2O4@Carbonitride LayersAssembled Microboxes for Enhanced Oxygen Evolution Reaction Yangde Ma, Xiaoping Dai, Mengzhao Liu, Jiaxi Yong, Hongyan Qiao, Axiang Jin, Zhanzhao Li, Xingliang Huang, Hai Wang, and Xin Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11821 • Publication Date (Web): 23 Nov 2016 Downloaded from http://pubs.acs.org on November 24, 2016

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ACS Applied Materials & Interfaces

Strongly Coupled FeNi Alloys/NiFe2O4@Carbonitride Layers-Assembled Microboxes for Enhanced Oxygen Evolution Reaction Yangde Maa, Xiaoping Daia*, Mengzhao Liua, Jiaxi Yonga, Hongyan Qiaoa, Axiang Jina, Zhanzhao Lia, Xingliang Huanga, Hai Wangb, Xin Zhanga*

a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249,

China b

National Institute of Metrology, Beijing 100013, China

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Abstract: Hydrogen produced from electro-catalytic water splitting is a promising route due to the sustainable powers derived from the solar and wind energy. However, the sluggish kinetics at the anode for water splitting makes the highly effective and inexpensive electrocatalysts desirable in oxygen evolution reaction (OER) by structure and composition modulations. Metal–organic frameworks (MOFs) have been intensively used as the templates/precursors to synthesize complex hollow structures for various energy-related applications. Herein, an effective and facile template-engaged strategy originated from bimetal MOFs is developed to construct hollow microcubes assembled by interconnected nanopolyhedron, consisting of intimately dominant FeNi alloys coupled with a small NiFe2O4 oxide, which was confined within carbonitride outer shell (denoted

as

FeNi/NiFe2O4@NC)

via

one-step

annealing

treatment.

The

optimized

FeNi/NiFe2O4@NC exhibits excellent electrocatalytic performances toward OER in alkaline media, showing 10 mA·cm-2 at η=316 mV, lower Tafel slope (60 mV·dec-1) and excellent durability without decay after 5000 CV cycles, which also surpasses the IrO2 catalyst and most of non-noble catalysts in the OER, demonstrating a great perspective. The superior OER performance is ascribed to the hollow interior for fast mass transport, in situ formed strong coupling between FeNi alloys and NiFe2O4 for electron transfer and the protection of carbonitride layers for long stability. Keywords: Iron–nickel alloy; NiFe2O4 oxide; carbonitride layers; microboxes; oxygen evolution reaction


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INTRODUCTION Hydrogen produced from electro-catalytic water splitting provides a promising way for utilizing renewable energy sources, such as solar, wind and tide energy. However, a high overpotential is required to drive this reaction, even with state-of-the-art IrO2 as anode catalysts and platinum as cathode catalysts.1 In anode, oxygen evolution reaction (OER) usually proceeds via four-electron transfer with slower kinetics as rate-limiting step in water splitting.2-5 Moreover, the scarcity and high cost of these noble metal catalysts restrict their large-scale application. Therefore, the themes are mainly focused on the development of efficient, economically and Earth-abundant catalysts in renewable energies.6,7 Among the Earth-abundant electrocatalysts toward OER, nickel (Ni)- and iron (Fe)-based catalysts are considered as the most promising candidates. To further improve the OER activity, the combined advantages between Ni and Fe should be a promising solution. Intensive research of NiFe-based composites have been focused on the different morphology to provide more active sites for OER, such as spindle, flowerlike, nanoparticles, nanosheet, nanodots, nanofibers and dendritic structures.8-15 Nevertheless, poor stability and limited conductivity in alkaline solutions prohibits their wider application. Currently, the growth of NiFe-based composites on conductive substrates is the most popular strategy to solve this problem, such as Ni foam,16,17 carbon nanofibers,18 carbon paper,19 graphene,20-22 and nanotube.23 Notably, the three-dimensional (3D) structure on conductive substrates should be the most promising candidate to improve dramatically the OER performance due to the fast mass and electron transfer. Han et al.24 synthesized 3D NiOx/Ni by the electrooxidation of Ni foam to obtain high OER activity (10 mA·cm-2 at 390 mV) and robust durability due to the partially reduction of NiOx and roughness of electrode surface. Lu et al.25 fabricated amorphous and interconnected mesoporous Ni-Fe nanosheets/macroporous Ni foam as an oxygen electrode by electrodepositing method, where the improvement in wetting properties ACS Paragon Plus Environment


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facilitated the detachment of bubbles from the electrode surface. FeNi3@GR@Fe-NiOOH was also designed as OER electrocatalyst by FeNi3 alloy nanodots encapsulated into Fe-doped NiOOH/graphene, which exhibited an excellent performance on Ni foam, affording a current density of 10.0 mA cm-2 at overpotential of 290 mV and long durability due to the high alloying degree and the protection of graphene sheets.13 Xiao et al.16 prepared a mesoporous and ultrathin NiFe nanosheets on the NiCo2O4 nanoflakes/nickel foams, which showed an extraordinary performance due to the efficient electron transfer by the integration between neighboring structures. Metallic FeNi alloy in the composites also play an essential role in OER by imparting the multicompositional synergism. Thus, the coupling between FeNi alloy and semiconductor metal oxide should be an effective strategy, and is highly desirable to dramatically improve OER activity.26 Recently, hollow structure with multicompositional synergism for OER has drawn much more attention. The fabrication of hollow structure with different compositions would exhibit enhanced OER performance because of their synergetic effect between different components.27-31 Owing to their diverse structure and compositional functionalities, metal-organic frameworks (MOFs) formed by connecting metal nodes with organic units, have been intensively employed as an emerging class of precursors/templates to construct hollow nanostructures with unique architectures in the last few years.29, 32-36 Nevertheless, the use of multicompositional synergism with hollow structure derived from MOFs for electrocatalytic OER is still in its infancy. For example, Han et al.

29

have

synthesized Ni–Co PBA cages by treating cubic precursor with ammonia, which exhibited the current density of 10 mA·cm-2 at overpotential 380 mV and good stability for OER in 1.0 M KOH, whereas the cubic Ni–Co mixed oxide afforded the same current density at 430 mV. Therefore, some materials with hollow structure and tunable physical and chemical properties by multicompositional synergism has become attractive to improve their performances. Despite the great progress made so far, novel materials with well-defined hollow micro-/nanoarchitectures and multicompositional ACS Paragon Plus Environment

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synergism for electrochemical applications is highly desirable in OER. To achieve these goals, we have designed the FeNi alloy/NiFe2O4@NC microboxes, followed by carbonization of the FeNi-containing bimetal MOFs with microcube structure under a N2 atmosphere, which forms a homogeneous distribution of FeNi alloy/NiFe2O4 as wall of each microcube and in situ formed carbonitride species without additional nitrogen doping. Our purpose is to accomplish better OER performance by combining distinctive properties of alloy, metal oxide and carbonitride components. Herein, the use of bimetal MOF as precursor represents a facile way of obtaining FeNi/NiFe2O4@NC microboxes with an interconnected porous structure. Remarkably, the optimized FeNi/NiFe2O4@NC combines the advantages of fast electron transfer from carbonitride layers and FeNi alloys, synergistic effects between FeNi alloys and NiFe2O4 semiconductor, and 3D porous architecture with hollow interior for fast mass transport, making FeNi/NiFe2O4@NC microboxes show excellent activity and stability toward the OER. EXPERIMENTAL SECTION Synthesis of FeNi-MOF microcubes: Typically, Ni(NO3)2·6H2O (1.0 mmol, 291 mg), Fe(NO3)3·9H2O (1.0 mmol, 404 mg), trimesic acid (2.0 mmol, 420 mg) and 2-methylimidazole (1.34 mmol, 110 mg) were fully dissolved in 30 mL of DMF to form a homogeneous solution under ultrasonic and agitated stirring. The homogeneous solution was placed into a sealed Teflon-lined autoclave (40 mL), and kept at 170 °C for 48 h. Then, the mixture were filtered to obtain dark brown precipitates, which were washed five times with ethanol. After that, the resulting precipitates were immersed into ethanol for 12 h, and the solvent was replaced with fresh ethanol for another 12 h, which was centrifuged to receive the crystals, and vacuum drying about 12 h at 80 °C, which was denoted as FeNi-MOF. Synthesis of FeNi/NiFe2O4/NC polyhedron-assembled microboxes (FeNi/NiFe2O4@NC): The resulting FeNi-MOF powder was heated to different temperatures (700 °C, 800 °C and 850 °C) ACS Paragon Plus Environment


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under flowing N2atmosphere (80 mL/min) with heating rate of 10 oC/min and carbonized about 2 h to obtain FeNi/NiFe2O4@NC polyhedron-assembled microboxes (FeNi/NiFe2O4@NC), which were denoted

as

FeNi/NiFe2O4@NC-700,

FeNi/NiFe2O4@NC-800

and

FeNi/NiFe2O4@NC-850,

respectively. Synthesis of Fe- and Ni-MOF and their derived composites: Fe- and Ni-MOF were synthesized by the same process as FeNi-MOF microcubes. For comparison, they are annealed under N2 atmosphere to prepare the Fe-MOF-800C and Ni-MOF-800C by the same annealing process as FeNi/NiFe2O4@NC-800. Materials characterization. The microstructures were analyzed by using scanning electron microscopy (SEM, FEI XL30 Sirion) and transmission electron microscopy (TEM, FEI Tecnai G2 F20). The composition and distribution were characterized by energy-dispersive spectroscopy (EDS) and elemental mapping images. X-ray diffraction patterns (XRD) were obtained by using Brüker AXS D8 Advance X-ray diffractometer. The chemical states of FeNi/NiFe2O4@NC were examined by X-ray photoelectron spectroscopy (XPS) with C 1s peak (284.6 eV) as an internal standard for calibrating the XPS-signals. The amount of Fe and Co was detected by Total-reflection X-ray fluorescence spectroscopy (TXRF) with Rigaku NANOHUNTER. Raman spectra were collected using a Renishaw Micro-Raman System 2000 spectrometer. The thermogravimetric analysis (TGA) was conducted on a Mettler TGA/DSC1 analyzer at a ramp rate of 10 oC min-1. Specific surface areas and pore volumes were determined by N2 adsorption–desorption isotherms on Micromeritics JW-BK222 at liquid-nitrogen temperature. Electrochemical Test. A three-electrode system (CHI660E) equipped with a gas flow controlling system was used for electrochemical tests. A commercial glassy carbon electrode (GCE, diameter: 3 mm, geometric area: 0.07065 cm2) as the working electrode covered by the as-prepared catalyst with Nafion as a binder. Platinum wire (Pt) and saturated calomel electrode (Hg/HgO) were used as ACS Paragon Plus Environment

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counter electrode and reference electrode, respectively. To make the electrode, the as-prepared catalyst or commercial IrO2 (2 mg) and carbon black (1 mg) were evenly dispersed in the mixture of water-ethanol (1.0 mL, 4:1, v/v) and Nafion solution (40 µL, 5 wt.%, Alfa Aesar). Afterwards, the homogeneous ink (5 µL) was spread out on the surface of GCE with the mass loading of about 0.131 mg/cm2. During electrochemical test, a flow of O2 was used to maintain the O2/H2O equilibrium. Before the test, the repeated scans were conducted from 0 to 0.6 V (vs. Hg/HgO) to obtain a steady voltammogram curves in 1.0 M KOH. Then the polarization curves were conducted with a scan rate of 100 mV·s -1. AC impedance measurements were carried out at η = 0.320 V from 106~0.01 Hz. The stability was obtained after repeated scans from 0.90 V to 1.50 V with a sweep rate of 100 mV·s-1 for a given number of cycles. The capacitive current was used to determine the electrochemical active surface areas (ECSA) of the as-prepared FeNi/NiFe2O4@NC with a scan window of 1.2-1.3 V vs. RHE. The current density differences at the 1.25 V against the scan rate (20 to 180 mV·s-1) were fitted to obtain the double-layer capacitance (Cdl) and ECSA.37,38 The measured potentials vs. Hg/HgO were converted to reversible hydrogen electrode by ERHE = EHg/HgO + 0.9024.39 The turnover frequency (TOF) the as-prepared FeNi/NiFe2O4@NC can be calculated according to the previous reference.23,40,41 Results and Discussion The synthesis process of the FeNi/NiFe2O4@NC polyhedron assembled microboxes with an optimized molar ratio is schematically illustrated in scheme 1. Uniform FeNi-based MOFs (FeNi-BTC) microcubes are facilely synthesized by using double ligands (trimesic acid, 2-methylimidazole) and double nodes (Fe3+, Ni2+) due to their matched lattice. The XRD patterns in Figure S1 represents the typical characteristic peaks of phase-pure Ni-BTC and Fe-BTC,42,43 despite some slight discrepancies were observed in small angles, suggesting that ACS Paragon Plus Environment


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bimetal-substituted variants possess identical framework topology. The panoramic SEM image in Figure S2 reveals a defined and smooth cubic structure of FeNi-BTC crystals with uniform size (~10 µm). The NiFe2O4 polyhedron are formed by the reaction between FeNi nodes and oxygen of ligand (trimesic acid) in FeNi-BTC at the initial stage under nitrogen atmosphere, which further partially reduced by carbon species in double ligands to produce FeNi alloys at under N2 at 800 °C according to TGA curve in nitrogen (Figure S3 and S4). During this process, FeNi/NiFe2O4/NC polyhedron-assembled microboxes with an interconnected porous structure are formed mainly because of the heterogeneous contraction with the non-equilibrium annealing under a high heating rate.44,45 As a result, unique microboxes with assembled polyhedron containing-FeNi alloys and NiFe2O4 as core, and carbonitride layers as shell are eventually obtained. Thus, the formed graphitic carbonitride layers effectively confines the metal particles from further growth. SEM images unravel the uniform microboxes of FeNi/NiFe2O4@NC with an even size of ~10 µm and 3D interconnected porous structure (Figure 1A-C and Figure S5). The wall of each microbox consist of many primary polyhedron with hundreds of nanometers (Figure 1C). The elemental mapping indicates that Fe, Ni, O, and N atoms are homogeneously distributed throughout the microbox (Figure 1D-G and Figure S6). HRTEM images further suggest that bimetal nanoparticles (NPs) are fully encapsulated into carbonitride layers (Figure 1H–I). The average size of bimetal NPs is 11.1±2.2 nm with lattice spacing of 0.218 nm and 0.256 nm, corresponding to the crystal plane (111) for FeNi alloys and (311) for NiFe2O4, respectively. Unique structure of the tight contact between FeNi alloys and NiFe2O4 by in situ method, as well as the coating of carbonitride layers is generally beneficial for electron transfer, synergistic electrocatalysis and long stability. The characteristic peaks in the XRD patterns (Figure 2A) also suggest the composition of dominant phases of FeNi alloys (JCPDS card nos. 47-1405 for Fe0.64Ni0.36, and JCPDS card nos. 38-0419 for ACS Paragon Plus Environment

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FeNi3), small fractional phases of Fe3C (JCPDS card nos. 35-0772) and cubic NiFe2O4 oxide (JCPDS card nos. 54-0964) in the as-prepared composites. Ni3C are not observed in the pyrolyzed product due to the structure metastable at 700 oC.46 All of these results manifest that the thermal annealing of FeNi-BTC precursor results in the formation of FeNi alloys/NiFe2O4@NC polyhedron assembled microboxes. The Fe, Ni, N, and O elements are confirmed as the main elements by XPS survey spectra (Figure S7), where the percentages of element N are all lower than 0.5 wt.%. The Fe/Ni ratio (atom) is 1.68 on the FeNi/NiFe2O4@NC-800, which is slightly higher than those of EDS characterization (1.06) (Figure S8) and TXRF (1.31) (Table S1). The high-resolution XPS scans in Figure 2B-C show the Fe3+ (707 eV) and metallic Fe (710.8 eV) in Fe 2p, as well as Ni2+ (852.7 eV) and metallic Ni (855.3 eV) in Ni 2p, respectively, indicating the coexistence of Ni2+ and Ni, as well as Fe3+ and Fe in the composites. The deconvolution of the magnified N1s XPS spectra (Figure S7C) yields four classic type peaks near 398.2 eV (pyridinic-N), 399.3 eV (pyrrolic-N), 400.9 eV (graphitic-N) and 403.0 eV (quaternary N) in the FeNi/NiFe2O4@NC-800, respectively. The pyridinic N species could coordinate with Fe ions to form Fe–N–C species, implying the possible interaction between NC and metal species.47 Raman spectra in Figure 2D differentiate the NiFe2O4 from other possible phases, such as Fe2O3 and Fe3O4, where the two strong bands around 488 and 690 cm-1 are assigned to the F2g(2) of the tetrahedral (Fe3+ at A) and the A1g of octahedral (Ni2+, Fe3+ at B) sites due to the FeO4 disordered tetrahedron in the NiFe2O4 oxide, respectively.48,49 The absence of peak at about 1160 cm-1 for Fe2O3 and double peaks for Fe3O4 confirm their absence in the as-prepared composites.50,51 The peaks at about 1340 and 1591 cm-1 indicate the presence of D and G band in carbonitride layers, which is also confirmed by TGA with two exothermic peaks indicating the two-step oxidation process (Figure 3A). In the temperature range of 40–450 oC, the weight loss of about 0.5 wt.% is observed due to the desorption of physically absorbed water as well as the combustion loss of ACS Paragon Plus Environment


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carbonitride layers, and the weight gain of about 18.4 wt.% is attributed to the oxidation of FeNi alloys during 450–860 oC, suggesting that the highest alloying degree on the FeNi/NiFe2O4/NC-800, which is not directly relevant with the amount of carbon in FeNi/NiFe2O4/NC. The above results verify the coexistence of FeNi alloys and NiFe2O4 oxides in the as-prepared composites. Figure 3B and Table S1 show the results of nitrogen adsorption/desorption. The drop of nitrogen adsorption/desorption isotherms at ~0.4-0.5 (p/p0) and the characteristic type IV with a H3-hysteresis loop could indicate the mesoporous structure with porous sheet in the FeNi/NiFe2O4/NC-800.52,53 The specific surface area is ∼26.7 m2/g with the most probable pore-size distribution from 2.1 to 4.4 nm with average size of 9.6 nm on the FeNi/NiFe2O4/NC-800, which can accelerate electron transfer, and OH- diffusion in OER due to porous structure.54 The OER activities of FeNi/NiFe2O4@NC with different annealing temperature were examined by electrochemical measurements in 1.0 M KOH, and for comparison, commercial IrO2 was also examined in Figure 4A. The peak at about 1.4 V in FeNi/NiFe2O4@NC is ascribed to the oxidation of FeNi alloy by the redox process in the hybrids, implying the FeNi alloy as one of the actual active sites for OER.55,56 Notably, the doped nitrogen, NiFe2O4 and NiFe alloy should be involved in OER on the FeNi/NiFe2O4@NC,2,5,56,57 which could highly improve the OER activity by multicomponent synergistic effect. Significantly, the overpotential of approximately 316 mV at 10 mA·cm-2 on the FeNi/NiFe2O4@NC-800 is much lower than those of FeNi/NiFe2O4@NC-700 (348 mV) and FeNi/NiFe2O4@NC-850 (364 mV). It has been previously reported that the electronic interaction between metal particles and carbon shells is beneficial to improve OER activity.52 Particularly, the overpotential required to achieve 10 mA cm-2 for IrO2 is ca. 360 mV, significantly decreasing to 316 mV on the FeNi/NiFe2O4@NC-800. The as-prepared FeNi/NiFe2O4@NC-800 is also comparable active catalyst for OER with the overpotential and potential difference at 10 mA·cm-2 between as-prepared catalyst and commercial IrO2 in the recent reports (Table S2), such as NiFe LDH,58 ACS Paragon Plus Environment

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MOF(Fe1-Co3)550N,59 Co3O4/CoMoO4 hollow nanospheres,30 FeNi@NC,20 NPCN/CoNi-NCNT,56 FeNi3@[email protected] Figure 4B illustrates that the Tafel slope of the FeNi/NiFe2O4@NC-800 is approximately 60 mV·dec.-1, considerably lowering than those of the IrO2 (ca. 122 mV·dec.-1), FeNi/NiFe2O4@NC-700 (ca. 78 mV·dec.-1) and FeNi/NiFe2O4@NC-850 (ca. 91 mV·dec.-1). Control experiments of Fe-MOF-800C and Ni-MOF-800C were then conducted to investigate the effects of Fe and Ni on the OER activity. Note that Fe-MOF-800C and Ni-MOF-800C derived from pure Feor Ni-MOF, used as control samples, exhibit poor activities toward OER (Figure S9) with a high overpotential of 392 mV and 499 mV to drive 10 mA/cm2, respectively. This result strongly suggests that the optimized composition and strong coupling between FeNi alloys and NiFe2O4 should be helpful to activate water-oxidation reaction kinetics, which is further demonstrated by electrochemical impedance spectroscopy (EIS) in Figure 4C. Nyquist plots were fitted to a simplified Randles circuit (R(C(R(Q(R(CR))))), inset in Figure 4C), where the Chi-squared (χ2) and the relative errors are chosen as the estimated indicators (Table S3). The chi-squared (χ2) below 10-4 suggests the good agreement between the fitting electrical equivalent circuit model and experimental results. The much lower charge transfer resistances (Rct, 3.93 Ω·cm2) and pore resistance (Rp1, 0.63 Ω·cm2) are observed on the FeNi/NiFe2O4@NC-800 than those of FeNi/NiFe2O4@NC-700 and FeNi/NiFe2O4@NC-850, further demonstrating the tremendous contributions from its high alloying degree for fast electron transfer by well electrode contact, and porous structure for smooth mass transport abilities of the reactant hydroxide ions for superior OER performance.60 This is further verified by ECSA of the as-prepared catalysts (Figure 4D and S10), where the ECSA of FeNi/NiFe2O4@NC-800 is 1.84 mF·cm-2, being 1.1 and 1.4 times higher than those of FeNi/NiFe2O4@NC-700 (1.31 mF·cm-2) and FeNi/NiFe2O4@NC-850 (1.68 mF·cm-2), respectively. The more accessible active sites for OER in the FeNi/NiFe2O4@NC-800 could be relevant with the hollow interior and porous structure for accelerating the diffusion of OH- and H2. Furthermore, the ACS Paragon Plus Environment


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lower limits for TOF can be calculated by assuming all the metal ions are available from TXRF characterization (Table S1). As displayed in Figure 5A, the TOF on the FeNi/NiFe2O4@NC-800 is apparently higher than those of FeNi/NiFe2O4@NC-700 and FeNi/NiFe2O4@NC-850. The TOF on the FeNi/NiFe2O4@NC-800 reaches 0.67 s–1 at η=400 mV, which is 1.53 and 2.91 times than those of FeNi/NiFe2O4@NC-700 (0.44 s-1) and FeNi/NiFe2O4@NC-850 (0.23 s-1), respectively, indicating the better intrinsic OER activity. In addition to the excellent OER activity, FeNi/NiFe2O4@NC-800 also exhibits excellent stability. After continuous 5000 cycles, the polarization curve of FeNi/NiFe2O4@NC-800 retains coincidence with the initial one, which possesses robust stability than the commercial IrO2 catalyst for OER (Figure 5B). The chronopotentiometry curve (inset of Figure 5B) at η=0.316 V with small loss (~13%) after 5000 s further verifies the long-term stability of FeNi/NiFe2O4@NC-800, which should be related to the FeNi/NiFe2O4 encapsulated into the carbonitride sheets, efficiently protecting them against agglomerating, pulverizing and dropping out. The synergistic effects between FeNi alloy, NiFe2O4 and carbonitride coating play a crucial role for the improvement in OER performance. CONCLUSION FeNi/NiFe2O4/NC polyhedron assembled microboxes are successfully fabricated through the manipulated carbonization of bimetal MOF. Owing to the compositional modulation and hollow microboxes with porous structure, the resulting FeNi/NiFe2O4/NC-800 shows pronounced electrocatalytic activity and durability toward OER in alkaline solution, outperforming a commercial IrO2 catalyst. The superior OER performance is ascribed to the combined effects of 3D hollow interior with interconnected porous wall for fast mass transport, strong coupling between intimately contacted FeNi alloy and NiFe2O4 oxide for easy electron transfer, and the protection of carbonitride layers for long stability. The facile process for the preparation of FeNi/NiFe2O4@NC microboxes with superior OER performance, may drive the urge to explore the hollow and nanostructured ACS Paragon Plus Environment

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hybrids for electrocatalytic water splitting. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:. SEM images, XRD patterns, TGA thermograms in Ar, BET surface area, pore size, TXRF, Nitrogen element mapping, XPS characterization, EDS, Polarization curves of Fe-MOF and Ni-MOF derived composites, Comparison of OER performance, Circuit modeling, Cyclic voltammograms scans for electrochemical active surface areas (PDF). AUTHOR INFORMATION Corresponding Author E–mail: [email protected]; [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (NSFC, Nos. 21576288, 21573286). REFERENCES (1) Ling, T.; Yan, D. Y.; Jiao, Y.; Wang, H.; Zheng, Y.; Zheng, X. L.; Mao, J.; Du, X. W.; Hu, Z. P.; Jaroniec, M.; et al. Engineering Surface Atomic Structure of Single-Crystal Cobalt (II) Oxide Nanorods for Superior Electrocatalysis. Nat. Commun., 2016, 7, 12876. (2) Ci, S. Q.; Mao, S.; Hou, Y.; Cui, S. M.; Kim, H.; Ren, R.; Wen, Z. H.; Chen, J. H. Rational Design of Mesoporous NiFe-Alloy-Based Hybrids for Oxygen Conversion Electrocatalysis. J. Mater. Chem. A 2015, 3, 7986–7993. (3) Gerken, J. B.; McAlpin, J. G.; Chen, J. Y. C.; Rigsby, M. L.; Casey, W. H.; Britt, R. D.; Stahl, S. ACS Paragon Plus Environment


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FIGURE CAPTIONS

Fe3+ Trimesic acid

Solvothermal

Annealing

in DMF

in N2

Ni2+ 2-methylimidazole

FeNi-BTC nanocubes

FeNi/NiFe2O4@NC nanoboxes

Scheme 1. Schematic illustration of the synthetic strategy of the FeNi alloys/NiFe2O4@carbonitride layers-assembled microboxes (FeNi/NiFe2O4@NC).



A

C

B

5 µm

20 µm

100 µm

D

E

G

O H

Fe F

50

Frequency (%)

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

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11.1±2.2 nm

40

Ni

I Carbonitride layers

30 20 10 0

8

9

10 11 12 13 14 15

0.256 nm (311)

Size (nm)

0.218 nm (111) Fe0.64Ni0.36

NiFe2O4

10 nm

2 nm

Figure 1. (A, B, C) SEM images. The magnified image in (C) clearly reveals the uniformly size polyhedron. (D, E, F, G) SEM images and corresponding elements mapping. (H, I) (HR) TEM images of FeNi/NiFe2O4@NC−800, showing the carbonitride shells and encapsulated FeNi alloy and NiFe2O4 spinel.


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B

♦ NiFe2O4

A

Fe2p

◊ Fe0.64Ni0.36

FeNi3 Fe3C

♦

♦ ♦

c Fe3+ 2p1/2

◊

◊

♦ ♦

c

b

Intensity (a.u.)

Intensity (a.u.)

◊

Fe0 2p1/2

Fe3+ 2p3/2

b

Fe0 2p3/2

a

a 20

30

40

50

60

70

80

732

726

720

714

708

702

ο

2θ ( )

Binding Energy (eV)

Ni2p

D

D

Intensity (a.u.)

C

G

c b a 1000

1200

1400

1600

1800 -1

c

Raman shiift (cm )

2+

Ni

Ni0

Satellite

b

Intensity (a.u.)

Intensity (a.u.)

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


c A1g

F2g(2)

b

E1g

a

a

F2g(1)

870

864

858

852

200

Binding Energy (eV)

400

600

800

1000

-1

Raman shift (cm )

Figure 2. (A) XRD patterns, (B, C) high–resolution XPS scans of Fe 2p and Ni 2p, (D) Raman spectra

of

(a)

FeNi/NiFe2O4@NC−700,

(b)

FeNi/NiFe2O4@NC−800

FeNi/NiFe2O4@NC−850.


and

(c)


120

B 140 b

120

115

c

110

a 105

10.3% 12.7%

0.5%

100

2.6%

4.1% 95

Volume adsorbed (cm 3/g)

18.4%

100 80

a b c

0.012

dV/dD (cm 3/g/nm)

A

Weight (%)

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

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0.009

c

0.006 0.003 0.000 1

10

60

b

100

Pore size (nm)

Adsorption Desorption

40

a

20 0

90 150

300

450

600

750

900

0.0

0.2

0.4

0.6

0.8

1.0

0

o

Relative pressure (P/P )

Temperature ( C)

Figure 3. (A) Thermogravimetric analysis in air, (B) Nitrogen absorption/desorption isotherm and the

corresponding

pore

size

distribution

of

(a)

FeNi/NiFe2O4@NC−800 and (c) FeNi/NiFe2O4@NC−850.


FeNi/NiFe2O4@NC−700,

(b)

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1.70

250 20

a

b

B

c

2

Current (mA/cm )

A

150

15

1.65 b

10

IrO2

5 1.52

100

1.56

1.60

a

1.64

IrO2

Potential (V vs. RHE)

c 50

0 1.2

1.4

1.6


2

Current density (mA/cm )

200

122 mV dec

1.60

1.55

6

Q

0.9

1.2

1.5

1.8

0.8

0.6 Rp2

0.6

FeNi-NiFe2O4/NC-800

Rct

FeNi-NiFe2O4/NC-850

2

2

Current (mA/cm )

Rp1

0.3

Cdl

Rs

4

d

Log (current density, mA/cm )

D 5

a

-1

2

Cp

C

b 60 mV dec

1.50

-1

-1

c

1.45 0.0

1.8

-1

78 mV dec 91 mV dec

Potential (V vs. RHE )

-Z'' (Ω cm )

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


3

2

0.4

FeNi-NiFe2O4/NC-700

0.2

1 a

b

c

0 0

2

4

6

8

0.0

10

0

2

40

Z' (Ω cm )

80

120

160

200

Scan rates (mV/s)

Figure 4. Electrochemical OER catalytic activity of FeNi/NiFe2O4 polyhedron−assembled microboxes, and IrO2. (A) Polarization curves. (B) Tafel plots derived from OER polarization curves. (C) EIS nyquist plots (symbol) at a η value of 0. 322 V (η=ERHE−1.23 V), ﬁtted data (solid line) by equivalent electrical circuit diagrams (inset), where Cp is the capacitance of catalyst coating, Rp1 is the pore resistance of catalyst, Rp2 is oxide resistance of catalyst, Rct is charge transfer resistance, Rs is solution resistance, Q is pseudo capacitance of catalyst coating, and Cdl is the double layer capacitance. (D) The differences in current density (∆J=Ja–Jc) at 1.15 V plotted against scan rate fitted to a linear regression allows for the estimation of Cdl, where the linear slope, equivalent to twice

the

double-layer

capacitance

Cdl,

was

used

to

represent

the

ECSA.

FeNi/NiFe2O4@NC−700, (b) FeNi/NiFe2O4@NC−800 and (c) FeNi/NiFe2O4@NC−850. ACS Paragon Plus Environment

(a)


B

160

20

0.4

a

0.2

c

2

b

0.6

Current density (mA/cm )

2

Current (mV/cm )

0.8

A

TOF (s-1)

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

120

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b

15 10 5 0 0

80

1000

2000

3000

4000

5000

Time (s)

Initial After 5000 cycles Initial for IrO2

40

After 5000 cycles 2

10 mA/cm

0

0.0 300

320

340

360

380

400

Overpotential (mV vs. RHE )

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7


Figure 5. (A) TOFs at different overpotentials from 300 to 400 mV by assuming that every Fe and Ni atoms are catalytically active (lower bound), (B)The LSV curves for the 1st and 5000th potential cycles of FeNi/NiFe2O4@NC−800 and commercial IrO2, and chronopotentiometry curve at a η value of 0.316 V (η=ERHE−1.23 V). (a) FeNi/NiFe2O4@NC−700, (b) FeNi/NiFe2O4@NC−800 and (c) FeNi/NiFe2O4@NC−850.


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Table of contents graphic Carbon layers OHAnnealing

NiFe2O4 FeNi alloys O2

FeNi-bimetal MOF

FeNi Alloys/NiFe2O4@NCAssembled Nanoboxes


NiFe2O4@Carbonitride Layers

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