FeNi cubic cage@N-doped Carbon coupled with N-doped graphene

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FeNi cubic cage@N-doped Carbon coupled with N-doped graphene towards efficient electrochemical water oxidation Xiao Zhang, Cheng Li, Tengfei Si, Heng Lei, Chengbo Wei, Yanfang Sun, Tianrong Zhan, Qingyun Liu, and Jinxue Guo ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00282 • Publication Date (Web): 18 May 2018 Downloaded from http://pubs.acs.org on May 18, 2018

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ACS Sustainable Chemistry & Engineering

FeNi cubic cage@N-doped Carbon coupled with N-doped graphene towards efficient electrochemical water oxidation Xiao Zhanga, Cheng Lib, Tengfei Sia, Heng Leia, Chengbo Weia, Yanfang Sunb, Tianrong Zhana, Qingyun Liuc, Jinxue Guoa,∗∗ a

State Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and Molecular

Engineering, Qingdao University of Science & Technology, No. 53 Zhengzhou Street, Qingdao 266042, China b

c

Institute of Science and Technology, Agricultural University of Hebei, Baoding 071000, China

College of Chemical and Environmental Engineering, Shandong University of Science and Technology,

Qingdao 266510, China ABSTRACT Oxygen evolution reaction (OER) is of great significance in electrochemical water splitting on industrial scale, which suffers from the slow kinetics and large overpotential, thus setting the main obstacle for efficient water electrolysis. To pursue cost-effective OER electrocatalysts with high activity and durable stability, we here set a facile strategy to prepare N-doped graphene supported core-shell FeNi alloy@N-doped carbon nanocages (FeNi@NC-NG) by annealing graphene oxides supported Prussian blue analogues under H2/Ar atmosphere. Based on the specific structural benefits, the present catalyst shows superior OER catalytic activity than precious metal catalyst of RuO2 and Ir/C, with a low overpotential of 270 mV for 10 mA cm-2, as well as high stability. The simple synthesis process and outstanding electrocatalytic performances show great potential of FeNi@NC-NG to

∗

Corresponding author. Phone: +86 532 84022681; Fax: +86 532 84023927. E-mail: [email protected] (J. Guo). 1

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replace the noble metal-based catalysts towards electrochemical water oxidation. Keywords: NiFe alloy; Nanocage; Nitrogen doping; Prussian blue analogues; Oxygen evolution

2


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Introduction

Facing the severe problems of increasing energy consumption and environmental pollution, it is crucial to explore renewable and clean energy sources to replace the fossil fuels. Electrochemical water splitting holds tremendous research interests as an ideal and clean technique towards energy conversion and storage, which generates H2 through the cathodic hydrogen evolution reaction (HER) and O2 via the anodic oxygen evolution reaction (OER).1-4 Among these two reactions, OER sets the major obstacle that hinders the development of high efficiency electrochemical water splitting cells, because of its intrinsic sluggish kinetics.5-7 Noble metal electrocatalysts (IrO2 and RuO2) are efficient for OER, while their applications suffer from their scarcity and high-cost. Consequently, numerous attentions have been paid on developing earth-abundant and efficient OER electrocatalysts, such as borides, phosphides, sulfides, metal-organic frameworks (MOFs), nitrides, oxides, and hydroxyls.8-15 Particularly, 3d transition metals (Fe, Co, and Ni) based electrocatalysts exhibit strong potential to replace precious metal catalysts because of their promising catalytic performances and low-cost.8-10,12,13,15 In recent years, 3d metal alloys show promising electrocatalytic activity due to the tuned electronic structure and moderate adsorption energy.16-24 Yang and co-workers synthesized ternary FeCoNi alloys-graphene electrocatalyst.21 Both computational and experimental studies showed that the tuned electronic structure in ternary alloys was responsible for the good HER and OER performances. Bare metal catalysts tend to degrade and cannot afford stable catalytic performance during the long-time operation under strong alkaline media and high potential. To address this, carbon/graphene coating techniques are employed to improve the electrocatalytic stability.21-26 Moreover, thin carbon layer on metal alloys could optimize their adsorption properties and thus boost the catalytic activity.22,26,27 3



Previous literatures reported that, M-N-C exhibits excellent electrocatalytic activity, because nitrogen doping could not only alter the electronic states of carbon but also supply C-N active sites.28-32 It is reasonably believed that electrocatalyst composited of N-doped carbon coated 3d metal alloy should possess good catalytic activity, therefore, the design and synthesis of such composite catalysts are desirable. Self-template method shows fascinating advantages for fabricating complex nanostructures towards applications in energy storage and conversion, because such nanostructures could supply additional opportunities to tune the physical and chemical features.9,33-37 Very recently, MOFs template engaged strategies have been developed to obtain nanostructured 3d metal-carbon electrocatalysts.20,21,27,30,34,39-41 Tao et al. reported the synthesis of FeNi@N-CNT using ZIF-8 as precursor via a simple pyrolysis approach, which showed active catalytic activity towards OER with a low overpotential of 300 mV at 10 mA cm-2.27 Feng and co-workers prepared NiFe nanoparticles@N-doped graphene by directly annealing NiFe-based Prussian blue analogues (NiFe-PBA, Ni3[Fe(CN)6]2), which delivered active OER performance (overpotential of 281 mV to afford 10 mA cm-2) and stable durability. Generally, most of the previous catalysts annealed from MOFs possess the typical structure of carbon coated particles. Hollow/core-shell nanostructures are believed to be ideal candidates for catalytic applications due to their complex structural advantages and large catalytic surface.5,6,9,20-22 Lou’s group reported the synthesis of hollow NiCoP/C nanoboxes from the precursors of ZIF-67 nanocubes, which delivered excellent oxygen evolution activity because of their hollow structures with high surface-to-volume ratios.9 Notwithstanding these intriguing progresses, the MOFs derived synthesis of hollow/core-shell nanostructured alloys@carbon for OER application has rarely been reported. Herein, the active and stable OER electrocatalyst based on hollow FeNi alloy cage@nitrogen-doped 4


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carbon coupled with nitrogen-doped graphene nanosheets (FeNi@NC-NG) is prepared via thermal treatment of bimetallic NiFe-PBA under H2/Ar. Ni3[Fe(CN)6]2 cubic precursor supplies bimetal FeNi compositions, assures in-situ formation of carbonitride species without additional nitrogen sources, as well as guides the cubic geometry. Due to the complex advantages of alloy synergism, hollow structure, thin N-doped carbon shell, Fe-N-C sites, and fast charge transfer, the obtained FeNi@NC-NG exhibits promising prospect for low-cost, highly efficient, and sustainable electrochemical water oxidation.

Experimental

Synthesis of N-doped carbon coated FeNi alloy nanocages coupled with N-doped graphene Graphene oxides (GO) are prepared using a reported method.42 GO (100 mg) are ultrasonic dispersed in 20 mL of deionized water, which is then added with 0.4 mmol of K3Fe(CN)6. The resultant dispersion is mixed with another aqueous solution (20 mL) containing 0.6 mmol of Ni(NO3)2·6H2O and 0.9 mmol of sodium citrate. After stirring for 24 h, the precipitate is collected, washed, and dried in vacuum at 60 o

C. To obtain the final sample of FeNi@NC-NG, the collected precipitate is sintered in a tube furnace

at 600 oC for 3 under H2/Ar (5%) atmosphere with a heating rate of 5 oC per minute. Characterizations of FeNi@NC-NG catalyst The morphology of FeNi@NC-NG is obtained by scanning electron microscope (SEM) with a JEOL JSM-7500F). Transmission electron microscope (TEM) and energy dispersive X-ray spectroscopy (EDX) are performed on a FEI Tecnai G2 F30. The crystallographic information is collected from powder X-ray diffraction (XRD) on Philips X’-pert X-ray diffactometer. Raman spectroscopy is recorded with confocal microprobe Raman system (LabRam-e010). X-ray photoelectron spectrum (XPS) is obtained from RBD upgraded PHI-5000c ESCA system (Perkin Elmer). The surface area of FeNi@NC-NG is determined by a Brunauer-Emmett-Teller (BET) method (TriStar 3000-BET/BJH). 5



Electrocatalytic activity measurements on FeNi@NC-NG towards water oxidation The electrochemical OER activity of FeNi@NC-NG is performed using a CHI760D (CH Instruments, Shanghai, China) with a three-electrode cell. An electrocatalyst modified glassy carbon (GC) electrode acts as working electrode. Graphite rod and saturated calomel electrodes (SCE) serve as counter electrode and reference electrode, respectively. The working and SCE electrodes are separated from the graphite rod electrode by a glass frit in an electrochemical H-cell. The catalyst slurry is the mixture of 4 mg of FeNi@NC-NG, 50 uL of Nafion solution (5 wt.%), and 1.25 mL water. One portion of the slurry is drop-casted onto a glassy carbon electrode with the catalyst loading of 0.5 mg cm-2. The OER activity is measured by linear sweep voltammetry (LSV) tests in 1.0 M KOH at a scan rate of 5 mV s-1. The potentials are calibrated by a reversible hydrogen electrode (RHE). Commercial Ru2O and Ir/C (20%) are tested as benchmark electrocatalyst. The electrochemical impedance spectroscopy (EIS) is obtained at the frequency ranging from 0.1 Hz to 100 kHz, which is measured at the open circuit voltage. The electrochemical double-layer capacitance (Cdl) is evaluated between the potential range where no faradic processes at varied scan rates. The electrochemically active surface area (ECSA) is determined based on the obtained specific capacitance.

Resultants and discussion

FeNi@NC-NG catalyst Fig. 1 shows the schematic synthesis process of FeNi@NC-NG. The NiFe-PBA nanocubes are formed and in-situ anchored onto the surface of GO nanosheets, guaranteeing the strong interaction between them, which is beneficial for charge exchange and could prevent the restacking of GO nanosheets. During the following sintering process, H2/Ar atmosphere is employed and plays a key role for the formation of NiFe alloy nanocages, which is different from the use of pure inert gas in previous 6


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reports.21,23 During the sintering process, metal cations on the surface of NiFe-PBA nanocubes are reduce by H2 and form alloy cubes along the scaffold of NiFe-PBA. Then the metal cations close to the newly formed NiFe cube interface are further reduced into NiFe alloys, which flow outward to NiFe cubic scaffold and lead to the formation of void space within the NiFe-PBA. On the other hand, CN ligands pyrolysis into nitrogen doped graphene and in-situ coat on NiFe alloy nanocages. Fig. 2a displays the SEM image of FeNi@NC-NG. The PBA-derived nanocages could maintain their cubic geometry after the heat-treatment, which are well dispersed onto the surface of NG nanosheets and prevent the restacking of NG nanosheets. The nanocages anchored NG assemble into sandwich structure with rich tunnels, which is beneficial for electrolyte infiltration and oxygen release, as well as supplies high surface area for catalyst/electrolyte contact. Meanwhile, the good couple of FeNi@NC nanocages with conductive NG affords rapid charge exchange for electrocatalytic reaction. Notably, as shown in the inset of Fig. 2a, the magnified SEM image of a cracked FeNi@NC nanocage clearly reveals the hollow interior. The microstructure of the present FeNi@NC-NG is further characterized by TEM. As shown in Fig. 2b, FeNi@NC nanocages show typical cubic shape, which are well dispersed on wrinkled and flexible NG nanosheets. The magnified TEM image of a broken nanocage is shown in the inset of Fig. 2b, further confirming the hollow structure. The N2 adsorption-desorption isotherm (Fig. S2) shows that, FeNi@NC-NG cages possess a high surface area of 29.5 m2 g-1, offering abundant active electrochemical sites and large contact area for OER. The HRTEM image (Fig. 2c) is obtained from the wall of nanocages, and an interface between NiFe/carbon and the good coupling of the hybrids can be clearly observed. The distinct lattice fringes can be seen on the inner layer, and the spacing is determined to be 0.21 nm, assigning to the (111) plane of NiFe alloy.20,41 The interlayer spacing of outer 7



layer is 0.34 nm, which is ascribed to graphene. It is reported that, the thickness of outer carbon shell is crucial for the catalytic performances of inner catalyst.21,22,26,27,43-46 For the present sample, the graphene shell consists of several graphene layers, which are ideal for electron transfer and catalytic durability.21,43-46 Fig. 2d shows the TEM image of FeNi@NC-NG and the corresponding EDX elemental mapping images of C, N, Fe, and Ni. Clearly, Fe and Ni elements are well overlapped with the cubic nanocages, and no Fe and Ni distribution is observed beside the location of nanocages. On the other hand, N element is well distributed, indicating the good doping of N into carbon species. Fig. 3a shows the powder XRD pattern of FeNi@NC-NG. The broad peak ranging from about 20° to 30° is related to the carbon in the composite. Except for this, all other sharp diffraction peaks agree well with cubic Fe0.64Ni0.36 (JCPDS No. 47-1405). Raman spectroscopy of FeNi@NC-NG is also recorded. In Fig. 3b, two prominent peaks appear at 1347 cm-1 and 1591 cm-1, corresponding to the well-documented D and G bands for carbonitride, respectively.20,24 The intensity ratio of D band to G band (ID/IG) is usually used to probe the graphitization degree of carbon materials. The ID/IG value is calculated to be 1.14 for FeNi@NC-NG, suggesting that there are plenty of defects and microstructural rearrangement, which should be assigned to the reduction of GO and nitrogen doping into carbon matrix. The FeNi@NC-NG is further characterized by XPS spectroscopy, which is a powerful technique for characterizing the surface elemental composition and valence states. The high-resolution XPS spectrum of C1s is fitted into four individual peaks and shown in Fig. 4a. The peak at 284.6 eV is assigned to the graphitic carbon of C=C and C-C bonds.25,47 The peak of 285.5 eV represents the carbon that is related to the formation of C-N, further confirming the N-doping into carbon.25 The peaks at higher binding energy of 286.3 eV and 289.2 eV are attributed to C-O and C=O bonds, which are derived from the 8


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functional groups of GO.25,47 In the N1s XPS spectrum of Fig. 4b, three individual peaks are observed at 398.3 eV, 400.5 eV, and 401.8 eV, corresponding to pyridinic-N, pyrrolic-N, and graphitic-N, respectively.25,27 Notably, pyridinic-N and graphitic-N are demonstrated to be advantageous for the electrocatalytic activity, suggesting that such doped nitrogen functional groups will boost the electrocatalytic performance of [email protected] Fig. 4c and d display the XPS spectra of Fe 2p and Ni 2P. XPS is surface-sensitive characterization technique, therefore, the show up of high-resolution Fe 2p and Ni 2p XPS spectra indicates that the coated carbon layer on FeNi nanocage is very thin, which is important for FeNi catalyst to show its catalytic activity.21,43-46 In Fig. 4c, two peaks located at the binding energies of ~ 707 eV and 725 eV are related to the metallic Fe,20,21 which is consistent with XRD and TEM. The peak of 711 eV is ascribed to the complex of Fe-Ox and Fe-Nx, which are due to the partially oxidization of surface metal and the formation of Fe-N.1,21,28,29 Notably, recent reports proved that, Fe-N-C could act as active sites for electrocatalytic reaction. Therefore, the formation of Fe-N-C in the present catalyst supplies additional active sites for electrocatalytic activity and may possibly induce synergism for OER.28-32 For Ni 2p XPS spectrum (Fig. 4d), the peak corresponding to metallic Ni is detected at about 852 eV, and the peak at 856 eV is assigned to the Ni-Ox.21 Electrochemical properties To get insight into the catalytic activity of FeNi@NC-NG towards electrochemical water oxidation, the LSV test is recorded in 1 M KOH. Fig. 5a displays the LSV polarization curves of FeNi@NC-NG, RuO2, and Ir/C. Impressively, FeNi@NC-NG exhibits excellent OER activity, which is even superior to the benchmark catalysts of RuO2 and Ir/C. For instance, FeNi@NC-NG generates a catalytic current density of 10 mA cm-2 at lower overpotential of 270 mV than RuO2 (280 mV) and Ir/C (340 mV). Moreover, with the potential further increasing, FeNi@NC-NG achieves negative shift polarization 9



curves in comparison with that of RuO2 and Ir/C. To afford a high current density of 50 mA cm-2, FeNi@NC-NG only needs a lower overpotential of 328 mV against that of 402 mV for RuO2 and 430 mV for Ir/C. Table 1 lists the OER catalytic activity of FeNi@NC-NG and previous state-of-the-art 3d metal/alloys and Ni/Fe-based electrocatalysts in alkaline environment, further confirming the excellent OER activity of FeNi@NC-NG. The value of 270 mV of FeNi@NC-NG is lower than most of the alloy electrocatalysts, including NiFe/CNx (360 mV),18 FeNi/NiFe2O4@NC (316 mV),20 FeCoNi@G (288 mV),21 FeNi@NC (280 mV),22 Ni-Fe@rGO (350 mV),24 FeNi@N-CNT (300 mV),27 Fe/C/N (360 mV),29 NiFe@NG (281 mV),41 and Ni@graphene (370 mV),48 which is close to the best value of 264 mV for NiFe nanosheets.16 Moreover, FeNi@NC-NG also shows better OER activity than NixB (380 mV),8 NiCoP/C (330 mV),9 NiCo LDH nanosheets (367 mV),12 and NiFeOH/NiFeP (290 mV).49 Moreover, the OER activity of present FeNi@NC-NG is compared with previous best water oxidation electrocatalysts reported so far. As shown in Table S1, NiFe double layer hydroxide coupled with defective graphene (NiFe LDH/DG) afforded a current density of 10 mA cm-2 at 210 mV overpotential. NiCo2O4 microcuboid achieved the same current density at 230 mV. Ni3S2/Ni foam needed 190 mV to deliver 10 mA cm-2. FeNi@NC-NG thus shows promising OER activity that is close to the best OER electrocatalysts. In addition, the Tafel slope value of FeNi@NC-NG is determined to be 72 mV dec-1, indicating favorable reaction kinetics. The electrochemically active surface area (Fig. 5c) and electrochemical impedance spectroscopy (Fig. 5d) are also recorded to further reveal the OER activity of FeNi@NC-NG. In Fig. 5c, the electrochemical double-layer capacitance of 44 mF cm-2 is obtained for FeNi@NC-NG, by calculating the slope of current density against the scan rate. This value (44 mF cm-2) is much higher than that of FeNi/NiFe2O4@NC (1.84 mF cm-2)20 and NiFe@NG (2.98 mF cm-2),41 indicating the huge ECSA of 10


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the present FeNi@NC-NG catalyst. The specific hollow structure with high surface area should be responsible for this. The additional Fe-N-C active sites and their possible synergism also contribute to the ECSA. The EIS measurement determines the ultralow charge-transfer resistance value of 5.8 Ω for FeNi@NC-NG, suggesting the fast electrode kinetics on FeNi@NC-NG catalyst. This should be attributed to the high conductivity of FeNi alloy and carbon matrix. Durability is another important criterion to assess electrocatalyst, which is evaluated by continuously recording the LSV polarization curves on FeNi@NC-NG electrode for over 2000 cycles. Fig. 6a shows the initial and 2000th polarization curves of FeNi@NC-NG. One can see that, no detectable changes on overpotential and current density are observed from these two LSV curves, indicating the high stability of FeNi@NC-NG. The powder XRD pattern is collected from FeNi@NC-NG catalyst after such durable OER test. As displayed in Fig. 6b, FeNi alloy keeps the strong and sharp diffraction peaks after 2000 cycles. Besides, the broad peak corresponding to carbon still exist in the tested FeNi@NC-NG catalyst. XRD characterization reveals that no crystallographic changes are detected for FeNi@NC-NG after 2000 cycles of LSV test, showing its good stability. HRTEM and SEM images are also obtained from FeNi@NC-NG after durability test to further probe the possible variation on catalyst. One can see in the SEM image (Fig. S3), most of the nanocages can keep their cubic structure and shows good structure stability of FeNi@NC-NG catalyst. As shown in Fig. 6c, the wall of cage is well maintained after the OER durability test, and the lattice spacing of 0.21 nm for FeNi (111) is determined for the main body of cage wall. Moreover, the coated graphene layers with interlayer spacing of 0.34 nm are still well maintained, indicating that the coated graphene shell can well protect the FeNi alloy catalyst during the OER process. Interestingly, Ni(OH)2 with the lattice spacing of 0.27 nm is also observed on the surface of FeNi alloy. However, oxidative Fe species are absent. Therefore, it can be reasonably 11



concluded that, the eventually electroactive species towards OER in the present FeNi@NC-NG catalyst are the Ni(OH)2/FeNi/NC heterostructure and Fe-N-C, as well as their possible synergism. The formation of Ni(OH)2 should be produced during the OER process, which can be seen from the oxidation peak at LSV polarization curve (Fig. 6d). The cyclic voltammetry curve of FeNi@NC-NG in Fig. 6d further confirms this. The oxidation wave centered at 1.48 V vs RHE is usually assigned to the oxidation of Ni species into Ni(OH)2 and further NiOOH.50,51 The absence of NiOOH in Fig. 6c might be due to its scarcity. A corresponding reduction wave is also detected at 1.34 V. Interestingly, the repeated CV curves (Fig. S4) show that, the peak intensity of the oxidation wave shows slight fade after the initial cycle. After that, no detectable decrease of peak intensity is observed during the following nine cycles, indicating that there is no loss of Ni to Ni(OH)2 state conversion during the repeated cycles. This result is consistent with the LSV test (Fig. 6a), further confirming the excellent stability of FeNi@NC-NG catalyst. This should be due to the excellent protection of N-doped carbon shell on the alloy. To further probe the existence of Ni(OH)2, XPS characterization on FeNi@NC-NG after OER stability test is conducted. Fig. S5 compares the Ni 2p XPS spectra of FeNi@NC-NG before and after the OER test. Clearly, the peak corresponding to Ni0 in the untested catalyst disappear after the OER stability test. In the XPS spectra of tested electrocatalyst, the typical Ni 2p3/2 XPS peaks of Ni(OH)2 are observed at 856.2 and 861.9 eV, confirming the show up of Ni(OH)2 in the electrocatalyst after OER. To further assess the potential application, a full cell with ion exchange membrane is assembled to measure the OER performance of FeNi@NC-NG electrode. The FeNi@NC-NG is evaluated as OER electrocatalyst in 1 M KOH, and commercial Pt/C is used as HER catalyst in 0.5 M H2SO4. KOH and H2SO4 are separated with an ion exchange membrane in an electrochemical H-cell. The OER LSV 12


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curve is shown in Fig. S6. Clearly, FeNi@NC-NG delivers good catalytic activity with low overpotential of 280 mV at a current density of 10 mA cm-2, making it a promising candidate for cheap and efficient OER electrocatalyst.

Conclusions

In conclusion, core-shell structured FeNi alloy@N-doped carbon nanocages coupled with N-doped graphene nanosheets have been synthesized for the first time via a facile annealing method under H2/Ar atmosphere, using GO supported Prussian blue analogues as precursor. The as-obtained FeNi@NC-NG shows combined advantages as OER electrocatalyst: 1) the sandwich structure and hollow feature not only provide large catalyst/electrolyte contact area, but also tunnels for electrolyte diffusion and oxygen release, 2) N-doped carbon shell protects FeNi catalyst for durable catalytic performance, 3) Fe-N-C supplies additional active sites for catalytic reaction, 4) the alloy and carbon matrix with high conductivity assure rapid charge transfer and fast reaction mechanism. Therefore, the obtained FeNi@NC-NG catalyst exhibits superior electrocatalytic activity towards water oxidation, as well as excellent stability. It is revealed that, the actual electroactive species of the present sample towards OER are Ni(OH)2, Fe-N-C, and their possible synergism. Our work not only paves a new way for synthesizing of hollow structured metal/carbon electrocatalysts from MOFs precursor, but also gets insight into the catalytic mechanism on such electrocatalysts for OER.

Acknowledgements

We thank for the financial support from Natural Science Foundation (2016GGX104019, 2017GGX20143) of Shandong Province.

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Table 1 Electrocatalytic activity towards OER of FeNi@NC-NG and previous state-of-the-art 3d metal/alloys and Ni/Fe-based electrocatalysts in alkaline media.

Samples

Overpotential/current

Ref.

-2

(mV/mA cm ) NixB

380/10

8

NiCoP/C

330/10

9

NiCo LDH nanosheets

367/10

12

NiFe nanosheets

264/20

16

NiFe/CNx

360/10

18

FeNi/NiFe2O4@NC

316/10

20

FeCoNi@G

288/10

21

FeNi@NC

280/10

22

Ni-Fe@rGO

350/10

24

FeNi@N-CNT

300/10

27

Fe/C/N

360/10

29

NiFe@NG

281/10

41

Ni@graphene

370/10

47

NiFeOH/NiFeP

290/10

48

FeNi@NC-NG

270/10

This work

21



Fig. 1 Schematic synthesis of FeNi@NC-NG.

Fig. 2 (a) SEM image of FeNi@NC-NG, the inset shows a cracked nanocage with exposed interior. The scale bar in the inset is 100 nm. (b) TEM image of FeNi@NC-NG, the inset shows a cracked nanocage with exposed interior. The scale bar is 0.2 µm. The scale bar in the inset is 100 nm. (c) High-resolution TEM image of FeNi@NC-NG, the scale bar is 5 nm. (d) TEM image and corresponding EDX elemental mapping images of C, N, Fe, and Ni of FeNi@NC-NG.

22


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Fig. 3 (a) Powder XRD pattern and (b) Raman spectrum of FeNi@NC-NG.

Fig. 4 XPS spectra of (a) C 1s, (b) N 1s, (c) Fe 2p, and (d) Ni 2p of FeNi@NC-NG.

23



Fig. 5 (a) Polarization curves of FeNi@NC-NG, RuO2, and Ir/C. (b) Corresponding Tafel plot of FeNi@NC-NG. (c) Measured capacitive currents plotted as a function of scan rate for FeNi@NC-NG. The inset shows the cyclic voltammograms. (d) The EIS spectrum of FeNi@NC-NG.

24


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Fig. 6 (a) Polarization curves at the 1st and 2000th cycles obtained from FeNi@NC-NG. (b) Powder XRD pattern of FeNi@NC-NG after OER durability test. (c) High-resolution TEM image of FeNi@NC-NG after the OER durability test, the scale bar is 5 nm. (d) LSV and CV curves of FeNi@NC-NG.

25



For Table of Contents Use Only

The core-shell FeNi alloy@N-doped carbon nanocages derived from NiFe-based Prussian blue analogues is developed as advanced electrocatalysts towards oxygen evolution for water splitting.

26


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FeNi cubic cage@N-doped Carbon coupled with N-doped graphene

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