Metallic Intermediate Phase Inducing Morphological Transformation in

Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 3699−3706

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Metallic Intermediate Phase Inducing Morphological Transformation in Thermal Nitridation: Ni3FeN-Based Three-Dimensional Hierarchical Electrocatalyst for Water Splitting Zhihe Liu,†,# Hua Tan,†,# Jianping Xin,† Jiazhi Duan,† Xiaowen Su,† Pin Hao,§ Junfeng Xie,§ Jie Zhan,† Jing Zhang,∥ Jian-Jun Wang,*,† and Hong Liu*,†,‡ ACS Appl. Mater. Interfaces 2018.10:3699-3706. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 01/23/19. For personal use only.

†

State Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100, China Institute for Advanced Interdisciplinary Research (IAIR), University of Jinan, Jinan, Shandong 250022, China § College of Chemistry, Chemical Engineering and Materials Science, Shandong Normal University, Jinan 250014, China ∥ Beijing Synchrotron Radiation Facility (BSRF), Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China ‡

S Supporting Information *

ABSTRACT: Transition-metal nitrides have attracted a great deal of interest as electrocatalysts for water splitting due to their super metallic performance, high efficiency, and good stability. Herein, we report a novel design of hierarchical electrocatalyst based on Ni3FeN, where the presence of carbon fiber cloth as a scaffold can effectively alleviate the aggregation of Ni3FeN nanostructure and form three-dimensional conducting networks to enlarge the surface area and simultaneously enhance the charge transfer. The composition and morphological variations of NiFe precursors during annealing in different atmospheres were investigated. Such Ni3FeN/CC hierarchical electrocatalyst shows much improved electrochemical properties for water splitting in terms of overpotentials (105 and 190 mV at 10 mA/cm2 for hydrogen evolution reaction and oxygen evolution reaction, respectively) and stability. KEYWORDS: electrochemical water splitting, intermediate, 3D conductive networks, hierarchical electrocatalyst, synergistic effect

1. INTRODUCTION Hydrogen from water splitting has been considered as a promising eco-friendly alternative to fossil energy for the everincreasing global energy crisis.1−6 Electrolysis of water with high-performance electrocatalysts has driven considerable attention because of its eco-friendliness, low costs, and convenience.7−11 A number of nanomaterials have been explored for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) such as Pt-based and IrO2-based electrocatalysts. 12−14 However, the high cost and low abundance limit their large-scale and widespread deployment. Recently, earth-abundant and non-noble-metal-based alternatives including metal oxides, carbides, phosphides, sulfides, selenides, and nitrides are increasingly attracting investigation for their inexpensiveness, high efficiency, and good stability.15−23 Transition-metal nitrides have been explored for electrocatalysis because of their superior metallic performance, remarkable stability, and the fact that they contain all earthabundant elements. The modification of the density of states in d-band of the metal atoms by introducing N atoms allows for fast electron transfer and corrosion resistance.17,24,25 Further© 2018 American Chemical Society

more, the introduction of additional cationic atom provides a powerful way to tune the valence and electronic states and can further boost the performance significantly.26−29 For instance, doping cobalt or nickel to molybdenum nitride has largely reduced overpotentials to enhance their electrocatalytic activity.30,31 Recently, Ni3N representing a new class of candidates has been demonstrated as efficient electrocatalyst for OER and HER.32−34 Incorporating Fe can further reduce overpotential and enhance the electrocatalysis performance, however, which will complicate the synthesis because of different reactivity and phase separation. Though great progress on transition-metal nitrides as electrocatalysts has been made, there are few reports to reveal the detailed synthesis mechanism of the thermal nitridation process.35 In general, a large surface area is highly desirable for catalysts to facilitate charge transfer for heterogeneous catalysis. To address this, various nanostructures have been reported, and, however, the less contact Received: December 7, 2017 Accepted: January 9, 2018 Published: January 9, 2018 3699

DOI: 10.1021/acsami.7b18671 ACS Appl. Mater. Interfaces 2018, 10, 3699−3706

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ACS Applied Materials & Interfaces

Figure 1. (a) Schematic of preparation process of Ni3Fe, Ni3FeN, and NiFeOx grown on carbon cloth. SEM images of (b, c) Ni3FeN/CC, (d, e) Ni3Fe/CC, and (f, g) NiFeOx/CC. and Pt/C (20 wt %) were provided by Sigma-Aldrich. Carbon cloth was purchased from Carbon Energy of Taiwai, China. All chemical reagents used in this work are of analytical grade. 2.2. Synthesis of Ni3FeN on Carbon Cloth (Ni3FeN/CC). A piece of carbon cloth (CC, 1 cm × 5 cm) was carefully treated with sonication for 10 min in acetone, ethanol, and deionized water, successively. The NiFe precursors were prepared using electrodeposition under −1.0 V (vs SCE) upon carbon cloth in a 6 mM Ni(NO3)2·6H2O and 2 mM Fe(NO3)3·9H2O solution, then washed several times with deionized water and absolute ethanol, and finally dried naturally. The composite was annealed at 400 °C for 2 h in a NH3 atmosphere. On average, 0.26 mg/cm2 of Ni3FeN was loaded on each carbon cloth. The control experiments for Ni precursors and Fe precursors were prepared using the same procedure. The effect of an annealing atmosphere was investigated in H2/N2, NH3, and N2. 2.3. Characterization. X-ray power diffraction (XRD) patterns were recorded on a Bruke D8 Advance Power X-ray diffractometer at 40 kV and 40 mA for monochromatized Cu Kα (λ = 0.15406 nm). Field-emission scanning electron microscopy (FESEM, Hitachi S4800) and NoVaTM Nano SEM 250 were used to study the morphology and size of the samples. The transmission electron microscopic (TEM) images were acquired with a JEOL JEM 2100 microscope operating at 200 kV. The chemical components were measured by the energy-dispersive X-ray spectroscopy (EDX). X-ray

between nanoparticles and collapse of the nanostructures lead to poor conductivity and stability. Herein, we propose and realize a rational design of Ni3FeN nanostructure on carbon cloth to provide a large surface area and address the aggregation issue. Carbon cloth was selected as a scaffold to form 3D conductive networks and promote electronic conductivity. Electrodeposition was used to prepare Ni3FeN for the composite, which is easy and efficient for upscale production. The formation process of bimetallic Ni3FeN has been studied, indicating that the Ni3Fe as the intermediate during the nitridation process determined the final morphology of Ni3FeN nanoframework and a possible mechanism is proposed. Compared with Ni3N and Fe2N, the synergistic effect of both cationic atoms contributes to enhancing electrocatalysis performance. Electrochemical measurement revealed the composite not only delivers improved electrocatalytic activity but also shows remarkable stability.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. Fe(NO3)3·9H2O, Ni(NO3)2·6H2O, and KOH were purchased from National Reagent Company. RuO2 3700


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Figure 2. Structural and composition characterization: (a) Typical TEM image of Ni3FeN. (b) HRTEM image of Ni3FeN (inset of corresponding FFT pattern). (c) XRD pattern of Ni3FeN/CC. High-resolution XPS spectra of (d) N 1s, (e) Fe 2p, and (f) Ni 2p. photoelectron spectroscopy (XPS) was performed on an ESCALAB 250. 2.4. Electrochemical Measurements. The electrochemical measurements were performed on a CHI660E electrochemical workstation in a three-electrode cell with platinum plate as counter electrode and saturated Ag/AgCl as reference electrode. The linear sweep voltammetry curves were recorded at a scan rate of 5 mV s−1 with deaerated 1 M KOH solution as the electrolyte without IR correction. The electrochemical impedance spectroscopy (EIS) measurements were conducted over a frequency range of 0.01−105 Hz. The long-term durability tests were carried out at −0.18 and 1.48 V vs RHE, respectively, using the chronopotentiometric measurements. The CV cycling test was conducted at different sweep rate. The measured potentials were calibrated to RHE according to the following equation: E(RHE) = E (Ag/AgCl) + 0.197 + 0.059 pH. One milliliter of homogeneous catalyst ink was obtained by sonication for 20 min, consisting of 5 mg of Pt/C or RuO2, 20 μL of Nafion solution (5%), and 980 μL of absolute ethanol. All the catalyst ink was dropped on the glassy carbon electrode (5 mm diameter with loading of ∼0.5 mg cm−2).

the NiFe precursor formed nanoflakes vertically standing on the surface of the carbon fiber and connected to each other to form flower-like networks (thickness of ∼10 nm). Ni3FeN was obtained by treating NiFe precursors in a NH3 atmosphere at 400 °C where the precursors tend to decompose indicated by TG curves (Figure S3). Figure 1b,c shows the nanoflakes were transformed into interlaced sheets consisting of interconnected nanospheres forming three-dimensional (3D) porous nanoframework, endowing the electrode large surface area for electrochemical reactions. The morphology changes from the intercrossed wall networks to porous nanoframework draw our attention to the mechanism of the transformation from NiFe precursors to NiFe3N because the spherical morphology is supposed to be derived from a droplet state, while the melting points of both NiFe oxides and NiFe nitrides are over 1000 °C. Therefore, it signifies that there is some intermediate with low melting point forming during the nitridation process. As the melting point of metal nanoparticles decreases significantly because of the nanosize effect, we speculate that the alloy of Ni and Fe is the intermediate.36 To investigate the mechanism and process of the transformation and confirm the intermediate between NiFe precursors and Ni3FeN, the NiFe precursors were treated at the same temperature in N2 and N2/H2,

3. RESULTS AND DISCUSSION NiFe precursors were in situ grown on carbon fiber cloth with a diameter of ∼800 nm (Figure S1) through a facile electrodeposition as illustrated in Figure 1a. Figure S2a,b show that 3701


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Figure 3. HER and OER performance of Ni3FeN/CC and Ni3Fe/CC in 1 M KOH. (a) Linear sweep voltammetry (LSV) of HER. (b) LSV of OER.

Figure 4. HER and OER performance in 1 M KOH. (a, b) LSV curves for HER without IR correction and the corresponding Tafel plots of all samples in 1 M KOH. (c) EIS Nyquist plots at −0.32 V. (d) Overpotential and Tafel slope of some recently reported non-noble electrocatalysts for HER. (e, f) LSV curves for OER without IR correcttion and the corresponding Tafel plots. (g) EIS at 1.48 V for different electrocatalysts. (h) Overpotential and Tafel slope of some recently reported non-noble electrocatalysts for OER.

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The performance of hybrid electrodes based on the parent monometallic metal nitrides of Ni3N/CC, Fe2N/CC, commercial RuO2, and Pt/C (20 wt %) loaded on glassy carbon electrode was also studied for comparison. (Figures S10−S13). Figure 4a shows that the Ni3N/CC and Fe2N/CC electrodes have the overpotentials of 178 and 172 mV, respectively. Ni3FeN/CC displays the lowest overpotential (105 mV at the current density of 10 mA cm−2) which is only 63 mV higher than that of Pt/C. The Tafel slop was fitted to assess HER kinetics according to the LSV curves. Figure 4b shows that Ni3FeN/CC has a smaller Tafel slope of 61 mV dec−1 than that of the Ni3N/CC (87 mV dec−1) and Fe2N/CC (121 mV dec−1), which is close to that of Pt/C (38 mV dec−1). The low Tafel slop implies the efficient charge transportation and transfer. To understand the mechanism of the improved electrochemical properties of Ni3FeN/CC, an electrochemical impedance spectroscope (EIS) was used to investigate the charge transportation and transfer. As shown in Figure 4c, the EIS Nyquist plot of Ni3FeN/CC measured at −0.33 V shows an arc with a smaller diameter than that of Ni3N/CC and of Fe2N/CC, indicating a smaller charge-transport resistance. The EIS results signify that Ni3FeN/CC has the fastest chargetransfer process, coincident with the best HER performance and the smallest Tafel plot. In general, the OER process is considered as the limiting step for the efficiency of overall water splitting. We further investigated the OER activity of Ni3FeN/CC along with Ni3N/CC, Fe2N/CC electrodes, RuO2, and pure carbon cloth for comparison. Figure 4e shows that the Ni3FeN/CC displays the highest OER activity with an overpotential of 190 mV at the current density of 10 mA cm−2, which is much less than that of Ni3N/CC (340 mV) and Fe2N/CC (400 mV). In addition, the RuO2 displayed an overpotential of 364 mV at 10 mA cm−2. Therefore, the Ni3FeN/CC exhibited better performance for OER than RuO2 in our study. From the analysis of the Tafel plot, the Tafel slope of Ni3FeN/CC (72 mV dec−1) was the smallest, compared with Ni3N/CC (112 mV dec−1), Fe2N/CC (128 mV dec−1), and RuO2 (94 mV dec−1), indicating the fastest charge-transfer process (Figure 4f). As shown in Figure 4g, Ni3FeN/CC has an arc with the smallest diameter, further confirming the lowest charge-transfer resistance during the electrochemical reactions. These results manifest that the combination of Fe and Ni-based nitrides can strongly improve the activity of the electrocatalysts for HER and OER because of the synergistic effect. To understand the improved performance of Ni3FeN/CC, the electrochemically active surface area (EASA) was studied by measuring the electrochemical double-layer capacitance (Cdl). As shown in Figure S14, the Ni3FeN/CC has a higher Cdl (52.55 mF cm−2) than that of Ni3N/CC (44.13 mF cm−2) and Fe2N/CC (39.32 mF cm−2), which is proportional to EASA, indicating the increased electrochemical surface area leads to higher exposure of active sites for reactions and contributes to the improved performance. To clarify the effect of carbon substrate, we also tested the HER and OER performance of pure carbon fiber cloth electrode in 1 M KOH. The results in Figure 4a,b indicate that the carbon cloth has slight contribution to the performance and confirm that the active component of the hybrid electrode is dominated by Ni3FeN. For HER, the hybrid electrode of Ni3FeN/CC shows a smaller overpotential than that of some typical Ni-based electrocatalysts (Figure 4d) because of the optimizing of the electrical properties by introducing Fe atoms,

respectively. The morphology of the product treated in H2/N2 was similar to that of Ni3FeN/CC (Figure 1d,e), while the product treated in N2 keeps the flower-like morphology (Figure 1f,g). The XRD results in Figure S4 indicated that the NiFe precursor has been reduced to Ni3Fe in H2/N2. These results suggest that NH3 reduced the NiFe precursors to form Ni3Fe intermediate, and further reacted to form Ni3FeN nanostructures. The structure of Ni3FeN was further confirmed by the analysis of transmission electron microscopy (TEM) and X-ray diffraction (XRD). A typical TEM image of Ni3FeN in Figure 2a demonstrates that the Ni3FeN nanoframework consists of interconnected nanoparticles, consistent with the results of SEM. The high-resolution TEM (HRTEM) image in Figure 2b taken from a single nanosphere marked by the red square in Figure 2a displays clear lattice fringes, corresponding to the (111) plane of cubic Ni3FeN, calculated from the corresponding fast Fourier transformation pattern (inset of Figure 2b). To identify the element distribution of the as-synthesized Ni3FeN, the typical high-angle annular dark-field scanning TEM (HAADF-STEM) element maps of Ni3FeN were recorded. Representative results in Figure S5 show that the three elements of Ni, Fe, and N were distributed homogeneously within all the nanocrystals and no apparent element separation or aggregation was observed. The XRD pattern in Figure 2c confirms that the peaks at 41.52°, 48.34°, and 70.81° can be indexed to (111), (200), and (220) planes of the cubic Ni3FeN (JCPDS card no. 50-1434).37 X-ray photoelectron spectroscopy (XPS) analysis was used to study the chemical states of the elements. For the N 1s spectrum in Figure 2d, the typical peak at 397.6 eV is in good agreement with the reported value for nitrides.18 The Fe 2p spectrum in Figure 2e was split into two regions, Fe 2p1/2 and Fe 2p3/2, with four peaks: the peaks at 706.8 and 719.6 eV are ascribed to Fe0.38 The peaks at 711.3 and 723.6 eV can be assigned to Fe3+. For the Ni 2p spectrum in Figure 2f, the peaks at 853.3 and 873.9 eV corresponding to Ni2+ are dominant with two satellite peaks located at 862.6 and 880.5 eV.39 A couple of weak peaks for Ni0 was also observed. The presence of the metallic species can effectively facilitate the charge transportation and enhance the electrical conductivity. To evaluate their potential application in water splitting, the HER and OER performance of the electrodes made from the composites was studied in a standard three-electrode system in 1 M KOH. As shown in Figure 3a, the Ni3FeN/CC displays a smaller overpotential of 105 mV to reach a current density of 10 mA cm−2 than that of the Ni3Fe/CC (180 mV), and for OER the Ni3FeN/CC exhibits a reduced overpotential of 190 mV at 10 mA cm−2, much better than that of the Ni3Fe/CC (270 mV) (Figure 3b). These results indicated that the introduction of N atoms into the metal structures can facilitate the HER and OER process. Furthermore, the HER and OER performance of NiFe-precursor/CC and NiFeOx/CC were also conducted in 1 M KOH. By contrast, Ni3FeN/CC exhibits better performance for HER and OER than that of NiFeprecursor/CC and NiFeOx/CC. (For more details see Figures S6−S9.) Therefore, the improved performance can be ascribed to the nitridation. The introduction of N atoms modulated the electronic structure and contributed to adsorption of protons to the surface of metal nitrides during the electrochemical process and the strong interaction between the d orbital of the metal and the continuous p orbital of N atoms also protected the surface of the electrodes from corrosion and promoted stability. 3703


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Figure 5. (a) Current−potential curve measured in 1 M KOH using Ni3FeN/CC as both anode and cathode. (b) Theoretical and measured amount of H2 and O2 over reaction time.

and after the stability test, Figure S17 shows that Ni3FeN maintained good crystallinity. On the other hand, the SEM image of Ni3FeN after stability test shows that the original 3D structures of Ni3FeN were maintained (Figure S18). All the above results manifest that the Ni3FeN/CC hybrid electrode outperforms the parent monometallic nitrides and other samples studied here as electrodes for HER and OER, respectively, and shows several advantages from fabrication to morphology and to properties. First, the hybrid electrode was prepared via electrodeposition, which is readily efficient and can greatly reduce the cost for large-scale production. Second, the in situ growth of Ni3FeN on carbon cloth allows for extremely superior contact between the active component and the conductive substrate, avoiding employing the nonconductive polymer as binder and promotes the charge transportation and transfer. Third, the 3D hierarchical nanostructure of the hybrid Ni3FeN/CC offers extremely large surface area for HER and OER to take place. Finally, the combination of Ni and Fe in the structure provides the flexibility to tune their electrical and chemical properties. The high electrochemical performance and environmental stability make the hybrid Ni3FeN/CC electrode a promising electrode material for overall water splitting.

while the overpotentials and the Tafel slope still need improving compared with typical electrocatalysts like Co−P and MoO2. For OER, Ni3FeN/CC exhibits promising electrochemical properties with a smaller overpotential than that of the typical electrocatalysts of Co−P and MoO2 (Figure 4h). To evaluate the potential application of the designed Ni3FeN/CC hybrid electrode for overall water splitting, Ni3FeN/CC electrodes were employed as both anode and cathode in a single electrochemical cell (Figure 5). The hydrogen and oxygen bubbles can be clearly seen on the surface of anode and cathode when the applied potential approached 1.4 V. As the applied potential increased, the amount of gas bubbles significantly increased accordingly. The amount of H2 and O2 was recorded by automatic online trace gas analysis system-gas chromatography (the equipment diagram in Figure S15). The electrochemical cell generates H2 and O2 in a steady monitored rate with a ratio of 1.98, close to the theoretical value of 2. The ratios between the measured and theoretical gas evolution rates gave a faradaic efficiency of ∼100% for HER and OER, respectively. Finally, for practical application, the environmental durability is a key parameter. The stability of Ni3FeN/CC electrode was examined for HER and OER by continuous chronoamperometric response (i-t) in 1 M KOH. For HER, at −0.18 V vs RHE, an initial current of −10.9 mA cm−2 was measured, and the current was kept nearly the same without any decay after 20 h. For OER, an initial current of 11.6 mA cm−2 was obtained at 1.48 V vs RHE, and the current stayed at 11.6 mA cm−2 after 20 h. These results indicate Ni3FeN/CC as electrodes for water splitting exhibits remarkable stability (Figure 6). The TEM image of Ni3FeN after OER stability test shows that the clear interface between NiFeOx and Ni3FeN can be seen in Figure S16. Compared with the XRD pattern of Ni3FeN/CC before

4. CONCLUSION In summary, a new facile method for Ni3FeN-based 3D hierarchical hybrid electrode for HER and OER has been developed. We have shown how the annealing atmosphere affects the growth and ultimately the phase morphology and the performance of the electrodes. XRD, TEM, EDS, and XPS measurements confirmed that the active component of the hybrid electrode was pure single phase of Ni3FeN. The investigation of the formation mechanism suggests that the NiFe precursors were reduced first by NH3 to form Ni3Fe as the intermediate, leading to the morphology change, and further reacted to form Ni3FeN. These insights allow for a greater understanding of growth and morphology change in nitridation process to form nitrides for a number of potential applications from electronics to energy storage devices. The Ni3FeN/CC electrode exhibits improved electrochemical performance with much less overpotentials for HER and OER, respectively, because of the synergistic effect of Fe and Ni. The high electrochemical performance and environmental stability demonstrate the ability of the hybrid Ni3FeN/CC electrode for overall water splitting. The facile synthesis procedure opens pathways for extension to other compositions of nitrides, phosphides, and oxides where the change of the morphology can be explored.

Figure 6. Stability of Ni3FeN/CC as anode and cathode for HER and OER. 3704


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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b18671. SEM images of carbon cloth, NiFe precursors, Fe precursor, Fe2N on carbon cloth, Ni precursor, and Ni3N on carbon cloth; XRD patterns of Ni3Fe, Fe2N, and Ni3N; TG curves analysis; TEM elemental mapping of Ni3FeN; CV curves for electrochemical active surface area; diagram of the automatic online trace gas analysis system-gas chromatography (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Hong Liu: 0000-0003-1640-9620 Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the National Natural Science Foundation of China (Grant Nos. 51732007, 51372142, and 51372138), the Innovation Research Group (IRG: 51321091), the Fundamental Research Funds of Shandong University (2015JC017), and the support of QiLu Young Scientist Program of Shandong University.

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