Metallic intermediate phase inducing morphological transformation in

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Metallic intermediate phase inducing morphological transformation in thermal nitridation: Ni3FeN based 3D 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, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18671 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 11, 2018

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ACS Applied Materials & Interfaces

Metallic intermediate phase inducing morphological transformation in thermal nitridation: Ni3FeN based 3D 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*,†, Hong Liu*,†, ‡

† State

Key Laboratory of Crystal Materials, Shandong University, Jinan, Shandong 250100

(China). ‡Institute

for Advanced Interdisciplinary Research (IAIR), University of 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) * Corresponding author Email: [email protected](JJ.Wang)[email protected](H. Liu) ⊥These

authors contributed equally to this work

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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 scaffold can effectively alleviate the aggregation of Ni3FeN nanostructure and form 3D 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 atmosphere were investigated. Such Ni3FeN/CC hierarchical electrocatalyst shows much improved electrochemical properties for water splitting in terms of overpotentials (105 mV and 190 mV at 10 mA/cm2 for HER and OER, 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 ever-increasing global energy crisis.1-6 Electrolysis of water with high performance electrocatalysts has driven considerable attention due to its eco-friendless, 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, nitrides are increasingly attracting investigation for their inexpensiveness, high efficiency and good stability.15-23 Transition metal nitrides have been explored for electrocatalysis due to their superior metallic performance, remarkable stability and containing all earth-abundant 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 Furthermore, the introduction of additional cationic atom provides a powerful way to tune the 2


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valence and electronic states and can further boost the performance significantly.26-29 For instance, doping cobalt or nickel to molybdenum nitride has largely reduce 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 Incorperating Fe can further reduce overpotentials and enhance the electrocatalysis performance, however, which will complicate the synthesis due to different reactivity and phase separation. Though the great progress on transition metal nitrides as electrocatalysts has been made, there are few reports to reveal the detailed synthesis mechanism of thermal nitridation process.35 In general, 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 between nanoparticles and collapse of the nanostructures leads to poor conductivity and stability. Herein, we propose and realize a rational design of Ni3FeN nanostructure on carbon cloth to provide large surface area and address the aggregation issue. Carbon cloth was selected as scaffold to form 3D conductive networks and promote the electronic conductivity. Electrodeposition was used to prepare Ni3FeN for the composite, which is easy and efficient for up-scale 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 an 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 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 3



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work are of analytical grade. 2.2 Synthesis of Ni3FeN on carbon cloth (Ni3FeN/CC). A piece of carbon cloth (CC, 1cm×5cm) was carefully treated with sonication for 10 min in acetone, ethanol and deionized water, successively. The NiFe-precursors were prepared using electrodeposition under -1.0V (vs. SCE) upon carbon cloth in a 6 mM Ni(NO3)2·6H2O and 2 mM Fe(NO3)3·9H2O solution, and then washed several times with deionized water and absolute ethanol, and finally dried naturally. The composite was annealed at 400 oC for 2 h in NH3 atmosphere. In average, 0.26 mg/cm2 of Ni3FeN were loaded on each carbon cloth. The control experiments for Ni-precursors and Fe-precursors were prepared using the same procedure. The effect of 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 S-4800) 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 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 1M KOH solution as the electrolyte without

IR-corrected.

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 -0.18V and 1.48 V vs RHE, respectively, using the 4


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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. 1 mL homogeneous catalyst

ink was obtained by sonication for 20 min, consisting of 5mg Pt/C or RuO2, 20 µL Nafion solution (5%) and 980 µL and absolute ethanol. All the catalyst ink was dropped on the glassy carbon electrode (5 mm diameter with loading of ~0.5 mg cm-2)

3. Results and discussions

Figure 1 (a) Schematic of preparation process of Ni3Fe, Ni3FeN and NiFeOx grown 5



on carbon cloth. SEM images of (b, c) Ni3FeN/CC (d, e) Ni3Fe/CC (f, g) NiFeOx/CC. 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 and 2b show that the NiFe-precursor formed nanoflakes vertically standing on the surface of the carbon fiber and connected each other to form flower-like networks (thickness of ~10 nm). Ni3FeN was obtained by treating NiFe-precursors in NH3 atmosphere at 400 oC where the precursors tend to decompose indicated by TG curves (Figure S3). Figure 1b and 1c show the nanoflakes were transformed into interlaced sheets consist of interconnected nanospheres forming 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 derive from a droplet state, while the melting points of both NiFe-oxides and NiFe-nitrides are over 1000 oC. 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 due to nanosize effect, we speculate that the alloy of Ni and Fe is the intermediate.36 In order 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, respectively. The morphology of the product treated in H2/N2 was similar to that of Ni3FeN/CC (Figure 1d and 1e), while the product treated in N2 keep the flower-like morphology (Figure 1f and 1g). 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 6


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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 a 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 homogenously 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

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Figure 2 Structural and composition characterization (a) 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 (f) Ni 2p.

X-ray photoelectron spectroscopy (XPS) analysis was used to study the chemical states of the elements. For the N1s 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 eV and 719.6 eV are ascribed to Fe0.38 The peaks at 711.3 eV and 723.6 eV 8


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can be assigned to Fe3+. For Ni 2p spectrum in Figure 2f, the peaks at 853.3 eV and 873.9 eV corresponding to Ni2+ are dominant with two satellite peaks located at 862.6 eV 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.

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

In order 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 1M KOH. By contrast, Ni3FeN/CC exhibits better performance for HER and OER than that of NiFe-precursor/CC and NiFeOx/CC (For more details see Figure S6-9). Therefore, the improved performance can be ascribed to the nitridation. The introduction of N atoms modulated the electronic structure and contributed to adsorb 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 9



atoms also protected the surface of the electrodes from corrosion and promoted stability.

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

The performance of hybrid electrodes based on the parent monometallic metal nitrides 10


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of Ni3N/CC, Fe2N/CC, commercial RuO2 and Pt/C (20 wt%) loaded on glassy carbon electrode was also studied for comparison. (Figure S10-S13). Figure 4a shows that The

Ni3N/CC and Fe2N/CC electrodes have the overpotential of 178 mV and 172 mV, respectively. Ni3FeN/CC displays the lowest overpotential (105 mV at the current density of 10 mA cm−2) which is only 63mV higher than that of Pt/C. The Tafel slop was fitted to assess HER kinetic 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, 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 -0.33V 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 charge transfer 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 11



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 ECSA, indicating the increased electrochemical surface area leads to higher exposure of active sites for reactions and contributes to the improved performance.

In order to clarify the effect of carbon substrate, we also tested the HER and OER performance of pure carbon fiber cloth electrode in 1M KOH. The results in Figure 4a and 4b 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, 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)

Figure 5 (a) Current−potential curve measured in 1M KOH using Ni3FeN/CC as both anode and cathode. (b) The theoretical and measured amount of H2 and O2 over reaction time. 12


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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. The hydrogen and oxygen bubbles can be clearly seen on the surface of anode and cathode when the applied potential approached 1.4V. 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.

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

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 -0.18V 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 20h. For OER, an initial current of 11.6 mA cm-2 was obtained at 1.48V vs RHE, and the current stayed at 11.6 mA cm-2 after 20h. 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 13



clear interface between NiFeOx and Ni3FeN can be seen in Figure S16. Compared with the XRD pattern of Ni3FeN/CC before and after the stability test, Figure S17 shows that Ni3FeN kept the good crystalline. 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, morphology, 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 non-conductive 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 as a promising electrode material for overall water splitting. 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 was 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 14


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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, due to 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. ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publications website. 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. The TEM elemental mapping of Ni3FeN. CV curves for Electrochemical active surface area. The diagram of the Automatic online trace gas analysis system-gas chromatography.

AUTHOR INFORMATION Corresponding Author *Email: [email protected] *Email: [email protected] ORCID Zhihe Liu 0000-0001-5921-4351 Hong Liu 0000-0002-4110-6333 15



Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are thankful for the National Natural Science Foundation of China (Grant No. 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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A 3D Ni3FeN based hierarchical bifunctional electrocatalyst with much improved electrochemical performance mediated by the intermediate and remarkable stability has been designed and synthesized 416x250mm (300 x 300 DPI)


Metallic intermediate phase inducing morphological transformation in

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