NixFe1âxOOH Nanotube Arrays

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In-Situ Derived NixFe1-xOOH/NiFe/NixFe1-xOOH Nanotube Arrays from NiFe Alloys as Efficient Electrocatalysts for Oxygen Evolution An-Liang Wang, Yu-Tao Dong, Mei Li, Chaolun Liang, and Gao-Ren Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b10609 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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ACS Applied Materials & Interfaces

In-Situ Derived NixFe1-xOOH/NiFe/NixFe1-xOOH Nanotube Arrays from NiFe Alloys as Efficient Electrocatalysts for Oxygen Evolution An-Liang Wang,† Yu-Tao Dong,† Mei Li,‡ Chaolun Liang,‡ and Gao-Ren Li†* †

MOE Laboratory of Bioinorganic and Synthetic Chemistry, The Key Lab of Low-carbon Chemistry &

Energy Conservation of Guangdong Province, School of Chemistry, Sun Yat-sen University, Guangzhou

510275, China ‡

Instrumental Analysis and Research Centre, Sun Yat-Sen University, Guangzhou 510275, China

E-mail: [email protected]

ABSTRACT Herein, NixFe1-xOOH/NiFe/NixFe1-xOOH sandwich-structured nanotube arrays (SNTAs) supported on carbon fiber cloth (CFC) (NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC) have been developed as flexible high-performance OER catalysts by a facile in-situ electrochemical oxidation of NiFe metallic alloy nanotube arrays (ANTAs) during oxygen evolution process. Benefiting from the advantages of high conductivity, hollow nanotube array and porous strucuture, NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC exhibited a low overpotential of ~220 mV at the current density of 10 mA cm-2 and a small Tafel slope of 52 mV dec-1 in alkaline solution, which both are smaller than those of most OER electrocatalysts. Furthermore, NixFe1-xOOH-/NiFe/NixFe1-xOOH SNTAs-CFC exhibits excellent stability at 100 mA cm-2 for more than 30 h. It is believed that the present work can provide a valuable route for the design and synthesis of inexpensive and efficient OER electrocatalysts.

Keywords: NiFe alloy; NiFe-oxyhydroxide; electrocatalyst; flexible; oxygen evolution reaction

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INTRODUCTION Oxygen evolution reaction (OER) is the bottleneck or rate-determining step of water splitting for a sustainable hydrogen economy.1-4 What‘s more, OER is also a key half-reaction of metal-air batteries and CO2 redcution raction.5-9 However, the four electron process of OER (4OH− - 4e- → 2H2O + O2, in base) is not favored kinetically, which needs catalysts to expedite the reaction and reduce the overpotential.10-16 Up to now, the state-of-art electrocatalysts for OER are usually IrO2 and RuO2. However, the scarcity on the earth and high cost make them not suitable for the industrial applications. Enormous studies have focused on seeking earth-abundant and low-cost electrocatalysts to replace IrO2 and RuO2.17-22 Among various non-precious electrocatalysts, NiFe-based oxides and (oxy)hydroxides have been known to be electroactive for OER.22-27 However, there are two main obstacles hindering the application of NiFe-based electroctalysts. Firstly, the semiconductor property of these NiFe-based electrocatalysts will cause large resistance and need the additional overpotentials to overcome the energy barriers of OER, which largely blocks the application of NiFe oxides and (oxy)hydroxides.32-34 Up to now, many strategies, such as doping another metal into electrocatalysts35-36 or combining carbon materials with electrocatalysts.18,30,37-39 have been taken to solve this problem, However, combined with the carbon material or doping strategy, the part of the active site of catalyst is often blocked and the performance enhancement is uaually limited. Secondly, most reported NiFe-based electrocatalysts exhibits small specific surface area,which causes less exposed active sites and low untilization efficiency of active sires. Therefore, it is ugrent to explore a new strategy for the development of high-performance NiFe-based OER electrocatalysts with more exposed active sites. Based on the above facts, here we designed novel NixFe1-xOOH/NiFe/NixFe1-xOOH sandwich-structured nanotube arrays (SNTAs) supported on carbon fiber cloth (CFC) (NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC) as high-performance OER electrocatalysts by facile in-situ electrochemical oxidation of NiFe alloy nanotube arrays (ANTAs)/CFC during the oxygen evolution. The NixFe1-xOOH/NiFe/NixFe1xOOH

as electrocatalysts show following advantages: (i) the metallic NiFe alloys can well solve the poor

conductivity of NixFe1-xOOH SNTAs and will bring efficient and fast transfer of electrons between the 2 Plus Environment ACS Paragon

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catalyst surface and current collector; (ii) the composite SNTAs grown on the surface of CFC directly can avoid the usage of binder and the existence of dead volume thus will enhance the utilization ratio of catalysts; (iii) the large surface area provided by SNTAs and porous nantobe wall will be beneficial for the improvement of catalyst utilization, electrolyte penetration and electroactive species diffusion. Here NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC exhibits excellent electrocatalytic activity and stability for OER in alkaline meida, such as a low overpotential of 220 mV at 10 mA cm-2, a small Tafel slope of 52 mV dec-1, and a high TOF value of 0.0453 s-1 at the overpotential of 350 mV. Our work would open a new route for the design of highly efficient OER electrocatalysts based on alloy materials.

RESULTS AND DISCUSSION The fabrication procedure of NiFe ANTAs-CFC is shown in Scheme 1, and the details are described in the experimental section. SEM image of NiFe ANTAs (Figure 1a) shows NiFe ANTAs uniformly stand on CFC and they are seperate each other. The inset in Figure 1a shows SEM image of a broken NiFe alloy nanotube, and the hollow structure is clearly seen. TEM image of a typical NiFe alloy nanotube is shown in Figure 1b. The wall thickness of nanotube is about 20 nm. In addition, it is clearly seen that the nanotube wall is porous and consisted with nanoparticles. The nanotube arrays with porous structure can provide large specific area. Energy dispersive specturm (EDS) pattern of NiFe ANTAs in Figure S2 shows the peaks of Ni and Fe, and Ni/Fe atomic ratio is determined to be about 1:1. HAADF-STEM image of a typical NiFe nanotube is shown in Figure 1c, which also shows NiFe nanotube is consisted of nanoparticles. HAADF-STEM mappings in Figure 1e and 1f confirm the homogenous distributions of Ni and Fe, respectively. High-resoultion TEM (HRTEM) image in Figure 1g clearly shows lattice fringes and they are measured to be 2.03, 0.144 and 0.117 Å, which are consistent with (110), (200) and (211) lattice planes of NiFe alloys, respectively. SAED pattern in Figure 1h indicates that the NiFe alloys are polycrystalline, and the diffraction pattern can be indexed to (110), (200), and (211) planes. The phases of NiFe ANTAs are charaterized by XRD as shown in Figure 1i, which shows all the diffraction peaks match well with NiFe alloy phases except for the peak at 26 oC that corresponds to carbon fiber cloth 3 Plus Environment ACS Paragon


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(PDF 37-0474). The above results indicate NiFe alloys were sucessfully synthesized. In order to further investigate the chemical state of NiFe ANTAs-CFC, X-ray photoelectron spectroscopy (XPS) of NiFe ANTAs-CFC was measured. In the high resolution Ni 2p spectrum in Figure 2a, the peaks at 870.29 eV and 853.05 eV corresponds to Ni 2p1/2 and 2p3/2, respectively, which indicates the metal Ni in NiFe ANTAs-CFC. In XPS spectrum of Fe 2p in Figure 2b, the peaks at 707.10 and 720.35 eV indicate the metal Fe in NiFe ANTAs-CFC. Therefore, all the above results clearly suggest the successful fabriction of NiFe ANTAs-CFC. According to the well-accepted OER mechanism, oxyhydroxides are the active sites for transition metal oxides, hydroxides, sulfides, nitrides, phosphides. Therefore, we wonder if NiFe alloys on the surface of catalyst can be converted to NixFe1-xOOH during the OER catalysis and the NixFe1xOOH/NiFe/NixFe1-xOOH

SNTAs can be formed. Bringing this question, the SEM, TEM, XPS

measurements of NiFe ANTAs after many polarization curve scans were performed. Figure 2c shows the polarization curves after different cycles, which shows the polarization curves almost keep unchangeable after 30 cycles, indicating the NixFe1-xOOH layer keeps unchangeable after 30 cycles. SEM image shows a similar surface morphology to NiFe ANTAs-CFC as shown in Figure 3a. In order to further investigate the structure, TEM measurements were conducted as shown in the Figure 3b, which shows that the surfaces of NiFe alloy nanotubes are oxidized to NixFe1-xOOH. The nanotube wall marked with red rectangle in Figure 3b is further magnified, and it shows the obvious sandwiched structure and the thickness of NixFe1-xOOH layer is about 10 nm as shown in Figure 3c. The middle layer still is NiFe alloys with thickenss of about 30 nm. HRTEM image of NixFe1-xOOH layer marked with yellow in Figure 3c clearly exhibits lattice fingers as shown in Figure 3d. The interplanar distances of 0.245 and 0.225 nm are indexed to FeOOH (301) and NiOOH (101) planes, respectively. The SAED in Figure 3d also proves the existences of NiOOH and FeOOH. HAADF-STEM and element mappings further indicate the change of compositions after in-situ transformation as shown in Figure 3e-h. From NiFe to NiFe-NixFe1-xOOH, Ni and Fe remain homogeneous distributions as shown in Figure 3f and 3g, respectively, and the oxygen was uniformly incorporated into nanotube as shown in Figure 3h. The formation of NixFe1-xOOH was 4 Plus Environment ACS Paragon

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further confirmed by the high-resolution XPS spectra of catalysts after activation test as shown in Figure 2a-b (red curves) (XPS result is usually achieved from surface layer of ~10 nm). In Ni 2p spectrum, the peaks at 856.05 eV and 873.72 eV, which refer to Ni 2p3/2 and 2p1/2, respectively, indicates the existence of NiOOH.41 In Fe 2p spectrum, the peaks of Fe 2p1/2 at 725.40 eV and Fe 2p3/2 at 711.63 eV indicates the existence of FeOOH.41 So XPS results also confirmed the formation of NixFe1-xOOH. However, XRD pattern in Figure S3 cannot detect the peak of NixFe1-xOOH, indicating very thin layers of NixFe1-xOOH formed on the surfaces of NiFe ANTAs, and NiFe alloys still are major phase in NiFe-NixFe1-xOOH SNTAs-CFC. Therefore, the above measurements confirm the formation of NixFe1-xOOH/NiFe/NixFe1xOOH

SNTAs with NiFe alloy as core and double thin NixFe1-xOOH layers as shells during the OER

process. OER catalytic activity of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC was measured by linear sweep voltammetry in 1.0 M KOH at 2 mV s-1 as shown in Figure 4a. For comparisive studies, Ni/NiOOH NTAs-CFC, Fe/FeOOH NTAs-CFC and NiFeOx NTAs-CFC with the same loadings of 1.0 mg cm-2 were also fabricated as shown in Figure S4, S5, and S6, resepctively, and they were tested for OER in the same solution. It is apparent that the NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC exhibits the highest OER activity in all the catalysts. When the current density is 10 mA cm-2, NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC exhibits the overpotential of ~220 mV (the potential is 1.45 V) as shown in Figure 4b, while Ni/NiOOH NTAs-CFC, Fe/FeOOH NTAs-CFC and NiFeOx NTAs-CFC showed much larger overpotentials of ~362, 417 and 327 mV, respectively. Moreover, it is worth noting that the overpotential of 220 mV of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC at 10 mA cm-2 is also smaller than most of ever reported NiFe-based OER electrocatalysts (see Table S1). In addition, at the overpotential of 350 mV, the current density of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC is 303.8 mA cm-2, which is 39, 190 and 17 times higher than those of Ni/NiOOH NTAs-CFC, Fe/FeOOH NTAs-CFC and NiFeOx NTAsCFC, respectively (Figure 4c). In order to further evaluate the electrocatalytic performance of NixFe1xOOH/NiFe/NixFe1-xOOH

SNTAs-CFC, the turnover frequency (TOF) was calculated and the result is

plotted in Figure 4d. It is obviously observed that the NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC 5 Plus Environment ACS Paragon


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shows the highest TOF of 0.0436 s-1, which is much larger than that of Ni/NiOOH NTAs-CFC (0.00091 s-1), Fe/FeOOH NTAs-CFC (0.000125 s-1) and NiFeOx NTAs-CFC (0.0026 s-1). To further insight into NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC, the Tafel slopes of all catalysts were investigated. The Tafel slope of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC is ~52 mV/dec, which is much smaller than those of Ni/NiOOH NTAs-CFC (~117 mV/dec), Fe/FeOOH NTAs-CFC (~70 mV/dec) and NiFeOx NTAs-CFC (~83 mV/dec), as shown in Figure 4e, indicating more rapid OER rates can be achieved for NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC. Therefore, all the above data indicate that the NixFe1xOOH/NiFe/NixFe1-xOOH SNTAs-CFC

exhibits excellent OER catalytic activity in alkaline media.

Besides the activity, the stability is also an important parameter to evaluate the performance of catalysts. The stability of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC was tested by chronopotentiome-try method at the current densities of 10 mA cm-2, 20 mA cm-2 and 100 mA cm-2, respectively, in 1.0 M KOH solution. It is seen that the potentials remain almost constant for more than 30 h at the different current densities as shown in Figure 4f, indicating the excellent electrocatalytic stability of NixFe1xOOH/NiFe/NixFe1-xOOH SNTAs-CFC

even if at a high current density of 100 mA cm-2. In addition, the

nanotube array morphology of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC were well retained after stability testing as shown in Figure S7. Here high conductivity and large electrochemically active surface area of NixFe1-xOOH/NiFe/NixFe1xOOH

SNTAs-CFC are important for the enhancement of catalytic performance. The electrochemical

impedance spectroscopy (EIS) of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC were measured as shown in Figure 4g. The semicircle in high-frequency section corresponds to the electron transfer resistance (Rct) that is correlated with the elctrocatalytic kinetics. A small Rct corresponds to a fast reaction rate, which enables simple and effective electrical integration that can minimize parasitic Ohmic resistance and is beneficial to acheive efficient charge transport.43-45 From Nyquist plots in Figure 4g, it is clearly observed that NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC owns a smallest Rct among the various catalysts. In addition, the electrochemically active surface area is very important for high-performance electrocatalyst and that of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC was evaluated from the electrochemical double 6 Plus Environment ACS Paragon

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layer capacitance (Cdl) by cyclic voltammetry (CV) method. CV measurements of various catalysts were carried out in 1.0 M KOH solution at the scan rate of 2~10 mV s-1 in the potential range of 1.37~1.43 V (the overpotential range of 200~260 mV), where there is no Faraday current, as shown in the Figure S8. The double layer capacitance is estimated by plotting ∆J(Ja-Jc) at the overpotential of 230 mV against the scan rate as shown in Figure 4h. NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC shows much higher Cdl (128 mF cm-2) than Ni-NiOOH NTAs (110 mF cm-2), Fe/FeOOH NTAs-CFC (8.4 mF cm-2) and NiFeOx SNTAs-CFC (50 mF cm-2). As we all know, the Cdl serves as an approximate guide for electrochemically active surface area within an order-of-magnitude accuracy.46 So the NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC own high electrochemically active surface area and accordingly achieve the enhanced electrocatalytic activity. Here CFC is a good flexible current collector and accordingly the NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC owns the potential as flexible electrode for OER as shown in Figure S9 (the flexible OER electrode will be important for the development of flexible energy devices, such as flexible fuel cells or metal-air batteries). The effects of flexibility of electrode on the electrocatalytic activity and durability are studied. The electrocatalytic activity of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC under the different distorted states was investigated by LSVs in 1.0 M KOH solution as shown in Figure 5a, which shows the catalytic activity keeps almost unchanged under the normal, bending or twisting states and after 500 bending and twisting tests. In addition, the chronopotentiometry meaurements of NixFe1-xOOH/NiFe/Nix Fe1-xOOH SNTAs-CFC were performed at 10 mA cm-2 under the different distorted states and after 500 bending and twisting tests as shown in Figure 5b, which shows almost the same curves. Therefore, the NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC is a good flexible electrode for OER, and its catalytic activity and stability almost are not affacted by the distorted state of electrode.

CONCLUSIONS In conclusion, using NiFe as a representative example of alloys, we showed that alloys were converted into composite metal oxyhydroxides under OER catalysis, and NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs7 Plus Environment ACS Paragon


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CFC were fabricated as flexible high-performance OER electrocatalyst by a facile in-situ electrochemical oxidation of NiFe alloy SNTAs. The NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC exhibited high performance for OER in alkaline media, such as a small overpotential of 220 mV at 10 mA cm-2, a small Tafel slope of 52 mV dec-1, and excellent stability at 100 mA cm-2 for more than 30 h. Furthermore, NixFe1xOOH/NiFe/NixFe1-xOOH

SNTAs-CFC exhibited excellent flexible performance, and its OER catalytic

performance remains almost unchange after 500 bending and twisting tests. The in-situ electrochemical transformation approach described here might be a promising strategy for the synthesis of other metal oxyhydroxide-based electrocatalysts.

ACKNOWLEDGEMENTS This work was supported by National Basic Research Program of China (2015CB932304 and 2016YFA0202603), NSFC (91645104), Science and Technology Program of Guangzhou (201704030019), Natural Science Foundation of Guangdong Province (2016A010104004 and 2017A010103007), and Fundamental Research Fund for the Central Universities (16lgjc67).

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.******. Details of the characterizations and electrochemical data (PDF)

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] Author Contributions A.L.W and G.R.L conceived the project and contributed to the design of the experiments and analysis of the data. A.L.W wrote the manuscript. All authors have given approval to the ﬁnal version of the paper. 8 Plus Environment ACS Paragon

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Notes The authors declare no competing ﬁnancial interest.

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2843-2850. 24. Zhu, J.; Xiao, M.; Zhang, Y.; Jin, Z.; Peng, Z.; Liu, C.; Chen, S.; Ge, J.; Xing, W. Metal-Organic Framework-Induced Synthesis of Ultrasmall Encased NiFe Nanoparticles Coupling with Graphene as an Efficient Oxygen Electrode for a Rechargeable Zn–Air Battery. ACS Catal. 2016, 6, 6335-6342. 25. Feng, Y.; Zhang, H.; Fang, L.; Mu, Y.; Wang, Y. Uniquely Monodispersing NiFe Alloyed Nanoparticles in Three-Dimensional Strongly Linked Sandwiched Graphitized Carbon Sheets for High-Efficiency Oxygen Evolution Reaction. ACS Catal. 2016, 6, 4477-4485. 26. Hou, Y.; Wen, Z.; Cui, S.; Feng, X.; Chen, J. Strongly Coupled Ternary Hybrid Aerogels of NDeficient Porous Graphitic-C3N4 Nanosheets/N-Doped Graphene/NiFe-Layered Double Hydroxide for Solar-Driven Photoelectrochemical Water Oxidation. Nano Lett. 2016, 16, 2268-2277. 27. Nurlaela, E.; Shinagawa, T.; Qureshi, M.; Dhawale, D. S.; Takanabe, K. Temperature Dependence of Electrocatalytic and Photocatalytic Oxygen Evolution Reaction Rates Using NiFe oxide. ACS Catal. 2016, 6, 1713-1722. 28. Chen, J. Y.; Dang, L.; Liang, H.; Bi, W.; Gerken, J. B.; Jin, S.; Alp, E. E.; Stahl, S. S. Operando Analysis of NiFe and Fe Oxyhydroxide Electrocatalysts for Water Oxidation: Detection of Fe4+ by Mossbauer Spectroscopy. J. Am. Chem. Soc. 2015, 137, 15090-15093. 29. Wang, D.; Zhou, J.; Hu, Y.; Yang, J.; Han, N.; Li, Y.; Sham, T.-K. In Situ X-ray Absorption Near-edge Structure Study of Advanced NiFe(OH)x Electrocatalyst on Carbon Paper for Water Oxidation. J. Phys. Chem. C 2015, 119, 19573-19583. 30. Tang, C.; Wang, H. S.; Wang, H. F.; Zhang, Q.; Tian, G. L.; Nie, J. Q.; Wei, F. Spatially Confined Hybridization of Nanometer‐Sized NiFe Hydroxides into Nitrogen‐Doped Graphene Frameworks Leading to Superior Oxygen Evolution Reactivity. Adv. Mater. 2015, 27, 4516-4522. 31. Jia, X.; Zhao, Y.; Chen, G.; Shang, L.; Shi, R.; Kang, X.; Waterhouse, G. I.; Wu, L. Z.; Tung, C. H.; Zhang, T. Ni3FeN Nanoparticles Derived from Ultrathin NiFe‐Layered Double Hydroxide Nanosheets: An Efficient Overall Water Splitting Electrocatalyst. Adv. Energy Mater. 2016, 6, 1502585. 32. Xiao, C.; Li, Y.; Lu, X.; Zhao, C. Bifunctional Porous NiFe/NiCo2O4/Ni Foam Electrodes with Triple Hierarchy and Double Synergies for Efficient Whole Cell Water Splitting. Adv. Funct. Mater. 2016, 26, 3515-3523. 33. Tang, D.; Liu, J.; Wu, X.; Liu, R.; Han, X.; Han, Y.; Huang, H.; Liu, Y.; Kang, Z. Carbon Quantum Dot/NiFe Layered Double-Hydroxide Composite as a Highly Efficient Electrocatalyst for Water Oxidation. ACS Appl. Mater. Interfaces 2014, 6, 7918-7925. 34. Zhang, C.; Shao, M.; Zhou, L.; Li, Z.; Xiao, K.; Wei, M. Hierarchical NiFe Layered Double Hydroxide Hollow Microspheres with Highly-Efficient Behavior Toward Oxygen Evolution Reaction. 11 Plus Environment ACS Paragon


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Scheme 1. Representation of the fabrication process of NiFe ANTAs-CFC.



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Figure 1. (a) SEM image of NiFe ANTAs-CFC (inset is a typical broken nanotube); (b) TEM image of a typical NiFe alloy nanotube; (c-f) STEM-HADDF images and elemental mappings of a typical NiFe alloy nanotube; (d) HRTEM; (e) SAED; and (i) XRD pattern of NiFe alloy nanotube.


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Figure 2. XPS spectra of (a) Ni 2p and (b) Fe 2p regions of NiFe ANTAs-CFC before and after activation test; (c) polarization curves of initial NiFe nanotube and those after different activation times (10th, 20th, 40th and 100th).



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200 nm Figure 3. (a) SEM image of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC (inset is TEM image of a nanotube); (b) TEM image of a typical broken nanotube; (c) TEM image of nanotube wall; (d) HRTEM image and SAED of the place marked with yellow block in (c) (in SAED, the green planes represent the crystal planes of NiOOH and the yellow planes represent the crystal planes of FeOOH); (e) STEMHAADF image and (f-g) element mapping images of the place marked with yellow block in (e).


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Figure 4. (a) Polarization curves of various catalysts in 1.0 M KOH solution at 2 mV s-1: (1) NixFe1-xOOH/NiFe/

NixFe1-xOOH SNTAs-CFC, (2) NiFeOx NTAs-CFC, (3) Ni/NiOOH NTAs-CFC, (4) Fe/FeOOH NTAs-CFC {Here (1), (2), (3) and (4) are the same in all the figures in Figure 4}; (b) Comparisons of the potentials required to reach the current density of 10 mA cm-2; (c) Comparisons of current densities of various catalysts at the overpotential of 350 mV; (d) Comparisons of TOF values of various catalysts at the overpotential of 350 mV; (e) Tafel plots of various catalysts; (f) Chronopotentiometric measurements of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC at the current densities of 10, 20, 100 mA cm-2; (g) Electrochemical impedance spectra of various catalysts; (h) Capacitive current densities as a function of scan rate for various catalysts.



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Figure 5. (a) Palorzation curves and (b) chronopotentiometric curves of NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC in 1.0 M KOH at 2 mV s-1 under the bending and twisting states.


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TOC NixFe1-xOOH -NiFe-NixFe1-xOOH

Current density (mA cm-2)

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


300

NixFe1-xOOH/NiFe/NixFe1-xOOH SNTAs-CFC NiFeOx NTAs-CFC

250

Ni-NiOOH NTAs-CFC

200

Fe-FeOOH NTAs-CFC

150 100 50 0 1.3

1.4

1.5

1.6

E / V (vs.RHE)


1.7

1.8

NixFe1âxOOH Nanotube Arrays

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NixFe1âxOOH Nanotube Arrays