NiOOH-Decorated Î±-FeOOH Nanosheet Array on Stainless Steel for

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NiOOH-decorated #-FeOOH nanosheet array on stainless steel for applications in oxygen evolution reaction and supercapacitor Dongbin Zhang, Xianggui Kong, Meihong Jiang, Deqiang Lei, and Xiaodong Lei ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06386 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 25, 2019

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ACS Sustainable Chemistry & Engineering

NiOOH-decorated α-FeOOH nanosheet array on stainless steel for applications in oxygen evolution reaction and supercapacitor

Dongbin Zhang†, Xianggui Kong†,*, Meihong Jiang†, Deqiang Lei‡, Xiaodong Lei†, *.

†State

Key Laboratory of Chemical Resource Engineering, Beijing University

of Chemical Technology, PO Box 98, No. 15 of North Three Ring East Road, Beijing 100029, People’s Republic of China.

‡Department

of Neurosurgery, Union Hospital, Tongji Medical College,

Huazhong University of Science and Technology, 1277 Jiefang Avenue, Wuhan 430022, People’s Republic of China.

*E-mail: [email protected]; [email protected]

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ABSTRACT The development of advanced electrode materials with high performance and low-cost for energy conversion and storage techniques, such as water splitting and supercapacitor, is significant in terms of future renewable energy systems. In this work, a unique NiOOH-decorated α-FeOOH nanosheet array (ASF) has been fabricated using the sintered 316L stainless steel felt (SF) as substrate with the help of alkaline oxidative etchant solution (AOES), which can be viewed as ‘pancake’, while α-FeOOH nanosheet as the skeleton and other elements uniformly embedded in. The formation mechanism of ASF was discussed in detail based on XRD, XPS, SEM, STEM tests and density functional theory (DFT) calculation. The results indicate that the oriented growth of α-FeOOH was influenced by the surface adsorption for reducing the surface energy of facet and electrostatic repulsion effect, to form the unexpected nanosheet structure. The 2-dimensional nanosheets could afford large specific surface area, and the active components (including Fe and Ni elements) distributed uniformly on α-FeOOH nanosheets, enabling the sufficient exposure of electrochemical active sites and facilitating the effective contact between them with electrolyte. As a consequence, the ASF shows the outstanding oxygen evolution reaction (OER, the overpotential is only 256 mV at 10 mA cm-2, with low Tafel slope of 45 mV dec-1) and supercapacitor (the specific capacity is 748.13 mF cm-2 at 1.5 mA cm-2 (554.17 F g-1, 1.1 A g-1), with good rate capacity and cyclic stability) performances. KEYWORDS: NiOOH-decorated α-FeOOH nanosheet array, alkaline oxidative etchant solution, sintered 316L stainless steel felt, oxygen evolution reaction, supercapacitor.


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INTRODUCTION Actuated by the demand to find alternatives for fossil fuels in the light of rapid growth in energy consumption and environmental pollution problems, there has been rapid progress in the development of energy conversion and storage devices.1-4 As we all know, electrochemically splitting water into hydrogen and oxygen is an attractive approach for producing clean and renewable energy.5, 6 However, the overall water splitting efficiency is largely limited by the kinetically sluggish oxygen evolution reaction (OER) involving the transfer of four electrons process.7, 8 Therefore, a highly efficient OER catalyst/electrode is required urgently to expedite the reaction kinetics and lower the overpotential, thereby improving the overall water splitting efficiency.911

The technology of energy storage is another key to relax the energy crisis.

Supercapacitor, which has much superiority, such as high power density, good rate capacity, ultra-long cyclic stability, is considered as one of the best candidates for new generation energy storage device.12-15 In the past decades, although the specific capacitance of supercapacitor positive electrodes can reach up to ~3000 F g-1, that of the negative electrodes is less than 600 F g-1 regretfully, leading to capacity unbalance (Q+ ≠ Q-), resulting the energy density is limited less than 60 Wh Kg-1, far lower than that of Li-ion battery (100~200 Wh Kg-1).16, 17 Obviously, it is meaningful to design and develop advanced negative electrode materials with ideal electrochemical performance, which can match well with the positive electrodes, to obtain high energy density supercapacitor. In general, FeOOH (including α-, β-, γ-FeOOH) is considered as one candidate for electrochemical electrode materials, due to its low-toxic, high abundance and redox activity. However, the morphology of FeOOH is nanorod or spindle usually that is unfavourable to realize satisfactory electrochemical performance, because of its low 3 ACS Paragon Plus Environment


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specific surface area and poor conductivity. The exposure of electrochemical active sites is also limited, which results the unsatisfactory OER and/or supercapacitor performance.18, 19 Instead, the 2-dimensional nanosheet structure could afford many advantages, such as large specific surface area, providing more electrochemical active sites and facilitating the effective contact between electrode with electrolyte.15 There is no wonder that if the morphology of FeOOH can be controlled and transformed from nanorod/spindle to 2-dimensional nanosheet, it will exhibit more excellent electrochemical performance. However, how to design and synthesize the FeOOH nanosheet by a simple and efficient method still remains a severe challenge. There are many effective approaches to construct electrochemical active material with nanoarray structure (including nanorod20, nanosheet21,

22

and multi-level

nanostructure23-25, etc.), such as chemical bath deposition25, chemical vapor deposition26, electrochemical deposition20, etc. Compared with the other methods, the in-situ etching method with alkaline oxidative etchant solution (AOES) is one of the best approaches to fabricate binder-free electrodes by using the mixed alkali (NaOH, KOH, etc.) and oxidant ((NH4)2S2O8, Na2S2O8, KBrO3, NaBrO3, NaClO, etc.), also based on its convenience, efficient and low-cost. In the past few years, although some metal current collectors (copper foam

14, 27, 28,

nickel foam13, 302 stainless steel

sheet29, 30, etc.) used as substrates for fabricating electrode materials with excellent electrochemical performance with AOES have been reported, that have been applied in the fields of Li-ion battery, supercapacitor13,

14, 28

and OER29,

30,

the forming

mechanism of nanostructure on the surface of metal current collectors is remaining unclear, especially for stainless steel. Meanwhile, compared with other metal current collectors, stainless steel contains different kinds of elements (Fe, Ni, Mn, Cr, C, etc.), which can take place various redox reactions,31-33 providing more electrochemical


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active sites, thus may show more excellent electrochemical activity. Bearing the discuss above, herein, the sintered 316L stainless steel felt (SF) was used as current collector and substrate to synthesize a unique NiOOH-decorated αFeOOH nanosheet array (ASF) using a simple one-step method with AOES. The formation mechanism of this unique nanosheet array has been investigated with the help of material characterizations and density functional theory (DFT) calculation. The surface adsorption and electrostatic repulsion effect lead to the formation of unexpected nanosheet structure, the decoration of NiOOH, not only increases the OER catalyst active sites, but also improves the conductivity, which endows ASF with excellent electrochemical performance and potential applications in fields of OER and supercapacitor.



EXPERIMENT SECTION

Materials. All the reactants in the experiments were of analytical grade and used without any further puriﬁcation. The SF (with 3-dimensional open celled network structure, Figure S1) was pretreated by ultrasound in 1 M HCl aqueous solution, ethanol and deionized water for 5 min, respectively, and finally dried at 333 K. AOES was obtained by mixing 6.0 g sodium hydroxide (NaOH) and different amounts of ammonium persulphates (APS) (0 g, 2.8 g, 8.4 g and 16.8 g, respectively) in 50 ml deionized water. Fabrication of ASF. The pretreated SF (3.0 cm × 2.0 cm) was immersed into AOES for 24 h at room temperature. Then the SF was removed and washed three times with deionized water, and dried at 333 K to obtain the ASF samples. The massloading of active material was 1.35 mg cm-2, see Table S1. Meanwhile, the powder of sample is obtained by dropping the Fe(NO3)3 9H2O (0.606 g) solution into AOES (6.0 g NaOH, 2.8 g APS and 50 ml deionized water), with stirring for 24 h at room temperature. For comparision, some ASF samples were obtained by immersing pretreated SF into different kinds of AOESs. In detail, the AOESs were prepared by mixing 6.0 g NaOH and 2.8 g sodium persulfates (SPS) for SPS-AOES, and 2.8 g NaBrO3 for NaBrO3-AOES and 2.8 g KBrO3 for KBrO3-AOES, respectively, in 50 ml deionized water. The NaClO-AOES was prepared according to the literature,29 while the amount of NaClO solution (8%) is 2, 5 and 20 ml, respectively (the amounts of NaOH and deionized water were the same as above). The ASF samples were obtained by immersing pretreated SF into these different kinds of AOESs for 24 h at 353 K. Material characterization. X-ray diffraction (XRD) patterns were collected on a


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Rigaku XRD-6000 diffractometer using Cu Kα radiation, from 10° to 80°, with the scan rate of 10° min-1. The morphology of samples was investigated using a scanning electron microscope (SEM; Zeiss SUPRA 55) with an accelerating voltage of 20 kV, combined with energy dispersive X-ray spectroscopy (EDS). Scanning transmission electron microscopy (STEM) images were recorded using a JEOL JEM-3200FS Field emission transmission electron microscope with an accelerating voltage of 200 kV. Xray photoelectron spectroscopy (XPS) measurements were performed on a Thermo VG ESCALAB 250 X-ray photoelectron spectrometer with Al Kα radiation at a pressure of about 2 × 10-9 Pa. Electrochemical measurements. Electrochemical measurements were performed on an electrochemical workstation (CHI 660E, CH Instruments Inc., Chenghua, Shanghai) using a three-electrode mode in 1 M KOH aqueous solution at 298 K. ASF was used as the working electrode, a platinum electrode as the counter electrode and saturated calomel electrode (SCE) electrode as the reference electrode. All the current density was calculated with the geometric area of the electrode as 1.0 × 1.0 cm2. The OER performance measurements were carried out as follows: the polarization (LSV) curves were measured from 0 V to 1.0 V vs. SCE at the scan rate of 5 mV s-1. The Tafel slope was derived from LSV curves without IR compensation, based on the equation: η = b log j + a, where η, b and j are the overpotential, Tafel slope and current density, respectively. The chronopotentiometry measurements for long-time stability were performed at different current density for 30 h under vigorous stirring. Electrochemical impedance spectroscopy (EIS) was conducted at 0.46 V by applying an AC voltage with 5 mV amplitude in the frequency range from 100 kHz to 0.01 Hz. The supercapacitor performance measurements were carried out as follows: the cyclic voltammetry (CV) experiments were performed at different scanning rates



(from 5 to 50 mV s-1). The galvanostatic charge/discharge (GCD) measurements were carried out within the potential window from 0 V to -1.2 V at various current densities (from 1.5 to 30 mA cm-2). The cyclic stability was tested by the repeated 4000 GCD cycles at the current density of 30 mA cm-2. The EIS was carried out by applying an AC voltage with 5 mV amplitude in the frequency range from 100 kHz to 0.01 Hz. During the testing, the samples were used as negative electrodes. Theoretical calculation. Plane-wave density functional theory (DFT) + U calculations of the electronic properties of α-FeOOH systems with exposed (100), (010) and (001) facets were carried out using the CASTEP module in Materials Studio. GGA with a PBE functional was employed for the DFT exchange correlation energy, and a cut off of 520 eV was assigned to the plane-wave basis set. The selfconsistent field (SCF) tolerance was 1×10−4 eV. The Brillouin zone was sampled by 1×1×1 k-points. The core electrons were replaced with ultrasoft pseudo-potentials.


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RESULTS AND DISCUSSION

Figure 1. The digital photo images (a) and XRD spectra (b) of SF and ASF; the SEM images of SF (c) and ASF (d). The pretreated SF was immersed into the AOES about 24 h, resulted to change the color of surface of SF from silver to brown, as shown in Figure 1a (the chemical reaction equations see SI. However, when the AOES consists without oxidative agent or alkali, this reaction can’t take place (Figure S2)). Although the composition of SF is complicated, according to the XRD results (Figure 1b), it is not doubt that the surface of SF was covered with a layer of α-FeOOH (JCPDS No. 29-0713) after immersion. In order to confirm this result, the powder of product was obtained by dropping the Fe3+ solution into AOES, see Figure S3. It is worth noting that the Ni contained in SF was oxidized to NiOOH, presenting the NiOOH phase signal (JCPDS 9 ACS Paragon Plus Environment


No. 06-0075). For investigated the morphology conversion of SF before and after this reaction, the SEM images of SF and ASF are shown in Figure 1c and 1d. Consistent with the XRD results, the unique NiOOH-decorated α-FeOOH nanosheets were formed on the relatively smooth surface of SF after reaction. The average size of nanosheet was about 400 nm. And, the ~400 nm thickness of the layer of nanosheet can be calculated by cross-section SEM images (Figure S4). According to the EDS (Figure S5) and mapping spectrum (Figure S6), the amount of O element is increased, because of the formation of α-FeOOH and NiOOH. Meanwhile, all the other elements (Fe, Ni, O and C, etc.) in SF are maintained after the reaction process, and distributed on the surface of ASF uniformly.

Figure 2. The STEM images of ASF nanosheets. The low magnification image (a), The high magnification image (b), Selected area electron diffraction image (c), Drift corrected spectrum image scanning area (d), the EDS image (e), Corresponding mapping results: Fe (f), Ni (g), O (h) and C (i). For investigated the morphological and structural information, the STEM measurements were carried out (Figure 2). As shown in Figure 2a, the fabricated ASF 10 ACS Paragon Plus Environment

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shows nanosheet structure with an average size ca. 400 nm. The interplanar distance of 0.249 nm in Figure 2b can be ascribed to the (040) plane of α-FeOOH phase, which is corresponding to XRD results, the selected area electron diffraction image shows the ASF nanosheets are polycrystal structure and include complex components (Figure 2c). In order to verify the validity of this unique structure, the drift corrected spectrum image scanning measurement was provided as shown in Figure 2d. After the immersion process, the Fe and Ni elements were remained and formed α-FeOOH and NiOOH, respectively, while the Mn and Cr elements may erode into the AOES,30 resulting hardly detected in ASF nanosheets (EDS result, Figure 2e). According to the mapping results (Figure 2f-i), it is no doubt that the NiOOH was distributed onto αFeOOH nanosheets uniformly, which was confirmed by Ni element mapping visually (Figure 2g). In addition, the carbon in the nanosheets (resourced from SF, atomic ratio = 10.48%, Figure S5) can improve the conductivity of the ASF, endowing a good electrochemical performance (Figure 2i). Obviously, this complex nanosheet (ASF) can be viewed as ‘pancake’, while α-FeOOH nanosheet as the skeleton and other elements (especially NiOOH) uniformly embedded in, since the main component of ASF is α-FeOOH (atomic ratio of Ni:Fe = 1:73.6, Table S2).



Figure 3. The XPS spectra of SF and ASF. Wide scan spectra, inset C 1s (a), Fe 2p (b), Ni 2p (c), Mn 2p (d), Cr 2p (e) and O 1s (f). XPS was employed to detect the chemical states and interaction between different kinds of compositions, as shown in Figure 3. The wide scan spectra of SF and ASF (Figure 3a) show mainly containing the peaks of Fe, Ni, Mn, Cr, O and C. Obviously, there are slightly different characteristics in which the metallic Fe (Fe0, 706.8 eV) feature does not appear in the ASF sample, because of the formation of FeOOH. Two peaks at banding energy of 711.0 eV and 724.4 eV belonging to Fe 2p3/2 and Fe 2p1/2 for ASF, respectively.30 The peak position variation (∆E) of Fe 2p3/2 for ASF and SF is 0.6 eV, which may be caused by the interaction between α-FeOOH, NiOOH and SF (Figure 3b). Contrasting the Ni 2p spectra of SF and ASF (Figure 3c), the peaks of Ni0 on SF disappear, instead, the Ni 2p3/2 (855.1 eV) and Ni 2p1/2 (871.0 eV) appear, indicating the transformation from metallic Ni to NiOOH. Meanwhile, the Mn 2p (Figure 3d) and Cr 2p (Figure 3e) spectra can be explained that the Mn and Cr metallic were oxidized and eroded into AOES.30 As shown in Figure 3f, the O 1s peak


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can be divided into three peaks located at 529.7 eV, 531.2 eV and 533.0 eV, corresponding to oxygen in metal oxides (O2-), hydroxyl (OH-) and absorbed water (H2O), respectively.14

Figure 4. The SEM images of ASF with different immersion times: 0 h (a), 2 h (b), 6 h (c), 10 h (d), 16 h (e) and 24 h (f). To our knowledge, the morphology of FeOOH (including α-, β-, γ-FeOOH) is nanorod or spindle usually.18,

34, 35

There are few reports about nanosheet-shape

FeOOH and/or FeOOH-based materials before, unless by electrodeposition method. In order to understand the growth mechanism of this unique nanosheet structure, in which the main component is α-FeOOH (atomic ratio of Ni:Fe = 1:73.6), the growth process was investigated by SEM, as shown in Figure 4. Before immersed into the AOES (0 h, Figure 4a), the surface of SF was relatively smooth. When the time prolonged to 2 h, there were some small particles or nanosheets attaching to the surface of SF (Figure 4b), these small seeds grew gradually to form relatively large nanosheets (6 h, Figure 4c), and were growing perpendicularly on the SF substrate (10



h, Figure 4d). As the reaction proceeded (16 h, Figure 4e), these nanosheets were growing with curved shape. Finally, after 24 h reaction, the perpendicularly oriented and interconnected nanosheets array was obtained (Figure 4f).

Figure 5. The theoretical calculation of α-FeOOH surface energy of (001) (a), (010) (b) and (100) (c) facets; the adsorption energy of (001) (d), (010) (e) and (100) (f) facets for SO42- and the possible formation mechanism of NiOOH-decorated αFeOOH nanosheet (g). In order to explain the growth mechanism, the DFT calculations were performed, as shown in Figure 5. Actually, as for α-FeOOH (the crystal structure, see Figure S7), due to the surface energy of (010) facet of α-FeOOH is higher than that of (100) and/or (001) facet, causing the growth rate along with y axis orientation is relatively


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fast to form nanorod or spindle structure as usual (Figure 5a-c).15, 18, 34, 35 However, the adsorption (including molecule and ions, etc.) on the facet may reduce the surface energy, enhancing its stability and resulting the change of the growth orientation.12 Taking this into account, the possible formation mechanism of α-FeOOH nanosheet may be postulated as follows: in the initial stage, metallic Fe was oxidized by S2O82to Fe3+, and then formed small α-FeOOH seeds without obvious orientation on the surface of SF. These α-FeOOH seeds with high surface energy have good absorption affinity, including adsorb Fe3+, NH4+, S2O82-, SO42-, Cl-, OH-, etc.19, 36-38 As a result, the surface energy of (010) facet decreased and became more stable. The adsorption energy of (001), (010) and (100) facets for SO42- were calculated, as shown in Figure 5d-f. The Eads(010) is less than Eads(001) and/or Eads(100), showing it is more easy take happen adsorption for SO42- on (010) facet.39 Meanwhile, the adsorption hinders the small seeds to combined together, because of the electrostatic repulsion effect.12 Due to the factors discussed above, two or three small α-FeOOH seeds grow along with xoz coordinate plane, and then self-assemble into one nanosheet perpendicularly through an oriented process. As last, these small seeds ultimately evolved into the lager interconnected α-FeOOH nanosheets with perpendicular orientation. During this process, the other elements embed in α-FeOOH uniformly to form the final ASF nanosheets (Figure 5g).



Figure 6. The SEM images of ASF, obtained by AOES containing different kinds of oxidative agents: Na2S2O8 (a), KBrO3 (b), NaBrO3 (c) and NaClO (d-f) with different concentrations. The formation mechanism can be confirmed by taking contrast experiments using different kinds of AOESs, as shown in Figure 6. Obviously, when the oxidative agent is Na2S2O8, KBrO3 or NaBrO3, respectively, the morphology of ASF was nanosheets with different sizes (Figure 6a-c). This phenomenon can be explained well as discussed above. It is worth to be reminded that the cations also can affect the morphology of the obtained FeOOH, such as K+ and Na+, the size of FeOOH nanosheets obtained by NaBrO3 was larger than that of samples by KBrO3 (Figure 6b and 6c). Interestingly, when used NaClO as oxidative agent, the different kinds of morphologies of ASF were observed on the surface of SF, including nanosheets and nanoparticles (Figure 6d-f). With high concentration NaClO-AOES, the ASF was likely to be formed as nanosheets, while with low concentration, particle-shape. Similarly, this phenomenon indicates the morphology of FeOOH was also influenced 16 ACS Paragon Plus Environment

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by the adsorption of Cl- that resourced from oxidant NaClO, further confirmed our postulated formation mechanism. Differently, The E[ads(010), 2SO4 ],

Cl ]

is less than E[ads(010),

meaning the adsorption effect of Cl- is weaker than SO42- slightly, which causes

the formation of particle-shape FeOOH in low concentration. (the corresponding DFT calculation see Figure S8).

Figure 7. Electrocatalytic performance of OER. (a) LSV curves of SF and ASF, without IR compensation; (b) Tafel plots for SF and ASF derived from LSV curves; (c) Plots of current density vs. the scan rate to determine the electrochemical active surface area (ECSA) value; (d) Nyquist plots of EIS; (e) Chronopotentiometry measurement for long-time stability of ASF; (f) LSV curves of ASF before and after stability test. To assess the OER performance, the ASF and SF were used as working electrodes, Pt wire as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode in 1 M KOH as the electrolyte (Figure S9). In order to make the results close to the practical situation, the OER measurements were carried out 17 ACS Paragon Plus Environment


without IR compensation. As shown in Figure 7a, to achieve 10 mA cm-2 current density, it only needed ~256 mV overpotential for ASF, which is better than that of SF (10 mA cm-2, η ~357 mV). More importantly, when the current density was up to 300 mA cm-2, the overpotential was only 612 mV (The detail data, see Table S3). Moreover, the ASF samples were obtained by immersed SF in different concentration AOESs (Figure S10) and the OER performance are shown in Figure S11. Obviously, the ASF sample obtained with 2.8 g APS has the best OER activity. While the concentration increased, the OER activity decreased, even worse than that of SF when the amount of APS was up to 16.8 g, which is probably caused by the surface passivation of SF under the high concentration of AOES (Figure S12). Tafel slope is another key parameter for OER catalysts. The Tafel slopes for SF and ASF were derived from LSV curves (Figure 7b). The Tafel slope for ASF was lower (45 mV dec-1) than that of SF (70 mV dec-1) when the current density ranges from 5 mA cm-2 to 10 mA cm-2. Even at high current density (ranges from 100 mA cm-2 to 110 mA cm-2), the ASF also had the lower Tafel slope value (305 mV dec-1, but the Tafel slope for SF was 409 mV dec-1 under the same condition). The excellent OER performance of ASF may be due to its high electrochemical active surface area (ECSA), as shown in Figure 7c (The CV curves are shown in Figure S13). Because of the unique microstructure, the ECSA value of ASF was about 2 times than that of SF, indicating the electrochemical active sites were fully exposed, lead to the outstanding water oxidation activity (see Table S4).30, 40-47 Meanwhile, The EIS plot (Figure 7d, measured at η ~267 mV) of ASF shows low charge-transfer resistance (Rct). But, the EIS plot of SF was a line under the same condition, indicating there was hard to take place water oxidation reaction at such low overpotential. The low Tafel slope, high ECSA and low Rct values suggest that the ASF


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electrocatalyst exhibited favorable kinetics during electrochemical processes, which is actually responsible for its outstanding OER activity. Additionally, in order to research the stability of ASF, the chronopotentiometry measurement was carried out at different current densities (Figure 7e). The overpotential at 10 mA cm-2 almost remains the same (between the first hour and last hour), indicating the excellent stability according to the LSV curves (Figure 7f). At the same time, EIS of ASF after the stability test exhibited no difference that also shows its excellent stability (Figure S14).

Figure 8. The electrochemical performance of ASF as a supercapacitor negative electrode material. (a) CV curves, at the scan rate of 50 mV s-1; (b) GCD curves, at the current density of 1.5 mA cm-2; (c) GCD curves of ASF, at the different charge and discharge current density; (d) The rate capacity; (e) The cyclic stability; (d) The plots of EIS. In addition, the electrochemical performance of the ASF that used as a supercapacitor negative electrode material was tested in a three-electrode electrochemical cell at 298 K with a platinum wire as the counter electrode and a SCE



as the reference electrode in 1 M KOH aqueous solution as the electrolyte. Figure 8a shows the CVs of ASF and SF at the scan rate of 50 mV s-1 (the CVs at different scan rates shown in Figure S15 and S16). Comparing with the CVs of ASF and SF, two distinct pairs of redox peaks (feature of faradic character) can be seen in the CV curves clearly, which can be attributed to Fe3+/Fe2+ and Fe2+/Fe0 (Ni can’t take place redox reaction under this potential window). The peak current density is proportional to the scan rate, suggesting that the redox reaction is a surface-confined process (Figure S17). The enclosed area of CV curve of ASF is much larger than that of SF, indicating its high specific capacity, which can be confirmed by GCD measurements as is shown in Figure 8b. At the current density of 1.5 mA cm-2 (1.1 A g-1), the discharge time of ASF reached to ~598 s, resulting in an excellent specific capacity of 748.13 mF cm-2 (554.17 F g-1) compared with that of the SF (27.50 mF cm-2, 1.5 mA cm-2), better than the values reported before48 (stainless steel-based/FeOOH-based materials, see Table S5). The GCD curves at different current density of ASF are shown in Figure 8c. When the current density increased from 1.5 mA cm-2 to 30 mA cm-2, the specific capacity decreased from 748.13 mF cm-2 to 402.50 mF cm-2 (see Table S6 for detail data), thus retaining 53.80% of the initial speciﬁc capacity, confirming its good rate capacity (Figure 8d). Under the same condition, the SF only retains 18.18% of the initial specific capacity, decreased from 27.50 mF cm-2 to 5.00 mF cm-2 (Figure S16). The cyclic stability of ASF was investigated by repeated charge-discharge processes at a high current density (30 mA cm-2, Figure 8e), the specific capacity of ASF retained 82.61% of initial capacity after 4000 chargedischarge cycles. Additionally, after long-term cycling, the morphology and structure of ASF were well preserved (nanosheet structure) with no obvious damage (Figure S18). And the EIS result (Figure 8f) shows the resistance (charge transfer and ion


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diffusion resistance) of ASF was low (The Rct is 0.1174 Ω, see Figure S19 and Table S7), showing the electrode material has good electroconductibility. Even after longtime for 4000 cycles test, the Rct only increases to 0.2398 Ω, which indicates its good stability. The excellent electrochemical performance (both used as OER catalyst and supercapacitor negative electrode material) of ASF can be attributed to the following factors: (1) the vertically ordered 2-dimensional complex nanosheets can provide plentiful electrochemical active sites (including Fe and Ni elements) for electrochemical reactions; (2) In-suit growth mechanism without usage of PTFE ensures nanosheet array with well mechanical adhesion and electrical connection to the current collector (SF) which provides short and straight electron pathways, leading to fast charge/discharge capabilities and good cyclic stability; (3) The NiOOHdecorated α-FeOOH is of good electrical conductivity (even better than α-FeOOH), showing a half-metallic character (Figure S20), decreases the resistance of the system 49-51;

(4) ASF is of unique 3-dimensional open-celled network and superhydrophilic

character

(Figure

S21),

enables

effective

electrode-electrolyte

contact

electrochemical reactions, leading to more efficient charge/ion transportation.


for


CONCLUSION In summary, a unique NiOOH-decorated α-FeOOH nanosheet array electrode material, which can be viewed as ‘pancake’, while α-FeOOH nanosheet as the skeleton and other elements embedded in uniformly, was fabricated by AOES method. Different from the traditional morphology of FeOOH-based materials, the ASF shows the unexpected nanosheet structure with the effect of surface adsorption. This unique morphology and structure endow the material much superiorities: exposure of electrochemical activity sufficiently, facilitating the contact between electrode and electrolyte, high electrochemical active surface area. Therefore, the fabricated ASF exhibits excellent OER and supercapacitor performance, showing its potential applications in fields of energy conversion and storage.

ASSOCIATED CONTENT Supporting information The weight of ASF before and after reduction; the SEM images of 316L SF; the digital photos of SF, after immersion in APS, NaOH and APS+NaOH aqueous solutions; the XRD, SEM and TEM of powder of α-FeOOH, obtained by dropping Fe3+ into AOES; the SEM images of the cross section of ASF; the EDS mapping images of SF and ASF; the composition of NiOOH-decorated α-FeOOH nanosheets; the crystal structure of α-FeOOH; the electrocatalystic activity of SF and ASF; comparison of the electrocatalystic activity of some stainless steel-based OER catalysts; comparison of the supercapacitor performance of some stainless steelbased/FeOOH-based negative electrode materials; the equivalent circuit and initial


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EIS curves with corresponding fitting curves; P-DOS of α-FeOOH and NiOOHdecorated α-FeOOH and contact angel test of SF and ASF.

AUTHOR INFORMATION Corresponding Author Tel: +86-10-64455357; E-mail: [email protected]; E-mail: [email protected]

ORCID Xianggui Kong: 0000-0002-2195-268X; Xiaodong Lei: 0000-0001-5818-9561

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (no: 2177060378, U1707603 and 21521005) and the Program for Changjiang Scholars, Innovative Research Teams in Universities (no. IRT1205).



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TOC

A unique NiOOH-decorated α-FeOOH nanosheet array, which can be viewed as ‘pancake’, while α-FeOOH nanosheet as the skeleton and other elements embedded in uniformly, was fabricated by a simple one-step method with the alkaline oxidative etchant solution, and exhibited outstanding OER and supercapacitor performance.



Figure 1. The digital photo images (a) and XRD spectra (b) of SF and ASF; the SEM images of SF (c) and ASF (d). 313x257mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Figure 2. The STEM images of ASF nanosheets. The low magnification image (a), The high magnification image (b), Selected area electron diffraction image (c), Drift corrected spectrum image scanning area (d), the EDS image (e), Corresponding mapping results: Fe (f), Ni (g), O (h) and C (i). 827x483mm (96 x 96 DPI)



Figure 3. The XPS spectra of SF and ASF. Wide scan spectra, inset C 1s (a), Fe 2p (b), Ni 2p (c), Mn 2p (d), Cr 2p (e) and O 1s (f).


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Figure 4. The SEM images of ASF with different immersion times: 0 h (a), 2 h (b), 6 h (c), 10 h (d), 16 h (e) and 24 h (f). 80x43mm (300 x 300 DPI)



Figure 5. The theoretical calculation of α-FeOOH surface energy of (001) (a), (010) (b) and (100) (c) facets; the adsorption energy of (001) (d), (010) (e) and (100) (f) facets for SO42- and the possible formation mechanism of NiOOH-decorated α-FeOOH nanosheet (g). 531x476mm (96 x 96 DPI)


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Figure 6. The SEM images of ASF, obtained by AOES containing different kinds of oxidative agents: Na2S2O8 (a), KBrO3 (b), NaBrO3 (c) and NaClO (d-f) with different concentrations.



Figure 7. Electrocatalytic performance of OER. (a) LSV curves of SF and ASF, without IR compensation; (b) Tafel plots for SF and ASF derived from LSV curves; (c) Plots of current density vs. the scan rate to determine the electrochemical active surface area (ECSA) value; (d) Nyquist plots of EIS; (e) Chronopotentiometry measurement for long-time stability of ASF; (f) LSV curves of ASF before and after stability test.


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Figure 8. The electrochemical performance of ASF as a supercapacitor negative electrode material. (a) CV curves, at the scan rate of 50 mV s-1; (b) GCD curves, at the current density of 1.5 mA cm-2; (c) GCD curves of ASF, at the different charge and discharge current density; (d) The rate capacity; (e) The cyclic stability; (d) The plots of EIS. 475x262mm (96 x 96 DPI)


NiOOH-Decorated Î±-FeOOH Nanosheet Array on Stainless Steel for

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