NiOOH Nanosheets as Highly Active

Jan 24, 2018 - Herein, we report Mo and Fe modification as a synergistic effect to enhance both activity and stability for OER. The Mo- and Fe-modifie...
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Mo- and Fe-modified Ni(OH)2/NiOOH nanosheets as highly active and stable electrocatalysts for oxygen evolution reaction Yanshuo Jin, Shangli Huang, Xin Yue, Hongyu Du, and Pei Kang Shen ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04226 • Publication Date (Web): 24 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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ACS Catalysis

Mo- and Fe-modified Ni(OH)2/NiOOH nanosheets as highly active and stable electrocatalysts for oxygen evolution reaction Yanshuo Jina‡, Shangli Huangb‡, Xin Yuea, Hongyu Duc, Pei Kang Shen*,a,b a

School of Materials Science and Engineering, Sun Yat-sen University, Guangzhou 510275, P. R. China. Collaborative Innovation Center of Sustainable Energy Materials, Guangxi University, Nanning, 530004, P. R. China. c School of Physics, Sun Yat-sen University, Guangzhou 510275, P. R. China. b

ABSTRACT: Highly active and stable electrocatalysts for oxygen evolution reaction (OER) are required for industrial hydrogen production. Herein, we report Mo- and Fe-modification is a synergistic effect to enhance both activity and stability for OER. The Mo- and Fe-modified Ni(OH)2/NiOOH nanosheets needs an overpotential of only about 280 mV to achieve current density of 100 mA cm-2 and shows no evidence of degradation after 50 hours at such high current density, outperforming all OER catalysts reported to date. This work may provide options for the design and preparation of promising OER electrocatalysts. KEYWORDS: Mo and Fe dual- modified; nickel (oxy)hydroxide; nanosheets; water splitting; oxygen evolution.

Water electrolysis is an appealing way to produce clean hydrogen fuel with zero emission of greenhouse gas.1-3 From both thermodynamic and kinetic points of view, the huge overpotential loss of the oxygen evolution reaction (OER) is a key difficulty for improving the electrolytic efficiency.4-7 Highly efficient electrocatalysts to improve the OER performance is extremely essential.8,9 The benchmark electrocatalysts for OER are Ir/Ru-based compounds, however, many efforts have focused on nonprecious-metal-based electrocatalysts for OER due to the high cost of noble metals.10-12 Among all of the reported nonprecious-metal-based electrocatalysts, first-row (3d) transition-metal oxide, hydroxide or oxyhydroxide electrocatalysts have attracted much attention due to their low cost and earth abundance.13,14 Nickel oxyhydroxide (NiOOH) or bimetallic oxyhydroxides (FeNiOOH, FeCoOOH, et al.) has been attracting significant interest because of their outstanding OER activity in alkaline electrolytes.15-17 However, greater efforts are still needed to improve the OER activity. Moreover, a major challenge associated with the use of bimetallic oxyhydroxides as OER electrocatalysts is their usually poor long-term OER stability due to poor structure stability based on thermodynamics.18,19 Remarkably, an important method to improve the electrocatalytic performance is modification, such as P-modified WN, et al.20 Based on this method, we began with computational studies aimed at improving activity and structure stability by modification. Our density functional theory plus Hubbard U (DFT+U) calculations revealed that Mo- and Fe-modified NiOOH has good adsorption energies for OER intermediates and a concomitant increase in OER activity.21 Our DFT+U calculations revealed that Mo- and Fe-modified NiOOH is more stable than Fe-modified NiOOH on the structure stability, maybe leading to better OER stability.22 Thus it is of great interest to develop Mo- and Fe-modified NiOOH for OER, however, this material for OER is very few reported to date.23 Apart from modification, the material morphology needs to be optimized to have a large surface area to develop highly efficient electrocatalysts.24-26 Thus we focus on two-

dimensional (2D) ultrathin nanosheets because their high specific surface area and rapid interfacial charge transfer.27-29 Herein, we report our recent efforts in developing Mo- and Fe-modified Ni(OH)2/NiOOH (MoFe:Ni(OH)2/NiOOH) nanosheets synthesized on nickel foam directly. The MoFe: Ni(OH)2/NiOOH nanosheets were directly grown on commercial nickel foam by hydrothermal treatments first and then with an electrochemical oxidation treatment. The experiment results prove that Mo- and Fe-modification is a synergistic effect to enhance both activity and stability. MoFe:Ni(OH)2/NiOOH nanosheets show more active and more stable for OER than Fe:Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH and Ni(OH)2/NiOOH. The MoFe: Ni(OH)2/NiOOH nanosheets need an overpotential of only ~280 mV to achieve current density of 100 mA cm-2 and shows no evidence of degradation after 50 hours at such high current density, outperforming all OER catalysts reported to date (Table S1).30-41 The MoFe:Ni(OH)2/NiOOH nanosheets were directly grown on commercial nickel foam by hydrothermal treatments first and then with an electrochemical oxidation treatment, as schematically elucidated in Figure 1a. In the first step, MoFe:Ni(OH)2 were directly grown on nickel foam by hydrothermal treatments. X-ray diffraction (XRD) technique was applied to examine the crystalline phase. As shown in Figure 1b, three obvious peaks of MoFe:Ni(OH)2 are well attributed to the diffraction peaks (003), (101) and (110) of Ni(OH)2 crystalline phase.42 After electrochemical oxidation treatment, Figure 1b shows three obvious peaks of MoFe:Ni(OH)2/NiOOH are still well attributed to Ni(OH)2 crystalline phase. Raman spectroscopy was applied to examine the composition of the surface layer. The Raman spectra (Figure 1c & Figure S1a) show MoFe:Ni(OH)2 nanosheets have two peaks at ~460 cm−1 (Ni–O vibrations) and ~3580 cm−1 (O–H vibrations) that are consistent with Ni(OH)2, however, MoFe:Ni(OH)2/NiOOH nanosheets have two peaks at ~474 cm−1 and ~554 cm−1, which are assigned as Ni-O bending and stretching vibrations corresponding to

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NiOOH.43,44 Based on XRD patterns and Raman spectra, after electrochemical oxidation treatment the crystalline phase was still MoFe:Ni(OH)2, but all the surfaces had fully transformed into MoFe:NiOOH due to disappeared peak at ~3580 cm−1 (Figure S1b).45

Figure 1. (a) The synthetic steps of MoFe:Ni(OH)2/NiOOH nanosheets on nickel foam directly. (b) XRD patterns of MoFe:Ni(OH)2 and MoFe:Ni(OH)2/NiOOH nanosheets. (c) Raman spectra of MoFe:Ni(OH)2 and MoFe:Ni(OH)2/NiOOH nanosheets. (d-g) TEM images and HRTEM images of MoFe:Ni(OH)2 and MoFe:Ni(OH)2/NiOOH nanosheets. The morphology of the MoFe:Ni(OH)2/NiOOH nanosheets were characterized by using scanning electron microscopy (SEM), as shown in Figure S2. The SEM images show that the MoFe:Ni(OH)2/NiOOH nanosheets were constructed by many ultrathin nanosheets. The transmission electron microscopic (TEM) images (Figure 1d,f and Figure S3 and S4) further show both MoFe:Ni(OH)2 nanosheets and MoFe:Ni(OH)2/NiOOH nanosheets were constructed by many ultrathin nanosheets. A high-resolution transmission electron microscopy (HRTEM) image of MoFe:Ni(OH)2 nanosheets is shown in Figure 1e, and the interplanar spacing of MoFe:Ni(OH)2 is determined to be ~0.78 nm (Figure S5), which is identical with the lattice planes of Ni(OH)2 (003). A HRTEM image of MoFe:Ni(OH)2/NiOOH nanosheets is shown in Figure 1g, the lattice fringes of MoFe:Ni(OH)2 is obvious, while the lattice fringes of MoFe:NiOOH is blurry due to poor crystallinity, which is consistent with XRD pattern (the presence of only MoFe:Ni(OH)2) and Raman spectrum (the presence of MoFe:NiOOH) of MoFe:Ni(OH)2/NiOOH. The TEM images (Figure S6-S8) show Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH, and Fe:Ni(OH)2/NiOOH all were constructed by many ultrathin nanosheets. High-angle annular dark-field scanning transmission electron microscopy (HAADF–STEM) images and the corresponding energydispersive X-ray (EDX) mappings (Figure 2a-2d) show different elements were distributed homogeneously. EDX spectrum of MoFe:Ni(OH)2/NiOOH (Figure S9) show the atom ratio of Mo, Fe and Ni was about 1:1:18. The Raman spectra (Figure 2e) show Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH, Fe:Ni(OH)2/NiOOH and MoFe:Ni(OH)2/NiOOH all have two peaks at ~474 cm−1 and ~554 cm−1, which are assigned as NiO bending and stretching vibrations corresponding to NiOOH.43-45

CV analyzing (Figure 2f) showed that, these redox couples are corresponding to the oxidation/reduction of Ni in Ni(OH)2/NiOOH. Alexis T. Bell et al. reported the Ni(OH)2/NiOOH redox peaks shift to higher potentials with increasing Fe content.46 Comparing the redox peaks of Ni(OH)2/NiOOH and Fe:Ni(OH)2/NiOOH in Figure 2f, the redox peaks shift from 1.380 V to 1.388 V vs. RHE, which reveals Fe has been successfully modified for Fe:Ni(OH)2/NiOOH. Comparing the redox peaks of Ni(OH)2/NiOOH and Mo:Ni(OH)2/NiOOH in Figure 2f, the redox peaks shift from 1.380 V to 1.333 V vs. RHE, which reveals Mo has been successfully modified for Mo:Ni(OH)2/NiOOH. As further proof, CV analyzing (Figure S10) showed the potential of the redox peak of commercial MoO3 (Figure S11) was 1.287 V vs. RHE and there was nearly no current at 1.333 V vs. RHE, which rules out the existence of molybdenum oxide. Raman spectrum of Mo:Ni(OH)2/NiOOH (Figure 2e) shows the presence of NiOOH, however, the potential of the redox peak of Mo:Ni(OH)2/NiOOH is 1.380 V vs. RHE, at which there is nearly no current for Ni(OH)2/NiOOH. Therefore, it is concluded that the Ni(OH)2/NiOOH redox peak shifts to lower potential is owing to Mo-modification. Because MoFe: Ni(OH)2/NiOOH was synthesized by Mo-modification hydrothermal treament first and then Fe-modification hydrothermal treatment, whether Fe has been successfully modified could be judged by comparing the redox peaks of MoFe:Ni(OH)2/NiOOH and Mo:Ni(OH)2/NiOOH. As shown in Figure 2f & Figure S12, the potential of the redox peak of MoFe:Ni(OH)2/NiOOH was 1.369 V vs. RHE and there was nearly no current at 1.333 V vs. RHE. Therefore, it is concluded that Fe has been successfully modified for MoFe:Ni(OH)2/NiOOH, and the redox peak shifts to higher potential compared with Mo:Ni(OH)2/NiOOH is owing to the modification of Fe. These results confirm that Mo and Fe have been successfully modified for MoFe:Ni(OH)2/NiOOH. X-ray photoelectron spectroscopy (XPS, Figure 2g) was used to further confirmed the Mo- and (or) Fe-modification.31 By comparing Fe:Ni(OH)2/NiOOH and Ni(OH)2/NiOOH, the Femodification leads the XPS peak of Ni shifts partly to higher binding energy. It means a part of Ni has higher interaction strength with OER intermediates and a concomitant increase in OER activity. Figure S13 shows the Mo 3d spectra of MoFe:Ni(OH)2/NiOOH, and the peaks at 231.5 and 234.7 eV are belonged to Mo6+, which is a structurally versatile coordination host, leading to good adsorption energies for OER intermediates and a concomitant increase in OER activity. Furthermore, by comparing MoFe:Ni(OH)2/NiOOH and Ni(OH)2/NiOOH, the modification of Mo and Fe to Ni(OH)2/NiOOH leads the XPS peak of Ni shifts partly to higher binding energy, which means a part of Ni has higher interaction strength with OER intermediates.

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ACS Catalysis Fe:Ni(OH)2/NiOOH (1.07 F cm−2, Figure S17) and MoFe:Ni(OH)2/NiOOH (0.97 F cm−2, Figure S18) are similar, thus the specific activity of MoFe:Ni(OH)2/NiOOH for the OER is much higher than that of the others, confirming its high intrinsic activity. MoFe:Ni(OH)2/NiOOH showed the best OER activity among all investigated bimetallic and trimetallic oxide, hydroxide and oxyhydroxide electrocatalysts. In addition, the corresponding activity data (Figure S19) of Ni foam show the OER activity current density @ 1.53 V vs. RHE of Ni foam (0.5 mA cm−2) is much smaller than that of Ni(OH)2/NiOOH (12.8 mA cm−2), Mo:Ni(OH)2/NiOOH (18.5 mA cm−2), Fe:Ni(OH)2/NiOOH (49.2 mA cm−2) and MoFe:Ni(OH)2/NiOOH (134.5 mA cm−2). The electrochemical double-layer capacitance of Ni foam (0.10 F cm−2, Figure S20) is much smaller than that of Ni(OH)2/NiOOH (1.01 F cm−2), Mo:Ni(OH)2/NiOOH (0.95 F cm−2), Fe:Ni(OH)2/NiOOH (1.07 F cm−2) and MoFe:Ni(OH)2/NiOOH (0.97 F cm−2). Therefore, the similar capacity determined for the various materials is due to the similar electrochemical surface area. Moreover, the Faradaic efficiency (Figure S21) using amount of O2 detection is very close to 100%.

Figure 2. (a-d) HAADF–STEM images and the corresponding EDX mappings of Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH, Fe:Ni(OH)2/NiOOH and MoFe:Ni(OH)2/NiOOH. (e-g) Raman spectra, CV curves and XPS spectra (Ni 2p) for Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH, Fe:Ni(OH)2/NiOOH and MoFe:Ni(OH)2/NiOOH. Figure 3a shows the polarization curves of Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH, Fe:Ni(OH)2/NiOOH and MoFe:Ni(OH)2/NiOOH. Compared to Ni(OH)2/NiOOH, all of the other samples showed significantly enhanced electrocatalytic activity in the OER and MoFe:Ni(OH)2/NiOOH is the best catalyst. At a potential of 1.53 V vs. RHE, MoFe:Ni(OH)2/NiOOH gave a current density of 134.5 mA cm−2 for the OER, which is much higher than that of Fe:Ni(OH)2/NiOOH (49.2 mA cm−2), Mo:Ni(OH)2/NiOOH (18.5 mA cm−2) and Ni(OH)2/NiOOH (12.8 mA cm−2). The OER performance of the different electrocatalysts was further tested in terms of their Tafel slopes (Figure 3b). The Tafel slope of MoFe:Ni(OH)2/NiOOH is much lower than that of Fe:Ni(OH)2/NiOOH (90 mV dec−1), Mo:Ni(OH)2/NiOOH (75 mV dec−1) and Ni(OH)2/NiOOH (104 mV dec−1), implying a more rapid OER rate for MoFe:Ni(OH)2/NiOOH. The Nyquist plots (Figure S14) show that the semicircular diameter of MoFe:Ni(OH)2/NiOOH is obviously smaller than those of Fe:Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH and Ni(OH)2/NiOOH, implying that the MoFe:Ni(OH)2/NiOOH possesses the smallest charge transfer impedance.47 ECSA (electrochemical surface area) was used to rule out the contribution regarding the total amount or surface area. The ECSA can be estimated from the electrochemical double-layer capacitance.8 As shown in Figure 3c, the electrochemical double-layer capacitance of Ni(OH)2/NiOOH (1.01 F cm−2, Figure S15), Mo:Ni(OH)2/NiOOH (0.95 F cm−2, Figure S16),

Figure 3. (a,b) Polarization curves and Tafel plots of Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH, Fe:Ni(OH)2/NiOOH and MoFe:Ni(OH)2/NiOOH in 1M KOH for OER. (c) Current density at the overpotential of 300 mV and electrochemical capacitance of Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH, Fe:Ni(OH)2/NiOOH and MoFe:Ni(OH)2/NiOOH. (d) Chronopotentiometric curves with constant current density of 100 mA cm−2. All of the potentials are iR-corrected. Catalyst durability is fundamental for practical applications. The chronopotentiometry was conducted under steady-state current density of 100 mA cm-2. As shown in Figure 3d, MoFe:Ni(OH)2/NiOOH needs overpotential of only ~280 mV to afford the current densities of 100 mA cm-2 and the timedependent potential curve shows no obvious degradation under steady-state current density of 100 mA cm-2 even after 50 h. Meanwhile, Mo:Ni(OH)2/NiOOH needs overpotential of ~370 mV to afford the current densities of 100 mA cm-2 and the time-dependent potential curve shows no obvious degradation under steady-state current density of 100 mA cm-2 even after 50 h. By contrast, in the beginning Fe:Ni(OH)2/NiOOH needs overpotential of only ~320 mV to afford the current densities of 100 mA cm-2, however, after 30h, needs overpotential of 480 mV (additional 160mV) to

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afford the current densities of 100 mA cm-2. By comparing Fe:Ni(OH)2/NiOOH and Ni(OH)2/NiOOH, the addition of Fe to Ni(OH)2/NiOOH enhances the OER activity, however, the OER stability is nearly unchanged. By comparing Mo:Ni(OH)2/NiOOH and Ni(OH)2/NiOOH, the modification of Mo to Ni(OH)2/NiOOH enhances the OER stability significantly. MoFe:Ni(OH)2/NiOOH nanosheets is more efficient and more stable for OER than all the others, which proves that Mo and Fe dual modification is a synergistic effect to enhance both activity and stability. The postelectrocatalytic characterizations (Figure S22-S29) show MoFe:Ni(OH)2/NiOOH nearly maintained unchanged after the chronopotentiometric reaction.

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outperforming all OER catalysts reported to date. XPS studies revealed that after addition of Mo and Fe, the peak of Ni shifts to higher binding energy, which means Mo and Fe dual modification is a synergistic effect to increase the interaction strength with OHad and a concomitant increase in OER activity. DFT+U calculations revealed that MoFe:NiOOH is enhanced stable on the structure stability. This work may open a avenue to explore the dual modification to further improve both activity and stability for OER.

ASSOCIATED CONTENT Supporting Information Details of synthesis, more characterization, and theoretical calculations; Figures S1-S30; Tables S1-S2 are included as Supporting Information.This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] ; [email protected]

Author Contributions Figure 4. (a) Activity trends towards oxygen evolution, for NiOOH, Fe:NiOOH and MoFe:NiOOH. (b) Dopant formation energies and model structures of NiOOH, Mo:NiOOH, Fe:NiOOH and MoFe:NiOOH. Previous computational studies show that the O to OH adsorption energy difference (∆GO-∆GOH) is the main descriptor for the observed activity trends towards oxygen evolution, as indicated by the volcano-type curve shown in Figure 4a.21,46 The data of NiOOH and Fe:NiOOH are acquired from Alexis T. Bell’s work.21 Based on their method, our calculations revealed that MoFe:NiOOH (Figure S30) has the best adsorption energies for OER intermediates and a concomitant increase in OER activity.21 Both experimental and computation works agree with each other. Although there is no direct theoretic calculation about OER stability, previous computational studies show that calculated dopant formation energy could affect the structure stability, which is a necessary condition for electrochemical stability.22 A significant, fundamental, interesting challenge associated with the use of bimetallic oxyhydroxides as OER electrocatalysts is their usually poor long-term OER stability due to poor structure stability based on thermodynamics.30,31 Our density functional theory plus Hubbard U (DFT+U) calculations revealed that MoFe:NiOOH is more stable than Fe:NiOOH on the structure stability, maybe leading to better OER stability. The calculated dopant formation energy of MoFe:NiOOH (-4.51 eV) and Mo:NiOOH (-2.41 eV) are much smaller than Fe:Ni(OH)2/NiOOH (-0.16 eV), which shows the Mo-modification decreases the energy dramatically and improves the structure stability obviously. In conclusion, we discovered Mo- and Fe-modification is a synergistic effect for OER activity and stability. MoFe:Ni(OH)2/NiOOH nanosheets show a more efficient and more stable electrocatalysts for OER than Fe:Ni(OH)2/NiOOH, Mo:Ni(OH)2/NiOOH and Ni(OH)2/NiOOH. The MoFe:Ni(OH)2/NiOOH nanosheets needs an overpotential of only about ~280 mV to achieve current density of 100 mA cm-2 and shows no evidence of degradation after 50 hours at such high current density,

‡These authors contributed equally.

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by the Major International (Regional) Joint Research Project (51210002), the National Basic Research Program of China (2015CB932304), the Natural Science Foundation of Guangdong Province (2015A030312007) and Guangxi Science and Technology Project (AB16380030). PKS acknowledge the support from the Danish project of Initiative toward Non-precious Metal Polymer Fuel Cells (4106-000012B).

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