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Oxygen Vacancies Confined in Nickel Molybdenum Oxide Porous Nanosheets for Promoted Electrocatalytic Urea Oxidation Yun Tong, Pengzuo Chen, Mengxing Zhang, Tianpei Zhou, Lidong Zhang, Wangsheng Chu, Changzheng Wu, and Yi Xie ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03177 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Oxygen Vacancies Confined in Nickel Molybdenum Oxide Porous Nanosheets for Promoted Electrocatalytic Urea Oxidation Yun Tong 1‡, Pengzuo Chen 1‡, Mengxing Zhang 2, Tianpei Zhou 1, Lidong Zhang 2, Wangsheng Chu2, Changzheng Wu1*, and Yi Xie1 1. Hefei National Laboratory for Physical Sciences at the Microscale, iChEM (Collaborative Innovation Center of Chemistry for Energy Materials), and CAS Key Laboratory of Mechanical Behavior and Design of Materials, University of Science and Technology of China, Hefei 230026, PR China. 2. National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, China.

Abstract Direct urea fuel cell (DUFC) as an efficient technology of generating power from urea, shows great potential for energy sustainable developments, but greatly hindered by the slow kinetics of urea oxidation reaction (UOR). Herein, we highlighted a defect engineering strategy to design oxygen-vacancies rich NiMoO4 nanosheets as a promising platform to study the relationship between O-vacancy and UOR activity. Experimental/theoretical results confirm that the rich Ovacancies confined in NiMoO4 nanosheets successfully brings synergetic effects of higher

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exposed active sites, faster electron transport and lower adsorption energy of urea molecules, giving rise to largely improved UOR activity. As expected, the r-NiMoO4/NF 3D electrode exhibits higher current density of 249.5 mA cm-2, which is about 1.9 and 5.0 times larger than that of p-NiMoO4/NF and Ni-Mo precursor/NF at the potential of 0.6 V. Our finding will be a promising pathway to develop non-noble materials as high-efficient UOR catalysts.

Keywords: oxygen vacancy, porous nanosheet, 3D configuration, urea oxidation, nickel molybdenum oxide

Introduction Electrocatalytic system plays a key role in coping with growing energy and environmental crises derived from the excessive utilization of fossil fuels.1-5 Urea electrolysis, which utilizes urea from wastewater or fertilizer urea to generate electricity, offers great potential for the development of sustainable alternative energy sources and simultaneously alleviating water contamination.6-9 Due to the complex 6e- transfer process, the half-reaction of anodic urea oxidation reaction (denoted as UOR, CO(NH2)2+6OH-→N2+5H2O+CO2+6e-) suffers the intrinsically sluggish kinetics, and requires expensive noble metals catalysts to promote this electrocatalytic oxidation process.10-12 However, the high cost and scarce crustal abundance of these noble metals severely limit their commercial viability for UOR.13 Therefore, developing highly-efficient and economical catalysts to realizing superior UOR catalytic performance is actively being perused. Despite tremendous efforts have been devoted to obtaining high catalytic active nonprecious-metal electrocatalysts for UOR process, such as S-doped Ni(OH)2 nanosheets,14 spinel Ni1.5Mn1.5O4 particles15 and perovskite-type LaNiO3,16 their UOR catalytic


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performance is still largely hindered by the low active sites exposure and poor intrinsic electrical conductivity, which is far from the requirements of practical applications. Two-dimensional (2D) nanosheet materials have attracted explosive attention in electrocatalytic field due to their unprecedented functionalities. 2D nanosheets possessed high exposure of surface area to provide abundant active sites for electrocatalytic process, as well as large area contact with the electrolyte to endow faster interfacial electron transfer, catering for improving the UOR kinetics.17-19 Most importantly, benefiting from the dimensionally reduced configuration, most of interior atoms can expose on surface for offering high chemical activity, which is in favor of further modification by various strategies, including defect engineering,20-21 surface/interface treatment22-23 and structure disorder.24-26 Among them, defect engineering, especially for oxygen vacancies for 2D transition metal oxides (TMOs) materials, has been regarded as effective strategy to regulate their electronic structure, catalytic active sites and adsorption energy of reactive species. Inducing oxygen vacancies with structure disorder on the surface of TMOs nanosheets could serve as highly active sites for catalytic reaction, increase the density of states near the Fermi level for more efficient electron transfer and weaken the metaloxygen bond to realize a faster intermediate exchange effect, resulting in a superior electrocatalytic performance.27-28 However, although oxygen vacancies engineering strategy show important influence on the widely electrocatalytic field, the comprehensive understanding of the underlying correlations between oxygen vacancies and intrinsic UOR catalytic activity has rarely been reported. Herein, inspired by this consideration, abundant O-vacancies confined in NiMoO4 nanosheets is proposed as an ideal platform for studying the O-vacancy-UOR performance relationship. As an example, we highlight a defect engineering strategy to achieve oxygen-vacancies rich NiMoO4


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(r-NiMoO4) nanosheets by regulating the electronic strucutre, representing the advanced electrocatalyst for UOR process. The formation of O-vacancies in NiMoO4 nanosheets successfully endows more active sites, better electron transport capacity and optimal adsorption energy, realizing greatly enhanced UOR performance. This work shed light on developing highactive electrocatalysts for energy storage and conversion system.

Results and discussion In this work, the oxygen-vacancies rich NiMoO4 porous nanosheets grown on Ni foam substrate were synthesized through topochemical transformations from the corresponding Ni–Mo hydroxide/NF precursor. The morphological feature of as-obtained three samples (Ni-Mo precursor/NF, p-NiMoO4/NF and r-NiMoO4/NF) was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 1a,b and Figure S4, both the SEM images of p-NiMoO4/NF and r-NiMoO4/NF electrodes suggests the NiMoO4 nanosheets can uniformly grow on the substrate and maintain the structural stability after annealing treatment. Meanwhile, the corresponding TEM images of r-NiMoO4 and pNiMoO4 exhibit the typical hierarchical nanosheet morphology with abundant porous feature(Figure 1c,d). Furthermore, high-resolution transition electron microscopy (HRTEM) and element mapping technique have been carried out to study their phase and composition. As shown in Figure 1e, the HRTEM image of r-NiMoO4 nanosheet further confirm the porous character of as-prepared NiMoO4 nanosheet and exhibit a distinct inter-planar distance of 2.03 Å that matched well with the (222) lattice plane of standard NiMoO4. In addition, element mapping images of r-NiMoO4 and p-NiMoO4 samples in Figure 1f and Figure S5 display the


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homogeneous spatial distribution of Ni (indicated by green color), Mo (indicated by blue color) and O (indicated by red color) in the whole nanosheet. X-ray diffraction (XRD) has been performed to investigate the structural information of the as-obtained electrode materials. As shown in Figure 2a and Figure S2, except for the two peaks (marked as dark rhombus) derived from Ni foam substrates, the other diffraction peaks can be well indexed to the NiMoO4 phase with the standard card of JCPDS No. 860361, indicating that the crystal structure of r-NiMoO4/NF and p-NiMoO4/NF were not influenced by oxygen defects generation. In order to further study the valence state information of as-prepared r-NiMoO4/NF and p-NiMoO4/NF, X-ray photoelectron spectroscopy (XPS) has been performed. In Figure S7, the survey XPS spectrum clearly confirms that r-NiMoO4/NF and p-NiMoO4/NF are both consisted of Mo, Ni, O elements. Moreover, as shown in Figure 2b, the Ni 2p core level spectra of r-NiMoO4/NF and p-NiMoO4/NF both include one pair of spin-orbit doublets and their correspondent two shakeup satellites (denoted as “sat”). The spin-orbit doublets locating at 856.1 eV and 873.9 eV can be ascribed to Ni 2p1/2 and Ni 2p3/2, respectively. Moreover, the energy difference between the absolute positions of Ni 2p1/2 and Ni 2p3/2 was 17.8, which is a signature of the Ni2+ oxidation state.29 In Figure 2c of Mo 3d XPS spectrum, the best deconvolution of the Mo 3d profile exhibits two sets of doublets which could be ascribed to Mo6+ and Mo5+.30-31 The Mo5+/Mo6+ intensity ratio in r-NiMoO4/NF sample shows obvious enhancement of Mo5+, indicating the greatly increasing content of oxygen defects. Moreover, the O 1s spectra was shown in Figure 2d, there are three peaks can be clearly identified, in which the peak at 529.7 eV and 532.6 eV can be ascribed to metal-oxygen bonds and surface-adsorbed water molecules, respectively. The peak located at 531.4 eV can be attributed to O-atoms in the vicinity of an O-vacancy.32-33 In addition, the proportion of oxygen defects of r-NiMoO4/NF is


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obvious larger than that of p-NiMoO4/NF, indicating that r-NiMoO4/NF obtained in N2 possess many more oxygen defects than p-NiMoO4/NF. X-ray absorption spectroscopy (XAS) has been used to detect the local chemical configuration around Mo and Ni atoms. As shown in Figure 3a, both the r-NiMoO4 and pNiMoO4 samples exhibit the same FT curves at the Ni K-edges, indicating the number of the oxygen coordinator is preserved around Ni atoms. As a contrast, the FTs curves of Mo-edges show three distinguish shells at around 1.24 Å, 1.65 Å and 2.96 Å (Figure 3b). The first and second peaks in the FT curves are corresponding to the Mo-O bond, which is divided into two sub-bonds denoted by Mo-O1 and Mo-O2. The third peak arises from the photoelectron scattering paths between the metals (Ni and Mo) and the outside oxygen atoms, as well as the multiplescattering paths of Mo-O-O. Notably, the Mo-O1 shells of the two samples present the almost identical intensity, suggesting that the coordination number and the distortion of the first nearest neighbored oxygen atoms is almost the same. And for the Mo-O2, the intensity of r-NiMoO4 is much lower than the p-NiMoO4, which reflects the absorbed Mo atoms in the r-NiMoO4 bind less oxygen coordinators, indicating more oxygen vacancies were generated. The loss of the oxygen coordinators also induced the intensity decrease of the third shell, due to the two reasons: 1) The first nearest neighbored oxygen atoms around one absorber Mo contribute to the third FT shell of the another nearest Mo. 2) The multiple scattering paths involving the second type of oxygen atom, Mo-O-O paths, contribute to the third shell. Based on above analysis, the location of oxygen vacancy in porous NiMoO4 nanosheet can be determined by the corresponding EXAFS fitting results (Figure 3c, d). To investigate the UOR electrocatalytic activity, the electrochemical measurements have been conducted. The cyclic voltammetry (CV) curves of r-NiMoO4/NF electrode were firstly


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collected in 1 M KOH solution in the absence and presence of 0.5 M urea. As shown in Figure 4a, comparing to the CV curve recorded only in KOH solution, the anodic current density of rNiMoO4/NF increase largely after adding urea, demonstrating its high catalytic response activity for electro-oxidation of urea. Moreover, the similar measurements were also performed over bare Ni foam, p-NiMoO4/NF and Ni-Mo precursor/NF electrodes for comparison. As shown in Figure 4b, the bare Ni foam shows a negligible catalytic activity for UOR, while a significantly enhanced electrocatalytic performance can be observed after growing the oxygen-vacancies-rich NiMoO4 porous nanosheets, which can be confirmed by a smaller onset potential and higher current density of r-NiMoO4/NF electrodes. For example, at the potential of 0.6 V vs. Ag/AgCl, the current density of r-NiMoO4/NF electrode is 249.5 mA cm-2, which is obviously higher than that of p-NiMoO4/NF (130.5 mA cm-2) and Ni-Mo precursor/NF (49.8 mA cm-2) materials. Furthermore, the UOR kinetics of as-obtained catalysts was investigated by the corresponding Tafel plots. As shown in Figure 4c, the Tafel slope of r-NiMoO4/NF electrode can be calculated about 32.5 mV/dec, which is much lower than that of p-NiMoO4/NF (59.4 mV/ dec) and marginally smaller than that of Ni-Mo precursor/NF (89 mV/ dec). Meanwhile, the further evaluation of as-prepared electrodes for UOR kinetics has been conducted by the electrochemical impedance spectroscopy (EIS). As shown in Figure 4d, the charge transfer resistance (Rct) for Ni-Mo precursor/NF, p-NiMoO4/NF and r-NiMoO4/NF are 11.0 Ω, 4.8 Ω and 2.6 Ω, respectively. A smallest interfacial charge transfer resistance of r-NiMoO4/NF can be observed comparing to the other two electrodes, indicating its largely promoted reaction kinetic during the UOR process.34-35 Stability is another important requirement to evaluate the long-term UOR catalytic activity of the electrocatalyst. To evaluate the stability of as-obtained r-NiMoO4/NF electrode material,


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long-term CV cycling and chronoamperometric response tests were further conducted. As shown in Figure 4e, the as-prepared r-NiMoO4/NF shows superior catalytic stability with a slight degradation in current density after 1000 CV tests and a long period of tests (Figure S12), suggesting the superior durability of r-NiMoO4/NF for the UOR process under alkaline medium. Moreover, the different CV curves of r-NiMoO4/NF electrode recorded at the different scan rate from 5 to 100 mV s-1 have also been carried out. As shown in Figure 4f and Figure S13, the rNiMoO4/NF electrode exhibits an insignificant change of current density during the evolutional process of scan rates, indicating the superior capability for UOR process under alkaline condition. Furthermore, the electrochemical double-layer capacitance (Cdl) measurements were further performed to evaluate the active surface areas of as-prepared electrode materials.36-39 As shown in Figure 5a and Figure S14, the Cdl of r-NiMoO4/NF is calculated to be 149.5 µF cm−2, which is much higher than those of Ni-Mo precursor/NF (Cdl = 58.3 µF cm−2) and p-NiMoO4/NF (Cdl = 119.4 µF cm−2), suggesting the larger electrochemical active surface areas of rNiMoO4/NF deriving from the formation of more oxygen vacancies. Based on above results, the as-prepared r-NiMoO4/NF electrode material has been proved to be a superior electrocatalyst for electro-oxidation of urea. In order to understand the role of oxygen vacancies during the UOR catalytic process, density-functional-theory (DFT) calculations have been performed. Comparing to the pristine NiMoO4 surface, the smaller adsorption energy of urea molecules on the r-NiMoO4 surface can be achieved (Figure 5b), indicating the initial catalyst-urea state can easily occur on the surface of the r-NiMoO4 catalyst to accelerate the whole UOR process. Moreover, the total densities of states (TDOSs) of both pristine NiMoO4 and oxygen-vacancy rich NiMoO4 have been plotted to further study the effect of oxygen vacancies on the electronic structure. As displayed by the DOS


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in Figure 5c,d, the Femi surface of NiMoO4 with rich oxygen vacancies is distinctly shifting to the conduction band edge compared with the pristine NiMoO4, which suggests that more carriers can be effectively transferred to the conduction band minimum of the r-NiMoO4 sample.40-41 This result can also be confirmed by the obviously increased DOS around Femi surface with respect to the pristine NiMoO4 sample, indicating the r-NiMoO4 possesses higher carrier density and conductivity to facilitate the electron transfer during electrocatalytic process. The evaluation of UOR catalytic activity of NiMoO4/NF electrode has proved the superior activity of NiMoO4 nanosheets, and the presence of O-vacancies can further promoted its UOR performance. The enhanced activity of r-NiMoO4/NF electrode may be associated with morphology and electronic structural properties. First, the morphology of nanosheets with porous structure guaranteed the larger surface active area, providing abundant active centers for UOR process. Second, the abundant oxygen vacancies confined in the NiMoO4 nanosheet endows its higher carrier concentration and electrical conductivity, which is in favor of the faster charge transfer process. Further, the electrons neighboring the oxygen vacancy will become delocalization, activating the metal-ion center becomes more active towards the adsorption of urea. Finally, the unique 3D conductive network combining with the highly porous structure can facilitate the mass transport and the penetration of electrolyte, thereby improving the UOR kinetic process.

Conclusions In conclusion, we highlighted that oxygen-vacancies confined in NiMoO4 nanosheets are firstly presented as an ideal material model for revealing the role of O-vacancies in electrooxidation of urea. As a proof-of–concept prototype, oxygen-vacancies rich NiMoO4 nanosheets


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grown on Ni foam are successfully synthesized by a facile defect engineering strategy, representing as a new high-active UOR electrocatalyst under alkaline condition. Thanks to the increased surface areas, improved charge transfer capacity, favorable reaction kinetics, optimal adsorption of urea molecular and superior structural stability, the r-NiMoO4/NF 3D electrode exhibits greatly enhanced UOR performance with a lower Tafel slope and higher current density. Both theoretical simulations and experimental results confirmed that the introduction of oxygen vacancies into the NiMoO4 nanosheets will bring a synergistic effect for improved electrochemical UOR performance. This work provides a guideline for the rational design of advanced electrode materials as high-efficiency electrocatalysts for UOR.

Methods Synthesis of Ni–Mo hydroxide precursor, Vo-rich and -poor NiMoO4 nanosheet grown on Ni foam (Precursor/NF, r-NiMoO4/NF and p-NiMoO4/NF): All of the chemicals were of analytical grade and used without further purification. Ni foam was firstly cleaned carefully by diluted hydrochloric acid, and then washed in succession with deionized water, acetone, and ethanol for several times. In a typical process, 1.5 mmol Ni(NO3)2·6H2O, and 1.5 mmol Na2MoO4·6H2O were firstly dissolved in 40 ml mixed solution consisted of 20 ml deionized water and 20 ml absolute ethanol and stirred vigorously for 1 h. After that, a cleaned Ni foam substrate (2 cm × 4 cm) was immersed into above solution for 20 min and the solution was transferred into a 50 ml Teflon-lined stainless-steel autoclave and maintained at 130 oC for 12 h. After the autoclave was allowed to cool down to room temperature, a light yellow product was formed on the Ni foam and then the precursor was wished with water, ethanol for several times and dried in vacuum. The r-NiMoO4/NF and p-NiMoO4/NF samples were prepared by heating above Ni–Mo


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hydroxide precursor/Ni foam product at 450 oC under N2 and O2 atmosphere, respectively. The obtained products were collected for further characterization and electrochemical measurement. Structural Characterization: The field-emission scanning electron microscopy (FE-SEM) images and corresponding energy-dispersive spectroscopy (EDS) mapping analyses were performed on a JEOL JSM-6700F SEM. The transmission electron microscopy (TEM) was carried out on a JEM-2100F field-emission electron microscope operated at an acceleration voltage of 200 kV. The high-resolution TEM (HRTEM) images were carried out on a JEOL JEM-ARF200F TEM/STEM with a spherical aberration corrector. X-ray powder diffraction (XRD) was performed using a Philips X’Pert Pro Super diffractometer with Cu-Kα radiation (λ = 1.54178 Å). X-ray photoelectron spectra (XPS) were obtained on an ESCALAB MK II X-ray photoelectron spectrometer with Mg Kα as the excitation source. The binding energies achieved in the XPS spectral analysis were corrected for specimen charging by referencing C 1s to 284.5 eV. The absorption spectra of Ni K-edge were collected in transmission mode using a Si (111) double-crystal monochromator at the X-ray absorption fine structure (XAFS) station of the 1W1B beamline of the Beijing Synchrotron Radiation Facility (BSRF) at room temperature. Calculation details: All the density functional theory (DFT) calculations were carried out with the code VASP5.2.42-43 using the generalized gradient approximation (GGA) with the PerdewBurke-Ernzerhof (PBE) functional.44 The studied model consists 1 × 1 × 1 unit cell, where the vacuum space of 15 Å were applied to the two layers in nearest-neighbor unit cells. A planewave basis set with cut-off energy 500 eV was employed within the framework of the projector augmented-wave (PAW) method. The Brillouin zone was sampled with a 9 × 7 × 2 k-points sample with Monkhorst-Pack method.


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Electrochemical tests: The electrochemical performance of UOR was tested in a three-electrode system on an electrochemical workstation (CHI660B). Ag/AgCl (saturated KCl solution) electrode was used as the reference electrode while the counter electrode was a graphite rod electrode. For 3D electrode, the as-obtained r-NiMoO4/NF was directly used as the working electrodes for the electrochemical tests in the solution (1 M KOH + 0.5 M Urea). The cyclic voltammogram (CVs) plots were recorded at the scan rates of 5-100 mV s-1. Before data collection, the working electrodes were firstly activated using CV test for several times until stabilization was reacthed. Electrochemical impedance spectroscopy (EIS) measurements of the catalysts were performed at the potential of 0.4V vs RHE by using an AC voltage with 5 mV amplitude in a frequency range from 100 KHz to 10 mHz. The electrode stability was tested by both CVs and chronoamperometric response. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Author Contributions ‡

These authors contributed equally.

Notes The authors declare no competing financial interests. ASSOCIATED CONTENT Supporting Information The following files are available free of charge.


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Characterizations and comparison experiments results. (PDF) ACKNOWLEDGMENT This work was financially supported by the National Basic Research Program of China (2015CB932302), Natural Science Foundation of China (U1432133 21501164, 1162163, U1632154), National Program for support of Top-notch Yong Professionals, the Anhui Provincial Natural Science Foundation (No. 1608085QA08), the Fundamental Research Funds for the Central Universities (no. WK2060190080). We also appreciate the support from the Major/Innovative Program of Development Foundation of Hefei Center for physical Science and Technology.

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19. Tan, C.; Zhang, H., Chem. Soc. Rev. 2015, 44, 2713-2731. 20. Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y., Adv. Mater. 2013, 25, 5807-5813. 21. Yin, Y.; Han, J.; Zhang, Y.; Zhang, X.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X.; Wang, Y.; Zhang, Z.; Zhang, P.; Cao, X.; Song, B.; Jin, S., J. Am. Chem. Soc. 2016, 138, 7965-7972. 22. Guo, Y.; Xu, K.; Wu, C.; Zhao, J.; Xie, Y., Chem. Soc. Rev. 2015, 44, 637-646. 23. Wang, F.; He, P.; Li, Y.; Shifa, T. A.; Deng, Y.; Liu, K.; Wang, Q.; Wang, F.; Wen, Y.; Wang, Z.; Zhan, X.; Sun, L.; He, J., Adv. Funct. Mater. 2017, 27, 1605802. 24. Xie, J.; Zhang, J.; Li, S.; Grote, F.; Zhang, X.; Zhang, H.; Wang, R.; Lei, Y.; Pan, B.; Xie, Y., J. Am. Chem. Soc. 2013, 135, 17881-17888. 25. Xu, K.; Chen, P.; Li, X.; Tong, Y.; Ding, H.; Wu, X.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y., J. Am. Chem. Soc. 2015, 137, 4119-4125. 26. Stern, L.-A.; Feng, L.; Song, F.; Hu, X., Energy Environ. Sci. 2015, 8, 2347-2351. 27. Guo, Y.; Tong, Y.; Chen, P.; Xu, K.; Zhao, J.; Lin, Y.; Chu, W.; Peng, Z.; Wu, C.; Xie, Y., Adv. Mater. 2015, 27, 5989-5994. 28. Lei, F.; Sun, Y.; Liu, K.; Gao, S.; Liang, L.; Pan, B.; Xie, Y., J. Am. Chem. Soc. 2014, 136, 6826-6829. 29. Xiao, K.; Xia, L.; Liu, G.; Wang, S.; Ding, L.-X.; Wang, H., J. Mater. Chem. A 2015, 3, 6128-6135.


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Figure 1 a) Low-magnification and b) high-magnification SEM images of r-NiMoO4/NF sample. c) Low-magnification and d) high-magnification TEM images of r-NiMoO4 porous nanosheets. (e) HRTEM image of r-NiMoO4 porous nanosheets. (f) The corresponding elemental mapping images of r-NiMoO4/NF electrode.


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Figure 2 a) XRD pattern of r-NiMoO4/NF. XPS spectra of (b) Ni 2p, (c) Mo 3d and (d) O 1s for obtained r-NiMoO4/NF and p-NiMoO4/NF electrodes.


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Figure 3 FT-EXAFS curves of a) Ni-edge and b) Mo-edge for r-NiMoO4 and pNiMoO4 products. c) The schematic diagram of oxygen vacancy. d) The EXAFS fitting results for r-NiMoO4 and p-NiMoO4 products.


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Figure 4 (a) Cyclic voltammetry (CV) curves of r-NiMoO4/NF in 1 M KOH electrolyte with and without 0.5 M urea. (b) CV curves and (c) Tafel plots of rNiMoO4/NF, p-NiMoO4/NF, Ni-Mo precursor/NF and bare Ni foam in 1 M KOH electrolyte with 0.5 M urea. (d) Nyquist plots of r-NiMoO4/NF, p-NiMoO4/NF and Ni-Mo precursor/NF. (e) The CV curves of r-NiMoO4/NF before and after 1000 CV testing. (f) The corresponding data plotted as the current density at different scan rates.


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Figure 5 (a) The Cdl linear fitting and calculation of r-NiMoO4/NF, p-NiMoO4/NF and Ni-Mo precursor/NF, plot of the current density vs the scan rate. (b) The adsorption energy of urea molecules on the p-NiMoO4 and r-NiMoO4 samples. Calculated density of states (DOS) of (c) pristine NiMoO4 and (d) oxygen-defect NiMoO4 samples.


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Oxygen Vacancies Confined in Nickel ... - ACS Publications

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