Oxygen Vacancy Determined Highly Efficient Oxygen Reduction in

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Oxygen Vacancy Determined Highly Efficient Oxygen Reduction in NiCo2O4/Hollow Carbon Spheres Hui Yuan, Jiantao Li, Wei Yang, Zechao Zhuang, Yan Zhao, Liang He, Lin Xu, Xiaobin Liao, Ruiqi Zhu, and Liqiang Mai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 25 Apr 2018 Downloaded from http://pubs.acs.org on April 25, 2018

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ACS Applied Materials & Interfaces

Oxygen

Vacancy

Determined

Highly

Efficient

Oxygen

Reduction in NiCo2O4/Hollow Carbon Spheres †

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Hui Yuan, Jiantao Li, Wei Yang, Zechao Zhuang, Yan Zhao, *, Liang He,*, , Lin Xu, Xiaobin Liao,† Ruiqi Zhu,† and Liqiang Mai*,† †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing,

International School of Materials Science and Engineering, Wuhan University of Technology, Luoshi Road 122, Wuhan 430070, People’s Republic of China ‡

Department of Materials Science and NanoEngineering, Rice University, Houston, TX 77005,

United States KEYWORDS: Oxygen reduction reaction, oxygen vacancies, mesoporous nanosheets, NiCo2O4, hollow carbon spheres, density functional theory

ABSTRACT: Rationally generating oxygen vacancies in electrocatalysts is an important approach to modulate the electrochemical activity of catalyst. Herein, we report a remarkable enhancement in oxygen reduction reaction (ORR) activity of NiCo2O4 supported on hollow carbon spheres achieved through generating abundant oxygen vacancies within the surface lattice. This catalyst exhibits enhanced ORR activity (larger limiting current density of ~-5.8 mA cm-2) and higher stability (~90% retention after 40,000 s) compared with those of NiCo2O4/HCS and NiCo2O4. The results of X-ray photoelectron spectroscopy (XPS) characterizations suggest that the introduction of oxygen vacancies optimizes valence state of active sites. Furthermore, we carried out density functional theory (DFT) calculations to further confirm the mechanism of

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oxygen vacancies, and results show that oxygen vacancies enhance the density of states (DOS) near Fermi level, decrease work function and lower the calculated overpotential of NiCo2O4.

INTRODUCTION

With the increasingly highlighted energy issue, developing high-capacity, high electrochemically active, inexpensive, pollution-free energy conversion and energy storage systems is in urgent need.1,

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As an important but sluggish cathodic reaction, oxygen reduction reaction (ORR)

employed in fuel cells and metal-air batteries, requires efficient catalysts to overcome its slow kinetics and high overpotential.3-5 Although platinum (Pt)-based catalysts display super electrocatalytic activity both in acid and alkaline medias for ORR, their commercial applications are limited by their scarcity and poor durability.6-8 Therefore, developing electrocatalysts with high efficiency, low price and, more importantly, long-term stability for ORR is urgently necessary. 9,10 As alternative ORR catalysts, transition metal oxides (TMOs), such as MnO2,11 Co3O4,12 CoMn2O4,13 and NiCo2O4,14 have gained considerable interests owing to their low price, abundance, and environmental friendliness.15,16 Among them, the spinel NiCo2O4 displays high activity for oxygen catalysis17,18 or supercapacitors19, stemming from the existence of solid-state redox couples Ni3+/Ni2+ and Co3+/Co2+.20 However, the poor electrical conductivity, low durability and insufficient active sites of NiCo2O4 greatly hinder its large-scale applications for ORR.21 To deal with these problems, many efforts have been devoted to enhancing the electron transport efficiency and exposing more catalytic active sites, including nanoarchitecture engineering22, conductive materials hybridization23-27 and heterogeneous structuring28, etc. For instance, Wu et al. reported a high-activity bi-functional catalyst based on three-dimension (3D)


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porous NiCo2O4 foam for ORR and oxygen evolution reaction (OER), in which the 3D structure provides suitable tunnels to facilitate continuous oxygen to interior substances.29 Lou et al. reported graphene-supported NiCo2O4 nanocrystals as efficient electrocatalysts for ORR, showing a positive onset potential of 0.88 V and a transferred electron number of ~4.0.30 Employing conductive substrates as current collectors is an effective strategy to shorten the electron transport pathways and increase the specific surface area of TMOs, thus improving their electrochemical performance.31,32 Nanocarbon with low price, easy manufacture, and low compactness, is one of the most common and highly qualified supports.33 Meanwhile, the strong couplings between catalyst and carbon support could dramatically change the interface charge density and coordination environment of active sites, which also lead to the enhancement of catalytic performance.34,35 However, the local electron states of these active sites and electron transport efficiency in solids still need further optimization. Tuning oxygen vacancy is emerging as an efficient approach to finely modify the electron and phonon structure of TMOs, and recently has been utilized to improve the electrocatalytic activity of spinel oxides for oxygen catalysis,36,37 supercapacitors38 and batteries.39 To the best of our knowledge, to date, the prospects and roles of oxygen vacancies in NiCo2O4 on ORR catalysis have not been evidently clarified yet. Herein, we synthesize mesoporous NiCo2O4 nanosheets assembled on conductive hollow carbon spheres (NiCo2O4/HCS) via a facile hydrothermal treatment and a subsequent annealing process. The synthesized NiCo2O4/HCS exhibits enhanced electrical conductivity, higher surface area and more edge sites compared with the pristine NiCo2O4. Moreover, abundant oxygen vacancies are introduced through annealing in an oxygen-deficient atmosphere, which can optimize the local electron states and coordinate the active sites of NiCo2O4/HCS-V.


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Specifically, such a catalyst with abundant pores and oxygen vacancies shows an outstanding ORR activity and long-term durability. RESULTS AND DISCUSSION The synthesis process of NiCo2O4/HCS is schematically shown in Figure 1. First, the negatively charged HCS were synthesized and dispersed in aqueous solution. Then the positively charged Co2+ and Ni2+ ions were introduced through dissolving cobalt acetate and nickel acetate into the solution. Due to of electrostatic adsorption, NiCo precursor was grown on HCS (NiCo/HCS precursor) through a hydrothermal treatment. Finally, mesoporous nanosheets assembled microflowers were formed after annealing NiCo/HCS precursor in air at 260, 320 and 450 °C, which were denoted as NiCo2O4/HCS-260, NiCo2O4/HCS-320 and NiCo2O4/HCS-450 microflowers, respectively. To introduce oxygen vacancies, NiCo/HCS precursor was annealed at 320 °C in oxygen-deficient atmosphere (molar ratio of air/N2 = 1:9), and NiCo2O4/HCS-320-V microflowers were obtained.

Figure 1. Schematic diagram of the preparation of NiCo2O4/HCS.


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Crystallographic structures of samples were characterized by X-Ray diffraction (XRD). As presented in Figure 2a, all peaks at 2θ of 18.92°, 31.15°, 36.70°, 44.63°, 59.11° and 64.96° are well indexed to the (111), (220), (311), (400), (511) and (440) crystal planes of spinel NiCo2O4 (JCPDS card No. 01-073-1702), respectively, indicating the pure phases of these samples. Pure HCS were also measured and a peak valley at around 26° is observed, which is characteristic of carbon materials.41 The peaks intensities in XRD patterns of NiCo2O4-320 and NiCo2O4/HCS260 are relatively lower than those of other microflowers, due to their low degree of crystallization.29 Raman spectra are presented in Figure S1, and two peaks of HCS at 1350 and 1600 cm-1 are observed, corresponding to characteristic D-band (disorder induced band) and Gband (graphitic band) of carbon materials. However, such two peaks in the Raman spectra of NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V are not observed, because the HCS are fully coated by NiCo2O4 nanosheets and the content of HCS is much smaller compared with NiCo2O4. Thermogravimetric analysis (TGA) was measured to investigate the thermostability of samples. As shown in Figure S2a, the TGA curve of NiCo/HCS precursor shows a slight weight loss of 2.5% before 160 °C because of the evaporation of its surface water,42 and the main weight loss occurs during 160-320 °C indicating the decomposition of NiCo/HCS precursor and formation of NiCo2O4/HCS. Therefore, the samples were annealed at 260, 320 and 450 °C in this study. The HCS were well preserved after annealing at 320 °C in air according to the TGA result in Figure S2b. To observe the morphologies of different samples, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) characterizations were conducted. Figure S3a reveals the synthesized HCS with homogeneous shape and size of 300-500 nm. NiCo2O4/HCS-320-V microflowers assembled with ultrathin and waving nanosheets were obtained, in which the HCS


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are completely coated by NiCo2O4 nanosheets, as indicated in Figure 2b. Compared with NiCo/HCS precursor (Figure S3b and Figure S3c), the morphologies of microflowers (Figure 2b and Figure S3d and Figure S3e) are well preserved without structural destruction after annealing at 260 and 320 °C. However, some microflowers collapse (Figure S3f and Figure S3g) due to the decomposition of HCS at 450 °C. Additionally, NiCo2O4/HCS microflowers assembled by thinner nanosheets than those of NiCo precursor (Figure S3h) or NiCo2O4 microflowers (Figure S3i) could pose positive effects on achieving larger specific surface area and more catalytic surface sites.43 Figure 2c shows that the ultrathin NiCo2O4 nanosheets are uniformly and seamlessly grown on the surfaces of HCS, which would lead to high specific surface area. Besides, the intimate contact between NiCo2O4 nanosheets and HCS would result in enhanced conductivity. Clear TEM image and HRTEM image of NiCo2O4/HCS are shown in Figure S4a and Figure S4b. Several spheres with the size of 300-500 nm are coated by NiCo2O4 nanosheets, which exhibit the same morphology as HCS, indicating the growth of NiCo2O4 nanosheets on HCS. Figure 2d confirms that the NiCo2O4 nanosheets are sufficiently porous, in which the mesopores were formed owing to the shrinkage of structure during annealing process.32 Such porous structure could provide channels for efficient diffusion of electrolytes and O2 in full contact with catalyst, offering more edges to facilitate ORR.7,45 Also, the polycrystalline feature of NiCo2O4/HCS-320V microflowers was characterized by selected area electron diffraction (SAED) (Figure 2d). The electron diffraction rings of NiCo2O4/HCS-320-V are well defined, and the results are in accordance with crystal parameters of NiCo2O4 (JCPDS card No. 01-073-1702). A lattice spacing of 0.24 nm corresponding to the (311) crystal plane of NiCo2O4 is shown in highresolution TEM (HRTEM) image presented in Figure 2e. Elemental disperse spectroscopy (EDS)


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mapping images (Figure 2f) reveal that the elements of Ni, Co, O and C are uniformly distributed in NiCo2O4/HCS microflowers, in which the C is difficult to be observed because of the ultrathin feature of HCS walls. The element analysis result of N, C, H and S (Table S1) shows only 1.42 wt.% of C in NiCo2O4/HCS-320-V. The results of nitrogen adsorption-desorption isotherm are shown in Figure S5a. BrunauerEmmett-Teller (BET) specific surface areas of NiCo2O4/HCS-320-V and NiCo2O4/HCS-320 are 99 and 90 m2 g-1, respectively, almost 200% higher than that of pristine NiCo2O4-320 without hybridizing with HCS (32 m2 g-1). It is noted that NiCo2O4/HCS-260 and NiCo2O4/HCS-450 exhibit the highest (144 m2 g-1) and lowest (39 m2 g-1) specific surface area among all the samples hybridized with HCS, since high temperature annealing at 450 °C would lead to structural collapse. Furthermore, through analyzing the pore size distributions of all samples (Figure S5b), it can be concluded that the average pore diameter is 6 to 16 nm, and the average pore diameter becomes larger with the increase of annealing temperature (Figure S6a and Figure S6b). Besides, the electrochemically active specific surface area of samples are provided in Figure S7- Figure S9.

The CVs of HCS, NiCo2O4, NiCo2O4/HCS-260, NiCo2O4/HCS-450,

NiCo2O4/HCS-320, NiCo2O4/HCS-320-V and Pt/C were evaluated in 0.1 M KOH electrolyte. The double layer capacitance values (CdI) of samples are calculated from CVs, proportionating to their electrochemically active specific surface area. The effective active specific surface area of NiCo2O4 is improved by introduction of HCS and oxygen vacancies as shown in Figure S7. Besides, from the Figure S8, we can conclude that NiCo/HCS annealing at 320 °C displays the highest active specific surface area, contributing to high ORR activity.


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Figure 2. (a) XRD patterns of samples. (b) SEM image of NiCo2O4/HCS-320-V microflowers. (c) TEM image of NiCo2O4/HCS-320-V microflowers. (d) TEM image of NiCo2O4/HCS-320-V microflowers, inset: the corresponding SAED pattern, (e) HRTEM image of NiCo2O4/HCS-320V microflowers. (f) EDS mapping of NiCo2O4/HCS-320-V microflowers The electrocatalytic activities of different catalysts (HCS, NiCo2O4, NiCo2O4/HCS-260, NiCo2O4/HCS-450, NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V) were studied. The cyclic voltammograms (CVs) were firstly evaluated in 0.1 M KOH and O2-saturated electrolyte. As shown in Figure S10a, these samples show various ORR activities, and NiCo2O4/HCS-320-V exhibits more positive oxygen reduction peak potential and higher capacitive current density than those of other catalysts, indicating its best electrocatalytic activity. No oxygen reduction peak in Ar-saturated solution is observed (Figure S10b).


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Figure 3. (a) LSV curves of samples at a scan rate of 5 mV s−1. (b) Numbers of transferred electron n of samples at the potential of 0.50 V. (c) HO-2 yields and n values of different samples at the potentials of 0.20-0.55 V. (d) Chronoamperometric responses of NiCo2O4/HCS-320, NiCo2O4/HCS-320-V and Pt/C catalysts at 0.50 V. Linear scan voltammetry (LSV) curves of samples were measured on a rotating disk electrode (RDE) at 1600 rpm, as presented in Figure 3a. Obviously, NiCo2O4/HCS-320 shows better ORR performance than the catalysts annealed at 260 and 450 °C by comparing their onset potential, half-wave potential (E1/2) and diffusion-limited current density. The enhancement of ORR catalysis by HCS was also verified by comparing the ORR performances of HCS, NiCo2O4-320 and NiCo2O4/HCS-320. Without HCS support, NiCo2O4-320 demonstrates a much lower ORR activity than NiCo2O4/HCS-320, confirming that employing HCS as support is an effective


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method to enhance ORR catalysis. In addition, NiCo2O4/HCS-320-V exhibits the same onset potential of 0.90 V but more positive E1/2 of 0.78 V and larger diffusion-limited current density of ~-5.8 mA cm-2 compared with NiCo2O4/HCS-320, supposing that the enhancement of ORR performance would be ascribed to the introduction of abundant oxygen vacancies. The specific activity

Tafel

plots

of

HCS,

NiCo2O4-320,

NiCo2O4/HCS-260,

NiCo2O4/HCS-450,

NiCo2O4/HCS-320, NiCo2O4/HCS-320-V and Pt are shown in Figure S11a, respectively, and the NiCo2O4/HCS-320-V exhibits the smallest slope, indicating the fastest ORR kinetics. The LSV curves of NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V at various rotation speeds (400, 625, 900, 1225 and 1600 rpm) are shown in Figure S11b and Figure S11c. It is obvious that the diffusionlimited current increase linearly with the increase of rotation speeds, indicating a first-order O2dependence kinetics. The kinetics of ORR catalysts is also reflected by the Koutecky-Levich (K-L) curves (Figure S11d). The transferred electron numbers n of different catalysts at the potential of 0.50 V are figured out from the K-L curves (Figure 3b). The n values of NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V are ~4.0, while those of HCS, NiCo2O4-320, NiCo2O4/HCS-260 and NiCo2O4/HCS-450 are just ~2.9, 3.7, 3.5 and ~3.9 respectively, suggesting that the ORR processes of NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V are mainly controlled by four-electron mechanism, and the oxygen is reduced to OH-.30 To further confirm the electron transfer numbers and catalytic pathways of those catalysts, the yields of peroxide species were detected by conducting the rotating ring-disk electrode (RRDE) tests (Figure S12 and Figure 3c). The percentages of yields of NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V are below ~8.0 and 3.5 over the potential range of 0.20-0.55 V, while the n values are ~3.9 and ~4.0 respectively. In conclusion, the ORR kinetics of NiCo2O4/HCS-320 and


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NiCo2O4/HCS-320-V can be contributed to four-electron reactions. Besides, OER performance is also enhanced by the introduction of oxygen vacancies, as shown in Figure S13a and Figure S13b. NiCo2O4/HCS-320-V also displays an outstanding long-term durability for ORR. Figure 3d shows the chronoamperometric responses at the potential of 0.50 V. The response current density of NiCo2O4/HCS-320-V exhibits a higher retention rate of ~90% than that of NiCo2O4/HCS-320 (82%) after 40,000 s. More importantly, the stability of HCS-supported NiCo2O4 is higher than that of Pt/C (68% retention after 40,000 s). The synthesized NiCo2O4/HCS-320-V also exhibits outstanding ORR performance among many reported NiCo2O4-based electrocatalysts (Table S2).

Figure 4. XPS spectra of (a) Co 2p and (b) O 1s. To study the elemental states and the oxygen vacancies of samples, X-ray photoelectron spectroscopy (XPS) measurement was performed (Figure 4). The survey spectrum of NiCo2O4/HCS-320-V (Figure S14a) confirms the existence of Co, Ni, O and C elements, and without other impurities are observed. The Co 2p core level spectra in Figure 4a can be well fitted with two spin-orbit doublets corresponding to Co2+ and Co3+, respectively.46 Notably, the atomic ratio of Co2+/Co3+ of NiCo2O4/HCS-320-V (0.96) is higher than that of NiCo2O4/HCS320 (0.79), suggesting more oxygen vacancies of NiCo2O4/HCS-320-V.47 Similar to Co 2p, Ni


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2p core level spectra (Figure S14b) are divided into four peaks belonging to Ni2+ and Ni3+. Besides, two prominent shake-up satellites is observed, which is denoted as “sat”.44 In O 1s core level spectra (Figure 4b), there exist three major peaks of O1, O2, O3 located at ~529.4, 531.5 and 532.9 eV respectively, in conformity with the bonding of oxygen atoms and metals, defect sites with abundant low oxygen coordination and hydroxyl species of water molecules adsorbed on surface.36,43,48 The peak area percentage attributed to O2 of NiCo2O4/HCS-320-V is much larger than that of NiCo2O4/HCS-320, indicating that the sample annealed in oxygen-deficient atmosphere possesses more oxygen vacancies. Photoluminescence spectroscopy (PL) measurement was also carried out to further confirm this result (Figure S15), because the luminescence intensity of NiCo2O4/HCS-320-V is higher than NiCo2O4/HCS-320, The PL emission peak at ~410 nm could be assigned to recombination of the photogenerated holes with two-electron-trapped O-vacancy.43 Besides, the NiCo2O4-320 with low luminescence intensity exhibits its poor oxygen vacancies. The HCS with a weak PL emission peak at ~410 nm is associated with poor oxygen defect for oxygen functional groups of HCS. To understand the origin of enhanced ORR activity of NiCo2O4/HCS with oxygen vacancies, density function theory (DFT) calculations were carried out to study electronic structure, density of states (DOS), work function and free energy. The NiCo2O4 crystal with (311) surface was chosen as our calculation model, due to the fact that it has the highest exposure percentage (Figure S16). Figure 5a shows the DOS of NiCo2O4 coupled with HCS and oxygen vacancies, while the 3D charge distributions are also presented in Figure S17. The Fermi level of NiCo2O4 increases after coupling with HCS; when oxygen vacancies are introduced, the level is further slightly increased. The inset of Figure 5a clearly shows that the DOS near Fermi level of NiCo2O4/HCS-V is enhanced, indicating that more electrons are available to participate in ORR


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process. Figure S18 shows the calculated work functions, with 6.083, 5.210 and 5.197 eV for NiCo2O4, NiCo2O4/HCS and NiCo2O4/HCS-V, respectively. NiCo2O4/HCS-V possesses the lowest work function, thus, electrons can be more easily transferred from the catalyst surface to reactants. Another model is proposed in which the C20 fullerene instead of graphene sheet is used to simulate HCS at same conditions. The models and 3D charge distributions of samples are shown in Figure S19 and Figure S20. The calculated results of density of states (DOS) and work functions of NiCo2O4, NiCo2O4/HCS, and NiCo2O4/HCS-V using C20 fullerene as HCS are in coincidence well with results using graphene sheet as HCS, as shown in Figure S21a and Figure S21b. Ultraviolet photoelectron spectroscopy (UPS) of NiCo2O4/HCS-320 and NiCo2O4/HCS320-V were studied, and the results are in coincidence well with DFT calculation results of NiCo2O4/HCS and NiCo2O4/HCS-V (Figure S22). The ORR free energy changes ∆G of each step are calculated at the zero electrode potential (Φ = 0) presented in Table S4. The first-step values ∆G1 of NiCo2O4/HCS and NiCo2O4/HCS-V are more negative than that of pristine NiCo2O4, which means the coupling with HCS can facilitate the adsorption of O2. NiCo2O4/HCS-V possesses the lowest value of ∆G4, indicating that the introduction of oxygen vacancies could further facilitates OH- desorption. The free energy changes calculated at equilibrium potential (Φ = Φeq) and ORR potential (Φ = ΦORR) 49 are shown in Figure 5b, and the calculated ORR overpotentials were also obtained. The calculated overpotentials of NiCo2O4, NiCo2O4/HCS and NiCo2O4/HCS-V are 0.79, 0.79, and 0.34 V, respectively. NiCo2O4/HCS-V shows the lowest ORR overpotential, illustrating that the introduction of oxygen vacancies enhance the ORR activity, which is in agreement with the electrocatalytical results.


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Figure 5. DFT calculations: (a) DOS and (b) free-energy diagram of ORR on NiCo2O4, NiCo2O4/HCS, and NiCo2O4/HCS-V. The dashed and solid lines in (b) reflect the reactions respectively at equilibrium potential (Φ = Φeq) and ORR potential (Φ = ΦORR). The reasons for the improvement of ORR performance of NiCo2O4/HCS-V could be summarized as follows. First, serving as conductive support, HCS shorten the transfer pathways of both electrons and ions, thus, improving the conductivity of catalyst. Second, the NiCo2O4/HCS with high specific surface due to the prevention of NiCo2O4 nanosheets agglomeration by HCS, has increased contact area with electrolyte during the reaction process, which is supported by the BET result. Besides, porous structure presents more sites edges, and provides channels to allow fast transfer of electrolytes and O2 to inner electrodes. Such ultrathin and abundant porous structure offers more exposed active sites. Most importantly, the oxygen vacancies introduced by annealing NiCo/HCS precursor in an oxygen-deficient atmosphere could optimize the local electron states of active sites and electron transfer efficiency. Thus ORR based on NiCo2O4/HCS-V catalyst could proceed through a faster kinetic process and exhibit lower overpotential as well as higher diffusion-limited current density. CONCLUSIONS


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In summary, we synthesize an efficient ORR electrocatalyst by assembling mesoporous NiCo2O4 nanosheets with oxygen vacancies on HCS. The NiCo2O4/HCS-320-V demonstrates a high ORR activity with a high onset potential (~0.90 V), large limiting current (~-5.8 mA cm-2), low peroxides yield (~2% @ 0.40 V) and favorable stability (~90% retention after 40000 s @ 0.50 V). The excellent performance is attributed not only to the hybrid structure, but also to the introduction of oxygen vacancies. XPS results show that the confined oxygen vacancies lead to optimized valence state of active sites. DFT theoretical calculations further demonstrate that NiCo2O4/HCS-320-V possesses enhanced DOS near Fermi level and lower work function, enabling more efficient electron transfer during catalysis. Besides, free energy calculations indicate a lower ORR overpotential by introducing oxygen vacancies. Our work will offer more guiding principles for other metal oxides-based materials to achieve the rational designed ORR catalysts with low cost and high efficiency. EXPERIMENTAL SECTION Synthesis of NiCo2O4 microflowers. To synthesize NiCo2O4 microflowers, 1 mmol C4H6O4Co·4H2O, 0.5 mmol C4H6O4Ni·4H2O, 2.4 mmol hexamethylenetetramine (HMTA) and 2 mmol NH4F were dissolved in 20 mL deionized (DI) water and 10 mL ethanol. After stirring 20 minutes, the solution was poured into a Teflon-lined stainless autoclave (100 mL), then reserved at 140 °C for 6 h, followed by washing with ethanol and DI water for several times. Afterwards, the precursor of NiCo microflowers was dried at 70 °C for 12 h, and NiCo2O4 microflowers were finally obtained by annealing NiCo precursor at 320 °C for 2 h in air with the heating rate of 5 °C min-1. Synthesis of HCS. The HCS were synthesized by one-step synthesis.40 20 mL DI water, 60 mL ethanol and 3 mL ammonium hydroxide (28 wt.%) were mixed in a 100 mL beaker, then 0.4 g


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resorcinol was added into the solution. 2.8 mL tetraethoxysilane (TEOS) was mixed in the above solution, followed by 10 minutes of stirring, then 0.56 mL formaldehyde (36 wt.%) was injected into the solution, and then continuous stirring was conducted for 24 h. Afterwards, the uniform solution was poured into a Teflon-lined stainless autoclave (100 mL) and reserved at 100 °C for 24 h. After washing several times by ethanol and DI water, the SiO2/HCS powder was obtained after carbonization at high temperature (800 °C) for 5 h in N2 atmosphere. Finally, the SiO2 in SiO2/HCS powder was etched by a HF solution (28 wt.%) for 24 h, and HCS powder was obtained after washing by ethanol/DI water for several times and drying at 70 °C for 12 h. Growth of NiCo2O4 microflowers on HCS. 0.01 g HCS were dissolved in 30 mL solution of ethanol and DI water (v/v of 1:2) by ultrasonication, then 1 mmol C4H6O4Co·4H2O, 0.5 mmol C4H6O4Ni·4H2O and 2.2 mmol HMTA were dissolved in the suspension liquid by continuous stirring. Afterwards, the suspension was poured into a Teflon-lined stainless autoclave (100 mL), then reserved at 140 °C for 6 h. Until the hydrothermal reaction was finished and the samples were cooled down in air naturally, the sample was washed alternately by ethanol and DI water for several times and dried at 70 °C for 12 h to obtain NiCo/HCS precursor. The precursor was annealed at 260, 320 and 450 °C for 2 h in air with the heating rate of 5 °C min-1, and the NiCo2O4/HCS-260, NiCo2O4/HCS-320 and NiCo2O4/HCS-450 were obtained, respectively. In addition, the NiCo/HCS precursor was annealed in oxygen-deficient atmosphere (molar ratio of air/N2 = 1:9) at 320 °C for 2 h with the same heating rate to synthesize NiCo2O4/HCS-320-V. Characterizations. A Bruker D8 Discover X-ray diffractometer was used to measure XRD patterns by Cu Kα source. Morphologies of samples were observed by SEM (JEOL-7100F) and TEM (JEM-2100F). HRTEM images and selected area electron diffraction (SAED) were recorded on a JEM-2100F microscopy. A VG MultiLab 2000 instrument was used to carry out


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XPS. The UPS measurements were conducted using an ESCALAB 250Xi instrument with He-I (21.2 eV) radiation. BET surface area and porous features were characterized by nitrogen adsorption-desorption isotherm test by Tristar II 3020 instrument. TGA result was recorded by a simultaneous thermal analyzer (Netzsch STA 449C), and a heating rate of 10 °C min-1 in air was utilized during the measured process. Electrochemical measurements. All the electrochemical measurements were carried out on a CHI760D electrochemical workstation (a three-electrode cell system), which employed a saturated calomel electrode (SCE) and Pt film as the reference electrode and counter electrode, respectively, and 10 µL of ink located onto RDE as the working electrode. The ink was prepared by dispersing 5 mg catalyst and 5 mg Vulcan XC72R (VXC72R) into 1 mL mixed solution (v/v/v of isopropanol/water/5 wt.% Nafion = 700:250:50) by ultrasonication for more than 30 minutes. O2 was bubbled into 0.1 M KOH electrolyte for at least 30 minutes, and the working electrode was activated at a scan rate of 100 mV s-1 for at least 20 cycles. All the potential units were transferred to reversible hydrogen electrode (RHE). The numbers of electrons transfer are calculated from K-L equation using RDE electrode (Equation 1):50

1 1 1 1 1 = + = + 2 / 3 -1/ 6 1/ 2 nFkC0 j jL jK 0.62nFC0 ( D0 ) ν ω

(1)

where j, jL and jK represent the current densities of measurement, diffusion limiting and kinetics, respectively; n represents the entire numbers of transferred electrons; F represents the faraday constant; C0 represents the O2-saturated concentration in 0.1 M KOH; D0 represents the diffusion coefficient of O2; v represents the kinetic viscosity of electrolyte and ω represents the speed of rotation (rad s-1) for the electrode; k represents the electron-transfer rate constant. The intermediate product of HO-2 was characterized by RRDE in O2-saturated 0.1 M KOH


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electrolyte. 12.5 µL catalyst was coated on RRDE electrode. The HO-2 % and electron reduction numbers during whole ORR are determined by equations 2 and 3:

HO-2 = 200

n=4

Ir / N Id + Ir / N

(2)

Id Id + Ir / N

(3)

where Ir represents the ring current, Id represents the disk current, and N represents the collection efficiency and the value is 0.37. ASSOCIATED CONTENT

Supporting Information More details are provided: synthesis details, materials characterizations (SEM images, nitrogen adsorption-desorption isotherm curves and the corresponding pore size distributions) of NiCo2O4-320, NiCo2O4/HCS-260, NiCo2O4/HCS-320, NiCo2O4/HCS-450 and NiCo2O4/HCS320-V, TGA curves of NiCo/HCS precursor and HCS, electrochemical measurements of NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V, high-resolution XPS and PL spectra of NiCo2O4/HCS-320 and NiCo2O4/HCS-320-V, DFT calculation details, UPS measurements of NiCo2O4/HCS and NiCo2O4/HCS-V. AUTHOR INFORMATION

Corresponding Author * E-mail: [email protected] (L. He)

* E-mail: [email protected] (Y. Zhao) * E-mail: [email protected] (L. Mai)

Author Contributions


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All the authors contribute equally to this manuscript, and the final version is approved by all authors.

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Fund for Distinguished Young Scholars (51425204), National Key Research and Development Program of China (2016YFA0202604, 2016YFA0202603), the National Natural Science Foundation of China (51579198, 51502227, 51521001), the Programme of Introducing Talents of Discipline to Universities (B17034), Wuhan Morning Light Plan of Youth Science and Technology (2017050304010316), Yellow Crane Talent (Science & Technology) Program of Wuhan City. REFERENCES (1) Gao, S.; Geng, K.; Liu, H.; Wei, X.; Zhang, M.; Wang, P.; Wang, J. Transforming OrganicRich Amaranthus Waste into Nitrogen-Doped Carbon with Superior Performance of the Oxygen Reduction Reaction. Energy Environ. Sci. 2015, 8, 221-229. (2) Sivanantham, A.; Ganesan, P.; Shanmugam, S. Hierarchical NiCo2S4 Nanowire Arrays Supported on Ni Foam: An Efficient and Durable Bifunctional Electrocatalyst for Oxygen and Hydrogen Evolution Reactions. Adv. Funct. Mater. 2016, 26, 4661-4672. (3) Guo, D. H.; Shibuya, R.; Akiba, C.; Saji, S.; Kondo, T.; Nakamura, J. Active Sites of Nitrogen-Doped Carbon Materials for Oxygen Reduction Reaction Clarified using Model Catalysts. Science 2016, 351, 361-365. (4) Nie, Y.; Li, L.; Wei, Z. Recent Advancements in Pt and Pt-Free Catalysts for Oxygen Reduction Reaction. Chem. Soc. Rev. 2015, 46, 2168-2201.


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TOC


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Oxygen Vacancy Determined Highly Efficient Oxygen Reduction in

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