Oxygen Vacancies Boost Î´-Bi2O3 as a High-Performance Electrode

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Energy, Environmental, and Catalysis Applications

Oxygen Vacancies Boost #-Bi2O3 as High-Performance Electrode for Rechargeable Aqueous Batteries Tingting Qin, Xiaoyu Zhang, Dong Wang, Ting Deng, Haoxiang Wang, Xiaofei Liu, Xiaoyuan Shi, Zhengming Li, Hong Chen, Xiangmin Meng, Wei Zhang, and Weitao Zheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19575 • Publication Date (Web): 13 Dec 2018 Downloaded from http://pubs.acs.org on December 14, 2018

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ACS Applied Materials & Interfaces

Oxygen Vacancies Boost δ-Bi2O3 as High-Performance Electrode for Rechargeable Aqueous Batteries Tingting Qin,1 Xiaoyu Zhang,1 Dong Wang,1 Ting Deng,1 Haoxiang Wang,1 Xiaofei Liu,1 Xiaoyuan Shi,1 Zhengming Li,1 Hong Chen,1 Xiangmin Meng,3 Wei Zhang,1,2,* Weitao Zheng1,* 1

State Key Laboratory of Automotive Simulation and Control, and School of Materials Science &

Engineering, and Electron Microscopy Center, and International Center of Future Science, Jilin University, Changchun 130012, China 2

CIC Energigune, Albert Einstein 48, 01510 Miñano, and IKERBASQUE, Basque Foundation for

Science, Bilbao 48013, Spain 3

Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute

of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, China

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

Abstract Metal oxides as electrode materials are of great potential for rechargeable aqueous batteries. However ， they suffer from inferior cycle stability and rate capability because of poor electronic and ionic conductivities. Herein, taking vertically-orientated Bi2O3 nanoflakes on Ti substrates as examples, we find that δ-Bi2O3 electrode with plenty of positively oxygen defects show remarkably higher specific capacity (264 mAh g-1) and far superior rate capability than that of α-Bi2O3 with less oxygen vacancies. Through pinpointing the existence form and the role of oxygen vacancies within the electrochemical processes, we demonstrate that oxygen vacancies in δ-Bi2O3 can not only promote electrical conductivity but also serve as central entrepots collecting OHˉ groups via electrostatic force effect, which has boosted the oxidation reaction and enhanced the electrochemical properties. Our work merits an excellent Bi2O3 negative electrode material via giving full play to the role of oxygen vacancies in electrochemical energy storage.

Keywords: Oxygen vacancies; Bi2O3; Aqueous battery; Electrostatic force.

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1. Introduction The majority of commercially negative electrodes employed in rechargeable batteries are carbon-based materials which possess long cycle lifetime but low specific capacity.1,2,3 It is a matter of the utmost urgency to seek comprehensive negative electrodes to meet ever-growing need for peak-power assistance in electric vehicles. Metal oxides hold high specific capacity as promising electrode materials for batteries and supercapacitors. Among these low-cost Bi2O3 have attracted much attention owning to their wide voltage window (~1.8 V in neutral electrolyte, ~1.0 V in aqueous electrolyte)4,

5, 6

and high theoretical specific capacity (six-electrons

transfer process).5,7 However, they suffer from inferior specific capacities and rate capabilities because of the poor electronic conductivity and ionic diffusion.8 9 Providing convenient channels for electron conduction and ion diffusion or compositing materials with high conductivity are two effective measurements to boost capacity and rate capability of electrode material. Increasing specific surface area via morphology tailoring10,11,12,13 (e.g. porosity14 and nanosizing15,16, 17,18) can offer more electrode-electrolyte contact areas, and thus effectively shorten ionic diffusion distance and enhance diffusion kinetics. Vertical low-tortuosity growth of active materials on a substrate enables promoting the charge transfer and ion transport. Besides, blending with conductive metal and/or carbon materials (graphene, carbon nanotubes, nanofibers and graphene oxide)19,20,21 can remarkably promote electronic conduction and inhibit the aggregation of active materials. To ultimately upgrade the overall performance of materials, increasing the intrinsic electronic and ionic diffusion highways is a key step. The notion is well accepted that introducing oxygen vacancies22,23,24,25 through atomic substitution or reducing agent (e.g. H2 and NaBH4) treatment to electrodes can enhance the activity of Faradaic reaction via facilitating electron and ion migration.26 There remains a grand challenge, however, to pinpoint how oxygen defects affect in a Faradaic reaction process. To address such fundamental issue will thereof forge delicate architecture of metal oxide electrode towards rationally affording high cycle stability and rate performance. 2


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Herein, we have synthesized vertically oriented Bi2O3 on Ti substrate and used a simple annealing approach to regulate the oxygen vacancy content in Bi2O3. They are used as negative electrodes in an aqueous battery. δ-Bi2O3 has larger oxygen vacancy concentration which exhibits a packaging-enhance performance (264 mAh g-1 at 0.2 A g-1) than α-Bi2O3 with less oxygen vacancies. Such superiority is attributed to higher concentration of intrinsic positively-charged oxygen vacancies of δ-Bi2O3 which can motivate electron conductivity and adsorb OH- via electrostatic force to promote oxidation reaction. Focusing on how oxygen vacancies are functionalized in a Faradaic reaction, our work initiates a new avenue for searching high-performance electrode materials within the catalogue of metal oxides.

2. Results and discussion 2.1 Characterization of Bi2O3 electrode Vertically-orientated δ-Bi2O3 (V-δ-Bi2O3) were prepared on a commercial titanium foil (Ti) using a simple solvothermal method. Vertically-orientated α-Bi2O3 (V-α-Bi2O3-Air) samples were prepared by annealing the V-δ-Bi2O3 electrodes at 250 °C in air. Vertically-orientated stable δ-Bi2O3 (V-δ-Bi2O3-Ar) electrodes were prepared by annealing the V-δ-Bi2O3 electrodes at 250 °C in Ar. Since there is the abundance of intrinsic oxygen vacancies in the crystal lattice of δ-Bi2O3,27,28 through air thermal treatment, some oxygen vacancies could be passivated with the assistance of O2. It will be discussed later. The crystal structures of the three electrodes were confirmed by using X-ray diffraction (XRD) pattern. As shown in Figure 1(a)/S1, we found the peaks for both V-δ-Bi2O3 and V-δ-Bi2O3-Ar are well indexed to face-centered cubic δ-Bi2O3 (JCPDS card no.27-0052). Some peaks that belong to the titanium foil substrate (JCPDS card no.44-1294) were also observed. It indicated that V-δ-Bi2O3 can be stabilized after an annealing treatment in Ar at 250℃. The typical crystal structure of δ-Bi2O3 (space group: Fm-3m) was given as an inset of Figure. 1(a). In a unit cell, Bi atoms form a face-centered cubic (fcc) lattice, whereas O atoms occupy 8c sites (centers of octants) or 32f sites or other positions.29,30, 31 It is well accepted that the total occupancy of O 3



atoms are only 75% and the 25% absence can account for its high O2- conductivity, 31

although there is still controversial about the concrete location and order of oxygen

vacancies. A scanning electron microscopy (SEM) image confirms that the δ-Bi2O3 film was in the form of interlinked and homogeneously oriented nanoflakes. Figures S2a/b/c/d show the morphology of V-δ-Bi2O3 electrode and corresponding elemental maps of Bi/Ti/O. It suggested that Bi and O were uniformly distributed throughout the nanoflakes on Ti substrates. TEM image of V-δ-Bi2O3 electrode (Figure S3) shows a uniform distribution of ultrathin nanoflakes. High-resolution transmission electron microscopy (HRTEM) image was shown in the inset of Figure S4a. We identified (111) plane of δ-Bi2O3 with an interplanar spacing of 0.319 nm. The highly uneven contrast of lattice fringes and dislocation of δ-Bi2O3, usually observed from the whole sample, could be correlated with the aforementioned oxygen vacancies.

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Figure 1 Structural elucidation of Bi2O3-based electrodes. XRD patterns and standard XRD patterns with insets of the fluorite-based unit cell (left insets) and SEM images (right insets) for (a) Ti substrate, V-δ-Bi2O3 and (b) V-α-Bi2O3-Air; (c) O1s XPS spectra collected for V-δ-Bi2O3 and V-α-Bi2O3-Air after etching under Ar at 2 kV for 10 mins; (d) Raman spectra of δ-Bi2O3 and α-Bi2O3; (e) EPR spectra of δ-Bi2O3 and α-Bi2O3 samples collected at room temperature, where H 5



represents the magnetic field; (f) PL emission spectrum of α- and δ-Bi2O3 at λex=420nm; Nitrogen adsorption–desorption isotherm and the corresponding pore size distribution (insets) of (g) δ-Bi2O3 and (h) α-Bi2O3.

Figure 1(b) showed the XRD pattern of V-α-Bi2O3-Air nanoflakes. All the diffraction peaks were indexed to monoclinic α-Bi2O3 coinciding with JCPDS card no. 41-1449 (space group: P21/c(14)). Bi and O are uniformly distributed on Ti substrates (Figures S2e/f/g/h). α-Bi2O3 has a perfect lattice of (200) with an interplanar spacing of 0.346 nm free of any obvious defects in Figure S4b. The monoclinic α-Bi2O3 unit cell is demonstrated in inset of Figure. 1(b). Obviously, the coordination numbers of Bi atoms in α-Bi2O3 is 6, only 4 in δ-Bi2O3; such difference renders more intrinsic oxygen vacancies in δ than α. V-α-Bi2O3 also grew with the morphology of nanoflakes, but the pores were much larger than V-δ-Bi2O3 from the inset of SEM. Though porosity can govern the contact between the electrolyte and electrode and then promote ionic diffusion, it remains insufficient to afford excellent specific capacity. Considerable active sites and favorable integrative electronic/ionic conductivity are also essential factors for excellent performance. We will illustrate this notion later. Figures S5/6 presented the Bi4f X-ray photoelectron spectra (XPS) of the V-δand V-α-Bi2O3-Air electrodes, respectively. Tables S1/2 presented the peak area ratios of the two electrodes. Both Bi3+ and Bi2+ appear in the two samples. Peaks centered at the binding energies of ~164.0 and ~158.6 eV are attributed to Bi4f5/2 and 4f7/2 of Bi2+ in Bi2O3, respectively. Peaks centered at the binding energies of ~164.6 and ~159.0 eV are attributed to Bi4f5/2 and 4f7/2 of Bi3+ in Bi2O3, respectively. All the data analysis coincided with aforementioned XRD patterns. Figure 1(c) exhibited the O1s XPS of V-α-Bi2O3-Air and V-δ-Bi2O3 after etching under Ar at 2 kV for 10 mins. This treatment could exclude the influence of oxygen and water vapor in air. Three peaks can be clearly identified. The peak centered at ~529.4 eV is attributed to the lattice oxygen of Bi2O3, the peak located at ~531.0 eV corresponds to the oxygen defects in the metal oxides,32 and the last one at 532.0 eV belongs to TiO2. The peak areas ratios were summarized in Tables S3/4, revealing that there are more oxygen deficiencies in 6


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V-δ-Bi2O3 electrode than V-α-Bi2O3-Air. The strong Ti-O bonds were detected between Ti substrates and δ-Bi2O3/α-Bi2O3. As shown in Figures S7/8, a peak at 464.9 eV attributed to Ti4+2p1/2 with a high concentration (according to its high proportion of peak area) was observed.33,34 The peak at 460.2 eV was assigned to Ti0 2p1/2 obtained from the Ti substrate. The presence of Ti4+ at the interface indicates an isomorphic substitution of Ti4+ for Bi3+, promoting the formation of some Bi-O-Ti type bonds. This strong bonding enhanced the Bi2O3 stability, and accelerated the electron conduction. Raman spectra were performed to confirm the structural differences (defects) between δ-Bi2O3 and α-Bi2O3 phases. As given in Figure 1(d), all the Raman peaks can be indexed well to the characteristic Raman peaks of α- and δ-Bi2O3, respectively.35,36,37,38,39 Obviously, the peaks of δ-Bi2O3 were more broadened than those of α-Bi2O3, indicating a more disordered structure of δ than α.39 It suggested that α-Bi2O3 (annealed from δ-Bi2O3) restored the lattice symmetry and reduced the lattice distortion. The mode at 719 cm-1 is correlated to the oxygen defects in the structure.35 Similar oxygen defects were also observed in other fluorite structure.40 As proved by the vibration frequency changing due to the dipole moment of the residual charge, and intensity variation from phonon vibration, the existence of oxygen vacancies in δ-Bi2O3 was further confirmed. Another evidence was obtained for the existence of oxygen vacancies in δ-Bi2O3: the electron paramagnetic resonance (EPR) spectra as shown in Figure 1(e). Ions including Bi3+, Bi+, and O2- do not contribute to EPR signals due to the absence of unpaired electrons, and the doubly positively charged oxygen vacancies (VO2+) without any electron trapping diamagnetic also do not contribute to EPR signals. A resonance line centered at about g = 2.00366 was observed, which is attributed to Bi2+ ions in δ-Bi2O3. No such signal was observed in α-Bi2O3, further demonstrating that δ-Bi2O3 possesses more Bi2+ ions than α-Bi2O3 which corresponds to the content statistics of aforementioned XPS. The XRD result has shown that neither other phase nor impurity has been detected. As a result of Bi2+ ions substituted the positions of Bi3+ ion in δ-Bi2O3, the average oxygen coordination was reduced around Bi elements. 7



Thus, it induced the appearance of oxygen vacancies in order to maintain the crystal structure of the cubic-fluorite δ-Bi2O3. Therefore, it demonstrated that more oxygen defects existed in δ-Bi2O3 than that of α-Bi2O3. It was reported the spontaneous radiative recombination between trap states generally leads to a lower energy emission, compared with that from the band edge transition.41 Thus, the photoluminescence (PL) emission spectra could offer information for internal structures and the oxygen vacancy intermediated defects. Figure 1(f) presents the PL emission spectra of δ- and α-Bi2O3 with an excitation wavelength (λex) of 420 nm. Both δ- and α-Bi2O3 exhibited broad emission peaks, which are typical PL spectra for metal oxides. The PL peaks were located at 596 and 598 nm for α-Bi2O3 and δ-Bi2O3, respectively. Since the PL emission reflects the energy structures, a 2 nm red-shift in the PL peaks indicates that there were more defect states (oxygen vacancies) in the energy structure of δ-Bi2O3 than α-Bi2O3.42 43 Figures. 1(g) and 1(h) showed the nitrogen adsorption-desorption isotherms of δ-Bi2O3 and α-Bi2O3 with the corresponding pore size distribution curves as insets. The comparisons between δ-Bi2O3 and α-Bi2O3 of the specific surface area, pore volume and average pore size were summarized in Table. S5. Both δ-Bi2O3 and α-Bi2O3 have type IV isotherms and very narrow hysteresis loops at relative pressures close to unity, indicating the presence of large mesopores (width distribution from 2 to 50 nm). Moreover, the adsorption branch of nitrogen isotherms showed a steady increase at P/P0 approaching unity, which resembled type II isotherms, indicating that the high external surface area is originated from the presence of macropores (width distribution from 50 to 110 nm). Both δ-Bi2O3 and α-Bi2O3 had mesopores and macropores, which also coincided with their unique nanoscale-structures. The BET specific surface area of α-Bi2O3 (39.8 m2 g-1) was more than 3 times larger than that of δ-Bi2O3 (12.3 m2 g-1). And the average pore size of α-Bi2O3 (27.9 nm) was two times larger than that of δ-Bi2O3 (13.3 nm). Thus, they perfectly tallied with the SEM results. From the aforementioned discussions, we demonstrate that both V-δ-Bi2O3 and V-α-Bi2O3-Air are phase-pure products free of any detectable impurities. V-δ-Bi2O3 8


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holds more oxygen vacancies but smaller specific surface area and pore size than that of V-α-Bi2O3-Air. Porous structures are extremely important in electrochemistry process because they guarantee efficient ion transport and contact between the electrolytes and active substance. It seems that V-α-Bi2O3-Air will show better electrochemical behavior and performance. We will check out whether this universally accepted deduction is correct later.

2.2 Electrochemical behavior and performance

Figure 2 Electrochemical behavior and performance of Bi2O3-based electrodes. (a) CV curves of V-δ-Bi2O3 and V-α-Bi2O3-Air at 1 mV s-1 in KOH (1M) electrolyte; (b & c) Bi4f XPS spectrum 9



analysis of V-δ-Bi2O3 electrode at -0.4 and -0.8 V, respectively; Galvanostatic charge/discharge curves of (d) V-δ-Bi2O3 and V-α-Bi2O3-Air at current densities of 0.2 A g-1 and (e) V-δ-Bi2O3 at current densities of 0.5, 1 and 2 A g-1; (f) Cycling performances of the V-δ-Bi2O3-Ar, V-δ-Bi2O3 and V-α-Bi2O3-Air electrodes at a current density of 0.2 A g-1 for 1500 cycles.

The electrochemical performance of the V-δ-Bi2O3-Ar, V-δ-Bi2O3 and V-α-Bi2O3-Air have been investigated by employing them as working electrodes in a three-electrode system in KOH (1M) solution as electrolyte, with a Pt counter-electrode and saturated calomel electrode (SCE) as reference electrode. The loading mass of δ- and α-Bi2O3 was controlled to 1.5~2.0 mg cm-2. Considering that the Ti foil has only a tiny contribution (CV of Ti substrate is demonstrated in Figure S9), its influence could be neglected for the performance of Bi2O3. Figure 2(a) compares the cyclic voltammogram (CV) curves of the V-δ-Bi2O3 and V-α-Bi2O3-Air electrodes at the scan rate of 1 mV s-1. The CV curves at different scan rates of V-δ-Bi2O3 and V-α-Bi2O3-Air electrodes were demonstrated in Figures S10/S11. The current densities increased with the increase of scan rates. Obviously, V-δ-Bi2O3 electrode exhibited substantially higher current densities in overall potentials than V-α-Bi2O3-Air, indicating higher redox reaction activity of V-δ-Bi2O3 than V-α-Bi2O3-Air. To gain more sights into electrochemical process, the V-δ-Bi2O3 electrode at different potentials was studied by using XPS. Two types of reaction mechanisms may exist in Bi2O3 electrode. One is K+ intercalation/ deintercalation mechanism20 by which Bi2O3 reacts with K+; the other is redox reaction of Bi3+ to Bi0 via Bi2+ which offers the interaction between Bi2O3 and OH- groups. As shown in Figure S12, K 2s 4 peak (293.5 eV) was not detected from XPS spectra, indicative of the absence of K+ in the electrode. We thereby suggested that K+ in the electrolyte didn’t participate in the electrode reaction. So we analyzed Bi valence state evolution in Bi2O3 electrode at different potentials for the 10th cycle in detail. Figures 2(b/c) showed the Bi4f XPS spectrum analysis of the electrode swept at -0.4 and -0.8 V, respectively. There were three valence states of Bi element for these two potentials. The XPS peaks of Bi3+ in 10


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Bi2O3 were located at binding energies of ~163.6 and ~158.2 eV. The lower binding energy peaks should be attributed to Bi2+, while the lowest binding energy peaks at 162.6 and 157.3 eV correspond to Bi0.44,45 The peak area can partially reflect the proportion of corresponding contents. The detailed statistics of proportions of each Bi state was listed in Tables S6/7. The presence of Bi2+ and Bi0 indicated a redox reaction mechanism among the different valence states of bismuth. Bi3+ was firstly reduced to Bi2+ and then to Bi0 in the reduction process. Calculated by the proportion of peak area, the content of Bi2+ at -0.8 V (82.9%) sharply decreased to 41.7% at -0.4 V (an oxidation process from -0.8 to -0.4V). Successively, Bi0 increased from 9.5% to 37.6% (a reduction process from -0.4 to -0.8V). These data manifested a clear trend that the content of reduction state ((Bi0+Bi2+) is 92.4% totally) is higher at -0.8 V and the one of oxidation state ((Bi2++Bi3+) reached 73.4% in total is higher at -0.4 V. These data fully support our mechanism of redox reaction, by which the reduction of Bi3+ to Bi0 bridgs with Bi2+ via charge transfer. Contrast to the CV curve of V-δ-Bi2O3 (Figure 2(a)), the detected prominent peaks at high current are corresponding to the reduction (-0.75 and -0.9V), whereas during the oxidation, two peaks at -0.6 and -0.45 V were obtained. Herein, combined with the previous XPS analysis, the reduction peaks are ascribed to the reduction of Bi3+ to Bi0, whereas the two oxidation peaks are due to the oxidation of Bi0 to Bi3+ bridged with Bi2+. Nevertheless, the intermediate Bi2+ should exist in a metastable phase (e.g. Bi2O2). The possible mechanism behind the reduction and oxidation process is described by the following equation, which is coincided with some other reported work46: Bi2O3+H2O+2e-↔Bi2O2+2OHBi2O2+2H2O+4e-↔2Bi+4OH-

(1) (2)

The galvanostatic charge/discharge (GCD) curves at different current densities of V-δ-Bi2O3 and V-α-Bi2O3 electrodes were displayed in Figures 2(d)/(e) and Figure S13, respectively. Obviously, at the same current density, V-δ-Bi2O3 exhibited higher capacities than V-α-Bi2O3-Air. For the V-δ-Bi2O3 electrode, the specific capacity is calculated to be 264 mAh g-1 at a current density of 0.2 A g-1. When the current 11



density increases up to 2 A g-1, the specific capacity remains 150 mAh g-1. It demonstrated a remarkable rate capability of the V-δ-Bi2O3 electrode. With regard to V-α-Bi2O3-Air, the specific capacity is only 127 mAh g-1 at 0.2 A g-1 and only 77 mAh g-1 at 2 A g-1, which are much lower than V-δ-Bi2O3. It suggests that the V-δ-Bi2O3 electrode affords higher specific capacity and rate capability than V-α-Bi2O3. Such considerable electrochemical performance exceeds some other reported Bi2O3 electrode material.5,9,47,48,49 The cycling performances of the three kinds of electrodes were tested by using GCD at a current density of 0.2 A g-1 for 1500 cycles in Figure 2(f). Obviously, V-δ-Bi2O3-Ar held the most superior specific capacity of 348 mAh g-1 for the first cycle, with an ultra-high specific capacity of 298 mAh g-1 (the capacity retention rate is 85.6%) maintained after 1500 cycles. With regard to V-α-Bi2O3-Air, it exhibited a lowest specific capacity (~127 mAh g-1) for the first cycle, with only 98 mAh g-1 (the capacity retention rate is 77.2%) maintained after 1500 cycles. V-δ-Bi2O3 (free of any annealing treatment) initially exhibited a relatively high specific capacity (264 mAh g-1), but its performance deteriorated significantly with only 128 mAh g-1 (the capacity retention rate is only 48.5%) after 1500 cycles. The decrease in specific capacity during the charge-discharge process is attributed to the reduction of ionic diffusion and electron conductivity resulted from a structural reorganization in the cycling process. We will discuss the cause of electrode performance degradation in the next section in detail. On one hand, it showed that annealing can stabilize the structures of α-Bi2O3 and δ-Bi2O3, so both V-α-Bi2O3-Air and V-δ-Bi2O3-Ar exhibited improved cycling stability. On the other hand, both V-δ-Bi2O3 and V-δ-Bi2O3-Ar exhibited higher specific capacities than V-α-Bi2O3-Air, indicating the superiority of δ-Bi2O3 with oxygen vacancies than α-Bi2O3. Annealing V-δ-Bi2O3 under Ar can not only retain the fcc-fluorite structure of δ-Bi2O3, but also hold oxygen vacancies further to improve the cyclic stability of the electrode. However, the oxygen vacancies were annihilated and δ-Bi2O3 transformed into α-Bi2O3 after annealing in air. Thus, V-α-Bi2O3-Air with less oxygen vacancies had poor electrochemical properties. According to the previous analysis, V-α-Bi2O3-Air possessed larger specific 12


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surface area and pore size than that of V-δ-Bi2O3, however, it demonstrated quite poorer specific capacity and rate performance. More oxygen vacancies are abundant in V-δ-Bi2O3 and V-δ-Bi2O3-Ar rather than V-α-Bi2O3-Air. How did oxygen vacancies as active sites play an important role in Faradic reaction process? We will verify the notion it next. Ion transport and electron conduction are two important factors to improve the specific capacity and rate properties of electrode materials. In the charging and discharging process, it is accepted that larger specific surface area can promote the contact between electrolyte and active substances. When the electrolyte reacts fully with the active material, fast ionic diffusion and electron conduction are significant to achieve high specific capacity and rate performance. Comparing the electrochemical behavior of the two electrodes, δ-Bi2O3 had shown higher specific capacity than α-Bi2O3. It indicated that there are more active sites (here the oxygen vacancies) within the Bi2O3 lattice, the more smooth ion transport and electrical conductivity benefited from oxygen vacancies within the electrode, and thus the faster Faradaic reaction will be achieved. Since the metal ions (K+) within the electrolyte do not participate in the electrode reaction here, it becomes critical how the metal oxides provide fast and convenient ion channel to guarantee diffusion and sufficient reaction of OH- within Bi2O3 lattice. We will focus on how the oxygen vacancies functionalize and improve the electrochemical process in the following section.

2.3 The roles of oxygen vacancies in redox reaction

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Figure 3 Mechanistic clarification of high-performance δ-Bi2O3 electrodes. (a) MS plots of V-α-Bi2O3-Ar, V-δ-Bi2O3 electrodes before and after 500 cycles at a current density of 0.2 A g-1 obtained for each potential with 10 kHz frequency in the dark in 1M KOH electrolyte; (b) Nyquist plots of V-α-Bi2O3-Ar and V-δ-Bi2O3 electrodes; the bottom insets present contact angle images for Ti, V-δ-Bi2O3 and V-α-Bi2O3-Ar electrodes; (c) HRTEM image of V-δ-Bi2O3 electrode operated after 500 cycles at a current density of 0.2 A g-1; (d) SEM image of V-δ-Bi2O3 electrode operated after 1000 cycles at a current density of 0.2 A g-1; (e) Schematic diagram of oxygen vacancies effecting on OHˉ groups via electrostatic force in oxidation process.

To further gain insights into the effect of oxygen vacancies on the electrical properties of Bi2O3, electrochemical impedance measurements were conducted for the V-δ-Bi2O3

and

V-α-Bi2O3-Air

electrodes.

Mott–Schottky

(MS)

plots

and

electrochemical impedance spectroscopy (EIS) measurements enable effectively evaluating the capacitance and resistance values. We firstly check out how the amount of oxygen defects affects the charge transport process and thus the electrode performance. Capacitance measurements were conducted in order to obtain MS plots (Figure. 3(a)) at each potential with 10 kHz frequency in the dark in KOH (1M) electrolyte. The carrier type and density of the semiconductor film can be estimated 14


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from the MS equation.50,51,52,53 N = (2/eε0ε) [d(1/C2)/dV]-1

(3)

where N is acceptor density (for p-type semiconductor is NA) or donor density (for n-type semiconductor is ND),54 e is electron charge, ε is dielectric constant of Bi2O3, ε0 is the permittivity of vacuum, C is the space charge capacitance in F cm -2, and V the potential applied at the electrode. The carrier types of semiconductor are judged via the slopes of MS curves. A positive slope corresponds to n-type semiconductor, and a negative slope corresponds to p-type semiconductor. As shown in Figure. 3(a), both V-α-Bi2O3-Air and V-δ-Bi2O3 electrodes showed distinctive semiconductor features. Obviously, the V-δ-Bi2O3 electrode exhibited the p-type behavior with a negative slope and maintains the semiconductor property after 500 cycles. The acceptor densities for the V-δ-Bi2O3 electrodes before and after 500 cycles are calculated to be 1.2371019 and 6.181018 cm-3, respectively. The increasing slope of the MS plots is indicative of a decrease of positive holes. The decrease of acceptor density after 500 cycles indicated a reduction of oxygen vacancies accompanying with the decay of specific capacity in Figure. 2f. This can explain the degradation of specific capacity of V-δ-Bi2O3 in essence. With regard to V-α-Bi2O3-Air, it exhibited the n-type semiconductor behavior with a positive slope, and the donor density (ND) was calculated to be 7.7131018 cm-3.50,51,53,55 Obviously, V-δ-Bi2O3 electrode held more than an order-of-magnitude of carrier concentration than that of V-α-Bi2O3-Air, indicating higher electronic conductivity of V-δ-Bi2O3. As far, many related work26,56,57, 58,59 reported that oxygen vacancies can promote electron conductivity. After 500 cycles, the charge density of V-δ-Bi2O3 decreased to the same level as that of V-α-Bi2O3-Air, but the specific capacity of V-δ-Bi2O3 remained higher than that of initial V-α-Bi2O3-Air. Since they have similar electronic conductivity but totally different electrochemical performances, there must be other reasons affecting the electrode properties. Taking the possible mechanism behind the oxidation and reduction process into consideration, OH- ions had played important roles in the whole electrochemical process. Thus, it can be deduced that the difference in ion conduction (OH- diffusion) may be another factor determining the specific capacity of Bi2O3. 15



Till now, several factors including charge density, pore size, and specific surface area have been checked, and they were found to have influences but cannot play a decisive role. The factor left to be analyzed are the ion diffusion and electrical conductivity, which are controlled by the amount of oxygen defects within the crystal structures. The existence of those positive oxygen vacancies within Bi2O3 could not only promote electrical conductivity but also act as active sites to absorb OH-, providing short and efficient channels for the migration of OH- inside Bi2O3. As a result,

the

oxidation

(Ovacancy+2Bi+4OH--4e-→Bi2O2+2H2O,

reactions

Ovacancy+Bi2O2+2OH--2e-→Bi2O3+H2O) were promoted. Fast ion diffusion and electron conductivity are keys to high rate performance and specific capacity for electrode material. Therefore, V-δ-Bi2O3 showed more superior rate performance and higher specific capacity than V-α-Bi2O3-Air, which are benefited from the abundance of oxygen vacancies. The analysis of contact angles and EIS measurements further proved this view. The contact angles of Ti (88.4°), V-δ-Bi2O3/Ti (29.1°) and V-α-Bi2O3/Ti (8.3°) were illustrated as insets at the bottom in Figure. 3b. The contact angle of V-α-Bi2O3-Air was smaller than that of V-δ-Bi2O3, demonstrating that V-α-Bi2O3-Air has a stronger hydrophilia. This was due to that V-α-Bi2O3-Air had larger pores than that of V-δ-Bi2O3, which was coincided with the previous SEM and BET analysis. However, better sufficient wettability can only guarantee larger surface contact area between the active substance and the electrolyte, but cannot ensure electrolyte ions diffuse into the Bi2O3 lattice sufficiently. That is why V-δ-Bi2O3 with larger contact angle has shown better electrode performance than V-α-Bi2O3-Air, further demonstrating that the higher capacity and rate performance of V-δ-Bi2O3 than V-α-Bi2O3-Air is attributed to the existence of more lattice oxygen vacancies which can provide more OH- channels. The Nyquist plots are presented in Figure 3(b). Both the plots are composed of two arcs in the high frequency region. The diameter of the semicircle is related to the charge-transfer resistance (Rct) of the electrodes.60,61 The two semicircles represented two reactions in the electrochemical process accompanying with two time constants. 16


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Two peaks in Bode diagram in Figure S14 are proofs to exhibit the two time constants corresponding to two redox reactions of Bi2O3, respectively. Both V-α-Bi2O3-Air and V-δ-Bi2O3 belong to powder electrodes which can cause irregular semicircles (like oblate shape) and the rotation of semicircles. So it is difficult to obtain the intersection value (RL+RP-2σ2Cd) with a real axis and then further calculation of σ. Obviously, both the two charge-transfer resistances (Rct) of the V-δ-Bi2O3 electrode are smaller than that of V-α-Bi2O3-Air electrode, suggesting that V-δ-Bi2O3 is beneficial in providing faster charge transport than V-α-Bi2O3-Air owing to abundant oxygen vacancies. This result is well consistent with the aforementioned MS test. We further investigated the cause of electrode performance decline. No obvious change of morphology was observed when the constituting electrode was operated after 500 cycles at a current density of 0.2 A g-1 as given in Figure S15. The co-existence of δ-Bi2O3 and α-Bi2O3 phases (HRTEM in Figure 3(c)) confirmed the δ→α transformation occurred in the cycling process. This phenomenon also coincided with other work.4 Remarkably, local structure reorganization derived from the transition from the nanoflakes to nanorods under the same working conditions after 1000 cycles was shown in Figure 3(d). This phase transformation may be caused by the oxygen vacancy population decline, leading to worse charge and ion transport properties. O1s XPS spectrum of V-δ-Bi2O3 after 1000 cycles at a density of 0.2 A/g was exhibited in Figure S16. The proportion statistics of every O state was thereof listed in Table S8. Compared with the original content of oxygen vacancies in Table S3, oxygen vacancies decreased to 29.2%. Such occurrence is also in good agreement with Figure 2f and MS plot in Figure 3a. Thus, phase transformation and local structural reorganization are two important factors to correlate to the reduction of oxygen vacancies, which accounts for the performance degradation of V-δ-Bi2O3. Furthermore, it illustrated that the higher electrochemical activity of δ-Bi2O3 than α-Bi2O3 is also originated from the intrinsic oxygen vacancies. In sum, both of carrier concentration and ion transport have positive influence 17



Page 18 of 25

on the electrode performance. Oxygen vacancies within Bi2O3 can not only promote electrical conductivity but also act as active sites to absorb OH- via electrostatic force, providing short and efficient channels for the migration of OH- inside the metal oxides, and thus promote the electrochemical reactions. However, oxygen vacancies are also unfavorable for structural stability, as they provide channels for ion diffusion and electron transport, with which a possible structural collapsing may occur (Figure 2f). Controlling the content of oxygen vacancies to some extents can contribute largely to improved electrochemical performances. It is a promising direction to explore how to inhibit the agglomeration and morphology transition of the nanoflakes of V-δ-Bi2O3 electrode to stabilize oxygen vacancies.

3. Conclusions V-δ-Bi2O3 nanoflakes electrode exhibited high specific capacity of 264 mAh g-1 at a current density of 0.2 A g-1 and superior rate capability. These considerable electrochemical performances are benefited from positive oxygen vacancies in δ-Bi2O3. The oxygen vacancies can promote excellent electrical conductivity and act as central entrepots to collect and disperse OHˉ groups via electrostatic force, which are also beneficial for fast ion transportation. Our study opens up a new perspective for understanding the role of oxygen vacancies in an electrochemical energy storage device.

Experimental procedures Chemicals Bismuth

nitrate

pentahydrate

(Bi(NO3)3·5H2O)

was

purchased

from

Sigma-Aldrich and used as received without further purification. Ethylene glycol (C2H6O2) and acetone (CH3COCH3) were purchased from Aladdin. Titanium foil was bought from Sigma-Aldrich then cut into slices in certain size of 11.5 cm2. Then the titanium sheets were washed in acetone, alcohol and deionized water by ultrasonic for 15mins then dried at 60 °C in vacuum drying box for 6 h. 18


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Synthesis of Bi2O3 electrode The V-Bi2O3 electrode was prepared using a simple solvothermal method. Typically, Bi(NO3)3·5H2O (0.6 g) was dissolved in a mixture consisting of glycol (6 ml) and ethanol (12 ml) by 30 min-magnetic stirring and 15 min-ultrasonication. Well-cleaned pieces of titanium substrate (1×1.5 cm2) were immersed into the solution and then transferred it into 20 ml Teﬂon-lined stainless at 160 °C for 5 h when Bi(NO3)3·5H2O was dissolved thoroughly. After cooling to room temperature, the δ-Bi2O3 covered titanium substrate was ultrasonicated in deionized water and finally dried at 60°C for 1 h. V-α-Bi2O3-Air and V-δ-Bi2O3-Ar electrodes were prepared by annealing the V-δ-Bi2O3 electrode at 250 °C in air and Ar atmosphere, respectively, with a heating rate of 5 °C min-1 and then cooled to room temperature naturally.

Characterizations X-ray powder diffraction (XRD) measurements were performed in the reaction mode (Cu Kα radiation, λ=1.5418 Å) on a Rigaku D/max2600 X-ray diffractometer. FESEM (Hitachi S-8010) and TEM (JEM-2100 F) were conducted to examine the morphology of all samples. EDS was obtained on an electron microscope, using a 20 kV accelerating voltage. X-ray photoelectron spectroscopy (XPS) was carried out using a K-alpha X-ray spectrometer (Thermofisher Scientific Company) with an Al source, the standard binding energy of C1s is 285 eV. Raman spectra were recorded with thin wafer on a Renishaw spectrometer using a 513 nm Ar+ laser as the excitation source at room temperature. The laser beam intensity and the spectrum slit width are 2 mW and 3.5 cm-1, respectively. The EPR spectra were obtained in the X-band (9.45GHz) and a magnetic field modulation of 100 kHz using a Bruker EMX-8/2.7 EPR Spectrometer at room temperature. The specific surface areas were determined using a surface analyzer (BEL Sorp-mini II, BEL Japan Co., Japan) through nitrogen adsorption and desorption isotherms at 77 K by the Brunauer–Emmett–Teller (BET) method. The photoluminescence of the samples were measured under an excitation wavelength of 420 nm (diodelaser) on a Fluorolog-3 spectrofluoromater. 19



Page 20 of 25

Electrochemical measurements Electrochemical measurements were performed by CHI660E electrochemical workstation. For individual paper electrode test, a conventional three electrode system was utilized, in which Pt foil and SCE were used as counter and reference electrodes, respectively. KOH (1M) solution served as electrolyte. The electrochemical performance of the samples was characterized by cyclic voltammetry (CV) and galvanostatic charge/discharge (GCD) tests. Moreover, the electrochemical impedance spectroscopy (EIS) measurements were performed in a frequency range from 0.01 Hz to 100 kHz at an open circuit potential with an AC perturbation of 5 mV. Mott–Schottky plots were operated at each potential with 10 kHz frequency in 1 M KOH electrolyte in the dark.

Supporting Information The name abbreviations of samples, Figures S1-S16 and Tables S1-S8 are included.

Acknowledgements This research is supported by the National Natural Science Foundation of China (51872115), National Key R&D Program of China (2016YFA0200400), the Jilin Province/Jilin University co-Construction Project-Funds

for New Materials

(SXGJSF2017-3, Branch-2/440050316A36), Strategic Priority Research Program” of Chinese Academy of Sciences (XDA09040203, the Program for JLU Science and Technology Innovative Research Team (JLUSTIRT, 2017TD-09), and “Double-First Class” Discipline for Materials Science & Engineering.

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25


Oxygen Vacancies Boost Î´-Bi2O3 as a High-Performance Electrode

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