Energy Storage Properties of Topochemically Synthesized Blue TiO₂ Nanostructures in Aqueous and Organic Electrolyte

Abstract

This book chapter discusses the topochemical synthesis of blue titanium oxide (b-TiO2) and their application as electrode material for supercapacitor devices in aqueous and organic electrolytes. The formation mechanism of b-TiO2 via topochemical synthesis and their characterization using X-ray diffraction, UV–visible, photoluminescence, electron spin resonance spectroscopy, laser Raman spectrum, X-ray photoelectron spectroscopy, and morphological studies (FESEM and HR-TEM) are discussed in detail. The supercapacitive properties of b-TiO2 electrode were studied using both aqueous (Na2SO4) and organic (TEABF4) electrolytes. The b-TiO2 based symmetric-type supercapacitor (SC) device using TEABF4 works over a wide voltage window (3 V) and delivered a high specific capacitance (3.58 mF cm−2), possess high energy density (3.22 μWh cm−2) and power density (8.06 mW cm−2) with excellent cyclic stability over 10,000 cycles. Collectively, this chapter highlighted the use of b-TiO2 sheets as an advanced electrode for 3.0 V supercapacitors.

1. Introduction

The invent of graphene and graphene-like two dimensional materials has created great interest in the exploration of other two-dimensional materials family which includes hexagonal boron nitride, graphitic carbon nitride, transition metal dichalcogenides (TMDCs), layered metal-oxides/double hydroxides, MXenes (transition metal carbides/nitrides), metal–organic frameworks (MOFs), covalent-organic frameworks (COFs), polymers, black phosphorus (BP), siloxene/silicenes, and metallenes [1, 2, 3, 4, 5, 6, 7]. In this scenario, the preparation of the 2D materials is of great interest for the development of the new range of materials [8, 9, 10]. The preparation methodologies/strategies can be categorized into top-down technique and bottom-up technique [8, 11]. In general, the top-down technique mainly depends on the breakage of the bulk materials to the micro- and nano-scale, whereas in the bottom-up technique, the growth of the materials is from atomic level to the macro-scale structure [12, 13, 14]. Amongst the preparation of the materials, bottom-up techniques such as molecular beam epitaxy, chemical vapor deposition and so on produced high purity and efficient layered materials with desired crystal structure [15, 16, 17]. This technique involves in building the nano materials from atomic/molecular scale using heterogeneous/homogeneous chemical reactions, which tend to produce thermodynamically stable materials where the morphology/structure is controlled by both reaction kinetics and thermodynamics [8]. In addition, the building of nanomaterials with desired morphology/structure and composition in a single step is very challenging due to their inherent characteristics of the materials [8, 18]. Moreover, the synthesis of two-dimensional layered materials (metal nitrides/carbides) are hindered due to their non-layered crystal structure. And also, the preparation methods employed in the bottom-up approach is of high cost and the utilization of the prepared materials also requires highly sophisticated instrumentation for characterizing the same. In this regard, top-down technique is of great interest as the cost of production of the materials is less compared to the former with high amount of yield. Some of the top-down approaches for the production of the 2D materials are photolithographic technique, exfoliation (wet and solid phase), mechanical ball milling (wet and solid phase), chemical etching, and topochemical reaction [8, 11]. Amongst the other methods for the top-down technique, topochemical reaction methods is the promising strategy for the preparation of the 2D layered materials [8].

The topochemical route can be classified depends on the reaction methodologies such as adding, extracting or substituting elements to/from the source materials to form the new materials with the retention of the structure/morphology of source material [11]. This type of preparation technique adopts the “corner-overtaking” route, which circumvents the hindrances of direct synthesis through chemical reaction to multi-level steps synthesis [11]. In this aspect, the topochemical preparation route is utilized to synthesize the high value added 2D materials which are difficult to process. Compared to that of the direct synthesis/bottom up approach, top down method has higher advantage to prepare the 2D materials with controlled morphology, composition and material structure. The topochemical reaction route can be performed through various methodologies which includes the (i) selective etching of the elements [19, 20]; (ii) electrochemical methods [21, 22] (iii) high temperature treatment reaction [23, 24] and (iv) liquid phase reaction [25, 26]. In the selective etching methods, the 2D layered materials can be easily obtained by from the bulk counterpart and also it is possible to engineer the interface and surface of the prepared materials. However, the chemical used as etchant is toxic which also affect the quality of the raw materials for the preparation of materials. In this case, hydrofluoric acid is used as the etchant for the preparation of MXenes from the MAX phase element, similarly hydrochloric acid is used for the preparation of siloxene from calcium silicide [27, 28, 29, 30]. In case of the electrochemical methods for the preparation, mild and controllable reaction is carried out with the application of electric field to promote the fast reaction [21, 22]. The limitation in this process is that it requires complex instrumentation and the mass production is very limited in the electrochemical method. In the high temperature treatment reaction, the preparation is associated with the controllable atmosphere induced chemical reaction and high temperature condition is employed for the preparation with the limitation of heterogeneity of gas/solid interface reaction, explosion risk at higher temperature and toxic gases as a byproduct of the reaction [23, 24]. Amongst the other techniques in the topochemical reaction, liquid phase exfoliation as the great advantages of high mass yield, wide range of controllable reaction parameters such as solvents, rich experimental/theoretical foundation and produce uniform and good crystalline materials. Hence the liquid phase topochemical methods attains more merits compared to the other methods.

In this book chapter, we have focused on preparing the 2D sheet like blue titanium oxide via liquid phase topochemical reaction on titanium boride. The TiO₂ prepared technique has electron-rich property which shows enormous property in the field of electrochemical energy storage. This electron-rich TiO₂ will be refereed as blue/black TiO₂ which inherits exceptional physical and chemical properties to that of other forms TiO₂ due to their disordered surface structure and Ti³⁺/oxygen vacancies which leads to high conductivity, magnetic properties, and better chemical properties; yet to be explored for the energy storage application. Herein, the preparation, characterization and the electrochemical properties of blue titanium oxide with sheet-like nanostructures is investigated in detail in this chapter.

2. Preparation of blue titanium oxide (TiO₂) nanosheets

The blue TiO₂ nanosheets was prepared using simple hydrothermal assisted topochemical process as reported in the literature [31, 32]. Initially an appropriate amount of titanium boride was dissolved in 25 mL of hydrofluoric acid and transferred to Teflon lined autoclave for hydrothermal process at 180°C for 12 h. Upon completion of the hydrothermal process, the blue color precipitate is washed with double distilled water/ethanol and dried at 80°C for 12 h to obtain the blue TiO₂ nanosheets.

3. Physicochemical characterization of the blue TiO₂ nanosheets

The blue TiO₂ (blue titanium oxide) nanosheets were prepared using layered TiB₂ and HF as starting materials via hydrothermal process in acidic medium [31]. The mechanism for the formation of layer like blue titanium oxide from the precursor TiB₂ is due to the topochemical reaction as seen in the preparation of siloxenes and MXenes under acidic medium [19, 33]. The TiB₂ is a layered material with titanium and boride arranged in alternating layers. In this case the TiB2 reacts with the HF, which topochemically dissolves boron with the release of hydrogen gas. This generated hydrogen gas creates the oxygen vacancies via H₂ reduction of Ti⁴⁺ on the surface [31, 34]. The digital micrograph of the precursor and the final product is depicted in the Figure 1(A), which shows the change in from gray TiB₂ to blue colored TiO₂. The X-ray diffraction pattern of the precursor TiB₂ and the prepared TiO₂ were provided in Figure 1(B), which confirms the complete transformation of TiB₂ to TiO₂ and matched with the anatase TiO₂ (JCPDS No: 021-1272) [35]. The implication of the bule colored TiO₂ can be corelated with the vacancy of oxygen, disordered surface and deficiency of oxygen which can be confirmed using the spectroscopic analysis such as electron spin resonance spectroscopy (ESR), UV–visible (UV–vis) and photoluminescence (PL) respectively. The X-band electron spin resonance spectrum of the blue titanium oxide was obtained under the temperature of 77 K with the frequency of 9364 GHz and provided in Figure 1(C). The presence of Ti³⁺ centers in the g- region (ranging from 1.87 to 1.99) which is associated to the resonation of Ti³⁺ species [36]. In addition, the presence of a number of partially overlapping signals in the ESR spectrum is matched well to the different types of Ti³⁺ centers present in the blue titanium oxide. The obtained ESR results is in agreement with the previous finding on the partially reduced TiO₂ [31]. Figure 1(D) shows the comparative UV–vis spectra of commercial TiO₂ and blue titanium oxide and the peak observed at 354 nm corresponds to the intrinsic absorption edge of TiO₂ which is red-shifted for the blue titanium oxide to that of the commercial TiO₂ confirming the increase in electron concentration on the surface of blue titanium oxide. Furthermore, the blue titanium oxide shows increased absorbance compared to the commercial TiO₂ over 400–800 nm which is due to the absorbance of low-energy-photon and/or thermal-excitations of trapped electrons in localized states of defects [37, 38, 39, 40]. Figure 1(E) shows the comparative PL spectra of commercial and prepared TiO₂. The PL spectra depicts the presence of peak over 510–540 nm for the blue titanium oxide compared to that of the commercial TiO₂ which shows a broad band over the region of 400–800 nm [41, 42]. The blue titanium oxide shows higher luminescence intensity to that of the commercial TiO₂ suggesting the low electron–hole recombination rate of blue titanium oxide [43, 44]. These studies confirming the higher electronic conductive states of blue titanium oxide compared to the commercial TiO₂. The laser Raman spectrum of the blue TiO₂ is presented in Figure 1(F) which discloses the occurrence of a major vibration band at 148.25 cm⁻¹ corresponds to the E_g mode of anatase TiO₂ [45]. In addition to the major vibrational band, the occurrence of the three minor vibrational bands occurred at 396, 517, and 640 cm⁻¹ which correspond to the B_1g, A_1g, and E_g vibrational modes of anatase TiO₂, respectively [38, 46].

Figure 1.

(A) Digital micrograph of titanium boride and blue titanium oxide. (B) X-ray diffraction pattern of the titanium boride and blue titanium oxide. (C) Electron spin resonance spectrum of blue titanium oxide. (D) UV–visible spectrum of commercial anatase titanium oxide and blue titanium oxide. (E) Photoluminescence spectrum of commercial anatase TiO₂ and blue titanium oxide. (F) Raman spectrum of blue titanium oxide. (G) XPS survey spectrum of TiB₂ and blue titanium oxide. XPS core level spectrum of(H) Ti 2p and (I) O 1 s state in blue titanium oxide.

The comparative X-ray photoelectron survey spectrum of TiB₂ and blue titanium oxide is provided in Figure 1(G), which shows the presence of Ti 2p, B 1 s, C 1 s and O 1 s states [47]. The existence of oxygen and carbon peaks in the spectrum is due to atmospheric interaction with the sample. From the comparative XPS spectra of TiB₂ and blue titanium oxide, exhibits the complete removal of the boron from TiB₂ which is evident from the XPS spectrum. This study proves the compete dissolution of the boron during the hydrothermal reaction [47]. The XPS spectrum of Ti and O oxidation states of elements present in the blue titanium oxide nanostructures were provided in Figure 1(H and I). The XPS core level spectrum of Ti 2p state reveals the existence of the 2p_3/2 and 2p_1/2 at 458, and 464 eV respectively. In addition, the observance of small-satellite peak at 472.2 eV corresponding to the defect of oxygen in the blue titanium oxide [38, 48]. Overall, the core-level spectrum of the titanium highlights the Ti⁴⁺ oxidation state with littleTi³⁺ state due to oxygen deficiency in blue titanium oxide [49]. Figure 1(I) represents the O 1 s states of blue titanium oxide at 530 eV respectively [38, 48]. The calculation of the O/Ti atomic ratio of blue titanium oxide nanostructures is found to be 2.47 from XPS analysis which is in agreement with the literature [50].

To analyze the surface morphology of blue titanium oxide nanostructures field emission scanning electron micrograph (FE-SEM) with energy dispersive spectra (EDS) and high-resolution transmission electron microscope (HR-TEM) were performed. The FE-SEM of blue titanium oxide is provided in Figure 2(A); reveals the presences of nano-sheets/plate-like blue titanium oxide as result of topochemical reaction. The elemental-mapping of Ti & O in the blue titanium oxide is shown in Figure 2(B and C) and EDS spectrum (Figure 2(D)) indicated the homogeneous distributions of Ti and O throughout the blue titanium oxide sheet/plate. With an O/Ti atomic ratio of about 3.92 present in the blue titanium oxide nanostructures [50, 51]. The HR-TEM micrographs of blue titanium oxide obtained under various levels of magnifications are shown in Figure 2(E–H). The HR-TEM micrograph of blue titanium oxide (Figure 2(E–F) revealed the presence of sheet-like structures with the lateral size ranging from 200 nm in length and 250 nm in breadth. The magnified portion of Figure 2(F) is provided in Figure 2(G) reveals the presence of thinner blue titanium oxide sheets with few-layers. The lattice fringes (Figure 2(H)) of blue titanium oxide sheets exhibits d = 0.35 nm (an interplanar-spacing) corresponds to (101) plane and SAED pattern (inset of Figure 2(H)) reveals the clear diffraction spots signifying the crystallinity-nature of blue titanium oxide [37].

Figure 2.

FESEM and HR-TEM analysis of blue titanium oxide nanostructures.

4. Electrochemical characterization of blue titanium oxide electrode

4.1 Preparation of electrode for electrochemical characterization

The working electrode was prepared by mixing the active material (blue titanium oxide) with carbon black and PVDF in an appropriate ratio of 80:15:5 using NMP as solvent. The slurry was spin coated on stainless steel substrate for the three electrode and two electrode characterizations. For the fabrication of symmetric supercapacitor, CR2032 coin cell configuration is utilized and the TiO₂-coated stainless-steel substrates with an area of 1.86 cm² as electrodes separated by a Celgard membrane and 1 M TEABF₄ in acetonitrile as the electrolyte. All the electrochemical measurements were performed using an Autolab PGSTAT302N electrochemical workstation.

4.2 Half-cell characterization of blue titanium oxide electrode

The electrochemical properties of the blue titanium oxide electrodes in aqueous electrolyte (half-cell) were inspected using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge–discharge (CD) analysis, respectively. The half-cell electrochemical characterization of the blue titanium oxide electrode was measured in 1 M sodium sulfate electrolyte and provided in Figure 3. The cyclic voltammogram provided in Figure 3(A) exhibits the existence of the quasi-rectangular behavior suggesting the pseudocapacitive nature of charge-storage in the blue titanium oxide electrode [52, 53]. The indication of the pseudocapacitance nature in blue titanium oxide due to ion-intercalation/de-intercalation phenomenon in addition to the electric double layer/surface capacitance, we quantify the contribution on the overall capacitance using Dunn’s method [54]. The plot ofslope of log (i) versus log (ν) obtained from the power law is provided in Figure 3(B) [55, 56]:

Figure 3.

Three electrode characterization of the blue titanium oxide electrode (A) cyclic voltammetry, (B) plot of log (I) vs. scan rate. The inset in (B) shows the plot of b-values vs. potential. (C) CV profile of blue titanium oxide electrode with the shaded part revealing the capacitive current. (D) Capacitance contribution plot. (E) Galvanostatic charge discharge analysis and the (F) plot of specific capacitance of the blue titanium oxide electrode.

$$\mathrm{i}=\mathrm{a}{\nu }^b$$E1

where, “v”,“a” and “b” are scan rate (mV s⁻¹), adjustable parameters, respectively. The value of “b” is calculated from the slope of log i versus log ν. From the obtained b-value, the nature of the capacitance is evaluated as when b value is 0.5, reveals the diffusion mediated ion-intercalation/de-intercalation whereas the b is 1 represents the surface capacitive nature in the electrode. The contribution of the capacitance related to the surface and diffusion process can be quantified using the relation [54]:

$$\mathrm{I}\left(\mathrm{V}\right)={\mathrm{k}}_1\nu +{\mathrm{k}}_2{\nu }^{1/2}$$E2

Were “k₁ ν” and “k₂ ν^1/2” related to the contributions from surface- and diffusion-mediated intercalation/de-intercalation process, respectively. The slope and intercept of the plot between I(V)/v^1/2 and v^1/2, provides the value of k₁ and k₂ to determine the capacitance contributions [54]. The overall charge stored and their contribution related to the diffusion and surface is calculated using the CV curve in Figure 3(C) and the contribution plot is provided in Figure 3(D). From the Figure 3(D), the diffusion capacitance is increasing from 42.10 to 77.29% when the scan rate is decreased from 50 to 5 mV s⁻¹. The diffusion capacitance is higher at the low scan rate which is due to the fact that at high scan rates, the electrolyte ions faces time constraints which limits the diffusion of electrolyte ions whereas low scan rates provide sufficient time for the electrolyte ions to diffuse into the interior surface of the blue titanium oxide electrode [57]. The charge discharge profile shows sloppy symmetric type curves which is evident from Figure 3(E) and delivered a high gravimetric specific capacitance of 19 F g-1 and areal capacitance of 19 mF cm⁻² obtained at a constant current of 0.5 mA as seen in Figure 3(F). The excellent electrochemical performance of the blue titanium oxide in the neutral electrolyte is due to the reason such as sheet-like structure, enhanced electrical conductivity with more oxygen vacant sites in blue titanium oxide.

4.3 Full-cell characterization of blue titanium oxide symmetric supercapacitor

In the view point of the practical application, a symmetric supercapacitor is fabricated using blue titanium oxide as electrode and organic liquid (TEABF₄) is used as electrolyte. The electrochemical performance of the fabricated blue titanium oxide SSC was tested for the electrochemical stability of the operating potential window by measuring the CV profiles over the increasing voltage from 0 to 3 3.0 V at a scan rate of 100 mV s⁻¹ as shown in Figure 4, which shows the distorted rectangular shaped curves and also suggested that the blue titanium oxide SSC device can operate at a window of 3.0 V without any sign of evolution. The cyclic voltammogram of the fabricated SSC displayed typical rectangular shaped curves for all the CV scans at different rates ranging from 5 to 1000 mV s⁻¹ (Figure 5(A and B)) suggesting the pseudocapacitive nature of charge-storage in blue titanium oxide via intercalation/de-intercalation phenomenon [58, 59]. The plot of the scan rate verses specific capacitance is provided in Figure 5(C), depicts a high capacitance of 6.67 F g⁻¹ measured at a scan rate of 5 mV s⁻¹. The EIS analysis of the blue titanium oxide SSC is provided in Figure 5(D–F) and represented in the form of Nyquist and Bode plots [60]. The high- and mid- frequency region can be used to determine the solution resistance (R_s) or equivalent series resistance (ESR) whereas the low- frequency region directly related to the frequency dependent ion diffusion kinetics of electrolyte ions to the electrode [19]. The values of R_s and R_ct from the Nyquist plot are determined to be 1.60 and 15.4 Ω, respectively. The presence of the Warburg element in the model circuit related to the frequency dependent ion diffusion kinetics of electrolyte ions to the electroactive surface as evident from Figure 5(D) [61]. Figure 5(E) shows the Bode phase angle plot of blue titanium oxide SSCs which tails the phase angle at the low- frequency region (0.01 Hz) is about 64.94°, thus, suggesting the pseudocapacitive nature of the blue titanium oxide SSCs [33]. Figure 5(F) presents the dependence of specific capacitance as a function of applied frequency which is about 3.24 F g⁻¹ for the blue titanium oxide SSCs, and the capacitance decreases with respect to an increase in frequency [62].

Figure 4.

CV profile ofblue titanium oxide symmetric supercapacitor measured with increasing in voltage window.

Figure 5.

Full cell characterization of the blue titanium oxide SSC. (A and B) CV profiles and (C) plot of specific capacitance of blue titanium oxide SSC. (D) Nyquist plot and the enlarged portion is provided in the inset; (E) bode phase angle plot and (F) effect of frequency on capacitance blue titanium oxide SSC.

The charge–discharge profiles of the fabricated SSC measured using various constant currents (0.5 to 10 mA) is provided in Figure 6(A). The profiles indicate the existence of the quasi symmetric profiles suggesting the pseudocapacitive behavior of the blue titanium oxide SSCs [63] and the dependance of current on the capacitance is given in Figure 6(B). A high specific capacitance of the 4.80 F g⁻¹ is obtained from the GCD profile measured at a constant current of 0.5 mA. The rate capability of the blue titanium oxide SSC device is provided in Figure 6(C) which indicating the better capacitance retention when switched from low- to high- current suggesting superior rate capability [19]. The energy and power density plot is represented in the form of Ragone plot (Figure 6(D)) which holds a high-energy-density of 6.0 Wh kg⁻¹ with a power of about 750 W kg⁻¹, respectively. The long tern cyclic life of the blue titanium oxide SSC depicts the capacitance retention of about 90.2% after 10,000 cycles (Figure 6(E)), highlighting the better cycle life [25]. The impedance analysis after stability shows the increase in the ESR and charge transfer resistance such as 1.60 to 4.17 Ω and 15.4 to 24.4 Ω. Figure 6(F) shows the practical applications of the blue titanium oxide SSC glowing a blue lamp over 10 seconds after charged upto 3 V using a constant current of 0.5 mA.

Figure 6.

Full cell characterization of the blue titanium oxide SSC. (A) CD profiles; (B) plot of specific capacitance; (C) rate capability; (D) Ragone plot; (E) cycling stability; the inset in (E) shows the Nyquist plot before and after 10000 cycles of blue titanium oxide SSC device. (F) Real time application of the blue titanium oxide SSC device.

5. Conclusion

In this book chapter we have demonstrated the preparation of the blue titanium oxide via topochemical reduction of titanium boride in hydrofluoric acid via hydrothermal process. In this we have discussed the formation of 2D layered titanium oxide with oxygen deficiency and the utilization in the energy storage sector. The electrochemical energy storage properties of the blue titanium oxide is tested via symmetric configuration type supercapacitor operated over a wide potential range of 3.0 V with good electrochemical stability and higher rate. The prepared blue titanium oxide shows higher output than those of other 2D electrode materials-based supercapacitors. From overall studies, the prepared materials shows better electrochemical performance which paves the way as a promising electrode material for next-generation smart energy storage sectors.

Acknowledgments

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2019R1A2C3009747, 2020R1A2C2007366, 2021R1I1A1A01049635 & 2021R1A4A2000934).

Conflict of interest

“The authors declare no conflict of interest.”