Preparation of Siloxene-Graphene 2D/2D Heterostructures for High-Performance Supercapacitors in Electric Vehicles

Abstract

The development of wide temperature tolerance supercapacitors (SCs) with high specific energy without compromising specific power is an area of emerging interest owing to the increasing demands for electrochemical energy storage system (EES). This chapter discusses the preparation of siloxene-graphene (rGO) 2D/2D heterostructures (via chemical methods) and examines their potential utility toward SCs for electric vehicles (EVs). The electrochemical characterization of the siloxene-rGO SC showed that they possess high specific energy (55.79 Wh kg⁻¹), and specific power (15, 000 W kg⁻¹). And their ability to operate over a wide temperature range (−15 to 80°C), ensuring their suitability as an EES in EVs. The additional experimental studies suggested the ability of the solar-charged siloxene-rGO SC to drive an electric car, and it can capture the regenerative braking energy during the braking process. This chapter provides a new avenue toward the use of siloxene-rGO SC as a suitable EES for next-generation EVs.

1. Introduction

The demands for the development of electric vehicles (EVs) are very rapidly increasing due to the depletion of fossil fuels worldwide [1]. Notably, the use of EVs instead of conventional system possesses the merits of zero carbon emission, thereby reduces environmental pollution [2]. Therefore, electric energy harvesting from renewable methods such as solar, wind, and hydro-power as well as developing high-performance electrochemical energy storage system (EES) are required to develop highly efficient EVs [3, 4, 5]. To date, Li-ion batteries (LIBs) are considered as a suitable choice for EES for EVs and other electronic devices owing to their high energy density [6, 7]. However, the major drawback/issues of LIBs for EVs application are their thermal degradation issues (that results in loss of specific capacity) and the deterioration of battery electrode due to the flow of reverse current from the electric motor during regenerative braking or deceleration of EVs [8, 9]. Additionally, the power density of LIBs is low, which limits their ability for rapid acceleration. In this scenario, the supercapacitors (SCs) are proposed as an alternate EES system wherever peak power demands are required such as storage of braking energy regenerated in electric vehicles (EVs), elevators, etc., [10, 11]. The merits or specific features of SCs such as high specific power, and long cycle-life made them as an attractive candidate for not only EVs (new EVs and hybrid EVs) but even as a maintenance-free EES for the wind-patch control systems [12].

Recent research has been focused on developing high-performance SCs to gain energy density to reach the level of battery without reducing their power density [10, 13]. Herein, the discovery of graphene sheets resulted in the achievement of high capacitance surpassing the previous generation carbon such as activated carbon, carbon nanotubes, fullerenes that mainly originated from the two-dimensional (2D) structures [14], high surface-to-volume ratio [15], and their intrinsic high electrical conductivity [16]. The emergence of graphene and its attractive energy storage properties, researchers focused on studying the energy storage properties of various 2D materials from the transition metal dichalcogenide (MoS₂, MoSe₂, and WS₂) [17, 18, 19] MXenes (2D metal carbides, metal nitrides, and metal carbonitrides) [20, 21, 22], metalenes (boron, antimonene, and phosphorene) [23, 24, 25], and from silicon family such as siloxene, and their derivatives [26, 27], respectively. Benefitting from the structural merits (for ion-intercalation/de-intercalation) [28], presence of transition metal (contributing electrochemical redox reactions) [29], in-plane electrical conductivity [30], these materials showed better electrochemical charge-storage performance than their bulk counterparts and other morphologies [31]. Herein, it is worth mentioning that siloxene sheets and their derivatives possess maximal power density than the reported silicon-based SCs signifying their applications in on-chip SCs for convenient integration with silicon technology [32]. However, the mechanism of charge-storage in these materials arises from ion-intercalation-mediated capacitance that significantly differs from the electric double-layer capacitance (EDLC) seen in graphene sheets [33, 34]. The major limitation of utilizing these 2D materials as electrodes of SCs in commercialization aspects is their ability to re-stack with the adjacent sheets, thereby results in a capacitance fading effect with an increase in cycle life [35]. To solve this issue, various strategies, like growing nanocrystals of 2D sheets, and developing binder-free electrode configurations directly grown on current collectors, have been investigated recently [14, 36].

In this chapter, we are discussing about the synthesis of siloxene-graphene(rGO) (2D/2D) heterostructures as a novel electrode for SCs that is capable of harvesting regenerative braking energy from EVs. Hybridizing siloxene sheets with graphene sheets results not only in the prevention of re-stacking of individual sheets but also offers two dissimilar charge-storage properties in the fabricated electrode for gaining much better capacitance and energy density without losing their specific power.

2. Results and discussion

Figure 1 schemes the synthesis of siloxene-rGO 2D/2D nanohybrids using a multi-step chemical reaction. The calcium silicide (CaSi₂) powders were immersed in an ice-cold HCl solution in which graphene oxide (GO) powders were already predispersed. The reaction is carried out for nearly 4 days which result in the production of siloxene layers via the de-intercalation of calcium ions by releasing hydrogen bubbles [32], which is responsible for the partial reduction of GO into rGO sheets. Finally, the 2D/2D heterostructures were re-dispersed in an aqueous solution containing hydrazine hydrate and subject to ultrasound irradiation for the reduction of GO into rGO sheets. The bare siloxene sheets were prepared by the same reaction without the addition of GO, and the bare rGO was prepared by sonochemical reaction, as mentioned in our earlier works [32, 37].

Figure 1.

Schematic illustration of the synthesis of siloxene-rGO heterostructures. Reproduced with permission: Copyright 2021, Wiley-VCH.

Figure 2(a) presents the X-ray diffraction (XRD) pattern of the siloxene-rGO heterostructures in comparison with the siloxene and GO sheets. The presence of peaks at the lower diffraction angles, i.e., 11.3° and 10.4° confirmed the formation of siloxene and GO sheets by the topochemical de-intercalation reaction and modified Hummer’s method, respectively [10, 27, 38]. The XRD pattern of the siloxene-rGO powders shows the presence of a broad peak at 23.3°, indicating the formation of the siloxene-rGO hybrids via random interstratification. Further, the absence of the peaks observed in bare siloxene and GO sheets confirmed the removal of oxygen-bonded functional groups from their basal planes and edges, respectively. Figure 2(b) shows the Fourier transform-infra red (FT-IR) spectrum of siloxene, GO, and siloxene-rGO powders. The presence of oxygenated functional groups like carbonyl, carboxyl, and epoxide, in addition to the C-C vibration in the GO, closely matched well with the literature [39]. The presence of Si-H, Si-O-Si, Si-Si, Si-OH, and O-Si₂ = Si-H bands in the FT-IR spectrum of siloxene powders confirms their Kautsky-type nature [32, 40]. The FT-IR spectrum of siloxene-rGO powders shows the presence of Si-O-Si, and O-Si₂ = Si-H bands raised from the siloxene and C-C and carbonyl groups of rGO, suggesting the formation of siloxene-rGO powers via the removal of surface oxygen groups present in the individual sheets. The comparative X-ray photoelectron spectroscopic (XPS) Si 2p spectra of the CaSi₂, siloxene, and siloxene-rGO powders (Figure 2(c)) displayed pronounced changes in their surface states. The Si-Si states observed at 99.5 eV disappeared after hydrazine reaction, whereas the Si-O-Si states (102.5 eV) were preserved. This suggested that the hydrazine treatment not only reduces the GO sheets but also reacts with siloxene sheets which needed to be studied in detail in the near future. The XPS C 1 s analysis of rGO (control sample) and the siloxene-rGO heterostructures (data not shown) revealed that the hydrazine treatment under ultrasound irradiation effectively reduces the GO into rGO [10]. Figure 2(d) shows the laser Raman spectra of the CaSi₂ precursor, siloxene sheets, and siloxene-rGO heterostructures. All of the Raman signatures of the siloxene powders such as from Si-H, and were Si-Si disappeared in the Raman spectrum of siloxene-rGO powders whereas Si-O-Si is retained with slight peak shift toward a higher wavenumber [41]. The Raman spectra of the siloxene-rGO powders (Figure 2(e)) displayed the presence of G band (1582 cm⁻¹) and D band (1350 cm⁻¹) of rGO sheets at all ranges of input laser power. At the same time, the bands raised from siloxene sheets (Si-O-Si) were seen only at the high powers [10]. Figure 3(a-d) depicts the morphological analysis of the siloxene-rGO heterostructures using high resolution transmission electron microscopic (HR-TEM) analysis with various magnification levels. It confirms the presence of siloxene sheets on rGO’s surface. The observation of hexagonal diffraction spots in the selected area electron diffraction (SAED) pattern (inset of Figure 3(d)) was raised from the rGO sheets present in the heterostructures. The elemental mapping analysis given in Figure 3(e-h) confirms the homogeneous distribution of Si element, O element, and C element in the prepared siloxene-rGO heterostructures in which both sheets are randomly stacked with each other. Further, the FE-SEM micrograph of the siloxene-rGO powders in the form of SC electrode fabricated using the Doctor blade method (Figure 3(i-j)) indicated the lateral orientation of the individual sheets. The elemental spectrum of the fabricated SC electrode (Figure 3(k)) confirms that they are made of Si, O, and C elements.

Figure 2.

(a) XRD patterns of siloxene-, GO-, and siloxene-rGO powders. (b) FT-IR spectra of siloxene-, GO-, and siloxene-rGO powders. (c) Si 2p core-level spectra of CaSi₂-, siloxene-, and siloxene-rGO powders. (d) Laser Raman spectra of CaSi₂-, siloxene-, and siloxene-rGO at lower wavenumber ranges. (e) Laser Raman spectrum of siloxene-rGO powders with respect to laser power levels. Reproduced with permission: Copyright 2021, Wiley-VCH.

Figure 3.

(a-d) HR-TEM images of siloxene-rGO powders under different magnifications; the inset in (d) shows their SAED pattern. (e-h) Elemental maps of siloxene-rGO powders with (e) overlay micrograph, and (f-h) represents the Si, O and C maps. (i-j) field emission-scanning electron microscopic (FE-SEM) images of roll pressed siloxene-rGO SC electrodes. (k) energy-dispersive X-ray spectroscopy (EDX) spectrum of roll pressed siloxene-rGO SC electrode. Reproduced with permission: Copyright 2021, Wiley-VCH.

In order to study the supercapacitive properties of the siloxene-rGO heterostructures, a symmetric type SC in the form of coin-cell configuration was constructed using 1 M TEABF₄ electrolyte. The standard practice methods viz. cyclic voltammetry (CV), charge-discharge (CD) analysis, and electrochemical impedance spectroscopy (EIS) were used to determine the performance metrics of the siloxene-rGO SC in comparison with the bare siloxene and rGO SCs. Figure 4(a) compares the CV profiles of the fabricated SCs measured at 50 mV s⁻¹. It showed that all the SCs operate over 3.0 V without any evolution; however, the current ranges of the siloxene-rGO SC are higher than the bare siloxene and rGO SCs, indicating the better energy storage performance of siloxene-rGO SC. The scan rate effect on the CV profiles of the siloxene-rGO SC is shown in Figure 4(b-c), which evidences no significant deviation in the rectangular shape of their profiles, suggesting good conductivity of the heterostructures. Figure 4(d) compares the device capacitance of the fabricated SCs at different sweep rates. It confirmed the superior charge-storage performance of the siloxene-rGO SC with a high device capacitance of 56.17 F g⁻¹ compared to the rGO SSC (37.55 F g⁻¹) and siloxene SSC (4.89 F g⁻¹) at 5 mV s⁻¹. Further, Figure 4(e) confirms the exceptional performance of the siloxene-rGO SC over the reported 2D materials/their hybrid-based SCs [10]. Figure 5(a) displays the comparative CD profiles of the fabricated SCs measured by using 1 mA current. It showed that the ultra-fast charge-discharge properties of siloxene SC due to the ion-intercalation capacitance, whereas rGO SC possesses better charge-storage properties compared to siloxene SC due to the EDLC nature of rGO [39]. However, the siloxene-rGO SC possesses higher charging and discharging time compared to the others indicating the role of the synergetic effects of ion-intercalation and EDL properties of the heterostructures [10]. The CD profiles given in Figure 5(b) revealed symmetric triangular-shaped profiles of the siloxene-rGO SC that were retained even after an increased in applied current ranges (Figure 5(c)). The plot of device capacitance with respect to the applied current of the fabricated SCs is compared in Figure 5(d). It showed that the siloxene-rGO SC possesses a capacitance of 44.63 F g^−1, which is higher than the siloxene SC (1.79 F g⁻¹) and rGO SC (30.46 F g⁻¹) from the CD profiles (@1 mA). Further, Figure 5(e) demonstrates the excellent power ratings of the siloxene-rGO SC to handle various currents. Figure 5(f) highlights the better long-term electrochemical stability of the siloxene-rGO SC over 10,000 cycles of continuous CD profiles with excellent capacitance retention of 112%. The initial increase in capacitance during the initial cycles (upto 2500 cycles) is due to the electro-activation process involved in the siloxene-rGO electrodes [10].

Figure 4.

(a) CV profiles of siloxene, rGO, and siloxene-rGO SCs recorded at 50 mV s⁻¹ over 3.0 V. (b-c) CVs of siloxene-rGO SC recorded at different scan rates. (d) Effect of applied scan rates on the device capacitance of the fabricated SCs. (e) Comparison of the siloxene-rGO SC’s capacitance with recently reported ones. Reproduced with permission: Copyright 2021, Wiley-VCH.

Figure 5.

(a) The CD profiles of siloxene, rGO, and siloxene-rGO SCs recorded using 1 mA. (b) Continuous CD profiles of siloxene-rGO SCs measured using 2 mA. (c) CD profiles of siloxene-rGO SCs measured using different applied current ranges. (d) Effect of applied current on the capacitance of the fabricated SCs. (e) Rate capability of siloxene-rGO SC. (f) Long-term stability of siloxene-rGO SC. (g) Ragone chart of siloxene-rGO SCs in comparison with reported SCs. (h) Practical applications of fully charged siloxene-rGO SC used to power a multifunctional electronic display (MFED) over 30 minutes. The references in Figure 5(g) can be seen from ref [10]. Reproduced with permission: Copyright 2021, Wiley-VCH.

Figure 5(g) provides the Ragone plot of the fabricated SCs for understanding the improvements in electrochemical performances of the heterostructures compared to individual counterparts. The siloxene-rGO SC possesses an extra-ordinary specific energy of 55.79 Wh kg⁻¹, which is better than the rGO SC (38.07 Wh kg⁻¹) and siloxene SC (2.23 Wh kg⁻¹) at a specific power of 1500 W kg⁻¹. The specific energy of the siloxene-rGO SC varies from 55.7 to 16.2 Wh kg⁻¹, when their specific power increases from 1.5 to 15 kW kg⁻¹. From Figure 5(g), it is evident that the performances of the siloxene-rGO SC are higher compared to the state-of-the-art SCs reported in literature [42, 43, 44]. Strikingly, the specific energy of the siloxene-rGO SC surpasses the commercial SCs and reaches the level of lead–acid batteries [32, 45]. The excellent metrics of siloxene-rGO SC originate from the structural benefit of 2D/2D heterostructure and differential electrochemical properties of the individual sheets of the heterostructure [46, 47]. Figure 5(h) portrays the applications of the siloxene-rGO SC as a sustainable energy storage and delivery system. The siloxene-rGO SC was charged upto 3.0 V, and the stored charges were used for continuous powering of a multifunctional electronic display (MFED) over 30 min in which all of their functionalities (humidity, temperature, and clock) can be monitored. Figure 6 shows the supercapacitive properties of the siloxene-rGO SC measured over a broad temperature range (−15 to 80°C). Figure 6(a) depicts the CV profiles of the siloxene-rGO SC measured at 100 mV s⁻¹ under various temperature (−15 to 80°C). It shows the change in the current ranges without distorting the rectangular shaped CV profiles. As seen from Figure 6(b), the siloxene-rGO SC possess a capacitance of 9.23 and 16.45 F g⁻¹ at a low temperature of −15 and 5°C whereas they tend to rise from 16.45 to 34.86 F g⁻¹ when the temperature reaches to 80°C. The temperature-dependent Nyquist plot of siloxene-rGO SC (Figure 6(c)) revealed that the high charge-transfer resistance (R_ct) at low temperatures since ion diffusion kinetics are limited at extremely low temperatures, and they tend to be minimal at high temperatures [48]. The Warburg line shifts toward the imaginary axis with the change in temperature (−15 to 80°C), suggesting increased capacitive nature for the siloxene-rGO SC device at high temperatures. The Bode modulus plot of the siloxene-rGO SC (Figure 6(d)) shows the low impedance at high temperatures and vice versa. The Bode phase angle plot of the siloxene-rGO SC (inset in Figure 6(d)) revealed the phase angle at 0.01 Hz shifts from −50.57° (−15°C) to −66.13° (80°C), highlighting their better capacitive properties at high temperatures [49].

Figure 6.

(a) CV profiles of siloxene-rGO SC over broad temperature range. (b) Effect of temperature on the siloxene-rGO SC’s capacitance. (c) Nyquist plot of siloxene-rGO SC, and (d) bode modulus plot of siloxene-rGO SC. The inset in figure (d) presents bode phase angle plot of siloxene-rGO SC. Reproduced with permission: Copyright 2021, Wiley-VCH.

The practical applicability of the siloxene-rGO SCs as an ideal EES system for the recovery of regenerative braking energy (RRBE) process in EVs is demonstrated experimentally, as seen in Figure 7. The main condition which makes the SC an ideal candidate for regenerative braking systems (RBSs) is their power density greater than15 000 W kg⁻¹, an essential power target suggested by the Partnership for New Generation of Vehicles (PNGVs) [50]. Based on the schematic representation of the RBS in EVs (Figure 7(a)), a prototype RBS system was designed (Figure 7(d)) comprising a wheel with two circular brakes (controlled by piston) and electric motors (act as a generator to convert kinetic energy into electrical energy) to which the siloxene-rGO SC was connected. The amount of charge stored with the various levels of braking condition is provided in Figure 7(b). It shows at the initial 25 s when the RBS is in the rest state, whereas when the brake is applied, the siloxene-rGO SC was charged to 0.8 V (@ 30 s) and 1.2 V (@ 90 s) and with the rapid braking, they charged up to 2.2 V (Figure 7(c)), confirming their capability toward an efficient EES for EVs [51]. Figure 7(e-f) shows the effective use of siloxene-rGO SC combined with the RBS system to light up the braking lights. In addition to the RBS, two parallelly connected siloxene-rGO SC was charged via an EV power station using solar cells and used for driving a toy EV over ≈30 s (Figure 7(g-h)) at stationary conditions. Similarly, the on-road test (Figure 7i) shows the ability of the siloxene-rGO SCs to drive the EV over 11 feet within 9 s. Overall, the experimental results demonstrate that the siloxene-rGO SSC can function as a powerful and reliable power source for portable electronics and EVs.

Figure 7.

(a) Principle of the RRBE process in EVs. (b) Voltage vs. time profile of siloxene-rGO SC under driving and braking conditions. (c) Magnified portion of the region marked in (c) that indicated the SCs was charged up to 2.2 V under high braking conditions. (d) Digital photograph of the RRBE prototype. (e-f) Practical utilization of the siloxene-rGO SC-based RBS system for powering up braking light with different LEDs. (g) EVs charging station, (h) digital photograph of charging using EV station (left) and siloxene-rGO SC’s ability to drive an EV for 30 seconds in stand-by mode (right). (i) On-road test of EV driven by siloxene-rGO SC over 11 feet within 9 seconds. Reproduced with permission: Copyright 2021, Wiley-VCH.

3. Conclusions

This chapter demonstrated the synthesis of siloxene-rGO heterostructures and their applications for supercapacitors in EVs. The 2D/2D heterostructures were prepared via multi-step chemical reactions (viz (i) topochemical de-intercalation and (ii) the ultrasound-assisted reduction reaction., respectively. Further, the physico-chemical characterizations suggested that heterostructures were formed via random interstification kinetics. The siloxene-rGO SCs using ionic liquid hold high device capacitance, specific energy, and specific power with long cycle life. Their performance metrics were higher compared to the individual rGO, siloxene sheets, and many of the reported 2D materials-based SCs. More importantly, the fabricated siloxene-rGO SCs possess excellent temperature tolerance, thereby ensuring their utilization at various temperatures. Owing to their high power density, the experimental results demonstrated the ability of the siloxene-rGO SCs to store energy generated from the regenerative braking in EV prototypes, thus, ensuring their potential applications as an EES system in next-generation EVs.

Acknowledgments

The Basic Science Research Program supported this research through the National Research Foundation of Korea (NRF) grant funded by the Korean Government (MSIT) (2021R1A4A2000934 and 2023R1A2C3004336).

Conflict of interest

The authors declare no conflict of interest.