**2. Electrochemical application of siloxene**

Due to the unique 2D structure and the abundant functional groups of siloxene, it can be applied in various applications such as optoelectronics, catalysis, water splitting, etc. Theoretical investigations of the siloxene have shown the high possibilities in different electrochemical applications [20]. However, because of limited knowledge of siloxene's electrochemistry, only a few works have been reported on the electrochemical application of siloxene so far. The siloxene has been mainly employed in supercapacitors and batteries as an electrode material and detection of biomarkers in electrochemical biosensors.

#### **2.1 Supercapacitors**

*Novel Nanomaterials*

sensors [1, 11–13].

and sp3

mixed sp2

The limitation in the bandgap of these materials has hindered their performance in practical applications. Therefore, exploring a new novel 2D material is highly recommended, especially for the future electrochemical energy conversion, storage, and biosensors applications. Recently, silicon (Si) based one-atom-thick layered material named siloxene has been investigated for electrochemical energy and sensing applications, including supercapacitors, batteries, and dopamine

Siloxene is a direct bandgap material that was discovered by Wohler in 1863. It can be obtained through the deintercalation of calcium and exfoliation from the Zintl phase of calcium silicide (CaSi2) powder [14–16]. Different from the graphene planner structure, siloxene possesses a low-buckled structure due to its double band role. As a result of the surface-terminated functional groups with Si chain and the

Siloxene is prepared by deintercalation of Ca2+ from CaSi2 under concentrated hydrochloric acid. Briefly, the required amount of CaSi2 powder and HCl acid stirred in the ice-cold condition under the inert gas atmosphere for 2-4 days (**Figure 1**). During this reaction, the deintercalation of Ca layers and functionalization of Si

electrochemical energy and sensor applications [1, 11, 12, 17].

*Siloxene synthesis process (reproduced from [11] with permission from Elsevier).*

**1.1 Synthesis of siloxene and its structural types**

hybridization, siloxene can provide several advantages in the

**158**

**Figure 2.**

**Figure 1.**

*Different types of Siloxene structure [19].*

#### *2.1.1 Siloxene based supercapacitors*

Though siloxene was discovered in 1863, it has recently received considerable attention in the electrochemical energy storage application. The researchers have been focused on siloxene based electrode materials for energy storage and conversion application. Due to the increases in energy consumption and the non-renewable sources decreasing gradually, the development of high-efficiency energy storage devices is highly demanded. Electrochemical or supercapacitors are the perfect choice for high-performance devices as the results of its high-power density and long cyclic lifetime [21]. Compared with the commercial activated carbon-based supercapacitors, the integration of Si-based materials with the current microelectronic technology can lead to higher performance in energy storage devices because of its high theoretical capacity (3579 mA hg−1). However, Si-based materials such as silicon carbide (SiC), Si nanowire, porous silicon have been employed as electrode materials in supercapacitor application, the functionalization of the one-atom-thick Si layers with interconnected Si6 rings can accommodate the better performance in supercapacitors [1].

Krishnamoorthy et al. have reported the siloxene based symmetric supercapacitor (SSC) application in 2018 [1]. The Kautsky-type of siloxene structure prepared by deintercalation of calcium from CaSi2 and confirmed its Si-O-Si bridges Si6 rings interconnection by Fourier transform infrared spectroscopy.

The capacitance behavior of the siloxene has been studied in tetraethylammonium tetrafluoroborate (TEABF4) electrolyte under optimal conditions. Interestingly, the operating potential window (OPW) of the siloxene-SSC device was determined from 0 to 3.0 V. This result confirms the excellent electrochemical stability of the SSC device even at a higher voltage window. The fabricated SSC device showed unique capacitance behavior with an energy density of 5.08 W h kg−1 (areal energy density of 9.82 mJ cm−2) and about 98% of capacitance retention even after 10 k cycles (**Figure 3**). The ion diffusion and the electron transfer rate were significantly enhanced by the conductive hexagonal Si frameworks in the siloxene during the electrochemical redox reactions. Also, the high surface area and the larger interlayer spacing between the siloxene sheets were enabled fast ion transport and improved the electrochemical performance of the SSC device.

It is well known that the reduced graphene oxide (rGO) can increase the electroactive sites for the electrochemical reactions than bare graphene oxide (GO) because of its higher electronic conductivity [22]. The electrical conductivity of siloxene sheets may decrease when a higher amount of the oxygen functional groups is attached on its edge/basal surface; thus, the reduction of oxygen functional groups in siloxene enhances the active sites for electrochemical redox reactions due to its better conductivity. In this scenario, Parthiban et al. have investigated the removal of oxygen functional groups in pristine siloxene (p-siloxene) at high temperatures and obtained reduced siloxene sheets (denoted as HT-siloxene). Calcinating siloxene sheets removed the functional groups at edge/basal planes of siloxene at 900°C, which led to the formation of reduced siloxene sheets [23]. Interestingly, the calcination process has decomposed the oxygen functional groups at edge/basal planes of siloxene and preserved the Si6 rings' connection with oxygen atom without affecting the 2D layer structure. The obtained HT-siloxene possessed a higher electrical conductivity than p-siloxene resulting in improved

#### **Figure 3.**

*(a, b) Ragone plot and cyclic stability of p-siloxene SSC device; (c) structure of p-siloxene [1]; (d, e) Ragone plot and cyclic stability of HT- siloxene SSC device; (f) structure of HT-siloxene (reproduced from [23] with permission from ACS).*

**161**

*Novel Two-Dimensional Siloxene Material for Electrochemical Energy Storage and Sensor…*

of the SSC (96.3% after 10000 cycles), as shown in **Figure 3**.

electrochemical performance. The specific capacitance of the HT-siloxene increased almost 1.71 times higher than that of p-siloxene. The maximum energy density of the HT-siloxene SSC device has been achieved by about 6.64 Wh kg−1, higher than p-siloxene (3.89 Wh kg−1) due to its lower equivalent series resistance and better electrical conductivity. The complete removal of the oxygen functional groups in p-siloxene enhanced the energy density of SSC. It also increased the cyclic stability

Another fascinating strategy has been developed by Kim and co-workers recently that dry reforming methane (DRM) recycled siloxene/Ni foam catalyst towards supercapacitor applications. The siloxene coated Ni foam was initially utilized for DRM reactions for producing H2 and CO gas by CO2 reduction. After the DRM reaction, the siloxene/Ni foam catalyst has employed as electrode material in SSC [24]. The regeneration of carbon during the DRM reactions deposited on the siloxene/Ni foam catalyst and could improve the electrochemical performance. Compared to the p-siloxene and HT-siloxene, the carbon-coated siloxene/Ni foam exhibited superior performance in the supercapacitor. A maximum energy density of 30.81 Wh kg−1 was achieved for carbon/siloxene/Ni foam-based SSC, indicates the remarkable performance enhancement. Thus, utilizing spent siloxene catalysts to supercapacitor can be an effective approach for waste-to-energy applications. Besides, the direct use of the siloxene in a supercapacitor, siloxene was also confirmed as a flexible template for fabricating silicon oxy-carbide (SiOC). Carbothermal conversion of siloxene to SiOC has been proposed by Pazhamali and co-works [12]. Mixing siloxene and sodium alginate at 900°C led to the formation of SiOC. Since the SiC-based electrodes can intensify the cycling stability and areal capacitance in supercapacitors, the SiOC electrodes were expected to improve the stability of the SSC device than siloxene based SSC. The SiOC based SSC device delivered an excellent electrochemical performance with an energy density of 20.89 Wh kg−1, which is higher than that of p-siloxene. However, the cyclic stability of SiOC supercapacitor decreased to 92.8% after 5000 cycles. As pointed out in the previous paragraph, the removal of oxygen functional groups can improve the SSC performance; the complete reduction of oxygen in SiOC may help to facilitate the fast ion transport and wettability of the electrode during the long cyclic time.

Making composite electrodes is an efficient approach to increase the supercapacitor's electrochemical performance due to its synergistic behavior [25]. The specific capacitance of siloxene is restricted because of its aggregation effect; consequently, the layers agglomerations generate poor utilization of the pores and the lower specific surface area. Thus, introducing a spacer material such as metal oxides or carbon between the siloxene sheets can enhance the accessible sites for the electrochemical reactions. Meng and co-works have reported the construction of a three-dimensional (3D) architecture of siloxene-reduced graphene oxide hydrogel

The hybrid structure of SGH has increased the specific surface area and facilitated the electrolyte ions transportation, resulting in improved capacitive performance. As compared to bare siloxene electrode specific capacitance (23 F g−1), the SGH with 1:3 ratio composite electrode exhibited a maximum specific capacitance of 520 F g−1 at a current density of 1 A g−1. However, the EDLC of the graphene in SGH has contributed significantly to the capacitive enhancement of siloxene-graphene composite. Though graphene could facilitate the capacitance performance, the surface oxygen-functional groups of siloxene provided pseudocapacitance and improved the wettability of the electrode, results in an excellent rate capability and outstanding cyclic stability.

(SGH) through a simple hydrothermal method (**Figure 4**) [26].

*DOI: http://dx.doi.org/10.5772/intechopen.93958*

*2.1.2 Siloxene composite supercapacitor*

#### *Novel Two-Dimensional Siloxene Material for Electrochemical Energy Storage and Sensor… DOI: http://dx.doi.org/10.5772/intechopen.93958*

electrochemical performance. The specific capacitance of the HT-siloxene increased almost 1.71 times higher than that of p-siloxene. The maximum energy density of the HT-siloxene SSC device has been achieved by about 6.64 Wh kg−1, higher than p-siloxene (3.89 Wh kg−1) due to its lower equivalent series resistance and better electrical conductivity. The complete removal of the oxygen functional groups in p-siloxene enhanced the energy density of SSC. It also increased the cyclic stability of the SSC (96.3% after 10000 cycles), as shown in **Figure 3**.

Another fascinating strategy has been developed by Kim and co-workers recently that dry reforming methane (DRM) recycled siloxene/Ni foam catalyst towards supercapacitor applications. The siloxene coated Ni foam was initially utilized for DRM reactions for producing H2 and CO gas by CO2 reduction. After the DRM reaction, the siloxene/Ni foam catalyst has employed as electrode material in SSC [24]. The regeneration of carbon during the DRM reactions deposited on the siloxene/Ni foam catalyst and could improve the electrochemical performance. Compared to the p-siloxene and HT-siloxene, the carbon-coated siloxene/Ni foam exhibited superior performance in the supercapacitor. A maximum energy density of 30.81 Wh kg−1 was achieved for carbon/siloxene/Ni foam-based SSC, indicates the remarkable performance enhancement. Thus, utilizing spent siloxene catalysts to supercapacitor can be an effective approach for waste-to-energy applications. Besides, the direct use of the siloxene in a supercapacitor, siloxene was also confirmed as a flexible template for fabricating silicon oxy-carbide (SiOC). Carbothermal conversion of siloxene to SiOC has been proposed by Pazhamali and co-works [12]. Mixing siloxene and sodium alginate at 900°C led to the formation of SiOC. Since the SiC-based electrodes can intensify the cycling stability and areal capacitance in supercapacitors, the SiOC electrodes were expected to improve the stability of the SSC device than siloxene based SSC. The SiOC based SSC device delivered an excellent electrochemical performance with an energy density of 20.89 Wh kg−1, which is higher than that of p-siloxene. However, the cyclic stability of SiOC supercapacitor decreased to 92.8% after 5000 cycles. As pointed out in the previous paragraph, the removal of oxygen functional groups can improve the SSC performance; the complete reduction of oxygen in SiOC may help to facilitate the fast ion transport and wettability of the electrode during the long cyclic time.

#### *2.1.2 Siloxene composite supercapacitor*

Making composite electrodes is an efficient approach to increase the supercapacitor's electrochemical performance due to its synergistic behavior [25]. The specific capacitance of siloxene is restricted because of its aggregation effect; consequently, the layers agglomerations generate poor utilization of the pores and the lower specific surface area. Thus, introducing a spacer material such as metal oxides or carbon between the siloxene sheets can enhance the accessible sites for the electrochemical reactions. Meng and co-works have reported the construction of a three-dimensional (3D) architecture of siloxene-reduced graphene oxide hydrogel (SGH) through a simple hydrothermal method (**Figure 4**) [26].

The hybrid structure of SGH has increased the specific surface area and facilitated the electrolyte ions transportation, resulting in improved capacitive performance. As compared to bare siloxene electrode specific capacitance (23 F g−1), the SGH with 1:3 ratio composite electrode exhibited a maximum specific capacitance of 520 F g−1 at a current density of 1 A g−1. However, the EDLC of the graphene in SGH has contributed significantly to the capacitive enhancement of siloxene-graphene composite. Though graphene could facilitate the capacitance performance, the surface oxygen-functional groups of siloxene provided pseudocapacitance and improved the wettability of the electrode, results in an excellent rate capability and outstanding cyclic stability.

*Novel Nanomaterials*

The capacitance behavior of the siloxene has been studied in tetraethylammonium tetrafluoroborate (TEABF4) electrolyte under optimal conditions. Interestingly, the operating potential window (OPW) of the siloxene-SSC device was determined from 0 to 3.0 V. This result confirms the excellent electrochemical stability of the SSC device even at a higher voltage window. The fabricated SSC device showed unique capacitance behavior with an energy density of 5.08 W h kg−1 (areal energy density of 9.82 mJ cm−2) and about 98% of capacitance retention even after 10 k cycles (**Figure 3**). The ion diffusion and the electron transfer rate were significantly enhanced by the conductive hexagonal Si frameworks in the siloxene during the electrochemical redox reactions. Also, the high surface area and the larger interlayer spacing between the siloxene sheets were enabled fast ion transport and improved the electrochemical performance of the SSC device. It is well known that the reduced graphene oxide (rGO) can increase the electroactive sites for the electrochemical reactions than bare graphene oxide (GO) because of its higher electronic conductivity [22]. The electrical conductivity of siloxene sheets may decrease when a higher amount of the oxygen functional groups is attached on its edge/basal surface; thus, the reduction of oxygen functional groups in siloxene enhances the active sites for electrochemical redox reactions due to its better conductivity. In this scenario, Parthiban et al. have investigated the removal of oxygen functional groups in pristine siloxene (p-siloxene) at high temperatures and obtained reduced siloxene sheets (denoted as HT-siloxene). Calcinating siloxene sheets removed the functional groups at edge/basal planes of siloxene at 900°C, which led to the formation of reduced siloxene sheets [23]. Interestingly, the calcination process has decomposed the oxygen functional groups at edge/basal planes of siloxene and preserved the Si6 rings' connection with oxygen atom without affecting the 2D layer structure. The obtained HT-siloxene possessed a higher electrical conductivity than p-siloxene resulting in improved

*(a, b) Ragone plot and cyclic stability of p-siloxene SSC device; (c) structure of p-siloxene [1]; (d, e) Ragone plot and cyclic stability of HT- siloxene SSC device; (f) structure of HT-siloxene (reproduced from [23] with* 

**160**

**Figure 3.**

*permission from ACS).*

**Figure 4.** *Synthesis of siloxene-reduced graphene oxide hydrogel and its specific capacitance plot [26].*
