Two-dimensional Siloxene Research Articles

Enabling Uniform and Stable Lithium-Ion Diffusion at the Ultrathin Artificial Solid-Electrolyte Interface in Siloxene Anodes.

Constructing a stable and robust solid electrolyte interphase (SEI) has a decisive influence on the charge/discharge kinetics of lithium-ion batteries (LIBs), especially for silicon-based anodes which generate repeated destruction and regeneration of unstable SEI films. Herein, a facile way is proposed to fabricate an artificial SEI layer composed of lithiophilic chitosan on the surface of two-dimensional siloxene, which has aroused wide attention as an advanced anode for LIBs due to its special characteristics. The optimized chitosan-modified siloxene anode exhibits an excellent reversible cyclic stability of about 672.6 mAh g-1 at a current density of 1000mA g-1 after 200 cycles and 139.9 mAh g-1 at 6000mA g-1 for 1200 cycles. Further investigation shows that a stable and LiF-rich SEI film is formed and can effectively adhere to the surface during cycling, redistribute lithium-ion flux, and enable a relatively homogenous lithium-ion diffusion. This work provides constructive guidance for interface engineering strategy of nano-structured silicon anodes.

Ni clusters immobilized on oxygen-rich siloxene nanosheets for efficient electrocatalytic oxygen reduction toward H2O2 synthesis.

Hydrogen peroxide (H2O2) electrosynthesis via the two-electron oxygen reduction reaction (2e- ORR) represents a green alternative to the energy-intensive anthraquinone process. However, the practical application of this method is limited by the lack of cost-effective and high-performance electrocatalysts. Reported here is a hybrid catalyst composed of nickel (Ni) clusters immobilized onto the surface of two-dimensional siloxene nanosheets (Ni/siloxene), which exhibits excellent efficiency and selectivity in electrocatalytic oxygen reduction to H2O2 in an alkaline medium, demonstrating a standard 2e- pathway with >95% H2O2 selectivity across a wide potential range. Experimental results disclose that the high performance of Ni/siloxene can be traced to a synergy of the Ni clusters and the oxygen-rich surface of siloxene. Density functional theory (DFT) calculations further reveal a weakened interaction between Ni/siloxene and *OOH and the consequently reduced energy barrier for the *OOH protonation toward H2O2 desorption, thus leading to a high 2e- ORR reactivity and selectivity. This work provides a valuable and practical guidance for designing high-performance 2e- ORR electrocatalysts based on the rational engineering of the metal-support interaction.

Two-Dimensional Siloxene Nanosheets: Impact of Morphology and Purity on Electrochemistry.

Two-dimensional (2D) siloxene (Si6O3H6) has shown promise as a negative electrode material for Li-ion batteries due to its high gravimetric capacity and superior mechanical properties under (de)lithiation compared to bulk Si. In this work, we prepare purified siloxene nanosheets through the removal of bulk Si contaminants, use ultrasonication to control the lateral size and thickness of the nanosheets, and probe the effects of the resulting morphology and purity on the electrochemistry. The thin siloxene nanosheets formed after 4 h of ultrasonication deliver an average capacity of 810 mA h/g under a 1000 mA/g rate over 200 cycles with a capacity retention of 76%. Interestingly, the purified siloxene shows lower initial capacity but superior capacity retention over extended cycling. The 2D morphology benefit is illustrated where the parent siloxene nanosheet morphology and structure were largely maintained based on operando optoelectrochemistry, in situ Raman, ex situ scanning electron microscopy, and ex situ transmission electron microscopy. Furthermore, the purified siloxene-based electrode free from crystalline Si impurity experiences the least expansion upon (de)lithiation as visualized by cross-section electron microscopy of samples recovered post-cycling.

Two-dimensional siloxene as an advanced support of platinum for superior hydrogen evolution and methanol oxidation electrocatalysis

To improve the utilization efficiency of versatile platinum (Pt) in electrochemical energy conversion, precisely choosing support materials is a significantly promising strategy. Herein, we demonstrate the competitive advantages of novel two-dimensional Siloxene as the advanced support of Pt toward enhancing bifunctional electrocatalysis for hydrogen evolution (HER) and methanol oxidation (MOR) based on unusual metal–support interactions. The Pt NPs/Siloxene electrocatalyst by supporting Pt nanoparticles on Siloxene sheets is successfully synthesized via simple wet-chemical method. Compared to commercial Pt/C (20 wt%), Pt NPs/Siloxene with ultra-low Pt content (1.3 wt%) exhibits superior electrocatalytic activity with good stability toward HER and MOR in acidic media. For HER, a small overpotential of 24 mV is required to drive 10 mA cm−2 current with a low Tafel slope (24.6 mV dec−1). Meanwhile, preeminent Pt mass activity of 1957 mA mg−1 for MOR is achieved, which is 12.6 times higher than that of Pt/C. Theoretical calculations indicate that the binding energies are optimized to facilitate desorption or adsorption of related adsorbates during HER and MOR electrocatalysis, and thus enhancing Pt utilization efficiency. Specially, this support engineering can trigger hydrogen spillover from Pt to Siloxene to promote hydrogen generation, attributed to a unique hydrogen spillover-based HER pathway with favorable proton adsorption on Pt sites and efficient hydrogen desorption from Siloxene.

Controlled prelithiation of siloxene nanosheet anodes enables high performance 5 V-class lithium-ion batteries

Two-dimensional siloxene nanosheet has attracted great attention as an advanced anode for lithium-ion batteries (LIBs) due to its special characters. However, the high surface area leads to a low initial Coulombic efficiency (ICE) and large irreversible capacity, impeding the further application of siloxene. In this work, controlled chemical prelithiation is designed to overcome the issue. The influences of prelithiated conditions on physical and electrochemical performance are researched. Uniform SEI film and high coulombic efficiency is obtained with a 15 min prelithiation process. The prelithiated siloxene anode shows high ICE, superior rate performance and stable cycling performance. When coupled with 5 V-class LiNi0.5Mn1.5O4 cathodes, an enhanced capacity retention of 94.3 % is achieved after 200 cycles. This work may be useful for designing advanced anodes for rechargeable batteries.

Electrocatalytic studies of siloxene sheets encrusted with cobalt chalcogenides (S, Se) for water splitting

Two-dimensional siloxene sheets were superficially coated with cobalt chalcogenides to optimize interfacial properties for broad applications in the field of catalysis. These catalytic composites were investigated for electrochemical water splitting in an alkaline electrolyte medium. The synthesis of siloxene sheet-cobalt chalcogenides composites was confirmed by X-ray diffraction, field emission scanning electron microscopy, high-resolution transmission electron microscopy, adsorption studies, and X-ray photoelectron spectroscopy analyses. Potentiometric and impedimetric experiments were performed to understand the inherent electrocatalytic activity of the developed catalysts. Variations in the onset potential and overpotential at a constant current density of ±10 mA/cm2 for hydrogen and oxygen evolution reactions—HER and OER, respectively—were evaluated with respect to a reversible hydrogen electrode (RHE). The catalysts exhibit superior current and catalytic activity due to interfacial kinetics, retaining lower Tafel slopes of ∼30 mV/dec for the OER and HER; they also exhibited improved, long-term stability for 12 h, indicating potential utility in commercial applications.

Carbothermal conversion of siloxene sheets into silicon-oxy-carbide lamellae for high-performance supercapacitors

This work describes the carbothermal preparation of silicon-oxy-carbide (SiOC) lamellae using two-dimensional siloxene sheets and alginic acid as precursors. X-ray photoelectron spectra, X-ray diffraction, Fourier-transform infrared spectra, high-resolution transmission electron micrographs, and Raman spectra revealed the formation of lamella-like SiOC nanostructures. Symmetric supercapacitors (SSCs) were fabricated using SiOC nanostructures as electrodes and evaluated in aqueous (1 M Li2SO4) and organic (1 M TEABF4) electrolytes. SiOC SSC fabricated with Li2SO4 electrolyte operated over a voltage window of 2.0 V, with an energy density of 14.2 Wh kg−1 and a power density of 6666 W kg−1. SiOC SSC fabricated using TEABF4 electrolyte operates over a voltage window of 3.0 V and delivered a device capacitance of about 16.71 F g−1, energy density of 20.89 Wh kg−1, with excellent cyclic stability and superior rate capability. Strikingly, the high-power density of the TEABF4-based SiOC SSC (15,000 W kg−1) reached the required power target for next-generation electric vehicles and is suitable for high-performance supercapacitor devices.

Understanding the Thermal Treatment Effect of Two-Dimensional Siloxene Sheets and the Origin of Superior Electrochemical Energy Storage Performances.

Two-dimensional siloxene sheets are an emerging class of materials with an eclectic range of potential applications including electrochemical energy conversion and storage sectors. Here, we demonstrated the dehydrogenation/dehydroxylation of siloxene sheets by thermal annealing at high temperature (HT) and investigated their supercapacitive performances using ionic liquid electrolyte. The X-ray diffraction analysis, spectroscopic (Fourier transform infrared, laser Raman, and X-ray photoelectron spectroscopy) studies, and morphological analysis of HT-siloxene revealed the removal of functional groups at the edges/basal planes of siloxene, and preservation of oxygen-interconnected Si6 rings with sheet-like structures. The HT-siloxene symmetric supercapacitor (SSC) operates over a wide potential window (0-3.0 V), delivers a high specific capacitance (3.45 mF cm-2), high energy density of about 15.53 mJ cm-2 (almost 2-fold higher than that of the as-prepared siloxene SSC), and low equivalent series resistance (compared to reported silicon-based SSCs) with excellent rate capability and long cycle life over 10 000 cycles.

Two-dimensional siloxene nanosheets: novel high-performance supercapacitor electrode materials

A symmetric supercapacitor constructed using two dimensional siloxene sheets in an ionic liquid electrolyte exhibits high areal capacitance and energy density.

Photocatalytic NO removal over calcium-bridged siloxenes under ultraviolet and visible light irradiation.

Ca-Bridged siloxenes (Ca-siloxenes) composed of two-dimensional siloxene planes with Ca bridging were prepared and their photocatalytic properties for nitrogen oxide (NO) removal were investigated. Ca-Siloxenes were synthesized via a solid-state metathesis reaction using TaCl5 to extract Ca from CaSi2 with different Cl2/Ca molar ratios of 0.25, 1.25 and 2.5 (CS0.25, CS1.25 and CS2.5, respectively) in an attempt to control the extent of Ca extraction. Ca-Siloxenes have a wide optical absorption band from the visible to ultraviolet region with absorption edges of 1.5, 2.9, and 3.1 eV for CS0.25, CS1.25, and CS2.5, respectively. Ca-Siloxenes exhibited photocatalytic activity for NO removal under irradiation with visible (λ > 400 nm (<3.10 eV)) and ultraviolet light (λ > 290 nm (<4.28 eV)). The photocatalytic activity was particularly improved by mixing the Ca-siloxene with acetylene black as a conductive material, which might have inhibited the recombination of photogenerated electrons and holes. The mixture of Ca-siloxene and acetylene black exhibited improved photocatalytic activity in the presence of 1O2 as one of the active oxygen species formed under ultraviolet light irradiation.

Transport properties and electroluminescence of siloxene

The transport and electroluminescence properties of siloxene (Si6O3H6) in the as-prepared and annealed states are investigated. As-prepared siloxene is found to show a low dark conductivity (≈10−19 Ω cm−1) at room temperature which is thermally activated with an activation energy that is approximately half of the optical gap of siloxene (≈2.6 eV). The presence of ambient gases can increase the dark conductivity due to intercalation of gas molecules between the two-dimensional siloxene polymer sheets. The photoconductivity of siloxene is investigated with respect to its spectral and temperature dependences. Electroluminescence of siloxene is found for sufficiently high current levels.

Electronic structure of siloxene model compounds

The electronic structures of one-dimensional and two-dimensional siloxene (Si6O3H6) model compounds have been examined theoretically, using the semiempirical tight-binding self-consistent field crystal orbital (SCF-CO) method. These compounds are formed by silicon-based chain and planar structures containing a regular array of oxygen atoms. Results show that the two-dimensional polysilane in which OH groups are substituted for H atoms possesses a relatively smaller direct gap than other siloxenes. It is assumed that the electronic structures of siloxenes are affected not only by the dimensionality of Si-Si σ-conjugational networks due to an array of oxygen atoms, but also by the diminishing of the electron population in the Si-Si bonding orbitals caused by oxygen atoms with large electronegativity.