High-performance electrode material can improve the overall output of an electrochemical cell. In an article recently published in the journal ACS Applied Energy Materials, the researchers synthesized porous flower-like tin sulfide (SnS2) nanostructures to use them as electrode material in developing high-performance electrochemical cells.
Study: 2D Flower-like Porous Nanostructures of Layered SnS2 for High-Performance Supercapacitors: Correlating Theoretical and Experimental Studies. Image Credit: sakkmesterke/Shutterstock.com
The developed electrode materials with porous morphology have better performance than those with solid morphology.
Two-Dimensional (2D) Materials for Energy Storage
The weak van der Waals forces and covalent bonds between layers make the layered transition metal dichalcogenides (LTMDs) useful for constructing batteries (used as high energy devices) and supercapacitors (used as power devices).
The future energy storage devices will have combined advantages of batteries and supercapacitors. These hybrid devices will be composed of layered structures with an electrode made of nanostructures. After graphene discovery, extensive research led to the development of cost-effective layered materials.
Recently, materials with good electrochemical activity, eco-friendly nature, low cost, tunable morphologies, and layered metal sulfide's high stability have received considerable attention. Among metal-based sulfides, SnS2 nanostructures have high carrier mobility, large theoretical capacitance, and redox activity. Hence, it is critical to explore SnS2 nanostructures for pseudocapacitor applications.
SnS2 is a TMD with a trigonal omega-like crystal structure having a band gap of approximately 1.562 electronvolts. SnS2 has a layered structure with a layer of Sn between two layers of S, bonded together through strong covalent bonds. The individual monolayers are held together by weak van der Waals forces.
2D materials can facilitate uptake of electrolyte-ion, yielding an improved storage capacity. Tunable porous structures and the high surface area of 2D materials contribute to the specific capacitance. The 2D homogenous structures with mesoporous monolayer and controllable pore diameters reduce the active site's transfer resistance of reactant and product, thus hosting foreign functional particles and enhancing the energy storage capacity.
SnS2 Nanosheets to Enhance Electrochemical Performance
In the present study, the researchers demonstrated a cost-effective and facile synthesis of SnS2 with 2D sheet-like morphology. Comparing the electrochemical activity of 2D SnS2 with conventional solid SnS2 resulted in an increase in electrochemical activity and an approximately 46% increase in specific capacitance compared to traditional solid SnS2. A mathematical model was used to establish the material's electrochemical performance.
More accessible active sites and enhanced ion transportation channels resulted in a higher diffusion coefficient in sheet-like structures. Thus, SnS2 nanosheets can achieve high stability to commercialize them for large-scale use.
Crystallographic structures of as-prepared solid and porous SnS2 nanostructures were confirmed from X-ray diffraction (XRD) patterns. In both the XRD patterns, the characteristic peaks observed for 2θ at 15, 29, 33, and 51 degrees correspond to the monoclinic phase’s (001), (100), (101), and (110) planes, respectively.
Raman analysis of SnS2 showed a strong peak around 310-centimeter inverse corresponding to A1g mode. The researchers observed that the strong peak observed at 310-centimeter inverse shifted to approximately 315-centimeter inverse for porous structure. The synthesized materials' pore size distribution and specific surface area determined the electrochemical response; these were further investigated using nitrogen (N2) adsorption-desorption isotherms.
For the SnS2 sample, a type II isotherm was observed for porous SnS2 with a volume of 40 cubic centimeters per gram adsorbed at a normalized pressure ratio (P/P0) of 0.995, which was twice the volume of solid morphology and revealed the higher porosity of SnS2 porous structures. The specific area of solid and porous SnS2 structures was estimated to be 25 and 42 square meters per gram, respectively.
The mesoporous structure with 2.8 and 2-nanometer pore radius for porous and solid SnS2 was confirmed from Barrett, Joyner, and Halenda (BJH) pore size distribution curves. The pore radius and greater surface area of porous SnS2 contributed to higher access to electrolyte ions during electrochemical analysis.
Images from a scanning electron microscope (SEM) confirmed the homogenous particle size distribution of porous SnS2 with uniform flower-like nanoparticles. Transmission electron microscope (TEM) images of porous SnS2 showed that the thin nanopetals were arranged randomly into nanoflower structures.
The energy-dispersive X-ray (EDX) spectroscopy of porous SnS2 revealed the homogenous distribution of Sn and S throughout the material. During the charge−discharge processes, the internal void spaces in 2D nanoflowers of SnS2 helped in volume expansion. In addition, owing to the short diffusion distance, the thin nanosheets facilitated the fast transport of electrons.
In conclusion, the authors demonstrated that in comparison to solid structures, porous nanostructures helped achieve higher specific capacitance values in transition metal sulfides. SnS2 flakes showed an approximately 50% increase in specific capacitance value at a current density of 1 ampere per gram.
Moreover, the flake morphology facilitated interactions with larger surface area and ion flow, resulting in low resistance and better rate capabilities. Due to the enhanced electrochemical behavior of SnS2 with flake-like morphology, SnS2 nanostructures could be promising candidates for applications in supercapacitors.
Debabrata Mandal, Joyanti Halder, Puja De, Ananya Chowdhury, Sudipta Biswas, and Amreesh Chandra. (2022) 2D Flower-like Porous Nanostructures of Layered SnS2 for High-Performance Supercapacitors: Correlating Theoretical and Experimental Studies. ACS Applied Energy Materials. https://pubs.acs.org/doi/full/10.1021/acsaem.2c01215