Seenu Ravi

Carbon nanotube/metal-sulfide composite flexible electrodes for high-performance quantum dot-sensitized solar cells and supercapacitors

Carbon nanotubes (CNT) and metal sulfides have attracted considerable attention owing to their outstanding properties and multiple application areas, such as electrochemical energy conversion and energy storage. Here we describes a cost-effective and facile solution approach to the preparation of metal sulfides (PbS, CuS, CoS, and NiS) grown directly on CNTs, such as CNT/PbS, CNT/CuS, CNT/CoS, and CNT/NiS flexible electrodes for quantum dot-sensitized solar cells (QDSSCs) and supercapacitors (SCs). X-ray photoelectron spectroscopy, X-ray diffraction, and transmission electron microscopy confirmed that the CNT network was covered with high-purity metal sulfide compounds. QDSSCs equipped with the CNT/NiS counter electrode (CE) showed an impressive energy conversion efficiency (η) of 6.41% and remarkable stability. Interestingly, the assembled symmetric CNT/NiS-based polysulfide SC device exhibited a maximal energy density of 35.39 W h kg−1 and superior cycling durability with 98.39% retention after 1,000 cycles compared to the other CNT/metal-sulfides. The elevated performance of the composites was attributed mainly to the good conductivity, high surface area with mesoporous structures and stability of the CNTs and the high electrocatalytic activity of the metal sulfides. Overall, the designed composite CNT/metal-sulfide electrodes offer an important guideline for the development of next level energy conversion and energy storage devices.

Solution processed low-cost and highly electrocatalytic composite NiS/PbS nanostructures as a novel counter-electrode material for high-performance quantum dot-sensitized solar cells with improved stability

The key challenges in boosting the power conversion efficiency (η) of quantum dot-sensitized solar cells (QDSSCs) are efficiently achieving charge separation at the photoanode and enhancing the charge transfer, which is limited by the interface between the polysulfide electrolyte and the counter-electrode (CE). We designed and fabricated new catalytic electrodes by combining a PbS nanoparticle catalyst with NiS nanoparticles by a facile chemical bath deposition method and optimizing the reaction conditions. These were used as CEs for polysulfide redox reactions in CdS/CdSe/ZnS QDSSCs. The PbS nano-morphologies were tuned from nanoparticles to nanospheres by controlling the PbS deposition time on the NiS surface. As the deposition time is increased, the surface morphology, the ratio of Ni:Pb:S, and the thickness of NiS and PbS are affected. The increase in the amount of PbS deposited on the NiS surface could improve the charge transfer at the CE/electrolyte interface. The optimized NiS/PbS composite CE shows a charge transfer resistance (Rct) as low as 10.06 Ω, which is an order of magnitude lower than those of bare NiS (39.65 Ω), PbS (42.12 Ω) and Pt (99.71 Ω) CEs. Therefore, the NiS/PbS composite CEs show much higher catalytic activity for the polysulfide electrolyte than NiS, PbS and Pt CEs. As a result, the QDSSC using this newly synthesized NiS/PbS as a CE achieves a higher power conversion efficiency of 4.52% than the one applying NiS (3.26%) or PbS (3.06%) or Pt (1.29%) CEs. There was no degradation of the efficiency over 10 h under room conditions. This enhancement is mainly attributed to the improved electrocatalytic activity and improved absorption of NiS/PbS, which resulted in the absorption of residual long-wavelength solar irradiation by the PbS CE. This irradiation may have excited the PbS and facilitated the injection of electrons from the conduction band into the polysulfide electrolyte, leading to higher Voc, Jsc, and FF. Cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements revealed that the composite CEs had better electrocatalytic activity, which improved the rate of polysulfide reduction compared to bare NiS and Pt CEs.

Organic sulphonate salts tethered to mesoporous silicas as catalysts for CO2 fixation into cyclic carbonates

A series of mesoporous silica materials tethered with the sulphonate salts of organic bases were synthesized and their catalytic activity for CO2–epoxide cycloaddition was investigated. The sulphonate group was supported on silica through the functionalization and subsequent oxidation of 3-mercaptopropyltrimethoxysilane (3-MPS) by a post-grafting method. All the synthesized materials have been characterized using various physicochemical techniques such as SAXS, BET, SEM, TEM, FTIR and XPS. The as-formed SBA-15(SO3H) was neutralized with different organic bases such as 4-dimethylaminopyridine, triethanolamine and triethylamine, such that the SO3H group ionizes to become the SO3− ion. These materials along with KI displayed promising CO2 conversion yields with excellent selectivity towards the desired product, cyclic carbonates, with a turnover frequency (TOF) as high as 1900 h−1. The catalysts were thermally stable and their reusability studies were also performed. The synergistic play between SO3− and KI is supposed to be the reason behind the good catalytic rates exhibited by this catalytic system. All the parameter studies have also been carried out.

Novel hierarchically dispersed mesoporous silica spheres: effective adsorbents for mercury from wastewater and a thermodynamic study

Novel hierarchically dispersed spherical mesoporous silica (HSMS) was synthesized using a surfactant mixture of fluorocarbon (FC-4), cetyl-trimethyl ammonium bromide (CTAB), and block copolymer Pluronic F127 (PF127). To prepare the thiol-functionalized HSMS (T-HSMS), the synthesized HSMS was functionalized by a co-condensation method using 3-mercaptotrimethoxysilane (3-MPS) as a thiol reagent. The obtained HSMS and T-HSMS possess a cubic morphology with a lmm space group and a particle diameter of 75–200 nm. The HSMS and T-HSMS materials exhibit large free surface areas exceeding 844 and 663 m2 g−1 and pore radii of approximately 3.2 and 3.1 nm, respectively. The synthesized materials were characterized using small angle X-ray scattering, N2-physisorption studies, scanning electron microscopy, transmission electron microscopy, and Fourier transform infrared spectroscopy studies. The efficacy of mercury adsorption by T-HSMS was studied at different temperatures, 283, 298, and 313 K. The obtained results were fitted with a Langmuir adsorption isothermal plot. The changes in the negative Gibbs free energy values for the spontaneity of the process were calculated. Mercury could be successfully desorbed using thiourea in a 2 M HCl solution, and the adsorbents could be subsequently reused without severe loss of their activity after repeated adsorption tests.

Mesoporous silica-giant particle with slit pore arrangement as an adsorbent for heavy metal oxyanions from aqueous medium

The incessant formation of chain-like IBN-4 mesoporous silica (ICMS-N) with long channels running along the length of hexagonal rods have been directly achieved by the co-condensation of 3-aminopropyltriethoxysilane (APTES) and tetraethylorthosilicate as the silica source in the presence of Pluronic P123 and fluorocarbon (FC-4) surfactants as structure directing agents. The newly synthesized materials were successfully characterized using SAXS, N2-physisorption, FT-IR, XPS, 29Si NMR, TEM, SEM and elemental analysis. TEM images confirmed the chain-like morphologies, whereas SEM studies showed that the synthesized ICMS-N particles possess length >10 μm. Moreover, the as-formed material was tested for the removal of oxyanions such as arsenate and chromate, and it exhibited the maximum adsorption capacity of 55 and 35 mg g−1 for ICMS-N15, respectively. The formation of quaternary species of amine on the surface of the ICMS-N enhanced the adsorption capacity towards arsenate and chromate under acidic pH and the adsorption isotherm fits with Langmuir monolayer adsorption. Naturally, existing anions such as nitrate, sulfate, phosphate and chlorides with 1 × 10−2 M have no significant effect on the adsorption of heavy metal ions; moreover, reusability tests were also carried out.