Ravi Kumar N V

Tantalum oxynitride (TaOxN1−x) fibers were synthesized and evaluated for their electrocatalytic hydrogen activity using an in-house developed centrifugal spinning setup. By tailoring the composition of the spinning solution and optimizing collector distance and rotation speed of the spinneret, bead-free TaOxN1−x fibers with a diameter of 800 nm were obtained. The fibers were structurally characterized through phase and elemental analysis, confirming the formation of monoclinic TaOxN1−x with clear splitting of the X-ray photoelectron spectroscopy peaks indicating Ta was in +5 oxidation state. The resulting oxynitride fibers exhibited superior electrocatalytic performance with low overpotentials (250 mV) to generate 10 mA/cm2 compared to Ta2O5 oxide fibers. Interestingly, the enhanced activity of oxynitride fibers was observed to be suppressed in basic medium due to the high oxophilicity of tantalum ions and a negative Gibbs adsorption-free energy, leading to poisoning of the active sites. This work demonstrates a facile pathway for the fabrication of high-performance electrocatalysts, based on TaOxN1−x fibers, from a cost-effective and energy-efficient centrifugal spinning technique.

TiNb2O7/carbon nanocomposites synthesized through a simple, surfactant assisted precursor route is reported as a promising alternative anode material for lithium-ion batteries (LIBs). The carbon component of the nanocomposites is derived from an inexpensive and sustainable keratin rich biological source. The reinforcement of carbon in TiNb2O7 facilitated the formation of non-stoichiometric (Ti0.712Nb0.288)O2 crystalline phase, in addition to the stoichiometric TiNb2O7 phase. It also yielded a high specific surface area (~90 ​m2 ​g−1) and reduced crystallite size (~4 ​nm). Electrochemical results exemplified high reversible capacity of 356 mAh g−1 at 0.1 ​C and remarkable rate capability of ~26 mAh g−1 at ultra-high current rate of 32C. TiNb2O7/carbon nanocomposites also demonstrated remarkable cyclic stability with large capacity retention of 85% even after 50 cycles at 1 ​C. The experimental data attests the potential of TiNb2O7/keratin derived carbon nanocomposites as economically and environmentally viable promising anode material for LIBs.

Chemical vapor deposited (CVD) amorphous tantalum-oxy nitride film on porous three-dimensional (3D) nickel foam (TaNx(Oy)/NF) utilizing tantalum precursor, tris(diethylamino)(ethylimino)tantalum(V), ([Ta(NEt)(NEt2)3]) with preformed Ta-N bonds is reported as a potential self-supported electrocatalyst for hydrogen evolution reaction (HER). The morphological analyses revealed the formation of thin film of core-shell structured TaNx(Oy) coating (ca. 236 nm) on NF. In 0.5 M H2SO4, TaNx(Oy)/NF exhibited enhanced HER activity with a low onset potential as compared to the bare NF (−50 mV vs. −166 mV). The TaNx(Oy)/NF samples also displayed higher current density (−11.08 mA cm−2vs. −3.36 mA cm−2 at 400 mV), lower Tafel slope (151 mV dec−1vs. 179 mV dec−1) and lower charge transfer resistance exemplifying the advantage of TaNx(Oy) coating towards enhanced HER performance. The enhanced HER catalytic activity is attributed to the synergistic effect between the amorphous TaNx(Oy) film and the nickel foam.

This study demonstrates the synthesis and characterisation of electrospun Ta3N5-(O) 1D-nanofibers for electrocatalytic hydrogen evolution reaction (HER) and its performance in a proton exchange membrane (PEM) water electrolyser. 1D nanofibers were synthesized by electrospinning of tantalum ethoxide/polyvinylpyrrolidone (PVP) sol followed by ammonolysis at varied temperatures (800–1000 °C). Elemental distribution of the nanofibers analysed through XPS, and bulk-EDS studies revealed an increase in surface oxygen concentration with an increase in nitridation temperature (from 900 °C to 1000 °C). The nanofibers were characterized to exhibit high electrocatalytic activity for hydrogen evolution reaction (HER) with a low overpotential of 320 mV to deliver 10 mA/cm2 in 0.5 M H2SO4 electrolyte. The Ta3N5-(O) 1D nanofibers were employed as novel electrocatalyst without any conducting supports in a PEM water electrolyser. A current density of 0.1 A/cm2 was achieved at an applied voltage of 2 V which is on par with earth-abundant electrocatalysts like MoS2. Furthermore, the electrospun nano fibers showed excellent stability with negligible losses over 6 h of prolonged operation. The study demonstrates the advantage of nanostructuring the electrocatalysts in enhancing the applicability of Ta3N5 and paves further a path for the development of high performance 1-D electrocatalysts for hydrogen evolution reactions (HER).

In this work, a novel ultra-high temperature resistant precursor-derived ceramic containing Zr, La, B, and C was synthesized through precursor modification of phenol formaldehyde resin. The thermal stability and resistance to crystallization of the ceramic at a temperature of 1600 °C was investigated and was found to be profoundly influenced by the boron content in the starting precursors. The ceramics remained amorphous at 1600 °C for 2 h in argon and upon sustained heat-treatment for up to 16 h resulted in nano-crystalline ultra-high temperature phases such as ZrB2, ZrC, LaB6 and La2Zr2O7. Thermodynamic equilibrium phase calculations show that even longer durations of heat treatment may be required to achieve thermodynamic equilibrium. High-resolution transmission electron microscopy revealed encapsulation of nanocrystals (<5 nm) in an amorphous matrix surrounded by turbostratic layers of carbon inhibiting its growth. Spectrochemical techniques confirmed the presence of boron substituted carbon in the amorphous matrix of the ceramic. The unique nature of the amorphous matrix lends the ceramic resistance to crystallization and chemical degradation that can surpass the likes of classical silicon-based precursor-derived ceramics.

The low-frequency dielectric behavior of in situ crystallized and stabilized nanocrystalline t-NbO2 in amorphous silicon oxycarbide (SiOC) ceramic nanocomposites derived from niobium-modified poly(hydridomethylsiloxane) (PHMS) is reported. Commercially available PHMS is modified using Nb(OC2H5)5 via chemical and mechanical mixing route. It is observed that the synthesis route and heat-treatment temperatures significantly affect the crystallization of the ceramic phases, where c-NbC and t-NbO2 in amorphous SiOC are formed through chemical and mechanical modification route, respectively. The variation in dielectric permittivity and loss at room temperature with respect to ceramic phases is comprehensively investigated. In contrast to amorphous SiOC ceramics derived from PHMS (ε′ = 100.0 at 1 Hz), t-NbO2 in amorphous SiOC ceramic exhibits colossal dielectric permittivity (ε′ = 5.0 × 105 at 1 Hz) with low loss (tan δ < 3.0). This is attributed to an interfacial charge polarization and in situ growth of insulator/semiconductor/insulator ceramic phases in amorphous SiOC ceramic nanocomposites.

Ablation behaviour of poly(hydridomethylsiloxane) derived open and closed porous structured SiOC ceramic foams was evaluated using oxy-acetylene flame at 1500 °C for various time durations. X-ray diffraction and scanning electron microscopy analyses of ablated SiOC ceramic foams revealed the formation of a thin protective SiO2 layer inhibiting further oxidation. The closed porous structured SiOC ceramic foams exhibited very low mass ablation rate in contrast to open porous structured SiOC ceramic foams owing to the differences in thermal energy dissipation mechanism. The feasibility of the plausible foam reduction reactions pertaining to the ablation mechanism was further investigated by computing the Gibbs energy and HR-TEM analysis. The study corroborated the significance of tailoring the microporous structured SiOC ceramic foams as potential thermal protection material for high temperature applications.