Showing papers by "Chandran Sudakar published in 2021"

Kinetic Insights into the Mechanism of Oxygen Reduction Reaction on Fe2O3/C Composites.

Abstract: Since the inception of cobalt phthalocyanine for oxygen reduction reaction (ORR), non-platinum group metals have been the central focus in the area of fuel-cell electrocatalysts. Besides Fe-Nx active sites, a large variety of species are formed during the pyrolysis, but studies related to their ORR activity have been given less importance in the literature. Fe2O3 is one among them, and this study describes the role of Fe2O3 in the ORR. The Fe2O3 is carefully synthesized on various carbon supports and characterized using X-ray photoelectron spectroscopy (XPS) spectra, high-resolution transmission electron microscopy (HRTEM) images, and surface area analysis. The characterization techniques reveal that the Fe2O3 nanoparticles are present in the pores of the carbon supports, having a particle size ranging from 4 to 15 nm. The current density of the ORR on Fe2O3/C catalysts is increased compared with bare carbon supports, as discerned from the rotating ring-disk electrode (RRDE) voltammetry experiments, demonstrating the role of size-confined Fe2O3 nanoparticles. The overall number of electrons in the ORR is increased by the introduction of Fe2O3 on the carbon support. Based on the kinetic analysis, the ORR on Fe2O3/C follows a pseudo-4-electron or 2+2-electron ORR, where the first 2-electron ORR to H2O2 and second 2-electron H2O2 reduction reaction (HPRR) to H2O are assigned to the graphitic carbon (carbon defects) and Fe2O3 active sites, respectively. Theoretical studies indicate that the role of Fe2O3 is to decrease the free energy of O2 adsorption and reduce the energy barrier for the reduction of *OOH to OH-. The onset potential estimated from the free energy diagram is 0.42 V, matching with the HPRR activity demonstrated using the potential-dependent rate constants plot. Fe2O3/C shows higher stability by retaining 95% of the initial activity even after 20 000 cycles.

Bandgap engineering and sublattice distortion driven bandgap bowing in Cs2Ag1-xNaxBiCl6 double perovskites

Abstract: Bandgap engineering in lead-free Cs2Ag1-xNaxBiCl6 (x = 0 to 1) double perovskite alloys synthesized through solution-based approach is investigated. The bandgap is shown to vary from 2.64 eV to 3.01 eV as Ag+ at B′ site gets replaced with Na+ cation. Despite a linear change in the lattice parameter according to Vegard's law, bandgap (Eg) changes in a nonlinear fashion for x = 0 to 1 with much lower Eg values observed than predicted by Vegard's rule. Further, we show the bandgap bowing effect in Cs2Ag1-xNaxBiCl6. Raman spectroscopic studies reveal that the changes in the vibrational mode positions arise due to the systematic variations in local distortions of [BiCl6]3– and [AgCl6]5– octahedra. The bandgap change, Raman mode frequency shift, Raman peak width, and the ratio of intensities of Raman modes all show a similar trend as a function of Na substitution concentration (x). The changes are minimal and linear for x from 0 to ∼0.6 and deviate sharply for higher Na concentration (x > 0.6). These observations strongly suggest that the sublattice distortion in the A2B′B″X6 lattice arises due to a mismatch in the octahedra. This imparts a nonlinear change in the bandgap. Thus, a strong interplay between the [Ag(Na)Cl6]5− and [BiCl6]3– octahedra is shown to have a significant influence on the deviation of bandgap from Vegard's rule and further enforces the bandgap bowing effect in Cs2Ag1-xNaxBiCl6.

Coexistence of large negative and positive magnetodielectric response in Bi 1 − x Ca x Fe 1 − y Ti y O 3 − δ nanoparticle ceramics

Abstract: Magnetodielectric (MD) properties of as-prepared (AP) and air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ nanoparticle ceramics made by spark plasma sintering process are investigated as a function of temperature. Aliovalent ${\mathrm{Ca}}^{2+}$ substitution at ${\mathrm{Bi}}^{3+}$ site creates oxygen vacancies (${\mathrm{V}}_{\mathrm{O}}$) in the lattice disrupting the intrinsic spin cycloid of ${\mathrm{BiFeO}}_{3}$, which are suppressed when the charge compensating ${\mathrm{Ti}}^{4+}$ is co-substituted. In addition, cation substitution reduces the grain size and increases surface oxygen vacancies. These lattice and surface ${\mathrm{V}}_{\mathrm{O}}$ defects play a significant role in enhancing the magnetic properties. Zero-field-cooled magnetization curves of all AP samples show a sharp Verwey-like transition at \ensuremath{\sim}120 K, which weakens on air-annealing. A coexistence of positive and negative MD [MD = $\frac{\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)}{\ensuremath{\varepsilon}(H=0)}$; $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)=\ensuremath{\varepsilon}(H)\ensuremath{-}\ensuremath{\varepsilon}(H=0)$] response is observed, with the former dominating at 300 K and the latter at 10 K. As-prepared 5 at.% (10 at.%) Ca and Ca-Ti substituted ${\mathrm{BiFeO}}_{3}$ ceramics exhibit a maximum MD response of --10% (\ensuremath{\sim}+3%) at 10 K (300 K). Negative MD response diminishes for air-annealed ${\mathrm{Bi}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Fe}}_{1\ensuremath{-}y}{\mathrm{Ti}}_{y}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$ ceramics due to the reduction in ${\mathrm{V}}_{\mathrm{O}}$ concentration. Samples exhibiting dominant positive MD response show a similar trend for MD $vs$ H and ${M}^{2}$ vs H plots. This agreement between ${M}^{2}$ and $\mathrm{\ensuremath{\Delta}}\ensuremath{\varepsilon}(H)$ demonstrates a strong inherent MD coupling. On the contrary, negative MD does not follow this trend yet shows a linear relationship of MD vs ${M}^{2}$, suggesting a strong coupling between the magnetic and dielectric properties. Temperature-dependent MD studies carried out at 5 T show a gradual change from negative to positive values. Negative MD at low temperatures could be activated by the spin-lattice coupling, which dominates even at high frequency (1 MHz) under the applied field. Other contributions, including Verwey-like transition, magnetoresistance, and Maxwell-Wagner effects, do not influence the observed MD response. A prominent role of oxygen vacancies in altering the MD behavior of ${\mathrm{BiFeO}}_{3}$ is discussed in detail.

Influence of Morphology and Compositional Mixing on the Electrochemical Performance of Li-Rich Layered Oxides Derived from Nanoplatelet-Shaped Transition Metal Oxide–Hydroxide Precursors

Abstract: The influence of morphology and compositional mixing on the electrochemical performances of Li-rich layered oxides (LLOs), specifically to address the high rate capability, is investigated. LLOs of composition xLi₂MnO₃·(1 – x)LiMn₀.₂₅Ni₀.₃₈Co₀.₃₇O₂ (LMNC, x = 0, 0.2, 0.4, and 0.6), lying in the plane NMC(640)–LCO–LMO, are synthesized in nanoplatelet morphology, and the results are compared to the same compounds prepared by a conventional solid-state reaction (SSR). Hexagonal-shaped thin (∼50 nm) flakes of transition metal oxide–hydroxide [TMO(OH)], prepared by the hydrothermal process, are reacted with Li carbonate to derive nanoplatelet morphology of LMNC by topotactic conversion. Structural and compositional evolutions of LLOs are analyzed with Rietveld refinement. The composite nature of LMNC comprising of monoclinic Li₂MnO₃ and rhombohedral LiMO₂ phases is evidenced. High-resolution transmission electron microscopy studies show the existence of a monoclinic Li₂MnO₃ phase embedded within the rhombohedral layered oxide phase. A uniform compositional distribution of all elements is discerned from EDS mapping, strongly suggesting that metal cations in both TMO/OH and LMNC are highly intermixed. Electrochemical properties become better with the larger fraction of the Li₂MnO₃ phase in LiMO₂. Among four compositions examined, LMNC (x = 0.6) shows the best electrochemical performance, with a capacity of ∼240 mAh g–¹ (∼173 mAh g–¹) at 0.1 C (1 C) current rate. Cycling stability studies, carried out at 1 C rate for 100 cycles, show a high capacity retention of 86%. Capacity at 3 C (5 C) is ∼140 mAh g–¹ (∼80 mAh g–¹) in LMNC (x = 0.6). LMNC (x = 0 and 0.6) prepared by SSR show inferior properties, suggesting that morphology and thorough intermixing of monoclinic Li₂MnO₃ and rhombohedral LiMO₂ phases are shown to play a significant role. Although enhanced performance is generally attributed to the extra capacity contribution from the Li₂MnO₃ phase, this study unequivocally brings out the influence of morphology on the electrochemical properties.

Enabling high-rate capability by combining sol-gel synthesis and solid-state reaction with PTFE of 4.2 V cathode material LiVPO4F/C

Abstract: LiVPO4F (LVPF) is a promising high voltage (4.2 V vs. Li/Li+) and high energy density (655 Wh kg−1) cathode material for lithium ion batteries. The challenge is to prepare phase pure and conducting material suitable for battery application. In this work carbon coated LiVPO4F (LVPF/C) is synthesized by sol-gel method, and the effect of carbon coating and control on F content on the structure, morphology, electrochemical properties of LVPF/C are studied comprehensively. Li3V2(PO4)3 and Li9V3(P2O7)3(PO4)2 form as minor secondary phases when LVPF is prepared without carbon source. In contrast Li3V2(PO4)3 and V2O3 are found when lauric acid is used as carbon source. Using optimal concentration of polytetrafluoroethylene (35 wt. % PTFE) and lauric acid (27 wt. % LA) in the synthesis yields carbon coated LVPF (LVPF/C-35) with best electrochemical performance. Lattice parameters of LiVPO4F are consistent with the reported values. Electrochemical properties of the LVPF/C-X cathodes (X = 25, 35 and 45 wt. % PTFE) show the discharge capacities of ∼ 85.4 (∼ 7), ∼ 114.7 (∼ 84.1), ∼ 102.7 (∼ 14.2) mA h g−1 at 0.1C (10C) rates, when scanned in the voltage range of 3.0–4.5 V vs. Li/Li+. A flat potential profile at ∼ 4.2 V vs. Li/Li+ is seen during charging-discharging profiles of the LVPF/C-X samples. From CV studies, the diffusion coefficient is found to be of the order of 10−16 cm2s−1 during oxidation and reduction. BET surface area is more (∼ 75.95 m2 g−1) for LVPF/C-35 compared to other two samples. From Electrochemical impedance spectroscopy, the influence of charge-transfer resistance resulting from the cathode electrolyte interface layer on electrochemical properties is studied.

Enhanced rate capability of interconnected LiNi1/3Mn1/3Co1/3O2 nanoparticle cathode

Abstract: Enhanced rate capability of interconnected LiNi1/3Mn1/3Co1/3O2 (NMC) nanoparticles fabricated via electrospinning method is presented. Interconnected NMC nanoparticles have a discharge capacity of 175 mAh g−1 at 0.1C and 94 mAh g−1 at 1C. Similar nanoparticular NMC cathode without interconnections, fabricated by co-precipitation method, has a capacity of 22 mAh g−1 at 1C. Interconnected NMC nanoparticles have good cycling stability after 500 cycles at 1C with a capacity of ~17 mAh g−1. Though the rate capability of Interconnected NMC nanoparticles is superior, cycling stability is similar across cathodes, indicating that capacity fading is intrinsic to the cathode.