Morphological control for high proton conduction in robust Co3O4-diethylmethylamine (metal-organic framework) membrane.

Metal-organic framework (MOF) based proton conductors are synthesized by the Avrami model (time-temperature modalities). Our objective here is to obtain a material with high proton conductivity in anhydrous conditions, improved catalytic behaviour and morphology control of conductivity, band gap and catalysis. For this purpose, we try to understand the role of morphology on mass transportation using computational fluid dynamics and the experimental realisation using the synthesis of MOF membranes with high protonic conductivity. In order to alter the morphology, the membranes are synthesized from protic ionic liquid (dimethyl ethyl amine H2PO4) and metal ion (Co3O4) at different temperatures and duration. A high protonic conductivity of 0.0286 S cm-1 with a high transference number >0.99 is observed in anhydrous conditions with the change in morphology. Furthermore, catalyst properties along with high activity (Tafel slope = 39 mV decade-1) with the alteration in morphology are also investigated in detail and observed adsorption governed conduction. This adsorption governed conduction is verified using computational fluid dynamics simulations with the alteration in morphology. This study suggests that morphology not only plays a pivotal role in obtaining a robust proton exchange membrane, it also improves the catalytic functionality and stability of the membrane.

Overlapping large polaron tunnelling in lanthanum silicate oxyapatite.

Amongst the various fast ion conductors, lanthanum excess lanthanum silicate oxyapatite (La10-α(SiO4)6O2+δ) has shown higher oxide ion conductivity with lower activation energy. On the other hand, the activation energy increases with La vacancies (La at 4f site). In the present work, La site is altered with Ca to form (La1-xCax)9.67(SiO4)6O2+δ(x=0.0,0.05,0.10and 0.15) with minimum oxygen non-stoichiometry and studied the hopping/tunnelling mechanism with the Ca substitution. The elemental content obtained from Rietveld refinement of the x-ray diffractograms suggests La deficiency with minimum oxygen deficiency. Further, XPS and TGA studies confirm the formation of La deficient samples. Temperature and frequency dependent ac conductivity in the temperature range (548-973 K) suggests that the conduction takes place via overlapping large polaron tunnelling. Further, the tunnelling distance and polaron radii as a function of temperature and frequency are observed to be altered with Ca and affecting the ion conducting channel through the elongation of La(6 h) triangles. Our study suggests the phononic contribution play a pivotal role in ionic transport.

Influence of the crystalline phase on the electrocatalytic behaviour of Sm2-xSrxNiO4-δ (x = 0.4 to 1.0) Ruddlesden-Popper-based systems: a comparative study of bulk and thin electrocatalysts.

Herein, the influence of compositional engineering via active site alternation on catalytic behaviour has been studied for the Ruddlesden-Popper-based system Sm2-xSrxNiO4-δ. A phase change from orthorhombic (x = 0.6) to tetragonal (x = 1.0) in bulk Sm2-xSrxNiO4-δ is confirmed by Rietveld (XRD) analysis, thermogravimetric analysis (TGA) and X-ray photoelectron spectroscopy (XPS). To alter the active sites, we fabricated thin films for x = 0.6 and x = 1.0 using a pulsed laser deposition technique. The electrocatalytic behaviour has been studied in an environmentally friendly medium, i.e., a neutral medium (pH = 7), for both bulk and thin films, and parameters such as transient response, electrochemical reversibility and oxygen evolution reactivity are measured. The cyclic voltammetry curves suggest that electrochemical reversibility for thin films is governed by adsorption as opposed to the diffusion observed for bulk samples. Our investigation further suggests that moderate electroactivity can be achieved with an increase in active sites on miniaturization with the phase change.

Hysteresis in centrosymmetric CuPbI3perovskite halide: apolar dielectric or orientable dielectric?

We demonstrated the change in polarization behaviour at the surface/interface before and after light through Havriliak-Negami equation of lesser known CuPbI3. We have synthesized CuPbI3through cold sintering technique and the polarization mechanisms are altered by increasing (cold) sintering temperature. The structure of CuPbI3was not known and we predicted it to be hexagonal (R3̄m) with 21R prototype representation. The hysteresis is reported to be affected by ferroelectricity (reorientable dipoles with non-centrosymmetry), to inspect this a centrosymmetric CuPbI3is taken. In spite of centrosymmetry, we observed that the hysteresis area and shape ofIVcurve in AM 1.5 G sunlight shows the drastic variation with the change in polarization behaviour. Our experimental results suggest that apolar dielectric behaviour is the cause ofI-Vhysteresis rather than robust ferroelectric polarization (which was absent in the present case).

Microstructural dependent oxygen reduction reaction in a Ruddlesden-Popper perovskite (SmSr)NiO4-δ.

Rare earth nickelate perovskites have very wide uses, as magnetic insulators, as well as being conducting materials for the various components of solid oxide fuel cells (SOFCs) due to them undergoing an insulator to metal transition below operating temperature. In SOFCs, the microstructural design of electrode materials is an important aspect for electron and oxygen ion conduction at the electrolyte-electrode and electrode-air interfaces. To investigate this feature, a Ruddlesen-Popper structured layered perovskite, (SmSr)NiO4-δ, was synthesized at different sintering temperatures using a solid-state reaction technique. Porous and dense microstructures were obtained at sintering temperatures of 1250 and 1425 °C, respectively. The influence of electrocatalysis on the structures of both surfaces was studied comprehensively. Post cyclic voltammetry structural studies show the presence of Ni-(OH)2 and Ni-OOH species for the samples, respectively, suggesting that they undergo different oxygen reduction reaction mechanisms.

Porous and highly conducting cathode material PrBaCo2O6-δ: bulk and surface studies of synthesis anomalies.

The paradigm that chemical synthesis reduces the sintering temperature as compared to solid state synthesis seems to be violated in the case of the PrBaCo2O6-δ double perovskite. The sintering temperatures for pure phase samples synthesized through the solid state route (P-SSR) and the auto-combustion route (P-ACR) were found to be 1050 and 1150 °C, respectively. The porous microstructure of P-SSR is suitable for SOFC cathode materials while that of P-ACR is pore free. High-resolution transmission electron microscopy, Raman and scanning tunneling microscopy studies reveal that there is crystal growth on a smooth surface with a preferred orientation. Our results show that this anomalous synthesis behaviour is due to anisotropic surface nucleation growth. Thermodynamically, the higher decomposition temperature in the chemical route is due to stronger electron-phonon coupling and the higher value of change in entropy. The variation in the Co-O-Co bond angle reveals Jahn-Teller vibrational anisotropy in the-b plane leading to the anisotropic synthesis behaviour. This anisotropy is the reason for the violation of the paradigm.

Sm/Ti co-substituted bismuth ferrite multiferroics: reciprocity between tetragonality and piezoelectricity.

BiFeO3 (BFO) systems co-modified with Ti, Sm and Sm-Ti have been investigated for piezoelectricity together with dielectric and multiferroic properties. Structural studies revealed the coexistence of orthorhombic and rhombohedral (R3c) phases for x > 0.12. Impurity phases were shown to have hardly any effect on the remanent magnetization, which rather depends on the Fe-O-Fe bond angle. The dielectric loss was reduced considerably by substitution. A correlation between the piezoelectric coefficient and tetragonality was observed in these samples. BFO co-substituted with Sm-Ti exhibited a high piezoelectric coefficient with better ferroic properties, which revealed a unique combination of green piezoelectricity and multiferroicity.