Paper & Patents

Mechanical, structural, electronic, magnetic and thermoelectric properties of multi functional CoFeMnSb quaternary Heusler alloy have been studied using spin polarized Density Functional and semiclassical Boltzmann transport theory. Ab-initio study reveals halfmetallic ferromagnetic characteristics of the alloy leading to high spin polarization. The calculated value of total magnetic moment (mt = 4.99 μB) almost follow the Slater-Pauling rule which is another prerequisite of halfmetallicity. Study of hydrostatic pressure shows a significant effect on the electronic structure of the alloy however, along with the robust mechanical stability there is no magnetic phase transition seen as such in the whole range of pressure. Due to the occurrence of flat bands near the Fermi level(EF) in the minority spin channel, thermoelectric properties are also studied for both the spin polarized channel using BoltzTraP code under constant relaxation time approximation of the charge carriers. Halfmetallic properties, robust mechanical stability and significantly high value of Seebeck coefficient (∼800μV/K for p-type and ∼−900μV/K for n-type) makes the material promising for spintronic as well as thermoelectric applications.

The role of spin orientation on the reactivity of oxygen reduction reaction (ORR) intermediates (O, OH) on a ferromagnetic electrode surface is studied using constrained density functional theory formalism. We show that the strength of the binding of these reaction intermediates depend on their relative spin orientations with respect to the magnetization of the electrode. This suggests that oxygen-based electrochemical reactions on ferromagnetic catalyst surfaces can be controlled through the applied magnetic field. In the present study, we demonstrate such a possibility through the study of an oxygen reduction reaction on a PdFe (001) surface by introducing a new concept: spin orientation dependent overpotential. Also, we have explained the origin of lower dissociation barrier for the O2 molecule on ferromagnetic surfaces when its spin moment is anti-parallel to the surface magnetization as reported in the recent experiments.

A coupled atomistic spin and lattice dynamics approach is developed which merges the dynamics of these two degrees of freedom into a single set of coupled equations of motion. The underlying microscopic model comprises local exchange interactions between the electron spin and magnetic moment and the local couplings between the electronic charge and lattice displacements. An effective action for the spin and lattice variables is constructed in which the interactions among the spin and lattice components are determined by the underlying electronic structure. In this way, expressions are obtained for the electronically mediated couplings between the spin and lattice degrees of freedom, besides the well known interatomic force constants and spin-spin interactions. These former susceptibilities provide an atomistic ab initio description for the coupled spin and lattice dynamics. It is important to notice that this theory is strictly bilinear in the spin and lattice variables and provides a minimal model for the coupled dynamics of these subsystems and that the two subsystems are treated on the same footing. Questions concerning time-reversal and inversion symmetry are rigorously addressed and it is shown how these aspects are absorbed in the tensor structure of the interaction fields. By means of these results regarding the spin-lattice coupling, simple explanations of ionic dimerization in double-antiferromagnetic materials, as well as charge density waves induced by a nonuniform spin structure, are given. In the final parts, coupled equations of motion for the combined spin and lattice dynamics are constructed, which subsequently can be reduced to a form which is analogous to the Landau-Lifshitz-Gilbert equations for spin dynamics and a damped driven mechanical oscillator for the ionic motion. It is important to notice, however, that these equations comprise contributions that couple these descriptions into one unified formulation. Finally, Kubo-like expressions for the discussed exchanges in terms of integrals over the electronic structure and, moreover, analogous expressions for the damping within and between the subsystems are provided. The proposed formalism and types of couplings enable a step forward in the microscopic first principles modeling of coupled spin and lattice quantities in a consistent format.

Being a member of van der Waals class of solids, bilayer MoS2 exhibits polytypism due to different possible stacking arrangements, namely, 2Hc, 2Ha and 3R-polytypes. Unlike monolayer MoS2, these bilayers exhibit indirect band gap. Band extrema states originate from linear combination of Mo-(d) and S-(p) orbitals which are sensitive to the interlayer interactions. We have studied the impact of stacking pattern on electronic structure and electron/hole transport property in these polytypes. Based on first principles computations coupled with Boltzmann Transport formalism, we find that a strong electron-hole anisotropy can be realised in 2Ha-MoS2 polytype unlike in monolayer which is isotropic in nature. A staggered arrangement between two layers results in higher relaxation time for electron compared to hole leading to anisotropy which is of importance in device engineering.

Although Mo2C is used as an alternative catalyst material for biomass conversion, the dependence of its reaction on the type of surface termination has not been studied. In the present study, we performed density functional theory calculations for the deoxygenation of propanoic acid on the two types of surface terminations possible for orthorhombic Mo2C (100), namely, on Mo- and C-terminated surfaces. The reaction energetics of the three possible deoxygenation pathways, namely, those for hydrodeoxygenation, decarbonylation, and decarboxylation, were compared by calculating the activation energies for their key surface reactions, such as hydrogenation and the scission of CC, CO, and OH bonds. The Mo-terminated surface was advantageous in the case of the hydrodeoxygenation pathway because of its low kinetic energy barrier for the hydrogenation of the oxygen species, while the C-terminated surface preferred the decarboxylation pathway because of its low kinetic energy barrier for OH bond scission.

A-site cation doping in perovskite-based materials with a general ABO3 formula has a significant effect on the bifunctional oxygen activity (oxygen evolution and reduction reactions) of the electrocatalysts with chemical stability, enabling the design of highly active, durable, and cost-effective catalysts. In particular, the oxygen activity of double perovskite-structured NdBa0.5Sr0.5Co1.5Fe0.5O5+δ (NBSCF) is 0.973 V, which is much greater than that of previously reported transition metal-based nanostructures. This result is verified by examination of the electronic structure, oxidation state, and electrical properties of perovskite-based materials using density functional theory (DFT) calculations, iodometric titration method, x-ray photon spectroscopy (XPS), and electrochemical impedance spectroscopy (EIS) analysis. Further improvements of NBSCF for bifunctional oxygen activity are made by incorporating these synergistic hybrid structures with nitrogen doped-reduced graphene-based (N-rGO) nanostructures (NBSCF/N-rGO). The NBSCF/N-rGO has an oxygen electrode activity of 0.766 V, which is superior to that of other previously reported transition metal-based nanostructures and compares favorably to precious metal electrocatalysts. Furthermore, strong N-rGO provides considerably greater electrochemical long-term stability and integrity of NBSCF/N-rGO hybrid catalysts under continuous chronopotentiometric and long-term potential sweep testing conditions for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR).

The d-band center model of Hammer and N{\o}rskov is widely used in understanding and predicting catalytic activity on transition metal (TM) surfaces. Here, we demonstrate that this model is inadequate for capturing the complete catalytic activity of the magnetically polarized TM surfaces and propose its generalization. We validate the generalized model through comparison of adsorption energies of the NH$_3$ molecule on the surfaces of 3d TMs (V, Cr, Mn, Fe, Co, Ni, Cu and Zn) determined with spin-polarized density functional theory (DFT)-based methods with the predictions of our model. Compared to the conventional d-band model, where the nature of the metal-adsorbate interaction is entirely determined through the energy and the occupation of the d-band center, we emphasize that for the surfaces with high spin polarization, the metal-adsorbate system can be stabilized through a competition of the spin-dependent metal-adsorbate interactions.

We report solutions (durable material and degradation prevention method) to minimize the performance degradation of cell components occurring in the solid oxide fuel cell (SOFC) operation. Reliability testing is carried out with the Ni[single bond]Nd0.1Ce0.9O2-? (NDC) anode-supported intermediate temperature-SOFCs. For the cathode materials, single perovskite structured Ba0.5Sr0.5Co0.8Fe0.2O3-? (BSCF) and double perovskite structured NdBa0.5Sr0.5Co1.5Fe0.5O5+? (NBSCF) are prepared and evaluated under harsh SOFC operating conditions. The double perovskite NBSCF cathode shows excellent stability in harsh SOFC environments of high humidity and low flow rate of air. Furthermore, we propose the concurrent fuel and air starvation mode, in which the cell potential is temporarily reduced due to the formation of both fuel-starvation (in the anode) and air-depletion (in the cathode) concurrently under a constant load. This is carried out in order to minimize the performance decay of the stable NBSCF-cell through the periodic and extra reduction of (and ) in the anode. The operating-induced degradation of SOFCs, which are ordinarily assumed to be unrecoverable, can be completely circumvented by the proposed periodical operation logic to prevent performance degradation (concurrent fuel-starvation and air-depletion mode).