Engineering topics

Rojas, Tomas

Publications and source records attributed to Rojas, Tomas.

5 records

DOE OSTI2024

Systematic Improvement of Quantum Monte Carlo Calculations in Transition Metal Oxides: sCI-Driven Wavefunction Optimization for Reliable Band Gap Prediction

Accurate determination of the electronic properties of correlated oxides remains a significant challenge for computational theory. Traditional Hubbard-corrected density functional theory (DFT+U) frequently encounters limitations in precisely capturing electron correlation, particularly in predicting band gaps. We introduce a systematic methodology to enhance the accuracy of diffusion Monte Carlo (DMC) simulations for both ground and excited states, focusing on LiCoO 2 as a case study. By employing a selected configuration interaction (sCI) approach, we demonstrate the capability to optimize wavefunctions beyond the constraints of single-reference DFT+U trial wavefunctions. Here, we show that the sCI framework enables accurate prediction of band gaps in LiCoO 2 , closely aligning with experimental values and substantially improving traditional computational methods. The study uncovers a nuanced mixed state of t 2g and e g orbitals at the band edges that is not captured by conventional single-reference methods, further elucidating the limitations of PBE+U in describing d-d excitations. Our findings advocate for the adoption of beyond-DFT methodologies, such as sCI, to capture the essential physics of excited-state wavefunctions in strongly correlated materials. The improved accuracy in band gap predictions and the ability to generate more reliable trial wavefunctions for DMC calculations underscore the potential of this approach for broader applications in the study of correlated oxides. This work not only provides a pathway for more accurate simulations of electronic structures in complex materials but also suggests a framework for future investigations of the excited states of other challenging systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗

DOE OSTI2023

Characterization of just one atom using synchrotron X-rays

Since the discovery of X-rays by Roentgen in 1895, its use has been ubiquitous, from medical and environmental applications to materials sciences. X-ray characterization requires a large number of atoms and reducing the material quantity is a long-standing goal. Here we show that X-rays can be used to characterize the elemental and chemical state of just one atom. Using a specialized tip as a detector, X-ray-excited currents generated from an iron and a terbium atom coordinated to organic ligands are detected. The fingerprints of a single atom, the L 2,3 and M 4,5 absorption edge signals for iron and terbium, respectively, are clearly observed in the X-ray absorption spectra. The chemical states of these atoms are characterized by means of near-edge X-ray absorption signals, in which X-ray-excited resonance tunnelling (X-ERT) is dominant for the iron atom. The X-ray signal can be sensed only when the tip is located directly above the atom in extreme proximity, which confirms atomically localized detection in the tunnelling regime. Finally, our work connects synchrotron X-rays with a quantum tunnelling process and opens future X-rays experiments for simultaneous characterizations of elemental and chemical properties of materials at the ultimate single-atom limit.

74 ATOMIC AND MOLECULAR PHYSICS↗

DOE OSTI2023

A High‐Rate Li–CO 2 Battery Enabled by 2D Medium‐Entropy Catalyst

Abstract Lithium‐air batteries based on CO 2 reactant (Li–CO 2 ) have recently been of interest because it has been found that reversible Li/CO 2 electrochemistry is feasible. In this study, a new medium‐entropy cathode catalyst, (NbTa) 0.5 BiS 3 , that enables the reversible electrochemistry to operate at high rates is presented. This medium entropy cathode catalyst is combined with an ionic liquid‐based electrolyte blend to give a Li–CO 2 battery that operates at high current density of 5000 mA g −1 and capacity of 5000 mAh g −1 for up to 125 cycles, far exceeding reported values in the literature for this type of battery. The higher rate performance is believed to be due to the greater stability of the multi‐element (NbTa) 0.5 BiS 3 catalyst because of its higher entropy compared to previously used catalysts with a smaller number of elements with lower entropies. Evidence for this comes from computational studies giving very low surface energies (high surface stability) for (NbTa) 0.5 BiS 3 and transmission electron microscopystudies showing the structure being retained after cycling. In addition, the calculations indicate that Nb‐terminated surface promotes Li–CO 2 electrochemistry resulting in Li 2 CO 3 and carbon formation, consistent with the products found in the cell. These results open new direction to design and develop high‐performance Li–CO 2 batteries.

25 ENERGY STORAGE↗

DOE OSTI2022

A KMnO 4 -Generated Colloidal Electrolyte for Redox Mediation and Anode Protection in a Li–Air Battery

The rechargeable lithium-oxygen (Li-O 2 ) battery has the highest theoretical specific energy density of any rechargeable batteries and could transform energy storage systems if a practical device could be attained. However, among numerous challenges, which are all interconnected, are polarization due to sluggish kinetics, low cycle life, small capacity, and slow rates. Here, in this study, we report on use of KMnO 4 to generate a colloidal electrolyte made up of MnO 2 nanoparticles. The resulting electrolyte provides a redox mediator for reducing the charge potential and lithium anode protection to increase cycle life. This electrolyte in combination with a stable binary transition metal dichalcogenide alloy, Nb 0.5 Ta 0.5 S 2 , as the cathode enables the operation of a Li-O 2 battery at a current density of 1 mA center dot cm -2 and specific capacity ranging from 1000 to 10000 mA center dot h center dot g -1 (corresponding to 0.1-1 mA center dot h center dot cm -2 ) in a dry air environment with a cycle life of up to 150. This colloidal electrolyte provides a robust approach for advancing Li-air batteries.

25 ENERGY STORAGE↗

DOE OSTI2022

Novel Co‐Catalytic Activities of Solid and Liquid Phase Catalysts in High‐Rate Li‐Air Batteries

Abstract Li‐air batteries are considered strong candidates for the next‐generation energy storage systems designed for electrical transportation. However, low cyclability and current rates are two major drawbacks that hinder them from further realization. These issues necessitate the discovery of novel materials to significantly enhance the redox process of discharge products. In this study, a novel catalytic system comprised of tin sulfide (SnS) nanoflakes as a solid catalyst and tin iodide (SnI 2 ) as a dual‐functional electrolyte additive is discovered. This system enables operating the battery at high current rates up to 10 000 mA g −1 (corresponding to 1 mA cm −2 ). The SnS catalyst shows outstanding catalytic activity for both oxygen reduction and evolution reactions compared to carbon, noble metals, and other transition metal dichalcogenides. It also exhibits good structural integrity at high rates. The computations indicate numerous possible oxygen reduction sites without oxygen dissociations on the SnS surface through solution mechanism that is likely responsible for the formation of Li 2 O 2 . The calculations also indicate that the role of the SnI 2 is not only reacting with the lithium anode to provide protection but reducing the charge potential by promoting catalytic decomposition of the Li 2 O 2 . This work provides new novel additives for designing high‐rate Li‐air batteries.

25 ENERGY STORAGE↗