Theoretical Study for Adsorption Behaviors in Catalysis and Energy Applications via Multi-Scale Simulation

Department of Chemical EngineeringWith increasing energy demands and environmental concerns related to the greenhouse effect, one of the most central themes of research society is the development of sustainable and environmental-friendly energy system. In this regard, catalysts play a pivotal role in producing the fossil-free industrial chemicals (e.g., ammonia, hydrogen, and hydrocarbon) from earth-abundant substances (e.g., nitrogen, water) through (electro-)chemical conversion process. Moreover, energy storage technologies, such as lithium-ion batteries and fuel cells, are equally important for practical applications. However, the current levels of energy conversion and storage technologies are inadequate; that is, more advanced design and fundamental understanding of these processes is now pursued. Adsorption has an important meaning in the (electro-)chemical process in that the energy states of adsorbed reaction intermediates in the mechanistic pathways can determine the reaction mechanisms. Many studies have focused on the adsorption behaviors and corresponding energetics to predict the catalytic activity or battery performance. In this context, molecular simulation approach can be a suitable tool to investigate the adsorption behaviors in the atomistic and molecular level. Particularly, multi-scale molecular simulation methods are useful to properly elucidate the physicochemical phenomena in experimental (or realistic) systems crossing over different temporal and spatial scales. In this dissertation, theoretical studies on adsorption behaviors in catalysis and energy applications have been conducted via multi-scale molecular simulation approach. In Chapter 2, we theoretically demonstrated that atomically dispersed Pt catalysts on carbon nanotube (Pt1/CNT) could catalyze the chlorine evolution reaction (CER) with excellent activity and selectivity. From ab initio Pourbaix diagram, the active adsorbate structures for the Pt1/CNT were initially found. Subsequently, the mechanistic pathways for the CER were thoroughly investigated by combining the experimental (for kinetics) and theoretical data (for thermodynamics). Among Pt???N4 sites, PtN4C12 was identified as the most plausible active site structure for the CER. Moreover, the excellent selectivity of Pt1/CNT was evidenced by the large differences in thermodynamic overpotentials for respective CER and oxygen evolution reaction, which can be determined from the adsorption free energies of the reaction intermediates. We envision that this type of catalysts may be exploited as an alternative CER catalyst instead of precious metal-based mixed metal oxides (MMOs), which have suffered from concomitant generation of oxygen. In Chapter 3, we investigated the thermodynamics of ion adsorption on the amorphous intermediate phases of calcium carbonates. Amorphous calcium carbonate (ACC) have received enormous attentions because their local order in the short-range can affect the subsequent pathways for phase transformation. Using molecular dynamic simulation, we theoretically elucidated the precise role of ion adsorption in controlling the local structures and stability of ACC phases. Starting from the nucleation clusters in aqueous solution, the hydrated and anhydrous forms of ACC were systematically examined by varying the hydration levels and molar composition of additive ions (e.g. Mg2+, Fe2+, Sr2+, and Ba2+). Our results revealed that each ion can exert promoting or inhibiting effect by tuning the local order and stability of ACC phases depending on their hydrophilicity and ionic radii. More importantly, our findings suggested that the thermodynamic spontaneity of the overall phase transition process can be determined by the balance between two opposing factors ??? endothermic dehydration and exothermic crystallization. Chapters 4 and 5 commonly describe the investigation of adsorption phenomena, related to the next-generation rechargeable batteries such as lithium-sulfur (Li-S) and lithium-oxygen (Li-O2) battery. The adsorption of reaction intermediates for Li-S and Li-O2 battery, which are polysulfides (i.e., Li2Sx, 1"???" x"???" 8) and superoxide species (i.e., O2???- or LiO2), respectively, is a decisive step for these systems because these floating reaction intermediates can hamper the cell performance by triggering the unwanted side reactions such as shuttle phenomena of Li2Sx, and electrolyte degradation by O2???- species. In Chapter 4, we investigated the polysulfide adsorption on molecularly designed chemical trap in Li-S battery. A microporous covalent organic framework (COF) net on mesoporous carbon nanotube (CNT) net hybrid architecture was introduced as a new class of chemical trap for polysulfides. Two COFs with different micropore sizes (COF-1 and COF-5) were selected as model systems. Using density functional theory calculation and grand canonical Monte Carlo simulation, the pore-size-enabled selective adsorption of Li2S in COF-1 was theoretically demonstrated. The results also revealed that COF-1 possesses a well-designed micropore size and (boron-mediated) chemical affinity suitable for selective adsorption of Li2S, which can significantly improve the electrochemical performance of Li-S battery. In Chapter 5, we investigated the superoxide adsorption and subsequent disproportionation mechanism in Li-O2 battery. Reactive O2???- species can trigger the side reactions, which are serious hurdles hampering the performance of Li-O2 battery. To resolve this issue, malonic-acid-decorated fullerene (MA-C60) was employed as a superoxide disproportionation chemo-catalyst. Using multi-scale molecular simulation methods including density functional theory and molecular dynamics, we theoretically evidenced the preference to solution mechanism over surface mechanism in the presence of MA-C60 catalyst. Additionally, from the free energy diagram along reaction pathway of the solution mechanism, we identified the beneficial role of MA-C60 to significantly reduce the thermodynamic barrier of the disproportionation step.clos

A single-ion conducting covalent organic framework for aqueous rechargeable Zn-ion batteries

Despite their potential as promising alternatives to current state-of-the-art lithium-ion batteries, aqueous rechargeable Zn-ion batteries are still far away from practical applications. Here, we present a new class of single-ion conducting electrolytes based on a zinc sulfonated covalent organic framework (TpPa-SO3Zn0.5) to address this challenging issue. TpPa-SO3Zn0.5 is synthesised to exhibit single Zn2+ conduction behaviour via its delocalised sulfonates that are covalently tethered to directional pores and achieve structural robustness by its beta-ketoenamine linkages. Driven by these structural and physicochemical features, TpPa-SO3Zn0.5 improves the redox reliability of the Zn metal anode and acts as an ionomeric buffer layer for stabilising the MnO2 cathode. Such improvements in the TpPa-SO3Zn0.5-electrode interfaces, along with the ion transport phenomena, enable aqueous Zn-MnO2 batteries to exhibit long-term cyclability, demonstrating the viability of COF-mediated electrolytes for Zn-ion batteries

Chiral self-sorted multifunctional supramolecular biocoordination polymers and their applications in sensors

Chiral supramolecules have great potential for use in chiral recognition, sensing, and catalysis. Particularly, chiral supramolecular biocoordination polymers (SBCPs) provide a versatile platform for characterizing biorelated processes such as chirality transcription. Here, we selectively synthesize homochiral and heterochiral SBCPs, composed of chiral naphthalene diimide ligands and Zn ions, from enantiomeric and mixed R-ligands and S-ligands, respectively. Notably, we find that the chiral self-sorted SBCPs exhibit multifunctional properties, including photochromic, photoluminescent, photoconductive, and chemiresistive characteristics, thus can be used for various sensors. Specifically, these materials can be used for detecting hazardous amine materials due to the electron transfer from the amine to the SBCP surface and for enantioselectively sensing a chiral species naproxen due to the different binding energies with regard to their chirality. These results provide guidelines for the synthesis of chiral SBCPs and demonstrate their versatility and feasibility for use in various sensors covering photoactive, chemiresistive, and chiral sensors

Colossal optical anisotropy from atomic-scale modulations

In modern optics, materials with large birefringence ({\Delta}n, where n is the refractive index) are sought after for polarization control (e.g. in wave plates, polarizing beam splitters, etc.), nonlinear optics and quantum optics (e.g. for phase matching and production of entangled photons), micromanipulation, and as a platform for unconventional light-matter coupling, such as Dyakonov-like surface polaritons and hyperbolic phonon polaritons. Layered "van der Waals" materials, with strong intra-layer bonding and weak inter-layer bonding, can feature some of the largest optical anisotropy; however, their use in most optical systems is limited because their optic axis is out of the plane of the layers and the layers are weakly attached, making the anisotropy hard to access. Here, we demonstrate that a bulk crystal with subtle periodic modulations in its structure -- Sr9/8TiS3 -- is transparent and positive-uniaxial, with extraordinary index n_e = 4.5 and ordinary index n_o = 2.4 in the mid- to far-infrared. The excess Sr, compared to stoichiometric SrTiS3, results in the formation of TiS6 trigonal-prismatic units that break the infinite chains of face-shared TiS6 octahedra in SrTiS3 into periodic blocks of five TiS6 octahedral units. The additional electrons introduced by the excess Sr subsequently occupy the TiS6 octahedral blocks to form highly oriented and polarizable electron clouds, which selectively boost the extraordinary index n_e and result in record birefringence ({\Delta}n > 2.1 with low loss). The connection between subtle structural modulations and large changes in refractive index suggests new categories of anisotropic materials and also tunable optical materials with large refractive-index modulation and low optical losses.Comment: Main text + supplementar

Giant Modulation of Refractive Index from Correlated Disorder

Correlated disorder has been shown to enhance and modulate magnetic, electrical, dipolar, electrochemical and mechanical properties of materials. However, the possibility of obtaining novel optical and opto-electronic properties from such correlated disorder remains an open question. Here, we show unambiguous evidence of correlated disorder in the form of anisotropic, sub-angstrom-scale atomic displacements modulating the refractive index tensor and resulting in the giant optical anisotropy observed in BaTiS3, a quasi-one-dimensional hexagonal chalcogenide. Single crystal X-ray diffraction studies reveal the presence of antipolar displacements of Ti atoms within adjacent TiS6 chains along the c-axis, and three-fold degenerate Ti displacements in the a-b plane. 47/49Ti solid-state NMR provides additional evidence for those Ti displacements in the form of a three-horned NMR lineshape resulting from low symmetry local environment around Ti atoms. We used scanning transmission electron microscopy to directly observe the globally disordered Ti a-b plane displacements and find them to be ordered locally over a few unit cells. First-principles calculations show that the Ti a-b plane displacements selectively reduce the refractive index along the ab-plane, while having minimal impact on the refractive index along the chain direction, thus resulting in a giant enhancement in the optical anisotropy. By showing a strong connection between correlated disorder and the optical response in BaTiS3, this study opens a pathway for designing optical materials with high refractive index and functionalities such as a large optical anisotropy and nonlinearity.Comment: 24 pages, 3 figure

Unconventional Charge-density-wave Order in a Dilute d-band Semiconductor

Electron-lattice coupling effects in low dimensional materials give rise to charge density wave (CDW) order and phase transitions. These phenomena are critical ingredients for superconductivity and predominantly occur in metallic model systems such as doped cuprates, transition metal dichalcogenides, and more recently, in Kagome lattice materials. However, CDW in semiconducting systems, specifically at the limit of low carrier concentration region, is uncommon. Here, we combine electrical transport, synchrotron X-ray diffraction and optical spectroscopy to discover CDW order in a quasi-one-dimensional (1D), dilute d-band semiconductor, BaTiS3, which suggests the existence of strong electron-phonon coupling. The CDW state further undergoes an unusual transition featuring a sharp increase in carrier mobility. Our work establishes BaTiS3 as a unique platform to study the CDW physics in the dilute filling limit to explore novel electronic phases

Reversibly controlled ternary polar states and ferroelectric bias promoted by boosting square???tensile???strain

Interaction between dipoles often emerges intriguing physical phenomena, such as exchange bias in the magnetic heterostructures and magnetoelectric effect in multiferroics, which lead to advances in multifunctional heterostructures. However, the defect-dipole tends to be considered the undesired to deteriorate the electronic functionality. Here, we report deterministic switching between the ferroelectric and the pinched states by exploiting a new substrate of cubic perovskite, BaZrO3, which boosts square-tensile-strain to BaTiO3 and promotes four-variants in-plane spontaneous polarization with oxygen vacancy creation. First-principles calculations propose a complex of an oxygen vacancy and two Ti3+ ions coins a charge-neutral defect-dipole. Cooperative control of the defect-dipole and the spontaneous polarization reveals ternary in-plane polar states characterized by biased/pinched hysteresis loops. Furthermore, we experimentally demonstrate that three electrically controlled polar-ordering states lead to switchable and non-volatile dielectric states for application of non-destructive electro-dielectric memory. This discovery opens a new route to develop functional materials via manipulating defect-dipoles and offers a novel platform to advance heteroepitaxy beyond the prevalent perovskite substrates

Molecular Dynamics Study for the Effect of Additive Ions on Phase Transformation of Amorphous Precursor Phases of CaCO3

Calcium carbonate is an earth-abundant material that exists in a variety of natural environments, including soils and sediments. In recent years, amorphous calcium carbonate (ACC) phases has increasingly received scientific attention because their local orders in the short-range can determine the subsequent pathways for phase transformation. However, the fundamental understanding about the structure-property relationship for these amorphous precursor phases is still lacking. As a fundamental work to elucidate this issue, we introduced additive metal ions, which are usually soluble in aqueous solution and can tailor the local orders of ACC with different hydrophilicity. With molecular dynamics simulation, we investigated the effect of additive ions on the phase transformation process of the ACC model system by varying the hydration levels and molar compositions of additive ions (i.e., Mg2+, Fe2+, Ba2+ and Sr2+). Starting from the cluster nucleation in aqueous solution, the hydrated and anhydrous forms of ACC were systematically examined following the dehydration scheme. Our results revealed that additive ions can exert promoting or inhibiting effect depending on their different hydration strengths and ionic radius. These results can provide a valuable information for controlling the structure and stability of amorphous precursor phases, which can shed light on the mineral carbonation processes for CO2 capture