Novel Multiphysics Phenomena in a New Generation of Energy Storage and Conversion Devices

The swelling demand for storing and using energy at diverse scales has stimulated the exploration of novel materials and design strategies applicable to energy storage systems. The most popular electrochemical energy storage systems are batteries, fuel cells and capacitors. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors have emerged to be particularly promising. Besides exhibiting high cycle life, they combine the best attributes of capacitors (high power density) and batteries (high energy storage density). Consequently, they are expected to be in high demand for applications requiring peak power such as hybrid electric vehicles and uninterruptible power supplies (UPS). This dissertation aims to make advancements on the following two topics in supercapacitor research with the aid of modeling and experimental tools: applying various thermophysical effects to design supercapacitor devices with novel functionalities and studying degradation mechanisms upon continuous cycling of conventional supercapacitors. The prime drawback of conventional supercapacitors is their low energy density. Most research in the last decade has focused on synthesizing novel electrode materials. Although such novel electrodes lead to high energy density, they often involve complicated synthesis process and result in high cost and low power density. A new concept of inducing pseudocapacitance developed in recent years is by introducing redox additives in the electrolyte that engage in redox reactions at the electrode/electrolyte interface during charge/discharge. The first section of this dissertation reports the performance of fabricated solid-state supercapacitors composed of redox-active gel electrolyte (PVA-K3Fe(CN)6-K4Fe(CN)6). The electrochemical performance has been studied extensively using cyclic voltammetry, constant current charge/discharge and impedance spectroscopy techniques, and then the results are compared with similar devices composed of conventional gel electrolytes such as PVA-H3PO4 and PVA-KOH on the basis of capacitance, internal resistance and stable voltage window. The second section explores the utility of the thermogalvanic property of the same redox-active gel electrolyte, PVA-K3Fe(CN)6-K4Fe(CN)6 in the construction of a thermoelectric supercapacitor. The integrated device is capable of being electrically charged by applying a temperature gradient across its two electrodes. In the absence of available temperature gradient, the device can be discharged electrically through an external circuit. Therefore, such a device can be used to harvest waste heat from intermittent heat sources. An equivalent circuit elucidating the mechanisms of energy conversion and storage applicable to thermally chargeable supercapacitors is developed. A fitting analysis aids in the evaluation of model circuit parameters providing good agreement with experimental voltage and current measurements. The latter part of the dissertation investigates the factors influencing aging in conventional supercapacitors. In the first part, a new imaging technique based on the electroreflectance property of gold has been developed and applied to characterize the aging characteristics of a microsupercapacitor device. Previous aging studies were performed through traditional electrical characterization techniques such as cyclic voltammetry, constant charge/discharge, and electrochemical impedance spectroscopy. These methods, although simple, measure an average of the structures’ internal performance, providing little or no information about microscopic details inside the device. The electroreflectance imaging method, developed in this work is demonstrated as a high-resolution imaging technique to investigate charge distribution, and thus to infer aging characteristics upon continuous cycling at high scan rates. The technique can be used for non-intrusive spatial analysis of other electrochemical systems in the future. In addition, we investigate heat generation mechanisms that are responsible for accelerated aging in supercapacitors. A modeling framework has been developed for heat generation rates and resulting temperature evolution in porous electrode supercapacitors upon continuous cycling. Past thermal models either neglected spatial variations of heat generation within the cell or considered electrodes as flat plates that led to inaccuracies. Here, expressions for spatiotemporal variation of heat generation rate are rigorously derived on the basis of porous electrode theory. Detailed numerical simulations of temperature evolution are performed for a real-world device, and the results resemble past measurements both qualitatively and quantitatively. In the last chapter of the thesis, a rare thermoelectric effect called the Nernst effect has been investigated in single-layer periodic graphene with the aid of a modified Boltzmann transport equation. Detailed formulations of the transport coefficients from the BTE solution are developed in order to relate the Nernst coefficient to the amount of impurity density, temperature, band gap and applied magnetic field. Detailed knowledge of the variation of the thermoelectric and thermomagnetic properties of graphene shown in this work will prove helpful for improving the performance of magnetothermoelectric coolers and sensors

Double Soft Graviton Theorems and BMS Symmetries

It is now well understood that Ward identities associated to the (extended) BMS algebra are equivalent to single soft graviton theorems. In this work, we show that if we consider nested Ward identities constructed out of two BMS charges, a class of double soft factorization theorems can be recovered. By making connections with earlier works in the literature, we argue that at the sub-leading order, these double soft graviton theorems are the so-called consecutive double soft graviton theorems. We also show how these nested Ward identities can be understood as Ward identities associated to BMS symmetries in scattering states defined around (non-Fock) vacua parametrized by supertranslations or superrotations.Comment: 29 pages, minor changes added, footnote 3 removed, consistency check with Ref:22 settle

Quantum vibronic effects on the electronic properties of molecular crystals

We present a study of molecular crystals, focused on the effect of nuclear quantum motion and anharmonicity on their electronic properties. We consider a system composed of relatively rigid molecules, a diamondoid crystal, and one composed of floppier molecules, NAI-DMAC, a thermally activated delayed fluorescence compound. We compute fundamental electronic gaps at the DFT level of theory, with the PBE and SCAN functionals, by coupling first-principles molecular dynamics with a nuclear quantum thermostat. We find a sizable zero-point-renormalization (ZPR) of the band gaps, which is much larger in the case of diamondoids (~ 0.6 eV) than for NAI-DMAC (~ 0.22 eV). We show that the frozen phonon (FP) approximation, which neglects inter-molecular anharmonic effects, leads to a large error (~ 50%) in the calculation of the band gap ZPR. Instead, when using a stochastic method, we obtain results in good agreement with those of our quantum simulations for the diamondoid crystal. However, the agreement is worse for NAI-DMAC where intra-molecular anharmonicities contribute to the ZPR. Our results highlight the importance of accurately including nuclear and anharmonic quantum effects to predict the electronic properties of molecular crystals.Comment: 54 pages, 20 figure

Interpolating from Bianchi Attractors to Lifshitz and AdS Spacetimes

We construct classes of smooth metrics which interpolate from Bianchi attractor geometries of Types II, III, VI and IX in the IR to Lifshitz or $AdS_2 \times S^3$ geometries in the UV. While we do not obtain these metrics as solutions of Einstein gravity coupled to a simple matter field theory, we show that the matter sector stress-energy required to support these geometries (via the Einstein equations) does satisfy the weak, and therefore also the null, energy condition. Since Lifshitz or $AdS_2 \times S^3$ geometries can in turn be connected to $AdS_5$ spacetime, our results show that there is no barrier, at least at the level of the energy conditions, for solutions to arise connecting these Bianchi attractor geometries to $AdS_5$ spacetime. The asymptotic $AdS_5$ spacetime has no non-normalizable metric deformation turned on, which suggests that furthermore, the Bianchi attractor geometries can be the IR geometries dual to field theories living in flat space, with the breaking of symmetries being either spontaneous or due to sources for other fields. Finally, we show that for a large class of flows which connect two Bianchi attractors, a C-function can be defined which is monotonically decreasing from the UV to the IR as long as the null energy condition is satisfied. However, except for special examples of Bianchi attractors (including AdS space), this function does not attain a finite and non-vanishing constant value at the end points.Comment: 37 pages, 12 figures, The comment regarding the behavior of C-function for general Bianchi Types appearing in IR or UV clarified, the relation of Type IX with $AdS_2 \times S^3$ for $\lambda=1$ made more precise and a comment regarding type V added in the conclusio

Path Integral Complexity for Perturbed CFTs

In this work, we formulate a path-integral optimization for two dimensional conformal field theories perturbed by relevant operators. We present several evidences how this optimization mechanism works, based on calculations in free field theories as well as general arguments of RG flows in field theories. Our optimization is performed by minimizing the path-integral complexity functional that depends on the metric and also on the relevant couplings. Then, we compute the optimal metric perturbatively and find that it agrees with the time slice of the hyperbolic metric perturbed by a scalar field in the AdS/CFT correspondence. Last but not the least, we estimate contributions to complexity from relevant perturbations

Nanoscale Multiphysics Phenomena for a New Generation of Energy Storage and Conversion Devices

The swelling demand for storing and using energy at diverse scales has stimulated the exploration of novel materials and design strategies applicable to energy storage systems. The most popular electrochemical energy storage systems are batteries, fuel cells and capacitors. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors have emerged to be particularly promising. Besides exhibiting high cycle life, they combine the best attributes of capacitors (high power density) and batteries (high energy storage density). Consequently, they are expected to be in high demand for applications requiring peak power such as hybrid electric vehicles and uninterruptible power supplies (UPS). This dissertation aims to make advancements on the following two topics in supercapacitor research with the aid of modeling and experimental tools: applying various thermophysical effects to design supercapacitor devices with novel functionalities and studying degradation mechanisms upon continuous cycling of conventional supercapacitors. The prime drawback of conventional supercapacitors is their low energy density. Most research in the last decade has focused on synthesizing novel electrode materials. Although such novel electrodes lead to high energy density, they often involve complicated synthesis process and result in high cost and low power density. A new concept of inducing pseudocapacitance developed in recent years is by introducing redox additives in the electrolyte that engage in redox reactions at the electrode/electrolyte interface during charge/discharge. The first section of this dissertation reports the performance of fabricated solid-state supercapacitors composed of redox-active gel electrolyte (PVA-K3Fe(CN)6-K4Fe(CN)6). The electrochemical performance has been studied extensively using cyclic voltammetry, constant current charge/discharge and impedance spectroscopy techniques, and then the results are compared with similar devices composed of conventional gel electrolytes such as PVA-H3PO4 and PVA-KOH on the basis of capacitance, internal resistance and stable voltage window. The second section explores the utility of the thermogalvanic property of the same redox-active gel electrolyte, PVA-K3Fe(CN)6-K4Fe(CN)6 in the construction of a thermoelectric supercapacitor. The integrated device is capable of being electrically charged by applying a temperature gradient across its two electrodes. In the absence of available temperature gradient, the device can be discharged electrically through an external circuit. Therefore, such a device can be used to harvest waste heat from intermittent heat sources. An equivalent circuit elucidating the mechanisms of energy conversion and storage applicable to thermally chargeable supercapacitors is developed. A fitting analysis aids in the evaluation of model circuit parameters providing good agreement with experimental voltage and current measurements. The latter part of the dissertation investigates the factors influencing aging in conventional supercapacitors. In the first part, a new imaging technique based on the electroreflectance property of gold has been developed and applied to characterize the aging characteristics of a microsupercapacitor device. Previous aging studies were performed through traditional electrical characterization techniques such as cyclic voltammetry, constant charge/discharge, and electrochemical impedance spectroscopy. These methods, although simple, measure an average of the structures\u27 internal performance, providing little or no information about microscopic details inside the device. The electroreflectance imaging method, developed in this work is demonstrated as a high-resolution imaging technique to investigate charge distribution, and thus to infer aging characteristics upon continuous cycling at high scan rates. The technique can be used for non-intrusive spatial analysis of other electrochemical systems in the future. In addition, we investigate heat generation mechanisms that are responsible for accelerated aging in supercapacitors. A modeling framework has been developed for heat generation rates and resulting temperature evolution in porous electrode supercapacitors upon continuous cycling. Past thermal models either neglected spatial variations of heat generation within the cell or considered electrodes as flat plates that led to inaccuracies. Here, expressions for spatiotemporal variation of heat generation rate are rigorously derived on the basis of porous electrode theory. Detailed numerical simulations of temperature evolution are performed for a real-world device, and the results resemble past measurements both qualitatively and quantitatively. In the last chapter of the thesis, a rare thermoelectric effect called the Nernst effect has been investigated in single-layer periodic graphene with the aid of a modified Boltzmann transport equation. Detailed formulations of the transport coefficients from the BTE solution are developed in order to relate the Nernst coefficient to the amount of impurity density, temperature, band gap and applied magnetic field. Detailed knowledge of the variation of the thermoelectric and thermomagnetic properties of graphene shown in this work will prove helpful for improving the performance of magnetothermoelectric coolers and sensors

Combined first-principles calculations of electron-electron and electron-phonon self-energies in condensed systems

We present a method to efficiently combine the computation of electron-electron and electron-phonon self-energies, which enables the evaluation of electron-phonon coupling at the $G_0W_0$ level of theory for systems with hundreds of atoms. In addition, our approach, which is a generalization of a method recently proposed for molecules [J. Chem. Theory Comput. 2018, 14, 6269-6275], enables the inclusion of non-adiabatic and temperature effects at no additional computational cost. We present results for diamond and defects in diamond and discuss the importance of numerically accurate $G_0W_0$ band structures to obtain robust predictions of zero point renormalization (ZPR) of band gaps, and of the inclusion of non-adiabatic effect to accurately compute the ZPR of defect states in the band gap