Direct usage of thermal energy, instead of electricity, is intriguing to diminish total energy consumption with a benefit of reducing the number of energy conversions by directly utilizing such solar power and waste heat. Although various thermally driven systems have been suggested to overcome the problem of electricity-dependent systems, materials for the mechanisms limit the realization and effective operation of the systems. A more complete theoretical understanding of materials for thermal applications can suggest guidance to determine the bottlenecks of current materials and then ways to develop new materials. Among many computational methods, molecular dynamics (MD) simulations especially provide a molecular insight into structures of materials and phononic heat transfer at nanoscale.

In this dissertation, I discuss unconventional phase transition materials for thermal applications. Here, I focus on two types of phase transition materials. The lower critical solution temperature (LCST) liquid-liquid transition of aqueous ionic liquids is a thermally responsive phenomenon with phase separation. The phase separation can be used for forward osmosis desalination which directly utilizes a low-grade of thermal energy. By using MD simulations, I describe the local molecular environment during the transition and the driving force. The unusual trend of osmotic strength of the ionic liquids is explained with a suggested definition of an apparent free ion ratio. MD simulations are also used to study the amorphous phases and their phase transition of sugar alcohols in nanopores. The nano-encapsulation is proposed to shift the melting temperature and here I suggest a methodology to determine the phase transition temperature of encapsulated materials. This thesis also uses a theoretical approach to explore the thermal conductivity of the nanoconfined composites. Phonon frequency analysis verifies the enhancement of heat transfer by the infusion of phase change materials to the frameworks due to additional heat pathways between the nanoporous structures. Finally, I propose a practical design for a thermal rectifier that can be realized by phase change materials and derive a general theory for the design. The general theory indicates the possibility to obtain a high thermal rectification from the design and introduces the critical parameters for theoretical maximal performance.

The dramatic rise in atmospheric carbon concentrations and consequences thereof has spurred a renewed effort to develop strategies to mitigate consumption of fossil fuels and emission of carbon dioxide. Membranes occupy a small niche within the larger strategy of carbon capture and storage as an effective medium to curb carbon dioxide emissions. However, to become competitive with incumbent technologies such as amine scrubbing, the performance of membranes needs to be greatly improved. Separation membranes are engineered to selectively permeate one molecular species over others through a combination of chemical and physical interactions between the molecule and gas. The performance limitations of current commercial polymeric membranes are due to an intrinsic trade-off between its permeability and selectivity. A novel and emerging approach to bypass traditional polymeric transport limitations is through the addition of inorganic nanomaterials. Inorganic materials often have inherently higher selective chemical and size sieving properties than polymers, but their material brittleness limits their large-scale adoption. However, we can harvest the selective properties of inorganics in hybrid polymer/inorganic composites to design membranes whose performance exceeds that of conventional polymers.

While promising, the primary obstacle towards developing high-performing hybrid membranes is to understand the role of polymer/inorganic interactions on molecular transport properties. These interactions can be highly complex and more often than not, the combination of a polymer and inorganic creates a non-selective membrane due to poor adhesion between phases. As a result, only a few systematic studies aimed to understand the complex nature of the interface have been reported in hybrid systems. Here, I describe the development of a model system using silica nanoparticles to examine contributions from the interface on transport properties. Utilizing this model system, I develop design rules for hybrid membranes and provide a scientific foundation on the role of inorganic size and surface chemistry on molecular transport properties. Polymer dynamics and structure, features that largely govern molecular transport properties, sensitively depend on three key variables (size, surface functionalization, and total volume loading) that are all linked by the total available interfacial surface area of the nanomaterials. From insights gained in these model studies, I explore the design of a completely new class of dual transport membranes using metal-organic frameworks (MOFs) as the inorganic material. By pushing the boundaries of previously achievable MOF loadings, I demonstrate the ability to create a secondary transport pathway through a percolating MOF network, which bypasses transport inefficiencies of the polymer. The new transport channel through the MOF greatly enhances the total CO2 permeability without sacrificing selectivity. Finally, I demonstrate the ability to design a flexible MOF membrane on a polymer support, leading to a new paradigm in inorganic membranes.

The findings presented in this dissertation offer a stronger understanding of fundamental transport properties in hybrid membranes and strategies to design membranes to overcome traditional limitations of polymer membranes. The utilization of dual transport membranes can lead to marked improvement in separation performance, which extends beyond carbon capture applications.

Thermoelectric devices have the unique ability to interconvert heat and electricity directly. Soft thermoelectric materials, including conjugated polymers and organic-inorganic hybrids, now demonstrate figures of merit approaching those of inorganic materials. These breakthroughs in materials development enable the design of thermoelectric devices that exhibit appropriate efficiencies for commercial use, while simultaneously leveraging the unique processing and mechanical advantages of soft materials. Such technology opens the door to a suite of new thermoelectric applications, including power generation for biomedical implants and the Internet of Things, or wearable heating and cooling devices. However, in order to realize deployment of such technologies, there is a fundamental need for deeper understanding of the complex transport physics underlying thermoelectric transport in soft materials.

The central focus of this dissertation is investigating the fundamental physical phenomena critical to carrier transport in hybrid organic-inorganic thermoelectric material. Due to the complex nature of this class of multiphase material, there remains a problematic lack of consensus in the field regarding transport in hybrid materials. The mechanisms of carrier transport, key physics responsible for high thermoelectric performance, and even how to model transport in these materials are all subjects of debate within the field. Here, I describe the design, synthesis, and characterization of a prototypical PEDOT:PSS-Te hybrid nanomaterial with the goal of performing careful study of the carrier physics and relevant molecular-scale phenomena in this material. A novel technique for patterning alloy nanophases is demonstrated, resulting in well-controlled PEDOT:PSS-Te-Cu1.75Te heterowires. The Te-Cu1.75Te energetics are well aligned to leverage the carrier filtering effects proposed in literature. Using a full suite of experimental, theoretical, and modeling tools, we reveal the key physics responsible for dictating carrier transport and thermoelectric properties in this material, testing each of the major hypothesis in the field. Contrary to popular belief in the field, it is revealed that energy filtering does not play a major role in the carrier transport and high thermoelectric performance of these materials; rather, organic structural effects at the hard-soft interface and interfacial charge transport emerge as the key phenomena underlying transport.

In a complementary study, I describe a platform approach for the synthesis of new solution-based, air stable n-type soft thermoelectrics. Using this approach, a composite perylene diimide-Te nanowire thermoelectric ink is prepared, demonstrating up to 20-fold enhancement over the individual components. The performance of these materials is competitive with the best-in-class for fully solution-processed, air stable n-type thermoelectric inks. We find experimental evidence linking reorganization of the perylene diimide molecules on the Te surface to enhanced electrical conductivity in the composite, further emphasizing the importance of structural effects in the organic phase to the overall thermoelectric properties of hybrid materials. Finally, leveraging the best materials from among the work in this dissertation, we demonstrate power generation in an all-ink flexible thermoelectric module with an innovative folded geometry.

The findings in this dissertation provide critical insight into the physics underlying carrier transport and high thermoelectric performance in hybrid organic-inorganic nanomaterials. This work highlights the importance of developing hybrid design strategies capable of leveraging molecular-level effects at the hard-soft interface. In furthering the field’s fundamental understanding of this material class, we drive progress towards the realization of flexible thermoelectric modules compatible with applications such as implantable medical devices, wearable technologies, and the Internet of Things.