Title: Thermal, Structural and Transport Behaviors of Nanoparticle Organic Hybrid Materials Enabling the Integrated Capture and Electrochemical Conversion of Carbon Dioxide
Description: Owing to the increased anthropogenic CO₂ emissions over the last several decades, there have been tremendous global efforts in the deployment of renewable energy technologies. However, due to intermittency issues of renewable energy generation and a current lack of reliable long-term energy storage solutions, the development of innovative electrolytes for sustainable energy storage and chemical reactions is an emerging research area. In particular, materials that can host multiple reactions and separations, such as the integrated capture and conversion of CO₂, are highly desired. The direct coupling of renewable energy generation with electrochemical CO₂ conversion to chemicals and fuels is one of the transformative pathways that can aid the global transition to carbon-neutrality, depending on the source of CO₂. However, the current solubility of CO₂ in aqueous electrolytes is quite low (34 mM), thus limiting overall reaction performance. Liquid-like Nanoscale Organic Hybrid Materials (NOHMs) consist of a polymer tethered to a nanoparticle surface and possess a number of favorable properties which are highly desirable in electrochemical applications, including negligible vapor pressure, chemical tunability, oxidative thermal stability and high conductivity.

To date, NOHMs have been successfully demonstrated for use as water-lean CO₂ capture solvents, as the polymer canopy can be tuned to capture CO₂ under various sets of operating conditions. Thus, in this dissertation, we have explored the thermal, transport and structural properties of NOHMs in their application as electrolytes enabling the integrated capture and conversion of CO₂. Liquid-like NOHMs functionalized with an ionic bond have been shown to display greatly enhanced oxidative thermal stability compared to the untethered polymer. However, our previous studies were limited in terms of reaction conditions and the detailed mechanisms of the oxidative thermal degradation were not reported. In this study, a kinetic thermal degradation analysis was performed on NOHM-I-HPE and the neat polymer, Jeffamine M2070 (HPE), in both non-oxidative and oxidative conditions. NOHM-I-HPE displayed similar thermal stability to the untethered polymer in a nitrogen environment, but interestingly, the thermal stability of the ionically tethered polymer was significantly enhanced in the presence of air.

This observed enhancement of oxidative thermal stability is attributed to the orders of magnitude larger viscosity of the liquid-like NOHMs compared to untethered polymer and the bond stabilization of the ionically tethered polymer in the NOHMs canopy. This study illustrated that NOHMs can serve as functional materials for sustainable energy storage applications because of their excellent oxidative thermal stability, when compared to the untethered polymer. Though NOHMs composed of an ionic bond have demonstrated a high conductivity and an enhanced oxidative thermal stability, their practical application in the neat state is limited by an inherently high viscosity. Thus, when incorporating NOHMs in electrolytes for CO₂ capture and conversion applications, it will be necessary to mix them with a secondary fluid. In this study, a series of binary mixtures of NOHM-I-HPE with five different secondary fluids – water, chloroform, toluene, acetonitrile, and ethyl acetate – were prepared to reduce the fluid viscosity and investigate the effects of secondary fluid properties (i.e., hydrogen bonding ability, polarity, and molar volume) on their transport behaviors including viscosity and diffusivity.

Our results revealed that the molecular ratio of secondary fluid to the ether groups of Jeffamine M2070 (λSF) was able to describe the effect that secondary fluid has on transport properties. Our findings also suggest that in solution, the Jeffamine M2070 molecules exist in different nano-scale environments, where some are more strongly associated with the nanoparticle surface than others, and the conformation of the polymer canopy was dependent on the secondary fluid. This understanding of the polymer conformation in NOHMs can allow for the better design of an electrolyte capable of capturing and releasing small gaseous or ionic species. To further investigate the effect of the bond type on the thermal stability as well as the structural and transport properties of the tethered HPE, NOHMs were synthesized by tethering HPE to SiO₂ nanocores via ionic (NOHM-I-HPE) and covalent (NOHM-C-HPE) bonding at two grafting densities. In the neat state, NOHM-C-HPE displayed the highest thermal stability in a nitrogen atmosphere, while NOHM-I-HPE was the most thermally stable under oxidative conditions.

Small-angle neutron scattering (SANS) revealed the presence of multiple types of Jeffamine M2070 (HPE) polymers in aqueous solutions of NOHM-I-HPE (i.e., tethered, interacting and free), whereas only tethered HPE chains were observed in NOHM-C-HPE systems. Moreover, the SANS profiles identified clustering of NOHM-C-HPE in dilute aqueous solutions, but not in the corresponding NOHM-I-HPE samples, suggesting that the different types of HPE chains in solutions of NOHM-I-HPE may be crucial to the uniform NOHMs dispersion. Additionally, our investigation of the viscosity and conductivity of different NOHM-based electrolytes revealed that in response to ionic stimulus, the covalently tethered HPE remained fixed at the nanoparticle surface, whereas there was a partial disassociation of HPE chains from the nanoparticle in NOHM-I-HPE. Overall, the results of this study highlight that NOHMs are highly tunable materials whose properties can be strategically altered by changing the bond type linking the polymer to the nanoparticle, as well as grafting density.

Finally, two types of aqueous NOHM-based electrolytes were prepared to study the effect of CO₂ Though NOHMs composed of an ionic binding energy (i.e., chemisorption vs. physisorption) on the CO₂ reduction reaction (CO₂RR) over a silver nanoparticle catalyst for the production of syngas, a mixture of H₂ and CO, at various ratios. Poly(ethylenimine) (PEI) and Jeffamine M2070 (HPE) were ionically tethered to SiO₂ nanoparticles to form the amine-containing NOHM-I-PEI and ether containing NOHM-I-HPE, respectively. At less negative applied potentials, PEI and NOHM-I-PEI based electrolytes produced CO at higher rates than 0.1 molal. KHCO₃ due to their enhanced conductivity, while at more negative applied potentials, H₂ production was significantly favored because of the electrochemical inactivity of carbamates and catalyst-electrolyte interactions affecting the selectivity of CO₂RR. Conversely, due to their lower ionic conductivity, HPE and NOHM-I-HPE electrolytes displayed poor CO₂RR performance at less negative applied potentials.

At more negative applied potentials, their performance approached that of 0.1 molal. KHCO₃, highlighting how the polymer functional groups of NOHMs are critical to the tunable production of syngas. The results of this study illustrate that more conductive polymer canopies with intermediate binding energies for CO₂ should be explored to improve the performance of NOHM-mediated CO₂ reduction. Altogether, the results of this dissertation demonstrate the ability of NOHM-based electrolytes to be used for systems enabling the integrated capture and electrochemical conversion of CO₂. The polymer grafting density, polymer canopy functionalities, bond type linking the polymer to the nanoparticle, secondary fluid selection and ionic stimulus were all found to play an important role in determining the thermal stability of NOHMs and/or the structural and transport properties of the corresponding NOHM-based fluids/electrolytes, thus highlighting the tunable nature of this class of materials. Additionally, the findings from this dissertation can be applicable to a wide range of energy and environmental applications that require the design and development of novel electrolytes.