2018 Theses Doctoral

Novel Liquid-Like Nanoscale Hybrid Materials with Tunable Chemical and Physical Properties as Dual-Purpose Reactive Media for Combined Carbon Capture and Conversion

Gao, Ming

In order to address the global challenges of climate change caused by the increasing concentration of carbon dioxide (CO2), Carbon Capture, Utilization and Storage (CCUS) has been proposed as a promising strategy in carbon management. In parallel with the target of zero emission in fossil-fired power plants, negative emission has also drawn a great deal of attention in other chemical sectors, including cement making and steel production industries. Thanks to the recent reduction in the cost of renewable energy sources, such as wind and solar, a paradigm shifting concept has emerged to directly convert the captured carbon into chemicals and fuels. In this way, decarbonization in various chemical sectors can be achieved with a reduced carbon footprint.
A variety of carbon dioxide conversion pathways have been investigated, including thermochemical, biological, photochemical, electrochemical and inorganic carbonation methods. Electrochemical conversion of carbon dioxide has been thoroughly investigated with great progress in electrocatalysts and reaction mechanisms. However, fewer studies have been taken to tackle the constraint of the low solubility of CO2 in conventional aqueous electrolytes.
In an effort to improve the solubility of CO2, various novel electrolytes have been designed with a higher uptake of CO2 and a compatibility with electrochemical conversion, including Nanoparticle Organic Hybrid Materials (NOHMs)-based fluids. NOHMs are a unique liquid-like nanoscale hybrid material, comprising of polymers grafted onto nanoparticles (e.g., silica). NOHMs have demonstrated an excellent thermal stability and a high chemical tunability. Two types of NOHMs with ionic bonding (I) between the polymers and nanoparticles were selected in this study: NOHM-I-PEI incorporating polyethylenimine polymer (PEI) and NOHM-I-HPE consisting of polyetheramine polymer (HPE), illustrative of two modes of carbon capture (e.g., chemisorption and physisorption). The NOHMs-based fluids were synthesized with different secondary fluids and salt to tune the viscosity and conductivity.

As the first liquid hybrid solvent system for combined carbon capture and conversion, the physical, chemical and electrochemical properties of NOHMs-based fluids were systematically investigated. It was found that NOHMs-based aqueous fluids have exhibited a lower specific heat capacity than that of the 30 wt.% monoethanolamine (MEA) solvents. In addition, upon CO2 loading, the increase in specific heat capacity and the reduction of the viscosity of the NOHM-I-PEI based aqueous fluids can be attributed to the formation of intra-molecular hydrogen bonds.
The different chemistries of the two NOHMs can be reflected by the viscosity-based mixing behavior. The smaller critical concentration and the higher intrinsic viscosity of NOHM-I-HPE based aqueous fluids implied a more significant contribution of viscosity to the system by the addition of NOHM-I-HPE. The viscosity of NOHM-I-HPE (30 wt.%) in water was measured to be 395 cP, an order of magnitude higher than that of NOHM-I-PEI (30 wt.%) in water, which was determined to be 22.6 cP. It was also discovered that the addition of N-methyl-2-pyrrolidone (NMP) has resulted in a dramatic increase of the viscosity of NOHM-I-PEI based aqueous fluids, hypothesized to be due to a possible formation of a complex between NMP and NOHM-I-PEI. On the other hand, the presence of 0.1 M potassium bicarbonate (KHCO3) salt greatly reduced the viscosity of NOHM-I-HPE based aqueous fluids.
The electrochemical properties of NOHMs-based fluids were also characterized and an excellent electrochemical stability has been demonstrated. The conductivities of NOHMs-based fluids witnessed an unexpected enhancement from the corresponding untethered polymer-based solutions. At 50 wt.% loading, the conductivity was 15 mS/cm for NOHM-I-PEI based aqueous fluids doped by 1 M bis(trifluoromethylsulfonyl)amine lithium salt (LiTFSI), while it was 0.91 mS/cm for PEI based aqueous solutions. Even after the viscosities of the two solutions were converted to the same value, there was still a large gap between the conductivities of the NOHMs-based fluids and polymer-based fluids. The relative tortuosity of ion transport in NOHMs-based fluids compared to untethered polymer-based solutions was less than 1. This result was indicative of a shorter pathway of ion transport in NOHMs-based fluids than in polymer-based fluids. Thus, it is suggested that in addition to a viscosity effect, unique multi-scale structures were also formed, enabling an enhanced ion transport in the NOHMs-based fluids.
With this hypothesis, ultra-small-angle X-ray scattering (USAXS) technique was utilized to construct the structures of NOHMs morphology in secondary fluids, from agglomerates at large scale to aggregates at mid-scale, and to the interparticle distance at small scale. The sizes of the aggregates and the interparticle distance were highly tunable by varying the concentrations of NOHMs, and the types of NOHMs and secondary fluids. For example, the aggregate size was (32.30 ± 0.3) nm and (153.9 ± 1.5) nm for 50 wt.% loading of NOHM-I-PEI and NOHM-I-HPE in mPEG, respectively. This hierarchical structure was hypothesized to give ions unique channels and pathways to migrate, resulting in the surprising conductivity enhancement. Cryogenic electron microscopy (CryoEM) was also employed to image such multi-scale fractal structures.

The diffusion behavior under this hierarchical structure was studied subsequently. To our surprise, in certain NOHMs-based fluids, such as 10 wt.% NOHM-I-HPE in water at 25℃, the diffusion coefficient of water was 3.43×(10)^(-9) m2/s, higher than that of deionized water, 2.99×(10)^(-9) m2/s. This is evident of the channels created by NOHMs in the secondary fluids to allow faster local diffusion of water and ions. Meanwhile, the diffusion coefficient of NOHM-I-HPE was higher with the presence of 0.1 M KHCO3 salt compared to the salt-free case in water. Though counter-intuitive, this was because salt would interact with the ionic bonding sites of NOHMs, facilitating the dynamic hopping of polymers on the nanoparticle surface, and thus improving the fluidity of the NOHM-I-HPE based aqueous fluids.
This investigation of multi-scale structures and diffusion behavior of NOHMs-based fluids was insightful in understanding how the ions move in the system, and in explaining the enhanced conductivity of NOHMs-based fluids compared to the corresponding untethered polymer-based solutions. It is believed that ions move in two regions of the NOHMs-based fluids, the NOHMs-rich region and secondary fluids-rich region, in the mechanisms of translational movement, and coupled and decoupled ion migration with structural relaxation of NOHMs and secondary fluids.
With the understanding of the fundamental properties and the construction of hierarchical structures, the carbon capture performance was evaluated for NOHMs-based fluids. The carbon capture behavior can be tuned by the concentration of NOHMs, and the presence of salt and physical solvents. The carbon capture kinetics was determined by both the amount of the capture material and the viscosity of the fluids. It was determined that 30 wt.% NOHM-I-PEI based aqueous fluids exhibited an optimal balance between capture capacity and sorption kinetics. As the concentration of NOHMs further increased, the elevated viscosity of the system limited the mass transfer of carbon capture. It was also found that salt induced a minimal impact on carbon capture in the initial 100 min for 5 wt.% NOHMs loading, but would negatively impact the capture capacity and kinetics at higher NOHMs loadings. Meanwhile, the addition of physical solvent (NMP) reduced carbon capture capacity and kinetics.
Various existing forms of CO2 have been identified in NOHMs-based fluids, including carbamate, bicarbonate, and physisorbed CO2. Carbamate came from the reaction between CO2 and the amine functional groups on NOHM-I-PEI. Physisorbed CO2 was identified as the electroactive species for electrochemical conversion of CO2. In the combined carbon capture and conversion experiments using 5 wt.% NOHM-I-HPE based aqueous electrolyte, carbon monoxide (CO) production was enhanced on polycrystalline silver by 60%, and selectivity was changed on a pyridinic-N doped carbon-based electrode, in comparison with conventional 0.1 M KHCO3 electrolyte.
The roles of NOHMs in carbon capture and conversion were also explored. The addition of NOHMs was able to improve the solubility of CO2 with a tunable pH change. It is hypothesized that NOHMs can complex with the electrochemical reaction species,CO2 (CO2^-), and this complex formation can be tunable by the concentration and types of NOHMs.
In the end, an alternative approach of utilizing NOHMs-based fluids has also been proposed through encapsulation. The encapsulation of NOHMs-based fluids has enabled a higher specific surface area for CO2 uptake, and an enhancement in capture kinetics has been observed compared to the non-encapsulated NOHMs-based fluids.
In summary, a novel nanoscale hybrid solvent system has been developed for combined carbon capture and conversion. The insight into the chemistry of this hybrid solvent system is not beneficial to the advancement in carbon capture and conversion, but it is also enlightening for the interdisciplinary development of various areas involving nanoscale hybrid materials.