2022 Theses Doctoral

Development of Nanoscale Hybrid Material-Based Electrolytes for Energy Storage and Tandem CO₂ Capture and Conversion

Hamilton, Sara

The recent growth in worldwide installation of renewable energy and the need for CO₂ capture and utilization to meet climate targets requires large-scale electrochemical energy storage systems, including redox flow batteries and electrolyzers enabling CO₂ reduction (CO₂R) to dense energy carriers. Electrolyte design and selection plays a critical role in the development of these electrochemical devices, dictating transport and solubility of the reactive species to ultimately deliver high energy, current and power density systems.

Nanoparticle organic hybrid materials (NOHMs) are a new class of materials with promising properties for electrochemical energy applications, including high tunability, thermal oxidative stability and negligible vapor pressure. NOHMs have primarily been studied in the neat state, but their incorporation as electrolyte components requires an understanding of their behavior in electrolyte solution and at the electrode/electrolyte interface. Here, it is shown that NOHMs-based electrolytes can complex electroactive species and enable tandem CO₂ capture and conversion, offering improvements in electrolyte design of interest in electrochemical systems such as redox flow batteries and CO₂ electrolyzers. These electrolytes are found to be electrochemically stable, have tunable bulk physicochemical properties, and have interfacial effects that impact reaction and transport of electroactive species.

Electrochemical cycling in oxidative and reductive environments and spectroscopic analyses were performed to assess electrochemical stability of NOHMs-based electrolyte materials. NOHMs-based electrolytes were found to have wider electrochemical voltage windows than water and to be robust to hours-long voltage holds, highlighting their ability to be integrated in electrochemical devices with continuous operation for extended time periods, including CO₂ electrolyzers and flow batteries.

The physicochemical properties of NOHMs-based electrolytes, including conductivity and viscosity, were studied in solution. Controlling transport properties is particularly important in NOHMs-based electrolytes mixtures, as they are challenged by inherently high viscosities, impacting charge transport critical in electrochemical performance. NOHMs were found to be highly responsive to ionic stimulus, with the addition of even low salt concentrations inducing large reductions in the viscosity of NOHMs-based electrolyte mixtures. Alterations in the intermolecular hydrogen bonding interactions, degree of polymer swelling, and the conformational structure of the NOHMs’ polymer canopy with ionicity were discerned via light scattering and NMR techniques. These insights provide a mechanistic understanding of the effect of salt ions on measured bulk physicochemical properties and could be ultimately employed to tailor transport properties for a range of electrochemical applications.

The ability of NOHMs to reversibly bind electroactive species was also investigated. NOHM-I-HPE (NOHMs based on ether functional groups) were found to complex zinc and their addition in the electrolyte alters diffusion and reaction pathways. NOHMs were found to selectively adsorb at the electrode interface, impacting achievable current densities and the morphology of metal deposits (reducing dendrite formation) during the electrochemical deposition reaction of zinc. The interfacial adsorption of NOHMs was characterized with electrochemical and spectroscopic measurements and suggests differences in structuring of tethered and untethered polymer near the electrode surface, which may play a role in electrochemical conversion mechanisms. These findings provide insights into how structured electrolyte additives such as NOHMs can allow for advancements in electrolyte design for controlled deposition of metal species from energy-dense electrolytes or for other electrochemical reactions.

Tandem CO₂ capture and conversion in NOHM-I-PEI-based electrolytes (NOHMs based on amine functional groups) was achieved using an Ag electrocatalyst. The addition of NOHMs and polymers was found to impact product distribution of CO₂ R and the polymer/electrode interface was probed ex-situ and in operando. PEI polymers chains were found to adsorb on the electrocatalyst surface and EC-AFM measurements revealed changes in conformation at the interface under applied polarization during CO₂R, with PEI chains aligning with the electrode due to electrostatic effects. The addition of supporting electrolyte salt ions and carbamate bond formation upon CO₂ saturation of the solution impacted the reconfiguration of polymeric chains on the electrode surface, as revealed by trends in surface modulus. This study highlights the potential of new functionalized electrolytes for integrated CO₂ capture and conversion and reveals that the choice of polymer material and electrolyte composition impacts the near-electrode environment, which has implications for product distribution and reaction rates in CO₂R.