PhD Oral Exam - Bahareh Raisi, Chemical and Materials Engineering

When studying for a doctoral degree (PhD), candidates submit a thesis that provides a critical review of the current state of knowledge of the thesis subject as well as the student’s own contributions to the subject. The distinguishing criterion of doctoral graduate research is a significant and original contribution to knowledge.

Once accepted, the candidate presents the thesis orally. This oral exam is open to the public.

Abstract

Increasing demand for lighter, smaller, more flexible, and multi-task devices has motivated the scientific community to fabricate novel two-dimensional (2D) materials for advanced and high-tech energy applications. Nowadays, 2D MXene materials have attracted significant attention due to their unique structural, physical, and chemical properties. As a rising star within the 2D material family, MXenes exhibit a unique combination of multi-functional groups and possess a high surface area. Numerous investigations in recent years have shed light on the creation of MXene-polymer composites tailored for applications in the realm of energy storage. These explorations underscore the significant role and potential of MXene-based materials in the domain of electrochemical energy storage devices. The particular significance are rechargeable batteries and supercapacitors, as they bear substantial promise for making substantial strides in the pursuit of sustainable energy solutions. Furthermore, the intriguing 2D structure of MXenes positions them as excellent nanofillers for polymer matrices. Incorporating MXenes as fillers within a polymer enhances mechanical and chemical properties and significantly improves thermal and electrical conductivity. However, the production of fully exfoliated /delaminated 2D MXene nano-sheets is still a major challenge, limiting their potential for practical applications. Thus far, MXene flakes display colloidal stability only in a few polar solvents. Conversely, they are unstable in nonpolar or low polarity solvents, while critically important to significantly expand the spectrum of applications for MXenes. For example, it will facilitate the preparation of tailor-made MXene nanocomposites with a variety of nonpolar hydrophobic polymer matrices. Furthermore, MXenes tend to get oxidized in the presence of oxygen and water during storage, handling, processing, delamination, and applications under ambient conditions. So far, very little research has been done on the oxidation prevention, dispersion, and stabilization of MXene in nonpolar/low-polarity organic solvents.

In the present study, a self-assembly method to prepare ionomer-modified MXenes has been developed for the first time. Hyperbranched polyethylene ionomers containing quaternary ammonium ions are designed to prepare stable and highly concentrated modified Ti3C2Tx MXene dispersions in various nonpolar and low-polarity organic solvents. Constructed with a nonpolar hydrophobic hyperbranched polyethylene backbone, the ionomers contain covalently tethered quaternary ammonium cations, which can bind tightly onto negatively charged Ti3C2Tx MXene surface by ionic interactions. Simple mixing renders the ionomer-modified MXenes, with the hyperbranched polyethylene ionomers efficiently intercalated within the MXene sheets. As a result, the interlayer spacing of Ti3C2Tx MXene is expanded to more than 5 nm ( 9.33 nm based on TEM results) with at least a 400% increase compared to the original spacing of 1 nm and can be delaminated to a few-layer or single-layer nano-flakes. The surface modification effectively changes the surface property of MXene sheets from hydrophilic to hydrophobic and facilitates their compatibility with various nonpolar or low-polarity organic media, rendering dispersions that are stable for 100 days. Meanwhile, high dispersion concentrations (as high as 30 mg mL-1 in THF) can be obtained by simple sonication/shaking or even without the need for a sonication. The modification also markedly improves the oxidation stability of MXene sheets due to the presence of the tightly surface-bound hydrophobic hyperbranched polyethylene protecting layer. This strategy thus uniquely facilitates the processing of MXenes in the form of highly stable high-concentration dispersions in nonpolar/low-polarity organic solvents, which greatly expands their opportunities for new applications in various areas.

Batteries and supercapacitor electrodes represent two of the most promising applications for MXenes. Rechargeable magnesium-ion batteries (MIBs) with Mg metal anodes are noted for their high theoretical energy density, potential safety, earth abundance, dendrite-free, and low cost. Nevertheless, owing to the large polarity of divalent Mg2+ ions, the insertion of Mg2+ into MXene layers suffers from sluggish kinetics, which limits the performance for storage of Mg2+ ions in Mg batteries. Herein, an ionomer self-assembly strategy has been demonstrated to achieve high magnesium ion storage capability with pillar-structured Ti3C2Tx MXene. With the expansion of interlayer spacing of Ti3C2Tx MXene upon the ionomer intercalation and the affinity of the hyperbranched polyethylene-skeleton of the ionomers to THF-based electrolyte, the delaminated ionomer-modified MXene shows significantly improved electrochemical performance as a cathode material for Mg batteries. A reversible intercalation charge-storage mechanism is demonstrated. It demonstrates a promising cycling stability with a capacity retention of 71% after 1000 cycles, as well as the outstanding high-rate performance with 110 mAh g−1 at 1 A g−1 compared to the initial capacity of 213 mAh g−1 at 0.02 A g−1. Subsequently, a symmetrical two-electrode supercapacitor made from an I-MXene suspension in THF exhibits a capacitance of 220 F g−1 at a scan rate of 2 mV s−1.

This thesis offers insights into the engineering of MXene electrode materials for applications in electrochemical energy storage.