Author: Li, Hao
Title: Interlayer engineering and surface modification of Ti3C2Tx MXene for high-performance supercapacitors
Advisors: Huang, Haitao (AP)
Degree: Ph.D.
Year: 2023
Subject: Supercapacitors
Supercapacitors -- Materials
Hong Kong Polytechnic University -- Dissertations
Department: Department of Applied Physics
Pages: xvii, 198 pages : color illustrations
Language: English
Abstract: Pseudocapacitors are a kind of promising energy storage devices due to simultaneous possession of high-power densities, high energy densities and robust cyclic stability. Their energy storage behaviors and ability are critically determined by electrode materials that should be redox-active, highly conductive and strong for cycling test. Ti3C2Tx MXene has aroused an upsurge of worldwide research on its various applications including energy storage since its discovery in 2011. This novel two-dimensional material exhibits pseudo-capacitance in aqueous acid electrolytes and has metallic conductivity, which make it a good candidate for supercapacitor electrode material. However, its wide application is restricted by the sluggish ion diffusion within the interlayer channel of assembled MXene film, unsatisfactory capacitance, and low oxidation resistance in air. This research is focused on solving the above mentioned issues.
Firstly, the restacking of MXene sheets is inevitable when they are assembled into a freestanding film electrode, which hinders electrolyte ion diffusion to active sites and results in sluggish energy storage kinetics. Herein, the volumetrically expanded ester reaction between ethanol and phosphoric acid is exploited to improve interlayer path within Ti3C2Tx film. These two kinds of molecules jointly intercalate into interlayer space of Ti3C2Tx film and then react to produce phosphate under heating, leading to molecular scale expanded but uniform interlayer gallery. The optimized film shows enhancement in both gravimetric and volumetric capacitances along with better rate capability. It exhibits a capacitance of 297 F g-1/965 F cm-3 at 2 mV s-1 and retains 108 F g-1/300 F cm-3 at 200 mV s-1, which are greatly superior to those of the pristine film without the treatments. The assembled symmetric and asymmetric supercapacitors with optimized film structure can deliver an energy density of 6.33 and 7 Wh Kg-1, respectively. This work demonstrates a novel yet simple method to ameliorate restacking of MXene sheets for its better supercapacitor application.
Secondly, Ti3C2Tx MXene is regarded as a promising supercapacitor electrode material due to its high conductivity and pseudocapacitive nature. Its capacitance can be improved via nitrogen or sulfur heteroatom doping due to the ability to donate surface electrons. Phosphorus possesses a lower electronegativity than nitrogen and sulfur so that it is believed to enable greater performance enhancement once being doped. Herein, phosphorus is incorporated onto Ti3C2Tx to boost its electrochemical performance and the underlying enhancement mechanism is revealed. The results show that doped phosphorus exists with the formation of Ti-O-P terminations on MXene surface, and 2.1 at. % P-doped Ti3C2Tx delivers a capacitance enhancement of 30% (328 F g-1 at 2 mV s-1) in comparison with pristine MXene and outstanding cyclic stability, comparable to N- and S-doped MXene. Multiple in situ and ex situ characterization methods along with DFT calculations collectively reveal that the formed Ti-O-P species are new active sites for a two-proton bonding-debonding process, contributing to the enhanced charge storage and capacitive performance in MXene. However, higher surface phosphorus doping would destroy crystal integrity of MXene and leads to performance deterioration.
Finally, Ti3C2Tx MXene delivers high pseudo-capacitance in acid solution via surface -O terminations bonding with protons. Numerous efforts have been devoted to engineer more -O for improved charge storage or expand interlayer gallery of MXene to boost ion intercalation. However, narrow interlayer space favors proton transport with Grotthuss mechanism and strengthens the interaction between intercalated protons and material interface, which is commonly neglected in optimizing MXene supercapacitive performance. Herein, a novel strategy combining surface modification and interlayer shrinkage of MXene is proposed to boost its energy storage ability. The modified MXene is rich in -O surface groups along with substantial removal of detrimental -F species, and the interlayer space becomes narrower than its pristine counterpart. It exhibits not only a high capacitance of 375 F g-1 (338 C g-1), but also much enhanced oxidation resistance and excellent cyclic stability. Multiple in situ and ex situ characterization results reveal that only monolayer interstitial water is formed during its charge/charge process, which boosts proton transport into interlayer space and strengthens host-ion interaction, contributing to the charge storage. And the rich oxygen groups are responsible for its superior oxidation resistance. The assembled asymmetric supercapacitor with modified MXene and Prussian blue analogue can deliver a high energy density of 19.7 Wh Kg-1. This work proposes a novel strategy to simultaneously enhance the energy storage capability and oxidation resistance of MXene for supercapacitor applications.
Rights: All rights reserved
Access: open access

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