Acoustic Platform for MXene Synthesis and Electrochemical Behaviour of i-MXenes in Aqueous Electrolytes

• Climate Change is believed to be the greatest global challenge and on its forefront is the topic of energy. While being of extreme importance, debates over energy have become a normality. The related field of material synthesis for energy storage applications has also been growing, as well as the demand for industrial electrification from renewable sources of energy. Water-based supercapacitors are a type of energy storage devices that can deliver high power densities while maintaining long term cyclability in an environmen-tally friendly media. However, their challenges include maintaining high per-formance in term of energy density, safety, and low cost of electrode manu-facturing. MXene is family of two-dimensional transition metal carbides/nitrides that are terminated with H, OH and F groups. The material demonstrates superior physical and chemical properties related to energy applications in compari-son to its 3D parent material, the MAX phase. Since its discovery in 2011, MXene, such as Ti3C2Tz, has been widely investigated in the field of energy storage due to its high conductivity (20,000 S.cm-1) and a volumetric capac-itance that can reach 900 Fcm-3. However, reported synthesis processes for MXene are fraught with hazardous procedures that are time consuming. The first section of this thesis presents a new innovative method for Ti3C2Tz MXene synthesis, in which MXene was synthesized in a few milliseconds with the assistance of 30 MHz frequency surface acoustic waves (SAW) and 0.05M of LiF. The aluminium element in the Ti3AlC2 MAX phase was etched by so called “localized HF”, and the powder was converted to 2D Ti3C2Tz. This method showed resulting MXene comparable to that of previ-ously reported synthesis techniques, as demonstrated by the material’s elec-trochemical performance.  The second section of the thesis focuses on investigating the electrochemical performance of a comparatively new family of MXene, coined i-MXene, in aqueous electrolyte. i-MXene, reported in 2017, has the chemical formula Mo1.33CTz and is a product of chemical etching of the in-plane chemically ordered (Mo2/3Sc1/3)2AlC i-MAX phase. The Mo1.33CTz was studied in a sul-phuric acid electrolyte. This electrolyte sets a limit for the electrode potential window and capacitance, and therefore, post-synthesis treatment protocols was used to enhance the electrochemical performance. The Mo1.33CTz recorded a volumetric capacitance of 1050 Fcm-3 and1600 Fcm-3 for hydrogel treatment and heat-treated electrodes, respectively. Moreover, mixing Mo1.33CTz with MoS2 and graphene improved both the specific capacitance and the electrode stability even further.  The electrochemical properties of Mo1.33CTz were thereafter explored in dif-ferent sulfate-based aqueous electrolytes with univalent (Li+, Na+, and K+) and divalent (Mg2+ Mn2+ or Zn2+) cations. Mo1.33CTz exhibited a wider po-tential window without degradation, expanding the previously reported limit in sulphuric acid for both symmetric and asymmetric devices. Lithium chlo-ride gave the best results, being an electrolyte based on a natural salt that has high solubility at room temperature. It presented a large potential window, -1.2 to +0.3V (vs. Ag/AgCl), and a volumetric capacitance of ~800 Fcm−3 at a scan rate of 2 mVs−1. In addition, the performance of a Mo1.33CTz //MnxOn asymmetric device was tested in 5M LiCl electrolyte. The results showed a potential window of 2 V, a volumetric energy density of 58 mWhcm-3, and a 100% columbic efficiency after 10,000 charge/discharge cycles. A cyclic sta-bility is crucial for practical applications, and altogether, the promising re-sults motivate further exploration of i-MXenes for energy storage and be-yond.

Ämnesord

NATURVETENSKAP  -- Kemi -- Materialkemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Materials Chemistry (hsv//eng)

Nyckelord

Surface Acoustic Waves

MXene

i-MXene

Supercapacitor

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