MXenes and their Derivatives for Energy Storage and Conversion

Abstract

Ultra-capacitors or supercapacitors are in demand due to the growing market of flexible wearable electronics, the exigence of off-grid energy storage devices, and electrical vehicles. Pseudocapacitive charge storage mechanism coupled with fast redox reactions facilitate the devices to store more energy as compared to conventional EDLCs in a rapid manner, unlike batteries that fail to deliver it. Only a few transitions-metal oxides have been investigated and attained the high-rate capability due to their low intrinsic ionic and electronic conductivity. Since their discovery, 2D MXenes are harnessed in energy storage and conversion devices due to their unique intrinsic physical and chemical properties as compared to their counterpart MAX phases. Their 2D structure high volume to-surface ratio, hydrophilicity, and controllable surface chemistry furnish them with rapid redox reaction for outstanding electrochemical performance. To date, many researchers from around the globe have investigated their potential for storage and conversion purpose. Despite all the substantial investigations the research is still in its infancy phase and needs to be explored in many areas. Especially addressing the issues related to material stability in aqueous electrolytes. This thesis aimed to develop high-efficiency electrode material for supercapacitors with enhanced energy density without comprising the power density. V2CTx MXene was intensively explored for this purpose. A hybrid with MWCNT was synthesized to address the reaggregation of sheets and to provide conductive channels for the fast reaction kinetics and to prevent oxidation as well. The addition of MWCNTs significantly increases the capacitance from 230.5 Fg-1 of pristine to double 576 Fg-1 for hybrid. As delaminated MXene is vulnerable to oxidation and re-aggregation greatly affecting the stability of MXene to address this issue pillaring was performed via ion intercalation to increase the stability and storage capacity utilizing alkali metals (Li, K, Na, Mg). It was found that the Li-intercalated MXene outstands the other alkali metals and a maximum of 890Fg-1 was achieved @ 5mVs-1 scan rate. It is also observed that Na-intercalated V2CTx performs better than Li-Intercalation specially at higher scan rates as in previous studies on Ti3C2Tx. A hybrid with zirconia intercalation was also fabricated in continuity of previous projects for the enhanced electrochemical activityV2CTx. Zirconia decorated V2CTx was hydrothermally synthesized and a maximum of 1200Fg-1 specific capacitance @ 5mVs 1 was achieved in 3M H2SO4 electrolyte. The other part of the thesis revolves around the development of bifunctional electrocatalyst and hydrogen production. The MWCNT@V2CTx exhibit an outstanding HER activity and needs only 27mV for a benchmark of 10mAcm-2 with a Tafel slope of 41mVdec-1 comparable to commercially available platinum catalyst. This was the first effort to check the overall water splitting using V2CTx MXene as electrocatalyst Double transition metal carbides were investigated, and a comparative study was carried out to explore their potential as a bifunctional catalyst for overall water splitting and it was found that DTM with a lower n value performs better than higher-order DTMs. As Mo2TiC2Tx\\ Mo2TiC2Tx symmetric device needs 1.57V while the Mo2T2iC3Tx\\ Mo2Ti2C3Tx configuration requires 2.1V for the overall water splitting in 1M KOH. A novel DTM carbonitride MAX Mo2TiAlCN and Mo2TiCNTx MXene was synthesized, electrochemical profiling was carried out in various acidic basic and neutral electrolytes to explore its potential for various storage systems. This Ph.D. dissertation sheds light on the fabrication of high efficiency electrode materials and addressed some of the basic issues related to the MXene material. This will directly contribute to the ongoing research on 2D MXenes for future consideration.