High-Capacitance Mechanism for Ti3C2Tx MXene by in Situ Electrochemical Raman Spectroscopy Investigation.

The production of hydrogen fuel from water splitting through the hydrogen evolution reaction (HER) has been investigated for more than a century; yet, there exists no commercial water splitting device because of the lack of an efficient and cost-effective electrocatalyst. This scientific gap can be filled by the new class of two-dimensional carbides and nitrides, known as MXenes, if their mechanisms of electrocatalysis can be understood. Herein, we report on the electrocatalytic properties of MXenes and elucidate their mechanisms of electrocatalysis and charge storage. More specifically, we report on the detailed mechanisms of HER electrocatalysis and charge storage on the benchmark Ti3C2 carbide MXene in acidic (HCl) and neutral (Na2SO4) electrolytes using an in-situ/operando Raman spectroelectrochemical approach. This approach allows us to monitor, in real time, the structural behavior of the materials under operating conditions. The correlation between the Raman shifts and the current density provides insights into the mechanism of HER. In acidic electrolyte, we found that the HER mechanism proceeds through a surface protonation mechanism due to termination group changes, whereas neutral electrolyte HER undergoes an overcharging mechanism prior to splitting water to form hydrogen gas. For capacitive charge storage, we found that acidic electrolytes allow for pseudocapacitive behavior to take place in a similar surface protonation mechanism as for HER. In neutral electrolytes, the charge storage mechanism follows a purely double layer capacitance mechanism with sodium ions adsorbing and desorbing from the surface. The new knowledge from this work can be applied to a broad class of materials and systems to advance the field of energy conversion and storage.

MXenes have been extensively explored as supercapacitor electrodes, especially in acidic aqueous electrolytes, where ultrahigh specific capacitance can be achieved; however, their narrow working potential window (≤ 1.0V) limits the acquisition of high energy. Neutral and alkaline electrolytes can be used to extend the working potential window but MXenes in these electrolytes are less pseudocapacitive active, which leads to reduced charge storage. In this study, it is shown that Ti3 C2 Tx MXene in a mild AlCl3 aqueous electrolyte can operate at a wide potential range from 0 to -1.3V versus Hg/Hg2 SO4 and retain high pseudocapacitive activity. Thus, a high capacity of up to 85mAhg-1 is achieved, surpassing its performance in H2 SO4 electrolyte of 78mAhg-1 . More interestingly, most of the capacity is released at a more negative potential range than that in acidic electrolytes, making it more suitable as a negative electrode material. In situ electrochemical quartz crystal microbalance results suggest that the high capacity originates from the pseudocapacitive intercalation/deintercalation of H+ instead of Al3+ , providing the possibility of coupling MXene anodes with proton redox active cathodes to achieve high-energy and high-power devices.

Capacitance performance of Ti3C2Tx MXene nanosheets on alkaline and neutral electrolytes

High electrical conductivity and breakdown current density of individual monolayer Ti3C2Tx MXene flakes

Charge storage improvement in uniformly grown TiO2 on Ti3C2Tx MXene surface

MXenes are promising materials for energy storage due to their high electrical conductivity and unique layered structure. This study demonstrates the dual role of 1-Butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) as both an etching agent and an intercalation medium for synthesizing Ti3C2Tx MXene with enhanced electrochemical performance. In-situ Raman and XRD analyses show that BMIm+ ions intercalate between MXene layers, stabilizing the nanoscale (002) interlayer spacing, preventing restacking and improving ion transport. When tested in the H2SO4–BMImBF4 hybrid electrolyte, the Ti3C2Tx MXene delivers a capacitance of 228.7 F g–1 at a scan rate of 1 mV s–1, compared to 179.9 F g–1 in pure H2SO4, measured on the same batch of materials. This demonstrates that incorporating BMImBF4 into H2SO4 significantly enhances the charge storage capability of Ti3C2Tx MXene. These findings highlight the effectiveness of BMImBF4 in optimizing charge storage and reinforce the potential of ionic liquids for advanced MXene-based energy storage systems.

Effect of Co content on electrochemical hydrogen kinetics properties of single-phase BCC-type MgAlTiCoxNi high entropy alloys used as a negative electrode in basic and acidic electrolyte

Alkali ions pre-intercalated 3D crinkled Ti3C2Tx MXene architectures for advanced sodium storage

C/MoS2@Ti3C2Tx composite flexible films for high performance supercapacitors

Two-dimensional Ti3C2Tx MXenes have been extensively studied as pseudocapacitive electrode materials. This Letter aims at providing further insights into the charge storage mechanism of the Ti3C2Tx MXene electrode in the acidic electrolyte by combining experimental and simulation approaches. Our results show that the presence of H2O molecules between the MXene layers plays a critical role in the pseudocapacitive behavior, providing a pathway for proton transport to activate the redox reaction of the Ti atoms. Also, thermal annealing of the samples at different temperatures suggests that the presence of the confined H2O molecules is mainly controlled by the surface termination groups. These findings pave the way for alternative strategies to enhance the high-rate performance of MXenes electrodes by optimizing their surface termination groups.

In this study, we used 2‐Dimmensionnal Ti3C2 MXene as model materials to understand how the surface groups affect their electrochemical performance. By adjusting the nature of the surface terminations (Cl‐, N/O‐, and O‐) of Ti3C2 MXene through a molten salt approach, we could change the spacing between MXene layers and the level of water confinement, resulting in significant modifications of the electrochemical performance in acidic electrolyte. Using a combination of techniques including in‐operando X‐ray diffraction and electrochemical quartz crystal microbalance (EQCM) techniques, we found that the presence of confined water results in a drastic transition from an almost electrochemically inactive behavior for Cl‐terminated Ti3C2 to an ideally fast pseudocapacitive signature for N,O‐terminated Ti3C2 MXene. This experimental work not only demonstrates the strong connection between surface terminations and confined water but also reveals the importance of confined water on the charge storage mechanism and the reaction kinetics in MXene.

Heterostructured bimetallic–sulfide@layered Ti3C2Tx–MXene as a synergistic electrode to realize high-energy-density aqueous hybrid-supercapacitor

MXene, notable for its excellent electrical conductivity and tunable surface groups, has garnered widespread attention in the field of electrochemical energy storage. Here, Ti3C2Tx MXene was synthesized by a Lewis acid molten salt‐shielded synthesis (MS3). The surface groups (−Cl, −O) were modified by washing Ti3C2Tx samples with various solutions (deionized water, 0.5 M hydrochloric acid (HCl), 0.5 M ammonium persulfate solution (APS)) and/or thermal treatments under an argon atmosphere at 300 °C, 500 °C, and 700 °C. It is shown that deionized water and HCl solution washing have minimal impact on the surface groups, while APS washing can increase the content of −O surface group. Conversely, thermal treatment may remove the −O. Electrochemical charge storage behavior of these Ti3C2Tx variants were further investigated in a 1 M acetate electrolyte buffered at pH=5.0. It is indicated that the −Cl surface group is electrochemically inert, whereas the −O may significantly improve the charge storage performance. Ti3C2Tx with high −O content delivered an impressive maximum capacity of 155 C g−1. This research underscores the crucial role of surface groups on the electrochemical performance of Ti3C2Tx in mild aqueous electrolytes, offering valuable insights for future modifications and applications of Ti3C2Tx in energy storage technologies.

In line with current research goals involving water splittingforhydrogen production, this work aims to develop a noble-metal-freeelectrocatalyst for a superior hydrogen evolution reaction (HER).A single-step interfacial activation of Ti3C2Tx MXene layers was employed by uniformlygrowing embedded WS2 two-dimensional (2D) nanopetal-likesheets through a facile solvothermal method. We exploited the interactionsbetween WS2 nanopetals and Ti3C2Tx nanolayers to enhance HER performance. Amuch safer method was adopted to synthesize the base material, Ti3C2Tx MXene, by etchingits MAX phase through mild in situ HF formation. Consequently, WS2 nanopetals were grown between the MXene layers and on edgesin a one-step solvothermal method, resulting in a 2D–2D nanocompositewith enhanced interactions between WS2 and Ti3C2Tx MXene. The resulting2D–2D nanocomposite was thoroughly characterized using X-raydiffraction (XRD), scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), Raman, Fourier transform infrared (FTIR),and X-ray photoelectron spectroscopy (XPS) analyses before being utilizedas working electrodes for HER application. Among various loadingsof WS2 into MXene, the 5% WS2–Ti3C2Tx MXene sample exhibitedthe best activity toward HER, with a low overpotential value of 66.0mV at a current density of −10 mA cm–2 ina 1 M KOH electrolyte and a remarkable Tafel slope of 46.7 mV·dec–1. The intercalation of 2D WS2 nanopetalsenhances active sites for hydrogen adsorption, promotes charge transfer,and helps attain an electrochemical stability of 50 h, boosting HERreduction potential. Furthermore, theoretical calculations confirmedthat 2D–2D interactions between 1T/2H-WS2 and Ti3C2Tx MXene realignthe active centers for HER, thereby reducing the overpotential barrier.

Capacitive characteristics and electrosorption of hydrogen in microporous aсtivated carbon fibers