MXenes, a class of two-dimensional transition metal carbides and nitrides, have garnered significant attention in recent years as promising materials for energy-related applications. Their unique combination of properties, including exceptional electronic conductivity, impressive ionic conductivity, a large specific surface area, and highly tunable surface chemistries, make them highly versatile for a wide range of advanced energy technologies. These properties not only enhance energy storage and conversion efficiency but also allow for precise control over their performance characteristics by tailoring their preparation and material characteristics.

In this presentation, I will highlight the latest advancements from our group that leverage the exceptional properties of MXenes to develop innovative energy harvesting and storage devices. Our research explores a broad spectrum of applications, demonstrating the versatility of MXenes in addressing challenges across various domains of energy technology.

Specifically, I will discuss the integration of MXenes into commercial-scale silicon heterojunction solar cells, where we show they can replace expensive silver back contacts. I will also present our work on MXene use in salinity gradient energy harvesters, which exploit ionic conductivity and surface functionality for efficient energy extraction from saltwater gradients. Furthermore, our work in thermoelectric nanogenerators showcases how MXenes contribute to converting waste heat into electrical energy by optimizing thermal and electronic transport properties.

Additionally, MXenes have been demonstrator in triboelectric nanogenerators, where their mechanical robustness and electronic conductivity enable efficient energy harvesting from mechanical motion. Their role in ultrasound energy devices will also be highlighted, with a focus on their unique ability to convert acoustic energy into usable electrical energy.

Finally, I will present our advancements in integrating MXenes into microsupercapacitors, which combine high energy density and power density for next-generation compact energy storage solutions. These integrated devices demonstrate the ability to pair energy harvesting with storage, paving the way for self-powered electronic systems.

This comprehensive exploration underscores the transformative potential of MXenes in energy applications and highlights their role as key enablers of sustainable and efficient energy technologies. Through the examples presented, I aim to showcase the versatility and impact of MXenes in shaping the future of energy harvesting and storage devices.

The field of battery research continually seeks to improve energy storage capabilities while addressing sustainability concerns. This applies in particular to the exploration and development of novel materials, such as the promising material group of MXenes. MXenes, known for their two-dimensional morphology, vast chemical composition range, and excellent electrical conductivity, require the synergistic integration of conversion or alloying materials to achieve high charge storage capacities.

The presentation highlights the development of high-performance sodium-ion batteries using MXene / antimony hybrid electrodes. By carefully optimizing synthesis parameters and material design strategies, researchers achieved an optimized electrode composition. This hybrid material exhibited a high reversible capacity of 450 mAh/g at 0.1 A/g, along with excellent cycling stability and rate capability. We also explore the combination of MXenes and SnO₂, a conversion material, for enhanced lithium-ion battery performance of over 500 mAh/g for 700 cycles at 0.1 A/g. The researchers synthesized a nanocomposite consisting of a 50/50 mixture of SnO₂ and MXene. The resulting nanocomposite demonstrated high-capacity retention over numerous cycles and excellent rate capability.

Additionally, we demonstrate MXene electrode recycling and upcycling. With binder- and additive-free MXene paper electrodes, we show the significance of finding sustainable and efficient approaches to recycle spent batteries. Researchers investigated the use of annealed delaminated MXene electrodes, obtained through vacuum-assisted filtration and annealing processes, in lithium-ion and sodium-ion batteries. The electrodes exhibited good electrochemical performance and were easily recovered through direct recycling processes, achieving high capacity recovery rates. Moreover, the cycled MXene electrodes could be transformed into TiO₂/C hybrids with adjustable carbon content, providing opportunities for their utilization in various battery and electrocatalysis applications.

Collectively, we emphasize the potential of MXenes and MXene hybrid materials for enhancing charge storage capabilities in batteries. They also underline the significance of developing sustainable recycling and upcycling approaches for MXene electrodes, contributing to the overall advancement of battery technology.

The oxygen evolution reaction (OER) is critical for water electrolysis, a key technology for the production of green hydrogen. While non-noble metal oxides show promise as OER catalysts, their application is limited by poor conductivity and stability.[1] This study demonstrates the functionalization of CuCo based hydroxides with V₂CT_x MXene to address these challenges, resulting in significantly improved OER performance in alkaline media.[2]
The CuCo@V₂CT_x composites were synthesized via a hydrothermal process, with varying MXene content (1%, 10%, 25%, 50%). The optimized CC50 catalyst achieved a lower overpotential and superior stability, outperforming the pure Co and CuCo hydroxides. Post-mortem analyses, including XPS and ICP-OES, revealed that the MXene not only enhanced hydrophilicity and charge transfer properties but also reduced Cu leaching during OER, improving the overall stability.
Characterization using SEM/TEM/EDX, XRD, and XPS confirmed that MXene mitigates aggregation and stabilizes CuCo on the electrode surface. Furthermore, the preferential leaching of V₂CT_x over Cu during stability tests preserved a higher concentration of active CuCo species, contributing to long-term catalytic performance. This work highlights the potential of MXene based hybrids as high-performance electrocatalysts for sustainable energy applications.

MXene, a novel class of two-dimensional materials, holds immense potential for revolutionizing energy storage systems due to its exceptional electrical conductivity, large surface area, and ability to accommodate various ions, thereby enhancing the performance of batteries and supercapacitors. [1-2]

In this pioneering study, state-of-the-art operando microscopy – interferometric scattering microscopy (iSCAT) – is employed to reveal the intricacies of ion diffusion within MXene nanostructures at the single-nanoparticle level (Fig. 1). iSCAT has already provided unique insights in various fields, from quantitative mass imaging of single macromolecules in biology [3-4] to elucidating ultrafast mechanisms in materials synthesis, such as the early-stage formation of covalent organic frameworks.[5] This research delivers an in-depth analysis of ion transport and charge transfer dynamics within 2D MXene structures, influenced by factors such as sheet size and thickness. The use of non-invasive operando microscopy for real-time tracking has provided critical insights into how these variables affect ion mobility within MXene frameworks. Notably, the study identifies how ion diffusion contributes to power density at the nanoparticle level, offering precise measurements of charge transport boundaries in MXene films.

These breakthroughs are crucial for advancing energy storage technologies, offering deeper insights into ultrafast nanoscale processes and the impact of nanoparticle heterogeneity on device performance at a larger scale.

The surface chemistry, structure, and morphology of MXenes play a critical role in their applications. Various methods of covalent functionalization have been reported, demonstrating significant improvements over pristine MXenes terminated with hydroxyl and fluorine functional groups. Furthermore, the direct conversion of MXenes into various chalcogenides, while retaining their original structure, can significantly enhance properties such as hydrogen and oxygen evolution overpotentials, as well as the capacitance of the material for electrochemical energy storage. Chemical functionalization, such as the introduction of charged zwitterionic compounds and defect engineering, has been shown to significantly improve the performance of MXenes in energy storage applications. Treating MXenes with different reagents, such as elemental chalcogens or pnictogens, effectively modifies their surface functionalization and can lead to the formation of composite materials. These composites, consisting of functionalized MXenes and transition metal dichalcogenides, exhibit significant enhancements in capacitance for supercapacitors and lithium-ion batteries. By leveraging specific chemical transformations, MXenes containing vanadium, niobium, and molybdenum can be converted into chalcogenides or vanadates/niobates. These processes pave the way for the development of advanced materials with promising applications in photocatalysis, supercapacitors, and batteries.

Single-molecule detection represents the highest sensitivity in analytical techniques, enabling precise studies of individual molecular events. Methods based on single-molecule fluorescence energy transfer offer nanoscale distance measurements with significant implications for structural biology, biophysics, and material-biomolecule interfaces. Förster Resonance Energy Transfer (FRET) operates in the 3–10 nm range, typically involving two organic fluorophores as energy donor and acceptor. Graphene-based energy transfer (GET) extends this range to 10–40 nm, with a fluorophore as the energy donor and graphene acting as a broadband, unbleachable energy acceptor. FRET has been used predominantly to study protein conformational changes, but its measurements are often qualitative due to challenges in resolving nanometer-scale distances accurately. GET is a more straightforward quantitative method that enables precise z-localization of fluorophores. Notably, it has recently enabled real-time monitoring of DNA-protein interactions at the structural level [1]. However, graphene's hydrophobic nature and the strong influence of thickness on fluorescence lifetime measurements limit its biocompatibility to non-DNA systems and overall reproducibility.

Our research focuses on titanium carbide (Ti₃C₂Tₓ) MXene as a complementary, powerful energy transfer tool. MXenes possess unique advantages for studying biological assemblies that require hydrophilic surfaces, combining FRET-like short-distance sensitivity (1–10 nm) with GET's quantitative protocol, while offering unique material advantages such as hydrophilicity, biocompatibility, and thickness-independent energy transfer efficiencies.

This research line began by exploring MXene-DNA interactions with ensemble fluorescence spectroscopy measurements and molecular dynamics simulations. The interaction between MXene and DNA was found to be driven by salt bridges, coinciding with prior reports [2], with ssDNA and dsDNA lying horizontally on the surface while maintaining their native structure, approximately 1 and 1.3 nm away from the surface, respectively. In real-time hybridization experiments, we observed a fluorescence increase as ssDNA converted to dsDNA on the surface, suggesting that MXenes possess nanoscale sensitivity to sub-nanometer changes in the fluorophore position [3].

Next, we investigated MXenes’ fluorescence quenching mechanism and distance dependency of the energy transfer with single-molecule fluorescence microscopy and DNA origami nanopositioners using MXene thin films on glass. We positioned a single dye (ATTO 542) at defined distances from the surface (1–8 nm) via glycine-immobilized DNA origami nanostructures on MXene surfaces. Fluorescence quenching was observed between 1 to 8 nm, following a cubic distance dependence, consistent with the Förster mechanism observed in transparent conductors at the bulk level. These findings not only supported the hypothesis of MXenes' sub-nanometer sensitivity but also established them as hydrophilic, short-distance spectroscopic nanorulers uniquely positioned to operate in a distance regime inaccessible to GET. Additionally, the energy transfer efficiency was nearly independent of material thickness, enhancing robustness for sensing applications. 

Finally, we demonstrated the utility of MXene's sensitivity, robustness and hydrophilic, salt-driven interactions for leaflet-resolved, single-molecule biosensing of fluorescently-labeled, 5-nm thick supported lipid bilayers (SLBs) as model cell membranes, directly fused on the MXene surface [4]. Similar to typical SLBs formed on mica surfaces—where a thin hydration layer separates the bilayer from the surface to maintain biomimetic fluidity—MXenes also supported biomimetic SLBs, offering an advantage over other metallic surfaces and graphene. At the single-molecule level, we determined the SLB thickness by independently resolving each leaflet of the bilayer and the hydration layer separating it from the MXene surface. This capability to discern such fine structures highlights MXene substrates as uniquely suited to studying lipid-protein interactions, membrane organization, and dynamic hydration phenomena, potentially offering new insights into lipid biology and biomembrane research.

MXenes having Metal-surface functional groups as well as structural defects offer much potential as solid catalysts, similarly as it happens with other metal carbides/nitrides and metal oxides. In addition, MXenes are also suitable supports for single atom and small metal clusters. The presentation will show that NH₃-thermoprogrammed desorption and pyridine adsorption/desorption monitored by IR spectroscopy are suitable techniques to measure the acidity of these materials. It has been found that the actual density of Brönsted and Lewis acid sites and their strength depends on the MXene type, preparation procedure and post-synthetic treatments. MXenes have been found to catalyze a series of organic reactions, including hydrocarbon oxidation, electrophilic additions to C-C triple and double bonds and guanylation reactions among other. In many of these cases, the turnover frequencies achieved by MXenes are among the highest reported for these reactions using solid catalysts. This is the case of 1-butanamine addition to 1-hexyne for which turnover frequencies over 10⁴ h^-1 have been measured. This illustrates the potential that MXenes can offer in the field.

MXenes dots as photocatalysts

Ruben Ramirez Grau¹, María Cabrero-Antonino¹, Hermenegildo García¹*, Ana Primo¹*¹Instituto de Tecnología Química, Consejo Superior de Investigaciones científicas-Universitat Politècnica de Valencia, Avda. de los Naranjos s/n, 46022, Valencia, Spain

Introduction

MXenes are 2D nanomaterials constituted by alternating one-atom thick layers of an early transition metal of the 3, 4, 5 or 6 groups of the Periodic Table (denoted as “M”) and a carbide, nitride or carbonitride layer (denoted as “X”) and have a general formula of M_n+1X_n.¹ We recently reported that MXene dots (MDs) prepared by liquid-phase laser ablation of the corresponding MAX phase exhibit intrinsic photocatalytic hydrogen generation activity in the absence of any other photoresponsive component.² Decrease of lateral size of MXenes to a few nanometers results in the operation of quantum confinement effects reflected in optical absorption and photoluminescence of MDs, these MDs exhibiting photocatalytic activity. Theoretical DFT calculations for Ti₃C₂ models indicates that the bandgap depends on the diameter of the MDs and on the nature of the surface functional groups. The compositional and structural versatility of MXenes will allow band engineering to align the energy values of the valence and conduction band to the values required to promote a given chemical reaction.

Continuing with this line of research, it is of interest to further explore the photocatalytic activity of these MDs. Specifically, it would be important to determine whether or not MDs promote the photocatalytic CO₂ reduction, the efficiency of the process and the products formed. Herein it is reported that MDs are active in the photocatalytic CO₂ reduction using H₂ as electron and proton donor.

Materials and Methods

3 mg of MAX phase were suspended in 3 ml of MilliQ water and the suspension was submitted to ultrasounds in order to prepare a homogenous dispersion. Then, the system was irradiated for 3 h using the second harmonic of a Q switched Nd:YAG laser (Quantel Brilliant, 532 nm, 5 mJ/pulse, 5 ns fwhm) operating at 1 Hz. Finally, the resultant material was centrifuged for 4 h at 4000 rpm and, then, 1 h more at 13 000 rpm to isolate from the bulk MAX and other phases that sediment in the process the small MXene QD that remains in the supernatant.

Results and Discussion

Four MDs, namely Ti₃C₂, Ti₂C, V₂C and Nb₂C were prepared from the corresponding Al MAX phases by 522 nm laser ablation in aqueous suspension, as previously reported.² After preparation, the four MD samples were characterized by different techniques, including AFM and transmission optical microscopy. Table 1 lists the corresponding average thickness and average particle lateral size values for each sample. The activity of the four MD samples was evaluated for the photocatalytic CO₂ hydrogenation under UV-Vis irradiation at 200 ^oC at 1 bar pressure, using a H₂/CO₂ ratio of 1/3. For the four samples, evolution of CO and methane as the only products was observed in different proportions. Fig. 1 shows the temporal product evolution upon irradiation of each MD, while Table 1 indicates the CO and methane production rates determined from these plots.

Significance

Photocatalytic CO₂ reduction is a clean technology that converts CO₂ into high-value-added products. The interest of the present study is to show that, MXenes by themselves, can exhibit an intrinsic photocatalytic activity that can be modulated depending on the composition. Since MXenes allow a wide range of compositions, including bimetallic solid solutions and a wide range of surface functional groups, there is still much room for improvement in the intrinsic photocatalytic efficiency by adjusting these parameters and also to achieve photocatalytic stability.

The pseudocapacitive behavior of Ti₃C₂T_x MXenes promises high power and energy densities thanks to redox reactions occurring during the (de-)protonation of the MXene surface in acidic electrolyte. Nevertheless, local electrochemical processes occurring at the MXene-electrolyte interface and the associated changes of the MXene surface chemistry are currently largely unexplored. To this aim, soft X-ray spectroscopies are particularly relevant as they enable the selective characterization of either the electrolyte or the material of interest thanks to their element specificity.¹ Furthermore, the high spatial resolution (<30 nm) offered by X-ray Microscopy can provide precious information about local inhomogeneities at the nanoscale,^2,3 enabling the characterization of single MXene flakes.

In this talk, I will introduce X-ray spectromicroscopy for monitoring the surface chemistry of Ti₃C₂T_x MXenes as well as intercalated species at the single flake level. In situ cells for the characterization of MXenes at controlled temperature or in liquid will be introduced. The effect of proton and alkali cations intercalation on the electronic structure of Ti and O atoms in the MXene will be discussed, as well as the electronic signature of water confined in the MXene interlayer. Perspectives towards correlative optical spectromicroscopy techniques will also be discussed.

MXenes are highly conductive materials with a high surface area, offering potential as promising catalyst supports in proton exchange membrane fuel cells (PEMFCs), a technology recognized for sustainable energy conversion. This study centers on a comparative analysis of various platinum (Pt) loadings uniformly distributed across MXene sheets to enhance catalytic performance. The synthesized materials were comprehensively characterized using X-ray diffraction (XRD), X-Ray Photoelectron Spectrscopy, scanning electron microscopy (SEM) and Raman spectroscopy to investigate their structural attributes and morphology and functional groups, providing insights into their stability and interaction with Pt particles. Electrochemical tests were conducted in rotating disc electrode (RDE) and half-cell setup to assess the efficiency of the catalysts in oxygen reduction reaction (ORR), a crucial step in PEMFC performance due to its role in limiting cell output. Results reveal that Pt particles on MXenes not only function effectively as ORR electrocatalysts but also improve reaction kinetics, positioning MXene-supported Pt catalysts as a promising alternative for PEMFC applications.

MXenes – transition metal-based carbides and (carbo)nitrides with the general chemical composition M_n+1X_nT_x (M = early-to-mid transition metals, X = C and/or N, T_x = surface groups, n = 1-4) are an incredibly fascinating class of 2D materials that have left almost no potential application area untouched. As much research as is dedicated to exploring their unique properties, for example in the context of biomedicine, catalysis and energy storage, especially of Ti-MXenes, as little focus is dedicated to non-Ti-MXenes as well as their synthesis science.

In this talk, I will discuss the synthesis of different V-MXenes derived from their MAX phase siblings. The preparation of the precursor MAX phases is not trivial as shown for MAX phase examples V₄AlC₃ and (V/Mo)₅AlC₄. MAX phases with minimal amounts of side phases as well as knowledge of their chemical composition are key to obtaining high-quality MXenes that allow for meaningful discussions of their properties. These aspects will be covered during my discussion of the “higher n” MXenes V₄C₃T_x and (V/Mo)₅C₄T_x including discussion of their electrocatalytic performance in the hydrogen evolution reaction (HER). Besides, I will highlight key synthetic chemistries during the exfoliation of V₂AlC as well as its Mo-substituted analogs and show how the catalytic properties depend on the V/Mo ratio.

We take advantage of diverse synthesis and exfoliation tools ranging from conventional to microwave heating and aqueous acid as well as Lewis acid (molten salt) etching. All materials are carefully characterized by diffraction (synchrotron and lab X-ray), microscopy and spectroscopy (lab and hard-X-ray photoelectron spectroscopy) techniques.

Producing hydrogen (H₂) through electrocatalytic water splitting in an electrolyzer offers a clean and sustainable pathway to generate an eco-friendly fuel and energy carrier. The oxygen evolution reaction (OER) plays a critical role in this process, though its efficiency is bottlenecked by sluggish kinetics [1-3]. Noble-metal-based catalysts, particularly IrO₂ and RuO₂, are the state-of-the-art OER electrocatalysts available today due to their excellent activity but are hindered by high costs and instability issues. In particular, a major obstacle to the large-scale application of Ru lies in its tendency to leach in alkaline media [4, 5].

To address these limitations, extensive research has focused on improving the stability of noble-metal-based catalysts, reducing noble metal usage while retaining high performance, and exploring non-noble metal alternatives with strong OER activity. Transition-metal-based catalysts are promising, affordable alternatives as they are stable in alkaline conditions and have low OER potentials [6, 7]. However, their limited conductivity restricts their catalytic performance in OER applications. To overcome both the instability issues and the OER activity, we propose using MXenes, a rapidly advancing family of 2D transition metal carbides and nitrides, as conductive supports to form NiRu@Ti₃C₂T_x hybrids [8-12]. While many studies have explored MXene-based materials for applications in water splitting, OER remains relatively less-explored compared to the hydrogen evolution reaction (HER). Moreover, there is limited information on how MXenes influence the stability of metal/oxide during alkaline OER.

In this study, the potential of MXenes to enhance the stability of Ru-based catalysts, which are known for their substantial dissolution in alkaline media during OER, was investigated. Here, a bimetallic NiRu compound was synthesized and incorporated onto the Ti₃C₂T_x MXene surface at varying concentrations (1%-25%) using a facile hydrothermal method. The resulting NiRu@Ti₃C₂T_x composites were tested for OER activity in 1 M KOH to examine the impact of Ti₃C₂T_x content on Ru dissolution and overall catalytic performance. After incorporating Ru into a stable Ni-based environment and functionalizing Ti₃C₂ with NiRu, the composites showed reduced Ru leaching and increased OER activity compared to pure Ru. Higher Ti₃C₂T_x content further stabilized Ru. These results highlight the positive impact of Ti₃C₂T_x functionalization on the stability and efficiency of Ru-based OER catalysts.

Two-dimensional (2D) materials have attracted significant attention for their high surface area, mechanical flexibility, and tunable electronic properties, enabling applications in energy storage, flexible electronics, optoelectronics, and biomedicine. The discovery of graphene's exceptional properties spurred interest in exfoliating other 2D materials from bulk crystals, uncovering diverse chemical structures and functionalities. Electrochemical exfoliation (ECE) offers a promising approach for mass-producing graphene and other 2D materials due to its mild conditions, fast processing, simplicity, and high yield. While ECE of layered van der Waals (L-vdW) crystals has advanced, research on layered non-van der Waals (L-NvdW) materials is still in its infancy. This work investigates ECE for producing 2D nanoplatelets from L-NvdW crystals, comparing exfoliation techniques for L-vdW and L-NvdW materials and reviewing recent progress in ECE methods for L-NvdW crystals. Key topics include selective extraction of 'M' and 'A' layers from MAX phases, decalcification of Zintl phases, and oxide delocalization from metal oxides. We also present our latest research on the electrochemical exfoliation of layered TM2SC-type MAX phases.

The growing global demand for sustainable energy has heightened the need to develop efficient electrocatalysts for energy conversion processes, particularly the oxygen evolution reaction (OER), which is a key part of water splitting for green hydrogen production. Although a wide range of catalysts has been investigated for use in electrolyser devices, challenges such as cost, conductivity, and stability continue to drive ongoing research in this area.[1]

Transition metal oxide (TMO) materials, despite their great potential as OER catalysts, suffer from limited conductivity, which hampers effective charge transfer during electrochemical processes, and can lead to deactivation due to structural transformations during the OER.MXenes, with their exceptional surface area and conductivity, are promising supports for enhancing the electrocatalytic performance of TMO-based OER catalysts.[2] However, MXenes are susceptible to oxidation under applied potentials, which can alter their structure and properties. By synergistic combination of TMOs with MXenes, a highly efficient OER catalyst could be developed, combining the enhanced conductivity, hydrophilicity, and increased surface area of MXenes with the active OER sites of TMOs.[3] While there have been numerous successful attempts in producing highly performing TMO/MXene composites, the overwhelming number of such reported catalysts is based on the first found and best known Ti₃C₂T_x MXene.[4] This leaves a wide range of potentially highly performing OER catalysts based on MXenes other than Ti₃C₂T_x yet to explore.

In our recent investigations we found that coprecipitation of CoFe layered double hydroxides (LDH) with V₂CT_x MXene at various weight percentages resulted in CoFe/ V₂CT_x composites that outperformed the separate components and physically mixed components as OER catalysts. Furthermore, we could show that the presence of V₂CT_x during the synthesis promotes the formation of a highly active CoFe-LDH phase as opposed to the pure CoFe sample. We further investigated the materials’ oxidation states under OER conditions via synchrotron based in-situ XAS to detect the catalytically active species for the OER.

These findings demonstrate the great potential of TMO/ V₂CT_x composites as OER electrocatalysts and are a step towards understanding the reasons for their improved catalytic activity.

As the world increasingly turns towards sustainable energy solutions, the efficient storage and transport of hydrogen has become critical. Ti₃C₂ MXenes, with their unique 2D structure and exceptional properties, are at the forefront of this technological revolution. For Ti₃C₂ MXene as energy storage material using hydrogen it is crucial to understand hydrogen bonding and diffusion, since unexpected properties may arise in terms of interaction with lattice atoms.

In this paper, it is shown that the chemical bonding of hydrogen atoms and molecules extends far beyond the simple picture of covalent, ionic and multicenter bonds. Density functional theory was used for the calculations. The Ti₃C₂ layers and surfaces were modelled in a slab geometry. The properties of H and H₂ were further analyzed by calculating the vibrational eigenmodes and their intensities by employing density-functional perturbation theory.

The results clearly show that in Ti₃C₂ hydrogen atoms and H₂ molecules form multicenter bonds. On the surface and between two Ti₃C₂ sheets, this bonding is restricted to the formation of Ti−H bonds. However, at interstitial sites, both H and H₂ form multicenter bonds. This includes the nearest neighbor Ti atoms and also C atoms. Notably, C−H bonds involve the formation of s−p hybrid orbitals. For H₂ molecules the formation of multicenter bonds results in an increase in bond length to 2.07 Å on the surface and to 1.85 Å at the interstitial site. When H₂ is accommodated between two Ti₃C₂ sheets the molecule dissociates. The vibrational eigenmodes for all complexes have been calculated and vibrational frequencies ranging from 890 to 1610 cm⁻¹ were obtained. This indicates that the hydrogen multicenter bonds are stable.

For efficient hydrogen storage and release the understanding of H transport in MXenes is vital for optimizing their performance in applications. Hence, the diffusion properties of H in Ti₃C₂ MXene were determined from ab-initio calculations. For this purpose, migration barriers, hopping frequencies, vacancy formation enthalpies, and the vibrational entropy were calculated to determine the temperature dependent diffusion coefficients. The data reveal that hydrogen diffusion predominantly occurs via interstitial sites, while vacancy-mediated diffusion plays a negligible role.

The future of electronics, wearables, healthcare, and smart environments is being shaped by the integration of IoT (Internet of Things) with flexible technologies. A plethora of printed electronic components, including OLED displays, OPVs, TFTs, antennas, and diodes, among others, have already been developed [1], [2], [3]. These can then be integrated into technological platforms with a multitude of applications. When building such platforms, it is essential to address the question of energy autonomy. For flexible electronics, energy is normally harvested from the environment (solar energy, heat, movement, etc.) and stored in supercapacitors (SC) [4] ⁴. These energy storage devices are the optimal choice for printed and wearable electronics. They offer a long shelf life, seamless integration, and rapid charge and discharge capabilities. From many available options, MXenes have been regarded a good choice for SC manufacturing thanks to their optimal chemical properties. MXenes have a hydrophilic surface due to the presence of hydroxyl, oxygen, or fluorine functional groups (-OH, =O, -F), high electrical conductivity up to ~10,000 S/cm, especially for Ti₃C₂Tx, high flexibility, high capacitance (~1500 F/cm³ for Ti₃C₂Tx), fast ion transport, chemical stability, and more. As such, for the last couple of years, there has been a tremendous focus on MXene synthesis and implementation as SC electrodes [5], [6], [7].

This presentation will prove the vital importance of MXenes in the fabrication of SC electrodes for printed and flexible electronics. It will also show how they compare to other promising technologies and highlight their advantages and shortcomings. [8], [9]. It will also be discussed some strategies for MXene-based SC development to meet different applications requirements, such as voltage window, usable area and stability. However, the full potential of MXenes will only be exploited once the significant hurdles that currently stand in the way have been overcome, namely long-term stability and green synthesis. Nevertheless, due to their rich chemistry and optimal properties for electrochemical energy storage, MXenes are poised to play a significant role on powering up future and printed and flexible electronics.

Electrolytic water splitting is a clean and sustainable way of producing hydrogen gas, which has huge potential as a fuel source. There is a vast range of materials that can replace expensive state-of-the-art catalysts, and Transition metal oxides (TMOs)-based catalysts are currently under consideration due to their cost-efficient nature and abundance. However, their poor conductivity presents a major challenge. To address this issue, MXenes have emerged as a promising option to be integrated with TMOs thanks to numerous beneficial properties, such as high conductivity, hydrophilicity, and tunneling capacity, which makes them an excellent choice as a mechanically stable and conductive material [1]. To enhance the electrochemical activity and stability of Ti₃C₂T_x MXenes, the control of surface functionalization and area is paramount. In this regard, this study utilizes HF in situ generation to remove Al-elements for the direct production of delaminated Ti₃C₂T_x, and, consequently, exposing their high surface area. In this study, Ni-based MXene composites were fabricated with varying MXene content to investigate the effect of the MXene on the OER. The findings of this study provide a comprehensive understanding of the Ti3C2Tx MXenes influence on OER activity in terms of Ni dissolution, and electro-mechanic-chemical-stability, thus providing valuable insights for their implementation and the importance of MXenes content optimization in hybrid electrocatalysts for OER.

As a carbon-free energy source and courier, ammonia (NH₃) plays an irreplaceable role in agriculture, medicine, and future renewable energy technologies [1,2]. Traditionally, industrial production of NH₃ relies on the energy-intensive and large CO₂ emission Haber-Bosch process, whereby nitrogen (N₂) and hydrogen (H2) are the precursor materials [3]. As a result, a wide-spread of alternative sources and mechanisms, other than the use of N₂ for the synthesis of NH₃, have recently been explored. Amongst the various routes, electrochemical nitrate reduction reaction (NO₃-RR) has received extensive attention due to its efficient synthesis of NH₃ under mild conditions. While taking into consideration NO₃-RR as a conducive and energy efficient route for synthesizing NH₃, identification of non-precious metals and high-efficiency electrocatalysts remains the major challenge towards commercial practicality of NO₃-RR and its sister mechanisms. As an emerging member of the two-dimensional materials family, MXenes have received increasing attention for electrochemical energy conversion because of their superior metallic conductivity, large specific surface area, hydrophilicity, and controllable surface functional groups [4,5], thereby possessing great application potential for NO₃-RR. Herein, we report the potential application of molybdenum-based Mxene (Mo₂TiC₂T_x) materials in the field of NO₃-RR. At low applied potential of -0.8 V (vs. Ag/AgCl) and in 0.1 M KOH electrolyte containing KNO₃, Mo₂TiC₂T_x exhibited excellent NO₃ reduction performance with high Faradaic Efficiency (FE) of ~18.3%, good selectivity for NO₃-reduction to N₂ (~87.9%), good hinderance to parasitic hydrogen evolution reaction (HER), along with displaying outstanding NH₃ yield rate of 176.1 µg.h^-1.mg_cat^-1. The high activity of the Mo₂TiC₂T_x MXenes was attributed to the abundant surface defects at low coordinated Mo and/or Ti sites, as well as the electrical conductivity and large surface area of the MXenes [4,5]. In general, this study provides a practical pathway into tailoring nanoengineered materials for their potential application in wastewater treatment and environmental remediation.

In recent years, Single-Junction Perovskite Solar Cells (SJ-PSCs) have emerged as a competitive alternative to silicon based solar cells in terms of efficiency. However, their stability under humidity and high-temperature conditions remains a major challenge [1]. In this context, the incorporation of 2D nanomaterials, such as MXenes, into SJ-PSCs offers a promising route to enhance both stability and efficiency [2]. Among these materials, Ti₃C₂T_x MXene stands out due to its excellent electrical conductivity and tunable electronic structure. The latter, along with its work function, can be controlled via surface terminal groups, which depend on synthesis methods and post-treatments [3]. In order to integrate Ti₃C₂T_x MXene into SJ-PSCs, morphological, chemical, and structural characterization, as well as a dispersibility study in typical organic solvents used during photovoltaic device fabrication, are required.

In this study, Ti₃C₂T_x MXene (T_x are -F, -O, and -OH terminal groups) was synthesized via wet chemistry under mild conditions from Ti₃AlC₂ (MAX phase). The resulting highly concentrated aqueous dispersion was used to prepare freestanding films via vacuum assisted filtration. X-ray Diffraction (XRD) analysis of the synthesized material revealed the successful etching of the MAX phase, since a typical pattern with a series of evenly spaced peaks corresponding to the characteristic (00X) (X = 2, 4, 6,…) family of planes of Ti₃C₂T_x was observed. Control of Ti₃C₂T_X flake thickness was achieved by optimizing centrifugation procedure. As a result, a suspension of single-layer or few-layered structure was obtained (where each layer shows a thickness of 1-1.5 nm and lateral dimensions in the micrometer range (1-10 μm)) as revealed using Atomic Force Microscopy. When studying dispersibility of Ti₃C₂T_x MXene in solvents involved to SJ-PSC fabrication, it was found that polar solvents promote dispersion, in contrast to non-polar ones, which correlates with the presence of -O and -OH as MXene-terminal groups. Additionally, aprotic solvents showed better dispersibility than protic solvents. UV-Vis Spectrophotometry characterization showed the characteristic 750-800 nm absorption band, attributed to Ti₃C₂T_x MXene localized surface plasmon resonance (LSPR). The LSPR absorbance as a function of concentration was analyzed in selected organic solvents, enabling the determination of absorption coefficients.

The obtained results lay the basis for obtaining Ti₃C₂T_x MXene dispersions with controlled concentrations, facilitating their further integration into SJ-PSCs. In addition, the challenges of creating Ti₃C₂T_x nanocomposites with metal nanoparticles for modulating the work function will be discussed.