Value addition of MXenes as photo-/electrocatalysts in water splitting for sustainable hydrogen production

The energy transition from fossil fuel-based to renewable energy is a global agenda. At present, a major concern in the green hydrogen economy is the demand for clean fuels and non-noble materials to produce hydrogen through water splitting. Researchers are focusing on addressing this concern with the help of the development of appropriate non-noble-based photo-/electrocatalytic materials. A new class of two-dimensional materials, MXenes, have recently shown tremendous potential for water splitting to produce H2via a photoelectrochemical process. The unique properties of emerging 2D MXene materials, such as hydrophilic surface functionalities, higher surface-to-volume ratios, and inherent flexibility, present these materials as appropriate photo-/electrocatalytic materials. Unique value addition and innovative strategies such as the introduction of end-group modification, heterojunctions, and nanostructure engineering have shown the potential of MXene materials as emerging photo-/electrocatalysts for water splitting. When integrated with conventional noble metal catalysts, MXene-based catalysts demonstrated a lower overpotential for hydrogen and oxygen evolution reactions and a remarkable boost in performance for enhanced H2 production rates surpassing those of pristine noble metal-based catalysts. These promote future perspectives for the utilization of chemically synthesized MXenes as alternative photo-/electrocatalysts. Future research direction should focus on MXene synthesis and utilization for surface modification, composite formation, stabilization, and optimization in synthesis methods and post-synthesis treatments. This review highlights the progress in the understanding of fundamental mechanisms and issues associated with water splitting, influencing factors of MXenes, their value addition role, and application strategies for water splitting, including performance, challenges, and outlook of MXene-based photo-/electrocatalysts, in the last five years.

MXene Contact Engineering for Printed Electronics

MXenes emerging as an amazing class of 2D layered materials, have drawn great attention in the past decade. Recent progress suggest that MXene-based materials have been widely explored as conductive electrodes for printed electronics, including electronic and optoelectronic devices, sensors, and energy storage systems. Here, the critical factors impacting device performance are comprehensively interpreted from the viewpoint of contact engineering, thereby giving a deep understanding of surface microstructures, contact defects, and energy level matching as well as their interaction principles. This review also summarizes the existing challenges of MXene inks and the related printing techniques, aiming at inspiring researchers to develop novel large-area and high-resolution printing integration methods. Moreover, to effectually tune the states of contact interface and meet the urgent demands of printed electronics, the significance of MXene contact engineering in reducing defects, matching energy levels, and regulating performance is highlighted. Finally, the printed electronics constructed by the collaborative combination of the printing process and contact engineering are discussed.

Theoretical Insights into the Hydrogen Evolution Reaction on VGe2N4 and NbGe2N4 Monolayers

Catalytically active sites at the basal plane of two-dimensional monolayers for hydrogen evolution reaction (HER) are important for the mass production of hydrogen. The structural, electronic, and catalytic properties of two-dimensional VGe₂N₄ and NbGe₂N₄ monolayers are demonstrated using the first-principles calculations. The dynamical stability is confirmed through phonon calculations, followed by computation of the electronic structure employing the hybrid functional HSE06 and PBE+U. Here, we introduced two strategies, strain and doping, to tune their catalytic properties toward HER. Our results show that the HER activity of VGe₂N₄ and NbGe₂N₄ monolayers are sensitive to the applied strain. A 3% tensile strain results in the adsorption Gibbs free energy (ΔG _H*) of hydrogen for the NbGe₂N₄ monolayer of 0.015 eV, indicating better activity than Pt (-0.09 eV). At the compressive strain of 3%, the ΔG _H* value is -0.09 eV for the VGe₂N₄ monolayer, which is comparable to that of Pt. The exchange current density for the P doping at the N site of the NbGe₂N₄ monolayer makes it a promising electrocatalyst for HER (ΔG _H* = 0.11 eV). Our findings imply the great potential of the VGe₂N₄ and NbGe₂N₄ monolayers as electrocatalysts for HER activity.

Oxygen-Terminated Nb2CO2 MXene with Interfacial Self-Assembled COF as a Bifunctional Catalyst for Durable Zinc-Air Batteries

The desirable air cathode in Zn-air batteries (ZABs) that can effectively balance oxygen evolution and oxygen reduction reactions not only needs to adjust the electronic structure of the catalyst but also needs a unique physical structure to cope with the complex gas-liquid environment. In this work, first-principles calculations were carried out to prove that oxygen-terminated Nb₂CO₂ MXene played an active role in enhancing the sluggish reaction of oxygen intermediates. Nb₂CO₂ MXene could also stimulate the spatial accumulation of discharge products, which was beneficial to improve the stability of secondary ZABs. Molecular dynamics simulation was used to show that the confinement effect of COF could effectively regulate the concentration of O₂ on the surface of Nb₂CO₂@COF, which was conducive to an efficient and durable reaction. COF-LZU1 was self-assembled on the interface of Nb₂CO₂ MXene (Nb₂CO₂@COF) for the first time. The Nb₂CO₂@COF electrode had excellent OER/ORR overpotentials with the potential difference (ΔE) of 0.79 V. When applied to the configuration of ZABs, Nb₂CO₂@COF showed a power density of 75 mW cm^-2 and favorable long-term charge/discharge stability, so it could be used as a potential candidate cathode for noble-metal-based catalysts. This idea of combining MXenes and COFs sheds some light on the design of ZABs.

Boosting the photocatalytic H2 evolution activity of type-II g-GaN/Sc2CO2 van der Waals heterostructure using applied biaxial strain and external electric field

Two-dimensional (2D) van der Waals (vdW) heterostructures are a new class of materials with highly tunable bandgap transition type, bandgap energy and band alignment. Herein, we have designed a novel 2D g-GaN/Sc2CO2 heterostructure as a potential solar-driven photocatalyst for the water splitting process and investigate its catalytic stability, interfacial interactions, and optical and electronic properties, as well as the effects of applying an electric field and biaxial strain using first-principles calculation. The calculated lattice mismatch and binding energy showed that g-GaN and Sc2CO2 are in contact and may form a stable vdW heterostructure. Ab initio molecular dynamics and phonon dispersion simulations show thermal and dynamic stability. g-GaN/Sc2CO2 has an indirect bandgap energy with appropriate type-II band alignment relative to the water redox potentials. Meanwhile, the interfacial charge transfer from g-GaN to Sc2CO2 can effectively separate electron-hole pairs. Moreover, a potential drop of 3.78 eV is observed across the interface, inducing a built-in electric field pointing from g-GaN to Sc2CO2. The heterostructure shows improved visible-light optical absorption compared to the isolated g-GaN and Sc2CO2 monolayers. Our study demonstrates that tunable electronic and structural properties can be realised in the g-GaN/Sc2CO2 heterostructure by varying the electric field and biaxial strain. In particular, the compressive strain and negative electric field are more effective for promoting hydrogen production performance. Since it is challenging to tune the electric field and biaxial strain experimentally, our research provides strategies to boost the performance of MXene-based heterojunction photocatalysts in solar harvesting and optoelectronic devices.

Hierarchical MXene/transition metal chalcogenide heterostructures for electrochemical energy storage and conversion

MXenes have gained rapidly increasing attention owing to their two-dimensional (2D) layered structures and unique mechanical and physicochemical properties. However, MXenes have some intrinsic limitations (e.g., the restacking tendency of the 2D structure) that hinder their practical applications. Transition metal chalcogenide (TMC) materials such as SnS, NiS, MoS₂, FeS₂, and NiSe₂ have attracted much interest for energy storage and conversion by virture of their earth-abundance, low costs, moderate overpotentials, and unique layered structures. Nonetheless, the intrinsic poor electronic conductivity and huge volume change of TMC materials during the alkali metal-ion intercalation/deintercalation process cause fast capacity fading and poor-rate and poor-cycling performances. Constructing heterostructures based on metallic conductive MXenes and highly electrochemically active TMCs is a promising and effective strategy to solve these problems and enhance the electrochemical performances. This review highlights and discusses the recent research development of MXenes and hierarchical MXene/TMC heterostructures, with a focus on the synthesis strategies, surface/heterointerface engineering, and potential applications for lithium-ion batteries, sodium-ion batteries, lithium-sulfur batteries, supercapacitors, electrocatalysis, and photocatalysis. The critical challenges and perspectives of the future development of MXenes and hierarchical MXene/TMC heterostructures for electrochemical energy storage and conversion are forecasted.