Porous 3D carbon-based materials:An emerging platform for efficient hydrogen production

Due to their unique properties and uninterrupted breakthrough in a myriad of clean energy-related applications,carbon-based materials have received great interest.However,the low selectivity and poor conductivity are two primary difficulties of traditional carbon-based materials(zero-dimensional(0D)/one-dimensional(1D)/two-dimensional(2D)),enerating inefficient hydrogen production and impeding the future commercialization of carbon-based materials.To improve hydrogen production,attempts are made to enlarge the surface area of porous three-dimensional(3D)carbon-based materials,achieve uniform interconnected porous channels,and enhance their stability,especially under extreme conditions.In this review,the structural advantages and performance improvements of porous carbon nanotubes(CNTs),g-C_(3)N_(4),covalent organic frameworks(COFs),metal-organic frameworks(MOFs),MXenes,and biomass-derived carbon-based materials are firstly summarized,followed by discussing the mechanisms involved and assessing the performance of the main hydrogen production methods.These include,for example,photo/electrocatalytic hydrogen production,release from methanolysis of sodium borohydride,methane decomposition,and pyrolysis-gasification.The role that the active sites of porous carbon-based materials play in promoting charge transport,and enhancing electrical conductivity and stability,in a hydrogen production process is discussed.The current challenges and future directions are also discussed to provide guidelines for the development of next-generation high-efficiency hydrogen 3D porous carbon-based materials prospected.

Carbonitride MXene Ti_(3)CN(OH)_(x)@MoS_(2)hybrids as efficient electrocatalyst for enhanced hydrogen evolution

Renewable energy powered electrocatalytic water splitting is a promising strategy for hydrogen generation,and the design and development of high-efficiency and earth-abundant electrocatalysts for hydrogen evolution reaction(HER)are highly desirable.Herein,MoS2 nanoflowers decorated two-dimensional carbonitride-based MXene Ti3CN(OH)x hybrids have been constructed by etching and post-hydrothermal methods.The electrochemical performance of the as-obtained Ti_(3)CN(OH)_(x)@MoS_(2)hybrids having a quasi core-shell structure is fascinating:An overpotential of 120 mV and a Tafel slope of 64 mV∙dec^(−1)can be delivered at a current density of 10 mA∙cm^(−2).And after 3,000 cyclic voltammetry cycles,it can be seen that there is no apparent attenuation.Both the experimental results and density functional theory(DFT)calculations indicate that the synergetic effects between Ti_(3)CN(OH)x and MoS_(2)are responsible for the robust electrochemical HER performance.The electrons of-OH group in Ti_(3)CN(OH)x are transferred to MoS_(2),making the adsorption energy of the composite for H almost vanish.The metallic Ti_(3)CN(OH)x is also beneficial to the fast charge transfer kinetics.The construction of MXene-based hybrids with optimal electronic structure and unique morphology tailored to the applications can be further used in other promising energy storage and conversion devices.

Improving stability of MXenes

Due to their superior hydrophilicity and conductivity,ultra-high volumetric capacitance,and rich surface-chemistry properties,MXenes exhibit unique and excellent performance in catalysis,energy storage,electromagnetic shielding,and life sciences.Since they are derived from ceramics(MAX phase)through etching,one of the challenges in MXenes preparation is the inevitable exposure of metal atoms on their surface and embedding of anions and cations.Because the as-obtained MXenes are always in a thermodynamically metastable state,they tend to react with trace oxygen or oxygen-containing groups to form metal oxides or degrade,leading to sharply declined activity and impaired performance.Therefore,improving the stability of MXenesbased materials is of practical significance in relevant applications.Unfortunately,there lacks a comprehensive review in the literature on relevant topics.To help promote the wide applications of MXenes,we review from the following aspects:(i)insights into the factors affecting the stability of MXenes-based materials,including oxidation of MXenes flakes,stability of MXenes colloidal solutions,and swelling and degradation of MXenes thin-film,(ii)strategies for enhancing the stability of MXenes-based materials by optimizing MAX phase synthesis and modifying the MXenes preparation,and(iii)techniques for further increasing the stability of freshly prepared MXenes-based materials via controlling the storage conditions,and forming shielding on the surface and/or edge of MXenes flakes.Finally,some outlooks are proposed on the future developments and challenges of highly active and stable MXenes.We aim to provide guidance for the design,preparation,and applications of MXenes-based materials with excellent stability and activity.

Three-dimensional MXenes heterostructures and their applications

Due to their unique surface chemistry,highly adjustable metal components,hydrophilicity,and high carrier concentrations,MXenes are applied in a variety of scenarios.Similar to other two-dimensional(2D)materials,building heterostructures with additional materials to form a 3D porous architecture for MXenes can significantly enhance their functionality and reactivity.Notably,the open structures and well-defined pathways of these 3D structured MXenes can improve ionic and electronic transport,thereby promoting their applications in electrochemical energy storage,sensing,catalysis,and environment.In this review,the recent efforts made on preparing 3D porous MXenes with heterostructures,focusing on MXenes/C,MXenes/inorganics,and MXenes/polymers were summarized.The discussion covers aspects ranging from the design to synthesis of 3D porous MXenes,and their applications in photocatalysis,environmental monitoring and electrochemical energy storage.This review is concluded by presenting the prospects and insights on exploring the relationships between the porosity formation mechanisms,properties and applications of the 3D porous MXenes heterostructures.This review can provide meaningful guidance for the design,fabrication and application of 3D porous MXenes in high-performance materials and devices.