This review addresses the recent developments and progress in the synthesis, structure and properties of MXenes, as well as their energy conversion and storage and related applications.

Highly efficient and low-cost electrocatalysts are essential for water spitting via electrolysis in an economically viable fashion. However, the best catalytic performance is found with noble metal-based electrocatalysts, which presents a formidable obstacle for the commercial success of electrolytic water splitting-based H₂ production due to their relatively high cost and scarcity. Therefore, the development of alternative inexpensive earth-abundant electrode materials with excellent electrocatalytic properties is of great urgency. In general, efficient electrocatalysts must possess several key characteristics such as low overpotential, good electrocatalytic activity, high stability, and low production costs. Direct synthesis of nanostructured catalysts on a conducting substrate may potentially improve the performance of the resultant electrocatalysts because of their high catalytic surface areas and the synergistic effect between the electrocatalyst and the conductive substrate. In this regard, three dimensional (3D) nickel foams have been advantageously utilized as electrode substrates as they offer a large active surface area and a highly conductive continuous porous 3D network. In this review, we discuss the most recent developments in nanostructured materials directly synthesized on 3D nickel foam as potential electrode candidates for electrochemical water electrolysis, namely, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). We also provide perspectives and outlooks for catalysts grown directly on 3D conducting substrates for future sustainable energy technologies.

Various two-dimensional (2D) materials have demonstrated unique structure-dependent characteristics that are conducive to energy-harvesting applications. Among them, the family of layered MXenes has found a wide range of applications in batteries, supercapacitors, photo- and electrocatalysis, water purification, biosensors, electromagnetic interference shielding, structural composites, etc., owing to their well-defined structure, large surface area, large interlayer distance, and excellent thermal and electrical conductivity. However, layer restacking due to hydrogen bonding or van der Waals forces between the layers considerably impedes the utility of MXenes. To tackle the restacking issues, transition metal dichalcogenides (TMDs) such as MoS₂, WS₂, and MoSe₂ nanosheets have been uniformly dispersed on the surface of MXenes, which not only mitigates the restacking of the MXenes but also improves the electrochemical performance due to the synergistic interaction between MXenes and TMDs. This review describes recent advances in the synthesis of MXene/TMD heterostructures and the nature of the synergistic interactions between TMDs and MXenes in energy-related applications. We further highlight future research directions for MXene/TMD-based materials for energy storage applications.

Human urine, otherwise potentially polluting waste, is an universal unused resource in organic form disposed by the human body. We present for the first time “proof of concept” of a convenient, perhaps economically beneficial and innovative template-free route to synthesize highly porous carbon containing heteroatoms such as N, S, Si and P from human urine waste as a single precursor for carbon and multiple heteroatoms. High porosity is created through removal of inherently-present salt particles in as-prepared “Urine Carbon” (URC) and multiple heteroatoms are naturally doped into the carbon, making it unnecessary to employ troublesome expensive pore-generating templates as well as extra costly heteroatom-containing organic precursors. Additionally, isolation of rock salts is an extra bonus of present work. The technique is simple, but successful, offering naturally doped conductive hierarchical porous URC, which leads to superior electrocatalytic ORR activity comparable to state of the art Pt/C catalyst along with much improved durability and methanol tolerance, demonstrating that the URC can be a promising alternative to costly Pt-based electrocatalyst for ORR. The ORR activity can be addressed in terms of heteroatom doping, surface properties and electrical conductivity of the carbon framework.