2D metals, metallenes, feature exciting opportunities at the forefront of electrocatalysis. We bring to attention metallene preparation techniques and modification strategies for the derivation of highly functional metallenes in key electrocatalytic applications.

Transition metal dichalcogenides (TMDs) are promising materials for use in electrocatalytic and electrochemical energy-storage systems owing to their exceptional physicochemical properties, including large surface area, remarkable mechanical properties, high catalytic activity, chemical stability, and low cost. In further improving material properties tailored to meet application-specific requirements, heterostructure construction holds significant advantages, benefiting from the synergistic effect between constituents involved. TMDbased heterostructures have been widely explored recently, giving rise to diverse materials with desirable characteristics such as significantly increased interfacial contact of low resistance for efficient electron transfers, constituent-dependent electronic structure, tunable layer distances facilitating easily intercalation of redox species, and increased surface area for effective interaction with electrolyte. In this review, TMD-based heterostructures are assessed for performance in electrocatalytic conversion (hydrogen evolution reaction) and electrochemical energy-storage systems (NiB/LiB/supercapacitors). The impactful strategies employed in overcoming key challenges are evaluated, and finally, future directions for TMD-based heterostructure construction are presented.

Heterostructured catalysts are hybrid materials that contain interfaces between their constituents formed through combinations of multiple solid‐state materials. The presence of multiple constituents institutes a synergistic effect that endows the catalyst with superior performance and appreciable potential in a diverse range of catalytic applications, including electrocatalytic and photocatalytic reduction of carbon dioxide. These promising catalysts can support a feasible method for large‐scale processing of valuable carbonaceous feedstock or fuel generation and alleviation of atmospheric carbon dioxide levels. Such technologies will serve as the much‐needed remedy for the global energy and environmental crisis. A broad spectrum of recently developed heterostructured catalysts pertaining to electrocatalytic and photocatalytic carbon dioxide reduction is evaluated. The insights included are of relevance to refresh fundamentals pertaining to the electron transfer processes leading to carbon dioxide reduction and the mechanistic reduction pathways yielding a possible multitude of carbonaceous products. Detailed discussions provide a rational understanding of how the hybrid and resultant properties from various combinations are useful in enhancing catalytic function. Lastly, the performance profiles of various catalyst structures together with modification strategies employed are of interest to highlight the current challenges to and directions for future catalyst development.

via the overall reaction of H 2 with O 2 , only giving rise to water without pollution. Beyond that, hydrogen fuel cells have also high safety, quick-refueling, fast-charging, power grid compatibility, and high energy conversion efficiency (40-70%) even restricted by polarization in practical conditions, which make them potential for long-range and high-utilization transportation as electric vehicles. [2] The energy conversion efficiency mainly depends on the cathodic oxygen reduction reaction (ORR) due to its naturally complex and sluggish kinetics as compared to the anodic hydrogen oxidation reduction during electron transfer. [3] It is, as such, highly desirable to employ efficient electrocatalysts, capable in alleviating overpotentials of electrocatalytic ORR while also, promoting the slow transformation process. Currently, the expensive platinum (Pt) electrocatalysts still exhibit unparalleled catalytic performance for ORR; [4] however, the scarcity of resources, sluggish ORR kinetics, inferior operational stability, and susceptibility to poisoning of the Pt catalysts result in high cost, which remains to be one of key factors that hamper commercial applications of fuel cells. [1a,5] At this juncture, the replacement of costly Pt-based catalysts with other cost-effective, high-performing and durable nonprecious metal catalysts (NPMCs) for ORR is urgently needed. [6] In pursuit of a diverse class NPMCs, M/N codoped M-N-C (M = Fe, Co, Ni, etc.) especially Fe-N-C catalysts, widely prepared by annealing a compound of carbon carriers, nitrogen-containing precursors and transition metal salts or without carbon supports under inert or reactive gas, attract substantial interest due to comparable activity with that of benchmark Pt/C for ORR in alkaline media. [7] However, their volumetric activity is still far from that of Pt/C. Furthermore, their stability remains poor in acid because of the dissolution of Fe-based species associated with active sites. [4b,8] As such, Fe-N-C catalysts demonstrate low competency for proton exchange membrane fuel cell (PEMFC) application at present. It is greatly challenging but yet, extremely desirable to exploit novel NPMCs having outstanding activity and durability in ORR. Elucidation of the active site nature and pursuit of unprecedented high ORR activity and stability motivates design of unique NPMCs. Recently, transition-metal carbides, such as Fe 3 C, Fuel cells represent the most suitable energy conversion, capable of addressing energy crises and environmental pollution. Recently, as one of nonprecious metal catalysts (NPMCs), the MC@N-C (M = Fe, Co, Ni, Mo, W) catalysts, especially for Fe 3 C encased in carbon layer (denoted as Fe 3 C@N-C) have emerged as promising replacements for costly Pt-based catalysts for oxygen reduction reaction (ORR). This review highlights the synthetic strategies undertaken such as hard template, soft template, and template-free methods for deriving enhancements in electrocatalytic activity and durability. It also provides a comparison on the synth...

renewable electricity into hydrogen fuel, while the latter plays an indispensable role in eco-friendly fuel cells and metalair batteries. [2] However, the fundamental electrode reactions that lie at the heart of these technologies, including the hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), and oxygen evolution reaction (OER), are inherently slow. Thus, high-performance electrocatalysts are inevitably required to accelerate the reaction kinetics, enhance the Faradaic efficiency, and minimize the Ohmic losses. Extensive research has revealed numerous electrocatalysts that drive these critical reactions. One such group of electrocatalysts-Platinum group metals (PGMs)-based materialsis extensively recognized for their optimal functionality, close-to-zero overpotentials, and excellent intrinsic activities. [3] Nevertheless, the high cost associated with using the resource-scarce PGMs significantly hinders the large-scale implementation of the technologies. [4] Therefore, it is crucial to improve the efficiency of PGM-based electrocatalysts via optimizing the intrinsic activity, maximizing atomic utilization, and enhancing operational stability. Furthermore, developing an electrocatalyst with multifunctionality is highly desirable to Developing highly efficient multifunctional electrocatalysts is crucial for future sustainable energy pursuits, but remains a great challenge. Herein, a facile synthetic strategy is used to confine atomically thin Pd-PdO nanodomains to amorphous Ru metallene oxide (RuO 2 ). The as-synthesized electrocatalyst (Pd 2 RuOx-0.5 h) exhibits excellent catalytic activity toward the pH-universal hydrogen evolution reaction (η 10 = 14 mV in 1 m KOH, η 10 = 12 mV in 0.5 m H 2 SO 4 , and η 10 = 22 mV in 1 m PBS), alkaline oxygen evolution reaction (η 10 = 225 mV), and overall water splitting (E 10 = 1.49 V) with high mass activity and operational stability. Further reduction endows the material (Pd 2 RuOx-2 h) with a promising alkaline oxygen reduction activity, evidenced by high halfway potential, four-electron selectivity, and excellent poison tolerance. The enhanced catalytic activity is attributed to the rational integration of favorable nanostructures, including 1) the atomically thin nanosheet morphology, 2) the coexisting amorphous and defective crystalline phases, and 3) the multi-component heterostructural features. These structural factors effectively regulate the material's electronic configuration and the adsorption of intermediates at the active sites for favorable reaction energetics.

No abstract

Designing efficient bifunctional electrocatalysts with excellent activity and robust stability presents a central challenge for the large‐scale commercialization of water electrolysis. Herein, a facile approach is reported for the construct of atomically thin amorphous RuM (MCo, Fe, or Ni) bimetallenes as high‐performance electrocatalysts toward both electrochemical hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). The RuCo bimetallene manifests excellent bifunctional activity characterized by low required overpotentials, superior price activity, robust electrochemical durability as well as a low cell potential water splitting performance, outperforming Pt/C and RuO2 benchmark catalysts. Combined operando X‐ray absorption spectroscopy  investigation and theoretical simulations reveal the synergism taking place between binary constituents, in which Co serves a promotive role along the HER/OER reaction pathway, contributing via optimal binding to *OH for facile water dissociation as well as modulating the Ru electronic structure favorably, hence rendering high activity catalytic centers for both the alkaline HER and OER.

Highly efficient and durable electrocatalysts are of the utmost importance for the sustainable generation of clean hydrogen by water electrolysis. Here, we present a report of an atomically thin rhodium metallene incorporated with oxygenbridged single atomic tungsten (Rh−O−W) as a high-performance electrocatalyst for pH-universal hydrogen evolution reaction. The Rh−O−W metallene delivers ascendant electrocatalytic HER performance, characterized by exceptionally low overpotentials, ultrahigh mass activities, excellent turnover frequencies, and robust stability with negligible deactivation, in pH-universal electrolytes, outperforming that of benchmark Pt/ C, Rh/C and numerous other reported precious-metal HER catalysts. Interestingly, the promoting feature of −O−W single atomic sites is understood via operando X-ray absorption spectroscopy characterization and theoretical calculations. On account of electron transfer and equilibration processes take place between the binary components of Rh−O−W metallenes, fine-tuning of the density of states and electron localization at Rh active sites is attained, hence promoting HER via a near-optimal hydrogen adsorption.