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.

provides a promising alternative to hydrocarbon sources for petrochemical feedstock. Thanks to the electrochemical nature of the CO 2 conversion to fuels and chemicals, the electrical energy invested to convert CO 2 in the form of a chemical fuel can be stored and redistributed using established supply chains for future use. Additionally, the integration of renewable energy systems (green electricity) into the grid can potentially create a carbon neutral energy cycle, and when driven by solar energy, offers a completely renewable energy cycle or negative carbon technology. Converting CO 2 electrochemically into compounds with high energy densities, such as alcohols (methanol, ethanol), formates, and CO, represents a form of energy storage and is also adaptable to demand response or energy arbitrage technologies. [3][4][5][6][7] The recoverable energy density of the chemicals that can be converted from CO 2 is substantially higher than most battery technologies.Given CO 2 electroreduction's immense economic and environmental potential, it has been the subject of much research activity over the past decades. [8][9][10][11] One of the greatest challenges of reducing CO 2 in an electrochemical cell is overcoming the immense energy barrier required to do so; the single electron reduction of CO 2 to CO 2 −˙, a common step in many CO 2 reduction mechanisms, requires an applied potential of −1.97 V measured versus standard hydrogen electrode (SHE). To alleviate this problem, many catalytic cathodes have been developed to avoid this intermediate and to reroute the reduction mechanism through alternate pathways requiring much lower applied potentials (a necessity if CO 2 electroreduction is to be implemented at industrial scale). [12][13][14] However, despite the low potentials offered by catalytic electrodes, the cost of electrodes is not always viable when upscaled to industrial levels. [15,16] Therefore, recent research trends in electrocatalytic reduction of CO 2 have been shifting toward the development of molecular catalysts that can exist as either solutes in electrolytes or can be surface-confined on electrodes. [13,16] The tunable nature and electronic characteristics of molecular catalysts give access to a large variety of catalysts with high activity, selectivity, and durability, as well as their ability to be integrated into sophisticated nanoassemblies. [17][18][19] Thanks to the aforementioned proprieties, performing CO 2 reduction using molecularly defined compounds offer several advantages compared to classical solid-state counterparts. Molecular catalysts usually exhibit well-defined homogeneous and/or CO 2 reduction using molecular catalysts is a key area of study for achieving electrical-to-chemical energy storage and feedstock chemical synthesis. Compared to classical metallic solid-state catalysts, these molecular catalysts often result in high performance and selectivity, even under unfavorable aqueous environments. This review considers the recent state-of-the-art molecular catalysts for CO 2 electro...

Efficient bifunctional electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are required for metal air batteries, to replace costly metals, such as Pt and Ir/Ru based compounds, which are typically used as benchmarks for ORR and OER, respectively. Isolated single atomic sites coordinated with nitrogen on carbon supports (M‐N‐C) have promising performance for replacement of precious metal catalysts. However, most of monometallic M‐N‐C catalysts demonstrate unsatisfactory bifunctional performance. Herein, a facile way of preparing bimetallic Fe and Co sites entrapped in nitrogen‐doped hollow carbon nanospheres (Fe,Co‐SA/CS) is explored, drawing on the unique structure and pore characteristics of Zeolitic imidazole frameworks and molecular size of Ferrocene, an Fe containing species. Fe,Co‐SA/CS showed an ORR onset potential and half wave potential of 0.96 and 0.86 V, respectively. For OER, (Fe,Co)‐SA/CS attained its anodic current density of 10 mA cm–2 at an overpotential of 360 mV. Interestingly, the oxygen electrode activity (ΔE) for (Fe,Co)‐SA/CS and commercial Pt/C‐RuO2 is calculated to be 0.73 V, exhibiting the bifunctional catalytic activity of (Fe,Co)‐SA/CS. (Fe,Co)‐SA/CS evidenced desirable specific capacity and cyclic stability than Pt/C‐RuO2 mixture when utilized as an air cathode in a homemade Zinc‐air battery.

with thermodynamic oxygen to water conversion potential of 1.23 V (vs reversible hydrogen electrode (RHE)). [1] Especially so in recent years, platinum (Pt)-based catalysts are widely utilized as the state of the art catalyst for ORR. [2] However, they are known to suffer from ineluctable concerns with regards to scarcity, extreme cost, poor stability in long term usage and poisoning effects from methanol crossover. [3,4] The potential of rechargeable metal-air batteries (metal = Zn, Li, Al, Mg, etc.) in future energy applications can be simply revealed from its high theoretical energy density values, in the range of 1086-11 140 Wh kg -1 . [5,6] Metal anodes, such as Zn, are earth abundant, cheap, safe for handling, and environmentally benign, favoring suitability of metal-air batteries for domestic and industrial applications. [7,8] Despite the intensive research being undertaken in the field of Zn-air batteries, major degradation issues remain with regards to the following: a) degradation of cathode materials-catalyst, carbon support and binders and b) morphological degradation of Zn anode during repeated cycles. [9] Thus, in order to realize and leverage on the full potential of metal air batteries, the primary task would be to develop highly efficient and stable air cathodes that favor oxygen chemistry, mainly the ORR.Earth abundant metal-based catalysts, like metal oxides, [10] perovskites, [11] spinel type, [12] MXenes , [13,14] and mixed metal oxides [15,16] have been investigated as ORR catalysts. In addition, heteroatom-doped carbon structures containing metallic elements, like cobalt, nickel, and iron, have emerged as promising candidates to replace Pt. Despite intensive research in metal pnictogens and chalcogens over the years, there is a lack in exploration of metal-metalloids, such as amorphous metal borides for the ORR. Transition metal borides show superior oxygen evolution reaction (OER) performance in alkaline solution compared with noble metal catalysts, metal oxides, and metal alloy counterparts. [17] In contrast, transition metal borides undergo severe oxidation, limiting activities that may be derived and consequently, their ORR performance. [18] To date, only J. Ma et al [19] and K. Elumeeva et al [20] reported the usage of CoB as an ORR electrocatalyst, but the performance obtained was not satisfying with those the state-of-the-art catalysts. CoB nanosheets are expected to be highly beneficial for electrochemical applications (for example, ORR) because 1) the synthesis of metal borides by chemical reduction is fast and facile compared with the time consuming synthesis of many other Compositional and structural engineering of metal-metalloid materials can boost their electrocatalytic performance. Herein, a highly efficient and stable electrocatalytic system for the oxygen reduction reaction is obtained by creating heterointerfaces between N-doped carbon and cobalt boride nanosheets. Furthermore, a detailed investigation on the effect of annealing temperature as well as the amount of carbon ...

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201901503. Oxygen EvolutionElectrochemical energy conversion and storage devices including metal-air batteries, regenerative fuel cells, and watersplitting cells are critical to satisfy the future energy demand of human society. Oxygen evolution reaction (OER) is the key reaction in these technologies and accounts for the major performance loss due to its sluggish kinetics. Precious metal-based catalysts are used predominantly, but the scarcity and low stability limit their application at large scale. [1] As a consequence, intensive efforts have been devoted to developing cost-effective catalysts with superior oxygen-evolving activity and stability. [2] Earth-abundant metal chalcogens, pnictogens, and metalloids have emerged as potential materials for water oxidation in

This review comprehensively describes the latest synthesis techniques for producing carbon-based hydrogels and their recent energy applications.

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