Over the past few decades, the design and development of advanced electrocatalysts for efficient energy conversion technologies have been subjects of extensive study. With the discovery of graphene, two-dimensional (2D) nanomaterials have emerged as some of the most promising candidates for heterogeneous electrocatalysts due to their unique physical, chemical, and electronic properties. Here, we review 2D-nanomaterial-based electrocatalysts for selected electrocatalytic processes. We first discuss the unique advances in 2D electrocatalysts based on different compositions and functions followed by specific design principles. Following this overview, we discuss various 2D electrocatalysts for electrocatalytic processes involved in the water cycle, carbon cycle, and nitrogen cycle from their fundamental conception to their functional application. We place a significant emphasis on different engineering strategies for 2D nanomaterials and the influence these strategies have on intrinsic material performance, such as electronic properties and adsorption energetics. Finally, we feature the opportunities and challenges ahead for 2D nanomaterials as efficient electrocatalysts. By considering theoretical calculations, surface characterization, and electrochemical tests, we describe the fundamental relationships between electronic structure, adsorption energy, and apparent activity for a wide variety of 2D electrocatalysts with the goal of providing a better understanding of these emerging nanomaterials at the atomic level.

This perspective highlights the rational design of efficient electrocatalysts and photo(electro)catalysts for N2 reduction to ammonia (NH3) under ambient conditions.

The hydrogen evolution reaction (HER) is a fundamental process in electrocatalysis and plays an important role in energy conversion for the development of hydrogen-based energy sources. However, the considerably slow rate of the HER in alkaline conditions has hindered advances in water splitting techniques for high-purity hydrogen production. Differing from well documented acidic HER, the mechanistic aspects of alkaline HER are yet to be settled. A critical appraisal of alkaline HER electrocatalysis is presented, with a special emphasis on the connection between fundamental surface electrochemistry on single-crystal models and the derived molecular design principle for real-world electrocatalysts. By presenting some typical examples across theoretical calculations, surface characterization, and electrochemical experiments, we try to address some key ongoing debates to deliver a better understanding of alkaline HER at the atomic level.

Organometallic complexes with metal-nitrogen/carbon (M-N/C) coordination are the most important alternatives to precious metal catalysts for oxygen reduction and evolution reactions (ORR and OER) in energy conversion devices. Here, we designed and developed a range of molecule-level graphitic carbon nitride (g-CN) coordinated transition metals (M-CN) as a new generation of M-N/C catalysts for these oxygen electrode reactions. As a proof-of-concept example, we conducted theoretical evaluation and experimental validation on a cobalt-CN catalyst with a desired molecular configuration, which possesses comparable electrocatalytic activity to that of precious metal benchmarks for the ORR and OER in alkaline media. The correlation of experimental and computational results confirms that this high activity originates from the precise M-N coordination in the g-CN matrix. Moreover, the reversible ORR/OER activity trend for a wide variety of M-CN complexes has been constructed to provide guidance for the molecular design of this promising class of catalysts.

Electrochemical reduction of CO 2 to highenergy-density oxygenates and hydrocarbons beyond CO is important for long-term and large-scale renewable energy storage. However, the key step of the C−C bond formation needed for the generation of C 2 products induces an additional barrier on the reaction. This inevitably creates larger overpotentials and greater variety of products as compared to the conversion of CO 2 to C 1 products. Therefore, an in-depth understanding of the catalytic mechanism is required for advancing the design of efficient electrocatalysts to control the reaction pathway to the desired products. Herein, we present a critical appraisal of reduction of CO 2 to C 2 products focusing on the connection between the fundamentals of reaction and efficient electrocatalysts. An in-depth discussion of the mechanistic aspects of various C 2 reaction pathways on copper-based catalysts is presented together with consideration of practical factors under electrocatalytic operating conditions. By providing some typical examples illustrating the benefit of merging theoretical calculations, surface characterization, and electrochemical measurements, we try to address the key issues of the ongoing debate toward better understanding electrochemical reduction of CO 2 at the atomic level and envisioning the roadmap for C 2 products generation.

The electrochemical CO 2 reduction reaction (CO 2 RR) can couple carbon-capture storage with renewable energy to convert CO 2 into chemical feedstocks. For this process, copper is the only metal known to catalyze the CO 2 RR to hydrocarbons with adequate efficiency, but it suffers from poor selectivity. Copper bimetallic materials have recently shown an improvement in CO 2 RR selectivity compared with that of copper, such that the secondary metal is likely to play an important role in altering inherent adsorption energetics. This review explores the fundamental role of the secondary metal with a focus on how oxygen (O) and hydrogen (H) affinity affect selectivity in bimetallic electrocatalysts. Here, we identify four metal groups categorized by O and H affinities to determine their CO 2 RR selectivity trends. By considering experimental and computational studies, we link the effects of extrinsic chemical composition and physical structure to intrinsic intermediate adsorption and reaction pathway selection. After this, we summarize some general trends and propose design strategies for future electrocatalysts.

great potential to meet our ever-growing demand for energy. These devices appear most promising due to their high energyconversion and storage efficiencies, portability, which can mitigate the intermittent distribution of the aforementioned energy in space and time, and environmental benignancy. [6][7][8][9][10] Fundamentally, these electrochemical systems or devices involve different energy-conversion reactions, such as the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER) and electrochemical CO 2 reduction (ECR), which provide energy by directly converting chemical energy (e.g., methanol, ethanol, zinc, and hydrogen) into electrical energy, or store energy vice versa. [6][7][8][9][10][11][12] The intrinsically sluggish kinetics of these reactions highlights the critical role of electrocatalysts. In this regard, the development of advanced electrocatalysts has always been the technological bottleneck that hinders the practical applications of these energy techniques at any appreciable scale. [7][8][9][10][11][12] Currently, noble-metal-based materials (e.g., Pt, IrO 2 , RuO 2 , and Au) are considered to be state-of-the-art catalysts for a variety of energy devices, [13][14][15][16] such as proton exchange membrane fuel cells (PEMFCs). [13] In spite of their outstanding catalytic performance for many electrochemical reactions, the high cost and scarcity of these noble metals severely hamper their large-scale commercialization. [17] Further, precious metal catalysts face issues with poisoning when exposed to a range of chemical compounds like methaol and carbon monoxide, leading to significant activity loss. [18] Therefore, extensive studies have focused on the development of alternative electrocatalysts with high activity, long stability, low cost, widespread availability, and facile synthesis. [8][9][10]19,20] To replace precious-metal-based catalysts, a wide range of non-noble-metal materials, especially transition-metal-based materials and metal-free carbon materials, have been intensively investigated as electrocatalysts for different applications. [21] Most of these electrocatalyst candidates show great promise in energy-conversion reactions. Nevertheless, very few alternative materials have yet to achieve superior electrochemical properties to those of noble-metal-based catalysts. Fortunately, the accumulation of experimental and theoretical studies have significantly advanced our knowledge on the chemical and physical nature of various candidate materials. [10][11][12] For example, our group has shed light on the activity origin of The key challenge to developing renewable and clean energy technologies lies in the rational design and synthesis of efficient and earth-abundant catalysts for a wide variety of electrochemical reactions. This review presents materials design strategies for constructing improved electrocatalysts based on MOF precursors/templates, with special emphasis on component manipulation, morphology control, and structure engineering. G...

efforts have been devoted to mimicking the successful material strategies used for LIBs. This is especially the case regarding suitable electrode materials that enable reversible Na + intercalation and deintercalation. [1,6] In this regard, a series of promising SIB electrode materials has been discovered in the past decade. [7][8][9][10][11][12][13][14] In terms of anode materials, however, the standard graphite anodes in LIBs exhibit extremely limited sodium storage properties. [15,16] Because Na + (1.16 Å) has larger ionic radius than Li + (0.90 Å), [17] which greatly hinders its reversible insertion into and extraction from conventional graphite anodes. Other non-carbon candidates, including transition metal compounds (e.g., oxides, [18] sulfides, [19] and phosphides [20] ) and alloys (e.g., Sn [21] and Sb [22] ) usually suffer from poor cycling durability because of their low intrinsic conductivity and significant volume change, [23] making them being far from real application in SIBs. Therefore, it is very urgent to develop efficient anode materials for advancing SIB systems.Although traditional graphite fails to provide sufficient interlayer spacing (≈0.34 nm) to host larger Na + , [24,25] both experimental results and theoretical calculations show that the energy barrier for Na + insertion into graphite layers can be reduced to a surmountable level once the interlayer distance is increased to 0.37 nm. [24] Accordingly, a rational material design strategy lies in enlarging the carbon interlayer distance to unlock its Na + storage capability. [24,26] To this end, different carbon materials with expanded interlayer spacing have emerged as improved anodes for SIBs. [13,[26][27][28][29] For instance, Wen et al. prepared expanded graphite with an enlarged interlayer distance of 0.43 nm by introducing oxygen-containing functional groups via a two-step oxidation-reduction treatment of graphite. [27] The as-prepared expanded graphite exhibited high reversible capacity, but its rate capability needed to be improved. Notably, among the reported carbon materials, heteroatom (e.g., N, P, and S) doping has also been demonstrated as one of the most effective ways to expand the carbon interlayer distance. [26,28,29] As an example, Hou and coworkers reported large-area carbon nanosheets doped with phosphorus, which achieved an expanded interlayer spacing of 0.42 nm, allowing for highly reversible Na + storage with excellent rate performance and Heteroatom-doped carbon materials with expanded interlayer distance have been widely studied as anodes for sodium-ion batteries (SIBs). However, it remains unexplored to further enlarge the interlayer spacing and reveal the influence of heteroatom doping on carbon nanostructures for developing more efficient SIB anode materials. Here, a series of N-rich few-layer graphene (N-FLG) with tuneable interlayer distance ranging from 0.45 to 0.51 nm is successfully synthesized by annealing graphitic carbon nitride (g-C 3 N 4 ) under zinc catalysis and selected temperature (T = 700, 800, an...