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.