A fundamental change has been achieved in understanding surface electrochemistry due to the profound knowledge of the nature of electrocatalytic processes accumulated over the past several decades and to the recent technological advances in spectroscopy and high resolution imaging. Nowadays one can preferably design electrocatalysts based on the deep theoretical knowledge of electronic structures, via computer-guided engineering of the surface and (electro)chemical properties of materials, followed by the synthesis of practical materials with high performance for specific reactions. This review provides insights into both theoretical and experimental electrochemistry toward a better understanding of a series of key clean energy conversion reactions including oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). The emphasis of this review is on the origin of the electrocatalytic activity of nanostructured catalysts toward the aforementioned reactions by correlating the apparent electrode performance with their intrinsic electrochemical properties. Also, a rational design of electrocatalysts is proposed starting from the most fundamental aspects of the electronic structure engineering to a more practical level of nanotechnological fabrication.

Design of electrocatalysts for oxygen- and hydrogen-involving energy conversion reactions

Since the discovery of mechanically exfoliated graphene in 2004, research on ultrathin two-dimensional (2D) nanomaterials has grown exponentially in the fields of condensed matter physics, material science, chemistry, and nanotechnology. Highlighting their compelling physical, chemical, electronic, and optical properties, as well as their various potential applications, in this Review, we summarize the state-of-art progress on the ultrathin 2D nanomaterials with a particular emphasis on their recent advances. First, we introduce the unique advances on ultrathin 2D nanomaterials, followed by the description of their composition and crystal structures. The assortments of their synthetic methods are then summarized, including insights on their advantages and limitations, alongside some recommendations on suitable characterization techniques. We also discuss in detail the utilization of these ultrathin 2D nanomaterials for wide ranges of potential applications among the electronics/optoelectronics, electrocat...

Recent Advances in Ultrathin Two-Dimensional Nanomaterials

Heteroatom doping can endow graphene with various new or improved electromagnetic, physicochemical, optical, and structural properties. This greatly extends the arsenal of graphene materials and their potential for a spectrum of applications. Considering the latest developments, we comprehensively and critically discuss the syntheses, properties and emerging applications of the growing family of heteroatom-doped graphene materials. The advantages, disadvantages, and preferential doping features of current synthesis approaches are compared, aiming to provide clues for developing new and controllable synthetic routes. We emphasize the distinct properties resulting from various dopants, different doping levels and configurations, and synergistic effects from co-dopants, hoping to assist a better understanding of doped graphene materials. The mechanisms underlying their advantageous uses for energy storage, energy conversion, sensing, and gas storage are highlighted, aiming to stimulate more competent applications.

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Heteroatom-doped graphene materials: syntheses, properties and applications

Notably, many significant breakthroughs for a new generation of supercapacitors have been reported in recent years, related to theoretical understanding, material synthesis and device designs. Herein, we summarize the state-of-the-art progress toward mechanisms, new materials, and novel device designs for supercapacitors. Firstly, fundamental understanding of the mechanism is mainly focused on the relationship between the structural properties of electrode materials and their electrochemical performances based on some in situ characterization techniques and simulations. Secondly, some emerging electrode materials are discussed, including metal–organic frameworks (MOFs), covalent organic frameworks (COFs), MXenes, metal nitrides, black phosphorus, LaMnO3, and RbAg4I5/graphite. Thirdly, the device innovations for the next generation of supercapacitors are provided successively, mainly emphasizing flow supercapacitors, alternating current (AC) line-filtering supercapacitors, redox electrolyte enhanced supercapacitors, metal ion hybrid supercapacitors, micro-supercapacitors (fiber, plane and three-dimensional) and multifunctional supercapacitors including electrochromic supercapacitors, self-healing supercapacitors, piezoelectric supercapacitors, shape-memory supercapacitors, thermal self-protective supercapacitors, thermal self-charging supercapacitors, and photo self-charging supercapacitors. Finally, the future developments and key technical challenges are highlighted regarding further research in this thriving field.

Latest advances in supercapacitors: from new electrode materials to novel device designs.

The stability of two-dimensional (2D) layers and membranes is subject of a long standing theoretical debate. According to the so called Mermin-Wagner theorem, long wavelength fluctuations destroy the long-range order for 2D crystals. Similarly, 2D membranes embedded in a 3D space have a tendency to be crumpled. These dangerous fluctuations can, however, be suppressed by anharmonic coupling between bending and stretching modes making that a two-dimensional membrane can exist but should present strong height fluctuations. The discovery of graphene, the first truly 2D crystal and the recent experimental observation of ripples in freely hanging graphene makes these issues especially important. Beside the academic interest, understanding the mechanisms of stability of graphene is crucial for understanding electronic transport in this material that is attracting so much interest for its unusual Dirac spectrum and electronic properties. Here we address the nature of these height fluctuations by means of straightforward atomistic Monte Carlo simulations based on a very accurate many-body interatomic potential for carbon. We find that ripples spontaneously appear due to thermal fluctuations with a size distribution peaked around 70 \AA which is compatible with experimental findings (50-100 \AA) but not with the current understanding of stability of flexible membranes. This unexpected result seems to be due to the multiplicity of chemical bonding in carbon.

Intrinsic ripples in graphene

Graphene has attracted increasing attention in different scientific fields including catalysis. Via modification with foreign metal-free elements such as nitrogen, its unique electronic and spin structure can be changed and these doped graphene sheets have been successfully employed in some catalytic reactions recently, showing them to be promising catalysts for a wide range of reactions. In this review, we summarize the recent advancements of these new and interesting catalysts, with an emphasis on the universal origin of their catalytic mechanisms. We are full of hope for future developments, such as more precisely controlled doping methods, atom-scale surface characterization technology, generating more active catalysts via doping, and finding wide applications in many different fields.

Doped graphene for metal-free catalysis.

Co3O4 nanoparticles have been prepared by a facile strategy, which involves the thermal decomposition of nanoparticles of cobalt-based Prussian blue analogues at different temperatures. The nanoparticles prepared at 450, 550, 650, 750, and 850 °C exhibited a high discharge capacity of 800, 970, 828, 854, and 651 mAhg–1, respectively, after 30 cycles at a current density of 50 mAg–1. The nanocages produced at 550 °C show the highest lithium storage capacity. It is found that the nanocages display nanosize grains, hollow structure, a porous shell, and large specific surface area. At the temperature higher than 650 °C, the samples with larger grains, better crystallinity, and lower specific surface area can be obtained. It is found that the size, crystallinity, and morphology of nanoparticles have different effects on electrochemical performance. Better crystallinity is able to enhance the initial discharge capacity, while porous structure can reduce the irreversible loss. Therefore, the optimal size, crysta...

Co3O4 Nanocages for High-Performance Anode Material in Lithium-Ion Batteries

The metal-free catalytic reduction of 4-nitrophenol (Nip) to 4-aminophenol (Amp) mediated by N-doped graphene (NG) was reported. Nip could be reduced to Amp completely without any by-product generation. The activity of the NG is comparable with some of the previously reported metallic catalysts. Interestingly, the NG sheet catalyzed reaction follows pseudo-zero-order kinetics, while all the metallic catalysts follow pseudo-first-order kinetics. The in situ FTIR experiment demonstrated that Nip ions will combine with NG via the O atoms of their hydroxyl groups. Theoretical calculations verified this adsorption model, and confirmed that the adsorption of Nip ions is the critical step, leading to the pseudo-zero-order kinetics. Moreover, only the carbon atoms next to the doped N atoms on NG surface can be activated, serving as the active sites. As expected, all four kinds of the doped N atoms are beneficial to the adsorption and activation of Nip, contributing to the catalytic reduction reaction.

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Metal-free catalytic reduction of 4-nitrophenol to 4-aminophenol by N-doped graphene

The great success of graphene has encouraged the fast development of other two-dimensional (2D) nanosheets, which have attracted extensive attention in different scientific fields encompassing field effect transistors, lithium-ion batteries, and catalysis. With atomic-scale thickness, almost all of the atoms are exposed on the surface, providing an extremely high specific surface area, in conjunction with special physical, chemical, and electronic properties, owing to the quantum confinement effects, which enable their surface phase to be as important as the bulk counterparts. In this review, we have summarized and discussed the recent advancements of 2D nanomaterials beyond graphene, with an emphasis on their basic fundamentals, preparation strategies, and applications. We believe that this review supplies critical insights for exploring and understanding 2D nanomaterials and puts forward the challenges and opportunities for further developments, such as more precise morphology control, foreign atom doping and surface modification technologies, atomic-scale characterization, and finding wide applications in many different fields.

Xiangkai Kong

Papers

Doped graphene for metal-free catalysis.

Co3O4 Nanocages for High-Performance Anode Material in Lithium-Ion Batteries

Metal-free catalytic reduction of 4-nitrophenol to 4-aminophenol by N-doped graphene

Elemental two-dimensional nanosheets beyond graphene

Multifunctional Fe3O4@C@Ag hybrid nanoparticles as dual modal imaging probes and near-infrared light-responsive drug delivery platform