Two-dimensional MXenes are promising for various energy-related applications such as energy storage devices and electrocatalysis of water-splitting. MXenes prepared from hydrofluoric (HF) acid etching have been widely reported. Nonetheless, the acute toxicity of HF acid impedes the large-scale fabrication of MXenes and their wide utilization in energy-related applications. It is thus greatly encouraging to explore a more innocuous protocol for MXenes synthesis. Thereby, a universal strategy based on thermal-assisted electrochemical etching route is developed to synthesize MXenes (e.g., Ti2CT x , Cr2CT x , and V2CT x ). Furthermore, the cobalt ion doped MXenes show an exceptionally enhanced capability of hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) activity, demonstrating their multifunctionalities, which is comparable to the commercialized catalysts. Moreover, we successfully exploited our MXenes as cathodes for the novel aqueous rechargeable battery, with proficient retention and excellent electrical output performance. This work paves a nontoxic and HF-free route to prepare various MXenes and demonstrates practical applications of the materials.

A major challenge that prohibits the practical application of single/double-transition metal (3d-M) oxides as oxygen evolution reaction (OER) catalysts is the high overpotentials during the electrochemical process. Herein, our theoretical calculation shows that Fe will be more energetically favorable in the tetrahedral site than Ni and Co, which can further regulate their electronic structure of binary NiCo spinel oxides for optimal adsorption energies of OER intermediates and improved electronic conductivity and hence boost their OER performance. X-ray absorption spectroscopy study on the assynthesized NiCoFe oxide catalysts indicates that Fe preferentially dopes into tetrahedral sites of the lattice, which induces high proportions of Ni 3+ and Co 2+ on the octahedral sites (the active sites in OER). Consequently, this material exhibits a significantly enhanced OER performance with an ultralow overpotential of 201 mV cm −2 at 10 mA cm −2 and a small Tafel slope of 39 mV dec −1 , which are much superior to state-of-the-art Ni−Co based catalysts.

The development of optoelectronic synapses can provide an important breakthrough toward creating a sophisticated and adaptable artificial visual system analogous to that of humans. However, it remains a great challenge to implement the various functions of the biological visual neuromorphic system at the single device level. Intriguingly, 2D van der Waals (vdW) heterostructure may offer a platform to address the issue. Here, a novel multifunctional synaptic device based on ferroelectric α-In 2 Se 3 /GaSe vdW heterojunction is proposed to emulate the entire biological visual system. Essential synaptic behaviors are observed in response to light and electrical stimuli; additionally, the retina-like selectivity for light wavelengths and the achievement of Pavlov's dog experiment demonstrate the device's capacity for processing complex electrical and optical inputs. Beyond the optoelectronic synaptic behaviors, the device incorporates memory and logic functions analogous to those in the brain's visual cortex. The results of artificial neural network simulations show that the vdW heterojunction-based device is completely capable of performing logic operations and recognizing images with a high degree of accuracy. The study indicates that versatile devices with a rationally designed construction have great potential for efficiently processing complex visual information and may simplify the design of artificial visual systems.

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The development of renewable energy conversion and storage devices, aiming at high efficiency, stable operation, environmental friendliness, and low-cost goals, provides a promising approach to resolve the global energy crisis. Recently, two-dimensional (2D) layered materials have drawn enormous attention due to their unique layered structure and intriguing electrical characteristics, which brings the unprecedented board applications in the fields ranging from electronic, optical, optoelectronic, thermal, magnetic, quantum devices to energy storage and catalysis. Graphene-based 2D layered materials show promising applications in energy storage and conversion owing to their high specific surface area, which have been used for supercapacitor electrode materials based on the electrical double-layer capacitance model. However, graphene has a limited value of theoretical electrical double-layer capacitance when the whole surface area is fully utilized. Among several classes of 2D layered materials beyond graphene, transition metal dichalcogenides, transition metal carbides, and nitrides may exhibit excellent electrochemical properties due to the distinctive features of these 2D materials, such as large specific surface area, good hydrophilic nature, highly exposed active edge sites, and ease of intercalation and modification. Therefore, careful design and construction of these 2D compounds make them become potential candidates used for electrochemical supercapacitors and electrocatalytic hydrogen evolution. This review emphasizes the recent important advances of the 2D layered materials composed of transition metal dichalcogenides, transition metal carbides, and nitrides for supercapacitors and electrocatalysts. Furthermore, we discuss the challenges and perspectives in this energy field in terms of the classes of two-dimensional layered materials.