First-row transition metal-based catalysts, such as metal oxides, sulfides, selenides, phosphides, and hydroxides of Co, Ni, Fe, and Mo have been studied intensively owing to high intrinsic activities and low cost. [9,10] However, most of the reported catalysts exhibited satisfactory activity and stability toward either HER or OER, which complicates the configuration of an integrated electrolyzer. [11] To simplify the system and reduce the overall cost for industrial applications, a transition metalbased bifunctional electrocatalyst that can catalyze both the HER and OER process is essential.Ni-Fe based layered double hydroxides (NiFe LDHs) are considered to be competitive OER catalyst candidates in alkaline media. [12,13] In contrast, the catalytic performance of NiFe LDHs in HER is unsatisfactory owing to the sluggish HER kinetics resulting from a large energy barrier for the Volmer step. [14,15] Additionally, NiFe LDHs suffer from the inherent problems associated with LDH-based catalysts, such as poor stability caused by self-aggregation and a low electrochemically-active surface area. [16,17] Therefore, the design of NiFe-based LDHs, with improved HER catalytic properties and maintained OER performance, to serve as effective bifunctional catalysts for electrolysis of water is an important challenge. In this regard, the controlled integration of NiFe LDHs and HERactive catalysts into a hetero-nanostructure may be a reasonable strategy to fabricate highly efficient bifunctional catalysts for water splitting. [18] Because of their high surface area and open structure, hollow-structured HER catalysts are believed to be ideal templates for NiFe LDH growth. [19,20] Previous studies have confirmed that growing LDHs on hollow-structured HER catalysts can increase the electrochemically-active surface area and inhibit self-aggregation of LDHs. [21] Recently, metal-organic frameworks (MOFs) have been considered as efficient precursors for fabricating porous and/ or hollow-structured materials based on their tunable physicochemical properties. [22,23] Consequently, numerous research groups have attempted to develop MOF-derived hollow-structured materials as highly efficient electrocatalysts for the HER process. [24,25] For example, Wang et al. fabricated a Ni-Co bimetallic sulfide catalyst with a multi-shell hollow structure (derived from the MOF component of the catalyst) for enhanced HER For effective hydrogen production by water splitting, it is essential to develop earth-abundant, highly efficient, and durable electrocatalysts. Herein, the authors report a bifunctional electrocatalyst composed of hollow CoS x and Ni-Fe based layered double hydroxide (NiFe LDH) nanosheets for efficient overall water splitting (OWS). The optimized heterostructure is obtained by the electrodeposition of NiFe LDH nanosheets on metal-organic framework-derived hollow CoS x nanoarrays, which are supported on nickel foam (H-CoS x @NiFe LDH/NF). The unique structure of the hybrid material not only provides ample active sites, but also facilit...

Two-dimensional (2D) MXenes are promising as electrode materials for energy storage, owing to their high electronic conductivity and low diffusion barrier. Unfortunately, similar to most 2D materials, MXene nanosheets easily restack during the electrode preparation, which degrades the electrochemical performance of MXene-based materials. A novel synthetic strategy is proposed for converting MXene into restacking-inhibited three-dimensional (3D) balls coated with iron selenides and carbon. This strategy involves the preparation of Fe2O3@carbon/MXene microspheres via a facile ultrasonic spray pyrolysis and subsequent selenization process. Such 3D structuring effectively prevents interlayer restacking, increases the surface area, and accelerates ion transport, while maintaining the attractive properties of MXene. Furthermore, combining iron selenides and carbon with 3D MXene balls offers many more sites for ion storage and enhances the structural robustness of the composite balls. The resultant 3D structured microspheres exhibit a high reversible capacity of 410 mAh g−1 after 200 cycles at 0.1 A g−1 in potassium-ion batteries, corresponding to the capacity retention of 97% as calculated based on 100 cycles. Even at a high current density of 5.0 A g−1, the composite exhibits a discharge capacity of 169 mAh g−1.