A long ZnO nanorod array with good verticality and thin diameter is synthesized from a single solution by the hydrothermal route. Prior to the growth, a ZnO seed layer with the c-axis texturing and the monolayer distribution is deposited on the substrate by a modified sol-gel spin coating process. A molecule adsorption stabilization mechanism is proposed to explain the seed orientation. Factors affecting the ZnO nanorod growth are systematically investigated. The critical conditions for the rod growth are obtained by changing the polyethyleneimine amount and the pH. The results suggest that the verticality of the array depends heavily on the seed orientation. The atomic force microscopy and X-ray diffraction measurements reveal that the crystallinity and the initial strain relaxation determine the growth activation energy and the rod diameter size.

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Figure 4. Chronopotentiometry tests for Ni 0.75 Fe 0.25 BDC and RuO 2 (with a constant current density of 10 mA cm À2 ).

Owing to the high specific capacities, high electrochemical activity, and various electronic properties, transition metal selenides are considered as promising anodes for lithium‐ and sodium‐ion storage. However, poor electronic conductivity and huge volume expansion during cycling are still responsible for their restricted electrochemical performance. Herein, CoSe hollow polyhedron anchoring onto graphene (CoSe/G) is synthesized by self‐assembly and subsequent selenization. In CoSe/G composites, the CoSe nanoparticles, obtained by in situ selenization of metal–organic frameworks (MOFs) in high temperature, are distributed among graphene sheets, realizing N element doping, developing robust heterostructures with a chemical bond. The unique architecture ensures the cohesion of the structure and endorses the reaction kinetics for metal ions, identified by in situ and ex situ testing techniques, and kinetics analysis. Thus, the CoSe/G anodes achieve excellent cycling performance (1259 mAh g−1 at 0.1 A g−1 after 300 cycles for lithium storage; 214 mAh g−1 at 2 A g−1 after 600 cycles for sodium storage) and rate capability (732 mAh g−1 at 5 A g−1 for lithium storage; 290 mAh g−1 at 5 A g−1 for sodium storage). The improved electrochemical performance for alkali‐ion storage provides new insights for the construction of MOFs derivatives toward high‐performance storage devices.

application. Therefore, electrochemical energy storage still lacks a suitable battery system with high energy density, highlevel safety, high environmental friendliness and low cost to satisfy the actual demands of large-scale energy storage. As shown in Table 1, compared with lithium, sodium and other metals, zinc is characterized by low cost and abundant resources. The small ion radius (0.74 Å) is conducive to the migration and storage in the cathode phase during charge and discharge. Meanwhile, metallic zinc shows numerous advantages, such as low redox potential (−0.76 V relative to standard hydrogen electrode (SHE)) and very high theoretical capacity (820 mAh g −1 , or 5851 mAh cm −3 ). In addition, the cheap aqueous electrolytes with superior ionic conductivity (about 1 S cm −1 ) are selected as electrolytes [12] instead of nonaqueous solvents (about 1-10 mS cm −1 ), and thereby aqueous zinc-ion batteries (AZIBs) become the novel generation of large-scale energy storage units and receive great concern and research interests.Remarkably, there is still lacking of related critical reviews that could provide a complete summary of cathode materials from the perspective of mechanisms and structural characteristics to offer comprehensive scientific guidance for modification strategies. Accordingly, with the aims of filling up such gap, this review summarizes the existing cathode materials of AZIBs from these three aspects of the reaction mechanism, material structure design as well as modification strategies and the approaches for advanced cathode structural engineering are also systematically discussed (Scheme 1). Besides, the key technical challenges and future prospects of materials are proposed. It is expected that this review can provide meaningful theories and inspiration for the development of high energy density and long cycle life cathode materials for AZIBs in the near future, and steadily promote their further practical application. Storage Mechanism of Cathode MaterialsAZIBs contain the merits of both ion batteries and metal batteries, so that they exhibit good safety, high rate performance, high energy density, and long cycle life. [12] By contrast, owing to the change of structure and properties of the cathode, the process of ion intercalation during the reaction will be different, By virtue of low cost, eco-friendliness, competitive gravimetric energy density, and intrinsic safety, more and more attention has increasingly focused on aqueous zinc ion batteries (AZIBs) as a promising alternative for scalable energy storage. However, plagued by a complex interfacial process, sluggish dynamics, lability of electrodes and electrolytes, insufficient energy density, and poor cycle life heavily restrict practical applications of AZIBs, indicating that profound understandings on cathode storage chemistry are necessarily needed. Hence, this paper comprehensively summarizes recent advance in cathodes with critical insight on the energy storage mechanism. Furthermore, the issues and challenges for high-performan...

Nd-doped ZnO nanoparticles with different Nd contents were synthesized and characterized by X-ray power diffraction, UV-vis, X-ray photoelectron spectroscopy, infrared spectra, and high-resolution transmission electron microscope. The results revealed that the particle size of Nd-doped ZnO is much smaller than that of pure ZnO, which is critical for enhancing the photocatalytic activity. The photocatalytic activity of Nd-doped ZnO with different Nd loading was compared with pure ZnO, and the effects of the solution pH in the degradation of C.I. Reactive Blue 4 were studied. It was observed that the rate of degradation of C.I. Reactive Blue 4 over Nd-doped ZnO increases with increasing Nd loading up to 2.5 mol % and then decreases. The rate of degradation reaches a maximum value approximately at pH 5 11 and then decreases. Among the catalyst studied, the 2.5 mol % Nd-doped ZnO was the most active, and the degradation rate of C.I. Reactive Blue 4 could reach 91.25% in 90 min when the concentration of the catalyst was 0.1 g/L.

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Prussian blue (PB) and its analogues are considered as promising cathode materials for sodium-ion batteries (SIBs) owing to their low cost and high capacity. However, it is still a huge challenge to avoid obvious capacity decay during cycling due to the structural collapse. Herein, we design a method to replace parts of Fe ion sites in PB with Ni ions to prepare fluffy-like nickel PB (PB-Ni) by cationic solution immersion, which improves cycling stability for sodium storage. The content of Ni in PB-Ni is explored by regulating the soaking time in the Ni-containing solution, which results in different effects on the electrochemical performance as cathodes of SIBs. Especially, PB-Ni-1d (soaking in NiCl2 solution for 1 day) exhibits an initial capacity of 114.2 mA h g–1 at 50 mA g–1 and a stable cycling performance of 800 cycles at 300 mA g–1. Furthermore, the reversible phase transformation and small volume variation for PB-Ni-1d are revealed by in situ X-ray diffraction characterization. The nickel hexacyanoferrate in outer layer maintains the cubic phase to stabilize the crystal structure. The cation-exchange strategy provides a facile idea to fabricate high-quality PB cathodes with superior stability for high-performance SIBs.