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Water electrolysis is an energy conversion technology to provide green and clean hydrogen energy. Developing a high-efficient and durable electrocatalyst is a critical material for water electrolysis. Therefore, we synthesize a series of iron-doped metal-organic frameworks (MOFs) by a facile one-pot hydrothermal method. In the conventional three-electrode-cell, the Co/Fe (1:1)-MOF catalyst exhibits an overpotential of 317 mV at a current density of 10 mA cm−2 in the oxygen evolution reaction (OER). Furthermore, the electrolysis performance of Co/Fe (1:1)-MOF catalyst is further evaluated in a home-made anion-exchange-membrane water electrolysis cell. With the Co/Fe (1:1)-MOF as the OER catalyst and commercial Pt/C as the hydrogen-evolution-reaction catalyst, the cell presents an overpotential of 490 mV at a large current density of 500 mA cm−2, which is superior to the benchmark cell with commercial IrO2 as the OER catalyst in the alkaline media. Theoretical calculation demonstrates that the introduction of Fe dopant into MOFs significantly reduces the binding energy of ∗O and ∗OOH intermedium during the OER progress. Consequently, the electrocatalytic activity is increased, which is perfectly consistent with the experimental results. This work suggests that the iron-doped MOFs structure significantly improves the electrocatalytic activity and provides a facile strategy to produce hydrogen at a large current density for industrial water electrolysis.

Aqueous rechargeable zinc ion batteries (ARZIBs) have received unprecedented attention owing to the low cost and high-safety merits. However, their further development and application are hindered by the issues of electrodes such as cathode dissolution, zinc anode dendrite, passivation, as well as sluggish reaction kinetics. Designing heterostructure electrodes is a powerful method to improve the electrochemical performance of electrodes by grafting the advantages of functional materials onto the active materials. In this review, various modified heterostructure electrodes with optimized electrochemical performance and wider applications are introduced. Moreover, the synergistic effect between active materials and functional materials are also in-depth analyzed. The specific modification methods are divided into interphase modification (electrode-electrolyte interphase and electrode-current collector interphase) and structure optimization. Finally, the conclusion and future perspective on the optimization mechanism of functional materials, and the cost issue of practical heterostructure electrodes in ARZIBs are also proposed. It is expected that this review can promote the further development of ARZIBs towards practical utility.

Benefiting from loose assembly conditions, a high level of safety and environmentally friendly characteristics, rechargeable aqueous Zn-ion batteries (AZIBs) have attracted significant attention. The electrochemical kinetics and performance of the AZIBs are greatly affected by water in electrolytes or electrode materials. The corrosion and passivation of the Zn electrode caused by the inevitable solvation process of water molecules can lead to the growth of dendrites, thus resulting in a limited cycle life. Moreover, water in the electrode material, whether in the form of structural water or co-intercalated hydrated cations, can greatly affect the electrochemical behavior due to its small size, high polarity and hydrogen bonding. Unlike previous reports, this review focuses on the roles of water molecules during electrochemical processes in AZIBs. We comprehensively summarize the influencing mechanisms of water molecules during the energy storage process from the perspectives of the electrolyte, Zn anode, and cathode materials, and further include the basic theory, modification methods, and practical applications. The mystery concerning the water molecules and the electrochemical performance of AZIBs is revealed herein, and we also propose novel insights and actionable methods regarding the potential future directions in the design of high-performance AZIBs.

A rechargeable aqueous zinc-ion battery (ZIB) is one of the attractive candidates for large-scale energy storage. Its further application relies on the exploitation of a high-capacity cathode and the understanding of an intrinsic energy storage mechanism. Herein, we report a novel layered K2V3O8 cathode material for the ZIB, adopting a strategy of charging first to extract part of K-ions from vanadate in initial few cycles, which creates more electrochemically active sites and lowers charge-transfer resistance of the ZIB system. As a result, a considerable specific capacity of 302.8 mA h g–1 at 0.1 A g–1, as well as a remarkable cycling stability (92.3% capacity retention at 4 A g–1 for 2000 cycles) and good rate capability, are achieved. Besides, the energy storage mechanism was studied by in situ X-ray diffraction, in situ Raman spectroscopy, X-ray photoelectron spectroscopy, and inductively coupled plasma mass spectroscopy. An irreversible K-ion deintercalation in the first charge process is proved. It...

Sodium-ion batteries (SIBs) are regarded as a promising candidate for large-scale energy storage applications, however, its sluggish sodiation kinetics limit high power capabilities. First-principles density functional theory (DFT) calculations demonstrates that the replacement of large anions, within a Co3X4 (X = O, S, Se) complex, can facilitate electron transfer through the reduction of the energy band gap (Eg) and the regulation of structural, electronic and bonding properties of Co-X, suggesting that the substituted Co3X4 in the following order O Na2S > Na2Se) with the increased active sites, which are paramount for electrochemical properties. The work in this article sheds light on the in-depth understanding of the improved sodium-storage behavior with the advancing VI group anions and provides an effective strategy for the design of high-rate electrode materials for SIBs.

Subtle structural changes during electrochemical processes often relate to the degradation of electrode materials. Characterizing the minutevariations in complementary aspects such as crystal structure, chemical bonds, and electron/ion conductivity will give an in-depth understanding on the reaction mechanism of electrode materials, as well as revealing pathways for optimization. Here, vanadium pentoxide (V2O5), a typical cathode material suffering from severe capacity decay during cycling, is characterized by in-situ X-ray diffraction (XRD) and in-situ Raman spectroscopy combined with electrochemical tests. The phase transitions of V2O5 within the 0–1 Li/V ratio are characterized in detail. The V–O and V–V distances became more extended and shrank compared to the original ones after charge/discharge process, respectively. Combined with electrochemical tests, these variations are vital to the crystal structure cracking, which is linked with capacity fading. This work demonstrates that chemical bond changes between the transition metal and oxygen upon cycling serve as the origin of the capacity fading.

Preventing the aggregation of nanosized electrode materials is a key point to fully utilize the advantage of the high capacity. In this work, a facile and low-cost surface solvation treatment is developed to synthesize Fe2 VO4 hierarchical porous microparticles, which efficiently prevents the aggregation of the Fe2 VO4 primary nanoparticles. The reaction between alcohol molecules and surface hydroxy groups is confirmed by density functional theory calculations and Fourier transform infrared spectroscopy. The electrochemical mechanism of Fe2 VO4 as lithium-ion battery anode is characterized by in situ X-ray diffraction for the first time. This electrode material is capable of delivering a high reversible discharge capacity of 799 mA h g-1 at 0.5 A g-1 with a high initial coulombic efficiency of 79%, and the capacity retention is 78% after 500 cycles. Moreover, a remarkable reversible discharge capacity of 679 mA h g-1 is achieved at 5 A g-1 . Furthermore, when tested as sodium-ion battery anode, a high reversible capacity of 382 mA h g-1 can be delivered at the current density of 1 A g-1 , which still retains at 229 mA h g-1 after 1000 cycles. The superior electrochemical performance makes it a potential anode material for high-rate and long-life lithium/sodium-ion batteries.

The development of manganese dioxide as the cathode for aqueous Zn-ion battery (ZIB) is limited by the rapid capacity fading and material dissolution. Here, a highly reversible aqueous ZIB using graphene scroll-coated α-MnO2 as the cathode is proposed. The graphene scroll is uniformly coated on the MnO2 nanowire with an average width of 5 nm, which increases the electrical conductivity of the MnO2 nanowire and relieves the dissolution of the cathode material during cycling. An energy density of 406.6 Wh kg-1 (382.2 mA h g-1 ) at 0.3 A g-1 can be reached, which is the highest specific energy value among all the cathode materials for aqueous Zn-ion battery so far, and good long-term cycling stability with 94% capacity retention after 3000 cycles at 3 A g-1 are achieved. Meanwhile, a two-step intercalation mechanism that Zn ions first insert into the layers and then the tunnels of MnO2 framework is proved by in situ X-ray diffraction, galvanostatic intermittent titration technique, and X-ray photoelectron spectroscopy characterizations. The graphene scroll-coated metallic oxide strategy can also bring intensive interests for other energy storage systems.