Oxygen evolution electrocatalysts are central to overall water splitting, and they should meet the requirements of low cost, high activity, high conductivity, and stable performance. Herein, a general, selenic‐acid‐assisted etching strategy is designed from a metal–organic framework as a precursor to realize carbon‐coated 3d metal selenides MmSen (Co0.85Se1−x, NiSe2−x, FeSe2−x) with rich Se vacancies as high‐performance precious metal‐free oxygen evolution reaction (OER) electrocatalysts. Specifically, the as‐prepared Co0.85Se1−x@C nanocages deliver an overpotential of only 231 mV at a current density of 10 mA cm−2 for the OER and the corresponding full water‐splitting electrolyzer requires only a cell voltage of 1.49 V at 10 mA cm–2 in alkaline media. Density functional theory calculation reveals the important role of abundant Se vacancies for improving the catalytic activity through improving the conductivity and reducing reaction barriers for the formation of intermediates. Although phase change after long‐term operation is observed with the formation of metal hydroxides, catalytic activity is not obviously affected, which strengthens the important role of the carbon network in the operating stability. This study provides a new opportunity to realize high‐performance OER electrocatalysts by a general strategy on selenic acid etching assisted vacancy engineering.

batteries have attracted great attention. [2,3] Among them, aqueous Zn-ion batteries (AZIBs) shows apparent merits of abundant zinc sources, low development cost, high security, and environmental friendliness. More importantly, the chemical stability of metal zinc in a water-oxygen environment is much superior than other alkali metals and polyvalent metals, thus in favor of the design of aqueous batteries. [4,5] However, the revolution of AZIBs is still at its early stage and far from the practical application because of the lack of suitable cathode materials. The relatively high redox potential of ≈−0.76 V (vs NHE) of Zn 2+ /Zn generally calls for high-voltage cathodes to construct an aqueous battery with satisfying energy density. [6] Recently, polyanionic compounds consisting of alternately linked [XO 4 ] (X = P, S, Si, etc.) tetrahedron and [MO 6 ] (M = V, Mn, Fe, Co, Cr, etc.) octahedron have been developed as very promising candidates for AZIBs cathode. Profiting from the inductive effect on the MO bond and the binding effect on oxygen atoms originating from the presence of [XO 4 ], polyanionic compounds generally combine the disadvantages of a high discharged plateau and excellent thermal stability. [7][8][9] Typically, a series of V-based phosphate based polyanionic compounds have been reported for the construction of ZIBs cathode materials such as layered hydrated VOPO 4 and NASICON-typed Na 3 V 2 PO 4 . [10,11] For instance, Huang et al. revealed that NASICON-typed Na 3 V 2 (PO 4 ) 3 displays a specific capacity of ≈100 mAh g −1 and a satisfying plateau of ≈1.1 V when served as a ZIBs cathode. [11] Our previous work also revealed layered hydrated VOPO 4 .2H 2 O as high voltage cathode materials originated from the aforementioned inductive effect. [12][13][14] Nevertheless, in spite of the high voltage plateau and decent cyclability, relative low specific capacity (generally < 150 mAh g −1 ) of V-based phosphate based polyanionic compounds is still a huge obstacle for the realization of high energy density cathode. [12,15,16] It is known that V and Mn are two important active metal elements in most of AZIBs cathode materials. This inspired us to consider whether one can incorporate V and Mn elements simultaneously into polyanionic compounds hosts to Aqueous Zn-ion battery (AZIB) has attracted huge attention due to its distinct advantages such as abundant zinc sources, low development cost, high security, and environmental friendliness compared to traditional commercialized batteries. Developing novel cathode materials with high energy density and long lifespan is very important for the practical application of AZIB in the coming years. Herein, V and Mn elements are simultaneously incorporated into the NASICON host to make use of the electrochemical redox reaction in both V and Mn transition metal elements for aqueous Zn-ion storage. A significant increase in both capacity and working voltage has been observed after Mn doping into Na 3 V 2 (PO 4 ) 3 to form Na 4 VMn(PO 4 ) 3 . Taking advantage of the me...

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Aqueous Zinc‐ion Batteries In article number 2200654, Linfeng Hu and co‐workers develop NASICON‐typed V‐Mn transition metal phosphate, Na4VMn(PO4)3, as a cathode material for robust aqueous Zn‐ion storage. Taking advantage of its high‐voltage properties that originate from the inductive effect and two‐step electron transfer mechanism between V4+/V3+ and Mn3+/Mn2+ redox couple, the Na4VMn(PO4)3 cathode exhibits excellent overall battery performance with an energy density of 309.7 Wh kg−1, and good rate capacity and cycling stability (89.1% capacity retention after 3000 cycles).

The non-metal NH 4 + carrier has attracted tremendous interests for aqueous energy storage owing to its light molar mass and fast diffusion in aqueous electrolytes. Previous study inferred that NH 4 + ion storage in layered VOPO 4 •2 H 2 O is impossible due to the removal of NH 4 + from NH 4 VOPO 4 leads to a phase change inevitably. Herein, we update this cognition and demonstrated highly reversible intercalation/de-intercalation behavior of NH 4 + in layered VOPO 4 •2 H 2 O host. Satisfactory specific capacity of 154.6 mAh g À 1 at 0.1 A g À 1 and very stable discharge potential plateau at 0.4 V based on reference electrode was achieved in VOPO 4 •2 H 2 O. A rocking-chair ammonium-ion full cell with the VOPO 4 •2 H 2 O//2.0 M NH 4OTf//PTCDI configuration exhibited a specific capacity of 55 mAh g À 1 , an average operating voltage of about 1.0 V and excellent long-term cycling stability over 500 cycles with a coulombic efficiency of � 99 %. Theoretical DFT calculations suggest a unique crystal water substitution process by ammonium ion during the intercalation process. Our results provide new insight into the intercalation/deintercalation of NH 4 + ions in layered hydrated phosphates through crystal water enhancement effect.