A simple strategy to synthesize ultrathin, amorphous and alloyed structural cobalt–vanadium hydr(oxy)oxide catalysts with enhanced water oxidation catalytic activity.

properties, e.g., mechanics and catalysis.[ 10 ] Particularly, a small but growing number of micro-and nanoscale noncrystalline materials are steadily capturing scientifi c attention, though only recently the well-characterized examples of amorphous nanomaterials have started to appear. [ 11 ] Owing to enhanced ionic conductivity as well as robust electrochemical and mechanical stability, [ 12 ] these amorphous nanomaterials have showed remarkable performance in electrochemical applications, e.g., Li-ion batteries, pseudo-capacitors, water oxidation, and sensors. [ 11b-f ] Nevertheless, it still remains as signifi cant challenges to develop the simple methods with broad applicability for synthesis of amorphous nanomaterials and study the relation between the amorphous structures and their properties.In this work, we explore, for the fi rst time to our knowledge, the feasibility of the amorphous hollow nanomaterials for effi cient electrochemical water oxidation. A unique template-engaged approach was employed here for fabrication of amorphous hollow nanomaterials. Ni-Co amorphous double hydroxides (ADHs) nanocages are chosen to demonstrate this feasibility, in view of the limited success in synthesizing cagelike structure of these materials, and emerging application of the hydroxides in water oxidation. [ 2h-j ] Furthermore, the relation between the compositions of amorphous catalysts and their OER activity is studied via both the experimental investigation and computational simulation.A two-step process was developed to synthesize the Ni-Co ADHs nanocages with well-defi ned shapes and structures. Cu 2 O nanocrystals, as the templates, were fi rst obtained via a facile wet-chemical route ( Figure S1, Supporting Information). Ni-Co ADHs nanocages were then synthesized by templating against these Cu 2 O nanocrystals at room temperature, as illustrated in Figure 1 . Briefl y, as-synthesized Cu 2 O nanocrystals are gradually removed by forming a soluble [Cu 2 (S 2 O 3 ) x ] 2−2 x complex when reacting continuously with Na 2 S 2 O 3 via a "coordinating etching" process in the synthesis solution, and then abundant amount of OH − ions are released at the etching interface. [ 11a,b ] Simultaneously, Ni 2+ and Co 2+ species can coprecipitate with these OH − ions at the etching interface to generate Ni-Co ADHs by perfectly inheriting the geometries of Cu 2 O templates and presenting cage-like nanostructures. Herein the impact of Na 2 S 2 O 3 on the precipitation of Ni 2+ and Co 2+ ions can be negligible since the interaction between Ni 2+ (Co 2+ ) ions (borderline acid) and S 2 O 3 2− ions (soft base) is not very strong on the basis of Pearson's hard and softacid-base principle. [ 11a ] The size, morphology, and structure of typical resulting nanomaterials were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM image ( Figure 2 a) shows that the products are exclusively high-quality nanocages, which inherit the geometrics The electrochemical water splitting into hydroge...

Potassium-ion batteries (KIBs) are promising electrochemical energy storage systems because of their low cost and high energy density. However, practical exploitation of KIBs is hampered by the lack of high-performance cathode materials. Here we report a potassium manganese hexacyanoferrate (K2Mn[Fe(CN)6]) material, with a negligible content of defects and water, for efficient high-voltage K-ion storage. When tested in combination with a K metal anode, the K2Mn[Fe(CN)6]-based electrode enables a cell specific energy of 609.7 Wh kg−1 and 80% capacity retention after 7800 cycles. Moreover, a K-ion full-cell consisting of graphite and K2Mn[Fe(CN)6] as anode and cathode active materials, respectively, demonstrates a specific energy of 331.5 Wh kg−1, remarkable rate capability, and negligible capacity decay for 300 cycles. The remarkable electrochemical energy storage performances of the K2Mn[Fe(CN)6] material are attributed to its stable frameworks that benefit from the defect-free structure.

Structural flexibility can be a desirable trait of an operating catalyst because it adapts itself to a given catalytic process for enhanced activity. Here, amorphous cobalt hydroxide nanocages are demonstrated to be a promising electrocatalyst with an overpotential of 0.28 V at 10 mA cm , far outperforming the crystalline counterparts and being in the top rank of the catalysts of their kind, under the condition of electrocatalytic oxygen evolution reaction. From the direct experimental in situ and ex situ results, this enhanced activity is attributed to its high structural flexibility in terms of 1) facile and holistic transformation into catalytic active phase; 2) hosting oxygen vacancies; and 3) structure self-regulation in a real-time process. Significantly, based on plausible catalytic mechanism and computational simulation results, it is disclosed how this structural flexibility facilitates the kinetics of oxygen evolution reaction. This work deepens the understanding of the structure-activity relationship of the Co-based catalysts in electrochemical catalysis, and it inspires more applications that require flexible structures enabled by such amorphous nanomaterials.