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.

Developing a reliable strategy for the modulation of the texture, composition, and electronic structure of electrocatalyst surfaces is crucial for electrocatalytic performance, yet still challenging. Herein, we develop a facile and universal strategy, quenching, to precisely tailor the surface chemistry of metal oxide nanocatalysts by rapidly cooling them in a salt solution. Taking NiMoO 4 nanocatalysts an example, we successfully produce the quenched nanocatalysts offering a greatly reduced oxygen evolution reaction (OER) overpotential by 85 mV and 135 mV at 10 mA cm −2 and 100 mA cm −2 respectively. Through detailed characterization studies, we establish that quenching induces the formation of numerous disordered stepped surfaces and the near-surface metal ions doping, thus regulating the local electronic structures and coordination environments of Ni, Mo, which promotes the formation of the dualsite active and thereby affords a low energy pathway for OER. This quenching strategy is also successfully applied to a number of other metal oxides, such as spinel-type Co 3 O 4 , Fe 2 O 3 , LaMnO 3 , and CoSnO 3 , with similar surface modifications and gains in OER activity. Our finding provides a new inspiration to activate metal oxide catalysts and extends the use of quenching chemistry in catalysis.

Development of novel and robust oxygen evolution reaction (OER) catalysts with well-modulated atomic and electronic structure remains a challenge. Compared to the well-known metal hydroxides or (oxyhydr)oxides with lamellar structure, delafossites (ABO2) are characterized by alternating layers of A cations and edge-sharing BO2 octahedra, but are rarely used in OER due to their poor electron conductivity and intrinsic activity. Here, we propose a delafossite analog by mutation of metal oxyhydroxide and delafossite based on first-principles calculations. Modulation on the electronic structure due to distortion of the original crystal field of the BO2 layers is calculated to enhance electron conductivity and catalytic activity. Inspired by the theoretical design, we have experimentally realized the delafossite analog by electrochemical self-reconstruction (ECSR). Operando X-ray absorption spectroscopy and other experimental techniques reveal the formation of delafossite analog with Ag intercalated into bimetallic cobalt–iron (oxyhydr)oxide layers from a metastable precursor through amorphization. Benefitting from the featured local electronic and geometric structures, the delafossite analog shows superior OER activity, affording a current density of 10 mA⋅cm−2 at an overpotential of 187 mV and an excellent stability (300 h) in alkaline conditions.

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