The slow kinetics of oxygen evolution reaction (OER) causes high power consumption for electrochemical water splitting. Various strategies have been attempted to accelerate the OER rate, but there are few studies on regulating the transport of reactants especially under large current densities when the mass transfer factor dominates the evolution reactions. Herein, NixFe1–x alloy nanocones arrays (with ≈2 nm surface NiO/NiFe(OH)2 layer) are adopted to boost the transport of reactants. Finite element analysis suggests that the high‐curvature tips can enhance the local electric field, which induces an order of magnitude higher concentration of hydroxide ions (OH−) at the active sites and promotes intrinsic OER activity by 67% at 1.5 V. Experimental results show that a fabricated NiFe nanocone array electrode, with optimized alloy composition, has a small overpotential of 190 mV at 10 mA cm−2 and 255 mV at 500 mA cm−2. When calibrated by electrochemical surface area, the nanocones electrode outperforms the state‐of‐the‐art OER electrocatalysts. The positive effect of the tip‐enhanced local electric field in promoting mass transfer is also confirmed by comparing samples with different tip curvature radii. It is suggested that this local field enhanced OER kinetics is a generic effect to other OER catalysts.

Metal oxides with a tunnelled structure are attractive as charge storage materials for rechargeable batteries and supercapacitors, since the tunnels enable fast reversible insertion/extraction of charge carriers (for example, lithium ions). Common synthesis methods can introduce large cations such as potassium, barium and ammonium ions into the tunnels, but how these cations affect charge storage performance is not fully understood. Here, we report the role of tunnel cations in governing the electrochemical properties of electrode materials by focusing on potassium ions in α-MnO2. We show that the presence of cations inside 2 × 2 tunnels of manganese dioxide increases the electronic conductivity, and improves lithium ion diffusivity. In addition, transmission electron microscopy analysis indicates that the tunnels remain intact whether cations are present in the tunnels or not. Our systematic study shows that cation addition to α-MnO2 has a strong beneficial effect on the electrochemical performance of this material.

While 3D printing of rechargeable batteries has received immense interest in advancing the next generation of 3D energy storage devices, challenges with the 3D printing of electrolytes still remain. Additional processing steps such as solvent evaporation were required for earlier studies of electrolyte fabrication, which hindered the simultaneous production of electrode and electrolyte in an all-3D-printed battery. Here, a novel method is demonstrated to fabricate hybrid solid-state electrolytes using an elevated-temperature direct ink writing technique without any additional processing steps. The hybrid solid-state electrolyte consists of solid poly(vinylidene fluoride-hexafluoropropylene) matrices and a Li -conducting ionic-liquid electrolyte. The ink is modified by adding nanosized ceramic fillers to achieve the desired rheological properties. The ionic conductivity of the inks is 0.78  × 10 S cm . Interestingly, a continuous, thin, and dense layer is discovered to form between the porous electrolyte layer and the electrode, which effectively reduces the interfacial resistance of the solid-state battery. Compared to the traditional methods of solid-state battery assembly, the directly printed electrolyte helps to achieve higher capacities and a better rate performance. The direct fabrication of electrolyte from printable inks at an elevated temperature will shed new light on the design of all-3D-printed batteries for next-generation electronic devices.

Amid the growing interest in rechargeable aqueous zinc-based batteries, tunnel-structured α-MnO2 has emerged as a promising cathode material owing to its low cost, high capacity and high safety.However, the precise charge storage mechanism, possibly involving proton and/or Zn ion insertion, has not been fully characterized especially at the atomistic level. Here, we report new insights through a combined investigation of atomic-scale electron microscopy, electrochemical analysis and ab initio simulations. We find that reversible Zn 2+ insertion into α-MnO2 framework is unlikely in the aqueous system, and that the charge storage process is dominated by H + insertion into the tunnel structures which are maintained upon discharging to HMnO2. Furthermore, we identify the local lattice positions for the hydroxyl (OH) groups in HxMnO2 as a function of H content. We reveal the consequent anisotropic structural change proceeding from the particle surface into the bulk, and thus account for the structural failure and capacity decay of the electrode upon cycling.Future work should consider optimizing proton insertion kinetics with enhanced host stability.

Correctly establishing a structure-property relationship is necessary to rationally develop energy materials for performance optimization. Bulk characterizations fall short of deciphering localized structural features at nanoscale and atomic scale. This work atomically resolves structural heterogeneity existing in single MnO 2 nanoparticles and demonstrates its significant effect on energy storage property, which was neglected by traditional bulk characterizations. Attention should thus be paid to controllable synthesis toward structural homogeneity with predictable/ tunable energy storage property and the proper choice of structural characterization tools. SUMMARY[MnO 6 ] octahedra are the structural units for a large family of manganese dioxides (MnO 2 ) possessing one-dimensional tunnel structures with extensive applications in catalysis and energy storage. Despite the long-range [MnO 6 ] ordering confirmed by conventional diffraction tools, surprisingly, the functional properties of a specific MnO 2 tunnel phase still vary significantly in literature with unclear structural origins. Here, we demonstrate the existence of tunnel heterogeneity featuring localized tunnel intergrowths within single MnO 2 nanoparticles via atomically resolved imaging. The degree of tunnel heterogeneity increases with the size increase of tunnels from b-MnO 2 (1 3 1 tunnel) to a-MnO 2 (2 3 2 tunnel), and to todorokite MnO 2 (3 3 3 tunnel). Furthermore, the tunnel heterogeneity within one MnO 2 nanoparticle significantly affects the energy storage kinetics even down to sub-nanometer scale. These findings are expected to call for renewed attention to the controlled synthesis of homogeneous tunnel-specific phases with predictable properties and to yield a more precise structure-property relationship in polymorphic materials.

In lithium–oxygen batteries, the solubility of LiO2 intermediates in the electrolyte regulates the formation routes of the Li2O2 discharge product. High-donor-number electrolytes with a high solubility of LiO2 tend to promote the formation of Li2O2 large particles following the solution route, which eventually benefits the cell capacity and cycle life. Here, we propose that facet engineering of cathode catalysts could be another direction in tuning the formation routes of Li2O2. In this work, β-MnO2 crystals with high occupancies of {111} or {100} facets were adopted as cathode catalysts in Li–O2 batteries with a tetra­(ethylene)­glycol dimethyl ether electrolyte. The {111}-dominated β-MnO2 catalyzed the formation of the Li2O2 discharge product into large toroids following the solution routes, while {100}-dominated β-MnO2 facilitated the formation of Li2O2 thin films through the surface routes. Further computational studies indicate that the different formation routes of Li2O2 could be related to different adsorption energies of LiO2 on the two facets of β-MnO2. Our results demonstrate that facet engineering of cathode catalysts could be a new way to tune the formation route of Li2O2 in a low-donor-number electrolyte. We anticipate that this new finding would offer more choices for the design of lithium–oxygen batteries with high capacities and ultimately a long cycle life.

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Wrinkle structures are commonly seen on graphene grown by the chemical vapor deposition (CVD) method due to the different thermal expansion coefficient between graphene and its substrate. Despite the intensive investigations focusing on the electrical properties, the nanotribological properties of wrinkles and the influence of wrinkle structures on the wrinkle-free graphene remain less understood. Here, we report the observation of anisotropic nanoscale frictional characteristics depending on the orientation of wrinkles in CVD-grown graphene. Using friction force microscopy, we found that the coefficient of friction perpendicular to the wrinkle direction was ∼194% compare to that of the parallel direction. Our systematic investigation shows that the ripples and "puckering" mechanism, which dominates the friction of exfoliated graphene, plays even a more significant role in the friction of wrinkled graphene grown by CVD. The anisotropic friction of wrinkled graphene suggests a new way to tune the graphene friction property by nano/microstructure engineering such as introducing wrinkles.