Facet-Dependent Surface Restructuring on Nickel (Oxy)hydroxides: A Self-Activation Process for Enhanced Oxygen Evolution Reaction.

Unraveling the catalyst surface structure and behavior during reactions is essential for both mechanistic understanding and performance optimization. Here we report a phenomenon of facet-dependent surface restructuring intrinsic to ß-Ni(OH)2 catalysts during oxygen evolution reaction (OER), discovered by the correlative ex situ and operando characterization. The ex situ study after OER reveals ß-Ni(OH)2 restructuring at the edge facets to form nanoporous Ni1-xO, which is Ni deficient containing Ni3+ species. Operando liquid transmission electron microscopy (TEM) and Raman spectroscopy further identify the active role of the intermediate ß-NiOOH phase in both the OER catalysis and Ni1-xO formation, pinpointing the complete surface restructuring pathway. Such surface restructuring is shown to effectively increase the exposed active sites, accelerate Ni oxidation kinetics, and optimize *OH intermediate bonding energy toward fast OER kinetics, which leads to an extraordinary activity enhancement of ∼16-fold. Facilitated by such a self-activation process, the specially prepared ß-Ni(OH)2 with larger edge facets exhibits a 470-fold current enhancement than that of the benchmark IrO2, demonstrating a promising way to optimize metal-(oxy)hydroxide-based catalysts.

Unlocking Liquid Sulfur Chemistry for Fast-Charging Lithium-Sulfur Batteries.

A recent study of liquid sulfur produced in an electrochemical cell has prompted further investigation into regulating Li-S oxidation chemistry. In this research, we examined the liquid-to-solid sulfur transition dynamics by visually observing the electrochemical generation of sulfur on a graphene-based substrate. We investigated the charging of polysulfides at various current densities and discovered a quantitative correlation between the size and number density of liquid sulfur droplets and the applied current. However, the areal capacities exhibited less sensitivity. This observation offers valuable insights for designing fast-charging sulfur cathodes. By incorporating liquid sulfur into Li-S batteries with a high sulfur loading of 4.2 mg cm-2, the capacity retention can reach ∼100%, even when increasing the rate from 0.1 to 3 C. This study contributes to a better understanding of the kinetics involved in the liquid-solid sulfur growth in Li-S chemistry and presents viable strategies for optimizing fast-charging operations.

Hollow-Carbon Confinement Annealing: A New Synthetic Approach to Make High-Entropy Solid-Solution and Intermetallic Nanoparticles.

High-entropy alloy (HEA) nanoparticles (NPs) have been emerging with superior compositional tunability and multielemental synergy, presenting a unique platform for material discovery and performance optimization. Here we report a synthetic approach utilizing hollow-carbon confinement in the ordinary furnace annealing to achieve the nonequilibrium HEA-NPs such as Pt0.45Fe0.18Co0.12Ni0.15Mn0.10 with uniform size ∼5.9 nm. The facile temperature control allows us not only to reveal the detailed reaction pathway through ex situ characterization but also to tailor the HEA-NP structure from the crystalline solid solution to intermetallic. The preconfinement of metal precursors is the key to ensure the uniform distribution of metal nanoparticles with confined volume, which is essential to prevent the thermodynamically favored phase separation even during the ordinary furnace annealing. Besides, the synthesized HEA-NPs exhibit remarkable activity and stability in oxygen reduction catalysis. The demonstrated synthetic approach may significantly expand the scope of HEA-NPs with uncharted composition and performance.

Enabling Polymer Single Crystals to Be High-Performance Dielectric.

Semicrystalline polymer dielectrics (SPDs) are highly sought-after state-of-the-art dielectric materials. As the disorder in SPDs degrades their electrical properties, homogeneously ordered SPDs are desired. However, complex crystallization behaviors of polymers make such homogeneity elusive. Polymer lamellar single crystals (PLSCs), the most regularly-ordered form of SPDs possible under mild crystallizing conditions, are ideal platforms for understanding and developing high-performance dielectric materials. Here, a typical and widely used SPD, polyethylene (PE) is selected as the model material. We successfully obtained, large, uniform, and high-quality PE PLSCs and devised a non-destructive strategy to construct PE PLSC-based vertical capacitors. These nanometer-thick capacitors exhibit exceptional dielectric properties, with a high breakdown strength of 6.95 MV/cm and a low dielectric constant of 2.14±0.07, that outperform the properties of any existing neat PE. This work provides novel insights into exploring the performance possibility of ordered SPDs and reveals the PLSCs as potential high-performance dielectric materials.

Surface Reconstruction on Metal Nitride during Photo-oxidation.

The efficient conversion and storage of solar energy for chemical fuel production presents a challenge in sustainable energy technologies. Metal nitrides (MNs) possess unique structures that make them multi-functional catalysts for water splitting. However, the thermodynamic instability of MNs often results in the formation of surface oxide layers and ambiguous reaction mechanisms. Herein, we present on the photo-induced reconstruction of a Mo-rich@Co-rich bi-layer on ternary cobalt-molybdenum nitride (Co3 Mo3 N) surfaces, resulting in improved effectiveness for solar water splitting. During a photo-oxidation process, the uniform initial surface oxide layer is reconstructed into an amorphous Co-rich oxide surface layer and a subsurface Mo-N layer. The Co-rich outer layer provides active sites for photocatalytic oxygen evolution reaction (POER), while the Mo-rich sublayer promotes charge transfer and enhances the oxidation resistance of Co3 Mo3 N. Additionally, the surface reconstruction yields a shortened Co-Mo bond length, weakening the adsorption of hydrogen and resulting in improved performance for both photocatalytic hydrogen evolution reaction (PHER) and POER. This work provides insight into the surface structure-to-activity relationships of MNs in solar energy conversion, and is expected to have significant implications for the design of metal nitride-based catalysts in sustainable energy technologies.

Flexible and Robust Triboelectric Nanogenerators with Chemically Prepared Metal Electrodes and a Plastic Contact Interface Based on Low-Cost Pressure-Sensitive Adhesive.

Triboelectric nanogenerators (TENGs) are devices that can harvest energy from mechanical motions; such devices can be used to power wearable sensors and various low-power electronics. To increase the lifetime of the device, scientists mainly use the method of making TENG in a hard skeleton to simplify the complex possible relative movements between two triboelectric parts. However, the hard skeletons cannot be embedded in soft and lightweight clothing. To make matters worse, the materials used in the garments must be able to withstand high mechanical forces when worn, such as the pressure of more than 100 KPa exerted by body pressure or everyday knocks. Notably, the TENGs are usually made of fragile materials, such as vacuum-evaporated metal electrodes and nano-sized coatings, on the contact interface; these electrodes and coatings often chip or wear off under the action of external loads. In this work, we succeeded in creating a thin, light-weight, but extremely robust garment-integrated triboelectric nanogenerator (G-TENG) that can be embedded in clothing and pass the water wash test. First, we chemically deposited a durable electrode with flexible properties for G-TENG using a novel technique called polymer-assisted metal deposition (PAMD). The as-formed metal electrodes are firmly bonded to the plastic substrate by a sub-10 nm adhesive polymer brush and can withstand a pressure of 22.5 MPa and a tear force of 0.7 MPa. We then removed the traditionally used fragile nanoparticle materials and the non-durable poly-dimethylsiloxane (PDMS) layer at the triboelectric interface, and then used a cost-effective, durable and slightly flowable pressure-sensitive adhesive to form a plastic contact interface. Such a soft plastic interface can ensure full contact of the triboelectric materials, which is excellent in complex environments and ultimately improves the power generation efficiency of the devices. The as-formed low-cost energy harvesting device could become an industry standard for future smart clothing.

Boosting Oxygen Reduction for High-Efficiency H2 O2 Electrosynthesis on Oxygen-Coordinated CoNC Catalysts.

Atomically dispersed CoNC is a promising material for H2 O2 selective electrosynthesis via a two-electron oxygen reduction reaction. However, the performance of typical CoNC materials with routine CoN4 active center is insufficient and needs to be improved further. This can be done by fine-tuning its atomic coordination configuration. Here, a single-atom electrocatalyst (Co/NC) is reported that comprises a specifically penta-coordinated CoNC configuration (OCoN2 C2 ) with Co center coordinated by two nitrogen atoms, two carbon atoms, and one oxygen atom. Using a combination of theoretical predictions and experiments, it is confirmed that the unique atomic structure slightly increases the charge state of the cobalt center. This optimizes the adsorption energy towards *OOH intermediate, and therefore favors the two-electron ORR relevant for H2 O2 electrosynthesis. In neutral solution, the as-synthesized Co/NC exhibits a selectivity of over 90% over a potential ranging from 0.36 to 0.8 V, with a turnover frequency value of 11.48 s-1 ; thus outperforming the state-of-the-art carbon-based catalysts.

Freestanding 2D NiFe Metal-Organic Framework Nanosheets: Facilitating Proton Transfer via Organic Ligands for Efficient Oxygen Evolution Reaction.

The oxygen evolution reaction (OER) is crucial to electrochemical hydrogen production. However, designing and fabricating efficient electrocatalysts still remains challenging. By confinedly coordinating organic ligands with metal species in layered double hydroxides (LDHs), an innovative LDHs-assisted approach is developed to facilely synthesize freestanding bimetallic 2D metal-organic framework nanosheets (2D MOF NSs), preserving the metallic components and activities in OER. Furthermore, the research has demonstrated that the incorporation of carboxyl organic ligands coordinated with metal atoms as proton transfer mediators endow 2D MOF NSs with efficient proton transfer during the electrochemical OHads  â†’ Oads transition. These freestanding NiFe-2D MOF NSs require a small overpotential of 260 mV for a current density of 10 mA cm-2 . When this strategy is applied to LDH nanosheets grown on nickel foam, the overpotential can be reduced to 221 mV. This outstanding OER activity supports the capability of multimetallic organic frameworks for the rational design of water oxidation electrocatalysts. This strategy provides a universal path to the synthesis of 2D MOF NSs that can be used as electrocatalysts directly.

Direct Observation of Oxygen Evolution and Surface Restructuring on Mn2O3 Nanocatalysts Using In Situ and Ex Situ Transmission Electron Microscopy.

Direct observation of oxygen evolution reaction (OER) on catalyst surface may significantly advance the mechanistic understanding of OER catalysis. Here, we report the first real-time nanoscale observation of chemical OER on Mn2O3 nanocatalyst surface using an in situ liquid holder in a transmission electron microscope (TEM). The oxygen evolution process can be directly visualized from the development of oxygen nanobubbles around nanocatalysts. The high spatial and temporal resolution further enables us to unravel the real-time formation of a surface layer on Mn2O3, whose thickness oscillation reflects a partially reversible surface restructuring relevant to OER catalysis. Ex situ atomic-resolution TEM on the residual surface layer after OER reveals its amorphous nature with reduced Mn valence and oxygen coordination. Besides shedding light on the dynamic OER catalysis, our results also demonstrate a powerful strategy combining in situ and ex situ TEM for investigating various chemical reaction mechanisms in liquid.

Modulating Built-In Electric Field via Variable Oxygen Affinity for Robust Hydrogen Evolution Reaction in Neutral Media.

Work function strongly impacts the surficial charge distribution, especially for metal-support electrocatalysts when a built-in electric field (BEF) is constructed. Therefore, studying the correlation between work function and BEF is crucial for understanding the intrinsic reaction mechanism. Herein, we present a Pt@CoOx electrocatalyst with a large work function difference (ΔΦ) and strong BEF, which shows outstanding hydrogen evolution activity in a neutral medium with a 4.5-fold mass activity higher than 20 % Pt/C. Both experimental and theoretical results confirm the interfacial charge redistribution induced by the strong BEF, thus subtly optimizing hydrogen and hydroxide adsorption energy. This work not only provides fresh insights into the neutral hydrogen evolution mechanism but also proposes new design principles toward efficient electrocatalysts for hydrogen production in a neutral medium.

Non-Periodic Epsilon-Near-Zero Metamaterials at Visible Wavelengths for Efficient Non-Resonant Optical Sensing.

Epsilon-near-zero (ENZ) materials offer unique properties for applications including optical clocking, nonlinear optics, and telecommunication. To date, the fabrication of ENZ materials at visible wavelengths relies mostly on the use of periodic structures, providing some manufacturing and material challenges. Here, we present the engineering of nonperiodic sodium tungsten bronzes (NaxWO3) metamaterials featuring ENZ properties in the visible spectrum. We showcase their use as efficient optical sensors, demonstrating a nonresonant sensing mechanism based on refractive index matching. Our optimized ENZ metamaterials display an unconventional blue-shift of the transmittance maximum to increasing refractive index of the surrounding environment, achieving sensitivity as high as 150 nm/RIU. Our theoretical and experimental investigations provide first insights on this sensing mechanism, establishing guidelines for the future engineering and implementation of efficient ENZ sensors. The unique optoelectronic properties demonstrated by this class of tunable NaxWO3 materials bear potential for various applications ranging from light-harvesting to optical photodetectors.

Surface Functionalized Sensors for Humidity-Independent Gas Detection.

Semiconducting metal oxides (SMOXs) are used widely for gas sensors. However, the effect of ambient humidity on the baseline and sensitivity of the chemiresistors is still a largely unsolved problem, reducing sensor accuracy and causing complications for sensor calibrations. Presented here is a general strategy to overcome water-sensitivity issues by coating SMOXs with a hydrophobic polymer separated by a metal-organic framework (MOF) layer that preserves the SMOX surface and serves a gas-selective function. Sensor devices using these nanoparticles display near-constant responses even when humidity is varied across a wide range [0-90 % relative humidity (RH)]. Furthermore, the sensor delivers notable performance below 20 % RH whereas other water-resistance strategies typically fail. Selectivity enhancement and humidity-independent sensitivity are concomitantly achieved using this approach. The reported tandem coating strategy is expected to be relevant for a wide range of SMOXs, leading to a new generation of gas sensors with excellent humidity-resistant performance.

Bulk-Heterojunction with Long-Range Ordering: C60 Single-Crystal with Incorporated Conjugated Polymer Networks.

Bulk-heterojunction (BHJ) blends are commonly used as active materials for optoelectronics. Ordering of molecular packing in blends is critical to their electronic properties, spurring investigation on how to obtain BHJ with long-range ordering. However, the difficulty in controlling crystallization during blending limits the crystallinity. Developing a new strategy instead of conventional blending is, thus, needed. Inspired by biomineralization, here, C60 single-crystals are prepared in organogel matrix of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenvinylene] (MEH-PPV) to form MEH-PPV:C60 composites. Essentially, networks of MEH-PPV are incorporated into growing C60 crystals and penetrate throughout the crystals, resulting in crystal/gel-network interpenetrating composites. Despite the coexistence of MEH-PPV, the C60 crystalline component maintains single-crystallinity and the composite exhibits as BHJ with long-range ordering. Furthermore, compared with blends, the long-range ordered BHJ shows a higher efficiency of charge dissociation and better performance in photodetection, exemplifying the advantage of ordering on organic electronics. Hence, this work provides a new platform to study BHJ with long-range ordering.

Two-Dimensional Antiferroelectricity in Nanostripe-Ordered In_{2}Se_{3}.

Two-dimensional (2D) layered materials have been an exciting frontier for exploring emerging physics at reduced dimensionality, with a variety of exotic properties demonstrated at 2D limit. Here, we report the first experimental discovery of in-plane antiferroelectricity in a 2D material ß^{'}-In_{2}Se_{3}, using optical and electron microscopy consolidated by first-principles calculations. Different from conventional 3D antiferroelectricity, antiferroelectricity in ß^{'}-In_{2}Se_{3} is confined within the 2D layer and generates the unusual nanostripe ordering: the individual nanostripes exhibit local ferroelectric polarization, whereas the neighboring nanostripes are antipolar with zero net polarization. Such a unique superstructure is underpinned by the intriguing competition between 2D ferroelectric and antiferroelectric ordering in ß^{'}-In_{2}Se_{3}, which can be preserved down to single-layer thickness as predicted by calculation. Besides demonstrating 2D antiferroelectricity, our finding further resolves the true nature of the ß^{'}-In_{2}Se_{3} superstructure that has been under debate for over four decades.

A Surface-Oxide-Rich Activation Layer (SOAL) on Ni2 Mo3 N for a Rapid and Durable Oxygen Evolution Reaction.

The oxygen evolution reaction (OER) is key to renewable energy technologies such as water electrolysis and metal-air batteries. However, the multiple steps associated with proton-coupled electron transfer result in sluggish OER kinetics and catalysts are required. Here we demonstrate that a novel nitride, Ni2 Mo3 N, is a highly active OER catalyst that outperforms the benchmark material RuO2 . Ni2 Mo3 N exhibits a current density of 10 mA cm-2 at a nominal overpotential of 270 mV in 0.1 m KOH with outstanding catalytic cyclability and durability. Structural characterization and computational studies reveal that the excellent activity stems from the formation of a surface-oxide-rich activation layer (SOAL). Secondary Mo atoms on the surface act as electron pumps that stabilize oxygen-containing species and facilitate the continuity of the reactions. This discovery will stimulate the further development of ternary nitrides with oxide surface layers as efficient OER catalysts for electrochemical energy devices.

Phosphorus Incorporation into Co9 S8 Nanocages for Highly Efficient Oxygen Evolution Catalysis.

The improvement of activity of electrocatalysts lies in the increment of the density of active sites or the enhancement of intrinsic activity of each active site. A common strategy to realize dual active sites is the use of bimetal compound catalysts, where each metal atom contributes one active site. In this work, a new concept is presented to realize dual active sites with tunable electron densities in monometal compound catalysts. Dual Co2+ tetrahedral (Co2+ (Td )) and Co3+ octahedral (Co3+ (Oh )) coordination active sites are developed and adjustable electron densities on the Co2+ (Td ) and Co3+ (Oh ) are further achieved by phosphorus incorporation (P-Co9 S8 ). The experimental results and density functional theory calculations show that the nonmetal P doping can systematically modulate charge density of Co2+ (Td ) and Co3+ (Oh ) in P-Co9 S8 and simultaneously improve the electrical conductivity of Co9 S8 , which substantially enhances oxygen evolution reaction performance of P-Co9 S8 .

Antioxidant Grain Passivation for Air-Stable Tin-Based Perovskite Solar Cells.

Tin-based perovskites with excellent optoelectronic properties and suitable band gaps are promising candidates for the preparation of efficient lead-free perovskite solar cells (PSCs). However, it is challenging to prepare highly stable and efficient tin-based PSCs because Sn2+ in perovskites can be easily oxidized to Sn4+ upon air exposure. Here we report the fabrication of air-stable FASnI3 solar cells by introducing hydroxybenzene sulfonic acid or its salt as an antioxidant additive into the perovskite precursor solution along with excess SnCl2 . The interaction between the sulfonate group and the Sn2+ ion enables the in situ encapsulation of the perovskite grains with a SnCl2 -additive complex layer, which results in greatly enhanced oxidation stability of the perovskite film. The corresponding PSCs are able to maintain 80 % of the efficiency over 500 h upon air exposure without encapsulation, which is over ten times longer than the best result reported previously. Our results suggest a possible strategy for the future design of efficient and stable tin-based PSCs.

Concave Ni(OH)2 Nanocube Synthesis and Its Application in High-Performance Hybrid Capacitors.

The controlled synthesis of hollow structure transition metal compounds has long been a very interesting and significant research topic in the energy storage and conversion fields. Herein, an ultrasound-assisted chemical etching strategy is proposed for fabricating concave Ni(OH)2 nanocubes. The morphology and composition evolution of the concave Ni(OH)2 nanocubes suggest a possible formation mechanism. The as-synthesized Ni(OH)2 nanostructures used as supercapacitor electrode materials exhibit high specific capacitance (1624 F g-1 at 2 A g-1) and excellent cycling stability (77% retention after 4000 cycles) due to their large specific surface area and open pathway. In addition, the corresponding hybrid capacitor (Ni(OH)2//graphene) demonstrates high energy density (42.9 Wh kg-1 at a power density of 800 W kg-1) and long cycle life (78% retention after 4000 cycles at 5 A g-1). This work offers a simple and economic approach for obtaining concave Ni(OH)2 nanocubes for energy storage and conversion.

Mitigating Lattice Distortion of High-Voltage LiCoO2 via Core-Shell Structure Induced by Cationic Heterogeneous Co-Doping for Lithium-Ion Batteries.

Inactive elemental doping is commonly used to improve the structural stability of high-voltage layered transition-metal oxide cathodes. However, the one-step co-doping strategy usually results in small grain size since the low diffusivity ions such as Ti4+ will be concentrated on grain boundaries, which hinders the grain growth. In order to synthesize large single-crystal layered oxide cathodes, considering the different diffusivities of different dopant ions, we propose a simple two-step multi-element co-doping strategy to fabricate core-shell structured LiCoO2 (CS-LCO). In the current work, the high-diffusivity Al3+/Mg2+ ions occupy the core of single-crystal grain while the low diffusivity Ti4+ ions enrich the shell layer. The Ti4+-enriched shell layer (~ 12 nm) with Co/Ti substitution and stronger Ti-O bond gives rise to less oxygen ligand holes. In-situ XRD demonstrates the constrained contraction of c-axis lattice parameter and mitigated structural distortion. Under a high upper cut-off voltage of 4.6 V, the single-crystal CS-LCO maintains a reversible capacity of 159.8 mAh g-1 with a good retention of ~ 89% after 300 cycles, and reaches a high specific capacity of 163.8 mAh g-1 at 5C. The proposed strategy can be extended to other pairs of low- (Zr4+, Ta5+, and W6+, etc.) and high-diffusivity cations (Zn2+, Ni2+, and Fe3+, etc.) for rational design of advanced layered oxide core-shell structured cathodes for lithium-ion batteries.

2D metal-organic frameworks for ultraflexible electrochemical transistors with high transconductance and fast response speeds.

Electrochemical transistors (ECTs) have shown broad applications in bioelectronics and neuromorphic devices due to their high transconductance, low working voltage, and versatile device design. To further improve the device performance, semiconductor materials with both high carrier mobilities and large capacitances in electrolytes are needed. Here, we demonstrate ECTs based on highly oriented two-dimensional conjugated metal-organic frameworks (2D c-MOFs). The ion-conductive vertical nanopores formed within the 2D c-MOFs films lead to the most convenient ion transfer in the bulk and high volumetric capacitance, endowing the devices with fast speeds and ultrahigh transconductance. Ultraflexible device arrays are successfully used for wearable on-skin recording of electrocardiogram (ECG) signals along different directions, which can provide various waveforms comparable with those of multilead ECG measurement systems for monitoring heart conditions. These results indicate that 2D c-MOFs are excellent semiconductor materials for high-performance ECTs with promising applications in flexible and wearable electronics.