Engineering Fe-N4 Electronic Structure with Adjacent Co-N2C2 and Co Nanoclusters on Carbon Nanotubes for Efficient Oxygen Electrocatalysis.

Regulating the local configuration of atomically dispersed transition-metal atom catalysts is the key to oxygen electrocatalysis performance enhancement. Unlike the previously reported single-atom or dual-atom configurations, we designed a new type of binary-atom catalyst, through engineering Fe-N4 electronic structure with adjacent Co-N2C2 and nitrogen-coordinated Co nanoclusters, as oxygen electrocatalysts. The resultant optimized electronic structure of the Fe-N4 active center favors the binding capability of intermediates and enhances oxygen reduction reaction (ORR) activity in both alkaline and acid conditions. In addition, anchoring M-N-C atomic sites on highly graphitized carbon supports guarantees of efficient charge- and mass-transports, and escorts the high bifunctional catalytic activity of the entire catalyst. Further, through the combination of electrochemical studies and in-situ X-ray absorption spectroscopy analyses, the ORR degradation mechanisms under highly oxidative conditions during oxygen evolution reaction processes were revealed. This work developed a new binary-atom catalyst and systematically investigates the effect of highly oxidative environments on ORR electrochemical behavior. It demonstrates the strategy for facilitating oxygen electrocatalytic activity and stability of the atomically dispersed M-N-C catalysts.

Boosting Oxygen-Evolving Activity via Atom-Stepped Interfaces Architected with Kinetic Frustration.

A highly active interface is extremely critical for the catalytic efficiency of an electrocatalyst; however, facilely tailoring its atomic packing characteristics remains challenging. Herein, a simple yet effective strategy is reported to obtain copious high-energy atomic steps at the interface via controlling the solidification behavior of glass-forming metallic liquids. By adjusting the chemical composition and cooling rate, highly faceted FeNi3 nanocrystals are in situ formed in an FeNiB metallic glass (MG) matrix, leading to the creation of order/disorder interfaces. Benefiting from the catalytically active and stable atomic steps at the jagged interfaces, the resultant free-standing FeNi3 nanocrystal/MG composite exhibits a low oxygen-evolving overpotential of 214 mV at 10 mA cm-2 , a small Tafel slope of 32.4 mV dec-1 , and good stability in alkaline media, outperforming most state-of-the-art catalysts. This approach is based on the manipulation of nucleation and crystal growth of the solid-solution nanophases (e.g., FeNi3 ) in glass-forming liquids, so that the highly stepped interface architecture can be obtained due to the kinetic frustration effect in MGs upon undercooling. It is envisaged that the atomic-level stepped interface engineering via the physical metallurgy method can be easily extended to other MG systems, providing a new and generic paradigm for designing efficient yet cost-effective electrocatalysts.

Real-space measurement of orbital electron populations for Li1-xCoO2.

The operation of lithium-ion batteries involves electron removal from and filling into the redox orbitals of cathode materials, experimentally probing the orbital electron population thus is highly desirable to resolve the redox processes and charge compensation mechanism. Here, we combine quantitative convergent-beam electron diffraction with high-energy synchrotron powder X-ray diffraction to quantify the orbital populations of Co and O in the archetypal cathode material LiCoO2. The results indicate that removing Li ions from LiCoO2 decreases Co t2g orbital population, and the intensified covalency of Co-O bond upon delithiation enables charge transfer from O 2p orbital to Co eg orbital, leading to increased Co eg orbital population and oxygen oxidation. Theoretical calculations verify these experimental findings, which not only provide an intuitive picture of the redox reaction process in real space, but also offer a guidance for designing high-capacity electrodes by mediating the covalency of the TM-O interactions.

Self-supported efficient hydrogen evolution catalysts with a core-shell structure designed via phase separation.

The development of cost-effective, high-performance and flexible electrocatalysts for hydrogen production is of scientific and technological importance. Catalysts with a core-shell structure for water dissociation have been extensively investigated. However, most of them are nanoparticles and thus their catalytic properties are inevitably limited by the use of binders in practice. Herein, this work reports a physical-metallurgy-based structural design strategy to develop a self-supported and unique nanoporous structure with core-shell-like ligaments, i.e., a Cu core surrounded by a NiO shell, formed on a metallic glass (MG) substrate. These newly developed noble metal-free catalysts exhibit outstanding HER performance; the overpotential reaches 67 mV at a current density of 10 mA cm-2, accompanied by a low Tafel slope of 40 mV dec-1 and good durability. More importantly, the current strategy could be readily applied to fabricate other nanoporous metals, which opens a new space for designing advanced catalysts as cost-effective electrode materials.

Atomically Dispersed Fe-Co Bimetallic Catalysts for the Promoted Electroreduction of Carbon Dioxide.

The electroreduction reaction of CO2 (ECO2RR) requires high-performance catalysts to convert CO2 into useful chemicals. Transition metal-based atomically dispersed catalysts are promising for the high selectivity and activity in ECO2RR. This work presents a series of atomically dispersed Co, Fe bimetallic catalysts by carbonizing the Fe-introduced Co-zeolitic-imidazolate-framework (C-Fe-Co-ZIF) for the syngas generation from ECO2RR. The synergistic effect of the bimetallic catalyst promotes CO production. Compared to the pure C-Co-ZIF, C-Fe-Co-ZIF facilitates CO production with a CO Faradaic efficiency (FE) boost of 10%, with optimal FECO of 51.9%, FEH2 of 42.4% at - 0.55 V, and CO current density of 8.0 mA cm-2 at - 0.7 V versus reversible hydrogen electrode (RHE). The H2/CO ratio is tunable from 0.8 to 4.2 in a wide potential window of - 0.35 to - 0.8 V versus RHE. The total FECO+H2 maintains as high as 93% over 10 h. The proper adding amount of Fe could increase the number of active sites and create mild distortions for the nanoscopic environments of Co and Fe, which is essential for the enhancement of the CO production in ECO2RR. The positive impacts of Cu-Co and Ni-Co bimetallic catalysts demonstrate the versatility and potential application of the bimetallic strategy for ECO2RR.

PGM-Free Fe/N/C and Ultralow Loading Pt/C Hybrid Cathode Catalysts with Enhanced Stability and Activity in PEM Fuel Cells.

In this work, the stability behaviors of the state-of-the-art Fe/N/C and Pt/C catalysts (as well as the activation time of the latter) were first systematically investigated, under different cathode catalyst loadings, in the membrane electrode assemblies (MEA) in PEM fuel cells. Based on that, two types of cathode electrodes with the combination of Fe/N/C and Pt/C catalysts were developed (type I: layered hybrid catalysts with Pt/C next to the membrane and type II: uniformly mixed catalysts). In this way, the shortcomings of the Fe/N/C catalyst (the fast decay) and the Pt/C catalyst (the long activation time) can be compensated at the same time. The hybrid catalysts also showed a very short activation time (a few hours vs over 10 h for Pt/C with the same Pt loading). Comparing the two types of hybrid catalysts, type I shows a much higher current density. The loadings of the Fe/N/C and Pt/C catalysts in the hybrid electrode were systematically studied, with optimal values of 1.0 mg cm-2 for Fe/N/C and 0.035 mgPt cm-2 for Pt/C. The Pt loading of this hybrid catalyst (type I) at the cathode only takes ca. 30% of the U.S. Department of Energy (DOE) target of Pt usage (0.100 mgPt cm-2), while its mass activity of Pt (in H2/O2 PEMFC) is 0.22 A mgPt-1 at 0.9iR-free V, reaching half of the DOE activity target (0.44 A mgPt-1), which is among the best performances reported so far. Via both half-cell and single-cell electrochemical evaluations together with other characterizations, the origin of the improved activity and stability is believed to be the synergistic effect between Pt/C and Fe/N/C catalysts to ORR. This work provides an effective strategy for engineering highly performing MEA for the industrialization of PEM fuel cells.

An Ordered Ni6 -Ring Superstructure Enables a Highly Stable Sodium Oxide Cathode.

Sodium-based layered oxides are among the leading cathode candidates for sodium-ion batteries, toward potential grid energy storage, having large specific capacity, good ionic conductivity, and feasible synthesis. Despite their excellent prospects, the performance of layered intercalation materials is affected by both a phase transition induced by the gliding of the transition metal slabs and air-exposure degradation within the Na layers. Here, this problem is significantly mitigated by selecting two ions with very different MO bond energies to construct a highly ordered Ni6 -ring superstructure within the transition metal layers in a model compound (NaNi2/3 Sb1/3 O2 ). By virtue of substitution of 1/3 nickel with antimony in NaNiO2 , the existence of these ordered Ni6 -rings with super-exchange interaction to form a symmetric atomic configuration and degenerate electronic orbital in layered oxides can not only largely enhance their air stability and thermal stability, but also increase the redox potential and simplify the phase-transition process during battery cycling. The findings reveal that the ordered Ni6 -ring superstructure is beneficial for constructing highly stable layered cathodes and calls for new paradigms for better design of layered materials.

Corrosion Engineering To Synthesize Ultrasmall and Monodisperse Alloy Nanoparticles Stabilized in Ultrathin Cobalt (Oxy)hydroxide for Enhanced Electrocatalysis.

Two-dimensional (2D) nanomaterials decorated with ultrasmall and well-alloyed bimetallic nanoparticles (NPs) have many important applications. Developing a facile and scalable 2D material/hybrid synthesis strategy is still a big challenge. Herein, a top-down corrosion strategy is developed to prepare ultrathin cobalt (oxy)hydroxide nanosheets decorated with ultrasmall (∼1.6 nm) alloy NPs. The formation of ultrathin (oxy)hydroxide nanosheets has a restrain effect to prevent the growth of small NPs into bigger ones. Thanks to the ultrathin 2D nature and strong electronic interaction between Co(OH)2 and alloy NPs, the Pt-based binary alloy NPs are greatly stabilized by the Co(OH)2 nanosheets and the hybrids exhibit much enhanced electrocatalytic performance for water splitting. Especially, the mass activities of the PtPd- and PtCu-decorated samples for hydrogen evolution are ∼8 times that of Pt/C. When used as both cathode and anode electrocatalysts to split water, the hybrid nanosheets outperform the commercial Pt/C-RuO2 combination. At 10 mA cm-2, the needed potential is only 1.53 V. This work provides us a highly controllable and scalable means to produce clean 2D nanomaterials decorated with a series of alloy NPs such as PtPd, PtCu, AuNi, and so forth.

High-content ductile coherent nanoprecipitates achieve ultrastrong high-entropy alloys.

Precipitation-hardening high-entropy alloys (PH-HEAs) with good strength-ductility balances are a promising candidate for advanced structural applications. However, current HEAs emphasize near-equiatomic initial compositions, which limit the increase of intermetallic precipitates that are closely related to the alloy strength. Here we present a strategy to design ultrastrong HEAs with high-content nanoprecipitates by phase separation, which can generate a near-equiatomic matrix in situ while forming strengthening phases, producing a PH-HEA regardless of the initial atomic ratio. Accordingly, we develop a non-equiatomic alloy that utilizes spinodal decomposition to create a low-misfit coherent nanostructure combining a near-equiatomic disordered face-centered-cubic (FCC) matrix with high-content ductile Ni3Al-type ordered nanoprecipitates. We find that this spinodal order-disorder nanostructure contributes to a strength increase of ~1.5 GPa (>560%) relative to the HEA without precipitation, achieving one of the highest tensile strength (1.9 GPa) among all bulk HEAs reported previously while retaining good ductility (>9%).

Na+/vacancy disordering promises high-rate Na-ion batteries.

As one of the most fascinating cathode candidates for Na-ion batteries (NIBs), P2-type Na layered oxides usually exhibit various single-phase domains accompanied by different Na+/vacancy-ordered superstructures, depending on the Na concentration when explored in a limited electrochemical window. Therefore, their Na+ kinetics and cycling stability at high rates are subjected to these superstructures, incurring obvious voltage plateaus in the electrochemical profiles and insufficient battery performance as cathode materials for NIBs. We show that this problem can be effectively diminished by reasonable structure modulation to construct a completely disordered arrangement of Na-vacancy within Na layers. The combined analysis of scanning transmission electron microscopy, ex situ x-ray absorption spectroscopy, and operando x-ray diffraction experiments, coupled with density functional theory calculations, reveals that Na+/vacancy disordering between the transition metal oxide slabs ensures both fast Na mobility (10-10 to 10-9 cm2 s-1) and a low Na diffusion barrier (170 meV) in P2-type compounds. As a consequence, the designed P2-Na2/3Ni1/3Mn1/3Ti1/3O2 displays extra-long cycle life (83.9% capacity retention after 500 cycles at 1 C) and unprecedented rate capability (77.5% of the initial capacity at a high rate of 20 C). These findings open up a new route to precisely design high-rate cathode materials for rechargeable NIBs.

New Nanoconfined Galvanic Replacement Synthesis of Hollow Sb@C Yolk-Shell Spheres Constituting a Stable Anode for High-Rate Li/Na-Ion Batteries.

In the current research project, we have prepared a novel Sb@C nanosphere anode with biomimetic yolk-shell structure for Li/Na-ion batteries via a nanoconfined galvanic replacement route. The yolk-shell microstructure consists of Sb hollow yolk completely protected by a well-conductive carbon thin shell. The substantial void space in the these hollow Sb@C yolk-shell particles allows for the full volume expansion of inner Sb while maintaining the framework of the Sb@C anode and developing a stable SEI film on the outside carbon shell. As for Li-ion battery anode, they displayed a large specific capacity (634 mAh g-1), high rate capability (specific capabilities of 622, 557, 496, 439, and 384 mAh g-1 at 100, 200, 500, 1000, and 2000 mA g-1, respectively) and stable cycling performance (a specific capacity of 405 mAh g-1 after long 300 cycles at 1000 mA g-1). As for Na-ion storage, these yolk-shell Sb@C particles also maintained a reversible capacity of approximate 280 mAh g-1 at 1000 mA g-1 after 200 cycles.

Analytical ABF-STEM imaging of Li ions in rechargeable batteries.

Rechargeable batteries are being intensively investigated in an attempt to solve the energy issues while meeting the environmental demands. Even though Li-ion batteries (LIB) with high energy and light weight have been commercialized within the last 20 years, these devices currently require higher energy density, output power and sustainability characteristics. The atomic behavior of Li ion that determines LIB's performance is hardly characterized by transmission electron microscopy (TEM) owing to its weak electron-scattering power. In this sense, annular bright-field (ABF) scanning TEM (STEM), in which the contrast has a low scaling rate with the atomic number, has been proven to be a robust technique for simultaneous imaging of light and heavy elements. The s-state model, in which electron channeling along the atomic column allows the intensity to be focusing in the forward direction, has successfully explained the theory of ABF contrast. Furthermore, the detector angle range, the defocus-thickness dependence and the accelerating voltage (among other parameters) were discussed for optimized imaging conditions. ABF-STEM has shown powerful capabilities in resolving the atomic structure and the chemistry of electrodes (e.g. Li-ion occupation and diffusion, phase transformation and interface reaction), thereby providing critical insights into the physical properties, the battery performance and the design guidance of LIB. The future directions of ABF imaging for the characterization of LIB materials were also reviewed.

Visualizing Under-Coordinated Surface Atoms on 3D Nanoporous Gold Catalysts.

The intricate 3D geometric shape and surface atomic structure of nanoporous gold catalysts are investigated using aberration-corrected scanning transmission electron microscopy in combination with discrete tomography. The real-space 3D atomic configurations illustrate geometrically necessary surface defects on the curved surface of the NPG, offering atomic insights into the catalysis of the nanoporous catalyst.

Investigating hybridization schemes of coupled split-ring resonators by electron impacts.

We present a comprehensive theoretical and experimental investigation of the plasmon hybridization of coupled split-ring resonators by means of the electron energy-loss spectroscopy. Split-ring resonator is a key element in design of negative refractive index metamaterials, and has been therefore intensively studied in the literature. Here, our aim is the study of hybridization effects for higher-order non-dipolar modes, which have been not investigated beforehand. We provide a complete scheme of the multimodal distribution of the coupled and single-element split-ring resonators, with a precise attention to the hybridization of those modes according to the induced moments. Our study suggests a clear dominance of electric and magnetic dipole moments over higher-order modes in the far-field radiation spectrum.

Eutectic nano-droplet template injection into bulk silicon to construct porous frameworks with concomitant conformal coating as anodes for Li-ion batteries.

Building porosity in monolithic materials is highly desired to design 3D electrodes, however ex-situ introduction or in-situ generation of nano-scale sacrificial template is still a great challenge. Here Al-Si eutectic droplet templates are uniformly injected into bulk Si through Al-induced solid-solid convection to construct a highly porous Si framework. This process is concomitant with process-inherent conformal coating of ion-conductive oxide. Such an all-in-one method has generated a (continuously processed) high-capacity Si anode integrating longevity and stable electrolyte-anode diaphragm for Li-ion batteries (e.g. a reversible capacity as large as ~1800 mAh/g or ~350 µAh/cm(2)-µm with a CE of ~99% at 0.1 C after long-term 400 cycles).

Nanosheets of earth-abundant jarosite as novel anodes for high-rate and long-life lithium-ion batteries.

Nanosheets of earth-abundant jarosite were fabricated via a facile template-engaged redox coprecipitation strategy at room temperature and employed as novel anode materials for lithium-ion batteries (LIBs) for the first time. These 2D materials exhibit high capacities, excellent rate capability, and prolonged cycling performance. As for KFe3(SO4)2(OH)6 jarosite nanosheets (KNSs), the reversible capacities of above 1300 mAh g(-1) at 100 mA g(-1) and 620 mAh g(-1) after 4000 cycles at a very high current density of 10 A g(-1) were achieved, respectively. Moreover, the resulting 2D nanomaterials retain good structural integrity upon cycling. These results reveal great potential of jarosite nanosheets as low-cost and high-performance anode materials for next-generation LIBs.

3D V6O13 nanotextiles assembled from interconnected nanogrooves as cathode materials for high-energy lithium ion batteries.

Three-dimensional (3D) hierarchical nanostructures have been demonstrated as one of the most ideal electrode materials in energy storage systems owing to the synergistic combination of the advantages of both nanostructures and microstructures. In this work, 3D V6O13 nanotextiles built from interconnected 1D nanogrooves with diameter of 20-50 nm were fabricated via a facile solution-redox-based self-assembly route at room temperature, and the mesh size in the textile structure can be controllably tuned by adjusting the precursor concentration. It is suggested that the formation of 3D fabric structure built from nanogrooves is attributed to the rolling and self-assembly processes of produced V6O13 nanosheet intermediates. When evaluated as cathodes for lithium ion batteries (LIBs), the products delivered reversible capacities of 326 mAh g(-1) at 20 mA g(-1) and 134 mAh g(-1) at 500 mA g(-1), and a capacity retention of above 80% after 100 cycles at 500 mA g(-1). Importantly, the resulting textiles exhibit a specific energy as high as 780 Wh kg(-1), 44-56% higher than those of conventional cathodes, that is, LiMn2O4, LiCoO2, and LiFePO4. Furthermore, the 3D architectures retain good structural integrity upon cycling. Such findings reveal a great potential of V6O13 nanotextiles as high-energy cathode materials for LIBs.

Rapid and up-scalable fabrication of free-standing metal oxide nanosheets for high-performance lithium storage.

Free-standing α-Fe2 O3 nanosheets, SnO2 mesoporous nanosheets and sandwich-like polyaniline (PAN)/SnO2 /PAN nanosheets are fabricated at very mild conditions (room temperature or 60 °C) via a galvanic replacement method for the first time. These nanosheets show excellent high-rate capability and long-term durability as anodes for lithium-ion batteries.

Facile synthesis of highly porous Ni-Sn intermetallic microcages with excellent electrochemical performance for lithium and sodium storage.

Highly porous Ni3Sn2 microcages composed of tiny nanoparticles were synthesized by a facile template-free solvothermal method (based on Ostwald ripening and etching mechanism) for use as anode materials for high-capacity and high-rate-capability Li-ion and Na-ion batteries. The Ni3Sn2 porous microcages exhibit highly stable and substantial discharge capacities of the amount to 700 mA h g(-1) after 400 cycles at 0.2C and 530 mA h g(-1) after 1000 cycles at 1C for Li-ion battery anode. For Na-ions storage performance, a reversible capacity of approximate 270 mA h g(-1) is stably maintained at 1C during the first 300 cycles.