Dynamic Reconstruction of Two-Dimensional Defective Bi Nanosheets for Efficient Electrocatalytic Urea Synthesis.

Catalyst surface dynamics drive the generation of active species for electrocatalytic reactions. Yet, the understanding of dominant site formation and reaction mechanisms is limited. In this study, we thoroughly investigate the dynamic reconstruction of two-dimensional defective Bi nanosheets from exfoliated Bi2Se3 nanosheets under electrochemical CO2 and nitrate (NO3 -) reduction conditions. The ultrathin Bi2Se3 nanosheets obtained by NaBH4-assisted cryo-mediated liquid-phase exfoliation are more easily reduced and reconstructed to Bi nanosheets with high-density grain boundaries (GBs; GB-rich Bi). The reconstructed GB-rich Bi catalyst affords a remarkable yield rate of 4.6 mmol h-1 mgcat. -1 and Faradaic efficiency of 32 % for urea production at -0.40 V vs. RHE. Notably, this yield rate is 2 and 8.2 times higher than those of the low-GB Bi and bulk Bi catalysts, respectively. Theoretical analysis demonstrates that the GB sites significantly reduce the *CO and *NH2 intermediate formation energy and C-N coupling energy barrier, enabling selective urea electrosynthesis on the GB-rich Bi catalyst. This work will trigger further research into the structure-activity interplay in dynamic processes using in situ techniques.

Accelerating Industrial-Level NO3 - Electroreduction to Ammonia on Cu Grain Boundary Sites via Heteroatom Doping Strategy.

Although the electrocatalytic nitrate reduction reaction (NO3 - RR) is an attractive NH3 synthesis route, it suffers from low yield due to the lack of efficient catalysts. Here, this work reports a novel grain boundary (GB)-rich Sn-Cu catalyst, derived from in situ electroreduction of Sn-doped CuO nanoflower, for effectively electrochemical converting NO3 - to NH3 . The optimized Sn1% -Cu electrode achieves a high NH3 yield rate of 1.98 mmol h-1 cm-2 with an industrial-level current density of -425 mA cm-2 at -0.55 V versus a reversible hydrogen electrode (RHE) and a maximum Faradaic efficiency of 98.2% at -0.51 V versus RHE, outperforming the pure Cu electrode. In situ Raman and attenuated total reflection Fourier transform infrared spectroscopies reveal the reaction pathway of NO3 - RR to NH3 by monitoring the adsorption property of reaction intermediates. Density functional theory calculations clarify that the high-density GB active sites and the competitive hydrogen evolution reaction (HER) suppression induced by Sn doping synergistically promote highly active and selective NH3 synthesis from NO3 - RR. This work paves an avenue for efficient NH3 synthesis over Cu catalyst by in situ reconstruction of GB sites with heteroatom doping.

Enhanced Charge Separation for Efficient Photocatalytic H2 Production by Long-Lived Trap-State-Induced Interfacial Charge Transfer.

Photogeneration of charge carriers in semiconductors provides the scientific fundamental for photocatalytic water splitting. However, an ongoing challenge is the development of a new mechanism promoting charge carrier separation. Here we propose a trap-state-induced interfacial charge-transfer transition mechanism (TSICTT), in which electrons in long-lived trap states recombine with holes on the valence band (VB) of the semiconductor, thus prolonging the electron lifetime. We demonstrate this concept in the Sr4Al14O25:Eu2+, Dy3+/CdS (SAO/CdS) heterostructure, where trapped electrons with a lifetime of up to several hours in the SAO persistent luminescence phosphor (PLP) can continuously consume holes on the VB of CdS nanoparticles (NPs). We discover that the interfacial interaction and the work function difference between SAO and CdS are crucial for the TSICTT, which finally contributes to the improved H2 production from 34.4 to 1212.9 µmol gCdS-1 h-1 under visible-light irradiation. This model introduces a new strategy to manipulate charge carrier transport for the effective utilization of solar energy.

Improving the Catalytic Activity of Carbon-Supported Single Atom Catalysts by Polynary Metal or Heteroatom Doping.

Single atom catalysts (SACs) are widely researched in various chemical transformations due to the high atomic utilization and catalytic activity. Carbon-supported SACs are the largest class because of the many excellent properties of carbon derivatives. The single metal atoms are usually immobilized by doped N atoms and in some cases by C geometrical defects on carbon materials. To explore the catalytic mechanisms and improve the catalytic performance, many efforts have been devoted to modulating the electronic structure of metal single atomic sites. Doping with polynary metals and heteroatoms has been recently proposed to be a simple and effective strategy, derived from the modulating mechanisms of metal alloy structure for metal catalysts and from the donating/withdrawing heteroatom doping for carbon supports, respectively. Polynary metals SACs involve two types of metal with atomical dispersion. The bimetal atom pairs act as dual catalytic sites leading to higher catalytic activity and selectivity. Polynary heteroatoms generally have two types of heteroatoms in which N always couples with another heteroatom, including B, S, P, etc. In this Review, the recent progress of polynary metals and heteroatoms SACs is summarized. Finally, the barriers to tune the activity/selectivity of SACs are discussed and further perspectives presented.

Coupling ultrafine TiO2 within pyridinic-N enriched porous carbon towards high-rate and long-life sodium ion capacitors.

Coupling TiO2 within N-doped porous carbon (NPC) is essential for enhancing its Na+ storage performance. However, the role of different N configurations in NPC in improving the electrochemical performance of TiO2 is currently unknown. In this study, melamine is deliberately incorporated as a pore-forming agent in the self-assembly process of metal organic framework precursors (NH2-MIL-125(Ti)). This intentional inclusion of melamine leads to the one-pot and in-situ formation of highly active edge-N, which is vital for the development of TiO2/NPC with exceptional reactivity. Electrochemical performance characterization and density functional theory (DFT) calculation indicate that the interaction between TiO2 and pyridinic-N enriched NPC can effectively narrow the bandgap of TiO2/NPC, thereby significantly improving electron/ion transfer. Additionally, the abundant mesoporous channels, high N content and oxygen vacancies also contribute to the fast reaction kinetics of TiO2/NPC. As a result, the optimized TiO2/NPC-M, with high proportion of pyridinic-N (44.1 %) and abundant mesoporous channels (97.8 %), delivers high specific capacity of 282.1 mA h-1 at 0.05 A g-1, superior rate capability of 177.3 mA h-1 at 10 A g-1, and prominent capacity retention of 89.3 % over 5000 cycles even under ultrahigh 10 A g-1. Furthermore, the TiO2/NPC-M//AC sodium ion capacitors (SIC) device achieves a high energy density of 136.7 Wh kg-1 at 200 W kg-1. This research not only offers fresh perspectives on the production of high-performance TiO2-based anodes, but also paves the way for customizing other active materials for energy storage and beyond.

Template-assisted fabrication of gold nanowire arrays for ethanol electro-oxidation.

Gold nanowires are considered a promising candidate in the area of electrocatalysis because of their extremely high surface-to-volume ratios and other special properties. In this paper, highly ordered gold nanowire arrays have been fabricated by direct electrodeposition utilizing highly ordered anodic aluminum oxide (AAO) templates. The morphology and microstructure of AAO templates and gold nanowire arrays were characterized by field emission scanning electron microscopy (FESEM) and X-ray diffraction (XRD), respectively. The results showed that the average diameter of gold nanowires was on the order of 80 nm, which was in accordance with the diameter of AAO pores. The electro-oxidation of ethanol on gold nanowire arrays in alkaline medium was measured by cyclic voltammetry. The results indicated that gold nanowire arrays showed excellent electrocatalytic activity and stability towards the electro-oxidation of ethanol, which made gold nanowire arrays an excellent electrocatalyst in the applications of direct ethanol fuel cell.

Ag nanoparticles decorated TiO2 nanotube arrays for ultrasensitive gas sensing.

Anodic TiO2 nanotube arrays have attracted extensive interests in the past decade, especially for monitoring or sensing of the gas. In this paper, an ameliorated two-step anodization process was adopted for the preparation of highly ordered and vertically aligned TiO2 nanotube arrays on the Ti metal substrate. The as-prepared TiO2 nanotube arrays were successfully decorated with uniformly distributed Ag nanoparticles through a wetting-ultraviolet radiation deoxidation technology using the AgNO3 solution as precursor and source for Ag element. Field-emission scanning electron microscope (FESEM) was employed for the structural and morphological characterization of the Ag nanoparticles decorated TiO2 nanotube arrays. Then the as-prepared TiO2 nanotube arrays and Ag-decorated TiO2 nanotube arrays were both evaluated for room temperature ethanol gas sensing and the results showed that the Ag-decorated TiO2 nanotube arrays exhibited much higher sensitivity, as well as the reponse speed and recovery rate, than the pure TiO2 nanotube arrays. This phenomenon indicated that the well dispersed Ag nanoparticles onto the TiO2 nanotube arrays could effectively optimize their gas sensing properties. The mechanism of the improvement of gas sensitivity was also discussed in this paper.

Rapid Fabrication of Flexible Cu@Ag Flake/SAE Composites with Exceptional EMIS and Joule Heating Performance.

High electromagnetic interference shielding (EMIS) effectiveness and good thermal management properties are both required to meet the rapid development of integrated electronic components. However, it remains challenging to obtain environmentally friendly and flexible films with high EMIS and thermal management performance in an efficient and scalable way. In this paper, an environmentally friendly strategy is proposed to synthesize multifunctional waterborne Cu@Ag flake conductive films using water as the solvent and silicone-acrylic emulsion (SAE) as a matrix. The obtained films show high electrical conductivity and exceptional EMI SE and electrothermal conversion properties. The EMI SE in the X-band is higher than 76.31 dB at a thickness of 60 µm owing to the ultrahigh electrical conductivity of 1073.61 S cm-1. The film warms up quickly to 102.1 °C within 10 s under a low voltage of 2.0 V. In addition, the shielding coating is sufficiently flexible to retain a conductivity of 93.4% after 2000 bending-release cycles with a bending radius of 3 mm. This work presents an alternative strategy to produce high EMIS effectiveness and Joule heating films for highly integrated and flexible electronic components in a green, scalable, and highly efficient way.

In-situ fabrication of AAO template without oxide barrier layer and its applications.

Porous anodic aluminum oxide (AAO) film is the most widely used template in combination with electrodeposition (ED) method to fabricate one-dimensional nanostructures such as nanowires, nanotubes and nanorods. However, the existing oxide barrier layer after the anodization blocks the application of AAO template in synthesis of nanostructures via direct electrodeposition. In this paper, AAO template without oxide barrier layer was successfully fabricated by stepwise voltage decrement; influence of two types of stepwise voltage decrement on the removal of oxide barrier layer was introduced. Field Emission Scanning Electron Microscopy (FESEM) images indicated that stepwise voltage decrement could make the oxide layer thin effectively. Meanwhile, highly ordered gold nanowire arrays were fabricated by using direct electrodeposition method based on AAO template with the second anodization process with stepwise voltage decrement of 1 V/min, FESEM image showed that as-prepared gold nanowires are uniform in diameter and the diameter is in accordance with the diameter of AAO template pores. XRD pattern revealed that gold nanowires were indexed as face-centered cubic phase.

π-d Electron-Coupled PBDIT/CdS Heterostructure Enables Hole Extraction for Efficient Photocatalytic Hydrogen Production.

Construction of heterostructures is one of the most promising strategies for designing photocatalysts for highly efficient solar hydrogen (H2) production because the introduction of an electron-donating counterpart contributes to more effective photon absorption, while the heterostructures benefit spatial carrier separation. However, the hole-transfer rate is usually 2-3 orders of magnitude slower than that of the electron-transfer rate within the heterostructures, ensuing serious charge recombination. Here, we find the energy band offset-driven charge-transfer behavior in a donor-acceptor (D-A)-conjugated polymer/CdS organic/inorganic heterostructure and realize hole-transfer improvement in cooperation with a further hole removal motif of poly(3,4-ethylenedioxythiophene) polystyrene sulfonate. The photocatalytic H2 production activity is increased by nearly 2 orders of magnitude with the apparent quantum yield hitting ca. 80% at 450 nm without co-catalysts. Ultrafast transient absorption together with surface photovoltage characterizations consolidates the hole extraction mechanism. The intimate bond formed at the interface between the polymer and the inorganic semiconductor acts as an interpenetrating network at the nanoscale level, thus providing a charge-transfer freeway for boosting charge separation.

New insights into the key bifunctional role of sulfur in Fe-N-C single-atom catalysts for ORR/OER.

Sulfur-doping of non-noble metal Fe-N-C single-atom catalysts (SACs) shows a key bifunctional role in promoting ORR and OER activity. The controversial claims about the enhanced ORR activity and the ambiguity of the OER activity brought about by S-doping demand in-depth investigation. Here, systematic theoretical investigation was carried out. Unlike previously believed, coordinative S-doping gives rise to a precisely regulated OOH* stabilization effect, which is revealed to be the origin of the bifunctional ORR/OER activity. The fine regulation is reflected in two aspects: (1) Compared with other intermediates, the regulation of OOH* adsorption is more obvious. (2) More sulfur-doping leads to excessive strong or weak stabilization, which is not conducive to ORR/OER. The single S doping elevates the charge density and opens the metallic spin channels of Fe-N3|S, moves the d-band center towards the Fermi level, all contributing to moderate OOH* stabilization. It is hoped that these results will promote the development of heteroatom-doped bifunctional SACs.

TiO2 nanotubular arrays decorated with ultrafine Ag nanoseeds enabling a stable and dendrite-free lithium metal anode.

To exploit next-generation high-energy Li metal batteries, it is vitally important to settle the issue of dendrite growth accompanied by interfacial instability of the Li anode. Applying 3D current collectors as hosts for Li deposition emerges as a prospective strategy to achieve uniform Li nucleation and suppress Li dendrites. Herein, well-aligned and spaced TiO2 nanotube arrays grown on Ti foil and surface decorated with dispersed Ag nanocrystals (Ag@TNTAs/Ti) were constructed and employed as a 3D host for regulating Li stripping/plating behaviors and suppressing Li dendrites, and also relieving volume fluctuation during repetitive Li plating/stripping. Uniform TiO2 nanotubular structures with a large surface allow fast electron/ion transport and uniform local current density distribution, leading to homogeneous Li growth on the nanotube surface. Moreover, Ag nanocrystals and TiO2 nanotubes have good Li affinity, which facilitates Li+ capture and reduces the Li nucleation barrier, achieving uniform nucleation and growth of Li metal over the 3D Ag@TNTAs/Ti host. As a result, the as-fabricated Ag@TNTAs/Ti electrode exhibits dendrite-free plating morphology and long-term cycle stability with coulombic efficiency maintained over 98.5% even after 1000 cycles at a current density of 1 mA cm-2 and cycling capacity of 1 mA h cm-2. In symmetric cells, the Ag@TNTAs/Ti-Li electrode shows a much lower hysteresis of 40 mV over an ultralong cycle period of 2600 h at a current density of 1 mA cm-2 and cycling capacity of 1 mA h cm-2. Moreover, the full cell with the Ag@TNTAs/Ti-Li anode and LiFePO4 cathode achieves a high capacity of 155.2 mA h g-1 at 0.5C and retains 77.9% capacity with an average CE of ≈99.7% over 200 cycles.

Electron coupled FeS2/MoS2 heterostructure for efficient electrocatalytic ammonia synthesis under ambient conditions.

Developing efficient ammonia synthesis technology under ambient conditions is of vital importance. In this work, an FeS2 coupled MoS2 heterostructure with ultrathin features was designed by a one-step hydrothermal process for the electrochemical nitrogen reduction reaction. Density functional theory calculations reveal that the electronic structure of MoS2 greatly changes with the introduction of FeS2. The modulated electronic structure of MoS2 not only exhibits enhanced conductivity but also facilitates the activation of N2 molecules due to its abundant electronic region. The optimized FeS2/MoS2 nanosheet heterostructure achieves a high NH3 yield rate of 2.59 µmol h-1 mg-1 and a FE of 4.63% at -0.3 V vs. RHE. Besides, the well-designed nanocomposite also shows excellent selectivity without N2H4 by-products and exhibits good stability after electrocatalysis for 48 hours.

Al doped Ni-Co layered double hydroxides with surface-sulphuration for highly stable flexible supercapacitors.

In this work, a strategy of using fast co-precipitation method has been developed to synthesize stable Al doped α-phase NiCo layered double hydroxides (NCA-LDH). A subsequent surface sulphuration of NCA-LDH (S-NCA-LDH) was carried out to improve the conductivity. The as-synthesized S-NCA-LDH exhibits a good electrochemical performance with a specific capacity of 727.1C g-1 at 1 A g-1. At 20 A g-1, the specific capacity still remains 556C g-1. The capacity retention reaches 95.1 % after 10,000 cycles. Moreover, S-NCA-LDH as positive electrode, activated carbons (AC) as negative electrode and KOH doped polybenzimidazole (PBI-KOH) as solid polymer electrolyte have been employed to assemble flexible all-solid-state supercapacitors (S-NCA-LDH//PBI-KOH//AC). The device exhibits a specific capacitance of 182.6F g-1 at 0.5 A g-1 within 1.8 V. It corresponds to a specific energy of 82.2 Wh kg-1 at 450 W kg-1. It also shows a good cycling stability with capacitance retention of about 92.5 % after 10,000 cycles. Further considering the good mechanical performance under bending and folding deformation conditions, the S-NCA-LDH based flexible devices demonstrate the superiority in applications of flexible wearable electronics.

High-yielding preparation of hierarchically branched carbon nanotubes derived from zeolitic imidazolate frameworks for enhanced electrochemical K+ storage.

Branched carbon nanotubes are regarded as very promising anode materials of K-ion batteries, while the high-yielding preparation still remains challenging. We here demonstrate a facile approach for synthesizing N-doped branched carbon nanotubes (br-CNTs) in macroscopic quantity, via one-step carbonization of ZnCo-containing zeolitic imidazolate framework (ZnCo-ZIF) nanotubes. At a high current density of 2 A g-1, the as-synthesized br-CNTs could exhibit 147.2 mA h g-1 specific capacity and retain 84.5% of the initial value after 1300 cycles for electrochemical K+ storage, which is better than commercial carbon nanotubes and other carbon counterparts derived from ZnCo-ZIF particles and ZnCo-BTC nanowires. The excellent K+ storage performance of ZnCo-ZIF-derived br-CNTs actually results from the unique branched architecture and N doping, by taking advantage of more active sites and desired electrochemical kinetics as well as structural integrity. Our proposed approach would give a significant example for the scalable preparation of other complex nanostructures, and the prepared br-CNT is expected to be a very competitive candidate for high-efficiency electrochemical K+ storage.

Layer-by-Layer Assembly of CeO2-x@C-rGO Nanocomposites and CNTs as a Multifunctional Separator Coating for Highly Stable Lithium-Sulfur Batteries.

Commercialization of high-energy Li-S batteries is greatly restricted by their unsatisfactory cycle retention and poor cycling life originated from the notorious "shuttling effect" of lithium polysulfides. Modification of a commercial separator with a functional coating layer is a facile and efficient strategy beyond nanostructured composite cathodes for suppressing polysulfide shuttling. Herein, a multilayered functional CeO2-x@C-rGO/CNT separator was successfully achieved by alternately depositing conductive carbon nanotubes (CNTs) and synthetic CeO2-x@C-rGO onto the surface of the commercial separator. The cooperation of multiple components including Ce-MOF-derived CeO2-x@C, rGO, and CNTs enables the as-built CeO2-x@C-rGO/CNT separator to perform multifunctions from the separator surface: (i) to hinder the diffusion of polysulfide species through physical blocking or chemical adsorption, (ii) to accelerate the sluggish redox reactions of sulfur species, and (iii) to enhance the conductivity for sulfur re-activation and efficient utilization. Serving as a multilayer and powerful barrier, the CeO2-x@C-rGO/CNT separator greatly constrains and reutilizes the polysulfide species. Thus, the Li-S battery assembled with the CeO2-x@C-rGO/CNT separator demonstrates an excellent combination of capacity, rate capability, and cycling performances (an initial capacity of 1107 mA h g-1 with a low decay rate of 0.060% per cycle over 500 cycles at 1 C, 651 mA h g-1 at 5 C) together with remarkably mitigated self-discharge and anode corrosion. This work provides guidelines for functional separator design as well as rare-earth material applications for Li-S batteries and other energy storage systems.

Pseudocapacitive TiNb2O7/reduced graphene oxide nanocomposite for high-rate lithium ion hybrid capacitors.

Lithium ion hybrid capacitors (LIHCs) have a capacitor-type cathode and a battery-type anode and are a prospective energy storage device that delivers high energy/power density. However, the kinetic imbalance between the cathode and the anode is a key obstacle to their further development and application. Herein, we prepared TiNb2O7 nanoparticles through a facile solvothermal method and annealing treatment. Then a homogeneous three-dimensional (3D) self-supported reduced graphene oxide (rGO)-coated TiNb2O7 (TiNb2O7/rGO) nanocomposite was constructed by freeze-drying, followed by a high-temperature reduction, which demonstrates an enhanced pseudocapacitive lithium ions storage performance. Benefiting from the improved electrical conductivity, ultrashort ions diffusion paths, and 3D architecture, the TiNb2O7/rGO nanocomposite exhibits a high specific capacity of 285.0 mA h g-1, excellent rate capability (73.6% capacity retention at 8 A g-1), and superior cycling stability. More importantly, quantitative kinetics analysis reflects that the capacity of TiNb2O7/rGO is mainly dominated by capacitive behavior, making it perfectly match with the capacitor-type activated carbon (AC) cathode. By using pre-lithiated TiNb2O7/rGO as anode material and AC as cathode material, a high-rate TiNb2O7/rGO//AC LIHC device can be fabricated, which delivers an ultrahigh energy density of 127 Wh kg-1 at the power density of 200 W kg-1, a maximum power density of 10 kW kg-1 at the energy density of 56.4 Wh kg-1, and durable service life.

In situ grown hierarchical NiO nanosheet@nanowire arrays for high-performance electrochromic energy storage applications.

Electrodes with hierarchical nanoarchitectures could promote electrochemical properties due to their largely exposed active sites and quick charge transfer. Herein, in situ grown hierarchical NiO nanosheet@nanowire films are reported by a one-step hydrothermal process followed by heat treatment. The unique NiO hierarchical nanostructures, which are composed of NiO nanowires grown on the surface of a nanosheet array, show improved electrochromic properties such as large optical modulation in different light regions (95% at 550 nm and 50.6% at 1000 nm), fast color change (9.8/5.4 s) and better coloring efficiency (91.2 cm2 C-1) with long-term cycling properties (82.2% after 700 cycles). Simultaneously, the hierarchical nanostructures possess optimal areal capacitance (117.2 mF cm-2), rate performance and cycling properties. The enhanced electrochemical properties are due to the pretreated seed layer and the synergistic effect between the unique in situ grown ultrathin nanowire and the underlying vertical nanosheet layer which can strengthen the mechanical adhesion of the nanoarray film to the substrate and make both nanosheets and nanowires more exposed to the electrolyte, enhancing charge transfer and mass diffusion. This work provides a promising pathway towards developing high quality electrochromic energy storage devices.

Li2O-2B2O3 coating decorated Li4Ti5O12 anode for enhanced rate capability and cycling stability in lithium-ion batteries.

Li2O-2B2O3 (LBO) ionic conductor with high conductivity plays an important role in boosting the rate performance and cycling stability of Li4Ti5O12 (LTO) anode for lithium-ion batteries by preventing direct exposure of LTO to the electrolyte. Herein, the effect of LBO coating layer on lithium ion (Li+) storage performance is investigated in detail by adjusting the adding amount of LBO precursor dispersion. LTO coated with 2 wt% LBO achieves an optimum performance with a specific capacity of 172.9 mA h g-1 at a current density of 0.1 A g-1, an improved rate capability (specific capacity of 127.9 mA h g-1 is maintained when the current density is 20 times than 0.1 A g-1) and a remarkable cycling stability (capacity retention of 94.2% after 4000 cycles at 2.0 A g-1). These LBO-LTO composites are competitive and promising candidates for electrochemical energy storage and other applications.

Directly Exfoliated Ultrathin Silicon Nanosheets for Enhanced Photocatalytic Hydrogen Production.

Here we present direct exfoliation of ultrathin silicon nanosheets from commercial silicon powders through an improved liquid phase exfoliation procedure. The feasibility of exfoliation was ascribed to the intrinsic anisotropic lattice structure, which allowed the oriented propagations of cryo-mediation-induced quenching cracks with the assistance of sonication. It was also revealed that the solid-solvent interface played a critical role in determining the morphology of exfoliated pieces as well as the exfoliation efficiency. Moreover, due to its superior morphology, enlarged surface area, and improved photon absorption, the resulting ultrathin silicon nanosheets presented enhanced and visible light responsive photocatalytic hydrogen generation performance, even without applying any co-catalyst.