Regulating Reconstruction-Engineered Active Sites for Accelerated Electrocatalytic Conversion of Urea.

Reconstruction-engineered electrocatalysts with enriched high active Ni species for urea oxidation reaction (UOR) have recently become promising candidates for energy conversion. However, to inhibit the over-oxidation of urea brought by the high valence state of Ni, tremendous efforts are devoted to obtaining low-value products of nitrogen gas to avoid toxic nitrite formation, undesirably causing inefficient utilization of the nitrogen cycle. Herein, we proposed a mediation engineering strategy to significantly boost high-value nitrite formation to help close a loop for the employment of a nitrogen economy. Specifically, platinum-loaded nickel phosphides (Pt-Ni2P) catalysts exhibit a promising nitrite production rate (0.82 mol kWh-1 cm-2), high stability over 66 h of Zn-urea-air battery operation, and 135 h of co-production of nitrite and hydrogen under 200 mA cm-2 in a zero-gap membrane electrode assembly (MEA) system. The in situ spectroscopic characterizations and computational calculations demonstrated that the urea oxidation kinetics is facilitated by enriched dynamic Ni3+ active sites, thus augmenting the "cyanate" UOR pathway. The C-N cleavage was further verified as the rate-determining step for nitrite generation.

Atomically dispersed asymmetric cobalt electrocatalyst for efficient hydrogen peroxide production in neutral media.

Electrochemical hydrogen peroxide (H2O2) production (EHPP) via a two-electron oxygen reduction reaction (2e- ORR) provides a promising alternative to replace the energy-intensive anthraquinone process. M-N-C electrocatalysts, which consist of atomically dispersed transition metals and nitrogen-doped carbon, have demonstrated considerable EHPP efficiency. However, their full potential, particularly regarding the correlation between structural configurations and performances in neutral media, remains underexplored. Herein, a series of ultralow metal-loading M-N-C electrocatalysts are synthesized and investigated for the EHPP process in the neutral electrolyte. CoNCB material with the asymmetric Co-C/N/O configuration exhibits the highest EHPP activity and selectivity among various as-prepared M-N-C electrocatalyst, with an outstanding mass activity (6.1 × 105 A gCo-1 at 0.5 V vs. RHE), and a high practical H2O2 production rate (4.72 mol gcatalyst-1 h-1 cm-2). Compared with the popularly recognized square-planar symmetric Co-N4 configuration, the superiority of asymmetric Co-C/N/O configurations is elucidated by X-ray absorption fine structure spectroscopy analysis and computational studies.

Recent Advances in Ultralow-Pt-Loading Electrocatalysts for the Efficient Hydrogen Evolution.

Hydrogen production from water electrolysis provides a green and sustainable route. Platinum (Pt)-based materials have been regarded as efficient electrocatalysts for the hydrogen evolution reaction (HER). However, the large-scale commercialization of Pt-based catalysts suffers from the high cost. Therefore, ultralow-Pt-loading electrocatalysts, which can reach the balance of low cost and high HER performance, have attracted much attention. In this review, representative promising synthetic strategies, including wet chemistry, annealing, electrochemistry, photochemistry, and atomic layer deposition are summarized. Further, the interaction between different electrocatalyst components (transition metals and their derivatives) and Pt is discussed. Notably, this interaction can effectively accelerate the kinetics of the HER, enhancing the catalytic activity. At last, current challenges and future perspectives are briefly discussed.

Multilayered Molybdate Microflowers Fabricated by One-Pot Reaction for Efficient Water Splitting.

The development of high-performance, low-cost and rapid-production bifunctional electrocatalysts towards overall water splitting still poses huge challenges. Herein, the authors utilize a facile hydrothermal method to synthesize a novel structure of Co-doped ammonium lanthanum molybdate on Ni foams (Co-ALMO@NF) as self-supported electrocatalysts. Owing to large active surfaces, lattice defect and conductive channel for rapid charge transport, Co-ALMO@NF exhibits good electrocatalytic performances which requires only 349/341 mV to achieve a high current density of 600 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), respectively. Besides, a low cell voltage of 1.52 V is required to reach the current density of 10 mA cm-2 in alkaline medium along with an excellent long-term stability for two-electrode configurations. Density functional theory calculations are performed to reveal the reaction mechanism on Co-ALMO@NF, which shows that the Mo site is the most favorable ones for HER, while the introduction of Co is beneficial to reduce the adsorption intensity on the surface of Co-ALMO@NF, thus accelerating OER process. This work highlighted the importance of the structural design for self-supporting electrocatalysts.

Spectroscopic Identification of Active Sites of Oxygen-Doped Carbon for Selective Oxygen Reduction to Hydrogen Peroxide.

The electrochemical synthesis of hydrogen peroxide (H2 O2 ) via a two-electron (2 e- ) oxygen reduction reaction (ORR) process provides a promising alternative to replace the energy-intensive anthraquinone process. Herein, we develop a facile template-protected strategy to synthesize a highly active quinone-rich porous carbon catalyst for H2 O2 electrochemical production. The optimized PCC900 material exhibits remarkable activity and selectivity, of which the onset potential reaches 0.83 V vs. reversible hydrogen electrode in 0.1 M KOH and the H2 O2 selectivity is over 95 % in a wide potential range. Comprehensive synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with electrocatalytic characterizations reveals the positive correlation between quinone content and 2 e- ORR performance. The effectiveness of chair-form quinone groups as the most efficient active sites is highlighted by the molecule-mimic strategy and theoretical analysis.

A highly efficient atomically thin curved PdIr bimetallene electrocatalyst.

The multi-metallene with an ultrahigh surface area has great potential in precise tuning of surface heterogeneous d-electronic correlation by surface strain effect for the distinctive surface electronic structure, which is a brand new class of promising 2D electrocatalyst for sustainable energy device application. However, achieving such an atomically thin multi-metallene still presents a great challenge. Herein, we present a new synthetic method for an atomic-level palladium-iridium (PdIr) bimetallene with an average thickness of only ∼1.0 nm for achieving superior catalysis in the hydrogen evolution reaction (HER) and the formic acid oxidation reaction (FAOR). The curved PdIr bimetallene presents a top-ranked high electrochemical active area of 127.5 ± 10.8 m2 gPd+Ir -1 in the reported noble alloy materials, and exhibits a very low overpotential, ultrahigh activity and improved stability for HER and FAOR. DFT calculation reveals that the PdIr bimetallene herein has a unique lattice tangential strain, which can induce surface distortion while concurrently creating a variety of concave-convex featured micro-active regions formed by variously coordinated Pd sites agglomeration. Such a strong strain effect correlates the abnormal on-site active 4d10-t2g-orbital Coulomb correlation potential and directly elevates orbital-electronegativity exposure within these active regions, resulting in a preeminent barrier-free energetic path for significant enhancement of FAOR and HER catalytic performance.

A Metal-Organic-Framework-Derived (Zn0.95Cu0.05)0.6Cd0.4S Solid Solution as Efficient Photocatalyst for Hydrogen Evolution Reaction.

Photocatalytic water splitting taking the advantage of using solar energy directly is one of the most effective strategies for hydrogen evolution. The development of facile methods for synthesizing highly efficient and stable photocatalysts for hydrogen production still remains a great challenge. Herein, a metal-organic framework (MOF)-templated strategy was designed for the synthesis of solid solutions of (Zn0.95Cu0.05)1-xCdxS that exhibit outstanding photocatalytic hydrogen production reaction activity. More importantly, efficient light capturing ability and photogenerated charges separation were accomplished via fine-tuning the composition of the photocatalysts by adjusting the concentrations of doping metals in the template MOFs. Under visible light (λ > 420 nm), an optimized nanocatalyst, (Zn0.95Cu0.05)0.6Cd0.4S, exhibited a higher durability and satisfied photocatalytic hydrogen evolution rate of 4150.1 µmol g-1 h-1 of water splitting.

Q-Carbon: A New Carbon Allotrope with a Low Degree of s-p Orbital Hybridization and Its Nucleation Lithiation Process in Lithium-Ion Batteries.

A novel metallic carbon allotrope, Q-carbon, was discovered using first-principles calculations. The named Q-carbon possessed a three-dimensional (3D) cage structure formed by carbon atoms with three ligands. The energy distribution of electrons in different orbitals revealed that Q-carbon has a low degree of s-p orbital hybridization. The calculated Li+ binding energies suggested Li+ aggregation inside Q-carbon during lithiation. As a result, a Li8C32 phase was formed and gradually expanded in Q-carbon, implying a typical two-phase transition. This allowed Q-carbon to have a constant theoretical voltage of 0.40 V, which effectively inhibited Li dendrite formation. A stable Li8C32/C32 two-phase interface was confirmed by stress-strain analysis, and a calculated Li+ diffusion barrier of ∼0.50 eV ensured effective Li+ diffusion along a 3D pathway. This study was of great significance for the understanding of two-phase transition of Li+ storage materials and provided a new insight into the design of new carbon materials for energy storage applications.

Multidimensional Integrated Chalcogenides Nanoarchitecture Achieves Highly Stable and Ultrafast Potassium-Ion Storage.

Potassium-ion batteries (KIBs) have come into the spotlight in large-scale energy storage systems because of cost-effective and abundant potassium resources. However, the poor rate performance and problematic cycle life of existing electrode materials are the main bottlenecks to future potential applications. Here, the first example of preparing 3D hierarchical nanoboxes multidimensionally assembled from interlayer-expanded nano-2D MoS2 @dot-like Co9 S8 embedded into a nitrogen and sulfur codoped porous carbon matrix (Co9 S8 /NSC@MoS2 @NSC) for greatly boosting the electrochemical properties of KIBs in terms of reversible capacity, rate capability, and cycling lifespan, is reported. Benefiting from the synergistic effects, Co9 S8 /NSC@MoS2 @NSC manifest a very high reversible capacity of 403 mAh g-1 at 100 mA g-1 after 100 cycles, an unprecedented rate capability of 141 mAh g-1 at 3000 mA g-1 over 800 cycles, and a negligible capacity decay of 0.02% cycle-1 , boosting promising applications in high-performance KIBs. Density functional theory calculations demonstrate that Co9 S8 /NSC@MoS2 @NSC nanoboxes have large adsorption energy and low diffusion barriers during K-ion storage reactions, implying fast K-ion diffusion capability. This work may enlighten the design and construction of advanced electrode materials combined with strong chemical bonding and integrated functional advantages for future large-scale stationary energy storage.

Formation of bonding interface in explosive welding-a molecular dynamics approach.

The bonding between copper (Cu) and iron (Fe) to form a bi-layer composite using explosive welding is investigated through molecular dynamics simulation. Three stages in the joining process, including loading, unloading and cooling, are sequentially considered in modelling the formation of the bonding interface. The results demonstrate that three types of bonding interfaces can be obtained, based on whether melting happens. The morphologies and the atomic structures of the three types bonding interfaces in each stage are analyzed. The formation of nanograins near the bonding interface is mainly due to the melting and subsequent cooling process. Atomic simulations of tensile tests reveal that melting is not a necessary factor to form the bonding interface. What's more, depending on whether melting occurs, the joining mechanism can be regarded as pressure welding or fusion-diffusion welding.

Nucleation and Conversion Transformations of the Transition Metal Polysulfide VS4 in Lithium-Ion Batteries.

Transition metal polysulfides with high S content, such as VS4, TiS4, and MoS3, have high specific Li+ capacities, but their reaction mechanisms for lithium-ion batteries remain unclear due to unknown intermediate products. In this work, first-principles calculations based on the density functional theory were performed to reveal the electrochemical properties of VS4 for lithium-ion batteries. The results demonstrated multiple phase transformations during Li+ insertion, starting with nucleation transformation from VS4 to Li3VS4 and followed by gradual decomposition reactions. Enthalpy-driven long-range migration of Li2S molecules resulted in crystalline to amorphous transformation during decomposition. S and V successively behaved as redox centers for Li xVS4 before and after x = 3. Moreover, low activation energy and high Li+ diffusivity were observed at room temperature, revealing superior rate capability of the material.

Deeply Nesting Zinc Sulfide Dendrites in Tertiary Hierarchical Structure for Potassium Ion Batteries: Enhanced Conductivity from Interior to Exterior.

Transition metal sulfides are deemed as attractive anode materials for potassium-ion batteries (KIBs) due to their high theoretical capacities based on conversion and alloying reaction. However, the main challenges are the low electronic conductivity, huge volume expansion, and consequent formation of unstable solid electrolyte interphase (SEI) upon potassiation/depotassiation. Herein, zinc sulfide dendrites deeply nested in the tertiary hierarchical structure through a solvothermal-pyrolysis process are designed as an anode material for KIBs. The tertiary hierarchical structure is composed of the primary ultrafine ZnS nanorods, the secondary carbon nanosphere, and the tertiary carbon-encapsulated ZnS subunits nanosphere structure. The architectural design of this material provides a stable diffusion path and enhances effective conductivity from the interior to exterior for both K+ ions and electrons, buffers the volume expansion, and constructs a stable SEI during cycling. A stable specific capacity of 330 mAh g-1 is achieved after 100 cycles at the current density of 50 mA g-1 and 208 mAh g-1 at 500 mA g-1 over 300 cycles. Using density functional theory calculations, we discover the interactions between ZnS and carbon interface can effectively decrease the K+ ions diffusion barrier and therefore promote the reversibility of K+ ions storage.

Constructing Heterointerface of Metal Atomic Layer and Amorphous Anode Material for High-Capacity and Fast Lithium Storage.

Interfacial engineering plays an important role in tuning the intrinsic property of electrode materials for energy applications such as lithium-ion batteries (LIBs), which however is rarely realized to amorphous electrode materials, despite a set of characteristics of amorphous materials desirable for LIBs. Here, Au atomic cluster layer-interfaced amorphous porous CoSnO3 nanocubes were fabricated by galvanic replacement and employed as a superior LIB anode, showing high reversible capacity (1615 mAh g-1 at 0.2 A g-1), good rate capability (1059 mAh g-1 with a 61.3% capacity retention upon the dramatic current variation from 0.1 to 5 A g-1), and excellent cycling stability. The amorphous nature, interconnected mesopores, and especially the thin Au atomic cluster layer on the surface/pore walls of CoSnO3 nanocubes can not only improve electron transport and ion diffusion in the electrode and electrolyte but also release the volume strain. Most significantly, density functional theory calculations reveal that the CoSnO3∥Au heterointerface can induce the atomic polarization of CoSnO3 and lower Li ion diffusion barriers in CoSnO3 near the heterointerface, endowing it with a significantly enhanced theoretical lithium storage capacity and outstanding rate capability. This study provides a model of a heterointerface for amorphous materials with desired properties for high-performance LIBs and future energy applications.

Heterostructure-Promoted Oxygen Electrocatalysis Enables Rechargeable Zinc-Air Battery with Neutral Aqueous Electrolyte.

Neutral aqueous zinc-air batteries (ZABs) are an emerging type of energy devices with substantially elongated lifetime and improved recyclability compared to conventional alkaline ZABs. However, their development is impeded by the lack of robust bifunctional catalyst at the air-electrode for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR). Here, we report the controlled synthesis of NiFe2O4/FeNi2S4 heterostructured nanosheets (HNSs) that are highly efficient in catalyzing OER and ORR, therefore enabling neutral rechargeable ZABs. Associated with the formation of abundant oxide/sulfide interfaces over NiFe2O4/FeNi2S4 HNSs' surfaces, the catalyst's oxygen binding energy can be effectively tuned to enhance the OER and ORR activities, as revealed by the density functional theory calculations. In 0.2 M phosphate buffer solution, the optimized NiFe2O4/FeNi2S4 HNSs present an excellent oxygen electrocatalytic activity and stability, with much lower OER and ORR overpotentials than single-component FeNi2S4 or NiFe2O4 and with negligible performance decay in accelerated durability testing. When used as an air-electrode, the NiFe2O4/FeNi2S4 HNSs can deliver a power density of 44.4 mW cm-2 and a superior cycling stability (only 0.6% decay after 900 cycles at 0.5 mA cm-2), making the resultant ZAB the most efficient and robust one with a neutral aqueous electrolyte reported to date. This work highlights the essential function of the heterostructure interface in oxygen electrocatalysis, opening a new avenue to advanced neutral metal-air batteries.

Iridium-Tungsten Alloy Nanodendrites as pH-Universal Water-Splitting Electrocatalysts.

The development of highly efficient and durable electrocatalysts for high-performance overall water-splitting devices is crucial for clean energy conversion. However, the existing electrocatalysts still suffer from low catalytic efficiency, and need a large overpotential to drive the overall water-splitting reactions. Herein, we report an iridium-tungsten alloy with nanodendritic structure (IrW ND) as a new class of high-performance and pH-universal bifunctional electrocatalysts for hydrogen and oxygen evolution catalysis. The IrW ND catalyst presents a hydrogen generation rate ∼2 times higher than that of the commercial Pt/C catalyst in both acid and alkaline media, which is among the most active hydrogen evolution reaction (HER) catalysts yet reported. The density functional theory (DFT) calculations reveal that the high HER intrinsic catalytic activity results from the suitable hydrogen and hydroxyl binding energies, which can accelerate the rate-determining step of the HER in acid and alkaline media. Moreover, the IrW NDs show superb oxygen evolution reaction (OER) activity and much improved stability over Ir. The theoretical calculation demonstrates that alloying Ir metal with W can stabilize the formed active iridium oxide during the OER process and lower the binding energy of reaction intermediates, thus improving the Ir corrosion resistance and OER kinetics. Furthermore, the overall water-splitting devices driven by IrW NDs can work in a wide pH range and achieve a current density of 10 mA cm-2 in acid electrolyte at a low potential of 1.48 V.

Sulfur-Deficient Bismuth Sulfide/Nitrogen-Doped Carbon Nanofibers as Advanced Free-Standing Electrode for Asymmetric Supercapacitors.

The use of free-standing carbon-based hybrids plays a crucial role to help fulfil ever-increasing energy storage demands, but is greatly hindered by the limited number of active sites for fast charge adsorption/desorption processes. Herein, an efficient strategy is demonstrated for making defect-rich bismuth sulfides in combination with surface nitrogen-doped carbon nanofibers (dr-Bi2 S3 /S-NCNF) as flexible free-standing electrodes for asymmetric supercapacitors. The dr-Bi2 S3 /S-NCNF composite exhibits superior electrochemical performances with an enhanced specific capacitance of 466 F g-1 at a discharge current density of 1 A g-1 . The high performance of dr-Bi2 S3 /S-NCNF electrodes originates from its hierarchical structure of nitrogen-doped carbon nanofibers with well-anchored defect-rich bismuth sulfides nanostructures. As modeled by density functional theory calculation, the dr-Bi2 S3 /S-NCNF electrodes exhibit a reduced OH- adsorption energy of -3.15 eV, compared with that (-3.06 eV) of defect-free bismuth sulfides/surface nitrogen-doped carbon nanofiber (df-Bi2 S3 /S-NCNF). An asymmetric supercapacitor is further fabricated by utilizing dr-Bi2 S3 /S-NCNF hybrid as the negative electrode and S-NCNF as the positive electrode. This composite exhibits a high energy density of 22.2 Wh kg-1 at a power density of 677.3 W kg-1 . This work demonstrates a feasible strategy to construct advanced metal sulfide-based free-standing electrodes by incorporating defect-rich structures using surface engineering principles.

Aggregation Strength Tuning in Difluorobenzoxadiazole-Based Polymeric Semiconductors for High-Performance Thick-Film Polymer Solar Cells.

High-performance polymer solar cells (PSCs) with thick active layers are essential for large-scale production. Polymer semiconductors exhibiting a temperature-dependent aggregation property offer great advantages toward this purpose. In this study, three difluorobenzoxadiazole (ffBX)-based donor polymers, PffBX-T, PffBX-TT, and PffBX-DTT, were synthesized, which contain thiophene (T), thieno[3,2- b]thiophene (TT), and dithieno[3,2- b:2',3'- d]thiophene (DTT) as the π-spacers, respectively. Temperature-dependent absorption spectra reveal that the aggregation strength increases in the order of PffBX-T, PffBX-TT, and PffBX-DTT as the π-spacer becomes larger. PffBX-TT with the intermediate aggregation strength enables well-controlled disorder-order transition in the casting process of blend film, thus leading to the best film morphology and the highest performance in PSCs. Thick-film PSCs with an average power conversion efficiency (PCE) of 8.91% and the maximum value of 9.10% are achieved using PffBX-TT:PC71BM active layer with a thickness of 250 nm. The neat film of PffBX-TT also shows a high hole mobility of 1.09 cm2 V-1 s-1 in organic thin-film transistors. When PffBX-DTT and PffBX-T are incorporated into PSCs utilizing PC71BM acceptor, the average PCE decreases to 6.54 and 1.33%, respectively. The performance drop mainly comes from reduced short-circuit current, as a result of nonoptimal blend film morphology caused by a less well-controlled film formation process. A similar trend was also observed in nonfullerene type thick-film PSCs using IT-4F as the electron acceptor. These results show the significance of polymer aggregation strength tuning toward optimal bulk heterojunction film morphology using ffBX-based polymer model system. The study demonstrates that adjusting π-spacer is an effective method, in combination with other important approaches such as alkyl chain optimization, to generate high-performance thick-film PSCs which are critical for practical applications.

An Efficient, Visible-Light-Driven, Hydrogen Evolution Catalyst NiS/Znx Cd1-x S Nanocrystal Derived from a Metal-Organic Framework.

Photocatalytic water splitting for hydrogen production using sustainable sunlight is a promising alternative to industrial hydrogen production. However, the scarcity of highly active, recyclable, inexpensive photocatalysts impedes the development of photocatalytic hydrogen evolution reaction (HER) schemes. Herein, a metal-organic framework (MOF)-template strategy was developed to prepare non-noble metal co-catalyst/solid solution heterojunction NiS/Znx Cd1-x S with superior photocatalytic HER activity. By adjusting the doping metal concentration in MOFs, the chemical compositions and band gaps of the heterojunctions can be fine-tuned, and the light absorption capacity and photocatalytic activity were further optimized. NiS/Zn0.5 Cd0.5 S exhibits an optimal HER rate of 16.78 mmol g-1 h-1 and high stability and recyclability under visible-light irradiation (λ>420 nm). Detailed characterizations and in-depth DFT calculations reveal the relationship between the heterojunction and photocatalytic activity and confirm the importance of NiS in accelerating the water dissociation kinetics, which is a crucial factor for photocatalytic HER.

Pistachio-Shuck-Like MoSe2 /C Core/Shell Nanostructures for High-Performance Potassium-Ion Storage.

Potassium-ion batteries (KIBs) have recently attracted intensive attention because of the abundant potassium resources and their low cost and high safety. However, the major challenge faced by KIBs lies in the lack of stable and high-capacity materials for the intercalation/deintercalation of large-size potassium ions. A unique pistachio-shuck-like MoSe2 /C core/shell nanostructure (PMC) is synthesized herein as an advanced anode for boosting the performance of KIBs. This PMC is featured with a few layers of molybdenum selenide as the core with an expanded interlayer spacing of ≈0.85 nm, facilitating the intercalation/deintercalation of K ions, and a thin amorphous carbon as the shell, which can confine the active molybdenum selenide nanosheets during cycling for maintaining the high structural stability. Most importantly, as a whole, the PMC has the advantages of reducing the surplus hollow interior space for improving its packing density and buffering the volume expansion during the K-ion intercalation for further enhancing the stability. As a consequence, the PMC shows a very high capacity of 322 mAh g-1 at 0.2 A g-1 over 100 cycles, and can still remain 226 mAh g-1 at 1.0 A g-1 for a long period of 1000 cycles, which is among the best-reported KIBs anodes.