Core-Shell CoS2@MoS2 with Hollow Heterostructure as an Efficient Electrocatalyst for Boosting Oxygen Evolution Reaction.

It is imperative to develop an efficient catalyst to reduce the energy barrier of electrochemical water decomposition. In this study, a well-designed electrocatalyst featuring a core-shell structure was synthesized with cobalt sulfides as the core and molybdenum disulfide nanosheets as the shell. The core-shell structure can prevent the agglomeration of MoS2, expose more active sites, and facilitate electrolyte ion diffusion. A CoS2/MoS2 heterostructure is formed between CoS2 and MoS2 through the chemical interaction, and the surface chemistry is adjusted. Due to the morphological merits and the formation of the CoS2/MoS2 heterostructure, CoS2@MoS2 exhibits excellent electrocatalytic performance during the oxygen evolution reaction (OER) process in an alkaline electrolyte. To reach the current density of 10 mA cm-2, only 254 mV of overpotential is required for CoS2@MoS2, which is smaller than that of pristine CoS2 and MoS2. Meanwhile, the small Tafel slope (86.9 mV dec-1) and low charge transfer resistance (47 Ω) imply the fast dynamic mechanism of CoS2@MoS2. As further confirmed by cyclic voltammetry curves for 1000 cycles and the CA test for 10 h, CoS2@MoS2 shows exceptional catalytic stability. This work gives a guideline for constructing the core-shell heterostructure as an efficient catalyst for oxygen evolution reaction.

Hierarchically Structured Graphene Aerogel Supported Nickel-Cobalt Oxide Nanowires as an Efficient Electrocatalyst for Oxygen Evolution Reaction.

The rational design of a heterostructure electrocatalyst is an attractive strategy to produce hydrogen energy by electrochemical water splitting. Herein, we have constructed hierarchically structured architectures by immobilizing nickel-cobalt oxide nanowires on/beneath the surface of reduced graphene aerogels (NiCoO2/rGAs) through solvent-thermal and activation treatments. The morphological structure of NiCoO2/rGAs was characterized by microscopic analysis, and the porous structure not only accelerates the electrolyte ion diffusion but also prevents the agglomeration of NiCoO2 nanowires, which is favorable to expose the large surface area and active sites. As further confirmed by the spectroscopic analysis, the tuned surface chemical state can boost the catalytic active sites to show the improved oxygen evolution reaction performance in alkaline electrolytes. Due to the synergistic effect of morphology and composition effect, NiCoO2/rGAs show the overpotential of 258 mV at the current density of 10 mA cm-2. Meanwhile, the small values of the Tafel slope and charge transfer resistance imply that NiCoO2/rGAs own fast kinetic behavior during the OER test. The overlap of CV curves at the initial and 1001st cycles and almost no change in current density after the chronoamperometric (CA) test for 10 h confirm that NiCoO2/rGAs own exceptional catalytic stability in a 1 M KOH electrolyte. This work provides a promising way to fabricate the hierarchically structured nanomaterials as efficient electrocatalysts for hydrogen production.

Synergistic Engineering of CoO/MnO Heterostructures Integrated with Nitrogen-Doped Carbon Nanofibers for Lithium-Ion Batteries.

The integration of heterostructures within electrode materials is pivotal for enhancing electron and Li-ion diffusion kinetics. In this study, we synthesized CoO/MnO heterostructures to enhance the electrochemical performance of MnO using a straightforward electrostatic spinning technique followed by a meticulously controlled carbonization process, which results in embedding heterostructured CoO/MnO nanoparticles within porous nitrogen-doped carbon nanofibers (CoO/MnO/NC). As confirmed by density functional theory calculations and experimental results, CoO/MnO heterostructures play a significant role in promoting Li+ ion and charge transfer, improving electronic conductivity, and reducing the adsorption energy. The accelerated electron and Li-ion diffusion kinetics, coupled with the porous nitrogen-doped carbon nanofiber structure, contribute to the exceptional electrochemical performance of the CoO/MnO/NC electrode. Specifically, the as-prepared CoO/MnO/NC exhibits a high reversible specific capacity of 936 mA h g-1 at 0.1 A g-1 after 200 cycles and an excellent high-rate capacity of 560 mA h g-1 at 5 A g-1, positioning it as a competitive anode material for lithium-ion batteries. This study underscores the critical role of electronic and Li-ion regulation facilitated by heterostructures, offering a promising pathway for designing transition metal oxide-based anode materials with high performances for lithium-ion batteries.

Realizing Ultrafast and Robust Sodium-Ion Storage of Iron Sulfide Enabled by Heteroatomic Doping and Regulable Interface Engineering.

Fe-based sulfides are a promising type of anode material for sodium-ion batteries (SIBs) due to their high theoretical capacities and affordability. However, these materials often suffer from issues such as capacity deterioration and poor conductivity during practical application. To address these challenges, an N-doped Fe7S8 anode with an N, S co-doped porous carbon framework (PPF-800) was synthesized using a template-assisted method. When serving as an anode for SIBs, it delivers a robust and ultrafast sodium storage performance, with a discharge capacity of 489 mAh g-1 after 500 cycles at 5 A g-1 and 371 mAh g-1 after 1000 cycles at 30 A g-1 in the ether-based electrolyte. This impressive performance is attributed to the combined influence of heteroatomic doping and adjustable interface engineering. The N, S co-doped carbon framework embedded with Fe7S8 nanoparticles effectively addresses the issues of volumetric expansion, reduces the impact of sodium polysulfides, improves intrinsic conductivity, and stimulates the dominant pseudocapacitive contribution (90.3% at 2 mV s-1). Moreover, the formation of a stable solid electrolyte interface (SEI) film by the effect of uniform pore structure in ether-based electrolyte produces a lower transfer resistance during the charge-discharge process, thereby boosting the rate performance of the electrode material. This work expands a facile strategy to optimize the electrochemical performance of other metal sulfides.

Identifying the Transfer Kinetics of Adsorbed Hydroxyl as a Descriptor of Alkaline Hydrogen Evolution Reaction.

The key descriptor that dominates the kinetics of the alkaline hydrogen evolution reaction (HER) has not yet been unequivocally identified. Herein, we focus on the adsorbed hydroxyl (OHad ) transfer process (OHad + e- ⇄ OH- ) and reveal its crucial role in promoting the overall kinetics of alkaline HER based on Ni/Co-modified MoSe2 model catalysts (Ni-MoSe2 and Co-MoSe2 ) that feature almost identical water dissociation and hydrogen adsorption energies, but evidently different activity trends in alkaline (Ni-MoSe2 ≫ Co-MoSe2 ) and acidic (Co-MoSe2 ≥ Ni-MoSe2 ) media. Experimental and theoretical calculation results demonstrate that tailoring MoSe2 with Ni not only optimizes the hydroxyl adsorption, but also promotes the desorption of OH- and the electron-involved conversion of OHad to OH- , all of which synergistically accelerate the kinetics of OHad + e- ⇄ OH- and thereby the overall kinetics of the alkaline HER.

Solubility-Parameter-Guided Solvent Selection to Initiate Ostwald Ripening for Interior Space-Tunable Structures with Architecture-Dependent Electrochemical Performance.

Despite significant advancement in preparing various hollow structures by Ostwald ripening, one common problem is the intractable uncontrollability of initiating Ostwald ripening due to the complexity of the reaction processes. Here, a new strategy on Hansen solubility parameter (HSP)-guided solvent selection to initiate Ostwald ripening is proposed. Based on this comprehensive principle for solvent optimization, N,N-dimethylformamide (DMF) was screened out, achieving accurate synthesis of interior space-tunable MoSe2 spherical structures (solid, core-shell, yolk-shell and hollow spheres). The resultant MoSe2 structures exhibit architecture-dependent electrochemical performances towards hydrogen evolution reaction and sodium-ion batteries. This pre-solvent selection strategy can effectively provide researchers great possibility in efficiently synthesizing various hollow structures. This work paves a new pathway for deeply understanding Ostwald ripening.

Trace Cu+-dominated band structure engineering in CuxIn0.25ZnSy for promoting photocatalytic H2 evolution.

As an attractive semiconductor photocatalyst, (CuInS2)x-(ZnS)y has been intensively studied in photocatalysis, due to its unique layered structure and stability. Here, we synthesized a series of CuxIn0.25ZnSy photocatalysts with different trace Cu+-dominated ratios. The results show that doping with Cu+ ions leads to an increase in the valence state of In and the formation of a distorted S structure, simultaneously inducing a decrease in the semiconductor bandgap. When the doping amount of Cu+ ions is 0.04 atomic ratio to Zn, the optimized Cu0.04In0.25ZnSy photocatalyst with a bandgap of 2.16 eV shows the highest catalytic hydrogen evolution activity (191.4 µmol.h-1). Subsequently, among the common cocatalysts, Rh loaded Cu0.04In0.25ZnSy gives the highest activity of 1189.8 µmol·h-1, corresponding to an apparent quantum efficiency of 49.11 % at 420 nm. Moreover, the internal mechanism of photogenerated carrier transfer between semiconductors and different cocatalysts is analyzed by the band bending phenomenon.

Constructing Abundant Oxygen-Containing Functional Groups in Hard Carbon Derived from Anthracite for High-Performance Sodium-Ion Batteries.

Hard carbon is regarded as one of the greatest potential anode materials for sodium-ion batteries (SIBs) because of its affordable price and large layer spacing. However, its poor initial coulombic efficiency (ICE) and low specific capacity severely restrict its practical commercialization in SIBs. In this work, we successfully constructed abundant oxygen-containing functional groups in hard carbon by using pre-oxidation anthracite as the precursor combined with controlling the carbonization temperature. The oxygen-containing functional groups in hard carbon can increase the reversible Na+ adsorption in the slope region, and the closed micropores can be conducive to Na+ storage in the low-voltage platform region. As a result, the optimal sample exhibits a high initial reversible sodium storage capacity of 304 mAh g-1 at 0.03 A g-1, with an ICE of 67.29% and high capacitance retention of 95.17% after 100 cycles. This synergistic strategy can provide ideas for the design of high-performance SIB anode materials with the intent to regulate the oxygen content in the precursor.

Synergistic Structure and Iron-Vacancy Engineering Realizing High Initial Coulombic Efficiency and Kinetically Accelerated Lithium Storage in Lithium Iron Oxide.

Transition metal oxides with high capacity still confront the challenges of low initial coulombic efficiency (ICE, generally <70%) and inferior cyclic stability for practical lithium-storage. Herein, a hollow slender carambola-like Li0.43 FeO1.51 with Fe vacancies is proposed by a facile reaction of Fe3+ -containing metal-organic frameworks with Li2 CO3 . Synthesis experiments combined with synchrotron-radiation X-ray measurements identify that the hollow structure is caused by Li2 CO3 erosion, while the formation of Fe vacancies is resulted from insufficient lithiation process with reduced Li2 CO3 dosage. The optimized lithium iron oxides exhibit remarkably improved ICE (from 68.24% to 86.78%), high-rate performance (357 mAh g-1 at 5 A g-1 ), and superior cycling stability (884 mAh g-1 after 500 cycles at 0.5 A g-1 ). Paring with LiFePO4 cathodes, the full-cells achieve extraordinary cyclic stability with 99.3% retention after 100 cycles. The improved electrochemical performances can be attributed to the synergy of structural characteristics and Fe vacancy engineering. The unique hollow structure alleviates the volume expansion of Li0.43 FeO1.51 , while the in situ generated Fe vacancies are powerful for modulating electronic structure with boosted Li+ transport rate and catalyze more Li2 O decomposition to react with Fe in the first charge process, hence enhancing the ICE of lithium iron oxide anode materials.

Improvement of Lithium Storage Performance of Silica Anode by Using Ketjen Black as Functional Conductive Agent.

In this paper, SiO2 aerogels were prepared by a sol-gel method. Using Ketjen Black (KB), Super P (SP) and Acetylene Black (AB) as a conductive agent, respectively, the effects of the structure and morphology of the three conductive agents on the electrochemical performance of SiO2 gel anode were systematically investigated and compared. The results show that KB provides far better cycling and rate performance than SP and AB for SiO2 anode electrodes, with a reversible specific capacity of 351.4 mA h g-1 at 0.2 A g-1 after 200 cycles and a stable 311.7 mA h g-1 at 1.0 A g-1 after 500 cycles. The enhanced mechanism of the lithium storage performance of SiO2-KB anode was also proposed.

Regulating the electronic structure of MoO2/Mo2C/C by heterostructure and oxygen vacancies for boosting lithium storage kinetics.

The electronic structure regulation of electrode materials can improve the ion/electron kinetics, which is beneficial to the cyclic performance and rate capability for lithium ion batteries (LIBs). Herein, we propose a facile strategy to achieve a MoO2/Mo2C/C heterostructure with abundant oxygen vacancies. Density functional theory calculations indicate that the heterostructure of MoO2/Mo2C/C can significantly promote the Li+/charge transfer and reduce the Li adsorption energy, and the abundant oxygen vacancies in MoO2/Mo2C/C can improve the intrinsic electronic conductivity and reduce the Li+ diffusion barrier. Benefiting from the multiscale coordinated regulation, the obtained MoO2/Mo2C/C film exhibits outstanding high rate capability (454.7 mA h g-1 at 5 A g-1) and remarkable cyclic performance (retaining 569 mA h g-1 over 1000 cycles at 2 A g-1). The insightful findings in this study can shed light on the behavior of the electron/ion structure regulation by the heterostructure and oxygen vacancies, which can guide future studies on designing other electrode materials with high-performance lithium-ion storage.

Ni activated Mo2C by regulating the interfacial electronic structure for highly efficient lithium-ion storage.

Regulating the electronic structure plays a positive role in improving the ion/electron kinetics of electrode materials for lithium ion batteries (LIBs). Herein, an effective approach is demonstrated to achieve Ni/Mo2C hybrid nanoparticles embedded in porous nitrogen-doped carbon nanofibers (Ni/Mo2C/NC). Density functional theory calculations indicate that Ni can activate the interface of Ni/Mo2C by regulating the electronic structure, and accordingly improve the electron/Li-ion diffusion kinetics. The charge at the interface transfers from Ni atoms to Mo atoms on the surface of Mo2C, illustrating the formation of an interfacial electric field in Ni/Mo2C. The formed interfacial electric field in Ni/Mo2C can improve the intrinsic electronic conductivity, and reduce the Li adsorption energy and the Li+ diffusion barrier. Thus, the obtained Ni/Mo2C/NC shows an excellent high-rate capability of 344.1 mA h g-1 at 10 A g-1, and also displays a superior cyclic performance (remaining at 412.7 mA h g-1 after 1800 cycles at 2 A g-1). This work demonstrates the important role of electronic structure regulation by assembling hybrid materials and provides new guidance for future work on designing novel electrode materials for LIBs.

Controlled Synthesis of Ultrafine ß-Mo2C Nanoparticles Encapsulated in N-Doped Porous Carbon for Boosting Lithium Storage Kinetics.

Rational construction of anode material architecture to afford excellent cycling stability, fast rate capacity, and large specific capacity is essential to promote further development of lithium-ion batteries in commercial applications. In this work, we propose a facile strategy to anchor ultrafine ß-Mo2C nanoparticles in N-doped porous carbon skeleton (ß-Mo2C@NC) using a scalable salt-template method. The well-defined and abundant hierarchical porous structure of ß-Mo2C@NC can not only significantly enhance the electron/ion transfer but also markedly increase the specific surface area to effectively expose the electrochemically accessible active sites. Besides, the N-doped carbon matrix can turn the d-orbital electrons of the Mo to boost the electron transportation as well as distribute active sites to buffer the volume change of Mo2C and provide conductive pathways during discharge/charge cycles. As a result, the as-prepared ß-Mo2C@NC displays excellent lithium storage performance in terms of 1701.6 mA h g-1 at 0.1 A g-1 after 100 cycles and a large capacity of 816.47 mA h g-1 at 2.0 A g-1 after 500 cycles. The above results distinctly demonstrate that the ß-Mo2C@NC composite has potential application as anode materials in high-performance energy storage devices.

Green and Scalable Fabrication of Sandwich-like NG/SiOx/NG Homogenous Hybrids for Superior Lithium-Ion Batteries.

SiOx is considered as a promising anode for next-generation Li-ions batteries (LIBs) due to its high theoretical capacity; however, mechanical damage originated from volumetric variation during cycles, low intrinsic conductivity, and the complicated or toxic fabrication approaches critically hampered its practical application. Herein, a green, inexpensive, and scalable strategy was employed to fabricate NG/SiOx/NG (N-doped reduced graphene oxide) homogenous hybrids via a freeze-drying combined thermal decomposition method. The stable sandwich structure provided open channels for ion diffusion and relieved the mechanical stress originated from volumetric variation. The homogenous hybrids guaranteed the uniform and agglomeration-free distribution of SiOx into conductive substrate, which efficiently improved the electric conductivity of the electrodes, favoring the fast electrochemical kinetics and further relieving the volumetric variation during lithiation/delithiation. N doping modulated the disproportionation reaction of SiOx into Si and created more defects for ion storage, resulting in a high specific capacity. Deservedly, the prepared electrode exhibited a high specific capacity of 545 mAh g-1 at 2 A g-1, a high areal capacity of 2.06 mAh cm-2 after 450 cycles at 1.5 mA cm-2 in half-cell and tolerable lithium storage performance in full-cell. The green, scalable synthesis strategy and prominent electrochemical performance made the NG/SiOx/NG electrode one of the most promising practicable anodes for LIBs.

Hydrogel-derived VPO4/porous carbon framework for enhanced lithium and sodium storage.

Vanadium phosphate (VPO4) is attracting extensive attention because of its advantages of low cost, stable structure and high theoretical capacity. However, similar to other phosphates, VPO4 suffers from low electrical conductivity and large volume expansion, adversely influencing its electrochemical performance and thus limiting its application as an anode in lithium and sodium ion batteries. Herein, we propose a novel, facile strategy based on the organic-inorganic network of a nanostructured hybrid hydrogel for immobilizing VPO4 in a hierarchically porous carbon framework (3DHP-VPO4@C). VPO4 chemically interacts with the carbon framework via a P-C bond, functioning as a buffer layer to maintain structural stability during charge/discharge cycles. The carbon framework offers an efficient pathway for electron and Li+/Na+ transport to ensure high electronic conductivity of the electrode. The 3DHP-VPO4@C anode exhibits excellent lithium and sodium storage performances, and notably high capacities of 957 mA h g-1 at 0.1 A g-1 and 345.3 mA h g-1 at 5 A g-1 for lithium ion batteries. Full cells consisting of a LiFePO4 cathode and the 3DHP-VPO4@C anode also prove to have superior cycling stability and rate performance for LIBs.

V3S4 Nanosheets Anchored on N, S Co-Doped Graphene with Pseudocapacitive Effect for Fast and Durable Lithium Storage.

Construction of a suitable hybrid structure has been considered an important approach to address the defects of metal sulfide anode materials. V3S4 nanosheets anchored on an N, S co-coped graphene (VS/NSG) aerogel were successfully fabricated by an efficient self-assembled strategy. During the heat treatment process, decomposition, sulfuration and N, S co-doping occurred. This hybrid structure was not only endowed with an enhanced capability to buffer the volume expansion, but also improved electron conductivity as a result of the conductive network that had been constructed. The dominating pseudocapacitive contribution (57.78% at 1 mV s-1) enhanced the electrochemical performance effectively. When serving as anode material for lithium ion batteries, VS/NSG exhibits excellent lithium storage properties, including high rate capacity (480 and 330 mAh g-1 at 5 and 10 A g-1, respectively) and stable cyclic performance (692 mAh g-1 after 400 cycles at 2 A g-1).

Cr2O3 nanosheet/carbon cloth anode with strong interaction and fast charge transfer for pseudocapacitive energy storage in lithium-ion batteries.

Flexible lithium-ion batteries have attracted considerable interest for next-generation bendable, implantable and wearable electronics devices. Here, we have successfully grown Cr2O3 nanosheets on carbon cloth (CC) as freestanding anodes for Li-ion batteries (LIBs). Density functional theory (DFT) calculations verify an optimal structure of two-dimensional Cr2O3 nanosheets on the carbon fiber surface and a strong interaction between the O edges of Cr2O3 and the carbon. The interconnected Cr2O3 nanosheets with a large surface area enable fast charge transfer by efficient contact with electrolyte while the flexible CC substrate accommodates the volume change during cycles, leading to excellent rate capability and cyclic stability through psuedocapacitance-dominant energy storage. Full cells are assembled using the Cr2O3-CC anode and a LiFePO4 cathode, which deliver excellent capacity retention and rate capability. The fully-charged cell is demonstrated to power a red light-emitting diode (LED), verifying the potential of Cr2O3-CC as a promising anode material for LIBs.

Powder exfoliated MoS2 nanosheets with highly monolayer-rich structures as high-performance lithium-/sodium-ion-battery electrodes.

Due to their low yield and easy aggregation during the electrode preparation process, exfoliated MoS2 monolayers cannot fulfill the requirements of alkali-metal-ion battery tests. Hence, we have developed a facile process to fabricate powder exfoliated MoS2 nanosheets capable of large-scale production and having highly monolayer-rich structures. This process contains two steps: liquid-phase exfoliation of the edge-rich MoS2 precursor and a freeze-drying procedure. The proposed MoS2 precursors contain rich edge fractions that are easily exfoliated by this method, and the freeze-drying procedure can maintain the unique monolayer-rich structure of MoS2 in the powder phase. The electrochemical evaluations of both lithium- and sodium-ion batteries reveal that the proposed powder exfoliated monolayer-rich MoS2 electrode exhibits remarkable specific capacities and stable cyclic performances. In particular, the monolayer-rich MoS2 nanosheet electrode delivers a superior lithium-storage capacity of ∼1400 mA h g-1. The exfoliated MoS2 nanosheet electrode can withstand over 1000 cycles even at 1 A g-1. The mechanism reveals that these unique MoS2 nanosheets not only have a large surface area but also their inclusive monolayer structures exhibit much higher charge mobility than those of bulk MoS2.

Co9S8@MoS2 Core-Shell Heterostructures as Trifunctional Electrocatalysts for Overall Water Splitting and Zn-Air Batteries.

The development of efficient non-noble-metal electrocatalysts is of critical importance for clean energy conversion systems, such as fuel cells, metal-air batteries, and water electrolysis. Herein, uniform Co9S8@MoS2 core-shell heterostructures have been successfully prepared via a solvothermal approach, followed by an annealing treatment. Transmission electron microscopy, X-ray absorption near-edge structure, and X-ray photoelectron spectroscopy measurements reveal that the core-shell structure of Co9S8@MoS2 can introduce heterogeneous nanointerface between Co9S8 and MoS2, which can deeply influence its charge state to boost the electrocatalytic performances. Besides, due to the core-shell structure that can promote the synergistic effect of Co9S8 and MoS2 and provide abundant catalytically active sites, Co9S8@MoS2 exhibits a superior hydrogen evolution reaction performance with a small overpotential of 143 mV at 10 mA cm-2 and a small Tafel slope value of 117 mV dec-1 under alkaline solution. Meanwhile, the activity of Co9S8@MoS2 toward oxygen evolution reaction is also impressive with a low operating potential (∼1.57 V vs reversible hydrogen electrode) at 10 mA cm-2. By using Co9S8@MoS2 catalyst for full water splitting, an alkaline electrolyzer affords a cell voltage as low as 1.67 V at a current density of 10 mA cm-2. Also, Co9S8@MoS2 reveals robust oxygen reduction reaction performance, making it an excellent catalyst for Zn-air batteries with a long lifetime (20 h). This work provides a new means for the development of multifunctional electrocatalysts of non-noble metals for the highly demanded electrochemical energy technologies.