The redox reactions occurring in the Li-S battery positive electrode conceal various and critical electrocatalytic processes, which strongly influence the performances of this electrochemical energy ...storage system. Here, we report the development of a single-dispersed molecular cluster catalyst composite comprising of a polyoxometalate framework (Co
(PW
O
)
) and multilayer reduced graphene oxide. Due to the interfacial charge transfer and exposure of unsaturated cobalt sites, the composite demonstrates efficient polysulfides adsorption and reduced activation energy for polysulfides conversion, thus serving as a bifunctional electrocatalyst. When tested in full Li-S coin cell configuration, the composite allows for a long-term Li-S battery cycling with a capacity fading of 0.015% per cycle after 1000 cycles at 2 C (i.e., 3.36 A g
). An areal capacity of 4.55 mAh cm
is also achieved with a sulfur loading of 5.6 mg cm
and E/S ratio of 4.5 μL mg
. Moreover, Li-S single-electrode pouch cells tested with the bifunctional electrocatalyst demonstrate a specific capacity of about 800 mAh g
at a sulfur loading of 3.6 mg cm
for 100 cycles at 0.2 C (i.e., 336 mA g
) with E/S ratio of 5 μL mg
.
Polyoxometalates (POMs) are a series of molecular metal oxide clusters, which span the two domains of solutes and solid metal oxides. The unique characters of POMs in structure, geometry, and ...adjustable redox properties have attracted widespread attention in functional material synthesis, catalysis, electronic devices, and electrochemical energy storage and conversion. This review is focused on the links between the intrinsic charge carrier behaviors of POMs from a chemistry‐oriented view and their recent ground‐breaking developments in related areas. First, the advantageous charge transfer behaviors of POMs in molecular‐level electronic devices are summarized. Solar‐driven, thermal‐driven, and electrochemical‐driven charge carrier behaviors of POMs in energy generation, conversion and storage systems are also discussed. Finally, present challenges and fundamental insights are discussed as to the advanced design of functional systems based upon POM building blocks for their possible emerging application areas.
The links between the intrinsic charge‐carrier behaviors of polyoxometalates (POMs) from a chemistry‐oriented view are discussed and their recent ground‐breaking developments in related areas, including molecular‐level electronic devices, solar‐driven, thermally driven, and electrochemically driven energy generation, conversion, and storage systems are reviewed. Finally, present challenges and the fundamental insights for advanced design and self‐assembly of POM building blocks are also discussed.
We present strategies to tune the redox properties of polyoxometalate clusters to enhance the electron‐coupled proton‐buffer‐mediated water splitting process, in which the evolution of hydrogen and ...oxygen can occur in different forms and is separated in time and space. By substituting the heteroatom template in the Keggin‐type polyoxometalate cluster, H6ZnW12O40, it is possible to double the number of electrons and protonation in the redox reactions (from two to four). This increase can be achieved with better matching of the energy levels as indicated by the redox potentials, compared to the ones of well‐studied H3PW12O40 and H4SiW12O40. This means that H6ZnW12O40 can act as a high‐performance redox mediator in an electrolytic cell for the on‐demand generation of hydrogen with a high decoupling efficiency of 95.5 % and an electrochemical energy efficiency of 83.3 %. Furthermore, the H6ZnW12O40 cluster also exhibits an excellent cycling behaviour and redox reversibility with almost 100 % H2‐mediated capacity retention during 200 cycles and a high coulombic efficiency >92 % each cycle at 30 mA cm−2.
The molecular structure of keggin‐type tungsten polyoxometalates (POMs) were tuned with different heteroatoms to modify their redox properties. Compared to the two electrons of well‐studied H3PW12O40 and H4SiW12O40, H6ZnW12O40 exhibits double reversible electrons with protonation in the redox reactions and the more desirable redox potentials, which enhance its ECPB performance (see scheme).
An electron conductive matrix, or collector, facilitates electron transport in an electrochemical device. It is stationary and does not change during the entire operation once it is built. The ...interface of this matrix and an electrode is constructed at a 2D level at the micro‐scale, and naturally limits the breadth and depth of electrochemical reactions. Herein, the idea of an enhanced electrode coupled with a conducting molecule that can extend interfacial reactions is first introduced. With a spatialized interspace, this electrode can change the present understanding of the electrode process and opens up a new realm of electrode‐based reaction chemistry. A lithium–sulfur (Li–S) battery is used as the target for implementing the enhanced electrode owing to the complex multi‐electron reaction. Through the interaction of π–π stacking between graphite‐based carbon and iron (II) phthalocyanine (FePc), soluble FePc can be decorated on the surface of an electrode that has the capability of transporting electrons. The scanning tunneling microscope break junction characterization and density functional theory indicate that FePc has a strong molecular electronic conductivity. The reactants obtain electrons more easily from the conducting molecule than from the collector directly. As a result, the performance of the corresponding Li–S battery considerably improves.
An enhanced electrode via decoration of a special electron conductive molecule (FePc) on a conductive matrix surface is reported that allows extended interfacial reactions. The electron transport path from electrode to electrolyte is then stretched into the body phase rather than limited to the interface, thus spatializing the interface into the interspace. This technique is expected to significantly improve the performance of lithium‐sulfur batteries.
Through a facile impregnation and pyrolysis treatment, a nitrogen-doped carbon nanoflake array rich in uniform and abount Co quantum dots (CoQD) on 3D glass fibers can been successfully prepared, ...which possess the superior lithophilicity and conductivity. Thus, the small nucleation barrier and evenly/rapidly Li deposition could be achieved.
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•GFLA anode is prepared via a two-step method and as the Li-host for the first time.•CoQD@NC nanoflakes can effectively improve the lithiophilicity and conductivity.•GFLA anode performs small nucleation barrier and evenly/rapidly Li deposition.•GFLA anode exhibits superior performance in half/symmetrical/full-cell.
Nonconducting textile materials will be the skeleton of electrode for flexible battery in growing widespread applications in portable and wearable electronics, a fiber-based lithium anode is facing more challenges and has more important sense. Herein, a glass fiber was employed as supporting skeleton on which the nitrogen-doped carbon nanoflake array has been modified first to form a conductive layer, then the cobalt quantum dots were embedded in the layer to establish lithophilic sites, finally, a unique glass-fiber-based lithium anode (GFLA) has been achieved. Furthermore, the cross-linked fiber framework not only dramatically lessen the local current density, but the richer cavity also greatly alleviates the volume expansion problem. Thus, the GFLA electrode demonstrated a highly reversible and stable long-term cycling behavior with nondendritic. As expected, the GFLA electrode exhibits a high CE (∼98.64%) at 0.5 mA cm−2 upon 1480 h in half-cell and a durable cycling with a small voltage hysteresis (20 mV) for 470 h at 4 mA cm−2/4 mAh cm−2 in symmetrical-cell. Moreover, the GFLA@Li||LiFePO4 full-cell displays a capacity retention rate of up to 90.3% after 400 cycles, while the carbon fiber@Li||LiFePO4 is only 36.4%.
Metallic sodium is considered the most likely anode material to replace metallic lithium owing to its high theoretical capacity, abundant reserves, and low cost. However, the uneven deposition and ...agglomerate deposition of Na often result in low coulombic efficiency and inferior lifetime during cycling. Here, by phosphorizing treatment, a sodiophilic phosphorized copper mesh (PCM) has been achieved as the metallic sodium-host current collector for the first time; then through
in situ
electrochemical reaction construct, sodiophilic Na-Cu-P composite layer, which has a fast electronic/ionic conductivity and strong adsorption ability with sodium, thereby greatly mitigating electrodeposition overpotential for improving Na plating/stripping behaviors. Meanwhile, the cross-linked mesh skeleton significantly diminishes the local current density, thus achieving highly reversible Na plating/stripping behavior with dendrite-free and "dead Na"-free. Consequently, the PCM electrode can maintain a high coulombic efficiency (∼99.96%) over 1000 cycles at 5 mA cm
−2
and exhibit an ultra-low electrodeposition overpotential from 0.5 mA cm
−2
to 10 mA cm
−2
in a half-cell. Similarly, the symmetrical cell displays superior cycling stability with low overpotential. Furthermore, the PCM@Na anode delivers excellent cycling/rate performance when paired with Prussian blue (PB) cathode in full-cell.
Herein, we successfully introduce a sodiophilic Na-Cu-P composites
via in situ
alloying reaction, which can greatly mitigate the tip/growth/nucleation overpotential during Na deposition, thereby to realize a stable Na plating/stripping behaviors.
Sulfur and polysulfides play important roles on the environment and energy storage systems, especially in the recent hot area of high energy density of lithium–sulfur (Li–S) batteries. However, the ...further development of Li–S battery is still retarded by the lack of complete mechanistic understanding of the sulfur redox process. Herein we introduce a conductive Lewis base matrix which has the ability to enhance the battery performance of Li–S battery, via the understanding of the complicated sulfur redox chemistry on the electrolyte/carbon interface by a combined in operando Raman spectroscopy and density functional theory (DFT) method. The higher polysulfides, Li2S8, is found to be missing during the whole redox route, whereas the charging process of Li–S battery is ended up with the Li2S6. DFT calculations reveal that Li2S8 accepts electrons more readily than S8 and Li2S6 so that it is thermodynamically and kinetically unstable. Meanwhile, the poor adsorption behavior of Li2S n on carbon surface further prevents the oxidization of Li2S n back to S8 upon charging. Periodic DFT calculations show that the N-doped carbon surface can serve as conductive Lewis base “catalyst” matrix to enhance the adsorption energy of Li2S n (n = 4–8). This approach allows the higher Li2S n to be further oxidized into S8, which is also confirmed by in operando Raman spectroscopy. By recovering the missing link of Li2S8 in the whole redox route, a significant improvement of the S utilization and cycle stability even at a high sulfur loading (70%, m/m) in the composite on a simple super P carbon.
Highly concentrated electrolytes (HCEs) are promising for the construction of advanced lithium metal secondary batteries through inhibiting lithium dendrites in kinetics and lithium protection with ...an anion-derived solid electrolyte interphase (SEI). The mixed-solvent strategy is effective for obtaining electrolytes with the required properties, and the tuning of their related proportions has great impact on the solvation structures and interface properties. In this work, to break through the limitation of the lithium salt solubility in fluoroethylene carbonate (FEC) and build the anion-derived SEI, acetonitrile (AN) was employed as an occupying agent to further decrease the mole ratio of FEC to anions. Thus, FEC-AN mixed solvent-based hybrid HCEs were systematically investigated. The reduced viscosity, improved ionic conductivity and enhanced oxidation stability of HCEs were obtained. At the optimal proportion, the expected SEI was achieved, ensuring high coulombic efficiency and long cycle stability of the lithium metal anode. This work shows the necessity of the mixed solvent regulation strategy for HCE improvement.
The mixed solvent strategy was applied to construct optimized highly concentrated electrolytes (HCEs). Hybrid HCEs based on FEC and AN were investigated systematically. The accelerated ion transport and enhanced anions-derived SEI were achieved.
Selective catalytic oxidation (SCO) of ammonia to a nitrogen molecule is an important process involved in many applications such as removing NH3 slip in selective catalytic reduction (SCR) of NO x , ...reducing the NH3 concentration from biomass-derived fuels, etc. Here we perform density functional theory calculations in conjunction with cluster models to investigate the SCO mechanisms on V2O5 surfaces. Our calculations show that, at the initial stage, NH3 can be activated by transferring an electron to the metal oxide surfaces, giving rise to an NH3 + intermediate. We disclose that the subsequent pathways are strongly dependent on the availability of the gaseous species. When oxygen is limited or absent, N2H4 can be produced from NH3 + reacting with a second NH3 or from two activated intermediates (e.g., NH2 + NH2 or ONH2 + NH2), and oxidation of N2H4 into N2 by VO is viable. On the other hand, when oxygen is abundant, NH3 + will react with O2 to make a NH3 +···O2 complex. Such a species will quickly decompose into NO, which switches on the selective catalytic reduction (SCR) reaction, eventually leading to the formation of N2. We propose that the combination of an efficient Ostwald reaction catalyst for NH3 to NO transformation with a capable SCR reaction catalyst for NO reduction by NH3 to N2 can lead to a good candidate catalyst for SCO at the high O2/NH3 ratio condition.
Novel chiral macrocyclic cobalt(II) complexes could be conveniently prepared and their structures were determined by X-ray diffraction method. The ATH of ketones catalyzed by cobalt(II) complex ...proceeded smoothly with high ee. Furthermore, DFT calculation accounts for the high catalytic performances of cobalt(II) catalyst.
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•Novel chiral macrocyclic cobalt(II) complexes could be conveniently prepared.•The structures of cobalt(II) complexes were determined by X-ray diffraction method.•ATH of ketones catalyzed by cobalt(II) complex proceeded smoothly with high ee.•DFT calculation accounts for the high catalytic performances of cobalt(II) catalyst.•It represents an attractive direction for development of novel cheap metal catalyst.
Using easily available CoBr2 and chiral cyclic PxNy-type ligands as starting materials, novel chiral cyclic cobalt(II) complexes could be conveniently prepared. Furthermore, we obtained the single crystals suitable for X-ray diffraction to confirm the structures of these cobalt(II) complexes. The asymmetric transfer hydrogenation (ATH) of ketones catalyzed by these well-designed cobalt(II) complexes was investigated. Among them, cobalt(II) complex containing chiral macrocyclic iminophosphine ligand CyP2N4 exhibited high catalytic activity and enantioselectivity (up to 99% ee). Density functional theory (DFT) calculations suggested that cobalt(II) complex (R,R,R',R')-CyP2N4-Co(II) (C1) is easier to form metal hydride, which is the key intermediate during the enantioselective transfer hydrogenation reaction. Study results also revealed that the unique macrocyclic structure of complex C1 could form a special microenvironment around cobalt ion. Therefrom the substrate coordinated with the central metal along this specific reaction channel during the asymmetric catalytic reaction, resulting high enantioselectivity.
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