γ-MnO2 as an Electron Reservoir for RuO2 Oxygen Evolution Catalyst in Acidic Media.

Developing highly active and durable catalysts in acid conditions remains an urgent issue due to the sluggish kinetics of oxygen evolution reaction (OER). Although RuO2 has been a state-of-the-art commercial catalyst for OER, it encounters poor stability and high cost. In this study, the electronic reservoir regulation strategy is proposed to promote the performance of acidic water oxidation via constructing a RuO2/MnO2 heterostructure supported on carbon cloth (CC) (abbreviated as RuO2/MnO2/CC). Theoretical and experimental results reveal that MnO2 acts as an electron reservoir for RuO2. It facilitates electron transfer from RuO2, enhancing its activity prior to OER, and donates electrons to RuO2, improving its stability after OER. Consequently, RuO2/MnO2/CC exhibits better performance compared to commercial RuO2, with an ultrasmall overpotential of 189 mV at 10 mA cm-2 and no signs of deactivation even after 800 h of electrolysis in 0.5 m H2SO4 at 10 mA cm-2. When applied as the anode in a proton exchange membrane water electrolyzer, the cost-efficient RuO2/MnO2/CC catalyst only requires a cell voltage of 1.661 V to achieve the water-splitting current of 1 A cm-2, and the noble metal cost is as low as US$ 0.00962 cm-2, indicating potential for practical applications.

Computational chemistry for water-splitting electrocatalysis.

Electrocatalytic water splitting driven by renewable electricity has attracted great interest in recent years for producing hydrogen with high-purity. However, the practical applications of this technology are limited by the development of electrocatalysts with high activity, low cost, and long durability. In the search for new electrocatalysts, computational chemistry has made outstanding contributions by providing fundamental laws that govern the electron behavior and enabling predictions of electrocatalyst performance. This review delves into theoretical studies on electrochemical water-splitting processes. Firstly, we introduce the fundamentals of electrochemical water electrolysis and subsequently discuss the current advancements in computational methods and models for electrocatalytic water splitting. Additionally, a comprehensive overview of benchmark descriptors is provided to aid in understanding intrinsic catalytic performance for water-splitting electrocatalysts. Finally, we critically evaluate the remaining challenges within this field.

Composite Quasi-Solid-State Electrolytes with Organic-Inorganic Interface Engineering for Fast Ion Transport in Dendrite-Free Sodium Metal Batteries.

Quasi-solid-state electrolytes (QSSE) are a promising candidate for addressing the limitations of liquid and solid electrolytes. However, different ion transport capacities between liquid solvents and polymers can cause localized heterogeneous distribution of Na+ fluxes. In addition, the continuous side reactions occurring at the interface between QSSE and sodium anode lead to uncontrollable dendrites growth. Herein, a novel strategy is designed to integrate the composite electrospun membrane of Na3Zr2Si2PO12 and poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) into QSSE, aiming to introduce new fast ion conducting channels at the organic-inorganic interface. The efficient ion transfer pathways can effectively promote the homogenization of ion migration, enabling composite QSSE to achieve an ultrahigh ionic conductivity of 4.1 mS cm-1 at room temperature, with a Na+ transference number as high as 0.54. Moreover, the PVDF-HFP is preferentially reduced upon contact with the sodium anode to form a "NaF-rich" solid electrolyte interphase, which effectively suppresses the growth of dendrites. The synergistic combination of multiple strategies can realize exceptional long-term cycling stability in both sodium symmetric batteries (≈700 h) and full batteries (2100 cycles). This study provides a new insight for constructing high performance and dendrite-free solid-state sodium metal batteries.

Inorganic-Organic Hybrid Multifunctional Solid Electrolyte Interphase Layers for Dendrite-Free Sodium Metal Anodes.

Constructing a stable and robust solid electrolyte interphase (SEI) is crucial for achieving dendrite-free sodium metal anodes and high-performance sodium batteries. However, maintaining the integrity of SEI during prolonged cycle life under high current densities poses a significant challenge. In this study, we propose an integrated multifunctional SEI layer with inorganic/organic hybrid construction (IOHL-Na) to enhance the durability of sodium metal anode during reduplicative plating/stripping processes. The inorganic components with high mechanical strength and strong sodiophilicity demonstrate optimized ionic conduction efficiency and dendrite inhibition ability. Simultaneously, the organic component contributes to the formation of a dense and elastic membrane structure, preventing fracture and delamination issues during volume fluctuations. The symmetrical batteries of IOHL-Na achieve stable cycling over 2000 hours with an extremely low voltage hysteresis of around 15.8 mV at a high current density of 4 mA cm-2 . Moreover, the Na-O2 batteries sustain exceptional long-term stability and impressive capacity retention, exploiting a promising approach for constructing durable SEI and dendrite-free sodium metal anodes.

Non-flammable, dilute, and hydrous organic electrolytes for reversible Zn batteries.

Rechargeable Zn batteries hold great practicability for cost-effective sustainable energy storage but suffer from irreversibility of the Zn anode in aqueous electrolytes due to parasitic H2 evolution, corrosion, and dendrite growth. Herein, we report a non-flammable, dilute, and hydrous organic electrolyte by dissolving low-cost hydrated Zn(ClO4)2·6H2O in trimethyl phosphate (TMP), which homogenizes plating/stripping and enables in situ formation of a Zn3(PO4)2-ZnCl2-rich interphase to stabilize the Zn anode. A dilute 0.5 m Zn(ClO4)2·6H2O/TMP electrolyte featuring a H2O-poor Zn2+-solvation sheath and low water activity enables significantly enhanced Zn reversibility and a wider electrochemical window than the concentrated counterpart. In this formulated electrolyte, the Zn anode exhibits a high efficiency of 99.5% over 500 cycles, long-term cycling for 1200 h (5 mA h cm-2 at 5 mA cm-2) and stable operation at 50 °C. The results would guide the design of hydrous organic electrolytes for practical rechargeable batteries employing metallic electrode materials.

Aqueous Electrolytes with Hydrophobic Organic Cosolvents for Stabilizing Zinc Metal Anodes.

Rechargeable aqueous zinc (Zn) batteries are promising for large-energy storage because of their low cost, high safety, and environmental compatibility, but their implementation is hindered by the severe irreversibility of Zn metal anodes as exemplified by water-induced side reactions (H2 evolution and Zn corrosion) and dendrite growth. Here, we find that the introduction of a hydrophobic carbonate cosolvent into a dilute aqueous electrolyte exhibits a much stronger ability to address the reversible issues facing Zn anodes than that with hydrophilic ones. Among the typical carbonates (ethylene carbonate, propylene carbonate, dimethyl carbonate, and diethyl carbonate (DEC)), DEC as the most hydrophobic additive enables the strongest breaking of water's H-bond network and replaces the solvating H2O in a Zn2+-solvation sheath, which significantly reduces the water activity and its decomposition. Additionally, DEC molecules preferentially adsorb onto the Zn surface to create an H2O-poor electrical double layer and render a dendrite-free Zn2+-plating behavior. The formulated hybrid 2 m Zn(OTf)2 + 7 m DEC electrolyte endows the Zn electrode with an ability to achieve high cycling stability (over 3500 h at 5 mA cm-2 with 2.5 mA h cm-2) and supports the stable operation of Zn||V2O5·nH2O full battery. This efficient strategy with hydrophobic cosolvent suggests a promising direction for designing aqueous battery chemistries.

Engineering Functional Interface with Built-in Catalytic and Self-Oxidation Sites for Highly Stable Lithium-Sulfur Batteries.

Lithium-sulfur (Li-S) batteries have attracted great attention due to their high theoretical energy density. The rapid redox conversion of lithium polysulfides (LiPS) is effective for solving the serious shuttle effect and improving the utilization of active materials. The functional design of the separator interface with fast charge transfer and active catalytic sites is desirable for accelerating the conversion of intermediates. Herein, a graphene-wrapped MnCO3 nanowire (G@MC) was prepared and utilized to engineer the separator interface. G@MC with active Mn2+ sites can effectively anchor the LiPS by forming the Mn-S chemical bond according to our theoretical calculation results. In addition, the catalytic Mn2+ sites and conductive graphene layer of G@MC could accelerate the reversible conversion of LiPS via the spontaneous "self-redox" reaction and the rapid electron transfer in electrochemical process. As a result, the G@MC-based battery exhibits only 0.038 % capacity decay (per cycle) after 1000 cycles at 2.0 C. This work affords new insights for designing the integrated functional interface for stable Li-S batteries.

Non-concentrated aqueous electrolytes with organic solvent additives for stable zinc batteries.

Rechargeable aqueous zinc batteries (RAZBs) are promising for large-scale energy storage because of their superiority in addressing cost and safety concerns. However, their practical realization is hampered by issues including dendrite growth, poor reversibility and low coulombic efficiency (CE) of Zn anodes due to parasitic reactions. Here, we report a non-concentrated aqueous electrolyte composed of 2 m zinc trifluoromethanesulfonate (Zn(OTf)2) and the organic dimethyl carbonate (DMC) additive to stabilize the Zn electrochemistry. Unlike the case in conventional aqueous electrolytes featuring typical Zn[H2O]6 2+ solvation, a solvation sheath of Zn2+ with the co-participation of the DMC solvent and OTf- anion is found in the formulated H2O + DMC electrolyte, which contributes to the formation of a robust ZnF2 and ZnCO3-rich interphase on Zn. The resultant Zn anode exhibits a high average CE of Zn plating/stripping (99.8% at an areal capacity of 2.5 mA h cm-2) and dendrite-free cycling over 1000 cycles. Furthermore, the H2O + DMC electrolytes sustain stable operation of RAZBs pairing Zn anodes with diverse cathode materials such as vanadium pentoxide, manganese dioxide, and zinc hexacyanoferrate. Rational electrolyte design with organic solvent additives would promote building better aqueous batteries.

Exploring the Interfacial Chemistry between Zinc Anodes and Aqueous Electrolytes via an In Situ Visualized Characterization System.

Direct monitoring of dendrite growth, hydrogen evolution, and surface passivation can enrich the chemical and morphological understanding of the unstable Zn/electrolyte interface and provide guidelines for rational design of Zn anodes; however, the on-line observation with high precision is hitherto lacking. Herein, we present a real-time comprehensive characterization system, including in situ atomic force microscopy, optical microscopy, and electrochemical quartz crystal microbalance (referred to as the "3M" system), to provide multiscale views on the semisphere nuclei and growth of bump-like dendrites and the potential-dependent chemical and morphological structures of passivated products in a mild acidic electrolyte. It is revealed that the poor interfacial properties can be attributed to the sparse nucleation sites and direct contact of Zn with the electrolyte. The 3M system further visualizes and confirms that the additive polyethylene glycol acts as a Zn2+ distribution promoter and physical barrier and merits stable electrochemical performance.

A 3D Hydroxylated MXene/Carbon Nanotubes Composite as a Scaffold for Dendrite-Free Sodium-Metal Electrodes.

Sodium metal is a promising anode, but uneven Na deposition with a dendrite growth seriously impedes its application. Herein, a fibrous hydroxylated MXene/carbon nanotubes (h-Ti3 C2 /CNTs) composite is designed as a scaffold for dendrite-free Na metal electrodes. This composite displays fast Na+ /electron transport kinetics and good thermal conductivity and mechanical properties. The h-Ti3 C2 contains abundant sodiophilic functional groups, which play a significant role in inducing homogeneous nucleation of Na. Meanwhile, CNTs provide high tensile strength and ease of film-forming. As a result, h-Ti3 C2 /CNTs exhibit a high average Coulombic efficiency of 99.2 % and no dendrite after 1000 cycles. The h-Ti3 C2 /CNTs/Na based symmetric cells show a long lifespan over 4000 h at 1.0 mA cm-2 with a capacity of 1.0 mAh cm-2 . Furthermore, Na-O2 batteries with a h-Ti3 C2 /CNTs/Na anode exhibit a low potential gap of 0.11 V after an initial 70 cycles.

Molecular Design Strategy for High-Redox-Potential and Poorly Soluble n-Type Phenazine Derivatives as Cathode Materials for Lithium Batteries.

The n-type phenazine (PZ) derivatives represent an emerging class of cathode materials in lithium batteries for low-cost and sustainable energy storage. However, their low redox potential (<2 V) and high solubility hinder their application to battery systems. To explore and solve such problems in lithium batteries, we investigate the redox characteristics of 13 n-type PZ derivatives and their dissolution behavior in seven organic electrolytes systematically by using DFT calculations. Two decisive factors are observed to tune the redox potentials for these molecules: the first is the electron density around the N active sites and the second is the chelation on lithium by both the active N and the substituent group. Specific approaches that include the reduction of aromatic rings and the introduction of functional groups at ß sites in n-type PZ derivatives can improve the redox potential to approximately 3 V. In addition, we develop a new index denoted as Ediff to investigate the solubility of n-type PZ derivatives. The most effective way to reduce the dissolution of electrodes in solvents is to improve intermolecular attraction between the electrode molecules by introducing π-π stacking and hydrogen bonds. Such all-around guidelines should promote the application of n-type PZ-based organic cathodes with a high redox potential and low electrode solubility for lithium batteries.

Pore size effect of graphyne supports on CO2 electrocatalytic activity of Cu single atoms.

The instinctive chemical inertia of CO2 impedes its electrochemical reduction by high energy input. Single atom catalysts (SACs) on supports are considered as a class of excellent electrocatalysts with high activity, selectivity and atomic efficiency for CO2 electrochemical reduction. Supports for single atoms are believed to greatly impact the electrocatalytic activity of SACs. However, further research on the relationship between the structure of supports for SACs and CO2 electroreduction is still needed. Herein, density functional theory (DFT) calculations are performed to investigate the role of supports in tuning the CO2 electrocatalytic activity of SACs. Graphynes with different pore sizes (graphyne, graphdiyne, graphyne-3 and graphyne-4) are taken into account to unveil the effect of their skeleton structure on the anchored Cu single atoms. We found that support skeletons could greatly impact the coordination configuration of metal atoms and the steric repulsion of support skeletons to intermediates. These two factors jointly result in different electrocatalytic performances of SACs. The comparative analysis proves that the graphynes with large pores are appropriate supports for Cu adatoms for CO2 electroreduction due to the low-coordinated Cu atoms and weak-steric-repulsion carbon skeleton. Such SACs exhibit much enhanced activity and selectivity as compared with the Cu(111) surface and monoatomic Cu on nitrogen-doped graphene. This work provides a new insight into the rational design of supports for SACs.

Layered H0.68Ti1.83O4/reduced graphene oxide nanosheets as a novel cathode for rechargeable magnesium batteries.

Rechargeable magnesium batteries (RMBs) are promising devices for energy storage owing to their high volumetric energy density and high safety, but they still suffer from the lack of suitable cathodes. Herein, we report the development of ultrathin hydrated H0.68Ti1.83O4/reduced graphene oxide nanosheet composite (HTO/rGO) as a novel cathode for RMBs. The HTO/rGO, strongly combining ultrathin HTO nanosheets (5 nm in thickness with large interlayer spacing of 0.93 nm) and high electronic conductivity of rGO performs quick Mg2+ kinetics with high reversibility and considerable capacity of 182 mA h g-1, undergo an intercalation type reaction.

Theoretical study on lithiation mechanism of benzoquinone-based macrocyclic compounds as cathode for lithium-ion batteries.

Benzoquinone (BQ)-based macrocyclic compounds have shown great potential as cathode materials for lithium-ion batteries (LIBs) owing to their high redox potential and specific capacity. However, such materials usually have complex structures, which impede the investigation of lithiation mechanisms. Herein, we take Calix[4]quinone (C4Q) molecule as an example to develop a viable mechanism investigation method for such materials. The lithiation profile of C4Q is determined by condensed Fukui function which provides the reaction sites and orders. A correction of redox potential is proposed by leaving out the ion-transfer effect during the redox reaction based on Gibbs free energy change. The redox potential obtained by this approach shows high consistency with the experimental results. Moreover, this method can also be well extended to study the lithiation mechanism of another BQ-based macrocyclic compound (Pillar[5]quinone). Our results are promising to more deeply understand the reaction mechanism and predict the redox potential of new BQ-based macrocyclic compounds for LIBs.

Cyclohexanehexone with Ultrahigh Capacity as Cathode Materials for Lithium-Ion Batteries.

Organic carbonyl compounds show potential as cathode materials for lithium-ion batteries (LIBs) but the limited capacities (<600 mA h g-1 ) and high solubility in electrolyte restrict their further applications. Herein we report the synthesis and application of cyclohexanehexone (C6 O6 ), which exhibits an ultrahigh capacity of 902 mA h g-1 with an average voltage of 1.7 V at 20 mA g-1 in LIBs (corresponding to a high energy density of 1533 Wh kg-1 C 6 O 6 ). A preliminary cycling test shows that C6 O6 displays a capacity retention of 82 % after 100 cycles at 50 mA g-1 because of the limited solubility in high-polarity ionic liquid electrolyte. Furthermore, the combination of DFT calculations and experimental techniques, such as Raman and IR spectroscopy, demonstrates the electrochemical active C=O groups during discharge and charge processes.

An Alternative to Lithium Metal Anodes: Non-dendritic and Highly Reversible Sodium Metal Anodes for Li-Na Hybrid Batteries.

Highly reversible, stable, and non-dendritic metal anode (Li, Na etc.) is a crucial requirement for next-generation high-energy batteries. Herein, we have built a Li-Na hybrid battery (LNHB) based on Na plating/stripping, which features a high and stable coulombic efficiency of 99.2 % after 100 cycles, low voltage hysteresis (42 mV at 2 mA cm-2 ), and fast charge transfer. As a result of the Li+ electrostatic shield layer, the Na deposition showed cubic morphology rather than dendritic, even at high current density of 5 mA cm-2 . The solvation/desolvation of Li+ and Na+ were modelled by density functional theory calculations, demonstrating the fast desolvation kinetics of Na+ . Owing to the superior performance of the Na metal anode, the LNHB coupled with LiFePO4 cathode exhibited low voltage hysteresis and stable cycling performance that demonstrates its feasibility in practical applications.

Molecular Electrostatic Potential: A New Tool to Predict the Lithiation Process of Organic Battery Materials.

This work is pioneering to introduce molecular electrostatic potential (MESP) to investigate the interaction between lithium ions and organic electrode molecules. The electrostatic potential on the van der Waals surface of the electrode molecule is calculated, and then the coordinates and relative values of the local minima of MESP can be correlated to the Li binding sites and sequence on an organic small molecule, respectively. This suggests a gradual lithiation process. Similar calculations are extended to polymers and even organic crystals. The operation process of MESP for these systems is explained in detail. Through providing accurate and visualizable lithium binding sites, MESP can give precise prediction of the lithiated structures and reaction mechanism of organic electrode materials. It will become a new theoretical tool for determining the feasibility of organic electrode materials for alkali metal ion batteries.

The structure-electrochemical property relationship of quinone electrodes for lithium-ion batteries.

Quinones are promising electrode materials for lithium-ion batteries (LIBs), but their structure-electrochemical property relationship remains unclear. The aim of this study is to unravel the structural influence on the electrochemical properties of different quinones in LIBs. Through density functional theory calculations, redox potentials of 20 parent quinone isomers were examined, which revealed an increasing order of redox potentials as para-quinones < discrete-quinones < ortho-quinones. Two new methods were introduced to calculate and design organic electrode materials rationally. One is the vertical electron affinity in consideration of solvation effect, which was used to estimate the number of electron accommodation for quinones during lithiation. The other is a new index denoted as ΔA2Li used in para- and ortho-quinones, which was introduced to reveal the relationship between aromaticity and redox potential, establishing the theoretical basis for the design of analogous high-voltage organic electrode materials of LIBs.

Porphyrin-Based Symmetric Redox-Flow Batteries towards Cold-Climate Energy Storage.

Electrochemical energy storage with redox-flow batteries (RFBs) under subzero temperature is of great significance for the use of renewable energy in cold regions. However, RFBs are generally used above 10 °C. Herein we present non-aqueous organic RFBs based on 5,10,15,20-tetraphenylporphyrin (H2 TPP) as a bipolar redox-active material (anode: [H2 TPP]2- /H2 TPP, cathode: H2 TPP/[H2 TPP]2+ ) and a Y-zeolite-poly(vinylidene fluoride) (Y-PVDF) ion-selective membrane with high ionic conductivity as a separator. The constructed RFBs exhibit a high volumetric capacity of 8.72 Ah L-1 with a high voltage of 2.83 V and excellent cycling stability (capacity retention exceeding 99.98 % per cycle) in the temperature range between 20 and -40 °C. Our study highlights principles for the design of RFBs that operate at low temperatures, thus offering a promising approach to electrochemical energy storage under cold-climate conditions.