New Engineering Science Insights into the Electrode Materials Pairing of Electrochemical Energy Storage Devices.

Pairing the positive and negative electrodes with their individual dynamic characteristics at a realistic cell level is essential to the practical optimal design of electrochemical energy storage devices. However, the complex relationship between the performance data measured for individual electrodes and the two-electrode cells used in practice often makes an optimal pairing experimentally challenging. Taking advantage of the developed tunable graphene-based electrodes with controllable structure, experiments with machine learning are successfully united to generate a large pool of capacitance data for graphene-based electrode materials with varied slit pore sizes, thicknesses, and charging rates and numerically pair them into different combinations for two-electrode cells. The results show that the optimal pairing parameters of positive and negative electrodes vary considerably with the operation rate of the cells and are even influenced by the thickness of inactive components. The best-performing individual electrode does not necessarily result in optimal cell-level performance. The machine learning-assisted pairing approach presents much higher efficiency compared with the traditional trial-and-error approach for the optimal design of supercapacitors. The new engineering science insights observed in this work enable the adoption of artificial intelligence techniques to efficiently translate well-developed high-performance individual electrode materials into real energy storage devices.

Surface-Enriched Room-Temperature Liquid Bismuth for Catalytic CO2 Reduction.

Bismuth-based electrocatalysts are effective for carbon dioxide (CO2) reduction to formate. However, at room temperature, these materials are only available in solid state, which inevitably suffers from surface deactivation, declining current densities, and Faradaic efficiencies. Here, the formation of a liquid bismuth catalyst on the liquid gallium surface at ambient conditions is shown as its exceptional performance in the electrochemical reduction of CO2 (i.e., CO2RR). By doping a trace amount of bismuth (740 ppm atomic) in gallium liquid metal, a surface enrichment of bismuth by over 400 times (30 at%) in liquid state is obtained without atomic aggregation, achieving 98% Faradic efficiency for CO2 conversion to formate over 80 h. Ab initio molecular simulations and density functional theory calculations reveal that bismuth atoms in the liquid state are the most energetically favorable sites for the CO2RR intermediates, superior to solid Bi-sites, as well as joint GaBi-sites. This study opens an avenue for fabricating high-performing liquid-state metallic catalysts that cannot be reached by elementary metals under electrocatalytic conditions.

Ligand Modulation of Active Sites to Promote Electrocatalytic Oxygen Evolution.

Rationally designed catalysts hold the key to address the sluggish kinetics of oxygen evolution reaction (OER). However, engineering the active sites of such catalysts still faces grand challenges. This study proposes a feasible ligand modulation strategy to boost the OER catalytic activity of cobalt-iron oxyhydroxide ((Fe,Co)OOH). The 2-methylimidazole (MI) ligand coordination on (Fe,Co)OOH reduces the orbital overlap between the Fe/Co 3d and O 2p, which weakens the adsorption to oxygen-containing intermediates and thus facilitates the unfavorable O2 desorption. As a result, the MI ligand modulated (Fe,Co)OOH achieves an excellent OER performance with low overpotentials (230/290 mV at 10/100 mA cm-2 ) and excellent durability (>155 h). This study provides a novel ligand modulation strategy for the design of OER catalysts.

Self-Assembly of Ir-Based Nanosheets with Ordered Interlayer Space for Enhanced Electrocatalytic Water Oxidation.

Iridium (Ir)-based electrocatalysts are widely explored as benchmarks for acidic oxygen evolution reactions (OERs). However, further enhancing their catalytic activity remains challenging due to the difficulty in identifying active species and unfavorable architectures. In this work, we synthesized ultrathin Ir-IrOx/C nanosheets with ordered interlayer space for enhanced OER by a nanoconfined self-assembly strategy, employing block copolymer formed stable end-merged lamellar micelles. The interlayer distance of the prepared Ir-IrOx/C nanosheets was well controlled at ∼20 nm and Ir-IrOx nanoparticles (∼2 nm) were uniformly distributed within the nanosheets. Importantly, the fabricated Ir-IrOx/C electrocatalysts display one of the lowest overpotential (η) of 198 mV at 10 mA cm-2geo during OER in an acid medium, benefiting from their features of mixed-valence states, rich electrophilic oxygen species (O(II-δ)-), and favorable mesostructured architectures. Both experimental and computational results reveal that the mixed valence and O(II-δ)- moieties of the 2D mesoporous Ir-IrOx/C catalysts with a shortened Ir-O(II-δ)- bond (1.91 Å) is the key active species for the enhancement of OER by balancing the adsorption free energy of oxygen-containing intermediates. This strategy thus opens an avenue for designing high performance 2D ordered mesoporous electrocatalysts through a nanoconfined self-assembly strategy for water oxidation and beyond.

Oxygen evolution reaction dynamics monitored by an individual nanosheet-based electronic circuit.

The oxygen evolution reaction involves complex interplay among electrolyte, solid catalyst, and gas-phase and liquid-phase reactants and products. Monitoring catalysis interfaces between catalyst and electrolyte can provide valuable insights into catalytic ability. But it is a challenging task due to the additive solid supports in traditional measurement. Here we design a nanodevice platform and combine on-chip electrochemical impedance spectroscopy measurement, temporary I-V measurement of an individual nanosheet, and molecular dynamic calculations to provide a direct way for nanoscale catalytic diagnosis. By removing O2 in electrolyte, a dramatic decrease in Tafel slope of over 20% and early onset potential of 1.344 V vs. reversible hydrogen electrode are achieved. Our studies reveal that O2 reduces hydroxyl ion density at catalyst interface, resulting in poor kinetics and negative catalytic performance. The obtained in-depth understanding could provide valuable clues for catalysis system design. Our method could also be useful to analyze other catalytic processes.Electrocatalysis offers important opportunities for clean fuel production, but uncovering the chemistry at the electrode surface remains a challenge. Here, the authors exploit a single-nanosheet electrode to perform in-situ measurements of water oxidation electrocatalysis and reveal a crucial interaction with oxygen.

Low-crystalline iron oxide hydroxide nanoparticle anode for high-performance supercapacitors.

Carbon materials are generally preferred as anodes in supercapacitors; however, their low capacitance limits the attained energy density of supercapacitor devices with aqueous electrolytes. Here, we report a low-crystalline iron oxide hydroxide nanoparticle anode with comprehensive electrochemical performance at a wide potential window. The iron oxide hydroxide nanoparticles present capacitances of 1,066 and 716 F g-1 at mass loadings of 1.6 and 9.1 mg cm-2, respectively, a rate capability with 74.6% of capacitance retention at 30 A g-1, and cycling stability retaining 91% of capacitance after 10,000 cycles. The performance is attributed to a dominant capacitive charge-storage mechanism. An aqueous hybrid supercapacitor based on the iron oxide hydroxide anode shows stability during float voltage test for 450 h and an energy density of 104 Wh kg-1 at a power density of 1.27 kW kg-1. A packaged device delivers gravimetric and volumetric energy densities of 33.14 Wh kg-1 and 17.24 Wh l-1, respectively.

A facile synthesis of three dimensional graphene sponge composited with sulfur nanoparticles for flexible Li-S cathodes.

Compared with a two dimensional graphene sheet, a three dimensional (3D) graphene sponge has a continuous conductive structure and numerous pores, which are beneficial for sulfur utilization and anchoring. However, strategies for the construction of 3D graphene sponges composited with sulfur nanoparticles (3DGS) are either energy consuming or involve toxic reagents. Herein, a 3DGS is fabricated via a reduction induced self-assembly method, which is simple but facile and scalable. The structural design of this 3DGS promises fast Li(+) transport, superior electrolyte absorbability and effective electrochemical redox reactions of sulfur. As a result, this 3DGS achieves a stable capacity of 580 mA h g(-1) after 500 cycles at a high rate of 1.5 A g(-1), which corresponds to a low fading rate of 0.043% per cycle. The present study effectively demonstrates that the 3D construction strategy is propitious for obtaining flexible high performance Li-S batteries.

Acetylene Black Induced Heterogeneous Growth of Macroporous CoV2O6 Nanosheet for High-Rate Pseudocapacitive Lithium-Ion Battery Anode.

Metal vanadates suffer from fast capacity fading in lithium-ion batteries especially at a high rate. Pseudocapacitance, which is associated with surface or near-surface redox reactions, can provide fast charge/discharge capacity free from diffusion-controlled intercalation processes and is able to address the above issue. In this work, we report the synthesis of macroporous CoV2O6 nanosheets through a facile one-pot method via acetylene black induced heterogeneous growth. When applied as lithium-ion battery anode, the macroporous CoV2O6 nanosheets show typical features of pseudocapacitive behavior: (1) currents that are mostly linearly dependent on sweep rate and (2) redox peaks whose potentials do not shift significantly with sweep rate. The macroporous CoV2O6 nanosheets display a high reversible capacity of 702 mAh g(-1) at 200 mA g(-1), excellent cyclability with a capacity retention of 89% (against the second cycle) after 500 cycles at 500 mA g(-1), and high rate capability of 453 mAh g(-1) at 5000 mA g(-1). We believe that the introduction of pseudocapacitive properties in lithium battery is a promising direction for developing electrode materials with high-rate capability.

Three-Dimensional Crumpled Reduced Graphene Oxide/MoS2 Nanoflowers: A Stable Anode for Lithium-Ion Batteries.

Recently, layered transition-metal dichalcogenides (TMDs) have gained great attention for their analogous graphite structure and high theoretical capacity. However, it has suffered from rapid capacity fading. Herein, we present the crumpled reduced graphene oxide (RGO) decorated MoS2 nanoflowers on carbon fiber cloth. The three-dimensional framework of interconnected crumpled RGO and carbon fibers provides good electronic conductivity and facile strain release during electrochemical reaction, which is in favor of the cycling stability of MoS2. The crumpled RGO decorated MoS2 nanoflowers anode exhibits high specific capacity (1225 mAh/g) and excellent cycling performance (680 mAh/g after 250 cycles). Our results demonstrate that the three-dimensional crumpled RGO/MoS2 nanoflowers anode is one of the attractive anodes for lithium-ion batteries.

Interwoven three-dimensional architecture of cobalt oxide nanobrush-graphene@Ni(x)Co(2x)(OH)(6x) for high-performance supercapacitors.

Development of pseudocapacitor electrode materials with high comprehensive electrochemical performance, such as high capacitance, superior reversibility, excellent stability, and good rate capability at the high mass loading level, still is a tremendous challenge. To our knowledge, few works could successfully achieve the above comprehensive electrochemical performance simultaneously. Here we design and synthesize one interwoven three-dimensional (3D) architecture of cobalt oxide nanobrush-graphene@Ni(x)Co(2x)(OH)(6x) (CNG@NCH) electrode with high comprehensive electrochemical performance: high specific capacitance (2550 F g(-1) and 5.1 F cm(-2)), good rate capability (82.98% capacitance retention at 20 A g(-1) vs 1 A g(-1)), superior reversibility, and cycling stability (92.70% capacitance retention after 5000 cycles at 20 A g(-1)), which successfully overcomes the tremendous challenge for pseudocapacitor electrode materials. The asymmetric supercapacitor of CNG@NCH//reduced-graphene-oxide-film exhibits good rate capability (74.85% capacitance retention at 10 A g(-1) vs 0.5 A g(-1)) and high energy density (78.75 Wh kg(-1) at a power density of 473 W kg(-1)). The design of this interwoven 3D frame architecture can offer a new and appropriate idea for obtaining high comprehensive performance electrode materials in the energy storage field.

Manganese oxide/carbon yolk-shell nanorod anodes for high capacity lithium batteries.

Transition metal oxides have attracted much interest for their high energy density in lithium batteries. However, the fast capacity fading and the low power density still limit their practical implementation. In order to overcome these challenges, one-dimensional yolk-shell nanorods have been successfully constructed using manganese oxide as an example through a facile two-step sol-gel coating method. Dopamine and tetraethoxysilane are used as precursors to obtain uniform polymer coating and silica layer followed by converting into carbon shell and hollow space, respectively. As anode material for lithium batteries, the manganese oxide/carbon yolk-shell nanorod electrode has a reversible capacity of 660 mAh/g for initial cycle at 100 mA/g and exhibits excellent cyclability with a capacity of 634 mAh/g after 900 cycles at a current density of 500 mA/g. An enhanced capacity is observed during the long-term cycling process, which may be attributed to the structural integrity, the stability of solid electrolyte interphase layer, and the electrochemical actuation of the yolk-shell nanorod structure. The results demonstrate that the manganese oxide is well utilized with the one-dimensional yolk-shell structure, which represents an efficient way to realize excellent performance for practical applications.

Novel Li2MnO3 nanowire anode with internal Li-enrichment for use in a Li-ion battery.

Anode materials which undergo a conversion reaction can achieve larger specific capacities than conventional carbon-based materials. They can even achieve higher energy densities when used at low voltages. However, the large amounts of Li2O generated in the interior of these structures when Li ions are inserted can cause volume expansion and mechanical fracturing from the inside out. This leads to a poor cycling performance and limits their commercial application. To overcome this limitation, we introduced Li ions into the interior of the cells of manganese oxide materials and successfully synthesized a novel Li-rich anode material (Li2MnO3). The reversible capacity reached 1279 mA h g(-1) after 500 cycles, much higher than that of pure MnO2 or other commercial anodes. This optimization of the internal Li-enrichment and its application in Li2MnO3 nanowires used as low voltage anodes in Li-ion batteries have rarely been reported. Further investigations by X-ray diffraction and photoelectron spectroscopy suggested that the strategy of optimizing the internal Li-enrichment of this novel Li2MnO3 anode is a promising development for Li-ion batteries.