Tianxi Liu

Conducting polymer composites: material synthesis and applications in electrochemical capacitive energy storage

In recent years, high efficiency, low cost and environmental friendly energy storage has drawn attention to meet the constantly escalating energy crisis. Conducting polymers in their pristine form have difficulty in achieving satisfying characteristics required for practical applications in electrochemical capacitive energy storage. Considering that conducting polymer composites have emerged as pertinent and beneficial resources for electrochemical capacitive energy storage, this review investigates the relevant topics by presenting the approaches in the design and fabrication of conducting polymer composites as electrode materials for electrochemical capacitive energy storage. The key issues for achieving optimized supercapacitive performances, such as fabricating nanostructured electrodes and tailoring microstructures of conducting polymer composites, are described and concisely discussed in this review. Finally, an outlook of the prospects and challenges in terms of synthesis and applications of conducting polymer composites for supercapacitors is presented.

General solution-processed formation of porous transition-metal oxides on exfoliated molybdenum disulfides for high-performance asymmetric supercapacitors

The combination of hierarchical porous transition-metal oxides with ultrathin two-dimensional (2D) transition-metal dichalcogenides (TMDs) with a favorable electrochemical performance beyond single-component materials is still very challenging. The present work demonstrates the general and targeted synthesis of hybrid heterostructures by the integration of porous transition-metal oxides (TMOs, e.g. NiO, Co3O4 and Fe2O3) and 2D MoS2 nanosheets. The as-prepared vertically aligned MoS2–NiO hybrids exhibit an excellent pseudocapacitive performance, such as a high specific capacitance of 1080.6 at 1 A g−1 and long cycling durability with 101.9% capacitance retention after 9000 cycles at 2 A g−1. This facile strategy using low-cost precursors is regarded as a general method to hybridize 2D MoS2 with other porous TMOs, such as Co3O4 and Fe2O3, with largely improved pseudocapacitive performances due to a favorable synergistic effect between MoS2 and TMOs with an enhanced electronic/ionic transport. Asymmetric supercapacitors using MoS2–TMO hybrids as both positive and negative electrodes are also demonstrated. As a proof-of-concept, the as-assembled MoS2–NiO//MoS2–Fe2O3 asymmetric supercapacitor operating within the potential window of 0–1.8 V delivers a high energy density of 39.6 W h kg−1 with a long cycle life and excellent rate capability.

Highly ordered graphene architectures by duplicating melamine sponges as a three-dimensional deformation-tolerant electrode

In this study, macroscopic graphene-wrapped melamine foams (MF-G) were fabricated by an MF-templated layer-by-layer (LBL) assembly using graphene oxide as building blocks, followed by solution-processed reduction. By concisely duplicating sponge-like, highly ordered three-dimensional architectures from MF, the resulting MF-G with an interconnected graphene-based scaffold and tunable nanostructure was explored as compressible, robust electrodes for efficient energy storage. A thin layer of pseudocapacitive polypyrrole (PPy) was then attached and uniformly coated on MF-G, resulting in a well-defined core–double-shell configuration of the MF-G-PPy ternary composite sponges. The as-assembled devices exhibited enhancement of supercapacitor performance, with a high specific capacitance of 427 F·g−1 under a compressive strain of 75% and an excellent cycling stability with only 18% degradation after 5,000 charge–discharge cycles. Besides, the MF-G-PPy electrode maintained stable capacitance up to 100 compression–release cycles, with a compressive strain of 75%. These encouraging results thus provide a new route towards the low-cost, easily scalable fabrication of lightweight and deformation-tolerant electrodes.

Leaf-inspired interwoven carbon nanosheet/nanotube homostructures for supercapacitors with high energy and power densities

The rational design and scalable synthesis of hierarchical porous carbon as an electrode material for high-energy-density supercapacitors has drawn wide interest. Herein, we report an environmentally friendly one-pot strategy for the synthesis of interwoven carbon nanotube (CNT)/carbon nanosheet (CNS) sandwiches in a molten salt. Green and cheap biomass glucose was chosen as the precursor for producing CNSs at a pyrolysis temperature of 700 °C in a conventional inorganic salt mixture (LiCl and KCl eutectic), while commercially available CNTs were introduced as veins sandwiched with CNS laminae. Relying on the compositional and structural superiorities benefitting from mimicking net-veined leaves, the unique CNT/CNS sandwiches manifest excellent electrochemical capacitive performance in terms of high specific energy (23.6 W h kg−1), excellent rate capacitance retention (∼75% at 10 A g−1) and exceptional cycling stability (capacitance retention of ∼100% after 5000 cycles).

MoSe2 Nanosheet Array with Layered MoS2 Heterostructures for Superior Hydrogen Evolution and Lithium Storage Performance

Engineering heterostructures of transition metal disulfides through low-cost and high-yield methods instead of using conventional deposition techniques still have great challenges. Herein, we present a conveniently operated and low-energy-consumption solution-processed strategy for the preparation of heterostructures of MoSe2 nanosheet array on layered MoS2, among which the two-dimensional MoS2 surface is uniformly covered with high-density arrays of vertically aligned MoSe2. The unique compositional and structural features of the MoS2-MoSe2 heterostructures not only provide more exposed active sites for sequent electrochemical process, but also facilitate the ion transfer due to the open porous space within the nanosheet array serving as well-defined ionic reservoirs. As a proof of concept, the MoS2-MoSe2 heterostructures serve as promising bifunctional electrodes for both energy conversions and storages, which exhibit an active and acid-stable activity for catalyzing the hydrogen evolution reaction, high specific capacity of 728 F g-1 at 0.1 A g-1, and excellent durability with a remained capacity as high as 676 mA h g-1 after 200 cycles.

Octahedral Tin Dioxide Nanocrystals Anchored on Vertically Aligned Carbon Aerogels as High Capacity Anode Materials for Lithium-Ion Batteries.

A novel binder-free graphene - carbon nanotubes - SnO2 (GCNT-SnO2) aerogel with vertically aligned pores was prepared via a simple and efficient directional freezing method. SnO2 octahedrons exposed of {221} high energy facets were uniformly distributed and tightly anchored on multidimensional graphene/carbon nanotube (GCNT) composites. Vertically aligned pores can effectively prevent the emersion of “closed” pores which cannot load the active SnO2 nanoparticles, further ensure quick immersion of electrolyte throughout the aerogel, and can largely shorten the transport distance between lithium ions and active sites of SnO2. Especially, excellent electrical conductivity of GCNT-SnO2 aerogel was achieved as a result of good interconnected networks of graphene and CNTs. Furthermore, meso- and macroporous structures with large surface area created by the vertically aligned pores can provide great benefit to the favorable transport kinetics for both lithium ion and electrons and afford sufficient space for volume expansion of SnO2. Due to the well-designed architecture of GCNT-SnO2 aerogel, a high specific capacity of 1190 mAh/g with good long-term cycling stability up to 1000 times was achieved. This work provides a promising strategy for preparing free-standing and binder-free active electrode materials with high performance for lithium ion batteries and other energy storage devices.

Hierarchical Nanostructures of Nitrogen-Doped Porous Carbon Polyhedrons Confined in Carbon Nanosheets for High-Performance Supercapacitors

Interconnected close-packed nitrogen-doped porous carbon polyhedrons (NCPs) confined in two-dimensional carbon nanosheets (CNSs) have been prepared through a sustainable one-pot pyrolysis of a simple solid mixture of zeolitic imidazolate framework-8 (ZIF-8) crystals and with organic potassium as the precursors. The hierarchically organized framework of the NCP-CNS composites enables NCPs and CNSs to act as well-defined electrolyte reservoirs and mechanical buffers accommodating large volume expansions of NCPs, respectively. Among the unique composite nanostructures, the NCPs with vast micropores provide electric double-layer capacitances, while the CNSs bridge the individual NCPs to form a conductive pathway with a hierarchical porosity. As a result, the NCP-CNS composites with high electrical integrity and structural stability are used as electrode materials for high-performance supercapacitors, which exhibit excellent electrochemical capacitive characteristics in terms of an outstanding capacitance of 300 F g-1 at 1 A g-1, large energy density of 20.9 W h kg-1, and great cycling performance of 100% retention after 6000 cycles. This work therefore presents a one-pot and efficient strategy to prepare an ordered arrangement of ZIF-8-derived porous carbons toward new electrode materials in promising energy storage systems.

A rechargeable metal-free full-liquid sulfur–bromine battery for sustainable energy storage

The broad application of lithium–sulfur technology is far from viable unless the obstacles associated with the dissolution of the sulfur cathode and the dendrite-growth related battery failure arising from the use of a metallic lithium anode are addressed. Taking advantage of the highly soluble sulfur species, this work explores the possibility of using redox-active species with highly positive potential to couple with a sulfur anolyte for a redox flow battery. When paired with an aqueous bromide catholyte, a sulfur–bromine (S–Br2) battery with the desired metal-free characteristic is successfully demonstrated. The battery exhibits a cell voltage exceeding 1.8 V, a specific capacity of ∼1600 mA h g−1, coulombic efficiency approaching 100% and decent cycling efficiencies over 100 cycles. A full-liquid flow-through mode is able to be realized with a controlled depth of charge. Moreover, a high energy density can be expected with highly concentrated electrolytes, guaranteeing a promising sustainable energy storage technology candidate for both stationary and mobile applications.

A biomimetic Setaria viridis-inspired electrode with polyaniline nanowire arrays aligned on MoO3@polypyrrole core–shell nanobelts

Inspired by multi-scale structures mimicking Setaria viridis, herein, arrays of vertically aligned polyaniline (PANI) nanowires on MoO3/polypyrrole (PPy) core–shell nanobelts have been successfully synthesized via a two-step wet-chemistry strategy, including a simple in situ oxidative polymerization of pyrrole on MoO3 nanobelts, followed by an in situ oxidative polymerization of aniline on the MoO3/PPy core–shell nanobelts. By mimicking the hierarchically multi-scale topography of Setaria viridis for tailoring the nanostructures and functions of the electrode materials, in the resultant MoO3/PPy/PANI composites, the MoO3 nanobelt core acts as the “stalk” surrounded by conducting polymers, whereas the intermediate PPy functions as the buffer “grain” connecting the MoO3 and PANI nanowires, which provides good structural stability as well as an efficient electron transfer pathway. Moreover, the outermost PANI nanowire arrays act as the “bristles” allowing fast transport of ions and electrons between the electrodes. Due to their compositional and structural superiority, the as-obtained MoO3/PPy/PANI composites deliver excellent electrochemical energy storage performance including a high specific capacitance of 1315 F g−1 at 0.5 A g−1, a high energy density of 63 W h kg−1 and an excellent cycling stability (capacitance retention of 86% at 10 A g−1 after 20u2006000 cycles). The easy synthesis and excellent electrode performance of the MoO3/PPy/PANI composites make them attractive candidates as promising electrode materials for high-performance supercapacitors.

Assessment on the Self-Discharge Behavior of Lithium–Sulfur Batteries with LiNO3-Possessing Electrolytes

It is generally understood that the reduction of nitrate on the metallic Li surface aids in the formation of a solid-electrolyte interphase. LiNO3 is, therefore, frequently used as an electrolyte additive to help suppress the polysulfide redox shuttle in lithium-sulfur (Li-S) batteries. Although LiNO3 enables cycling of cells with considerably improved Coulombic efficiency and cyclic performance, the self-discharge behavior has largely been neglected. We present in this work a basic but systematic study to assess self-discharge of Li-S batteries with electrolytes possessing LiNO3. Comparative electrochemical tests and interfacial analysis reveal that the redox shuttle is fast enough to cause cells to self-discharge at a relatively rapid rate with limited concentration of the LiNO3 additive. Despite the capacity loss of a full-charged cell under rest for one day can be controlled to 2% with LiNO3 concentration as high as 0.5 M,xa0the development of a practically viable Li-S technology looks like a daunting challenge. Further increasing LiNO3 would potentially cause more irreversible reduction of LiNO3 on the cathode during the first discharge. Therefore, a possible pathway for a long shelf life and low self-discharge is offered as well by the synergic protection of the separator and stabilization of the Li anode surface. The cell using a nanosized Al2O3-coated microporous membrane and a LiNO3-possessing electrolyte exhibits an extremely suppressed self-discharge, providing an alternative perspective for the practical use of Li-S batteries.