Wee Siang Vincent Lee

Mn3O4/reduced graphene oxide based supercapacitor with ultra-long cycling performance

In this work, highly stable Mn3O4 nanoparticles were synthesized via an air oxidation procedure with the aid of an anionic surfactant, which showed durable performance with 100% capacitance retention after 10 000 cycles in 1 M Na2SO4. An annealed film was subsequently fabricated in which the nanostructured Mn3O4 was uniformly intercalated between reduced graphene oxide (RGO) sheets. To complement the Mn3O4/RGO film, a hybrid film consisting of RGO and carbon nanotubes was employed as the negative electrode. Benefiting from the thermodynamically stable Mn3O4 nanoparticles, specific layered structural design, and highly conductive RGO scaffolds, the assembled supercapacitor exhibited a high volumetric capacitance of 52.2 F cm−3 at 0.2 A cm−3, which translated to remarkable volumetric energy and power density (18 mW h cm−3 and 3.13 W cm−3). More importantly, the assembled device was able to demonstrate an outstanding cycling performance of 115% capacitance retention after 60 000 cycles. The bottom-up approach proposed in this study involving the synthesis of durable nanoparticles followed by composite construction could pave the way towards designing ultra-stable supercapacitors as next-generation energy storage devices.

Indole-based conjugated macromolecules as a redox-mediated electrolyte for an ultrahigh power supercapacitor

Balancing energy density and power density has been a critical challenge since the inception of supercapacitors. Introducing redox-active additives in the supporting electrolyte has been shown to increase the energy density, however the power density and cycling stability are severely hampered in the process. Herein, an extensively conjugated indole-based macromolecule consisting of 5,6-dihydroxyindole/5,6-quinoneindole motifs, prepared by electrochemical polymerization of dopamine under acidic conditions, was employed as a redox-active additive. By utilizing the conjugation effect, the HOMO–LUMO gap (HLG) of the extensively conjugated indole-based macromolecule was reduced to ca. 2.08 eV, which enhanced the electronic transfer kinetics, in turn improving the power density and reversibility of redox reactions. When coupled with a porous honeycomb-like carbon (PHC) electrode, the assembled supercapacitor delivered an excellent rate performance with a high specific capacitance of 205 F g−1 at 1000 A g−1. This work reports one of the highest power densities recorded at 153 kW kg−1 for redox-mediated electrolyte systems with a respectable energy density of 8.8 W h kg−1. In addition to an excellent cycling stability of 97.1% capacitance retention after 20 000 charge/discharge cycles, the conjugation degree has to be considered when engineering the redox-active electrolyte so as to improve the power density and stability.

Mechanically robust glucose strutted graphene aerogel paper as a flexible electrode

With the advent of next generation wearable technologies, energy storage devices at present not only have to achieve high energy densities they also need to possess reasonable mechanical robustness. Traditional graphene paper is mechanically robust which is ideal for a flexible electrode. However, the restacking issue and the degradation in performance with increasing thickness, limit the usage of graphene paper as a functional electrode. In this paper, porous, flexible, and free-standing graphene aerogel paper was prepared from an acid-treated glucose-strutted graphene aerogel via mechanical compression. The sulphur groups on the glucose struts served as a strengthening function due to thiol-carboxylic acid esterification and the increase in hydrogen bonding. The resulting nanostructured composite was able to exhibit a high ultimate tensile strength of 0.6 MPa which is 3 times that of the graphene aerogel paper without the glucose struts (0.2 MPa). It was also able to withstand 100 cyclic loadings of 0.13 MPa without failure. When electrochemically tested, it was able to discharge with a specific capacitance of 311 F g−1 at a current density of 1 A g−1, and a modest specific capacitance of 262 F g−1 at a high current density of 20 A g−1. Therefore, such results validate its promising application as a flexible electrode in energy storage devices.

High lithium insertion voltage single-crystal H2Ti12O25 nanorods as high capacity and high rate lithium-ion battery anode material

H2 Ti12 O25 holds great promise as a high-voltage anode material for advanced lithium-ion battery applications. To enhance its electrochemical performance, control of the crystal orientation and morphology is an effective way to cope with slow Li+ -ion diffusion inside H2 Ti12 O25 with severe anisotropy. In this report, Na2 Ti6 O13 nanorods, prepared from Na2 CO3 and anatase TiO2 in molten NaCl medium, were used as a precursor in the synthesis of long single-crystal H2 Ti12 O25 nanorods with reactive facets. The as-prepared H2 Ti12 O25 nanorods with a diameter of 100-200 nm showed higher charge (extraction) specific capacity and better rate performance than previously reported systems. The reversible capacity of H2 Ti12 O25 was 219.8 mAh g-1 at 1C after 100 cycles, 172.1 mAh g-1 at 10C, and 144.4 mAh g-1 at 20C after 200 cycles; these values are higher than those of H2 Ti12 O25 prepared by the conventional soft-chemical method. Moreover, the as-prepared H2 Ti12 O25 nanorods exhibited superior cycle stability with more than 94 % retention of capacity with nearly 100 % coulombic efficiency after 100 cycles at 1C. On the basis of the above results, long single-crystal H2 Ti12 O25 nanorods synthesized in molten NaCl with outstanding electrochemical characteristics hold a significant amount of promise for hybrid electric vehicles and energy-storage systems.

Low Li+ Insertion Barrier Carbon for High Energy Efficient Lithium-Ion Capacitor

Lithium-ion capacitor (LIC) is an attractive energy-storage device (ESD) that promises high energy density at moderate power density. However, the key challenge in its design is the low energy efficient negative electrode, which barred the realization of such research system in fulfilling the current ESD technological inadequacy due to its poor overall energy efficiency. Large voltage hysteresis is the main issue behind high energy density alloying/conversion-type materials, which reduces the electrode energy efficiency. Insertion-type material though averted in most research due to the low capacity remains to be highly favorable in commercial application due to its lower voltage hysteresis. To further reduce voltage hysteresis and increase capacity, amorphous carbon with wider interlayer spacing has been demonstrated in the simulation result to significantly reduce Li+ insertion barrier. Hence, by employing such amorphous carbon, together with disordered carbon positive electrode, a high energy efficient LIC with round-trip energy efficiency of 84.3% with a maximum energy density of 133 Wh kg-1 at low power density of 210 W kg-1 can be achieved.

Bioinspired Dual-Tier Coalescence for Water Collection Efficiency Enhancement

Directly harvesting water from the atmosphere could aid in negating the issue of fresh water scarcity, garnering increased research interest in recent years. Typically, atmospheric water collection occurs via three main steps: accumulation, transportation, and collection. Although multiple studies have been published on bioinspired structures with enhanced directional fluid transportation, there is a significant lack of designs for enhancing water droplet coalescence. Long mean times before coalescence result in the re-evaporation of microdroplets, severely impeding the efficiency of atmospheric water collection. Herein, a water accumulator derived from a synergistic combination of inspiration from cacti spines and Tillandsia trichomes has been designed to encourage rapid coalescence. The drip-off volume measured in a fog chamber was found to be 220% that of a flat surface within 15 min, suggesting that improving the coalescence efficiency will be important in the future development of water-collection devices.

o-Benzenediol-Functionalized Carbon Nanosheets as Low Self-Discharge Aqueous Supercapacitors

Widening the voltage window is often proposed as a way to increase the energy density of aqueous supercapacitors. However, attempting to operate beyond the aqueous supercapacitor stability region can undermine the supercapacitor reliability due to pronounced electrolyte decomposition, which can lead to a significant self-discharge process. To minimize this challenge, charge injection by grafting o-benzenediol onto the carbon electrode is proposed through a simple electrochemical cycling technique. Due to charge injection from o-benzenediol into the carbon electrode, the equilibrium potential of the individual electrode can be reduced. In addition, due to its small molecular size, charge distribution, which is commonly faced by bulk pseudocapacitive materials, is also avoided. The assembled supercapacitor based on the o-benzenediol-grafted carbon demonstrated a maximum energy density of 24 Wh kg-1 and a maximum power density of 69 kW kg-1 , with a retention of 89 % after 10 000 cycles at 10 A g-1 . A low self-discharge of about 4 h was recorded; this could be attributed to the low driving force arising from the lower equilibrium potential. Thus, the proposed technique may provide insight towards the tuning of the equilibrium potential to attain reliable, high-performing supercapacitors with a low self-discharge process.