Chengjun Xu

Recent progress on manganese dioxide based supercapacitors

The increasing worldwide interest in MnO 2 for supercapacitor applications is based on anticipation that MnO 2 -based high-voltage aqueous supercapacitors will ultimately serve as a safe and low-cost alternative to state-of-the-art commercial organic-based electrochemical double-layer capacitors or RuO 2 -based acid systems. In this paper, the physicochemical features, synthesis methods, and charge storage mechanism of MnO 2 as well as the current status of MnO 2 -based supercapacitors are summarized and discussed in detail. The future opportunities and challenges related to MnO 2 -based supercapacitors have also been proposed.

Flexible electrodes and supercapacitors for wearable energy storage: a review by category

Supercapacitors are important energy storage devices capable of delivering energy at a very fast rate. With the increasing interest in portable and wearable electronic equipment, various flexible supercapacitors (FSCs) and flexible electrodes (FEs) have been investigated widely and constantly in recent years. Currently-developed FEs/FSCs exhibit myriad physical forms and functional features and form a complicated and extensive system. Herein, we summarize the recent results about FEs/FSCs and present this review by categories. According to different micro-structures and macroscopic patterns, the existing FEs/FSCs can be divided into three types: fiber-like FEs/FSCs; paper-like FEs/FSCs; and three-dimensional porous FEs (and corresponding FSCs). Subsequently each type of the FEs/FSCs is further sub-classified based on their construction rules, and mechanical and electrochemical properties. To our best knowledge, this is the first time such a hierarchical and detailed classification strategy has been propose. We believe it will be beneficial for researchers around the world to understand FEs/FSCs. In addition, we bring up some fresh ideas for the future development of wearable energy storage devices.

Asymmetric Activated Carbon-Manganese Dioxide Capacitors in Mild Aqueous Electrolytes Containing Alkaline-Earth Cations

Manganese dioxide exhibits the ideal capacitive behavior in aqueous electrolytes containing alkaline-earth cations (Mg 2+ , Ca 2+ , or Ba 2+ ). A specific capacitance value as high as 325 F g -1 was obtained in 0.1 mol L -1 Mg(NO 3 ) 2 electrolyte at 2 mV s -1 . The ideal capacitive behavior, high specific capacitance, good coulombic efficiency, and rate ability of manganese dioxides in aqueous electrolytes containing bivalent cations (Mg 2+ , Ca 2+ , or Ba 2+ ) indicate that these alkaline-earth cations might be good alternatives for the state-of-the-art univalent alkaline cations. Moreover, activated carbon/MnO 2 asymmetric capacitors with 2 V operating voltage were built up based on the aqueous electrolytes containing these alkaline-earth cations. The energy density of the AC/MnO 2 asymmetric capacitor with Ca 2+ cation at a current density of 0.3 A g -1 was found to be 21 Wh Kg -1 .

Co-electro-deposition of the MnO2–PEDOT:PSS nanostructured composite for high areal mass, flexible asymmetric supercapacitor devices

To meet the rapidly growing demand, it is necessary to develop novel flexible energy storage devices with a high energy density in a limited area, a fast charging ability, a low cost for mass production and a miniaturized device size. To address the above issues, here we introduce the co-electro-deposition strategy, which is able to prepare an electrode material with a high areal capacitance (1670 mF cm−2 at 0.5 mA cm−2), a high areal mass (8.5 mg cm−2), an excellent mechanical robustness, a high through-put and great convenience even on a piece of a ubiquitous stainless steel mesh current collector. Based on this advancement, we are able to obtain an ultrathin (less than 200 μm) aqueous asymmetric supercapacitor device with a high energy density (1.8 × 10−3 W h cm−3), a high power density (0.38 W cm−3 at 3.62 × 10−4 W h cm−3) and an excellent rate capability. This energy storage device is integrated into a prototype smart card to drive a light emitting diode (LED) indicator, which is charged for 5 seconds and can light up the indicator for more than 2 hours, demonstrating great promise in miniaturized novel flexible energy storage devices.

Flexible asymmetric supercapacitors based on ultrathin two-dimensional nanosheets with outstanding electrochemical performance and aesthetic property

Flexible asymmetric supercapacitors with excellent electrochemical performance and aesthetic property are realized by using ultrathin two-dimensional (2D) MnO2 and graphene nanosheets as cathode and anode materials, respectively. 2D MnO2 nanosheets (MSs) with a thickness of ca. 2 nm are synthesized with a soft template method for the first time, which achieve a high specific capacitance of 774 F g−1 even after 10000 cycles. Asymmetric supercapacitors based on ultrathin MSs and graphene exhibit a very high energy density up to 97.2 Wh kg−1 with no more than 3% capacitance loss after 10000 cycles in aqueous electrolyte. Most interestingly, we show that the energy storage device can have an aesthetic property. For instance, a “Chinese panda” supercapacitor is capable of lighting up a red light emitting diode. This work has another, quite different aspect that a supercapacitor is no longer a cold industry product, but could have the meaning of art.

Breathable and Wearable Energy Storage Based on Highly Flexible Paper Electrodes.

Breathable and wearable energy storage is achieved based on an innovative design solution. Carbon nanotube/MnO2 -decorated air-laid paper electrodes, with outstanding flexibility and good electrochemical performances, are prepared. They are then assembled into solid-state supercapacitors. By making through-holes on the supercapacitors, breathable and flexible supercapacitors are successfully fabricated.

High-performance compressible supercapacitors based on functionally synergic multiscale carbon composite textiles

High-performance compressible supercapacitors were realized based on functionally synergic multiscale carbon composite textiles. The composite textiles were fabricated by introducing nanoscale carbon fillers, i.e., carbon nanotube (CNT) or graphene (GN), on activated carbon fiber felt (ACFF) body material using a “simple impregnation and freeze-drying method”. The prepared CNT/ACFF and GN/ACFF composite textiles preserved the advantages of the ACFF body material in structure, such as being foldable and windable, whereas functionally they showed a synergic effect of the body material and nano-fillers, by integrating the excellent electrical conductivity of the nano-fillers with the high specific surface area and appropriate pore structures of ACFF. Thus, their electrochemical performances were significantly enhanced in the assembled symmetric supercapacitors, compared with those of the existing studies on textile electrodes. Areal capacitance, energy density and power density could be as high as 3350 mF cm−2, 112 μW h cm−2 and 4155 μW h cm−2, respectively. Most interestingly, our composite textiles can be compressed into a much smaller-sized and windable supercapacitive device, providing opportunities for their application as portable textile supercapacitors.

Reversible Insertion Properties of Zinc Ion into Manganese Dioxide and Its Application for Energy Storage

The reversible intercalation of Zn 2+ ions into manganese dioxide was first reported in an aqueous system and a large capacity (210 mAh g -1 ) was measured. A cycle life test was performed at 1 A g -1 , and after 50 cycles, no capacity fading was found, which indicates the good cycling properties of manganese dioxide toward the insertion of zinc ions. X-ray photoelectron spectrum measurements indicated that Zn 2+ ions in the electrolyte are involved in the charge storage process of manganese dioxide. X-ray diffraction results exhibited the kinetic stability of the host structure in allowing high-capacity, single-phase, and reversible zinc-ion intercalation.

Secondary batteries with multivalent ions for energy storage

The use of electricity generated from clean and renewable sources, such as water, wind, or sunlight, requires efficiently distributed electrical energy storage by high-power and high-energy secondary batteries using abundant, low-cost materials in sustainable processes. American Science Policy Reports state that the next-generation “beyond-lithium” battery chemistry is one feasible solution for such goals. Here we discover new “multivalent ion” battery chemistry beyond lithium battery chemistry. Through theoretic calculation and experiment confirmation, stable thermodynamics and fast kinetics are presented during the storage of multivalent ions (Ni2+, Zn2+, Mg2+, Ca2+, Ba2+, or La3+ ions) in alpha type manganese dioxide. Apart from zinc ion battery, we further use multivalent Ni2+ ion to invent another rechargeable battery, named as nickel ion battery for the first time. The nickel ion battery generally uses an alpha type manganese dioxide cathode, an electrolyte containing Ni2+ ions, and Ni anode. The nickel ion battery delivers a high energy density (340 Wh kg−1, close to lithium ion batteries), fast charge ability (1 minute), and long cycle life (over 2200 times).

High-performance supercapacitors based on graphene/MnO2/activated carbon fiber felt composite electrodes in different neutral electrolytes

Graphene/MnO2 composites are introduced into activated carbon fiber felt (ACFF) to fabricate composite textile electrodes. Their micro-structure, electrical properties and electrochemical performance for supercapacitor applications in different neutral electrolytes (1 M NaNO3 and Ca(NO3)2 aqueous solutions) have been studied. The composite electrodes have similar pore features to original ACFF textiles, but show notably enhanced electrical and electrochemical performance. The composite textile electrodes show low electrical resistance, high specific capacitance (up to 1516 mF cm−2 in neutral electrolytes) and excellent cycling stability (no capacitance decay after 5000 charge–discharge cycles). Besides, electrochemical capacitance of composite textile electrodes in Ca(NO3)2 electrolyte is higher than that in NaNO3 electrolyte at low scan rates (1–5 mV s−1), but the situation is reversed when scan rates are higher than 10 mV s−1. Above all, the results show that our low-cost composite textile electrodes are high-performance in neutral electrolytes, which is helpful for developing large-scale energy storage devices.