Mild Aqueous Electrolyte Research Articles

Recent Progress of Rechargeable Batteries Using Mild Aqueous Electrolytes

AbstractGiven the low cost, ease of fabrication, high safety, and environmental‐friendly characteristics, aqueous rechargeable batteries using mild aqueous solutions as electrolytes (pH is close to 7) and a monovalent/multivalent metal ion as charge carrier, are attracting extensive attention for energy storage. However, accompanied by advantages of mild aqueous electrolyte mentioned above, there are some challenges that stand in the way of the development of these aqueous rechargeable batteries, such as the narrow stable electrochemical window of water, instability of electrode materials, undesired side reactions, etc. In recent years, a massive effort is devoted to overcoming the drawbacks, and some encouraging works have arisen. In this review, the latest advances of electrolyte and electrode materials in aqueous batteries based on monovalent ion (Li+, Na+, K+) and multivalent ion (Zn2+, Mg2+, Ca2+, Al3+) are briefly reviewed.

An Environmentally Friendly and Flexible Aqueous Zinc Battery Using an Organic Cathode.

Rechargeable batteries have been used to power various electric devices and store energy from renewables, but their toxic components (namely, electrode materials, electrolyte, and separator) generally cause serious environment issues when disused. Such toxicity characteristic makes them difficult to power future wearable electronic devices. Now an environmentally friendly and highly safe rechargeable battery, based on a pyrene-4,5,9,10-tetraone (PTO) cathode and zinc anode in mild aqueous electrolyte is presented. The PTO-cathode shows a high specific capacity (336 mAh g-1 ) for Zn2+ storage with fast kinetics and high reversibility. Thus, the PTO//Zn full cell exhibits a high energy density (186.7 Wh kg-1 ), supercapacitor-like power behavior and long-term lifespan (over 1000 cycles). Moreover, a belt-shaped PTO//Zn battery with robust mechanical durability and remarkable flexibility is first fabricated to clarify its potential application in wearable electronic devices.

An Environmentally Friendly and Flexible Aqueous Zinc Battery Using an Organic Cathode

AbstractRechargeable batteries have been used to power various electric devices and store energy from renewables, but their toxic components (namely, electrode materials, electrolyte, and separator) generally cause serious environment issues when disused. Such toxicity characteristic makes them difficult to power future wearable electronic devices. Now an environmentally friendly and highly safe rechargeable battery, based on a pyrene‐4,5,9,10‐tetraone (PTO) cathode and zinc anode in mild aqueous electrolyte is presented. The PTO‐cathode shows a high specific capacity (336 mAh g−1) for Zn2+ storage with fast kinetics and high reversibility. Thus, the PTO//Zn full cell exhibits a high energy density (186.7 Wh kg−1), supercapacitor‐like power behavior and long‐term lifespan (over 1000 cycles). Moreover, a belt‐shaped PTO//Zn battery with robust mechanical durability and remarkable flexibility is first fabricated to clarify its potential application in wearable electronic devices.

Polyaniline-intercalated manganese dioxide nanolayers as a high-performance cathode material for an aqueous zinc-ion battery

Rechargeable zinc–manganese dioxide batteries that use mild aqueous electrolytes are attracting extensive attention due to high energy density and environmental friendliness. Unfortunately, manganese dioxide suffers from substantial phase changes (e.g., from initial α-, β-, or γ-phase to a layered structure and subsequent structural collapse) during cycling, leading to very poor stability at high charge/discharge depth. Herein, cyclability is improved by the design of a polyaniline-intercalated layered manganese dioxide, in which the polymer-strengthened layered structure and nanoscale size of manganese dioxide serves to eliminate phase changes and facilitate charge storage. Accordingly, an unprecedented stability of 200 cycles with at a high capacity of 280 mA h g−1 (i.e., 90% utilization of the theoretical capacity of manganese dioxide) is achieved, as well as a long-term stability of 5000 cycles at a utilization of 40%. The encouraging performance sheds light on the design of advanced cathodes for aqueous zinc-ion batteries.

Nanoporous MnO2 Nanoflakes Modified Carbon Cloth Material for Efficient Removal of Heavy Metal Ions in Water by Capacitive Deionization

Increasing demand for the limited resource of fresh water for the large urban populations and development of agriculture and industry draws public concern. Removal of heavy metals such as lead, cadmium, chromium and mercury is crucial in environmental improvement of water and industrial wastewater treatment. Great efforts have been made through chemical precipitation, adsorption, ion exchange, filtration and electrochemical treatment. However, a large volume of sludge residue, expensive and complex matrix materials and low efficiency are still problems that need to be improved. Capacitive deionization (CDI) is a promising energy-efficient technology for water desalination, which is easy to handle and environmentally friendly producing no secondary contaminants through the water purifying process [1]. In order to effectively remove ions, the porous electrodes with large surface area, good chemical stability, high electronic conductivity, and hydrophility are key factors in the selection of CDI materials. Highly porous carbon materials represent the typical electrodes to store the ions through surface ion adsorption/desorption, which is generally categorized as electrochemical double layer. By contrast, pseudocapacitors that consist of conducting polymers and transition metals, store more charge through redox reactions. Among the alternative candidates, the natural abundant and environmental benign MnO2 is of particular interest for research, due to its high theoretical specific capacitance and the ability to be use in mild aqueous electrolytes which expand its practical application [2-3]. MnO2 can be fabricated easily and its morphology can be controlled during simple hydrothermal growth processes. Direct growth on carbon cloth, which is an excellent flexible and conductive substrate, could enhance the regeneration and reuse property of MnO2 as an ideal CDI electrode. Porous MnO2@cabon cloth composites were prepared via a facile hydrothermal method (Figure a). The BET result showed that the average pore width is 18.2 nm. To investigate the CDI property of removing the heavy metal ions, one piece of MnO2@CC and one piece of activated carbon@graphite paper were assembled as working and counter electrodes respectively. This work confirmed the potential of using MnO2@CC as a good CDI electrode material for removal of heavy metal ions from water (Figure b). References S. Porada, R. Zhao, A. Wal, V. Presser, and P. M. Biesheuvel, Prog. Mater. Sci., 58, 1388 (2013).W. Wei, X. Cui, W. Chen, and D. G. Ivey, Chem. Soc. Rev., 40, 1697 (2011).J. Wang, F. Kang, and B. Wei, Prog. Mater. Sci., 74, 51 (2015). Figure 1

Rechargeable Zinc-Ion Batteries Based on Deep Eutectic Solvent

The intermittent characteristic of renewable energy sources leads to a significant challenge in energy generation. Considerable efforts have been made to explore viable solutions, with rechargeable batteries standing out among various energy storage systems. Recently, lithium-ion batteries (LIBs) are widely used in various energy storage devices. However, the high cost, inherent safety risks, and limited lithium resources seem to restrict their large-scale application. Therefore, different new rechargeable battery systems such as aluminum-ion, magnesium-ion, and zinc-ion batteries (ZIBs) have been developed. Among these, rechargeable ZIBs with high safety, abundant electrode materials, high theoretical capacity, environmentally benignity, and low-cost, have received extensive attention. Typically, a ZIB system consists of a host material capable of storing zinc ions as the cathode, zinc metal as the anode. The cathode material becomes the primary limitation for battery performance and manganese dioxide (MnO2) is considered the most promising candidate due to its natural abundance, environmental benignity and low-cost. Generally, mild aqueous electrolyte made of sodium sulfate (Na2SO4) solution are implemented. However, the narrow electrochemical stability window of aqueous electrolytes has limited the operating voltage of the host materials. Thus, deep eutectic solvents are introduced to address this issue. Nevertheless, the mechanism of zinc ion storage and the reason for the capacity fading in ZIB system in deep eutectic solvents have not been thoroughly clarified. In this work, we present a rechargeable ZIB that is composed of a zinc anode prepared from nickel foam supported zinc, a cathode made of stainless steel mesh supported MnO2/carbon film and an electrolyte made of a non-flammable deep eutectic solvent (ethylene glycol/(2-hydroxyethyl) trimethylammonium chloride). ZnCl2 additive in the electrolyte was examined, and the optimal ratio of ZnCl2 was determined. The fabricated battery operated through the electrochemical deposition and dissolution of zinc at the anode, and intercalation/de-intercalation of zinc ions in the MnO2 structure. The fabricated battery exhibited a large capacity of about 200 mAhg-1 and a charge efficiency of approximately 98%. The cathode was found to withstand more than 2,000 cycles without significant capacity decay. The results demonstrated a deep eutectic solvent-based ZIB and offered a fundamental basis for understanding the electrochemical intercalation chemistry of zinc ions in a deep eutectic solvent. Figure 1

Low-cost birnessite as a promising cathode for high-performance aqueous rechargeable batteries

A highly reversible aqueous rechargeable battery system is assembled by using a Zn anode, a room-temperature synthesized MnO2-birnessite cathode and a mild ZnSO4-based aqueous electrolyte with MnSO4 as an additive. Structural analysis reveal that MnO2-birnessite undergoes a structural transformation to poorly crystalline MnO2. It offers an operating voltage of 1.39 V, an energy density of 364 Wh kg−1 (cathode only) at 200 mA g−1 and shows a capacity retention of 100% over 400 cycles at 500 mA g−1. This low-cost battery system is a promising candidate for large-scale energy storage.

Electrochemically induced spinel-layered phase transition of Mn3O4 in high performance neutral aqueous rechargeable zinc battery

Mn3O4 is an electrochemically inactive in alkaline Zn–MnO2 battery. Here, we show that Mn3O4 shows superior electrochemical performance in mild aqueous zinc sulfate electrolytes. Mn3O4 can be a new cathode material for neutral aqueous rechargeable zinc battery. Results reveal that spinel Mn3O4 transforms to intermediate Mn5O8 and finally to Zn-birnessite. The initial charge cycle proceeds without participation of zinc ions, while zinc ions participate in the subsequent reaction. The intercalation of zinc ions into the interlamination of birnessite leads to the generation of Zn-birnessite in discharge process, accompanied by the reduction of Mn4+ to Mn3+. Our results demonstrate a new manganese oxide cathode with average Mn valence state lower than four for rechargeable zinc batteries.

Solvothermal synthesis of molybdenum oxide on liquid-phase exfoliated graphene composite electrodes for aqueous supercapacitor application

MoO3–graphene nanocomposites have been prepared through a green and facile synthesis method and serve as the main electrode materials for electrochemical capacitor in mild aqueous electrolyte. First, graphene sheets were synthesized via mild sonication treatment of graphite flakes in an optimum ratio of ethanol and de-ionized water, followed by deposition of MoO3 onto graphene sheets via low-temperature solvothermal treatment using ethylene glycol- de-ionized water as mixed solvent. The composites were characterised using X-ray diffraction, Raman spectroscopy and electron diffraction and the results revealed that α-MoO3 particles were successfully synthesized and anchored homogeneously onto graphene sheets. The electrochemical capacitance properties of MoO3–graphene nanocomposites were measured by cyclic voltammetry, galvanostatic charge discharge method and electrochemical impedance spectroscopy. The results showed that the specific capacitance of MoO3–graphene nanocomposites is 148 F/g, which is much higher than that of pure MoO3 electrodes (84 F/g) in neutral Na2SO3 electrolyte. The specific capacitance is superior to those reported in the literatures using neutral aqueous electrolytes. Moreover, over 81% of the original capacitance was retained after 1600 cycles, indicating a good cycle stability of composite materials. The high specific capacitance and excellent cyclic stability of the electrode is believed originated from the synergistic effect of the highly conductive graphene material and the pseudocapacitive behavior of the MoO3 nanoparticles in neutral electrolyte.

Electrochemical Characterization of Na-Ion Charge-Storage Properties for Nanostructured NaTi2(PO4)3 as a Function of Crystalline Order

We evaluate a series of nanoparticulate NaTi2(PO4)3 (NTP) powders as Na+-insertion hosts in either nonaqueous or aqueous electrolyte, correlating electrochemical properties such as capacity and electrode kinetics (in the form of powder–composite electrodes) with the degree of crystallinity in NTP. Starting with amorphous NTP powders prepared using the Pechini method, calcination from 500 to 800°C was used to induce varying degrees of crystallinity and to remove carbonaceous species. Poorly crystalline NTP powders derived by heating at 500–600°C exhibit low specific capacities and broad voltammetric features for Na+-insertion, characteristic of surface-limited processes. Heating at higher temperatures (700–800°C) yields the nanocrystalline form with the NASICON structure. Nanocrystalline NTP exhibits sharp voltammetric peaks and diffusion-limited kinetics in both aqueous and nonaqueous electrolytes. The electrochemical performance of nanocrystalline NTP is further enhanced when integrated with reduced graphene oxide (rGO) to increase local electronic conductivity; theoretical specific capacity for NTP (133 mAh g−1) is achieved when NTP-rGO is cycled in a nonaqueous electrolyte, and 100 mAh g−1 in a mild aqueous electrolyte. This nanocomposite also exhibits long-term stability (86% capacity retention after 1000 charge/discharge cycles) in a nonaqueous electrolyte.

Highly Reversible Aqueous Zn/MnO2 Energy Storage System from Chemical Conversion Reactions

Rechargeable aqueous batteries are highly desirable for large scale energy storage due to their low cost and high safety. Lead acid batteries are low cost aqueous systems and much easy to scale up for stationary applications. Nevertheless the use of lead causes significant environmental concern besides the low energy density and limited life span, even though lead-acid batteries still account for more than half of the global battery market. An alternative, low cost and high safety aqueous energy storage system is highly desirable. Herein, we demonstrated a highly reversible chemical conversion reaction mechanism of Zn/MnO2 systems in optimized mild aqueous electrolytes. This conversion reaction provides an operating voltage of 1.44 V, high capacity of 285 mAh g-1 (MnO2), excellent rate, and capacity retention of 92% over 5000 cycles. Zn metal anode also shows high reversibility. The highly reversible and stable chemistry in aqueous Zn/MnO2 batteries opens new opportunities and can be a viable alternate to the widely used lead acid batteries.

A rechargeable Na-Zn hybrid aqueous battery fabricated with nickel hexacyanoferrate and nanostructured zinc

Rechargeable aqueous batteries are very attractive as a promising alternative energy storage system, although their reversible capacity is typically limited. A new rechargeable Na-Zn hybrid aqueous battery with nickel hexacyanoferrate (NiHCF) cathode and the nanostructured zinc anode is fabricated. The rational combination of two materials with mild aqueous electrolyte renders the devices with an average operating voltage close to 1.5 V, higher specific capacity of 76.2 mAh g−1, and a good cycling stability with 81% capacity retention for 1000 cycles. These remarkable features can provide guidance for the development of rechargeable batteries from the naturally abundant electrode materials with neutral aqueous electrolytes.

Valence‐Optimized Vanadium Oxide Supercapacitor Electrodes Exhibit Ultrahigh Capacitance and Super‐Long Cyclic Durability of 100 000 Cycles

Vanadium oxides (VOx) have been intensely investigated as cathode materials for SCs due to the multiple stable oxidation states (III–V) of vanadium in its oxides and typical layered structure. Nevertheless, fast capacity fading is always observed for VOx upon cycling in aqueous electrolyte. Developing an efficient strategy to essentially promote the durability of VOx in mild aqueous electrolyte remains a crucial challenge. Here, an innovative and effective method is reported to significantly boost the durability and capacitance of VOx through tuning the valence state of vanadium. The valence state of vanadium is optimized through a very facile electrochemical oxidation method. A superior electrochemical performance and an ultralong cyclic stability of 100 000 cycles are obtained for these electrodes. An in‐depth study on the variation for the valence state of vanadium during the oxidation process and the cyclic stability test indicates that the long cyclic stability has an important relationship with the distribution of the valence state of vanadium.

Extending Electrochemical Quartz Crystal Microbalance Techniques to Macroscale Electrodes: Insights on Pseudocapacitance Mechanisms in MnOx-Coated Carbon Nanofoams

Electrochemical quartz crystal microbalance studies of MnOx-coated carbon nanofoams reveal that charge-compensation mechanisms associated with MnOx pseudocapacitance in mild aqueous electrolytes are dominated by anion insertion rather than more commonly reported cation ejection. Specific charge-compensation behavior depends on such factors as electrolyte composition, nanofoam pore size, and polarization amplitude. For example, MnOx–carbon nanofoams with average pore sizes of 5–20 nm, cycled in 2.5 M LiNO3, reveal a kinetically-hindered, mixed anion-cation charge-compensation mechanism, whereas the same nanofoam cycled in 2.5 M NaNO3 shows only anion association. Nanofoams with larger pores (10–200 nm) that are cycled in 2.5 M LiNO3, reveal anion-only charge compensation. Our results demonstrate that critical new insights on charge-storage mechanisms are achieved using EQCM methods, even when analyzing practical, macroscale electrodes such as carbon nanofoams.

Nanostructured Multilayer Manganese Dioxide/Nickel/Copper Sulfite (MnO2/Ni/CuS) Composite Films for Electrochemical Applications

Nanostructured multilayer manganese dioxide/nickel/copper sulfite (MnO2/Ni/CuS) composite films were successfully deposited on supporting polyethylene terephathalate (PET) substrate through the sequential deposition of CuS, Ni and MnO2 thin films by chemical bath deposition, electrodeposition, and horizontal submersion deposition techniques, respectively. Deposition of each thin-film layer was optimized through varying deposition parameters and conditions associated with specific deposition technique. Both CuS and Ni thin films were optimized for their electrical conductivity whereas MnO2 thin film was optimized for its microstructure and charge capacity. The electrochemical properties of nanostructured multilayer MnO2/Ni/CuS composite films were evaluated by cyclic voltammetry as electrode materials of electrochemical capacitor prototype in a dual planar device configuration. Cyclic voltammograms in mild Na2SO4 aqueous electrolyte exhibited a featureless and almost rectangular shape which was indicative of ideal capacitive behavior and high cycling reversibility for prolonged cycling up to 1,500 cycles. Nanostructured multilayer MnO2/Ni/CuS composite films on supporting polyethylene terephathalate (PET) substrate could potentially be utilized as electrode materials for the fabrication of high performance electrochemical capacitors.

Invited: Probing Pseudocapacitance Mechanisms in Metal-Oxide–Functionalized Carbon Nanoarchitectures with an Electrochemical Quartz Microbalance

Transition-metal oxides that exhibit “pseudo-capacitance” are challenging high-surface-area carbons as the active material of choice for next-generation electrochemical capacitors (ECs).1 The electrochemical performance of such EC-relevant metal oxides as MnOx has been extensively reported,2 yet the underlying mechanisms responsible for pseudocapacitance are still a subject of debate in the scientific literature. Spectroscopic techniques have been used to verify reversible oxidation-state cycling of metal sites in these oxides (Mn(III/IV) in MnOx; Fe(II/III) in FeOx),3,4 but less is known about the participation of charge-compensating cations from the contacting electrolyte. In the case of mild aqueous electrolytes, redox reactions that give rise to pseudocapacitance may be coupled with insertion/ deinsertion of cations from the electrolyte salt (e.g., Li+, Na+, K+), protons that are available from the H2O solvent, or combinations thereof. Electrochemical quartz crystal microbalance (EQCM) techniques provide information on changes in electrode mass during electrochemical cycling that can address questions regarding ion insertion.5 Until recently the use of EQCM has been limited to well-defined thin-film electrodes, but advancements in EQCM methods now permit analysis of a wider range of substrates, including powder-composite electrodes of EC relevance.6,7 In this paper, we will describe the application of EQCM analysis to a class of electrode architectures comprising carbon nanofoam papers8 whose exterior and interior surfaces are exhaustively coated with nanometers-thick metal oxides that exhibit pseudocapacitance.4 ,9,10,11 Carbon nanofoam papers are freestanding, device-ready electrode architectures of controllable macroscale dimensions (1–102 cm2 in area; 100–500 mm thickness) in which nanoscale dimensions of pore and solid can also be readily adjusted over a wide range (10 nm to several mm). The addition of a nanoscale metal-oxide coating to nanofoam substrates amplifies charge-storage capacity via pseudocapacitance, while the high-frequency response inherent to the nanofoam architecture is retained by ensuring that the metal oxide coating is conformal and self-limiting and that the pore network stays open in three dimensions.For EQCM analysis of these nanofoam-based electrode architectures, we attach disk-shaped pieces of native or oxide-coated carbon nanofoam paper to Au-coated quartz crystals using a carbon-based conductive epoxy. Voltammetric cycling of the resulting electrodes in an EQCM cell yields the expected pseudocapacitance response in terms of both shape and magnitude, demonstrating effective electrical contact to the nanofoam paper. Coupled to the electrochemical response, we monitor mass changes during electrochemical cycling while also varying such parameters as aqueous electrolyte composition (Li+ vs. Na+ vs. K+), average pore size of the nanofoam (10–200 nm), and the type of metal-oxide coating (MnOx vs. FeOx). Preliminary EQCM experiments show that “depletion” effects6 are observed in the presence of Li+-containing electrolytes and for MnOx-coated nanofoams with pores <40 nm, in which the pseudocapacitance mechanism switches from Li+ to NO3 – insertion/deinsertion over the most positive regions of the active potential window of MnOx. The EQCM-observed depletion effect corresponds with the slowest time response of this series of MnOx–carbon nanofoams, as previously reported.12 We compare EQCM results obtained from nanofoam-based electrode architectures with those for thin-film versions of the respective metal oxides. Monitoring mass changes during electrochemical cycling will also allow us determine the conditions under which deleterious dissolution reactions may occur.

Novel iron oxyhydroxide lepidocrocite nanosheet as ultrahigh power density anode material for asymmetric supercapacitors.

A simple one-step electroplating route is proposed for the synthesis of novel iron oxyhydroxide lepidocrocite (γ-FeOOH) nanosheet anodes with distinct layered channels, and the microstructural influence on the pseudocapacitive performance of the obtained γ-FeOOH nanosheets is investigated via in situ X-ray absorption spectroscopy (XAS) and electrochemical measurement. The in situ XAS results regarding charge storage mechanisms of electrodeposited γ-FeOOH nanosheets show that a Li(+) can reversibly insert/desert into/from the 2D channels between the [FeO6 ] octahedral subunits depending on the applied potential. This process charge compensates the Fe(2+) /Fe(3+) redox transition upon charging-discharging and thus contributes to an ideal pseudocapacitive behavior of the γ-FeOOH electrode. Electrochemical results indicate that the γ-FeOOH nanosheet shows the outstanding pseudocapacitive performance, which achieves the extraordinary power density of 9000 W kg(-1) with good rate performance. Most importantly, the asymmetric supercapacitors with excellent electrochemical performance are further realized by using 2D MnO2 and γ-FeOOH nanosheets as cathode and anode materials, respectively. The obtained device can be cycled reversibly at a maximum cell voltage of 1.85 V in a mild aqueous electrolyte, further delivering a maximum power density of 16 000 W kg(-1) at an energy density of 37.4 Wh kg(-1).

Preparation and Characterization of MnO2/acid-treated CNT Nanocomposites for Energy Storage with Zinc Ions

The rod-like manganese dioxide (MnO2)/acid-treated carbon nanotube (a-CNT) nanocomposites have been synthesized by a simple co-precipitation method. The MnO2 nanorod with the diameter of about 10nm and length of 50∼120nm is in-situ deposited on the surface of acid-treated CNT through the redox reaction of KMnO4 and Mn(CH3COO)2·4H2O. Results show that the electron transfer efficiency of the MnO2/a-CNT nanocomposites is greatly enhanced and results in a high electrical conductivity. The energy storage mechanism of as-prepared MnO2/a-CNT in mild aqueous electrolyte (ZnSO4 and MnSO4) is associated with the insertion/extraction of zinc ion into/from the tunnels of crystalline α-MnO2. The composites display both excellent storage properties with zinc ions (∼400 mAh·g−1 at 1 A·g−1) and reversibility at various current rates (∼100% coulombic efficiency after 500 charge/discharge cycles). The MnO2/a-CNT nanocomposites are rather promising cathode material for high performance rechargeable zinc ion batteries.

Liquid precipitation synthesis of Co3O4 for high-performance electrochemical capacitors

The amorphous Co3O4 nanostructure, which adopted sodium hexametaphosphate as structure-directing agent, has been successfully synthesized in large scale via two steps: preparation of the precursor and the calcination process. The results of X-ray diffraction indicate that the prepared materials are mainly composed of Co3O4; the formless Co3O4 nanoplate with loose structures is observed by scanning electron microscopy. Cyclic voltammetry, chronopotentiometry, and electrochemical impedance measurements are applied in a mild aqueous electrolyte (2 mol L−1 KOH) to investigate the performance of the Co3O4, which show a high specific capacitance (SC) of 482.61 F g−1 at 5 mA cm−2. Besides, the SC degradation is only 10.05 % after 250 continuous charge–discharge cycles at 5 mA cm−2, indicating an excellent electrochemical stability. The improved performance is reasonably ascribed to their irregular structure for ionic transport during the electrochemical reaction, which presents as promising candidates for supercapacitors.

Preparation of MnO2 nanostructures by controlled crystal growth and its pseudocapacitive properties

Abstract MnO2 nanostructures were synthesized via a controlled crystal growth method. The results showed that the feeding speed of the starting material affected the physical characterization of the product but did not affect its microcrystal structure. MnO2 nanostructures, composed of interleaving nanoplates, exhibited a mesoporous characteristic and the morphologies were clew-like. Cyclic voltammetry in a mild aqueous electrolyte showed that the charge-storage process was poorly related to double-layer charging processes and was mainly Faradaic. The linear relationship between the response current density and scan rate indicated that the adsorption of cations on the MnO2 surface was the controlling process in the Faradaic reaction. The MnO2 with a mean pore size of 10.6 nm had a high specific capacitance of 404.1 F g− 1 in 1 mol L− 1 Na2SO4 solution at a 2 mV s− 1 scan rate and 2% capacitance loss after 800 cycles at a 10 mV s− 1 scan rate. The MnO2 with good electrochemical performance is a promising supercapacitor material.