For efficient use of metal oxides, such as MnO2 and RuO2, in pseudocapacitors and other electrochemical applications, the poor conductivity of the metal oxide is a major problem. To tackle the ...problem, we have designed a ternary nanocomposite film composed of metal oxide (MnO2), carbon nanotube (CNT), and conducting polymer (CP). Each component in the MnO2/CNT/CP film provides unique and critical function to achieve optimized electrochemical properties. The electrochemical performance of the film is evaluated by cyclic voltammetry, and constant-current charge/discharge cycling techniques. Specific capacitance (SC) of the ternary composite electrode can reach 427 F/g. Even at high mass loading and high concentration of MnO2 (60%), the film still showed SC value as high as 200 F/g. The electrode also exhibited excellent charge/discharge rate and good cycling stability, retaining over 99% of its initial charge after 1000 cycles. The results demonstrated that MnO2 is effectively utilized with assistance of other components (fFWNTs and poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) in the electrode. Such ternary composite is very promising for the next generation high performance electrochemical supercapacitors.
Long-term instability of Li–S batteries is one of their major disadvantages compare to other secondary batteries. The reasons for the instability include dissolution of polysulfide intermediates and ...mechanical instability of the electrode film caused by volume changes during charging/discharging cycles. In this paper, we report a novel graphene–sulfur–carbon nanofibers (G-S-CNFs) multilayer and coaxial nanocomposite for the cathode of Li–S batteries with increased capacity and significantly improved long-cycle stability. Electrodes made with such nanocomposites were able to deliver a reversible capacity of 694 mA h g–1 at 0.1C and 313 mA h g–1 at 2C, which are both substantially higher than electrodes assembled without graphene wrapping. More importantly, the long-cycle stability was significantly improved by graphene wrapping. The cathode made with G-S-CNFs with a initial capacity of 745 mA h g–1 was able to maintain ∼273 mA h g–1 even after 1500 charge–discharge cycles at a high rate of 1C, representing an extremely low decay rate (0.043% per cycle after 1500 cycles). In contrast, the capacity of an electrode assembled without graphene wrapping decayed dramatically with a 10 times high rate (∼0.40% per cycle after 200 cycles). These results demonstrate that the coaxial nanocomposites are of great potential as the cathode for high-rate rechargeable Li–S batteries. Such improved rate capability and cycle stability could be attributed to the unique coaxial architecture of the nanocomposite, in which the contributions from graphene and CNFs enable electrodes with improved electrical conductivity, better ability to trap soluble the polysulfides intermediate and accommodate volume expansion/shrinkage of sulfur during repeated charge/discharge cycles.
Flexible and lightweight energy storage systems have received tremendous interest recently due to their potential applications in wearable electronics, roll-up displays, and other devices. To ...manufacture such systems, flexible electrodes with desired mechanical and electrochemical properties are critical. Herein we present a novel method to fabricate conductive, highly flexible, and robust film supercapacitor electrodes based on graphene/MnO2/CNTs nanocomposites. The synergistic effects from graphene, CNTs, and MnO2 deliver outstanding mechanical properties (tensile strength of 48 MPa) and superior electrochemical activity that were not achieved by any of these components alone. These flexible electrodes allow highly active material loading (71 wt % MnO2), areal density (8.80 mg/cm2), and high specific capacitance (372 F/g) with excellent rate capability for supercapacitors without the need of current collectors and binders. The film can also be wound around 0.5 mm diameter rods for fabricating full cells with high performance, showing significant potential in flexible energy storage devices.
Silver nanoparticles (AgNPs) are increasingly used as antimicrobial additives in consumer products and may have adverse impacts on organisms when they inadvertently enter ecosystems. This study ...investigated the uptake and toxicity of AgNPs to the common grass, Lolium multiflorum. We found that root and shoot Ag content increased with increasing AgNP exposures. AgNPs inhibited seedling growth. While exposed to 40 mg L−1 GA-coated AgNPs, seedlings failed to develop root hairs, had highly vacuolated and collapsed cortical cells and broken epidermis and rootcap. In contrast, seedlings exposed to identical concentrations of AgNO3 or supernatants of ultracentrifuged AgNP solutions showed no such abnormalities. AgNP toxicity was influenced by total NP surface area with smaller AgNPs (6 nm) more strongly affecting growth than did similar concentrations of larger (25 nm) NPs for a given mass. Cysteine (which binds Ag+) mitigated the effects of AgNO3 but did not reduce the toxicity of AgNP treatments. X-ray spectro-microscopy documented silver speciation within exposed roots and suggested that silver is oxidized within plant tissues. Collectively, this study suggests that growth inhibition and cell damage can be directly attributed either to the nanoparticles themselves or to the ability of AgNPs to deliver dissolved Ag to critical biotic receptors.
The environmental and health impacts of nanomaterials are becoming important topics of research in recent years. The unique advantages offered by these nanomaterials in wide range of applications ...cannot be realized until these concerns are resolved. Among all the nanomaterials, Ag nanoparticles, due to their existing extensive uses in commercial products, demand immediate attention. Since the nanoparticle suspensions will be exposed to environmental conditions different from a research lab setting, many factors, including light, temperature, salinity, etc., are suspected to affect the stability of the nanoparticle and also their toxicity. In this study, we examined the effect of sunlight on the stability and toxicity of 6 and 25 nm Ag nanoparticles coated with gum arabic (GA) and polyvinylpyrrolidone (PVP). Under sunlight irradiation, all of these nanoparticles irreversibly aggregated to different degrees depending on the surface coating. The UV content of the sunlight is identified to be the driving force of nanoparticle aggregation, and the strong oscillating dipole−dipole interaction is believed to be the origin of the destabilization. Toxicity examinations of the nanoparticles to a wetland plant, Lolium multiflorum, indicate that their toxicity is greatly reduced after sunlight irradiation.
Rechargeable magnesium batteries have attracted wide attention for energy storage. Currently, most studies focus on Mg metal as the anode, but this approach is still limited by the properties of the ...electrolyte and poor control of the Mg plating/stripping processes. This paper reports the synthesis and application of Bi nanotubes as a high-performance anode material for rechargeable Mg ion batteries. The nanostructured Bi anode delivers a high reversible specific capacity (350 mAh/gBi or 3430 mAh/cm3 Bi), excellent stability, and high Coulombic efficiency (95% initial and very close to 100% afterward). The good performance is attributed to the unique properties of in situ formed, interconnected nanoporous bismuth. Such nanostructures can effectively accommodate the large volume change without losing electric contact and significantly reduce diffusion length for Mg2+. Significantly, the nanostructured Bi anode can be used with conventional electrolytes which will open new opportunities to study Mg ion battery chemistry and further improve its properties.
The propensity of silver nanoparticles (AgNPs) having two different polymer coatings (poly(vinylpyrrolidone), PVP, or gum arabic, GA) to aggregate, or to deposit to a reference surface (silica), was ...explored as a basis for differentiating the effect of surface coating on the stability of nanoparticles in aggregation and in deposition. Surface polymeric coatings stabilize nanoparticles against aggregation as shown by either an increased critical coagulation concentration as for PVP-coated AgNPs (AgPVP) or the absence of observable aggregation even at a high ionic strength as for GA-coated AgNPs (AgGA). In experiments of AgNPs deposition in a silica porous medium, dissimilar surfaces favored deposition, such as the case where polymer coatings were present on the AgNPs but were absent on the porous medium. The increased affinity of the AgNPs for the porous medium in this case may be explained by a shifted contact frontier where electrical double layer interaction is weaker. When coating polymers were introduced to the porous medium and allowed to preadsorb to the silica surfaces, the attachment efficiencies for both the AgPVP and AgGA were reduced due to steric and electrosteric stabilization, respectively. The results suggest that polymeric coatings that are usually deemed as stabilizers (as they indeed are in the case of autoaggregation) might not necessarily stabilize nanoparticles against deposition unless the collector surfaces are also coated with polymer.
The solubility of Ag NPs can affect their toxicity and persistence in the environment. We measured the solubility of organic-coated silver nanoparticles (Ag NPs) having particle diameters ranging ...from 5 to 80 nm that were synthesized using various methods, and with different organic polymer coatings including poly(vinylpyrrolidone) and gum arabic. The size and morphology of Ag NPs were characterized by transmission electron microscopy (TEM). X-ray absorption fine structure (XAFS) spectroscopy and synchrotron-based total X-ray scattering and pair distribution function (PDF) analysis were used to determine the local structure around Ag and evaluate changes in crystal lattice parameters and structure as a function of NP size. Ag NP solubility dispersed in 1 mM NaHCO3 at pH 8 was found to be well correlated with particle size based on the distribution of measured TEM sizes as predicted by the modified Kelvin equation. Solubility of Ag NPs was not affected by the synthesis method and coating as much as by their size. Based on the modified Kelvin equation, the surface tension of Ag NPs was found to be ∼1 J/m2, which is expected for bulk fcc (face centered cubic) silver. Analysis of XAFS, X-ray scattering, and PDFs confirm that the lattice parameter, a, of the fcc crystal structure of Ag NPs did not change with particle size for Ag NPs as small as 6 nm, indicating the absence of lattice strain. These results are consistent with the finding that Ag NP solubility can be estimated based on TEM-derived particle size using the modified Kelvin equation for particles in the size range of 5–40 nm in diameter.
PEM fuel cells and redox flow batteries are two very similar technologies which share common component materials and device design. Electrocatalysts are the key components in these two devices. In ...this Review, we discuss recent progress of electrocatalytic materials for these two technologies with a focus on our research activities at Pacific Northwest National Laboratory (PNNL) in the past years. This includes (1) nondestructive functionalization of graphitic carbon as Pt support to improve its electrocatalytic performance, (2) triple-junction of metal–carbon–metal oxides to promote Pt performance, (3) nitrogen-doped carbon and metal-doped carbon (i.e., metal oxides) to improve redox reactions in flow batteries. A perspective on future research and the synergy between the two technologies are also discussed.
The effect of composition and microstructure of NiO on the electrochemical properties of NiO/reduced graphene oxide-based supercapacitor electrodes prepared by solvothermal methods were investigated ...systematically. The findings reveal that the influences of reduced graphene oxide (RGO) coating on electrode systems are essentially 2-fold. One is to increase the electrical conductivity of the electrodes, especially when the electrodes are not very conductive. However, this influence is not that obvious when the electrodes are already highly conductive to begin with. The second influence is to improve the mechanical stability of the electrode, thus increasing the cyclability of the electrodes. Moreover, adding glucose during the electrode synthesis reduces the particle size as well as the thickness of deposited active material (NiO) on the substrate under similar mass loading. These thinner but denser NiO structures exhibited much improved electrochemical performance. These understandings will be important in the designing of high performance energy storage devices, especially from materials with limited electrical conductivity.
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