Stability In Aqueous Electrolyte Research Articles

Supercritical CO2 processing to improve the electrochemical properties of graphene oxide

Porous graphene oxide (GO) structures at different oxidation levels have been obtained by a SC-CO2 assisted process performed at 200bar, 35°C and different durations up to 24h. Supercritical CO2 processing was aimed at exfoliating and reducing graphene oxide.Different techniques were used to characterize the materials obtained: transmission electron microscopy (TEM), Raman and FT-IR Spectroscopy, X-ray diffraction analysis and N2 adsorption-desorption at 77K. The materials with high surface area up to 930m2/g, obtained after SC-CO2 treatment, exhibited unique mesoporous structure based on curve graphene sheets and a high capacitive performance with values of 253F/g at 1A/g and of 210F/g at 16A/g; moreover, a good long cycle stability in aqueous electrolytes (higher than 90% after 2ÿ103 cycles) has been also obtained.SC-CO2 processing proved to be successful and simple to be realized, producing high performance materials, characterized by high specific surface areas and supercapacitive performances.

Hydrogen‐Bonded Organic Semiconductors as Stable Photoelectrocatalysts for Efficient Hydrogen Peroxide Photosynthesis

Research on semiconductor photocatalysts for the conversion of solar energy into chemical fuels has been at the forefront of renewable energy technologies. Water splitting to produce H2 and CO2 reduction to hydrocarbons are the two prominent approaches. A lesser‐known process, the conversion of solar energy into the versatile high‐energy product H2O2 via reduction of O2 has been proposed as an alternative concept. Semiconductor photoelectrodes for the direct photosynthesis of H2O2 from O2 have not been applied up to now. Photoelectrocatalytic oxygen reduction to peroxides in aqueous electrolytes by hydrogen‐bonded organic semiconductor is observed photoelectrodes. These materials have been found to be remarkably stable operating in a photoelectrochemical cell converting light into H2O2 under constant illumination for at least several days, functioning in a pH range from 1 to 12. This is the first report of a semiconductor photoelectrode for H2O2 production, with catalytic performance exceeding prior reports on photocatalysts by one to two orders of magnitude in terms of peroxide yield/catalyst amount/time. The combination of a strongly reducing conduction band energy level with stability in aqueous electrolytes opens new avenues for this widely available materials class in the field of photo(electro) catalysis.

Three-Dimensional Porous Carbon Materials with High Surface Area for Electrochemical Energy Storage

Supercapacitors are widely recognized as an important class of energy storage devices because they can provide a high specific power and long cycle life. Electrical double-layer capacitors (EDLCs) store charge and release electric energy through the transfer of ions to and from the electrode/electrolyte interface without a Faradaic reaction. Therefore, the amount of energy stored by EDLCs is dependent on the surface area of the electrodes available for electrolyte ion access. However, for the use of the commercial carbon materials such as activated carbon and CNT etc., a poor energy density is observed due to the lack of porous structure which limits the active area for charge storage. Thus, great efforts have been focused on preparation of 3D-porous structure carbons for EDLCs to improve upon these factors. In this work, a new type of advanced 3D macroporous carbon material derived from PIM-1 with a high surface area and electrical conductivity is proposed and anticipated for energy storage applications. PIM-1 (polymer of intrinsic microporosity) is a polymer with a large fractional free-volume with a high surface area of 760 m2/g and chosen as precursor material for carbonization. 3D macroporous PIM-1 films (NPIMs) with continuously interconnected structure were fabricated by applying the nonsolvent-induced phase inversion method. Finally the carbonized NPIMs (cNPIMs) were obtained through direct high temperature pyrolysis of NPIMs without any external activation agent. The cNPIMs presented a very large surface area (2101.1 m2/g) with narrow micropore size distribution (0.75 nm) as shown in Fig. 1. The SEM analysis revealed that cNPIMs also have a unique 3D macroporous structure having both dense skin layer and gradient pore structure, which is expected to grant a smooth and easy ion transfer capability as an electrode material. Consequently, the cNPIM exhibits a high capacitance (405.8 F/g) and long-term stability in aqueous electrolyte. We believe that our approach can provide a variety of new 3D macroporous carbon materials for the application in a electrochemical energy storage. Figure 1

Nano-composite LiMnPO4 as new insertion electrode for electrochemical supercapacitors

Nano-composite olivine LiMnPO4 (nC-LMP) was found to exhibit facile pseudo-capacitive characteristics in aqueous as well as non-aqueous electrolytes. We demonstrated employing nC-LMP as positive electrode in hybrid electrochemical capacitors namely Li-Ion hybrid capacitors (LIC). Adapting a simple CVD technique, nano-crystallites of LiMnPO4 were coated with carbon monolayers of ∼2 nm thick to circumvent its poor intrinsic electronic conductivity. The novelty is that the single crystallites were intimately covered with carbon ring and networked to the neighboring crystallites via the continuous carbon wire-like connectivity as revealed from HRTEM analysis. Single electrode faradic capacitance of 3025 Fg−1 (versus standard calomel reference electrode) was deduced for carbon coated LMP, the highest reported hitherto in Li+ aqueous electrolytes. Employing nC-LMP as working electrode versus an activated carbon (AC), we obtained a high specific energy of 28.8 Wh kg−1 with appreciable stability in aqueous electrolytes whereas in nonaqueous electrolyte there is an obvious increase in energy density (35 Wh kg−1) due to wider potential window. That is, a full cell version of LIC, AC|Li+|LMP, was fabricated and demonstrated its facile cycling characteristics via removal/insertion of Li+ within nC-LMP (positive electrode) and the electrosorption of Li+ into mesoporous carbon (AC) (negative electrode). Such cells ensured a typical battery-like charging and EDLC-like discharging characteristics of LIC type electrochemical capacitors (ECs) which are desired to enhance safety and energy densities.

Platinum-Enhanced Electron Transfer and Surface Passivation through Ultrathin Film Aluminum Oxide (Al₂O₃) on Si(111)-CH₃ Photoelectrodes.

We report the preparation, stability, and utility of Si(111)-CH3 photoelectrodes protected with thin films of aluminum oxide (Al2O3) prepared by atomic layer deposition (ALD). The photoelectrodes have been characterized by X-ray photoelectron spectroscopy (XPS), photoelectrochemistry (Fc in MeCN, Fc-OH in H2O), electrochemical impedance spectroscopy (EIS), and cyclic voltammetry (CV) simulation. XPS analysis of the growing Al2O3 layer affords both the thickness, and information regarding two-dimensional versus three-dimensional mode of growth. Impedance measurements on Si(111)|CH3|Al2O3 devices reveal that the nascent films (5-30 Å) exhibit significant capacitance, which is attenuated upon surpassing the bulk threshold (∼30 Å). The Al2O3 layer provides enhanced photoelectrochemical (PEC) stability evidenced by an increase in the anodic window of operation in MeCN (up to +0.5 V vs Ag) and enhanced stability in aqueous electrolyte (up to +0.2 V vs Ag). XPS analysis before and after PEC confirms the Al2O3 layer is persistent and prevents surface corrosion (SiOx). Sweep-rate dependent CVs in MeCN at varying thicknesses exhibit a trend of increasingly broad features, characteristic of slow electron transport kinetics. Simulations were modeled as slow electron transfer through a partially resistive and electroactive Al2O3 layer. Lastly, we find that the Al2O3 ultrathin film serves as a support for the ALD deposition of Pt nanoparticles (d ≈ 8 nm) that enhance electron transfer through the Al2O3 layer. Surface recombination velocity (SRV) measurements on the assembled Si(111)|CH3|Al2O3-15 device affords an S value of 4170 cm s(-1) (τ = 4.2 μs) comparable to the bare Si(111)-CH3 surface (3950 cm s(-1); τ = 4.4 μs). Overall, the results indicate that high electronic quality and low surface defect densities can be retained throughout a multistep assembly of an integrated and passivated semiconductor|thin-film|metal device.

Using Intimate Carbon to Enhance the Performance of NaTi2(PO4)3 Anode Materials: Carbon Nanotubes vs Graphite

The effect of intermixed carbon nanotubes and graphite on the electrochemical functionality of NaTi2(PO4)3 was examined. Specifically, the performance of NaTi2(PO4)3 made with “intimate carbons” containing carbon nanotubes (or graphite) introduced at the precursor stage was compared to the performance of similar materials made with the carbons added post synthesis. Specifically, different combinations of carbon nanotubes and graphite were used as carbon sources both in the precursor blend prior to sintering as well as conductive additives in the test electrodes. Graphite-coated NaTi2(PO4)3 with additional carbon nanotubes added (post synthesis) as conductive agent exhibited the best overall performance: the first cycle discharge specific capacity is 130 mAh/g (close to theoretical value 133 mAh/g) at 0.1C rate. The combination also exhibits good cycling stability in aqueous electrolyte: 86% capacity retention under continuous charge/discharge without relaxation at 1C rate for 100 cycles. This suggests that fully coating the precursor material with carbon is critical for performance, and that a graphite additive in the precursor phase is more effective than nanotubes at performing this function.

Sulfur-doped porous reduced graphene oxide hollow nanosphere frameworks as metal-free electrocatalysts for oxygen reduction reaction and as supercapacitor electrode materials.

Chemical doping with foreign atoms is an effective approach to significantly enhance the electrochemical performance of the carbon materials. Herein, sulfur-doped three-dimensional (3D) porous reduced graphene oxide (RGO) hollow nanosphere frameworks (S-PGHS) are fabricated by directly annealing graphene oxide (GO)-encapsulated amino-modified SiO2 nanoparticles with dibenzyl disulfide (DBDS), followed by hydrofluoric acid etching. The XPS and Raman spectra confirmed that sulfur atoms were successfully introduced into the PGHS framework via covalent bonds. The as-prepared S-PGHS has been demonstrated to be an efficient metal-free electrocatalyst for oxygen reduction reaction (ORR) with the activity comparable to that of commercial Pt/C (40%) and much better methanol tolerance and durability, and to be a supercapacitor electrode material with a high specific capacitance of 343 F g(-1), good rate capability and excellent cycling stability in aqueous electrolytes. The impressive performance for ORR and supercapacitors is believed to be due to the synergistic effect caused by sulfur-doping enhancing the electrochemical activity and 3D porous hollow nanosphere framework structures facilitating ion diffusion and electronic transfer.

Electrochemical Characterization of a Carbon Nanofiber Electrode System Hybridized with PEDOT-PSS

In this work, electrochemical properties of a bilayer electrode system prepared from an electrically conducting polymer, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), PEDOT-PSS coated carbon nanofibers (CNFs), have been investigated. The CNFs were used as supports for the deposition of PEDOT-PSS by a dip-coating technique to yield a bilayer electrode system. Electrodes prepared by such a method were used in supercapacitors operating in acidic (1 M H2SO4) electrolytes. The capacitance values were estimated by voltammetry and galvanostatic techniques with a three-electrode cell configuration. Due to the CNF's graphitic structure and the presence of exterior walls with numerous edges, a high specific surface area and easily accessible electrode/electrolyte interface were obtained, thus yielding good capacitance in the bilayer active materials. The capacitance for PEDOT-PSS coated CNF bilayer electrodes ranged from 80 to 180 F/g and the fabricated materials showed good cycling performance with high stability in aqueous electrolytes. This was probably due to enhanced access to the CNFs, leading to the generation of a double layer and, ultimately, higher values of the capacitance.

The stability of nonporous and macroporous titania thin films in aqueous electrolyte solutions

A simple electrochemical approach was used to evaluate the stability and porosity of titania and silica thin films spin coated on electrode surfaces. This approach involved monitoring the magnitude of the Faradaic current of diffusing redox probes at the modified electrode surfaces over the course of a week to 4.5 months. Relatively nonporous films were examined as well as films templated with polystyrene latex spheres. The results show that templated titania films were significantly more porous compared to non-templated films. After the defect sites in the templated films were blocked, their long-term stability in aqueous electrolyte was evaluated. For titania, blocking was done by spin coating a dilute titania sol on the top of the film whereas for silica, the film was soaked in octyltrimethoxysilane. Both types of titania films (templated and non-templated) were found to be significantly more stable than the corresponding silica films, showing no signs of deterioration in simple electrolyte solutions during the entire evaluation period. In contrast, silica films showed significant deterioration in as little as 3 days. The enhanced stability of the titania films relative to silica films in near neutral electrolyte solutions was attributed to the differences in the point of zero charge of the oxide films.

Investigation on capacity fading of LiFePO4 in aqueous electrolyte

The capacity loss rate of LiFePO4 in aqueous electrolyte was found to be much faster than it in organic electrolyte. The cycling stability in aqueous electrolyte with various dissolved oxygen content and pH value was extensively studied by cyclic voltammetry. It was found that both high OH− and dissolve O2 content can accelerate the cycling fading of LiFePO4. It has been proved that the capacity fading of LiFePO4 is due to not only chemical instability but also electrochemical instability. Mössbauer spectroscopy demonstrated that the Fe(III)-containing species was formed in the active materials arisen from the irreversible side reaction. The carbon-coated LiFePO4 prepared by chemical vapor decomposition method shows significantly improvement in cycling stability. The carbon coating technology provides an effective approach to enhance cycling performance in aqueous electrolyte as well as proof of proposed fading mechanism.

Electrochemical performance of a high cation-deficiency Li 2Mn 4O 9/active carbon supercapacitor in LiNO 3 electrolyte

A hybrid supercapacitor based on spinel Li 2Mn 4O 9 and activated carbon (AC) was fabricated. The electrochemical performance of the capacitor was studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge in different aqueous electrolytes such as 1 M LiNO 3, Li 2SO 4, NaNO 3 and KNO 3 solution. A maximum specific capacitance of 261 F g −1 was obtained for the Li 2Mn 4O 9 single electrode between 0 and 1.4 V. The Li 2Mn 4O 9/AC hybrid supercapacitor showed a sloping voltage profile from 0 to 1.4 V and delivered an energy density of 53 Wh kg −1 based on the total weight of the active electrode materials. The hybrid capacitor exhibited a desirable profile and maintained over 80% of its initial energy density after 1000 cycles, indicating that Li 2Mn 4O 9 has excellent cycling performance and structural stability in aqueous electrolyte. The hybrid supercapacitor also exhibited an excellent rate capability, even at a power density of 1250 W kg −1, it had a specific energy 29 Wh kg −1 compared with 48 Wh kg −1 at the power density of about 417 W kg −1.

Aqueous Lithium‐ion Battery LiTi2(PO4)3/LiMn2O4 with High Power and Energy Densities as well as Superior Cycling Stability**

AbstractPorous, highly crystalline Nasicon‐type phase LiTi2(PO4)3 has been prepared by a novel poly(vinyl alcohol)‐assisted sol–gel route and coated by a uniform and continuous nanometers‐thick carbon thin film using chemical vapor deposition technology. The as‐prepared LiTi2(PO4)3 exhibits excellent electrochemical performance both in organic and aqueous electrolytes, and especially shows good cycling stability in aqueous electrolytes. An aqueous lithium‐ion battery consisting of a combination of LiMn2O4 cathode, LiTi2(PO4)3 anode, and a 1 M Li2SO4 electrolyte has been constructed. The cell delivers a capacity of 40 mA h g–1 and a specific energy of 60 W h kg–1 with an output voltage of 1.5 V based on the total weight of the active electrode materials. It also exhibits an excellent cycling stability with a capacity retention of 82 % over 200 charge/discharge cycles, which is much better than any aqueous lithium‐ion battery reported.

Electrochemical stabilization of crystalline silicon with aromatic self‐assembled monolayers in aqueous electrolytes

AbstractWe report stable chemical engineering of hydrogen‐terminated Si[111] surfaces in aqueous electrolytes by electrochemical grafting of aromatic monolayers. The topography and free energy of the engineered surface obtained from AFM and contact angle measurements confirmed homogeneous coating of the surface with a monolayer. Grafting of monolayers actually resulted in a clear suppression of the surface defect densities, demonstrated by photoluminescence lifetime. Changes in the surface chemical identities after grafting and post‐treatments were followed by X‐ray photoelectron spectroscopy (XPS). The electrochemical stability in aqueous electrolytes was assessed by impedance spectroscopy, revealing an improved stabilization of the Si/electrolyte interface by the grafted monomolecular film. This protocol was further applied for another aromatic compound, where the impact of 4‐substituent functions could clearly be detected by photovoltage measurements. The chemical and electrochemical stability achieved here is promising for the successive deposition of biocompatible polymer films and lipid membranes. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The Role of Adsorbed Water Molecules in Bulk Restructuring of Amorphous Tungsten Trioxide Films

AbstractThe influence of water molecules adsorbed at the surface of electrochromic WO3 films on film stability in aqueous electrolyte has been studied by scanning electron microscopy and high energy electron diffraction.