Graphite–graphite oxide composite electrode for vanadium redox flow battery

A graphite/graphite oxide (GO) composite electrode for vanadium redox battery (VRB) was prepared successfully in this paper. The materials were characterized with X-ray diffraction, X-ray photoelectron spectroscopy and scanning electron microscopy. The specific surface area was measured by the Brunauer–Emmett–Teller method. The redox reactions of [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ were studied with cyclic voltammetry and electrochemical impedance spectroscopy. The results indicated that the electrochemical performances of the electrode were improved greatly when 3 wt% GO was added into graphite electrode. The redox peak currents of [VO₂]⁺/[VO]²⁺ and V³⁺/V²⁺ couples on the composite electrode were increased nearly twice as large as that on the graphite electrode, and the charge transfer resistances of the redox pairs on the composite electrode are also reduced. The enhanced electrochemical activity could be ascribed to the presence of plentiful oxygen functional groups on the basal planes and sheet edges of the GO and large specific surface areas introduced by the GO.     

Synthesis and electrocatalytic oxygen reduction activity of graphene-supported Pt3Co and Pt3Cr alloy nanoparticles

Graphene-supported Pt and Pt₃M (M = Co and Cr) alloy nanoparticles are prepared by ethylene glycol reduction method and characterized with X-ray diffraction and transmission electron microscopy. X-ray diffraction depicted the face-centered cubic structure of Pt in the prepared materials. Electron microscopic images show the high dispersion of metallic nanoparticles on graphene sheets. Electrocatalytic activity and stability of the materials is investigated by rotating-disk electrode voltammetry. Oxygen reduction activity of the Pt₃M/graphene is found to be 3–4 times higher than that of Pt/graphene. In addition, Pt₃M/graphene electrodes exhibited overpotential 45–70 mV lower than that of Pt/graphene. The high catalytic performance of Pt₃M alloys is ascribed to the inhibition of formation of (hydr) oxy species on Pt surface by the alloying elements. The fuel cell performance of the catalysts is tested at 353 K and 1 atm. Maximum power densities of 790, 875, and 985 mW/cm² are observed with graphene-supported Pt, Pt₃Co, and Pt₃Cr cathodes, respectively. The enhanced electrocatalytic performance of the Pt₃M/graphene (M = Co and Cr) compared to that of Pt/graphene makes them a viable alternative to the extant cathodes for energy conversion device applications.     

One-pot electrochemical synthesis of graphene by the exfoliation of graphite powder in sodium dodecyl sulfate and its decoration with platinum nanoparticles for methanol oxidation

We demonstrate a method which directly grows large areas of graphene on carbon paper and glassy carbon (GC) substrates from graphite powder and anionic surfactant, sodium dodecyl sulfate, assisted electrochemical exfoliation. The electrochemically reduced graphene has been carefully characterized by scanning electron microscopy (SEM) and electrochemical techniques. Particularly, SEM images show enhanced growth of graphene structures formed of ‘urchin’ objects. The CV spectra illustrate that a variety of the oxygen-containing functional groups has been thoroughly removed from the graphite plane via electrochemical reduction. Potential peak (Ep) of graphene electrode in [Fe(CN)₆]^3−/4− solution is as small as 212 mV which is 168 mV smaller than that of graphite electrode. This could be attributed to the high quality graphene accelerating the electron transfer rate in [Fe(CN)₆]^3−/4− electrochemistry. Finally, platinum was electro reduced onto the GC and graphene modified GC based electrodes for use in methanol oxidation. The catalytic activities of graphene-supported Pt nanoparticles and Pt-GC electrocatalysts for methanol oxidation were 1900 and 915.5 A g⁻¹ Pt, which can reveal the particular properties of the exfoliated graphene supports.     

rGO Sheets/ZnFe2O4 Nanocomposities as an Efficient Electro Catalyst Material for I3−/I− Reaction for High Performance DSSCs

In this paper, Reduced Graphene Oxide (rGO)/ZnFe₂O₄ (rZnF) nanocomposite is synthesized by a simple hydrothermal method and employed as a counter electrode (CE) material for tri-iodide redox reactions in Dye sensitized solar cells (DSSC) to replace the traditional high cost platinum (Pt) CE. X-ray diffraction analysis and High resolution Transmission electron microscopy, clearly indicated the formation of rZnF nanocomposite and also amorphous rGO sheets were smoothly distributed on the surface of ZnFe₂O₄ (ZnF) nanostructure. The rZnF-50 CE shows excellent electro catalytic activity toward I₃^? reduction, which has simultaneously been confirmed by cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization measurements. A DSSC developed by rZnF-50 CE (η?=?8.71%) obtained quite higher than the Pt (η?=?8.53%) based CE under the same condition. The superior performances of rZnF-50 CE due to addition of graphene in to Spinel (ZnF) nanostructure results in creation of highly active electrochemical sites, fast electron transport linkage between CE and electrolyte. Thus it’s a promising low cost CE material for DSSCs.

     

Acceleration of the redox kinetics of VO2+/VO2 + and V3+/V2+ couples on carbon paper

The redox kinetics of VO²⁺/VO₂ ⁺ and V³⁺/V²⁺ couples on a carbon paper (CP, HCP030 N, Shanghai Hesen, Ltd., China) electrode were investigated in terms of their standard rate constant (k ₀) and reaction mechanism. The values determined for k ₀ for VO²⁺ ?? VO₂ ⁺ and V³⁺ ?? V²⁺ using the CP electrode are 1.0 × 10^?3 and 1.1 × 10^?3 cm s^?1, respectively. The value of k ₀ increases by one or two order(s) of magnitude compared with values obtained using electrodes composed of pyrolytic graphite and glassy carbon. The acceleration of the redox kinetics of vanadium ions is a result of the large surface area of the CP electrode. An inner-sphere mechanism for the reaction on the surface of the electrode is proposed. The kinetic features of vanadium redox reactions on the CP electrode reveal that CP is suitable for use as the electrodes in vanadium redox-flow batteries.     

Low temperature preparation of a high performance Pt/SWCNT counter electrode for flexible dye-sensitized solar cells

A platinum/single-wall carbon nanotube (Pt/SWCNT) film was sprayed onto a flexible indium-doped tin oxide coated polyethylene naphthalate (ITO/PEN) substrate to form a counter electrode for use in a flexible dye-sensitized solar cell using a vacuum thermal decomposition method at low temperature (120 °C). The obtained Pt/SWCNT electrode showed good chemical stability and light transmittance and had lower charge transfer resistance and higher electrocatalytic activity for the I₃⁻/I⁻ redox reaction compared to the flexible Pt electrode or a commercial Pt/Ti electrode. The light-to-electric energy conversion efficiency of the flexible DSSC based on the Pt/SWCNT/ITO/PEN counter electrode and the TiO₂/Ti photoanode reached 5.96% under irradiation with a simulated solar light intensity of 100 mW cm⁻². The efficiency was increased by 25.74% compared to the flexible DSSC with an unmodified Pt counter electrode.     

Direct electrochemistry of horseradish peroxidase on graphene-modified electrode for electrocatalytic reduction towards H2O2

Graphene was synthesized by a chemical method to reduce graphite oxide and well characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), powder X-ray diffraction (PXRD) and Fourier transform infrared (FTIR) spectra. Horseradish peroxidase (HRP) immobilized on a graphene film glassy carbon electrode was found to undergo direct electron transfer and exhibited a fast electron transfer rate constant of 4.63 s⁻¹. The HRP-immobilized electrode was investigated by electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV). The CV results showed that the modified electrode gave rise to well-defined peaks in phosphate buffer, corresponding to the electrochemical redox reaction between HRP–Fe(III) and HRP–Fe(II). The obtained electrode also displayed an electrocatalytic reduction behavior towards H₂O₂. The new H₂O₂ sensor shows a linear range of 0.33–14.0 μM (R² = 0.9987) with a calculated detection limit of 0.11 μM (S/N = 3). Furthermore, the biosensor exhibits both good operational storage and storage stability.     

Reduced graphene oxide with tunable C/O ratio and its activity towards vanadium redox pairs for an all vanadium redox flow battery

Electrochemically reduced graphene oxides (ERGO) are obtained under various reducing potentials in the phosphate buffer solution (PBS). Different characterization methods are used to analyse the changes of structure and surface chemical condition for graphene oxide (GO). The results show that GO could be reduced controllably to certain degree and its electrochemical activity towards VO₂⁺/VO²⁺ and V³⁺/V²⁺ redox couples is also tunable using this environmentally friendly method. The catalytic mechanism of the ERGO is discussed in detail, the CO functional groups other than the C–O functional groups on the surface of ERGO more likely provide reactive sites for those redox couples, leading to a more comprehensive understanding about the catalytic process than previous relevant researches. This controllable modification method and the ERGO as electrode reaction catalyst with enhanced battery performance are supposed to have promising applications in the all vanadium redox flow battery.     

Detection of oxygen ion drift in Pt/Al2O3/TiO2/Pt RRAM using interface-free single-layer graphene electrodes

Resistive switching random access memory (RRAM) with oxygen ion drift under electric (E)-field has been intensively studied. However, the findings are insufficient because redox reaction by oxygen ion drift occurs beneath the top electrode, and it is difficult to analyze with a nondestructive method. Therefore, an effective method to circumvent this difficulty is suggested in this study with a Pt/Al₂O₃/TiO₂/Pt device using a single layer graphene (SLG) top electrode. Based on results from spectroscopic analyses, the SLG serves as not only an interface free electrode, but also as a highly effective indicator for proving O⁻ ion drift motion in response to the E-field in RRAM. The origin of asymmetric resistive switching is due to a redox reaction at the interface by oxygen ion drift. The endurance and operation-current distribution are significantly improved with increased thickness of the Al₂O₃ insertion layer, which provides carrier tunneling barrier height. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) is greater than one order of magnitude in a log scale within 1800 cycles. This result demonstrates that control of a localized charge tunneling barrier is a key factor for reliable resistive switching of the scaled-down RRAM.     

Excellent electrocatalytic performance of Pt nanoparticles on reduced graphene oxide nanosheets prepared by a direct redox reaction between Na2PtCl4 and graphene oxide

A simple and environment-friendly method was used to prepare Pt/reduced graphene oxide (Pt/RGO) hybrids. This approach used a redox reaction between Na₂PtCl₄ and graphene oxide (GO) nanosheets and a subsequent thermal reduction of the material at 200 °C for 24 h in a vacuum oven. In contrast to other methods that use an additional reductant to prepare Pt nanoparticles, the Pt²⁺ was directly reduced to Pt⁰ in the GO solution. GO was used as the reducing agent, the stabilizing agent and the carrier. The resulting Pt/RGO hybrid was characterized by X-ray diffraction, thermo-gravimetric analysis, X-ray photoelectron spectroscopy, transmission electron microscopy and energy-dispersive X-ray spectroscopy. Electrochemical measurements showed that the Pt/RGO hybrids exhibit good activity as catalysts for the electro-oxidation of methanol and ethanol in acid media. Interestingly, the Pt/RGO hybrids showed better electrocatalytic activity and stability for the oxidation of methanol than Pt/C and Pt/RGO hybrids made from other Pt precursors. This indicates that the Pt/RGO hybrids should have great potential applications in direct methanol and ethanol fuel cells.     

Integrated urea sensor module based on poly(3-methylthiophene)-modified p-type porous silicon substrate

Integrated three-electrode system module composed of a porous silicon (PS)-based sensing electrode, an Ag/AgCl thin film reference electrode (RE_tf), and a Pt thin film counter electrode (CE) was fabricated for monitoring urea level of artificially-prepared body fluid. After thermal evaporation of the 200 nm thick Ag on the Ti-underlayered planar p-type silicon (p-Si) substrate, the Ag Film was oxidized in a FeCl₃ solution to obtain the Ag/AgCl RE_tf. Multi-layered RE_tf was clearly shown from results of the auger electron spectroscopy (AES) depth profile. The nernstian slope of the RE_tf also showed good reproducibility. The PS layer was formed by electrochemical anodization with applying constant current to the p-Si substrate in an ethanolic HF solution and the macro PS (2 μm diameter and 10 μm depth) was obtained. The electrochemical active area (A_ea) of the PS-based Pt thin film electrode was determined from the cyclovoltammetric result of redox reactions of K₃Fe(CN)₆ on the electrode surface and compared with the A_ea of the planar silicon (PLS)-based Pt film substrate. After electropolymerization of the conductive poly(3-methylthiophene) (P3MT) on the PS-based Pt thin film, urease (isoelectric point ≈ 4.1) molecules were electrostatically doped into the P3MT film by applying positive bias. Amperometric calibration curves for both PS- and PLS-based sensing electrodes were compared in the range of 0.1–125 mM urea concentrations and the cross-sectional scanning electron microscopy (SEM) image of the PS-based sensing electrode was also shown.     

Mn3O4 nanoparticles embedded into graphene nanosheets: Preparation, characterization, and electrochemical properties for supercapacitors

Mn₃O₄/graphene nanocomposites were synthesized by mixing graphene suspension in ethylene glycol with MnO₂ organosol, followed by subsequent ultrasonication processing and heat treatment. The as-prepared product consists of nanosized Mn₃O₄ particles homogeneously distributed on graphene nanosheets, which has been confirmed by field emission scanning electron microscopy and transmission electron microscopy analysis. Atomic force microscope analysis further identified the distribution of dense Mn₃O₄ nanoparticles on graphene nanosheets. When used as electrode materials in supercapacitors, Mn₃O₄/graphene nanocomposites exhibited a high specific capacitance of 175 F g⁻¹ in 1 M Na₂SO₄ electrolyte and 256 F g⁻¹ in 6 M KOH electrolyte, respectively. The enhanced supercapacitance of Mn₃O₄/graphene nanocomposites could be ascribed to both electrochemical contributions of Mn₃O₄ nanoparticles, functional groups attached to graphene nanosheets, and significantly increased specific surface area.     

Studies on synergetic effect of rGO-NiCo2S4 nanocomposite as an effective counter electrode material for DSSC

Nanocomposites of reduced graphene oxide (rGO) and NiCo₂S₄ with different amount of graphene oxide (GO) are synthesized through a one- step solvothermal method and their catalytic activity towards I^-/I₃^- redox electrolyte for dye-sensitized solar cell (DSSC) application are reported. The growth mechanism of the pristine hierarchical marigold like microspheres of NiCo₂S₄ that formed without rGO and the nanocomposite of rGO-NiCo₂S₄ are also proposed. Electrochemical studies confirmed the synergetic effect of nickel and cobalt ions with the high electrical conductive rGO networks that enhance the electrocatalytic activity of NiCo₂S₄ nanostructures. The synergistic effect between NiCo₂S₄ and rGO may be attributed to the higher conductivity of rGO and the inverse spinel crystal structure of NiCo₂S₄ that have more octahedral catalytic active sites of Co³⁺. The amount of graphene oxide plays the important role of controlling the DSSC performance and the power conversion efficiency. The efficiency achieved for the rGO-NiCo₂S₄ counter electrode (CE) based DSSC is 8.15%, which is remarkably higher than that of pristine NiCo₂S₄ (7.36%), and Pt (7.23%) under the same experimental conditions.     

Sulfonated polyimide/chitosan composite membrane for vanadium redox flow battery: Membrane preparation,characterization, and single cell performance

A novel sulfonated polyimide/chitosan (SPI/CS) composite membrane was prepared from self‐made SPI (50% of sulfonation degree) through an immersion and self‐assembly method, which was successfully applied in vanadium redox flow battery (VRB). The proton conductivity of SPI/CS composite membrane is effectively improved compared to the plain SPI membrane. The VO²⁺ permeability coefficient across SPI/CS composite membrane is 1.12 × 10^?7 cm² min^?1, which is only one tenth of that of Nafion® 117 membrane. Meanwhile, the proton selectivity of SPI/CS composite membrane is about eight times higher than that of Nafion® 117 membrane. In addition, the oxidative stability SPI/CS composite membrane is superior to that of pristine SPI membrane. The VRB single cell using SPI/CS composite membrane showed higher energy efficiency (88.6%) than that using Nafion® 117 membrane, indicating that SPI/CS composite membrane is a promising proton conductive membrane for VRB application. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Influences of graphene oxide support on the electrochemical performances of graphene oxide-MnO2 nanocomposites

MnO₂ supported on graphene oxide (GO) made from different graphite materials has been synthesized and further investigated as electrode materials for supercapacitors. The structure and morphology of MnO₂-GO nanocomposites are characterized by X-ray diffraction, X-ray photoemission spectroscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, and Nitrogen adsorption-desorption. As demonstrated, the GO fabricated from commercial expanded graphite (denoted as GO(1)) possesses more functional groups and larger interplane gap compared to the GO from commercial graphite powder (denoted as GO(2)). The surface area and functionalities of GO have significant effects on the morphology and electrochemical activity of MnO₂, which lead to the fact that the loading amount of MnO₂ on GO(1) is much higher than that on GO(2). Elemental analysis performed via inductively coupled plasma optical emission spectroscopy confirmed higher amounts of MnO₂ loading on GO(1). As the electrode of supercapacitor, MnO₂-GO(1) nanocomposites show larger capacitance (307.7 F g^-1) and better electrochemical activity than MnO₂-GO(2) possibly due to the high loading, good uniformity, and homogeneous distribution of MnO₂ on GO(1) support.     

The effect of Sn content on the electrocatalytic properties of Pt–Sn nanoparticles dispersed on graphene nanosheets for the methanol oxidation reaction

Direct methanol fuel cell (DMFC) electrode catalysts with improved electrochemical properties have been prepared by dispersing platinum–tin (Pt–Sn) nanoparticles onto graphene sheets. During the deposition, a majority of the oxygenated functional groups on the graphene oxide nanosheets were removed, resulting in the formation of graphene. Microstructural characterization shows that metallic Pt, Pt–Sn alloy and tin dioxide (SnO₂) nanoparticles were distributed on the graphene sheets, representing different lattice planes during the synthetic process. In terms of the electrocatalytic properties, graphene-supported Pt–Sn and graphene-supported Pt catalysts exhibited much higher current densities compared with that of commercial carbon black-supported Pt catalysts. Graphene-supported Pt–Sn increased the electrocatalytic activity, which is strongly influenced by the addition of Sn in its alloyed and oxidized forms, boosting the reaction more readily because of the lower oxidation potential.     

A facile one-step synthesis and fabrication of hexagonal palladium-carbon nanocubes (H-Pd/C NCs) and their application as an efficient counter electrode for dye-sensitized solar cell (DSSC)

Hexagonal palladium-carbon nanocubes (H-Pd/C NCs) were prepared using a simple one-step chemical synthesis protocol and subsequently the prepared materials were used as the counter electrode (CE) in dye sensitized solar cell (DSSC) to replace the platinum (Pt) electrode. The H-Pd/C NCs were characterized by a variety of suitable analytical techniques including powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis to evaluate the crystalline, structural, morphological, compositional, chemical state and surface area. The BET nitrogen adsorption /desorption analysis shows that the as-prepared H-Pd/C NCs sample had a large surface area (568.8 m² g⁻¹) with average pore size of ∼3 nm. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses indicate that the H-Pd/C NCs have low charge-transfer resistance on the electrolyte/electrode interface and high electrocatalytic activity for the reduction of triiodide to iodide redox electrolyte and hence it is used as a CE in DSSC. The H-Pd/C NCs showed an overall power conversion efficiency (PCE) of 4.1% which performance is comparable with the conventional Pt CE (4.0%) under the identical condition.     

全钒液流电池石墨毡电极的Ga2O3修饰

为提高全钒液流电池石墨毡电极的电化学性能, 采用热分解法将纳米Ga₂O₃沉积在全钒液流电池石墨毡电极表面。通过循环伏安测试、动电位极化曲线测试、扫描电镜测试(SEM)、X射线光电子能谱分析(XPS)和充放电实验考察了Ga₂O₃对石墨毡电化学性能和表面形貌的影响。研究结果表明:Ga₂O₃对电极反应具有显著的催化作用, 当用Ga₂O₃电极修饰石墨毡时, 电池正负极反应活性较未处理前分别提高13%和18%, 同时也导致全钒液流电池的容量更大, 电流效率和能量效率更高。     

Graphene-supported SnO2 nanoparticles prepared by a solvothermal approach for an enhanced electrochemical performance in lithium-ion batteries

SnO₂ nanoparticles were dispersed on graphene nanosheets through a solvothermal approach using ethylene glycol as the solvent. The uniform distribution of SnO₂ nanoparticles on graphene nanosheets has been confirmed by scanning electron microscopy and transmission electron microscopy. The particle size of SnO₂ was determined to be around 5 nm. The as-synthesized SnO₂/graphene nanocomposite exhibited an enhanced electrochemical performance in lithium-ion batteries, compared with bare graphene nanosheets and bare SnO₂ nanoparticles. The SnO₂/graphene nanocomposite electrode delivered a reversible lithium storage capacity of 830 mAh g⁻¹ and a stable cyclability up to 100 cycles. The excellent electrochemical properties of this graphene-supported nanocomposite could be attributed to the insertion of nanoparticles between graphene nanolayers and the optimized nanoparticles distribution on graphene nanosheets.     

全钒液流电池开路电压模型

建立全钒液流电池六参数开路电压计算模型,揭示开路电压由电池总电势和VO₂⁺/VO²⁺电极电势决定,并与电解液中钒离子透膜扩散行为密切相关。模型计算得到开路电压存在两个电压平台和一个转折点,与实验结果较为一致。利用该模型计算了电池开路时电解液中4种钒离子浓度随时间的变化关系,指出电池电解液不均衡性是由不同价态钒离子在膜相的Donnan平衡和透膜扩散系数不同产生的。该模型可为优化电池操作提供理论指导,并可为电池长期运行时电解液管理提供工程指导。