Understanding RuO2·xH2O/carbon nanofibre composites as supercapacitor electrodes

Composites made from RuO₂·xH₂O particles supported on carbon nanofibres (CNF) have been prepared for supercapacitor electrodes. CNF, produced by Grupo Antolin Ing. SA. using a floating catalyst procedure was treated either in HCl or in HNO₃. Then the composites were obtained by impregnation of CNF with an aqueous RuCl₃·0.5H₂O solution followed by filtering and alkali solution treatment. Heat treatment at 150 °C for 2 h was done. Specific capacitance of the composites has been measured and discussed on the basis of their RuO₂·xH₂O content and RuO₂·xH₂O particle size. The composites having RuO₂·xH₂O contents below 11 wt% show RuO₂·xH₂O particles, which grow from 2 to 4 nm as the RuO₂·xH₂O content increases. The specific capacitance of supported RuO₂·xH₂O, which can be very high (up to 840 F g⁻¹), decreases as the RuO₂·xH₂O content increases and RuO₂·xH₂O particles grow. The composites having RuO₂·xH₂O contents above 11 wt% show RuO₂·xH₂O particles of nearly constant size (4 nm); the effect of increasing the RuO₂·xH₂O content is to increase the amount of particles but not the size of the particles. In these composites the specific capacitance of supported RuO₂·xH₂O is nearly constant (440 F g⁻¹) and close to bare RuO₂·xH₂O (460 F g⁻¹).     

Carbon nanofibre/hydrous RuO2 nanocomposite electrodes for supercapacitors

Amorphous RuO₂·xH₂O and a VGCF/RuO₂·xH₂O nanocomposite (VGCF = vapour-grown carbon fibre) are prepared by thermal decomposition. The morphology of the materials is investigated by means of scanning electron microscopy. The electrochemical characteristics of the materials, such as specific capacitance and rate capability, are investigated by cyclic voltammetry over a voltage range of 0–1.0 V at various scan rates and with an electrolyte solution of 1.0 M H₂SO₄. The specific capacitance of RuO₂·xH₂O and VGCF/RuO₂·xH₂O nanocomposite electrodes at a scan rate of 10 mV s⁻¹ is 410 and 1017 F g⁻¹, respectively, and at 1000 mV s⁻¹ are 258 and 824 F g⁻¹, respectively. Measurements of ac impedance spectra are made on both the electrodes at various bias potentials to obtain a more detailed understanding of their electrochemical behaviour. Long-term cycle-life tests for 10⁴ cycles shows that the RuO₂·xH₂O and VGCF/RuO₂·xH₂O electrodes retain 90 and 97% capacity, respectively. These encouraging results warrant further development of these electrode materials towards practical application.     

Solid state synthesis of hydrous ruthenium oxide for supercapacitors

A novel solid state route has been successfully developed for the synthesis of nano-scale hydrous ruthenium oxide (denoted as RuO₂·xH₂O). The procedure involves directly mixing RuCl₂·xH₂O with alkali to form RuO₂·xH₂O in a mortar at room temperature. Transmission electron microscopy (TEM) and N₂ adsorption–desorption measurement indicate that the RuO₂·xH₂O particle is approximately 30–40 nm with mesoporous structure. The crystalline structure and the electrochemical properties of RuO₂·xH₂O have been systematically explored as a function of annealing temperature. At lower temperatures, the RuO₂·xH₂O powder was found in an amorphous phase and the maximum capacitance of 655 F g⁻¹ was obtained by annealing at 150 °C. Higher temperatures (exceeding 175 °C) presumably converted amorphous phase into crystalline one and the corresponding specific capacitance dropped rapidly from 547 F g⁻¹ at 175 °C to 87 F g⁻¹ at 400 °C. Also, the dependence of electrochemical performance on annealing conditions of RuO₂·xH₂O was investigated by electrical impedance spectroscopy (EIS) study.     

New RuO2 and carbon–RuO2 composite diffusion layer for use in direct methanol fuel cells

A RuO₂ diffusion layer is examined for use in direct methanol fuel cells (DMFC) by comparison with acetylene black and Vulcan XC-72R. In the test with a DMFC unit cell, the RuO₂ diffusion layer is superior to the other two materials. The difference in performance is interpreted in terms of structural and electrical properties which are evaluated by porosity, scanning electron microscopy and resistance measurements. The RuO₂ diffusion layer displays different behaviors at the anode and cathode sides. These characteristics can be attributed to a reduced loss of catalyst in the active catalyst layer, which leads to increased methanol diffusion at the anode and prevention of water flooding in the cathode. The effect of the RuO₂ diffusion layer on cell performance becomes more pronounced at lower temperatures and during operation in the presence of air. Finally, a carbon–RuO₂ composite is evaluated as a diffusion layer material for a DMFC.     

Controllable proton and CO2 photoreduction over Cu2O with various morphologies

Simultaneous photocatalytic reduction of water to H₂ and CO₂ to CO was observed over Cu₂O photocatalyst under both full arc and visible light irradiation (>420 nm). It was found that the photocatalytic reduction preference shifts from H₂ (water splitting) to CO (CO₂ reduction) by controlling the exposed facets of Cu₂O. More interestingly, the low index facets of Cu₂O exhibit higher activity for CO₂ photoreduction than high index facets, which is different from the widely-reported in which the facets with high Miller indices would show higher photoactivity. Improved CO conversion yield could be further achieved by coupling the Cu₂O with RuO_x to form a heterojunction which slows down fast charge recombination and relatively stabilises the Cu₂O photocatalyst. The RuO_x amount was also optimised to maximise the junction's photoactivity.     

Sulfonated polyether ether ketone and hydrated tin oxide proton conducting composites for direct methanol fuel cell applications

Composite membranes based on sulfonated polyether ether ketone (SPEEK) and hydrated tin oxide (SnO₂·nH₂O) were prepared and characterized. The formation of the composite substantially modified the properties of SPEEK in terms of durability and electrochemical performance. The structural and electrochemical performance of the samples were investigated using scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, electrochemical impedance spectroscopy (EIS), water and methanol uptake (WU, MU), and direct methanol fuel cell (DMFC) tests. The polymer electrolyte membrane doped with 50 wt% SnO₂·n(H₂O) possess good proton transport characteristics, reduced methanol uptake and improved stability with respect to a reference unfilled membrane and it is then suitable for application as electrolyte in DMFCs.     

LiFePO4 with enhanced performance synthesized by a novel synthetic route

Pure LiFePO₄ was synthesized by heating an amorphous LiFePO₄. The amorphous LiFePO₄ obtained through lithiation of FePO₄·xH₂O by using oxalic acid as a novel reducing agent at room temperature. FePO₄·xH₂O was prepared through co-precipitation by employing FeSO₄·7H₂O and H₃PO₄ as raw materials. X-ray diffraction (XRD), scanning electron microscopy (SEM) observations showed that LiFePO₄ composites with fine particle sizes between 100 nm and 200 nm, and with homogenous sizes distribution. The electrochemical performance of LiFePO₄ powder synthesized at 500 °C were evaluated using coin cells by galvanostatic charge/discharge. The synthesized LiFePO₄ composites showed a high electrochemical capacity of 166 mAh g⁻¹ at the 0.1C rate, and possessed a favorable capacity cycling maintenance at the 0.1C, 0.2C, 0.5C and 1C rate.     

Electrochemical property of NH4V3O8·0.2H2O flakes prepared by surfactant assisted hydrothermal method

NH₄V₃O₈·0.2H₂O is synthesized by sodium dodecyl sulfonate (SDS) assisted hydrothermal method and its electrochemical performance is investigated. The as-prepared material is characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), infrared (IR) spectrum, differential scanning calorimetry and thermal gravimetry (DSC/TG), cyclic voltammetry (CV), and charge-discharge cycling test. The results show a pure NH₄V₃O₈·0.2H₂O phase with flake-like morphology is obtained and the average flake thickness is about 150 nm. The NH₄V₃O₈·0.2H₂O electrode has a good lithium ion insertion/extraction ability with the highest discharge capacity of 225.9 mAh g⁻¹ during 1.8-4.0 V versus Li at the constant current density of 15 mA g⁻¹. After 30 cycles, it still maintains a high discharge capacity of 209.4 mAh g⁻¹, demonstrating good cyclic stability. Interestingly, at the discharge process a new (NH₄)Li_xV₃O₈·0.2H₂O compound is formed due to the new lithium ion from lithium metal anode.     

Effect of conditioning method on direct methanol fuel cell performance

We investigated the effect of the conditioning methods on improving the direct methanol fuel cell (DMFC) performance. The DMFC performance after the conditioning was measured using a newly developed single cell having an Ag/Ag₂SO₄ reference electrode, which is not influenced by methanol. As a result, we succeeded in developing an original two-step conditioning method in which the conditioning by fueling H₂ gas is conducted prior to a conventional DMFC conditioning. The anode and cathode characteristics after the two-step conditioning were measured with respect to a reference electrode. Based on the obtained i-E curves, the two-step conditioning is found to improve the methanol oxidation performance at the anode and also suppress the decline of the O₂ reduction performance at the cathode. The high DMFC performance based on the two-step conditioning is well explained by the anode and cathode characteristics.     

Design of a MEA with multi-layer electrodes for high concentration methanol DMFCs

According to the conventional MEA test, methanol and water crossover are the main factors to determine performance of a passive DMFC. Thus, to ensure the high cell performance of a passive DMFC using high concentration methanol of 50–95 vol%, the MEA in this study introduces the barrier layer to limit the crossover of high concentration methanol, a hydrophobic layer to reduce water crossover, and a hydrophilic layer to enhance the water recovery from the cathode to the anode. The functional layers of the MEA have the effect of improving the performance of the passive DMFC by decreasing the methanol and water crossover. In spite of the operation with 95 vol% methanol, the MEA with multi-layer electrodes for high concentration methanol DMFCs shows a maximum power density of 35.1 mW cm⁻² and maintains a high power density of 30 mW cm⁻² (0.405 V) under constant current operation.     

The study on dynamic response performance of PEMFC with RuO2•xH2O/CNTs and Pt/C composite electrode

Dynamic response performance of Proton Exchange Membrane (PEM) fuel cells affects its durability and reliability significantly. In this study, in order to promote the PEM fuel cell dynamic response performance, RuO₂•xH₂O/CNTs was prepared by sol–gel method and then sprayed onto the Pt/C electrode surface to form the cathode. The RuO₂•xH₂O/CNTs prepared was characterized by Transmission Electron Microscopy (TEM), which shows the average particle size of RuO₂•xH₂O is 3 nm and particulates are well distributed. Performance of single cells with and without RuO₂•xH₂O/CNTs at the cathode under various operating conditions such as different gases pressure, air stoichiometry and relative humidity was studied using cyclic voltammetry, electrochemical impedance spectra (EIS) and polarization curve techniques. When the fuel cell modified with RuO₂•xH₂O/CNTs was operated at lower pressure and higher air stoichiomotery, a faster and more stable dynamic response could be found. It was also found that the humidity of cathode inlet gas had a significant effect on fuel cell performance. Adding 0.1 mg/cm² RuO₂•xH₂O/CNTs (RuO₂•xH₂O 11%) composite material not only slightly increases the single cell performance but also dramatically improves the dynamic response performance, revealing that RuO₂•xH₂O/CNTs can buffer the voltage undershoot whenever the current increases instantly.     

A synthesis of CO-tolerant Nb2O5-promoted Pt/C catalyst for direct methanol fuel cell; its physical and electrochemical characterization

A Pt-Nb₂O₅/C electrocatalyst was synthesized by a two-step process as an anode material in direct methanol fuel cell (DMFC). The Pt-Nb₂O₅/C catalysts heat-treated at different temperatures (400 and 500 °C) in flowing N₂ were characterized by various methods such as inductively coupled plasma-atomic emission spectroscopy, X-ray diffraction, transmission electron microscopy, and X-ray photoemission spectroscopy (XPS). The heat-treated Pt-Nb₂O₅/C catalyst at 400 °C showed the best electrochemical activity for CO and methanol oxidations among the prepared catalysts. The XPS results showed the electronic structure change of Pt, indicating a formation of interaction between Pt and Nb₂O₅. It is suggested that a synergistic effect between Pt and Nb₂O₅ enhances the electrocatalytic activity for CO and methanol oxidations. We believe that Nb₂O₅-promoted Pt/C catalyst may be regarded as one of the attractive candidates as an anode material in DMFC.     

Tailoring of RuO2 nanoparticles by microwave assisted “Instant method” for energy storage applications

RuO₂ nanoparticles are synthesized by Instant method using Li₂CO₃ as stabilizing agent, under microwave irradiation at 60 °C and investigated for the anodic oxygen evolution reaction (OER) and for their supercapacitance properties in 0.5 M H₂SO₄ medium. Structural and morphological characterizations of RuO₂ are investigated by in situ X-ray diffraction (XRD), thermogravimetric analysis (TG-DTA), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDS) and Raman spectroscopy. The TEM images of as prepared material show the uniform distribution of RuO₂ nanoparticles with mean diameter of ca. 1.5 nm. Analysis on as prepared material indicates the structural formula as [RuO₂·2.6H₂O] 0.7H₂O with low crystallinity. The influence of annealing temperature on RuO₂ is studied in light of electrocatalytic activity for oxygen evolution reaction (OER) and capacitance. Electrochemical performances of RuO₂ electrodes are followed by current-potential curves, galvanostatic charge-discharge cycles and evolved oxygen measurements. The amount of oxygen gas evolved during the OER by the crystalline RuO₂ is found to be consistent with the electrical energy supplied to the catalyst. The cyclic voltammogram of RuO₂ exhibits the typical capacitance behavior with highly reversible nature. The specific capacitance of hydrous RuO₂ is found to be 737 F g⁻¹ at the scan rate of 2 mV s⁻¹, by the balanced transport of proton through the structural water and electron transport along dioxo bridges, which makes a suitable material for energy storage. The specific capacitance decreases with increase in the crystallinity of RuO₂. The present study shows the potential method to synthesize rapid and uniform nano particles of RuO₂ for water electrolysis and supercapacitors.     

Microwave-assisted hydrothermal synthesis of crystalline WO3-WO3·0.5H2O mixtures for pseudocapacitors of the asymmetric type

Crystalline tungsten oxide mixtures, WO₃-WO₃·0.5H₂O, prepared by microwave-assisted hydrothermal (MAH) synthesis at 180 °C for various periods, show capacitive-like behavior at 200 mV s⁻¹ and C_S ≈ 290 F g⁻¹ at 25 mV s⁻¹ in 0.5 M H₂SO₄ between −0.6 and 0.2 V. Oxide rods can be obtained via the MAH process even when the synthesis time is only 0.75 h while WO₃·0.5H₂O sheets with poor capacitive performances are obtained by a normal hydrothermal synthesis process at the same temperature for 24 h. The aspect ratio of tungsten oxide rods is found to increase with prolonging the MAH time while all oxides consist of WO₃ and WO₃·0.5H₂O. The oxide mixtures prepared by the MAH method with annealing in air at temperatures ≤400 °C show promising performances for electrochemical capacitors (ECs). Due to the narrow working potential window of the oxide mixtures, an aqueous EC of the asymmetric type, consisting of a WO₃-WO₃·0.5H₂O anode and a RuO₂·xH₂O cathode, with a potential window of 1.6 V is demonstrated in this work, which shows the device energy and power densities of 23.4 W kg⁻¹ and 5.2 kW kg⁻¹, respectively.     

Flexible graphite-based integrated anode plate for direct methanol fuel cells at high methanol feed concentration

An integrated anode plate suitable for operating direct methanol fuel cells (DMFCs) at a high methanol feed concentration is reported. This anode structure which was made of flexible graphite materials not only provides dual role of liquid diffusion layer and flow field plate, but also serves as a methanol blocker by decreasing methanol flux to the interface of catalyst and membrane electrolyte. DMFCs incorporating this new anode structure exhibited a much higher open circuit voltage (OCV) (∼0.51 V) than that (∼0.42 V) of a conventional DMFC at a 10 M methanol feed. Cell polarization data show that this new anode structure significantly improves the cell performance at high methanol concentration scenarios (e.g. 12 M or above). Moreover, this new design greatly simplifies the anode structure and offers a promising approach in running passive-mode DMFC at high methanol feed concentrations.     

Photocatalytic hydrogen production over CuGa2−xFexO4 spinel

Recovery of hydrogen from industrial H₂S waste using spinel photocatalyst was studied. Spinel metal oxide photocatalysts (CuGa_2−xFe_xO₄ for x = 0.8, 0.6 and 0.4) were synthesized by ceramic route. They were loaded with 0.5 and 1 wt% noble metal oxide, RuO_2. Their XRD pattern revealed a single phase cubic spinel crystalline structure for all the catalysts. SEM displayed small size cubic particles with the particle size decreasing with the decrease in iron content. 1 wt% RuO₂ loaded CuGa_1.6Fe_0.4O₄ decomposed H₂S in aqueous 0.5 M KOH solution under visible light (λ ≥ 420 nm) irradiation and generated H₂ to the tune of 10,045 μmol/h, giving rise to a high quantum efficiency of 21% at 510 nm.     

Water management in a passive direct methanol fuel cell

Passive direct methanol fuel cells (DMFCs) are under development for use in portable applications because of their enhanced energy density in comparison with other fuel cell types. The most significant obstacles for DMFC development are methanol and water crossover because methanol diffuses through the membrane generating heat but no power. The presence of a large amount of water floods the cathode and reduces cell performance. The present study was carried out to understand the performance of passive DMFCs, focused on the water crossover through the membrane from the anode to the cathode side. The water crossover behaviour in passive DMFCs was studied analytically with the results of a developed model for passive DMFCs. The model was validated with an in‐house designed passive DMFC. The effect of methanol concentration, membrane thickness, gas diffusion layer material and thickness and catalyst loading on fuel cell performance and water crossover is presented. Water crossover was lowered with reduction on methanol concentration, reduction of membrane thickness and increase on anode diffusion layer thickness and anode and cathode catalyst layer thickness. It was found that these conditions also reduced methanol crossover rate. A membrane electrode assembly was proposed to achieve low methanol and water crossover and high power density, operating at high methanol concentrations. The results presented provide very useful and actual information for future passive DMFC systems using high concentration or pure methanol. Copyright © 2012 John Wiley & Sons, Ltd.     

An effective layout of polyaniline nanofibers incorporated in membrane-electrode assembly as methanol transport regulator for direct methanol fuel cells

This study reports a novel strategy by using polyaniline nanofibers (PANFs) to modify membrane-electrode assembly (MEA) for improving direct methanol fuel cell (DMFC) performance. First of all, a series of PANFs emeraldine salt was synthesized and characterized. Then, we investigated the effect of PANFs layout in MEA on DMFC performance. Three different placements to incorporate the as-synthesized PANFs in anodes include (1) placing a layer of PANFs between catalyst layer (CL) and proton exchange membrane (PEM), (2) mixing with catalyst slurry and coating onto gas diffusion layer (GDL), and (3) placing a layer of PANFs between CL and GDL. Polarization curves indicate that the third method is superior to the others and is adopted as the incorporation layout thereafter. Both methanol transport resistance and methanol crossover of the PANFs-modified MEA are studied further. The DMFC incorporated with H₂SO₄-doped PANFs obtained after the re-doping process with 2 mol L⁻¹ H₂SO₄ performs a power density as high as 53 mW cm⁻², about 20% higher than that of the pristine one without PANFs incorporation. However, an excessive doping level may result in a higher methanol transport resistance due to PANFs aggregation and thus deteriorate DMFC performance. This study provides a simple and effective way by placing a layer of PANFs between CL and GDL in anode to act as methanol transport regulator and improve DMFC performance consequently.     

Stabilized zirconia-based sensor utilizing SnO2-based sensing electrode with an integrated Cr2O3 catalyst layer for sensitive and selective detection of hydrogen

A highly selective hydrogen (H₂) sensor has been successfully developed by using an yttria-stabilized zirconia (YSZ)-based mixed-potential-type sensor utilizing SnO₂ (+30 wt.% YSZ) sensing electrode (SE) with an intermediate Al₂O₃ barrier layer which was coated with a catalyst layer of Cr₂O₃. The sensor utilizing SnO₂ (+30 wt.% YSZ)-SE was found to be capable of detecting H₂ and propene (C₃H₆) sensitively at 550 °C. In order to enhance the selectivity towards H₂, a selective C₃H₆ oxidation catalyst was employed to minimize unwanted responses caused by interfering gases. Among the examined metal oxides, Cr₂O₃ facilitated the selective oxidation of C₃H₆. However, the addition or lamination of Cr₂O₃ to SnO₂ (+30 wt.% YSZ)-SE was found to diminish the sensing responses to all examined gases. Therefore, an intermediate layer of Al₂O₃ was sandwiched between the SE layer and the catalyst layer to prevent the penetration of Cr₂O₃ particles into the SE layer. The sensor using SnO₂ (+30 wt.% YSZ)-SE coated with a catalyst layer of Cr₂O₃ as well as an intermediate layer of Al₂O₃ exhibited a sensitive response toward H₂, with only minor responses toward other examined gases at 550 °C under humid conditions (21 vol.% O₂ and 1.35 vol.% H₂O in N₂ balance). A linear relationship was observed between sensitivity and H₂ concentration in the range of 20–800 ppm on a logarithmic scale. The results of sensing performance evaluation and polarization curve measurements indicate that the sensing mechanism is based on the mixed-potential model.     

One-compartment electrochemical H2 generator from borohydride

A one-compartment membrane-less electrochemical H₂ generator from borohydride was realized using a Rh porphyrin and RuO₂ as the anode and cathode, respectively. H₂ generation from this cell was successfully controlled electrochemically by varying the potential applied. The regulation of H₂ generation was based on the selectivity of the anode and cathode. We found that RuO₂ exhibits H₂O electro-reduction activity without electro-oxidation or chemical decomposition of borohydride, and used the catalyst as a selective cathode in the electrochemical H₂ generator. Anode and cathode potentials of the electrochemical H₂ generator were measured separately. The both potentials were discussed in terms of the catalytic activities of a Rh porphyrin and RuO₂.