Anodic deposition of porous RuO2 on stainless steel for supercapacitor studies at high current densities

Ruthenium dioxide is deposited on stainless steel (SS) substrate by galvanostatic oxidation of Ru³⁺. At high current densities employed for this purpose, there is oxidation of water to oxygen, which occurs in parallel with Ru³⁺ oxidation. The oxygen evolution consumes a major portion of the charge. The oxygen evolution generates a high porosity to RuO₂ films, which is evident from scanning electron microscopy studies. RuO₂ is identified by X-ray photoelectron spectroscopy. Cyclic voltammetry and galvanostatic charge–discharge cycling studies indicate that RuO₂/SS electrodes possess good capacitance properties. Specific capacitance of 276 F g⁻¹ is obtained at current densities as high as 20 mA cm⁻² (13.33 A g⁻¹). Porous nature of RuO₂ facilitates passing of high currents during charge–discharge cycling. RuO₂/SS electrodes are thus useful for high power supercapacitor applications.     

Electrochemical supercapacitor behavior of Ni3(Fe(CN)6)2(H2O) nanoparticles

Nanosized Ni₃(Fe(CN)₆)₂(H₂O) was prepared by a simple co-precipitation method. The electrochemical properties of the sample as the electrode material for supercapacitor were studied by cyclic voltammetry (CV), constant charge/discharge tests and electrochemical impedance spectroscopy (EIS). A specific capacitance of 574.7 F g⁻¹ was obtained at the current density of 0.2 A g⁻¹ in the potential range from 0.3 V to 0.6 V in 1 M KNO₃ electrolyte. Approximately 87.46% of specific discharge capacitance was remained at the current density of 1.4 A g⁻¹ after 1000 cycles.     

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).     

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.     

Synthesis of nano-crystalline Sr2MgMoO6−δ anode material by a sol–gel thermolysis method

Nano-crystalline Sr₂MgMoO_6−δ (SMMO) powders were synthesized successfully by a novel sol–gel thermolysis method using a unique combination of polyvinyl alcohol (PVA) and urea. The decomposition behavior of gel precursor was studied by thermogravimetric-differential thermal analysis (TG/DTA) and the results showed that the double-perovskite phase of SMMO began to form at 1000 °C. The microstructure of the samples had been investigated by X-ray diffraction (XRD), transmission electron microscope (TEM), selected area electron diffraction (SAED), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). XRD patterns confirmed that well-crystalline double-perovskite SMMO powders were obtained by calcining at 1450 °C for 12 h. TEM morphological analysis showed that SMMO powders had a mean particle size around 50–100 nm. The SAED pattern and Raman spectroscopy showed that the SMMO powders were nano-polycrystalline well-developed A(B′_0.5B″_0.5)O₃ type perovskite material. The XPS results demonstrated that the Mo ions in SMMO had been reduced after exposure to H₂. The electric property was studied by four-probe method. The results showed that conductivity was 8.64 S cm⁻¹ in 5.0% H₂/Ar at 800 °C and the activation energies at low temperatures (400–640 °C) and high temperatures (640–800 °C) are about 21.43 and 6.59 kJ mol⁻¹, respectively.     

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.     

Low Pt content on the Pd45Pt5Sn50 cathode catalyst for PEM fuel cells

Pd₄₅Pt₅Sn₅₀ electrocatalyst was prepared by a NaBH₄ reduction of PdCl₂, H₂PtCl₆ and SnCl₂ in THF at 0 °C. This catalyst was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS) microanalysis and hydrodynamic electrochemical technique. XRD, SEM and TEM results demonstrate that the borohydrate reduction methodology enable the synthesis of conglomerated particles nanometric in size ranging from 1 to 6 nm. Oxygen reduction reaction (ORR) activity was investigated on carbon dispersed catalyst by rotating disk electrode (RDE) technique in H₂SO₄ 0.5 M. The effect of temperature on the kinetics was analyzing resulting in an apparent activation energy of 42.54 ± 1 kJ mol⁻¹, value which is less than the obtained for the nanostructured bimetallic PdSn electrocatalyst under the same experimental condition. The Pd₄₅Pt₅Sn₅₀ electrocatalyst dispersed on a carbon powder was tested as cathode electrocatalyst in a membrane-electrode assembly (MEA) arriving to a power density of 210 mW cm⁻² at 0.35 V and 80 °C.     

A cheap asymmetric supercapacitor with high energy at high power: Activated carbon//K0.27MnO2·0.6H2O

Studies of the electrochemical behavior of K_0.27MnO₂·0.6H₂O in K₂SO₄ show the reversible intercalation/deintercalation of K⁺-ions in the lattice. An asymmetric supercapacitor activated carbon (AC)/0.5 mol l⁻¹ K₂SO₄/K_0.27MnO₂·0.6H₂O was assembled and tested successfully. It shows an energy density of 25.3 Wh kg⁻¹ at a power density of 140 W kg⁻¹; at the same time it keeps a very good rate behavior with an energy density of 17.6 Wh kg⁻¹ at a power density of 2 kW kg⁻¹ based on the total mass of the active electrode materials, which is higher than that of AC/0.5 mol l⁻¹ Li₂SO₄/LiMn₂O₄. In addition, this asymmetric supercapacitor shows excellent cycling behavior without the need to remove oxygen from the electrolyte solution. This can be ascribed in part to the stability of the lamellar structure of K_0.27MnO₂·0.6H₂O. This asymmetric aqueous capacitor has great promise for practical applications due to high energy density at high power density.     

Enhanced photocatalytic water splitting activity of carbon-modified TiO2 composite materials synthesized by a green synthetic approach

We report a green and facile approach for the preparation of carbon-modified (C-modified) TiO₂ composite materials by hydrothermal synthesis followed by pyrolytic treatment. The resultant materials were characterized by powder X-ray diffraction (XRD), nitrogen physisorption studies, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), diffuse reflectance spectroscopy (DRS), photoluminescence (PL) spectroscopy, and transmission electron microscopy (TEM). The photocatalytic performances of these materials were evaluated by calculating the amount of hydrogen evolved from the decomposition of water under solar simulated irradiation conditions. An improvement was achieved from no H₂ evolution at all with the bare TiO₂, to an evolution of 0.21 mL g⁻¹ h⁻¹ from a composite material modified with an optimum carbon loading of 3.62%. These results suggested that the interaction of carbon with predominantly rutile form of TiO₂ can promote shallow trapping of photogenerated electrons in the oxygen vacancies. This phenomenon consequently enhances the photocatalytic activity by minimizing charge carrier recombination, a characteristic demonstrated by fluorescence quenching of the TiO₂ emission.     

Effect of a reducing agent for silver on the electrochemical activity of an Ag/Ba0.5Sr0.5Co0.8Fe0.2O3−δ electrode prepared by electroless deposition technique

Silver-modified Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (Ag/BSCF) electrodes were prepared using an electroless deposition technique. The morphology, microstructure and oxygen reduction reaction activity of the resulted Ag/BSCF electrodes were comparatively studied using Fourier transform infrared spectra, environmental scanning electron microscopy, temperature-programmed oxygen desorption, X-ray diffraction, and electrochemical impedance spectroscopy. An area-specific resistance as low as 0.038 Ω cm² was achieved for N₂H₄-reduced Ag/BSCF cathode at 600 °C. Carbonates were detected over the BSCF surface during the reduction of silver, which deteriorated both the charge-transfer process and diffusion process of HCHO-reduced Ag/BSCF cathode for the oxygen electrochemical reduction reaction. An anode-supported single cell with an N₂H₄-reduced Ag/BSCF cathode showed a peak power of 826 mW cm⁻² at 600 °C. In comparison, only 672 mW cm⁻² was observed with the HCHO-reduced Ag/BSCF cathode.     

A pilot-scale study of biohydrogen production from distillery effluent using defined bacterial co-culture

We evaluated the feasibility of improving the scale of hydrogen (H₂) production from sugar cane distillery effluent using co-cultures of Citrobacter freundii 01, Enterobacter aerogenes E10 and Rhodopseudomonas palustris P2 at 100 m³ scale. The culture conditions at 100 ml and 2 L scales were optimized in minimal medium and we observed that the co-culture of the above three strains enhanced H₂ productivity significantly. Results at the 100 m³ scale revealed a maximum of 21.38 kg of H₂, corresponding to 10692.6 mol, which was obtained through batch method at 40 h from reducing sugar (3862.3 mol) as glucose. The average yield of H₂ was 2.76 mol mol⁻¹ glucose, and the rate of H₂ production was estimated as 0.53 kg/100 m³/h. Our results demonstrate the utility of distillery effluent as a source of clean alternative energy and provide insights into treatment for industrial exploitation.     

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.     

Synthesis and electrochemical properties of Li1−xFe0.8Ni0.2O2–LixMnO2 (Mn/(Fe + Ni + Mn) = 0.8) material

A new type of Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ (Mn/(Fe + Ni + Mn) = 0.8) material was synthesized at 350 °C in air atmosphere using a solid-state reaction. The material had an XRD pattern that closely resembled that of the original Li_1−xFeO₂–Li_xMnO₂ (Mn/(Fe + Mn) = 0.8) with much reduced impurity peaks. The Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell showed a high initial discharge capacity above 192 mAh g⁻¹, which was higher than that of the parent Li/Li_1−xFeO₂–Li_xMnO₂ (186 mAh g⁻¹). We expected that the increase of initial discharge capacity and the change of shape of discharge curve for the Li/Li_1−xFe_0.8Ni_0.2O₂–Li_xMnO₂ cell is the result from the redox reaction from Ni²⁺ to Ni³⁺ during charge/discharge process. This cell exhibited not only a typical voltage plateau in the 2.8 V region, but also an excellent cycle retention rate (96%) up to 45 cycles.     

Ion dynamics in the pseudocapacitive reaction of hydrous ruthenium oxide. Effect of the temperature pre-treatment

Pseudocapacitive redox reaction of hydrous ruthenium oxide was investigated by the combined electrochemical and quartz crystal nanobalance measurements on gold support in H₂SO₄ and Na₂SO₄ solutions. The results show that the pseudocapacitance arises from at least two different Faradaic reactions with significant influence of double layer capacitance. All three processes simultaneously take place during charging/discharging reaction, but their contribution vary depending on the electrolyte used, the temperature pre-treatment and on the potential range. One Faradaic reaction releases protons during oxidation reaction resulting in the electrode mass decrease, while another Faradaic reaction results in the chemical binding of water leading to the mass gain during the oxidation reaction. The former reaction is favoured in acidic electrolyte and at lower anodic potentials, and the latter reaction proceeds predominantly in neutral media and at higher anodic potentials. The influence of annealing temperatures on the characteristics of the redox reaction of hydrous ruthenium oxide, as well as on its capacitance, was confirmed. It was demonstrated that specific capacitances of hydrous ruthenium oxide could achieve values as high as 1500 F g⁻¹, provided that conditions of good electronic conductivity among RuO₂ particles, as well as good electrical contact between gold and RuO₂, are met.     

Electrochemical capacitance of NiO/Ru0.35V0.65O2 asymmetric electrochemical capacitor

A designed asymmetric hybrid electrochemical capacitor was presented where NiO and Ru_0.35V_0.65O₂ as the positive and negative electrode, respectively, both stored charge through reversible faradic pseudocapacitive reactions of the anions (OH⁻) with electroactive materials. And the two electrodes had been individually tested in 1 M KOH aqueous electrolyte to define the adequate balance of the active materials in the hybrid system as well as the working voltage of the capacitor based on them. The electrochemical tests demonstrated that the maximum specific capacitance and energy density of the asymmetric hybrid electrochemical capacitor were 102.6 F g⁻¹ and 41.2 Wh kg⁻¹, respectively, delivered at a current density of 7.5 A cm⁻². And the specific energy density decreased to 23.0 Wh kg⁻¹ when the specific power density increased up to 1416.7 W kg⁻¹. The hybrid electrochemical capacitor also exhibited a good electrochemical stability with 83.5% of the initial capacitance over consecutive 1500 cycle numbers.     

Thermal stability and kinetics of delithiated LiCoO2

The thermal stability of the materials that comprise the battery has been one of the important issues. By using temperature programmed desorption-mass spectrometry (TPD-MS) and XRD, the thermal decomposition reaction of delithiated Li_xCoO₂ (x = 1, 0.81, 0.65) was quantitatively analyzed. Delithiated Li_xCoO₂ samples were metastable and liberated oxygen at a temperature of above 250 °C. Liberated oxygen gas was quantified by TPD-MS. Structural changes of the samples were confirmed by XRD. We identified the stoichiometry of the thermal decomposition reaction of Li_xCoO₂. Furthermore, to analyze the heating rate dependence of the oxygen generation, we calculated the activation energy (E_a) of the thermal decomposition reaction. The average E_a through the reaction of Li_0.81CoO₂ is 130 kJ mol⁻¹, and that of Li_0.65CoO₂ is 97 kJ mol⁻¹. The Li content decreased as the activation energy increased.     

Biomolecule-assisted synthesis of cobalt sulfide nanowires for application in supercapacitors

A biomolecule-assisted hydrothermal process is developed to synthesize cobalt sulfide (CoS), in which l-cysteine is used as the sulfide source and directing molecule. By controlling the synthesis conditions, CoS nanospheres and nanowires can be assembled. The as-synthesized samples are characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) are used to study the effects of microstructure and morphology of the samples on their capacitance and conductivity. A specific capacitance, as high as 508 F g⁻¹, is achieved for CoS nanowires. This is very competitive with the best supercapacitor material, RuO₂ (720–760 F g⁻¹), but its cost is remarkably lower than RuO₂. Thus the nanowires are a promising material for low-cost, high-performance supercapacitors. This method could provide a universal green chemistry approach to synthesize other metal sulfides.     

Visible light induced hydrogen evolution on new hetero-system ZnFe2O4/SrTiO3

The physical properties and photoelectrochemical characterization of the spinel ZnFe₂O₄, elaborated by chemical route, have been investigated for the hydrogen production under visible light. The forbidden band is found to be 1.92 eV and the transition is indirectly allowed. The electrical conduction occurs by small polaron hopping with activation energy of 0.20 eV. p-type conductivity is evidenced from positive thermopower and cathodic photocurrent. The flat band potential (0.18 V_SCE) determined from the capacitance measurements is suitably positioned with respect to H₂O/H₂ level (−0.85 V_SCE). Hence, ZnFe₂O₄ is found to be an efficient photocatalyst for hydrogen generation under visible light. The photoactivity increases significantly when the spinel is combined with a wide band gap semiconductor. The best performance with a hydrogen rate evolution of 9.2 cm³ h⁻¹ (mg catalyst)⁻¹ occurs over the new hetero-system ZnFe₂O₄/SrTiO₃ in Na₂S₂O₃ (0.025 M) solution.     

Evaluation of nano-structured Ir0.5Mn0.5O2 as a potential cathode for intermediate temperature solid oxide fuel cell

A novel Ir_0.5Mn_0.5O₂ cathode has been synthesized by thermal decomposition of mixed H₂IrCl₆ and Mn(NO₃)₂ water solution. The Ir_0.5Mn_0.5O₂ cathode has been characterized by XRD, field emission SEM (FESEM) and AC impedance spectroscopy. XRD result shows that rutile-structured Ir_0.5Mn_0.5O₂ phase is formed by thermal decomposition of mixed H₂IrCl₆ and Mn(NO₃)₂ water solution. FESEM micrographs show that a porous structure with well-necked particles forms in the cathode after sintering at 1000 °C. The average grain size is between 20 and 30 nm. Two depressed arcs appear in the medium-frequency and low-frequency region, indicating that there are at least two different processes in the cathode reaction: charge transfer and molecular oxygen dissociation followed by surface diffusion. The minimum area specific resistance (ASR) is 0.67 Ω cm² at 800 °C. The activation energy for the total oxygen reduction reaction is 93.7 kJ mol⁻¹. The maximum power densities of the Ir_0.5Mn_0.5O₂/LSGM/Pt cell are 43.2 and 80.7 mW cm⁻² at 600 and 700 °C, respectively.     

Effect of synthetic conditions on the electrochemical properties of LiMn0.4Fe0.6PO4/C synthesized by sol–gel technique

Carbon-coated LiMn_0.4Fe_0.6PO₄ (LMFP) was synthesized by sol–gel technique using citric acid as foaming agent and carbon precursor. To evaluate the effect of synthetic conditions on the electrochemical properties of LMFP for use as cathode active material, the carbon-coated olivines were synthesized by a two-step thermal treatment at different temperatures. The composites were characterized by elemental analysis, XRD, SEM, TEM, Raman microprobe spectroscopy and their electrochemical properties were also studied. The composite that shows the better electrochemical performance has more porous structure, lower D/G band ratio in Raman spectra, and charge and discharge capacities of same 155 mAh g⁻¹ with higher material utilization of 97% at 0.1 C-rate (0.05 mA cm⁻²). The material exhibiting the better performance was also incorporated in a polymer electrolyte hosted in an electrospun P(VdF-HFP) membrane. The lithium polymer battery composed of LiMn_0.4Fe_0.6PO₄ cathode and polymer electrolyte showed a good cycling performance with the initial discharge capacity of 146 mAh g⁻¹.