Electrocatalytic activity of IrO2-RuO2 supported on Sb-doped SnO2 nanoparticles

Electrocatalytic IrO₂-RuO₂ supported on Sb-doped SnO₂ (ATO) nanoparticles is very active towards the oxygen evolution reaction. The IrO₂-RuO₂ material is XRD amorphous and exists as clusters on the surface of the ATO. Systematic changes to the surface chemical composition of the ATO as a function of the IrO₂:RuO₂ ratio suggests an interaction between the IrO₂-RuO₂ and ATO. Cyclic voltammetry indicates that the electrochemically active surface area of IrO₂-RuO₂ clusters is maximised when the composition is 75 mol% IrO₂-25 mol% RuO₂. Decreasing the loading of IrO₂-RuO₂ on ATO reduces the electrochemically active surface area, although there is evidence to support a decrease in the clusters size with decreased loading. Tafel slope analysis shows that if the clusters are too small, the kinetics of the oxygen evolution reaction are reduced. Overall, clusters of IrO₂-RuO₂ on ATO have similar or better performance for the oxygen evolution reaction than many previously reported materials, despite the low quantity of noble metals used in the electrocatalysts. This suggests that these oxides may be of economic advantage if used as PEM water electrolysis anodes.     

Electrochemical oxidation of organics at metal oxide electrodes: The incineration of oxalic acid at IrO2-Ta2O5 (DSA-O2) anode

The electrochemical incineration of oxalic acid (OA) at Ti/IrO₂-Ta₂O₅ (DSA-O₂) anode was investigated to find the influence of the operative parameters on the performances of the process. Polarization curves and chronoamperometric measurements indicate the probable occurrence of a direct electrochemical oxidation of OA at the surface of the DSA anode. In incineration electrolyses, the performances of the process in terms of OA conversion and current efficiency dramatically depend on the adopted operative conditions. Interestingly, very high OA removal and current efficiency were obtained when the process was performed at relatively high temperatures (50 °C) or in the presence of NaCl. The experimental results are in good agreement with the previsions of a simple theoretical model previously developed.     

Characterization of surface fouling of Ti/IrO2 electrodes in 4-chlorophenol aqueous solutions by electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was used as the main technique coupled with cycling voltammetry (CV) to characterize the surface fouling of a conventional Ti/IrO₂ in 4-CP aqueous solutions caused by the electropolymerization of chlorinated phenol. Capacitive information of polymeric films formed was successfully derived from both the on-line and off-line impedance measurements and was used to characterize the surface fouling of IrO₂ electrodes. Results showed that the fouling extent at IrO₂ electrode decreased when its heating temperature was increased. With increasing the anodic potential, the surface fouling was enhanced firstly and then weakened, reaching the highest extent at 0.9 V. More positive potentials were believed to further oxidize the formed films and thereby to reactivate the deactivated electrode surface. With the increase of positive potential, the regeneration was enhanced, but no entire recovering could be achieved after the reactivation even at very high potentials.     

A comparative study on IrO2-Ta2O5 coated titanium electrodes prepared with different methods

The electrodes of IrO₂-Ta₂O₅ coated titanium were prepared using conventionally thermal decomposition procedure and polymer sol-gel (Pechini) method, respectively. The microstructure and electrochemical properties of the electrodes were studied with scanning electron microscope (SEM), energy dispersive X-ray (EDX), atomic force microscope (AFM), potentiodynamic polarization, cyclic voltammetry, electrochemical impedance spectroscopy and accelerated life test. As compared with the electrode formed using the traditional method of thermal decomposition, the oxide electrode prepared by Pechini method presents morphology of higher nano-scale roughness and more uniform surface composition with little precipitates. It also has larger electrochemically active surface area, better electrocatalytic activity for oxygen evolution and higher stability.     

Investigation of formic acid oxidation on Ti/IrO2 electrodes

A model has been proposed according to which the voltammetric charge involved in the Ti/IrO₂ electrodes is due to two contributions: a faradaic contribution due to surface redox activities at the IrO₂ coating and a non-faradaic contribution due to the charging of electrical double layer (). The later has been proposed as a tool for the estimation of the relative surface area of the Ti/IrO₂ electrodes.Differential electrochemical mass spectrometry (DEMS) measurements using H₂¹⁸O has demonstrated that we are dealing with an active electrode in which the surface redox couple IrO₃/IrO₂ acts as mediator in the oxidation of formic acid (FA).From the voltammetric measurements using different IrO₂ loading and FA concentrations, the kinetic parameters of FA oxidation via the surface redox couple IrO₃/IrO₂ have been determined.Finally a model has been proposed considering that FA oxidation at Ti/IrO₂ anodes is controlled by mass transfer. The good agreement between the experimental results and the model indicates that the surface reaction between FA and the electrogenerated IrO₃ is a fast reaction.     

Electrochemical oxidation of ammonia (NH4/NH3) on thermally and electrochemically prepared IrO2 electrodes

The electrochemical oxidation of ammonia (NH₄⁺/NH₃) in sodium perchlorate was investigated on IrO₂ electrodes prepared by two techniques: the thermal decomposition of H₂IrCl₆ precursor and the anodic oxidation of metallic iridium. The electrochemical behaviour of Ir(IV)/Ir(III) surface redox couple differs between the electrodes indicating that on the anodic iridium oxide film (AIROF) both, the surface and the interior of the electrode are electrochemically active whereas on the thermally decomposed iridium oxide films (TDIROF), mainly the electrode surface participates in the electrochemical processes.On both electrodes, ammonia is oxidized in the potential region of Ir(V)/Ir(IV) surface redox couple activity, thus, may involve Ir(V). During ammonia oxidation, TDIROF is deactivated, probably by adsorbed products of ammonia oxidation. To regenerate TDIROF, it is necessary to polarize the electrode in the hydrogen evolution region. On the contrary, AIROF seems not to be blocked during ammonia oxidation indicating its fast regeneration during the potential scan. The difference between both electrodes results from the difference in the activity of the iridium oxide surface redox couples.     

IrO2-SiO2 binary oxide films: Preparation, physiochemical characterization and their electrochemical properties

Mixed IrO₂-SiO₂ oxide films were prepared on titanium substrate by the thermo-decomposition of hexachloroiridate (H₂IrCl₆) and tetraethoxysilane (TEOS) mixed precursors in organic solvents. The solution chemistry and thermal decomposition kinetics of the mixed precursors were investigated by ultra violet/visible (UV/vis) spectroscopy and thermogravimetry (TGA) and differential thermal analysis (DTA), respectively. The physiochemical characterization of the resulting materials was conducted by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical measurements. It is shown from the UV/vis spectra that the electronic absorption intensity of IrCl₆²⁻ complexes in the precursors decreases in the presence of TEOS, indicating the interaction between these two components. Thermal analysis shows the decomposition reaction of H₂IrCl₆ is inhibited by TEOS in the low temperature range, but the further oxidation reaction at high temperatures of formed intermediates is independent of the presence of silane component. Physical measurements show a restriction effect of silica on the crystallization and crystal growth processes of IrO₂, leading to the formation of finer oxide particles and the porous morphology of the binary oxide films. The porous composite films exhibit high apparent electrocatalytic activity toward the oxygen evolution reaction. In addition, the long-term stability of Ti-supported IrO₂ electrodes is found to apparently improve with appropriate amount of SiO₂ incorporation, as tested under galvanostatic electrolysis.     

Electrochemical impedance characterization of FeSn2 electrodes for Li-ion batteries

This work reports the electrochemical characterization of a micro-scale FeSn₂ electrode in a lithium battery. The electrode is proposed as anode material for advanced lithium ion batteries due to its characteristics of high capacity (500 mAh g⁻¹) and low working voltage (0.6 V vs. Li). The electrochemical alloying process is studied by cyclic voltammetry and galvanostatic cycling while the interfacial properties are investigated by electrochemical impedance spectroscopy. The impedance measurements in combination with the galvanostatic cycling tests reveal relatively low overall impedance values and good electrochemical performance for the electrode, both in terms of delivered capacity and cycling stability, even at the higher C-rate regimes.     

Electrochemical degradation of Ibuprofen on Ti/Pt/PbO2 and Si/BDD electrodes

The electrochemical oxidation of Ibuprofen (Ibu) was performed using a Ti/Pt/PbO₂ electrode as the anode, prepared according to literature, and a boron doped diamond (BDD) electrode, commercially available at Adamant Technologies. Tests were performed with model solutions of Ibu, with concentrations ranging from 0.22 to 1.75 mM for the Ti/Pt/PbO₂ electrode and 1.75 mM for the BDD electrode, using 0.035 M Na₂SO₄ as the electrolyte, in a batch cell, at different current densities (10, 20 and 30 mA cm⁻²). Absorbance measurements, Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) tests were conducted for all samples. The results have shown a very good degradation of Ibu, with COD removals between 60 and 95% and TOC removals varying from 48 to 92%, in 6 h experiments, with higher values obtained with the BDD electrode. General Current Efficiency and Mineralization Current Efficiency, determined for both electrodes, show a similar behaviour for 20 mA cm⁻² but a very different one at 30 mA cm⁻². The combustion efficiency was also determined for both anodes, and found to be slightly higher with BDD at lower current density and equal to 100% for both anodes at 30 mA cm⁻².     

Effectiveness factor of isopropanol oxidation on IrO2 based electrodes of different loading

The influence of IrO₂ loading on the effectiveness factor E_f of the electrochemical oxidation of isopropanol was investigated. A model has been proposed based on three main reactions: electrochemical IrO₂ oxidation to IrO₃, chemical oxidation of the organic compound via IrO₃ and O₂ evolution via decomposition of IrO₃. It has been found that the relative effectiveness factor E_f for the electrochemical oxidation of IrO₂ to IrO₃ is loading independent contrary to the chemical reaction which decreases with increasing IrO₂ loading.     

Electrochemical quartz crystal microbalance studies of thin-solid films of higher fullerenes: C76, C78 and C84

Thin-solid films of higher fullerenes, viz. C₇₆, C₇₈ and C₈₄, were prepared by the drop coating technique and characterized by simultaneous cyclic voltammetry and piezoelectric microgravimetry with the use of an electrochemical quartz crystal microbalance. Properties of the films were compared with those reported earlier for the C₆₀ and C₇₀ thin-solid films. The effect of nature of the counter cation on electrochemical properties of the films has been probed by employing acetonitrile solutions of two different 0.1 M supporting electrolytes, namely tetra(n-butyl)ammonium (TBA⁺) hexafluorophosphate and potassium hexafluorophosphate. Stability of the films with respect to dissolution depends on the fullerene oxidation state as well as on the nature of both the fullerene in the film and the counter cation in the supporting electrolyte. The TBA⁺ counter cation ingress to the film for compensation of the negative charge of the reduced fullerene is accompanied by the acetonitrile solvent intake. The number of acetonitrile molecules per TBA⁺ counter cation entering the film is higher the higher the fullerene. Also, the Langmuir films of higher fullerenes were prepared at the air-water interface and the film morphology was characterized by the Brewster angle microscopy.     

Electrochemical behavior of LiCoO2 in a saturated aqueous Li2SO4 solution

The electrochemical behavior of a commercial LiCoO₂ with spherical shape in a saturated Li₂SO₄ aqueous solution was investigated with cyclic voltammetry and electrochemical impedance spectroscopy. Three redox couples at E_SCE = 0.87/0.71, 0.95/0.90 and 1.06/1.01 V corresponding to those found at ELi/Li⁺=4.08/3.83, 4.13/4.03 and 4.21/4.14 V in organic electrolyte solutions were observed. The diffusion coefficient of lithium ions is 1.649 × 10⁻¹⁰ cm² s⁻¹, close to the value in organic electrolyte solutions. The results indicate that the intercalation and deintercalation behavior of lithium ions in the Li₂SO₄ solution is similar to that in the organic electrolyte solutions. However, due to the higher ionic conductivity of the aqueous solution, current response and reversibility of redox behavior in the aqueous solution are better than in the organic electrolyte solutions, suggesting that the aqueous solution is favorable for high rate capability. The charge transfer resistance, the exchange current and the capacitance of the double layer vary with the charge voltage during the deintercalation process. At the peak of the oxidation (0.87 V), the charge transfer resistance is the lowest. These fundamental results provide a good base for exploring new safe power sources for large scale energy storage.     

Study of carbon-supported IrO2 and RuO2 for use in the hydrogen evolution reaction in a solid polymer electrolyte electrolyzer

Carbon-supported IrO₂ and RuO₂ were prepared using an incipient wetness method and were then calcinated at various temperatures. IrO₂/C and RuO₂/C are less expensive than the conventional Pt/C material and more stable than metal Ni in an acidic electrolyte. Moreover, IrO₂/C and RuO₂/C are not influenced by under potential deposition (UPD) and show lower sensitivity to poisoning by Ni or Fe impurities. The physical properties of IrO₂/C and RuO₂/C were investigated via XRD and TEM. Cyclic voltammograms (CV) and Tafel plots were used to provide information regarding surface redox reaction and electrocatalytic activity. The activity and durability of IrO₂/C and RuO₂/C were studied after prolonged potential cycling between −0.3 and 0.3 V_SCE. After comparison of Tafel plots of Pt/C and IrO₂/C after activation, it was observed that they have similar electrocatalytic activities in a hydrogen evolution reaction (HER). A single cell test with solid polymer electrolyte (SPE) proved that the performance of IrO₂/C (0.5 mg cm⁻²) was similar to that of Pt/C (0.5 mg cm⁻²).     

Physical characteristics, electrochemical behavior, and stability of BaFeO4

In this study, BaFeO₄ was prepared from K₂FeO₄ and characterized by SEM, XRD, IR, and TG. BaFeO₄ is insoluble in pure water and its solubility increases with the increase in the concentration of KOH solution. The measured solubility of BaFeO₄ is higher than that of K₂FeO₄ in saturated KOH solution at 25 °C, and may be due to the presence of a higher concentration of carbonate in the concentrated KOH solution. BaFeO₄ has been found to decompose both in dry environment and in KOH solution. The instability of BaFeO₄ is studied in detail by means of constant current discharge, XPS, XRD, and IR spectra. The reason why BaFeO₄ is not stable is discussed in this paper.     

Similarity and differences in the oxidative dehydrogenation of C2–C4 alkanes over nano-sized VOx species using N2O and O2

The effect of O₂ and N₂O on alkane reactivity and olefin selectivity in the oxidative dehydrogenation of ethane, propane, n-butane, and iso-butane over highly dispersed VO_x species (0.79 V/nm²) supported on MCM-41 has been systematically investigated. For all the reactions studied, olefin selectivity was significantly improved upon replacing O₂ with N₂O. This is due to suppressing CO_x formation in the presence of N₂O. The most significant improving effect of N₂O was observed for iso-butane dehydrogenation: S(iso-butene) was ca. 67% at X(iso-butane) of 25%.Possible origins of the superior performance of N₂O were derived from transient experiments using ¹⁸O₂ traces. ¹⁸O¹⁶O species were detected in ¹⁸O₂ and ¹⁸O₂–C₃H₈ transient experiments indicating reversible oxygen chemisorption. In the presence of alkanes, the isotopic heteroexchange of O₂ strongly increased. Based on the distribution of labeled oxygen in CO_x and in O₂ as well as on the increased CO_x formation in sequential O₂–C₃H₈ experiments, it is suggested that non-lattice oxygen species (possibly of a bi-atomic nature) originating from O₂ are non-selective ones and responsible for CO_x formation. These species are not formed from N₂O.     

Kinetic study of formic acid oxidation on Ti/IrO2 electrodes prepared using the spin coating deposition technique

In the first part of this paper, IrO₂ electrodes produced by thermal decomposition of H₂IrCl₆ precursor were manufactured using the spin coating deposition technique, where centrifugal forces spread the precursor solution with simultaneous evaporation of the solvent on the rotating Ti substrate. It was found using this technique, that it is possible to obtain thin and uniform IrO₂ coatings with controlled loadings. The influence of the concentration of iridium salt in the precursor solution (c₀) as well as the influence of the rotation speed at which the substrate spins (ω) on the IrO₂ loading have been studied using voltammetric charge measurements. From these results, a simple relation has been proposed for the estimation of the IrO₂ loading for a given c₀ and ω.In the second part of this paper and from measurements performed using different IrO₂ loadings and formic acid concentrations, the kinetic parameters of the oxidation of formic acid have been quantitatively determined using a model that involves the redox couple IrO₃/IrO₂ as mediator of this reaction. Furthermore, using the kinetic parameters obtained together with the Nernst equation and the I-V curves of the supporting electrolyte (1 M HClO₄), theoretical I-V curves could be constructed for different concentrations of formic acid and different IrO₂ loadings.     

Effect of Zr-ion substitution in CeO2 on H2O2-assisted degradation of orange G

A series of Pd ion-substituted CeO₂–ZrO₂ solid solutions were synthesized using the solution combustion technique. H₂O₂-assisted degradation of orange G was carried out in the presence of the catalysts. The activity of the catalysts was found to increase with the introduction of the second component in the solid solution, as signified by an increase in the rate constants and lowering of activation energy. The study showed the involvement of lattice oxygen and the importance of reducibility of the compound for the reaction.     

A novel LiTi2(PO4)3/MnO2 hybrid supercapacitor in lithium sulfate aqueous electrolyte

In this work, carbon-coated lithium-ion intercalated compound LiTi₂(PO₄)₃ and MnO₂ have been synthesized and they deliver a capacity of 90 and 60 mAh/g in 1 M Li₂SO₄ neutral aqueous electrolyte within safe potentials without O₂ and H₂ evolution, respectively. The novel hybrid supercapacitor in which MnO₂ was used as a positive electrode and carbon-coated LiTi₂(PO₄)₃ as a negative electrode was assembled and the LiTi₂(PO₄)₃/MnO₂ hybrid supercapacitor showed a sloping voltage profile from 0.7 to 1.9 V, at an average voltage near 1.3 V, and delivers a capacity of 36 mAh/g and an energy density of 47 Wh/kg based on the total weight of the active electrode materials. It exhibits a desirable profile and maintains over 80% of its initial energy density after 1000 cycles. The hybrid supercapacitor also exhibit an excellent rate capability, even at a power density of 1000 W/kg, it has a specific energy 25 Wh/kg compared with 43 Wh/kg at the power density about 200 W/kg.     

A study of lithium insertion into MnO2 containing TiS2 additive a battery material in aqueous LiOH solution

The electrochemical behavior and surface characterization of manganese dioxide (MnO₂) containing titanium disulphide (TiS₂) as a cathode in aqueous lithium hydroxide (LiOH) electrolyte battery have been investigated. The electrode reaction of MnO₂ in this electrolyte is shown to be lithium insertion rather than the usual protonation. MnO₂ shows acceptable rechargeability as the battery cathode. The influence of TiS₂ (1, 3 and 5 wt%) additive on the performance of MnO₂ as a cathode has been determined. The products formed on reduction of the cathode material have been characterized by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), fourier transform infrared spectroscopy (IR) and transmission electron microscopy (TEM). It is found that the presence of TiS₂ to ≤3 wt% improves the discharge capacity of MnO₂. However, increasing the additive content above this amount causes a decrease in its discharge capacity.     

Electrochemical properties of TiP2O7 and LiTi2(PO4)3 as anode material for lithium ion battery with aqueous solution electrolyte

Some polyanionic compounds, e.g. TiP₂O₇ and LiTi₂(PO₄)₃ with 3D framework structure were proposed to be used as anodes of lithium ion battery with aqueous electrolyte. The cyclic voltammetry properties TiP₂O₇ and LiTi₂(PO₄)₃ suggested that Li-ion de/intercalation reaction can occur without serious hydrogen evolution in 5 M LiNO₃ aqueous solution. The TiP₂O₇ and LiTi₂(PO₄)₃ give capacities of about 80 mAh/g between potentials of −0.50 V and 0 V (versus SHE) and 90 mAh/g between −0.65 V and −0.10 V (versus SHE), respectively. A test cell consisting of TiP₂O₇/5 M LiNO₃/LiMn₂O₄ delivers approximately 42 mAh/g (weight of cathode and anode) at average voltage of 1.40 V, and LiTi₂(PO₄)₃/5 M LiNO₃/LiMn₂O₄ delivers approximately 45 mAh/g at average voltage of 1.50 V. Both as-assembled cells suffered from short cycle life. The capacity fading may be related to deterioration of anode material.