Mixed Valence Spinel Research Articles

Zn-substituted MnCo2O4 nanostructure anchored over rGO for boosting the electrocatalytic performance towards methanol oxidation and oxygen evolution reaction (OER)

Mixed valence spinel oxides have emerged as an attractive and inexpensive anode electrocatalyst for water oxidation to replace noble metals based electrocatalysts. The present work demonstrates the facile synthesis of Zn substituted MnCo2O4 supported on 3D graphene prepared by simple hydrothermal technique and its application as an electrocatalyst for water oxidation and methanol oxidation. The physico-chemical properties of the nanocatalyst were studied using various microscopic, spectroscopic and diffraction analyses confirming the formation of the composite. The electrocatalytic performance of the prepared electrocatalyst was evaluated using potentiodynamic, potentiostatic and impedance techniques. The synthesized Zn1-xMnxCo2O4/rGO electrocatalyst with x = 0.2 and 0.4 offered the same onset potential and overpotential at 10 mA/cm2. However, catalyst x = 0.4 delivered a higher current density indicating the superiority of the same over other compositions which is attributed to better kinetics that it possessed for OER as revealed by the smallest Tafel slope (80.6 mV dec−1). The prepared electrocatalysts were tested for methanol oxidation in which electrocatalyst Zn1-xMnxCo2O4/rGO with x = 0.4 shows a better electrochemical performance in oxidizing methanol with the higher current density of 142.3 mA/cm2. The above catalyst also revealed excellent stability and durability during both MOR and OER, suggesting that it can be utilized in practical applications.

Oxygen Electrocatalyst Based on Non-Precious Bifunctional Hybrid Material

Synthesis of new functional materials for the development of energy conversion and storage devices like fuel cells, metal-air batteries received significant interest in the last decade. The limited availability of traditional fossil fuel demands renewable energy resources. The electrochemical oxygen reduction and oxygen evolution reaction are the key processes in energy conversion and storage devices like metal-air batteries. The development of rechargeable metal-air batteries requires efficient bifunctional electrocatalyst for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Traditionally, Pt-, Ru- and Ir-based materials have been used for ORR and OER.1 The large overpotential due to the slow electron transfer kinetics, high cost and the electrochemical instability of these traditional precious metal-based electrocatalysts demands for active non-precious bifunctional catalysts. Moreover, the use of these traditional catalysts as an oxygen electrode is limited as they are not able to catalyse both the ORR and OER simultaneously. In the recent past, the oxides of transition metals such as Co, Mn, etc. including mixed valence spinel and perovskites have been proposed as alternate catalyst for these reactions.2 However, the poor electronic conductivity and the recyclability of these oxides limits their potential application.3 Recently, the hybrid electrocatalyst based on carbon and transition metal catalyst is emergingfor ORR and OER reactions.4 Our group is interested in the development of new functional materials for fuel cell and supercapacitor applications.2a,5 In continuation of our earlier efforts, herein we demonstrate the synthesis of two types of low-cost transition metal (Co, Fe) based hybrid bifunctional electrocatalyst. The two catalysts cobalt and nitrogen dual-doped carbon (Co-N-C) and reduced graphene oxide-CoFe (rGO-CoFe) were synthesized by chemical method followed by thermal annealing at an optimized temperature. The Co-N-C catalyst was obtained from cobalt tetra-aza macrocycle (Co-TET-A) whereas the rGO-CoFe catalyst was synthesized from multimetal complex, cobalt hexacyanoferrate. Both the catalysts were characterized by analytical techniques including XPS, TEM and XRD. The electrocatalytic response towards ORR and OER was evaluated by hydrodynamic voltammetry. The Co-N-C and rGO-CoFe catalysts show the limiting current density of -4.37 mA/cm2 and -5.58 mA/cm2, respectively and both the catalysts promote the 4-electron pathway for the reduction of oxygen. The OER activity was evaluated by measuring the overpotential at which 10 mA/cm2 current density was observed. Interestingly both Co-N-C and rGO-CoFe catalysts show the benchmark current density of 10 mA/cm2 at a low overpotential of 0.28 V and 0.22 V, respectively. The overall oxygen electrode activities were evaluated by calculating the difference in potential (ΔE) between the OER current density at 10 mA/cm2 and the ORR current density at −3 mA/cm2. We could achieve a ΔE of 0.92 V and 0.82 V on Co-N-C and rGO-CoFe catalyst respectively, confirming the efficient bifunctional activity of these catalysts. It is demonstrated that the synergistic effect between the metal and carbon and the porous nature of the catalysts plays an important role in the bifunctional activity. The detailed synthetic and bifunctional activity will be discussed.

OXYGEN REDUCTION ON Cu1 ,Mn16O4 SPINEL PARTICLES COMPOSITE ELECTRODES EFFECT OF PARTICLES SIZE

ABSTRACT The oxygen reduction reaction (orr) was studied on CG/PPy/PPy(Ox)/PPy and CG/paint/CC(Ox)PPy composite electrodes, CG representing conducting glass, PPy polypyrrole, CC carbon cloth and Ox mixed valence spinel oxide particles of transition metals Cu 1.4 Mn 1.6 O 4 in 0.2 M KCl pH = 9.2, at room temperature. PPy is electrochemically polymerized and serves as a protective layer to dispersed oxide particles in the carbon cloth. Oxide particles size varied from 7.6 to 11.2 µm. The orr takes place on the oxide particles with formation of hydrogen peroxide which diffuses to electrolyte solution. The amount of peroxide ions produced was detected using the rotating ring-disk electrode technique and determined indirectly by iodine spectroscopy. The peroxide ions production depends strongly on the oxide particle size. Keywords: Copper manganites, Spinel oxide, Composite electrode, Polypyrrole, Oxygen electroreduction. INTRODUCTION The importance of the cathodic oxygen reduction reaction (orr) to water either directly or indirectly via hydrogen peroxide is widely recognized due to their applications such as oxygen electrode in fuel cells, lithium-air batteries, wastewater treatments and, organic and inorganic synthesis. In fuel cells the orr is the only employed reaction to accept electrons from chemicals such as methanol, ethanol or other molecules

XRD and XPS characterization of mixed valence Mn 3O 4 hausmannite thin films prepared by chemical spray pyrolysis technique

Spray pyrolysis technique has been employed successfully for the synthesis of single phase mixed valence spinel hausmannite (Mn 3O 4) thin films using alcoholic start solution of manganese acetate (Mn(CH 3COO) 2·4H 2O) on pyrex glass substrates at atmospheric pressure using air as a carrier gas. Thermal decomposition of the precursor in the temperature range 320–490 °C led to the formation of Mn 3O 4 phase as revealed from the thermogravimetry analysis. Prepared samples are characterized by X-ray diffraction that shows spinel structure with space group I4 1/ amd. Pure and well crystallized specimen is subjected to X-ray photoelectron spectroscopy for the surface chemistry investigation of these systems at a molecular level. Surface Mn/O ratio is compared to the bulk composition of the sample. Atomic force micrographs revealed that the morphology and the surface grains of the films largely influenced by the substrate temperature.

Oxygen reduction on high-area carbon cloth-supported oxide nanoparticles/polypyrrole composite electrodes

Composite electrodes on glassy carbon (GC) having the structure GC/CC(Ox)/PPy, with CC representing carbon cloth, PPy polypyrrole and Ox mixed valence spinel oxide nanoparticles of transition metals Cu 1.4Mn 1.6O 4, exhibit high reactivity and stability towards the oxygen reduction reaction (ORR), when the ORR proceeds electrocatalytically on the oxide particles dispersed throughout the carbon cloth. PPy here is electrochemically polymerized and serves as a protective layer to make the oxide sandwiched between PPy and CC fiber backbone. In oxygen saturated 0.15 M KCl solution, the composite electrodes show a good performance with current density up to 10 mA cm − 2 under gentle stirring. The enhancements of the composite electrodes for ORR result from the oxide nanoparticles dispersed on high-area CC. Optimal oxide loadings were encountered at 3.5 mg cm − 2 .

Structure property relationships in the ATi 2O 4 ( A=Na, Ca) family of reduced titanates

Reduced titanates in the ATi 2O 4 ( A=Li, Mg) spinel family exhibit a variety of interesting electronic and magnetic properties, most notably superconductivity in the mixed-valence spinel, Li 1+ x Ti 2− x O 4. The sodium and calcium analogs, NaTi 2O 4 and CaTi 2O 4, each differ in structure, the main features of which are double rutile-type chains composed of edge-sharing TiO 6 octahedra. We report for the first time, the properties and band structures of these two materials. XANES spectroscopy at the Ti K-edge was used to probe the titanium valence. The absorption edge position and the pre-edge spectral features observed in the XANES data confirm the assignment of Ti 3+ in CaTi 2O 4 and mixed-valence Ti 3+/Ti 4+ in NaTi 2O 4. Temperature-dependent resistivity and magnetic susceptibility studies are consistent with the classification of both NaTi 2O 4 and CaTi 2O 4 as small band-gap semiconductors, although changes in the high-temperature magnetic susceptibility of CaTi 2O 4 suggest a possible insulator–metal transition near 700 K. Band structure calculations agree with the observed electronic properties of these materials and indicate that while Ti–Ti bonding is of minimal importance in NaTi 2O 4, the titanium atoms in CaTi 2O 4 are weakly dimerized at room temperature.

Mn 3O 4 and γ-MnOOH powders, preparation, phase composition and XPS characterisation

The paper reports data relative to the preparation of Mn 3O 4 and γ-MnOOH powders by following hydrothermal procedures. The mixed valence spinel was prepared starting either from manganous sulphate or chloride and oxidising in suspension (by either air or oxygen) the alkaline precipitate. Manganite was obtained by oxidising manganous sulphate solutions with H 2O 2, subsequent alkalisation and refluxing. The samples were characterised for phase composition by X-ray diffraction. Pure phase oxides were obtained in the case of both Mn 3O 4 and γ-MnOOH when Mn(II) sulphate was used as the precursor. The pure phase samples were submitted to X-ray photoelectron spectroscopy (XPS) investigation, in the case of manganese, for both 2p and 3s spectral regions. The binding energies of the un-fitted experimental Mn peaks compare well with the relative values reported in the literature for the compounds. With the aim of assessing the actual Mn speciation, in the external layers, the fitting procedure of the experimental XPS peaks has been performed by modulating the relative intensities of the spectral components on the grounds of distinct hypotheses consistent with the oxide formulas. The possibility of extending the outcome of this elaboration to the bulk composition of the samples is also discussed.

TiK XANES in superconducting LiTi2O4 and related compounds

The mixed valence spinel Li1+xTi2-xO4 (O>or=x>or=0.33) compounds are superconducting for x<or=0.12 (Tcmax approximately=12.7 K) and present a metal or semiconductor-like transition for x approximately=0.2. In order to study the context of such interesting electronic properties, the authors performed TiK edge measurements in these compounds and in the related systems MgTi2O4 and TiO2. The evolution of the intensity, position and shape of the pre-edge and main edge features is reported. They show that these data do not support a one-electron description and suggest that LiTi2O4 is a correlated d electron metal (U approximately=W). Modifications in the pre-edge spectra as a function of temperature have been observed for LiTi2O4, Li1.33Ti1.67O4 and TiO2. They discuss possible interpretations of this effect (vibronic coupling or thermally induced lattice deformation).