José Manuel Amarilla

Nanosize LiNiyMn2 −yO4(0 < y≤ 0.5) spinels synthesized by a sucrose-aided combustion method. Characterization and electrochemical performance

Nanosize crystalline cathode materials of LiNiyMn2 − yO4 (0 < y ≤ 0.5) composition and spinel-type structure have been obtained by a single-step sucrose-aided self-combustion method. The as-prepared samples contained some amorphous organic impurities that were removed after a short period of heating at 500 °C. The pure single-phase spinels have been characterized by X-ray diffraction, transmission electron microscopy, chemical analysis, and nitrogen sorption isotherms. The samples consist of particles (ca. 24 nm size) that are aggregated in clusters (ca. 1 µm size) in which mesopores (10–80 nm size) appear among the particles. Additional heating at 800° and 1000 °C produces a slight increase in the cubic lattice parameter and a pronounced increase in particle size (>100 nm). Electrical conductivity decreases as the Ni content increases in accordance with an electron hopping mechanism between Mn3+ and Mn4+ ions. The 500 °C- and 800 °C-heated LiNi0.5Mn1.5O4 samples show good electrochemical behaviour at 4.7 V as cathode materials. The capacity (132.7 mA h g−1) found is close to the nominal capacity (146.7 mA h g−1) and remains constant for current densities in the range C/24–2C (where C = 2.6 mA cm−2). At higher current densities (2C–10C) the capacity decreases progressively. The cyclability at the C current density is ca. 99.7% for both samples.

Multifunctional Response of Anatase Nanostructures Based on 25 nm Mesocrystal‐Like Porous Assemblies

Financial support from Spain Ministerio de Ciencia e Innovacion under projects MAT2008-03224 and MAT2008-03182 is acknowledged.

Electrochemical characteristics of cobalt-doped LiCoyMn2−yO4 (0≤y≤0.66) spinels synthesized at low temperature from CoxMn3−xO4 precursors

Abstract A series of Co-doped LiMn 2 O 4 spinels of general formula LiCo y Mn 2− y O 4 (0≤ y ≤0.66) has been synthesized by a new procedure, i.e. by reacting Co x Mn 3− x O 4 (0≤ x ≤0.99) precursors with LiOH·H 2 O at 600°C for 4 h. Structural and electrochemical characterization has been carried out. The spinels have been obtained as single-phase compounds, with lattice parameters decreasing from 8.2090(4) A for the LiMn 2 O 4 to 8.0664(4) A for LiCo 0.66 Mn 1.34 O 4 . The LiCo y Mn 2− y O 4 compounds have small crystallite size, that ranges from 250 to 400 A. The first cycle of the Li//LiCo y Mn 2− y O 4 cells has been studied by step potential electrochemical spectroscopy (SPECS). The lithium extraction/insertion mechanism has been studied. A linear diminution of the spinel capacity on increasing the Co-dopant content is observed for the first cycle. For the doped spinels a remarkable enhancement of the cyclability with a retention of the initial capacity >90% after 12 cycles at low current density (C/30) is observed.

Antimonic acid and sulfonated polystyrene proton-conducting polymeric composites

Abstract Polymer composites formed of a proton conductor and polyvinylidene fluoride (PVDF) are prepared. Sulfonated polystyrene (SPS) and antimonic acid (AAc) are used as organic and inorganic proton conductor, respectively. PVDF is the insulating matrix used as a binder to improve the mechanical stability of the composites compared with the pure proton conductors. The effect of the SPS and/or AAc on both the glass transition temperature, T g , and the melting temperature, T m , of the PVDF is studied. The ionic conductivity of the composites, after water vapor exposure at r.t., and after water immersion at 50°C, is measured at room temperature. The overall conductivity of the composite increases with time up to saturation, which is attained: (i) after 17 h for the water vapor exposure experiment and (ii) after ≈10 min, for the water immersion experiment. The ionic conductivity of the composites is smaller than that of the pure protonic conductors, but the composites show high-dimensional stability. The shape of the conductivity curve depends on the proton conductor chosen.

Influence of KOH concentration on the γ-MnO2 redox mechanism

The redox mechanism of γ-MnO2 was studied in KOH electrolytic solutions of increasing concentration ranging from 1 to 9N. The reduction process of the first cycle, carried out at i = 0.33 Am−2, shows a one-stage reduction in KOH 1N and two stage-reduction in [KOH] > 3N. In all cases the first stage of reduction is the H+/e− insertion process. For [KOH] > 3N, an intermediate oxyhydroxide, tentatively identified by electrochemical measurement as MnO1.69 was formed during this process. The flat part of the E versus capacity curve observed for [KOH] > 3N is attributed to the Mn(III) dissolution mechanism. The H+/e− insertion process decreases while the second heterogeneous stage increases, with increasing KOH concentration. The oxidation process of the first cycle and the cycling behaviour was studied by Step Potential ElectroChemical Spectroscopy (SPECS). In KOH 1N, one main anodic peak is observed in the voltammogram of the first oxidation process. For [KOH] > 3N, two main oxidation peaks are observed. XRD and chronoamperometric data indicate that these are different steps of the oxidation process. During the redox cycling, different electrochemical behaviour is observed depending on the KOH concentration. In 1N KOH, the voltamperometric and XRD data show that the redox mechanism of the γ-MnO2 can be described as a H+/e− insertion/desinsertion process, with good reversibility. For 3N > [KOH], after the first cycle, a different redox mechanism is observed and a loss of electrochemical activity of γ-MnO2 is also noticed.

Effects of architecture on the electrochemistry of binder-free inverse opal carbons as Li–air cathodes in an ionic liquid-based electrolyte

In this work, different electrodes consisting of a layer of nanostructured binderless carbon supported on a stainless steel (SS) mesh have been developed and tested as cathodes for Li–air batteries. Inverse opal (IO) carbons were developed using poly(styrene-co-methacrylic acid) (PS-MAA) spheres of different sizes as templates via a resorcinol–formaldehyde sol–gel process. The resulting electrodes, which were mechanically stable and easy to manipulate, were electrochemically tested at both 25 and 60 °C by galvanostatic cycling in an ionic liquid-based electrolyte (0.3 M LiTFSI in PYR14TFSI). Different ratios of co-monomers used in the preparation of the template polymeric spheres to control their size significantly influenced the resulting surface area, pore volume and pore distribution in IO carbons of different macropore size. From the electrochemical characterisation, transverse trends in reversibility and rate capability were identified depending on the macropore size of the inverse opal carbon. Smaller pores favor a better charge–discharge reversibility. Large pores contribute to an improved rate capability and large capacity, which are likely due, respectively, to deeper oxygen diffusion into the electrode, and to larger pore bottlenecks.

Electrochemical behaviour of natural and synthetic ramsdellite

Five forms of MnO2, two stoichiometric [β-MnO2 and ramsdellite from New Mexico (NM)] and three synthetic ones [Sedema WSA, IBA 11 and a new synthetic (S) ramsdellite], were characterized by XRD, FTIR and slow voltammetry. NM-ramsdellite is found to be a mixture of ramsdellite, pyrolusite and groutellite. The new S-ramsdellite has the lowest fraction of pyrolusite defects (de Wolff disorder) compared with other synthetic samples. Slow potentiostatic reduction in 1 mol–1 KOH shows that NM-ramsdellite and β-MnO2 are reduced at a lower potential and have a lower electrochemical activity than synthetic, defect compounds, while S-ramsdellite is reduced at an intermediate potential. The 3400 cm–1 OH bending mode associated with Mn3+ is sharp and well resolved in groutellite; it extends progressively until 1200 cm–1 with increasing stoichiometric defects. Structural and chemical disorders appear to be necessary for electrochemical activity by (i) simultaneously increasing the Fermi level energy in the oxides, (ii) levelling the insertion sites energy, and (iii) increasing the mixed valence state instability, which improves the H+e–insertion/reduction kinetics.

Electrochemical reduction of βMnO2, ramsdellite, γ- and εMnO2

Abstract The electrochemical reduction of four different forms of MnO 2 (ramsdellite, βMnO 2 , γMnO 2 and eMnO 2 was made in KOH 1N and 7N, under potentiostatic control. The structural disorder (De Wolffs defauts and microtwinning) and stoichiometry defects are the origin of higher reduction potential observed for γ- and eMnO 2 . The wide peak observed in the KOH 1N voltammogram for γ- and eMnO 2 is assigned to H + /e − insertion process, and not observed for ordered and stoichiometric forms ramsdellite and βMnO 2 . For KOH 7N, a step is defined in all cases and related to the dissolution of MnO 2 .

Organosilicic membranes doped with crown-ethers

Composite membranes have been prepared via the sol–gel process by controlled hydrolysis of ethyltriethoxysilane incorporating in the sol phase a selected crown-ether. The membranes are supported on borosilicate porous discs forming a continuous network as shown by scanning electron microscopy. The crown-ether (12-crown-4, 15-crown-5, 18-crown-6) remains homogeneously distributed in the organosilicic matrix as evidenced by IR and 13C NMR spectroscopies. A.C. impedance plots vary with the nature of the doped crown-ether as well as with the electrolyte used (alkali-metal chloride aqueous solutions).

TayNb1−yVO5 (0<y<1) mixed oxides synthesized by sol–gel method: electrochemical Li+-insertion

Mixed oxides of general formulae Ta y Nb 1-y O 5 (0 < y < 1) have been synthesized by a modified sol-gel method. Characterization of the samples has been carried out by x-ray diffraction, SEM/EDX, FTIR spectroscopy and thermal analysis. The electrochemical properties have been studied in a lithium cell. The first discharge capacity decrease from 206 (y = 0.25) to 136 mA h g -1 (y = 0.75). Ta y Nb 1-y O 5 samples undergo an irreversible structural changes induced by electrochemical Li + -insertion. For all compositions, the new compounds formed after the first discharge has a very high cyclability, as shown the low capacity loss < 1% per cycle.