Juan Ortiz

INDIRECT OXIDATION OF PHENOL ON GRAPHITE ON NI0.3 Co2.7O4 SPINEL ELECTRODES IN ALKALINE MEDIUM

We studied the oxidation of phenol by HO2- ions obtained from the oxygen reduction reaction (ORR) on pyrolytic graphite and on Ni0.3Co2.7 O4 spinel oxide in 1M KOH at 25C. Phenol degradation was achieved by using the controlled potential electrolysis of saturated O2 solutions under batch and laminar flow conditions. In both electrodes, the electro- generation of HO2- occurs between -0.2 V and -0.4V, with the graphite being more efficient than the NiO.3CO2.7 O4 electrode. The phenol degradation conversions for the batch electrolysis were 32% on graphite and 21% on the oxide, whereas with the flow mode they were 7% and 22% respectively

LITHIUM COBALT SPINEL OXIDE: A STRUCTURAL AND ELECTROCHEMICAL STUDY

Lithium cobaltite, LiCo 2 O 4 , prepared by thermal decomposition of nitrates salts was synthesized at 400 oC. X-ray Rietveld refinement showed the presence of two phases: a cubic spinel (Co 3 O 4 ) and other corresponding to spinel-like, which could not be classified as LiCo 2 O 4 or LiCoO 2 since both crystallized in Fd3m S.G. Electrochemical determinations (charge-discharge curves and impedance spectroscopy measurements) showed that Li+ ion is inserted into the spinel lattice, with D = 8 10 -16 cm s-1 at 2.00-2.25 V potential range whereas lithium extraction occurred at 3.2-3.8 V potential range.

Oxygen reduction on bornite (Cu5FeS4) in alkaline medium

The electroreduction of molecular oxygen is investigated between −0.1 and −0.5V vs SHE on bornite, Cu5FeS4, at pH9.2 and 14, by means of cyclic voltammetry (CV) and stationary voltammetry (SV), using a double channel electrode flow cell (DCEFC). Using an E/pH diagram established in this work, the CV results suggest that the bornite surface is stable between −0.1 and −0.5V then oxidized to CuS and Fe(OH)3 above −0.1V whereas, below −0.5V the mineral reduces to metal sulphides: Cu2S and FeS. The SV results show that oxygen is reduced to peroxide ions, HO2−. At pH9.2 the generated sulphide ions hinder the oxidation of HO2− on the collector electrode of the DCEFC, due to the formation of a blocking surface layer of elemental sulphur, S, impeding the determination of the kinetic parameters, k1 (direct way) and k 2 (indirect way) of the oxygen electroreduction reaction. In contrast, at pH14, as soluble polysulphides are formed, it was possible to determine these parameters, showing that the bornite is a poor catalyst for oxygen reduction. At pH14, in the presence of potassium ethylxanthate, generally used as a flotation collector, the ethylxanthate ions, C2H5OCSSO−, are oxidized by HO2− to perxanthates, ROCSSO−, while at pH9.2 the oxygen reduction is inhibited due to ethylxanthate chemisoption on the bornite surface.

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

SUBSTITUTION EFFECTS ON THE MAGNETIC PROPERTIES OF NI(ME,MN)2O4 AND LI(ME,MN)2O4: COMPARISON BETWEEN ME = CR AND FE

Manganese oxides of spinel structure, based on nickel (NiMn2O4) or lithium (LiMn2O4), were studied by magnetic measurements. Manganese was partially substituted by either chromium or iron, showing substantial variations of the magnetic properties with respect to the non-substituted compounds. Chromium enhances the antiferromagnetic interactions in both NiMn2-xCrxO4 and LiMn2-xCrxO4, while iron enhances the ferromagnetic ones, increasing Tc well above room temperature in NiMn2xFexO4 and LiMn2-xFexO4 (0 £ x £ 1). Titration analyses allowed us to estimate the Mnn+/Mnn+1 ratios in most cases, confirming the importance of the Mn3+-O-Mn4+ double-exchange ferromagnetism and the controlled-valence conduction mechanism in these materials

SPINEL LiFe xCo2-xO4 (0.25 ≤ x ≤ 1) AS CATHODES IN LITHIUM BATTERIES: RELATIONSHIP BETWEEN IONIC DISTRIBUTION AND LITHIUM ION INSERTION

ABSTRACT Spinel LiFe x Co 2-x O4 samples with 0.25≤x≤1 were synthesized by the sol-gel Pechini method at 300 o C in order to study the impact on their structural and electrochemical properties due to the substitution of Co by Fe. The specific capacity for lithium insertion into the electrode materials depend on lithium diffusion coefficient DLi which in turn depends on the Fe 3+ /(Fe +Co 3+ +Co 4+ ) octahedral cationic ratio. Keywords: Lithium- Ferrites, spinels, diffusion coefficient, Li-ion batteries, sol-gel 1.INTRODUCTION Li-ion batteries possess high energy density compared with Ni-Cd, NiMH, LiAl-FeS2 batteries, they are currently the most popular type of battery for portable electronic devices and they are growing in popularity for defense, automotive (EV,HEV,PHEV), storage systems using solar or wind powers and aerospace applications.Recently published reviews about Li-ion batteries and lithium-air batteries [1-6] emphasize that the energy densities of the current lithium-ion batteries are limited mainly by the inherent low energy density of the available conventional cathode materials. Transition metal oxides consisting of highly oxidized redox couples (Co

LONG CHAIN 1-ALKYL-3-METHYLIMIDAZOLIUM TETRAFLUOROBORATES AS ELECTROLYTES IN Li-ION BATTERIES

ABSTRACT We have determined the electrochemical behavior of a series of long chain 1-alkyl-3-methylimidazolium ionic liquids [(R-mim)BF 4 ], with alkyl groups ranging from pentyl to octyl, and tested them as electrolytes in lithium ion batteries. These ionic liquids present good conductivities and electrochemical windows which make them likely solvents of choice for Li-ion batteries using LiMn 2 O 4 as the cathode. Charge/discharge properties of a Li/ R-mim)BF 4 /LiMn 2 O 4 cell have been evaluated both with the IL as the sole electrolyte and with IL and conducting support. The Li- ion insertion results in IL can be explained on the basis of self aggregation of each long chain 1-alkyl-3-methylimidazolium cation. INTRODUCTION It is well known that progress in lithium-ion batteries relies much on improvements in the electrolyte. Ionic liquids (IL) have become a viable alternative as electrolytes for these devices [1]. In general, the most common electrolytes for Li-ion batteries are a mixture of 1:1 EC/EMC (ethylene carbonate and ethyl methyl carbonate) and LiPF

REDUCCION DEL OXIGENO SOBRE CALCOSITA, PIRITA Y GALENA EN MEDIO ALCALINO

La electroreduccion del oxigeno fue investigada a pH = 14 sobre electrodos de calcosita, pirita y galena mediante voltamperometria ciclica y por voltamperometria estacionaria usando una celda electroquimica de flujo de doble canal (CEFDC). La superficie de los electrodos fue caracterizada mediante consideraciones termodinamicas y cineticas. En todos lo casos la reduccion del O2 ocurre con formacion de iones HO2-. Sobre calcosita la reaccion de reduccion del O2 (RRO) ocurre sobre una superficie que contiene Cu2S y sobre la superficie de pirita que soporta una pelicula de Fe(OH)3. En el caso del electrodo de galena, la formacion de azufre tanto en el electrodo generador (por oxidacion quimica) como sobre el electrodo colector de la celda de flujo (por oxidacion de los iones sulfuro), dificulta la determinacion de los parametros cineticos de la RRO, k1 (via directa) y k2 (via indirecta). De los valores de k1 y k2, se infiere que la calcosita es el mejor catalizador de la RRO, pues genera menos iones peroxido considerando que los iones HO2- oxidan a los colectores de flotacion.