Papers by Author: Diogo M.F. Santos

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Authors: Diogo M.F. Santos, Yun Chen, César A.C. Sequeira
Abstract: Anodic polarisation curves of Mm(Ni3.6Co0.7Mn0.4Al0.3)x (Mm = mischmetal, 0.85  x  1.15) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen (H/M), potential sweeping rates (v), temperatures (T), and amounts of reducing agent (y = [NaBH4]) in alkaline solution. Anodic peak current (Ip) at the Mm(B5)x electrodes increased with an increase in T, x and y values. In addition, the Ip value depended linearly on initial hydrogen concentration and square root of potential sweeping rate, irrespective of T, x and y values. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in x and y values. From these results, it is considered that the surface reduction treatment of the Mm(B5)x alloys, performed in alkaline solution with sodium borohydride, the nonstoichiometry and the initial concentration of absorbed hydrogen, are important factors for improving the charge-discharge performance of negative electrodes for metal-hydrogen energy systems.
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Authors: Diogo M.F. Santos, César A.C. Sequeira
Abstract: Chronoamperometry is carried out with a Au disc electrode in alkaline sodium borohydride solutions of varying concentrations and temperatures. By selecting conditions for which the borohydride oxidation is controlled by diffusion, it was possible to deduce diffusion coefficients and activation energies for borohydride concentrations ranging from 0.03 M to 0.12 M, and temperatures ranging from 25 to 65 °C. The estimated parameters were compared with other values reported in the open literature.
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Authors: Diogo M.F. Santos, César A.C. Sequeira
Abstract: Borohydrides present interesting options for electrochemical power generation acting either as hydrogen sources or anodic fuels for direct borohydride fuel cells and batteries. Though there have been several papers concerning electrochemical determination of relevant thermodynamic and kinetic parameters to the borohydride system, a number of its basic aspects have not been yet systematically studied. In this paper we report chronopotentiometric studies of the electrooxidation of sodium borohydride at a gold sphere electrode in 2M NaOH solutions, at temperatures ranging from 25 to 55 °C. Gold displayed a rather good BH4 - oxidation activity, and the overall oxidation process was shown to be irreversible involving a number of electrons very close to the theoretically expected value of 8. The results suggested that the rate-determining step is an irreversible, diffusion controlled, one-electron oxidation step, for which the transfer coefficients were calculated.
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Authors: César A.C. Sequeira, Diogo M.F. Santos
Abstract: In this paper, current-voltage (i-Vg) results from different kinds of n-type InP Schottky diodes are reported. The diodes were fabricated on an unintentionally doped n-type (100) indium phosphide substrate, and the i-Vg characteristics were measured in the temperature range 100 300 K. For the ideality factor, n always exhibited a small (1) but continuous increase with the voltage. At higher forward voltage, slightly higher values of n were due to series resistance effect; in other words, the interface state density always remained small. However, it was possible to obtain some information in the case of discrete interface traps. It was shown that i-Vg measurements can be used as a fast method to determine the densities of the interface states when they equilibrate with the semiconductor.
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Authors: César A.C. Sequeira, Yun Chen, Diogo M.F. Santos
Abstract: Anodic polarisation curves of treated and untreated MmNi3.6Mn0.4Al0.3Co0.7 (Mm = mischmetal) electrodes were measured under the conditions of various initial concentrations of absorbed hydrogen (H/M), rates of potential sweep and temperatures. The alloy powders of the electrodes were treated with 6 M NaOH solution containing y M NaBH4 (y = 0.0, 0.005, 0.01, 0.02, 0.03 and 0.05). The initial concentration of absorbed hydrogen in the electrodes varied between 0 and 0.06 H/M, and the working temperature range was 15 °C to 50 °C. Anodic peak current (Ip) at the MmNi3.6Mn0.4Al0.3Co0.7 electrodes increased with an increase in the y value. In addition, the Ip value depended linearly on initial hydrogen concentration and square root of potential rate irrespective of y value. Furthermore, the activation energy for hydrogen diffusion decreased with an increase in y value. From these results, it is considered that the hydrogen diffusion in the MmNi3.6Mn0.4Al0.3Co0.7 alloys and the electrocatalytic activity at their surfaces influence the Ip value and that they are important factors for improving the charge-discharge performance of the negative hydride electrode. Its low temperature (15-30 °C) performances are also improved by the simple powder treatment with the 6 M NaOH solution containing NaBH4 as reducing agent.
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Authors: Diogo M.F. Santos, César A.C. Sequeira
Abstract: The electrocatalysis of the oxygen reduction reaction by Lanthanum Barium Manganate, (La0.5Ba0.5MnO3) (LBM) has been studied by cyclic voltammetry using the rotating ring-disc electrode technique (RRDE) in alkaline medium. From the ring-disc data and other kinetic parameters it has been assumed that the oxygen reduction occurs by dissociative chemisorption at low overpotentials. At higher overpotentials, the formation of hydrogen peroxide (HO2 - in this case) has been observed on this electrocatalyst. The apparent exchange current density value for oxygen reduction on LBM has been found to be 4 x 10-8 Acm-2, while the corresponding Tafel slope is 0.115 V per decade. The possible reaction mechanism for electroreduction of oxygen on this oxide catalyst has been discussed.
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Authors: Yun Chen, Diogo M.F. Santos, César A.C. Sequeira
Abstract: The electrochemical permeation technique was used to evaluate the hydrogen diffusion, and analyse its interdependent solubility and trapping behaviour in 9 austenitic stainless steels. The hydrogen diffusion of all alloys obeyed Arrhenius relationships in the temperature range between 25 °C to 100 °C. The apparent diffusion coefficients were determined and the relevant activation energies were estimated. It was observed that Ni, Mo, Mn and N, among other alloying elements, clearly effect the hydrogen diffusion.
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Authors: Diogo M.F. Santos, Rui F. M. Lobo, César A.C. Sequeira
Abstract: Ultramicroelectrodes offer several unique characteristics which enable new types of electrochemical measurements. These include: 1) small size; 2) minimisation of iR effects; 3) rapid response; and 4) steady-state response at moderate times. These features enable experiments as diverse as in vivo electrochemistry, electrochemistry in pharmacology, nanoelectrochemistry, electrochemistry in solvents such as benzene, microsecond electrochemistry, and flow-rate independent electrochemistry. Thus, it is apparent that the use of ultramicroelectrodes has become a rapidly growing area of interest. In this paper, the attributes of ultramicroelectrodes, its construction, the equations of diffusion, and key applications of electrochemistry at ultramicroelectrodes, are analysed.
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Authors: L.P.S. Araújo, Diogo M.F. Santos, César A.C. Sequeira
Abstract: In this work, the Scanning Reference Electrode Technique (SRET) was used to visualise the pyrite corrosion by atmospheric oxygen and its inhibition by a thymol derivative. SRET has the advantage over the potentiostatic and galvanostatic techniques of enabling in situ operation without imposing any potential or current to the sample under study. SRET revealed that the inhibiting effect is effective only if the sample is oxidised beforehand. It also revealed that pyrite corrosion by oxygen is a localised process whereas that by Fe(III) is not local, thus the oxidation of pyrite by atmospheric oxygen is not only due to the oxidative role of Fe(III). Adsorption of oxygen plays probably a determining role. Studies are now carried on to evidence the intervention of galvanic processes or catalysis phenomena occurring during the corrosion of natural conducting minerals and industrial wastes. Their aim is to better understand the mechanisms and to propose remediation processes.
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