Papers by Author: César A.C. Sequeira

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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, Fernand D.S. Marquis
Abstract: The effects of aluminising on the hot corrosion resistance of two nickel-chromium alloys containing 10 and 30 weight percent chromium, respectively, were examined. The Ni/Cr specimens were aluminised by pack cementation in Ar and underwent cyclic hot corrosion testing in Na2SO4- NaCl molten salt. XRD analysis indicated that a NiAl phase formed between the coating layer and substrate. Cyclic hot corrosion test results indicated that the effects of aluminising are more pronounced in the case of the 10 % Cr than in the case of 30 % Cr. The ductile NiAl phase suppressed the potential for crack initiation during thermal cycling of the 10 % Cr specimens, and reinforced the hot cyclic corrosion resistance in molten salt for the 30 % Cr specimens.
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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: Yun Chen, César A.C. Sequeira
Abstract: With the goal to find hydride electrodes, the phase structure and electrochemical characteristics of the as-cast and ball-milled La2Mg17 alloy with Ni additions were investigated. The addition of Ni was found to be an important factor for the earlier transformation to an amorphous alloy. When the addition of Ni reached 100 wt%, the ball-milled La2Mg17 alloy transformed into a homogeneous phase with high discharge capacity of 1060 mAh/g(La2Mg17) [530 mAh/g(La2Mg17+100 wt% Ni)]. The exchange current density and the kinetics of the amorphous alloy were also found experimentally noticeably higher than those of corresponding crystal alloy.
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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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