C.A.C. Sequeira

Hydrogen production by alkaline water electrolysis

Water electrolysis is one of the simplest methods used for hydrogen production. It has the advantage of being able to produce hydrogen using only renewable energy. To expand the use of water electrolysis, it is mandatory to reduce energy consumption, cost, and maintenance of current electrolyzers, and, on the other hand, to increase their efficiency, durability, and safety. In this study, modern technologies for hydrogen production by water electrolysis have been investigated. In this article, the electrochemical fundamentals of alkaline water electrolysis are explained and the main process constraints (e.g., electrical, reaction, and transport) are analyzed. The historical background of water electrolysis is described, different technologies are compared, and main research needs for the development of water electrolysis technologies are discussed.

Zinc Anode for Direct Borohydride Fuel Cells

Zinc is evaluated as a negative electrode for the direct electrochemical oxidation of sodium borohydride (NaBH 4 ). Open-circuit potential measurements, cyclic voltammetry, chronoamperometry, and chronopotentiometry are used to characterize the electrode behavior, namely, the oxidation reaction at the Zn/borohydride interface. Two consecutive oxidation steps are identified, and a possible working mechanism is proposed. A relatively low electrocatalytic activity of Zn for borohydride oxidation is indicated. A laboratory direct sodium borohydride/hydrogen peroxide (NaBH 4 /H 2 O 2 ) fuel cell using a Zn anode is studied at room temperature. Cell voltages of 2.14 V and short-circuit currents of 1.05 A cm ―2 are reported but cell stability is limited to 6 h operation. Energy densities and maximum specific capacities of 2799 Wh kg ―1 and 1577 Ah kg ―1 , respectively, are obtained. A power density of about 470 mW cm ―2 at a cell voltage of 1.10 V and a current density of 426 mA cm ―2 is reported but it cannot be sustained for appreciable operation times.

Thin film solid state polymer electrolytes containing silver, copper and zinc ions as charge carriers

Abstract The ionic conductivities, decomposition voltages and limiting discharge potentials were measured in a series of poly(ethylene-oxide)-complex salts of silver, copper and zinc. The ionic conductivities range from 1×10−5 (Ω cm)−1 for (PEO)4AgClO4 to 1.2×10−9 (Ω cm)−1 for (PEO)8CuCl2 at room temperature and the stability window widths range from 1.3V for (PEO)xAgClO4 to 3.3V for (PEO)4.KI.2ZnCl2.

Metal hydride beds and hydrogen supply tanks as minitype PEMFC hydrogen sources

Abstract In order to improve the properties of hydrogen tanks for hydrogen fuel cells, we studied the effect of two different matrix configurations, one being pellet pressed by Cu coating hydrogen storage alloy powders and the other foam nickel sheets filled with hydrogen storage alloy powders, on the heat and mass transfer properties of metal hydride beds. AB5-type alloy Ml0.85Ca0.15Ni5 and AB2-type alloy Ti0.9Zr0.15Mn1.6Cr0.2V0.2 were used as hydrogen storage materials, respectively. Comparison with general metal hydride powder beds shows that the heat transfer property of the beds with these two new matrix configurations is well improved. Based on these two new matrix configurations of metal hydride beds, hydrogen tanks with different structures and capacities were designed and prepared. For the prototype tanks of 500 l hydrogen storage capacity, the gravimetric and volumetric density of the tanks with AB5-type and AB2-type materials can attain values of the order of 1.1 wt % , 25 kg H 2 / m 3 and 1.3 wt % , 40 kg H 2 / m 3 , respectively.

Electrochemical properties of the ball-milled La1.8Ca0.2Mg14Ni3+xwt%Ni composites (x=0, 50, 100 and 200)

Abstract The electrochemical characteristics of the as-cast and ball-milled La 2 Mg 17 type alloy La 1.8 Ca 0.2 Mg 14 Ni 3 + x wt%Ni ( x =0, 50, 100 and 200) were investigated. After 50 h ball-milling the amorphous La 1.8 Ca 0.2 Mg 14 Ni 3 +100wt%Ni composite exhibited a large discharge capacity of 1004 mAh g (La 1.8 Ca 0.2 Mg 14 Ni 3 ) −1 at 298 K. Compared with the as-cast La 1.8 Ca 0.2 Mg 14 Ni 3 alloy, the remarkable improvement in electrochemical capacity of the ball-milled composite is due to the formation of an amorphous composite. The addition of Ni is an important factor for the earlier formation of an amorphous alloy. The cycling degradation of the composite is influenced by the degree of amorphorization and the amount of Ni added.

Electrochemical routes for industrial synthesis

Esta revisao examina as razoes que justificam um interesse crescente da industria quimica pelos processos eletroliticos. Reveem-se as industrias quimicas, nas quais compostos orgânicos e inorgânicos sao processados e descrevem-se os avancos tecnologicos mais relevantes. Inicialmente, abordam-se processos bem estabelecidos, como as industrias cloroalcalinas de producao de aluminio, p-aminofenol, adiponitrila, etileno glicol, antraquinona, produtos perfluorados, acido glioxilico e L-cisteina. Face a grande quantidade de informacao disponivel, o assunto e tratado com base cientifica, mas de modo bastante simplificado. Posteriormente, descrevem-se processos orgânicos e inorgânicos emergentes, nomeadamente processos eletroquimicos mediados e sintese em liquidos ionicos. Estabelecem-se paralelos entre sintese quimica e sintese eletroquimica. Estimula-se o interesse nos processos de eletrossintese, particularmente em liquidos ionicos, nao desmerecendo a importância das vias puramente quimicas de sintese orgânica e inorgânica. This review examines the reasons for increasing interest towards electrolyses by the chemical industry, reviews the electrochemical industries as most of them now exist, and provides a status report on the key technological advances which are occurring to meet present and future needs. Classical industries like those of chloroalkali, aluminium, p-aminophenol, adiponitrile, ethylene glycol, anthraquinone, perfluorinated products, glyoxylic acid and L-cysteine are initially covered. Considering the large amount of available publications in these topics of electrochemical engineering, the covered relevant information is treated at a scientific level, although in a simplified way. Then the paper deals with emerging inorganic and organic processes, e.g. electrosynthesis in ionic liquids and mediated electrochemical processes, and finishes by assessing what the future development trend might be given the electrochemical and non electrochemical competing influences. With this approach it is hoped to stimulate the interest of chemical engineers and scientists non-specialised in electrochemical routes, and to review some cutting edge research, particularly as far as electrosynthesis in ionic liquids is concerned.

Stability domain of a complexed lithium salt-poly(ethylene oxide) polymer electrolyte

Abstract The redox stability domain of a polyethylene oxide-lithium trifluoromethanesulphonate polymer electrolyte has been investigated using low-sweep-rate voltammetry, in the range 100–170°C. Below 140°C, its voltage stability window is in excess of 3.3 volts, making it a candidate for use with practical electrochemical couples. The electrolyte is not, however, a pure cation conductor, and problems associated with the observed significant anion mobility must be overcome in order to improve the polymers suitability as a solid-state electrolyte for battery applications.

Chronopotentiometric Investigation of Borohydride Oxidation at a Gold Electrode

Sodium borohydride (NaBH 4 ) presents interesting options for electrochemical power generation acting either as a hydrogen source or an anodic fuel for direct borohydride fuel cells. Though there have been several papers concerning electrochemical determination of relevant thermodynamic and kinetic parameters to the borohydride (BH 4 ) system, there are many basic aspects that have not yet been systematically studied. This paper reports chronopotentiometric studies of the electro-oxidation of BH4 at a gold electrode in 2 M NaOH solutions at temperatures ranging from 25 to 65°C. Gold displays a rather good BH4 oxidation activity, and the overall oxidation process is shown to be irreversible involving a number of exchanged electrons close to the theoretically expected value of 8. For the specific potential and concentration ranges observed, the results suggest that the rate-determining step is an irreversible, diffusion-controlled, one-electron oxidation step for which several key kinetic parameters (α, k s , and E 0 ct ) are calculated.

Cathodic oxygen reduction on noble metal and carbon electrodes

Abstract Because of their high electrocatalytic activity and stability noble metals were thoroughly investigated for the cathodic oxygen reduction. Emphasis has also been placed on the study of oxygen reduction on carbon electrodes in alkaline media due to their relative stability, easy modification of surface properties, large availability and low price. The observed behaviour of oxygen reduction on noble metal and carbon electrocatalysts is surveyed here. The common mechanisms under Langmuir conditions of adsorption are also examined in an attempt to account for the reaction kinetics of oxygen reduction on metaldoped polymer carbons in alkaline solutions.

Physics of Electrolytic Gas Evolution

A brief analysis of the physics and effects of electrolytic gas evolution is presented. Aspects considered include bubble nucleation, growth and detachment; enhancement of mass and heat transfer; and decrease of apparent electrical conductivity of bubble containing electrolytes. This analysis is mainly oriented to hydrogen-/oxygen-evolving electrodes.