Diogo M.F. Santos

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.

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.

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.

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.

Molybdenum Carbide Nanoparticles on Carbon Nanotubes and Carbon Xerogel: Low-Cost Cathodes for Hydrogen Production by Alkaline Water Electrolysis.

Low-cost molybdenum carbide (Mo2 C) nanoparticles supported on carbon nanotubes (CNTs) and on carbon xerogel (CXG) were prepared and their activity for the hydrogen evolution reaction (HER) was evaluated in 8 m KOH aqueous electrolyte at 25-85 °C. Measurements of the HER by linear scan voltammetry allowed us to determine Tafel slopes of 71 and 74 mV dec(-1) at 25 °C for Mo2 C/CNT and Mo2 C/CXG, respectively. Stability tests were also performed, which showed the steady performance of the two electrocatalysts. Moreover, the HER kinetics at Mo2 C/CNT was enhanced significantly after the long-term stability tests. The specific activity of both materials was high, and a higher stability was obtained for the activated Mo2 C/CNT (40 A g(-1) at -0.40 V vs. the reversible hydrogen electrode).

Design of dedicated instrumentation for temperature distribution measurements in solid oxide fuel cells

A thermocouple was designed for temperature distribution measurements in solid oxide fuel cells. A theoretical model, based on mixed convective–radiative heat transfer was used to predict the thermocouple response. The proposed flat type thermocouple was shown to be a high sensitive, low error temperature sensor, capable of satisfying the requirements for solid oxide fuel cell thermal behaviour research. Thereafter, a purpose-built, thin, flat-type thermocouple has been used for temperature distribution measurements at the cathode side of a planar solid oxide fuel cell. High temperature conditions of 1223K have been tested. Beside temperature mapping, local hot spots have been easily located.

Hydrogen evolution on nanostructured Ni-Cu foams

Three-dimensional (3D) nickel–copper (Ni–Cu) nanostructured foams were prepared by galvanostatic electrodeposition, on stainless steel substrates, using the dynamic hydrogen bubble template. These foams were tested as electrodes for the hydrogen evolution reaction (HER) in 8 M KOH solutions. Polarisation curves were obtained for the Ni–Cu foams and for a solid Ni electrode, in the 25–85 °C temperature range, and the main kinetic parameters were determined. It was observed that the 3D foams have higher catalytic activity than pure Ni. HER activation energies for the Ni–Cu foams were lower (34–36 kJ mol−1) than those calculated for the Ni electrode (62 kJ mol−1). The foams also presented high stability for HER, which makes them potentially attractive cathode materials for application in industrial alkaline electrolysers.

Anion- or Cation-Exchange Membranes for NaBH4/H2O2 Fuel Cells?

Direct borohydride fuel cells (DBFC), which operate on sodium borohydride (NaBH4) as the fuel, and hydrogen peroxide (H2O2) as the oxidant, are receiving increasing attention. This is due to their promising use as power sources for space and underwater applications, where air is not available and gas storage poses obvious problems. One key factor to improve the performance of DBFCs concerns the type of separator used. Both anion- and cation-exchange membranes may be considered as potential separators for DBFC. In the present paper, the effect of the membrane type on the performance of laboratory NaBH4/H2O2 fuel cells using Pt electrodes is studied at room temperature. Two commercial ion-exchange membranes from Membranes International Inc., an anion-exchange membrane (AMI-7001S) and a cation-exchange membrane (CMI-7000S), are tested as ionic separators for the DBFC. The membranes are compared directly by the observation and analysis of the corresponding DBFC’s performance. Cell polarization, power density, stability, and durability tests are used in the membranes’ evaluation. Energy densities and specific capacities are estimated. Most tests conducted, clearly indicate a superior performance of the cation-exchange membranes over the anion-exchange membrane. The two membranes are also compared with several other previously tested commercial membranes. For long term cell operation, these membranes seem to outperform the stability of the benchmark Nafion membranes but further studies are still required to improve their instantaneous power load.

Effects of Temperature on the Performance of the MmNi3.6Co0.7Mn0.4Al0.3 Metal Hydride Electrode in Alkaline Solution

In the present study, the electrochemical and hydrogen transport kinetic performance of the MmNi 3.6 Co 0.7 Mn 0.4 Al 0.3 metal hydride electrode has been systematically investigated in the temperature range 0-60°C. The results show that the electrode reaction becomes more reversible and gets faster at higher temperatures. At low-discharge current densities, the charge transfer overpotential is greater than the diffusion overpotential, whereas at high-discharge current densities the diffusion overpotential becomes dominant. At medium-discharge current densities, there is a mixed control with a predominance of the mass-transport conditions at lower temperatures. The calculated kinetic parameters also show that the hydrogen desorption on the metal hydride electrode is easier than the hydrogen absorption into its alloy bulk. Activation energies for charge transfer and for hydrogen diffusion, as well as the hydrogen diffusion coefficients in the electrode, have also been estimated.