Francisco Alcaide

Oxygen Reduction on Uncatalyzed Carbon-PTFE Gas Diffusion Cathode in Alkaline Medium

The reduction of O 2 to hydroperoxide ion HO - 2 on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) gas diffusion cathode has been studied in aqueous solutions with KOH concentrations ranging between 1.0 and 6.0 mol dm -3 . The O 2 /HO - 2 couple behaves as reversible in open circuit and its apparent standard electrode potential, calculated from the Nernst equation. becomes less negative when raising the OH concentration. Under linear sweep voltammetric conditions, the process is independent of both OH and HO 2 concentrations and a first-order reaction with respect to O 2 feed is found. In each alkaline medium the apparent activation energy for O 2 reduction at several potentials has been determined from Arrhenius-type plots at temperatures ranging between 5 and 45°C. While the Tafel slope is proportional to absolute temperatures the cathodic transfer coefficient is always close to unity A reaction mechanism involving the initial reduction of O 2 on a located active site of the carbon black surface by a reversible one-electron transfer, followed by the irreversible protonation of the resulting adsorbed superoxide ion by water as the rate-determining step, is proposed.

Hydrogen Oxidation Reaction in a Pt-Catalyzed Gas Diffusion Electrode in Alkaline Medium

The oxidation of H 2 to water on commercial platinum catalyzed carbon-polytetrafluoroethylene gas diffusion electrode (GDE) has been studied in deoxygenated aqueous solutions with KOH concentrations in the range 1.0-6.0 mol dm - 3 . The H 2 O‖H 2 couple behaves reversibly in open circuit, closely following the Nernst equation. A semiempirical equation in agreement with a Tafel-Volmer mechanism, accounting for diffusion, adsorption, inhibition, charge transfer, and ohmic-type drop in the wetted electrode pores, has been developed to quantitatively explain the steady-state linear sweep voltammograms for the different media, temperatures, and feeding hydrogen partial pressures tested. Charge transfer is also slow at high overpotentials except for 1.0 mol dm - 3 KOH. The micropolarization in the current-potential region around the reversible potential has been interpreted considering the Tafel reaction as the rate-determining step. The decay in exchange current density and the increase in activation energy both, at the reversible potential and high overpotentials, with raising KOH concentration has been ascribed to the increasing blocking and energetic effect of adsorbed hydroxyl species. The results of the activation energies suggest that the reactive Pt(110) faces are dominant in the dispersed Pt particles of the GDE.

Impedance study of the evolution of a HO2−-generating hydrophobic gas diffusion electrode

Abstract The behaviour of a recently developed O 2 -diffusion electrode for HO 2 − generation has been studied by electrochemical impedance spectroscopy, diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy. It has been submitted to a cathodic reduction of 100 mA cm −2 in 1.0 and 6.0 M KOH to analyse its evolution with electrolysis time. An increase in the double layer capacity and a decrease in the charge transfer resistance with increasing time were found. The electrode wetting is higher in 6.0 M KOH, but only about 6% of its total surface area is electroactive after 50 h. The charge transfer control for O 2 reduction, the low functionalisation of the electrode surface by oxygen-containing groups and its high hydrophobicity have been related to its high durability.

An Impedance Study of the O 2 | HO 2 − System in Equilibrium on a Gas Diffusion Electrode

The behavior of the O 2 |HO 2 redox couple at equilibrium on a commercial uncatalyzed carbon-polytetrafluoroethylene (PTFE) oxygen-diffusion electrode, fed with O 2 partial pressures between 0.21 and 1.0 atm, has been studied by electrochemical impedance spectroscopy. Measurements have been made in the open-circuit potential using aqueous solutions with KOH concentrations in the range 1.0-6.0 mol dm 3 and HO 2 concentrations up 50 mmol dm 3 at 25.0°C. Under these conditions, the system is controlled by activation, the charge-transfer resistance being much higher than ohmic, adsorption, and diffusion resistive elements. The double-layer capacities show that the wetted electroactive areas, much smaller than the total area, depend on the KOH concentration used for activation. True exchange current densities of about I μA cm 2 are obtained, while their apparent values are two orders of magnitude greater, since the sluggish reaction is compensated by the high electroactive area. The cathodic process is a first-order reaction with respect to the O 2 feed, and Independent of HO 2 and OH concentrations. For the anodic one, a zero order for O 2 and a first order for HO 2 and OH are calculated. These results agree with the previously proposed mechanism for the O 2 reduction to HO 2 on the same electrode from voltammetric studies. Indirect evidence on a weak adsorption of HO 2 is found from the impedance diagrams.

Effect of Gas Diffusion Layer Composition on the Performance of Direct Methanol Fuel Cells

The effect of the cathode gas diffusion layer compositions on the performance of direct methanol fuel cells has been investigated. Morphological and textural properties of different diffusion layers have been characterized. Experiments in a single cell show that the use of a microporous layer (MPL) made with acetylene black instead of furnace carbon black results in higher cell voltages and power densities. This has been explained by a smaller mass-transfer resistance in the cathode diffusion layer due to a more suitable structure of the MPL when using acetylene black.

Effect of Gas Diffusion Layer Composition on the Performance of Liquid-Direct Methanol Fuel Cells

Introduction Direct liquid methanol fuel cells (liquid-DMFC) are promising energy sources for mobile and portable applications, mainly because of the methanol advantages against hydrogen, such as its higher solubility in liquid electrolytes, its availability at low cost, easier handling, transport and storage, as well as its higher energy density (6 kWh kg). From a practical perspective, there is a wide range of devices covering several operational conditions. For example, cell phones and PDA’s (small power devices) will operate at 40 oC or lower, whereas laptops, power tools and battery chargers for army or remote site applications (intermediate power devices) will operate at 60-80 oC.

APPLICATIONS - STATIONARY | Cogeneration of Energy and Chemicals: Fuel Cells

Electricity generation is the main application of fuel cells (FCs). However, chemical compounds and heat are also cogenerated in them, thus opening the way to participate in combined-cycle processes. Specific FCs produce chemicals that are valuable intermediates in other processes or have important industrial applications. Such cells then behave as reactors in which hydrogenations, dehydrogenations, halogenations, and oxidations take place. Although the number of related papers in the literature is limited, their potential economical interest is demonstrated in some cases, thus encouraging the development of FCs able to cogenerate useful chemicals with high added value together with electricity. In this article, the generation of electricity together with useful chemicals in FCs is examined. The application of heat cogeneration to obtain more electricity is also considered. Finally, a survey about processes that, although not electrochemical in origin, result in the production of electricity together with chemicals, which can be used in electrochemical power sources, is performed.