THREE-DIMENSIONAL SIMULATIONS OF TRANSIENT RESPONSE OF PEM FUEL CELLS

Transients have utmost importance in the lifetime and performance degradation of PEM fuel cells. Recent studies show that cyclic transients can induce hygro-thermal fatigue. In particular, the amount of water in the membrane varies significantly during transients, and determines the ionic conductivity and the structural properties of the membrane. In this work, we present three-dimensional time-dependent simulations and analysis of the transport in PEM fuel cells. U-sections of anode and cathode serpentine flow channels, anode and cathode gas diffusion layers, and the membrane sandwiched between them are modeled using incompressible Navier-Stokes equations in the gas flow channels, Maxwell-Stefan equations in the channels and gas diffusion layers, advection-diffusion-type equation for water transport in the membrane and Ohm’s law for ionic currents in the membrane and electric currents in gas diffusion electrodes. Transient responses to step changes in load, pressure and the relative humidity of the cathode are obtained from simulations, which are conducted by means of a third party finite-element package, COMSOL

A study of Na(x)Pt3O4 as an O2 electrode bifunctional electrocatalyst

The present study suggests that polytetrafluoroethylene (PTFE) bonded Na(X)Pt3O4 gas porous diffusion electrodes may be a viable candidate for bifunctional O2 reduction and evolution activity. The electrodes exhibited Tafel slopes of about 0.06 V/decade for both O2 reduction an evolution. For O2 reduction, the 0.06 slope doubled to 0.12 V/decade at larger current densities. Preliminary stability testing at 24 C suggest that the Na(x)Pt3O4 electrodes were relatively stable at reducing and oxidizing potentials typically encountered at the O2 electrodes in a regenerative fuel cell

Response behaviour of oxygen sensing solid electrolytes

The response time (t r) after a step change in oxygen partial pressure was investigated for some solid electrolytes used in Nernst type oxygen sensors. The electrolyte as well as the (porous) electrode material affect the value oft r. Stabilized Bi2O3 materials exhibit slower response rates (largert r values) than stabilized ZrO2. Introduction of Bi2O3 in stabilized ZrO2 increases the response time. Gold electrodes show a higher response rate than platinum in the oxygen partial pressure and temperature region used.\u

Modeling Nucleation and Growth of Zinc Oxide During Discharge of Primary Zinc-Air Batteries

Metal-air batteries are among the most promising next-generation energy storage devices. Relying on abundant materials and offering high energy densities, potential applications lie in the fields of electro-mobility, portable electronics, and stationary grid applications. Now, research on secondary zinc-air batteries is revived, which are commercialized as primary hearing aid batteries. One of the main obstacles for making zinc-air batteries rechargeable is their poor lifetime due to the degradation of alkaline electrolyte in contact with atmospheric carbon dioxide. In this article, we present a continuum theory of a commercial Varta PowerOne button cell. Our model contains dissolution of zinc and nucleation and growth of zinc oxide in the anode, thermodynamically consistent electrolyte transport in porous media, and multi-phase coexistance in the gas diffusion electrode. We perform electrochemical measurements and validate our model. Excellent agreement between theory and experiment is found and novel insights into the role of zinc oxide nucleation and growth and carbon dioxide dissolution for discharge and lifetime is presented. We demonstrate the implications of our work for the development of rechargeable zinc-air batteries.Comment: 16 pages, 8 figures, Supplementary Information uploaded as ancillary fil

LaNiO3-based catalyst in gas diffusion electrodes: activity and stability for oxigen reactions

Perovskite-type oxides are potential catalysts for next generation of regenerative fuel cells. In particular, LaNiO3 has been recognised as one of the most promising oxygen electrodes. In this work LaNiO3 perovskite-type oxides, prepared by a self-combustion method [1, 2], have been used for the preparation of porous gas-diffusion electrodes (GDE). Electrodes were prepared on Toray carbon paper (CP) substrates, consisting of a diffusion layer, a catalyst layer and a Nafion® layer. The gas diffusion layers were prepared using Vulcan XC-72R. The catalyst ink was prepared by suspending the material in isopropanol, stirring the mixture in an ultrasonic bath to thoroughly disperse it. Ink slurries were also pasted onto glassy carbon discs and used as working electrodes for full kinetic studies at potential domains for the oxygen reduction (ORR) and oxygen evolution (OER) reactions. A systematic study on the effect of the oxide loading (OL) on the electrodes surface area was done by cyclic voltammetry. It was found a quasi linear variation between the electrodes surface area and the oxide loading. Roughness values varying from 106±3 to 307±6 were obtained for OL between 1 and 5 mg cm-2 respectively. The results show that the peak current density increases with the increasing on oxide loading as shown in Fig. 1. Higher current densities for ORR were obtained for the electrodes prepared using LaNiO3-based perovskite with partial substitution of Ni by Cu. Stability studies of the GDEs, performed using a pre-defined cycling protocol in 1M KOH, will be discussed together with catalytic activity parameters relevant for their potential use as bifunctional oxygen electrodes

Simulation of a wire-cylinder-plate positive corona discharge in nitrogen gas at atmospheric pressure

In this work we are going to perform a simulation of a wire-cylinder-plate positive corona discharge in nitrogen gas, and compare our results with already published experimental results in air for the same structure. We have chosen to simulate this innovative geometry because it has been established experimentally that it can generate a thrust per unit electrode length transmitted to the gas of up to 0.35 N/m and is also able to induce an ion wind top velocity in the range of 8-9 m/s in air. In our model, the used ion source is a small diameter wire, which generates a positive corona discharge in nitrogen gas directed to the ground electrode, after which the generated positive ions are further accelerated in the acceleration channel between the ground and cathode. By applying the fluid dynamic and electrostatic theories all hydrodynamic and electrostatic forces that act on the considered geometries will be computed in an attempt to theoretically confirm the generated ion wind profile and also the thrust per unit electrode length. These results are important to establish the validity of this simulation tool for the future study and development of this effect for practical purposes.Comment: 11 pages, 7 figures, 2 tables, submitted for publication. arXiv admin note: substantial text overlap with arXiv:1102.425

Preparation and evaluation of advanced electrocatalysts for phosphoric acid fuel cells

Results are presented for hydrogen oxidation and hydrogen oxidation poisoned by carbon monoxide at levels between 0 and 30%. Due to the high activities that are now being observed for our platinum based electrocatalysts, the hydrogen concentrations were reduced to 10% levels in the gas supplies. Perturbation techniques were used to determine that a mechanism for the efficient operation of our porous gas diffusion electrodes is diffusion of the carbon monoxide out of the electrode structure through the electrolyte film on the electro-catalyst. A survey of the literature on platinum group materials (PGM) was carried out so that an identification of successful electrocatalysts could be made. Two PGM electrocatalysts were prepared and performance data for hydrogen oxidation in hot phosphoric acid in the presence of high carbon monoxide concentrations showed that they matched the best platinum on carbon electrocatalysts but with an electrocatalyst cost that was half of the platinum catalyst cost

Electrocatalytic phenomena in gas phase reactions in solid electrolyte electrochemical cells

The recent literature on electrocatalysis and electrocatalytic phenomena occurring in gas phase reactions on solid, oxygen conducting electrolytes is reviewed. In this field there are a number of different subjects which are treated separately. These are: the use of electrochemical methods to study catalytic phenomena, electrocatalysis proper, the transfer of oxygen at the electrodes or electrolyte, and the (electro)catalytic properties of mixed, electronic and ionic, conducting materials

Impurity intrusion in radio-frequency micro-plasma jets operated in ambient air

Space and time resolved concentrations of helium metastable atoms in an atmospheric pressure radio-frequency micro-plasma jet were measured using tunable diode laser absorption spectroscopy. Spatial profiles as well as lifetime measurements show significant influences of air entering the discharge from the front nozzle and of impurities originating from the gas supply system. Quenching of metastables was used to deduce quantitative concentrations of intruding impurities. The impurity profile along the jet axis was determined from optical emission spectroscopy as well as their dependance on the feed gas flow through the jet.Comment: Journal of Physics D: Applied Physics (accepted), 6 page