Cyclone flow cell for the investigation of gas-diffusion electrodes

Electrochemical gas–liquid reactions can be efficiently carried out at porous gas diffusion electrodes (GDE). These electrodes are simultaneously in contact with a gas phase and a liquid phase. For the design and scale-up of electrochemical reactors based on these GDE their macrokinetic behaviour (i.e., the interaction of reaction and internal mass transport phenomena) must be investigated under well defined external mass transfer conditions and controlled wetting conditions. To meet these requirements, a novel cyclone cell has been designed in which two vortex flow fields are realised on either side of a horizontally positioned GDE. The external mass transfer coefficients in this cell are determined from limiting current measurements for the oxidation of Fe(CN₆)^4–.     

Macroscopic analysis of polarization characteristics of gas-diffusion electrodes in contact with liquid electrolytes: Part I: First order reactions

In this paper principles of gas-liquid chemical reaction engineering are applied to analyse the current-potential characteristics of gas-diffusion electrodes (GDE) in contact with liquid electrolytes. A macroscopic electrode model is formulated which accounts for mass transfer in the external diffusion films, in the gas layer and in the flooded layer. The set of model equations accounts for material balances, mass transport kinetics and Butler-Volmer polarization kinetics. Several dimensionless parameter groups are introduced which allow a compact reformulation of the proposed model. For first order reactions its solution can be derived analytically. The introduced parameter groups allow a classification of the different operating modes of a GDE, that is, slow reaction, fast reaction and instantaneous reaction.     

A model for Kolbe electrolysis in a parallel plate reactor

A parallel-plate reactor model is developed for the Kolbe electrolysis of acetate to ethane and carbon dioxide with hydrogen evolution as the counterelectrode reaction. The parallel-plate reactor is considered to consist of three zones: a turbulent bulk region in which streamwise convection is the dominant mass-transport mechanism (plug-flow model) and a thin diffusion layer at each electrode where diffusion and migration mass transport are dominant (Nernst diffusion-layer model). The acetic acid solution is supported with sodium hydroxide, and the reactor is under steady cell-potential control. Gaseous products are tracked by a hypothetical gas layer which increases in thickness in the streamwise direction. The gas phase is assumed to be an ideal, three-component mixture of hydrogen, carbon dioxide and ethane; the liquid phase consists of acetate, proton, acetic acid, and sodium and hydroxyl ions. The model predicts streamwise profiles of concentration, current density, gas-void fraction, and gas and liquid velocities in addition to reactant conversion, and cell-polarization characteristics. The average current density exhibits a maximum at a base-to-acid ratio of 0.96 due to the weak-acid/strong-base chemistry and a broad maximum at an interelectrode spacing of 0.37 cm resulting from minimized ohmic losses.     

Reduction of metal ions in dilute solutions using a GBC-reactor Part II: Theoretical model for the hydrogen oxidation in a gas diffusion electrode at relatively low current densities

The GBC-reactor is based on the combination of a gas diffusion anode and a porous cathode. A theoretical model for gas diffusion electrode, valid at relatively low current densities, is derived. This is based on the pseudohomogeneous film model including an approximation of the Volmer–Tafel mechanism for the hydrogen oxidation kinetics. Results show a severe mass transfer limitation of the hydrogen oxidation reaction inside the active layer of the gas diffusion electrode, even at low current densities. Empirical formulae are given to estimate whether leakage of dissolved hydrogen gas into the bulk electrolyte occurs at specific process conditions. A simplified version of the model, the reactive plane approximation, is presented.     

PEMFC流道横截面二维两相流数学模型(Ⅰ)模型建立

建立了质子交换膜燃料电池（PEMFC）流道和脊横截面的二维两相流数学模型（across-the-channel model）。模型描述了PEMFC主要的传递和反应过程,包括阴、阳两极反应气的质量传递、动量传递、电子和质子的传递以及电化学反应等。模型细致地描述了水（液态和气态）在扩散层、催化层以及质子交换膜中的传递过程。模型可以用来研究流场、扩散层、催化层以及膜等对电池性能的影响,进而达到优化电池结构的目的。     

Anomalous behaviour of hydrogen extraction from hydride-forming metals and alloys under impermeable boundary conditions

Hydrogen injection into and extraction from hydride-forming electrodes has been routinely investigated under the assumption that the electrode is homogeneous in structure and hydrogen transport through such electrodes is purely controlled by hydrogen diffusion. However, various kinds of abnormal behaviours in hydrogen transport, which could hardly be explained in terms of the diffusion-control model, have been quite frequently reported by many researchers. This review provides a comprehensive survey of the anomalous behaviours of hydrogen transport observed in such hydride-forming electrodes as Pd and metal-hydrides, with particular emphasis on hydrogen extraction under the impermeable boundary conditions during the potentiostatic current transient measurement involving the potential stepping. After a brief discourse on the conventional diffusion-control model, the topics related to the boundary conditions at the electrode surface during hydrogen extraction are extensively reviewed. In particular, it is shown that the diffusion-controlled constraint should be no longer valid at the electrode surface for hydrogen extraction in case hydrogen diffusion is influenced by either the interfacial charge transfer reaction or the hydrogen transfer reaction between adsorbed state on the electrode surface and absorbed state at the electrode sub-surface. Subsequently, the atypical behaviours of current transient due to hydrogen diffusion in the presence of traps and in the coexistence of two hydride phases are treated in detail. Each of the hydrogen extraction models suggested is discussed with the aid of the anodic current transients numerically calculated based upon the theoretical electrode potential curve, and then it is exemplified by hydrogen extraction from Pd and metal-hydrides in aqueous solutions.     

One‐dimensional Model of Oxygen Transport Impedance Accounting for Convection Perpendicular to the Electrode

A one‐dimensional (1D) model of oxygen transport in the diffusion media of proton exchange membrane fuel cells (PEMFC) is presented, which considers convection perpendicular to the electrode in addition to diffusion. The resulting analytical expression of the convecto‐diffusive impedance is obtained using a convection–diffusion equation instead of a diffusion equation in the case of classical Warburg impedance. The main hypothesis of the model is that the convective flux is generated by the evacuation of water produced at the cathode which flows through the porous media in vapor phase. This allows the expression of the convective flux velocity as a function of the current density and of the water transport coefficient α (the fraction of water being evacuated at the cathode outlet). The resulting 1D oxygen transport impedance neglects processes occurring in the direction parallel to the electrode that could have a significant impact on the cell impedance, like gas consumption or concentration oscillations induced by the measuring signal. However, it enables us to estimate the impact of convection perpendicular to the electrode on PEMFC impedance spectra and to determine in which conditions the approximation of a purely diffusive oxygen transport is valid. Experimental observations confirm the numerical results.     

Benefits of Membrane Electrode Assemblies with Asymmetrical GDL Configurations for PEM Fuel Cells

Membrane electrode assemblies (MEAs), based on commercial catalyst‐coated membranes combined with various gas diffusion layers (GDLs) on anode and cathode, were studied in terms of their specific advantages for different operations regimes of proton exchange membrane fuel cells (PEMFCs.) It is verified that MEAs with optimized gas diffusion layer designs (backing and micro‐porous layers) on anode and cathode are able to provide improved cell performance combined with a largely reduced sensitivity towards changes in the relative humidity as compared to MEAs with symmetrical gas diffusion layer configuration.     

A three-dimensional numerical simulation of the transport phenomena in the cathodic side of a PEMFC

A three-dimensional numerical model is developed to simulate the transport phenomena on the cathodic side of a polymer electrolyte membrane fuel cell (PEMFC) that is in contact with parallel and interdigitated gas distributors. The computational domain consists of a flow channel together with a gas diffusion layer on the cathode of a PEMFC. The effective diffusivities according to the Bruggman correlation and Darcy's law for porous media are used for the gas diffusion layer. In addition, the Tafel equation is used to describe the oxygen reduction reaction (ORR) on the catalyst layer surface. Three-dimensional transport equations for the channel flow and the gas diffusion layer are solved numerically using a finite-volume-based numerical technique. The nature of the multi-dimensional transport in the cathode side of a PEMFC is illustrated by the fluid flow, mass fraction and current density distribution. The interdigitated gas distributor gives a higher average current density on the catalyst layer surface than that with the parallel gas distributor under the same mass flow rate and cathode overpotential. Moreover, the limiting current density increased by 40% by using the interdigitated flow field design instead of the parallel one.     

Carbon-supported platinum metal catalysts for hydrogenation reactions. Mass transport effects in liquid phase hydrogenation over Pd/C

The efficient use of palladium charcoal catalysts in liquid phase hydrogenation processes is shown to depend on hydrogen transport limitations from the bulk liquid phase to catalyst particles, and on the diffusion of hydrogen through the porous structure of the support to the active metal crystallites. These processes impose a strict limitation on the maximum rate of hydrogenation, and can effect the selectivity of reaction processes. The factors which control the output of a reactor in terms of operating conditions, and catalyst design are examined.     

双极Electro-Fenton法降解水中苯酚的研究

设计了一种双极电化学芬顿方法(Bipolar Electro—Fenton;BEF)并对水中的苯酚进行了降解研究。采用热压法制备了多孔气体扩散碳电极。在无分隔槽反应装置中,以多孔气体扩散碳电极为阴极将氧气还原产生过氧化氢,铁板作阳极产生Fe2 ,直接利用两电极产物发生芬顿(Fenton)反应对苯酚进行降解。TOC、COD的检测结果表明,BEF法中所采用的气体扩散电极对苯酚的降解程度较通常废水处理中以石墨为阴极的电芬(Electro-Fenton;EF)更为彻底;而且BEF法对苯酚的降解速率比传统芬顿法更快。     

Model‐Based Ex Situ Diagnostics of Water Fluxes in Catalyst Layers of Polymer Electrolyte Fuel Cells

The ability to predict the electrochemical performance of the cathode catalyst layer in a polymer electrolyte fuel cell hinges on a precise knowledge of water distribution and fluxes. Water transport mechanisms that must be accounted for include vapor diffusion, liquid water permeation and vaporization exchange. In order to facilitate experimental efforts to this effect, we propose an ex situ model of water fluxes in catalyst layers. The model formulation is similar to transmission line models that are widely used in the analysis of electrochemical impedance spectra of porous composite electrodes. Focusing in this article on steady state and isothermal conditions, we rationalize the response function between defined environmental conditions, i.e. gas pressures, partial vapor pressures and temperature, which are defined at the boundaries of the catalyst layer, and the net water flux. This response function provides diagnostic capabilities to isolate and extract water transport parameters of catalyst layers from measurements of water fluxes through membrane electrode assemblies or half cell systems. An important asset of the model is the ability to analyze catalyst layer transport properties under partial saturation.     

Two-phase transport and the role of micro-porous layer in polymer electrolyte fuel cells

Two-phase transport of reactants and products constitutes an important limit in performance of polymer electrolyte fuel cells (PEFC). Particularly, at high current densities and/or low gas flow rates, product water condenses in open pores of the cathode gas diffusion layer (GDL) and limits the effective oxygen transport to the active catalyst sites. Furthermore, liquid water covers some of the active catalytic surface, rendering them inactive for electrochemical reaction. Traditionally, these two-phase transport processes in the GDL are modeled using so-called unsaturated flow theory (UFT), in which a uniform gas-phase pressure is assumed across the entire porous layer, thereby ignoring the gas-phase flow counter to capillarity-induced liquid motion. In this work, using multi-phase mixture (M²) formalism, the constant gas pressure assumption is relaxed and the effects of counter gas-flow are studied and found to be a new oxygen transport mechanism. Further, we analyze the multi-layer diffusion media, composed of two or more layers of porous materials having different pore sizes and/or wetting characteristics. Particularly, the effects of porosity, thickness and wettability of a micro-porous layer (MPL) on the two-phase transport in PEFC are elucidated.     

Recirculating feed operation of bipolar packed-bed and trickle-bed electrode cells equipped with mesh spacers

Unsteady state changes in ion concentration, liquid conductivity and electric current via different pathways in a bipolar packed-bed electrode cell were investigated by using the electrochemical reduction of Cu(II) ions. Ferrite pellets were used as particle electrodes and were packed in layers separated by plastic spacers. The electrode reaction rate was controlled by the diffusion of Cu(II) ions. The faradaic current passing through the pellets decreased with time, while the bypass current through the liquid bulk phase increased with time because of the production of hydrogen ions. The faradaic and bypass currents and the overall current efficiency were well simulated by a proposed reactor model. The overall current efficiency unavoidably decreased with time when the electrolyte was recirculated. However, the employment of trickle flow reduced the liquid hold-up in the bed and consequently reduced the bypass current. The overall current efficiency in the trickle-bed electrode cell was 20–30% higher than that in the flooded packed-bed electrode cell.     

Oberflächendiffusion auf oxidiertem Nickel

A porous electrode of sintered nickel powder was used as a separating diaphragm between two electrolyte-solution spaces and its flow resistance for the solution and the electrical diaphragm resistance was measured. By galvanostatic or potentiostatic oxidation the electrode was oxidized on the surface to Ni(OH)₂ and NiOOH, causing a decrease of pore volume indicated by an increase of flow resistance. In contrast under certain conditions the electrical diaphragm resistance simultaneously diminishes. Thus the electric current is now carried not only by the ions of the electrolyte in the pores of the electrode; there is now an additional charge transport, which is assumed to be proton diffusion on the oxidized nickel surface. The thickness of the diffusion layer, the diffusion coefficient and the temperature dependence of proton diffusion were measured.     

Electrochemical impedance spectroscopy evidence of dimethyl-silicon-oil enhancing O2 transport in a porous electrode

Faster oxygen transport is critical to guarantee reliable power output of polymer electrolyte membrane fuel cells (PEMFCs). In order to enhance oxygen transfer in a porous electrode especially in the case of water flooding, water-proof oil (dimethyl-silicon-oil (DMS)) was introduced into the conventional Pt/C electrode. Owing to the capability of electrochemical impedance spectroscopy (EIS) in discriminating individual contribution of ohmic, kinetic, and mass transport from all PEMFC processes, EIS was carried out to evaluate the effect of the DMS on the oxygen reduction reaction (ORR). The equivalent circuits corresponding to the EIS spectra were employed. The parameters in the equivalent circuits were obtained by curve fitting to the EIS spectra with the aid of the frequency response analysis software (FRA) attached in the electrochemical station Autolab PGSYAT302. The EIS analysis has shown that the introduction of DMS reduces the oxygen diffusion resistance as well as the charge transfer resistance in the flooded state. The single cell tests show that even in the case of normal operating condition the accumulated water with PEMFC operation also worsens the oxygen transfer in the conventional Pt/C gas diffusion electrode (GDE) with more and more water produced at the cathode. GDE containing DMS, which is defined as a flooding tolerant electrode (FTE), is fortunately quite good at alleviating water flooding. Success of the FTE in alleviating water flooding is ascribed to (1) its high oxygen transfer flux due to the higher solubility of oxygen in DMS than in water as long as parts of pores are occupied beforehand by DMS rather than by water, and (2) enhanced hydrophobic property of the FTE with DMS adsorption on the walls of the pores, which makes more hydrophobic pores be open to oxygen transport.     

An evaluation of the macro-homogeneous and agglomerate model for oxygen reduction in PEMFCs

The kinetics of oxygen reduction reaction on platinum/carbon powders in a Nafion film were evaluated with rotating disk electrode and gas diffusion electrode. The effects of the activation, mass transport and ohmic overpotentials were simulated via an “effectiveness factor” approach. The macro-homogeneous model was suitable to simulate the ORR kinetics at the RDE. On the other hand, it was found that the macro-homogeneous model does not simulate the operation of a porous gas diffusion cathode in PEMFC. With this model, the diffusion overpotential in the cathode is considerably overestimated. Conversely, the good agreement between calculated and experimental Tafel plots demonstrates the validity of the agglomerate model, even though the active layers of the PEMFC electrodes were thin and contained no PTFE. These results provided evidence for a two step transport process in the active layer of PEMFC electrodes.     

FACILITATED TRANSPORT OF HNO3 THROUGH A SUPPORTED LIQUID MEMBRANE CONTAINING A TERTIARY AMINE AS CARRIER

ABSTRACT

The facilitated transport of HN0₃ through a supported liquid membrane consisting of a porous polypropylene film containing a solution of trllaurylamine 1n diethyl benzene as carrier was studied as a function of the stirring speed of the aqueous solutions and the membrane composition. A physico-chemical model which takes Into account diffusion through an aqueous boundary layer, a fast Interfaclal chemical reaction leading to the formation of a membrane soluble alkylammonium salt and diffusion through the membrane was proposed. In this way, equations were derived which describe how composition changes, occurring In the course of the permeation process, Influence the membrane permeability. The experimental data were quantitatively explained by the derived equations. The results indicate that the monomerlc form of the trllaurylammonium nitrate salt 1s the species which 1s mainly responsible for the acid transport through the membrane. The diffusion coefficient of the permeating species and the order of magnitude of the thickness of the aqueous boundary layer were evaluated.     

Preparation of a Gas Diffusion Electrode by Ozone Gas Pretreatment and an Ion-exchange Method

Surface oxidation using ozone gas, produced by an electrolytic ozone generator, was applied for preparation of a gas-diffusion electrode (GDE) for an electrochemical energy conversion system. An uncatalyzed carbon sheet containing poly(tetrafluoroethylene) binder was first placed into contact with ozone gas to form active functional groups on the surface of the carbon; then ion-exchange between a weakly bound hydrogen of the functional groups and a platinum cation complex was performed. A GDE having highly dispersed particles of a platinum metal deposited on a porous carbon sheet ws developed by this method. The fuel cell using this GDE showed high performance.