Cathode Diffusion Layer Research Articles

Tubular Cathode Prepared by a Dip‐Coating Method for Low Temperature DMFC

AbstractA novel tubular cathode for the direct methanol fuel cell (DMFC) is proposed, based on a tubular titanium mesh. A dip‐coating method has been developed for its fabrication. The tubular cathode is composed of titanium mesh, a cathode diffusion layer, a catalyst layer, and a recast Nafion® film. The titanium mesh is present at the inner circumference of the diffusion layer, while the recast Nafion® film is at the outer circumference of the catalyst layer. A DMFC single cell with a 3.5 mgPt cm–2 tubular cathode was able to perform as well, in terms of power density, as a conventional planar DMFC. A peak power density of 9 mW cm–2 was reached under atmospheric air at 25 °C.

Performance/design formulation for a solid polymer based acid electrolyte hydrogen/air fuel cell

Mathematical development of preliminary performance/design equations for a hydrogen/air, solid polymer acid electrolyte based fuel cell is presented. The development is based on the principles of transport phenomena, intrinsic electrochemical kinetics, and classical thermodynamics. The developed formulation is intended to quantitatively describe the mass fraction profiles of the chemical species, hydrogen and oxygen, in the cell anode and cathode diffusion and electrocatalytic reaction layers as a function of the distance in the proton transport direction at an axial distance parallel to the cell anode or cathode channel flow. Given the cell geometry, chemical species and charge transport, and intrinsic electrochemical kinetic parameters, the developed formulation can be employed to compute the species local mass fluxes and predict the cell anode and cathode cell overvoltages for a desired geometric current density. The presented single cell performance predictive formulation has also been linked to the formulation needed to predict the performance of a stack of a number of identical PEMFCs connected in series.

A mathematical model for simulating methanol permeation and the mixed potential effect in a direct methanol fuel cell

A mathematical model for simulating methanol permeation and the pertinent mixed potential effect in a direct methanol fuel cell (DMFC) is presented. In this model a DMFC is divided into seven compartments namely the anodic flow channel, the anodic diffusion layer, the anodic catalyst layer, the proton exchange membrane (PEM), the cathodic catalyst layer, the cathodic diffusion layer and the cathodic flow channel. All compartments are considered to have finite thickness, and within every one of them a set of governing equations are given to stipulate methanol transport and oxygen transport. For the flow channels, fluid dynamics, which could substantially lower the local methanol concentration within catalyst layers is taken into account. With the knowledge of local concentrations of the species, the electrochemical reaction rates within both catalyst layers can be quantified by a kinetic Tafel expression. For the anodic catalyst layer the local external current generated by methanol oxidation is computed; for the cathodic catalyst layer, in addition to the local external current generated by oxygen reduction, the local internal current as a result of methanol permeation is also computed. With the information of the local internal current, the mixed potential effect, which is responsible for adversely lowering the cell voltage can be analyzed.

A tubular direct methanol fuel cell with Ti mesh anode

A novel tubular cell structure for a direct methanol fuel cell (DMFC) is proposed based on a tubular Ti mesh and a Ti mesh anode. A dip coating method has been developed to fabricate the cell. The characterization of the tubular MEA has been analyzed by scanning electron microscopy (SEM), energy dispersive X-ray (EDX), half cell and single cell testing. The tubular DMFC single cell comprises: a Ti mesh, a cathode diffusion layer and catalyst layer, a Nafion recast membrane and a PtRuO x /Ti anode. Half cell tests show that the optimum catalyst loading, Ru/(Ru + Pt) atomic ratio and the Nafion loading of a PtRuO x /Ti mesh anode are: 4 mg cm −2, 38% and 0.6 mg cm −2, respectively. Single cell tests show that the Nafion loading of the recast Nafion membrane and the concentration of the methanol in the electrolyte have a major influence on cell performance.

Effect of operational parameters of mini-direct methanol fuel cells operating at ambient temperature

There is currently increased interest in small-size direct methanol fuel cells for portable applications. This work presents results of the influence of operational parameters on the performance of a mini-direct methanol fuel cell. The effects of methanol concentration, Pt load, membrane thickness and PTFE content in the cathode diffusion layer on the performance were studied. Two anodic materials were prepared, PtRu 75:25 at.% and PtRu 90:10 at.%, as nanoparticles supported on Vulcan XC-72 carbon, while for the cathodes Pt/C E-TEK catalysts were used. The materials were characterized physically by EDX and DRX and electrochemically in a half-cell. The results with single cells showed better performances with cells operating with 3 mg Pt cm−2, 5 mol l−1 methanol solution, Nafion® 112 membrane and with 30 wt.% PTFE in the cathode diffusion layer deposited on only one face of the electrode support.

A mathematical model for PEMFC in different flow modes

A two-dimensional, steady state model for proton exchange membrane fuel cell (PEMFC) is presented. The model is used to describe the effect of flow mode (coflow and counterflow), operation conditions and membrane thickness on the water transport, ohmic resistance and water distribution in the membrane, current density distribution along the channel and performance of PEMFC. Effect of liquid water on the transport in the two-phase region of cathode diffusion layer was considered. Water transport in the membrane by electro-osmosis drag, diffusion and convection were combined in this model. The model predicts that the dry reactant gases can be well internally humidified and maintain high performance when PEMFC is operated in the counterflow mode without external humidification. Counterflow mode does not show any advantageous while the reactant gases are high humidified or saturated. Compared to the coflow mode, counterflow mode improves the current density distribution with dry or low humidity gases. The higher the anode is humidified, the more water will migrate from anode to cathode. The modeling results compare very well with experimental results.

Influence of heat transfer on gas and water transport in fuel cells

An analysis of transport phenomena in a proton exchange membrane fuel cell (PEMFC) is presented, with a focus on the modelling and assessment of non-isothermal and non-isobaric effects that have been neglected in previous studies. A model is formulated for a complete fuel cell taking into account diffusion through the porous electrodes of the humidified fuel (H 2, CO 2 and H 2O ( v) ) and oxidant gases (O 2, N 2 and H 2O ( v) ); the convective and electro-osmotic transport of liquid water in the electrodes and the membrane; and heat generation and transfer in the fuel cell. The thermodynamic equilibrium potential is calculated using the Nernst equation, and reaction kinetics is determined using the Butler–Volmer equation. Non-uniform distribution of gas pressure in the porous gas-diffusing electrodes and micro-hydrodynamics in very small pores (Knudsen diffusion) are also taken into account. The model is solved numerically to analyze fuel cell performance and water transport over a range of operating current densities. Non-uniform temperature and pressure distributions are found to have a large impact on the predicted liquid water and vapour fluxes in the anode and cathode diffusion layers. In particular, the results indicate that water management requirements (i.e., humidification or water removal) to prevent potential membrane dehydration or electrode flooding are much more conservative than predicted assuming isothermal conditions. Finally, it is found that, in the range of permeabilities of the porous electrodes used in PEMFCs (10 −16–10 −17 m 2), Knudsen diffusion has to be taken into account in modelling gas transport.

On the zinc-nickel alloy electrodeposition mechanism

It is shown that during the zinc–nickel alloy electrodeposition in sulfate–chloride electrolytes, the predominant zinc deposition into the alloy can be caused by the generation of a film in the cathode diffusion layer formed of highly-disperse colloid zinc compounds.

The influence of boric acid and sulfate ions on the hydrogen formation in NiFe plating electrolytes

The current efficiency for electrodeposition of iron group metals and alloys is limited by concurrent hydrogen evolution. In the present paper the effect of dissociation equilibria in the electrolyte on the rate of the hydrogen formation is studied both experimentally and theoretically. Cathodic polarization experiments are performed in sulfate, chloride and perchlorate electrolytes of pH 3 to determine the limiting current for proton reduction. The effect of boric acid on the pH at the cathode surface is studied for sulfate electrolytes with and without nickel. Experimental results and theoretical simulations using a finite difference code were used to identify the relative importance of different electrolyte equilibria for the value of the limiting current for proton reduction and for the apparent onset of the reduction of water. The presence of boric acid leads to a release of protons in the cathodic diffusion layer and as a result, the potential region of proton reduction is extended to more negative potentials. This retards the pH increase at the cathode surface associated with the onset of water reduction. As a consequence, plating of iron group metals can be performed within a wider potential range without interference by hydroxide precipitation.

Reduction of Ti IV species in aqueous hydrochloric and sulfuric acids II. ECE model of the behaviour in sulfate media

The reduction kinetics of Ti IV species in aqueous H 2SO 4 and HCl was studied using an amalgamated lead rotating disc electrode. Whereas a one electron Ti IV reduction mechanism was operative in aqueous HCl, Ti IV reduction in aqueous H 2SO 4 appeared to involve a Ti IV-Ti III-sulfate intermediate, which absorbed visible light strongly, and probably proceeded via an ECE mechanism. A potentiostatically controlled amalgamated lead plate cathode was used to reduce Ti IV species in acidic sulfate electrolytes in a membrane-divided reactor operated in batch recycle mode with a continuously stirred tank reservoir. The kinetic behaviour was modelled mathematically as an ECE process, both with a second order irreversible chemical reaction and with a chemical equilibrium between the Ti IV-Ti III-sulfate intermediate, Ti IV and Ti III. Analysis of the predicted concentration profiles in the cathode diffusion layer showed that the mass transport controlled Ti IV reduction current density was independent of the rate of the second order chemical reaction, enabling the Ti IV diffusion coefficient to be extracted from rotating disc electrode data, but precluding derivation of kinetic information. The concentration-time and current density-time behaviour of the ECE reaction in a plug flow reactor operated in batch recycle mode with a continuously stirred tank reservoir was predicted for various values of the chemical reaction rate coefficient. For an irreversible chemical reaction, the concentration of the intermediate was greater, the larger the value of the chemical rate coefficient for formation of the intermediate. Predicted current densities were initially smaller, and at longer times, greater, than predicted for a simple, mass transport controlled process.

Pulsed electrodeposition of zinc in magnetic fields: diffusion layer relaxation followed by laser interferometry

The pulsed galvanostatic deposition of zinc was carried out in the presence and absence of an externally applied magnetic field of 0.43 T. The concentration time profiles and the relaxation of the cathodic diffusion layer during the pulsing sequence have been determined by using laser interferometry. When, in the pulsing mode, longer ‘off’ periods compared to the ‘on’ periods were used, significantly greater relaxation of the diffusion layer was observed. The fringe shift time profiles for the electrodes kept in the vertical or anode over cathode positions are influenced by the hydrodynamic effect; the magnetic field effect on the relaxation of the diffusion layer has been observed. The electrodeposited Zn has been analysed by scanning electron microscopy; the Zn deposit obtained by pulsed current electrolysis in the cathode over anode position is dense and uniform.

Concentration Profile of CuSO<sub>4</sub> in the Cathodic Diffusion Layer during Unsteady Copper Electrodeposition

Concentration Profile of CuSO<sub>4</sub> in the Cathodic Diffusion Layer during Unsteady Copper Electrodeposition

Profile of the Refractive Index in the Cathodic Diffusion Layer of an Electrolyte Containing CuSO4 and H 2 SO 4

The profile of the refractive index in the cathodic diffusion layer of a solution containing and was measured by holographic interferometry. Measurement was carried out at various heights from the lower edge of the cathode and also at various cathodic current densities up to the limiting value. The theoretical expression for the difference of the refractive index between the cathode surface and the bulk electrolyte was derived assuming a uniform distribution of the local cathodic current density. Experimental results obtained at the current densities below one‐half of the limiting value were in good agreement with the theoretical value. Measurement was also conducted with the solutions containing and , respectively, at their cathodic limiting current densities. concentration at the cathode surface was estimated from the interferogram and concentration at the cathode surface was presumed to be virtually zero at the limiting current. The estimated concentration was compared with the theoretical values, and it was found that they are in good agreement with each other.

Concentration Profile of CuSO4 in the Cathodic Diffusion Layer

The concentration profile of in the cathodic diffusion layer which is accompanied by the upward natural convection along the plane vertical cathode was measured by holographic interferometry in aqueous solutions containing. The measurement was conducted at various heights from the lower edge of cathode. The cathodic current density was also varied. The theoretical expressions of the concentration difference between the cathode surface and the bulk electrolyte and the thickness of the diffusion layer were derived under the assumption of uniform cathodic current density distribution. Experimental results obtained at the current densities far lower than the limiting value were in good agreement with the theoretical value.

Change in pH near the Cathode During the Electrodeposition of a Bivalent Metal. Analysis

An important factor in the cathodic electrodeposition of a base metal (M) is the increase in pH (“alkalization”) of the electrolyte near the cathode.Chemical equilibria and transport processes in the cathodic diffusion layer are analyzed by numerical integration of the transport equations. The analysis shows that alkalization is caused by the production of OH− at the cathode and is prevented when the hydroxyl ions so produced react to form a soluble species (e.g., MOH+ or ).When complex ions are not formed, alkalization occurs if , where is the hydrogen reduction current, δ is the thickness of the diffusion layer, is the bulk hydrogen ion concentration, and is the ionic diffusion coefficient for hydrogen. Formation of metal‐hydroxy or metal‐anion complexes can prevent alkalization even when this limit is exceeded. This is illustrated by specific numerical examples.