Polymer Electrolyte Fuel Cell Cathode Research Articles

Effect of Agglomeration of Pt/C Catalyst on Hydrogen Peroxide Formation

Various amounts of 20 wt % Pt/C catalysts (56.7-5.7 μg c a r b o n cm - 2 ) were loaded on glassy carbon (GC) disk electrode, and the effect of agglomeration on hydrogen peroxide formation in oxygen reduction was investigated by the rotating ring-disk electrode technique. The formation of H 2 O 2 was enhanced with a decrease in agglomeration of Pt/C. Even in the operating potential range of polymer electrolyte fuel cell cathodes (0.6-0.8 V), 10% hydrogen peroxide was formed at 5.7 μg c a r b o n cm - 2 Pt/C loaded on GC. These results revealed that series two-electron reduction pathway, which is negligible on clean bulk Pt surface, does exist on Pt particles supported on carbon.

Performance equations of polymer electrolyte fuel cells

Abstract Performance equations that describe the dependence of cell potential on current density for polymer electrolyte fuel cells (PEFCs) have been developed in algebraic form. The equations are derived from the reduction of a one-dimensional model that takes into account, in detail, the limitations of reactant transport, proton migration and electron conduction, as well as electrochemical reactions within the cathode, anode and membrane electrolyte. Reduction of the one-dimensional model is implemented by approximating the profiles of reactant concentration and ionomer potential with appropriate functions and by lumping the overall reaction rates at the reaction centres of the catalyst layers. Since the performance equations originate from a mechanistic one-dimensional model, all parameters appearing in the equations have a precise physical significance. In addition, individual potential losses caused by the various limiting processes can be clearly quantified in the equation. Particularly, the equations for potential losses relevant to the anode limiting processes are first revealed by the present work. Thus, they can be used as a diagnostic tool for PEFC performance. Computational results show that the performance equations agree well with the original one-dimensional model over an extensive parameter range. The present performance equations allow for an efficient evaluation of PEFC performance since the complexities of the one-dimensional model and the procedures for the numerical solutions are completely avoided. As compared with previously developed performance equations dealing with PEFC cathodes only, the present equations are able to provide accurate interpretation on the polarization behaviour of a complete PEFC.

Transient Techniques for Investigating Mass-Transport Limitations in Gas Diffusion Electrodes : II. Experimental Characterization of the PEFC Cathode

The current-interrupt technique and electrochemical impedance spectroscopy were employed in order to study the behavior of a polymer electrolyte fuel cell (PEFC) cathode containing 30 wt % Nafion and 70 wt % Pt/C. The steady-state polarization curves were also recorded. The experimental results were analyzed with help of the mathematical models developed in Part I of this paper. The effect of a varying oxygen pressure and humidity on the dynamic response of the cathode was investigated. The double-layer capacitance, Tafel slope, oxygen solubility, a group containing the effective diffusion coefficient and agglomerate size, and finally, the effective proton conductivity in the cathode were obtained. The parameter values were reasonable and attest the robustness of the agglomerate model for describing the PEFC cathode. At low humidity, a second, low-frequency loop was observed that was attributed to the membrane behavior. © 2003 The Electrochemical Society. All rights reserved.

Analysis of a Model for Multicomponent Mass Transfer in the Cathode of a Polymer Electrolyte Fuel Cell

A chief factor that is thought to limit the performance of polymer electrolyte fuel cells (PEFCs) is the hydrodynamics associated with the cathode. In this paper, a two-dimensional model for three-component (oxygen, nitrogen, water) gaseous flow in a PEFC cathode is derived, nondimensionalized, and analyzed. The fact that the geometry is slender allows the use of a narrow-gap approximation leading to a simplified formulation. In spite of the highly nonlinear coupling between the velocity variables and the mole fractions, an asymptotic treatment of the problem indicates that oxygen consumption and water production can be described rather simply in the classical lubrication theory limit with the reduced Reynolds number as a small parameter. In general, however, the reduced Reynolds number is O(1), requiring a numerical treatment; this is done using the Keller--Box discretization scheme. The analytical and numerical results are compared in the limit mentioned above, and further results are generated for varying inlet velocity and gas composition, channel width and porous backing thickness, pressure and current density. Also, a novel, compact way to present fuel cell performance, which takes into account geometrical, hydrodynamical, and electrochemical features, is introduced.

Analysis of Oxygen Reduction on Pt Microelectrode with Polymer Electrolytes of Various Exchange Capacities

In order to obtain an improved design of a polymer electrolyte fuel cell cathode, the oxygen reduction reaction on a Pt microelectrode with proton exchange membranes of various exchange capacities has been investigated. The measurements were performed by chronoamperometry for determination of the diffusion coefficient and oxygen solubility, and by slow scan voltammetry for reaction kinetics with the microelectrode in the temperature range 20 to 70°C. The results of chronoamperometry and slow scan voltammetry were analyzed by the fitting of analytic equations and digital simulation. The diffusion coefficients increased with temperature and exchange capacity. On the other hand, the solubilities increased with a decrease in temperature and exchange capacity. The dependences of the diffusion coefficient on temperature and exchange capacity were larger than those of the solubility. Accordingly, the permeabilities, which are the product of the diffusion coefficient and the solubility, increased with temperature and exchange capacity. Although the dependence of the exchange current densities on the exchange capacity was not clear, the exchange current densities were to in the high potential region and about in the low potential region. © 2002 The Electrochemical Society. All rights reserved.

Pt Inclusion Compounds as Oxygen Reduction Catalysts in Polymer‐Electrolyte Fuel Cells

Pt inclusions in high‐specific‐area graphite (HS300, from Lonza) and Vulcan (XC‐72R, from Cabot) have been synthesized by the reduction of , salts intercalated in these hosts. These materials perform well as catalysts for the reduction of oxygen in polymer‐electrolyte fuel cells (PEFCs). After an activation time of 48 h during which the cathode is held at 0.5 V vs. the reference hydrogen electrode, it is found that catalysts based on Pt inclusions and commercial catalysts of the same Pt content and loading have similar mass and specific activities. However, in the same conditions, catalysts based on Pt inclusions perform better in the high‐current region than the commercial catalysts. It has been shown by X‐ray photoelectron spectroscopy analysis that, at the beginning of the activation time, a fraction of the Pt oxides and/or hydroxides present in the materials is reduced to Pt metal. This reduction improves the catalytic activities of the materials prepared in this study and also those of commercial catalysts. Further improvement is probably due to hydration of the catalyst layer and/or the washing out of insidious contaminants (e.g., chloride ions). Because higher current densities can be obtained with Pt included in graphite catalyst at a loading of 0.13 mg Pt/cm2 than with commercial Pt supported on Vulcan catalyst at a loading of 0.15 mg Pt/cm2, the use of cathode catalysts containing Pt inclusions is therefore a way of reducing the Pt loading at the cathode of PEFCs. However, a limitation of the new materials is the fact that the maximum amount of Pt that can be included in Vulcan is only about 5 wt %, whereas it is only 10 wt % in graphite.