High Catalyst Utilization Research Articles

Distribution of Nafion Ionomer in PEFC Electrodes with High Catalyst Utilization

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Electron Tomography of Nafion Ionomer Coated on Pt/Carbon Black in High Utilization Electrode for PEFCs

To confirm the superiority of newly developed electrocatalyst layer (ECL) for polymer electrolyte fuel cells, three-dimensional dispersion states of Nafion ionomer in Pt/carbon black agglomerates were analyzed by electron tomography based on multiple TEM images taken at different tilt angles. Uniform distribution of the ionomer has been first observed, proving the high catalyst utilization in the new ECL distinctive from that of the conventional one.

Preparation of High Catalyst Utilization Electrodes for Polymer Electrolyte Fuel Cells

We have developed a novel preparation procedure for an electrocatalyst layer with high utilization of catalyst for polymer electrolyte fuel cells. A commercial Pt catalyst supported on high surface area carbon black (Pt/CB) and Nafion ionomer solution was heated in an autoclave at 200 degrees C, followed by quenching to form the ink of the mixture. It was found that the cathode prepared with the new catalyst ink exhibited very high performance, i.e., high catalyst utilization and improved gas diffusivity. The microstructure analysis indicated that the autoclave treatment promoted an effective introduction of Nafion ionomer into primary pores of Pt/CB agglomerates, in which ca. 90% of Pt catalysts were supported. It was clearly observed by scanning transmission electron microscopy that Nafion ionomer was distributed more uniformly inside Pt/CB agglomerates, compared with those simply mixed with a ball mill in a conventional manner.

Gas diffusion electrodes for polymer electrolyte fuel cell using sulfonated polyimide

Gas diffusion electrodes for high temperature polymer electrolyte fuel cells (PEFCs) have been prepared by using a novel proton conductive sulfonated polyimide (SPI) electrolyte. The catalyst layer was composed of Pt-loaded carbon black (Pt-CB) and SPI ionomer. The polarization properties and the microstructure of the catalyst layer were investigated as a function of the SPI/CB weight ratio. The anodic polarization was found to be negligibly small for all the compositions examined. The highest cathode performance was obtained at SPI/CB = 0.5 (by weight), where the best balance of high catalyst utilization and oxygen gas diffusion rate through the ionomer was obtained.

Catalyst Improvement of Utilization for Direct Methanol Fuel Cell Using Silane Coupling Agents

A method to prepare improved Pt-based catalyst for direct methanol fuel cell is described in this study. Silane coupling agents (3-aminopropyl) trimethoxysilane (APTMS) (3-glycidyloxy propyl) trimethoxysilane, and N-[3(trimethoxysilyl) propyl] ethylenediamine (3-glycidyloxy) were used to graft perfluorosulfonic ionomer on Pt to stabilize the dispersion and therefore to obtain an electrode with fine Pt cluster and high catalyst utilization. From extended X-ray absorption fine structure examination, Pt nano-particle can be successfully confined to smaller size with the help of APTMS. Polarization test shows improved membrane electrode assembly (MEA) performance while using APTMS. From hydrogen adsorption/desorption cyclic voltammotric analysis, about 15% catalyst utilization increase can be achieved.

Numerical analysis of Pt utilization in PEMFC catalyst layer using random cluster model

A random cluster model was proposed to simulate the catalyst layer of PEMFC. The cluster model consists of a random distribution of three kinds of particles, i.e., Pt/C catalyst, Nafion and poly-tetra-fluoro-ethylene (PTFE), which were generated on a computer by means of Monte Carlo method. Based on such a cluster model, the catalyst utilization was calculated through counting the number of Pt/C clusters and Nafion particles clusters. It was assumed that the effective Pt/C clusters are those that not only have electron channels via carbon particles to current collector but also have proton channels via Nafion polymer particles to the Nafion membrane. The factors influencing catalyst utilization was thoroughly discussed. For the case of high catalyst utilization, numerical results showed that there is a threshold for ratio of Pt/C catalyst loading to Nafion. Beyond this threshold, the catalyst utilization may drop dramatically. The results also showed that the Pt/C catalyst with higher Pt content could allow a larger range of the ratio of the Pt/C catalyst loading to Nafion. Generally, there is high catalyst utilization around the ratio of 1. Results also showed that the lower the Teflon loading in the catalyst layer, the higher the catalyst utilization will be. However, the Pt/C catalyst with higher Pt content can tolerate relatively high Teflon loading than that with a lower Pt content.

Properties of Gas Diffusion Electrodes Containing Sulfonated Poly(ether ether ketone)

The use of sulfonated poly(ether ether ketone) as the proton-conducting medium within the electrocatalytic layer of fuel cell gas diffusion electrodes (GDEs) is described. An electrochemically active surface area determined by cyclic voltammetry and electrochemical impedance spectroscopy was studied as a function of ion exchange capacity. Maximum activity is related to the concentration of sulfonate groups within the catalyst layer. Despite their relatively high electrochemical surface area and catalyst utilization, fuel cell performance is rather poor compared to Nafion-based GDEs. Fuel cell performance is greatly enhanced when polytetrafluoroethylene (PTFE) is incorporated into the catalyst layer. The addition of PTFE results in GDEs with larger pore diameters that are less prone to flooding. The incorporation of an ionomer of similar chemical structure to the proton-conducting membrane employed is shown to reduce interfacial resistance. Electrode structures of this type may prove useful for investigating other novel proton-conducting materials.

Structure and performance of different types of agglomerates in cathode catalyst layers of PEM fuel cells

In this work, we investigate transport and reaction kinetics in agglomerates of cathode catalyst layers used in proton-exchange membrane fuel cells. Two types of spherical agglomerates are evaluated, which represent limiting structures that can be obtained by distinct synthetic procedures. One type consists of a mixture of carbon/catalyst particles and a proton conducting perfluorosulfonated ionomer (PFSI). The other type consists of carbon/catalyst particles and water-filled pores. The performance of the former type is rationalized on the basis of the well-known Thiele-modulus. Characteristics of the latter type are studied using Nernst–Planck and Poisson equations. Aspects of current conversion, reactant and current distributions and catalyst utilization are explored. In general, the PFSI-filled agglomerates exhibit more homogeneous distributions of reaction rates. Effectiveness factors for these are close to one. However, it was found that proton penetration depths in water-flooded agglomerates could be quite substantial under certain conditions, resulting in unexpectedly high catalyst utilization. The effects of agglomerate radius and of boundary conditions on the agglomerate surface are studied. An approximate analytical solution was obtained for a planar geometry of agglomerates. The significance of these results for the optimization catalyst layers in the light of operation conditions and synthesis methods is discussed.

The effect of the MEA preparation procedure on both ethanol crossover and DEFC performance

In the present work, the changes of Nafion ®-115 membrane porosity in the presence of ethanol aqueous solutions of different concentrations were determined by weighing vacuum-dried and ethanol solution-equilibrated membranes. It was found that membrane porosity increases as ethanol concentration increases. Membrane electrode assemblies (MEAs) have been prepared by following both the conventional and the decal transfer method. The ethanol crossover through these two MEAs was electrochemically quantified by a voltammetric method. A 10 h stability test of direct ethanol fuel cell (DEFC) at a current density of 50 mA cm −2 was carried out. It was found that the electrode preparation procedure has an obvious effect on ethanol crossover and direct ethanol fuel cell's performance and stability. The single DEFC test results showed that about 15 and 34% of the original peak power density was lost after 10 h of life test for the MEAs prepared by the decal transfer method and the conventional method, respectively. Electrochemical impedance spectrum (EIS) results of the MEAs showed that, in the case of the membrane electrode assembly prepared by the following decal transfer method, the internal cell resistance was almost the same, 0.236 Ω cm 2 before the life test and 0.239 Ω cm 2 after 10 h of life test, while the respective values for the membrane electrode assembly by the conventional method are 0.289 and 0.435 Ω cm 2. It is supposed that the improved cell performance with MEA by the decal transfer method could be resorted to both a better contact between the catalyst layer and the electrolyte membrane and higher catalyst utilization. Furthermore, based on the experimental results, the increased internal cell resistance and the degraded single DEFC performance could be attributed to the delamination of the catalyst layer from the electrolyte membrane.

Preparation of Membrane Electrode Assembly for Fuel Cell by Using Electrophoretic Deposition Process

A membrane electrode assembly (MEA) was prepared by electrophoretic deposition (EPD) process onto Nafion 117 membrane. A suspension consisting of ethanol, carbon powders with Pt catalyst, and Nafion polymer was used in the EPD process to obtain a stable dispersed solution. A thickness of the prepared catalyst layer was easily controlled by EPD duration or concentration of suspension. The prepared catalyst layer was well attached to the Nafion membrane. This layer was similar to that prepared by a hot press method. However, the catalyst layer consisted of only fine carbon particles was smaller and more uniform than that prepared by a hot press method. Such a uniform structure provided a high utilization of Pt catalyst. A 56% utilization of Pt catalyst loaded to the catalyst layer was successfully achieved by using the EPD method. A hydrogen-oxygen fuel cell was constructed using the MEA prepared by the EPD process. The cell exhibited a higher electrochemical performance, compared with ordinary cells. © 2004 The Electrochemical Society. All rights reserved.

Structure and performance of different types of agglomerates in cathode catalyst layers of PEM fuel cells

In this work, we investigate transport and reaction kinetics in agglomerates of cathode catalyst layers used in proton-exchange membrane fuel cells. Two types of spherical agglomerates are evaluated, which represent limiting structures that can be obtained by distinct synthetic procedures. One type consists of a mixture of carbon/catalyst particles and a proton conducting perfluorosulfonated ionomer (PFSI). The other type consists of carbon/catalyst particles and water-filled pores. The performance of the former type is rationalized on the basis of the well-known Thiele-modulus. Characteristics of the latter type are studied using Nernst–Planck and Poisson equations. Aspects of current conversion, reactant and current distributions and catalyst utilization are explored. In general, the PFSI-filled agglomerates exhibit more homogeneous distributions of reaction rates. Effectiveness factors for these are close to one. However, it was found that proton penetration depths in water-flooded agglomerates could be quite substantial under certain conditions, resulting in unexpectedly high catalyst utilization. The effects of agglomerate radius and of boundary conditions on the agglomerate surface are studied. An approximate analytical solution was obtained for a planar geometry of agglomerates. The significance of these results for the optimization catalyst layers in the light of operation conditions and synthesis methods is discussed.

Gas diffusion electrodes for polymer electrolyte fuel cells using borosiloxane electrolytes

Gas diffusion electrodes for high temperature polymer electrolyte fuel cells (PEFCs) have been prepared by using a proton conductive borosiloxane (BSO) electrolyte. The catalyst layer was composed of Pt loaded carbon black (Pt-CB) and BSO ionomer. The polarization properties and the microstructure of the catalyst layer were investigated as a function of the composition. The anodic polarization was found to be negligibly small for all the compositions examined. The highest cathode performance was obtained at BSO/CB=1.5 (by weight), where the best balance of high catalyst utilization and oxygen gas diffusion rate through the ionomer was obtained.

Surface Area Loss of Supported Platinum in Polymer Electrolyte Fuel Cells

Life tests of polymer‐electrolyte fuel cells using supported Pt catalyst in thin film catalyst layers are run for up to 4000 h at maximum power. Particle ripening is readily evident using these types of electrodes in which the high catalyst utilization efficiency apparently subjects the majority of the platinum to conditions that sustain particle growth. X‐ray diffraction analyses indicate that the initial platinum specific surface areas of 100 m2/g Pt eventually stabilize to about 40 to 50 m2/g in the cathode and 60 to 70 m2/g in the anode. Interestingly, this loss in surface area does not affect the apparent catalytic activity of these fuel cell electrodes. A crystallite migration particle growth mechanism is suggested by the shape of the particle size distribution curves. Since the presence of liquids is known to lower the activation energy for particle growth, the particle size difference between the two electrodes may possibly be attributable to the different hydration levels at the anode and the cathode in operating polymer electrolyte fuel cells.