Novel carbon aerogel-supported catalysts for PEM fuel cell application

https://doi.org/10.1016/j.ijhydene.2004.04.014Get rights and content

Abstract

Novel carbon aerogel supported Pt catalysts with different pore size distributions and Pt content have been synthesized and tested in a proton exchange membrane fuel cell (PEMFC) operation. Characterization of the aerogel supported Pt catalyst has been performed in respect to the total surface area of Pt using HRTEM and BET methods, and was compared to the electrochemical surface area of Pt in a cathode layer of the PEMFC by means of cyclic voltammetry. The effect of pore size distribution of the novel aerogel supported Pt catalyst on the performance of the PEMFCs, and kinetic parameters of the catalysts at different temperatures, is discussed in terms of the microstructure of the support and perfluorosulfonate-ionomer distribution. The PEMFC with a low Pt loading (0.1mg/cm2) of a new porous aerogel catalyst has shown high power densities up to 0.8mW/cm2 in fuel cell operation conditions in air and at ambient pressure.

Introduction

Tremendous effort has been made in the past 10 years to develop catalysts for proton-exchange membrane fuel cells (PEMFCs). It has been shown [1], that PEMFCs can be used as very efficient chemical energy converters in vehicles. They particularly suite automotive applications primarily because of their relatively low operating temperature, high efficiency, and high power density, which is superior to existing internal combustion technology [2]. A remaining problem is the high production cost for the fuel cell units, which, in comparison to the internal combustion engine, is far more expensive per kW [3]. Cost reduction can be achieved by using more effective unsupported or supported catalysts with low precious metal loadings [1], [4].

Fuel cell reactions on the surface of the catalyst involve both electrons and protons, which require high electrical and proton conductivity respectively. The incorporation of the proton conductor, NafionTM, significantly increases the membrane conductivity in the catalyst layer and improves the catalyst utilization [5].

It is well known that the catalytic activity of the catalyst depends upon the contact area between the catalyst and the electrolyte, and that only the catalyst in contact with the electrolyte is active. Therefore, the pore size of the supported catalyst available for electrolyte molecules could be very significant in achieving high electrochemical catalytic activity.

The platinum catalyst supported on high surface area carbon, e.g. Vulcan XC 72, contains primary pores, which cannot be filled with the polymer electrolyte because of their small size in comparison to the size of a single polymer particle [6]. This does not allow for the efficient use of the precious metal catalyst, thus decreasing the cell performance, and increasing its cost.

The size of the pores cannot be changed in common carbon used as a support for PEMFC catalysts, however it can be changed in aerogels. Aerogel materials possess a wide variety of exceptional properties, which include controlled chemical synthesis and design of structure and porosity of organic and inorganic materials [7]. The porosity of the aerogel can be changed via chemical synthesis modification with further distribution of Pt particles on its surface allowing good contact with the ionically conducting polymer electrolyte.

In this paper, we describe the behavior of carbon aerogel-supported Pt catalysts. These supports are produced by aqueous polycondensation of resorcinol with formaldehyde [8] followed by pyrolysis to form the porous carbon network structure. The metallic catalyst is introduced into via organometallic precursors using supercritical carbon dioxide, followed by secondary pyrolysis to form Pt nanoparticles on the surface of aerogel carbon support. Details of these processes and the characteristics of the structures which result have been presented elsewhere [9].

The paper outlines the progress that has been made on “in situ” characterization of novel aerogel supported catalysts in PEMFC operation. The goal of this work was to develop, synthesize, and characterize catalysts with various pore size distributions, Pt content, and Pt surface area, which can be used in PEMFCs.

The cell performance for the membrane electrode assemblies (MEAs) under different operating conditions will be discussed in relation to the aerogel structure, composition, and Pt loadings in the cathode catalyst layer ranging from 0.6 to 0.06mg/cm2. The results of the electrochemical surface area calculations (ESA) calculations for aerogel-supported catalysts used in this work will be compared with similar data obtained for commercially available supported and unsupported catalysts.

Section snippets

Characterization of the aerogel supported Pt catalysts

Two aerogel-supported Pt catalysts have been used for the manufacture of cathode catalyst layers. The catalysts differ by Pt content (37% sample 1 and 20% Pt sample 2) and porosity (16 and 22nm), estimated by the Brunauer–Emmett–Teller (BET) method. Adsorption and desorption isotherms of nitrogen were measured using a Sorptomatic 1990, a fully automatic Gas Adsorption Analyzer from Horiba. The mesopore size distributions and mesopore volumes were estimated for the pore diameter range 2–50nm by

Structure and properties of the aerogel supported Pt catalyst

Morphological characterization and estimation of nano-scale structure of two aerogel supported Pt samples used in this work have been made after each stage of catalyst ink preparation. The size of the Pt particles after grinding was still too big for the preparation of a catalyst paste and corresponded to ca. 100–150μm. Consequently, this procedure was followed by continuous homogenization with short intermittent stirring to prevent the material from settling. As a result, the size of the

Conclusions

Aerogel supported Pt catalysts have been successfully applied in the preparation of cathode catalyst layers using screen-printing technique and tested as cathode catalysts in PEMFCs at ambient pressure, and fully saturated conditions. The novel catalysts demonstrated good catalytic properties, relatively high OCV, close to theoretical Tafel slope, and high electrochemical surface area, which is twice higher than the ESA of the commercial catalysts.

The PEMFC performance has been estimated for

Acknowledgements

The authors acknowledge Aerogel Composite, LLC that supported this work through the Grant with Connecticut Global Fuel Center.

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