Stainless Steel Wire Mesh Flowfields For Polymer Electrolyte Fuel Cells

Christine Zawodzinski, Mahlon S. Wilson and Shimshon Gottesfeld Electronic and Electrochemical Materials and Devices Research Group, MS D429 Los Alamos National Laboratory, Los Alamos, N.M. 87545

Introduction

The advantages offered by the PEM fuel cell, efficient power generation with little or no environmental emissions, low operating temperature, and non-liquid/non-corrosive electrolyte, make it attractive as a potential power source for transportation and for portable and stationary power generation applications. Fuel cells have been successfully demonstrated in a number of aerospace, utility and military applications, however, the high cost of fuel cells compared to conventional power generation technologies has delayed their potential widespread use. Stack manufacturers have historically used high-platinum loading membrane/electrode assemblies (MEAs) and intricately machined graphite bipolar plates, which have made the cost too high for most commercial applications. We have thus focused our efforts on decreasing the cost of these components in order to demonstrate an inexpensive, yet high performance PEM fuel cell. Here, we describe the design and demonstration of a 100 cm2 (active area) cell that utilizes ultra-low platinum loading MEAs and inexpensive, stainless steel wire screen flow-fields.

Membrane/Electrode Assembly

Our efforts in the design of this cell involved adapting ultra-low platinum loading technology developed at Los Alamos [1] to reproducibly fabricate larger-scale, high performance MEAs. This was achieved using a computer-controlled chart-recorder process that has been previously described [2], A typical MEA consists of a 100 cm2 active area catalyzed Nation™ membrane (Nafion 112 from DuPont) and two E-TEK (Natick, MA) carbon cloth backings. The platinum loading is approximately 0.14 mg Pt/cm2/electrode, which corresponds to a dramatic decrease in total stack platinum content and significantly reduces the cost of the MEA.

FLOW-FELD FNE

SCREEN SCREEN

FLOW-FELD FNE

SCREEN SCREEN

Figure 1. Components of a unit cell utilizing wire-screen flow-fields, metal foil separators, and a Picture Frame MEA.

Wire Screen Flow-Fields and Unit Cell Configuration

More recently, we have focused on replacing machined graphite or machined metal flow-fields with inexpensive, off-the-shelf wire screens and foils [2]. The flow-fields are based on simple woven wire-mesh screens of various stainless steels, which can be sandwiched around a thin metal plate of the same material to create a bipolar plate/flow-field configuration for use in a stack. Major advantages of using stainless steel wire screens include the elimination of expensive raw materials as well as machining and/or other special fabrication costs. Many types of screens are readily available in a variety of thicknesses and mesh sizes. The screens are also relatively light-weight in comparison to thick graphite or solid metal plates. Another advantage of metal screen hardware is that the screens and foils are not brittle, thus very thin unit cells may be possible. The wire screen flow-field consists of a sandwich of two screens, a coarse mesh and a fine mesh, that sit within a compressible gasket "frame" backed by a thin, metal foil (Figure 1). The fine mesh screen, which is situated between the coarse mesh screen and the MEA, allows adequate reactant access to the MEA while protecting the carbon cloth backing from squeezing into the coarser screen. The MEA is encased within a thin metal "picture frame" which simplifies sealing and manifolding and has the added benefits of making the MEA easy to handle and align within the cell during assembly. Cooling plates (not depicted in the figure) can be provided as well by sandwiching two unit cells around a wire-screen flow-field.

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