Effect of different carbon fillers on the properties of graphite composite bipolar plate

Abstract

A bipolar plate is one of the key components of a proton exchange membrane fuel cell. Development of a suitable material for use as bipolar plate is scientifically and technically important due to the need to maintain high-electrical conductivity, better mechanical properties and low-manufacturing cost. A replacement of the conventional graphite bipolar plate is reported which is twice as strong as the conventional monolithic graphite plate. These plates have been produced by compression molding technique using natural graphite, synthetic graphite, carbon fiber and carbon black as reinforcing constituents and phenolic resin as a binder matrix. A judicious combination and their respective proportions, could produce a composite plate with bulk density 1.8–1.90 g/cm³, electrical resistivity between 0.002 and 0.007 Ω cm, shore hardness ∼65, flexural strength ∼45 MPa and flexural modulus ∼12 GPa. The characteristics and the performance of the composite plate developed by us are compared with the commercially available bipolar plates. A power density of ∼500 mW/cm² was achieved at 1400 mA/cm² current density when the above composite plate was used as a bipolar plate in a unit fuel cell.

Introduction

Due to expected shortage of fuel resources within the next few decades and increasing environmental pollution by the use of fossil fuels, it is necessary to develop alternative form of energy generation. At present fuel cell constitutes one of the most promising sources of energy (Joon, 1998). Bipolar plate is an important component of the low-temperature fuel cell stack, such as PEMFC, PAFC, and AFC. The one considered viable for personal transportation technology as electric current generator in electrical cars is the proton exchange membrane (PEM) fuel cell (Lemons, 1990). In this cell proton from dissociated hydrogen atoms, permeate through a nonconductive membrane while the corresponding electron generates current by flowing through external circuit.

The biggest challenge for the development of PEM fuel cell type for automotive application is the weight reduction and its cost. The conventional graphite bipolar plate is the heaviest part of PEM fuel cell and leads up to 80% of the total weight of the fuel cell. Further, high-cost bipolar plate prevents current economic feasibility of fuel cells. Recent cost analysis of PEM fuel cell (Kamarudin et al., 2006) indicates that, the highest cost is incurred by the bipolar plate, i.e., 38%, followed by the cost for the electrode, membrane, and catalyst (platinum), i.e., 32%, 12% and 11%, respectively. It is therefore obvious that cost of bipolar plate can be mainly minimized by minimizing the thickness and use of low-cost and high-efficient materials like advanced composite materials. For materials to qualify for bipolar plate US Department of Energy has suggested (Onischak et al., 1999) a target value of number of competing parameters as shown in Table 1.

The most commonly used bipolar plate material is graphite. Among the advantages of graphite are, its excellent corrosion resistance and low-bulk resistivity. On the other hand, the disadvantages are difficulties in machining, and its brittleness (Kamarudin et al., 2006, Onischak et al., 1999, Larminie and Dicks, 2001) due to which bipolar plate requires a thickness of the order of several millimeters, and causes the fuel cell stack to be heavy and voluminous (Makkus et al., 2000a). Thin metal sheet is also a good material for a bipolar plate, it offers the attributes of good electrical conductivity, low-cost, excellent mechanical strength and ease to fabrication, but suffers from low-corrosion resistance (Hornung and Kappelt, 1998, Makkus et al., 2000b, Wilson, 2000, Bisaria, 2000, Wind et al., 2002, Davies et al., 2000, Wang and Turner, 2004). A composite bipolar plate is a promising alternative to both metal and graphite, and has the advantages of low-cost, ease in machining or in situ molding of complex flow fields during processing, good corrosion resistance and light weight. A carbon/polymer combination was thought to be an ideal component for producing such plates. The advantage with carbon is its light weight, availability in different structural forms to provide varied electrical, thermal, mechanical and hardness properties.

Different types of bipolar plates are being developed which makes use of carbon materials like, electro graphite, flexible graphite, carbon–carbon composites, graphite–polymer, etc. While individual plate has some advantages over other in certain respect, it fails to meet one or the other requirements listed in Table 1. There is hardly any detailed study in the open literature, which could combine processing, characterization and performance evaluation of a bipolar plate in a fuel cell stack. There are specific grades available commercially but they do differ in their respective characteristics. Following these studies we feel that there is still further scope to improve the quality of the bipolar plates especially to achieve light weight, high-strength and low-electrical resistivity. In the present study we discuss processing of advanced composite bipolar plate with characteristics close to the one typified by DOE and reproduced in Table 1. Influence of the individual carbon filler on the characteristics of the plate has been investigated in depth. High-strength of the plate makes it possible to scale down its thickness from 6 to 4 mm without sacrificing any desired property in a bipolar plate. We will show how the study enabled us to select optimum material composition. The performance of the plate developed is also compared with the commercial Schunk plate in a unit fuel cell assembly.