Corrosion behavior of polyphenylene sulfide–carbon black–graphite composites for bipolar plates of polymer electrolyte membrane fuel cells
Introduction
The growing interest in renewable energy sources has driven the development of polymer electrolyte membrane (PEM) fuel cells during the last three decades. Bipolar plates have a central role in the evolution of this technology [1]. These components play functions related to the mechanical stability, thermal and electrical management of PEM fuel cells [2]. To properly perform such functions bipolar plates have to meet stringent technical targets based on different materials properties such as electrical conductivity, mechanical strength, corrosion resistance and thermal stability [3].
Traditional materials for manufacturing bipolar plates are graphite, metals or polymer-graphite composites [4]. Each material class has intrinsic advantages and drawbacks which must be carefully considered to carry out a successful materials selection of such components [5]. Polymer-graphite composites have spread out as commercial products due to a suitable combination of easy manufacturing, proper electrical conductivity and reasonable mechanical and thermal stabilities at a relatively low weight [6].
Corrosion resistance is a major concern for metallic bipolar plates due to the well-known and undesirable increase of electrical resistance due to the formation of a less conductive oxide film on the surface of the metallic material in typical PEM fuel cell environments [7], [8]. Composite bipolar plates, though, are considered to be little affected by corrosion in PEM fuel cells. Thus, the corrosion resistance of polymer-graphite composites is often neglected in the literature. Notwithstanding, there are several reports showing that carbon-based bipolar plates are affected by corrosion in PEM fuel cells. Graphite, carbon black and carbon nanotubes have been shown to participate in corrosion reactions in simulated PEM fuel cell environments [9], [10]. Kakati et al. [11] have found that carbon black and carbon fiber additions to phenolic resin-graphite composites increase the chemical instability of the compound, leading to higher corrosion current densities in simulated PEM fuel cell environments. Recently, Park et al. [12] investigated the corrosion of carbon black supports for platinum-based catalysts in PEM fuel cells under start-up/shutdown cycling conditions. They observed that carbon black particles were prone to corrosion and this diminished the cell performance. Studies by Hung et al. [13] and Hsieh et al. [14] showed that carbon nanotubes are electrochemically stable when used as carbon supports for catalysts in PEM fuel cells. In a previous work [15], our group has shown that the corrosion resistance of acrylonitrile-butadiene-styrene-graphite-carbon nanotube composites decreased as the carbon nanotube content increased whereas the electrical conductivity was not significantly affected by the corrosion tests. Carbon black particles are often reported as presenting lower electrochemical stability than carbon nanotubes [16], [17]. However, the effect of corrosion tests on the electrical conductivity of polymer-graphite-carbon black composite bipolar plates is not encountered in the literature.
Polyphenylene sulfide (PPS) is a thermoplastic polymer with excellent chemical and thermal stabilities allied with superior mechanical properties [18]. These characteristics have been explored to produce composite bipolar plates. Some reports have shown the suitability of using PPS-based composite bipolar plates with respect to their mechanical and electrical behaviors [19], [20], [21], [22]. In spite of the excellent mechanical and electrical performance of PPS-graphite composite bipolar plates, their corrosion behavior has not been reported yet. Moreover, the effect of incorporating carbon black particles into these composites on both the corrosion resistance and electrical conductivity is also unexplored.
This work aims at shedding some light into this scenario. PPS-graphite composites were prepared with different contents of carbon black particles using hot pressure molding. The corrosion behavior of the composites has been evaluated by electrochemical impedance spectroscopy, potentiostatic and potentiodynamic polarization curves. The electrical conductivity of the composites was also determined before and after the corrosion tests. Scanning electron microscopy (SEM) was used to reveal the morphology of the composites.
Section snippets
Experimental details
Powder commercial PPS resin was supplied by Ticona (Fortron 0214). The major conductive filler were synthetic graphite particles supplied by Asbury Carbons. Carbon black (CB) particles were supplied by Cabot Corporation (Vulcan XC 72).
Six different compositions were prepared as shown in Table 1. The components were firstly mixed in a powder blender for 30 min (Turbula T10B). Next, compression molding was used to prepare the composite plates using a hydraulic press. The processing parameters
Composites morphology
SEM cross-sectional views of fractured PPS–CB–graphite composites are shown in Fig. 1. The importance of evaluating the fracture surface morphology resides in the identification of the inner distribution of the conductive particles within the PPS matrix. Characteristic features such as internal voids and wetting of the conductive particles by the polymeric resin can be observed. One can promptly realize that the composite without carbon black addition (CB0) has a layered structure typical of
Directions to the development of carbon black-containing composite bipolar plates
The results obtained in this work can give useful directions to the development of carbon black-containing composite bipolar plates. Through-plane electrical conductivity determination pointed that the incorporation of carbon black was only effective at enhancing the electrical properties of the composites at 5 wt%. The increment of electrical conductivity was only marginal below this content. It is important to be aware, though, that the composites were more susceptible to oxidation in the
Conclusions
The SEM micrographs obtained from the fractured surfaces of the composites have not permitted to observe significant differences of morphological features. Notwithstanding, they confirmed the presence of internal voids which can act as preferential pathways to the penetration of electrolyte, leading to corrosion. Moreover, porosity measurements revealed that there was a gradual and slight increase of the porosity level with the carbon black content. Additionally, the through-plane electrical
Acknowledgments
The authors are thankful to CNPq (The Brazilian Research Council) for the financial support to this work (Project 558134/2008-4). Mara Cristina Lopes de Oliveira is thankful for the post-doctoral grant (Proc. 150396/2009-0).
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