Literature review of bipolar graphite plates (2) –material selection

With all test stands built and operating procedures determined, the next important factor for testing is the materials for the bipolar plates. Continuing with Hentall et al., the materials under scrutiny are divided into metallic and non-metallic current collectors and compared to the baseline graphite. The metallic current collectors include aluminum, stainless steel, and titanium, whereas the non-metallic current collectors include exfoliated graphite. All parts were manufactured to the exact dimensions of the graphite baseline. The aluminum and stainless steel plates were machined, whereas the titanium plates were made by diffusion bonding. Both gold-plated and regular aluminum and stainless steel plates were tested to observe any difference in overall cell performance and long-term stability.

For a cost-competitive bipolar plate, Makkus et al investigated fuel cell operation using five different stainless steels, two proprietary stainless steel alloys, and a baseline graphite sample. These alloys were chosen and subjected to three selection criteria for fuel cell application: (1) Cell current density degrades less than 10% after 5000 hours of operation; (2) Total contact resistance lower than 50 S/cm2 after 5000 hours of operation; (3) Destructive actions from bipolar plate materials are limited to 8*10-7 mol/cm2. The bipolar plates were also subjected to pre-treatment to influence the surface of the plate in hopes of increasing conductivity and reducing contact resistance. Furthermore, to investigate corrosion of the stainless steels, several tests were run with direct contact between the plate and membrane without any backing layer.

Other related work depicts investigation of three more stainless steel bipolar plates by Davies et al. Bipolar plates were machined from 904L, 310, and 316 stainless steel and subjected to the fuel cell environment. Davies concluded the performance, corrosion resistance and conductivity depended on the composite on the alloy. The nickel and chromium content has an influence on how the plate will react in an acidic environment. Previous work has shown that uncoated stainless steel can perform in long-term durability testing without corrosion evidence. In their latest work, a proprietary coating for the steel to reduce contact resistance is also investigated.

Several other researchers have displayed long-term performance data, corrosion resistance, and high electrical conductivity of new materials for bipolar plates. Hodgson et at. presented tests using titanium in single cell configurations. However, during formation of the plates, a passive film forms on the surface of the machined plate that increases the contact resistance. To alleviate this problem, the surface of the plate is modified and a special coating is applied to reduce ohmic losses and increase cell performance. The newly built cells with titanium plates were compared to similar cell configurations using 316 stainless steel machined plates.

Busick and Wilson described in-cell testing as well as out-of-cell corrosion testing for composite materials for bipolar plates. The in-cell baseline tests were conducted using a composite material consisting of a synthetic graphite powder embedded in a thermosetting vinyl ester resin matrix. Development of this baseline continued by analyzing the effects of adding short fiber reinforcements, long fibers and chopped fibers of different materials. Both conductivity and mechanical strength of the plates were tested using reinforcing fibers such as graphite, glass, polyester and cotton. The composite materials were subjected to corrosion testing consisting of methanol or sulfuric acid immersion and monitoring the weight of the samples. Further leaching of ions was investigated using x-ray fluorescence spectroscopy. In-cell testing was conducted on the graphite composite baseline and compared to previously tested stainless steel and composite plates.

Development for a new iron-based alloy without a special surface coating for bipolar plates was introduced by Hornung and Kappelt. The main drawback of iron-based plates is a passive oxide layer on the plate surface that aids in corrosion resistance but further decreases cell performance due to ohmic losses caused by high contact resistance. The Fe-based plates were manufactured using different compositions of Cr, Mo, and Ni. Using in-cell testing, these combinations of iron-based plates were compared to gold-plated nickel-based alloys.

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