Design and testing criteria for bipolar plate materials for PEM fuel cell applications Page: 3 of 8
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This report estimates bipolar plate design parameters, and evaluates possible
material routes for useful bipolar plates. The relevant reactive conditions for bipolar plates
evaluation are defined, and appropriate testing parameters established. The design
constraints are used to estimate materials that could be useful for current, large scale
markets, such as those required for transportation applications.
1. Electronic Conductivity: The bipolar plate conducts electricity with minimum
voltage lost. Operating at a current of 1 amp/cm2, voltage loss of < 10 mV/plate is
minimally acceptable for high efficiency. Thus the plate resistance must be less than 0.01
Q-cm2 for each plate, independent of plate thickness. For example, a thickness of 0.100
cm (0.040 inch) sets bulk material resistivity of less than 1.0 x 10-1 0-cm. This value is
readily met with metals and some semiconductors. For a thin coating; assuming that the
substrate is a conductive metal (R @ 0), then for a coating thickness of 25 pm (0.001
inch), the coating resistivity must b. less than 4 a-cm.
2. Corrosion: The sulfonic acid cation exchange membranes utilized in PEM fuel
cells foul when contaminated by most cations and by some organic compounds. Metal
cations exchange with protons resulting in increased membrane resistance and raobile
aqueous acids. Corrosion processes that yield halide ions, such as Cl-, poison
electrochemical catalysts. Corrosion processes also can result in formation of resistive
surface layers, reducing the plate conductivity. For example, aluminum plates are covered
by resistive alumina. In graphite hardware, slow carbon corrosion results in gaseous CO2,
which exhausts from the device with no adverse effect. Consequently, zero corrosion rates
are not the constraints. Rather the task is to design materials that present no adverse
consequences of corrosion processes.
One consideration is the number of metal ions that can be allowed to adsorb into the
proton exchange membrane before performance loss is evident. For a 5,000 hour
operating life an acceptable bipolar corrosion rate is approximately 0.016 mA/cm2. If 99%
of the metallic ions generated by this corrosion rate exhaust the cell as part of the product
water, then a corrosion current as high as 1 mA/cm2 may be acceptable. Corrosion which
leads to ions or molecules that do not adversely affect the fuel cell, such as CO2, mentioned
above, are permissible. Thus the corrosion products may need to be analyzed via a
technique such as XRF or atomic absorbance spectroscopy to determine elemental
composition so that detrimental corrosion products are identified.
3. Gas Diffusivity: Each plate separates reacting hydrogen and oxygen streams.
High leakage of either reactant presents an immediate safety concern as well as an
efficiency loss. At 1 A/cm2 current density, each cm2 of active fuel cell electrode area
processes 2.1 x 10-5 moles/second of hydrogen. At 90*C and 2 atm, this corresponds to a
volume of 0.31 cm3 hydrogen processed each second. Assuming a leakage rate of 0.5%
could be acceptable, the maximum average plate gas permeability is 1.0 x 104 cm3/s-cm2.
This bulk permeability can not be compromised by localized 'holes'.
4. Chemical Compatibility: The bipolar plate, although separated by graphite current
collectors, is essentially in chemical contact with either the anode side or cathode side of the
fuel cell. The anode and cathode have different chemical and electrochemical
environments-the anode has a reducing atmosphere of H2 and CO2, with dissolved CO2
forming carbonic acid, with a potential from 0.0 to possibly 0.6 VRHE; the cathode has an
02 oxidizing atmosphere, and potentials of up to 1.23 VRHE. The two surfaces of the
bipolar plate experience dissimilar chemical environments and most probably require
different materials for the cathode and anode. The anode face cannot be susceptible to
degradation by the hydrogen environment. One important consideration is chemical
hydriding. Formation of disrtuptive hydride layers is of concern. However, the relatively
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Borup, R.L. & Vanderborgh, N.E. Design and testing criteria for bipolar plate materials for PEM fuel cell applications, article, May 1, 1995; New Mexico. (digital.library.unt.edu/ark:/67531/metadc691543/m1/3/: accessed November 19, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.