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Highly conductive and strong graphite-phenolic resin composite for bipolar plate applications Kang Yao, Daniel Adams, Ayou Hao, Jim P. Zheng, Zhiyong Liang, and Nam Nguyen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02678 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 30, 2017
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Energy & Fuels
Highly conductive and strong graphite-phenolic resin composite for bipolar plate applications Kang Yao,*†‡§ Daniel Adams,‡∇ Ayou Hao,§║ Jim P. Zheng,†‡⊥ Richard Liang,†§║ and Nam Nguyen§║ †Materials Science & Engineering, ‡Aero-propulsion, Mechatronics and Energy Center (AME), and §High-Performance Materials Institute (HPMI), Florida State University; ⊥Department of Electrical & Computer Engineering, ║Department of Industrial and Manufacturing Engineering, and ∇Department of Mechanical Engineering, Florida A&M University-Florida State University College of Engineering, Tallahassee, FL 32310, USA *Corresponding author e-mail:
[email protected] Abstract
Composite materials for bipolar plate applications in proton exchange membrane fuel cells were fabricated from synthetic graphite (SG), natural graphite (NG), or expanded graphite (EG) and novolac phenolic resin using compression molding. In comparing the resultant samples, the EG composite exhibited the best properties with a density of ~1.55 g/cm3, flexural strength of 109 MPa and modulus of 24 GPa even with a high graphite loading of 80 wt% and a low plate thickness of ~0.9 mm, as well as in-plane conductivity of 182 S/cm. The EG content was further varied to find the optimal composition. The effects of using carbon nanotube sheets
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(buckypapers) or multi-walled carbon nanotubes as reinforcements to the EG-resin composite were investigated. A modified approach of measuring through-plane electrical conductivity capable of separated analysis of bulk resistance and interfacial contact resistance was attempted. Corrosion resistance, thermal conductivity, and gas permeability of EG-based composites were also assessed.
Keywords: expanded graphite, phenolic resin, composite bipolar plates, carbon nanotubes, buckypaper, through-plane conductivity, corrosion resistance, fuel cells 1. Introduction Featuring high efficiency and power density, relatively low operating temperature (60-80 °C), quiet operation, and environmental friendliness, proton exchange membrane fuel cells (PEMFCs) can potentially become promising power sources for transportation, stationary, and portable applications.1-3 Bipolar plates are a vital component in PEMFCs, accounting for 60-80% of fuel cell stack weight and 30-45% of stack cost.4 The plates serve multiple functions: distributing uniform gas flow, facilitating water management, conducting current between adjacent cells, maintaining impermeable hydrogen and oxygen barrier, providing stack structural support, and enabling heat transfer. Therefore, the bipolar plate must meet a variety of property requirements, as shown in Table 1.5-8 Conventional bipolar plate materials are graphite or metal. The advantages of graphite plates over metallic plates include high corrosion resistance and low density. However, graphite suffers from its brittleness and difficulties in machining, which results in a thick (5-6 mm) bipolar plate and therefore, a big and heavy fuel cell stack.1, 9
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Table 1. U.S. DOE technical targets for bipolar plates.5-8 Characteristic
Unit
Target
Flexural strength
MPa
>25
In-plane electrical conductivity
S/cm
>100
Through-plane electrical conductivity
S/cm
>20
Areal specific resistance
Ohm·cm2
10
Corrosion resistance
µA/cm2
