Water management in polymer electrolyte membrane fuel cells (PEMFCs) is a crucial aspect which must be properly addressed in order to have high and constant efficiency during device operation. In this respect, gas diffusion medium (GDM) is a fundamental component for a PEMFC because it is inserted between the flow field and the catalytic layer aiming to guarantee a correct balance between the inlet (with humidified gases) and generated water (by the oxygen reduction reaction). Indeed, an excessive accumulation of water could rise mass transfer limitations and lead to flooding, above all at high current densities values. Therefore GDM needs to be made hydrophobic [1, 2].

GDM is formed by a carbon cloth or paper macro-porous substrate (gas diffusion layer, GDL) and a micro-porous layer (MPL) made from an ink and deposited onto GDL. Such a coating improves the smoothness of the GDL surface allowing a better contact with the catalytic layer [2].

MPLs are mainly prepared from inks containing carbon conductive particles (carbon black, nanotubes or graphene) and PTFE, the latter used as a hydrophobic agent. For some years now, our research group has demonstrated the effectiveness of replacing PTFE with fluorinated ethylene propylene (FEP) in order to improve hydrophobic properties of MPLs and, consequently, the water removal [3].

In this work, graphene nanoplatelets (GNPs) and carbon black were mixed in order to compare resulting MPLs with known optimized pure carbon black-based MPLs [3]. GNPs were thought to be beneficial for the enhancement of mass transport and removal of the generated liquid water. GNPs are nanocarbonaceous materials with a mean surface area of 350 m²/g and functional groups, like carbonyl and carboxyl, at the edges of the platelets. The resistivity of GNPs as a function of applied pressure shows values from 0.59 to 0.11 Ωcm. A typical carbon black powder used to prepare MPLs, such as Vulcan XC72, has a mean surface area of 262 m²/g; furthermore, its electrical resistivity varies from 0.67 to 0.10 Ωcm. So it could be useful to replace Vulcan XC72 with GNPs in MPL design; a higher surface area and a better resistivity than carbon powder should guarantee higher performances in both mass transport and generated power.

An optimized amount of FEP was employed (12 wt %) [3]. MPLs were deposited onto GDLs and then they were thermal treated together (260 °C for 30 minutes).

Durability is still a critical issue to be faced in fuel cells field in order to have a real competition with conventional energy generators. While for membranes, catalysts and bipolar plates many standard protocols for testing their durability through accelerated stress tests (ASTs) have been designed, this is not the case for GDLs and MPLs. Thus, ad-hoc chemical, mechanical and thermal ASTs were developed in this work.

Morphological (SEM analysis, mercury intrusion porosimetry and static contact angle) and electrochemical tests were carried out on GDMs upon different times of ASTs treatment, up to 1000 h. Moreover, weight and carbon loading variation were evaluated.

Electrochemical tests were performed in a single cell with Nafion 212, a platinum loading of 0.2 mg/cm² at the anode and 0.4 mg/cm² at the cathode, and an active area of 23 cm². The cell testing was run at two temperatures (60 °C and 80 °C) with different relative humidities (RH 80/100 and 80/60, hydrogen/air). Electrochemical Impedance Spectroscopy (EIS) was carried out using a Frequency Response Analyzer.

The use of GNPs in MPLs allowed to obtain a more homogeneous surface with less cracks than conventional MPLs and a consequent lower overall ohmic resistance of the running fuel cell, due to a better contact with the electrode.

MPLs with GNPs showed the highest performance under the point of view of both output power density and mass transfer resistance. Moreover, they revealed to be more resistant than conventional MPLs against mechanical, chemical and thermal degradation, since the electrochemical analysis upon ASTs showed only small changes of the parameters related to MPLs quality, namely ohmic and diffusion resistances. Therefore, graphene-based MPLs seem to be a valuable solution to further improve the performance of PEMFC devices.

References

[1] S. Park, J.W. Lee, B.N. Popov, International Journal of Hydrogen Energy, 37, 7, 2012, 5850-5865.

[2] M.B. Ji, Z.D. Wei, Energies, 2, 4, 2009, 1057-1106.

[3] S. Latorrata, P. Gallo Stampino, C. Cristiani, G. Dotelli, International Journal of Hydrogen Energy, 39, 10, 2014, 5350-5357.