Hybrids Materials Based on Carbon Nanotubes Rooted on Clays Platelets for Nafion Nanocomposite Membranes in Pemfcs: Investigation on Water and Methanol Transport Properties
Authors: Nicotera Isabella , Simari Cataldo , Coppola Luigi , Ranieri Giuseppe Antonio , Oliviero Rossi Cesare 
Published: February 2020

In PEMFC, the polymer electrolyte membrane (PEM) is one of the crucial components which strongly determine the cell performance. Nafion is the electrolyte that been most extensively studied.  Despite to its high thermal and chemical stability, it manifests a strong dependence of proton conductivity on hydration level, indeed conductivity dramatically declines as temperature rises above 80 °C and relative humidity/hydration decreases. It is thus highly important to enhance the proton conductivity of the Nafion electrolyte membrane under low RH in order to accomplish higher PEMFCs performance. Additionally, in polymer nanocomposite research, the basic aim is to enhance several properties of polymeric components by using molecular or nanoscale reinforcements in order to increase tensile strength, modulus, heat resistance and impact resistance.  One area of research has focused on using carbon nanotubes (CNTs) which, under the electronic percolation level, offer many opportunities for new composites thanks to their superior properties. However, CNTs are generally insoluble in common solvents and polymers, and they agglomerate easily, forming entangled bundles which lead to many defect sites in the composites.

In this study, a new class of hybrid materials based on carbon nanotubes (CTNs) rooted on aluminosilicate smectite clays, was synthesized by catalytic chemical vapor deposition (CCVD) method and proposed as nanoadditives in a perfluorosulfonic acid (Nafion®) membrane.

The clay platelets were used as alternative substrate for immobilizing the catalytic metal centres necessary for the growth of carbon nanotubes, owing to their unique swelling, ion-exchange and intercalation properties.

Side-wall chemical oxidation and organo-functionalization of the CNTs was performed using organic ester molecules containing hydrophilic groups (-RSO3H). SWy−fCNTs nanoadditives were incorporated in the polymer by solution-precipitation method, producing highly homogeneous nanocomposite membranes with outstanding mechanical properties and  functionalities obtained by combining both the clays and nanotubes features.

The intent is to favour a "branched" structure among the hydrophilic pores inside the membrane, in order to facilitate the proton transport by a more efficient Grotthuss-type mechanism, from one acid site to another, in the dehydration state. Additionally, the presence of hydrophilic 2D platelike layers in the polymeric matrix is of great interest due to the significant gains in thermal stability, mechanical and barrier properties of the resulting nanocomposites. This last property may be exploited in the Direct Methanol Fuel Cells (DMFCs) to reduce the methanol crossover through obstruction effect and increased tortuosity of the fuel diffusion path.

Synthesized materials are characterized by a combination of techniques (TGA, Raman, FT-IR, SEM, TEM and DMA), while a deep investigation on the water and methanol transport properties are performed by NMR spectroscopy (PFG and relaxation times). The results demonstrated that membranes containing the organo-functionalized hybrid nanoadditives are able to guarantee a very high proton diffusion in “quasi-anhydrous” conditions, i.e. in the region of high temperatures (above 100 °C) under very low RH, and unveiling a reduction of the methanol diffusion. In accordance, proton conductivity has gained one order of magnitude respect to recast polymer when the cell operating conditions become more drastic.

Concerning the mechanical investigation, the adding of such hybrid nanoadditives to Nafion polymer produce membranes more structured and with higher stiffness with higher solidity and extending their thermal stability, which are highly advantageous characteristics for use in fuel cell applications.