3: Experimental Methods
3.2 Materials
3.2.1 Nafion
Nafion is a copolymer membrane with polytetraflouroethylene (PTFE) backbone. It is well known for the best ion conductivity and is the leading PEMFC membrane at this moment. The Nafion precursor, i.e. a perfluorosulfonyl fluoride copolymer resin (Nafion R–1100 resin) was purchased from Ion Power Inc., Bear, DE, USA in the sulfonyl fluoride (–SO2F) form. In this form, the polymer does not have the cation-exchange properties, and was chemically treated to make it suitable for cation-exchange applications.
Figure 3.1 Hydrolysis of the Nafion precursor
n 2) CF (
)m
CF
( O CF2 CF O
CF3
CF2 CF2 SO2F
n 2) CF (
)m
CF
( O CF2 CF O
CF3
CF2 CF2 SO3K
n 2) CF (
)m
CF
( O CF2 CF O
CF3
CF2 CF2 SO3H DMSO
/ 2O H / KOH
) M 5 3( HNO
Nafion resin was converted to the salt form (K+) and acid form (H+) using the following processes as shown in Figure 3.1. The polymer resins were immersed in a solution containing 15% potassium hydroxide (KOH), 50% deionised water and 35% dimethyl-sulfoxide (wt/wt) at 80°C for 2h. Afterwards the resin was thoroughly washed with deionised water to remove all traces of unreacted KOH. The Nafion resin was now in the K+ form. The K+ form Nafion resin was converted to the H+ form by exchanging K+ for H+ ions using a 5 M nitric acid (HNO3).
Since this is an equilibrium exchange, this step was repeated twice with fresh HNO3 to complete protonation, including a rinse in fresh deionised water after each acid treatment.
3.2.2 Nanofillers
Nanofillers are primary particle compounds used as fillers or reinforcers in polymers with very small particle size (in the nanometer range). There are different materials that are used as fillers.
In this study we used carbon nanotubes, titanium dioxide nanotubes, and montmorillonite as fillers as mentioned below.
3.2.2.1 Carbon Nanotubes (CNTs)
CNTs are classified as advanced filler materials with interesting properties and capacity of improving thermo-mechanical properties of a polymer, as reported some years ago. [3–6] This is due to their high aspect ratio, extraordinary mechanical and thermal properties, exceptional strength and flexibility and excellent electrical conductivity. CNTs can be classified as single- walled (consisting of a single graphene sheet rolled up to form a tube) and multi-walled (multiple graphite sheets rolled up to form a tube). Multi-walled CNTs (MWCNTs) were used in this study because they are relatively cheaper and possess more mechanical stability than single-walled CNTs (SWCNTs). Their electrical conductivity is also lower compared to SWCNTs, and this was thought to be an advantage in avoiding short circuiting in PEMFC.
MWCNTs were purchased by Sigma Aldrich with average outer diameter of 10 nm and purity of +95%. The surface of MWCNTs was modified to improve its compatibility and dispersion
within polymer matrix. Modification of carbon nanotubes is based on the previously reported method. [7]
3.2.2.1.1 Surface Modification of CNTs
The as-received neat MWCNTs (pCNTs) were first refluxed in 5 M nitric acid for 1 h to introduce carboxylic acid groups on the surface of MWCNT (COOH–CNTs). Carboxylic acid groups are expected to improve dispersion of CNTs within a polymer matrix. The mixture was then filtered and washed with distilled water until no residual acid was present. The sample was then dried in a vacuum oven at 110°C over night. The as-synthesised COOH–CNTs were then refluxed with hexadecylamine (HAD) for 4 days to attach the amine functional group. Treatment with HDA leads to flexibility and solubility of tubes in organic solvents. The product was first washed with methanol and then extensively washed with deionised water to remove excess amine. Amine functionalized MWCNTs (fCNTs) were then dried in a vacuum oven at 110°C overnight. All pCNTs, COOH–CNTs and fCNTs were then used as fillers and their effect on Nafion properties were compared.
3.2.2.2 Layered Silicates
The most commonly used layered silicate to improve the Nafion membrane properties is montmorillonite (MMT). MMT has been used to improve the properties of Nafion membranes for direct methanol fuel cell applications. [8–15] Most promising layered silicate is a natural MMT modified with methyl tallow bis(2-hydroxylethyl) quaternary ammonium salt (commercially know as Cloisite® 30B, C30B) (Figure 3.2). This is an additive for plastics to improve various plastic physical properties, such as reinforcement and barrier. C30B has shown much improvement on methanol permeability compared to other types of multi-layered silicates.
However this improvement was achieved at the expense of proton conductivity of Nafion membranes. In this study, a different method (melt-mixing) will be used to prepare nanocomposite membranes with C30B (purchased from Southern Clay Products, USA) as filler.
This method is expected to improve dispersion of clay within polymer matrix. This might balance both methanol permeability and proton conductivity properties of Nafion.
Figure 3.2 Chemical structure of methyl tallow bis(2-hydroxylethyl) quaternary ammonium salt used to modify MMT, where T is Tallow (~65% C18; ~30% C16; ~5% C14)
3.2.2.3 Titanium Dioxide Nanotubes (TNTs)
Beginning with the discovery of carbon nanotubes,[16–18] a series of approaches to produce nanotubular structures other than carbon, such as the well–known cases of alumina or Titania, have been explored. Among them special attention has been directed to TiO2 due to its unique functional properties. Their photo-corrosion resistance, high photo-conversion efficiency, and the suitability as a material for purification of water and air have attracted more attention in many research fields. In this study, a new application for TiO2 nanotubes is investigated. TiO2 is used as filler in Nafion membranes and its photo-corrosion resistance property is expected to improve Nafion mechanical stability at high temperatures.
3.2.2.3.1 Synthesis of TNTs