As discussed above, the Nafion®:Pt ratio was found to have very little influence on the size of the colloidal Pt particles produced by the synthesis. Therefore an alternative method was developed for the control of Pt nanoparticle size. Experiments were carried out in which the composition of
Figure 4.8: Pt particle size distributions and TEM micrographs for colloidal Pt synthesised at three batch scales 5mg, 50mg and 100mg
glycerol, with water making up the balance in each case. The Pt:Nafion® mass ratio was 1:30 and the concentrations of all solutes were maintained identical to a control experiment using water alone.
Polyols are commonly employed in the synthesis of colloidal metal nanoparticles, where they can serve as both a solvent and reducing agent [8-14]. At ambient temperature the reduction kinetics are slow, so in a typical polyol synthesis the reaction is performed at high temperature (80 – 150 ºC) and pH (8 - 10) in order to initiate and sustain nucleation. These conditions have been found to be unsuitable for the production of Nafion®-stabilised colloidal Pt, so in these experiments the Pt4+ → Pt0 reduction reaction is facilitated by sodium borohydride. Under the conditions employed in the Nafion®-Pt synthesis (22 ºC, pH < 3), the polyol additives make a negligible additional contribution to the rate of Pt0 formation. Instead, the polyol additives are intended to reduce the particle size of the colloidal product via two alternative modes of action:
1. Increased solution viscosity lowers particle growth rate. As shown in Table 4.3, the mixtures of water and polyol additives have significantly higher viscosities than pure water alone. A higher solution viscosity will lead to a decrease in the diffusion coefficient D of monatomic Pt0, which from the growth rate equation (4.5) will effect a proportional decrease in growth rate.
2. By coordination with the surface of metal nuclei during formation, the polyol additives can reduce the solid-liquid interfacial tension σSL [135-136], thereby lowering both the critical
nucleation radius and the activation energy for cluster formation in accordance with equations (4.4) and (4.1).
Importantly, from Table 4.3 it should be noted that the dielectric constants of the water-polyol mixtures are well within the range over which Nafion® is soluble (ε > 10) [137].
4 Preparation of Nafion®-Pt/C electrocatalysts
Pure form Mixture 50/50 vol with water Kinematic viscosity 20 ºC (cSt)[138] Dielectric constant ε 20 ºC [138] Kinematic viscosity 20 ºC (cSt) Dielectric constant ε 20 ºC Water 1 80 1 80 Ethylene glycol 17.8 37 3.66 [139] 58.5 [139] Propane-1,2-diol (α-propylene glycol) 52 32 Not available 56 [140] Glycerol 648 46 5.29 [141] 63 [142]
Table 4.3: Comparison of physical properties of polyol additives for size control of Nafion®-stabilised colloidal Pt
The TEM micrographs and particle size distributions in Figure 4.9 show the effect of each of the polyol additives on the morphology of the as-prepared colloidal Pt product. The polyol additives led to a decrease in average particle size as summarised in Table 4.4. From Figure 4.9, the morphologies of the polyol-assisted products appear rather agglomerated compared to the control sample in Figure 4.9 (a), but this is probably an artefact of slower drying of the TEM specimens for samples containing high-boiling additives. As-prepared colloidal dispersions were stable against precipitation for several days. The addition of ethylene glycol yielded the smallest average particle size (4.6 nm). On this basis, and in light of concerns over the difficulty of removal of significantly less volatile propane-1,2-diol and glycerol additives from the reaction product, the product of the ethylene glycol-assisted synthesis was selected for further investigation.
H O H HO OH HO HO OH OH OH
Solvent composition Phys. surface area Aphys / m2/g
Pt
Sauter diameter / nm
Water only (no additives) 37 7.8
50% ethylene glycol 62 4.6
50% propylene glycol 41 6.9
50% glycerol 57 5.0
Table 4.4: Comparison of physical surface areas (Aphys) and average particle diameters for colloidal Pt nanoparticles synthesised in the presence of various polyol additives (50 mg scale)
4.3.2 Preparation of supported catalysts
The extent of Pt nanoparticle agglomeration on the final supported catalyst could be varied by addition of the carbon support either prior to or following purification of colloidal Pt. The products of these two preparation routes are designated as Nafion®-Pt/C B and Nafion®-Pt/C A, respectively, and the routes are illustrated in Figure 4.10.
4 Preparation of Nafion®-Pt/C electrocatalysts
The TEM micrographs in Figure 4.12 illustrate the dramatic variation in morphology that can be produced simply by adding the carbon support at different stages during the purification.
Nafion®-Pt/C A
In this route, the as-synthesised colloidal Pt was purified by centrifugation to remove reaction by-products and excess Nafion® prior to addition of the carbon support. Centrifugation resulted in complete precipitation of the colloidal Pt. The clear, colourless supernatant liquid containing soluble impurities and excess Nafion® was removed and discarded. The precipitate was easily re-dispersed under sonication in 50/50 v/v water/acetone, and the centrifugation process was repeated. The concentrations of Pt and Nafion® in Nafion®-stabilised colloidal Pt catalysts was determined by thermogravimetric analysis. In an initial experiment, a recast Nafion® film was found to undergo thermal decomposition between 320 – 550 ºC in air (see Figure 4.15 on page 80). In experiments on dried films of colloidal Nafion®-Pt, mass losses within this temperature region were attributed to the removal of Nafion®, whilst the residual mass at 800 ºC was attributed to Pt. The Nafion® content, determined thermogravimetrically, was found to decrease with successive centrifugation steps as shown in Figure 4.11.
Figure 4.10: Comparison of preparation routes for Nafion®-Pt/C A and B. Pt particles can be made to agglomerate (A) or disperse evenly (B) on the carbon support depending on whether the support is added after (A) or before (B) removal of excess Nafion® by centrifugation .