Effect of pore-directing agents and silanol groups in mesoporous silica nanoparticles as Nafion fillers on the performance of DMFCs

Abstract

Two kinds of nanoparticles of mesoporous silica, namely SBA-15n and MSN, were prepared using P123 and CTMABr as pore-directing agents, respectively, and loaded into Nafion® to form composite membranes by solvent casting method. The physico-chemical properties of these nanoparticles were examined with powder-XRD, N₂ sorption, TGA, EA, SEM and ²⁹Si MAS NMR. The methanol permeability, proton conductivity, and cell performance of the resultant composite membranes were compared in terms of the amount of nanoparticles, whether the pore-directing agents were removed and the different kinds of nanoparticles of mesoporous silica. It was found that both pore-directing agents present inside the pores of nanoparticles could contribute to resisting methanol crossover to the cathode. However, only P123 inside SBA-15n could assist proton transfer, probably through the ether groups on the P123 co-polymer. The proton conductivity of the composite membrane containing extracted SBA-15n was lower than the membrane that contained P123. Nevertheless, the highest proton conductivity was obtained on the composite membranes filled with ethanol extracted MSN (Ex-MSN) particles. This is attributed to the larger amount of silanol groups present in Ex-MSN than in extracted SBA-15n. The optimal proton conductivity of 4.01 × 10⁻² S cm⁻¹ was obtained on the composite membrane filled with 5% Ex-MSN, and the single cell assembled with this composite membrane gave a highest power density of 131 mW cm⁻² at 60 °C, which was about 2 times higher than the cell with recasting plain Nafion membrane and 36% higher than that with commercial Nafion® 117 membrane.