Assimilation of highly porous sulfonated carbon nanospheres into Nafion^® matrix as proton and water reservoirs

A unique form of carbon nanospheres possessing an immense number of micropores and pendant surface sulfonic acid groups was synthesized and used as an effective filler to enhance proton transfer in Nafion^® membrane at elevated temperatures. The synthesis of the filler involved the formation of polypyrrole nanoparticles and pyrolysis of them to generate carbon nanospheres (CN). Alkaline etching was then carried out to create the porous structure, and the resulting porous carbon nanospheres were then sulfonated to attain the sulfonated porous carbon nanospheres (sPCN, 1300 m²/g, 6.9 mmol-SO₃H/g). Dispersion of a substantially small amount of sPCN in a Nafion matrix brought about a cross-adsorption between the hydrophilic side-chain of Nafion molecules and sPCN. This causes the formation of a cross-linking network with sPCN junctions. The scope of this network, however, decreased with the increase in the sPCN loading from 1 to 2 wt% due to a reduction in extent of the cross-adsorption. The sPCN loading of 1 wt% reached the highest crosslinking degree that displayed the maximum enhancement on proton transport. It can be attributed to the role of the sPCN crosslinking junctions in keeping moisture and supplying protons. The characterizations of glass transition behaviour, hydrophilic microenvironments, and proton conductivity under low humidity levels reflected the impact of crosslinking extent. In the single H₂-PEMFC test at 70 °C using dry H₂/O ₂, 1 wt%-sPCN Nafion composite membrane manifested a power density of 571 mW/cm² as compared to the pristine Nafion membrane that showed uppermost value of 388 mW/cm².