Development and characterization of highly macroporous carbon nanotube foams by freeze-drying method

Abstract

Macroporous bulk foams made of multi-walled carbon nanotubes (MWCNTs), and having porosity of up to 97% were produced by freezing aqueous solutions consisting of dispersed MWCNTs and a carboxymethyl cellulose (CMC) sodium salt (surfactant), followed by freeze-drying. The solutions were frozen via two different freezing processes (contact freezing with a plate heat exchanger vs. immersion freezing in a cryo-bath) and then lyophilized to obtain dry specimens. Besides the ordered macropores of the produced CNT foam specimens, mesoporosity also developed inside the thin foam walls. The average macropore size was mainly determined by both the initial formulation and freezing condition. A faster freezing rate led to a smaller macropore network in the bulk matrix, and so did a higher surfactant concentration or a higher MWCNT content. However, the morphology of the macropores was strongly dependent on the freezing method. A rather uniform monolithic honeycomb structure was produced by the immersion freezing technique, whereas the contact freezing method typically gave a rather random macroporous structure. Interestingly, the electrical resistance of the CNT foams was found to depend on the preparation condition and to rapidly respond to a gas pressure change. The macropore morphology played a critical role in the identification of gas type. A larger average macropore in the bulk specimen led to high mass transfer capability during usage, thereby resulting in a faster response in the electric resistance. Moreover, the pattern of the normalized gas-uptake electric resistance response towards each gas species was affected by the macropore morphology as well as the average macropore size. It was found that the produced CNT foams displayed hybrid characteristics of the original MWCNTs and the resulting macroporous bulk matrix. These preliminary results suggest that the produced CNT foams have reasonable potential as a gas identifier.