Ice crystal formation in the carbon nanotube suspension: A modelling approach

Freeze-dried porous solid foams were prepared from carbon nanotube (CNT) suspensions via either contact freezing with a heat exchanger or immersion freezing into a cryo-bath. Microstructures in the freeze-dried foam cast by ice crystals formed during the freezing step. It was found that domains of the carbon nanotubes in the freeze-dried solid foams were fairly well interconnected, and the microstructures were largely influenced by the freezing condition and freezing method. A mathematical model was proposed to simulate the observed thermal history during the freezing step; then theoretical predictions of the microstructural features were attempted. The simulated thermal history was in good agreement with experimental result. The resulting mean ice crystal sizes L* could be estimated from the calculated freezing front velocity (R) and the temperature gradient in the frozen zone (G). Interestingly, it was found that a correlation based on the power law (L*∝R^?0.2G^?0.2) was applicable to the present system for both the contact and immersion freezing methods. Though the fundamental ice crystallization phenomena were essentially the same for both freezing methods, actual temperature distribution and movement of the freezing front through the whole freezing bulk could control the morphology of ice crystal in the frozen matrix. In other words, good control of thermal flow in the freezing system would greatly contribute to rational design of microstructures of the CNT foam.     

Freeze-dried solid foams prepared from carbon nanotube aqueous suspension: Application to gas diffusion layers of a proton exchange membrane fuel cell

Freeze-dried macroporous solid foams were prepared from the multi-walled carbon nanotube (MWCNT) aqueous suspensions dispersed by chitosan. Thin film shaped CNT solid foams were prepared, and applied to the gas diffusion layers (GDLs) of a laboratory scale proton exchange membrane fuel cell (PEMFC). It was demonstrated that the prepared carbon foams in this study were useful to a fuel cell GDL material. The prepared cell performances were fairly comparable to the cell prepared with conventional carbon paper for GDL material. The microstructures of the prepared carbon foams were found to affect on the PEMFC performances. It was suggested that the interconnected carbon networks formed during the freezing step closely link to the cell performances. Hence, the defection of the interconnected microstructure lead degradation of the GDL quality. The impedance measurement made clear that the prepared foam materials were also advantageous for reducing the ohmic resistance in PEMFC assembly. The kinetic resistance values and the thermal conductive characteristics suggested that the freezing process would also control the degree of overlaps among single CNTs in a freeze-dried bulk that influenced on the electrochemical properties.