Methanol has high volumetric energy density and is relatively easy to handle, thus methanol fuel cells have potential applications in automobiles and portable devices. There are two types of methanol fuel cells. The direct type cell oxidizes methanol for power generation while the indirect type first converts methanol to hydrogen and uses it as fuel. Indirect methanol fuel cell has several advantages. It has a higher efficiency compared to the direct type, and does not need hydrogen supply and storage as in the hydrogen fuel cell. However, its size is larger and it is heavier as reforming methanol and removing CO are required. Meanwhile, the catalyst with mechanical support such as ceramic foam or honeycomb accounts for a significant part of its total weight. Thus, its applications are limited, especially in portable devices. As a remedy, we have developed a high surface area/weight ratio, low cost and self-supported catalyst for the indirect methanol fuel cell by using porous biomorphic Cu/CeO2/Al2O3. This ternary system is produced via a chemical method by using cotton as templates. Cu/CeO2 is a catalyst for reforming methanol and for oxidation of CO, and the Al2O3 is to prevent segregation of the Cu and CeO2 fine particles.
To prepare this biomorphic Cu/CeO2/Al2O3, a 0.5 M Boehmite solution was prepared with C9H21O3Al via the Yoldas process. It was then mixed with 0.5 M Cu(NO3)2 and 0.5 M Ce(NO3)3 solutions. The weight percentage ratio of Cu:Ce:Al was 3:2:5 in the mixture. Cotton balls were soaked in the mixture, and they were air dried before sintered in air at temperatures ranged from 400°C to 800°C. The sintered samples were further reduced with diluted hydrogen at 250°C for 2 hours. The final products were characterized via SEM, TEM, XRD, BET, DTA and TGA.
We found that the products maintained the original macroscopic shape and size of the cotton ball and no obvious shrinkage was observed. The SEM images showed that the original internal fibrous networks in the raw cotton was retained in the biomorphic product, however, the fibers became hollow. The XRD result confirmed that the Cu/CeO2/Al2O3 compound was produced when the sintering temperatures between 500°C and 700°C. The sizes of the Cu and CeO2 particles in the products were in nanometer scale. When the sintering temperature was higher than 700°C, CuAl2O4 was produced. When temperature was below 500°C, Cu2O was produced. From the DTA and TGA results, the pyrolysis of soaked cotton occurred between 190°C and 380°C. The sample weight decreased to about 16% of its original at 380°C, and kept almost constant at higher temperatures. The BET area of our products was about 100m2 per gram, which was comparable to those of similar products synthesized by other methods. Our products are self supported and mechanical support was not required. It is expected that the weight of catalysts can be greatly reduced and the energy utilization of the indirect methanol fuel cell can be improved.