Preparation of Carbonaceous Fiber Sheets Derived from Bamboo and Application to Gas Diffusion Layer of Proton Exchange Membrane Fuel Cells

Bamboo is one of sustainable natural resources with high CO₂ immobilization capacity; on the other hand, its relatively high vitality induces several environmental serious problems recently.  To solve the problems, new and some favorable applications of bamboo are widely being developed and investigated.  To utilize the feature of bamboo is very important for the application development.  The microstructure of bamboo is fibrous and is composed mainly of cellulose, giving us a cellulose fiber easily.  Moreover, its fibrous morphology was even kept after carbonization at relatively high temperature.  We then attempted to prepare the carbonaceous fiber sheet by using bamboo as a starting material and employ it as gas diffusion layer of proton exchange membrane fuel cells (PEMFCs) [1].

In this presentation, the detail regarding to preparation procedure of bamboo fiber, its carbonization and typical character is demonstrated.

Bamboo was cut down to 80 mm in length by a metallic chopper and the inner and outer skins were carefully descaled by specially developed cutting machine (Silver-loy Co. Ltd., Japan).  Then, it was pressed repeatedly by specially developed roll press machine (Silver-loy Co. Ltd., Japan) to form several breaches.  Thus obtained bamboo strips were employed for further study. 

The specimens were boiled in 0.1 mol dm^-3 sodium hydroxide aqueous solution for 3 h by using commercially available electric pot (Panasonic, SR-P37).  After that, the samples were washed repeatedly by water and treated by an electric blender at 37000 rpm for 1 min for defiberize.  Thus obtained bamboo mono filaments were molded and treated at various temperatures under argon atmosphere to obtain carbonaceous fiber sheets.

The size and morphology of the fibers in the sheets were investigated by an optical microscope (OM) and field emission scanning electron microscope (FE-SEM).  X-ray diffraction (XRD) and Raman spectroscopy was used to evaluate the carbonization degree. 

The electrochemical behavior of carbonaceous fiber sheets was investigated in 0.1 mol dm^-3 perchloric acid aqueous solution under nitrogen atmosphere by cyclic voltammetry.  Furthermore, a gas diffusion electrode was fabricated by using carbonaceous fiber sheet as a gas diffusion layer and Pt/C catalyst.  PEMFC operation test was carried out by using Nafion117 membrane as the electrolyte.

Figure 1 shows FE-SEM image of carbonaceous bamboo fiber after heated at 3000ºC.   The diameter of fiber was ca. 6 μm, which is similar to carbon fiber that used in the fuel cell.

Figure 2 shows XRD patterns of carbonaceous bamboo fiber after heated at various temperatures. The peak corresponding to 002 diffraction of graphite was appeared and become to be sharpened with increasing in the heat treatment temperature.

Both of the in-plane electrical conductivity, which is measured by a four-probe method, and that measured under consolidation condition is lower than that of the carbon paper composed by carbon fibers.

Figure 3 shows the cyclic voltammograms of carbonaceous fiber sheets after the heat treatment at various temperatures. The double layer capacity was decreased as an increase in the heat treatment temperature, suggesting that carbonization developed at higher temperature whereas the surface area decreased.  The results of PEMFC operation test will be mentioned in the presentation.

Reference

[1] T. Kinumoto et al., ECS Trans., accepted for publication at 2013.

Acknowledgement

This study is supported by The Environment Research and Technology Development Fund (ERTDF) from Ministry of the Environment, JAPAN.