(a) A picture of the kapok tree. (b) An image of the kapok fibers and the seeds from the kapok seed pods. (c) The schematic shows the hollow kapok fiber as the electrode support for the microbial growth and provides double surfaces for microbial colonization through both internal and external surfaces.
(a) A picture of the kapok tree. (b) An image of the kapok fibers and the seeds from the kapok seed pods. (c) The schematic shows the hollow kapok fiber as the electrode support for the microbial growth and provides double surfaces for microbial colonization through both internal and external surfaces.

Light, fluffy fibers from the Kapok tree, which grows widely in tropical regions, could make the ideal electrode for a new generation of microbial fuel cells (MFCs), say researchers [Zhu, H., et al., Nano Energy (2014), DOI: 10.1016/j.nanoen.2014.08.014].

MFCs break down organic matter, using electrochemically active bacteria as a biocatalyst, to generate electricity. Such devices could be used to treat wastewater and simultaneously produce clean energy. The crucial reaction takes places on the surface of electrodes, which need to have a high surface area and porous structure compatible with bacterial colonization. The material must also be electrically conductive to enable electron transfer. Various carbon-based conductive porous materials have been tested in MFCs to date, but tend to be high in cost, weight, and unsustainable. More unusual materials have also been investigated, including natural loofah, biochar made from wood chippings, and plant stems.

Now researchers from the Universities of Maryland and Colorado Boulder led by Zhiyong Ren and Liangbing Hu propose that carbonized kapok fibers could offer an ideal alternative. The kapok tree produces abundant fibers, typically 10-20 µm in diameter and several centimeters long. As the fibers are hollow, they provide double the surface area for bacterial colonization.

Raw kapok fibers are simply formed into a paper-like arrangement by vacuum filtration and then carbonized in a tube furnace in an atmosphere of Ar (95%) and H2 (5%) to render them conductive. The researchers used the carbonized and uncarbonized fibers as electrodes in single-chamber air-cathode MFCs and compared the performance to traditional carbon cloth.

“The direct carbonization of low-cost and naturally available hollow Kapok fiber provides an advantageous alternative to non-renewable solid macroporous carbon cloth electrodes,” explains Hu. “It is much cheaper, lighter, and has much higher gravity energy density.”

While the power density production using carbonized kapok and expensive carbon cloth are comparable (27.9 W/m3 versus 27.1 W/m3, respectively), when normalized to the mass of the electrode the potential benefits become apparent. According to the researchers’ calculations, the power density production of carbonized kapok fiber is around twenty times higher than carbon cloth (104.1 mW/g versus 5.5 mW/g).

SEM showing bacteria growing on the surface and inside a hollow Kapok fiber, which will facilitate charge transport. (Courtesy of Liangbing Hu.)
SEM showing bacteria growing on the surface and inside a hollow Kapok fiber, which will facilitate charge transport. (Courtesy of Liangbing Hu.)

The hollow structure of kapok fiber could be a significant benefit to real devices because of the potential weight saving. The density of carbon cloth, for example, is 1.75 g/cm3 while kapok is as low as 0.384 g/cm3.

“Traditional graphite- or carbon-based electrodes are from non-renewable sources, and they only provide a 2D surface for electron transfer,” says Ren. “With kapok, we can pack these new 3D-structured fibers into electrodes for MFCs, which can significantly improve energy production during wastewater treatment, especially on weight basis – something that is very important in larger scale applications.”

The researchers are already discussing some further steps in this effort, they told Materials Today.

To download the article related to this news story, please click here.