A Novel Approach to the Sustainable Synthesis of Carbon Fibers
Authors: Keck Devin Monroe , Islam Monsur , Martinez-Duarte Rodrigo 
Published: February 2020

Presented here are preliminary experiments facilitating the use of the cellulose-extruding bacteria Gluconoacetobacter xylinus as a sustainable tool for the production of carbon fibers. G. xylinus has the natural ability to convert different kinds of sugars from its environment into highly crystalline cellulose nanofibrils. When grown in culture, the random movement of the bacteria leads to a porous cellulosic scaffold with no apparent order. Compared to the cellulose extracted from plants, bacterial cellulose has higher purity and exhibits superior mechanical properties [1].

Results presented last year in this same conference showed how heat treatment of these cellulose fibers that were functionalized with metal precursors yielded a porous tungsten carbide. Here, we focus on the positioning of G. xylinus cells using Dielectrophoresis (DEP) to grow cellulose fibers from specific locations, which we then carbonize by pyrolysis. DEP refers to the movement of electrically polarized cells in response to a non-uniform electric field of specific frequency and magnitude. Specifically, positive DEP will be used here to position the G. xylinus cells on the electrodes while they are continuously perfused with nutrient-rich media, allowing for the continuous extrusion of straight fibers. The ultimate goal is to develop a scalable platform for the continuous production of cellulose nanofibers that can then be converted into carbon. This can become a more sustainable alternative to current practices of cellulose production by eliminating the cost and energy required for forest management and cellulose extraction and purification. Most importantly, it eliminates the use of oil in the production of carbon fibers.

In this work, we present initialcharacterization of the response of G. xylinus to electrical stimuli of varying frequencies as well as the optimized protocol to convert bacterial cellulose into carbon. Dielectrophoresis experiments were completed using AC signals of 1, 5, 10, 15, and 20MHz frequencies at a constant voltage of 20Vpp. The results indicate a positive DEP response across the spectrum of 1-20MHz with the highest trapping at a frequency in the range of 15-20 MHz (figure 1 and 2). Additional work focused on the carbonization of bacterial cellulose nanofibers. Amorphous carbon was successfully achieved (figure 3) through heat treatment of bacterial cellulose to 900 ºC in a nitrogen atmosphere following a 5 ºC/min heating ramp. A dwell time of 30 minutes at a temperature of 300 ºC was included in the heat treatment to induce as much oxygen release from the sample as possible to prevent burning the piece when increasing the temperature. Such step is also necessary because the removal of the hydroxyl groups stabilizes the carbon polymer by producing conjugated double bonds, forming an aromatic structure [2]. An optimal heating rate was determined to be 5 ºC/min to achieve high carbon yield without making the process prohibitively long. Although we observed cellulose to carbonize at around 400 ºC, 900 ºC was used as the final temperature to ensure a high degree of carbonization [3].

Ongoing work is on characterizing the fiber properties and dimensions depending on the suspending media and its perfusion rate over the cells. The goal is to understand how the conditions in the microreactor lead to the tailoring of the fiber properties and a maximized throughput.

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*****CAPTION****Figure 1: Characterizing the response of G. xylinus to electrical stimuli using Carbon DEP

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*****CAPTION****Figure 2: A 2D carbon electrode showing indications of inducing a positive dielectrophoretic force on the G. xylinus bacterium (A.) Electrode is electrically polarized (B.) Same electrode immediately after electric field is switched off

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*****CAPTION****Figure 3: Carbonized Cellulose Nanofibrils

References

1. Huang, Y., Zhu, C., Yang, J., Nie, Y., Chen, C., & Sun, D. (2014). Recent advances in bacterial cellulose. Cellulose, 21(1), 1-30.

2. McGrath, T. E., Chan, W. G., & Hajaligol, M. R. (2003). Low temperature mechanism for the formation of polycyclic aromatic hydrocarbons from the pyrolysis of cellulose. Journal of Analytical and Applied Pyrolysis, 66(1), 51-70.

3. Rhim, Y. R., Zhang, D., Rooney, M., Nagle, D. C., Fairbrother, D. H., Herman, C., & Drewry, D. G. (2010). Changes in the thermophysical properties of microcrystalline cellulose as function of carbonization temperature. Carbon, 48(1), 31-40.

Figure 1