Nafion has been the industry standard membrane for polymer electrolyte membrane fuel cells (PEFCs) for decades due to its high conductivity, mechanical stability and durability. However, it has several drawbacks such as poor performance at high temperatures / low humidity, use of harmful fluorine in its production, and as its high price. Therefore research into new membrane materials is necessary. Basic requirements for next-generation fuel cell membranes are sufficiently high proton conductivity, high mechanical strength, good hydrogen gas barrier, and low cost.

Nanocellulose is a biopolymer produced in bulk from abundant cheap cellulosic materials such as wood or cotton. Depending on its source, nanocellulose fiber dimensions can range from a few to several tens of nanometers in diameter, and are several micrometers in length. Nanocellulose paper with high mechanical strength and thermal stability up to 150°C has been reported.^1,2 Previously, we showed that the hydrogen barrier of nanocellulose membranes is approximately 1000 times higher than Nafion.³ In addition, reasonable proton conductivity at high temperature (~5 mS/cm at 120°C) was shown by impedance spectroscopy. Based on these results we fabricated the world’s first nanocellulose “paper fuel cells” and successfully operated them as hydrogen fuel cells. The performance was stable, with no degradation over 24 hours.

Here, in order to increase the proton conductivity, we perform chemical modification of nanocellulose fibers. This chemical treatment resulted in an increase in the number of acidic functional groups, as well as modifying the microstructure. A strong increase in the proton conductivity was observed, with an associated increase in fuel cell performance. In addition, nanocellulose was composited with more established ionomer materials and the resulting membranes were investigated for their fuel cell properties.

References

1. Henriksson, M. et al. Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 9,1579–1585 (2008).

2. Nogi, M. et al. High thermal stability of optical transparency in cellulose nanofiber paper. Appl. Phys. Lett. 102,181911 (2013).

3. Bayer, T. et al. High Temperature Proton Conduction in Nanocellulose Membranes: Paper Fuel Cells. Chem. Mater. 28, 4805–4814 (2016).