(PEFC&E 1st Place Best Poster Winner) Nanocellulose Crosslinked with Sulfonic Acid As an Alternative Proton Conductive Membrane for Hydrogen Fuel Cells

Abstract

Polymer electrolyte membrane fuel cells (PEMFCs) using hydrogen as fuel are efficient and clean energy generating devices. Commercialization of fuel cells is however slow, in particular due to high cost of the materials used to fabricate proton exchange membrane (PEM). Nafion®, a benchmark fluorinated polymer used as PEM, contributes significantly to the cost of fuel cell devices and therefore alternative materials are needed. The high proton conductivity of Nafion (i.e. ~100 mS/cm) has made this polymer the main material for PEMs, since its discovery in the late 1960s. However, it has number of disadvantages additionally to high cost, such as degradation under corrosive conditions and difficulty in recycling. Numerous research groups work on synthesis of proton conducting polymers that could compete with Nafion. Among the available techniques sulfonation of hydrocarbon polymers is recognized as an effective approach to create new PEMs [Peckham, T. J. et al. Adv. Mater. 2010, 22, 4667–4690]. Nanocellulose (NC) is an environmentally friendly, very low-cost material that can be produced by processing of conventional cellulose, which is plant-derived, renewable, and most abundant, material resource in the world. Research into nanocellulose for various applications is accelerating rapidly. Both cellulose nanofibers (CNF) and cellulose nanocrystals (CNC) are attractive due to their intrinsic properties, such as mechanical strength, high gas barrier, and a chemical structure suitable for various modifications. Being a biodegradable polymer, it has an additional advantage over fluorinated polymers (such as Nafion) The majority of works that considered the use of nanocellulose in PEMFCs have investigated composites with conventional ionomers such as Nafion, with varying results [Jiang, G. P. et al. J. Power Sources 2015, 273, 697–706]. However, the use of non-composited nanocellulose membranes for PEMFCs is a completely novel topic for research. Nanocellulose is an attractive polymer platform for PEMs, however its application for this purpose is not widely explored. The use of nanocellullose membranes in PEMFCs was pioneered by our group and previous works have demonstrated that both CNF and CNC membranes can work as a PEM in fuel cells. However, membranes made of pristine nanocellulose lacked sufficient proton conductivity, and required better stability in high temperature and humid environments. Namely, the inherent proton conductivity of nanocellulose is quite low: i.e. ~0.05 mS/cm for cellulose nanofiber (CNF) and ~4 mS/cm for cellulose nanocrystals (CNC) membrane [Bayer, T. et al., Chem. Mater. 2016, 28, 4805–4814] and thus requires significant improvement. Conventional approaches for nanocellulose modification (e.g. TEMPO-oxidation, backbone sulfonation) can achieve better proton conductivity only by sacrificing other properties (i.e. the mechanical, aqueous, and chemical stability) [Isogai A. et al. Nanoscale, 2011, 3, 71-85]. In contrast, crosslinking of nanocellulose was shown to provide improved mechanical properties and reduced swelling in water [Quellmalz et al. ACS Biomater. Sci. Eng. 2015, 1, 271−276], as well as better proton conductivity when an appropriate crosslinking agent is used (i.e. strong acid and good proton donor) [Seo J. A. et al. Ionics 2009 15, 555–560]. In this work we used chemical crosslinking of nanocellulose with sulfonic acid in order to achieve the required properties for PEM applications. Sulfosuccinic acid (SSA) was used as a crosslinking agent. Crosslinking of cellulose nanofibers (CNF) and cellulose nanocrystals (CNC) membranes was performed in a hot-press, to form SSA-x-CNF and SSA-x-CNC membranes. Initial tests resulted in membranes of < 10 µm thickness; significantly decreased swelling; stability in boiling water; and increased through-plane conductivity by 2 orders of magnitude. The findings so far confirm the hypothesis that a proper crosslinking agent can turn nanocellulose into an effective proton conductor as well as improving other relevant properties. Investigation into the effect on mechanical strength, chemical stability and fuel cell performance is still underway and will be also reported.