Cellulose is derived from biomass and is useful in a wide range of applications across society, most notably in paper and cardboard. Nanocellulose is a relatively newly discovered variant of cellulose with much smaller fibril size, leading to unique properties such as high mechanical strength. Meanwhile, electrochemical energy conversion in fuel cells will be a key technology in the development of the hydrogen economy, but new lower cost proton exchange membrane (PEM) materials are needed. Nanocellulose has emerged as a potential candidate for this important application. In this review we summarize scientific developments in the area of cellulosic materials with special emphasis on the proton conductivity, which is the most important parameter for application in PEMs. We cover conventional cellulose and nanostructured cellulose materials, polymer composites or blends, and chemically modified cellulose. These developments are critically reviewed, and we identify interesting trends in the literature data. Finally, we speculate on future directions for this field.

In this work, we deposited a CO2-selective block copolymer, Pebax-1657, as a selective layer with a thickness of 2–20 nm on the oxygen plasma-activated surface of poly­(dimethylsiloxane) (PDMS) used as a gutter layer (thickness ∼400 nm). This double-layered structure was subsequently transferred onto the polyacrylonitrile (PAN) microporous support and studied for CO2/N2 separation. The effect of interfacial molecular arrangements between the selective and gutter layers on CO2 permeance and selectivity has been investigated. We have revealed that the gas permeance and selectivity do not follow the conventional theoretical predictions for the multilayer membrane (resistance in series transport model); specifically, more selective CO2/N2 separation membranes were achieved with ultrathin selective layers. Detailed characterization of the chemical structure of the outermost membrane surface suggests that nanoscale blending of the ultrathin Pebax-1657 layer with O2 plasma-activated PDMS chains on the surface takes place. This nanoblending at the interface between the selective and gutter layers played a critical role in enhancing the CO2/N2 selectivity. CO2 permeances in the developed thin-film composite membranes (TFCM) were between 1200 and 3500 gas permeance units (GPU) and the respective CO2/N2 selectivities were between 72 and 23, providing the gas separation performance suitable for CO2 capture in postcombustion processes. This interpenetrating polymer interface enhanced the overall selectivity of the membrane significantly, exceeding the separation ability of the pristine Pebax-1657 polymer.

Nanocellulose is a sustainable material which holds promise for many energy-related applications. Here, nanocrystalline cellulose is used to prepare proton exchange membranes (PEMs). Normally, this nanomaterial is highly dispersible in water, preventing its use as an ionomer in many electrochemical applications. To solve this, we utilized a sulfonic acid crosslinker to simultaneously improve the mechanical robustness, water-stability, and proton conductivity (by introducing -SO3−H+ functional groups). The optimization of the proportion of crosslinker used and the crosslinking reaction time resulted in enhanced proton conductivity up to 15 mS/cm (in the fully hydrated state, at 120 °C). Considering the many advantages, we believe that nanocellulose can act as a sustainable and low-cost alternative to conventional, ecologically problematic, perfluorosulfonic acid ionomers for applications in, e. fuel cells and electrolyzers.

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.