The polymer electrolyte membrane fuel cells (PEMFCs) (Scofield, Liu, & Wong, 2015; Hwang et al., 2022) are energy-conversion devices based on the phenomenon of proton conduction. They are an attractive alternative to electrical devices that generate energy based on fossil fuels. The growing awareness of societies worldwide regarding the need to protect the environment for future generations causes a constant increase in interest in this type of energy source. PEMFCs are expected to become the leading technology to power vehicles, especially heavy-duty and long-range ones (Zhao & Li, 2020). Many laboratories are working on obtaining polymer electrolyte membranes, which are the heart of PEMC, with dedicated properties such as mechanical, chemical, and thermal stability over a long time and high proton conductivity under anhydrous conditions. Currently used membranes are mostly based on perfluorosulfonic acid polymers containing sulfonic groups, such as Nafion, Aciplex, and Flemion (Kusoglu & Weber, 2017). Despite the very good conductive properties, and thermal and mechanical resistance of such membranes, PEMFCs with their use have a limited operating temperature of about 90 oC because their electrical conductivity depends on the proton conductivity of the hydrated membrane. The proton conductivity of Nafion-based membranes falls in the range from 1 to 20 S/m, but the highest value is only achieved under highly hydrated conditions and drops drastically due to membrane dehydration at high temperatures (Liu, Chen, & Li, 2016). In addition to the above-mentioned issue, the serious disadvantages of Nafion-based membranes are the high cost of their production, the limited lifetime associated with operation below 100 oC, and the lack of environmental friendliness. The latter factor is now extremely important as we observe the global fight to stop climate change. Thus, there is still a need for further development of membrane material with thermal, mechanical, and conductivity properties similar to or better than Nafion but environmentally friendly and proton-conductive in anhydrous conditions. The last property will allow the fuel cells to work above 100 oC, preferably up to 150 oC, and thus, eliminate the problems with a low-temperature operation, such as poor electrode performance due to the slow rate of oxygen reduction at the cathode, low tolerance of platinum catalyst in the electrode to the fuel impurities (e.g., CO), low heat rate transfer, and water management (Zhao & Li, 2020; Maheshwari, Sharma, Sharma, & Verma, 2018; Chandan et al., 2013). To meet the condition of membrane proton conductivity in anhydrous conditions, water, which is a proton carrier in Nafion-type membranes, must be replaced, for example, with nitrogen-containing heterocyclic molecules (Kreuer, 1997). Heterocycles can act as donors or acceptors because of the conductive charge with intermolecular proton transfer. Besides, heterocyclic molecules have a high boiling point, form hydrogen bond networks, and are characterized by a high degree of self-dissociation, which is beneficial for proton transfer (Kreuer, 1996). To ensure the thermal stability of such molecules at the high-temperature application of PEFCs they must be incorporated into a polymer matrix, preferably a biopolymer, to make the new membrane environmentally friendly. In composites, the heterocycle attached to the biopolymer backbone acts as a solid protic solvent that does not evaporate at high temperatures, while the biopolymer acts as a matrix stabilizing the heterocycle, but at the same time ensuring high local dynamics of heterocycles and protons. Cellulose deserves special attention among biopolymers as the most common naturally occurring polymer, environmentally friendly, easy, and cheap in production and recycling, characterized by unique properties including biodegradability, biocompatibility, and high thermal stability, high strength, and a great possibility of surface modifications. All these features have led to an interest in cellulose and cellulosic materials in terms of their possible applications as membranes in fuel cells (Selyanchyn, Selyanchyn, & Lyth, 2020; Vilela, Silvestre, Figueiredo, & Freire, 2019; Shojaeiarani, Bajwa, & Chanda, 2021; Asandulesa, Chibac-Scutaru, Culica, & Melinte, 2023). It has been found that the proton conductivity of pure cellulosic materials is too low (order of 10 − 11 to 10− 5 S/m at 90 oC (Jankowska et al., 2018) for real applications. To obtain a proton conductivity in cellulose that is satisfactory for potential applications in electrical devices, it must be chemically modified or mixed with other polymers or small molecules (Selyanchyn, Selyanchyn, & Lyth, 2020; Vilela, Silvestre, Figueiredo, & Freire, 2019; Shojaeiarani, Bajwa, & Chanda, 2021). In our group, we have experience in the functionalization of cellulose and cellulose nanocrystals (CNC) with imidazole (Im) and triazole (Tri) (Tritt-Goc, Lindner, Bielejewski, Markiewicz, & Pankiewicz, 2019; Smolarkiewicz, Rachocki, Pogorzelec-Glaser, & Pankiewicz, 2015; Tritt-Goc, Lindner, Bielejewski, Markiewicz, & Pankiewicz, 2020; Lindner et al., 2021). CNC-Im composites of CNC functionalized with Im show conductivity up to 10− 1 S/m at 160 oC under anhydrous conditions, but their thermal stability is unsatisfactory (Tritt-Goc, Lindner, Bielejewski, Markiewicz, & Pankiewicz, 2019). While CNC-Tri composites are characterized by appropriate thermal properties and the required service life, their highest conductivity value of 10− 4 S/m at 160 oC is much lower than CNC-Im and insufficient for potential applications as membranes in fuel cells (Lindner, Bielejewski, Markiewicz, & Pankiewicz, 2020; Lindner et al., 2021).
Protic ionic liquids (PILs) are receiving increasing interest as another proton transfer media that can be used as an alternative to water as a charge carrier in PEMFCs (Martinelli et al., 2007; Armand, Endres, Macfarlane, Ohno, & Scrosat, 2009; Shaari, Ahmad, Bahru, & Leo, 2021). They are a subclass of ionic liquids, which, in addition to properties such as low vapor pressure, high thermal stability, high ionic conductivity, and relatively low viscosity, also have an exchangeable proton, usually residing on the cation (Armand, Endres, Macfarlane, Ohno, & Scrosat, 2009). Unfortunately, ionic liquids alone cannot form membranes in PEMFCs. They must be absorbed by a material with adequate strength and chemical and thermal stability to function in the PEMFC. In the context of biopolymers, ionic liquids are best known as solvents (Swatloski, Spear, Holbrey, & Rogers, 2002), but under certain conditions, they can form composites with biopolymers, sometimes also called ionogels (Hopson et al., 2021; Takada & Kadokawa, 2015; Kaszynska, Rachocki, Bielejewski, & Tritt-Goc, 2017). Among them, cellulose ionogels are considered the most promising solid electrolytes of the future due to their high conductivity and wide electrochemical potential window. So far, they have been prepared with aprotic ionic liquids (APIL) (Zhu et al., 2019; Zhang et al., 2022).
Only recently Danyliv et al. (Danyliv, Strach, Nechyporchuk, Nypelö, & Martinelli, 2021) published the first example of CNC-based membranes embedded with protic ionic liquids such as 1-hexylimidazolium trifluoromethylsulfonate ([HC6Im][TfO]) and 1-hexylimidazolium bis(trifluoromethylsulfonyl) imide ([HC6Im][TFSI. The composites obtained are characterized by good thermal stability up to 200 oC and good conductivity values between 10− 2 to 10− 1 S/m in the temperature range between 120 and 160 oC under anhydrous conditions. The use of imidazole-based ionic liquids for the preparation of cellulose/PIL composites was motivated by their higher thermal stability compared to ammonia-based ionic liquids (MacFarlane, Forsyth, Golding, & Deacon, 2002) and immiscibility with water (Danyliv & Martinelli, 2019). The latter property will prevent possible washing out of the ionic liquid from the membrane by water which is a product of the oxygen reduction reaction at the cathode in the PEMFC, even in the case of a conductive electrolyte in anhydrous conditions. The properties of CNC-based membranes with protic ionic liquids are very promising in their use as membranes in fuel cells. Therefore research on this type of material is worth continuing.
Having extensive experience in the synthesis and study of the properties of cellulose membranes functionalized with heterocyclic compounds (Tritt-Goc, Lindner, Bielejewski, Markiewicz, & Pankiewicz, 2019; Smolarkiewicz, Rachocki, Pogorzelec-Glaser, & Pankiewicz, 2015; Tritt-Goc, Lindner, Bielejewski, Markiewicz, & Pankiewicz, 2020), we decided to investigate composites of CNC with chosen proton ionic liquid 1-methylimidazolium bis(trifluoromethylsulfonyl) imide ([MIm][TFSI]) and contribute to the still little explored field of biopolymer/PIL composites.
We hypothesize that protic ionic liquid will be a better replacement for heterocycles as charge carriers in CNC-based membranes, leading to better performance in terms of thermal and conductivity properties.
Two composites with different soaking times of cellulose with ionic liquid were prepared, resulting in different liquid content in the cellulose matrix. The composites were obtained in the form of a film. The synthesis of composites is described in detail and explained why an imidazole-functionalized CNC matrix (CNC-Im) was used for soaking with ionic liquid instead of a pure CNC matrix. The thermal and conductivity properties of the composites were characterized by thermogravimetric analysis (TGA + DTA) and electrical impedance spectroscopy (EIS). The microstructure and CNC-ionic liquid interactions were determined based on solid-state nuclear magnetic resonance (NMR) spectroscopy in the magic angle spinning (MAS) conditions. The combination of EIS and NMR data allowed us to propose a conductivity mechanism in composites and to distinguish two contributions to conductivity in one of the composites. The properties of the new composites were compared with each other and with the properties of previously studied CNC-Im/PILs (Danyliv, Strach, Nechyporchuk, Nypelö, & Martinelli, 2021). The current work presents the results of the conducted research. Composites of synthetic polymers with ionic liquids are known in the literature, but the reports of biopolymers with protic ionic liquids are unique. To our knowledge, our paper gives a second example of such compositions.