Study of Nanocellulose Crosslinking with Organic Acids for Improved Proton Conductivity in Nanocellulose Paper-Based Proton Exchange Membranes

The utilization of sustainable material feedstocks (such as cellulose, naturally produced by plants) to fabricate advanced materials is the vital area of the research needed to achieve the future independent of fossil fuels and related greenhouse gas emissions. Japan focuses on the energy transition toward a hydrogen society to improve its sustainability profile – a conceptually different energy system with hydrogen (H2) as the primary energy carrier. Fuel cells and specifically polymer electrolyte membrane fuel cells (PEMFCs) – the devices used for efficient transformation of chemical energy into electrical energy are the key technology to achieve this transition. PEMFCs are particularly suited for the transportation sector due to their high power density, durability, and fast start-up time.<br/>PEMFCs are built around an essential core element – the proton exchange membrane (PEM). The main feature of a PEM is the ability to conduct protons through its structure selectively, enabling the controlled reaction between hydrogen and oxygen. In addition, the PEM acts as a barrier to gases and electrons. Currently, the specific class of materials – sulfonated perfluoropolymers (or perfluorosulfonic acids) are benchmarks for PEM. Among them, the most recognized polymer is Nafion® (Du Pont). Its chemical structure and microscopic morphology are responsible for its high proton conductivity of &gt; 100 mS/cm in the fully hydrated state [1]. However, these materials' high-cost and non-recyclable nature are among the main drawbacks that prevented their use on a larger scale and enabled the lower cost of hydrogen fuel cell devices. These factors motivate the research to develop novel materials to substitute the PFSA materials for PEM applications.<br/>Most mainstream research efforts relate to the synthesis of polymers with architectures similar to Nafion®, namely, including acid-bearing functional groups in conventional polymers [2]. Recently, however, considering the synthetic origin of most polymers (made of fossil fuels), it is strongly desired to use more sustainable polymer feedstocks. Therefore, various bio-polymers naturally produced by the biosphere are considered a good option. Nanostructured forms of cellulose (often referred to as nanocellulose) are particularly attractive. Cellulose is a biodegradable polymer; it has numerous advantages over fluorinated polymers, such as exceptional mechanical strength, high gas barrier, and chemical structure convenient for modifications. The utilization of nanocellulose in the form of PEMFCs and the concept of "paper-based" fuel cells was introduced by our research group [3]. However, low proton conductivity and the aqueous stability of pristine nanocellulose membrane are limiting factors for the fuel cell's performance and application.<br/><br/>This work will report a systematic study of organic-acid-based crosslinking of various nanocelluloses to create the material suitable for PEMs for hydrogen fuel cells. Namely, we demonstrate that crosslinking of nanocelluloses with acid could deliver improved proton conductivity, provide mechanical reinforcement allowing the fabrication of thinner membranes, and enhance the stability in aqueous media. Ultimately these improvements would result in suitable PEMFC performance leading to the expansion of the concept of "paper-based" fuel cells that are low-cost and biodegradable. From the fundamental point of view, this study also aims to clarify what is more important for the proton conduction: "backbone sulfonation" or "sulfonation via added crosslinker". Finally, from the practical point of view – the research aims to create proton exchange membranes that can be used as a low-cost alternative in hydrogen fuel cells.