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Li, C.; Kasuga, T.; Uetani, K.; Koga, H.; Nogi, M. High-Speed Fabrication of Clear Transparent Cellulose Nanopaper by Applying Humidity-Controlled Multi-Stage Drying Method. Nanomaterials 2020, 10, 2194. https://doi.org/10.3390/nano10112194., As a renewable nanomaterial, transparent nanopaper is one of the promising materials for electronic devices. Although conventional evaporation drying method endows nanopaper with superior optical properties, the long fabrication time limits its widely use. In this work, we propose a multi-stage drying method to achieve high-speed fabrication of clear transparent nanopaper. Drying experiments reveal that nanopaper’s drying process can be separated into two periods. For the conventional single-stage evaporation drying, the drying condition is kept the same. In our newly proposed multi-stage drying, the relative humidity (RH), which is the key parameter for both drying time and haze, is set differently during these two periods. Applying this method in a humidity-controllable environmental chamber, the drying time can be shortened by 35% (from 11.7 h to 7.6 h) while maintaining the same haze level as that from single-stage drying. For a conventional humidity-uncontrollable oven, a special air flow system is added. The air flow system enables decrease of RH by removing water vapor at the water/air interface during the earlier period, thus fabricating clear transparent nanopaper in a relatively short time. Therefore, this humidity-controlled multi-stage drying method will help reduce the manufacturing time and encourage the widespread use of future nanopaper-based flexible electronics.

Uetani, K.; Koga, H.; Nogi, M. Checkered Films of Multiaxis Oriented Nanocelluloses by Liquid-Phase Three-Dimensional Patterning. Nanomaterials 2020, 10, 958. https://doi.org/10.3390/nano10050958., It is essential to build multiaxis oriented nanocellulose films in the plane for developing thermal or optical management films. However, using conventional orientation techniques, it is difficult to align nanocelluloses in multiple directions within the plane of single films rather than in the thickness direction like the chiral nematic structure. In this study, we developed the liquid-phase three-dimensional (3D) patterning technique by combining wet spinning and 3D printing. Using this technique, we produced a checkered film with multiaxis oriented nanocelluloses. This film showed similar retardation levels, but with orthogonal molecular axis orientations in each checkered domain as programmed. The thermal transport was enhanced in the domain with the oriented pattern parallel to the heat flow. This liquid-phase 3D patterning technique could pave the way for bottom-up design of differently aligned nanocellulose films to develop sophisticated optical and thermal materials.

Uetani, K.; Izakura, S.; Kasuga, T.; Koga, H.; Nogi, M. Self-Alignment Sequence of Colloidal Cellulose Nanofibers Induced by Evaporation from Aqueous Suspensions. Colloids Interfaces 2018, 2, 71. https://doi.org/10.3390/colloids2040071., Cellulose nanopapers fabricated by drying aqueous colloidal suspensions of cellulose nanofibers (CNFs) have characteristic hierarchic structures, which cause the problem that their optical properties, including their transparency or haze, vary due to the drying processes affecting CNF alignment. It is unclear when and how the colloidal CNFs align in the evaporation–condensation process from the randomly dispersed suspension to form the nanopaper. In this study, we found that the CNFs undergo a self-alignment sequence during the evaporation–condensation process to form chiral nematic nanopaper by observing the birefringence of the drying suspensions from both the top and side for two suspensions with different initial CNF concentrations. The layer structures of the CNFs first form on the surface by condensation of the suspension, owing to water evaporation from the surface. The thickness of the layered structure then increases and the CNFs begin to align within each layer plane, finally forming chiral nematic structures. A birefringence difference also occurs for dried nanopapers with similar transparency or haze because of the initial CNF concentration.

This is the peer reviewed version of the following article: Inui, T., Koga, H., Nogi, M., Komoda, N. and Suganuma, K. (2015), A Miniaturized Flexible Antenna Printed on a High Dielectric Constant Nanopaper Composite. Adv. Mater., 27: 1112-1116., which has been published in final form at https://doi.org/10.1002/adma.201404555. This article may be used for non-commercial purposes in accordance with Wiley Terms and Conditions for Self-Archiving., A high-dielectric-constant and flexible cellulose nanopaper composite is prepared by mixing a small amount of silver nanowires with cellulose nanofibers. The nanopaper antenna is downsized by about a half when using the nanopaper substrate. The nanopaper antenna has potential in wearable wireless communication devices.

Glass has been widely used as a substrate for display, because of its high transparency and low thermal expansion coefficient. However, glass is not suitable for stretchable displays because it is easily broken and has no ductility. Instead of glass, synthetic polymers can be alternative. However, most synthetic polymers have low transparency and high thermal expansion coefficient. Therefore, as a substitute for glass or synthetic polymer, cellulose nano-fiber was used to make transparent, stretchable and thermally stable film.1 Cellulose is composed of β-(1,4) linked D-glucose unit. Glucose has three hydroxyl groups on surfaces and it can form hydrogen bonds with Nitrogen, Oxygen and Fluorine. Cellulose composed of glucose has many hydroxyl groups. Due to these hydroxyl groups, they are dense with strong hydrogen bonds between the fibers. The cellulose is formed by bundles of many nano-fibers. And it can be used to make a flexible film like paper. However, there is no elasticity because of the intense hydrogen bonding between the molecules.2 In order to impart elasticity to the cellulose, it is necessary to loosen the molecular structure and a network structure. Loosening the molecular structure is possible by modifying the hydroxyl group to another functional group so that the cellulose does not allow hydrogen bonding, or by using a solvent that interrupts and breaks hydrogen bonds between molecules. It is not enough to break hydrogen bonds to give elasticity, so that cross-linking was used to form a network structure. Prior to cross-linking, cellulose fiber was treated by homogenizer to increase the surface area and reaction efficiency of the cellulose. In this process, cellulose are nano fibrillized. Then, polymeric aldehyde was added as a cross-linking agent to the cellulose nano fiber. In this work, the properties of the stretchable cellulose will be presented. To investigate thermal stability, DSC and TGA analyzes were performed. Tg was measured by DSC analysis and thermal decomposition temperature of cellulose film was measured by TGA anaylsis. Transparency was measured by the transmittance at various wavelengths. To determine the strength and elongation, tensile strength and strain were also measured. REFERENCES (1) Nogi, M; Iwamoto, S; Nakagaito, A; Yano, H. Optically Transparent Nanofiber Paper. 2009, Adv. Mater. 21, 1595–1598 (2) Striegel, A. Theory and applications of DMAc/LiCl in the analysis of polysaccharides. 1997, Carbohydrate Polymers 34, 267-214

Kasuga, T.; Isobe, N.; Yagyu, H.; Koga, H.; Nogi, M. Clearly Transparent Nanopaper from Highly Concentrated Cellulose Nanofiber Dispersion Using Dilution and Sonication. Nanomaterials 2018, 8, 104. https://doi.org/10.3390/nano8020104., Nanopaper prepared from holocellulose pulp is one of the best substrates for flexible electronics because of its high thermal resistance and high clear transparency. However, the clearness of nanopaper decreases with increasing concentration of the starting cellulose nanofiber dispersion—with the use of a 2.2 wt % dispersion, for example—resulting in translucent nanopaper with a high haze of 44%. To overcome this problem, we show that the dilution of this high-concentration dispersion with water followed by sonication for 10 s reduces the haze to less than 10% while maintaining the high thermal resistance of the nanopaper. Furthermore, the combination of water dilution and a short sonication treatment improves the clearness of the nanopaper, which would translate into cost savings for the transportation and storage of this highly concentrated cellulose nanofiber dispersion. Finally, we demonstrate the improvement of the electrical conductivity of clear transparent nanopaper prepared from an initially high-concentration dispersion by dropping and heating silver nanowire ink on the nanopaper. These achievements will pave the way toward the realization of the mass production of nanofiber-based flexible devices.

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). Figure 1

Fuel cells are the key technology towards establishing a hydrogen energy based society, as they convert the chemical energy of hydrogen into electricity with high efficiency, and without emitting harmful exhaust gases. Polymer electrolyte membrane fuel cells (PEMFCs) are already successfully commercialized and are being used in e.g. fuel cell vehicles such as the Toyota MIRAI and the Honda Clarity Fuel Cell. However, the aim for next-generation PEMFCs is to operate at temperatures above the boiling point of water, therefore new membrane materials are needed. Stable operation at high temperature, sufficient high proton conductivity, high gas barrier as well as low cost are essential requirements. Cellulose is the most abundant biopolymer on earth, and has been utilized in various forms by mankind for thousands of years. Cellulose polymer units form nanofibrils during biosynthesis, with diameters of several tens of nanometers and lengths of up to a few microns. These group together to form micro- and macrofibrils. In 1977 nanofibrils were successfully observed for the first time by Turbak et al., kick-starting the field of nanocellulose science. Paper made from nanocellulose has intriguing properties such as high tensile strength (214 MPa), high gas barrier and thermal stability up to 150°C.1–3 During the fabrication process of nanocellulose, acid group functionalities (i.e. carboxyl and sulfonic acid groups) are introduced,4,5which increase hydrophilicity, and additionally act as proton donors and acceptors, thus enabling proton conduction. We investigated nanocellulose for its suitability as fuel cell membrane in PEMFCs for the first time. Two different nanocellulose materials were characterized for their morphology and chemical composition, as well as mechanical properties and gas barrier. Nanocellulose membranes showed four times higher tensile strength compared to the industry standard membrane Nafion and a roughly three orders of magnitude lower hydrogen permeability. A maximum proton conductivity of 4.7 mS/cm at 120°C and 100% RH was observed. For the first time, nanocellulose membranes were assembled into membrane electrode assemblies and operated as hydrogen fuel cell. We will report latest results of our research. References: 1. Henriksson, M., Berglund, L. A., Isaksson, P., Lindstro, T. & Nishino, T. Cellulose Nanopaper Structures of High Toughness. Biomacromolecules9, 1579–1585 (2008). 2. Nair, S. S., Zhu, J., Deng, Y. & Ragauskas, A. J. High performance green barriers based on nanocellulose. Sustain. Chem. Process.2, 1–7 (2014). 3. Nogi, M. et al. High thermal stability of optical transparency in cellulose nanofiber paper. Appl. Phys. Lett.102, 181911 (2013). 4. Lin, N. & Dufresne, A. Nanocellulose in biomedicine: Current status and future prospect. Eur. Polym. J.59, 302–325 (2014). 5. Barbosa, L. et al. A Rapid Method for Quantification of Carboxyl Groups in Cellulose Pulp. BioResources 8, 1043–1054 (2013). Figure 1

Koga, H., Nogi, M., Komoda, N. et al. Uniformly connected conductive networks on cellulose nanofiber paper for transparent paper electronics. NPG Asia Mater 6, e93 (2014). https://doi.org/10.1038/am.2014.9., We demonstrate the fabrication of highly transparent conductive networks on a cellulose nanofiber paper, called cellulose nanopaper. Uniform coating of the conductive nanomaterials, such as silver nanowires (AgNWs) and carbon nanotubes, is achieved by simple filtration of their aqueous dispersions through the cellulose nanopaper, which acts as both filter and transparent flexible substrate. The as-prepared AgNW networks on the nanopaper offer sheet resistance of 12Xsq.Ω1 with optical transparency of 88%, which is up to 75 times lower than the sheet resistance on a polyethylene terephthalate film prepared by conventional coating processes. These results indicate that the 'filtration coating' provides uniformly connected conductive networks because of drainage in the perpendicular direction through paper-specific nanopores, whereas conventional coating processes inevitably cause self-aggregation and uneven distribution of the conductive nanomaterials because of the hard-to-control drying process, as indicated by the well-known coffee-ring effect. Furthermore, the conductive networks are embedded in the surface layer of the nanopaper, showing strong adhesion to the nanopaper substrate and providing foldability with negligible changes in electrical conductivity. This filtration process is thus expected to offer an effective coating approach for various conductive materials, and the resulting transparent conductive nanopaper is a promising material for future paper electronics.