Soft and flexible nanocomposite films for batteries, super-capacitors, and solar cells

This is a collaborative project with the Deutsche Elektronen Synchrotron (DESY), Hamburg, Germany and the Kungliga Tekniska högskolan (KTH) in Stockholm, Sweden.

This is a collaborative project with the German Aerospace Center (DLR) (Institute for Material Physics in Space), Cologne, Germany, and the Université Grenoble (SIMAP), France.

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is a water-soluble and electrically conductive polymer that is increasingly applied as organic electronics in numerous fields such as batteries, super-capacitors, and solar cells. However, pure PEDOT:PSS films absorb a significant amount of water (over 50 V%), which is accompanied with a pronounced swelling process (swelling ratio of up to 1.6). As a result, PEDOT:PSS films exposed to humid environments, experience strong mechanical stress, which eventually results in a decreased device performance.

One approach to overcome this, is the integration and mechanical re-enforcement of soft PEDOT:PSS films with organic nanofibers. While there are several options regarding the use of nanofibers e.g., made from carbon, silver or glass, cellulose nanofibrils (CNFs) are gaining more and more attention due to their properties: they feature mechanical stiffness, lightweight, and flexibility. In addition, CNFs can be chemically modified, which results in different surface charges, and thus, surface properties, while CNFs are prepared from sustainable resources, featuring biodegradability.

As nanocomposite material PEDOT:PSS/CNF exhibits a complex film structure: in a dry state, CNF self-assembly into bundles, while PEDOT:PSS is distributed on the CNF surface but also forms larger clusters in-between the CNF bundles. Furthermore, the films feature a certain microporosity. Upon humidifying, water diffuses into these empty pores, PEDOT:PSS de-wets from the CNF surface, and overall, the CNF bundles as well as the PEDOT:PSS clusters become larger. The nano-structure can be directly correlated with the conductivity of the films i.e., control over the nanostructure offers a possibility to optimize the films conductivity.

Figure 1: (left) Schematic representation of the CNF agglomerates (rods)/PEDOT:PSS (red coating and spherical appearance) under initially dry (state I), humidified (state II), and re-dried (state III). State II and III are cyclic repetitive, where the blue arrow marks applied humidity and the yellow arrow the drying. (right) Conductivity (blue bars) as a function of exposure to low and high humidity. A direct correlation of the measured conductivity and nanostructure of the PEDOT:PSS/CNF films could be established. (Figure taken from C. J. Brett, O. K. Forslund, E. Nocerino, L. P. Kreuzer, T. Widmann, L. Porcar, N. L. Yamada, N. Matsubara, M. Månsson, P. Müller-Buschbaum, L. D. Söderberg, S. V. Roth, Humidity-Induced Nanoscale Restructuring in PEDOT:PSS and Cellulose Nanofibrils Reinforced Biobased Organic Electronics. Adv. Electron. Mater. 2021, 7, 2100137. https://doi.org/10.1002/aelm.202100137)

Water dynamics are believed to play a crucial role for structure evolution. So far, we know there are different water species present in the film: Unbound (or weakly bound) water that diffuses more or less freely within the film, and strongly bound hydration water that forms a hydration layer around CNF and PEDOT:PSS. Still, many questions remain unanswered: i) What is the ratio between bulk and hydration water? ii) How does this ratio change with film thickness, CNF surface charges, counter-ion concentration, and temperature? iii) Is there a possibility to control/manipulate the water-cellulose interaction with external (temperature, pressure, light, ultrasound, salt concentration) or internal (chemical modification) trigger points?

Both, the structural and dynamical features are investigated with neutron scattering techniques (mainly GISANS and QENS). Other techniques such as FTIR, Raman, and conductivity measurements are used to characterize hydration effects and electrical properties of the PEDOT:PSS/CNF nanocomposite films.

Please see these papers for more information

https://onlinelibrary.wiley.com/doi/full/10.1002/aelm.202100137

https://pubs.acs.org/doi/10.1021/acs.macromol.9b00531

If you are interested in the origin of water-cellulose interactions, and the structure-dynamics relationship in conductive polymer films, feel free to reach out to me via lucas.kreuzer@frm2.tum.de, currently there are open positions for master and bachelor students.