Highly Conductive Nanoassemblies for Electrochemical Energy Applications
The global need for sustainable energy is ever increasing and electrochemical devices such as batteries, fuel cells, and solar cells show great potential. A major component of these devices is an electrolyte which enables fast charge transport between electrodes. Traditionally, liquid or gel electrolytes are used which prohibit widespread use of these devices due to stability and safety concerns. The challenges presented in development of new polymer electrolyte systems are high conductivity, mechanical strength, and nanometer level control to optimize materials properties. Although the desired properties of polymer electrolytes depend on the device application, fast ion conduction is essential to reduce electrical resistance and power losses. Furthermore, it would be desirable to utilize approaches that are cost effective and environmentally friendly.
Alternating charge-based layer-by-layer (LBL) film assembly, combined with the commercial availability of polyelectrolytes, allow for constructing highly conducting polymeric conductors targeted towards energy conversion devices such as dye-sensitized solar cells (DSSCs) and proton-exchange membrane fuel cells (PEMFCs). This approach presents strong advantages as it allows the incorporation of many different functional materials within a single thin film at a full range of compositions with exceptional homogeneity. For example, by pairing a sulfonated poly(2,6-dimethyl 1,4-phenylene oxide) (sPPO) with poly(diallyl dimethyl ammonium chloride) (PDAC), we obtain ionic conductivity values of up to 35 mS/cm, the highest ever obtained from an LBL assembled thin film.1 These multilayer systems also exhibit low liquid methanol permeability, which provides a promising application in direct-methanol fuel cells (DMFCs). Uniformly coating traditional fuel cell membranes with these LBL films (see figure below) improves the power output of a DMFC by over 50%.
Conventional electrolytes for many electrochemical energy conversion and storage devices consist of a polar liquid capable of solvating ions. The need for a safe and lightweight solid state electrolyte has driven extensive research to replace caustic or flammable liquid electrolytes to circumvent problems associated with leakage. Poly(ethylene oxide) (PEO) has been one of the most thoroughly investigated polymer electrolytes because it bears cation-solvating ether groups and a flexible backbone for facile ion mobility. However, its crystallinity, and limited chemical stability are major limitations for realistic applications. To address these issues, a hybrid organic-inorganic polymer, poly[bis(methoxyethoxyethoxy) phosphazene] (MEEP), is designed by functionalizing a polyphosphazene backbone with ethylene oxide chains. The phosphazene backbone has numerous advantages over that of PEO, such as higher chain flexibility and thermo-oxidative stability. In collaboration with the Allcock Group from Penn State, we have investigated the LbL assembly method to create homogenous blends of MEEP, a hydrogen bonding acceptor, and poly (acrylic acid) (PAA), a hydrogen bonding donor, with controlled film growth, high ionic conductivity, and excellent hydrolytic stability.2 This is the first incorporation of a phosphazene based polymer into a multilayer structured thin film. By simply varying the assembly conditions, it is possible to tune these multilayers as candidates for various electrochemical devices such as batteries, fuel cells, and dye-sensitized solar cells.
Conjugated polymer-based electrochromic materials have attracted great attention because of their low cost, ease of processing, and relatively high optical contrast and response speed, which are essential for their applications in electrochromic devices. PANI is the most widely used electrochromic polymer, which possesses good environmental stability and exhibits multicolor electrochromism in visible region as well as in infrared region. An attractive approach to the electrochromic thin film fabrication method is the LBL assembly technique that utilizes the positive charges on the PANI backbone. In collaboration with the Nanyang Technology University of Singapore, we have demonstrated the first incorporation of a POSS-PANI copolymer into electrochromic thin films via LBL assembly.3 Under dynamic switching conditions, the LBL assembled POSS-PANI/PAMPS multilayer thin films exhibit higher optical contrast and faster switching kinetics compared to their linear PANI/PAMPS counterparts because of the synergistic combination of the starlike molecular architecture and the ultimate morphology control provied by the LBL assembly method. The switching kinetics of the POSS-PANI/PAMPS multilayer films is also much faster than that of spin-coated thin films of POSS-PANI/polymeric acid complexes.
2-) Argun, A. A.; Ashcraft, J. N.; Herring, M. K.; Lee, D. K. Y.; Allcock, H. R.; Hammond, P. T. "Ion Conduction and Water Transport in Polyphosphazene-Based Multilayers". Chemistry of Materials 2010, 22, 226-232.
3-) Jia, P. T.; Argun, A. A.; Xu, J. W.; Xiong, S. X.;
Ma, J.; Hammond, P. T.; Lu, X. H. "Enhanced Electrochromic Switching in
Multilayer Thin Films of Polyaniline-Tethered Silsesquioxane Nanocage". Chemistry of Materials 2009, 21,