Section: Batteries & Fuel Cells

Depletion Aggregation

We are exploring a number of synthetic strategies to produce nanohybrids systems that carry great promise as structural materials in fuel cells and other electrochemical devices. To that end, we have prepared highly oriented arrays of several inorganic (silicate, graphene and graphitic) nanosheets well dispersed within the Nafion matrix (Figure 1a), without the need of any external field1,2. By virtue of their impermeable and plate-like nature, inorganic nanolayers can significantly enhance the barrier properties of the polyelectrolyte membranes. A recent advance of that methodology refers to a two-step procedure to produce graphene nanolayers well aligned within the polyelectrolyte matrix by in situ reduction of Graphite Oxide precursor nanocomposites (Figure 1b). 

Figure 1. Nafion based  nanohybrids; a) H+ exchanged clay/Nafion membranes, b) graphene/Nafion  composites, c) water-Nafion-clay gel with clays showing a network  structure, d) cryogenically dried Nafion with a porous microstructure.

Following an alternative approach based on concepts of depletion aggregation we have produced hybrid membranes with a network structure3. The interaction diagrams of Nafion aqueous solutions in the presence of negatively charged nanoparticles (clay, silica, carbon black) are used as guidelines to identify the development of percolated networks of nanofiller clusters (Figure 1c). Subsequent drying of those physical gels (under controlled conditions) can give rise to nano and/or submicron composites that have the potential to be integrated to various parts of an electrochemical device. We are currently applying cryogenic techniques to prepare well aligned Nafion based scaffolds that posses a multi level of porosity in both nano and micro scale (Figure 1d). Incorporation of carbon black, carbon nanotubes, carbon nanofibers or graphene sheets can confer electrical conduction to those complex structures, making them promising candidates for membrane electrode assemblies in fuel cell applications.  

References
1. R. H. Alonso, L. Estevez, H. Lian, A. Kelarakis, E. P. Giannelis, Polymer 50, 2402 (2009)
2. S. Ansari, A. Kelarakis, L. Estevez, and E. P. Giannelis, Small 2, 205 (2010)
3. E. Burgaz, H. Lian, R. H. Alonso, L. Estevez, A. Kelarakis,  E. P. Giannelis  Polymer 50, 2384 (2009)