459708 Bipolar Plates for Redox Flow Batteries: Relating Conductivity to Morphology of Carbon-Based Polymer Composites

Thursday, November 17, 2016: 9:40 AM
Golden Gate 5 (Hilton San Francisco Union Square)
Jiri Vrana1, Martin Kroupa1, Petr Mazur2, Jan Dundalek1, Jaromi­r Pocedic2 and Juraj Kosek1, (1)Department of Chemical Engineering, University of Chemistry and Technology Prague, Prague 6, Czech Republic, (2)New Technologies – Research Center, University of West Bohemia, Pilsen, Czech Republic

The increasing expansion of renewable sources of electricity stimulates the need for a large-scale storage of energy. A very promising candidate in this respect is the family of redox flow batteries (RFB) due to their good scalability and the possibility to decouple the capacity and power. For the broader commercialization, the price per kilowatt-hour of the energy stored by these devices need to be competitive with other technologies, such as pumped storage hydroelectricity, conventional rechargeable batteries or hydrogen-based storage. The relatively high price of the state-of-art RFB technologies is caused by their expensive components. Therefore, there is a large motivation for the development of cheaper materials with high performance and durability for the application in RFB. This work is focused on the composite bipolar plates used in RFB and their development and optimization using the combination of experiments and mathematical modelling.
Carbon-based polymer composites represent highly conductive materials with corrosion stability (even in acidic environment) and good mechanical properties at a reasonable price. Due to these properties they are widely used as bipolar plates in various electrochemical energy convertors, such as RFB and polymer electrolyte fuel cells. These composites consist of a polymer matrix and carbon fillers. Carbon fillers are responsible for the transport of electrons through the material. The resulting conductivity of the composite is strongly influenced by the nature of the carbon-based fillers and their content in the material. In order to obtain good mechanical properties and processability of the material it is crucial to minimize the total content of the fillers, while preserving high conductivity of the composite.
The composites of polypropylene with various fillers (graphite, black, fibers) were prepared using laboratory compounder Plasticorder Brabender PL2000. The effect of the filler content on the electrical conductivity of the composite plate was observed by the measurements of in-plane and through-plane conductivity. The morphology of the composites was observed using AFM, SEM and X-ray micro tomography. The mathematical model is based on the Poisson’s equation and computes the effective conductivity of the composite plate from different conductivities of the matrix and fillers and from the morphology of the plate. This morphology can be either obtained from 3D-reconstructed images of the experimental samples or generated artificially using algorithms for the sphere or fiber deposition.
Our results indicate that a significant increase of the plate conductivity can be achieved by the careful tuning of its composition. For example, the addition of the carbon black into polymer-graphite composites resulted in a dramatic decrease of the percolation threshold due to synergetic effect of the fillers. Moreover, even with the same total filler content, different morphologies of the composite can further increase the conductivity, an effect observed in both our experimental and simulation results. Namely, the shape of the filler particles largely determines the effective conductivity. Anisotropic particles were found to be more efficient in achieving high effective conductivities, but at the same time their orientation becomes an important issue leading to the anisotropy of the whole composite.
The basic idea of this work is to improve the properties of a complex composite material through the understanding and subsequent tuning of its morphology. The developed combination of the experimental and theoretical approach offers a promising tool for the further optimization of carbon-based composite materials.

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