(774e) Electrostatically Assembled Carbon Nanoparticle Electrodes for Vanadium Redox Flow Batteries

Efficient charge and mass transfer at the surface of the electrode is critical to overall redox flow battery (RFB) performance.  Fundamental structure-property correlations for RFB electrodes cannot presently be obtained using electrodes produced from carbon felts or carbon nanoparticle slurries and pastes due to the ill-defined distribution of active sites in these materials. For example, a recent study using high surface area multi-wall carbon nanotubes as electrode materials yielded only a two to three-fold increase in peak current density relative to glassy carbon.  We here report on the first use of electrostatic layer by layer (LbL) assembly to fabricate RFB electrodes.  Since its introduction by Iler over four decades ago, LbL assembly has been repeatedly shown capable of producing well-defined arrangements of nanoparticles and the method is now ubiquitous in scientific research.  However, the approach has yet to be applied to produce well-defined electrode structures for RFB electrodes and has only rarely been explored in the general field of electrocatalysis.  Key areas of concern include: 1) the impact of the polymer counter-ion layer that is typically required for assembly on ohmic resistance, 2) accessibility of active sites in tightly packed layered structures, and 3) durability.  In this investigation, we demonstrate how LbL assembly of graphite nanoplatelets may be used to achieve unprecedented high current densities for the all-vanadium system through fine control over the thickness and morphology of the electrode. In particular, we show how nanoparticle packing density, size, and size distribution in addition to local polymer binder concentration and crystalline order all impact active site concentration and promote reagent accessibility to these sites in a manner that has not been previously recognized.  Such basic understanding permits rational optimization of electrode chemistry and structure using carbon-based electrocatalysts as well as detailed interpretation of the promotional effects of inorganic electrode additives.  Finally, we demonstrate through electrochemical cycling how these coatings are exceptionally stable relative to electrodes fabricated without LbL assembly.