Electrostatic Layer-by-Layer (LbL) assembly of nanoplatelets of stacked graphene sheets and bismuth metal nanoparticles having negative surface charge on cationic polymer binder was investigated as an electrode fabrication method. The bismuth nanoparticles were synthesized utilizing a novel aqueous-based autoreduction scheme wherein SnCl2 serves as both reducing and stabilizing agent. Correlation of well-defined electrode nanostructure to fundamental electrocatalytic activity was evaluated using Scanning Electron Microscopy (SEM), Energy- Dispersive Spectroscopy (EDS), Rotating Disk Electrode (RDE), Cyclic Voltammetry (CV), and Electrochemical Impedance Spectroscopy (EIS). LbL assembly was found to be a facile method of obtaining large electrochemically active mass-specific surface areas for the positive VO2+/VO 2 + and the negative V3+/V2+ redox reactions crucial to Vanadium Redox Flow Battery (VRFB) application for large-scale energy storage. Redox exchange currents obtained using RDE were found to increase with the number of carbon nanoplatelet layers deposited. This process thereby provides a convenient means by which energy storage can be systematically scaled to differing power requirements.

LbL assemblies for the positive VO2+/VO2 + electrode consisted of horizontally oriented stacked graphene nanoplatelets deposited onto cationic polymer binder that yielded a constant exchange current density, i0, of 3.36 mA cm-2 per layer. Nanoplatelet agglomeration did not impact the electrochemically active area with the deposition of successive layers. In the case of the negative V3+/V2+ redox reaction, layered structures consisted of co-adsorbed bismuth metal and stacked graphene nanoplatelets in horizontal orientations exhibited i0 that increased from 35.81 mA cm-2 for 4 layers to 52.63 mA cm-2 for 20 layers. Increasing CV currents at peak potential with number of layers were also observed on more commercially viable bipolar substrate materials. Identically oriented structures to those on glassy carbon substrates yielded a constant peak mass-specific CV current of 135 A g-1 for the positive VO2+/VO2 + electrode obtained at a scan rate of 0.05 V s-1. In the case of negative V3+/V2+ redox reaction, the peak current increased from 20 mA cm-2 to 55 mA cm-2 when Bi was incorporated into the electrode nanostructure. Separation of the onset potential of the hydrogen evolution reaction (HER) from that of the V3+/V2+ redox couple was found to be the mechanism by which such high activities were obtained using these hybrid nanostructures. The LbL assemblies were also found to be exceptionally durable. Negligible loss in activity was observed over 100 electrochemical cycles from -.1 V (vs Ag/AgCl) to 1.2 V (vs Ag/AgCl).

Pseudocapacitive behavior of Bi2O3-based graphite thin-film structures was also evaluated for asymmetric electrode-type supercapacitor conditions. The area-specific capacitance improved linearly with increasing number of hybrid nanoparticle layers (stacked graphene sheets with incorporated Bi nanoparticles) on graphene substrates. Correspondingly, a nearly-constant massspecific capacitance was observed at different applied current densities for all electrode assemblies recorded at same electrolyte conditions. Typical values obtained for the mass-specific capacitances were 550, 485, and 410 F g-1 at current densities of 1, 2 and 4 A g-1, respectively. This result provides an increase of two orders of magnitude over alternate approaches that do not employ LbL assembly.