Pseudocapacitance of Copper-Nickel Nanowires for Transparent Electrochemical Capacitor Electrodes
Our study demonstrates the potential to utilize raw materials and processing techniques that avoid many of these cost barriers associated with transparent electrodes to build a more viable transparent and flexible supercapacitor electrode. By optimizing the treatment and deposition of randomly distributed networks of solution-synthesized core-shell copper-nickel nanowires, we achieve a pseudocapacitive material that is a formidable candidate for use as a supercapacitor anode. Copper is chosen for our nanowires since it is roughly 1000 times more abundant, 1% the cost of silver, and only a exhibits a 6% loss in conductivity. Moreover, the synthesis method used to produce these nanowire structures is highly compatible with roll-to-roll manufacturing. This copper core serves as the conductive medium while the electrolessly deposited nickel shell protects the copper from oxidation. The nickel film can then be easily electro-oxidized in an electrolyte solution to form the nickel hydroxide species that are responsible for the pseudocapacitive charge storage mechanism. This study examines the trade-off of transparency vs. surface area/capacitance for transparent supercapacitors to facilitate the choice of the application-specific performance. We find that a thin layer of nanowires can exhibit areal capacitances greater than 1000 µF/cm2 at a sweep rate of 20 mV/s while maintaining roughly 80% transmittance with good cycle stability in 1 M KOH electrolyte. This is commensurate with many of the latest efforts devoted to developing transparent supercapacitors. Furthermore, we systematically characterize the material properties of electro-oxidized nickel coatings of various thicknesses to determine the optimum ratios for capacitance versus nanowire density and nickel composition for such a nano-architecture.
Figure - Cyclic voltammetry scans at various sweep rates showing pseudocapacitive redox peaks due to Ni(OH)2 outer shell. Inset shows scanning electron micrograph displaying network of nanowires composed of a 20% Ni shell.