Polypyrrole⁻Nickel Hydroxide Hybrid Nanowires as Future Materials for Energy Storage

Hybrid materials play an essential role in the development of the energy storage technologies since a multi-constituent system merges the properties of the individual components. Apart from new features and enhanced performance, such an approach quite often allows the drawbacks of single components to be diminished or reduced entirely. The goal of this paper was to prepare and characterize polymer-metal hydroxide (polypyrrole-nickel hydroxide, PPy-Ni(OH)₂) nanowire arrays demonstrating good electrochemical performance. Nanowires were fabricated by potential pulse electrodeposition of pyrrole and nickel hydroxide into nanoporous anodic alumina oxide (AAO) template. The structural features of as-obtained PPy-Ni(OH)₂ hybrid nanowires were characterized using FE-SEM and TEM analysis. Their chemical composition was confirmed by energy-dispersive x-ray spectroscopy (EDS). The presence of nickel hydroxide in the synthesized PPy-Ni(OH)₂ nanowire array was investigated by X-ray photoelectron spectroscopy (XPS). Both FE-SEM and TEM analyses confirmed that the obtained nanowires were composed of a polymer matrix with nanoparticles dispersed within. EDS and XPS techniques confirmed the presence of PPy-Ni(OH)₂ in the nanowire array obtained. Optimal working potential range (i.e., available potential window), charge propagation, and cyclic stability of the electrodes were determined with cyclic voltammetry (CV) at various scan rates. Interestingly, the electrochemical stability window for the aqueous electrolyte at PPy-Ni(OH)₂ nanowire array electrode was remarkably wider (ca. 2 times) in comparison with the non-modified PPy electrode. The capacitance values, calculated from cyclic voltammetry performed at 20 mV s^-1, were 25 F cm^-2 for PPy and 75 F cm^-2 for PPy-Ni(OH)₂ array electrodes. The cyclic stability of the PPy nanowire array electrode up to 100 cycles showed a capacitance fade of about 13%.