Realization of cost-effective energy storage that is both safe and scalable relies on the ease with which it can be integrated into existing manufacturing technology. Roll-to-roll and solution-based processes are two such technologies that offer the ability to deposit flexible films of controlled thickness for a low cost device. Our work uses one such approach to build a transparent and flexible pseudocapacitive electrode using earth-abundant and low cost materials. The electrode is comprised of a core/shell Cu/Ni(OH)₂ structure of nanowires (NWs) on a glass slide. In order to understand the effects of both NW network density and the thickness of the Ni(OH)₂ shell, we utilize Meyer rod and electroless plating techniques to control the network density and shell thickness, respectively. Our study uses NW networks with resistance of <50 Ω/sq. and >60% transmittance to investigate the performance of Ni(OH)₂shells with 20, 30, and 40 mol% Ni, with respect to the conductive Cu core.

Controlling the Cu NW density and Ni(OH)₂ shell thickness yields capacitances of 800 F/g and 2550 μF/cm² at 0.02 mA/cm² charge-discharge current density with >70% transmittance. The 30 mol% Ni on Cu NW structure exhibits higher gravimetric capacitances than the 20 and 40 mol% Ni structures. We propose that these differences are due to diffusion limitations limiting the amount of electrochemically active material and to the density of active NWs, which impacts the sheet resistance of the underlying conductive NW network. In addition, the quasi-1D Ni(OH)₂ structure of our NW devices enabled highly reversible pseudocapacitive intercalation and remarkably stable performance under fast charge-discharge rates, exhibiting coulombic efficiencies of >90% with only a 13% decrease in capacitance after 10,000 cycles for the thicker 40 mol% Ni(OH)₂ shell.

Our continuing and future work in exploring the Cu/Ni(OH)₂ NW material is to further stabilize and improve both areal and gravimetric capacitance while we investigate the use of a flexible polypropylene (PPE) substrate. Preliminary results show that the electrode can be bent at a 90° angle and retain capacitive behavior beyond 12 hrs in solution. Additionally, we seek to match our electrode’s characteristics with a suitable counter electrode and solid-state electrolyte to construct a fully flexible, hybrid supercapacitor device.