Vacuum-deposited thin film porous ZnO-metal oxide hybrid systems for microsupercapacitor applications with Ir/IrO₂in ZnO as a new, high-performance electrode

We fabricate porous nanostructured 1 μm thick ZnO-metal/metal oxide hybrid material thin films using a unique approach utilizing physical vapor deposition with postdeposition annealing. We study Pt, Pd, Ru, Ir and Sn as the metals and find they all form hybrid structures, however with differing physical and electrochemical properties. We investigate their applicability in microsupercapacitor electrodes in a LiCl aqueous electrolyte and find that the ZnO hybrid with Ir exhibits the highest capacitances. We follow with optimization and more detailed material studies of the ZnO-Ir hybrid showing that a significant amount of Ir is present in the material in the form of metallic Ir and indiffused Ir, while IrO2 is also present in the nanoscale. We obtain electrodes with 5.25 mF • cm-2 capacitance with 90% retention over 10 000 charge/discharge cycles in an aqueous LiCl electrolyte, which is better than the reported values for other Ir-based hybrids. Finally, we showed that the electrodes provide 2.64 mF • cm-2 in a symmetric device with an operating voltage of 0.8 V. With this report, we discuss the influence of both Ir and IrO2 on the capacitance, underlining the synergistic effect, and show them as promising inorganic matterials for integration with other supercapacitor electrodes.