Supercapacitors are considered optimal storage devices for electric vehicles and wireless technology due to their superior power density, cycle stability, and charge-discharge efficiency in comparison to traditional batteries [1]. Their inability to act as primary energy sources, however, is hindered by their low energy density. Research efforts are thus concentrated on nanostructured materials that exhibit enhanced specific capacitance, with a focus on carbon-based materials such as activated carbon, carbon nanotubes (CNTs),  carbon aerogels and graphene [2]. Supercapacitors operate via electrochemical double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs store energy at the electrode/electrolyte interface through ion adsorption, relying on carbon-based electrodes [3]. Although graphene has superior electrical and thermal conductivity, its theoretical capacitance leads to limitations, despite showing promise. Adjusting the alignment of graphene from horizontal to vertical, as discovered in previous studies, leads to a 38% increase in capacitance, thereby overcoming some of the limitations [4]. Vertical graphene nanowalls (GNWs), manufactured via plasma-enhanced chemical vapor deposition (PECVD), provide superior electrical conductivity and a three-dimensional structure that makes them ideal scaffolds for supercapacitor electrodes [5], [6]. Pseudocapacitors, comprising of metal oxides and polymers, demonstrate a higher specific capacitance in comparison to EDLCs but necessitate a carbon substrate for better charge transfer [7]. The amalgamation of carbon materials and metal oxides manifests a synergistic effect, fulfilling vital requisites for novel energy storage devices. Nevertheless, the elevated cost and toxicity linked with most metal oxides present obstacles. Zinc oxide (ZnO) is a promising electrode material for supercapacitors due to its natural abundance, eco-friendliness, cost-effectiveness, and long cycling life [8]. The specific capacitance of ZnO can be improved by preserving oxygen vacancies, which can be attained by annealing in inert atmospheres [9]. Although integrating ZnO with graphene-based structures has exhibited notable advancements, it remains a complicated and expensive process.

In this study, GNWs represent a promising porous structure, offering a substantial surface area for active sites and facilitating rapid ion diffusion. To enhance their specific capacitance, we present a supercapacitive improvement through a hierarchical arrangement of defect-engineered ZnO nanorods (ZNRs) anchored onto the GNWs. This hierarchical structure is synthesized through a multi-step process involving inductively coupled plasma-chemical vapor deposition, magnetron sputtering, and hydrothermal methods. The presence of oxygen vacancy (OV) defects within the ZNRs, induced by argon annealing, has been characterized utilizing X-ray photoelectron spectroscopy. The emergence of OV defects below the conduction band of the ZNRs results in a narrowing of the bandgap within the hybrid structure, thereby enhancing its conductivity and increasing the reaction sites. In the capacity of supercapacitor electrodes, the ZNRs/GNWs hybrids were evaluated in an aqueous KOH electrolyte solution, operating within a voltage range of 0.5 V and at a current density of 0.1 mA cm^-2. This assessment yielded an area capacitance of 21.45 mF cm^-2, signifying a 1.5-fold increase in capacitance compared to GNWs grown on graphite sheets. The ZNRs/GNWs hybrid demonstrates remarkable electrochemical performance and exhibits substantial potential for energy storage applications. Our work is expected to offer valuable insights for the enhancement of electrochemical properties in various composite and hybrid materials.