Role of Defects on the Particle Size–Capacitance Relationship of Zn–Co Mixed Metal Oxide Supported on Heteroatom‐Doped Graphenes as Supercapacitors

Supercapacitors are considered among the most promising electrical energy storage devices, there being a need to achieve the highest possible energy storage density. Herein small mixed Zn-Co metal oxide nanoparticles are grown on doped graphene (O-, N- and, B-doped graphenes). The electrochemical properties of the resulting mixed Zn-Co metal oxide nanoparticles (4 nm) grown on B-doped graphene exhibit an outstanding specific capacitance of 2568 F g(-1) at 2 A g(-1), ranking this B-doped graphene composite among the best performing electrodes. The energy storage capacity is also remarkable even at large current densities (i.e., 640 F g(-1) at 40 A g(-1)). In contrast, larger nanoparticles are obtained using N- and O-doped graphenes as support, the resulting materials exhibiting lower performance. Besides energy storage, the Zn-Co oxide on B-doped graphene shows notable electrochemical performance and stability obtaining a maximum energy density of 77.6 W h Kg(-1) at 850 W Kg(-1), a power density of 8500 W Kg(-1) at 28.3 W h Kg(-1), and a capacitance retention higher than 85% after 5000 cycles. The smaller nanoparticle size and improved electrochemical performance on B-doped graphene-based devices are attributed to the higher defect density and nature of the dopant element on graphene.