Carbon-Based Flexible and All-Solid-State Micro-supercapacitors Fabricated by Inkjet Printing with Enhanced Performance

Significant advances in nanotechnology have greatly prompted the development of maintenance-free microelectronic devices such as wireless microsensors, implantable medical devices, nanorobotics, and active radio frequency identification tags, which have ultimately stimulated the rapid development of new concept and sufficiently compact energy storage systems [ 1 – 3 ]. In this respect, miniaturized supercapacitors (SCs), also called micro-SCs (MSCs), which can be fabricated directly on plastic or paper substrates and integrated with other microelectronic device, are attracting more and more attention as an important class of energy storage devices [ 4 – 11 ]. According to the charge storage mechanism as well as active materials used, SCs/MSCs generally have three types. The first is called electrochemical double-layer capacitors (EDLCs), in which the charge is stored by the surface charge separation at the electrode/electrolyte interface. The second is pseudo-capacitors or redox-capacitors, which is using fast and reversible surface faradic redox reactions for charge storage. The last are hybrid capacitors which consist of both EDLCs and pseudo-capacitors in a single device [ 4 , 5 ]. Because neither of these two surface charge storages involve diffusion of ions within the inner bulk region of electrode active materials, SCs/MSCs possess a higher power density which is an order of magnitude larger (10,000 W kg −1 ) than that of regular batteries (e.g., lithium-ion batteries). In spite of the relatively lower energy density, SCs/MSCs can deliver an energy density in two orders of magnitude higher (10 Wh kg −1 ) than that of conventional capacitors. Furthermore, unlike batteries, due to their highly reversible charge storage process, SCs/MSCs have longer life-cycle that can often achieve up to millions of charge–discharge cycles without energy storage capacity loss. With these excellent performances, SCs/MSCs have been widely acknowledged as a promising compatible power source to complement or replace micro-batteries to provide the required power and energy for maintenance-free microelectronic devices [ 12 – 16 ].