All about supercapacitors explained in this app note from Abracon. Link here (PDF)
While batteries have been a mature technology for over a century, the need for energy storage solutions with faster charging and discharging cycles than traditional batteries has led to the search for a new alternative. Although conventional capacitors offer the fastest charging and discharging cycles among energy storage solutions, they lack the high energy densities that batteries feature. Technological research in the domain of energy storage has given birth to a new class of solution that bridges the gap between the properties of both batteries and capacitors: supercapacitors.