Metrics for Fast Supercapacitors as Energy Storage Devices

Supercapacitors are investigated as an energy storage device alternative to batteries, but their electrochemical performance is usually inspected with the metrics of classic capacitors. The resulting inconsistency in the literature has caused confusion about the potentials and limitations of supercapacitors. First, the average power density of a supercapacitor cannot be directly compared with the relatively constant power density of counterpart batteries. Second, specific capacitance is the capability of capacitors for charge separation by the potential perturbation and does not represent the capacity for energy storage when the delivered charge has a nonlinear dependency on the potential. Third, many new supercapacitors are not even faster than their counterpart batteries to justify practical development, but the problem is buried under the shield of inappropriate metrics. This paper clarifies that employing the appropriate metrics for energy storage can lead us in the designing of faster supercapacitors for practical applications.