Electrochemical capacitors (i.e., supercapacitors) as energy storage technologies have attracted a lot of attention because of the increasing demand for efficient high-power delivery. Over the past decades, various advanced electrode materials and cell design have been developed to improve the performance of electrochemical capacitors. Hybrid capacitors and pseudocapacitors achieve much higher energy density due to their fast surface redox reactions. However, although all of them can be classified as electrochemical capacitors, different charge storage mechanisms alter their electrochemical characteristics. All the speculation and potential uncertainties, in the absence of standard performance metrics and test method, make it rather challenging to reliably evaluate the performance of various electrochemical systems, which hinder the development of advanced energy storage technologies and make the comparison between different energy storage systems (batteries, electrochemical capacitors, and hybrid devices) not so meaningful. In this regard, the uncertainties related to the performance metrics and evaluation of electrochemical capacitors are summarized and the ways in which the uncertainties can be minimized for both electric double layer capacitors (EDLCs) and hybrid supercapacitors is analyzed. It is hoped this effort will provide an accurate assessment method for determining the device properties to a high level of confidence and promote the standardization process of performance evaluation for the electrochemical energy storage systems.

Key words: Electrochemical capacitors, energy storage technologies, hybrid capacitors