Electrochemical impedance spectroscopy (EIS) has been widely used in the field of electrochemical energy technologies such as fuel cells, batteries, supercapacitors, electrolysis of H 2 O and CO 2 . This paper systematically reviews the basic theory of EIS, measurement technology, development trend, and application in electrochemical energy technology, and clarifies the accurate understanding of the following four core issues: (1) What is EIS? (2) Why do you need EIS? (3) How to implement EIS diagnosis?
(4) What are capability boundaries for EIS? In order to appropriately and accurately use EIS technique, it must be clear about its
capability boundaries: what EIS can do, what EIS can’t do, and what EIS is good at. EIS is obviously not omnipotent, however its
value and potential will be fully reflected if used correctly, especially for electrochemical energy technologies.