Determining realistic electrochemical stability windows of electrolytes for electrical double layer capacitors

Electrical double-layer capacitors are a key building block for energy storage applications, including renewable energies, wherever high power is needed. Most research on electrolytes in this field focuses on improving their electrochemical stability. This improves the energy density as it scales with the square of the maximum operative voltage. However, the decisive criteria to assess the electrochemical stability window of electrolytes are unclear. Consequently, new electrolyte candidates are often presented with unrealistic high stability windows and their performance is difficult to compare. In this Minireview, an overview of electrochemical stability window determination methods is presented. It is argued that constant voltage lifetime tests are needed to confirm the electrochemical stability window determined by any other method. Also, the importance of using realistic working electrodes, reference electrodes, and cycling protocols are highlighted. Finally, an industrial perspective on what is necessary to yield results relevant to applications is given.